{"notes":[{"id":"p72","n":"72","slug":"9-curl-commands-for-debugging-websites","title":"9 curl commands for debugging websites","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["Linux","curl","WebDevelopment","SysAdmin","Troubleshooting"],"series_name":"Linux","design":"terminal","script":"","date":"2027-03-14","files":{"it":"notes/p072-9-curl-commands-for-debugging-websites.md"},"md":"# 9 curl commands for debugging websites\n\nWhen a website misbehaves, the browser hides most of the story. curl shows all of it. Nine commands I would teach anyone who runs a website.\n\n## Quick reference\n\n### What did the server answer?\n\n```bash\n# -I: headers only. Status code and redirects at a glance\ncurl -I https://shop.example.com\n# only the final status code. -L follows redirects\ncurl -sL -o /dev/null -w '%{http_code}\\n' https://shop.example.com\n# -v: connection, TLS and every header, step by step\ncurl -v https://shop.example.com\n```\n\n### Where is the time going?\n\n```bash\n# total time in seconds\ncurl -so /dev/null -w '%{time_total}\\n' https://shop.example.com\n# time to first byte. High means a slow backend\ncurl -so /dev/null -w '%{time_starttransfer}\\n' https://shop.example.com\n# fail fast instead of hanging forever\ncurl --connect-timeout 5 -m 10 https://api.example.com\n```\n\n### Test before DNS, with auth, without a UI\n\n```bash\n# test the new server before you change DNS\ncurl -I --resolve shop.example.com:443:203.0.113.20 https://shop.example.com\n# send any header: tokens, Host, User-Agent\ncurl -H \"Authorization: Bearer $TOKEN\" https://api.example.com/me\n# test a form or API call without the frontend\ncurl -X POST -d 'sku=BA-101' https://api.example.com/cart\n```\n\n## Explained\n\n### What did the server answer?\n\n- `curl -I https://shop.example.com` — headers only. You see the status code, redirects, cache headers and the server in one screen.\n- `curl -sL -o /dev/null -w '%{http_code}\\n' URL` — just the final status code after following redirects. Perfect for scripts and health checks.\n- `curl -v URL` — the full conversation: which IP it connected to, the TLS handshake, every request and response header.\n\n### Where is the time going?\n\n- `curl -so /dev/null -w '%{time_total}\\n' URL` — the total time of the request, in seconds.\n- `curl -so /dev/null -w '%{time_starttransfer}\\n' URL` — time to first byte. If this is high and the rest is fast, the network is fine and the backend is slow.\n- `curl --connect-timeout 5 -m 10 URL` — give up after 5 seconds to connect and 10 seconds in total. Scripts should fail fast, not hang.\n\n### Test before DNS, with auth, without a UI\n\n- `curl -I --resolve shop.example.com:443:203.0.113.20 URL` — send the request to a new server IP while keeping the real host name. Test a migration before you change DNS.\n- `curl -H \"Authorization: Bearer $TOKEN\" URL` — send any header you need: API tokens, a Host header, a User-Agent.\n- `curl -X POST -d 'sku=BA-101' URL` — test a form or an API call directly, without clicking through the frontend.\n\n> **One honest caveat:** -I sends a HEAD request, and a few apps answer HEAD differently from GET. If the result looks strange, compare with -v.\n\n## In one line\n\n> **Read the response, not the page.** The answer is in the headers more often than you think.\n\n**Try it**\n\n```bash\ncurl -sI https://shop.example.com | head -1\n```\n"},{"id":"p73","n":"73","slug":"9-checks-after-every-linux-reboot","title":"9 checks after every Linux reboot","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","ITOperations","DevOps","Troubleshooting"],"series_name":"Linux","design":"checklist","script":"","date":"2027-03-16","files":{"it":"notes/p073-9-checks-after-every-linux-reboot.md"},"md":"# 9 checks after every Linux reboot\n\nA reboot that \"worked\" and a server that works are not the same thing. Nine checks to run every time a Linux server comes back up.\n\n## Quick reference\n\n### The system\n\n```bash\n# Nothing failed\nsystemctl --failed\n# Booted the right kernel\nuname -r\n# Every disk is mounted\ndf -h\n```\n\n### The network\n\n```bash\n# IP and route look right\nip -br a; ip r\n# Services are listening\nss -tlnp\n# Clock is in sync\ntimedatectl\n```\n\n### The workload\n\n```bash\n# No new errors\njournalctl -p err -b\n# The app answers\ncurl -sI localhost\n# Scheduled jobs still there\nsystemctl list-timers\n```\n\n## Explained\n\n### The system\n\n- `systemctl --failed` — any unit that did not start. An empty list is what you want to see.\n- `uname -r` — the kernel that is actually running. After a kernel update, this confirms the new one booted, not the old one.\n- `df -h` — every filesystem you expect is mounted. A data disk that did not mount means your app may be writing into an empty folder on the root disk.\n\n### The network\n\n- `ip -br a; ip r` — the right IP addresses on the right interfaces, and a default route. Short output, easy to read.\n- `ss -tlnp` — the services that should be listening are listening, on the ports you expect.\n- `timedatectl` — \"System clock synchronized: yes\". A wrong clock breaks TLS, logs and anything with tokens.\n\n### The workload\n\n- `journalctl -p err -b` — errors since this boot only. New errors after a reboot deserve a look before anyone reports them.\n- `curl -sI localhost` — the app actually answers, not just the process running. Use your real health endpoint if you have one.\n- `systemctl list-timers` — backups and scheduled jobs are still there and have a next run time.\n\nThis takes about five minutes. The alternative is finding out from a user, or from a backup that silently did not run.\n\nReboot is not the last step. Checking is.\n\n## In one line\n\n> **Reboot is not the last step.** Checking is. Five minutes now saves a long night later.\n\n**A one-line first look**\n\n```bash\nsystemctl --failed; df -h; journalctl -p err -b\n```\n"},{"id":"p74","n":"74","slug":"what-happens-when-you-press-enter-in-the-shell","title":"What happens when you press Enter in the shell","sections":["it"],"topic":"Linux","tags":["Linux","Bash","CommandLine","SysAdmin","DevOps"],"series_name":"Linux","design":"pipeline","script":"","date":"2027-03-18","files":{"it":"notes/p074-what-happens-when-you-press-enter-in-the-shell.md"},"md":"# What happens when you press Enter in the shell\n\nYou type ls and press Enter. Before a single file is listed, the shell does six things. Knowing them explains most strange shell errors.\n\n## Quick reference\n\n1. **The shell reads the line** — Splits it into words and expands aliases. · `read`\n2. **Expansion** — Variables, ~ and \\*.log turn into real values and file names. · `$VAR  *.log`\n3. **Find the program** — A function or builtin? If not, search each folder in `$PATH`. · `$PATH`\n4. **fork()** — The shell makes a copy of itself: a new child process. · `fork()`\n5. **exec()** — The child replaces itself with the program, like `/usr/bin/ls`. · `execve()`\n6. **Exit code** — The program ends, the shell waits, and the result goes in `$?`. · `$? = 0`\n\n## Explained\n\n1. **The shell reads the line.** It splits it into words: the command and its arguments. Aliases are expanded here, which is why ll can mean `ls -lah`.\n\n2. **Expansion.** `$HOME` becomes `/home/you`, ~ becomes your home folder, and \\*.log becomes the list of matching file names. This happens before the command runs, which is why quoting matters. rm $file with a space in the name becomes two arguments.\n\n3. **Find the program.** Is it a shell function or a builtin like cd? If not, the shell searches every folder in `$PATH`, in order, and uses the first match. \"command not found\" means it searched them all. type ls tells you what the shell will actually run.\n\n4. **fork().** The shell creates a copy of itself, a child process. The parent shell keeps running and waits.\n\n5. **exec().** The child replaces itself with the real program, for example `/usr/bin/ls`. Same process ID, completely new code.\n\n6. **Exit code.** The program finishes, the shell collects its exit status and stores it in `$?`. Zero means success, anything else is an error. This is what && and || in scripts are checking.\n\n> **Why this matters in practice:** a script that works in your terminal but not in cron is usually step 3, a different PATH. A command that breaks on file names with spaces is step 2. And builtins like cd cannot be a separate program, because a child process cannot change the parent's directory.\n\n## In one line\n\n> **Every command is fork, exec, wait.** Know the steps, and shell errors start to make sense.\n\n**See it yourself**\n\n```bash\ntype ls; echo $PATH; ls /nope; echo $?\n```\n"},{"id":"p75","n":"75","slug":"5-commands-that-take-servers-down","title":"5 commands that take servers down","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","DevOps","BestPractices","CommandLine"],"series_name":"Linux","design":"dontdo","script":"","date":"2027-03-21","files":{"it":"notes/p075-5-commands-that-take-servers-down.md"},"md":"# 5 commands that take servers down\n\nFive commands that look harmless and have taken real servers down. Each one has a safer version that costs nothing.\n\n## Quick reference\n\n**1. Don't**\n\n```bash\nrm -rf $APP_DIR/\n```\n\nIf APP_DIR is empty, this becomes `rm -rf /`.\n\n**Do instead**\n\n```bash\nrm -rf \"${APP_DIR:?}\"/\n```\n\n`${VAR:?}` stops the command if the variable is empty.\n\n**2. Don't**\n\n```bash\ncrontab -r\n```\n\nOne key away from -e. Deletes the whole crontab, no question.\n\n**Do instead**\n\n```bash\ncrontab -l > ~/crontab.bak\n```\n\nBack it up first, then edit with `crontab -e`.\n\n**3. Don't**\n\n```bash\necho '10.0.0.5 db' > /etc/hosts\n```\n\n\\> replaces the whole file. localhost is gone too.\n\n**Do instead**\n\n```bash\necho '10.0.0.5 db' >> /etc/hosts\n```\n\n\\>> appends. Better still, copy the file first.\n\n**4. Don't**\n\n```bash\nsystemctl restart sshd\n```\n\nOne typo in sshd_config and nobody can log in again.\n\n**Do instead**\n\n```bash\nsshd -t && systemctl reload sshd\n```\n\nTest the config first. Keep a second session open.\n\n**5. Don't**\n\n```bash\nreboot\n```\n\nIn the wrong terminal tab, that is production.\n\n**Do instead**\n\n```bash\napt install molly-guard\n```\n\nAsks for the host name before a `reboot` over SSH.\n\n## Explained\n\n1. `rm -rf $APP_DIR/`\n   If APP_DIR is empty or unset, the shell runs `rm -rf /`. Instead: `rm -rf \"${APP_DIR:?}\"/` — the :? makes the shell stop with an error if the variable is empty.\n\n2. `crontab -r`\n   r and e sit right next to each other on the keyboard. -e edits. -r deletes the whole crontab without asking. Instead: `crontab -l > ~/crontab.bak` before you touch it, and keep that habit.\n\n3. `echo '10.0.0.5 db' > /etc/hosts`\n   A single > replaces the entire file. Your localhost line is gone, and strange things start to break. Instead: >> appends. And for any system file, cp it to a .bak first.\n\n4. `systemctl restart sshd`\n   After editing sshd_config, one typo means the SSH daemon does not come back, and on a remote server that can mean no way in. Instead: `sshd -t` to test the config, then reload, with a second session still open.\n\n5. `reboot`\n   Typed into the wrong terminal tab, that is a production outage. Instead: install molly-guard on Debian or Ubuntu. Over SSH, it asks you to type the host name before a `reboot` or shutdown goes through.\n\n> **The pattern behind all five:** the dangerous version is the fast one. The safe version adds one check that takes a second.\n\nBefore anything risky, three commands: hostname, whoami, pwd. Where am I, who am I, and where exactly will this run?\n\n## In one line\n\n> **Slow down for one second.** Most outages start with a command that ran too fast.\n\n**Before anything risky**\n\n```bash\nhostname; whoami; pwd\n```\n"},{"id":"p76","n":"76","slug":"9-apt-and-dpkg-commands-every-ubuntu-admin-uses","title":"9 apt and dpkg commands every Ubuntu admin uses","sections":["it"],"topic":"Linux","tags":["Linux","Ubuntu","Debian","SysAdmin","PatchManagement"],"series_name":"Linux","design":"terminal","script":"","date":"2027-03-23","files":{"it":"notes/p076-9-apt-and-dpkg-commands-every-ubuntu-admin-uses.md"},"md":"# 9 apt and dpkg commands every Ubuntu admin uses\n\nMost Ubuntu admins know apt install. These nine commands are what turn patching from a gamble into a routine.\n\n## Quick reference\n\n### Keep it current\n\n```bash\n# refresh the package lists. Installs nothing\nsudo apt update\n# exactly what would change\napt list --upgradable\n# upgrade, after you have read the list\nsudo apt upgrade\n```\n\n### What is installed, and why?\n\n```bash\n# installed vs available, and from which repository\napt-cache policy nginx\n# ii means installed and healthy\ndpkg -l | grep nginx\n# which package owns this file?\ndpkg -S /usr/sbin/nginx\n```\n\n### Stay in control\n\n```bash\n# freeze a version. Upgrades will skip it\nsudo apt-mark hold nginx\n# remove unused dependencies and old kernels\nsudo apt autoremove --purge\n# what changed, before you install it\napt changelog nginx | head -3\n```\n\n## Explained\n\n### Keep it current\n\n- `sudo apt update` — refreshes the list of available packages from the repositories. It installs nothing. Run it before anything else.\n- `apt list --upgradable` — exactly which packages would change, and from which version to which. This is the list to read before you upgrade a production server.\n- `sudo apt upgrade` — upgrades everything on that list. With the list already read, there are no surprises.\n\n### What is installed, and why?\n\n- `apt-cache policy nginx` — the installed version, the candidate version, and which repository each comes from. Essential when you have added third-party repos.\n- `dpkg -l | grep nginx` — installed packages with their status. ii means installed and healthy. Anything else deserves a closer look.\n- `dpkg -S /usr/sbin/nginx` — which package owns a file. Useful when you find a binary or config and need to know where it came from.\n\n### Stay in control\n\n- `sudo apt-mark hold nginx` — freezes a package at its current version. Upgrades skip it until you run apt-mark unhold. apt-mark showhold lists everything on hold.\n- `sudo apt autoremove --purge` — removes dependencies nothing needs anymore, including old kernels that fill up `/boot`.\n- `apt changelog nginx` — what changed in the new version, from the package maintainers. Security fix or a big behaviour change? Now you know before you install.\n\nThe habit worth keeping: update, read the list, then upgrade. Not `apt upgrade -y` in the dark.\n\n## In one line\n\n> **Read the list before you upgrade.** Thirty seconds of reading beats an hour of rollback.\n\n**Try it**\n\n```bash\nsudo apt update && apt list --upgradable\n```\n"},{"id":"p77","n":"77","slug":"9-checks-before-you-patch-a-production-server","title":"9 checks before you patch a production server","sections":["it"],"topic":"Linux","tags":["Linux","PatchManagement","SysAdmin","ITOperations","CyberSecurity"],"series_name":"Linux","design":"checklist","script":"","date":"2027-03-25","files":{"it":"notes/p077-9-checks-before-you-patch-a-production-server.md"},"md":"# 9 checks before you patch a production server\n\nPatching is not optional. Patching blind is. Nine checks before you update a production Linux server.\n\n## Quick reference\n\n### Know what changes\n\n```bash\n# See what will update\napt list --upgradable\n# Space in /var and /boot\ndf -h /var /boot\n# Held packages known\napt-mark showhold\n```\n\n### Have a way back\n\n- **Backup or snapshot done** — and a tested restore\n- **Console access works** — provider console or iLO\n- **Window agreed** — low traffic, people awake\n\n### Confirm after\n\n```bash\n# Reboot needed?\nls /var/run/reboot-required\n# Services came back\nsystemctl --failed\n# The app answers\ncurl -sI http://localhost\n```\n\n## Explained\n\n### Know what changes\n\n- `apt list --upgradable` — read what will update. A kernel, a database or a web server deserves more attention than a small library.\n- `df -h /var /boot` — packages are downloaded into `/var`, and kernels go to `/boot`. A full `/boot` halfway through a kernel update is a bad afternoon.\n- `apt-mark showhold` — packages someone froze on purpose. Know why before you unhold anything.\n\n### Have a way back\n\n- **Backup or snapshot** — taken today, and from a system where you have tested a restore. A snapshot of a VM takes seconds.\n- **Console access** — if the server does not come back after a reboot, SSH will not help. Make sure the provider console, iLO or iDRAC login works before you start.\n- **Window agreed** — low traffic, and the people who know the application are awake and reachable.\n\n### Confirm after\n\n- `ls /var/run/reboot-required` — on Ubuntu, this file exists when an update needs a reboot. `/var/run/reboot-required.pkgs` lists which packages asked for it.\n- `systemctl --failed` — every service came back after the update or reboot.\n- `curl -sI http://localhost` — the application actually answers, not just the process. Use your real health check if you have one.\n\nNone of this is complicated. It is the difference between a patch that nobody notices and one everybody remembers.\n\n## In one line\n\n> **Patch with a plan, not with hope.** The patch takes five minutes. The plan makes it boring.\n\n**The first command**\n\n```bash\napt list --upgradable && df -h /boot\n```\n"},{"id":"p78","n":"78","slug":"how-linux-boots-step-by-step","title":"How Linux boots, step by step","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","systemd","ITInfrastructure","Troubleshooting"],"series_name":"Linux","design":"pipeline","script":"","date":"2027-03-28","files":{"it":"notes/p078-how-linux-boots-step-by-step.md"},"md":"# How Linux boots, step by step\n\nA server that will not boot is stuck at one specific stage. Knowing the six stages tells you where to look.\n\n## Quick reference\n\n1. **Firmware: UEFI or BIOS** — Checks the hardware and finds a disk it can boot from. · *POST*\n2. **Bootloader: GRUB** — Shows the menu and loads the chosen kernel into memory. · `grub.cfg`\n3. **Kernel** — Starts the CPU, memory and drivers. Hardware is detected. · *vmlinuz*\n4. **initramfs** — A small temporary root with the drivers to mount the real one. · `initrd.img`\n5. **systemd, PID 1** — The first process. It starts everything else. · *PID 1*\n6. **Targets and services** — Disks, network, SSH and your apps. Then the login. · `multi-user.target`\n\n## Explained\n\n1. **Firmware: UEFI or BIOS.** The machine checks its hardware and looks for a disk it can boot from. Problems here look like no disk found, or a boot order that points at the wrong device.\n\n2. **Bootloader: GRUB.** It shows the boot menu and loads the selected kernel and initramfs into memory. A broken GRUB config drops you to a grub> prompt. This is also where you can pick an older kernel if a new one fails.\n\n3. **Kernel.** The kernel starts the CPUs, sets up memory and detects hardware. A kernel panic here is often a driver or hardware problem.\n\n4. **initramfs.** A small temporary root filesystem with just enough drivers and tools to find and mount the real root filesystem, for example on LVM, RAID or an encrypted disk. If you land in an initramfs or emergency shell, the real root could not be mounted.\n\n5. **systemd, PID 1.** The first real process. Everything else on the system is started by it, which is why it always has process ID 1.\n\n6. **Targets and services.** systemd mounts the filesystems in `/etc/fstab`, brings up the network, starts SSH and your applications, and finally the login. A wrong line in fstab stops the boot right here.\n\nTwo commands worth knowing after any slow or strange boot: systemd-analyze blame shows which services took the longest, and `journalctl -b -1` shows the logs of the previous boot, the one that failed.\n\n## In one line\n\n> **Know the stage, find the problem.** A boot problem is stuck at one of these six steps.\n\n**What slowed the last boot?**\n\n```bash\nsystemd-analyze blame | head\n```\n"},{"id":"p79","n":"79","slug":"5-bash-script-mistakes-that-bite-in-production","title":"5 Bash script mistakes that bite in production","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","DevOps","Automation","ShellScripting"],"series_name":"Linux","design":"dontdo","script":"","date":"2027-03-30","files":{"it":"notes/p079-5-bash-script-mistakes-that-bite-in-production.md","devsecops":"notes/p079-5-bash-script-mistakes-that-bite-in-production.md"},"md":"# 5 Bash script mistakes that bite in production\n\nBash scripts run backups, deployments and cleanups on almost every server. Five mistakes that work fine in testing and fail in production.\n\n## Quick reference\n\n**1. Don't**\n\n```bash\nrm $file\n```\n\nA name with a space becomes two names. Or worse.\n\n**Do instead**\n\n```bash\nrm -- \"$file\"\n```\n\nQuote every variable. -- stops names starting with -.\n\n**2. Don't**\n\n```bash\ncd /opt/app; rm -rf build\n```\n\nIf cd fails, rm runs in whatever folder you are in.\n\n**Do instead**\n\n```bash\ncd /opt/app || exit 1\n```\n\nStop the script the moment cd fails.\n\n**3. Don't**\n\n```bash\nfor f in $(ls *.log); do\n```\n\nBreaks on spaces and unusual file names.\n\n**Do instead**\n\n```bash\nfor f in *.log; do\n```\n\nLet the shell expand the glob. It handles every name.\n\n**4. Don't**\n\n```bash\ntmp=/tmp/backup.tmp\n```\n\nA fixed name can collide with another run, or be abused.\n\n**Do instead**\n\n```bash\ntmp=$(mktemp)\n```\n\nA unique temp file. Remove it with a trap on EXIT.\n\n**5. Don't**\n\n```bash\n./deploy.sh\n```\n\nNever checked. Bugs show up in production first.\n\n**Do instead**\n\n```bash\nshellcheck deploy.sh\n```\n\nFinds quoting bugs and traps before they run.\n\n## Explained\n\n1. `rm $file`\n   Without quotes, a file name with a space becomes two arguments, and a name like -rf becomes an option. Instead: `rm -- \"$file\"`. Quote every variable, every time. The -- tells the command that options have ended.\n\n2. `cd /opt/app; rm -rf build`\n   If the folder does not exist or cannot be entered, cd fails and the next command runs in whatever directory the script is in. Instead: `cd /opt/app || exit 1`, or `set -e` at the top of the script.\n\n3. for f in $(ls \\*.log)\n   Parsing ls output breaks on spaces, newlines and odd characters in file names. Instead: `for f in *.log; do`. The shell expands the glob itself and handles every name correctly.\n\n4. `tmp=/tmp/backup.tmp`\n   A fixed temp file name collides when two runs overlap, and on a shared system another user can create that file first. Instead: `tmp=$(mktemp)`, and trap '`rm -f \"$tmp\"'` EXIT so it is cleaned up even if the script fails.\n\n5. `./deploy.sh, never checked`\n   Most of the mistakes above are found automatically. Instead: `shellcheck deploy.sh`. It is free, fast, and explains every warning with a link.\n\nAnd at the top of every script: `set -euo` pipefail. Stop on errors, stop on unset variables, and do not hide failures inside pipes.\n\n## In one line\n\n> **Scripts are code. Treat them like it.** Quote everything, check it, and fail early.\n\n**Add to every script**\n\n```bash\nset -euo pipefail; trap 'rm -f \"$tmp\"' EXIT\n```\n"},{"id":"p80","n":"80","slug":"9-tar-and-compression-commands","title":"9 tar and compression commands","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","Backup","CommandLine","DevOps"],"series_name":"Linux","design":"terminal","script":"","date":"2027-04-01","files":{"it":"notes/p080-9-tar-and-compression-commands.md"},"md":"# 9 tar and compression commands\n\nEvery admin uses tar. Most people know one command by heart and search for the rest. Nine that cover nearly every real situation.\n\n## Quick reference\n\n### Pack it\n\n```bash\n# c = create, z = gzip, f = the file name\ntar -czf site.tar.gz /var/www/site\n# leave logs and caches out of the archive\ntar -czf site.tar.gz --exclude='*.log' /var/www/site\n# zstd: much faster than gzip. -T0 uses every core\ntar -cf - /var/www/site | zstd -T0 > site.tar.zst\n```\n\n### Look before you unpack\n\n```bash\n# list the contents. Note: no leading /\ntar -tzf site.tar.gz | head -3\n# check the size before you copy it anywhere\nls -lh site.tar.gz\n# test that the archive is not corrupt\ngzip -t site.tar.gz && echo OK\n```\n\n### Unpack it safely\n\n```bash\n# -C: extract into a folder, not where you stand\ntar -xzf site.tar.gz -C /tmp/restore\n# extract one file only\ntar -xzf site.tar.gz var/www/site/wp-config.php\n# zip files: list first, then unzip -d folder\nunzip -l theme.zip\n```\n\n## Explained\n\n### Pack it\n\n- `tar -czf site.tar.gz /var/www/site` — c creates an archive, z compresses it with gzip, f names the file. tar removes the leading / so the archive can be extracted anywhere.\n- `tar -czf site.tar.gz --exclude='*.log' /var/www/site` — leave logs, caches and build folders out. Smaller archives, faster copies.\n- `tar -cf - /var/www/site | zstd -T0 > site.tar.zst` — zstd compresses much faster than gzip at a similar size, and -T0 uses every CPU core. Recent tar versions also support `--zstd` directly.\n\n### Look before you unpack\n\n- `tar -tzf site.tar.gz | head` — list the contents without extracting. You see the folder structure and whether paths start where you expect.\n- `ls -lh site.tar.gz` — check the size before you copy a large archive over the network or into a nearly full disk.\n- `gzip -t site.tar.gz && echo OK` — test that the compressed file is not corrupt. Better to know now than during a restore.\n\n### Unpack it safely\n\n- `tar -xzf site.tar.gz -C /tmp/restore` — extract into a specific folder. Without -C, everything lands in the directory you are standing in, which may be the live site.\n- `tar -xzf site.tar.gz var/www/site/wp-config.php` — restore a single file. Use the path exactly as `tar -t` shows it.\n- `unzip -l theme.zip` — zip files work the same way: list first, then `unzip -d` folder to extract somewhere safe.\n\nThe habit that matters: list an archive before you trust it, and extract into a temporary folder before you copy anything back.\n\nA backup you have never opened is only a hope.\n\n## In one line\n\n> **List it before you trust it.** A backup you have never opened is only a hope.\n\n**Try it**\n\n```bash\ntar -tzf backup.tar.gz | head\n```\n"},{"id":"p81","n":"81","slug":"9-firewall-checks-with-ufw","title":"9 firewall checks with ufw","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Linux","Firewall","CyberSecurity","SysAdmin","Ubuntu"],"series_name":"Linux","design":"checklist","script":"","date":"2027-04-04","files":{"it":"notes/p081-9-firewall-checks-with-ufw.md","devsecops":"notes/p081-9-firewall-checks-with-ufw.md"},"md":"# 9 firewall checks with ufw\n\nufw makes the Linux firewall simple. Simple is not the same as safe. Nine checks for a firewall you can trust.\n\n## Quick reference\n\n### Before you enable\n\n```bash\n# Allow SSH first\nufw allow OpenSSH\n# Deny incoming by default\nufw default deny incoming\n# Allow outgoing by default\nufw default allow outgoing\n```\n\n### Open only what is needed\n\n```bash\n# Web ports only\nufw allow 80,443/tcp\n# DB for the app only\nufw allow from 10.0.0.5 to any port 5432\n# Rate-limit SSH\nufw limit OpenSSH\n```\n\n### Verify\n\n```bash\n# Rules look right\nufw status numbered\n# Logging is on\nufw logging on\n# Test from outside\nnc -zv 203.0.113.10 5432\n```\n\n## Explained\n\n### Before you enable\n\n- `ufw allow OpenSSH` — first, always. Enable the firewall without this rule on a remote server and you have locked yourself out.\n- `ufw default deny incoming` — nothing comes in unless a rule allows it. This is the whole point of a firewall.\n- `ufw default allow outgoing` — the server can still reach updates, DNS and APIs. Tighten this later if you need to.\n\n### Open only what is needed\n\n- `ufw allow 80,443/tcp` — a web server needs these two ports, and nothing else from the internet.\n- `ufw allow from 10.0.0.5 to any port 5432` — the database only accepts connections from the app server's IP. The rest of the world never sees it.\n- `ufw limit OpenSSH` — blocks an IP that opens too many SSH connections in a short time. A simple brake on brute-force attempts.\n\n### Verify\n\n- `ufw status numbered` — read every rule. Numbers make it easy to delete the wrong one with ufw delete 3.\n- `ufw logging on` — blocked connections are logged, so you can see what is knocking.\n- `nc -zv 203.0.113.10 5432` — test from another machine. A rule only counts when you have confirmed it from the outside.\n\n> **One honest caveat:** Docker writes its own iptables rules, and published container ports can bypass ufw completely. If you run Docker, check your exposed ports from outside, not just ufw status.\n\n## In one line\n\n> **Allow SSH before you enable.** Order matters. The firewall does exactly what you tell it.\n\n**The safe start**\n\n```bash\nufw allow OpenSSH && ufw enable\n```\n"},{"id":"p82","n":"82","slug":"how-a-request-reaches-your-app-through-nginx","title":"How a request reaches your app through nginx","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["Nginx","Linux","WebDevelopment","DevOps","SysAdmin"],"series_name":"Linux","design":"pipeline","script":"","date":"2027-04-06","files":{"it":"notes/p082-how-a-request-reaches-your-app-through-nginx.md"},"md":"# How a request reaches your app through nginx\n\nA user opens your website. Before your application runs a single line of code, the request passes through six stages. Each one can fail in its own way.\n\n## Quick reference\n\n1. **DNS lookup** — The browser turns shop.example.com into an IP address. · *A record*\n2. **TCP connection** — A connection opens to port 443 on your server. · `:443`\n3. **TLS handshake** — nginx shows the certificate and the link is encrypted. · `ssl_certificate`\n4. **server block match** — nginx picks the site by the host name in the request. · `server_name`\n5. **location and proxy_pass** — The path decides: serve a static file, or pass to the app. · `proxy_pass`\n6. **App answers, nginx logs** — The app replies, nginx returns it and writes the log line. · `access.log`\n\n## Explained\n\n1. **DNS lookup.** The browser asks DNS for the IP address of shop.example.com. Wrong or stale records send users to the wrong server, or nowhere. `dig shop.example.com +short` shows what the world sees.\n\n2. **TCP connection.** The browser connects to port 443 on that IP. A firewall rule, a security group or nginx not listening shows up here as a timeout or \"connection refused\". `ss -tlnp` shows what is listening.\n\n3. **TLS handshake.** nginx presents the certificate from ssl_certificate. An expired certificate, a missing intermediate or a name that does not match fails here, before any HTTP is sent.\n\n4. **server block match.** nginx reads the Host header and picks the server block whose server_name matches. If none match, the default server answers, which is why people sometimes see the wrong site.\n\n5. **location and proxy_pass.** Inside that server block, the URL path is matched against location blocks. A static file is served directly. Everything else is passed to the application, for example a PHP-FPM socket or an app on 127.0.0.1:8000.\n\n6. **App answers, nginx logs.** The application responds, nginx sends the response back, and writes one line to `access.log`. Errors from the app connection go to `error.log`, with messages like \"upstream timed out\" or \"connect() failed\".\n\n> **The practical use:** when someone says \"the site is down\", these six stages are your checklist. `curl -v` walks through most of them in one command.\n\n## In one line\n\n> **Every hop can fail. Each one has a log.** Know the hop, and you know where to look.\n\n**Follow one request**\n\n```bash\ncurl -v https://shop.example.com/health\n```\n"},{"id":"p83","n":"83","slug":"5-disk-mistakes-that-lose-data","title":"5 disk mistakes that lose data","sections":["it"],"topic":"Storage & infrastructure","tags":["Linux","Storage","SysAdmin","DataProtection","DevOps"],"series_name":"Linux","design":"dontdo","script":"","date":"2027-04-08","files":{"it":"notes/p083-5-disk-mistakes-that-lose-data.md"},"md":"# 5 disk mistakes that lose data\n\nDisk commands do exactly what you type, instantly, with no undo. Five mistakes that lose data, and the check that prevents each one.\n\n## Quick reference\n\n**1. Don't**\n\n```bash\ndd if=ubuntu.iso of=/dev/sda\n```\n\nOne wrong letter and you overwrite the system disk.\n\n**Do instead**\n\n```bash\nlsblk -o NAME,SIZE,MODEL,MOUNTPOINT\n```\n\nConfirm the target first: size, model, mounted or not.\n\n**2. Don't**\n\n```bash\nmkfs.ext4 /dev/sdb1\n```\n\nFormats instantly. The wrong partition means data gone.\n\n**Do instead**\n\n```bash\nblkid /dev/sdb1\n```\n\nAlready has a filesystem? Stop and look before you format.\n\n**3. Don't**\n\n```bash\n/dev/sdb1  /data  ext4  defaults  0 2\n```\n\nDevice names can change after a `reboot`.\n\n**Do instead**\n\n```bash\nUUID=3f2a...  /data  ext4  defaults,nofail  0 2\n```\n\nUUIDs do not change. nofail keeps the boot going.\n\n**4. Don't**\n\n```bash\nreboot\n```\n\nRight after editing fstab. A typo can stop the boot.\n\n**Do instead**\n\n```bash\nmount -a && findmnt --verify\n```\n\nTest fstab now, while you can still fix it.\n\n**5. Don't**\n\n```bash\nrm /var/log/app/huge.log\n```\n\nThe space stays used while the app keeps the file open.\n\n**Do instead**\n\n```bash\ntruncate -s 0 /var/log/app/huge.log\n```\n\nEmpty it instead. The space comes back at once.\n\n## Explained\n\n1. `dd if=ubuntu.iso of=/dev/sda`\n   Writing an image to the wrong device overwrites it completely. sda and sdb are one letter apart. Instead: `lsblk -o NAME,SIZE,MODEL,MOUNTPOINT` first. The size and model tell you which disk is which. A mounted device is almost never the one you want to overwrite.\n\n2. `mkfs.ext4 /dev/sdb1`\n   Formatting is immediate. Pick the wrong partition and its data is gone. Instead: `blkid /dev/sdb1` before you format. If it already shows a filesystem type and a UUID, stop and find out what is on it.\n\n3. `/dev/sdb1` in `/etc/fstab`\n   Device names are assigned at boot and can change when disks are added or moved. Instead: use UUID= from blkid, and add nofail for data disks so a missing disk does not stop the server from booting.\n\n4. Reboot right after editing fstab\n   A single typo in fstab can drop the server into emergency mode at the next boot, far from SSH. Instead: `mount -a` to mount everything as the next boot would, and `findmnt --verify` to check the file for errors. Fix it now, not from a console at 2 AM.\n\n5. rm on a huge log that is still open\n   The file disappears from the folder, but the space stays used until the process closes it. Instead: `truncate -s 0` empties the file in place, and the space comes back immediately.\n\nBefore any disk command: `lsblk -f`. Five seconds, and you know exactly what you are about to touch.\n\n## In one line\n\n> **Look twice. Write once.** Disk commands do exactly what you type. There is no undo.\n\n**Before any disk command**\n\n```bash\nlsblk -f\n```\n"},{"id":"p84","n":"84","slug":"9-openssl-commands-for-ssl-certificate-problems","title":"9 openssl commands for SSL certificate problems","sections":["it","devsecops"],"topic":"Web servers, TLS & HTTP","tags":["SSL","TLS","Linux","CyberSecurity","SysAdmin"],"series_name":"Linux","design":"terminal","script":"","date":"2027-04-11","files":{"it":"notes/p084-9-openssl-commands-for-ssl-certificate-problems.md","devsecops":"notes/p084-9-openssl-commands-for-ssl-certificate-problems.md"},"md":"# 9 openssl commands for SSL certificate problems\n\n\"Your connection is not private.\" When a certificate breaks, the browser tells users there is a problem but not what it is. openssl tells you exactly. Nine commands for every certificate problem.\n\n## Quick reference\n\n### What certificate is live?\n\n```bash\n# connect like a browser. -servername sends SNI\nopenssl s_client -connect shop.example.com:443 -servername shop.example.com\n# valid from, valid until\nopenssl x509 -in cert.pem -noout -dates\n# the names it covers. Browsers check these\nopenssl x509 -in cert.pem -noout -ext subjectAltName\n```\n\n### Why does the browser complain?\n\n```bash\n# expires within 30 days? Exit code 1 means yes\nopenssl x509 -in cert.pem -noout -checkend 2592000\n# is the chain complete? A missing intermediate fails here\nopenssl verify -CAfile chain.pem cert.pem\n# run the same on the key (rsa). The hashes must match\nopenssl x509 -noout -modulus -in cert.pem | openssl md5\n```\n\n### A new key and request\n\n```bash\n# new private key + CSR for the certificate authority\nopenssl req -new -newkey rsa:2048 -nodes -keyout shop.key -out shop.csr\n# check the CSR before you submit it\nopenssl req -in shop.csr -noout -subject\n# Let's Encrypt: every certificate and its expiry\ncertbot certificates\n```\n\n## Explained\n\n### What certificate is live?\n\n- `openssl s_client -connect shop.example.com:443 -servername shop.example.com` — connects like a browser and shows the certificate the server really sends. The -servername part sends SNI, which matters when one server hosts many sites.\n- `openssl x509 -in cert.pem -noout -dates` — when the certificate starts and when it expires.\n- `openssl x509 -in cert.pem -noout -ext subjectAltName` — the names it is valid for. Browsers check this list, not the old Common Name field.\n\n### Why does the browser complain?\n\n- `openssl x509 -in cert.pem -noout -checkend 2592000` — does it expire within the next 30 days? The exit code makes this easy to use in a monitoring script.\n- `openssl verify -CAfile chain.pem cert.pem` — is the chain complete? A missing intermediate certificate is the classic \"works in Chrome, fails on Android and in curl\".\n- `openssl x509 -noout -modulus -in cert.pem | openssl md5` — run the same with openssl rsa on the key file. If the two hashes differ, the key and certificate do not belong together, and nginx will refuse to start.\n\n### A new key and request\n\n- `openssl req -new -newkey rsa:2048 -nodes -keyout shop.key -out shop.csr` — a new private key and a certificate signing request to send to your certificate authority.\n- `openssl req -in shop.csr -noout -subject` — check the names in the CSR before you submit it.\n- `certbot certificates` — for Let's Encrypt: every certificate and the days it has left.\n\n## In one line\n\n> **Expiry is a date. Watch it.** Monitor it, renew it automatically, and never be surprised.\n\n**Try it**\n\n```bash\nopenssl x509 -in cert.pem -noout -enddate\n```\n"},{"id":"p85","n":"85","slug":"9-checks-for-a-backup-you-can-trust","title":"9 checks for a backup you can trust","sections":["it"],"topic":"Backup & recovery","tags":["Backup","Linux","DataProtection","SysAdmin","DisasterRecovery"],"series_name":"Linux","design":"checklist","script":"","date":"2027-04-13","files":{"it":"notes/p085-9-checks-for-a-backup-you-can-trust.md"},"md":"# 9 checks for a backup you can trust\n\nAlmost every company has backups. Far fewer have backups they have actually restored. Nine checks for a backup you can trust.\n\n## Quick reference\n\n### The copies\n\n- **3 copies, 2 media, 1 offsite** — the 3-2-1 rule\n- **One copy off the server** — `rsync -a /backups/ offsite:/b/`\n- **Encrypted at rest** — restic and borg encrypt\n\n### The process\n\n- **Runs on a schedule** — `systemctl list-timers`\n- **Alerts when it fails** — on any non-zero exit code\n- **Retention is set** — keep 7 daily, 4 weekly\n\n### The proof\n\n```bash\n# A restore was tested\ntar -xzf db.tar.gz -C /tmp/r\n# Databases dumped properly\nmysqldump --single-transaction\n# Config backed up too\n/etc, crontabs, certificates\n```\n\n## Explained\n\n### The copies\n\n- **The 3-2-1 rule** — three copies of the data, on two different types of storage, with one copy offsite. A backup on the same disk as the data is not a backup.\n- **One copy off the server** — `rsync -a /backups/ offsite:/b/` or a tool like restic or borg to another location. If the server is lost, encrypted by ransomware or deleted, the backup must survive it.\n- **Encrypted at rest** — an offsite backup is a full copy of your data somewhere else. restic and borg encrypt by default. Keep the keys somewhere that is not the server itself.\n\n### The process\n\n- **Runs on a schedule** — `systemctl list-timers` or `crontab -l` shows it is really scheduled, and when it last ran.\n- **Alerts when it fails** — a backup job that exits with an error and tells nobody is the most common silent failure. Alert on any non-zero exit code, and on a job that did not run at all.\n- **Retention is set** — for example 7 daily, 4 weekly and 12 monthly. Without retention, the disk fills up and the newest backups fail first.\n\n### The proof\n\n- **A restore was tested** — extract to a temporary folder like `/tmp/r` and check the files open. Do it monthly, not on the day you need it.\n- **Databases dumped properly** — copying the files of a running database can give a broken backup. `mysqldump --single-transaction` or pg_dump give a consistent one.\n- **Config backed up too** — `/etc, crontabs, certificates` and keys. Rebuilding a server from memory takes far longer than restoring its config.\n\n## In one line\n\n> **A backup is only a restore you tested.** Test one restore every month. Put it in the calendar.\n\n**Check the last run**\n\n```bash\njournalctl -u backup.service -n 20\n```\n"},{"id":"p86","n":"86","slug":"add-a-new-disk-to-a-linux-server","title":"Add a new disk to a Linux server","sections":["it"],"topic":"Storage & infrastructure","tags":["Linux","Storage","SysAdmin","ITInfrastructure","DevOps"],"series_name":"Linux","design":"pipeline","script":"","date":"2027-04-15","files":{"it":"notes/p086-add-a-new-disk-to-a-linux-server.md"},"md":"# Add a new disk to a Linux server\n\nAdding a disk to a Linux server is a six-step job. Done in the right order, it is ten minutes. Done in the wrong order, it is a server that does not boot.\n\n## Quick reference\n\n1. **Find the disk** — The new disk has no partitions and no mount point. · `lsblk`\n2. **Partition it** — One GPT partition across the whole disk. · `parted /dev/sdb`\n3. **Make a filesystem** — Format the new partition. Double-check the name first. · `mkfs.ext4 /dev/sdb1`\n4. **Mount it** — Create a folder and mount the partition there. · `mount /dev/sdb1 /data`\n5. **Make it permanent** — Add it to `/etc/fstab` by UUID, with nofail. · `blkid /dev/sdb1`\n6. **Test before reboot** — Mount everything from fstab and confirm it worked. · `mount -a && df -h`\n\n## Explained\n\n1. **Find the disk.** lsblk lists every block device. The new disk is the one with the right size, no partitions and no mount point, for example `/dev/sdb`. Never guess this from the name alone.\n\n2. **Partition it.** `parted /dev/sdb`, then mklabel gpt and mkpart primary ext4 0% 100% creates one partition across the whole disk: `/dev/sdb1`. GPT handles disks larger than 2 TB, which old MBR tables cannot.\n\n3. **Make a filesystem.** `mkfs.ext4 /dev/sdb1` formats the partition. Check the device name one more time before you press Enter. This step erases whatever is on it. XFS with mkfs.xfs is a common alternative for large data disks.\n\n4. **Mount it.** `mkdir /data`, then `mount /dev/sdb1 /data`. The disk is usable now, but only until the next reboot.\n\n5. **Make it permanent.** `blkid /dev/sdb1` shows the UUID. Add a line to `/etc/fstab`: `UUID=<uuid> /data ext4 defaults,nofail 0 2`. The UUID never changes, unlike the device name, and nofail means the server still boots if this disk is ever missing.\n\n6. **Test before reboot.** `umount /data`, then `mount -a` mounts everything from fstab the same way the next boot will. `df -h` confirms `/data` is there with the right size. `findmnt --verify` checks fstab for mistakes.\n\nThat last step is the one people skip, and the one that saves you from an emergency console session.\n\n## In one line\n\n> **Test fstab before you reboot.** A bad fstab line is found now, or at 2 AM on a console.\n\n**The fstab line**\n\n```bash\nUUID=<uuid>  /data  ext4  defaults,nofail  0 2\n```\n"},{"id":"p87","n":"87","slug":"5-network-changes-that-lock-you-out","title":"5 network changes that lock you out","sections":["it"],"topic":"Networking & DNS","tags":["Linux","Networking","SysAdmin","Ubuntu","DevOps"],"series_name":"Linux","design":"dontdo","script":"","date":"2027-04-18","files":{"it":"notes/p087-5-network-changes-that-lock-you-out.md"},"md":"# 5 network changes that lock you out\n\nOn a remote server, a network mistake does not just break something. It removes your only way to fix it. Five changes that lock admins out, and the safe version of each.\n\n## Quick reference\n\n**1. Don't**\n\n```bash\nnetplan apply\n```\n\nOne typo in the new IP, and your SSH session is gone.\n\n**Do instead**\n\n```bash\nnetplan try\n```\n\nRolls back after 120 seconds unless you confirm.\n\n**2. Don't**\n\n```bash\niptables -F\n```\n\nWith a DROP policy, flushing the rules blocks everything.\n\n**Do instead**\n\n```bash\niptables -P INPUT ACCEPT && iptables -F\n```\n\nSet the policy to ACCEPT first, then flush.\n\n**3. Don't**\n\n```bash\nvi /etc/resolv.conf\n```\n\nsystemd-resolved rewrites it. Your change disappears.\n\n**Do instead**\n\n```bash\nresolvectl status\n```\n\nSee who manages DNS, and change it there.\n\n**4. Don't**\n\n```bash\nip route add 10.8.0.0/24 via 10.0.0.1\n```\n\nWorks now. Gone after the next reboot.\n\n**Do instead**\n\nroutes: in `/etc/netplan/*.yaml`\n\nPut it in the network config so it survives a reboot.\n\n**5. Don't**\n\n```bash\nping db01\n```\n\nICMP is often blocked. No reply does not mean down.\n\n**Do instead**\n\n```bash\nnc -zv db01 5432\n```\n\nTest the real port that the app uses.\n\n## Explained\n\n1. `netplan apply`\n   It applies the new network config immediately. One wrong IP, gateway or interface name, and your SSH session is gone. Instead: `netplan try`. It applies the change and rolls it back automatically after 120 seconds unless you press Enter to confirm.\n\n2. `iptables -F`\n   Flushing the rules sounds like \"open everything\". But if the default policy is DROP, removing the rules that allowed SSH blocks everything, including you. Instead: `iptables -P INPUT` ACCEPT first, then flush.\n\n3. Editing `/etc/resolv.conf` directly\n   On most modern distributions, systemd-resolved or NetworkManager manages this file and rewrites it. Your change works for a while, then quietly disappears. Instead: `resolvectl status` shows who is in charge of DNS. Change it there, for example in netplan.\n\n4. ip route add for a permanent route\n   It works immediately, so it feels finished. After the next reboot it is gone, and nobody remembers why the VPN subnet stopped working. Instead: add the route to the network configuration, like a routes: section in netplan.\n\n5. ping to test a service\n   Many networks block ICMP. No ping reply does not mean the server is down, and a reply does not mean the service works. Instead: `nc -zv db01 5432` tests the actual port the application uses.\n\n> **The rule behind all five:** before any network change on a remote server, have a way back. A rollback timer, a provider console, or a second session open.\n\n## In one line\n\n> **Always have a way back.** A timer, a console or a second session. Before every change.\n\n**The safest network command**\n\n```bash\nnetplan try --timeout 60\n```\n"},{"id":"p88","n":"88","slug":"9-commands-to-know-any-server-in-60-seconds","title":"9 commands to know any server in 60 seconds","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","ITSupport","DevOps","CommandLine"],"series_name":"Linux","design":"terminal","script":"","date":"2027-04-20","files":{"it":"notes/p088-9-commands-to-know-any-server-in-60-seconds.md"},"md":"# 9 commands to know any server in 60 seconds\n\nA new job, a new client, or a server nobody documented. Before you change anything, spend sixty seconds finding out what it is. Nine commands.\n\n## Quick reference\n\n### What is this machine?\n\n```bash\n# name, OS, kernel, VM or bare metal\nhostnamectl\n# the kernel that is running right now\nuname -r\n# time since the last reboot, and the load\nuptime\n```\n\n### What does it run on?\n\n```bash\n# CPU model and number of cores\nlscpu | grep -E 'Model name|^CPU\\(s\\)'\n# memory. Read the available column\nfree -h\n# disks, partitions and where they are mounted\nlsblk -o NAME,SIZE,TYPE,MOUNTPOINT\n```\n\n### What does it actually do?\n\n```bash\n# every service that is running\nsystemctl list-units --type=service --state=running\n# what listens on which port\nss -tlnp\n# what runs on a schedule\ncrontab -l; ls /etc/cron.d\n```\n\n## Explained\n\n### What is this machine?\n\n- `hostnamectl` — the host name, operating system, kernel, and whether it is a virtual machine or bare metal. One command, a lot of answers.\n- `uname -r` — the kernel that is actually running. Compare it with the installed kernels to see if a reboot is overdue.\n- `uptime` — how long since the last reboot, and the load average for the last 1, 5 and 15 minutes. 400 days of `uptime` also tells you something about patching.\n\n### What does it run on?\n\n- `lscpu | grep -E 'Model name|^CPU\\(s\\)'` — the CPU model and number of cores. Now the load average has context: a load of 4 is busy on 4 cores and quiet on 32.\n- `free -h` — total memory and, more importantly, the available column. Low \"free\" is normal on Linux. Low \"available\" is not.\n- `lsblk -o NAME,SIZE,TYPE,MOUNTPOINT` — the disks, their partitions, and where each one is mounted. Spot the data disk, LVM or RAID at a glance.\n\n### What does it actually do?\n\n- `systemctl list-units --type=service --state=running` — every running service. This is the real job description of the server.\n- `ss -tlnp` — what is listening on which port, and which process owns it. Run with sudo to see the process names.\n- `crontab -l; ls /etc/cron.d` — what runs on a schedule. Check other users' crontabs and systemctl list-timers too. Hidden jobs are where surprises come from.\n\n## In one line\n\n> **Understand it before you change it.** Sixty seconds of looking saves a day of guessing.\n\n**Try it**\n\n```bash\nhostnamectl; uptime; ss -tlnp\n```\n"},{"id":"p89","n":"89","slug":"9-checks-before-a-server-goes-live","title":"9 checks before a server goes live","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["Linux","SysAdmin","DevOps","ITOperations","CyberSecurity"],"series_name":"Linux","design":"checklist","script":"","date":"2027-04-22","files":{"it":"notes/p089-9-checks-before-a-server-goes-live.md","devsecops":"notes/p089-9-checks-before-a-server-goes-live.md"},"md":"# 9 checks before a server goes live\n\nThe app works, the demo went well, and the server is about to go live. Nine checks first, because after today, real people depend on it.\n\n## Quick reference\n\n### Access\n\n- **Named admin accounts** — no shared root logins\n- **SSH keys, no passwords** — `PasswordAuthentication no`\n- **Only needed ports open** — `ufw status verbose`\n\n### Operations\n\n- **Monitoring and alerts** — CPU, disk, memory, uptime\n- **Backups and a restore** — a tested restore, not a job\n- **Logs rotate** — `logrotate -d /etc/logrotate.conf`\n\n### Basics\n\n- **Time in sync** — `timedatectl`\n- **Security updates automatic** — `apt install unattended-upgrades`\n- **Written down** — owner, purpose, how to restart\n\n## Explained\n\n### Access\n\n- **Named admin accounts** — every admin logs in as themselves and uses sudo. No shared root password, so you always know who did what.\n- **SSH keys, no passwords** — `PasswordAuthentication no` in sshd_config, after key login is tested. Automated password guessing becomes useless.\n- **Only needed ports open** — `ufw status verbose` should show the web ports and SSH, and nothing you cannot explain.\n\n### Operations\n\n- **Monitoring and alerts** — CPU, memory, disk space and an external uptime check, with alerts that reach a person. If it breaks at night, you hear about it before the users do.\n- **Backups and a restore** — not just a backup job, but one restore you have actually tested on this server.\n- **Logs rotate** — `logrotate -d /etc/logrotate.conf` is a dry run. Make sure the application logs are included, or the disk fills up in a few months.\n\n### Basics\n\n- **Time in sync** — `timedatectl` should say the clock is synchronized. Wrong time breaks TLS, tokens and every log timeline.\n- **Security updates automatic** — `apt install unattended-upgrades` on Ubuntu or Debian, so security patches do not wait for someone to remember.\n- **Written down** — who owns the server, what it does, how to restart it, and where the backups are. The person reading it at 3 AM may not be you.\n\nLive is not the finish line. It is the moment someone else starts to depend on it.\n\n## In one line\n\n> **Live is not the finish line.** It is the moment someone else starts to depend on it.\n\n**The 10-second audit**\n\n```bash\nufw status; timedatectl; systemctl --failed\n```\n"},{"id":"p90","n":"90","slug":"the-life-of-a-log-line","title":"The life of a log line","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","Logging","SysAdmin","CyberSecurity","DevOps"],"series_name":"Linux","design":"pipeline","script":"","date":"2027-04-25","files":{"it":"notes/p090-the-life-of-a-log-line.md","devsecops":"notes/p090-the-life-of-a-log-line.md"},"md":"# The life of a log line\n\nEvery log line on a Linux server has a life: it is written, collected, stored, rotated, maybe shipped, and eventually deleted. Knowing the path tells you where to look, and how long you have.\n\n## Quick reference\n\n1. **The app writes it** — To stdout, to a file, or to the syslog socket. · `logger`\n2. **journald collects it** — systemd captures the output of every service. · `journalctl -u`\n3. **rsyslog files it** — Gets a copy from the journal and writes classic log files. · `/var/log/syslog`\n4. **logrotate rotates it** — Daily or by size: rename, compress, start a fresh file. · `app.log.1.gz`\n5. **Shipped off the box** — Optional: sent to a central log server or SIEM. · `@@siem:514`\n6. **Retention deletes it** — After N rotations, the oldest file is removed. · *rotate 14*\n\n## Explained\n\n1. **The app writes it.** To standard output, to its own file in `/var/log`, or to the syslog socket. logger \"test\" writes a line yourself, which is a handy way to test the whole chain.\n\n2. **journald collects it.** For every service started by systemd, its output goes into the journal. `journalctl -u nginx` shows one service, `journalctl -b` the current boot. By default the journal may live only in memory and vanish at reboot unless `/var/log/journal` exists or Storage=persistent is set.\n\n3. **rsyslog files it.** On Debian and Ubuntu, rsyslog receives the messages and writes the classic text files like `/var/log/syslog` and `/var/log/auth.log`. On RHEL-based systems, the equivalents are `/var/log/messages` and `/var/log/secure`.\n\n4. **logrotate rotates it.** Usually daily, or when a file reaches a size limit, the current log is renamed to `app.log`.1, older ones are compressed to .gz, and a fresh file starts.\n\n5. **Shipped off the box.** Optionally, rsyslog forwards everything to a central server or SIEM, for example \\*.\\* @@siem:514 over TCP. If an attacker cleans the local logs, the central copy survives.\n\n6. **Retention deletes it.** With rotate 14, the fifteenth oldest file is deleted. That number decides how far back you can investigate.\n\nThe practical question to answer today: if an incident started three weeks ago, would your logs still be there?\n\n## In one line\n\n> **Know where logs go, and when they go away.** During an incident, a deleted log is a missing witness.\n\n**See one line travel**\n\n```bash\nlogger \"test from $(hostname)\" && journalctl -n 1\n```\n"},{"id":"p91","n":"91","slug":"5-troubleshooting-habits-to-unlearn","title":"5 troubleshooting habits to unlearn","sections":["it"],"topic":"IT operations & incidents","tags":["Linux","Troubleshooting","SysAdmin","DevOps","IncidentResponse"],"series_name":"Linux","design":"dontdo","script":"","date":"2027-04-27","files":{"it":"notes/p091-5-troubleshooting-habits-to-unlearn.md"},"md":"# 5 troubleshooting habits to unlearn\n\nMost troubleshooting time is not lost on hard problems. It is lost on habits that hide the real cause. Five to unlearn.\n\n## Quick reference\n\n**1. Don't**\n\n```bash\nsystemctl restart app\n```\n\nRestarting first wipes out the evidence.\n\n**Do instead**\n\n```bash\njournalctl -u app --since '10 min ago'\n```\n\nRead what happened first. Then restart.\n\n**2. Don't**\n\n```bash\ntail -f /var/log/app.log\n```\n\nAfter log rotation, -f keeps watching the old file.\n\n**Do instead**\n\n```bash\ntail -F /var/log/app.log\n```\n\n-F follows the file name, even after rotation.\n\n**3. Don't**\n\n```bash\ngrep '03:00' app.log\n```\n\nApp logs often use UTC. You may read the wrong hour.\n\n**Do instead**\n\n```bash\ntimedatectl\n```\n\nCheck the server time zone before you compare times.\n\n**4. Don't**\n\n```bash\nvi nginx.conf php.ini my.cnf\n```\n\nThree changes at once. Which one fixed it, or broke it?\n\n**Do instead**\n\n```bash\ngit -C /etc commit -am 'one change'\n```\n\nOne change at a time, with a record of each.\n\n**5. Don't**\n\n```bash\njournalctl -u app\n```\n\nThe app log only shows the app. The cause may be below it.\n\n**Do instead**\n\n```bash\ndmesg -T | tail -20\n```\n\nDisk errors, OOM kills and hardware faults live here.\n\n## Explained\n\n1. Restarting first\n   `systemctl restart app` often makes the symptom go away, and takes the explanation with it. Instead: `journalctl -u app --since '10 min ago'` first. Thirty seconds of reading tells you whether this will happen again tonight.\n\n2. `tail -f` on a log that rotates\n   When logrotate renames the file, `tail -f` keeps watching the old one. The screen goes quiet and you think the problem stopped. Instead: `tail -F` follows the file name, and picks up the new file after rotation.\n\n3. Ignoring time zones\n   The app logs in UTC, the server shows local time, and the monitoring tool uses a third. You end up comparing events that are hours apart. Instead: check `timedatectl` first, and note which time zone each log uses before you build a timeline.\n\n4. Changing several things at once\n   Three config files edited, one restart, and the problem is gone. Which change fixed it? Or which one will break something next week? Instead: one change, one test, and a record of it. With `/etc` in git, every change is one commit.\n\n5. Only reading the app's log\n   The application log only knows about the application. A failing disk, an OOM kill or a network driver problem happens below it. Instead: `dmesg -T | tail -20` shows the kernel's side of the story, with readable timestamps.\n\nThe first two commands for any failing service: systemctl status app, then `journalctl -xeu` app.\n\n## In one line\n\n> **Slow is smooth. Smooth is fast.** Read first, change one thing, write it down.\n\n**The first two commands**\n\n```bash\nsystemctl status app; journalctl -xeu app\n```\n"},{"id":"p92","n":"92","slug":"quiz-3-linux-questions-most-admins-get-wrong","title":"Quiz: 3 Linux questions most admins get wrong","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","ITCareer","DevOps","CommandLine"],"series_name":"Linux + IT","design":"quiz","script":"","date":"2027-04-29","files":{"it":"notes/p092-quiz-3-linux-questions-most-admins-get-wrong.md"},"md":"# Quiz: 3 Linux questions most admins get wrong\n\nThree Linux questions that catch out experienced admins. Answer each one before you read the explanation.\n\n## Quick reference\n\n**Q1. You delete a 10 GB log file, but `df -h` still shows the same usage. Why?**\n\n- a) The filesystem needs a reboot to update\n- b) A process still has the file open\n- c) Linux keeps deleted files in a trash folder\n\n> **Answer:** b) A process still has the file open. The space is freed only when the last process closes the file. Find it with `lsof +L1`, then restart that process.\n\n**Q2. A 4-core server shows a load average of 4.00. What does that mean?**\n\n- a) It is overloaded at 400%\n- b) It is about fully busy, not yet overloaded\n- c) Four users are logged in\n\n> **Answer:** b) It is about fully busy, not yet overloaded. Load counts tasks running or waiting. On 4 cores, 4.00 means every core is busy. Above that, work starts to queue.\n\n**Q3. Which file decides whether `/etc/hosts` is checked before DNS?**\n\n- a) `/etc/resolv.conf`\n- b) `/etc/nsswitch.conf`\n- c) `/etc/hostname`\n\n> **Answer:** b) `/etc/nsswitch.conf`. The hosts: line in `nsswitch.conf` sets the order, usually files then dns. `resolv.conf` only lists the DNS servers.\n\n## Explained\n\n1. You delete a 10 GB log file, but `df -h` still shows the same disk usage. Why?\n\n> **Answer:** a process still has the file open. Deleting a file removes its name, but the data stays on disk until the last process holding it closes the file. `lsof +L1` lists these deleted-but-open files. Restart the process and the space comes back. Next time, empty the file with `truncate -s 0` instead of deleting it.\n\n2. A 4-core server shows a load average of 4.00. What does that mean?\n\n> **Answer:** it is roughly fully busy, but not overloaded. The load average counts tasks that are running or waiting to run, including tasks waiting on disk I/O. On 4 cores, 4.00 means every core has work. A load of 8 on the same machine means tasks are queuing. The number only means something next to the core count, which nproc shows.\n\n3. Which file decides whether `/etc/hosts` is checked before DNS?\n\n> **Answer:** `/etc/nsswitch.conf`. The hosts: line sets the lookup order, usually files dns, meaning the hosts file first and DNS second. `/etc/resolv.conf` only says which DNS servers to ask. This is why an old entry in `/etc/hosts` can override a DNS change and make one server see a different site than everyone else.\n\nNone of these are rare situations. All three show up in real tickets every month.\n\n## In one line\n\n> **Fundamentals never expire.** Tools change every year. These answers do not.\n\n**Try the first one**\n\n```bash\nlsof +L1\n```\n"},{"id":"p93","n":"93","slug":"9-windows-commands-for-network-problems","title":"9 Windows commands for network problems","sections":["it"],"topic":"Windows & Active Directory","tags":["Windows","Networking","ITSupport","Troubleshooting","SysAdmin"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-05-02","files":{"it":"notes/p093-9-windows-commands-for-network-problems.md"},"md":"# 9 Windows commands for network problems\n\n\"The internet is down\" is a symptom, not a diagnosis. Nine built-in Windows commands take you from the complaint to the exact cause.\n\n## Quick reference\n\n### This pc — What does the machine think?\n\n```powershell\n# IP, gateway, DNS servers and DHCP lease for each adapter.\nipconfig /all\n# Clear the local DNS cache after a record changes.\nipconfig /flushdns\n# The routing table. Check the default route is there.\nroute print\n```\n\n### The path — Where does it break?\n\n```bash\n# Reach an IP first. This rules DNS in or out.\nping -n 4 8.8.8.8\n# Every hop on the way. Shows where packets stop.\ntracert shop.example.com\n# What DNS answers, and which server answered.\nnslookup shop.example.com\n```\n\n### The service — Is the port really open?\n\n```powershell\n# Tests one TCP port. Look at TcpTestSucceeded.\nTest-NetConnection shop.example.com -Port 443\n# What uses a port, with the process ID.\nnetstat -ano | findstr :3389\n# Repairs a broken network stack. Needs a restart.\nnetsh winsock reset\n```\n\n## Explained\n\n### This PC\n\n- `ipconfig /all` — the IP address, gateway, DNS servers and DHCP lease for every adapter. An address starting with 169.254 means the PC never got one from DHCP.\n- `ipconfig /flushdns` — clears the local DNS cache. Use it after a DNS record changes and one PC still goes to the old address.\n- `route print` — the routing table. No default route (0.0.0.0) means nothing outside the local network is reachable, and a VPN client can quietly change this.\n\n### The path\n\n- `ping -n 4 8.8.8.8` — test an IP address first. If this works but names do not, the problem is DNS, not the connection.\n- `tracert shop.example.com` — every router on the way to the destination. Where the replies stop is where to look next.\n- `nslookup shop.example.com` — what DNS answers, and which DNS server gave that answer. Compare it with `nslookup shop.example.com 1.1.1.1`.\n\n### The service\n\n- `Test-NetConnection shop.example.com -Port 443` — tests whether a specific TCP port is reachable. Many firewalls block ping, so this is the real test for a website, RDP or a database.\n- `netstat -ano | findstr :3389` — what is listening on or connected to a port, with the process ID. Match the PID in Task Manager.\n- `netsh winsock reset` — resets the Windows network stack when it is corrupted, often by old VPN or security software. It needs a restart.\n\n> **The pattern:** name, path, port. Test all three and the cause is almost always clear.\n\n## In one line\n\n> **Ping is only the first question.** Name, path, port. Test all three before you call it fixed.\n\n**The one that answers most**\n\n```powershell\nTest-NetConnection host -Port 443\n```\n"},{"id":"p94","n":"94","slug":"6-port-numbers-every-it-person-should-know","title":"6 port numbers every IT person should know","sections":["it"],"topic":"Networking & DNS","tags":["Networking","CyberSecurity","ITSupport","SysAdmin","CCNA"],"series_name":"Linux + IT","design":"numbers","script":"","date":"2027-05-04","files":{"it":"notes/p094-6-port-numbers-every-it-person-should-know.md"},"md":"# 6 port numbers every IT person should know\n\nSix port numbers that come up in almost every IT job. Knowing them turns a firewall rule or a scan result into something you can read at a glance.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **22** | SSH | Remote shell for Linux. Never open it to the world without keys. | TCP |\n| **53** | DNS | UDP for normal lookups, TCP for big answers and transfers. | UDP/TCP |\n| **443** | HTTPS | Encrypted web traffic. Almost everything runs over it now. | TCP |\n| **3389** | RDP | Windows Remote Desktop. A favourite target for ransomware. | TCP |\n| **3306** | MySQL / MariaDB | Database port. Keep it on localhost or a private network. | TCP |\n| **587** | SMTP submission | Where mail clients send email, with login and TLS. | TCP |\n\n## Explained\n\n- **22** — SSH. The remote shell for Linux and network devices. It is scanned constantly, so it should only accept key-based logins, and ideally only from known IPs or a VPN.\n\n- **53** — DNS. Normal lookups use UDP. Large responses and zone transfers between DNS servers use TCP on the same port. A firewall that only allows UDP 53 can cause strange, intermittent DNS failures.\n\n- **443** — HTTPS. Encrypted web traffic. Today it carries websites, APIs, VPNs and much more. If only one inbound port is open on a web server, this is usually it, plus port 80 to redirect to it.\n\n- **3389** — RDP. Windows Remote Desktop. Open to the internet, it is one of the most common ways ransomware groups get in. Put it behind a VPN or an RD Gateway with MFA.\n\n- **3306** — MySQL and MariaDB. A database should almost never be reachable from the internet. Bind it to 127.0.0.1 or a private network, and allow only the application servers.\n\n- **587** — SMTP submission. This is where mail clients and applications send email, with authentication and STARTTLS. Port 25 is for server-to-server delivery, and many cloud providers block it outbound.\n\nA good habit: run `ss -tlnp` on Linux or `netstat -ano` on Windows and explain every listening port. Anything you cannot explain should not be open.\n\n## In one line\n\n> **Know the port, know the risk.** Every open port is a door. Count yours.\n\n**What is open here?**\n\n```bash\nsudo ss -tlnp\n```\n"},{"id":"p95","n":"95","slug":"vlan-vs-subnet","title":"VLAN vs Subnet","sections":["it"],"topic":"Networking & DNS","tags":["Networking","CCNA","VLAN","ITInfrastructure","NetworkSecurity"],"series_name":"Linux + IT","design":"compare","script":"","date":"2027-05-06","files":{"it":"notes/p095-vlan-vs-subnet.md"},"md":"# VLAN vs Subnet\n\nVLANs and subnets are often mixed up because they are almost always used together. They solve the problem at two different layers.\n\n## Quick reference\n\n| | **VLAN** | **SUBNET** |\n|:--|:--|:--|\n| OSI layer | Layer 2 (data link) | Layer 3 (network) |\n| Configured on | Switches, 802.1Q tags | IP addresses and masks |\n| Separates | Broadcast domains | IP networks |\n| Talk across | Router or L3 switch | Router or L3 switch |\n| Example | VLAN 20 = Finance | 192.168.20.0/24 |\n| Usual design | One subnet per VLAN | One VLAN per subnet |\n\n**VLAN** (Layer 2 separation): Splits one physical switch into separate broadcast domains.\n\nExample: `switchport access vlan 20`\n\n**SUBNET** (Layer 3 addressing): Splits an IP range into networks that need a router to talk.\n\nExample: `192.168.20.0/24`\n\n- VLAN → separates devices on the switch.\n- Subnet → separates addresses in IP. Use them together.\n\n## Explained\n\nA VLAN works at Layer 2, on the switch. switchport access vlan 20 puts a port into VLAN 20. Devices in VLAN 20 can only see each other's broadcasts, as if they were on a separate physical switch. Between switches, a trunk carries many VLANs over one cable, with an 802.1Q tag on each frame saying which VLAN it belongs to.\n\nA subnet works at Layer 3, in IP addressing. 192.168.20.0/24 is a range of 254 usable addresses. Devices in different subnets cannot talk directly. Their traffic has to go through a router or a Layer 3 switch.\n\n### How they fit together\n\nThe usual design is one subnet per VLAN. VLAN 10 is 192.168.10.0/24 for staff, VLAN 20 is 192.168.20.0/24 for finance, VLAN 30 for printers, VLAN 40 for guest Wi-Fi. The router or Layer 3 switch has an interface in each one, and that is where you control what can talk to what.\n\n### Where people get confused\n\nPutting two subnets in one VLAN works, but broadcasts still reach everyone, so you gain addresses, not separation.  \nCreating VLANs without firewall rules between them separates broadcasts, but the router may still forward everything. The security comes from the rules on the router, not from the VLAN alone.\n\nThe practical win: printers, cameras and guest devices in their own VLAN and subnet means a compromised camera cannot reach the finance PCs.\n\nVLAN separates on the switch. Subnet separates in IP. Use both.\n"},{"id":"p96","n":"96","slug":"ext4-vs-xfs-vs-btrfs","title":"ext4 vs XFS vs Btrfs","sections":["it"],"topic":"Storage & infrastructure","tags":["Linux","Storage","SysAdmin","Filesystem","ITInfrastructure"],"series_name":"Linux + IT","design":"matrix","script":"","date":"2027-05-09","files":{"it":"notes/p096-ext4-vs-xfs-vs-btrfs.md"},"md":"# ext4 vs XFS vs Btrfs\n\nThree Linux filesystems you will meet on servers. Each one is the right answer somewhere.\n\n## Quick reference\n\n| | **ext4** | **XFS** | **Btrfs** |\n|:--|:--|:--|:--|\n| Maturity | ✅ Very mature | ✅ Very mature | 🟡 RAID 5/6 not advised |\n| Default on | Debian, Ubuntu | RHEL, Rocky, Alma | Fedora, openSUSE |\n| Shrink | ✅ Yes, offline | ❌ Not possible | ✅ Yes, online |\n| Snapshots | ❌ Use LVM | ❌ Use LVM | ✅ Built in |\n| Checksums | 🟡 Metadata only | 🟡 Metadata only | ✅ Data and metadata |\n| Huge files, big disks | 🟡 Good | ✅ Excellent | 🟡 Good |\n\n**How to pick**\n\n- **ext4** → the safe default for most servers and VMs.\n- **XFS** → large disks, big files, heavy parallel I/O.\n- **Btrfs** → when you want snapshots and data checksums.\n\n## Explained\n\next4 is the long-time default on Debian and Ubuntu. Extremely mature, well understood, and every rescue tool supports it. It can grow online and shrink while unmounted. It checksums its metadata but not your data, and it has no snapshots of its own, so people pair it with LVM when they need them.\n\nXFS is the default on RHEL, Rocky and Alma Linux. It shines with very large disks, very large files and many parallel writes, which is why it is common on file and database servers. Its one hard limit: an XFS filesystem cannot be shrunk. Plan your sizes, or be ready to copy the data to a new volume.\n\nBtrfs is the default on Fedora and openSUSE. It has snapshots built in, checksums on data as well as metadata so silent corruption is detected, compression, and it can grow and shrink online. The honest caveat: its RAID 5 and 6 modes are still not recommended for important data. RAID 1 and 10 are fine.\n\n### The practical choice\n\next4 for most servers and VMs, especially if your team already knows it.  \nXFS for big storage and heavy I/O, on the RHEL family where it is the default anyway.  \nBtrfs when snapshots before upgrades and data checksums matter more than familiarity.\n\nThe most important feature of a filesystem is that someone on your team knows how to repair it at 3 AM.\n\n`df -Th` shows what each of your filesystems uses.\n\n## In one line\n\n> **Boring is a feature here.** Pick the filesystem you know how to repair.\n\n**Check what you run**\n\n```bash\ndf -Th\n```\n"},{"id":"p97","n":"97","slug":"9-dig-commands-for-dns-problems","title":"9 dig commands for DNS problems","sections":["it"],"topic":"Networking & DNS","tags":["DNS","Networking","Linux","SysAdmin","Troubleshooting"],"series_name":"Linux + IT","design":"terminal","script":"","date":"2027-05-11","files":{"it":"notes/p097-9-dig-commands-for-dns-problems.md"},"md":"# 9 dig commands for DNS problems\n\n\"It is always DNS.\" Sometimes it is. dig tells you for sure. Nine commands that answer almost every DNS question.\n\n## Quick reference\n\n### What does DNS say?\n\n```bash\n# just the answer, nothing else\ndig shop.example.com +short\n# mail servers, with priority\ndig example.com MX +short\n# SPF, site verification and other text records\ndig example.com TXT +short\n```\n\n### Who says what?\n\n```bash\n# ask one specific resolver\ndig @1.1.1.1 shop.example.com +short\n# ask the authoritative server. Different = cache\ndig @ns1.example.com shop.example.com +short\n# 2843 = seconds left before the cache expires\ndig shop.example.com +noall +answer\n```\n\n### Follow the whole chain\n\n```bash\n# from the root servers down to the answer\ndig shop.example.com +trace\n# which servers are authoritative\ndig example.com NS +short\n# reverse lookup: IP to name (PTR)\ndig -x 203.0.113.10 +short\n```\n\n## Explained\n\n### What does DNS say?\n\n- `dig shop.example.com +short` — only the answer, the IP address, without the rest of the output.\n- `dig example.com MX +short` — the mail servers for a domain and their priority. Lower number, higher priority.\n- `dig example.com TXT +short` — text records: SPF, DKIM selectors, domain verification codes. Most email delivery problems start here.\n\n### Who says what?\n\n- `dig @1.1.1.1 shop.example.com +short` — ask one specific resolver instead of your default. Compare 1.1.1.1, 8.8.8.8 and your company DNS to see who is out of date.\n- `dig @ns1.example.com shop.example.com +short` — ask the authoritative nameserver directly. If it gives a different answer than the public resolvers, they are still serving a cached value.\n- `dig shop.example.com +noall +answer` — shows the TTL. That number is how many seconds the cached answer will live. It tells you how long you have to wait after a change.\n\n### Follow the whole chain\n\n- `dig shop.example.com +trace` — follows the lookup from the root servers, to .com, to your nameservers, to the final answer. Broken delegation shows up clearly here.\n- `dig example.com NS +short` — the authoritative nameservers. After a migration, check they point where you think.\n- `dig -x 203.0.113.10 +short` — the reverse lookup, IP to name. Mail servers with no matching PTR record often land in spam.\n\nThe three facts that solve most DNS mysteries: the answer, which server gave it, and the TTL.\n\n## In one line\n\n> **Check the answer, the server, the TTL.** Three facts that solve most DNS mysteries.\n\n**Try it**\n\n```bash\ndig shop.example.com +short\n```\n"},{"id":"p98","n":"98","slug":"defense-in-depth-6-layers-of-security","title":"Defense in depth: 6 layers of security","sections":["it","devsecops"],"topic":"Security & hardening","tags":["CyberSecurity","InfoSec","BlueTeam","ITSecurity","DefenseInDepth"],"series_name":"Linux + IT","design":"stack","script":"","date":"2027-05-13","files":{"it":"notes/p098-defense-in-depth-6-layers-of-security.md","devsecops":"notes/p098-defense-in-depth-6-layers-of-security.md"},"md":"# Defense in depth: 6 layers of security\n\nNo security control is perfect. Defense in depth means stacking layers so that when one fails, the next one stops the attack. Six layers, from the data outward.\n\n## Quick reference\n\n*From the data outward*\n\n1. **Data** — Encryption, least access, backups you can restore. · *encrypt + 3-2-1*\n2. **Application** — Updates, strong logins, input checks, a WAF. · *MFA + WAF*\n3. **Server** — Patching, hardening, only the services you need. · *patch + harden*\n4. **Network** — Firewalls, segmentation, VPN or zero trust. · *VLAN + firewall*\n5. **Endpoint** — EDR, disk encryption, no local admin rights. · *EDR + BitLocker*\n6. **People** — Phishing awareness and an easy way to report. · *training*\n\n## Explained\n\n- **Layer 1** — Data. The thing attackers actually want. Encrypt it at rest and in transit, give access only to people who need it, and keep backups that are offline or immutable and have been restored at least once.\n\n- **Layer 2** — Application. Keep it updated, require strong authentication with MFA, validate input, and put a web application firewall in front of anything public.\n\n- **Layer 3** — Server. Patch on a schedule, remove services you do not use, harden SSH and RDP, and log to somewhere the attacker cannot easily erase.\n\n- **Layer 4** — Network. Firewalls that deny by default, segmentation so a compromised printer cannot reach the finance server, and remote access through a VPN or a zero-trust gateway rather than open ports.\n\n- **Layer 5** — Endpoint. EDR on laptops and servers, full disk encryption, and no local admin rights for daily work. Most attacks start on an endpoint.\n\n- **Layer 6** — People. Phishing awareness, and just as important, an easy and blame-free way to report a suspicious email or a mistake within minutes.\n\nWhy layers matter: a phishing email gets past the filter. The user clicks. EDR blocks the payload. Even if it did not, segmentation limits the spread, and offline backups make the ransom note irrelevant.\n\nIf you can only start with three things: MFA everywhere, patching on a schedule, and backups you have tested.\n\n## In one line\n\n> **No single layer is enough.** Attackers need one gap. Make them find six.\n\n**Start with these three:** MFA + patching + tested backups\n"},{"id":"p99","n":"99","slug":"fix-502-bad-gateway","title":"Fix: 502 Bad Gateway","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["Nginx","Linux","WebDevelopment","Troubleshooting","SysAdmin"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-05-16","files":{"it":"notes/p099-fix-502-bad-gateway.md"},"md":"# Fix: 502 Bad Gateway\n\n502 Bad Gateway. nginx is running fine. What failed is the application behind it: PHP-FPM, Node, Gunicorn or whatever nginx passes requests to.\n\n## Quick reference\n\n### Diagnose — What is nginx trying to reach?\n\n```bash\n# The upstream error: refused, timed out, or no socket.\ntail -f /var/log/nginx/error.log\n# Is the app server running at all?\nsystemctl status php8.3-fpm\n# Which socket or port nginx expects to find.\ngrep -r fastcgi_pass /etc/nginx/sites-enabled/\n```\n\n### Fix — Reconnect the two\n\n```bash\n# Bring the backend back. Then read its own log.\nsystemctl restart php8.3-fpm\n# The socket nginx expects must exist, with that exact name.\nls -l /run/php/\n# After fixing the path, test and reload.\nnginx -t && systemctl reload nginx\n```\n\n### Prevent — Stop the next one\n\n- `pm.max_children` — Too few PHP workers queue up and time out under load.\n- **Monitor the backend** — Check the app port or socket, not only nginx.\n- **Version in the socket name** — A PHP upgrade changes php8.2 to php8.3. Update nginx.\n\n## Explained\n\n### Diagnose\n\n- `tail -f /var/log/nginx/error.log` — nginx writes exactly what went wrong when it tried to reach the backend. \"Connection refused\" means nothing is listening. \"No such file or directory\" means the socket does not exist. \"upstream timed out\" is a 504, a slow backend rather than a missing one.\n- `systemctl status php8.3-fpm` — is the backend service running? If it crashed, its own log or `journalctl -u php8.3-fpm` explains why.\n- `grep -r fastcgi_pass /etc/nginx/sites-enabled/` — which socket or port nginx expects. For apps behind proxy_pass, search for that instead.\n\n### Fix\n\n- `systemctl restart php8.3-fpm` — get the backend running again, then read its log so it does not happen again tonight.\n- `ls -l /run/php/` — the socket in the nginx config must exist with exactly that name, and nginx must have permission to use it.\n- `nginx -t && systemctl reload nginx` — after correcting the path, test the config and reload. No downtime.\n\n### Prevent\n\n- `pm.max_children` — when all PHP-FPM workers are busy, new requests wait and eventually fail. Size it to your RAM and traffic, and watch the log for \"server reached `pm.max_children`\".\n- **Monitor the backend** — an uptime check that only tests nginx can stay green while the app is dead. Check a page that needs the application.\n- **Version in the socket name** — a PHP upgrade from 8.2 to 8.3 often changes the socket path. The upgrade succeeds, and the site returns 502 until nginx is updated.\n\n## In one line\n\n> **502 means look behind nginx.** The error shows in nginx. The cause almost never lives there.\n\n**The first command**\n\n```bash\ntail -20 /var/log/nginx/error.log\n```\n"},{"id":"p100","n":"100","slug":"9-active-directory-hygiene-checks","title":"9 Active Directory hygiene checks","sections":["it"],"topic":"Windows & Active Directory","tags":["ActiveDirectory","WindowsServer","CyberSecurity","SysAdmin","ITSecurity"],"series_name":"Linux + IT","design":"checklist","script":"","date":"2027-05-18","files":{"it":"notes/p100-9-active-directory-hygiene-checks.md"},"md":"# 9 Active Directory hygiene checks\n\nActive Directory does not break all at once. It drifts: old accounts, extra admins, forgotten computers. Nine checks to run every quarter.\n\n## Quick reference\n\n### Accounts\n\n- **Inactive users disabled** — `Search-ADAccount -AccountInactive`\n- **No passwords that never expire** — check PasswordNeverExpires\n- **Few Domain Admins** — `Get-ADGroupMember 'Domain Admins'`\n\n### Machines\n\n- **Stale computers removed** — inactive for 90+ days\n- **LAPS on every PC** — unique local admin passwords\n- **Replication healthy** — `repadmin /replsummary`\n\n### The domain\n\n- **DCs pass dcdiag** — `dcdiag /q`\n- **AD backup tested** — system state + a test restore\n- **krbtgt rotated** — on a schedule, twice each time\n\n## Explained\n\n### Accounts\n\n- **Inactive users disabled** — `Search-ADAccount -AccountInactive -UsersOnly` finds accounts that have not logged in for a while. Former employees and old test accounts are a favourite entry point.\n- **No passwords that never expire** — check for PasswordNeverExpires on user accounts. Service accounts are the usual culprits. Group managed service accounts (gMSA) solve this properly.\n- **Few Domain Admins** — `Get-ADGroupMember 'Domain Admins'`. Every name on that list should be a person who needs it, using a separate admin account. Fewer is better.\n\n### Machines\n\n- **Stale computers removed** — computer accounts that have not been active for 90 days or more. Disable first, delete later.\n- **LAPS on every PC** — Windows LAPS gives each machine a unique, rotating local admin password. Without it, one stolen local admin password often works on every PC.\n- **Replication healthy** — `repadmin /replsummary` shows replication failures between domain controllers. Silent replication problems turn into strange login issues.\n\n### The domain\n\n- **DCs pass dcdiag** — `dcdiag /q` prints only the errors. A clean run is the goal.\n- **AD backup tested** — a system state backup of at least one DC, and a test restore in an isolated lab. AD is the one system everything else depends on.\n- **krbtgt rotated** — the password of the krbtgt account signs every Kerberos ticket. Rotate it on a schedule, twice each time with replication in between, using Microsoft's documented process.\n\n## In one line\n\n> **AD drifts. Check it on purpose.** Every stale account is a door nobody is watching.\n\n**Find inactive users**\n\n```powershell\nSearch-ADAccount -AccountInactive -UsersOnly\n```\n"},{"id":"p101","n":"101","slug":"how-an-email-gets-from-sender-to-inbox","title":"How an email gets from sender to inbox","sections":["it"],"topic":"Networking & DNS","tags":["Email","DNS","CyberSecurity","ITSupport","SysAdmin"],"series_name":"Linux + IT","design":"pipeline","script":"","date":"2027-05-20","files":{"it":"notes/p101-how-an-email-gets-from-sender-to-inbox.md"},"md":"# How an email gets from sender to inbox\n\n\"The email was sent\" and \"the email arrived in the inbox\" are two very different things. Six stages sit between them.\n\n## Quick reference\n\n1. **Client submits the mail** — Outlook or your app sends it to the outgoing server. · `:587`\n2. **Sender looks up MX** — DNS says which server accepts mail for the domain. · *MX record*\n3. **SMTP handoff with TLS** — The two mail servers connect and talk SMTP. · *STARTTLS*\n4. **SPF and DKIM checks** — Is this server allowed? Is the signature valid? · *SPF + DKIM*\n5. **DMARC decides** — If both checks fail, the domain policy says what to do. · `p=reject`\n6. **Spam filter, then inbox** — Content and reputation decide: inbox or junk. · *inbox*\n\n## Explained\n\n1. **The client submits the mail.** Outlook, a phone or your application sends the message to its outgoing mail server on port 587, with a login and TLS.\n\n2. **The sending server looks up MX.** It asks DNS for the MX records of the recipient's domain. These say which server accepts mail for that domain. A wrong MX record means mail goes nowhere, or to the wrong place.\n\n3. **SMTP handoff with TLS.** The sending server connects to the receiving server on port 25 and, ideally, upgrades the connection with STARTTLS. The message is now handed over.\n\n4. **SPF and DKIM checks.** The receiving server checks SPF: is this sending IP listed in the domain's SPF record? And DKIM: does the cryptographic signature in the header match the public key published in DNS?\n\n5. **DMARC decides.** DMARC requires that SPF or DKIM pass and match the visible From domain. If they do not, the domain's DMARC policy says what to do: p=none to only report, p=quarantine for the junk folder, p=reject to refuse the message.\n\n6. **Spam filter, then inbox.** Even fully authenticated mail is scored on content, links, sending reputation and user behaviour. That decides inbox or junk.\n\nThe practical lesson: a new website or system that sends email from your domain must be added to SPF and set up with DKIM. Otherwise its mail fails DMARC, and the customer never sees the order confirmation.\n\n## In one line\n\n> **Delivered is not the same as inbox.** SPF, DKIM and DMARC decide which one you get.\n\n**Check your domain**\n\n```bash\ndig example.com TXT +short\n```\n"},{"id":"p102","n":"102","slug":"5-windows-admin-mistakes-to-stop","title":"5 Windows admin mistakes to stop","sections":["it"],"topic":"Windows & Active Directory","tags":["WindowsServer","ActiveDirectory","CyberSecurity","SysAdmin","ITSecurity"],"series_name":"Linux + IT","design":"dontdo","script":"","date":"2027-05-23","files":{"it":"notes/p102-5-windows-admin-mistakes-to-stop.md"},"md":"# 5 Windows admin mistakes to stop\n\nFive Windows admin habits that save a few minutes and cost a lot when an attacker finds them.\n\n## Quick reference\n\n**1.** ❌ **Domain Admin for daily work** — One phished login gives away the whole domain.  \n✅ **a normal account + an admin account** — Daily work as a user. Admin rights only when needed.\n\n**2. Don't**\n\n```powershell\nnetsh advfirewall set allprofiles state off\n```\n\nTurns the firewall off for every profile at once.\n\n**Do instead**\n\nopen one port, not the whole firewall\n\nAdd a rule for the single port the app needs.\n\n**3.** ❌ **same local admin password on every PC** — One stolen password or hash opens them all.  \n✅ **Windows LAPS** — A unique, rotating local admin password per machine.\n\n**4.** ❌ **restore a DC from an old snapshot** — Can roll AD back and break replication.  \n✅ **AD-aware system state restore** — Use a backup method that knows it is restoring AD.\n\n**5.** ❌ **RDP open to the internet** — Port 3389 is scanned all day by ransomware groups.  \n✅ **VPN or RD Gateway + MFA** — Never expose RDP directly. Put a gate in front.\n\n## Explained\n\n1. Using a Domain Admin account for daily work\n   Email, browsing and documents as Domain Admin means one phishing click hands over the entire domain. Instead: a normal account for daily work, and a separate admin account used only for admin tasks, ideally from a dedicated admin workstation.\n\n2. Turning the whole firewall off\n   `netsh advfirewall set allprofiles state off` makes an app work by removing all protection at once. Instead: add one inbound rule for the exact port and program the application needs.\n\n3. The same local admin password on every PC\n   Attackers who get that password, or its hash, from one machine can move to every other machine with it. Instead: Windows LAPS gives each computer a unique local admin password that rotates automatically and is stored in AD or Entra ID.\n\n4. Restoring a domain controller from an old snapshot\n   Rolling back a DC with an unsupported snapshot can cause USN rollback and break replication. Modern hypervisors with VM-Generation ID protect against much of this, but the safe path is still an AD-aware system state backup and a documented restore process.\n\n5. RDP open to the internet\n   Port 3389 is scanned around the clock, and exposed RDP is one of the most common ransomware entry points. Instead: reach RDP only through a VPN or an RD Gateway, with MFA.\n\nEvery one of these has been the first step in a real breach.\n\n## In one line\n\n> **Convenience is the attack surface.** Every shortcut above has been used in a real breach.\n\n**Who is domain admin today?**\n\n```powershell\nGet-ADGroupMember 'Domain Admins'\n```\n"},{"id":"p103","n":"103","slug":"quiz-3-networking-questions-from-it-interviews","title":"Quiz: 3 networking questions from IT interviews","sections":["it"],"topic":"Networking & DNS","tags":["Networking","CCNA","ITCareer","InterviewPrep","ITSupport"],"series_name":"Linux + IT","design":"quiz","script":"","date":"2027-05-25","files":{"it":"notes/p103-quiz-3-networking-questions-from-it-interviews.md"},"md":"# Quiz: 3 networking questions from IT interviews\n\nThree networking questions that come up in IT interviews again and again. Try to answer before you read on.\n\n## Quick reference\n\n**Q1. How many usable host addresses does a /26 subnet have?**\n\n- a) 64\n- b) 62\n- c) 30\n\n> **Answer:** b) 62. A /26 has 64 addresses. One is the network address and one is broadcast, so 62 are usable.\n\n**Q2. DNS normally uses UDP port 53. When does it use TCP 53 instead?**\n\n- a) Never, DNS is UDP only\n- b) For large answers and zone transfers\n- c) Only for DNS over HTTPS\n\n> **Answer:** b) For large answers and zone transfers. When a reply is too big for UDP, and for zone transfers between servers, DNS uses TCP on the same port.\n\n**Q3. What does ARP do on a local network?**\n\n- a) Finds the MAC address for an IP\n- b) Gives out IP addresses\n- c) Turns names into IP addresses\n\n> **Answer:** a) Finds the MAC address for an IP. ARP asks who has 192.168.1.1 and gets the MAC address back. DHCP gives out IPs. DNS resolves names.\n\n## Explained\n\n1. How many usable host addresses does a /26 subnet have?\n\n> **Answer:** \\62. A /26 leaves 6 bits for hosts, which is 2 to the power of 6, so 64 addresses. The first is the network address and the last is the broadcast address, so 62 can be given to devices. A quick way to remember: /24 gives 254, and each extra bit halves it. /25 gives 126, /26 gives 62, /27 gives 30.\n\n2. **DNS normally uses UDP port 53.** When does it use TCP 53?\n\n> **Answer:** for large responses and zone transfers. When a reply does not fit in a UDP packet, the server marks it as truncated and the client asks again over TCP. Zone transfers between DNS servers always use TCP. This is why a firewall that only allows UDP 53 causes failures that are rare and hard to explain, especially with DNSSEC and big TXT records.\n\n3. What does ARP do on a local network?\n\n> **Answer:** it finds the MAC address for an IP address. Before a PC can send a frame to 192.168.1.1, it broadcasts \"who has 192.168.1.1?\" and the owner replies with its MAC address. The result is cached in the ARP table, which `ip neigh` shows on Linux and `arp -a` on Windows. Two devices answering for the same IP is how you spot an IP conflict, or ARP spoofing.\n\nInterviews test the basics because real outages are so often caused by them.\n\n## In one line\n\n> **Interviews test the basics.** Because outages are usually caused by them.\n\n**See your arp table**\n\n```bash\nip neigh\n```\n"},{"id":"p104","n":"104","slug":"9-nginx-settings-worth-knowing","title":"9 nginx settings worth knowing","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["Nginx","Linux","WebPerformance","WebSecurity","DevOps"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-05-27","files":{"it":"notes/p104-9-nginx-settings-worth-knowing.md"},"md":"# 9 nginx settings worth knowing\n\nnginx works out of the box. These nine settings are what make it work well, safely, and in front of a real application.\n\n## Quick reference\n\n### Safety — Hide and limit\n\n```bash\n# Hide the nginx version in headers and error pages.\nserver_tokens off;\n# Maximum upload size. Too small gives a 413 error.\nclient_max_body_size 64m;\n# Rate-limit logins. Needs a limit_req_zone first.\nlimit_req zone=login burst=5;\n```\n\n### Speed — Faster responses\n\n```bash\n# Compress text responses. Often half the size or less.\ngzip on;\n# Let browsers cache static files for 30 days.\nexpires 30d;\n# Reuse client connections instead of opening new ones.\nkeepalive_timeout 65;\n```\n\n### Proxy — In front of an app\n\n```bash\n# How long to wait for a slow backend before a 504.\nproxy_read_timeout 120s;\n# Pass the real client IP to the app.\nproxy_set_header X-Real-IP $remote_addr;\n# Send every plain HTTP request to HTTPS.\nreturn 301 https://$host$request_uri;\n```\n\n## Explained\n\n### Safety\n\n- `server_tokens off;` — hides the nginx version number in response headers and error pages. It will not stop a real attacker, but there is no reason to advertise it.\n- `client_max_body_size 64m;` — the maximum size of a request body. The default is only 1 MB, which is why file uploads and big WordPress imports fail with \"413 Request Entity Too Large\".\n- `limit_req zone=login burst=5;` — rate limiting for sensitive paths like login pages. It needs a matching limit_req_zone in the http block, which defines the key and the rate.\n\n### Speed\n\n- `gzip on;` — compresses HTML, CSS, JavaScript and JSON. Text responses often shrink to a third of their size. Add gzip_types for anything beyond HTML.\n- `expires 30d;` — in a location for static files, tells browsers to cache images, CSS and JS for 30 days instead of asking again on every page.\n- `keepalive_timeout 65;` — keeps client connections open so the next request skips a new TCP and TLS handshake.\n\n### Proxy\n\n- `proxy_read_timeout 120s;` — how long nginx waits for a response from the backend. Too short and slow reports or exports fail with 504. Too long and stuck requests pile up.\n- `proxy_set_header X-Real-IP $remote_addr;` — passes the real client IP to the application. Without it, every visitor looks like 127.0.0.1 in your app logs.\n- `return 301 https://$host$request_uri;` — in the port 80 server block, sends every HTTP request to HTTPS.\n\nAfter every change: `nginx -t && systemctl reload nginx`.\n\n## In one line\n\n> **Defaults are a starting point.** Five minutes in the config saves a day of tickets.\n\n**Always after editing**\n\n```bash\nnginx -t && systemctl reload nginx\n```\n"},{"id":"p105","n":"105","slug":"6-numbers-every-sysadmin-should-know","title":"6 numbers every sysadmin should know","sections":["it"],"topic":"Linux","tags":["Linux","SysAdmin","DevOps","ITInfrastructure","CommandLine"],"series_name":"Linux + IT","design":"numbers","script":"","date":"2027-05-30","files":{"it":"notes/p105-6-numbers-every-sysadmin-should-know.md"},"md":"# 6 numbers every sysadmin should know\n\nSix numbers that keep coming up in Linux and server work. Each one explains a whole category of problems.\n\n## Quick reference\n\n| Number | What | Why it matters |\n|:--|:--|:--|\n| **0** | Exit code for success | Anything else means an error. Check it with `echo $?`. |\n| **1** | PID of the init system | systemd is process 1. Everything else starts from it. |\n| **1024** | Privileged port limit | Ports below 1024 need root, or a capability, to bind. |\n| **255** | Highest exit code | Exit codes run from 0 to 255. exit 256 becomes 0. |\n| **8.76 h** | Downtime a year at 99.9% | Three nines still allows almost nine hours of outage. |\n| **127.0.0.1** | Loopback address | Traffic that never leaves the machine. Bind internal services here. |\n\n## Explained\n\n- **0** — the exit code for success. Every command returns a number when it finishes. Zero means it worked, anything else is an error. `echo $?` shows the last one, and it is what && and || in scripts and cron monitoring are checking.\n\n- **1** — the process ID of the init system. On modern Linux that is systemd. It starts every other service and adopts orphaned processes. That is why ps shows PID 1 on every Linux system, and why a container's main process runs as PID 1 inside its own namespace.\n\n- **1024** — the privileged port limit. Ports below 1024, like 22, 80 and 443, need root or the CAP_NET_BIND_SERVICE capability to bind. This is why web servers start as root and then drop privileges, or sit behind a proxy.\n\n- **255** — the highest exit code. Exit codes are stored in 8 bits, so they run from 0 to 255. A script that does exit 256 actually returns 0, success. Codes above 128 usually mean the process was killed by a signal: 137 is 128 + 9, SIGKILL, often the OOM killer.\n\n- **8.76 hours** — the downtime per year that 99.9% availability still allows. Three nines sounds strict. It is almost nine hours of outage a year, or about 43 minutes a month.\n\n- **127.0.0.1** — the loopback address. Traffic to it never leaves the machine. Databases, caches and admin panels that only local apps use should listen here, not on 0.0.0.0.\n\n## In one line\n\n> **Numbers make things concrete.** Know these and many error messages explain themselves.\n\n**Check the last exit code**\n\n```bash\necho $?\n```\n"},{"id":"p106","n":"106","slug":"nat-vs-port-forwarding","title":"NAT vs Port forwarding","sections":["it"],"topic":"Networking & DNS","tags":["Networking","CCNA","Firewall","ITInfrastructure","NetworkSecurity"],"series_name":"Linux + IT","design":"compare","script":"","date":"2027-06-01","files":{"it":"notes/p106-nat-vs-port-forwarding.md"},"md":"# NAT vs Port forwarding\n\nNAT and port forwarding are configured in the same router menu, and they are often confused. They work in opposite directions.\n\n## Quick reference\n\n| | **NAT** | **PORT FWD** |\n|:--|:--|:--|\n| Direction | Outbound, inside to out | Inbound, outside to in |\n| Purpose | Share one public IP | Publish one service |\n| Who starts it | The inside device | The outside client |\n| Default | On, on every router | Off until you add it |\n| Risk | Low | Exposes that service |\n| Example | Office PCs browsing | A web server on 443 |\n\n**NAT** (Many devices, one public IP): Private devices go out to the internet sharing one public address.\n\nExample: `10.0.0.0/24 -> 203.0.113.5`\n\n**PORT FWD** (Let one service in): Sends traffic for one public port to one server inside.\n\nExample: `203.0.113.5:443 -> 10.0.0.20`\n\n- NAT → lets the inside reach out.\n- Port forwarding → lets the outside reach in, one port at a time.\n\n## Explained\n\nNAT, or more precisely source NAT and PAT, lets many private devices share one public IP address. When a PC at 10.0.0.15 opens a website, the router replaces its private address with the public one, 203.0.113.5, and remembers the connection so the reply finds its way back to the right PC. It is on by default on every home and office router, and it only works for connections started from the inside.\n\nPort forwarding, or destination NAT, works the other way. It tells the router: traffic arriving on public port 443 should go to the server at 10.0.0.20. It is off until you add it, because it deliberately lets the outside world reach something inside.\n\n### Where people get confused\n\n\"NAT is my firewall.\" NAT does block unsolicited inbound traffic as a side effect, but it is not a security policy. A real firewall rule set is still needed, and IPv6 usually has no NAT at all.  \n\"I forwarded the port but it still does not work.\" Check that the server is listening on that port, that its own firewall allows it, and that the ISP does not use carrier-grade NAT, which means you do not have a real public IP to forward.\n\nThe risk is very different. NAT exposes nothing new. Every port forward is a service on the internet that needs patching, monitoring and ideally a reverse proxy or VPN in front.\n\nNAT lets the inside reach out. Port forwarding lets the outside reach in.\n"},{"id":"p107","n":"107","slug":"ubuntu-lts-vs-debian-vs-rocky-linux-for-servers","title":"Ubuntu LTS vs Debian vs Rocky Linux for servers","sections":["it"],"topic":"Linux","tags":["Linux","Ubuntu","Debian","RockyLinux","SysAdmin"],"series_name":"Linux + IT","design":"matrix","script":"","date":"2027-06-03","files":{"it":"notes/p107-ubuntu-lts-vs-debian-vs-rocky-linux-for-servers.md"},"md":"# Ubuntu LTS vs Debian vs Rocky Linux for servers\n\nThree Linux distributions run a large share of the world's servers. They are all good. They are good at different things.\n\n## Quick reference\n\n| | **Ubuntu LTS** | **Debian** | **Rocky** |\n|:--|:--|:--|:--|\n| Support length | 5 years, 10 with Pro | About 5 years | 10 years |\n| Package tool | apt | apt | dnf |\n| New release | Every 2 years | About every 2 years | Follows RHEL |\n| Paid support | ✅ Canonical | 🟡 Third parties | ✅ CIQ and others |\n| Cloud images | ✅ Everywhere | ✅ Most clouds | ✅ Most clouds |\n| Best known for | Most tutorials | Stable, minimal | RHEL compatible |\n\n**How to pick**\n\n- **Ubuntu LTS** → most docs, most cloud images, easy start.\n- **Debian** → stable, minimal and community run.\n- **Rocky** → RHEL compatibility and the longest support.\n\n## Explained\n\nUbuntu LTS releases come every two years, with five years of standard updates and up to ten with Ubuntu Pro. It has the most tutorials, the most cloud images and the largest community, which makes it the easiest place to start. Canonical sells commercial support.\n\nDebian is the base Ubuntu is built on. It is community run, minimal by default, and known for stability over new features. A stable release gets about three years of regular support plus around two years of LTS. Commercial support comes from third parties rather than one vendor.\n\nRocky Linux is a free rebuild of Red Hat Enterprise Linux, made to be compatible with it. It uses dnf, SELinux by default, and follows RHEL's ten-year lifecycle. If your software vendor certifies RHEL, or your team comes from the Red Hat world, Rocky feels like home. AlmaLinux fills the same role.\n\n### How to choose\n\nSupport length: ten years on Rocky means fewer major upgrades over a server's life.  \nTeam skills: apt and Debian-style configuration, or dnf and the RHEL way? Retraining costs more than any feature.  \nVendor requirements: some commercial software only supports RHEL-compatible systems. Others test mostly on Ubuntu.\n\nThe worst choice is mixing all three without a reason. Pick one for most servers and know it well.\n\n`cat /etc/os-release` tells you what you are standing on.\n\n## In one line\n\n> **The best distro is the one you can run.** Pick for support length and your team's skills.\n\n**Which one is this?**\n\n```bash\ncat /etc/os-release\n```\n"},{"id":"p108","n":"108","slug":"9-docker-commands-for-when-a-container-fails","title":"9 docker commands for when a container fails","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","Linux","Troubleshooting"],"series_name":"Linux + IT","design":"terminal","script":"","date":"2027-06-06","files":{"devsecops":"notes/p108-9-docker-commands-for-when-a-container-fails.md"},"md":"# 9 docker commands for when a container fails\n\nA container that exits or restarts in a loop is frustrating until you know where to look. Nine docker commands that find the cause.\n\n## Quick reference\n\n### What state is it in?\n\n```bash\n# -a includes stopped ones. Exited (1) = it crashed\ndocker ps -a --filter name=api\n# the last lines usually explain the crash\ndocker logs --tail 50 api\n# exit 137 + OOMKilled = out of memory\ndocker inspect -f '{{.State.ExitCode}} {{.State.OOMKilled}}' api\n```\n\n### Look inside\n\n```bash\n# a shell inside the running container\ndocker exec -it api sh\n# the config the app really sees\ndocker exec api env | grep DB_\n# CPU and memory right now\ndocker stats --no-stream\n```\n\n### Network, disk and events\n\n```bash\n# who is on the same network?\ndocker network inspect app_net -f '{{range .Containers}}{{.Name}} {{end}}'\n# disk used by images, containers and volumes\ndocker system df\n# what happened, in order\ndocker events --since 10m --filter container=api\n```\n\n## Explained\n\n### What state is it in?\n\n- `docker ps -a --filter name=api` — -a includes stopped containers. \"Exited (1)\" means the app crashed with an error. \"Exited (0)\" means it finished normally, which is just as wrong for a service that should keep running.\n- `docker logs --tail 50 api` — the last lines of output. Most crashes explain themselves here: a missing variable, a refused database connection, a bad config file.\n- `docker inspect -f '{{.State.ExitCode}} {{.State.OOMKilled}}' api` — exit code 137 with OOMKilled true means the container hit its memory limit and was killed.\n\n### Look inside\n\n- `docker exec -it api sh` — a shell inside a running container. Check files, permissions and whether the app can reach its dependencies.\n- `docker exec api env | grep DB_` — the environment variables the app actually received. A typo in a compose file shows up immediately.\n- `docker stats --no-stream` — CPU and memory per container right now, next to the memory limit.\n\n### Network, disk and events\n\n- `docker network inspect app_net` — which containers share a network. Two containers on different networks cannot reach each other by name.\n- `docker system df` — disk used by images, containers, volumes and build cache, with how much can be reclaimed. A full disk stops containers from starting.\n- `docker events --since 10m --filter container=api` — a timeline of what happened to the container: start, die, restart, OOM.\n\nThe order that works: status, logs, exit code. Only then rebuild or change anything.\n\n## In one line\n\n> **The container already told you.** Read the logs and the exit code before you rebuild anything.\n\n**Try it**\n\n```bash\ndocker logs --tail 50 api\n```\n"},{"id":"p109","n":"109","slug":"what-is-inside-a-docker-image","title":"What is inside a Docker image","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","CloudComputing","SoftwareEngineering"],"series_name":"Linux + IT","design":"stack","script":"","date":"2027-06-08","files":{"devsecops":"notes/p109-what-is-inside-a-docker-image.md"},"md":"# What is inside a Docker image\n\nA Docker image is not one big file. It is a stack of read-only layers, one for each step in the Dockerfile. Understanding the stack is how you get small images and fast builds.\n\n## Quick reference\n\n*Bottom to top: one layer per step*\n\n1. **Base image** — OS files plus the runtime. Pin an exact version. · `FROM python:3.12-slim`\n2. **System packages** — Extra libraries in one RUN, cache cleaned. · `RUN apt-get install`\n3. **Dependency list** — Copied before the code so this layer is cached. · `COPY requirements.txt`\n4. **Install dependencies** — Rebuilt only when the list changes. · `RUN pip install`\n5. **Application code** — Changes every build. That is why it goes last. · `COPY . /app`\n6. **Container layer** — Thin, writable, deleted with the container. · `docker run`\n\n## Explained\n\n- **L1** — Base image. FROM python:3.12-slim gives you a minimal set of OS files and the language runtime. Pin an exact version. A smaller base means fewer packages, which means fewer vulnerabilities to patch.\n\n- **L2** — System packages. Extra libraries the app needs, installed in a single RUN step that also cleans the package cache. Split across several steps, the cache stays in the image forever.\n\n- **L3** — Dependency list. COPY `requirements.txt` on its own, before the code. Docker reuses a cached layer when its input has not changed, so this layer is rebuilt only when the list of dependencies changes.\n\n- **L4** — Install dependencies. RUN pip install, or npm ci, or composer install. This is usually the slowest step, and thanks to L3 it is skipped on most builds.\n\n- **L5** — Application code. COPY . `/app`. It changes on almost every build, so it goes last. Put it earlier and every code change rebuilds everything above it.\n\n- **L6** — Container layer. When you run the image, Docker adds a thin writable layer on top. Anything written there disappears when the container is removed, which is why databases and uploads belong in volumes.\n\n`docker history myapp:1.0` shows every layer and its size. The biggest one is where to start optimizing.\n\n## In one line\n\n> **Order the layers. Builds get fast.** What changes often goes on top. Data goes in volumes.\n\n**See the layers**\n\n```bash\ndocker history myapp:1.0\n```\n"},{"id":"p110","n":"110","slug":"fix-the-trust-relationship-between-this-workstation-and-the-domain-fai","title":"Fix: The trust relationship between this workstation and the domain failed","sections":["it"],"topic":"Windows & Active Directory","tags":["ActiveDirectory","Windows","ITSupport","Troubleshooting","SysAdmin"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-06-10","files":{"it":"notes/p110-fix-the-trust-relationship-between-this-workstation-and-the-domain-fai.md"},"md":"# Fix: The trust relationship between this workstation and the domain failed\n\n\"The trust relationship between this workstation and the primary domain failed.\" The user cannot log in, and the classic fix, leaving and rejoining the domain, is slow and resets the computer's group memberships. There is a faster way.\n\n## Quick reference\n\n### Diagnose — Is the secure channel broken?\n\n```powershell\n# False: the PC and the domain disagree on its password.\nTest-ComputerSecureChannel\n# The same check, with the error code and the DC used.\nnltest /sc_verify:corp.local\n# Check the clock. Kerberos fails beyond 5 minutes off.\nw32tm /query /status\n```\n\n### Fix — Repair it without rejoining\n\n```powershell\n# Resets the machine password. Log in locally first.\nTest-ComputerSecureChannel -Repair -Credential corp\\admin\n# Another way to reset the computer account password.\nReset-ComputerMachinePassword -Credential corp\\admin\n# Restart, then log in with a domain account.\nRestart-Computer\n```\n\n### Prevent — Why it happened\n\n- **Old snapshots of domain PCs** — A restored VM brings back an old machine password.\n- **Duplicate computer names** — Two PCs with one name fight over one account.\n- **Computer account reset in AD** — Check AD history before you blame the PC.\n\n## Explained\n\nEvery domain computer has its own account in Active Directory with a password that the computer changes automatically. When the computer and AD disagree on that password, the secure channel breaks and domain logins fail.\n\n### Diagnose\n\n- `Test-ComputerSecureChannel` — returns False when the secure channel is broken. Run it from a local administrator account, or with cached credentials if they still work.\n- `nltest /sc_verify:corp.local` — the same check with more detail: which domain controller was used and the exact error code.\n- `w32tm /query /status` — check the time too. Kerberos fails when the clock is more than five minutes off, and that can look like a trust problem.\n\n### Fix\n\n- `Test-ComputerSecureChannel -Repair -Credential corp\\admin` — resets the computer account password on both sides, using a domain account with rights to do it. No rejoin, no lost group memberships.\n- `Reset-ComputerMachinePassword -Credential corp\\admin` — another built-in way to do the same reset.\n- `Restart-Computer` — then log in with a domain account again.\n\n### Prevent\n\n- **Old snapshots** — restoring a VM or a disk image of a domain PC brings back an old machine password. This is the most common cause.\n- **Duplicate computer names** — two machines joined with the same name share one AD account and keep breaking each other.\n- **Computer account reset in AD** — someone reset or deleted the account. Check before you blame the PC.\n\n## In one line\n\n> **Repair the channel. Skip the rejoin.** One PowerShell line, run as local admin, fixes most cases.\n\n**The fix**\n\n```powershell\nTest-ComputerSecureChannel -Repair -Credential corp\\admin\n```\n"},{"id":"p111","n":"111","slug":"9-woocommerce-checks-before-a-big-sale","title":"9 WooCommerce checks before a big sale","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["WooCommerce","WordPress","Ecommerce","WebPerformance","SysAdmin"],"series_name":"Linux + IT","design":"checklist","script":"","date":"2027-06-13","files":{"it":"notes/p111-9-woocommerce-checks-before-a-big-sale.md"},"md":"# 9 WooCommerce checks before a big sale\n\nI look after a bilingual WooCommerce store with about 1,400 products. A big sale is when small problems become expensive, because every minute of a broken checkout is lost orders. Nine checks for the week before.\n\n## Quick reference\n\n### The store\n\n- **Stock and prices right** — export, then spot-check\n- **Test order end to end** — every payment method\n- **Coupons tested** — dates, limits, exclusions\n\n### The server\n\n- **Checkout not cached** — exclude cart, checkout, account\n- **Fresh backup** — files + database, restore tested\n- **Room to grow** — CPU, RAM and PHP workers\n\n### The day itself\n\n- **Updates frozen** — no plugin updates on sale day\n- **Uptime monitor on** — the checkout page, every minute\n- **Someone on call** — with hosting and payment contacts\n\n## Explained\n\n### The store\n\n- **Stock and prices right** — export the products, check the sale prices and stock levels against the plan. A wrong price at scale is hard to undo.\n- **Test order end to end** — place a real order with every payment method you offer, and follow it all the way: payment, confirmation email, order status, stock reduction.\n- **Coupons tested** — start and end dates, usage limits, product exclusions. A coupon that stacks with the sale price by accident can wipe out the margin.\n\n### The server\n\n- **Checkout not cached** — page caching makes a store fast, but the cart, checkout and my-account pages must be excluded. A cached cart shows one customer another customer's items.\n- **Fresh backup** — files and database, taken right before the sale, from a setup where a restore has been tested.\n- **Room to grow** — CPU, memory, and the number of PHP workers. Traffic during a sale can be many times normal. If workers run out, customers see errors instead of slow pages.\n\n### The day itself\n\n- **Updates frozen** — no plugin, theme or WordPress updates on sale day. Update a week before, test, then freeze.\n- **Uptime monitor on** — check the checkout page itself, every minute, not just the homepage.\n- **Someone on call** — with the hosting provider's and payment gateway's support contacts ready.\n\n## In one line\n\n> **Sale day is not test day.** Every check this week is one less surprise on the day.\n\n**Watch the database live**\n\n```bash\nwp db query 'SHOW PROCESSLIST'\n```\n"},{"id":"p112","n":"112","slug":"how-a-ransomware-attack-unfolds-and-where-to-stop-it","title":"How a ransomware attack unfolds, and where to stop it","sections":["it"],"topic":"Security & hardening","tags":["Ransomware","CyberSecurity","IncidentResponse","BlueTeam","ITSecurity"],"series_name":"Linux + IT","design":"pipeline","script":"","date":"2027-06-15","files":{"it":"notes/p112-how-a-ransomware-attack-unfolds-and-where-to-stop-it.md"},"md":"# How a ransomware attack unfolds, and where to stop it\n\nI have handled a LockBit Black ransomware incident end to end: containment, a report to the NCA, a working decryptor from nomoreransom.org, and recovery. One lesson stays with me: the encryption is the last step, not the first. Most attacks follow the same six stages, and each one is a chance to stop it.\n\n## Quick reference\n\n1. **Initial access** — Phishing, exposed RDP, or an unpatched VPN. · *phish + RDP*\n2. **Foothold** — Malware runs and connects back to the attacker. · *C2 beacon*\n3. **Privilege escalation** — Stolen or cracked passwords lead to admin rights. · *admin creds*\n4. **Lateral movement** — Spreads to servers, file shares and backups. · *RDP + SMB*\n5. **Data theft** — Copies data out first, for double extortion. · *exfiltration*\n6. **Encryption and ransom** — Files locked, backups targeted, a demand made. · *ransom note*\n\n## Explained\n\n1. **Initial access.** A phishing email, RDP open to the internet, or an unpatched VPN appliance. Stop it here with email filtering, MFA on every remote access, and fast patching of anything internet-facing.\n\n2. **Foothold.** The malware runs and connects back to the attacker's server. EDR on endpoints and blocking unusual outbound traffic catch this stage.\n\n3. **Privilege escalation.** The attacker collects passwords, often from memory or from a reused local admin password, until they reach admin rights. Separate admin accounts, LAPS and few Domain Admins slow this down.\n\n4. **Lateral movement.** With admin rights, they move to servers, file shares and, very deliberately, the backup server. Network segmentation and restricting RDP and SMB between machines limit how far they get.\n\n5. **Data theft.** Many groups copy data out before encrypting, so they can threaten to publish it. Large unusual uploads are a warning sign worth alerting on.\n\n6. **Encryption and ransom.** Files are encrypted, online backups are deleted or encrypted, and the ransom note appears. Offline or immutable backups are what make this stage survivable.\n\nIf an incident does happen: isolate affected machines, preserve logs, report to the national authority, and check nomoreransom.org before anything else. Free decryptors exist for some families.\n\n## In one line\n\n> **Break the chain early.** Every stage you block makes the last one harder.\n\n**The short list:** MFA + patched VPN/RDP + offline backups\n"},{"id":"p113","n":"113","slug":"5-docker-mistakes-in-production","title":"5 Docker mistakes in production","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","DevOps","Containers","CyberSecurity","Linux"],"series_name":"Linux + IT","design":"dontdo","script":"","date":"2027-06-17","files":{"devsecops":"notes/p113-5-docker-mistakes-in-production.md"},"md":"# 5 Docker mistakes in production\n\nDocker makes it easy to run anything. These five habits are fine on a laptop and risky on a production server.\n\n## Quick reference\n\n**1.** ❌ **FROM node:latest** — The next rebuild can pull a very different version.  \n✅ **FROM node:22.11.0-alpine** — Pin the version. Update on purpose, not by accident.\n\n**2. Don't**\n\nno USER line (runs as root)\n\nAny breakout starts with full root rights.\n\n**Do instead**\n\n```bash\nUSER app\n```\n\nRun the process as an unprivileged user.\n\n**3. Don't**\n\n```bash\nENV DB_PASSWORD=secret\n```\n\nBaked into the image. Anyone who pulls it can read it.\n\n**Do instead**\n\n```bash\ndocker run --env-file .env app\n```\n\nPass secrets at runtime, or use a secrets manager.\n\n**4. Don't**\n\n```bash\n-p 5432:5432\n```\n\nPublishes the database on every interface, past ufw.\n\n**Do instead**\n\n```bash\n-p 127.0.0.1:5432:5432\n```\n\nBind to localhost, or keep it on an internal network.\n\n**5. Don't**\n\n```bash\ndocker run -d app\n```\n\nAfter a crash or a reboot, it simply stays down.\n\n**Do instead**\n\n```bash\ndocker run -d --restart unless-stopped app\n```\n\nComes back after crashes and host reboots.\n\n## Explained\n\n1. FROM node:latest\n   latest is not a version, it is whatever was newest when you built. Next month's rebuild can bring a new major version and break the app without a single code change. Instead: pin an exact tag like node:22.11.0-alpine, and update it deliberately.\n\n2. No USER line, so the app runs as root\n   By default the process inside the container is root. If the app is compromised and the attacker breaks out of the container, they start with root rights. Instead: create a user in the Dockerfile and add `USER app`.\n\n3. `ENV DB_PASSWORD=secret` in the Dockerfile\n   Everything in a Dockerfile ends up in the image layers. Anyone who can pull the image can read it with docker history or docker inspect. Instead: pass secrets at runtime with `--env-file` or a secrets manager, and keep .env files out of git.\n\n4. `-p 5432:5432`\n   This publishes the port on every interface. Docker writes its own iptables rules, so the port can be reachable from the internet even when ufw says it is blocked. Instead: `-p 127.0.0.1:5432:5432`, or no published port at all and an internal Docker network between app and database.\n\n5. `docker run -d app`, with no restart policy\n   If the app crashes, or the server reboots for an update, the container stays down until someone notices. Instead: `--restart unless-stopped`, or restart: unless-stopped in compose.\n\nNone of these take more than a minute to fix, and each one has caused real outages or breaches.\n\n## In one line\n\n> **Containers are easy. Production is not.** Pin it, drop root, hide secrets, bind tight, restart.\n\n**Check what is published**\n\n```bash\ndocker ps --format '{{.Names}} {{.Ports}}'\n```\n"},{"id":"p114","n":"114","slug":"quiz-3-active-directory-questions","title":"Quiz: 3 Active Directory questions","sections":["it"],"topic":"Windows & Active Directory","tags":["ActiveDirectory","WindowsServer","SysAdmin","ITSupport","ITCareer"],"series_name":"Linux + IT","design":"quiz","script":"","date":"2027-06-20","files":{"it":"notes/p114-quiz-3-active-directory-questions.md"},"md":"# Quiz: 3 Active Directory questions\n\nThree Active Directory questions that come up in real work all the time. Try them before you read the answers.\n\n## Quick reference\n\n**Q1. Which FSMO role is the main time source for the whole domain?**\n\n- a) RID Master\n- b) PDC Emulator\n- c) Schema Master\n\n> **Answer:** b) PDC Emulator. The PDC Emulator in the forest root domain syncs with a reliable outside source. Every other machine follows it.\n\n**Q2. A user was just added to a group but still cannot open the share. Fastest fix?**\n\n- a) Restart the file server\n- b) Log off and log on again\n- c) Run gpupdate /force\n\n> **Answer:** b) Log off and log on again. Group membership is in the Kerberos ticket from logon. A new logon, or klist purge, gets a fresh ticket.\n\n**Q3. Which command shows a summary of AD replication health?**\n\n- a) `repadmin /replsummary`\n- b) ipconfig /registerdns\n- c) netdom resetpwd\n\n> **Answer:** a) `repadmin /replsummary`. `repadmin /replsummary` lists failures and the longest replication delay for each domain controller.\n\n## Explained\n\n1. Which FSMO role is the main time source for the whole domain?\n\n> **Answer:** the PDC Emulator. In a domain, member computers sync their clocks with a domain controller, domain controllers sync with the PDC Emulator, and the PDC Emulator of the forest root domain should sync with a reliable external source. This matters because Kerberos fails when clocks differ by more than five minutes. If logins break after a DC change, check w32tm /query /status before anything else.\n\n2. A user was just added to a group but still cannot open the share. What is the fastest fix?\n\n> **Answer:** log off and log on again. Group memberships are stored in the Kerberos ticket the user receives at logon. Until they get a new ticket, the file server still sees the old list of groups. Logging off and on works, and so does klist purge followed by accessing the share again. gpupdate /force does not refresh the ticket.\n\n3. Which command shows a summary of AD replication health?\n\n> **Answer:** `repadmin /replsummary`. It lists every domain controller, how many replication attempts failed, and the largest delay since the last successful replication. Broken replication causes strange problems: a password change that works on one DC but not another, or a new user who cannot log in at one site.\n\nAll three come back to the basics: time, tickets and replication.\n\n## In one line\n\n> **AD rewards those who know the basics.** Time, tickets and replication explain most AD tickets.\n\n**Check replication now**\n\n```powershell\nrepadmin /replsummary\n```\n"},{"id":"p115","n":"115","slug":"9-kubectl-commands-for-troubleshooting","title":"9 kubectl commands for troubleshooting","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","DevOps","CloudNative","Containers","SRE"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-06-22","files":{"devsecops":"notes/p115-9-kubectl-commands-for-troubleshooting.md"},"md":"# 9 kubectl commands for troubleshooting\n\nKubernetes tells you a lot about why something failed, if you know which command to ask. Nine kubectl commands for when pods misbehave.\n\n## Quick reference\n\n### See — What is the cluster saying?\n\n```bash\n# Every pod in every namespace, with status and restarts.\nkubectl get pods -A\n# The events at the bottom explain most failures.\nkubectl describe pod api-7d9f8c6b5-x2kqp\n# Recent cluster events, newest at the bottom.\nkubectl get events --sort-by=.lastTimestamp\n```\n\n### Debug — Look inside the pod\n\n```bash\n# Logs of the crashed container, not the new one.\nkubectl logs api-7d9f8c6b5-x2kqp --previous\n# A shell inside a running pod.\nkubectl exec -it web-5c8d7f9d4-mm7tz -- sh\n# CPU and memory per pod. Needs metrics-server.\nkubectl top pods -n shop\n```\n\n### Change — Roll forward or back\n\n```bash\n# Watch a deployment finish, or see it get stuck.\nkubectl rollout status deploy/api\n# Back to the previous version in seconds.\nkubectl rollout undo deploy/api\n# Reach a service from your laptop, no ingress needed.\nkubectl port-forward svc/api 8080:80\n```\n\n## Explained\n\n### See\n\n`kubectl get pods -A` shows every pod in every namespace, with status and restart count. A high restart count is a clue even when the status says Running.  \nkubectl describe pod \\<name> has an Events section at the bottom that explains most problems: image pull errors, failed health checks, OOMKilled, or \"0/3 nodes are available\" for a Pending pod.  \n`kubectl get events --sort-by=.lastTimestamp` shows recent events across the namespace in time order. Useful when you do not know which pod to look at yet.\n\n### Debug\n\n`kubectl logs <pod> --previous`. In a crash loop, the current container has just started and has nothing useful to say. `--previous` shows the logs of the one that crashed.  \n`kubectl exec -it <pod>` -- sh opens a shell inside a running pod. Check environment variables, config files and whether it can reach its database.  \n`kubectl top pods -n shop` shows CPU and memory per pod, compared with what you set in requests and limits. It needs metrics-server installed.\n\n### Change\n\n`kubectl rollout status deploy/api` lets you watch a deployment roll out. If new pods never become ready, it stops and tells you.  \n`kubectl rollout undo deploy/api` goes back to the previous version. The fastest fix for a bad deploy, while you investigate.  \n`kubectl port-forward svc/api 8080:80` reaches a service from your laptop through the API server, without exposing it publicly.\n\nThe order that saves the most time: describe, events, logs `--previous`. Then change something.\n\nCrashLoopBackOff is a symptom, not a cause.\n\n## In one line\n\n> **CrashLoopBackOff is a symptom.** The reason is in describe, events and the previous logs.\n\n**The first two**\n\n```bash\nkubectl describe pod <name>; kubectl logs <name> --previous\n```\n"},{"id":"p116","n":"116","slug":"uptime-math-what-each-nine-really-allows","title":"Uptime math: what each nine really allows","sections":["it","devsecops"],"topic":"Observability & SRE","tags":["SRE","ITInfrastructure","CloudComputing","DevOps","ITManagement"],"series_name":"Linux + IT","design":"numbers","script":"","date":"2027-06-24","files":{"it":"notes/p116-uptime-math-what-each-nine-really-allows.md","devsecops":"notes/p116-uptime-math-what-each-nine-really-allows.md"},"md":"# Uptime math: what each nine really allows\n\nWe need 99.99% uptime. Before agreeing, it helps to know what that number actually allows. One year has 525,600 minutes.\n\n## Quick reference\n\n| Number | What | Why it matters |\n|:--|:--|:--|\n| **99%** | 3.65 days a year | Two nines. Fine for internal tools, not for a shop. |\n| **99.9%** | 8.76 hours a year | Three nines. About 43 minutes of downtime a month. |\n| **99.95%** | 4.38 hours a year | A common cloud SLA for a single service. |\n| **99.99%** | 52.6 minutes a year | Four nines. Needs redundancy and automatic failover. |\n| **99.999%** | 5.26 minutes a year | Five nines. One bad deploy uses up the whole year. |\n| **43.8 min** | A month at 99.9% | That includes maintenance, unless the SLA excludes it. |\n\n## Explained\n\n99% is 3.65 days of downtime a year. Two nines sounds high, but it allows a whole long weekend of outage. Acceptable for an internal tool, painful for an online shop.\n\n99.9% is 8.76 hours a year, or about 43 minutes a month. Three nines is a realistic target for a well-run single server with good monitoring and backups.\n\n99.95% is 4.38 hours a year. A common SLA for a single cloud service or VM. Check whether it covers one instance or only a group across zones.\n\n99.99% is 52.6 minutes a year. Four nines usually means no single points of failure: two or more servers, a load balancer, a replicated database, and failover that happens automatically, because a human cannot respond fast enough.\n\n99.999% is 5.26 minutes a year. Five nines is a telecom and banking target. One bad deploy or one slow failover can use up the entire year's budget.\n\n43.8 minutes is a month at 99.9%. Read the SLA carefully: does planned maintenance count? Is it measured per month or per year? Does a slow site count as down?\n\nThe practical lesson: every extra nine roughly multiplies the cost and complexity. The right question is not how many nines can we get, but what does an hour of downtime actually cost us.\n\ndowntime = (1 - uptime) x 525,600 minutes.\n\n## In one line\n\n> **Every nine costs ten times more.** Pick the target the business needs, then build for it.\n\n**The formula:** downtime = (1 - uptime) x 525,600 minutes\n"},{"id":"p117","n":"117","slug":"hashing-vs-encryption","title":"Hashing vs Encryption","sections":["it","devsecops"],"topic":"Security & hardening","tags":["CyberSecurity","Cryptography","InfoSec","ITSecurity","WebSecurity"],"series_name":"Linux + IT","design":"compare","script":"","date":"2027-06-27","files":{"it":"notes/p117-hashing-vs-encryption.md","devsecops":"notes/p117-hashing-vs-encryption.md"},"md":"# Hashing vs Encryption\n\nHashing and encryption are both called scrambling data, and mixing them up leads to real security mistakes.\n\n## Quick reference\n\n| | **HASHING** | **ENCRYPTION** |\n|:--|:--|:--|\n| Reversible | No | Yes, with the key |\n| Output size | Fixed (256 bits) | About the input size |\n| Key needed | No | Yes |\n| Used for | Integrity, passwords | Privacy of data |\n| Examples | SHA-256, bcrypt | AES, RSA, TLS |\n| Passwords | Yes, slow and salted | Never |\n\n**HASHING** (One way, no key): Turns any data into a fixed fingerprint. It cannot be turned back.\n\n```bash\nsha256sum backup.tar.gz\n```\n\n**ENCRYPTION** (Two way, with a key): Locks data so only someone with the key can read it again.\n\n```bash\ngpg -c backup.tar.gz\n```\n\n- Hashing → prove data has not changed, store passwords.\n- Encryption → keep data secret, and get it back later.\n\n## Explained\n\nHashing is one way. sha256sum backup.tar.gz turns a file of any size into a fixed 256-bit fingerprint. The same input always gives the same hash, and changing a single byte changes it completely. There is no key, and no way to turn the hash back into the original.\n\nEncryption is two way. `gpg -c backup.tar.gz` locks the file with a key or passphrase. Anyone with the key can decrypt it and get the original data back. Without the key, it is unreadable.\n\n### Where each one belongs\n\nIntegrity: publish the SHA-256 hash of a download or a backup. Anyone can check the file has not been changed or corrupted.  \nPrivacy: encrypt backups before they leave the server, disks with BitLocker or LUKS, and traffic with TLS. The data must be readable again later, so it has to be encryption.  \nPasswords: hash them, never encrypt them. If passwords are encrypted, whoever steals the key gets every password. But use a slow, salted password hash like bcrypt or Argon2, not plain SHA-256, which is fast enough to brute force billions of guesses.\n\nA quick test: do you ever need the original data back? Then you need encryption. Do you only need to check that something matches? Then you need a hash.\n\nHashing proves. Encryption hides.\n"},{"id":"p118","n":"118","slug":"vmware-vs-hyper-v-vs-proxmox","title":"VMware vs Hyper-V vs Proxmox","sections":["it"],"topic":"Storage & infrastructure","tags":["Virtualization","VMware","HyperV","Proxmox","ITInfrastructure"],"series_name":"Linux + IT","design":"matrix","script":"","date":"2027-06-29","files":{"it":"notes/p118-vmware-vs-hyper-v-vs-proxmox.md"},"md":"# VMware vs Hyper-V vs Proxmox\n\nSince VMware's licensing changes under Broadcom, many IT teams are asking the same question: stay, or move? Three common options, compared honestly.\n\n## Quick reference\n\n| | **VMware** | **Hyper-V** | **Proxmox** |\n|:--|:--|:--|:--|\n| Licensing | Paid subscription | With Windows Server | ✅ Free, paid support optional |\n| Management | vCenter | WAC, SCVMM | ✅ Web UI built in |\n| Containers | ❌ Not built in | ❌ Not built in | ✅ LXC built in |\n| Clusters and HA | ✅ Yes, with vCenter | ✅ Failover clustering | ✅ Yes, built in |\n| Ecosystem | ✅ Largest | ✅ Large | 🟡 Growing fast |\n| Backup tools | ✅ Many vendors | ✅ Many vendors | ✅ PBS and others |\n\n**How to pick**\n\n- **VMware** → large estates with existing skills and budget.\n- **Hyper-V** → Windows shops that already license Windows Server.\n- **Proxmox** → open source, small to mid-size, homelab to production.\n\n## Explained\n\nVMware vSphere is still the most complete platform, with the largest ecosystem of backup, storage and monitoring tools, and many admins who know it. It is now sold as a paid subscription, and for many smaller customers the cost went up significantly. Most advanced features need vCenter.\n\nHyper-V is included with Windows Server. If you already license Windows Server Datacenter, unlimited Windows VMs on that host are covered. Management comes through Hyper-V Manager, Windows Admin Center or System Center VMM, and failover clustering provides high availability. It fits naturally in Windows shops.\n\nProxmox VE is open source and free to use, with optional paid subscriptions for support and the stable update repository. It runs KVM virtual machines and LXC containers side by side, has clustering and HA built in, a web interface out of the box, and its own Proxmox Backup Server. It has moved from homelabs into many small and mid-size production environments.\n\n### How to decide\n\nCount the real cost over three to five years: licences, support, and the time to migrate.  \nCheck your backup software supports the target platform before you move anything.  \nConsider your team. A platform nobody knows how to troubleshoot at night is not cheaper.\n\nThe hypervisor is the easy part. Storage, backups and people decide whether it works.\n\n## In one line\n\n> **The hypervisor is the easy part.** Backups, storage and skills decide if it works.\n\n**Check your next renewal:** licence cost x hosts x years\n"},{"id":"p119","n":"119","slug":"9-nmap-commands-to-check-your-own-network","title":"9 nmap commands to check your own network","sections":["it","devsecops"],"topic":"Security & hardening","tags":["CyberSecurity","Networking","BlueTeam","Nmap","InfoSec"],"series_name":"Linux + IT","design":"terminal","script":"","date":"2027-07-01","files":{"it":"notes/p119-9-nmap-commands-to-check-your-own-network.md","devsecops":"notes/p119-9-nmap-commands-to-check-your-own-network.md"},"md":"# 9 nmap commands to check your own network\n\nAttackers scan your network whether you do or not. Scanning it yourself, regularly, means you see the forgotten open port first. One rule before anything else: only scan networks you own or have written permission to test.\n\n## Quick reference\n\n### What is on the network?\n\n```bash\n# host discovery only, no port scan\nnmap -sn 192.168.10.0/24\n# the 100 most common ports, fast\nnmap --top-ports 100 192.168.10.20\n# all 65535 ports. Finds the forgotten ones\nnmap -p- 192.168.10.20\n```\n\n### What exactly is running?\n\n```bash\n# service versions: what needs patching\nnmap -sV -p 22,443 192.168.10.20\n# operating system guess. Needs root\nsudo nmap -O 192.168.10.20\n# UDP ports. SNMP open is worth a look\nsudo nmap -sU -p 53,161 192.168.10.1\n```\n\n### Check and keep the results\n\n```bash\n# which TLS versions and ciphers are offered\nnmap --script ssl-enum-ciphers -p 443 shop.example.com\n# skip ping. Filtered = a firewall answers for it\nnmap -Pn -p 3389 203.0.113.5\n# save results. Compare next month: what changed?\nnmap -sV 192.168.10.0/24 -oN scan-may.txt\n```\n\n## Explained\n\n### What is on the network?\n\n`nmap -sn 192.168.10.0/24` does host discovery only. Which IP addresses answer, without scanning ports. Unknown devices are the first thing to investigate.  \n`nmap --top-ports 100 192.168.10.20` checks the 100 most common ports. Fast, and catches most services.  \n`nmap -p- 192.168.10.20` checks all 65,535 TCP ports. Slower, but this is where forgotten admin panels and test services show up.\n\n### What exactly is running?\n\n`nmap -sV -p 22,443 192.168.10.20` does service and version detection. The output is your patch list: old OpenSSH, old nginx, old anything.  \n`sudo nmap -O 192.168.10.20` guesses the operating system. Useful for finding devices nobody documented.  \n`sudo nmap -sU -p 53,161 192.168.10.1` scans UDP ports, which a normal scan skips. SNMP with a default community string is a classic finding on printers and switches.\n\n### Check and keep the results\n\n`nmap --script ssl-enum-ciphers -p 443 shop.example.com` shows which TLS versions and ciphers a server offers. TLS 1.0 and 1.1 should be gone.  \n`nmap -Pn -p 3389 203.0.113.5` skips the ping check, for hosts that block it. filtered means a firewall is dropping the traffic; open from the internet on 3389 is an urgent fix.  \n`nmap -sV 192.168.10.0/24 -oN scan-may.txt` saves the results. Next month, compare. New open ports are changes someone should be able to explain.\n\n## In one line\n\n> **See what attackers see. First.** Scan your own network on a schedule, and compare the results.\n\n**Try it**\n\n```bash\nnmap --top-ports 100 your-public-ip\n```\n"},{"id":"p120","n":"120","slug":"the-stack-behind-a-wordpress-woocommerce-store","title":"The stack behind a WordPress / WooCommerce store","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["WooCommerce","WordPress","WebPerformance","Linux","Ecommerce"],"series_name":"Linux + IT","design":"stack","script":"","date":"2027-07-04","files":{"it":"notes/p120-the-stack-behind-a-wordpress-woocommerce-store.md"},"md":"# The stack behind a WordPress / WooCommerce store\n\nI look after a bilingual WooCommerce store with about 1,400 products. When someone says the site is slow, the useful question is always: which layer? A typical store runs on six of them.\n\n## Quick reference\n\n*Bottom to top: each layer can be the slow one*\n\n1. **Server or VM** — CPU, RAM and fast NVMe disks. The foundation. · `4 vCPU / 8 GB`\n2. **Linux** — Updates, firewall, time sync, enough free disk. · *Ubuntu LTS*\n3. **nginx** — TLS, static files, compression, page cache. · `nginx`\n4. **PHP-FPM** — Runs WordPress. Workers and OPcache matter. · `pm.max_children`\n5. **MariaDB / MySQL** — Orders, products, sessions. Tune the buffer pool. · `buffer_pool`\n6. **WordPress + Woo** — Theme, plugins and an object cache like Redis. · *Redis cache*\n\n## Explained\n\nL1, server or VM. CPU, memory and disk speed. If the server is out of RAM or the disk is slow, nothing above it can be fast. uptime, `free -h` and `df -h` are the first check.\n\nL2, Linux. Security updates, a firewall, correct time, and free disk space. A full disk breaks sessions, uploads and the database at the same moment.\n\nL3, nginx. Handles TLS, serves images, CSS and JavaScript directly, compresses responses, and can cache full pages for visitors who are not logged in. Static files should never reach PHP.\n\nL4, PHP-FPM. Runs WordPress itself. pm.max_children decides how many requests can run at once. Too few and visitors queue during busy hours. Too many and the server runs out of memory. OPcache should always be on.\n\nL5, MariaDB or MySQL. Products, orders, customers and sessions all live here. innodb_buffer_pool_size should be large enough to keep the active data in memory, and the slow query log shows which queries and plugins hurt.\n\nL6, WordPress and WooCommerce. The theme, the plugins, and an object cache like Redis that saves repeated database queries. One badly written plugin can slow every page.\n\nThe method that works: measure from the bottom up. Server resources, then nginx and PHP-FPM logs, then slow queries, then plugins. Fix the slowest layer first.\n\n## In one line\n\n> **Slow is a layer, not a mystery.** Find the layer first. Then tune that one.\n\n**Start at the bottom**\n\n```bash\nuptime; free -h; df -h\n```\n"},{"id":"p121","n":"121","slug":"fix-mysql-too-many-connections","title":"Fix: MySQL Too many connections","sections":["it"],"topic":"Databases & SQL","tags":["MySQL","MariaDB","Databases","Linux","Troubleshooting"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-07-06","files":{"it":"notes/p121-fix-mysql-too-many-connections.md"},"md":"# Fix: MySQL Too many connections\n\nERROR 1040: Too many connections. The database server is running, but the website, the app and even you cannot connect. Here is how to get back in and find the cause.\n\n## Quick reference\n\n### Diagnose — Who is using the connections?\n\n```sql\n# The limit. The default is only 151.\nSHOW VARIABLES LIKE 'max_connections';\n# Every connection: user, host, state and query.\nSHOW FULL PROCESSLIST;\n# How many right now. Watch it over time.\nSHOW STATUS LIKE 'Threads_connected';\n```\n\n### Fix — Get the app connecting again\n\n```sql\n# End a stuck long query that holds everything up.\nKILL 48213;\n# Raise the limit now. Check memory first.\nSET GLOBAL max_connections = 300;\n# Also in my.cnf, or it resets on restart.\nmax_connections = 300\n```\n\n### Prevent — Find the real cause\n\n- **Many Sleep connections?** — The app opens connections and never closes them.\n- `wait_timeout = 300` — Close idle connections after 5 minutes.\n- **Use a connection pool** — Reuse a few connections instead of hundreds.\n\n## Explained\n\nOne useful detail first: MySQL keeps one extra connection for an account with admin rights, so root can usually still log in when everyone else is refused.\n\n### Diagnose\n\nSHOW VARIABLES LIKE 'max_connections' shows the limit. The default is only 151, which a busy application can reach quickly.  \nSHOW FULL PROCESSLIST lists every connection: user, host, how long it has been there, and its query. Hundreds in Sleep state point to the application. A few long queries with everything else waiting point to a slow query or a lock.  \nSHOW STATUS LIKE 'Threads_connected' gives the current count. Watching it over time shows whether it climbs steadily, which is a leak, or spikes with traffic.\n\n### Fix\n\nKILL 48213 ends a stuck query that is holding locks and making every other connection wait.  \nSET GLOBAL `max_connections = 300` raises the limit immediately, without a restart. Every connection uses memory, so check you have room first.  \n`max_connections = 300` in my.cnf, otherwise the setting is lost on the next restart.\n\n### Prevent\n\nMany Sleep connections means the application opens connections and does not close them, or keeps far too many open. That is a code or configuration fix, not a database one.  \n`wait_timeout = 300` closes idle connections after five minutes instead of the default eight hours.  \nA connection pool lets the application reuse a small number of connections instead of opening a new one per request.\n\nRaising the limit buys time. Finding out who holds the connections fixes it.\n\n## In one line\n\n> **More connections is not the fix.** Find out who holds them, and why they never let go.\n\n**The first look**\n\n```bash\nmysql -e 'SHOW FULL PROCESSLIST'\n```\n"},{"id":"p122","n":"122","slug":"9-checks-before-handing-a-laptop-to-a-new-employee","title":"9 checks before handing a laptop to a new employee","sections":["it"],"topic":"IT operations & incidents","tags":["ITSupport","Onboarding","EndpointSecurity","Windows","ITManagement"],"series_name":"Linux + IT","design":"checklist","script":"","date":"2027-07-08","files":{"it":"notes/p122-9-checks-before-handing-a-laptop-to-a-new-employee.md"},"md":"# 9 checks before handing a laptop to a new employee\n\nOnboarding is part of supporting 600 employees across 9 branches. The laptop a new hire receives on day one sets the standard for security and for how they see IT. Nine checks before handing it over.\n\n## Quick reference\n\n### Security\n\n```powershell\n# Disk encryption on\nmanage-bde -status\n# Antivirus / EDR active\nGet-MpComputerStatus\n# No local admin for the user\nnet localgroup administrators\n```\n\n### Management\n\n- **Joined to domain or Intune** — `dsregcmd /status`\n- **Fully updated** — Windows, drivers, Office\n- **Asset tag recorded** — serial, user, date\n\n### Ready to work\n\n- **MFA enrolled** — on day one, with the user\n- **VPN and Wi-Fi tested** — from outside the office too\n- **Files syncing** — OneDrive or the file share\n\n## Explained\n\n### Security\n\nDisk encryption on. `manage-bde -status` should show BitLocker protection on. A lost laptop without encryption is a data breach. With encryption, it is just a lost laptop.  \nAntivirus or EDR active. `Get-MpComputerStatus` for Defender, or your EDR console. Check it is running, up to date, and reporting to the central console.  \nNo local admin for the user. `net localgroup administrators`. Daily work as a standard user blocks a large share of malware. Admin rights, if really needed, should be separate and temporary.\n\n### Management\n\nJoined to the domain or Intune. `dsregcmd /status` shows whether the device is joined and managed. An unmanaged laptop gets no policies and no updates you control.  \nFully updated. Windows, drivers and Office, before it leaves IT. Otherwise the user's first morning is spent waiting for updates.  \nAsset tag recorded. Serial number, user, date. When the person leaves, or the laptop is lost, you need to know exactly what to recover.\n\n### Ready to work\n\nMFA enrolled. Set it up with the user on day one, in person if possible. It is also the moment to explain how to report a suspicious email.  \nVPN and Wi-Fi tested, including from outside the office if they will work remotely.  \nFiles syncing. OneDrive or the right file shares, so work is backed up from the first day.\n\n## In one line\n\n> **Day one sets the standard.** A secure laptop on day one is easier than fixing it later.\n\n**A quick security look**\n\n```powershell\nmanage-bde -status; dsregcmd /status\n```\n"},{"id":"p123","n":"123","slug":"how-a-lets-encrypt-certificate-is-issued-and-renewed","title":"How a Let's Encrypt certificate is issued and renewed","sections":["it","devsecops"],"topic":"Web servers, TLS & HTTP","tags":["SSL","TLS","LetsEncrypt","Linux","WebSecurity"],"series_name":"Linux + IT","design":"pipeline","script":"","date":"2027-07-11","files":{"it":"notes/p123-how-a-lets-encrypt-certificate-is-issued-and-renewed.md","devsecops":"notes/p123-how-a-lets-encrypt-certificate-is-issued-and-renewed.md"},"md":"# How a Let's Encrypt certificate is issued and renewed\n\nLet's Encrypt made HTTPS free for everyone. Understanding the six steps behind it explains why renewals sometimes fail, and how to make sure they do not.\n\n## Quick reference\n\n1. **Account and order** — certbot asks Let's Encrypt for a certificate. · `certbot`\n2. **Challenge token** — Let's Encrypt replies: prove you control this domain. · *HTTP-01*\n3. **Token served on port 80** — certbot places it under /.well-known/. · `:80`\n4. **Let's Encrypt validates** — It fetches the token from several locations. · *validated*\n5. **Certificate issued** — Currently valid for 90 days. nginx uses it. · *90 days*\n6. **Automatic renewal** — A timer renews with 30 days left, then reloads. · `certbot.timer`\n\n## Explained\n\n1. **Account and order.** certbot, or another ACME client, registers an account and asks Let's Encrypt for a certificate for shop.example.com.\n\n2. **Challenge token.** Let's Encrypt does not take your word for it. It replies with a challenge: prove you control this domain. The most common type is HTTP-01.\n\n3. **Token served on port 80.** certbot places a token file under /.well-known/acme-challenge/ on your web server. This is why port 80 must be reachable from the internet, even if the site redirects everything to HTTPS.\n\n4. **Let's Encrypt validates.** It fetches the token over HTTP, from several network locations at once. If the file is there and correct, the domain is proven. A firewall blocking port 80, a wrong DNS record, or a redirect rule that sends the challenge somewhere else makes this step fail.\n\n5. **Certificate issued.** The certificate is currently valid for 90 days, with shorter lifetimes announced for the future. certbot saves it and configures nginx to use it.\n\n6. **Automatic renewal.** A systemd timer or cron job runs certbot renew twice a day. It renews when a certificate has 30 days or less left, then a deploy hook reloads nginx so the new certificate is actually served.\n\nMost expired certificate incidents come from step 3, 4 or 6: port 80 closed, DNS moved, or nginx never reloaded.\n\n`certbot renew --dry-run` tests the whole chain without issuing anything.\n\nFree certificates, automated renewals, as long as port 80 and the timer keep working.\n\n## In one line\n\n> **Free certificates. Automated renewals.** As long as port 80 and the timer keep working.\n\n**Test renewal now**\n\n```bash\ncertbot renew --dry-run\n```\n"},{"id":"p124","n":"124","slug":"5-password-and-access-mistakes","title":"5 password and access mistakes","sections":["it","devsecops"],"topic":"Security & hardening","tags":["CyberSecurity","IAM","PasswordSecurity","MFA","ITSecurity"],"series_name":"Linux + IT","design":"dontdo","script":"","date":"2027-07-13","files":{"it":"notes/p124-5-password-and-access-mistakes.md","devsecops":"notes/p124-5-password-and-access-mistakes.md"},"md":"# 5 password and access mistakes\n\nMost breaches do not start with clever code. They start with a login. Five access habits that make it easy for attackers, and what to do instead.\n\n## Quick reference\n\n**1.** ❌ **one shared admin account** — Nobody knows who did what. Leavers keep access.  \n✅ **named accounts + MFA** — Every action has a name. Remove one person, not a password.\n\n**2.** ❌ **passwords.xlsx on the file share** — One compromised PC exposes every system.  \n✅ **a password manager with sharing** — Encrypted, logged, and access can be removed.\n\n**3.** ❌ **SMS codes for admin accounts** — SIM swaps and fake login pages can steal them.  \n✅ **authenticator app or passkey** — Phishing-resistant MFA for anyone with admin rights.\n\n**4.** ❌ **service account, password never expires** — Set once, known by many, never changed.  \n✅ **gMSA or a secrets vault** — Passwords rotate automatically. No human knows them.\n\n**5.** ❌ **admin / admin on a new device** — Default logins are the first thing bots try.  \n✅ **change defaults on day one** — Before the device ever touches the network.\n\n## Explained\n\n1. One shared admin account\n   When five people use the same admin login, the logs cannot tell you who did what, and someone who leaves the company still knows the password. Instead: named accounts for every person, with MFA. Removing access means disabling one account.\n\n2. passwords.xlsx on the file share\n   A spreadsheet of passwords is the first thing an attacker searches for after getting into one PC. Instead: a password manager with shared vaults for teams. It is encrypted, every access is logged, and you can remove someone's access in one click.\n\n3. SMS codes for admin accounts\n   SMS is better than nothing, but SIM swap attacks and fake login pages that relay the code in real time can defeat it. Instead: an authenticator app with number matching, or a passkey or hardware key for anyone with admin rights.\n\n4. Service accounts whose passwords never expire\n   They are created once, the password is shared with whoever set up the integration, and it never changes. Instead: group managed service accounts in Active Directory, or a secrets vault, so passwords rotate automatically and no person needs to know them.\n\n5. Default logins on new devices\n   Printers, cameras, switches and NAS boxes ship with admin and admin or similar. Bots try these within minutes of a device appearing online. Instead: change default credentials on day one, before the device is connected to the network.\n\n## In one line\n\n> **Identity is the new perimeter.** Protect logins like you protect servers.\n\n**Start here:** MFA for every admin, this week\n"},{"id":"p125","n":"125","slug":"quiz-3-security-questions-everyone-in-it-should-answer","title":"Quiz: 3 security questions everyone in IT should answer","sections":["it"],"topic":"Security & hardening","tags":["CyberSecurity","SecurityAwareness","InfoSec","ITSecurity","Phishing"],"series_name":"Linux + IT","design":"quiz","script":"","date":"2027-07-15","files":{"it":"notes/p125-quiz-3-security-questions-everyone-in-it-should-answer.md"},"md":"# Quiz: 3 security questions everyone in IT should answer\n\nThree security questions that everyone in IT should be able to answer instantly. They sound simple. Getting them wrong is expensive.\n\n## Quick reference\n\n**Q1. What is the right way to store user passwords in a database?**\n\n- a) Encrypt them with AES\n- b) Hash them with SHA-256\n- c) Hash them with bcrypt or Argon2\n\n> **Answer:** c) Hash them with bcrypt or Argon2. Slow, salted password hashes make brute force expensive. Fast hashes like SHA-256 are cracked quickly.\n\n**Q2. Your CEO emails asking you to buy gift cards urgently. What first?**\n\n- a) Reply to confirm the amount\n- b) Check with the CEO another way\n- c) Buy them, it is the CEO\n\n> **Answer:** b) Check with the CEO another way. Call or message the CEO on a known number. Urgency plus an unusual request is the classic sign of fraud.\n\n**Q3. Which of these counts as multi-factor authentication?**\n\n- a) Password + a security question\n- b) Password + a code from an app\n- c) Two different passwords\n\n> **Answer:** b) Password + a code from an app. MFA needs different types: something you know plus something you have. Two passwords are both know.\n\n## Explained\n\n1. What is the right way to store user passwords in a database?\n\n> **Answer:** hash them with a slow, salted password hashing algorithm like bcrypt or Argon2. Encryption is wrong, because whoever steals the key can decrypt every password. Plain SHA-256 is also wrong, because it is designed to be fast, and modern GPUs can test billions of guesses per second against it. Slow password hashes make every guess expensive, and a unique salt per user stops attackers from cracking everyone at once.\n\n2. Your CEO emails asking you to buy gift cards urgently. What do you do first?\n\n> **Answer:** verify through a different channel. Call or message the CEO on a number you already know, not one from the email. This is business email compromise, and the pattern is always the same: a person with authority, urgency, secrecy, and an unusual request involving money. Real executives are rarely offended by a quick check.\n\n3. **Which of these counts as multi-factor authentication?**\n\n> **Answer:** a password plus a code from an authenticator app. MFA means combining different types of factors: something you know, something you have, or something you are. A password and a security question are both something you know, so that is still a single factor. Two passwords, the same.\n\nNone of this needs expensive tools. It needs people who know the basics and use them every day.\n\n## In one line\n\n> **Security is mostly good habits.** The basics stop most real attacks.\n\n**Today's habit:** verify unusual requests on a second channel\n"},{"id":"p126","n":"126","slug":"9-aws-cli-commands-every-admin-should-know","title":"9 AWS CLI commands every admin should know","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudComputing","DevOps","SysAdmin","Automation"],"series_name":"Linux + IT","design":"steps","script":"","date":"2027-07-18","files":{"devsecops":"notes/p126-9-aws-cli-commands-every-admin-should-know.md"},"md":"# 9 AWS CLI commands every admin should know\n\nThe AWS console is fine for exploring. For daily work and anything repeatable, the CLI is faster and scriptable. Nine commands worth knowing.\n\n## Quick reference\n\n### Who and where — Know which account you are in\n\n```bash\n# Which account and user you are. Run it first, always.\naws sts get-caller-identity\n# Separate profiles for each account or environment.\naws configure --profile prod\n# Switch profile for this shell session.\nexport AWS_PROFILE=prod\n```\n\n### Storage — Work with S3\n\n```bash\n# List objects, like ls for a bucket.\naws s3 ls s3://my-bucket/\n# Upload only changes. --delete removes extras.\naws s3 sync ./site s3://my-bucket --delete\n# Copy a single file, for example a backup.\naws s3 cp db.sql.gz s3://backups/\n```\n\n### Servers and logs — EC2 and CloudWatch\n\n```bash\n# Every instance in the region, readable.\naws ec2 describe-instances --output table\n# Stop a server. Its EBS disks stay.\naws ec2 stop-instances --instance-ids i-0abc\n# Follow CloudWatch logs live, like tail -f.\naws logs tail /aws/lambda/api --follow\n```\n\n## Explained\n\n### Who and where\n\n`aws sts get-caller-identity` shows which account and which user or role you are using right now. Run it before anything that changes resources. Most CLI accidents happen in the wrong account.  \n`aws configure --profile prod` stores credentials and a default region under a named profile. One profile per account or environment.  \n`export AWS_PROFILE=prod` switches profile for the current shell session, so you do not need `--profile` on every command.\n\n### Storage\n\n`aws s3 ls s3://my-bucket/` lists objects in a bucket, like ls for a folder.  \n`aws s3 sync ./site s3://my-bucket --delete` uploads only new and changed files. `--delete` removes files from the bucket that no longer exist locally, so run it with `--dryrun` first.  \n`aws s3 cp db.sql.gz s3://backups/` copies a single file. A simple way to get backups off the server, ideally into a bucket with versioning turned on.\n\n### Servers and logs\n\n`aws ec2 describe-instances --output table` lists every instance in the current region in a readable table. Add `--query` to pick only the columns you need.  \n`aws ec2 stop-instances --instance-ids i-0abc` stops an instance. The EBS volumes stay, so it can be started again. Terminate is permanent.  \n`aws logs tail /aws/lambda/api --follow` follows CloudWatch logs live, like `tail -f`, without opening the console.\n\nThe habit that matters most: check which account you are in before you press Enter. The CLI is fast, and that includes mistakes.\n\n## In one line\n\n> **Check the account before you press Enter.** The CLI is fast. That includes mistakes.\n\n**Always first**\n\n```bash\naws sts get-caller-identity\n```\n"},{"id":"p127","n":"127","slug":"subnet-sizes-cheat-sheet","title":"Subnet sizes cheat sheet","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","CCNA","Subnetting","ITSupport","ITInfrastructure"],"series_name":"Linux + IT","design":"numbers","script":"","date":"2027-07-20","files":{"it":"notes/p127-subnet-sizes-cheat-sheet.md","devsecops":"notes/p127-subnet-sizes-cheat-sheet.md"},"md":"# Subnet sizes cheat sheet\n\nSix subnet sizes worth knowing by heart. They come up in network design, firewall rules, cloud VPCs and almost every networking interview.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **254** | /24  255.255.255.0 | The classic office or home network. | /24 |\n| **126** | /25  255.255.255.128 | Half a /24. Two departments from one range. | /25 |\n| **62** | /26  255.255.255.192 | A floor, a branch, or a server VLAN. | /26 |\n| **30** | /27  255.255.255.224 | A small team, printers, or cameras. | /27 |\n| **14** | /28  255.255.255.240 | A few servers or a DMZ. | /28 |\n| **2** | /30  255.255.255.252 | A link between two routers. | /30 |\n\n## Explained\n\n254 usable hosts, a /24, mask 255.255.255.0. The classic office or home network, like 192.168.1.0/24. 256 addresses minus the network address and the broadcast address.\n\n126 usable hosts, a /25, mask 255.255.255.128. Half of a /24. One /24 split into two networks, for example staff and printers.\n\n62 usable hosts, a /26, mask 255.255.255.192. A good size for one floor, a small branch, or a server VLAN.\n\n30 usable hosts, a /27, mask 255.255.255.224. A small team, a group of printers, or security cameras in their own segment.\n\n14 usable hosts, a /28, mask 255.255.255.240. A handful of servers or a DMZ. Cloud providers often reserve a few extra addresses in every subnet, so check before you size tightly.\n\n2 usable hosts, a /30, mask 255.255.255.252. Traditionally used for the link between two routers. A /31 is also allowed for point-to-point links and saves two addresses.\n\n> **The pattern that makes it easy:** every extra bit in the prefix halves the number of addresses, and you subtract two for the network and broadcast addresses.\n\nusable hosts = 2^(32 - prefix) - 2.\n\nSize subnets for growth. Renumbering a full network later is far harder than leaving room now.\n\n## In one line\n\n> **Each bit halves it. Minus two.** Network address and broadcast are not usable.\n\n**The formula:** usable = 2^(32 - prefix) - 2\n"},{"id":"p128","n":"128","slug":"active-directory-vs-entra-id","title":"Active Directory vs Entra ID","sections":["it","devsecops"],"topic":"Windows & Active Directory","tags":["ActiveDirectory","EntraID","Microsoft365","IAM","CloudComputing"],"series_name":"Linux + IT","design":"compare","script":"","date":"2027-07-22","files":{"it":"notes/p128-active-directory-vs-entra-id.md","devsecops":"notes/p128-active-directory-vs-entra-id.md"},"md":"# Active Directory vs Entra ID\n\nActive Directory and Entra ID, formerly Azure AD, share part of a name. They are very different systems, and most companies today use both.\n\n## Quick reference\n\n| | **AD DS** | **ENTRA ID** |\n|:--|:--|:--|\n| Runs on | Your domain controllers | Microsoft's cloud |\n| Protocols | Kerberos, NTLM, LDAP | OAuth 2.0, OIDC, SAML |\n| Structure | OUs and Group Policy | Flat, admin units |\n| Device policy | Group Policy | Intune |\n| Access rules | Network location | Conditional Access |\n| Together | Synced with Entra Connect | Hybrid identity |\n\n**AD DS** (On-premises Active Directory): Domain controllers in your network. Kerberos, LDAP and Group Policy.\n\n```powershell\nGet-ADUser jsmith\n```\n\n**ENTRA ID** (Microsoft cloud identity): Identity for Microsoft 365 and cloud apps. Modern sign-in protocols.\n\n```powershell\nGet-MgUser -UserId jsmith\n```\n\n- AD DS → on-prem servers, file shares and classic Windows apps.\n- Entra ID → Microsoft 365, SaaS apps and modern device management.\n\n## Explained\n\nActive Directory Domain Services runs on domain controllers in your own network. Computers join the domain, users log in with Kerberos, applications query it over LDAP, and Group Policy configures every machine. It has a deep structure of organizational units, and it was designed for a world where everyone worked inside the office network.\n\nEntra ID is Microsoft's cloud identity service. It is what you sign in to for Microsoft 365, and it connects to thousands of SaaS applications using OAuth 2.0, OpenID Connect and SAML. There are no OUs or Group Policy. Devices are managed with Intune, and access is controlled by Conditional Access: who you are, which device, from where, and how risky the sign-in looks.\n\n### How they work together\n\nMost organizations run hybrid identity. Entra Connect, or the newer Cloud Sync, synchronizes users and groups from AD to Entra ID, so people have one account for both the file server and Microsoft 365.\n\n### Where people get confused\n\nEntra ID is not AD in the cloud. You cannot point an old application that needs LDAP or Kerberos at it without extra services.  \nMoving to the cloud does not remove AD overnight. File servers, legacy apps and on-premises systems often keep it alive for years.\n\nAD DS for the on-premises world. Entra ID for the cloud. A hybrid bridge between them.\n"},{"id":"p129","n":"129","slug":"block-vs-file-vs-object-storage","title":"Block vs File vs Object storage","sections":["it","devsecops"],"topic":"Storage & infrastructure","tags":["Storage","CloudComputing","ITInfrastructure","AWS","SysAdmin"],"series_name":"Linux + IT","design":"matrix","script":"","date":"2027-07-25","files":{"it":"notes/p129-block-vs-file-vs-object-storage.md","devsecops":"notes/p129-block-vs-file-vs-object-storage.md"},"md":"# Block vs File vs Object storage\n\nBlock, file and object storage are the three building blocks behind almost every server and cloud setup. Choosing the wrong one makes things slow, expensive, or both.\n\n## Quick reference\n\n| | **Block** | **File** | **Object** |\n|:--|:--|:--|:--|\n| Looks like | A raw disk | Folders and files | Objects in a bucket |\n| Accessed by | One server | Many, NFS/SMB | Anyone, HTTPS |\n| Examples | EBS, iSCSI | NFS, SMB, EFS | S3, MinIO, Blob |\n| Edit in place | ✅ Yes | ✅ Yes | ❌ Replace object |\n| Scales to | 🟡 One volume | 🟡 Large | ✅ Near unlimited |\n| Best for | Databases, VMs | Shared files | Backups, media |\n\n**How to pick**\n\n- **Block** → databases and VM disks that need low latency.\n- **File** → shared folders that people and apps use together.\n- **Object** → backups, archives and huge amounts of static data.\n\n## Explained\n\nBlock storage looks like a raw disk. The server formats it with a filesystem and mounts it. AWS EBS, an iSCSI LUN from a SAN, and the virtual disk of a VM are all block storage. It gives the lowest latency and lets you change any byte in place, which is exactly what databases need. Usually one server uses one volume at a time.\n\nFile storage looks like folders and files, shared over the network with NFS or SMB. Many servers and users can read and write the same files at once. File shares, home directories and AWS EFS are file storage. It is the natural choice for shared team files and applications that expect a normal filesystem path.\n\nObject storage stores whole objects in a bucket, each with a key and metadata, accessed over HTTPS with an API. Amazon S3, MinIO and Azure Blob are object storage. You cannot edit part of an object, you replace it. In exchange, it scales to practically unlimited size, is very durable, and is usually the cheapest per gigabyte.\n\n### How to choose\n\nA database or VM disk: block.  \nFolders that several people or servers share: file.  \nBackups, archives, logs, images and video, website assets: object. With versioning and object lock, it is also a strong defence against ransomware deleting your backups.\n\nMatch the storage to how the data is accessed.\n\n## In one line\n\n> **Match the storage to the access.** How the data is used decides where it should live.\n\n**See your block devices**\n\n```bash\nlsblk -f\n```\n"},{"id":"p130","n":"130","slug":"9-windows-commands-to-diagnose-and-repair-a-pc","title":"9 Windows commands to diagnose and repair a PC","sections":["it"],"topic":"Windows & Active Directory","tags":["Windows","ITSupport","Troubleshooting","SysAdmin","PowerShell"],"series_name":"Linux + IT","design":"terminal","script":"","date":"2027-07-27","files":{"it":"notes/p130-9-windows-commands-to-diagnose-and-repair-a-pc.md"},"md":"# 9 Windows commands to diagnose and repair a PC\n\nA PC that is slow, crashing, or just acting strange does not always need a reinstall. These nine built-in Windows commands fix or diagnose most of it first. Run them in an elevated terminal.\n\n## Quick reference\n\n### Repair Windows itself\n\n```powershell\n# check and repair protected system files\nsfc /scannow\n# repair the image sfc relies on. Run sfc after\nDISM /Online /Cleanup-Image /RestoreHealth\n# scan the disk online. /f schedules a full fix\nchkdsk C: /scan\n```\n\n### Reset the network\n\n```powershell\n# clear stale DNS entries\nipconfig /flushdns\n# repair a broken network stack\nnetsh winsock reset\n# confirm the machine can reach the internet\nTest-NetConnection 8.8.8.8\n```\n\n### Check the hardware and logs\n\n```powershell\n# is the disk healthy?\nGet-PhysicalDisk | ft FriendlyName, HealthStatus\n# the last 20 system events\nGet-WinEvent -LogName System -MaxEvents 20\n# laptop battery health, in one file\npowercfg /batteryreport\n```\n\n## Explained\n\n### Repair Windows itself\n\n`sfc /scannow` checks every protected system file against a known-good copy and repairs what is corrupt. The first thing to try for strange errors and crashes.  \n`DISM /Online /Cleanup-Image /RestoreHealth` repairs the component store that sfc relies on. If sfc cannot fix something, run DISM, then run sfc again.  \n`chkdsk C: /scan` scans the system disk online without a reboot. Add /f to schedule a full repair at the next restart if it finds problems.\n\n### Reset the network\n\n`ipconfig /flushdns` clears cached DNS entries, the fix when one PC keeps reaching the old address after a DNS change.  \n`netsh winsock reset` repairs a corrupted network stack, often left behind by old VPN or security software. It needs a restart.  \n`Test-NetConnection 8.8.8.8` confirms the machine can actually reach the internet, and the -Port form tests a specific service.\n\n### Check the hardware and logs\n\n`Get-PhysicalDisk | ft FriendlyName, HealthStatus` shows whether a disk reports itself as healthy. A failing disk explains a lot of slowness and freezing.  \n`Get-WinEvent -LogName System -MaxEvents 20` shows the most recent system events. Repeated errors near a crash time point straight at the cause.  \n`powercfg /batteryreport` writes a battery health report for a laptop, including how much capacity it has lost.\n\nThe order that saves a reinstall: DISM, then sfc, then check the disk and the event log.\n\n## In one line\n\n> **Repair before you reinstall.** Most strange Windows behaviour is a repairable file or a tired disk.\n\n**Try it**\n\n```powershell\nDISM ... /RestoreHealth && sfc /scannow\n```\n"},{"id":"p131","n":"131","slug":"how-your-data-is-wrapped-to-cross-the-network-tcp-ip","title":"How your data is wrapped to cross the network (TCP/IP)","sections":["it"],"topic":"Networking & DNS","tags":["Networking","CCNA","TCPIP","SysAdmin","ITInfrastructure"],"series_name":"Linux + IT","design":"stack","script":"","date":"2027-07-29","files":{"it":"notes/p131-how-your-data-is-wrapped-to-cross-the-network-tcp-ip.md"},"md":"# How your data is wrapped to cross the network (TCP/IP)\n\nWhen you load a web page, your data does not travel as-is. Each layer of the TCP/IP stack wraps it in another header on the way out, and the receiver unwraps it in reverse. Understanding this explains most networking tools and most networking problems.\n\n## Quick reference\n\n*Top to bottom: each layer adds a header*\n\n1. **Application** — Your actual data: an HTTP request, an email, a query. · `HTTP / DNS`\n2. **Transport** — Adds a TCP or UDP header with port numbers. · `TCP :443`\n3. **Internet** — Adds an IP header with source and destination IPs. · *IP addresses*\n4. **Link** — Wraps it in a frame with source and destination MACs. · `Ethernet / MAC`\n5. **Physical** — The frame becomes signals on copper, fibre or radio. · *bits on the wire*\n\n## Explained\n\nApplication. Your actual data: an HTTP request for a page, a DNS query, an email. At this point it is just the message your program wants to send.\n\nTransport. A TCP or UDP header is added. It carries the source and destination port numbers, so the receiving machine knows which program the data is for: 443 for HTTPS, 53 for DNS. TCP also adds sequence numbers so lost pieces can be resent.\n\nInternet. An IP header is added, with the source and destination IP addresses. This is what routers read to move the packet across networks toward its destination. TTL lives here, which is how traceroute works.\n\nLink. The packet is wrapped in a frame with source and destination MAC addresses, for the next hop on the local network. Unlike the IP address, the MAC changes at every hop. ARP is how a device finds the MAC for an IP.\n\nPhysical. The frame becomes actual signals: electricity on copper, light on fibre, or radio on Wi-Fi.\n\nWhy it matters in practice: a firewall port rule works at the transport layer, routing works at the internet layer, a switch and ARP work at the link layer, and a dead cable is physical. Knowing which layer your problem is on tells you which tool to reach for.\n\n`sudo tcpdump -n -i any port 443` lets you watch these headers on a real connection.\n\n## In one line\n\n> **Every layer adds, then removes, a header.** The receiver unwraps it in the exact reverse order.\n\n**Watch it happen**\n\n```bash\nsudo tcpdump -n -i any port 443\n```\n"},{"id":"p132","n":"132","slug":"quiz-3-git-questions-every-developer-should-know","title":"Quiz: 3 Git questions every developer should know","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","DevOps","SoftwareDevelopment","VersionControl","ITCareer"],"series_name":"IT","design":"quiz","script":"","date":"2027-08-01","files":{"devsecops":"notes/p132-quiz-3-git-questions-every-developer-should-know.md"},"md":"# Quiz: 3 Git questions every developer should know\n\nThree Git questions that separate people who memorised commands from people who understand Git. Try each before reading on.\n\n## Quick reference\n\n**Q1. You committed to the wrong branch. Nothing is pushed yet. Best fix?**\n\n- a) `git reset --hard`, then redo it\n- b) `git switch -c right-branch` (moves the commit)\n- c) Delete the folder and clone again\n\n> **Answer:** b) `git switch -c right-branch` (moves the commit). Branching from where you are keeps the commit, then reset the old branch. `--hard` would throw the work away.\n\n**Q2. What is the difference between git reset and git revert?**\n\n- a) Nothing, they are aliases\n- b) reset moves the branch pointer; revert adds an undo commit\n- c) revert deletes history\n\n> **Answer:** b) reset moves the branch pointer; revert adds an undo commit. revert is safe on shared branches: it adds a new commit that undoes the old one. reset rewrites history.\n\n**Q3. git pull is shorthand for which two commands?**\n\n- a) git fetch + git merge\n- b) git clone + git checkout\n- c) git add + git commit\n\n> **Answer:** a) git fetch + git merge. git pull fetches the remote changes, then merges (or rebases) them. Knowing that explains most pull surprises.\n\n## Explained\n\n1. You committed to the wrong branch, nothing is pushed yet. Best fix?\n\n> **Answer:** create the correct branch from where you are, then move the old branch back. `git switch -c right-branch` keeps your commit, then git switch main and `git reset --hard origin/main` cleans the branch you were on. Reaching straight for `git reset --hard` throws the work away before you have saved it anywhere.\n\n2. What is the difference between git reset and git revert?\n\n> **Answer:** reset moves the branch pointer, rewriting history. revert adds a new commit that undoes a previous one, leaving history intact. On a shared branch, use revert, because rewriting history that others have already pulled causes painful conflicts. On your own local work, reset is fine.\n\n3. git pull is shorthand for which two commands?\n\n> **Answer:** git fetch followed by git merge, or a rebase if you configure it. Fetch downloads the remote changes, merge combines them with yours. This is why an unexpected merge commit or conflict appears after a pull: it is the merge step doing its job. git fetch first, then look, then merge, gives you more control.\n\nThe common thread: Git almost never truly loses committed work. git reflog remembers where every branch has been for weeks, so even a bad reset can usually be undone.\n\n## In one line\n\n> **Git rewards those who understand it.** Most scary Git moments are undoable once you know how.\n\n**See what changed**\n\n```bash\ngit status && git log --oneline -5\n```\n"},{"id":"p133","n":"133","slug":"anatomy-of-a-docker-run-command","title":"Anatomy of a docker run command","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","Linux","CloudComputing"],"series_name":"IT","design":"anatomy","script":"","date":"2027-08-03","files":{"devsecops":"notes/p133-anatomy-of-a-docker-run-command.md"},"md":"# Anatomy of a docker run command\n\n`docker` run is the command everyone copies and few read closely. Taken apart, it tells you exactly what kind of container you are about to start.\n\n## Quick reference\n\n```text\ndocker run -d --name api -p 8080:80 myapp:1.4\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `docker` | The client | The Docker CLI, sending your command to the daemon. |\n| `run` | The action | Create a new container from an image and start it. |\n| `-d` | Detached | Run in the background and print the container ID. |\n| `--name api` | A name | So you refer to it as api, not a random hash. |\n| `-p 8080:80` | Port map | Host port 8080 forwards to container port 80. |\n| `myapp:1.4` | The image | Name and tag. Pin a real tag, never latest. |\n\n## Explained\n\n`docker` is the client. It takes your command and sends it to the Docker daemon, which does the real work. That split is why Docker commands can target a remote host too.\n\nrun means create a new container from an image and start it. It is really two steps, create and start, in one. If the image is not on the machine, Docker pulls it first.\n\n-d is detached mode. The container runs in the background and Docker prints its ID instead of streaming the logs. Leave it off and your terminal is attached to the container, which is handy for a quick test.\n\n`--name api` gives the container a readable name. Without it, Docker invents a random one, and every later command has to use the ID. A clear name makes `docker` logs, `docker` exec and `docker` stop much easier.\n\n`-p 8080:80` publishes a port. Traffic to port 8080 on the host is forwarded to port 80 inside the container. The order is host:container, and getting it backwards is a classic mistake. For safety, bind to a specific address: -p 127.0.0.1:8080:80.\n\n`myapp:1.4` is the image and tag. The tag is the part people skip, and latest is not a version: it is whatever was newest when you pulled. Pin a real tag so the same command gives the same container next month.\n\n## In one line\n\n> **Read the command, know the container.** Each flag is a decision you can explain.\n\n**See what is running**\n\n```bash\ndocker ps\n```\n"},{"id":"p134","n":"134","slug":"9-git-commands-to-undo-mistakes","title":"9 git commands to undo mistakes","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","DevOps","VersionControl","SoftwareDevelopment","Programming"],"series_name":"IT","design":"steps","script":"","date":"2027-08-05","files":{"devsecops":"notes/p134-9-git-commands-to-undo-mistakes.md"},"md":"# 9 git commands to undo mistakes\n\nThe scariest Git moments are almost always recoverable. Nine commands to undo mistakes, from a wrong edit to a commit you thought was gone.\n\n## Quick reference\n\n### Not committed yet — Changes still in the working tree\n\n```bash\n# Throw away local changes to one file. It is gone, so be sure.\ngit restore file.txt\n# Unstage a file but keep your edits.\ngit restore --staged file.txt\n# Park all changes to deal with later. git stash pop brings them back.\ngit stash\n```\n\n### Already committed — Fix the last commit or go back\n\n```bash\n# Fix the last commit message or add a forgotten file.\ngit commit --amend\n# Undo the last commit, keep the changes staged.\ngit reset --soft HEAD~1\n# Undo a commit by adding an opposite one. Safe when pushed.\ngit revert <hash>\n```\n\n### When it looks lost — The safety nets\n\n```bash\n# Every place HEAD has been. Your time machine.\ngit reflog\n# Restore one file from any past commit.\ngit checkout <hash> -- file\n# Pull one commit from another branch to here.\ngit cherry-pick <hash>\n```\n\n## Explained\n\n### Not committed yet\n\n`git restore file.txt` throws away local changes to one file, back to the last commit. The change is gone, so be sure first.  \n`git restore --staged file.txt` unstages a file but keeps your edits. The modern replacement for git reset HEAD file.  \n`git stash` parks all your uncommitted changes and gives you a clean tree. `git stash` pop brings them back. Perfect when you need to switch branches in a hurry.\n\n### Already committed\n\n`git commit --amend` fixes the most recent commit: a typo in the message, or a file you forgot to add. Only before you push, since it rewrites that commit.  \n`git reset --soft HEAD~1` undoes the last commit but keeps the changes staged, ready to recommit differently. `--mixed` keeps them unstaged; `--hard` throws them away.  \n`git revert <hash>` undoes a commit by creating a new one that reverses it. This is the safe choice on a shared branch, because it does not rewrite history.\n\n### When it looks lost\n\n`git reflog` shows every position HEAD has had, even commits no longer on any branch. This is how you recover from a bad reset.  \n`git checkout <hash> -- file` restores a single file from any past commit without touching anything else.  \n`git cherry-pick <hash>` copies one commit from another branch onto your current one.\n\n> **The rule that calms everyone down:** once something is committed, Git keeps it for weeks in the reflog, even if no branch points to it.\n\n## In one line\n\n> **Committed work is rarely lost.** reflog remembers for weeks. Breathe, then recover.\n\n**When you panic**\n\n```bash\ngit reflog\n```\n"},{"id":"p135","n":"135","slug":"what-a-healthy-linux-server-looks-like","title":"What a healthy Linux server looks like","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SysAdmin","Monitoring","DevOps","Observability"],"series_name":"IT","design":"dashboard","script":"","date":"2027-08-08","files":{"it":"notes/p135-what-a-healthy-linux-server-looks-like.md","devsecops":"notes/p135-what-a-healthy-linux-server-looks-like.md"},"md":"# What a healthy Linux server looks like\n\nWhen someone asks \"is the server OK?\", the useful answer is numbers, not a feeling. Here is what a healthy Linux box looks like, and why each number matters.\n\n## Quick reference\n\n**Status:** All systems normal\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| CPU | **34%** | 4 cores, load 1.3 |\n| Memory | **61%** | 4.7 of 7.7 GiB used |\n| Disk / | **48%** | 19 of 40 GB used |\n| Swap | **4%** | Barely touched. Good. |\n| Uptime | **87d** | since last reboot |\n| Failed units | **0** | `systemctl --failed` |\n\n## Explained\n\nCPU at 34%, load 1.3 on 4 cores. The load average matters more than the percentage. On 4 cores, a load under 4 means no task is waiting for CPU. If load climbs well above the core count while CPU sits low, tasks are waiting on disk or network, not compute.\n\nMemory at 61%, 4.7 of 7.7 GiB. Read the available figure, not free. Linux uses spare RAM as cache and hands it back when needed, so low free is normal and healthy. Low available is the real warning.\n\nDisk at 48%. The number to watch is the trend, not the value. A disk at 48% that grows 1% a day gives you weeks of warning. Alert at 80%, not 100%, because a full disk breaks logging, databases and uploads at once.\n\nSwap barely touched. A little swap use is fine. Heavy, constant swapping means the server is short on RAM and everything slows down.\n\nUptime 87 days. Useful context, but very long uptime also hints a kernel update and reboot are overdue.\n\nFailed units: 0. `systemctl --failed` should be empty. One failed unit is the thread to pull before it becomes an outage.\n\nThe point of knowing the normal numbers is simple: when something spikes, you notice immediately instead of guessing.\n\nHealthy is boring, and boring is the goal.\n\n## In one line\n\n> **Healthy is boring.** Know your baseline, and problems announce themselves.\n\n**One-line snapshot**\n\n```bash\nuptime; free -h; df -h\n```\n"},{"id":"p136","n":"136","slug":"latency-numbers-every-engineer-should-know","title":"Latency numbers every engineer should know","sections":["devsecops"],"topic":"DevOps basics","tags":["SoftwareEngineering","Performance","SystemDesign","DevOps","CloudComputing"],"series_name":"IT","design":"numbers","script":"","date":"2027-08-10","files":{"devsecops":"notes/p136-latency-numbers-every-engineer-should-know.md"},"md":"# Latency numbers every engineer should know\n\nYou do not need to memorise exact nanoseconds. You need the orders of magnitude, because they explain nearly every performance decision you will make. These are the classic \"latency numbers every engineer should know\", rounded for memory.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **1 ns** | L1 cache | The CPU's fastest memory. The baseline for everything. | ~ |\n| **100 ns** | Main memory (RAM) | About 100x slower than L1. Still very fast. | RAM |\n| **16 us** | SSD read | Solid-state storage. Microseconds, not milliseconds. | NVMe |\n| **2 ms** | Disk seek (HDD) | A spinning disk. Thousands of times slower than RAM. | HDD |\n| **0.5 ms** | Same datacenter | A network round trip between nearby servers. | LAN |\n| **150 ms** | Across the world | A round trip between continents. Physics, not your code. | WAN |\n\n## Explained\n\n1 ns, L1 cache. The CPU's fastest memory, right next to the core. Everything else is measured against this.\n\n100 ns, main memory. Reading from RAM is roughly 100 times slower than L1 cache. Still astonishingly fast, which is why keeping data in memory beats fetching it again.\n\n16 us, an SSD read. Microseconds. Modern NVMe drives are thousands of times faster than spinning disks, which is why moving a database onto SSD can feel like a different machine.\n\n2 ms, a disk seek on a spinning HDD. The mechanical arm has to move. This is why random reads on HDDs are slow and sequential reads are fine.\n\n0.5 ms, a round trip inside the same datacenter. Local network calls are cheap, but not free. A thousand of them in a loop still adds up.\n\n150 ms, a round trip between continents. This is the speed of light over distance, not slow code. No optimisation removes it. You hide it by putting data and servers closer to users, with CDNs and regions.\n\n> **The lesson behind the numbers:** reach for cache before RAM, RAM before disk, and local before remote. Most performance work is just moving data up this ladder.\n\n## In one line\n\n> **Know the orders of magnitude.** They explain why caching and location matter so much.\n\n**Measure one**\n\n```bash\nping -c 4 your-server\n```\n"},{"id":"p137","n":"137","slug":"terraform-vs-ansible","title":"Terraform vs Ansible","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","Ansible","DevOps","IaC","CloudComputing"],"series_name":"IT","design":"compare","script":"","date":"2027-08-12","files":{"devsecops":"notes/p137-terraform-vs-ansible.md"},"md":"# Terraform vs Ansible\n\nTerraform and Ansible are both \"infrastructure as code\", and teams argue about which to use. The honest answer is that they do different jobs and work well together.\n\n## Quick reference\n\n| | **TERRAFORM** | **ANSIBLE** |\n|:--|:--|:--|\n| Main job | Build infrastructure | Configure servers |\n| Model | Declarative, keeps state | Mostly procedural |\n| State file | Yes, tracks reality | No state |\n| Language | HCL | YAML playbooks |\n| Agent | Agentless (APIs) | Agentless (SSH) |\n| Best at | Cloud resources | OS and app setup |\n\n**TERRAFORM** (Provisioning): Creates and tracks infrastructure: servers, networks, databases.\n\n```bash\nterraform apply\n```\n\n**ANSIBLE** (Configuration): Configures what is already there: packages, files, services.\n\n```bash\nansible-playbook site.yml\n```\n\n- Terraform → create the servers and networks.\n- Ansible → set up what runs on them. Often used together.\n\n## Explained\n\nTerraform provisions infrastructure. You describe the servers, networks, load balancers and databases you want, and terraform apply makes the cloud match that description. Its superpower is state: Terraform keeps a file that records what it created, so it knows the difference between what you asked for and what exists, and changes only what is needed. Destroy an environment and recreate it identically with one command.\n\nAnsible configures machines. Once a server exists, an Ansible playbook installs packages, writes config files, starts services and deploys your app. It connects over SSH, needs no agent on the target, and reads as a readable list of steps in YAML.\n\n### The clearest way to see the split\n\nTerraform answers \"what infrastructure should exist?\" It is declarative and tracks state.  \nAnsible answers \"what should be true on this machine?\" It is mostly procedural, running tasks in order, and keeps no state file.\n\nWhere they overlap: Terraform can run some configuration, and Ansible can create cloud resources. But each is clumsy at the other's job. Terraform managing files on a server, or Ansible tracking hundreds of cloud resources without state, both get painful.\n\nThe common pattern: Terraform builds the servers and networks, then hands off to Ansible to configure them. Many teams run exactly this.\n\nBuild with Terraform. Configure with Ansible.\n"},{"id":"p138","n":"138","slug":"the-first-30-minutes-of-an-outage","title":"The first 30 minutes of an outage","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["IncidentResponse","SRE","DevOps","OnCall","ITOperations"],"series_name":"IT","design":"timeline","script":"","date":"2027-08-15","files":{"it":"notes/p138-the-first-30-minutes-of-an-outage.md","devsecops":"notes/p138-the-first-30-minutes-of-an-outage.md"},"md":"# The first 30 minutes of an outage\n\nWhen production breaks, the difference between a 10-minute blip and a 2-hour outage is rarely the fix itself. It is how the first 30 minutes are handled.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **00:00** | Alert fires | Error rate crosses the threshold. The pager goes off. |\n| **00:02** | Acknowledge | One person owns it. Open the incident channel. |\n| **00:05** | Assess | What changed? Last deploy, config, or a traffic spike. |\n| **00:10** | Stop the bleeding | Roll back or scale out before you root-cause. |\n| **00:20** | Confirm recovery | Metrics back to normal. Users are unblocked. |\n| **00:30** | Write it down | Capture the timeline while it is still fresh. |\n\n## Explained\n\n00:00 - Alert fires. The error rate, latency or an uptime check crosses a threshold and the pager goes off. Good alerts are few and meaningful, so this one is trusted.\n\n00:02 - Acknowledge. One person takes ownership and opens the incident channel. Clear ownership stops the two worst failure modes: everyone assuming someone else has it, or five people changing things at once.\n\n00:05 - Assess. The first question is almost always \"what changed?\" The last deploy, a config change, an expired certificate, or a traffic spike. git log, the deploy history and the dashboards answer this fast.\n\n00:10 - Stop the bleeding. This is the key mindset: restore service before you fully understand the cause. Roll back the deploy, scale out, fail over, or disable the broken feature. Users do not care why it broke, they care that it works.\n\n00:20 - Confirm recovery. Watch the metrics return to normal and verify from the outside, as a user would. Do not declare victory from a single green check.\n\n00:30 - Write it down. Capture the timeline and the actions while memory is fresh. This becomes the blameless postmortem that stops it happening again.\n\nThe hardest discipline is resisting the urge to debug deeply while users are down. Stabilise first. Root cause after.\n\nCalm is a technical skill, and it can be practised.\n\n## In one line\n\n> **Calm is a technical skill.** Stabilise first. Understand the root cause after.\n\n**The first move:** acknowledge, then stop the bleeding\n"},{"id":"p139","n":"139","slug":"aws-vs-azure-vs-gcp-core-services","title":"AWS vs Azure vs GCP: core services","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","Azure","GCP","CloudComputing","DevOps"],"series_name":"IT","design":"matrix","script":"","date":"2027-08-17","files":{"devsecops":"notes/p139-aws-vs-azure-vs-gcp-core-services.md"},"md":"# AWS vs Azure vs GCP: core services\n\nThe three big clouds look overwhelming until you notice they offer the same building blocks under different names. Learn the concept once and you can work on any of them.\n\n## Quick reference\n\n| | **AWS** | **Azure** | **GCP** |\n|:--|:--|:--|:--|\n| Virtual machines | EC2 | Virtual Machines | Compute Engine |\n| Object storage | S3 | Blob Storage | Cloud Storage |\n| Managed Kubernetes | EKS | AKS | GKE |\n| Serverless functions | Lambda | Functions | Cloud Functions |\n| Managed SQL | RDS | SQL Database | Cloud SQL |\n| Identity | IAM | Entra ID | Cloud IAM |\n\n**How to pick**\n\n- **Learn one deeply** → the concepts transfer to the others.\n- **AWS** → biggest, most services. Azure  ->  Microsoft shops.\n- **GCP** → strong Kubernetes and data. Names differ, ideas match.\n\n## Explained\n\nVirtual machines: EC2 on AWS, Virtual Machines on Azure, Compute Engine on GCP. A server you rent by the hour. The starting point for lift-and-shift.\n\nObject storage: S3, Blob Storage, Cloud Storage. Cheap, near-unlimited storage for backups, media and static files, accessed over HTTPS. S3 is so common its API is a de facto standard.\n\nManaged Kubernetes: EKS, AKS, GKE. The cloud runs the Kubernetes control plane so you do not have to. GKE is often praised as the most polished, since Google created Kubernetes.\n\nServerless functions: Lambda, Azure Functions, Cloud Functions. Run code on an event without managing servers. Great for glue and automation, and you pay per execution.\n\nManaged SQL: RDS, Azure SQL Database, Cloud SQL. A database where the cloud handles backups, patching and failover.\n\nIdentity: IAM, Entra ID, Cloud IAM. Who can do what. This is the part people underestimate, and the part attackers love when it is too permissive.\n\nThe practical advice: pick one cloud and learn it deeply. The hard concepts, networking, IAM, storage classes and cost, transfer almost directly. After that, a new cloud is mostly learning new names.\n\nOne warning on all three: data egress, moving data out, is where surprise bills come from.\n\n## In one line\n\n> **Different names, same building blocks.** Learn the concept once; the console is just labels.\n\n**Know your bill:** compute + storage + egress\n"},{"id":"p140","n":"140","slug":"9-jq-commands-for-working-with-json","title":"9 jq commands for working with JSON","sections":["it","devsecops"],"topic":"Linux","tags":["jq","JSON","DevOps","CommandLine","API"],"series_name":"IT","design":"terminal","script":"","date":"2027-08-19","files":{"it":"notes/p140-9-jq-commands-for-working-with-json.md","devsecops":"notes/p140-9-jq-commands-for-working-with-json.md"},"md":"# 9 jq commands for working with JSON\n\nAlmost every API and cloud CLI returns JSON, and reading it by eye is painful. jq filters, reshapes and extracts it from the command line. Nine patterns cover most of what you will ever need.\n\n## Quick reference\n\n### Pull out what you need\n\n```bash\n# pretty-print any JSON. The first thing you run\ncurl -s api/users | jq '.'\n# one field. jq '.user.email' goes deeper\njq '.name' user.json\n# the first element of an array\njq '.items[0]' data.json\n```\n\n### Narrow it down\n\n```bash\n# one field from every array element\njq '.items[].sku' order.json\n# keep only objects that match a condition\njq '.[] | select(.active)' users.json\n# count elements\njq '.items | length' order.json\n```\n\n### Change the shape\n\n```bash\n# build a smaller object\njq '.items[] | {sku, qty}'  order.json\n# -r: raw strings, no quotes. Great for scripts\njq -r '.users[].email' u.json\n# map to numbers, then sum them\njq '[.items[].qty] | add' o.json\n```\n\n## Explained\n\n### Read\n\n`curl -s api/users | jq '.'` pretty-prints any JSON with colour and indentation. The first thing to run on an ugly response.  \n`jq '.name' user.json` pulls one field. Go deeper with jq '.user.email', and quote keys with spaces or dashes as jq '.[\"content-type\"]'.  \n`jq '.items[0]' data.json` gets the first element of an array.\n\n### Filter\n\n`jq '.items[].sku' order.json` pulls one field from every element of an array. The [] means \"for each\".  \n`jq '.[] | select(.active)' users.json` keeps only the objects where a condition is true. select is how you filter.  \n`jq '.items | length' order.json` counts the elements. Simple and constantly useful.\n\n### Reshape\n\njq '.items[] | {sku, qty}' `order.json` builds a smaller object with only the fields you care about.  \n`jq -r '.users[].email' u.json` prints raw strings without the surrounding quotes. Essential when feeding the output into a loop or another command.  \njq '[.items[].qty] | add' `order.json` collects all the quantities into an array and sums them. jq does maths too.\n\nThe mental model: jq is a pipeline, like the shell. Each filter transforms the JSON and passes it to the next, separated by |.\n\nJSON becomes readable, and scriptable.\n\n## In one line\n\n> **JSON becomes readable.** One tool turns API noise into the one value you wanted.\n\n**Try it**\n\n```bash\ncurl -s api/health | jq .\n```\n"},{"id":"p141","n":"141","slug":"devops-jargon-decoded","title":"DevOps jargon, decoded","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","CloudComputing","SRE","ITCareer","SoftwareEngineering"],"series_name":"IT","design":"reveal","script":"","date":"2027-08-22","files":{"devsecops":"notes/p141-devops-jargon-decoded.md"},"md":"# DevOps jargon, decoded\n\nDevOps is full of acronyms that get used as if everyone already knows them. Here are six, in plain English, so the next standup makes more sense.\n\n## Quick reference\n\n- **CI** — Continuous Integration: every code push is built and tested automatically, so bugs are caught early.\n- **CD** — Continuous Delivery or Deployment: tested code ships to production with little or no manual work.\n- **IaC** — Infrastructure as Code: your servers and networks are defined in files you version, not clicks in a console.\n- **SLO** — Service Level Objective: the reliability target you aim for, like 99.9% of requests succeed.\n- **Idempotent** — Running it again changes nothing new. Safe to repeat, which is why automation relies on it.\n- **Rollback** — Returning to the last known-good version fast, when a release goes wrong.\n\n## Explained\n\nCI, Continuous Integration. Every time someone pushes code, a pipeline automatically builds the project and runs the tests. The point is to catch problems within minutes of writing them, while the change is small and fresh, instead of during a painful merge weeks later.\n\nCD, Continuous Delivery or Deployment. Once code passes CI, it moves toward production automatically. Continuous Delivery means it is always ready to release with one click. Continuous Deployment goes further and ships it with no human step at all.\n\nIaC, Infrastructure as Code. Instead of clicking through a console to create servers, you describe them in files, like Terraform. Those files are versioned, reviewed and repeatable, so an environment can be rebuilt identically.\n\nSLO, Service Level Objective. The reliability target you commit to, such as 99.9% of requests succeeding. It turns \"the site should be reliable\" into a number you can measure and make decisions against.\n\nIdempotent. A fancy word for a simple idea: running something again makes no new change. Automation depends on this, because a script must be safe to re-run after a failure.\n\nRollback. Returning quickly to the last version that worked when a release breaks. A tested, one-command rollback is what makes frequent deploys safe.\n\nClear words build trust. If a term stays jargon, it stays a black box for the people around you.\n\n## In one line\n\n> **Clear words build trust.** If you cannot explain it simply, it stays a black box.\n\n**Teach one today:** explain CI to someone new\n"},{"id":"p142","n":"142","slug":"the-cloud-native-stack-layer-by-layer","title":"The cloud-native stack, layer by layer","sections":["devsecops"],"topic":"Kubernetes","tags":["CloudNative","Kubernetes","DevOps","Containers","CloudComputing"],"series_name":"IT","design":"stack","script":"","date":"2027-08-24","files":{"devsecops":"notes/p142-the-cloud-native-stack-layer-by-layer.md"},"md":"# The cloud-native stack, layer by layer\n\nPeople say \"cloud-native\" as if it is one thing. It is really a stack of layers, each running on the one below. Knowing them tells you where a problem lives and where to harden.\n\n## Quick reference\n\n*Bottom to top: each layer runs on the one below*\n\n1. **Cloud / hardware** — The datacenter: compute, storage and network you rent. · *IaaS*\n2. **Linux host** — The operating system on each node. Patched and hardened. · `Ubuntu / RHEL`\n3. **Container runtime** — Runs the containers. Images become processes. · *containerd*\n4. **Kubernetes** — Schedules containers across nodes, heals and scales them. · *orchestration*\n5. **Service mesh** — Secures and routes traffic between services. Optional. · *mTLS + routing*\n6. **Your app** — Containers, config and secrets. The part users touch. · *pods + config*\n\n## Explained\n\nCloud or hardware. The foundation: the compute, storage and network you rent from a provider, or run yourself. This is IaaS, and everything above depends on it being available and correctly configured.\n\nLinux host. Every node runs an operating system, almost always Linux. It still needs patching, hardening, time sync and enough disk. People forget this layer exists once Kubernetes is on top, and that is where quiet problems start.\n\nContainer runtime. The piece that actually runs containers, turning an image into an isolated process. These days it is usually containerd. You rarely touch it directly, but when images will not start, this is a layer to check.\n\nKubernetes. The orchestrator. It schedules containers across nodes, restarts them when they crash, scales them with load, and reschedules them when a node dies. This is the layer that makes the system self-healing.\n\nService mesh. Optional, but common at scale. It handles secure, encrypted traffic between services, retries, and fine-grained routing, without each app implementing it. mTLS between services lives here.\n\nYour app. Containers, their config and their secrets. The only layer your users actually experience.\n\nThe key insight: each layer trusts the one beneath it. A compromised node or an unpatched host undermines every clever thing Kubernetes does above it. Security and reliability are only as strong as the lowest layer.\n\n## In one line\n\n> **Each layer trusts the one below.** A weak lower layer undermines everything above it.\n\n**See the bottom layers**\n\n```bash\nkubectl get nodes -o wide\n```\n"},{"id":"p143","n":"143","slug":"fix-kubernetes-imagepullbackoff","title":"Fix: Kubernetes ImagePullBackOff","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","DevOps","Containers","Troubleshooting","CloudNative"],"series_name":"IT","design":"steps","script":"","date":"2027-08-26","files":{"devsecops":"notes/p143-fix-kubernetes-imagepullbackoff.md"},"md":"# Fix: Kubernetes ImagePullBackOff\n\nImagePullBackOff means Kubernetes tried to pull your container image, failed, and is backing off before trying again. The pod status tells you something is wrong; the event tells you what.\n\n## Quick reference\n\n### Diagnose — Why can't it pull the image?\n\n- `kubectl describe pod <name>` — The Events at the bottom give the real reason.\n- **Check the tag** — manifest unknown means the name or tag is wrong.\n- **Check auth** — 401 or 403 means the pull secret is missing or wrong.\n\n### Fix — Match the name and the access\n\n- **Correct image + tag** — Fix a typo or a tag that was never pushed.\n- `kubectl create secret docker-registry regcred ...` — Credentials for a private registry.\n- **imagePullSecrets: in the spec** — Point the pod at that secret, then re-apply.\n\n### Prevent — Stop it recurring\n\n- **Pin real tags** — Avoid latest. Use an immutable tag or a digest.\n- **Verify in CI** — Confirm the image was pushed before you deploy.\n- `imagePullPolicy: IfNotPresent` — Do not re-pull an image the node already has.\n\n## Explained\n\n### Diagnose\n\n`kubectl describe pod <name>` and read the Events at the bottom. This is where the real error lives, and it is almost always one of three things.  \nA wrong name or tag. \"manifest unknown\" or \"not found\" means the image name, tag or registry path is wrong, or that tag was never pushed. Typos and tags that only exist on a developer's laptop are the usual cause.  \nAn access problem. \"401 Unauthorized\" or \"403\" means the registry is private and the cluster has no valid credentials, or the pull secret is wrong.\n\n### Fix\n\nCorrect the image and tag first if it is a typo or a missing push. Compare exactly with what is in the registry.  \nFor a private registry, create the credentials: kubectl create secret docker-registry regcred with the registry, username and token.  \nReference that secret with imagePullSecrets in the pod or service account, then re-apply. Kubernetes retries on its own once the problem is gone.\n\n### Prevent\n\nPin real, immutable tags or digests instead of latest, so a deploy cannot point at a tag that moved or vanished.  \nVerify in CI that the image was actually built and pushed before the deploy step runs.  \nSet `imagePullPolicy: IfNotPresent` for stable tags so nodes do not re-pull an image they already have, which also survives a registry outage.\n\nThe habit: when a pod will not start, describe it before anything else. The status is the symptom; the event is the diagnosis.\n\n## In one line\n\n> **Read the event, not the status.** ImagePullBackOff is the symptom. describe has the cause.\n\n**The first command**\n\n```bash\nkubectl describe pod <name>\n```\n"},{"id":"p144","n":"144","slug":"9-checks-before-a-service-is-production-ready","title":"9 checks before a service is production-ready","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","SRE","CloudNative","Kubernetes","BestPractices"],"series_name":"IT","design":"checklist","script":"","date":"2027-08-29","files":{"devsecops":"notes/p144-9-checks-before-a-service-is-production-ready.md"},"md":"# 9 checks before a service is production-ready\n\n\"It works on my machine\" and \"it is ready for production\" are far apart. Nine checks that close the gap, whether you deploy to Kubernetes, a VM or a platform.\n\n## Quick reference\n\n### It runs well\n\n- **Health endpoint** — liveness and readiness\n- **Config from env** — no secrets in the image\n- **Resource limits set** — CPU and memory requests\n\n### It survives\n\n- **Restarts cleanly** — stateless, or state elsewhere\n- **Graceful shutdown** — finishes in-flight requests\n- **Two or more replicas** — no single point of failure\n\n### You can see it\n\n- **Structured logs** — to stdout, with levels\n- **Metrics exposed** — /metrics or an agent\n- **Alerts wired up** — to a person, not a void\n\n## Explained\n\n### It runs well\n\nHealth endpoint. A liveness check so the platform can restart a hung process, and a readiness check so traffic only arrives once the app can actually serve it.  \nConfig from the environment. Settings and secrets come from environment variables or a secrets manager, never baked into the image. The same image then runs in staging and production unchanged.  \nResource limits set. CPU and memory requests and limits, so the scheduler can place it and one hungry service cannot starve the rest.\n\n### It survives\n\nRestarts cleanly. The app can be killed and restarted at any moment with no data loss. State lives in a database or cache, not on the local disk.  \nGraceful shutdown. On a stop signal it finishes in-flight requests and closes connections, instead of dropping users mid-request.  \nTwo or more replicas. One instance is a single point of failure. Two or more, spread across nodes, survive a crash or a node dying.\n\n### You can see it\n\nStructured logs to stdout, with log levels, so the platform collects them and you can search them.  \nMetrics exposed, through a /metrics endpoint or an agent, so you can see request rate, errors and latency.  \nAlerts wired to a person. An alert that reaches a dashboard nobody watches is not monitoring.\n\nThe fastest honesty test: kill the instance in staging and watch whether it recovers on its own.\n\n## In one line\n\n> **Works on my machine is not ready.** Production-ready means it survives and tells you how it is.\n\n**The quick test:** kill the pod and watch it recover\n"},{"id":"p145","n":"145","slug":"how-a-git-push-becomes-a-running-deploy","title":"How a git push becomes a running deploy","sections":["devsecops"],"topic":"Git & CI/CD","tags":["CICD","DevOps","Automation","GitOps","CloudComputing"],"series_name":"IT","design":"pipeline","script":"","date":"2027-08-31","files":{"devsecops":"notes/p145-how-a-git-push-becomes-a-running-deploy.md"},"md":"# How a git push becomes a running deploy\n\nBetween a developer pushing code and that code serving users, a good pipeline does six things automatically. Each step is a gate, so a broken change stops before it reaches customers.\n\n## Quick reference\n\n1. **git push** — You push a branch and open a pull request. · `git push`\n2. **CI runs** — Build, tests and linting start automatically. · *CI checks*\n3. **Review and merge** — A human approves; the change lands on main. · *merge*\n4. **Build the artifact** — CD builds a container image and tags it. · `docker build`\n5. **Deploy to staging** — Rolled out, smoke tests run automatically. · *staging*\n6. **Promote to production** — Health checks pass, with rollback ready. · *production*\n\n## Explained\n\n1. **git push.** The developer pushes a branch and opens a pull request. This is the trigger for everything that follows, and the last fully manual step.\n\n2. **CI runs.** The moment the PR opens, Continuous Integration kicks off: it builds the project, runs unit and integration tests, and checks formatting and linting. The result is reported back on the pull request as pass or fail. A failed build blocks the merge.\n\n3. **Review and merge.** A teammate reviews the change. Once approved and green, it merges into the main branch. Branch protection rules enforce that nothing reaches main without passing CI and review.\n\n4. **Build the artifact.** Now Continuous Delivery takes over. It builds the deployable artifact, usually a container image, tags it with the commit, and pushes it to a registry. The exact thing that was tested is the thing that ships.\n\n5. **Deploy to staging.** The image rolls out to a staging environment that mirrors production. Smoke tests run automatically to confirm the basics work with real infrastructure.\n\n6. **Promote to production.** With staging green, the image is promoted. A health check watches the rollout, and an automatic rollback stands ready if error rates climb.\n\nThe payoff: a fix goes from a laptop to production in minutes, verified at every step. No manual builds, no \"works on my machine\", no deployment Fridays.\n\n## In one line\n\n> **Code to production, verified at each step.** No manual builds, no deploy Fridays, no guessing.\n\n**Where did it stop?:** check the pipeline, newest run first\n"},{"id":"p146","n":"146","slug":"5-terraform-mistakes-to-avoid","title":"5 Terraform mistakes to avoid","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","BestPractices"],"series_name":"IT","design":"dontdo","script":"","date":"2027-09-02","files":{"devsecops":"notes/p146-5-terraform-mistakes-to-avoid.md"},"md":"# 5 Terraform mistakes to avoid\n\nTerraform is powerful because it tracks the real state of your infrastructure. That same state is where most team pain comes from. Five mistakes, and the safer habit for each.\n\n## Quick reference\n\n**1.** ❌ **`terraform.tfstate` in git** — It holds secrets and causes conflicts between people.  \n✅ **remote state (S3 + lock)** — Shared, encrypted, and locked so two applies cannot clash.\n\n**2.** ❌ **edit resources in the console** — Terraform and reality drift. The next apply fights you.  \n✅ **change it in code, then apply** — The code stays the single source of truth.\n\n**3. Don't**\n\n```bash\nterraform apply (no plan)\n```\n\nYou find out what it changes while it changes it.\n\n**Do instead**\n\nterraform plan, read, then apply\n\nSee every create, change and destroy first.\n\n**4.** ❌ **hard-coded secrets in .tf** — They end up in state and in git history.  \n✅ **variables + a secrets manager** — Inject at apply time; keep secrets out of code.\n\n**5.** ❌ **one giant `main.tf` for everything** — One mistake can destroy prod and dev together.  \n✅ **separate state per environment** — Blast radius stays inside one environment.\n\n## Explained\n\n1. Committing `terraform.tfstate` to git\n   The state file records everything Terraform manages, including secrets in plain text, and two people committing it causes ugly conflicts. Instead: use remote state in S3, Azure Blob or Terraform Cloud, with state locking so two applies cannot run at once.\n\n2. Editing resources by hand in the console\n   Click-changing something Terraform manages creates drift: the code and reality no longer match, and the next apply tries to \"fix\" your manual change or fails. Instead: make every change in code and apply it. If you must touch the console in an emergency, reconcile the code straight after.\n\n3. Running terraform apply without a plan\n   apply will happily destroy and recreate resources. Running it blind means you discover what it does while it is doing it. Instead: `terraform plan -out=tfplan`, read every line, especially anything that says destroy, then apply that exact plan.\n\n4. Hard-coding secrets in .tf files\n   Secrets written into Terraform files end up in the state file and in git history forever. Instead: use variables and pull secrets from a manager like Vault, AWS Secrets Manager or environment variables at apply time.\n\n5. One giant configuration for everything\n   When dev, staging and prod share one state, a single mistake can damage all of them. Instead: separate state per environment, so the blast radius of any change stays contained.\n\nThe thread through all five: state is shared, and plan before apply is not optional.\n\n## In one line\n\n> **State is shared. Treat it with care.** Most Terraform pain is state and plan discipline.\n\n**Always before apply**\n\n```bash\nterraform plan -out=tfplan\n```\n"},{"id":"p147","n":"147","slug":"quiz-3-container-questions","title":"Quiz: 3 container questions","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","CloudNative","ITCareer"],"series_name":"IT","design":"quiz","script":"","date":"2027-09-05","files":{"devsecops":"notes/p147-quiz-3-container-questions.md"},"md":"# Quiz: 3 container questions\n\nThree container questions that sort people who run docker run from people who understand what is happening. Try each first.\n\n## Quick reference\n\n**Q1. Where should a database's data live in Docker?**\n\n- a) Inside the container\n- b) In a named volume\n- c) In the image\n\n> **Answer:** b) In a named volume. Containers are disposable. A named volume keeps the data when the container is removed or replaced.\n\n**Q2. What is the difference between an image and a container?**\n\n- a) Nothing, they are the same\n- b) An image is the template; a container is a running instance\n- c) An image runs, a container stores\n\n> **Answer:** b) An image is the template; a container is a running instance. An image is a read-only template. A container is a running instance of it, with a thin writable layer on top.\n\n**Q3. Why avoid the latest tag in production?**\n\n- a) It is slower\n- b) It is not a fixed version; it can change under you\n- c) It costs more\n\n> **Answer:** b) It is not a fixed version; it can change under you. latest just means the newest at pull time. A rebuild can bring a different version and break things silently.\n\n## Explained\n\n1. Where should a database's data live in Docker?\n\n> **Answer:** in a named volume. Containers are designed to be disposable. Anything written inside a container's own filesystem disappears when that container is removed or replaced, which happens on every update. A named volume lives outside the container's lifecycle, so the data survives. The same applies to uploads and anything else you cannot afford to lose.\n\n2. What is the difference between an image and a container?\n\n> **Answer:** an image is the read-only template; a container is a running instance of it. One image can start many containers, each with its own thin writable layer on top. This is why changes inside a running container do not change the image, and why those changes vanish when the container is gone. docker images lists templates; docker ps lists instances.\n\n3. **Why avoid the latest tag in production?**\n\n> **Answer:** because latest is not a version. It simply points to whatever was most recent when you pulled. A rebuild next month can pull a newer, different version and break your app with no code change and no obvious cause. Pin a specific tag, or better, an image digest, so the same deploy is always the same bytes.\n\nThe thread: containers are cattle, not pets. Treat them as disposable, keep state outside them, and pin what you run.\n\n## In one line\n\n> **Containers are simple, until they are not.** The basics prevent most production surprises.\n\n**See image vs container**\n\n```bash\ndocker images; docker ps -a\n```\n"},{"id":"p148","n":"148","slug":"anatomy-of-a-url","title":"Anatomy of a URL","sections":["it"],"topic":"Web servers, TLS & HTTP","tags":["WebDevelopment","Networking","API","DevOps","ITSupport"],"series_name":"IT","design":"anatomy","script":"","date":"2027-09-07","files":{"it":"notes/p148-anatomy-of-a-url.md"},"md":"# Anatomy of a URL\n\nYou type and read URLs all day. Taken apart, a URL is a precise instruction, and each part tells a different system what to do.\n\n## Quick reference\n\n```text\nhttps://api.example.com:8443/v2/orders?status=open#details\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `https://` | Scheme | The protocol. https means the connection is encrypted. |\n| `api.example.com` | Host | The server's name, resolved to an IP by DNS. |\n| `:8443` | Port | Which service on the host. 443 for https is implied. |\n| `/v2/orders` | Path | Which resource on the server you are asking for. |\n| `?status=open` | Query | Parameters: filters, pages, search terms. |\n| `#details` | Fragment | A spot on the page. The browser keeps it; the server never sees it. |\n\n## Explained\n\nhttps:// is the scheme, the protocol. https means the connection is encrypted with TLS; http is plain text. Other schemes exist too, like ssh:// or postgres:// in connection strings. This part decides how the client talks to the server.\n\n`api.example.com` is the host. DNS resolves this name to an IP address so the request can be routed. This is the part that moves when you migrate servers, and the part a wrong DNS record breaks.\n\n:8443 is the port, which service on that host to reach. It is usually hidden because 443 is implied for https and 80 for http. You only see it when a service runs somewhere non-standard.\n\n`/v2/orders` is the path, which resource you are asking for. On an API this maps to an endpoint; on a site it maps to a page or file.\n\n?status=open is the query string, extra parameters after the question mark. Filters, pagination, search terms and tracking tags all live here, joined by &.\n\n\\#details is the fragment. It points at a spot within the page, and crucially, the browser keeps it to itself. The server never receives the fragment, which surprises people debugging why a value never arrives.\n\nWhy it matters: when a request misbehaves, knowing which part does what tells you where to look, from DNS for the host to the query string for a missing parameter.\n\n## In one line\n\n> **Every part tells the server something.** Read a URL and you know exactly what is being asked.\n\n**Inspect one**\n\n```bash\ncurl -v 'https://api.example.com/v2/orders?status=open'\n```\n"},{"id":"p149","n":"149","slug":"9-terraform-commands-every-engineer-should-know","title":"9 terraform commands every engineer should know","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","Automation"],"series_name":"IT","design":"steps","script":"","date":"2027-09-09","files":{"devsecops":"notes/p149-9-terraform-commands-every-engineer-should-know.md"},"md":"# 9 terraform commands every engineer should know\n\nTerraform has many commands, but nine cover almost all real work, from setting up a project to getting out of trouble safely.\n\n## Quick reference\n\n### The core loop — Init, plan, apply\n\n```bash\n# Download providers and set up the backend. Run it first.\nterraform init\n# Show every change and save it. Read before applying.\nterraform plan -out=tfplan\n# Apply exactly what you reviewed. No surprises.\nterraform apply tfplan\n```\n\n### Stay safe — See and check before you change\n\n```bash\n# Format every file to the standard style.\nterraform fmt -recursive\n# Catch syntax and type errors without touching the cloud.\nterraform validate\n# Everything Terraform currently manages.\nterraform state list\n```\n\n### Out of trouble — Targeted and careful\n\n```bash\n# Plan one resource when debugging.\nterraform plan -target=aws_instance.web\n# Bring an existing resource under management.\nterraform import aws_s3_bucket.b my-bucket\n# Tear it all down. Read the plan twice.\nterraform destroy\n```\n\n## Explained\n\n### The core loop\n\n`terraform init` downloads the providers and sets up the backend where state is stored. Run it first in any new project, and again after adding a provider or module.  \n`terraform plan -out=tfplan` shows exactly what will be created, changed or destroyed, and saves that plan to a file. Reading it is the single most important habit in Terraform.  \n`terraform apply tfplan` applies the saved plan, so what runs is exactly what you reviewed. Running apply with no saved plan re-plans and asks for confirmation, which is fine, but applying a saved plan removes any gap between review and action.\n\n### Stay safe\n\n`terraform fmt -recursive` formats every file to the canonical style, so diffs stay clean and reviews focus on real changes.  \n`terraform validate` checks syntax and types without contacting the cloud. Fast feedback, ideal in CI.  \n`terraform state list` shows everything Terraform currently manages. Your map of reality.\n\n### Out of trouble\n\n`terraform plan -target=aws_instance.web` limits the plan to one resource, useful when debugging a specific change. Use it sparingly, not as a habit.  \nterraform import brings an existing, manually created resource under Terraform's management without recreating it.  \n`terraform destroy` tears everything down. Powerful and permanent, so read that plan twice, especially the count.\n\nThe thread through all of it: plan is the command that matters. apply should never surprise you.\n\n## In one line\n\n> **Plan is the most important command.** Read it every time. apply should never surprise you.\n\n**The safe loop**\n\n```bash\nterraform plan -out=tfplan && terraform apply tfplan\n```\n"},{"id":"p150","n":"150","slug":"the-four-golden-signals-of-monitoring","title":"The four golden signals of monitoring","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","Monitoring","Observability","DevOps","CloudComputing"],"series_name":"IT","design":"dashboard","script":"","date":"2027-09-12","files":{"devsecops":"notes/p150-the-four-golden-signals-of-monitoring.md"},"md":"# The four golden signals of monitoring\n\nGoogle's SRE book boils monitoring down to four golden signals. If you can only watch a few things on a service, watch these, because together they tell you whether users are having a good time.\n\n## Quick reference\n\n**Status:** What good monitoring measures\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| LATENCY | **182 ms** | p95 response time. How slow? |\n| TRAFFIC | **1.2k/s** | requests per second. How much? |\n| ERRORS | **0.4%%** | share of failed requests |\n| SATURATION | **68%** | how full the system is |\n| Watch together | **4** | one hides, four reveal |\n| Alert on | **SLO** | symptoms users feel |\n\n## Explained\n\nLatency: how long requests take. Measure it at a percentile, like p95 or p99, not an average, because averages hide the slow tail that real users feel. And separate successful from failed requests, since a fast error is still an error.\n\nTraffic: how much demand the system is under, such as requests per second. On its own it is just a number, but next to the others it gives meaning. A spike in latency during a traffic spike is capacity; a spike in latency with flat traffic is a problem.\n\nErrors: the rate of failed requests, as a share of the total. This is the most direct signal that something is broken. Watch both obvious failures, like HTTP 500s, and silent ones, like wrong responses with a 200 status.\n\nSaturation: how full the system is, the resource closest to its limit, whether that is CPU, memory, disk or a connection pool. Saturation is a leading indicator: it climbs before latency and errors do, giving you warning.\n\nWhy four and not forty: a wall of CPU and memory graphs tells you how the machine feels. These four tell you how the user feels, and that is what an alert should be about.\n\nThe practical rule: alert on symptoms users notice, like latency and errors against your SLO, and use saturation to see trouble coming.\n\n## In one line\n\n> **Measure what users feel.** CPU graphs are nice. These four tell you if users are happy.\n\n**The four:** latency, traffic, errors, saturation\n"},{"id":"p151","n":"151","slug":"data-size-and-speed-units-decoded","title":"Data size and speed units, decoded","sections":["it"],"topic":"Networking & DNS","tags":["Networking","Storage","ITSupport","SysAdmin","DevOps"],"series_name":"IT","design":"numbers","script":"","date":"2027-09-14","files":{"it":"notes/p151-data-size-and-speed-units-decoded.md"},"md":"# Data size and speed units, decoded\n\nHalf of all arguments about \"why is it so slow\" come down to mixing up units. Six that trip people up constantly.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **1 KB** | vs 1 KiB | KB is 1000 bytes. KiB is 1024. Disks use KB, RAM uses KiB. | kilo |\n| **8 bits** | = 1 byte | Networks count bits, storage counts bytes. Divide by 8. | b vs B |\n| **100 Mbps** | = 12.5 MB/s | A 100 megabit link moves 12.5 megabytes a second. | link |\n| **1 TB** | = 1000 GB | A 1 TB drive shows ~931 GiB in the OS. Same disk, two units. | disk |\n| **1 Gbps** | = 125 MB/s | Gigabit ethernet, at best. Real throughput is lower. | LAN |\n| **ms vs us** | 1000x apart | A millisecond is a thousand microseconds. Know which one. | time |\n\n## Explained\n\n1 KB vs 1 KiB. A kilobyte (KB) is 1000 bytes. A kibibyte (KiB) is 1024. Storage vendors use the decimal KB, MB, TB; operating systems and RAM use the binary KiB, MiB, GiB. This is why a \"1 TB\" drive shows up as about 931 GiB. Nothing is missing; they are two different units.\n\n8 bits = 1 byte. The single most useful fact here. Networks are measured in bits per second (lowercase b), storage and files in bytes (uppercase B). To compare them, divide the network number by 8.\n\n100 Mbps = 12.5 MB/s. A 100 megabit internet connection moves at most 12.5 megabytes per second. So a 1 GB download takes around 80 seconds at full speed, not 10. The \"slow\" download is often just bits-versus-bytes.\n\n1 TB = 1000 GB in marketing, about 931 GiB in the OS. Same disk, two unit systems.\n\n1 Gbps = 125 MB/s. Gigabit ethernet tops out around there, and real-world throughput is lower due to overhead.\n\nms vs us. A millisecond is a thousand microseconds. An SSD at 100 us and a disk at 10 ms are a hundred times apart. Read the unit before you compare the number.\n\nThe one rule to remember: check whether it is bits or bytes. Lowercase b, uppercase B. That alone clears up most confusion.\n\n## In one line\n\n> **Bits or bytes? Always check.** Half of all speed confusion is b versus B.\n\n**The rule:** divide Mbps by 8 for MB/s\n"},{"id":"p152","n":"152","slug":"docker-compose-vs-kubernetes","title":"Docker Compose vs Kubernetes","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Kubernetes","DevOps","Containers","CloudNative"],"series_name":"IT","design":"compare","script":"","date":"2027-09-16","files":{"devsecops":"notes/p152-docker-compose-vs-kubernetes.md"},"md":"# Docker Compose vs Kubernetes\n\nDocker Compose and Kubernetes both run multi-container apps from a file, so teams ask which to use. They are built for very different scales, and starting with the wrong one wastes a lot of time.\n\n## Quick reference\n\n| | **COMPOSE** | **KUBERNETES** |\n|:--|:--|:--|\n| Scope | One machine | A cluster of nodes |\n| Self-healing | Restart only | Reschedules, scales |\n| Scaling | Manual | Automatic (HPA) |\n| Load balancing | Basic | Built in |\n| Learning curve | An afternoon | Weeks |\n| Best for | Dev, small apps | Production at scale |\n\n**COMPOSE** (One host): Runs a set of containers on a single machine from one file.\n\n```bash\ndocker compose up\n```\n\n**KUBERNETES** (A cluster): Runs containers across many machines, and keeps them running.\n\n```bash\nkubectl apply -f .\n```\n\n- Compose → dev and single-host apps. Start here.\n- Kubernetes → many machines, scale and self-healing.\n\n## Explained\n\nDocker Compose describes a set of containers in one YAML file and runs them on a single machine. docker compose up starts the whole stack: app, database, cache, all wired together on one host. It is simple enough to learn in an afternoon, and perfect for local development and small single-server apps.\n\nKubernetes runs containers across a cluster of machines and, crucially, keeps them running. If a container crashes it restarts it; if a node dies it reschedules the work elsewhere; under load it can add more copies automatically. That power comes with real complexity: pods, services, deployments, ingress and more.\n\n### The honest comparison\n\nCompose restarts a failed container. Kubernetes reschedules it onto a healthy node and can scale it with demand.  \nCompose load-balances crudely. Kubernetes has services and ingress built for it.  \nCompose takes an afternoon to learn. Kubernetes takes weeks, plus ongoing operations.\n\nThe trap to avoid: reaching for Kubernetes because it is what big companies use. For a single app on one or two servers, Compose or a managed platform is faster to run and far cheaper to operate. Kubernetes earns its complexity when you genuinely need many services, many machines, automatic scaling and self-healing.\n\nA common path is Compose in development and a managed Kubernetes service in production, sharing similar container definitions.\n\nCompose to start. Kubernetes when you outgrow one machine.\n"},{"id":"p153","n":"153","slug":"a-zero-downtime-deployment-minute-by-minute","title":"A zero-downtime deployment, minute by minute","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","Kubernetes","CICD","SRE","CloudNative"],"series_name":"IT","design":"timeline","script":"","date":"2027-09-19","files":{"devsecops":"notes/p153-a-zero-downtime-deployment-minute-by-minute.md"},"md":"# A zero-downtime deployment, minute by minute\n\nShipping a new version without any downtime is not luck. It is a sequence of steps, each protecting users from the change happening underneath them. Here is how a rolling deployment works, minute by minute.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T-0** | New version built | CI produces an image, tagged with the commit. |\n| **T+1** | Rolling update starts | New pods start alongside the old ones. |\n| **T+2** | Readiness checks | Traffic only reaches a new pod once it is ready. |\n| **T+4** | Old pods drain | They finish in-flight requests, then stop. |\n| **T+6** | All traffic on new | The rollout completes. Users noticed nothing. |\n| **T+7** | Rollback stays ready | One command returns to the old version if needed. |\n\n## Explained\n\nT-0, new version built. CI finishes and produces a container image tagged with the exact commit. The thing that was tested is the thing that will ship.\n\nT+1, rolling update starts. Instead of stopping the old version and starting the new one, the platform starts new pods alongside the old ones. For a moment both versions run together, which is why your app must tolerate that.\n\nT+2, readiness checks. Each new pod must pass its readiness probe before it receives any traffic. A pod that is still starting up, connecting to the database or warming a cache gets no users until it says it is ready. This single mechanism prevents most \"deploy broke the site\" moments.\n\nT+4, old pods drain. As new pods become ready, old ones are told to stop. A graceful shutdown lets them finish the requests already in flight and close connections cleanly, rather than dropping users mid-request.\n\nT+6, all traffic on new. The last old pod is gone, every request now hits the new version, and users noticed nothing. No maintenance page, no dropped connections.\n\nT+7, rollback stays ready. The previous version is one command away. If error rates climb, kubectl rollout undo returns to it in seconds.\n\nThe two things that make it work: readiness checks so traffic waits for healthy pods, and graceful shutdown so leaving pods finish their work.\n\n## In one line\n\n> **Deploys should be boring.** Health checks and rolling updates make downtime optional.\n\n**Watch it roll**\n\n```bash\nkubectl rollout status deploy/api\n```\n"},{"id":"p154","n":"154","slug":"jenkins-vs-github-actions-vs-gitlab-ci","title":"Jenkins vs GitHub Actions vs GitLab CI","sections":["devsecops"],"topic":"Git & CI/CD","tags":["CICD","DevOps","Jenkins","GitHubActions","GitLab"],"series_name":"IT","design":"matrix","script":"","date":"2027-09-21","files":{"devsecops":"notes/p154-jenkins-vs-github-actions-vs-gitlab-ci.md"},"md":"# Jenkins vs GitHub Actions vs GitLab CI\n\nThree of the most common CI/CD tools, compared honestly. The right choice usually follows where your code already lives.\n\n## Quick reference\n\n| | **Jenkins** | **GH Actions** | **GitLab CI** |\n|:--|:--|:--|:--|\n| Hosting | You run it | ✅ Cloud or self | ✅ Cloud or self |\n| Config | UI + Groovy | ✅ YAML in repo | ✅ YAML in repo |\n| Setup effort | ❌ High | ✅ Low | 🟡 Medium |\n| Ecosystem | ✅ Huge plugins | ✅ Marketplace | 🟡 Built in |\n| Best fit | Custom, on-prem | GitHub repos | GitLab repos |\n| Maintenance | ❌ You own it | ✅ Managed | 🟡 Depends |\n\n**How to pick**\n\n- **Jenkins** → full control, on-prem, complex custom pipelines.\n- **GitHub Actions** → already on GitHub, want it simple.\n- **GitLab CI** → all-in-one with your GitLab repos.\n\n## Explained\n\nJenkins is the veteran. You run it yourself, which means full control and a massive plugin ecosystem that can automate almost anything. The cost is ownership: you patch it, secure it, scale the agents and maintain the pipelines, often written in Groovy. It shines for complex, on-premises or highly custom pipelines where control matters more than convenience.\n\nGitHub Actions is built into GitHub. Pipelines are YAML files in your repository, triggered by pushes and pull requests, and the Marketplace has a ready-made action for almost everything. There is nothing to host, and the free tier is generous. If your code is on GitHub, this is the path of least resistance.\n\nGitLab CI is built into GitLab the same way: YAML in the repo, runners you can host or use GitLab's, and the CD side integrated tightly with the rest of GitLab. If you use GitLab for repos and issues, the CI is right there.\n\n### How to choose\n\nAlready on GitHub and want simple: GitHub Actions.  \nUsing GitLab for everything: GitLab CI.  \nNeed full control, on-prem, or deeply custom pipelines, and have people to run it: Jenkins.\n\nThe honest trade-off is managed convenience versus control. Managed tools remove maintenance but tie you to a platform. Jenkins gives total freedom and total responsibility.\n\nThe best CI is the one that lives next to your code.\n\n## In one line\n\n> **The best CI is the one next to your code.** Pick where your repos already live, then grow.\n\n**The rule of thumb:** CI should live where your code lives\n"},{"id":"p155","n":"155","slug":"9-git-commands-for-reading-history","title":"9 git commands for reading history","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","DevOps","VersionControl","Debugging","SoftwareDevelopment"],"series_name":"IT","design":"terminal","script":"","date":"2027-09-23","files":{"devsecops":"notes/p155-9-git-commands-for-reading-history.md"},"md":"# 9 git commands for reading history\n\nGit history is not just a record, it is a debugging tool. When something breaks, the answer to \"when did this change and why\" is a few commands away.\n\n## Quick reference\n\n### See what happened\n\n```bash\n# a compact, readable history\ngit log --oneline -10\n# branches and merges, drawn as a tree\ngit log --oneline --graph --all\n# filter by time and author\ngit log --since='1 week ago' --author=sara\n```\n\n### Track down a line\n\n```bash\n# who last changed each line, and when\ngit blame app.py\n# the full diff of one commit\ngit show a1b2c3d\n# the history of one file, with diffs\ngit log -p -- app.py\n```\n\n### Search the past\n\n```bash\n# find the commit that added or removed a string\ngit log -S 'apply_coupon'\n# commits on feature that are not on main\ngit log --oneline main..feature\n# binary-search history to find a bug\ngit bisect start\n```\n\n## Explained\n\n### See what happened\n\n`git log --oneline -10` gives a compact, readable list of recent commits. Far easier to scan than the full log.  \n`git log --oneline --graph --all` draws branches and merges as a tree, so you can see how work came together. The clearest picture of a messy history.  \n`git log --since='1 week ago' --author=sara` filters by time and person. Useful for \"what changed before this started failing\".\n\n### Who and why\n\n`git blame app.py` shows who last changed each line and in which commit. Not for blame in the human sense, but for context: find the commit, then read why it was made.  \ngit show \\<hash> displays one commit in full, message and diff. The natural next step after blame hands you a hash.  \n`git log -p -- app.py` shows the history of a single file, each change with its diff. The story of how one file became what it is.\n\n### Find it\n\n`git log -S 'apply_coupon'` finds the commit that added or removed that string. Perfect for \"where did this function come from, or go?\"  \ngit log main..feature lists commits on one branch that are not on another. Exactly what a pull request contains.  \n`git bisect start` begins a binary search through history. You mark a known-good and known-bad commit, and Git checks out the midpoint repeatedly until it finds the commit that introduced a bug. Powerful on long histories.\n\n## In one line\n\n> **History is a debugging tool.** The commit that introduced a bug is a few commands away.\n\n**Try it**\n\n```bash\ngit log --oneline --graph --all\n```\n"},{"id":"p156","n":"156","slug":"kubernetes-terms-decoded","title":"Kubernetes terms, decoded","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","DevOps","Containers","ITCareer"],"series_name":"IT","design":"reveal","script":"","date":"2027-09-26","files":{"devsecops":"notes/p156-kubernetes-terms-decoded.md"},"md":"# Kubernetes terms, decoded\n\nKubernetes is not conceptually hard. The vocabulary is what makes it feel impossible at first. Six terms you will meet on day one, in plain English.\n\n## Quick reference\n\n- **Pod** — The smallest unit you run: one or more containers that share a network and live and die together.\n- **Deployment** — Describes the desired state: which image and how many replicas. It keeps reality matching that.\n- **Service** — A stable address and load balancer for a set of pods, even as pods come and go.\n- **Ingress** — Routes outside HTTP traffic to services by host and path. The front door of the cluster.\n- **ConfigMap** — Configuration kept outside the image, injected into pods as files or variables.\n- **Namespace** — A way to divide one cluster into separate areas for teams or environments.\n\n## Explained\n\nPod. The smallest thing Kubernetes runs. Usually one container, sometimes a few that belong together and share a network address and storage. Pods are disposable: they are created and destroyed constantly, and you rarely manage them directly.\n\nDeployment. A description of what you want: this image, this many replicas. Kubernetes then works to make reality match, creating pods, replacing crashed ones, and rolling out new versions. You change the Deployment; it handles the pods.\n\nService. Pods come and go and get new IP addresses, so you cannot rely on a pod's address. A Service gives a stable name and address, and load-balances across whichever pods currently match it. This is how parts of your app find each other.\n\nIngress. Routes outside HTTP and HTTPS traffic into the cluster, choosing a Service based on the host name and URL path. It is the front door, and where TLS certificates usually live.\n\nConfigMap. Configuration kept outside the container image and injected into pods as environment variables or files. Its sibling, a Secret, does the same for sensitive values. This keeps one image usable across environments.\n\nNamespace. Divides a single cluster into separate areas, for teams or for dev, staging and prod, each with its own names and limits.\n\nOnce these six click, most Kubernetes documentation suddenly reads easily.\n\n## In one line\n\n> **The words are the hard part.** The concepts are simple once the names click.\n\n**See them all**\n\n```bash\nkubectl get pods,svc,ingress -A\n```\n"},{"id":"p157","n":"157","slug":"what-runs-inside-a-kubernetes-node","title":"What runs inside a Kubernetes node","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","Linux","DevOps","Containers"],"series_name":"IT","design":"stack","script":"","date":"2027-09-28","files":{"devsecops":"notes/p157-what-runs-inside-a-kubernetes-node.md"},"md":"# What runs inside a Kubernetes node\n\nKubernetes can feel like magic until you look inside a single worker node. It is just a Linux machine running a few agents. Here is what sits where.\n\n## Quick reference\n\n*Bottom to top: each layer runs the one above*\n\n1. **The node** — A Linux machine, VM or bare metal, with CPU and RAM. · *Linux host*\n2. **kubelet** — The agent that talks to the control plane and runs pods. · `kubelet`\n3. **Container runtime** — Pulls images and runs containers for the kubelet. · *containerd*\n4. **Pods** — Groups of containers the kubelet keeps alive. · *pods*\n5. **Containers** — Your actual processes, isolated from each other. · *your app*\n6. **kube-proxy** — Wires up Service networking so pods are reachable. · *networking*\n\n## Explained\n\nThe node. At the bottom is an ordinary Linux machine, virtual or physical, with CPU, memory and disk. Everything above is software running on it, which is why node-level basics like patching, disk space and time sync still matter.\n\nkubelet. The agent that makes the machine part of the cluster. It talks to the control plane, receives the list of pods it should run, starts and stops them, and reports their health back. If the kubelet is down, the node goes NotReady.\n\nContainer runtime. The kubelet does not run containers itself. It asks a runtime, usually containerd, to pull images and start containers. This is the layer that turns an image into a running process.\n\nPods. The runtime's containers are grouped into pods. The kubelet keeps each pod matching what the control plane asked for, restarting containers that crash.\n\nContainers. Inside the pods are your actual processes, isolated from each other by Linux namespaces and cgroups, the same kernel features that power Docker.\n\nkube-proxy. Finally, kube-proxy programs the node's networking rules so that Service addresses route to the right pods, wherever they run in the cluster.\n\nOnce you see that a node is Linux plus kubelet plus a runtime plus kube-proxy, kubectl describe node and node problems stop being mysterious.\n\n## In one line\n\n> **A node is just Linux with agents.** Demystify the node and Kubernetes stops being magic.\n\n**Look at a node**\n\n```bash\nkubectl describe node <name>\n```\n"},{"id":"p158","n":"158","slug":"fix-terraform-state-lock","title":"Fix: Terraform state lock","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","Troubleshooting"],"series_name":"IT","design":"steps","script":"","date":"2027-09-30","files":{"devsecops":"notes/p158-fix-terraform-state-lock.md"},"md":"# Fix: Terraform state lock\n\nError acquiring the state lock. Terraform is refusing to run because the state is locked, and that lock is doing its job: it stops two applies from running at once and corrupting your infrastructure's state. The fix is to understand the lock, not to smash it.\n\n## Quick reference\n\n### Diagnose — Who holds the lock, and why?\n\n- **Read the Lock Info** — Who, when, and the operation. The error prints it.\n- **Ask that person** — Is an apply actually running right now?\n- **Check CI** — A pipeline mid-apply holds the lock too.\n\n### Fix — Release it the right way\n\n- **Wait if it is live** — A real apply will release the lock when it finishes.\n- `terraform force-unlock <ID>` — Only when you are sure nothing is running.\n- **Re-run plan** — Confirm the state is sane before applying again.\n\n### Prevent — Fewer locks, fewer clashes\n\n- **One apply at a time** — Serialise applies in CI, not on laptops.\n- **Remote state + DynamoDB lock** — A proper backend manages locks for you.\n- **Small, separate states** — Less chance two people touch the same one.\n\n## Explained\n\n### Diagnose\n\nRead the Lock Info. The error prints who holds the lock, when it was created, and which operation. This tells you most of what you need.  \nAsk that person. If sara's laptop holds a lock from three minutes ago, an apply may genuinely be running. Message her before doing anything.  \nCheck CI. A pipeline mid-apply holds the lock too. A lock with no obvious owner is often a CI job, or a previous one that crashed.\n\n### Fix\n\nWait if it is live. A real, running apply releases the lock automatically when it finishes. Patience is often the whole fix.  \n`terraform force-unlock <ID>`, but only when you are certain nothing is running. This happens when an apply crashed or lost network before it could release the lock. Forcing it while another apply is live can corrupt state badly.  \nRe-run terraform plan afterwards to confirm the state matches reality before you apply again.\n\n### Prevent\n\nOne apply at a time. Run applies through CI, serialised, not from several laptops.  \nUse a backend with locking. Remote state in S3 with a DynamoDB lock table, or Terraform Cloud, manages locks properly.  \nKeep states small and separate, so two people rarely need the same one.\n\nThe lock is protecting you. force-unlock is a last resort, used knowingly.\n\n## In one line\n\n> **The lock is protecting you.** Never force-unlock blindly. Two applies at once corrupt state.\n\n**Only when sure**\n\n```bash\nterraform force-unlock <LOCK_ID>\n```\n"},{"id":"p159","n":"159","slug":"9-checks-for-a-new-cloud-account","title":"9 checks for a new cloud account","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["CloudComputing","CyberSecurity","AWS","DevOps","ITSecurity"],"series_name":"IT","design":"checklist","script":"","date":"2027-10-03","files":{"devsecops":"notes/p159-9-checks-for-a-new-cloud-account.md"},"md":"# 9 checks for a new cloud account\n\nA brand-new cloud account is at its most vulnerable on day one, before anyone has thought about security. Nine checks to do before you build anything real.\n\n## Quick reference\n\n### Lock the root\n\n- **MFA on the root user** — hardware or app, not SMS\n- **Stop using root** — create an admin user instead\n- **Rotate root keys away** — delete root access keys\n\n### See everything\n\n- **Audit logging on** — CloudTrail / activity log\n- **Billing alerts set** — a budget with email alerts\n- **Default region chosen** — and unused regions restricted\n\n### Least privilege\n\n- **No long-lived keys** — use roles and short tokens\n- **Groups, not per-user grants** — assign policies to groups\n- **Encrypt by default** — storage and volumes\n\n## Explained\n\n### Lock the root\n\nMFA on the root user. The root or global-admin account can do anything, including closing the account. Protect it with a hardware key or authenticator app, never SMS.  \nStop using root. Create a separate admin user or role for daily work, and lock root away for the rare tasks that truly need it.  \nRotate root access keys away. Root should have no long-lived access keys at all. Delete them.\n\n### See everything\n\nAudit logging on. Turn on CloudTrail, Azure activity logs or the equivalent from the start, stored where it cannot be quietly deleted. You cannot investigate what you never recorded.  \nBilling alerts set. A budget with email alerts is your early warning for both runaway resources and a compromised account mining crypto on your bill.  \nDefault region chosen, and regions you do not use restricted, so resources cannot be spun up unnoticed on the other side of the world.\n\n### Least privilege\n\nNo long-lived keys. Prefer roles and short-lived tokens over access keys that live in files forever.  \nGroups, not per-user grants. Attach policies to groups and put users in them, so access stays reviewable.  \nEncrypt by default. Turn on default encryption for object storage and disks.\n\nThe economics are simple: an hour securing a fresh account is far cheaper than responding to a breached one.\n\n## In one line\n\n> **Day one is the cheapest day.** Securing a fresh account beats cleaning up a breached one.\n\n**First two moves:** MFA on root, then a budget alert\n"},{"id":"p160","n":"160","slug":"how-a-request-reaches-a-pod-in-kubernetes","title":"How a request reaches a pod in Kubernetes","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CloudNative","DevOps","SRE"],"series_name":"IT","design":"pipeline","script":"","date":"2027-10-05","files":{"devsecops":"notes/p160-how-a-request-reaches-a-pod-in-kubernetes.md"},"md":"# How a request reaches a pod in Kubernetes\n\nA user opens your app running in Kubernetes. Between the browser and the container, the request passes through several layers. When something is \"not reachable\", knowing them turns a vague problem into a specific one.\n\n## Quick reference\n\n1. **DNS and the load balancer** — The name resolves to the cluster's entry point. · *LB*\n2. **Ingress controller** — Matches host and path, terminates TLS. · *ingress*\n3. **Service** — A stable address for a changing set of pods. · *service*\n4. **kube-proxy rules** — Picks one healthy pod and routes to it. · *endpoints*\n5. **The pod** — The request arrives at a container. · *pod*\n6. **Readiness decides** — Only ready pods are ever chosen. · *readiness*\n\n## Explained\n\n1. **DNS and the load balancer.** The domain resolves to the cluster's entry point, usually a cloud load balancer in front of the cluster. If DNS is wrong or the load balancer has no healthy targets, nothing after this matters.\n\n2. **Ingress controller.** The load balancer forwards to the ingress controller, which reads the host name and URL path and decides which Service should handle the request. This is also where TLS is usually terminated. A missing or wrong ingress rule sends traffic nowhere, or to the wrong app.\n\n3. **Service.** The ingress targets a Service, which is a stable address for a set of pods that are constantly being created and destroyed. The Service does not care which pods exist right now; it tracks them by label.\n\n4. **kube-proxy rules.** On each node, kube-proxy has programmed networking rules that pick one healthy pod behind the Service and route the request to it, load-balancing across them.\n\n5. **The pod.** The request finally reaches a container inside a pod, where your code runs and responds.\n\n6. **Readiness decides.** Throughout, only pods that pass their readiness probe are in the Service's list of endpoints. This is why a pod can be Running but get no traffic: it is not ready.\n\nDebugging tip: walk the chain with kubectl get ingress, svc and endpoints. An empty endpoints list means no ready pods, and that points straight at the problem.\n\n## In one line\n\n> **Many hops, one request.** Know the hops, and 'it is not reachable' gets specific.\n\n**Trace the chain**\n\n```bash\nkubectl get ingress,svc,endpoints\n```\n"},{"id":"p161","n":"161","slug":"5-kubernetes-mistakes-in-production","title":"5 Kubernetes mistakes in production","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","DevOps","SRE","BestPractices"],"series_name":"IT","design":"dontdo","script":"","date":"2027-10-07","files":{"devsecops":"notes/p161-5-kubernetes-mistakes-in-production.md"},"md":"# 5 Kubernetes mistakes in production\n\nKubernetes is forgiving in a demo and unforgiving at scale. Five mistakes that run fine until real traffic, a node failure or a rollback exposes them.\n\n## Quick reference\n\n**1.** ❌ **no resource requests or limits** — One pod can starve a whole node of CPU or memory.  \n✅ **set requests and limits** — The scheduler places pods well; noisy neighbours are capped.\n\n**2.** ❌ **no readiness probe** — Traffic hits pods before they can serve it.  \n✅ **readiness + liveness probes** — Traffic waits for ready; hung pods get restarted.\n\n**3.** ❌ **one replica for a service** — A node dies and the service goes with it.  \n✅ **2+ replicas, spread out** — Survive a node failure and rolling updates.\n\n**4. Don't**\n\n```bash\nimage: myapp:latest\n```\n\nRollouts and rollbacks become unpredictable.\n\n**Do instead**\n\npin a tag or digest\n\nYou always know exactly what is running.\n\n**5.** ❌ **secrets in plain env vars in git** — Anyone with the repo has your credentials.  \n✅ **Secrets + a secrets manager** — Kept out of the manifest and out of git.\n\n## Explained\n\n1. No resource requests or limits\n   Without requests, the scheduler cannot place pods sensibly. Without limits, one pod can consume all the CPU or memory on a node and take its neighbours down. Instead: set requests so the scheduler packs pods well, and limits so no single pod can starve the rest.\n\n2. No readiness probe\n   Kubernetes assumes a pod can serve traffic the moment it starts. Without a readiness probe, requests hit pods that are still booting or connecting to the database, and users see errors during every deploy. Instead: add a readiness probe so traffic waits, and a liveness probe so a hung pod gets restarted.\n\n3. One replica for a service\n   A single pod means a single point of failure. The node it sits on reboots or dies, and the service is gone. Instead: run two or more replicas, spread across nodes with anti-affinity, so a node failure and rolling updates are both survivable.\n\n4. `image: latest`\n   latest makes rollouts and rollbacks unpredictable, because the same tag can point at different images over time. Instead: pin a specific tag or, better, a digest, so you always know exactly what is deployed.\n\n5. Secrets as plain env vars committed to git\n   Putting credentials straight into manifests in the repository hands them to anyone with read access. Instead: use Kubernetes Secrets, ideally backed by a real secrets manager, and keep them out of git.\n\n> **The pattern:** probes, limits and replicas are what separate a demo from production.\n\n## In one line\n\n> **Demos hide what scale reveals.** Probes, limits and replicas are not optional in production.\n\n**Audit one deployment**\n\n```bash\nkubectl describe deploy <name>\n```\n"},{"id":"p162","n":"162","slug":"quiz-3-kubernetes-questions","title":"Quiz: 3 Kubernetes questions","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","DevOps","InterviewPrep","ITCareer"],"series_name":"IT","design":"quiz","script":"","date":"2027-10-10","files":{"devsecops":"notes/p162-quiz-3-kubernetes-questions.md"},"md":"# Quiz: 3 Kubernetes questions\n\nThree Kubernetes questions that come up in interviews and in real incidents. Try each before the answer.\n\n## Quick reference\n\n**Q1. A pod is Running but gets no traffic. Most likely cause?**\n\n- a) It is out of memory\n- b) Its readiness probe is failing\n- c) The node is down\n\n> **Answer:** b) Its readiness probe is failing. Running means the container started. Only pods passing readiness are added to a Service, so a failing probe means no traffic.\n\n**Q2. What does a Deployment actually manage?**\n\n- a) Nodes\n- b) ReplicaSets, which manage pods\n- c) Services\n\n> **Answer:** b) ReplicaSets, which manage pods. A Deployment creates ReplicaSets, and a ReplicaSet keeps the right number of pods running. Rollouts swap ReplicaSets.\n\n**Q3. Where should you store a database password for a pod?**\n\n- a) In the container image\n- b) In a Secret\n- c) In the Deployment YAML in git\n\n> **Answer:** b) In a Secret. A Secret keeps it out of the image and the manifest. Back it with a real secrets manager for production.\n\n## Explained\n\n1. A pod is Running but gets no traffic. Most likely cause?\n\n> **Answer:** its readiness probe is failing. Running only means the container process started. A pod receives traffic through a Service, and the Service only sends traffic to pods that pass their readiness probe. So a pod can sit there Running, healthy-looking, and get nothing, because it never reported ready. kubectl get pods shows READY as 0/1 in that case. This one difference explains a huge share of \"why is my app not reachable\" tickets.\n\n2. **What does a Deployment actually manage?**\n\n> **Answer:** ReplicaSets, which in turn manage pods. You edit a Deployment, it creates a ReplicaSet, and the ReplicaSet keeps the requested number of pods alive. A rolling update works by creating a new ReplicaSet and scaling it up while scaling the old one down. Knowing this explains what you see during kubectl rollout and why old ReplicaSets linger for rollbacks.\n\n3. Where should you store a database password for a pod?\n\n> **Answer:** in a Secret. It keeps the value out of the container image and out of the Deployment YAML you commit to git. On its own a Kubernetes Secret is only base64-encoded, not encrypted, so for production back it with a real secrets manager and enable encryption at rest.\n\nThe thread: Running is not the same as ready, and configuration is not the same as secrets.\n\n## In one line\n\n> **Running is not the same as ready.** Most Kubernetes gotchas come back to that difference.\n\n**Check readiness**\n\n```bash\nkubectl get pods -o wide\n```\n"},{"id":"p163","n":"163","slug":"anatomy-of-a-container-image-reference","title":"Anatomy of a container image reference","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","CloudNative","Security"],"series_name":"IT","design":"anatomy","script":"","date":"2027-10-12","files":{"devsecops":"notes/p163-anatomy-of-a-container-image-reference.md"},"md":"# Anatomy of a container image reference\n\nA container image name looks simple, but each part answers a different question, and the last part is the one that guarantees you run exactly what you think you run.\n\n## Quick reference\n\n```text\nregistry.io/team/api:1.4.2@sha256:9f3c\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `registry.io` | Registry | Where the image lives. Defaults to Docker Hub if omitted. |\n| `/team` | Namespace | The owner or organisation on that registry. |\n| `/api` | Repository | The image's name within that namespace. |\n| `:1.4.2` | Tag | A human label for a version. It can be moved. |\n| `@sha256:9f3c` | Digest | The exact content hash. Immutable. This is the real identity. |\n\n## Explained\n\n`registry.io` is the registry: where the image is stored. Leave it off and the tool defaults to Docker Hub, which is why docker pull nginx quietly reaches out to a public registry. In production you usually name a private or cloud registry explicitly.\n\n`/team` is the namespace: the user or organisation that owns the image on that registry. It keeps names from colliding between teams and projects.\n\n`/api` is the repository: the image's own name within that namespace. Registry, namespace and repository together point at a place.\n\n:1.4.2 is the tag: a human-friendly label for a version. The catch is that a tag is just a pointer and can be moved. Someone can push a new image to :1.4.2, or to latest, and the name now points somewhere different. Same name, different bytes.\n\n@sha256:9f3c... is the digest: a cryptographic hash of the exact image content. It never changes, because changing the content changes the hash. This is the image's true identity.\n\nWhy it matters: a tag answers \"roughly which version?\", a digest answers \"exactly which bytes?\". For reproducible, tamper-evident deploys, pin the digest. Many teams tag for humans and deploy by digest, getting both readability and certainty.\n\n## In one line\n\n> **Pin the digest for certainty.** Tags are convenient. Digests are exact.\n\n**See the digest**\n\n```bash\ndocker image inspect api:1.4.2 --format '{{.Id}}'\n```\n"},{"id":"p164","n":"164","slug":"9-docker-compose-commands-for-daily-work","title":"9 docker compose commands for daily work","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","DockerCompose","DevOps","Containers","Linux"],"series_name":"IT","design":"steps","script":"","date":"2027-10-14","files":{"devsecops":"notes/p164-9-docker-compose-commands-for-daily-work.md"},"md":"# 9 docker compose commands for daily work\n\nDocker Compose runs a whole multi-container app from a single file. Nine commands cover almost all daily work, from starting the stack to looking inside it. Modern Docker uses docker compose, two words; older setups use docker-compose.\n\n## Quick reference\n\n### Run it — Start, stop, rebuild\n\n```bash\n# Start the whole stack in the background.\ndocker compose up -d\n# Stop and remove it. Add -v to delete volumes too.\ndocker compose down\n# Rebuild images, then start. After code changes.\ndocker compose up -d --build\n```\n\n### Look inside — What is going on?\n\n```bash\n# Which services are up, and their status and ports.\ndocker compose ps\n# Follow one service's logs live.\ndocker compose logs -f api\n# A shell inside a running service.\ndocker compose exec api sh\n```\n\n### Manage — Scale, config, clean\n\n```bash\n# Run three copies of one service.\ndocker compose up -d --scale worker=3\n# Show the final, merged configuration.\ndocker compose config\n# Fetch newer images before the next up.\ndocker compose pull\n```\n\n## Explained\n\n### Run it\n\n`docker compose up -d` starts every service defined in the file, in the background. App, database, cache and more come up wired together.  \n`docker compose down` stops and removes the containers and network. Add -v to also delete the named volumes, which wipes the data, so use it deliberately.  \n`docker compose up -d --build` rebuilds the images first. This is the one people forget after changing code, then wonder why nothing changed.\n\n### Look inside\n\n`docker compose ps` shows which services are running, their status, and the ports they expose.  \n`docker compose logs -f api` follows one service's logs live. Leave off the name to tail everything at once.  \n`docker compose exec api sh` opens a shell inside a running service, to check files, config or connectivity.\n\n### Manage\n\n`docker compose up -d --scale worker=3` runs three copies of a service, handy for testing a queue worker.  \n`docker compose config` prints the final configuration after merging files and variables. It catches YAML and variable mistakes before they bite.  \n`docker compose pull` fetches newer versions of the images before your next up.\n\nThe daily rhythm is simple: up -d to start, logs -f to watch, down to stop.\n\n## In one line\n\n> **One file, the whole stack.** Compose is the fastest way to run a real app locally.\n\n**The daily two**\n\n```bash\ndocker compose up -d && docker compose logs -f\n```\n"},{"id":"p165","n":"165","slug":"an-slo-and-error-budget-at-a-glance","title":"An SLO and error budget at a glance","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","Observability","DevOps","CloudComputing","Reliability"],"series_name":"IT","design":"dashboard","script":"","date":"2027-10-17","files":{"devsecops":"notes/p165-an-slo-and-error-budget-at-a-glance.md"},"md":"# An SLO and error budget at a glance\n\nAn SLO turns \"the service should be reliable\" into a number, and an error budget turns that number into something you can actually manage. Here is how to read one.\n\n## Quick reference\n\n**Status:** This month, so far\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| SLO TARGET | **99.9%** | of requests succeed |\n| Actual | **99.95%%** | better than target |\n| Budget used | **45%** | of this month's allowance |\n| Budget left | **24 min** | downtime you can still spend |\n| Errors (30d) | **0.05%** | share of all requests |\n| If spent | **freeze** | slow down risky releases |\n\n## Explained\n\nThe SLO target: 99.9% of requests succeed. This is the promise. It sounds absolute, but the interesting part is the 0.1% that is allowed to fail.\n\nThe actual: 99.95% this month. Better than target, which means there is room to spare. Measuring the real number is the whole point; a target you do not measure is a wish.\n\nBudget used: 45%. The error budget is the allowed failure, the 0.1%. Over a 30-day month, 99.9% allows about 43 minutes of downtime. Using 45% of it means you have spent roughly 20 of those minutes.\n\nBudget left: 24 minutes. This is the key idea. Reliability is not \"never fail\", it is a budget you can spend. While budget remains, you can ship bold changes, run risky migrations, do chaos testing. The budget existing is permission to move fast.\n\nErrors over 30 days: 0.05%. Comfortably inside the 0.1% allowance.\n\nIf spent: freeze. When the budget runs out, the policy kicks in: slow down, stop risky releases, and focus on reliability until the next window. This is what makes an SLO a decision tool, not just a dashboard number.\n\nThe maths: error budget = (1 - SLO) times total requests, or expressed as allowed downtime over the period.\n\nReliability is a budget. Spend it wisely.\n\n## In one line\n\n> **Reliability is a budget.** Spend it on bold releases while there is room; slow down when it runs low.\n\n**The math:** budget = (1 - SLO) x total requests\n"},{"id":"p166","n":"166","slug":"6-timeouts-every-devops-should-set","title":"6 timeouts every DevOps should set","sections":["devsecops"],"topic":"Observability & SRE","tags":["DevOps","SRE","Reliability","CloudComputing","SysAdmin"],"series_name":"IT","design":"numbers","script":"","date":"2027-10-19","files":{"devsecops":"notes/p166-6-timeouts-every-devops-should-set.md"},"md":"# 6 timeouts every DevOps should set\n\nThe default timeout for most things is either far too long or infinite. A single call with no timeout can tie up a worker forever, and enough of them take the whole service down. Six timeouts worth setting on purpose.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **5 s** | TCP connect | Fail fast when a host is unreachable, not after a minute. | connect |\n| **30 s** | HTTP client | Cap how long a call to another service can hang. | request |\n| **10 s** | Liveness probe | How long before the platform restarts a stuck pod. | k8s |\n| **30 s** | Graceful shutdown | Time to finish in-flight work before a hard kill. | SIGTERM |\n| **60 s** | Load balancer idle | Must exceed your app's keep-alive, or you get resets. | LB |\n| **30 s** | DB statement | Kill a runaway query before it blocks everything. | query |\n\n## Explained\n\n5 s, TCP connect. How long to wait to establish a connection before giving up. If a host is down or a firewall is dropping packets, you want to fail in seconds, not after the operating system's default minute or more.\n\n30 s, HTTP client. The total time a call to another service or API may take. Without it, one slow dependency makes your app hang too, and the hang spreads upstream. This is the classic cause of cascading failures.\n\n10 s, liveness probe. In Kubernetes, how long a pod can be unresponsive before it is restarted. Too short and healthy-but-busy pods get killed; too long and users suffer before recovery.\n\n30 s, graceful shutdown. The time a process gets to finish in-flight requests after a stop signal before it is force-killed. Too short and you drop requests on every deploy.\n\n60 s, load balancer idle. The idle timeout on the load balancer should be longer than your app's keep-alive timeout. If the app closes connections first, users get confusing resets.\n\n30 s, database statement. A statement timeout kills a runaway query before it holds locks and blocks every other connection.\n\n> **The rule behind all of them:** every network call needs a timeout. A call that can wait forever will, eventually, on the worst possible day.\n\n## In one line\n\n> **A missing timeout is an outage waiting.** Defaults are often minutes, or infinite. Set them on purpose.\n\n**The rule:** every network call needs a timeout\n"},{"id":"p167","n":"167","slug":"horizontal-vs-vertical-scaling","title":"Horizontal vs Vertical scaling","sections":["devsecops"],"topic":"DevOps basics","tags":["CloudComputing","SystemDesign","DevOps","Scalability","SRE"],"series_name":"IT","design":"compare","script":"","date":"2027-10-21","files":{"devsecops":"notes/p167-horizontal-vs-vertical-scaling.md"},"md":"# Horizontal vs Vertical scaling\n\nWhen a system is overloaded, there are two ways to give it more capacity: make the machine bigger, or add more machines. Each fits a different problem.\n\n## Quick reference\n\n| | **VERTICAL** | **HORIZONTAL** |\n|:--|:--|:--|\n| How | Upgrade the machine | Add more machines |\n| Limit | The biggest box | Almost none |\n| Downtime | Often a reboot | None, add live |\n| Redundancy | Still one machine | Survives one failing |\n| Needs | Simple | Load balancer, stateless app |\n| Best for | Databases, quick wins | Web and app tiers |\n\n**VERTICAL** (A bigger box): Give the existing server more CPU, memory or disk.\n\nExample: `2 vCPU -> 8 vCPU`\n\n**HORIZONTAL** (More boxes): Add more servers and spread the load across them.\n\nExample: `1 server -> 5 servers`\n\n- Vertical → fast and simple, until you hit the ceiling.\n- Horizontal → scales far and survives failures. Design for it.\n\n## Explained\n\nVertical scaling means giving the existing server more resources: more CPU, more memory, a faster disk. 2 vCPUs become 8. It is the simplest option, needs no change to your application, and is often the fastest fix. The limits are real, though. There is a biggest box you can buy, it is still a single point of failure, and resizing usually means a reboot and a moment of downtime.\n\nHorizontal scaling means adding more servers and spreading the load across them with a load balancer. One app server becomes five. There is almost no ceiling, you can add or remove capacity while the system is live, and if one server dies the others carry on. The cost is design: your app must be stateless, or keep its state in a shared database or cache, so any server can handle any request.\n\n### How to choose\n\nVertical: a database, or a quick win under sudden load. Simple, but temporary.  \nHorizontal: web and application tiers, where you need to scale far and survive failures. The cloud-native default.\n\nA common pattern combines them: scale the stateless web and app tiers horizontally behind a load balancer, and scale the database vertically, because databases are harder to spread across machines.\n\nThe deeper point: horizontal scaling is as much an application design decision as an infrastructure one. Build stateless, and scaling out becomes easy later.\n\nVertical is fast. Horizontal goes far.\n"},{"id":"p168","n":"168","slug":"a-zero-downtime-database-migration","title":"A zero-downtime database migration","sections":["it","devsecops"],"topic":"Databases & SQL","tags":["Databases","DevOps","SRE","Migration","CloudComputing"],"series_name":"IT","design":"timeline","script":"","date":"2027-10-24","files":{"it":"notes/p168-a-zero-downtime-database-migration.md","devsecops":"notes/p168-a-zero-downtime-database-migration.md"},"md":"# A zero-downtime database migration\n\nChanging a database schema on a live system sounds scary because the naive way, altering a column in place, locks tables and breaks the running app. The safe way is a sequence that never requires downtime. It is called expand and contract.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **Step 1** | Add, do not change | Add the new column or table. Nothing reads it yet. |\n| **Step 2** | Write to both | Deploy code that writes old and new side by side. |\n| **Step 3** | Backfill | Copy existing rows into the new shape, in batches. |\n| **Step 4** | Read from new | Switch reads to the new column, still writing both. |\n| **Step 5** | Stop writing old | Once new is trusted, drop the old writes. |\n| **Step 6** | Clean up | Remove the old column, long after the switch. |\n\n## Explained\n\nStep 1, add, do not change. Add the new column or table alongside the old one. Nothing reads it yet, so this change is invisible and safe. Crucially, you never rename or retype a column in place while code depends on it.\n\nStep 2, write to both. Deploy application code that writes to both the old and the new column at once. Now every new row is correct in both shapes, and the old code still works because the old column is still there.\n\nStep 3, backfill. Copy the existing rows into the new shape, in small batches so you do not lock the table or overload the database. This can run for hours in the background without anyone noticing.\n\nStep 4, read from new. Switch the application to read from the new column, while still writing to both. If something is wrong, you can switch reads back instantly.\n\nStep 5, stop writing old. Once the new column is trusted in production, remove the code that writes the old one.\n\nStep 6, clean up. Much later, when you are certain nothing depends on it, drop the old column.\n\nThe principle: expand the schema, migrate the data and traffic gradually, then contract. Each step is independently deployable and reversible, so there is never a moment where a rollback breaks.\n\n## In one line\n\n> **Expand, migrate, then contract.** Never change a column in place on a live database.\n\n**The pattern:** add -> dual-write -> backfill -> switch -> drop\n"},{"id":"p169","n":"169","slug":"terraform-vs-cloudformation-vs-pulumi","title":"Terraform vs CloudFormation vs Pulumi","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["IaC","Terraform","CloudFormation","Pulumi","DevOps"],"series_name":"IT","design":"matrix","script":"","date":"2027-10-26","files":{"devsecops":"notes/p169-terraform-vs-cloudformation-vs-pulumi.md"},"md":"# Terraform vs CloudFormation vs Pulumi\n\nInfrastructure as code means defining your servers and networks in files instead of clicking a console. Three popular tools do it in different styles.\n\n## Quick reference\n\n| | **Terraform** | **CloudFormation** | **Pulumi** |\n|:--|:--|:--|:--|\n| Language | HCL | YAML / JSON | Python, TS, Go |\n| Clouds | ✅ Many | ❌ AWS only | ✅ Many |\n| State | You manage it | ✅ AWS manages it | You manage it |\n| Learning curve | 🟡 Medium | 🟡 Medium | 🟡 Know a language |\n| Maturity | ✅ Very mature | ✅ Very mature | 🟡 Newer |\n| Best fit | Multi-cloud default | All-in on AWS | Devs who want code |\n\n**How to pick**\n\n- **Terraform** → the multi-cloud default, huge community.\n- **CloudFormation** → all-in on AWS, no state to manage.\n- **Pulumi** → teams who want real code, not a DSL.\n\n## Explained\n\nTerraform, by HashiCorp, is the de facto standard. You write in HCL, a purpose-built configuration language, and it works across many clouds and services through providers. You manage the state file yourself, usually in a remote backend with locking. Its huge community means there is a module or example for almost anything. It is the usual default, especially for multi-cloud.\n\nCloudFormation is AWS's native tool. You write YAML or JSON, and AWS runs and tracks it for you, so there is no state file to manage and no locking to set up. The trade-off is that it is AWS-only, and the templates get verbose. If you are all-in on AWS, the tight integration and managed state are real advantages.\n\nPulumi takes a different path: you define infrastructure in a real programming language, Python, TypeScript, Go or others. That means loops, functions, tests and your editor's tooling, which developers love. It is newer and the community is smaller, and you manage state as with Terraform.\n\n### How to choose\n\nMulti-cloud, or want the biggest community: Terraform.  \nEntirely on AWS and value managed state: CloudFormation.  \nA team that would rather write code than a DSL: Pulumi.\n\nThey all achieve the same goal: infrastructure that is version-controlled, reviewable and repeatable. The biggest win is adopting any of them, not which one.\n\n## In one line\n\n> **Same idea, different language.** They all make infrastructure repeatable. Pick for your cloud and team.\n\n**Whatever you pick:** plan and review before every apply\n"},{"id":"p170","n":"170","slug":"9-ansible-commands-to-start-with","title":"9 ansible commands to start with","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Ansible","DevOps","Automation","Linux","SysAdmin"],"series_name":"IT","design":"terminal","script":"","date":"2027-10-28","files":{"devsecops":"notes/p170-9-ansible-commands-to-start-with.md"},"md":"# 9 ansible commands to start with\n\nAnsible configures many servers at once, over SSH, with no agent to install on them. You can start with one-line commands today and grow into full playbooks later. Nine to begin with.\n\n## Quick reference\n\n### Can I reach the hosts?\n\n```bash\n# check SSH and Python on every host\nansible all -m ping\n# gather facts: OS, IP, memory, and more\nansible all -m setup\n# run one command on a group of hosts\nansible web -a 'uptime'\n```\n\n### Make changes\n\n```bash\n# install a package. -b runs with sudo\nansible web -b -m apt -a 'name=nginx state=present'\n# restart a service everywhere at once\nansible web -b -m service -a 'name=nginx state=restarted'\n# push a file to every host\nansible web -b -m copy -a 'src=app.conf dest=/etc/app/'\n```\n\n### Repeatable runs\n\n```bash\n# dry run: see what would change\nansible-playbook site.yml --check\n# run against one host first\nansible-playbook site.yml --limit web01\n# keep secrets encrypted at rest\nansible-vault edit secrets.yml\n```\n\n## Explained\n\n### Can I reach the hosts?\n\n`ansible all -m ping` is the first test. It confirms Ansible can reach each host over SSH and run Python there. It is not a network ping; it is an end-to-end check.  \n`ansible all -m setup` gathers facts about each host: OS, IP addresses, memory, disks. These facts are what playbooks use to make decisions.  \n`ansible web -a 'uptime'` runs a single command on every host in the web group. Great for quick checks across a fleet.\n\n### Make changes\n\n`ansible web -b -m apt -a 'name=nginx state=present'` installs a package everywhere. -b means become, running with sudo. Modules like apt are idempotent: run them again and nothing changes if nginx is already there.  \n`ansible web -b -m service -a 'name=nginx state=restarted'` restarts a service across the whole group at once.  \n`ansible web -b -m copy -a 'src=app.conf dest=/etc/app/'` pushes a file to every host.\n\n### Repeatable runs\n\n`ansible-playbook site.yml --check` is a dry run. It shows what would change without changing anything. Always run this first.  \n`ansible-playbook site.yml --limit web01` runs a playbook against one host before the whole fleet.  \n`ansible-vault edit secrets.yml` keeps secrets encrypted at rest, so credentials never sit in plain text in your repo.\n\nThe path: ad-hoc commands to learn, playbooks to make it repeatable.\n\n## In one line\n\n> **One command, every server.** Start with ad-hoc commands, then grow into playbooks.\n\n**Try it**\n\n```bash\nansible all -m ping\n```\n"},{"id":"p171","n":"171","slug":"cloud-acronyms-decoded","title":"Cloud acronyms, decoded","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["CloudComputing","AWS","DevOps","ITCareer","CloudNative"],"series_name":"IT","design":"reveal","script":"","date":"2027-10-31","files":{"devsecops":"notes/p171-cloud-acronyms-decoded.md"},"md":"# Cloud acronyms, decoded\n\nThe cloud drowns newcomers in acronyms. Six that come up constantly, in plain English, so the rest start to make sense.\n\n## Quick reference\n\n- **IaaS** — Infrastructure as a Service: you rent raw servers, storage and network, and manage the rest.\n- **PaaS** — Platform as a Service: you push code; the platform runs it, scales it and patches the servers.\n- **SaaS** — Software as a Service: finished software you just log into, like email or a CRM.\n- **VPC** — Virtual Private Cloud: your own isolated network inside the cloud, with your own subnets.\n- **IAM** — Identity and Access Management: who can do what to which resources.\n- **CDN** — Content Delivery Network: copies of your content near users, so it loads fast everywhere.\n\n## Explained\n\nIaaS, Infrastructure as a Service. You rent the raw building blocks, virtual servers, storage and networking, and you manage everything on top: the OS, the runtime, your app. EC2 is the classic example. Most control, most responsibility.\n\nPaaS, Platform as a Service. You push your code and the platform runs it: it provisions the servers, scales them, and patches the OS for you. You focus on the app, not the machines. App Engine, Heroku and managed container platforms fit here.\n\nSaaS, Software as a Service. Finished software you simply log into, with nothing to run yourself. Gmail, Salesforce, Microsoft 365. You manage only your data and users.\n\nThese three form a ladder: IaaS to PaaS to SaaS, each step handing more responsibility to the provider and leaving you less to manage.\n\nVPC, Virtual Private Cloud. Your own isolated network inside the cloud, where you define subnets, routing and firewall rules. It is how you keep your resources private and segmented.\n\nIAM, Identity and Access Management. Who can do what to which resources. The most important service to get right, and the one attackers exploit when it is too permissive.\n\nCDN, Content Delivery Network. Copies of your static content cached in locations near users worldwide, so pages and files load fast everywhere and your origin server is spared the load.\n\nThe cloud is learnable. Behind the acronyms are a few simple ideas.\n\n## In one line\n\n> **The cloud is learnable.** Behind the acronyms are a few simple ideas.\n\n**The ladder:** IaaS -> PaaS -> SaaS: more managed each step\n"},{"id":"p172","n":"172","slug":"a-disk-space-alert-in-15-lines-of-python","title":"A disk-space alert in 15 lines of Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SysAdmin","DevOps","Automation"],"series_name":"Cloud / DevSecOps","design":"code","script":"disk_alert.py","date":"2027-11-02","files":{"it":"notes/p172-a-disk-space-alert-in-15-lines-of-python.md","devsecops":"notes/p172-a-disk-space-alert-in-15-lines-of-python.md"},"md":"# A disk-space alert in 15 lines of Python\n\nPhase 1 of my Cloud/DevSecOps plan is Linux and automation, so here is the first of a few real Python scripts you can use at work today. This one warns you before a disk fills up. The full file is attached and runs on any server with Python, no libraries to install.\n\n## Quick reference\n\nThe full script is at the end of this note: `disk_alert.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 disk_alert.py\n```\n\n**Example output**\n\n```text\nALERT / at 91%\n```\n\n## Explained\n\n### What it does\n\nIt checks the usage of each path you list. If any crosses the threshold, it prints an ALERT and exits with code 1, so cron, a monitoring agent or a CI step all understand that something is wrong. Exit 0 means all good.\n\n### Why the standard library only\n\nshutil.disk_usage comes with Python. No pip install, nothing to break on an old server. That matters when you are dropped onto a machine you do not control.\n\n### How to use it\n\nSave it, then schedule it: `*/30 * * * * /usr/bin/python3 /opt/scripts/disk_alert.py >> /var/log/disk_alert.log 2>&1`. Now you get a line in a log the moment a disk gets tight, instead of finding out when the database stops writing.\n\nThe real version, attached, adds command-line options for the threshold and paths, shows human-readable sizes, and includes the hostname so one log can cover many servers.\n\n> **The lesson behind it:** a full disk is never sudden. It is unwatched. Fifteen lines and a cron entry turn \"unwatched\" into \"I knew yesterday\".\n\n## In one line\n\n> **Automate the boring checks.** A cron job and a few lines beat a 2 a.m. outage.\n\n**Run it every 30 min**\n\n```bash\n*/30 * * * * python3 disk_alert.py\n```\n\n## Script: disk_alert.py\n\nDownload: [scripts/disk_alert.py](../scripts/disk_alert.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"disk_alert.py - warn before a disk fills up.\n\nChecks the usage of one or more paths and prints an ALERT for any that\ncross a threshold. Exit code is 1 if anything alerted, so it works in cron\nand in monitoring. Standard library only.\n\nUsage:\n    python3 disk_alert.py                 # checks / and /var at 85%\n    python3 disk_alert.py -t 90 / /data   # custom threshold and paths\n\nCron (every 30 min):\n    */30 * * * * /usr/bin/python3 /opt/scripts/disk_alert.py >> /var/log/disk_alert.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport shutil\nimport socket\nimport sys\n\n\ndef pct_used(path):\n    total, used, _free = shutil.disk_usage(path)\n    return round(used / total * 100), total, used\n\n\ndef human(n):\n    for unit in (\"B\", \"KB\", \"MB\", \"GB\", \"TB\"):\n        if n < 1024 or unit == \"TB\":\n            return f\"{n:.0f}{unit}\"\n        n /= 1024\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Alert when a disk crosses a usage threshold.\")\n    ap.add_argument(\"-t\", \"--threshold\", type=int, default=85, help=\"percent, default 85\")\n    ap.add_argument(\"paths\", nargs=\"*\", default=[\"/\", \"/var\"], help=\"mount points to check\")\n    args = ap.parse_args()\n\n    host = socket.gethostname()\n    alerted = False\n    for path in args.paths:\n        try:\n            used, total, used_bytes = pct_used(path)\n        except FileNotFoundError:\n            print(f\"[skip] {path} not found\")\n            continue\n        status = \"ALERT\" if used >= args.threshold else \"OK\"\n        if used >= args.threshold:\n            alerted = True\n        print(f\"[{status}] {host}:{path} {used}% used \"\n              f\"({human(used_bytes)} of {human(total)})\")\n\n    sys.exit(1 if alerted else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p173","n":"173","slug":"9-selinux-commands-that-fix-permission-denied","title":"9 SELinux commands that fix Permission denied","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","RHEL","SELinux","RHCSA","SysAdmin"],"series_name":"Cloud / DevSecOps","design":"terminal","script":"","date":"2027-11-04","files":{"it":"notes/p173-9-selinux-commands-that-fix-permission-denied.md","devsecops":"notes/p173-9-selinux-commands-that-fix-permission-denied.md"},"md":"# 9 SELinux commands that fix Permission denied\n\nOn RHEL and its family, the most confusing error is the one where the file permissions look perfect and the app still gets Permission denied. The usual culprit is SELinux, and the RHCSA exam expects you to handle it. Nine commands that turn it from a mystery into a routine.\n\n## Quick reference\n\n### Is SELinux the cause?\n\n```bash\n# Enforcing means SELinux can block things\ngetenforce\n# -Z shows the security context, the key field\nls -Z /var/www/html\n# the actual denials, in plain-ish text\nausearch -m avc -ts recent\n```\n\n### Fix the context\n\n```bash\n# reset files to their correct default context\nrestorecon -Rv /var/www\n# set a new default context for a path\nsemanage fcontext -a -t httpd_sys_rw_content_t '/srv/app(/.*)?'\n# change one file's context (temporary, test only)\nchcon -t httpd_sys_content_t file.html\n```\n\n### Booleans and ports\n\n```bash\n# toggles for common app behaviour\ngetsebool -a | grep httpd\n# -P makes it permanent. Let the app reach the DB\nsetsebool -P httpd_can_network_connect on\n# let a service listen on a non-standard port\nsemanage port -a -t http_port_t -p tcp 8080\n```\n\n## Explained\n\n### Is SELinux the cause?\n\n`getenforce` tells you if it is Enforcing, Permissive or Disabled. In Enforcing mode it can block actions that normal permissions would allow.  \n`ls -Z` shows the security context of files, the part normal ls hides. The context, not just rwx, decides what a labelled service may touch.  \n`ausearch -m avc -ts recent` lists the actual denials. This is the evidence, and it usually names the fix.\n\n### Fix the context\n\n`restorecon -Rv` resets files to their correct default context. This single command solves a huge share of SELinux problems, especially after moving files into a web root.  \nsemanage fcontext adds a new default context for a path, so it survives a relabel. Pair it with restorecon.  \nchcon changes one file's context immediately, but it is not permanent, so use it only to test a theory.\n\n### Booleans and ports\n\ngetsebool and `setsebool -P` toggle built-in behaviours, like letting the web server make network connections. The -P makes it stick across reboots.  \nsemanage port lets a service listen on a non-standard port, like nginx on 8080.\n\n> **The one habit that matters:** do not reach for setenforce 0. Turning SELinux off hides the problem and removes a real layer of defence. Read the denial, fix the label.\n\n## In one line\n\n> **Don't disable it. Label it.** setenforce 0 hides the problem. restorecon fixes it.\n\n**Try it**\n\n```bash\nausearch -m avc -ts recent\n```\n"},{"id":"p174","n":"174","slug":"anatomy-of-an-etc-fstab-line","title":"Anatomy of an /etc/fstab line","sections":["it","devsecops"],"topic":"Storage & infrastructure","tags":["Linux","RHCSA","Storage","SysAdmin","RHEL"],"series_name":"Cloud / DevSecOps","design":"anatomy","script":"","date":"2027-11-07","files":{"it":"notes/p174-anatomy-of-an-etc-fstab-line.md","devsecops":"notes/p174-anatomy-of-an-etc-fstab-line.md"},"md":"# Anatomy of an /etc/fstab line\n\n`/etc/fstab` decides which disks get mounted at boot and how. RHCSA tests it, and one wrong line can stop a server from booting, so it is worth reading field by field. Each line has six, separated by spaces or tabs.\n\n## Quick reference\n\n```text\nUUID=3f2a... /data ext4 defaults,nofail 0 2\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `UUID=3f2a...` | What to mount | The device, by UUID. Stable, unlike `/dev/sdb1`. |\n| `/data` | Where | The mount point. The folder it appears at. |\n| `ext4` | Type | The filesystem: ext4, xfs, vfat, nfs. |\n| `defaults,nofail` | Options | Mount flags. nofail lets the server boot if it is missing. |\n| `0` | Dump | Backup flag for the old dump tool. Almost always 0. |\n| `2` | fsck order | Boot-time check order. 1 for root, 2 for others, 0 to skip. |\n\n## Explained\n\nField 1, what to mount. The device. Use UUID= from blkid, not `/dev/sdb1`, because device names can change when disks are added or reordered, while a UUID never does.\n\nField 2, where. The mount point: the existing folder where the filesystem appears, like `/data`. Create the folder first.\n\nField 3, type. The filesystem type: ext4, xfs (the RHEL default), vfat for EFI, or nfs for a network share. auto lets the system guess, but naming it is clearer.\n\nField 4, options. Mount flags, comma-separated. defaults covers the common set. The one worth knowing is nofail: with it, a missing or failed disk does not stop the boot, which is exactly what you want for a secondary data disk.\n\nField 5, dump. A flag for the ancient dump backup tool. On modern systems it is almost always 0.\n\nField 6, fsck order. The order filesystems are checked at boot. 1 for the root filesystem, 2 for other permanent disks, and 0 to skip the check, which you use for network or removable mounts.\n\n> **The rule that saves you:** after editing fstab, never reboot blindly. Run `mount -a` to mount everything the way boot would, and `findmnt --verify` to catch mistakes. Fix them now, not from a rescue console.\n\n## In one line\n\n> **Test it before you reboot.** A bad fstab line drops the server to emergency mode.\n\n**The safe check**\n\n```bash\nmount -a && findmnt --verify\n```\n"},{"id":"p175","n":"175","slug":"9-lvm-commands-every-rhcsa-needs","title":"9 LVM commands every RHCSA needs","sections":["it","devsecops"],"topic":"Storage & infrastructure","tags":["Linux","RHCSA","LVM","RHEL","SysAdmin"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2027-11-09","files":{"it":"notes/p175-9-lvm-commands-every-rhcsa-needs.md","devsecops":"notes/p175-9-lvm-commands-every-rhcsa-needs.md"},"md":"# 9 LVM commands every RHCSA needs\n\nLVM is the reason you can add space to a running Linux server without downtime, and it is a guaranteed part of the RHCSA. Nine commands take you from a raw disk to a volume you can grow on demand.\n\n## Quick reference\n\n### Build the stack — Disk to volume group\n\n```bash\n# Mark a disk as an LVM physical volume.\npvcreate /dev/sdb\n# Create a volume group from one or more PVs.\nvgcreate vg_data /dev/sdb\n# Carve a logical volume out of the group.\nlvcreate -L 10G -n lv_app vg_data\n```\n\n### Grow it live — The reason to use LVM\n\n```bash\n# Add another disk to the group for more space.\nvgextend vg_data /dev/sdc\n# Grow the LV and the filesystem in one step (-r).\nlvextend -r -L +5G /dev/vg_data/lv_app\n# Confirm the new size. No unmount, no reboot.\ndf -h /app\n```\n\n### Inspect — See what you have\n\n```bash\n# Physical volumes and their free space.\npvs\n# Volume groups and how much is left.\nvgs\n# Logical volumes and their sizes.\nlvs\n```\n\n## Explained\n\n### Build the stack\n\n`pvcreate /dev/sdb` marks a whole disk or partition as an LVM physical volume, the raw material.  \n`vgcreate vg_data /dev/sdb` pools one or more physical volumes into a volume group, a single flexible space.  \n`lvcreate -L 10G -n lv_app vg_data` carves a logical volume out of that group. You format and mount this, not the disk directly.\n\nThe layers are simple: physical volumes feed a volume group, and logical volumes are cut from the group.\n\n### Grow it live\n\n`vgextend vg_data /dev/sdc` adds another disk to the group when it runs low.  \n`lvextend -r -L +5G /dev/vg_data/lv_app` is the command that matters. It grows the logical volume and, thanks to -r, resizes the filesystem on top in the same step. No unmount, no reboot, no downtime for users.  \n`df -h` confirms the new size immediately.\n\n### Inspect\n\n`pvs`, `vgs` and `lvs` give a quick summary of physical volumes, groups and logical volumes, with free space at each level. Run them before and after any change.\n\nThe exam tip and the real-world tip are the same: practise `lvextend -r` until it is muscle memory. Running out of space on a production server at 2 a.m. is exactly when you want this to be automatic.\n\n## In one line\n\n> **Grow storage without downtime.** That one ability is why LVM is on every RHEL server.\n\n**The one-liner**\n\n```bash\nlvextend -r -L +5G /dev/vg/lv\n```\n"},{"id":"p176","n":"176","slug":"fix-the-app-works-with-selinux-off-fails-with-it-on","title":"Fix: the app works with SELinux off, fails with it on","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SELinux","RHEL","Troubleshooting","RHCSA"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2027-11-11","files":{"it":"notes/p176-fix-the-app-works-with-selinux-off-fails-with-it-on.md","devsecops":"notes/p176-fix-the-app-works-with-selinux-off-fails-with-it-on.md"},"md":"# Fix: the app works with SELinux off, fails with it on\n\nEvery RHEL admin meets this moment: the app fails, someone runs setenforce 0, it works, and SELinux gets blamed and left off. That is not a fix. It switches off a real security layer to avoid fixing one label. Here is the proper way.\n\n## Quick reference\n\n### Diagnose — Confirm it is SELinux\n\n```bash\n# If it works now, SELinux is the cause. Turn it back on.\nsetenforce 0  (test only!)\n# The denial names the process, action and context.\nausearch -m avc -ts recent\n# Wrong context? default_t instead of an app type.\nls -Z /srv/app/uploads\n```\n\n### Fix — Give the path the right label\n\n```bash\n# Set the correct default context.\nsemanage fcontext -a -t <web_rw_type> '/srv/app(/.*)?'\n# Apply it to the existing files.\nrestorecon -Rv /srv/app/uploads\n# Back to Enforcing. Test again: it should pass.\nsetenforce 1\n```\n\n### Prevent — Work with SELinux, not against it\n\n- `audit2why < /var/log/audit/audit.log` — Explains a denial in plain language.\n- **Check booleans first** — `getsebool -a` often has the toggle you need.\n- **Label new paths up front** — Add fcontext rules when you deploy, not after.\n\n## Explained\n\n### Diagnose\n\nsetenforce 0 is fine as a one-minute test: if the problem disappears, SELinux is involved. Then `setenforce 1` immediately, because leaving it off is the mistake.  \n`ausearch -m avc -ts recent` shows the denial: which process, which action (read, write, connect) and the context it was denied. This is your evidence.  \n`ls -Z` on the path usually reveals the issue: files sitting with a generic type like default_t instead of the type the service is allowed to use.\n\n### Fix\n\n`semanage fcontext -a -t` ... sets the correct default context for the path. For a writable app folder served by the web server, that is often httpd_sys_rw_content_t.  \n`restorecon -Rv` applies that label to the files already there.  \n`setenforce 1` returns to Enforcing, and the app now works with SELinux fully on.\n\n### Prevent\n\naudit2why reads a denial and explains it in plain language, often suggesting the exact fix.  \nCheck booleans first: `getsebool -a` frequently has a ready-made toggle, like letting the web server connect to a database.  \nLabel new paths when you deploy, not after they break.\n\nThe mindset shift: SELinux is not in your way, it is telling you a label is wrong. Fix the label and you keep both the app and the protection.\n\n## In one line\n\n> **setenforce 0 is not a fix.** It switches off a whole security layer to hide one label.\n\n**The real fix**\n\n```bash\nsemanage fcontext + restorecon\n```\n"},{"id":"p177","n":"177","slug":"find-risky-file-permissions-with-python","title":"Find risky file permissions with Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","CyberSecurity","DevSecOps","SysAdmin"],"series_name":"Cloud / DevSecOps","design":"code","script":"perms_audit.py","date":"2027-11-14","files":{"it":"notes/p177-find-risky-file-permissions-with-python.md","devsecops":"notes/p177-find-risky-file-permissions-with-python.md"},"md":"# Find risky file permissions with Python\n\nStill in the Linux and security phase of my plan, here is a script that reflects exactly what RHCSA and CKS care about: permissions. It walks a folder and flags the classic risks, world-writable files, SUID and SGID binaries, and sensitive files like .env or keys that others can read. The full version is attached and uses only the standard library.\n\n## Quick reference\n\nThe full script is at the end of this note: `perms_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 perms_audit.py /var/www\n```\n\n**Example output**\n\n```text\n[!] world-writable /var/www/uploads/tmp\n[FAIL] 1 risky item(s) found\n```\n\n## Explained\n\n### Why it is useful\n\nAfter a deploy, a careless chmod 777, or inheriting a server you did not set up, risky permissions hide in plain sight. This turns \"I hope the perms are fine\" into a short, specific list you can act on. It exits with code 1 when it finds something, so it also works as a CI gate before a release.\n\n### What it checks\n\nWorld-writable files and directories, which anyone on the box can overwrite. SUID and SGID binaries, which run with elevated rights and are a favourite for privilege escalation. And sensitive filenames that are readable by group or others, because a key only you should read is a key only you should read.\n\n### How to use it\n\n`python3 perms_audit.py /etc /home /var/www`. Run it on a schedule, or before going live, and compare the output over time. New findings are changes someone should be able to explain.\n\nIt is a first-line check, not a replacement for a full scanner or a CIS benchmark tool like kube-bench. But it is instant, portable, and catches the mistakes that cause real incidents.\n\n## In one line\n\n> **Audit before an attacker does.** A hardening check you can run in seconds, anywhere.\n\n**Run it on a server**\n\n```bash\npython3 perms_audit.py /etc /home /var/www\n```\n\n## Script: perms_audit.py\n\nDownload: [scripts/perms_audit.py](../scripts/perms_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"perms_audit.py - find risky file permissions before an attacker does.\n\nWalks a folder and flags the classics: world-writable files and directories,\nSUID/SGID binaries, and sensitive files (.env, keys, .pem) that others can\nread. A fast hardening check for any Linux server. Standard library only.\n\nUsage:\n    python3 perms_audit.py /var/www\n    python3 perms_audit.py /etc /home\n\nExit code is 1 if anything risky is found.\n\nBy s0sta.com\n\"\"\"\nimport os\nimport stat\nimport sys\n\nSENSITIVE = (\".env\", \".pem\", \".key\", \"id_rsa\", \"id_ed25519\", \".htpasswd\")\n\n\ndef main():\n    roots = sys.argv[1:] or [\".\"]\n    findings = 0\n    for root in roots:\n        for dirpath, dirs, files in os.walk(root):\n            for name in files + dirs:\n                path = os.path.join(dirpath, name)\n                try:\n                    mode = os.lstat(path).st_mode\n                except OSError:\n                    continue\n                if stat.S_ISLNK(mode):\n                    continue\n                perm = stat.S_IMODE(mode)\n                # world-writable\n                if perm & stat.S_IWOTH:\n                    findings += 1\n                    print(f\"[!] world-writable  {oct(perm)[-3:]}  {path}\")\n                # SUID / SGID\n                if perm & (stat.S_ISUID | stat.S_ISGID) and os.path.isfile(path):\n                    findings += 1\n                    print(f\"[!] SUID/SGID       {oct(perm)[-4:]}  {path}\")\n                # sensitive and readable by group/other\n                if name.endswith(SENSITIVE) or name in SENSITIVE:\n                    if perm & (stat.S_IRGRP | stat.S_IROTH):\n                        findings += 1\n                        print(f\"[!] readable secret {oct(perm)[-3:]}  {path}\")\n\n    print(f\"\\n{'[FAIL]' if findings else '[OK]'} {findings} risky item(s) found\")\n    sys.exit(1 if findings else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p178","n":"178","slug":"9-checks-to-harden-a-fresh-rhel-server","title":"9 checks to harden a fresh RHEL server","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Linux","RHEL","CyberSecurity","RHCSA","Hardening"],"series_name":"Cloud / DevSecOps","design":"checklist","script":"","date":"2027-11-16","files":{"it":"notes/p178-9-checks-to-harden-a-fresh-rhel-server.md","devsecops":"notes/p178-9-checks-to-harden-a-fresh-rhel-server.md"},"md":"# 9 checks to harden a fresh RHEL server\n\nA fresh RHEL server is at its safest before it holds anything valuable, which is exactly when hardening is cheapest. Nine checks to run on day one, all within RHCSA scope.\n\n## Quick reference\n\n### Access\n\n```bash\n# SSH keys, not passwords\nPasswordAuthentication no\n# A sudo user, root login off\nPermitRootLogin no\n# Firewall on, least open\nfirewall-cmd --list-all\n```\n\n### The system\n\n```bash\n# SELinux enforcing\ngetenforce\n# Patched and registered\ndnf update; subscription-manager status\n# Auto security updates\ndnf-automatic\n```\n\n### Visibility\n\n- **Time in sync** — `timedatectl`\n- **Auditd running** — `systemctl is-active auditd`\n- **Logs shipped off-box** — to a central server\n\n## Explained\n\n### Access\n\nSSH keys, not passwords. Set `PasswordAuthentication no` in sshd_config once key login works, and automated password guessing becomes pointless.  \nA sudo user, root login off. Create an admin user with sudo, then `PermitRootLogin no`. Every action now has a name behind it.  \nFirewall on, least open. firewalld is on by default on RHEL. Check `firewall-cmd --list-all` and open only the services you actually serve.\n\n### The system\n\nSELinux enforcing. `getenforce` should say Enforcing. It is a real layer of defence, and turning it off to make something work is a habit worth breaking.  \nPatched and registered. Register with subscription-manager for updates, then dnf update. An unpatched box is the easiest target there is.  \nAutomatic security updates. `dnf-automatic` applies security patches without waiting for someone to remember.\n\n### Visibility\n\nTime in sync. `timedatectl` should show the clock synchronized. Wrong time breaks TLS, Kerberos and every log timeline.  \nAuditd running. The audit daemon records security-relevant events, including the SELinux denials you will need later.  \nLogs shipped off-box. Send logs to a central server so an attacker cannot simply erase the local evidence.\n\nNone of this is advanced. It is the baseline that separates a server you can trust from one you merely hope about.\n\n## In one line\n\n> **Harden it before it has data.** Day one on a clean box beats cleanup after a breach.\n\n**A 10-second look**\n\n```bash\ngetenforce; firewall-cmd --list-all; sshd -T | grep -i root\n```\n"},{"id":"p179","n":"179","slug":"firewalld-vs-iptables","title":"firewalld vs iptables","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Linux","RHEL","Firewall","RHCSA","Networking"],"series_name":"Cloud / DevSecOps","design":"compare","script":"","date":"2027-11-18","files":{"it":"notes/p179-firewalld-vs-iptables.md","devsecops":"notes/p179-firewalld-vs-iptables.md"},"md":"# firewalld vs iptables\n\nOn RHEL you will meet both firewalld and iptables, and knowing how they relate removes a lot of confusion. They are not really competitors: firewalld is a friendlier front end, and nftables does the work underneath both today.\n\n## Quick reference\n\n| | **FIREWALLD** | **IPTABLES** |\n|:--|:--|:--|\n| Model | Zones + services | Ordered rule chains |\n| Live changes | Yes, then `--permanent` | Yes, but not saved by default |\n| Persistence | Built in | Needs a save step |\n| Backend | nftables today | nftables or legacy |\n| RHEL default | Yes | Underneath |\n| Best for | Servers, RHCSA | Fine-grained custom rules |\n\n**FIREWALLD** (The modern front end): Zones and services, changes you can make live and then persist.\n\n```bash\nfirewall-cmd --add-service=https\n```\n\n**IPTABLES** (The classic rule engine): Raw, ordered rules. Powerful, but you manage every detail.\n\n```bash\niptables -A INPUT -p tcp --dport 443 -j ACCEPT\n```\n\n- firewalld → the default on RHEL, and what RHCSA tests.\n- iptables → know it, but let firewalld manage the rules.\n\n## Explained\n\nfirewalld organises the firewall around zones and services. A zone is a trust level for an interface (public, internal, trusted), and a service is a named bundle of ports, like https or ssh. `firewall-cmd --add-service=https` opens a service in the live config, and adding `--permanent` saves it across reboots. This is the model RHCSA tests, and it is comfortable for day-to-day server work.\n\niptables is the classic tool: ordered chains of rules that each packet is checked against, top to bottom, until one matches. `iptables -A INPUT -p tcp --dport 443 -j ACCEPT` appends a rule. It is extremely precise, but you manage order, state and persistence yourself, and by default rules are lost on reboot unless you save them.\n\n### How they relate\n\nOn modern RHEL, firewalld is the default and runs on top of nftables. Writing raw iptables rules at the same time as firewalld can cause confusing conflicts, because they are both trying to manage the same engine.\n\n### The practical advice\n\nLet firewalld manage the firewall on RHEL. Learn iptables so you can read older systems and understand what is happening underneath, but do not mix the two on the same box.\n\nFor RHCSA, know firewall-cmd cold: zones, services, ports, `--permanent` and `--reload`.\n\nfirewalld manages. iptables is underneath.\n"},{"id":"p180","n":"180","slug":"the-layers-that-decide-who-can-touch-a-file-on-rhel","title":"The layers that decide who can touch a file on RHEL","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","RHEL","SELinux","RHCSA","SysAdmin"],"series_name":"Cloud / DevSecOps","design":"stack","script":"","date":"2027-11-21","files":{"it":"notes/p180-the-layers-that-decide-who-can-touch-a-file-on-rhel.md","devsecops":"notes/p180-the-layers-that-decide-who-can-touch-a-file-on-rhel.md"},"md":"# The layers that decide who can touch a file on RHEL\n\nOn RHEL, \"can this user open this file?\" is answered by several layers stacked on top of each other, and access is granted only if every one of them agrees. Understanding the stack explains the most confusing permission errors.\n\n## Quick reference\n\n*Bottom to top: each layer can still say no*\n\n1. **The user and groups** — Who you are, and which groups you belong to. · `id`\n2. **File ownership** — Every file has an owner and a group. · `chown`\n3. **Permission bits** — The rwx for owner, group and others. · `chmod`\n4. **ACLs** — Extra per-user or per-group rules beyond the basics. · `setfacl`\n5. **SELinux** — Labels and policy, checked even when rwx allows it. · `ls -Z`\n6. **The result** — Access is granted only if every layer agrees. · `allow / deny`\n\n## Explained\n\nThe user and groups. It starts with identity: your user ID and the groups you belong to, which id shows. Group membership is why adding someone to a group can grant access, but only after they log in again.\n\nFile ownership. Every file has an owner and a group, set with chown. The permission check depends on whether you are the owner, in the group, or neither.\n\nPermission bits. The familiar rwx for owner, group and others, set with chmod. This is the layer most people stop at, and most of the time it is the answer.\n\nACLs. When the basic three are not enough, Access Control Lists add extra rules for specific users or groups with setfacl and getfacl. A small + at the end of `ls -l` output means a file has ACLs.\n\nSELinux. On top of all of that, SELinux checks labels and policy. It can deny an action even when the ownership, bits and ACLs would allow it. This is why a file with perfect rwx can still give Permission denied.\n\nThe result. Access is granted only if every layer agrees. Any one of them can say no.\n\n> **The debugging order that works:** when rwx looks correct and it still fails, go up the stack. Check `ls -Z` for the SELinux context and getfacl for ACLs, not just `ls -l`.\n\n## In one line\n\n> **Every layer can still say no.** 'Permission denied' with perfect rwx usually means SELinux.\n\n**Check top-down**\n\n```bash\nls -Z; getfacl file; ls -l\n```\n"},{"id":"p181","n":"181","slug":"quiz-3-rhcsa-questions","title":"Quiz: 3 RHCSA questions","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","RHCSA","RHEL","SysAdmin","ITCareer"],"series_name":"Cloud / DevSecOps","design":"quiz","script":"","date":"2027-11-23","files":{"it":"notes/p181-quiz-3-rhcsa-questions.md","devsecops":"notes/p181-quiz-3-rhcsa-questions.md"},"md":"# Quiz: 3 RHCSA questions\n\nRHCSA is the foundation of my Cloud/DevSecOps plan, because everything above it, Azure, Kubernetes, security, runs on Linux. Three questions that test whether the fundamentals are solid. Try each before the answer.\n\n## Quick reference\n\n**Q1. An app has perfect rwx permissions but still gets 'Permission denied' on RHEL. Why?**\n\n- a) The disk is full\n- b) SELinux is denying it\n- c) The user is in too many groups\n\n> **Answer:** b) SELinux is denying it. SELinux checks labels on top of normal permissions. `ls -Z` and ausearch reveal it; restorecon usually fixes it.\n\n**Q2. Which command grows a logical volume and its filesystem in one step?**\n\n- a) `lvextend -r`\n- b) resize2fs\n- c) vgextend\n\n> **Answer:** a) `lvextend -r`. `lvextend -r` grows the LV and resizes the filesystem together. vgextend adds a disk to the group.\n\n**Q3. You edited `/etc/fstab`. What do you run before rebooting?**\n\n- a) systemctl daemon-reload\n- b) `mount -a`\n- c) `fsck -A`\n\n> **Answer:** b) `mount -a`. `mount -a` mounts everything in fstab as boot would, so you catch a bad line now, not from a rescue console.\n\n## Explained\n\n1. An app has perfect rwx permissions but still gets Permission denied on RHEL. Why?\n\n> **Answer:** SELinux is denying it. On RHEL-family systems, SELinux checks a security label on every file in addition to the normal permission bits, and it can deny an action that rwx would allow. `ls -Z` shows the context, `ausearch -m avc` shows the denial, and restorecon usually fixes it. Reaching for setenforce 0 only hides it.\n\n2. Which command grows a logical volume and its filesystem in one step?\n\n> **Answer:** `lvextend -r`. The -r resizes the filesystem on top at the same time as growing the logical volume, with no unmount and no downtime. vgextend is different: it adds a disk to the volume group. resize2fs resizes only the filesystem, `which lvextend -r` calls for you.\n\n3. **You edited /etc/fstab.** What do you run before rebooting?\n\n> **Answer:** `mount -a`. It mounts everything in fstab exactly as boot would, so a typo shows up as an error you can fix now, instead of dropping the server into emergency mode on the next reboot. `findmnt --verify` adds a sanity check.\n\nAll three are things you will actually do on real servers, which is the point of RHCSA: it is hands-on, not multiple choice.\n\n## In one line\n\n> **Fundamentals first. Then certs.** RHCSA is the base the whole cloud path stands on.\n\n**Practise it:** break a VM, then fix it\n"},{"id":"p182","n":"182","slug":"a-tls-certificate-expiry-checker-in-python","title":"A TLS certificate expiry checker in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","TLS","CyberSecurity","DevOps","Monitoring"],"series_name":"Cloud / DevSecOps","design":"code","script":"ssl_expiry_check.py","date":"2027-11-25","files":{"it":"notes/p182-a-tls-certificate-expiry-checker-in-python.md","devsecops":"notes/p182-a-tls-certificate-expiry-checker-in-python.md"},"md":"# A TLS certificate expiry checker in Python\n\nAzure phase of the plan means running web apps and APIs, and the fastest way to take one down is an expired TLS certificate. This script checks expiry for any list of domains and alerts before it is too late. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `ssl_expiry_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ssl_expiry_check.py -w 14 shop.example.com\n```\n\n**Example output**\n\n```text\n[ALERT] shop.example.com:443 expires in 9 days\n```\n\n## Explained\n\n### What it does\n\nIt connects to each host, reads the certificate the server actually presents, and works out how many days are left. If any cert is inside your warning window, it prints an ALERT and exits 1, so cron or a pipeline can act on it.\n\n### Why read the live cert\n\nChecking the real certificate the server serves catches problems a file on disk cannot: the wrong cert deployed, a missing intermediate, or a renewal that happened but was never reloaded. You test what the world sees.\n\n### How to use it\n\nPut your domains in a file and run it daily: `0 8 * * * python3 ssl_expiry_check.py -w 14 -f domains.txt`. Fourteen days of warning is plenty of time for even a manual renewal, and far better than an alert from an angry customer.\n\nThe attached version takes a warning window, reads domains from a file, supports host:port, and handles unreachable hosts gracefully instead of crashing.\n\nEven with auto-renewal like Let's Encrypt or Azure-managed certs, keep this running. Renewals fail quietly: port 80 closed, DNS moved, a reload that never happened. This is the backstop that catches it.\n\n## In one line\n\n> **Watch expiry like a hawk.** An expired cert is a full outage that a cron job prevents.\n\n**Daily check**\n\n```bash\n0 8 * * * python3 ssl_expiry_check.py -f domains.txt\n```\n\n## Script: ssl_expiry_check.py\n\nDownload: [scripts/ssl_expiry_check.py](../scripts/ssl_expiry_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"ssl_expiry_check.py - warn before a TLS certificate expires.\n\nConnects to each host, reads the certificate, and prints how many days are\nleft. Alerts (exit 1) if any cert expires within the warning window. Great\nfor a daily cron so a renewal never surprises you. Standard library only.\n\nUsage:\n    python3 ssl_expiry_check.py example.com shop.example.com\n    python3 ssl_expiry_check.py -w 21 -f domains.txt\n\ndomains.txt: one host (or host:port) per line.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport socket\nimport ssl\nimport sys\n\n\ndef days_left(host, port=443, timeout=5):\n    ctx = ssl.create_default_context()\n    with socket.create_connection((host, port), timeout=timeout) as sock:\n        with ctx.wrap_socket(sock, server_hostname=host) as ssock:\n            cert = ssock.getpeercert()\n    expires = dt.datetime.strptime(cert[\"notAfter\"], \"%b %d %H:%M:%S %Y %Z\")\n    return (expires - dt.datetime.utcnow()).days, expires.date()\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Check TLS certificate expiry.\")\n    ap.add_argument(\"hosts\", nargs=\"*\", help=\"host or host:port\")\n    ap.add_argument(\"-w\", \"--warn\", type=int, default=14, help=\"warn if days left <= this\")\n    ap.add_argument(\"-f\", \"--file\", help=\"file with one host per line\")\n    args = ap.parse_args()\n\n    hosts = list(args.hosts)\n    if args.file:\n        with open(args.file) as fh:\n            hosts += [ln.strip() for ln in fh if ln.strip() and not ln.startswith(\"#\")]\n    if not hosts:\n        ap.error(\"give at least one host, or -f a file\")\n\n    alerted = False\n    for entry in hosts:\n        host, _, port = entry.partition(\":\")\n        port = int(port) if port else 443\n        try:\n            left, when = days_left(host, port)\n        except Exception as exc:\n            print(f\"[ERR ] {host}:{port} - {exc}\")\n            alerted = True\n            continue\n        tag = \"ALERT\" if left <= args.warn else \"OK\"\n        if left <= args.warn:\n            alerted = True\n        print(f\"[{tag}] {host}:{port} expires in {left} days ({when})\")\n\n    sys.exit(1 if alerted else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p183","n":"183","slug":"9-azure-cli-commands-for-az-104","title":"9 Azure CLI commands for AZ-104","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","AZ104","CloudComputing","DevOps","CLI"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2027-11-28","files":{"devsecops":"notes/p183-9-azure-cli-commands-for-az-104.md"},"md":"# 9 Azure CLI commands for AZ-104\n\nAZ-104 is phase 2 of my plan, and the fastest way to learn Azure is the CLI, not clicking through the portal. Nine az commands cover most of the lab work, and they are repeatable, which the portal is not.\n\n## Quick reference\n\n### Set up — Sign in and scope\n\n```bash\n# Sign in. Add --use-device-code on a headless box.\naz login\n# Pick the subscription you are working in.\naz account set --subscription <id>\n# A resource group to hold your lab.\naz group create -n rg-lab -l westeurope\n```\n\n### Build — A tiny environment\n\n```bash\n# Create a VM with sensible defaults.\naz vm create -g rg-lab -n vm1 --image Ubuntu2404\n# A virtual network for your resources.\naz network vnet create -g rg-lab -n vnet1\n# Storage, with a globally unique name.\naz storage account create -g rg-lab -n st$RANDOM\n```\n\n### Inspect & clean — See it, then delete it\n\n```bash\n# Everything in the group, readable.\naz resource list -g rg-lab -o table\n# Public and private IPs at a glance.\naz vm list-ip-addresses -o table\n# Delete the group and stop the billing.\naz group delete -n rg-lab --yes\n```\n\n## Explained\n\n### Set up\n\n`az login` signs you in through the browser; add `--use-device-code` on a server with no browser.  \n`az account set --subscription <id>` picks the subscription, so you do not accidentally build in the wrong one. az account show confirms where you are.  \n`az group create -n rg-lab -l westeurope` makes a resource group, a container that lets you delete everything at once later.\n\n### Build\n\n`az vm create -g rg-lab -n vm1 --image Ubuntu2404` spins up a VM with sensible defaults and returns its public IP.  \naz network vnet create makes a virtual network, the backbone your resources sit in.  \naz storage account create needs a globally unique name, which is why `$RANDOM` is handy in a lab.\n\n### Inspect and clean\n\n`az resource list -g rg-lab -o table` shows everything in the group in a readable table. -o table is the flag that makes the CLI pleasant.  \n`az vm list-ip-addresses -o table` gives you IPs without hunting through the portal.  \n`az group delete -n rg-lab --yes` removes the whole group, which is the single most important habit on a free account: it stops the billing.\n\n> **The rule that saves money:** build in a resource group, and delete the group when the lab is done. A forgotten VM is the classic surprise on the Azure bill.\n\n## In one line\n\n> **Delete the lab when you finish.** A forgotten VM is the most common surprise on the bill.\n\n**Always after a lab**\n\n```bash\naz group delete -n rg-lab --yes --no-wait\n```\n"},{"id":"p184","n":"184","slug":"azure-blob-storage-tiers-hot-vs-cool-vs-archive","title":"Azure Blob storage tiers: Hot vs Cool vs Archive","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","AZ104","CloudStorage","CloudComputing","CostOptimization"],"series_name":"Cloud / DevSecOps","design":"matrix","script":"","date":"2027-11-30","files":{"devsecops":"notes/p184-azure-blob-storage-tiers-hot-vs-cool-vs-archive.md"},"md":"# Azure Blob storage tiers: Hot vs Cool vs Archive\n\nAzure Blob storage has access tiers, and picking the right one is both an AZ-104 topic and a real way to cut the cloud bill. The trade-off is always between storage cost and access cost.\n\n## Quick reference\n\n| | **Hot** | **Cool** | **Archive** |\n|:--|:--|:--|:--|\n| Storage cost | ❌ Highest | 🟡 Lower | ✅ Lowest |\n| Access cost | ✅ Lowest | 🟡 Higher | ❌ Highest |\n| Retrieval time | ✅ Instant | ✅ Instant | ❌ Hours (rehydrate) |\n| Min retention | None | 30 days | 180 days |\n| Best for | Live data | Backups, monthly | Compliance, cold |\n| Example | Website assets | Last quarter's logs | 7-year archives |\n\n**How to pick**\n\n- **Hot** → data you read often. Cheap to access, costly to store.\n- **Cool** → backups you rarely touch but may need soon.\n- **Archive** → cold data for years. Cheapest, but slow to get back.\n\n## Explained\n\nHot tier has the highest storage cost but the lowest access cost, and data is available instantly. It is for data you read often: website images, active app data, anything served to users right now.\n\nCool tier lowers the storage cost in exchange for higher access costs and a 30-day minimum retention. It suits backups and data you rarely touch but might need soon, like last quarter's logs. Reading it a lot in Cool can cost more than just keeping it Hot, so it is for genuinely infrequent access.\n\nArchive tier is the cheapest to store and the most expensive and slowest to read. Data is offline, and retrieving it means a rehydration that can take hours. The minimum retention is 180 days. It is for compliance copies and cold data you must keep for years but almost never open.\n\n### How to choose\n\nRead often: Hot. Rarely, but need it fast when you do: Cool. Keep for years, fine to wait: Archive.\n\nThe feature that makes it automatic: lifecycle management rules. You can move blobs Hot to Cool to Archive based on age, so old data drifts to cheaper tiers without anyone remembering to do it.\n\nThe two mistakes to avoid: frequently reading Cool or Archive data (the access cost bites), and leaving everything in Hot forever (the storage cost bites).\n\n## In one line\n\n> **Match the tier to the access.** Wrong tier means a surprise bill or a slow restore.\n\n**Set a lifecycle rule:** auto-move blobs Hot -> Cool -> Archive by age\n"},{"id":"p185","n":"185","slug":"anatomy-of-an-azure-resource-id","title":"Anatomy of an Azure resource ID","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","AZ104","CloudComputing","IAM","DevOps"],"series_name":"Cloud / DevSecOps","design":"anatomy","script":"","date":"2027-12-02","files":{"devsecops":"notes/p185-anatomy-of-an-azure-resource-id.md"},"md":"# Anatomy of an Azure resource ID\n\nEverything in Azure has a resource ID: one long path that uniquely identifies it. Once you can read it, the CLI, role assignments and policies all make more sense, because they all point at this path. It reads left to right, from broadest to most specific.\n\n## Quick reference\n\n```text\n/subscriptions/8f3a../resourceGroups/rg-prod/providers/Microsoft.Compute/virtualMachines/vm-web-01\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `/subscriptions/8f3a..` | Subscription | Which subscription pays for and owns it. |\n| `/resourceGroups/rg-prod` | Resource group | The container that holds related resources. |\n| `/providers/Microsoft.Compute` | Provider | The Azure service namespace, here compute. |\n| `/virtualMachines` | Type | The kind of resource within that provider. |\n| `/vm-web-01` | Name | The resource's own name. The part you chose. |\n\n## Explained\n\n`/subscriptions/8f3a..`. is the subscription. The top-level billing and ownership boundary. Every resource lives inside exactly one, and the GUID is how automation targets the right one.\n\n`/resourceGroups/rg-prod` is the resource group. A container for related resources that share a lifecycle, so you can deploy, manage and delete them together. Good naming here keeps an account readable.\n\n`/providers/Microsoft.Compute` is the resource provider, the Azure service namespace. Compute for VMs, Storage for storage accounts, Network for networking. This is Azure's catalogue of services.\n\n`/virtualMachines` is the resource type within that provider. A provider offers many types; this one is VMs.\n\n`/vm-web-01` is the resource name, the part you chose when you created it.\n\n### Why it matters in practice\n\nRole-based access control and Azure Policy are applied at a scope, and the scope is one of these path segments. Grant a role at the subscription and it covers everything; grant it at a resource group and it covers just that group; grant it at a single resource ID and it covers only that one. Reading the ID tells you exactly how broad a permission really is.\n\n## In one line\n\n> **One ID names everything.** Every CLI call, role and policy points at this path.\n\n**See an id**\n\n```bash\naz resource list -o tsv --query '[].id'\n```\n"},{"id":"p186","n":"186","slug":"6-azure-numbers-worth-knowing","title":"6 Azure numbers worth knowing","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","AZ104","CloudComputing","CostOptimization","CloudCareer"],"series_name":"Cloud / DevSecOps","design":"numbers","script":"","date":"2027-12-05","files":{"devsecops":"notes/p186-6-azure-numbers-worth-knowing.md"},"md":"# 6 Azure numbers worth knowing\n\nA few numbers make Azure, and the AZ-104 exam, much less intimidating. Six worth keeping in your head.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **$200** | Free account credit | Spend it on a 30-day lab, then use always-free services. | free |\n| **700** | AZ-104 pass score | Out of 1000. Scaled, so not a simple percentage. | exam |\n| **3** | Availability Zones | Most regions have 3, for surviving a datacenter failure. | HA |\n| **99.99%** | VM SLA (multi-AZ) | Spread VMs across zones to earn the higher SLA. | SLA |\n| **65+** | Always-free services | Services with a free tier beyond the 30-day credit. | free |\n| **5 min** | Delete a lab | The habit that keeps your free credit alive. | cost |\n\n## Explained\n\n$200, the free account credit. A new Azure account gives you $200 to spend in the first 30 days. That is more than enough for a full AZ-104 lab: VMs, networking, storage and identity. After the 30 days, you drop to the always-free services.\n\n700, the AZ-104 pass score. Microsoft exams are scored from 1 to 1000, and 700 is the pass mark. It is scaled, not a raw percentage, so do not assume 70% of questions. Practice assessments are the best gauge.\n\n3, availability zones. Most major Azure regions have three physically separate zones. Spreading resources across them is how you survive a single datacenter failure, and it is a core AZ-104 concept.\n\n99.99%, the VM SLA. You only get the top availability SLA when VMs are spread across zones. A single VM has a much lower guarantee. The architecture earns the SLA, not the VM type.\n\n65+, always-free services. Beyond the 30-day credit, Azure has dozens of services with a standing free tier, enough to keep learning without spending.\n\n5 minutes, to delete a lab. The single habit that protects your credit and your wallet: put everything in a resource group and delete it when you are done.\n\n> **The theme:** respect the free limits, and nothing on this list costs you money.\n\n## In one line\n\n> **Know the limits, keep the credit.** Most Azure bill shocks are a lab nobody deleted.\n\n**Guard the bill:** set a budget alert on day one\n"},{"id":"p187","n":"187","slug":"a-website-uptime-and-latency-checker-in-python","title":"A website uptime and latency checker in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","DevOps","Monitoring","SRE","Automation"],"series_name":"Cloud / DevSecOps","design":"code","script":"url_uptime.py","date":"2027-12-07","files":{"it":"notes/p187-a-website-uptime-and-latency-checker-in-python.md","devsecops":"notes/p187-a-website-uptime-and-latency-checker-in-python.md"},"md":"# A website uptime and latency checker in Python\n\nMonitoring runs through the whole plan, from Linux to Kubernetes, so here is a tiny uptime checker you can run anywhere Python exists. It requests each URL, reports the status code and response time, and alerts on failures or slow responses. Full file attached, no dependencies.\n\n## Quick reference\n\nThe full script is at the end of this note: `url_uptime.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 url_uptime.py https://shop.example.com\n```\n\n**Example output**\n\n```text\n[OK] https://shop.example.com 200 in 182 ms\n```\n\n## Explained\n\n### What it does\n\nFor each URL it records the HTTP status and how long the request took. If the status is an error, or the response is slower than your threshold, it flags it and exits 1, so cron or a pipeline treats it as a failure.\n\n### Why this beats a simple ping\n\nping only tells you a server answers ICMP. It says nothing about whether the website actually works. This checks the real endpoint, the page or health route users hit, so a 500 error or a 5-second page both show up.\n\n### How to use it\n\nKeep your URLs in a file and run it every few minutes: `*/5 * * * * python3 url_uptime.py -s 2.0 -f urls.txt`. Point it at a real health endpoint, not just the homepage, so it exercises the app and not just the web server.\n\nThe attached version adds a slow threshold, reads URLs from a file, sets a sane User-Agent, and handles timeouts and connection errors cleanly instead of crashing on the first one.\n\nIt is not a replacement for a full monitoring stack, but it is a dependency-free check you can drop on any box in two minutes, and it has caught real outages for me before a customer noticed.\n\n## In one line\n\n> **Check the thing users actually use.** Ping the real page, not just the server.\n\n**Watch your sites**\n\n```bash\n*/5 * * * * python3 url_uptime.py -f urls.txt\n```\n\n## Script: url_uptime.py\n\nDownload: [scripts/url_uptime.py](../scripts/url_uptime.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"url_uptime.py - check that your sites and APIs are up and fast.\n\nRequests each URL, reports the status code and response time, and alerts\n(exit 1) on any failure or slow response. No dependencies, so it runs on any\nserver with Python. Good as a cron check or a simple status page feeder.\n\nUsage:\n    python3 url_uptime.py https://shop.example.com https://api.example.com/health\n    python3 url_uptime.py -s 2.0 -f urls.txt\n\n-s is the slow threshold in seconds.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nimport time\nimport urllib.request\n\n\ndef check(url, timeout=10):\n    req = urllib.request.Request(url, headers={\"User-Agent\": \"s0sta-uptime/1.0\"})\n    start = time.perf_counter()\n    with urllib.request.urlopen(req, timeout=timeout) as resp:\n        resp.read(1)  # touch the body\n        code = resp.status\n    return code, time.perf_counter() - start\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Check URL uptime and latency.\")\n    ap.add_argument(\"urls\", nargs=\"*\")\n    ap.add_argument(\"-s\", \"--slow\", type=float, default=2.0, help=\"slow threshold seconds\")\n    ap.add_argument(\"-f\", \"--file\", help=\"file with one URL per line\")\n    args = ap.parse_args()\n\n    urls = list(args.urls)\n    if args.file:\n        with open(args.file) as fh:\n            urls += [ln.strip() for ln in fh if ln.strip() and not ln.startswith(\"#\")]\n    if not urls:\n        ap.error(\"give at least one URL, or -f a file\")\n\n    bad = False\n    for url in urls:\n        try:\n            code, secs = check(url)\n        except Exception as exc:\n            print(f\"[DOWN] {url} - {exc}\")\n            bad = True\n            continue\n        slow = secs > args.slow\n        tag = \"OK\" if 200 <= code < 400 and not slow else (\"SLOW\" if slow else \"WARN\")\n        if tag != \"OK\":\n            bad = True\n        print(f\"[{tag}] {url} {code} in {secs * 1000:.0f} ms\")\n\n    sys.exit(1 if bad else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p188","n":"188","slug":"nsg-vs-azure-firewall","title":"NSG vs Azure Firewall","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Azure","AZ104","NetworkSecurity","CloudComputing","CyberSecurity"],"series_name":"Cloud / DevSecOps","design":"compare","script":"","date":"2027-12-09","files":{"devsecops":"notes/p188-nsg-vs-azure-firewall.md"},"md":"# NSG vs Azure Firewall\n\nAzure gives you two very different tools for controlling network traffic, and AZ-104 expects you to know when each applies. They are not either/or; most real designs use both.\n\n## Quick reference\n\n| | **NSG** | **AZ FIREWALL** |\n|:--|:--|:--|\n| Layer | 3-4 (IP, port) | 3-7 (incl. FQDN) |\n| Scope | Subnet or NIC | Whole virtual network |\n| Cost | Free | Paid, per hour + data |\n| Filter by domain | No | Yes (FQDN rules) |\n| Threat intel | No | Yes |\n| Best for | Basic segmentation | Central egress control |\n\n**NSG** (Network Security Group): Free, basic allow/deny rules attached to subnets and NICs.\n\nExample: `allow 443 from 10.0.0.0/16`\n\n**AZ FIREWALL** (Managed firewall service): A paid, managed firewall for the whole network, with more features.\n\nExample: `allow *.windowsupdate.com`\n\n- NSG → free, basic rules on every subnet. Always use them.\n- Azure Firewall → central, advanced control when you need it.\n\n## Explained\n\nA Network Security Group (NSG) is a set of basic allow and deny rules, evaluated by priority, that you attach to a subnet or a network interface. It filters by source and destination IP, port and protocol, which is layers 3 and 4. It is free, built in, and the right way to do basic segmentation: let the web subnet reach the database subnet on 5432, and nothing else. There is no reason not to use NSGs everywhere.\n\nAzure Firewall is a managed, stateful firewall service for an entire virtual network. It does everything an NSG does and more: filtering by fully qualified domain name (allow traffic to \\*.windowsupdate.com but not the wider internet), threat intelligence feeds, and central logging. It is a paid service, billed per hour plus data processed, and it usually sits at the edge of a hub network to control all outbound traffic.\n\n### How to choose\n\nNSGs for basic, free, per-subnet rules. Use them as the baseline on every network.  \nAzure Firewall when you need central control, especially outbound: filtering which domains your VMs may reach, applying threat intelligence, and logging everything in one place.\n\nThe common pattern: NSGs on every subnet for segmentation, and an Azure Firewall in a hub for centralised egress filtering. The NSG is your fence between rooms; the firewall is the guarded main gate.\n\nFor the exam, remember: NSG is L3-L4 and free; Firewall adds FQDN filtering and threat intel for a cost.\n"},{"id":"p189","n":"189","slug":"how-a-request-reaches-a-vm-in-azure","title":"How a request reaches a VM in Azure","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","AZ104","Networking","CloudComputing","CyberSecurity"],"series_name":"Cloud / DevSecOps","design":"pipeline","script":"","date":"2027-12-12","files":{"devsecops":"notes/p189-how-a-request-reaches-a-vm-in-azure.md"},"md":"# How a request reaches a VM in Azure\n\nWhen you cannot reach a VM in Azure, the problem is almost always one specific layer between the user and the app. Knowing the path turns guesswork into a checklist, and it is core AZ-104 networking.\n\n## Quick reference\n\n1. **DNS resolves** — The name points to a public IP or load balancer. · *DNS*\n2. **Public IP / Load Balancer** — Traffic enters the Azure network here. · *frontend*\n3. **NSG on the subnet** — Allow/deny rules filter by IP and port. · *NSG*\n4. **NSG on the NIC** — A second NSG can filter per VM. · *NIC rules*\n5. **The VM's OS firewall** — ufw or firewalld still applies inside. · `host firewall`\n6. **The app listens** — Only now does your service see the request. · `:443`\n\n## Explained\n\n1. **DNS resolves.** The domain name points to a public IP or a load balancer's address. A wrong or missing DNS record means nothing after this matters, and it is the first thing to check.\n\n2. **Public IP or Load Balancer.** This is where traffic enters the Azure network. A load balancer also has its own health probes and rules; if the backend is marked unhealthy, it stops sending traffic there.\n\n3. **NSG on the subnet.** The first set of allow and deny rules, applied to the whole subnet. If there is no rule allowing your port from the source, the packet is dropped here, silently.\n\n4. **NSG on the NIC.** Azure lets you attach a second NSG directly to a VM's network interface. Both the subnet and NIC NSGs must allow the traffic, which catches people out: the subnet allows it, but the NIC does not.\n\n5. **The VM's OS firewall.** Inside the VM, ufw or firewalld still applies. The Azure layers can all say yes and the host firewall still say no.\n\n6. **The app listens.** Only now does your service actually receive the request, and only if it is bound to the right address and port.\n\nThe debugging order: DNS, then the load balancer health, then both NSGs, then the host firewall, then the app. Walk it in order and the broken gate reveals itself.\n\n## In one line\n\n> **Many gates, one request.** 'Can't reach the VM' is one of these layers saying no.\n\n**Trace the layers:** check DNS, then NSG rules, then the host firewall\n"},{"id":"p190","n":"190","slug":"9-checks-to-secure-an-azure-subscription","title":"9 checks to secure an Azure subscription","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Azure","AZ104","CloudSecurity","CyberSecurity","CloudGovernance"],"series_name":"Cloud / DevSecOps","design":"checklist","script":"","date":"2027-12-14","files":{"devsecops":"notes/p190-9-checks-to-secure-an-azure-subscription.md"},"md":"# 9 checks to secure an Azure subscription\n\nSecuring an Azure subscription is both an AZ-104 topic and the thing that keeps a cloud account from becoming a headline. Nine checks, grouped by what attackers and surprise bills actually exploit.\n\n## Quick reference\n\n### Identity\n\n- **MFA for every user** — Conditional Access\n- **Few Global Admins** — and use PIM for just-in-time\n- **No standing owner on resources** — grant roles, not ownership\n\n### Governance\n\n- **Budget + cost alerts** — on day one\n- **Azure Policy baseline** — deny public IPs, enforce tags\n- **Resource locks on prod** — CanNotDelete on key groups\n\n### Visibility\n\n- **Defender for Cloud on** — free tier at least\n- **Activity logs retained** — and exported\n- **NSG flow logs** — see what is really talking\n\n## Explained\n\n### Identity\n\nMFA for every user, enforced through Conditional Access. Identity is the real perimeter in the cloud; a password alone is not enough for anyone.  \nFew Global Admins, and use Privileged Identity Management so even admins get their rights just-in-time, not permanently. Fewer standing admins, smaller blast radius.  \nNo standing ownership on resources. Grant specific roles at the right scope instead of handing out Owner, which can do anything including grant more access.\n\n### Governance\n\nBudget and cost alerts on day one. A compromised account mining crypto, or a forgotten cluster, both show up on the bill first.  \nAn Azure Policy baseline: deny public IPs where they are not needed, enforce tags, restrict regions. Policy stops mistakes before they happen.  \nResource locks on production. A CanNotDelete lock on critical resource groups prevents an accidental deletion.\n\n### Visibility\n\nMicrosoft Defender for Cloud on, at least the free tier, for a secure-score and recommendations.  \nActivity logs retained and exported, so you can investigate later. You cannot analyse what you never recorded.  \nNSG flow logs, to see what is actually talking to what on the network.\n\n> **The theme is simple:** least privilege on identity, guardrails on governance, and enough logging to see trouble.\n\n## In one line\n\n> **Least privilege, full visibility.** Identity and cost are where cloud accounts get hurt.\n\n**First two moves:** MFA everywhere, then a budget alert\n"},{"id":"p191","n":"191","slug":"azure-cost-at-a-glance","title":"Azure cost at a glance","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","CloudComputing","FinOps","CostOptimization","CloudCareer"],"series_name":"Cloud / DevSecOps","design":"dashboard","script":"","date":"2027-12-16","files":{"devsecops":"notes/p191-azure-cost-at-a-glance.md"},"md":"# Azure cost at a glance\n\nCloud bills do not explode from one big thing. They drift upward from small, forgotten resources. Watching cost like any other metric is part of running Azure well, and it is cheaper than the alternative. Here is the view that matters.\n\n## Quick reference\n\n**Status:** This month, so far\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| SPEND | **$142** | month to date |\n| FORECAST | **$310** | projected by month end |\n| Budget used | **62%** | of a $500 budget |\n| Top service | **VMs** | 48% of the bill |\n| Idle resources | **3** | unused disks + IPs |\n| Alert at | **80%** | email before you blow it |\n\n## Explained\n\nSpend, month to date. What you have used so far. On its own it is just a number; it needs the next tile for meaning.\n\nForecast. Azure projects the month-end total from your current run rate. This is the early warning: if the forecast is above your budget halfway through the month, act now, not on the last day.\n\nBudget used. The percentage of your budget consumed. Set a real budget per subscription or resource group, because a budget is what turns cost into something you can alert on.\n\nTop service. Knowing which service dominates the bill tells you where to look. Usually it is compute, VMs or containers, which is where right-sizing and shutting down idle resources pays off most.\n\nIdle resources. The quiet bill-builders: unattached managed disks, unused public IP addresses, old snapshots, stopped-but-not-deallocated VMs. These cost money while doing nothing. Azure Advisor flags many of them.\n\nAlert at 80%. A budget alert that emails you before you hit the limit, not after. This single setting prevents most bill shocks.\n\nThe habit: a budget with an alert on day one, and a monthly look at idle resources. Treat cost as a signal, like latency or errors.\n\n## In one line\n\n> **Watch the cost like a metric.** Idle disks, old snapshots and public IPs quietly add up.\n\n**The first step:** create a budget with an 80% alert\n"},{"id":"p192","n":"192","slug":"a-self-cleaning-backup-script-in-python","title":"A self-cleaning backup script in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","DevOps","Backup","Linux","Automation"],"series_name":"Cloud / DevSecOps","design":"code","script":"backup_rotate.py","date":"2027-12-19","files":{"it":"notes/p192-a-self-cleaning-backup-script-in-python.md","devsecops":"notes/p192-a-self-cleaning-backup-script-in-python.md"},"md":"# A self-cleaning backup script in Python\n\nA backup that fills the disk is a backup that eventually fails. This script makes a compressed, timestamped archive of a folder and then deletes the oldest ones so only the newest N remain. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `backup_rotate.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 backup_rotate.py /var/www /backups --keep 7\n```\n\n**Example output**\n\n```text\n[OK] created www-20261019-020001.tar.gz (84.2 MB)\n[OK] keeping 7 most recent backups\n```\n\n## Explained\n\n### What it does\n\nIt creates prefix-YYYYMMDD-HHMMSS.tar.gz in your backup folder, then lists all backups with that prefix, sorted oldest to newest, and removes everything except the most recent N. The naming means they sort chronologically by default, which is why the cleanup is one line.\n\n### Why it is useful\n\nMost people set up a backup and forget the cleanup, so the disk fills months later at the worst time. This handles both halves: make the backup, and prune the old ones, every run.\n\n### How to use it\n\n`0 2 * * * python3 backup_rotate.py /var/www /backups --keep 7` gives you a week of nightly backups that never grow without bound. Change the path and `--keep` for different folders and retention.\n\nThe attached version takes the keep count and a custom prefix, creates the destination if needed, prints the size, and lists what it removed, so your log shows exactly what happened each night.\n\nOne honest note: this is a local backup. Follow the 3-2-1 rule and also copy these archives off the server, to object storage or another machine, so a dead disk does not take the backups with it. And test a restore, because a backup you have never opened is only a hope.\n\n## In one line\n\n> **Backups that clean up after themselves.** No full disk, no manual pruning, no forgotten old files.\n\n**Run it nightly**\n\n```bash\n0 2 * * * python3 backup_rotate.py /var/www /backups --keep 7\n```\n\n## Script: backup_rotate.py\n\nDownload: [scripts/backup_rotate.py](../scripts/backup_rotate.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"backup_rotate.py - timestamped backups that clean up after themselves.\n\nCreates a compressed tar.gz of a source folder into a backup folder, named\nwith the date and time, then deletes the oldest backups so only the newest N\nremain. Standard library only, so it runs anywhere Python does.\n\nUsage:\n    python3 backup_rotate.py /var/www /backups\n    python3 backup_rotate.py /etc /backups --keep 14 --prefix etc\n\nCron (daily at 02:00):\n    0 2 * * * /usr/bin/python3 /opt/scripts/backup_rotate.py /var/www /backups --keep 7\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport sys\nimport tarfile\nfrom pathlib import Path\n\n\ndef make_backup(src, dest_dir, prefix):\n    stamp = dt.datetime.now().strftime(\"%Y%m%d-%H%M%S\")\n    out = dest_dir / f\"{prefix}-{stamp}.tar.gz\"\n    with tarfile.open(out, \"w:gz\") as tar:\n        tar.add(src, arcname=Path(src).name)\n    return out\n\n\ndef rotate(dest_dir, prefix, keep):\n    backups = sorted(dest_dir.glob(f\"{prefix}-*.tar.gz\"))\n    removed = []\n    while len(backups) > keep:\n        old = backups.pop(0)\n        old.unlink()\n        removed.append(old.name)\n    return removed\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Create and rotate tar.gz backups.\")\n    ap.add_argument(\"source\", help=\"folder to back up\")\n    ap.add_argument(\"dest\", help=\"folder to store backups in\")\n    ap.add_argument(\"--keep\", type=int, default=7, help=\"how many backups to keep\")\n    ap.add_argument(\"--prefix\", help=\"name prefix, default = source folder name\")\n    args = ap.parse_args()\n\n    src = Path(args.source)\n    if not src.exists():\n        sys.exit(f\"source not found: {src}\")\n    dest = Path(args.dest)\n    dest.mkdir(parents=True, exist_ok=True)\n    prefix = args.prefix or src.name\n\n    out = make_backup(str(src), dest, prefix)\n    size_mb = out.stat().st_size / 1024 / 1024\n    print(f\"[OK] created {out.name} ({size_mb:.1f} MB)\")\n\n    removed = rotate(dest, prefix, args.keep)\n    for name in removed:\n        print(f\"[--] removed old backup {name}\")\n    print(f\"[OK] keeping {args.keep} most recent backups\")\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p193","n":"193","slug":"anatomy-of-a-terraform-resource-block","title":"Anatomy of a Terraform resource block","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","Azure"],"series_name":"Cloud / DevSecOps","design":"anatomy","script":"","date":"2027-12-21","files":{"devsecops":"notes/p193-anatomy-of-a-terraform-resource-block.md"},"md":"# Anatomy of a Terraform resource block\n\nEvery Terraform configuration is built from resource blocks, and once you can read the first line of one, the rest of HCL falls into place. A block has four parts.\n\n## Quick reference\n\n```text\nresource \"azurerm_linux_vm\" \"web\" { ... }\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `resource` | Block type | Declares something Terraform will create and manage. |\n| `\"azurerm_linux_vm\"` | Resource type | Provider name + what to make. Sets the arguments allowed. |\n| `\"web\"` | Local name | Your label in code: azurerm_linux_vm.web.id. Not the cloud name. |\n| `{ ... }` | The body | Arguments inside: name, size, location, tags. |\n\n## Explained\n\nresource is the block type. It tells Terraform this is something to create and manage in the real world, as opposed to a data source (which only reads) or a variable. It is the most common block you will write.\n\n\"azurerm_linux_virtual_machine\" is the resource type. The first part names the provider (azurerm for Azure, aws for AWS), and the rest names what to build. The type decides which arguments are valid inside the body, which is why the docs are organised by resource type.\n\n\"web\" is the local name, your label for this resource within the configuration. It is how you reference the resource elsewhere, like azurerm_linux_virtual_machine.web.id. Important: this is not the name of the resource in the cloud. The cloud name is an argument inside the body. People mix these up constantly.\n\n{ ... } is the body, where the arguments live: the actual name, size, location, network and tags. This is the configuration itself.\n\n### Why the type-and-name pair matters\n\nTogether, the type and local name form a unique address inside your configuration, and that address is how everything connects. A subnet references its virtual network by address; an output exports a resource's attribute by address. State tracks resources by this address too, which is why renaming one can make Terraform want to destroy and recreate it.\n\n## In one line\n\n> **Type plus name is the address.** That pair is how every other resource refers to this one.\n\n**See the plan**\n\n```bash\nterraform plan -out=tfplan\n```\n"},{"id":"p194","n":"194","slug":"9-docker-commands-for-building-and-shipping-images","title":"9 docker commands for building and shipping images","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","DevOps","Containers","CICD","CloudNative"],"series_name":"Cloud / DevSecOps","design":"terminal","script":"","date":"2027-12-23","files":{"devsecops":"notes/p194-9-docker-commands-for-building-and-shipping-images.md"},"md":"# 9 docker commands for building and shipping images\n\nBefore Kubernetes, you need to be fluent with images: building, tagging, shipping and cleaning them up. Nine docker commands cover the whole loop.\n\n## Quick reference\n\n### Build and tag\n\n```bash\n# build from the Dockerfile here, with a tag\ndocker build -t api:1.4.2 .\n# add a second name, pointing at a registry\ndocker tag api:1.4.2 registry.io/team/api:1.4.2\n# rebuild from scratch when cache hides a change\ndocker build --no-cache -t api:1.4.2 .\n```\n\n### Push and pull\n\n```bash\n# authenticate to a private registry\ndocker login registry.io\n# upload the image\ndocker push registry.io/team/api:1.4.2\n# fetch it on the server\ndocker pull registry.io/team/api:1.4.2\n```\n\n### See inside, free space\n\n```bash\n# every layer and its size. Find the fat one\ndocker history api:1.4.2\n# the exact digest, for pinning\ndocker image inspect api:1.4.2 --format '{{.Id}}'\n# delete unused images. Reclaim the disk\ndocker image prune -a\n```\n\n## Explained\n\n### Build and tag\n\n`docker build -t api:1.4.2 .` builds from the Dockerfile in the current directory and gives the image a name and tag. Always tag with a real version, never leave it as latest.  \n`docker tag api:1.4.2 registry.io/team/api:1.4.2` adds a second name pointing at your registry. The same image can carry several tags.  \n`docker build --no-cache` forces a clean rebuild when a cached layer is hiding a change, like an apt package that updated upstream.\n\n### Push and pull\n\ndocker login authenticates to a private registry.  \ndocker push uploads the image and prints its digest, the content hash that identifies it exactly.  \ndocker pull fetches it on the target server.\n\n### Inspect and clean\n\n`docker history api:1.4.2` shows every layer and its size. This is how you find the one bloated layer to optimise.  \n`docker image inspect --format '{{.Id}}'` gives the exact digest, which you use to deploy by digest instead of a movable tag.  \n`docker image prune -a` deletes images nothing is using. On a busy build server, images and build cache quietly eat the disk; this reclaims gigabytes.\n\nThe two habits that matter: tag deliberately so you always know what you shipped, and prune on a schedule so the disk does not fill. docker system df shows how much space images, containers and cache are using right now.\n\n## In one line\n\n> **Tag on purpose. Prune on schedule.** Images pile up fast and quietly fill the disk.\n\n**Try it**\n\n```bash\ndocker image prune -a\n```\n\n**Pin what you ship**\n\n```bash\ndocker build -t api:1.4.2 . && docker push ...\n```\n"},{"id":"p195","n":"195","slug":"fix-a-failing-github-actions-job","title":"Fix: a failing GitHub Actions job","sections":["devsecops"],"topic":"Git & CI/CD","tags":["CICD","GitHubActions","DevOps","Troubleshooting","Automation"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2027-12-26","files":{"devsecops":"notes/p195-fix-a-failing-github-actions-job.md"},"md":"# Fix: a failing GitHub Actions job\n\nA red X on your pipeline looks alarming and is usually a five-minute fix, because CI failures are loud: the log names the step and the error. Here is how to read it and move on.\n\n## Quick reference\n\n### Diagnose — Read the log, top to bottom\n\n- **Open the failed step** — The red step, not the whole job. The error is there.\n- **Reproduce locally** — Run the exact command on your machine.\n- **Check the runner** — ubuntu-latest can differ from your laptop.\n\n### Fix — The usual causes\n\n- **Lockfile out of sync** — Commit `package-lock.json`. npm ci needs it exact.\n- **Missing secret or env** — Set it in repo Settings > Secrets, not in code.\n- **Wrong versions** — Pin node/python versions in the workflow.\n\n### Prevent — Fewer red X's\n\n- **Pin action versions** — uses: actions/checkout@v4, not @main.\n- **Cache dependencies** — Faster, and fails less on network blips.\n- **Run the same checks locally** — pre-commit hooks catch it before CI.\n\n## Explained\n\n### Diagnose\n\nOpen the failed step, not the whole job. GitHub Actions marks the exact step that failed in red. Expand it and read the last 20 lines; the real error is almost always there, above the generic \"exit code 1\".  \nReproduce locally. Run the exact command from the workflow on your machine. Most failures reproduce immediately, and local debugging is far faster than pushing commits to re-trigger CI.  \nCheck the runner. ubuntu-latest is not your laptop. Different OS, different default versions, a clean checkout with no local files. Many \"works on my machine\" failures live here.\n\n### Fix\n\nLockfile out of sync. npm ci (and its equivalents) demand that the lockfile matches `package.json` exactly. Commit the updated `package-lock.json`.  \nMissing secret or environment variable. Set it in the repository's Settings, Secrets, not in the code, and reference it in the workflow. A blank secret is a common silent failure.  \nWrong versions. Pin the Node or Python version in the workflow so CI matches what you tested.\n\n### Prevent\n\nPin action versions: uses: actions/checkout@v4, not @main, so an upstream change cannot break your build overnight.  \nCache dependencies, which makes runs faster and less fragile on network hiccups.  \nRun the same checks locally with pre-commit hooks, so most failures never reach CI.\n\nThe habit: read the failed step first. The log already told you what broke.\n\n## In one line\n\n> **The log already told you.** Open the failed step, read the error, reproduce it locally.\n\n**The first move:** open the red step and read the last 20 lines\n"},{"id":"p196","n":"196","slug":"alpine-vs-debian-base-images","title":"Alpine vs Debian base images","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","CloudNative","CICD"],"series_name":"Cloud / DevSecOps","design":"compare","script":"","date":"2027-12-28","files":{"devsecops":"notes/p196-alpine-vs-debian-base-images.md"},"md":"# Alpine vs Debian base images\n\nChoosing a base image is one of the first decisions in a Dockerfile, and the Alpine-versus-Debian debate comes up every time. Both are good; they trade size for compatibility.\n\n## Quick reference\n\n| | **ALPINE** | **DEBIAN** |\n|:--|:--|:--|\n| Size | ~5 MB base | ~30 MB (slim) |\n| C library | musl | glibc (standard) |\n| Package manager | apk | apt |\n| Compatibility | Occasional issues | Just works |\n| Build pain | Sometimes high | Low |\n| Best for | Small, simple apps | Most apps, especially Python |\n\n**ALPINE** (Tiny base image): A few MB. Smaller images, smaller attack surface, faster pulls.\n\nExample: `FROM python:3.12-alpine`\n\n**DEBIAN** (Standard base (slim)): Bigger, but familiar, compatible, and painless to build on.\n\nExample: `FROM python:3.12-slim`\n\n- Alpine → smallest images, when your deps cooperate.\n- Debian slim → the safe default. Small enough, no surprises.\n\n## Explained\n\nAlpine is tiny, a base of about 5 MB, because it uses musl libc and busybox instead of the standard GNU tools. Smaller images mean faster pulls, less storage, and a smaller attack surface, which security people love. Its package manager is apk.\n\nDebian slim is the trimmed-down Debian base, around 30 MB. It uses glibc, the standard C library almost everything is built and tested against, and apt for packages. It is bigger than Alpine but still far smaller than the full image, and it rarely surprises you.\n\n### The catch with Alpine\n\nBecause Alpine uses musl instead of glibc, some software behaves differently or needs to be recompiled. In Python especially, packages with C extensions (data science, image processing, database drivers) may have no prebuilt Alpine wheel, so pip compiles from source. That makes builds slow and sometimes fails outright, and it can introduce subtle runtime differences. The small image you wanted ends up costing hours of build pain.\n\n### How to choose\n\nAlpine when the app is simple, the dependencies are pure or known to work, and image size really matters.  \nDebian slim as the default, especially for Python apps with compiled dependencies. It is small enough for almost everyone and it just works.\n\nA good middle path: start on Debian slim, ship, and only move to Alpine later if image size becomes a real problem and your dependencies cooperate. Measure, do not assume.\n\nSmaller is not always better.\n"},{"id":"p197","n":"197","slug":"a-secret-scanner-in-python-for-your-ci","title":"A secret scanner in Python for your CI","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DevSecOps","CyberSecurity","CICD","Git"],"series_name":"Cloud / DevSecOps","design":"code","script":"secret_scanner.py","date":"2027-12-30","files":{"devsecops":"notes/p197-a-secret-scanner-in-python-for-your-ci.md"},"md":"# A secret scanner in Python for your CI\n\nDevSecOps means shifting security left, catching problems before they ship. This script scans a folder for likely secrets, API keys, tokens, passwords, private keys, so you catch a leak before it reaches git, where it lives forever. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `secret_scanner.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 secret_scanner.py .\n```\n\n**Example output**\n\n```text\n[!] config/settings.py:7  Generic API key\n[FAIL] 1 potential secret(s) found\n```\n\n## Explained\n\n### What it does\n\nIt walks the folder, reads text files, and flags lines matching patterns for common secret formats: AWS access keys, generic api_key or token assignments, Slack tokens, private key blocks. It skips binaries and folders like .git and node_modules, and exits 1 when it finds something, so it can fail a CI step or a pre-commit hook.\n\n### Why it matters\n\nA secret committed to git is not fixed by deleting it in the next commit; it stays in the history, and anyone with the repo can recover it. The only real fix is to rotate the key. Catching it before the push avoids all of that.\n\n### How to use it\n\nLocally before a commit, or as a CI step: run it, and if it exits non-zero, block the merge. `python3 secret_scanner.py .` scans the current project.\n\nThe attached version has more patterns, skips the folders and file types that cause noise, and prints the file, line number and a snippet so you can jump straight to it.\n\n> **Honest caveat:** this is a first line of defence, not a complete solution. Dedicated tools like gitleaks or trufflehog go deeper, and a pre-commit hook plus branch protection makes it hard to bypass. But a dependency-free script you fully understand is a great place to start.\n\n## In one line\n\n> **Catch the leak before the push.** A key in git history is a key you must rotate.\n\n**Add it to ci:** run it as a step; exit 1 fails the build\n\n## Script: secret_scanner.py\n\nDownload: [scripts/secret_scanner.py](../scripts/secret_scanner.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"secret_scanner.py - find leaked secrets before you commit them.\n\nWalks a folder and flags lines that look like API keys, passwords, tokens or\nprivate keys. Run it before a push, or in CI to block a commit. It is a\nfirst line of defence, not a replacement for a full scanner. Stdlib only.\n\nUsage:\n    python3 secret_scanner.py .\n    python3 secret_scanner.py /path/to/repo\n\nExit code is 1 if anything is found, so it fails a CI step.\n\nBy s0sta.com\n\"\"\"\nimport re\nimport sys\nfrom pathlib import Path\n\nPATTERNS = {\n    \"AWS access key\": re.compile(r\"AKIA[0-9A-Z]{16}\"),\n    \"Private key block\": re.compile(r\"-----BEGIN (?:RSA |EC |OPENSSH )?PRIVATE KEY-----\"),\n    \"Generic API key\": re.compile(r\"(?i)(api[_-]?key|secret|token)\\s*[:=]\\s*['\\\"][0-9a-zA-Z/+_\\-]{16,}['\\\"]\"),\n    \"Password assignment\": re.compile(r\"(?i)password\\s*[:=]\\s*['\\\"][^'\\\"]{6,}['\\\"]\"),\n    \"Slack token\": re.compile(r\"xox[baprs]-[0-9A-Za-z-]{10,}\"),\n}\nSKIP_DIRS = {\".git\", \"node_modules\", \"venv\", \".venv\", \"__pycache__\", \"dist\", \"build\"}\nSKIP_EXT = {\".png\", \".jpg\", \".jpeg\", \".gif\", \".pdf\", \".zip\", \".gz\", \".lock\"}\n\n\ndef scan_file(path):\n    hits = []\n    try:\n        text = path.read_text(errors=\"ignore\")\n    except OSError:\n        return hits\n    for i, line in enumerate(text.splitlines(), 1):\n        for name, rx in PATTERNS.items():\n            if rx.search(line):\n                hits.append((i, name, line.strip()[:80]))\n    return hits\n\n\ndef main():\n    root = Path(sys.argv[1] if len(sys.argv) > 1 else \".\")\n    found = 0\n    for path in root.rglob(\"*\"):\n        if path.is_dir() or path.suffix in SKIP_EXT:\n            continue\n        if any(part in SKIP_DIRS for part in path.parts):\n            continue\n        for lineno, name, snippet in scan_file(path):\n            found += 1\n            print(f\"[!] {path}:{lineno}  {name}\\n    {snippet}\")\n    print(f\"\\n{'[FAIL]' if found else '[OK]'} {found} potential secret(s) found\")\n    sys.exit(1 if found else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p198","n":"198","slug":"how-a-github-actions-workflow-is-structured","title":"How a GitHub Actions workflow is structured","sections":["devsecops"],"topic":"Git & CI/CD","tags":["GitHubActions","CICD","DevOps","Automation","Git"],"series_name":"Cloud / DevSecOps","design":"stack","script":"","date":"2028-01-02","files":{"devsecops":"notes/p198-how-a-github-actions-workflow-is-structured.md"},"md":"# How a GitHub Actions workflow is structured\n\nA GitHub Actions workflow looks like a wall of YAML until you see that it is just levels nested inside each other. Here is the structure, top to bottom.\n\n## Quick reference\n\n*Top to bottom: each level contains the next*\n\n1. **Workflow** — One YAML file in .github/workflows. The whole pipeline. · `ci.yml`\n2. **Trigger (on:)** — What starts it: push, pull_request, schedule. · `on: push`\n3. **Jobs** — Independent units. They run in parallel by default. · `jobs:`\n4. **Runner** — The machine each job runs on. A fresh VM every time. · *runs-on*\n5. **Steps** — The ordered commands in a job: actions or run. · `steps:`\n6. **Actions** — Reusable building blocks, pinned by version. · `uses: @v4`\n\n## Explained\n\nWorkflow. One YAML file in .github/workflows/, like `ci.yml`. It is the whole pipeline, and a repo can have many, each for a different purpose: tests, deploys, scheduled jobs.\n\nTrigger (on:). What starts the workflow: a push, a pull_request, a schedule (cron), a manual workflow_dispatch, or an event from another workflow. This is the when.\n\nJobs. A workflow contains one or more jobs. By default they run in parallel, on separate machines, which is a common surprise. Use needs: to make one job wait for another.\n\nRunner (runs-on:). Each job runs on a runner, a fresh virtual machine such as ubuntu-latest, or a self-hosted one. Fresh means no leftover state from last time, which is why you install dependencies every run.\n\nSteps. Inside a job, steps run in order, top to bottom, on the same runner. A step is either a shell command (run:) or an action (uses:).\n\nActions. Reusable building blocks that steps call, like actions/checkout to get your code or actions/setup-node to install Node. Pin them to a version (@v4), never @main, so an upstream change cannot break your build.\n\nThe nesting in one line: a workflow has jobs, a job runs on a runner and contains steps, and a step can use an action. Once that clicks, reading and writing CI becomes straightforward.\n\n## In one line\n\n> **Workflow, job, step, action.** Know the nesting and the YAML stops being a mystery.\n\n**Start small:** one workflow, one job, a few steps\n"},{"id":"p199","n":"199","slug":"5-dockerfile-mistakes-to-avoid","title":"5 Dockerfile mistakes to avoid","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","Containers","DevOps","CICD","BestPractices"],"series_name":"Cloud / DevSecOps","design":"dontdo","script":"","date":"2028-01-04","files":{"devsecops":"notes/p199-5-dockerfile-mistakes-to-avoid.md"},"md":"# 5 Dockerfile mistakes to avoid\n\nA working Dockerfile and a good Dockerfile are different things. Five mistakes that make images huge, slow to build or insecure, with the fix for each.\n\n## Quick reference\n\n**1. Don't**\n\nCOPY . . early in the file\n\nAny code change busts the cache for every later step.\n\n**Do instead**\n\n```sql\nCOPY deps first, code last\n```\n\nDependencies cache; only code rebuilds on a change.\n\n**2.** ❌ **no .dockerignore** — You copy .git, node_modules and secrets into the image.  \n✅ **a .dockerignore file** — Smaller, cleaner, safer images.\n\n**3. Don't**\n\n```bash\nRUN apt-get install ... (no clean)\n```\n\nThe package cache stays baked into the layer.\n\n**Do instead**\n\ninstall, then `rm -rf /var/lib/apt/lists/*`\n\nSame step, smaller image.\n\n**4.** ❌ **one big image for build + run** — Compilers and dev tools ship to production.  \n✅ **multi-stage build** — Build in one stage, copy only the result to a slim one.\n\n**5.** ❌ **FROM ubuntu  (full)** — Hundreds of MB and packages you never use.  \n✅ **a slim or distroless base** — Less to download, patch and attack.\n\n## Explained\n\n1. **COPY .** . early in the file\n   Docker caches layers, and a layer is invalidated when its input changes. Copy all your code near the top and every tiny code change busts the cache for everything below it, so dependencies reinstall every build. Instead: copy the dependency manifest first, install, then copy the code last. Now only the code layer rebuilds on a change.\n\n2. No .dockerignore\n   Without it, COPY . . drags .git, node_modules, local env files and build artifacts into the image, making it bigger and sometimes leaking secrets. Instead: add a .dockerignore, just like .gitignore.\n\n3. `RUN apt-get install without cleaning`\n   The downloaded package lists stay baked into that layer forever. Instead: in the same RUN, finish with `rm -rf /var/lib/apt/lists/*`. It must be the same step, because a later cleanup does not shrink an earlier layer.\n\n4. One big image for building and running\n   If you compile in the final image, the compiler, headers and dev tools all ship to production, bloating the image and widening the attack surface. Instead: a multi-stage build. Compile in a build stage, then copy only the finished artifact into a slim runtime stage.\n\n5. A full base image\n   FROM ubuntu pulls hundreds of MB and packages you never use. Instead: a -slim base, or distroless for compiled apps, so there is less to download, patch and attack.\n\nThe thread: order layers for caching, and ship only what runs.\n\n## In one line\n\n> **Order layers. Ship less.** A good Dockerfile is small, cached and has no build tools in it.\n\n**Measure it**\n\n```bash\ndocker history <image>  |  docker image ls\n```\n"},{"id":"p200","n":"200","slug":"the-dora-metrics-4-numbers-that-measure-delivery","title":"The DORA metrics: 4 numbers that measure delivery","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","DORA","SRE","CICD","EngineeringMetrics"],"series_name":"Cloud / DevSecOps","design":"numbers","script":"","date":"2028-01-06","files":{"devsecops":"notes/p200-the-dora-metrics-4-numbers-that-measure-delivery.md"},"md":"# The DORA metrics: 4 numbers that measure delivery\n\nHow do you know if your delivery is actually good? The DORA research, from years of State of DevOps reports, boils it down to four metrics. Two measure speed, two measure stability, and the big finding is that the best teams score well on all four at once.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **Deploy freq** | How often you ship | Elite teams deploy on demand, many times a day. | speed |\n| **Lead time** | Commit to production | Elite: under an hour. Low: over a month. | speed |\n| **Change fail %** | Deploys that break | Elite: 0-15%. The quality side of speed. | quality |\n| **MTTR** | Time to recover | Elite: under an hour. How fast you bounce back. | quality |\n| **2** | Speed vs stability | The myth is you trade them. The data says you don't. | insight |\n| **DORA** | The research | From the State of DevOps reports. The industry standard. | source |\n\n## Explained\n\nDeployment frequency: how often you release to production. Elite teams deploy on demand, many times a day; lower performers release weekly or monthly. Frequent, small deploys are safer than rare, big ones.\n\nLead time for changes: how long from a commit to that code running in production. Elite teams measure this in under an hour; the slowest in weeks or months. It reflects how much friction sits between an idea and users.\n\nChange failure rate: the share of deploys that cause a problem needing a fix or rollback. Elite teams keep it in the 0 to 15% range. This is the quality counterweight to deploy frequency.\n\nMean time to recovery (MTTR): when something does break, how fast you restore service. Elite teams recover in under an hour. Fast recovery matters more than never failing, because everything fails eventually.\n\nThe insight that changed the industry: speed and stability are not a trade-off. The data shows the same teams that deploy most often also fail least and recover fastest, because small, frequent, automated changes are easier to test, ship and reverse.\n\nWhere to start: you do not need a fancy tool. Begin by tracking deployment frequency and MTTR, and the conversations they start will improve the rest.\n\n## In one line\n\n> **Speed and stability rise together.** Good teams ship more often AND break less. Measure both.\n\n**Start measuring:** track deploy frequency and MTTR first\n"},{"id":"p201","n":"201","slug":"quiz-3-terraform-questions","title":"Quiz: 3 Terraform questions","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","InterviewPrep"],"series_name":"Cloud / DevSecOps","design":"quiz","script":"","date":"2028-01-09","files":{"devsecops":"notes/p201-quiz-3-terraform-questions.md"},"md":"# Quiz: 3 Terraform questions\n\nTerraform Associate is phase 3 of my plan. Three questions that test whether you really understand how Terraform thinks, not just the commands. Try each before the answer.\n\n## Quick reference\n\n**Q1. What does the Terraform state file actually store?**\n\n- a) Your HCL code\n- b) A map of your config to real resources\n- c) Your cloud credentials\n\n> **Answer:** b) A map of your config to real resources. State maps each resource in your config to the real object, so Terraform knows what exists and what to change.\n\n**Q2. You run terraform apply with no saved plan. What happens?**\n\n- a) It applies instantly\n- b) It shows a plan and asks you to confirm\n- c) It only validates\n\n> **Answer:** b) It shows a plan and asks you to confirm. apply re-plans and prompts for yes. Applying a saved plan file skips the prompt and runs exactly what you reviewed.\n\n**Q3. Which command previews changes without making them?**\n\n- a) terraform plan\n- b) terraform refresh\n- c) terraform show\n\n> **Answer:** a) terraform plan. plan shows what would be created, changed or destroyed. Always read it, especially anything marked destroy.\n\n## Explained\n\n1. What does the Terraform state file actually store?\n\n> **Answer:** a map from your configuration to the real resources. State records which real-world object (by ID) corresponds to each resource in your code, plus their current attributes. This is how Terraform knows the difference between what you asked for and what already exists, so it can change only what is needed. It is not your HCL (that is your code), and it should never contain plaintext credentials you put there, though it can capture sensitive attributes, which is why state must be stored securely and remotely.\n\n2. You run terraform apply with no saved plan. What happens?\n\n> **Answer:** it creates a plan, shows it, and asks you to confirm with yes. Many people think apply runs immediately. It does not, unless you pass -auto-approve or give it a saved plan file. Applying a saved plan (terraform apply tfplan) skips the prompt and runs exactly what you already reviewed, which is the safest pattern in automation.\n\n3. **Which command previews changes without making them?**\n\n> **Answer:** terraform plan. It shows everything that would be created, changed or destroyed, and changes nothing. Reading it, especially any line marked destroy, is the single most important habit in Terraform. refresh updates state from reality; show displays existing state.\n\nThe core of Terraform is two ideas: state (what exists) and plan (what will change). Most mistakes come from misunderstanding one of them.\n\n## In one line\n\n> **State and plan. That is the core.** Most Terraform mistakes come from misunderstanding those two.\n\n**The safe loop**\n\n```bash\nterraform plan -out=tfplan && terraform apply tfplan\n```\n"},{"id":"p202","n":"202","slug":"a-kubernetes-pod-health-summary-in-python","title":"A Kubernetes pod health summary in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","Kubernetes","DevOps","SRE","CloudNative"],"series_name":"Cloud / DevSecOps","design":"code","script":"pod_health.py","date":"2028-01-11","files":{"devsecops":"notes/p202-a-kubernetes-pod-health-summary-in-python.md"},"md":"# A Kubernetes pod health summary in Python\n\nKubernetes phase, CKA on the horizon. kubectl get pods is fine for 10 pods and useless for 500. This script reads kubectl's JSON and prints only the pods that need attention: not Running, not fully ready, or restarting a lot. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `pod_health.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nkubectl get pods -A -o json | python3 pod_health.py\n```\n\n**Example output**\n\n```text\napi-7d9f  0/1  7x\nworker-2  0/1  12x\n2 pods need attention\n```\n\n## Explained\n\n### What it does\n\nYou pipe kubectl get pods -o json into it. It walks every pod, checks the phase, how many containers are ready, and the total restart count, and prints a short list of just the problem pods. Everything healthy is hidden, so what remains is your to-do list. It exits 1 if anything is unhealthy, so it also works in CI and alerts.\n\n### Why read JSON\n\nkubectl's -o json output is a stable, structured contract. Parsing it with Python is far more reliable than scraping the text table, and it lets you build exactly the view you want.\n\n### How to use it\n\n`kubectl get pods -A -o json | python3 pod_health.py` across all namespaces, or point it at a saved `pods.json` file. Run it from a laptop, a bastion, or a scheduled job.\n\nThe attached version shows the namespace, phase, ready count and restarts in a clean table, and flags high restart counts even when a pod currently reads Running, because a pod that keeps crashing and recovering is still a problem.\n\nIt is not a replacement for Prometheus and alerts, but it is a dependency-free triage tool you can run the moment something feels off in a cluster.\n\n## In one line\n\n> **Surface the problems only.** A long pod list becomes a short to-do list.\n\n**Pipe it in**\n\n```bash\nkubectl get pods -A -o json | python3 pod_health.py\n```\n\n## Script: pod_health.py\n\nDownload: [scripts/pod_health.py](../scripts/pod_health.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"pod_health.py - a quick health summary of your Kubernetes pods.\n\nReads `kubectl get pods -o json` and prints only what is wrong: pods that are\nnot Running/Succeeded, containers that are not ready, and restart counts.\nTurns a long pod list into a short problem list. Standard library only.\n\nUsage:\n    kubectl get pods -A -o json | python3 pod_health.py\n    python3 pod_health.py pods.json          # from a saved file\n\nExit code is 1 if any pod is unhealthy, so it works in CI and alerts.\n\nBy s0sta.com\n\"\"\"\nimport json\nimport sys\n\n\ndef load(args):\n    if args and args[0] != \"-\":\n        with open(args[0]) as fh:\n            return json.load(fh)\n    return json.load(sys.stdin)\n\n\ndef main():\n    data = load(sys.argv[1:])\n    items = data.get(\"items\", [])\n    unhealthy = []\n    for pod in items:\n        meta = pod.get(\"metadata\", {})\n        name = meta.get(\"name\", \"?\")\n        ns = meta.get(\"namespace\", \"default\")\n        status = pod.get(\"status\", {})\n        phase = status.get(\"phase\", \"Unknown\")\n        conts = status.get(\"containerStatuses\", []) or []\n        ready = sum(1 for c in conts if c.get(\"ready\"))\n        total = len(conts)\n        restarts = sum(c.get(\"restartCount\", 0) for c in conts)\n        bad = phase not in (\"Running\", \"Succeeded\") or (total and ready < total) or restarts >= 5\n        if bad:\n            unhealthy.append((ns, name, phase, f\"{ready}/{total}\", restarts))\n\n    print(f\"{len(items)} pods checked, {len(unhealthy)} need attention\\n\")\n    if unhealthy:\n        print(f\"{'NAMESPACE':<16}{'POD':<40}{'PHASE':<12}{'READY':<8}RESTARTS\")\n        for ns, name, phase, ready, restarts in unhealthy:\n            print(f\"{ns:<16}{name:<40}{phase:<12}{ready:<8}{restarts}\")\n    else:\n        print(\"[OK] all pods healthy\")\n    sys.exit(1 if unhealthy else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p203","n":"203","slug":"anatomy-of-a-kubectl-command","title":"Anatomy of a kubectl command","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","DevOps","CloudNative","CLI"],"series_name":"Cloud / DevSecOps","design":"anatomy","script":"","date":"2028-01-13","files":{"devsecops":"notes/p203-anatomy-of-a-kubectl-command.md"},"md":"# Anatomy of a kubectl command\n\nAlmost every `kubectl` command follows the same shape. Once you see it, the whole CLI becomes predictable, which is exactly what you need under time pressure in the CKA exam.\n\n## Quick reference\n\n```text\nkubectl get pods -n shop -o wide\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `kubectl` | The tool | The CLI that talks to the cluster's API server. |\n| `get` | The verb | The action: get, describe, delete, apply, logs. |\n| `pods` | The resource | What kind of object: pods, svc, deploy, nodes. |\n| `-n shop` | Namespace | Which namespace. Without it, the default one. |\n| `-o wide` | Output | The format: wide, yaml, json, or name. |\n\n## Explained\n\n`kubectl` is the tool: the command-line client that talks to your cluster's API server, using the context and credentials in your kubeconfig. `kubectl` config current-context tells you which cluster you are pointed at, worth checking before anything destructive.\n\nget is the verb, the action you want: get to list, describe for detail, logs to read output, apply to create or update, delete to remove, exec to run a command inside a container. Learn the verbs and you can act on anything.\n\npods is the resource type, the kind of object. It could be pods, svc (services), deploy (deployments), nodes, ns (namespaces) and dozens more. `kubectl` api-resources lists them all, with their short names.\n\n`-n shop` is the namespace flag. Kubernetes divides a cluster into namespaces, and most commands default to the default namespace. Forgetting -n is why people \"cannot find\" a pod that is really in another namespace. -A targets all namespaces at once.\n\n`-o wide` is the output format. wide adds columns like node and IP; yaml and json give the full object (great for learning and scripting); name gives just names for piping into other commands.\n\n> **The pattern in one line:** `kubectl` verb resource with a namespace and an output format. Master that template and most of `kubectl` is just filling in the blanks.\n\n## In one line\n\n> **Verb, resource, flags.** Every `kubectl` command follows the same shape.\n\n**Learn the shape**\n\n```bash\nkubectl <verb> <resource> -n <ns> -o <format>\n```\n"},{"id":"p204","n":"204","slug":"9-imperative-kubectl-commands-that-save-cka-exam-time","title":"9 imperative kubectl commands that save CKA exam time","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","DevOps","CloudNative","CLI"],"series_name":"Cloud / DevSecOps","design":"terminal","script":"","date":"2028-01-16","files":{"devsecops":"notes/p204-9-imperative-kubectl-commands-that-save-cka-exam-time.md"},"md":"# 9 imperative kubectl commands that save CKA exam time\n\nThe CKA is hands-on and timed, so speed matters. Writing YAML from scratch is slow; generating it with imperative commands is fast. Nine commands that save minutes in the exam and in real work.\n\n## Quick reference\n\n### Make objects in one line\n\n```bash\n# a pod, instantly\nkubectl run nginx --image=nginx\n# a deployment with replicas\nkubectl create deploy web --image=nginx --replicas=3\n# a service in front of it\nkubectl expose deploy web --port=80\n```\n\n### Let kubectl write the YAML\n\n```bash\n# print the YAML instead of creating it\nkubectl run nginx --image=nginx --dry-run=client -o yaml\n# save it, edit it, then apply\nkubectl create deploy web --image=nginx --dry-run=client -o yaml > web.yaml\n# apply your edited version\nkubectl apply -f web.yaml\n```\n\n### Scale, update, roll back\n\n```bash\n# change replica count live\nkubectl scale deploy web --replicas=5\n# roll out a new image version\nkubectl set image deploy/web nginx=nginx:1.27\n# back to the previous version\nkubectl rollout undo deploy/web\n```\n\n## Explained\n\n### Make objects in one line\n\n`kubectl run nginx --image=nginx` creates a pod instantly.  \n`kubectl create deploy web --image=nginx --replicas=3` creates a deployment with replicas, the normal way to run something.  \n`kubectl expose deploy web --port=80` puts a service in front of it, so other pods can reach it by name.\n\n### Let kubectl write the YAML\n\n`kubectl run nginx --image=nginx --dry-run=client -o yaml` prints the YAML instead of creating anything. This is the single biggest time-saver: let kubectl generate a correct skeleton.  \nRedirect it to a file, edit the few fields you need, and `kubectl apply -f web.yaml`. Far faster and less error-prone than typing apiVersion and kind by hand.\n\n### Change fast\n\n`kubectl scale deploy web --replicas=5` changes the replica count live.  \n`kubectl set image deploy/web nginx=nginx:1.27` rolls out a new image version, triggering a rolling update.  \n`kubectl rollout undo deploy/web` returns to the previous version in seconds, the fastest fix for a bad deploy.\n\nThe exam tip that applies everywhere: do not write YAML from a blank file. Generate it with `--dry-run=client -o yaml`, then edit. Also set an alias (alias k=kubectl) and learn kubectl explain for field names, since the docs are allowed in the exam.\n\n## In one line\n\n> **Generate it, don't type it.** `--dry-run=client -o yaml` is the CKA time-saver.\n\n**Try it**\n\n```bash\nkubectl run p --image=nginx --dry-run=client -o yaml\n```\n"},{"id":"p205","n":"205","slug":"fix-kubernetes-crashloopbackoff","title":"Fix: Kubernetes CrashLoopBackOff","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","DevOps","Troubleshooting","CloudNative"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2028-01-18","files":{"devsecops":"notes/p205-fix-kubernetes-crashloopbackoff.md"},"md":"# Fix: Kubernetes CrashLoopBackOff\n\nCrashLoopBackOff means the container starts, crashes, and Kubernetes keeps restarting it with an increasing delay. The status tells you it is failing; the logs and events tell you why. This is the most common pod problem, and a CKA staple.\n\n## Quick reference\n\n### Diagnose — Why did it crash?\n\n- `kubectl logs <pod> --previous` — Logs of the crashed container, not the new one.\n- `kubectl describe pod <pod>` — Events: OOMKilled, failed probe, mount error.\n- **Check the exit code** — 137 = OOM/killed, 1 = app error, 0 = exited cleanly.\n\n### Fix — Match the cause\n\n- **App error in logs** — Fix the config, missing env, or bad connection string.\n- **OOMKilled (137)** — Raise the memory limit, or fix the leak.\n- **Failing liveness probe** — The probe is too strict or the port is wrong.\n\n### Prevent — Fewer crash loops\n\n- **Realistic probes** — Give slow apps an initialDelaySeconds.\n- **Right-size memory** — Set requests and limits from real usage.\n- **Fail fast with a clear log** — Make the app say why it quit.\n\n## Explained\n\n### Diagnose\n\n`kubectl logs <pod> --previous` is the key command. In a crash loop the current container has just started and has nothing useful; `--previous` shows the output of the one that actually crashed. The real error is almost always here: a missing environment variable, a refused database connection, a bad config file.  \n`kubectl describe pod <pod>` shows the Events at the bottom, which catch the causes that are not in the app log: OOMKilled, a failing liveness probe, or a volume that would not mount.  \nThe exit code is a strong hint. 137 means killed, usually out of memory; a code of 1 or 2 is an application error; 0 means it exited cleanly, which is wrong for a long-running service.\n\n### Fix\n\nAn app error in the logs: fix the config, the missing secret, or the connection string it cannot reach.  \nOOMKilled: raise the memory limit, or fix the leak that makes it grow.  \nA failing liveness probe: the probe may be too strict, or pointing at the wrong port or path, killing a healthy-but-slow app.\n\n### Prevent\n\nRealistic probes with an initialDelaySeconds for apps that take time to start.  \nRight-sized memory requests and limits, based on real usage.  \nAn app that fails fast and logs clearly why, so the next crash loop takes 30 seconds to diagnose.\n\n## In one line\n\n> **Read --previous, not the restart count.** The crashed container's log holds the answer.\n\n**The first command**\n\n```bash\nkubectl logs <pod> --previous\n```\n"},{"id":"p206","n":"206","slug":"deployment-vs-statefulset","title":"Deployment vs StatefulSet","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","CloudNative","DevOps","Databases"],"series_name":"Cloud / DevSecOps","design":"compare","script":"","date":"2028-01-20","files":{"devsecops":"notes/p206-deployment-vs-statefulset.md"},"md":"# Deployment vs StatefulSet\n\nDeployment and StatefulSet both run pods, and choosing the wrong one causes real pain. The difference is whether your app cares which pod it is.\n\n## Quick reference\n\n| | **DEPLOYMENT** | **STATEFULSET** |\n|:--|:--|:--|\n| Pod names | Random hash | Ordered: app-0, app-1 |\n| Identity | Interchangeable | Stable and sticky |\n| Storage | Shared or none | Own volume per pod |\n| Scaling | Any order, parallel | Ordered, one by one |\n| Use for | Web, API, workers | Databases, Kafka, etcd |\n| DNS | One service | Stable per-pod name |\n\n**DEPLOYMENT** (For stateless apps): Interchangeable pods with random names. Scale and replace freely.\n\n```bash\nkind: Deployment\n```\n\n**STATEFULSET** (For stateful apps): Pods with stable identity and their own storage. For databases.\n\n```bash\nkind: StatefulSet\n```\n\n- Deployment → stateless apps. The default for most workloads.\n- StatefulSet → apps that need stable identity and storage.\n\n## Explained\n\nA Deployment runs stateless pods: interchangeable, with random names like web-7d9f8c6b5-x2kqp. Any pod can handle any request, so Kubernetes can create, destroy, scale and replace them freely and in parallel. This is the default for web servers, APIs and workers, anything that keeps no important local state.\n\nA StatefulSet runs pods that have a stable identity. They get ordered, predictable names (db-0, db-1, db-2), each keeps the same name across restarts, and each gets its own persistent volume that follows it. Pods are created and scaled in order, one at a time, and each has a stable DNS name. This is for apps where the instances are not interchangeable: databases, message brokers like Kafka, and clustered systems like etcd.\n\n### Why it matters\n\nRun a database as a Deployment and you get chaos: pods with random names sharing or losing storage, no stable address for replication, and scaling that does not respect the order a cluster needs. Run a stateless web app as a StatefulSet and you add complexity and slow, ordered rollouts for no benefit.\n\n### The quick test\n\nDoes each instance need its own persistent data and a stable identity? StatefulSet. Are the instances interchangeable? Deployment.\n\nMost of your workloads are stateless, so Deployment is the common case. Reach for StatefulSet deliberately, for the specific apps that need it. And remember: stateful apps on Kubernetes also need a StorageClass for their volumes.\n\nIdentity is the deciding question.\n"},{"id":"p207","n":"207","slug":"a-systemd-service-watchdog-in-python","title":"A systemd service watchdog in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SysAdmin","DevOps","Automation"],"series_name":"Cloud / DevSecOps","design":"code","script":"service_watchdog.py","date":"2028-01-23","files":{"it":"notes/p207-a-systemd-service-watchdog-in-python.md","devsecops":"notes/p207-a-systemd-service-watchdog-in-python.md"},"md":"# A systemd service watchdog in Python\n\nSometimes a service dies between monitoring checks, and you want it back before anyone notices. This script checks systemd services and, if one is down, restarts it and records what happened. Full file attached, standard library only (it drives systemctl).\n\n## Quick reference\n\nThe full script is at the end of this note: `service_watchdog.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nsudo python3 service_watchdog.py --restart nginx\n```\n\n**Example output**\n\n```text\n[OK] nginx active\n[FIXED] php8.3-fpm was down, restarted\n```\n\n## Explained\n\n### What it does\n\nFor each service you name, it runs systemctl is-active. If the service is up, it logs OK. If it is down and you passed `--restart`, it restarts it and logs FIXED or, if the restart fails, FAIL with the reason. Every line is timestamped, so the log is an honest record of what went down and when.\n\n### The safe default\n\nWithout `--restart` it only reports. That is deliberate: run it in dry mode first to see what it would do, then add `--restart` once you trust it. Restarting services needs root, so it runs under sudo or as a root cron job.\n\n### How to use it\n\n`*/5 * * * * /usr/bin/python3 /opt/scripts/service_watchdog.py --restart nginx php8.3-fpm` mariadb >> `/var/log/watchdog.log 2>&1`. Every five minutes it makes sure your key services are alive.\n\n> **An honest caveat:** this is a backstop, not real monitoring. If a service keeps dying, auto-restarting it hides the real problem. The log is there so you notice the pattern. systemd itself can also do this with Restart=on-failure in the unit; this script is handy when you want one scheduled check across several services with a single log.\n\n## In one line\n\n> **A safety net between checks.** Not a replacement for monitoring, but a cheap backstop.\n\n**Run it often**\n\n```bash\n*/5 * * * * python3 service_watchdog.py --restart nginx\n```\n\n## Script: service_watchdog.py\n\nDownload: [scripts/service_watchdog.py](../scripts/service_watchdog.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"service_watchdog.py - restart a dead systemd service, and log it.\n\nChecks each systemd service with `systemctl is-active`. If one is down, it\ntries to restart it and records what happened. Run it from cron as a safety\nnet between proper monitoring checks. Standard library only (uses systemctl).\n\nUsage:\n    python3 service_watchdog.py nginx php8.3-fpm mariadb\n    sudo python3 service_watchdog.py --restart nginx     # actually restart\n\nWithout --restart it only reports (safe dry run). Needs root to restart.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport subprocess\nimport sys\n\n\ndef is_active(service):\n    r = subprocess.run([\"systemctl\", \"is-active\", service],\n                       capture_output=True, text=True)\n    return r.stdout.strip() == \"active\"\n\n\ndef restart(service):\n    r = subprocess.run([\"systemctl\", \"restart\", service],\n                       capture_output=True, text=True)\n    return r.returncode == 0, r.stderr.strip()\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Watch systemd services and restart if down.\")\n    ap.add_argument(\"services\", nargs=\"+\")\n    ap.add_argument(\"--restart\", action=\"store_true\", help=\"actually restart (default: report only)\")\n    args = ap.parse_args()\n\n    now = dt.datetime.now().strftime(\"%Y-%m-%d %H:%M:%S\")\n    problems = False\n    for svc in args.services:\n        if is_active(svc):\n            print(f\"{now} [OK] {svc} active\")\n            continue\n        problems = True\n        if not args.restart:\n            print(f\"{now} [DOWN] {svc} is not active (dry run, use --restart)\")\n            continue\n        ok, err = restart(svc)\n        if ok and is_active(svc):\n            print(f\"{now} [FIXED] {svc} was down, restarted successfully\")\n        else:\n            print(f\"{now} [FAIL] {svc} down, restart failed: {err}\")\n\n    sys.exit(1 if problems else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p208","n":"208","slug":"9-skills-to-have-ready-for-the-cka-exam","title":"9 skills to have ready for the CKA exam","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","DevOps","CloudNative","Certification"],"series_name":"Cloud / DevSecOps","design":"checklist","script":"","date":"2028-01-25","files":{"devsecops":"notes/p208-9-skills-to-have-ready-for-the-cka-exam.md"},"md":"# 9 skills to have ready for the CKA exam\n\nThe CKA is hands-on and timed: you fix real clusters in a terminal, with the Kubernetes docs open. Theory is not enough; speed is the skill. Nine things to have in muscle memory before exam day.\n\n## Quick reference\n\n### Speed\n\n- **alias k=kubectl** — and k get po -A by reflex\n- **Generate YAML** — `--dry-run=client -o yaml`\n- **Find docs fast** — kubernetes.io/docs is open in the exam\n\n### Core tasks\n\n- **Fix a broken pod** — logs `--previous`, describe\n- **Expose a deployment** — `kubectl expose + test`\n- **RBAC + service accounts** — create role, rolebinding\n\n### Cluster\n\n```bash\n# Drain and cordon a node\nkubectl drain <node>\n# etcd backup and restore\netcdctl snapshot save\n# Set the right context\nkubectl config use-context\n```\n\n## Explained\n\n### Speed\n\nalias k=kubectl, set at the start, saves keystrokes on every command. k get po -A should be automatic.  \nGenerate YAML with `--dry-run=client -o yaml` instead of writing it by hand. This alone saves minutes per question.  \nFind docs fast. kubernetes.io is allowed during the exam, so practise searching it quickly for the one YAML snippet you need, rather than memorising everything.\n\n### Core tasks\n\nFix a broken pod: `kubectl logs --previous` and kubectl describe, then correct the cause. Troubleshooting is the biggest chunk of the exam.  \nExpose a deployment with a service and verify it works from another pod.  \nRBAC: create a role and rolebinding, and a service account, and test what it can and cannot do.\n\n### Cluster\n\nDrain and cordon a node for maintenance with kubectl drain, then uncordon it.  \netcd backup and restore with `etcdctl snapshot save` and restore. A classic, high-value question.  \nSet the right context first. Every task tells you which cluster to use; `kubectl config use-context` is the first thing you run, every question, or you will fix the wrong cluster.\n\nThe mindset: the CKA rewards the person who has done these tasks a hundred times, not the one who has read about them. Build a kind or k3s cluster and break it on purpose.\n\n## In one line\n\n> **The CKA tests speed, not theory.** It is hands-on and timed. Practise until it is reflex.\n\n**Always first**\n\n```bash\nkubectl config use-context <q>\n```\n"},{"id":"p209","n":"209","slug":"the-kubernetes-control-plane-part-by-part","title":"The Kubernetes control plane, part by part","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","CloudNative","DevOps","SRE"],"series_name":"Cloud / DevSecOps","design":"stack","script":"","date":"2028-01-27","files":{"devsecops":"notes/p209-the-kubernetes-control-plane-part-by-part.md"},"md":"# The Kubernetes control plane, part by part\n\nKubernetes feels like magic until you see the control plane, the handful of components that make a cluster self-healing. Each has one job, and together they turn your desired state into reality. This is core CKA material.\n\n## Quick reference\n\n*The brain of the cluster*\n\n1. **API server** — The front door. Everything talks to it, nothing bypasses it. · *kube-apiserver*\n2. **etcd** — The database. The cluster's entire state lives here. · *etcd*\n3. **Scheduler** — Decides which node each new pod runs on. · *kube-scheduler*\n4. **Controller manager** — Drives reality toward your desired state. · *controllers*\n5. **Cloud controller** — Talks to the cloud for load balancers and disks. · *cloud-cm*\n6. **kubelet (on nodes)** — The agent that actually runs the pods. · `kubelet`\n\n## Explained\n\nAPI server (kube-apiserver). The front door. Every command, every component, every kubectl call goes through it, and nothing talks to etcd directly. It validates requests, handles authentication and authorisation, and is the only component that reads and writes the datastore. If the API server is down, the cluster is effectively frozen.\n\netcd. The database. The entire state of the cluster, every object, its spec and status, lives in etcd as key-value data. This is why backing up etcd is backing up the cluster, and why etcdctl snapshot save is a classic exam task.\n\nScheduler (kube-scheduler). When a new pod has no node yet, the scheduler picks one, based on resources, taints, affinity and constraints. It decides where, not how.\n\nController manager. A bundle of controllers, each watching for a difference between desired and actual state and acting to close it: the deployment controller creates ReplicaSets, the node controller notices dead nodes. This control loop is the heart of Kubernetes.\n\nCloud controller manager. Talks to the cloud provider for things like load balancers, routes and persistent disks, keeping cloud-specific logic separate.\n\nkubelet. On each worker node, the agent that actually starts and watches the pods the control plane assigned.\n\nThe whole idea in one line: you declare desired state, and the control plane continuously works to make reality match.\n\n## In one line\n\n> **Desired state in, reality out.** The control plane's whole job is to close that gap.\n\n**Back up the brain**\n\n```bash\netcdctl snapshot save snap.db\n```\n"},{"id":"p210","n":"210","slug":"kubernetes-networking-terms-decoded","title":"Kubernetes networking terms, decoded","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","Networking","CloudNative","DevOps"],"series_name":"Cloud / DevSecOps","design":"reveal","script":"","date":"2028-01-30","files":{"devsecops":"notes/p210-kubernetes-networking-terms-decoded.md"},"md":"# Kubernetes networking terms, decoded\n\nKubernetes networking is where most people get lost, and it is a big part of the CKA. Six terms, in plain English, that make cluster traffic make sense.\n\n## Quick reference\n\n- **ClusterIP** — The default Service type: a stable internal address, reachable only inside the cluster.\n- **NodePort** — Opens the same port on every node, so traffic from outside can reach a Service.\n- **LoadBalancer** — Asks the cloud for an external load balancer in front of a Service.\n- **Ingress** — Routes outside HTTP traffic to Services by host and path. One entry point, many apps.\n- **CNI** — The plugin (Calico, Cilium, Flannel) that gives every pod an IP and wires the network.\n- **NetworkPolicy** — A firewall for pods: which pods may talk to which. Default is allow-all until you add one.\n\n## Explained\n\nClusterIP. The default Service type. It gives a set of pods one stable internal IP and DNS name, load-balanced, reachable only from inside the cluster. This is how your services find each other: the frontend talks to backend, not to a pod IP that changes.\n\nNodePort. Opens the same high port (30000 to 32767) on every node, forwarding to a Service. It is the simplest way to reach something from outside the cluster, and common in labs, though rarely the final answer in production.\n\nLoadBalancer. Asks the cloud provider to create an external load balancer with a public IP in front of a Service. This is the usual production way to expose a single service to the internet.\n\nIngress. Routes outside HTTP and HTTPS traffic to different Services based on host name and URL path, so one entry point and one IP can serve many apps, with TLS in one place. An Ingress needs an ingress controller running to do the work.\n\nCNI. The Container Network Interface plugin, like Calico, Cilium or Flannel, that actually gives every pod an IP address and connects them. Your choice of CNI affects performance and which features, like NetworkPolicy, you get.\n\nNetworkPolicy. A firewall for pods: rules for which pods may talk to which, on which ports. The catch: by default Kubernetes allows all pod-to-pod traffic. A NetworkPolicy only takes effect once you add one, and your CNI must support it.\n\n## In one line\n\n> **Pods, Services, and the rules between.** Networking is where most Kubernetes confusion lives.\n\n**See the services**\n\n```bash\nkubectl get svc,ingress -A\n```\n"},{"id":"p211","n":"211","slug":"what-happens-when-you-run-kubectl-apply","title":"What happens when you run kubectl apply","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CKA","CloudNative","DevOps","SRE"],"series_name":"Cloud / DevSecOps","design":"timeline","script":"","date":"2028-02-01","files":{"devsecops":"notes/p211-what-happens-when-you-run-kubectl-apply.md"},"md":"# What happens when you run kubectl apply\n\nYou run `kubectl apply -f app.yaml` and a pod appears. Between those two moments, the control plane does a precise sequence of work. Understanding it explains why Kubernetes is declarative, and where to look when a pod never starts.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **1** | kubectl sends it | Your YAML goes to the API server, authenticated. |\n| **2** | Validated and stored | The API server checks it and writes it to etcd. |\n| **3** | Controller notices | The deployment controller sees the desired state. |\n| **4** | Scheduler picks a node | It chooses where the new pod should run. |\n| **5** | kubelet starts it | The node's agent pulls the image and runs the container. |\n| **6** | Status flows back | Health is reported up to etcd. kubectl get shows Running. |\n\n## Explained\n\n1. **kubectl sends it.** Your YAML is sent to the API server as a request, authenticated with your kubeconfig credentials and checked against RBAC. kubectl itself does not create anything; it just talks to the API server.\n\n2. **Validated and stored.** The API server validates the object (is the YAML well-formed, do the fields exist, are you allowed?) and writes the desired state into etcd. At this point the object exists in the cluster's brain, but nothing is running yet.\n\n3. **Controller notices.** The relevant controller, for a Deployment that is the deployment controller, is watching for changes. It sees the new desired state and creates the objects below it, a ReplicaSet, which in turn wants pods.\n\n4. **Scheduler picks a node.** The new pods have no node assigned. The scheduler evaluates resources, taints, affinity and constraints, and binds each pod to a suitable node. A pod stuck in Pending usually means this step could not find a fit.\n\n5. **kubelet starts it.** The kubelet on the chosen node sees a pod assigned to it, tells the container runtime to pull the image and start the container, and runs the probes.\n\n6. **Status flows back.** The kubelet reports health up through the API server into etcd, and kubectl get pods shows Running.\n\nThe whole idea: you declare the goal, and independent controllers continuously converge reality toward it.\n\n## In one line\n\n> **You declare. The cluster converges.** apply sets the goal; controllers make it real.\n\n**Watch it happen**\n\n```bash\nkubectl get pods -w\n```\n"},{"id":"p212","n":"212","slug":"find-ssh-brute-force-attempts-with-python","title":"Find SSH brute-force attempts with Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","CyberSecurity","Linux","BlueTeam","DevSecOps"],"series_name":"Cloud / DevSecOps","design":"code","script":"ssh_failed_logins.py","date":"2028-02-03","files":{"it":"notes/p212-find-ssh-brute-force-attempts-with-python.md","devsecops":"notes/p212-find-ssh-brute-force-attempts-with-python.md"},"md":"# Find SSH brute-force attempts with Python\n\nSecurity phase of the plan. Your SSH log is full of brute-force noise; the useful question is who, and how much. This script parses an auth log and prints the top source IPs and the usernames they tried. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `ssh_failed_logins.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ssh_failed_logins.py /var/log/auth.log\n```\n\n**Example output**\n\n```text\n  4821  203.0.113.42\n   912  198.51.100.7\n  2 unique IPs, 5733 attempts\n```\n\n## Explained\n\n### What it does\n\nIt reads the auth log line by line, matches the \"Failed password ... from IP\" pattern with a regex, and counts attempts per IP and per username. The output is a short, ranked list: the IPs hammering you most, and whether they are guessing root, admin or real usernames.\n\n### Why it is useful\n\nA raw log is thousands of lines no one reads. This turns it into a decision: the top IPs go into a firewall drop rule or a fail2ban jail, and seeing attempts on a real username tells you that account name has leaked.\n\n### How to use it\n\n`python3 ssh_failed_logins.py /var/log/auth.log`, or pipe journalctl into it: `journalctl -u ssh | python3 ssh_failed_logins.py` -. On RHEL the log is `/var/log/secure`.\n\nThe attached version reads from a file or stdin, shows totals and unique IP counts, and lets you choose how many to display.\n\nThe deeper lesson, straight from the security path: the best fix is to make brute force pointless. Disable password authentication and use SSH keys, and these thousands of attempts become thousands of harmless failures. This script is how you see the threat and confirm your defences are working.\n\n## In one line\n\n> **Make the log tell you who.** The top IPs are your firewall or fail2ban list.\n\n**Run it on a server**\n\n```bash\npython3 ssh_failed_logins.py /var/log/auth.log\n```\n\n## Script: ssh_failed_logins.py\n\nDownload: [scripts/ssh_failed_logins.py](../scripts/ssh_failed_logins.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"ssh_failed_logins.py - who is trying to brute-force your SSH?\n\nParses an auth log for failed SSH logins and prints the top source IPs and\nthe usernames they tried. Turns a noisy log into a short list you can act on\n(firewall, fail2ban, or just awareness). Standard library only.\n\nUsage:\n    python3 ssh_failed_logins.py /var/log/auth.log\n    python3 ssh_failed_logins.py -n 20 /var/log/auth.log\n    journalctl -u ssh | python3 ssh_failed_logins.py -   # read from stdin\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\nfrom collections import Counter\n\nFAIL = re.compile(r\"Failed password for (?:invalid user )?(\\S+) from (\\d+\\.\\d+\\.\\d+\\.\\d+)\")\n\n\ndef parse(lines):\n    ips, users = Counter(), Counter()\n    for line in lines:\n        m = FAIL.search(line)\n        if m:\n            users[m.group(1)] += 1\n            ips[m.group(2)] += 1\n    return ips, users\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Summarise failed SSH logins.\")\n    ap.add_argument(\"logfile\", help=\"path to auth log, or - for stdin\")\n    ap.add_argument(\"-n\", \"--top\", type=int, default=10, help=\"how many to show\")\n    args = ap.parse_args()\n\n    src = sys.stdin if args.logfile == \"-\" else open(args.logfile, errors=\"ignore\")\n    ips, users = parse(src)\n    if src is not sys.stdin:\n        src.close()\n\n    total = sum(ips.values())\n    print(f\"{total} failed SSH attempts from {len(ips)} unique IPs\\n\")\n    print(f\"Top {args.top} source IPs:\")\n    for ip, n in ips.most_common(args.top):\n        print(f\"  {n:>6}  {ip}\")\n    print(f\"\\nTop {args.top} usernames tried:\")\n    for user, n in users.most_common(args.top):\n        print(f\"  {n:>6}  {user}\")\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p213","n":"213","slug":"9-commands-for-kubernetes-security-cks","title":"9 commands for Kubernetes security (CKS)","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","CyberSecurity","DevSecOps","CloudNative"],"series_name":"Cloud / DevSecOps","design":"steps","script":"","date":"2028-02-06","files":{"devsecops":"notes/p213-9-commands-for-kubernetes-security-cks.md"},"md":"# 9 commands for Kubernetes security (CKS)\n\nThe CKS is the security end of my Kubernetes path, and it is practical: scan images, harden the cluster against a benchmark, and watch what runs. Nine commands and tools that cover the core, all free and in the CKS curriculum.\n\n## Quick reference\n\n### Scan images — Find vulnerabilities before they ship\n\n```bash\n# Scan an image for known CVEs, by severity.\ntrivy image api:1.4.2\n# Scan source and dependencies in a repo.\ntrivy fs .\n# Scan Dockerfiles and manifests for misconfig.\ntrivy config .\n```\n\n### Harden the cluster — Check against the benchmark\n\n```bash\n# Check nodes against the CIS Kubernetes benchmark.\nkube-bench run\n# What can this account actually do? Audit RBAC.\nkubectl auth can-i --list\n# Pod Security Admission levels per namespace.\nkubectl get psa -A\n```\n\n### Watch & enforce — Runtime and policy\n\n```bash\n# Alert on suspicious runtime behaviour live.\nfalco\n# See which pods are actually isolated.\nkubectl get networkpolicy -A\n# Enforce a strict pod baseline (PSA: pod-security.kubernetes.io/enforce).\nkubectl label ns prod enforce=restricted\n```\n\n## Explained\n\n### Scan images\n\n`trivy image api:1.4.2` scans a container image for known vulnerabilities, grouped by severity, and tells you which version fixes each one. Run it in CI and fail the build on criticals.  \n`trivy fs .` scans a repo's source and dependencies.  \n`trivy config .` scans Dockerfiles and Kubernetes manifests for misconfigurations, like running as root or a missing resource limit.\n\n### Harden the cluster\n\n`kube-bench run` checks your nodes against the CIS Kubernetes benchmark, the official hardening list, and tells you exactly what to fix.  \n`kubectl auth can-i --list` shows what an account can really do. RBAC that is too broad is a classic finding; this is how you audit it.  \nkubectl get psa, or the namespace labels, shows the Pod Security Admission level: privileged, baseline or restricted.\n\n### Watch and enforce\n\n`falco` watches system calls at runtime and alerts on suspicious behaviour, like a shell spawning in a container or a write to a sensitive path.  \n`kubectl get networkpolicy -A` shows which pods are actually isolated. Remember, without a policy, all pod traffic is allowed.  \nLabelling a namespace with `pod-security.kubernetes.io/enforce=restricted` enforces a strict baseline: no privileged pods, no host access.\n\nThe CKS mindset: shift left with scanning, lock down with benchmarks and RBAC, and keep watch at runtime with Falco.\n\n## In one line\n\n> **Scan, harden, then watch.** CKS is build-time scanning plus run-time defence.\n\n**Start here**\n\n```bash\ntrivy image <your-image>\n```\n"},{"id":"p214","n":"214","slug":"argo-cd-vs-flux-gitops","title":"Argo CD vs Flux (GitOps)","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","Kubernetes","ArgoCD","Flux","DevOps"],"series_name":"Cloud / DevSecOps","design":"compare","script":"","date":"2028-02-08","files":{"devsecops":"notes/p214-argo-cd-vs-flux-gitops.md"},"md":"# Argo CD vs Flux (GitOps)\n\nGitOps is the end of my roadmap, and the Dutch government's Haven standard uses it, so it is directly relevant to where I am heading. The idea is simple: git is the single source of truth, and a controller continuously makes the cluster match it. Two tools dominate, and both are excellent.\n\n## Quick reference\n\n| | **ARGO CD** | **FLUX** |\n|:--|:--|:--|\n| Interface | Rich web UI | CLI and CRDs |\n| Model | Application objects | Kustomizations + sources |\n| Multi-tenancy | Projects, RBAC, SSO | Namespaces + RBAC |\n| Footprint | Heavier | Lightweight |\n| Both do | Git is the source of truth | Auto-sync + drift repair |\n| Dutch gov (Haven) | Supported | Supported |\n\n**ARGO CD** (GitOps with a UI): A visual dashboard of what git says versus what the cluster runs.\n\n```bash\nargocd app sync myapp\n```\n\n**FLUX** (GitOps, CLI-first): Lightweight controllers that sync git to the cluster, no UI needed.\n\n```bash\nflux reconcile kustomization app\n```\n\n- Argo CD → when teams want a UI and visibility.\n- Flux → when you want it lean and GitOps-native. Both are great.\n\n## Explained\n\nArgo CD is GitOps with a strong web UI. It shows, visually, the difference between what git declares and what the cluster is actually running, which makes drift and failed syncs obvious at a glance. It organises work into Application objects, has Projects for multi-tenancy, and integrates SSO and RBAC. Teams love the visibility, and argocd app sync pushes changes on demand.\n\nFlux is GitOps, CLI-first and lightweight. It is a set of small controllers that watch git sources and apply them with Kustomize or Helm, no central UI required. It leans on Kubernetes-native objects (CRDs) and feels like a natural extension of the cluster. flux reconcile forces a sync. It is lean and composable.\n\n### What they share\n\nBoth make git the source of truth, both continuously sync, and both repair drift: change something by hand in the cluster and they pull it back to what git says. Both are CNCF projects and both are supported by the Dutch Haven standard.\n\n### How to choose\n\nArgo CD when teams want a dashboard and clear visibility, especially across many apps and people.  \nFlux when you want a minimal, GitOps-native footprint driven from config and CLI.\n\nHonestly, either is a strong choice. The bigger win is adopting GitOps at all: reviewed, versioned, auto-reconciled infrastructure.\n\nGit is the source of truth.\n"},{"id":"p215","n":"215","slug":"the-secure-software-supply-chain","title":"The secure software supply chain","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["DevSecOps","SupplyChainSecurity","Kubernetes","CICD","CloudNative"],"series_name":"Cloud / DevSecOps","design":"pipeline","script":"","date":"2028-02-10","files":{"devsecops":"notes/p215-the-secure-software-supply-chain.md"},"md":"# The secure software supply chain\n\nSupply-chain security is 20 percent of the CKS and the fastest-growing area of DevSecOps, because the biggest recent breaches came not from the app but from the pipeline that built it. The defence is a chain where every step proves the next can trust it.\n\n## Quick reference\n\n1. **Write & commit code** — It all starts in git, reviewed and versioned. · `source`\n2. **Build the image** — From a trusted, pinned base image, in CI. · *build*\n3. **Scan for CVEs** — Trivy fails the build on critical vulnerabilities. · *scan*\n4. **Sign the image** — cosign signs it, so its origin is provable. · *sign*\n5. **Admission check** — The cluster rejects any image that is unsigned or unscanned. · *verify*\n6. **Run with least privilege** — Non-root, read-only filesystem, dropped capabilities. · *run*\n\n## Explained\n\n### Write and commit code\n\nIt starts in git: reviewed, versioned, with no secrets committed. If the source is not trustworthy, nothing after it is.\n\n### Build the image\n\nBuild in CI from a trusted, pinned base image, not latest. Pinning a digest means you get the exact bytes you tested, not whatever was pushed since.\n\n### Scan for CVEs\n\nTrivy scans the built image for known vulnerabilities and the pipeline fails on criticals. This is the shift-left gate: catch it before it ships, not in production.\n\n### Sign the image\n\ncosign signs the image so its origin is provable. A signature answers the question an attacker hates: did this image really come from our pipeline, unchanged?\n\n### Admission check\n\nAn admission controller in the cluster rejects any image that is unsigned or unscanned. This is the enforcement point. Policy stops being a document and becomes a gate.\n\n### Run with least privilege\n\nFinally the pod runs non-root, with a read-only root filesystem and dropped Linux capabilities, so even a compromised container can do little.\n\nEach stage assumes the previous one could be attacked, and verifies instead of trusting. That is the whole idea: provenance you can check at every hop.\n\n## In one line\n\n> **Trust nothing unproven.** A supply-chain attack hides in a step you did not check.\n\n**Add one gate:** scan images in CI before they ever reach the registry\n"},{"id":"p216","n":"216","slug":"5-kubernetes-security-mistakes","title":"5 Kubernetes security mistakes","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","CyberSecurity","DevSecOps","CloudNative"],"series_name":"Cloud / DevSecOps","design":"dontdo","script":"","date":"2028-02-13","files":{"devsecops":"notes/p216-5-kubernetes-security-mistakes.md"},"md":"# 5 Kubernetes security mistakes\n\nKubernetes ships with convenient defaults, not safe ones. Most CKS findings, and most real incidents, come down to the same five. Here is each mistake and the one-line fix.\n\n## Quick reference\n\n**1. Don't**\n\ncontainers running as root\n\nA container escape becomes root on the node.\n\n**Do instead**\n\n```bash\nrunAsNonRoot: true\n```\n\nThe pod refuses to start as root. Escapes get a nobody user.\n\n**2.** ❌ **no NetworkPolicy** — Every pod can talk to every other pod, cluster-wide.  \n✅ **a default-deny NetworkPolicy** — Deny all, then allow only the traffic each app needs.\n\n**3.** ❌ **wide-open RBAC (cluster-admin)** — One leaked token owns the whole cluster.  \n✅ **least-privilege Roles** — Grant only the verbs and resources an account truly uses.\n\n**4.** ❌ **secrets as plain env vars** — They leak into logs, crashes and kubectl describe.  \n✅ **mounted Secrets, encrypted at rest** — Enable etcd encryption; mount as files, not env.\n\n**5.** ❌ **no resource limits** — One pod can starve a node. A DoS from inside.  \n✅ **requests and limits on every pod** — Cap CPU and memory so one workload can't take the node down.\n\n## Explained\n\n### Running as root\n\nBy default a container can run as root, so a container escape lands as root on the node. Set `runAsNonRoot: true` in the pod's securityContext and the pod refuses to start as root. An escape then gets an unprivileged user instead of the keys to the host.\n\n### No NetworkPolicy\n\nWithout a NetworkPolicy, every pod can reach every other pod across the whole cluster. One compromised pod can scan and attack everything. Apply a default-deny policy, then allow only the specific traffic each app needs.\n\n### Wide-open RBAC\n\nHanding out cluster-admin \"to make it work\" means one leaked token owns the cluster. Grant least-privilege Roles: only the verbs and resources an account actually uses. Audit with kubectl auth can-i.\n\n### Secrets as plain env vars\n\nSecrets passed as environment variables leak into logs, crash dumps and kubectl describe. Mount them as files instead, and enable encryption at rest for etcd so they are not stored in plaintext.\n\n### No resource limits\n\nA pod with no limits can consume a whole node's CPU and memory, a denial of service from inside. Set requests and limits on every pod so one workload cannot take a node down.\n\nNone of these are exotic. They are defaults left unchanged. Securing them is most of what hardening a cluster means.\n\n## In one line\n\n> **Secure the defaults.** Kubernetes ships open for convenience. Production is your job.\n\n**Audit now:** kubectl get pods -A -o jsonpath on securityContext\n"},{"id":"p217","n":"217","slug":"check-a-sites-security-headers-with-python","title":"Check a site's security headers with Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","WebSecurity","DevSecOps","CyberSecurity","AppSec"],"series_name":"Cloud / DevSecOps","design":"code","script":"http_security_headers.py","date":"2028-02-15","files":{"it":"notes/p217-check-a-sites-security-headers-with-python.md","devsecops":"notes/p217-check-a-sites-security-headers-with-python.md"},"md":"# Check a site's security headers with Python\n\nThe cheapest security win on any website is its HTTP response headers. They cost nothing to set and close whole classes of attack, yet most sites miss them. This script checks the important ones in a second. Full file attached, standard library only.\n\n## Quick reference\n\nThe full script is at the end of this note: `http_security_headers.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 http_security_headers.py https://example.com\n```\n\n**Example output**\n\n```text\nOK    Strict-Transport-Security\nMISS  Content-Security-Policy\n2 of 4 present\n```\n\n## Explained\n\n### What it does\n\nIt makes one request to a URL and inspects the response headers, reporting which security headers are present and which are missing: HSTS (forces HTTPS), Content-Security-Policy (limits what scripts can load, blunting XSS), X-Content-Type-Options (stops MIME sniffing) and X-Frame-Options (prevents clickjacking). The attached version also flags a Server header that leaks version numbers and returns a non-zero exit code when something is missing, so it fits in CI.\n\n### Why it is useful\n\nThese headers are set once in your web server or load balancer and then protect every page. A missing CSP or HSTS is a real, scannable weakness, and this is how you confirm yours are in place, before an attacker checks for you.\n\n### How to use it\n\n`python3 http_security_headers.py https://yoursite.com`. Run it against staging after a config change, or in a pipeline so a regression fails the build.\n\nThis is DevSecOps in miniature: a small, automatable check that turns a security best practice into something you verify every deploy instead of hoping for.\n\n## In one line\n\n> **Audit the headers.** Missing headers are free vulnerabilities, fixed in config.\n\n**Check your site**\n\n```bash\npython3 http_security_headers.py https://yoursite.com\n```\n\n## Script: http_security_headers.py\n\nDownload: [scripts/http_security_headers.py](../scripts/http_security_headers.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"http_security_headers.py - grade the security headers of your sites.\n\nRequests each URL and checks for the headers that protect visitors:\nHSTS, Content-Security-Policy, X-Content-Type-Options, X-Frame-Options and\nReferrer-Policy. Prints a simple score per site. Great before a launch or an\naudit. Standard library only.\n\nUsage:\n    python3 http_security_headers.py https://shop.example.com\n    python3 http_security_headers.py -f sites.txt\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nimport urllib.request\n\nWANTED = [\n    (\"Strict-Transport-Security\", \"HSTS: forces HTTPS\"),\n    (\"Content-Security-Policy\", \"CSP: limits what can load\"),\n    (\"X-Content-Type-Options\", \"stops MIME sniffing\"),\n    (\"X-Frame-Options\", \"blocks clickjacking\"),\n    (\"Referrer-Policy\", \"controls the Referer header\"),\n]\n\n\ndef fetch_headers(url, timeout=10):\n    req = urllib.request.Request(url, headers={\"User-Agent\": \"s0sta-headers/1.0\"})\n    with urllib.request.urlopen(req, timeout=timeout) as resp:\n        return {k.lower(): v for k, v in resp.headers.items()}\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Check HTTP security headers.\")\n    ap.add_argument(\"urls\", nargs=\"*\")\n    ap.add_argument(\"-f\", \"--file\", help=\"file with one URL per line\")\n    args = ap.parse_args()\n\n    urls = list(args.urls)\n    if args.file:\n        with open(args.file) as fh:\n            urls += [ln.strip() for ln in fh if ln.strip() and not ln.startswith(\"#\")]\n    if not urls:\n        ap.error(\"give at least one URL, or -f a file\")\n\n    worst = 100\n    for url in urls:\n        try:\n            headers = fetch_headers(url)\n        except Exception as exc:\n            print(f\"[ERR ] {url} - {exc}\\n\")\n            worst = 0\n            continue\n        have = [(h, d) for h, d in WANTED if h.lower() in headers]\n        score = round(len(have) / len(WANTED) * 100)\n        worst = min(worst, score)\n        print(f\"{url}  ->  {score}/100 ({len(have)}/{len(WANTED)} headers)\")\n        for h, d in WANTED:\n            mark = \"+\" if h.lower() in headers else \"-\"\n            print(f\"   [{mark}] {h}  ({d})\")\n        print()\n\n    sys.exit(1 if worst < 100 else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p218","n":"218","slug":"trivy-vs-falco-vs-opa-gatekeeper","title":"Trivy vs Falco vs OPA Gatekeeper","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","DevSecOps","CloudNative","CyberSecurity"],"series_name":"Cloud / DevSecOps","design":"matrix","script":"","date":"2028-02-17","files":{"devsecops":"notes/p218-trivy-vs-falco-vs-opa-gatekeeper.md"},"md":"# Trivy vs Falco vs OPA Gatekeeper\n\nThree open-source tools cover most Kubernetes security, and they are easy to confuse because all three are about safety. The clearest way to tell them apart is the question each one answers and the moment it acts. All three are in the CKS.\n\n## Quick reference\n\n| | **Trivy** | **Falco** | **OPA/Gatekeeper** |\n|:--|:--|:--|:--|\n| Question | What CVEs are here? | What is happening now? | Is this allowed? |\n| When | Build time | Runtime | Admission |\n| Watches | Images & files | System calls | API requests |\n| Catches | Known vulns | Live attacks | Bad config |\n| Acts by | Failing CI | Alerting | Rejecting |\n| CKS domain | Supply chain | Runtime | Hardening |\n\n**How to pick**\n\n- **Trivy** → scan images and code before they ship.\n- **Falco** → detect suspicious behaviour while pods run.\n- **OPA/Gatekeeper** → block non-compliant resources at the door.\n\n## Explained\n\nTrivy asks: what vulnerabilities are in here? It scans container images, filesystems and config for known CVEs and misconfigurations, at build time. It acts by failing CI so a vulnerable image never reaches the registry. This is the supply-chain and shift-left tool.\n\nFalco asks: what is happening right now? It watches system calls at runtime and alerts on suspicious behaviour, a shell opening inside a container, an unexpected write to a sensitive path, a connection to a strange IP. It acts by alerting, so you see an attack in progress. This is the runtime-detection tool.\n\nOPA Gatekeeper asks: is this allowed? It evaluates every request to the Kubernetes API against your policies at admission time and rejects anything non-compliant, a privileged pod, an image from an untrusted registry, a missing label. It acts by blocking the resource at the door. This is the policy-enforcement tool.\n\n### How they fit together\n\nThey are not competitors; they cover different moments. Trivy scans before, Falco watches during, Gatekeeper enforces at the gate. A hardened cluster runs all three: scan images in CI, enforce policy on admission, detect anomalies at runtime.\n\n## In one line\n\n> **Three tools, three moments.** Scan before, watch during, enforce at the gate.\n\n**Layer them:** Trivy in CI, Gatekeeper on admission, Falco at runtime\n"},{"id":"p219","n":"219","slug":"6-kubernetes-security-terms","title":"6 Kubernetes security terms","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","CyberSecurity","CloudNative","DevSecOps"],"series_name":"Cloud / DevSecOps","design":"reveal","script":"","date":"2028-02-20","files":{"devsecops":"notes/p219-6-kubernetes-security-terms.md"},"md":"# 6 Kubernetes security terms\n\nKubernetes security has a vocabulary problem: the concepts are not hard, but the names hide them. Learn these six terms and most CKS material suddenly reads clearly. Plain definitions, no jargon.\n\n## Quick reference\n\n- **securityContext** — The block on a pod or container that sets who it runs as and what it can do: user, capabilities, read-only filesystem.\n- **Pod Security Admission** — The built-in gate that enforces a baseline per namespace: privileged, baseline or restricted.\n- **RBAC** — Role-Based Access Control: who (subjects) can do what (verbs) to which resources. Least privilege lives here.\n- **NetworkPolicy** — A firewall for pods, choosing which pods may talk to which. Default is allow-all until you add one.\n- **Admission Controller** — Code that inspects every API request and can mutate or reject it before it is stored. Where OPA and signatures enforce.\n- **CIS Benchmark** — The industry checklist for hardening a cluster. kube-bench scores your nodes against it.\n\n## Explained\n\nsecurityContext is the block on a pod or container that decides who it runs as and what it is allowed to do: the user and group ID, whether it can run as root, which Linux capabilities it keeps, and whether its root filesystem is read-only. Most pod hardening is a few lines here.\n\nPod Security Admission is the built-in gate that enforces a security baseline per namespace. It has three levels: privileged (no restrictions), baseline (sensible minimum) and restricted (locked down). You set it with namespace labels.\n\nRBAC, Role-Based Access Control, defines who can do what to which resources. A Role lists verbs and resources; a binding attaches it to a user or service account. Least privilege is implemented here, and over-broad RBAC is the most common finding.\n\nNetworkPolicy is a firewall for pods, choosing which pods may communicate. Crucially, the default is allow-all: until you add a policy, everything can talk to everything.\n\nAdmission Controller is code that inspects every request to the API server and can change or reject it before it is saved. This is where image-signature checks and OPA policies actually enforce.\n\nCIS Benchmark is the industry checklist for hardening a cluster, and kube-bench scores your nodes against it automatically.\n\n## In one line\n\n> **Know the words, own the cluster.** Security is mostly vocabulary you have not met yet.\n\n**See yours**\n\n```bash\nkubectl get psa,networkpolicy,roles -A\n```\n"},{"id":"p220","n":"220","slug":"the-cks-exam-by-the-numbers","title":"The CKS exam by the numbers","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","CyberSecurity","CloudNative","DevSecOps"],"series_name":"Cloud / DevSecOps","design":"numbers","script":"","date":"2028-02-22","files":{"devsecops":"notes/p220-the-cks-exam-by-the-numbers.md"},"md":"# The CKS exam by the numbers\n\nThe Certified Kubernetes Security Specialist is the security capstone of my roadmap, and like the CKA it is entirely hands-on: you fix and harden a real cluster against the clock. Knowing how it is weighted tells you where to spend your study time. Here it is in numbers.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **6** | Domains | The exam is split into six weighted areas. Know the weights. | exam |\n| **20%** | Supply chain | Tied for the biggest domain: scanning, signing, SBOMs. | weight |\n| **20%** | Microservice vuln | Secrets, mTLS, pod security, container sandboxing. | weight |\n| **20%** | Runtime security | Falco, behavioural analytics, immutable containers. | weight |\n| **2 hr** | Time limit | Hands-on, in a real cluster. Speed matters. | format |\n| **67%** | Pass score | And you must already hold a valid CKA to sit it. | pass |\n\n## Explained\n\nSix domains. The exam is divided into six areas, each with a published weight, and ignoring the weights is the classic mistake. Three of them dominate.\n\nTwenty percent, supply chain security. Tied for the largest domain: minimising base images, scanning with tools like Trivy, signing images, and working with SBOMs. This reflects where the industry is most worried right now.\n\nTwenty percent, microservice vulnerabilities. Managing secrets properly, mutual TLS between services, pod security standards, and container sandboxing with runtimes like gVisor.\n\nTwenty percent, runtime security. Behavioural detection with Falco, immutable containers, and auditing. Catching attacks while they happen.\n\nThose three alone are 60 percent of the exam. The remaining 40 is cluster setup (NetworkPolicies, CIS benchmarks), cluster hardening (RBAC, restricting API access) and system hardening (minimising the host, AppArmor, seccomp).\n\nTwo hours, hands-on, in a live cluster. Speed and muscle memory matter as much as knowledge.\n\nSixty-seven percent to pass, and you must already hold a valid CKA to even sit it. CKS builds on CKA; you cannot skip the operations layer.\n\n## In one line\n\n> **Study the weights.** Three domains are 60 percent of the exam. Start there.\n\n**Practise live:** harden a real cluster, not slides\n"},{"id":"p221","n":"221","slug":"3-kubernetes-security-questions","title":"3 Kubernetes security questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CKS","CyberSecurity","DevSecOps","CloudNative"],"series_name":"Cloud / DevSecOps","design":"quiz","script":"","date":"2028-02-24","files":{"devsecops":"notes/p221-3-kubernetes-security-questions.md"},"md":"# 3 Kubernetes security questions\n\nThe final post of this 50-part series, and a fitting place to end: the security layer the whole path has been climbing toward, from Linux fundamentals to the CKS. Three questions. Answer before the reveal.\n\n## Quick reference\n\n**Q1. By default, which pods can a new pod talk to in a cluster?**\n\n- a) Only pods in its namespace\n- b) All pods in the cluster\n- c) None until RBAC allows it\n\n> **Answer:** b) All pods in the cluster. Without a NetworkPolicy, Kubernetes allows all pod-to-pod traffic. Add a default-deny policy, then allow only what each app needs.\n\n**Q2. What proves a container image really came from your pipeline, unchanged?**\n\n- a) A scan with Trivy\n- b) A signature with cosign\n- c) A read-only root filesystem\n\n> **Answer:** b) A signature with cosign. Signing with cosign gives provable origin. Scanning finds known CVEs but says nothing about who built the image or whether it was tampered with.\n\n**Q3. Which setting stops a container escape from landing as root on the node?**\n\n- a) runAsNonRoot: true\n- b) a resource limit\n- c) an Ingress rule\n\n> **Answer:** a) runAsNonRoot: true. runAsNonRoot: true makes the pod refuse to start as root, so an escape gets an unprivileged user instead of control of the host.\n\n## Explained\n\nQuestion one: by default, which pods can a new pod talk to? The answer is all pods in the cluster. This surprises people: Kubernetes networking is allow-all out of the box. Until you add a NetworkPolicy, any pod can reach any other pod, across every namespace. The fix is a default-deny policy, then explicit allows for the traffic each app actually needs.\n\nQuestion two: what proves an image really came from your pipeline, unchanged? Signing it with cosign. A scan with Trivy is valuable, but it only finds known vulnerabilities; it says nothing about origin or tampering. A signature answers \"did this come from us, unmodified?\", which is the heart of supply-chain security.\n\nQuestion three: what stops a container escape from landing as root on the node? Setting runAsNonRoot: true in the securityContext. The pod then refuses to start as root, so even an escape gets an unprivileged user instead of control of the host. Resource limits and Ingress rules solve different problems.\n\nIf you got these, you are thinking the way the CKS wants: assume the defaults are open, and close them deliberately.\n\nThat is the series. Fifty posts, from mounting a filesystem to hardening a cluster, plus ten Python scripts you can run today. Thank you for following along.\n\n## In one line\n\n> **Security is the last mile.** From RHCSA to CKS, this is where the path has been heading.\n\n**Keep going:** pick one weak default and fix it this week\n"},{"id":"p222","n":"222","slug":"9-argo-cd-commands-for-daily-gitops-work","title":"9 Argo CD commands for daily GitOps work","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","DevOps","CICD"],"series_name":"Platform","design":"steps","script":"","date":"2028-02-27","files":{"devsecops":"notes/p222-9-argo-cd-commands-for-daily-gitops-work.md"},"md":"# 9 Argo CD commands for daily GitOps work\n\nGitOps means the cluster follows Git, not your terminal history. Argo CD is the tool most teams use to make that true, and its CLI covers the whole loop: sync, inspect, recover.\n\n## Quick reference\n\n### Sync — Send Git to the cluster\n\n```bash\n# Apply the Git state now, on demand.\nargocd app sync guestbook\n# Also delete resources removed from Git.\nargocd app sync guestbook --prune\n# Block until Synced and Healthy. Use it in CI.\nargocd app wait guestbook\n```\n\n### Inspect — See before you change\n\n```bash\n# Every app with its sync and health status.\nargocd app list\n# Spec, live state, and what differs.\nargocd app get guestbook\n# The drift, printed. Nothing is applied.\nargocd app diff guestbook\n```\n\n### Recover — Go back, safely\n\n```bash\n# Deployed revisions, newest first.\nargocd app history guestbook\n# Reapply revision 3 in seconds.\nargocd app rollback guestbook 3\n# Stop a sync that is stuck or wrong.\nargocd app terminate-op guestbook\n```\n\n## Explained\n\n### Sync\n\nargocd app sync applies the Git state now, on demand. It is the same action the controller takes when auto-sync fires.  \n`argocd app sync --prune` also deletes resources that were removed from Git. Without it, deleted manifests leave orphans running for ever.  \nargocd app wait blocks until the app is Synced and Healthy, which is exactly what you want as the last line of a CI job.\n\n### Inspect\n\n`argocd app list` shows every app with its sync and health status: the fastest overview of a cluster.  \nargocd app get shows the spec, the live state, and the differences between them.  \nargocd app diff prints the drift without touching the cluster. This is the command to run before a production sync, not after.\n\n### Recover\n\nargocd app history lists the deployed revisions, newest first.  \nargocd app rollback redeploys an earlier revision in seconds. It works even when the repo is broken, because it replays what was applied.  \nargocd app terminate-op stops a sync that is stuck, or running by mistake.\n\nThe habit that matters: diff before sync in production. A sync is a deployment, so treat it like one and look first.\n\n## In one line\n\n> **Sync is a decision, not a habit.** Diff first in production. A sync is a deployment.\n\n**Diff, then sync**\n\n```bash\nargocd app diff api\n```\n"},{"id":"p223","n":"223","slug":"anatomy-of-a-helm-upgrade","title":"Anatomy of a helm upgrade","sections":["devsecops"],"topic":"GitOps & Helm","tags":["Helm","Kubernetes","GitOps","DevOps","CICD"],"series_name":"Platform","design":"anatomy","script":"","date":"2028-02-29","files":{"devsecops":"notes/p223-anatomy-of-a-helm-upgrade.md"},"md":"# Anatomy of a helm upgrade\n\nHelm is how most teams package Kubernetes manifests, and `helm upgrade` is the command that ships a release. Short enough to type from memory, long enough that every part matters when something goes wrong.\n\n## Quick reference\n\n```text\nhelm upgrade api ./chart --set image.tag=1.4.2 --atomic\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `helm upgrade` | The verb | Creates a release or updates one. On a first install it fails; `--install` fixes that. |\n| `api` | Release name | How Helm tracks this deployment. One release, one name, one history. |\n| `./chart` | The chart | A local folder, a repo chart, or an oci:// reference. The templates to render. |\n| `--set image.tag=1.4.2` | One value | Fast for a test. For anything repeated, use -f `values-prod.yaml`. |\n| `--atomic` | The safety net | Waits for readiness, then rolls the release back if it fails. |\n\n## Explained\n\nThe verb. `helm upgrade` creates a release or updates one that already exists. On a first install it fails, which is why most pipelines use upgrade `--install` so the same command works both ways.\n\nThe release name. api is how Helm tracks this deployment inside the cluster: one release, one name, one history. Reusing a name across environments is how people overwrite production by accident.\n\nThe chart. `./chart` is a local folder. It could also be a repo name or an OCI reference like oci://registry/charts/api. It tells Helm which templates and default values to render.\n\nThe inline override. `--set image.tag=1.4.2` changes one value quickly. It is fine for a test. For anything a pipeline repeats, put the value in a values file with -f, where it can be reviewed and diffed.\n\nThe safety net. `--atomic` makes Helm wait for the new pods to become ready, and rolls the whole release back if they do not. Without it, a failed upgrade leaves the release half-applied and the next deploy inherits the mess.\n\n> **The lesson:** the last flag is what turns an upgrade from a gamble into a rollback plan.\n\n## In one line\n\n> **Upgrade is a rollback plan.** Without `--atomic`, a failed release stays half-applied.\n\n**Dry run first**\n\n```bash\nhelm upgrade api ./chart --dry-run\n```\n"},{"id":"p224","n":"224","slug":"a-gitops-drift-detector-in-python","title":"A GitOps drift detector in Python","sections":["devsecops"],"topic":"Python scripts","tags":["GitOps","Kubernetes","Python","DevSecOps","SRE"],"series_name":"Platform","design":"code","script":"drift_check.py","date":"2028-03-02","files":{"devsecops":"notes/p224-a-gitops-drift-detector-in-python.md"},"md":"# A GitOps drift detector in Python\n\nGitOps has one promise: what is in Git is what runs in the cluster. Drift is when that stops being true, usually because someone ran kubectl edit at 2 a.m. This script finds it, and fails a pipeline when it does.\n\n## Quick reference\n\nThe full script is at the end of this note: `drift_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 drift_check.py deploy\n```\n\n**Example output**\n\n```text\nALERT 2 drifted\n```\n\n## Explained\n\n### What it does\n\nIt walks a folder of manifests, runs kubectl diff against each one, and counts the files that no longer match the live cluster. Anything with a non-zero exit fails the run, including a kubectl error, so a broken kubeconfig cannot pass silently.\n\n### Why kubectl diff\n\nkubectl diff sends each file to the API server, does a server-side dry run, and prints the difference without applying anything. That is the same comparison Argo CD and Flux make during a sync, and it costs nothing but a read.\n\n### How to use it\n\nSave it beside your manifests and point it at the folder: `python3 drift_check.py deploy`. Run it nightly from cron, or as a step after apply in CI. A clean run prints OK and exits 0.\n\n> **The honest caveat:** kubectl diff needs permission to run a server-side dry run, and it is noisy on objects that controllers mutate constantly, like status fields. Scope it to the manifests you own.\n\n> **The lesson:** auto-sync keeps the cluster current, but a drift check catches everything GitOps does not manage yet.\n\n## In one line\n\n> **Drift is a silent change.** Git did not change. The cluster did.\n\n**Run it nightly**\n\n```bash\n0 2 * * * python3 drift_check.py\n```\n\n## Script: drift_check.py\n\nDownload: [scripts/drift_check.py](../scripts/drift_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"drift_check.py - find what changed in the cluster behind Git's back.\n\nWalks a folder of Kubernetes manifests and runs `kubectl diff` against each\none. A manifest that no longer matches the live cluster is drift. Exit code\nis 1 when drift (or a kubectl error) is found, so it drops into cron or a CI\nstep. Standard library only; needs kubectl on PATH and rights for a\nserver-side dry run.\n\nUsage:\n    python3 drift_check.py                     # scans ./deploy\n    python3 drift_check.py path/to/manifests   # scans another folder\n    python3 drift_check.py -n shop deploy      # fixed namespace\n    python3 drift_check.py -q                  # only the summary\n\nCron (nightly at 02:00):\n    0 2 * * * cd /srv/gitops && python3 drift_check.py -q >> /var/log/drift.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport subprocess\nimport sys\nfrom pathlib import Path\n\nSUFFIXES = (\".yaml\", \".yml\")\n\n\ndef kubectl_diff(path, namespace=None, timeout=60):\n    \"\"\"Return (drifted, detail). detail is the diff or the error text.\"\"\"\n    cmd = [\"kubectl\", \"diff\", \"-f\", str(path)]\n    if namespace:\n        cmd += [\"-n\", namespace]\n    try:\n        r = subprocess.run(cmd, capture_output=True, text=True, timeout=timeout)\n    except FileNotFoundError:\n        sys.exit(\"kubectl not found on PATH\")\n    except subprocess.TimeoutExpired:\n        return True, \"timeout after %ss\" % timeout\n    if r.returncode == 0:\n        return False, \"\"\n    # kubectl diff: 0 = same, 1 = differences, >1 = error\n    text = (r.stdout or \"\") + (r.stderr or \"\")\n    return True, text.strip()\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Detect drift between manifests and the live cluster.\")\n    ap.add_argument(\"root\", nargs=\"?\", default=\"deploy\", help=\"folder of manifests, default deploy\")\n    ap.add_argument(\"-n\", \"--namespace\", help=\"namespace for every file\")\n    ap.add_argument(\"-q\", \"--quiet\", action=\"store_true\", help=\"summary only\")\n    ap.add_argument(\"--timeout\", type=int, default=60, help=\"seconds per file, default 60\")\n    args = ap.parse_args()\n\n    root = Path(args.root)\n    if not root.exists():\n        sys.exit(\"not found: %s\" % root)\n    files = sorted(p for p in root.rglob(\"*\") if p.suffix in SUFFIXES and p.is_file())\n    if not files:\n        sys.exit(\"no manifests under %s\" % root)\n\n    drifted, clean = [], 0\n    for f in files:\n        bad, detail = kubectl_diff(f, args.namespace, args.timeout)\n        if bad:\n            drifted.append(f)\n            if not args.quiet:\n                print(\"[DRIFT] %s\" % f)\n                for line in detail.splitlines()[:12]:\n                    print(\"        \" + line)\n                if len(detail.splitlines()) > 12:\n                    print(\"        ...\")\n        else:\n            clean += 1\n\n    print()\n    print(\"checked %d, clean %d, drifted %d\" % (len(files), clean, len(drifted)))\n    if drifted:\n        print(\"ALERT \" + \", \".join(str(p) for p in drifted))\n        sys.exit(1)\n    print(\"OK no drift\")\n    sys.exit(0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p225","n":"225","slug":"helm-vs-kustomize","title":"Helm vs Kustomize","sections":["devsecops"],"topic":"GitOps & Helm","tags":["Helm","Kustomize","Kubernetes","GitOps","DevOps"],"series_name":"Platform","design":"compare","script":"","date":"2028-03-05","files":{"devsecops":"notes/p225-helm-vs-kustomize.md"},"md":"# Helm vs Kustomize\n\nHelm and Kustomize both stop you copying YAML between environments, and they are often framed as rivals. In practice they solve two different problems, and many teams run both.\n\n## Quick reference\n\n| | **HELM** | **KUSTOMIZE** |\n|:--|:--|:--|\n| Approach | Templates + values | Patches on plain YAML |\n| Packaging | Charts, versions, registries | None built in |\n| Learning curve | A day for the basics | An hour for the basics |\n| Per environment | `values-prod.yaml` | overlays/prod |\n| Readable output | Render first: helm template | Plain YAML by design |\n| Best for | Third-party and shared charts | Your own app's env changes |\n\n**HELM** (Charts with version history): Renders templates with your values, packaged as a versioned chart.\n\n```bash\nhelm upgrade -f values.yaml\n```\n\n**KUSTOMIZE** (Plain YAML, patched per env): Applies plain YAML with per-environment patches layered on top.\n\n```bash\nkubectl apply -k overlays/prod\n```\n\n- Helm → package once, install anywhere.\n- Kustomize → one base, an overlay per environment.\n\n## Explained\n\n### How they differ\n\nHelm is a template engine plus a package format. You write templates with variables, set defaults in `values.yaml`, and override per environment. The result is a chart: a versioned artefact you can publish to a registry and install anywhere.\n\nKustomize is not a template engine. You keep plain YAML and describe the changes as patches and overlays: a base, then overlays/prod that edits it. `kubectl apply -k` reads it directly, so nothing extra is installed.\n\n### Where Helm wins\n\nPackaging. A chart carries templates, defaults, version history and a rollback. If you install third-party software, Helm is the standard.  \nReview. A values file is small and readable, and the template logic stays in the chart where it belongs.\n\n### Where Kustomize wins\n\nPlain YAML. What you apply is what you can read, with no templating language to debug.  \nSmall differences. Changing replicas, image tags or limits per environment is a three-line patch.\n\nThe honest answer: use Helm to package and ship, Kustomize to adapt a base per environment. Helm can render to stdout, and Kustomize can consume the result.\n\n> **The rule:** choose per layer, not per company.\n\nTemplates or patches.\n"},{"id":"p226","n":"226","slug":"9-checks-before-you-enable-auto-sync","title":"9 checks before you enable auto-sync","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","DevSecOps","SRE"],"series_name":"Platform","design":"checklist","script":"","date":"2028-03-07","files":{"devsecops":"notes/p226-9-checks-before-you-enable-auto-sync.md"},"md":"# 9 checks before you enable auto-sync\n\nAuto-sync is the part of GitOps everyone wants and the part that bites hardest. It means every merge to main reaches the cluster with no human in between, so the checks that used to happen at deploy time have to happen before the merge.\n\n## Quick reference\n\n### Safety\n\n```bash\n# Prune is deliberate\nprune: true\n# Self-heal is on purpose\nselfHeal: true\n# Rollback works today\nargocd app rollback\n```\n\n### Access\n\n- **Repo key is read-only** — deploy key: read-only\n- **No secrets in Git** — SOPS / Sealed Secrets\n- **RBAC is namespace-scoped** — argocd-manager role\n\n### Visibility\n\n```bash\n# Failed syncs alert\nargocd_app_sync_total\n# Health, not just Synced\nargocd app wait --health\n# Drift is checked nightly\nkubectl diff\n```\n\n## Explained\n\n### Safety\n\nPrune is a decision. With prune on, removing a manifest deletes the resource. Turn it on deliberately, and know what the first prune will remove.  \nSelf-heal is a decision too. With selfHeal on, manual cluster edits are reverted within minutes, including the hotfix someone applied at 2 a.m. That is the point, but someone must know it.  \nRollback exists. History and rollback must work before auto-sync is enabled, not discovered afterwards.\n\n### Access\n\nThe repo key is read-only. Argo CD only needs to read Git. A write-capable deploy key is a credential an attacker can use to change what you deploy.  \nSecrets stay out of Git. SOPS, Sealed Secrets or External Secrets, with the decryption key in the cluster, not the repo.  \nCluster RBAC is scoped. argocd-manager should not be cluster-admin everywhere. Give it what it needs, namespace by namespace.\n\n### Visibility\n\nFailed syncs alert. A failed sync nobody sees is a silent outage, so alert on the Argo CD metrics, not on a dashboard someone remembers to open.  \nHealth is checked. Synced is not the same as Healthy. An app can be Synced and still CrashLooping.  \nDrift is watched. Anything GitOps does not manage still drifts. Run a diff nightly.\n\n> **The lesson:** auto-sync does not remove the checks, it moves them earlier, into the pull request.\n\n## In one line\n\n> **Auto-sync is a promise.** It only holds if a bad commit can be undone in minutes.\n\n**The first check**\n\n```bash\nargocd app diff api\n```\n"},{"id":"p227","n":"227","slug":"how-a-commit-reaches-the-cluster","title":"How a commit reaches the cluster","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","CICD","DevOps"],"series_name":"Platform","design":"pipeline","script":"","date":"2028-03-09","files":{"devsecops":"notes/p227-how-a-commit-reaches-the-cluster.md"},"md":"# How a commit reaches the cluster\n\nA merge to main passes CI, and four minutes later the cluster runs a different image. The path between those two moments is short, mostly automatic, and worth knowing end to end.\n\n## Quick reference\n\n1. **A commit lands on main** — The merge to main is the only trigger that matters. · `git`\n2. **CI builds an image** — Tests pass, then the image is tagged with the commit SHA. · *ci*\n3. **Scan and sign** — Trivy checks for CVEs; cosign signs the digest. · *scan+sign*\n4. **Manifest is updated** — CI writes the new tag into the manifests repo. · *manifest*\n5. **Argo CD detects it** — It polls Git every three minutes, or gets a webhook. · `argocd`\n6. **Apply and check health** — The diff is applied, then health gates the result. · *health*\n\n## Explained\n\n### The pipeline\n\n1. **A commit is pushed.** The merge to main is the only trigger that matters.\n   \\2. CI builds an image. Tests run, then a multi-stage build tags it with the commit SHA.\n   \\3. Scan and sign. Trivy checks the image for CVEs, and cosign signs the digest.\n   \\4. The manifest is updated. CI writes the new tag into the manifests repo, usually as a pull request.\n   \\5. Argo CD detects it. It polls Git every three minutes by default, or receives a webhook, then computes the diff.\n   \\6. Apply and check health. Argo CD applies the diff and waits for health before the sync is called done.\n\n### Where it breaks\n\nMost failures are not in the cluster. A mutable tag means CI ships something different from what it tested. A manifest repo without review means the wrong change is already in Git. An app that reports Synced while pods CrashLoop is a health check nobody read.\n\n### The habit\n\nThe habit that matters: move the image digest, not the tag, between stages. A digest cannot be repointed, so every step above can be audited afterwards.\n\n> **The honest caveat:** this chain is only as strong as the manifest repo's permissions. If CI can push straight to main there, the review step is decoration.\n\n## In one line\n\n> **Git is the source. The cluster is a copy.** Every step after the commit is mechanical and repeatable.\n\n**Watch it happen**\n\n```bash\nargocd app get api\n```\n"},{"id":"p228","n":"228","slug":"5-gitops-mistakes-and-the-fix-for-each","title":"5 GitOps mistakes and the fix for each","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","DevSecOps","SRE"],"series_name":"Platform","design":"dontdo","script":"","date":"2028-03-12","files":{"devsecops":"notes/p228-5-gitops-mistakes-and-the-fix-for-each.md"},"md":"# 5 GitOps mistakes and the fix for each\n\nGitOps fails in the same five ways in almost every cluster I have seen. None is exotic, and all are fixable before the next merge.\n\n## Quick reference\n\n**1.** ❌ **Hand edits in the cluster** — Self-heal reverts them, or worse, they survive until the next sync.  \n✅ **Every change through Git** — A pull request is the review, the record and the rollback point.\n\n**2.** ❌ **Deploying the :latest tag** — A rollback replays the tag, not the bits it pointed at that day.  \n✅ **Pin the image digest** — A digest cannot move, so history and rollback mean something.\n\n**3.** ❌ **Prune enabled by default** — One deleted manifest can take a database or a namespace with it.  \n✅ **Prune after an inventory** — List what will be deleted, then enable prune deliberately.\n\n**4.** ❌ **Secrets committed to Git** — Git history keeps the secret long after the file is deleted.  \n✅ **Sealed Secrets or SOPS** — The encrypted value is safe to commit; the key lives in the cluster.\n\n**5.** ❌ **Auto-sync with no rollback** — A bad commit reaches production with no human and no way back.  \n✅ **Test rollback first** — argocd app rollback must work before you hand over the keys.\n\n## Explained\n\n### The mistakes\n\n1. **Hand edits.** kubectl edit at midnight is the oldest habit in operations. With self-heal on it is reverted; with self-heal off it survives until the next sync. Either way Git no longer describes production. Every change goes through a pull request, even the urgent one.\n\n2. **The :latest tag.** It looks convenient until a rollback replays the tag and pulls whatever latest means today. Pin the digest. Then history and rollback point at the exact bits that ran.\n\n3. **Prune, blind.** prune: true deletes every tracked resource that is no longer in Git, including the database someone removed from a chart by accident. Run a diff, list what will go, then enable it.\n\n4. **Secrets in Git.** Deleting a secret from Git does not delete it from history or from every clone. Use Sealed Secrets, SOPS or External Secrets, and keep the key out of the repo.\n\n5. **Auto-sync, no rollback.** The commit reaches production with no human and no way back. Run argocd app rollback once before enabling auto-sync, not during the incident.\n\n### The common thread\n\n> **The lesson:** each mistake removes a review, a pin or a way back. GitOps is not safe because it is automatic; it is safe when those three survive.\n\n> **The honest caveat:** self-heal will also revert the fix you apply by hand during an incident, so know how to pause auto-sync before you need to.\n\n## In one line\n\n> **Automate the deploy, not the mistake.** If a change did not come through a pull request, it is drift.\n\n**Test rollback now**\n\n```bash\nargocd app rollback api 3\n```\n"},{"id":"p229","n":"229","slug":"an-argo-cd-sync-report-in-python","title":"An Argo CD sync report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["GitOps","ArgoCD","Python","DevOps","SRE"],"series_name":"Platform","design":"code","script":"argocd_report.py","date":"2028-03-14","files":{"devsecops":"notes/p229-an-argo-cd-sync-report-in-python.md"},"md":"# An Argo CD sync report in Python\n\nA sync dashboard is fine until you need a number for a change record. Then you are scrolling the Argo CD UI during an incident, counting red rows by hand.\n\n## Quick reference\n\nThe full script is at the end of this note: `argocd_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nargocd app list -o json | python3 argocd_report.py -\n```\n\n**Example output**\n\n```text\nshop-api  OutOfSync  Healthy\nALERT 3 of 12 need attention\n```\n\n## Explained\n\n### What it does\n\nThe script reads `argocd app list -o json` from stdin or a file. For every app it compares status.sync.status and status.health.status, prints a line for anything that is not Synced and Healthy, then a count. Exit 1 means something needs attention, 0 means clean, 2 means bad JSON or a usage error. It is read-only: it never syncs, rolls back or writes to the cluster.\n\n### Why JSON\n\nThe CLI's JSON is a stable contract, and it carries more than the table: labels, revisions, operation state. Text tables change between CLI versions; the JSON shape is what people script against. One pipe works on a laptop, in cron and in CI, with no screen scraping and no extra dependency.\n\n### How to use it\n\n`argocd app list -o json | python3 argocd_report.py -`. Or save the output and pass the file. In CI, run it after argocd app sync api, which waits by default, so the pipeline fails on the same state a human would see.\n\n> **The honest caveat:** the script only knows what Argo CD knows. An app with no readiness probe can be Synced and Healthy while returning 500s, so pair the report with a real health signal.\n\n> **The lesson:** a report turns a dashboard into an exit code, and an exit code can fail a pipeline.\n\n## In one line\n\n> **A dashboard is not an alert.** An app can be Synced and still be down.\n\n**Pipe it in**\n\n```bash\nargocd app list -o json\n```\n\n## Script: argocd_report.py\n\nDownload: [scripts/argocd_report.py](../scripts/argocd_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"argocd_report.py - a short GitOps sync report from Argo CD's JSON.\n\nReads `argocd app list -o json` from a file or stdin ('-'), prints every\nApplication that is not Synced and Healthy, and exits 1 if any need\nattention. Read-only: it never syncs, rolls back or writes to the cluster.\n\nUsage:\n    argocd app list -o json | python3 argocd_report.py -\n    argocd app list -o json > apps.json && python3 argocd_report.py apps.json\n    python3 argocd_report.py -q apps.json      # summary only\n\nCI (after a sync):\n    argocd app sync api\n    argocd app list -o json | python3 argocd_report.py -\n\nCron (every 15 minutes):\n    */15 * * * * argocd app list -o json | python3 /opt/scripts/argocd_report.py -q - \\\n        >> /var/log/argocd_report.log 2>&1\n\nExit codes: 0 all Synced and Healthy, 1 something needs attention, 2 usage\nor input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\ndef load(path):\n    \"\"\"Return a list of Application objects from Argo CD's JSON.\"\"\"\n    if path == \"-\":\n        raw = sys.stdin.read()\n    else:\n        with open(path, encoding=\"utf-8\") as fh:\n            raw = fh.read()\n    if not raw.strip():\n        raise ValueError(\"empty input\")\n    data = json.loads(raw)\n    if isinstance(data, dict):\n        data = data.get(\"items\")\n        if not isinstance(data, list):\n            raise ValueError(\"JSON object has no items list\")\n    if not isinstance(data, list):\n        raise ValueError(\"expected a JSON array of Applications\")\n    if not all(isinstance(app, dict) for app in data):\n        raise ValueError(\"expected Application objects in the array\")\n    return data\n\n\ndef state(app):\n    \"\"\"Return (sync, health) for one Application, tolerating missing fields.\"\"\"\n    status = app.get(\"status\") or {}\n    sync = (status.get(\"sync\") or {}).get(\"status\", \"Unknown\")\n    health = (status.get(\"health\") or {}).get(\"status\", \"Unknown\")\n    return sync, health\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Report Argo CD apps that are not Synced and Healthy.\",\n        epilog=\"Read-only. Exit 0 clean, 1 attention needed, 2 input error.\",\n    )\n    ap.add_argument(\"path\", nargs=\"?\", default=\"-\",\n                    help=\"JSON file from 'argocd app list -o json', or - for stdin\")\n    ap.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                    help=\"print the summary only\")\n    args = ap.parse_args()\n\n    try:\n        apps = load(args.path)\n    except (OSError, ValueError) as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    rows = []\n    for app in apps:\n        sync, health = state(app)\n        if sync != \"Synced\" or health != \"Healthy\":\n            name = (app.get(\"metadata\") or {}).get(\"name\", \"?\")\n            rows.append((name, sync, health))\n    rows.sort()\n\n    out_of_sync = sum(1 for _, sync, _ in rows if sync != \"Synced\")\n    unhealthy = sum(1 for _, _, health in rows if health != \"Healthy\")\n    attention = len(rows)\n\n    if not args.quiet and rows:\n        print(\"%-28s %-12s %s\" % (\"APP\", \"SYNC\", \"HEALTH\"))\n        for name, sync, health in rows:\n            print(\"%-28s %-12s %s\" % (name, sync, health))\n        print()\n\n    print(\"checked %d apps: %d out of sync, %d unhealthy\"\n          % (len(apps), out_of_sync, unhealthy))\n    if attention:\n        print(\"ALERT %d of %d apps need attention\" % (attention, len(apps)))\n        return 1\n    print(\"OK %d apps Synced and Healthy\" % len(apps))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p230","n":"230","slug":"gitops-terms-decoded","title":"GitOps terms, decoded","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","PlatformEngineering","DevOps"],"series_name":"Platform","design":"reveal","script":"","date":"2028-03-16","files":{"devsecops":"notes/p230-gitops-terms-decoded.md"},"md":"# GitOps terms, decoded\n\nGitOps arguments usually end with two people meaning different things by the same word. Six terms, decoded, so the next design review is shorter.\n\n## Quick reference\n\n- **drift** — The live cluster no longer matches Git: a hand edit, a deleted resource, a changed image.\n- **sync** — Argo CD applies the Git state to the cluster and records the revision it applied.\n- **prune** — Deleting live resources that are no longer in Git. Off by default, and dangerous blind.\n- **self-heal** — Argo CD reverts manual changes to the resources it tracks, within one reconcile loop.\n- **ApplicationSet** — A controller that generates Applications from a template and a generator, one per target.\n- **app-of-apps** — One Application whose manifests are other Applications. A tree you manage as a unit.\n\n## Explained\n\n### The basics\n\ndrift. The live cluster no longer matches Git: a hand edit, a deleted resource, a changed image. Some drift is invisible until a diff runs. A diff is the only reliable way to see it.  \nsync. Argo CD applies the Git state to the cluster and records the revision it applied. Synced means the diff is empty, nothing more.  \nprune. Deleting live resources that are no longer in Git. Off by default, and dangerous when it is turned on without a look.  \nself-heal. Argo CD reverts manual changes to the resources it tracks, usually within one reconciliation loop. It is not a rollback; it restores what Git already says.\n\n### At scale\n\nApplicationSet. A controller that generates Applications from a template and a generator: one per cluster, per pull request or per tenant.  \napp-of-apps. One Application whose manifests are other Applications. A tree you manage as a unit, instead of a list you maintain by hand.\n\n> **The lesson:** the vocabulary is simple; the defaults are where the arguments live. Most disagreements are about prune, self-heal and who owns the generated apps. Ask which one each person means before the argument starts.\n\n> **The honest caveat:** self-heal only touches resources Argo CD tracks. Anything created outside the app is invisible to it, and stays until someone looks.\n\n## In one line\n\n> **Learn the words, read the diff.** Most GitOps arguments are two definitions of the same term.\n\n**The first term**\n\n```bash\nargocd app diff api\n```\n"},{"id":"p231","n":"231","slug":"quiz-3-gitops-questions","title":"Quiz: 3 GitOps questions","sections":["devsecops"],"topic":"GitOps & Helm","tags":["GitOps","ArgoCD","Kubernetes","SRE","DevOps"],"series_name":"Platform","design":"quiz","script":"","date":"2028-03-19","files":{"devsecops":"notes/p231-quiz-3-gitops-questions.md"},"md":"# Quiz: 3 GitOps questions\n\nThree GitOps questions I ask in interviews, because the answers separate people who run auto-sync from people who understand it. No CLI trivia, just the trade-offs.\n\n## Quick reference\n\n**Q1. Why does a :latest tag break rollbacks?**\n\n- a) It pulls slower\n- b) The tag can point somewhere new\n- c) Kubernetes refuses it\n\n> **Answer:** b) The tag can point somewhere new. A rollback replays the manifest, not the image. If latest moved, you deploy different bits. Pin the digest.\n\n**Q2. What does prune actually do?**\n\n- a) Deletes resources removed from Git\n- b) Deletes old ReplicaSets\n- c) Clears the Argo CD cache\n\n> **Answer:** a) Deletes resources removed from Git. Prune deletes live resources no longer in Git. Off by default. With it on, one deleted manifest deletes the resource.\n\n**Q3. Why diff before sync in production?**\n\n- a) It is faster\n- b) It shows the change before applying it\n- c) It skips the health check\n\n> **Answer:** b) It shows the change before applying it. A sync is a deployment. The diff shows the exact change while nothing has been applied yet.\n\n## Explained\n\n### Three questions\n\n1. **Why does a :latest tag break rollbacks?**\n\n> **Answer:** the tag can point somewhere new. A rollback replays the manifest, not the image, so if latest moved, the rollback deploys different bits or nothing at all. Pin the digest and the history means something. That is why the digest is the only safe reference.\n\n2. **What does prune actually do?**\n\n> **Answer:** it deletes live resources that are no longer in Git. It is off by default. With it on, deleting a manifest deletes the resource, including a database that was removed by accident. Diff first, then prune. And prune runs after the diff, never instead of it.\n\n3. **Why diff before sync in production?**\n\n> **Answer:** a sync is a deployment. The diff shows the exact change before anything is applied. It is the cheapest review you will ever do, and the only one that sees the live cluster. It is also the point where a surprise shows up before a user does.\n\n### The pattern\n\n> **The lesson:** the commands are simple; the consequences are not. A quiz is cheap; an incident is not.\n\n> **The honest caveat:** a clean diff does not prove the app works. Only health checks and real traffic do.\n\n## In one line\n\n> **Know the why, not the verb.** GitOps is a set of trade-offs, not a list of commands.\n\n**Check the history**\n\n```bash\nargocd app history api\n```\n"},{"id":"p232","n":"232","slug":"9-promql-queries-for-real-incidents","title":"9 PromQL queries for real incidents","sections":["devsecops"],"topic":"Observability & SRE","tags":["Prometheus","PromQL","SRE","Observability","Kubernetes"],"series_name":"Platform","design":"terminal","script":"","date":"2028-03-21","files":{"devsecops":"notes/p232-9-promql-queries-for-real-incidents.md"},"md":"# 9 PromQL queries for real incidents\n\nAn alert fires at 02:14. Before you open a dashboard, answer three questions in order: is traffic normal, is it failing, is it slow. Nine PromQL lines do that.\n\n## Quick reference\n\n### Is it traffic?\n\n```promql\n# per-series request rate, last 5 minutes\nrate(http_reqs_total[5m])\n# one number for the whole service\nsum(rate(http_reqs_total[5m]))\n# split by status. Spikes show up here\nsum by (code) (rate(http_reqs_total[5m]))\n```\n\n### Is it failing?\n\n```promql\n# 5xx per second, the headline number\nsum(rate(http_reqs_total{code=~\"5..\"}[5m]))\n# errors in the last hour, for the note\nincrease(http_reqs_total{code=~\"5..\"}[1h])\n# 2.1% failing, from a recording rule\njob:http_error_ratio:rate5m\n```\n\n### Is it slow?\n\n```promql\n# p50: the typical request\nhistogram_quantile(.5,rate(lat_bucket[5m]))\n# p95: what most users feel\nhistogram_quantile(.95,rate(lat_bucket[5m]))\n# p99 above 1s. This is the page\nhistogram_quantile(.99,rate(lat_bucket[5m]))\n```\n\n## Explained\n\n### Rate\n\n`rate(http_reqs_total[5m])` gives requests per second for each series.  \n`sum(rate(http_reqs_total[5m]))` collapses them into one number for the service.  \n`sum by (code) (rate(http_reqs_total[5m]))` splits it by status, which is where a spike first shows. A flat line means traffic is not your problem.\n\n### Errors\n\n`sum(rate(http_reqs_total{code=~\"5..\"}[5m]))` is the 5xx rate per second. Compare it with the same hour last week.  \n`increase(http_reqs_total{code=~\"5..\"}[1h])` is the count for the incident note. It is the number you paste into the incident channel.  \n`job:http_error_ratio:rate5m` is the ratio from a recording rule: 0.021 is 2.1% failing.\n\n### Latency\n\n`histogram_quantile(.5,rate(lat_bucket[5m]))` is p50, the typical request. A 41ms p50 with a 1.2s p99 is a bimodal service.  \n`histogram_quantile(.95,rate(lat_bucket[5m]))` is p95, what most users feel. If p95 moves, they feel it.  \n`histogram_quantile(.99,rate(lat_bucket[5m]))` is p99, and the one that pages.\n\n### The habit\n\nThe habit that matters: start with rate, then errors, then latency. It stops you tuning a slow endpoint that is simply receiving a traffic spike, and answers the three questions in the order that matters. It is a triage order, not a tour of the metrics.\n\n> **The honest caveat:** rate() needs a counter and at least two samples. A 5m window holds 20 scrapes at a 15s interval, so do not shorten it below a minute.\n\n## In one line\n\n> **Ask the metric, not the dashboard.** Every incident starts with one query and a timestamp.\n\n**Try it**\n\n```bash\npromtool query instant http://localhost:9090 up\n```\n"},{"id":"p233","n":"233","slug":"the-observability-stack-layer-by-layer","title":"The observability stack, layer by layer","sections":["devsecops"],"topic":"Observability & SRE","tags":["Observability","Prometheus","SRE","OpenTelemetry","DevOps"],"series_name":"Platform","design":"stack","script":"","date":"2028-03-23","files":{"devsecops":"notes/p233-the-observability-stack-layer-by-layer.md"},"md":"# The observability stack, layer by layer\n\nMost monitoring stacks are drawn as boxes, which hides the only question that matters during an incident: where did the signal stop?\n\n## Quick reference\n\n*Bottom to top*\n\n1. **Collection** — node_exporter, kubelet and app /metrics expose the raw numbers. · *agents*\n2. **Pipeline** — The OpenTelemetry Collector batches, retries, samples and relabels. · *OTel Collector*\n3. **Storage** — Prometheus writes 2-hour blocks; Thanos or Mimir keeps history. · *TSDB*\n4. **Query** — PromQL reads the TSDB; Grafana draws it, but the query is the truth. · *PromQL*\n5. **Alerting** — Alertmanager deduplicates, inhibits and routes to the right channel. · *Alertmanager*\n6. **Action** — A runbook, a dashboard link and a rollback command, not just a page. · *runbooks*\n\n## Explained\n\n### Collect\n\nCollection. node_exporter reads the host, kubelet exposes pod and container metrics, and every serious app publishes its own /metrics endpoint. Collection is pull by default: Prometheus scrapes, so a dead target shows up as down instead of silently missing.  \nPipeline. The OpenTelemetry Collector sits between the app and the store. It batches, retries, samples and relabels, and turns many formats into one. Useful, and one more thing to monitor.\n\n### Store and query\n\nStorage. Prometheus writes two-hour blocks into a local TSDB. That is fine for weeks, not years, so Thanos or Mimir takes the long-term copy.  \nQuery. PromQL reads the TSDB. Grafana draws the result, but the query is the truth, and a dashboard is only one view of it.\n\n### Alert and act\n\nAlerting. Alertmanager deduplicates, inhibits and routes. It decides who is woken up, so its config deserves review like code.  \nAction. A page needs a runbook, a dashboard link and a rollback command. Without those, the alert is just a notification.\n\n> **The lesson:** each layer has one job, and a wrong alert usually means one layer lost or changed the signal.\n\n> **The honest caveat:** every added layer is a new failure point. A Collector between the app and Prometheus can drop or delay metrics, so monitor the Collector itself.\n\n## In one line\n\n> **Follow the signal, not the diagram.** Every wrong alert is a layer that lost or changed the signal.\n\n**Monitor the collector**\n\n```bash\ncurl localhost:8888/metrics\n```\n"},{"id":"p234","n":"234","slug":"nginx-latency-percentiles-in-python","title":"nginx latency percentiles in Python","sections":["devsecops"],"topic":"Python scripts","tags":["nginx","Python","SRE","Observability","DevOps"],"series_name":"Platform","design":"code","script":"log_latency.py","date":"2028-03-26","files":{"devsecops":"notes/p234-nginx-latency-percentiles-in-python.md"},"md":"# nginx latency percentiles in Python\n\nYour nginx access log already holds the latency data you need. You do not need a metrics stack to answer the question that follows every deploy: is p99 worse than it was this morning?\n\n## Quick reference\n\nThe full script is at the end of this note: `log_latency.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 log_latency.py access.log\n```\n\n**Example output**\n\n```text\np50 48ms  p95 164ms\np99 210ms  5xx 5.00%\n```\n\n## Explained\n\n### What it does\n\nThe script reads an access log line by line, takes the status code and the $request_time field in seconds, converts the time to milliseconds, and keeps them in a list. Nearest-rank keeps it simple: sort, then take the value at the 99th percentile position. It prints p50, p95, p99 and the 5xx rate, then exits 1 if p99 passes 1000ms or the 5xx rate passes 5%. The 5xx rate uses the same lines, so both numbers describe one window.\n\n### Why percentiles\n\nAn average hides the requests that hurt. One slow checkout among a hundred fast pages barely moves the mean, but it is the request the customer remembers. p99 names the tail, and the tail is where timeouts and abandoned carts live.\n\n### How to use it\n\n`python3 log_latency.py access.log`, or pipe it: `cat access.log | python3 log_latency.py` -. Add it after a deploy, or run it from cron over yesterday's log. The thresholds are flags, not constants.\n\n> **The honest caveat:** the log only covers requests that reached nginx. A timeout at the load balancer never appears, and $request_time includes upstream time but not the queue before nginx.\n\n> **The lesson:** an average describes the system; a percentile describes the user.\n\n## In one line\n\n> **The tail is the user.** The average hides the request that timed out.\n\n**Check after deploy**\n\n```bash\npython3 log_latency.py access.log\n```\n\n## Script: log_latency.py\n\nDownload: [scripts/log_latency.py](../scripts/log_latency.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"log_latency.py - p50/p95/p99 and 5xx rate from an nginx access log.\n\nParses the nginx combined log format with $request_time appended as the last\nfield. The matching log_format is:\n\n    log_format main '$remote_addr - $remote_user [$time_local] \"$request\" '\n                    '$status $body_bytes_sent \"$http_referer\" '\n                    '\"$http_user_agent\" $request_time';\n\nReads a file or stdin ('-'), prints p50/p95/p99 in milliseconds and the 5xx\nrate, and exits 1 when p99 or the 5xx rate is over the thresholds. Read-only.\n\nUsage:\n    python3 log_latency.py access.log\n    cat access.log | python3 log_latency.py -\n    python3 log_latency.py access.log --p99-ms 800 --max-5xx 2\n\nCI (after a deploy):\n    cat /var/log/nginx/access.log | python3 log_latency.py - --p99-ms 800\n\nCron (yesterday's log, 06:00):\n    0 6 * * * python3 /opt/scripts/log_latency.py /var/log/nginx/access.log.1 \\\n        >> /var/log/latency.log 2>&1\n\nExit codes: 0 within thresholds, 1 over a threshold, 2 usage or no data.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport math\nimport re\nimport sys\n\n# Closing quote of the request line, status, body bytes, referer, user agent,\n# then $request_time in seconds at the end of the line.\nLINE = re.compile(\n    r'\"\\s+(?P<status>[1-5]\\d{2})\\s+\\d+\\s+\"[^\"]*\"\\s+\"[^\"]*\"\\s+'\n    r'(?P<rt>\\d+(?:\\.\\d+)?)\\s*$'\n)\n\n\ndef percentile(values, q):\n    \"\"\"Nearest-rank percentile of a sorted list. q is 0-100.\"\"\"\n    if not values:\n        return 0.0\n    rank = max(1, math.ceil(q / 100.0 * len(values)))\n    return values[rank - 1]\n\n\ndef report(total, skipped, statuses, times, p99_ms, max_5xx):\n    n = len(times)\n    bad = sum(1 for s in statuses if s.startswith(\"5\"))\n    rate = 100.0 * bad / n\n    times.sort()\n    p50 = percentile(times, 50)\n    p95 = percentile(times, 95)\n    p99 = percentile(times, 99)\n\n    print(\"parsed %d  skipped %d  5xx %d (%.2f%%)\" % (total, skipped, bad, rate))\n    print(\"p50 %.0fms  p95 %.0fms  p99 %.0fms\" % (p50, p95, p99))\n\n    problems = []\n    if p99 > p99_ms:\n        problems.append(\"p99 %.0fms over %.0fms\" % (p99, p99_ms))\n    if rate > max_5xx:\n        problems.append(\"5xx %.2f%% over %.1f%%\" % (rate, max_5xx))\n    if problems:\n        for problem in problems:\n            print(\"ALERT \" + problem)\n        return 1\n    print(\"OK p99 %.0fms within %.0fms, 5xx %.2f%% within %.1f%%\"\n          % (p99, p99_ms, rate, max_5xx))\n    return 0\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"p50/p95/p99 and 5xx rate from an nginx access log.\",\n        epilog=\"Read-only. Exit 0 clean, 1 over a threshold, 2 usage or no data.\",\n    )\n    ap.add_argument(\"log\", nargs=\"?\", default=\"-\",\n                    help=\"access log file, or - for stdin (default)\")\n    ap.add_argument(\"--p99-ms\", type=float, default=1000.0,\n                    help=\"p99 threshold in milliseconds (default 1000)\")\n    ap.add_argument(\"--max-5xx\", type=float, default=5.0,\n                    help=\"5xx rate threshold in percent (default 5)\")\n    args = ap.parse_args()\n\n    if args.p99_ms < 0 or args.max_5xx < 0:\n        print(\"error: thresholds must not be negative\", file=sys.stderr)\n        return 2\n\n    if args.log == \"-\":\n        fh = sys.stdin\n        close = False\n    else:\n        try:\n            fh = open(args.log, encoding=\"utf-8\", errors=\"replace\")\n        except OSError as exc:\n            print(\"error: %s\" % exc, file=sys.stderr)\n            return 2\n        close = True\n\n    total = skipped = 0\n    statuses, times = [], []\n    try:\n        for line in fh:\n            total += 1\n            match = LINE.search(line)\n            if not match:\n                skipped += 1\n                continue\n            statuses.append(match.group(\"status\"))\n            times.append(float(match.group(\"rt\")) * 1000.0)\n    finally:\n        if close:\n            fh.close()\n\n    if not times:\n        print(\"error: no timed requests found; add $request_time to the \"\n              \"nginx log_format\", file=sys.stderr)\n        return 2\n\n    return report(total, skipped, statuses, times, args.p99_ms, args.max_5xx)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p235","n":"235","slug":"6-numbers-that-set-your-monitoring","title":"6 numbers that set your monitoring","sections":["devsecops"],"topic":"Observability & SRE","tags":["Prometheus","SRE","Observability","Monitoring","DevOps"],"series_name":"Platform","design":"numbers","script":"","date":"2028-03-28","files":{"devsecops":"notes/p235-6-numbers-that-set-your-monitoring.md"},"md":"# 6 numbers that set your monitoring\n\nMonitoring defaults are not opinions. Six numbers decide how fast you notice, how often you page, and how far back you can look.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **15s** | Scrape interval | A common target: four samples a minute, enough for rate(). | scrape |\n| **5m** | Alert for: duration | A starting point that skips one bad scrape and still pages fast. | alert |\n| **14.4x** | Burn rate that pages | 1h at 14.4x spends 2% of a 30-day budget. Pair it with 5m. | SLO |\n| **15d** | Retention default | Local Prometheus blocks last 15 days unless you raise it. | storage |\n| **p99** | Not the average | The slowest 1% complains; the mean hides them. | latency |\n| **30d** | SLO window | A month of requests makes a 99.9% target measurable. | window |\n\n## Explained\n\n### Scrape\n\n15s, the scrape interval. Many teams scrape every 15 seconds: four samples a minute, enough for rate() and small enough to catch a short spike. Prometheus's own default is one minute, which is slower than most symptom alerts want.\n\n### Alert\n\n5m, the for: duration. A starting point for symptom alerts: long enough to skip a single bad scrape, short enough that five minutes of pain still gets a response. It is a choice, not a default.\n\n14.4x, the burn rate that pages. At a 30-day budget, burning 14.4 times the normal rate for one hour spends 2% of the budget. That is the multi-window alert from the SRE workbook, paired with a 5m short window.\n\n### Retain and read\n\n15d, the retention default. Prometheus keeps local blocks for 15 days unless you raise it. That covers most incident reviews; it does not cover a quarterly trend, which is what remote storage is for.\n\np99, not the average. The slowest 1% is who complains, and the mean hides them. Put the percentile on the dashboard and the average in the appendix.\n\n30d, the SLO window. A month of requests makes a 99.9% target measurable and stops a quiet week from flattering the number.\n\n> **The honest caveat:** none of these is universal. A 15s scrape is wrong for an hourly batch job, and 15 days of retention is useless if reviews happen quarterly.\n\n> **The lesson:** pick the number that matches the user-visible threshold, then configure the tool.\n\n## In one line\n\n> **Pick the number, then the tool.** A default nobody measured is a rumour with a config file.\n\n**Set it in code**\n\n```bash\nscrape_interval: 15s\n```\n"},{"id":"p236","n":"236","slug":"logs-vs-metrics-vs-traces","title":"Logs vs metrics vs traces","sections":["devsecops"],"topic":"Observability & SRE","tags":["Observability","SRE","Prometheus","DevOps","Monitoring"],"series_name":"Platform","design":"compare","script":"","date":"2028-03-30","files":{"devsecops":"notes/p236-logs-vs-metrics-vs-traces.md"},"md":"# Logs vs metrics vs traces\n\nA page fires at 02:40 and checkout is slow. Three signals can explain it, and the team opens logs, as always.\n\n## Quick reference\n\n| | **METRICS** | **EVENTS** |\n|:--|:--|:--|\n| Answers | How much, how fast? | What happened, where? |\n| Cost | Cheap per series | Per GB and per span |\n| Cardinality | Bounded by labels | Unbounded by default |\n| Best at | Trends and alerts | One request, end to end |\n| Worst at | Explaining one request | Long-term trends |\n| When to use | Always on | When you need detail |\n\n**METRICS** (Numbers over time): Aggregates with labels: cheap, bounded, queried in milliseconds.\n\n```promql\nrate(http_requests_total[5m])\n```\n\n**EVENTS** (Logs and traces, one by one): Every event, kept for days: expensive per GB and per span.\n\n```bash\n{job=\"api\"} |= \"error\"\n```\n\n- Metrics → keep them always, they are cheap.\n- Logs + traces → open them when a page fires.\n\n## Explained\n\n### Three questions\n\nMetrics answer how much and how fast: request rate, error ratio, p99 latency. Numbers with labels, cheap enough to keep for years.  \nLogs answer what happened, in the words your code wrote: one line per event, with the fields you chose to print.  \nTraces answer where the time went, span by span, across every service the request touched.\n\n### Two families\n\nThe three split in two. Metrics are aggregates: one series per label set, and that set is the unit of cost. Logs and traces are events, priced per GB or per span.  \nCardinality is where the bill comes from: a user ID label turns one metric into millions of series and slows every query. Event data carries that detail by design, which is why it is sampled or kept for days.  \nMetrics are best at trends and alerts, worst at explaining one request. Logs and traces are best at one request, and worst at cheap long-term trends.\n\n### When to use each\n\nPage on metrics, because they are always on. Open traces to find the slow hop, and logs to read the error in full.\n\n> **The honest caveat:** traces are sampled at the SDK or the collector, so the slow request you hunt may not have been kept.\n\n> **The lesson:** pick the signal by the question, not the tool you already have open.\n\n## In one line\n\n> **Metrics page. Events explain.** Metrics tell you something broke. Events tell you what.\n\n**Add a trace:** trace_id in every log line\n"},{"id":"p237","n":"237","slug":"how-a-metric-becomes-an-alert","title":"How a metric becomes an alert","sections":["devsecops"],"topic":"Observability & SRE","tags":["Prometheus","Alerting","SRE","DevOps","Observability"],"series_name":"Platform","design":"pipeline","script":"","date":"2028-04-02","files":{"devsecops":"notes/p237-how-a-metric-becomes-an-alert.md"},"md":"# How a metric becomes an alert\n\nThe page lands at 03:12. By then the metric has been scraped, stored, evaluated, fired, grouped and routed, and any of those six stages can be the reason it is late.\n\n## Quick reference\n\n1. **Scrape** — Prometheus pulls /metrics on a schedule, 1m by default. · *pull, not push*\n2. **Store** — Samples go to the TSDB, one series per label set. · *tsdb*\n3. **Evaluate** — The rule engine runs the PromQL expression on a timer. · *1m default*\n4. **Fire** — True for the whole for: duration, then the alert fires. · `pending/firing`\n5. **Route and group** — Alertmanager dedupes, groups and routes by labels. · `group_by`\n6. **Notify** — A receiver delivers it: Slack, email, PagerDuty, webhook. · *receiver*\n\n## Explained\n\n### The path\n\nEach stage has its own clock and its own way to fail.  \nScrape. Prometheus pulls /metrics from each target on the scrape interval, one minute by default.  \nStore. Every sample is appended to the TSDB under its full label set, and that set is the series identity.  \nEvaluate. On each evaluation_interval, also one minute by default, the rule engine runs the expression in expr.  \nFire. When the expression returns a result for the whole for: duration, the alert moves from pending to firing.  \nRoute and group. Prometheus sends firing alerts to Alertmanager, which groups and routes them by label.  \nNotify. The receiver delivers: Slack, email, PagerDuty or a webhook, and the text is whatever the annotation said.\n\n### Four clocks\n\nFour clocks run here: scrape, evaluation, for: and Alertmanager's group_wait, 30 seconds by default. Add them up and a page can sit two or three minutes behind the event while every part works. The first notification of an incident is the slowest, because Alertmanager is still collecting siblings.\n\n> **The honest caveat:** no stage knows what the user feels. The rule can be green while checkout is broken.\n\n> **The lesson:** when a page is late or noisy, find the stage, not the person.\n\n## In one line\n\n> **An alert is a pipeline, not a bang.** Six stages stand between the metric and the page.\n\n**Watch it fire**\n\n```bash\nALERTS{alertstate=\"firing\"}\n```\n"},{"id":"p238","n":"238","slug":"5-observability-mistakes","title":"5 observability mistakes","sections":["devsecops"],"topic":"Observability & SRE","tags":["Observability","SRE","Monitoring","DevOps","CloudNative"],"series_name":"Platform","design":"dontdo","script":"","date":"2028-04-04","files":{"devsecops":"notes/p238-5-observability-mistakes.md"},"md":"# 5 observability mistakes\n\nEvery observability stack starts the same way: install Prometheus, add Grafana, feel modern. A year later the pager is noise and the bill is real, and the causes are almost always these five.\n\n## Quick reference\n\n**1.** ❌ **Alert on CPU above 80%** — CPU is a cause. Users feel latency and errors.  \n✅ **Alert on latency and errors** — Those are the symptoms your SLO names.\n\n**2.** ❌ **Dashboards nobody owns** — No owner means no one fixes it when it lies.  \n✅ **One owner on every board** — A name in the title, checked each quarter.\n\n**3.** ❌ **No SLO, only gut feel** — You cannot prioritise what you never defined.  \n✅ **One SLO per user journey** — The error budget decides ship or fix.\n\n**4.** ❌ **Log everything at debug** — Volume grows, signal does not, and the bill does.  \n✅ **Info default, debug sampled** — Keep the fields you query, drop the rest.\n\n**5.** ❌ **user_id as a metric label** — Every value is a new series, kept for ever.  \n✅ **Labels from a bounded set** — Per-user detail belongs in logs and traces.\n\n## Explained\n\n### The five\n\n1. **Alerting on CPU.** CPU at 80 percent is a cause, not a symptom; the symptom is a slow checkout. Page on latency and error rate, and keep CPU for diagnosis.\n   \\2. Unowned dashboards. A board with no owner stops being true and nobody notices for a quarter. Put a name in the title, or delete it.\n   \\3. No SLO. Without one, every alert is equally urgent and every priority argument is opinion. One SLO per user journey turns that into an error budget.\n   \\4. Logging everything. Debug lines multiply volume without adding signal, and the bill grows. Log at info, sample debug, and keep the fields you query.\n   \\5. High-cardinality labels. A user ID label creates a series per value, and Prometheus has to track every one. Put per-user detail in logs and traces.\n\n### What they share\n\nEach one buys a little short-term convenience and sends the bill to the person on call. None of them looks wrong on the day it is chosen.\n\n> **The honest caveat:** an SLO is a negotiation, not a number you pick alone. Write it with the team that carries the pager.\n\n> **The lesson:** observability is a budget, and every label spends from it.\n\n## In one line\n\n> **Observability is a budget.** Every label, log line and alert costs money and attention.\n\n**Count your series**\n\n```bash\ncount by (__name__) ({__name__=~\".+\"})\n```\n"},{"id":"p239","n":"239","slug":"a-prometheus-health-check-in-python","title":"A Prometheus health check in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Prometheus","Python","SRE","Observability","DevOps"],"series_name":"Platform","design":"code","script":"prom_check.py","date":"2028-04-06","files":{"devsecops":"notes/p239-a-prometheus-health-check-in-python.md"},"md":"# A Prometheus health check in Python\n\nA Grafana dashboard is on the wall screen and nobody has looked at it since Tuesday. A firing alert and a dead scrape target are facts you can check from a pipeline, and this script does exactly that.\n\n## Quick reference\n\nThe full script is at the end of this note: `prom_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 prom_check.py http://prometheus:9090\n```\n\n**Example output**\n\n```text\nALERT 2 firing, 1 down\n```\n\n## Explained\n\n### What it does\n\nIt calls two read-only Prometheus endpoints: /api/v1/alerts for the alerts Prometheus is holding, and /api/v1/targets?state=active for everything it is scraping. It counts the alerts in state firing and the targets whose health is not up, prints them, and exits 1 if either list is non-empty. The full script is attached: standard library only, read-only requests.\n\n### Why these two\n\nAlerts prove a rule has been true long enough to fire. Targets prove the monitoring itself is working, which is the failure nobody alerts on: a down target is a blind spot where the metrics you rely on stop arriving. A dashboard cannot fail a deploy, and a green board is not evidence.\n\n### How to use it\n\n`python3 prom_check.py http://prometheus:9090`, from cron every five minutes or as the last step of a deploy. It exits 0 when clean, 1 when something is firing or down, and 2 when the request fails, which is not the same as healthy.\n\n> **The honest caveat:** it reports what Prometheus knows. A rule with a long for: or a scrape interval of five minutes will still look green while users wait.\n\n> **The lesson:** monitor the monitor, with something that can fail a build.\n\n## In one line\n\n> **A dashboard is not a check.** If nobody reads it at 3 a.m., it is decoration.\n\n**Run it in ci**\n\n```bash\npython3 prom_check.py $PROM_URL\n```\n\n## Script: prom_check.py\n\nDownload: [scripts/prom_check.py](../scripts/prom_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"prom_check.py - fail when Prometheus has firing alerts or targets that are down.\n\nReads two read-only endpoints from the Prometheus HTTP API:\n\n    /api/v1/alerts                every alert Prometheus is holding\n    /api/v1/targets?state=active  every target it is scraping\n\nExit codes: 0 clean, 1 firing alerts or targets not up, 2 usage or request\nerror. GET requests only; nothing is written. Standard library only.\n\nUsage:\n    python3 prom_check.py\n    python3 prom_check.py http://prometheus:9090\n    python3 prom_check.py -t 10 -q\n    python3 prom_check.py --alerts alerts.json --targets targets.json\n    cat alerts.json | python3 prom_check.py --alerts - --targets targets.json\n    python3 prom_check.py --json\n\nCron / CI (a non-zero exit is the signal):\n    */5 * * * * python3 prom_check.py -q >> /var/log/prom_check.log || \\\n        mail -s \"prometheus unhealthy\" oncall@example.com < /var/log/prom_check.log\n    python3 prom_check.py \"$PROM_URL\" || exit 1\n\nThe default base URL is http://localhost:9090, or $PROM_URL when it is set.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport os\nimport sys\nimport urllib.error\nimport urllib.request\n\nDEFAULT_URL = \"http://localhost:9090\"\n\n\nclass CheckError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source, timeout):\n    \"\"\"Read one JSON document from a URL, a file, or '-' for stdin.\"\"\"\n    if source == \"-\":\n        try:\n            return json.load(sys.stdin)\n        except ValueError as exc:\n            raise CheckError(\"stdin is not JSON: %s\" % exc)\n\n    if \"://\" in source:\n        try:\n            with urllib.request.urlopen(source, timeout=timeout) as response:\n                return json.load(response)\n        except urllib.error.HTTPError as exc:\n            raise CheckError(\"%s returned HTTP %s\" % (source, exc.code))\n        except urllib.error.URLError as exc:\n            raise CheckError(\"%s unreachable: %s\" % (source, exc.reason))\n        except OSError as exc:\n            raise CheckError(\"%s failed: %s\" % (source, exc))\n        except ValueError as exc:\n            raise CheckError(\"%s did not return JSON: %s\" % (source, exc))\n\n    try:\n        with open(source) as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise CheckError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise CheckError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef data_of(doc, what):\n    \"\"\"Return the data object of a Prometheus API response, or fail.\"\"\"\n    if not isinstance(doc, dict) or doc.get(\"status\") != \"success\":\n        raise CheckError(\"%s: API did not return status=success\" % what)\n    data = doc.get(\"data\")\n    if not isinstance(data, dict):\n        raise CheckError(\"%s: response has no data object\" % what)\n    return data\n\n\ndef label(item, name, fallback=\"(none)\"):\n    labels = item.get(\"labels\") or {}\n    return labels.get(name) or fallback\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check the Prometheus API for firing alerts and targets that are not up.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or request error.\")\n    parser.add_argument(\"url\", nargs=\"?\", default=None,\n                        help=\"Prometheus base URL, default $PROM_URL or %s\" % DEFAULT_URL)\n    parser.add_argument(\"-t\", \"--timeout\", type=float, default=5.0,\n                        help=\"seconds per request, default 5\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON report instead of text\")\n    parser.add_argument(\"--alerts\", metavar=\"SOURCE\",\n                        help=\"read the /api/v1/alerts response from a URL, file or -\")\n    parser.add_argument(\"--targets\", metavar=\"SOURCE\",\n                        help=\"read the /api/v1/targets response from a URL, file or -\")\n    args = parser.parse_args(argv)\n\n    if args.alerts == \"-\" and args.targets == \"-\":\n        parser.error(\"--alerts and --targets cannot both read from stdin\")\n\n    base = (args.url or os.environ.get(\"PROM_URL\") or DEFAULT_URL).rstrip(\"/\")\n    alerts_src = args.alerts or base + \"/api/v1/alerts\"\n    targets_src = args.targets or base + \"/api/v1/targets?state=active\"\n\n    try:\n        alerts_data = data_of(read_json(alerts_src, args.timeout), \"alerts\")\n        targets_data = data_of(read_json(targets_src, args.timeout), \"targets\")\n    except CheckError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    alerts = alerts_data.get(\"alerts\") or []\n    targets = targets_data.get(\"activeTargets\") or []\n    if not isinstance(alerts, list) or not isinstance(targets, list):\n        print(\"ERR API data is not shaped as expected\", file=sys.stderr)\n        return 2\n    alerts = [a for a in alerts if isinstance(a, dict)]\n    targets = [t for t in targets if isinstance(t, dict)]\n\n    firing = [a for a in alerts if a.get(\"state\") == \"firing\"]\n    pending = [a for a in alerts if a.get(\"state\") == \"pending\"]\n    down = [t for t in targets if t.get(\"health\") != \"up\"]\n\n    if args.json:\n        report = {\n            \"ok\": not (firing or down),\n            \"firing\": [{\"alertname\": label(a, \"alertname\"),\n                        \"severity\": label(a, \"severity\", \"\"),\n                        \"activeAt\": a.get(\"activeAt\", \"\")} for a in firing],\n            \"down\": [{\"job\": label(t, \"job\"),\n                      \"instance\": label(t, \"instance\"),\n                      \"scrapeUrl\": t.get(\"scrapeUrl\", \"\"),\n                      \"health\": t.get(\"health\", \"\")} for t in down],\n            \"pending\": len(pending),\n            \"targets\": len(targets),\n        }\n        print(json.dumps(report, indent=None if args.quiet else 2, sort_keys=True))\n    else:\n        if firing or down:\n            print(\"ALERT %d firing, %d down\" % (len(firing), len(down)))\n        else:\n            print(\"OK no firing alerts, %d targets up\" % len(targets))\n        if not args.quiet:\n            for alert in firing:\n                print(\"  firing  %s  severity=%s\" %\n                      (label(alert, \"alertname\"), label(alert, \"severity\", \"-\")))\n            for target in down:\n                print(\"  down    %s  %s  (%s)\" %\n                      (label(target, \"job\"), label(target, \"instance\"),\n                       target.get(\"health\", \"unknown\")))\n\n    return 1 if (firing or down) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p240","n":"240","slug":"anatomy-of-an-alert-rule","title":"Anatomy of an alert rule","sections":["devsecops"],"topic":"Observability & SRE","tags":["Prometheus","Alerting","SRE","DevOps","CloudNative"],"series_name":"Platform","design":"anatomy","script":"","date":"2028-04-09","files":{"devsecops":"notes/p240-anatomy-of-an-alert-rule.md"},"md":"# Anatomy of an alert rule\n\nA Prometheus alert rule has six fields that matter, and most bad pages come from two of them: for: and labels.\n\n## Quick reference\n\n```text\nalert: expr: for: labels: annotations: runbook_url:\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `alert:` | The name | A short name, e.g. HighErrorRate. |\n| `expr:` | The query | Any PromQL vector: results mean it is true. |\n| `for:` | The delay | True this long before firing. Stops flapping. |\n| `labels:` | The routing | Route and group on these. severity: page. |\n| `annotations:` | The message | summary one line, description with numbers. |\n| `runbook_url:` | The link | A convention in annotations, not a field. |\n\n## Explained\n\n### What fires\n\nThree fields decide whether the rule fires at all.  \nalert: is the name. It becomes the alertname label and the first word on the phone, so make it specific.  \nexpr: is the query. Any PromQL that returns a vector; the rule fires while that vector has results. One clear ratio beats a compound condition.  \nfor: is the delay. The expression must stay true for the whole duration before it fires. This is what stops one spike paging anyone.\n\n### Who hears it\n\nlabels: route it. severity: page or severity: ticket is a convention, not a reserved word, and Alertmanager routes and groups on it.  \nannotations: explain it. summary is the one-liner, description carries the numbers, and both can use {{ $labels.instance }} and {{ $value }}.  \nrunbook_url: is the link. Not a Prometheus field, a convention inside annotations, and the first thing on-call should click.  \nAn alert with none of these is a text from a stranger: it says something broke and leaves you to guess what.\n\n> **The honest caveat:** for: delays the page, it does not make it correct. A bad expression with a long for: is still a bad expression.\n\n> **The lesson:** write the rule for the person who gets woken, not for the query editor.\n\n## In one line\n\n> **A rule is a promise to wake up.** Write it for the person who gets the page at 3 a.m.\n\n**Check it first**\n\n```bash\npromtool check rules alert.yml\n```\n"},{"id":"p241","n":"241","slug":"quiz-3-observability-questions","title":"Quiz: 3 observability questions","sections":["devsecops"],"topic":"Observability & SRE","tags":["Observability","SRE","Prometheus","CloudNative","DevOps"],"series_name":"Platform","design":"quiz","script":"","date":"2028-04-11","files":{"devsecops":"notes/p241-quiz-3-observability-questions.md"},"md":"# Quiz: 3 observability questions\n\nThree questions separate a dashboard someone built once from alerting that survives an on-call week. Answer before the reveal, then check the rule that caught you.\n\n## Quick reference\n\n**Q1. What does for: 5m change about an alert rule?**\n\n- a) It fires only after 5 minutes of truth\n- b) It retries the query every 5 minutes\n- c) It silences the alert for 5 minutes\n\n> **Answer:** a) It fires only after 5 minutes of truth. The rule stays pending while the expression is true and only fires after the whole duration, so one spike never pages anyone.\n\n**Q2. What does a high-cardinality label do to Prometheus?**\n\n- a) Nothing, labels are free\n- b) Multiplies series, memory and disk\n- c) Compresses samples more tightly\n\n> **Answer:** b) Multiplies series, memory and disk. Every unique label value is a new series. A user ID label can take one metric to millions of series and slow every query until it times out.\n\n**Q3. Why page on p99 latency instead of the average?**\n\n- a) The average hides the slow tail\n- b) p99 is cheaper to compute\n- c) The average ignores failed requests\n\n> **Answer:** a) The average hides the slow tail. Most requests are fast, so the mean stays low while the slowest one in a hundred waits. The tail is where timeouts and users give up.\n\n## Explained\n\n### The delay\n\nQuestion one: what does for: 5m change? The rule fires only after the expression has been true for five minutes. Prometheus holds it in pending, re-evaluates each interval, and moves it to firing when the duration is done, so one failed scrape sets nothing off.\n\n### Cardinality\n\nQuestion two: what does a high-cardinality label do? Every unique label value is a new series, and every series costs memory and index space. A user ID or request ID label can take a single metric to millions of series and slow every query that touches it.\n\n### The tail\n\nQuestion three: why page on p99 instead of the average? Because the average is dragged down by the majority of fast requests. If 99 requests take 100ms and one takes 10 seconds, the mean still looks fine while one user waited ten seconds. The tail is where timeouts and retries live.\n\n> **The honest caveat:** p99 over a one-minute window is noisy, and a slow tail on a quiet endpoint may be one user. Pair it with a longer window and a minimum request rate, or you will chase noise.\n\n> **The lesson:** a number you cannot explain is a number you cannot page on.\n\n## In one line\n\n> **Ask what the number hides.** Three answers that change how you build dashboards.\n\n**Test your rules**\n\n```bash\npromtool test rules test.yml\n```\n"},{"id":"p242","n":"242","slug":"platform-engineering-terms-decoded","title":"Platform engineering terms, decoded","sections":["devsecops"],"topic":"Platform engineering","tags":["PlatformEngineering","DevOps","Kubernetes","SRE","CloudNative"],"series_name":"Platform","design":"reveal","script":"","date":"2028-04-13","files":{"devsecops":"notes/p242-platform-engineering-terms-decoded.md"},"md":"# Platform engineering terms, decoded\n\nPlatform engineering is the rename of the year: the same infrastructure work, now with users who can say no. The vocabulary is where most conversations go wrong.\n\n## Quick reference\n\n- **IDP** — Internal Developer Platform: the self-service layer over Kubernetes and cloud, so developers touch neither.\n- **Golden path** — The supported route for a new service: template, CI, deploy, dashboards and alerts, already wired.\n- **Service catalog** — The index of every service, its owner, repo and dependencies. Backstage is the usual home for it.\n- **Scorecard** — An automatic per-service checklist: tests, SLO, runbook, owner. It shows gaps, not grades.\n- **Tenant** — One team's slice of the platform: namespace, quota, RBAC and cost, fenced off from the others.\n- **Self-service** — A team creates what it needs through a form or a pull request, without a ticket to the platform team.\n\n## Explained\n\n### The layer\n\nAn IDP, Internal Developer Platform, is the self-service layer a platform team runs over Kubernetes and cloud. Its job is to keep developers out of both.  \nA golden path is the supported route for a new service: template, CI, deploy, dashboards and alerts, already wired. A default, not a mandate: teams can leave it, but they pay the support cost.  \nA service catalog is the index of every service, its owner, repo and dependencies. Backstage is the usual home for it.  \nA scorecard checks each service against tests, SLO, runbook and owner, automatically. It shows gaps, not grades.  \nA tenant is one team's slice of the platform: namespace, quota, RBAC and cost, fenced off from the others.  \nSelf-service means a team creates what it needs through a form or a pull request, without a ticket to the platform team.\n\n### Why the words matter\n\nEach one names a boundary: what the platform owns, what the team owns, and where the ticket queue used to be. Get them wrong and you build a portal nobody uses.\n\n> **The honest caveat:** a platform without a golden path is a portal with links. The path is the product.\n\n> **The lesson:** platform teams ship products, and developers are the users.\n\n## In one line\n\n> **A platform is a product.** Its users are developers, and its competition is a ticket queue.\n\n**Read the docs:** backstage.io/docs\n"},{"id":"p243","n":"243","slug":"monorepo-vs-polyrepo","title":"Monorepo vs polyrepo","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Monorepo","DevOps","CICD","Git","SoftwareEngineering"],"series_name":"Platform","design":"compare","script":"","date":"2028-04-16","files":{"devsecops":"notes/p243-monorepo-vs-polyrepo.md"},"md":"# Monorepo vs polyrepo\n\nThe monorepo argument is rarely about Git. It is about how often two services must change in the same commit, and what breaks when they cannot.\n\n## Quick reference\n\n| | **MONOREPO** | **POLYREPO** |\n|:--|:--|:--|\n| Coupling | Atomic across services | Versioned contracts |\n| CI speed | Affected-only builds | Small, fast pipelines |\n| Tooling | One build system for all | Best tool per repo |\n| Ownership | CODEOWNERS per path | A repo per team |\n| Versioning | One version for everything | Semver per package |\n| Best for | Tightly coupled services | Independent teams |\n\n**MONOREPO** (One repo, one version): One checkout, one CI graph, and a commit that changes API and client together.\n\n```bash\nnx affected -t test\n```\n\n**POLYREPO** (A repo per service): Each service ships alone, with a versioned package as the contract between them.\n\n```bash\nnpm i @org/lib@1.4.2\n```\n\n- Monorepo → one change, one commit, one CI run.\n- Polyrepo → one version per service, released alone.\n\n## Explained\n\n### The trade\n\nCoupling. A monorepo lets one commit change an API and every caller, so CI sees the whole blast radius. A polyrepo puts a versioned package between teams: slower, and much harder to break silently.  \nCI speed. A big monorepo is only fast with affected-only builds; Nx, Bazel and Turborepo can build only what changed. Without that graph, every push runs everything and the queue becomes the bottleneck.  \nTooling. One repo means one build system, one lint config and one upgrade path, paid for in migrations. Polyrepos let each team choose, and then let them drift.\n\n### The rest\n\nOwnership. CODEOWNERS maps paths to teams in a monorepo. In a polyrepo the repo boundary is the ownership boundary.  \nVersioning. A monorepo usually versions the whole repo together, which is simple until a shared library needs a breaking change. A polyrepo gives each package its own semver and release.  \nBest for. Monorepo when services change together and the tooling keeps CI honest. Polyrepo when teams are independent and the contract between them is stable.\n\n> **The honest caveat:** a monorepo without a build graph runs everything on every push, and a polyrepo without version discipline is a dependency maze.\n\n> **The lesson:** choose by coupling, then invest in the tooling that makes it bearable.\n\n## In one line\n\n> **Layout follows coupling, not fashion.** The repo layout should follow how the code changes together.\n\n**See the graph**\n\n```bash\nnx graph\n```\n"},{"id":"p244","n":"244","slug":"a-terraform-plan-gate-in-python","title":"A Terraform plan gate in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Terraform","DevOps","Python","IaC","DevSecOps"],"series_name":"Platform","design":"code","script":"tf_plan_gate.py","date":"2028-04-18","files":{"devsecops":"notes/p244-a-terraform-plan-gate-in-python.md"},"md":"# A Terraform plan gate in Python\n\nA Terraform plan is a wall of text, and the one line that matters is often a delete. This script reads the plan's JSON and fails the pipeline before anyone applies it.\n\n## Quick reference\n\nThe full script is at the end of this note: `tf_plan_gate.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nterraform show -json tfplan | python3 tf_plan_gate.py -\n```\n\n**Example output**\n\n```text\nALERT delete aws_s3_bucket.old\nALERT replace aws_db_instance.main\n```\n\n## Explained\n\n### What it does\n\nIt reads the JSON from `terraform show -json` and walks resource_changes. For each entry it reads change.actions, which is a list: create, update, no-op, delete, or delete plus create for a replacement. Anything containing delete is a finding. No cloud call and no state write, so it is safe in a read-only CI job. The full script is attached, standard library only.\n\n### Why actions\n\nActions are the plan's own summary of intent, so you do not parse the diff. A replacement is always a delete plus a create; Terraform only chooses the order. Catching delete catches both, and the order tells you which one it is. The shape is provider-agnostic, so one gate covers every cloud in the repo.\n\n### How to use it\n\n`terraform plan -out=tfplan`, then `terraform show -json tfplan | python3 tf_plan_gate.py -`. Run it before apply, and on pull requests with a saved plan. Exit 0 clean, 1 when something is destroyed, 2 when the input is not a plan.\n\n> **The honest caveat:** some deletes are intended, like a bucket you are retiring on purpose. The gate makes the exception explicit; `--allow` keeps it in the pipeline file.\n\n> **The lesson:** a plan nobody reads is a change nobody reviewed.\n\n## In one line\n\n> **The plan is the review.** A delete in the plan is a decision. Read it before apply.\n\n**Read the actions**\n\n```bash\nterraform show -json tfplan\n```\n\n## Script: tf_plan_gate.py\n\nDownload: [scripts/tf_plan_gate.py](../scripts/tf_plan_gate.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"tf_plan_gate.py - fail a pipeline when a Terraform plan deletes or replaces.\n\nReads the JSON printed by `terraform show -json` and walks\nresource_changes[].change.actions. Terraform writes:\n\n    [\"no-op\"] [\"create\"] [\"update\"] [\"read\"]\n    [\"delete\"]                     a plain destroy\n    [\"delete\", \"create\"]           a replacement\n    [\"create\", \"delete\"]           a replacement with create_before_destroy\n\nAny action list containing \"delete\" is a finding, so replacements are caught\nwhichever order Terraform chooses. No cloud calls, no state writes, no\nnetwork: it only reads the plan JSON.\n\nExit codes: 0 clean, 1 destructive change found, 2 usage or parse error.\n\nUsage:\n    terraform plan -out=tfplan\n    terraform show -json tfplan | python3 tf_plan_gate.py -\n    terraform show -json tfplan > plan.json && python3 tf_plan_gate.py plan.json\n    python3 tf_plan_gate.py plan.json --allow 'aws_autoscaling_group.*'\n    python3 tf_plan_gate.py plan.json --json\n\nCI step, before apply:\n    terraform plan -out=tfplan\n    terraform show -json tfplan > plan.json\n    python3 tf_plan_gate.py plan.json || exit 1\n\nWith no argument the plan is read from stdin, so a pipe is enough.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport fnmatch\nimport json\nimport sys\n\n\nclass PlanError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_plan(path):\n    \"\"\"Read the plan JSON from a file, or '-' for stdin.\"\"\"\n    if path == \"-\":\n        try:\n            return json.load(sys.stdin)\n        except ValueError as exc:\n            raise PlanError(\"stdin is not JSON: %s\" % exc)\n    try:\n        with open(path) as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise PlanError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise PlanError(\"%s is not JSON: %s\" % (path, exc))\n\n\ndef changes_of(doc):\n    \"\"\"Return the resource_changes list, or fail when this is not a plan.\"\"\"\n    if not isinstance(doc, dict):\n        raise PlanError(\"top level is not a JSON object\")\n    if \"resource_changes\" in doc:\n        return doc.get(\"resource_changes\") or []\n    if \"values\" in doc and \"planned_values\" not in doc:\n        raise PlanError(\"this looks like a state file; pass `terraform show -json <planfile>`\")\n    if \"planned_values\" not in doc and \"format_version\" not in doc:\n        raise PlanError(\"no resource_changes; is this `terraform show -json` output?\")\n    return []\n\n\ndef collect(changes, allow):\n    \"\"\"Split resource changes into (findings, allowed) on destructive actions.\"\"\"\n    findings, allowed = [], []\n    for change in changes:\n        if not isinstance(change, dict):\n            raise PlanError(\"resource_changes contains something that is not an object\")\n        address = change.get(\"address\") or \"(no address)\"\n        detail = change.get(\"change\")\n        if not isinstance(detail, dict):\n            raise PlanError(\"%s has no change object\" % address)\n        actions = detail.get(\"actions\") or []\n        if not isinstance(actions, list):\n            raise PlanError(\"%s has a non-list actions field\" % address)\n        if \"delete\" not in actions:\n            continue\n        kind = \"replace\" if \"create\" in actions else \"delete\"\n        record = {\"address\": address, \"kind\": kind, \"actions\": actions}\n        if any(fnmatch.fnmatch(address, pattern) for pattern in allow):\n            allowed.append(record)\n        else:\n            findings.append(record)\n    return findings, allowed\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Fail when a Terraform plan deletes or replaces resources.\",\n        epilog=\"Exit codes: 0 clean, 1 destructive change, 2 usage or parse error.\")\n    parser.add_argument(\"plan\", nargs=\"?\", default=\"-\",\n                        help=\"plan JSON from terraform show -json, default - (stdin)\")\n    parser.add_argument(\"--allow\", action=\"append\", default=[], metavar=\"PATTERN\",\n                        help=\"exempt addresses matching a shell wildcard, repeatable\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON report instead of text\")\n    args = parser.parse_args(argv)\n\n    try:\n        doc = load_plan(args.plan)\n        changes = changes_of(doc)\n        findings, allowed = collect(changes, args.allow)\n    except PlanError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    deletes = [f for f in findings if f[\"kind\"] == \"delete\"]\n    replaces = [f for f in findings if f[\"kind\"] == \"replace\"]\n\n    if args.json:\n        report = {\n            \"ok\": not findings,\n            \"resource_changes\": len(changes),\n            \"findings\": findings,\n            \"allowed\": allowed,\n            \"deletes\": len(deletes),\n            \"replaces\": len(replaces),\n        }\n        print(json.dumps(report, indent=None if args.quiet else 2, sort_keys=True))\n    else:\n        if not args.quiet:\n            for item in findings:\n                print(\"%-7s %s  %s\" % (item[\"kind\"].upper(), item[\"address\"],\n                                       \",\".join(item[\"actions\"])))\n            for item in allowed:\n                print(\"allow   %s  %s\" % (item[\"address\"], \",\".join(item[\"actions\"])))\n        if findings:\n            print(\"ALERT %d destructive: %d delete, %d replace\"\n                  % (len(findings), len(deletes), len(replaces)))\n        else:\n            print(\"OK no deletes or replaces in %d resource changes\" % len(changes))\n\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p245","n":"245","slug":"9-checks-for-a-golden-path-template","title":"9 checks for a golden-path template","sections":["devsecops"],"topic":"Platform engineering","tags":["PlatformEngineering","DevOps","Kubernetes","CICD","DevSecOps"],"series_name":"Platform","design":"checklist","script":"","date":"2028-04-20","files":{"devsecops":"notes/p245-9-checks-for-a-golden-path-template.md"},"md":"# 9 checks for a golden-path template\n\nEvery platform team promises a golden path. Most ship a template repo and a wiki page, then wonder why teams copy the old service. A golden path is nine checks, not a folder.\n\n## Quick reference\n\n### Build\n\n- **Tests run from day one** — `make test` in CI\n- **Dependencies are pinned** — lockfile committed\n- **Pipeline is included** — CI config in the repo\n\n### Ship\n\n- **Security scan is wired** — Trivy fails on critical\n- **Deploy path is real** — staging then production\n- **Rollback is tested** — one command, run once\n\n### Own\n\n- **Observability is built in** — metrics, logs, traces\n- **Docs live with the code** — README plus a runbook\n- **An owner is named** — team in CODEOWNERS\n\n## Explained\n\n### Build\n\nTests run from day one. The template ships a test target that CI calls, so a new service cannot start with zero tests.  \nDependencies are pinned. A lockfile is committed and CI installs from it, so two builds a week apart match.  \nThe pipeline is included. The CI config lives in the template, not in a platform ticket. The first push runs lint, test and build.\n\n### Ship\n\nA security scan is wired. Trivy or Grype fails the build on a fixable critical CVE. Scanning after deploy is an audit, not a gate.  \nThe deploy path is real. Staging and production manifests exist, and the first deploy uses the same Argo CD app as every later one.  \nRollback is documented and tested. One command returns the previous version, and someone has run it before an incident.\n\n### Own\n\nObservability is built in. Metrics, logs and traces are exported from the first commit, not added after an outage.  \nDocs live with the code. README, runbook and architecture note sit in the repo and are reviewed with it.  \nAn owner is named. CODEOWNERS points at a team, and the catalog shows the same team.\n\n> **The lesson:** a golden path is a product, and these nine checks are its acceptance test. The honest caveat: a template nobody maintains becomes a trap, so version it and track adoption.\n\n## In one line\n\n> **The path must be faster than the detour.** Nine checks turn a template into a product teams choose.\n\n**Test the template**\n\n```bash\nmake test\n```\n"},{"id":"p246","n":"246","slug":"terraform-vs-crossplane-vs-pulumi","title":"Terraform vs Crossplane vs Pulumi","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","Crossplane","Pulumi","PlatformEngineering","DevOps"],"series_name":"Platform","design":"matrix","script":"","date":"2028-04-23","files":{"devsecops":"notes/p246-terraform-vs-crossplane-vs-pulumi.md"},"md":"# Terraform vs Crossplane vs Pulumi\n\nTerraform, Crossplane and Pulumi all turn infrastructure into code, and the argument between them is usually about taste. The useful question is where the control plane lives and who owns the API.\n\n## Quick reference\n\n| | **Terraform** | **Crossplane** | **Pulumi** |\n|:--|:--|:--|:--|\n| Model | HCL and state | Kubernetes CRDs | General-purpose code |\n| Control plane | CLI plus state | Runs in the cluster | CLI plus backend |\n| Drift | 🟡 plan shows it | ✅ reconciles it | 🟡 preview shows it |\n| Day-2 ops | 🟡 apply and import | ✅ kubectl and GitOps | 🟡 code and pipelines |\n| Learning curve | 🟡 HCL plus state | ❌ Kubernetes plus YAML | ✅ a language you know |\n| Best for | ✅ broad cloud estates | ✅ platform APIs | ✅ app teams |\n\n**How to pick**\n\n- **Terraform** → provision anything, anywhere, from one state.\n- **Crossplane** → expose platform APIs as Kubernetes objects.\n- **Pulumi** → let app teams use the language they already know.\n\n## Explained\n\n### Three models\n\nTerraform describes infrastructure in HCL and keeps a state file that maps resources to the real world. The CLI is the control plane: plan, apply, import.  \nCrossplane runs controllers inside a Kubernetes cluster. Its resources are CRDs, and its control plane is the cluster itself, reconciling continuously.  \nPulumi uses a general-purpose language such as TypeScript, Python, Go or .NET and a state backend. The CLI or an operator runs the program.\n\n### Where they differ\n\nDrift. Terraform plan shows it when you run it; Crossplane reconciles it without being asked; Pulumi preview and refresh show it.  \nDay-2 operations. Terraform is apply and import; Crossplane is kubectl and GitOps; Pulumi is code and pipelines.  \nLearning curve. HCL plus state is its own language; Crossplane adds Kubernetes and composition, the steepest; Pulumi is fastest if you already write code.\n\n### How to choose\n\nTerraform for a broad estate across clouds and SaaS. Crossplane when you want to offer platform APIs as Kubernetes objects. Pulumi when application teams own their infrastructure in a language they already use.\n\n> **The honest caveat:** each has a sharp edge. Terraform has remote state to secure and lock; Crossplane adds a control plane to operate; Pulumi needs a language runtime and a state backend.\n\n> **The lesson:** pick by the interface you want to support, not by the syntax you like.\n\n## In one line\n\n> **Model the API, not the YAML.** The tool matters less than who owns the interface.\n\n**Prove the diff**\n\n```bash\nterraform plan\n```\n"},{"id":"p247","n":"247","slug":"a-developers-first-deploy-hour-by-hour","title":"A developer's first deploy, hour by hour","sections":["devsecops"],"topic":"Platform engineering","tags":["PlatformEngineering","DevOps","GitOps","CICD","Kubernetes"],"series_name":"Platform","design":"timeline","script":"","date":"2028-04-25","files":{"devsecops":"notes/p247-a-developers-first-deploy-hour-by-hour.md"},"md":"# A developer's first deploy, hour by hour\n\nA new developer joins on Monday. By lunch they have a service in production, and nobody opened a ticket. This is what a real golden path buys, hour by hour.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **0:00** | Repo from the template | Clone it. Tests, CI and manifests already exist. |\n| **1:00** | First pull request | A small change. The pipeline runs tests, a build and a scan. |\n| **2:00** | Staging deploy | Merge to main. Argo CD syncs the staging namespace. |\n| **2:30** | First real error | A missing env var. The runbook names the fix, not the person. |\n| **3:30** | Promote to production | The same image by tag. No rebuild, no drift. |\n| **4:00** | First alert fires | The dashboard and the alert existed before the user did. |\n\n## Explained\n\n### The first hour\n\nHour zero. They clone the template. The repo already has a test target, CI and manifests for staging and production. The first commit is a one-line change to the README.  \nOne hour in. They open a pull request. CI runs the tests, builds an image tagged with the commit SHA, and scans it. The pipeline fails on an unpinned dependency: the template teaching the standard, not a reviewer.\n\n### The middle\n\nTwo hours in. The merge to main triggers Argo CD. The staging namespace syncs, the pod starts, the health check passes. No kubectl from a laptop.  \nHalf an hour later. The first real error: a missing environment variable. The pod CrashLoops, the alert fires, and the runbook names the exact secret to add. The fix is a pull request, not a shell command on a running pod.  \nThree thirty. The same image is promoted to production by tag. No rebuild: what ran in staging runs in production.\n\n### The last hour\n\nFour hours. The first alert reaches the on-call channel. It is noisy, but real, and the dashboard already shows the four golden signals.\n\nThe habit that matters: make the first deploy boring, and every later one inherits the path. The honest caveat: this is a walkthrough, not a stopwatch. The first deploy is fast because the template carries the boring parts, not because production is simple.\n\n## In one line\n\n> **Four hours to first value.** The template carries the boring parts, not the risk.\n\n**Read the runbook**\n\n```bash\ncat docs/runbook.md\n```\n"},{"id":"p248","n":"248","slug":"6-kubernetes-numbers-that-decide-capacity","title":"6 Kubernetes numbers that decide capacity","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","SRE","DevOps","CapacityPlanning"],"series_name":"Platform","design":"numbers","script":"","date":"2028-04-27","files":{"devsecops":"notes/p248-6-kubernetes-numbers-that-decide-capacity.md"},"md":"# 6 Kubernetes numbers that decide capacity\n\nKubernetes capacity is not one number. It is a handful of defaults that decide how many pods fit, when they are evicted and how many failures you survive. Six numbers cover most of the argument.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **110** | Pods per node | The kubelet default maxPods. Plan pod CIDR and IPs around it. | default |\n| **100m** | One CPU request | A tenth of a core. Requests, not limits, decide where a pod fits. | request |\n| **100Mi** | Eviction threshold | The default memory.available floor before the kubelet evicts. | memory |\n| **2** | Replicas minimum | One pod is a single point of failure. Two survive a rollout. | HA |\n| **80%** | HPA CPU target | The default target. Above it, the HPA adds pods up to max. | HPA |\n| **3** | Zones to spread | Three failure domains keep etcd quorum through one zone loss. | zones |\n\n## Explained\n\n### How many fit\n\n110 pods per node. That is the kubelet default maxPods, and it bounds the pod CIDR, the IP plan and the room left for DaemonSets. A node with 110 pods needs addresses for all of them.  \n100m CPU. A request of 100m is a tenth of a core. The scheduler adds requests, not limits, when it decides where a pod fits, so inflated requests waste a node before it is full.\n\n### When it breaks\n\n100Mi. The kubelet's default hard eviction threshold is memory.available<100Mi. Below that, it evicts pods to reclaim memory, starting with the lowest-priority ones.  \n2 replicas. One pod is a single point of failure. Two survive a rolling update and a node drain; three survive a zone.\n\n### How it scales\n\n80 percent. The default HPA target for CPU utilisation. Above it, the HPA adds pods, up to maxReplicas and no further. A bad request makes the percentage lie.  \n3 zones. Spread the control plane and workers across three failure domains. A three-member etcd cluster keeps quorum through one zone loss; two do not.\n\n> **The honest caveat:** 110 is a kubelet default, not a promise. CNI plugins, IP prefixes and instance size all cap it lower.\n\n> **The lesson:** capacity is a sum of defaults, so read them before you size a node.\n\n## In one line\n\n> **Defaults decide capacity.** Read them before you size a node or a namespace.\n\n**Check a node**\n\n```bash\nkubectl describe node\n```\n"},{"id":"p249","n":"249","slug":"a-repository-standards-checker-in-python","title":"A repository standards checker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DevOps","PlatformEngineering","CICD","DevSecOps"],"series_name":"Platform","design":"code","script":"repo_standards.py","date":"2028-04-30","files":{"devsecops":"notes/p249-a-repository-standards-checker-in-python.md"},"md":"# A repository standards checker in Python\n\nEvery new service starts with good intentions and an empty repo. Six weeks later it has no README, no owner and a CI file that only the original author understands. This script makes those omissions visible on day one.\n\n## Quick reference\n\nThe full script is at the end of this note: `repo_standards.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 repo_standards.py .\n```\n\n**Example output**\n\n```text\nscore 43/100\nmissing: SECURITY, CI, CODEOWNERS, TESTS\n```\n\n## Explained\n\n### What it checks\n\nSeven files and folders: README, licence, `SECURITY.md`, a CI workflow, CODEOWNERS, tests and a lockfile. Each one is a proxy for a promise. README means someone can run it. Licence means legal can ship it. `SECURITY.md` means there is a way to report a vulnerability. CI means the checks run without a human. CODEOWNERS means a team is accountable. Tests mean the code can change. A lockfile means builds repeat.\n\n### How it scores\n\nEach check is worth the same, so the score is the percentage present. Seven of seven is 100. Six is 86. The script prints the score and the missing names, then exits 1 if the score is below `--min-score`, which defaults to 100.\n\n### How to use it\n\nRun it against a new repo, or add it as the first CI step in the template. `python3 repo_standards.py . --min-score 100`. For an existing estate, start at 70 and ratchet the threshold up as teams fix the gaps.\n\n> **The honest caveat:** presence is not quality. An empty `SECURITY.md` passes, so review the content once and link the template.\n\n> **The lesson:** standards that are not checked are suggestions.\n\n## In one line\n\n> **A repo is a contract.** The checks are the contract new services sign.\n\n**Gate new repos**\n\n```bash\npython3 repo_standards.py .\n```\n\n## Script: repo_standards.py\n\nDownload: [scripts/repo_standards.py](../scripts/repo_standards.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"repo_standards.py - score a repository against seven baseline standards.\n\nChecks that a repository root has the files a service needs before its first\ndeploy: a README, a licence, a security policy, CI, CODEOWNERS, tests and a\nlockfile. Prints a score out of 100 and exits 1 when the score is below the\nthreshold, so a new service cannot start life without the basics.\n\nUsage:\n    python3 repo_standards.py                  # checks the current directory\n    python3 repo_standards.py path/to/repo     # checks another repository\n    python3 repo_standards.py . --json         # machine-readable result\n    python3 repo_standards.py . --min-score 80 # ratchet, do not block yet\n\nCI (GitHub Actions step):\n    - run: python3 repo_standards.py . --min-score 100\n\nCron (weekly, Monday 07:00):\n    0 7 * * 1 cd /srv/repos && python3 repo_standards.py . >> /var/log/repo.log 2>&1\n\nRead-only: it only looks for files and never writes to the repository.\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom pathlib import Path\n\nREADME = (\"README.md\", \"README.rst\", \"README.txt\", \"README\", \"readme.md\")\nLICENCE = (\"LICENSE\", \"LICENSE.md\", \"LICENSE.txt\", \"LICENCE\", \"LICENCE.md\",\n           \"COPYING\", \"COPYING.md\")\nSECURITY = (\"SECURITY.md\", \".github/SECURITY.md\", \"SECURITY.rst\",\n            \"docs/SECURITY.md\")\nOWNERS = (\".github/CODEOWNERS\", \"CODEOWNERS\", \"docs/CODEOWNERS\")\nLOCKFILES = (\"package-lock.json\", \"yarn.lock\", \"pnpm-lock.yaml\", \"bun.lockb\",\n             \"poetry.lock\", \"Pipfile.lock\", \"uv.lock\", \"go.sum\", \"Cargo.lock\",\n             \"composer.lock\", \"Gemfile.lock\", \"mix.lock\", \"pubspec.lock\",\n             \"packages.lock.json\", \"flake.lock\")\nTEST_DIRS = (\"tests\", \"test\", \"spec\", \"__tests__\")\nTEST_FILES = (\"test_*.py\", \"*_test.go\", \"*.test.js\", \"*.test.ts\", \"*.spec.ts\",\n              \"*_test.rs\")\nCI_FILES = (\".gitlab-ci.yml\", \"Jenkinsfile\", \".circleci/config.yml\",\n            \"azure-pipelines.yml\", \"bitbucket-pipelines.yml\", \".drone.yml\",\n            \".buildkite/pipeline.yml\")\n\n\ndef first_file(root, names):\n    \"\"\"Return the first existing relative path, or None.\"\"\"\n    for name in names:\n        if (root / name).is_file():\n            return name\n    return None\n\n\ndef find_ci(root):\n    workflows = root / \".github\" / \"workflows\"\n    try:\n        if workflows.is_dir():\n            for p in sorted(workflows.iterdir()):\n                if p.is_file() and p.suffix in (\".yml\", \".yaml\"):\n                    return p.relative_to(root).as_posix()\n    except OSError:\n        pass\n    return first_file(root, CI_FILES)\n\n\ndef find_tests(root):\n    for name in TEST_DIRS:\n        if (root / name).is_dir():\n            return name + \"/\"\n    for pattern in TEST_FILES:\n        for p in sorted(root.glob(pattern)):\n            if p.is_file():\n                return p.name\n    return None\n\n\ndef find_lockfile(root):\n    return first_file(root, LOCKFILES)\n\n\ndef collect(root):\n    checks = [\n        (\"README\", first_file(root, README)),\n        (\"LICENCE\", first_file(root, LICENCE)),\n        (\"SECURITY\", first_file(root, SECURITY)),\n        (\"CI\", find_ci(root)),\n        (\"CODEOWNERS\", first_file(root, OWNERS)),\n        (\"TESTS\", find_tests(root)),\n        (\"LOCKFILE\", find_lockfile(root)),\n    ]\n    rows = []\n    for name, found in checks:\n        rows.append({\"name\": name, \"ok\": found is not None,\n                     \"detail\": found or \"not found\"})\n    passed = sum(1 for r in rows if r[\"ok\"])\n    score = int(round(100.0 * passed / len(rows)))\n    return rows, score\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        prog=\"repo_standards.py\",\n        description=\"Score a repository against seven baseline standards.\")\n    ap.add_argument(\"root\", nargs=\"?\", default=\".\",\n                    help=\"repository root to check (default: current directory)\")\n    ap.add_argument(\"--json\", action=\"store_true\",\n                    help=\"print the result as JSON\")\n    ap.add_argument(\"--min-score\", type=int, default=100, metavar=\"N\",\n                    help=\"minimum passing score, 0-100 (default: 100)\")\n    args = ap.parse_args()\n\n    if not 0 <= args.min_score <= 100:\n        ap.error(\"--min-score must be between 0 and 100\")\n\n    root = Path(args.root).expanduser()\n    if not root.exists():\n        ap.error(\"not found: %s\" % root)\n    if not root.is_dir():\n        ap.error(\"not a directory: %s\" % root)\n\n    rows, score = collect(root)\n    missing = [r[\"name\"] for r in rows if not r[\"ok\"]]\n    ok = score >= args.min_score\n\n    if args.json:\n        print(json.dumps({\n            \"root\": str(root.resolve()),\n            \"score\": score,\n            \"passed\": len(rows) - len(missing),\n            \"total\": len(rows),\n            \"checks\": rows,\n            \"missing\": missing,\n            \"ok\": ok,\n        }, indent=2))\n    else:\n        print(\"repo standards  %s\" % root.resolve())\n        print()\n        for r in rows:\n            mark = \"ok  \" if r[\"ok\"] else \"MISS\"\n            print(\"[%s] %-11s %s\" % (mark, r[\"name\"], r[\"detail\"]))\n        print()\n        print(\"score %d/100 (%d/%d)\" % (score, len(rows) - len(missing), len(rows)))\n        if missing and ok:\n            print(\"PASS at min-score %d, still missing: %s\"\n                  % (args.min_score, \", \".join(missing)))\n        elif missing:\n            print(\"FAIL missing: \" + \", \".join(missing))\n        else:\n            print(\"OK all standards present\")\n\n    return 0 if ok else 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p250","n":"250","slug":"anatomy-of-a-backstage-catalog-info-yaml","title":"Anatomy of a Backstage catalog-info.yaml","sections":["devsecops"],"topic":"Platform engineering","tags":["Backstage","PlatformEngineering","DevOps","Kubernetes","CloudNative"],"series_name":"Platform","design":"anatomy","script":"","date":"2028-05-02","files":{"devsecops":"notes/p250-anatomy-of-a-backstage-catalog-info-yaml.md"},"md":"# Anatomy of a Backstage catalog-info.yaml\n\nA `catalog-info.yaml` is a short YAML file, and it decides whether a service exists to everyone outside its repo. Backstage reads it, indexes it and shows it on a page. A missing field fails at processing, not at build.\n\n## Quick reference\n\n```text\napiVersion kind metadata.name spec.type spec.owner spec.lifecycle\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `apiVersion` | The schema | `backstage.io/v1alpha1` is the current version for catalog entities. |\n| `kind` | The entity type | Component for a service or website. Also API, System, Domain, Resource. |\n| `metadata.name` | The unique name | Unique for its `kind`. Other entities use it to reference this one. |\n| `spec.type` | The type | service, website or library. A custom type is a taxonomy decision. |\n| `spec.owner` | The owner | Prefer a group, for example group:default/platform. |\n| `spec.lifecycle` | The lifecycle | production, experimental or deprecated. It drives scorecards. |\n\n## Explained\n\n### The six fields\n\napiVersion. `backstage.io/v1alpha1` is the current version for catalog entities. It is not v1; the alpha is part of the string.  \n`kind`. Component for a service, website or library. The same schema covers API, System, Domain and Resource.  \n`metadata.name`. The unique name of the entity within its `kind`. Other entities use it to reference this one, so renaming it breaks those references.  \n`spec.type`. service, website or library. The catalog accepts any string, but a custom type is a taxonomy decision.  \n`spec.owner`. The owning group, for example group:default/platform. Prefer a group to a person; groups outlive staff changes.  \n`spec.lifecycle`. production, experimental or deprecated. It tells readers whether the service is safe to depend on.\n\n### What goes wrong\n\nAn owner that does not resolve shows up as an orphan, and a missing required field keeps the entity out of the catalog. Both pass the commit and surface when Backstage processes the entity.\n\n> **The honest caveat:** Backstage validates the entity when it processes it, not when you push. A typo in an owner can sit in Git for weeks.\n\n> **The lesson:** the catalog is only as true as the owner and lifecycle you put in it.\n\n## In one line\n\n> **A file is not a service.** The catalog only works when owner and lifecycle are true.\n\n**Lint the file**\n\n```bash\nyamllint catalog-info.yaml\n```\n"},{"id":"p251","n":"251","slug":"quiz-3-platform-engineering-questions","title":"Quiz: 3 platform engineering questions","sections":["devsecops"],"topic":"Platform engineering","tags":["PlatformEngineering","DevOps","SRE","CloudNative","CICD"],"series_name":"Platform","design":"quiz","script":"","date":"2028-05-04","files":{"devsecops":"notes/p251-quiz-3-platform-engineering-questions.md"},"md":"# Quiz: 3 platform engineering questions\n\nPlatform engineering is easy to describe and hard to judge. Three questions separate a real platform from a rebranded ticket queue. Answer before you read on.\n\n## Quick reference\n\n**Q1. What makes a golden path succeed?**\n\n- a) It is mandatory for every team\n- b) It is the fastest supported way to ship\n- c) It replaces the platform team\n\n> **Answer:** b) It is the fastest supported way to ship. A golden path is a product. If it is slower or harder than the free-for-all, teams route around it, and the platform loses the signal it needs to improve.\n\n**Q2. Why keep a service catalog instead of a spreadsheet?**\n\n- a) It stores more columns per service\n- b) It links owner, docs and runtime\n- c) It replaces incident management\n\n> **Answer:** b) It links owner, docs and runtime. A catalog makes the owner, API, runbook and dependencies discoverable in one entity. A spreadsheet copies stale facts with no link to the running system.\n\n**Q3. When should you build a platform tool instead of buying it?**\n\n- a) Always, to avoid licence fees\n- b) When it is core to how you differentiate\n- c) Never, managed services are cheaper\n\n> **Answer:** b) When it is core to how you differentiate. Build where your constraints are different and the capability is strategic. Buy the commodity layers, where a vendor carries upgrades and on-call.\n\n## Explained\n\n### Question one\n\nWhat makes a golden path succeed? It is the fastest supported way to ship, not the only allowed one. A golden path is a product with users. If it is slower or harder than the free-for-all, teams route around it, and the platform team loses the telemetry it needs to improve. Making it mandatory hides that signal; making it faster earns adoption.\n\n### Question two\n\nWhy keep a service catalog instead of a spreadsheet? Because it links the owner, the docs, the API and the runtime in one entity. A spreadsheet copies facts that go stale, and it cannot answer \"who owns this and is it healthy?\" from the same place as the alert. Backstage's catalog is a graph, not a table.\n\n### Question three\n\nWhen should you build a platform tool instead of buying it? When it is core to how the company differentiates, or when the constraint is genuinely unusual. Build the thin layer that encodes your rules; buy the commodity underneath, where a vendor carries upgrades and on-call. Building a database because you dislike the licence is not platform engineering.\n\n> **The honest caveat:** all three answers depend on adoption. A catalog nobody updates and a path nobody uses are both expensive decoration.\n\n> **The lesson:** a platform is judged by what teams do, not by what it ships.\n\n## In one line\n\n> **Make the right way the easy way.** A platform is judged by what teams do, not what it ships.\n\n**Ship one path:** template + CI + docs\n"},{"id":"p252","n":"252","slug":"9-aws-iam-commands-for-access-reviews","title":"9 AWS IAM commands for access reviews","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["AWS","IAM","CloudSecurity","DevSecOps","CloudComputing"],"series_name":"Platform","design":"terminal","script":"","date":"2028-05-07","files":{"devsecops":"notes/p252-9-aws-iam-commands-for-access-reviews.md"},"md":"# 9 AWS IAM commands for access reviews\n\nAn access review ends with a decision about every key, role and policy. Doing it from the console is slow and easy to miss. Nine AWS CLI commands cover the loop, and they change no permissions.\n\n## Quick reference\n\n### Who has access\n\n```bash\n# identity and age\naws iam get-user --user-name alice\n# long-lived keys\naws iam list-access-keys --user-name alice\n# what the user can do\naws iam list-attached-user-policies --user-name alice\n```\n\n### What the policy allows\n\n```bash\n# the document in force\naws iam get-policy-version --policy-arn $ARN --version-id v3\n# every role in the account\naws iam list-roles\n# who can assume it\naws iam get-role --role-name deploy-prod\n```\n\n### Prove it, then export\n\n```bash\n# model one action\naws iam simulate-principal-policy --policy-source-arn $P --action-names s3:GetObject\n# every human account\naws iam list-users\n# password and MFA status\naws iam generate-credential-report\n```\n\n## Explained\n\n### Who\n\n`aws iam get-user --user-name alice` shows the identity and when it was created.  \n`aws iam list-access-keys --user-name alice` shows every access key, its status and age. A three-year-old active key is a finding.  \n`aws iam list-attached-user-policies --user-name alice` shows the managed policies attached to the user, one part of why alice can do that.\n\n### What\n\n`aws iam get-policy-version --policy-arn $ARN --version-id v3` prints the policy document. Replace v3 with the default version, the one in force.  \n`aws iam list-roles` lists roles in the account. The interesting ones trust a human or a wildcard.  \n`aws iam get-role --role-name deploy-prod` shows the trust policy, which answers who can assume the role, and the maximum session duration.\n\n### Audit\n\n`aws iam simulate-principal-policy --policy-source-arn $P --action-names s3:GetObject` evaluates one action for one principal. It is a model, not a log: what the policy allows, not what happened.  \n`aws iam list-users` lists IAM users, including the ones that should have been deleted.  \n`aws iam generate-credential-report` requests the CSV: password age, MFA status and key rotation for every user.\n\n> **The honest caveat:** the credential report is sensitive and takes minutes to generate. Download it with get-credential-report, store it privately and delete it after the review.\n\n> **The lesson:** access reviews are a query, not a meeting.\n\n## In one line\n\n> **Access reviews beat intentions.** Nine commands, one account, before the auditor asks.\n\n**Try it**\n\n```bash\naws iam get-credential-report\n```\n"},{"id":"p253","n":"253","slug":"irsa-vs-gke-workload-identity-vs-azure-workload-identity","title":"IRSA vs GKE Workload Identity vs Azure Workload Identity","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","AWS","Azure","GCP","DevSecOps"],"series_name":"Platform","design":"matrix","script":"","date":"2028-05-09","files":{"devsecops":"notes/p253-irsa-vs-gke-workload-identity-vs-azure-workload-identity.md"},"md":"# IRSA vs GKE Workload Identity vs Azure Workload Identity\n\nIRSA, GKE Workload Identity and Azure Workload Identity solve one problem: a pod needs a cloud credential, and a static key is not acceptable. All three exchange a Kubernetes service account token for short-lived credentials.\n\n## Quick reference\n\n| | **IRSA** | **GKE WI** | **Azure WI** |\n|:--|:--|:--|:--|\n| Mechanism | OIDC token to STS | OIDC token to IAM | OIDC token to Entra |\n| Trust anchor | EKS OIDC provider | GKE workload pool | AKS OIDC issuer |\n| Scope | Per service account | KSA to GSA | Per federated cred |\n| Setup | 🟡 Role, trust, annotate | 🟡 Binding, annotate | 🟡 Credential, annotate |\n| Portability | ❌ AWS only | ❌ GCP only | ❌ Azure only |\n| Best for | ✅ EKS clusters | ✅ GKE clusters | ✅ AKS clusters |\n\n**How to pick**\n\n- **IRSA** → the EKS way: a KSA annotated with an IAM role.\n- **GKE** → map a KSA to a Google service account.\n- **Azure** → federate a KSA with a managed identity.\n\n## Explained\n\n### Mechanism and trust\n\nEach pod gets a projected token for its Kubernetes service account. The cloud STS validates it against the cluster's OIDC issuer and returns temporary credentials. No secret is stored in the pod.  \nThe trust anchor is what differs. IRSA trusts the EKS cluster's OIDC provider. GKE trusts the workload identity pool of the project. Azure trusts the AKS cluster's OIDC issuer through a federated identity credential.\n\n### Scope and setup\n\nIRSA maps one Kubernetes service account to one IAM role. GKE maps a KSA to a Google service account with an IAM binding. Azure maps a KSA to a managed identity through a federated credential.  \nSetup has the same shape everywhere: enable the cluster issuer, create the cloud identity, annotate the Kubernetes service account.\n\n### Portability and fit\n\nNone of them ports. The annotation key, trust policy and token audience are provider-specific, so moving cloud means rebuilding the identity layer.  \nEach is native to its cluster: IRSA on EKS, Workload Identity on GKE, federated identity on AKS. Use the managed option, not a custom token broker.\n\n> **The honest caveat:** the trust policy is the security boundary. A wildcard subject or audience lets any service account in the cluster assume the role.\n\n> **The lesson:** the mechanism is portable; the trust anchor is not.\n\n## In one line\n\n> **One identity, three clouds.** The mechanism is the same; the trust anchor is not.\n\n**Check the issuer**\n\n```bash\naws eks describe-cluster --name prod\n```\n"},{"id":"p254","n":"254","slug":"an-iam-wildcard-scanner-in-python","title":"An IAM wildcard scanner in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","IAM","CloudSecurity","Python","DevSecOps"],"series_name":"Platform","design":"code","script":"iam_wildcard.py","date":"2028-05-11","files":{"devsecops":"notes/p254-an-iam-wildcard-scanner-in-python.md"},"md":"# An IAM wildcard scanner in Python\n\nAn IAM policy can grant every action on every resource in one line. Most teams find out during a review, or after an incident. This script reads the policy JSON and fails on the wildcards.\n\n## Quick reference\n\nThe full script is at the end of this note: `iam_wildcard.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 iam_wildcard.py policies/\n```\n\n**Example output**\n\n```text\nALERT 3 wildcard grants\n```\n\n## Explained\n\n### What it does\n\nIt walks a folder of policy documents and reads every statement. It flags an Action of \"\\*\", an action ending in \":\\*\" such as s3:\\* or iam:\\*, and a Resource of \"\\*\". Deny statements are skipped, because a wildcard deny restricts rather than grants. Each hit prints the file, the statement number and the value, then exits 1 so CI stops.\n\n### Why those three\n\n\"Action\": \"\\*\" grants every action; paired with \"Resource\": \"\\*\" that is full administrator, unless a Condition narrows it. \"s3:\\*\" is every S3 call in the service, including DeleteBucket and PutBucketPolicy. \"Resource\": \"\\*\" turns a narrow action into an account-wide one: s3:GetObject on \\* reads every bucket you own.\n\n### How to use it\n\nPoint it at a folder, a single file or stdin: `python3 iam_wildcard.py policies/`. In CI, run it before terraform plan and let a non-zero exit block the merge. The scan is read-only; it never calls AWS.\n\n> **The honest caveat:** a wildcard is not automatically wrong. Some AWS actions only work with \"Resource\": \"\\*\", and a tightly conditioned wildcard can be defensible. The script shows you where to look; a human still decides.\n\n> **The lesson:** least privilege is not a policy you write once. It is a check that runs on every change.\n\n## In one line\n\n> **A wildcard is a permission you forgot.** Every \\* is a grant someone will eventually find.\n\n**List local policies**\n\n```bash\naws iam list-policies --scope Local\n```\n\n## Script: iam_wildcard.py\n\nDownload: [scripts/iam_wildcard.py](../scripts/iam_wildcard.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"iam_wildcard.py - find wildcard grants in AWS IAM policy JSON.\n\nReads IAM policy documents and flags the three patterns that quietly widen\npermissions:\n\n    \"Action\": \"*\"        every action\n    \"Action\": \"s3:*\"     every action in one service\n    \"Resource\": \"*\"      every resource the action can touch\n\nStatements with \"Effect\": \"Deny\" are skipped, because a wildcard in a Deny\nrestricts access instead of granting it. NotAction and NotResource are out of\nscope. The tool is read-only: it never calls AWS and never edits a policy.\n\nUsage:\n    python3 iam_wildcard.py policies/\n    python3 iam_wildcard.py policy.json\n    cat policy.json | python3 iam_wildcard.py -\n\nExit codes: 0 clean, 1 wildcard grant found, 2 usage or read error.\n\nCI:\n    python3 iam_wildcard.py infra/policies || exit 1\nCron (weekly, Monday 06:00):\n    0 6 * * 1 python3 iam_wildcard.py /srv/policies >> /var/log/iam-wildcard.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom pathlib import Path\nfrom urllib.parse import unquote\n\nENVELOPES = (\"Document\", \"PolicyDocument\", \"AssumeRolePolicyDocument\")\n\n\ndef unwrap(doc):\n    \"\"\"Return the policy document inside common API envelopes.\"\"\"\n    if not isinstance(doc, dict):\n        return doc\n    if \"Statement\" in doc:\n        return doc\n    for key in ENVELOPES:\n        inner = doc.get(key)\n        if isinstance(inner, str):\n            try:\n                inner = json.loads(unquote(inner))\n            except (ValueError, TypeError):\n                continue\n        if isinstance(inner, dict) and \"Statement\" in inner:\n            return inner\n    version = doc.get(\"PolicyVersion\")\n    if isinstance(version, dict):\n        inner = version.get(\"Document\")\n        if isinstance(inner, str):\n            try:\n                inner = json.loads(unquote(inner))\n            except (ValueError, TypeError):\n                return doc\n        if isinstance(inner, dict) and \"Statement\" in inner:\n            return inner\n    return doc\n\n\ndef statements(doc):\n    \"\"\"Yield (number, statement) for each statement in the document.\"\"\"\n    doc = unwrap(doc)\n    if not isinstance(doc, dict):\n        return\n    body = doc.get(\"Statement\")\n    if isinstance(body, dict):\n        body = [body]\n    if not isinstance(body, list):\n        return\n    for i, st in enumerate(body, 1):\n        if isinstance(st, dict):\n            yield i, st\n\n\ndef as_list(value):\n    if isinstance(value, str):\n        return [value]\n    if isinstance(value, list):\n        return [v for v in value if isinstance(v, str)]\n    return []\n\n\ndef scan(doc):\n    \"\"\"Return a list of (statement number, kind, value, condition keys).\"\"\"\n    hits = []\n    for i, st in statements(doc):\n        effect = str(st.get(\"Effect\", \"?\"))\n        if effect.lower() == \"deny\":\n            continue\n        cond = st.get(\"Condition\")\n        cond_keys = \", \".join(sorted(cond)) if isinstance(cond, dict) else \"\"\n        for action in as_list(st.get(\"Action\")):\n            if action in (\"*\", \"*:*\"):\n                hits.append((i, \"Action\", action, \"every action\", cond_keys))\n            elif action.endswith(\":*\") and action.count(\":\") == 1:\n                service = action.split(\":\", 1)[0]\n                hits.append((i, \"Action\", action, \"every action in \" + service, cond_keys))\n        for resource in as_list(st.get(\"Resource\")):\n            if resource == \"*\":\n                hits.append((i, \"Resource\", resource, \"every resource\", cond_keys))\n    return hits\n\n\ndef candidates(paths, recursive):\n    \"\"\"Return (files, errors); stdin is represented by None.\"\"\"\n    files, errors = [], []\n    for raw in paths:\n        if raw == \"-\":\n            files.append(None)\n            continue\n        path = Path(raw)\n        if path.is_dir():\n            found = path.rglob(\"*.json\") if recursive else path.glob(\"*.json\")\n            batch = sorted(p for p in found if p.is_file())\n            if not batch:\n                errors.append(\"no .json files under %s\" % path)\n            files.extend(batch)\n        elif path.is_file():\n            files.append(path)\n        else:\n            errors.append(\"not found: %s\" % path)\n    return files, errors\n\n\ndef load(path):\n    if path is None:\n        return json.load(sys.stdin)\n    return json.loads(Path(path).read_text(encoding=\"utf-8\"))\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Flag wildcard actions and resources in AWS IAM policy JSON.\")\n    ap.add_argument(\"paths\", nargs=\"+\", metavar=\"PATH\",\n                    help=\"policy .json file, a directory, or - for stdin\")\n    ap.add_argument(\"--no-recursive\", action=\"store_true\",\n                    help=\"scan only the top level of a directory\")\n    ap.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                    help=\"print the summary only\")\n    args = ap.parse_args(argv)\n\n    files, errors = candidates(args.paths, not args.no_recursive)\n    checked = 0\n    total = 0\n    hit_files = set()\n    for path in files:\n        label = \"<stdin>\" if path is None else str(path)\n        try:\n            doc = load(path)\n        except (OSError, ValueError) as exc:\n            errors.append(\"%s: %s\" % (label, exc))\n            continue\n        checked += 1\n        hits = scan(doc)\n        if hits:\n            hit_files.add(label)\n        total += len(hits)\n        if hits and not args.quiet:\n            for num, kind, value, note, cond in hits:\n                extra = \"  [condition: %s]\" % cond if cond else \"\"\n                print(\"[WILDCARD] %s:statement %d  %s=\\\"%s\\\"  (%s)%s\"\n                      % (label, num, kind, value, note, extra))\n    for err in errors:\n        print(\"error: %s\" % err, file=sys.stderr)\n    print(\"%d file(s) checked, %d wildcard grant(s) in %d file(s)\"\n          % (checked, total, len(hit_files)))\n    if total:\n        print(\"ALERT %d wildcard grant%s\" % (total, \"\" if total == 1 else \"s\"))\n    if errors:\n        return 2\n    return 1 if total else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p255","n":"255","slug":"networkpolicy-vs-service-mesh-mtls","title":"NetworkPolicy vs service mesh mTLS","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","NetworkPolicy","MTLS","DevSecOps","CloudNative"],"series_name":"Platform","design":"compare","script":"","date":"2028-05-14","files":{"devsecops":"notes/p255-networkpolicy-vs-service-mesh-mtls.md"},"md":"# NetworkPolicy vs service mesh mTLS\n\nA NetworkPolicy and a service mesh both promise to secure service-to-service traffic, so teams pick one and assume the other is covered. They answer different questions: who may connect, and who is calling.\n\n## Quick reference\n\n| | **NETPOL** | **mTLS** |\n|:--|:--|:--|\n| Layer | L3/L4 packet rules | L4 identity, L7 policy |\n| Identity | Namespace and pod labels | SPIFFE / service account |\n| Blocks | Unlisted pod traffic | Calls without a valid cert |\n| Encryption | None | mTLS on every hop |\n| Ops cost | Low, built into the API | High: proxies and certs |\n| Best for | Default-deny east-west | Encrypted service calls |\n\n**NETPOL** (L3/L4 rules, pod to pod): Allows or blocks traffic by pod, namespace, port and protocol.\n\n```bash\nkubectl apply -f netpol.yaml\n```\n\n**mTLS** (Identity per workload): Both sides prove identity and encrypt every request.\n\nExample: `linkerd viz edges deployment`\n\n- NetworkPolicy → who may connect.\n- mTLS → who is actually calling.\n\n## Explained\n\n### The layer\n\nNetworkPolicy is a Kubernetes object enforced by the CNI at layer 3 and 4. It selects pods by label and allows traffic by namespace, pod selector, port and protocol. Once selected, the policy drops what it does not allow. Identity is a label: allowed means trusted.\n\nThe mesh adds identity at layer 4 and policy at layer 7. Each workload gets a cryptographic identity tied to its ServiceAccount, and both sides present certificates before a request. Traffic is encrypted and the caller is authenticated.\n\n### What each blocks\n\nNetworkPolicy blocks unlisted traffic. It does not encrypt, and cannot tell two callers on one port apart. In strict mode, mTLS blocks calls without a valid certificate, proving the caller, not the intent.\n\n### The cost\n\nNetworkPolicy is one object, enforced where the CNI runs. mTLS brings a control plane, certificate rotation and a proxy or eBPF agent on every node. That is real weight.\n\n### Where each fits\n\nDefault-deny segmentation is NetworkPolicy work. Encryption and workload identity are mesh work. Mature platforms run both, and the mesh still needs authorisation rules.\n\n> **The honest caveat:** a NetworkPolicy is only as strong as its CNI. Some plugins accept the object and ignore it, so test a connection that should fail, not a YAML review.\n\n> **The lesson:** NetworkPolicy decides who may talk; mTLS proves who is talking.\n\nWho may connect, who is calling.\n"},{"id":"p256","n":"256","slug":"5-kubernetes-rbac-mistakes","title":"5 Kubernetes RBAC mistakes","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","RBAC","CloudSecurity","DevSecOps","CKS"],"series_name":"Platform","design":"dontdo","script":"","date":"2028-05-16","files":{"devsecops":"notes/p256-5-kubernetes-rbac-mistakes.md"},"md":"# 5 Kubernetes RBAC mistakes\n\nRBAC is the easiest control to grant and the hardest one to take back. These five turn a stolen token into a cluster-wide incident.\n\n## Quick reference\n\n**1.** ❌ **cluster-admin for CI** — The pipeline can delete any namespace, secret or CRD it can reach.  \n✅ **A namespaced Role per app** — CI writes to one namespace and cannot see the rest.\n\n**2.** ❌ **Wildcard verbs and resources** — '\\*' on '\\*' is cluster-admin with extra steps, and nobody reviews it.  \n✅ **Only the verbs the job calls** — get, list and watch on the three resources it actually uses.\n\n**3.** ❌ **Default ServiceAccount token** — Every pod mounts a token the API server accepts, needed or not.  \n✅ **Disable token automount** — Set automountServiceAccountToken: false unless the pod calls the API.\n\n**4.** ❌ **One binding for all namespaces** — A ClusterRoleBinding grants the same rights in kube-system too.  \n✅ **RoleBinding per namespace** — A RoleBinding scopes the same Role to one namespace at a time.\n\n**5.** ❌ **Bindings nobody removes** — A contractor's access outlives the contract; the token still works.  \n✅ **Review bindings quarterly** — `kubectl auth can-i --list` shows what each account still holds.\n\n## Explained\n\n### The grants\n\n1. **cluster-admin for CI.** The pipeline that deploys everything is often bound to cluster-admin, because it made the first deploy work. It can delete any namespace, read any Secret and change any CRD. Give it a namespaced Role per app instead.\n\n2. **Wildcard verbs and resources.** verbs: [\"\\*\"] on resources: [\"\\*\"] is cluster-admin with extra steps, and nobody reviews what it includes. The grant grows with every new resource, without a commit. List the verbs the job calls.\n\n### The leftovers\n\n3. **Default ServiceAccount tokens.** Every pod mounts a token unless you turn it off, and the API server accepts it. Anything that can read the pod's filesystem can act as that ServiceAccount. Set automountServiceAccountToken: false unless the pod calls the API.\n\n4. **One binding for all namespaces.** A ClusterRoleBinding grants the same rights everywhere, including kube-system. A RoleBinding keeps the same Role inside one namespace, which is usually the intent.\n\n5. **Bindings nobody removes.** Access has no expiry. A contractor's RoleBinding outlives the contract, and the token still works a year later. Review bindings quarterly and delete what no account uses.\n\n> **The honest caveat:** RBAC only guards the API server. A pod that can mount hostPath / or run privileged reaches the node credentials, and no Role stops that.\n\n> **The lesson:** every binding is a door. Grant the smallest one, then check who still has a key.\n\n## In one line\n\n> **Every binding is a door left open.** RBAC is additive: nothing expires on its own.\n\n**Audit the defaults**\n\n```bash\nkubectl get clusterrolebindings -o wide\n```\n"},{"id":"p257","n":"257","slug":"how-a-pod-becomes-cluster-admin","title":"How a pod becomes cluster admin","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","RBAC","CloudSecurity","DevSecOps","CKS"],"series_name":"Platform","design":"pipeline","script":"","date":"2028-05-18","files":{"devsecops":"notes/p257-how-a-pod-becomes-cluster-admin.md"},"md":"# How a pod becomes cluster admin\n\nA pod does not need a vulnerability to become cluster admin. It needs a ServiceAccount token, a permissive RoleBinding and a namespace that still allows privileged pods. Here is the path.\n\n## Quick reference\n\n1. **A token is stolen** — A compromised pod reads its mounted ServiceAccount token. · *token*\n2. **List secrets** — It reads Secrets in its namespace, including credentials. · *secrets*\n3. **Find a privileged SA** — A RoleBinding shows which ServiceAccount can create pods. · *rbac*\n4. **Create a pod as that SA** — The API server runs the new pod with the stronger identity. · *pod create*\n5. **Mount the node** — hostPath / exposes the node filesystem and kubelet creds. · *hostPath*\n6. **Take the cloud role** — Node instance profile gives the pod cloud rights. · *cloud*\n\n## Explained\n\n### The path\n\n1. **A token is stolen.** Every pod mounts a projected token at `/var/run/secrets/kubernetes.io/serviceaccount/token` unless automount is disabled. A compromised container reads it first.\n\n2. **List secrets.** If the ServiceAccount can list Secrets in its namespace, credentials come back in plaintext: database passwords, API keys, sometimes a cloud key. It is the most common RBAC over-grant.\n\n3. **Find a privileged SA.** RoleBindings show which accounts can create pods. The CI account often can, and its Role may also allow Secrets or pods/exec. `kubectl auth can-i --list --as=system:serviceaccount:shop:api` answers it.\n\n4. **Create a pod as that SA.** Anyone who can create a pod can usually set serviceAccountName in the spec. The API server admits it; the kubelet mounts the stronger token.\n\n5. **Mount the node.** With hostPath: /, the container reads the node's filesystem, often including the kubelet kubeconfig.\n\n6. **Take the cloud role.** The metadata endpoint at 169.254.169.254 may return the node's instance profile credentials. The cluster admin is now the cloud account.\n\n### Where it stops\n\n> **The honest caveat:** Pod Security Admission at baseline or restricted rejects hostPath and privileged pods, so this chain needs a namespace that allows them. Inventory it first.\n\n> **The lesson:** the token is not the breach; the binding behind it is.\n\n## In one line\n\n> **One pod is the whole cluster.** A stolen token is a foothold; weak RBAC makes it the account.\n\n**Prove the path**\n\n```bash\nkubectl auth can-i create pods\n```\n"},{"id":"p258","n":"258","slug":"cloud-security-terms-decoded","title":"Cloud security terms, decoded","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["CloudSecurity","AWS","IAM","DevSecOps","CloudNative"],"series_name":"Platform","design":"reveal","script":"","date":"2028-05-21","files":{"devsecops":"notes/p258-cloud-security-terms-decoded.md"},"md":"# Cloud security terms, decoded\n\nCloud security has a vocabulary problem: the words sound like features, but they are controls. Learn these six and half the architecture diagrams start making sense.\n\n## Quick reference\n\n- **OIDC federation** — A CI job trades a short-lived OIDC token for cloud credentials, so no static key is stored.\n- **Workload ID** — A cloud identity bound to a Kubernetes ServiceAccount, so the pod calls the cloud as itself.\n- **CIEM** — Cloud Infrastructure Entitlement Management: it maps which identity can do what, and finds unused admin rights.\n- **CSPM** — Cloud Security Posture Management: it scans the account for public buckets, open ports and missing logs.\n- **Least privilege** — Only the actions a job needs, on only the resources it needs, for only as long as it runs.\n- **Break-glass** — A sealed emergency account, kept out of daily use, with every login alerted and reviewed.\n\n## Explained\n\n### Identity\n\nOIDC federation lets a CI job present a signed, short-lived token to the cloud and receive temporary credentials in return. Nothing long-lived sits in the pipeline, which removes the static key that leaks in a log.\n\nWorkload identity is the same idea inside Kubernetes: a cloud identity is bound to a ServiceAccount, so a pod calls the cloud as itself, not as the node.\n\nCIEM, Cloud Infrastructure Entitlement Management, maps which identity can do what across accounts, and finds the admin permission nobody uses.\n\n### Posture\n\nCSPM, Cloud Security Posture Management, scans the account for misconfiguration: public buckets, open security groups, disabled logging, unencrypted volumes. It answers what is exposed, not who can reach it.\n\nLeast privilege is the rule that ties them together: only the actions a job needs, on only its resources, for only as long as it runs. Permission boundaries enforce it; Access Analyzer finds where it slipped.\n\n### The exception\n\nBreak-glass is the account kept for the day normal access fails. It is sealed, out of daily use, with MFA, a vaulted credential and an alert on every login.\n\n> **The honest caveat:** CIEM and CSPM are product categories, not guarantees. Both are only as good as the permissions they can read and the rules someone maintains.\n\n> **The lesson:** the vocabulary is a checklist. Federation, identity, entitlement, posture, least privilege, break-glass.\n\n## In one line\n\n> **Know the term, know the control.** A vocabulary is a checklist you can run.\n\n**Check a role**\n\n```bash\naws iam get-role --role-name api\n```\n"},{"id":"p259","n":"259","slug":"a-trivy-gate-in-python","title":"A Trivy gate in Python","sections":["devsecops"],"topic":"Python scripts","tags":["DevSecOps","Trivy","Kubernetes","Python","CloudNative"],"series_name":"Platform","design":"code","script":"trivy_gate.py","date":"2028-05-23","files":{"devsecops":"notes/p259-a-trivy-gate-in-python.md"},"md":"# A Trivy gate in Python\n\nA vulnerability report that nobody fails a build on is just a file. Trivy can produce clean JSON for every image; this script turns that JSON into a gate with an allowlist and an exit code.\n\n## Quick reference\n\nThe full script is at the end of this note: `trivy_gate.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\ntrivy image -f json api:1.4.2 | python3 trivy_gate.py -\n```\n\n**Example output**\n\n```text\nALERT 3 blocking, 0 allowlisted\n```\n\n## Explained\n\n### What it reads\n\nTrivy puts findings under Results, one entry per target, and each entry has a Vulnerabilities list. Every finding carries Severity, PkgName, InstalledVersion, FixedVersion and VulnerabilityID. The script walks both levels and keeps CRITICAL and HIGH.\n\n### What it does\n\nAnything left after the allowlist is printed with the package and the fixed version, then the run exits 1. Allowlisted IDs are counted separately and can be printed, so a temporary exception stays visible instead of silent. A clean image exits 0.\n\n### How to use it\n\nRun Trivy and pipe the output: `trivy image -f json api:1.4.2 | python3 trivy_gate.py -`. In CI, that is the step that blocks the merge. The allowlist file uses one ID per line, the same shape as .trivyignore.\n\n### The exception list\n\nTrivy can already fail a build with `--exit-code` and `--severity`. The difference here is the allowlist: one place to record why a finding is allowed, and a count that keeps the decision honest.\n\n> **The honest caveat:** Trivy only knows what is in its database at scan time, and a HIGH with no fixed version will fail every build. Filter unfixed findings deliberately with `--ignore-unfixed` rather than allowlisting them for ever.\n\n> **The lesson:** a scan is evidence; a gate is the decision.\n\n## In one line\n\n> **A scan is not a gate.** A report nobody fails the build on is a PDF.\n\n**Gate in ci**\n\n```bash\ntrivy image --exit-code 1 api:1.4.2\n```\n\n## Script: trivy_gate.py\n\nDownload: [scripts/trivy_gate.py](../scripts/trivy_gate.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"trivy_gate.py - fail a build on the Trivy findings that matter.\n\nReads the JSON that `trivy image -f json` writes, keeps the severities you\ncare about, subtracts an allowlist, and exits 1 if anything is left. It reads\nthe report only: it does not run Trivy, pull an image or change anything.\n\nThe fields used are the real Trivy schema:\n    Results[].Target\n    Results[].Vulnerabilities[].VulnerabilityID\n    Results[].Vulnerabilities[].PkgName\n    Results[].Vulnerabilities[].InstalledVersion\n    Results[].Vulnerabilities[].FixedVersion\n    Results[].Vulnerabilities[].Severity\n\nUsage:\n    trivy image -f json api:1.4.2 > trivy.json\n    python3 trivy_gate.py trivy.json\n    trivy image -f json api:1.4.2 | python3 trivy_gate.py -\n    python3 trivy_gate.py trivy.json --allow-file .trivyignore\n    python3 trivy_gate.py trivy.json --ignore-unfixed --severity CRITICAL\n\nThe allowlist file takes one vulnerability ID per line, # comments allowed,\nthe same shape as .trivyignore.\n\nExit codes: 0 clean, 1 blocking finding, 2 usage or read error.\n\nCI:\n    trivy image -f json \"$IMAGE\" | python3 trivy_gate.py -\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom pathlib import Path\n\nSEVERITIES = (\"CRITICAL\", \"HIGH\", \"MEDIUM\", \"LOW\", \"UNKNOWN\")\n\n\ndef severities(value):\n    chosen = {s.strip().upper() for s in value.split(\",\") if s.strip()}\n    bad = sorted(chosen - set(SEVERITIES))\n    if not chosen or bad:\n        raise argparse.ArgumentTypeError(\n            \"choose from %s\" % \", \".join(SEVERITIES))\n    return chosen\n\n\ndef allowlist(paths, inline):\n    ids = {item.strip() for item in inline if item.strip()}\n    for path in paths:\n        for line in Path(path).read_text(encoding=\"utf-8\").splitlines():\n            line = line.strip()\n            if line and not line.startswith(\"#\"):\n                ids.add(line.split()[0])\n    return ids\n\n\ndef read_report(path):\n    text = sys.stdin.read() if path == \"-\" else Path(path).read_text(encoding=\"utf-8\")\n    doc = json.loads(text)\n    if not isinstance(doc, dict):\n        raise ValueError(\"report is not a JSON object\")\n    if not any(k in doc for k in (\"Results\", \"SchemaVersion\", \"ArtifactName\")):\n        raise ValueError(\"not a Trivy JSON report\")\n    return doc\n\n\ndef collect(doc, wanted, ignore_unfixed):\n    found, below, unfixed = [], 0, 0\n    seen = set()\n    for result in doc.get(\"Results\") or []:\n        if not isinstance(result, dict):\n            continue\n        target = result.get(\"Target\", \"?\")\n        for vuln in result.get(\"Vulnerabilities\") or []:\n            if not isinstance(vuln, dict):\n                continue\n            severity = str(vuln.get(\"Severity\", \"\")).upper()\n            if severity not in wanted:\n                below += 1\n                continue\n            fixed = vuln.get(\"FixedVersion\") or \"\"\n            if ignore_unfixed and not fixed:\n                unfixed += 1\n                continue\n            key = (vuln.get(\"VulnerabilityID\", \"?\"),\n                   vuln.get(\"PkgName\", \"?\"),\n                   vuln.get(\"InstalledVersion\", \"?\"))\n            if key in seen:\n                continue\n            seen.add(key)\n            found.append({\n                \"id\": key[0], \"pkg\": key[1], \"installed\": key[2],\n                \"fixed\": fixed, \"severity\": severity, \"target\": target,\n            })\n    return found, below, unfixed\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Gate a build on Trivy image scan JSON.\")\n    ap.add_argument(\"report\", nargs=\"?\", default=\"-\", metavar=\"FILE\",\n                    help=\"Trivy JSON report, or - for stdin (default)\")\n    ap.add_argument(\"-s\", \"--severity\", type=severities, default=severities(\"CRITICAL,HIGH\"),\n                    help=\"comma list, default CRITICAL,HIGH\")\n    ap.add_argument(\"-a\", \"--allow\", action=\"append\", default=[], metavar=\"ID\",\n                    help=\"allowlist one vulnerability ID (repeatable)\")\n    ap.add_argument(\"--allow-file\", action=\"append\", default=[], metavar=\"FILE\",\n                    help=\"file of IDs to allowlist, one per line\")\n    ap.add_argument(\"--ignore-unfixed\", action=\"store_true\",\n                    help=\"skip findings with no FixedVersion\")\n    ap.add_argument(\"--show-allowlisted\", action=\"store_true\",\n                    help=\"print the IDs that the allowlist removed\")\n    args = ap.parse_args(argv)\n\n    try:\n        allowed = allowlist(args.allow_file, args.allow)\n        doc = read_report(args.report)\n    except (OSError, ValueError) as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    found, below, unfixed = collect(doc, args.severity, args.ignore_unfixed)\n    blocking = [f for f in found if f[\"id\"] not in allowed]\n    skipped = [f for f in found if f[\"id\"] in allowed]\n\n    if args.show_allowlisted:\n        for f in skipped:\n            print(\"ALLOW  %s  %s  %s\" % (f[\"id\"], f[\"pkg\"], f[\"severity\"]))\n    for f in blocking:\n        fixed = f[\"fixed\"] if f[\"fixed\"] else \"no fix\"\n        print(\"BLOCK  %-16s %-12s %-12s -> %-12s %s\"\n              % (f[\"id\"], f[\"pkg\"], f[\"installed\"], fixed, f[\"severity\"]))\n    print(\"%d blocking, %d allowlisted, %d below threshold, %d unfixed\"\n          % (len(blocking), len(skipped), below, unfixed))\n    if blocking:\n        print(\"ALERT %d blocking, %d allowlisted\" % (len(blocking), len(skipped)))\n        print(\"FAIL %s\" % (doc.get(\"ArtifactName\") or args.report))\n        return 1\n    print(\"OK 0 blocking findings\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p260","n":"260","slug":"anatomy-of-an-iam-policy-statement","title":"Anatomy of an IAM policy statement","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["AWS","IAM","CloudSecurity","DevSecOps","CloudNative"],"series_name":"Platform","design":"anatomy","script":"","date":"2028-05-25","files":{"devsecops":"notes/p260-anatomy-of-an-iam-policy-statement.md"},"md":"# Anatomy of an IAM policy statement\n\nAn IAM policy is a JSON document, but the behaviour comes from six fields. Read them in order and a policy stops being a wall of text.\n\n## Quick reference\n\n```text\n\"Sid\" \"Effect\" \"Principal\" \"Action\" \"Resource\" \"Condition\"\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `\"Sid\"` | Statement ID | Optional label for one statement, so a review can point at it. |\n| `\"Effect\"` | Allow or Deny | Allow or Deny. An explicit Deny always wins over any Allow. |\n| `\"Principal\"` | Who | Who it applies to: account, role, user or federated identity. Resource-based policies only. |\n| `\"Action\"` | What | The API calls allowed or denied, like s3:GetObject. A wildcard widens it fast. |\n| `\"Resource\"` | Which thing | The ARN the action applies to. \\* means every resource in the account. |\n| `\"Condition\"` | When | Optional tests: MFA present, source IP, tag, VPC endpoint. They narrow a grant. |\n\n## Explained\n\n### The six fields\n\nSid is an optional statement ID. It changes nothing, but gives reviews a name to point at.\n\nEffect is Allow or Deny, and it is case-sensitive. An explicit Deny always wins over any Allow. A guardrail policy can therefore override an admin grant.\n\nPrincipal is who the statement applies to: an account, role, user, federated identity or service. It appears in resource-based policies and role trust policies. An identity policy has no Principal because the identity owns the policy.\n\nAction is the API call, written service:operation, like s3:GetObject. It accepts wildcards: s3:\\* is every S3 call. List actions explicitly unless you mean the service.\n\nResource is the ARN it applies to. \\* means every resource, and some actions only work with \\*. Scope by ARN where possible.\n\nCondition is the when: MFA present, source IP, a tag, a VPC endpoint. All conditions must pass; several values for one key are ORed. A wildcard with a tight Condition is narrower than it looks.\n\n### The version\n\nThe Version field at the top, 2012-10-17 in current policies, tells IAM which grammar to parse, not when the policy was created.\n\n> **The honest caveat:** this statement is one layer. Permissions boundaries, SCPs and session policies can remove what it allows, and an explicit Deny anywhere still wins.\n\n> **The lesson:** read the six fields, then ask what the policy does not say.\n\n## In one line\n\n> **Read the fields, not the name.** A policy called ReadOnly can still allow s3:\\*.\n\n**Dump the account**\n\n```bash\naws iam get-account-authorization-details\n```\n"},{"id":"p261","n":"261","slug":"quiz-3-cloud-security-questions","title":"Quiz: 3 cloud security questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["CloudSecurity","IAM","AWS","DevSecOps","CloudNative"],"series_name":"Platform","design":"quiz","script":"","date":"2028-05-28","files":{"devsecops":"notes/p261-quiz-3-cloud-security-questions.md"},"md":"# Quiz: 3 cloud security questions\n\nThree questions from the cloud security work in this phase. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why is a wildcard action like s3:\\* riskier than it looks?**\n\n- a) It is slower to evaluate.\n- b) It grants every call in that service.\n- c) It only works in the console.\n\n> **Answer:** b) It grants every call in that service.. s3:\\* covers GetObject, DeleteObject and PutBucketPolicy. One line grants the whole service, including the calls that destroy data.\n\n**Q2. What does workload identity remove from a CI pipeline?**\n\n- a) The need for a cluster.\n- b) Long-lived static cloud keys.\n- c) The need for a build step.\n\n> **Answer:** b) Long-lived static cloud keys.. The pipeline presents a short-lived OIDC token, the cloud returns temporary credentials, and no access key sits in a secret store.\n\n**Q3. Why should a break-glass account be audited and alerted?**\n\n- a) It is the only account with MFA.\n- b) It bypasses normal access paths by design.\n- c) It costs more than a normal role.\n\n> **Answer:** b) It bypasses normal access paths by design.. Break-glass exists for the day normal access fails. Every use should page someone and be reviewed, because it is the account an attacker wants.\n\n## Explained\n\n### Question one\n\nWhy is a wildcard action such as s3:\\* riskier than it looks? It grants every call in the service, not the one call you needed. s3:\\* includes GetObject, but also DeleteObject, PutBucketPolicy and DeleteBucket. One line of policy can hand over the data and the controls around it.\n\n### Question two\n\nWhat does workload identity remove from a CI pipeline? The long-lived static key. The pipeline presents a short-lived OIDC token, the cloud exchanges it for temporary credentials, and no access key sits in a secret store waiting to leak. That is what OIDC federation and workload identity give you.\n\n### Question three\n\nWhy should a break-glass account be audited and alerted? It bypasses the normal access path by design. It exists for the day the identity provider fails, so every use should page someone, log the session and be reviewed afterwards. It is the account an attacker wants most.\n\nEach answer is a control you can check. Wildcard actions are an IAM review. Static keys are replaced by an OIDC trust policy. Break-glass is an alert and a session log.\n\n> **The honest caveat:** knowing the answer is not the same as enforcing it. Check your own account before you grade someone else's.\n\n> **The lesson:** the why is the part that transfers to the next incident.\n\n## In one line\n\n> **Know the why, not the word.** Three answers that outlast the exam.\n\n**Review one role**\n\n```bash\naws iam list-role-policies --role-name api\n```\n"},{"id":"p262","n":"262","slug":"the-llm-application-stack-layer-by-layer","title":"The LLM application stack, layer by layer","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["AI","LLM","DevSecOps","MLOps","CloudNative"],"series_name":"Platform","design":"stack","script":"","date":"2028-05-30","files":{"devsecops":"notes/p262-the-llm-application-stack-layer-by-layer.md"},"md":"# The LLM application stack, layer by layer\n\nAn LLM feature that works in a demo is a model call. One that works in production is a stack of six layers, and the model is only the bottom one.\n\n## Quick reference\n\n*Bottom to top: each layer changes the answer*\n\n1. **Model** — The foundation model: a hosted API, open weights or a fine-tune. · *model*\n2. **Gateway** — One endpoint in front: keys, quotas, retries, routing and logs. · *gateway*\n3. **Retrieval** — Chunks, embeddings and a vector index, so answers use your data. · *retrieval*\n4. **Orchestration** — The loop that plans, calls tools and assembles the final prompt. · *agent loop*\n5. **Guardrails** — Input and output checks: PII, injection, schema and policy. · *guardrails*\n6. **Evals** — Scored test sets that gate a prompt or model change before release. · *evals*\n\n## Explained\n\n### The lower layers\n\nModel. A hosted API, an open-weight model or a fine-tune. The choice sets cost, latency, context window and what data leaves your network.\n\nGateway. One endpoint in front of every model: keys, quotas, retries, routing and logs. Without it, every team hard-codes a different provider key.\n\nRetrieval. Chunks, embeddings and a vector index, so the model answers from your documents instead of its training data. The quality of the chunks matters more than the model choice.\n\n### The upper layers\n\nOrchestration. The loop that plans, calls tools and assembles the prompt. It is where an agent lives, and where most latency and most failure modes live too.\n\nGuardrails. Input and output checks: PII, prompt injection, schema validation, policy. They sit around the model, not inside it.\n\nEvals. Scored test sets that gate a prompt or model change before release. Without them, every change is a vibe check. Start with thirty real questions and a pass mark.\n\n> **The honest caveat:** evals are the weakest layer in most teams. An LLM judge is useful, but it correlates with quality rather than proving it, so keep a small human-reviewed set.\n\n> **The lesson:** debug the layer, not the model.\n\n## In one line\n\n> **The model is one layer.** Every layer above it can change the answer.\n\n**Start with evals:** pick 30 real questions and score them\n"},{"id":"p263","n":"263","slug":"rag-vs-fine-tuning","title":"RAG vs fine-tuning","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["RAG","LLM","AIEngineering","MachineLearning","DevOps"],"series_name":"Platform","design":"compare","script":"","date":"2028-06-01","files":{"devsecops":"notes/p263-rag-vs-fine-tuning.md"},"md":"# RAG vs fine-tuning\n\nRetrieval-augmented generation and fine-tuning both make a general model useful for your product, and teams argue about them as if one replaces the other. They change different things, at different costs.\n\n## Quick reference\n\n| | **RAG** | **FINE-TUNE** |\n|:--|:--|:--|\n| What changes | Prompt and context only | The model's weights |\n| Data needed | Docs plus an index | Labelled examples |\n| Freshness | Reindex, no retrain | Retrain to update |\n| Cost | Retrieval plus tokens | Training plus hosting |\n| Latency | One extra hop | No retrieval hop |\n| Best for | Changing, cited facts | Tone, format, one task |\n\n**RAG** (Retrieval at query time): Search an index, put the best chunks in the prompt, nothing else changes.\n\nExample: `retrieve then prompt`\n\n**FINE-TUNE** (Weights changed by training): Train on your examples, ship new weights, serve a model that behaves your way.\n\nExample: `train, evaluate, deploy`\n\n- RAG → reindex, no retrain.\n- Fine-tune → retrain, then serve.\n\n## Explained\n\n### How they differ\n\nRAG leaves the weights alone. At query time you embed the question, search an index, and put the best chunks in the prompt. The model still knows only what it was trained on; the answer is grounded in documents you control.\n\nFine-tuning changes the weights. You show the model hundreds or thousands of input-output pairs and it learns a pattern: a format, a tone, a narrow task. The knowledge is baked in, so updating it means training again.\n\n### Where RAG wins\n\nFreshness. Reindex a changed document and the next answer reflects it, with no retraining and no redeploy.  \nProvenance. You can return the source chunk with the answer, which is what a reviewer asks for.  \nCost. Indexing is cheap next to a training run.\n\n### Where fine-tuning wins\n\nLatency. No retrieval hop and a shorter prompt, so the first token arrives sooner.  \nConsistency. A small model tuned on your examples can match a much larger general model on one narrow task.  \nCost at volume. A tuned small model is often cheaper per request than a large model with a long context.\n\n> **The honest caveat:** fine-tuning does not reliably add facts, and RAG cannot teach a new output format. Most teams that need both start with RAG, then fine-tune for shape.\n\n> **The rule:** retrieve for knowledge, train for behaviour.\n\nContext is cheaper than weights.\n"},{"id":"p264","n":"264","slug":"an-llm-gateway-log-audit-in-python","title":"An LLM gateway log audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["LLM","Python","DevSecOps","AIEngineering","CloudNative"],"series_name":"Platform","design":"code","script":"llm_audit.py","date":"2028-06-04","files":{"devsecops":"notes/p264-an-llm-gateway-log-audit-in-python.md"},"md":"# An LLM gateway log audit in Python\n\nEvery prompt through your gateway becomes a log line, and most teams keep those lines for debugging and never read them. That log is a security control: it shows what users asked, what the model answered, and what your filters missed.\n\n## Quick reference\n\nThe full script is at the end of this note: `llm_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 llm_audit.py gateway.jsonl\n```\n\n**Example output**\n\n```text\nALERT PII 2 INJECTION 1 TOKENS 203\n```\n\n## Explained\n\n### What it does\n\nThe script reads a JSONL export where each line is one request and response. It runs PII regexes over both fields, looks for injection markers such as \"ignore previous instructions\", totals the token counts, and computes the refusal rate from the response text.\n\n### Why audit it\n\nA prompt that reaches the model with an email address in it has already left your boundary, whether or not the answer repeats it. Injection markers show attempts that your input filter did not stop. The refusal rate is a cheap drift signal: if it jumps, a prompt change or a model change broke something.\n\n### How to use it\n\nPoint it at an export from the gateway: `python3 llm_audit.py gateway.jsonl`. It prints one summary and a few sample hits, and exits 1 when it finds a policy problem, so a nightly job can alert on it.\n\n> **The honest caveat:** a regex catches patterns, not intent, so treat a hit as a lead to review, not a verdict. A prompt log is also a second copy of the leak, so redact before you archive it.\n\n> **The lesson:** if you log prompts, audit them like production data, because that is what they are.\n\n## In one line\n\n> **A log is a control.** You cannot review what you never recorded.\n\n**Run it weekly**\n\n```bash\npython3 llm_audit.py gateway.jsonl\n```\n\n## Script: llm_audit.py\n\nDownload: [scripts/llm_audit.py](../scripts/llm_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"llm_audit.py - audit an LLM gateway log for PII, injection and refusals.\n\nReads a JSONL export from an LLM gateway: one JSON object per line, with the\nprompt and the response. Reports the four things a policy review needs:\n\n    PII patterns        email, phone and card numbers in either field\n    injection markers   \"ignore previous instructions\", \"system prompt\", ...\n    tokens              total from usage, or prompt plus completion counts\n    refusal rate        responses that read as a refusal\n\nPrompts are read from \"prompt\"/\"input\" or a \"messages\" list; responses from\n\"response\"/\"completion\"/\"output\". Nothing is written and no provider is\ncalled. Exit codes: 0 clean, 1 a policy finding, 2 usage or read error.\n\nUsage:\n    python3 llm_audit.py gateway.jsonl\n    cat gateway.jsonl | python3 llm_audit.py -\n    python3 llm_audit.py logs/*.jsonl --max-refusal 0.15\n    python3 llm_audit.py gateway.jsonl --json --quiet\n\nCI:\n    python3 llm_audit.py logs/export.jsonl --max-refusal 0.2 || exit 1\nCron (nightly at 01:30):\n    30 1 * * * cd /srv/gateway && python3 llm_audit.py logs/export.jsonl >> /var/log/llm_audit.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\nfrom contextlib import nullcontext\n\nEMAIL = re.compile(r\"[\\w.+-]+@[\\w-]+\\.[\\w.-]+\")\nPHONE = re.compile(r\"\\+\\d[\\d ()-]{7,}\\d\")\nCARD = re.compile(r\"\\b(?:\\d[ -]?){15}\\d\\b\")\n\nINJECTION = (\n    \"ignore previous\", \"ignore all previous\", \"ignore the above\",\n    \"disregard previous\", \"disregard the above\", \"forget your instructions\",\n    \"system prompt\", \"you are now\", \"new instructions\", \"reveal your\",\n    \"print your instructions\", \"jailbreak\",\n)\n\nREFUSAL = (\n    \"i can't assist\", \"i cannot assist\", \"i can't help with\",\n    \"i cannot help with\", \"i'm sorry, but i can't\", \"i must decline\",\n    \"i can't provide\", \"i cannot provide\", \"i am unable to\",\n    \"i'm not able to\",\n)\n\n\ndef luhn(number):\n    \"\"\"True when the digits pass the Luhn checksum.\"\"\"\n    digits = [int(c) for c in number if c.isdigit()]\n    total = 0\n    for i, digit in enumerate(reversed(digits)):\n        if i % 2:\n            digit *= 2\n            if digit > 9:\n                digit -= 9\n        total += digit\n    return total % 10 == 0\n\n\ndef mask(kind, value):\n    \"\"\"Return a value that is safe to print in a report.\"\"\"\n    if kind == \"email\":\n        user, _, domain = value.partition(\"@\")\n        return (user[:1] + \"***@\" + domain) if user else \"***@\" + domain\n    digits = \"\".join(c for c in value if c.isdigit())\n    return \"***\" + digits[-4:] if len(digits) >= 4 else \"***\"\n\n\ndef pii_hits(text):\n    \"\"\"Yield (kind, masked value) for every PII match in text.\"\"\"\n    for match in EMAIL.finditer(text):\n        yield \"email\", mask(\"email\", match.group(0))\n    for match in PHONE.finditer(text):\n        yield \"phone\", mask(\"phone\", match.group(0))\n    for match in CARD.finditer(text):\n        value = match.group(0)\n        if luhn(value):\n            yield \"card\", mask(\"card\", value)\n\n\ndef injection_hits(text):\n    \"\"\"Return the injection markers present in text, in list order.\"\"\"\n    low = text.lower()\n    return [marker for marker in INJECTION if marker in low]\n\n\ndef refused(response):\n    \"\"\"True when the response reads as a refusal.\"\"\"\n    low = response.lower()\n    return any(phrase in low for phrase in REFUSAL)\n\n\ndef prompt_of(record):\n    for key in (\"prompt\", \"input\"):\n        value = record.get(key)\n        if isinstance(value, str) and value:\n            return value\n    messages = record.get(\"messages\")\n    if isinstance(messages, list):\n        parts = [str(m.get(\"content\", \"\")) for m in messages if isinstance(m, dict)]\n        return \" \".join(part for part in parts if part)\n    return \"\"\n\n\ndef response_of(record):\n    for key in (\"response\", \"completion\", \"output\"):\n        value = record.get(key)\n        if isinstance(value, str):\n            return value\n    return \"\"\n\n\ndef tokens_of(record):\n    \"\"\"Total tokens from usage, preferring total_tokens.\"\"\"\n    usage = record.get(\"usage\")\n    if not isinstance(usage, dict):\n        return 0\n    total = usage.get(\"total_tokens\")\n    if isinstance(total, int):\n        return total\n    return sum(v for k, v in usage.items()\n               if k.endswith(\"_tokens\") and isinstance(v, int))\n\n\ndef iter_records(paths, open_errors, bad_lines):\n    \"\"\"Yield one dict per JSONL record; collect unreadable input.\"\"\"\n    for path in paths:\n        label = \"<stdin>\" if path == \"-\" else path\n        try:\n            cm = nullcontext(sys.stdin) if path == \"-\" else open(\n                path, \"r\", encoding=\"utf-8\", errors=\"replace\")\n        except OSError as exc:\n            open_errors.append(\"%s: %s\" % (label, exc))\n            continue\n        with cm as handle:\n            for line_no, line in enumerate(handle, 1):\n                line = line.strip()\n                if not line:\n                    continue\n                try:\n                    record = json.loads(line)\n                except ValueError:\n                    bad_lines.append(\"%s:%d not JSON\" % (label, line_no))\n                    continue\n                if not isinstance(record, dict):\n                    bad_lines.append(\"%s:%d not an object\" % (label, line_no))\n                    continue\n                yield record\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Audit an LLM gateway JSONL log for PII, injection, \"\n                    \"token totals and refusals.\")\n    ap.add_argument(\"paths\", nargs=\"*\", default=[\"-\"], metavar=\"PATH\",\n                    help=\"JSONL export, or - for stdin (default)\")\n    ap.add_argument(\"--max-refusal\", type=float, metavar=\"RATE\",\n                    help=\"fail when the refusal rate is above this, e.g. 0.2\")\n    ap.add_argument(\"--show\", type=int, default=3, metavar=\"N\",\n                    help=\"sample hits to print per category, default 3\")\n    ap.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                    help=\"print the summary only\")\n    ap.add_argument(\"--json\", action=\"store_true\",\n                    help=\"print one machine-readable JSON object\")\n    args = ap.parse_args(argv)\n\n    if args.max_refusal is not None and not 0.0 <= args.max_refusal <= 1.0:\n        ap.error(\"--max-refusal must be between 0 and 1\")\n    if args.show < 0:\n        ap.error(\"--show must be 0 or more\")\n\n    open_errors, bad_lines = [], []\n    records = responses = tokens = refusals = 0\n    pii = {\"email\": 0, \"phone\": 0, \"card\": 0}\n    samples = {\"email\": [], \"phone\": [], \"card\": []}\n    injection_records = 0\n    injection_samples = []\n\n    for record in iter_records(args.paths, open_errors, bad_lines):\n        prompt = prompt_of(record)\n        response = response_of(record)\n        text = prompt + \"\\n\" + response\n        records += 1\n        if response:\n            responses += 1\n            if refused(response):\n                refusals += 1\n        tokens += tokens_of(record)\n        for kind, masked in pii_hits(text):\n            pii[kind] += 1\n            if len(samples[kind]) < args.show:\n                samples[kind].append(masked)\n        markers = injection_hits(text)\n        if markers:\n            injection_records += 1\n            if len(injection_samples) < args.show:\n                injection_samples.append((records, markers))\n\n    for message in open_errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if open_errors:\n        return 2\n    if not records:\n        print(\"no JSONL records in %s\" % \" \".join(args.paths), file=sys.stderr)\n        return 2\n    for message in bad_lines[:5]:\n        print(\"warn: skipped %s\" % message, file=sys.stderr)\n\n    rate = (refusals / responses) if responses else 0.0\n    pii_total = sum(pii.values())\n    findings = pii_total + injection_records\n    over_limit = args.max_refusal is not None and rate > args.max_refusal\n    if over_limit:\n        findings += 1\n\n    if args.json:\n        print(json.dumps({\n            \"records\": records,\n            \"responses\": responses,\n            \"skipped\": len(bad_lines),\n            \"tokens\": tokens,\n            \"refusals\": refusals,\n            \"refusal_rate\": round(rate, 4),\n            \"pii\": pii,\n            \"pii_total\": pii_total,\n            \"injection_records\": injection_records,\n            \"findings\": findings,\n        }, sort_keys=True))\n        return 1 if findings else 0\n\n    print(\"%d record(s), %d response(s), %d skipped\"\n          % (records, responses, len(bad_lines)))\n    print(\"tokens %d\" % tokens)\n    print(\"refusals %d  rate %.1f%%\" % (refusals, rate * 100))\n    print(\"PII %d  email %d  phone %d  card %d\"\n          % (pii_total, pii[\"email\"], pii[\"phone\"], pii[\"card\"]))\n    print(\"INJECTION %d record(s)\" % injection_records)\n\n    if not args.quiet:\n        for kind in (\"email\", \"phone\", \"card\"):\n            for value in samples[kind]:\n                print(\"  PII %-5s %s\" % (kind, value))\n        for number, markers in injection_samples:\n            print(\"  INJECTION record %d: %s\" % (number, \", \".join(markers)))\n\n    if over_limit:\n        print(\"REFUSAL %.1f%% above the %.1f%% limit\"\n              % (rate * 100, args.max_refusal * 100))\n    print(\"ALERT %d finding(s)\" % findings if findings else \"OK clean\")\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p265","n":"265","slug":"5-llm-app-security-mistakes","title":"5 LLM app security mistakes","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLMSecurity","GenAI","CyberSecurity","AppSec","DevSecOps"],"series_name":"Platform","design":"dontdo","script":"","date":"2028-06-06","files":{"devsecops":"notes/p265-5-llm-app-security-mistakes.md"},"md":"# 5 LLM app security mistakes\n\nThe model is not a security boundary. It runs on text, it cannot tell an instruction from data, and it will follow the last convincing sentence in its context. These five mistakes all come from forgetting that.\n\n## Quick reference\n\n**1.** ❌ **The model holds permissions** — A prompt can spend any credential the model was given.  \n✅ **A tool with a narrow scope** — Your code calls the tool. The model never sees the credential.\n\n**2.** ❌ **Secrets in the system prompt** — The prompt is logged, echoed and extractable.  \n✅ **Keys in the runtime only** — The model gets a placeholder, never the key itself.\n\n**3.** ❌ **Unbounded agency** — A shell, a browser and write access, driven by text.  \n✅ **Small tool set, human approval** — State changes wait for a person. Reads can run alone.\n\n**4.** ❌ **Rendering output as HTML** — An answer becomes script in your page: stored XSS.  \n✅ **Render as text or sanitise** — Model output is untrusted input to the DOM.\n\n**5.** ❌ **Logging raw prompts** — PII and keys get copied to weaker storage for months.  \n✅ **Redact on write** — Store a hash or a redacted copy, not the raw text.\n\n## Explained\n\n### Permissions\n\nTrusting the model with permissions. Give the model a database credential or an admin token and a prompt can spend it. The fix is a tool with a narrow scope, called by your code, not a credential handed to the model.\n\n### Secrets and agency\n\nSecrets in the system prompt. The system prompt is not private: it is logged, echoed back, and extractable with the right question. Keep keys in the runtime and out of every prompt.\n\nUnbounded agency. A model with a shell, a browser and write access can be talked into using them. Limit the tools to the smallest set, and require approval for anything that changes state.\n\n### Output and logs\n\nRendering output as HTML. Model output is untrusted input. Render it as text or sanitise it, or an answer becomes stored XSS.\n\nLogging raw prompts. Prompts carry PII, keys and customer data, and logs get copied to cheaper storage with weaker access. Redact on write.\n\n> **The honest caveat:** a better prompt is not a fix for any of these. It is a mitigation that fails open when the model is persuaded.\n\nThe habit that matters: every time you are tempted to fix one of these with a prompt, ask whether a permission or a validator would hold when the prompt fails.\n\n## In one line\n\n> **A prompt is advice. A permission is a limit.** Every fix moves a decision out of the prompt.\n\n**Scope the tool:** one tool, one verb, no admin\n"},{"id":"p266","n":"266","slug":"a-prompts-journey-through-a-gateway","title":"A prompt's journey through a gateway","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLM","GenAI","AIEngineering","DevOps","SRE"],"series_name":"Platform","design":"timeline","script":"","date":"2028-06-08","files":{"devsecops":"notes/p266-a-prompts-journey-through-a-gateway.md"},"md":"# A prompt's journey through a gateway\n\nA prompt leaves your app and reaches a model in under a second. Between those two points a gateway can do six things, and every one of them is a place to fail safely.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **1** | The request arrives | App sends prompt, tenant and trace IDs; the gateway normalises. |\n| **2** | Auth and quota | The key resolves to a tenant, then rates and quotas are checked. |\n| **3** | The guardrail runs | Input filters catch PII, blocked topics and injection patterns. |\n| **4** | Retrieval | Top-k chunks from the tenant index, as data, not instructions. |\n| **5** | The model call | Provider call with a timeout, a retry budget and a fallback model. |\n| **6** | Check and log | Schema check, secret scan, redaction, then one trace-ID log line. |\n\n## Explained\n\n### Before the model\n\n1. **The request arrives.** Your app sends the prompt, a tenant ID and a trace ID. The gateway normalises the call so every provider looks the same.\n\n2. **Auth and quota.** The key resolves to a tenant, then a rate limit checks requests per minute and tokens per minute. Over quota returns 429, not a queue that grows all night.\n\n3. **The guardrail runs.** Input filters check for PII, blocked topics and injection patterns. A hit is logged and returned as a policy refusal, before a token is spent.\n\n### At the model\n\n4. **Retrieval.** The gateway embeds the question, fetches the top-k chunks from the tenant's index, and inserts them as data, not instructions.\n\n5. **The model call.** The prompt goes to the provider with a timeout, a retry budget and a fallback model. Streaming starts as soon as the first token arrives.\n\n### After the model\n\n6. **Check and log.** The response is validated against a schema, scanned for secrets, redacted, then logged with the trace ID. Only then does your app see it.\n\n> **The honest caveat:** each hop costs milliseconds, and a gateway that checks everything becomes the reason your app feels slow.\n\n> **The lesson:** a gateway is not a proxy. It is the one place where every prompt and every answer can be inspected in a single code path.\n\n## In one line\n\n> **Every hop is a control.** A gateway is where a prompt becomes auditable.\n\n**Trace one prompt**\n\n```bash\ngrep trace_id gateway.jsonl\n```\n"},{"id":"p267","n":"267","slug":"9-checks-before-you-expose-an-llm-app","title":"9 checks before you expose an LLM app","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLM","AISecurity","DevSecOps","CloudNative","CyberSecurity"],"series_name":"Platform","design":"checklist","script":"","date":"2028-06-11","files":{"devsecops":"notes/p267-9-checks-before-you-expose-an-llm-app.md"},"md":"# 9 checks before you expose an LLM app\n\nAn LLM feature is an endpoint that takes free text and can call tools. That is a wider attack surface than most, and the checks that make it safe are boring.\n\n## Quick reference\n\n### Input\n\n- **Test injections in CI** — prompt plus poisoned doc\n- **Rate limit per tenant** — requests and tokens\n- **Redact PII on the way in** — email, card, phone\n\n### Tools\n\n- **Scope every tool call** — verbs, not admin\n- **Approve state changes** — human in the loop\n- **Keep one kill switch** — tested, not assumed\n\n### Output\n\n- **Validate every output** — schema and allow-list\n- **Log with a trace ID** — redacted prompt\n- **Eval before you ship** — a fixed test set\n\n## Explained\n\n### Input\n\nTest injections in CI. Keep a set of prompts and poisoned documents that try to override your instructions, and run them on every prompt change. If one succeeds, the build fails.  \nRate limit per tenant. Requests and tokens both. Without it, one user with a loop can spend your whole budget and starve everyone else.  \nRedact PII before the model sees it. Email, card and phone patterns are cheap to catch, and a prompt that never carried the data cannot leak it.\n\n### Tools\n\nScope every tool. The model should call a function with the verbs it needs, not hold a credential. A read-only search tool cannot delete a table, however the prompt is written.  \nApprove what changes state. Payments, emails, deletions and deploys get a human confirmation. Read-only actions can run unattended.  \nKeep one flag that stops the agent. A kill switch that drops tool calls and returns a safe message, tested.\n\n### Output\n\nValidate output against a schema and an allow-list. Never pass model output straight into SQL, a shell or the DOM.  \nLog with a trace ID: redacted prompt, response, model version, latency, token counts.  \nEvaluate before you ship. A fixed test set rerun on every model or prompt change.\n\n> **The honest caveat:** a checklist is not a control until someone runs it.\n\n> **The lesson:** the checks are not about the model. They are about everything around it.\n\n## In one line\n\n> **Expose the app, not the model.** Nine checks that keep an LLM feature inside its lane.\n\n**First check:** add one injection test to CI\n"},{"id":"p268","n":"268","slug":"6-gpu-numbers-for-platform-teams","title":"6 GPU numbers for platform teams","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["GPU","NVIDIA","Kubernetes","H100","CloudNative"],"series_name":"Platform","design":"numbers","script":"","date":"2028-06-13","files":{"devsecops":"notes/p268-6-gpu-numbers-for-platform-teams.md"},"md":"# 6 GPU numbers for platform teams\n\nA GPU node is not a bigger CPU node. It has its own scheduling unit, memory ceiling and power budget, and these six numbers decide capacity.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **1** | GPU per request | H100 SXM: one `nvidia.com/gpu` request is a whole card. | k8s |\n| **8** | GPUs per HGX board | H100 SXM: an HGX baseboard carries eight cards on NVLink. | hardware |\n| **80 GB** | HBM3 per GPU | H100 SXM: the memory ceiling for one model shard. | memory |\n| **3.35** | TB/s bandwidth | H100 SXM: HBM3 bandwidth, the limit for token speed. | hbm3 |\n| **700 W** | Board power | H100 SXM: plan cooling for eight of these in one node. | power |\n| **7** | MIG slices | H100 SXM: split one card into seven small instances. | sharing |\n\n## Explained\n\n### Scheduling\n\nOne GPU per request. Kubernetes schedules GPUs as whole devices. A container asks for `nvidia.com/gpu`: 1 and the device plugin hands it one card. There is no fractional request unless you configure sharing.\n\nEight per HGX board. An HGX H100 baseboard carries eight SXM GPUs joined by NVLink, which is why GPU nodes are sold in eights and why one node failure takes eight cards with it.\n\n### Memory and bandwidth\n\n80 GB of HBM3. That is the memory on one H100 SXM, and it sets the largest model shard you can fit before you need tensor parallelism across cards.\n\n3.35 TB/s. The HBM3 bandwidth on the same card. Inference on a large model is memory-bound, so this number, not the FLOPs, often decides tokens per second.\n\n### Power and sharing\n\n700 W. The board power of an H100 SXM. Eight of them plus CPUs and fans is a multi-kilowatt node, and the rack and cooling plan has to exist before the order does.\n\n7 MIG slices. One H100 can be split into up to seven MIG instances, which is how small jobs share a card without time-slicing.\n\n> **The honest caveat:** these are H100 SXM figures. The PCIe card is 350 W with lower bandwidth, and H200 changes the memory numbers again. Read the spec sheet for the part you actually buy.\n\n> **The lesson:** size the node from the card's real limits, not from the model's parameter count.\n\n## In one line\n\n> **Size the node, not the GPU.** These are H100 SXM figures. Check the card you buy.\n\n**Read the spec**\n\n```bash\nnvidia-smi -q -d MEMORY\n```\n"},{"id":"p269","n":"269","slug":"a-gpu-workload-audit-in-python","title":"A GPU workload audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["GPU","Kubernetes","Python","DevSecOps","SRE"],"series_name":"Platform","design":"code","script":"gpu_audit.py","date":"2028-06-15","files":{"devsecops":"notes/p269-a-gpu-workload-audit-in-python.md"},"md":"# A GPU workload audit in Python\n\nYou run `kubectl get pods -A -o json` and get a page of JSON. Somewhere in it are the pods holding the expensive cards, and some of them are privileged, run latest, or mount the node filesystem.\n\n## Quick reference\n\nThe full script is at the end of this note: `gpu_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nkubectl get pods -A -o json | python3 gpu_audit.py -\n```\n\n**Example output**\n\n```text\nALERT GPUS 13 FLAGS 2\n```\n\n## Explained\n\n### What it does\n\nThe script reads that dump, finds every container that requests an `nvidia.com/gpu`, and checks the pod around it: is the image pinned to a tag that is not latest, do the containers have CPU and memory limits, is anything privileged, and does the pod mount a hostPath volume. It prints the GPU inventory, one line per finding, and exits 1 if there are any.\n\n### Why these four\n\nA GPU pod holds model weights, which are data you may not want on a shared host. A hostPath mount turns that into host access, privileged mode removes the container boundary, and no limits let one pod starve the node. An unpinned image means the code that runs tomorrow is not the code you reviewed.\n\n### How to use it\n\nExport the pods and pipe them in: `kubectl get pods -A -o json | python3 gpu_audit.py -`. Run it against a saved dump in CI, or nightly against the live cluster. A clean run prints OK and exits 0.\n\n> **The honest caveat:** the device plugin is the source of truth for allocation, and a pod that requests a GPU may sit Pending. This audits configuration, not utilisation, so pair it with DCGM metrics for the other half.\n\n> **The lesson:** a GPU is a node-level asset. Treat the pods that hold one like node-level workloads.\n\n## In one line\n\n> **A GPU pod is still a pod.** The same hardening rules, plus an expensive queue.\n\n**Audit now**\n\n```bash\nkubectl get pods -A -o json\n```\n\n## Script: gpu_audit.py\n\nDownload: [scripts/gpu_audit.py](../scripts/gpu_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"gpu_audit.py - audit the Kubernetes pods that request GPUs.\n\nReads `kubectl get pods -A -o json` from a file or stdin. For every pod that\nasks for a GPU it reports the card count, then checks the four things that\nturn a GPU workload into a host-wide risk:\n\n    image pinned        a digest, or a tag that is not :latest\n    cpu and memory      limits on every container\n    privileged          container securityContext.privileged is false\n    hostPath            no volume mounts the node filesystem\n\nRead-only: it only parses JSON and never talks to the cluster. Exit codes:\n0 clean, 1 finding, 2 usage or read error.\n\nUsage:\n    kubectl get pods -A -o json | python3 gpu_audit.py -\n    python3 gpu_audit.py pods.json\n    python3 gpu_audit.py pods.json --namespace ml\n    python3 gpu_audit.py pods.json --all --json\n\nCron (nightly at 03:00):\n    0 3 * * * kubectl get pods -A -o json | python3 gpu_audit.py - >> /var/log/gpu_audit.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nGPU_KEYS = (\"nvidia.com/gpu\", \"amd.com/gpu\")\n\n\ndef gpu_count(container):\n    \"\"\"Cards requested by one container, 0 when it requests none.\"\"\"\n    resources = container.get(\"resources\") or {}\n    best = 0\n    for part in (\"requests\", \"limits\"):\n        for key in GPU_KEYS:\n            value = (resources.get(part) or {}).get(key)\n            try:\n                best = max(best, int(value))\n            except (TypeError, ValueError):\n                pass\n    return best\n\n\ndef pinned(image):\n    \"\"\"True when the image is a digest or a tag that is not latest.\"\"\"\n    if \"@sha256:\" in image:\n        return True\n    tail = image.rsplit(\"/\", 1)[-1]\n    return \":\" in tail and not tail.endswith(\":latest\")\n\n\ndef has_limits(container):\n    limits = (container.get(\"resources\") or {}).get(\"limits\") or {}\n    return bool(limits.get(\"cpu\")) and bool(limits.get(\"memory\"))\n\n\ndef is_privileged(container):\n    context = container.get(\"securityContext\") or {}\n    return context.get(\"privileged\") is True\n\n\ndef host_paths(pod):\n    \"\"\"Names of volumes that mount a path from the node.\"\"\"\n    spec = pod.get(\"spec\") or {}\n    return [volume.get(\"name\", \"?\") for volume in spec.get(\"volumes\") or []\n            if \"hostPath\" in volume]\n\n\ndef check_pod(pod, scan_all=False):\n    \"\"\"Return (namespace, name, cards, findings) or None if skipped.\"\"\"\n    meta = pod.get(\"metadata\") or {}\n    namespace = meta.get(\"namespace\", \"default\")\n    name = meta.get(\"name\", \"?\")\n    spec = pod.get(\"spec\") or {}\n    containers = spec.get(\"containers\") or []\n    cards = sum(gpu_count(container) for container in containers)\n    if cards == 0 and not scan_all:\n        return None\n\n    findings = []\n    for container in containers:\n        cname = container.get(\"name\", \"?\")\n        image = str(container.get(\"image\", \"\"))\n        if not pinned(image):\n            findings.append((namespace, name, cname, \"unpinned image\", image))\n        if not has_limits(container):\n            findings.append((namespace, name, cname, \"no cpu/memory limits\", \"\"))\n        if is_privileged(container):\n            findings.append((namespace, name, cname, \"privileged container\", \"\"))\n    for volume in host_paths(pod):\n        findings.append((namespace, name, \"-\", \"hostPath volume\", volume))\n    return namespace, name, cards, findings\n\n\ndef load_pods(path, errors):\n    \"\"\"Load the items list from one kubectl JSON dump.\"\"\"\n    label = \"<stdin>\" if path == \"-\" else path\n    try:\n        if path == \"-\":\n            doc = json.load(sys.stdin)\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                doc = json.load(handle)\n    except OSError as exc:\n        errors.append(\"%s: %s\" % (label, exc))\n        return []\n    except ValueError as exc:\n        errors.append(\"%s: not JSON: %s\" % (label, exc))\n        return []\n    items = doc.get(\"items\") if isinstance(doc, dict) else None\n    if not isinstance(items, list):\n        errors.append('%s: expected {\"items\": [...]} from kubectl -o json' % label)\n        return []\n    return items\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Audit GPU pods for unpinned images, missing limits, \"\n                    \"privileged containers and hostPath mounts.\")\n    ap.add_argument(\"paths\", nargs=\"*\", default=[\"-\"], metavar=\"PATH\",\n                    help=\"kubectl JSON dump, or - for stdin (default)\")\n    ap.add_argument(\"-n\", \"--namespace\", help=\"only this namespace\")\n    ap.add_argument(\"-a\", \"--all\", action=\"store_true\",\n                    help=\"audit every pod, not only GPU pods\")\n    ap.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                    help=\"findings and summary only\")\n    ap.add_argument(\"--json\", action=\"store_true\",\n                    help=\"print one machine-readable JSON object\")\n    args = ap.parse_args(argv)\n\n    errors = []\n    audited = []\n    for path in args.paths:\n        for pod in load_pods(path, errors):\n            if not isinstance(pod, dict):\n                continue\n            result = check_pod(pod, args.all)\n            if result is None:\n                continue\n            namespace = result[0]\n            if args.namespace and namespace != args.namespace:\n                continue\n            audited.append(result)\n\n    for message in errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if errors:\n        return 2\n\n    findings = [item for _, _, _, items in audited for item in items]\n    cards = sum(item[2] for item in audited)\n    pods_with_findings = sum(1 for _, _, _, items in audited if items)\n\n    if args.json:\n        print(json.dumps({\n            \"pods\": len(audited),\n            \"cards\": cards,\n            \"pods_with_findings\": pods_with_findings,\n            \"findings\": [\n                {\"namespace\": ns, \"pod\": pod, \"container\": container,\n                 \"check\": check, \"detail\": detail}\n                for ns, pod, container, check, detail in findings\n            ],\n        }, sort_keys=True))\n        return 1 if findings else 0\n\n    if not audited:\n        print(\"no GPU pods found\" if not args.all else \"no pods found\")\n        return 0\n\n    if not args.quiet:\n        for namespace, name, pod_cards, _ in audited:\n            print(\"%-44s %4d GPU\" % (namespace + \"/\" + name, pod_cards))\n        for namespace, name, container, check, detail in findings:\n            where = \"%s/%s\" % (namespace, name)\n            if container != \"-\":\n                where += \" \" + container\n            extra = \" (%s)\" % detail if detail else \"\"\n            print(\"[FINDING] %s: %s%s\" % (where, check, extra))\n        print()\n\n    print(\"%d pod(s) audited, %d card(s) requested, %d finding(s)\"\n          % (len(audited), cards, len(findings)))\n    if findings:\n        print(\"ALERT %d of %d pod(s) need review\"\n              % (pods_with_findings, len(audited)))\n        return 1\n    print(\"OK clean\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p270","n":"270","slug":"llm-security-terms-decoded","title":"LLM security terms, decoded","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLMSecurity","AISecurity","GenAI","CyberSecurity","CloudNative"],"series_name":"Platform","design":"reveal","script":"","date":"2028-06-18","files":{"devsecops":"notes/p270-llm-security-terms-decoded.md"},"md":"# LLM security terms, decoded\n\nA model that only talks is a small risk. The moment it reads your documents and calls your tools, it is a system, and systems fail in named ways. Six terms cover most of it.\n\n## Quick reference\n\n- **prompt injection** — Text in the input tells the model to ignore its instructions. Indirect injection arrives through a document, a page or a tool result.\n- **jailbreak** — A prompt that gets around the model's safety training: role-play, encoding, or a long story that walks it past a refusal.\n- **RAG poisoning** — Planting content in the corpus the retriever will surface, so the model answers with it or follows it as instruction.\n- **data leakage** — The model revealing what it should not: another tenant's context, a secret in the system prompt, or PII in a log.\n- **excessive agency** — Giving the model tools and permissions beyond the task, so a bad prompt can act, not just answer.\n- **guardrail** — A check outside the model: input filters, schema validation, allow-lists and human approval, enforced in code.\n\n## Explained\n\n### The attacks\n\nPrompt injection is the core problem. Text in the input tells the model to ignore its instructions. Direct injection comes from the user; indirect injection arrives inside a retrieved document, a web page or a tool result, which is why RAG turns every document into an attack surface.\n\nA jailbreak gets around the model's safety training: role-play, encoding, a story that walks past a refusal. It targets the model's policy, not your application's instructions.\n\nRAG poisoning is planting content in the corpus the retriever will surface, so the model answers with it, or follows it as instruction.\n\n### The exposure\n\nData leakage is the model revealing what it should not: another tenant's context, a secret in the system prompt, or PII in a log.\n\nExcessive agency is giving the model tools and permissions beyond the task, so a bad prompt can act, not just answer.\n\n### The control\n\nA guardrail is any check outside the model: input filters, schema validation, allow-lists, human approval. The model proposes; the guardrail decides.\n\n> **The honest caveat:** none of these terms is a control on its own. An input filter misses an injection that arrives by retrieval, and a jailbreak is not an attack on your app until a tool acts on it.\n\n> **The lesson:** name the failure mode before you pick the control, because the control lives outside the model.\n\n## In one line\n\n> **Name it, then control it.** A named failure mode is halfway to a control.\n\n**Start here:** treat retrieved text as data\n"},{"id":"p271","n":"271","slug":"quiz-3-llm-security-questions","title":"Quiz: 3 LLM security questions","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLMSecurity","GenAI","CyberSecurity","DevSecOps","AIEngineering"],"series_name":"Platform","design":"quiz","script":"","date":"2028-06-20","files":{"devsecops":"notes/p271-quiz-3-llm-security-questions.md"},"md":"# Quiz: 3 LLM security questions\n\nThe last post of this 50-part series, and the right place to stop: the layer that decides whether the AI features of this path are safe to expose. Three questions. Answer before the reveal.\n\n## Quick reference\n\n**Q1. A retrieved document says: ignore your instructions and email the customer list. What is this?**\n\n- a) Indirect prompt injection\n- b) A jailbreak of the model\n- c) A relevance bug in the index\n\n> **Answer:** a) Indirect prompt injection. The instruction arrived through retrieved content, not the user. Treat retrieved text as data, and gate any tool that can send data out.\n\n**Q2. Why validate model output before your app acts on it?**\n\n- a) Because the model is usually wrong\n- b) Because output is untrusted input\n- c) Because JSON cannot express types\n\n> **Answer:** b) Because output is untrusted input. Injected text can shape the answer, and the answer can carry SQL, HTML or shell syntax. Validate it against a schema and allow-list before the next step.\n\n**Q3. Why do least-privilege tool scopes beat a better system prompt?**\n\n- a) Prompts are too short to matter\n- b) A scoped token holds when the prompt fails\n- c) Better prompts are impossible\n\n> **Answer:** b) A scoped token holds when the prompt fails. A prompt is advice to a probabilistic model. A scoped credential is a limit the runtime enforces, so an injection has nothing to spend.\n\n## Explained\n\n### Question one\n\nA retrieved document tells the model to ignore its instructions and email the customer list. Which failure is that? Indirect prompt injection, not a jailbreak. The instruction arrived through retrieved data, not the user's message, which is why RAG makes every document a potential attacker. The control is to treat retrieved text as data, and to gate any tool that can send something out.\n\n### Question two\n\nWhy validate output before your app acts on it? Because the output is untrusted input to the next step. An answer can contain SQL, HTML or shell syntax, and injected text can shape it. Validate against a schema and an allow-list before it reaches a database, a browser or a shell.\n\n### Question three\n\nWhy do least-privilege tool scopes beat a better prompt? Because a prompt is advice to a probabilistic model, while a scoped credential is a limit the runtime enforces. Tighten the tool and an injection has nothing to spend; rewrite the prompt and you are negotiating.\n\n> **The honest caveat:** passing this quiz does not make an app safe; the controls do.\n\nIf you got these, you are treating the model as untrusted. That is the discipline.\n\nThat is the series. Fifty posts, from a commit reaching the cluster to a prompt reaching a model, plus ten Python scripts you can run today. Thank you for following along.\n\n## In one line\n\n> **The last mile is the model.** Fifty posts, one idea: trust the system, not the output.\n\n**Keep going:** add one output check this week\n"},{"id":"p272","n":"272","slug":"the-3-tier-architecture-on-aws-drawn","title":"The 3-tier architecture on AWS, drawn","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudArchitecture","DevOps","CloudNative","SystemDesign"],"series_name":"Visual","design":"architecture","script":"","date":"2028-06-22","files":{"devsecops":"notes/p272-the-3-tier-architecture-on-aws-drawn.md"},"md":"# The 3-tier architecture on AWS, drawn\n\nMost AWS bills and outages start with a diagram nobody drew. The classic web app still has three tiers, and every request walks the same path through them.\n\n## Quick reference\n\n- **Tier 1 · Edge:** Route 53 (DNS) · CloudFront (CDN) · AWS WAF (WAF)\n- **Tier 2 · Application:** Load balancer (ALB) · EC2 auto scaling (APP) · IAM role (IAM)\n- **Tier 3 · Data:** Aurora (DB) · ElastiCache (CACHE) · S3 objects (OBJ)\n\n**Request flow:** Route 53 → CloudFront → AWS WAF → Load balancer → EC2 auto scaling → Aurora; EC2 auto scaling → ElastiCache; EC2 auto scaling → S3 objects\n\n**Around it:** CloudWatch (LOG) · CloudTrail (AUDIT) · KMS (KEYS) · Backup (BCK)\n\n## Explained\n\nEdge. Route 53 answers DNS, CloudFront caches at the edge, and AWS WAF filters the request before it reaches your account. The edge tier exists to absorb traffic you never want to see again.\n\nApplication. An Application Load Balancer spreads connections across EC2 instances in an auto scaling group. The group adds capacity when CPU climbs and removes it when traffic drops. An IAM role gives the instances permission to call other services without a stored key.\n\nData. Aurora holds the transactional data, ElastiCache takes the hot reads, and S3 stores media and documents. Each tier fails differently: edge failures are DNS or certificate problems, application failures are capacity or code, data failures are locks and disk.\n\nAround the three tiers sit the services that keep them honest: CloudWatch for metrics and alarms, CloudTrail for the audit trail, KMS for encryption keys, Backup for recovery, and a load balancer that only sends traffic to instances that pass their health check.\n\n> **The lesson:** the value of the diagram is the arrows. A box tells you what you bought; an arrow tells you what breaks when it dies.\n\n## In one line\n\n> **Draw it before you build it.** A diagram is the cheapest design review you will ever run.\n\n**Trace one request**\n\n```bash\ncurl -sv https://example.com\n```\n"},{"id":"p273","n":"273","slug":"raid-0-1-5-and-10-drawn","title":"RAID 0, 1, 5 and 10, drawn","sections":["it"],"topic":"Storage & infrastructure","tags":["Linux","Storage","RAID","SysAdmin","DataProtection"],"series_name":"Visual","design":"explainer","script":"","date":"2028-06-25","files":{"it":"notes/p273-raid-0-1-5-and-10-drawn.md"},"md":"# RAID 0, 1, 5 and 10, drawn\n\nRAID combines disks so that one failure is survivable, or so that several disks behave like one fast one. The level you choose is a trade between speed, capacity and how many drives you can lose.\n\n## Quick reference\n\n### RAID 0 - Striping\n\n- Data split across 2+ drives\n- Full capacity, best speed\n- No redundancy: one loss, all lost\n- Good for scratch, caches\n\n### RAID 1 - Mirroring\n\n- Every write copied to both\n- Survives one drive failure\n- Half the capacity\n- Good for boot and small data\n\n### RAID 5 - Striping + parity\n\n- Minimum 3 drives, parity spread\n- Tolerates 1 drive failure\n- Capacity of n-1 drives\n- Good for file servers\n\n### RAID 10 - Mirrored stripe\n\n- Minimum 4 drives, pairs then stripes\n- Tolerates one loss per pair\n- Half the capacity, fast rebuild\n- Good for databases\n\n> **Note:** RAID is not a backup: keep an external or cloud copy, and test the restore.\n\n## Explained\n\nRAID 0 stripes data across drives. You get the full capacity and the best speed, and you get no protection at all: lose one drive and the set is gone. It belongs on scratch space, never on data you cannot rebuild.\n\nRAID 1 mirrors every write to two drives. One drive can fail and the server keeps running, at the cost of half your capacity. It is the standard choice for boot disks and small datasets.\n\nRAID 5 stripes data with parity spread across the set. It needs at least three drives, survives one failure, and gives you the capacity of all but one drive. Rebuilds are slow and put the remaining drives under load, which is why large drives make RAID 5 uncomfortable.\n\nRAID 10 mirrors pairs and stripes across them. It needs at least four drives, survives one failure per pair, and rebuilds fast. For databases, the speed and rebuild time are usually worth the 50 percent capacity cost.\n\n> **The rule that outranks all four:** RAID is not a backup. It protects against a dead disk, not against a deleted table, ransomware or a bad deploy.\n\n## In one line\n\n> **Redundancy is not backup.** A mirrored disk survives a disk, not a delete.\n\n**Check the array**\n\n```bash\ncat /proc/mdstat\n```\n"},{"id":"p274","n":"274","slug":"the-linux-command-cheat-sheet","title":"The Linux command cheat sheet","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","CommandLine","SysAdmin","DevOps","CheatSheet"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-06-27","files":{"it":"notes/p274-the-linux-command-cheat-sheet.md","devsecops":"notes/p274-the-linux-command-cheat-sheet.md"},"md":"# The Linux command cheat sheet\n\nMost days on a Linux server need a small, boring set of commands. Not fifty flags you found on a blog, but five per area that you can type without thinking.\n\n## Quick reference\n\n### Files\n\n```bash\nls -lah\ncd /var/log\ncp -a src/ dst/\nmv old.txt new.txt\nfind / -name '*.log' 2>/dev/null\n```\n\n### Processes\n\n```bash\nps aux | grep nginx\ntop -o %CPU\nkill -TERM <pid>\nsystemctl status nginx\njournalctl -u nginx -f\n```\n\n### Disk\n\n```bash\ndf -h\ndu -sh /var/log/*\nlsblk -f\nfree -h\nmount | column -t\n```\n\n### Network\n\n```bash\nss -tlnp\nip -br addr\ncurl -I https://example.com\ndig example.com +short\ntcpdump -i any port 443\n```\n\n## Explained\n\nFiles. `ls -lah` gives you the listing you actually want: hidden files, human sizes, permissions. `cp -a` preserves the things you forget: timestamps, symlinks, ownership. find with -name is still the fastest way to answer \"where is that log\".\n\nProcesses. ps aux | grep is crude and works. `top -o %CPU` answers \"what is eating the box\" without a dashboard. systemctl status and `journalctl -u` are the pair you use on any systemd host, and `kill -TERM` asks politely before -9 ends the argument.\n\nDisk. `df -h` shows the filesystem, `du -sh` shows the folder, and the gap between them is usually a deleted-but-open file. `lsblk -f` shows what is actually mounted where. `free -h` is the first thing to check when a process disappears at night.\n\nNetwork. `ss -tlnp` shows what is listening, `ip -br addr` is the short address view, `curl -I` proves the service answers, dig answers DNS, and tcpdump is the last resort that settles arguments.\n\n> **The lesson:** a cheat sheet is not a reference. It is the set of commands you can recall under pressure.\n\n## In one line\n\n> **Know the command, skip the search.** Five per area beats fifty you cannot recall.\n\n**A first look**\n\n```bash\nuptime; df -h; free -h\n```\n"},{"id":"p275","n":"275","slug":"a-vpc-subnet-planner-in-python","title":"A VPC subnet planner in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","AWS","Networking","DevOps","CloudNative"],"series_name":"Visual","design":"code","script":"subnet_plan.py","date":"2028-06-29","files":{"devsecops":"notes/p275-a-vpc-subnet-planner-in-python.md"},"md":"# A VPC subnet planner in Python\n\nThe VPC is built, the peering is up, and then someone notices the new subnet sits inside a range that already exists. Nothing errors; traffic just takes the wrong path.\n\n## Quick reference\n\nThe full script is at the end of this note: `subnet_plan.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 subnet_plan.py 10.0.0.0/20 --check 10.0.8.0/23\n```\n\n**Example output**\n\n```text\nOVERLAP 10.0.8.0/24 10.0.8.0/23\nOVERLAP 10.0.9.0/24 10.0.8.0/23\n```\n\n## Explained\n\n### What the script does\n\nIt takes a base CIDR and an optional target prefix, then uses the standard library ipaddress module to list every subnet. Each row prints the network and its usable count. Any CIDR passed with `--check` is compared against every row with overlaps(), and each collision is printed as an OVERLAP line.\n\n### The arithmetic\n\nEvery prefix bit doubles the number of subnets and halves their size. A /16 holds 65,536 addresses; a /24 holds 256, of which 254 are usable. Divide and you get 256 /24s in a /16, 16 in a /20, and four /26s in a /24.\n\n### How to run it\n\n`python3 subnet_plan.py 10.0.0.0/20 --check 10.0.8.0/23` prints the plan and flags the two /24s inside the checked range. Exit 0 means no collision, 1 means at least one overlap, and 2 means a bad CIDR or a target prefix shorter than the base.\n\n> **The honest caveat:** this is IPv4 only, and it knows nothing about route tables, security groups or the five addresses AWS reserves in every subnet. In AWS a /24 hands out 251, not 254.\n\n> **The lesson:** address space is the cheapest thing to plan and the most expensive thing to fix.\n\n## In one line\n\n> **Plan the range before the build.** Address space is the cheapest thing to plan and the most expensive to fix.\n\n**See the plan**\n\n```bash\npython3 subnet_plan.py 10.0.0.0/20\n```\n\n## Script: subnet_plan.py\n\nDownload: [scripts/subnet_plan.py](../scripts/subnet_plan.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"subnet_plan.py - plan an IPv4 range and catch overlapping subnets.\n\nSplits a base CIDR into a target prefix (a /24 by default), prints one row\nper subnet with its network, first and last host, broadcast and usable\ncount, then checks every --check CIDR against the plan with\nipaddress.overlaps(). IPv4 only. Read-only: it opens no sockets and writes\nno files.\n\nExit codes: 0 clean, 1 overlap found, 2 usage or input error.\n\nUsage:\n    python3 subnet_plan.py 10.0.0.0/16\n    python3 subnet_plan.py 10.0.0.0/16 --prefix 20\n    python3 subnet_plan.py 10.0.0.0/20 --check 10.0.8.0/23\n    python3 subnet_plan.py 10.0.0.0/20 --check 10.0.8.0/23 -q\n    python3 subnet_plan.py 10.0.0.0/16 --prefix 24 --json\n\nCron or CI, fail the job when a planned range collides:\n    python3 subnet_plan.py 10.0.0.0/16 --check 10.0.8.0/23 -q || exit 1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport json\nimport sys\n\n# A /12 split into /24s is 4,096 rows; refuse anything larger than that.\nMAX_SUBNETS = 4096\n\n\nclass PlanError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef network_of(text, label):\n    \"\"\"Parse an IPv4 network, clearing host bits, or raise PlanError.\"\"\"\n    try:\n        net = ipaddress.ip_network(text, strict=False)\n    except ValueError as exc:\n        raise PlanError(\"%s is not a CIDR: %s\" % (label, exc))\n    if net.version != 4:\n        raise PlanError(\"%s is IPv6; this planner is IPv4 only\" % label)\n    return net\n\n\ndef usable(net):\n    \"\"\"Host addresses: /31 is a point-to-point pair (RFC 3021), /32 is one.\"\"\"\n    if net.prefixlen >= 31:\n        return net.num_addresses\n    return net.num_addresses - 2\n\n\ndef bounds(net):\n    \"\"\"Return (first host, last host, broadcast) as strings.\"\"\"\n    if net.prefixlen >= 31:\n        first = net.network_address\n        last = net.network_address + net.num_addresses - 1\n        return str(first), str(last), \"n/a\"\n    return (str(net.network_address + 1),\n            str(net.broadcast_address - 1),\n            str(net.broadcast_address))\n\n\ndef build_nets(base, prefix):\n    \"\"\"List the target subnets, refusing a split that would flood the output.\"\"\"\n    count = 2 ** (prefix - base.prefixlen)\n    if count > MAX_SUBNETS:\n        raise PlanError(\"that split makes %s subnets; the limit is %s\"\n                        % (format(count, \",\"), format(MAX_SUBNETS, \",\")))\n    return list(base.subnets(new_prefix=prefix))\n\n\ndef find_overlaps(nets, checks):\n    \"\"\"Every (planned subnet, checked range) pair that overlaps.\"\"\"\n    return [(net, check) for net in nets for check in checks if net.overlaps(check)]\n\n\ndef print_table(base, prefix, nets):\n    \"\"\"The human-readable plan.\"\"\"\n    print(\"PLAN %s as /%d: %d subnets, %d addresses\"\n          % (base, prefix, len(nets), base.num_addresses))\n    print(\"%-5s %-18s %-15s %-15s %-15s %6s\"\n          % (\"#\", \"network\", \"first\", \"last\", \"broadcast\", \"usable\"))\n    for i, net in enumerate(nets, 1):\n        first, last, bcast = bounds(net)\n        print(\"%-5d %-18s %-15s %-15s %-15s %6d\"\n              % (i, str(net), first, last, bcast, usable(net)))\n\n\ndef emit(args, base, nets, checks, overlaps):\n    \"\"\"Print the table or the JSON summary, then the findings.\"\"\"\n    if args.json:\n        report = {\n            \"base\": str(base),\n            \"prefix\": args.prefix,\n            \"count\": len(nets),\n            \"addresses\": base.num_addresses,\n            \"checks\": [str(check) for check in checks],\n            \"overlaps\": [{\"network\": str(net), \"check\": str(check)}\n                         for net, check in overlaps],\n            \"ok\": not overlaps,\n        }\n        print(json.dumps(report, indent=None if args.quiet else 2, sort_keys=True))\n        return\n\n    if not args.quiet:\n        print_table(base, args.prefix, nets)\n    for net, check in overlaps:\n        print(\"OVERLAP %s %s\" % (net, check))\n    if overlaps:\n        hit = len({str(net) for net, _ in overlaps})\n        print(\"ALERT %d of %d subnets overlap %d checked range(s)\"\n              % (hit, len(nets), len(checks)))\n    else:\n        extra = \"\" if not checks else \" with %d checked range(s)\" % len(checks)\n        print(\"OK %d subnets, no overlap%s\" % (len(nets), extra))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Plan an IPv4 subnet split and flag ranges that overlap it.\",\n        epilog=\"Exit codes: 0 clean, 1 overlap, 2 usage or input error.\")\n    parser.add_argument(\"base\", help=\"base CIDR, for example 10.0.0.0/16\")\n    parser.add_argument(\"--prefix\", type=int, default=24, metavar=\"N\",\n                        help=\"target prefix length, default 24 (max 4096 rows)\")\n    parser.add_argument(\"--check\", action=\"append\", default=[], metavar=\"CIDR\",\n                        help=\"range to test against the plan, repeatable\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print findings and the summary only\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON summary instead of the table\")\n    args = parser.parse_args(argv)\n\n    try:\n        base = network_of(args.base, \"base\")\n        if not 0 <= args.prefix <= 32:\n            raise PlanError(\"--prefix must be between 0 and 32\")\n        if args.prefix < base.prefixlen:\n            raise PlanError(\"--prefix /%d is shorter than the base /%d\"\n                            % (args.prefix, base.prefixlen))\n        checks = [network_of(text, \"--check\") for text in args.check]\n        nets = build_nets(base, args.prefix)\n        overlaps = find_overlaps(nets, checks)\n    except PlanError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    emit(args, base, nets, checks, overlaps)\n    return 1 if overlaps else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p276","n":"276","slug":"the-osi-model-grouped","title":"The OSI model, grouped","sections":["it"],"topic":"Networking & DNS","tags":["Networking","OSI","CCNA","ITInfrastructure","DevOps"],"series_name":"Visual","design":"explainer","script":"","date":"2028-07-02","files":{"it":"notes/p276-the-osi-model-grouped.md"},"md":"# The OSI model, grouped\n\nA request fails and three people blame three layers: the cable, the firewall, the application. The OSI model is not academic; it is a way to split that argument into seven testable questions.\n\n## Quick reference\n\n### Layers 1-2 - Link\n\n- Cable, Wi-Fi, MAC addresses, frames\n- Switches forward by MAC, not by IP\n- Ethernet, ARP, VLANs and PPP\n- Everything above assumes it works\n\n### Layer 3 - Network\n\n- IP addresses, packets, routing\n- A router chooses the next hop\n- ICMP, ping and traceroute\n- NAT and IPsec sit here too\n\n### Layer 4 - Transport\n\n- TCP and UDP, ports and segments\n- TCP numbers bytes, re-sends loss\n- UDP sends datagrams, keeps no state\n- Firewalls filter ports at layer 4\n\n### Layers 5-7 - Session and up\n\n- Session: starts and ends the conversation\n- Presentation: TLS, compression, encoding\n- Application: HTTP, DNS, SMTP, SSH\n- Most tickets land in this group\n\n> **Note:** TCP/IP uses four layers: link, internet, transport, application.\n\n## Explained\n\n### Link and network\n\nThe first two layers are the link: cable, Wi-Fi, MAC addresses and frames. A switch forwards by MAC, not by IP. Ethernet, ARP, VLANs and PPP live here, and everything above assumes this works. Layer 3 is the network: IP addresses, packets and routing. A router chooses the next hop, ICMP and traceroute report the path, and NAT and IPsec rewrite or protect packets here.\n\n### Transport and above\n\nLayer 4 is transport: TCP and UDP, ports and segments. TCP numbers bytes and re-sends what is lost; UDP sends datagrams and keeps no state. Firewalls filter ports here. Layers 5 to 7 are the upper group: session starts and ends the conversation, presentation handles TLS, compression and encoding, application is HTTP, DNS, SMTP and SSH. Most tickets land in this group.\n\n### How to use it\n\nStart low. If ping fails, the problem is layer 3 or below. If ping works and the port refuses, it is layer 4. If the port answers and the page is wrong, it is layer 7. The panel order in the drawing is the order you test.\n\n> **The honest caveat:** this is a teaching model, not an implementation; TLS is drawn at layer 6 by some people and between layers 4 and 7 by others.\n\n> **The lesson:** name the layer before you name the culprit.\n\n## In one line\n\n> **Know the layer, know the suspect.** The layer that failed narrows the tool you reach for.\n\n**Name the layer**\n\n```bash\nping, dig, curl -v, ss -tlnp\n```\n"},{"id":"p277","n":"277","slug":"the-docker-command-cheat-sheet","title":"The Docker command cheat sheet","sections":["devsecops"],"topic":"Docker & containers","tags":["Docker","DevOps","Containers","CheatSheet","CloudNative"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-07-04","files":{"devsecops":"notes/p277-the-docker-command-cheat-sheet.md"},"md":"# The Docker command cheat sheet\n\nYou need to rebuild one container, read its logs and then delete a week of images. The commands are all in your history, somewhere between a kubectl line and a typo.\n\n## Quick reference\n\n### Images\n\n```bash\ndocker images\ndocker pull nginx:1.27\ndocker build -t api:1.0 .\ndocker history api:1.0\ndocker inspect nginx:1.27\n```\n\n### Containers\n\n```bash\ndocker ps -a\ndocker run -d -p 8080:80 nginx\ndocker logs -f --tail 100 api\ndocker exec -it api sh\ndocker stats --no-stream\n```\n\n### Compose\n\n```bash\ndocker compose up -d\ndocker compose ps\ndocker compose logs -f api\ndocker compose config\ndocker compose down -v\n```\n\n### Cleanup\n\n```bash\ndocker system df\ndocker container prune\ndocker image prune -a\ndocker volume prune -a\ndocker builder prune\n```\n\n## Explained\n\n### Images\n\nStart with images. `docker images` lists what is local, `docker pull nginx:1.27` fetches a pinned tag, and `docker build -t api:1.0 .` builds from the Dockerfile in the current directory. `docker history api:1.0` shows the layers, and `docker inspect nginx:1.27` prints the JSON behind the tag.\n\n### Containers and Compose\n\n`docker ps -a` shows running and stopped containers, and `docker run -d -p 8080:80 nginx` starts one in the background with a port mapped. `docker logs -f --tail 100 api` follows the last hundred lines, `docker exec -it api sh` opens a shell, and `docker stats --no-stream` gives one snapshot of CPU and memory. Compose uses the same verbs with a project: `docker compose up -d`, ps, logs -f api, config and down -v.\n\n### Cleanup\n\nCleanup is where the disk comes back: `docker system df` shows what is using space, and container prune, image prune -a, volume prune -a and builder prune each target one kind of leftover.\n\n> **The honest caveat:** prune commands delete data. `docker volume prune -a` removes named volumes that no container is using, and the database inside one will not ask twice.\n\n> **The lesson:** five commands per area, typed from memory, beat a search engine and a guess.\n\n## In one line\n\n> **Five per area beats fifty.** You need the five you type weekly, not every flag in the manual.\n\n**Check the disk**\n\n```bash\ndocker system df\n```\n"},{"id":"p278","n":"278","slug":"inside-a-kubernetes-cluster","title":"Inside a Kubernetes cluster","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Architecture","DevOps","CloudNative","SRE"],"series_name":"Visual","design":"architecture","script":"","date":"2028-07-06","files":{"devsecops":"notes/p278-inside-a-kubernetes-cluster.md"},"md":"# Inside a Kubernetes cluster\n\nkubectl apply returns and the pod sits in Pending. The API server accepted the object, so the control plane looks fine; the work of turning that object into a running container is somewhere else.\n\n## Quick reference\n\n- **Control plane:** scheduler (SCHED) · API server (API) · etcd (KV) · controller manager (CTRL)\n- **Workers:** kubelet (NODE) · container runtime (CRI) · kube-proxy (PROXY)\n- **Add-ons:** CoreDNS (DNS) · CNI plugin (CNI)\n\n**Request flow:** scheduler → API server → etcd; API server → kubelet → container runtime; kubelet → CoreDNS; kubelet → CNI plugin; API server → kube-proxy\n\n## Explained\n\n### Control plane\n\nThe control plane is four components. The API server is the front door and the only component that talks to etcd, the key-value store that holds cluster state. The scheduler watches for pods with no node and writes the binding back through the API server. The controller manager runs the controllers: Deployment, ReplicaSet, Node, each comparing desired state with actual.\n\n### Workers\n\nEach worker runs three things. The kubelet is the agent: it watches the API server for pods bound to its node and asks the container runtime to start them through the CRI. The runtime, usually containerd, pulls the image and runs the container. kube-proxy programs the iptables or IPVS rules that make Service virtual IPs work.\n\n### Add-ons\n\nBelow the workers, the add-ons give the cluster names and addresses. CoreDNS runs on the workers and resolves service names inside the cluster. The CNI plugin assigns pod IPs and wires the pod network, and some plugins enforce network policy.\n\n> **The honest caveat:** this is a logical map. A managed control plane hides those four boxes, CoreDNS runs on the workers rather than in its own tier, and the CNI is a set of binaries and daemons, not one service.\n\n> **The lesson:** every cluster change is a write to the API server; every stuck object is a reconcile loop that cannot finish.\n\n## In one line\n\n> **Every pod starts at the API server.** Cluster state is a set of API writes, and etcd keeps them.\n\n**Check the api**\n\n```bash\nkubectl get --raw /readyz\n```\n"},{"id":"p279","n":"279","slug":"tcp-vs-udp","title":"TCP vs UDP","sections":["it"],"topic":"Networking & DNS","tags":["Networking","TCP","UDP","DevOps","SRE"],"series_name":"Visual","design":"compare","script":"","date":"2028-07-09","files":{"it":"notes/p279-tcp-vs-udp.md"},"md":"# TCP vs UDP\n\nA video call freezes for half a second and the SSH session stays perfect. Both crossed the same network; the difference is what each transport promised before the first packet.\n\n## Quick reference\n\n| | **TCP** | **UDP** |\n|:--|:--|:--|\n| Handshake | 3-way SYN, SYN-ACK, ACK | None |\n| Delivery | Acknowledged, re-sent | Best effort, dropped |\n| Order | Sequence numbers | Whatever arrives |\n| Header | 20 bytes minimum | 8 bytes fixed |\n| Speed | Slower: round trips | Faster: no setup |\n| Best for | Web, SSH, file transfer | DNS, video, games, VoIP |\n\n**TCP** (Reliable, ordered byte stream): Handshake, order, sequence numbers, retransmission.\n\n```bash\nss -tan state established\n```\n\n**UDP** (Connectionless datagrams): No handshake, no order, no retransmission.\n\n```bash\ndig +notcp example.com\n```\n\n- TCP → reliable and ordered, at the cost of round trips.\n- UDP → no promises, no waiting, lower latency.\n\n## Explained\n\n### Handshake and delivery\n\nTCP opens with a three-way handshake: SYN, SYN-ACK, ACK. That is one round trip before data, which is why a new TCP connection feels slower. UDP has no handshake and no connection. TCP then acknowledges what arrives and re-sends what does not; UDP sends each datagram and forgets it.\n\n### Order, header, speed\n\nTCP numbers the bytes, so the application reads an ordered stream even when packets arrive out of order. UDP preserves no order at all. A TCP header is at least 20 bytes before options; a UDP header is 8. TCP pays for its guarantees in round trips and state, so it is slower to start; UDP has no setup and no state, so it starts immediately.\n\n### Where each fits\n\nTCP is the default for web, SSH and file transfer, where every byte matters. UDP is the default for DNS, video, games and VoIP, where a late packet is worse than a missing one.\n\n> **The honest caveat:** the boundary is a default, not a law. DNS falls back to TCP when a reply is truncated, and QUIC puts reliability and ordering back on top of UDP.\n\n> **The lesson:** choose the promise you need, then pay for it.\n\nTCP waits for proof. UDP sends and moves on.\n"},{"id":"p280","n":"280","slug":"an-http-redirect-chain-checker-in-python","title":"An HTTP redirect chain checker in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","HTTP","WebSecurity","DevOps","Networking"],"series_name":"Visual","design":"code","script":"redirect_chain.py","date":"2028-07-11","files":{"it":"notes/p280-an-http-redirect-chain-checker-in-python.md","devsecops":"notes/p280-an-http-redirect-chain-checker-in-python.md"},"md":"# An HTTP redirect chain checker in Python\n\nYou move a site to a new domain, add the www, force HTTPS, and the old URL now takes four hops to arrive. Every hop works, and every hop is a round trip you pay for on every visit.\n\n## Quick reference\n\nThe full script is at the end of this note: `redirect_chain.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 redirect_chain.py http://github.com\n```\n\n**Example output**\n\n```text\nhttp://github.com -> https://github.com/\nhsts max-age=31536000; includeSubdomains; preload\n```\n\n## Explained\n\n### What it checks\n\nThe script opens the URL with urllib, installs a redirect handler that records every redirect and its target, and prints the chain in order. It then looks at the final URL, the final status and the Strict-Transport-Security header. A chain longer than `--max-hops`, three by default, is a finding. So is an http start that never reaches https.\n\n### HSTS and the end of the chain\n\nHSTS is reported, not enforced: urllib does not remember it, and a browser ignores the header when it arrives over plain HTTP. Add `--require-hsts` if a missing header should fail the run. A final 4xx or 5xx is also a finding, because a redirect that lands on a 404 is just a slower error.\n\n### How to run it\n\n`python3 redirect_chain.py http://github.com` prints one hop and the HSTS max-age. Exit 0 is clean, 1 is a finding, and 2 is a bad URL or a connection that never opened. It sends a GET, follows at most ten redirects, and writes nothing.\n\n> **The honest caveat:** a GET is still a request to production, and urllib stops after ten redirects, so a very long chain is reported rather than counted.\n\n> **The lesson:** every redirect is a tax you charge your own visitors.\n\n## In one line\n\n> **Every hop costs a round trip.** A chain of five redirects is five chances to lose the visitor.\n\n**Count the hops**\n\n```bash\ncurl -sIL http://github.com\n```\n\n## Script: redirect_chain.py\n\nDownload: [scripts/redirect_chain.py](../scripts/redirect_chain.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"redirect_chain.py - follow an HTTP redirect chain and audit the hops.\n\nOpens a URL with urllib, records every redirect and its target, prints the\nchain, then checks the hops, the http to https upgrade and the HSTS header.\nA final 4xx or 5xx is also a finding, because a redirect that lands on an\nerror page is a slower error.\n\nExit codes: 0 clean, 1 finding, 2 usage or input error.\n\nUsage:\n    python3 redirect_chain.py http://github.com\n    python3 redirect_chain.py http://example.com --max-hops 2\n    python3 redirect_chain.py https://example.com --require-hsts\n    python3 redirect_chain.py http://example.com --json\n\nCron or CI, fail when the chain is too long or never upgrades:\n    python3 redirect_chain.py http://shop.example.com --max-hops 2 || exit 1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport http.client\nimport json\nimport sys\nimport urllib.error\nimport urllib.parse\nimport urllib.request\n\nREDIRECT_CODES = (301, 302, 303, 307, 308)\nUSER_AGENT = \"s0sta-redirect-check/1.0\"\nHSTS = \"Strict-Transport-Security\"\n\n\nclass FetchError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\nclass Recorder(urllib.request.HTTPRedirectHandler):\n    \"\"\"An opener handler that records each redirect before following it.\"\"\"\n\n    def __init__(self):\n        super().__init__()\n        self.hops = []\n\n    def redirect_request(self, req, fp, code, msg, headers, newurl):\n        self.hops.append({\"code\": code, \"from\": req.full_url, \"to\": newurl})\n        return super().redirect_request(req, fp, code, msg, headers, newurl)\n\n\ndef valid_url(text):\n    \"\"\"Only http and https, and only with a host.\"\"\"\n    parts = urllib.parse.urlsplit(text)\n    return parts.scheme in (\"http\", \"https\") and bool(parts.netloc)\n\n\ndef fetch(start, timeout):\n    \"\"\"Follow the chain and return hops, the final URL, status and headers.\"\"\"\n    rec = Recorder()\n    rec.max_redirections = 10\n    opener = urllib.request.build_opener(rec)\n    request = urllib.request.Request(start, headers={\"User-Agent\": USER_AGENT})\n    try:\n        resp = opener.open(request, timeout=timeout)\n        return {\"hops\": rec.hops, \"final\": resp.geturl(),\n                \"status\": resp.getcode(), \"headers\": resp.headers, \"stuck\": False}\n    except urllib.error.HTTPError as exc:\n        if exc.code in REDIRECT_CODES:\n            final = rec.hops[-1][\"to\"] if rec.hops else start\n            return {\"hops\": rec.hops, \"final\": final, \"status\": exc.code,\n                    \"headers\": exc.headers, \"stuck\": True}\n        return {\"hops\": rec.hops, \"final\": exc.geturl(), \"status\": exc.code,\n                \"headers\": exc.headers, \"stuck\": False}\n    except http.client.HTTPException as exc:\n        raise FetchError(\"malformed HTTP response: %s\" % exc)\n    except OSError as exc:\n        raise FetchError(str(exc))\n\n\ndef hsts_value(result):\n    \"\"\"HSTS only counts when the final response arrived over HTTPS.\"\"\"\n    final = result[\"final\"] or \"\"\n    if urllib.parse.urlsplit(final).scheme != \"https\":\n        return None\n    headers = result[\"headers\"]\n    value = headers.get(HSTS) if headers else None\n    return value.strip() if value else None\n\n\ndef audit(start, result, max_hops, require_hsts):\n    \"\"\"Return (findings, warnings, hsts) for one fetched chain.\"\"\"\n    findings, warnings = [], []\n    hops = len(result[\"hops\"])\n    if result[\"stuck\"]:\n        findings.append(\n            \"redirect not followed: loop, missing Location, or over 10 hops\")\n    if hops > max_hops:\n        findings.append(\"%d hops exceeds --max-hops %d\" % (hops, max_hops))\n\n    start_scheme = urllib.parse.urlsplit(start).scheme\n    final = result[\"final\"] or start\n    final_scheme = urllib.parse.urlsplit(final).scheme\n    if start_scheme == \"http\" and final_scheme != \"https\":\n        findings.append(\"no http -> https upgrade\")\n    if start_scheme == \"https\" and final_scheme == \"http\":\n        findings.append(\"https downgraded to http\")\n    if final_scheme not in (\"http\", \"https\"):\n        findings.append(\"redirect leaves HTTP: %s\" % final)\n    status = result[\"status\"] or 0\n    if status >= 400:\n        findings.append(\"final status %d\" % status)\n    elif 300 <= status < 400 and not result[\"stuck\"]:\n        findings.append(\"final status %d is an unfollowed redirect\" % status)\n\n    hsts = hsts_value(result)\n    if final_scheme == \"https\":\n        if not hsts:\n            if require_hsts:\n                findings.append(\"HSTS required but missing\")\n            else:\n                warnings.append(\"no Strict-Transport-Security header\")\n    return findings, warnings, hsts\n\n\ndef report(start, result, findings, warnings, hsts, max_hops, as_json):\n    \"\"\"Print the chain and the verdict, or one JSON object.\"\"\"\n    hops = result[\"hops\"]\n    start_scheme = urllib.parse.urlsplit(start).scheme\n    final = result[\"final\"] or \"\"\n    final_scheme = urllib.parse.urlsplit(final).scheme\n    if as_json:\n        print(json.dumps({\n            \"start\": start,\n            \"hops\": hops,\n            \"final\": result[\"final\"],\n            \"status\": result[\"status\"],\n            \"upgraded\": start_scheme == \"http\" and final_scheme == \"https\",\n            \"hsts\": hsts,\n            \"findings\": findings,\n            \"warnings\": warnings,\n            \"ok\": not findings,\n        }, indent=2, sort_keys=True))\n        return\n\n    print(\"CHAIN %s  max %d hops\" % (start, max_hops))\n    for i, hop in enumerate(hops, 1):\n        print(\"  %d  %d  %s -> %s\" % (i, hop[\"code\"], hop[\"from\"], hop[\"to\"]))\n    if result[\"final\"]:\n        number = len(hops) + 1 if hops else 1\n        print(\"  %d  %d  %s\" % (number, result[\"status\"], result[\"final\"]))\n\n    if start_scheme == \"https\":\n        print(\"UPGRADE n/a  already https\")\n    elif final_scheme == \"https\":\n        print(\"UPGRADE yes  http -> https\")\n    else:\n        print(\"UPGRADE no  still http\")\n    if final_scheme == \"https\":\n        print(\"HSTS %s\" % (hsts or \"missing\"))\n    for warning in warnings:\n        print(\"WARN %s\" % warning)\n    for finding in findings:\n        print(\"ALERT %s\" % finding)\n\n    if findings:\n        print(\"ALERT %d finding(s), %d warning(s)\" % (len(findings), len(warnings)))\n        return\n    if start_scheme == \"https\":\n        state = \"already https\"\n    elif final_scheme == \"https\":\n        state = \"upgraded\"\n    else:\n        state = \"not upgraded\"\n    if final_scheme == \"https\":\n        security = \"HSTS present\" if hsts else \"no HSTS\"\n    else:\n        security = \"no HTTPS\"\n    print(\"OK %d hop(s), %s, %s, final %d\"\n          % (len(hops), state, security, result[\"status\"]))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Follow an HTTP redirect chain and audit each hop.\",\n        epilog=\"Exit codes: 0 clean, 1 finding, 2 usage or input error.\")\n    parser.add_argument(\"url\", help=\"http:// or https:// URL to check\")\n    parser.add_argument(\"--max-hops\", type=int, default=3, metavar=\"N\",\n                        help=\"fail when the chain has more than N hops, default 3\")\n    parser.add_argument(\"--timeout\", type=float, default=10.0, metavar=\"SEC\",\n                        help=\"socket timeout in seconds, default 10\")\n    parser.add_argument(\"--require-hsts\", action=\"store_true\",\n                        help=\"fail when the final HTTPS response has no HSTS header\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON report instead of text\")\n    args = parser.parse_args(argv)\n\n    if not valid_url(args.url):\n        parser.error(\"URL must start with http:// or https:// and include a host\")\n    if not 0 <= args.max_hops <= 10:\n        parser.error(\"--max-hops must be between 0 and 10\")\n    if args.timeout <= 0:\n        parser.error(\"--timeout must be greater than zero\")\n\n    try:\n        result = fetch(args.url, args.timeout)\n    except (FetchError, ValueError) as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings, warnings, hsts = audit(args.url, result, args.max_hops, args.require_hsts)\n    report(args.url, result, findings, warnings, hsts, args.max_hops, args.json)\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p281","n":"281","slug":"quiz-3-infrastructure-questions","title":"Quiz: 3 infrastructure questions","sections":["it"],"topic":"Networking & DNS","tags":["ITInfrastructure","Networking","RAID","TCP","Quiz"],"series_name":"Visual","design":"quiz","script":"","date":"2028-07-13","files":{"it":"notes/p281-quiz-3-infrastructure-questions.md"},"md":"# Quiz: 3 infrastructure questions\n\nThree questions show up in almost every infrastructure interview, and the wrong answer is usually the one that sounds clever.\n\n## Quick reference\n\n**Q1. A RAID 5 array loses one drive. What is still true?**\n\n- a) It survives: parity is spread\n- b) It is gone: no parity here\n- c) It survives with only two drives\n\n> **Answer:** a) It survives: parity is spread. RAID 5 spreads parity across at least three drives and tolerates one failure; capacity is n-1. A second failure during the rebuild ends the set.\n\n**Q2. How many /24 subnets fit inside one /16?**\n\n- a) 16\n- b) 256\n- c) 65,536\n\n> **Answer:** b) 256. A /16 holds 65,536 addresses and a /24 holds 256, so 65,536 / 256 = 256 subnets. Each extra prefix bit doubles the count.\n\n**Q3. A live video call drops packets. Which transport fits?**\n\n- a) TCP, because it re-sends loss\n- b) UDP, because late data is useless\n- c) Either; the port decides\n\n> **Answer:** b) UDP, because late data is useless. Video needs low latency, and a re-sent frame arrives too late to show. UDP drops it and moves on; WebRTC uses UDP and adds a jitter buffer and congestion control.\n\n## Explained\n\n### Parity\n\nQuestion one: a RAID 5 array loses one drive. It survives, because parity is striped across the set and the missing data is rebuilt from the remaining drives. RAID 5 needs at least three drives, gives the capacity of n-1, and tolerates exactly one failure. A second failure before the rebuild finishes ends the array.\n\n### Prefixes\n\nQuestion two: how many /24s fit in a /16? A /16 holds 65,536 addresses; a /24 holds 256, of which 254 are usable. Divide and the answer is 256 subnets, 65,536 addresses in total. Each extra prefix bit doubles the subnet count and halves the size.\n\n### Transport\n\nQuestion three: a live video call drops packets. UDP is the right default, because a re-sent frame arrives too late to show. TCP would hold the stream while it retransmits, which is worse for a call. WebRTC uses UDP, adds a jitter buffer and its own congestion control.\n\n> **The honest caveat:** RAID 5's one-drive tolerance is a promise about drives, not about rebuild time; on large disks a rebuild can run for days, and RAID 6 or RAID 10 is often the safer answer.\n\n> **The lesson:** the basics are not the beginner questions; they are the ones that decide the outage.\n\n## In one line\n\n> **The basics still catch the outage.** Parity, prefixes and ports explain most of the pager.\n\n**Check the array**\n\n```bash\nmdadm --detail /dev/md0\n```\n"},{"id":"p282","n":"282","slug":"a-serverless-web-app-on-aws-drawn","title":"A serverless web app on AWS, drawn","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","Serverless","CloudArchitecture","DevOps","CloudNative"],"series_name":"Visual","design":"architecture","script":"","date":"2028-07-16","files":{"devsecops":"notes/p282-a-serverless-web-app-on-aws-drawn.md"},"md":"# A serverless web app on AWS, drawn\n\nThe first serverless architecture you draw looks empty: six managed services and not one server to patch. The request path still has three tiers, and each tier fails differently.\n\n## Quick reference\n\n- **Tier 1 · Edge:** Route 53 (DNS) · API Gateway (API)\n- **Tier 2 · Compute:** Lambda (FN) · SQS queue (SQS)\n- **Tier 3 · Data:** DynamoDB (DB) · S3 objects (OBJ)\n\n**Request flow:** Route 53 → API Gateway → Lambda → DynamoDB; Lambda → S3 objects; API Gateway → SQS queue → Lambda\n\n## Explained\n\n**Edge**  \nRoute 53 answers DNS and API Gateway is the front door. API Gateway can check the caller, applies throttling and sends the request to an integration; it can invoke Lambda directly or put a message on a queue. Neither service is a machine you own, which is the point.\n\n**Compute**  \nLambda runs the request. Fifteen minutes is the hard ceiling per invocation, so it suits short work rather than a nightly batch. When work can wait, API Gateway writes to SQS and Lambda polls the queue in batches through an event source mapping, adding concurrency as the backlog grows. The queue is the part that survives a traffic spike.\n\n**Data**  \nDynamoDB stores the session and the order, S3 stores the uploads, and the execution role is what lets Lambda write to either. Permissions live in that role, not in the function code. A VPC is optional here: add it only when the function must reach a private database, and then give it a NAT gateway for outbound internet calls.\n\n> **The honest caveat:** serverless moves the operational work rather than deleting it. Cold starts, concurrency limits and per-request pricing become your capacity plan, and a retry storm on a queue can bill you twice for the same failure.\n\n> **The lesson:** in a serverless diagram the arrows are permissions and queues, not cables.\n\n## In one line\n\n> **Draw the arrows, not the servers.** In serverless, the arrows are permissions and queues.\n\n**Follow the function**\n\n```bash\naws logs tail /aws/lambda/fn --follow\n```\n"},{"id":"p283","n":"283","slug":"the-aws-cli-cheat-sheet","title":"The AWS CLI cheat sheet","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudComputing","DevOps","CommandLine","CloudNative"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-07-18","files":{"devsecops":"notes/p283-the-aws-cli-cheat-sheet.md"},"md":"# The AWS CLI cheat sheet\n\nThe console is fine until the same question arrives for the tenth account, or you need the answer inside a pipeline. The AWS CLI is the same API with a shorter path.\n\n## Quick reference\n\n### Identity\n\n```bash\naws sts get-caller-identity\naws iam list-users\naws iam list-roles\naws iam get-role --role-name app\naws configure list\n```\n\n### Compute\n\n```bash\naws ec2 describe-instances\naws ec2 describe-instance-status\naws lambda list-functions\naws lambda invoke --function-name fn out\naws ecs list-clusters\n```\n\n### Storage\n\n```bash\naws s3 ls\naws s3 cp file.txt s3://bucket/\naws s3 sync ./dist s3://bucket/\naws s3 presign s3://bucket/key\naws s3api get-bucket-policy --bucket b\n```\n\n### Network\n\n```bash\naws ec2 describe-vpcs\naws ec2 describe-subnets\naws ec2 describe-route-tables\naws ec2 describe-security-groups\naws elbv2 describe-load-balancers\n```\n\n## Explained\n\n**Identity**  \n`aws sts get-caller-identity` is the first command in any account: it tells you who you are and which account you are in. `aws iam list-users` and `aws iam list-roles` show what exists, `aws iam get-role --role-name app` shows one role, and `aws configure list` shows the profile, key and region the CLI is actually using.\n\n**Compute**  \n`aws ec2 describe-instances` lists the fleet, and `aws ec2 describe-instance-status` adds the health checks. `aws lambda list-functions` and aws lambda invoke are enough for most function work, and `aws ecs list-clusters` finds the container clusters.\n\n**Storage**  \n`aws s3 ls` is still the fastest bucket inventory. aws s3 cp and aws s3 sync move data, aws s3 presign makes a temporary URL without changing the object, and aws s3api get-bucket-policy shows the policy that can open the bucket to the world.\n\n**Network**  \n`aws ec2 describe-vpcs`, describe-subnets, describe-route-tables and describe-security-groups read the four parts of a VPC, and `aws elbv2 describe-load-balancers` shows what sits in front of the app.\n\n> **The honest caveat:** the CLI silently takes the profile and region from your shell. A command that works on your laptop can hit the wrong account in CI, so run `aws sts get-caller-identity` first, every time.\n\n> **The lesson:** the console is a view, and the CLI is the same view that you can put in a script.\n\n## In one line\n\n> **Type it once, then script it.** Twenty commands beat a browser tab and a memory.\n\n**Who am i**\n\n```bash\naws sts get-caller-identity\n```\n"},{"id":"p284","n":"284","slug":"the-shared-responsibility-model-drawn","title":"The shared responsibility model, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["AWS","CloudSecurity","CloudArchitecture","DevOps","CloudNative"],"series_name":"Visual","design":"explainer","script":"","date":"2028-07-20","files":{"devsecops":"notes/p284-the-shared-responsibility-model-drawn.md"},"md":"# The shared responsibility model, drawn\n\nThe shared responsibility model fits in one sentence: AWS secures the cloud, you secure what you put in it. The useful version names four layers, because the line is not in the same place for every service.\n\n## Quick reference\n\n### Identity and access\n\n- Users, roles and policies are yours to write\n- MFA on the root account, with no exceptions\n- Least privilege, reviewed, not guessed\n- AWS runs IAM; you decide who gets in\n\n### Data\n\n- Classification, encryption and backups are yours\n- AWS encrypts the media; you choose the key\n- Bucket and database policies decide access\n- Deletion protection is a setting you own\n\n### Platform and configuration\n\n- AWS patches the hypervisor and managed runtime\n- On EC2, you patch the guest operating system\n- On Lambda and RDS, AWS patches the OS\n- Security groups and open ports are your call\n\n### Physical infrastructure\n\n- Buildings, power and cooling belong to AWS\n- Hardware and the network fabric are theirs\n- Media disposal is their problem, not yours\n- You still choose the region your data sits in\n\n> **Note:** The line moves with the service: on EC2 you patch, on Lambda AWS does.\n\n## Explained\n\n**Identity and access**  \nUsers, roles and policies are yours to write, and so is MFA on the root account. AWS runs IAM, but it has no idea who should be an administrator in your organisation. Least privilege is a decision you make and review, not a checkbox the provider ticks for you.\n\n**Data**  \nClassification, encryption and backups are your responsibility. AWS encrypts the physical media; you choose the key, its rotation and who can use it. Bucket and database policies decide who reads the data, and deletion protection is a setting only you can turn on.\n\n**Platform and configuration**  \nThis layer is genuinely shared. AWS patches the hypervisor and the runtime behind managed services such as Lambda and RDS. On EC2, you patch the guest operating system, and security groups and open ports are your configuration whichever service sits behind them.\n\n**Physical infrastructure**  \nBuildings, power, cooling, hardware and the network fabric belong to AWS. Media disposal is their problem, not yours. You still choose the region, and that choice is often the most consequential control you have.\n\n> **The honest caveat:** the line moves with the service, and audits fail where a team assumed the provider had covered a shared control such as patching or configuration management.\n\n> **The lesson:** write down which layer you own before you argue about who forgot it.\n\n## In one line\n\n> **The line moves with the service.** Every managed service redraws it, so read it again.\n\n**Read the model:** aws.amazon.com/compliance\n"},{"id":"p285","n":"285","slug":"an-s3-public-access-checker-in-python","title":"An S3 public access checker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","S3","CloudSecurity","Python","DevSecOps"],"series_name":"Visual","design":"code","script":"s3_public_check.py","date":"2028-07-23","files":{"devsecops":"notes/p285-an-s3-public-access-checker-in-python.md"},"md":"# An S3 public access checker in Python\n\nA bucket is public for two reasons that have nothing to do with each other: its policy and its ACL. Fix one and the other can still serve your objects.\n\n## Quick reference\n\nThe full script is at the end of this note: `s3_public_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 s3_public_check.py pol.json acl.json\n```\n\n**Example output**\n\n```text\nPUBLIC policy PublicRead: Principal *\nPUBLIC acl READ: AllUsers\n```\n\n## Explained\n\n**What it checks**  \nThe script reads a policy file and an ACL file. For each statement it looks for Effect Allow, a Principal containing \"\\*\", and no Condition, then prints the statement ID. For each grant it looks for the AllUsers or AuthenticatedUsers group and prints the permission. It exits 1 when either list is non-empty.\n\n**Why the Condition matters**  \nPrincipal \"\\*\" with a Condition is not automatically public: a condition on aws:PrincipalOrgID can keep the world out while allowing your whole organisation. The check treats any Condition as a restriction, which is deliberately conservative.\n\n**How to run it**  \nExport both documents with aws s3api, then run the attached script with the policy path and `--acl`. Exit 0 means neither file exposes the bucket; exit 2 means a missing or unreadable file, which is a broken check rather than a clean result. Standard library only, read-only.\n\n> **The honest caveat:** it reads the files you give it, not Block Public Access, and it will not flag a weak Condition. Use it as a CI smoke test, not your only control.\n\n> **The lesson:** two doors, one lock. Check both or you have checked neither.\n\n## In one line\n\n> **Check both doors, not one.** A policy and an ACL are two separate ways to go public.\n\n**Run it in ci**\n\n```bash\npython3 s3_public_check.py p.json a.json\n```\n\n## Script: s3_public_check.py\n\nDownload: [scripts/s3_public_check.py](../scripts/s3_public_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"s3_public_check.py - flag public S3 bucket policies and ACL grants.\n\nReads two JSON documents and never calls AWS:\n\n    the bucket policy   aws s3api get-bucket-policy --bucket b --query Policy \\\n                            --output text\n    the bucket ACL      aws s3api get-bucket-acl --bucket b\n\nA policy statement is public when Effect is Allow, Principal is \"*\" (also\n{\"AWS\": \"*\"} or a list containing \"*\") and the statement has no Condition.\nAn ACL grant is public when its Grantee URI is the AllUsers or\nAuthenticatedUsers group.\n\nUsage:\n    aws s3api get-bucket-policy --bucket demo --query Policy \\\n        --output text > /tmp/policy.json\n    aws s3api get-bucket-acl --bucket demo > /tmp/acl.json\n    python3 s3_public_check.py /tmp/policy.json --acl /tmp/acl.json\n    python3 s3_public_check.py /tmp/policy.json\n    python3 s3_public_check.py --acl /tmp/acl.json -q\n    cat /tmp/policy.json | python3 s3_public_check.py -\n\nExit codes: 0 clean, 1 public access found, 2 usage or input error.\n\nCI / cron (a non-zero exit fails the job):\n    aws s3api get-bucket-policy --bucket demo --query Policy \\\n        --output text > /tmp/policy.json && \\\n    aws s3api get-bucket-acl --bucket demo > /tmp/acl.json && \\\n    python3 s3_public_check.py /tmp/policy.json --acl /tmp/acl.json\n\nRead-only: the script opens local files (or stdin) and writes nothing.\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass CheckError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source, label):\n    \"\"\"Read one JSON document from a file or '-' for stdin.\"\"\"\n    if source == \"-\":\n        try:\n            return json.load(sys.stdin)\n        except ValueError as exc:\n            raise CheckError(\"%s from stdin is not JSON: %s\" % (label, exc))\n    try:\n        with open(source) as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise CheckError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise CheckError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef unwrap_policy(doc):\n    \"\"\"Accept a raw policy document or the get-bucket-policy response.\"\"\"\n    if not isinstance(doc, dict):\n        raise CheckError(\"policy must be a JSON object\")\n    inner = doc.get(\"Policy\")\n    if isinstance(inner, str):\n        try:\n            doc = json.loads(inner)\n        except ValueError as exc:\n            raise CheckError(\"the Policy field is not JSON: %s\" % exc)\n    elif isinstance(inner, dict):\n        doc = inner\n    if not isinstance(doc, dict):\n        raise CheckError(\"policy must be a JSON object\")\n    return doc\n\n\ndef statements_of(doc):\n    \"\"\"Return the policy statements as a list of objects.\"\"\"\n    statements = unwrap_policy(doc).get(\"Statement\")\n    if statements is None:\n        return []\n    if isinstance(statements, dict):\n        return [statements]\n    if not isinstance(statements, list):\n        raise CheckError(\"Statement must be an object or a list\")\n    for statement in statements:\n        if not isinstance(statement, dict):\n            raise CheckError(\"every Statement must be an object\")\n    return statements\n\n\ndef principal_is_public(principal):\n    \"\"\"True for \"*\", {\"AWS\": \"*\"}, [\"*\"] and similar shapes.\"\"\"\n    if principal == \"*\":\n        return True\n    if isinstance(principal, list):\n        return any(principal_is_public(item) for item in principal)\n    if isinstance(principal, dict):\n        return any(principal_is_public(value) for value in principal.values())\n    return False\n\n\ndef has_condition(statement):\n    \"\"\"True when the statement carries a non-empty Condition block.\"\"\"\n    condition = statement.get(\"Condition\")\n    return isinstance(condition, dict) and bool(condition)\n\n\ndef check_policy(doc):\n    \"\"\"Return one finding per public Allow statement without a Condition.\"\"\"\n    findings = []\n    for statement in statements_of(doc):\n        if statement.get(\"Effect\") != \"Allow\":\n            continue\n        if not principal_is_public(statement.get(\"Principal\")):\n            continue\n        if has_condition(statement):\n            continue\n        sid = statement.get(\"Sid\") or \"(no sid)\"\n        findings.append(\"policy %s: Principal * with no Condition\" % sid)\n    return findings\n\n\ndef check_acl(doc):\n    \"\"\"Return one finding per grant to AllUsers or AuthenticatedUsers.\"\"\"\n    if not isinstance(doc, dict):\n        raise CheckError(\"ACL must be a JSON object\")\n    grants = doc.get(\"Grants\") or []\n    if not isinstance(grants, list):\n        raise CheckError(\"Grants must be a list\")\n    findings = []\n    for grant in grants:\n        if not isinstance(grant, dict):\n            raise CheckError(\"every Grant must be an object\")\n        grantee = grant.get(\"Grantee\")\n        uri = \"\"\n        if isinstance(grantee, dict):\n            uri = grantee.get(\"URI\") or \"\"\n        if not isinstance(uri, str):\n            continue\n        if uri.endswith(\"/AllUsers\") or uri.endswith(\"/AuthenticatedUsers\"):\n            name = uri.rsplit(\"/\", 1)[-1]\n            findings.append(\"acl %s: %s\" % (grant.get(\"Permission\", \"?\"), name))\n    return findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Flag public statements in an S3 bucket policy and public ACL grants.\",\n        epilog=\"Exit codes: 0 clean, 1 public access found, 2 usage or input error.\")\n    parser.add_argument(\"policy\", nargs=\"?\",\n                        help=\"bucket policy JSON file, or - for stdin\")\n    parser.add_argument(\"--acl\", metavar=\"FILE\",\n                        help=\"bucket ACL JSON file\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    if not args.policy and not args.acl:\n        parser.error(\"give a policy file, an --acl file, or both\")\n    if args.policy == \"-\" and args.acl == \"-\":\n        parser.error(\"policy and --acl cannot both read from stdin\")\n\n    try:\n        findings = []\n        if args.policy:\n            findings += check_policy(read_json(args.policy, \"policy\"))\n        if args.acl:\n            findings += check_acl(read_json(args.acl, \"ACL\"))\n    except CheckError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if findings:\n        if not args.quiet:\n            for finding in findings:\n                print(\"PUBLIC \" + finding)\n        plural = \"\" if len(findings) == 1 else \"s\"\n        print(\"PUBLIC %d finding%s\" % (len(findings), plural))\n        return 1\n    print(\"OK no public policy statements or ACL grants\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p286","n":"286","slug":"multi-region-active-active-drawn","title":"Multi-region active-active, drawn","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudArchitecture","SRE","DevOps","CloudNative"],"series_name":"Visual","design":"architecture","script":"","date":"2028-07-25","files":{"devsecops":"notes/p286-multi-region-active-active-drawn.md"},"md":"# Multi-region active-active, drawn\n\nActive-active is easy to draw and hard to finish. The application tier multiplies cleanly; the database is where the diagram starts lying.\n\n## Quick reference\n\n- **Tier 1 · Edge:** Route 53 latency (DNS) · Health checks (HC)\n- **Tier 2 · Region a:** ALB (ALB) · App servers (APP) · Aurora writer (DB)\n- **Tier 3 · Region b:** ALB (ALB) · App servers (APP) · Aurora replica (DB)\n\n**Request flow:** Health checks → Route 53 latency → ALB → App servers → Aurora writer → Aurora replica; Route 53 latency → ALB → App servers → Aurora replica\n\n## Explained\n\n**Edge**  \nRoute 53 holds latency records for both regions, so each user is answered by the region with the lowest measured latency. Latency is measured per resolver, so a user in Riyadh and a user in Dublin can land in different regions. Health checks watch each regional endpoint, and when one fails, Route 53 stops returning it. That is the whole failover for reads and for stateless requests.\n\n**Region A and Region B**  \nEach region has its own load balancer, its own application servers and its own Aurora endpoint. The load balancers are regional, the application servers hold no session state, and either region can serve a request without asking the other. Reads are served locally in both.\n\n**The database**  \nAurora Global Database keeps a read replica in Region B. Replication is asynchronous, typically under a second, and the replica stays read-only while the writer in Region A lives. Promotion of the secondary takes about a minute and can lose the last few transactions, so plan for a small window of lost writes.\n\n> **The honest caveat:** this design is active-active for reads and active-passive for writes. Calling it active-active without saying that is how teams discover the difference during a failover.\n\n> **The lesson:** availability is a property of each tier, not of the diagram.\n\n## In one line\n\n> **Active-active is a per-tier claim.** The application can be active in both regions; the writer cannot.\n\n**Check the lag**\n\n```bash\naws rds describe-global-clusters\n```\n"},{"id":"p287","n":"287","slug":"vpc-anatomy-drawn","title":"VPC anatomy, drawn","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","Networking","VPC","CloudArchitecture","DevOps"],"series_name":"Visual","design":"explainer","script":"","date":"2028-07-27","files":{"devsecops":"notes/p287-vpc-anatomy-drawn.md"},"md":"# VPC anatomy, drawn\n\nA VPC looks like a diagram and behaves like a set of rules. Four parts decide whether a packet gets in, gets out or goes nowhere.\n\n## Quick reference\n\n### Subnets\n\n- One subnet lives in one availability zone\n- Public means a route to an internet gateway\n- AWS reserves five addresses in every subnet\n- A private subnet has no inbound route\n\n### Route tables\n\n- The local route for the VPC CIDR cannot be deleted\n- Longest prefix match chooses the next hop\n- Each subnet follows exactly one table\n- 0.0.0.0/0 is the route that makes it public\n\n### Internet and NAT\n\n- An internet gateway attaches to the VPC\n- A NAT gateway gives outbound access only\n- Nothing unsolicited gets in through NAT\n- The NAT gateway lives in a public subnet\n\n### Security groups and NACLs\n\n- Security groups are stateful and allow-only\n- NACLs are stateless and can deny traffic\n- A group sits on an ENI, a NACL on a subnet\n- NACL rules run in order, first match wins\n\n> **Note:** Stateful means the reply is allowed; stateless means you write both directions.\n\n## Explained\n\n**Subnets**  \nA subnet is one CIDR block in one availability zone. A public subnet is public because its route table sends 0.0.0.0/0 to an internet gateway. AWS reserves five addresses in every subnet, so a /24 gives you 251 usable addresses, not 256.\n\n**Route tables**  \nEach subnet follows exactly one route table. The local route for the VPC CIDR cannot be deleted. For everything else, the longest prefix match picks the next hop, so a /32 route can override the default.\n\n**Internet and NAT**  \nAn internet gateway attaches to the VPC and gives public subnets a path in and out. A NAT gateway gives private subnets outbound access only: it translates their addresses to its own public IP and drops unsolicited inbound traffic. It sits in a public subnet and holds an elastic IP.\n\n**Security groups and NACLs**  \nSecurity groups are stateful and allow-only: the reply to an allowed connection is allowed automatically. NACLs are stateless and evaluated in numbered order, first match wins, so both directions need a rule. A group attaches to an elastic network interface; a NACL attaches to a subnet.\n\n> **The honest caveat:** most VPC outages are a rule in the wrong place, such as a NACL deny or a private route table with no NAT route.\n\n> **The lesson:** read the four together, because a correct rule in the wrong layer still drops the packet.\n\n## In one line\n\n> **Four layers, one packet path.** A correct rule in the wrong layer still drops the packet.\n\n**Trace the route**\n\n```bash\naws ec2 describe-route-tables\n```\n"},{"id":"p288","n":"288","slug":"the-azure-cli-cheat-sheet","title":"The Azure CLI cheat sheet","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Azure","CloudComputing","DevOps","CommandLine","CloudNative"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-07-30","files":{"devsecops":"notes/p288-the-azure-cli-cheat-sheet.md"},"md":"# The Azure CLI cheat sheet\n\nThe Azure CLI has one advantage over the portal: every click you make can become a command in a pipeline.\n\n## Quick reference\n\n### Auth\n\n```bash\naz login\naz account show\naz account list --output table\naz account set --subscription prod\naz ad signed-in-user show\n```\n\n### Resources\n\n```bash\naz group list\naz group create -n rg -l uksouth\naz group delete -n rg --no-wait\naz resource list -g rg -o table\naz deployment group list -g rg\n```\n\n### Compute\n\n```bash\naz vm list -o table\naz vm show -n web -g rg\naz vm start -n web -g rg\naz vm stop -n web -g rg\naz vm list-ip-addresses -g rg -n web\n```\n\n### Storage\n\n```bash\naz storage account list\naz storage account show -n st -g rg\naz storage account create -n st -g rg\naz storage account keys list -n st -g rg\naz storage account check-name -n st\n```\n\n## Explained\n\n**Auth**  \n`az login` opens the session, `az account show` tells you which subscription you are actually in, and `az account set --subscription prod` switches it. `az account list --output table` shows the choices, and `az ad signed-in-user show` confirms the identity behind the token.\n\n**Resources**  \n`az group list` finds the resource groups, and `az group create -n rg -l uksouth` makes one in a named region. `az resource list -g rg -o table` is the inventory, `az deployment group list -g rg` shows what was deployed into a group, and `az group delete -n rg --no-wait` starts the cleanup.\n\n**Compute**  \n`az vm list -o table` is the fleet, `az vm show -n web -g rg` is one machine, and start and stop are the two commands you will type most. `az vm list-ip-addresses -g rg -n web` finds the address without opening the portal.\n\n**Storage**  \n`az storage account list` is the inventory. Show and create work on one account, keys list fetches the two access keys, and check-name tells you whether a name is free before you create anything.\n\n> **The honest caveat:** the CLI is only as honest as the subscription you selected. Run `az account show` before a destructive command, and never assume the portal and the terminal point at the same tenant.\n\n> **The lesson:** the portal teaches you the shape, and the CLI lets you repeat it.\n\n## In one line\n\n> **Select the account before the command.** The wrong subscription makes every command wrong.\n\n**Who am i**\n\n```bash\naz account show\n```\n"},{"id":"p289","n":"289","slug":"5-cloud-architecture-mistakes","title":"5 cloud architecture mistakes","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudArchitecture","DevOps","CloudSecurity","SRE"],"series_name":"Visual","design":"dontdo","script":"","date":"2028-08-01","files":{"devsecops":"notes/p289-5-cloud-architecture-mistakes.md"},"md":"# 5 cloud architecture mistakes\n\nNone of these decisions looks wrong on the day it is made. Each one saves a little time or money, and sends the bill to the person on call.\n\n## Quick reference\n\n**1.** ❌ **Everything in one subnet** — One route table means nothing is private.  \n✅ **Split public and private** — Each subnet gets only the route it needs.\n\n**2.** ❌ **A single availability zone** — One zone outage takes the whole app down.  \n✅ **Two zones from day one** — A zone failure becomes a failover, not an outage.\n\n**3.** ❌ **Long-lived access keys** — A leaked key outlives the person who made it.  \n✅ **Roles and OIDC federation** — Credentials expire in minutes, not years.\n\n**4.** ❌ **No tags, no budgets** — The bill arrives with no way to split it.  \n✅ **Owner and cost tags** — Every resource names who pays for it.\n\n**5.** ❌ **Backups never tested** — An untested backup is a hope, not a plan.  \n✅ **A restore drill each quarter** — You learn the restore time before an incident.\n\n## Explained\n\n### The five\n\n1. **Everything in one subnet.** One subnet means one route table, and a route that reaches the internet makes the subnet public. Split public from private and route the private side through a NAT gateway.\n\n2. **A single availability zone.** One zone outage takes the whole app with it. Two zones turn that outage into a failover; cross-zone traffic costs a little, downtime costs more.\n\n3. **Long-lived access keys.** A key in a CI variable outlives the person who made it. Roles and OIDC federation give workloads credentials that expire in minutes.\n\n4. **No tags, no budgets.** The bill arrives with no way to split it and nobody can name the team behind the expensive queue. Owner and cost-centre tags cost nothing now and hurt to retrofit.\n\n5. **Backups never tested.** An untested backup is a hope. A quarterly restore drill gives you the restore time before an incident needs it, and proves the backup exists.\n\n### What they share\n\nEach one is a shortcut in the architecture, not in the code, which is why a code review never catches it.\n\n> **The honest caveat:** two zones and a tested restore still do not cover a bad deploy or a deleted table; for that you need versioning, point-in-time recovery and a rollback you have practised.\n\n> **The lesson:** the cheapest architecture decision is the one you review before you build.\n\n## In one line\n\n> **Small shortcuts, large outages.** The bill and the incident both start with the same decision.\n\n**Check your subnets**\n\n```bash\naws ec2 describe-subnets\n```\n"},{"id":"p290","n":"290","slug":"a-terraform-tag-auditor-in-python","title":"A Terraform tag auditor in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Terraform","IaC","AWS","DevOps","FinOps"],"series_name":"Visual","design":"code","script":"tf_tag_audit.py","date":"2028-08-03","files":{"devsecops":"notes/p290-a-terraform-tag-auditor-in-python.md"},"md":"# A Terraform tag auditor in Python\n\nA tag policy that lives in a wiki is a suggestion. A tag check that fails the pipeline is a rule.\n\n## Quick reference\n\nThe full script is at the end of this note: `tf_tag_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nterraform show -json plan.out | python3 tf_tag_audit.py -\n```\n\n**Example output**\n\n```text\nMISSING aws_instance.web CostCenter\nFAIL 1 resources\n```\n\n## Explained\n\n**What it reads**  \n`terraform show -json plan.out` prints a saved plan as JSON. Every resource the plan touches appears in resource_changes, with its actions, the object before and the object after. The auditor reads the after object, because that is the resource you are about to have.\n\n**What it checks**  \nFor each changed managed resource it takes tags, or tags_all when the provider adds default tags, and compares the keys with Owner, Environment and CostCenter. A missing key is printed with the resource address. Resource types that do not support tags are counted and skipped rather than failed, because an IAM policy attachment has nowhere to put them.\n\n**Where it fits**  \nRun it after terraform plan and before apply, in the same job. Exit 1 fails the build. Exit 2 means the file is missing, is state rather than a plan, or is not JSON, which is a broken check rather than a clean result. The attached script is standard library only and read-only.\n\n> **The honest caveat:** it only sees resources in this plan. Drift, resources created outside Terraform and tags added by hand after apply are invisible, so pair it with a state-wide audit.\n\n> **The lesson:** a tag you cannot enforce is a tag you do not have.\n\n## In one line\n\n> **No tag, no apply.** A tag check that fails the pipeline is a rule that exists.\n\n**Audit the plan**\n\n```bash\nterraform show -json plan.out\n```\n\n## Script: tf_tag_audit.py\n\nDownload: [scripts/tf_tag_audit.py](../scripts/tf_tag_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"tf_tag_audit.py - fail a Terraform plan when changed resources lack tags.\n\nReads the JSON that `terraform show -json` prints for a saved plan and checks\nevery managed resource the plan creates or replaces or updates. A resource\npasses when its planned tags (or tags_all, which includes provider default\ntags) carry every required name. Resource types that do not expose tags in\nthe plan are counted and skipped.\n\nUsage:\n    terraform plan -out plan.out\n    terraform show -json plan.out > /tmp/plan.json\n    python3 tf_tag_audit.py /tmp/plan.json\n    terraform show -json plan.out | python3 tf_tag_audit.py -\n    python3 tf_tag_audit.py /tmp/plan.json --require Owner,Team,Environment\n    python3 tf_tag_audit.py /tmp/plan.json -q\n\nExit codes: 0 clean, 1 required tags missing, 2 usage or input error.\n\nCI (run it after terraform plan, before apply):\n    terraform plan -out plan.out && \\\n        terraform show -json plan.out > /tmp/plan.json && \\\n        python3 tf_tag_audit.py /tmp/plan.json\n\nRead-only: it reads one JSON document and writes nothing. Standard library\nonly, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nREQUIRED = (\"Owner\", \"Environment\", \"CostCenter\")\nCHANGED = (\"create\", \"update\")\n\n\nclass CheckError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    \"\"\"Read one JSON document from a file or '-' for stdin.\"\"\"\n    if source == \"-\":\n        try:\n            return json.load(sys.stdin)\n        except ValueError as exc:\n            raise CheckError(\"stdin is not JSON: %s\" % exc)\n    try:\n        with open(source) as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise CheckError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise CheckError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef audit(doc, required):\n    \"\"\"Return (findings, checked, skipped) for one plan document.\"\"\"\n    if not isinstance(doc, dict):\n        raise CheckError(\"plan must be a JSON object\")\n    changes = doc.get(\"resource_changes\")\n    if changes is None:\n        raise CheckError(\"no resource_changes: pass plan JSON, not state JSON\")\n    if not isinstance(changes, list):\n        raise CheckError(\"resource_changes must be a list\")\n\n    findings = []\n    checked = 0\n    skipped = 0\n    for item in changes:\n        if not isinstance(item, dict):\n            raise CheckError(\"every resource_changes entry must be an object\")\n        if item.get(\"mode\", \"managed\") != \"managed\":\n            continue\n        change = item.get(\"change\")\n        if not isinstance(change, dict):\n            raise CheckError(\"resource_changes entry has no change object\")\n        actions = change.get(\"actions\") or []\n        if not isinstance(actions, list):\n            raise CheckError(\"change.actions must be a list\")\n        if not set(CHANGED) & set(actions):\n            continue\n        after = change.get(\"after\")\n        if not isinstance(after, dict):\n            continue\n        if \"tags\" not in after and \"tags_all\" not in after:\n            skipped += 1\n            continue\n        tags = after.get(\"tags_all\")\n        if not isinstance(tags, dict):\n            tags = after.get(\"tags\")\n        if tags is None:\n            tags = {}\n        if not isinstance(tags, dict):\n            raise CheckError(\"tags for %s are not an object\" % item.get(\"address\", \"?\"))\n        checked += 1\n        missing = sorted(set(required) - set(tags))\n        if missing:\n            address = item.get(\"address\") or item.get(\"type\") or \"(unknown)\"\n            findings.append((address, missing))\n    return findings, checked, skipped\n\n\ndef parse_required(raw):\n    \"\"\"Turn 'Owner, Environment' into ('Owner', 'Environment').\"\"\"\n    names = tuple(name.strip() for name in raw.split(\",\") if name.strip())\n    if not names:\n        raise ValueError(\"no tag names given\")\n    return names\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Fail when a Terraform plan changes resources without the required tags.\",\n        epilog=\"Exit codes: 0 clean, 1 missing tags, 2 usage or input error.\")\n    parser.add_argument(\"plan\", help=\"plan JSON from terraform show -json, or - for stdin\")\n    parser.add_argument(\"-r\", \"--require\", default=\",\".join(REQUIRED), metavar=\"TAGS\",\n                        help=\"comma-separated tag names, default %s\" % \",\".join(REQUIRED))\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    try:\n        required = parse_required(args.require)\n    except ValueError as exc:\n        parser.error(str(exc))\n\n    try:\n        findings, checked, skipped = audit(read_json(args.plan), required)\n    except CheckError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if findings:\n        if not args.quiet:\n            for address, missing in findings:\n                print(\"ERR %s missing %s\" % (address, \",\".join(missing)))\n        plural = \"\" if len(findings) == 1 else \"s\"\n        print(\"ERR %d resource%s missing required tags (%d checked, %d untaggable)\"\n              % (len(findings), plural, checked, skipped))\n        return 1\n\n    print(\"OK %d changed resource%s carry %s (%d untaggable skipped)\"\n          % (checked, \"\" if checked == 1 else \"s\", \", \".join(required), skipped))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p291","n":"291","slug":"quiz-3-cloud-architecture-questions","title":"Quiz: 3 cloud architecture questions","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","CloudArchitecture","CloudSecurity","DevOps","CloudNative"],"series_name":"Visual","design":"quiz","script":"","date":"2028-08-06","files":{"devsecops":"notes/p291-quiz-3-cloud-architecture-questions.md"},"md":"# Quiz: 3 cloud architecture questions\n\nThree questions separate an architecture you can explain from one you have only copied. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why fix availability across two AZs before adding a region?**\n\n- a) AZ failure is common; region failure is rare\n- b) A second region is cheaper than a second AZ\n- c) Multi-region removes the need for backups\n\n> **Answer:** a) AZ failure is common; region failure is rare. AZs are separate data centres in one region. An AZ failure is realistic, a region failure is rare, and multi-region costs far more in money and complexity.\n\n**Q2. What does a NAT gateway actually do?**\n\n- a) Gives private subnets outbound access only\n- b) Publishes private addresses to the internet\n- c) Removes the need for a route table\n\n> **Answer:** a) Gives private subnets outbound access only. It translates private addresses to its own public IP for outbound traffic and drops unsolicited inbound. It belongs in a public subnet with a private route.\n\n**Q3. Why is the shared responsibility model split at all?**\n\n- a) No single party can secure every layer\n- b) AWS owns the data, the customer owns the metal\n- c) Compliance requires exactly one owner\n\n> **Answer:** a) No single party can secure every layer. The provider secures facilities, hardware and the platform; the customer owns data, access and configuration. Gaps appear where each side assumes the other.\n\n## Explained\n\n**The zone and the region**  \nQuestion one: why two availability zones before a second region? Because an AZ is a data centre and a region is a group of them. Zone failures happen, region failures are rare, and two zones buy the common failure for the cost of duplicated capacity. A second region buys the rare one for far more money, data transfer and operational work.\n\n**The middle box**  \nQuestion two: what does a NAT gateway do? It lets a private subnet start outbound connections by translating their addresses to the gateway's public IP, and it drops unsolicited inbound. It is not a reverse proxy and it does not make a private subnet public. It sits in a public subnet and needs a route from the private one.\n\n**The moving line**  \nQuestion three: why is shared responsibility split? Because no single party can secure every layer. AWS owns the buildings, hardware and managed platform; you own the data, the identities and the configuration. The split moves with the service, which is why the same control can be yours on EC2 and AWS's on Lambda.\n\n> **The honest caveat:** the model is a map, not an audit. A shared control such as patching needs a named owner on your side, or both teams will assume the other one has it.\n\n> **The lesson:** know where your responsibility starts, and write it down.\n\n## In one line\n\n> **Know where your responsibility starts.** Three answers that decide what you own.\n\n**Audit your side**\n\n```bash\naws iam generate-credential-report\n```\n"},{"id":"p292","n":"292","slug":"the-kubectl-command-cheat-sheet","title":"The kubectl command cheat sheet","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Kubectl","DevOps","CloudNative","SRE"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-08-08","files":{"devsecops":"notes/p292-the-kubectl-command-cheat-sheet.md"},"md":"# The kubectl command cheat sheet\n\nA pod is restarting, the deploy is stuck, and someone types kubectl get pods for the fourth time. Four groups of commands answer most of that, and none of them needs a dashboard.\n\n## Quick reference\n\n### Inspect\n\n```bash\nkubectl get pods -A -o wide\nkubectl describe pod api-7d9f\nkubectl get deploy api -o yaml\nkubectl get events -A --watch\nkubectl explain deployment.spec.strategy\n```\n\n### Logs\n\n```bash\nkubectl logs deploy/api --tail=100\nkubectl logs api-7d9f -f\nkubectl logs api-7d9f --previous\nkubectl logs -l app=api --prefix\nkubectl logs api-7d9f --since=1h\n```\n\n### Debug\n\n```bash\nkubectl exec -it api-7d9f -- sh\nkubectl port-forward svc/api 8080:80\nkubectl debug -it api-7d9f --image=busybox\nkubectl top pod api-7d9f\nkubectl run tmp --rm -it --image=busybox\n```\n\n### Apply\n\n```bash\nkubectl apply -f k8s/\nkubectl apply -k overlays/prod\nkubectl diff -f k8s/\nkubectl rollout status deploy/api\nkubectl rollout undo deploy/api\n```\n\n## Explained\n\n### Inspect\n\n`kubectl get pods -A -o wide` shows every pod with its node and IP. describe pod api-7d9f gives the events at the bottom, which is usually the reason. get deploy api -o yaml shows what the cluster holds, get events -A `--watch` follows a live incident, and explain settles a field name.\n\n### Logs\n\n`kubectl logs deploy/api --tail=100` is the first look. Add -f to follow, or `--previous` for the container that died. logs -l app=api `--prefix` tags every pod's lines with its name, and `--since=1h` cuts a noisy service to the window that matters.\n\n### Debug\n\n`kubectl exec -it api-7d9f -- sh` gets a shell. port-forward svc/api 8080:80 reaches the service from your laptop. debug -it api-7d9f `--image=busybox` adds a container when the image has no shell. top pod shows usage, and run tmp `--rm -it --image=busybox` gives you a disposable pod inside the cluster network.\n\n### Apply\n\n`kubectl apply -f k8s/` or apply -k overlays/prod. diff -f k8s/ shows the change before you make it, rollout status blocks until it is done, and rollout undo is the way back.\n\n> **The honest caveat:** kubectl debug needs ephemeral containers, beta from 1.23, and kubectl top needs metrics-server; on a stripped cluster both fail rather than degrade.\n\n> **The lesson:** read before you write. Get, describe and logs answer most incidents before any apply.\n\n## In one line\n\n> **Look first. Then apply.** The cluster answers more questions than you ask it.\n\n**Diff before apply**\n\n```bash\nkubectl diff -f k8s/\n```\n"},{"id":"p293","n":"293","slug":"how-a-commit-reaches-production-drawn","title":"How a commit reaches production, drawn","sections":["devsecops"],"topic":"Git & CI/CD","tags":["CICD","GitOps","DevOps","Kubernetes","DevSecOps"],"series_name":"Visual","design":"architecture","script":"","date":"2028-08-10","files":{"devsecops":"notes/p293-how-a-commit-reaches-production-drawn.md"},"md":"# How a commit reaches production, drawn\n\nA pull request merges at 14:02 and someone asks whether it is live. Between that merge and the answer sit three tiers, and each one hands over a different object.\n\n## Quick reference\n\n- **Source:** Git repository (GIT) · Pull request (PR)\n- **Build:** CI runner (CI) · Image registry (REG) · Scan and sign (SCAN)\n- **Deploy:** Argo CD (SYNC) · Kubernetes cluster (K8S)\n\n**Request flow:** Git repository → Pull request → CI runner → Image registry → Scan and sign → Argo CD → Kubernetes cluster\n\n## Explained\n\n### Source\n\nThe source tier is the Git repository and the pull request. The branch lives in Git, the review lives in the pull request, and branch protection is what makes a second pair of eyes mandatory. The merge is the event everything after it reacts to.\n\n### Build\n\nThe build tier is the CI runner, the image registry and the scan-and-sign step. CI runs the tests and builds the image; the registry stores it under a tag and a digest; the scanner reads the pushed digest and cosign signs it. After this tier, the artefact carries evidence of what it is.\n\n### Deploy\n\nThe deploy tier is Argo CD and the cluster. Argo CD compares the manifests in Git with the live cluster, shows the diff, and syncs it. The cluster pulls the image by digest, so the manifest decides which bytes run, not a person.\n\n> **The honest caveat:** the chain only holds if the manifest pins the digest. Pin a tag and the same manifest can deploy different bytes next week.\n\n> **The lesson:** production is three handoffs, and each one can swap the artefact. The digest is the only thing that survives all three.\n\n## In one line\n\n> **A commit is not a release.** Production runs a digest, not a branch.\n\n**Trace one commit**\n\n```bash\ngit log --oneline -1\n```\n"},{"id":"p294","n":"294","slug":"git-merge-vs-rebase-drawn","title":"Git merge vs rebase, drawn","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","VersionControl","DevOps","CICD","SoftwareEngineering"],"series_name":"Visual","design":"explainer","script":"","date":"2028-08-13","files":{"devsecops":"notes/p294-git-merge-vs-rebase-drawn.md"},"md":"# Git merge vs rebase, drawn\n\nA branch is forty commits behind main and the pull request is a zigzag of merge commits. The reviewer asks for a rebase and the argument starts. Four shapes explain what each side means.\n\n## Quick reference\n\n### Fast-forward\n\n- main has not moved since you branched\n- Git slides the label forward, no new commit\n- History stays linear and unchanged\n- Only possible when nothing diverged\n\n### Merge commit\n\n- Both branches keep every commit\n- One new commit with two parents\n- Conflict resolved once, in the merge\n- History shows what actually happened\n\n### Rebase\n\n- Your commits replay on the new base\n- New hashes, same changes\n- Linear history, no merge commit\n- Never rebase what others have pulled\n\n### Squash merge\n\n- Every branch commit becomes one\n- Main gets a single tidy change\n- The branch history is lost\n- Good for a messy feature branch\n\n> **Note:** Rebase your own branch. Merge the shared one. The rule is about who else has the commits.\n\n## Explained\n\n### Fast-forward\n\nIf main has not moved since you branched, Git slides the main label forward. No merge commit appears, history stays linear, and your commits land unchanged. The result is indistinguishable from having committed straight to main.\n\n### Merge commit\n\nIf both sides moved, git merge main creates one commit with two parents. Every commit survives, the conflict is resolved once, and the history shows what actually happened, mess included.\n\n### Rebase\n\ngit rebase main replays your commits on top of main, one at a time. You get linear history and new hashes: the changes are the same, the commits are not. Anyone who already pulled your branch now has different history to reconcile.\n\n### Squash merge\n\nThe pull request becomes one commit on main. The feature history disappears, main gets one readable change, and the individual steps are gone, which is sometimes the point. Many teams enable only this button on main, because it keeps the log readable.\n\n> **The honest caveat:** none of these is wrong. Rebasing your own unpushed branch is clean; rebasing a shared one forces everyone else to recover.\n\n> **The lesson:** merge records what happened, rebase edits how it reads. Pick the one your team needs and write it down.\n\n## In one line\n\n> **Rewrite your own. Merge the shared.** The history other people pulled is not yours to rewrite.\n\n**Before you rebase**\n\n```bash\ngit branch backup && git rebase main\n```\n"},{"id":"p295","n":"295","slug":"a-dockerfile-linter-in-python","title":"A Dockerfile linter in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Docker","Python","DevSecOps","CICD","Containers"],"series_name":"Visual","design":"code","script":"dockerfile_lint.py","date":"2028-08-15","files":{"devsecops":"notes/p295-a-dockerfile-linter-in-python.md"},"md":"# A Dockerfile linter in Python\n\nEvery Dockerfile review produces the same six comments, and they are mechanical enough that a script should make them instead of a tired reviewer at the end of a sprint.\n\n## Quick reference\n\nThe full script is at the end of this note: `dockerfile_lint.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 dockerfile_lint.py Dockerfile\n```\n\n**Example output**\n\n```text\nDockerfile:1: FROM without a tag\n3 findings in Dockerfile\n```\n\n## Explained\n\n### What it checks\n\nThe linter reads the file line by line, joins continuation lines, and applies six rules. FROM must carry a tag or a digest. A USER must exist and must not be root. ADD is flagged where COPY would do. A HEALTHCHECK must exist. apt-get install must pass `--no-install-recommends`. An ENV or ARG whose name looks like a secret must not carry a value.\n\n### How it works\n\nIt ignores comments and blank lines, tracks the last USER and whether a HEALTHCHECK appeared, and prints each finding with a rule code and a line number when there is one. It also handles FROM `--platform` and multi-stage references, so a build stage is not mistaken for an unpinned image. Exit 0 is clean, 1 is findings, 2 is a file it could not read. The full script is attached: standard library only, no daemon, no writes.\n\n> **The honest caveat:** this is a text linter. It cannot see what a tag resolves to today, or a secret copied in with COPY; it catches the cheap mistakes so a human can look at the expensive ones.\n\n> **The lesson:** a rule you can write down is a rule you can automate. Put it in CI and the review is about design, not punctuation.\n\n## In one line\n\n> **Catch it before the build does.** A Dockerfile review is six checks and a decision.\n\n**Add it to ci**\n\n```bash\npython3 dockerfile_lint.py Dockerfile\n```\n\n## Script: dockerfile_lint.py\n\nDownload: [scripts/dockerfile_lint.py](../scripts/dockerfile_lint.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"dockerfile_lint.py - six line-based checks on a Dockerfile.\n\nChecks:\n    DF001  FROM without a tag or digest\n    DF002  no USER instruction, or USER is root\n    DF003  ADD where COPY would do\n    DF004  no HEALTHCHECK, or HEALTHCHECK NONE\n    DF005  apt-get install without --no-install-recommends\n    DF006  a secret-looking ENV or ARG that carries a value\n\nUsage:\n    python3 dockerfile_lint.py Dockerfile\n    python3 dockerfile_lint.py deploy/Dockerfile api/Dockerfile\n    python3 dockerfile_lint.py --quiet Dockerfile\n    python3 dockerfile_lint.py --list-rules\n\nCI (before the build):\n    python3 dockerfile_lint.py Dockerfile || exit 1\n    docker build -t api:1.0 .\n\nCron (weekly review of every Dockerfile in a tree):\n    0 6 * * 1 find /srv/app -name Dockerfile -print0 | xargs -0 python3 /opt/scripts/dockerfile_lint.py -q\n\nExit codes: 0 clean, 1 findings, 2 usage or input error.\nRead-only: every file is opened for reading and nothing is written.\nLine-based: continuation lines are joined, and comments are skipped.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nRULES = (\n    (\"DF001\", \"FROM without a tag or digest\"),\n    (\"DF002\", \"no USER instruction, or USER is root\"),\n    (\"DF003\", \"ADD where COPY would do\"),\n    (\"DF004\", \"no HEALTHCHECK, or HEALTHCHECK NONE\"),\n    (\"DF005\", \"apt-get install without --no-install-recommends\"),\n    (\"DF006\", \"a secret-looking ENV or ARG that carries a value\"),\n)\n\nSECRET = re.compile(\n    r\"SECRET|TOKEN|PASSWORD|PASSWD|(?:^|_)PASS(?:$|_)|API_?KEY|ACCESS_KEY\"\n    r\"|PRIVATE_KEY|CREDENTIAL\", re.I)\nAPT = re.compile(r\"\\bapt(?:-get)?\\b.*\\binstall\\b\", re.I)\nINSTRUCTION = re.compile(r\"(\\S+)\\s*(.*)\")\n\n\ndef logical_lines(path):\n    \"\"\"Yield (first line number, instruction) with continuations joined.\"\"\"\n    with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n        buffer, start = \"\", None\n        for number, raw in enumerate(handle, 1):\n            stripped = raw.rstrip(\"\\r\\n\").strip()\n            if start is None:\n                if not stripped or stripped.startswith(\"#\"):\n                    continue\n                start = number\n                buffer = stripped\n            else:\n                if not stripped or stripped.startswith(\"#\"):\n                    continue\n                buffer = buffer + \" \" + stripped\n            if buffer.endswith(\"\\\\\"):\n                buffer = buffer[:-1].rstrip()\n                continue\n            yield start, buffer\n            buffer, start = \"\", None\n        if start is not None:\n            yield start, buffer\n\n\ndef parse(instruction):\n    \"\"\"Return (KEYWORD, rest) for one instruction.\"\"\"\n    match = INSTRUCTION.match(instruction.strip())\n    if not match:\n        return \"\", \"\"\n    return match.group(1).upper(), match.group(2).strip()\n\n\ndef image_and_stage(rest):\n    \"\"\"Return (image, stage) from a FROM line, ignoring --platform flags.\"\"\"\n    parts = rest.split()\n    image = \"\"\n    for part in parts:\n        if not part.startswith(\"-\"):\n            image = part\n            break\n    stage = \"\"\n    for index, part in enumerate(parts):\n        if part.upper() == \"AS\" and index + 1 < len(parts):\n            stage = parts[index + 1]\n    return image, stage\n\n\ndef is_pinned(image, stages):\n    \"\"\"True when image is a digest, a local stage name, or scratch.\"\"\"\n    if not image:\n        return True\n    if image.lower() == \"scratch\" or image in stages:\n        return True\n    if \"@\" in image:\n        return True\n    return \":\" in image.rsplit(\"/\", 1)[-1]\n\n\ndef assignments(rest):\n    \"\"\"Yield (name, value) from the arguments of ENV or ARG.\"\"\"\n    tokens = rest.split()\n    if not tokens:\n        return\n    if \"=\" not in tokens[0]:\n        yield tokens[0], \" \".join(tokens[1:])\n        return\n    for token in tokens:\n        if \"=\" not in token:\n            continue\n        name, _, value = token.partition(\"=\")\n        yield name, value\n\n\ndef add_is_copy(rest):\n    \"\"\"True when ADD only moves local files, so COPY would do.\"\"\"\n    parts = [part for part in rest.split() if not part.startswith(\"--\")]\n    if len(parts) < 2:\n        return False\n    for source in parts[:-1]:\n        low = source.lower()\n        if low.startswith((\"http://\", \"https://\")):\n            return False\n        if low.endswith((\".tar\", \".tar.gz\", \".tgz\", \".tar.bz2\", \".tar.xz\", \".zip\")):\n            return False\n    return True\n\n\ndef secret_finding(number, rest, keyword):\n    \"\"\"Return one DF006 finding, or None.\"\"\"\n    for name, value in assignments(rest):\n        if not value.strip().strip(\"'\\\"\"):\n            continue\n        if SECRET.search(name):\n            return (number, \"DF006\",\n                    \"%s %s carries a value; use a BuildKit secret\" % (keyword, name))\n    return None\n\n\ndef check_file(path):\n    \"\"\"Return a list of (line or None, code, message) for one Dockerfile.\"\"\"\n    findings = []\n    stages = set()\n    user, user_line = None, None\n    health = None\n\n    for number, instruction in logical_lines(path):\n        keyword, rest = parse(instruction)\n        flat = \" \".join(rest.split())\n\n        if keyword == \"FROM\":\n            image, stage = image_and_stage(rest)\n            if not is_pinned(image, stages):\n                findings.append((number, \"DF001\",\n                                 \"FROM without a tag or digest: %s\" % image))\n            if stage:\n                stages.add(stage)\n        elif keyword == \"USER\":\n            user, user_line = flat, number\n        elif keyword == \"HEALTHCHECK\":\n            health = (number, flat.upper() == \"NONE\")\n        elif keyword == \"ADD\" and add_is_copy(rest):\n            findings.append((number, \"DF003\",\n                             \"ADD where COPY would do: %s\" % flat[:40]))\n        elif keyword in (\"ENV\", \"ARG\"):\n            hit = secret_finding(number, rest, keyword)\n            if hit:\n                findings.append(hit)\n        elif APT.search(flat) and \"--no-install-recommends\" not in flat:\n            findings.append((number, \"DF005\",\n                             \"apt-get install without --no-install-recommends\"))\n\n    if health is None:\n        findings.append((None, \"DF004\", \"no HEALTHCHECK instruction\"))\n    elif health[1]:\n        findings.append((health[0], \"DF004\", \"HEALTHCHECK NONE disables the check\"))\n\n    if user is None:\n        findings.append((None, \"DF002\",\n                         \"no USER instruction; the container runs as root\"))\n    elif user.split(\":\")[0].strip().lower() in (\"root\", \"0\"):\n        findings.append((user_line, \"DF002\", \"USER is root: %s\" % user))\n\n    findings.sort(key=lambda item: (item[0] is None, item[0] or 0, item[1]))\n    return findings\n\n\ndef show(path, findings, quiet):\n    \"\"\"Print findings and the per-file summary.\"\"\"\n    if not quiet:\n        for line, code, message in findings:\n            where = \"%s:%d\" % (path, line) if line else path\n            print(\"%s: %s %s\" % (where, code, message))\n    if findings:\n        print(\"%d finding%s in %s\"\n              % (len(findings), \"\" if len(findings) == 1 else \"s\", path))\n    else:\n        print(\"%s: OK\" % path)\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        prog=\"dockerfile_lint.py\",\n        description=\"Six line-based checks on a Dockerfile.\",\n        epilog=\"Read-only. Exit 0 clean, 1 findings, 2 usage or input error.\",\n    )\n    parser.add_argument(\"files\", nargs=\"*\", metavar=\"DOCKERFILE\",\n                        help=\"Dockerfile to check\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    parser.add_argument(\"--list-rules\", action=\"store_true\",\n                        help=\"print the rule table and exit\")\n    args = parser.parse_args()\n\n    if args.list_rules:\n        for code, title in RULES:\n            print(\"%s  %s\" % (code, title))\n        return 0\n    if not args.files:\n        parser.error(\"give at least one Dockerfile, or --list-rules\")\n\n    total, broken = 0, False\n    for path in args.files:\n        try:\n            findings = check_file(path)\n        except OSError as exc:\n            print(\"error: %s: %s\" % (path, exc.strerror or exc), file=sys.stderr)\n            broken = True\n            continue\n        show(path, findings, args.quiet)\n        total += len(findings)\n\n    if broken:\n        return 2\n    return 1 if total else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p296","n":"296","slug":"the-terraform-command-cheat-sheet","title":"The Terraform command cheat sheet","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["Terraform","IaC","DevOps","CloudComputing","CICD"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-08-17","files":{"devsecops":"notes/p296-the-terraform-command-cheat-sheet.md"},"md":"# The Terraform command cheat sheet\n\nThe plan says three to add, none to change and one to destroy, and everyone scrolls to the bottom. Terraform is four verbs and twenty commands, and the state file decides whether you sleep.\n\n## Quick reference\n\n### Init\n\n```bash\nterraform init\nterraform init -upgrade\nterraform init -reconfigure\nterraform providers\nterraform version\n```\n\n### Plan\n\n```bash\nterraform plan -out=tfplan\nterraform plan -var-file=prod.tfvars\nterraform show -no-color tfplan\nterraform validate\nterraform fmt -check -recursive\n```\n\n### State\n\n```bash\nterraform state list\nterraform state show aws_instance.api\nterraform state pull\nterraform state mv module.old module.new\nterraform apply -refresh-only\n```\n\n### Import\n\n```bash\nterraform import aws_s3_bucket.logs logs\nterraform plan -generate-config-out=gen.tf\nterraform state list | grep logs\nterraform plan -target=aws_s3_bucket.logs\nterraform state rm aws_s3_bucket.logs\n```\n\n## Explained\n\n### Init\n\nInit prepares the directory: downloads providers, writes the lock file. -upgrade moves to the newest allowed versions, -reconfigure forgets a moved backend. `terraform providers` lists what is installed, and `terraform version` names the binary.\n\n### Plan\n\nPlan shows what would change. -out=tfplan records the decision for the apply, -var-file picks production values. show -no-color prints it for a pull request, validate catches type errors, and fmt -check -recursive fails CI on unformatted files.\n\n### State\n\nState maps configuration to real resources. state list names the addresses, state show reads one, state pull prints the file. state mv moves an address without touching the cloud, and apply -refresh-only updates state from reality without changing infrastructure.\n\n### Import\n\nImport adopts what already exists. `terraform import aws_s3_bucket.logs logs` writes it into state, plan -generate-config-out=`gen.tf` writes matching HCL, state list | grep logs confirms it, and state rm drops it from state without deleting the bucket.\n\n> **The honest caveat:** -target is for recovery, not routine work. A targeted plan ignores everything outside the target, so a green run says little about the whole configuration.\n\n> **The lesson:** the plan is the review and the state is the database. Treat both with the care that implies.\n\n## In one line\n\n> **Plan first. Then apply.** The plan is the review; the state is the database.\n\n**Review the plan**\n\n```bash\nterraform show -no-color tfplan\n```\n"},{"id":"p297","n":"297","slug":"the-observability-pipeline-drawn","title":"The observability pipeline, drawn","sections":["devsecops"],"topic":"Observability & SRE","tags":["Observability","OpenTelemetry","Prometheus","SRE","DevOps"],"series_name":"Visual","design":"architecture","script":"","date":"2028-08-20","files":{"devsecops":"notes/p297-the-observability-pipeline-drawn.md"},"md":"# The observability pipeline, drawn\n\nAn alert fires, the dashboard is green, and the trace nobody kept would have answered it in a minute. The observability pipeline is three tiers, and the signal can die in any of them.\n\n## Quick reference\n\n- **Signals:** App /metrics (APP) · node exporter (NODE) · kubelet (KUBE)\n- **Pipeline:** OTel Collector (OTEL) · Prometheus (TSDB) · Loki (LOGS) · Tempo (TRACE)\n- **Action:** Grafana (UI) · Alertmanager (PAGE)\n\n**Request flow:** App /metrics → OTel Collector → Prometheus → Grafana; Prometheus → Alertmanager; node exporter → OTel Collector; kubelet → OTel Collector; Loki → Grafana; Tempo → Grafana\n\n## Explained\n\n### Signals\n\nSignals come from three places. The app exposes /metrics on its own port, node exporter reports the host, and kubelet reports pods and containers. All three are pull endpoints, which is why a missing signal looks like a down target rather than silence. Each is scraped on a fixed interval, so the freshest number you have is always one scrape old.\n\n### Pipeline\n\nThe pipeline starts at the OTel Collector, which receives every signal and batches it before the stores. Prometheus stores metrics, Loki stores logs, Tempo stores traces. Each keeps its own retention and query language, which is why one incident needs three different queries.\n\n### Action\n\nAction is Grafana and Alertmanager. Grafana queries all three and can put a trace ID next to the log line that explains it. Alertmanager takes what Prometheus fires, deduplicates, inhibits and routes it to a person.\n\n> **The honest caveat:** sampling decides what you can answer later. A one percent trace sample keeps the shape of the tail, not the one request the customer rang about.\n\n> **The lesson:** metrics say something is wrong, traces say where, logs say why. A pipeline that loses the link between them is three separate tools.\n\n## In one line\n\n> **Every signal has a leak.** The gap between a metric and a page is where incidents hide.\n\n**Check the collector**\n\n```bash\ncurl -s localhost:8888/metrics\n```\n"},{"id":"p298","n":"298","slug":"the-four-kubernetes-objects-you-actually-use-drawn","title":"The four Kubernetes objects you actually use, drawn","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","CloudNative","DevOps","CKA","PlatformEngineering"],"series_name":"Visual","design":"explainer","script":"","date":"2028-08-22","files":{"devsecops":"notes/p298-the-four-kubernetes-objects-you-actually-use-drawn.md"},"md":"# The four Kubernetes objects you actually use, drawn\n\nA new team asks where to put the port, the TLS certificate and the database URL. Four objects answer most of it, and every other kind is a variation.\n\n## Quick reference\n\n### Deployment\n\n- Runs pods and replaces them when they die\n- Rolling update, then a clean rollback\n- Replicas live here, not in the pod\n- Pod names are random on purpose\n\n### Service\n\n- One stable name and IP for the pods\n- Label selector decides the endpoints\n- ClusterIP inside, LoadBalancer outside\n- kube-proxy programs the routing rules\n\n### Ingress\n\n- Routes HTTP requests by host and path\n- Needs a controller, or it does nothing\n- TLS usually ends at the controller\n- One entry point for many services\n\n### ConfigMap / Secret\n\n- ConfigMap holds plain settings\n- Secret holds base64, not encryption\n- Mount as files or inject as env vars\n- Editing one does not restart pods\n\n> **Note:** A Secret is base64, not encryption. Turn on encryption at rest and keep RBAC tight.\n\n## Explained\n\n### Deployment\n\nA Deployment runs pods and replaces them. You declare an image and a replica count; the controller creates a ReplicaSet, which creates pods with random names, and a rolling update swaps them without downtime. Rollback is a revision, not a rebuild.\n\n### Service\n\nA Service gives those pods one stable name and IP. The label selector decides which pods are endpoints, and kube-proxy programs the routing. ClusterIP is internal, NodePort opens a port on every node, and LoadBalancer asks the cloud for one.\n\n### Ingress\n\nAn Ingress routes HTTP by host and path to services. It is only a spec: without a controller running, it does nothing. TLS usually ends at the controller, not at the pod.\n\n### ConfigMap and Secret\n\nA ConfigMap holds plain settings; a Secret holds base64 values Kubernetes treats more carefully. Both mount as files or inject as environment variables, and changing either does not restart a running pod.\n\n> **The honest caveat:** a Secret is encoded, not encrypted. Encryption at rest and RBAC are separate settings, and a readable namespace turns a Secret into a password with extra steps.\n\n> **The lesson:** each object answers one question. What runs, how it is reached, how it is routed, and what it is told.\n\n## In one line\n\n> **Four objects. Most of the cluster.** Learn these and every other kind is a variation.\n\n**List the four**\n\n```bash\nkubectl get deploy,svc,ing,cm -A\n```\n"},{"id":"p299","n":"299","slug":"5-ci-cd-mistakes-and-the-fix-for-each","title":"5 CI/CD mistakes and the fix for each","sections":["devsecops"],"topic":"Git & CI/CD","tags":["CICD","DevOps","DevSecOps","GitOps","SRE"],"series_name":"Visual","design":"dontdo","script":"","date":"2028-08-24","files":{"devsecops":"notes/p299-5-ci-cd-mistakes-and-the-fix-for-each.md"},"md":"# 5 CI/CD mistakes and the fix for each\n\nThe pipeline is green, the release is broken, and the post-mortem finds the same five shortcuts. None of them looked like a mistake on the day.\n\n## Quick reference\n\n**1.** ❌ **Hotfix built on a laptop** — Nothing records the compiler, the versions or the person.  \n✅ **One pipeline for every change** — Run the emergency through it; a bypass is untested.\n\n**2.** ❌ **Rebuild the image per stage** — Staging tested a different binary from the one that ships.  \n✅ **Build once, promote the digest** — The same artefact moves through every environment.\n\n**3.** ❌ **Secrets echoed in job logs** — A debug echo puts the token in a log kept for ninety days.  \n✅ **Mask every secret value** — Mask values in CI and keep them in the secret store.\n\n**4.** ❌ **Deploy job runs as cluster-admin** — The runner can delete every namespace it can see.  \n✅ **Namespace-scoped deploy role** — One namespace and one verb set is enough to deploy.\n\n**5.** ❌ **Retry flaky tests until green** — The retry hides a race that appears under production load.  \n✅ **Quarantine and fix the test** — Quarantine it, fix it, or delete it; do not normalise red.\n\n## Explained\n\n### The five\n\n1. **A hotfix built on a laptop.** Nothing records the compiler, the dependency versions or the person, so the binary cannot be rebuilt. Run the emergency through the pipeline; a bypass is a second process nobody tests.\n   \\2. The image rebuilt per stage. Staging built one image and production built another from the same commit, and only one was tested. Build once, promote the digest.\n   \\3. Secrets echoed in job logs. A debug step prints the environment, the token lands in a log kept for ninety days, and the leak is one search away. Mask values and use the secret store.\n   \\4. A deploy job with cluster-admin. The runner is the most valuable credential in the building and can delete every namespace. Scope it to one namespace and one verb set.\n   \\5. Flaky tests retried until green. The retry hides a race that appears under load in production. Quarantine the test, fix it or delete it, but do not teach the team that green means nothing.\n\n### Why they survive\n\nEach one saves a minute today and spends an hour in the incident review later.\n\n> **The honest caveat:** a retry is right for a network timeout. The mistake is a retry with no ticket, no owner and no count.\n\n> **The lesson:** a pipeline is a series of promises. Every shortcut breaks one, and production collects the debt.\n\n## In one line\n\n> **The shortcut is the incident.** Every skipped step is a future 2 a.m. page.\n\n**Trace the run**\n\n```bash\ngh run list --limit 5\n```\n"},{"id":"p300","n":"300","slug":"a-git-repository-health-report-in-python","title":"A git repository health report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Git","Python","DevOps","SRE","Engineering"],"series_name":"Visual","design":"code","script":"git_health.py","date":"2028-08-27","files":{"devsecops":"notes/p300-a-git-repository-health-report-in-python.md"},"md":"# A git repository health report in Python\n\nOne person knows where the deploy scripts live, and they are on holiday. A repository's health is measurable: who commits, how often, and how long since anyone did.\n\n## Quick reference\n\nThe full script is at the end of this note: `git_health.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 git_health.py\n```\n\n**Example output**\n\n```text\nbus factor 1 of 3 authors\nFAIL: bus factor is 1\n```\n\n## Explained\n\n### What it reports\n\nThe script runs git log with `--pretty=format:'%an|%ad' --date=short`, or reads the same text from a file. The full script is attached: standard library only, read-only. It prints commits per month for the last six months, the top three authors with their share, the bus factor, and the age of the last commit.\n\n### Bus factor\n\nThe bus factor is the smallest number of authors whose commits together exceed half the total. One name above fifty percent means a bus factor of one, and that is where a hiring plan should start.\n\n### Exit codes\n\nExit 0 when the bus factor is at least two and the last commit is inside ninety days, exit 1 when either fails, exit 2 when git cannot read the repository. Exit 2 is not a finding: it means the tool could not answer, and a pipeline should treat it as broken. Run it monthly from cron, or in CI on the default branch. The age limit is yours to set with `--stale-days`.\n\n> **The honest caveat:** commit counts measure activity, not knowledge. The person who reviews every change may never appear in git log, and merge commits inflate whoever pressed the button.\n\n> **The lesson:** a repository with one author is a single point of failure that no dashboard shows.\n\n## In one line\n\n> **One author is a single point.** A repository is a team asset, not a personal one.\n\n**Run it monthly**\n\n```bash\n0 9 1 * * python3 git_health.py /srv/repo\n```\n\n## Script: git_health.py\n\nDownload: [scripts/git_health.py](../scripts/git_health.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"git_health.py - a one-page health report for a Git repository.\n\nRuns `git log --pretty=format:'%an|%ad' --date=short` (or reads the same\ntext from a saved file) and reports:\n\n    - commits per month for the last N months\n    - the top authors and their share of commits\n    - the bus factor: the smallest number of authors whose commits\n      together exceed half of the total\n    - the age of the last commit\n\nExit codes: 0 healthy, 1 bus factor is 1 or the repository is stale,\n2 usage or input error (including a repository with no commits).\nRead-only: it runs git log and reads one file; nothing is written.\n\nUsage:\n    python3 git_health.py\n    python3 git_health.py /srv/app\n    python3 git_health.py /srv/app --months 12 --stale-days 120\n    python3 git_health.py --log saved.log --top 5\n    python3 git_health.py /srv/app --quiet\n\nSave the log once and report on it later:\n    git log --pretty=format:'%an|%ad' --date=short > /tmp/git.log\n    python3 git_health.py --log /tmp/git.log\n\nCI (fail a job on a lone maintainer or a stale default branch):\n    python3 git_health.py . || exit 1\n\nCron (first of the month, 09:00):\n    0 9 1 * * python3 /opt/scripts/git_health.py /srv/app >> /var/log/git_health.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport os\nimport subprocess\nimport sys\nfrom collections import Counter\nfrom datetime import date\n\nLOG_FORMAT = \"%an|%ad\"\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_git_log(repo):\n    \"\"\"Return the raw git log text for repo, or raise InputError.\"\"\"\n    command = [\"git\", \"-C\", repo, \"log\",\n               \"--pretty=format:\" + LOG_FORMAT, \"--date=short\"]\n    try:\n        proc = subprocess.run(command, capture_output=True, text=True, timeout=120)\n    except FileNotFoundError:\n        raise InputError(\"git is not installed or not on PATH\")\n    except OSError as exc:\n        raise InputError(\"could not run git: %s\" % exc)\n    except subprocess.TimeoutExpired:\n        raise InputError(\"git log timed out after 120 seconds\")\n    if proc.returncode != 0:\n        message = (proc.stderr or \"\").strip().splitlines()\n        raise InputError(message[-1] if message else \"git log failed\")\n    return proc.stdout\n\n\ndef parse_log(text):\n    \"\"\"Return [(author, ISO date)] from author|date lines, or raise InputError.\"\"\"\n    rows = []\n    for number, line in enumerate(text.splitlines(), 1):\n        if not line.strip():\n            continue\n        if \"|\" not in line:\n            raise InputError(\"line %d is not author|date: %s\" % (number, line[:60]))\n        author, _, when = line.rpartition(\"|\")\n        author, when = author.strip(), when.strip()[:10]\n        if not author:\n            raise InputError(\"line %d has no author\" % number)\n        try:\n            date.fromisoformat(when)\n        except ValueError:\n            raise InputError(\"line %d has no ISO date: %s\" % (number, when))\n        rows.append((author, when))\n    if not rows:\n        raise InputError(\"no commits found\")\n    return rows\n\n\ndef months_back(today, count):\n    \"\"\"Return the last count months as YYYY-MM strings, oldest first.\"\"\"\n    year, month = today.year, today.month\n    out = []\n    for _ in range(count):\n        out.append(\"%04d-%02d\" % (year, month))\n        month -= 1\n        if month == 0:\n            year, month = year - 1, 12\n    return list(reversed(out))\n\n\ndef bus_factor(counts, total):\n    \"\"\"Smallest number of top authors whose commits exceed half of total.\"\"\"\n    running = 0\n    for index, (_, count) in enumerate(counts, 1):\n        running += count\n        if running * 2 > total:\n            return index\n    return 0\n\n\ndef age_text(days):\n    \"\"\"Return a short English age for the last commit.\"\"\"\n    if days <= 0:\n        return \"today\"\n    if days == 1:\n        return \"1 day ago\"\n    return \"%d days ago\" % days\n\n\ndef report(args, rows):\n    \"\"\"Print the report and return the exit code.\"\"\"\n    total = len(rows)\n    counts = Counter(author for author, _ in rows).most_common()\n    last = max(when for _, when in rows)\n    age = (date.today() - date.fromisoformat(last)).days\n    if age < 0:\n        age = 0\n    bus = bus_factor(counts, total)\n\n    bus_line = \"%d of %d author%s\" % (bus, len(counts),\n                                      \"\" if len(counts) == 1 else \"s\")\n    source = args.log if args.log else os.path.abspath(args.repo)\n    print(\"git_health: %s\" % source)\n    for label, value in ((\"commits\", total),\n                         (\"authors\", len(counts)),\n                         (\"last commit\", \"%s (%s)\" % (last, age_text(age))),\n                         (\"bus factor\", bus_line)):\n        print(\"%-11s %s\" % (label, value))\n\n    if not args.quiet:\n        per_month = Counter(when[:7] for _, when in rows)\n        window = months_back(date.today(), args.months)\n        peak = max([per_month.get(month, 0) for month in window] or [1]) or 1\n        print(\"\\nCommits per month (last %d)\" % args.months)\n        for month in window:\n            count = per_month.get(month, 0)\n            bar = \"#\" * max(1, round(20 * count / peak)) if count else \"\"\n            print(\"%s  %-20s %d\" % (month, bar, count))\n\n        print(\"\\nTop authors\")\n        for index, (author, count) in enumerate(counts[:args.top], 1):\n            share = round(100 * count / total)\n            print(\"%d. %-22s %4d  %3d%%\" % (index, author[:22], count, share))\n\n    failures = []\n    if bus < 2:\n        failures.append(\"bus factor is %d\" % bus)\n    if age > args.stale_days:\n        failures.append(\"last commit is %d days old (limit %d)\"\n                        % (age, args.stale_days))\n    if failures:\n        for failure in failures:\n            print(\"FAIL: %s\" % failure)\n        return 1\n    print(\"OK: bus factor %d, last commit %s\" % (bus, age_text(age)))\n    return 0\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        prog=\"git_health.py\",\n        description=\"Report commits per month, top authors, bus factor and last commit age.\",\n        epilog=\"Read-only. Exit 0 healthy, 1 bus factor 1 or stale, 2 input error.\",\n    )\n    parser.add_argument(\"repo\", nargs=\"?\", default=\".\",\n                        help=\"repository path (default: current directory)\")\n    parser.add_argument(\"-l\", \"--log\", metavar=\"FILE\",\n                        help=\"read a saved git log (author|date lines) instead of running git\")\n    parser.add_argument(\"-m\", \"--months\", type=int, default=6,\n                        help=\"months to show (default 6)\")\n    parser.add_argument(\"-t\", \"--top\", type=int, default=3,\n                        help=\"authors to list (default 3)\")\n    parser.add_argument(\"-s\", \"--stale-days\", type=int, default=90,\n                        help=\"fail when the last commit is older than this (default 90)\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args()\n\n    if args.months < 1:\n        parser.error(\"--months must be at least 1\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n    if args.stale_days < 0:\n        parser.error(\"--stale-days cannot be negative\")\n    if not args.log and not os.path.isdir(args.repo):\n        print(\"error: not a directory: %s\" % args.repo, file=sys.stderr)\n        return 2\n\n    try:\n        if args.log:\n            try:\n                with open(args.log, encoding=\"utf-8\", errors=\"replace\") as handle:\n                    raw = handle.read()\n            except OSError as exc:\n                raise InputError(\"cannot read %s: %s\" % (args.log, exc.strerror or exc))\n        else:\n            raw = read_git_log(args.repo)\n        rows = parse_log(raw)\n    except InputError as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n    return report(args, rows)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p301","n":"301","slug":"quiz-3-toolchain-questions","title":"Quiz: 3 toolchain questions","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","Kubernetes","Git","CICD","SRE"],"series_name":"Visual","design":"quiz","script":"","date":"2028-08-29","files":{"devsecops":"notes/p301-quiz-3-toolchain-questions.md"},"md":"# Quiz: 3 toolchain questions\n\nThree questions come up in every toolchain review, and the answers are shorter than the arguments. Each one is really about ownership.\n\n## Quick reference\n\n**Q1. Your branch is shared and already pushed. Merge or rebase onto main?**\n\n- a) Rebase, then force-push\n- b) Merge main into the branch\n- c) Delete the branch and start again\n\n> **Answer:** b) Merge main into the branch. Rebase rewrites hashes other people already pulled. Merge adds a commit; nobody has to recover, and the conflict is resolved once.\n\n**Q2. Why pin an image by digest instead of a tag?**\n\n- a) Digests pull faster\n- b) A digest cannot be repointed\n- c) Tags expire after thirty days\n\n> **Answer:** b) A digest cannot be repointed. A tag can move to a new image. The digest is the exact bytes you scanned, signed and ran, so a rollback returns to those bytes.\n\n**Q3. What does a readiness probe actually protect?**\n\n- a) The node from too many pods\n- b) Traffic from a pod that cannot serve\n- c) The container from high CPU\n\n> **Answer:** b) Traffic from a pod that cannot serve. A failing readiness probe removes the pod from Service endpoints until it passes. Liveness is the one that restarts the container.\n\n## Explained\n\n### Shared branches\n\nA shared branch, already pushed: merge or rebase? Merge. The branch is public, so its commits belong to other people too, and a rebase rewrites hashes they already pulled. Merge main into the branch, resolve once, push. Rebase your own unpushed work as much as you like, because nobody else has those hashes.\n\n### Image digests\n\nWhy pin an image by digest? Because a tag is a pointer that can move. The digest is the exact bytes: the ones you scanned, signed and ran. Repoint the tag tomorrow and every signature stays valid for a different image, which is why a tag is not an identity.\n\n### Readiness probes\n\nWhat does a readiness probe protect? Traffic, not the container. A failing readiness probe removes the pod from Service endpoints, so no request arrives until it can serve. A liveness probe restarts the container; confusing the two turns a slow start into a restart loop that never finishes.\n\n> **The honest caveat:** digest pinning needs an update process. Pinned images go stale, so pair the pin with a bot that opens a pull request when the tag moves.\n\n> **The lesson:** each answer is about who owns the consequence. The shared branch, the signed bytes, the traffic.\n\n## In one line\n\n> **Know the why, not just the flag.** Three answers that explain most toolchain arguments.\n\n**Read the field**\n\n```bash\nkubectl explain pod.spec.readinessProbe\n```\n"},{"id":"p302","n":"302","slug":"zero-trust-architecture-drawn","title":"Zero-trust architecture, drawn","sections":["it","devsecops"],"topic":"Security & hardening","tags":["ZeroTrust","CyberSecurity","CloudSecurity","Identity","DevSecOps"],"series_name":"Visual","design":"architecture","script":"","date":"2028-08-31","files":{"it":"notes/p302-zero-trust-architecture-drawn.md","devsecops":"notes/p302-zero-trust-architecture-drawn.md"},"md":"# Zero-trust architecture, drawn\n\nA laptop on hotel Wi-Fi asks for the finance database. The VPN says yes because the laptop is on the network, and that is the whole problem with perimeter security.\n\n## Quick reference\n\n- **Tier 1 · Identity:** Identity provider (IDP) · MFA (MFA) · Device posture (DEV)\n- **Tier 2 · Policy:** Policy decision point (PDP) · Policy enforcement (PEP)\n- **Tier 3 · Resources:** Applications (APP) · Data stores (DATA) · APIs (API)\n\n**Request flow:** Identity provider → MFA → Device posture → Policy decision point → Policy enforcement → Applications → Data stores; Applications → APIs\n\n## Explained\n\n### Identity\n\nThe identity provider authenticates the user and issues a short-lived token. MFA proves the human at the keyboard. Device posture answers a different question: is this machine patched, encrypted and enrolled? A stolen password on an unmanaged laptop fails that second check without anyone reading a log.\n\n### Policy\n\nThe policy decision point evaluates the token, the device, the resource and the context, then returns allow or deny. The policy enforcement point sits in front of the resource and applies the answer. The decision is made per request, not once per session, so a revoked device loses access on the next call.\n\n### Resources\n\nApplications, data stores and APIs are the resources. Each one is reached through an enforcement point, not through a flat network. The arrows matter: identity feeds policy, policy guards resources, and the resource tier is no longer a trusted segment. Every arrow is a control point.\n\n> **The honest caveat:** this moves the single point of failure to the identity provider. If it is down, nothing authenticates; if it is compromised, every policy decision is made by the attacker.\n\n> **The lesson:** the network stops being the trust boundary, and identity takes its place.\n\n## In one line\n\n> **The network is not the boundary.** Identity is the control plane for every request.\n\n**Trace one request**\n\n```bash\ncurl -sv https://example.com\n```\n"},{"id":"p303","n":"303","slug":"the-tls-handshake-drawn","title":"The TLS handshake, drawn","sections":["it","devsecops"],"topic":"Web servers, TLS & HTTP","tags":["TLS","Networking","CyberSecurity","HTTPS","DevOps"],"series_name":"Visual","design":"explainer","script":"","date":"2028-09-03","files":{"it":"notes/p303-the-tls-handshake-drawn.md","devsecops":"notes/p303-the-tls-handshake-drawn.md"},"md":"# The TLS handshake, drawn\n\nYou reload nginx with a new certificate and curl returns a handshake error. The failure is in one of four steps, and the error text usually names which. The sequence below is TLS 1.2, the easiest to read in a capture.\n\n## Quick reference\n\n### ClientHello\n\n- Client random, versions, cipher suites\n- SNI carries the hostname in the clear\n- Server picks the version and cipher\n- One round trip before any secret\n\n### Certificate\n\n- Server sends its certificate chain\n- Client checks root, dates, hostname\n- OCSP or CRL answers on revocation\n- A bad chain stops the handshake\n\n### Key exchange\n\n- ECDHE shares are ephemeral\n- Both sides derive the same secret\n- Certificate key signs the exchange\n- Session keys are symmetric\n\n### Finished\n\n- ChangeCipherSpec switches the keys\n- Finished covers the handshake transcript\n- TLS 1.3 does it in one round trip\n- Records are encrypted from here\n\n## Explained\n\n### ClientHello\n\nThe client sends a random value, the TLS versions it supports, its cipher suites, and the server name in SNI. The server answers with its own random value, version and cipher. Nothing is encrypted yet, so the hostname is visible on the wire.\n\n### Certificate\n\nThe server sends its certificate chain. The client checks the chain to a trusted root, the dates, the hostname, and the revocation status through OCSP or a CRL. A missing intermediate or an expired leaf ends the connection before any key is derived.\n\n### Key exchange\n\nWith ECDHE, each side sends an ephemeral public share and both derive the same secret. The certificate signature proves the server holds the private key behind the certificate. Session keys are symmetric, because public-key operations are slow and symmetric ciphers are not.\n\n### Finished\n\nEach side sends ChangeCipherSpec, then a Finished message that covers the handshake transcript, so a modified hello or cipher list breaks the check. From there, every record is encrypted. TLS 1.3 collapses this into one round trip and encrypts the certificate, but the four jobs are the same.\n\n> **The honest caveat:** a valid certificate proves the domain, not the business behind it. Phishing sites have padlocks too.\n\n> **The lesson:** every TLS error belongs to hello, certificate, key exchange or finished.\n\n## In one line\n\n> **A padlock is a finished handshake.** Encryption is the result, not the whole story.\n\n**Inspect a chain**\n\n```bash\nopenssl s_client -connect example.com:443\n```\n"},{"id":"p304","n":"304","slug":"the-security-command-cheat-sheet","title":"The security command cheat sheet","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Linux","CyberSecurity","CommandLine","SysAdmin","DevSecOps"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-09-05","files":{"it":"notes/p304-the-security-command-cheat-sheet.md","devsecops":"notes/p304-the-security-command-cheat-sheet.md"},"md":"# The security command cheat sheet\n\nAt 03:00 the incident channel asks three things at once: what is listening, which key is on the box, and who logged in an hour ago. Twenty commands cover the answers.\n\n## Quick reference\n\n### TLS\n\n```bash\nopenssl s_client -connect example.com:443\nopenssl x509 -in cert.pem -noout -dates\nopenssl verify -CAfile ca.pem cert.pem\nopenssl s_client -tls1_2 -connect host:443\nopenssl s_client -connect host:443 -brief\n```\n\n### Network\n\n```bash\nss -tlnp\nss -tnp state established\nnmap -sT -p- 10.0.0.5\nnmap -sV --top-ports 100 10.0.0.5\ntcpdump -i any -n port 443\n```\n\n### Keys\n\n```bash\nssh-keygen -t ed25519 -C 'me@host'\nssh-keygen -lf id_ed25519.pub\nssh-keygen -y -f id_ed25519\nssh-keygen -p -f id_ed25519\nssh-copy-id -i id_ed25519.pub host\n```\n\n### Audit\n\n```bash\nlastb | head\nwho -a\nlast -a | head -20\njournalctl -u sshd -n 50\ngrep -i 'failed' /var/log/auth.log\n```\n\n## Explained\n\n### TLS\n\n`openssl s_client -connect example.com:443` shows the chain the server actually sends. Add -brief when you only want the summary, or -tls1_2 to prove the old protocol still works. x509 -dates reads the expiry from a file, and verify -CAfile checks a chain against your own CA.\n\n### Network\n\n`ss -tlnp` lists listening sockets with the process that owns them. `ss -tnp state established` shows what is connected now. `nmap -sT -p-` scans every port, and -sV `--top-ports 100` fingerprints the common ones. tcpdump is the last word when the two hosts disagree.\n\n### Keys\n\n`ssh-keygen -t ed25519` creates the key. -lf prints the fingerprint you compare out of band, -y recovers the public key from the private one, and -p changes the passphrase. ssh-copy-id installs it on the host without hand-editing authorized_keys.\n\n### Audit\n\nlastb reads failed logins, `who -a` shows who is on the box now, and `last -a` is the history. `journalctl -u sshd` gives the daemon's own view, and grep on `auth.log` is the fast answer when the journal is gone.\n\n> **The honest caveat:** lastb needs root and `/var/log/btmp`, and on RHEL the log is `/var/log/secure`, not `auth.log`.\n\n> **The lesson:** four areas, five commands each, and most incident questions are answered before the first coffee.\n\n## In one line\n\n> **Type the command, read the answer.** Five per area beats fifty you cannot recall.\n\n**Check a cert**\n\n```bash\nopenssl x509 -in cert.pem -noout -dates\n```\n"},{"id":"p305","n":"305","slug":"an-ssh-configuration-auditor-in-python","title":"An SSH configuration auditor in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["SSH","Python","CyberSecurity","Linux","DevSecOps"],"series_name":"Visual","design":"code","script":"ssh_audit.py","date":"2028-09-07","files":{"it":"notes/p305-an-ssh-configuration-auditor-in-python.md","devsecops":"notes/p305-an-ssh-configuration-auditor-in-python.md"},"md":"# An SSH configuration auditor in Python\n\nYou inherit a jump host and a ~/.ssh directory nobody has read in three years. The config still allows passwords, and one key is an ssh-rsa blob with no comment.\n\n## Quick reference\n\nThe full script is at the end of this note: `ssh_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ssh_audit.py --home /home/deploy\n```\n\n**Example output**\n\n```text\nconfig:4: PasswordAuthentication yes\nauthorized_keys:2: no comment or options\n```\n\n## Explained\n\n### What it checks\n\nThe script reads ~/.ssh/config and ~/.ssh/authorized_keys. It checks the directory at 0700 and both files at 0600, because a world-readable config or key list is a finding on its own. It flags PasswordAuthentication yes and PermitRootLogin yes, DSA and ssh-rsa keys, and entries with no comment or options.\n\n### How it runs\n\n`python3 ssh_audit.py --home /home/deploy` prints one line per finding and exits 1 if there are any, which is what CI needs. Run it weekly from cron, or as the first step of a host build. The default `--home` is your own, so a local run audits the machine you are on.\n\n### What a finding means\n\nA weak key type or an open mode is a fact, not an opinion. A config that permits passwords is a preference the client will offer; the server still decides. Fix the mode with chmod, replace the key, and remove the password fallback from both the client and the server.\n\n> **The honest caveat:** PasswordAuthentication in a client config is a preference, and PermitRootLogin belongs in sshd_config, so a finding is a prompt to check the server file, not proof.\n\n> **The lesson:** your SSH directory is a security control, and nobody reviews it.\n\n## In one line\n\n> **Read the config before you trust it.** The one file nobody reads until an incident.\n\n**Audit a host**\n\n```bash\npython3 ssh_audit.py --home /home/deploy\n```\n\n## Script: ssh_audit.py\n\nDownload: [scripts/ssh_audit.py](../scripts/ssh_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit an SSH client configuration for weak settings and key hygiene.\n\nUsage:\n    python3 ssh_audit.py\n    python3 ssh_audit.py --home /home/deploy\n    python3 ssh_audit.py --home /tmp/ssh_fixture\n\nChecks:\n    - ~/.ssh, ~/.ssh/config and ~/.ssh/authorized_keys permissions\n    - PasswordAuthentication yes and PermitRootLogin yes in the config\n    - DSA (ssh-dss) and ssh-rsa keys in authorized_keys\n    - authorized_keys entries with no comment, including bare keys that\n      carry no options either\n\nExit codes:\n    0  no findings\n    1  at least one finding\n    2  usage or input error\n\nRun it weekly from cron, or as a CI step before a deploy:\n\n    0 6 * * 1 python3 /opt/s0sta/ssh_audit.py --home /home/deploy\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport os\nimport re\nimport sys\n\nKEY_TYPES = (\n    \"ssh-ed25519\",\n    \"ssh-rsa\",\n    \"ssh-dss\",\n    \"ecdsa-sha2-nistp256\",\n    \"ecdsa-sha2-nistp384\",\n    \"ecdsa-sha2-nistp521\",\n    \"sk-ssh-ed25519@openssh.com\",\n    \"sk-ecdsa-sha2-nistp256@openssh.com\",\n)\nWEAK = (\"ssh-rsa\", \"ssh-dss\")\nWEAK_PREFIXES = (\"ssh-rsa-cert-\", \"ssh-dss-cert-\")\n\n\ndef split_entry(raw):\n    \"\"\"Return (options, key_type, comment) for one authorized_keys line.\n\n    OpenSSH allows a comma-separated options field, which may contain quoted\n    strings and spaces, before the key type. Blank lines and comments return\n    None. A malformed line returns whatever could be read.\n    \"\"\"\n    line = raw.strip()\n    if not line or line.startswith(\"#\"):\n        return None\n\n    # Walk to the first unquoted space: that ends the first token.\n    in_quote = False\n    i = 0\n    while i < len(line):\n        ch = line[i]\n        if ch == '\"':\n            in_quote = not in_quote\n        elif ch == \"\\\\\" and in_quote:\n            i += 1\n        elif ch.isspace() and not in_quote:\n            break\n        i += 1\n    first = line[:i]\n    rest = line[i:].strip()\n\n    if first in KEY_TYPES or first.startswith((\"ssh-\", \"ecdsa-\", \"sk-\")):\n        options, key_type, tail = \"\", first, rest\n    else:\n        options = first\n        parts = rest.split(None, 1)\n        if not parts:\n            return options, \"\", \"\"\n        key_type = parts[0]\n        tail = parts[1].strip() if len(parts) > 1 else \"\"\n\n    # tail is \"base64 [comment]\"; the comment may contain spaces.\n    parts = tail.split(None, 1)\n    comment = parts[1].strip() if len(parts) > 1 else \"\"\n    return options, key_type, comment\n\n\ndef check_mode(path, want, findings):\n    try:\n        mode = os.stat(path).st_mode & 0o777\n    except OSError as exc:\n        findings.append(\"{}: cannot stat ({})\".format(path, exc.strerror))\n        return\n    if mode & 0o077:\n        findings.append(\"{}: mode {:04o}, expected {:04o}\".format(path, mode, want))\n\n\ndef audit(home):\n    ssh = os.path.join(home, \".ssh\")\n    findings = []\n    check_mode(ssh, 0o700, findings)\n\n    cfg_path = os.path.join(ssh, \"config\")\n    keys_path = os.path.join(ssh, \"authorized_keys\")\n\n    if os.path.isfile(cfg_path):\n        check_mode(cfg_path, 0o600, findings)\n        with open(cfg_path, \"r\", errors=\"replace\") as fh:\n            for n, raw in enumerate(fh, 1):\n                line = raw.strip()\n                if not line or line.startswith(\"#\"):\n                    continue\n                parts = re.split(r\"\\s*=\\s*|\\s+\", line, 1)\n                if len(parts) != 2:\n                    continue\n                key = parts[0].lower()\n                value = parts[1].split(\"#\", 1)[0].strip().lower()\n                if key == \"passwordauthentication\" and value in (\"yes\", \"true\"):\n                    findings.append(\"config:{}: PasswordAuthentication yes\".format(n))\n                elif key == \"permitrootlogin\" and value in (\"yes\", \"true\"):\n                    findings.append(\"config:{}: PermitRootLogin yes\".format(n))\n\n    if os.path.isfile(keys_path):\n        check_mode(keys_path, 0o600, findings)\n        with open(keys_path, \"r\", errors=\"replace\") as fh:\n            for n, raw in enumerate(fh, 1):\n                entry = split_entry(raw)\n                if entry is None:\n                    continue\n                options, key_type, comment = entry\n                if not key_type:\n                    findings.append(\"authorized_keys:{}: unreadable entry\".format(n))\n                    continue\n                if key_type in WEAK or key_type.startswith(WEAK_PREFIXES):\n                    findings.append(\n                        \"authorized_keys:{}: weak key type {}\".format(n, key_type))\n                if not comment:\n                    if options:\n                        findings.append(\n                            \"authorized_keys:{}: key has no comment\".format(n))\n                    else:\n                        findings.append(\n                            \"authorized_keys:{}: no comment or options\".format(n))\n\n    return findings\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Audit an SSH client configuration for weak settings and \"\n                    \"key hygiene.\")\n    ap.add_argument(\"--home\", default=\"~\",\n                    help=\"home directory to audit (default: ~)\")\n    args = ap.parse_args()\n\n    home = os.path.abspath(os.path.expanduser(args.home))\n    if not os.path.isdir(home):\n        ap.error(\"not a directory: {}\".format(home))\n    ssh = os.path.join(home, \".ssh\")\n    if not os.path.isdir(ssh):\n        ap.error(\"no .ssh directory under {}\".format(home))\n    if not (os.path.isfile(os.path.join(ssh, \"config\"))\n            or os.path.isfile(os.path.join(ssh, \"authorized_keys\"))):\n        ap.error(\"no config or authorized_keys under {}\".format(ssh))\n\n    try:\n        findings = audit(home)\n    except OSError as exc:\n        print(\"error: {}\".format(exc), file=sys.stderr)\n        return 2\n    for line in findings:\n        print(line)\n    if findings:\n        print(\"{} finding(s) in {}\".format(len(findings), ssh))\n        return 1\n    print(\"clean: {}\".format(ssh))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p306","n":"306","slug":"the-software-supply-chain-drawn","title":"The software supply chain, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["SupplyChain","DevSecOps","CICD","SBOM","CloudNative"],"series_name":"Visual","design":"architecture","script":"","date":"2028-09-10","files":{"devsecops":"notes/p306-the-software-supply-chain-drawn.md"},"md":"# The software supply chain, drawn\n\nThe build that shipped on Tuesday pulled a package published that morning from an account with no MFA. The miner ran for six hours before anyone looked at egress.\n\n## Quick reference\n\n- **Tier 1 · Source:** Git commit (GIT) · Dependencies (DEP)\n- **Tier 2 · Build:** CI pipeline (CI) · SBOM (SBOM) · Signing (SIGN)\n- **Tier 3 · Deploy:** Verify signature (VFY) · Admission control (ADMIT) · Runtime monitor (RUN)\n\n**Request flow:** Git commit → Dependencies → CI pipeline → SBOM → Signing → Verify signature → Admission control → Runtime monitor; Git commit → CI pipeline\n\n## Explained\n\n### Source\n\nEvery commit is an input, and every dependency is code you did not write. The source tier has two nodes: the git commit and the dependency set. Pin versions, commit the lockfile, and review a new package the way you review a new colleague. A commit is cheap to make and hard to take back.\n\n### Build\n\nThe CI pipeline runs on that commit, builds the artefact, writes an SBOM, and signs the result. The three nodes are CI, SBOM and signing. The signature is only worth something if the signing identity belongs to the pipeline, not to a laptop. If the SBOM is written after signing, the signature covers less than you think.\n\n### Deploy\n\nVerification checks the signature against the identity you expect. The admission controller refuses an artefact that fails the check, and the runtime monitor watches what the container does after it starts. Verify, admit, run: three nodes and three chances to stop the same attack. The runtime monitor is the only node that watches behaviour, not metadata.\n\n> **The honest caveat:** signing proves who built the artefact, not that the code is safe. A signed malicious dependency is still malicious.\n\n> **The lesson:** trust the artefact you verified, not the network you built it on.\n\n## In one line\n\n> **Build it, sign it, verify it.** Three tiers, and every artefact carries proof.\n\n**Verify an image**\n\n```bash\ncosign verify ghcr.io/acme/api:1.4\n```\n"},{"id":"p307","n":"307","slug":"four-kubernetes-attack-paths-drawn","title":"Four Kubernetes attack paths, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Kubernetes","CyberSecurity","CloudNative","CKS","DevSecOps"],"series_name":"Visual","design":"explainer","script":"","date":"2028-09-12","files":{"devsecops":"notes/p307-four-kubernetes-attack-paths-drawn.md"},"md":"# Four Kubernetes attack paths, drawn\n\nA namespace boundary, a network policy and a non-root user feel like enough. Then one pod is compromised, and four well-known paths decide how far the attacker gets. The cluster looks contained until you draw the arrows.\n\n## Quick reference\n\n### Stolen token\n\n- A token copied from a pod or a repo\n- The API server sees a real identity\n- Bound tokens expire; legacy ones do not\n- No exploit needed, just the token\n\n### Privileged pod\n\n- securityContext.privileged: true\n- Host devices and every capability\n- Container escape gets much easier\n- One manifest flag, root on the node\n\n### hostPath mount\n\n- A hostPath of / mounts the node root\n- Write a static pod manifest\n- The kubelet runs it as root\n- A volume, not an exploit\n\n### Cloud metadata\n\n- Pods reach 169.254.169.254\n- IMDSv1 needs no token at all\n- The node IAM role is the prize\n- IMDSv2 asks for a PUT first\n\n## Explained\n\n### Stolen token\n\nEvery pod mounts a service account token by default. A bound token expires, but a legacy token does not, and a token copied from a repo or a log is a working identity. The API server cannot tell it from the real pod.\n\n### Privileged pod\n\nA pod with securityContext.privileged: true gets host devices and every capability. Escape to the node is a recipe away, and one manifest flag is all it takes. A privileged pod is a node shell with extra steps.\n\nhostPath mount\n\nA hostPath of / mounts the node root inside the container. From there you can write a static pod manifest, and the kubelet runs it as root. That is not an exploit, it is a volume.\n\n### Cloud metadata\n\nFrom the pod, 169.254.169.254 answers. IMDSv1 returns credentials with no token, and IMDSv2 asks for a PUT first. Either way, the node's IAM role is the prize, and it is usually wider than the pod's own service account.\n\n> **The honest caveat:** EKS blocks pod access to IMDS on current AMIs, but self-managed nodes and other clouds often do not.\n\n> **The lesson:** every path ends at the node's identity or the node's filesystem.\n\n## In one line\n\n> **The pod is not the boundary.** Four paths, one destination: the node.\n\n**Check a pod**\n\n```bash\nkubectl get pod api -o yaml\n```\n"},{"id":"p308","n":"308","slug":"the-aws-security-cheat-sheet","title":"The AWS security cheat sheet","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["AWS","CloudSecurity","IAM","DevSecOps","CloudNative"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-09-14","files":{"devsecops":"notes/p308-the-aws-security-cheat-sheet.md"},"md":"# The AWS security cheat sheet\n\nAn alert fires at 02:00, and the first question is not what happened but who did it and with which key. Five commands per area take you from alert to evidence.\n\n## Quick reference\n\n### Identity\n\n```bash\naws sts get-caller-identity\naws iam list-access-keys\naws iam list-mfa-devices --user-name bob\naws iam get-account-password-policy\naws iam get-account-summary\n```\n\n### Keys\n\n```bash\naws kms list-keys\naws kms describe-key --key-id $KEY_ID\naws kms list-aliases --output table\naws kms enable-key-rotation --key-id $ID\naws kms list-grants --key-id $KEY_ID\n```\n\n### Detection\n\n```bash\naws guardduty list-detectors\naws securityhub get-findings\naws cloudtrail lookup-events --max-items 5\naws cloudtrail describe-trails\naws cloudtrail get-trail-status --name $T\n```\n\n### Policy\n\n```bash\naws iam simulate-principal-policy\naws iam get-policy --policy-arn $A\naws iam list-policies --scope Local\naws iam get-role --role-name api\naws iam list-role-policies --role-name x\n```\n\n## Explained\n\n### Identity\n\n`aws sts get-caller-identity` tells you which principal the shell is using, which is often not the one you think. list-access-keys and list-mfa-devices show the credentials attached to a user. get-account-password-policy shows the account rules, and get-account-summary gives the quotas and counts.\n\n### Keys\n\nkms list-keys lists the key IDs, and describe-key shows the key manager and the state. list-aliases maps those IDs to names. enable-key-rotation turns annual rotation on for a symmetric customer managed key, and list-grants shows who else has access.\n\n### Detection\n\nguardduty list-detectors finds the detector IDs, and securityhub get-findings pulls the aggregated findings. cloudtrail lookup-events answers what was called recently; describe-trails and get-trail-status prove logging is on and current. If the trail is off, every other answer is a guess.\n\n### Policy\n\nsimulate-principal-policy answers whether a principal can do an action, though the full call needs `--policy-source-arn` and `--action-names`, so the sheet stops at the subcommand. get-policy reads the metadata behind a policy ARN, list-policies `--scope` Local hides the AWS-managed noise, and get-role plus list-role-policies show what a role carries.\n\n> **The honest caveat:** CloudTrail records management events by default, and S3 object reads need data events, which cost extra.\n\n> **The lesson:** the account already knows what happened, so ask it in the right order.\n\n## In one line\n\n> **Ask the account what happened.** Four areas, five calls, one trail.\n\n**Who am i**\n\n```bash\naws sts get-caller-identity\n```\n"},{"id":"p309","n":"309","slug":"secrets-management-vault-vs-cloud-kms-vs-sops","title":"Secrets management: Vault vs cloud KMS vs SOPS","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["SecretsManagement","Vault","KMS","SOPS","DevSecOps"],"series_name":"Visual","design":"matrix","script":"","date":"2028-09-17","files":{"devsecops":"notes/p309-secrets-management-vault-vs-cloud-kms-vs-sops.md"},"md":"# Secrets management: Vault vs cloud KMS vs SOPS\n\nA service needs a database password. Three teams answer with three tools, and the argument is usually about taste rather than trust boundaries.\n\n## Quick reference\n\n| | **Vault** | **Cloud KMS** | **SOPS** |\n|:--|:--|:--|:--|\n| What it stores | Any secret or key | Keys, not secrets | Encrypted files |\n| Encryption | Barrier, AES-256-GCM | HSM-backed CMK | Age, PGP or KMS |\n| Access model | Token and policy | IAM policy | Key holder only |\n| Audit | Audit devices | CloudTrail | Git diffs |\n| Rotation | Leases and rotation | Annual or manual | Re-encrypt files |\n| Best for | Dynamic secrets | Cloud-native keys | GitOps secrets |\n\n**How to pick**\n\n- **Vault** → dynamic secrets, leases, run it yourself.\n- **KMS** → keys stay in the HSM; you keep the envelope.\n- **SOPS** → encrypted files in git, decrypted at deploy.\n\n## Explained\n\n### What each one is\n\nVault stores secrets and hands them out with a lease. A cloud KMS stores keys and never gives the key material back; you call it to encrypt or decrypt. SOPS encrypts a file so the ciphertext can live in git.\n\n### The six rows\n\nStorage: Vault holds any secret or key, KMS holds keys, SOPS holds encrypted files.  \nEncryption: Vault uses an AES-256-GCM barrier, KMS uses an HSM-backed key, SOPS uses age, PGP or a cloud KMS.  \nAccess: Vault checks a token and policy, KMS checks IAM, SOPS trusts the key holder.  \nAudit: Vault writes to audit devices, KMS to CloudTrail, SOPS to the git diff.  \nRotation: Vault has leases and rotation, KMS rotates yearly or manually, SOPS re-encrypts.  \nFit: Vault for dynamic secrets, KMS for cloud keys, SOPS for GitOps.\n\n### How to choose\n\nStart from where the secret is used. If it is a database credential that should expire, you want leases. If it is a key that must never leave a boundary, you want KMS. If it is a config file that must ship with the app, SOPS is enough.\n\n> **The honest caveat:** SOPS moves the problem to the key holder. If the age key sits next to the repository, the encryption is decoration.\n\n> **The lesson:** pick the trust boundary first, then the tool that fits it.\n\n## In one line\n\n> **The key is the boundary.** Three tools, three different trust boundaries.\n\n**Check the key**\n\n```bash\naws kms describe-key --key-id $ID\n```\n"},{"id":"p310","n":"310","slug":"a-git-history-secret-scanner-in-python","title":"A git history secret scanner in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Git","CyberSecurity","Python","DevSecOps","AppSec"],"series_name":"Visual","design":"code","script":"git_secret_history.py","date":"2028-09-19","files":{"devsecops":"notes/p310-a-git-history-secret-scanner-in-python.md"},"md":"# A git history secret scanner in Python\n\nThe key was removed in a later commit, so the repository looks clean. It is still there in the history, one git log away from anyone who clones.\n\n## Quick reference\n\nThe full script is at the end of this note: `git_secret_history.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 git_secret_history.py --repo .\n```\n\n**Example output**\n\n```text\n2f9c1a3b4c5d  app/settings.py  aws-access-key\ne7d6c5b4a321  deploy/id_rsa  private-key\n```\n\n## Explained\n\n### What it scans\n\nThe script runs `git log -p --all` and reads the diff, not the working tree. It tracks the commit and the file, then checks only added lines for three shapes: an AWS access key ID, a PEM private key block, and a bearer token of twenty characters or more. A match is reported as a rule name and never printed, so the log does not become the leak.\n\n### How to run it\n\n`python3 git_secret_history.py --repo /srv/app` prints the commit, the file and the rule, then exits 1 if anything matched. In CI, run it on every pull request, because a scan of the working tree does not see it. `--patch` reads a saved diff when you cannot run git.\n\n### When it hits\n\nTreat the key as compromised the moment it is found. Rotate it first, then rewrite history with git filter-repo, force-push, and tell the team to re-clone. Rewriting without rotation protects nothing, because the old key is already in someone's clone.\n\n> **The honest caveat:** a regex cannot catch a secret with no recognisable shape, so this finds the common leaks, not every leak.\n\n> **The lesson:** deleting a secret in a new commit does not delete the commit.\n\n## In one line\n\n> **Rotate the key, then rewrite.** A finding means the secret is already compromised.\n\n**Scan a repo**\n\n```bash\npython3 git_secret_history.py --repo .\n```\n\n## Script: git_secret_history.py\n\nDownload: [scripts/git_secret_history.py](../scripts/git_secret_history.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Scan git history for secrets that were committed and later removed.\n\nUsage:\n    python3 git_secret_history.py\n    python3 git_secret_history.py --repo /srv/app\n    python3 git_secret_history.py --patch /tmp/change.patch\n\nHow it works:\n    Runs \"git log -p --all --no-color\" (or reads a patch file), tracks the\n    current commit and file, and scans only added lines for:\n      - AWS access key IDs (AKIA, ASIA and the other IAM prefixes)\n      - PEM private key blocks\n      - bearer tokens of 20 characters or more\n    Matched values are never printed; the commit, the file and the rule are.\n\nExit codes:\n    0  no secrets found\n    1  at least one secret found\n    2  usage or input error\n\nRun it in CI on every pull request, or weekly from cron:\n\n    0 3 * * 0 python3 /opt/s0sta/git_secret_history.py --repo /srv/app\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport os\nimport re\nimport subprocess\nimport sys\n\nRULES = (\n    (\"aws-access-key\",\n     re.compile(r\"\\b(?:AKIA|ASIA|AGPA|AIDA|AROA|ANPA|ANVA|ASCA)[0-9A-Z]{16}\\b\")),\n    (\"private-key\",\n     re.compile(r\"-----BEGIN (?:[A-Z0-9]+ )?PRIVATE KEY(?: BLOCK)?-----\")),\n    (\"bearer-token\",\n     re.compile(r\"\\bBearer\\s+[A-Za-z0-9._~+/=-]{20,}\", re.IGNORECASE)),\n)\nCOMMIT_RE = re.compile(r\"^commit ([0-9a-f]{7,40})\\b\")\nFROM_RE = re.compile(r\"^From ([0-9a-f]{7,40}) \")\nFILE_RE = re.compile(r\"^\\+\\+\\+ b/(.+?)\\s*$\")\n\n\ndef scan(text, source):\n    \"\"\"Yield (commit, path, rule) for every added line that matches a rule.\"\"\"\n    commit, path = source, \"?\"\n    for line in text.splitlines():\n        match = COMMIT_RE.match(line) or FROM_RE.match(line)\n        if match:\n            commit = match.group(1)\n            continue\n        match = FILE_RE.match(line)\n        if match:\n            path = match.group(1)\n            continue\n        if not line.startswith(\"+\") or line.startswith(\"+++\"):\n            continue\n        for name, pattern in RULES:\n            if pattern.search(line):\n                yield commit, path, name\n\n\ndef read_git(repo):\n    cmd = [\"git\", \"-C\", repo, \"--no-pager\", \"log\", \"-p\", \"--all\", \"--no-color\"]\n    try:\n        proc = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE,\n                              text=True)\n    except OSError as exc:\n        print(\"cannot run git: {}\".format(exc), file=sys.stderr)\n        return None\n    if proc.returncode != 0:\n        message = proc.stderr.strip() or \"git log failed\"\n        print(message, file=sys.stderr)\n        return None\n    return proc.stdout\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Scan git history for AWS keys, private key blocks and \"\n                    \"bearer tokens.\")\n    ap.add_argument(\"--repo\", default=\".\",\n                    help=\"git repository to scan (default: .)\")\n    ap.add_argument(\"--patch\", metavar=\"FILE\",\n                    help=\"read a patch file instead of running git log\")\n    args = ap.parse_args()\n\n    if args.patch:\n        if not os.path.isfile(args.patch):\n            ap.error(\"no such patch file: {}\".format(args.patch))\n        try:\n            with open(args.patch, \"r\", errors=\"replace\") as fh:\n                text = fh.read()\n        except OSError as exc:\n            ap.error(\"cannot read {}: {}\".format(args.patch, exc.strerror))\n        source = \"patch\"\n    else:\n        if not os.path.isdir(args.repo):\n            ap.error(\"not a directory: {}\".format(args.repo))\n        text = read_git(args.repo)\n        if text is None:\n            return 2\n        source = \"worktree\"\n\n    found = list(scan(text, source))\n    for commit, path, name in found:\n        print(\"{:<12}  {:<40}  {}\".format(commit[:12], path, name))\n    if found:\n        print(\"{} secret(s) in history\".format(len(found)))\n        return 1\n    print(\"clean: no secrets in the scanned history\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p311","n":"311","slug":"quiz-3-security-questions","title":"Quiz: 3 security questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["CyberSecurity","IAM","Kubernetes","CloudSecurity","DevSecOps"],"series_name":"Visual","design":"quiz","script":"","date":"2028-09-21","files":{"devsecops":"notes/p311-quiz-3-security-questions.md"},"md":"# Quiz: 3 security questions\n\nThree questions from the security work in this phase. Answer before you read the reveal. Three answers, each with a control behind it.\n\n## Quick reference\n\n**Q1. What makes s3:\\* worse than s3:GetObject on one bucket?**\n\n- a) It is slower for IAM to evaluate.\n- b) It grants every call in the service.\n- c) It only works from the console.\n\n> **Answer:** b) It grants every call in the service.. s3:\\* covers GetObject, PutObject, DeleteObject and PutBucketPolicy. One line can hand over the data and the controls around it, including the bucket policy.\n\n**Q2. What does mTLS prove that a NetworkPolicy does not?**\n\n- a) That the pod runs on the right node.\n- b) That both ends hold a trusted certificate.\n- c) That the traffic is rate limited.\n\n> **Answer:** b) That both ends hold a trusted certificate.. A NetworkPolicy filters traffic, not identities. mTLS demands a client certificate from a trusted CA, so a compromised pod cannot impersonate the service.\n\n**Q3. Why should every break-glass login page the on-call?**\n\n- a) It has no password.\n- b) It bypasses normal access paths by design.\n- c) It is shared by the whole team.\n\n> **Answer:** b) It bypasses normal access paths by design.. Break-glass exists for the day the identity provider fails, so its use is rare. A page and a session log make sure the exception is reviewed, not routine.\n\n## Explained\n\n### Question one\n\nWhat makes s3:\\* worse than s3:GetObject on one bucket? It grants every action in the service, not the one call you needed. The wildcard includes GetObject, but also PutObject, DeleteObject and PutBucketPolicy. One line of policy can hand over the data and the controls around it.\n\n### Question two\n\nWhat does mTLS prove that a NetworkPolicy does not? A NetworkPolicy filters traffic by namespace, pod selector and port. It never authenticates the caller. mTLS requires a certificate from a trusted CA on both ends, so a compromised pod cannot simply claim to be the payments service.\n\n### Question three\n\nWhy should every break-glass login page the on-call? The account bypasses the normal access path by design, which is exactly why its use must be reviewed. It is the account an attacker wants, and the one nobody watches.\n\nEach answer is a control. Wildcards are an IAM review, mTLS is an identity check, and break-glass is an alert plus a session log. None of them works without someone reading the result.\n\n> **The honest caveat:** a control you cannot test is a rumour, so run the wildcard query and the handshake yourself.\n\n> **The lesson:** the why is the part that transfers to the next incident.\n\n## In one line\n\n> **Explain the control or remove it.** Three answers and the controls behind them.\n\n**Review one role**\n\n```bash\naws iam list-role-policies --role-name api\n```\n"},{"id":"p312","n":"312","slug":"rag-architecture-drawn","title":"RAG architecture, drawn","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["RAG","LLM","VectorDatabase","AIEngineering","SystemDesign"],"series_name":"Visual","design":"architecture","script":"","date":"2028-09-24","files":{"devsecops":"notes/p312-rag-architecture-drawn.md"},"md":"# RAG architecture, drawn\n\nA RAG feature looks like one prompt and is really a pipeline. When the answer is wrong, the model is the last place to look.\n\n## Quick reference\n\n- **Tier 1 · Ingest:** Documents (DOCS) · Chunker (CHUNK) · Embeddings (EMBED)\n- **Tier 2 · Store:** Vector index (VEC) · Metadata store (META)\n- **Tier 3 · Serve:** API gateway (GW) · Retriever (TOPK) · Model (LLM)\n\n**Request flow:** Documents → Chunker → Embeddings → Vector index → Retriever → Model; Metadata store → Retriever; API gateway → Retriever\n\n**Around it:** Document store (RAW) · Embedding cache (CACHE) · Eval set (EVAL) · Access control (ACL)\n\n## Explained\n\n### Ingest\n\nDocuments arrive from a wiki, a share or a ticket queue. A parser turns them into text, a chunker cuts the text into overlapping passages, and an embedding model turns each passage into a vector. Chunk size is the first decision you will regret: too small and a passage loses its context, too large and the vector blurs several ideas into one.\n\n### Store\n\nThe vectors go into a vector index that answers one question fast: which passages are closest to this query. Beside it sits a metadata store with source, owner, timestamp and access labels, because a vector without provenance cannot be cited or deleted.\n\n### Serve\n\nA request reaches the API gateway, which authenticates it and applies a quota. The retriever embeds the question, asks the index for the top passages, filters them by metadata, and passes them to the model. The citation is the only proof the answer has a source.\n\nAround the tiers sit the parts that keep it honest: a document store for the originals, an embedding cache so the same text is not paid twice, an eval set that catches regressions, and access control on the passages each user may see.\n\n> **The honest caveat:** no chunk size fixes a corpus that is out of date, and a high retrieval score is not confidence.\n\n> **The lesson:** when RAG answers badly, read the retrieved chunks before you touch the prompt.\n\n## In one line\n\n> **RAG is a pipeline, not a prompt.** The model is the last box; everything before it decides what it sees.\n\n**Check the chunks:** print the top 5 retrieved chunks\n"},{"id":"p313","n":"313","slug":"anatomy-of-an-agent-drawn","title":"Anatomy of an agent, drawn","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["AIAgents","LLM","GenAI","AIEngineering","SystemDesign"],"series_name":"Visual","design":"explainer","script":"","date":"2028-09-26","files":{"devsecops":"notes/p313-anatomy-of-an-agent-drawn.md"},"md":"# Anatomy of an agent, drawn\n\nA chat model answers. An agent acts, and the difference is four parts bolted around the same model. Take one away and it fails in a way you can predict.\n\n## Quick reference\n\n### Planner - the loop\n\n- Turns one goal into ordered steps\n- Picks the next call, reads the result\n- Stops on success, error or budget\n- The loop is the product, not the prompt\n\n### Tools - typed calls\n\n- A tool is a function with a schema\n- The model proposes the arguments\n- Your code validates and runs them\n- Results are untrusted input\n\n### Memory - two speeds\n\n- Short-term: the messages in this run\n- Long-term: a store it can search\n- Both grow until something breaks\n- Write less than you read, then expire\n\n### Guardrails - the limits\n\n- Allow-list the tools, scope the token\n- Validate arguments before the call\n- Cap steps, spend and wall-clock time\n- The only part you can test with certainty\n\n> **Note:** A model with tools and no guardrail is a remote shell with a chat interface.\n\n## Explained\n\n### The planner\n\nThe planner turns a goal into steps, picks the next tool call, reads the result and decides again. That loop needs a stopping rule: success, an error it cannot recover from, or a budget cap. Without the cap, a confused agent keeps buying tokens.\n\n### The tools\n\nA tool is a function with a typed schema: a name, arguments, and what it returns. The model proposes the arguments; your code validates them and makes the call. That split is the point, because the model never executes anything itself.\n\n### The memory\n\nShort-term memory is the message list of the current run, and it grows until it meets the context window. Long-term memory is a store the agent can search, written deliberately and read rarely. Both need a rule for what to forget.\n\n### The guardrails\n\nGuardrails live outside the model: an allow-list of tools, a scoped token per tool, schema validation on every argument, and caps on steps, spend and wall-clock time. They are the part you can test with certainty.\n\n> **The honest caveat:** a better planner does not fix a tool with too much permission, and a guardrail cannot make a bad plan good.\n\n> **The lesson:** build the loop, then build the limits around it.\n\n## In one line\n\n> **An agent is a loop with limits.** The plan is probabilistic; the limits around it are not.\n\n**Watch one run:** log every tool call and its result\n"},{"id":"p314","n":"314","slug":"the-llm-api-cheat-sheet","title":"The LLM API cheat sheet","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLM","API","GenAI","CheatSheet","AIEngineering"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-09-28","files":{"devsecops":"notes/p314-the-llm-api-cheat-sheet.md"},"md":"# The LLM API cheat sheet\n\nThe OpenAI-compatible request shape is now the common one, so the useful cheat sheet is not per vendor. It is one endpoint and the four checks around it.\n\n## Quick reference\n\n### Chat\n\n```bash\ncurl -s $BASE/v1/chat/completions ...\ncurl -s $BASE/v1/models | jq '.data[].id'\njq -r '.choices[0].message.content'\njq -r '.choices[0].finish_reason'\nexport BASE=https://api.example.com\n```\n\n### Tokens\n\n```bash\njq '.usage.prompt_tokens'\njq '.usage.completion_tokens'\njq '.usage.total_tokens'\nwc -c prompt.txt  # ~4 chars per token\n```\n\n### Tools\n\n```bash\njq '.tools[].function.name' req.json\njq '.choices[0].message.tool_calls'\njq '.choices[0].message.tool_calls[0]'\njq -r '.choices[0].finish_reason'\n```\n\n### Evals\n\n```bash\npython3 -m pytest evals -q\njq -r .score out/*.json | sort -n\njq '[.[] | select(.pass)] | length'\ngit diff --stat evals/baseline.json\n```\n\n## Explained\n\n### Chat\n\nThe call is a POST to `$BASE`/v1/chat/completions with a model and a messages array. `curl -s` proves the endpoint answers, and a GET to `$BASE`/v1/models lists what you can call. Read choices[0].message.content for the answer and finish_reason for why it stopped: stop, length or tool_calls.\n\n### Tokens\n\nEvery response carries a usage object. `jq '.usage.prompt_tokens'` and .completion_tokens give the two numbers that bill differently, and total_tokens is their sum. For a rough pre-flight estimate, run `wc -c` on the prompt and divide by four.\n\n### Tools\n\nYou describe tools in the request as JSON schemas, each with a name and parameters. The model does not call them; it returns tool_calls with a name and an arguments string. Parse it, validate it against the schema, run your function, and send the result back as a tool message.\n\n### Evals\n\nKeep a folder of cases with expected answers and a score per case. Run them on every prompt change with `python3 -m pytest evals -q`, diff the scores against the baseline in git, and count the passes rather than reading a few outputs. An eval that is not in CI is an opinion.\n\n> **The honest caveat:** compatible endpoints agree on the shape, not on the details. Tool-call formats, streaming chunks and token counts still differ between providers.\n\n> **The lesson:** learn the shape once, then measure tokens and evals on every call.\n\n## In one line\n\n> **The API is small. The bill is not.** One endpoint, four checks, and a bill that grows quietly.\n\n**Save the shape**\n\n```bash\ncurl -s $BASE/v1/models\n```\n"},{"id":"p315","n":"315","slug":"a-token-budget-estimator-in-python","title":"A token budget estimator in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","LLM","FinOps","AIEngineering","CostControl"],"series_name":"Visual","design":"code","script":"token_budget.py","date":"2028-10-01","files":{"devsecops":"notes/p315-a-token-budget-estimator-in-python.md"},"md":"# A token budget estimator in Python\n\nPrompt work stops at the invoice. Someone asks what the feature costs a month, and the honest answer is that you never multiplied it out.\n\n## Quick reference\n\nThe full script is at the end of this note: `token_budget.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 token_budget.py cfg.json reqs.jsonl -b 500\n```\n\n**Example output**\n\n```text\nMONTHLY $612.00 BUDGET $500.00\nALERT over by $112.00\n```\n\n## Explained\n\n### What it does\n\nThe script takes a JSON config with a price per million input tokens, a price per million output tokens, and a characters-per-token estimate. It reads a JSONL file where each line is one workload: a name, a monthly request count, and the input and output characters of a call.\n\n### The arithmetic\n\nTokens are estimated as characters divided by four, close enough for English prose. Input tokens are priced at the input rate, output tokens at the output rate, and the two are added per request before the monthly count is applied. It totals the month, prints the total, and compares it with a budget.\n\n### Why the split matters\n\nOutput is the expensive direction, so a long prompt with a short answer and a short prompt with a long answer can move the same number of tokens and cost different money. Pricing them together hides the one you can fix.\n\n### How to use it\n\nRun it with the config, the JSONL and a budget. It exits 1 when the month is over, so it works as a CI gate.\n\n> **The honest caveat:** four characters per token is a rule for English prose. Code, JSON, other languages and long numbers tokenise worse, so read real usage from the provider once and calibrate.\n\n> **The lesson:** a cost estimate you can rerun is worth more than a spreadsheet you cannot.\n\n## In one line\n\n> **Count tokens before they count you.** A budget you cannot estimate is a bill you cannot approve.\n\n**Price one feature**\n\n```bash\npython3 token_budget.py cfg.json reqs.jsonl\n```\n\n## Script: token_budget.py\n\nDownload: [scripts/token_budget.py](../scripts/token_budget.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"token_budget.py - estimate LLM tokens and project a monthly bill.\n\nEstimates tokens from character counts (about four characters per token by\ndefault), prices input and output separately from a JSON config, and projects\na monthly bill from a JSONL file of request counts.\n\nConfig JSON, one object:\n    {\"model\": \"example-large\", \"chars_per_token\": 4,\n     \"price_in\": 3.0, \"price_out\": 15.0}\n\nRequest JSONL, one workload per line:\n    {\"name\": \"chat\", \"requests\": 120000,\n     \"input_chars\": 1600, \"output_chars\": 600}\n\nRead-only: it reads files and never writes one. Exit codes: 0 within budget\n(or no budget given), 1 over budget, 2 usage or input error.\n\nUsage:\n    python3 token_budget.py budget.json requests.jsonl --budget 500\n    python3 token_budget.py budget.json requests.jsonl\n    cat requests.jsonl | python3 token_budget.py budget.json -\n    python3 token_budget.py budget.json requests.jsonl --budget 500 --json\n    python3 token_budget.py budget.json requests.jsonl --chars-per-token 3.5\n\nCron (monthly, first day at 07:00):\n    0 7 1 * * python3 token_budget.py budget.json requests.jsonl --budget 500\n\nCI gate (fails the build when the month is over budget):\n    python3 token_budget.py budget.json requests.jsonl --budget 500\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nDEFAULT_CHARS_PER_TOKEN = 4.0\n\n\ndef is_number(value):\n    \"\"\"True for an int or float that is not a bool.\"\"\"\n    return isinstance(value, (int, float)) and not isinstance(value, bool)\n\n\ndef load_config(path, errors):\n    \"\"\"Read and validate the price config. Returns a dict or None.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            config = json.load(handle)\n    except OSError as exc:\n        errors.append(\"%s: %s\" % (path, exc))\n        return None\n    except ValueError as exc:\n        errors.append(\"%s: not JSON: %s\" % (path, exc))\n        return None\n    if not isinstance(config, dict):\n        errors.append(\"%s: expected a JSON object\" % path)\n        return None\n    for key in (\"price_in\", \"price_out\"):\n        value = config.get(key)\n        if not is_number(value) or value < 0:\n            errors.append(\"%s: %s must be a number >= 0\" % (path, key))\n            return None\n    cpt = config.get(\"chars_per_token\", DEFAULT_CHARS_PER_TOKEN)\n    if not is_number(cpt) or cpt <= 0:\n        errors.append(\"%s: chars_per_token must be a number > 0\" % path)\n        return None\n    return config\n\n\ndef load_rows(path, errors):\n    \"\"\"Read one JSONL file of workloads. Returns a list of dicts.\"\"\"\n    label = \"<stdin>\" if path == \"-\" else path\n    try:\n        if path == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        errors.append(\"%s: %s\" % (label, exc))\n        return []\n\n    rows = []\n    for lineno, line in enumerate(text.splitlines(), 1):\n        line = line.strip()\n        if not line or line.startswith(\"#\"):\n            continue\n        try:\n            row = json.loads(line)\n        except ValueError as exc:\n            errors.append(\"%s:%d: not JSON: %s\" % (label, lineno, exc))\n            continue\n        if not isinstance(row, dict):\n            errors.append(\"%s:%d: expected a JSON object\" % (label, lineno))\n            continue\n        name = str(row.get(\"name\", \"workload-%d\" % lineno))\n        requests = row.get(\"requests\", 1)\n        input_chars = row.get(\"input_chars\")\n        output_chars = row.get(\"output_chars\")\n        if not is_number(requests) or requests < 0:\n            errors.append(\"%s:%d: requests must be a number >= 0\" % (label, lineno))\n            continue\n        if not is_number(input_chars) or input_chars < 0:\n            errors.append(\"%s:%d: input_chars missing or < 0\" % (label, lineno))\n            continue\n        if not is_number(output_chars) or output_chars < 0:\n            errors.append(\"%s:%d: output_chars missing or < 0\" % (label, lineno))\n            continue\n        rows.append({\"name\": name, \"requests\": requests,\n                     \"input_chars\": float(input_chars),\n                     \"output_chars\": float(output_chars)})\n    return rows\n\n\ndef price_rows(rows, config, cpt):\n    \"\"\"Add token counts and a monthly USD cost to each workload.\"\"\"\n    price_in = float(config[\"price_in\"])\n    price_out = float(config[\"price_out\"])\n    priced = []\n    for row in rows:\n        in_tokens = row[\"input_chars\"] / cpt\n        out_tokens = row[\"output_chars\"] / cpt\n        cost = row[\"requests\"] * (in_tokens * price_in\n                                  + out_tokens * price_out) / 1e6\n        item = dict(row)\n        item[\"in_tokens\"] = in_tokens\n        item[\"out_tokens\"] = out_tokens\n        item[\"cost\"] = cost\n        priced.append(item)\n    return priced\n\n\ndef print_table(priced):\n    print(\"%-24s %10s %11s %12s %13s\"\n          % (\"WORKLOAD\", \"REQUESTS\", \"INPUT TOK\", \"OUTPUT TOK\", \"USD/MONTH\"))\n    for item in priced:\n        print(\"%-24s %10d %11.0f %12.0f %13.2f\"\n              % (item[\"name\"][:24], item[\"requests\"], item[\"in_tokens\"],\n                 item[\"out_tokens\"], item[\"cost\"]))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Estimate tokens from characters and project a monthly \"\n                    \"LLM bill. Exits 1 when the month is over budget.\")\n    parser.add_argument(\"config\", help=\"JSON file with price_in and price_out\")\n    parser.add_argument(\"requests\", nargs=\"+\", metavar=\"REQUESTS\",\n                        help=\"JSONL file(s) of workloads, or - for stdin\")\n    parser.add_argument(\"-b\", \"--budget\", type=float,\n                        help=\"monthly budget in USD; exit 1 when over it\")\n    parser.add_argument(\"-c\", \"--chars-per-token\", type=float,\n                        help=\"override chars_per_token from the config\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only, no table\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one machine-readable JSON object\")\n    args = parser.parse_args(argv)\n\n    if args.budget is not None and args.budget < 0:\n        parser.error(\"--budget must be >= 0\")\n    if args.chars_per_token is not None and args.chars_per_token <= 0:\n        parser.error(\"--chars-per-token must be > 0\")\n\n    errors = []\n    config = load_config(args.config, errors)\n    rows = []\n    for path in args.requests:\n        rows.extend(load_rows(path, errors))\n\n    for message in errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if errors:\n        return 2\n    if not rows:\n        print(\"error: no workload rows found\", file=sys.stderr)\n        return 2\n\n    cpt = (args.chars_per_token if args.chars_per_token is not None\n           else float(config.get(\"chars_per_token\", DEFAULT_CHARS_PER_TOKEN)))\n    priced = price_rows(rows, config, cpt)\n    total = sum(item[\"cost\"] for item in priced)\n    budget = args.budget\n\n    if args.json:\n        print(json.dumps({\n            \"model\": config.get(\"model\"),\n            \"chars_per_token\": cpt,\n            \"monthly_usd\": round(total, 2),\n            \"budget_usd\": budget,\n            \"over\": budget is not None and total > budget,\n            \"workloads\": [{\"name\": item[\"name\"],\n                           \"requests\": item[\"requests\"],\n                           \"input_tokens\": round(item[\"in_tokens\"]),\n                           \"output_tokens\": round(item[\"out_tokens\"]),\n                           \"usd_month\": round(item[\"cost\"], 2)}\n                          for item in priced],\n        }, sort_keys=True))\n        return 1 if budget is not None and total > budget else 0\n\n    if not args.quiet:\n        print_table(priced)\n        print()\n\n    if budget is None:\n        print(\"MONTHLY $%.2f\" % total)\n        return 0\n    print(\"MONTHLY $%.2f BUDGET $%.2f\" % (total, budget))\n    if total > budget:\n        print(\"ALERT over by $%.2f\" % (total - budget))\n        return 1\n    print(\"OK under by $%.2f\" % (budget - total))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p316","n":"316","slug":"a-gpu-training-cluster-drawn","title":"A GPU training cluster, drawn","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["GPU","AIInfrastructure","HPC","InfiniBand","MachineLearning"],"series_name":"Visual","design":"architecture","script":"","date":"2028-10-03","files":{"devsecops":"notes/p316-a-gpu-training-cluster-drawn.md"},"md":"# A GPU training cluster, drawn\n\nYou rent a rack of GPUs and the job finishes no faster than a single node. The cards are fine; the path between them is not.\n\n## Quick reference\n\n- **Tier 1 · Control:** API server (API) · Scheduler (SCHED)\n- **Tier 2 · Fabric:** InfiniBand (IB) · NVLink (NV)\n- **Tier 3 · Compute:** GPU nodes (GPU) · Storage (IO)\n\n**Request flow:** API server → Scheduler → InfiniBand → GPU nodes → Storage; NVLink → GPU nodes\n\n**Around it:** Checkpoint store (CKPT) · Telemetry (DCGM) · Registry (REG) · Identity (IAM)\n\n## Explained\n\n### Control\n\nA job reaches the cluster through the API server, which authenticates the user and records the request. The scheduler places it: which nodes, how many GPUs each, where the ranks land. Placement is the performance decision, because a rank on the wrong side of the fabric waits for its pair.\n\n### Fabric\n\nInfiniBand connects the nodes, and NVLink connects the GPUs inside one node. Both matter, and they are not the same: a collective that stays inside a node uses NVLink, and every step that crosses nodes pays the InfiniBand hop. A cluster with fast cards and a thin fabric is a set of fast islands.\n\n### Compute\n\nThe GPU nodes run the job. Local storage holds the dataset shard and the checkpoints in flight; the shared storage tier holds what must survive the node. A node that cannot feed its cards is idle with a full queue behind it.\n\nAround the tiers: a checkpoint store for restarts, telemetry that samples the cards, a registry for the image, and identity for every job. These are the parts nobody demos and every long job needs.\n\n> **The honest caveat:** a faster fabric will not fix a data loader that cannot keep up, and utilisation alone does not tell you whether the maths is useful.\n\n> **The lesson:** size the fabric and the storage for the job, not just the cards on the invoice.\n\n## In one line\n\n> **The network is the cluster.** You buy GPUs; the fabric decides whether they act like one machine.\n\n**Check the links**\n\n```bash\nnvidia-smi topo -m\n```\n"},{"id":"p317","n":"317","slug":"the-gpu-and-kubernetes-cheat-sheet","title":"The GPU and Kubernetes cheat sheet","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["GPU","Kubernetes","DevOps","SRE","CheatSheet"],"series_name":"Visual","design":"cheatsheet","script":"","date":"2028-10-05","files":{"devsecops":"notes/p317-the-gpu-and-kubernetes-cheat-sheet.md"},"md":"# The GPU and Kubernetes cheat sheet\n\nA GPU pod stops training at 2 a.m. and the dashboard says everything is healthy. The commands that answer the question are four views of the same stack.\n\n## Quick reference\n\n### GPUs\n\n```bash\nnvidia-smi\nnvidia-smi -L\nnvidia-smi topo -m\nnvidia-smi --query-gpu=name,memory.used\nwatch -n 2 nvidia-smi\n```\n\n### Nodes\n\n```bash\nkubectl describe node gpu-0\nkubectl describe node gpu-0 | grep -i gpu\nkubectl get node gpu-0 -o yaml\nkubectl label node gpu-0 pool=a100\nkubectl describe node gpu-0 | tail -20\n```\n\n### Pods\n\n```bash\nkubectl get pods -A -o wide\nkubectl exec -it gpu-pod -- nvidia-smi\nkubectl logs -f gpu-pod\nkubectl describe pod gpu-pod\nkubectl get pod gpu-pod -o yaml\n```\n\n### Checks\n\n```bash\nkubectl get events -A\nkubectl get events -A --watch\nkubectl get pods -A | grep Pending\nkubectl top pod -A --sort-by=memory\nkubectl get events -A -o wide\n```\n\n## Explained\n\n### GPUs\n\nStart at the card. `nvidia-smi` shows utilisation, memory and temperature; `nvidia-smi -L` lists the devices; `nvidia-smi topo -m` prints the topology, which tells you whether two GPUs talk over NVLink or PCIe. The query form, `nvidia-smi --query-gpu=name,memory.used`, is the one to remember.\n\n### Nodes\n\nThen the node. `kubectl describe node gpu-0` shows capacity and allocatable, including `nvidia.com/gpu`, and the events at the bottom explain why a card is not schedulable. `kubectl get node gpu-0 -o yaml` is the full record, and `kubectl label node gpu-0 pool=a100` is how you pin a workload to a class of card.\n\n### Pods\n\nThen the workload. `kubectl get pods -A -o wide` shows where it landed. `kubectl exec -it gpu-pod -- nvidia-smi` shows what the container can actually see, and `kubectl logs -f gpu-pod` or `kubectl describe pod gpu-pod` explains a crash or an eviction.\n\n### Checks\n\nThen the cluster. `kubectl get events -A` is the history, and `kubectl get pods -A | grep Pending` finds the jobs waiting for a card. `kubectl top pod -A --sort-by=memory` needs metrics-server, and it catches the pod that is running but not working.\n\n> **The honest caveat:** `nvidia-smi` inside a container shows the cards the container was given, not the whole node, so check the node before you blame the pod.\n\n> **The lesson:** read the card, the node, the pod and the event, in that order.\n\n## In one line\n\n> **The pod is fine. The node is not.** Four views: the card, the node, the pod, the event.\n\n**Start at the card**\n\n```bash\nkubectl exec -it gpu-pod -- nvidia-smi\n```\n"},{"id":"p318","n":"318","slug":"tokens-context-and-cost-drawn","title":"Tokens, context and cost, drawn","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["LLM","Tokens","GenAI","AIEngineering","FinOps"],"series_name":"Visual","design":"explainer","script":"","date":"2028-10-08","files":{"devsecops":"notes/p318-tokens-context-and-cost-drawn.md"},"md":"# Tokens, context and cost, drawn\n\nYou paste a forty-page PDF into a prompt and the bill lands well above the estimate. Nothing was wrong with the model; the four numbers behind the price were never measured.\n\n## Quick reference\n\n### Tokens\n\n- Text becomes tokens before the model sees it\n- English prose runs at about four chars per token\n- Code, JSON and other languages cost more\n- The tokeniser is part of the contract\n\n### Context window\n\n- One budget shared by prompt and answer\n- History, retrieved text and output all count\n- Past the limit: truncated or rejected\n- Every turn re-sends the history\n\n### Input vs output\n\n- Both billed per million tokens\n- Output is the more expensive direction\n- Input is one parallel pass; output is a loop\n- A long answer is the cost you control least\n\n### Caching\n\n- A hit skips the repeated prompt prefix\n- Providers bill the hit at a lower input rate\n- It rewards a stable prefix\n- Put the variable part last\n\n> **Note:** The context window is a budget, and the bill is per token.\n\n## Explained\n\n### Tokens\n\nText is cut into tokens before the model sees it: words, parts of words, punctuation. English prose runs at roughly four characters per token, so a thousand words is about fifteen hundred tokens. Code, JSON and other languages cost more, because their patterns are less predictable.\n\n### The context window\n\nThe window is one budget shared by the prompt, the conversation so far, retrieved text and the answer. Past the limit, content is truncated or the request is rejected outright. Every turn re-sends the history, which is why a long chat grows more expensive.\n\n### Input versus output\n\nBoth directions are billed per million tokens, at different prices, and output is the more expensive one. Output is generated one token at a time, each step reading the weights, while input is processed in one parallel pass. A long answer is the cost you control least once it starts.\n\n### Caching\n\nA cache hit skips the recomputation of a prompt prefix you have sent before, and providers bill that prefix at a lower input rate. It rewards a stable prefix: instructions and reference document first, the variable question last.\n\n> **The honest caveat:** token counts differ by tokeniser, and cache rules differ by provider, so the number that settles an argument is your own usage report.\n\n> **The lesson:** measure tokens, context and cache hits before you argue about the model.\n\n## In one line\n\n> **Every token is on the bill.** Context is a budget; output is the expensive half.\n\n**Measure a prompt**\n\n```bash\njq .usage resp.json\n```\n"},{"id":"p319","n":"319","slug":"ai-infrastructure-terms-decoded","title":"AI infrastructure terms, decoded","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["AIInfrastructure","GPU","LLM","Inference","MachineLearning"],"series_name":"Visual","design":"reveal","script":"","date":"2028-10-10","files":{"devsecops":"notes/p319-ai-infrastructure-terms-decoded.md"},"md":"# AI infrastructure terms, decoded\n\nThe dashboard says the GPU is busy and users still complain. Utilisation is not latency, and these six words are how you tell the difference.\n\n## Quick reference\n\n- **TTFT** — Time to first token: how long the user stares at nothing before the answer starts. It covers the queue and the prefill pass.\n- **tokens/s** — Decode throughput: how fast tokens arrive after the first one. Batching raises the total while a single request slows down.\n- **batching** — Packing several requests into one forward pass. Static batching waits for a full batch; continuous swaps requests every step.\n- **KV cache** — The attention keys and values kept from earlier tokens so they are not recomputed. It buys speed and costs memory, one per request.\n- **quantisation** — Weights stored in fewer bits: FP16 down to INT8 or FP8. Less memory, faster maths, and an accuracy loss to measure.\n- **MIG** — Multi-Instance GPU: one card split into isolated slices, each with its own memory and compute. For sharing, not for one big job.\n\n## Explained\n\n### TTFT\n\nTime to first token is how long the user stares at nothing before the answer starts. It covers the queue plus prefill, the pass over the prompt, so it grows with prompt length.\n\ntokens/s\n\nDecode throughput is how fast tokens arrive after the first one. It is measured per request or across the batch, and the two move in opposite directions: more requests raise the total while one request slows down.\n\n### Batching\n\nPacking several requests into one forward pass. Static batching waits for a full batch; continuous batching adds and removes requests at every step, which is why serving engines hold a queue.\n\n### KV cache\n\nThe attention keys and values kept from earlier tokens so they are not recomputed. It buys speed and costs memory, one cache per concurrent request, which is the real ceiling on users per card.\n\n### Quantisation\n\nStoring weights in fewer bits, from FP16 down to INT8 or FP8. Less memory and faster matrix maths, at the risk of a small accuracy loss you measure rather than assume.\n\n### MIG\n\nMulti-Instance GPU splits one card into isolated slices, each with its own memory and compute. Good for sharing a card, not for making one large job fit.\n\n> **The honest caveat:** every term is a trade, none is a target. A high tokens per second figure on an idle cluster is worth nothing.\n\n> **The lesson:** name the number before you tune anything.\n\n## In one line\n\n> **Name the number before you tune.** Each term is a lever, and every lever has a cost.\n\n**Watch one card**\n\n```bash\nnvidia-smi dmon -s mu -d 2\n```\n"},{"id":"p320","n":"320","slug":"a-gpu-utilisation-report-in-python","title":"A GPU utilisation report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["GPU","Python","Observability","AIInfrastructure","SRE"],"series_name":"Visual","design":"code","script":"gpu_report.py","date":"2028-10-12","files":{"devsecops":"notes/p320-a-gpu-utilisation-report-in-python.md"},"md":"# A GPU utilisation report in Python\n\nThe invoice says the cluster ran all month. The cards tell a different story: a queue that waited and cards that were not doing the work.\n\n## Quick reference\n\nThe full script is at the end of this note: `gpu_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 gpu_report.py < gpus.csv\n```\n\n**Example output**\n\n```text\nIDLE 1 OF 4 GPUS\nALERT 1 of 4 GPUs under 10%\n```\n\n## Explained\n\n### What it runs\n\nThe script parses the CSV from `nvidia-smi --query-gpu=index,name,utilization.gpu,memory.used,memory.total --format=csv,noheader,nounits`. Feed it a file, or pipe the live query in on stdin.\n\n### What it prints\n\nOne row per GPU: the index, the name, utilisation as a percentage, and memory used as a share of total. A card below the threshold, ten percent by default, counts as idle. At the end it prints the count, and the exit code is 1 when any card is idle, so cron can log it and CI can fail on it.\n\n### Why the memory column\n\nA card at zero utilisation with most of its memory held is a stuck process, not an idle one. A card at high utilisation with almost no memory is usually a small kernel spinning. Neither shows up in a cluster-wide utilisation average.\n\n### How to use it\n\nRun it against a saved sample in CI, or every ten minutes from cron on each GPU node, appending to a log. Add `--json` for a dashboard, `--threshold` to match the shape of the workload, and -q when you only want the summary.\n\n> **The honest caveat:** one nvidia-smi sample is one moment. A card at zero may be waiting on data, and a MIG instance reports its own slice, so sample over time before you call a GPU wasted.\n\n> **The lesson:** utilisation is a time series, not a status field.\n\n## In one line\n\n> **An idle card is a finding.** A card at zero is either waiting or wasted; the log tells you which.\n\n**Collect a sample**\n\n```bash\nnvidia-smi --query-gpu=utilization.gpu\n```\n\n## Script: gpu_report.py\n\nDownload: [scripts/gpu_report.py](../scripts/gpu_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"gpu_report.py - report per-GPU utilisation from nvidia-smi and flag idle cards.\n\nParses the CSV from:\n\n    nvidia-smi --query-gpu=index,name,utilization.gpu,memory.used,memory.total --format=csv,noheader,nounits\n\ngiven as one or more files, or on stdin. For every GPU it prints the\nutilisation, the memory used as a percentage of total, and a state. A card is\nIDLE when its utilisation is below --threshold percent (default 10). The exit\ncode is 1 when any card is idle, so a cron job or a CI gate can act on it.\n\nRead-only: it parses text and never talks to the driver. Exit codes: 0 every\nGPU at or above the threshold, 1 at least one idle GPU, 2 usage or input\nerror.\n\nUsage:\n    nvidia-smi --query-gpu=index,name,utilization.gpu,memory.used,memory.total --format=csv,noheader,nounits | python3 gpu_report.py -\n    python3 gpu_report.py gpus.csv\n    python3 gpu_report.py gpus.csv --threshold 20 --json\n    python3 gpu_report.py a.csv b.csv --quiet\n\nCron (every ten minutes):\n    */10 * * * * nvidia-smi --query-gpu=index,name,utilization.gpu,memory.used,memory.total --format=csv,noheader,nounits | python3 gpu_report.py - >> /var/log/gpu_report.log 2>&1\n\nCI gate (fails when a card sits idle):\n    python3 gpu_report.py gpus.csv --threshold 20\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nPLACEHOLDERS = (\"\", \"n/a\", \"[n/a]\", \"na\", \"[not supported]\", \"[unknown error]\")\n\n\ndef as_float(value):\n    \"\"\"A number from an nvidia-smi field, or None for a placeholder.\"\"\"\n    if value is None:\n        return None\n    text = str(value).strip()\n    if text.lower() in PLACEHOLDERS:\n        return None\n    try:\n        return float(text)\n    except ValueError:\n        return \"bad\"\n\n\ndef parse_rows(text, label, errors):\n    \"\"\"Rows of (index, name, util, used, total) from one nvidia-smi dump.\"\"\"\n    rows = []\n    for lineno, line in enumerate(text.splitlines(), 1):\n        line = line.strip()\n        if not line:\n            continue\n        parts = [part.strip() for part in line.split(\",\")]\n        if len(parts) < 5:\n            errors.append(\"%s:%d: expected 5 comma-separated fields\"\n                          % (label, lineno))\n            continue\n        if parts[0].lower() == \"index\":\n            continue  # header, when the query ran without noheader\n        if not parts[0].isdigit():\n            errors.append(\"%s:%d: index is not a number: %s\"\n                          % (label, lineno, parts[0]))\n            continue\n        name = \", \".join(parts[1:-3]) or \"?\"\n        util = as_float(parts[-3])\n        used = as_float(parts[-2])\n        total = as_float(parts[-1])\n        if util == \"bad\" or used == \"bad\" or total == \"bad\":\n            errors.append(\"%s:%d: non-numeric memory or utilisation\"\n                          % (label, lineno))\n            continue\n        if total is not None and total <= 0:\n            errors.append(\"%s:%d: memory.total must be > 0\" % (label, lineno))\n            continue\n        rows.append({\"index\": int(parts[0]), \"name\": name, \"util\": util,\n                     \"used\": used, \"total\": total})\n    return rows\n\n\ndef state_of(util, threshold):\n    \"\"\"busy, IDLE or unknown for one utilisation reading.\"\"\"\n    if util is None:\n        return \"unknown\"\n    return \"IDLE\" if util < threshold else \"busy\"\n\n\ndef memory_pct(used, total):\n    \"\"\"Memory used as a percentage of total, or None when either is missing.\"\"\"\n    if used is None or not total:\n        return None\n    return 100.0 * used / total\n\n\ndef cards(count):\n    \"\"\"GPU or GPUs, so the summary line reads correctly for one card.\"\"\"\n    return \"GPU\" if count == 1 else \"GPUs\"\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report per-GPU utilisation from nvidia-smi CSV and flag \"\n                    \"cards below a threshold. Exits 1 when one is idle.\")\n    parser.add_argument(\"paths\", nargs=\"*\", default=[\"-\"], metavar=\"PATH\",\n                        help=\"nvidia-smi CSV file(s), or - for stdin (default)\")\n    parser.add_argument(\"-t\", \"--threshold\", type=float, default=10.0,\n                        help=\"idle below this utilisation percentage (default 10)\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only, no table\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one machine-readable JSON object\")\n    args = parser.parse_args(argv)\n\n    if args.threshold < 0 or args.threshold > 100:\n        parser.error(\"--threshold must be between 0 and 100\")\n\n    errors = []\n    gpus = []\n    for path in args.paths:\n        label = \"<stdin>\" if path == \"-\" else path\n        try:\n            if path == \"-\":\n                text = sys.stdin.read()\n            else:\n                with open(path, \"r\", encoding=\"utf-8\") as handle:\n                    text = handle.read()\n        except OSError as exc:\n            errors.append(\"%s: %s\" % (label, exc))\n            continue\n        gpus.extend(parse_rows(text, label, errors))\n\n    for message in errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if errors:\n        return 2\n    if not gpus:\n        print(\"error: no GPU rows found\", file=sys.stderr)\n        return 2\n\n    gpus.sort(key=lambda item: item[\"index\"])\n    for gpu in gpus:\n        gpu[\"mem_pct\"] = memory_pct(gpu[\"used\"], gpu[\"total\"])\n        gpu[\"state\"] = state_of(gpu[\"util\"], args.threshold)\n    idle = sum(1 for gpu in gpus if gpu[\"state\"] == \"IDLE\")\n    count = len(gpus)\n    limit = (\"%g\" % args.threshold)\n\n    if args.json:\n        print(json.dumps({\n            \"count\": count,\n            \"idle\": idle,\n            \"threshold\": args.threshold,\n            \"ok\": idle == 0,\n            \"gpus\": [{\"index\": gpu[\"index\"], \"name\": gpu[\"name\"],\n                      \"util\": gpu[\"util\"],\n                      \"mem_pct\": (round(gpu[\"mem_pct\"], 1)\n                                  if gpu[\"mem_pct\"] is not None else None),\n                      \"state\": gpu[\"state\"]} for gpu in gpus],\n        }, sort_keys=True))\n        return 1 if idle else 0\n\n    if not args.quiet:\n        print(\"%-4s %-28s %5s %7s   %s\"\n              % (\"GPU\", \"NAME\", \"UTIL\", \"MEMORY\", \"STATE\"))\n        for gpu in gpus:\n            util = \"-\" if gpu[\"util\"] is None else \"%d%%\" % round(gpu[\"util\"])\n            mem = (\"-\" if gpu[\"mem_pct\"] is None\n                   else \"%d%%\" % round(gpu[\"mem_pct\"]))\n            print(\"%-4d %-28s %5s %7s   %s\"\n                  % (gpu[\"index\"], gpu[\"name\"][:28], util, mem, gpu[\"state\"]))\n        print()\n    print(\"IDLE %d OF %d %s\" % (idle, count, cards(count).upper()))\n    if idle:\n        print(\"ALERT %d of %d %s under %s%%\"\n              % (idle, count, cards(count), limit))\n        return 1\n    print(\"OK all %d %s at or above %s%%\" % (count, cards(count), limit))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p321","n":"321","slug":"quiz-3-ai-infrastructure-questions","title":"Quiz: 3 AI infrastructure questions","sections":["devsecops"],"topic":"AI infrastructure & LLM security","tags":["AIInfrastructure","GPU","LLM","GenAI","SystemDesign"],"series_name":"Visual","design":"quiz","script":"","date":"2028-10-15","files":{"devsecops":"notes/p321-quiz-3-ai-infrastructure-questions.md"},"md":"# Quiz: 3 AI infrastructure questions\n\nThe last post of this 50-part series, and the right place to stop: the layer that decides what the AI systems of this path actually cost. Three questions. Answer before the reveal.\n\n## Quick reference\n\n**Q1. A GPU shows 100 percent utilisation but few tokens per second. Why?**\n\n- a) Decode is bound by memory bandwidth\n- b) The card has too few CUDA cores\n- c) The model is too small to fill the card\n\n> **Answer:** a) Decode is bound by memory bandwidth. Each generated token reads the weights once, so bytes moved, not maths, set the pace. Batching spreads that read, and HBM bandwidth is the number to compare.\n\n**Q2. What does TTFT measure?**\n\n- a) Time from request to the first token\n- b) Time to finish the whole answer\n- c) Time the request waits in the queue\n\n> **Answer:** a) Time from request to the first token. Queueing plus prefill, the pass over your prompt. It grows with prompt length and with the number of requests ahead, and it is the latency the user feels first.\n\n**Q3. Why do output tokens cost more than input tokens?**\n\n- a) Decode generates one token per step\n- b) Output is stored for longer\n- c) Output tokens are larger\n\n> **Answer:** a) Decode generates one token per step. Prefill processes the prompt in parallel; decode generates a token per step and reads the weights each step, so a generated token costs more than a read one.\n\n## Explained\n\n### Question one\n\nA GPU reports 100 percent utilisation but few tokens per second. Why? Decode is bound by memory bandwidth, not by maths. Each generated token reads the weights once, so the bytes moved set the pace; batch more requests and the same weights serve more tokens.\n\n### Question two\n\nWhat does TTFT measure? The time from request to the first token, which is queueing plus prefill. Prefill processes the whole prompt in one pass, so a long prompt and a busy queue both push the number up, and the user feels every millisecond of it.\n\n### Question three\n\nWhy do output tokens cost more than input tokens? Because prefill handles the prompt in parallel, while decode generates one token per step and re-reads the weights each time. A generated token takes more GPU time than a read one, and the price follows the time.\n\n> **The honest caveat:** passing this quiz does not make a cluster efficient; the measurements do.\n\nIf you got these, you are reading the system, not the demo. That is the discipline.\n\nThat is the series. Fifty posts, from a cable to a GPU cluster, drawn one page at a time, plus ten Python scripts you can run today. Thank you for following along.\n\n## In one line\n\n> **Draw the whole system.** Fifty posts, one idea: measure the system, not the demo.\n\n**Keep measuring**\n\n```bash\nnvidia-smi dmon -s mu -d 5\n```\n"},{"id":"p322","n":"322","slug":"how-a-sql-query-runs-drawn","title":"How a SQL query runs, drawn","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","DataEngineering","Performance"],"series_name":"Data","design":"architecture","script":"","date":"2028-10-17","files":{"it":"notes/p322-how-a-sql-query-runs-drawn.md"},"md":"# How a SQL query runs, drawn\n\nYou press enter on a SELECT and the rows come back in milliseconds. Between the two, the query crosses three tiers, and each one can be the reason it is slow.\n\n## Quick reference\n\n- **Client:** driver (DRV) · connection pool (POOL)\n- **Engine:** parser (PARSE) · planner (PLAN) · executor (EXEC)\n- **Storage:** buffer cache (CACHE) · heap files (HEAP) · indexes (IDX) · WAL (WAL)\n\n**Request flow:** driver → connection pool → parser → planner → executor → buffer cache → heap files; buffer cache → indexes; executor → WAL\n\n## Explained\n\n**Client**  \nThe driver speaks the database protocol, binds your parameters and returns typed rows. A connection pool keeps a handful of physical connections open, so the next query does not pay for TCP, TLS and authentication again.\n\n**Engine**  \nThe parser turns the text into a syntax tree and checks it against the system tables; a typo fails here. The planner estimates how many rows each step will touch, using the statistics the last ANALYZE collected, then chooses a seq scan or an index, a nested loop or a hash join. The executor pulls rows through that plan, one node at a time, and asks the buffer cache for pages.\n\n**Storage**  \nThe buffer cache holds hot 8 kB pages in shared memory. A miss reads from the heap files that hold the table, or from an index, and a write first appends to the WAL so a crash can replay it. The heap is where the rows live; the index is only a map to them.\n\n> **The honest caveat:** this is PostgreSQL's shape. MySQL, SQL Server and SQLite do the same work under different names, and some engines fuse the parser and planner into one step.\n\n> **The lesson:** when a query is slow, ask which tier is doing the work, not which tier looks busy.\n\n## In one line\n\n> **Parse it, plan it, then read pages.** A query is a path through three tiers, and the slowest step owns the latency.\n\n**Read the plan**\n\n```sql\nEXPLAIN ANALYZE SELECT * FROM t;\n```\n"},{"id":"p323","n":"323","slug":"the-four-joins-drawn","title":"The four joins, drawn","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","DataEngineering","Joins"],"series_name":"Data","design":"explainer","script":"","date":"2028-10-19","files":{"it":"notes/p323-the-four-joins-drawn.md"},"md":"# The four joins, drawn\n\nA join is where two tables meet, and where the row count stops being obvious. Four shapes cover most queries you will write.\n\n## Quick reference\n\n### INNER JOIN\n\n- Only pairs where the ON clause is true\n- Unmatched rows disappear from both sides\n- A many-to-many join can grow the result\n- The default when you write JOIN\n\n### LEFT JOIN\n\n- Every row from the left table survives\n- Unmatched right columns come back NULL\n- Count left rows before and after to check\n- WHERE b.id IS NULL finds the orphans\n\n### RIGHT and FULL\n\n- RIGHT keeps the right table, a mirror of LEFT\n- FULL keeps both sides, NULL where no match\n- MySQL has no FULL OUTER JOIN; use UNION\n- Most RIGHT JOINs read better as LEFT\n\n### CROSS JOIN\n\n- Every left row pairs with every right row\n- n rows times m rows, with no ON clause\n- Useful for date grids and test matrices\n- A missing WHERE gives the same set\n\n> **Note:** A join is a filter on pairs: the ON clause decides which pairs survive.\n\n## Explained\n\n**Inner and left**  \nINNER JOIN keeps only the pairs where the ON clause is true; unmatched rows from either side disappear. LEFT JOIN keeps every row from the left table and fills the right columns with NULL when there is no match. Count the left rows before and after, or add WHERE b.id IS NULL to find the ones that never matched.\n\n**Right and full**  \nRIGHT JOIN is the mirror of LEFT: every right row survives. Most people rewrite it as a LEFT JOIN with tables swapped, because the reading order then matches the data flow. FULL JOIN keeps both sides and puts NULLs wherever a match is missing. MySQL has no FULL OUTER JOIN, so there you union a LEFT and a RIGHT.\n\n**Cross**  \nCROSS JOIN pairs every left row with every right row: n times m rows, with no ON clause. It is the right tool for a date grid or a test matrix, and the wrong tool by accident. FROM a, b with no WHERE is the same set.\n\n> **The honest caveat:** a join does not filter rows in place; it builds pairs first. A missing or too-wide ON clause multiplies the result before any WHERE runs, which is how a report grows from 200 rows to 2 million.\n\n> **The lesson:** draw the Venn diagram, then count the rows; the picture and the count disagree more than you think.\n\n## In one line\n\n> **The ON clause decides the rows.** A join multiplies rows before it filters them, so the count is the first check.\n\n**See the plan**\n\n```sql\nEXPLAIN SELECT * FROM a JOIN b USING (id);\n```\n"},{"id":"p324","n":"324","slug":"the-sql-cheat-sheet","title":"The SQL cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","DataEngineering","CheatSheet"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-10-22","files":{"it":"notes/p324-the-sql-cheat-sheet.md"},"md":"# The SQL cheat sheet\n\nYou know SQL until someone asks for a running total, a top row per group, or the difference between LEFT and FULL. The syntax is small; the habits are not.\n\n## Quick reference\n\n### Select\n\n```sql\nSELECT id, email FROM users LIMIT 10;\nSELECT * FROM orders WHERE total > 100;\nSELECT DISTINCT country FROM users;\nSELECT count(*) FROM orders;\nSELECT id, email FROM users ORDER BY id;\n```\n\n### Join\n\n```sql\nSELECT * FROM a JOIN b ON a.id = b.id;\nSELECT * FROM a LEFT JOIN b USING (id);\nSELECT * FROM a FULL JOIN b USING (id);\nSELECT * FROM a CROSS JOIN b;\nSELECT * FROM a JOIN b ON TRUE;\n```\n\n### Group\n\n```sql\nSELECT c, count(*) FROM t GROUP BY c;\nSELECT a,b,count(*) FROM t GROUP BY a,b;\nSELECT c, sum(x) FROM t GROUP BY c;\nSELECT c, avg(x) FROM t GROUP BY c;\nSELECT c, max(ts) FROM t GROUP BY c;\n```\n\n### Window\n\n```sql\nSELECT sum(x) OVER (ORDER BY id) FROM t;\nSELECT rank() OVER (ORDER BY x) FROM t;\nSELECT lag(x) OVER (ORDER BY id) FROM t;\nSELECT count(*) OVER () FROM t;\nSELECT row_number() OVER () FROM t;\n```\n\n## Explained\n\n**Select**  \nStart with five statements: `SELECT id, email FROM users LIMIT 10;` `SELECT * FROM orders WHERE total > 100;` `SELECT DISTINCT country FROM users;` `SELECT count(*) FROM orders;` and the same select with ORDER BY id. They cover the shape of a read.\n\n**Join**  \nThe join block is four shapes and one trick: INNER JOIN with ON, LEFT JOIN with USING (id), FULL JOIN with USING (id), CROSS JOIN, and JOIN b ON TRUE, which is a cross join in disguise. USING (id) needs the same column name on both sides.\n\n**Group**  \nGROUP BY collapses rows into groups. `SELECT c, count(*) FROM t GROUP BY c;` counts them, sum and avg add and average a numeric column, and GROUP BY a,b groups on a pair. Every selected column is either grouped or aggregated.\n\n**Window**  \nA window function keeps the rows and adds a value over a frame. sum(x) OVER (ORDER BY id) is a running total, rank() numbers ties the same, lag(x) reaches the previous row, and count(\\*) OVER () totals every row.\n\n> **The honest caveat:** the cheat sheet is PostgreSQL syntax. USING, FILTER and window functions are widely supported, but the clause order in your engine may differ, and a window cannot go in WHERE.\n\n> **The lesson:** the clause order is fixed, so learn the shape and the words stop mattering.\n\n## In one line\n\n> **Describe the rows, not the steps.** The clause order is fixed; the words are the only thing that changes.\n\n**See the shape**\n\n```sql\nEXPLAIN (FORMAT JSON) SELECT 1;\n```\n"},{"id":"p325","n":"325","slug":"a-query-plan-differ-in-python","title":"A query plan differ in Python","sections":["it"],"topic":"Python scripts","tags":["SQL","PostgreSQL","Python","QueryPlans","Performance"],"series_name":"Data","design":"code","script":"plan_diff.py","date":"2028-10-24","files":{"it":"notes/p325-a-query-plan-differ-in-python.md"},"md":"# A query plan differ in Python\n\nA query gets slower after a deploy, and the first question is whether the plan changed. Reading two EXPLAIN outputs by eye is how you miss the join that moved.\n\n## Quick reference\n\nThe full script is at the end of this note: `plan_diff.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 plan_diff.py old.json new.json\n```\n\n**Example output**\n\n```text\nPLAN DIFF\n35.5 -> 182.4 WORSE\n```\n\n## Explained\n\n**What it reads**  \nEXPLAIN (FORMAT JSON) returns an array with one object, and that object holds a Plan. Every plan node has a Node Type, a Total Cost, a Plan Rows estimate, and usually a Plans array of children. The script loads both files, takes the first element, and walks the tree.\n\n**What it compares**  \nIt records the node type of every node in order, so a changed join or scan shows up as a different sequence. It reads the root Total Cost and Plan Rows from both plans. The new plan is worse when its cost is over 1.2 times the old one, or its estimated rows are more than double; either makes the exit code 1.\n\n**How to run it**  \nSave each plan with `psql -c \"EXPLAIN` (FORMAT JSON) SELECT ...\" -o `old.json`, then run the script with both files. A node change alone prints PLAN DIFF but does not fail the run; a cost or row regression does. Exit 2 means a file is missing or is not an EXPLAIN array.\n\n> **The honest caveat:** Total Cost is an estimate in arbitrary units, not milliseconds, and it only means something on the same server with the same settings. For real time, use EXPLAIN (ANALYZE, BUFFERS).\n\n> **The lesson:** compare the plan before the release, not the wall clock after the incident.\n\n## In one line\n\n> **Cost is an estimate, not a promise.** A plan change is a code change, and it deserves the same review.\n\n**Diff two plans**\n\n```bash\npython3 plan_diff.py old.json new.json\n```\n\n## Script: plan_diff.py\n\nDownload: [scripts/plan_diff.py](../scripts/plan_diff.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"plan_diff.py - compare two PostgreSQL EXPLAIN (FORMAT JSON) plans.\n\nReads the saved output of EXPLAIN (FORMAT JSON) and reports what changed:\nthe node types in order, the root Total Cost and the root Plan Rows. A node\ntype change is reported but does not fail the run. The new plan is worse when\nits root cost is over --cost-factor times the old cost, or its root row\nestimate is over --row-factor times the old estimate; then the exit code is 1\nso a CI job can fail on the regression.\n\nRead-only: it opens two files and writes nothing.\n\nExit codes: 0 clean or unchanged, 1 the new plan is worse, 2 usage or input\nerror (missing file, invalid JSON, or not an EXPLAIN array).\n\nUsage:\n    psql -c \"EXPLAIN (FORMAT JSON) SELECT * FROM orders WHERE id = 1\" -o old.json\n    psql -c \"EXPLAIN (FORMAT JSON) SELECT * FROM orders WHERE id = 1\" -o new.json\n    python3 plan_diff.py old.json new.json\n    python3 plan_diff.py old.json new.json --cost-factor 1.5\n    python3 plan_diff.py old.json new.json --row-factor 3\n    python3 plan_diff.py old.json new.json --json\n\nCI, fail the job when the new plan regresses:\n    python3 plan_diff.py old.json new.json || exit 1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass PlanError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_plan(path):\n    \"\"\"Load the root Plan object from one EXPLAIN (FORMAT JSON) file.\"\"\"\n    try:\n        with open(path) as fh:\n            doc = json.load(fh)\n    except OSError as exc:\n        raise PlanError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise PlanError(\"%s is not JSON: %s\" % (path, exc))\n    if isinstance(doc, list):\n        if len(doc) != 1 or not isinstance(doc[0], dict):\n            raise PlanError(\"%s is not an EXPLAIN array with one object\" % path)\n        doc = doc[0]\n    if not isinstance(doc, dict) or \"Plan\" not in doc:\n        raise PlanError(\"%s has no Plan object\" % path)\n    plan = doc[\"Plan\"]\n    if not isinstance(plan, dict):\n        raise PlanError(\"%s: Plan is not an object\" % path)\n    return plan\n\n\ndef check_node(plan):\n    \"\"\"Reject a plan node that is missing the fields or types we need.\"\"\"\n    if not isinstance(plan.get(\"Node Type\"), str):\n        raise PlanError(\"a plan node has no string Node Type\")\n    for key in (\"Total Cost\", \"Plan Rows\"):\n        value = plan.get(key)\n        if isinstance(value, bool) or not isinstance(value, (int, float)):\n            raise PlanError(\"a plan node has no numeric %r\" % key)\n\n\ndef walk(plan, out):\n    \"\"\"Append (node type, total cost, plan rows) for every node, in order.\"\"\"\n    check_node(plan)\n    out.append((plan[\"Node Type\"], plan[\"Total Cost\"], plan[\"Plan Rows\"]))\n    children = plan.get(\"Plans\", [])\n    if not isinstance(children, list):\n        raise PlanError(\"a plan node has a non-list Plans value\")\n    for child in children:\n        if not isinstance(child, dict):\n            raise PlanError(\"a Plans entry is not an object\")\n        walk(child, out)\n    return out\n\n\ndef pct(old, new):\n    \"\"\"Percentage change, or n/a when the old value is zero.\"\"\"\n    if old == 0:\n        return \"n/a\"\n    return \"%+.1f%%\" % ((new - old) / old * 100.0)\n\n\ndef compare(old_path, new_path, cost_factor, row_factor):\n    \"\"\"Build the report dictionary for two plan files.\"\"\"\n    old_root = load_plan(old_path)\n    new_root = load_plan(new_path)\n    old_nodes = walk(old_root, [])\n    new_nodes = walk(new_root, [])\n\n    old_cost, old_rows = old_root[\"Total Cost\"], old_root[\"Plan Rows\"]\n    new_cost, new_rows = new_root[\"Total Cost\"], new_root[\"Plan Rows\"]\n    cost_worse = new_cost > old_cost * cost_factor\n    row_worse = new_rows > old_rows * row_factor\n\n    old_types = [node[0] for node in old_nodes]\n    new_types = [node[0] for node in new_nodes]\n    changes = []\n    for i, (a, b) in enumerate(zip(old_types, new_types)):\n        if a != b:\n            changes.append({\"position\": i + 1, \"old\": a, \"new\": b})\n    return {\n        \"old_file\": old_path,\n        \"new_file\": new_path,\n        \"old_nodes\": old_types,\n        \"new_nodes\": new_types,\n        \"node_count\": [len(old_types), len(new_types)],\n        \"changes\": changes,\n        \"old_cost\": old_cost,\n        \"new_cost\": new_cost,\n        \"old_rows\": old_rows,\n        \"new_rows\": new_rows,\n        \"cost_factor\": cost_factor,\n        \"row_factor\": row_factor,\n        \"cost_worse\": cost_worse,\n        \"row_worse\": row_worse,\n        \"worse\": cost_worse or row_worse,\n    }\n\n\ndef print_report(rep):\n    \"\"\"The human-readable report.\"\"\"\n    print(\"PLAN DIFF  %s -> %s\" % (rep[\"old_file\"], rep[\"new_file\"]))\n    print(\"NODES      %d -> %d\" % (rep[\"node_count\"][0], rep[\"node_count\"][1]))\n    for change in rep[\"changes\"][:10]:\n        print(\"CHANGED    node %d: %s -> %s\"\n              % (change[\"position\"], change[\"old\"], change[\"new\"]))\n    if len(rep[\"changes\"]) > 10:\n        print(\"CHANGED    ... and %d more\" % (len(rep[\"changes\"]) - 10))\n    if not rep[\"changes\"]:\n        print(\"CHANGED    none\")\n    print(\"COST       %.2f -> %.2f  (%s)\"\n          % (rep[\"old_cost\"], rep[\"new_cost\"],\n             pct(rep[\"old_cost\"], rep[\"new_cost\"])))\n    print(\"ROWS       %.0f -> %.0f  (%s)\"\n          % (rep[\"old_rows\"], rep[\"new_rows\"],\n             pct(rep[\"old_rows\"], rep[\"new_rows\"])))\n    if rep[\"worse\"]:\n        reasons = []\n        if rep[\"cost_worse\"]:\n            reasons.append(\"cost over %.2fx\" % rep[\"cost_factor\"])\n        if rep[\"row_worse\"]:\n            reasons.append(\"rows over %.2fx\" % rep[\"row_factor\"])\n        print(\"VERDICT    WORSE (%s)\" % \", \".join(reasons))\n    else:\n        print(\"VERDICT    OK (within %.2fx cost and %.2fx rows)\"\n              % (rep[\"cost_factor\"], rep[\"row_factor\"]))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Compare two PostgreSQL EXPLAIN (FORMAT JSON) plans.\",\n        epilog=\"Exit codes: 0 clean, 1 the new plan is worse, 2 usage or input error.\")\n    parser.add_argument(\"old\", help=\"old plan JSON file\")\n    parser.add_argument(\"new\", help=\"new plan JSON file\")\n    parser.add_argument(\"--cost-factor\", type=float, default=1.2, metavar=\"X\",\n                        help=\"fail when new cost is over X times the old cost (default 1.2)\")\n    parser.add_argument(\"--row-factor\", type=float, default=2.0, metavar=\"X\",\n                        help=\"fail when new rows are over X times the old rows (default 2.0)\")\n    parser.add_argument(\"--json\", action=\"store_true\", help=\"print one JSON report\")\n    args = parser.parse_args(argv)\n\n    if args.cost_factor <= 0 or args.row_factor <= 0:\n        parser.error(\"--cost-factor and --row-factor must be positive\")\n    try:\n        rep = compare(args.old, args.new, args.cost_factor, args.row_factor)\n    except PlanError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n    if args.json:\n        print(json.dumps(rep, indent=2, sort_keys=True))\n    else:\n        print_report(rep)\n    return 1 if rep[\"worse\"] else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p326","n":"326","slug":"acid-in-four-panels","title":"ACID, in four panels","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","Transactions","DataEngineering"],"series_name":"Data","design":"explainer","script":"","date":"2028-10-26","files":{"it":"notes/p326-acid-in-four-panels.md"},"md":"# ACID, in four panels\n\nA payment leaves one account and never reaches the other, and the database says the transaction committed. ACID is the four-part answer to how that is supposed to be impossible.\n\n## Quick reference\n\n### Atomicity - all or nothing\n\n- A transaction is all or nothing\n- Any error aborts the whole transaction\n- ROLLBACK undoes every statement\n- Savepoints allow a partial rollback\n\n### Consistency - rules hold\n\n- Constraints reject invalid rows\n- NOT NULL, UNIQUE, CHECK, FOREIGN KEY\n- Triggers and rules run inside the transaction\n- Your application invariants are still yours\n\n### Isolation - no interference\n\n- Concurrent transactions cannot see uncommitted work\n- READ COMMITTED is the PostgreSQL default\n- SERIALIZABLE adds predicate locking (SSI)\n- Higher isolation costs retries and locks\n\n### Durability - it stays\n\n- COMMIT returns only after the WAL is flushed\n- synchronous_commit=on is the default\n- fsync=off trades durability for speed\n- A replica is not a substitute for a backup\n\n> **Note:** Atomicity and durability are guarantees; isolation is the level you choose.\n\n## Explained\n\n**Atomicity and consistency**  \nAtomicity means all or nothing: any error aborts the transaction, ROLLBACK undoes every statement, and a savepoint lets you undo part of it. Consistency means the database only moves between valid states: NOT NULL, UNIQUE, CHECK and FOREIGN KEY reject bad rows, and triggers run inside the same transaction. The invariants your application checks are still your job.\n\n**Isolation and durability**  \nIsolation keeps concurrent transactions from seeing each other's uncommitted work. READ COMMITTED is the PostgreSQL default; SERIALIZABLE adds SSI and can abort a transaction that would break a serial order. Durability means a committed change survives a crash: COMMIT returns after the WAL is flushed, synchronous_commit is on by default, and fsync=off trades that guarantee for speed.\n\n**What it costs**  \nHigher isolation means more locks or more retries. A serialisable transaction can fail with a serialisation error and must be retried, so the application needs a retry loop, not just a setting.\n\n> **The honest caveat:** consistency in ACID is about constraints, not about your business rules. If no constraint encodes a rule, the database will happily commit a state your code calls impossible.\n\n> **The lesson:** atomicity and durability are guarantees; isolation is a dial; consistency is only as strong as the constraints you wrote.\n\n## In one line\n\n> **A commit is a promise.** Atomicity and durability are guaranteed; isolation is the setting you choose.\n\n**Check the level**\n\n```sql\nSHOW transaction_isolation;\n```\n"},{"id":"p327","n":"327","slug":"the-b-tree-behind-your-index-drawn","title":"The B-tree behind your index, drawn","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","Indexes","Performance"],"series_name":"Data","design":"architecture","script":"","date":"2028-10-29","files":{"it":"notes/p327-the-b-tree-behind-your-index-drawn.md"},"md":"# The B-tree behind your index, drawn\n\nAn index lookup feels like magic: one key in, one row out, even in a table with fifty million rows. Underneath, it is a small tree of pages and a short walk down.\n\n## Quick reference\n\n- **Root:** meta page (META) · root page (ROOT)\n- **Internal:** internal page (INNER) · separator keys (KEY) · downlinks (PTR)\n- **Leaf:** leaf page (LEAF) · row pointers (TID) · heap files (HEAP)\n\n**Request flow:** meta page → root page → internal page → separator keys → downlinks → leaf page → row pointers → heap files\n\n## Explained\n\n**Root and internal**  \nBlock 0 of a PostgreSQL B-tree index is the metapage; it stores the root block number and the tree level. The root page is a B-tree page like any other. Below it, each internal page holds separator keys and downlinks: compare your key with the separators, follow one downlink, and repeat at the next level.\n\n**Leaf**  \nLeaf pages hold the index tuples in key order: the key and a row pointer, the six-byte address of the heap row. The leaves are also linked left and right, which is what makes a range scan cheap: find the first key, then walk the leaf chain. The heap files hold the actual rows, and an index-only scan still has to check visibility.\n\n**Why it matters**  \nFanout is the trick. A page holds hundreds of keys, so three or four levels cover millions of rows. One lookup reads one page per level, usually three or four pages, instead of scanning the table.\n\n> **The honest caveat:** PostgreSQL's B-tree is a Lehman-Yao variant with high keys and sibling links, not the textbook B+tree. The search shape is the same; the details are not.\n\n> **The lesson:** the index is small because each page points to many others; the tree is shallow for the same reason.\n\n## In one line\n\n> **Down the tree, then to the heap.** The tree stays shallow because every page points to many others.\n\n**See the depth**\n\n```sql\nSELECT * FROM bt_metap('idx');\n```\n"},{"id":"p328","n":"328","slug":"the-psql-cheat-sheet","title":"The psql cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","psql","Databases","CommandLine","DataEngineering"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-10-31","files":{"it":"notes/p328-the-psql-cheat-sheet.md"},"md":"# The psql cheat sheet\n\nYou open a terminal, connect to a database you last touched six months ago, and need to know what is in it before you change anything. psql answers most of that without a browser.\n\n## Quick reference\n\n### Connect\n\n```bash\npsql -h db -U app -d appdb\npsql \"postgresql://app@db/app\"\npsql -l\n\\c appdb\n\\conninfo\n```\n\n### Inspect\n\n```text\n\\l\n\\dt\n\\d+ orders\n\\di\n\\df\n```\n\n### Query\n\n```text\n\\timing\n\\x auto\n\\watch 2\nSELECT * FROM orders LIMIT 10;\n\\e\n```\n\n### Admin\n\n```text\n\\du\n\\password\nSELECT pg_terminate_backend(123);\nVACUUM (VERBOSE) orders;\nANALYZE orders;\n```\n\n## Explained\n\n**Connect**  \n`psql -h db -U app -d appdb` opens a session; the URL form, `psql \"postgresql://app@db/app\"`, does the same in one string. `psql -l` lists the databases, `\\c appdb` switches to one, and `\\conninfo` tells you which host, port, user and database you are actually on.\n\n**Inspect**  \nThe backslash commands read the system tables. \\\\l lists databases, `\\dt` lists tables, `\\d+ orders` shows the columns, indexes, constraints and storage for one table, `\\di` lists indexes and `\\df` lists functions. None of them change anything.\n\n**Query**  \n`\\timing` turns on the duration line, `\\x auto` switches to expanded output when a row is wide, and `\\watch 2` re-runs the last query every two seconds. `SELECT * FROM orders LIMIT 10;` is the query you will type most, and \\\\e opens the last statement in your editor.\n\n**Admin**  \n`\\du` lists roles, `\\password` changes the current role's password, and `SELECT pg_terminate_backend(123);` ends one session by PID. `VACUUM (VERBOSE) orders;` reclaims dead rows and `ANALYZE orders;` refreshes the statistics the planner uses.\n\n> **The honest caveat:** psql is a client, not a backup tool. \\\\copy moves data, but it is not pg_dump, and the backslash commands are not SQL, so they cannot go inside a migration.\n\n> **The lesson:** learn ten backslash commands and you stop opening a GUI to answer a question the prompt already knows.\n\n## In one line\n\n> **The database is one prompt away.** The prompt answers most questions before a GUI finishes loading.\n\n**Reconnect**\n\n```bash\npsql -h db -U app -d appdb\n```\n"},{"id":"p329","n":"329","slug":"oltp-vs-olap","title":"OLTP vs OLAP","sections":["it"],"topic":"Databases & SQL","tags":["Databases","OLTP","OLAP","DataEngineering","SQL"],"series_name":"Data","design":"compare","script":"","date":"2028-11-02","files":{"it":"notes/p329-oltp-vs-olap.md"},"md":"# OLTP vs OLAP\n\nThe dashboard query takes forty seconds and the checkout query takes four milliseconds, and both run on the same server. That gap is the OLTP and OLAP split, and it starts in how the data is stored.\n\n## Quick reference\n\n| | **OLTP** | **OLAP** |\n|:--|:--|:--|\n| Workload | Short writes, point reads | Long scans, aggregates |\n| Typical query | One row by primary key | Millions of rows, grouped |\n| Indexes | Many, narrow, covering | Few, columnar, compressed |\n| Storage | Row store, 8 kB pages | Column store, compressed |\n| Latency | Milliseconds per query | Seconds to minutes |\n| Scale by | More connections, sharding | More workers, partitions |\n\n**OLTP** (Many short reads and writes): The checkout database: many short transactions, one row at a time.\n\n```bash\npgbench -c 32 -T 60\n```\n\n**OLAP** (Few large scans, many columns): The warehouse: few long scans, many columns, one answer.\n\n```sql\nSELECT count(*) FROM events;\n```\n\n- OLTP → write fast, read one row, stay correct.\n- OLAP → read everything, once, and forgive latency.\n\n## Explained\n\n**The workload**  \nOLTP is the database behind checkout: many short transactions, each reading or writing one row by primary key. Latency is the product, so it is measured in milliseconds and the schema is normalised to keep writes cheap. OLAP is the warehouse behind the dashboard: few queries, each scanning millions of rows and grouping them. Throughput matters more than per-query latency.\n\n**The storage**  \nA row store keeps a whole row together, so a point lookup reads one page. A column store keeps each column apart and compresses it, so a scan of three columns never touches the other thirty. That is why an OLTP index is narrow and many, and an OLAP table is wide, partitioned and columnar.\n\n**Where the line blurs**  \nSome managed warehouses now offer row-store tables for small dimensions, and PostgreSQL extensions add columnar storage to an OLTP database. The labels describe a workload, not a product.\n\n> **The honest caveat:** a small warehouse on a row store can beat a badly modelled column store, and an HTAP system that claims both usually pays for it somewhere. Measure your queries before you move them.\n\n> **The lesson:** choose the workload first, then the storage; the label is a shortcut, not a specification.\n\nOLTP writes the row.  \nOLAP reads the world.\n"},{"id":"p330","n":"330","slug":"a-slow-query-log-report-in-python","title":"A slow-query log report in Python","sections":["it"],"topic":"Python scripts","tags":["SQL","PostgreSQL","Python","Logging","Performance"],"series_name":"Data","design":"code","script":"slow_query_report.py","date":"2028-11-05","files":{"it":"notes/p330-a-slow-query-log-report-in-python.md"},"md":"# A slow-query log report in Python\n\nThe pager goes off at 02:00 because the checkout is slow, and the dashboard shows a flat line. The database log has been recording every slow statement for weeks.\n\n## Quick reference\n\nThe full script is at the end of this note: `slow_query_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 slow_query_report.py postgres.log\n```\n\n**Example output**\n\n```text\n1800.000 ms  SELECT * FROM orders WHERE total > 100\nSLOW 2 of 5\n```\n\n## Explained\n\n**What it reads**  \nThe script scans a PostgreSQL log for lines containing duration: 1234.567 ms. It takes the number after duration:, strips the statement: or execute \\<unnamed>: prefix that follows, and keeps the text. Lines without a duration are skipped, and parse and bind lines are ignored because they are steps, not query executions.\n\n**What it ranks**  \nEach statement becomes a row of milliseconds and text. The report sorts them slowest first, prints the slowest rows, and counts how many crossed the threshold. The exit code is 1 when at least one statement is over it, so a CI job fails on the same evidence you would read.\n\n**How to run it**  \n`python3 slow_query_report.py postgres.log --threshold 1000` prints the report. The threshold defaults to 1000 ms; `--top` changes the list length and `--json` prints one object for a dashboard. Exit 2 means the file could not be read or the arguments were wrong, which is a broken check, not a clean log.\n\n> **The honest caveat:** the report only sees what the log wrote. log_min_duration_statement filters at the server, and a statement containing newlines is written across several lines, of which the script reads only the first.\n\n> **The lesson:** the slow query is usually already in the log, waiting for someone to rank it.\n\n## In one line\n\n> **The log knows what dashboards hide.** The log already has the evidence; the report just ranks it.\n\n**Rank the log**\n\n```bash\npython3 slow_query_report.py postgres.log\n```\n\n## Script: slow_query_report.py\n\nDownload: [scripts/slow_query_report.py](../scripts/slow_query_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"slow_query_report.py - rank slow statements in a PostgreSQL log.\n\nScans a PostgreSQL server log for lines containing \"duration: 1234.567 ms\",\nkeeps the statement text after \"statement:\" or \"execute ...:\", ranks the\nslowest statements and exits 1 when one is over --threshold.\n\nRead-only: it reads a log file (or stdin with \"-\") and writes nothing.\n\nExit codes: 0 no statement over the threshold, 1 at least one statement over\nit, 2 usage or input error.\n\nUsage:\n    python3 slow_query_report.py postgresql.log\n    python3 slow_query_report.py postgresql.log --threshold 500 --top 20\n    tail -n 5000 postgresql.log | python3 slow_query_report.py -\n    python3 slow_query_report.py postgresql.log --json\n\nCron or CI, fail the job when a slow statement appears:\n    python3 slow_query_report.py /var/log/postgresql/postgresql.log || exit 1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\nDURATION_RE = re.compile(r\"duration:\\s*([0-9]+(?:\\.[0-9]+)?)\\s*ms\\b\", re.IGNORECASE)\nMAX_STATEMENT = 110\n\n\nclass ReportError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef statement_after(line, match):\n    \"\"\"The statement text after a duration match, or None for parse/bind lines.\"\"\"\n    rest = line[match.end():].strip()\n    if not rest:\n        return None\n    low = rest.lower()\n    if low.startswith(\"statement:\"):\n        return rest[len(\"statement:\"):].strip() or None\n    if low.startswith(\"execute \"):\n        if \":\" not in rest:\n            return None\n        return rest.split(\":\", 1)[1].strip() or None\n    return None\n\n\ndef parse_lines(lines):\n    \"\"\"Every (duration_ms, statement) pair in an iterable of log lines.\"\"\"\n    rows = []\n    for line in lines:\n        match = DURATION_RE.search(line)\n        if not match:\n            continue\n        stmt = statement_after(line, match)\n        if stmt is None:\n            continue\n        rows.append((float(match.group(1)), \" \".join(stmt.split())))\n    return rows\n\n\ndef read_rows(path):\n    \"\"\"Read the log file (or stdin for \"-\") and return the duration rows.\"\"\"\n    try:\n        if path == \"-\":\n            return parse_lines(sys.stdin)\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as fh:\n            return parse_lines(fh)\n    except OSError as exc:\n        raise ReportError(\"cannot read %s: %s\" % (path, exc))\n\n\ndef build_report(path, rows, threshold, top):\n    \"\"\"The report dictionary, ready for text or JSON output.\"\"\"\n    ranked = sorted(rows, key=lambda row: -row[0])[:top]\n    over = [row for row in rows if row[0] > threshold]\n    slowest = max((row[0] for row in rows), default=0.0)\n    return {\n        \"file\": path,\n        \"threshold_ms\": threshold,\n        \"statements\": len(rows),\n        \"over\": len(over),\n        \"slowest_ms\": slowest,\n        \"top\": [{\"duration_ms\": ms, \"statement\": stmt} for ms, stmt in ranked],\n        \"ok\": not over,\n    }\n\n\ndef print_report(rep):\n    \"\"\"The human-readable report.\"\"\"\n    print(\"SLOW QUERY REPORT  %s\" % rep[\"file\"])\n    print(\"threshold %.3f ms | %d statements | %d over\"\n          % (rep[\"threshold_ms\"], rep[\"statements\"], rep[\"over\"]))\n    if rep[\"top\"]:\n        print(\"\")\n        print(\"%3s  %12s  %s\" % (\"#\", \"DURATION ms\", \"STATEMENT\"))\n        for i, item in enumerate(rep[\"top\"], 1):\n            stmt = item[\"statement\"]\n            if len(stmt) > MAX_STATEMENT:\n                stmt = stmt[:MAX_STATEMENT - 3] + \"...\"\n            print(\"%3d  %12.3f  %s\" % (i, item[\"duration_ms\"], stmt))\n        print(\"\")\n    if rep[\"ok\"]:\n        print(\"OK %d statements, none over %.3f ms\"\n              % (rep[\"statements\"], rep[\"threshold_ms\"]))\n    else:\n        print(\"ALERT %d statements over %.3f ms\"\n              % (rep[\"over\"], rep[\"threshold_ms\"]))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank the slowest statements in a PostgreSQL log.\",\n        epilog=\"Exit codes: 0 clean, 1 slow statement, 2 usage or input error.\")\n    parser.add_argument(\"logfile\", nargs=\"?\", default=\"-\",\n                        help=\"log file to read, or - for stdin (default -)\")\n    parser.add_argument(\"--threshold\", type=float, default=1000.0, metavar=\"MS\",\n                        help=\"flag statements over this many milliseconds (default 1000)\")\n    parser.add_argument(\"--top\", type=int, default=10, metavar=\"N\",\n                        help=\"rows to print (default 10)\")\n    parser.add_argument(\"--json\", action=\"store_true\", help=\"print one JSON report\")\n    args = parser.parse_args(argv)\n\n    if args.threshold < 0:\n        parser.error(\"--threshold must be zero or positive\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n    try:\n        rows = read_rows(args.logfile)\n    except ReportError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n    rep = build_report(args.logfile, rows, args.threshold, args.top)\n    if args.json:\n        print(json.dumps(rep, indent=2, sort_keys=True))\n    else:\n        print_report(rep)\n    return 0 if rep[\"ok\"] else 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p331","n":"331","slug":"quiz-3-sql-questions","title":"Quiz: 3 SQL questions","sections":["it"],"topic":"Databases & SQL","tags":["SQL","PostgreSQL","Databases","QueryPlans","Quiz"],"series_name":"Data","design":"quiz","script":"","date":"2028-11-07","files":{"it":"notes/p331-quiz-3-sql-questions.md"},"md":"# Quiz: 3 SQL questions\n\nThree SQL questions show up in every interview, and the wrong answer is usually the one that sounds like a rule.\n\n## Quick reference\n\n**Q1. Why can a sequential scan beat an index on a small table?**\n\n- a) The whole table fits in a few pages\n- b) Indexes are ignored below 1,000 rows\n- c) A sequential scan uses less CPU\n\n> **Answer:** a) The whole table fits in a few pages. An index scan pays a root-to-leaf descent plus a heap fetch per row. When the table fits in a few pages, reading every page once is cheaper; the planner prices both.\n\n**Q2. In a composite index on (a, b), what does the leftmost prefix mean?**\n\n- a) A filter on a, or on a and b, can use it\n- b) A filter on b alone can use it\n- c) Column order does not matter\n\n> **Answer:** a) A filter on a, or on a and b, can use it. The index is sorted by a, then by b. A filter on b alone cannot seek, because b is only ordered inside each value of a; a range on a also limits the b part.\n\n**Q3. Which isolation level stops a phantom read?**\n\n- a) SERIALIZABLE\n- b) READ COMMITTED\n- c) READ UNCOMMITTED\n\n> **Answer:** a) SERIALIZABLE. A phantom is a new row in a repeated range query. READ COMMITTED allows it; SERIALIZABLE does not. PostgreSQL's REPEATABLE READ also hides phantoms.\n\n## Explained\n\n**The small table**  \nQuestion one: why can a sequential scan beat an index on a small table? Because the planner compares costs, not rules. An index scan pays for a descent from root to leaf and then a heap fetch per row; when the table fits in a handful of pages, reading all of them sequentially is cheaper. The index is not ignored; it is priced.\n\n**The prefix**  \nQuestion two: in a composite index on (a, b), what does the leftmost prefix mean? The index is sorted by a, then by b. A filter on a, or on a and b, can use it; a filter on b alone cannot seek, because b is only ordered inside each a. A range on a limits how much of b the scan can use.\n\n**The phantom**  \nQuestion three: which isolation level stops a phantom read? A phantom is a row that appears between two reads of the same range. READ COMMITTED allows it; SERIALIZABLE rules it out. PostgreSQL's REPEATABLE READ also hides phantoms because it reads one snapshot, but SERIALIZABLE is the level that rules out every anomaly.\n\n> **The honest caveat:** the planner's choice depends on statistics, cost settings and the correlation of the column, so the same query can flip plans after an ANALYZE or a version upgrade.\n\n> **The lesson:** the planner is always costing; learn what it prices, and the three answers become obvious.\n\n## In one line\n\n> **Read the plan, not the rule.** Three answers that decide how you read a query plan.\n\n**Try it**\n\n```sql\nEXPLAIN (ANALYZE, BUFFERS) SELECT 1;\n```\n"},{"id":"p332","n":"332","slug":"postgresql-architecture-drawn","title":"PostgreSQL architecture, drawn","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","Databases","Architecture","SRE","DataEngineering"],"series_name":"Data","design":"architecture","script":"","date":"2028-11-09","files":{"it":"notes/p332-postgresql-architecture-drawn.md"},"md":"# PostgreSQL architecture, drawn\n\nA slow query is rarely one process. PostgreSQL splits the work across a supervisor, a process per connection, and a few background workers that all write the same files.\n\n## Quick reference\n\n- **Tier 1 · Client:** Application (APP) · Connection pooler (POOL)\n- **Tier 2 · Processes:** Postmaster (POST) · Backend process (BACK) · WAL writer (WAL) · Checkpointer (CKPT)\n- **Tier 3 · Storage:** Heap files (HEAP) · WAL segments (WAL) · WAL archive (ARCH)\n\n**Request flow:** Application → Connection pooler → Postmaster → Backend process → WAL writer → WAL segments → WAL archive; Backend process → Heap files; Checkpointer → Heap files\n\n## Explained\n\n**Client**  \nThe application opens a connection, and a pooler keeps a small set of server connections warm. PostgreSQL forks a backend process per connection, so each idle connection still costs memory; the pooler exists to stop that cost growing with every application worker.\n\n**Processes**  \nThe postmaster listens on the port and forks a backend for each client. The backend parses, plans and executes the query, reads pages through shared buffers, and writes WAL buffers before the change reaches the heap. The WAL writer flushes those buffers into WAL segments. The checkpointer writes dirty shared buffers back to the data files and updates the control file.\n\n**Storage**  \nHeap files hold table pages in 8 kB blocks, and indexes live in their own files. WAL segments hold the redo stream, 16 MB each by default, and the WAL archive keeps completed segments for point-in-time recovery and for rebuilding a standby. The archive is written by archive_command, not by the checkpointer.\n\n> **The honest caveat:** a transaction-mode pooler breaks session state such as advisory locks and SET, so use session mode when the application depends on it.\n\n> **The lesson:** every query is a process reading pages and writing WAL, and durability is the WAL reaching disk.\n\n## In one line\n\n> **Processes write, WAL decides.** The diagram is a hierarchy of processes over two kinds of file.\n\n**Check the archiver**\n\n```sql\nSELECT * FROM pg_stat_archiver;\n```\n"},{"id":"p333","n":"333","slug":"backups-physical-logical-pitr-and-the-restore-test","title":"Backups: physical, logical, PITR and the restore test","sections":["it"],"topic":"Databases & SQL","tags":["Backups","PostgreSQL","PITR","SRE","DataEngineering"],"series_name":"Data","design":"explainer","script":"","date":"2028-11-12","files":{"it":"notes/p333-backups-physical-logical-pitr-and-the-restore-test.md"},"md":"# Backups: physical, logical, PITR and the restore test\n\nA backup you have never restored is a rumour with a timestamp. Four panels separate what you copy from the test that proves it.\n\n## Quick reference\n\n### Physical backup\n\n- Copies data files block by block\n- pg_basebackup or a filesystem snapshot\n- Whole cluster, same major version\n- Fast restore; no table cherry-picking\n\n### Logical backup\n\n- pg_dump writes SQL or a custom archive\n- One database, one schema or one table\n- Portable to a newer major version\n- Slow restore: every index is rebuilt\n\n### Point in time\n\n- A base backup plus every WAL segment since\n- Replay stops at a time, LSN or restore point\n- Needs archive_mode and a working archive\n- Undoes a bad migration, not one table\n\n### Restore test\n\n- Restore into an isolated instance\n- Measure the time, not just the success\n- Check row counts and application queries\n- Schedule it; a drill you skip is a guess\n\n> **Note:** A physical backup restores the cluster; a logical one restores an object.\n\n## Explained\n\n**Physical backup**  \nThree disks stand for the data files copied block by block: pg_basebackup, a filesystem snapshot or a storage clone. It captures the whole cluster at one consistent point, keeps the same major version, and restores quickly because nothing is rebuilt from SQL.\n\n**Logical backup**  \nA stack of statements stands for the object copy. pg_dump writes SQL or a custom archive for one database, schema or table; pg_dumpall adds roles and tablespaces. The output loads into a newer major version and restores one table, at the cost of rebuilding every index and replaying every row.\n\n**Point in time**  \nThe shield in the third panel is the archive. PITR needs a base backup and every WAL segment written since it started. Restore the base, point recovery at the archive, and replay stops at a time, an LSN or a restore point. Lose one segment and the replay stops early.\n\n**Restore test**  \nThe last panel is a check mark. Restore into an isolated instance, measure the time, run the application's real queries, and compare row counts. A drill you skip is a guess.\n\n> **The honest caveat:** PITR restores the whole cluster to a moment, not one table, and it only works if every WAL segment reached the archive.\n\n> **The lesson:** copy the cluster, copy the objects, replay the WAL, then prove all three with a restore.\n\n## In one line\n\n> **A backup is a tested restore.** A copy is not a backup until a restore has opened it.\n\n**Book the drill**\n\n```bash\npg_basebackup -D /restore -Fp -X stream\n```\n"},{"id":"p334","n":"334","slug":"the-mysql-cheat-sheet","title":"The MySQL cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["MySQL","Databases","SQL","DevOps","CheatSheet"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-11-14","files":{"it":"notes/p334-the-mysql-cheat-sheet.md"},"md":"# The MySQL cheat sheet\n\nMySQL sits behind a lot of PHP, WordPress and custom applications, and most of the work happens in the client. Four blocks cover the commands you type when something is slow or locked.\n\n## Quick reference\n\n### Connect\n\n```bash\nmysql -u root -p\nmysql -h db1 -u app -p shop\nmysql --defaults-file=/etc/my.cnf\nmysql -e 'SELECT VERSION();'\nmysqladmin -u root -p ping\n```\n\n### Inspect\n\n```sql\nSHOW DATABASES;\nSHOW TABLES;\nSHOW TABLE STATUS\\G\nSHOW INDEX FROM orders;\nEXPLAIN SELECT * FROM orders;\n```\n\n### Users\n\n```sql\nSELECT user, host FROM mysql.user;\nSHOW GRANTS FOR 'app'@'%';\nCREATE USER 'app'@'%';\nGRANT SELECT ON shop.* TO 'app'@'%';\nREVOKE ALL ON shop.* FROM 'app'@'%';\n```\n\n### Replication\n\n```sql\nSHOW REPLICA STATUS\\G\nSHOW REPLICAS;\nSHOW BINARY LOGS;\nSTART REPLICA;\nSTOP REPLICA;\n```\n\n## Explained\n\n**Connect**  \n`mysql -u root -p` prompts for a password instead of putting it in your shell history. `mysql -h db1 -u app -p shop` names host, user and database in one line. `mysql --defaults-file=/etc/my.cnf` reads a client file, `mysql -e` runs one statement and exits, and `mysqladmin -u root -p ping` answers whether the server is alive.\n\n**Inspect**  \nSHOW DATABASES and SHOW TABLES answer the first two questions. `SHOW TABLE STATUS\\G` gives engine, collation, row estimate and data length. SHOW INDEX FROM orders shows the indexes and their cardinality, and EXPLAIN SELECT \\* FROM orders shows the plan the optimiser chose.\n\n**Users**  \nSELECT user, host FROM mysql.user lists the accounts. SHOW GRANTS FOR 'app'@'%' shows what one account can do, CREATE USER 'app'@'%' makes an account, and GRANT and REVOKE change its rights on the shop schema. Account and host are one identity: 'app'@'localhost' is not 'app'@'%'.\n\n**Replication**  \n`SHOW REPLICA STATUS\\G` reads the replica, SHOW REPLICAS lists replicas on the source, and SHOW BINARY LOGS shows the files a replica needs. START REPLICA and STOP REPLICA start and stop replication.\n\n> **The honest caveat:** most of the replica block uses the 8.0.22 names; on 5.7 their equivalents say SLAVE, and SHOW TABLE STATUS row counts are estimates for InnoDB.\n\n> **The lesson:** know which account you are, which plan you have, and whether the replica is behind.\n\n## In one line\n\n> **Know the plan before the query.** The client is the shortest path to the answer.\n\n**See the plan**\n\n```sql\nEXPLAIN SELECT * FROM orders;\n```\n"},{"id":"p335","n":"335","slug":"a-backup-verifier-in-python","title":"A backup verifier in Python","sections":["it"],"topic":"Python scripts","tags":["Python","Backups","PostgreSQL","SRE","Automation"],"series_name":"Data","design":"code","script":"backup_verify.py","date":"2028-11-16","files":{"it":"notes/p335-a-backup-verifier-in-python.md"},"md":"# A backup verifier in Python\n\nThe backup job has been green for a year, and nobody has opened one of its files. A manifest turns that tick into a check: every backup has a path, a timestamp, a size and a hash.\n\n## Quick reference\n\nThe full script is at the end of this note: `backup_verify.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 backup_verify.py manifest.json\n```\n\n**Example output**\n\n```text\nFAIL db-2026-10-18.dump\nFAIL 1\n```\n\n## Explained\n\n**What it reads**  \nThe manifest is JSON: a backups list, each entry with path, created_at, bytes and sha256. Paths are resolved relative to the manifest, so the directory can move with it. created_at is ISO 8601; a trailing Z is accepted, and a timestamp without a zone is treated as UTC.\n\n**What it checks**  \nThe script parses each timestamp and computes its `age against --max-age-hours`, 26 by default. It compares the size with bytes, streams the file through SHA-256 in 1 MiB chunks, and compares the digest. A missing file, a stale timestamp, a wrong size and a wrong hash are all findings; the summary counts findings and backups.\n\n**How to run it**  \nRun it from cron after the backup job, or as a CI job that fails the pipeline. Exit 0 means every backup is present, recent and intact; exit 1 means at least one finding; exit 2 means the manifest is missing, is not JSON, or has a bad entry. The attached script is standard library only and read-only.\n\n> **The honest caveat:** a checksum proves the file is the file you recorded, not that the dump restores. Pair it with a restore drill; this tool catches truncation and staleness, not a broken schema.\n\n> **The lesson:** a backup is a claim until a verifier and a restore both agree with it.\n\n## In one line\n\n> **Trust the hash, not the folder.** A checksum catches corruption; only a restore catches regret.\n\n**Run it in ci**\n\n```bash\npython3 backup_verify.py manifest.json\n```\n\n## Script: backup_verify.py\n\nDownload: [scripts/backup_verify.py](../scripts/backup_verify.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Verify a backup manifest: age, size and SHA-256.\n\nReads a JSON manifest, checks that every listed backup is recent enough\nand matches its recorded byte count and checksum, then exits 1 if anything\nis stale or corrupt. It never writes to the backups or the manifest.\n\nUsage:\n    python3 backup_verify.py /srv/backups/manifest.json\n    python3 backup_verify.py manifest.json --max-age-hours 8\n    python3 backup_verify.py manifest.json --now 2026-10-20T09:00:00Z\n\nManifest:\n    {\"backups\": [\n        {\"path\": \"db-2026-10-20.dump\",\n         \"created_at\": \"2026-10-20T01:00:00Z\",\n         \"bytes\": 4194304,\n         \"sha256\": \"0123...cdef\"}\n    ]}\n\nRelative paths are resolved against the directory that holds the manifest.\ncreated_at is ISO 8601; a trailing Z is accepted, and a timestamp without\na zone is treated as UTC. A bare JSON list of entries is also accepted.\n\nExit codes:\n    0  every backup is present, recent, the right size and the right hash\n    1  at least one finding (missing, stale, wrong size or wrong hash)\n    2  usage error, or a manifest that is missing, unreadable, invalid,\n       has no entries, or has a malformed entry\n\nCron: 30 3 * * * python3 backup_verify.py /srv/backups/manifest.json\nCI:   python3 backup_verify.py manifest.json --max-age-hours 26\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport hashlib\nimport json\nimport os\nimport re\nimport sys\nfrom datetime import datetime, timezone\n\nCHUNK = 1024 * 1024\nREQUIRED = (\"path\", \"created_at\", \"bytes\", \"sha256\")\nSHA256_RE = re.compile(r\"^[0-9a-fA-F]{64}$\")\n\n\nclass ManifestError(Exception):\n    \"\"\"The manifest cannot be used as a manifest.\"\"\"\n\n\ndef parse_when(value):\n    \"\"\"Parse an ISO 8601 timestamp and return an aware UTC datetime.\"\"\"\n    text = str(value).strip()\n    if text.endswith(\"Z\"):\n        text = text[:-1] + \"+00:00\"\n    try:\n        when = datetime.fromisoformat(text)\n    except ValueError:\n        raise ValueError(\"bad timestamp: %r\" % (value,))\n    if when.tzinfo is None:\n        when = when.replace(tzinfo=timezone.utc)\n    return when.astimezone(timezone.utc)\n\n\ndef sha256_file(path):\n    \"\"\"Return the lowercase hex SHA-256 of a file, read in 1 MiB chunks.\"\"\"\n    digest = hashlib.sha256()\n    with open(path, \"rb\") as handle:\n        while True:\n            block = handle.read(CHUNK)\n            if not block:\n                break\n            digest.update(block)\n    return digest.hexdigest()\n\n\ndef load_manifest(path):\n    \"\"\"Return the list of backup entries from a JSON manifest.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            raw = json.load(handle)\n    except OSError as exc:\n        raise ManifestError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ManifestError(\"%s is not valid JSON: %s\" % (path, exc))\n\n    if isinstance(raw, dict):\n        raw = raw.get(\"backups\")\n    if not isinstance(raw, list):\n        raise ManifestError(\"manifest must be a list or an object with a backups list\")\n    if not raw:\n        raise ManifestError(\"manifest lists no backups\")\n    return raw\n\n\ndef check_entry(index, entry):\n    \"\"\"Validate one manifest entry and return a normalised copy.\"\"\"\n    if not isinstance(entry, dict):\n        raise ManifestError(\"entry %d is not an object\" % index)\n    missing = [key for key in REQUIRED if key not in entry]\n    if missing:\n        raise ManifestError(\"entry %d is missing %s\" % (index, \", \".join(missing)))\n\n    name = entry[\"path\"]\n    if not isinstance(name, str) or not name.strip():\n        raise ManifestError(\"entry %d has an empty path\" % index)\n\n    created = entry[\"created_at\"]\n    if not isinstance(created, str):\n        raise ManifestError(\"entry %d has a non-string created_at\" % index)\n    try:\n        when = parse_when(created)\n    except ValueError as exc:\n        raise ManifestError(\"entry %d has a %s\" % (index, exc))\n\n    size = entry[\"bytes\"]\n    if isinstance(size, bool) or not isinstance(size, int) or size < 0:\n        raise ManifestError(\"entry %d has an invalid bytes value\" % index)\n\n    digest = entry[\"sha256\"]\n    if not isinstance(digest, str) or not SHA256_RE.match(digest):\n        raise ManifestError(\"entry %d has an invalid sha256 value\" % index)\n\n    return {\"path\": name, \"created_at\": when, \"bytes\": size,\n            \"sha256\": digest.lower()}\n\n\ndef verify(manifest_path, max_age_hours, now):\n    \"\"\"Check every backup and return (checked, findings, failed_count).\"\"\"\n    entries = load_manifest(manifest_path)\n    base = os.path.dirname(os.path.abspath(manifest_path))\n    findings = []\n    failed = 0\n\n    for index, raw in enumerate(entries, 1):\n        entry = check_entry(index, raw)\n        name = entry[\"path\"]\n        target = name if os.path.isabs(name) else os.path.join(base, name)\n        before = len(findings)\n\n        age = (now - entry[\"created_at\"]).total_seconds() / 3600.0\n        if age < 0:\n            findings.append(\"FAIL %s timestamp in the future\" % name)\n        elif age > max_age_hours:\n            findings.append(\"FAIL %s stale: %.1fh > %.1fh\"\n                            % (name, age, max_age_hours))\n\n        try:\n            size = os.path.getsize(target)\n        except FileNotFoundError:\n            findings.append(\"FAIL %s missing\" % name)\n            failed += 1\n            continue\n        except OSError as exc:\n            raise ManifestError(\"cannot stat %s: %s\" % (target, exc))\n\n        if size != entry[\"bytes\"]:\n            findings.append(\"FAIL %s size %d != %d\"\n                            % (name, size, entry[\"bytes\"]))\n\n        try:\n            digest = sha256_file(target)\n        except OSError as exc:\n            raise ManifestError(\"cannot read %s: %s\" % (target, exc))\n        if digest != entry[\"sha256\"]:\n            findings.append(\"FAIL %s sha256 mismatch\" % name)\n\n        if len(findings) > before:\n            failed += 1\n\n    return len(entries), findings, failed\n\n\ndef plural(count, word):\n    \"\"\"Return a count and a correctly pluralised noun.\"\"\"\n    return \"%d %s%s\" % (count, word, \"\" if count == 1 else \"s\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Verify backup age, size and SHA-256 from a JSON manifest.\",\n        epilog=\"Exit 0 clean, 1 finding, 2 usage or manifest error. By s0sta.com\")\n    parser.add_argument(\"manifest\", help=\"path to the JSON manifest\")\n    parser.add_argument(\"--max-age-hours\", type=float, default=26.0, metavar=\"H\",\n                        help=\"maximum age in hours (default: 26)\")\n    parser.add_argument(\"--now\", metavar=\"ISO\",\n                        help=\"override the current UTC time (for tests)\")\n    args = parser.parse_args(argv)\n\n    if not (args.max_age_hours >= 0):\n        parser.error(\"--max-age-hours must be zero or positive\")\n\n    if args.now is None:\n        now = datetime.now(timezone.utc)\n    else:\n        try:\n            now = parse_when(args.now)\n        except ValueError as exc:\n            parser.error(\"--now has a %s\" % exc)\n\n    try:\n        checked, findings, failed = verify(args.manifest, args.max_age_hours, now)\n    except ManifestError as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    for line in findings:\n        print(line)\n    if findings:\n        print(\"FAIL %s, %d of %s affected\"\n              % (plural(len(findings), \"finding\"),\n                 failed, plural(checked, \"backup\")))\n        return 1\n    print(\"OK %s checked\" % plural(checked, \"backup\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p336","n":"336","slug":"replication-topologies-drawn","title":"Replication topologies, drawn","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","Replication","Databases","SRE","DataEngineering"],"series_name":"Data","design":"architecture","script":"","date":"2028-11-19","files":{"it":"notes/p336-replication-topologies-drawn.md"},"md":"# Replication topologies, drawn\n\nThe primary is easy. The hard part of replication is deciding which replica answers a read, and what the application is allowed to assume when it does.\n\n## Quick reference\n\n- **Tier 1 · Writer:** Primary (PRIM) · WAL sender (WALS)\n- **Tier 2 · Replicas:** Sync replica (SYNC) · Async replica (ASYNC) · Cascading replica (CASC)\n- **Tier 3 · Read path:** Application (APP) · Read router (RTR)\n\n**Request flow:** Primary → Sync replica → Read router; Primary → Async replica → Cascading replica; Async replica → Read router; Application → Read router\n\n## Explained\n\n**Writer**  \nThe writer tier is the primary and its WAL sender. The primary accepts every write and writes WAL; a WAL sender process per replica streams those records. There is still exactly one writer, which is why replication scales reads and not writes.\n\n**Replicas**  \nThe sync replica acknowledges the WAL before the primary reports a commit, so a committed transaction survives its failure, at the cost of a network round trip on every write. The async replica acknowledges nothing, so it lags and a failover can lose the last few transactions. The cascading replica streams from the async replica rather than the primary, which removes load from the writer and adds the upstream delay to its own.\n\n**Read path**  \nUnderneath, the application talks only to the read router, and the router chooses a replica for each read-only query. The router needs a policy for lag: send reads to the sync replica when freshness matters, spread them across the others when throughput matters, and send read-after-write traffic to the primary.\n\n> **The honest caveat:** synchronous replication waits for the standby to flush WAL, not to apply it, so a sync replica can still return a slightly stale snapshot unless you use remote_apply.\n\n> **The lesson:** replication gives you copies, not consistency; the router is where you decide which copy is good enough.\n\n## In one line\n\n> **One writer, many copies.** Copies are cheap; knowing which copy to read is not.\n\n**Read the lag**\n\n```sql\nSELECT * FROM pg_stat_replication;\n```\n"},{"id":"p337","n":"337","slug":"isolation-levels-and-the-anomalies-they-stop","title":"Isolation levels and the anomalies they stop","sections":["it"],"topic":"Databases & SQL","tags":["Databases","SQL","PostgreSQL","Transactions","DataEngineering"],"series_name":"Data","design":"explainer","script":"","date":"2028-11-21","files":{"it":"notes/p337-isolation-levels-and-the-anomalies-they-stop.md"},"md":"# Isolation levels and the anomalies they stop\n\nTwo sessions read the same row and both are sure they are right. Isolation levels say which of them may be surprised, and the fourth panel is its bill.\n\n## Quick reference\n\n### Dirty read\n\n- One transaction reads an uncommitted row\n- The writer rolls back and the row never existed\n- Stopped by READ COMMITTED and above\n- PostgreSQL never allows it at all\n\n### Non-repeatable read\n\n- You read a row, then someone commits an update\n- The same SELECT returns a different value\n- READ COMMITTED takes a new snapshot per statement\n- REPEATABLE READ holds one snapshot throughout\n\n### Phantom read\n\n- A re-run query returns rows that were not there\n- Another transaction inserted and committed a match\n- PostgreSQL REPEATABLE READ stops it with a snapshot\n- InnoDB uses next-key locks for locking reads\n\n### Serialization failure\n\n- SERIALIZABLE checks whether the order was possible\n- A dangerous cycle aborts one transaction with 40001\n- The fix is a retry, not a longer timeout\n- It replaces a wrong answer with a visible failure\n\n> **Note:** SERIALIZABLE is the level that refuses to guess, and the one that asks you to retry.\n\n## Explained\n\n**Dirty read**  \nOne transaction updates a row, another reads the uncommitted value, and the first rolls back. The reader acted on a row that never existed. READ COMMITTED and above prevent it; PostgreSQL maps READ UNCOMMITTED to READ COMMITTED, so it never happens there.\n\n**Non-repeatable read**  \nYou read a row, another transaction commits an update, and you read it again in the same transaction and get a different value. READ COMMITTED takes a new snapshot for each statement, so it allows this. REPEATABLE READ holds one snapshot for the whole transaction and stops it.\n\n**Phantom read**  \nYou run a query, another transaction inserts a matching row and commits, and the same query returns an extra row. A snapshot at REPEATABLE READ prevents it in PostgreSQL; MySQL InnoDB uses next-key locks for locking reads and a consistent snapshot for plain SELECTs. SERIALIZABLE closes the remaining gaps.\n\n**Serialization failure**  \nThe fourth panel is not an anomaly; it is what a serializable engine does when it cannot prove a safe order for two transactions. One is aborted with SQLSTATE 40001, and the correct response is to retry the transaction, not to raise the timeout.\n\n> **The honest caveat:** PostgreSQL REPEATABLE READ is snapshot isolation, and it still allows write skew; only SERIALIZABLE prevents that class of bug.\n\n> **The lesson:** pick the weakest level that keeps your invariants, then handle the retries the stronger one demands.\n\n## In one line\n\n> **Choose the level, accept the cost.** Stronger isolation does not remove the anomaly; it changes its shape.\n\n**Check the level**\n\n```sql\nSHOW transaction_isolation;\n```\n"},{"id":"p338","n":"338","slug":"the-redis-cheat-sheet","title":"The Redis cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["Redis","Databases","Caching","DevOps","DataEngineering"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-11-23","files":{"it":"notes/p338-the-redis-cheat-sheet.md"},"md":"# The Redis cheat sheet\n\nRedis is fast because it does one thing at a time, which is also why one careless command can stall every client. Four blocks cover what matters.\n\n## Quick reference\n\n### Keys\n\n```text\nSCAN 0 MATCH user:* COUNT 100\nEXISTS user:42\nTYPE user:42\nDEL user:42\nUNLINK user:42\n```\n\n### Strings\n\n```sql\nSET user:42:name Ada\nGET user:42:name\nSET lock:job 1 NX PX 30000\nINCR page:views\nMSET a 1 b 2\n```\n\n### Hashes\n\n```text\nHSET user:42 name Ada\nHGET user:42 name\nHGETALL user:42\nHINCRBY user:42 logins 1\nHDEL user:42 name\nHSCAN user:42 0\n```\n\n### Streams/TTL\n\n```text\nXADD events * type signup\nXLEN events\nXREAD COUNT 10 STREAMS events 0\nEXPIRE user:42 3600\nTTL user:42\nPERSIST user:42\n```\n\n## Explained\n\n**Keys**  \n`SCAN 0 MATCH user:* COUNT 100` walks the keyspace in slices instead of blocking on the whole thing. `EXISTS user:42` answers whether a key is there, `TYPE user:42` says what it is, DEL removes it synchronously, UNLINK removes it and frees memory in the background.\n\n**Strings**  \n`SET user:42:name Ada` stores a value, GET reads it, and `SET lock:job 1 NX PX 30000` is the atomic lock: it only writes when the key is absent and expires after thirty seconds. `INCR page:views` is atomic, and `MSET a 1 b 2` writes several keys in one round trip.\n\n**Hashes**  \n`HSET user:42 name Ada` sets a field, HGET reads one, HGETALL reads the whole hash, and `HINCRBY user:42 logins 1` increments a counter without a read-modify-write race. HDEL removes a field, and HSCAN walks a large hash in slices.\n\n**Streams and TTL**  \n`XADD events * type signup` appends an entry with an auto ID, XLEN counts entries, and `XREAD COUNT 10 STREAMS events 0` reads from the beginning. `EXPIRE user:42 3600` sets a TTL, TTL reads it, and PERSIST removes it. TTL returns -1 for no expiry and -2 for a missing key.\n\n> **The honest caveat:** expiry is lazy and sampled, so a key can outlive its deadline slightly, and a replica waits for the primary's delete.\n\n> **The lesson:** the command you choose is the complexity you accept; every O(N) command pauses everyone.\n\n## In one line\n\n> **Every command has a cost.** The single thread is the feature and the limit.\n\n**Find the hot key**\n\n```bash\nredis-cli --bigkeys\n```\n"},{"id":"p339","n":"339","slug":"5-database-mistakes-and-the-fix-for-each","title":"5 database mistakes and the fix for each","sections":["it"],"topic":"Databases & SQL","tags":["Databases","PostgreSQL","SRE","Backups","DataEngineering"],"series_name":"Data","design":"dontdo","script":"","date":"2028-11-26","files":{"it":"notes/p339-5-database-mistakes-and-the-fix-for-each.md"},"md":"# 5 database mistakes and the fix for each\n\nEvery database incident review ends with a short list that was visible for months. Each is cheap to fix before the incident and expensive to explain after it.\n\n## Quick reference\n\n**1.** ❌ **Backups never restored** — A backup that fails to restore is a file, not a backup.  \n✅ **A restore drill each month** — The drill measures RTO and proves the files open.\n\n**2.** ❌ **Indexes added and never reviewed** — Every index slows writes and costs disk.  \n✅ **Review slow queries monthly** — Add what the plan needs; drop what nothing uses.\n\n**3.** ❌ **Transactions held open for hours** — Old snapshots stop vacuum cleaning dead rows.  \n✅ **Keep transactions short** — Short transactions free space and locks sooner.\n\n**4.** ❌ **The app connects as superuser** — One injection can drop every database.  \n✅ **Least-privilege app role** — The app cannot drop what it cannot reach.\n\n**5.** ❌ **SELECT \\* in every query** — It moves columns the code never reads.  \n✅ **Name the columns you use** — Covering indexes can answer from the index.\n\n## Explained\n\n### The five\n\n1. **Backups never restored.** The job writes a file nightly, the dashboard stays green, and the restore fails at 2 a.m. because the WAL archive stopped three weeks ago. Run a restore drill each month, measure the time, and run a real query.\n\n2. **Indexes added and never reviewed.** Every index speeds one read, slows every write and takes disk. Review the slow query log, add what the workload needs, drop what nothing uses.\n\n3. **Transactions held open for hours.** A long transaction keeps an old snapshot alive, and in PostgreSQL that stops vacuum removing dead rows. It also holds locks. Commit before the next network call.\n\n4. **The app connects as superuser.** One injection or one bad migration then has the rights to drop every database. Give the app a least-privilege role and a separate migration role.\n\n5. **SELECT \\* in every query.** It moves columns the code never reads, defeats covering indexes and breaks when a column is renamed or reordered. Name the columns you use.\n\n### What they share\n\nEach is a convenience that became a default, invisible to a single code review.\n\n> **The honest caveat:** SELECT \\* in an ad hoc psql query is fine; the rule is about code paths that run a million times a day.\n\n> **The lesson:** the database keeps the receipt for every shortcut, and it presents the bill during an incident.\n\n## In one line\n\n> **Small habits, large outages.** The bill arrives in the incident review, not the invoice.\n\n**Count the roles**\n\n```sql\nSELECT rolname FROM pg_roles;\n```\n"},{"id":"p340","n":"340","slug":"a-table-bloat-report-in-python","title":"A table bloat report in Python","sections":["it"],"topic":"Python scripts","tags":["PostgreSQL","Python","Databases","SRE","Automation"],"series_name":"Data","design":"code","script":"bloat_report.py","date":"2028-11-28","files":{"it":"notes/p340-a-table-bloat-report-in-python.md"},"md":"# A table bloat report in Python\n\nBloat does not announce itself: queries get a little slower, the disk fills, and autovacuum reports that it ran while the table keeps its dead rows. A CSV from pg_stat_user_tables is enough to find the worst tables before a page does.\n\n## Quick reference\n\nThe full script is at the end of this note: `bloat_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 bloat_report.py stats.csv\n```\n\n**Example output**\n\n```text\nBLOAT orders 84.0\nFAIL 2 tables\n```\n\n## Explained\n\n**What it reads**  \nThe report takes the output of a query on pg_stat_user_tables: relname, n_live_tup, n_dead_tup, and last_vacuum. The query uses GREATEST(last_vacuum, last_autovacuum), so the column shows the last manual or automatic vacuum. Empty means never.\n\n**The ratio**  \nFor each table it computes dead divided by live plus dead. A table with 42,000 dead rows and 8,000 live ones is 84 percent dead, and that is the number that matters, not the raw count. The default threshold is 0.20, with a floor of 1,000 dead rows so a tiny table cannot trigger the report on four rows.\n\n**What it reports**  \nTables over the threshold are printed worst first, with live and dead counts, the ratio and the last vacuum date. The summary counts flagged tables. Exit 1 means at least one table is over the line, exit 0 means none, and exit 2 means the CSV is missing, has no header, or holds a value that is not an integer.\n\n> **The honest caveat:** n_dead_tup is an estimate, and a high ratio is a signal, not proof of physical bloat; VACUUM marks space reusable inside the table, and VACUUM FULL rewrites it and returns free pages to the operating system.\n\n> **The lesson:** measure dead rows before you reach for VACUUM FULL, because the lock is the expensive part.\n\n## In one line\n\n> **Dead rows are a debt.** A ratio is a signal; the lock is the cost.\n\n**Run it weekly**\n\n```bash\npython3 bloat_report.py stats.csv\n```\n\n## Script: bloat_report.py\n\nDownload: [scripts/bloat_report.py](../scripts/bloat_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report PostgreSQL table bloat from a pg_stat_user_tables CSV.\n\nReads a CSV with the columns relname, n_live_tup, n_dead_tup and\nlast_vacuum, then flags tables whose dead-tuple ratio is above a\nthreshold. Read-only: it reads the CSV, never the database.\n\nProduce the CSV with:\n    psql -d shop -c \"COPY (SELECT relname, n_live_tup, n_dead_tup,\n        GREATEST(last_vacuum, last_autovacuum) AS last_vacuum\n        FROM pg_stat_user_tables) TO STDOUT CSV HEADER\" > stats.csv\n    python3 bloat_report.py stats.csv\n    python3 bloat_report.py - < stats.csv\n    python3 bloat_report.py stats.csv --threshold 0.3 --min-dead 500\n\nThe ratio is n_dead_tup / (n_live_tup + n_dead_tup). Tables with no\ntuples are counted but skipped, and --min-dead keeps tiny tables out of\nthe report. Flagged tables are printed worst first.\n\nExit codes:\n    0  no table is over the threshold\n    1  at least one table is over the threshold\n    2  usage error, or a CSV that is missing, has no header, lacks a\n       required column, or contains a value that is not an integer\n\nCron: 15 6 * * * python3 bloat_report.py /var/lib/reports/stats.csv\nCI:   python3 bloat_report.py stats.csv --threshold 0.2\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport csv\nimport io\nimport sys\n\nFIELDS = (\"relname\", \"n_live_tup\", \"n_dead_tup\", \"last_vacuum\")\n\n\nclass InputError(Exception):\n    \"\"\"The CSV cannot be used as a stats report.\"\"\"\n\n\ndef as_int(value, field, row):\n    \"\"\"Parse a non-negative integer from one CSV cell.\"\"\"\n    try:\n        number = int(str(value).strip())\n    except (TypeError, ValueError):\n        raise InputError(\"row %d: %s is not an integer\" % (row, field))\n    if number < 0:\n        raise InputError(\"row %d: %s is negative\" % (row, field))\n    return number\n\n\ndef read_rows(path):\n    \"\"\"Yield normalised rows from a CSV path, or '-' for stdin.\"\"\"\n    handle = None\n    try:\n        if path == \"-\":\n            text = sys.stdin.read()\n            if text.startswith(\"\\ufeff\"):\n                text = text[1:]\n            reader = csv.DictReader(io.StringIO(text), strict=True)\n        else:\n            handle = open(path, \"r\", encoding=\"utf-8-sig\", newline=\"\")\n            reader = csv.DictReader(handle, strict=True)\n\n        if not reader.fieldnames:\n            raise InputError(\"CSV has no header\")\n        missing = [field for field in FIELDS if field not in reader.fieldnames]\n        if missing:\n            raise InputError(\"CSV is missing columns: %s\" % \", \".join(missing))\n\n        rows = []\n        for line, raw in enumerate(reader, 2):\n            name = (raw.get(\"relname\") or \"\").strip()\n            if not name:\n                raise InputError(\"row %d: relname is empty\" % line)\n            live = as_int(raw.get(\"n_live_tup\"), \"n_live_tup\", line)\n            dead = as_int(raw.get(\"n_dead_tup\"), \"n_dead_tup\", line)\n            vacuum = (raw.get(\"last_vacuum\") or \"\").strip() or \"never\"\n            rows.append({\"relname\": name, \"live\": live, \"dead\": dead,\n                         \"last_vacuum\": vacuum})\n        return rows\n    except (OSError, UnicodeError, csv.Error) as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    finally:\n        if handle is not None:\n            handle.close()\n\n\ndef report(rows, threshold, min_dead):\n    \"\"\"Return (considered, flagged) where flagged is sorted worst first.\"\"\"\n    considered = 0\n    flagged = []\n    for row in rows:\n        considered += 1\n        total = row[\"live\"] + row[\"dead\"]\n        if total == 0:\n            continue\n        ratio = row[\"dead\"] / total\n        if ratio > threshold and row[\"dead\"] >= min_dead:\n            flagged.append({\"relname\": row[\"relname\"], \"live\": row[\"live\"],\n                            \"dead\": row[\"dead\"], \"ratio\": ratio,\n                            \"last_vacuum\": row[\"last_vacuum\"]})\n    flagged.sort(key=lambda item: (-item[\"ratio\"], -item[\"dead\"]))\n    return considered, flagged\n\n\ndef plural(count, word):\n    \"\"\"Return a count and a correctly pluralised noun.\"\"\"\n    return \"%d %s%s\" % (count, word, \"\" if count == 1 else \"s\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Flag PostgreSQL tables with a high dead-tuple ratio.\",\n        epilog=\"Exit 0 clean, 1 finding, 2 usage or input error. By s0sta.com\")\n    parser.add_argument(\"csv\", help=\"CSV path, or - for stdin\")\n    parser.add_argument(\"--threshold\", type=float, default=0.20, metavar=\"R\",\n                        help=\"dead-tuple ratio to flag, between 0 and 1 (default: 0.20)\")\n    parser.add_argument(\"--min-dead\", type=int, default=1000, metavar=\"N\",\n                        help=\"minimum dead tuples to flag (default: 1000)\")\n    args = parser.parse_args(argv)\n\n    if not 0 < args.threshold < 1:\n        parser.error(\"--threshold must be greater than 0 and less than 1\")\n    if args.min_dead < 0:\n        parser.error(\"--min-dead must be zero or positive\")\n\n    try:\n        rows = read_rows(args.csv)\n    except InputError as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    considered, flagged = report(rows, args.threshold, args.min_dead)\n    for item in flagged:\n        print(\"BLOAT %s dead=%d live=%d ratio=%.1f%% last_vacuum=%s\"\n              % (item[\"relname\"], item[\"dead\"], item[\"live\"],\n                 item[\"ratio\"] * 100.0, item[\"last_vacuum\"]))\n\n    if flagged:\n        print(\"FAIL %d of %s over %.1f%% (dead >= %d)\"\n              % (len(flagged), plural(considered, \"table\"),\n                 args.threshold * 100.0, args.min_dead))\n        return 1\n    print(\"OK %s checked, none over %.1f%%\"\n          % (plural(considered, \"table\"), args.threshold * 100.0))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p341","n":"341","slug":"quiz-3-database-operations-questions","title":"Quiz: 3 database operations questions","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","Replication","Backups","Databases","DataEngineering"],"series_name":"Data","design":"quiz","script":"","date":"2028-11-30","files":{"it":"notes/p341-quiz-3-database-operations-questions.md"},"md":"# Quiz: 3 database operations questions\n\nThree questions separate a team that has run a database from one that has only read about it. Each answer is one mechanism, and each mechanism has a cost.\n\n## Quick reference\n\n**Q1. A read replica lags by two seconds. Why can a user see stale data?**\n\n- a) It applies the primary's WAL after the commit\n- b) The replica caches every query for two seconds\n- c) The primary sends reads to the replica first\n\n> **Answer:** a) It applies the primary's WAL after the commit. Replication is asynchronous by default: the primary commits, then ships WAL, and the replica applies it later. A read in that window sees the older snapshot.\n\n**Q2. What does point-in-time recovery need to restore to 09:15?**\n\n- a) A base backup plus every WAL segment since it\n- b) A nightly logical dump and the current schema\n- c) A replica promoted at 09:15\n\n> **Answer:** a) A base backup plus every WAL segment since it. PITR restores the base backup, then replays archived WAL to the target time. Without the WAL archive you can only return to the backup's moment.\n\n**Q3. Why does PostgreSQL need VACUUM after heavy updates?**\n\n- a) Dead row versions still occupy pages\n- b) It compresses every table and index\n- c) It rewrites the WAL so recovery is faster\n\n> **Answer:** a) Dead row versions still occupy pages. An UPDATE writes a new row version and leaves the old one dead. VACUUM marks that space reusable and updates the visibility map; it does not rewrite the table.\n\n## Explained\n\n**The stale replica**  \nWhy can a replica serve stale reads? Because replication is asynchronous by default: the primary commits and returns, then ships WAL, and the replica applies it later. A read that arrives in that window sees the older snapshot. The fix is not to trust every replica for every read: route read-after-write traffic to the primary, or track the replay position.\n\n**The recovery chain**  \nWhat does point-in-time recovery need? A base backup and every WAL segment written since it. Restore the base, point recovery at the archive, and replay stops at the target time. Without the archive you can only return to the moment of the backup, which is rarely the moment you need.\n\n**The vacuum debt**  \nWhy does PostgreSQL need VACUUM after heavy updates? An UPDATE writes a new row version and leaves the old one dead. VACUUM marks that space reusable and updates the visibility map; it does not rewrite the table. Without it, dead rows pile up, scans read more pages and the table grows even though the live row count is flat.\n\n> **The honest caveat:** PITR is only as good as the archive, so check pg_stat_archiver for failed segments before you need a restore.\n\n> **The lesson:** know what each operation promises, and know the gap it leaves.\n\n## In one line\n\n> **Know the copy, know the gap.** Every operation has a promise and a gap.\n\n**Check the archive**\n\n```sql\nSELECT * FROM pg_stat_archiver;\n```\n"},{"id":"p342","n":"342","slug":"a-read-path-with-a-cache-drawn","title":"A read path with a cache, drawn","sections":["it"],"topic":"Databases & SQL","tags":["Databases","Redis","Caching","Backend","SystemDesign"],"series_name":"Data","design":"architecture","script":"","date":"2028-12-03","files":{"it":"notes/p342-a-read-path-with-a-cache-drawn.md"},"md":"# A read path with a cache, drawn\n\nThe dashboard is slow at nine in the morning, and the database is not the first place to look. The read path has three tiers, and each one is allowed to answer without the next.\n\n## Quick reference\n\n- **Tier 1 · App:** Application (APP) · Connection pool (POOL)\n- **Tier 2 · Cache:** Local cache (L1) · Redis (L2)\n- **Tier 3 · Data:** Read replica (RO) · Primary (RW)\n\n**Request flow:** Application → Connection pool → Primary; Application → Local cache → Redis → Read replica\n\n## Explained\n\n**Application**  \nThe request arrives at the application, which asks its connection pool for a session. The pool decides whether to reuse an open connection or pay for a new one, and that wait happens before any SQL runs. On a busy box the pool is the queue, not the database.\n\n**Cache**  \nThe application checks its local cache first: an in-process map with no network hop and no shared state. A miss goes to Redis, one round trip away and shared by every instance. A hit there returns in a fraction of a millisecond; a miss costs the second hop and sends the query onward.\n\n**Data**  \nOnly the third miss reaches PostgreSQL. Reads go to the read replica, writes go through the pool to the primary, and replication is asynchronous, so a read immediately after a write can return the old row. That is the gap the cache hides, not the gap it closes.\n\n> **The honest caveat:** a cache is a second copy of the truth. Every entry needs a time to live, an invalidation rule on write, and a decision about what stale means for the page.\n\n> **The lesson:** each tier removes a round trip, and each round trip you remove is a failure mode you no longer have to debug.\n\n## In one line\n\n> **Cache the read, not the write.** The read path is where the latency lives, and where the stale data hides.\n\n**See the hit rate**\n\n```bash\nredis-cli info stats | grep keyspace\n```\n"},{"id":"p343","n":"343","slug":"normalisation-in-four-panels","title":"Normalisation in four panels","sections":["it"],"topic":"Databases & SQL","tags":["SQL","DatabaseDesign","DataModelling","PostgreSQL","Backend"],"series_name":"Data","design":"explainer","script":"","date":"2028-12-05","files":{"it":"notes/p343-normalisation-in-four-panels.md"},"md":"# Normalisation in four panels\n\nThe table has one column holding three phone numbers, and the reporting query has to split them on every run. That column is the first normal form problem, and it is the easiest one to see.\n\n## Quick reference\n\n### 1NF - Atomic columns\n\n- One value per cell, no lists in a column\n- A repeating group becomes its own table\n- Each row needs a key you can point at\n- Split the phone numbers before the query does\n\n### 2NF - No partial keys\n\n- Only matters with a composite key\n- Every non-key column needs the whole key\n- A column that needs half the key moves out\n- Order lines depend on order and product\n\n### 3NF - No transitive deps\n\n- Non-key columns depend on the key alone\n- A city stored next to a postcode is a dependency\n- Move the lookup to its own table\n- One fact, one place, one row to update\n\n### When to denormalise\n\n- After a measured query, never before\n- Copy a column to remove a join on a hot read\n- Accept the write cost and the drift risk\n- Write down the rule that keeps the copy true\n\n> **Note:** Normal forms describe a fact once. Denormalising copies it back on purpose.\n\n## Explained\n\n**First normal form**  \nEvery cell holds one value. No comma-separated list, no repeating group, no array stuffed into a text column. A repeating group becomes its own table with one row per value, and every row gets a key so you can point at it later.\n\n**Second normal form**  \nThis one only applies when the key is composite. Every non-key column must depend on the whole key, not half of it. If a column depends on the order id alone when the key is order id plus product id, it belongs in the order table.\n\n**Third normal form**  \nNo transitive dependency: a non-key column must not depend on another non-key column. The postcode determines the city, so storing both in the orders table means two rows can disagree about the same city. Move the pair to a lookup table and keep the postcode as the reference.\n\n**When to denormalise**  \nAfter a measured query, copy a column or keep a summary table to remove a join. You pay on every write and you own the drift.\n\n> **The honest caveat:** normal forms are a design discipline, not a performance plan. A fully normalised schema is still slow if the indexes and the statistics are wrong.\n\n> **The lesson:** normalise until the model is honest, then denormalise only where a query proves it.\n\n## In one line\n\n> **Normalise first, then measure.** Denormalise for a query you have timed, never for a feeling.\n\n**Find the duplicate**\n\n```bash\npg_dump --schema-only db | grep -c city\n```\n"},{"id":"p344","n":"344","slug":"the-explain-cheat-sheet","title":"The EXPLAIN cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","SQL","QueryTuning","DatabasePerformance","Backend"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-12-07","files":{"it":"notes/p344-the-explain-cheat-sheet.md"},"md":"# The EXPLAIN cheat sheet\n\nThe query is slow, and the first useful thing you can do is ask the planner what it intends to do. EXPLAIN answers that without running the statement.\n\n## Quick reference\n\n### Plan\n\n```sql\nEXPLAIN SELECT * FROM orders;\nEXPLAIN (FORMAT JSON) SELECT 1;\nEXPLAIN (VERBOSE) SELECT * FROM t;\nEXPLAIN (COSTS OFF) SELECT * FROM t;\nEXPLAIN SELECT * FROM t WHERE id = 1;\n```\n\n### Analyse\n\n```sql\nEXPLAIN ANALYZE SELECT * FROM orders;\nEXPLAIN (ANALYZE, TIMING OFF) SELECT 1;\nEXPLAIN (ANALYZE, BUFFERS) SELECT 1;\nEXPLAIN (ANALYZE, WAL) SELECT 1;\nEXPLAIN (ANALYZE) SELECT count(*) FROM t;\n```\n\n### Buffers\n\n```sql\nEXPLAIN (ANALYZE, BUFFERS) SELECT 1;\nSET track_io_timing = on;\nSELECT * FROM pg_statio_user_tables;\nSELECT * FROM pg_statio_user_indexes;\nSHOW track_io_timing;\n```\n\n### Read\n\n```sql\nEXPLAIN (FORMAT JSON) SELECT * FROM t;\nEXPLAIN (ANALYZE, FORMAT JSON) SELECT 1;\nEXPLAIN (VERBOSE, FORMAT JSON) SELECT 1;\n\\x auto\n\\timing on\n```\n\n## Explained\n\n**Plan**  \nEXPLAIN prints the plan tree with an estimated cost and row count per node. FORMAT JSON returns the tree as data you can parse, VERBOSE adds output columns, and COSTS OFF strips the estimates when you want only the shape. A Seq Scan on a large table is the line to look for.\n\n**Analyse**  \nEXPLAIN ANALYZE runs the statement and adds the actual time and rows per node. It really executes, so a DELETE deletes; wrap it in BEGIN and ROLLBACK. TIMING OFF removes the per-node timing overhead and can make a loop cheaper to measure.\n\n**Buffers**  \nEXPLAIN (ANALYZE, BUFFERS) adds shared hit, read, dirtied and written block counts. A hit is a page found in shared buffers; a read is a page fetched from the operating system. pg_statio_user_tables shows the same story per table.\n\n**Read**  \nRead the tree from the inside out. The four fields worth naming first are Node Type, Relation Name, Filter and Total Cost. In psql, `\\x auto` and \\\\timing on make the JSON readable, and a Filter on a large scan is a missing index.\n\n> **The honest caveat:** the estimates come from statistics, so a stale ANALYZE makes the planner look wrong for a reason you can fix. Run ANALYZE before you blame the plan.\n\n> **The lesson:** the plan is the query's own account of the work.\n\n## In one line\n\n> **Read the plan before the index.** A plan you cannot read is a query you cannot fix.\n\n**Plan without running**\n\n```sql\nEXPLAIN (COSTS OFF) SELECT * FROM t;\n```\n"},{"id":"p345","n":"345","slug":"an-index-advisor-in-python","title":"An index advisor in Python","sections":["it"],"topic":"Python scripts","tags":["PostgreSQL","Python","DatabasePerformance","QueryTuning","SQL"],"series_name":"Data","design":"code","script":"index_advisor.py","date":"2028-12-10","files":{"it":"notes/p345-an-index-advisor-in-python.md"},"md":"# An index advisor in Python\n\nEvery slow query review starts the same way: someone reads a plan, spots a Seq Scan, and guesses an index. The guess is the part worth automating.\n\n## Quick reference\n\nThe full script is at the end of this note: `index_advisor.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 index_advisor.py plan.json --rows 250000\n```\n\n**Example output**\n\n```text\nCREATE INDEX ON orders (customer_id);\nindex_advisor: 1 candidate\n```\n\n## Explained\n\n**What it reads**  \nEXPLAIN (FORMAT JSON) returns a list with one object whose Plan is the root node. Each node has a Node Type and a Total Cost, plus a Relation Name and Filter where they apply, and children in Plans. Save the output with `psql -Atc`.\n\n**What it finds**  \nThe script walks the tree with a stack. A node counts when its Node Type ends in Seq Scan, it names a relation, and it has a Filter. A small regular expression pulls column names out of the filter, skipping casts, and the columns become a CREATE INDEX line.\n\n**The table size**  \nA seq scan on 800 rows is the right plan. Pass `--rows` with the table's row count and the script skips candidates below `--min-rows`, default 1000, because an index there costs writes.\n\n**Running it**  \nSave one plan per file and pass several, or pipe psql output on stdin. Exit 0 means no candidate, 1 means at least one, and 2 means unreadable JSON or no Plan object.\n\n> **The honest caveat:** a filter column is a candidate, not a verdict. A low-selectivity filter, a function around the column, or a table that fits in memory may still choose a seq scan. Build the index on a copy and read the second plan.\n\n> **The lesson:** let the plan name the index, and let a second plan prove it.\n\n## In one line\n\n> **The plan tells you the index.** A seq scan on a large table with a filter is an index asking to exist.\n\n**Plan json, saved**\n\n```bash\npsql -Atc 'EXPLAIN (FORMAT JSON) SELECT 1'\n```\n\n## Script: index_advisor.py\n\nDownload: [scripts/index_advisor.py](../scripts/index_advisor.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"index_advisor.py - read EXPLAIN (FORMAT JSON) plans and suggest indexes.\n\nThe script reads one or more saved plans, for example:\n\n    psql -Atc \"EXPLAIN (FORMAT JSON) SELECT * FROM orders WHERE customer_id = 42\" > plan.json\n\nand walks every node of every plan. A node is a candidate when its \"Node Type\"\nis a sequential scan (Seq Scan or Parallel Seq Scan), it has a \"Relation Name\",\nand it carries a \"Filter\" - a condition evaluated after the scan. Columns named\nin the filter are collected and printed as a suggested CREATE INDEX statement.\nA function call around a column, such as lower(name), is reported as the column\nname; a functional index is the real fix for that case.\n\n--rows is the table size you are auditing, in rows. When it is given and the\ntable is below --min-rows (default 1000), the candidate is skipped: a small\ntable is usually cheaper to scan than to index.\n\nRead-only: it reads JSON and prints suggestions. It never connects to a\ndatabase and never creates an index. Exit codes: 0 no candidate, 1 at least\none candidate, 2 usage or input error.\n\nUsage:\n    python3 index_advisor.py plan.json\n    python3 index_advisor.py plan.json --rows 250000\n    psql -Atc \"EXPLAIN (FORMAT JSON) SELECT * FROM orders WHERE status = 'open'\" | python3 index_advisor.py -\n    python3 index_advisor.py a.json b.json --min-rows 5000 --json\n\nCron (weekly index review, Monday 06:00):\n    0 6 * * 1 psql -Atc \"EXPLAIN (FORMAT JSON) SELECT * FROM orders WHERE customer_id = 42\" > /tmp/plan.json && python3 index_advisor.py /tmp/plan.json --rows 250000 >> /var/log/index_advisor.log 2>&1\n\nCI gate (fails when a plan still scans a large table):\n    python3 index_advisor.py plan.json --rows 250000\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\n# A filter looks like \"(customer_id = 42)\" or \"((status)::text = 'open'::text)\".\n# The lookbehind keeps the \"text\" of a cast from being read as a column name.\nCOL_RE = re.compile(\n    r\"(?<![:A-Za-z0-9_'\\\"])(?:\\\"([A-Za-z_][A-Za-z0-9_]*)\\\"|([A-Za-z_][A-Za-z0-9_]*))\"\n    r\"\\s*\\)?\\s*(?:::\\s*[A-Za-z_][A-Za-z0-9_ ]*?)?\"\n    r\"\\s*(?:=|<>|!=|>=|<=|>|<|~~\\*?|!~~\\*?)\"\n)\n\nSTOP = {\"and\", \"or\", \"not\", \"is\", \"null\", \"true\", \"false\", \"any\", \"all\",\n        \"select\", \"where\", \"case\", \"when\", \"then\", \"else\", \"end\"}\n\n\ndef columns_in(text):\n    \"\"\"Column names compared in a filter, unique, in order.\n\n    Unquoted names are folded to lower case the way PostgreSQL folds them;\n    a quoted identifier keeps its case and its quotes in the suggestion.\n    \"\"\"\n    seen = []\n    for match in COL_RE.finditer(text or \"\"):\n        quoted, plain = match.group(1), match.group(2)\n        name = quoted or plain\n        if name.lower() in STOP or name.isdigit():\n            continue\n        name = '\"%s\"' % quoted if quoted else name.lower()\n        if name not in seen:\n            seen.append(name)\n    return seen\n\n\ndef plan_roots(doc, label, errors):\n    \"\"\"Plan objects from one decoded EXPLAIN document.\"\"\"\n    if isinstance(doc, dict):\n        doc = [doc]\n    if not isinstance(doc, list):\n        errors.append(\"%s: expected a JSON array or object\" % label)\n        return []\n    roots = []\n    for item in doc:\n        if isinstance(item, dict) and isinstance(item.get(\"Plan\"), dict):\n            roots.append(item[\"Plan\"])\n        elif isinstance(item, dict) and \"Node Type\" in item:\n            roots.append(item)\n        else:\n            errors.append(\"%s: no Plan object in the document\" % label)\n    return roots\n\n\ndef scan_node(node):\n    \"\"\"Candidates in one plan node and its children, depth first.\"\"\"\n    found = []\n    stack = [node]\n    while stack:\n        current = stack.pop()\n        if not isinstance(current, dict):\n            continue\n        kind = str(current.get(\"Node Type\", \"\"))\n        relation = current.get(\"Relation Name\")\n        condition = current.get(\"Filter\")\n        if kind in (\"Seq Scan\", \"Parallel Seq Scan\") and relation and condition:\n            found.append({\"relation\": relation, \"node\": kind,\n                          \"filter\": condition,\n                          \"columns\": columns_in(condition)})\n        children = current.get(\"Plans\") or []\n        stack.extend(reversed(children))\n    return found\n\n\ndef read_document(path):\n    \"\"\"Decoded JSON from a path, or '-' for stdin.\"\"\"\n    if path == \"-\":\n        return json.loads(sys.stdin.read())\n    with open(path, \"r\", encoding=\"utf-8\") as handle:\n        return json.load(handle)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Suggest CREATE INDEX statements from EXPLAIN (FORMAT JSON) \"\n                    \"plans that still use a sequential scan with a filter.\")\n    parser.add_argument(\"paths\", nargs=\"*\", default=[\"-\"], metavar=\"PATH\",\n                        help=\"saved plan JSON file(s), or - for stdin (default)\")\n    parser.add_argument(\"--rows\", type=int, default=None, metavar=\"N\",\n                        help=\"table size in rows; small tables are skipped\")\n    parser.add_argument(\"--min-rows\", type=int, default=1000, metavar=\"N\",\n                        help=\"skip candidates below this size (default 1000)\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one machine-readable JSON object\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only, no per-scan detail\")\n    args = parser.parse_args(argv)\n\n    if args.rows is not None and args.rows < 0:\n        parser.error(\"--rows cannot be negative\")\n    if args.min_rows < 0:\n        parser.error(\"--min-rows cannot be negative\")\n\n    errors = []\n    candidates = []\n    for path in args.paths:\n        label = \"<stdin>\" if path == \"-\" else path\n        try:\n            doc = read_document(path)\n        except (OSError, ValueError) as exc:\n            errors.append(\"%s: %s\" % (label, exc))\n            continue\n        for root in plan_roots(doc, label, errors):\n            candidates.extend(scan_node(root))\n\n    for message in errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if errors:\n        return 2\n\n    unique = []\n    seen = set()\n    for item in candidates:\n        key = (item[\"relation\"], tuple(item[\"columns\"]), item[\"filter\"])\n        if key not in seen:\n            seen.add(key)\n            unique.append(item)\n\n    skipped = []\n    if args.rows is not None:\n        for item in list(unique):\n            if args.rows < args.min_rows:\n                unique.remove(item)\n                skipped.append(item)\n\n    if args.json:\n        print(json.dumps({\n            \"rows\": args.rows,\n            \"min_rows\": args.min_rows,\n            \"count\": len(unique),\n            \"skipped\": [item[\"relation\"] for item in skipped],\n            \"candidates\": [{\"relation\": item[\"relation\"],\n                            \"node\": item[\"node\"],\n                            \"filter\": item[\"filter\"],\n                            \"columns\": item[\"columns\"]} for item in unique],\n        }, sort_keys=True))\n        return 1 if unique else 0\n\n    if not unique:\n        for item in skipped:\n            print(\"SKIP %s: %d rows is below %d, a seq scan is fine\"\n                  % (item[\"relation\"], args.rows, args.min_rows))\n        print(\"index_advisor: no seq scan with a filter to report\")\n        return 0\n\n    width = max(len(item[\"relation\"]) for item in unique)\n    for item in unique:\n        if not args.quiet:\n            print(\"%-*s  %s\" % (width, item[\"relation\"], item[\"node\"]))\n            print(\"%-*s  filter  %s\" % (width, \"\", item[\"filter\"]))\n        if item[\"columns\"]:\n            print(\"%-*s  CREATE INDEX ON %s (%s);\"\n                  % (width, \"\", item[\"relation\"], \", \".join(item[\"columns\"])))\n        else:\n            print(\"%-*s  no column parsed; review the filter by hand\"\n                  % (width, \"\"))\n    print(\"index_advisor: %d candidate%s\"\n          % (len(unique), \"\" if len(unique) == 1 else \"s\"))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p346","n":"346","slug":"partitioning-a-big-table-drawn","title":"Partitioning a big table, drawn","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","DatabaseDesign","Partitioning","DataEngineering","Backend"],"series_name":"Data","design":"architecture","script":"","date":"2028-12-12","files":{"it":"notes/p346-partitioning-a-big-table-drawn.md"},"md":"# Partitioning a big table, drawn\n\nThe orders table passed four hundred million rows, and the delete at the end of the month now takes longer than the month. Partitioning changes what the delete has to touch.\n\n## Quick reference\n\n- **Tier 1 · Planner:** Query (SQL) · Partition key (KEY)\n- **Tier 2 · Partitions:** Range (RANGE) · Hash (HASH) · List (LIST)\n- **Tier 3 · Archive:** Cold store (COLD) · Retention (TTL)\n\n**Request flow:** Query → Partition key → Range → Cold store → Retention; Partition key → Hash; Partition key → List\n\n## Explained\n\n**Planner**  \nA partitioned table is one logical table with many physical children. The query names the partition key, and the planner prunes: it opens only the partitions whose bounds can match the WHERE clause. A query without the key opens every partition, which is the failure this design invites.\n\n**Partitions**  \nRange partitioning by month suits time-series data, because each child has known bounds and a new month is a new child. Hash spreads rows evenly when there is no natural range and you only need a smaller unit of maintenance. List gives one partition per known value, such as region or status. Range answers when, list answers what value, and hash answers neither.\n\n**Archive**  \nOld partitions are detached, not deleted row by row. The cold store holds them compressed, and the retention rule drops or archives the oldest. A detach is a metadata change; a DELETE is a long transaction that leaves bloat behind.\n\n> **The honest caveat:** partitioning helps when queries filter on the key. Every unique index must include the partition key, planning time grows with the number of partitions, and a default partition quietly collects everything you forgot to route.\n\n> **The lesson:** partitioning is a routing decision, and the key is the route.\n\n## In one line\n\n> **Cut the table, keep the query.** The win is the partitions the planner never opens.\n\n**See the partitions**\n\n```sql\nSELECT * FROM pg_partition_tree('orders');\n```\n"},{"id":"p347","n":"347","slug":"the-n-1-query-problem-drawn","title":"The N+1 query problem, drawn","sections":["it"],"topic":"Databases & SQL","tags":["Databases","ORM","Backend","Performance","SQL"],"series_name":"Data","design":"explainer","script":"","date":"2028-12-14","files":{"it":"notes/p347-the-n-1-query-problem-drawn.md"},"md":"# The N+1 query problem, drawn\n\nThe page takes four seconds and the database reports a hundred and one queries, each one fast. That gap is the N+1 problem, and it is easier to see in a count than a profile.\n\n## Quick reference\n\n### The loop\n\n- One query for the list, one per row\n- A hundred orders mean a hundred and one trips\n- Each trip pays latency, not CPU\n- Nothing in the loop looks expensive\n\n### The ORM\n\n- Lazy loading fetches on first access\n- It looks like a property, it costs a query\n- The loop hides it in plain sight\n- Turn on statement logging and watch\n\n### The fix\n\n- Load the relation with the first query\n- A join, or a second query with IN\n- Batch the ids, keep one round trip\n- Then the loop touches memory only\n\n### The measurement\n\n- Count queries per request, not seconds\n- Log every statement in development\n- Assert the count in a test\n- A budget of five catches the next one\n\n> **Note:** Seconds move with the machine. The query count does not.\n\n## Explained\n\n**The loop**  \nThe code loads a hundred orders, then loops over them and asks each order for its customer. One query for the list and one per row: a hundred and one round trips. Each trip is short, the latency between them is not, and a remote database turns milliseconds into seconds.\n\n**The ORM**  \nLazy loading makes the association look like a property. Writing order.customer reads like a field and costs a query the first time it is touched. Nothing in the loop mentions a query, which is why a code review walks past it and a fast local database hides it.\n\n**The fix**  \nLoad the relation with the first query: a join, or a second query that fetches every customer with WHERE id IN. Two queries, one round trip each, and the loop then touches memory. The second often wins because it keeps the row count honest.\n\n**The measurement**  \nCount statements per request. Log every statement in development, add a counter around the request, and assert the count in a test. Seconds move with the machine; the count does not.\n\n> **The honest caveat:** eager loading everything turns a small N+1 into one large join with duplicated rows. Measure the query count and the row count before you call it fixed.\n\n> **The lesson:** N+1 is a loop you can see once you count the queries.\n\n## In one line\n\n> **Count the queries, not the seconds.** A hundred and one round trips is the bug, and it is measurable.\n\n**Log every query**\n\n```sql\nSET log_statement = 'all';\n```\n"},{"id":"p348","n":"348","slug":"the-psql-performance-cheat-sheet","title":"The psql performance cheat sheet","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","SQL","DatabasePerformance","psql","SRE"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-12-17","files":{"it":"notes/p348-the-psql-performance-cheat-sheet.md"},"md":"# The psql performance cheat sheet\n\nA report is slow and nobody changed the code. Before you read a plan, read what the server is doing right now, because the plan will not tell you who is blocking the table.\n\n## Quick reference\n\n### Activity\n\n```sql\nSELECT * FROM pg_stat_activity;\nSELECT pid, state FROM pg_stat_activity;\nSELECT count(*) FROM pg_stat_activity;\n\\watch 2\n```\n\n### Locks\n\n```sql\nSELECT * FROM pg_locks WHERE NOT granted;\nSELECT locktype, mode FROM pg_locks;\nSELECT * FROM pg_blocking_pids(1234);\nSELECT locktype, relation FROM pg_locks;\n```\n\n### Statements\n\n```sql\nCREATE EXTENSION pg_stat_statements;\nSET pg_stat_statements.track = 'all';\nSELECT * FROM pg_stat_statements LIMIT 5;\nSELECT * FROM pg_stat_user_functions;\n```\n\n### Sizes\n\n```sql\nSELECT pg_database_size('db');\nSELECT pg_total_relation_size('t');\nSELECT pg_relation_size('t');\nSELECT pg_size_pretty(123456789);\n\\dt+\n```\n\n## Explained\n\n**Activity**  \npg_stat_activity has one row per connection: state, query, wait event and how long the current query has run. A session idle in transaction is the one holding locks while doing nothing. `\\watch 2` re-runs the query every two seconds, which is enough to watch a queue build.\n\n**Locks**  \npg_locks shows every lock, granted or not. A row with granted false is a queue, and pg_blocking_pids names the sessions in front of it. Cancel the blocker with pg_cancel_backend before you terminate it, because a terminate rolls the transaction back and can take longer.\n\n**Statements**  \npg_stat_statements aggregates by query shape: calls, total time, mean time and rows. Create the extension, set track to all to include statements inside functions, and read the top five by total time rather than by mean.\n\n**Sizes**  \npg_database_size, pg_relation_size and pg_total_relation_size return bytes; pg_size_pretty makes them readable. `\\dt+` lists tables with their size, which is the fastest way to find the one behind the slow count.\n\n> **The honest caveat:** these views are cumulative since the last reset, and pg_stat_statements needs shared_preload_libraries plus a restart, so a managed service may not offer it at all.\n\n> **The lesson:** the server will tell you where the time goes if you ask the right view.\n\n## In one line\n\n> **Ask the server, not the hunch.** The views know what the plan only estimates.\n\n**Find the long query**\n\n```sql\nSELECT * FROM pg_stat_activity;\n```\n"},{"id":"p349","n":"349","slug":"postgresql-vs-mysql-vs-mongodb","title":"PostgreSQL vs MySQL vs MongoDB","sections":["it"],"topic":"Databases & SQL","tags":["PostgreSQL","MySQL","MongoDB","Databases","DataModelling"],"series_name":"Data","design":"matrix","script":"","date":"2028-12-19","files":{"it":"notes/p349-postgresql-vs-mysql-vs-mongodb.md"},"md":"# PostgreSQL vs MySQL vs MongoDB\n\nThe schema argument is usually a brand argument, and the brand is rarely the reason the query is slow. Three engines, six rows, and the row that decides the rest is the first one.\n\n## Quick reference\n\n| | **PostgreSQL** | **MySQL** | **MongoDB** |\n|:--|:--|:--|:--|\n| Data model | Relational, extensible | Relational, InnoDB | Document, BSON |\n| Joins | Full SQL joins | Full SQL joins | Limited, via $lookup |\n| Transactions | Full ACID, MVCC | ACID on InnoDB | Multi-doc since 4.0 |\n| Replication | Streaming, logical | Binlog, group repl | Replica sets |\n| Scaling | Read replicas | Read replicas | Sharding built in |\n| Best for | Complex queries, GIS | Simple web apps | Flexible documents |\n\n**How to pick**\n\n- **PostgreSQL** → complex queries, strong SQL.\n- **MySQL** → simple read-heavy web apps.\n- **MongoDB** → flexible documents at scale.\n\n## Explained\n\n**Data model**  \nPostgreSQL and MySQL are relational and give you tables, constraints and SQL. MongoDB stores BSON documents, so a record is a tree you read whole rather than a row you join.\n\n**Joins and transactions**  \nSQL joins are first class in both relational engines. MongoDB has $lookup, a left outer join in the aggregation pipeline with more limits. PostgreSQL and MySQL on InnoDB give full ACID; MongoDB has multi-document transactions since 4.0 on replica sets, and since 4.2 across shards.\n\n**Replication and scaling**  \nPostgreSQL streams WAL and offers logical replication; MySQL uses the binlog and group replication; MongoDB uses replica sets with automatic failover. Both relational engines scale reads with replicas and otherwise want a bigger machine, while MongoDB shards on a key from the start.\n\n**Picking**  \nStart with the shape of the data and the queries. Known schema and ad hoc reporting point at PostgreSQL, which is also the strongest on complex queries and GIS; a simple CRUD app with a big read load fits MySQL; documents that change shape per tenant fit MongoDB.\n\n> **The honest caveat:** every engine has moved. MySQL 8 has window functions, PostgreSQL has JSONB, MongoDB has transactions, so the version in front of you matters more than the logo.\n\n> **The lesson:** pick the model the data has, then the engine that runs it best.\n\n## In one line\n\n> **Pick the model the data has.** The engine is a data-shape decision before it is a brand decision.\n\n**See a plan**\n\n```sql\nEXPLAIN (FORMAT JSON) SELECT 1;\n```\n"},{"id":"p350","n":"350","slug":"a-table-size-report-in-python","title":"A table size report in Python","sections":["it"],"topic":"Python scripts","tags":["PostgreSQL","Python","DatabasePerformance","Observability","SRE"],"series_name":"Data","design":"code","script":"table_sizes.py","date":"2028-12-21","files":{"it":"notes/p350-a-table-size-report-in-python.md"},"md":"# A table size report in Python\n\nThe table is eight hundred megabytes and the indexes are one point two gigabytes, which nobody noticed until the restore took an hour. Size is the cheapest performance fact to collect.\n\n## Quick reference\n\nThe full script is at the end of this note: `table_sizes.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 table_sizes.py sizes.csv 1.0\n```\n\n**Example output**\n\n```text\naudit_log        50.0  150.0\nTOTAL 1726.0 MB, 1 over\n```\n\n## Explained\n\n**What it reads**  \nA CSV with relname, table_bytes and index_bytes, straight from `psql --csv`. pg_table_size counts the table's heap plus TOAST, the free space map and the visibility map, and pg_indexes_size counts every index attached to it. Both numbers are bytes on disk, not row counts.\n\n**What it prints**  \nOne row per table, largest total first: table megabytes, index megabytes, the ratio between them, and a state. The summary adds the grand total and names every table over the ratio.\n\n**The threshold**  \nThe second argument is the ratio; the attached script exposes it as `--ratio` and defaults to 1.0, so an index bigger than its table is reported. `--min-mb`, default 1, keeps an eight kilobyte table with a sixteen kilobyte index out of the report. Exit 1 when anything is flagged.\n\n> **The honest caveat:** a large index can be correct. A covering index on a narrow table, or a primary key on a UUID column, is big by design, so the ratio is a prompt to look rather than a verdict.\n\n> **The lesson:** an index is a second copy of the data, and you pay for it on every write and every restore.\n\n## In one line\n\n> **An index bigger than its table.** That is not speed, that is a second copy with overhead.\n\n**Export the sizes**\n\n```bash\npsql --csv -c 'SELECT relname ...'\n```\n\n## Script: table_sizes.py\n\nDownload: [scripts/table_sizes.py](../scripts/table_sizes.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"table_sizes.py - rank table and index sizes, and flag index overhead.\n\nReads a CSV with the columns relname, table_bytes and index_bytes, as produced\nby:\n\n    psql --csv -c \"SELECT relname, pg_table_size(oid) AS table_bytes, pg_indexes_size(oid) AS index_bytes FROM pg_class WHERE relkind = 'r' ORDER BY 2 DESC\" > sizes.csv\n\nBlank size cells count as zero. For every table it prints the table size, the\nindex size and the index ratio\n(index bytes divided by table bytes), largest total first. A table is flagged\nwhen its indexes are larger than --ratio times the table (default 1.0). Tables\nbelow --min-mb (default 1) are left out of the flagging, so an 8 kB table with\none index does not win the report. Exit code 1 when anything is flagged.\n\nRead-only: it reads a CSV and prints a report. It never connects to a database.\nExit codes: 0 nothing over the ratio, 1 at least one table over it, 2 usage or\ninput error.\n\nUsage:\n    psql --csv -c \"SELECT relname, pg_table_size(oid) AS table_bytes, pg_indexes_size(oid) AS index_bytes FROM pg_class WHERE relkind = 'r'\" > sizes.csv\n    python3 table_sizes.py sizes.csv\n    python3 table_sizes.py sizes.csv --ratio 0.5 --min-mb 10\n    python3 table_sizes.py sizes.csv --top 10 --json\n\nCron (weekly size review, Monday 07:00):\n    0 7 * * 1 psql --csv -c \"SELECT relname, pg_table_size(oid) AS table_bytes, pg_indexes_size(oid) AS index_bytes FROM pg_class WHERE relkind = 'r'\" > /tmp/sizes.csv && python3 table_sizes.py /tmp/sizes.csv --ratio 1.0 >> /var/log/table_sizes.log 2>&1\n\nCI gate (fails when an index is bigger than its table):\n    python3 table_sizes.py sizes.csv --ratio 1.0\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport json\nimport sys\n\nFIELDS = (\"relname\", \"table_bytes\", \"index_bytes\")\nMIB = 1024.0 * 1024.0\n\n\ndef as_bytes(value):\n    \"\"\"An integer byte count, or None when the cell is blank.\"\"\"\n    text = (value or \"\").strip().replace(\",\", \"\")\n    if not text:\n        return None\n    try:\n        number = int(text)\n    except ValueError:\n        return \"bad\"\n    return number if number >= 0 else \"bad\"\n\n\ndef read_rows(handle, label, errors):\n    \"\"\"Table rows from one CSV handle, with the three required columns.\"\"\"\n    reader = csv.DictReader(handle)\n    missing = [name for name in FIELDS if name not in (reader.fieldnames or [])]\n    if missing:\n        errors.append(\"%s: missing column(s): %s\" % (label, \", \".join(missing)))\n        return []\n    rows = []\n    for lineno, record in enumerate(reader, 2):\n        name = (record.get(\"relname\") or \"\").strip()\n        table = as_bytes(record.get(\"table_bytes\"))\n        index = as_bytes(record.get(\"index_bytes\"))\n        if not name:\n            errors.append(\"%s:%d: empty relname\" % (label, lineno))\n            continue\n        if table == \"bad\" or index == \"bad\":\n            errors.append(\"%s:%d: table_bytes and index_bytes must be integers\"\n                          % (label, lineno))\n            continue\n        rows.append({\"relname\": name, \"table\": table or 0,\n                     \"index\": index or 0})\n    return rows\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank table and index sizes from a CSV and flag tables \"\n                    \"whose indexes are larger than --ratio times the table.\")\n    parser.add_argument(\"path\", metavar=\"PATH\",\n                        help=\"CSV with relname, table_bytes, index_bytes\")\n    parser.add_argument(\"--ratio\", type=float, default=1.0, metavar=\"R\",\n                        help=\"flag when index bytes > R x table bytes (default 1.0)\")\n    parser.add_argument(\"--min-mb\", type=float, default=1.0, metavar=\"MB\",\n                        help=\"ignore tables below this size for flagging (default 1)\")\n    parser.add_argument(\"--top\", type=int, default=0, metavar=\"N\",\n                        help=\"print only the N largest tables (default all)\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one machine-readable JSON object\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only, no table\")\n    args = parser.parse_args(argv)\n\n    if args.ratio < 0:\n        parser.error(\"--ratio cannot be negative\")\n    if args.min_mb < 0:\n        parser.error(\"--min-mb cannot be negative\")\n    if args.top < 0:\n        parser.error(\"--top cannot be negative\")\n\n    errors = []\n    try:\n        with open(args.path, \"r\", encoding=\"utf-8\", newline=\"\") as handle:\n            rows = read_rows(handle, args.path, errors)\n    except OSError as exc:\n        errors.append(\"%s: %s\" % (args.path, exc))\n        rows = []\n\n    for message in errors:\n        print(\"error: %s\" % message, file=sys.stderr)\n    if errors:\n        return 2\n    if not rows:\n        print(\"error: no table rows in %s\" % args.path, file=sys.stderr)\n        return 2\n\n    floor = args.min_mb * MIB\n    for row in rows:\n        row[\"total\"] = row[\"table\"] + row[\"index\"]\n        row[\"ratio\"] = row[\"index\"] / float(row[\"table\"]) if row[\"table\"] else 0.0\n        row[\"over\"] = (row[\"index\"] > args.ratio * row[\"table\"]\n                       and row[\"total\"] >= floor)\n    rows.sort(key=lambda item: (-item[\"total\"], item[\"relname\"]))\n    shown = rows[:args.top] if args.top else rows\n    over = [row for row in rows if row[\"over\"]]\n    grand = sum(row[\"total\"] for row in rows)\n\n    if args.json:\n        print(json.dumps({\n            \"tables\": len(rows),\n            \"total_bytes\": grand,\n            \"ratio\": args.ratio,\n            \"min_mb\": args.min_mb,\n            \"over\": len(over),\n            \"flagged\": [row[\"relname\"] for row in over],\n            \"rows\": [{\"relname\": row[\"relname\"],\n                      \"table_bytes\": row[\"table\"],\n                      \"index_bytes\": row[\"index\"],\n                      \"ratio\": round(row[\"ratio\"], 3),\n                      \"over\": row[\"over\"]} for row in shown],\n        }, sort_keys=True))\n        return 1 if over else 0\n\n    if not args.quiet:\n        print(\"%-24s %9s %9s %7s  %s\"\n              % (\"TABLE\", \"TABLE_MB\", \"INDEX_MB\", \"RATIO\", \"STATE\"))\n        for row in shown:\n            print(\"%-24s %9.1f %9.1f %7.2f  %s\"\n                  % (row[\"relname\"][:24], row[\"table\"] / MIB,\n                     row[\"index\"] / MIB, row[\"ratio\"],\n                     \"OVER\" if row[\"over\"] else \"ok\"))\n        print()\n    print(\"TOTAL %.1f MB across %d table%s; %d over ratio %g\"\n          % (grand / MIB, len(rows), \"\" if len(rows) == 1 else \"s\",\n             len(over), args.ratio))\n    for row in over:\n        print(\"OVER %s: %.1f MB of index on %.1f MB of table\"\n              % (row[\"relname\"], row[\"index\"] / MIB, row[\"table\"] / MIB))\n    return 1 if over else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p351","n":"351","slug":"quiz-3-modelling-questions","title":"Quiz: 3 modelling questions","sections":["it"],"topic":"Databases & SQL","tags":["SQL","DataModelling","DatabaseDesign","PostgreSQL","Backend"],"series_name":"Data","design":"quiz","script":"","date":"2028-12-24","files":{"it":"notes/p351-quiz-3-modelling-questions.md"},"md":"# Quiz: 3 modelling questions\n\nThree questions separate a schema that was designed from one that grew a table at a time. Answer them before the reveal, then check the reasoning.\n\n## Quick reference\n\n**Q1. What does third normal form remove from a table?**\n\n- a) Columns that depend on another non-key column\n- b) Every column that is not a primary key\n- c) All duplicate rows\n\n> **Answer:** a) Columns that depend on another non-key column. 3NF removes a transitive dependency: a non-key column that depends on another non-key column, such as a city that follows from a postcode.\n\n**Q2. When is denormalising a table the right call?**\n\n- a) After a measured query needs fewer joins\n- b) Before launch, to save a join later\n- c) Whenever the schema has more than five tables\n\n> **Answer:** a) After a measured query needs fewer joins. Denormalising is a trade, not a default: it removes a join from a read you have timed and adds a write cost and a drift risk you must own.\n\n**Q3. Why does the order of columns in an index matter?**\n\n- a) The leftmost column must match the query\n- b) Indexes sort on the last column first\n- c) Order only changes the storage size\n\n> **Answer:** a) The leftmost column must match the query. A B-tree sorts by the first column, then the second. A query filtering only on the second column cannot seek the tree, so pick the leftmost column from the query.\n\n## Explained\n\n**The first question**  \nWhat does third normal form remove? A transitive dependency: a non-key column that depends on another non-key column. The postcode determines the city, so a city column in the orders table can disagree with the city column in another row. Third normal form removes the redundancy that lets two rows tell different stories.\n\n**The second question**  \nWhen is denormalising the right call? After a measured query needs fewer joins, and the write cost is understood. Before launch it is a guess, and the guess usually copies the column that changes most often.\n\n**The third question**  \nWhy does the order of columns in a composite index matter? A B-tree is sorted by the first column, then the second. A query that filters only on the second column cannot seek through the tree, so the leftmost column should be the one your queries use most.\n\n**What they share**  \nAll three are decisions about where a fact lives and how it is found, not about which database you bought.\n\n> **The honest caveat:** these are defaults, not laws. A reporting table can be denormalised on purpose, and a partial index can beat the textbook column order when it matches the query exactly.\n\n> **The lesson:** model for the facts, index for the queries, and measure both.\n\n## In one line\n\n> **Model for the query you run.** Three answers that decide the schema and the index.\n\n**Check the plan**\n\n```sql\nEXPLAIN (ANALYZE, BUFFERS) SELECT 1;\n```\n"},{"id":"p352","n":"352","slug":"kafka-architecture-drawn","title":"Kafka architecture, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["Kafka","DataEngineering","Streaming","Architecture","Backend"],"series_name":"Data","design":"architecture","script":"","date":"2028-12-26","files":{"devsecops":"notes/p352-kafka-architecture-drawn.md"},"md":"# Kafka architecture, drawn\n\nA customer updates an address and four teams want it before the commit returns. Kafka turns one write into an event without four integrations.\n\n## Quick reference\n\n- **Tier 1 · Producers:** Order service (APP) · CDC connector (CDC)\n- **Tier 2 · Brokers:** KRaft controller (META) · Broker 1 (B1) · Broker 2 (B2) · Broker 3 (B3)\n- **Tier 3 · Consumers:** Consumer group (CG) · Sink connector (SINK)\n\n**Request flow:** KRaft controller → Broker 1 → Consumer group; Order service → Broker 1; Order service → Broker 2 → Sink connector; CDC connector → Broker 3 → Consumer group\n\n**Around it:** Schema Registry (SR) · Kafka Connect (KC) · Monitoring (OPS)\n\n## Explained\n\n**Producers**  \nThe order service is a producer: it sends a record with a key and a value, and the key chooses the partition. The CDC connector is one too, but its source is the database transaction log, so a table change becomes a record without a second write.\n\n**Brokers**  \nThree brokers hold the partitions and their replicas. The order service writes to broker 1 and broker 2, which lead its partitions, and the CDC connector writes to broker 3. Every record is appended to a log and kept for a retention window you set, not for the slowest consumer. The KRaft controller sits beside them and holds the cluster metadata: which broker leads which partition. It carries no records, which is why it gets its own box.\n\n**Consumers**  \nThe consumer group splits the partitions between its members, so a fourth member takes work off the other three. The sink connector reads the same topic and writes each record into the warehouse.\n\n**Around the tiers**  \nSchema Registry holds the record contract, Kafka Connect runs the connectors, and monitoring watches the lag.\n\n> **The honest caveat:** a topic is not a queue or a database. Retention deletes old records whether or not your consumer read them, and a consumer that falls behind the window loses data without an error.\n\n> **The lesson:** producers and consumers never meet. The topic is the contract, and its retention is part of it.\n\n## In one line\n\n> **The log is the contract.** Producers and consumers never meet; the topic is the agreement.\n\n**Follow the lag**\n\n```bash\nkafka-consumer-groups --describe --group g\n```\n"},{"id":"p353","n":"353","slug":"kafka-delivery-semantics-drawn","title":"Kafka delivery semantics, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["Kafka","DataEngineering","Streaming","DistributedSystems","Backend"],"series_name":"Data","design":"explainer","script":"","date":"2028-12-28","files":{"devsecops":"notes/p353-kafka-delivery-semantics-drawn.md"},"md":"# Kafka delivery semantics, drawn\n\nAn invoice is charged twice after a consumer restarts mid-batch, and the review asks which delivery guarantee you chose. Kafka offers three, and none of them is free.\n\n## Quick reference\n\n### At-most-once\n\n- Commit the offset before the work\n- A crash loses the record, never repeats it\n- Fine for metrics and sampled events\n- No retry, no dedupe, no ceremony\n\n### At-least-once\n\n- Commit the offset after the work succeeds\n- A crash repeats the last batch\n- The shape of most consumers\n- So the write has to be idempotent\n\n### Exactly-once\n\n- Idempotent producer plus transactions\n- Offsets and output commit together\n- Consumers read with read_committed\n- Kafka Streams: exactly_once_v2\n\n### Ordering per partition\n\n- One partition is one ordered log\n- The key picks the partition, every time\n- More partitions, more parallel readers\n- No order across two partitions\n\n> **Note:** At-least-once plus an idempotent write is the exactly-once most teams need.\n\n## Explained\n\n**At-most-once**  \nCommit the offset before the work. A crash between the commit and the write loses the record, and nothing retries it. It suits metrics and sampled events, where a missing point is cheaper than a duplicate.\n\n**At-least-once**  \nCommit after the work succeeds. A crash between the write and the commit replays the record, so the work runs twice. That is the shape of most consumers, and it is why the write must be idempotent: an insert keyed by the event id.\n\n**Exactly-once**  \nAn idempotent producer removes duplicates from retries, and transactions commit offsets and output together. A consumer with read_committed sees only committed records, and Kafka Streams switches the whole pattern on with processing.guarantee=exactly_once_v2.\n\n**Ordering per partition**  \nOne partition is one log with one reader order. The key chooses the partition, so every event for one order stays in order while different orders run in parallel. Across partitions there is no order at all.\n\n> **The honest caveat:** exactly-once is a Kafka-to-Kafka promise. The moment the consumer writes to Postgres or S3, you are back to at-least-once plus your own idempotency.\n\n> **The lesson:** the guarantee belongs to the whole path, not to the broker.\n\n## In one line\n\n> **Pick the guarantee before the code.** The setting is cheap; the missing dedupe is not.\n\n**Read the config**\n\n```bash\ngrep isolation.level consumer.properties\n```\n"},{"id":"p354","n":"354","slug":"the-kafka-cli-cheat-sheet","title":"The Kafka CLI cheat sheet","sections":["devsecops"],"topic":"Data engineering","tags":["Kafka","DataEngineering","CommandLine","Streaming","DataOps"],"series_name":"Data","design":"cheatsheet","script":"","date":"2028-12-31","files":{"devsecops":"notes/p354-the-kafka-cli-cheat-sheet.md"},"md":"# The Kafka CLI cheat sheet\n\nYou need the lag for one group, the topic name is spelled two ways, and the documentation page is 400 lines. Four Kafka tools answer most of the questions you actually ask. The t and g in the commands are your topic and your group.\n\n## Quick reference\n\n### Topics\n\n```bash\nkafka-topics --list\nkafka-topics --describe --topic t\nkafka-topics --create --topic t\nkafka-topics --delete --topic t\n```\n\n### Produce\n\n```bash\nkafka-console-producer --topic t\nkafka-console-producer --topic t --sync\nkafka-console-producer --topic t < e.json\necho hi|kafka-console-producer --topic t\n```\n\n### Consume\n\n```bash\nkafka-console-consumer --topic t\nkafka-console-consumer --topic t --group g\nkafka-console-consumer --include t\nkafka-console-consumer --topic t|head -1\n```\n\n### Groups\n\n```bash\nkafka-consumer-groups --list\nkafka-consumer-groups --describe --group g\nkafka-consumer-groups --delete --group g\nkafka-get-offsets --topic t --time -1\n```\n\n## Explained\n\n**Topics**  \n`kafka-topics --list` shows every topic. `--describe` adds the partitions, the leader and the replicas, `--create` makes one, and `--delete` removes it and its data. This is the tool you use before you touch anything else.\n\n**Produce**  \n`kafka-console-producer --topic t` opens a prompt where each line is a record. Add `--sync` to wait for the acknowledgement, redirect a file to send a batch, or pipe one line in from a script.\n\n**Consume**  \n`kafka-console-consumer --topic t` reads from the end by default. Add `--group g` to start from that group's committed offsets, use `--include` when the topic name is a family, and pipe through head when you only want one record. The group is what makes the read repeatable: a restart resumes where the last commit left off.\n\n**Groups**  \n`kafka-consumer-groups --list` finds the groups. `--describe --group g` is the lag report, `--delete --group g` removes the offsets, and `kafka-get-offsets --time -1` gives the broker's end offset, the number lag is measured against.\n\n> **The honest caveat:** `--describe` shows state, not history. A lag of zero proves the consumer committed an offset, not that it processed the record correctly.\n\n> **The lesson:** the flag you cannot remember is the one worth writing down.\n\n## In one line\n\n> **The flag is the whole job.** Four tools, one broker, and the flags that matter.\n\n**See the lag**\n\n```bash\nkafka-consumer-groups --describe --group g\n```\n"},{"id":"p355","n":"355","slug":"a-consumer-lag-report-in-python","title":"A consumer lag report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Kafka","Python","DataEngineering","SRE","Observability"],"series_name":"Data","design":"code","script":"consumer_lag.py","date":"2029-01-02","files":{"devsecops":"notes/p355-a-consumer-lag-report-in-python.md"},"md":"# A consumer lag report in Python\n\nOne consumer group is three days behind and the dashboard is green. `kafka-consumer-groups --describe` prints the lag, but it prints one row per partition, and nobody adds those up by hand.\n\n## Quick reference\n\nThe full script is at the end of this note: `consumer_lag.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nkafka-consumer-groups --describe --group g|./consumer_lag.py\n```\n\n**Example output**\n\n```text\nbilling              invoices                 1190\nbilling 1190 OVER\n```\n\n## Explained\n\n**What it reads**  \nThe script takes the plain describe output on stdin. Each row is a group, a topic, a partition, the current offset, the log-end offset and the lag. Rows for a group with no committed offsets carry a dash instead of a number, the header starts with GROUP, and the parser skips both.\n\n**What it sums**  \nEvery row with a numeric lag is added to a per-group-and-topic total and to a per-group total. The report prints the topic rows first, then one line per group marked OVER or ok.\n\n**The exit code**  \nExit 0 means every group is inside the budget, exit 1 means at least one is over it, and exit 2 means the input was unreadable or held no lag rows. Exit 2 is a broken check, not a clean result, and CI should treat it that way. The attached script is standard library only, read-only, and takes `--limit` and `--group`.\n\n> **The honest caveat:** describe is a snapshot from one broker. A group with no active members still reports its last committed offsets, and a recreated group starts again at zero, so the trend matters more than one number.\n\n> **The lesson:** a lag number you have to assemble by hand is a lag number nobody checks.\n\n## In one line\n\n> **Lag is the only honest metric.** A green dashboard with a growing lag is a dashboard lying.\n\n**Run it in ci**\n\n```bash\npython3 consumer_lag.py lag.txt --limit 500\n```\n\n## Script: consumer_lag.py\n\nDownload: [scripts/consumer_lag.py](../scripts/consumer_lag.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report Kafka consumer lag from kafka-consumer-groups --describe output.\n\nReads the plain (non-verbose) output of\n\n    kafka-consumer-groups --bootstrap-server localhost:9092 \\\n        --describe --group api-consumers\n\nfrom a file or from stdin, adds up LAG per group and topic, and exits 1\nwhen a group's total lag is over the limit.\n\nUsage:\n    kafka-consumer-groups --describe --group api-consumers > lag.txt\n    python3 consumer_lag.py lag.txt --limit 500\n\n    kafka-consumer-groups --describe --group api-consumers \\\n        | python3 consumer_lag.py - --limit 500\n\nExit codes:\n    0  every group is at or under the limit\n    1  at least one group is over it\n    2  unreadable input, bad arguments or no lag rows at all\n\nCron/CI:\n    0 * * * * kafka-consumer-groups --describe --group api-consumers \\\n        | python3 /srv/checks/consumer_lag.py - --limit 500\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\n\nHEADER = \"GROUP\"\n\n\ndef lag_rows(lines):\n    \"\"\"Return (group, topic, lag) for each partition row with a numeric LAG.\"\"\"\n    rows = []\n    for line in lines:\n        parts = line.split()\n        # group, topic, partition, current-offset, log-end-offset, lag, ...\n        if len(parts) < 6 or parts[0] == HEADER:\n            continue\n        lag = parts[5]\n        if not lag.isdigit():\n            continue\n        rows.append((parts[0], parts[1], int(lag)))\n    return rows\n\n\ndef report(rows, limit):\n    \"\"\"Print the report and return 0 when inside the limit, 1 when over.\"\"\"\n    by_topic, by_group = {}, {}\n    for name, topic, lag in rows:\n        by_topic[(name, topic)] = by_topic.get((name, topic), 0) + lag\n        by_group[name] = by_group.get(name, 0) + lag\n\n    print(\"%-24s %-24s %8s\" % (\"GROUP\", \"TOPIC\", \"LAG\"))\n    for (name, topic), lag in sorted(by_topic.items()):\n        print(\"%-24s %-24s %8d\" % (name, topic, lag))\n\n    over = [(g, n) for g, n in sorted(by_group.items()) if n > limit]\n    print(\"\")\n    for name, lag in sorted(by_group.items()):\n        print(\"%-24s %8d  %s\" % (name, lag, \"OVER\" if lag > limit else \"ok\"))\n    print(\"%d of %d groups over %d\" % (len(over), len(by_group), limit))\n    return 1 if over else 0\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"consumer_lag.py\",\n        description=\"Sum Kafka consumer lag per group and topic from the output \"\n                    \"of kafka-consumer-groups --describe.\")\n    parser.add_argument(\"source\", nargs=\"?\", default=\"-\",\n                        help=\"describe output file, or - for stdin (default: -)\")\n    parser.add_argument(\"--limit\", type=int, default=1000, metavar=\"N\",\n                        help=\"lag a group may carry before failing (default: 1000)\")\n    parser.add_argument(\"--group\", metavar=\"NAME\",\n                        help=\"only report this consumer group\")\n    args = parser.parse_args(argv)\n\n    if args.limit < 0:\n        parser.error(\"--limit must be zero or more\")\n\n    try:\n        if args.source == \"-\":\n            lines = sys.stdin.read().splitlines()\n        else:\n            with open(args.source) as handle:\n                lines = handle.read().splitlines()\n    except OSError as exc:\n        print(\"error: cannot read %s: %s\" % (args.source, exc), file=sys.stderr)\n        return 2\n\n    rows = lag_rows(lines)\n    if not rows:\n        print(\"error: no lag rows found; is this kafka-consumer-groups \"\n              \"--describe output?\", file=sys.stderr)\n        return 2\n\n    if args.group:\n        rows = [r for r in rows if r[0] == args.group]\n        if not rows:\n            print(\"error: group %s is not in the input\" % args.group,\n                  file=sys.stderr)\n            return 2\n\n    return report(rows, args.limit)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p356","n":"356","slug":"etl-and-elt-drawn","title":"ETL and ELT, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","ETL","DataWarehouse","Analytics","SQL"],"series_name":"Data","design":"architecture","script":"","date":"2029-01-04","files":{"devsecops":"notes/p356-etl-and-elt-drawn.md"},"md":"# ETL and ELT, drawn\n\nA report says revenue is up 4 percent and the warehouse says it is flat. Both read the same source; one of them read it before the transform.\n\n## Quick reference\n\n- **Tier 1 · Sources:** OLTP database (OLTP) · SaaS APIs (API) · Files and logs (FILE)\n- **Tier 2 · Pipeline:** Extract (EX) · Transform (TX) · Load (LD)\n- **Tier 3 · Warehouse:** Warehouse (WH) · Data marts (MART) · BI and dashboards (BI)\n\n**Request flow:** OLTP database → Extract → Transform → Load → Warehouse → Data marts → BI and dashboards; SaaS APIs → Extract; Files and logs → Extract\n\n## Explained\n\n**Sources**  \nThree kinds of source feed the pipeline: an OLTP database that holds the orders, SaaS APIs that hold billing and the CRM, and files and logs that arrive from everywhere else. Extract reads each one in its own way, and that is already the hardest part.\n\n**Pipeline**  \nExtract pulls the rows into a staging area. Transform cleans, joins and aggregates them, and load writes the result into the warehouse. ETL runs the transform before the load, so the heavy work happens on a separate engine and only clean rows land in the warehouse.\n\nELT keeps the same boxes and moves the letter: extract loads the raw rows into the warehouse, and the transform runs there, in SQL, next to the data. The warehouse, the marts and the BI layer then read from those tables.\n\n**Warehouse**  \nThe warehouse holds the modelled tables. The marts narrow them for one team or one question, and the BI layer is where a human finally sees a number.\n\n> **The honest caveat:** ELT is not free. The transform now competes with every dashboard for warehouse compute, and the raw layer grows without a retention rule.\n\n> **The lesson:** the order of the letters is a decision about where the work runs and who pays for it.\n\n## In one line\n\n> **Transform where the data lives.** The order of the letters decides what pays for the compute.\n\n**Count the rows**\n\n```sql\nselect count(*) from orders\n```\n"},{"id":"p357","n":"357","slug":"the-lakehouse-layers-drawn","title":"The lakehouse layers, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","Lakehouse","Databricks","DeltaLake","Analytics"],"series_name":"Data","design":"explainer","script":"","date":"2029-01-07","files":{"devsecops":"notes/p357-the-lakehouse-layers-drawn.md"},"md":"# The lakehouse layers, drawn\n\nA dashboard says revenue is 4 percent up, finance says it is flat, and both are reading the lakehouse. The argument is not about the number; it is about which layer each query touched.\n\n## Quick reference\n\n### Bronze - raw\n\n- The source rows, appended as they arrived\n- Schema as it came, quirks included\n- Never edited in place, only added to\n- Replay starts here\n\n### Silver - cleaned\n\n- Typed, deduplicated and conformed\n- One row per entity, keys enforced\n- Joins and business rules applied once\n- Where most of the work actually lives\n\n### Gold - modelled\n\n- Star schemas and wide tables\n- Named for the business, not the source\n- What BI and finance actually read\n- Rebuilt from silver, never patched\n\n### Catalogue and lineage\n\n- Every table has an owner and a description\n- Lineage shows what a dropped column breaks\n- Freshness and row counts sit beside it\n- Without it, gold is only a rumour\n\n> **Note:** The layers are a contract: each one is rebuildable from the one below.\n\n## Explained\n\n**Bronze**  \nBronze is the raw copy: the source rows appended as they arrived, with the quirks, the duplicates and the string dates intact. Nothing is edited in place, only added to, which is what makes a replay possible.\n\n**Silver**  \nSilver is the cleaned layer: typed, deduplicated and conformed. One row per entity, keys enforced, the joins and business rules applied once instead of in every query. It is the layer most teams underestimate, because it is where the work lives.\n\n**Gold**  \nGold is modelled for consumption: star schemas, wide tables and names a finance analyst recognises. It is rebuilt from silver rather than patched, so the definition of revenue lives in one place.\n\n**Catalogue and lineage**  \nThe catalogue gives every table an owner, a description and a freshness timestamp. Lineage shows what a dropped column would break, and a table with neither is a table nobody will trust.\n\n> **The honest caveat:** the medallion is a convention, not a product. Bronze grows forever unless something deletes it, and naming the layers adds no lineage by itself.\n\n> **The lesson:** a layer is only real if it can be rebuilt from the one below it.\n\n## In one line\n\n> **Each layer earns the next one.** Bronze is cheap; silver is where the work is.\n\n**Check the catalogue**\n\n```sql\nselect * from information_schema.tables\n```\n"},{"id":"p358","n":"358","slug":"the-window-functions-cheat-sheet","title":"The window functions cheat sheet","sections":["devsecops"],"topic":"Data engineering","tags":["SQL","DataEngineering","Analytics","Databases","DataWarehouse"],"series_name":"Data","design":"cheatsheet","script":"","date":"2029-01-09","files":{"devsecops":"notes/p358-the-window-functions-cheat-sheet.md"},"md":"# The window functions cheat sheet\n\nA running total that resets halfway down the report is the classic window function bug. The function was right; the frame was not.\n\n## Quick reference\n\n### Rank\n\n```bash\nrow_number() over (order by ts)\nrank() over (order by score desc)\ndense_rank() over (order by score)\nntile(4) over (order by score)\n```\n\n### Aggregate\n\n```bash\nsum(amount) over (partition by uid)\navg(amount) over (partition by uid)\ncount(*) over (partition by uid)\nmax(ts) over (partition by uid)\n```\n\n### Offset\n\n```bash\nlag(ts, 1) over (order by ts)\nlead(ts, 1) over (order by ts)\nfirst_value(ts) over (order by ts)\nlast_value(ts) over (order by ts)\n```\n\n### Frame\n\n```bash\nsum(x) over (order by d)\nsum(x) over (partition by g)\nsum(x) over (order by d rows 6 preceding)\nsum(x) over ()\n```\n\n## Explained\n\n**Rank**  \nrow_number() numbers every row. rank() leaves a gap after a tie, and dense_rank() does not. ntile(4) splits the partition into quarters. All four need an order, and none of them collapse rows.\n\n**Aggregate**  \nsum(), avg(), count() and max() with an OVER clause keep every row and add the aggregate beside it. Partition by the customer and you get a per-customer total on each order line, without a join or a group by.\n\n**Offset**  \nlag() reads the previous row and lead() reads the next, both inside the window. first_value() and last_value() reach the edges of the frame, and last_value() is the one that surprises people.\n\n**Frame**  \nThe frame decides which rows the function sees. `sum(x) over (order by d)` is a running total, partition by g gives the whole partition, and rows 6 preceding gives a seven-row window. With ORDER BY and no frame, the default is range unbounded preceding to current row, which is why last_value() returns the current row.\n\n> **The honest caveat:** frame and null ordering differ between engines. Postgres and BigQuery disagree about where nulls sort, and a query that works on one warehouse can rank differently on another.\n\n> **The lesson:** the OVER clause is a frame, and the frame is usually the answer.\n\n## In one line\n\n> **The frame is the query.** Most window bugs are a missing frame, not a missing function.\n\n**Test the frame**\n\n```sql\nselect sum(x) over (order by d) from t\n```\n"},{"id":"p359","n":"359","slug":"5-data-pipeline-mistakes-and-the-fix-for-each","title":"5 data pipeline mistakes and the fix for each","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","DataPipelines","ETL","DataQuality","Analytics"],"series_name":"Data","design":"dontdo","script":"","date":"2029-01-11","files":{"devsecops":"notes/p359-5-data-pipeline-mistakes-and-the-fix-for-each.md"},"md":"# 5 data pipeline mistakes and the fix for each\n\nEvery pipeline in the incident channel has the same five decisions behind it, and each one saved an afternoon when it was made.\n\n## Quick reference\n\n**1.** ❌ **Retries with no idempotency** — A retried batch inserts every row a second time.  \n✅ **Write by key, upsert** — The second run overwrites the first instead of doubling it.\n\n**2.** ❌ **No schema contract** — A renamed column breaks the dashboard at 2am.  \n✅ **A registry or a typed table** — Producer and consumer agree on the shape before the load.\n\n**3.** ❌ **Backfills break consumers** — Reprocessing old dates overwrites the live tables.  \n✅ **Replay into a new partition** — Write history beside the live table, then swap it in.\n\n**4.** ❌ **No freshness checks** — A stalled job looks exactly like a quiet day.  \n✅ **Check the maximum timestamp** — A table that has not moved in an hour fails the run.\n\n**5.** ❌ **Local timezones in the data** — The DST hour repeats or vanishes every year.  \n✅ **Store UTC, render local** — One instant on disk, one timezone at the edge.\n\n## Explained\n\n### The five\n\n1. **Retries with no idempotency.** A batch fails halfway, the scheduler retries it, and every row that already landed lands again. Write by key with an upsert, so the second run overwrites the first instead of doubling it.\n\n2. **No schema contract.** A producer renames a column, the load succeeds, and the dashboard breaks at 2am. A registry or a typed table catches the change at the producer, before it reaches the consumer.\n\n3. **Backfills that break consumers.** Reprocessing last month in place overwrites the table the live dashboard reads. Write history into a new partition, check it, then swap it in.\n\n4. **No freshness check.** A stalled job looks exactly like a quiet day: the table is there, the rows are old, and nobody notices. Fail the run when the maximum timestamp has not moved.\n\n5. **Local timezones in the data.** Store the instant in UTC and render it in the user's zone. A timestamp with no zone is a timestamp with two meanings.\n\n### What they share\n\nEach one is a decision about the contract between two teams, which is why no single job's tests catch it.\n\n> **The honest caveat:** an idempotent write still needs a key you trust. A natural key that changes, such as an email address, turns an upsert into a duplicate.\n\n> **The lesson:** the pipeline is the contract, and every shortcut rewrites it without telling the other side.\n\n## In one line\n\n> **The pipeline is the contract.** Every shortcut rewrites it without telling the other side.\n\n**Check the clock**\n\n```sql\nselect max(updated_at) from orders\n```\n"},{"id":"p360","n":"360","slug":"a-data-freshness-checker-in-python","title":"A data freshness checker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["DataEngineering","Python","DataQuality","DataOps","Analytics"],"series_name":"Data","design":"code","script":"freshness_check.py","date":"2029-01-14","files":{"devsecops":"notes/p360-a-data-freshness-checker-in-python.md"},"md":"# A data freshness checker in Python\n\nThe orders table stopped updating at 2am and the dashboard looks normal, because yesterday's numbers are still there. Freshness is the check nobody writes until the first silent failure.\n\n## Quick reference\n\nThe full script is at the end of this note: `freshness_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 freshness_check.py fresh.json --max-age 60\n```\n\n**Example output**\n\n```text\nSTALE 720 warehouse.customers\n1 of 2 stale\n```\n\n## Explained\n\n**What it reads**  \nThe script takes a JSON map of table name to last-updated timestamp, from a file or stdin. The timestamps are ISO 8601, and a trailing Z or an explicit offset both work.\n\n**How it decides**  \nFor each table it subtracts the timestamp from now, in minutes. A table older than `--max-age` prints as STALE and joins the list; the rest print ok. Both sides of the subtraction are UTC, so a server in another zone does not move the line.\n\n**Exit codes**  \nExit 0 means every table is inside the window, exit 1 means at least one is stale, and exit 2 means the file was unreadable, the JSON was malformed or a timestamp would not parse. Exit 2 is a broken check, not a clean result. The attached script is standard library only, read-only, and takes `--max-age` and `--now`.\n\n> **The honest caveat:** a timestamp written by the job that loads the table proves the job ran, not that it wrote new rows. Point the check at a heartbeat the pipeline updates itself, or at the maximum updated_at in the table.\n\n> **The lesson:** a table that stopped moving looks exactly like a quiet day, unless something counts the minutes.\n\n## In one line\n\n> **A table that stopped moving.** A silent failure looks exactly like a quiet day.\n\n**Run it in ci**\n\n```bash\npython3 freshness_check.py fresh.json\n```\n\n## Script: freshness_check.py\n\nDownload: [scripts/freshness_check.py](../scripts/freshness_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Fail when a table in a JSON freshness map has not been updated in time.\n\nThe map is one JSON object of table name to ISO 8601 timestamp:\n\n    {\"warehouse.orders\": \"2026-10-20T20:30:00Z\",\n     \"warehouse.customers\": \"2026-10-19T09:00:00Z\"}\n\nUsage:\n    python3 freshness_check.py fresh.json --max-age 60\n\n    cat fresh.json | python3 freshness_check.py - --max-age 60\n\n    python3 freshness_check.py fresh.json --now 2026-10-20T21:00:00Z\n\nExit codes:\n    0  every table is inside the window\n    1  at least one table is stale\n    2  unreadable input, bad JSON or an unparsable timestamp\n\nCron/CI:\n    15 * * * * python3 /srv/checks/freshness_check.py \\\n        /srv/checks/fresh.json --max-age 60\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\n\ndef parse_stamp(text):\n    \"\"\"Parse an ISO 8601 timestamp and return it in UTC.\"\"\"\n    value = text.strip().replace(\"Z\", \"+00:00\")\n    stamp = datetime.fromisoformat(value)\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=timezone.utc)\n    return stamp.astimezone(timezone.utc)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"freshness_check.py\",\n        description=\"Flag tables whose last_updated timestamp is older than \"\n                    \"--max-age, from a JSON map of table to timestamp.\")\n    parser.add_argument(\"source\", nargs=\"?\", default=\"-\",\n                        help=\"JSON map file, or - for stdin (default: -)\")\n    parser.add_argument(\"--max-age\", type=int, default=60, metavar=\"MINUTES\",\n                        help=\"minutes before a table counts as stale (default: 60)\")\n    parser.add_argument(\"--now\", metavar=\"ISO8601\",\n                        help=\"evaluate against this time instead of the clock\")\n    args = parser.parse_args(argv)\n\n    if args.max_age < 0:\n        parser.error(\"--max-age must be zero or more\")\n\n    try:\n        if args.source == \"-\":\n            raw = sys.stdin.read()\n        else:\n            with open(args.source) as handle:\n                raw = handle.read()\n    except OSError as exc:\n        print(\"error: cannot read %s: %s\" % (args.source, exc), file=sys.stderr)\n        return 2\n\n    try:\n        tables = json.loads(raw)\n    except ValueError as exc:\n        print(\"error: not valid JSON: %s\" % exc, file=sys.stderr)\n        return 2\n    if not isinstance(tables, dict) or not tables:\n        print(\"error: expected a non-empty JSON object of table to timestamp\",\n              file=sys.stderr)\n        return 2\n\n    try:\n        now = parse_stamp(args.now) if args.now else datetime.now(timezone.utc)\n    except ValueError as exc:\n        print(\"error: bad --now value: %s\" % exc, file=sys.stderr)\n        return 2\n\n    stale = []\n    for name in sorted(tables):\n        try:\n            stamp = parse_stamp(str(tables[name]))\n        except ValueError:\n            print(\"error: %s has an unparsable timestamp: %r\"\n                  % (name, tables[name]), file=sys.stderr)\n            return 2\n        minutes = int((now - stamp).total_seconds() // 60)\n        if minutes < 0:\n            print(\"%-6s %5dm %s\" % (\"FUTURE\", -minutes, name))\n            stale.append(name)\n            continue\n        flag = \"STALE\" if minutes > args.max_age else \"ok\"\n        print(\"%-6s %5dm %s\" % (flag, minutes, name))\n        if minutes > args.max_age:\n            stale.append(name)\n\n    print(\"%d of %d tables stale after %d minutes\"\n          % (len(stale), len(tables), args.max_age))\n    return 1 if stale else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p361","n":"361","slug":"quiz-3-pipeline-questions","title":"Quiz: 3 pipeline questions","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","Kafka","DataPipelines","DataQuality","Analytics"],"series_name":"Data","design":"quiz","script":"","date":"2029-01-16","files":{"devsecops":"notes/p361-quiz-3-pipeline-questions.md"},"md":"# Quiz: 3 pipeline questions\n\nThree questions separate a pipeline you can operate from one you can only restart and hope.\n\n## Quick reference\n\n**Q1. Why must a consumer's write be idempotent before it retries?**\n\n- a) Kafka can deliver the same record twice\n- b) The broker rewrites records on retry\n- c) Offsets are never committed twice\n\n> **Answer:** a) Kafka can deliver the same record twice. At-least-once means a crash after the write and before the offset commit replays the record. An upsert keyed by the event id makes the replay harmless.\n\n**Q2. What does a schema registry actually prevent?**\n\n- a) A producer shipping a breaking change\n- b) A topic running out of disk space\n- c) A consumer reading too slowly\n\n> **Answer:** a) A producer shipping a breaking change. It stores the schema per subject and checks each new version against a compatibility rule. A change compatible in shape but wrong in meaning still gets through.\n\n**Q3. Why does a backfill need replayable history?**\n\n- a) To rebuild a range without double counting\n- b) Because storage is cheaper than compute\n- c) So the dashboard refreshes faster\n\n> **Answer:** a) To rebuild a range without double counting. A backfill re-reads the past. If the pipeline overwrote those rows, the input is gone; an append-only or partitioned history lets you recompute and swap.\n\n## Explained\n\n**Why idempotent writes**  \nQuestion one: a consumer retries a batch after a crash. At-least-once delivery means the record written before the crash arrives again, so the write must be safe to repeat. An upsert on a natural key, or an insert that ignores a duplicate event id, makes the replay a no-op instead of a second charge.\n\n**What a registry prevents**  \nQuestion two: a producer ships a field rename, a type change or a removed column. The registry stores the schema per subject and checks the new version against a compatibility rule before the consumer sees it. It cannot stop a change that is structurally compatible but semantically wrong, such as a currency field that switches from cents to dollars.\n\n**Why backfills need history**  \nQuestion three: a bug corrupts ninety days of a derived table. If the pipeline overwrote rows in place, the input for those days is gone with them. With an append-only raw layer or partitioned history, you can recompute the range and swap it in, because the past still exists.\n\n> **The honest caveat:** replay is only as good as the code that wrote the history. A raw table with no schema and no partition key is a pile of files, not a replayable log.\n\n> **The lesson:** the pipeline is only as good as what it can rebuild.\n\n## In one line\n\n> **Replay decides what you can fix.** Three answers that separate a pipeline from a script.\n\n**Read the subject**\n\n```bash\ncurl -s localhost:8081/subjects\n```\n"},{"id":"p362","n":"362","slug":"the-feature-store-drawn","title":"The feature store, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","FeatureStore","MachineLearning","MLOps","DataArchitecture"],"series_name":"Data","design":"architecture","script":"","date":"2029-01-18","files":{"devsecops":"notes/p362-the-feature-store-drawn.md"},"md":"# The feature store, drawn\n\nThe model that scored 0.91 in the notebook scored 0.74 in production, and nobody had retrained it. The same features were being computed twice, by two jobs that had drifted apart.\n\n## Quick reference\n\n- **Sources:** Event stream (EVT) · Orders database (DB) · Nightly dumps (DUMP)\n- **Transform:** Batch job (BATCH) · Streaming job (FLOW) · Validation (CHECK)\n- **Serve:** Online store (ONLN) · Offline store (OFFLN) · Training pipeline (TRAIN)\n\n**Request flow:** Event stream → Streaming job → Online store; Orders database → Batch job → Validation → Offline store → Training pipeline; Nightly dumps → Batch job\n\n## Explained\n\n### Sources\n\nThe event stream carries clicks and orders as they happen, with a timestamp on every event. The orders database holds the current state and forgets what the row looked like last week. The nightly dumps are a full copy, once a day, and already hours old when they land.\n\n### Transform\n\nThe batch job reads the database and the dumps and computes features over history. The streaming job reads the event stream and recomputes the last few seconds. Validation sits after the batch job and before the offline store: null counts, ranges, and a row count that has not collapsed, because a job that writes nothing should fail loudly.\n\n### Serve\n\nThe online store answers one entity at a time on the request path, in milliseconds, keyed by user or order. The offline store keeps every historical value, so a training set can be assembled as of the moment each label happened. The training pipeline reads the offline store only, which is the point: training and serving share a definition, not a copy of the code.\n\n> **The honest caveat:** a feature store does not fix a label that was written late, and a point-in-time join is only as correct as the event timestamp you feed it. A perfect feature from a wrong clock is still skew.\n\n> **The lesson:** one definition, two stores, and a clock you can defend.\n\n## In one line\n\n> **One definition, two stores.** Online for the request, offline for the training set.\n\n**Register features**\n\n```bash\nfeast apply\n```\n"},{"id":"p363","n":"363","slug":"vector-search-in-four-steps","title":"Vector search in four steps","sections":["devsecops"],"topic":"Data engineering","tags":["VectorSearch","Embeddings","AISearch","Postgres","MachineLearning"],"series_name":"Data","design":"explainer","script":"","date":"2029-01-21","files":{"devsecops":"notes/p363-vector-search-in-four-steps.md"},"md":"# Vector search in four steps\n\nA search box that returns the wrong five results usually has a good model and a bad second half. Most failures live in the last two steps.\n\n## Quick reference\n\n### Step 1 - Embed\n\n- One model for documents and queries\n- A 1,536-dimension float per row\n- Normalise to make inner product cosine\n- Re-embed everything when the model changes\n\n### Step 2 - Index (HNSW)\n\n- A multi-layer graph of near neighbours\n- Search descends from a sparse top layer\n- m=16 and ef_construction=64 by default\n- ef_search trades recall for latency\n\n### Step 3 - Similarity\n\n- Cosine: 1 minus the distance, higher is closer\n- Inner product arrives negated in pgvector\n- L2 when magnitude should count\n- Top k candidates, not final answers\n\n### Step 4 - Rerank\n\n- A cross-encoder rescores the top fifty\n- Better ordering, milliseconds per batch\n- Rerank only what the index returned\n- Measure recall at k before and after\n\n> **Note:** Brute force is exact and slow; every index is a promise about recall.\n\n## Explained\n\n### Embed\n\nOne model turns each document into a vector, and the query into one. A 1,536-dimension float per row is normal. If you rank by inner product, normalise both sides: inner product rewards a long vector as much as a similar one.\n\n### Index\n\nComparing a query to every row is a full scan: fine at ten thousand rows, hopeless at ten million. HNSW builds a multi-layer graph, and the search descends from a sparse top layer into denser ones. The defaults are m=16 and ef_construction=64; ef_search, 40 by default, is the dial at query time.\n\n### Similarity\n\nCosine distance, written <=>, is one minus cosine similarity, so lower is closer. Inner product, <#>, comes back negated so an ascending index scan works. L2, <->, is the plain distance. The index returns candidates, not answers, and returns them approximately.\n\n### Rerank\n\nA cross-encoder reads the query and each candidate together and rescores the top fifty. It costs milliseconds per batch and buys more accuracy than another index parameter. Rerank only what the index returned, and measure recall at k before and after.\n\n> **The honest caveat:** every HNSW or IVFFlat index trades recall for speed, so the same query can return different rows than exact search. That is not a bug; it is a number you have to measure.\n\n> **The lesson:** the index finds candidates, and the reranker decides the order.\n\n## In one line\n\n> **The index is an approximation.** Recall is a number you measure, not a promise the database makes.\n\n**Tune the search**\n\n```sql\nSET hnsw.ef_search = 100;\n```\n"},{"id":"p364","n":"364","slug":"the-pgvector-cheat-sheet","title":"The pgvector cheat sheet","sections":["devsecops"],"topic":"Data engineering","tags":["Postgres","pgvector","VectorSearch","SQL","DataEngineering"],"series_name":"Data","design":"cheatsheet","script":"","date":"2029-01-23","files":{"devsecops":"notes/p364-the-pgvector-cheat-sheet.md"},"md":"# The pgvector cheat sheet\n\nYou have Postgres, you have embeddings, and no appetite for a second database. pgvector is one extension, one column type, two index types and four operators.\n\n## Quick reference\n\n### Setup\n\n```sql\nCREATE EXTENSION IF NOT EXISTS vector;\nSELECT extversion FROM pg_extension;\n\\dx vector\nSHOW server_version;\n```\n\n### Store\n\n```sql\nCREATE TABLE t (id int, v vector(3));\nINSERT INTO t VALUES (1, '[1,2,3]');\nSELECT id, v::text FROM t;\nUPDATE t SET v = '[3,2,1]';\nALTER TABLE t ADD v2 vector(3);\n```\n\n### Index\n\n```sql\nCREATE INDEX ON t USING ivfflat (v);\nCREATE INDEX ON t USING hnsw (v\n  vector_l2_ops) WITH (m = 16);\nSET hnsw.ef_search = 100;\nSET ivfflat.probes = 10;\n```\n\n### Query\n\n```sql\nSELECT * FROM t ORDER BY v <-> $1 LIMIT 5\nSELECT v <=> '[1,2,3]' AS cos FROM t;\nSELECT v <#> '[1,2,3]' AS ip FROM t;\nSELECT v <+> '[1,2,3]' AS l1 FROM t;\nSELECT 1 - (v <=> $1) AS sim;\n```\n\n## Explained\n\n### Setup\n\n`CREATE EXTENSION IF NOT EXISTS vector;` runs once per database, not once per cluster. `SELECT extversion FROM pg_extension;` shows the version, `\\dx vector` lists it in psql, and `SHOW server_version;` tells you what you run against.\n\n### Store\n\nThe column is the schema. `CREATE TABLE t (id int, v vector(3));` pins the dimension, and `INSERT INTO t VALUES (1, '[1,2,3]');` takes a string Postgres casts for you. Read it with `SELECT id, v::text FROM t;` and update it with `UPDATE t SET v = '[3,2,1]';`.\n\n### Index\n\nivfflat has a default operator class, so `CREATE INDEX ON t USING ivfflat (v);` works, and lists default to 100. HNSW has no default, so the statement wraps to name vector_l2_ops and m=16. At query time `SET hnsw.ef_search = 100;` and `SET ivfflat.probes = 10;` buy recall with latency.\n\n### Query\n\n`SELECT * FROM t ORDER BY v <-> $1 LIMIT 5` is the shape that uses an index: a distance operator in ORDER BY, ascending, with a LIMIT. The vector arrives as a bind parameter, which is why the sheet writes $1. The other lines show <=>, <#> and <+>.\n\n> **The honest caveat:** the index is used only for that ORDER BY shape. Put the distance in a SELECT list, or sort descending, and you get a full scan instead.\n\n> **The lesson:** the operator picks the metric, and the index only makes it fast.\n\n## In one line\n\n> **Store the vector, query the distance.** The operator picks the metric; the index only makes it fast.\n\n**Show the plan**\n\n```sql\nEXPLAIN ANALYZE SELECT * FROM t\n```\n"},{"id":"p365","n":"365","slug":"a-pii-column-scanner-in-python","title":"A PII column scanner in Python","sections":["devsecops"],"topic":"Python scripts","tags":["DataGovernance","PII","Python","Privacy","DataEngineering"],"series_name":"Data","design":"code","script":"pii_columns.py","date":"2029-01-25","files":{"devsecops":"notes/p365-a-pii-column-scanner-in-python.md"},"md":"# A PII column scanner in Python\n\nThe security review asks one question you cannot answer from memory: which columns hold personal data? The schema knows, if you read it properly.\n\n## Quick reference\n\nThe full script is at the end of this note: `pii_columns.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 pii_columns.py schema.json\n```\n\n**Example output**\n\n```text\nPII: users.email, users.dob, events.ip\ncolumns: 3 tables: 2\n```\n\n## Explained\n\n### What it runs\n\nThe schema is JSON: a tables key, and each table a list of columns with a name and a type. It splits every column name into tokens and compares them to a list such as email, phone, ssn, dob, address, name, passport, iban, card and ip. A match is a hit.\n\n### What the types add\n\nSome columns are personal whatever the name. inet is an IP address, json and jsonb can hold a free-text form, and bytea can hold a scanned document. Flagged for review, not as proof.\n\n### What it prints\n\nOne line of flagged columns, then a summary line. Exit 1 when anything is flagged, 0 when clean, and 2 when the file is missing or not JSON, because a check that cannot run is not a pass.\n\n### How to use it\n\nRun it in CI against the schema dump, or weekly from cron and diff it: a new column is the event you want to see. The full script adds `--json` and one line per column.\n\n> **The honest caveat:** a name list is a guess. A column called notes can hold an email address and pass, a name in another language is invisible, and a bare name column is flagged whether it holds a person or a product.\n\n> **The lesson:** start with the schema, then sample the values. Neither one alone is discovery.\n\n## In one line\n\n> **The schema is the short list.** A name pattern is a hypothesis; the schema is where you start.\n\n**Run it on a dump**\n\n```bash\npython3 pii_columns.py schema.json\n```\n\n## Script: pii_columns.py\n\nDownload: [scripts/pii_columns.py](../scripts/pii_columns.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Scan a JSON schema for columns that probably hold personal data.\n\nUsage:\n    python3 pii_columns.py schema.json\n    python3 pii_columns.py schema.json --json\n    python3 pii_columns.py schema.json --quiet\n    python3 pii_columns.py --help\n\nThe schema file is JSON, either wrapped in a \"tables\" key or bare:\n\n    {\"tables\": {\"users\": [{\"name\": \"email\", \"type\": \"text\"},\n                          {\"name\": \"date_of_birth\", \"type\": \"date\"}]}}\n\nA column may also be written as the string \"email text\".  Column names are\nsplit on punctuation and camel case, then matched against a short list of\npersonal-data tokens.  Types that can hide data whatever the name (inet, json,\njsonb, bytea) are reported for review, not as proof.\n\nExit codes: 0 no findings, 1 findings, 2 usage or input error.\nRead-only: the script never writes to the schema or to a database.\n\nCI:   python3 pii_columns.py schema.json || exit 1\nCron: 0 6 * * 1 python3 pii_columns.py schema.json >> /var/log/pii.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\n# token -> category. Keep it short and boring; every entry is a guess.\nNAME_HINTS = {\n    \"email\": \"email\",\n    \"mail\": \"email\",\n    \"phone\": \"phone\",\n    \"mobile\": \"phone\",\n    \"tel\": \"phone\",\n    \"telephone\": \"phone\",\n    \"ssn\": \"national id\",\n    \"nino\": \"national id\",\n    \"passport\": \"national id\",\n    \"licence\": \"national id\",\n    \"license\": \"national id\",\n    \"tax\": \"national id\",\n    \"dob\": \"date of birth\",\n    \"birth\": \"date of birth\",\n    \"birthdate\": \"date of birth\",\n    \"address\": \"address\",\n    \"street\": \"address\",\n    \"postcode\": \"address\",\n    \"zip\": \"address\",\n    \"postal\": \"address\",\n    \"name\": \"name\",\n    \"surname\": \"name\",\n    \"forename\": \"name\",\n    \"given\": \"name\",\n    \"family\": \"name\",\n    \"card\": \"payment card\",\n    \"pan\": \"payment card\",\n    \"cvv\": \"payment card\",\n    \"iban\": \"bank account\",\n    \"bic\": \"bank account\",\n    \"ip\": \"ip address\",\n    \"ipv4\": \"ip address\",\n    \"ipv6\": \"ip address\",\n    \"latitude\": \"location\",\n    \"longitude\": \"location\",\n    \"geo\": \"location\",\n    \"location\": \"location\",\n    \"health\": \"special category\",\n    \"diagnosis\": \"special category\",\n    \"religion\": \"special category\",\n    \"ethnicity\": \"special category\",\n    \"biometric\": \"special category\",\n    \"salary\": \"financial\",\n}\n\n# type -> category. These hold personal data whatever the column is called.\nTYPE_HINTS = {\n    \"inet\": \"ip address\",\n    \"json\": \"free-form json\",\n    \"jsonb\": \"free-form json\",\n    \"bytea\": \"binary blob\",\n}\n\n# Metadata columns that contain a hint token but are not personal data.\nIGNORE = {\n    \"table_name\",\n    \"column_name\",\n    \"schema_name\",\n    \"index_name\",\n    \"file_name\",\n    \"constraint_name\",\n    \"product_name\",\n    \"company_name\",\n    \"model_name\",\n}\n\n\ndef tokens(name):\n    \"\"\"Split a column name into lowercase tokens: dateOfBirth -> date, of, birth.\"\"\"\n    spaced = re.sub(r\"(?<=[a-z0-9])(?=[A-Z])\", \"_\", str(name))\n    return [t for t in re.split(r\"[^a-z0-9]+\", spaced.lower()) if t]\n\n\ndef base_type(typ):\n    \"\"\"text[] and varchar(255) both reduce to their base type.\"\"\"\n    return str(typ).split(\"(\")[0].strip().lower().rstrip(\"[]\").strip()\n\n\ndef load_schema(path):\n    \"\"\"Read the schema file and return the table mapping.\"\"\"\n    with open(path, \"r\") as fh:\n        data = json.load(fh)\n    if not isinstance(data, dict):\n        raise ValueError(\"schema must be a JSON object\")\n    tables = data.get(\"tables\", data.get(\"schema\", data))\n    if not isinstance(tables, dict):\n        raise ValueError(\"tables must be a JSON object of table: columns\")\n    return tables\n\n\ndef iter_columns(tables):\n    \"\"\"Yield (table, column, type) for every column in the schema.\"\"\"\n    for table, cols in tables.items():\n        if isinstance(cols, dict) and \"columns\" in cols:\n            cols = cols[\"columns\"]\n        if isinstance(cols, dict):\n            cols = [{\"name\": k, \"type\": v} for k, v in cols.items()]\n        if not isinstance(cols, list):\n            raise ValueError(\"table %r must be a list of columns\" % table)\n        for col in cols:\n            if isinstance(col, str):\n                parts = col.split(None, 1)\n                if not parts:\n                    raise ValueError(\"empty column in table %r\" % table)\n                yield table, parts[0], parts[1] if len(parts) > 1 else \"unknown\"\n            elif isinstance(col, dict):\n                name = col.get(\"name\") or col.get(\"column\")\n                if not name:\n                    raise ValueError(\"column without a name in table %r\" % table)\n                yield table, str(name), str(col.get(\"type\", \"unknown\"))\n            else:\n                raise ValueError(\"column in table %r must be an object or string\" % table)\n\n\ndef scan(tables):\n    \"\"\"Return the findings, sorted by table and column.\"\"\"\n    findings = []\n    for table, column, typ in iter_columns(tables):\n        lower = str(column).lower()\n        hit = None\n        if lower not in IGNORE:\n            for token in tokens(column):\n                if token in NAME_HINTS:\n                    hit = (\"name\", NAME_HINTS[token])\n                    break\n        if hit is None:\n            base = base_type(typ)\n            if base in TYPE_HINTS:\n                hit = (\"type\", TYPE_HINTS[base])\n        if hit:\n            findings.append({\n                \"table\": table,\n                \"column\": column,\n                \"type\": typ,\n                \"kind\": hit[0],\n                \"category\": hit[1],\n            })\n    findings.sort(key=lambda f: (str(f[\"table\"]), str(f[\"column\"])))\n    return findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Flag columns that probably hold personal data, from a schema JSON.\",\n        epilog=\"Exit 0 clean, 1 findings, 2 usage or input error. Read-only.\",\n    )\n    parser.add_argument(\"schema\", help=\"path to the schema JSON file\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON object instead of lines\")\n    parser.add_argument(\"--quiet\", action=\"store_true\",\n                        help=\"print only the summary line\")\n    args = parser.parse_args(argv)\n\n    try:\n        tables = load_schema(args.schema)\n    except json.JSONDecodeError as exc:\n        print(\"error: %s is not valid JSON: %s\" % (args.schema, exc), file=sys.stderr)\n        return 2\n    except (OSError, ValueError) as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    try:\n        findings = scan(tables)\n    except ValueError as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    if args.json:\n        print(json.dumps({\n            \"schema\": args.schema,\n            \"tables\": len(tables),\n            \"count\": len(findings),\n            \"findings\": findings,\n        }, indent=2, sort_keys=True))\n    else:\n        if not args.quiet:\n            for f in findings:\n                label = (\"name:\" if f[\"kind\"] == \"name\" else \"type:\") + f[\"category\"]\n                print(\"%-34s %-12s %s\" % (f[\"table\"] + \".\" + f[\"column\"], f[\"type\"], label))\n        print(\"PII columns: %d in %d tables\" % (len(findings), len(tables)))\n\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p366","n":"366","slug":"lakehouse-governance-drawn","title":"Lakehouse governance, drawn","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","Lakehouse","DataGovernance","ApacheIceberg","Analytics"],"series_name":"Data","design":"architecture","script":"","date":"2029-01-28","files":{"devsecops":"notes/p366-lakehouse-governance-drawn.md"},"md":"# Lakehouse governance, drawn\n\nThree teams wrote to the same bucket, and the only map was a wiki page from 2023. When the revenue model broke, the fix took three days because nobody could say what fed it.\n\n## Quick reference\n\n- **Sources:** OLTP database (DB) · File drops (FILES) · Change streams (CDC)\n- **Lake:** Raw zone (RAW) · Curated zone (CUR) · Table format (TBL)\n- **Governance:** Catalogue (CAT) · Lineage (LINE) · Access policies (POL)\n\n**Request flow:** OLTP database → Raw zone → Curated zone → Table format; Curated zone → Catalogue; Curated zone → Lineage; Curated zone → Access policies; File drops → Raw zone; Change streams → Raw zone\n\n## Explained\n\n### Sources\n\nThe OLTP database holds the current state of orders. File drops arrive from partners as CSV, on their schedule. Change streams read the database log and emit every insert and update as an event. All three land in the raw zone untouched, because the first rule of a lake is that you can replay it.\n\n### Lake\n\nThe raw zone is immutable and cheap. The curated zone is where deduplication, typing and joins happen, and where the table format earns its keep: Iceberg, Delta or Hudi keeps a transaction log, so a reader sees a consistent snapshot while a writer adds files. The catalogue registers each table, so an engine finds it without a path.\n\n### Governance\n\nThe catalogue holds names, schemas and owners. Lineage records which table and which column fed which, so an incident review becomes a walk downstream instead of a search. Access policies add row filters and column masks, so an analyst sees the region they own and not the national margin.\n\n> **The honest caveat:** policies are enforced by the engine that reads the table. A role with direct access to the bucket can read the files and skip every mask, which is why the storage credentials matter as much as the policy.\n\n> **The lesson:** the lake is storage, and the catalogue is the contract.\n\n## In one line\n\n> **Storage is easy. The catalogue is not.** Raw and curated zones are storage; the catalogue makes them answerable.\n\n**Check snapshots**\n\n```sql\nSELECT * FROM db.orders.snapshots\n```\n"},{"id":"p367","n":"367","slug":"data-quality-four-checks","title":"Data quality, four checks","sections":["devsecops"],"topic":"Data engineering","tags":["DataQuality","DataEngineering","Analytics","SQL","DataGovernance"],"series_name":"Data","design":"explainer","script":"","date":"2029-01-30","files":{"devsecops":"notes/p367-data-quality-four-checks.md"},"md":"# Data quality, four checks\n\nThe pipeline was green, the dashboard loaded, and the revenue number was forty percent low. Nothing had failed. The data was simply incomplete.\n\n## Quick reference\n\n### Completeness\n\n- Expected rows against rows landed\n- Nulls in the columns that matter\n- Counts per partition, not just the total\n- A missing partition is a failed load\n\n### Uniqueness\n\n- One row per key, enforced by a test\n- A duplicate doubles every sum after it\n- Test the natural key, not the surrogate id\n- Late data is the usual culprit\n\n### Validity\n\n- Types, ranges, enums and regex\n- 1970-01-01 is a sentinel, not a birthday\n- Reject at the edge, not in the report\n- Null is not the same as zero\n\n### Freshness\n\n- The age of the newest partition\n- A staleness SLA per table\n- Alert on lag, not only on failure\n- Fresh but wrong is still wrong\n\n> **Note:** A check you did not write is a failure you will not see.\n\n## Explained\n\n### Completeness\n\nExpected rows against rows landed, nulls in the columns that matter, and counts per partition rather than a total. A missing partition is the most common failure, and a healthy total can hide it, because the other partitions grew.\n\n### Uniqueness\n\nOne row per key. A duplicate doubles every sum that follows, and the natural key is the one to test, not the surrogate id, which is unique by construction and tells you nothing.\n\n### Validity\n\nTypes, ranges, enums and regular expressions. A date of 1970-01-01 is a sentinel, not a birthday, and a negative quantity is a refund or a bug. Reject at the edge, where the message can still name the source file.\n\n### Freshness\n\nThe age of the newest partition, measured against a staleness SLA per table. Measure it from the newest event, not from the job's exit time. Alert on lag rather than on failure, because a job that succeeds and writes nothing is the failure that pages nobody.\n\n> **The honest caveat:** these four checks test structure, not meaning. A currency conversion applied twice passes all of them, and so does a correct table joined to the wrong one.\n\n> **The lesson:** test the data, not the job.\n\n## In one line\n\n> **Test the data, not the job.** A green pipeline can still write an empty table.\n\n**Check freshness**\n\n```sql\nSELECT max(loaded_at) FROM orders\n```\n"},{"id":"p368","n":"368","slug":"the-dbt-cheat-sheet","title":"The dbt cheat sheet","sections":["devsecops"],"topic":"Data engineering","tags":["dbt","DataEngineering","AnalyticsEngineering","SQL","DataPipelines"],"series_name":"Data","design":"cheatsheet","script":"","date":"2029-02-01","files":{"devsecops":"notes/p368-the-dbt-cheat-sheet.md"},"md":"# The dbt cheat sheet\n\nA dbt project is a graph, and every command chooses part of it. Four blocks cover the working day.\n\n## Quick reference\n\n### Build\n\n```bash\ndbt build\ndbt build --select staging+\ndbt run --full-refresh\ndbt seed\ndbt snapshot\n```\n\n### Test\n\n```bash\ndbt test\ndbt test --select +orders\ndbt test --store-failures\ndbt test --select source:raw\ndbt source freshness\n```\n\n### Run\n\n```bash\ndbt run\ndbt run --select +orders\ndbt run --select orders+\ndbt run --exclude tag:slow\ndbt compile --select orders\n```\n\n### Docs\n\n```bash\ndbt docs generate\ndbt docs serve --port 8080\ndbt ls --select tag:nightly\ndbt debug\ndbt parse\n```\n\n## Explained\n\n### Build\n\n`dbt build` runs models, tests, seeds and snapshots in dependency order, and tests each node as it is built, so a failure stops everything downstream. `dbt build --select staging+` adds everything downstream, and `dbt run --full-refresh` rebuilds a model whose SQL changed shape.\n\n### Test\n\n`dbt test` runs the schema tests and the singular tests you wrote. `--select +orders` tests orders and everything upstream, `--store-failures` writes the failing rows to a table you can query, `--select source:raw` tests the sources themselves, and `dbt source freshness` compares loaded_at with the declared thresholds.\n\n### Run\n\n`dbt run` executes models only, which is faster and less safe. `--select +orders` builds upstream first, `--select orders+` builds downstream, `--exclude tag:slow` removes a tagged set, and dbt compile renders the SQL without running it, which checks a Jinja change before it costs a warehouse.\n\n### Docs\n\n`dbt docs generate` writes the manifest and catalogue the docs site reads, and `dbt docs serve --port 8080` opens it. dbt ls shows what a selector would run without running it. `dbt debug` checks the connection and the profile; `dbt parse` validates the project.\n\n> **The honest caveat:** dbt tests are SQL you wrote, so a project can pass them all and still publish a wrong number. The test only checks the rule somebody thought of.\n\n> **The lesson:** selection is the skill, and the command is just the verb.\n\n## In one line\n\n> **The DAG decides the order.** You name the nodes; dbt works out what runs before what.\n\n**See the selection**\n\n```bash\ndbt ls --select staging+\n```\n"},{"id":"p369","n":"369","slug":"data-terms-decoded","title":"Data terms, decoded","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","CDC","Lakehouse","OLAP","DataGovernance"],"series_name":"Data","design":"reveal","script":"","date":"2029-02-04","files":{"devsecops":"notes/p369-data-terms-decoded.md"},"md":"# Data terms, decoded\n\nSix words come up in every data meeting, and half the room nods at a different definition. Here is what each one buys and what it costs.\n\n## Quick reference\n\n- **CDC** — Change data capture: read the database log, not the table. Every change becomes an event, and the warehouse sees it in seconds.\n- **ELT** — Extract, load, transform: land the raw data first, then transform it in SQL inside the warehouse. The raw copy stays replayable.\n- **OLAP** — Online analytical processing: column stores that scan a few columns across billions of rows. Built for sums, not for one order by id.\n- **Lakehouse** — Object storage plus an open table format (Iceberg, Delta or Hudi) plus a catalogue. Cheap like a lake, transactional like a warehouse.\n- **Lineage** — The graph of which table and column fed which. It turns 'this report is wrong' into a list of downstream tables.\n- **Cardinality** — How many distinct values a column holds. user_id is high, status is low, and the estimate decides join order.\n\n## Explained\n\n### CDC\n\nChange data capture reads the database log, the WAL in Postgres or the binlog in MySQL, instead of polling the table. Every insert, update and delete becomes an event.\n\n### ELT\n\nExtract, load, transform. Land the raw data first, then transform it with SQL in the warehouse, so the raw copy stays replayable when the logic changes.\n\n### OLAP\n\nOnline analytical processing: column stores built to scan a few columns across billions of rows. A sum over a month is fast; fetching one order by id is not.\n\n### Lakehouse\n\nObject storage plus an open table format (Iceberg, Delta or Hudi) plus a catalogue. Cheap like a lake, transactional like a warehouse.\n\n### Lineage\n\nThe graph of which table and which column fed which. In an incident it names the upstream cause and everything downstream.\n\n### Cardinality\n\nThe number of distinct values in a column. user_id is high, status is low, and the planner's estimate of both decides join order and index choice.\n\n> **The honest caveat:** every one of these is a trade, and the bill arrives later. Replay costs storage, lineage costs instrumentation, and a lakehouse costs metadata discipline.\n\n> **The lesson:** the words decide the design before the tool does.\n\n## In one line\n\n> **The words decide the design.** Each term is a cost you agree to when you pick the tool.\n\n**Check cardinality**\n\n```sql\nSELECT count(DISTINCT country) FROM users\n```\n"},{"id":"p370","n":"370","slug":"a-data-quality-checker-in-python","title":"A data quality checker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DataQuality","DataEngineering","DataPipelines","Analytics"],"series_name":"Data","design":"code","script":"data_quality.py","date":"2029-02-06","files":{"devsecops":"notes/p370-a-data-quality-checker-in-python.md"},"md":"# A data quality checker in Python\n\nThe nightly load finished at 02:14 and the table was empty. Nothing raised an error, because nothing was checking the data itself.\n\n## Quick reference\n\nThe full script is at the end of this note: `data_quality.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 data_quality.py rows.jsonl rules.json\n```\n\n**Example output**\n\n```text\nrow 4: missing email\nfindings: 1 in 5 rows\n```\n\n## Explained\n\n### What it runs\n\nThe script takes JSONL rows, one object per line, and a rules file with required fields, unique keys and numeric ranges. Every row is checked against all three, and a failed row prints once, with its problems on one line.\n\n### What it prints\n\nThe failed rows, then a count against the rows read. Exit 1 when anything failed, 0 when clean, and 2 when a file is missing or a rule is malformed, because a check that cannot run is not a pass.\n\n### Why JSONL and JSON\n\nOne object per line streams, so the script reads a file larger than memory, and a truncated line is an error you can see rather than a silent gap. The rules stay in JSON because whoever owns the rules may not own the code.\n\n### How to use it\n\nRun it after the load and before the models, from cron or in the pipeline. The full script adds `--json` and counts findings by rule, so a broken range shows up as a broken range, not a bad night.\n\n> **The honest caveat:** required, unique and range checks are the three you can write in an afternoon. They will not catch a payment counted twice or a currency converted twice; those need a business rule from someone who knows the number.\n\n> **The lesson:** a load that succeeds is not a load that worked.\n\n## In one line\n\n> **Count the rows, not the exit code.** A job that returns zero can still write zero.\n\n**Run it after load**\n\n```bash\npython3 data_quality.py rows.jsonl\n```\n\n## Script: data_quality.py\n\nDownload: [scripts/data_quality.py](../scripts/data_quality.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check JSONL rows against a small rules file and report the violations.\n\nUsage:\n    python3 data_quality.py rows.jsonl rules.json\n    python3 data_quality.py rows.jsonl rules.json --json\n    python3 data_quality.py rows.jsonl rules.json --limit 5\n    python3 data_quality.py --help\n\nrows.jsonl is one JSON object per line.  The rules file is JSON:\n\n    {\"required\": [\"id\", \"email\"],\n     \"unique\":   [\"id\"],\n     \"ranges\":   {\"amount\": [0, 10000]}}\n\nEvery row is checked for missing required fields, duplicate unique keys and\nout-of-range numbers.  Findings print one per row, then the count of findings\nagainst the number of rows read.  A malformed line is a finding, not a crash.\n\nExit codes: 0 no findings, 1 findings, 2 usage or input error.\nRead-only: the script never writes to the rows or to the rules file.\n\nCI:   python3 data_quality.py rows.jsonl rules.json || exit 1\nCron: 15 3 * * * python3 data_quality.py rows.jsonl rules.json >> /var/log/dq.log 2>&1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nEMPTY = (None, \"\")\n\n\ndef load_rules(path):\n    \"\"\"Read and validate the rules file. Raises ValueError on a bad rule.\"\"\"\n    with open(path, \"r\") as fh:\n        rules = json.load(fh)\n    if not isinstance(rules, dict):\n        raise ValueError(\"rules must be a JSON object\")\n\n    for key in (\"required\", \"unique\"):\n        value = rules.get(key, [])\n        if not isinstance(value, list) or not all(isinstance(v, str) for v in value):\n            raise ValueError(\"%s must be a list of field names\" % key)\n\n    ranges = rules.get(\"ranges\", {})\n    if not isinstance(ranges, dict):\n        raise ValueError(\"ranges must be an object of field: [min, max]\")\n    clean = {}\n    for field, spec in ranges.items():\n        if (not isinstance(spec, list) or len(spec) != 2\n                or any(isinstance(v, bool) or not isinstance(v, (int, float)) for v in spec)):\n            raise ValueError(\"range for %r must be [min, max] numbers\" % field)\n        low, high = spec\n        if low > high:\n            raise ValueError(\"range for %r has min above max\" % field)\n        clean[field] = (low, high)\n\n    return {\n        \"required\": rules.get(\"required\", []),\n        \"unique\": rules.get(\"unique\", []),\n        \"ranges\": clean,\n    }\n\n\ndef kind_of(message):\n    \"\"\"Map a message to a rule name for the JSON summary.\"\"\"\n    if message.startswith(\"missing \"):\n        return \"required\"\n    if message.startswith(\"duplicate \") or message.startswith(\"unhashable \"):\n        return \"unique\"\n    return \"range\"\n\n\ndef check_row(row, rules, seen):\n    \"\"\"Return the list of problems for one row, and remember unique values.\"\"\"\n    problems = []\n\n    for field in rules[\"required\"]:\n        if row.get(field) in EMPTY:\n            problems.append(\"missing \" + field)\n\n    for field in rules[\"unique\"]:\n        value = row.get(field)\n        if value is None:\n            continue\n        try:\n            hash(value)\n        except TypeError:\n            problems.append(\"unhashable \" + field)\n            continue\n        if value in seen[field]:\n            problems.append(\"duplicate %s=%r\" % (field, value))\n        else:\n            seen[field].add(value)\n\n    for field, (low, high) in rules[\"ranges\"].items():\n        value = row.get(field)\n        if value is None:\n            continue\n        if isinstance(value, bool) or not isinstance(value, (int, float)):\n            problems.append(\"%s is not numeric\" % field)\n        elif not low <= value <= high:\n            problems.append(\"%s %s out of range %s..%s\" % (field, value, low, high))\n\n    return problems\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check JSONL rows for missing fields, duplicates and out-of-range numbers.\",\n        epilog=\"Exit 0 clean, 1 findings, 2 usage or input error. Read-only.\",\n    )\n    parser.add_argument(\"rows\", help=\"path to the JSONL rows file\")\n    parser.add_argument(\"rules\", help=\"path to the rules JSON file\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON object instead of lines\")\n    parser.add_argument(\"--limit\", type=int, default=50,\n                        help=\"maximum findings to print, 0 for all (default 50)\")\n    args = parser.parse_args(argv)\n    if args.limit < 0:\n        print(\"error: --limit cannot be negative\", file=sys.stderr)\n        return 2\n\n    try:\n        rules = load_rules(args.rules)\n    except json.JSONDecodeError as exc:\n        print(\"error: %s is not valid JSON: %s\" % (args.rules, exc), file=sys.stderr)\n        return 2\n    except (OSError, ValueError) as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    seen = {field: set() for field in rules[\"unique\"]}\n    findings = []\n    rows = 0\n\n    try:\n        fh = open(args.rows, \"r\")\n    except OSError as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    with fh:\n        for line_no, line in enumerate(fh, 1):\n            line = line.strip()\n            if not line:\n                continue\n            rows += 1\n            try:\n                row = json.loads(line)\n            except json.JSONDecodeError as exc:\n                findings.append({\"row\": rows, \"line\": line_no, \"rule\": \"parse\",\n                                 \"message\": \"invalid json: %s\" % exc.msg})\n                continue\n            if not isinstance(row, dict):\n                findings.append({\"row\": rows, \"line\": line_no, \"rule\": \"parse\",\n                                 \"message\": \"not a JSON object\"})\n                continue\n            for message in check_row(row, rules, seen):\n                findings.append({\"row\": rows, \"line\": line_no,\n                                 \"rule\": kind_of(message), \"message\": message})\n\n    if args.json:\n        by_rule = {}\n        for f in findings:\n            by_rule[f[\"rule\"]] = by_rule.get(f[\"rule\"], 0) + 1\n        print(json.dumps({\n            \"rows\": rows,\n            \"findings\": len(findings),\n            \"by_rule\": by_rule,\n            \"violations\": findings,\n        }, indent=2, sort_keys=True))\n    else:\n        shown = findings if args.limit == 0 else findings[:args.limit]\n        for f in shown:\n            print(\"row %d: %s\" % (f[\"row\"], f[\"message\"]))\n        if len(findings) > len(shown):\n            print(\"... %d more findings\" % (len(findings) - len(shown)))\n        print(\"findings: %d in %d rows\" % (len(findings), rows))\n\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p371","n":"371","slug":"quiz-3-data-questions","title":"Quiz: 3 data questions","sections":["devsecops"],"topic":"Data engineering","tags":["DataEngineering","DataGovernance","SQL","Analytics","MachineLearning"],"series_name":"Data","design":"quiz","script":"","date":"2029-02-08","files":{"devsecops":"notes/p371-quiz-3-data-questions.md"},"md":"# Quiz: 3 data questions\n\nThe last post of this 50-part series, and the right place to stop: the data itself. Three questions. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why does a table of a million embeddings need a vector index?**\n\n- a) A scan compares every vector per query\n- b) The database cannot store floats without one\n- c) Embeddings are only valid inside an index\n\n> **Answer:** a) A scan compares every vector per query. Without an index every query compares the vector to every row, then sorts: a million calculations per request. HNSW or IVFFlat visits a small candidate set instead.\n\n**Q2. What does lineage give an incident review?**\n\n- a) The downstream tables and reports to fix\n- b) The exact SQL error from the failed job\n- c) A copy of the data before the bad load\n\n> **Answer:** a) The downstream tables and reports to fix. Lineage turns 'which dashboards are wrong' into a graph walk: find the bad table, then everything downstream of it. That list is the blast radius.\n\n**Q3. Why start PII discovery with the schema, not the values?**\n\n- a) Names, types and owners are cheap to read\n- b) Values are always encrypted at rest\n- c) A sample of every row is fast enough\n\n> **Answer:** a) Names, types and owners are cheap to read. The schema is small, complete and already owned. Scanning values is slower, misses encrypted columns and free text, and a sample can miss the one row that matters.\n\n## Explained\n\n### Question one\n\nWhy does a table of a million embeddings need a vector index? Without one, every query compares the query vector to every row, a million calculations and a sort, per request. HNSW or IVFFlat visits a small candidate set instead and trades recall for milliseconds. A full scan is correct, just too slow to search.\n\n### Question two\n\nWhat does lineage give an incident review? The list of everything downstream of the bad table, and usually the one upstream table that caused it. Without it the review is four teams comparing screenshots.\n\n### Question three\n\nWhy start PII discovery with the schema rather than the data? Because names, types and owners are complete, cheap to read and already maintained. Scanning values is slower, misses encrypted columns and free text, and a sample can miss the one row that matters.\n\n> **The honest caveat:** passing this quiz does not make a warehouse trustworthy; the tests, the lineage and a named owner do.\n\nIf you got these, you are reading the data, not the dashboard. That is the discipline.\n\nThat is the series. Fifty posts, from a query plan to a feature store, five phases, and ten Python scripts you can run today. Thank you for following along.\n\n> **The lesson:** the number is only as good as the table behind it.\n\n## In one line\n\n> **Own the data, not the dashboard.** Fifty posts, one rule: the number is only as good as the table behind it.\n\n**Rerun the check**\n\n```bash\npython3 data_quality.py rows.jsonl\n```\n"},{"id":"p372","n":"372","slug":"the-secure-sdlc-drawn","title":"The secure SDLC, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["DevSecOps","SecureSDLC","AppSec","CyberSecurity","CloudNative"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-02-11","files":{"devsecops":"notes/p372-the-secure-sdlc-drawn.md"},"md":"# The secure SDLC, drawn\n\nMost teams can draw their build pipeline. Far fewer can point to the three places where security work actually happens, or name the person who owns each one.\n\n## Quick reference\n\n- **Design:** Security requirement (REQ) · Threat model (TM)\n- **Build:** Code review (CR) · SAST scan (SAST)\n- **Run:** Monitoring (MON) · Incident response (IR)\n\n**Request flow:** Threat model → Code review → SAST scan → Monitoring → Incident response → Security requirement → Code review; Code review → Monitoring\n\n## Explained\n\n**Design**  \nA security requirement turns a policy into something a pull request can satisfy: no card data in logs, no admin route without MFA. The threat model asks what can go wrong before code exists, so the fix costs a sentence instead of a sprint.\n\n**Build**  \nCode review looks for the flaw the author cannot see, and SAST scans every commit for the patterns a reviewer misses at 5 p.m. Neither replaces the other: the scanner has no runtime context, and the reviewer has no stamina.\n\n**Run**  \nMonitoring watches for the behaviour the first two tiers predicted: a spike in 401s, a new outbound host, a query that reads too much. Incident response is the tier with a name and a phone number at 3 a.m.\n\n**The loop**  \nThe arrow back from response to design is what makes this a loop rather than a list. Every incident should end as a new requirement or a new test.\n\n> **The honest caveat:** a three-tier diagram is not a control. If nobody owns the gate, each tier becomes a meeting that approves the last one.\n\n> **The lesson:** security is a path through the SDLC, and every tier needs an owner, an artefact and an exit condition.\n\n## In one line\n\n> **Secure the path, not the stage.** A gate that nobody owns is a poster on a wall.\n\n**Run the scan**\n\n```bash\nsemgrep --config auto .\n```\n"},{"id":"p373","n":"373","slug":"the-security-review-path-drawn","title":"The security review path, drawn","sections":["devsecops"],"topic":"Application & API security","tags":["DevSecOps","AppSec","ThreatModelling","CyberSecurity","SecureSDLC"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-02-13","files":{"devsecops":"notes/p373-the-security-review-path-drawn.md"},"md":"# The security review path, drawn\n\nA feature request arrives with a deadline already attached. Security gets two days to comment, and the comment is a list of worries with no owner and no date.\n\n## Quick reference\n\n- **Request:** Feature request (REQ) · Data flow (DFD)\n- **Review:** Threat model (TM) · Design review (DR)\n- **Decision:** Accept (ACC) · Mitigate (MIT) · Block (BLK)\n\n**Request flow:** Feature request → Threat model → Design review → Accept; Design review → Mitigate; Design review → Block; Data flow → Threat model; Data flow → Design review\n\n## Explained\n\n**Request**  \nA feature request and a data flow diagram are the entry ticket. The diagram does not need to be beautiful; it needs to show every place data crosses a trust boundary and which identity carries it.\n\n**Review**  \nThe threat model turns that diagram into named risks. The design review checks the answers: is the token validated, is the tenant check on every query, what happens when the queue is replayed. Both happen before code, where a change costs a sentence.\n\n**Decision**  \nThe decision tier has three exits. Accept records the risk, the owner and the date it is reviewed again. Mitigate names the control and the test that proves it. Block stops the work and says what would unblock it.\n\n**Why it stalls**  \nMost review processes have the first two tiers and no third, so the same risks return every quarter with better slides.\n\n> **The honest caveat:** the diagram is a model, not the system. If it is drawn once and never updated, the review is theatre with a colour scheme, and nobody can point to the decision.\n\n> **The lesson:** a review is finished when someone with authority writes down a decision, not when the comments are resolved.\n\n## In one line\n\n> **No decision, no review.** A review that ends in 'looks fine' has not ended.\n\n**Ask for a decision:** accept, mitigate or block?\n"},{"id":"p374","n":"374","slug":"the-threat-modelling-cheat-sheet","title":"The threat modelling cheat sheet","sections":["devsecops"],"topic":"Application & API security","tags":["ThreatModelling","DevSecOps","CyberSecurity","AppSec","SecureSDLC"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-02-15","files":{"devsecops":"notes/p374-the-threat-modelling-cheat-sheet.md"},"md":"# The threat modelling cheat sheet\n\nThe blank threat-model page is why most teams skip threat modelling. They open a template, read the headings, and close it again.\n\n## Quick reference\n\n### STRIDE\n\n```bash\nSpoof or tamper: who can act or alter?\nRepudiate: what is never logged?\nDisclose: what leaks in an error?\nDeny: what has no rate limit?\nElevate: what runs as root?\n```\n\n### Boundaries\n\n```bash\nDraw the edge: browser, API, data\nMark every place trust changes\nList each entry point and its auth\nName the asset behind each flow\nGive every boundary an owner\n```\n\n### Data flows\n\n```bash\nTrace one request end to end\nLabel the data on every arrow\nMark where it is encrypted\nNote where it is logged\nFind the flow with no auth check\n```\n\n### Mitigations\n\n```bash\nValidate input at the boundary\nParameterise every SQL query\nRate limit by identity and route\nLog the deny and alert on it\nAdd one test per named threat\n```\n\n## Explained\n\n**STRIDE**  \nSTRIDE gives the questions their names: spoofing, tampering, repudiation, information disclosure, denial of service and elevation of privilege. The sheet pairs them with prompts you can ask out loud: who can act or alter, what is never logged, what leaks in an error, what has no rate limit, what runs as root. Spoofing and tampering share one line because both ask who can act or alter what.\n\n**Boundaries**  \nBoundaries asks where trust changes. Draw the edge from browser to API to database, mark every boundary, list the entry points and their authentication, name the asset behind each flow, and give every boundary an owner.\n\n**Data flows**  \nData flows asks what moves. Trace one request end to end, label the data on every arrow, mark where it is encrypted and where it is logged, and find the flow with no auth check.\n\n**Mitigations**  \nMitigations asks what you will do. Validate at the boundary, parameterise every query, rate limit by identity and route, log the deny and alert on it, and add one test per named threat.\n\n> **The honest caveat:** these are prompts, not answers. A prompt becomes a threat model only when a named person writes down the mitigation and the test that proves it.\n\n> **The lesson:** the quality of a threat model is the quality of its questions, and every question needs an owner.\n\n## In one line\n\n> **Ask better questions.** A threat model is a list of questions with owners.\n\n**Start with a diagram:** draw the data flow first\n"},{"id":"p375","n":"375","slug":"the-secure-code-review-cheat-sheet","title":"The secure code review cheat sheet","sections":["devsecops"],"topic":"Application & API security","tags":["DevSecOps","AppSec","CodeReview","CyberSecurity","SecureCoding"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-02-18","files":{"devsecops":"notes/p375-the-secure-code-review-cheat-sheet.md"},"md":"# The secure code review cheat sheet\n\nThe code review is the last place a cheap fix is still possible, and the diff is where everyone looks. The flaw is usually one line outside it, in the call path nobody opened.\n\n## Quick reference\n\n### Input\n\n```bash\ngrep -rn 'eval(' --include=*.py .\ngrep -rn 'shell=True' --include=*.py .\ngrep -rn 'innerHTML' --include=*.js .\ngrep -rn 'pickle.loads' .\ngrep -rn 'execute(f' .\n```\n\n### Auth\n\n```bash\ngrep -rn 'verify=False' .\ngrep -rn 'jwt.decode' .\ngrep -rn 'algorithm=none' .\ngrep -rn '== password' .\ngrep -rn 'is_admin = True' .\n```\n\n### Secrets\n\n```bash\ngrep -rnE '(api_key|secret)=' .\ngrep -rn 'BEGIN RSA PRIVATE KEY' .\ngitleaks detect --no-git -v\ntrufflehog filesystem . --only-verified\ngit log -p -S 'password' | head\n```\n\n### Crypto\n\n```bash\ngrep -rnE 'md5|sha1' .\ngrep -rn 'MODE_ECB' .\ngrep -rn 'random.random' .\ngrep -rn 'AES.new(' .\nopenssl x509 -in cert.pem -text -noout\n```\n\n## Explained\n\n**Input**  \nInput first. eval, pickle.loads and shell=True turn data into code; innerHTML turns a string into markup; execute(f marks an f-string query. Grep for each, then read the path from the request to the call.\n\n**Auth**  \nAuth next. verify=False disables certificate checks, a jwt.decode call that trusts the token's algorithm is where the none algorithm gets in, a password comparison with == leaks timing, and a hard-coded is_admin is a privilege bug with a friendly name.\n\n**Secrets**  \nSecrets. The regex finds assignments, gitleaks and trufflehog find the history, and `git log -p -S` finds a password removed from the file but not from the commit. Rotate first, rewrite history second.\n\n**Crypto**  \nCrypto last. md5 and sha1 are broken for passwords, MODE_ECB leaks patterns, random.random is not a key, and a certificate you never inspect is a trust decision you did not make. openssl x509 prints the dates and the key size.\n\n> **The honest caveat:** grep finds spellings, not logic. A clean run means the obvious mistakes are absent, not that the code is safe.\n\n> **The lesson:** a review is a reading exercise, and a grep is only the index.\n\n## In one line\n\n> **Read the code, not the diff.** The dangerous line is usually the caller, not the change.\n\n**Grep the diff**\n\n```bash\ngit diff -U0 | grep '^+'\n```\n"},{"id":"p376","n":"376","slug":"the-four-questions-of-threat-modelling","title":"The four questions of threat modelling","sections":["devsecops"],"topic":"Application & API security","tags":["ThreatModelling","DevSecOps","AppSec","CyberSecurity","SecureSDLC"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-02-20","files":{"devsecops":"notes/p376-the-four-questions-of-threat-modelling.md"},"md":"# The four questions of threat modelling\n\nA threat model that stops after \"what can go wrong\" is a list of fears. The four questions are a sequence, and the last one is the only proof the first three were real.\n\n## Quick reference\n\n### What are we building?\n\n- A data flow diagram with every boundary\n- Actors, assets and entry points named\n- The assumptions written down, not implied\n\n### What can go wrong?\n\n- Walk STRIDE across every boundary\n- Name the attacker and the asset\n- Write the abuse case, not the CVE\n\n### What do we do?\n\n- Accept, mitigate or block, with an owner\n- One control per named threat\n- One test that proves the control works\n\n### Did we do it?\n\n- Re-read the model against the built system\n- Check the tests exist and pass\n- Record what changed and why\n\n> **Note:** A model nobody revisits is a snapshot of last quarter's assumptions.\n\n## Explained\n\nWhat are we building?  \nDraw the data flow, name the actors, assets and entry points, and write down the assumptions. If the diagram cannot show where trust changes, the next question has no surface to land on.\n\nWhat can go wrong?  \nWalk STRIDE across every boundary, name the attacker and the asset, and write the abuse case in the user's language. \"An attacker reads another tenant's invoice\" is a threat; \"injection\" is a category.\n\nWhat do we do?  \nEach threat gets one of three answers: accept, mitigate or block. Mitigate names the control and the test that proves it. Accept names the owner and the review date. Block names what would unblock it.\n\nDid we do it?  \nRe-read the model against the built system, check the tests exist and pass, and record what changed. This is the step that turns a document into a programme, and the one most teams skip.\n\n> **The honest caveat:** the four questions take an afternoon, not an hour. Rushing the first one is how you get a beautiful model of the wrong system.\n\n> **The lesson:** a threat model is a loop, not a workshop.\n\n## In one line\n\n> **Ask all four, or none count.** The fourth question turns a document into a programme.\n\n**Book the review:** diary: re-read in 90 days\n"},{"id":"p377","n":"377","slug":"a-risk-register-scorer-in-python","title":"A risk register scorer in Python","sections":["devsecops"],"topic":"Python scripts","tags":["DevSecOps","RiskManagement","Python","AppSec","CyberSecurity"],"series_name":"DevSecOps","design":"code","script":"risk_score.py","date":"2029-02-22","files":{"devsecops":"notes/p377-a-risk-register-scorer-in-python.md"},"md":"# A risk register scorer in Python\n\nMost risk registers are spreadsheets sorted by whoever shouted last. A score will not fix the politics, but it does make the queue visible.\n\n## Quick reference\n\nThe full script is at the end of this note: `risk_score.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 risk_score.py register.json --appetite 15\n```\n\n**Example output**\n\n```text\nALERT 1 over appetite 15\ntop 25 R-001 customer-api\n```\n\n## Explained\n\n**What it does**  \nThe script reads a JSON register, takes each risk's likelihood and impact on a 1 to 5 scale, multiplies them, and sorts the result from 25 down to 1. It prints the score, id, asset, owner and status, then compares every score with the appetite you pass on the command line. It is read-only: the register is never rewritten.\n\n**Why multiply**  \nMultiplication is crude and it is honest: a 5 by 5 is 25, a 4 by 4 is 16, and a 1 by 5 is 5. It forces the two arguments into one number, which is the only way a queue can be ordered. Ties break on id, so two runs agree.\n\n**Exit codes**  \nZero means every risk is at or below the appetite; one means at least one is above it; two means the usage, the file or a field is wrong. Missing fields, a likelihood outside 1 to 5, or a duplicate id stop the run with exit 2 rather than a guess. That makes the script a CI step or a cron job, not just a report.\n\n> **The honest caveat:** the score knows nothing about status. An accepted risk still scores 20, so the status column is there for the human who has to argue with the number.\n\n> **The lesson:** a register that cannot fail a build is a diary.\n\n## In one line\n\n> **Score the risk, then own it.** A number ranks the queue; it does not assign the owner.\n\n**Run it in ci**\n\n```bash\npython3 risk_score.py register.json\n```\n\n## Script: risk_score.py\n\nDownload: [scripts/risk_score.py](../scripts/risk_score.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Score a JSON risk register and fail when a risk is over the appetite.\n\nUsage:\n    python3 risk_score.py register.json\n    python3 risk_score.py register.json --appetite 12\n    python3 risk_score.py --help\n\nThe register is a JSON object with a \"risks\" list. A bare list also works.\nEach risk needs: id, asset, likelihood (1-5), impact (1-5), owner, status.\nThe score is likelihood x impact, so it runs from 1 to 25. The script ranks\nevery risk, prints the table, and exits 1 when any score is above --appetite.\n\nExit codes:\n    0  every score is at or below the appetite\n    1  at least one score is above the appetite\n    2  usage error, unreadable file, invalid JSON or invalid field\n\nThe register is opened read-only and is never modified.\n\nCron/CI:\n    0 7 * * 1 python3 risk_score.py /etc/risk/register.json --appetite 15\n    python3 risk_score.py register.json --appetite 15 || exit 1\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom pathlib import Path\n\nFIELDS = (\"id\", \"asset\", \"likelihood\", \"impact\", \"owner\", \"status\")\n\n\nclass RegisterError(Exception):\n    \"\"\"The register or one of its fields is not usable.\"\"\"\n\n\ndef parse_args(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Score a JSON risk register (likelihood x impact) and gate on an appetite.\",\n    )\n    parser.add_argument(\"register\", help=\"path to the JSON register\")\n    parser.add_argument(\n        \"--appetite\",\n        type=int,\n        default=15,\n        metavar=\"N\",\n        help=\"highest score allowed without failing, 1-25 (default: 15)\",\n    )\n    return parser.parse_args(argv)\n\n\ndef load_register(path):\n    try:\n        raw = Path(path).read_text(encoding=\"utf-8\")\n    except (OSError, UnicodeError) as exc:\n        raise RegisterError(\"cannot read %s: %s\" % (path, exc))\n    try:\n        document = json.loads(raw)\n    except ValueError as exc:\n        raise RegisterError(\"invalid JSON in %s: %s\" % (path, exc))\n    if isinstance(document, dict):\n        document = document.get(\"risks\")\n    if not isinstance(document, list):\n        raise RegisterError('register must be a list or an object with a \"risks\" list')\n    return document\n\n\ndef validate(rows):\n    scored = []\n    seen = set()\n    for index, row in enumerate(rows, 1):\n        where = \"risk %d\" % index\n        if not isinstance(row, dict):\n            raise RegisterError(\"%s: expected an object\" % where)\n        missing = [field for field in FIELDS if field not in row]\n        if missing:\n            raise RegisterError(\"%s: missing %s\" % (where, \", \".join(missing)))\n        risk_id = row[\"id\"]\n        if not isinstance(risk_id, str) or not risk_id.strip():\n            raise RegisterError(\"%s: id must be a non-empty string\" % where)\n        if risk_id in seen:\n            raise RegisterError(\"%s: duplicate id %s\" % (where, risk_id))\n        seen.add(risk_id)\n        for field in (\"likelihood\", \"impact\"):\n            value = row[field]\n            if isinstance(value, bool) or not isinstance(value, int) or not 1 <= value <= 5:\n                raise RegisterError(\"%s: %s must be an integer 1-5\" % (where, field))\n        for field in (\"asset\", \"owner\", \"status\"):\n            if not isinstance(row[field], str) or not row[field].strip():\n                raise RegisterError(\"%s: %s must be a non-empty string\" % (where, field))\n        scored.append(\n            {\n                \"score\": row[\"likelihood\"] * row[\"impact\"],\n                \"id\": risk_id,\n                \"asset\": row[\"asset\"],\n                \"owner\": row[\"owner\"],\n                \"status\": row[\"status\"],\n            }\n        )\n    scored.sort(key=lambda item: (-item[\"score\"], item[\"id\"]))\n    return scored\n\n\ndef print_table(scored):\n    asset_width = max(len(item[\"asset\"]) for item in scored)\n    owner_width = max(8, max(len(item[\"owner\"]) for item in scored))\n    print(\"%5s  %-8s  %-*s  %-*s  %s\" % (\n        \"SCORE\", \"ID\", asset_width, \"ASSET\", owner_width, \"OWNER\", \"STATUS\"))\n    for item in scored:\n        print(\"%5d  %-8s  %-*s  %-*s  %s\" % (\n            item[\"score\"], item[\"id\"], asset_width, item[\"asset\"],\n            owner_width, item[\"owner\"], item[\"status\"]))\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    if not 1 <= args.appetite <= 25:\n        print(\"risk_score.py: error: --appetite must be 1-25\", file=sys.stderr)\n        return 2\n    try:\n        scored = validate(load_register(args.register))\n    except RegisterError as exc:\n        print(\"risk_score.py: error: %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not scored:\n        print(\"OK no risks in register\")\n        return 0\n\n    print_table(scored)\n    over = [item for item in scored if item[\"score\"] > args.appetite]\n    if over:\n        print(\"ALERT %d over appetite %d\" % (len(over), args.appetite))\n        print(\"top %d %s %s\" % (over[0][\"score\"], over[0][\"id\"], over[0][\"asset\"]))\n        return 1\n    print(\"OK %d risk%s, all at or below appetite %d\" % (\n        len(scored), \"\" if len(scored) == 1 else \"s\", args.appetite))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p378","n":"378","slug":"anatomy-of-a-cvss-vector","title":"Anatomy of a CVSS vector","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CVSS","CyberSecurity","RiskManagement","DevSecOps","AppSec"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-02-25","files":{"devsecops":"notes/p378-anatomy-of-a-cvss-vector.md"},"md":"# Anatomy of a CVSS vector\n\nTwo findings arrive on the same day, both called critical. One needs a click from an admin; the other needs nothing but a browser. The vector is where that difference lives, and six segments carry it.\n\n## Quick reference\n\n```text\nCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `CVSS:3.1` | Version | The scoring system in use. v3.1 and v4.0 do not score the same flaw alike. |\n| `AV:N` | Attack vector | Network: reached over a network, not from a keyboard in the lobby. |\n| `AC:L` | Attack complexity | Low: no race to win, no special configuration, no lucky timing. |\n| `PR:N` | Privileges required | None: the attacker starts unauthenticated. |\n| `UI:N` | User interaction | None: no link to click, no file to open, no prompt to approve. |\n| `S:U/C:H/I:H/A:H` | Scope and impact | Scope unchanged; confidentiality, integrity and availability all High. |\n\n## Explained\n\n**The prefix**  \nCVSS:3.1 names the scoring system. Version 3.1 and version 4.0 do not produce the same number from the same flaw, so a score without a version is a rumour. It looks like decoration; it is the first thing to check.\n\n**The attack path**  \nAV:N is network: the attacker reaches the service over a network, not from a keyboard in the lobby. Adjacent, local and physical are the other three values.  \nAC:L is low complexity: no race to win, no special configuration, no lucky timing. High complexity means the attacker must meet a condition you do not control.  \nPR:N is no privileges: the attacker starts unauthenticated.  \nUI:N is no user interaction: nobody has to click a link, open a file or approve a prompt.\n\n**Scope and impact**  \nS:U is scope unchanged, so the flaw stays inside one security authority. C:H, I:H and A:H are high impact on confidentiality, integrity and availability. This vector scores 9.8 and lands in the critical band.\n\n> **The honest caveat:** the base vector ignores your environment. A network-only service with no data behind it can carry a 9.8 that means nothing in your estate. Temporal and environmental metrics exist; most teams never fill them in.\n\n> **The lesson:** read the vector before you argue about the score; the fields are the argument.\n\n## In one line\n\n> **A vector is a sentence.** Every field answers a different question about the same flaw.\n\n**Read the vector:** nvd.nist.gov/vuln-metrics/cvss\n"},{"id":"p379","n":"379","slug":"security-posture-at-a-glance","title":"Security posture at a glance","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["DevSecOps","CyberSecurity","Metrics","RiskManagement","AppSec"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-02-27","files":{"devsecops":"notes/p379-security-posture-at-a-glance.md"},"md":"# Security posture at a glance\n\nA security dashboard with forty tiles is wallpaper. The quarterly conversation needs six numbers, and each one has to be able to change a decision.\n\n## Quick reference\n\n**Status:** This quarter\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Critical open | **12** | of 214 open findings |\n| Mean time to fix | **9 days** | critical: 3 days |\n| Scan coverage | **94%%** | repos scanned in CI |\n| Secrets found | **7** | all rotated, none live |\n| Patch compliance | **88%%** | within 14 days |\n| Risks accepted | **3** | reviewed this quarter |\n\n## Explained\n\n**Critical open and time to fix**  \nCritical open: 12 of 214 open findings are critical. On its own that is a count; next to mean time to fix, 9 days overall and 3 for criticals, it becomes a rate. A backlog growing faster than the fix rate is the signal that matters.\n\n**Coverage and secrets**  \nScan coverage: 94 percent of repos run a scan in CI. The missing 6 percent is where the next finding will come from. Secrets found: 7 this quarter, all rotated, none live. The number to watch is time to rotate, not the count.\n\n**Compliance and acceptance**  \nPatch compliance: 88 percent of hosts are inside the 14-day window. Risks accepted: 3, reviewed this quarter. Accepted is a decision, so it needs an owner and a review date; an expired acceptance is an open risk with better paperwork.\n\n**Why six**  \nThe six mix three counts, one duration and two percentages. That mix stops the page becoming a single number that hides the rest.\n\n> **The honest caveat:** every one of these numbers is defined by someone. If the definition changes between quarters, the trend is fiction.\n\n> **The lesson:** a posture dashboard is a set of decisions waiting to be made, not a scoreboard.\n\n## In one line\n\n> **A dashboard is a question.** Six tiles are only useful if one of them changes a decision.\n\n**Pick one metric:** critical open / all open\n"},{"id":"p380","n":"380","slug":"from-vulnerability-report-to-fix","title":"From vulnerability report to fix","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["DevSecOps","VulnerabilityManagement","CyberSecurity","SRE","AppSec"],"series_name":"DevSecOps","design":"timeline","script":"","date":"2029-03-01","files":{"devsecops":"notes/p380-from-vulnerability-report-to-fix.md"},"md":"# From vulnerability report to fix\n\nA vulnerability report lands in a shared inbox on Friday. The fix closes ten days later, and nobody can say which of the six steps lost the time.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0** | Report received | The clock starts and the reporter gets a ticket ID. |\n| **T+1** | Triaged | Severity set, owner assigned, duplicate checked. |\n| **T+2** | Reproduced | A failing test proves the bug before any fix. |\n| **T+4** | Patch shipped | The fix goes through the normal pipeline, not around it. |\n| **T+6** | Fix verified | The reporter confirms, or the regression test does. |\n| **T+10** | Closed | Postmortem filed; the lesson becomes a new test. |\n\n## Explained\n\n**T+0 and T+1**  \nT+0: the report is received and the clock starts. The reporter gets a ticket ID, which is the difference between a report and a rumour. T+1: triage sets severity, assigns an owner and checks for a duplicate. This is where most programmes stall: the queue is real, but nobody owns the front of it.\n\n**T+2 and T+4**  \nT+2: reproduction proves the bug with a failing test. Without it, the fix is a guess and verification has nothing to check. T+4: the patch ships through the normal pipeline. A hotfix that bypasses the tests is a second incident waiting for a quiet week.\n\n**T+6 and T+10**  \nT+6: the reporter confirms, or the regression test does. If verification waits on the reporter, say so and set a date. T+10: close with a postmortem and a new test. The date is not the point; the test is what stops the same report arriving next quarter.\n\n**Why the clock**  \nEach timestamp gives one step a budget and a name. Without them, the only measured number is the total, and the total cannot tell you whether triage or verification is the problem.\n\n> **The honest caveat:** the times are a target, not a measurement. Track the median per step, or the timeline becomes a story you tell auditors.\n\n> **The lesson:** measure each step, because the total hides the queue.\n\n## In one line\n\n> **The clock is the control.** A timeline without owners is a wish list with dates.\n\n**Start the clock**\n\n```bash\ndate -u +%Y-%m-%dT%H:%M:%SZ\n```\n"},{"id":"p381","n":"381","slug":"devsecops-terms-decoded","title":"DevSecOps terms, decoded","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["DevSecOps","CyberSecurity","AppSec","CloudNative","ITSecurity"],"series_name":"DevSecOps","design":"reveal","script":"","date":"2029-03-04","files":{"devsecops":"notes/p381-devsecops-terms-decoded.md"},"md":"# DevSecOps terms, decoded\n\nEvery DevSecOps pitch uses the same six words, and half the room nods without knowing which one changes a decision.\n\n## Quick reference\n\n- **Shift left** — Move the check to the pull request, not the release.\n- **Threat model** — A short exercise that names what you build and how it can fail.\n- **SAST** — Static analysis of your own source, run before the code executes.\n- **DAST** — Dynamic testing against a running app, probing it like an attacker.\n- **SBOM** — A machine-readable list of every component and version in a build.\n- **Security gate** — A pipeline step that stops a release when a threshold is crossed.\n\n## Explained\n\n**The early checks**  \nShift left means moving the check to the pull request. It is a location, not a tool: if the check still runs only before release, nothing shifted.  \nA threat model is a short exercise that names what you are building, how it can fail and what you will do about it. It is a meeting with an artefact, not a document you buy.\n\n**The scanners**  \nSAST reads your source before it runs and finds patterns: a dangerous call, a missing check. DAST attacks a running app and finds behaviour: a reflected parameter, an open redirect. One sees the code, the other sees the result.\n\n**The list and the gate**  \nAn SBOM is a machine-readable list of every component and version in a build. It does not make you secure; it makes the next Log4Shell a query instead of a week of grep.  \nA security gate is a pipeline step that stops a release when a threshold is crossed. The threshold is the policy; the gate is only the enforcement.\n\n> **The honest caveat:** every one of these can be bought as a product and still fail, because the control is the decision, not the dashboard.\n\n> **The lesson:** learn the words to find the decision each one hides.\n\n## In one line\n\n> **Know the term, own the control.** A word is not a control until someone owns the decision.\n\n**Read the sbom**\n\n```bash\nsyft . -o spdx-json\n```\n"},{"id":"p382","n":"382","slug":"the-dependency-supply-chain-drawn","title":"The dependency supply chain, drawn","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["DevSecOps","SupplyChain","AppSec","CyberSecurity","SCA"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-03-06","files":{"devsecops":"notes/p382-the-dependency-supply-chain-drawn.md"},"md":"# The dependency supply chain, drawn\n\nA package you never chose runs with the permissions of the service that imported it. Between the registry and production it passes seven stops, and the weakest one decides the risk.\n\n## Quick reference\n\n- **Source:** Public registry (PUB) · Private registry (PRIV) · Lockfile (LOCK)\n- **Build:** SCA scan (SCA) · SBOM (SBOM)\n- **Consume:** Admission check (GATE) · Runtime workload (RUN)\n\n**Request flow:** Public registry → SCA scan → SBOM → Admission check → Runtime workload; Private registry → SCA scan; Lockfile → SCA scan\n\n## Explained\n\n### Source\n\nThe first tier is where code enters. The public registry serves open-source packages. The private registry holds the packages your organisation builds and does not publish. The lockfile records the exact version and hash of every fetched package, so the same install reproduces on a laptop, a runner and a container.\n\n### Build\n\nThe second tier turns that tree into evidence. The SCA scan reads the lockfile, matches each version against advisory data and flags the ones with known CVEs. The SBOM records every component, including the transitive packages nobody named, so the next question is a query, not an archaeology project.\n\n### Consume\n\nThe third tier is where the package finally runs. The admission check refuses an image whose signature or provenance does not match policy. The runtime workload is where the imported code executes with your service account and your network access.\n\nRead the arrows as trust boundaries. Both registries feed the SCA scan, the lockfile feeds it too, and the scan produces the SBOM. The SBOM is what admission checks against, and nothing reaches runtime without crossing that line.\n\n> **The honest caveat:** an SBOM proves what is inside a build, not that any of it is safe, and a lockfile only protects you when CI runs npm ci, not npm install.\n\n> **The lesson:** the supply chain is only as strong as the stop nobody reviews.\n\n## In one line\n\n> **Trust the hash, not the name.** A package is a claim until its hash and its source agree.\n\n**Audit the lockfile**\n\n```bash\npython3 lockfile_audit.py package-lock.json\n```\n"},{"id":"p383","n":"383","slug":"secret-management-architecture-drawn","title":"Secret management architecture, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["SecretsManagement","DevSecOps","Vault","CloudSecurity","AppSec"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-03-08","files":{"devsecops":"notes/p383-secret-management-architecture-drawn.md"},"md":"# Secret management architecture, drawn\n\nA static cloud key in a CI variable is a password with no expiry. Secret management replaces it with three tiers and a credential that dies on time.\n\n## Quick reference\n\n- **Apps:** Application (APP) · CI job (CI)\n- **Vault:** Secret store (VAULT) · Dynamic credentials (DYN)\n- **Cloud:** KMS (KMS) · Target service (TGT)\n\n**Request flow:** Application → Secret store → Dynamic credentials → Target service; Secret store → KMS; Application → Target service; CI job → Secret store\n\n## Explained\n\n### Apps\n\nThe first tier is the caller. The application authenticates to the secret store with its workload identity, not with a root token in a config file. The CI job proves which repository and branch it is through OIDC, and gets a short-lived token back.\n\n### Vault\n\nThe second tier is the store and the credentials it mints. The store keeps static values encrypted and audits every read. Dynamic credentials are generated on request: a database user with a TTL, an IAM session, an SSH certificate. When the lease expires the credential stops working, whether or not the job cleaned up.\n\n### Cloud\n\nThe third tier is where the secret is used. KMS wraps the root key, so that key never sits in the store's memory. The target service accepts the short-lived credential and enforces its own authorisation. The application still connects to the target, but never owns a long-lived password.\n\nThe arrows carry the design. Both callers point at the store, the store points at dynamic credentials and KMS, the credentials point at the target, and the application's arrow carries the borrowed credential.\n\n> **The honest caveat:** a short TTL limits the damage after a leak, but not a compromised workload asking for a credential while its identity is valid. Scope the role, not just the clock.\n\n> **The lesson:** not a bigger vault, but a credential that expires before an attacker uses it.\n\n## In one line\n\n> **Borrow the key, never store it.** A credential that expires is worth more than a vault that grows.\n\n**Check a lease**\n\n```bash\nvault read database/creds/app\n```\n"},{"id":"p384","n":"384","slug":"the-sast-dast-and-sca-cheat-sheet","title":"The SAST, DAST and SCA cheat sheet","sections":["devsecops"],"topic":"Application & API security","tags":["DevSecOps","AppSec","SAST","DAST","SCA"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-03-11","files":{"devsecops":"notes/p384-the-sast-dast-and-sca-cheat-sheet.md"},"md":"# The SAST, DAST and SCA cheat sheet\n\nA pull request adds a dependency and nobody reads the lockfile. Three scanners in the same pipeline can catch that, and each answers a different question.\n\n## Quick reference\n\n### SAST\n\n```bash\nsemgrep --config=auto --error .\nsemgrep --config p/ci --sarif -o s.sarif\nbandit -r app/ -ll\ngitleaks dir . -v\n```\n\n### SCA\n\n```bash\ntrivy fs --scanners vuln,secret .\nnpm audit --audit-level=high\nnpm audit --omit=dev\npip-audit -r requirements.txt\n```\n\n### DAST\n\n```bash\nnuclei -u https://staging.local\nzap-baseline.py -t https://staging.local\nnikto -h https://staging.local\nsqlmap -u 'https://app/?id=1' --batch\n```\n\n### Triage\n\n```bash\njq '.runs[].results | length' s.sarif\njq -r '.runs[].results[].ruleId' s.sarif\nsort findings.txt | uniq -c | sort -rn\ngrype sbom:sbom.json --fail-on critical\n```\n\n## Explained\n\n### SAST\n\nSAST reads the code you wrote. `semgrep --config=auto --error .` runs the community rules and fails on a match; `semgrep --config p/ci --sarif -o s.sarif` writes a SARIF file for the code scanning tab. bandit -r app/ -ll reports medium and high Python issues, and `gitleaks dir . -v` looks for hardcoded secrets.\n\n### SCA\n\nSCA reads the code you imported. `trivy fs --scanners vuln,secret .` scans the tree, `npm audit --audit-level=high` fails on high advisories, `npm audit --omit=dev` ignores dev-only packages, and pip-audit -r `requirements.txt` checks a requirements file.\n\n### DAST\n\nDAST attacks the running app. nuclei -u https://staging.local runs the template set against one URL, `zap-baseline.py -t https://staging.local` runs the ZAP baseline scan, nikto -h https://staging.local checks server configuration, and sqlmap -u 'https://app/?id=1' `--batch` tests one parameter without prompting.\n\n### Triage\n\nTriage turns output into a decision. `jq '.runs[].results | length' s.sarif` counts SARIF results, `jq -r '.runs[].results[].ruleId' s.sarif` lists the rules, `sort findings.txt | uniq -c | sort -rn` groups duplicates, and `grype sbom:sbom.json --fail-on critical` gates an SBOM.\n\n> **The honest caveat:** a scanner only reports what its rules know. A clean DAST run against staging says nothing about a route that only exists in production, and a semgrep pass says nothing about business logic.\n\n> **The lesson:** run all three, then triage them in one queue with one owner.\n\n## In one line\n\n> **Scan early, triage once.** A finding without an owner is a log line.\n\n**Fail on high**\n\n```bash\ntrivy fs --exit-code 1 --severity HIGH .\n```\n"},{"id":"p385","n":"385","slug":"sast-vs-dast-vs-sca-drawn","title":"SAST vs DAST vs SCA, drawn","sections":["devsecops"],"topic":"Application & API security","tags":["AppSec","SAST","DAST","SCA","CyberSecurity"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-03-13","files":{"devsecops":"notes/p385-sast-vs-dast-vs-sca-drawn.md"},"md":"# SAST vs DAST vs SCA, drawn\n\nThree scanners, three views, and most teams buy one and call it coverage. The panels are the questions each tool answers.\n\n## Quick reference\n\n### White box - SAST\n\n- Reads source and builds an abstract syntax tree\n- Follows tainted data from a parameter to a sink\n- Names the file and line before the merge\n- Blind to runtime config and live routes\n\n### Black box - DAST\n\n- Attacks a running app through its own interfaces\n- Finds auth gaps, headers, CORS and injection\n- No source needed, so any language works\n- Cannot point at the line that caused it\n\n### Dependencies - SCA\n\n- Reads lockfiles and SBOMs, not your source\n- Matches versions to advisories and licences\n- Sees the transitive package four levels down\n- Cannot tell whether the vulnerable path runs\n\n### When each runs\n\n- SAST on commit and every pull request\n- SCA when the lockfile changes, and nightly\n- DAST against staging after each deploy\n- All three gate the merge, not a monthly PDF\n\n> **Note:** SAST reads code, DAST attacks a running app, SCA reads the packages both depend on.\n\n## Explained\n\n### White box - SAST\n\nStatic analysis reads the source you wrote. It parses the source into an abstract syntax tree, follows tainted data from a request parameter to a sink, and names the file and line before the merge. It is fast enough for every pull request and blind to what exists only at runtime: environment variables, feature flags, routes registered by convention.\n\n### Black box - DAST\n\nDynamic analysis attacks a running app through its interfaces. It needs no source, so it works on any language or framework, and it finds what a code reader misses: a missing authorisation check, a permissive CORS header and injection. It cannot point at the line that caused it.\n\n### Dependencies - SCA\n\nSoftware composition analysis reads the lockfile and SBOM, not your source. It matches versions against advisory databases and licence rules, and it alone sees the transitive package four levels down. It cannot tell whether the vulnerable function is ever called.\n\n### When each runs\n\nSAST runs on commit and every pull request. SCA runs when the lockfile changes, and nightly for new advisories. DAST runs against staging after each deploy. All three should gate the same merge, not three PDFs for a monthly meeting.\n\n> **The honest caveat:** a finding from any of them is a lead, not a verdict. Reachability, configuration and exploitability decide whether it is a real risk.\n\n> **The lesson:** the three tools answer different questions, so one of them is never coverage.\n\n## In one line\n\n> **Three lenses, one merge gate.** Different questions, one decision.\n\n**See the panels**\n\n```bash\nsemgrep --config=auto --error .\n```\n"},{"id":"p386","n":"386","slug":"dependency-confusion-and-typosquatting-drawn","title":"Dependency confusion and typosquatting, drawn","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","DevSecOps","AppSec","CyberSecurity","NPM"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-03-15","files":{"devsecops":"notes/p386-dependency-confusion-and-typosquatting-drawn.md"},"md":"# Dependency confusion and typosquatting, drawn\n\nA developer installs a package with a familiar name and its postinstall script runs. Nobody chose it, and no source review would show it.\n\n## Quick reference\n\n### The public name\n\n- Anyone can publish an unscoped name on npm\n- The install script runs with your permissions\n- One swapped letter is enough to catch a team\n- Reserve your names before someone else does\n\n### The private scope\n\n- An internal name left unclaimed in public\n- A fallback index can answer for it instead\n- The attacker publishes a higher version\n- Scope ownership and index pinning are the control\n\n### The lockfile\n\n- npm ci installs exactly the resolved URLs\n- A changed hash is visible in the diff\n- A new transitive entry is a review event\n- Never commit a lockfile you did not read\n\n### Verification\n\n- npm audit signatures checks registry signatures\n- Provenance ties a tarball to a commit and build\n- Pin the registry host, not just the package name\n- Test a fresh install in a clean directory\n\n> **Note:** The attack is not a clever exploit; it is a name that resolved somewhere you did not expect.\n\n## Explained\n\n### The public name\n\nAnyone can publish an unscoped name on the public registry. Attackers pick names close to popular packages: one letter swapped, a hyphen moved, a plural added. The script runs with the developer's permissions and can read environment variables, tokens and keys.\n\n### The private scope\n\nThe second attack targets names used internally. If an internal name is unclaimed on the public registry, an attacker can claim it and publish a higher version. When the resolver falls back to the public index, or compares versions, the public copy can win. On npm the scope is the control; on pip, `--extra-index-url` creates the same fallback.\n\n### The lockfile\n\nThe lockfile is where substitution becomes visible. npm ci installs exactly the resolved URLs and hashes in it, and a changed hash or a new host appears in the diff. A new transitive entry is a review event, not noise.\n\n### Verification\n\nVerification closes the loop. npm audit signatures checks the registry signatures on installed packages, provenance ties a tarball to a repository and a build, and pinning the registry host stops a fallback you did not intend. A fresh install in a clean directory proves the lockfile resolves where you think.\n\n> **The honest caveat:** none of this stops an attacker who controls a legitimate package; it only makes the name and source visible before the install.\n\n> **The lesson:** a package name is a claim, and the lockfile is the evidence.\n\n## In one line\n\n> **Claim the name, pin the source.** The attack is a name that resolved where you did not look.\n\n**Check a name**\n\n```bash\nnpm view @acme/utils version\n```\n"},{"id":"p387","n":"387","slug":"a-lockfile-audit-in-python","title":"A lockfile audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","SupplyChain","DevSecOps","AppSec","NPM"],"series_name":"DevSecOps","design":"code","script":"lockfile_audit.py","date":"2029-03-18","files":{"devsecops":"notes/p387-a-lockfile-audit-in-python.md"},"md":"# A lockfile audit in Python\n\nThe lockfile says what will actually install, and it is the file a dependency attack edits. A short audit turns that diff into three checks.\n\n## Quick reference\n\nThe full script is at the end of this note: `lockfile_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 lockfile_audit.py package-lock.json\n```\n\n**Example output**\n\n```text\nRISK node_modules/left-pad:no integrity\nFAIL 5\n```\n\n## Explained\n\n### What it reads\n\nThe script loads a `package-lock.json` with lockfileVersion 2 or 3, takes the packages map, and skips the root entry for your own project. It also accepts a generic JSON mapping, so the same checks work on a hand-built inventory. Nothing is written back.\n\n### What it checks\n\nIt looks for three things. An entry with no integrity string is a package npm cannot verify: normal for a local link, suspicious everywhere else. A resolved URL whose host is neither registry.npmjs.org nor a host you allow with `--allow-host` points at a mirror, a tarball server or a lookalike. A resolved value starting with file: or git is outside the registry entirely. Each finding names the package path and the reason.\n\n### How to run it\n\n`python3 lockfile_audit.py package-lock.json` prints one RISK line per package and exits 1 if there is any finding. Exit 0 means every entry is hash-pinned and registry-hosted; exit 2 means the file is missing, is not JSON, or holds a non-object entry. The `--json` flag prints the same result for a pipeline.\n\n> **The honest caveat:** workspace links, bundled packages and local links have no integrity hash by design, so expect them in the report or filter them downstream. The audit proves where an entry came from, not that it is safe.\n\n> **The lesson:** review the lockfile diff like code, because it decides what runs.\n\n## In one line\n\n> **Read the lockfile like a diff.** An attacker edits the lockfile because that is what installs.\n\n**Run it in ci**\n\n```bash\npython3 lockfile_audit.py package-lock.json\n```\n\n## Script: lockfile_audit.py\n\nDownload: [scripts/lockfile_audit.py](../scripts/lockfile_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit a lockfile for supply-chain risk: hashes, hosts and sources.\n\nReads a package-lock.json (lockfileVersion 2 or 3) or a generic JSON\nmapping package paths to entries, then flags:\n  - an entry with no integrity hash (the SRI string npm writes\n    for registry packages)\n  - a resolved URL whose host is not an allowed registry\n  - a resolved source that is a file: or git: reference\n\nUsage:\n    python3 lockfile_audit.py package-lock.json\n    python3 lockfile_audit.py package-lock.json --allow-host npm.acme.internal\n    python3 lockfile_audit.py package-lock.json --json\n\nThe default registry host is registry.npmjs.org. Add private registries\nwith --allow-host; the flag may be repeated. The lockfile is only read,\nnever changed. Bundled dependencies, workspace links and local links have\nno integrity hash by design, so they are reported rather than skipped.\n\nExit codes:\n    0  every entry has an integrity hash and an allowed registry source\n    1  at least one finding\n    2  usage error, or a lockfile that is missing, unreadable or invalid\n\nCron: 0 6 * * 1 python3 lockfile_audit.py package-lock.json\nCI:   python3 lockfile_audit.py package-lock.json --allow-host \"$NPM_HOST\"\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport json\nimport sys\nfrom urllib.parse import urlparse\n\nDEFAULT_HOSTS = (\"registry.npmjs.org\",)\nFILE_PREFIX = (\"file:\",)\nGIT_PREFIXES = (\"git+\", \"git:\", \"github:\", \"gitlab:\", \"bitbucket:\", \"gist:\")\n\n\nclass LockfileError(Exception):\n    \"\"\"The input cannot be used as a lockfile.\"\"\"\n\n\ndef load_json(path):\n    \"\"\"Return the parsed JSON document, or raise LockfileError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise LockfileError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise LockfileError(\"%s is not valid JSON: %s\" % (path, exc))\n\n\ndef walk_dependencies(deps, prefix=\"\"):\n    \"\"\"Yield (path, entry) for a v1 dependencies tree, depth first.\"\"\"\n    for name, entry in deps.items():\n        if not isinstance(entry, dict):\n            raise LockfileError(\"%s is not an object\" % name)\n        if prefix:\n            path = prefix + \"/node_modules/\" + name\n        else:\n            path = \"node_modules/\" + name\n        yield path, entry\n        nested = entry.get(\"dependencies\")\n        if isinstance(nested, dict):\n            for item in walk_dependencies(nested, path):\n                yield item\n\n\ndef package_entries(doc):\n    \"\"\"Return a list of (path, entry) pairs from any supported document.\"\"\"\n    if isinstance(doc, list):\n        items = []\n        for index, entry in enumerate(doc, 1):\n            if not isinstance(entry, dict):\n                raise LockfileError(\"entry %d is not an object\" % index)\n            path = entry.get(\"path\") or entry.get(\"name\")\n            if not isinstance(path, str) or not path:\n                raise LockfileError(\"entry %d has no path or name\" % index)\n            items.append((path, entry))\n    elif isinstance(doc, dict):\n        packages = doc.get(\"packages\")\n        if isinstance(packages, dict):\n            items = list(packages.items())\n        elif isinstance(doc.get(\"dependencies\"), dict):\n            items = list(walk_dependencies(doc[\"dependencies\"]))\n        else:\n            items = list(doc.items())\n    else:\n        raise LockfileError(\"expected a JSON object or list\")\n\n    entries = []\n    for path, entry in items:\n        if path == \"\":\n            continue\n        if not isinstance(path, str) or not path:\n            raise LockfileError(\"invalid package path %r\" % (path,))\n        if not isinstance(entry, dict):\n            raise LockfileError(\"%s is not an object\" % path)\n        entries.append((path, entry))\n    if not entries:\n        raise LockfileError(\"no package entries found\")\n    return entries\n\n\ndef source_kind(resolved):\n    \"\"\"Classify a resolved value as file, git, url or other.\"\"\"\n    text = resolved.strip().lower()\n    if text.startswith(FILE_PREFIX):\n        return \"file\"\n    if text.startswith(GIT_PREFIXES):\n        return \"git\"\n    if text.startswith((\"http://\", \"https://\")):\n        return \"url\"\n    return \"other\"\n\n\ndef audit_entry(path, entry, allowed_hosts):\n    \"\"\"Return the list of reasons this entry is risky.\"\"\"\n    reasons = []\n    integrity = entry.get(\"integrity\")\n    if not isinstance(integrity, str) or not integrity.strip():\n        reasons.append(\"no integrity hash\")\n\n    resolved = entry.get(\"resolved\")\n    if resolved is not None and not isinstance(resolved, str):\n        raise LockfileError(\"%s has a non-string resolved value\" % path)\n    if resolved:\n        kind = source_kind(resolved)\n        if kind == \"file\":\n            reasons.append(\"file: source\")\n        elif kind == \"git\":\n            reasons.append(\"git source\")\n        elif kind == \"url\":\n            host = urlparse(resolved).hostname\n            if not host:\n                reasons.append(\"URL without a host\")\n            elif host.lower() not in allowed_hosts:\n                reasons.append(\"non-registry host: %s\" % host)\n        else:\n            reasons.append(\"unrecognised source: %s\" % resolved)\n    return reasons\n\n\ndef audit(path, allowed_hosts):\n    \"\"\"Return (entry_count, findings) for a lockfile path.\"\"\"\n    doc = load_json(path)\n    entries = package_entries(doc)\n    findings = []\n    for package_path, entry in entries:\n        reasons = audit_entry(package_path, entry, allowed_hosts)\n        if reasons:\n            findings.append((package_path, reasons))\n    findings.sort(key=lambda item: item[0])\n    return len(entries), findings\n\n\ndef plural(count, word):\n    \"\"\"Return a count and a correctly pluralised noun.\"\"\"\n    return \"%d %s%s\" % (count, word, \"\" if count == 1 else \"s\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit a package-lock.json for supply-chain risk.\",\n        epilog=\"Exit 0 clean, 1 finding, 2 usage or input error. By s0sta.com\")\n    parser.add_argument(\"lockfile\", help=\"package-lock.json or generic JSON\")\n    parser.add_argument(\"--allow-host\", action=\"append\", default=[], metavar=\"HOST\",\n                        help=\"extra registry host to accept (repeatable)\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print the findings as JSON\")\n    args = parser.parse_args(argv)\n\n    allowed = {host.lower() for host in DEFAULT_HOSTS}\n    for host in args.allow_host:\n        host = host.strip().lower()\n        if not host:\n            parser.error(\"--allow-host cannot be empty\")\n        allowed.add(host)\n\n    try:\n        checked, findings = audit(args.lockfile, allowed)\n    except LockfileError as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    if args.json:\n        payload = {\n            \"lockfile\": args.lockfile,\n            \"packages\": checked,\n            \"ok\": not findings,\n            \"findings\": [{\"path\": p, \"reasons\": r} for p, r in findings],\n        }\n        print(json.dumps(payload, indent=2, sort_keys=True))\n    else:\n        for package_path, reasons in findings:\n            print(\"RISK %s: %s\" % (package_path, \", \".join(reasons)))\n        if findings:\n            issues = sum(len(reasons) for _, reasons in findings)\n            print(\"FAIL %s with findings, %s\"\n                  % (plural(len(findings), \"package\"), plural(issues, \"issue\")))\n        else:\n            print(\"OK %s checked\" % plural(checked, \"package\"))\n\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p388","n":"388","slug":"anatomy-of-a-dependency-path","title":"Anatomy of a dependency path","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","AppSec","NPM","DevSecOps","CyberSecurity"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-03-20","files":{"devsecops":"notes/p388-anatomy-of-a-dependency-path.md"},"md":"# Anatomy of a dependency path\n\nA scanner reports a critical CVE in a package you have never heard of. Before anyone patches, one question decides the urgency: can the vulnerable code be reached from your application?\n\n## Quick reference\n\n```text\napp > express > body-parser > qs > 6.11.0\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `app` | Your app | The service you ship; the path starts here. |\n| `express` | Direct dependency | You chose it and pinned it in `package.json`. |\n| `body-parser` | Transitive dep | Express pulls it in; you never named it. |\n| `qs` | Vulnerable package | The advisory names this package, not your code. |\n| `6.11.0` | Installed version | The exact version the lockfile resolves today. |\n\n## Explained\n\n### The path\n\nThe path starts at app, your application: the service you ship. Everything after this point is code you did not write, running with the permissions you gave it.\n\nNext is express, your direct dependency, the package you chose and pinned in `package.json`. You asked for it, you upgrade it, and its release notes are yours to read.\n\nThen body-parser, a transitive dependency pulled in by express. You never named it, never reviewed it, and probably cannot say which version is installed without asking the lockfile.\n\n### The finding\n\nqs is the vulnerable package, where the advisory lands. The CVE names this package and a version range, not your application, which is why the first alert feels remote and the eventual patch feels urgent.\n\n6.11.0 is the installed version, the one the lockfile resolves today. Change one line in the lockfile or one range in `package.json` and the whole path changes, which is why the path is the unit of triage, not the package name.\n\n> **The honest caveat:** a path proves the package is installed, not that the vulnerable function is called. `npm ls qs` shows the tree; only a trace or a code search shows whether the bad branch runs.\n\n> **The lesson:** triage the path, not the package, and patch the version that actually resolves.\n\n## In one line\n\n> **A CVE needs a path to run.** A finding on a package is only a risk if the path reaches it.\n\n**Trace the path**\n\n```bash\nnpm ls qs\n```\n"},{"id":"p389","n":"389","slug":"dependency-risk-at-a-glance","title":"Dependency risk at a glance","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["DevSecOps","SupplyChain","Metrics","AppSec","CyberSecurity"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-03-22","files":{"devsecops":"notes/p389-dependency-risk-at-a-glance.md"},"md":"# Dependency risk at a glance\n\nA dependency review is six numbers and a decision. The dashboard is not there to look healthy; it is there to show which number changed since the last sprint.\n\n## Quick reference\n\n**Status:** Sprint 14 review\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Outdated direct | **23%** | of 61 direct deps |\n| Critical CVEs | **4** | 2 reachable from prod |\n| Unmaintained | **7** | no release in 24 months |\n| Licence issues | **3** | GPL in a closed product |\n| Images pinned | **82%%** | of 45 by digest |\n| Mean age | **14 mo** | median is 9 months |\n\n## Explained\n\n### The tiles\n\nOutdated direct dependencies: 23 of 61 direct packages are behind their latest release. Direct means you chose them, so each one has an owner and a changelog. Critical CVEs: four, two of them reachable from production code. The other two are deferred with a reason. Unmaintained packages: seven with no release in 24 months. Some are finished, not abandoned, which is why this is a conversation and not a gate.\n\nLicence issues: three, all GPL-family packages inside a closed product. The fix may be legal, architectural or a replacement, but it is not a patch. Digest-pinned images: 82 percent of 45 images are pinned by digest rather than a mutable tag. A tag can be moved; a digest cannot. That is the difference between a pointer and a promise. Mean age: 14 months, with a median of nine. The mean is pulled up by a few old dependencies that dominate the tail.\n\n### What it means\n\n> **The honest caveat:** every one of these numbers is a snapshot with a definition behind it. Change the definition and the dashboard changes, so write the definition next to the number.\n\n> **The lesson:** six numbers are useful only when each one has an owner and a threshold.\n\n## In one line\n\n> **Count the risk, own the list.** A dashboard is a decision list, not a wall of badges.\n\n**Export the list**\n\n```bash\nnpm outdated --json > outdated.json\n```\n"},{"id":"p390","n":"390","slug":"from-cve-publish-to-patch-deployed","title":"From CVE publish to patch deployed","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","DevSecOps","SupplyChain","AppSec","CyberSecurity"],"series_name":"DevSecOps","design":"timeline","script":"","date":"2029-03-25","files":{"devsecops":"notes/p390-from-cve-publish-to-patch-deployed.md"},"md":"# From CVE publish to patch deployed\n\nA critical CVE is published on a Friday. The patch exists by Tuesday, but the service is not fixed until the deploy lands and the scan agrees, which is a different clock from the one in the advisory.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0** | CVE published | The advisory names a package and a version range. |\n| **T+1** | Alert fires | Dependabot or a nightly SCA job opens a ticket. |\n| **T+2** | Triaged | Is the package reachable from production code? |\n| **T+4** | Patch available | An upstream release, or a pinned override. |\n| **T+6** | Deployed | The lockfile changes and the image rebuilds. |\n| **T+7** | Verified | Scan clean, exploit test fails, ticket closes. |\n\n## Explained\n\n### The clock\n\nT+0 is publication. The advisory names a package, a version range and a severity, and the clock starts whether or not your team has read it.\n\nT+1 is the alert. Dependabot, a nightly SCA job or a registry webhook opens a ticket. The alert is not triage; it is a notification that a version in your lockfile now has a name.\n\nT+2 is triage. Someone answers whether the package is reachable from production code, which service imports it and whether the vulnerable function is on a live path. This is where most of the seven days is spent, not on the patch.\n\nT+4 is the patch. An upstream release is the clean route; a pinned override or a temporary fork is the route when the maintainer is silent.\n\nT+6 is the deploy. The lockfile changes, the image rebuilds, the admission check passes the new digest and the rollout completes.\n\nT+7 is verification. The scan is clean, the exploit test fails to land against the new build, and the ticket closes with evidence.\n\n### The target\n\n> **The honest caveat:** the seven days are a policy target, not a guarantee. Reachability triage can take longer than the patch, and some packages never get an upstream fix.\n\n> **The lesson:** publish a clock, then measure every event against it.\n\n## In one line\n\n> **Seven days is a policy, not luck.** The clock starts at publication, not at your first alert.\n\n**Check the clock**\n\n```bash\nnpm audit --audit-level=high\n```\n"},{"id":"p391","n":"391","slug":"supply-chain-terms-decoded","title":"Supply chain terms, decoded","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","SLSA","SBOM","DevSecOps","CyberSecurity"],"series_name":"DevSecOps","design":"reveal","script":"","date":"2029-03-27","files":{"devsecops":"notes/p391-supply-chain-terms-decoded.md"},"md":"# Supply chain terms, decoded\n\nSix words do most of the work in supply-chain conversations. Learn what each proves and the sales deck stops sounding like magic.\n\n## Quick reference\n\n- **Provenance** — A record of where and how an artefact was built, tied to a source commit.\n- **SBOM** — A machine-readable inventory of every component and version inside a build.\n- **Attestation** — A signed statement about a build, a test or a scan, bound to an artefact digest.\n- **SLSA** — A build-hardening scale: Build L1 to L3, with L3 the strongest.\n- **Typosquatting** — A lookalike package name, one keystroke away from the real one.\n- **Dep confusion** — A public package published under an internal name, hoping a resolver picks it.\n\n## Explained\n\n### The build\n\nProvenance is a record of where and how a build ran: the repository, the commit, the builder and the steps. It answers who built this, not whether the code is safe.\n\nAn SBOM is a machine-readable inventory of every component and version inside a build, usually SPDX or CycloneDX. It answers what is inside, a question a Dockerfile cannot answer.\n\nAn attestation is a signed statement about an artefact, bound to its digest: a build ran, a test passed, a scan found nothing. It answers what was claimed, and by whom.\n\nSLSA's build track is a hardening scale. Build L1 means provenance exists; L2 means a hosted platform signs it; L3 means a hardened platform isolates the build and protects the signing key. It answers how far you can trust the builder.\n\n### The name\n\nTyposquatting is a lookalike name: one letter changed, a hyphen moved, a scope added. It answers whether you installed the package you meant to install.\n\nDependency confusion publishes a package under an internal name, hoping the resolver prefers the public copy. It answers whether your registry precedence is a control or an accident.\n\n> **The honest caveat:** a signature proves who signed, not that the signed thing is good. Provenance and attestations move trust to the builder; they do not remove it.\n\n> **The lesson:** every term is a question you can ask of an artefact before deploying it.\n\n## In one line\n\n> **Prove the build, not the badge.** A signature is only useful when you know what it covers.\n\n**Verify one image**\n\n```bash\ncosign verify --key cosign.pub $IMAGE\n```\n"},{"id":"p392","n":"392","slug":"hardening-the-ci-cd-pipeline-drawn","title":"Hardening the CI/CD pipeline, drawn","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","SupplyChain","CyberSecurity","CloudNative"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-03-29","files":{"devsecops":"notes/p392-hardening-the-ci-cd-pipeline-drawn.md"},"md":"# Hardening the CI/CD pipeline, drawn\n\nA fork pull request reaches your runner in under a minute, and the runner holds the keys. Hardening the pipeline is deciding which tier may hold what.\n\n## Quick reference\n\n- **Trigger:** Pull request (PR) · Fork PR (FORK)\n- **Runner:** Isolated runner (CI) · Secrets broker (OIDC)\n- **Deploy:** Signed artifact (SIG) · Admission check (GATE)\n\n**Request flow:** Pull request → Isolated runner → Signed artifact → Admission check; Fork PR → Isolated runner\n\n**Around it:** Audit log (LOG)\n\n## Explained\n\n### Three tiers\n\nTrigger. A pull request from a branch in your repository is already trusted. A fork pull request is the untrusted edge: the workflow file comes from your default branch, the code comes from a stranger, and the token is read-only with no secrets.\n\nRunner. Give every job a fresh runner that is destroyed afterwards; a self-hosted runner reused across jobs keeps whatever the last job wrote. The secrets broker mints credentials with OIDC: the job asks GitHub for a signed token, the cloud checks its claims, and returns credentials that expire in an hour by default.\n\nDeploy. The build signs the artifact digest, so the signature covers bytes, not a tag. The admission check verifies that signature and the identity behind it before anything reaches the cluster. The audit log records both decisions.\n\n### Why the boundaries matter\n\nAn attacker who owns the runner owns the build, and an attacker who owns the build can sign anything. The runner must not push to production directly: it hands an artifact to a gate that checks a signature it cannot forge.\n\n> **The honest caveat:** signing proves who built the artifact, not that the code is safe. A compromised build signs malicious bytes with valid credentials.\n\n> **The lesson:** put the trust in the artifact and the check, not in the job that ran.\n\n## In one line\n\n> **Trust the artifact, not the job.** A runner is a boundary; treat it like one.\n\n**Check the pin**\n\n```bash\ngrep -rn 'uses:' .github/\n```\n"},{"id":"p393","n":"393","slug":"the-ci-cd-security-cheat-sheet","title":"The CI/CD security cheat sheet","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","GitHubActions","CyberSecurity","CheatSheet"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-04-01","files":{"devsecops":"notes/p393-the-ci-cd-security-cheat-sheet.md"},"md":"# The CI/CD security cheat sheet\n\nMost CI security work is a grep away. The failures are visible in the workflow files before a runner starts, and four blocks of commands cover the ground.\n\n## Quick reference\n\n### Workflows\n\n```bash\ngrep -rn 'pull_request_target' .github/\ngrep -rn 'allow-unsafe' .github/\ngrep -rn 'uses:.*@v[0-9]' .github/\nactionlint .github/workflows/*.yml\nzizmor .github/workflows/\n```\n\n### Permissions\n\n```bash\ngrep -rn 'permissions:' .github/\ngrep -rn 'write-all' .github/\ngh api repos/$R/actions/permissions\ngrep -rn 'GITHUB_TOKEN' .github/\ngh api orgs/$O/actions/permissions\n```\n\n### Pinning\n\n```bash\ngrep -rn 'uses:.*@' .github/workflows/\ngrep -rnE '@(main|master)' .github/\npinact run\nratchet pin .github/workflows/ci.yml\ngrep -c 'uses:' .github/workflows/*.yml\n```\n\n### Secrets\n\n```bash\ngrep -rn 'secrets\\.' .github/workflows/\ngh secret list\ngh api repos/$R/actions/secrets\ngitleaks dir . -v\ntrufflehog filesystem . --only-verified\n```\n\n## Explained\n\n### Workflows\n\nRead the files first. `grep -rn 'pull_request_target' .github/` finds the event that runs with the base repository's token, and `grep -rn 'allow-unsafe' .github/` finds the checkout flag that lets a fork's code into that job. actionlint .github/workflows/\\*.yml parses the YAML and catches expressions that would fail at runtime, and zizmor .github/workflows/ reads the same files for security patterns.\n\n### Permissions\n\n`grep -rn 'permissions:' .github/` shows which workflows set them at all; a missing block means the repository default applies. `grep -rn 'write-all' .github/` finds the broadest setting. `gh api repos/$R/actions/permissions` prints the repository policy for R as owner/repo, including whether SHA pinning is required.\n\n### Pinning\n\n`grep -rn 'uses:.*@' .github/workflows/` lists every action reference. A tag like @v4 can be moved; a 40-character SHA cannot. pinact run rewrites the files to SHAs, and ratchet pin .github/workflows/ci.yml does the same for one workflow.\n\n### Secrets\n\n`grep -rn 'secrets\\.' .github/workflows/` shows what a workflow can reach. `gh secret list` shows what exists. `gitleaks dir . -v` scans the working tree for committed credentials, and trufflehog filesystem . `--only-verified` checks the hits against the provider.\n\n> **The honest caveat:** a grep proves nothing. It points at a line a human still has to read, and pull_request_target is not always wrong.\n\n> **The lesson:** the cheapest pipeline audit is the one that reads the files before the runner does.\n\n## In one line\n\n> **The files are the evidence.** Read them before the runner does.\n\n**Start with the event**\n\n```bash\ngrep -rn 'pull_request_target' .github/\n```\n"},{"id":"p394","n":"394","slug":"the-iac-scanning-cheat-sheet","title":"The IaC scanning cheat sheet","sections":["devsecops"],"topic":"Terraform, Ansible & IaC","tags":["DevSecOps","IaC","Terraform","Kubernetes","CloudNative"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-04-03","files":{"devsecops":"notes/p394-the-iac-scanning-cheat-sheet.md"},"md":"# The IaC scanning cheat sheet\n\nTerraform, Kubernetes and a Dockerfile can all be wrong in ways a plan will not show. Four scanners, run in order, cover the infrastructure before it is applied.\n\n## Quick reference\n\n### Terraform\n\n```bash\ncheckov -d . --framework terraform\ntrivy config .\nterraform validate\nterraform plan -out tfplan\ncheckov -d . --compact\n```\n\n### Kubernetes\n\n```bash\nkubectl apply --dry-run=server -f .\nkubectl auth can-i --list\nkubectl get sa -A -o wide\nkubectl get pods -A\nkube-score score deploy.yaml\n```\n\n### Containers\n\n```bash\ntrivy image api:1.4.2\ntrivy image --severity HIGH api:1.4.2\ntrivy image --exit-code 1 api:1.4.2\nhadolint Dockerfile\ndocker scout cves api:1.4.2\n```\n\n### Policy\n\n```bash\nconftest test -p policy deploy.yaml\nconftest verify\nopa eval -d policy 'data.main.deny'\ncheckov -d . --framework kubernetes\nkyverno apply policy/ -r .\n```\n\n## Explained\n\n### Terraform\n\nStart with `terraform validate`, which parses the configuration without touching state. `checkov -d . --framework terraform` checks the resources against its policy set: public buckets, open security groups, unencrypted disks. `trivy config .` scans the same files and prints the offending line.\n\n### Kubernetes\n\n`kubectl apply --dry-run=server -f .` asks the API server to validate the manifest without persisting it, which catches schema errors a local lint misses. `kubectl auth can-i --list` shows what the current identity can do. kube-score score `deploy.yaml` grades the workload on probes, limits and security context.\n\n### Containers\n\n`trivy image api:1.4.2` lists CVEs in the image layers. `trivy image --exit-code 1 api:1.4.2` turns the same scan into a gate. `hadolint Dockerfile` checks the build file itself, and `docker scout cves api:1.4.2` gives a second opinion on the same image.\n\n### Policy\n\n`conftest test -p policy deploy.yaml` runs your own Rego rules against the manifest. `conftest verify` checks those rules before they check anything else. `opa eval -d policy 'data.main.deny'` shows what the policy returns, and `checkov -d . --framework kubernetes` adds its built-in checks.\n\n> **The honest caveat:** a scanner only finds the rules someone wrote, and a clean report is not proof of a safe configuration.\n\n> **The lesson:** scan the source, test the policy, and gate the apply.\n\n## In one line\n\n> **Scan before you apply.** A clean plan can still be an unsafe configuration.\n\n**Scan the source**\n\n```bash\ntrivy config .\n```\n"},{"id":"p395","n":"395","slug":"four-ways-a-ci-pipeline-leaks","title":"Four ways a CI pipeline leaks","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","CyberSecurity","CloudNative","AppSec"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-04-05","files":{"devsecops":"notes/p395-four-ways-a-ci-pipeline-leaks.md"},"md":"# Four ways a CI pipeline leaks\n\nA pipeline leak rarely looks like an attack. It looks like a workflow that echoes a variable, or a token with one scope too many.\n\n## Quick reference\n\n### Secrets in logs\n\n- echo `$TOKEN` writes it to a log anyone can read\n- Masking hides the exact string, not a base64 copy\n- Deleting the run does not delete the download\n\n### Fork pull requests\n\n- pull_request_target runs the base workflow with secrets\n- It is safe until a step runs the fork's code\n- Checkout now needs allow-unsafe-pr-checkout\n\n### Cache poisoning\n\n- A cache is restored, not verified, by the next job\n- GitHub made default-branch cache writes read-only\n- Self-hosted and package caches still cross lines\n\n### Over-privileged token\n\n- The default token can write contents in old repos\n- Set permissions: read-all, then grant one scope\n- A write token can push a tag or edit a release\n\n> **Note:** Every leak is a permission someone did not need.\n\n## Explained\n\n### Secrets in logs\n\nA step prints an environment variable and the value lands in a log anyone with repository access can download. GitHub masks registered secrets, but only the exact string, so a base64 copy goes through. Deleting the run does not delete the copy someone fetched.\n\n### Fork pull requests\n\npull_request_target runs the workflow from your default branch and gives it the base repository's token and secrets. That is safe until a step checks out the fork's head and runs it. The workflow file is yours; the Makefile it runs is not. GitHub now requires allow-unsafe-pr-checkout before checkout fetches a fork head.\n\n### Cache poisoning\n\nA cache is restored, not verified, so a job that can write an entry can hand the next job a tampered archive under a key it trusts. GitHub made default-branch cache writes read-only for untrusted triggers in June 2026; self-hosted and package caches still cross lines.\n\n### Over-privileged token\n\nA workflow with contents: write can push a commit or move a tag; actions: write can delete artifacts and caches. permissions: read-all at the top, one scope per job, removes most of that reach.\n\n> **The honest caveat:** these are defaults, not exotic bugs, and a scanner that greps only for pull_request_target misses the workflow that fetches fork code with git.\n\n> **The lesson:** every leak is a permission someone did not need.\n\n## In one line\n\n> **The pipeline has the keys.** The pull request has the vote. Never give it the keys.\n\n**Read the trigger**\n\n```bash\ngrep -rn 'pull_request_target' .github/\n```\n"},{"id":"p396","n":"396","slug":"a-github-actions-audit-in-python","title":"A GitHub Actions audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DevSecOps","CICD","GitHubActions","SupplyChain"],"series_name":"DevSecOps","design":"code","script":"gha_audit.py","date":"2029-04-08","files":{"devsecops":"notes/p396-a-github-actions-audit-in-python.md"},"md":"# A GitHub Actions audit in Python\n\nEvery workflow file in the repository is a small program with a token attached. This audit reads them as text, because the standard library has no YAML parser and the checks do not need one.\n\n## Quick reference\n\nThe full script is at the end of this note: `gha_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 gha_audit.py .github/workflows\n```\n\n**Example output**\n\n```text\nALERT ci.yml 12\nALERT ci.yml 40\n```\n\n## Explained\n\n### What it checks\n\nFive things, in order. A file with no permissions: block is counted once, because the repository default then decides what the token can do. Then each line is read and four checks can add a finding: pull_request_target, write-all, a secrets. reference inside a run block, tracked by indentation so the block's later lines count, and a uses: reference whose ref is not a 40-character SHA.\n\n### Why line by line\n\nPyYAML is not in the standard library, and adding a dependency to an audit tool defeats the point. Workflow YAML is regular enough for these checks: keys sit on their own lines, run blocks are indented, and uses values are single tokens. The full script skips local actions and docker:// references, which have no SHA to pin.\n\n### How to use it\n\n`python3 gha_audit.py .github/workflows` prints one line per finding and exits 1 if there are any. Put it in a job before the build, or run it from cron against every repository. Exit 0 means clean, 2 means a path was missing or unreadable.\n\n> **The honest caveat:** a line scan cannot evaluate expressions, anchors or reusable workflows, and a risky key inside a quoted string will still be flagged. Read the findings; do not just count them.\n\n> **The lesson:** a workflow is code, so review it like code and pin what it calls.\n\n## In one line\n\n> **A workflow is code.** The runner trusts it. Audit it like code.\n\n**Run it in ci**\n\n```bash\npython3 gha_audit.py .github/workflows\n```\n\n## Script: gha_audit.py\n\nDownload: [scripts/gha_audit.py](../scripts/gha_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit GitHub Actions workflows for five common configuration risks.\n\nEach workflow is read as text and checked for:\n\n  * a file that never sets permissions:\n  * the pull_request_target event\n  * permissions: write-all\n  * a secrets expression inside a run: block\n  * a uses: reference pinned to a tag or branch, not a full commit SHA\n\nUsage:\n    python3 gha_audit.py .github/workflows\n    python3 gha_audit.py .github/workflows ci.yml\n    python3 gha_audit.py . --quiet\n\nExit codes:\n    0  no findings\n    1  at least one finding\n    2  usage error, or a path that is missing, unreadable, or holds no\n       workflow files\n\nA directory is scanned recursively for .yml and .yaml files. The audit is\nread-only: files are opened for reading and nothing is written.\nIt is deliberately line-based, so it cannot evaluate expressions, aliases or\nreusable workflows; treat a finding as a line to review, not a verdict.\n\nCron: 0 7 * * 1 python3 gha_audit.py .github/workflows\nCI:   python3 gha_audit.py .github/workflows\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport os\nimport re\nimport sys\n\nEXTENSIONS = (\".yml\", \".yaml\")\nSHA_RE = re.compile(r\"^[0-9a-fA-F]{40}$\")\nRUN_RE = re.compile(r\"^(\\s*)(?:-\\s+)?run:\")\nUSES_RE = re.compile(r\"^\\s*(?:-\\s+)?uses:\\s*(\\S+)\")\nSECRET_EXPR_RE = re.compile(r\"\\$\\{\\{[^}]*\\bsecrets\\.\")\n\nKINDS = {\n    \"permissions-missing\": \"permissions: is not set, so the repository default applies\",\n    \"pull_request_target\": \"pull_request_target runs with the base repository's trust\",\n    \"write-all\": \"permissions: write-all grants every scope\",\n    \"secrets-in-run\": \"a secrets expression is used inside a run: block\",\n    \"unpinned-action\": \"an action is pinned to a tag or branch, not a commit SHA\",\n}\n\n\nclass AuditError(Exception):\n    \"\"\"The input cannot be audited.\"\"\"\n\n\ndef workflow_files(paths):\n    \"\"\"Return a sorted list of workflow files from files and directories.\"\"\"\n    found = []\n    for path in paths:\n        if os.path.isdir(path):\n            for root, dirs, names in os.walk(path):\n                dirs[:] = sorted(d for d in dirs if d != \".git\")\n                for name in sorted(names):\n                    if name.endswith(EXTENSIONS):\n                        found.append(os.path.join(root, name))\n        elif os.path.isfile(path):\n            found.append(path)\n        else:\n            raise AuditError(\"no such file or directory: %s\" % path)\n    if not found:\n        raise AuditError(\"no workflow files found in %s\" % \", \".join(paths))\n    return sorted(set(found))\n\n\ndef read_lines(path):\n    \"\"\"Read a workflow file and return its lines, or raise AuditError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n            return handle.read().splitlines()\n    except OSError as exc:\n        raise AuditError(\"cannot read %s: %s\" % (path, exc))\n\n\ndef code_part(line):\n    \"\"\"Return the line with a trailing comment removed (best effort).\"\"\"\n    stripped = line.lstrip()\n    if stripped.startswith(\"#\"):\n        return \"\"\n    quote = None\n    for index, char in enumerate(line):\n        if quote:\n            if char == quote:\n                quote = None\n        elif char in \"'\\\"\":\n            quote = char\n        elif char == \"#\" and index and line[index - 1] in \" \\t\":\n            return line[:index]\n    return line\n\n\ndef check_uses(line, number, path, findings):\n    \"\"\"Report a uses: reference that is not pinned to a 40-character SHA.\"\"\"\n    match = USES_RE.match(line)\n    if not match:\n        return\n    value = match.group(1).strip().strip(\"'\\\"\")\n    if value.startswith((\"./\", \"docker://\")) or \"@\" not in value:\n        return\n    ref = value.rsplit(\"@\", 1)[1]\n    if not SHA_RE.match(ref):\n        findings.append((path, number, \"unpinned-action\"))\n\n\ndef audit_file(path):\n    \"\"\"Return the findings for one workflow file.\"\"\"\n    lines = read_lines(path)\n    findings = []\n    code = [code_part(line) for line in lines]\n\n    if not any(\"permissions:\" in line for line in code):\n        findings.append((path, 0, \"permissions-missing\"))\n\n    run_indent = None\n    for number, line in enumerate(lines, 1):\n        text = code[number - 1]\n        if \"pull_request_target\" in text:\n            findings.append((path, number, \"pull_request_target\"))\n        if \"write-all\" in text:\n            findings.append((path, number, \"write-all\"))\n        check_uses(text, number, path, findings)\n\n        match = RUN_RE.match(text)\n        if match:\n            run_indent = text.index(\"run:\")\n            inline = text[match.end():]\n            if SECRET_EXPR_RE.search(inline):\n                findings.append((path, number, \"secrets-in-run\"))\n            continue\n\n        if run_indent is None:\n            continue\n        if not text.strip():\n            continue\n        indent = len(text) - len(text.lstrip())\n        if indent > run_indent:\n            if SECRET_EXPR_RE.search(text):\n                findings.append((path, number, \"secrets-in-run\"))\n        else:\n            run_indent = None\n\n    return findings\n\n\ndef audit(paths):\n    \"\"\"Audit every workflow and return (files, findings).\"\"\"\n    files = workflow_files(paths)\n    findings = []\n    for path in files:\n        findings.extend(audit_file(path))\n    findings.sort(key=lambda item: (item[0], item[1], item[2]))\n    return files, findings\n\n\ndef plural(count, word):\n    \"\"\"Return a count and a correctly pluralised noun.\"\"\"\n    return \"%d %s%s\" % (count, word, \"\" if count == 1 else \"s\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit GitHub Actions workflows for five configuration risks.\",\n        epilog=\"Exit 0 clean, 1 finding, 2 usage or input error. By s0sta.com\")\n    parser.add_argument(\"paths\", nargs=\"*\", default=[\".\"],\n                        help=\"workflow files or directories (default: .)\")\n    parser.add_argument(\"--quiet\", action=\"store_true\",\n                        help=\"print only the summary line\")\n    args = parser.parse_args(argv)\n\n    try:\n        files, findings = audit(args.paths or [\".\"])\n    except AuditError as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not args.quiet:\n        for path, number, kind in findings:\n            print(\"ALERT %s:%d %s\" % (path, number, kind))\n        for kind in sorted({kind for _, _, kind in findings}):\n            print(\"NOTE  %s\" % KINDS[kind])\n\n    if findings:\n        print(\"FAIL %s in %s\" % (plural(len(findings), \"finding\"),\n                                 plural(len(files), \"file\")))\n        return 1\n    print(\"OK %s checked\" % plural(len(files), \"workflow\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p397","n":"397","slug":"a-terraform-security-audit-in-python","title":"A Terraform security audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Terraform","DevSecOps","IaC","Python","CloudNative"],"series_name":"DevSecOps","design":"code","script":"iac_audit.py","date":"2029-04-10","files":{"devsecops":"notes/p397-a-terraform-security-audit-in-python.md"},"md":"# A Terraform security audit in Python\n\nA Terraform file can describe a public bucket, an open port and a wildcard policy in twenty lines. This audit reads the HCL for five of those smells and fails the build.\n\n## Quick reference\n\nThe full script is at the end of this note: `iac_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 iac_audit.py infra/\n```\n\n**Example output**\n\n```text\nALERT main.tf 14 acl\nALERT main.tf 22 ingress\n```\n\n## Explained\n\n### What it checks\n\nFive patterns. A public S3 ACL: acl = \"public-read\" or \"public-read-write\", which makes the bucket readable without a signed request. An open ingress: cidr_blocks with 0.0.0.0/0 or ::/0, which lets the internet reach the security group. An unencrypted volume: encrypted = false, also the default when the attribute is absent. A wildcard IAM action: Action = \"\\*\" or actions = [\"\\*\"]. A hard-coded secret: password or secret assigned to a literal string instead of a variable.\n\n### Why regex on HCL\n\nThe standard library has no HCL parser, and these five are single-line facts. The script reads one .tf file at a time, matches each line, and reports the file, line and rule. It ignores quoted values that start with a dollar sign, so an interpolated reference is not a finding.\n\n### How to use it\n\n`python3 iac_audit.py infra/` scans a directory recursively and exits 1 on a match. Run it before terraform plan, then let the plan review cover what a line scan cannot. Exit 2 means the path was missing or held no .tf files.\n\n> **The honest caveat:** an omitted encrypted attribute is not caught by a line scan, and account-level EBS encryption may already cover the volume the file leaves unset. The finding is a question, not a verdict.\n\n> **The lesson:** catch the obvious misconfiguration in review, and let the plan catch the rest.\n\n## In one line\n\n> **Read the source before the plan.** The scanner sees intent; the plan sees the account.\n\n**Scan the source**\n\n```bash\ntrivy config .\n```\n\n## Script: iac_audit.py\n\nDownload: [scripts/iac_audit.py](../scripts/iac_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit Terraform HCL for five common security smells.\n\nEach .tf file is read as text and checked for:\n\n  * a public S3 ACL (public-read or public-read-write)\n  * an ingress rule open to 0.0.0.0/0 or ::/0\n  * an EBS volume with encrypted = false\n  * an IAM policy with a wildcard action\n  * a hard-coded password, secret, key or token\n\nUsage:\n    python3 iac_audit.py infra/\n    python3 iac_audit.py infra/ main.tf\n    python3 iac_audit.py . --quiet\n\nExit codes:\n    0  no findings\n    1  at least one finding\n    2  usage error, or a path that is missing, unreadable, or holds no\n       .tf files\n\nA directory is scanned recursively for .tf files. The audit is read-only:\nfiles are opened for reading and nothing is written.\nIt is line-based, so it does not follow values across lines or modules; use\nterraform plan and a policy engine for the rest.\n\nCron: 30 6 * * 1 python3 iac_audit.py infra/\nCI:   python3 iac_audit.py infra/\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport os\nimport re\nimport sys\n\nRULES = [\n    (\"public-acl\",\n     re.compile(r'\\bacl\\s*=\\s*\"public-read(-write)?\"')),\n    (\"open-ingress\",\n     re.compile(r'(?:ipv6_)?cidr_blocks\\s*=\\s*\\[[^\\]]*\"'\n                r'(?:0\\.0\\.0\\.0/0|::/0)\"')),\n    (\"unencrypted-volume\",\n     re.compile(r'\\bencrypted\\s*=\\s*(?:false|\"false\")')),\n    (\"wildcard-iam\",\n     re.compile(r'\"?(?:[Aa]ctions?)\"?\\s*[:=]\\s*(?:\"\\*\"|\\[[^\\]]*\"\\*\")')),\n    (\"hardcoded-secret\",\n     re.compile(r'\\b(?:password|secret|secret_key|access_key|api_key|token)'\n                r'\\s*=\\s*\"[^\"$][^\"]*\"')),\n]\n\nKINDS = {name: name.replace(\"-\", \" \") for name, _ in RULES}\n\n\nclass AuditError(Exception):\n    \"\"\"The input cannot be audited.\"\"\"\n\n\ndef terraform_files(paths):\n    \"\"\"Return a sorted list of .tf files from files and directories.\"\"\"\n    found = []\n    for path in paths:\n        if os.path.isdir(path):\n            for root, dirs, names in os.walk(path):\n                dirs[:] = sorted(d for d in dirs if d != \".git\")\n                for name in sorted(names):\n                    if name.endswith(\".tf\"):\n                        found.append(os.path.join(root, name))\n        elif os.path.isfile(path):\n            found.append(path)\n        else:\n            raise AuditError(\"no such file or directory: %s\" % path)\n    if not found:\n        raise AuditError(\"no .tf files found in %s\" % \", \".join(paths))\n    return sorted(set(found))\n\n\ndef audit_file(path):\n    \"\"\"Return the findings for one .tf file.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n            lines = handle.read().splitlines()\n    except OSError as exc:\n        raise AuditError(\"cannot read %s: %s\" % (path, exc))\n\n    findings = []\n    for number, line in enumerate(lines, 1):\n        stripped = line.lstrip()\n        if stripped.startswith(\"#\") or stripped.startswith(\"//\"):\n            continue\n        for name, pattern in RULES:\n            if pattern.search(line):\n                findings.append((path, number, name))\n    return findings\n\n\ndef audit(paths):\n    \"\"\"Audit every file and return (files, findings).\"\"\"\n    files = terraform_files(paths)\n    findings = []\n    for path in files:\n        findings.extend(audit_file(path))\n    findings.sort(key=lambda item: (item[0], item[1], item[2]))\n    return files, findings\n\n\ndef plural(count, word):\n    \"\"\"Return a count and a correctly pluralised noun.\"\"\"\n    return \"%d %s%s\" % (count, word, \"\" if count == 1 else \"s\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit Terraform HCL for five common security smells.\",\n        epilog=\"Exit 0 clean, 1 finding, 2 usage or input error. By s0sta.com\")\n    parser.add_argument(\"paths\", nargs=\"*\", default=[\".\"],\n                        help=\".tf files or directories (default: .)\")\n    parser.add_argument(\"--quiet\", action=\"store_true\",\n                        help=\"print only the summary line\")\n    args = parser.parse_args(argv)\n\n    try:\n        files, findings = audit(args.paths or [\".\"])\n    except AuditError as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not args.quiet:\n        for path, number, name in findings:\n            print(\"ALERT %s:%d %s\" % (path, number, name))\n        for name in sorted({name for _, _, name in findings}):\n            print(\"NOTE  %s\" % KINDS[name])\n\n    if findings:\n        print(\"FAIL %s in %s\" % (plural(len(findings), \"finding\"),\n                                 plural(len(files), \"file\")))\n        return 1\n    print(\"OK %s checked\" % plural(len(files), \"file\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p398","n":"398","slug":"anatomy-of-a-signed-image-reference","title":"Anatomy of a signed image reference","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["DevSecOps","Containers","SupplyChain","Kubernetes","CloudNative"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-04-12","files":{"devsecops":"notes/p398-anatomy-of-a-signed-image-reference.md"},"md":"# Anatomy of a signed image reference\n\nA Kubernetes manifest says ghcr.io/acme/api:1.4.2, and the cluster may run something else tomorrow, because a tag is a pointer and pointers move.\n\n## Quick reference\n\n```text\nghcr.io/acme/api:1.4.2@sha256:9f2c7b1e\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `ghcr.io` | Registry | The host that stores the image. Docker Hub is implied when it is omitted. |\n| `/acme` | Organisation | The namespace or org that owns the repository. |\n| `/api` | Image | The repository name inside that namespace: what you push and pull. |\n| `:1.4.2` | Tag | A human label. It can be moved to a new digest at any time. |\n| `@sha256:9f2c7b1e` | Digest | The content hash, 64 hex characters. Immutable and verifiable. |\n\n## Explained\n\n### Five parts\n\nRegistry. `ghcr.io` is the host. If you write api:1.4.2 with no host, Docker and Kubernetes assume docker.io, so the short form hides where it lives.\n\nOrganisation. acme is the namespace or organisation that owns the repository. On GHCR it is usually the GitHub user or org, and access is granted per package.\n\nImage. api is the repository inside that namespace: the name you push to and pull from.\n\nTag. 1.4.2 is a human label. It is mutable: the same tag can point at a new digest an hour from now, and nothing in the reference records that it moved.\n\nDigest. sha256:9f2c7b1e is a shortened content hash: the full value is 64 hexadecimal characters after the algorithm. It is computed from the bytes, so it cannot change without a new digest. The full reference puts the tag before an @, the digest after.\n\n### Why the last part wins\n\nWhen a reference carries both a tag and a digest, the digest is what gets pulled. cosign signs the digest, not the tag, so verification is only meaningful against the digest. A tag that resolves to a different digest is a supply-chain event, even if it looks unchanged.\n\n> **The honest caveat:** a digest proves the bytes are the bytes someone signed, not that those bytes are safe. Scanning still has to happen before the signature means anything.\n\n> **The lesson:** deploy the digest, and let the tag be a comment for humans.\n\n## In one line\n\n> **The tag is a rumour.** The digest is the fact. Deploy the digest.\n\n**Resolve the tag:** crane digest ghcr.io/acme/api:1.4.2\n"},{"id":"p399","n":"399","slug":"pipeline-security-at-a-glance","title":"Pipeline security at a glance","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","Metrics","CyberSecurity","CloudNative"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-04-15","files":{"devsecops":"notes/p399-pipeline-security-at-a-glance.md"},"md":"# Pipeline security at a glance\n\nA security board that nobody owns is a wall decoration. These six tiles are the ones a pipeline team can change this sprint, each with a name and a number.\n\n## Quick reference\n\n**Status:** Last 30 days\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Unsigned images | **7** | of 42 in production |\n| Unpinned actions | **23** | of 61 use sites |\n| Failed scans | **3** | blocked the merge |\n| Secret hits | **1** | revoked in 11 minutes |\n| OIDC jobs | **90%%** | no stored cloud key |\n| Mean fix time | **2.4d** | critical: 1.1 days |\n\n## Explained\n\n### Coverage\n\nUnsigned images. Seven of forty-two images in production have no signature. Count them by listing the running digests and checking each against the cosign transparency log; an image built outside the pipeline is the usual gap.\n\nUnpinned actions. Twenty-three of sixty-one uses: references point at a tag, not a SHA. The count is mechanical: every uses: line that does not end in a 40-character ref.\n\nFailed scans. Three scans failed in the last thirty days and blocked the merge. A scan that fails without blocking is a dashboard, not a gate.\n\n### Exposure\n\nSecret hits. One credential was found in a workflow log and revoked in eleven minutes. The number that matters is the time from exposure to revocation, not the count.\n\nOIDC jobs. Ninety per cent of jobs get cloud credentials through OIDC, with no stored key. The remaining ten per cent are the ones to migrate first.\n\n### Response\n\nMean fix time. 2.4 days from finding to merge, and 1.1 days for critical findings. Track the tail, not the mean.\n\n> **The honest caveat:** every tile is only as good as its coverage; the unsigned tile counts what the registry knows, and mean fix time hides the one finding that has been open for a year.\n\n> **The lesson:** six numbers, six owners, one review every Monday.\n\n## In one line\n\n> **A board is not a control.** Six numbers, six owners, one review every Monday.\n\n**Count the pins**\n\n```bash\ngrep -rn 'uses:' .github/ | wc -l\n```\n"},{"id":"p400","n":"400","slug":"a-pipeline-attack-minute-by-minute","title":"A pipeline attack, minute by minute","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","CyberSecurity","IncidentResponse","SupplyChain"],"series_name":"DevSecOps","design":"timeline","script":"","date":"2029-04-17","files":{"devsecops":"notes/p400-a-pipeline-attack-minute-by-minute.md"},"md":"# A pipeline attack, minute by minute\n\nThe alert did not fire when the pull request opened. It fired nineteen days later, when someone read a bill.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **0m** | Push to a fork | The attacker forks the repo and opens a pull request. |\n| **1m** | Token issued | pull_request_target hands the job a write token and secrets. |\n| **2m** | Cache poisoned | A package cache the release build trusts now holds bad bytes. |\n| **4m** | Secrets read | The job prints its environment; the log is readable. |\n| **6m** | Artifact published | The write token pushes a new image tag to the registry. |\n| **19d** | Detection | A bill, an audit log or a scanner finally connects the dots. |\n\n## Explained\n\n### The first minutes\n\n0m. Push to a fork. The attacker forks the repository and opens a pull request from a branch. Nothing looks unusual: a fork, a commit and a green check requested.\n\n1m. Token issued. The workflow uses pull_request_target. It runs from the default branch, so the job receives the base repository's token and its secrets, and the token can write.\n\n2m. Cache poisoned. The job writes an entry into a package cache the release workflow trusts. A later restore unpacks it and treats the contents as its own build output.\n\n### The next minutes\n\n4m. Secrets read. A step prints its environment, or reads a file the token can reach. The run log is readable to anyone with repository access.\n\n6m. Artifact published. The write token pushes a new tag to the registry, or edits a release. The image carries the project's name and a plausible version.\n\n### The detection\n\n19d. Detection. A cloud bill, an audit log or a scanner finally connects the fork to the image. The mean time to detect is measured in weeks because nothing watches the pipeline itself.\n\n> **The honest caveat:** this is a composite of documented cases, and the timings are illustrative. GitHub's 2026 cache and checkout changes close the easiest version, but the chain still runs wherever secrets are reachable.\n\n> **The lesson:** the pipeline is a production system, so alert on it like one.\n\n## In one line\n\n> **Detection is the last step.** The first five happen before anyone looks.\n\n**Read the log**\n\n```bash\ngh run view --log\n```\n"},{"id":"p401","n":"401","slug":"ci-cd-security-terms-decoded","title":"CI/CD security terms, decoded","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevSecOps","CICD","CyberSecurity","CloudNative","AppSec"],"series_name":"DevSecOps","design":"reveal","script":"","date":"2029-04-19","files":{"devsecops":"notes/p401-ci-cd-security-terms-decoded.md"},"md":"# CI/CD security terms, decoded\n\nSix terms come up in every pipeline security review, and each one is a control you can test.\n\n## Quick reference\n\n- **Runner isolation** — A fresh, single-use runner per job, so nothing a job writes survives into the next one.\n- **OIDC** — A job swaps a signed GitHub token for short-lived cloud credentials. No stored key.\n- **Pinning** — Referencing an action by its full 40-character commit SHA, because tags can move.\n- **Provenance** — A signed record of where and how an artifact was built, checkable before deploy.\n- **Cache poisoning** — Seeding a cache entry that a more privileged job later restores and trusts.\n- **Least privilege** — A permissions: block that grants one job only the scopes it needs.\n\n## Explained\n\n### Identity\n\nRunner isolation. A fresh, single-use runner for every job. On self-hosted runners that is a configuration choice, and a runner reused across jobs carries the last job's files and credentials into the next one.\n\nOIDC. The job asks GitHub for a signed token, presents it to the cloud, and receives temporary credentials. The cloud checks the issuer, the audience and the subject claim. No long-lived key is stored.\n\n### Supply chain\n\nPinning. An action referenced by a full 40-character commit SHA cannot be moved. A tag can: the maintainer, or an attacker with the maintainer's account, can point v4 at new code tomorrow.\n\nProvenance. A signed record of what was built, where, from which commit, by which workflow. It answers what a signature cannot: was this built by the pipeline I trust?\n\nCache poisoning. Seeding a cache entry that a more privileged job later restores and trusts. The defence is to separate cache scopes and never restore a cache into a job that holds secrets.\n\n### Permission\n\nLeast privilege. A permissions: block that starts at read-all and grants one job the one scope it needs. The token is the pipeline's identity; every extra scope is a door.\n\n> **The honest caveat:** a vocabulary is not a control. Isolation means nothing without a test that proves the runner is fresh, and least privilege means nothing without a policy that fails the build.\n\n> **The lesson:** name the control, then write the check that proves it.\n\n## In one line\n\n> **Name the control, then check it.** A term you cannot test is a term you cannot enforce.\n\n**List the scopes**\n\n```bash\ngrep -A2 'permissions:' .github/\n```\n"},{"id":"p402","n":"402","slug":"the-request-security-path-drawn","title":"The request security path, drawn","sections":["devsecops"],"topic":"Application & API security","tags":["AppSec","APISecurity","DevSecOps","CyberSecurity","WebSecurity"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-04-22","files":{"devsecops":"notes/p402-the-request-security-path-drawn.md"},"md":"# The request security path, drawn\n\nA request arrives with a valid session, a plausible body and a familiar path. Nothing about it looks hostile. The only question is how many layers inspected it before your data.\n\n## Quick reference\n\n- **Edge:** WAF (WAF) · Rate limit (429)\n- **App:** Authentication (401) · Authorisation (403) · Input validation (VAL)\n- **Data:** Parameterised query (SQL) · Encrypted store (AES)\n\n**Request flow:** WAF → Rate limit → Authentication → Authorisation → Input validation → Parameterised query → Encrypted store\n\n## Explained\n\n### Edge\n\nThe first tier is the edge. A WAF matches the request against rules for known payloads and blocks the obvious attempts before they cost you a worker. A rate limit caps how many requests one client can send, so a credential-stuffing run or a scraper gets a 429 instead of your database. Neither knows your business logic.\n\n### Application\n\nThe second tier is the application. Authentication proves who is calling: a missing or invalid token gets a 401. Authorisation decides what that identity may touch, and a valid session asking for another tenant's order gets a 403. Input validation then rejects anything that does not match the expected shape, type and length. The three run in that order, because identity without permission is still a stranger.\n\n### Data\n\nThe third tier is the data. The parameterised query keeps the value separate from the SQL, so a quote in a name cannot become syntax. The encrypted store protects the rows if a backup or a disk leaves the building.\n\n> **The honest caveat:** layers do not add up to safety. A WAF rule can be bypassed, a rate limit is per key, and an internal service that trusts its caller skips the edge entirely.\n\n> **The lesson:** each layer answers a different question, and no layer answers all of them.\n\n## In one line\n\n> **Every layer says no in its own way.** A request that survives all three is the only one that reaches data.\n\n**Trace one request**\n\n```bash\ncurl -sv https://api.example.com/me\n```\n"},{"id":"p403","n":"403","slug":"the-api-security-testing-cheat-sheet","title":"The API security testing cheat sheet","sections":["devsecops"],"topic":"Application & API security","tags":["APISecurity","AppSec","CheatSheet","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-04-24","files":{"devsecops":"notes/p403-the-api-security-testing-cheat-sheet.md"},"md":"# The API security testing cheat sheet\n\nAn API test is not a scanner run. You have a token, a second account and a shell, and the goal is to prove what one user can do to another.\n\n## Quick reference\n\n### Auth\n\n```bash\ncurl -si -u user:pass $API/me\ncurl -si $API/me -b jar.txt\njwt decode $TOKEN\njwt decode $TOKEN | jq -r .exp\ncurl -si -X POST $API/login -d @a.json\n```\n\n### Authz\n\n```bash\ncurl -si $API/admin -b bob.txt\ncurl -si $API/users/2 -b bob.txt\ncurl -si $API/users/2 -b alice.txt\ncurl -si -X DELETE $API/u/2 -b bob.txt\n```\n\n### Input\n\n```bash\ncurl -si \"$API/q?x=%27\"\ncurl -si \"$API/q?x=%3Cscript%3E\"\ncurl -si \"$API/f?p=../../etc/passwd\"\ncurl -si $API/q?x=%00\ncurl -si --path-as-is $API/../admin\n```\n\n### Limits\n\n```bash\ncurl -si $API?page=999999999\ncurl -si -X POST $API/login -d @big.json\ncurl -si --max-time 1 $API/slow\nseq 100 | xargs -P20 -n1 curl -s $API\n```\n\n## Explained\n\n### Auth\n\nStart with authentication. `curl -si -u user:pass $API/me` establishes a baseline: status line and headers, no follow and no noise. Replay the session with -b `jar.txt`, then read the token itself. jwt decode `$TOKEN` shows the header and claims, and piping it to `jq -r .exp` turns the expiry into a number you can compare. A login POST with -d @`a.json` keeps the body out of history.\n\n### Authz\n\nAuthorisation needs two identities, and that is the only honest test. Request `$API`/users/2 as bob, then as alice, and compare both responses. If both return 200 with the same record, the object check is missing. Repeat on `$API`/admin and on a DELETE: read, role and write are three different failures.\n\n### Input\n\nInput tests ask whether data becomes syntax. Send a single quote, a script tag, a traversal path, a null byte, and a .. path. `--path-as-is` keeps the request honest.\n\n### Limits\n\nLimits are about cost. Ask for page 999999999, post a large body, hold a slow request, and send a small burst. A 429 or a timeout is a control; a 500 is a finding.\n\n> **The honest caveat:** a 403 on one ID proves nothing about the collection endpoint or the write path, and a WAF block may hide the bug rather than fix it.\n\n> **The lesson:** test with two accounts, or you are only testing one.\n\n## In one line\n\n> **Two identities, one bug.** A second account is the cheapest authorisation test you own.\n\n**Replay as alice**\n\n```bash\ncurl -si $API/users/2 -b alice.txt\n```\n"},{"id":"p404","n":"404","slug":"xss-in-four-panels-drawn","title":"XSS in four panels, drawn","sections":["devsecops"],"topic":"Application & API security","tags":["XSS","AppSec","WebSecurity","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-04-26","files":{"devsecops":"notes/p404-xss-in-four-panels-drawn.md"},"md":"# XSS in four panels, drawn\n\nAn XSS report usually arrives as a screenshot with an alert box. The screenshot proves a payload ran; it does not say which kind, and the fix differs for each.\n\n## Quick reference\n\n### Reflected XSS\n\n- Payload rides in the request and echoes back\n- Crafted link: ?q=\\<script>alert(1)\\</script>\n- Server-side encoding stops the echo\n\n### Stored XSS\n\n- Payload is saved: a comment, profile or ticket\n- Every reader runs it until the row is fixed\n- One write, many victims, no link needed\n\n### DOM XSS\n\n- The page's own JavaScript writes the payload\n- innerHTML with location.hash is the classic\n- The server never sees it in the response\n\n### The fix\n\n- Encode on output, for the context it lands in\n- CSP blocks inline script it did not allow\n- CSP is a backstop, not the fix\n\n> **Note:** Encode every output. CSP catches what encoding misses.\n\n## Explained\n\n### Reflected\n\nReflected XSS rides in the request and comes back in the response. A link like ?q=\\<script>alert(1)\\</script> is enough if the page prints q without encoding. The server stored nothing: the payload lives in the crafted URL and the chat app that carried it.\n\n### Stored\n\nStored XSS is saved first: a comment, a profile field, a support ticket. Every reader of that row runs the payload until the row is fixed, which is why one write can hit thousands of sessions and no link is needed.\n\n### DOM\n\nDOM XSS never reaches the server. The page's own JavaScript reads location.hash or a query parameter and writes it into innerHTML, so the response is clean and the browser still executes the payload. Server-side filters cannot see it.\n\n### The fix\n\nThe fix is contextual output encoding: HTML entities in HTML text, attribute encoding in attributes, JSON encoding in script blocks, URL encoding in URLs. Then add a CSP that disallows inline script and allows only the origins you name. CSP is the backstop for the encoder you missed, not a substitute for it.\n\n> **The honest caveat:** a CSP containing 'unsafe-inline' or a wildcard source gives you the header and none of the protection, and encoding the input instead of the output breaks data in ways users notice.\n\n> **The lesson:** encode where the data lands, not where it came from.\n\n## In one line\n\n> **Encode the output, not the input.** CSP catches what a missed encoder lets through.\n\n**Find the sink**\n\n```bash\ngrep -rn 'innerHTML' src/\n```\n"},{"id":"p405","n":"405","slug":"sql-injection-to-a-parameterised-query-drawn","title":"SQL injection to a parameterised query, drawn","sections":["devsecops"],"topic":"Application & API security","tags":["SQLInjection","AppSec","CyberSecurity","DevSecOps","WebSecurity"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-04-29","files":{"devsecops":"notes/p405-sql-injection-to-a-parameterised-query-drawn.md"},"md":"# SQL injection to a parameterised query, drawn\n\nThe query looks harmless in the repository: a string, a variable, and a WHERE clause. The database sees one string too, and that is the whole problem.\n\n## Quick reference\n\n### The string concat\n\n- Code and data share one string\n- WHERE name = '\" + name + \"'\n- The database cannot tell them apart\n\n### The payload\n\n- ' OR '1'='1' -- returns every row\n- ' UNION SELECT password FROM users --\n- The comment swallows the trailing quote\n\n### The prepared statement\n\n- SELECT \\* FROM users WHERE name = ?\n- The driver sends SQL and values separately\n- The value can never become syntax\n\n### The allow-list\n\n- ORDER BY name is not a bindable parameter\n- Map sort=name to a fixed column list\n- Reject anything not on the list\n\n> **Note:** Binding protects values; identifiers still need a list.\n\n## Explained\n\n### The string concat\n\nThe code builds SELECT \\* FROM users WHERE name = '\" + name + \"'. The value is pasted into the statement before the database parses it. The parser cannot tell the developer's quote from the attacker's quote, because by then there is no difference.\n\n### The payload\n\nSend ' OR '1'='1' -- and the WHERE clause becomes true for every row; the comment swallows the trailing quote the developer added. A UNION SELECT can pull a password column into the response. No special tool is needed, only a parameter the app trusts.\n\n### The prepared statement\n\nThe fix is to send the SQL and the values separately: SELECT \\* FROM users WHERE name = ?. The driver parses the statement with a placeholder, then binds the value. The value is data at every stage and can never become syntax, even if it contains quotes, comments or a UNION.\n\n### The allow-list\n\nBinding has a boundary: identifiers cannot be parameters. ORDER BY ? does not sort by the column you pass, and a table name cannot be bound. Map the user's sort value to a fixed list of allowed columns and directions, and reject everything else.\n\n> **The honest caveat:** prepared statements protect values, not identifiers, and an ORM that concatenates a column name reintroduces the same bug with nicer syntax.\n\n> **The lesson:** bind the values and allow-list the names.\n\n## In one line\n\n> **Never build SQL with a string.** A prepared statement protects values; identifiers are still code.\n\n**Test the param**\n\n```bash\ncurl -si \"$API/q?x=%27\"\n```\n"},{"id":"p406","n":"406","slug":"a-jwt-checker-in-python","title":"A JWT checker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["JWT","AppSec","Python","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"code","script":"jwt_check.py","date":"2029-05-01","files":{"devsecops":"notes/p406-a-jwt-checker-in-python.md"},"md":"# A JWT checker in Python\n\nA JWT is not a session; it is a signed claim. Before you trust one, read what it says. This script decodes the header and payload, checks the claims that cause trouble, and never verifies the signature.\n\n## Quick reference\n\nThe full script is at the end of this note: `jwt_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 jwt_check.py \"$TOKEN\"\n```\n\n**Example output**\n\n```text\nALERT no exp no aud\n```\n\n## Explained\n\n### What the GIF shows\n\nThe 14-line core pads and decodes the first two parts of the token. It lowercases alg, so alg none is caught whatever the case. It reads exp once, then reports a missing or expired exp, or a lifetime over 86,400 seconds. Finally it collects a missing iss and aud, prints ALERT and exits 1.\n\n### The checks\n\nalg none is the old bypass: a verifier that trusts the header can be told there is no signature. exp is a NumericDate in seconds since the epoch, and a token without it never expires. iss and aud tie a token to an issuer and an audience; without them, a token for one service may be accepted by another. A token valid for a week is a stolen token valid for a week.\n\n### Running it\n\n`python3 jwt_check.py \"$TOKEN\"`, from cron or a CI step over your token fixtures. The attached script adds argparse, `--max-hours`, file input and clear exit codes: 0 clean, 1 findings, 2 bad input. It is read-only.\n\n> **The honest caveat:** this is a claim linter, not an authenticator. It does not verify the signature, so a clean result says the claims look sane, not that the token is genuine; most libraries reject alg none by default.\n\n> **The lesson:** decode first, verify separately, and never accept a token because it parsed.\n\n## In one line\n\n> **A claim is not a promise.** Decode first; verify the signature somewhere else.\n\n**Run it in ci**\n\n```bash\npython3 jwt_check.py \"$TOKEN\"\n```\n\n## Script: jwt_check.py\n\nDownload: [scripts/jwt_check.py](../scripts/jwt_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"jwt_check.py - inspect JWT claims without verifying the signature.\n\nDecodes the header and payload of a JSON Web Token and reports the claim\nproblems reviewers look for: alg none, a missing or expired exp, a lifetime\nover the limit, and a missing iss or aud. It never checks the signature, so\nit must not be used to accept a token. Read-only: it decodes and makes no\nnetwork calls. Standard library only, Python 3.8+.\n\nUsage:\n    python3 jwt_check.py <token>\n    python3 jwt_check.py --file token.txt\n    python3 jwt_check.py --stdin < token.txt\n    python3 jwt_check.py --max-hours 12 <token>\n\nRun it from cron on a token you already hold, or as a CI step over the test\nfixtures that contain sample tokens.\n\nExit codes: 0 clean, 1 findings, 2 usage or input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport base64\nimport binascii\nimport json\nimport re\nimport sys\nimport time\n\nDEFAULT_MAX_HOURS = 24\nPART_RE = re.compile(r\"^[A-Za-z0-9_-]*$\")\n\n\ndef decode_segment(part, name):\n    \"\"\"Decode one base64url segment into a JSON object.\"\"\"\n    if not part:\n        raise ValueError(\"empty %s segment\" % name)\n    if not PART_RE.match(part):\n        raise ValueError(\"%s segment is not base64url\" % name)\n    padded = part + \"=\" * (-len(part) % 4)\n    try:\n        raw = base64.urlsafe_b64decode(padded.encode(\"ascii\"))\n        obj = json.loads(raw.decode(\"utf-8\"))\n    except (binascii.Error, UnicodeDecodeError, ValueError) as exc:\n        raise ValueError(\"cannot decode %s segment: %s\" % (name, exc))\n    if not isinstance(obj, dict):\n        raise ValueError(\"%s segment is not a JSON object\" % name)\n    return obj\n\n\ndef parse_token(token):\n    \"\"\"Split a JWT and return its header and payload as dicts.\"\"\"\n    parts = token.strip().split(\".\")\n    if len(parts) != 3:\n        raise ValueError(\"a JWT has three dot-separated parts, got %d\" % len(parts))\n    return decode_segment(parts[0], \"header\"), decode_segment(parts[1], \"payload\")\n\n\ndef find_claims(header, payload, max_hours, now=None):\n    \"\"\"Return a list of findings; an empty list means the claims look sane.\"\"\"\n    now = time.time() if now is None else now\n    bad = []\n    alg = str(header.get(\"alg\", \"\")).lower()\n    if alg == \"none\":\n        bad.append(\"alg=none\")\n    exp = payload.get(\"exp\")\n    if exp is None:\n        bad.append(\"no exp\")\n    elif isinstance(exp, bool) or not isinstance(exp, (int, float)):\n        bad.append(\"exp is not a number\")\n    elif exp < now:\n        bad.append(\"expired\")\n    elif exp - now > max_hours * 3600:\n        bad.append(\"long-lived\")\n    if not payload.get(\"iss\"):\n        bad.append(\"no iss\")\n    if not payload.get(\"aud\"):\n        bad.append(\"no aud\")\n    return bad\n\n\ndef read_token(args):\n    \"\"\"Return the token text from the positional, --file or --stdin.\"\"\"\n    if args.token:\n        return args.token\n    if args.file:\n        with open(args.file) as fh:\n            return fh.read().strip()\n    if args.stdin:\n        return sys.stdin.read().strip()\n    raise ValueError(\"give a token, or use --file or --stdin\")\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Decode a JWT and flag claim problems without verifying it.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\",\n    )\n    ap.add_argument(\"token\", nargs=\"?\", help=\"the JWT, as one argument\")\n    ap.add_argument(\"-f\", \"--file\", help=\"read the token from a file\")\n    ap.add_argument(\"--stdin\", action=\"store_true\", help=\"read the token from stdin\")\n    ap.add_argument(\"--max-hours\", type=float, default=DEFAULT_MAX_HOURS,\n                    help=\"flag lifetimes longer than this (default: %(default)s)\")\n    args = ap.parse_args()\n\n    try:\n        token = read_token(args)\n        if not token:\n            raise ValueError(\"the token is empty\")\n        if args.max_hours <= 0:\n            raise ValueError(\"--max-hours must be greater than zero\")\n        header, payload = parse_token(token)\n    except (OSError, ValueError) as exc:\n        print(\"ERROR: %s\" % exc, file=sys.stderr)\n        return 2\n\n    bad = find_claims(header, payload, args.max_hours)\n    if bad:\n        print(\"ALERT \" + \" \".join(bad))\n        return 1\n    print(\"OK alg=%s, claims look sane (signature not checked)\"\n          % header.get(\"alg\", \"missing\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p407","n":"407","slug":"anatomy-of-an-ssrf-target","title":"Anatomy of an SSRF target","sections":["devsecops"],"topic":"Application & API security","tags":["SSRF","AppSec","CloudSecurity","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-05-03","files":{"devsecops":"notes/p407-anatomy-of-an-ssrf-target.md"},"md":"# Anatomy of an SSRF target\n\nAn SSRF report rarely starts with a payload. It starts with a feature: fetch this image, import this URL, check this webhook. The server fetches, and it can reach places your browser cannot.\n\n## Quick reference\n\n```text\nhttp://169.254.169.254/latest/meta-data/iam/\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `http://` | Scheme | Plain HTTP: the metadata endpoint is not TLS. |\n| `169.254.169.254` | Link-local IP | The instance metadata service, reachable from the host network. |\n| `/latest` | API version | The versioned root; latest is an alias that can change. |\n| `/meta-data` | Metadata path | Instance identity, user data and the IAM branch live here. |\n| `/iam/` | Credential path | Lists the roles; one role returns temporary access keys. |\n\n## Explained\n\n### The string\n\nThe target is http://169.254.169.254/latest/meta-data/iam/. Read it in five parts. http:// is the scheme: plain HTTP, not HTTPS. 169.254.169.254 is a link-local address, and on AWS it is the metadata service.\n\n### The path\n\n`/latest` selects the API version. `/meta-data` is the metadata root, where instance identity and user data live. `/iam/` is the credential branch: list the roles, then read a role to receive temporary access keys.\n\n### Why it works\n\nThe attacker needs no shell access, only one parameter the server will fetch. A shortener, a redirect, a DNS name for the link-local range, or a webhook tester can point it inward. The response returns through that same feature.\n\n### The fix\n\nBlock link-local and private ranges at the egress layer, allow-list the hosts a fetch feature may call, and do not follow redirects into a different range. On AWS, require IMDSv2, which needs a PUT to get a session token, and set the hop limit to 1 so a container cannot reach the host metadata.\n\n> **The honest caveat:** IMDSv2 raises the bar rather than closing the hole. A proxy that allows PUT, or an SSRF with method control, can still mint the token; the egress allow-list is the control that lasts.\n\n> **The lesson:** any URL the server fetches is a request you did not write.\n\n## In one line\n\n> **The cloud trusts its own network.** A server that fetches URLs can fetch its own credentials.\n\n**Test the block**\n\n```bash\ncurl -m 2 http://169.254.169.254\n```\n"},{"id":"p408","n":"408","slug":"anatomy-of-a-set-cookie-header","title":"Anatomy of a Set-Cookie header","sections":["devsecops"],"topic":"Application & API security","tags":["WebSecurity","Cookies","AppSec","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-05-06","files":{"devsecops":"notes/p408-anatomy-of-a-set-cookie-header.md"},"md":"# Anatomy of a Set-Cookie header\n\nA session cookie is a bearer token with a browser attached. The Set-Cookie header is where the server sets the rules, and six attributes decide how far the secret travels.\n\n## Quick reference\n\n```text\nsession=abc; Secure; HttpOnly; SameSite=Lax; Max-Age=3600; Path=/\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `session=abc` | Name and value | The session ID is the secret: anyone who has it is the user. |\n| `Secure` | HTTPS only | The browser sends it only over TLS; plain HTTP never sees it. |\n| `HttpOnly` | No JavaScript | document.cookie cannot read it, so an XSS cannot steal it. |\n| `SameSite=Lax` | Cross-site rule | Not sent on cross-site POSTs; a top-level GET still carries it. |\n| `Max-Age=3600` | Lifetime | One hour in seconds. Expires is the absolute-date alternative. |\n| `Path=/` | Scope | Sent to every path on the host. Narrow it where you can. |\n\n## Explained\n\n### The six parts\n\n`session=abc` is the name and value. The value is the session identifier: anyone who has it is the user, so it should be random and long.\n\nSecure means the browser sends the cookie only over HTTPS. Without it, one plain-HTTP request leaks the session to anyone on the path.\n\nHttpOnly hides the cookie from document.cookie, so a script injected into the page cannot read it. It does not stop that script from making requests as the user.\n\nSameSite=Lax stops the browser sending the cookie on cross-site POSTs, while a top-level GET navigation still carries it. Lax is the common default now; Strict is tighter and None requires Secure.\n\n`Max-Age=3600` sets a lifetime of one hour in seconds. Expires does the same with an absolute date, and Max-Age wins when both appear.\n\nPath=/ scopes the cookie to every path on the host. Narrow it to `/app` when the session is only needed there.\n\n### What is missing\n\nDomain is absent, so the cookie is host-only. There is no __Host- prefix and no Partitioned attribute. Those are choices too.\n\n> **The honest caveat:** HttpOnly does not make XSS harmless, SameSite=Lax does not stop CSRF on GET side effects, and Secure protects the wire, not the browser or the server.\n\n> **The lesson:** every attribute removes one way to steal or misuse the cookie.\n\n## In one line\n\n> **The cookie is a policy.** Six attributes decide who can read it and where it travels.\n\n**Audit the header**\n\n```bash\ncurl -sI https://example.com | grep -i cookie\n```\n"},{"id":"p409","n":"409","slug":"application-security-at-a-glance","title":"Application security at a glance","sections":["devsecops"],"topic":"Application & API security","tags":["AppSec","DevSecOps","CyberSecurity","Metrics","RiskManagement"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-05-08","files":{"devsecops":"notes/p409-application-security-at-a-glance.md"},"md":"# Application security at a glance\n\nA dashboard is not a security programme, but it is how you notice one. Six tiles, read left to right and top to bottom, answer the questions a lead asks on a Monday.\n\n## Quick reference\n\n**Status:** This sprint\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Critical open | **7** | 2 past SLA |\n| Authz failures | **3** | IDOR tests failing |\n| WAF blocks | **12.4k** | last 24 hours |\n| Dependency age | **148 d** | median, direct deps |\n| Test coverage | **78%%** | authz and input tests |\n| Open exceptions | **5** | 2 expire this month |\n\n## Explained\n\n### Findings\n\nCritical open: seven, with two past their SLA. The age matters more than the count; a critical from last week is a decision, one from last quarter is a policy. Authz failures: three IDOR tests failing in the API suite. This is the number that predicts a breach, because a scanner rarely tests two accounts.\n\n### Exposure\n\nWAF blocks: 12.4k in the last day. A block is not a fix; a rising count can mean an attack, a noisy rule, or a scanner you own, so read the rule IDs before you celebrate.\n\n### Debt\n\nDependency age: a median of 148 days for direct dependencies. Age is a proxy for missing patches, not risk on its own; a pinned library with no advisories is less urgent than a fresh one with a critical CVE.\n\n### Assurance\n\nTest coverage: 78 percent on the authz and input tests. Coverage proves a test exists, not that it asserts the right thing. Open exceptions: five accepted risks, two expiring this month. An exception without an expiry is a permanent hole with a ticket number.\n\n> **The honest caveat:** every tile can improve while the product gets less safe. Fewer findings may mean better code or a scanner that stopped running, so plot the count against the scan volume.\n\n> **The lesson:** a board is a prompt for questions, not an answer.\n\n## In one line\n\n> **Count what could hurt you.** Six numbers, and the one that should worry you.\n\n**Export the board**\n\n```bash\ncurl -s $API/metrics | jq .appsec\n```\n"},{"id":"p410","n":"410","slug":"anatomy-of-an-api-attack","title":"Anatomy of an API attack","sections":["devsecops"],"topic":"Application & API security","tags":["APISecurity","AppSec","CyberSecurity","IncidentResponse","DevSecOps"],"series_name":"DevSecOps","design":"timeline","script":"","date":"2029-05-10","files":{"devsecops":"notes/p410-anatomy-of-an-api-attack.md"},"md":"# Anatomy of an API attack\n\nMost API breaches do not begin with a zero-day. They begin with an endpoint that answers a question it should have refused, and a script that keeps asking.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0** | Recon | Swagger, JS bundles and error messages map the API. |\n| **T+2h** | Auth bypass | A valid token asks for another tenant's object and gets it. |\n| **T+3h** | Data access | IDs are walked in order; each returns a full record. |\n| **T+5h** | Exfiltration | A script pages through 40k records on the normal endpoint. |\n| **T+9h** | Detection | A volume alert fires on one key, not on the endpoint. |\n| **T+11h** | Containment | The key is revoked, the object check ships, 403 follows. |\n\n## Explained\n\n### The attack\n\nT+0. Recon: the Swagger file, the JavaScript bundle and the error messages. Every field name and every ID format is a hint, and a public API document is a map.\n\nT+2h. Auth bypass: a valid token asks for another tenant's object and receives it. The token is genuine; the check that the object belongs to the caller is missing. This is broken object level authorisation, and it needs no exploit.\n\nT+3h. Data access: IDs are walked in order. Each response is a full record, so the attacker learns the shape of the data and which fields are sensitive.\n\nT+5h. Exfiltration: a script pages through 40,000 records on the normal endpoint at a rate that looks like a busy customer. No malware, no unusual port, just GETs.\n\n### The response\n\nT+9h. Detection: a volume alert fires on one API key. The endpoint never alerted, because it was doing exactly what it was built to do.\n\nT+11h. Containment: the key is revoked and the object check ships; the endpoint returns 403 for the other tenant. The log review then shows how far back the access goes.\n\n> **The honest caveat:** this timeline is compressed. Real dwell time is usually weeks, and detection often arrives from a customer or a partner before it arrives from the SIEM.\n\n> **The lesson:** an API does not need a vulnerability to leak; it needs a missing check and patience.\n\n## In one line\n\n> **The API answered every question.** Nothing was hacked. A missing check was used.\n\n**Test one id**\n\n```bash\ncurl -si $API/orders/1002 -b bob.txt\n```\n"},{"id":"p411","n":"411","slug":"appsec-terms-decoded","title":"AppSec terms, decoded","sections":["devsecops"],"topic":"Application & API security","tags":["AppSec","CyberSecurity","WebSecurity","DevSecOps","APISecurity"],"series_name":"DevSecOps","design":"reveal","script":"","date":"2029-05-13","files":{"devsecops":"notes/p411-appsec-terms-decoded.md"},"md":"# AppSec terms, decoded\n\nSix acronyms carry most of the application security conversation. Each one is a mechanism, not a mood, and knowing the mechanism tells you where to look.\n\n## Quick reference\n\n- **XSS** — Untrusted input runs as script in another user's browser, so it acts with their session.\n- **CSRF** — A third-party page makes the browser send a state-changing request the user never intended.\n- **SSRF** — The server is tricked into fetching an attacker-chosen URL, including internal addresses.\n- **IDOR** — An object ID is accepted without an ownership check, so changing it reads another user's data.\n- **CORS** — A browser rule that lets a page read another origin's response only when that origin allows it.\n- **CSP** — A response header that tells the browser which scripts, styles, frames and hosts may load.\n\n## Explained\n\n### In the browser\n\nXSS, cross-site scripting, is untrusted input running as script in another user's browser, so it acts with that session. CSRF, cross-site request forgery, is a third-party page making the browser send a state-changing request the user never intended; SameSite cookies and per-request tokens are the defence.\n\n### On the server\n\nSSRF, server-side request forgery, is the server itself fetching an attacker-chosen URL, including internal addresses and cloud metadata. IDOR, insecure direct object reference, is an object ID accepted without an ownership check, so changing it reads or writes another user's data. It is an authorisation failure, not an input bug.\n\n### Policy\n\nCORS, cross-origin resource sharing, is a browser rule that lets a page read another origin's response only when that origin allows it. It is not a server breach by itself; it controls what a malicious site can read with the user's browser. CSP, content security policy, is a response header naming the scripts, styles, frames and hosts a page may load; it mitigates XSS and cannot fix an injection alone.\n\n> **The honest caveat:** the acronyms overlap. IDOR is one kind of broken authorisation, CSRF is often blocked by SameSite, and a loose CORS rule is a finding only when credentials or sensitive data are involved.\n\n> **The lesson:** name the mechanism and the fix follows.\n\n## In one line\n\n> **Learn the word, see the bug.** Six acronyms that explain most API findings.\n\n**Check the header**\n\n```bash\ncurl -sI $URL | grep -i content-security\n```\n"},{"id":"p412","n":"412","slug":"the-detection-pipeline-drawn","title":"The detection pipeline, drawn","sections":["devsecops"],"topic":"Incident response & detection","tags":["DetectionEngineering","SIEM","IncidentResponse","DevSecOps","CyberSecurity"],"series_name":"DevSecOps","design":"architecture","script":"","date":"2029-05-15","files":{"devsecops":"notes/p412-the-detection-pipeline-drawn.md"},"md":"# The detection pipeline, drawn\n\nThe 02:00 page named a host, a user and a command, and the on-call engineer had a runbook open before the coffee finished. That is not luck. It is three tiers of plumbing that existed before anyone was attacked.\n\n## Quick reference\n\n- **Sources:** Application logs (APP) · Cloud audit trail (AUDIT) · CI logs (CI)\n- **Detect:** SIEM rules (RULE) · Correlation (CORR)\n- **Respond:** Alert (PAGE) · Runbook (RUN) · Containment (HOLD)\n\n**Request flow:** Application logs → SIEM rules → Correlation → Alert → Runbook → Containment; Cloud audit trail → SIEM rules; CI logs → Correlation\n\n## Explained\n\n### Sources\n\nApplication logs carry requests, authentication events and the errors the code chose to print. The cloud audit trail records every management API call with the identity, the source address and whether it succeeded. CI logs record deploys, token use and the jobs that touched production. If a source is missing, the tier above it has nothing to match.\n\n### Detect\n\nSIEM rules match one event: a root login, a policy change, a key created outside the change window. Correlation joins several events inside a window, so a burst of failed logins followed by one success becomes a signal rather than two hundred rows. Rules are code: versioned, reviewed and replayed against a fixture before they page anyone.\n\n### Respond\n\nThe alert carries severity, the evidence and a link to the runbook. The runbook names the first three moves: verify the finding, scope what else the identity touched, revoke or isolate. Containment is the last tier, and the one that buys the investigation its time.\n\n> **The honest caveat:** detection is bounded by what the application logs and how long it is kept. Buying a SIEM does not create the log line that was never written.\n\n> **The lesson:** detection is a pipeline, and its slowest tier sets the response time.\n\n## In one line\n\n> **A log line is a promise to act.** Three tiers stand between the event and the response.\n\n**Trace one alert**\n\n```bash\njq '.Records[0].eventName' trail.json\n```\n"},{"id":"p413","n":"413","slug":"the-detection-engineering-cheat-sheet","title":"The detection engineering cheat sheet","sections":["devsecops"],"topic":"Incident response & detection","tags":["DetectionEngineering","SIEM","Sigma","CyberSecurity","DevSecOps"],"series_name":"DevSecOps","design":"cheatsheet","script":"","date":"2029-05-17","files":{"devsecops":"notes/p413-the-detection-engineering-cheat-sheet.md"},"md":"# The detection engineering cheat sheet\n\nA detection engineer's day is four kinds of command: find the log, query it, write the rule, prove it fires. Twenty commands, five per block.\n\n## Quick reference\n\n### Logs\n\n```bash\ntail -f /var/log/auth.log\njournalctl -u sshd --since -1h\ngrep -c 'Failed password' auth.log\njq '.Records[] | .eventName' trail.json\naws cloudtrail lookup-events --max-items 5\n```\n\n### Queries\n\n```bash\nindex=web status=5xx | stats count by uri\nsearch errorCode=AccessDenied\n| where eventName=\"ConsoleLogin\"\nearliest=-24h index=cloudtrail\n| top limit=10 sourceIPAddress\n```\n\n### Rules\n\n```bash\nsigma list rules/\nsigma convert -t splunk rules/\nsigma check rules/\nyq '.detection.selection' rules/ssh.yml\ngrep -n 'level: high' rules/*.yml\n```\n\n### Testing\n\n```bash\nevtx_dump security.evtx | grep 4625\nchainsaw hunt evtx/ -s sigma/\ngrep 'ALERT' alerts.log | wc -l\njq -e '.Records | length' trail.json\npython3 -m json.tool trail.json\n```\n\n## Explained\n\n### Logs\n\nStart where evidence lands. `tail -f /var/log/auth.log` follows authentication live, `journalctl -u sshd --since -1h` asks systemd for the hour, and `grep -c 'Failed password'` counts the failures. For cloud events, `jq '.Records[] | .eventName' trail.json` lists what happened, and `aws cloudtrail lookup-events --max-items 5` asks the API when the local file is gone.\n\n### Queries\n\nA query turns events into a number. `index=web status=5xx | stats count by uri` groups errors by endpoint, search errorCode=AccessDenied finds refused calls, and | where eventName=\"ConsoleLogin\" narrows to sign-ins. `earliest=-24h index=cloudtrail` sets the window, and | top limit=10 sourceIPAddress ranks the loudest addresses.\n\n### Rules\n\nA rule is code. sigma list rules/ shows what you have, sigma convert -t splunk rules/ turns one format into another, and sigma check rules/ validates the schema. `yq '.detection.selection' rules/ssh.yml` reads the matching block, and `grep -n 'level: high' rules/*.yml` finds the rules that page.\n\n### Testing\n\nUntested rules are guesses. evtx_dump security.evtx | grep 4625 replays failed logons, and chainsaw hunt evtx/ -s sigma/ replays the rule set. `grep 'ALERT' alerts.log | wc -l` counts what fired, `jq -e '.Records | length' trail.json` proves the fixture parses, and `python3 -m json.tool trail.json` checks it before CI does.\n\n> **The honest caveat:** sigma check validates structure, not truth. A rule can pass every lint and still match nothing in your environment.\n\n> **The lesson:** the command is the easy part; the window and the field are the detection.\n\n## In one line\n\n> **Query the log, test the rule.** A rule you have not replayed is a rule you do not know.\n\n**Start with one event**\n\n```bash\njq '.Records[0]' trail.json\n```\n"},{"id":"p414","n":"414","slug":"four-siem-detection-patterns","title":"Four SIEM detection patterns","sections":["devsecops"],"topic":"Incident response & detection","tags":["SIEM","DetectionEngineering","CyberSecurity","CloudSecurity","DevSecOps"],"series_name":"DevSecOps","design":"explainer","script":"","date":"2029-05-20","files":{"devsecops":"notes/p414-four-siem-detection-patterns.md"},"md":"# Four SIEM detection patterns\n\nFour alerts fire on the same Tuesday, and only one is an incident. The difference is the pattern behind the rule, and every pattern needs a normal.\n\n## Quick reference\n\n### Brute force\n\n- Many failed logins, one account, short window\n- Ten 4625 events in five minutes is a start\n- The success after the burst is the real alert\n\n### Impossible travel\n\n- Two logins, two cities, less time than a flight\n- Compare the source IP, the clock and the distance\n- VPN egress and carrier NAT make this noisy\n\n### Privilege escalation\n\n- A policy attach outside the change window\n- CreateAccessKey, AttachRolePolicy, AddUserToGroup\n- Watch the first admin action after the grant\n\n### Data exfiltration\n\n- A read spike, then a transfer to a strange host\n- S3 GetObject volume, then a large egress bill\n- Baseline the account, then alert on the change\n\n> **Note:** Each pattern is a baseline plus a threshold; the baseline is the hard part.\n\n## Explained\n\n### Brute force\n\nMany failed logins, one account, a short window. Ten Windows 4625 events in five minutes is a start, and the same count of failed passwords in `auth.log` is the Linux version. The real alert is the success after the burst.\n\n### Impossible travel\n\nTwo logins, two cities, less time than a flight takes. Compare the source IP, the clock and the distance, and set a speed no human reaches. VPN egress and carrier NAT break the geography, so this pattern produces more false positives than the other three.\n\n### Privilege escalation\n\nThe signal is a policy attach outside the change window. CreateAccessKey, AttachRolePolicy and AddUserToGroup are the three calls that hand out or keep power, and the signal is the first admin action after the grant.\n\n### Data exfiltration\n\nA read spike, then a transfer to a strange host. S3 GetObject volume and the egress bill are the two numbers to baseline, per account and per hour. Alert on the change from the normal, because a large number on a busy account is not the same as a large number on a quiet one.\n\n> **The honest caveat:** impossible travel is the weakest of the four. A VPN makes one person look like two, and a shared office makes two people look like one.\n\n> **The lesson:** a pattern is a baseline plus a threshold, and the baseline is the part you have to earn.\n\n## In one line\n\n> **A pattern is a baseline first.** The rule is the easy half; the normal is the other half.\n\n**Check the window**\n\n```bash\ngrep -c 4625 security.log\n```\n"},{"id":"p415","n":"415","slug":"a-cloudtrail-detection-script-in-python","title":"A CloudTrail detection script in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","CloudTrail","DetectionEngineering","DevSecOps","CloudSecurity"],"series_name":"DevSecOps","design":"code","script":"cloudtrail_detect.py","date":"2029-05-22","files":{"devsecops":"notes/p415-a-cloudtrail-detection-script-in-python.md"},"md":"# A CloudTrail detection script in Python\n\nThe CloudTrail export was 40 MB and the answer was four events: a root login, a policy change, a bucket policy change and a new access key. Nobody had read the file, because nobody reads 40 MB.\n\n## Quick reference\n\nThe full script is at the end of this note: `cloudtrail_detect.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cloudtrail_detect.py trail.json\n```\n\n**Example output**\n\n```text\nALERT root login\nfindings: 4 of 6\n```\n\n## Explained\n\n### What it does\n\nIt loads the JSON and walks Records. Each record has an eventName, a userIdentity, a sourceIPAddress and, for a refused call, an errorCode. The script keeps the four shapes that matter: a ConsoleLogin whose userIdentity type is Root, an IAM policy call such as PutUserPolicy or AttachRolePolicy, an S3 policy call such as PutBucketPolicy or PutBucketAcl, and CreateAccessKey.\n\n### What it skips\n\nA record with an errorCode changed nothing, so the loop skips it. The full script adds `--include-denied`, which reports those attempts separately, because a denied root login is still a signal.\n\n### What it prints\n\nOne ALERT line per finding, then a count of findings against records read. Exit 1 when anything is found, 0 when clean, and 2 when the file is missing or not a CloudTrail export, because a check that cannot run is not a pass.\n\n### How to use it\n\n`python3 cloudtrail_detect.py trail.json`, from cron after the daily download or as a pipeline step. The attached script is standard library only, one read, no AWS call.\n\n> **The honest caveat:** CloudTrail records management events by default. S3 object reads are data events, priced separately and often off, so a quiet exfiltration may never appear here.\n\n> **The lesson:** the audit trail is evidence only when something reads it on a schedule.\n\n## In one line\n\n> **The trail knows who did what.** An audit log nobody reads is a file, not a control.\n\n**Read one event**\n\n```bash\njq '.Records[0]' trail.json\n```\n\n## Script: cloudtrail_detect.py\n\nDownload: [scripts/cloudtrail_detect.py](../scripts/cloudtrail_detect.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Flag risky events in a CloudTrail JSON export.\n\nUsage:\n    python3 cloudtrail_detect.py trail.json\n    python3 cloudtrail_detect.py trail.json --include-denied\n    python3 cloudtrail_detect.py trail.json --json\n    python3 cloudtrail_detect.py --help\n\nReads the Records[] array of a CloudTrail JSON export and reports four\nshapes that matter:\n\n    root console logins        ConsoleLogin with userIdentity.type == Root\n    IAM policy changes         Put*, Attach*, Detach*, Delete*Policy calls\n                               and policy version changes\n    S3 policy changes          PutBucketPolicy, DeleteBucketPolicy,\n                               PutBucketAcl, PutBucketPublicAccessBlock\n    access key creation        CreateAccessKey\n\nBy default a record with an errorCode is skipped: the call was refused and\nchanged nothing. --include-denied reports those attempts as well.\n\nExit codes: 0 clean, 1 findings, 2 usage or input error.\nRead-only: the trail file is opened for reading and never modified.\n\nCron: 15 6 * * * python3 /opt/s0sta/cloudtrail_detect.py /var/tmp/trail.json --json >> /var/log/cloudtrail.json\nCI:   python3 cloudtrail_detect.py trail.json || exit 1\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport json\nimport sys\n\nROOT_EVENT = \"ConsoleLogin\"\n\nIAM_EVENTS = frozenset({\n    \"PutUserPolicy\", \"PutRolePolicy\", \"PutGroupPolicy\",\n    \"AttachUserPolicy\", \"AttachRolePolicy\", \"AttachGroupPolicy\",\n    \"DetachUserPolicy\", \"DetachRolePolicy\", \"DetachGroupPolicy\",\n    \"DeleteUserPolicy\", \"DeleteRolePolicy\", \"DeleteGroupPolicy\",\n    \"CreatePolicyVersion\", \"SetDefaultPolicyVersion\",\n    \"UpdateAssumeRolePolicy\",\n})\n\nS3_EVENTS = frozenset({\n    \"PutBucketPolicy\", \"DeleteBucketPolicy\",\n    \"PutBucketAcl\", \"PutBucketPublicAccessBlock\",\n})\n\nKEY_EVENTS = frozenset({\"CreateAccessKey\"})\n\nLABELS = {\n    \"root\": \"root login\",\n    \"iam\": \"iam policy\",\n    \"s3\": \"s3 policy\",\n    \"key\": \"access key\",\n}\n\n\nclass InputError(Exception):\n    \"\"\"A trail that cannot be read or does not look like CloudTrail.\"\"\"\n\n\ndef identity_name(identity):\n    \"\"\"Best available name for the caller, in descending order of usefulness.\"\"\"\n    for key in (\"arn\", \"userName\", \"principalId\"):\n        value = identity.get(key)\n        if value:\n            return str(value)\n    return str(identity.get(\"type\") or \"unknown\")\n\n\ndef classify(record):\n    \"\"\"Return (kind, eventName) for a record, or None if it is not a hit.\"\"\"\n    event = str(record.get(\"eventName\") or \"\")\n    identity = record.get(\"userIdentity\")\n    if not isinstance(identity, dict):\n        identity = {}\n\n    if event == ROOT_EVENT and identity.get(\"type\") == \"Root\":\n        return \"root\", event\n    if event in IAM_EVENTS:\n        return \"iam\", event\n    if event in S3_EVENTS:\n        return \"s3\", event\n    if event in KEY_EVENTS:\n        return \"key\", event\n    return None\n\n\ndef load_records(path):\n    \"\"\"Read a CloudTrail export and return its records list.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            document = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n\n    if isinstance(document, dict):\n        records = document.get(\"Records\")\n    elif isinstance(document, list):\n        records = document\n    else:\n        records = None\n\n    if not isinstance(records, list):\n        raise InputError(\"%s has no Records[] array\" % path)\n    return records\n\n\ndef scan(records, include_denied=False):\n    \"\"\"Return (findings, denied_skipped, records_read).\"\"\"\n    findings = []\n    skipped = 0\n    read = 0\n\n    for record in records:\n        if not isinstance(record, dict):\n            continue\n        read += 1\n        hit = classify(record)\n        if hit is None:\n            continue\n        kind, event = hit\n        denied = bool(record.get(\"errorCode\"))\n        if denied and not include_denied:\n            skipped += 1\n            continue\n\n        identity = record.get(\"userIdentity\")\n        if not isinstance(identity, dict):\n            identity = {}\n        findings.append({\n            \"kind\": LABELS[kind],\n            \"event\": event,\n            \"time\": str(record.get(\"eventTime\") or \"\"),\n            \"identity\": identity_name(identity),\n            \"source\": str(record.get(\"sourceIPAddress\") or \"\"),\n            \"region\": str(record.get(\"awsRegion\") or \"\"),\n            \"denied\": denied,\n            \"error\": str(record.get(\"errorCode\") or \"\"),\n        })\n\n    return findings, skipped, read\n\n\ndef print_text(findings, skipped, read):\n    for item in findings:\n        line = \"ALERT %s %s %s %s %s\" % (\n            item[\"kind\"], item[\"time\"] or \"-\", item[\"identity\"],\n            item[\"source\"] or \"-\", item[\"region\"] or \"-\")\n        if item[\"denied\"]:\n            line += \" DENIED \" + (item[\"error\"] or \"\")\n        print(line)\n    print(\"cloudtrail: %d finding%s in %d record%s (%d denied skipped)\"\n          % (len(findings), \"\" if len(findings) == 1 else \"s\",\n             read, \"\" if read == 1 else \"s\", skipped))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"cloudtrail_detect.py\",\n        description=\"Flag root logins, IAM and S3 policy changes and new \"\n                    \"access keys in a CloudTrail JSON export.\")\n    parser.add_argument(\"trail\", help=\"CloudTrail JSON export (Records[] array)\")\n    parser.add_argument(\"--include-denied\", action=\"store_true\",\n                        help=\"also report refused calls that carry an errorCode\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON object instead of text lines\")\n    args = parser.parse_args(argv)\n\n    try:\n        records = load_records(args.trail)\n    except InputError as exc:\n        parser.error(str(exc))\n\n    findings, skipped, read = scan(records, args.include_denied)\n\n    if args.json:\n        print(json.dumps({\n            \"records\": read,\n            \"findings\": findings,\n            \"findings_count\": len(findings),\n            \"denied_skipped\": skipped,\n            \"clean\": not findings,\n        }, indent=2, sort_keys=True))\n    else:\n        print_text(findings, skipped, read)\n\n    return 1 if findings else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p416","n":"416","slug":"a-vulnerability-sla-tracker-in-python","title":"A vulnerability SLA tracker in Python","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","DevSecOps","Python","CyberSecurity","SLA"],"series_name":"DevSecOps","design":"code","script":"sla_tracker.py","date":"2029-05-24","files":{"devsecops":"notes/p416-a-vulnerability-sla-tracker-in-python.md"},"md":"# A vulnerability SLA tracker in Python\n\nThe critical finding was fixed on day nine. The policy said seven, and nobody noticed until the quarterly report, because the clock lived in a spreadsheet.\n\n## Quick reference\n\nThe full script is at the end of this note: `sla_tracker.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 sla_tracker.py findings.csv\n```\n\n**Example output**\n\n```text\nBREACH 1 AT RISK 1\n```\n\n## Explained\n\n### What it does\n\nIt reads a CSV with four columns: id, severity, found_at and fixed_at. Each severity has a window: critical 7 days, high 30, medium 90. The age is fixed_at minus found_at, or today minus found_at while it is open. Anything older than its window is a breach.\n\n### What at-risk means\n\nAn open finding that has used 80 percent of its window is at risk, not yet breached. For critical that is day 5.6, for high day 24, for medium day 72. Flagging it early is the point.\n\n### What it prints\n\nOne line with the breach count and the at-risk count. Exit 1 when either list is non-empty, 0 when every finding is inside its window, and 2 when a date or a severity does not parse, because a tracker that cannot read a row is not a pass.\n\n### How to use it\n\n`python3 sla_tracker.py findings.csv`, every morning from cron, with the file exported by the scanner. The full script adds `--now` and `--json`, so the report can be tested and fed to a dashboard.\n\n> **The honest caveat:** the clock starts at found_at, the moment the scanner saw it. A finding triaged a week late already has a week on the clock, and the person who fixed it in two days still looks slow.\n\n> **The lesson:** an SLA is real only when a script can fail on it.\n\n## In one line\n\n> **The clock starts when it is found.** An SLA with no script behind it is a promise, not a control.\n\n**Check one row**\n\n```bash\nawk -F, 'NR>1' findings.csv\n```\n\n## Script: sla_tracker.py\n\nDownload: [scripts/sla_tracker.py](../scripts/sla_tracker.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Track vulnerability fix SLAs from a CSV.\n\nUsage:\n    python3 sla_tracker.py findings.csv\n    python3 sla_tracker.py findings.csv --now 2026-10-21\n    python3 sla_tracker.py findings.csv --now 2026-10-21 --json\n    python3 sla_tracker.py --help\n\nThe CSV has four columns:\n\n    id,severity,found_at,fixed_at\n    VULN-1042,critical,2026-10-12,2026-10-21\n    VULN-1077,high,2026-09-26,\n\nseverity is critical, high or medium. The windows are 7, 30 and 90 days.\nfound_at and fixed_at are ISO dates or datetimes; fixed_at stays empty while\na finding is open. A finding is breached when its age is past the window, and\nat risk when it is open and has used 80 percent of the window (day 5.6 for\ncritical, 24 for high, 72 for medium).\n\nExit codes: 0 inside SLA, 1 breach or at-risk finding, 2 usage or input error.\nRead-only: the CSV is opened for reading and never modified.\n\nCron: 0 7 * * * python3 /opt/s0sta/sla_tracker.py /var/tmp/findings.csv\nCI:   python3 sla_tracker.py findings.csv --now \"$(date -u +%F)\"\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport csv\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nSLA_DAYS = {\"critical\": 7, \"high\": 30, \"medium\": 90}\nCOLUMNS = (\"id\", \"severity\", \"found_at\", \"fixed_at\")\nDAY = 86400.0\n\n\nclass InputError(Exception):\n    \"\"\"A CSV row or header that cannot be used.\"\"\"\n\n\ndef parse_when(value, label):\n    \"\"\"Parse an ISO date or datetime into an aware UTC datetime.\"\"\"\n    text = (value or \"\").strip()\n    if not text:\n        return None\n    try:\n        if len(text) == 10:\n            when = datetime.strptime(text, \"%Y-%m-%d\")\n        else:\n            when = datetime.fromisoformat(text.replace(\"Z\", \"+00:00\"))\n    except ValueError:\n        raise InputError(\"bad %s: %r (expected ISO date or datetime)\"\n                         % (label, value))\n    if when.tzinfo is None:\n        when = when.replace(tzinfo=timezone.utc)\n    return when.astimezone(timezone.utc)\n\n\ndef load_rows(path):\n    \"\"\"Read the CSV and return a list of dicts.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8-sig\", newline=\"\") as handle:\n            reader = csv.DictReader(handle)\n            fields = reader.fieldnames or []\n            missing = [name for name in COLUMNS if name not in fields]\n            if missing:\n                raise InputError(\"%s is missing column(s): %s\"\n                                 % (path, \", \".join(missing)))\n            return list(reader)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except csv.Error as exc:\n        raise InputError(\"%s is not valid CSV: %s\" % (path, exc))\n\n\ndef assess(row, now, ratio):\n    \"\"\"Return a dict describing one finding's SLA state.\"\"\"\n    row_id = (row.get(\"id\") or \"\").strip() or \"?\"\n    severity = (row.get(\"severity\") or \"\").strip().lower()\n    if severity not in SLA_DAYS:\n        raise InputError(\"row %s: unknown severity %r\" % (row_id, severity))\n\n    try:\n        found = parse_when(row.get(\"found_at\"), \"found_at\")\n        fixed = parse_when(row.get(\"fixed_at\"), \"fixed_at\")\n    except InputError as exc:\n        raise InputError(\"row %s: %s\" % (row_id, exc))\n    if found is None:\n        raise InputError(\"row %s: found_at is required\" % row_id)\n\n    limit = SLA_DAYS[severity]\n    end = fixed or now\n    age = (end - found).total_seconds() / DAY\n    if age < 0:\n        raise InputError(\"row %s: fixed_at is before found_at\" % row_id)\n\n    if age > limit:\n        state = \"breach\"\n    elif fixed is None and age >= ratio * limit:\n        state = \"at-risk\"\n    else:\n        state = \"inside\"\n\n    return {\n        \"id\": row_id,\n        \"severity\": severity,\n        \"found_at\": found.strftime(\"%Y-%m-%d\"),\n        \"fixed_at\": fixed.strftime(\"%Y-%m-%d\") if fixed else \"\",\n        \"open\": fixed is None,\n        \"age_days\": round(age, 1),\n        \"sla_days\": limit,\n        \"state\": state,\n    }\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"sla_tracker.py\",\n        description=\"Report breached and at-risk vulnerability SLAs from a CSV.\")\n    parser.add_argument(\"csv\", help=\"CSV with id,severity,found_at,fixed_at\")\n    parser.add_argument(\"--now\", default=None,\n                        help=\"evaluate at this ISO date or datetime \"\n                             \"(default: now, UTC)\")\n    parser.add_argument(\"--at-risk-ratio\", type=float, default=0.8,\n                        help=\"fraction of the window used before at-risk \"\n                             \"(default: 0.8)\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print one JSON object instead of text lines\")\n    args = parser.parse_args(argv)\n\n    if not 0 < args.at_risk_ratio <= 1:\n        parser.error(\"--at-risk-ratio must be greater than 0 and at most 1\")\n\n    try:\n        now = parse_when(args.now, \"--now\") if args.now else \\\n            datetime.now(timezone.utc)\n        # parse_when returns None only for an empty string.\n        if now is None:\n            parser.error(\"--now cannot be empty\")\n        rows = load_rows(args.csv)\n        results = [assess(row, now, args.at_risk_ratio) for row in rows]\n    except InputError as exc:\n        parser.error(str(exc))\n\n    breaches = [r for r in results if r[\"state\"] == \"breach\"]\n    at_risk = [r for r in results if r[\"state\"] == \"at-risk\"]\n    inside = len(results) - len(breaches) - len(at_risk)\n\n    if args.json:\n        print(json.dumps({\n            \"now\": now.strftime(\"%Y-%m-%dT%H:%M:%SZ\"),\n            \"rows\": len(results),\n            \"breaches\": breaches,\n            \"at_risk\": at_risk,\n            \"inside_sla\": inside,\n            \"clean\": not breaches and not at_risk,\n        }, indent=2, sort_keys=True))\n    else:\n        for item in breaches:\n            print(\"BREACH %s %s %.1fd of %dd\" % (\n                item[\"id\"], item[\"severity\"],\n                item[\"age_days\"], item[\"sla_days\"]))\n        for item in at_risk:\n            print(\"AT RISK %s %s %.1fd of %dd\" % (\n                item[\"id\"], item[\"severity\"],\n                item[\"age_days\"], item[\"sla_days\"]))\n        print(\"SLA: %d breached, %d at risk, %d inside, %d %s\"\n              % (len(breaches), len(at_risk), inside, len(results),\n                 \"row\" if len(results) == 1 else \"rows\"))\n\n    return 1 if breaches or at_risk else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p417","n":"417","slug":"anatomy-of-an-audit-event","title":"Anatomy of an audit event","sections":["devsecops"],"topic":"Incident response & detection","tags":["CloudTrail","AWS","DetectionEngineering","IncidentResponse","DevSecOps"],"series_name":"DevSecOps","design":"anatomy","script":"","date":"2029-05-27","files":{"devsecops":"notes/p417-anatomy-of-an-audit-event.md"},"md":"# Anatomy of an audit event\n\nOne line from the audit trail is enough to open an investigation, if you know which six fields to read. The line is normalised for the poster: raw CloudTrail spreads these facts across userIdentity, errorCode and awsRegion.\n\n## Quick reference\n\n```text\nAssumeRole user=deploy 10.0.4.7 mfa=false result=Denied eu-west-1\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `AssumeRole` | Event | The API call. A rule matches this field first. |\n| `user=deploy` | Identity | The role or IAM user behind the call. |\n| `10.0.4.7` | Source IP | Where the call came from. Compare it with the baseline. |\n| `mfa=false` | MFA used | Whether the session presented a second factor. |\n| `result=Denied` | Result | Denied means errorCode was set; nothing changed. |\n| `eu-west-1` | Region | The region targeted. Drift from the usual one is a signal. |\n\n## Explained\n\n### The six fields\n\nAssumeRole is the event name, the call that hands out temporary credentials, and the first field a rule matches. `user=deploy` is the identity: a role or an IAM user, and the difference matters, because a role can be assumed by anything that can reach its trust policy. 10.0.4.7 is the source address, a number on its own and a location against the baseline; this one sits in the build subnet, so a call from here at 02:00 is worth a look.\n\n`mfa=false` is the flag that turns a suspicious login into a finding, because the session presented no second factor. `result=Denied` means an errorCode was set and the call changed nothing, though a run of denied calls from one identity is a script learning the permissions it lacks. eu-west-1 is the region, and a call from a region the account has never used is the cheapest anomaly to alert on.\n\n### Why the line works\n\nA rule reads the fields together: AssumeRole, no MFA, denied, from a new region. Any single field is a fact; the combination is a detection.\n\n> **The honest caveat:** this is a display form, not the API shape. Build the parser on Records, userIdentity.type and errorCode, then format the line.\n\n> **The lesson:** a detection rule is only as good as the fields it can name.\n\n## In one line\n\n> **One line, six fields to check.** The fields separate a fact from a finding.\n\n**Read one record**\n\n```bash\njq '.Records[0].eventName' trail.json\n```\n"},{"id":"p418","n":"418","slug":"security-operations-at-a-glance","title":"Security operations at a glance","sections":["devsecops"],"topic":"Incident response & detection","tags":["SecurityOperations","SOC","DevSecOps","CyberSecurity","SRE"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-05-29","files":{"devsecops":"notes/p418-security-operations-at-a-glance.md"},"md":"# Security operations at a glance\n\nThe weekly review opened with a green dashboard and closed with two questions nobody could answer: how long did we take to notice, and how often were we wrong?\n\n## Quick reference\n\n**Status:** This week\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Open incidents | **7** | 3 high or critical |\n| MTTD | **18 min** | median, last 30 days |\n| MTTR | **4h 10m** | alert to service restored |\n| SLA breaches | **2** | one critical, overdue |\n| Detections fired | **146** | this week, all rules |\n| False positives | **21%%** | of 146 alerts |\n\n## Explained\n\n### The week\n\nOpen incidents: seven, three of them high or critical. A count is not a workload: two incidents with a named owner and a plan are lighter than seven nobody has touched.\n\nMTTD: eighteen minutes, median over thirty days. Mean time to detect runs from the first event to the alert, so a rule with a one-hour window can never score below an hour.\n\nMTTR: four hours ten minutes. Mean time to resolve runs from the alert to the service restored, and the definition is where meetings go wrong: resolve, respond and remediate are three different stop clocks.\n\nSLA breaches: two, one critical and overdue. A breach is a finding about the process, not about the person who was on call when it landed.\n\n### The signal\n\nDetections fired: one hundred and forty-six this week, across every rule. The number says how much the programme produces; it says nothing about how much of it was true.\n\nFalse positives: twenty-one percent of the 146 alerts. A fifth is tolerable if the queue stays short, and expensive if every one of those thirty-one alerts woke somebody.\n\n> **The honest caveat:** a mean hides the tail. One twenty-six-hour incident disappears inside a four-hour average, so read the median and the ninetieth percentile too.\n\n> **The lesson:** six numbers are a way to ask better questions, not the answer to the review.\n\n## In one line\n\n> **A dashboard is a starting point.** Six numbers open the review; they do not close it.\n\n**Read the queue**\n\n```bash\ngrep -c 'high' alerts.log\n```\n"},{"id":"p419","n":"419","slug":"compliance-posture-at-a-glance","title":"Compliance posture at a glance","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Compliance","GRC","CyberSecurity","DevSecOps","ISO27001"],"series_name":"DevSecOps","design":"dashboard","script":"","date":"2029-05-31","files":{"devsecops":"notes/p419-compliance-posture-at-a-glance.md"},"md":"# Compliance posture at a glance\n\nThe auditor asked for three things and the team produced a spreadsheet, a screenshot and an apology. Compliance is a maintenance programme, and these six tiles are what maintenance looks like.\n\n## Quick reference\n\n**Status:** Q4\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Controls passing | **92%%** | of 148 controls |\n| Evidence current | **88%%** | refreshed this quarter |\n| Open exceptions | **4** | 2 expire this month |\n| Last audit | **12 days** | external, ISO 27001 scope |\n| Findings closed | **37** | of 41 from the audit |\n| Owner coverage | **96%** | of assets with a named owner |\n\n## Explained\n\n### The register\n\nControls passing: ninety-two percent of 148, so 136 controls have evidence behind them this quarter. The twelve that do not are the agenda: a control that passes on paper is worse than a red one.\n\nEvidence current: eighty-eight percent refreshed this quarter. Evidence has a shelf life: a screenshot from February proves February, and an auditor will ask for the date on the file.\n\nOpen exceptions: four, two expiring this month. An exception is a decision with a date on it; one without an expiry is a permanent hole with a ticket number.\n\n### The audit\n\nLast audit: twelve days ago, external, ISO 27001 scope. The date matters less than the findings list it produced, because the list is the work plan for the next quarter.\n\nFindings closed: thirty-seven of forty-one. Closure rate is the honest measure of a compliance programme, and the four that remain are the ones to talk about first.\n\n### The owners\n\nOwner coverage: ninety-six percent of assets have a named owner. The last four percent are the assets nobody will claim, and those are the ones an attacker finds.\n\n> **The honest caveat:** a passing control is a snapshot of the day it was tested. Drift between audits is normal, which is why the exceptions register, not the pass rate, is where the truth lives.\n\n> **The lesson:** compliance is maintained between audits, not demonstrated during them.\n\n## In one line\n\n> **Evidence first, opinions later.** A control is a claim until the evidence is in the folder.\n\n**Check the register**\n\n```bash\ngrep -c 'open' register.csv\n```\n"},{"id":"p420","n":"420","slug":"from-alert-to-containment","title":"From alert to containment","sections":["devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","DFIR","CyberSecurity","DevSecOps","SOC"],"series_name":"DevSecOps","design":"timeline","script":"","date":"2029-06-03","files":{"devsecops":"notes/p420-from-alert-to-containment.md"},"md":"# From alert to containment\n\nThe alert arrived at 02:14. The credential was revoked at 02:39, and the review was written five days later. Between those two clocks sits the whole discipline.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0** | Detected | The rule fires and the alert reaches the on-call queue. |\n| **T+8m** | Triaged | Scope, severity and owner agreed in one thread. |\n| **T+25m** | Contained | The credential is revoked and the host isolated. |\n| **T+2h** | Eradicated | The entry point is closed and the implant removed. |\n| **T+6h** | Recovered | Clean capacity serves traffic and the backlog drains. |\n| **T+5d** | Reviewed | Timeline, cause and the control that changes. |\n\n## Explained\n\n### The first hour\n\nDetected, T+0. The rule fires and the alert reaches the on-call queue. This is the only phase where the clock is not yours: the log latency, the rule window and the paging path decide how long it took.\n\nTriaged, T+8m. Scope, severity and owner agreed in one thread. Triage answers three questions: what is affected, how bad is it, and who decides. Without an owner, the next four phases wait.\n\nContained, T+25m. The credential is revoked and the host isolated. Containment buys time and costs evidence: powering off loses volatile memory, so capture it first when the host matters.\n\n### The next day\n\nEradicated, T+2h. The entry point is closed and the implant removed. If you skip this and go straight to recovery, the attacker returns through the same door.\n\nRecovered, T+6h. Clean capacity serves traffic and the backlog drains. Recovery is a watched state: keep the heightened logging for a week, because the second attempt usually follows the first.\n\n### The review\n\nReviewed, T+5d. Timeline, cause and the control that changes. The review is the only phase that improves the next incident, and it is the one that gets cancelled when the week is busy.\n\n> **The honest caveat:** the phases overlap in practice. Containment starts before triage finishes, and recovery begins while eradication is still running.\n\n> **The lesson:** speed in the first hour is bought by the runbook written last quarter.\n\n## In one line\n\n> **The clock does not stop at containment.** Six phases, and the last one is where the next incident is prevented.\n\n**Time the phases**\n\n```bash\ndate -u +%FT%TZ\n```\n"},{"id":"p421","n":"421","slug":"detection-and-response-terms-decoded","title":"Detection and response terms, decoded","sections":["devsecops"],"topic":"Incident response & detection","tags":["DetectionEngineering","IncidentResponse","SOC","DevSecOps","CyberSecurity"],"series_name":"DevSecOps","design":"reveal","script":"","date":"2029-06-05","files":{"devsecops":"notes/p421-detection-and-response-terms-decoded.md"},"md":"# Detection and response terms, decoded\n\nSix terms come up in every incident review, and two of them are usually being defined differently by the people in the room. Definitions are the contract.\n\n## Quick reference\n\n- **MTTD** — Mean time to detect: from the first event to the alert, averaged over incidents.\n- **MTTR** — Mean time to resolve: from the alert to the service restored, averaged over incidents.\n- **SIEM** — Security information and event management: it collects logs, runs rules and queues the alerts.\n- **SOAR** — Security orchestration, automation and response: runbooks that enrich and act without a human.\n- **IOC** — Indicator of compromise: a hash, an IP or a domain seen in an attack; useful, never proof.\n- **Containment** — Stop the spread first: revoke the credential, isolate the host, then investigate.\n\n## Explained\n\n### The metrics\n\nMTTD, mean time to detect, runs from the first event to the alert. It is the metric the attacker influences most, and it depends on log latency, rule windows and retention.\n\nMTTR, mean time to resolve, runs from the alert to the service restored. Some teams use it for respond, so the definition goes in the report header.\n\n### The platform\n\nSIEM, security information and event management. It collects logs, normalises the fields, runs the rules and queues the alerts. The queue is the product, and retention is the bill.\n\nSOAR, security orchestration, automation and response. Runbooks that enrich an alert, check a reputation feed and revoke a credential without a human. It is only as good as its last review.\n\n### The incident\n\nIOC, indicator of compromise: a hash, an IP or a domain seen in an attack. Matching one is a lead, never proof, because a good attacker rotates them.\n\nContainment: stop the spread first. Revoke the credential, isolate the host, block the address, then investigate. Eradication comes after, not instead.\n\n> **The honest caveat:** every number here can be gamed. Closing an incident early improves MTTR, and a narrow MTTD definition flatters the detection team.\n\n> **The lesson:** define the stop clock before an incident.\n\nThat is the series. Fifty posts, from a threat model to a contained host, five phases and seven Python scripts. Thank you for following along.\n\n## In one line\n\n> **Detection is a programme, not a tool.** Fifty posts, one idea: the alert is the start of the work.\n\n**Name the metric**\n\n```bash\ngrep -i 'mttd' incident.md\n```\n"},{"id":"p422","n":"422","slug":"the-release-path-drawn","title":"The release path, drawn","sections":["devsecops"],"topic":"Release & build engineering","tags":["DevOps","ReleaseEngineering","CICD","Kubernetes","SRE"],"series_name":"DevOps","design":"architecture","script":"","date":"2029-06-07","files":{"devsecops":"notes/p422-the-release-path-drawn.md"},"md":"# The release path, drawn\n\nA release often starts with a rebuild, and that is the first mistake. The pipeline that tested the image should not build another one for production; it should promote what it already signed.\n\n## Quick reference\n\n- **Artifact:** Signed image (IMG) · Release manifest (MAN)\n- **Environment:** Staging (STG) · Pre-production (PRE)\n- **Traffic:** Canary (CAN) · Stable (STB)\n\n**Request flow:** Signed image → Release manifest → Staging → Pre-production → Canary → Stable\n\n## Explained\n\n### The artefact\n\nThe first tier is ARTIFACT: a signed image, with a digest that identifies the exact bytes, and a manifest that records the digest, the source commit and the build. Signing does not make the image safe; it proves who built it and that nobody swapped it afterwards.\n\n### The environment\n\nThe second tier is ENVIRONMENT: staging, then pre-production. Both receive the same digest, never a fresh build. Staging answers \"does it work\"; pre-production answers \"does it work with production-shaped configuration and data\". A rebuild here would quietly invalidate every test that came before.\n\n### The traffic\n\nThe third tier is TRAFFIC: a canary and a stable slice. The canary takes a small share, and you compare its error rate and latency against the stable version. Then it grows, or the traffic returns. The arrows run from the image to the manifest, into staging and pre-production, then to the canary and stable.\n\n> **The honest caveat:** a signature proves provenance, not quality. A signed image can still carry a bad migration.\n\n> **The lesson:** build once, sign once, then move the same digest down the path, and let each environment answer one question.\n\n## In one line\n\n> **Promote, do not rebuild.** One signed digest, every environment, no second build.\n\n**Promote the digest**\n\n```bash\nkubectl set image deploy/api api=$IMAGE\n```\n"},{"id":"p423","n":"423","slug":"the-git-release-cheat-sheet","title":"The git release cheat sheet","sections":["devsecops"],"topic":"Release & build engineering","tags":["Git","ReleaseEngineering","DevOps","CICD","SoftwareEngineering"],"series_name":"DevOps","design":"cheatsheet","script":"","date":"2029-06-10","files":{"devsecops":"notes/p423-the-git-release-cheat-sheet.md"},"md":"# The git release cheat sheet\n\nEvery release ends with the same four questions: what did we tag, what changed, how do we fix it, and how do we undo it? The commands are short; the order is what people get wrong.\n\n## Quick reference\n\n### Tag\n\n```bash\ngit tag -a v1.4.0 -m \"release 1.4.0\"\ngit tag --list 'v1.*' --sort=-v:refname\ngit push origin v1.4.0\ngit show v1.4.0 --stat\ngit tag -d v1.4.0\n```\n\n### Changelog\n\n```bash\ngit log --pretty=format:%s v1.3.0..v1.4.0\ngit log --oneline v1.3.0..v1.4.0\ngit shortlog -sn v1.3.0..v1.4.0\ngit log --grep='^feat' v1.3.0..v1.4.0\ngit log --format='%h %s' -n 20\n```\n\n### Hotfix\n\n```bash\ngit switch -c hotfix/1.4.1 v1.4.0\ngit commit -am \"fix: tax rounding\"\ngit tag -a v1.4.1 -m \"hotfix 1.4.1\"\ngit push origin hotfix/1.4.1 v1.4.1\ngit cherry-pick -x 9f2c1ab\n```\n\n### Revert\n\n```bash\ngit revert --no-edit 9f2c1ab\ngit revert -m 1 4d2e8f0\ngit revert --no-commit v1.4.0..HEAD\ngit revert --abort\ngit log --oneline -5\n```\n\n## Explained\n\n### Tag\n\nAnnotated tags carry a message and a date; lightweight tags do not. `git tag -a` tags the commit CI passed, git push origin publishes it, and `git tag --sort=-v:refname` lists the newest first.\n\n### Changelog\n\n`git log --pretty=format:'%s'` between two tags prints one subject per commit, which is the raw material for release notes. `--oneline` adds the short hash; shortlog -sn counts contributions per author; `--grep='^feat'` isolates the features. None of them writes the story, but they stop you inventing it.\n\n### Hotfix\n\nBranch from the running tag, not a later main: `git switch -c hotfix/1.4.1 v1.4.0`. Commit the smallest fix, tag it, push the branch and the tag, then merge the fix back so the next release contains it. cherry-pick -x copies a fix and records where it came from.\n\n### Revert\n\ngit revert creates a commit that undoes a change, which is safe on a shared branch because history stays intact. For a merge, -m 1 chooses the mainline parent. A range with `--no-commit` lets you read first; `--abort` exits.\n\n> **The honest caveat:** deleting and re-pushing a tag breaks every clone that already fetched it. A tag is a name people pin to, so cut a new patch version instead of moving one.\n\n> **The lesson:** a release cheat sheet is not about typing fast; it is about never guessing which commit is live.\n\n## In one line\n\n> **Tag what shipped, not what might.** The tag is the receipt; the changelog is the story.\n\n**Read the range**\n\n```bash\ngit log --oneline v1.3.0..v1.4.0\n```\n"},{"id":"p424","n":"424","slug":"canary-blue-green-and-rolling-drawn","title":"Canary, blue-green and rolling, drawn","sections":["devsecops"],"topic":"Release & build engineering","tags":["DevOps","ReleaseEngineering","Kubernetes","ProgressiveDelivery","SRE"],"series_name":"DevOps","design":"explainer","script":"","date":"2029-06-12","files":{"devsecops":"notes/p424-canary-blue-green-and-rolling-drawn.md"},"md":"# Canary, blue-green and rolling, drawn\n\nThe deployment strategy decides how much damage a bad build can do. Rolling, blue-green and canary answer that question differently.\n\n## Quick reference\n\n### Rolling update\n\n- Replaces instances in batches\n- No extra environment, needs surge capacity\n- Rollback is another rollout, not instant\n- The Kubernetes default for stateless apps\n\n### Blue-green\n\n- Two identical environments, one router\n- The switch sends all traffic at once\n- Rollback is a switch back in seconds\n- Costs double capacity at cutover\n\n### Canary\n\n- A small share of real traffic goes first\n- Compare errors and latency to stable\n- Raise the share only when the numbers hold\n- Needs traffic splitting and metrics\n\n### When each fits\n\n- Rolling: stateless, schema-compatible\n- Blue-green: instant switch, spare capacity\n- Canary: high traffic, risky change, metrics\n- All three need a signal you trust\n\n> **Note:** The strategy decides how much risk you take before you know.\n\n## Explained\n\n### Rolling\n\nA rolling update replaces instances in batches. Kubernetes starts new pods, waits for readiness, then removes old ones, so versions overlap. It needs no second environment, only surge capacity, and the default is a quarter of the replicas at a time. Rollback is another rolling update in reverse, which is safe but not instant.\n\n### Blue-green\n\nBlue-green runs two identical environments. Blue serves all traffic while green receives the new release and its smoke tests; then the router switches, and rollback is a switch back. The switch is instant, the cost is double capacity at cutover, and the shared database is where most plans meet reality.\n\n### Canary\n\nA canary sends a small share of real traffic to the new version, compares error rate and latency against the stable baseline, then raises the share. That comparison needs traffic splitting and metrics good enough to separate signal from noise.\n\n### Choosing\n\nRolling fits stateless services with a compatible schema. Blue-green fits when the switch must be instant and you can pay for two environments. Canary fits high-traffic changes where a small exposure is cheaper than a big one.\n\n> **The honest caveat:** a canary on a low-traffic endpoint can run for hours without enough requests to prove anything. It is only as good as the traffic it sees.\n\n> **The lesson:** choose by how fast you can see a failure, not by which one looks modern.\n\n## In one line\n\n> **Route traffic by evidence.** Three strategies, one question: how fast can you see a bad release?\n\n**Advance the canary**\n\n```bash\nkubectl argo rollouts promote api\n```\n"},{"id":"p425","n":"425","slug":"a-changelog-generator-in-python","title":"A changelog generator in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Git","Python","ReleaseEngineering","CICD","DevOps"],"series_name":"DevOps","design":"code","script":"changelog.py","date":"2029-06-14","files":{"devsecops":"notes/p425-a-changelog-generator-in-python.md"},"md":"# A changelog generator in Python\n\nRelease notes are usually written from memory at six on a Friday, which is why they read like a receipt. The commit subjects already contain the raw material, if the team writes conventional commits.\n\n## Quick reference\n\nThe full script is at the end of this note: `changelog.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\ngit log --pretty=format:'%s' | python3 changelog.py\n```\n\n**Example output**\n\n```text\n## Unreleased\n- **feat**: add SSO login\n```\n\n## Explained\n\n### What it does\n\nThe script reads the output of `git log --pretty=format:'%s'`, one subject per line. It matches the conventional form: a type, an optional scope, an optional exclamation mark for a breaking change, then a colon and the description. It keeps feat, fix, docs, refactor, perf and test, groups the subjects by type, and prints a markdown list under ## Unreleased. If nothing matches, it prints an error and exits 1.\n\n### Why subjects\n\nA subject is short, stable and already reviewed, because it travelled through the pull request. The type is the only taxonomy a first draft needs. Grouping by type puts the features above the chores, and the breaking marker tells the reader what to check before upgrading.\n\n### How to use it\n\n`git log --pretty=format:'%s' v1.3.0..v1.4.0 | python3 changelog.py` -. Run it in the release job, write the output to `CHANGELOG.md`, and edit the paragraphs by hand. Exit 0 means a changelog was written, 1 means there were no conventional commits, 2 means the input could not be read.\n\n> **The honest caveat:** a generated changelog records what changed, not why it matters, and a squash merge can hide twenty commits behind one vague subject.\n\n> **The lesson:** generate the list, then write the story.\n\n## In one line\n\n> **Subjects are not release notes.** Group the commits, then write the why by hand.\n\n**Read the range**\n\n```bash\ngit log --oneline v1.3.0..v1.4.0\n```\n\n## Script: changelog.py\n\nDownload: [scripts/changelog.py](../scripts/changelog.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Build release notes from conventional commit subjects.\n\nReads the output of:\n\n    git log --pretty=format:'%s' v1.3.0..v1.4.0\n\none subject per line, keeps the conventional types below, groups them by\ntype and prints markdown. Lines that are not conventional commits are\nignored, which is why a merge commit does not break the run.\n\nUsage:\n    git log --pretty=format:'%s' v1.3.0..v1.4.0 | python3 changelog.py -\n    python3 changelog.py commits.txt --version v1.4.0\n\nCI: run the first command as a step in the release job and redirect\nstdout to CHANGELOG.md. Read-only: this script writes no files.\n\nExit codes: 0 changelog written, 1 no conventional commits, 2 usage or\ninput error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\n# Order is the order of the sections in the output.\nSECTIONS = (\n    (\"feat\", \"Features\"),\n    (\"fix\", \"Bug fixes\"),\n    (\"docs\", \"Documentation\"),\n    (\"refactor\", \"Refactoring\"),\n    (\"perf\", \"Performance\"),\n    (\"test\", \"Tests\"),\n)\nKINDS = {kind for kind, _ in SECTIONS}\nSUBJECT = re.compile(\n    r\"^(?P<kind>[a-z]+)(?:\\((?P<scope>[^)]+)\\))?(?P<bang>!)?: (?P<text>.+)$\"\n)\n\n\ndef parse(line):\n    \"\"\"Return (kind, scope, breaking, text) for a kept subject, else None.\"\"\"\n    match = SUBJECT.match(line.strip())\n    if not match:\n        return None\n    kind = match.group(\"kind\")\n    if kind not in KINDS:\n        return None\n    breaking = bool(match.group(\"bang\")) or \"BREAKING CHANGE\" in line\n    return kind, match.group(\"scope\"), breaking, match.group(\"text\")\n\n\ndef read_lines(path):\n    \"\"\"Read the subject list from a file, or from stdin when path is '-'.\"\"\"\n    if path == \"-\":\n        return sys.stdin.read().splitlines()\n    with open(path, \"r\", encoding=\"utf-8\") as handle:\n        return handle.read().splitlines()\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Group conventional commit subjects into release notes.\"\n    )\n    parser.add_argument(\n        \"input\",\n        nargs=\"?\",\n        default=\"-\",\n        help=\"file of commit subjects, one per line (default: stdin)\",\n    )\n    parser.add_argument(\n        \"--version\",\n        default=\"Unreleased\",\n        metavar=\"LABEL\",\n        help=\"heading after '##' (default: Unreleased)\",\n    )\n    args = parser.parse_args(argv)\n\n    try:\n        lines = read_lines(args.input)\n    except OSError as exc:\n        print(\"ERR cannot read input: %s\" % exc, file=sys.stderr)\n        return 2\n    except UnicodeDecodeError:\n        print(\"ERR input is not UTF-8 text\", file=sys.stderr)\n        return 2\n\n    groups = {kind: [] for kind in KINDS}\n    total = 0\n    for line in lines:\n        parsed = parse(line)\n        if parsed is None:\n            continue\n        kind, scope, breaking, text = parsed\n        entry = \"- \" + text\n        if scope:\n            entry += \" (%s)\" % scope\n        if breaking:\n            entry = \"- **BREAKING** \" + text\n            if scope:\n                entry += \" (%s)\" % scope\n        groups[kind].append(entry)\n        total += 1\n\n    if total == 0:\n        print(\"ERR no conventional commits in input\", file=sys.stderr)\n        return 1\n\n    print(\"## %s\" % args.version)\n    for kind, heading in SECTIONS:\n        entries = groups[kind]\n        if not entries:\n            continue\n        print(\"\")\n        print(\"### %s\" % heading)\n        for entry in entries:\n            print(entry)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p426","n":"426","slug":"a-semantic-version-bumper-in-python","title":"A semantic version bumper in Python","sections":["devsecops"],"topic":"Python scripts","tags":["SemVer","Python","Git","CICD","DevOps"],"series_name":"DevOps","design":"code","script":"semver_bump.py","date":"2029-06-17","files":{"devsecops":"notes/p426-a-semantic-version-bumper-in-python.md"},"md":"# A semantic version bumper in Python\n\nThe next version number should be a decision made by the commits that caused it, not by whoever remembers to edit a file. This script reads the latest tag and the subjects since that tag, and prints the next one.\n\n## Quick reference\n\nThe full script is at the end of this note: `semver_bump.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 semver_bump.py release.txt\n```\n\n**Example output**\n\n```text\nv1.5.0\n```\n\n## Explained\n\n### What it does\n\nThe first line is the latest tag; every line after it is a commit subject. A subject with an exclamation mark before the colon, or the words BREAKING CHANGE, is a major change. A feat is a minor change. Every other conventional type counts as a patch. The script prints the next tag, keeping whatever v prefix the input used.\n\n### Why classify\n\nSemVer is a promise about compatibility. A breaking change raises the major and resets the rest; a feature raises the minor and resets the patch; a fix raises the patch. Deciding from the commits means the version cannot drift from the change that caused it, and a bot can do it in the release job while a human reviews the result.\n\n### How to use it\n\n`git describe --tags --abbrev=0 > /tmp/release.txt`, then `git log --no-merges --pretty=format:'%s' <tag>..HEAD >> /tmp/release.txt`, then `python3 semver_bump.py /tmp/release.txt`. Exit 0 prints the next tag, 1 lists commits it could not classify, 2 means the tag or the input is missing.\n\n> **The honest caveat:** the script only sees the subjects. A breaking change described in a pull request body, or in a footer below the subject, will ship as a patch unless the subject carries the exclamation mark.\n\n> **The lesson:** let the commits choose the number.\n\n## In one line\n\n> **The commits pick the number.** A version is a promise about compatibility, not a count of effort.\n\n**Read the commits**\n\n```bash\ngit log --pretty=format:'%s' v1.4..\n```\n\n## Script: semver_bump.py\n\nDownload: [scripts/semver_bump.py](../scripts/semver_bump.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Print the next SemVer tag from the latest tag and commit subjects.\n\nThe input is the last release tag on the first line, followed by one\ncommit subject per line:\n\n    v1.4.2\n    feat: add SSO login\n    fix(api): correct tax rounding\n\nA subject with '!' before the colon, or the text BREAKING CHANGE, raises\nthe major version. A feat raises the minor version. Every other\nconventional type is a patch. Unclassified subjects stop the run.\n\nA prerelease tag such as v1.4.2-rc.1 becomes v1.4.2 on a fix, and still\ngets a minor or major bump for a feat or a breaking change.\n\nUsage:\n    git describe --tags --abbrev=0 > /tmp/release.txt\n    git log --no-merges --pretty=format:'%s' v1.4.2..HEAD >> /tmp/release.txt\n    python3 semver_bump.py /tmp/release.txt\n\nCI: run it in the release job, then create and push the tag that it\nprints. Read-only: this script never creates or pushes a tag.\n\nExit codes: 0 next tag printed, 1 unclassified commits, 2 usage or\ninput error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nCONVENTIONAL = re.compile(\n    r\"^(?P<kind>[a-z]+)(?:\\((?P<scope>[^)]+)\\))?(?P<bang>!)?: (?P<text>.+)$\"\n)\nTAG = re.compile(\n    r\"^(?P<prefix>v?)(?P<major>\\d+)\\.(?P<minor>\\d+)\\.(?P<patch>\\d+)\"\n    r\"(?:-(?P<pre>[0-9A-Za-z.-]+))?(?:\\+(?P<build>[0-9A-Za-z.-]+))?$\"\n)\nKINDS = {\n    \"feat\", \"fix\", \"docs\", \"style\", \"refactor\", \"perf\",\n    \"test\", \"build\", \"ci\", \"chore\", \"revert\",\n}\n\n\ndef read_lines(path):\n    \"\"\"Read the tag and subjects from a file, or stdin when path is '-'.\"\"\"\n    if path == \"-\":\n        return sys.stdin.read().splitlines()\n    with open(path, \"r\", encoding=\"utf-8\") as handle:\n        return handle.read().splitlines()\n\n\ndef classify(subject):\n    \"\"\"Return 'major', 'minor', 'patch' or None for one commit subject.\"\"\"\n    if \"BREAKING CHANGE\" in subject or \"BREAKING-CHANGE\" in subject:\n        return \"major\"\n    match = CONVENTIONAL.match(subject.strip())\n    if not match:\n        return None\n    if match.group(\"kind\") not in KINDS:\n        return None\n    if match.group(\"bang\"):\n        return \"major\"\n    if match.group(\"kind\") == \"feat\":\n        return \"minor\"\n    return \"patch\"\n\n\ndef next_version(tag, subjects):\n    \"\"\"Return the next tag string, or raise ValueError for a bad tag.\"\"\"\n    match = TAG.match(tag)\n    if not match:\n        raise ValueError(\"tag is not SemVer: %s\" % tag)\n    major = int(match.group(\"major\"))\n    minor = int(match.group(\"minor\"))\n    patch = int(match.group(\"patch\"))\n    pre = bool(match.group(\"pre\"))\n\n    level = 0\n    unclassified = []\n    for subject in subjects:\n        if not subject.strip():\n            continue\n        kind = classify(subject)\n        if kind is None:\n            unclassified.append(subject.strip())\n        elif kind == \"major\":\n            level = 2\n        elif kind == \"minor\" and level < 1:\n            level = 1\n\n    if unclassified:\n        for subject in unclassified:\n            print(\"ERR unclassified: %s\" % subject, file=sys.stderr)\n        print(\"ERR use conventional commits and filter merges with --no-merges\",\n              file=sys.stderr)\n        return None\n\n    if level == 2:\n        major, minor, patch = major + 1, 0, 0\n    elif level == 1:\n        minor, patch = minor + 1, 0\n    elif pre:\n        pass                      # the release candidate becomes the release\n    else:\n        patch += 1\n    return \"%s%d.%d.%d\" % (match.group(\"prefix\"), major, minor, patch)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Print the next SemVer tag from commits since the last tag.\"\n    )\n    parser.add_argument(\n        \"input\",\n        nargs=\"?\",\n        default=\"-\",\n        help=\"file with the tag on line one and subjects after it (default: stdin)\",\n    )\n    args = parser.parse_args(argv)\n\n    try:\n        lines = read_lines(args.input)\n    except OSError as exc:\n        print(\"ERR cannot read input: %s\" % exc, file=sys.stderr)\n        return 2\n    except UnicodeDecodeError:\n        print(\"ERR input is not UTF-8 text\", file=sys.stderr)\n        return 2\n\n    while lines and not lines[0].strip():\n        lines.pop(0)\n    if not lines:\n        print(\"ERR no tag on the first line\", file=sys.stderr)\n        return 2\n    tag = lines[0].strip()\n    subjects = lines[1:]\n    if not any(subject.strip() for subject in subjects):\n        print(\"ERR no commit subjects after the tag\", file=sys.stderr)\n        return 2\n\n    try:\n        tag_out = next_version(tag, subjects)\n    except ValueError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n    if tag_out is None:\n        return 1\n    print(tag_out)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p427","n":"427","slug":"9-steps-to-a-repeatable-release","title":"9 steps to a repeatable release","sections":["devsecops"],"topic":"Release & build engineering","tags":["ReleaseEngineering","DevOps","CICD","Kubernetes","Git"],"series_name":"DevOps","design":"steps","script":"","date":"2029-06-19","files":{"devsecops":"notes/p427-9-steps-to-a-repeatable-release.md"},"md":"# 9 steps to a repeatable release\n\nA repeatable release is not a checklist someone remembers; it is the same nine actions, in the same order, every time. Here is the sequence that turns a tag into traffic.\n\n## Quick reference\n\n### Prepare — Freeze, tag, sign\n\n```bash\n# Tag the commit that passed CI, not the merge after it.\ngit tag -a v1.4.0 -m \"release 1.4.0\"\n# One push starts one build, from one commit.\ngit push origin v1.4.0\n# Sign the digest, then promote that digest.\ncosign sign --key cosign.key $IMAGE\n```\n\n### Ship — Promote the digest\n\n```bash\n# The same digest staging tested, no rebuild.\nkubectl set image deploy/api api=$IMAGE\n# Wait until every replica reports ready.\nkubectl rollout status deploy/api\n# The production canary starts at 10 percent.\nkubectl argo rollouts set image api api=$IMAGE\n```\n\n### Verify — Let the numbers decide\n\n```bash\n# Canary and stable, error rate and p99.\nkubectl argo rollouts get rollout api\n# Advance one step, then watch again.\nkubectl argo rollouts promote api\n# Record what runs, where the next incident starts.\nkubectl annotate rollout/api release=v1.4.0\n```\n\n## Explained\n\n### Prepare\n\nTag the commit that passed CI, push the tag so one build starts from one commit, then sign the image digest. The tag names the release; the signature proves the bytes did not change after the build. A rebuild here would make the signature a decoration.\n\n### Ship\n\nPromote the signed digest to staging, wait for every replica to report ready, then set the same digest on the production canary. No step rebuilds, and no step touches a pod by hand. If staging fails, the release stops before production sees a single request, and the tag remains a record of what was attempted.\n\n### Verify\n\nRead the canary against stable: error rate first, then p99 latency. Promote one step at a time, watch again, and annotate the workload with the version that is now running. The annotation is what the next incident starts from, and the next release can compare itself against it.\n\n> **The honest caveat:** these nine steps only hold if the pipeline is the only path to production. One manual kubectl edit and the next release becomes a surprise.\n\n> **The lesson:** the value of nine steps is that step ten, the emergency, looks the same.\n\n## In one line\n\n> **Same steps, every time.** A release you cannot repeat is a release you cannot trust.\n\n**Tag it first**\n\n```bash\ngit tag -a v1.4.0 -m 'release 1.4.0'\n```\n"},{"id":"p428","n":"428","slug":"trunk-based-vs-gitflow","title":"Trunk-based vs GitFlow","sections":["devsecops"],"topic":"Release & build engineering","tags":["Git","DevOps","ReleaseEngineering","CICD","SoftwareEngineering"],"series_name":"DevOps","design":"compare","script":"","date":"2029-06-21","files":{"devsecops":"notes/p428-trunk-based-vs-gitflow.md"},"md":"# Trunk-based vs GitFlow\n\nThe branch model argument is about how often code meets main. Trunk-based development merges every day; GitFlow defers the meeting to a release branch.\n\n## Quick reference\n\n| | **TRUNK-BASED** | **GITFLOW** |\n|:--|:--|:--|\n| Model | One trunk, short PRs | Many long-lived branches |\n| Integration | Merged daily to main | Merged at release time |\n| Release | A tag on main | A stabilised branch |\n| Hotfix | Fix on main, tag again | Branch, then merge twice |\n| Merge pain | Small and constant | Large and deferred |\n| Best for | Continuous delivery | Versioned, scheduled releases |\n\n**TRUNK-BASED** (One branch, merges every day): Everyone merges to main daily; a release is a tag on main.\n\n```bash\ngit switch -c feat/sso\n```\n\n**GITFLOW** (Long-lived release branches): Develop, release and hotfix branches, each merged twice.\n\n```bash\ngit flow release start 1.4\n```\n\n- Trunk → merge daily, release from one commit.\n- GitFlow → stabilise a branch, then merge twice.\n\n## Explained\n\n### The models\n\nTrunk-based: one long-lived branch, short-lived feature branches, and a merge to main at least daily. Releases are cut from a commit on main, usually with a tag. It depends on fast CI and flags, because unfinished work must ship dark safely.\n\nGitFlow: main, develop, feature, release and hotfix branches. Features merge into develop; a release branch is cut and stabilised; main only ever holds what shipped; a hotfix branches from main and merges back into both main and develop.\n\n### What changes\n\nIntegration. Trunk-based finds conflicts in hours; GitFlow finds them at release time, when the branch has drifted. Release. A trunk release is a tag on main; a GitFlow release is a branch that receives only fixes. Hotfix. Trunk fixes forward on main and tags again; GitFlow fixes on a hotfix branch and merges twice.\n\n### Choosing\n\nTrunk-based fits continuous delivery, a strong test suite and flags that hide unfinished work. GitFlow fits versioned software with several supported releases, or a cadence that needs a stabilisation window. Most teams adopting GitFlow for a web service keep the branches and lose the cadence.\n\n> **The honest caveat:** GitFlow is not wrong, but it is often chosen for the diagram rather than the release schedule, and long-lived branches make the merge the riskiest event in the quarter.\n\n> **The lesson:** choose the branch model that matches how often you want to integrate, not how the diagram looks.\n\n**Merge daily, or**  \nmerge in fear.\n"},{"id":"p429","n":"429","slug":"a-feature-flags-life","title":"A feature flag's life","sections":["devsecops"],"topic":"Release & build engineering","tags":["FeatureFlags","ReleaseEngineering","DevOps","CICD","ProgressiveDelivery"],"series_name":"DevOps","design":"timeline","script":"","date":"2029-06-24","files":{"devsecops":"notes/p429-a-feature-flags-life.md"},"md":"# A feature flag's life\n\nThe first feature flag is exciting; the hundredth is technical debt with a dashboard. A flag has a life cycle, and most teams stop halfway through it.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **01** | Create the flag | A boolean, off by default, with an owner and a removal date. |\n| **02** | Deploy dark | Code reaches production; no user takes the new path yet. |\n| **03** | 1 percent | A stable hash of the user ID picks the same cohort every time. |\n| **04** | 10 percent | Only after error rate and latency hold against the control. |\n| **05** | 100 percent | All traffic, and the old path is now dead code still shipped. |\n| **06** | Remove the flag | Delete the flag and the old branch in one pull request. |\n\n## Explained\n\n### Create and deploy\n\nThe flag starts as a boolean in the flag service, off by default, with an owner and a removal date. The code ships dark: the new path exists in production, but no user reaches it. That separates the risk of shipping code from the risk of exposing it.\n\n### Ramp\n\nThen the percentage rises. One percent first, chosen by a stable hash of the user ID so the same users stay in the cohort. Ten percent only after error rate and latency hold against the control group. At 100 percent the flag is still doing work: the old path is dead code that still compiles, still ships and still gets tested.\n\n### Remove\n\nThe last step is the one nobody schedules. Delete the flag and the old branch in the same pull request, and let the code default to the new behaviour. A flag left at 100 percent is not harmless; it is a permanent branch, a config dependency and a test case nobody remembers.\n\n> **The honest caveat:** a percentage rollout on a low-traffic surface can take days to produce a signal. A percentage is not a significance test.\n\n> **The lesson:** a feature flag is a temporary tool for separating deploy from release, not a permanent configuration system.\n\n## In one line\n\n> **Ship the code, release later.** A flag separates deploy from release; delete it when the release is done.\n\n**Count the debt**\n\n```bash\ngrep -rn featureFlag src/ | wc -l\n```\n"},{"id":"p430","n":"430","slug":"quiz-3-release-questions","title":"Quiz: 3 release questions","sections":["devsecops"],"topic":"Release & build engineering","tags":["DevOps","ReleaseEngineering","CICD","Git","FeatureFlags"],"series_name":"DevOps","design":"quiz","script":"","date":"2029-06-26","files":{"devsecops":"notes/p430-quiz-3-release-questions.md"},"md":"# Quiz: 3 release questions\n\nThree questions separate a team that deploys from a team that releases. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why build once and promote the same artefact through every environment?**\n\n- a) So staging tests the exact production digest\n- b) Because rebuilding is slower than promoting\n- c) Because registries charge per build\n\n> **Answer:** a) So staging tests the exact production digest. A rebuild can resolve a different base image or dependency, so the digest changes and the tests no longer describe what ships. Promotion moves one digest.\n\n**Q2. What does a merge queue guarantee before a pull request reaches main?**\n\n- a) That every reviewer has approved it\n- b) That it passes CI against the latest main\n- c) That it deploys to production immediately\n\n> **Answer:** b) That it passes CI against the latest main. The queue builds the pull request on top of the current main, in order, so two green pull requests cannot break main together. Review approval is a separate gate.\n\n**Q3. Why is a feature flag not a deployment?**\n\n- a) It deploys code without a pipeline\n- b) It replaces the need for rollback\n- c) It changes the code path, not the artefact\n\n> **Answer:** c) It changes the code path, not the artefact. Deploying puts code on the servers; the flag decides who reaches it. That separation lets you ship dark, ramp traffic and kill a feature without a rollback.\n\n## Explained\n\n### The artefact\n\nWhat does \"build once and promote\" actually protect? A rebuild is a new artefact. It can resolve a different base image, a newer dependency or a different compiler, and the digest changes. The moment it changes, staging tested something production will never run. Promotion moves the same digest, so the test result still describes the release.\n\n### The queue\n\nWhat does a merge queue guarantee? That the pull request passed CI on top of the current main, in the order it will merge. Two pull requests can each be green alone and broken together; the queue builds the combination before it lands. It does not guarantee review, and it does not deploy anything.\n\n### The flag\n\nWhy is a feature flag not a deployment? Deploying puts code on the servers. The flag decides who reaches it. That separation lets you ship dark, ramp a percentage and kill a feature without a rollback, while the artefact stays exactly the same. A flag is a release control, not a deployment mechanism.\n\n> **The honest caveat:** a merge queue serialises merges, so a busy repository can wait. The guarantee is a green main, not a fast one.\n\n> **The lesson:** deploy is mechanical; release is a decision, and you can reverse it.\n\n## In one line\n\n> **Deploy is not release.** Three answers that separate a pipeline from a release process.\n\n**Check the range**\n\n```bash\ngit log --oneline v1.4.0..HEAD\n```\n"},{"id":"p431","n":"431","slug":"anatomy-of-a-semantic-version","title":"Anatomy of a semantic version","sections":["devsecops"],"topic":"Release & build engineering","tags":["SemVer","ReleaseEngineering","DevOps","Git","SoftwareEngineering"],"series_name":"DevOps","design":"anatomy","script":"","date":"2029-06-28","files":{"devsecops":"notes/p431-anatomy-of-a-semantic-version.md"},"md":"# Anatomy of a semantic version\n\nA version number is the cheapest API your project has. Package managers, deploy scripts and tired people at two in the morning read it, and it means exactly what SemVer 2.0.0 says it means.\n\n## Quick reference\n\n```text\n2.4.1-rc.2+build.7\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `2.` | Major | Incompatible API change. Increment it, then reset minor and patch. |\n| `4.` | Minor | New functionality, backward compatible. The patch number resets. |\n| `1` | Patch | Backward-compatible bug fixes, and nothing else. |\n| `-rc.2` | Prerelease | Lower precedence than 2.4.1, so resolvers skip it unless asked. |\n| `+build.7` | Build metadata | Ignored when comparing versions: it names the build, not the release. |\n\n## Explained\n\n### Major, minor, patch\n\n2 is the major: increment it for an incompatible API change, and reset minor and patch. 4 is the minor: new functionality, backward compatible, with the patch reset. 1 is the patch: backward-compatible bug fixes and nothing else. In 0.y.z the rules relax and anything may change, which is why 1.0.0 is a promise rather than a milestone.\n\n### Prerelease\n\nThe `-rc.2` part is a prerelease. It has lower precedence than 2.4.1, so package managers will not choose it unless asked. Dot-separated identifiers compare numerically when they are numbers, so rc.2 is lower than rc.10. That ordering is the point of publishing a candidate before the release.\n\n### Build metadata\n\nThe +build.7 part is build metadata. SemVer says it must be ignored when determining precedence, so 2.4.1+build.7 and 2.4.1+build.8 are equal in precedence. It identifies the build; it does not order it.\n\n> **The honest caveat:** SemVer describes the API, not the operation. A config change or a database migration can break production without changing a single endpoint, and no version number will warn you.\n\n> **The lesson:** the number is a contract; change it when the contract changes.\n\n## In one line\n\n> **Break the promise, change the major.** A version number is a contract, not a counter.\n\n**Sort the tags**\n\n```bash\ngit tag --sort=-v:refname | head -3\n```\n"},{"id":"p432","n":"432","slug":"the-build-cache-drawn","title":"The build cache, drawn","sections":["devsecops"],"topic":"Release & build engineering","tags":["DevOps","BuildSystems","CICD","Performance","Engineering"],"series_name":"DevOps","design":"architecture","script":"","date":"2029-07-01","files":{"devsecops":"notes/p432-the-build-cache-drawn.md"},"md":"# The build cache, drawn\n\nEvery pipeline has a cache. The only question is whether it is a cache or a rumour.\n\n## Quick reference\n\n- **Source:** Repository (GIT) · Lockfile (LOCK)\n- **Cache:** Local cache (LOCAL) · Remote cache (S3)\n- **Output:** Artifact (TAR) · Registry (OCIR)\n\n**Request flow:** Repository → Local cache → Artifact; Lockfile → Remote cache → Registry\n\n**Around it:** Build log (LOG)\n\n## Explained\n\n### Source\n\nThe top tier is the input. The repository holds the commit and the tree you meant to build. The lockfile holds the exact versions of everything that commit pulls in. Those two are the first honest inputs to a cache key: the code, and the dependencies it resolved to.\n\n### Cache\n\nThe middle tier is where the time comes back. The local cache lives on the runner, survives between steps and dies with the machine. The remote cache lives in object storage, S3 for example, and every runner holding the same key can read it.\n\n### Output\n\nThe bottom tier is what you keep. The artifact is the tarball or binary the pipeline hands to the next stage. The registry keeps the image layers under a digest, so a deploy can name exactly the bits it shipped.\n\nThe four arrows follow the work: repository to local cache, lockfile to remote cache, local cache to artifact, remote cache to registry. The build log sits below the tiers and watches all three. It is the only part that shows you when a hit became a miss.\n\n> **The honest caveat:** a remote cache is shared state. A stale or poisoned entry can hand you a wrong artifact with a perfect hit rate, so scope the keys, expire the entries and sign what you store.\n\n> **The lesson:** a cache keyed on the wrong inputs only makes the wrong build faster.\n\n## In one line\n\n> **Cache the work, not the failure.** A cache miss should be the exception.\n\n**Measure it**\n\n```bash\ntime make build\n```\n"},{"id":"p433","n":"433","slug":"the-docker-build-cheat-sheet","title":"The Docker build cheat sheet","sections":["devsecops"],"topic":"Release & build engineering","tags":["Docker","DevOps","CICD","BuildSystems","Containers"],"series_name":"DevOps","design":"cheatsheet","script":"","date":"2029-07-03","files":{"devsecops":"notes/p433-the-docker-build-cheat-sheet.md"},"md":"# The Docker build cheat sheet\n\nA Docker build is a cache with a command line. These are the commands that decide what gets rebuilt and what gets reused.\n\n## Quick reference\n\n### Build\n\n```bash\ndocker build --progress=plain .\ndocker build --platform linux/amd64 .\ndocker build --target test .\ndocker build --no-cache .\ndocker build -f Dockerfile.api .\n```\n\n### Cache\n\n```bash\ndocker build --cache-from api:dev .\ndocker buildx build --cache-to=type=gha .\ndocker build --no-cache-filter=test .\ndocker buildx prune --filter until=24h\n```\n\n### Multi-stage\n\n```bash\ndocker build --target builder .\ndocker build --target runtime .\ndocker build --build-arg VERSION=2 .\ndocker build --ssh default .\ndocker build --platform linux/arm64 .\n```\n\n### Inspect\n\n```bash\ndocker buildx du\ndocker buildx imagetools inspect api:1.0\ndocker history api:1.0\ndocker image inspect api:1.0\ndocker buildx inspect --bootstrap\n```\n\n## Explained\n\n### Build\n\n`docker build --progress=plain .` prints one DONE line per step, and that line is the only per-step timing the build itself reports. `--platform linux/amd64` pins the target architecture. `--target` test stops at the test stage. `--no-cache` rebuilds everything when you suspect the cache, and -f Dockerfile.api names the file when a repository keeps several.\n\n### Cache\n\n`docker build --cache-from api:dev .` reuses an image's cache, provided that image carries inline cache metadata. `--cache-to=type=gha` writes the cache to the GitHub Actions cache. `--no-cache-filter=test` rebuilds only the test stage. `docker buildx prune --filter until=24h` stops the cache eating the disk; add `--force` in CI.\n\n### Multi-stage\n\n`--target` builder and `--target` runtime are how one Dockerfile serves two jobs: build in the fat stage, copy into the thin one. `--build-arg VERSION=2` passes a value. `--ssh default` gives the build a key without baking it in. `--platform linux/arm64` builds for the other architecture.\n\n### Inspect\n\n`docker buildx du` reports what the cache costs. `docker buildx imagetools inspect api:1.0` reads the manifest in the registry. `docker history api:1.0` shows layer sizes, `docker image inspect api:1.0` shows the config, and `docker buildx inspect --bootstrap` proves which builder you are actually using.\n\n> **The honest caveat:** `--cache-to=type=gha` only works inside GitHub Actions, where the cache API and its token exist. Anywhere else, use a registry cache or a local directory.\n\n> **The lesson:** the flag you forget is the one that costs an hour.\n\n## In one line\n\n> **Build once, reuse everywhere.** A cache you cannot inspect is a cache you cannot trust.\n\n**Plain progress**\n\n```bash\ndocker build --progress=plain .\n```\n"},{"id":"p434","n":"434","slug":"why-builds-are-slow","title":"Why builds are slow","sections":["devsecops"],"topic":"Release & build engineering","tags":["DevOps","CICD","BuildSystems","Performance","Engineering"],"series_name":"DevOps","design":"explainer","script":"","date":"2029-07-05","files":{"devsecops":"notes/p434-why-builds-are-slow.md"},"md":"# Why builds are slow\n\nA build that takes eleven minutes is not one problem. It is usually four, and the loudest one is rarely the expensive one.\n\n## Quick reference\n\n### Cold cache\n\n- Every layer rebuilt from scratch\n- Dependencies downloaded again\n- Usually a cache key problem\n\n### No parallelism\n\n- One job, one core, the rest idle\n- `make -j` is not the default\n- One test runner walks the suite\n\n### Network\n\n- Every run pulls base images\n- Registries throttle large pulls\n- A proxy adds a hop to each fetch\n\n### Test suite\n\n- The suite only ever grows\n- Integration tests wait on services\n- Nobody shards the slowest stage\n\n> **Note:** Measure before you optimise: a build without numbers is a rumour.\n\n## Explained\n\n### Cold cache\n\nEvery layer is rebuilt because nothing survived the previous run. Dependencies download again, and the first build on a new runner pays for it. Most of the time the cache is not missing; it is keyed on something that changes every run, such as a timestamp or a branch name.\n\n### No parallelism\n\nOne job runs the steps in order, on one core, while the rest of the machine idles. `make without -j` compiles one file at a time. A single test runner walks the suite serially. The fix is usually a flag and a larger runner, not a rewrite.\n\n### Network\n\nEvery run pulls base images and packages from registries that may be slow, throttled or proxied. A layer that installs four hundred packages pays that cost on every cold build, and the bill arrives as wall clock.\n\n### Test suite\n\nThe suite grows every sprint and nobody deletes a test. Integration tests wait on real services, and the whole suite gates every merge. Nobody shards it, and it is the phase teams optimise last.\n\nThe note under the panels is the rule: measure before you optimise, because a build without numbers is a rumour.\n\n> **The honest caveat:** a faster build is not a better one. Shaving minutes by skipping tests or caching aggressively just ships the bug sooner.\n\n> **The lesson:** fix the cause you can measure, not the one that sounds modern.\n\n## In one line\n\n> **Slow is a symptom, not a verdict.** The fix is a measurement away.\n\n**Time each step**\n\n```bash\ntime make -j8 build\n```\n"},{"id":"p435","n":"435","slug":"a-build-time-report-in-python","title":"A build time report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["CICD","Python","BuildSystems","Performance","DevOps"],"series_name":"DevOps","design":"code","script":"build_time.py","date":"2029-07-08","files":{"devsecops":"notes/p435-a-build-time-report-in-python.md"},"md":"# A build time report in Python\n\nThe build did not get slower on Tuesday. It had been getting slower for a month, and nobody had a number.\n\n## Quick reference\n\nThe full script is at the end of this note: `build_time.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 build_time.py jobs.json --baseline prev.json\n```\n\n**Example output**\n\n```text\n  612s integration-tests !\n  184s unit-tests\n```\n\n## Explained\n\n### What it does\n\n`build_time.py` reads a CI jobs export: a JSON list of jobs, each with a name, a duration in seconds and a status. It sorts them longest first and prints the ranking. With `--baseline` it reads an earlier export and flags any job that is more than twenty percent slower. `--threshold` sets a hard ceiling in seconds. If a job crosses either line, the script exits 1, so the pipeline fails on the build that caused it.\n\n### Why a baseline\n\nA threshold catches a build that is already too slow. A baseline catches the build that has just become slower, which is the one a developer can still fix. Keep yesterday's export as an artifact and pass it with `--baseline`.\n\n### How to use it\n\nExport the jobs, then run `python3 build_time.py jobs.json --baseline prev.json`. The run prints one line per job, slowest first, with ! beside a regression. In CI, save the export on every run so the next run has something to compare against. Exit 1 fails the job, 2 means bad JSON or bad arguments, and 0 means the build is still within budget.\n\n> **The honest caveat:** the export is only as good as the CI's own measurement. Queue time and setup are often excluded, so a job that looks steady can still be waiting forty minutes for a runner.\n\n> **The lesson:** rank before you optimise, and compare before you celebrate.\n\n## In one line\n\n> **A build has a budget, not a mood.** Compare against yesterday, not against a feeling.\n\n**Keep yesterday**\n\n```bash\ncp jobs.json prev.json\n```\n\n## Script: build_time.py\n\nDownload: [scripts/build_time.py](../scripts/build_time.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"build_time.py - rank CI jobs and fail on a slow build or a regression.\n\nReads a JSON export of CI jobs. The export is either a list of jobs or an\nobject with a \"jobs\" list. Each job needs:\n    name      the job name\n    duration  wall-clock seconds, as a number\n    status    the CI status, for example success, failed or cancelled\n\nDurations are compared per job name. When a name appears more than once, the\nbaseline keeps its slowest run, so a matrix leg is compared against a real run\nrather than an average.\n\nUsage:\n    python3 build_time.py jobs.json\n    python3 build_time.py jobs.json --threshold 600\n    python3 build_time.py jobs.json --baseline yesterday.json\n    python3 build_time.py jobs.json --baseline yesterday.json --regression 15\n    python3 build_time.py jobs.json --top 5 --quiet\n\nCI (after the build, keeping the export for the next run):\n    python3 build_time.py jobs.json --threshold 600 || exit 1\n\nCron (a weekly look at the slowest jobs):\n    0 7 * * 1 python3 /opt/scripts/build_time.py /var/lib/ci/jobs.json --top 10\n\nExit codes: 0 within budget, 1 over the threshold or a regression,\n2 usage or input error.\nRead-only: every file is opened for reading and nothing is written.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef load_jobs(path):\n    \"\"\"Return a list of jobs from one export, or raise ValueError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError as exc:\n        raise ValueError(\"%s is not UTF-8 text: %s\" % (path, exc.reason))\n    except json.JSONDecodeError as exc:\n        raise ValueError(\"%s is not valid JSON: line %d column %d\"\n                         % (path, exc.lineno, exc.colno))\n    if isinstance(data, dict):\n        data = data.get(\"jobs\")\n    if not isinstance(data, list):\n        raise ValueError(\"%s: expected a list of jobs or an object with a \"\n                         \"jobs list\" % path)\n    jobs = []\n    for index, item in enumerate(data, 1):\n        if not isinstance(item, dict):\n            raise ValueError(\"%s: job %d is not an object\" % (path, index))\n        name = item.get(\"name\")\n        duration = item.get(\"duration\")\n        status = item.get(\"status\", \"unknown\")\n        if not isinstance(name, str) or not name.strip():\n            raise ValueError(\"%s: job %d has no name\" % (path, index))\n        if isinstance(duration, bool) or not isinstance(duration, (int, float)):\n            raise ValueError(\"%s: job %s has no numeric duration\"\n                             % (path, name))\n        if duration < 0:\n            raise ValueError(\"%s: job %s has a negative duration\"\n                             % (path, name))\n        jobs.append({\"name\": name.strip(), \"duration\": float(duration),\n                     \"status\": str(status)})\n    if not jobs:\n        raise ValueError(\"%s: the export contains no jobs\" % path)\n    return jobs\n\n\ndef baseline_map(jobs):\n    \"\"\"Return {job name: slowest duration} for one export.\"\"\"\n    slowest = {}\n    for job in jobs:\n        name = job[\"name\"]\n        slowest[name] = max(slowest.get(name, 0.0), job[\"duration\"])\n    return slowest\n\n\ndef delta_text(duration, was):\n    \"\"\"Return a readable change against the baseline.\"\"\"\n    if not was:\n        return \"new\"\n    change = duration - was\n    return \"%+.0fs (%+.0f%%)\" % (change, change / was * 100.0)\n\n\ndef report(jobs, base, threshold, regression, top, quiet):\n    \"\"\"Print the ranking and the summary, and return the exit code.\"\"\"\n    rows = sorted(jobs, key=lambda job: -job[\"duration\"])\n    over, slower = set(), set()\n    for index, job in enumerate(rows):\n        was = base.get(job[\"name\"], 0.0)\n        if threshold is not None and job[\"duration\"] > threshold:\n            over.add(index)\n        if was and job[\"duration\"] > was * (1.0 + regression / 100.0):\n            slower.add(index)\n\n    if not quiet:\n        print(\"%-26s %7s  %-9s %9s  %-14s %s\"\n              % (\"JOB\", \"SECONDS\", \"STATUS\", \"BASELINE\", \"DELTA\", \"FLAG\"))\n        for index, job in enumerate(rows[:top]):\n            was = base.get(job[\"name\"], 0.0)\n            flags = []\n            if index in slower:\n                flags.extend([\"!\", \"REGRESSION\"])\n            if index in over:\n                flags.append(\"OVER\")\n            line = (\"%-26s %7.0f  %-9s %9s  %-14s %s\"\n                    % (job[\"name\"][:26], job[\"duration\"],\n                       job[\"status\"][:9], (\"%.0fs\" % was) if was else \"-\",\n                       delta_text(job[\"duration\"], was) if base else \"-\",\n                       \" \".join(flags)))\n            print(line.rstrip())\n\n    parts = [\"checked %d job%s\" % (len(rows), \"\" if len(rows) == 1 else \"s\")]\n    if threshold is not None:\n        parts.append(\"%d over %gs\" % (len(over), threshold))\n    if base:\n        parts.append(\"%d regression%s\"\n                     % (len(slower), \"\" if len(slower) == 1 else \"s\"))\n    print(\", \".join(parts))\n    return 1 if over or slower else 0\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        prog=\"build_time.py\",\n        description=\"Rank CI jobs by duration and fail on a threshold or a \"\n                    \"regression.\",\n        epilog=\"Read-only. Exit 0 clean, 1 over threshold or regression, \"\n               \"2 usage or input error.\",\n    )\n    parser.add_argument(\"export\", metavar=\"JOBS.json\",\n                        help=\"CI jobs JSON export\")\n    parser.add_argument(\"--baseline\", metavar=\"PREV.json\",\n                        help=\"earlier export to compare against\")\n    parser.add_argument(\"--threshold\", type=float, metavar=\"SECONDS\",\n                        help=\"fail when any job is slower than this\")\n    parser.add_argument(\"--regression\", type=float, default=20.0,\n                        metavar=\"PERCENT\",\n                        help=\"fail when a job is this much slower than the \"\n                             \"baseline (default 20)\")\n    parser.add_argument(\"--top\", type=int, default=10, metavar=\"N\",\n                        help=\"print only the N slowest jobs (default 10)\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args()\n\n    if args.threshold is not None and args.threshold < 0:\n        parser.error(\"--threshold cannot be negative\")\n    if args.regression < 0:\n        parser.error(\"--regression cannot be negative\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n\n    try:\n        jobs = load_jobs(args.export)\n        base = baseline_map(load_jobs(args.baseline)) if args.baseline else {}\n    except ValueError as exc:\n        print(\"error: %s\" % exc, file=sys.stderr)\n        return 2\n    return report(jobs, base, args.threshold, args.regression, args.top,\n                  args.quiet)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p436","n":"436","slug":"a-flaky-test-detector-in-python","title":"A flaky test detector in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Testing","CICD","Python","DevOps","FlakyTests"],"series_name":"DevOps","design":"code","script":"flaky_tests.py","date":"2029-07-10","files":{"devsecops":"notes/p436-a-flaky-test-detector-in-python.md"},"md":"# A flaky test detector in Python\n\nA test that fails once a week is worse than a test that fails every time: it teaches the team to press re-run.\n\n## Quick reference\n\nThe full script is at the end of this note: `flaky_tests.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 flaky_tests.py run-1.xml run-2.xml run-3.xml\n```\n\n**Example output**\n\n```text\ntests.test_api::test_timeout\ntests.test_auth::test_expiry\n```\n\n## Explained\n\n### What it does\n\n`flaky_tests.py` reads two or more JUnit XML files, one per run. It walks every testcase, treats a failure or error child as failed, skipped as neither, and anything else as passed. Any test that both passed and failed across the runs is printed. Exit 1 means at least one flaky test, 0 means the runs agree, and 2 means a file is missing or is not JUnit XML.\n\n### Why JUnit XML\n\nJUnit XML is the format most runners can emit: pytest with `--junitxml`, gotestsum, jest-junit, and the GitLab and Jenkins parsers. A testcase carries classname and name, and a failure is a child element, not an attribute. Reading that shape means one script works across languages and CI vendors.\n\n### How to use it\n\nRun it over the last few exports: `python3 flaky_tests.py run-1.xml run-2.xml run-3.xml`. In CI, download the JUnit reports from the last ten runs and fail a nightly job when the list is non-empty. Then quarantine the test, fix it, or delete it. Retrying until green is the one answer that makes it worse.\n\n> **The honest caveat:** two runs prove very little. A test that passes twice and fails once needs a third run, and a test that fails for an environmental reason looks exactly like a flaky one.\n\n> **The lesson:** a flaky test is a measurement you cannot trust, and an untrusted suite stops being a gate.\n\n## In one line\n\n> **A rerun is not a fix.** Two runs disagree; one of them is wrong.\n\n**Fail on flaky:** `flaky_tests.py run-*.xml`\n\n## Script: flaky_tests.py\n\nDownload: [scripts/flaky_tests.py](../scripts/flaky_tests.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"flaky_tests.py - find tests that both passed and failed across JUnit runs.\n\nReads two or more JUnit XML reports, one per run, and reports every test that\npassed in at least one run and failed in at least one other. A testcase counts\nas failed when it contains a <failure> or <error> child, as skipped when it\ncontains <skipped> and no failure, and as passed otherwise. A report may be a\nsingle <testsuite> or a <testsuites> wrapper.\n\nUsage:\n    python3 flaky_tests.py run-1.xml run-2.xml\n    python3 flaky_tests.py reports/*.xml\n    python3 flaky_tests.py --quiet run-1.xml run-2.xml run-3.xml\n\nCI (after downloading the JUnit reports of the last runs):\n    python3 flaky_tests.py junit/*.xml || exit 1\n\nCron (a nightly flake report over the week's reports):\n    0 3 * * * python3 /opt/scripts/flaky_tests.py /var/lib/junit/*.xml -q\n\nExit codes: 0 no flaky test, 1 at least one flaky test, 2 usage or input error.\nRead-only: every file is opened for reading and nothing is written.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nimport xml.etree.ElementTree as ET\n\nFAIL_TAGS = (\"failure\", \"error\")\n\n\ndef case_id(case, path):\n    \"\"\"Return classname::name for one testcase, or raise ValueError.\"\"\"\n    classname = (case.get(\"classname\") or \"\").strip()\n    name = (case.get(\"name\") or \"\").strip()\n    if not name:\n        raise ValueError(\"%s: a testcase has no name attribute\" % path)\n    if classname:\n        return \"%s::%s\" % (classname, name)\n    return name\n\n\ndef case_state(case):\n    \"\"\"Return failed, skipped or passed for one testcase element.\"\"\"\n    tags = {child.tag for child in case}\n    if tags.intersection(FAIL_TAGS):\n        return \"failed\"\n    if \"skipped\" in tags:\n        return \"skipped\"\n    return \"passed\"\n\n\ndef read_run(path):\n    \"\"\"Return {test id: state} for one JUnit report, or raise ValueError.\"\"\"\n    try:\n        tree = ET.parse(path)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc.strerror or exc))\n    except ET.ParseError as exc:\n        raise ValueError(\"%s is not well-formed XML: %s\" % (path, exc))\n    root = tree.getroot()\n    if root.tag not in (\"testsuite\", \"testsuites\"):\n        raise ValueError(\"%s: root element is <%s>, not <testsuite>\"\n                         % (path, root.tag))\n    run = {}\n    for case in root.iter(\"testcase\"):\n        key = case_id(case, path)\n        state = case_state(case)\n        if run.get(key) == \"failed\" or state == \"failed\":\n            run[key] = \"failed\"\n        elif run.get(key) == \"passed\" or state == \"passed\":\n            run[key] = \"passed\"\n        else:\n            run.setdefault(key, state)\n    if not run:\n        raise ValueError(\"%s: no testcase elements\" % path)\n    return run\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        prog=\"flaky_tests.py\",\n        description=\"Find tests that both passed and failed across JUnit XML \"\n                    \"runs.\",\n        epilog=\"Read-only. Exit 0 no flaky test, 1 flaky tests found, \"\n               \"2 usage or input error.\",\n    )\n    parser.add_argument(\"files\", nargs=\"+\", metavar=\"JUNIT.xml\",\n                        help=\"two or more JUnit XML reports, one per run\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args()\n\n    if len(args.files) < 2:\n        parser.error(\"give at least two JUnit XML files, one per run\")\n\n    runs, states, broken = [], {}, False\n    for path in args.files:\n        try:\n            run = read_run(path)\n        except ValueError as exc:\n            print(\"error: %s\" % exc, file=sys.stderr)\n            broken = True\n            continue\n        runs.append((path, run))\n        for key, state in run.items():\n            states.setdefault(key, set()).add(state)\n\n    if len(runs) < 2:\n        print(\"error: need at least two valid JUnit reports, got %d\"\n              % len(runs), file=sys.stderr)\n        return 2\n\n    flaky = sorted(key for key, seen in states.items()\n                   if \"passed\" in seen and \"failed\" in seen)\n\n    if not args.quiet:\n        for path, run in runs:\n            failed = sum(1 for state in run.values() if state == \"failed\")\n            skipped = sum(1 for state in run.values() if state == \"skipped\")\n            print(\"%s: %d test%s, %d failed, %d skipped\"\n                  % (path, len(run), \"\" if len(run) == 1 else \"s\",\n                     failed, skipped))\n        for key in flaky:\n            passed = sum(1 for _p, run in runs if run.get(key) == \"passed\")\n            failed = sum(1 for _p, run in runs if run.get(key) == \"failed\")\n            print(\"  %s  passed %d, failed %d\" % (key, passed, failed))\n\n    print(\"%d flaky test%s in %d run%s\"\n          % (len(flaky), \"\" if len(flaky) == 1 else \"s\",\n             len(runs), \"\" if len(runs) == 1 else \"s\"))\n    if broken:\n        return 2\n    return 1 if flaky else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p437","n":"437","slug":"9-checks-for-a-reproducible-build","title":"9 checks for a reproducible build","sections":["devsecops"],"topic":"Release & build engineering","tags":["BuildSystems","DevOps","CICD","ReproducibleBuilds","Security"],"series_name":"DevOps","design":"checklist","script":"","date":"2029-07-12","files":{"devsecops":"notes/p437-9-checks-for-a-reproducible-build.md"},"md":"# 9 checks for a reproducible build\n\nA reproducible build is not a tool you install. It is nine properties of the inputs, the toolchain and the output.\n\n## Quick reference\n\n### Inputs\n\n- **Lockfile is committed** — npm ci, not npm install\n- **Base image is pinned** — tag or digest, never latest\n- **Build args are declared** — no hidden env in CI\n\n### Toolchain\n\n- **Compiler version is pinned** — same in CI and locally\n- **Build runs in a container** — one image, every runner\n- **Timestamps normalised** — `set SOURCE_DATE_EPOCH`\n\n### Output\n\n- **Artifact hash is recorded** — `sha256sum release.tar`\n- **Image digest is signed** — `cosign sign <digest>`\n- **Rebuild is byte-identical** — diff two builds\n\n## Explained\n\n### Inputs\n\nCommit the lockfile and install from it: npm ci rather than npm install, pip install with hashes. Pin the base image by tag, or better by digest, because latest moves under you. Declare every build argument, so a hidden environment variable in CI cannot change the result. A lockfile regenerated on every install is not a lockfile; it is a suggestion.\n\n### Toolchain\n\nPin the compiler version in the repository, not on the runner: a .tool-versions file, a container image, a wrapper script. Build inside that container, so the laptop and CI run the same binaries. Normalise timestamps with SOURCE_DATE_EPOCH, the reproducible-builds convention, so archives do not differ by the second.\n\n### Output\n\nRecord a hash of every artifact: `sha256sum release.tar` gives the next stage something to verify. Sign the image digest with cosign, so a deploy can verify the bits. Rebuild twice and diff the results. If they differ, one build is lying about its inputs.\n\n> **The honest caveat:** bit-for-bit is not always reachable. Compilers embed paths and build IDs, and a signed image carries a timestamp, so reproducible often means every difference is explained rather than absent.\n\n> **The lesson:** reproducibility is a property you check, not a flag you set.\n\n## In one line\n\n> **Same input, same output.** If two builds disagree, one is lying about its inputs.\n\n**Hash the output**\n\n```bash\nsha256sum dist/*.tar.gz\n```\n"},{"id":"p438","n":"438","slug":"9-commands-to-debug-a-slow-build","title":"9 commands to debug a slow build","sections":["devsecops"],"topic":"Release & build engineering","tags":["Docker","Performance","CICD","DevOps","BuildSystems"],"series_name":"DevOps","design":"terminal","script":"","date":"2029-07-15","files":{"devsecops":"notes/p438-9-commands-to-debug-a-slow-build.md"},"md":"# 9 commands to debug a slow build\n\nThe build took six minutes fifty-two. The interesting part is where those minutes went, and the answer is in nine commands.\n\n## Quick reference\n\n### Where did it go?\n\n```bash\n# wall clock first: CPU or waiting?\ntime docker build -t api:dev .\n# the fattest layer is the first suspect\ndocker history --no-trunc api:dev\n# one DONE line per step, with its duration\ndocker build --progress=plain . | grep -E 'DONE|CACHED'\n```\n\n### What was it doing?\n\n```bash\n# a wall of futex is waiting, not working\nstrace -f -c -o /tmp/b.strace make -j8\n# under one CPU is a parallelism bug\nperf stat -d make -j8\n# throttled means the limit, not the code\ncat /sys/fs/cgroup/cpu.stat\n```\n\n### What was rebuilt?\n\n```bash\n# know the size before you prune\ndocker buildx du\n# add --force in CI; it prompts\ndocker buildx prune --filter until=24h\n# pull the image first, or it is ignored\ndocker build --cache-from api:dev .\n```\n\n## Explained\n\n### Time\n\nThe run starts with `docker build --progress=plain -t api:dev` . and finishes in 412.8s. `time docker build -t api:dev .` gives the wall clock that matters. `docker history --no-trunc api:dev` shows each layer's size, where the fat usually hides. `docker build --progress=plain . | grep -E 'DONE|CACHED'` shows which steps ran and which were reused, each with its own duration.\n\n### Trace\n\n`strace -f -c -o /tmp/b.strace make -j8` counts syscalls per process, and a wall of futex means waiting rather than working. `perf stat -d make -j8` reports CPUs utilised; a number near 1.0 on an eight-core box is a parallelism bug. `cat /sys/fs/cgroup/cpu.stat` shows nr_throttled, which means the limit, not the code.\n\n### Cache\n\n`docker buildx du` reports the cache size and what is reclaimable. `docker buildx prune --filter until=24h` clears the old entries, and `--force` stops it asking in CI. `docker build --cache-from api:dev .` reuses an image's cache, provided the image was built with inline cache metadata and pulled first.\n\nThe end card repeats `docker buildx du` with one number: 14.2GB reclaimable.\n\n> **The honest caveat:** strace slows a build several-fold and perf needs permissions a container may not have, so use them to find the shape and then measure without them.\n\n> **The lesson:** every slow build is waiting, working or rebuilding, and the command only tells you which.\n\n## In one line\n\n> **Time it, then trust it.** A build nobody measured is a build nobody can fix.\n\n**Try it**\n\n```bash\ndocker buildx du\n```\n"},{"id":"p439","n":"439","slug":"make-vs-bazel-vs-nx","title":"Make vs Bazel vs Nx","sections":["devsecops"],"topic":"Release & build engineering","tags":["BuildSystems","Bazel","Nx","DevOps","Monorepo"],"series_name":"DevOps","design":"matrix","script":"","date":"2029-07-17","files":{"devsecops":"notes/p439-make-vs-bazel-vs-nx.md"},"md":"# Make vs Bazel vs Nx\n\nMake, Bazel and Nx are not three versions of the same tool. They disagree about what a build even is.\n\n## Quick reference\n\n| | **Make** | **Bazel** | **Nx** |\n|:--|:--|:--|:--|\n| Model | File timestamps | Content hashes | Task graph |\n| Caching | None built in | Local + remote | Local + remote |\n| Incremental | Per file, by mtime | Action-level | Task-level |\n| Ecosystem | Everywhere | JVM, C++, Go, JS | JS/TS plus plugins |\n| Learning curve | An afternoon | Weeks | Days |\n| Best for | Small native trees | Polyglot monorepos | JS monorepos |\n\n**How to pick**\n\n- **Make** → the default for a small tree.\n- **Bazel** → pay the setup once, cache everything.\n- **Nx** → the JS monorepo answer, with a graph.\n\n## Explained\n\n### The model\n\nMake is a set of file rules: a target depends on prerequisites, and the timestamp decides. Bazel is an action graph over content hashes: every action declares its inputs and outputs, and the hash decides. Nx is a task graph over projects: it knows which tasks depend on which, and which projects a change touches.\n\n### Cache and incrementality\n\nMake has no cache; it has mtimes, so touching a file rebuilds its targets. Bazel caches action results locally and remotely, keyed by the inputs and the command. Nx caches task results, locally and on Nx Cloud, and replays them for an unchanged project. Incrementality is per file for Make, per action for Bazel and per task for Nx.\n\n### Ecosystem and effort\n\nMake is everywhere and takes an afternoon to learn. Bazel covers JVM, C++, Go and JavaScript, and costs weeks before a large monorepo pays it back. Nx is JavaScript and TypeScript first, with plugins for other stacks, and a team is productive in days. Small native trees stay on Make, polyglot monorepos go to Bazel, and JS monorepos go to Nx.\n\n> **The honest caveat:** Bazel's remote cache needs a server and a genuinely hermetic build, and Nx's remote cache is an Nx Cloud feature. Neither is free to run, and a cache nobody can clear is a liability.\n\n> **The lesson:** pick the graph you can explain, then the cache that fits it.\n\n## In one line\n\n> **Pick the graph, not the logo.** The build tool you can debug at 2 a.m. is the fast one.\n\n**List the targets**\n\n```bash\nbazel query 'kind(rule, //...)'\n```\n"},{"id":"p440","n":"440","slug":"quiz-3-build-questions","title":"Quiz: 3 build questions","sections":["devsecops"],"topic":"Release & build engineering","tags":["BuildSystems","CICD","Testing","DevOps","Engineering"],"series_name":"DevOps","design":"quiz","script":"","date":"2029-07-19","files":{"devsecops":"notes/p440-quiz-3-build-questions.md"},"md":"# Quiz: 3 build questions\n\nThree questions separate a build that is fast by accident from one that is fast on purpose.\n\n## Quick reference\n\n**Q1. Why must a cache key hash every input?**\n\n- a) So the key stays short\n- b) So changed inputs miss the cache\n- c) So the cache survives a reboot\n\n> **Answer:** b) So changed inputs miss the cache. A key that ignores an input can return an output built from different sources. Hash what changes the result, and a change forces a miss.\n\n**Q2. Why do hermetic builds catch missing dependencies?**\n\n- a) They cannot reach the network\n- b) They compile faster\n- c) They skip the lockfile\n\n> **Answer:** a) They cannot reach the network. A hermetic build cannot fetch what the manifest omits, so the missing dependency fails in CI instead of in production.\n\n**Q3. Why do parallel test shards need isolation?**\n\n- a) So each shard runs faster\n- b) So shards cannot share state\n- c) So the report is smaller\n\n> **Answer:** b) So shards cannot share state. Shards that share a database, port or temp folder fail each other at random, and the flake gets blamed on the tests.\n\n## Explained\n\n### The cache key\n\n1. Why must a cache key hash every input?\n\nBecause an output is only reusable for the inputs that produced it. If the key ignores the lockfile, a dependency bump returns yesterday's build with a perfect hit rate. Hash the source, the lockfile, the compiler, the flags and the environment that changes the output. A key that changes too often is expensive; a key that changes too rarely is wrong.\n\n### Hermetic builds\n\n2. **Why do hermetic builds catch missing dependencies?**\n\nBecause they cannot reach the network. If a build fetches a package at compile time, it passes on the machine that already has it and fails on a clean runner. Cut the network and the missing declaration fails in CI, where you can see it, instead of in a customer's environment.\n\n### Shards\n\n3. **Why do parallel test shards need isolation?**\n\nBecause shared state turns scheduling into luck. Two shards writing the same database row, binding the same port or reusing the same temp folder fail each other at random, and the flake gets blamed on the test. Give each shard its own database, port range and workspace.\n\n> **The honest caveat:** a hermetic build is a direction, not a binary. Vendored dependencies and a local proxy still hide the moment a package enters, so keep the manifest the only door.\n\n> **The lesson:** speed comes from trusting a boundary, and the boundary is what you design.\n\n## In one line\n\n> **Know the why, not the flag.** A build system rewards the person who can explain the miss.\n\n**Hash the inputs**\n\n```bash\nsha256sum package-lock.json\n```\n"},{"id":"p441","n":"441","slug":"6-numbers-in-a-build-pipeline","title":"6 numbers in a build pipeline","sections":["devsecops"],"topic":"Release & build engineering","tags":["CICD","BuildSystems","Performance","DevOps","SRE"],"series_name":"DevOps","design":"numbers","script":"","date":"2029-07-22","files":{"devsecops":"notes/p441-6-numbers-in-a-build-pipeline.md"},"md":"# 6 numbers in a build pipeline\n\nA build pipeline is a chain of waiting rooms, and the only one people time is the build.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **85%** | Cache hit rate | A working target; below 60% the key is usually wrong. | cache |\n| **3m** | Queue time | The wait before a runner starts; invisible in the log. | queue |\n| **6m** | Build time | First instruction to pushed image, at p50 and p95. | build |\n| **8m** | Test time | The suite that gates the merge; shard before rewriting. | test |\n| **180MB** | Artifact size | Pulled by every environment that deploys it. | image |\n| **2%** | Flake rate | Two runs in a hundred fail for no reason. Trust goes. | flaky |\n\n## Explained\n\n### Cache and queue\n\n85% cache hit rate is a working target: most steps replay, and the misses are the ones that changed. Below 60%, the key is usually wrong, not the cache. 3m queue time is the wait before a runner picks the job up. It is invisible in the build log and very visible to the developer who is waiting.\n\n### Build and test\n\n6m build time is the step from first instruction to pushed image, measured at p50 and p95. 8m test time is the suite that gates the merge. Shard it before you rewrite it, because a suite that long is mostly waiting, and a rewrite is the expensive way to find that out.\n\n### Size and flake\n\n180MB artifact size is pulled by every environment that deploys it, so a fat image is a tax on every node. 2% flake rate means two runs in a hundred fail for no reason, which is enough to teach the team to press re-run and stop reading failures.\n\n> **The honest caveat:** none of these is universal. A fifteen-minute build is fine for firmware and a scandal for a web API, so compare against your own last ten runs, not someone else's slide.\n\n> **The lesson:** the pipeline is the sum of the parts nobody timed.\n\n## In one line\n\n> **Measure the queue, not just the build.** The pipeline is the sum of the parts nobody timed.\n\n**Export the jobs**\n\n```bash\ngh run view 123 --json jobs\n```\n"},{"id":"p442","n":"442","slug":"environment-topology-drawn","title":"Environment topology, drawn","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","Environments","CICD","CloudNative","SRE"],"series_name":"DevOps","design":"architecture","script":"","date":"2029-07-24","files":{"devsecops":"notes/p442-environment-topology-drawn.md"},"md":"# Environment topology, drawn\n\nA pull request opens and ninety seconds later a URL answers with the change running. That is the first tier of an environment topology, and it is the one most teams never build.\n\n## Quick reference\n\n- **Dev · Ephemeral:** Pull request (PR) · Preview environment (ENV) · Preview URL (URL)\n- **Staging · Shared:** Shared staging (STG) · Seeded data (SEED) · Smoke tests (SMOKE)\n- **Prod · Multi-az:** Load balancer (LB) · App, multi-AZ (APP) · Read replica (READ)\n\n**Request flow:** Pull request → Preview environment → Preview URL; Preview environment → Shared staging → Smoke tests → Load balancer → App, multi-AZ → Read replica; Seeded data → Shared staging\n\n## Explained\n\n### Three tiers\n\nDEV is ephemeral. Each pull request gets its own namespace, its own deployment and its own preview URL, built from the same chart as production. Close or merge the request and the namespace goes with it, which is what keeps the tier cheap and honest.\n\nSTAGING is shared. One deployment per team, one known dataset loaded from fixtures, and a smoke test that runs after every deploy. Staging exists to catch integration, not to be a second production.\n\nPROD is multi-AZ. A load balancer spreads traffic across zones, the application runs in more than one, and a read replica takes the reporting queries. It differs by scale and topology, not by build.\n\n### One path\n\nThe same artefact is promoted: the image digest that passed the preview is the digest that reaches staging and production. Only the configuration differs between tiers, which is factor three of the twelve-factor app. The seeded staging data is synthetic, never a copy of customer records.\n\n> **The honest caveat:** a preview environment is only a preview if it runs the same services and versions. Add a different database engine or skip a queue and you are testing a demo.\n\n> **The lesson:** the three tiers are one system at different sizes, and the promotion path is the product.\n\n## In one line\n\n> **Same code, different config.** Promote the artefact, never rebuild it for the next tier.\n\n**Check the digest**\n\n```bash\ndocker image inspect api:1.4.2\n```\n"},{"id":"p443","n":"443","slug":"the-environment-debugging-cheat-sheet","title":"The environment debugging cheat sheet","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","Troubleshooting","Kubernetes","Linux","SRE"],"series_name":"DevOps","design":"cheatsheet","script":"","date":"2029-07-26","files":{"devsecops":"notes/p443-the-environment-debugging-cheat-sheet.md"},"md":"# The environment debugging cheat sheet\n\nThe deploy is green and the app still cannot reach the database. Before you read the code, read the environment: four blocks cover almost every case.\n\n## Quick reference\n\n### DNS\n\n```bash\ndig api.staging.example.com +short\ndig +trace api.example.com\ncat /etc/resolv.conf\ngetent hosts api.staging\nnslookup api.staging\n```\n\n### Ports\n\n```bash\nss -tlnp | grep 5432\nss -tn state established\ncurl -sv https://api.staging/health\nnc -zv db.staging 5432\nkubectl get svc api -n staging\nkubectl port-forward svc/api 8080:80\n```\n\n### Config\n\n```bash\nprintenv | sort\nenv | grep -i database\nkubectl exec deploy/api -- printenv\nkubectl get cm api -o yaml\ndiff .env.staging .env.prod\npython3 -m json.tool config.json\n```\n\n### Logs\n\n```bash\nkubectl logs deploy/api -f\nkubectl logs deploy/api --tail=100\njournalctl -u api -n 100 --no-pager\ndocker logs --tail 100 api\ngrep -i error /var/log/api.log\nkubectl logs -l app=api --since=1h\n```\n\n## Explained\n\n### DNS\n\nStart with names. dig with +short returns the address the resolver hands back. `dig +trace` walks the delegation from the root, which shows whether the record or the resolver is wrong. `cat /etc/resolv.conf` shows which resolver the host uses, and getent hosts follows the same path as the application; dig does not read `/etc/hosts`.\n\n### Ports\n\nThen check the path. `ss -tlnp` lists listening TCP sockets with the process that owns them. `ss -tn state established` shows what is actually connected. `curl -sv` prints the handshake and the status line, `nc -zv` tests one port, and kubectl port-forward proves the service works before you blame the network.\n\n### Config\n\nThen the values. printenv dumps the process environment, env | grep narrows it, and `kubectl exec deploy/api -- printenv` reads what the container really has. kubectl get cm shows the ConfigMap behind it, diff compares two env files, and json.tool catches a broken JSON config.\n\n### Logs\n\nThen the story. `kubectl logs -f` follows a pod, `--tail` keeps the last hundred lines, journalctl serves systemd units, docker logs serves containers, and a grep for error finds the line. Add `--since=1h` to ignore yesterday.\n\n> **The honest caveat:** slim images often ship without dig, ss or nc; keep a debug pod, or use kubectl debug, so the tools exist when you need them.\n\n> **The lesson:** the environment is a hypothesis, and each block is a cheap test.\n\n## In one line\n\n> **The environment is a hypothesis.** Four blocks turn a guess into a check.\n\n**Start with dns**\n\n```bash\ndig api.staging +short\n```\n"},{"id":"p444","n":"444","slug":"four-sources-of-environment-drift","title":"Four sources of environment drift","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","SRE","Configuration","CloudNative","CICD"],"series_name":"DevOps","design":"explainer","script":"","date":"2029-07-29","files":{"devsecops":"notes/p444-four-sources-of-environment-drift.md"},"md":"# Four sources of environment drift\n\nNothing changed, said the deploy log. The environment disagreed: the pod that worked on Tuesday failed on Thursday, and no commit explained it. Drift rarely announces itself.\n\n## Quick reference\n\n### Manual changes\n\n- A hotfix typed into a console at 2 a.m.\n- It works, so nobody writes it down\n- The next rebuild silently removes it\n\n### Config drift\n\n- A value edited in a dashboard, not in Git\n- Two services read the same key differently\n- The repo says one thing, the runtime another\n\n### Secret rotation\n\n- A key rotated in one environment only\n- The old credential still works somewhere\n- The new key never reached the batch job\n\n### Dependency versions\n\n- A base image tag moved under you\n- Two lockfiles resolved different versions\n- Staging and prod run different majors\n\n> **Note:** Drift is not one mistake. It is a gap you stopped measuring.\n\n## Explained\n\n### Manual changes\n\nSomeone edits a value in a console at 2 a.m., the fix works, and nobody writes it down. If the next apply rebuilds the resource, the change disappears and the bug returns. If nothing applies the manifest, the change stays and the definition is now a lie. Both endings are drift.\n\n### Config drift\n\nA value is edited in a dashboard rather than in Git. Two services read the same key and get different answers, because one reads a ConfigMap and the other reads a host env file. The repository says one thing and the runtime says another, and neither is wrong enough to fail a health check.\n\n### Secret rotation\n\nA key is rotated in production but not in staging, or the old credential is never revoked. A batch job keeps a static copy and keeps working until the day it does not. Rotation without revocation is a second secret, not a replacement.\n\n### Dependency versions\n\nA base image tag moves, a lockfile resolves differently on a rebuild, or staging installs the latest minor while production pins the previous one. The code is identical; the tree underneath it is not.\n\n> **The honest caveat:** some drift is a deliberate hotfix. The point is not to forbid the change, it is to record it before the next person inherits it.\n\n> **The lesson:** drift is not one mistake, it is a gap you stopped measuring.\n\n## In one line\n\n> **Not written down is still drift.** A change nobody recorded is a change nobody can repeat.\n\n**Diff the baseline**\n\n```bash\nterraform plan -detailed-exitcode\n```\n"},{"id":"p445","n":"445","slug":"an-environment-diff-in-python","title":"An environment diff in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DevOps","Configuration","CICD","SRE"],"series_name":"DevOps","design":"code","script":"env_diff.py","date":"2029-07-31","files":{"devsecops":"notes/p445-an-environment-diff-in-python.md"},"md":"# An environment diff in Python\n\nStaging has a key production does not, and nobody can say which one is right. Two JSON manifests and one Python script settle it in a second.\n\n## Quick reference\n\nThe full script is at the end of this note: `env_diff.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 env_diff.py staging.json prod.json\n```\n\n**Example output**\n\n```text\nMISSING API_KEY ***\nMISSING 2 EXTRA 1 CHANGED 2\n```\n\n## Explained\n\n### The code\n\nThe editor shows a short script. First the imports and a regex that matches secret-looking names: SECRET, TOKEN, KEY, PASS. The m function returns three stars for those keys and truncates every other value to forty characters, so the report fits a terminal. Then it loads both manifests from the command line and builds three sorted lists: keys missing from the target, keys only in the target, and keys present in both with different values. Three loops print each list, a summary prints the counts, and the exit call returns 1 if any list is non-empty.\n\n### The run\n\n`python3 env_diff.py staging.json prod.json` compares the two files. The output starts with a masked MISSING line and ends with the counts: two missing, one extra and two changed. Nothing leaves the machine and nothing is written.\n\n### How to use it\n\nExport each environment to a flat JSON object, keep the files beside the chart, and run the diff in CI before a deploy. Exit 0 means the manifests agree, 1 means they differ, 2 means a file is missing or not valid JSON.\n\n> **The honest caveat:** the diff compares declared values, not the running process. A variable injected at boot or overridden on one host will not appear; compare printenv dumps when the question is what is live.\n\n> **The lesson:** a readable diff is the cheapest review an environment gets.\n\n## In one line\n\n> **Parity is a diff you can read.** Run it before a deploy, not after the outage.\n\n**Run it in ci**\n\n```bash\npython3 env_diff.py a.json b.json\n```\n\n## Script: env_diff.py\n\nDownload: [scripts/env_diff.py](../scripts/env_diff.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"env_diff.py - compare two environment manifests and report the differences.\n\nEach manifest is a flat JSON object that maps a configuration key to a value.\nThe script reports keys missing from the second manifest, keys that exist only\nin the second, and keys whose value changed. Values under secret-looking keys\nare masked, so the report is safe to paste into a ticket or a CI log.\n\nUsage:\n    python3 env_diff.py base.json target.json\n    python3 env_diff.py staging.json production.json\n    python3 env_diff.py --help\n\nExit codes: 0 no differences, 1 differences found, 2 usage or input error.\nRead-only: both files are opened for reading and nothing is written.\n\nCI:   python3 env_diff.py staging.json prod.json || echo \"environments differ\"\nCron: 0 6 * * * python3 /opt/s0sta/env_diff.py /srv/app/staging.json /srv/app/prod.json\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\nSECRET_KEY = re.compile(\n    r\"(SECRET|TOKEN|PASSWORD|PASSWD|API_KEY|ACCESS_KEY|PRIVATE|CREDENTIAL\"\n    r\"|(^|_)(KEY|PASS)(_|$))\",\n    re.I,\n)\n\nURL_CREDS = re.compile(r\"(://[^/\\s:@]+:)[^/\\s@]+@\")\n\n\nclass InputError(Exception):\n    \"\"\"A manifest that cannot be read or does not look like an object.\"\"\"\n\n\ndef load_manifest(path):\n    \"\"\"Read one flat JSON object and return it as a dict.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise InputError(\"%s must contain a JSON object\" % path)\n    return data\n\n\ndef display(key, value):\n    \"\"\"Return a printable value, masked when the key or URL looks secret.\"\"\"\n    if SECRET_KEY.search(key):\n        return \"***\"\n    text = URL_CREDS.sub(r\"\\1***@\", str(value))\n    return text if len(text) <= 40 else text[:37] + \"...\"\n\n\ndef compare(base, target):\n    \"\"\"Return (missing, extra, changed), all sorted.\"\"\"\n    missing = sorted(set(base) - set(target))\n    extra = sorted(set(target) - set(base))\n    changed = sorted(\n        k for k in set(base) & set(target) if base[k] != target[k])\n    return missing, extra, changed\n\n\ndef report(base, target, missing, extra, changed):\n    \"\"\"Print the three sections and the summary line.\"\"\"\n    for key in missing:\n        print(\"MISSING %s = %s\" % (key, display(key, base[key])))\n    for key in extra:\n        print(\"EXTRA %s = %s\" % (key, display(key, target[key])))\n    for key in changed:\n        print(\"CHANGED %s: %s -> %s\"\n              % (key, display(key, base[key]), display(key, target[key])))\n    print(\"%d missing, %d extra, %d changed\"\n          % (len(missing), len(extra), len(changed)))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"env_diff.py\",\n        description=\"Compare two JSON environment manifests and report keys \"\n                    \"that are missing, extra or changed. Secret-looking \"\n                    \"values are masked.\")\n    parser.add_argument(\"base\", help=\"baseline manifest, a JSON object\")\n    parser.add_argument(\"target\",\n                        help=\"manifest to compare against the baseline\")\n    args = parser.parse_args(argv)\n\n    try:\n        base = load_manifest(args.base)\n        target = load_manifest(args.target)\n    except InputError as exc:\n        parser.error(str(exc))\n\n    missing, extra, changed = compare(base, target)\n    report(base, target, missing, extra, changed)\n    return 1 if (missing or extra or changed) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p446","n":"446","slug":"a-config-linter-in-python","title":"A config linter in Python","sections":["devsecops"],"topic":"Python scripts","tags":["DevOps","Python","Security","Configuration","CICD"],"series_name":"DevOps","design":"code","script":"config_lint.py","date":"2029-08-02","files":{"devsecops":"notes/p446-a-config-linter-in-python.md"},"md":"# A config linter in Python\n\nA production .env file is a small document with a large blast radius. Four mistakes account for most of the incidents it causes, and they are cheap to catch before a deploy.\n\n## Quick reference\n\nThe full script is at the end of this note: `config_lint.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 config_lint.py .env.prod --require DATABASE_URL\n```\n\n**Example output**\n\n```text\nDUP 1 EMPTY 1\nMISSING 1 SECRET 1\n```\n\n## Explained\n\n### Four checks\n\nDuplicate keys. The same key appears twice, and the last one wins silently. The value someone read at the top of the file is not the value the process gets.\n\nEmpty values. A key exists but its value is empty after quotes are stripped. It passes a presence check and fails at the first request.\n\nMissing required keys. The file never defines DATABASE_URL, or defines a near miss like DATABASE_URLS. The process starts and dies on the first connection.\n\nPlaintext secrets. A secret-looking key holds a production-looking value: not a placeholder, not an interpolation. If it is committed, assume it is leaked and rotate it before you remove it; Git remembers history.\n\n### The run\n\nRun `python3 config_lint.py .env.prod --require DATABASE_URL`. The editor shows the condensed checker: it splits each KEY=VALUE line and builds four lists: duplicates, empty values, missing required keys, and secret keys with values over seven characters. The two output lines give the counts, and the exit code is 1 when any list is non-empty.\n\nThe attached script adds line numbers, JSON duplicate detection, placeholder rules and a per-finding report.\n\n> **The honest caveat:** a secret that looks real may be a dead test key, and a strong production key may look ordinary. The linter finds commits, not live credentials; rotate first, then clean.\n\n> **The lesson:** a config file is code, so lint it like code.\n\n## In one line\n\n> **Declared is not the same as live.** A linter reads what is written; a diff reads what runs.\n\n**Lint before deploy**\n\n```bash\npython3 config_lint.py .env.prod\n```\n\n## Script: config_lint.py\n\nDownload: [scripts/config_lint.py](../scripts/config_lint.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"config_lint.py - lint .env files and JSON config for common mistakes.\n\nFour checks, per file:\n\n    duplicate  the same key defined more than once\n    empty      a key whose value is empty\n    missing    a key named by --require that is not defined\n    secret     a secret-looking key committed with a real value in plaintext\n\nUsage:\n    python3 config_lint.py .env\n    python3 config_lint.py .env.prod config.json --require DATABASE_URL\n    python3 config_lint.py config/*.json --require API_KEY --require REGION\n    python3 config_lint.py --help\n\nThe .env reader takes KEY=VALUE lines, skips blank lines and # comments, allows\nan export prefix, and strips one layer of matching quotes. The JSON reader\ntakes a flat or nested object, reports nested keys in dotted form, and detects\nduplicate keys with object_pairs_hook because json.load keeps only the last.\n\nExit codes: 0 clean, 1 findings, 2 usage or input error.\nRead-only: every file is opened for reading and nothing is written.\n\nPlaceholders are not findings: changeme, ${VAR}, <your-key>, ***, example,\nnone, null, todo and anything starting with your- or replace- are ignored.\n\nCI:   python3 config_lint.py .env.prod --require DATABASE_URL --require API_KEY\nCron: 30 7 * * 1 python3 /opt/s0sta/config_lint.py /srv/app/.env.prod --require DATABASE_URL\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\nfrom pathlib import Path\n\nSECRET_KEY = re.compile(\n    r\"(SECRET|TOKEN|PASSWORD|PASSWD|API_KEY|ACCESS_KEY|PRIVATE|CREDENTIAL\"\n    r\"|(^|_)(KEY|PASS)(_|$))\",\n    re.I,\n)\n\nTOKEN_SHAPE = re.compile(\n    r\"(AKIA[0-9A-Z]{16}\"\n    r\"|sk_live_[0-9A-Za-z]{8,}\"\n    r\"|ghp_[0-9A-Za-z]{20,}\"\n    r\"|github_pat_[0-9A-Za-z_]{20,}\"\n    r\"|xox[baprs]-[0-9A-Za-z-]{10,}\"\n    r\"|-----BEGIN [A-Z ]*PRIVATE KEY-----\"\n    r\"|://[^/\\s:@]+:[^/\\s@]+@)\"\n)\n\nPLACEHOLDERS = {\"***\", \"changeme\", \"change-me\", \"example\", \"none\", \"null\",\n                \"todo\", \"xxx\"}\nPLACEHOLDER_PREFIXES = (\"${\", \"<\", \"your-\", \"replace-\")\n\n\nclass InputError(Exception):\n    \"\"\"A file that cannot be read or does not parse.\"\"\"\n\n\ndef is_placeholder(value):\n    \"\"\"True for an empty or obviously fake value.\"\"\"\n    text = value.strip().lower()\n    return (not text or text in PLACEHOLDERS\n            or text.startswith(PLACEHOLDER_PREFIXES))\n\n\ndef looks_secret(key, value):\n    \"\"\"True when a key and value look like a plaintext secret.\"\"\"\n    text = str(value).strip()\n    if is_placeholder(text) or \"PUBLIC_KEY\" in key.upper():\n        return False\n    if TOKEN_SHAPE.search(text):\n        return True\n    return bool(SECRET_KEY.search(key)) and len(text) >= 8\n\n\ndef read_text(path):\n    \"\"\"Read one UTF-8 file, or raise InputError.\"\"\"\n    try:\n        return path.read_text(encoding=\"utf-8\")\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except UnicodeDecodeError as exc:\n        raise InputError(\"cannot decode %s: %s\" % (path, exc))\n\n\ndef parse_env(text):\n    \"\"\"Return (rows, duplicates) for a .env document.\n\n    rows is a list of (line number, key, value); duplicates is a list of\n    (line number, key, first line number).\n    \"\"\"\n    rows, duplicates, first = [], [], {}\n    for number, raw in enumerate(text.splitlines(), 1):\n        line = raw.strip()\n        if not line or line.startswith(\"#\") or \"=\" not in line:\n            continue\n        key, value = line.split(\"=\", 1)\n        key = key.strip()\n        if key.startswith(\"export \"):\n            key = key[len(\"export \"):].strip()\n        if not key:\n            continue\n        value = value.strip()\n        if len(value) >= 2 and value[0] == value[-1] and value[0] in \"\\\"'\":\n            value = value[1:-1]\n        if key in first:\n            duplicates.append((number, key, first[key]))\n        else:\n            first[key] = number\n        rows.append((number, key, value))\n    return rows, duplicates\n\n\ndef flatten(obj, prefix=\"\"):\n    \"\"\"Yield (dotted key, value) for a nested JSON object.\"\"\"\n    for key, value in obj.items():\n        full = prefix + key\n        if isinstance(value, dict):\n            yield from flatten(value, full + \".\")\n        else:\n            yield full, value\n\n\ndef parse_json(path, text):\n    \"\"\"Return (rows, duplicate keys) for a JSON object.\"\"\"\n    duplicates = []\n\n    def hook(pairs):\n        obj = {}\n        for key, value in pairs:\n            if key in obj:\n                duplicates.append(key)\n            obj[key] = value\n        return obj\n\n    try:\n        data = json.loads(text, object_pairs_hook=hook)\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise InputError(\"%s must contain a JSON object\" % path)\n    rows = [(None, key, value) for key, value in flatten(data)]\n    return rows, sorted(set(duplicates))\n\n\ndef lint_file(path, required):\n    \"\"\"Return a list of (line number or None, message) findings.\"\"\"\n    text = read_text(path)\n    findings = []\n    if path.suffix.lower() == \".json\":\n        rows, duplicate_keys = parse_json(path, text)\n        for key in duplicate_keys:\n            findings.append((None, \"duplicate key %s\" % key))\n    else:\n        rows, duplicates = parse_env(text)\n        for number, key, first in duplicates:\n            findings.append((number, \"duplicate key %s (first at line %d)\"\n                             % (key, first)))\n\n    keys = {key for _, key, _ in rows}\n    for key in required:\n        if key not in keys:\n            findings.append((None, \"missing required key %s\" % key))\n\n    seen = set()\n    for number, key, value in rows:\n        if key in seen:\n            continue\n        seen.add(key)\n        if str(value).strip() == \"\":\n            findings.append((number, \"empty value for %s\" % key))\n        elif looks_secret(key, value):\n            findings.append((number, \"plaintext secret in %s\" % key))\n    return findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        prog=\"config_lint.py\",\n        description=\"Lint .env files and JSON config for duplicate keys, \"\n                    \"empty values, missing required keys and plaintext \"\n                    \"secrets.\")\n    parser.add_argument(\"files\", nargs=\"+\", type=Path, metavar=\"FILE\",\n                        help=\".env or .json file to check\")\n    parser.add_argument(\"--require\", action=\"append\", default=[], metavar=\"KEY\",\n                        help=\"key that must exist; repeatable\")\n    args = parser.parse_args(argv)\n\n    required = list(dict.fromkeys(args.require))\n    total = 0\n    for path in args.files:\n        try:\n            findings = lint_file(path, required)\n        except InputError as exc:\n            parser.error(str(exc))\n        for number, message in findings:\n            where = \"%s:%d\" % (path, number) if number else str(path)\n            print(\"%s: %s\" % (where, message))\n        total += len(findings)\n\n    if total:\n        print(\"%d finding%s in %d file%s\"\n              % (total, \"\" if total == 1 else \"s\",\n                 len(args.files), \"\" if len(args.files) == 1 else \"s\"))\n        return 1\n    print(\"OK %d file%s clean\"\n          % (len(args.files), \"\" if len(args.files) == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p447","n":"447","slug":"9-steps-to-a-preview-environment","title":"9 steps to a preview environment","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","Kubernetes","CICD","GitOps","PlatformEngineering"],"series_name":"DevOps","design":"steps","script":"","date":"2029-08-05","files":{"devsecops":"notes/p447-9-steps-to-a-preview-environment.md"},"md":"# 9 steps to a preview environment\n\nA reviewer opens pull request 142 and asks for a demo. Ten minutes later the branch is running at its own URL, with data, and the link is in the thread. That is a preview environment: a lifecycle, not a server.\n\n## Quick reference\n\n### Create — Give the pull request a home\n\n```bash\n# The pipeline names the preview after the PR number.\ngit push origin feature/checkout\n# One namespace fences the preview from every other.\nkubectl create ns pr142\n# The staging chart, with a preview values file.\nhelm upgrade --install pr142 ./chart -n pr142\n```\n\n### Seed — Make it look like a real system\n\n```bash\n# Synthetic fixtures, never a production copy.\npg_restore -d preview seed.dump\n# The same schema version as staging, not a guess.\npython3 manage.py migrate\n# Fail the job if the app does not answer.\ncurl -fsS $PREVIEW_URL/health\n```\n\n### Destroy — Take it down when the PR closes\n\n```bash\n# Remove the release, not only the running pods.\nhelm uninstall pr142 -n pr142\n# The namespace takes secrets and volumes with it.\nkubectl delete ns pr142\n# Uploads and test artefacts live outside the cluster.\naws s3 rm s3://previews/pr142 --recursive\n```\n\n## Explained\n\n### Create\n\n1. **Push the branch.** The pipeline names the environment after the pull request number, so two previews never collide.\n   \\2. Create the namespace. One namespace per preview fences its deployment, service, ingress and secrets from other previews.\n   \\3. Install the chart. The same chart as staging, with a values file that points at the preview database and a lower replica count. The ingress prints the URL.\n\n### Seed\n\n4. **Restore the seed dump.** Synthetic fixtures, never a production copy; the point is realistic shapes without real people.\n   \\5. Run the migrations. The preview database starts at the same schema version as staging.\n   \\6. Hit the health endpoint. `curl -fsS` on /health fails the job if the app does not answer, so a broken preview is never announced.\n\n### Destroy\n\n7. **Uninstall the release.** helm uninstall removes the objects the chart owns.\n   \\8. Delete the namespace. It takes the service, ingress, secrets and volumes with it.\n   \\9. Delete the bucket prefix. Uploads and test artefacts live outside the cluster, so the sweep has to include object storage.\n\n> **The honest caveat:** previews cost money and abandoned ones rot. Add a scheduled sweeper that deletes any preview older than seven days, or the webhook will eventually fail and leave one behind.\n\n> **The lesson:** the value of a preview is that it disappears.\n\n## In one line\n\n> **A preview is a lease, not a server.** Create it, seed it, and take it down when the pull request closes.\n\n**Destroy on close**\n\n```bash\ngh pr close 142 --delete-branch\n```\n"},{"id":"p448","n":"448","slug":"environment-variables-vs-a-config-service","title":"Environment variables vs a config service","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","Configuration","CloudNative","SecretsManagement","SRE"],"series_name":"DevOps","design":"compare","script":"","date":"2029-08-07","files":{"devsecops":"notes/p448-environment-variables-vs-a-config-service.md"},"md":"# Environment variables vs a config service\n\nEvery platform eventually asks whether configuration belongs in environment variables or in a config service, and the argument usually starts with a strong opinion and no example.\n\n## Quick reference\n\n| | **ENV VARS** | **CONFIG SVC** |\n|:--|:--|:--|\n| Where it lives | In the process | In a remote service |\n| Change needs | A restart or redeploy | A restart or reload |\n| Secrets | Easy to leak in CI logs | Scoped, audited access |\n| Versioning | Git history, if at all | A version per change |\n| Drift risk | Per-host overrides | One source of truth |\n| Best for | Small, static settings | Many environments, audit |\n\n**ENV VARS** (Keys in the process): Set by the shell, a manifest or CI, and inherited by every child process.\n\n```bash\nexport DATABASE_URL=...\n```\n\n**CONFIG SVC** (Keys served at runtime): A managed API: versioned values, scoped per environment.\n\n```bash\naws appconfig get-configuration\n```\n\n- Env vars → local, cheap, easy to leak.\n- Config service → versioned, scoped, audited.\n\n## Explained\n\n### Where each one fits\n\nEnvironment variables live in the process. The shell, a Kubernetes manifest or a CI job sets them, every child process inherits them, and a change means a restart or a redeploy. That is cheap and local, and it is why twelve-factor apps put config there.\n\nA config service lives outside the process. A managed API versions values, scopes them per environment and records changes, and the app fetches them at boot or watches for updates. A change can reach a running service without a rebuild, and access is a permission rather than a file.\n\n### The rows in the picture\n\nWhere it lives is the process versus a remote service. A change needs a restart versus a reload. Secrets are easy to leak in CI logs versus scoped and audited. Versioning is Git history, if anything, versus a version per change. Drift risk is per-host overrides versus one source of truth. Best for is small static settings versus many environments with an audit requirement. The panel at the bottom repeats the split in one line each.\n\n> **The honest caveat:** a config service is another dependency on the boot path. Cache the last known good values and decide what happens when the service is unreachable, or a control plane outage becomes an application outage.\n\n> **The lesson:** use environment variables for the process, and a service for what must change without a deploy.\n\nConfig is a runtime concern.\n"},{"id":"p449","n":"449","slug":"environment-health-at-a-glance","title":"Environment health at a glance","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","SRE","PlatformEngineering","Observability","CloudNative"],"series_name":"DevOps","design":"dashboard","script":"","date":"2029-08-09","files":{"devsecops":"notes/p449-environment-health-at-a-glance.md"},"md":"# Environment health at a glance\n\nMonday stand-up starts with the same worry: something is about to break. Six tiles cover the part of it that numbers can, and the rest is a conversation.\n\n## Quick reference\n\n**Status:** This week\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Config drift | **3 envs** | declared vs live, 24h |\n| Failed deploys | **2** | of 41, last 7 days |\n| Stale previews | **33%%** | 6 of 18 open |\n| Missing env vars | **4** | found at boot, prod |\n| Secret age | **47 days** | median, oldest 210 |\n| Environment cost | **$3.8k** | staging and previews |\n\n## Explained\n\n### The tiles\n\nConfig drift: three environments no longer match their manifests, measured against the last applied baseline. A tile without a baseline measures nothing.\n\nFailed deploys: two of forty-one in the last week. A failed deploy that rolled back cleanly is a different event from one that left the environment half-applied, so read the count with the rollback log.\n\nStale previews: six of eighteen, a third, older than seven days. Each one still costs money and still holds a URL and a set of secrets.\n\nMissing env vars: four found at boot in production this week. The app failed fast, which is the good version; the bad version starts with a default and fails later.\n\nSecret age: forty-seven days at the median, with the oldest at two hundred and ten. Age is not compromise, but an old credential has had more chances to leak.\n\nEnvironment cost: three thousand eight hundred dollars for the month across staging and previews. A preview that no one has opened in a week is the easiest line to cut.\n\n### The review\n\n> **The honest caveat:** every tile is a trailing indicator, and a green board can be a week out of date. Check the timestamp before you quote it in the review.\n\n> **The lesson:** the dashboard starts the review; it does not replace it.\n\n## In one line\n\n> **A green tile is not a green env.** Every tile names a question; none of them answers it.\n\n**Count the drift**\n\n```bash\nterraform plan -detailed-exitcode\n```\n"},{"id":"p450","n":"450","slug":"quiz-3-environment-questions","title":"Quiz: 3 environment questions","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","SRE","Environments","CICD","CloudNative"],"series_name":"DevOps","design":"quiz","script":"","date":"2029-08-12","files":{"devsecops":"notes/p450-quiz-3-environment-questions.md"},"md":"# Quiz: 3 environment questions\n\nThree questions separate an environment you trust from one you hope is fine. Answer before the reveal, then check the question that caught you.\n\n## Quick reference\n\n**Q1. Why must staging never share the production secrets?**\n\n- a) A leak there becomes a prod leak\n- b) Staging is too slow for real keys\n- c) The values would not parse\n\n> **Answer:** a) A leak there becomes a prod leak. A shared secret collapses the boundary: a copied .env or a weaker staging host hands an attacker production access. Scope and rotate each secret per environment.\n\n**Q2. What must a preview environment destroy when its pull request closes?**\n\n- a) The namespace and its data\n- b) Only the preview URL\n- c) Nothing, to keep it warm\n\n> **Answer:** a) The namespace and its data. The namespace carries the deployment, service, ingress, secrets and volumes. A preview is a lease, so the close ends it; a sweeper catches missed webhooks.\n\n**Q3. Why is environment drift invisible without a baseline?**\n\n- a) Nothing records what correct was\n- b) Drift only happens at night\n- c) Baselines slow the deploy\n\n> **Answer:** a) Nothing records what correct was. Drift is a difference, so it needs a comparison: a manifest, a saved plan or a snapshot. Without one, whatever is running becomes the definition of correct.\n\n## Explained\n\n### The boundary\n\nWhy must staging never share production secrets? Because the boundary is the point. A copied .env, a debug log or a weaker staging host turns a staging compromise into a production one. Scope each secret to one environment, rotate it there, and give staging its own credentials with no production rights.\n\n### The lease\n\nWhat must a preview environment destroy when its pull request closes? The namespace and its data: deployment, service, ingress, secrets, volumes and the database rows. If the database lives outside the cluster, the teardown deletes its rows as well. A preview is a lease, not a server. Delete it on close, and run a scheduled sweeper for the webhooks that never arrive.\n\n### The baseline\n\nWhy is drift invisible without a baseline? Because drift is a difference, and a difference needs two sides. Without a manifest, a saved plan or a snapshot, whatever is running becomes the definition of correct. The first step of any drift check is to record what you expect.\n\n> **The honest caveat:** a baseline is only useful if it is current. A snapshot from six months ago records a state nobody wants any more, and every diff against it will be noise.\n\n> **The lesson:** parity, cleanup and a baseline are the three habits behind every calm environment.\n\n## In one line\n\n> **A baseline is how drift shows.** Three questions, one habit: compare before you trust.\n\n**Snapshot it**\n\n```bash\nterraform plan -out=tfplan\n```\n"},{"id":"p451","n":"451","slug":"5-environment-mistakes","title":"5 environment mistakes","sections":["devsecops"],"topic":"DevOps basics","tags":["DevOps","SRE","PlatformEngineering","CICD","CloudNative"],"series_name":"DevOps","design":"dontdo","script":"","date":"2029-08-14","files":{"devsecops":"notes/p451-5-environment-mistakes.md"},"md":"# 5 environment mistakes\n\nEvery environment problem I have seen started as a shortcut that saved an afternoon. Five of them are common enough to name.\n\n## Quick reference\n\n**1.** ❌ **Prod secrets in staging** — Staging is weaker, shared and logged, so a leak there reaches production.  \n✅ **One secret per environment** — Scoped values, separate rotation, separate blast radius.\n\n**2.** ❌ **Hand-edited config on a host** — Nobody records it, and the next deploy erases it or keeps a lie.  \n✅ **Change the manifest, then apply** — Reviewed, repeatable and visible in the diff.\n\n**3.** ❌ **A snowflake server** — One machine nobody can rebuild is one disk away from an outage.  \n✅ **Rebuild from code in minutes** — Replacement is routine, so failure is boring.\n\n**4.** ❌ **Previews that never expire** — They keep running, keep costing money and keep a door open.  \n✅ **Destroy on merge or close** — A sweeper deletes anything older than seven days.\n\n**5.** ❌ **No seed data** — An empty preview proves nothing about a real query.  \n✅ **Seeded, synthetic fixtures** — Every preview starts with the same known dataset.\n\n## Explained\n\n### The five\n\n1. **Production secrets in staging.** Staging is weaker, shared and logged, so a leak there becomes a production incident with a staging entry point. Give each environment its own credentials and its own rotation.\n\n2. **Hand-edited config on a host.** The change works, nobody records it, and the next deploy either erases it or preserves a state the repo does not describe. Change the manifest, then apply it.\n\n3. **A snowflake server.** One machine that nobody can rebuild is one disk failure from an outage, and the rebuild recipe lives in someone's memory. Replace it from code in minutes.\n\n4. **Previews that never expire.** They keep running, keep costing money and keep a URL and secrets reachable. Destroy on merge or close, and sweep anything older than seven days.\n\n5. **No seed data.** An empty preview proves nothing about a real query, so the first realistic request fails in front of a reviewer. Seed a small synthetic dataset on every create.\n\n### The pattern\n\n> **The honest caveat:** each shortcut is a reasonable decision on the day it is made. The cost arrives later, in the incident review, not in the pull request.\n\n> **The lesson:** environments are cleaned up as carefully as they are built, or they quietly become production.\n\n## In one line\n\n> **Clean up as carefully as you build.** A short-lived environment must actually be short-lived.\n\n**Set a ttl**\n\n```bash\nkubectl delete ns pr142\n```\n"},{"id":"p452","n":"452","slug":"the-on-call-system-drawn","title":"The on-call system, drawn","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","OnCall","IncidentResponse","DevOps","Reliability"],"series_name":"DevOps","design":"architecture","script":"","date":"2029-08-16","files":{"devsecops":"notes/p452-the-on-call-system-drawn.md"},"md":"# The on-call system, drawn\n\nAt 03:12 the pager goes off, and the person it reaches has the runbook open before the second alert lands. It is a system with three tiers, and each one answers a different question.\n\n## Quick reference\n\n- **Tier 1 · Signals:** SLO alert (SLO) · Synthetic check (SYNTH)\n- **Tier 2 · Rotation:** Primary on-call (P1) · Secondary on-call (P2)\n- **Tier 3 · Escalation:** Incident commander (IC) · Duty manager (MGR)\n\n**Request flow:** SLO alert → Primary on-call → Incident commander; Primary on-call → Duty manager; Synthetic check → Primary on-call; Secondary on-call → Incident commander\n\n**Around it:** Pager (PAGE) · Status page (STAT) · Incident channel (CHAT) · Runbook (RUN)\n\n## Explained\n\n### Signals\n\nAn SLO alert fires when the error budget is burning fast enough that a human should look now, not when one request fails. A synthetic check calls the login page from three regions every minute, so a total outage reaches you before a customer does. Both end in a page; neither is a dashboard someone has to notice.\n\n### Rotation\n\nPrimary carries the pager for the week. Secondary takes over when primary has not acknowledged in five minutes, and reviews the handover at every shift change. The rota is published, the swap is agreed in the channel, and secondary is a real name, not a line nobody checks.\n\n### Escalation\n\nThe incident commander declares severity, assigns roles and keeps the timeline. The duty manager is paged for a SEV1, for a customer-facing call, or when the IC needs authority the rota does not carry. Escalation is not punishment; it is a second pair of hands with a wider view.\n\nAround the tiers sit the tools: one pager that owns delivery and acknowledgement, a status page customers can read, an incident channel that holds the record, and a runbook linked from every alert.\n\n> **The honest caveat:** a rota with no handover is two people guessing in parallel. Write the state down at every change.\n\n> **The lesson:** the system that wakes people is designed, not inherited.\n\n## In one line\n\n> **Page a person, not a channel.** A rotation is a promise that someone is awake.\n\n**Test it monthly:** trigger a test page, ack it, close it\n"},{"id":"p453","n":"453","slug":"the-incident-command-cheat-sheet","title":"The incident command cheat sheet","sections":["devsecops"],"topic":"Observability & SRE","tags":["IncidentResponse","SRE","OnCall","DevOps","IncidentCommand"],"series_name":"DevOps","design":"cheatsheet","script":"","date":"2029-08-19","files":{"devsecops":"notes/p453-the-incident-command-cheat-sheet.md"},"md":"# The incident command cheat sheet\n\nAn incident starts as a strange graph and ends as a written record. In between, four blocks decide whether the next hour is calm or chaotic.\n\n## Quick reference\n\n### Declare\n\n```bash\n/incident declare sev2 checkout\ndate -u +%FT%TZ\npage IC: @ana, secondary: @sam\nchannel: #inc-2026-04-17-checkout\nSEV2 declared by @ana at 14:05Z\n```\n\n### Roles\n\n```bash\nIC: @ana  OPS: @sam  COMMS: @lee\nscribe: @dev  liaison: @ops\nrole swap at 16:00Z, both ack\nSEV3 to SEV2 by IC at 15:20Z\nhandover: IC @ana to @sam, 22:00Z\n```\n\n### Comms\n\n```bash\nStatus: investigating, next 14:30Z\nStatus: identified, rollback started\nStatus: monitoring, errors flat 10 min\nStatus: resolved at 14:52Z\nnext update in 30 min, even if quiet\n```\n\n### Close\n\n```bash\nkubectl rollout undo deploy/api\ngit revert --no-edit 3f2a1c9\nclose after 30 min clean, not 1 min\nreview booked within 48 hours\ntimeline exported to the incident doc\n```\n\n## Explained\n\n### Declare\n\nSay it out loud before you understand it. Declaring a SEV2 opens a channel, gives the work a name and tells people to stop guessing in DMs. Stamp the start with `date -u +%FT%TZ`, page the incident commander and the secondary, and publish the channel name and the first update in the same minute. A slow declaration is the easiest way to lose ten minutes.\n\n### Roles\n\nOne incident commander, one operations lead, one communications lead, one scribe. Post the names in the channel: IC: @ana OPS: @sam COMMS: @lee. The scribe writes timestamps and decisions, not opinions. When someone joins, they read the roles before they type.\n\n### Comms\n\nThe status page gets a sentence, not a paragraph: Status: investigating, next update 14:30Z. Then identified, monitoring and resolved, each with the same shape and a time. Update every 30 minutes even when nothing has changed, because silence reads as chaos to the customer.\n\n### Close\n\nUndo the change with `kubectl rollout undo deploy/api` and confirm the graphs. Close on evidence: thirty minutes clean, not one. Book the review within 48 hours while memory is fresh, and put the timeline in the incident document.\n\n> **The honest caveat:** templates feel bureaucratic at 3 a.m. They are the reason you can write a clear update while half awake.\n\n> **The lesson:** the channel is the record, so write for the person who was not there.\n\n## In one line\n\n> **Say it once, say it early.** The channel is the record. Write for the person who was not there.\n\n**Write the first update:** Status: investigating, 14:05Z\n"},{"id":"p454","n":"454","slug":"four-levels-of-incident-severity","title":"Four levels of incident severity","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","IncidentResponse","OnCall","DevOps","Reliability"],"series_name":"DevOps","design":"explainer","script":"","date":"2029-08-21","files":{"devsecops":"notes/p454-four-levels-of-incident-severity.md"},"md":"# Four levels of incident severity\n\nA SEV1 and a SEV4 both mean something is wrong. The difference is who is woken, how often you speak, and how much of the company stops to watch.\n\n## Quick reference\n\n### SEV1 - Total outage\n\n- Customer-facing outage, data loss or breach\n- Paged: IC, service owners, duty manager\n- War room, status page, update every 15 min\n- Fix first; the full review comes after\n\n### SEV2 - Major degradation\n\n- A core journey is broken or badly slow\n- Paged: IC and the owning team\n- Manager informed, not woken\n- Update every 30 min; decide rollback in 1h\n\n### SEV3 - Limited impact\n\n- One segment, feature or small customer set\n- Paged: on-call engineer, ticket to team\n- Handled in hours, update once a day\n- Escalate if the impact turns out wider\n\n### SEV4 - Cosmetic or internal\n\n- Cosmetic or internal, no customer impact\n- No page: a ticket in the backlog\n- Fixed in normal working hours\n- Re-triage quietly if the signal changes\n\n> **Note:** The names differ by company; the shape does not: impact decides who is woken.\n\n## Explained\n\n### SEV1: total outage\n\nCustomer-facing outage, data loss or a security breach. Page the incident commander, the service owners and the duty manager. Open a war room, update the status page, and post an update every 15 minutes until it is over. Fix first; the review comes after.\n\n### SEV2: major degradation\n\nA core journey is broken or badly slow, and a workaround may exist. Page the IC and the owning team, and inform the manager rather than paging them. Update every 30 minutes, and decide on a rollback within the first hour.\n\n### SEV3: limited impact\n\nOne segment, one feature or a small set of customers. Page the on-call engineer, open a ticket for the team, and handle it in working hours if nobody is blocked. Update daily, and escalate the moment the impact turns out wider.\n\n### SEV4: cosmetic or internal\n\nNo customer impact: a broken report, a noisy log, a typo in the console. No page. A ticket in the backlog, fixed in normal hours, and re-triaged if the signal changes.\n\nThe labels and the paging rules differ by company; the shape does not. Impact decides who is woken, and a written definition lets someone downgrade an incident without an argument.\n\n> **The honest caveat:** a severity matrix nobody has read at 3 a.m. is decoration. Put the four lines in the alert template.\n\n> **The lesson:** declare early, then let evidence move the level.\n\n## In one line\n\n> **Severity decides who wakes up.** Declare early, then downgrade it with evidence.\n\n**Define sev2:** major degradation, workaround exists\n"},{"id":"p455","n":"455","slug":"an-on-call-load-report-in-python","title":"An on-call load report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["SRE","OnCall","Python","DevOps","Reliability"],"series_name":"DevOps","design":"code","script":"oncall_load.py","date":"2029-08-23","files":{"devsecops":"notes/p455-an-on-call-load-report-in-python.md"},"md":"# An on-call load report in Python\n\nEvery rota has one person who takes twice the pages and never says so. The pager log knows. This script turns it into a count per person per week, and fails when the load is not shared.\n\n## Quick reference\n\nThe full script is at the end of this note: `oncall_load.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 oncall_load.py incidents.jsonl\n```\n\n**Example output**\n\n```text\n2026-W16  sam            8 pages   5 night\nworst hour 03:00 UTC, 5 pages\n```\n\n## Explained\n\n### What it does\n\nIt reads an incidents JSONL: one page per line, with paged_at, assignee and severity. For each person it counts pages by ISO week and how many landed between 22:00 and 07:00, then finds the hour of the day with the most pages. It prints the table and exits 1 when a person-week passes the threshold, eight by default.\n\n### Why weeks\n\nA night of four pages is noise. Five pages a week, every week, for one person is a retention problem that no retro will surface. ISO weeks put the Tuesday and the Sunday in the same bucket, so a bad rota shows up as a pattern rather than an anecdote.\n\n### How to use it\n\n`python3 oncall_load.py incidents.jsonl --threshold 6`, from cron on Monday morning or as a report step in CI. It exits 0 at or under the threshold, 1 over it, and 2 when the file is missing or a line will not parse. The full script is attached: standard library only, read-only.\n\n> **The honest caveat:** a page count says nothing about difficulty. One SEV1 at 03:00 can cost more than ten SEV3 tickets, and the count treats them the same. Read the severities before you judge the rota.\n\n> **The lesson:** measure the load before you argue about fairness.\n\n## In one line\n\n> **Share the pager, not the blame.** One person carrying every week is a rota bug, not a hero.\n\n**Run it monday**\n\n```bash\n0 8 * * 1 python3 oncall_load.py\n```\n\n## Script: oncall_load.py\n\nDownload: [scripts/oncall_load.py](../scripts/oncall_load.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report on-call pager load from an incidents JSONL file.\n\nUsage:\n    python3 oncall_load.py incidents.jsonl\n    python3 oncall_load.py incidents.jsonl --threshold 6\n    python3 oncall_load.py incidents.jsonl --night-start 21 --night-end 6\n    python3 oncall_load.py incidents.jsonl --json\n    python3 oncall_load.py --help\n\nEach line is one page:\n\n    {\"paged_at\": \"2026-04-14T03:12:00+00:00\", \"assignee\": \"ana\", \"severity\": \"sev2\"}\n\npaged_at is ISO 8601; a trailing Z is accepted. assignee is the person who\nwas paged. severity is validated and counted in the JSON summary; it does not\nchange the per-person count. A page is a night page when its hour is at or\nafter --night-start (default 22) or before --night-end (default 7), in UTC.\n\nThe report gives pages per person per ISO week, the night pages per row, the\ntotal night share and the hour of the day with the most pages. Ties go to the\nearliest hour, because 03:00 is a worse rota than 15:00.\n\nExit codes: 0 under the threshold, 1 one person-week over it, 2 usage or\ninput error (bad JSON, missing field, bad timestamp). Read-only: the file is\nopened for reading and never written.\n\nCron: 0 8 * * 1 python3 /opt/s0sta/oncall_load.py /var/tmp/incidents.jsonl\nCI:   python3 oncall_load.py incidents.jsonl --threshold 6\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport json\nimport sys\nfrom collections import Counter, defaultdict\nfrom datetime import datetime, timezone\n\nDEFAULT_THRESHOLD = 8\nDEFAULT_NIGHT_START = 22\nDEFAULT_NIGHT_END = 7\nFIELDS = (\"paged_at\", \"assignee\", \"severity\")\n\n\nclass InputError(Exception):\n    \"\"\"An input line or argument that cannot be used.\"\"\"\n\n\ndef parse_when(value, where):\n    \"\"\"Parse an ISO 8601 timestamp into an aware UTC datetime.\"\"\"\n    text = str(value or \"\").strip()\n    if not text:\n        raise InputError(\"%s: paged_at is empty\" % where)\n    try:\n        when = datetime.fromisoformat(text.replace(\"Z\", \"+00:00\"))\n    except ValueError:\n        raise InputError(\"%s: bad paged_at %r (expected ISO 8601)\"\n                         % (where, value))\n    if when.tzinfo is None:\n        when = when.replace(tzinfo=timezone.utc)\n    return when.astimezone(timezone.utc)\n\n\ndef load_pages(path):\n    \"\"\"Read the JSONL file and return one dict per page.\"\"\"\n    try:\n        handle = open(path, \"r\", encoding=\"utf-8\")\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    pages = []\n    with handle:\n        for number, line in enumerate(handle, 1):\n            if not line.strip():\n                continue\n            where = \"%s line %d\" % (path, number)\n            try:\n                row = json.loads(line)\n            except ValueError:\n                raise InputError(\"%s: not valid JSON\" % where)\n            if not isinstance(row, dict):\n                raise InputError(\"%s: expected a JSON object\" % where)\n            missing = [name for name in FIELDS\n                       if not str(row.get(name, \"\")).strip()]\n            if missing:\n                raise InputError(\"%s: missing %s\"\n                                 % (where, \", \".join(missing)))\n            pages.append({\n                \"assignee\": str(row[\"assignee\"]).strip(),\n                \"severity\": str(row[\"severity\"]).strip(),\n                \"when\": parse_when(row[\"paged_at\"], where),\n            })\n    return pages\n\n\ndef is_night(hour, start, end):\n    \"\"\"True when hour falls in the night window, which may cross midnight.\"\"\"\n    if start > end:\n        return hour >= start or hour < end\n    return start <= hour < end\n\n\ndef build_report(pages, night_start, night_end):\n    \"\"\"Group pages by person and ISO week, and find the worst hour.\"\"\"\n    buckets = defaultdict(lambda: {\"pages\": 0, \"night\": 0})\n    hours = Counter()\n    for page in pages:\n        when = page[\"when\"]\n        year, week, _day = when.isocalendar()\n        key = (page[\"assignee\"], year, week)\n        buckets[key][\"pages\"] += 1\n        hours[when.hour] += 1\n        if is_night(when.hour, night_start, night_end):\n            buckets[key][\"night\"] += 1\n    rows = [{\"assignee\": who, \"iso_year\": year, \"iso_week\": week,\n             \"pages\": counts[\"pages\"], \"night_pages\": counts[\"night\"]}\n            for (who, year, week), counts in buckets.items()]\n    rows.sort(key=lambda r: (r[\"iso_year\"], r[\"iso_week\"],\n                             -r[\"pages\"], r[\"assignee\"]))\n    worst = None\n    if hours:\n        hour, count = sorted(hours.items(), key=lambda kv: (-kv[1], kv[0]))[0]\n        worst = {\"hour\": hour, \"pages\": count}\n    return rows, worst\n\n\ndef print_table(rows, worst, threshold):\n    \"\"\"Print the table and return the person-weeks over the threshold.\"\"\"\n    total = sum(r[\"pages\"] for r in rows)\n    night = sum(r[\"night_pages\"] for r in rows)\n    for row in rows:\n        print(\"%04d-W%02d  %-12s %3d pages  %2d night\"\n              % (row[\"iso_year\"], row[\"iso_week\"], row[\"assignee\"],\n                 row[\"pages\"], row[\"night_pages\"]))\n    share = 100.0 * night / total if total else 0.0\n    print(\"\")\n    print(\"%d pages, %d at night (%.0f%%)\" % (total, night, share))\n    if worst:\n        print(\"worst hour %02d:00 UTC, %d pages\"\n              % (worst[\"hour\"], worst[\"pages\"]))\n    breaches = [r for r in rows if r[\"pages\"] > threshold]\n    for row in breaches:\n        print(\"ALERT %s %d pages in %04d-W%02d (threshold %d)\"\n              % (row[\"assignee\"], row[\"pages\"], row[\"iso_year\"],\n                 row[\"iso_week\"], threshold))\n    return breaches\n\n\ndef build_parser():\n    parser = argparse.ArgumentParser(\n        description=\"Report pages per person per week from an incidents \"\n                    \"JSONL file.\",\n        epilog=\"Exit 0 under the threshold, 1 over it, 2 usage or input \"\n               \"error. By s0sta.com\")\n    parser.add_argument(\"path\", help=\"incidents JSONL, one page per line\")\n    parser.add_argument(\"--threshold\", type=int, default=DEFAULT_THRESHOLD,\n                        metavar=\"N\",\n                        help=\"pages per person per week before exit 1 \"\n                             \"(default %d)\" % DEFAULT_THRESHOLD)\n    parser.add_argument(\"--night-start\", type=int, default=DEFAULT_NIGHT_START,\n                        metavar=\"H\",\n                        help=\"first night hour, 0-23 (default %d)\"\n                             % DEFAULT_NIGHT_START)\n    parser.add_argument(\"--night-end\", type=int, default=DEFAULT_NIGHT_END,\n                        metavar=\"H\",\n                        help=\"first day hour, 0-23 (default %d)\"\n                             % DEFAULT_NIGHT_END)\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print a JSON report instead of a table\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n    try:\n        if args.threshold < 1:\n            raise InputError(\"--threshold must be at least 1\")\n        for value, label in ((args.night_start, \"--night-start\"),\n                             (args.night_end, \"--night-end\")):\n            if not 0 <= value <= 23:\n                raise InputError(\"%s must be 0-23, got %d\" % (label, value))\n        if args.night_start == args.night_end:\n            raise InputError(\"night window is empty: --night-start equals \"\n                             \"--night-end\")\n        pages = load_pages(args.path)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    rows, worst = build_report(pages, args.night_start, args.night_end)\n    breaches = [r for r in rows if r[\"pages\"] > args.threshold]\n    total = len(pages)\n    night = sum(r[\"night_pages\"] for r in rows)\n    by_severity = Counter(page[\"severity\"] for page in pages)\n\n    if args.json:\n        print(json.dumps({\n            \"threshold\": args.threshold,\n            \"night_window\": [args.night_start, args.night_end],\n            \"total_pages\": total,\n            \"night_pages\": night,\n            \"worst_hour\": worst,\n            \"by_severity\": dict(sorted(by_severity.items())),\n            \"people\": rows,\n            \"breaches\": breaches,\n        }, indent=2, sort_keys=True))\n    else:\n        print_table(rows, worst, args.threshold)\n    return 1 if breaches else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p456","n":"456","slug":"an-incident-metrics-report-in-python","title":"An incident metrics report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["SRE","IncidentResponse","Python","DevOps","Reliability"],"series_name":"DevOps","design":"code","script":"incident_metrics.py","date":"2029-08-26","files":{"devsecops":"notes/p456-an-incident-metrics-report-in-python.md"},"md":"# An incident metrics report in Python\n\nAn incident ends and the channel goes quiet, but the timestamps stay behind. Read as a set, they say where the response was slow: detection, acknowledgement or recovery.\n\n## Quick reference\n\nThe full script is at the end of this note: `incident_metrics.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 incident_metrics.py incidents.jsonl\n```\n\n**Example output**\n\n```text\nsev2     3        6m      21m    1h20m  15m/4h\nBREACH sev2 MTTA 21m > 15m\n```\n\n## Explained\n\n### What it measures\n\nMTTD is the gap from impact (started_at) to detection (detected_at). MTTA is detection to acknowledgement (acked_at). MTTR is impact to service restored (mitigated_at), not to the moment the ticket closed (resolved_at). The script prints the median of each, per severity, because one long incident drags a mean somewhere useless.\n\n### Why medians\n\nA single 40-minute SEV1 among four five-minute ones moves the mean from five minutes to twelve and says nothing about the typical night. The median answers the question people actually ask: what a normal incident of this level looks like.\n\n### The SLA gate\n\nEach severity has an acknowledgement window and a restore window: five minutes and one hour for SEV1, fifteen minutes and four hours for SEV2, one hour and one day for SEV3. `python3 incident_metrics.py incidents.jsonl` exits 1 when a median misses its window. The frame runs the acknowledgement check; the attached tool runs the restore check too. Exit 0 means everything is inside, and 2 is an incomplete record or timestamps that run backwards.\n\n> **The honest caveat:** without started_at, MTTD cannot be measured at all, and a team that skips it reports a detection time of zero. Record when impact began, not only when the alert fired.\n\n> **The lesson:** a timestamp you never wrote is a metric you will never have.\n\n## In one line\n\n> **Measure the wait, not the story.** Four timestamps answer the question a retro argues about.\n\n**Stamp the start:** started_at, not just detected_at\n\n## Script: incident_metrics.py\n\nDownload: [scripts/incident_metrics.py](../scripts/incident_metrics.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report incident detection, acknowledgement and recovery medians from JSONL.\n\nUsage:\n    python3 incident_metrics.py incidents.jsonl\n    python3 incident_metrics.py incidents.jsonl --json\n    python3 incident_metrics.py incidents.jsonl --sla sev2=10,180\n    python3 incident_metrics.py --help\n\nEach line is one closed incident:\n\n    {\"severity\": \"sev2\",\n     \"started_at\": \"2026-04-14T09:05:00+00:00\",\n     \"detected_at\": \"2026-04-14T09:19:00+00:00\",\n     \"acked_at\": \"2026-04-14T09:24:00+00:00\",\n     \"mitigated_at\": \"2026-04-14T11:02:00+00:00\",\n     \"resolved_at\": \"2026-04-14T13:40:00+00:00\"}\n\nstarted_at is when customer impact began; without it MTTD cannot be\nmeasured. detected_at is when a signal or a person noticed, acked_at is the\nacknowledgement, mitigated_at is when service was restored, resolved_at is\nwhen the incident was closed.\n\nThe report prints, per severity, the median MTTD (started to detected), MTTA\n(detected to acknowledged) and MTTR (started to mitigated). The SLA gate uses\ntwo windows per severity: acknowledgement and restore, in minutes. Defaults:\n\n    sev1   5 / 60        sev2  15 / 240\n    sev3  60 / 1440      sev4 240 / 4320\n\nOverride one severity with --sla sev2=10,180 (ack_minutes,restore_minutes).\nAn unknown severity is reported without a gate.\n\nExit codes: 0 every median inside its window, 1 a median over a window,\n2 usage or input error (missing field, bad timestamp, times out of order).\nRead-only: the file is opened for reading and never written.\n\nCron: 0 7 * * 1 python3 /opt/s0sta/incident_metrics.py /var/tmp/closed.jsonl\nCI:   python3 incident_metrics.py incidents.jsonl --sla sev1=5,60\n\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport json\nimport statistics\nimport sys\nfrom collections import defaultdict\nfrom datetime import datetime, timezone\n\nFIELDS = (\"severity\", \"started_at\", \"detected_at\", \"acked_at\",\n          \"mitigated_at\", \"resolved_at\")\nSLA_MINUTES = {\n    \"sev1\": (5, 60),\n    \"sev2\": (15, 240),\n    \"sev3\": (60, 1440),\n    \"sev4\": (240, 4320),\n}\n\n\nclass InputError(Exception):\n    \"\"\"An input line or argument that cannot be used.\"\"\"\n\n\ndef parse_when(value, where, field):\n    \"\"\"Parse an ISO 8601 timestamp into an aware UTC datetime.\"\"\"\n    text = str(value or \"\").strip()\n    if not text:\n        raise InputError(\"%s: %s is empty\" % (where, field))\n    try:\n        when = datetime.fromisoformat(text.replace(\"Z\", \"+00:00\"))\n    except ValueError:\n        raise InputError(\"%s: bad %s %r (expected ISO 8601)\"\n                         % (where, field, value))\n    if when.tzinfo is None:\n        when = when.replace(tzinfo=timezone.utc)\n    return when.astimezone(timezone.utc)\n\n\ndef load_incidents(path):\n    \"\"\"Read the JSONL file and return one dict per incident, in minutes.\"\"\"\n    try:\n        handle = open(path, \"r\", encoding=\"utf-8\")\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    incidents = []\n    with handle:\n        for number, line in enumerate(handle, 1):\n            if not line.strip():\n                continue\n            where = \"%s line %d\" % (path, number)\n            try:\n                row = json.loads(line)\n            except ValueError:\n                raise InputError(\"%s: not valid JSON\" % where)\n            if not isinstance(row, dict):\n                raise InputError(\"%s: expected a JSON object\" % where)\n            missing = [name for name in FIELDS\n                       if not str(row.get(name, \"\")).strip()]\n            if missing:\n                raise InputError(\"%s: missing %s\"\n                                 % (where, \", \".join(missing)))\n            stamps = {name: parse_when(row[name], where, name)\n                      for name in FIELDS if name != \"severity\"}\n            started = stamps[\"started_at\"]\n            detected = stamps[\"detected_at\"]\n            acked = stamps[\"acked_at\"]\n            mitigated = stamps[\"mitigated_at\"]\n            resolved = stamps[\"resolved_at\"]\n            for first, second in ((\"started_at\", \"detected_at\"),\n                                  (\"detected_at\", \"acked_at\"),\n                                  (\"detected_at\", \"mitigated_at\"),\n                                  (\"mitigated_at\", \"resolved_at\")):\n                if stamps[second] < stamps[first]:\n                    raise InputError(\"%s: %s is before %s\"\n                                     % (where, second, first))\n            incidents.append({\n                \"severity\": str(row[\"severity\"]).strip().lower(),\n                \"mttd\": (detected - started).total_seconds() / 60.0,\n                \"mtta\": (acked - detected).total_seconds() / 60.0,\n                \"mttr\": (mitigated - started).total_seconds() / 60.0,\n                \"close\": (resolved - started).total_seconds() / 60.0,\n            })\n    return incidents\n\n\ndef parse_sla(values):\n    \"\"\"Start from the default windows and apply --sla overrides.\"\"\"\n    table = dict(SLA_MINUTES)\n    for item in values:\n        key, sep, rest = item.partition(\"=\")\n        key = key.strip().lower()\n        parts = rest.split(\",\")\n        if not sep or not key or len(parts) != 2:\n            raise InputError(\"--sla expects sev2=15,240, got %r\" % item)\n        try:\n            ack, restore = int(parts[0]), int(parts[1])\n        except ValueError:\n            raise InputError(\"--sla windows must be whole minutes: %r\" % item)\n        if ack < 0 or restore < 0:\n            raise InputError(\"--sla windows cannot be negative: %r\" % item)\n        table[key] = (ack, restore)\n    return table\n\n\ndef human(minutes):\n    \"\"\"Format a duration in minutes as 7m, 2h05m or 3d02h.\"\"\"\n    total = int(round(minutes))\n    if total < 60:\n        return \"%dm\" % total\n    hours, mins = divmod(total, 60)\n    if hours < 24:\n        return \"%dh\" % hours if mins == 0 else \"%dh%02dm\" % (hours, mins)\n    days, hours = divmod(hours, 24)\n    return \"%dd\" % days if hours == 0 else \"%dd%02dh\" % (days, hours)\n\n\ndef summarise(incidents):\n    \"\"\"Median MTTD, MTTA and MTTR per severity.\"\"\"\n    groups = defaultdict(lambda: {\"n\": 0, \"mttd\": [], \"mtta\": [], \"mttr\": [],\n                                 \"close\": []})\n    for incident in incidents:\n        group = groups[incident[\"severity\"]]\n        group[\"n\"] += 1\n        group[\"mttd\"].append(incident[\"mttd\"])\n        group[\"mtta\"].append(incident[\"mtta\"])\n        group[\"mttr\"].append(incident[\"mttr\"])\n        group[\"close\"].append(incident[\"close\"])\n    report = {}\n    for severity, group in groups.items():\n        report[severity] = {\n            \"n\": group[\"n\"],\n            \"mttd\": statistics.median(group[\"mttd\"]),\n            \"mtta\": statistics.median(group[\"mtta\"]),\n            \"mttr\": statistics.median(group[\"mttr\"]),\n            \"close\": statistics.median(group[\"close\"]),\n        }\n    return report\n\n\ndef find_breaches(report, sla):\n    \"\"\"Return a list of (severity, metric, median, window) breaches.\"\"\"\n    breaches = []\n    for severity in sorted(report):\n        windows = sla.get(severity)\n        if not windows:\n            continue\n        ack_window, restore_window = windows\n        row = report[severity]\n        if row[\"mtta\"] > ack_window:\n            breaches.append((severity, \"mtta\", row[\"mtta\"], ack_window))\n        if row[\"mttr\"] > restore_window:\n            breaches.append((severity, \"mttr\", row[\"mttr\"], restore_window))\n    return breaches\n\n\ndef print_table(report, breaches, sla):\n    print(\"%-6s %3s  %8s %8s %8s  %s\"\n          % (\"sev\", \"n\", \"mttd\", \"mtta\", \"mttr\", \"sla ack/restore\"))\n    for severity in sorted(report):\n        row = report[severity]\n        windows = sla.get(severity)\n        window_text = (\"%s/%s\" % (human(windows[0]), human(windows[1]))\n                       if windows else \"-\")\n        print(\"%-6s %3d  %8s %8s %8s  %s\"\n              % (severity, row[\"n\"], human(row[\"mttd\"]), human(row[\"mtta\"]),\n                 human(row[\"mttr\"]), window_text))\n    for severity, metric, median, window in breaches:\n        print(\"BREACH %s %s %s > %s\"\n              % (severity, metric.upper(), human(median), human(window)))\n\n\ndef build_parser():\n    parser = argparse.ArgumentParser(\n        description=\"Report median MTTD, MTTA and MTTR per severity from an \"\n                    \"incidents JSONL file.\",\n        epilog=\"Exit 0 inside SLA, 1 a breach, 2 usage or input. By s0sta.com\")\n    parser.add_argument(\"path\", help=\"incidents JSONL, one incident per line\")\n    parser.add_argument(\"--sla\", action=\"append\", default=[], metavar=\"SEV=A,R\",\n                        help=\"ack and restore windows in minutes, e.g. \"\n                             \"sev2=10,180; repeatable\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print a JSON report instead of a table\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n    try:\n        sla = parse_sla(args.sla)\n        incidents = load_incidents(args.path)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    report = summarise(incidents)\n    breaches = find_breaches(report, sla)\n\n    if args.json:\n        print(json.dumps({\n            \"incidents\": len(incidents),\n            \"sla_minutes\": {key: list(value) for key, value in sla.items()},\n            \"severity\": report,\n            \"breaches\": [{\"severity\": s, \"metric\": m, \"median_minutes\": v,\n                          \"window_minutes\": w}\n                         for s, m, v, w in breaches],\n        }, indent=2, sort_keys=True))\n    else:\n        print_table(report, breaches, sla)\n    return 1 if breaches else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p457","n":"457","slug":"9-checks-before-you-go-on-call","title":"9 checks before you go on call","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","OnCall","IncidentResponse","DevOps","Reliability"],"series_name":"DevOps","design":"checklist","script":"","date":"2029-08-28","files":{"devsecops":"notes/p457-9-checks-before-you-go-on-call.md"},"md":"# 9 checks before you go on call\n\nGoing on call for the first time feels like being handed a pager and a prayer. The first hour is not the moment to discover that your login expired or that nobody mentioned the migration. Nine checks, three groups, done before the handover ends.\n\n## Quick reference\n\n### Access\n\n- **Pager login works today** — send a test page, ack it\n- **Runbooks reachable offline** — copy in the repo\n- **Prod access is read-only** — write via the IC\n\n### Context\n\n- **Top three alerts, explained** — what each one means\n- **Dashboards bookmarked** — RED, one per service\n- **Recent changes in view** — `git log --since=yesterday`\n\n### Process\n\n- **Escalation path has names** — a person, not a number\n- **Severity one-pager** — sev2 vs sev3 in 10s\n- **Handover template ready** — open, watch, changed\n\n## Explained\n\n### Access\n\nPager login works today. Send a test page, acknowledge it, close it; a login that works only in a browser will fail at 03:00 on a phone.  \nRunbooks are reachable offline. If the wiki is behind SSO, keep a copy in the repo.  \nProduction access is read-only. Write access goes through the incident commander, and every use is logged.\n\n### Context\n\nKnow the top three alerts. For each, say what it means, what it does not mean, and the first command you would run.  \nDashboards are bookmarked. One per service, showing rate, errors and duration, not thirty panels.  \nRecent changes are in view. `git log --since=yesterday` on the deploy repo explains most new incidents before you open a terminal.\n\n### Process\n\nThe escalation path has names. A rota with a number and no name is half a plan.  \nSeverity definitions fit on one page. If you cannot tell a SEV2 from a SEV3 in ten seconds, you will guess under pressure.  \nThe handover template is ready: what is open, what to watch, what changed.\n\n> **The honest caveat:** nine checks do not make you ready for the incident you have not imagined. They remove the failures that have nothing to do with it.\n\n> **The lesson:** preparation is the part of on-call you control.\n\n## In one line\n\n> **Know the system before it pages.** Nine checks cost an hour; the first bad night costs more.\n\n**The first check:** page yourself, then ack it\n"},{"id":"p458","n":"458","slug":"the-layers-of-incident-response","title":"The layers of incident response","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","IncidentResponse","Reliability","DevOps","OnCall"],"series_name":"DevOps","design":"stack","script":"","date":"2029-08-30","files":{"devsecops":"notes/p458-the-layers-of-incident-response.md"},"md":"# The layers of incident response\n\nAn incident is not one event. It is a stack of six decisions, and each layer can be the one that turns a bad hour into a bad week.\n\n## Quick reference\n\n*Bottom to top: signal to learning*\n\n1. **Signal** — An SLO burn, a failed check or a customer report reaches a person. · `SLO / SYNTH`\n2. **Triage** — Severity, scope and an owner are decided, often in ten minutes. · `SEV / OWNER`\n3. **Mitigate** — Stop the bleeding: roll back, fail over, turn the feature off. · *ROLLBACK*\n4. **Resolve** — Service is stable, data is corrected, the queue is drained. · *STABLE*\n5. **Review** — The timeline is read; causes are separated from triggers. · *REVIEW*\n6. **Prevent** — Actions with owners and dates, merged as code. · *PREVENT*\n\n## Explained\n\n### Signal and triage\n\nSignal. Something is wrong and a machine says so: an SLO burn, a failed synthetic check, a customer report. Its job is to be specific and to reach a person, not a dashboard.\n\nTriage. Someone decides how bad it is and who owns it: severity, scope, a first hypothesis. This is where a wrong call costs the most time, and where a missing owner shows up first.\n\n### Mitigate and resolve\n\nMitigate. Stop the bleeding before you understand the cause: roll back, fail over, turn the feature off. The goal is a working service, not a complete explanation.\n\nResolve. Service is stable, data is corrected, the queue is drained. This is where incidents quietly become week-long projects, so write the exit condition down.\n\n### Review and prevent\n\nReview. Within a few days the timeline is read, and causes are separated from triggers. No blame, but no vague conclusions either.\n\nPrevent. Actions with owners and dates, merged as code. A review with no change to the system is a meeting with extra steps.\n\n> **The honest caveat:** the layers are not a waterfall. Most incidents loop between mitigate and triage while the cause stays unknown, and that is normal.\n\n> **The lesson:** move through the stack deliberately, and never skip the last layer.\n\n## In one line\n\n> **Signal, then learning.** An incident that changes nothing will happen again.\n\n**Check the exit:** did a merge change a default?\n"},{"id":"p459","n":"459","slug":"5-postmortem-mistakes","title":"5 postmortem mistakes","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","Postmortem","IncidentResponse","DevOps","Reliability"],"series_name":"DevOps","design":"dontdo","script":"","date":"2029-09-02","files":{"devsecops":"notes/p459-5-postmortem-mistakes.md"},"md":"# 5 postmortem mistakes\n\nMost postmortems are written by tired people who want to close the ticket. That is exactly why the same five mistakes appear in review after review.\n\n## Quick reference\n\n**1.** ❌ **Name the engineer** — It ends the conversation early.  \n✅ **Name the condition** — Fix the guardrail that allowed it.\n\n**2.** ❌ **Action with no owner** — A wish, not a task.  \n✅ **One named owner** — They accept it in the document.\n\n**3.** ❌ **No deadline** — It slips for a quarter.  \n✅ **A date in the tracker** — Even if the date later moves.\n\n**4.** ❌ **Fix lives in a wiki** — Nobody reads it at 3 a.m.  \n✅ **Change code or config** — A test that fails next time.\n\n**5.** ❌ **Review stays in the team** — Four people learn, nobody else.  \n✅ **Publish and discuss** — The organisation learns from one night.\n\n## Explained\n\n### The five\n\n1. **Blame.** Naming the engineer who ran the command ends the learning, because everyone else stops talking. Name the condition that made the command look safe: the missing guardrail, the alert nobody saw.\n   \\2. No owner. An action with no name is a wish. Every action gets one person, not a team, and that person accepts it in the document.\n   \\3. No deadline. An action with no date is a wish with a longer horizon. Put a date in the tracker, even if it slips later.\n   \\4. No code change. If the fix lives in a wiki page, it will not hold. The strongest action changes a pipeline, a default, a check or a test.\n   \\5. Not shared. A review read by four people fixes four people's habits. Post it where the organisation reads, and repeat the finding somewhere public.\n\n### What they share\n\nEach one is a way to finish the document without changing the system. The document looks complete, the incident returns, and the next review repeats the same paragraphs.\n\n> **The honest caveat:** a blameless review is not a consequence-free one. It removes blame from people so the facts can surface; performance problems are handled elsewhere, privately.\n\n> **The lesson:** a postmortem is finished when the system is different, not when the document is signed.\n\n## In one line\n\n> **Change the system, not the name.** A review is finished when the system is different.\n\n**Check the actions:** owner, date, and a merged change\n"},{"id":"p460","n":"460","slug":"quiz-3-operations-questions","title":"Quiz: 3 operations questions","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","IncidentResponse","OnCall","DevOps","Reliability"],"series_name":"DevOps","design":"quiz","script":"","date":"2029-09-04","files":{"devsecops":"notes/p460-quiz-3-operations-questions.md"},"md":"# Quiz: 3 operations questions\n\nThree questions from the on-call work in this phase. Answer them before you read the explanations, then check your own rota against the answers.\n\n## Quick reference\n\n**Q1. Why does a page need an owner before it needs a fix?**\n\n- a) An owner decides if it is real\n- b) Every fix needs approval first\n- c) Pages expire without an owner\n\n> **Answer:** a) An owner decides if it is real. An unowned page has nobody to decide whether it is real, who acts, or whether it can wait until morning. A fix is a task; ownership is the routing.\n\n**Q2. What does MTTA measure?**\n\n- a) Detection to acknowledgement\n- b) Detection to resolution\n- c) Alert to deploy\n\n> **Answer:** a) Detection to acknowledgement. MTTA starts when the incident is detected and stops when a human acknowledges. It measures the wait the rota controls, not the length of the fix.\n\n**Q3. Why does a blameless postmortem still name actions?**\n\n- a) Because actions need owners and dates\n- b) Because blame improves memory\n- c) Because reviews must name someone\n\n> **Answer:** a) Because actions need owners and dates. Blameless removes blame from people, not responsibility from the system. Actions still get an owner, a date and a code change, so the review alters behaviour.\n\n## Explained\n\n### Question one\n\nWhy does a page need an owner before it needs a fix? Because an unowned page has nobody to decide whether it is real, who should act, and whether it can wait until morning. The fix is a task; ownership is the routing that gets the task to a person with context. A page without an owner is a notification.\n\n### Question two\n\nWhat does MTTA measure? The time from detection to acknowledgement, not from detection to resolution. It measures how long the system waited for a human to say \"I have it\", which is the part a rota controls directly. A fast MTTA with a slow MTTR is usually a staffing and knowledge problem; a slow MTTA is a paging problem.\n\n### Question three\n\nWhy does a blameless postmortem still name actions? Because removing blame from people does not remove responsibility from the system. The review still assigns owners, dates and code changes. What it drops is the question of who to punish, which is the question that stops people from telling the truth.\n\n> **The honest caveat:** these answers describe a healthy process, not a guarantee. A team can recite all three and still page the same person every night.\n\n> **The lesson:** the question behind the question is who owns the response.\n\n## In one line\n\n> **Own the page, then fix it.** Three answers, one habit: route before you repair.\n\n**Check one page:** does it name a person?\n"},{"id":"p461","n":"461","slug":"on-call-terms-decoded","title":"On-call terms, decoded","sections":["devsecops"],"topic":"Observability & SRE","tags":["SRE","OnCall","IncidentResponse","DevOps","Reliability"],"series_name":"DevOps","design":"reveal","script":"","date":"2029-09-06","files":{"devsecops":"notes/p461-on-call-terms-decoded.md"},"md":"# On-call terms, decoded\n\nOn-call has a vocabulary of its own, and most of it is learned at the worst possible moment. Six terms, in the order you meet them.\n\n## Quick reference\n\n- **page** — A machine asking a human to act now. It goes to a pager, not a channel that can be muted.\n- **ack** — A human confirming they have the page. It stops the escalation timer, not the incident.\n- **escalation** — Waking the next person because the first did not answer, or cannot fix it.\n- **incident commander** — Runs the response: severity, roles, timeline and communication, not necessarily the fix.\n- **sev level** — The agreed label for impact. It decides who is paged and how often you update.\n- **postmortem** — The review after the fact: timeline, causes and actions with owners and dates.\n\n## Explained\n\n### The terms\n\nA page is a request for a human to act now, sent by the pager rather than a chat message that can be muted. An ack is the acknowledgement that a human has seen it; it stops the escalation timer, not the incident.\n\nEscalation is the next person in line being woken because the first did not answer or cannot fix it. It should be automatic and boring, not a judgement call made at 03:00.\n\nThe incident commander runs the response: severity, roles, timeline and communication. They are not necessarily the best debugger in the room, and on a SEV1 they should not be typing fixes.\n\nA sev level is the agreed label for impact, from total outage to cosmetic. It decides who is paged and how often you update, which is why the definition belongs in writing.\n\nA postmortem is the review after the fact: what happened, what made it worse, what changes. Blameless refers to people, not to actions.\n\n> **The honest caveat:** every team bends these words. The only thing that matters is that everyone in the rota means the same thing by them, and that the meanings are written down.\n\n> **The lesson:** shared words are the first tool of a shared response.\n\n## In one line\n\n> **Learn the words, share the pager.** A shared vocabulary is the first tool of a shared response.\n\n**Write them down:** page, ack, escalation, IC, sev, postmortem\n"},{"id":"p462","n":"462","slug":"the-developer-workflow-drawn","title":"The developer workflow, drawn","sections":["devsecops"],"topic":"Platform engineering","tags":["DevOps","DeveloperExperience","CICD","Git","PlatformEngineering"],"series_name":"DevOps","design":"architecture","script":"","date":"2029-09-09","files":{"devsecops":"notes/p462-the-developer-workflow-drawn.md"},"md":"# The developer workflow, drawn\n\nA change takes three hops before anyone uses it, and each hop has its own clock. The map is the same for a team of two and a platform group of two hundred.\n\n## Quick reference\n\n- **Local · Laptop:** Editor (ED) · Pre-commit hook (HOOK) · Test runner (TEST)\n- **Review · Pull request:** Pull request (PR) · CI pipeline (CI) · Reviewer (REV)\n- **Ship · Main:** Merge queue (MQ) · Deploy (CD) · Production (PROD)\n\n**Request flow:** Editor → Pre-commit hook → Test runner → Pull request → CI pipeline → Reviewer → Merge queue → Deploy → Production\n\n## Explained\n\n### Local\n\nIt starts in the editor, where the change is written. A pre-commit hook runs the fast checks before the commit exists: formatting, secrets, a lint pass. The test runner runs the suite on the laptop, so the first failure costs seconds instead of a queue slot.\n\n### Review\n\nThe branch becomes a pull request: diff, description and discussion in one place. The CI pipeline runs the workflow on the branch and reports status checks against the pull request. A reviewer reads the change for intent, not only for green, and asks for a smaller slice when the diff is too large to hold in mind.\n\n### Ship\n\nThe merge queue tests the merge result, not the branch alone, so two green pull requests cannot break main together. Deploy takes the merged commit to production, and production is where one clock stops and the next change starts.\n\n> **The honest caveat:** the diagram is a loop, not a line. A failure at deploy sends the work back to the editor, and the review step is usually the slowest hop because it waits for a person.\n\n> **The lesson:** measure each hop separately; an average across all three hides the queue that hurts.\n\n## In one line\n\n> **Fast alone, safe together.** Every tier has to hand off cleanly to the next one.\n\n**Time the handoff**\n\n```bash\ngh pr checks 412 --watch\n```\n"},{"id":"p463","n":"463","slug":"the-developer-workflow-cheat-sheet","title":"The developer workflow cheat sheet","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","GitHub","DeveloperExperience","DevOps","CommandLine"],"series_name":"DevOps","design":"cheatsheet","script":"","date":"2029-09-11","files":{"devsecops":"notes/p463-the-developer-workflow-cheat-sheet.md"},"md":"# The developer workflow cheat sheet\n\nThe loop between a thought and a green build is where a working day disappears. These commands shorten it.\n\n## Quick reference\n\n### Worktrees\n\n```bash\ngit worktree add ../hotfix main\ngit worktree list\ngit worktree remove ../hotfix\ngit worktree add -b fix/412 ../p412\ngit worktree prune\n```\n\n### Review\n\n```bash\ngh pr list --state open\ngh pr checkout 412\ngh pr view 412 --comments\ngh pr diff 412 --color=never\ngh pr checks 412 --watch\n```\n\n### CI locally\n\n```bash\nact pull_request\nact -j build\nact -l\nact -n\nact --secret-file .secrets\n```\n\n### Debug\n\n```bash\ntmux new -s debug\ntmux attach -t debug\ntmux ls\ntmux split-window -h\ntmux send-keys -t debug 'make test' Enter\n```\n\n## Explained\n\n### Worktrees\n\nA worktree is a second checkout of the same repository, so a hotfix does not disturb the branch you were reading. git worktree add puts it beside the main checkout, `git worktree list` shows every tree, git worktree remove clears a finished one, and prune tidies the records. A branch can live in one worktree at a time.\n\n### Review\n\ngh pr list shows the open queue, and `gh pr checkout 412` puts a branch in your working copy without a manual fetch. `gh pr view 412 --comments` reads the discussion, gh pr diff 412 shows the patch, and `gh pr checks 412 --watch` waits for CI in the terminal.\n\n### CI locally\n\nact runs the workflow on your machine in Docker. act pull_request runs the event you care about, act -j build runs one job, act -l lists them, act -n does a dry run, and act `--secret-file .secrets` loads the values CI would inject.\n\n### Debug\n\n`tmux new -s debug` starts a session that survives a dropped connection. Reattach with `tmux attach -t debug`, list sessions with `tmux ls`, split a pane with `tmux split-window -h`, and send a command with `tmux send-keys -t debug 'make test' Enter`. `tmux kill-session -t` debug ends it.\n\n> **The honest caveat:** act is close to Actions, not identical. The runner image, secrets and cache differ, so a green local run is evidence, not proof.\n\n> **The lesson:** the fastest pipeline is the one you can run before you push.\n\n## In one line\n\n> **Fewer tools, sharper loop.** A command you can recall beats a plugin you have to configure.\n\n**Start a worktree**\n\n```bash\ngit worktree add ../hotfix main\n```\n"},{"id":"p464","n":"464","slug":"where-developer-time-goes","title":"Where developer time goes","sections":["devsecops"],"topic":"Platform engineering","tags":["DevOps","DeveloperExperience","CICD","Productivity","Engineering"],"series_name":"DevOps","design":"explainer","script":"","date":"2029-09-13","files":{"devsecops":"notes/p464-where-developer-time-goes.md"},"md":"# Where developer time goes\n\nA developer's day is not spent typing. It is spent waiting, switching and setting up, and none of those costs appear on a sprint board.\n\n## Quick reference\n\n### Waiting for CI\n\n- A ten-minute run four times a day is forty minutes\n- Queue time in front of a runner is not build time\n- Track the median and p90 duration per job\n- Cache dependencies and split the slowest job\n\n### Review latency\n\n- The wait for a first reviewer, not the approval\n- A large diff gets postponed, then skimmed\n- Keep a review rota and a first-response target\n- Measure time to first review, not time to merge\n\n### Environment setup\n\n- A new laptop to a running stack can take days\n- Every manual step drifts from the documented one\n- One command should build, seed and run the stack\n- Containers make setup repeatable, not instant\n\n### Context switching\n\n- Every interruption costs a rebuilt mental model\n- An interrupted review restarts from the first line\n- Protect two-hour blocks and batch the pings\n- Fewer open pull requests means fewer switches\n\n> **Note:** Developer time is mostly queue time: measure the wait.\n\n## Explained\n\n### Waiting for CI\n\nWaiting for CI is the most visible queue. A ten-minute pipeline run four times a day is forty minutes at the desk, and queue time in front of a runner is not build time. Track the median and the ninetieth percentile per job, cache dependencies, and split the slow job that blocks everything else.\n\n### Review latency\n\nReview latency is the wait for a first reviewer, not the time to approve. A large diff is postponed until Friday and then skimmed, which is how a two-day wait becomes a shallow review. Keep a review rota, set a first-response target people can meet, and track time to first review.\n\n### Environment setup\n\nEnvironment setup is the cost nobody budgets. A new laptop to a running service can take days of undocumented steps, and every manual step drifts. One command should build, seed and run the stack; containers make that repeatable, not instant.\n\n### Context switching\n\nContext switching is the tax on all three. Every interruption costs a rebuild of the mental model, and an interrupted review restarts from the first line. Protect two-hour blocks, batch the pings, and keep fewer pull requests open per person.\n\n> **The honest caveat:** the four overlap. A slow environment makes CI feel slow, and a noisy chat tool turns every review into a context switch, so fix the queue you can see first.\n\n> **The lesson:** developer time is mostly queue time, so measure the wait.\n\n## In one line\n\n> **Time is lost in the handoffs.** Optimise the queue, and the work gets faster on its own.\n\n**Measure the wait**\n\n```bash\ngh pr list --json number,createdAt,reviews\n```\n"},{"id":"p465","n":"465","slug":"a-review-latency-report-in-python","title":"A review latency report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DeveloperExperience","CodeReview","CICD","DevOps"],"series_name":"DevOps","design":"code","script":"review_latency.py","date":"2029-09-16","files":{"devsecops":"notes/p465-a-review-latency-report-in-python.md"},"md":"# A review latency report in Python\n\nA pull request opened on Monday and reviewed on Wednesday costs more than the review itself. The delay is measurable from the API export you already have.\n\n## Quick reference\n\nThe full script is at the end of this note: `review_latency.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 review_latency.py prs.json\n```\n\n**Example output**\n\n```text\nERR #412 31.2h 780L\n6 of 42 breach\n```\n\n## Explained\n\n### What it reads\n\nThe export is a JSON list, one object per pull request: number, author, created_at, first_review_at, merged_at, additions and deletions. Time to first review runs from created_at to first_review_at in hours. Size is additions plus deletions, which is the number a reviewer actually faces.\n\n### What it does\n\nThe script parses the timestamps, computes the wait and the size, then flags a pull request as slow above twenty-four hours or large above four hundred changed lines. It lists every breach, prints a one-line summary and exits 1 when any threshold is crossed. No API token is involved: the export is the only input.\n\n### How to use it\n\nRun it on a weekday schedule or as a report step in CI: `python3 review_latency.py prs.json --first-review 24 --size 400`. The full script takes the thresholds as flags, prints a table and leaves the exit code for the pipeline. It only reads the export.\n\n> **The honest caveat:** first review is not review quality, and a team can hit the target with a one-line approval. Pair the number with a read of the comments, or it becomes a target that gets gamed.\n\n> **The lesson:** review latency is a queue, so make the queue visible before you ask anyone to go faster.\n\n## In one line\n\n> **A wait you cannot see is a default.** Time to first review is a service level, not a personality.\n\n**Run it on export**\n\n```bash\npython3 review_latency.py prs.json\n```\n\n## Script: review_latency.py\n\nDownload: [scripts/review_latency.py](../scripts/review_latency.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Flag pull requests that wait too long for a first review.\n\nUsage:\n    python3 review_latency.py prs.json\n    python3 review_latency.py prs.json --first-review 24 --size 400 --top 10\n\nThe export is a JSON list of objects with the keys number, author,\ncreated_at, first_review_at, merged_at, additions and deletions. Timestamps\nare ISO 8601; a trailing Z is accepted.\n\nTime to first review runs from created_at to first_review_at, in hours. An\nopen PR with no review yet is measured to now and marked waiting; a merged\nPR with no review is marked unreviewed and always counts as a breach. Size\nis additions plus deletions. The report exits 1 when any PR breaches\n--first-review hours or --size changed lines.\n\nCron (weekdays, 08:00):  0 8 * * 1-5  python3 review_latency.py prs.json\nCI step: python3 review_latency.py prs.json   # non-zero fails the job\n\nExit codes: 0 clean, 1 a threshold is breached, 2 usage or input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nDEFAULT_HOURS = 24.0\nDEFAULT_SIZE = 400\n\n\ndef parse_time(value):\n    \"\"\"Parse an ISO 8601 timestamp, or return None for an empty value.\"\"\"\n    if value in (None, \"\"):\n        return None\n    if not isinstance(value, str):\n        raise ValueError(\"timestamp is not a string: %r\" % (value,))\n    stamp = datetime.fromisoformat(value.replace(\"Z\", \"+00:00\"))\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=timezone.utc)\n    return stamp\n\n\ndef hours_between(start, end):\n    \"\"\"Hours from start to end, or None when either is missing.\"\"\"\n    if start is None or end is None:\n        return None\n    return (end - start).total_seconds() / 3600.0\n\n\ndef read_export(path):\n    \"\"\"Read the JSON export and insist that it is a list.\"\"\"\n    with open(path, encoding=\"utf-8\") as handle:\n        data = json.load(handle)\n    if not isinstance(data, list):\n        raise ValueError(\"export must be a JSON list of pull requests\")\n    return data\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Flag PRs with a slow first review or a large diff.\")\n    parser.add_argument(\"export\", help=\"JSON export of pull requests\")\n    parser.add_argument(\"--first-review\", type=float, default=DEFAULT_HOURS,\n                        metavar=\"HOURS\",\n                        help=\"breach above this time to first review (24)\")\n    parser.add_argument(\"--size\", type=int, default=DEFAULT_SIZE,\n                        metavar=\"LINES\",\n                        help=\"breach above this many changed lines (400)\")\n    parser.add_argument(\"--top\", type=int, default=10, metavar=\"N\",\n                        help=\"rows to print (10)\")\n    args = parser.parse_args(argv)\n\n    if args.first_review <= 0 or args.size <= 0 or args.top < 1:\n        print(\"ERROR thresholds and --top must be positive\", file=sys.stderr)\n        return 2\n\n    try:\n        prs = read_export(args.export)\n    except (OSError, ValueError) as exc:\n        print(\"ERROR cannot read %s: %s\" % (args.export, exc), file=sys.stderr)\n        return 2\n\n    now = datetime.now(timezone.utc)\n    rows = []\n    breaches = 0\n    for pr in prs:\n        if not isinstance(pr, dict):\n            print(\"ERROR every row must be a JSON object\", file=sys.stderr)\n            return 2\n        number = pr.get(\"number\", \"?\")\n        try:\n            created = parse_time(pr.get(\"created_at\"))\n            review = parse_time(pr.get(\"first_review_at\"))\n            merged = parse_time(pr.get(\"merged_at\"))\n            size = int(pr.get(\"additions\") or 0) + int(pr.get(\"deletions\") or 0)\n        except (TypeError, ValueError) as exc:\n            print(\"ERROR PR %s: %s\" % (number, exc), file=sys.stderr)\n            return 2\n        if created is None:\n            print(\"ERROR PR %s: created_at is required\" % number, file=sys.stderr)\n            return 2\n        if size < 0:\n            print(\"ERROR PR %s: negative size\" % number, file=sys.stderr)\n            return 2\n\n        wait = hours_between(created, review)\n        flags = []\n        if review is None:\n            if merged is None:\n                wait = hours_between(created, now)\n                if wait > args.first_review:\n                    flags.append(\"waiting\")\n            else:\n                flags.append(\"unreviewed\")\n        elif wait > args.first_review:\n            flags.append(\"slow\")\n        if size > args.size:\n            flags.append(\"large\")\n        if flags:\n            breaches += 1\n        rows.append((number, str(pr.get(\"author\", \"?\")), wait, size,\n                     \",\".join(flags) or \"-\"))\n\n    rows.sort(key=lambda row: -(row[2] if row[2] is not None else -1))\n    print(\"%6s  %-14s %7s %6s  flags\" % (\"PR\", \"author\", \"wait h\", \"size\"))\n    for number, author, wait, size, flags in rows[: args.top]:\n        shown = \"-\" if wait is None else \"%.1f\" % wait\n        print(\"%6s  %-14s %7s %6s  %s\"\n              % (number, author[:14], shown, size, flags))\n    if breaches:\n        print(\"ERR %d of %d pull requests breach a threshold\"\n              % (breaches, len(prs)))\n    else:\n        print(\"OK %d pull requests within thresholds\" % len(prs))\n    return 1 if breaches else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p466","n":"466","slug":"a-ci-cost-report-in-python","title":"A CI cost report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","CICD","FinOps","DevOps","CloudComputing"],"series_name":"DevOps","design":"code","script":"ci_cost.py","date":"2029-09-18","files":{"devsecops":"notes/p466-a-ci-cost-report-in-python.md"},"md":"# A CI cost report in Python\n\nThe CI bill arrives once a month, long after the pull request that doubled the minutes. Projecting it from a usage export turns a surprise into a weekly number.\n\n## Quick reference\n\nThe full script is at the end of this note: `ci_cost.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ci_cost.py usage.json --budget 200\n```\n\n**Example output**\n\n```text\napi                 96.00\nERR month 212.57\n```\n\n## Explained\n\n### What it reads\n\nThe export is a JSON list, one object per repository: repo, minutes and price per minute. If a row omits the price, the script uses the rate passed on the command line. Spend is minutes times price, which is the part of the bill a team controls.\n\n### What it does\n\nIt ranks repositories by spend, totals the month to date, and projects the full month from the days elapsed. Then it compares the projection with the budget and exits 1 when the projection is over. A clean run exits 0; a bad export exits 2, which is not the same as over budget. The top rows show which repository to fix first.\n\n### How to use it\n\nRun it daily from cron or as a scheduled job: `python3 ci_cost.py usage.json --budget 400`. The full script accepts `--price`, `--day`, `--days` and `--top`, so a mid-month run or a short month is not a special case. It reads one file and writes nothing.\n\n> **The honest caveat:** minutes are not the whole bill. Storage, larger runners, and Windows or macOS minutes are priced differently, so the projection is a floor, not an invoice.\n\n> **The lesson:** a budget you cannot see is a budget you will exceed.\n\n## In one line\n\n> **Minutes are money now.** A budget nobody projects is a surprise in December.\n\n**Set the budget**\n\n```bash\npython3 ci_cost.py usage.json --budget 400\n```\n\n## Script: ci_cost.py\n\nDownload: [scripts/ci_cost.py](../scripts/ci_cost.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank CI spend by repository and fail when the month is over budget.\n\nUsage:\n    python3 ci_cost.py usage.json --budget 400\n    python3 ci_cost.py usage.json --budget 400 --price 0.008 --top 5\n\nThe export is a JSON list of objects with the keys repo, minutes and,\noptionally, price_per_minute. A row's own price wins; --price is the\nfallback for rows that omit it. Spend is minutes times price. The month is\nprojected from --day and --days (defaults: today's day of the month and the\nlength of the current month), and the report exits 1 when the projection is\nover --budget.\n\nThe export should cover the current month to date. Prices differ by runner\nsize and operating system, so pass the rate on your own bill.\n\nCron (daily, 07:30):  30 7 * * *  python3 ci_cost.py usage.json --budget 400\nCI gate: python3 ci_cost.py usage.json --budget 400   # non-zero fails the job\n\nExit codes: 0 clean, 1 the budget is breached, 2 usage or input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport calendar\nimport json\nimport sys\nfrom datetime import date\n\nDEFAULT_PRICE = 0.008\n\n\ndef read_usage(path):\n    \"\"\"Read the JSON usage export and insist that it is a list.\"\"\"\n    with open(path, encoding=\"utf-8\") as handle:\n        data = json.load(handle)\n    if not isinstance(data, list):\n        raise ValueError(\"export must be a JSON list of usage rows\")\n    return data\n\n\ndef main(argv=None):\n    today = date.today()\n    parser = argparse.ArgumentParser(\n        description=\"Rank CI spend by repo and project the month.\")\n    parser.add_argument(\"usage\", help=\"JSON export of CI usage rows\")\n    parser.add_argument(\"--budget\", type=float, required=True, metavar=\"USD\",\n                        help=\"monthly budget in dollars\")\n    parser.add_argument(\"--price\", type=float, default=DEFAULT_PRICE,\n                        metavar=\"USD\",\n                        help=\"fallback price per minute (0.008)\")\n    parser.add_argument(\"--day\", type=int, default=today.day, metavar=\"N\",\n                        help=\"day of the month covered by the export\")\n    parser.add_argument(\"--days\", type=int,\n                        default=calendar.monthrange(today.year, today.month)[1],\n                        metavar=\"N\", help=\"days in the month\")\n    parser.add_argument(\"--top\", type=int, default=10, metavar=\"N\",\n                        help=\"rows to print (10)\")\n    args = parser.parse_args(argv)\n\n    if args.budget < 0:\n        print(\"ERROR --budget cannot be negative\", file=sys.stderr)\n        return 2\n    if args.price <= 0:\n        print(\"ERROR --price must be positive\", file=sys.stderr)\n        return 2\n    if not 1 <= args.day <= 31 or not 28 <= args.days <= 31:\n        print(\"ERROR --day must be 1-31 and --days 28-31\", file=sys.stderr)\n        return 2\n    if args.day > args.days:\n        print(\"ERROR --day cannot exceed --days\", file=sys.stderr)\n        return 2\n    if args.top < 1:\n        print(\"ERROR --top must be positive\", file=sys.stderr)\n        return 2\n\n    try:\n        usage = read_usage(args.usage)\n    except (OSError, ValueError) as exc:\n        print(\"ERROR cannot read %s: %s\" % (args.usage, exc), file=sys.stderr)\n        return 2\n\n    spend = []\n    for row in usage:\n        if not isinstance(row, dict):\n            print(\"ERROR every row must be a JSON object\", file=sys.stderr)\n            return 2\n        repo = row.get(\"repo\")\n        if not isinstance(repo, str) or not repo:\n            print(\"ERROR every row needs a repo name\", file=sys.stderr)\n            return 2\n        try:\n            minutes = float(row.get(\"minutes\", 0))\n            price = float(row.get(\"price_per_minute\", args.price))\n        except (TypeError, ValueError) as exc:\n            print(\"ERROR %s: %s\" % (repo, exc), file=sys.stderr)\n            return 2\n        if minutes < 0 or price <= 0:\n            print(\"ERROR %s: minutes >= 0 and price > 0\" % repo,\n                  file=sys.stderr)\n            return 2\n        spend.append((repo, minutes, price, minutes * price))\n\n    total = sum(row[3] for row in spend)\n    projected = total / args.day * args.days\n\n    spend.sort(key=lambda row: -row[3])\n    print(\"%-20s %8s %8s %10s\" % (\"repo\", \"minutes\", \"rate\", \"spend\"))\n    for repo, minutes, price, cost in spend[: args.top]:\n        print(\"%-20s %8.0f %8.4f %10.2f\" % (repo, minutes, price, cost))\n    print(\"spend to date %.2f, projected %.2f, budget %.2f\"\n          % (total, projected, args.budget))\n    if projected > args.budget:\n        print(\"ERR projection %.2f is over budget %.2f\"\n              % (projected, args.budget))\n        return 1\n    print(\"OK projection %.2f is within budget %.2f\"\n          % (projected, args.budget))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p467","n":"467","slug":"anatomy-of-a-commit-message","title":"Anatomy of a commit message","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Git","ConventionalCommits","CodeReview","DevOps","SoftwareEngineering"],"series_name":"DevOps","design":"anatomy","script":"","date":"2029-09-20","files":{"devsecops":"notes/p467-anatomy-of-a-commit-message.md"},"md":"# Anatomy of a commit message\n\nA commit message is read twice: by a reviewer in a hurry, and by a tool with no context. The line either gives both what they need or makes them guess.\n\n## Quick reference\n\n```text\nfix(api)!: reject expired tokens (#412)\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `fix` | The type | The kind of change: fix, feat, docs, refactor, perf or test. |\n| `(api)` | The scope | The area, in brackets. Optional, one noun, not a file name. |\n| `!` | Breaking marker | Sits before the colon. The footer must explain the migration. |\n| `: reject expired tokens` | The summary | Imperative, lower case, no full stop: what the change does. |\n| `(#412)` | PR reference | GitHub appends it on squash merge; you rarely type it. |\n\n## Explained\n\n### The type\n\nfix is the type: what kind of change this is. feat, docs, refactor, perf, test, build, ci and chore are the other common ones. The type is what a changelog groups by, so a fix filed as chore disappears from the release notes.\n\n### The scope\n\n(api) is the scope: the area of the codebase, not the file name. It is optional, and a new joiner should find it in the layout. One scope per commit is enough; a commit that touches three areas is usually three commits.\n\n### The breaking marker\n\n! is the breaking marker. It goes after the type or scope and before the colon, and it warns every consumer that the next release may break them. The marker starts the conversation; the BREAKING CHANGE footer must explain the migration, or the warning is noise.\n\n### The summary and the reference\n\n: reject expired tokens is the summary: imperative mood, lower case, no full stop, and what the change does, not what you felt. The trailing pull request reference, 412 in the example, is appended by GitHub on squash merge, so you rarely type it.\n\n> **The honest caveat:** Conventional Commits is a convention, not a standard, and nothing enforces the truth of a type. Lint the format in CI, review the meaning by hand.\n\n> **The lesson:** the message is the only part of the change that survives the review.\n\n## In one line\n\n> **A message is a contract.** Read by humans first, tooling second.\n\n**Show the change**\n\n```bash\ngit show --stat HEAD\n```\n"},{"id":"p468","n":"468","slug":"developer-experience-at-a-glance","title":"Developer experience at a glance","sections":["devsecops"],"topic":"Platform engineering","tags":["DevEx","DORA","PlatformEngineering","DevOps","SRE"],"series_name":"DevOps","design":"dashboard","script":"","date":"2029-09-23","files":{"devsecops":"notes/p468-developer-experience-at-a-glance.md"},"md":"# Developer experience at a glance\n\nThe monthly developer review had a green dashboard and one awkward question: why did the new hire take three weeks to merge anything?\n\n## Quick reference\n\n**Status:** This month\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Lead time | **2.4 days** | median, commit to production |\n| Review time | **14 h** | median, to first review |\n| CI duration | **9m 40s** | median pipeline, all jobs |\n| Flaky rate | **7%%** | failed, then green on retry |\n| Onboarding days | **11 days** | access to first merged PR |\n| Docs freshness | **82%%** | pages edited in 90 days |\n\n## Explained\n\n### Speed\n\nLead time is the median from commit to production, the DORA measure, not from the idea to the merge. Review time is the median wait for a first review, which is the number a pull request actually feels. CI duration is the median pipeline, because a mean is dragged by the slowest job.\n\n### Health\n\nFlaky rate is the share of failed runs that pass on a retry with no code change. It matters because a flaky suite teaches people to press rerun, and a rerun hides the real failure that follows. A suite that fails at random is not a fast suite; it is a coin toss with a log. The fix is a quarantine and an owner, not a rerun button.\n\n### The path\n\nOnboarding days run from access granted to the first merged pull request, which tests the golden path end to end. Docs freshness is the share of pages edited in the last ninety days; a stale runbook is worse than a missing one because it looks authoritative.\n\n> **The honest caveat:** every tile is a proxy. Medians hide the tail, and a survey explains what the numbers cannot. SPACE adds satisfaction and flow to the numbers; DORA keeps four delivery keys.\n\n> **The lesson:** six numbers should start one conversation, not end it.\n\n## In one line\n\n> **Six numbers, one conversation.** A DevEx board starts the retro; it does not replace it.\n\n**Read the median**\n\n```bash\ngh pr list --state merged\n```\n"},{"id":"p469","n":"469","slug":"github-vs-gitlab-vs-bitbucket","title":"GitHub vs GitLab vs Bitbucket","sections":["devsecops"],"topic":"Git & CI/CD","tags":["GitHub","GitLab","Bitbucket","DevOps","PlatformEngineering"],"series_name":"DevOps","design":"matrix","script":"","date":"2029-09-25","files":{"devsecops":"notes/p469-github-vs-gitlab-vs-bitbucket.md"},"md":"# GitHub vs GitLab vs Bitbucket\n\nThe forge you choose is a ten-year decision made in a two-week trial. The feature lists look similar; the differences are in how the work queues and who owns the escape hatch.\n\n## Quick reference\n\n| | **GitHub** | **GitLab** | **Bitbucket** |\n|:--|:--|:--|:--|\n| CI | Actions + runners | Built-in CI/CD | Pipelines, Docker |\n| Issues | Issues + Projects | Issues + boards | Issues, or Jira |\n| Permissions | Roles + teams | Roles + protected | Groups + branch |\n| Self-hosting | Enterprise Server | Self-managed CE/EE | Data Center |\n| Ecosystem | Largest marketplace | One integrated app | Atlassian stack |\n| Best for | Open source, reach | All-in-one DevOps | Jira-centric teams |\n\n**How to pick**\n\n- **GitHub** → reach: the community and the widest integrations.\n- **GitLab** → one application from issue to deploy.\n- **Bitbucket** → the Atlassian shop that lives in Jira.\n\n## Explained\n\n### CI and issues\n\nGitHub runs Actions, with hosted runners and a marketplace of reusable workflows. GitLab ships CI/CD in the same application, driven by `.gitlab-ci.yml`, so the pipeline needs no second product. Bitbucket uses Pipelines, a Docker-based YAML runner. For issues, GitHub pairs Issues with Projects, GitLab has issues and boards with epics on higher tiers, and Bitbucket has issues but many teams keep the real tracker in Jira.\n\n### Control and hosting\n\nPermissions follow the same shape: GitHub has read, triage, write, maintain and admin roles plus teams and rulesets; GitLab has Guest through Owner with protected branches; Bitbucket uses groups, branch permissions and merge checks. Self-hosting is Enterprise Server, self-managed GitLab, or Bitbucket Data Center.\n\n### Ecosystem and fit\n\nGitHub has the largest integration ecosystem and the open-source gravity. GitLab sells one application, one login and one data model. Bitbucket is the natural choice when the organisation already lives in Jira and Confluence. Best for: GitHub for reach, GitLab for consolidation, Bitbucket for the Atlassian stack.\n\n> **The honest caveat:** plan tiers decide the details. Merge queue, merge trains, required reviewers and included CI minutes all change with the plan, and list prices move, so check the edition you would actually buy.\n\n> **The lesson:** compare the migration out, not only the feature in.\n\n## In one line\n\n> **Pick the forge you can leave.** Migration cost is a feature; measure it before you commit.\n\n**Export the issues**\n\n```bash\ngh issue list --limit 1000\n```\n"},{"id":"p470","n":"470","slug":"quiz-3-developer-experience-questions","title":"Quiz: 3 developer experience questions","sections":["devsecops"],"topic":"Platform engineering","tags":["DevEx","DORA","CodeReview","PlatformEngineering","DevOps"],"series_name":"DevOps","design":"quiz","script":"","date":"2029-09-27","files":{"devsecops":"notes/p470-quiz-3-developer-experience-questions.md"},"md":"# Quiz: 3 developer experience questions\n\nThree questions separate a team that measures its delivery from one that measures how tired it is. Answer before you read on.\n\n## Quick reference\n\n**Q1. Why does lead time beat utilisation as a productivity measure?**\n\n- a) Busy people ship more\n- b) It counts the waiting\n- c) Utilisation is easier to chart\n\n> **Answer:** b) It counts the waiting. Utilisation counts busy time; lead time counts elapsed time to production. In a queue, ninety per cent busy means waits about nine times the work.\n\n**Q2. Why does pull request size predict how long the review takes?**\n\n- a) Large diffs get postponed\n- b) Large diffs contain more bugs\n- c) Reviewers prefer small teams\n\n> **Answer:** a) Large diffs get postponed. A reviewer batches a large diff, then skims it. Past roughly four hundred changed lines the wait grows and the reading gets shallower.\n\n**Q3. Why is onboarding time a platform metric, not a hiring metric?**\n\n- a) It measures the golden path\n- b) It measures the new hire\n- c) It measures the recruiter\n\n> **Answer:** a) It measures the golden path. Time to a first merged pull request exercises access, docs, environments and CI at once. A slow result is a finding about the platform.\n\n## Explained\n\n### Question one\n\nWhy does lead time beat utilisation? Utilisation counts the time people spend busy; lead time counts the time a change takes to reach production. In any queue, high utilisation means long waits: at ninety per cent busy, the wait is roughly nine times the work in the simplest model. A team at full capacity has no slack to absorb a review, an incident or a bad day.\n\n### Question two\n\nWhy does pull request size predict review delay? A reviewer batches a large diff, postpones it, then skims it. Past roughly four hundred changed lines, the wait grows and the reading gets shallower, so the review that was meant to catch defects catches fewer of them.\n\n### Question three\n\nWhy is onboarding time a platform metric? Time to a first merged pull request exercises access, documentation, environments and CI in one path. A slow result is a finding about the platform, not about the new hire's ability, and the fix is usually a missing template or a manual grant.\n\n> **The honest caveat:** these are rules of thumb, not laws. A small diff can wait for a missing reviewer, and a fast onboarding can hide a team that never gives the new hire real work.\n\n> **The lesson:** the queue is the system, and the person is rarely the bottleneck.\n\n## In one line\n\n> **Measure the wait, not the busy.** Three questions, one theme: queues beat effort.\n\n**Ask a new hire**\n\n```bash\ngh pr list --author @me\n```\n"},{"id":"p471","n":"471","slug":"devex-terms-decoded","title":"DevEx terms, decoded","sections":["devsecops"],"topic":"Platform engineering","tags":["DevEx","PlatformEngineering","DevOps","DORA","SoftwareEngineering"],"series_name":"DevOps","design":"reveal","script":"","date":"2029-09-30","files":{"devsecops":"notes/p471-devex-terms-decoded.md"},"md":"# DevEx terms, decoded\n\nSix terms come up in every developer productivity review, and two of them are usually being defined differently by the people in the room.\n\n## Quick reference\n\n- **Lead time** — Commit to production in DORA; request to delivery in lean. Name the clock before comparing.\n- **Cycle time** — Start of work to done. It leaves out the queue that lead time includes.\n- **WIP** — Work in progress: started, not finished. Every extra item makes every queue longer.\n- **Toil** — Manual, repetitive, automatable work that grows with the service, not with its value.\n- **Cognitive load** — The working memory a task demands. A platform should carry it, not the team.\n- **Inner source** — Open-source habits inside one company: internal repos, issues and pull requests anyone may send.\n\n## Explained\n\n### Time\n\nLead time is the clock from a change to production. DORA starts it at the commit; lean starts it at the request, which is why the same team can report two different numbers. Cycle time starts when work starts and ends when it is done, so it leaves out the queue that lead time includes.\n\n### Flow\n\nWIP, work in progress, is everything started and not finished. Every extra item makes every queue longer, which is the whole argument for a WIP limit. Toil is manual, repetitive, automatable work that grows with the service rather than with the value it delivers.\n\n### People\n\nCognitive load is the working memory a task demands; a platform should carry it so the team does not have to. Inner source is open-source practice inside one company: internal repositories, issues and pull requests that anyone may send.\n\n> **The honest caveat:** every definition here is a choice. Two teams can both say lead time and mean different clocks, so put the start and the stop in the metric name.\n\n> **The lesson:** a shared definition is worth more than a precise one nobody shares.\n\nThat is the series. Fifty posts, from a release strategy to a first merged pull request, five phases and ten Python scripts. Thank you for following along.\n\n## In one line\n\n> **Name the clock, then the number.** A metric without a definition is an opinion.\n\n**Define it first**\n\n```bash\ngrep -rn 'lead time' docs/\n```\n"},{"id":"p472","n":"472","slug":"the-tcp-handshake-in-four-panels","title":"The TCP handshake in four panels","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["TCP","Networking","DevOps","SRE","CloudNative"],"series_name":"Network","design":"explainer","script":"","date":"2029-10-02","files":{"it":"notes/p472-the-tcp-handshake-in-four-panels.md","devsecops":"notes/p472-the-tcp-handshake-in-four-panels.md"},"md":"# The TCP handshake in four panels\n\nA browser opens a connection and the first byte waits while three small packets cross the network. The handshake is the part of TCP nobody draws, and it decides how long every later request takes.\n\n## Quick reference\n\n### 1. SYN\n\n- Picks an initial sequence number (ISN)\n- Offers MSS, window scale and SACK\n- No payload: the packet is a proposal\n\n### 2. SYN-ACK\n\n- Server picks its own ISN\n- Acknowledges client ISN plus one\n- Returns its own options in the same packet\n\n### 3. ACK\n\n- Acknowledges server ISN plus one\n- Both directions are now established\n- This ACK may already carry request data\n\n### 4. Data\n\n- Sequence numbers count bytes, not packets\n- Acknowledgements are cumulative\n- One round trip spent before the URL leaves\n\n> **Note:** Three packets, one round trip, then the request can leave.\n\n## Explained\n\n### Four panels\n\nSYN. The client picks an initial sequence number, sets the SYN flag and offers its options: maximum segment size, window scale and selective ACK. There is no payload yet. The packet is a proposal, not a connection.\n\nSYN-ACK. The server picks its own initial sequence number, acknowledges the client's sequence number plus one, and returns its own options. Both sides now know the other is reachable and what each can send.\n\nACK. The client acknowledges the server's ISN plus one. The connection is established in both directions, and this ACK may already carry data.\n\nData. Sequence numbers count bytes, not packets, and acknowledgements are cumulative: each one names the next byte the sender expects. A retransmission fills a gap instead of repeating the whole stream.\n\n### What it costs\n\nThe handshake spends one full round trip before the request can leave, and TLS adds its own round trips on top. On a 40 ms link that is 40 ms of pure waiting for every new connection, before the server has seen a URL.\n\n> **The honest caveat:** TCP Fast Open can put data in the SYN, but middleboxes still drop it often enough that the first round trip remains the norm.\n\n> **The lesson:** count the round trips before you blame the server.\n\n## In one line\n\n> **One round trip before the request.** The first byte you send is not the first packet on the wire.\n\n**Count the rtt**\n\n```bash\ncurl -w '%{time_connect}' -sI URL\n```\n"},{"id":"p473","n":"473","slug":"the-network-debugging-cheat-sheet","title":"The network debugging cheat sheet","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Troubleshooting","Linux","DevOps","SRE"],"series_name":"Network","design":"cheatsheet","script":"","date":"2029-10-04","files":{"it":"notes/p473-the-network-debugging-cheat-sheet.md","devsecops":"notes/p473-the-network-debugging-cheat-sheet.md"},"md":"# The network debugging cheat sheet\n\nA request fails and everyone has an opinion: DNS, the firewall, the certificate, the server. Four blocks of commands settle it in the order the packet travels.\n\n## Quick reference\n\n### DNS\n\n```bash\ndig example.com +short\ndig example.com @1.1.1.1 +short\ndig +trace example.com\ndig -x 10.0.0.5 +short\n```\n\n### Reach\n\n```bash\nping -c 4 10.0.8.20\ntraceroute -n 10.0.8.20\nnc -vz 10.0.8.20 443\ncurl -sS -I https://example.com\n```\n\n### Sockets\n\n```bash\nss -tlnp\nss -tan state established\nss -s\nss -tanp | grep 10.0.8.20\n```\n\n### Capture\n\n```bash\ntcpdump -i any port 443 -nn\ntcpdump -nn port 53\ntcpdump -i eth0 -w /tmp/cap.pcap\ntcpdump -r /tmp/cap.pcap -nn\n```\n\n## Explained\n\n### DNS\n\nStart with the name. `dig example.com +short` returns the address your resolver would use. `dig example.com @1.1.1.1 +short` asks a different resolver, which separates a local problem from a global one. `dig +trace example.com` walks down from the root. `dig -x 10.0.0.5 +short` reverses an address you found in a log.\n\n### Reach\n\nThen the path. `ping -c 4 10.0.8.20` proves the host answers ICMP, though many firewalls block it, so a timeout is not proof of death. `traceroute -n 10.0.8.20` shows the hops. `nc -vz 10.0.8.20 443` tests one port. `curl -sS -I https://example.com` proves the service answers at the application layer.\n\n### Sockets\n\nOn the host, `ss -tlnp` shows what is listening and which process owns it. `ss -tan state established` shows live connections. `ss -s` gives the summary when the table is too long. `ss -tanp | grep 10.0.8.20` finds every socket talking to one peer.\n\n### Capture\n\nWhen the counters disagree with the story, capture. `tcpdump -i any port 443 -nn` shows the packets. `tcpdump -nn port 53` catches DNS. `tcpdump -i eth0 -w /tmp/cap.pcap` writes a file, and `tcpdump -r /tmp/cap.pcap -nn` reads it back later.\n\n> **The honest caveat:** a capture on one host cannot see packets dropped by a middlebox, and tcpdump needs root or the right capability.\n\n> **The lesson:** test the layers in order, and stop at the first one that disagrees.\n\n## In one line\n\n> **Follow the packet, not the guess.** The answer is in the path, not in the ticket.\n\n**Capture one flow**\n\n```bash\ntcpdump -nn -i any port 443\n```\n"},{"id":"p474","n":"474","slug":"http-timing-phases-in-python","title":"HTTP timing phases in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Networking","HTTP","Performance","DevOps"],"series_name":"Network","design":"code","script":"http_timing.py","date":"2029-10-07","files":{"it":"notes/p474-http-timing-phases-in-python.md","devsecops":"notes/p474-http-timing-phases-in-python.md"},"md":"# HTTP timing phases in Python\n\nA page takes 2.4 seconds and the dashboard says the server is fine. The time is in the phases before the request reaches the application, and a stopwatch around the whole URL hides all of them.\n\n## Quick reference\n\nThe full script is at the end of this note: `http_timing.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 http_timing.py https://example.com --budget 1.5\n```\n\n**Example output**\n\n```text\ndns 2.1ms tcp 18.4ms tls 41.2ms ttfb 96.0ms\ntotal 157 ms\n```\n\n## Explained\n\n### What it measures\n\nThe script resolves the hostname, opens a TCP connection, performs the TLS handshake, sends one GET and waits for the first byte of the response. Each phase gets its own timer, from the same monotonic clock, so the rows add up to something you can defend.\n\n### The table\n\nIt prints five rows: dns, tcp, tls, ttfb and total, each in milliseconds. A slow DNS row means the resolver or a search domain. A slow TCP row means the path or a full accept queue. A slow TLS row means a long certificate chain or a cold session cache. A slow TTFB row means the application, or a proxy waiting on it.\n\n### The budget\n\nPass `--budget 1.5` and the script exits 1 when the total exceeds 1.5 seconds. That makes it a check, not a dashboard: run it from cron against your own URL, or as a post-deploy step, and let the exit code fail the job. The full script is attached, standard library only and read-only.\n\n> **The honest caveat:** one sample from one machine measures that machine's path, not your users'. Run it from the region you care about, and compare distributions rather than single runs.\n\n> **The lesson:** a page is a sum of phases, so measure the phases.\n\n## In one line\n\n> **Measure the hop, not the whole.** A slow page is a sum of phases you never separated.\n\n**Run it in ci**\n\n```bash\npython3 http_timing.py https://example.com\n```\n\n## Script: http_timing.py\n\nDownload: [scripts/http_timing.py](../scripts/http_timing.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Measure the HTTP timing phases of one URL.\n\n    python3 http_timing.py https://example.com --budget 1.5\n    python3 http_timing.py http://127.0.0.1:8080/ --budget 0.25\n\nPrints dns, tcp, tls and ttfb, then total to the first response byte,\nin milliseconds. Exits 1 when total exceeds --budget, and 2 on a usage,\nURL or connection error. The request is one GET; nothing is written.\n\nCron/CI: python3 http_timing.py \"$URL\" --budget 2 || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport socket\nimport ssl\nimport sys\nimport time\nfrom urllib.parse import urlsplit\n\n\ndef parse_url(url):\n    \"\"\"Return scheme, host, port and request path for an http(s) URL.\"\"\"\n    parts = urlsplit(url)\n    if parts.scheme not in (\"http\", \"https\"):\n        raise ValueError(\"URL must start with http:// or https://\")\n    if not parts.hostname:\n        raise ValueError(\"URL has no hostname\")\n    try:\n        port = parts.port\n    except ValueError as exc:\n        raise ValueError(\"URL has an invalid port\") from exc\n    if port is None:\n        port = 443 if parts.scheme == \"https\" else 80\n    path = parts.path or \"/\"\n    if parts.query:\n        path += \"?\" + parts.query\n    return parts.scheme, parts.hostname, port, path\n\n\ndef tcp_connect(infos, timeout):\n    \"\"\"Connect to the first working address and return the socket.\"\"\"\n    last = None\n    for family, socktype, proto, _, sockaddr in infos:\n        sock = socket.socket(family, socktype, proto)\n        sock.settimeout(timeout)\n        try:\n            sock.connect(sockaddr)\n            return sock\n        except OSError as exc:\n            last = exc\n            sock.close()\n    raise last or OSError(\"no address to connect to\")\n\n\ndef measure(url, timeout, insecure):\n    \"\"\"Time each phase of one GET, in milliseconds.\"\"\"\n    scheme, host, port, path = parse_url(url)\n    started = time.perf_counter()\n    infos = socket.getaddrinfo(host, port, type=socket.SOCK_STREAM)\n    after_dns = time.perf_counter()\n    sock = tcp_connect(infos, timeout)\n    after_tcp = time.perf_counter()\n    if scheme == \"https\":\n        context = ssl.create_default_context()\n        if insecure:\n            context.check_hostname = False\n            context.verify_mode = ssl.CERT_NONE\n        sock = context.wrap_socket(sock, server_hostname=host)\n    after_tls = time.perf_counter()\n    default_port = 443 if scheme == \"https\" else 80\n    shown = \"[%s]\" % host if \":\" in host else host\n    host_header = shown if port == default_port else \"%s:%d\" % (shown, port)\n    request = (\n        \"GET %s HTTP/1.1\\r\\n\"\n        \"Host: %s\\r\\n\"\n        \"User-Agent: http-timing/1.0\\r\\n\"\n        \"Accept: */*\\r\\n\"\n        \"Connection: close\\r\\n\\r\\n\" % (path, host_header)\n    ).encode(\"ascii\", \"replace\")\n    sock.sendall(request)\n    first = sock.recv(1)\n    after_first = time.perf_counter()\n    sock.close()\n    if not first:\n        raise OSError(\"the server closed before the first byte\")\n    return {\n        \"dns\": (after_dns - started) * 1000,\n        \"tcp\": (after_tcp - after_dns) * 1000,\n        \"tls\": (after_tls - after_tcp) * 1000,\n        \"ttfb\": (after_first - after_tls) * 1000,\n        \"total\": (after_first - started) * 1000,\n    }\n\n\ndef report(phases, budget):\n    \"\"\"Print the phase table and return the process exit code.\"\"\"\n    total = phases[\"total\"]\n    print(\"%-8s%9s%8s\" % (\"phase\", \"ms\", \"share\"))\n    for name in (\"dns\", \"tcp\", \"tls\", \"ttfb\", \"total\"):\n        ms = phases[name]\n        share = (ms / total * 100) if total > 0 else 0.0\n        print(\"%-8s%9.1f%7.1f%%\" % (name, ms, share))\n    if total > budget * 1000:\n        print(\"OVER budget by %.1f ms\" % (total - budget * 1000))\n        return 1\n    print(\"within budget: %.1f ms of %.0f ms\" % (total, budget * 1000))\n    return 0\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Measure DNS, TCP, TLS, TTFB and total for one URL.\")\n    parser.add_argument(\"url\", help=\"http:// or https:// URL to time\")\n    parser.add_argument(\"--budget\", type=float, default=1.5,\n                        help=\"total budget in seconds (default: 1.5)\")\n    parser.add_argument(\"--timeout\", type=float, default=10.0,\n                        help=\"connect and read timeout in seconds (default: 10)\")\n    parser.add_argument(\"--insecure\", action=\"store_true\",\n                        help=\"skip TLS verification (test fixtures only)\")\n    args = parser.parse_args(argv)\n    if args.budget <= 0:\n        parser.error(\"--budget must be greater than zero\")\n    if args.timeout <= 0:\n        parser.error(\"--timeout must be greater than zero\")\n    try:\n        phases = measure(args.url, args.timeout, args.insecure)\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    except ssl.SSLError as exc:\n        print(\"ERROR TLS handshake failed: %s\" % exc, file=sys.stderr)\n        return 2\n    except OSError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    return report(phases, args.budget)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p475","n":"475","slug":"a-tls-inspector-in-python","title":"A TLS inspector in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["TLS","Security","Python","Networking","DevOps"],"series_name":"Network","design":"code","script":"tls_inspect.py","date":"2029-10-09","files":{"it":"notes/p475-a-tls-inspector-in-python.md","devsecops":"notes/p475-a-tls-inspector-in-python.md"},"md":"# A TLS inspector in Python\n\nA certificate expires on a Sunday and the first person to notice is a customer. Checking one host takes a handshake, and the facts are all in the certificate the server already sends.\n\n## Quick reference\n\nThe full script is at the end of this note: `tls_inspect.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 tls_inspect.py example.com --port 443\n```\n\n**Example output**\n\n```text\nTLSv1.3 TLS_AES_256_GCM_SHA384\ndays 212 SANs example.com, www.example.com\n```\n\n## Explained\n\n### What it prints\n\nThe script connects, negotiates TLS, and prints the protocol version, the cipher, the subject, the DNS names in the SAN extension, and the days left before notAfter. Those are the five things a support ticket usually needs.\n\n### What it flags\n\nTwo checks make it exit 1. A certificate with fewer than 30 days left is expiring, because renewal queues and approval windows are longer than people remember. A hostname that matches none of the SAN entries is a mismatch, which is what a browser turns into a full-page warning. Wildcards match one label only: \\*.example.com covers api.example.com, not a.b.example.com.\n\n### How to run it\n\n`python3 tls_inspect.py example.com`, or the host and `--port` for a non-standard listener. Add `--days 14` to tighten the expiry threshold, and `--cafile` for a private CA. Exit 0 means clean, 1 means a finding, 2 means the name, the port or the connection failed. The full script is attached: standard library only, one handshake, no writes.\n\n> **The honest caveat:** the script checks the certificate the server presents now. It cannot see the renewal job that will run next week, so put it in cron and let it page before the customer does.\n\n> **The lesson:** expiry is a date you can test, so test it.\n\n## In one line\n\n> **Trust is a date, not a feeling.** A certificate is a fact with an expiry attached.\n\n**Check one host**\n\n```bash\npython3 tls_inspect.py example.com\n```\n\n## Script: tls_inspect.py\n\nDownload: [scripts/tls_inspect.py](../scripts/tls_inspect.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Inspect the TLS certificate served by one host.\n\n    python3 tls_inspect.py example.com\n    python3 tls_inspect.py 127.0.0.1 --port 8443 --cafile /tmp/test-ca.pem\n    python3 tls_inspect.py example.com --days 14\n\nPrints the protocol, cipher, subject, SANs and days to expiry. Exits 1\nwhen the certificate expires within --days, does not match the hostname,\nor fails verification; 0 when clean; 2 on a usage or connection error.\nOne handshake; nothing is written.\n\nCron/CI: python3 tls_inspect.py \"$HOST\" --days 21 || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport socket\nimport ssl\nimport sys\nimport time\n\nNAMES = {\n    \"commonName\": \"CN\",\n    \"organizationName\": \"O\",\n    \"organizationalUnitName\": \"OU\",\n    \"countryName\": \"C\",\n    \"stateOrProvinceName\": \"ST\",\n    \"localityName\": \"L\",\n}\n\n\ndef format_subject(cert):\n    \"\"\"Turn the subject tuple into CN=..., O=... form.\"\"\"\n    parts = []\n    for rdn in cert.get(\"subject\", ()):\n        for key, value in rdn:\n            parts.append(\"%s=%s\" % (NAMES.get(key, key), value))\n    return \", \".join(parts) or \"(none)\"\n\n\ndef dns_match(host, names):\n    \"\"\"Match a hostname against SAN DNS names, one wildcard label only.\"\"\"\n    host = host.lower().rstrip(\".\")\n    for raw in names:\n        name = raw.lower().rstrip(\".\")\n        if name == host:\n            return True\n        if name.startswith(\"*.\"):\n            suffix = name[2:]\n            if host.count(\".\") == suffix.count(\".\") + 1 and host.endswith(\".\" + suffix):\n                return True\n    return False\n\n\ndef ip_match(host, names):\n    \"\"\"Match an IP address against SAN IP entries.\"\"\"\n    try:\n        address = ipaddress.ip_address(host)\n    except ValueError:\n        return False\n    for value in names:\n        try:\n            if ipaddress.ip_address(value) == address:\n                return True\n        except ValueError:\n            continue\n    return False\n\n\ndef host_matches(host, sans):\n    \"\"\"Check a hostname or IP against the certificate's SAN entries.\"\"\"\n    dns = [value for kind, value in sans if kind == \"DNS\"]\n    ips = [value for kind, value in sans if kind == \"IP Address\"]\n    try:\n        ipaddress.ip_address(host)\n    except ValueError:\n        return dns_match(host, dns)\n    return ip_match(host, ips)\n\n\ndef inspect(host, port, sni, timeout, cafile):\n    \"\"\"Connect once and return the certificate facts.\"\"\"\n    raw = socket.create_connection((host, port), timeout)\n    try:\n        context = ssl.create_default_context()\n        if cafile:\n            try:\n                context.load_verify_locations(cafile=cafile)\n            except OSError as exc:\n                raise ValueError(\"cannot load CA file: %s\" % exc)\n        context.check_hostname = False\n        context.verify_mode = ssl.CERT_REQUIRED\n        sock = context.wrap_socket(raw, server_hostname=sni)\n    except Exception:\n        raw.close()\n        raise\n    try:\n        cert = sock.getpeercert()\n        version = sock.version() or \"?\"\n        cipher = sock.cipher() or (\"?\", None, None)\n    finally:\n        sock.close()\n    if not cert:\n        raise ValueError(\"the server presented no certificate\")\n    not_after = cert.get(\"notAfter\")\n    if not not_after:\n        raise ValueError(\"the certificate has no notAfter field\")\n    expiry = ssl.cert_time_to_seconds(not_after)\n    days = (expiry - time.time()) / 86400\n    sans = cert.get(\"subjectAltName\", ())\n    return {\n        \"version\": version,\n        \"cipher\": cipher[0],\n        \"bits\": cipher[2] if cipher[2] is not None else \"?\",\n        \"subject\": format_subject(cert),\n        \"sans\": sans,\n        \"expiry\": time.strftime(\"%Y-%m-%d\", time.gmtime(expiry)),\n        \"days\": days,\n        \"match\": host_matches(sni, sans),\n    }\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Inspect one host's TLS certificate and expiry.\")\n    parser.add_argument(\"host\", help=\"hostname or IP address to inspect\")\n    parser.add_argument(\"--port\", type=int, default=443,\n                        help=\"TCP port (default: 443)\")\n    parser.add_argument(\"--sni\", default=None,\n                        help=\"hostname for SNI and matching (default: host)\")\n    parser.add_argument(\"--days\", type=float, default=30.0,\n                        help=\"expiry warning threshold in days (default: 30)\")\n    parser.add_argument(\"--timeout\", type=float, default=10.0,\n                        help=\"connect and handshake timeout (default: 10)\")\n    parser.add_argument(\"--cafile\", default=None,\n                        help=\"CA bundle to trust instead of the system store\")\n    args = parser.parse_args(argv)\n    if not 1 <= args.port <= 65535:\n        parser.error(\"--port must be between 1 and 65535\")\n    if args.days < 0:\n        parser.error(\"--days must not be negative\")\n    if args.timeout <= 0:\n        parser.error(\"--timeout must be greater than zero\")\n    check_name = args.sni or args.host\n    try:\n        info = inspect(args.host, args.port, check_name, args.timeout, args.cafile)\n    except ssl.SSLCertVerificationError as exc:\n        reason = getattr(exc, \"verify_message\", str(exc))\n        print(\"FINDING certificate verify failed: %s\" % reason)\n        return 1\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    except ssl.SSLError as exc:\n        print(\"ERROR TLS handshake failed: %s\" % exc, file=sys.stderr)\n        return 2\n    except OSError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    sans = info[\"sans\"]\n    shown = \", \".join(value for _, value in sans) or \"(none)\"\n    print(\"host      %s:%d\" % (args.host, args.port))\n    print(\"protocol  %s\" % info[\"version\"])\n    print(\"cipher    %s (%s bits)\" % (info[\"cipher\"], info[\"bits\"]))\n    print(\"subject   %s\" % info[\"subject\"])\n    print(\"SANs      %s\" % shown)\n    print(\"expires   %s (%.1f days)\" % (info[\"expiry\"], info[\"days\"]))\n\n    findings = []\n    if info[\"days\"] < 0:\n        findings.append(\"EXPIRED %.1f days ago\" % -info[\"days\"])\n    elif info[\"days\"] < args.days:\n        findings.append(\n            \"EXPIRING %.1f days left (threshold %.0f)\"\n            % (info[\"days\"], args.days))\n    if not info[\"match\"]:\n        findings.append(\"MISMATCH %s not in the SANs\" % check_name)\n    if findings:\n        for finding in findings:\n            print(\"FINDING   %s\" % finding)\n        return 1\n    print(\"OK        no findings\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p476","n":"476","slug":"http-1-1-vs-http-2-vs-http-3","title":"HTTP/1.1 vs HTTP/2 vs HTTP/3","sections":["it","devsecops"],"topic":"Web servers, TLS & HTTP","tags":["HTTP","Networking","Performance","WebPerformance","DevOps"],"series_name":"Network","design":"matrix","script":"","date":"2029-10-11","files":{"it":"notes/p476-http-1-1-vs-http-2-vs-http-3.md","devsecops":"notes/p476-http-1-1-vs-http-2-vs-http-3.md"},"md":"# HTTP/1.1 vs HTTP/2 vs HTTP/3\n\nThree versions of HTTP carry the same request, and the difference is not the syntax. It is what one lost packet does to everything else sharing the connection.\n\n## Quick reference\n\n| | **HTTP/1.1** | **HTTP/2** | **HTTP/3** |\n|:--|:--|:--|:--|\n| Transport | TCP | TCP | QUIC over UDP |\n| Multiplexing | No, six connections | Streams over one TCP | Streams over QUIC |\n| Head-of-line | Blocks the next | TCP loss stalls all | Stalls one stream |\n| Handshake | TCP + TLS, 2-3 RTT | TCP + TLS 1.3, 2 RTT | QUIC, 1 RTT or 0 |\n| Server push | No | Yes, but rarely used | Not in HTTP/3 |\n| Best for | Legacy and simple | Busy sites on TCP | Lossy mobile links |\n\n**How to pick**\n\n- **HTTP/1.1** → legacy clients and simple paths.\n- **HTTP/2** → many requests on a reliable network.\n- **HTTP/3** → mobile, lossy and high-latency links.\n\n## Explained\n\n### Transport\n\nHTTP/1.1 and HTTP/2 both run over TCP. HTTP/3 runs over QUIC, which is built on UDP and implements its own streams, loss detection and encryption. UDP is not a downgrade here; it is the layer QUIC needs to avoid TCP's ordering.\n\n### Multiplexing\n\nHTTP/1.1 handles one response at a time per connection, so browsers open about six. HTTP/2 puts many streams on one TCP connection, each with its own identifier. HTTP/3 keeps the streams but moves them onto QUIC, where they are independent by design.\n\n### Head-of-line\n\nHTTP/1.1 blocks the next response behind the current one. HTTP/2 removes the application-level queue but keeps the TCP one: a single lost segment stalls every stream until it is retransmitted. HTTP/3 stalls only the stream that lost the packet, which is why it wins on mobile and lossy links.\n\n### Handshake, push, fit\n\nHTTP/1.1 costs a TCP handshake plus TLS. HTTP/2 pays the same two handshakes, though TLS 1.3 cuts one round trip. HTTP/3 folds transport and crypto into one QUIC handshake, with 0-RTT resumption. Server push exists in HTTP/2 and is rarely used; HTTP/3 has no push. HTTP/1.1 suits legacy clients, HTTP/2 busy sites on reliable networks, HTTP/3 lossy mobile paths.\n\n> **The honest caveat:** HTTP/3 needs UDP 443 open end to end, and some corporate networks still block it, so a working HTTP/2 path is not wasted.\n\n> **The lesson:** framing decides what one lost packet costs.\n\n## In one line\n\n> **Framing decides what stalls.** The version matters less than what one lost packet blocks.\n\n**See the version**\n\n```bash\ncurl -sI https://example.com | head -1\n```\n"},{"id":"p477","n":"477","slug":"the-edge-path","title":"The edge path","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["CDN","Networking","CloudArchitecture","DevOps","Security"],"series_name":"Network","design":"architecture","script":"","date":"2029-10-14","files":{"it":"notes/p477-the-edge-path.md","devsecops":"notes/p477-the-edge-path.md"},"md":"# The edge path\n\nA user types a name and a page appears. Between those two events sit three tiers, and each one can add latency, cost or a failure mode.\n\n## Quick reference\n\n- **Client:** Browser (WEB) · DNS resolver (DNS)\n- **Edge:** CDN edge (CDN) · WAF (WAF)\n- **Origin:** Load balancer (LB) · Application (APP)\n\n**Request flow:** DNS resolver → Browser → CDN edge → WAF → Load balancer → Application\n\n## Explained\n\n### Client\n\nThe browser starts with DNS. It asks the resolver, which answers with an address and a TTL. Then it connects to the edge, not to your origin. The first lookup and the round trip to the nearest point of presence bound everything the client does.\n\n### Edge\n\nThe CDN terminates TLS near the user, looks up the object in its cache and answers a hit without touching your network. On a miss it forwards to the origin through a persistent connection. The WAF sits in the same path, inspecting the request before it passes; a block here never reaches the load balancer.\n\n### Origin\n\nThe load balancer receives the request, picks a healthy target and opens or reuses a connection to the application. The application does the work and returns the response, which the CDN may cache for the next user.\n\n### The path in order\n\nBrowser, resolver, CDN, WAF, load balancer, application. Every arrow is a place to measure. A long DNS TTL slows a failover; a broad WAF rule blocks a checkout; a shallow health check sends traffic to a dead app.\n\n> **The honest caveat:** the edge only helps what it can cache or terminate. A personalised POST still crosses the whole path, and a wrong TTL can serve stale content.\n\n> **The lesson:** draw the path, then measure each arrow on it.\n\n## In one line\n\n> **The shortest path is the edge.** Every hop you remove is a hop that cannot fail.\n\n**Check the edge**\n\n```bash\ncurl -sI https://example.com\n```\n"},{"id":"p478","n":"478","slug":"anatomy-of-a-connection-tuple","title":"Anatomy of a connection tuple","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["TCP","Networking","Linux","SysAdmin","SRE"],"series_name":"Network","design":"anatomy","script":"","date":"2029-10-16","files":{"it":"notes/p478-anatomy-of-a-connection-tuple.md","devsecops":"notes/p478-anatomy-of-a-connection-tuple.md"},"md":"# Anatomy of a connection tuple\n\nA server sees 10.0.4.7:49152 connect to 10.0.8.20:443, and those addresses and ports are the connection.\n\n## Quick reference\n\n```text\n10.0.4.7:49152 -> 10.0.8.20:443\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `10.0.4.7` | Source IP | The client. Private in the VPC, NATed before it reaches the internet. |\n| `:49152` | Source port | Ephemeral, chosen per connection. The half that makes the tuple unique. |\n| `->` | The arrow | Direction of the first packet. Every reply reverses it. |\n| `10.0.8.20` | Destination IP | The server the client resolved. Anycast may land on another machine. |\n| `:443` | Destination port | 443 means TLS. The listener that accepts the connection. |\n\n## Explained\n\n### The tuple\n\nSource IP. 10.0.4.7 is the client. It is private in the VPC. The server routes its reply here, so changing it changes the return path.\n\nSource port. 49152 is the ephemeral port. The client picks it from the kernel's range, usually 32768 to 60999 on Linux, to tell this connection from every other to the same server. Two connections to one host get two source ports.\n\nThe arrow. The arrow is direction, not a field. The first packet travels from source to destination, and every reply reverses the addresses and ports. A stateful firewall remembers the direction, so the reply needs no rule.\n\nDestination IP. 10.0.8.20 is the server. The client got it from DNS, and with anycast it may not be the machine that runs the code.\n\nDestination port. 443 is the listener. It tells the server which service receives the packet, and it rarely changes between connections.\n\n### What NAT does\n\nNAT rewrites the source address and port on the way out and keeps the mapping, so the reply finds its way back. The server sees the NAT address, not 10.0.4.7, which is why a connection can look healthy on the client and unknown on the server.\n\n> **The honest caveat:** the real key is a five-tuple, and the drawing leaves out the protocol. TCP and UDP can share the same addresses and ports.\n\n> **The lesson:** a connection is an address pair and a port pair, and NAT rewrites half of it.\n\n## In one line\n\n> **The tuple is the connection.** Change one field and it is a different connection.\n\n**See your tuples**\n\n```bash\nss -tanp\n```\n"},{"id":"p479","n":"479","slug":"the-life-of-a-tcp-connection","title":"The life of a TCP connection","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["TCP","Networking","SRE","DevOps","Linux"],"series_name":"Network","design":"timeline","script":"","date":"2029-10-18","files":{"it":"notes/p479-the-life-of-a-tcp-connection.md","devsecops":"notes/p479-the-life-of-a-tcp-connection.md"},"md":"# The life of a TCP connection\n\nA connection is born, works, idles and dies, and the kernel keeps a state for every step. Knowing the six states explains most of what ss shows you.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **0 ms** | SYN | Client sends SYN with its initial sequence number and options. |\n| **1 ms** | Established | SYN-ACK returns, ACK completes it; data flows both ways. |\n| **2 h** | Keep-alive | First probe after 7200 s idle; nine misses drop the socket. |\n| **2 h+** | Idle | No payload, but a file descriptor and a conntrack entry remain. |\n| **60 s** | TIME_WAIT | The active closer waits 2 x MSL before the tuple is reused. |\n| **60 s+** | Closed | The tuple is released and the ephemeral port can be reused. |\n\n## Explained\n\n### The start\n\nSYN. The client sends a SYN with an initial sequence number and options. The socket sits in SYN_SENT, and the server creates an entry in SYN_RECV.\n\nEstablished. The SYN-ACK returns and the ACK completes the handshake. Both sockets move to ESTABLISHED and data can flow both ways. This is the only state where the application reads and writes normally.\n\n### The idle\n\nKeep-alive. When enabled, after two hours idle by default on Linux, the kernel sends a probe with no payload. If the peer answers, the timer resets. If nine probes go unanswered, the connection is dropped.\n\nIdle. Between probes the connection is idle but still established, and it still consumes a file descriptor, a buffer and a conntrack entry. Thousands of idle connections are held open, not leaked.\n\n### The end\n\nTIME_WAIT. The side that closes first waits 60 seconds on Linux before the tuple can be reused. The wait absorbs delayed duplicates and lets the final ACK be retransmitted if it was lost.\n\nClosed. The tuple is released and the ephemeral port can be reused. The next connection may get the same port with a new sequence number.\n\n> **The honest caveat:** keep-alive does not survive every middlebox. A NAT or firewall may drop the idle mapping long before two hours, and the next write discovers a dead connection.\n\n> **The lesson:** read the state before you blame the application.\n\n## In one line\n\n> **Every state is a promise.** The kernel keeps it so your application does not have to.\n\n**Watch the state**\n\n```bash\nss -tan state time-wait\n```\n"},{"id":"p480","n":"480","slug":"quiz-3-packet-questions","title":"Quiz: 3 packet questions","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","SRE","DevOps","HTTP","CloudNative"],"series_name":"Network","design":"quiz","script":"","date":"2029-10-21","files":{"it":"notes/p480-quiz-3-packet-questions.md","devsecops":"notes/p480-quiz-3-packet-questions.md"},"md":"# Quiz: 3 packet questions\n\nThree questions come up in every network review, and the answers are more mechanical than most people expect.\n\n## Quick reference\n\n**Q1. Why does TIME_WAIT exist?**\n\n- a) To retransmit the request body\n- b) To absorb delayed duplicates\n- c) To reserve the port for ever\n\n> **Answer:** b) To absorb delayed duplicates. The active closer waits twice the maximum segment lifetime so delayed duplicates from the old connection expire, and a lost final ACK can be retransmitted.\n\n**Q2. A load balancer returns 502. What usually happened?**\n\n- a) The upstream refused or reset\n- b) The upstream was too slow\n- c) The client sent a bad header\n\n> **Answer:** a) The upstream refused or reset. 502 means the balancer could not get a valid response: connection refused, reset or malformed. 504 is the slow one, where the upstream was reachable but timed out.\n\n**Q3. Why does keep-alive cut latency?**\n\n- a) It compresses each request\n- b) It skips TCP and TLS setup\n- c) It caches the response\n\n> **Answer:** b) It skips TCP and TLS setup. Reusing a connection avoids a new TCP handshake and TLS handshake, and keeps slow start warm. On HTTP/2 the same connection also carries many streams.\n\n## Explained\n\n### Why TIME_WAIT\n\nQuestion one: why does TIME_WAIT exist? The side that closes first waits twice the maximum segment lifetime before the tuple can be reused. That wait does two jobs: it lets a delayed duplicate segment from the old connection expire, and it lets the final ACK be retransmitted if the peer never received it. Without it, a new connection on the same port pair could receive a stale packet. It is not a leak.\n\n### What a 502 means\n\nQuestion two: a load balancer returns 502. Usually the balancer could not get a valid response from the upstream: the connection was refused, reset, or the reply was malformed. The application may have crashed, the port may be wrong, or the upstream may have closed the connection early. A 504 is different: the upstream was reachable but too slow, and the balancer gave up waiting.\n\n### Why keep-alive helps\n\nQuestion three: why does keep-alive cut latency? A fresh HTTPS request pays a TCP handshake and a TLS handshake before the first byte of the request. Reusing the connection skips the handshakes and the slow start that follows them, so the second request leaves immediately. On HTTP/2, one connection also carries many concurrent streams.\n\n> **The honest caveat:** 502 is a symptom, not a cause. The balancer log names the upstream, and that upstream's log names the real error.\n\n> **The lesson:** the packet state and the status code are evidence, not opinion.\n\n## In one line\n\n> **The packet tells you the answer.** Three questions worth asking before the next incident.\n\n**Ask the kernel**\n\n```bash\nss -s\n```\n"},{"id":"p481","n":"481","slug":"5-network-mistakes","title":"5 network mistakes","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","DevOps","SRE","CloudNative","Troubleshooting"],"series_name":"Network","design":"dontdo","script":"","date":"2029-10-23","files":{"it":"notes/p481-5-network-mistakes.md","devsecops":"notes/p481-5-network-mistakes.md"},"md":"# 5 network mistakes\n\nA network fails at the defaults, not the clever parts. These five turn a blip into an outage.\n\n## Quick reference\n\n**1.** ❌ **No timeout on a request** — A hung peer holds workers until the pool is empty.  \n✅ **Connect and read timeouts** — Fail fast and keep capacity for healthy peers.\n\n**2.** ❌ **A new connection per call** — Every call pays TCP and TLS before the first byte.  \n✅ **Pool with keep-alive** — One handshake serves hundreds of requests.\n\n**3.** ❌ **MTU set too small** — More packets, more headers, and UDP gets fragmented.  \n✅ **Match the smallest link MTU** — Usually 1500 on the internet; jumbo stays inside.\n\n**4.** ❌ **Retry immediately, ten times** — A struggling service gets ten times the load.  \n✅ **Backoff with jitter** — Spread retries and give the service room to recover.\n\n**5.** ❌ **Health check that only pings** — The host is up while its database connection is dead.  \n✅ **Check the real dependency** — A shallow check sends traffic to a broken app.\n\n## Explained\n\n### The five\n\n1. **No timeouts.** A request with no timeout waits for ever. One hung peer holds a worker, then the next, until the pool is empty. Set connect and read timeouts below your caller's timeout.\n   \\2. No keep-alive. A new connection per call pays TCP and TLS handshakes before every request, plus slow start. A pool with keep-alive pays once, saving one or two round trips.\n   \\3. Tiny MTU. An MTU smaller than the path is not a safety net, it is overhead: more packets, more headers, and fragmented UDP. Path MTU discovery depends on ICMP, and a black hole still swallows the message that reports the limit. Match the smallest link, usually 1500 on the internet.\n   \\4. Retry storms. Retrying immediately, ten times, multiplies load on a service already struggling. Add backoff and jitter, cap the attempts and the total time, and retry only idempotent calls.\n   \\5. Health checks that lie. A health check that only pings says the machine is up, not that the app can serve. The process may be running with a dead database connection. Check the dependency the request needs, and keep the check cheap enough to run often.\n\n### What they share\n\nEach is cheap to leave alone and expensive to discover.\n\n> **The honest caveat:** a timeout is a guess about the tail. Measure your p99 before you pick five seconds, or you will cut slow but successful requests.\n\n> **The lesson:** the network punishes defaults you never chose.\n\n## In one line\n\n> **The network punishes defaults.** Every one of these is a default somewhere.\n\n**Set one timeout**\n\n```bash\ncurl --max-time 5 -sI URL\n```\n"},{"id":"p482","n":"482","slug":"how-dns-resolves-a-name","title":"How DNS resolves a name","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","DevOps","SRE","Infrastructure"],"series_name":"Network","design":"architecture","script":"","date":"2029-10-25","files":{"it":"notes/p482-how-dns-resolves-a-name.md","devsecops":"notes/p482-how-dns-resolves-a-name.md"},"md":"# How DNS resolves a name\n\nA deploy is green, the health check passes, and the browser still says the site cannot be found. The application is fine; the name is not.\n\n## Quick reference\n\n- **Stub · Application:** Application (APP) · Resolver library (STUB)\n- **Recursive:** Resolver cache (CACHE) · Recursive resolver (REC)\n- **Authoritative:** Root servers (ROOT) · TLD servers (TLD) · Zone server (ZONE)\n\n**Request flow:** Application → Resolver library → Recursive resolver → Resolver cache; Recursive resolver → Root servers → TLD servers → Zone server\n\n**Around it:** Browser cache (WEB) · OS stub cache (OS)\n\n## Explained\n\n### Three tiers\n\nThe stub resolver is the application's side: getaddrinfo, the resolver library, and the nameservers in `/etc/resolv.conf`. It sends a recursive query and waits.\n\nThe recursive resolver does the walking. It checks its cache, then asks a root server, which refers it to the .com TLD servers, which refer it to your zone.\n\nThe zone's authoritative server returns the real record with the AA flag. The root and the TLD answer with referrals, not the final address; each is authoritative only for its own piece.\n\nThere are thirteen root server identities, anycast across hundreds of sites, and you rarely reach one because a cache answered first.\n\n### The caches\n\nThe answer comes back with a TTL, and every cache on the path may keep it for that long. The second lookup stops at the recursive resolver and never reaches the root.\n\nBrowser cache and OS stub cache sit outside the tiers and answer before a packet leaves the machine.\n\n> **The honest caveat:** getaddrinfo can answer from `/etc/hosts`, mDNS or a local cache without asking any of these servers, so the diagram is not proof of what your process did.\n\n> **The lesson:** a DNS failure belongs to one tier, and the tier tells you who to call.\n\n## In one line\n\n> **Every lookup is a short walk down.** The answer is cached on the way back, so the second lookup stops early.\n\n**Trace the path**\n\n```bash\ndig +trace example.com\n```\n"},{"id":"p483","n":"483","slug":"the-dns-records-cheat-sheet","title":"The DNS records cheat sheet","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","SysAdmin","DevOps","CheatSheet"],"series_name":"Network","design":"cheatsheet","script":"","date":"2029-10-28","files":{"it":"notes/p483-the-dns-records-cheat-sheet.md","devsecops":"notes/p483-the-dns-records-cheat-sheet.md"},"md":"# The DNS records cheat sheet\n\nA record change went out at noon and the old address is still answering at four. Nobody is lying; the answer is cached.\n\n## Quick reference\n\n### Address\n\n```bash\ndig example.com A +short\ndig example.com AAAA +short\ndig example.com A +noall +answer\ndig @1.1.1.1 example.com A\ndig example.com AAAA +noall +answer\n```\n\n### Alias\n\n```bash\ndig www.example.com CNAME +short\ndig example.com NS +short\ndig www.example.com CNAME +noall +answer\ndig +trace example.com\ndig www.example.com A +short\n```\n\n### Mail\n\n```bash\ndig example.com MX +short\ndig example.com TXT +short\ndig _dmarc.example.com TXT +short\ndig default._domainkey.example.com TXT\ndig example.com MX +noall +answer\n```\n\n### Policy\n\n```bash\ndig example.com CAA +short\ndig example.com DS +short\ndig example.com DNSKEY +short\ndig +dnssec example.com A\ndig example.com SOA +short\n```\n\n## Explained\n\n### Address\n\nAn A record holds one IPv4 address, and AAAA holds one IPv6 address. Ask with `dig example.com A +short` and you get the value alone; drop +short when you need the TTL and the section it came from.\n\nA second resolver is a second opinion. `dig @1.1.1.1 example.com A` asks Cloudflare instead of your local resolver, which separates a zone problem from a cache problem.\n\n### Alias\n\nA CNAME points one name at another and must be the only data record at that name. `dig www.example.com CNAME +short` shows the target. NS shows the delegation, and `dig +trace example.com` walks root, TLD and zone in one command. `dig www.example.com A +short` shows what the chain resolves to.\n\n### Mail\n\nMX carries a preference and a mail host. TXT carries SPF and other policy text; DMARC lives at _dmarc.example.com, and DKIM at selector._domainkey.example.com.\n\n### Policy\n\nCAA names the certificate authorities allowed to issue for the name and its subdomains. DS and DNSKEY build the DNSSEC chain, and +dnssec asks for the signatures. SOA holds the serial and the negative-cache timer.\n\n> **The honest caveat:** +short prints only the answer data, so it hides the TTL, the class and the authority section; use the full answer when the question is why.\n\n> **The lesson:** read the record type before you read the record.\n\n## In one line\n\n> **Read the record, not the rumour.** A query is evidence; a dashboard screenshot is not.\n\n**Check the delegation**\n\n```bash\ndig +trace example.com\n```\n"},{"id":"p484","n":"484","slug":"dns-ttl-and-failover","title":"DNS TTL and failover","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","SRE","DevOps","Infrastructure"],"series_name":"Network","design":"explainer","script":"","date":"2029-10-30","files":{"it":"notes/p484-dns-ttl-and-failover.md","devsecops":"notes/p484-dns-ttl-and-failover.md"},"md":"# DNS TTL and failover\n\nYou lower the TTL, change the record, and the old address keeps serving traffic for another hour. The change worked; the caches did not hear about it.\n\n## Quick reference\n\n### TTL\n\n- The authoritative server sets it, per record\n- A resolver may cache for up to that long\n- 300 is five minutes; 86400 is one day\n\n### Caches\n\n- Browser, OS, stub and recursive resolver\n- Each keeps its own copy and its own timer\n- A cache hit never reaches your zone\n\n### Failover\n\n- Health checks swap the answer to a standby\n- Clients keep the old address until TTL ends\n- Live TCP connections are not moved by DNS\n\n### Propagation\n\n- No push exists; answers wait to expire\n- Wait one old TTL before you judge a change\n- Lower the TTL a day early, raise it after\n\n> **Note:** A record change is live only when the last cache that matters expires.\n\n## Explained\n\n### The timer\n\nTTL is a number of seconds on each record, set by the zone. It is the maximum time a resolver may cache the answer, so 300 means five minutes and 86400 means a day.\n\n### The caches\n\nA lookup can stop at the browser, the OS stub or the recursive resolver. Each keeps its own copy and its own countdown, and only a miss reaches your authoritative zone.\n\nThe authoritative server is the source, not a cache: it answers from the zone every time it is asked.\n\n### The failover\n\nA health check can swap the answer to a standby address. Clients that already hold the old address keep using it until its TTL expires, and a live TCP connection stays on the old address whatever DNS says next.\n\n### The wait\n\nPropagation is not a push. It is the time for caches to expire, so lower the TTL at least one old TTL before the change and raise it again after the cutover.\n\n> **The honest caveat:** lowering the TTL does not flush a cache that already holds the old record, and some resolvers enforce a minimum TTL while JVMs and browsers keep their own caches.\n\n> **The lesson:** a failover plan that ignores TTL is a hope with a diagram.\n\n## In one line\n\n> **The old answer has a timer.** Propagation is not a delay you can skip; it is a cache expiring.\n\n**Watch the ttl**\n\n```bash\ndig example.com +noall +answer\n```\n"},{"id":"p485","n":"485","slug":"a-dns-check-in-python","title":"A DNS check in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["DNS","Python","DevOps","SRE","Networking"],"series_name":"Network","design":"code","script":"dns_check.py","date":"2029-11-01","files":{"it":"notes/p485-a-dns-check-in-python.md","devsecops":"notes/p485-a-dns-check-in-python.md"},"md":"# A DNS check in Python\n\nThe container starts, the health check calls the API by name, and the name does not resolve. The image is fine; the resolver is not.\n\n## Quick reference\n\nThe full script is at the end of this note: `dns_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 dns_check.py localhost nope.invalid\n```\n\n**Example output**\n\n```text\n  A    127.0.0.1\n  AAAA ::1\n```\n\n## Explained\n\n### What it does\n\nThe script takes a list of hostnames and calls getaddrinfo for each one with a stream socket hint. It prints every A and AAAA address it gets back, catches a failed lookup, counts rows that repeat an address, and exits 1 if any name failed.\n\n### Why getaddrinfo\n\nIt is the same call the application makes, so it follows `/etc/hosts`, the search domains and the system resolver. dig answers a different question: what a DNS server says, not what your process will get.\n\nIt also returns addresses, not records. There is no TTL, no CNAME chain and no authority section in the result, which is why a DNS check built on it cannot tell you when the answer expires.\n\n### Running it\n\nPass names on the command line or a file with `--file`. Run it after a deploy, or from cron every ten minutes; 0 means every name resolved, 1 means at least one failed, and 2 means the arguments or the file were wrong.\n\n> **The honest caveat:** a v4-mapped IPv6 answer can look like AAAA until you check for it; the full script relabels it, the condensed version does not, and getaddrinfo has no timeout you can set, so a hung resolver hangs the check.\n\n> **The lesson:** test the lookup your app makes, not the one dig makes.\n\n## In one line\n\n> **Test the lookup your app makes.** A name that fails here fails before the first byte of HTTP.\n\n**Run it in ci**\n\n```bash\npython3 dns_check.py --file hosts.txt\n```\n\n## Script: dns_check.py\n\nDownload: [scripts/dns_check.py](../scripts/dns_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"dns_check.py - resolve hostnames and fail when one does not resolve.\n\nEvery hostname is resolved with socket.getaddrinfo, the same call an\napplication makes. The script prints each IPv4 and IPv6 address it gets back,\nreports a name that fails, and counts addresses that arrive more than once.\nIt exits 1 when any name fails, so a pipeline can stop before the app does.\n\ngetaddrinfo is not a DNS client. It follows the system name service switch\n(usually /etc/hosts, DNS and mDNS), applies search domains and returns\naddresses rather than records: no TTL, no CNAME chain, no authority section.\nThat is the point. It answers what the process will actually get.\n\nUsage:\n    python3 dns_check.py example.com\n    python3 dns_check.py example.com api.example.com\n    python3 dns_check.py --file hosts.txt\n    python3 dns_check.py --file hosts.txt example.com\n    python3 dns_check.py --help\n\nA host file has one name per line; blank lines and lines starting with # are\nignored.\n\nCI (after a deploy, with the names the service needs):\n    python3 dns_check.py --file /etc/s0sta/hosts.txt || exit 1\n\nCron (every ten minutes):\n    */10 * * * * python3 /opt/s0sta/dns_check.py --file /etc/s0sta/hosts.txt\n\nExit codes: 0 every name resolved, 1 at least one name failed, 2 usage or\ninput error.\nRead-only: no file is written and no connection is opened; the only traffic\nis the resolver lookups that getaddrinfo makes.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport socket\nimport sys\n\n\ndef read_hosts(path):\n    \"\"\"Return the hostnames in one file, or raise ValueError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            lines = handle.read().splitlines()\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError as exc:\n        raise ValueError(\"%s is not UTF-8 text: %s\" % (path, exc.reason))\n    hosts = []\n    for number, line in enumerate(lines, 1):\n        name = line.split(\"#\", 1)[0].strip()\n        if not name:\n            continue\n        if any(ch.isspace() for ch in name):\n            raise ValueError(\"%s line %d: a hostname cannot contain spaces\"\n                             % (path, number))\n        hosts.append(name)\n    return hosts\n\n\ndef clean(host):\n    \"\"\"Validate one command-line hostname and return it.\"\"\"\n    name = host.strip()\n    if not name:\n        raise ValueError(\"empty hostname\")\n    if any(ch.isspace() for ch in name):\n        raise ValueError(\"hostname %r contains a space\" % host)\n    return name\n\n\ndef classify(address):\n    \"\"\"Return (tag, shown, note) for one getaddrinfo address.\"\"\"\n    bare = address.split(\"%\", 1)[0]\n    try:\n        ip = ipaddress.ip_address(bare)\n    except ValueError:\n        return \"?\", address, \"\"\n    if ip.version == 6 and ip.ipv4_mapped is not None:\n        return \"A\", str(ip.ipv4_mapped), \"v4-mapped\"\n    if ip.version == 6:\n        return \"AAAA\", address, \"\"\n    return \"A\", address, \"\"\n\n\ndef dedupe(rows):\n    \"\"\"Return (unique rows, duplicate rows), keeping the first of each.\"\"\"\n    seen = set()\n    unique = []\n    duplicates = []\n    for row in rows:\n        key = (row[0], row[1])\n        if key in seen:\n            duplicates.append(row)\n            continue\n        seen.add(key)\n        unique.append(row)\n    return unique, duplicates\n\n\ndef resolve(host):\n    \"\"\"Resolve one name; return (unique rows, duplicate rows).\"\"\"\n    infos = socket.getaddrinfo(host, None, socket.AF_UNSPEC,\n                               socket.SOCK_STREAM)\n    rows = [classify(info[4][0]) for info in infos]\n    return dedupe(rows)\n\n\ndef build_parser():\n    \"\"\"Return the argument parser.\"\"\"\n    parser = argparse.ArgumentParser(\n        prog=\"dns_check.py\",\n        description=\"Resolve hostnames with socket.getaddrinfo, print each \"\n                    \"A and AAAA address, and exit 1 when a name fails. \"\n                    \"This tests the lookup the application will make, not \"\n                    \"what a public DNS resolver would answer.\")\n    parser.add_argument(\"hosts\", nargs=\"*\", metavar=\"HOST\",\n                        help=\"hostname to resolve; repeat for more\")\n    parser.add_argument(\"--file\", action=\"append\", default=[], metavar=\"PATH\",\n                        help=\"read hostnames from a file, one per line; \"\n                             \"can be given more than once\")\n    return parser\n\n\ndef main(argv=None):\n    parser = build_parser()\n    args = parser.parse_args(argv)\n    if not args.hosts and not args.file:\n        parser.error(\"give at least one HOST or --file PATH\")\n\n    hosts = []\n    try:\n        for host in args.hosts:\n            hosts.append(clean(host))\n        for path in args.file:\n            hosts.extend(read_hosts(path))\n    except ValueError as exc:\n        parser.error(str(exc))\n    if not hosts:\n        parser.error(\"no hostnames found\")\n\n    failed = 0\n    addresses = 0\n    duplicates = 0\n    for host in hosts:\n        try:\n            rows, dups = resolve(host)\n        except (socket.gaierror, UnicodeError, OSError) as exc:\n            reason = getattr(exc, \"strerror\", None) or str(exc)\n            print(\"FAIL  %-28s %s\" % (host, reason))\n            failed += 1\n            continue\n        if not rows:\n            print(\"FAIL  %-28s no addresses returned\" % host)\n            failed += 1\n            continue\n        addresses += len(rows)\n        duplicates += len(dups)\n        for tag, addr, note in rows:\n            extra = \"  (%s)\" % note if note else \"\"\n            print(\"%-5s %-28s %s%s\" % (tag, host, addr, extra))\n        for tag, addr, _note in dups:\n            print(\"%-5s %-28s %s  (duplicate)\" % (\"DUP\", host, addr))\n\n    print(\"%d/%d hosts resolved, %d addresses, %d duplicate, %d failed\"\n          % (len(hosts) - failed, len(hosts), addresses, duplicates, failed))\n    return 1 if failed else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p486","n":"486","slug":"a-tcp-port-check-in-python","title":"A TCP port check in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Networking","Python","DevOps","SRE","TCP"],"series_name":"Network","design":"code","script":"port_check.py","date":"2029-11-04","files":{"it":"notes/p486-a-tcp-port-check-in-python.md","devsecops":"notes/p486-a-tcp-port-check-in-python.md"},"md":"# A TCP port check in Python\n\nThe load balancer says healthy and the port is closed. A TCP connect answers that in milliseconds or one timeout, before any HTTP status exists.\n\n## Quick reference\n\nThe full script is at the end of this note: `port_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 port_check.py 1.1.1.1:443 127.0.0.1:5432\n```\n\n**Example output**\n\n```text\n1.1.1.1:443 open 11ms\n127.0.0.1:5432 closed 0ms\n```\n\n## Explained\n\n### Three answers\n\nOpen means the handshake completed and something is listening. Closed means the host sent a reset: the packet arrived and nothing was there. Filtered means no answer before the timeout, which is a dropped packet, a dead host or a service too slow to accept.\n\n### The code\n\nFor each host and port it starts a timer, calls create_connection with a two-second timeout, and prints the state with the elapsed time. A refused connection is closed; any other socket error in the short version is filtered. A refused connect returns in milliseconds; a filtered one costs the whole timeout. The exit code is 1 if a required port is not open.\n\n### Running it\n\nPut a + in front of a target to mark it required; with none marked, every target is required. Run it after a deploy or from cron, and read the timing as an upper bound because it includes name resolution and the handshake.\n\n> **The honest caveat:** a firewall can send a reset and look closed, a dead host and a slow service can both look filtered, and an open port proves the kernel accepted a connection, not that the application works.\n\n> **The lesson:** connect first, then read the error; it names the layer that failed.\n\n## In one line\n\n> **Connect first, read the error.** A closed port and a filtered port send you to different places.\n\n**Check after deploy**\n\n```bash\npython3 port_check.py +api:443\n```\n\n## Script: port_check.py\n\nDownload: [scripts/port_check.py](../scripts/port_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"port_check.py - is the service actually listening?\n\nTests TCP connectivity to host:port pairs. Unlike ping (ICMP is often\nblocked), this checks the real port your app uses, so it tells you whether\nSSH, a database or a web server is truly reachable. Standard library only.\n\nUsage:\n    python3 port_check.py db.internal:5432 web.internal:443 10.0.0.5:22\n    python3 port_check.py -t 3 -f targets.txt\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport socket\nimport sys\n\n\ndef is_open(host, port, timeout=3.0):\n    try:\n        with socket.create_connection((host, port), timeout=timeout):\n            return True\n    except OSError:\n        return False\n\n\ndef main():\n    ap = argparse.ArgumentParser(description=\"Check if TCP ports are open.\")\n    ap.add_argument(\"targets\", nargs=\"*\", help=\"host:port\")\n    ap.add_argument(\"-t\", \"--timeout\", type=float, default=3.0)\n    ap.add_argument(\"-f\", \"--file\", help=\"file with one host:port per line\")\n    args = ap.parse_args()\n\n    targets = list(args.targets)\n    if args.file:\n        with open(args.file) as fh:\n            targets += [ln.strip() for ln in fh if ln.strip() and not ln.startswith(\"#\")]\n    if not targets:\n        ap.error(\"give at least one host:port, or -f a file\")\n\n    closed = False\n    for entry in targets:\n        host, _, port = entry.rpartition(\":\")\n        if not host or not port.isdigit():\n            print(f\"[ERR ] bad target: {entry}  (use host:port)\")\n            closed = True\n            continue\n        ok = is_open(host, int(port), args.timeout)\n        print(f\"[{'OPEN' if ok else 'SHUT'}] {host}:{port}\")\n        if not ok:\n            closed = True\n\n    sys.exit(1 if closed else 0)\n\n\nif __name__ == \"__main__\":\n    main()\n```\n"},{"id":"p487","n":"487","slug":"ipv4-vs-ipv6","title":"IPv4 vs IPv6","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["IPv4","IPv6","Networking","DNS","DevOps"],"series_name":"Network","design":"compare","script":"","date":"2029-11-06","files":{"it":"notes/p487-ipv4-vs-ipv6.md","devsecops":"notes/p487-ipv4-vs-ipv6.md"},"md":"# IPv4 vs IPv6\n\nA new service gets an AAAA record and half the office cannot reach it. The other half cannot reach the A record. The two protocols are not two versions of the same address; they are two address spaces.\n\n## Quick reference\n\n| | **IPV4** | **IPV6** |\n|:--|:--|:--|\n| Address size | 32 bits, 4 octets | 128 bits, 16 octets |\n| Written as | 203.0.113.10 | 2001:db8::10 |\n| DNS record | A | AAAA |\n| Configuration | DHCP or manual | SLAAC, DHCPv6 optional |\n| NAT | Common, address shortage | Rare, no shortage |\n| Discovery | ARP and broadcast | NDP and multicast |\n\n**IPV4** (32-bit dotted quad): Four bytes, four decimal octets, and most of the internet.\n\n```bash\ndig example.com A +short\n```\n\n**IPV6** (128-bit hex groups): Sixteen bytes, eight hex groups, and :: for a run of zeros.\n\n```bash\ndig example.com AAAA +short\n```\n\n- IPv4 → everywhere, and out of addresses.\n- IPv6 → the same internet, with room.\n\n## Explained\n\n### Size and notation\n\nIPv4 is 32 bits, written as four decimal octets such as 203.0.113.10. IPv6 is 128 bits, written as eight groups of four hex digits, with :: allowed once for a run of zeros, as in 2001:db8::10.\n\n### DNS and configuration\n\nAn A record holds an IPv4 address and an AAAA record holds an IPv6 one. IPv4 hosts get an address from DHCP or a static file; IPv6 hosts usually build one from a router advertisement with SLAAC, and DHCPv6 is optional.\n\n### NAT and discovery\n\nIPv4 often sits behind NAT because the address space ran out, so many hosts share one public address. IPv6 has enough addresses that NAT is rare, though NPTv6 exists for translation. IPv4 finds neighbours with ARP and broadcast; IPv6 uses Neighbour Discovery over multicast, with no broadcast at all. The two run side by side on one host, and a client with a working IPv6 path will usually try it first.\n\n> **The honest caveat:** dual stack is the normal deployment, not a migration deadline, and an IPv6 path is not automatically faster; it is a different path that has to be measured.\n\n> **The lesson:** choose the record by what the client can reach, not by what the RFC prefers.\n\nTwo protocols,  \none address space.\n"},{"id":"p488","n":"488","slug":"anatomy-of-a-dns-record","title":"Anatomy of a DNS record","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","SysAdmin","DevOps","Infrastructure"],"series_name":"Network","design":"anatomy","script":"","date":"2029-11-08","files":{"it":"notes/p488-anatomy-of-a-dns-record.md","devsecops":"notes/p488-anatomy-of-a-dns-record.md"},"md":"# Anatomy of a DNS record\n\nAn answer line looks like five fields in a row: api.example.com. 300 IN A 203.0.113.10. Read them left to right and the record stops being a mystery.\n\n## Quick reference\n\n```text\napi.example.com. 300 IN A 203.0.113.10\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `api.example.com.` | The name | Fully qualified: the trailing dot stops the search list. |\n| `300` | The TTL | Seconds a cache may keep it. 300 is five minutes. |\n| `IN` | The class | Internet. The only class you will meet in practice. |\n| `A` | The type | An IPv4 address record. AAAA is the IPv6 version. |\n| `203.0.113.10` | The value | Documentation range, RFC 5737: never a real host. |\n\n## Explained\n\n### The five fields\n\nThe name is fully qualified because of the trailing dot. Without it, the resolver may append a search domain and ask for api.example.com.corp.example.\n\nThe TTL is 300 seconds: the longest a cache may keep this answer before asking again. The class is IN, the internet class, and the only one you will meet.\n\nThe type is A, which means one IPv4 address. The value is 203.0.113.10, from the documentation range, so it is safe to print and not a public host.\n\n### What the line does not say\n\nIt does not show the CNAME chain that produced the name, the signature that DNSSEC would add, or the resolver that answered. Those live in the full dig output, not in the answer line.\n\nIt also does not promise the TTL you see is the zone's TTL. A cache counts down, so a record with 300 in the zone can arrive with 42.\n\n> **The honest caveat:** a short answer is a snapshot, not a history; if you need to know why a name points where it does, read the whole response.\n\n> **The lesson:** five fields, one line, and every DNS argument gets shorter once you can name them.\n\n## In one line\n\n> **The dot ends the name.** A name without its trailing dot is a search-domain accident.\n\n**Read the answer**\n\n```bash\ndig api.example.com A\n```\n"},{"id":"p489","n":"489","slug":"dns-health-at-a-glance","title":"DNS health at a glance","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","SRE","Observability","Networking","DevOps"],"series_name":"Network","design":"dashboard","script":"","date":"2029-11-11","files":{"it":"notes/p489-dns-health-at-a-glance.md","devsecops":"notes/p489-dns-health-at-a-glance.md"},"md":"# DNS health at a glance\n\nMonday stand-up starts with a page about a site that is up for some users and gone for others. Six DNS numbers narrow the argument before anyone opens a laptop.\n\n## Quick reference\n\n**Status:** Production, last 24h\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Resolution failures | **0.42%** | SERVFAIL and timeouts |\n| Slow lookups | **2.1%** | over 200ms, p95 340ms |\n| TTL coverage | **38%%** | records at 300s or less |\n| Failing CAA | **2** | zones blocking the CA |\n| Cert mismatch | **1** | name or chain, 7 days |\n| Propagation | **11m** | median after a change |\n\n## Explained\n\n### The tiles\n\nResolution failures: 0.42 per cent of queries ended in SERVFAIL or a timeout in the last day.\n\nSlow lookups: 2.1 per cent took over 200 milliseconds, with a p95 of 340ms.\n\nTTL coverage: 38 per cent of records carry a TTL of 300 seconds or less, so only that share can move quickly in a failover.\n\nFailing CAA: two zones have a CAA policy that blocks the certificate authority they use, which turns into a renewal failure later.\n\nCert mismatch: one certificate does not match its name or its chain, found in the last seven days.\n\nPropagation: eleven minutes at the median between a record change and the new answer being seen.\n\n### What it misses\n\nThe resolver only sees queries that reach it. A client on DNS over HTTPS, a browser with its own cache or a host with a stale `/etc/hosts` entry is invisible here. A resolver log can be sampled, too, so a rare failure can hide between two quiet minutes.\n\n> **The honest caveat:** every tile is a snapshot, and a zero can mean no failures or no traffic; check the query count before you celebrate.\n\n> **The lesson:** the dashboard starts the DNS review; it does not replace the lookup.\n\n## In one line\n\n> **DNS fails before HTTP.** Six numbers, and the one question they cannot answer.\n\n**Check the ttl**\n\n```bash\ndig example.com +noall +answer\n```\n"},{"id":"p490","n":"490","slug":"quiz-3-dns-questions","title":"Quiz: 3 DNS questions","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","SRE","DevOps","CloudNative"],"series_name":"Network","design":"quiz","script":"","date":"2029-11-13","files":{"it":"notes/p490-quiz-3-dns-questions.md","devsecops":"notes/p490-quiz-3-dns-questions.md"},"md":"# Quiz: 3 DNS questions\n\nThree DNS questions come up in every incident review, and the answers are shorter than the arguments. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why can a CNAME not sit at the zone apex?**\n\n- a) CNAMEs are only for subdomains\n- b) SOA and NS already own that name\n- c) The apex has no TTL\n\n> **Answer:** b) SOA and NS already own that name. A CNAME must be the only data record at its name, and the apex must carry SOA and NS. Providers offer ALIAS or ANAME to map the apex.\n\n**Q2. Why lower a record's TTL before a migration?**\n\n- a) So the new record propagates instantly\n- b) So resolvers skip the root servers\n- c) So old cached answers expire sooner\n\n> **Answer:** c) So old cached answers expire sooner. Caches keep the old record for its old TTL. Lower it at least one old TTL before the change, then raise it after the cutover.\n\n**Q3. What does SERVFAIL usually mean?**\n\n- a) The name does not exist\n- b) The resolver could not get a valid answer\n- c) The record has no TTL\n\n> **Answer:** b) The resolver could not get a valid answer. An upstream timeout, a broken delegation or a failed DNSSEC check. NXDOMAIN is the name-does-not-exist answer; SERVFAIL says try again.\n\n## Explained\n\n### The apex\n\nWhy can a CNAME not sit at the zone apex? Because a CNAME must be the only data record at its name, and the apex must carry SOA and NS. Providers offer ALIAS, ANAME or CNAME flattening, which return an address at the apex without a real CNAME.\n\n### The timer\n\nWhy lower a record's TTL before a migration? Because caches keep the old answer for its old TTL. Lower it at least one old TTL before the change, wait, then move the record and raise the TTL after the cutover.\n\n### The failure\n\nWhat does SERVFAIL usually mean? That the resolver could not get a valid answer: an upstream timeout, a broken delegation or a failed DNSSEC validation. It also appears when the authoritative servers disagree or one of them is lame. NXDOMAIN is the name-does-not-exist answer, and the two send you to different places.\n\n> **The honest caveat:** CNAME flattening looks like a CNAME at the apex in a dashboard, but the provider is synthesising an A or AAAA answer; and a low TTL does not override a resolver's minimum or a client's own cache.\n\n> **The lesson:** the record, the cache and the response code answer three different questions.\n\n## In one line\n\n> **Check the answer, not the rumour.** Three answers that turn a failed lookup into a next step.\n\n**Ask the resolver**\n\n```bash\ndig example.com +comments\n```\n"},{"id":"p491","n":"491","slug":"dns-terms-decoded","title":"DNS terms, decoded","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["DNS","Networking","DevOps","SRE","Infrastructure"],"series_name":"Network","design":"reveal","script":"","date":"2029-11-15","files":{"it":"notes/p491-dns-terms-decoded.md","devsecops":"notes/p491-dns-terms-decoded.md"},"md":"# DNS terms, decoded\n\nA lookup that fails is usually a vocabulary problem before it is a technical one. Six words cover most of the conversation.\n\n## Quick reference\n\n- **A / AAAA** — Address records: A holds one IPv4 address, AAAA holds one IPv6 address, and a name can carry several of each.\n- **CNAME** — Canonical name: an alias pointing at another name. The resolver follows the chain and returns the target's address.\n- **TTL** — Time to live: the seconds a resolver may cache the answer. The zone sets it; every cache counts it down.\n- **Resolver** — The recursive server that walks root, TLD and zone for you, then caches what it finds for the TTL.\n- **Authoritative** — The nameserver that holds the zone and gives the real answer, with the AA flag set.\n- **Propagation** — Not a push: the wait for caches to expire. No update is pushed to caches; old answers simply time out.\n\n## Explained\n\n### The records\n\nA and AAAA are address records: A holds one IPv4 address, AAAA holds one IPv6 address, and a name can carry several of each. The AAAA record is pronounced quad A, and it does not replace the A record; the two usually sit side by side.\n\nCNAME is a canonical name, an alias pointing at another name. The resolver follows the chain and returns the target's address, while the URL in the browser stays the same.\n\n### The timer\n\nTTL is the time to live in seconds: the longest a resolver may cache the answer. The authoritative server sets it, and every cache counts it down.\n\n### The servers\n\nThe resolver is the recursive server that walks root, TLD and zone for you, then caches what it finds.\n\nThe authoritative server holds the zone and gives the real answer, with the AA flag set.\n\n### The wait\n\nPropagation is not a push. It is the wait for caches to expire; no update is pushed to them, and old answers simply time out. That is why one change looks live for some users and stale for others.\n\n> **The honest caveat:** TTL is a maximum, not a promise; resolvers can clamp it, and browsers and JVMs keep caches with their own TTLs.\n\n> **The lesson:** learn the words and the diagram draws itself.\n\n## In one line\n\n> **Learn the words, read the answer.** A vocabulary turns a failed lookup into a diagram.\n\n**Decode a lookup**\n\n```bash\ndig +trace example.com\n```\n"},{"id":"p492","n":"492","slug":"vpc-routing-drawn","title":"VPC routing, drawn","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","Networking","CloudComputing","DevOps"],"series_name":"Network","design":"architecture","script":"","date":"2029-11-18","files":{"devsecops":"notes/p492-vpc-routing-drawn.md"},"md":"# VPC routing, drawn\n\nA subnet is not public because of a checkbox. It is public because one route in its route table points at an internet gateway.\n\n## Quick reference\n\n- **Subnets:** Public subnet (PUB) · Private subnet (PRIV)\n- **Route table:** Local route (LOCAL) · Default route (0/0)\n- **Gateways:** Internet gateway (IGW) · NAT gateway (NAT)\n\n**Request flow:** Public subnet → Local route; Private subnet → Local route; Default route → Internet gateway; Default route → NAT gateway → Internet gateway\n\n## Explained\n\n### Subnets\n\nPublic subnet, private subnet: both are CIDR ranges tied to one Availability Zone. The difference is which route table is associated with them. A subnet with the public table can reach the internet gateway directly. A subnet with the private table cannot.\n\n### Route table\n\nEvery route table starts with a local route: 10.0.0.0/16 to local. AWS creates it, it cannot be deleted, and it keeps traffic for the VPC inside the VPC. The route that decides egress is the default route, 0.0.0.0/0. Point it at the internet gateway and the subnet is public. Point it at a NAT gateway and the subnet is private with outbound access. Longest prefix wins, so the local route always beats the default for VPC addresses.\n\n### Gateways\n\nThe internet gateway is attached to the VPC, not to a subnet. It is horizontally scaled and performs one-to-one NAT for instances with a public IPv4 address. The NAT gateway is different: it sits in a public subnet, holds an Elastic IP, and maps many private sources to its own address. It only passes connections initiated from inside.\n\n> **The honest caveat:** a zonal NAT gateway is per Availability Zone. Put one NAT in one zone and every other zone now crosses an AZ boundary, pays cross-AZ transfer and shares the failure.\n\n> **The lesson:** the route table is the policy. The gateways are just where the packets go.\n\n## In one line\n\n> **The route table is the network.** Public is a route, not a checkbox.\n\n**Trace the route**\n\n```bash\naws ec2 describe-route-tables\n```\n"},{"id":"p493","n":"493","slug":"the-vpc-troubleshooting-cheat-sheet","title":"The VPC troubleshooting cheat sheet","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","Troubleshooting","Networking","DevOps"],"series_name":"Network","design":"cheatsheet","script":"","date":"2029-11-20","files":{"devsecops":"notes/p493-the-vpc-troubleshooting-cheat-sheet.md"},"md":"# The VPC troubleshooting cheat sheet\n\nThe console is green, the health check is green, and the packet still dies. The VPC does not have one place to look; it has four.\n\n## Quick reference\n\n### Addresses\n\n```bash\naws ec2 describe-vpcs --output table\naws ec2 describe-subnets --output table\naws ec2 describe-addresses\nip -br addr\nip route get 10.0.1.7\n```\n\n### Routes\n\n```bash\naws ec2 describe-route-tables\naws ec2 describe-route-tables | jq .\nip route show table main\nip route get 8.8.8.8\nip route\n```\n\n### Firewall\n\n```bash\naws ec2 describe-security-groups\naws ec2 describe-security-group-rules\naws ec2 describe-network-acls\naws ec2 describe-network-acl-entries\naws ec2 describe-vpc-endpoints\n```\n\n### Flow logs\n\n```bash\naws ec2 describe-flow-logs\naws logs describe-log-groups\naws logs tail /aws/vpc/flowlogs\naws logs tail /aws/vpc/flowlogs --follow\naws s3 ls s3://my-flow-logs/\n```\n\n## Explained\n\n### Addresses\n\nStart with what exists. `aws ec2 describe-vpcs --output table` and `aws ec2 describe-subnets --output table` show the CIDR plan and the subnets. `aws ec2 describe-addresses` shows the Elastic IPs, and `ip -br addr` plus `ip route get 10.0.1.7` on the instance show what it believes.\n\n### Routes\n\n`aws ec2 describe-route-tables` prints the routes and their state. Read the local route, then the default route, then the association. A subnet with no explicit association uses the main table, which is a common surprise. `ip route show table main` and `ip route get 8.8.8.8` confirm the instance side, and `ip route` is the short view.\n\n### Firewall\n\n`aws ec2 describe-security-groups`, `aws ec2 describe-security-group-rules` and `aws ec2 describe-network-acls` list the rules. Remember the direction: security groups are stateful, NACLs are not. `aws ec2 describe-vpc-endpoints` answers the other half: whether the traffic was meant to stay inside AWS.\n\n### Flow logs\n\n`aws ec2 describe-flow-logs` says whether anyone is recording. `aws logs describe-log-groups` and `aws logs tail /aws/vpc/flowlogs --follow` read it back. If the logs go to S3, `aws s3 ls s3://my-flow-logs/` is the first command. A REJECT line turns a guess into a fact.\n\n> **The honest caveat:** a command that returns nothing is not proof of health. It may be the wrong Region, the wrong profile or the wrong account.\n\n> **The lesson:** check the layer the packet is on, not the layer the console draws.\n\n## In one line\n\n> **Find the hop that says no.** A green console is a claim, not a proof.\n\n**Check a subnet**\n\n```bash\naws ec2 describe-subnets\n```\n"},{"id":"p494","n":"494","slug":"four-reasons-a-packet-is-dropped","title":"Four reasons a packet is dropped","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","NetworkSecurity","CloudComputing","DevOps"],"series_name":"Network","design":"explainer","script":"","date":"2029-11-22","files":{"devsecops":"notes/p494-four-reasons-a-packet-is-dropped.md"},"md":"# Four reasons a packet is dropped\n\nA dropped packet is not a mystery. It met a rule, a route or a missing target, and the VPC has only four places for that to happen.\n\n## Quick reference\n\n### Security group\n\n- Stateful: the reply is allowed\n- Allow rules only, no deny\n- Checked at the network interface\n- No matching rule means drop\n\n### Network ACL\n\n- Stateless: each direction alone\n- Ordered rules, lowest wins\n- Needs an ephemeral-port rule\n- Applies to the whole subnet\n\n### Missing route\n\n- Longest prefix match decides\n- No match means a drop\n- Peering routes are not transitive\n- Add the route on both sides\n\n### Blackhole\n\n- Target deleted, route stayed\n- State shows as blackhole\n- Common after a NAT is removed\n- The log may show ACCEPT, then silence\n\n> **Note:** The order matters: interface, subnet, route, target. Stop at the first one that says no.\n\n## Explained\n\n### At the interface\n\nSecurity group first. It is stateful, allow-only and evaluated at the network interface. If no rule allows the traffic, it is dropped, and the reply to an allowed connection is allowed automatically.\n\nNetwork ACL second. It is stateless, ordered by rule number and attached to the subnet. Return traffic needs its own rule, which is why an inbound 443 rule without an outbound 1024-65535 rule produces a one-way connection.\n\n### On the path\n\nMissing route third. Every subnet uses one route table; the longest prefix wins, and no matching route means a drop. There is no fallback to the default when nothing matches. Peering routes are not transitive and must exist on both sides.\n\nBlackhole fourth. The route is still in the table but its target is gone, so its state turns to blackhole and the packet is dropped. A deleted NAT gateway or network interface leaves exactly this. The table still looks populated, which is why blackholes survive so many reviews.\n\n> **The honest caveat:** a flow log records what reached the interface, not why the router dropped it. A missing route or a blackhole can leave an ACCEPT record and no reply.\n\n> **The lesson:** the packet tells you where it died; the order tells you where to look.\n\n## In one line\n\n> **Every drop has a reason.** The packet did not vanish; a rule, a route or a deleted target did it.\n\n**Watch the drop**\n\n```bash\naws ec2 describe-flow-logs\n```\n"},{"id":"p495","n":"495","slug":"a-route-table-check-in-python","title":"A route table check in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","Python","VPC","Networking","Automation"],"series_name":"Network","design":"code","script":"route_check.py","date":"2029-11-25","files":{"devsecops":"notes/p495-a-route-table-check-in-python.md"},"md":"# A route table check in Python\n\nA route table is a list of promises, and most of them are never read until something times out.\n\n## Quick reference\n\nThe full script is at the end of this note: `route_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 route_check.py route_table.json\n```\n\n**Example output**\n\n```text\nALERT no default route\nALERT blackhole 10.0.5.0/24\n```\n\n## Explained\n\n### What it does\n\nThe script loads a route table JSON with the VPC CIDR and routes that carry destination, target and state. It checks three things: whether a default route exists, whether any route is in a blackhole state, and whether a non-local route falls inside the VPC CIDR. Each finding prints an ALERT line; any finding exits 1.\n\n### Why these three\n\nNo default route means no general internet egress, even when the NAT gateway is healthy. A blackhole means the target was deleted and the route stayed behind. An inside route is the quiet one: a non-local path for part of the VPC's own range can shadow the local route, which is rarely what anyone meant.\n\n### How to use it\n\nRun `python3 route_check.py route_table.json`, or pipe the JSON in with -. Pass `--allow-no-default` for a table that should stay isolated. In CI, run it before terraform apply and let the exit code stop the merge. It reads only; it never changes a route.\n\n> **The honest caveat:** this checks one table in isolation. It cannot see whether a subnet is associated with that table, or whether the other side of a peering has the return route.\n\n> **The lesson:** a route is only real when the target exists and the other side agrees.\n\n## In one line\n\n> **A route table is a list of promises.** A promise with a deleted target is a blackhole.\n\n**Run it in ci**\n\n```bash\npython3 route_check.py table.json\n```\n\n## Script: route_check.py\n\nDownload: [scripts/route_check.py](../scripts/route_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check an AWS route table export for the failures that break egress.\n\nUsage:\n    python3 route_check.py route_table.json\n    python3 route_check.py - --vpc-cidr 10.0.0.0/16\n    python3 route_check.py table.json --allow-no-default\n\nThe input is JSON, either an object:\n    {\"vpc_cidr\": \"10.0.0.0/16\",\n     \"routes\": [{\"destination\": \"0.0.0.0/0\",\n                 \"target\": \"igw-0123456789abcdef0\",\n                 \"state\": \"active\"}]}\nor a bare list of routes, in which case pass --vpc-cidr.\n\nChecks:\n    * a missing default route (0.0.0.0/0), unless --allow-no-default\n    * any route whose state is not active (a blackhole)\n    * a non-local route whose destination falls inside the VPC CIDR\n\nExit codes: 0 clean, 1 findings, 2 usage or input error.\nThe script is read-only: it never calls AWS and never writes a file.\n\nCron (daily at 06:00):\n    0 6 * * * /usr/bin/python3 /opt/s0sta/route_check.py /etc/s0sta/rtb.json\n\nCI:\n    python3 route_check.py route_table.json\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport json\nimport sys\n\n\ndef fail(message):\n    print(\"ERROR: \" + message, file=sys.stderr)\n    raise SystemExit(2)\n\n\ndef load(path):\n    try:\n        if path == \"-\":\n            data = json.load(sys.stdin)\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                data = json.load(handle)\n    except OSError as exc:\n        fail(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        fail(\"invalid JSON: %s\" % exc)\n\n    if isinstance(data, dict):\n        routes = data.get(\"routes\")\n        vpc_cidr = data.get(\"vpc_cidr\")\n    elif isinstance(data, list):\n        routes, vpc_cidr = data, None\n    else:\n        fail(\"expected a JSON object with 'routes' or a list of routes\")\n\n    if not isinstance(routes, list):\n        fail(\"expected 'routes' to be a list\")\n    for index, route in enumerate(routes):\n        if not isinstance(route, dict):\n            fail(\"route %d is not an object\" % index)\n        for key in (\"destination\", \"target\", \"state\"):\n            if key not in route:\n                fail(\"route %d has no %s\" % (index, key))\n    return routes, vpc_cidr\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check an AWS route table JSON for missing routes, \"\n                    \"blackholes and VPC CIDR overlaps.\")\n    parser.add_argument(\"path\", nargs=\"?\", default=\"-\",\n                        help=\"route table JSON file, or - for stdin (default: -)\")\n    parser.add_argument(\"--vpc-cidr\",\n                        help=\"VPC CIDR when the JSON does not carry vpc_cidr\")\n    parser.add_argument(\"--allow-no-default\", action=\"store_true\",\n                        help=\"do not report a missing 0.0.0.0/0 route\")\n    args = parser.parse_args(argv)\n\n    routes, embedded_cidr = load(args.path)\n    cidr = args.vpc_cidr or embedded_cidr\n    if not cidr:\n        fail(\"no VPC CIDR: add vpc_cidr to the JSON or pass --vpc-cidr\")\n    try:\n        vpc = ipaddress.ip_network(str(cidr), strict=False)\n    except ValueError as exc:\n        fail(\"invalid VPC CIDR %r: %s\" % (cidr, exc))\n\n    findings = []\n    if not args.allow_no_default and not any(\n            str(route[\"destination\"]).strip() == \"0.0.0.0/0\" for route in routes):\n        findings.append(\"no default route\")\n\n    for route in routes:\n        destination = str(route[\"destination\"]).strip()\n        target = str(route[\"target\"]).strip()\n        state = str(route[\"state\"]).strip().lower()\n        if state != \"active\":\n            if state == \"blackhole\":\n                findings.append(\"blackhole \" + destination)\n            else:\n                findings.append(\"not active (%s) %s\" % (state or \"no state\", destination))\n            continue\n        if target == \"local\":\n            continue\n        try:\n            network = ipaddress.ip_network(destination, strict=False)\n        except ValueError as exc:\n            fail(\"invalid destination %r: %s\" % (destination, exc))\n        if network.version != vpc.version:\n            continue\n        if network.subnet_of(vpc):\n            findings.append(\"overlaps VPC \" + destination)\n\n    if findings:\n        for finding in findings:\n            print(\"ALERT \" + finding)\n        return 1\n    print(\"OK route table\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p496","n":"496","slug":"a-vpc-flow-log-report-in-python","title":"A VPC flow log report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","Python","VPC","Networking","Observability"],"series_name":"Network","design":"code","script":"flow_log_report.py","date":"2029-11-27","files":{"devsecops":"notes/p496-a-vpc-flow-log-report-in-python.md"},"md":"# A VPC flow log report in Python\n\nA flow log is where a dropped packet leaves a receipt.\n\n## Quick reference\n\nThe full script is at the end of this note: `flow_log_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 flow_log_report.py flow.log\n```\n\n**Example output**\n\n```text\n10.0.1.5 -> 203.0.113.5: 12400 bytes\nREJECT 2 of 9\n```\n\n## Explained\n\n### What it does\n\nThe script reads version 2 flow log lines, the default 14-field format: version, account, interface, source, destination, source port, destination port, protocol, packets, bytes, start, end, action, status. It totals bytes per source and destination pair, counts destination ports, and counts REJECT records. It prints the top talkers, the top ports and the rejection rate, then exits 1 when that rate passes the threshold.\n\n### Why it works\n\nEvery record is one direction of one flow over an aggregation window, with an ACCEPT or REJECT verdict from the network interface. That makes it a cheap first pass: who is talking, on which ports, and what is being refused. It is not a packet capture, and it will not show payload.\n\n### How to use it\n\nPipe it from CloudWatch with aws logs tail /aws/vpc/flowlogs `--follow`, or point it at an exported file. Run it from cron over yesterday's log, or in CI after a network change. The threshold is a flag; the default is 5 per cent.\n\n> **The honest caveat:** a REJECT tells you the packet was refused, not which rule refused it. It also misses traffic the VPC never sees, and NODATA records are not zero traffic.\n\n> **The lesson:** count the rejects before you argue about the cause.\n\n## In one line\n\n> **A flow log is a receipt.** It says what left, where it went, and whether it was allowed.\n\n**Tally the rejects**\n\n```bash\npython3 flow_log_report.py flow.log\n```\n\n## Script: flow_log_report.py\n\nDownload: [scripts/flow_log_report.py](../scripts/flow_log_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report top talkers, top ports and rejected flows from AWS VPC flow logs.\n\nUsage:\n    python3 flow_log_report.py flow.log\n    aws logs tail /aws/vpc/flowlogs --follow | python3 flow_log_report.py -\n    python3 flow_log_report.py flow.log --top 10 --max-reject-rate 0.02\n\nReads the default version 2 record, 14 space-separated fields:\n    version account-id interface-id srcaddr dstaddr srcport dstport\n    protocol packets bytes start end action log-status\n\nExit codes: 0 clean, 1 rejection rate over the threshold, 2 usage or input\nerror. The script is read-only: it never calls AWS and never writes a file.\n\nCron (daily at 06:10):\n    10 6 * * * /usr/bin/python3 /opt/s0sta/flow_log_report.py /var/log/vpc-flow.log\n\nCI:\n    aws logs tail /aws/vpc/flowlogs --since 1h | python3 flow_log_report.py -\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nfrom collections import Counter\n\n\ndef fail(message):\n    print(\"ERROR: \" + message, file=sys.stderr)\n    raise SystemExit(2)\n\n\ndef open_input(path):\n    if path == \"-\":\n        return sys.stdin\n    try:\n        return open(path, \"r\", encoding=\"utf-8\", errors=\"replace\")\n    except OSError as exc:\n        fail(\"cannot read %s: %s\" % (path, exc))\n\n\ndef parse(lines):\n    talkers = Counter()\n    ports = Counter()\n    accepts = 0\n    rejects = 0\n    skipped = 0\n    for raw in lines:\n        line = raw.strip()\n        if not line or line.startswith(\"#\"):\n            continue\n        fields = line.split()\n        if len(fields) != 14 or fields[0] != \"2\":\n            skipped += 1\n            continue\n        action = fields[12].upper()\n        status = fields[13].upper()\n        if action not in (\"ACCEPT\", \"REJECT\") or status in (\"NODATA\", \"SKIPDATA\"):\n            skipped += 1\n            continue\n        try:\n            size = int(fields[9])\n        except ValueError:\n            skipped += 1\n            continue\n        talkers[(fields[3], fields[4])] += size\n        ports[fields[6]] += 1\n        if action == \"ACCEPT\":\n            accepts += 1\n        else:\n            rejects += 1\n    return talkers, ports, accepts, rejects, skipped\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Summarise AWS VPC flow log version 2 records: top talkers \"\n                    \"by bytes, top destination ports, and rejected flows.\")\n    parser.add_argument(\"path\", nargs=\"?\", default=\"-\",\n                        help=\"flow log file, or - for stdin (default: -)\")\n    parser.add_argument(\"--top\", type=int, default=5,\n                        help=\"rows to show in each ranking (default: 5)\")\n    parser.add_argument(\"--max-reject-rate\", type=float, default=0.05,\n                        help=\"fail above this REJECT share, 0 to 1 (default: 0.05)\")\n    args = parser.parse_args(argv)\n\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n    if not 0 <= args.max_reject_rate <= 1:\n        parser.error(\"--max-reject-rate must be between 0 and 1\")\n\n    stream = open_input(args.path)\n    try:\n        talkers, ports, accepts, rejects, skipped = parse(stream)\n    finally:\n        if stream is not sys.stdin:\n            stream.close()\n\n    total = accepts + rejects\n    if not total:\n        print(\"no usable version 2 records\")\n        return 0\n\n    print(\"TOP TALKERS BY BYTES\")\n    for (source, destination), size in talkers.most_common(args.top):\n        print(\"%s -> %s: %d bytes\" % (source, destination, size))\n\n    print(\"TOP DESTINATION PORTS\")\n    for port, count in ports.most_common(args.top):\n        print(\"  %s  %d\" % (port, count))\n\n    rate = rejects / total\n    print(\"REJECT %d of %d\" % (rejects, total))\n    print(\"reject rate %.1f%%\" % (rate * 100))\n    if skipped:\n        print(\"skipped %d non-v2 or malformed lines\" % skipped, file=sys.stderr)\n\n    if rate > args.max_reject_rate:\n        print(\"ALERT reject rate over %.1f%%\" % (args.max_reject_rate * 100))\n        return 1\n    print(\"OK reject rate within %.1f%%\" % (args.max_reject_rate * 100))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p497","n":"497","slug":"peering-vs-transit-gateway-vs-privatelink","title":"Peering vs Transit Gateway vs PrivateLink","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","Networking","TransitGateway","PrivateLink","CloudArchitecture"],"series_name":"Network","design":"matrix","script":"","date":"2029-11-29","files":{"devsecops":"notes/p497-peering-vs-transit-gateway-vs-privatelink.md"},"md":"# Peering vs Transit Gateway vs PrivateLink\n\nThree ways to connect VPCs, and the invoice explains the difference before the diagram does.\n\n## Quick reference\n\n| | **Peering** | **Transit Gateway** | **PrivateLink** |\n|:--|:--|:--|:--|\n| Model | VPC to VPC, one to one | Regional hub and spoke | Service to consumer |\n| Scale | 50 peers, 125 on request | 5,000 attachments | Endpoint ENIs per subnet |\n| Routing | Routes on both sides | Transitive, route tables | No routes, DNS endpoint |\n| Security | SG refs, same Region | Central inspection VPC | One-way, no CIDR overlap |\n| Cost | Per GB only | Attachment-hour + GB | Endpoint-hour + GB |\n| Best for | A few VPCs, low latency | Many VPCs, shared egress | SaaS and shared services |\n\n**How to pick**\n\n- **Peering** → two VPCs, two route tables, no transit.\n- **TGW** → one hub, many attachments, one route table.\n- **PrivateLink** → one service, one endpoint, no route.\n\n## Explained\n\n### Peering\n\nOne-to-one, two route tables, no transit. Each side adds a route for the other VPC, and neither can pass traffic to a third. Overlapping CIDRs are refused outright. The default is 50 active peers per VPC, adjustable to 125. You pay for data transfer, not for the connection. In the same Region, a security group can reference the peer's group. Peering is the simplest option only while the mesh is small.\n\n### Transit Gateway\n\nA regional hub. Attach VPCs, VPNs and Direct Connect gateways, and let the route tables decide what talks. It routes transitively, so a spoke reaches another spoke through the hub, and central egress or inspection becomes one attachment. The default is 5,000 attachments, and you pay per attachment-hour plus data.\n\n### PrivateLink\n\nNot a network path: a service endpoint. The consumer creates an interface endpoint in its own subnet, DNS resolves the service name to it, and traffic stays on the AWS network. The provider cannot initiate back. No route table entry, and overlapping CIDRs are not a problem, which is why it works between tenants that could never peer. You pay per endpoint-hour plus data.\n\n> **The honest caveat:** PrivateLink gives you a service, not a route. If you need the provider to initiate, or a full network path, it is the wrong tool.\n\n> **The lesson:** choose the shape of the relationship, then choose the product.\n\n## In one line\n\n> **Pick the shape before the bill.** The cheapest connection is the one that matches the shape of your estate.\n\n**Count the attachments**\n\n```bash\naws ec2 describe-transit-gateways\n```\n"},{"id":"p498","n":"498","slug":"anatomy-of-a-route-table-entry","title":"Anatomy of a route table entry","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","Networking","Routing","CloudComputing"],"series_name":"Network","design":"anatomy","script":"","date":"2029-12-02","files":{"devsecops":"notes/p498-anatomy-of-a-route-table-entry.md"},"md":"# Anatomy of a route table entry\n\nMost routing incidents are one line long. The route table is where the intent is written down, and one entry carries the whole story.\n\n## Quick reference\n\n```text\n10.0.0.0/16 -> local (active) [vpc]\n```\n\n| Part | Name | What it means |\n|:--|:--|:--|\n| `10.0.0.0/16` | Destination | The prefix to match. Longest match wins, so a /24 beats a /16. |\n| `->` | The arrow | Read it as: send this prefix to the target on the right. |\n| `local` | Target | The VPC itself. Created by AWS, and it cannot be deleted. |\n| `(active)` | State | The target exists. Blackhole means it was deleted and the route stayed. |\n| `[vpc]` | Scope and origin | This entry belongs to this VPC's table and applies only to associated subnets. |\n\n## Explained\n\n### Destination and arrow\n\n10.0.0.0/16 is the destination: the prefix this entry matches. The router compares every route and picks the longest match, so a /24 beats a /16. A /16 is a broad promise; a /24 is a narrow one, and the narrow one wins. The arrow is not decoration; it says send this prefix to the target on the right.\n\n### Target and state\n\nlocal is the target. It means the VPC itself, the route AWS creates in every table, and the one you cannot delete. (active) is the state: the target exists and the route can forward. If the target is deleted, the state becomes blackhole and the same line silently drops traffic.\n\n### Scope and origin\n\n[vpc] is the scope, the part the CLI does not print as a token. The entry belongs to this VPC and this table only, and the table applies only to the subnets associated with it. Every other subnet is reading a different list. Change the association and every subnet that reads this list changes with it.\n\n> **The honest caveat:** a correct route proves intent, not delivery. A security group, a NACL or a target that refuses the packet can still drop it after the route matched.\n\n> **The lesson:** read the line before you redraw the diagram.\n\n## In one line\n\n> **A route is a promise.** The table records intent; the packet still has to survive the path.\n\n**Read a table**\n\n```bash\naws ec2 describe-route-tables\n```\n"},{"id":"p499","n":"499","slug":"the-life-of-a-nat-connection","title":"The life of a NAT connection","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","NATGateway","Networking","VPC","CloudComputing"],"series_name":"Network","design":"timeline","script":"","date":"2029-12-04","files":{"devsecops":"notes/p499-the-life-of-a-nat-connection.md"},"md":"# The life of a NAT connection\n\nA NAT connection is a mapping with a clock on it. The instance sees a normal TCP session; the gateway sees a port, a timer and a table.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **0 ms** | Outbound SYN | The instance sends a SYN; its default route points at the NAT gateway. |\n| **1 ms** | Source NAT | The gateway rewrites the source to its own address and a free port. |\n| **2 ms** | Connection tracked | The five-tuple mapping is stored so returning packets can be matched. |\n| **40 ms** | The reply | The server answers the Elastic IP; the gateway maps it back. |\n| **350 s** | Idle timeout | Idle for 350 seconds, the mapping is removed; a late packet gets an RST. |\n| **351 s** | Port reuse | The port returns to the pool and the next connection can take it. |\n\n## Explained\n\n### The first milliseconds\n\n0 ms. The instance sends a SYN to a public address. Its default route points at the NAT gateway, so the packet goes there.\n\n1 ms. The gateway rewrites the source to its own private address and a free port; the internet gateway maps that to the Elastic IP. That is source NAT, and the port is the scarce resource: 55,000 connections per unique destination per address.\n\n2 ms. It stores the five-tuple mapping, so it can recognise the reply. The connection is now state on the gateway, not just on the instance.\n\n### The rest of its life\n\n40 ms. The server replies to the Elastic IP and the mapped port. The gateway translates it back and the instance never knows.\n\n350 s. If no packet crosses the mapping for 350 seconds, the idle timer removes it. If the instance keeps sending, it gets an RST, not a FIN.\n\n351 s. The port returns to the pool, and the next connection can reuse it.\n\n> **The honest caveat:** 350 seconds is the idle timeout, not a maximum lifetime. A keepalive under 350 seconds holds the mapping open, and a client that opens connections faster than they close can exhaust the ports.\n\n> **The lesson:** a NAT gateway is stateful, and every state has a timer.\n\n## In one line\n\n> **The mapping is the connection.** A NAT gateway remembers state so your instance does not have to.\n\n**Watch the idle count:** IdleTimeoutCount\n"},{"id":"p500","n":"500","slug":"quiz-3-cloud-network-questions","title":"Quiz: 3 cloud network questions","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","Networking","CloudSecurity","DevOps"],"series_name":"Network","design":"quiz","script":"","date":"2029-12-06","files":{"devsecops":"notes/p500-quiz-3-cloud-network-questions.md"},"md":"# Quiz: 3 cloud network questions\n\nThree questions that separate a VPC diagram from a VPC that works. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why can two VPCs with overlapping CIDRs not be peered?**\n\n- a) The route table cannot tell them apart\n- b) The peering limit is one per account\n- c) AWS blocks all RFC 1918 ranges\n\n> **Answer:** a) The route table cannot tell them apart. A route matches on prefix. If both VPCs own 10.0.0.0/16, a packet to 10.0.0.7 has two homes, so AWS refuses the peering connection outright.\n\n**Q2. What does a NAT gateway not allow?**\n\n- a) Inbound connections from the internet\n- b) Outbound connections to the internet\n- c) Return traffic for an outbound request\n\n> **Answer:** a) Inbound connections from the internet. It maps outbound connections and their replies. An unsolicited inbound SYN has no mapping, so it is dropped; publish the service behind a load balancer instead.\n\n**Q3. Why does a network ACL need a rule for the reply?**\n\n- a) Because it is stateless\n- b) Because security groups drop replies\n- c) Because routes rewrite the source port\n\n> **Answer:** a) Because it is stateless. A security group remembers the connection and allows the reply. A NACL checks each direction alone, so the reply needs its own rule, usually ports 1024-65535.\n\n## Explained\n\n### Question one\n\nWhy can two VPCs with overlapping CIDRs not be peered? Because a route matches on prefix. If both VPCs own 10.0.0.0/16, a packet to 10.0.0.7 has two possible homes, and no route table can choose. AWS refuses the peering connection outright, so the fix is to re-IP before you need the link, not after. The limit is not a policy choice; it is arithmetic.\n\n### Question two\n\nWhat does a NAT gateway not allow? Inbound connections initiated from the internet. It maps outbound connections and their replies; an unsolicited SYN has no mapping and is dropped. A NAT gateway is stateful for the connections it opened, and nothing else. Publish the service behind a load balancer if it must accept connections.\n\n### Question three\n\nWhy does a NACL need a rule for the reply? Because it is stateless. A security group remembers the connection and allows the return traffic; a NACL checks each direction alone, so the reply needs its own rule, usually an ephemeral port range of 1024-65535.\n\n> **The honest caveat:** these are the common answers, not every answer. A NAT instance, a firewall appliance or a PrivateLink endpoint changes the path, and the rules change with it.\n\n> **The lesson:** state is the difference between a rule and a session.\n\n## In one line\n\n> **One hop remembers, one does not.** State is the difference between a rule and a session.\n\n**Read the counters**\n\n```bash\naws ec2 describe-network-acls\n```\n"},{"id":"p501","n":"501","slug":"5-vpc-mistakes","title":"5 VPC mistakes","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["AWS","VPC","CloudSecurity","Networking","DevOps"],"series_name":"Network","design":"dontdo","script":"","date":"2029-12-09","files":{"devsecops":"notes/p501-5-vpc-mistakes.md"},"md":"# 5 VPC mistakes\n\nMost VPC incidents are decisions made on day one, when the diagram was simple and the deadline was real.\n\n## Quick reference\n\n**1.** ❌ **Overlapping CIDRs across VPCs** — Peering and Transit Gateway cannot route between them; re-IP later hurts.  \n✅ **Plan a global CIDR scheme** — One range per VPC, no overlap, documented before build.\n\n**2.** ❌ **0.0.0.0/0 on SSH and RDP** — The whole internet can reach the admin port; scanners find it in hours.  \n✅ **SSM Session Manager, no port** — No inbound rule, no bastion, and every session is logged.\n\n**3.** ❌ **Private subnet with no egress** — No NAT or endpoints: package installs and updates fail at 3 a.m.  \n✅ **NAT per AZ or VPC endpoints** — Egress works, and endpoints keep AWS traffic off the NAT.\n\n**4.** ❌ **One subnet, one AZ** — An AZ outage takes the service down; the SLA does not cover it.  \n✅ **Two AZs, one subnet each** — Failover is a routing change, not a rebuild.\n\n**5.** ❌ **No flow logs, no evidence** — You cannot prove who talked to what, or why a packet died.  \n✅ **Flow logs to S3 or CloudWatch** — Cheap evidence, and the first place to look after a drop.\n\n## Explained\n\n### The five\n\n1. **Overlapping CIDRs.** Two VPCs both on 10.0.0.0/16 can never peer. Re-IP later means touching every subnet, route and firewall rule; plan a global range before the second VPC exists.\n\n2. 0.0.0.0/0 on admin ports. SSH and RDP open to the world are found by scanners in hours. Use SSM Session Manager, or a bastion with a security group that names the source.\n\n3. **No egress path.** A private subnet with no NAT gateway, no VPC endpoints and no other route out cannot install a package or reach an API. The failure appears at the worst moment, not at build time.\n\n4. **One AZ.** One subnet in one Availability Zone is one power domain, one NAT gateway and one failure. Two subnets in two zones turn an outage into a routing change.\n\n5. **No flow logs.** Without them, a dropped packet is an opinion. Flow logs to S3 or CloudWatch are cheap evidence, and the first place to look.\n\n### The pattern\n\nEach one trades a small convenience today for a larger failure later.\n\n> **The honest caveat:** none of these is a security control on its own. They are the defaults that let the other controls do their job.\n\n> **The lesson:** the network you draw on day one is the one you debug at 3 a.m.\n\n## In one line\n\n> **Design the network for the bad day.** The bill for a shortcut arrives with interest.\n\n**Check the cidrs**\n\n```bash\naws ec2 describe-vpcs\n```\n"},{"id":"p502","n":"502","slug":"pod-to-pod-networking-drawn","title":"Pod-to-pod networking, drawn","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CNI","CloudNative","DevOps"],"series_name":"Network","design":"architecture","script":"","date":"2029-12-11","files":{"devsecops":"notes/p502-pod-to-pod-networking-drawn.md"},"md":"# Pod-to-pod networking, drawn\n\nTwo pods on the same node keep talking after the node loses its default route. Their packets never leave the host: the path is a veth pair and a bridge.\n\n## Quick reference\n\n- **Pod a:** Pod A (POD) · veth pair A (VETH)\n- **Node:** cni0 bridge (BRDG) · CNI plugin (CNI)\n- **Pod b:** veth pair B (VETH) · Pod B (POD)\n\n**Request flow:** Pod A → veth pair A → cni0 bridge → veth pair B → Pod B; CNI plugin → cni0 bridge\n\n## Explained\n\n### The pod side\n\nPod A has an eth0 inside its network namespace. That interface is one end of a veth pair; the other end sits in the host namespace and attaches to cni0. The pod gets an IP from the node's pod CIDR and a default route that points at the host.\n\n### The node\n\nOn the host, cni0 forwards frames by MAC address between the two host veths. No routing, no NAT and no kube-proxy: pod IP to pod IP is a layer 2 hop. The CNI plugin beside it did the wiring: it allocated the pod IP, created the veth pair, attached it to the bridge, and enforces NetworkPolicy at that boundary. If the pods sit on different nodes, the CNI encapsulates the packet or routes the pod CIDR; the bridge path stops at the node boundary.\n\n### The other end\n\nPod B mirrors Pod A: its own eth0, its own veth pair, and a host end on the same bridge. The reply walks the same path backwards.\n\n> **The honest caveat:** not every CNI uses a bridge. Cilium attaches eBPF programs to the veth and Calico can route pod IPs directly, so the cni0 box is the common diagram, not a law.\n\n> **The lesson:** the pod IP stays the same; the path under it is the CNI's choice.\n\n## In one line\n\n> **Two pods, one layer 2 hop.** The pod IP never changes; the path under it does.\n\n**Trace the path**\n\n```bash\nkubectl exec -it api -- ip route\n```\n"},{"id":"p503","n":"503","slug":"the-kubernetes-network-debugging-cheat-sheet","title":"The Kubernetes network debugging cheat sheet","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","Troubleshooting","DevOps","SRE"],"series_name":"Network","design":"cheatsheet","script":"","date":"2029-12-13","files":{"devsecops":"notes/p503-the-kubernetes-network-debugging-cheat-sheet.md"},"md":"# The Kubernetes network debugging cheat sheet\n\nA service is down, the pod is Running, and the logs are clean. The network is now a process of elimination, and these four blocks are the order I use.\n\n## Quick reference\n\n### DNS\n\n```bash\nkubectl exec -it api -- nslookup db\nkubectl exec api -- cat /etc/resolv.conf\nkubectl get svc -n kube-system kube-dns\nkubectl -n kube-system get pods\nkubectl exec api -- nslookup kubernetes\n```\n\n### Sockets\n\n```bash\nkubectl exec -it api -- ss -tlnp\nkubectl exec -it api -- ss -tan\nkubectl exec -it api -- ss -s\nkubectl exec -it api -- netstat -tulpn\nkubectl port-forward svc/api 8080:80\n```\n\n### Policy\n\n```bash\nkubectl get netpol -A\nkubectl describe netpol api\nkubectl get pods --show-labels\nkubectl exec -it api -- nc -vz db 5432\nkubectl get netpol api -o yaml\n```\n\n### Trace\n\n```bash\nkubectl exec -it api -- tcpdump -i eth0\nkubectl exec -it api -- tcpdump -n port 53\nkubectl exec -it api -- ping -c 3 db\nkubectl debug -it api --image=busybox\nkubectl exec -it api -- ip route\n```\n\n## Explained\n\n### DNS\n\nStart with the name. nslookup db from inside the pod proves the resolver and the search path; `cat /etc/resolv.conf` shows the nameserver and ndots the pod actually got. If the name fails, everything after it is noise.\n\n### Sockets\n\nIf the name resolves, check the listener. `ss -tlnp` shows what is bound and on which address; `ss -tan` adds the connection states. A process bound to 127.0.0.1 is unreachable from another pod, which is the most common \"the port is open\" lie.\n\n### Policy\n\nA refused connection can be a NetworkPolicy, not a dead app. List the policies, read the selectors, and compare them with the pod labels. Then test the exact pair the policy should allow; a policy that selects no pods enforces nothing.\n\n### Trace\n\nWhen the path is still unclear, capture. tcpdump on eth0 shows whether the SYN left the pod, and a reply proves the far side heard it. ping and ip route answer the layer 3 questions, and kubectl debug gives you a shell with the tools the app image lacks.\n\n> **The honest caveat:** many app images have no ss, dig or tcpdump. Run the checks from a debug container or a netshoot pod, and remember that a missing binary looks like a failed command.\n\n> **The lesson:** debug in the order the packet travels, not the order the tools appear.\n\n## In one line\n\n> **Find the hop, not the blame.** Four blocks, one order: name, socket, policy, packet.\n\n**Run it in the pod**\n\n```bash\nkubectl exec -it api -- ss -tlnp\n```\n"},{"id":"p504","n":"504","slug":"the-four-service-types","title":"The four service types","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CloudNative","DevOps","SRE"],"series_name":"Network","design":"explainer","script":"","date":"2029-12-16","files":{"devsecops":"notes/p504-the-four-service-types.md"},"md":"# The four service types\n\nYou need to expose a service and the YAML offers four types. One is the trunk, two build on it, and one is only a DNS record.\n\n## Quick reference\n\n### ClusterIP\n\n- A virtual IP from the service CIDR\n- Reachable inside the cluster only\n- The default type for every Service\n\n### NodePort\n\n- One port on every node, 30000-32767\n- Still builds a ClusterIP behind it\n- For demos and bare-metal edge cases\n\n### LoadBalancer\n\n- NodePort plus a cloud load balancer\n- A public or internal address, for a fee\n- One load balancer per Service by default\n\n### ExternalName\n\n- A DNS CNAME, not a proxy or a VIP\n- No endpoints and no kube-proxy rules\n- For a managed service outside the cluster\n\n> **Note:** Two types build on ClusterIP; ExternalName is only a DNS name.\n\n## Explained\n\n### Inside the cluster\n\nClusterIP is the default and the type NodePort and LoadBalancer build on. The API server assigns a virtual IP from the service CIDR, and kube-proxy installs rules that rewrite traffic to a ready pod. Nothing listens on the VIP, so a curl only works while endpoints exist.\n\n### From outside\n\nNodePort opens one port on every node, by default in 30000-32767, and forwards it to the same ClusterIP. It is quick for a demo, but it exposes the port on every node with a public address. LoadBalancer asks the cloud controller for an external load balancer that targets those NodePorts. It gives you an address and health checks, and it bills per Service. On bare metal it stays pending until MetalLB answers.\n\n### A name, not a proxy\n\nExternalName is the odd one: it returns a CNAME record and creates no endpoints, no VIP and no proxy. Clients resolve the name and connect directly, which is useful for a managed database and useless if you expect policy or TLS termination.\n\n> **The honest caveat:** a LoadBalancer Service is only as portable as its cloud controller. The same YAML on a laptop or a bare-metal cluster will sit in pending until a controller implements it.\n\n> **The lesson:** choose the smallest exposure that works, then put one ingress in front of the rest.\n\n## In one line\n\n> **Pick the smallest path that works.** Every type above ClusterIP adds a hop someone must operate.\n\n**Check the type**\n\n```bash\nkubectl get svc -o wide\n```\n"},{"id":"p505","n":"505","slug":"a-connection-state-report-in-python","title":"A connection state report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","Python","Networking","SRE","Linux"],"series_name":"Network","design":"code","script":"conn_state.py","date":"2029-12-18","files":{"devsecops":"notes/p505-a-connection-state-report-in-python.md"},"md":"# A connection state report in Python\n\nA pod restarts cleanly, the dashboard is green, and twenty minutes later the node cannot open new outbound connections. The socket table was filling the whole time.\n\n## Quick reference\n\nThe full script is at the end of this note: `conn_state.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nss -tan | python3 conn_state.py\n```\n\n**Example output**\n\n```text\nTIME-WAIT=843 CLOSE-WAIT=27 ESTAB=12\nFAIL close-wait=27\n```\n\n## Explained\n\n### What it counts\n\n`ss -tan` prints one line per TCP socket, with the state in the first column. The script reads that column, skips the header, and counts ESTAB, TIME-WAIT, CLOSE-WAIT, LISTEN and the rest. The version in the GIF prints the three biggest states, from a read-only parse of text.\n\n### The two flags\n\nCLOSE-WAIT means the peer sent FIN and the application never closed its side. One is normal; a number that climbs is a leak, usually a connection pool or a handler that forgets to close. TIME-WAIT means the local side closed first and is waiting out the timer. It is normal on a busy client, but a spike can exhaust ephemeral ports.  \nThe defaults are 10 for CLOSE-WAIT and 1000 for TIME-WAIT, both adjustable. The script exits 1 when either count reaches its limit, 0 when clean, and 2 when the input cannot be read.\n\n### How to use it\n\nRun `ss -tan | python3 conn_state.py` on the node, or save the output and pass `--input`. Put it in cron every minute, or in a CI step against a captured fixture, and let the exit code fail the run.\n\n> **The honest caveat:** ss is a snapshot. A leak is a trend, so keep the output and compare counts over time; a single TIME-WAIT number on a busy client proves very little.\n\n> **The lesson:** name the state before you restart the pod.\n\n## In one line\n\n> **A leak has a shape.** CLOSE-WAIT is usually an app bug; TIME-WAIT is usually load.\n\n**Run on the node**\n\n```bash\nss -tan | python3 conn_state.py\n```\n\n## Script: conn_state.py\n\nDownload: [scripts/conn_state.py](../scripts/conn_state.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report TCP socket states from `ss -tan` and flag leaks or spikes.\n\nUsage:\n    ss -tan | python3 conn_state.py\n    ss -tan > /tmp/ss.txt\n    python3 conn_state.py --input /tmp/ss.txt\n    python3 conn_state.py --input /tmp/ss.txt --close-wait 5 --time-wait 500\n\nReads a captured `ss -tan` table and prints a count per TCP state. It exits\n1 when CLOSE-WAIT sockets reach --close-wait (default 10) or TIME-WAIT\nsockets reach --time-wait (default 1000).\n\nExit codes: 0 clean, 1 finding, 2 usage or input error.\nRead-only: parses text, prints, and never changes a socket.\nCron: * * * * * ss -tan | python3 conn_state.py >> /var/log/conn-state.log\nCI:   ss -tan | python3 conn_state.py || exit 1\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport collections\nimport sys\n\nSTATES = frozenset({\n    \"ESTAB\", \"SYN-SENT\", \"SYN-RECV\", \"FIN-WAIT-1\", \"FIN-WAIT-2\",\n    \"TIME-WAIT\", \"CLOSE\", \"CLOSE-WAIT\", \"LAST-ACK\", \"LISTEN\", \"CLOSING\",\n})\n\n\ndef positive(text):\n    \"\"\"argparse type for a whole number of 1 or more.\"\"\"\n    try:\n        value = int(text)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"expected a whole number\")\n    if value < 1:\n        raise argparse.ArgumentTypeError(\"expected a number of 1 or more\")\n    return value\n\n\ndef parse(stream):\n    \"\"\"Return (Counter of states, number of socket rows) from ss -tan text.\"\"\"\n    counts = collections.Counter()\n    total = 0\n    for line in stream:\n        fields = line.split()\n        if len(fields) < 5:\n            continue\n        state = fields[0].upper()\n        if state not in STATES:\n            continue\n        counts[state] += 1\n        total += 1\n    return counts, total\n\n\ndef report(counts, args):\n    \"\"\"Print the counts and the verdict; return 0 or 1.\"\"\"\n    print(f\"{'total':<12}{sum(counts.values())}\")\n    for state, count in counts.most_common():\n        print(f\"{state:<12}{count}\")\n\n    findings = []\n    close_wait = counts.get(\"CLOSE-WAIT\", 0)\n    time_wait = counts.get(\"TIME-WAIT\", 0)\n    if close_wait >= args.close_wait:\n        findings.append(\n            f\"close-wait {close_wait} >= {args.close_wait}: an app is not closing sockets\")\n    if time_wait >= args.time_wait:\n        findings.append(\n            f\"time-wait {time_wait} >= {args.time_wait}: short-lived connections are piling up\")\n    if findings:\n        for finding in findings:\n            print(f\"FAIL {finding}\")\n        return 1\n    print(f\"OK close-wait {close_wait} < {args.close_wait}, \"\n          f\"time-wait {time_wait} < {args.time_wait}\")\n    return 0\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Count TCP states from `ss -tan` and fail on CLOSE-WAIT or TIME-WAIT.\")\n    parser.add_argument(\n        \"-i\", \"--input\", default=\"-\", metavar=\"PATH\",\n        help=\"file containing `ss -tan` output, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"--close-wait\", type=positive, default=10, metavar=\"N\",\n        help=\"fail when CLOSE-WAIT sockets reach N (default: 10)\")\n    parser.add_argument(\n        \"--time-wait\", type=positive, default=1000, metavar=\"N\",\n        help=\"fail when TIME-WAIT sockets reach N (default: 1000)\")\n    args = parser.parse_args(argv)\n\n    if args.input == \"-\":\n        counts, total = parse(sys.stdin)\n    else:\n        try:\n            with open(args.input, \"r\", encoding=\"utf-8\") as handle:\n                counts, total = parse(handle)\n        except OSError as exc:\n            print(f\"ERROR cannot read {args.input}: {exc}\", file=sys.stderr)\n            return 2\n\n    if total == 0:\n        print(\"ERROR no TCP states found; is this `ss -tan` output?\", file=sys.stderr)\n        return 2\n    return report(counts, args)\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p506","n":"506","slug":"a-latency-matrix-in-python","title":"A latency matrix in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Networking","SRE","DevOps","Performance"],"series_name":"Network","design":"code","script":"latency_matrix.py","date":"2029-12-20","files":{"it":"notes/p506-a-latency-matrix-in-python.md","devsecops":"notes/p506-a-latency-matrix-in-python.md"},"md":"# A latency matrix in Python\n\nA dashboard says the API is slow, and the p99 is fine. The problem is one target that takes 300 ms to accept a TCP connection, and nobody measured the connect step on its own.\n\n## Quick reference\n\nThe full script is at the end of this note: `latency_matrix.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 latency_matrix.py db:5432 api:443 5 250\n```\n\n**Example output**\n\n```text\ndb:5432 p50=0.4 p95=0.9\napi:443 p50=1.2 p95=18.4\n```\n\n## Explained\n\n### What it measures\n\nThe script takes a list of host:port targets, opens a TCP connection to each one, closes it, and times the handshake. It repeats that N times, five by default, then reports p50 and p95 per target in milliseconds. No HTTP request, no TLS, no payload: this is the network and the listen backlog, before the application says anything.\n\n### The verdict\n\nEach target's p95 is compared with a budget, 250 ms by default. The script exits 1 when one is over, or when a connection fails, and 0 when every target is inside it. The attached version also prints the failure count and names the slowest target, so a red build tells you where to look.\n\n### How to use it\n\nRun `python3 latency_matrix.py db:5432 api:443` from the same network segment as the caller. A laptop on VPN measures the VPN, not the cluster. Put it in a cron or a synthetic check, and keep a baseline for each target so a p95 of 40 ms is read against its own history, not against another service.\n\n> **The honest caveat:** a TCP connect measures reachability and backlog, not application latency. A fast handshake with a slow response is a different problem, so pair this with an HTTP check.\n\n> **The lesson:** measure the layer you blame.\n\n## In one line\n\n> **A fast connect is not a fast reply.** Measure the layer you blame, not the one you know.\n\n**Measure the connect**\n\n```bash\npython3 latency_matrix.py db:5432 --repeat 5\n```\n\n## Script: latency_matrix.py\n\nDownload: [scripts/latency_matrix.py](../scripts/latency_matrix.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Measure TCP connect latency to a list of hosts and enforce a budget.\n\nUsage:\n    python3 latency_matrix.py db:5432 api:443\n    python3 latency_matrix.py db:5432 api:443 --repeat 10 --budget 150\n    python3 latency_matrix.py db:5432 --timeout 1\n\nEach target is HOST:PORT. The tool opens a TCP connection, closes it, and\ntimes the handshake. It repeats --repeat times (default 5), prints p50 and\np95 in milliseconds per target, names the slowest target, and exits 1 when\na p95 is over --budget (default 250 ms) or a connection fails.\n\nExit codes: 0 clean, 1 finding, 2 usage or input error.\nRead-only: it connects and closes; it sends no application data.\nCron: */5 * * * * python3 latency_matrix.py db:5432 api:443 >> /var/log/latency.log\nCI:   python3 latency_matrix.py db:5432 api:443 || exit 1\nBy s0sta.com\n\"\"\"\n\nimport argparse\nimport socket\nimport sys\nimport time\n\n\ndef positive_int(text):\n    \"\"\"argparse type for a whole number of 1 or more.\"\"\"\n    try:\n        value = int(text)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"expected a whole number\")\n    if value < 1:\n        raise argparse.ArgumentTypeError(\"expected a number of 1 or more\")\n    return value\n\n\ndef positive_float(text):\n    \"\"\"argparse type for a number greater than zero.\"\"\"\n    try:\n        value = float(text)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"expected a number\")\n    if value <= 0:\n        raise argparse.ArgumentTypeError(\"expected a number greater than zero\")\n    return value\n\n\ndef target(text):\n    \"\"\"argparse type for HOST:PORT, returning (host, port, original).\"\"\"\n    host, separator, port_text = text.rpartition(\":\")\n    if not separator or not host or not port_text.isdigit():\n        raise argparse.ArgumentTypeError(f\"expected HOST:PORT, got {text!r}\")\n    port = int(port_text)\n    if not 1 <= port <= 65535:\n        raise argparse.ArgumentTypeError(f\"port out of range in {text!r}\")\n    if host.startswith(\"[\") and host.endswith(\"]\"):\n        host = host[1:-1]\n    return host, port, text\n\n\ndef measure(host, port, timeout, repeat):\n    \"\"\"Return (successful connect times in ms, number of failed attempts).\"\"\"\n    samples = []\n    failures = 0\n    for _ in range(repeat):\n        start = time.perf_counter()\n        try:\n            with socket.create_connection((host, port), timeout=timeout):\n                samples.append((time.perf_counter() - start) * 1000)\n        except OSError:\n            failures += 1\n    return samples, failures\n\n\ndef percentile(values, fraction):\n    \"\"\"Linear-interpolated percentile of a non-empty list.\"\"\"\n    ordered = sorted(values)\n    if len(ordered) == 1:\n        return ordered[0]\n    position = (len(ordered) - 1) * fraction\n    lower = int(position)\n    upper = min(lower + 1, len(ordered) - 1)\n    weight = position - lower\n    return ordered[lower] + (ordered[upper] - ordered[lower]) * weight\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Time TCP connects to HOST:PORT targets and fail over a p95 budget.\")\n    parser.add_argument(\"targets\", nargs=\"+\", type=target, metavar=\"HOST:PORT\",\n                        help=\"targets to measure, for example db:5432 api:443\")\n    parser.add_argument(\"--repeat\", type=positive_int, default=5, metavar=\"N\",\n                        help=\"connects per target (default: 5)\")\n    parser.add_argument(\"--budget\", type=positive_float, default=250.0, metavar=\"MS\",\n                        help=\"p95 budget per target in milliseconds (default: 250)\")\n    parser.add_argument(\"--timeout\", type=positive_float, default=2.0, metavar=\"S\",\n                        help=\"connect timeout per attempt in seconds (default: 2)\")\n    args = parser.parse_args(argv)\n\n    rows = []\n    for host, port, label in args.targets:\n        samples, failures = measure(host, port, args.timeout, args.repeat)\n        if samples:\n            p50 = percentile(samples, 0.50)\n            p95 = percentile(samples, 0.95)\n        else:\n            p50 = p95 = float(\"nan\")\n        rows.append((label, samples, failures, p50, p95))\n\n    print(f\"{'TARGET':<26}{'N':>3}{'FAIL':>6}{'P50':>9}{'P95':>9}\")\n    for label, samples, failures, p50, p95 in rows:\n        if samples:\n            print(f\"{label:<26}{len(samples):>3}{failures:>6}\"\n                  f\"{p50:>7.1f}ms{p95:>7.1f}ms\")\n        else:\n            print(f\"{label:<26}{0:>3}{failures:>6}{'FAIL':>9}{'FAIL':>9}\")\n\n    findings = []\n    for label, samples, failures, p50, p95 in rows:\n        if failures:\n            findings.append(f\"{label}: {failures}/{args.repeat} connections failed\")\n        if samples and p95 > args.budget:\n            findings.append(f\"{label}: p95 {p95:.1f} ms over budget {args.budget:g} ms\")\n\n    reachable = [row for row in rows if row[1]]\n    if reachable:\n        slowest = max(reachable, key=lambda row: row[4])\n        print(f\"SLOWEST {slowest[0]} p95 {slowest[4]:.1f} ms\")\n\n    if findings:\n        for finding in findings:\n            print(f\"FAIL {finding}\")\n        return 1\n    print(f\"OK every p95 under {args.budget:g} ms\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p507","n":"507","slug":"ingress-vs-gateway-api","title":"Ingress vs Gateway API","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Ingress","GatewayAPI","Networking","CloudNative"],"series_name":"Network","design":"compare","script":"","date":"2029-12-23","files":{"devsecops":"notes/p507-ingress-vs-gateway-api.md"},"md":"# Ingress vs Gateway API\n\nAn ingress is three lines of YAML and a controller you forgot you installed. Gateway API is the same job split across roles, and the migration is not a rename.\n\n## Quick reference\n\n| | **INGRESS** | **GATEWAY** |\n|:--|:--|:--|\n| API | One object, v1 | Role-based CRDs |\n| Protocols | HTTP and HTTPS | HTTP, gRPC, TCP, TLS |\n| Routes on | Host and path | Host, path, header, weight |\n| Ownership | One team owns it all | Infra and app split |\n| Controllers | Traefik, HAProxy, Kong | Envoy Gateway, Cilium |\n| Status | Frozen since 1.19 | GA, installed separately |\n\n**INGRESS** (The stable L7 entry point): One object for host and path rules, handled by a controller you install.\n\n```bash\nkubectl get ingress\n```\n\n**GATEWAY** (The role-oriented successor): A family of CRDs that splits infrastructure from application routing.\n\n```bash\nkubectl get gateway,httproute\n```\n\n- Ingress → one object, host and path rules.\n- Gateway → role-based routes, more protocols.\n\n## Explained\n\n### The API\n\nIngress is one stable object, `networking.k8s.io/v1` since Kubernetes 1.19, with host and path rules and a reference to an IngressClass. It handles HTTP and HTTPS. Gateway API is a family of CRDs, GatewayClass, Gateway and HTTPRoute among them, and it adds gRPC, TCP, UDP and TLS routes.\n\n### Routes and ownership\n\nIngress matches host and path. Gateway API matches host, path, header, method and query, and can split traffic by weight between backends. With Ingress, one team usually owns the controller, the certificate and every rule. Gateway API separates the infrastructure owner, who runs the Gateway, from the application owner, who attaches a route. That split is the real feature.\n\n### Controllers and status\n\nIngress controllers are mature: Traefik, HAProxy, Kong and others. Gateway API implementations are younger but real: Envoy Gateway, Cilium, Istio and others. Ingress is frozen, not deprecated; it will keep working. Gateway API is generally available but still installed as CRDs, so it is not in the core API server.\n\n> **The honest caveat:** Gateway API is more capable and has more moving parts. If one team owns one controller and needs only host and path, Ingress is still the smaller system.\n\n> **The lesson:** choose by ownership model, not by feature count.\n\nIngress -> one object, host and path rules.  \nGateway -> role-based routes, more protocols.\n"},{"id":"p508","n":"508","slug":"from-kubectl-apply-to-a-routable-service","title":"From kubectl apply to a routable service","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","KubeProxy","CloudNative","DevOps"],"series_name":"Network","design":"timeline","script":"","date":"2029-12-25","files":{"devsecops":"notes/p508-from-kubectl-apply-to-a-routable-service.md"},"md":"# From kubectl apply to a routable service\n\nYou run kubectl apply, the Service appears, and for a few seconds every connection is refused. The object exists; the path to a pod does not yet.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0s** | Service created | The API server assigns a ClusterIP and writes the object to etcd. |\n| **T+1s** | Endpoints populated | The controller adds ready pod IPs to an EndpointSlice. |\n| **T+2s** | kube-proxy notices | Every node watches Services and EndpointSlices, not just one. |\n| **T+3s** | Rules written | iptables or IPVS rewrites the VIP to a pod IP and targetPort. |\n| **T+4s** | First packet | The client hits the VIP; conntrack remembers the rewrite. |\n| **T+5s** | Service ready | Readiness changes move the pod in and out; the rules follow. |\n\n## Explained\n\n### The object\n\nService created. The API server validates the Service, assigns a ClusterIP from the service CIDR, and writes it to etcd. The object is real, but no node has a rule for it yet.  \nEndpoints populated. The endpoint slice controller, inside kube-controller-manager, watches matching pods and adds ready pod IPs to an EndpointSlice. A pod that is Running but not Ready stays out.\n\n### The rules\n\nKube-proxy notices. Normally every node runs kube-proxy, and every kube-proxy watches Services and EndpointSlices. The update reaches all of them, not just the node running the pod.  \nRules written. In iptables mode, kube-proxy writes a chain that matches the ClusterIP and DNATs to a backend pod IP and targetPort. In IPVS mode it creates virtual and real servers. With no endpoints, iptables mode rejects instead.\n\n### The packet\n\nFirst packet. The client connects to ClusterIP:port. The node's rules rewrite the destination to a pod IP, conntrack remembers the mapping, and the reply is rewritten back.  \nReady. After that, readiness changes move the pod in and out of the slice, and the rules follow. The Service never sees a packet.\n\n> **The honest caveat:** kube-proxy is not the only implementation. Cilium and Calico can replace it with eBPF or direct routing, so the rule order differs even when the result is the same.\n\n> **The lesson:** a Service is a promise the data plane has to keep.\n\n## In one line\n\n> **The VIP is a rule, not a process.** A Service is a promise the data plane has to keep.\n\n**Watch the slice**\n\n```bash\nkubectl get endpointslice -w\n```\n"},{"id":"p509","n":"509","slug":"the-layers-a-pod-packet-crosses","title":"The layers a pod packet crosses","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CNI","CloudNative","Linux"],"series_name":"Network","design":"stack","script":"","date":"2029-12-27","files":{"devsecops":"notes/p509-the-layers-a-pod-packet-crosses.md"},"md":"# The layers a pod packet crosses\n\nA pod on node A calls a pod on node B, and the packet crosses six layers. Most debugging stops early.\n\n## Quick reference\n\n*Bottom to top: the path a packet takes*\n\n1. **veth pair** — eth0 in the pod pairs with a host veth; every packet crosses it. · *VETH*\n2. **Linux bridge** — cni0 forwards by MAC to the host veth, or passes it to the host stack. · *BRIDGE*\n3. **CNI overlay** — VXLAN, IPIP or routed pod CIDRs carry it to the next node. · *OVERLAY*\n4. **node route** — The host table picks the interface, the next hop and the MTU. · *ROUTE*\n5. **underlay** — The physical or cloud network moves the packet, unaware of pods. · *UNDERLAY*\n6. **destination pod** — The far node decapsulates, bridges and passes it into the pod netns. · *NETNS*\n\n## Explained\n\n### The pod end\n\nVeth pair. Inside the pod, eth0 is one end of a veth pair; the peer sits in the host namespace and carries every packet out. The pod's default route points at the host side.  \nLinux bridge. On a typical CNI, the host veth is attached to cni0 or another Linux bridge. The bridge forwards by MAC to the right port, or passes the frame to the host stack.\n\n### The cluster network\n\nCNI overlay. When the destination is on another node, the CNI decides how to carry the packet. VXLAN and IPIP wrap it in an outer header; routed CNIs advertise the pod CIDR with BGP.  \nNode route. The host routing table chooses the interface and next hop. This is where the MTU is set, and where a wrong MTU makes large packets vanish while small pings work.\n\n### The far side\n\nUnderlay. The physical or cloud network forwards it without knowing what a pod is. Security groups and ACLs here can drop traffic the cluster allows.  \nDestination pod. The far node decapsulates, routes or bridges to the destination host veth, and the packet enters the pod netns. Only now does the application see it.\n\n> **The honest caveat:** eBPF CNIs collapse several layers. Cilium can attach to the veth and route directly, so the bridge may not exist; the failure modes move, they do not disappear.\n\n> **The lesson:** know which layer owns the drop before you change a manifest.\n\n## In one line\n\n> **Every layer has a failure mode.** Know which layer owns the drop before you change a manifest.\n\n**Check the mtu**\n\n```bash\nping -M do -s 1400 <pod-ip>\n```\n"},{"id":"p510","n":"510","slug":"quiz-3-kubernetes-network-questions","title":"Quiz: 3 Kubernetes network questions","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CKA","DevOps","CloudNative"],"series_name":"Network","design":"quiz","script":"","date":"2029-12-30","files":{"devsecops":"notes/p510-quiz-3-kubernetes-network-questions.md"},"md":"# Quiz: 3 Kubernetes network questions\n\nThree questions separate people who have debugged a cluster from people who have only read about one. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why does a ClusterIP Service pass no traffic until it has endpoints?**\n\n- a) The VIP only exists in node rules\n- b) The API server drops traffic\n- c) Pods must dial the VIP first\n\n> **Answer:** a) The VIP only exists in node rules. A ClusterIP is virtual: no interface owns it. kube-proxy installs DNAT rules to endpoint IPs; with none, there is nowhere to send the packet, so it is rejected.\n\n**Q2. What does kube-proxy actually program on each node?**\n\n- a) iptables, IPVS or nftables rules\n- b) A proxy process per Service\n- c) The pod's network namespace\n\n> **Answer:** a) iptables, IPVS or nftables rules. It writes DNAT and REJECT rules in iptables mode, or virtual and real servers in IPVS mode; nftables is the newer backend. It forwards no packets itself.\n\n**Q3. Why does a NetworkPolicy need a CNI that enforces it?**\n\n- a) The API server only stores the object\n- b) Policies are applied by kubelet\n- c) The scheduler reads the policy\n\n> **Answer:** a) The API server only stores the object. NetworkPolicy is an API object; the apiserver stores it and filters nothing. A CNI plugin or mesh must translate it into rules. One that ignores it leaves pods open.\n\n## Explained\n\n### Question one\n\nWhy does a ClusterIP Service pass no traffic until it has endpoints? Because the VIP is virtual: no interface owns it and no process listens on it. kube-proxy, or the CNI that replaced it, writes DNAT rules that point at endpoint IPs. With no endpoints there is nowhere to rewrite the packet, so iptables mode rejects it. The Service object was never a server.\n\n### Question two\n\nWhat does kube-proxy program? In iptables mode it writes DNAT and REJECT rules; in IPVS mode it creates virtual servers and real servers; nftables is the newer backend. It does not proxy packets through a user-space process, and it does not touch the pod's network namespace. It keeps node rules in sync with Services and EndpointSlices.\n\n### Question three\n\nWhy does a NetworkPolicy need a CNI that enforces it? Because the API server only stores the object. Policy is enforced where the packet is evaluated, at the veth, the bridge or an eBPF hook, and that is the CNI's code. A plugin that accepts the YAML and ignores it leaves every pod open, which is why a policy review is not a test.\n\n> **The honest caveat:** a passing quiz is not a working cluster. The only proof is a connection that should fail and does.\n\n> **The lesson:** know which component owns the drop.\n\n## In one line\n\n> **Rules need a reader.** A policy that nothing enforces is a comment with a schema.\n\n**Test the deny**\n\n```bash\nkubectl exec -it api -- nc -vz db 5432\n```\n"},{"id":"p511","n":"511","slug":"kubernetes-networking-terms","title":"Kubernetes networking terms","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","Networking","CNI","CloudNative","DevOps"],"series_name":"Network","design":"reveal","script":"","date":"2030-01-01","files":{"devsecops":"notes/p511-kubernetes-networking-terms.md"},"md":"# Kubernetes networking terms\n\nSix terms carry most Kubernetes networking conversations, and most arguments come from mixing two of them.\n\n## Quick reference\n\n- **CNI** — The plugin contract that gives a pod an interface, an IP and routes, then removes them.\n- **ClusterIP** — A virtual IP from the service CIDR; node rules rewrite traffic to a ready pod.\n- **NodePort** — A port on every node that forwards to the Service, still behind a ClusterIP.\n- **Ingress** — An L7 routing object: host and path rules handled by a controller, not the API server.\n- **NetworkPolicy** — A label-selected allow-list for pods; only a CNI that implements it enforces it.\n- **kube-proxy** — The node agent that programs iptables, IPVS or nftables rules for Services.\n\n## Explained\n\n### The plugin\n\nCNI is the interface between the kubelet and the network plugin: give this pod an interface, an IP and routes, and remove them when it dies. The plugin, not Kubernetes, owns the data plane.\n\n### The service path\n\nClusterIP is a virtual IP from the service CIDR. No process owns it; node rules rewrite traffic to a ready pod. NodePort opens a port on every node and forwards to that same VIP; it is another path, not another backend.\n\n### The entry and the policy\n\nIngress is an L7 routing object: host and path rules handled by a controller, not by the API server. It terminates or passes TLS, and it needs a controller and a Service behind it.  \nNetworkPolicy is a label-selected allow-list for pods. It is stored by the API server and enforced by the CNI, so a plugin that ignores it leaves the pods open.\n\n### The agent\n\nkube-proxy is the node agent that programs iptables, IPVS or nftables rules for Services. Many clusters now replace it with eBPF, but the job remains: turn Service and EndpointSlice objects into packet rules.\n\n> **The honest caveat:** these definitions are the common case, not the specification. Implementations differ, and the only reliable answer is the one your cluster's data plane actually enforces.\n\n> **The lesson:** name the layer before you argue about the fix.\n\n## In one line\n\n> **Know the term, then check the rules.** A named layer is easier to test than a vague suspicion.\n\n**Read the rules**\n\n```bash\nkubectl get netpol -A\n```\n"},{"id":"p512","n":"512","slug":"the-network-observability-path-drawn","title":"The network observability path, drawn","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Observability","SRE","DevOps","eBPF"],"series_name":"Network","design":"architecture","script":"","date":"2030-01-03","files":{"it":"notes/p512-the-network-observability-path-drawn.md","devsecops":"notes/p512-the-network-observability-path-drawn.md"},"md":"# The network observability path, drawn\n\nA latency alert fires and three dashboards disagree about which hop is slow. The observability path is how you stop guessing.\n\n## Quick reference\n\n- **Sources:** Flow logs (FLOW) · eBPF probes (EBPF) · Interface counters (NIC)\n- **Pipeline:** Collector (COLL) · Time-series store (TSDB)\n- **Action:** Alerts (PAGE) · Dashboards (DASH) · Runbooks (RUN)\n\n**Request flow:** Flow logs → Collector → Time-series store → Alerts; Time-series store → Dashboards; Time-series store → Runbooks; eBPF probes → Collector; Interface counters → Collector\n\n## Explained\n\n### Sources\n\nFlow logs record who talked to whom, how many bytes moved and whether the security group accepted the flow. eBPF probes read the kernel's socket and scheduler events without copying every packet, which is how retransmits and queue time become visible. Interface counters are the cheapest signal: bytes, packets, drops and errors straight from the wire.\n\n### Pipeline\n\nA collector receives all three streams, batches them and adds the labels that make them joinable: account, region, node and pod. The time-series store keeps those labels for as long as the retention budget allows, and cardinality, not volume, is what makes it expensive. Sampling is a budget decision, not a technical one.\n\n### Action\n\nAlerts fire on symptoms worth waking for. Dashboards put the path beside the number. Runbooks turn the page into three commands and a rollback. An alert without a runbook is only a notification.\n\nThe drawing builds top to bottom: the three source nodes feed the collector, the collector writes to the store, and the store fans out to alerts, dashboards and runbooks. A packet then travels each arrow in turn.\n\n> **The honest caveat:** flow logs aggregate, eBPF depends on the kernel, and counters cannot say which flow dropped. None of the three is complete alone.\n\n> **The lesson:** instrument the path, not the box.\n\n## In one line\n\n> **Measure the path, not the box.** A counter without a path is a number without an owner.\n\n**Read the counters**\n\n```bash\ncurl -s localhost:9100/metrics | head\n```\n"},{"id":"p513","n":"513","slug":"the-packet-capture-cheat-sheet","title":"The packet capture cheat sheet","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Tcpdump","Wireshark","Troubleshooting","SRE"],"series_name":"Network","design":"cheatsheet","script":"","date":"2030-01-06","files":{"it":"notes/p513-the-packet-capture-cheat-sheet.md","devsecops":"notes/p513-the-packet-capture-cheat-sheet.md"},"md":"# The packet capture cheat sheet\n\nAt 02:00 the API is slow, every dashboard is green, and the only evidence left is on the wire. Packet capture is the tool that turns an argument into a file.\n\n## Quick reference\n\n### Capture\n\n```bash\ntcpdump -i any -w /tmp/cap.pcap\ntcpdump -i eth0 -s 0 -w cap.pcap\ntshark -i eth0 -w /tmp/cap.pcapng\ntcpdump -i eth0 -c 100 -nn\ntcpdump -i eth0 -w cap.pcap -U\n```\n\n### Filter\n\n```bash\ntcpdump -i eth0 port 443\ntcpdump -i eth0 host 10.0.0.5\ntcpdump 'tcp[tcpflags] & tcp-syn != 0'\ntcpdump -i eth0 net 10.0.0.0/24\ntshark -i eth0 -f \"port 53\"\n```\n\n### Read\n\n```bash\ntcpdump -r cap.pcap -nn -tttt\ntshark -r cap.pcap -Y http.request\ntshark -r cap.pcap -q -z conv,tcp\ntcpdump -r cap.pcap -A port 80\ntshark -r cap.pcap -T fields -e ip.src\n```\n\n### Rotate\n\n```bash\ntcpdump -i eth0 -G 60 -w cap-%H%M.pcap\ntcpdump -i eth0 -C 100 -w cap.pcap\ntcpdump -i eth0 -W 5 -C 100 -w cap.pcap\ntcpdump -i eth0 -z gzip -w cap.pcap\ntshark -i eth0 -b duration:60 -w c.pcapng\n```\n\n## Explained\n\n### Capture\n\nStart with `tcpdump -i any -w /tmp/cap.pcap`, or `tshark -i eth0 -w /tmp/cap.pcapng` when you want the newer file format. Add -s 0 to keep the whole packet, -nn to stop the resolver turning addresses into names under load, and -U to write each packet as it arrives rather than in blocks.\n\n### Filter\n\nA capture without a filter fills the disk. Keep it narrow: `tcpdump -i eth0 port 443`, host 10.0.0.5, or 'tcp[tcpflags] & tcp-syn != 0' for handshakes. `tshark -f \"port 53\"` applies the same BPF before anything is written, which is cheaper than filtering later.\n\n### Read\n\nRead the file with `tcpdump -r cap.pcap -nn -tttt` for absolute timestamps, `tshark -Y http.request` for one protocol, and `tshark -q -z` conv,tcp for the conversation summary that shows who sent the most.\n\n### Rotate\n\nOn a busy host, rotate: -G 60 starts a file a minute, -C 100 at a hundred megabytes, -W 5 keeps five, and -z gzip compresses each one as it closes. `tshark -b duration:60` does the same with one file a minute.\n\n> **The honest caveat:** a capture on one host sees only traffic that reaches that host, and a pcap can hold credentials, so treat it like production data and delete it on a schedule.\n\n> **The lesson:** capture the smallest slice that can still answer the question.\n\n## In one line\n\n> **Capture first, argue later.** A pcap settles a disagreement faster than a dashboard.\n\n**Capture, then read**\n\n```bash\ntcpdump -i any -w /tmp/cap.pcap\n```\n"},{"id":"p514","n":"514","slug":"latency-bandwidth-and-throughput-drawn","title":"Latency, bandwidth and throughput, drawn","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Latency","Performance","SRE","DevOps"],"series_name":"Network","design":"explainer","script":"","date":"2030-01-08","files":{"it":"notes/p514-latency-bandwidth-and-throughput-drawn.md","devsecops":"notes/p514-latency-bandwidth-and-throughput-drawn.md"},"md":"# Latency, bandwidth and throughput, drawn\n\nA 1 Gbps link and a 100 ms round trip can feel slower than a home connection, and the reason is arithmetic, not hardware.\n\n## Quick reference\n\n### Round-trip time (RTT)\n\n- One packet there and back\n- TCP handshake: one RTT\n- TLS 1.3: one more RTT\n- Add one for the request\n\n### Bandwidth\n\n- Capacity of the narrowest link\n- Bits per second, not seconds\n- A wider pipe does not shorten RTT\n\n### Throughput\n\n- What actually arrives\n- Window divided by RTT\n- 64 KB at 100 ms is about 5 Mbps\n\n### The bottleneck\n\n- The slowest hop wins\n- Look for queues and firewalls\n- Loss makes the window smaller\n\n> **Note:** Latency and bandwidth are different budgets; a bigger pipe does not shorten the path.\n\n## Explained\n\n### Round trip\n\nRTT is the time a packet takes to go and come back. Ping reports it in milliseconds; the path holds two one-way trips, and only a symmetric path makes each half the RTT. A TCP handshake costs one RTT before data, TLS 1.3 costs another, and the request itself costs a third.\n\n### Bandwidth\n\nBandwidth is the pipe's capacity in bits per second. It belongs to the narrowest link, not to the server, and it says nothing about delay. Upgrading a circuit raises the ceiling without moving the floor.\n\n### Throughput\n\nThroughput is what arrives. With a 64 KB window and a 100 ms RTT, the sender can have 64 KB in flight, so throughput is 64 KB divided by 0.1 seconds, about 5 Mbps, however fast the link claims to be.\n\n### The bottleneck\n\nThe bottleneck is the slowest hop, and it is not always the one with the least bandwidth. A stateful firewall, a saturated queue or a resolver answering slowly can set the pace for a path with capacity to spare.\n\n> **The honest caveat:** ping measures ICMP, which some routers deprioritise; time a TCP handshake to the port your users actually call.\n\n> **The lesson:** latency and bandwidth are separate budgets, and you spend both.\n\n## In one line\n\n> **Distance sets the floor.** You can buy bandwidth; you cannot buy a shorter round trip.\n\n**Measure the rtt**\n\n```bash\nping -c 5 api.example.com\n```\n"},{"id":"p515","n":"515","slug":"an-mtu-calculator-in-python","title":"An MTU calculator in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Networking","MTU","Python","Linux","Troubleshooting"],"series_name":"Network","design":"code","script":"mtu_calc.py","date":"2030-01-10","files":{"it":"notes/p515-an-mtu-calculator-in-python.md","devsecops":"notes/p515-an-mtu-calculator-in-python.md"},"md":"# An MTU calculator in Python\n\nA VXLAN overlay works in staging and stalls in production, and the difference is fifty bytes of header. The MTU calculator makes that cost visible before the packet leaves.\n\n## Quick reference\n\nThe full script is at the end of this note: `mtu_calc.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 mtu_calc.py --profile vxlan --mtu 1500\n```\n\n**Example output**\n\n```text\nvxlan MTU 1450 MSS 1410\nFRAG 1500 > 1450\n```\n\n## Explained\n\n### What it computes\n\nThe script subtracts the encapsulation overhead from the underlay MTU and prints the usable MTU and TCP MSS. Plain Ethernet is 1500, and 1460 after the 20-byte IPv4 header and the 20-byte TCP header. VXLAN adds 50 bytes: 14 for the outer Ethernet header, 20 for the outer IP, 8 for UDP and 8 for the VXLAN header, so the inner MTU is 1450 and the MSS is 1410. GRE costs 24 bytes, leaving 1476 and 1436. IPsec is not one number: ESP overhead changes with the cipher and integrity algorithm, so the script takes a flag and defaults to 50.\n\n### Why it fragments\n\nIf the inner interface still claims 1500 and a packet leaves with the don't-fragment bit set, the underlay cannot forward it. A router that would fragment without DF drops it and returns ICMP fragmentation-needed; if a firewall eats it, the connection black-holes.\n\n### How to use it\n\nRun `python3 mtu_calc.py --profile vxlan --mtu 1500`. The output shows the profile, usable MTU and MSS, then the verdict: FRAG 1500 > 1450, exit 1. A clean profile prints OK and exits 0; a bad argument exits 2.\n\n> **The honest caveat:** the underlay may be 9001 in a cloud VPC, 1492 on PPPoE or lower on mobile, and IPsec overhead depends on the proposal, so measure the path.\n\n> **The lesson:** every tunnel spends bytes from the same budget.\n\n## In one line\n\n> **Leave room for the outer header.** A tunnel that ignores overhead fragments at the worst moment.\n\n**Check the path**\n\n```bash\nping -M do -s 1472 10.0.0.5\n```\n\n## Script: mtu_calc.py\n\nDownload: [scripts/mtu_calc.py](../scripts/mtu_calc.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Compute the usable MTU and TCP MSS under common tunnel overheads.\n\nUsage:\n    python3 mtu_calc.py\n    python3 mtu_calc.py --profile vxlan --mtu 1500\n    python3 mtu_calc.py --profile gre --underlay 9001\n    python3 mtu_calc.py --profile ipsec --ipsec-overhead 73\n\nProfiles and overhead, in bytes:\n    plain    0   no encapsulation\n    vxlan   50   outer Ethernet 14 + outer IPv4 20 + UDP 8 + VXLAN 8\n    gre     24   outer IPv4 20 + GRE 4\n    ipsec  varies, 50 by default; ESP and outer IP, NAT-T adds 8 more\n\nThe usable MTU is the underlay MTU minus the profile overhead. The TCP MSS is\nthe usable MTU minus 40 bytes, the 20-byte IPv4 header plus the 20-byte TCP\nheader. If the configured inner MTU is larger than the usable MTU, the profile\nwill fragment when the don't-fragment bit is clear, or black-hole when ICMP\nfragmentation-needed is filtered, so the exit code is 1.\n\nCron (daily, 08:15):  15 8 * * *  python3 mtu_calc.py --profile vxlan --mtu 1450\nCI step: python3 mtu_calc.py --profile vxlan --mtu 1450   # fails if it drifts\n\nExit codes: 0 clean, 1 the configured MTU does not fit, 2 usage or input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\n\nOVERHEAD = {\"plain\": 0, \"vxlan\": 50, \"gre\": 24}\nDEFAULT_IPSEC = 50\nIPV4_TCP = 40\nMIN_MTU = 68\nMAX_MTU = 65535\n\n\ndef parse_args(argv=None):\n    \"\"\"Parse the command line.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Compute usable MTU and TCP MSS for a tunnel profile.\")\n    parser.add_argument(\"--profile\", choices=(\"plain\", \"vxlan\", \"gre\", \"ipsec\"),\n                        default=\"plain\", help=\"encapsulation profile (plain)\")\n    parser.add_argument(\"--underlay\", type=int, default=1500, metavar=\"BYTES\",\n                        help=\"underlay MTU in bytes (1500)\")\n    parser.add_argument(\"--mtu\", type=int, default=1500, metavar=\"BYTES\",\n                        help=\"configured inner MTU to check (1500)\")\n    parser.add_argument(\"--ipsec-overhead\", type=int, default=DEFAULT_IPSEC,\n                        metavar=\"BYTES\",\n                        help=\"IPsec overhead for the ipsec profile (50)\")\n    return parser.parse_args(argv)\n\n\ndef overhead_for(profile, ipsec_overhead):\n    \"\"\"Return the encapsulation overhead in bytes for one profile.\"\"\"\n    if profile == \"ipsec\":\n        return ipsec_overhead\n    return OVERHEAD[profile]\n\n\ndef compute(profile, underlay, ipsec_overhead):\n    \"\"\"Return (overhead, usable MTU, TCP MSS) for one profile.\"\"\"\n    overhead = overhead_for(profile, ipsec_overhead)\n    usable = underlay - overhead\n    return overhead, usable, usable - IPV4_TCP\n\n\ndef main(argv=None):\n    \"\"\"Print one profile and return the exit code.\"\"\"\n    args = parse_args(argv)\n\n    if not MIN_MTU <= args.underlay <= MAX_MTU:\n        print(\"ERROR --underlay must be %d..%d bytes\" % (MIN_MTU, MAX_MTU),\n              file=sys.stderr)\n        return 2\n    if not MIN_MTU <= args.mtu <= MAX_MTU:\n        print(\"ERROR --mtu must be %d..%d bytes\" % (MIN_MTU, MAX_MTU),\n              file=sys.stderr)\n        return 2\n    if not 0 <= args.ipsec_overhead <= 200:\n        print(\"ERROR --ipsec-overhead must be 0..200 bytes\", file=sys.stderr)\n        return 2\n\n    overhead, usable, mss = compute(args.profile, args.underlay,\n                                    args.ipsec_overhead)\n    if mss <= 0:\n        print(\"ERROR underlay %d is too small for profile %s\"\n              % (args.underlay, args.profile), file=sys.stderr)\n        return 2\n\n    print(\"%-9s %s\" % (\"profile\", args.profile))\n    print(\"%-9s %d bytes\" % (\"overhead\", overhead))\n    print(\"%-9s %d bytes\" % (\"underlay\", args.underlay))\n    print(\"%-9s %d bytes\" % (\"usable\", usable))\n    print(\"%-9s %d bytes\" % (\"TCP MSS\", mss))\n\n    over = args.mtu - usable\n    if over > 0:\n        print(\"FRAG configured %d bytes is %d over the usable MTU\"\n              % (args.mtu, over))\n        return 1\n    print(\"OK configured %d bytes fits the usable MTU\" % args.mtu)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p516","n":"516","slug":"a-bandwidth-delay-product-in-python","title":"A bandwidth-delay product in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Networking","TCP","Python","Performance","SRE"],"series_name":"Network","design":"code","script":"bandwidth_delay.py","date":"2030-01-13","files":{"it":"notes/p516-a-bandwidth-delay-product-in-python.md","devsecops":"notes/p516-a-bandwidth-delay-product-in-python.md"},"md":"# A bandwidth-delay product in Python\n\nA file transfer between two regions runs at 12 Mbps on a 1 Gbps link, and the network is not broken. The bandwidth-delay product explains why.\n\n## Quick reference\n\nThe full script is at the end of this note: `bandwidth_delay.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 bandwidth_delay.py --rtt 40 --mbps 1000 --window 65535\n```\n\n**Example output**\n\n```text\nBDP 5000000 B, have 65535 B\n13.1 Mbps ERR undersized\n```\n\n## Explained\n\n### What it computes\n\nThe bandwidth-delay product is bandwidth times RTT. At 1 Gbps and 40 ms, the pipe holds 1,000,000,000 bits per second times 0.040 seconds, divided by 8, which is 5 MB. That is how much data must be in flight to fill the link.\n\n### Why the window matters\n\nTCP can only have as much unacknowledged data as its window allows. The classic 64 KB window at 40 ms gives 65,535 bytes divided by 0.04 seconds, about 1.6 MB per second or 13 Mbps. The window, not the link, is the limit; window scaling raises the ceiling but the arithmetic stays.\n\n### How to use it\n\nRun `python3 bandwidth_delay.py --rtt 40 --mbps 1000 --window 65535`. It prints the BDP, compares the configured window with it and reports the throughput that window buys. A window below the BDP prints ERR and exits 1; a window at or above it exits 0. Bad input exits 2.\n\n> **The honest caveat:** BDP is an upper bound. Loss, application stalls and the receive window all reduce the real rate, and a bigger window does nothing if the sender never fills it.\n\n> **The lesson:** on a long path, the window is the network.\n\n## In one line\n\n> **A pipe is only as wide as its window.** Throughput is a window divided by a round trip.\n\n**Raise the window**\n\n```bash\nsysctl -w net.core.rmem_max=8388608\n```\n\n## Script: bandwidth_delay.py\n\nDownload: [scripts/bandwidth_delay.py](../scripts/bandwidth_delay.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Compute the bandwidth-delay product and the window it demands.\n\nUsage:\n    python3 bandwidth_delay.py\n    python3 bandwidth_delay.py --rtt 40 --mbps 1000\n    python3 bandwidth_delay.py --rtt 40 --mbps 1000 --window 65535\n    python3 bandwidth_delay.py --rtt 150 --mbps 10000 --window 262144\n\nUnits: --rtt is milliseconds, --mbps is megabits per second (1,000,000 bits)\nand --window is bytes. The bandwidth-delay product is:\n\n    BDP = bandwidth in bits per second x RTT in seconds / 8\n\nThe result is in bytes: the data that must be in flight to fill the path.\nThe window needed for line rate is the BDP. The throughput of a window is:\n\n    window in bytes x 8 / RTT in seconds\n\nin bits per second, printed in megabits. A window below the BDP is\nwindow-limited: the path is not the limit, the receive window is, and the\nexit code is 1.\n\nCron (daily, 08:30):  30 8 * * *  python3 bandwidth_delay.py --window 5000000\nCI step: python3 bandwidth_delay.py --rtt 40 --window 5000000  # fails if small\n\nExit codes: 0 clean or no window to check, 1 the window is undersized,\n2 usage or input error.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport math\nimport sys\n\nDEFAULT_RTT_MS = 40.0\nDEFAULT_MBPS = 1000.0\nDEFAULT_MSS = 1460\n\n\ndef parse_args(argv=None):\n    \"\"\"Parse the command line.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Compute BDP in bytes and the window needed for line rate.\")\n    parser.add_argument(\"--rtt\", type=float, default=DEFAULT_RTT_MS,\n                        metavar=\"MS\", help=\"round-trip time in ms (40)\")\n    parser.add_argument(\"--mbps\", type=float, default=DEFAULT_MBPS,\n                        metavar=\"MBPS\", help=\"bandwidth in Mbps (1000)\")\n    parser.add_argument(\"--window\", type=int, default=None, metavar=\"BYTES\",\n                        help=\"TCP window to check in bytes (none)\")\n    parser.add_argument(\"--mss\", type=int, default=DEFAULT_MSS, metavar=\"BYTES\",\n                        help=\"MSS used to count segments (1460)\")\n    return parser.parse_args(argv)\n\n\ndef bdp_bytes(mbps, rtt_ms):\n    \"\"\"Return the bandwidth-delay product in bytes.\"\"\"\n    return mbps * 1_000_000 / 8 * (rtt_ms / 1000)\n\n\ndef window_mbps(window_bytes, rtt_ms):\n    \"\"\"Return the throughput of a window in Mbps.\"\"\"\n    return window_bytes * 8 / (rtt_ms / 1000) / 1_000_000\n\n\ndef main(argv=None):\n    \"\"\"Print the BDP, the line-rate window and one verdict; return an exit code.\"\"\"\n    args = parse_args(argv)\n\n    if args.rtt <= 0:\n        print(\"ERROR --rtt must be greater than zero\", file=sys.stderr)\n        return 2\n    if args.mbps <= 0:\n        print(\"ERROR --mbps must be greater than zero\", file=sys.stderr)\n        return 2\n    if args.window is not None and args.window < 0:\n        print(\"ERROR --window must be zero or more bytes\", file=sys.stderr)\n        return 2\n    if args.mss <= 0:\n        print(\"ERROR --mss must be greater than zero\", file=sys.stderr)\n        return 2\n\n    bdp = bdp_bytes(args.mbps, args.rtt)\n    need = math.ceil(bdp)\n    segments = math.ceil(bdp / args.mss)\n\n    print(\"RTT %.1f ms   bandwidth %.1f Mbps\" % (args.rtt, args.mbps))\n    print(\"BDP %d bytes (%.2f MB, %d segments of %d)\"\n          % (need, bdp / 1_000_000, segments, args.mss))\n    print(\"window for line rate %d bytes\" % need)\n\n    if args.window is None:\n        print(\"no --window given; nothing to check\")\n        return 0\n\n    rate = window_mbps(args.window, args.rtt)\n    share = 100.0 * args.window / bdp if bdp else 0.0\n    print(\"window %d bytes -> %.1f Mbps (%.1f%% of line rate)\"\n          % (args.window, rate, share))\n\n    if args.window < bdp:\n        print(\"ERR window is %d bytes below the BDP\" % (need - args.window))\n        return 1\n    print(\"OK window covers the BDP\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p517","n":"517","slug":"6-network-numbers","title":"6 network numbers","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Latency","TCP","SRE","Performance"],"series_name":"Network","design":"numbers","script":"","date":"2030-01-15","files":{"it":"notes/p517-6-network-numbers.md","devsecops":"notes/p517-6-network-numbers.md"},"md":"# 6 network numbers\n\nSix numbers explain most of the tickets that arrive as \"it is slow\". None of them is a speed test.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **1500** | Ethernet MTU | The payload a standard frame carries; 1460 is TCP's MSS. | mtu |\n| **1 RTT** | TCP handshake | SYN, SYN-ACK, ACK before data; the third leg can carry it. | tcp |\n| **1 RTT** | TLS 1.3 handshake | One round trip after TCP, so the first byte waits two. | tls |\n| **5 MB** | BDP at 1 Gbps, 40 ms | Bandwidth x RTT / 8: the data in flight to fill the pipe. | bdp |\n| **150 ms** | Cross-region RTT | Same continent 10-40 ms; across an ocean 150-250 ms. | rtt |\n| **1%** | Packet loss | Enough to cut a 40 ms path to single-digit Mbps. | loss |\n\n## Explained\n\n### Size\n\n1500 bytes is the Ethernet MTU, the payload a standard frame carries, and 1460 is what TCP gets after 20 bytes of IP and 20 of TCP header. Jumbo frames push the MTU to 9000 on a controlled LAN, and every hop must agree or the path falls back to 1500.\n\n### Round trips\n\nA TCP handshake costs 1 RTT before data, and TLS 1.3 costs 1 more, so the first byte waits 2 RTT and the request waits a third.\n\n### The pipe\n\nThe BDP at 1 Gbps and 40 ms is 5 MB, the data in flight needed to fill the link. A 64 KB window there allows about 13 Mbps, which is why a long path with a small window looks like a slow link.\n\n### Distance and loss\n\nCross-region RTT is 150 ms across an ocean, against 10-40 ms on the same continent, so three round trips cost 450 ms before the server has done any work. A 1% packet loss can pull a 40 ms path into single-digit Mbps, because TCP reads loss as congestion and cuts the window.\n\n> **The honest caveat:** these are round numbers from textbook paths. Real RTT depends on routing, peering and queueing, and your 150 ms may be 90 on a good day.\n\n> **The lesson:** put a number on the path before you blame the application.\n\n## In one line\n\n> **Distance costs round trips.** Every handshake, window and retransmit is paid in RTT.\n\n**Measure one path**\n\n```bash\nmtr -rw api.example.com\n```\n"},{"id":"p518","n":"518","slug":"network-health-at-a-glance","title":"Network health at a glance","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","Observability","SRE","Dashboard","DevOps"],"series_name":"Network","design":"dashboard","script":"","date":"2030-01-17","files":{"it":"notes/p518-network-health-at-a-glance.md","devsecops":"notes/p518-network-health-at-a-glance.md"},"md":"# Network health at a glance\n\nThe pager fires on latency, and the dashboard that explains it has six tiles, none of which is CPU.\n\n## Quick reference\n\n**Status:** Last 15 minutes\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| p95 latency | **48 ms** | edge to origin |\n| Packet loss | **0.2%%** | inter-AZ path |\n| Retransmits | **1.8%** | of TCP segments |\n| DNS failures | **0.4%%** | SERVFAIL and timeouts |\n| LB 5xx | **0.12%** | server-side requests |\n| Bandwidth headroom | **38%** | peak vs provisioned |\n\n## Explained\n\n### The path\n\np95 latency is 48 ms from edge to origin, which means one request in twenty is slower and the average is hiding it. A missing tile is a blind spot, not a zero. Packet loss is 0.2% on the inter-AZ path, low enough to look healthy and high enough to cost retransmits. Retransmits are 1.8% of TCP segments, the tile that usually explains the other two.\n\n### The edges\n\nDNS failures are 0.4%, and the split matters: NXDOMAIN is a client asking for a name that does not exist, while SERVFAIL and timeouts are yours. LB 5xx is 0.12% of requests, server-side failures only; a rise in 4xx is a client or a broken deploy, not a sick backend.\n\n### The margin\n\nBandwidth headroom is 38% at peak, measured against provisioned egress; below 20% a single large response can fill the pipe. Read the six together: loss explains retransmits, retransmits explain latency, and headroom explains why it got worse at noon. One large response at 38% headroom is a spike, not a capacity plan.\n\n> **The honest caveat:** p95 hides the p99 that a few users remember, and a tile is only comparable when its label names the window and the population.\n\n> **The lesson:** a network board should point at the next query, not answer it.\n\n## In one line\n\n> **Six tiles, one direction.** A board should point at the next query, not answer it.\n\n**Follow the drop:** nstat -az TcpRetransSegs\n"},{"id":"p519","n":"519","slug":"anatomy-of-a-network-incident","title":"Anatomy of a network incident","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","IncidentResponse","SRE","OnCall","DevOps"],"series_name":"Network","design":"timeline","script":"","date":"2030-01-20","files":{"it":"notes/p519-anatomy-of-a-network-incident.md","devsecops":"notes/p519-anatomy-of-a-network-incident.md"},"md":"# Anatomy of a network incident\n\nThe alert says latency, the graph says packet loss, and the last deploy was four hours ago. A network incident has six stages: alert, localise, mitigate, fix, verify and review. Most teams skip the last two.\n\n## Quick reference\n\n| Time | Step | What happens |\n|:--|:--|:--|\n| **T+0** | Alert | p95 crosses the SLO; the on-call opens the network board. |\n| **T+5** | Localise | Which region, which hop: mtr, the AZ split, the deploy log. |\n| **T+15** | Mitigate | Shift traffic, add capacity or roll back; stop the bleeding first. |\n| **T+1h** | Fix | Repair the cause: a route, a full queue, a health check that lied. |\n| **T+2h** | Verify | Watch p95 and loss through a full traffic cycle, not five quiet minutes. |\n| **T+1d** | Review | Blameless: what the alert missed, what the runbook lacked, who owns it. |\n\n## Explained\n\n### First hour\n\nAt T+0 the p95 alert crosses the SLO and the on-call opens the network board, not the service dashboard. The first question is not why; it is how many users and which regions. At T+5 the job is to localise: which region, which hop, which direction. mtr shows where the RTT steps up, the per-AZ split shows whether every zone is affected, and the deploy log says whether anything changed.\n\n### Repair\n\nAt T+15 mitigation starts before diagnosis ends: withdraw the bad route, shift traffic to the healthy region, add capacity or roll back. Stopping the bleeding is a separate skill from finding the cut. At T+1h the fix repairs the cause, whether that is a route, a full queue, a health check that never failed or an MTU that only breaks on one path.\n\n### After\n\nAt T+2h verification watches p95 and loss through a full traffic cycle, not five quiet minutes; a cache warm-up can hide a broken path for ten minutes. At T+1d the review asks what the alert missed and what the runbook lacked, and assigns both an owner and a date.\n\n> **The honest caveat:** the timeline is reconstructed from chat and alert timestamps that disagree by minutes, so record decisions as you make them.\n\n> **The lesson:** mitigation buys the time a root cause needs.\n\n## In one line\n\n> **Stop the bleeding, then find the cut.** Mitigation buys the time a root cause needs.\n\n**Check the path**\n\n```bash\nmtr -rw --report-cycles 10 10.0.0.5\n```\n"},{"id":"p520","n":"520","slug":"quiz-3-network-operations-questions","title":"Quiz: 3 network operations questions","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","TCP","Observability","SRE","CloudNative"],"series_name":"Network","design":"quiz","script":"","date":"2030-01-22","files":{"it":"notes/p520-quiz-3-network-operations-questions.md","devsecops":"notes/p520-quiz-3-network-operations-questions.md"},"md":"# Quiz: 3 network operations questions\n\nThree questions separate a team that watches the network from one that watches the graph. Answer before you read on.\n\n## Quick reference\n\n**Q1. Why do retransmits rise before latency does when loss begins?**\n\n- a) Retransmits make the RTT longer\n- b) TCP repairs loss below the request\n- c) Latency is only sampled hourly\n\n> **Answer:** b) TCP repairs loss below the request. A lost segment is resent, and the counter increments immediately. The request only slows when the repair takes a timeout or a second loss.\n\n**Q2. What does a rising bandwidth-delay product imply?**\n\n- a) The link is getting faster\n- b) The round trip is falling\n- c) More bytes must be in flight\n\n> **Answer:** c) More bytes must be in flight. BDP is bandwidth times RTT. More capacity or more distance means a larger window is needed to fill the pipe; a fixed window loses throughput.\n\n**Q3. Why do flow logs miss a packet dropped in the fabric?**\n\n- a) They capture only TCP traffic\n- b) They record flows, not packets\n- c) They sample one packet in ten\n\n> **Answer:** b) They record flows, not packets. A flow log aggregates counters after the fact. A packet dropped before its flow is recorded, or inside the network, leaves no entry; a REJECT entry is a policy drop.\n\n## Explained\n\n### Question one\n\nWhy do retransmits rise before latency does when loss begins? TCP repairs a lost segment below the request: the counter increments on every resend, while the request only slows when the repair takes a timeout or a second loss. The average barely moves because the repair succeeds; the counter sees every packet, including the ones that never reached the server.\n\n### Question two\n\nWhat does a rising BDP imply? Bandwidth times RTT is the data the path can hold. If either grows, the window must grow with it or throughput falls, which is why a new region needs a bigger window before it needs a faster link.\n\n### Question three\n\nWhy do flow logs miss a packet dropped in the fabric? A flow log records flows, not packets: it aggregates counters after the fact, so a packet dropped before its flow is accounted for never appears. A REJECT entry shows a policy drop; silence shows nothing at all. That is why a capture, not a dashboard, settles a drop argument.\n\n> **The honest caveat:** counters are per host and per direction, so a retransmit seen at the sender may be invisible at the receiver.\n\n> **The lesson:** loss shows up in counters before it shows up in percentiles.\n\n## In one line\n\n> **Watch the repair, not the average.** Loss shows up in counters before it shows up in percentiles.\n\n**Read tcp stats:** nstat -az TcpRetransSegs\n"},{"id":"p521","n":"521","slug":"network-terms-decoded","title":"Network terms, decoded","sections":["it","devsecops"],"topic":"Networking & DNS","tags":["Networking","TCP","SRE","Observability","Engineering"],"series_name":"Network","design":"reveal","script":"","date":"2030-01-24","files":{"it":"notes/p521-network-terms-decoded.md","devsecops":"notes/p521-network-terms-decoded.md"},"md":"# Network terms, decoded\n\nSix words get used in every network incident, and two of them are usually being defined differently by the people in the room.\n\n## Quick reference\n\n- **RTT** — A packet there and back. Ping measures it; half is one-way only on a symmetric path.\n- **BDP** — Bandwidth times RTT divided by 8: the bytes in flight needed to fill the pipe.\n- **MTU** — The largest payload one frame carries. Ethernet defaults to 1500 bytes.\n- **Anycast** — One IP announced from many places; BGP usually picks the nearest, so the path can change.\n- **Overlay** — A network built on another. VXLAN wraps frames in UDP and adds 50 bytes.\n- **Retransmit** — A segment sent again because no ACK arrived; it rises before latency does.\n\n## Explained\n\n### Path\n\nRTT is the round-trip time: a packet there and back. Ping measures it, and half of it is the one-way latency only when the path is symmetric. BDP is bandwidth times RTT divided by 8, the bytes that must be in flight to fill the pipe; at 1 Gbps and 40 ms it is 5 MB.\n\n### Frames\n\nMTU is the largest payload one frame carries; Ethernet defaults to 1500 bytes, and every tunnel header comes out of that budget. Anycast is one IP address announced from many places, so BGP usually picks the nearest and the path can change without the address changing.\n\n### Hidden layers\n\nOverlay is a network built on another: VXLAN wraps the frame in UDP, adds 50 bytes, and hides the underlay from the workload. Retransmit is a segment sent again because no ACK arrived, a counter that rises before latency does.\n\n> **The honest caveat:** a definition is not a measurement. Two teams can agree on RTT and still quote different numbers, because one timed a ping and the other timed a request.\n\n> **The lesson:** a network you can describe is a network you can debug.\n\nThat is the series. Fifty posts, from the first handshake to a network you can operate, five phases and ten Python scripts. Thank you for following along.\n\n## In one line\n\n> **Name the term, measure the path.** A network you can describe is a network you can debug.\n\n**Count the hops**\n\n```bash\nmtr -rw api.example.com\n```\n"},{"id":"p522","n":"522","slug":"the-security-automation-pipeline-drawn","title":"The security automation pipeline, drawn","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","Automation","CyberSecurity","DevOps"],"series_name":"PythonSec","design":"architecture","script":"","date":"2030-01-27","files":{"devsecops":"notes/p522-the-security-automation-pipeline-drawn.md"},"md":"# The security automation pipeline, drawn\n\nA nightly check turns red and three people open three terminals. One is reading a command, one is staring at a log, one is editing the rule that fired. They are all inside the same pipeline, just at different tiers.\n\n## Quick reference\n\n- **Collect:** Command output (CMD) · Log file (LOG)\n- **Parse:** JSON parser (JSON) · Regex pass (RE)\n- **Decide:** Rule set (RULE) · Report (OUT)\n\n**Request flow:** Command output → JSON parser → Rule set → Report; Log file → Regex pass → Rule set\n\n**Around it:** Exit code (EXIT)\n\n## Explained\n\n### Collect\n\nCommand output is the first source: the text `sshd -T` or systemctl show prints. It is easy to capture and easy to trust too much, because a locale change can rewrite its shape. A log file is richer: timestamped lines, repeated warnings, and the evidence of what happened before the check ran.\n\n### Parse\n\nA JSON parser reads structured output: field names, nested values, types. It fails loudly on malformed input, which is what you want in a check. A regex pass reads the files that never had a schema: sshd_config, sudoers, cron. Anchor each pattern to a line and test it against a fixture that is allowed to be ugly.\n\n### Decide\n\nA rule set turns parsed facts into findings. One rule, one condition, one message. The report is the only thing a human reads, so it names the file, the line and the check. The exit code is the last tier: 0 clean, 1 finding, 2 the check itself broke. That code is what the pipeline believes.\n\n> **The honest caveat:** a pipeline can parse its own output and still agree with a mistake. A fixture per tier is the only proof that the failure is visible.\n\n> **The lesson:** draw the three tiers, then test each one where it breaks.\n\n## In one line\n\n> **Three tiers, three places to fail.** A broken audit can fail before it finds anything.\n\n**Fail the job**\n\n```bash\npython3 audit.py || exit 1\n```\n"},{"id":"p523","n":"523","slug":"the-python-stdlib-for-devsecops","title":"The Python stdlib for DevSecOps","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DevSecOps","StandardLibrary","Automation","CyberSecurity"],"series_name":"PythonSec","design":"cheatsheet","script":"","date":"2030-01-29","files":{"devsecops":"notes/p523-the-python-stdlib-for-devsecops.md"},"md":"# The Python stdlib for DevSecOps\n\nA script needs to read a file, run a command, open a socket and hash a value. Four blocks, no pip install, no dependency review, no supply-chain ticket. All of them ship with Python, and none of them needs a lockfile. That is one fewer artefact to scan, pin and patch.\n\n## Quick reference\n\n### Files\n\n```bash\nfrom pathlib import Path\nPath(p).read_text(encoding=\"utf-8\")\np.stat().st_mode & 0o777\nsorted(p.parent.glob(\"*.conf\"))\n```\n\n### Processes\n\n```bash\nimport subprocess, shlex\nshlex.split(cmd, posix=True)\nsubprocess.run(argv, check=True)\nsubprocess.run(argv, capture_output=True)\n```\n\n### Network\n\n```bash\nimport socket, ssl\nsocket.create_connection((h, p), 5)\nssl.create_default_context()\nsock.getpeercert()\n```\n\n### Crypto\n\n```bash\nimport hashlib, hmac, secrets\nhashlib.sha256(data).hexdigest()\nhmac.compare_digest(a, b)\nsecrets.token_hex(32)\n```\n\n## Explained\n\n### Files\n\nfrom pathlib import Path starts it. `Path(p).read_text(encoding=\"utf-8\")` returns the whole file as a string. `p.stat().st_mode & 0o777` gives the permission bits without parsing ls. `sorted(p.parent.glob(\"*.conf\"))` lists the config files an audit should read.\n\n### Processes\n\nimport subprocess, shlex. `shlex.split(cmd, posix=True)` turns a command into an argument list without a shell. `subprocess.run(argv, check=True)` raises on a non-zero exit. `subprocess.run(argv, capture_output=True)` keeps stdout and stderr apart, which is the only way to report one without the other.\n\n### Network\n\nimport socket, ssl. `socket.create_connection((h, p), 5)` opens a TCP connection with a timeout. `ssl.create_default_context()` gives a verified TLS context. `sock.getpeercert()` returns the certificate as a dictionary of fields.\n\n### Crypto\n\nimport hashlib, hmac, secrets. `hashlib.sha256(data).hexdigest()` prints a digest. `hmac.compare_digest(a, b)` compares without a timing leak. `secrets.token_hex(32)` makes a token that is safe to use.\n\n> **The honest caveat:** the standard library gives you mechanism, not policy. It will read `/etc/shadow` if the process can, so permissions and scope are still your job.\n\n> **The lesson:** the shortest dependency list is the one already on the host.\n\n## In one line\n\n> **Import the tool, not the framework.** The standard library is already installed on the host.\n\n**Check one import**\n\n```bash\npython3 -c 'import ssl, hmac'\n```\n"},{"id":"p524","n":"524","slug":"four-rules-for-safe-automation","title":"Four rules for safe automation","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","Automation","CICD","CyberSecurity"],"series_name":"PythonSec","design":"explainer","script":"","date":"2030-01-31","files":{"devsecops":"notes/p524-four-rules-for-safe-automation.md"},"md":"# Four rules for safe automation\n\nA script that edits `/etc` while it audits `/etc` is not an audit. It is a change with a report attached. Four rules keep the difference obvious.\n\n## Quick reference\n\n### Read-only first\n\n- Open files with mode r, never w\n- No shell=True, no eval, no exec\n- A `--dry-run` path is the default\n\n### Exit codes are the API\n\n- 0 clean, 1 finding, 2 usage\n- Never exit 0 after a failed check\n- CI reads the code, not the prose\n\n### Idempotent by design\n\n- Run twice, get the same report\n- No append without a dedupe key\n- State stays outside the script\n\n### Log to stderr\n\n- stdout is the report, stderr the noise\n- One finding per line, same prefix\n- Mask values, never echo a secret\n\n## Explained\n\n### Read-only first\n\nOpen every file with mode r, never w. No shell=True, no eval, no exec: each turns data into code. A `--dry-run` path should be the default, with writes behind a flag someone has to type on purpose.\n\n### Exit codes are the API\n\n0 clean, 1 finding, 2 usage or unreadable input. A check that fails must never exit 0, or CI will believe it. The code is the contract; the prose is for the human who reads it afterwards.\n\n### Idempotent by design\n\nRun it twice and the report should match. Do not append without a dedupe key, and keep state outside the script: a lock file, a database, a ticket. The script becomes a function you can call again.\n\n### Log to stderr\n\nstdout is the report, stderr is the noise. One finding per line, same prefix, so grep and a parser agree. Mask values, never echo a secret, even when it is the thing that broke.\n\n> **The honest caveat:** read-only is a property of every code path, not a comment at the top. One subprocess call can undo it.\n\n> **The lesson:** the safest automation is the one you can run twice, on production, without asking.\n\n## In one line\n\n> **Read-only, predictable, quiet.** The rules are boring; the incident they prevent is not.\n\n**Dry run first**\n\n```bash\npython3 audit.py --dry-run\n```\n"},{"id":"p525","n":"525","slug":"an-sshd-config-audit-in-python","title":"An sshd_config audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","SSH","DevSecOps","Linux","CyberSecurity"],"series_name":"PythonSec","design":"code","script":"sshd_audit.py","date":"2030-02-03","files":{"it":"notes/p525-an-sshd-config-audit-in-python.md","devsecops":"notes/p525-an-sshd-config-audit-in-python.md"},"md":"# An sshd_config audit in Python\n\nA base image ships with PermitRootLogin yes because a build script needed it in 2021. Nobody remembers, and every host built from it still accepts root over SSH.\n\n## Quick reference\n\nThe full script is at the end of this note: `sshd_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 sshd_audit.py /tmp/sshd_config\n```\n\n**Example output**\n\n```text\nFINDING 4 checks failed\nALERT root-login, password-auth, 3des-cbc, port 2222\n```\n\n## Explained\n\n### What it checks\n\nFor most keywords the first value wins, as sshd_config documents. The script reports PermitRootLogin yes, PasswordAuthentication yes and PermitEmptyPasswords yes. It then splits Ciphers, MACs and KexAlgorithms and names every weak token, including CBC ciphers and SHA-1 MACs. A non-default Port is a finding unless a comment names that port and a firewall.\n\n### What the screen shows\n\nThe 14 visible lines read the file, check the three yes values, scan the weak list and collect Port numbers. On the dirty fixture they print two lines: FINDING 4 checks failed, then ALERT root-login, password-auth, 3des-cbc, port 2222. The full script extends those weak lists and keeps the same exit code.\n\n### How to run it\n\n`python3 sshd_audit.py /etc/ssh/sshd_config`, or a fixture in `/tmp`. `--allow-port 2222` records a port that is deliberately exposed. Exit 0 is clean, 1 is a finding, 2 is a usage, unreadable or empty input. Nothing is written and no service is reloaded, so it is safe in CI and in cron.\n\n> **The honest caveat:** this reads one file. Include directives and Match blocks are not merged, so audit the file sshd actually loads.\n\n> **The lesson:** an SSH default is a decision someone made, so make it visible.\n\n## In one line\n\n> **Root login is a policy, not a default.** sshd_config is a file you can read, so audit it like one.\n\n**Audit one file**\n\n```bash\npython3 sshd_audit.py /etc/ssh/sshd_config\n```\n\n## Script: sshd_audit.py\n\nDownload: [scripts/sshd_audit.py](../scripts/sshd_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"sshd_audit.py - audit an OpenSSH server configuration for weak settings.\n\nReads one sshd_config and reports the settings that widen the attack\nsurface: root login, password authentication, empty passwords, weak\nCiphers, MACs or key-exchange algorithms, and any non-default Port with\nno firewall note in the file.\n\nUsage:\n    python3 sshd_audit.py\n    python3 sshd_audit.py /etc/ssh/sshd_config\n    python3 sshd_audit.py /tmp/sshd_config --allow-port 2222\n    python3 sshd_audit.py --help\n\nCI (fail the job when the host template is weak):\n    python3 sshd_audit.py /etc/ssh/sshd_config || exit 1\n\nCron (weekly drift check):\n    0 6 * * 1 python3 /opt/s0sta/sshd_audit.py /etc/ssh/sshd_config\n\nExit codes: 0 no findings, 1 at least one finding, 2 usage error or\nunreadable, empty or directive-free input.\n\nRead-only: the file is opened for reading. Nothing is written, no\nservice is reloaded and no network connection is made.\n\nThe first value of a keyword wins, as sshd_config documents. Directives\ninside a Match block are conditional and are not scored; the script\nreports how many conditional blocks it skipped on stderr. A non-default\nport is treated as documented only when a comment names both the port\nand a firewall, ufw, iptables, nftables, a security group or an\nallowlist. Include directives are not followed: pass the merged file.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nDEFAULT_PATH = \"/etc/ssh/sshd_config\"\n\nNAME = {\n    \"permitrootlogin\": \"PermitRootLogin\",\n    \"passwordauthentication\": \"PasswordAuthentication\",\n    \"permitemptypasswords\": \"PermitEmptyPasswords\",\n    \"ciphers\": \"Ciphers\",\n    \"macs\": \"MACs\",\n    \"kexalgorithms\": \"KexAlgorithms\",\n}\nCHECK_ORDER = (\n    \"permitrootlogin\",\n    \"passwordauthentication\",\n    \"permitemptypasswords\",\n    \"ciphers\",\n    \"macs\",\n    \"kexalgorithms\",\n)\n\nWEAK_CIPHERS = {\n    \"3des-cbc\",\n    \"aes128-cbc\",\n    \"aes192-cbc\",\n    \"aes256-cbc\",\n    \"blowfish-cbc\",\n    \"cast128-cbc\",\n    \"rijndael-cbc@lysator.liu.se\",\n}\nWEAK_MACS = {\n    \"hmac-md5\",\n    \"hmac-md5-96\",\n    \"hmac-sha1\",\n    \"hmac-sha1-96\",\n    \"hmac-ripemd160\",\n    \"umac-64@openssh.com\",\n}\nWEAK_KEX = {\n    \"diffie-hellman-group1-sha1\",\n    \"diffie-hellman-group14-sha1\",\n    \"diffie-hellman-group-exchange-sha1\",\n}\n\nFIREWALL = re.compile(\n    r\"\\b(firewall|ufw|iptables|nftables|security group|allowlist|\"\n    r\"allow list)\\b\",\n    re.IGNORECASE,\n)\nDIRECTIVE = re.compile(r\"^([A-Za-z][A-Za-z0-9]*)\\s*(?:=\\s*|\\s+)(.+)$\")\n\n\nclass InputError(Exception):\n    \"\"\"An input file that cannot be audited.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return the configuration text, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not UTF-8 text\" % path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    return text\n\n\ndef parse(text):\n    \"\"\"Return first values, Port values, comments and skipped Match blocks.\"\"\"\n    first = {}\n    ports = []\n    comments = []\n    matches = 0\n    in_match = False\n    for number, raw in enumerate(text.splitlines(), start=1):\n        if \"#\" in raw:\n            comments.append(raw.split(\"#\", 1)[1])\n        body = raw.split(\"#\", 1)[0].strip()\n        if not body:\n            continue\n        found = DIRECTIVE.match(body)\n        if not found:\n            continue\n        key = found.group(1).lower()\n        value = found.group(2).strip()\n        if key == \"match\":\n            in_match = True\n            matches += 1\n            continue\n        if in_match:\n            continue\n        if key == \"port\":\n            for token in value.split():\n                try:\n                    ports.append(int(token))\n                except ValueError:\n                    continue\n            continue\n        # sshd_config uses the first obtained value for every keyword.\n        first.setdefault(key, (value, number))\n    if not first and not ports:\n        raise InputError(\"no directives found\")\n    return first, ports, \" \".join(comments), matches\n\n\ndef list_tokens(value):\n    \"\"\"Split a comma list and drop an append or prepend marker.\"\"\"\n    value = value.strip()\n    if value.startswith(\"-\"):\n        return []\n    if value[:1] in (\"+\", \"^\"):\n        value = value[1:]\n    return [\n        token.strip().lower()\n        for token in value.split(\",\")\n        if token.strip()\n    ]\n\n\ndef weak_ciphers(value):\n    \"\"\"Return the weak cipher names enabled by one Ciphers line.\"\"\"\n    return [\n        token\n        for token in list_tokens(value)\n        if (\n            token in WEAK_CIPHERS\n            or token.endswith(\"-cbc\")\n            or token.startswith(\"arcfour\")\n        )\n    ]\n\n\ndef weak_macs(value):\n    \"\"\"Return the weak MAC names enabled by one MACs line.\"\"\"\n    return [\n        token\n        for token in list_tokens(value)\n        if token in WEAK_MACS\n        or token.startswith(\"hmac-md5\")\n        or token.startswith(\"hmac-sha1\")\n        or token.startswith(\"umac-64\")\n    ]\n\n\ndef weak_kex(value):\n    \"\"\"Return the weak key-exchange names enabled by one KexAlgorithms line.\"\"\"\n    return [\n        token\n        for token in list_tokens(value)\n        if token in WEAK_KEX\n        or token.startswith(\"diffie-hellman-group1\")\n        or token.startswith(\"diffie-hellman-group14-sha1\")\n        or token.startswith(\"gss-group1-sha1\")\n        or token.startswith(\"gss-gex-sha1\")\n    ]\n\n\ndef audit(first, ports, comments, allow_ports):\n    \"\"\"Return a list of (kind, message) findings.\"\"\"\n    findings = []\n    for key in CHECK_ORDER:\n        if key not in first:\n            continue\n        value, line = first[key]\n        if key in (\n            \"permitrootlogin\",\n            \"passwordauthentication\",\n            \"permitemptypasswords\",\n        ):\n            if value.strip().lower() == \"yes\":\n                findings.append(\n                    (\"WEAK\", \"%s yes (line %d)\" % (NAME[key], line)))\n            continue\n        check = {\n            \"ciphers\": weak_ciphers,\n            \"macs\": weak_macs,\n            \"kexalgorithms\": weak_kex,\n        }[key]\n        for token in check(value):\n            findings.append(\n                (\"WEAK\", \"%s %s (line %d)\" % (NAME[key], token, line)))\n\n    if not ports:\n        ports = [22]\n    for port in ports:\n        if port == 22 or port in allow_ports:\n            continue\n        documented = FIREWALL.search(comments) and str(port) in comments\n        if not documented:\n            findings.append((\"PORT\", \"Port %d has no firewall note\" % port))\n    return findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit one sshd_config for weak settings (read-only).\")\n    parser.add_argument(\n        \"config\", nargs=\"?\", default=DEFAULT_PATH,\n        help=\"sshd_config to audit (default: %s)\" % DEFAULT_PATH)\n    parser.add_argument(\n        \"--allow-port\", type=int, action=\"append\", default=[], metavar=\"PORT\",\n        help=\"non-default port that is intentionally allowed (repeatable)\")\n    args = parser.parse_args(argv)\n\n    for port in args.allow_port:\n        if not 1 <= port <= 65535:\n            parser.error(\"--allow-port must be between 1 and 65535\")\n\n    try:\n        text = read_text(args.config)\n        first, ports, comments, matches = parse(text)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if matches:\n        print(\n            \"NOTE %d Match block(s) skipped: conditional settings \"\n            \"are not scored\" % matches,\n            file=sys.stderr,\n        )\n\n    findings = audit(first, ports, comments, set(args.allow_port))\n    if findings:\n        for kind, message in findings:\n            print(\"%-5s %s\" % (kind, message))\n        print(\"FINDING %d finding(s) in %s\" % (len(findings), args.config))\n        return 1\n    print(\"OK no findings in %s\" % args.config)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p526","n":"526","slug":"a-sudoers-audit-in-python","title":"A sudoers audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","DevSecOps","Sudo","CyberSecurity"],"series_name":"PythonSec","design":"code","script":"sudoers_audit.py","date":"2030-02-05","files":{"it":"notes/p526-a-sudoers-audit-in-python.md","devsecops":"notes/p526-a-sudoers-audit-in-python.md"},"md":"# A sudoers audit in Python\n\nThe deploy role needs one command, so someone adds NOPASSWD. Six months later the same rule covers every command on the host, and the person who wrote it has changed team.\n\n## Quick reference\n\nThe full script is at the end of this note: `sudoers_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 sudoers_audit.py /tmp/sudoers\n```\n\n**Example output**\n\n```text\nFINDING NOPASSWD, ALL=(ALL) ALL, wildcard\nALERT 2 users with full sudo: admin, deploy\n```\n\n## Explained\n\n### What it flags\n\nNOPASSWD anywhere in a rule is the first finding. A line with ALL=(ALL) ALL is the second: the user can run anything as anyone. A wildcard command path such as `/usr/bin/*` is the third, because the glob can match a binary you did not mean to grant. A blanket command set ALL without the ALL runas is still full sudo. The script expands User_Alias, Runas_Alias and Cmnd_Alias, then lists every user or group that can run everything.\n\n### What the screen shows\n\nThe 14 visible lines read the file, look for NOPASSWD, the ALL=(ALL) ALL shape and a wildcard path, then pull the users from lines with an ALL runas. They print FINDING NOPASSWD, ALL=(ALL) ALL, wildcard, then ALERT 2 users with full sudo: admin, deploy.\n\n### How to run it\n\n`python3 sudoers_audit.py /etc/sudoers`, or one file from `/etc/sudoers.d`. Includes and unparsed lines go to stderr, not into the report. Exit 0 is clean, 1 is a finding, 2 is unreadable or rule-free input. `visudo -c` still owns syntax; this script owns exposure.\n\n> **The honest caveat:** it is a line parser, not visudo. Include files and drop-ins are reported but not followed, so a clean result is not proof of a clean host.\n\n> **The lesson:** delegated privilege is still privilege, so count who holds all of it.\n\n## In one line\n\n> **Full sudo is a finding, not a favour.** A line parser will not replace visudo, but it shows the blast radius.\n\n**Check syntax**\n\n```bash\nvisudo -c -f /tmp/sudoers\n```\n\n## Script: sudoers_audit.py\n\nDownload: [scripts/sudoers_audit.py](../scripts/sudoers_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"sudoers_audit.py - audit a sudoers file for over-broad grants.\n\nReads one sudoers file and reports grants that turn delegated work into\nfull administrative access: any NOPASSWD tag, an ALL=(ALL) ALL rule, a\nwildcard command path, a blanket command set, and every user or group\nthat can run all commands.\n\nUsage:\n    python3 sudoers_audit.py\n    python3 sudoers_audit.py /etc/sudoers\n    python3 sudoers_audit.py --help\n\nCI (fail the build when a role grants full sudo):\n    python3 sudoers_audit.py /etc/sudoers.d/deploy || exit 1\n\nCron (weekly check of the managed sudoers tree):\n    30 6 * * 1 python3 /opt/s0sta/sudoers_audit.py /etc/sudoers\n\nExit codes: 0 no findings, 1 at least one finding, 2 usage error or\nunreadable, empty or rule-free input.\n\nRead-only: the file is opened for reading. Nothing is written, no user\nor group is changed and no command is executed. Reports name users and\ncommand paths, never a command argument, so a password passed on a\ncommand line is not echoed.\n\nThis is a line parser, not visudo. User_Alias, Runas_Alias and\nCmnd_Alias definitions are expanded, but #include and #includedir\ndirectives are reported on stderr and not followed: audit each included\nfile directly. A comma inside a command argument must be escaped, as\nsudoers itself requires. Run visudo -c for syntax, then this script for\nexposure.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nDEFAULT_PATH = \"/etc/sudoers\"\n\nALIAS = re.compile(\n    r\"^(User_Alias|Runas_Alias|Cmnd_Alias)\\s+\"\n    r\"([A-Za-z][A-Za-z0-9_]*)\\s*=\\s*(.+)$\",\n    re.IGNORECASE,\n)\nINCLUDE = re.compile(r\"^[#@]include(dir)?\\s+(\\S+)\", re.IGNORECASE)\nRUNAS = re.compile(r\"^\\(([^)]*)\\)\\s*(.*)$\")\nTAG = re.compile(\n    r\"\\b(?:NOPASSWD|PASSWD|NOEXEC|EXEC|SETENV|NOSETENV|LOG_INPUT|NOLOG_INPUT|\"\n    r\"LOG_OUTPUT|NOLOG_OUTPUT|MAIL|NOMAIL|FOLLOW|NOFOLLOW)\\s*:\\s*\",\n    re.IGNORECASE,\n)\nNOPASSWD = re.compile(r\"\\bNOPASSWD\\s*:\", re.IGNORECASE)\n\n\nclass InputError(Exception):\n    \"\"\"An input file that cannot be audited.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return the sudoers text, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not UTF-8 text\" % path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    return text\n\n\ndef logical_lines(text):\n    \"\"\"Yield (line number, joined line) for one logical line.\"\"\"\n    buffered = \"\"\n    start = 1\n    for number, raw in enumerate(text.splitlines(), start=1):\n        line = raw.rstrip()\n        if not buffered:\n            start = number\n        if line.endswith(\"\\\\\"):\n            buffered += line[:-1] + \" \"\n            continue\n        buffered += line\n        yield start, buffered\n        buffered = \"\"\n    if buffered:\n        yield start, buffered\n\n\ndef strip_comment(line):\n    \"\"\"Remove a # comment, but keep include directives intact.\"\"\"\n    stripped = line.strip()\n    if INCLUDE.match(stripped):\n        return stripped\n    if stripped.startswith((\"#\", \"@\")):\n        return \"\"\n    return stripped.split(\"#\", 1)[0].strip()\n\n\ndef split_escaped(text, separator=\",\"):\n    \"\"\"Split on a separator that is not backslash-escaped.\"\"\"\n    parts = []\n    current = \"\"\n    escaped = False\n    for char in text:\n        if escaped:\n            current += char\n            escaped = False\n        elif char == \"\\\\\":\n            escaped = True\n        elif char == separator:\n            parts.append(current)\n            current = \"\"\n        else:\n            current += char\n    parts.append(current)\n    return [part.strip() for part in parts if part.strip()]\n\n\ndef expand(items, aliases, seen=None):\n    \"\"\"Expand aliases in a list, keeping any ! negation prefix.\"\"\"\n    seen = set() if seen is None else set(seen)\n    out = []\n    for item in items:\n        raw = item.strip()\n        if not raw:\n            continue\n        negated = raw.startswith(\"!\")\n        name = raw[1:].strip() if negated else raw\n        if name in aliases and name not in seen:\n            inner = expand(aliases[name], aliases, seen | {name})\n        else:\n            inner = [name]\n        for value in inner:\n            out.append((\"!\" if negated else \"\") + value)\n    return out\n\n\ndef parse_aliases(lines):\n    \"\"\"Return (user aliases, runas aliases, command aliases, includes).\"\"\"\n    users = {}\n    runas = {}\n    commands = {}\n    includes = []\n    for _number, raw in lines:\n        line = strip_comment(raw)\n        if not line or line.lower().startswith(\"defaults\"):\n            continue\n        if INCLUDE.match(line):\n            includes.append(line)\n            continue\n        found = ALIAS.match(line)\n        if not found:\n            continue\n        kind = found.group(1).lower()\n        name = found.group(2)\n        values = split_escaped(found.group(3))\n        if kind == \"user_alias\":\n            users[name] = values\n        elif kind == \"runas_alias\":\n            runas[name] = values\n        else:\n            commands[name] = values\n    return users, runas, commands, includes\n\n\ndef parse_spec(line):\n    \"\"\"Return (users, runas, commands) for one user spec.\"\"\"\n    if \"=\" not in line:\n        return None\n    left, right = line.split(\"=\", 1)\n    parts = left.strip().split()\n    if not parts:\n        return None\n    user_chunks = [parts[0]]\n    index = 1\n    while user_chunks[-1].endswith(\",\") and index < len(parts):\n        user_chunks.append(parts[index])\n        index += 1\n    user_items = []\n    for chunk in user_chunks:\n        user_items.extend(split_escaped(chunk))\n    if not user_items:\n        return None\n\n    body = right.strip()\n    runas_items = [\"root\"]\n    found = RUNAS.match(body)\n    if found:\n        runas_items = split_escaped(found.group(1).replace(\":\", \",\"))\n        body = found.group(2).strip()\n    body = TAG.sub(\"\", body)\n    commands = split_escaped(body)\n    return user_items, runas_items, commands\n\n\ndef flatten_commands(commands, aliases, seen=None):\n    \"\"\"Expand Cmnd_Alias names into their command lists.\"\"\"\n    seen = set() if seen is None else set(seen)\n    out = []\n    for item in commands:\n        raw = item.strip()\n        name = raw[1:].strip() if raw.startswith(\"!\") else raw\n        if name in aliases and name not in seen:\n            expanded = flatten_commands(aliases[name], aliases, seen | {name})\n            if raw.startswith(\"!\"):\n                expanded = [\"!\" + value for value in expanded]\n            out.extend(expanded)\n        else:\n            out.append(raw)\n    return out\n\n\ndef audit(text):\n    \"\"\"Return a list of (kind, message) findings for one sudoers file.\"\"\"\n    lines = list(logical_lines(text))\n    user_alias, runas_alias, cmd_alias, includes = parse_aliases(lines)\n    findings = []\n    full_users = set()\n    specs = 0\n    skipped = 0\n\n    for number, raw in lines:\n        line = strip_comment(raw)\n        if (\n            not line\n            or INCLUDE.match(line)\n            or ALIAS.match(line)\n            or line.lower().startswith(\"defaults\")\n        ):\n            continue\n        parsed = parse_spec(line)\n        if parsed is None:\n            skipped += 1\n            continue\n        specs += 1\n        user_items, runas_items, command_items = parsed\n        users = expand(user_items, user_alias)\n        runas = expand(runas_items, runas_alias)\n        commands = flatten_commands(command_items, cmd_alias)\n        positive = [value for value in users if not value.startswith(\"!\")]\n        if not positive:\n            continue\n        shown_users = \", \".join(positive)\n\n        if NOPASSWD.search(raw):\n            findings.append(\n                (\"SUDO\", \"line %d: NOPASSWD for %s\" % (number, shown_users)))\n\n        command_paths = []\n        any_all = False\n        for command in commands:\n            raw_command = command.strip()\n            negated = raw_command.startswith(\"!\")\n            clean = raw_command[1:].strip() if negated else raw_command\n            path = clean.split()[0] if clean.split() else \"\"\n            if path == \"ALL\" and not negated:\n                any_all = True\n            if path:\n                command_paths.append((path, negated))\n\n        runas_all = \"ALL\" in [value.lstrip(\"!\") for value in runas]\n        exact = (\n            any_all\n            and runas_all\n            and len(command_paths) == 1\n            and not command_paths[0][1]\n        )\n        if exact:\n            findings.append(\n                (\"SUDO\",\n                 \"line %d: ALL=(ALL) ALL for %s\" % (number, shown_users)))\n        elif any_all:\n            findings.append(\n                (\"SUDO\", \"line %d: full command set ALL for %s\"\n                 % (number, shown_users)))\n\n        for path, negated in command_paths:\n            if (\n                not negated\n                and path.startswith(\"/\")\n                and (\"*\" in path or \"?\" in path)\n            ):\n                findings.append(\n                    (\"SUDO\", \"line %d: wildcard path %s for %s\"\n                     % (number, path, shown_users)))\n\n        if any_all:\n            full_users.update(positive)\n\n    if full_users:\n        findings.append(\n            (\"FULL\", \"users with full sudo: \"\n             + \", \".join(sorted(full_users))))\n    return findings, specs, skipped, includes\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit a sudoers file for over-broad grants (read-only).\")\n    parser.add_argument(\n        \"sudoers\", nargs=\"?\", default=DEFAULT_PATH,\n        help=\"sudoers file to audit (default: %s)\" % DEFAULT_PATH)\n    args = parser.parse_args(argv)\n\n    try:\n        text = read_text(args.sudoers)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings, specs, skipped, includes = audit(text)\n    if includes:\n        print(\n            \"NOTE %d include directive(s) not followed; \"\n            \"audit each file directly\" % len(includes),\n            file=sys.stderr,\n        )\n    if skipped:\n        print(\"NOTE %d line(s) not understood and skipped\" % skipped,\n              file=sys.stderr)\n    if not specs:\n        if includes:\n            print(\"ERROR no rules in %s: only includes found\" % args.sudoers,\n                  file=sys.stderr)\n            return 2\n        print(\"ERROR no sudoers rules found in %s\" % args.sudoers,\n              file=sys.stderr)\n        return 2\n\n    if findings:\n        for kind, message in findings:\n            print(\"%-5s %s\" % (kind, message))\n        print(\"FINDING %d finding(s) in %s\" % (len(findings), args.sudoers))\n        return 1\n    print(\"OK no findings in %s\" % args.sudoers)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p527","n":"527","slug":"9-steps-to-write-a-security-script","title":"9 steps to write a security script","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","Automation","CICD","CyberSecurity"],"series_name":"PythonSec","design":"steps","script":"","date":"2030-02-07","files":{"devsecops":"notes/p527-9-steps-to-write-a-security-script.md"},"md":"# 9 steps to write a security script\n\nA security script starts as a hunch: something in this file is wrong. Nine steps turn the hunch into a check that fails for the right reason and passes on a clean host. The terminal shows the interface first: usage, one run, one finding.\n\n## Quick reference\n\n### Spec — Decide what the check decides\n\n```bash\n# Write the help text first; it fixes the interface.\npython3 audit.py --help\n# One dirty fixture, smaller than the real file.\nprintf 'root ALL=(ALL) ALL\\n' > /tmp/f\n# Read the real line shape before the parser.\ngrep -n 'NOPASSWD' /tmp/f\n```\n\n### Build — Make the smallest correct pass\n\n```bash\n# Compile after every change; syntax is not a test.\npython3 -m py_compile audit.py\n# Run on the dirty fixture until exit 1 is stable.\npython3 audit.py /tmp/f\n# Then the clean one; it must exit 0 and print OK.\npython3 audit.py /tmp/clean\n```\n\n### Prove — Prove the failure paths\n\n```bash\n# Missing input must print 2, not a traceback.\npython3 audit.py /tmp/missing; echo $?\n# stdout is the report; the exit code is the result.\npython3 audit.py /tmp/f >/dev/null; echo $?\n# Run it the way production will run it.\ncrontab -l | python3 audit.py -\n```\n\n## Explained\n\n### Spec\n\n`python3 audit.py --help` comes first. Write the help text before the code, because it fixes the input, the flags and the exit codes. `printf 'root ALL=(ALL) ALL\\n' > /tmp/f` builds one dirty fixture, smaller than production and safe to break. `grep -n 'NOPASSWD' /tmp/f` reads the real line shape before a parser assumes it.\n\n### Build\n\n`python3 -m py_compile audit.py` catches syntax, not logic, so run it after every change. `python3 audit.py /tmp/f` runs on the dirty fixture until exit 1 is stable. `python3 audit.py /tmp/clean` then proves the clean path: exit 0 and one OK line. The two exits tell CI everything it needs.\n\n### Prove\n\n`python3 audit.py /tmp/missing; echo $?` must print 2, not a traceback. `python3 audit.py /tmp/f >/dev/null; echo $?` separates the report from the result, which is how CI reads it. `crontab -l | python3 audit.py -` runs it the way production will, through stdin.\n\n> **The honest caveat:** a fixture you wrote will agree with a parser you wrote. Add one real, redacted file from production before you trust the pass.\n\n> **The lesson:** a security script is a test, so write the assertion before the scanner.\n\n## In one line\n\n> **Write the check, then the fix.** A security script is a test with a docstring and an exit code.\n\n**Start with --help**\n\n```bash\npython3 audit.py --help\n```\n"},{"id":"p528","n":"528","slug":"9-commands-to-debug-a-python-automation-script","title":"9 commands to debug a Python automation script","sections":["devsecops"],"topic":"Python scripts","tags":["Python","Debugging","DevSecOps","Linux","Automation"],"series_name":"PythonSec","design":"terminal","script":"","date":"2030-02-10","files":{"devsecops":"notes/p528-9-commands-to-debug-a-python-automation-script.md"},"md":"# 9 commands to debug a Python automation script\n\nThe script works on your laptop and exits 0 in cron. Nothing changed except the environment, and the traceback is not helping. Three tabs, nine commands, one question at a time.\n\n## Quick reference\n\n### Where did it go?\n\n```bash\n# every line, in order\npython3 -m trace --trace audit.py /tmp/f\n# stop before the line you doubt\npython3 -m pdb audit.py /tmp/f\n# dev mode turns warnings on\npython3 -X dev audit.py /tmp/f\n```\n\n### Where did it spend the time?\n\n```bash\n# wall clock first, then CPU\ntime python3 audit.py /tmp/f\n# the function, not the guess\npython3 -m cProfile -s cumtime audit.py /tmp/f\n# slow imports hide here\npython3 -X importtime audit.py /tmp/f 2>&1 | tail -3\n```\n\n### What did it inherit?\n\n```bash\n# which file it really opened\nstrace -f -e trace=openat python3 audit.py /tmp/f\n# no HOME, no locale, same result?\nenv -i PATH=/usr/bin python3 audit.py /tmp/f\n# a shadow module changes everything\nPYTHONPATH=/tmp python3 audit.py /tmp/f\n```\n\n## Explained\n\n### Trace\n\n`python3 -m trace --trace audit.py /tmp/f` prints every line as it runs, so the last line before the silence is the suspect. `python3 -m pdb audit.py /tmp/f` stops where you ask: break at the line you doubt and inspect the values. `python3 -X dev audit.py /tmp/f` turns on development warnings, including the unclosed file that only appears under load.\n\n### Time\n\n`time python3 audit.py /tmp/f` gives wall clock and CPU before any theory. `python3 -m cProfile -s cumtime audit.py /tmp/f` ranks functions by cumulative time, which finds the loop that reads the file twice. `python3 -X importtime audit.py /tmp/f 2>&1 | tail -3` shows the imports that cost more than the work.\n\n### State\n\n`strace -f -e trace=openat python3 audit.py /tmp/f` shows the openat line for the file it really opened, not the one you meant. env -i PATH=`/usr/bin python3 audit.py /tmp/f` runs with no HOME and no locale, which is close to cron. PYTHONPATH=`/tmp python3 audit.py /tmp/f` proves that a shadow module is not changing the result; both still print FINDING 3 findings.\n\n> **The honest caveat:** strace and pdb are debugging tools, not production monitors. They slow the script and expose its arguments, so run them on a copy.\n\n> **The lesson:** debug the call and the environment, not the story you tell about them.\n\n## In one line\n\n> **Trace the call, not the guess.** The bug is in one line; find the line, not the story.\n\n**Try it**\n\n```bash\npython3 -m trace --trace audit.py /tmp/f | tail -5\n```\n"},{"id":"p529","n":"529","slug":"a-systemd-hardening-score-in-python","title":"A systemd hardening score in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Systemd","Linux","DevSecOps","CyberSecurity"],"series_name":"PythonSec","design":"code","script":"systemd_hardening.py","date":"2030-02-12","files":{"it":"notes/p529-a-systemd-hardening-score-in-python.md","devsecops":"notes/p529-a-systemd-hardening-score-in-python.md"},"md":"# A systemd hardening score in Python\n\nA unit file starts as ExecStart and a description. Three years later it runs as root with the whole filesystem writable, because nobody added the lines that take privilege away.\n\n## Quick reference\n\nThe full script is at the end of this note: `systemd_hardening.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 systemd_hardening.py /tmp/sshd.service\n```\n\n**Example output**\n\n```text\nFINDING 3 basics missing\nALERT score 55 LOW\n```\n\n## Explained\n\n### What it scores\n\nFour basics are worth 15 points each: NoNewPrivileges, ProtectSystem, PrivateTmp and RestrictAddressFamilies. Eight further directives are worth 5 each, including ProtectHome, PrivateDevices, ProtectKernelTunables and SystemCallFilter. A unit with all twelve scores 100; the default threshold is 60. Every missing directive is named in the output, so the score comes with a to-do list.\n\n### What the screen shows\n\nThe 14 visible lines read one unit, exit 2 on an empty file, check three exact strings and a RestrictAddressFamilies value, then subtract 15 points for each miss. On the weak fixture they print FINDING 3 basics missing, then ALERT score 55 LOW. The full script uses all twelve checks, counts the extras and prints one UNIT line per file.\n\n### How to run it\n\n`python3 systemd_hardening.py /etc/systemd/system` scans a tree, or pass one .service file. `--threshold 80` raises the bar for a new image; the default 60 suits an existing estate. Exit 0 means every unit passed, 1 means at least one fell below, 2 means the path could not be read. Nothing is written and no unit is reloaded.\n\n> **The honest caveat:** the score is a prompt, not a verdict. A network service may need AF_INET, and a drop-in can change everything the file says.\n\n> **The lesson:** hardening is a set of lines someone has to write, so score them or forget them.\n\n## In one line\n\n> **Hardening is a score you can raise.** A unit file is a policy; the score makes the policy visible.\n\n**Score one unit**\n\n```bash\npython3 systemd_hardening.py sshd.service\n```\n\n## Script: systemd_hardening.py\n\nDownload: [scripts/systemd_hardening.py](../scripts/systemd_hardening.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"systemd_hardening.py - score systemd service units on hardening.\n\nReads one .service file or a directory tree of units and gives each unit\na score out of 100 for the sandboxing and privilege directives it sets.\nFour basics are worth 15 points each: NoNewPrivileges, ProtectSystem,\nPrivateTmp and RestrictAddressFamilies. Eight further directives are\nworth 5 each, including ProtectHome, PrivateDevices, ProtectKernel*\nand SystemCallFilter.\n\nUsage:\n    python3 systemd_hardening.py /etc/systemd/system/sshd.service\n    python3 systemd_hardening.py /etc/systemd/system\n    python3 systemd_hardening.py /tmp/units --threshold 80\n    python3 systemd_hardening.py --help\n\nCI (fail a unit build when the score drops):\n    python3 systemd_hardening.py ./units --threshold 70 || exit 1\n\nCron (weekly drift report for the boot image):\n    15 5 * * 1 python3 /opt/s0sta/systemd_hardening.py /etc/systemd/system\n\nExit codes: 0 every unit is at or above the threshold, 1 at least one\nunit is below it, 2 usage error, unreadable input or a directory with no\n.service files.\n\nRead-only: unit files are opened for reading. Nothing is written, no\nunit is reloaded, started or stopped and no process is touched. Only\ndirective names are printed, never their values, so a credential in an\nEnvironment= line is not echoed.\n\nThe score is a prompt, not a verdict. A network service may legitimately\nskip PrivateNetwork and a unit that needs AF_NETLINK may list it in\nRestrictAddressFamilies. Raise --threshold when you want the extras to\ncount. Only the [Service] section is read; drop-ins under .d/ are not\nmerged, so audit a merged unit with systemctl cat first.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nfrom pathlib import Path\n\nTRUE = (\"yes\", \"true\", \"1\", \"on\")\nCHECKS = (\n    (\"NoNewPrivileges\", TRUE, 15, True),\n    (\"ProtectSystem\", (\"strict\", \"full\", \"yes\", \"true\"), 15, True),\n    (\"PrivateTmp\", TRUE, 15, True),\n    (\"RestrictAddressFamilies\", None, 15, True),\n    (\"ProtectHome\", (\"yes\", \"true\", \"read-only\", \"tmpfs\"), 5, False),\n    (\"PrivateDevices\", TRUE, 5, False),\n    (\"ProtectKernelTunables\", TRUE, 5, False),\n    (\"ProtectKernelModules\", TRUE, 5, False),\n    (\"ProtectControlGroups\", TRUE, 5, False),\n    (\"RestrictNamespaces\", TRUE, 5, False),\n    (\"LockPersonality\", TRUE, 5, False),\n    (\"SystemCallFilter\", None, 5, False),\n)\n\n\nclass InputError(Exception):\n    \"\"\"An input path that cannot be audited.\"\"\"\n\n\ndef unit_paths(path):\n    \"\"\"Return a sorted list of .service files for one file or directory.\"\"\"\n    root = Path(path)\n    if root.is_dir():\n        files = sorted(root.rglob(\"*.service\"))\n        if not files:\n            raise InputError(\"%s: no .service files found\" % path)\n        return files\n    if root.is_file():\n        if root.suffix != \".service\":\n            raise InputError(\"%s: not a .service file\" % path)\n        return [root]\n    raise InputError(\"%s: no such file or directory\" % path)\n\n\ndef parse_service(text):\n    \"\"\"Return the [Service] key/value pairs, last value winning.\"\"\"\n    values = {}\n    section = \"\"\n    for raw in text.splitlines():\n        line = raw.strip()\n        if not line or line[0] in \"#;\":\n            continue\n        if line.startswith(\"[\") and line.endswith(\"]\"):\n            section = line[1:-1].strip().lower()\n            continue\n        if section != \"service\" or \"=\" not in line:\n            continue\n        key, value = line.split(\"=\", 1)\n        key = key.strip()\n        if key:\n            values[key] = value.strip()\n    return values\n\n\ndef normalise(value):\n    \"\"\"Strip surrounding quotes and whitespace from a unit value.\"\"\"\n    value = value.strip()\n    if len(value) >= 2 and value[0] == value[-1] and value[0] in \"\\\"'\":\n        value = value[1:-1]\n    return value\n\n\ndef score(values):\n    \"\"\"Return (score, missing basics, missing extras).\"\"\"\n    earned = 0\n    missing_basics = []\n    missing_extras = []\n    for name, allowed, points, basic in CHECKS:\n        value = values.get(name)\n        if allowed is None:\n            present = value is not None and normalise(value) != \"\"\n        else:\n            present = value is not None and normalise(value).lower() in allowed\n        if present:\n            earned += points\n        elif basic:\n            missing_basics.append(name)\n        else:\n            missing_extras.append(name)\n    return earned, missing_basics, missing_extras\n\n\ndef audit(path, threshold):\n    \"\"\"Print one line per unit and return the number below the threshold.\"\"\"\n    low = 0\n    units = 0\n    for unit in unit_paths(path):\n        try:\n            text = unit.read_text(encoding=\"utf-8\")\n        except OSError as exc:\n            raise InputError(\"%s: %s\" % (unit, exc.strerror or exc))\n        except UnicodeDecodeError:\n            raise InputError(\"%s: not UTF-8 text\" % unit)\n        units += 1\n        values = parse_service(text)\n        points, missing_basics, missing_extras = score(values)\n        missing = missing_basics + missing_extras\n        shown = \", \".join(missing) if missing else \"none\"\n        print(\"UNIT  %-28s score %3d missing %s\" % (unit.name, points, shown))\n        if points < threshold:\n            low += 1\n    if low:\n        print(\"FINDING %d unit(s) below %d (%d scanned)\"\n              % (low, threshold, units))\n    else:\n        noun = \"unit\" if units == 1 else \"units\"\n        print(\"OK all %d %s at or above %d\" % (units, noun, threshold))\n    return low\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Score systemd .service units on hardening (read-only).\")\n    parser.add_argument(\n        \"path\", help=\"a .service file or a directory to scan\")\n    parser.add_argument(\n        \"--threshold\", type=int, default=60,\n        help=\"minimum passing score, 0-100 (default: 60)\")\n    args = parser.parse_args(argv)\n    if not 0 <= args.threshold <= 100:\n        parser.error(\"--threshold must be between 0 and 100\")\n\n    try:\n        low = audit(args.path, args.threshold)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    return 1 if low else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p530","n":"530","slug":"a-cron-audit-in-python","title":"A cron audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Cron","Linux","DevSecOps","CyberSecurity"],"series_name":"PythonSec","design":"code","script":"cron_audit.py","date":"2030-02-14","files":{"it":"notes/p530-a-cron-audit-in-python.md","devsecops":"notes/p530-a-cron-audit-in-python.md"},"md":"# A cron audit in Python\n\nA crontab is a list of commands that run without a terminal, often as root, often after everyone who wrote them has left. Five checks make the risk visible.\n\n## Quick reference\n\nThe full script is at the end of this note: `cron_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cron_audit.py /tmp/crontab\n```\n\n**Example output**\n\n```text\nFINDING 5 of 5 checks\nALERT no MAILTO /tmp/crontab\n```\n\n## Explained\n\n### What it flags\n\nA root job is the first: the user field says who runs it, and root means no permission boundary. A world-writable script is the second: another account can replace the code between runs. A relative path is the third, because cron's working directory is not yours. A missing MAILTO is the fourth: output goes to a local mailbox nobody reads. A curl or wget piped into a shell is the fifth, and it is remote code execution on a schedule.\n\n### What the screen shows\n\nThe 14 visible lines split environment lines from jobs, look for root, a curl pipe, a ./ path and MAILTO, then stat every script path for the other-write bit. On the fixture they print FINDING 5 of 5 checks, then ALERT no MAILTO `/tmp/crontab`.\n\n### How to run it\n\n`python3 cron_audit.py /etc/crontab /etc/cron.d` audits system files. `crontab -l | python3 cron_audit.py - --user-crontab --owner root` audits a user table and treats every job as root. `--cwd` resolves relative paths as cron would. Exit 0 is clean, 1 is a finding, 2 is unreadable or unparseable input. It only reads.\n\n> **The honest caveat:** a crontab can call a wrapper that hides everything. This reads the line, not the shell script it starts.\n\n> **The lesson:** the crontab is code that runs as root, so review it like code.\n\n## In one line\n\n> **Cron is a root shell.** Every line runs with the privileges of the account that owns it.\n\n**Read the crontab**\n\n```bash\ncrontab -l | python3 cron_audit.py -\n```\n\n## Script: cron_audit.py\n\nDownload: [scripts/cron_audit.py](../scripts/cron_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"cron_audit.py - audit crontab lines for privilege and injection risk.\n\nReads one or more crontab files, or stdin, and reports root jobs, scripts\nthat are world-writable, relative script paths, a missing MAILTO and a\ncurl or wget command piped into a shell.\n\nUsage:\n    python3 cron_audit.py\n    python3 cron_audit.py /etc/crontab /etc/cron.d/app\n    crontab -l | python3 cron_audit.py - --user-crontab --owner deploy\n    python3 cron_audit.py /tmp/crontab --cwd /srv\n    python3 cron_audit.py --help\n\nCI (fail the job when a generated crontab is unsafe):\n    python3 cron_audit.py build/crontab || exit 1\n\nCron (weekly check of the system tables):\n    45 6 * * 1 python3 /opt/s0sta/cron_audit.py /etc/crontab /etc/cron.d\n\nExit codes: 0 no findings, 1 at least one finding, 2 usage error or\nunreadable input.\n\nRead-only: files are opened for reading and stdin is consumed. Nothing\nis written, no crontab is installed and no job is run. Reports name a\nline, a user or a path, never a command body, so a credential in a\ncrontab is not echoed.\n\nA system crontab carries a user field after the five time fields. Pass\n--user-crontab for output from crontab -l, and --owner to name whose\ncrontab it is so root jobs can still be found. --cwd resolves relative\nscript paths; cron starts system jobs in / and user jobs in the owner's\nhome, so set it to match.\n\nWorld-writable means the regular file has the other-write bit. A missing\nscript is not reported: cron may run it from another mount or after a\ndeploy. MAILTO=\"\" counts as set and deliberately disables mail. The\ncurl check matches a pipe into sh, bash, zsh, dash or ksh; it does not\nprove the URL is trusted.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport os\nimport re\nimport shlex\nimport stat\nimport sys\n\nDEFAULT_PATH = \"/etc/crontab\"\n\nSPECIAL = {\n    \"@reboot\", \"@yearly\", \"@annually\", \"@monthly\", \"@weekly\", \"@daily\",\n    \"@midnight\", \"@hourly\",\n}\nENV = re.compile(r\"^([A-Za-z_][A-Za-z0-9_]*)\\s*=\\s*(.*)$\")\nSHELL = re.compile(r\"\\b(sh|bash|zsh|dash|ksh)\\b\")\nFETCH = re.compile(r\"\\b(curl|wget)\\b\")\nSCRIPT_SUFFIX = (\".sh\", \".bash\", \".py\", \".pl\", \".rb\")\nOPERATORS = {\"|\", \"||\", \"&&\", \";\", \">\", \">>\", \"<\", \"2>\", \"2>>\", \"&\"}\nMONTHS = {\n    \"jan\": 1, \"feb\": 2, \"mar\": 3, \"apr\": 4, \"may\": 5, \"jun\": 6,\n    \"jul\": 7, \"aug\": 8, \"sep\": 9, \"oct\": 10, \"nov\": 11, \"dec\": 12,\n}\nDAYS = {\"sun\": 0, \"mon\": 1, \"tue\": 2, \"wed\": 3, \"thu\": 4, \"fri\": 5, \"sat\": 6}\nFIELDS = (\n    (0, 59, None),\n    (0, 23, None),\n    (1, 31, None),\n    (1, 12, MONTHS),\n    (0, 7, DAYS),\n)\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return (label, text) for a file or - for stdin, or raise InputError.\"\"\"\n    if path == \"-\":\n        try:\n            return \"<stdin>\", sys.stdin.read()\n        except UnicodeDecodeError:\n            raise InputError(\"stdin: not UTF-8 text\")\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return path, handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not UTF-8 text\" % path)\n\n\ndef field_value(text, low, high, names):\n    \"\"\"True when one cron field component is in range or a known name.\"\"\"\n    value = text.strip()\n    if not value:\n        return False\n    if names is not None and value.lower() in names:\n        return True\n    if not value.isdigit():\n        return False\n    return low <= int(value) <= high\n\n\ndef valid_field(token, low, high, names):\n    \"\"\"True for *, steps, lists and ranges in one cron field.\"\"\"\n    for part in token.split(\",\"):\n        core = part.split(\"/\", 1)[0]\n        if \"/\" in part:\n            step = part.split(\"/\", 1)[1]\n            if not step.isdigit() or int(step) == 0:\n                return False\n        if core == \"*\":\n            continue\n        if \"-\" in core:\n            first, second = core.split(\"-\", 1)\n            if not field_value(first, low, high, names):\n                return False\n            if not field_value(second, low, high, names):\n                return False\n        elif not field_value(core, low, high, names):\n            return False\n    return True\n\n\ndef valid_time(tokens):\n    \"\"\"True when the five time fields look like a crontab schedule.\"\"\"\n    return all(\n        valid_field(token, low, high, names)\n        for token, (low, high, names) in zip(tokens, FIELDS)\n    )\n\n\ndef parse(text, user_mode, owner):\n    \"\"\"Return (environment, jobs, malformed count).\n\n    A job is (line number, user or None, command token list).\n    \"\"\"\n    environment = {}\n    jobs = []\n    malformed = 0\n    for number, raw in enumerate(text.splitlines(), start=1):\n        line = raw.strip()\n        if not line or line.startswith(\"#\"):\n            continue\n        assignment = ENV.match(line)\n        if assignment:\n            environment[assignment.group(1)] = assignment.group(2)\n            continue\n        try:\n            tokens = shlex.split(line, comments=False, posix=True)\n        except ValueError:\n            malformed += 1\n            continue\n        if not tokens:\n            continue\n        if tokens[0].lower() in SPECIAL:\n            if user_mode:\n                user, command = owner, tokens[1:]\n            else:\n                if len(tokens) < 3:\n                    malformed += 1\n                    continue\n                user, command = tokens[1], tokens[2:]\n        else:\n            if len(tokens) < 6 or not valid_time(tokens[:5]):\n                malformed += 1\n                continue\n            if user_mode:\n                user, command = owner, tokens[5:]\n            else:\n                if len(tokens) < 7:\n                    malformed += 1\n                    continue\n                user, command = tokens[5], tokens[6:]\n        if not command:\n            malformed += 1\n            continue\n        jobs.append((number, user, command))\n    return environment, jobs, malformed\n\n\ndef command_paths(command, cwd):\n    \"\"\"Return (kind, token) for every script-like path in a command.\"\"\"\n    found = []\n    for token in command:\n        if not token or token in OPERATORS or token.startswith(\"-\"):\n            continue\n        if \"://\" in token:\n            continue\n        if token.startswith(\"/\"):\n            found.append((\"absolute\", token))\n        elif token.startswith((\"./\", \"../\")) or (\n            \"/\" in token and token.endswith(SCRIPT_SUFFIX)\n        ):\n            found.append((\"relative\", token))\n    return found\n\n\ndef world_writable(path):\n    \"\"\"True when path is a regular file with the other-write bit set.\"\"\"\n    try:\n        info = os.stat(path)\n    except OSError:\n        return False\n    return stat.S_ISREG(info.st_mode) and bool(info.st_mode & stat.S_IWOTH)\n\n\ndef piped_to_shell(command):\n    \"\"\"True when a curl or wget in the command is piped into a shell.\"\"\"\n    text = \" \".join(command)\n    if \"|\" not in text:\n        return False\n    left, right = text.split(\"|\", 1)\n    if not FETCH.search(left):\n        return False\n    return bool(SHELL.search(right))\n\n\ndef audit_file(label, environment, jobs, cwd):\n    \"\"\"Return a list of (kind, message) findings for one crontab.\"\"\"\n    findings = []\n\n    for number, user, command in jobs:\n        if user and user.lower() == \"root\":\n            paths = command_paths(command, cwd)\n            shown = paths[0][1] if paths else \"inline command\"\n            findings.append((\"ROOT\", \"line %d: root job %s\" % (number, shown)))\n\n    for number, _user, command in jobs:\n        seen = set()\n        for kind, token in command_paths(command, cwd):\n            if token in seen:\n                continue\n            seen.add(token)\n            if kind == \"absolute\":\n                resolved = token\n            else:\n                resolved = os.path.normpath(os.path.join(cwd, token))\n            if world_writable(resolved):\n                findings.append(\n                    (\"WW\", \"line %d: %s is world-writable\" % (number, token)))\n\n    for number, _user, command in jobs:\n        seen = set()\n        for kind, token in command_paths(command, cwd):\n            if kind != \"relative\" or token in seen:\n                continue\n            seen.add(token)\n            findings.append(\n                (\"REL\", \"line %d: relative path %s\" % (number, token)))\n\n    if jobs and \"MAILTO\" not in environment:\n        findings.append((\"MAIL\", \"line %d: no MAILTO set\" % jobs[0][0]))\n\n    for number, _user, command in jobs:\n        if piped_to_shell(command):\n            findings.append(\n                (\"PIPE\", \"line %d: curl or wget piped to a shell\" % number))\n\n    return findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit crontab lines for privilege and injection risk.\")\n    parser.add_argument(\n        \"paths\", nargs=\"*\", default=[DEFAULT_PATH],\n        help=\"crontab files to audit, or - for stdin \"\n             \"(default: %s)\" % DEFAULT_PATH)\n    parser.add_argument(\n        \"--user-crontab\", action=\"store_true\",\n        help=\"input has no user field, as produced by crontab -l\")\n    parser.add_argument(\n        \"--owner\", default=None,\n        help=\"owner of a user crontab, so root jobs can be found\")\n    parser.add_argument(\n        \"--cwd\", default=\"/\",\n        help=\"directory used to resolve relative script paths (default: /)\")\n    args = parser.parse_args(argv)\n\n    if not os.path.isdir(args.cwd):\n        parser.error(\"--cwd must be an existing directory\")\n    if args.owner is not None and not args.owner.strip():\n        parser.error(\"--owner must not be empty\")\n\n    total = []\n    labels = []\n    malformed_total = 0\n    jobs_total = 0\n    for path in args.paths:\n        try:\n            label, text = read_text(path)\n        except InputError as exc:\n            print(\"ERROR %s\" % exc, file=sys.stderr)\n            return 2\n        labels.append(label)\n        if not text.strip():\n            continue\n        environment, jobs, malformed = parse(\n            text, args.user_crontab, args.owner)\n        malformed_total += malformed\n        jobs_total += len(jobs)\n        if not jobs and not environment and malformed:\n            print(\"ERROR %s: no crontab lines understood\" % label,\n                  file=sys.stderr)\n            return 2\n        total.extend(audit_file(label, environment, jobs, args.cwd))\n\n    if malformed_total:\n        print(\"NOTE %d line(s) not understood and skipped\" % malformed_total,\n              file=sys.stderr)\n    if not jobs_total:\n        print(\"OK no jobs found in %s\" % \", \".join(labels))\n        return 0\n    if total:\n        for kind, message in total:\n            print(\"%-5s %s\" % (kind, message))\n        print(\"FINDING %d finding(s) in %s\" % (len(total), \", \".join(labels)))\n        return 1\n    print(\"OK no findings in %s\" % \", \".join(labels))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p531","n":"531","slug":"quiz-3-automation-questions","title":"Quiz: 3 automation questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","Automation","CyberSecurity","CICD"],"series_name":"PythonSec","design":"quiz","script":"","date":"2030-02-17","files":{"devsecops":"notes/p531-quiz-3-automation-questions.md"},"md":"# Quiz: 3 automation questions\n\nThree questions come back in every review of an automation script, and the answers separate a tool from a liability.\n\n## Quick reference\n\n**Q1. Why exit 2 instead of 1 when the input is unreadable?**\n\n- a) 2 means the check itself failed\n- b) 2 is the standard for a finding\n- c) 1 is reserved for syntax errors\n\n> **Answer:** a) 2 means the check itself failed. 1 means the check ran and found something. 2 means it could not run. CI can separate a finding from a broken scanner, and a broken scanner is not a clean host.\n\n**Q2. Why is shell=True a vulnerability in a scanner?**\n\n- a) It is slower than an argument list\n- b) It passes data through `/bin/sh`\n- c) It hides the exit code\n\n> **Answer:** b) It passes data through `/bin/sh`. A list keeps every argument a value. With shell=True the joined string is parsed again, so a filename or a log line containing a semicolon becomes a command.\n\n**Q3. Why must a scanner be read-only?**\n\n- a) So it can run on production safely\n- b) Because writes are slower\n- c) Because cron runs as root\n\n> **Answer:** a) So it can run on production safely. A read-only check is safe to repeat on a live host and its result is reproducible. The moment it edits state, the check becomes a change with a rollback problem.\n\n## Explained\n\n### Exit 2\n\nQuestion one: why exit 2 instead of 1 when the input is unreadable? 1 means the check ran and found something. 2 means it could not run. CI can then tell a finding from a broken scanner, and a broken scanner is not a clean host. If a missing file exits 1, the pipeline reports a vulnerability that does not exist.\n\nshell=True\n\nQuestion two: why is shell=True a vulnerability in a scanner? A list keeps every argument a value. With shell=True the joined string is parsed again by `/bin/sh`, so a filename or a log line containing a semicolon or a dollar sign becomes syntax. The scanner reads untrusted text all day, which is exactly the input you must not hand to a shell.\n\n### Read-only\n\nQuestion three: why must a scanner be read-only? A check should be safe to repeat on production, and its result reproducible. The moment it edits state, it becomes a change with a rollback problem. Read-only is not a limitation; it is what lets you run the tool at 3 a.m.\n\n> **The honest caveat:** read-only is not passive. A scanner can still exhaust CPU, fill a log or hold a lock, so give it a timeout and a budget.\n\n> **The lesson:** the exit code and the write path are part of the security review.\n\n## In one line\n\n> **The code is the contract.** Three answers that decide whether the report can be trusted.\n\n**Check the exit**\n\n```bash\npython3 audit.py; echo $?\n```\n"},{"id":"p532","n":"532","slug":"the-sbom-pipeline-drawn","title":"The SBOM pipeline, drawn","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SBOM","SupplyChain","DevSecOps","Python","CICD"],"series_name":"PythonSec","design":"architecture","script":"","date":"2030-02-19","files":{"devsecops":"notes/p532-the-sbom-pipeline-drawn.md"},"md":"# The SBOM pipeline, drawn\n\nA base image moves a minor version and 40 packages change underneath you. The build stays green, the audit trail says nothing, and nobody can answer what actually shipped. An SBOM answers that only if it is generated, stored and compared in the same place.\n\n## Quick reference\n\n- **Generate:** Build (BUILD) · syft scan (SBOM)\n- **Store:** OCI registry (OCI) · Artefact store (STORE)\n- **Diff:** SBOM diff (DIFF) · Policy gate (GATE)\n\n**Request flow:** Build → syft scan → OCI registry → SBOM diff → Policy gate; syft scan → Artefact store\n\n**Around it:** Image digest (DIG) · Exit code (EXIT)\n\n## Explained\n\n### Generate\n\nThe scan belongs in the build, not on a laptop. The build produces the image and its digest, then syft reads that artefact and writes CycloneDX JSON listing every package and version. Because the scan runs after the image exists, the list belongs to the bytes that will ship.\n\n### Store\n\nThe file goes into the OCI registry beside the image, or into the artefact store the pipeline already uses. Tag it with the image digest, never the branch name. An SBOM nobody can find at 3 a.m. is a document, not evidence.\n\n### Diff\n\nBefore the gate, compare the candidate with the SBOM from production. The diff has three outcomes: components added, removed and version-changed. The version change is the one to read, because a patch bump can add a transitive package or remove one that carried a fix. The policy gate reads that diff, and the job exits non-zero when the inventory moved.\n\n> **The honest caveat:** an SBOM is an inventory, not a scan. It tells you what is inside, not whether any of it is vulnerable, and two scanners can produce different component lists for the same image.\n\n> **The lesson:** generate once, store with the digest, and diff every build.\n\n## In one line\n\n> **Generate once, diff every build.** An SBOM that is never compared is a document, not a control.\n\n**Diff two sboms**\n\n```bash\npython3 sbom_diff.py old.json new.json\n```\n"},{"id":"p533","n":"533","slug":"the-sbom-and-licence-cheat-sheet","title":"The SBOM and licence cheat sheet","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SBOM","SupplyChain","Security","DevOps","Python"],"series_name":"PythonSec","design":"cheatsheet","script":"","date":"2030-02-21","files":{"devsecops":"notes/p533-the-sbom-and-licence-cheat-sheet.md"},"md":"# The SBOM and licence cheat sheet\n\nA dependency review arrives as a spreadsheet, a Slack thread and a PDF, and nobody agrees on the package list. Four blocks of commands settle it.\n\n## Quick reference\n\n### Generate\n\n```bash\nsyft dir:. -o cyclonedx-json\nsyft registry:app:1.4 -o spdx-json\nsyft scan dir:. -o table\ntrivy fs -f cyclonedx -o sbom.json .\n```\n\n### Scan\n\n```bash\ngrype sbom:sbom.json\ngrype sbom:sbom.json --fail-on high\ntrivy sbom sbom.json\ntrivy image --scanners vuln app:1.4\n```\n\n### Licences\n\n```bash\npip-licenses --format=json\npip-licenses --format=csv --with-urls\npip-licenses --allow-only=\"MIT\"\ntrivy fs --scanners license .\n```\n\n### Compare\n\n```bash\ndiff <(jq . a.json) <(jq . b.json)\njq '.components|length' sbom.json\njq -r '.components[].name' s.json|sort\npython3 Scripts/sbom_diff.py a b\n```\n\n## Explained\n\n### Generate\n\n`syft dir:. -o cyclonedx-json` reads the working tree and writes CycloneDX. `syft registry:app:1.4 -o spdx-json` reads the image that actually ships. `syft scan dir:. -o table` is the quick human view, and `trivy fs -f cyclonedx -o sbom.json .` writes the same shape from the scanner you already run.\n\n### Scan\n\nA scan needs a target. `grype sbom:sbom.json` matches the file against its vulnerability database. Adding `--fail-on` high turns a finding into a failed job. `trivy sbom sbom.json` does the equivalent, and `trivy image --scanners vuln app:1.4` scans the image directly when no SBOM exists yet.\n\n### Licences\n\npip-licenses `--format=json` dumps the licence of every installed package. `--format=csv --with-urls` adds the project URL a reviewer will ask for. `--allow-only=\"MIT\"` fails the run when something else appears, and `trivy fs --scanners license .` does a similar pass over a directory.\n\n### Compare\n\nTwo SBOMs compare line by line. diff <(jq . `a.json`) <(jq . `b.json`) is the first look. `jq '.components|length' sbom.json` counts components, `jq -r '.components[].name' s.json|sort` lists them, and `python3 Scripts/sbom_diff.py a b` adds version changes and an exit code.\n\n> **The honest caveat:** every scanner has its own database and its own idea of a component, so two tools can disagree on the same image. Pick one as the gate and use the other as a second opinion.\n\n> **The lesson:** ask one question per block, and let the exit code end the argument.\n\n## In one line\n\n> **Inventory first, then argue.** Four commands per question, and one exit code.\n\n**Write the sbom**\n\n```bash\nsyft dir:. -o cyclonedx-json\n```\n"},{"id":"p534","n":"534","slug":"what-to-check-in-a-dependency","title":"What to check in a dependency","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","DevSecOps","SBOM","Security","Python"],"series_name":"PythonSec","design":"explainer","script":"","date":"2030-02-24","files":{"devsecops":"notes/p534-what-to-check-in-a-dependency.md"},"md":"# What to check in a dependency\n\nA package arrives in a pull request with a familiar name, a green CI run and a README you remember. Four questions decide whether it belongs in the build.\n\n## Quick reference\n\n### 1. Version\n\n- Pin an exact version, not a range\n- Commit the lockfile with hashes\n- Ranges resolve differently tomorrow\n- Pin the transitive packages too\n\n### 2. Licence\n\n- MIT and Apache-2.0 are permissive, with duties\n- GPL-3.0 can reach your own source\n- Read the identifier, not the README\n- No licence is not permission\n\n### 3. Provenance\n\n- Who published it, and from which commit\n- Attestations bind source to digest\n- Typosquats copy the name and the README\n- Check the namespace, not the logo\n\n### 4. Vulnerabilities\n\n- A CVE list is dated the hour it is made\n- Reachability decides what matters\n- Scan the SBOM, not just the manifest\n- A fix is not always a safe version\n\n> **Note:** Four checks, one pull request, in this order.\n\n## Explained\n\n### Version\n\nPin an exact version. Commit the lockfile with hashes. A range such as ~=2.0 promises compatibility and delivers whatever the resolver finds: a different tree next month. Pin the transitive packages too, because the one you did not name is the one that changed.\n\n### Licence\n\nMIT and Apache-2.0 are permissive, though Apache-2.0 adds a notice duty and a patent grant. GPL-3.0 can reach your own source if you distribute the combined work. Read the identifier, not the README. A package with no licence is not permission; it is an unanswered question.\n\n### Provenance\n\nAsk who published the artefact and from which commit. A signed attestation binds the source repository to the image digest, so a rebuilt artefact can be checked against the same source. Typosquats copy the name and the README, so check the registry namespace, not the logo.\n\n### Vulnerabilities\n\nA CVE list is dated the hour it is generated. Match it against the SBOM, then ask whether the vulnerable function is reachable from your code. A high score in a package you never call is noise; a medium in the parser that reads user input is not. Upgrading is not automatically safer, because a fix can arrive with a new dependency.\n\n> **The honest caveat:** provenance is only as strong as the ecosystem that signs it, and most packages still ship no attestation.\n\n> **The lesson:** trust is four checks, not one reputation.\n\n## In one line\n\n> **Four checks, not a feeling.** The file records a choice. The build should enforce it.\n\n**Check one package**\n\n```bash\ngrype sbom:sbom.json\n```\n"},{"id":"p535","n":"535","slug":"an-sbom-diff-in-python","title":"An SBOM diff in Python","sections":["devsecops"],"topic":"Python scripts","tags":["SBOM","SupplyChain","Python","DevSecOps","CICD"],"series_name":"PythonSec","design":"code","script":"sbom_diff.py","date":"2030-02-26","files":{"devsecops":"notes/p535-an-sbom-diff-in-python.md"},"md":"# An SBOM diff in Python\n\nA base image update lands as a one-line change in the Dockerfile, and the image gains three packages and loses one. The tag says nothing about that, so compare the contents instead.\n\n## Quick reference\n\nThe full script is at the end of this note: `sbom_diff.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 sbom_diff.py old.json new.json\n```\n\n**Example output**\n\n```text\nADDED requests 2.31.0\nCHANGED flask 2.2.5 -> 2.3.0\n```\n\n## Explained\n\n### What it compares\n\nThe script loads two CycloneDX JSON files into a name-to-version map. Added components are names in the new file that are not in the old one. Removed components are the reverse. Changed components keep their name and get a new version, which is the line most people skip and the one a transitive bump usually shows up in.\n\n### The exit code\n\nThe loops print added, removed and changed lines, each group sorted by name. When anything is different it exits 1; when the maps are identical it exits 0. That is the whole interface a pipeline needs: no parsing of prose, no grep for a warning, just a non-zero code that stops the promotion.\n\n### Running it\n\n`python3 sbom_diff.py old.json new.json`, where `old.json` is the SBOM stored beside the production image and `new.json` is the one from the build under test. Put it before the deploy step, not after, and keep the old file for as long as the rollback window.\n\n> **The honest caveat:** the key is the component name, so a package that changes its name, or two packages that share one, will look like a removal and an addition rather than a change.\n\n> **The lesson:** compare the inventory, not the tag.\n\n## In one line\n\n> **Diff the list, not the tag.** A tag is a name. The SBOM is what the name contains.\n\n**Gate the build**\n\n```bash\npython3 sbom_diff.py old.json new.json\n```\n\n## Script: sbom_diff.py\n\nDownload: [scripts/sbom_diff.py](../scripts/sbom_diff.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Diff two CycloneDX JSON SBOMs and report what changed.\n\n    python3 sbom_diff.py old.json new.json\n    python3 sbom_diff.py prod.json candidate.json --ignore flask\n\nPrints one line per component added, removed or version-changed, sorted by\nname. Exits 1 when the inventory changed, 0 when the two SBOMs match, and 2\non a missing file, invalid JSON or an unexpected shape. Read-only.\n\nCI/cron: python3 sbom_diff.py old.json new.json || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef load(path):\n    \"\"\"Return {component name: version} from a CycloneDX JSON file.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ValueError(\"invalid JSON in %s: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise ValueError(\"%s: top level must be a JSON object\" % path)\n    components = data.get(\"components\")\n    if not isinstance(components, list):\n        raise ValueError(\"%s: no components list\" % path)\n    found = {}\n    for index, component in enumerate(components):\n        if not isinstance(component, dict):\n            raise ValueError(\"%s: components[%d] is not an object\" % (path, index))\n        name = component.get(\"name\")\n        if not isinstance(name, str) or not name:\n            raise ValueError(\"%s: components[%d] has no name\" % (path, index))\n        version = component.get(\"version\", \"?\")\n        if not isinstance(version, str):\n            version = str(version)\n        version = version or \"?\"\n        if name in found and found[name] != version:\n            raise ValueError(\"%s: duplicate component %s\" % (path, name))\n        found[name] = version\n    return found\n\n\ndef main(argv=None):\n    \"\"\"Compare two SBOMs and return the process exit code.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Diff two CycloneDX JSON SBOMs by component name.\")\n    parser.add_argument(\"old\", help=\"baseline SBOM\")\n    parser.add_argument(\"new\", help=\"candidate SBOM\")\n    parser.add_argument(\"--ignore\", action=\"append\", default=[], metavar=\"NAME\",\n                        help=\"component name to ignore (repeatable)\")\n    args = parser.parse_args(argv)\n    ignore = set(args.ignore)\n    try:\n        old, new = load(args.old), load(args.new)\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    old_names, new_names = set(old) - ignore, set(new) - ignore\n    added = sorted(new_names - old_names)\n    removed = sorted(old_names - new_names)\n    changed = sorted(name for name in old_names & new_names\n                     if old[name] != new[name])\n\n    for name in added:\n        print(\"ADDED %s %s\" % (name, new[name]))\n    for name in removed:\n        print(\"REMOVED %s %s\" % (name, old[name]))\n    for name in changed:\n        print(\"CHANGED %s %s -> %s\" % (name, old[name], new[name]))\n    if not (added or removed or changed):\n        print(\"OK no component changes\")\n        return 0\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p536","n":"536","slug":"a-licence-audit-in-python","title":"A licence audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["SBOM","OpenSource","DevSecOps","Python","SupplyChain"],"series_name":"PythonSec","design":"code","script":"license_audit.py","date":"2030-02-28","files":{"devsecops":"notes/p536-a-licence-audit-in-python.md"},"md":"# A licence audit in Python\n\nA legal review asks for every licence in the product, and the answer is spread across three package managers and a container base image. An SBOM already has the licences; the missing part is the decision.\n\n## Quick reference\n\nThe full script is at the end of this note: `license_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 license_audit.py --allow MIT,BSD-3-Clause --deny GPL-3.0 licenses.json\n```\n\n**Example output**\n\n```text\nFINDING libfoo GPL-3.0\nFINDING mystery NO-LICENCE\n```\n\n## Explained\n\n### The lists\n\nThe script reads a CycloneDX file and takes two comma-separated sets: what the organisation allows and what it forbids. A licence on the deny list always fails, even if someone also put it on the allow list. When an allow list is present, anything outside it fails too, which is how most policies actually read.\n\n### The findings\n\nEach component contributes its licence IDs. A component with no licence entry gets NO-LICENCE, because missing metadata is a question, not a permission. Findings print one per line with the component name and the licence, then the process exits 1. A clean file exits 0.\n\n### The output\n\n`python3 license_audit.py --allow MIT,BSD-3-Clause --deny GPL-3.0 licenses.json` prints libfoo GPL-3.0 and mystery NO-LICENCE in the example, which is exactly the pair a reviewer needs to see. Wire it into the build after the SBOM is generated and before the image is pushed.\n\n> **The honest caveat:** it compares the identifier in the SBOM, so a package that declares Apache-2.0 in its metadata but ships a different file is invisible to this check.\n\n> **The lesson:** an allow list is a policy, and a policy needs an exit code.\n\n## In one line\n\n> **A licence is a decision.** The file records it. The gate enforces it.\n\n**Audit the sbom:** `license_audit.py sbom.json --allow` MIT\n\n## Script: license_audit.py\n\nDownload: [scripts/license_audit.py](../scripts/license_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit the licences in a CycloneDX SBOM against allow and deny lists.\n\n    python3 license_audit.py sbom.json --allow MIT,Apache-2.0\n    python3 license_audit.py sbom.json --deny GPL-3.0,AGPL-3.0\n    python3 license_audit.py sbom.json --allow MIT --deny GPL-3.0\n\nA component with no licence entry is reported as NO-LICENCE. A licence on\nthe deny list always fails, even when it is also on the allow list. Licence\nIDs are compared case-insensitively; an SPDX expression is treated as one\nstring. Exits 1 on a disallowed or unknown licence, 0 when clean, and 2 on a\nmissing file, invalid JSON or an unexpected shape. Read-only.\n\nCI/cron: python3 license_audit.py sbom.json --allow MIT,Apache-2.0 || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef parse_list(value):\n    \"\"\"Turn a comma-separated option into a set of lower-case names.\"\"\"\n    return {item.strip().lower() for item in value.split(\",\") if item.strip()}\n\n\ndef load(path):\n    \"\"\"Return [(component name, [licence, ...]), ...] from an SBOM.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ValueError(\"invalid JSON in %s: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise ValueError(\"%s: top level must be a JSON object\" % path)\n    components = data.get(\"components\")\n    if not isinstance(components, list):\n        raise ValueError(\"%s: no components list\" % path)\n    found = []\n    for index, component in enumerate(components):\n        if not isinstance(component, dict):\n            raise ValueError(\"%s: components[%d] is not an object\" % (path, index))\n        name = component.get(\"name\")\n        if not isinstance(name, str) or not name:\n            raise ValueError(\"%s: components[%d] has no name\" % (path, index))\n        entries = component.get(\"licenses\", [])\n        if not isinstance(entries, list):\n            raise ValueError(\"%s: %s has a non-list licenses field\" % (path, name))\n        licences = []\n        for entry in entries:\n            if not isinstance(entry, dict):\n                continue\n            licence = entry.get(\"license\")\n            value = None\n            if isinstance(licence, dict):\n                value = licence.get(\"id\") or licence.get(\"name\")\n            elif isinstance(entry.get(\"expression\"), str):\n                value = entry[\"expression\"]\n            if isinstance(value, str) and value.strip():\n                licences.append(value.strip())\n        found.append((name, licences))\n    return found\n\n\ndef main(argv=None):\n    \"\"\"Audit the SBOM and return the process exit code.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Audit CycloneDX SBOM licences against allow and deny lists.\")\n    parser.add_argument(\"sbom\", help=\"CycloneDX JSON SBOM to audit\")\n    parser.add_argument(\"--allow\", default=\"\", metavar=\"IDS\",\n                        help=\"comma-separated licences to allow\")\n    parser.add_argument(\"--deny\", default=\"\", metavar=\"IDS\",\n                        help=\"comma-separated licences to deny\")\n    args = parser.parse_args(argv)\n    allow, deny = parse_list(args.allow), parse_list(args.deny)\n    try:\n        components = load(args.sbom)\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings = []\n    for name, licences in components:\n        if not licences:\n            findings.append((name, \"NO-LICENCE\"))\n            continue\n        for licence in licences:\n            key = licence.lower()\n            if key in deny or (allow and key not in allow):\n                findings.append((name, licence))\n    findings = sorted(set(findings))\n    if not findings:\n        print(\"OK all licences allowed\")\n        return 0\n    for name, licence in findings:\n        print(\"FINDING %s %s\" % (name, licence))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p537","n":"537","slug":"sbom-vs-lockfile","title":"SBOM vs lockfile","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SBOM","SupplyChain","DevSecOps","Python","CICD"],"series_name":"PythonSec","design":"compare","script":"","date":"2030-03-03","files":{"devsecops":"notes/p537-sbom-vs-lockfile.md"},"md":"# SBOM vs lockfile\n\nA security review asks what is in the release and what will be installed on the next deploy. Those are two different documents, and teams routinely answer both with whichever file they have open.\n\n## Quick reference\n\n| | **SBOM** | **LOCKFILE** |\n|:--|:--|:--|\n| What it lists | Every component installed | Every dependency resolved |\n| Made by | A scan of the build | A resolve of the manifest |\n| Covers | OS and language packages | Language packages only |\n| Integrity | Digest of the artefact | Hash of each package |\n| Changes when | The build changes | The manifest changes |\n| Answers | What shipped? | What will install? |\n\n**SBOM** (What is installed): An inventory of the artefact that was built, down to the OS packages.\n\n```bash\nsyft dir:. -o cyclonedx-json\n```\n\n**LOCKFILE** (What will install): The resolved graph a package manager will reproduce on the next install.\n\nExample: `pip-sync requirements.txt`\n\n- SBOM → evidence of the build.\n- Lockfile → a promise before the build.\n\n## Explained\n\n### The two files\n\nAn SBOM is generated after the artefact exists. It lists every component that went into the image, including OS packages the language resolver never saw, and it is tied to the image digest. It is evidence of what shipped.\n\n### The lockfile\n\nA lockfile is generated before the artefact exists. The resolver walks the manifest, picks one version for every dependency, records a hash and freezes the graph. The next install reproduces that graph exactly, which is a promise about the future, not a record of the past.\n\n### Where they meet\n\nThe lockfile covers language packages. The SBOM covers those plus the base image, the OS libraries and anything copied in at build time. A lockfile can be clean while the image carries a vulnerable OpenSSL, because the base image is outside its view. An SBOM names that OpenSSL; the lockfile keeps the next install honest. You need both, and you need them tied to the same build. The SBOM changes when the build changes; the lockfile changes when someone re-resolves it.\n\n> **The honest caveat:** a lockfile proves what the resolver chose, not that the choice was safe or current, and an SBOM proves what was present, not that it was the only path into the image.\n\n> **The lesson:** the lockfile is a promise, the SBOM is evidence, and the release needs both.\n\nEvidence of the build,  \npromise before it.\n"},{"id":"p538","n":"538","slug":"9-checks-before-trusting-a-dependency","title":"9 checks before trusting a dependency","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","DevSecOps","Security","Python","CICD"],"series_name":"PythonSec","design":"checklist","script":"","date":"2030-03-05","files":{"devsecops":"notes/p538-9-checks-before-trusting-a-dependency.md"},"md":"# 9 checks before trusting a dependency\n\nA dependency arrives with a familiar name and a clean scan, and the review stops there. Nine checks in three groups take a few minutes and cover the parts a scanner cannot see.\n\n## Quick reference\n\n### Source\n\n- **Upstream is alive** — commits in last 90 days\n- **Name is not a typo** — check the namespace\n- **Release is signed** — sigstore or GPG\n\n### Build\n\n- **Version is exact** — == 2.3.0, not >=2.3\n- **Lockfile is committed** — hash for every package\n- **Install is repeatable** — `pip --require-hashes`\n\n### Runtime\n\n- **SBOM is generated** — `syft dir:. -o cyclonedx`\n- **Scan runs on the SBOM** — `grype sbom:sbom.json`\n- **Licence is allowed** — `license_audit.py --allow`\n\n## Explained\n\n### Source\n\nUpstream is alive. A package with no commits in a year is not abandoned automatically, but it is a risk you should name. The name is not a typo: check the namespace, because typosquats copy the spelling and the README. The release is signed: sigstore or GPG, and verify the signature against the identity you expect.\n\n### Build\n\nVersion is exact. == 2.3.0, not >=2.3, so a rebuild does not quietly move. The lockfile is committed with a hash for every package, including the transitive ones. Install is repeatable: `pip --require-hashes` refuses a package whose hash is not in the file.\n\n### Runtime\n\nThe SBOM is generated with `syft dir:. -o cyclonedx`. The scan runs on that SBOM with `grype sbom:sbom.json`, not on a manifest that may disagree with the image. The licence is allowed: run `license_audit.py --allow` against the SBOM, and let the exit code fail the build.\n\n> **The honest caveat:** none of these proves intent. A trusted maintainer can turn hostile, as the xz-utils backdoor showed in 2024, and no checklist sees that coming.\n\n> **The lesson:** trust is a list of checks you can run, not a feeling you can argue.\n\n## In one line\n\n> **Trust is a list of checks.** Nine questions that cost minutes and save a quarter.\n\n**Pin one today**\n\n```bash\npip install pkg==2.3.0\n```\n"},{"id":"p539","n":"539","slug":"a-terraform-module-pin-check-in-python","title":"A Terraform module pin check in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Terraform","SupplyChain","DevSecOps","Python","IaC"],"series_name":"PythonSec","design":"code","script":"tf_module_pins.py","date":"2030-03-07","files":{"devsecops":"notes/p539-a-terraform-module-pin-check-in-python.md"},"md":"# A Terraform module pin check in Python\n\nA module block with a source and no version is a dependency that updates itself. The next terraform init can pull a different tree, and the plan will not show why.\n\n## Quick reference\n\nThe full script is at the end of this note: `tf_module_pins.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 tf_module_pins.py infra/\n```\n\n**Example output**\n\n```text\nUNPINNED infra/main.tf vpc version\nUNPINNED infra/main.tf app ref\n```\n\n## Explained\n\n### What it reads\n\nThe script walks a directory for .tf files and finds every module block. It reads the source and the version, then applies two rules. A registry source needs a version constraint. A git source needs a ref that is a full commit SHA, not a branch or a tag. Local paths are skipped; the attached script adds a third rule for other remote sources that carry neither.\n\n### Why the SHA\n\nA branch is a pointer. The commit it names today is not the commit it names next week, so the module you reviewed is not the module that runs. A tag is better but still movable. A 40-character SHA names one tree, so a plan is reproducible and an upgrade becomes a visible diff.\n\n### The output\n\nEach finding prints the file, the module name and a reason: version or ref. `python3 tf_module_pins.py infra/` prints UNPINNED infra/main.tf vpc version when the registry module has no constraint, then exits 1. A clean tree exits 0.\n\n> **The honest caveat:** this is a text scan, not an HCL parser. Module blocks produced by Terragrunt or a template, or files generated at plan time, can be missed, so run it beside terraform validate, not instead of it.\n\n> **The lesson:** every module source is a dependency, and every dependency needs a pin.\n\n## In one line\n\n> **A module is a dependency too.** It runs with your credentials, in your account.\n\n**Check one module**\n\n```bash\npython3 tf_module_pins.py .\n```\n\n## Script: tf_module_pins.py\n\nDownload: [scripts/tf_module_pins.py](../scripts/tf_module_pins.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check that every Terraform module source is pinned.\n\n    python3 tf_module_pins.py infra/\n    python3 tf_module_pins.py . --quiet\n\nWalks a directory for .tf files and reads every module block. A registry\nmodule (namespace/name/provider) needs a version constraint; a git module\nneeds a ref that is a full commit SHA; any other remote source needs a\nversion or a SHA ref. Local paths are skipped. Exits 1 when a module is\nunpinned, 0 when every module is pinned, and 2 on a missing directory or an\nunreadable file. Read-only.\n\nCI/cron: python3 tf_module_pins.py infra/ || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport os\nimport re\nimport sys\n\nMODULE = re.compile(r'\\bmodule\\s+\"([A-Za-z0-9_-]+)\"\\s*\\{')\nSOURCE = re.compile(r'\\bsource\\s*=\\s*\"([^\"]*)\"')\nVERSION = re.compile(r'\\bversion\\s*=\\s*\"([^\"]*)\"')\nREGISTRY = re.compile(r\"^[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+$\")\nSHA = re.compile(r\"[?&]ref=([0-9a-fA-F]{40}|[0-9a-fA-F]{64})(?:[&#]|$)\")\n\n\ndef clean(text):\n    \"\"\"Blank comments and heredocs but keep strings, so braces still match.\"\"\"\n    out, index, size = [], 0, len(text)\n    while index < size:\n        char = text[index]\n        if char == \"<\" and text.startswith(\"<<\", index):\n            match = re.match(r\"<<-?([A-Za-z_][A-Za-z0-9_]*)\", text[index:])\n            if match:\n                token = re.escape(match.group(1))\n                start = index + match.end()\n                closing = re.compile(r\"(?m)^[ \\t]*%s[ \\t]*$\" % token).search(text, start)\n                end = closing.end() if closing else size\n                out.append(\"\\n\" * text.count(\"\\n\", index, end))\n                index = end\n                continue\n        if char == '\"':\n            out.append(char)\n            index += 1\n            while index < size:\n                out.append(text[index])\n                if text[index] == \"\\\\\" and index + 1 < size:\n                    out.append(text[index + 1])\n                    index += 2\n                    continue\n                if text[index] == '\"':\n                    index += 1\n                    break\n                index += 1\n            continue\n        if char == \"#\" or (char == \"/\" and index + 1 < size and text[index + 1] == \"/\"):\n            while index < size and text[index] != \"\\n\":\n                out.append(\" \")\n                index += 1\n            continue\n        if char == \"/\" and index + 1 < size and text[index + 1] == \"*\":\n            out.append(\"  \")\n            index += 2\n            while index < size and not (text[index] == \"*\" and index + 1 < size\n                                        and text[index + 1] == \"/\"):\n                out.append(\"\\n\" if text[index] == \"\\n\" else \" \")\n                index += 1\n            if index < size:\n                out.append(\"  \")\n                index += 2\n            continue\n        out.append(char)\n        index += 1\n    return \"\".join(out)\n\n\ndef close_brace(text, start):\n    \"\"\"Return the index of the brace closing the block opened at start.\"\"\"\n    depth, index, quoted = 0, start, False\n    while index < len(text):\n        char = text[index]\n        if quoted:\n            if char == \"\\\\\":\n                index += 2\n                continue\n            if char == '\"':\n                quoted = False\n        elif char == '\"':\n            quoted = True\n        elif char == \"{\":\n            depth += 1\n        elif char == \"}\":\n            depth -= 1\n            if depth == 0:\n                return index\n        index += 1\n    return -1\n\n\ndef blocks(path):\n    \"\"\"Yield (module name, line, body) for each module block in one file.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            text = clean(handle.read())\n    except (OSError, UnicodeDecodeError) as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    for match in MODULE.finditer(text):\n        start = match.end() - 1\n        end = close_brace(text, start)\n        if end < 0:\n            raise ValueError(\"%s: module %s has no closing brace\"\n                             % (path, match.group(1)))\n        line = text.count(\"\\n\", 0, match.start()) + 1\n        yield match.group(1), line, text[start + 1:end]\n\n\ndef is_git(source):\n    \"\"\"True for the git source forms Terraform accepts.\"\"\"\n    return (source.startswith((\"git::\", \"git@\", \"github.com/\", \"bitbucket.org/\"))\n            or \".git\" in source)\n\n\ndef pin_reason(source, body):\n    \"\"\"Return the reason a module is unpinned, or None when it is pinned.\"\"\"\n    if not source or source.startswith((\"./\", \"../\", \"/\")):\n        return None\n    version = VERSION.search(body)\n    has_version = bool(version and version.group(1).strip())\n    ref = SHA.search(source)\n    if REGISTRY.match(source):\n        return None if has_version else \"version\"\n    if is_git(source):\n        if not ref:\n            return \"ref\"\n        return None\n    if not has_version and not ref:\n        return \"source\"\n    return None\n\n\ndef main(argv=None):\n    \"\"\"Scan the directory and return the process exit code.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Find Terraform modules without a pin.\")\n    parser.add_argument(\"path\", nargs=\"?\", default=\".\",\n                        help=\"directory to scan (default: .)\")\n    args = parser.parse_args(argv)\n    if not os.path.isdir(args.path):\n        print(\"ERROR not a directory: %s\" % args.path, file=sys.stderr)\n        return 2\n\n    findings, files = [], 0\n    for root, dirs, names in os.walk(args.path):\n        dirs[:] = sorted(name for name in dirs if name not in (\".git\", \".terraform\"))\n        for name in sorted(names):\n            if not name.endswith(\".tf\"):\n                continue\n            files += 1\n            path = os.path.join(root, name)\n            try:\n                for module, line, body in blocks(path):\n                    source = SOURCE.search(body)\n                    source = source.group(1) if source else \"\"\n                    reason = pin_reason(source, body)\n                    if reason:\n                        findings.append((path, line, module, reason))\n            except ValueError as exc:\n                print(\"ERROR %s\" % exc, file=sys.stderr)\n                return 2\n\n    if not files:\n        print(\"OK no .tf files under %s\" % args.path)\n        return 0\n    findings.sort()\n    for path, _line, module, reason in findings:\n        print(\"UNPINNED %s %s %s\" % (path, module, reason))\n    if not findings:\n        print(\"OK every module source is pinned\")\n        return 0\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p540","n":"540","slug":"a-terraform-state-secret-scanner-in-python","title":"A Terraform state secret scanner in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Terraform","Security","DevSecOps","Python","IaC"],"series_name":"PythonSec","design":"code","script":"terraform_state_secrets.py","date":"2030-03-10","files":{"devsecops":"notes/p540-a-terraform-state-secret-scanner-in-python.md"},"md":"# A Terraform state secret scanner in Python\n\nA state file sits in a bucket with versioning and a wide read policy, and inside it are the passwords Terraform used to build the database. The state is a secret store that nobody chose.\n\n## Quick reference\n\nThe full script is at the end of this note: `terraform_state_secrets.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 terraform_state_secrets.py terraform.tfstate\n```\n\n**Example output**\n\n```text\nFINDING $.resources.0.instances.0.attributes.password ***\nFINDING $.resources.1.instances.0.attributes.user_data ***\n```\n\n## Explained\n\n### The walk\n\nThe script loads the JSON and walks every dict and list, building a path such as $.resources.0.instances.0.attributes.password. Keys and indices both become part of the path, so a finding names the exact attribute that holds the value.\n\n### The two tests\n\nA path that contains password, secret, token or key is a finding. A string longer than 23 characters with a Shannon entropy above 3.5 is also a finding, which catches base64 user data and connection strings that no key name would reveal. The attached script also requires three character classes and skips hashes, checksums, ARNs and IDs, because they look random and are not secrets.\n\n### The output\n\nOnly the path is printed, and the value is replaced with three asterisks. That is deliberate: a scanner that prints the secret has moved the secret into the CI log, the ticket and the screenshot. Every finding prints as FINDING followed by the path and \\*\\*\\*, and the process exits 1.\n\n> **The honest caveat:** a clean scan does not make the state safe. It still holds plaintext values the provider marked sensitive, so restrict who can read the bucket and encrypt it at rest.\n\n> **The lesson:** name the path, never the value.\n\n## In one line\n\n> **Name the path, not the value.** The finding is where it lives, not what it says.\n\n**Scan the state**\n\n```bash\npython3 terraform_state_secrets.py state\n```\n\n## Script: terraform_state_secrets.py\n\nDownload: [scripts/terraform_state_secrets.py](../scripts/terraform_state_secrets.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Scan a terraform.tfstate file for values that look like secrets.\n\n    python3 terraform_state_secrets.py terraform.tfstate\n    python3 terraform_state_secrets.py state.json --min-length 32\n\nWalks every value, finds strings whose key looks like a credential\n(password, secret, token, key) and long random-looking strings. Prints the\npath only and replaces the value with ***. Exits 1 on findings, 0 when\nclean, and 2 on a missing file, invalid JSON or a top-level that is not an\nobject. Read-only.\n\nCI/cron: python3 terraform_state_secrets.py terraform.tfstate || exit 1\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport math\nimport re\nimport sys\n\nKEY = re.compile(r\"(?i)(password|passwd|passphrase|secret|token|key)$\")\nSAFE = re.compile(r\"(?i)(public|map_key|foreign_key|partition_key|sort_key|\"\n                  r\"primary_key|key_name|hash|checksum|etag|digest|fingerprint|\"\n                  r\"uuid|_id|_arn|^id$)$\")\n\n\ndef entropy(value):\n    \"\"\"Shannon entropy in bits per character.\"\"\"\n    counts = {}\n    for char in value:\n        counts[char] = counts.get(char, 0) + 1\n    total = float(len(value))\n    return -sum(count / total * math.log2(count / total) for count in counts.values())\n\n\ndef random_looking(value, min_length):\n    \"\"\"True for a long string with mixed character classes and high entropy.\"\"\"\n    if len(value) < min_length:\n        return False\n    classes = sum((any(c.islower() for c in value),\n                   any(c.isupper() for c in value),\n                   any(c.isdigit() for c in value),\n                   any(not c.isalnum() for c in value)))\n    return classes >= 3 and entropy(value) >= 3.5\n\n\ndef walk(value, path=\"$\"):\n    \"\"\"Yield (path, string) for every string in the JSON tree.\"\"\"\n    if isinstance(value, dict):\n        for key, item in value.items():\n            yield from walk(item, \"%s.%s\" % (path, key))\n    elif isinstance(value, list):\n        for index, item in enumerate(value):\n            yield from walk(item, \"%s.%s\" % (path, index))\n    elif isinstance(value, str):\n        yield path, value\n\n\ndef main(argv=None):\n    \"\"\"Scan the state file and return the process exit code.\"\"\"\n    parser = argparse.ArgumentParser(\n        description=\"Find secret-looking values in a Terraform state file.\")\n    parser.add_argument(\"state\", help=\"terraform.tfstate or any state JSON\")\n    parser.add_argument(\"--min-length\", type=int, default=24,\n                        help=\"shortest random-looking string to report (default: 24)\")\n    args = parser.parse_args(argv)\n    if args.min_length < 8:\n        parser.error(\"--min-length must be at least 8\")\n    try:\n        with open(args.state, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        print(\"ERROR cannot read %s: %s\" % (args.state, exc), file=sys.stderr)\n        return 2\n    except ValueError as exc:\n        print(\"ERROR invalid JSON in %s: %s\" % (args.state, exc), file=sys.stderr)\n        return 2\n    if not isinstance(data, dict):\n        print(\"ERROR %s: top level must be a JSON object\" % args.state,\n              file=sys.stderr)\n        return 2\n\n    findings = []\n    for path, value in walk(data):\n        leaf = path.rsplit(\".\", 1)[-1]\n        if not value or SAFE.search(leaf):\n            continue\n        if KEY.search(leaf) or random_looking(value, args.min_length):\n            findings.append(path)\n    findings.sort()\n    if not findings:\n        print(\"OK no secret-looking values\")\n        return 0\n    for path in findings:\n        print(\"FINDING %s ***\" % path)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p541","n":"541","slug":"quiz-3-supply-chain-questions","title":"Quiz: 3 supply-chain questions","sections":["devsecops"],"topic":"Supply chain & SBOM","tags":["SupplyChain","SBOM","Terraform","DevSecOps","Security"],"series_name":"PythonSec","design":"quiz","script":"","date":"2030-03-12","files":{"devsecops":"notes/p541-quiz-3-supply-chain-questions.md"},"md":"# Quiz: 3 supply-chain questions\n\nThree questions separate a supply-chain control from a supply-chain document. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why is an SBOM not a vulnerability scan?**\n\n- a) It lists packages, not CVEs\n- b) It only covers containers\n- c) It is generated after deploy\n\n> **Answer:** a) It lists packages, not CVEs. An SBOM is an inventory of names, versions and licences. A scanner matches that inventory against advisories. Without it, the file is a manifest, not a finding.\n\n**Q2. What does a lockfile prove?**\n\n- a) That the build passed\n- b) That the graph resolved once\n- c) That no CVE exists\n\n> **Answer:** b) That the graph resolved once. It records the exact versions and hashes the resolver chose, so the same install reproduces them. It proves what was resolved, not that it is safe or current.\n\n**Q3. Why pin a git module to a SHA, not a branch?**\n\n- a) A branch has no version\n- b) A branch can move under you\n- c) Terraform rejects branches\n\n> **Answer:** b) A branch can move under you. A branch is a pointer: the next commit changes what runs, with no diff in your repo. A SHA names one tree, so the plan is reproducible and an upgrade is a review.\n\n## Explained\n\n### SBOM versus scan\n\nQuestion one: why is an SBOM not a vulnerability scan? Because it is an inventory. It lists the components, versions and licences it found, and stops. A scanner takes that list and matches it against advisories, which is where a CVE appears. An SBOM with no scan is a manifest; a scan with no SBOM is a guess about what is installed.\n\n### What a lockfile proves\n\nQuestion two: what does a lockfile prove? That the resolver ran once and produced a graph, and that a later install can reproduce it from the recorded versions and hashes. It proves the graph, not its safety. A locked package can be three years old, and the lockfile will happily reproduce the vulnerability.\n\n### Branch or SHA\n\nQuestion three: why pin a git module to a SHA, not a branch? A branch is a pointer that moves. The next commit changes what runs in your account, and your repository shows no diff. A commit SHA names one tree, so a plan is reproducible and an upgrade becomes a reviewable change.\n\n> **The honest caveat:** a SHA can still point at malicious code. Pinning gives you reproducibility and a review point, not trust.\n\n> **The lesson:** a list is not a verdict, and a pin is not a promise of safety.\n\n## In one line\n\n> **A list is not a verdict.** Three questions to close the supply-chain phase.\n\n**Ask the sbom**\n\n```bash\nsyft dir:. -o cyclonedx-json\n```\n"},{"id":"p542","n":"542","slug":"policy-as-code-in-three-tiers","title":"Policy as code, in three tiers","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["PolicyAsCode","OPA","DevSecOps","Kubernetes","CloudNative"],"series_name":"PythonSec","design":"architecture","script":"","date":"2030-03-14","files":{"devsecops":"notes/p542-policy-as-code-in-three-tiers.md"},"md":"# Policy as code, in three tiers\n\nA team writes its first Rego rule, runs it once in a terminal, and calls it policy as code. Six weeks later the rule is not in CI, the cluster carries a different version, and nobody can say which one is live.\n\n## Quick reference\n\n- **Source:** Policy files (REGO) · Policy tests (TEST)\n- **Evaluate:** OPA engine (OPA) · CI step (CI)\n- **Enforce:** Admission (ADMIT) · Report (OUT)\n\n**Request flow:** Policy files → OPA engine → CI step → Admission → Report; Policy tests → OPA engine\n\n## Explained\n\n### The three tiers\n\nSource. Two nodes: policy files and policy tests. The files are Rego; the tests are fixtures with expected allow and deny decisions. Both sit in the same repository and go through the same review as the application.\n\nEvaluate. The policy files and the tests feed the OPA engine. Its decision goes to a CI step, which runs the tests and evaluates the manifests in the pull request. A rule that fails its own tests never reaches a cluster.\n\nEnforce. The same bundle reaches admission, which sees every create and update, and a report, which records what was allowed and denied. The arrows run from files and tests into the engine, from the engine into CI, then from CI to admission, and from admission to the report. Admission is the gate; the report is the evidence, and without it the gate is a decision nobody can audit later.\n\n### The drift\n\nThe gap between source and enforce is where drift lives: a hotfix at admission, a deleted test, a bundle never rebuilt.\n\n> **The honest caveat:** an engine only enforces the rules someone wrote and tested. A selector typo can pass every test written for the wrong object.\n\n> **The lesson:** write the rule, test the decision, then enforce the same bundle in both places.\n\n## In one line\n\n> **Source, evaluate, then enforce.** A policy that is not tested is a wish with YAML around it.\n\n**Test one rule**\n\n```bash\nopa test policy/ -v\n```\n"},{"id":"p543","n":"543","slug":"querying-cloud-json-without-an-sdk","title":"Querying cloud JSON without an SDK","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["Python","AWS","Kubernetes","CloudCLI","DevSecOps"],"series_name":"PythonSec","design":"cheatsheet","script":"","date":"2030-03-17","files":{"devsecops":"notes/p543-querying-cloud-json-without-an-sdk.md"},"md":"# Querying cloud JSON without an SDK\n\nYou need every unencrypted volume, and the SDK wants credentials, a client and a paginator first. The CLI already prints JSON; the gap is the query.\n\n## Quick reference\n\n### Select\n\n```bash\naws sts get-caller-identity\njq '.items[].metadata.name' pods.json\nkubectl get cm -o jsonpath='{.data}'\npython3 -m json.tool ec2.json\naws s3api list-buckets --query 'Buckets'\n```\n\n### Filter\n\n```bash\njq '.Volumes[]|select(.Encrypted==false)'\njq '.Users[]|select(.PasswordLastUsed)'\njq '.Statement[]|select(.Effect==\"Allow\")'\njq '.Grants[]|select(.Grantee.URI)'\n```\n\n### Aggregate\n\n```bash\njq '.items | length' pods.json\njq '[.Volumes[].Size] | add' vols.json\njq '[.items[].spec.containers[]] | length'\njq 'group_by(.region) | map(length)'\n```\n\n### Diff\n\n```bash\njq -S . old.json > /tmp/a.json\njq -S . new.json > /tmp/b.json\ndiff -u /tmp/a.json /tmp/b.json\nkubectl diff -f deploy.yaml\njq -S . policy.json | sha256sum\n```\n\n## Explained\n\n### Four moves\n\nSelect. Ask for the field, not the object. `aws sts get-caller-identity` proves the account. `jq '.items[].metadata.name' pods.json` lists pod names, `kubectl get cm -o jsonpath='{.data}'` prints a config map, `python3 -m json.tool ec2.json` makes a response readable, and `aws s3api list-buckets --query 'Buckets'` lists buckets.\n\nFilter. Keep only matching objects. `jq '.Volumes[]|select(.Encrypted==false)'` finds unencrypted volumes, .Users[]|select(.PasswordLastUsed) finds console sign-ins, .Statement[]|select(.Effect==\"Allow\") keeps allow statements, and .Grants[]|select(.Grantee.URI) finds ACL grants.\n\nAggregate. Count and group. jq '.items | length' counts pods, [.Volumes[].Size] | add sums volume sizes, [.items[].spec.containers[]] | length counts containers, and group_by(.region) | map(length) counts objects per region.\n\nDiff. Sort both sides first. `jq -S . old.json > /tmp/a.json` and the same for the new file, then diff -u the two. `kubectl diff -f deploy.yaml` compares a manifest with the live object, and `jq -S . policy.json | sha256sum` checks equality.\n\n### The glue\n\nPython is the glue: subprocess.run captures the output, json.loads parses it, and a threshold becomes exit 1. Python keeps types honest: counts stay integers, booleans stay booleans, and an empty list is not a missing key.\n\n> **The honest caveat:** none of these reduces the payload. `--query` filters after the response arrives, jq is another dependency on the runner, and jsonpath prints text rather than JSON.\n\n> **The lesson:** query the shape at the edge, then let Python decide.\n\n## In one line\n\n> **Ask the JSON, not the SDK.** The query is cheap; the parsing is where the bugs live.\n\n**Pretty-print one**\n\n```bash\npython3 -m json.tool ec2.json\n```\n"},{"id":"p544","n":"544","slug":"four-policy-checks-that-catch-most-misconfigs","title":"Four policy checks that catch most misconfigs","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["CloudSecurity","Kubernetes","AWS","DevSecOps","PolicyAsCode"],"series_name":"PythonSec","design":"explainer","script":"","date":"2030-03-19","files":{"devsecops":"notes/p544-four-policy-checks-that-catch-most-misconfigs.md"},"md":"# Four policy checks that catch most misconfigs\n\nA review passes because the Terraform plan is short. The bucket is public, the security group is open, the role is admin and the pod has no limits; the plan was short because nobody read it.\n\n## Quick reference\n\n### Public storage\n\n- A bucket policy with Principal \\* and no Condition\n- Block Public Access off, or one of its four flags\n- An ACL grant to AllUsers or AuthenticatedUsers\n\n### Open ingress\n\n- 0.0.0.0/0 on 22, 3389, 3306 or 5432\n- ::/0 on the same ports, often forgotten\n- A default security group with an all-traffic rule\n\n### Wildcard IAM\n\n- Action \\* and Resource \\* in one statement\n- s3:\\* or iam:\\* where one verb was needed\n- iam:PassRole with a wildcard resource\n\n### Missing limits\n\n- No resources.limits on any container\n- QoS BestEffort: first evicted under pressure\n- One pod can starve the node it shares\n\n> **Note:** Four checks, run on every plan and every cluster.\n\n## Explained\n\n### Four checks\n\nPublic storage. A bucket policy with Principal \\* and no Condition, Block Public Access switched off even for one flag, or an ACL grant to AllUsers or AuthenticatedUsers. Any one is enough to expose the data.\n\nOpen ingress. 0.0.0.0/0 or ::/0 on 22, 3389, 3306 or 5432. A default security group with an all-traffic rule is the same finding with a friendlier name.\n\nWildcard IAM. Action \\* with Resource \\*, s3:\\* where one verb was needed, or iam:PassRole on a wildcard resource. The grant outlives the ticket that asked for it.\n\nMissing limits. No resources.limits on any container puts the pod in the BestEffort class: first to be evicted, and able to starve the node it shares.\n\n### Why these four\n\nEach one is a default that turns a small mistake into an exposure, and each is cheap to check on a plan, a saved JSON dump or a live account. They are not the only checks; they are the four that keep appearing in the same audit.\n\n> **The honest caveat:** each is a candidate, not a verdict. A public website bucket, port 443 and a break-glass admin role are legitimate. The check finds the ones nobody meant.\n\n> **The lesson:** check the four defaults before you argue about the exotic ones.\n\n## In one line\n\n> **Four checks, most misconfigs.** Not every finding is a bug, but every one deserves a look.\n\n**Run the four**\n\n```bash\ncheckov -d . --compact\n```\n"},{"id":"p545","n":"545","slug":"an-iam-policy-diff-in-python","title":"An IAM policy diff in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","IAM","Python","CloudSecurity","DevSecOps"],"series_name":"PythonSec","design":"code","script":"cloud_policy_diff.py","date":"2030-03-21","files":{"devsecops":"notes/p545-an-iam-policy-diff-in-python.md"},"md":"# An IAM policy diff in Python\n\nA pull request adds one line to an IAM policy. The reviewer sees a green check, approves, and the role can now read every bucket in the account.\n\n## Quick reference\n\nThe full script is at the end of this note: `cloud_policy_diff.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cloud_policy_diff.py /tmp/iam_old.json /tmp/iam_new.json\n```\n\n**Example output**\n\n```text\nADD 1 DEL 1 WIDE 1\nALERT\n```\n\n## Explained\n\n### What it compares\n\nThe script loads the old and new policy JSON and indexes each statement by Sid, or by position when there is none. It prints statements added, statements removed, and statements widened. A matched Allow statement is widened when a wildcard appears in its Action or Resource, when it grants a new action or resource, or when a Deny became an Allow; a removed Deny counts too.\n\n### The frame\n\nThe condensed version reads both documents, indexes them by Sid, counts the additions and removals, and compares the wildcard state of each matched statement. On the fixture it prints ADD 1 DEL 1 WIDE 1, then ALERT, and exits 1. The attached script prints the statement names and the reason for each widening.\n\n### How to run it\n\n`python3 cloud_policy_diff.py old.json new.json`, or pipe the new version from aws iam get-policy-version and keep the old file. Run it before apply, or in CI on the policy change. Exit 0 means no widening, 1 means a widened grant, 2 means bad input or usage.\n\n> **The honest caveat:** a diff sees the document, not the effective permission. A Condition, a permissions boundary or an SCP can narrow what the statement actually grants. The tool points at the change; a human still reads the context.\n\n> **The lesson:** review the permission that changed, not the policy that contains it.\n\n## In one line\n\n> **A wildcard is a quiet change.** The line that grants everything rarely looks like a line.\n\n**Diff before apply**\n\n```bash\npython3 cloud_policy_diff.py old new\n```\n\n## Script: cloud_policy_diff.py\n\nDownload: [scripts/cloud_policy_diff.py](../scripts/cloud_policy_diff.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Diff two IAM policy documents and report permissions that widened.\n\nCompares an old and a new AWS IAM policy JSON document and prints:\n\n    ADD   a statement only in the new document\n    DEL   a statement only in the old document\n    WIDE  a grant that became broader\n\nA matched Allow statement is widened when its new Action or Resource list\ncontains a wildcard the old list did not contain, or when it adds an action\nor a resource the old statement did not grant. An added Allow statement with\na wildcard, an Allow statement that was a Deny, and a removed Deny statement\nalso count as widening. A new Allow statement without a wildcard is printed\nas ADD and passes unless --strict-adds is used.\n\nStatements are matched by Sid. Statements without a Sid are matched by\nposition, so keep a Sid on every statement you review. NotAction and\nNotResource are out of scope. Conditions are not evaluated: a wildcard with\na Condition is still reported as widened, so read the reason before you\nfail the build.\n\nThe input can be an identity policy, a resource policy, or the output of\n`aws iam get-policy-version` (the URL-encoded Document is decoded).\n\nUsage:\n    python3 cloud_policy_diff.py old.json new.json\n    python3 cloud_policy_diff.py old.json new.json --quiet\n    python3 cloud_policy_diff.py old.json new.json --strict-adds\n    aws iam get-policy-version --policy-arn \"$ARN\" --version-id v7 \\\n        | python3 cloud_policy_diff.py /tmp/old.json -\n    python3 cloud_policy_diff.py - new.json < old.json\n\nExit codes: 0 no widening (additions and removals still print), 1 a\npermission widened, 2 usage or unreadable/invalid input.\n\nCI (block a merge that widens):\n    python3 cloud_policy_diff.py /tmp/old.json /tmp/new.json || exit 1\nCron (weekly drift check):\n    0 6 * * 1 python3 /opt/s0sta/cloud_policy_diff.py /srv/iam/old.json /srv/iam/new.json\n\nRead-only: opens the two documents and writes nothing.\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom collections import Counter\nfrom urllib.parse import unquote\n\n\nclass DiffError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source, label):\n    \"\"\"Read one JSON document from a file or '-' for stdin, then unwrap it.\"\"\"\n    try:\n        if source == \"-\":\n            doc = json.load(sys.stdin)\n        else:\n            with open(source, encoding=\"utf-8\") as handle:\n                doc = json.load(handle)\n    except OSError as exc:\n        raise DiffError(\"cannot read %s %s: %s\" % (label, source, exc))\n    except ValueError as exc:\n        raise DiffError(\"%s %s is not JSON: %s\" % (label, source, exc))\n    return unwrap(doc, label)\n\n\ndef unwrap(doc, label):\n    \"\"\"Accept a raw policy or the usual API envelopes around one.\"\"\"\n    if not isinstance(doc, dict):\n        raise DiffError(\"%s must be a JSON object\" % label)\n    if \"Statement\" in doc:\n        return doc\n    for field in (\"Policy\", \"Document\", \"PolicyDocument\", \"AssumeRolePolicyDocument\"):\n        inner = doc.get(field)\n        if isinstance(inner, str):\n            try:\n                inner = json.loads(unquote(inner))\n            except ValueError as exc:\n                raise DiffError(\"%s %s field is not JSON: %s\" % (label, field, exc))\n        if isinstance(inner, dict) and \"Statement\" in inner:\n            return inner\n    version = doc.get(\"PolicyVersion\")\n    if isinstance(version, dict):\n        return unwrap(version, label)\n    raise DiffError(\"%s has no Statement field\" % label)\n\n\ndef statements(doc, label):\n    \"\"\"Return the Statement list, checking the shape as it goes.\"\"\"\n    body = doc.get(\"Statement\")\n    if body is None:\n        return []\n    if isinstance(body, dict):\n        body = [body]\n    if not isinstance(body, list):\n        raise DiffError(\"%s Statement must be an object or a list\" % label)\n    out = []\n    for number, statement in enumerate(body, 1):\n        if not isinstance(statement, dict):\n            raise DiffError(\"%s statement %d is not an object\" % (label, number))\n        if not isinstance(statement.get(\"Effect\"), str):\n            raise DiffError(\"%s statement %d has no Effect\" % (label, number))\n        out.append(statement)\n    return out\n\n\ndef keyed(items):\n    \"\"\"Map each statement to a stable key: Sid when unique, position otherwise.\"\"\"\n    sids = Counter(str(st.get(\"Sid\")) for st in items if st.get(\"Sid\"))\n    out = []\n    for position, statement in enumerate(items, 1):\n        sid = statement.get(\"Sid\")\n        if isinstance(sid, str) and sid and sids[sid] == 1:\n            key = \"sid:\" + sid\n        else:\n            key = \"pos:%d\" % position\n        out.append((key, statement))\n    return dict(out)\n\n\ndef as_list(value):\n    \"\"\"Return a string field as a list, and drop non-string entries.\"\"\"\n    if isinstance(value, str):\n        return [value]\n    if isinstance(value, list):\n        return [item for item in value if isinstance(item, str)]\n    return []\n\n\ndef has_wildcard(values):\n    \"\"\"True when any value contains a wildcard.\"\"\"\n    return any(\"*\" in value for value in values)\n\n\ndef label_of(key):\n    \"\"\"Turn a matching key into something a reviewer can find in the file.\"\"\"\n    kind, value = key.split(\":\", 1)\n    if kind == \"sid\":\n        return value\n    return \"statement \" + value\n\n\ndef describe(statement):\n    \"\"\"One compact line for an added or removed statement.\"\"\"\n    effect = str(statement.get(\"Effect\", \"?\"))\n    actions = \",\".join(as_list(statement.get(\"Action\"))) or \"-\"\n    resources = \",\".join(as_list(statement.get(\"Resource\"))) or \"-\"\n    if len(actions) > 42:\n        actions = actions[:39] + \"...\"\n    if len(resources) > 42:\n        resources = resources[:39] + \"...\"\n    return \"%s %s on %s\" % (effect, actions, resources)\n\n\ndef widen_reason(old, new):\n    \"\"\"Return a reason string when the new statement grants more than the old.\"\"\"\n    old_effect = str(old.get(\"Effect\", \"\")).lower()\n    new_effect = str(new.get(\"Effect\", \"\")).lower()\n    if old_effect == \"deny\" and new_effect == \"allow\":\n        return \"Deny became Allow\"\n    if new_effect != \"allow\":\n        return None\n\n    old_actions = as_list(old.get(\"Action\"))\n    new_actions = as_list(new.get(\"Action\"))\n    old_resources = as_list(old.get(\"Resource\"))\n    new_resources = as_list(new.get(\"Resource\"))\n\n    reasons = []\n    wild_action = has_wildcard(new_actions) and not has_wildcard(old_actions)\n    wild_resource = has_wildcard(new_resources) and not has_wildcard(old_resources)\n    if wild_action:\n        reasons.append(\"Action wildcard\")\n    if wild_resource:\n        reasons.append(\"Resource wildcard\")\n    if not wild_action and not has_wildcard(old_actions):\n        added_actions = [a for a in new_actions if a not in old_actions]\n        if added_actions:\n            reasons.append(\"new action \" + \",\".join(added_actions[:2]))\n    if not wild_resource and not has_wildcard(old_resources):\n        added_resources = [r for r in new_resources if r not in old_resources]\n        if added_resources:\n            reasons.append(\"new resource \" + added_resources[0][:30])\n    return \"; \".join(reasons) or None\n\n\ndef compare(old_items, new_items, strict_adds=False):\n    \"\"\"Return (added, removed, widened, old_by_key, new_by_key).\"\"\"\n    old = keyed(old_items)\n    new = keyed(new_items)\n    added = [key for key in new if key not in old]\n    removed = [key for key in old if key not in new]\n    widened = []\n\n    for key in old:\n        if key in new:\n            reason = widen_reason(old[key], new[key])\n            if reason:\n                widened.append((key, reason))\n\n    for key in added:\n        statement = new[key]\n        if str(statement.get(\"Effect\", \"\")).lower() != \"allow\":\n            continue\n        has_star = has_wildcard(as_list(statement.get(\"Action\")))\n        has_star = has_star or has_wildcard(as_list(statement.get(\"Resource\")))\n        if has_star:\n            widened.append((key, \"new wildcard grant\"))\n        elif strict_adds:\n            widened.append((key, \"new Allow statement\"))\n\n    for key in removed:\n        if str(old[key].get(\"Effect\", \"\")).lower() == \"deny\":\n            widened.append((key, \"Deny removed\"))\n\n    return added, removed, widened, old, new\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Diff two AWS IAM policy documents and flag widened permissions.\",\n        epilog=\"Exit codes: 0 no widening, 1 widening found, 2 usage or input error.\")\n    parser.add_argument(\"old\", help=\"old policy JSON file, or - for stdin\")\n    parser.add_argument(\"new\", help=\"new policy JSON file, or - for stdin\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    parser.add_argument(\"--strict-adds\", action=\"store_true\",\n                        help=\"treat every added Allow statement as a widening\")\n    args = parser.parse_args(argv)\n\n    if args.old == \"-\" and args.new == \"-\":\n        parser.error(\"old and new cannot both read from stdin\")\n\n    try:\n        old_items = statements(read_json(args.old, \"old policy\"), \"old policy\")\n        new_items = statements(read_json(args.new, \"new policy\"), \"new policy\")\n        added, removed, widened, old, new = compare(old_items, new_items,\n                                                    args.strict_adds)\n    except DiffError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not args.quiet:\n        for key in added:\n            print(\"ADD  %s  %s\" % (label_of(key), describe(new[key])))\n        for key in removed:\n            print(\"DEL  %s  %s\" % (label_of(key), describe(old[key])))\n        for key, reason in widened:\n            print(\"WIDE %s  %s\" % (label_of(key), reason))\n\n    print(\"ADD %d DEL %d WIDE %d\" % (len(added), len(removed), len(widened)))\n    if widened:\n        print(\"ALERT %d widened permission%s\" % (len(widened),\n                                                 \"\" if len(widened) == 1 else \"s\"))\n        return 1\n    print(\"OK no widening\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p546","n":"546","slug":"an-s3-bucket-config-audit-in-python","title":"An S3 bucket config audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","S3","CloudSecurity","Python","DevSecOps"],"series_name":"PythonSec","design":"code","script":"s3_config_audit.py","date":"2030-03-24","files":{"devsecops":"notes/p546-an-s3-bucket-config-audit-in-python.md"},"md":"# An S3 bucket config audit in Python\n\nThe bucket has a policy, the policy looks narrow, and the review stops there. Encryption is off, versioning is suspended, logging is disabled and the ACL still grants AllUsers read.\n\n## Quick reference\n\nThe full script is at the end of this note: `s3_config_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 s3_config_audit.py /tmp/bucket.json\n```\n\n**Example output**\n\n```text\nALERT enc log\n```\n\n## Explained\n\n### Five settings\n\nEncryption: a default rule must exist, not just a KMS key somewhere in the account. Versioning: Status must be Enabled, because Suspended is not a backup. Public access block: all four flags must be true. Logging: LoggingEnabled needs a TargetBucket. ACL: no grant may go to AllUsers or AuthenticatedUsers.\n\n### The frame\n\nThe condensed version builds a dictionary of five booleans, collects the failed keys and prints them. On the fixture it prints ALERT enc log and exits 1: encryption and logging are the two missing sections. The attached script prints one FAIL line per finding and the same exit code.\n\n### How to run it\n\nCollect the five API responses, join them with `jq -s` into one file, then run `python3 s3_config_audit.py /tmp/bucket.json`. In CI, run it on a saved bundle; from cron, on a nightly export. Exit 0 is clean, 1 is a finding, 2 is a usage or input error.\n\n> **The honest caveat:** the audit reads configuration, not the objects. A bucket can be encrypted and still hold public data, and a missing section is treated as a finding rather than a pass.\n\n> **The lesson:** a bucket is safe when five settings agree, not when one policy looks narrow.\n\n## In one line\n\n> **A bucket is five settings.** Public is a state, not a checkbox you ticked once.\n\n**Audit one bucket**\n\n```bash\npython3 s3_config_audit.py /tmp/b.json\n```\n\n## Script: s3_config_audit.py\n\nDownload: [scripts/s3_config_audit.py](../scripts/s3_config_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit one S3 bucket configuration and flag missing or public settings.\n\nReads a JSON bundle with the five settings that decide whether a bucket is\nsafe by default:\n\n    encryption             aws s3api get-bucket-encryption\n    versioning             aws s3api get-bucket-versioning\n    public_access_block    aws s3api get-public-access-block\n    logging                aws s3api get-bucket-logging\n    acl                    aws s3api get-bucket-acl\n\nThe bundle is one object with those keys, for example:\n\n    {\n      \"bucket\": \"demo\",\n      \"encryption\": {\"ServerSideEncryptionConfiguration\": {\"Rules\": [...]}},\n      \"versioning\": {\"Status\": \"Enabled\"},\n      \"public_access_block\": {\"PublicAccessBlockConfiguration\": {...}},\n      \"logging\": {\"LoggingEnabled\": {\"TargetBucket\": \"logs\", ...}},\n      \"acl\": {\"Grants\": [...]}\n    }\n\nBuild it from the API responses (encryption and the public access block\nreturn an error when they are not configured, so keep an empty object):\n\n    aws s3api get-bucket-encryption --bucket demo > /tmp/enc.json 2>/dev/null \\\n        || echo '{}' > /tmp/enc.json\n    aws s3api get-bucket-versioning --bucket demo > /tmp/ver.json\n    aws s3api get-public-access-block --bucket demo > /tmp/pab.json 2>/dev/null \\\n        || echo '{}' > /tmp/pab.json\n    aws s3api get-bucket-logging --bucket demo > /tmp/log.json\n    aws s3api get-bucket-acl --bucket demo > /tmp/acl.json\n    jq -s '{encryption:.[0], versioning:.[1], public_access_block:.[2],\n            logging:.[3], acl:.[4]}' /tmp/enc.json /tmp/ver.json \\\n        /tmp/pab.json /tmp/log.json /tmp/acl.json > /tmp/bucket.json\n\nThe script checks:\n    encryption           a default encryption rule must exist\n    versioning           Status must be Enabled\n    public access block  all four block settings must be true\n    logging              LoggingEnabled with a TargetBucket must exist\n    acl                  no grant to AllUsers or AuthenticatedUsers\n\nUsage:\n    python3 s3_config_audit.py /tmp/bucket.json\n    cat /tmp/bucket.json | python3 s3_config_audit.py -\n    python3 s3_config_audit.py /tmp/bucket.json --quiet\n\nExit codes: 0 clean, 1 at least one finding, 2 usage or unreadable/invalid\ninput. A missing section is a finding, not a pass: absent configuration is\nwhat the audit exists to catch.\n\nCI (a non-zero exit fails the job):\n    python3 s3_config_audit.py /tmp/bucket.json || exit 1\nCron (daily at 06:20):\n    20 6 * * * python3 /opt/s0sta/s3_config_audit.py /srv/s3/bucket.json\n\nRead-only: opens one local file (or stdin) and writes nothing.\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nPAB_KEYS = (\"BlockPublicAcls\", \"IgnorePublicAcls\",\n            \"BlockPublicPolicy\", \"RestrictPublicBuckets\")\nPUBLIC_GROUP_SUFFIXES = (\"/AllUsers\", \"/AuthenticatedUsers\")\n\n\nclass AuditError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    \"\"\"Read the bundle from a file or '-' for stdin.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise AuditError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise AuditError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef section(doc, *names):\n    \"\"\"Return a named section, or an empty object when it is absent.\"\"\"\n    for name in names:\n        if name in doc:\n            value = doc[name]\n            if value is None:\n                return {}\n            if not isinstance(value, dict):\n                raise AuditError(\"%s must be a JSON object\" % name)\n            return value\n    return {}\n\n\ndef check_encryption(doc):\n    \"\"\"A default server-side encryption rule must exist.\"\"\"\n    sec = section(doc, \"encryption\", \"Encryption\")\n    config = sec.get(\"ServerSideEncryptionConfiguration\")\n    if config is None:\n        return [\"encryption: no ServerSideEncryptionConfiguration\"]\n    if not isinstance(config, dict):\n        raise AuditError(\"encryption ServerSideEncryptionConfiguration must be an object\")\n    rules = config.get(\"Rules\")\n    if not isinstance(rules, list) or not rules:\n        return [\"encryption: no default encryption rule\"]\n    for rule in rules:\n        if not isinstance(rule, dict):\n            continue\n        default = rule.get(\"ApplyServerSideEncryptionByDefault\")\n        if isinstance(default, dict) and default.get(\"SSEAlgorithm\"):\n            return []\n    return [\"encryption: rule has no SSEAlgorithm\"]\n\n\ndef check_versioning(doc):\n    \"\"\"Versioning must be Enabled, not Suspended or absent.\"\"\"\n    sec = section(doc, \"versioning\", \"Versioning\")\n    status = sec.get(\"Status\")\n    if status == \"Enabled\":\n        return []\n    return [\"versioning: %s\" % (status or \"missing\")]\n\n\ndef check_public_access_block(doc):\n    \"\"\"All four account-level block settings must be true.\"\"\"\n    sec = section(doc, \"public_access_block\", \"publicAccessBlock\",\n                  \"PublicAccessBlock\", \"public-access-block\")\n    config = sec.get(\"PublicAccessBlockConfiguration\")\n    if config is None:\n        return [\"public access block: missing\"]\n    if not isinstance(config, dict):\n        raise AuditError(\"PublicAccessBlockConfiguration must be an object\")\n    findings = []\n    for key in PAB_KEYS:\n        if config.get(key) is not True:\n            findings.append(\"public access block: %s=%s\"\n                            % (key, config.get(key)))\n    return findings\n\n\ndef check_logging(doc):\n    \"\"\"Server access logging must have a destination bucket.\"\"\"\n    sec = section(doc, \"logging\", \"Logging\")\n    enabled = sec.get(\"LoggingEnabled\")\n    if enabled is None:\n        return [\"logging: access logging disabled\"]\n    if not isinstance(enabled, dict):\n        raise AuditError(\"LoggingEnabled must be an object\")\n    if not enabled.get(\"TargetBucket\"):\n        return [\"logging: no TargetBucket\"]\n    return []\n\n\ndef check_acl(doc):\n    \"\"\"No grant may go to AllUsers or AuthenticatedUsers.\"\"\"\n    sec = section(doc, \"acl\", \"Acl\", \"ACL\")\n    grants = sec.get(\"Grants\")\n    if grants is None:\n        return [\"acl: no Grants list to inspect\"]\n    if not isinstance(grants, list):\n        raise AuditError(\"acl Grants must be a list\")\n    findings = []\n    for grant in grants:\n        if not isinstance(grant, dict):\n            raise AuditError(\"every acl Grant must be an object\")\n        grantee = grant.get(\"Grantee\")\n        uri = \"\"\n        if isinstance(grantee, dict):\n            uri = grantee.get(\"URI\") or \"\"\n        if isinstance(uri, str) and uri.endswith(PUBLIC_GROUP_SUFFIXES):\n            group = uri.rsplit(\"/\", 1)[-1]\n            findings.append(\"acl: %s %s\" % (grant.get(\"Permission\", \"?\"), group))\n    return findings\n\n\nCHECKS = (\n    (\"encryption\", check_encryption),\n    (\"versioning\", check_versioning),\n    (\"public access block\", check_public_access_block),\n    (\"logging\", check_logging),\n    (\"acl\", check_acl),\n)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit an S3 bucket config bundle: encryption, versioning, \"\n                    \"public access block, logging and ACL.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\")\n    parser.add_argument(\"config\", nargs=\"?\", default=\"-\",\n                        help=\"bucket config JSON file, or - for stdin (default: -)\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    try:\n        doc = read_json(args.config)\n        if not isinstance(doc, dict):\n            raise AuditError(\"the config must be a JSON object\")\n        present = (\"encryption\", \"Encryption\", \"versioning\", \"Versioning\",\n                   \"public_access_block\", \"publicAccessBlock\", \"PublicAccessBlock\",\n                   \"public-access-block\", \"logging\", \"Logging\", \"acl\", \"Acl\", \"ACL\")\n        if not any(key in doc for key in present):\n            raise AuditError(\"no S3 config sections found; expected encryption, \"\n                             \"versioning, public_access_block, logging and acl\")\n        findings = []\n        for _name, check in CHECKS:\n            findings.extend(check(doc))\n    except AuditError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    bucket = doc.get(\"bucket\")\n    if not args.quiet:\n        for finding in findings:\n            print(\"FAIL \" + finding)\n\n    if findings:\n        where = \" for %s\" % bucket if isinstance(bucket, str) and bucket else \"\"\n        print(\"ALERT %d finding%s%s\" % (len(findings),\n                                        \"\" if len(findings) == 1 else \"s\", where))\n        return 1\n    print(\"OK all five S3 checks passed\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p547","n":"547","slug":"5-scripting-mistakes-in-cloud-audits","title":"5 scripting mistakes in cloud audits","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["DevSecOps","Python","CloudSecurity","Automation","CICD"],"series_name":"PythonSec","design":"dontdo","script":"","date":"2030-03-26","files":{"devsecops":"notes/p547-5-scripting-mistakes-in-cloud-audits.md"},"md":"# 5 scripting mistakes in cloud audits\n\nThe audit script prints a beautiful table, the pipeline stays green, and the finding sits in a log nobody opens. The report was never the problem; the exit code was.\n\n## Quick reference\n\n**1.** ❌ **Hard-code the access key** — The script works, then the key leaks in a repo or a log.  \n✅ **Use a short-lived role** — OIDC or a role gives the job credentials it cannot keep.\n\n**2.** ❌ **Read only the first page** — A 1000-item API answers 1000 items; page two is invisible.  \n✅ **Follow NextToken to the end** — Loop until the token is empty, then count what you saw.\n\n**3.** ❌ **Missing key means clean** — A renamed field or a denied call turns unknown into green.  \n✅ **Missing key means unknown** — Fail, or report it separately; never count it as checked.\n\n**4.** ❌ **Print the whole response** — Tokens and emails land in CI logs kept for months.  \n✅ **Mask values, print a hash** — Show AKIA... or a digest; never the secret itself.\n\n**5.** ❌ **Print findings, exit 0** — The pipeline is green and the finding is a diary entry.  \n✅ **Exit 1 on a finding** — CI reads the exit code, not the report body.\n\n## Explained\n\n### The five\n\nCredentials. Hard-coding an access key makes the script work and the key leak. Use a role or OIDC, and let the job keep nothing.\n\nPagination. One call returns one page. Follow NextToken until it is empty, or the account you audit is only the first thousand resources.\n\nMissing data. A renamed field or a denied call is not a pass. Treat absent data as unknown, and count it separately from checked.\n\nSecrets in output. Printing the whole response copies tokens and emails into CI logs. Mask the value; show a prefix or a hash.\n\nExit code. Print findings and exit 0, and CI has nothing to act on. Exit 1 on a finding, 2 on an error, 0 only when clean.\n\n### What they share\n\nEach one makes the script feel finished while leaving the decision to a human who never sees it. None of the five looks wrong on the day it is written.\n\n> **The honest caveat:** a non-zero exit is a blunt instrument. One low finding fails the build until someone allowlists it. That friction is the point, but keep the allowlist reviewed.\n\n> **The lesson:** make the check fail, or it is not a check.\n\n## In one line\n\n> **A report is not a gate.** If the check cannot fail, it is a newsletter.\n\n**Return 1**\n\n```bash\nsys.exit(1 if bad else 0)\n```\n"},{"id":"p548","n":"548","slug":"opa-vs-kyverno-vs-a-python-script","title":"OPA vs Kyverno vs a Python script","sections":["devsecops"],"topic":"Kubernetes","tags":["OPA","Kyverno","Kubernetes","PolicyAsCode","DevSecOps"],"series_name":"PythonSec","design":"matrix","script":"","date":"2030-03-28","files":{"devsecops":"notes/p548-opa-vs-kyverno-vs-a-python-script.md"},"md":"# OPA vs Kyverno vs a Python script\n\nThree engineers describe policy as code and mean three different things: a Rego rule, a Kyverno YAML and a Python script in a cron job. They are not interchangeable; where the decision runs differs.\n\n## Quick reference\n\n| | **OPA** | **Kyverno** | **Python script** |\n|:--|:--|:--|:--|\n| Language | Rego | YAML and JMESPath | Python |\n| Where it runs | Sidecar, CLI, library | In-cluster webhook | CI, cron, laptop |\n| Admission | Only with Gatekeeper | Native | No, you build it |\n| Ecosystem | Broad, CNCF | Kubernetes only | Cloud SDKs and APIs |\n| Learning curve | Steep: Rego | Low: YAML | Low, but you own it |\n| Best for | Cross-system policy | Kubernetes guardrails | Bespoke audits, diffs |\n\n**How to pick**\n\n- **OPA** → policy across Kubernetes, Terraform and CI.\n- **Kyverno** → Kubernetes admission in YAML.\n- **Python** → a diff, a report or a one-off audit.\n\n## Explained\n\n### Language and place\n\nOPA speaks Rego, a general-purpose policy language, and runs as a sidecar, a CLI or a library. Kyverno speaks YAML with JMESPath and runs as an in-cluster webhook. Python runs wherever you start it: CI, cron or a laptop.\n\n### Admission and ecosystem\n\nOPA reaches admission through an admission controller such as Gatekeeper. Kyverno is native: it sees create and update, and can validate, mutate, generate and verify images. A Python script is not an admission controller; it audits a snapshot after the fact. OPA's ecosystem is broader: Kubernetes, Terraform, CI and Envoy. Kyverno is Kubernetes-only. Python has cloud SDKs and every API that prints JSON.\n\n### Learning curve and fit\n\nRego is a new language, and it is not YAML. Kyverno is YAML most Kubernetes engineers know, with JMESPath for the tricky parts. Python is easiest to start and hardest to own: tests, retries and reports are yours. OPA suits policy that spans systems. Kyverno suits a Kubernetes-only platform that wants YAML and mutation. Python suits a bespoke cross-cloud audit, a diff or a report that no engine ships.\n\n> **The honest caveat:** every one of them enforces only the rules someone wrote. Kyverno will happily mutate with a bad rule, and OPA will return deny for the wrong object.\n\n> **The lesson:** pick the engine by where the decision has to be made.\n\n## In one line\n\n> **Where the decision has to be made.** The engine follows the enforcement point, not the fashion.\n\n**Start with tests**\n\n```bash\nopa test policy/ -v\n```\n"},{"id":"p549","n":"549","slug":"a-pod-security-context-audit-in-python","title":"A pod security context audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","CloudNative","Python","DevSecOps","CKS"],"series_name":"PythonSec","design":"code","script":"k8s_security_context.py","date":"2030-03-31","files":{"devsecops":"notes/p549-a-pod-security-context-audit-in-python.md"},"md":"# A pod security context audit in Python\n\nThe YAML passed review because it was four hundred lines and the diff was two. The pod runs as root, shares the host network and mounts the node filesystem.\n\n## Quick reference\n\nThe full script is at the end of this note: `k8s_security_context.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\nkubectl get pods -A -o json | python3 k8s_security_context.py -\n```\n\n**Example output**\n\n```text\nFAIL default web\nALERT 1\n```\n\n## Explained\n\n### Six checks\n\nHostNetwork: the pod shares the node network namespace. HostPath: a volume mounts a path from the node. Privileged: a container with securityContext.privileged true. RunAsRoot: the effective runAsUser is 0, after container inheritance. RunAsNonRoot: neither the container nor the pod sets it true. Limits: a container has no resources.limits. A pod can fail several at once, so read the list, not just the count.\n\n### The frame\n\nThe condensed version merges the pod security context into each container, checks app and init containers, and prints one FAIL line per pod. On the fixture it prints FAIL default web, then ALERT 1, and exits 1.\n\n### How to run it\n\n`kubectl get pods -A -o json | python3 k8s_security_context.py -`, or save the dump and audit it in CI. Run it nightly against the live cluster and before a release. Exit 0 is clean, 1 is a finding, 2 is an input error.\n\n> **The honest caveat:** it checks configuration, not intent. A CNI or a system add-on may need hostNetwork or privileged, and a missing limit is a risk rather than a breach. Keep an allowlist with an owner.\n\n> **The lesson:** the pod spec is the contract; read it before the node does.\n\n## In one line\n\n> **Root is a choice.** The pod spec says who it runs as and what it can reach.\n\n**Audit the cluster**\n\n```bash\nkubectl get pods -A -o json\n```\n\n## Script: k8s_security_context.py\n\nDownload: [scripts/k8s_security_context.py](../scripts/k8s_security_context.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit pod security contexts in a kubectl JSON dump.\n\nReads the output of `kubectl get pods -A -o json` and flags six settings that\nturn a pod into a node-level risk:\n\n    privileged          a container runs with securityContext.privileged\n    runAsRoot           the effective runAsUser is 0\n    runAsNonRoot        neither the container nor the pod sets it to true\n    hostPath            a volume mounts a path from the node\n    hostNetwork         the pod shares the node network namespace\n    no limits           a container has no resources.limits\n\nThe effective runAsUser and runAsNonRoot values are inherited: a container\nsetting wins, otherwise the pod securityContext applies. Both app containers\nand init containers are checked. HostPath and hostNetwork are pod-level.\n\nUsage:\n    kubectl get pods -A -o json > /tmp/pods.json\n    python3 k8s_security_context.py /tmp/pods.json\n    kubectl get pods -A -o json | python3 k8s_security_context.py -\n    python3 k8s_security_context.py /tmp/pods.json --quiet\n\nExit codes: 0 no findings, 1 at least one finding, 2 usage, unreadable or\ninvalid input.\n\nCI (audit a saved dump, fail the job):\n    kubectl get pods -A -o json > /tmp/pods.json\n    python3 k8s_security_context.py /tmp/pods.json || exit 1\nCron (daily at 06:40):\n    40 6 * * * kubectl get pods -A -o json | python3 /opt/s0sta/k8s_security_context.py -\n\nThe audit is read-only: it reads one local file or stdin and never calls the\nAPI server. It checks configuration, not exploitability: a system add-on may\nneed privileged, and a missing limit is a risk, not a breach. Pair it with\nPod Security Admission for enforcement.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass AuditError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    \"\"\"Read the kubectl dump from a file or '-' for stdin.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise AuditError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise AuditError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef pod_items(doc):\n    \"\"\"Return the pod list from a List, a single Pod, or an items wrapper.\"\"\"\n    if not isinstance(doc, dict):\n        raise AuditError(\"the dump must be a JSON object\")\n    items = doc.get(\"items\")\n    if items is not None:\n        if not isinstance(items, list):\n            raise AuditError(\"items must be a list\")\n        return items\n    if doc.get(\"kind\") == \"Pod\" or (\"metadata\" in doc and \"spec\" in doc):\n        return [doc]\n    raise AuditError(\"no items list and not a single Pod\")\n\n\ndef as_dict(value, label):\n    \"\"\"Return a mapping, or raise when the field has the wrong shape.\"\"\"\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise AuditError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    \"\"\"Return a list, or raise when the field has the wrong shape.\"\"\"\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise AuditError(\"%s must be a list\" % label)\n    return value\n\n\ndef effective(container_sc, pod_sc, key):\n    \"\"\"Container securityContext wins; otherwise inherit from the pod.\"\"\"\n    if key in container_sc:\n        return container_sc[key]\n    return pod_sc.get(key)\n\n\ndef check_pod(pod, number):\n    \"\"\"Return (namespace, name, findings) for one pod.\"\"\"\n    if not isinstance(pod, dict):\n        raise AuditError(\"pod %d is not an object\" % number)\n    metadata = as_dict(pod.get(\"metadata\"), \"pod %d metadata\" % number)\n    spec = as_dict(pod.get(\"spec\"), \"pod %d spec\" % number)\n    namespace = metadata.get(\"namespace\") or \"default\"\n    name = metadata.get(\"name\") or \"pod-%d\" % number\n    pod_sc = as_dict(spec.get(\"securityContext\"), \"pod %s securityContext\" % name)\n\n    findings = []\n    if spec.get(\"hostNetwork\") is True:\n        findings.append((\"-\", \"hostNetwork\"))\n    for volume in as_list(spec.get(\"volumes\"), \"pod %s volumes\" % name):\n        if isinstance(volume, dict) and \"hostPath\" in volume:\n            findings.append((\"-\", \"hostPath %s\" % (volume.get(\"name\") or \"?\")))\n\n    containers = spec.get(\"containers\")\n    if not isinstance(containers, list) or not containers:\n        raise AuditError(\"pod %s has no containers list\" % name)\n    every = list(containers) + as_list(spec.get(\"initContainers\"),\n                                       \"pod %s initContainers\" % name)\n    for index, container in enumerate(every, 1):\n        if not isinstance(container, dict):\n            raise AuditError(\"pod %s container %d is not an object\" % (name, index))\n        cname = container.get(\"name\") or \"container-%d\" % index\n        sc = as_dict(container.get(\"securityContext\"),\n                     \"pod %s container %s securityContext\" % (name, cname))\n        resources = as_dict(container.get(\"resources\"),\n                            \"pod %s container %s resources\" % (name, cname))\n        if sc.get(\"privileged\") is True:\n            findings.append((cname, \"privileged\"))\n        if effective(sc, pod_sc, \"runAsUser\") == 0:\n            findings.append((cname, \"runAsRoot\"))\n        if effective(sc, pod_sc, \"runAsNonRoot\") is not True:\n            findings.append((cname, \"runAsNonRoot\"))\n        limits = resources.get(\"limits\")\n        if not isinstance(limits, dict) or not limits:\n            findings.append((cname, \"no limits\"))\n    return namespace, name, findings\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Audit pod securityContext, hostPath, hostNetwork and \"\n                    \"resource limits in a kubectl JSON dump.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\")\n    parser.add_argument(\"pods\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get pods -A -o json output, or - for stdin\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    try:\n        items = pod_items(read_json(args.pods))\n        total = 0\n        findings = 0\n        for number, pod in enumerate(items, 1):\n            namespace, name, pod_findings = check_pod(pod, number)\n            total += 1\n            findings += len(pod_findings)\n            if pod_findings and not args.quiet:\n                for container, finding in pod_findings:\n                    print(\"FAIL %s/%s %s %s\" % (namespace, name, container, finding))\n    except AuditError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if findings:\n        print(\"ALERT %d finding%s in %d pod%s\" % (\n            findings, \"\" if findings == 1 else \"s\",\n            total, \"\" if total == 1 else \"s\"))\n        return 1\n    print(\"OK no findings in %d pod%s\" % (total, \"\" if total == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p550","n":"550","slug":"a-networkpolicy-coverage-report-in-python","title":"A NetworkPolicy coverage report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","NetworkPolicy","CloudNative","Python","DevSecOps"],"series_name":"PythonSec","design":"code","script":"netpol_coverage.py","date":"2030-04-02","files":{"devsecops":"notes/p550-a-networkpolicy-coverage-report-in-python.md"},"md":"# A NetworkPolicy coverage report in Python\n\nThe cluster has forty NetworkPolicies, the team says default-deny, and two namespaces have none. The pods there can talk to everything.\n\n## Quick reference\n\nThe full script is at the end of this note: `netpol_coverage.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 netpol_coverage.py /tmp/pods.json /tmp/netpols.json\n```\n\n**Example output**\n\n```text\nNOPOLICY staging\nALERT 1\n```\n\n## Explained\n\n### What it reads\n\nIt reads the pods dump and the NetworkPolicy dump, groups the policies by namespace, and matches each pod against every selector in its own namespace. A policy in another namespace does not count. An empty selector selects all pods. matchLabels and all four matchExpressions operators are supported; the frame shows the matchLabels path.\n\n### What it reports\n\nNOPOLICY for a namespace with pods and no policy at all. UNSELECTED for a pod that no policy in its namespace selects. The frame prints NOPOLICY staging, then ALERT 1, and exits 1; the attached script also prints UNSELECTED for pods in covered namespaces. A namespace with a policy that selects no pods is not NOPOLICY; its pods appear as UNSELECTED, which is why both lines exist.\n\n### How to run it\n\nkubectl get pods -A -o json > `/tmp/pods.json`, the same for networkpolicy, then `python3 netpol_coverage.py /tmp/pods.json /tmp/netpols.json`. Use it before a default-deny rollout to find the namespaces that would break, and after it to prove coverage. Run it after a selector edit too, because a quiet change can drop a workload.\n\n> **The honest caveat:** coverage is not correctness. A policy can select a pod and still allow the wrong traffic, an egress-only policy leaves ingress open, and a CNI that ignores NetworkPolicy reports full coverage.\n\n> **The lesson:** a policy that selects nothing protects nothing.\n\n## In one line\n\n> **Selected is not protected.** A policy that selects nothing protects nothing.\n\n**Check coverage**\n\n```bash\nkubectl get networkpolicy -A\n```\n\n## Script: netpol_coverage.py\n\nDownload: [scripts/netpol_coverage.py](../scripts/netpol_coverage.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report NetworkPolicy coverage for every pod in a cluster dump.\n\nReads two JSON documents:\n\n    kubectl get pods -A -o json          the pods\n    kubectl get networkpolicy -A -o json the policies\n\nand reports two gaps:\n\n    NOPOLICY    a namespace with pods but no NetworkPolicy at all\n    UNSELECTED  a pod that no NetworkPolicy in its namespace selects\n\nPods in a namespace with no policy at all are covered by the NOPOLICY line\nand are not listed again as UNSELECTED. A policy that selects no pods still\nlets the namespace pass the first check; its pods then fail the second.\n\nA policy selects a pod when its spec.podSelector matches the pod labels in\nthe same namespace. An empty selector selects every pod in that namespace.\nmatchLabels and all four matchExpressions operators are supported: In, NotIn,\nExists and DoesNotExist.\n\nUsage:\n    kubectl get pods -A -o json > /tmp/pods.json\n    kubectl get networkpolicy -A -o json > /tmp/netpols.json\n    python3 netpol_coverage.py /tmp/pods.json /tmp/netpols.json\n    python3 netpol_coverage.py /tmp/pods.json /tmp/netpols.json --quiet\n\nExit codes: 0 every pod is selected, 1 at least one gap, 2 usage,\nunreadable or invalid input.\n\nCI (fail the job on an uncovered namespace or pod):\n    python3 netpol_coverage.py /tmp/pods.json /tmp/netpols.json || exit 1\nCron (daily at 07:00):\n    0 7 * * * python3 /opt/s0sta/netpol_coverage.py /srv/k8s/pods.json /srv/k8s/netpols.json\n\nCoverage is not correctness: a policy can select a pod and still allow the\nwrong traffic, a policy with only egress rules leaves ingress open, and the\nCNI must actually enforce NetworkPolicy. This report finds pods with no\npolicy in front of them, not pods with a bad policy.\n\nRead-only: opens two local files and writes nothing.\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass CoverageError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source, label):\n    \"\"\"Read one JSON document from a file or '-' for stdin.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise CoverageError(\"cannot read %s %s: %s\" % (label, source, exc))\n    except ValueError as exc:\n        raise CoverageError(\"%s %s is not JSON: %s\" % (label, source, exc))\n\n\ndef items_of(doc, label):\n    \"\"\"Return the item list from a List, a single object, or an items wrapper.\"\"\"\n    if not isinstance(doc, dict):\n        raise CoverageError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        if not isinstance(items, list):\n            raise CoverageError(\"%s items must be a list\" % label)\n        return items\n    if \"metadata\" in doc and \"spec\" in doc:\n        return [doc]\n    raise CoverageError(\"%s has no items list\" % label)\n\n\ndef as_dict(value, label):\n    \"\"\"Return a mapping, or raise when the field has the wrong shape.\"\"\"\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise CoverageError(\"%s must be an object\" % label)\n    return value\n\n\ndef match_selector(selector, labels, where):\n    \"\"\"Return True when a NetworkPolicy podSelector matches these labels.\"\"\"\n    if selector is None:\n        raise CoverageError(\"%s has no podSelector\" % where)\n    if not isinstance(selector, dict):\n        raise CoverageError(\"%s podSelector must be an object\" % where)\n\n    match_labels = selector.get(\"matchLabels\")\n    if match_labels is not None:\n        if not isinstance(match_labels, dict):\n            raise CoverageError(\"%s matchLabels must be an object\" % where)\n        for key, value in match_labels.items():\n            if not isinstance(key, str) or not isinstance(value, str):\n                raise CoverageError(\"%s matchLabels must map strings to strings\" % where)\n            if labels.get(key) != value:\n                return False\n\n    expressions = selector.get(\"matchExpressions\")\n    if expressions is not None:\n        if not isinstance(expressions, list):\n            raise CoverageError(\"%s matchExpressions must be a list\" % where)\n        for expression in expressions:\n            if not isinstance(expression, dict):\n                raise CoverageError(\"%s matchExpression must be an object\" % where)\n            key = expression.get(\"key\")\n            operator = expression.get(\"operator\")\n            values = expression.get(\"values\", [])\n            if not isinstance(key, str) or not isinstance(operator, str):\n                raise CoverageError(\"%s matchExpression needs key and operator\" % where)\n            if not isinstance(values, list) or not all(isinstance(v, str) for v in values):\n                raise CoverageError(\"%s matchExpression values must be a list of strings\" % where)\n            present = key in labels\n            if operator == \"In\":\n                if not values or labels.get(key) not in values:\n                    return False\n            elif operator == \"NotIn\":\n                if not values:\n                    raise CoverageError(\"%s NotIn needs at least one value\" % where)\n                if present and labels.get(key) in values:\n                    return False\n            elif operator == \"Exists\":\n                if not present:\n                    return False\n            elif operator == \"DoesNotExist\":\n                if present:\n                    return False\n            else:\n                raise CoverageError(\"%s has unknown operator %s\" % (where, operator))\n    return True\n\n\ndef load_pods(doc):\n    \"\"\"Return [(namespace, name, labels)] and the namespace set.\"\"\"\n    pods = []\n    namespaces = {}\n    for number, pod in enumerate(items_of(doc, \"pods\"), 1):\n        if not isinstance(pod, dict):\n            raise CoverageError(\"pod %d is not an object\" % number)\n        metadata = as_dict(pod.get(\"metadata\"), \"pod %d metadata\" % number)\n        namespace = metadata.get(\"namespace\") or \"default\"\n        name = metadata.get(\"name\") or \"pod-%d\" % number\n        labels = as_dict(metadata.get(\"labels\"), \"pod %s labels\" % name)\n        if not all(isinstance(k, str) and isinstance(v, str)\n                   for k, v in labels.items()):\n            raise CoverageError(\"pod %s labels must map strings to strings\" % name)\n        pods.append((namespace, name, labels))\n        namespaces[namespace] = namespaces.get(namespace, 0) + 1\n    return pods, namespaces\n\n\ndef load_policies(doc):\n    \"\"\"Return {namespace: [(name, podSelector)]}.\"\"\"\n    policies = {}\n    for number, policy in enumerate(items_of(doc, \"networkpolicies\"), 1):\n        if not isinstance(policy, dict):\n            raise CoverageError(\"policy %d is not an object\" % number)\n        metadata = as_dict(policy.get(\"metadata\"), \"policy %d metadata\" % number)\n        namespace = metadata.get(\"namespace\") or \"default\"\n        name = metadata.get(\"name\") or \"policy-%d\" % number\n        spec = as_dict(policy.get(\"spec\"), \"policy %s spec\" % name)\n        if \"podSelector\" not in spec:\n            raise CoverageError(\"policy %s has no podSelector\" % name)\n        selector = spec.get(\"podSelector\")\n        if not isinstance(selector, dict):\n            raise CoverageError(\"policy %s podSelector must be an object\" % name)\n        policies.setdefault(namespace, []).append((name, selector))\n    return policies\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report namespaces without a NetworkPolicy and pods that no \"\n                    \"NetworkPolicy selects.\",\n        epilog=\"Exit codes: 0 every pod is selected, 1 gaps found, 2 usage or input error.\")\n    parser.add_argument(\"pods\", help=\"kubectl get pods -A -o json file, or - for stdin\")\n    parser.add_argument(\"policies\", help=\"kubectl get networkpolicy -A -o json file, or - for stdin\")\n    parser.add_argument(\"-q\", \"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    if args.pods == \"-\" and args.policies == \"-\":\n        parser.error(\"pods and policies cannot both read from stdin\")\n\n    try:\n        pods, namespaces = load_pods(read_json(args.pods, \"pods\"))\n        policies = load_policies(read_json(args.policies, \"networkpolicies\"))\n\n        no_policy = sorted(ns for ns in namespaces if ns not in policies)\n        unselected = []\n        for namespace, name, labels in pods:\n            if namespace in no_policy:\n                continue\n            selected = False\n            for policy_name, selector in policies[namespace]:\n                where = \"policy %s/%s\" % (namespace, policy_name)\n                if match_selector(selector, labels, where):\n                    selected = True\n                    break\n            if not selected:\n                unselected.append((namespace, name))\n    except CoverageError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not args.quiet:\n        for namespace in no_policy:\n            print(\"NOPOLICY %s\" % namespace)\n        for namespace, name in unselected:\n            print(\"UNSELECTED %s/%s\" % (namespace, name))\n\n    gaps = len(no_policy) + len(unselected)\n    if gaps:\n        print(\"ALERT %d gap%s: %d namespace%s, %d pod%s\" % (\n            gaps, \"\" if gaps == 1 else \"s\",\n            len(no_policy), \"\" if len(no_policy) == 1 else \"s\",\n            len(unselected), \"\" if len(unselected) == 1 else \"s\"))\n        return 1\n    print(\"OK all pods selected: %d in %d namespace%s\" % (\n        len(pods), len(namespaces), \"\" if len(namespaces) == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p551","n":"551","slug":"quiz-3-policy-questions","title":"Quiz: 3 policy questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["PolicyAsCode","OPA","Kyverno","DevSecOps","Kubernetes"],"series_name":"PythonSec","design":"quiz","script":"","date":"2030-04-04","files":{"devsecops":"notes/p551-quiz-3-policy-questions.md"},"md":"# Quiz: 3 policy questions\n\nA policy quiz is easy until the policy is the thing standing between a typo and a production cluster. Three questions from this phase.\n\n## Quick reference\n\n**Q1. Why does a policy need a test before it enforces anything?**\n\n- a) So it returns deny for a known bad fixture\n- b) So the YAML parses in the editor\n- c) So the policy is longer than the app\n\n> **Answer:** a) So it returns deny for a known bad fixture. A policy is code: a typo in a selector can allow everything. A test pins the decision for a known object before a cluster or a pipeline relies on it.\n\n**Q2. Why run a policy in audit mode before enforce?**\n\n- a) To see what would fail without blocking it\n- b) To make the policy run faster\n- c) To skip the admission webhook\n\n> **Answer:** a) To see what would fail without blocking it. Audit records would-be violations on real traffic, so you find broken workloads and false positives before a deny stops a deploy.\n\n**Q3. Why is a Python script not an admission controller?**\n\n- a) It runs on a schedule, not in the request path\n- b) It cannot read JSON\n- c) It cannot return an exit code\n\n> **Answer:** a) It runs on a schedule, not in the request path. Admission needs a webhook in the API server path with TLS, latency and high availability. A script audits a snapshot; it cannot block a create.\n\n## Explained\n\n### Question one\n\nWhy does a policy need a test? Because a policy is code, and a selector typo can allow everything. A test pins the decision for a known object: this pod must be denied, this one must be allowed. opa test or kyverno test runs those fixtures before a cluster or a pipeline relies on the rule. A test that only checks the happy path is a demo.\n\n### Question two\n\nWhy run in audit mode before enforce? Audit records what would have been denied on real traffic without blocking it. You find the workloads that would break and the rules that would block too much, fix them, then switch to enforce. A deny that arrives with the first deploy is an outage.\n\n### Question three\n\nWhy is a Python script not an admission controller? Admission is a webhook in the API server request path, with TLS, a latency budget and high availability. A script audits a snapshot on a schedule. It can tell you what is wrong; it cannot stop the create that is wrong now.\n\n> **The honest caveat:** passing this quiz does not enforce anything. The test, the audit run and the webhook do.\n\n> **The lesson:** test the rule, audit the rollout, then let admission enforce.\n\n## In one line\n\n> **Test, audit, then enforce.** Three answers that keep policy out of production surprises.\n\n**Test a rule**\n\n```bash\nopa test policy/ -v\n```\n"},{"id":"p552","n":"552","slug":"the-api-security-check-in-three-tiers","title":"The API security check, in three tiers","sections":["devsecops"],"topic":"Application & API security","tags":["API","OpenAPI","DevSecOps","Python","CyberSecurity"],"series_name":"PythonSec","design":"architecture","script":"","date":"2030-04-07","files":{"devsecops":"notes/p552-the-api-security-check-in-three-tiers.md"},"md":"# The API security check, in three tiers\n\nThe API review starts with a list of endpoints somebody typed from memory. The OpenAPI file says one thing, the gateway route table says another, and the endpoint nobody listed is the one that never got reviewed.\n\n## Quick reference\n\n- **Spec:** OpenAPI file (SPEC) · Gateway routes (ROUTE)\n- **Checks:** Auth per operation (AUTH) · Input shape (INPUT) · Header rules (HDR)\n- **Report:** Findings list (FIND) · Exit gate (GATE)\n\n**Request flow:** OpenAPI file → Auth per operation → Findings list → Exit gate; OpenAPI file → Input shape → Findings list; Gateway routes → Header rules → Findings list\n\n**Around it:** CI job (CI)\n\n## Explained\n\n### Spec\n\nThe first tier is the spec: the OpenAPI file and the gateway route table. Diffing the two finds the shadow endpoint, live in the gateway and absent from the spec, which means absent from every review that trusted the spec.\n\n### Checks\n\nThe second tier runs three checks per operation. Auth asks whether the operation declares a security requirement, inherited from the root or set on the operation. Input shape asks whether the body and parameters have a schema with types, lengths and enums. Header rules ask whether the response carries the security headers the platform promises. Every finding keeps its method and path.\n\n### Report\n\nThe third tier is the findings list and the exit gate, read by the CI job. Exit 1 means at least one check failed, exit 0 means clean, exit 2 means the spec will not parse. That exit code is the point: a report nobody gates on is a PDF.\n\n> **The honest caveat:** a green gate proves the spec declares these controls, not that the running service enforces them. Compare the spec with live responses before you trust the badge.\n\n> **The lesson:** audit the spec, then prove the service matches it.\n\n## In one line\n\n> **Spec, check, gate. Nothing skips a tier.** A finding you cannot trace to a check is a finding nobody fixes.\n\n**Check the spec**\n\n```bash\npython3 openapi_auth_check.py api.json\n```\n"},{"id":"p553","n":"553","slug":"the-http-security-header-cheat-sheet","title":"The HTTP security header cheat sheet","sections":["it","devsecops"],"topic":"Application & API security","tags":["HTTP","Security","DevSecOps","WebSecurity","Curl"],"series_name":"PythonSec","design":"cheatsheet","script":"","date":"2030-04-09","files":{"it":"notes/p553-the-http-security-header-cheat-sheet.md","devsecops":"notes/p553-the-http-security-header-cheat-sheet.md"},"md":"# The HTTP security header cheat sheet\n\nA security review reads the code and skips the response headers. Four blocks of commands read what the browser actually receives, and each block answers a different question.\n\n## Quick reference\n\n### TLS\n\n```bash\ncurl -sI https://example.com | head -1\nopenssl s_client -connect example.com:443\nopenssl x509 -noout -dates -in cert.pem\ncurl -sI $URL | grep -i strict\n```\n\n### CSP\n\n```bash\ncurl -sI $URL | grep -i content-sec\ncurl -sI $URL | grep -i x-frame\njq -r .\"content-security-policy\" h.json\ncurl -sI $URL | grep -i report-only\n```\n\n### CORS\n\n```bash\ncurl -sI -H 'Origin: https://a.test' $URL\ncurl -sI -H 'Origin: null' $URL\ncurl -sI $URL | grep -i access-control\ncurl -sI $URL | grep -i vary\n```\n\n### Cookies\n\n```bash\ncurl -sI $URL | grep -i set-cookie\ncurl -sI $URL | grep -i httponly\ncurl -sI $URL | grep -i samesite\njq -r .\"set-cookie\" h.json\n```\n\n## Explained\n\n### TLS\n\nExport URL=https://example.com once. `curl -sI https://example.com | head -1` shows the status line, `openssl s_client -connect example.com:443` prints the chain, and `openssl x509 -noout -dates -in cert.pem` reads the dates from a saved certificate. `curl -sI $URL | grep -i strict` finds HSTS.\n\n### CSP\n\n`curl -sI $URL | grep -i content-sec` finds the policy, and `grep -i x-frame` checks framing. `jq -r .\"content-security-policy\" h.json` reads the same header from a dump, and `grep -i report-only` catches a policy that only reports.\n\n### CORS\n\n`curl -sI -H 'Origin: https://a.test' $URL` asks the server to answer a foreign origin. Repeat with Origin: null, which is what a sandboxed frame sends. `grep -i access-control` shows the answer, and `grep -i` vary shows whether caches will mix the two.\n\n### Cookies\n\n`curl -sI $URL | grep -i set-cookie` finds the header. `grep -i` httponly and `grep -i` samesite confirm the flags, and `jq -r .\"set-cookie\" h.json` lists every cookie from a dump.\n\n> **The honest caveat:** a CDN can add, rewrite or strip any of these headers, so test the hostname users hit, not the origin behind it, and run the blocks against staging before production.\n\n> **The lesson:** read the headers before you believe the diagram.\n\n## In one line\n\n> **Headers are the cheapest control.** Four blocks, one dump, five minutes before the next release.\n\n**Dump the headers**\n\n```bash\ncurl -sI $URL\n```\n"},{"id":"p554","n":"554","slug":"four-header-defences-and-what-each-stops","title":"Four header defences and what each stops","sections":["it","devsecops"],"topic":"Application & API security","tags":["HTTP","WebSecurity","DevSecOps","CyberSecurity","CSP"],"series_name":"PythonSec","design":"explainer","script":"","date":"2030-04-11","files":{"it":"notes/p554-four-header-defences-and-what-each-stops.md","devsecops":"notes/p554-four-header-defences-and-what-each-stops.md"},"md":"# Four header defences and what each stops\n\nA scanner gives the site a green tick and a session still leaks. Four response headers close four different doors, and each one is easy to configure badly.\n\n## Quick reference\n\n### 1. HSTS\n\n- Refuses plain HTTP after the first HTTPS visit\n- Closes SSL-strip on a hostile network\n- max-age 31536000 is one year\n- Preload is a browser-list commitment\n\n### 2. CSP\n\n- Allowlists where scripts may load from\n- Blocks inline script without a nonce or hash\n- Stops an injected tag from executing\n- Report-only first, enforce second\n\n### 3. CORS\n\n- Decides which origins may read a response\n- The browser enforces it, not the server\n- Wildcard plus credentials is invalid\n- It is not authentication\n\n### 4. Cookie flags\n\n- Secure keeps the cookie off plain HTTP\n- HttpOnly hides it from document.cookie\n- SameSite blocks the cross-site send\n- Stops script and CSRF from using the token\n\n> **Note:** Four headers, four attacks: none of them replaces another.\n\n## Explained\n\n### HSTS\n\nStrict-Transport-Security tells the browser to refuse plain HTTP for this host for the seconds in max-age. After one clean HTTPS visit it stops an SSL-strip attack, because the browser never offers HTTP again for that period. A max-age of 31536000 is one year, and preload is a separate commitment with the browser vendors.\n\n### CSP\n\nContent-Security-Policy limits where scripts may load from and whether inline script may run. With default-src 'self' and a nonce on the one inline block you need, an injected script tag reaches the page and does not execute.\n\n### CORS\n\nAccess-Control-Allow-Origin decides which origins a browser may read a response from. The browser enforces it, not the server: curl ignores CORS completely, so a permissive header is not access control for scripts. A wildcard with credentials is invalid, and browsers say so.\n\n### Cookie flags\n\nSecure keeps the cookie off plain HTTP, HttpOnly hides it from document.cookie, and SameSite stops another site from sending it on a cross-site request. Secure and HttpOnly do different jobs, and one does not imply the other. Together they shrink what an XSS foothold can steal.\n\n> **The honest caveat:** HSTS does nothing on the very first visit, CSP needs a report-only phase before enforcement, and cookie flags do not encrypt the value. Each control leaves a gap the next one does not fill.\n\n> **The lesson:** name the attack each header stops, then test that it does.\n\n## In one line\n\n> **Four headers, four attacks.** Each control closes a door the others leave open.\n\n**Check the headers**\n\n```bash\ncurl -sI $URL\n```\n"},{"id":"p555","n":"555","slug":"a-csp-audit-in-python","title":"A CSP audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["CSP","WebSecurity","Python","DevSecOps","HTTP"],"series_name":"PythonSec","design":"code","script":"csp_audit.py","date":"2030-04-14","files":{"devsecops":"notes/p555-a-csp-audit-in-python.md"},"md":"# A CSP audit in Python\n\nA Content-Security-Policy header arrives as one long string, and most reviews read the first directive and stop. A script can check every weak token in a second.\n\n## Quick reference\n\nThe full script is at the end of this note: `csp_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 csp_audit.py --headers /tmp/h.json\n```\n\n**Example output**\n\n```text\nFAIL 3 csp finding(s)\nunsafe-inline; *; no frame-ancestors\n```\n\n## Explained\n\n### What it checks\n\nThe script reads a headers JSON dump or a raw policy. It splits the semicolon-separated directives and keeps the effective script sources: script-src when present, default-src otherwise. It flags unsafe-inline, unsafe-eval, a bare \\*, a missing default-src and a missing frame-ancestors. Directives are matched case-insensitively, and a policy spread over several headers is checked one header at a time.\n\n### The run\n\nOn the sample dump it prints two lines: FAIL 3 csp finding(s), then unsafe-inline; \\*; no frame-ancestors. Exit 1 makes it a gate, exit 0 is clean, and exit 2 means the file or the JSON is unreadable. The full script is attached, standard library only and read-only.\n\n### The fix\n\nEach label maps to a change. Replace unsafe-inline with a nonce or a hash, remove unsafe-eval and the library that needs it, replace \\* with the hosts you actually use, add default-src 'self', and add frame-ancestors 'none'. A nonce has to be generated per response, not per deploy, or it becomes a new constant.\n\n> **The honest caveat:** a strict policy can still allow a compromised script from an allowlisted host, and a report-only policy will not fail the gate. Read the reports before you enforce.\n\n> **The lesson:** a policy is code, so test it like code.\n\n## In one line\n\n> **Read the policy, not the badge.** A header is only as good as the directives inside it.\n\n**Audit the policy**\n\n```bash\npython3 csp_audit.py --headers h.json\n```\n\n## Script: csp_audit.py\n\nDownload: [scripts/csp_audit.py](../scripts/csp_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit a Content-Security-Policy header for weak directives.\n\nReads a response-headers JSON dump, or a raw policy string, and flags:\n\n    unsafe-inline        inline script runs without a nonce or hash\n    unsafe-eval          string-to-code evaluation is allowed\n    *                    any host may serve script\n    no default-src       every unlisted fetch type falls back to defaults\n    no frame-ancestors   any site may frame the page\n\nThe script checks the effective script sources: script-src when it exists,\ndefault-src otherwise. A missing Content-Security-Policy header is reported\nas a finding, because no policy is not the same as a strict policy. A\nheaders dump may be an object of names to values, an object with a \"headers\"\nobject, or a list of {\"name\": ..., \"value\": ...} entries; names are matched\ncase-insensitively.\n\nUsage:\n    python3 csp_audit.py --headers /tmp/headers.json\n    python3 csp_audit.py --policy \"default-src 'self'; script-src 'self'\"\n    python3 csp_audit.py --headers /tmp/headers.json --verbose\n    python3 csp_audit.py --help\n\nCI (after a deploy):\n    python3 csp_audit.py --headers /tmp/headers.json || exit 1\n\nCron (daily at 06:00):\n    0 6 * * * python3 /opt/s0sta/csp_audit.py --headers /var/tmp/h.json\n\nExit codes: 0 clean, 1 at least one finding, 2 usage or unreadable input.\nRead-only: the file is read, nothing is written, nothing is fetched.\n\nThe honest limit: a policy can be strict and still allow an old bundled\nlibrary to run; this checks directives, not the code they load.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nHEADER = \"content-security-policy\"\n\n\ndef parse_policy(policy):\n    \"\"\"Return {directive: [sources]} for one policy string.\"\"\"\n    directives = {}\n    for part in policy.split(\";\"):\n        tokens = part.split()\n        if not tokens:\n            continue\n        name = tokens[0].lower()\n        directives.setdefault(name, []).extend(tokens[1:])\n    return directives\n\n\ndef load_headers(path):\n    \"\"\"Return a lowercase {header: [values]} map from a JSON dump.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except json.JSONDecodeError as exc:\n        raise ValueError(\"%s is not valid JSON: %s\" % (path, exc))\n\n    if isinstance(data, dict) and isinstance(data.get(\"headers\"), dict):\n        data = data[\"headers\"]\n\n    if isinstance(data, dict):\n        items = list(data.items())\n    elif isinstance(data, list):\n        items = []\n        for entry in data:\n            if not isinstance(entry, dict):\n                raise ValueError(\"each header entry must be an object\")\n            name = entry.get(\"name\")\n            if name is None:\n                raise ValueError(\"a header entry has no name\")\n            items.append((name, entry.get(\"value\", \"\")))\n    else:\n        raise ValueError(\"headers JSON must be an object or a list\")\n\n    headers = {}\n    for name, value in items:\n        key = str(name).lower()\n        values = value if isinstance(value, list) else [value]\n        clean = [str(item) for item in values if item is not None]\n        if clean:\n            headers.setdefault(key, []).extend(clean)\n    return headers\n\n\ndef audit(policy):\n    \"\"\"Return a list of (label, detail) findings for one policy string.\"\"\"\n    directives = parse_policy(policy)\n    script = directives.get(\"script-src\", directives.get(\"default-src\", []))\n    lowered = [source.lower() for source in script]\n    findings = []\n    if \"'unsafe-inline'\" in lowered:\n        findings.append((\"unsafe-inline\",\n                         \"the effective script sources allow inline code\"))\n    if \"'unsafe-eval'\" in lowered:\n        findings.append((\"unsafe-eval\",\n                         \"the effective script sources allow eval\"))\n    if \"*\" in lowered:\n        findings.append((\"*\", \"the effective script sources allow any host\"))\n    if \"default-src\" not in directives:\n        findings.append((\"no default-src\",\n                         \"unlisted fetch types use the browser defaults\"))\n    if \"frame-ancestors\" not in directives:\n        findings.append((\"no frame-ancestors\",\n                         \"CSP does not restrict framing\"))\n    return findings\n\n\ndef build_parser():\n    \"\"\"Return the argument parser.\"\"\"\n    parser = argparse.ArgumentParser(\n        prog=\"csp_audit.py\",\n        description=\"Flag weak Content-Security-Policy directives in a \"\n                    \"headers JSON dump or a raw policy string. Checks \"\n                    \"unsafe-inline, unsafe-eval, a bare *, default-src and \"\n                    \"frame-ancestors. Exits 1 on any finding.\")\n    group = parser.add_mutually_exclusive_group(required=True)\n    group.add_argument(\"--headers\", metavar=\"FILE\",\n                       help=\"JSON dump of response headers\")\n    group.add_argument(\"--policy\", metavar=\"CSP\",\n                       help=\"raw Content-Security-Policy value\")\n    parser.add_argument(\"--verbose\", action=\"store_true\",\n                        help=\"print one line of detail per finding\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n\n    try:\n        if args.headers:\n            headers = load_headers(args.headers)\n            policies = headers.get(HEADER) or [\"\"]\n        else:\n            policies = [args.policy]\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings = []\n    seen = set()\n    for policy in policies:\n        for label, detail in audit(policy):\n            if label in seen:\n                continue\n            seen.add(label)\n            findings.append((label, detail))\n\n    if findings:\n        print(\"FAIL %d csp finding(s)\" % len(findings))\n        print(\"; \".join(label for label, _ in findings))\n        if args.verbose:\n            for label, detail in findings:\n                print(\"  %s: %s\" % (label, detail))\n        return 1\n\n    print(\"OK csp clean\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p556","n":"556","slug":"a-cors-audit-in-python","title":"A CORS audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["CORS","API","WebSecurity","Python","DevSecOps"],"series_name":"PythonSec","design":"code","script":"cors_audit.py","date":"2030-04-16","files":{"devsecops":"notes/p556-a-cors-audit-in-python.md"},"md":"# A CORS audit in Python\n\nA CORS bug rarely looks like one. The header is present, the request works, and the response is readable from a domain that should never see it.\n\n## Quick reference\n\nThe full script is at the end of this note: `cors_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cors_audit.py /tmp/cors.json\n```\n\n**Example output**\n\n```text\nFAIL 2 cors finding(s)\nreflected origin + credentials; null origin\n```\n\n## Explained\n\n### What it reads\n\nThe script takes a JSON file with one response sample or a list of them. A sample has a request_origin and either a headers object or the headers at the top level. The script also accepts a configuration object with allow_origins and allow_credentials. Header names are matched case-insensitively, so a dump from nginx and a dump from curl both work.\n\n### What it flags\n\nThree patterns fail the run. A wildcard origin together with credentials set to true, which browsers reject but a proxy can rewrite into something worse. An origin the response echoes back, unless it appears in allowed_origins. A null origin, which any sandboxed frame can claim. A wildcard without credentials is not flagged, because public CORS is a valid design.\n\n### The run\n\nOn the sample file it prints two lines: FAIL 2 cors finding(s), then reflected origin + credentials; null origin. Exit 1 fails the job, a clean allowlist exits 0, and a broken file exits 2. The full script is attached, standard library only and read-only.\n\n> **The honest caveat:** one sample proves one response. Send two different Origins and compare, because a server that echoes both is reflecting whatever the config says. CORS is also not authentication: it only decides who may read.\n\n> **The lesson:** an echoed Origin is a wildcard with extra steps.\n\n## In one line\n\n> **Echo is not an allowlist.** An origin sent back unchanged is a wildcard with extra steps.\n\n**Test two origins**\n\n```bash\npython3 cors_audit.py cors.json\n```\n\n## Script: cors_audit.py\n\nDownload: [scripts/cors_audit.py](../scripts/cors_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit CORS headers or a CORS configuration for three common mistakes.\n\nReads a JSON file holding one response sample, a list of samples, or a\nconfiguration object, and flags:\n\n    wildcard * + credentials   any origin may send authenticated requests\n    reflected origin           the response echoes the caller's Origin\n    null origin                sandboxed frames and data: URLs get access\n\nA response sample has a \"headers\" object and an optional \"request_origin\".\nA flat object whose keys are header names also works, which is what a curl\ndump looks like. A configuration object uses \"allow_origins\" (a string or a\nlist) and \"allow_credentials\" (true or false).\n\nA wildcard without credentials is not flagged: public, unauthenticated CORS\nis a valid design. A reflected origin is only flagged when it is not listed\nin the sample's optional \"allowed_origins\" list, so a deliberate allowlist\nthat echoes one approved origin is not a finding.\n\nUsage:\n    python3 cors_audit.py /tmp/cors.json\n    python3 cors_audit.py /tmp/cors-config.json\n    python3 cors_audit.py /tmp/cors.json --verbose\n    python3 cors_audit.py --help\n\nCI (after a deploy):\n    python3 cors_audit.py /tmp/cors.json || exit 1\n\nCron (daily at 06:30):\n    30 6 * * * python3 /opt/s0sta/cors_audit.py /var/tmp/cors.json\n\nExit codes: 0 clean, 1 at least one finding, 2 usage or unreadable input.\nRead-only: the file is read, nothing is written, nothing is fetched.\n\nThe honest limit: one response sample proves what the server does for one\nOrigin. To prove reflection, send two different Origins and compare; this\nscript can only judge the samples it is given.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nALLOW_ORIGIN = \"access-control-allow-origin\"\nALLOW_CREDENTIALS = \"access-control-allow-credentials\"\n\n\ndef load_json(path):\n    \"\"\"Read one JSON document and return it.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except json.JSONDecodeError as exc:\n        raise ValueError(\"%s is not valid JSON: %s\" % (path, exc))\n\n\ndef normalize_headers(raw):\n    \"\"\"Return a lowercase {header: [values]} map.\"\"\"\n    headers = {}\n    for name, value in raw.items():\n        values = value if isinstance(value, list) else [value]\n        clean = [str(item).strip() for item in values if item is not None]\n        if clean:\n            headers[str(name).lower()] = clean\n    return headers\n\n\ndef first(headers, name):\n    \"\"\"Return the first value of a header, lowercased, or an empty string.\"\"\"\n    values = headers.get(name)\n    return values[0].lower() if values else \"\"\n\n\ndef truthy(value):\n    \"\"\"Read a JSON boolean that may also be the string true.\"\"\"\n    if isinstance(value, bool):\n        return value\n    return str(value).strip().lower() == \"true\"\n\n\ndef origin_list(value):\n    \"\"\"Return a lowercase list of origins from a string or a list.\"\"\"\n    if value is None:\n        return []\n    if isinstance(value, str):\n        value = [value]\n    if not isinstance(value, list):\n        raise ValueError(\"allow_origins must be a string or a list\")\n    return [str(item).strip().lower() for item in value]\n\n\ndef is_config(sample):\n    \"\"\"True when the object is a CORS configuration, not a response dump.\"\"\"\n    if \"allow_origins\" not in sample or \"headers\" in sample:\n        return False\n    keys = [str(key).lower() for key in sample]\n    return ALLOW_ORIGIN not in keys and ALLOW_CREDENTIALS not in keys\n\n\ndef audit_config(sample):\n    \"\"\"Return a list of (label, detail) findings for a configuration.\"\"\"\n    origins = origin_list(sample.get(\"allow_origins\"))\n    credentials = truthy(sample.get(\"allow_credentials\", False))\n    findings = []\n    if \"*\" in origins and credentials:\n        findings.append((\"wildcard * + credentials\",\n                         \"allow_origins is * while credentials are allowed\"))\n    if \"null\" in origins:\n        findings.append((\"null origin\",\n                         \"the null origin is allowed in the configuration\"))\n    return findings\n\n\ndef audit_response(sample):\n    \"\"\"Return a list of (label, detail) findings for one response sample.\"\"\"\n    if isinstance(sample.get(\"headers\"), dict):\n        raw = sample[\"headers\"]\n    else:\n        raw = {key: value for key, value in sample.items()\n               if str(key).lower() not in (\"request_origin\", \"allowed_origins\")}\n    headers = normalize_headers(raw)\n    origin = str(sample.get(\"request_origin\", \"\")).strip().lower()\n    allowed = origin_list(sample.get(\"allowed_origins\"))\n    acao = first(headers, ALLOW_ORIGIN)\n    credentials = first(headers, ALLOW_CREDENTIALS) == \"true\"\n    findings = []\n\n    if acao == \"*\" and credentials:\n        findings.append((\"wildcard * + credentials\",\n                         \"the response allows any origin with credentials\"))\n    if (acao and origin and acao == origin and origin != \"null\"\n            and acao != \"null\" and origin not in allowed):\n        label = \"reflected origin\"\n        if credentials:\n            label += \" + credentials\"\n        findings.append((label, \"the response echoes the request Origin\"))\n    if acao == \"null\":\n        findings.append((\"null origin\",\n                         \"the response allows the null origin\"))\n    return findings\n\n\ndef build_parser():\n    \"\"\"Return the argument parser.\"\"\"\n    parser = argparse.ArgumentParser(\n        prog=\"cors_audit.py\",\n        description=\"Flag wildcard-with-credentials, reflected origins and \"\n                    \"null origins in a CORS headers dump or configuration. \"\n                    \"Exits 1 on any finding.\")\n    parser.add_argument(\"file\", metavar=\"JSON\",\n                        help=\"response sample, list of samples, or config\")\n    parser.add_argument(\"--verbose\", action=\"store_true\",\n                        help=\"print one line of detail per finding\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n\n    try:\n        data = load_json(args.file)\n        samples = data if isinstance(data, list) else [data]\n        if not samples:\n            raise ValueError(\"the JSON file holds no samples\")\n        for sample in samples:\n            if not isinstance(sample, dict) or not sample:\n                raise ValueError(\"each sample must be a non-empty object\")\n            if \"headers\" in sample and not isinstance(sample[\"headers\"], dict):\n                raise ValueError(\"the headers field must be an object\")\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings = []\n    seen = set()\n    try:\n        for sample in samples:\n            found = (audit_config(sample) if is_config(sample)\n                     else audit_response(sample))\n            for label, detail in found:\n                if label in seen:\n                    continue\n                seen.add(label)\n                findings.append((label, detail))\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if findings:\n        print(\"FAIL %d cors finding(s)\" % len(findings))\n        print(\"; \".join(label for label, _ in findings))\n        if args.verbose:\n            for label, detail in findings:\n                print(\"  %s: %s\" % (label, detail))\n        return 1\n\n    print(\"OK cors clean\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p557","n":"557","slug":"6-status-codes-in-api-security","title":"6 status codes in API security","sections":["devsecops"],"topic":"Application & API security","tags":["API","HTTP","Security","DevSecOps","AppSec"],"series_name":"PythonSec","design":"numbers","script":"","date":"2030-04-18","files":{"devsecops":"notes/p557-6-status-codes-in-api-security.md"},"md":"# 6 status codes in API security\n\nAn API returns a status code and thinks the job is done. The code is also a disclosure decision, and the wrong one tells an attacker which objects exist.\n\n## Quick reference\n\n| Number | What | Why it matters | |\n|:--|:--|:--|:--|\n| **401** | Unauthenticated | Missing or bad credentials. Name the scheme, not the user. | auth |\n| **403** | Forbidden | Authenticated but not allowed. The missing role is usually safe to name. | authz |\n| **404** | Not found | Use it for another tenant's object. Do not confirm the id. | enum |\n| **405** | Method not allowed | Send Allow: GET, POST. It describes the route, not the code. | method |\n| **429** | Too many requests | Send Retry-After. Never echo the API key or the account id. | throttle |\n| **500** | Server error | Return the request id, never the stack trace or the SQL. | logging |\n\n## Explained\n\n### The six codes\n\n401 means no valid credentials. It should name the scheme in WWW-Authenticate and should not say whether the user exists. A 401 body that lists valid usernames is an enumeration oracle.\n\n403 means the caller is authenticated and still not allowed. Naming the missing role is usually safe; naming the object id confirms it exists.\n\n404 means not found. For another tenant's object, 404 is safer than 403 because a missing id and a forbidden one get the same answer.\n\n405 means the method is wrong for this path. Send Allow: GET, POST, which describes the route contract and not the internals. A 405 still confirms the path exists.\n\n429 means slow down. Send Retry-After and the rate-limit headers, and never echo the API key or the account id. A 429 without Retry-After invites a client to hammer the endpoint.\n\n500 means the server failed. Return the request id and log it; never return the stack trace, the SQL or the upstream hostname.\n\n### The trade-off\n\n> **The honest caveat:** hiding existence behind 404 costs support time and can confuse legitimate clients. Decide per object, and write the decision down.\n\n> **The lesson:** choose the status code for the caller you trust.\n\n## In one line\n\n> **The code is a contract, not a hint.** Every status is a decision about what the caller may know.\n\n**Read the allow header**\n\n```bash\ncurl -si -X OPTIONS $URL\n```\n"},{"id":"p558","n":"558","slug":"a-cookie-flag-audit-in-python","title":"A cookie flag audit in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Cookies","WebSecurity","Python","DevSecOps","HTTP"],"series_name":"PythonSec","design":"code","script":"cookie_audit.py","date":"2030-04-21","files":{"devsecops":"notes/p558-a-cookie-flag-audit-in-python.md"},"md":"# A cookie flag audit in Python\n\nA session cookie with no flags is a password in a paper bag. The browser will send it over plain HTTP, script can read it, and another site can borrow it. A framework default is not an audit, and these flags are often unset.\n\n## Quick reference\n\nThe full script is at the end of this note: `cookie_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cookie_audit.py --headers /tmp/h.json\n```\n\n**Example output**\n\n```text\nFAIL session: 4 issue(s)\nSecure; HttpOnly; SameSite; Max-Age 31536000 too long\n```\n\n## Explained\n\n### What it checks\n\nThe script parses Set-Cookie headers from a headers dump or one raw value. It never prints the cookie value, only the name. For each cookie it checks Secure, HttpOnly, SameSite, and a Max-Age above the ceiling, which defaults to thirty days. Expires is ignored, because Max-Age is the modern attribute. A cookie without Max-Age is a session cookie, so its lifetime is not flagged.\n\n### The run\n\nThe dirty sample prints two lines: FAIL session: 4 issue(s), then Secure; HttpOnly; SameSite; Max-Age 31536000 too long. Exit 1 fails the job, a clean cookie exits 0, and a malformed header exits 2. The full script is attached, standard library only and read-only.\n\n### Why each\n\nSecure keeps the cookie off plain HTTP. HttpOnly keeps it away from document.cookie. SameSite stops another origin from sending it on a cross-site request. The Max-Age check catches a token that outlives its welcome by a year.\n\n> **The honest caveat:** Chrome and Firefox now default to Lax, so a missing attribute is less dangerous than it was; the missing Secure and HttpOnly flags are the ones to fix first. A short Max-Age also does not shorten a server-side session.\n\n> **The lesson:** flags are cheap, and their absence is visible.\n\n## In one line\n\n> **A cookie is a policy, not a value.** The flags decide where the token may travel and who may read it.\n\n**Check the flags**\n\n```bash\npython3 cookie_audit.py --headers h.json\n```\n\n## Script: cookie_audit.py\n\nDownload: [scripts/cookie_audit.py](../scripts/cookie_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Audit Set-Cookie headers for missing flags and a long lifetime.\n\nReads a response-headers JSON dump, or one raw Set-Cookie value, and flags:\n\n    Secure       the cookie may travel over plain HTTP\n    HttpOnly     document.cookie can read the value\n    SameSite     cross-site requests may send the cookie\n    Max-Age      the lifetime is longer than the ceiling, default 30 days\n\nThe cookie value is never printed. Only the name, the flags and the Max-Age\nnumber appear in the output, so a session token in the fixture stays out of\nthe logs.\n\nUsage:\n    python3 cookie_audit.py --headers /tmp/headers.json\n    python3 cookie_audit.py --cookie \"session=abc; Path=/; HttpOnly\"\n    python3 cookie_audit.py --headers /tmp/headers.json --max-age 86400\n    python3 cookie_audit.py --headers /tmp/headers.json --verbose\n    python3 cookie_audit.py --help\n\nA headers dump may be an object of names to values, an object with a\n\"headers\" object, or a list of {\"name\": ..., \"value\": ...} entries. The\nset-cookie entry may be one string or a list of strings.\n\nCI (after a deploy):\n    python3 cookie_audit.py --headers /tmp/headers.json || exit 1\n\nCron (daily at 06:15):\n    15 6 * * * python3 /opt/s0sta/cookie_audit.py --headers /var/tmp/h.json\n\nExit codes: 0 clean, 1 at least one finding, 2 usage or unreadable input.\nRead-only: the file is read, nothing is written, nothing is fetched.\n\nExpires is not parsed: Max-Age is the modern attribute and the one that\nsurvives a browser restart consistently. A short Max-Age is not proof of a\nshort session, because the server can still expire the token.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nHEADER = \"set-cookie\"\n\n\ndef load_headers(path):\n    \"\"\"Return a lowercase {header: [values]} map from a JSON dump.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except json.JSONDecodeError as exc:\n        raise ValueError(\"%s is not valid JSON: %s\" % (path, exc))\n\n    if isinstance(data, dict) and isinstance(data.get(\"headers\"), dict):\n        data = data[\"headers\"]\n\n    if isinstance(data, dict):\n        items = list(data.items())\n    elif isinstance(data, list):\n        items = []\n        for entry in data:\n            if isinstance(entry, str):\n                items.append((HEADER, entry))\n                continue\n            if not isinstance(entry, dict):\n                raise ValueError(\"each header entry must be an object\")\n            name = entry.get(\"name\")\n            if name is None:\n                raise ValueError(\"a header entry has no name\")\n            items.append((name, entry.get(\"value\", \"\")))\n    else:\n        raise ValueError(\"headers JSON must be an object or a list\")\n\n    headers = {}\n    for name, value in items:\n        key = str(name).lower()\n        values = value if isinstance(value, list) else [value]\n        clean = [str(item) for item in values if item is not None]\n        if clean:\n            headers.setdefault(key, []).extend(clean)\n    return headers\n\n\ndef parse_cookie(raw):\n    \"\"\"Return (name, attributes) for one Set-Cookie value.\"\"\"\n    parts = raw.split(\";\")\n    pair = parts[0].strip()\n    if \"=\" not in pair:\n        raise ValueError(\"set-cookie value has no name=value pair\")\n    name = pair.split(\"=\", 1)[0].strip()\n    if not name:\n        raise ValueError(\"set-cookie value has an empty name\")\n    attrs = {}\n    for part in parts[1:]:\n        text = part.strip()\n        if not text:\n            continue\n        key, _, value = text.partition(\"=\")\n        attrs[key.strip().lower()] = value.strip()\n    return name, attrs\n\n\ndef audit(attrs, max_age):\n    \"\"\"Return a list of (label, detail) findings for one cookie.\"\"\"\n    findings = []\n    if \"secure\" not in attrs:\n        findings.append((\"Secure\", \"the cookie can travel over plain HTTP\"))\n    if \"httponly\" not in attrs:\n        findings.append((\"HttpOnly\", \"document.cookie can read the value\"))\n    if \"samesite\" not in attrs:\n        findings.append((\"SameSite\", \"cross-site requests may send the cookie\"))\n    raw_age = attrs.get(\"max-age\", \"\")\n    if raw_age:\n        try:\n            seconds = int(raw_age)\n        except ValueError:\n            findings.append((\"Max-Age invalid\",\n                             \"Max-Age %r is not an integer\" % raw_age))\n        else:\n            if seconds > max_age:\n                findings.append((\"Max-Age %d too long\" % seconds,\n                                 \"%d seconds is above the %d second ceiling\"\n                                 % (seconds, max_age)))\n    return findings\n\n\ndef positive_int(text):\n    \"\"\"argparse type: a whole number greater than zero.\"\"\"\n    try:\n        value = int(text)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"%r is not a whole number\" % text)\n    if value <= 0:\n        raise argparse.ArgumentTypeError(\"must be greater than zero\")\n    return value\n\n\ndef build_parser():\n    \"\"\"Return the argument parser.\"\"\"\n    parser = argparse.ArgumentParser(\n        prog=\"cookie_audit.py\",\n        description=\"Flag Set-Cookie headers without Secure, HttpOnly or \"\n                    \"SameSite, and cookies whose Max-Age is above the \"\n                    \"ceiling. The value is never printed. Exits 1 on any \"\n                    \"finding.\")\n    group = parser.add_mutually_exclusive_group(required=True)\n    group.add_argument(\"--headers\", metavar=\"FILE\",\n                       help=\"JSON dump of response headers\")\n    group.add_argument(\"--cookie\", metavar=\"VALUE\",\n                       help=\"one raw Set-Cookie value\")\n    parser.add_argument(\"--max-age\", type=positive_int, default=2592000,\n                        metavar=\"SECONDS\",\n                        help=\"longest acceptable Max-Age (default: 2592000, \"\n                             \"30 days)\")\n    parser.add_argument(\"--verbose\", action=\"store_true\",\n                        help=\"print one line of detail per finding\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n\n    try:\n        if args.headers:\n            headers = load_headers(args.headers)\n            cookies = headers.get(HEADER, [])\n            if not cookies:\n                print(\"OK 0 cookie(s), nothing to audit\")\n                return 0\n        else:\n            cookies = [args.cookie]\n        parsed = [parse_cookie(raw) for raw in cookies]\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    failed = 0\n    for name, attrs in parsed:\n        findings = audit(attrs, args.max_age)\n        if not findings:\n            continue\n        failed += 1\n        print(\"FAIL %s: %d issue(s)\" % (name, len(findings)))\n        print(\"; \".join(label for label, _ in findings))\n        if args.verbose:\n            for label, detail in findings:\n                print(\"  %s: %s\" % (label, detail))\n\n    if failed:\n        return 1\n    print(\"OK %d cookie(s) with all flags set\" % len(parsed))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p559","n":"559","slug":"an-openapi-auth-check-in-python","title":"An OpenAPI auth check in Python","sections":["devsecops"],"topic":"Python scripts","tags":["OpenAPI","API","Python","DevSecOps","AppSec"],"series_name":"PythonSec","design":"code","script":"openapi_auth_check.py","date":"2030-04-23","files":{"devsecops":"notes/p559-an-openapi-auth-check-in-python.md"},"md":"# An OpenAPI auth check in Python\n\nEvery API has one endpoint nobody remembers. It is in the gateway, missing from the spec, and it is the one that accepts anonymous writes.\n\n## Quick reference\n\nThe full script is at the end of this note: `openapi_auth_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 openapi_auth_check.py api.json\n```\n\n**Example output**\n\n```text\nFAIL 2 unauthenticated operation(s)\nGET /health, GET /v1/reports\n```\n\n## Explained\n\n### What it reads\n\nThe script loads an OpenAPI 3 JSON document and walks every path and method. For each operation it takes the security list from the operation itself, or inherits the root list when the operation has none. The output names the method and the path, so every fix has a one-line address. A path item behind a $ref is skipped, not guessed.\n\n### What it flags\n\nNo security at either level is a gap. So is security: [], a deliberate public endpoint, and security: [{}], an empty requirement that makes auth optional in the OpenAPI model. The `--allow` flag accepts the public endpoints you meant to ship, by path or by method and path.\n\n### The run\n\nOn the sample spec it prints two lines: FAIL 2 unauthenticated operation(s), then GET /health, GET /v1/reports, and exits 1. A spec where every operation inherits bearerAuth exits 0. A file that is not JSON, or not OpenAPI 3, exits 2. The full script is attached, standard library only and read-only.\n\n> **The honest caveat:** this checks declarations, not enforcement. An operation can declare bearerAuth and still accept anonymous traffic if the handler forgets the check, so pair it with a request test against a real token.\n\n> **The lesson:** a spec that hides an endpoint cannot secure it.\n\n## In one line\n\n> **Declared is not enforced.** A spec that hides an endpoint cannot protect it.\n\n**Allow the public**\n\n```bash\npython3 openapi_auth_check.py --allow /health\n```\n\n## Script: openapi_auth_check.py\n\nDownload: [scripts/openapi_auth_check.py](../scripts/openapi_auth_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check an OpenAPI 3 document for operations with no auth requirement.\n\nReads the JSON spec, walks every path and method, and reports each operation\nthat does not require authentication. Security can be declared once at the\ndocument root or per operation; per-operation security replaces the global\nlist, so an explicit empty list is a deliberate public endpoint and an empty\nrequirement object ({}) makes auth optional. Both are still reported.\n\nUse --allow to accept an intended public endpoint, by path (\"/health\") or by\nmethod and path (\"GET /health\"). Allowed endpoints are counted but not\nreported.\n\nUsage:\n    python3 openapi_auth_check.py /tmp/openapi.json\n    python3 openapi_auth_check.py /tmp/openapi.json --allow /health\n    python3 openapi_auth_check.py /tmp/openapi.json --allow \"POST /login\"\n    python3 openapi_auth_check.py /tmp/openapi.json --verbose\n    python3 openapi_auth_check.py --help\n\nCI (before publishing a spec):\n    python3 openapi_auth_check.py api/openapi.json || exit 1\n\nCron (daily at 07:00, to catch a spec committed after hours):\n    0 7 * * * python3 /opt/s0sta/openapi_auth_check.py /srv/api/openapi.json\n\nExit codes: 0 every operation requires auth, 1 at least one does not,\n2 usage or unreadable input.\nRead-only: the file is read, nothing is written, nothing is fetched.\n\nJSON only. Convert a YAML spec first with the tool your CI already uses;\nthe script rejects text that is not JSON with exit 2.\n\nThe honest limit: this reads declarations, not enforcement. An operation\ncan declare bearerAuth and still accept anonymous traffic if the handler\nforgets the check, so pair this with a request test.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nMETHODS = (\"get\", \"put\", \"post\", \"delete\", \"options\", \"head\", \"patch\", \"trace\")\n\n\ndef load_spec(path):\n    \"\"\"Read and shape-check one OpenAPI 3 JSON document.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            spec = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    except json.JSONDecodeError as exc:\n        raise ValueError(\"%s is not valid JSON (JSON only, not YAML): %s\"\n                         % (path, exc))\n\n    if not isinstance(spec, dict):\n        raise ValueError(\"the spec must be a JSON object\")\n    if \"swagger\" in spec and \"openapi\" not in spec:\n        raise ValueError(\"OpenAPI 2 (swagger) is not supported; use 3.x\")\n    version = spec.get(\"openapi\")\n    if not isinstance(version, str) or not version.startswith(\"3.\"):\n        raise ValueError(\"the spec has no openapi 3.x version field\")\n    if not isinstance(spec.get(\"paths\"), dict):\n        raise ValueError(\"the spec has no paths object\")\n    security = spec.get(\"security\")\n    if security is not None and not isinstance(security, list):\n        raise ValueError(\"the global security field must be a list\")\n    return spec\n\n\ndef check_security(security):\n    \"\"\"Validate one security list and return its gap reason, or None.\"\"\"\n    if not isinstance(security, list):\n        raise ValueError(\"a security field must be a list\")\n    for requirement in security:\n        if not isinstance(requirement, dict):\n            raise ValueError(\"each security requirement must be an object\")\n    if not security:\n        return \"security is an empty list\"\n    if any(not requirement for requirement in security):\n        return \"an empty requirement ({}) makes auth optional\"\n    return None\n\n\ndef collect(spec):\n    \"\"\"Return (operations, gaps) where gaps are (method, path, reason).\"\"\"\n    global_security = spec.get(\"security\")\n    operations = 0\n    gaps = []\n    refs = 0\n    for path, item in spec[\"paths\"].items():\n        if not isinstance(item, dict):\n            raise ValueError(\"path %r must be an object\" % path)\n        if \"$ref\" in item:\n            refs += 1\n            continue\n        for method in METHODS:\n            if method not in item:\n                continue\n            operation = item[method]\n            if not isinstance(operation, dict):\n                raise ValueError(\"%s %s must be an object\" % (method.upper(), path))\n            operations += 1\n            if \"security\" in operation:\n                security = operation[\"security\"]\n                source = \"operation\"\n            else:\n                security = global_security\n                source = \"global\"\n            if security is None:\n                gaps.append((method, path, \"no security at operation or global level\"))\n                continue\n            reason = check_security(security)\n            if reason:\n                gaps.append((method, path, \"%s: %s\" % (source, reason)))\n    return operations, gaps, refs\n\n\ndef allowed_match(allowed, method, path):\n    \"\"\"True when a gap is on the --allow list.\"\"\"\n    target = \"%s %s\" % (method.upper(), path)\n    return target in allowed or path in allowed\n\n\ndef build_parser():\n    \"\"\"Return the argument parser.\"\"\"\n    parser = argparse.ArgumentParser(\n        prog=\"openapi_auth_check.py\",\n        description=\"Report OpenAPI 3 operations that do not require \"\n                    \"authentication, from the global security list or the \"\n                    \"operation's own. Exits 1 when any operation is open.\")\n    parser.add_argument(\"spec\", metavar=\"JSON\",\n                        help=\"OpenAPI 3 document in JSON form\")\n    parser.add_argument(\"--allow\", action=\"append\", default=[],\n                        metavar=\"PATH\",\n                        help='intended public endpoint, by path or by '\n                             '\"METHOD /path\"; repeatable')\n    parser.add_argument(\"--verbose\", action=\"store_true\",\n                        help=\"print one line of detail per gap\")\n    return parser\n\n\ndef main(argv=None):\n    args = build_parser().parse_args(argv)\n    allowed = set()\n    for entry in args.allow:\n        parts = entry.split()\n        if len(parts) == 2:\n            allowed.add(\"%s %s\" % (parts[0].upper(), parts[1]))\n        elif len(parts) == 1:\n            allowed.add(parts[0])\n        else:\n            print(\"ERROR --allow takes /path or 'METHOD /path'\", file=sys.stderr)\n            return 2\n\n    try:\n        spec = load_spec(args.spec)\n        operations, gaps, refs = collect(spec)\n    except ValueError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    open_gaps = []\n    allowed_count = 0\n    for method, path, reason in gaps:\n        if allowed_match(allowed, method, path):\n            allowed_count += 1\n            continue\n        open_gaps.append((method, path, reason))\n\n    if open_gaps:\n        print(\"FAIL %d unauthenticated operation(s)\" % len(open_gaps))\n        print(\", \".join(\"%s %s\" % (method.upper(), path)\n                        for method, path, _ in open_gaps))\n        if args.verbose:\n            for method, path, reason in open_gaps:\n                print(\"  %s %s: %s\" % (method.upper(), path, reason))\n        return 1\n\n    note = \"\"\n    if allowed_count:\n        note = \"; %d allowed\" % allowed_count\n    if refs:\n        note += \"; %d path item(s) behind $ref\" % refs\n    print(\"OK %d operation(s), all require auth%s\" % (operations, note))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p560","n":"560","slug":"the-layers-of-web-request-defence","title":"The layers of web request defence","sections":["it","devsecops"],"topic":"Application & API security","tags":["WebSecurity","HTTP","TLS","DevSecOps","AppSec"],"series_name":"PythonSec","design":"stack","script":"","date":"2030-04-25","files":{"it":"notes/p560-the-layers-of-web-request-defence.md","devsecops":"notes/p560-the-layers-of-web-request-defence.md"},"md":"# The layers of web request defence\n\nA request crosses six layers before your code sees it, and each layer answers a different question. Remove one and the others still stand, but not as high. The order matters: each layer can only act on what the one below passed through.\n\n## Quick reference\n\n*Bottom to top: each layer assumes the one below*\n\n1. **TLS** — Encrypts the hop and gives the client a certificate to verify. · *transport*\n2. **HSTS** — Tells the browser to refuse plain HTTP for this host. · *browser policy*\n3. **WAF** — Filters known-bad requests at the edge before the app sees them. · *edge filter*\n4. **CSP** — Limits where scripts load from and whether inline code runs. · *response policy*\n5. **CORS** — Decides which origins may read a response in a browser. · *browser gate*\n6. **Cookies and auth** — Who the caller is, and what the token is allowed to do. · *identity*\n\n## Explained\n\n### Bottom to top\n\nTLS encrypts the hop and gives the client a certificate to verify. HSTS tells the browser to refuse plain HTTP for this host, so the next visit cannot be stripped.\n\nThe WAF filters known-bad requests at the edge before the application sees them. CSP limits where scripts load from and whether inline code runs.\n\nCORS decides which origins may read a response in a browser. Cookies and auth carry the identity: who the caller is and what the token may do.\n\n### What each cannot do\n\nTLS does not stop an injection. HSTS does not help the first visit. A WAF does not know your business logic. CSP does not fix a server-side bug. CORS does not stop curl. Cookie flags do not encrypt the value. A missing layer is not a finding on its own.\n\n> **The honest caveat:** the layers are not independent. A CDN that terminates TLS sees plaintext, and a WAF bypass says nothing about the code behind it. Each layer assumes the one below did its job.\n\n> **The lesson:** know which layer you are testing before you call it secure.\n\n## In one line\n\n> **Six layers, six ways to fail.** Remove one and the others still stand, but not as high.\n\n**Start at the top**\n\n```bash\ncurl -sI $URL | head -20\n```\n"},{"id":"p561","n":"561","slug":"quiz-3-web-security-questions","title":"Quiz: 3 web security questions","sections":["devsecops"],"topic":"Application & API security","tags":["WebSecurity","API","DevSecOps","CyberSecurity","HTTP"],"series_name":"PythonSec","design":"quiz","script":"","date":"2030-04-28","files":{"devsecops":"notes/p561-quiz-3-web-security-questions.md"},"md":"# Quiz: 3 web security questions\n\nThree questions from a web security review. The answers are short, and each one is a place where a control is trusted for more than it does.\n\n## Quick reference\n\n**Q1. Why is CSP not XSS-proof?**\n\n- a) It only blocks inline script\n- b) It restricts sources, not code\n- c) Browsers ignore it now\n\n> **Answer:** b) It restricts sources, not code. CSP limits where script can load from and whether inline code runs, not what an allowed script does. A JSONP host or a DOM sink in your own bundle still executes.\n\n**Q2. Why is a wildcard with credentials a bug?**\n\n- a) Browsers reject the pair\n- b) Any origin could read the data\n- c) It disables HTTPS\n\n> **Answer:** b) Any origin could read the data. The pair would let any origin read authenticated responses, so browsers reject it. The usual fix, echoing the Origin, turns the rejection into a working bypass.\n\n**Q3. Why can 404 be safer than 403?**\n\n- a) It hides whether the object exists\n- b) It skips authentication\n- c) It is faster to render\n\n> **Answer:** a) It hides whether the object exists. 403 confirms the object exists and the caller lacks permission. 404 gives the same answer for missing and forbidden ids, so enumeration gets slower.\n\n## Explained\n\n### Why CSP is not XSS-proof\n\nIt restricts where script comes from, not what a permitted script does. An allowlisted host can serve a JSONP endpoint, and a DOM sink inside your own bundle is still a sink. CSP raises the cost; it does not remove the class. That is why an XSS review still needs output encoding and a DOM audit.\n\n### Why the wildcard plus credentials is a bug\n\nThe pair would let any origin read authenticated responses, so browsers reject it. The bug is the intent: the usual fix echoes the request origin back, and that turns a rejected header into a working bypass. The safe pattern is an exact allowlist checked on the server, with credentials only for those origins.\n\n### Why 404 can be safer than 403\n\n403 confirms the object exists and the caller lacks permission. 404 gives the same answer for a missing id and a forbidden one, so an attacker cannot enumerate another tenant's objects. It turns a permission bug into a missing-object bug, which is harder to debug.\n\n> **The honest caveat:** CSP still earns its place, and returning 404 for everything can confuse legitimate users. Both are judgement calls, not defaults.\n\n> **The lesson:** test the control, not the badge.\n\n## In one line\n\n> **The gap is in the assumption.** Three answers worth checking against your own stack.\n\n**Re-read the policy**\n\n```bash\ncurl -sI $URL\n```\n"},{"id":"p562","n":"562","slug":"the-reporting-pipeline","title":"The reporting pipeline","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","SecurityReporting","Automation","CICD"],"series_name":"PythonSec","design":"architecture","script":"","date":"2030-04-30","files":{"devsecops":"notes/p562-the-reporting-pipeline.md"},"md":"# The reporting pipeline\n\nA scan finishes and the output is three files in three formats. The person who asked for a report gets none of them, and the findings live in a channel nobody owns.\n\n## Quick reference\n\n- **Collect:** Scanner output (SCAN) · Asset inventory (INV)\n- **Normalise:** Schema mapping (MAP) · Dedupe key (KEY)\n- **Publish:** Report (OUT) · Exit gate (GATE)\n\n**Request flow:** Scanner output → Schema mapping → Dedupe key → Report → Exit gate; Asset inventory → Schema mapping\n\n**Around it:** Exit code (EXIT) · Owner map (OWNER)\n\n## Explained\n\n### Collect\n\nTwo inputs, not one. The scanner output is whatever the tool prints: JSON, SARIF or a table in a log. The asset inventory supplies the context the scanner lacks: service, environment and owner. Collect both unchanged before any filtering.\n\n### Normalise\n\nMap every scanner field onto one schema: finding ID, asset, severity, evidence, first seen. Then dedupe on a fingerprint built from the rule and the resource, so the same issue from two tools counts once. A finding that cannot be mapped is logged, not dropped. A missing inventory row is a finding too, because an asset nobody owns cannot be fixed.\n\n### Publish\n\nOne report, generated from the normalised rows, and one gate. The report is for people: what changed, what is new, who owns it. The gate is for machines: exit 1 when a new critical appears, so the pipeline stops before the deploy. Both read the same rows, which is why the numbers agree.\n\n> **The honest caveat:** normalisation loses scanner-specific detail. Keep the raw output beside the report, or the first disputed finding sends someone back to the tool.\n\n> **The lesson:** a report is a pipeline, not a document.\n\n## In one line\n\n> **Collect once, publish once.** One schema, one report, one exit code.\n\n**Count the inputs**\n\n```bash\nfind scans -type f | wc -l\n```\n"},{"id":"p563","n":"563","slug":"the-log-redaction-cheat-sheet","title":"The log redaction cheat sheet","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Logging","Security","Python","DevSecOps","SRE"],"series_name":"PythonSec","design":"cheatsheet","script":"","date":"2030-05-02","files":{"devsecops":"notes/p563-the-log-redaction-cheat-sheet.md"},"md":"# The log redaction cheat sheet\n\nA log line with a bearer token in it is a credential, and pasting that line into a ticket publishes it. The fix is a short loop: find, redact, rotate, verify.\n\n## Quick reference\n\n### Find\n\n```bash\ngrep -nE 'Bearer|AKIA[0-9A-Z]{16}' log\ngrep -nE '[0-9a-f]{32,}' log\njournalctl -u api --since -1h\nawk '/@/ {print NR\": \"$0}' log\n```\n\n### Redact\n\n```bash\nsed -E 's/Bearer [^ ]+/Bearer X/g' log\nsed -E 's/[^ ]+@[^ ]+/EMAIL/g' log\nsed -E 's/([0-9]+\\.){3}[0-9]+/IP/g' log\nsed -E 's/[0-9a-f]{32,}/HEX/g' log\n```\n\n### Rotate\n\n```bash\nlogrotate -d /etc/logrotate.d/api\nlogrotate -f /etc/logrotate.d/api\njournalctl --vacuum-time=7d\njournalctl --vacuum-size=500M\n```\n\n### Verify\n\n```bash\ngrep -c 'AKIA' log\ngrep -c '@' log\nzgrep -c '@' log.1.gz\nlogrotate -d /etc/logrotate.d/api | head\n```\n\n## Explained\n\n### Find\n\n`grep -nE 'Bearer|AKIA[0-9A-Z]{16}' log` catches both shapes in one pass. `grep -nE '[0-9a-f]{32,}' log` finds long hex. `journalctl -u api --since -1h` reads the service journal directly. `awk '/@/ {print NR\": \"$0}' log` lists the lines with an at sign, the cheapest email lead.\n\n### Redact\n\n`sed -E 's/Bearer [^ ]+/Bearer X/g' log` keeps the scheme and masks the value. `sed -E 's/[^ ]+@[^ ]+/EMAIL/g' log` replaces an address. `sed -E 's/([0-9]+\\.){3}[0-9]+/IP/g' log` replaces an IPv4 address. `sed -E 's/[0-9a-f]{32,}/HEX/g' log` replaces long hex.\n\n### Rotate\n\n`logrotate -d /etc/logrotate.d/api` checks the rule without moving a file; -f runs it now. `journalctl --vacuum-time=7d` and `--vacuum-size=500M` bound the journal, so the redacted copy is not the only copy. Rotation is the step that makes the mask matter.\n\n### Verify\n\n`grep -c 'AKIA' log` and `grep -c '@' log` count what remains. zgrep -c '@' log.1.gz checks the rotated archive. `logrotate -d /etc/logrotate.d/api | head` shows the rule it would apply.\n\n> **The honest caveat:** redaction misses a secret split across lines or encoded in a blob, and masking a log does not revoke a key.\n\n> **The lesson:** redact at the edge, rotate at the source.\n\n## In one line\n\n> **Find it, mask it, then rotate it.** A redacted log is only useful if the key behind it is dead.\n\n**Check the archive:** zgrep -c '@' log.1.gz\n"},{"id":"p564","n":"564","slug":"four-properties-of-a-security-report","title":"Four properties of a security report","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","SecurityReporting","AppSec","CICD"],"series_name":"PythonSec","design":"explainer","script":"","date":"2030-05-05","files":{"devsecops":"notes/p564-four-properties-of-a-security-report.md"},"md":"# Four properties of a security report\n\nA report that says \"3 critical, 12 high\" and nothing else is a mood, not a finding. Four properties, checked in this order, separate a report a team can act on from a file nobody opens.\n\n## Quick reference\n\n### Accurate\n\n- Every claim names a file, a line and a command\n- No finding without evidence in the same report\n- Say what you did not test\n\n### Reproducible\n\n- The reader can rerun every command\n- Version, commit and date are pinned\n- Fixtures are small and included\n\n### Prioritised\n\n- Severity follows the impact, not the tool\n- One critical outranks ten lows\n- The first paragraph is the decision\n\n### Owned\n\n- Every finding has a name and a date\n- A gate fails the build, not an inbox\n- Closed means verified, not filed\n\n> **Note:** Four properties, checked in this order.\n\n## Explained\n\n### Accurate\n\nEvery claim names a file, a line and a command. No finding without evidence in the same report. Say what you did not test, because silence reads as coverage. A finding without a reproduction is a hypothesis, and the report should label it.\n\n### Reproducible\n\nThe reader can rerun every command. Version, commit and date are pinned. Fixtures are small and included, so the same input gives the same answer next quarter. The artefact digest and the tool version belong in the header, not in a comment.\n\n### Prioritised\n\nSeverity follows the impact, not the tool. One critical outranks ten lows. The first paragraph is the decision, so write it as one. A severity that disagrees with the impact teaches the reader to ignore the label.\n\n### Owned\n\nEvery finding has a name and a date. A gate fails the build, not an inbox. Closed means verified, not filed. An owner without a date is a name, not a commitment.\n\n> **The honest caveat:** a report can be accurate, reproducible and owned, and still be ignored when the reader disputes the scope. Put the scope on page one.\n\n> **The lesson:** a report is a decision document, not a scan dump.\n\n## In one line\n\n> **A report is a decision.** Four properties turn a scan dump into something a team can act on.\n\n**Ask one owner**\n\n```bash\ngrep -n 'Owner' report.md\n```\n"},{"id":"p565","n":"565","slug":"a-log-redactor-in-python","title":"A log redactor in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","Logging","Security","DevSecOps","SRE"],"series_name":"PythonSec","design":"code","script":"log_redact.py","date":"2030-05-07","files":{"devsecops":"notes/p565-a-log-redactor-in-python.md"},"md":"# A log redactor in Python\n\nA production log carries email addresses, client IPs and the occasional bearer token, and log shipping copies all of it to three systems with weaker access. Redaction is the step between the file and the archive.\n\n## Quick reference\n\nThe full script is at the end of this note: `log_redact.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 log_redact.py app.log\n```\n\n**Example output**\n\n```text\n2026-10-21T09:14:02Z api login [EMAIL] from [IP]\n2026-10-21T09:14:03Z api auth Bearer [TOKEN]\n```\n\n## Explained\n\n### What it does\n\nThe script reads log lines from a file or stdin and applies four patterns in order: email, IPv4, bearer token, long hex. Email and IPv4 come first because they are easy to over-match. The bearer pattern keeps the scheme and masks the value. Hex last catches commit SHAs and keys of 32 characters or more.\n\n### The run\n\n`python3 log_redact.py app.log` prints the same lines with the address as [EMAIL], the client as [IP] and the credential as Bearer [TOKEN]. A long hex run becomes [HEX] the same way. It writes only to stdout, so redirect it or pipe it into the next tool. A redaction is the job, so a file with hits still exits 0; exit 2 means the file or the input was unreadable.\n\n### Why the order\n\nA JWT can carry a hex run inside it. A hex regex run first would cut the token into pieces and leave the rest exposed, so the token pattern has to run first.\n\n> **The honest caveat:** these patterns miss a secret split across lines, encoded in a blob or written in an unfamiliar format. A redacted log is safer, not safe.\n\n> **The lesson:** redact before the log travels, not after it leaks.\n\n## In one line\n\n> **Redact the line, keep the evidence.** A log is evidence, so mask it before it travels.\n\n**Mask before ship**\n\n```bash\npython3 log_redact.py app.log > safe.log\n```\n\n## Script: log_redact.py\n\nDownload: [scripts/log_redact.py](../scripts/log_redact.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"log_redact.py - redact secrets and identifiers from log lines.\n\nReads log text from files or stdin, masks email addresses, IPv4 addresses,\nbearer tokens and long hexadecimal strings, and writes the result to stdout.\nThe input files are never modified.\n\n    EMAIL    [EMAIL]        anything shaped like user@example.com\n    IPv4     [IP]           10.0.0.7, 192.168.1.20\n    TOKEN    Bearer [TOKEN] the credential after a Bearer scheme\n    HEX      [HEX]          32 or more hexadecimal characters\n\nUsage:\n    python3 log_redact.py app.log\n    cat app.log | python3 log_redact.py\n    journalctl -u api --since -1h | python3 log_redact.py > safe.log\n    python3 log_redact.py access.log error.log\n    python3 log_redact.py --help\n\nCI (sanitise an artefact before upload):\n    python3 log_redact.py build.log > build.redacted.log || exit 2\n\nCron (a daily archive of one service log):\n    15 2 * * * journalctl -u api --since -1d | python3 /opt/s0sta/log_redact.py >> /var/log/api.redacted\n\nExit codes: 0 input read and redacted, 2 usage or unreadable input.\nA redaction is the job, not a failure, so a file with hits still exits 0.\n\nRead-only: it writes nothing except stdout and opens no network socket.\n\nThe patterns are deliberately narrow. They catch the common shapes and miss\nsecrets split across lines, encoded blobs and credentials in other formats.\nA redacted log is safer, not safe.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nPATTERNS = (\n    (re.compile(r\"[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}\"), \"[EMAIL]\"),\n    (re.compile(r\"\\b(?:\\d{1,3}\\.){3}\\d{1,3}\\b\"), \"[IP]\"),\n    (re.compile(r\"(?i)\\b(bearer\\s+)[A-Za-z0-9._~+/-]+=*\"), r\"\\1[TOKEN]\"),\n    (re.compile(r\"\\b[0-9A-Fa-f]{32,}\\b\"), \"[HEX]\"),\n)\n\n\ndef redact(line):\n    \"\"\"Return one line with every known shape replaced.\"\"\"\n    for pattern, replacement in PATTERNS:\n        line = pattern.sub(replacement, line)\n    return line\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Redact emails, IPv4 addresses, bearer tokens and long \"\n                    \"hex strings from log lines.\")\n    parser.add_argument(\"files\", nargs=\"*\", metavar=\"FILE\",\n                        help=\"log files; read stdin when none is given\")\n    args = parser.parse_args(argv)\n    for name in args.files or [\"-\"]:\n        try:\n            if name == \"-\":\n                for line in sys.stdin:\n                    sys.stdout.write(redact(line))\n            else:\n                with open(name, \"r\", encoding=\"utf-8\") as handle:\n                    for line in handle:\n                        sys.stdout.write(redact(line))\n        except (OSError, UnicodeError) as exc:\n            print(\"log_redact: %s: %s\" % (name, exc), file=sys.stderr)\n            return 2\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p566","n":"566","slug":"an-api-key-age-report-in-python","title":"An API key age report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","APIKeys","Security","DevSecOps","Automation"],"series_name":"PythonSec","design":"code","script":"key_age.py","date":"2030-05-09","files":{"devsecops":"notes/p566-an-api-key-age-report-in-python.md"},"md":"# An API key age report in Python\n\nEight API keys sit in an inventory, and the oldest was created 412 days ago and has never been used. Nobody notices until the next audit, because the inventory is a file and nobody reads a file.\n\n## Quick reference\n\nThe full script is at the end of this note: `key_age.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 key_age.py keys.json --max-days 180 --now 2026-10-21\n```\n\n**Example output**\n\n```text\nFLAG svc-legacy 412d never\nBUCKETS 3 2 1 2\n```\n\n## Explained\n\n### What it does\n\nThe script reads the inventory JSON, parses created_at and last_used, and computes the age of every key against a reference time. It flags a key when the age is above `--max-days` or when last_used is null, then prints four age buckets, thirty days each with the last one open-ended, so the shape of the estate is visible.\n\n### The output\n\nThe run above prints one FLAG line for the old key, with its age in days and never, then BUCKETS 3 2 1 2. Each bucket covers thirty days; the last one holds everything from 90 days up. A used key over the threshold prints its last-used date instead of never.\n\n### The exit code\n\n0 means no key is stale and none is unused, 1 means at least one flag, 2 means the file, the JSON or a record was bad. That makes the report a gate: a weekly job can fail the review instead of filling an inbox.\n\n> **The honest caveat:** last_used is only as good as the inventory. A key used by a quarterly batch job looks unused today, and a key marked used can still be live after its owner left.\n\n> **The lesson:** age is a proxy for risk, so use it to ask, not to assume.\n\n## In one line\n\n> **Old and unused is a finding.** Age is a question; the owner has to answer it.\n\n**Set the policy**\n\n```bash\n--max-days 90\n```\n\n## Script: key_age.py\n\nDownload: [scripts/key_age.py](../scripts/key_age.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"key_age.py - report stale and unused API keys from an inventory JSON.\n\nReads an inventory of key metadata (never key material) and prints the keys\nthat are older than a threshold or have never been used. Four age buckets\n(0-29, 30-59, 60-89 and 90+ days) summarise the inventory, so the report says\nwhat to fix and how the estate is ageing.\n\nInventory shape, a JSON object with a \"keys\" list (a bare list also works):\n\n    {\"keys\": [\n      {\"id\": \"svc-billing\", \"owner\": \"platform\",\n       \"created_at\": \"2026-01-04T09:00:00Z\",\n       \"last_used\": \"2026-10-01T12:00:00Z\"},\n      {\"id\": \"svc-ci\", \"owner\": \"build\",\n       \"created_at\": \"2024-08-30T09:00:00Z\", \"last_used\": null}\n    ]}\n\nUsage:\n    python3 key_age.py keys.json\n    python3 key_age.py keys.json --max-days 180\n    python3 key_age.py keys.json --now 2026-10-21T00:00:00Z\n    cat keys.json | python3 key_age.py -\n    python3 key_age.py --help\n\nCI (fail the job when a key is older than the policy):\n    python3 key_age.py keys.json --max-days 90 || exit 1\n\nCron (weekly report to the security channel):\n    0 7 * * 1 python3 /opt/s0sta/key_age.py /etc/s0sta/keys.json --max-days 90\n\nExit codes: 0 no key is stale and none is unused, 1 at least one flag,\n2 usage, unreadable input or a record that does not match the schema.\n\nRead-only: the inventory is opened for reading only. No key is created,\nrotated or revoked, and no network call is made.\n\nThe inventory holds identifiers and timestamps, not secrets. This script\nnever asks for, prints or stores a key value.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nBUCKET_DAYS = 30\nBUCKETS = 4\n\n\ndef parse_time(text, field, where):\n    \"\"\"Parse an ISO 8601 timestamp; a naive value is treated as UTC.\"\"\"\n    if not isinstance(text, str) or not text.strip():\n        raise ValueError(\"%s: %s is not a timestamp\" % (where, field))\n    raw = text.strip()\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        value = datetime.fromisoformat(raw)\n    except ValueError:\n        raise ValueError(\"%s: %s is not ISO 8601: %r\" % (where, field, text))\n    if value.tzinfo is None:\n        value = value.replace(tzinfo=timezone.utc)\n    return value.astimezone(timezone.utc)\n\n\ndef load_inventory(path):\n    \"\"\"Return the list of key records, or raise ValueError.\"\"\"\n    try:\n        if path == \"-\":\n            document = json.load(sys.stdin)\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                document = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    except (json.JSONDecodeError, UnicodeError) as exc:\n        raise ValueError(\"%s: not valid JSON: %s\" % (path, exc))\n    if isinstance(document, dict):\n        keys = document.get(\"keys\")\n    else:\n        keys = document\n    if not isinstance(keys, list):\n        raise ValueError(\"%s: expected a list or an object with a keys list\"\n                         % path)\n    return keys\n\n\ndef check_record(record, index):\n    \"\"\"Validate one record; return a label for error messages.\"\"\"\n    where = \"key %d\" % (index + 1)\n    if not isinstance(record, dict):\n        raise ValueError(\"%s is not an object\" % where)\n    for field in (\"id\", \"owner\", \"created_at\"):\n        if not isinstance(record.get(field), str) or not record[field].strip():\n            raise ValueError(\"%s: %s is missing\" % (where, field))\n    last_used = record.get(\"last_used\")\n    if last_used is not None and not isinstance(last_used, str):\n        raise ValueError(\"%s: last_used must be a string or null\" % where)\n    return where\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Report API keys that are too old or never used.\")\n    parser.add_argument(\"inventory\", nargs=\"?\", default=\"-\",\n                        help=\"inventory JSON file; stdin when omitted\")\n    parser.add_argument(\"--max-days\", type=int, default=90,\n                        help=\"flag keys older than this (default 90)\")\n    parser.add_argument(\"--now\",\n                        help=\"ISO 8601 reference time for the report\")\n    args = parser.parse_args(argv)\n    if args.max_days < 1:\n        parser.error(\"--max-days must be a positive integer\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        records = load_inventory(args.inventory)\n        if args.now:\n            as_of = parse_time(args.now, \"--now\", \"arguments\")\n        else:\n            as_of = datetime.now(timezone.utc)\n        buckets = [0] * BUCKETS\n        flags = 0\n        for index, record in enumerate(records):\n            where = check_record(record, index)\n            created = parse_time(record[\"created_at\"], \"created_at\", where)\n            age = (as_of - created).days\n            if age < 0:\n                raise ValueError(\"%s: created_at is in the future\" % where)\n            last_used = record.get(\"last_used\")\n            if last_used:\n                parse_time(last_used, \"last_used\", where)\n            buckets[min(age // BUCKET_DAYS, BUCKETS - 1)] += 1\n            if age > args.max_days or not last_used:\n                flags += 1\n                used = \"never\" if not last_used else \"last=%s\" % last_used[:10]\n                print(\"FLAG %s %dd %s\" % (record[\"id\"], age, used))\n        print(\"BUCKETS \" + \" \".join(str(count) for count in buckets))\n    except ValueError as exc:\n        print(\"key_age: %s\" % exc, file=sys.stderr)\n        return 2\n    return 1 if flags else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p567","n":"567","slug":"security-automation-at-a-glance","title":"Security automation at a glance","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","SecurityMetrics","Automation","SRE"],"series_name":"PythonSec","design":"dashboard","script":"","date":"2030-05-12","files":{"devsecops":"notes/p567-security-automation-at-a-glance.md"},"md":"# Security automation at a glance\n\nA monthly review opens with six tiles and a question: are the scripts doing anything useful? The tiles are not the answer, but each one can change the next decision.\n\n## Quick reference\n\n**Status:** Last 30 days\n\n| Metric | Value | Note |\n|:--|:--|:--|\n| Scripts run | **1,284** | in 42 scheduled jobs |\n| Findings open | **17** | 4 high, none critical |\n| False positives | **9%%** | of 186 findings |\n| Mean runtime | **38 s** | p95 is 2m 10s |\n| Coverage | **91%%** | of 63 repos gated |\n| Last failure | **2 days** | auth check, exit 2 |\n\n## Explained\n\n### What the six show\n\nScripts run: 1,284 across 42 scheduled jobs. A high count is activity, not coverage; 42 jobs that all scan the same repository would look busy and prove little.\n\nFindings open: 17, four high and none critical. The shape matters more than the total, because a queue of lows with no owner is a backlog with better branding.\n\n### The trust numbers\n\nFalse positives: 9 percent of 186 findings. That is the trust metric. Every false positive spends a reviewer's attention, and attention is the scarce resource.\n\nMean runtime: 38 seconds, with a p95 of 2 minutes 10. The mean hides the tail, so read both before you decide a nightly job is cheap.\n\nCoverage: 91 percent of 63 repositories are gated. The missing nine percent is where the next finding will come from.\n\nLast failure: 2 days ago, an auth check that exited 2. A failure with an exit code is a fact; a failure without one is a rumour. The second kind is the one that wakes you up.\n\n> **The honest caveat:** coverage counts repositories with a job, not repositories with a job that could fail. Check one gate by breaking it.\n\n> **The lesson:** six numbers are questions, not a score.\n\n## In one line\n\n> **Six numbers, one question.** A dashboard opens the review; it does not close it.\n\n**Read one tile**\n\n```bash\ngrep -c 'exit 2' cron.log\n```\n"},{"id":"p568","n":"568","slug":"python-security-scripting-terms","title":"Python security scripting terms","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","SecurityAutomation","CICD","SRE"],"series_name":"PythonSec","design":"reveal","script":"","date":"2030-05-14","files":{"devsecops":"notes/p568-python-security-scripting-terms.md"},"md":"# Python security scripting terms\n\nA script review can stall on a single word: someone says idempotent and two people in the room mean different things. Six terms cover most of the argument.\n\n## Quick reference\n\n- **idempotent** — Running it twice leaves the same state as running it once, so a retry is safe.\n- **exit code** — 0 clean, 1 finding, 2 bad input: the number a pipeline reads instead of prose.\n- **read-only** — It inspects and reports; it never writes the repo, the host or the network.\n- **parser** — Code that turns text into fields you can test, instead of regex guesses on raw output.\n- **fixture** — A small input with a known answer, kept beside the test so tomorrow runs the same.\n- **gate** — A check whose exit code blocks a merge or a deploy, not a report nobody opens.\n\n## Explained\n\n### The terms\n\nIdempotent: running it twice leaves the same state as running it once, so a retry is safe. If a second run creates a second ticket, the script is not idempotent.\n\nExit code: 0 clean, 1 finding, 2 bad input. The number is the interface a pipeline reads; the prose is for the human who arrives later.\n\nRead-only: it inspects and reports, and never writes the repo, the host or the network. A read-only script can run against production without a change window.\n\nParser: code that turns text into fields you can test, instead of regex guesses on raw output. A parser makes the check explainable when it fires.\n\nFixture: a small input file with a known answer, kept beside the test. Without one, a change to the script has nothing to disagree with.\n\nGate: a check whose exit code blocks a merge or a deploy, not a report nobody opens. A gate is a control; a report is a hope.\n\n### The contract\n\n> **The honest caveat:** a script can be idempotent, read-only and gated and still check the wrong thing. The terms describe the contract, not the correctness.\n\n> **The lesson:** a script is a contract of input, output and exit code.\n\n## In one line\n\n> **A script is a contract.** Input, output, exit code: the three things a reviewer checks first.\n\n**Read the docstring**\n\n```bash\npython3 script.py --help\n```\n"},{"id":"p569","n":"569","slug":"a-docker-cis-check-in-python","title":"A Docker CIS check in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Docker","Security","Python","DevSecOps","CIS"],"series_name":"PythonSec","design":"code","script":"docker_cis.py","date":"2030-05-16","files":{"devsecops":"notes/p569-a-docker-cis-check-in-python.md"},"md":"# A Docker CIS check in Python\n\ndocker info is a page of JSON and most of it is inventory: containers, images, storage. Five fields decide whether the daemon itself is set up the way the benchmark expects.\n\n## Quick reference\n\nThe full script is at the end of this note: `docker_cis.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 docker_cis.py docker-info.json\n```\n\n**Example output**\n\n```text\nlive=OK proxy=WEAK ulimit=WEAK logging=OK userns=WEAK\nWEAK 3 of 5\n```\n\n## Explained\n\n### The five checks\n\nLive restore: if the daemon restarts, containers should keep running. LiveRestoreEnabled must be true.\n\nUserland proxy: the userland proxy is extra surface and a known bottleneck. UserlandProxy must be false.\n\nDefault ulimits: an unset default lets one container exhaust file descriptors for the host. DefaultUlimits must carry a value.\n\nLogging driver: json-file keeps logs on the local disk and loses them with the container. The check rejects it, though it cannot tell a central driver from another local one.\n\nUser namespaces: with userns enabled, a root process inside a container is not root on the host. SecurityOptions must list userns. A missing key is treated as its default, so an older daemon cannot pass by accident.\n\n### The output\n\nThe run prints one status line and a count: live=OK proxy=WEAK ulimit=WEAK logging=OK userns=WEAK, then WEAK 3 of 5. It exits 1 if any setting is weak, so a host check can block the node before it joins the pool.\n\n> **The honest caveat:** docker info reports the daemon that is running, not the `daemon.json` you wrote. A setting can be correct in the file and absent at runtime; compare both.\n\n> **The lesson:** check the daemon that is running, not the file you wrote.\n\n## In one line\n\n> **Five settings, one daemon.** The daemon that is running is the one that matters.\n\n**Dump the info**\n\n```bash\ndocker info --format '{{json .}}'\n```\n\n## Script: docker_cis.py\n\nDownload: [scripts/docker_cis.py](../scripts/docker_cis.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"docker_cis.py - check a Docker daemon info dump against five CIS settings.\n\nReads the JSON printed by `docker info --format '{{json .}}'` and checks five\ndaemon settings that the CIS Docker Benchmark calls out. Every setting is\nreported as OK or WEAK, and the script exits 1 if any is weak.\n\n    live      LiveRestoreEnabled is true          live restore\n    proxy     UserlandProxy is false             userland proxy\n    ulimit    DefaultUlimits is set              default ulimit\n    logging   LoggingDriver is not json-file     centralised logging\n    userns    SecurityOptions includes userns    user namespace support\n\nThe check names follow the CIS Docker Benchmark. Its section numbers move\nbetween editions, so they are not quoted here.\n\nUsage:\n    docker info --format '{{json .}}' > /tmp/docker-info.json\n    python3 docker_cis.py /tmp/docker-info.json\n    docker info --format '{{json .}}' | python3 docker_cis.py -\n    python3 docker_cis.py --help\n\nCI (check the host before it joins the pool):\n    python3 docker_cis.py /tmp/docker-info.json || exit 1\n\nCron (every Monday at 06:00):\n    0 6 * * 1 docker info --format '{{json .}}' | python3 /opt/s0sta/docker_cis.py - >> /var/log/docker-cis.log\n\nExit codes: 0 all five settings are strong, 1 at least one is weak,\n2 usage or unreadable/invalid input.\n\nRead-only: the dump is read; the daemon is not contacted, reconfigured or\nrestarted. A missing key is treated as its default, which is weak here.\n\nThis is a configuration smoke test, not a full CIS benchmark. It does not\nread daemon.json, file permissions, audit rules or kernel settings that the\nbenchmark also requires.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef load(path):\n    \"\"\"Read one JSON document; raise ValueError on any input problem.\"\"\"\n    try:\n        if path == \"-\":\n            return json.load(sys.stdin)\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    except (json.JSONDecodeError, UnicodeError) as exc:\n        raise ValueError(\"%s: not valid JSON: %s\" % (path, exc))\n\n\ndef checks(info):\n    \"\"\"Return (name, is_weak) for the five settings, in report order.\"\"\"\n    raw = info.get(\"SecurityOptions\")\n    if isinstance(raw, list):\n        security = \" \".join(str(item) for item in raw)\n    else:\n        security = str(raw or \"\")\n    ulimits = info.get(\"DefaultUlimits\") or []\n    logging_driver = info.get(\"LoggingDriver\") or \"json-file\"\n    return [\n        (\"live\", info.get(\"LiveRestoreEnabled\") is not True),\n        (\"proxy\", info.get(\"UserlandProxy\") is not False),\n        (\"ulimit\", not ulimits),\n        (\"logging\", logging_driver == \"json-file\"),\n        (\"userns\", \"userns\" not in security),\n    ]\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check docker info JSON against five CIS-ish settings.\")\n    parser.add_argument(\"info\", nargs=\"?\", default=\"-\",\n                        help=\"docker info JSON file; stdin when omitted\")\n    args = parser.parse_args(argv)\n    try:\n        info = load(args.info)\n        if not isinstance(info, dict):\n            raise ValueError(\"%s: expected a JSON object\" % args.info)\n        results = checks(info)\n    except ValueError as exc:\n        print(\"docker_cis: %s\" % exc, file=sys.stderr)\n        return 2\n    print(\" \".join(name + (\"=WEAK\" if weak else \"=OK\")\n                   for name, weak in results))\n    weak = sum(1 for _, bad in results if bad)\n    print(\"WEAK %d of %d\" % (weak, len(results)))\n    return 1 if weak else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p570","n":"570","slug":"an-image-pin-report-in-python","title":"An image pin report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","Containers","Python","DevSecOps","SupplyChain"],"series_name":"PythonSec","design":"code","script":"image_pin_report.py","date":"2030-05-19","files":{"devsecops":"notes/p570-an-image-pin-report-in-python.md"},"md":"# An image pin report in Python\n\n`kubectl get pods -A -o json` returns every image in the cluster, and somewhere in that list is a latest tag that was meant to be temporary. The dump is the inventory; the check is a short parse.\n\n## Quick reference\n\nThe full script is at the end of this note: `image_pin_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 image_pin_report.py pods.json\n```\n\n**Example output**\n\n```text\nUNPINNED shop nginx:latest\nNS {'shop': 1} TOTAL 1\n```\n\n## Explained\n\n### What it does\n\nThe script walks every pod and every container. It extracts the tag from the last path segment, so registry ports do not confuse it. An image with a digest is pinned. An image with a tag other than latest is pinned. An image with latest, or with no tag at all, is unpinned and printed with its namespace. The last line counts findings per namespace.\n\n### The run\n\nThe output above names one image in shop and reports NS {'shop': 1} TOTAL 1. Exit 0 means every image is pinned, 1 means at least one is not, 2 means the JSON or the shape was wrong.\n\n### Why it matters\n\nlatest is a moving reference: the pod that starts tomorrow can run different bytes from the pod you reviewed. No tag means latest by default, which is the same problem with less typing. A digest names the bytes, and a version tag names a promise that the registry can still overwrite.\n\n> **The honest caveat:** a pinned tag can be moved, so this checks the reference, not the bytes. Pair it with a digest policy if you need the image itself to be immutable.\n\n> **The lesson:** pin the reference, then verify what it resolves to.\n\n## In one line\n\n> **A tag moves; a digest does not.** Pin the reference, then check what it resolves to.\n\n**Pin by digest**\n\n```bash\nkubectl get pods -A -o json\n```\n\n## Script: image_pin_report.py\n\nDownload: [scripts/image_pin_report.py](../scripts/image_pin_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"image_pin_report.py - report container images without a tag or digest.\n\nReads `kubectl get pods -A -o json`, walks every container, and prints each\nimage that is unpinned: `:latest` or no tag at all. An image with a digest\n(`@sha256:...`) is pinned, and so is an image with any other tag. It prints\none line per unpinned image, a per-namespace count, and exits 1 if it found\nany.\n\nUsage:\n    kubectl get pods -A -o json > /tmp/pods.json\n    python3 image_pin_report.py /tmp/pods.json\n    kubectl get pods -A -o json | python3 image_pin_report.py -\n    python3 image_pin_report.py --help\n\nCI (block a manifest that reintroduces latest):\n    python3 image_pin_report.py /tmp/pods.json || exit 1\n\nCron (weekly drift report):\n    0 8 * * 1 kubectl get pods -A -o json | python3 /opt/s0sta/image_pin_report.py - >> /var/log/image-pins.log\n\nExit codes: 0 every image is pinned, 1 at least one is unpinned,\n2 usage or unreadable/invalid input.\n\nRead-only: reads a JSON dump; it does not call the API server, pull an image\nor change a workload. A tag can be moved later, so this checks the reference,\nnot the bytes that run.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef load(path):\n    \"\"\"Read a kubectl JSON document and return its items list.\"\"\"\n    try:\n        if path == \"-\":\n            document = json.load(sys.stdin)\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                document = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    except (json.JSONDecodeError, UnicodeError) as exc:\n        raise ValueError(\"%s: not valid JSON: %s\" % (path, exc))\n    if not isinstance(document, dict):\n        raise ValueError(\"%s: expected a JSON object\" % path)\n    if not isinstance(document.get(\"items\"), list):\n        raise ValueError(\"%s: expected an items array\" % path)\n    return document[\"items\"]\n\n\ndef tag_of(image):\n    \"\"\"Return the tag of a reference, or an empty string when it has none.\"\"\"\n    if \"@\" in image:\n        return \"\"\n    last = image.rsplit(\"/\", 1)[-1]\n    if \":\" in last:\n        return last.rsplit(\":\", 1)[1]\n    return \"\"\n\n\ndef unpinned(image):\n    \"\"\"True when the reference is latest or has no tag at all.\"\"\"\n    if \"@\" in image:\n        return False\n    return tag_of(image) in (\"\", \"latest\")\n\n\ndef containers(pod):\n    \"\"\"Yield every container image in one pod, including init containers.\"\"\"\n    spec = pod.get(\"spec\")\n    if not isinstance(spec, dict):\n        raise ValueError(\"pod has no spec object\")\n    for field in (\"containers\", \"initContainers\"):\n        entries = spec.get(field) or []\n        if not isinstance(entries, list):\n            raise ValueError(\"pod %s is not a list\" % field)\n        for container in entries:\n            if not isinstance(container, dict):\n                raise ValueError(\"container entry is not an object\")\n            image = container.get(\"image\")\n            if isinstance(image, str) and image:\n                yield image\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report Kubernetes images that use latest or no tag.\")\n    parser.add_argument(\"pods\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl JSON file; stdin when omitted\")\n    args = parser.parse_args(argv)\n    try:\n        pods = load(args.pods)\n    except ValueError as exc:\n        print(\"image_pin_report: %s\" % exc, file=sys.stderr)\n        return 2\n    counts = {}\n    total = 0\n    try:\n        for pod in pods:\n            if not isinstance(pod, dict):\n                raise ValueError(\"pod entry is not an object\")\n            metadata = pod.get(\"metadata\")\n            if not isinstance(metadata, dict):\n                raise ValueError(\"pod has no metadata object\")\n            namespace = metadata.get(\"namespace\") or \"default\"\n            for image in containers(pod):\n                if unpinned(image):\n                    total += 1\n                    counts[namespace] = counts.get(namespace, 0) + 1\n                    print(\"UNPINNED %s %s\" % (namespace, image))\n    except ValueError as exc:\n        print(\"image_pin_report: %s\" % exc, file=sys.stderr)\n        return 2\n    summary = {name: counts[name] for name in sorted(counts)}\n    print(\"NS %s TOTAL %d\" % (summary, total))\n    return 1 if total else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p571","n":"571","slug":"quiz-3-reporting-questions","title":"Quiz: 3 reporting questions","sections":["devsecops"],"topic":"DevSecOps & cloud security","tags":["Python","DevSecOps","SecurityReporting","Automation","CICD"],"series_name":"PythonSec","design":"quiz","script":"","date":"2030-05-21","files":{"devsecops":"notes/p571-quiz-3-reporting-questions.md"},"md":"# Quiz: 3 reporting questions\n\nThree questions close the series, and each one is a decision you make when you publish a report.\n\n## Quick reference\n\n**Q1. Why does a report need an exit code?**\n\n- a) So the pipeline can fail on a finding\n- b) So the report can be signed\n- c) So the log stays short\n\n> **Answer:** a) So the pipeline can fail on a finding. The exit code is the one machine-readable verdict. 0, 1 and 2 let CI gate without parsing prose, and the number survives every log format.\n\n**Q2. Why is a false positive a product bug?**\n\n- a) It spends the reviewer's trust\n- b) It proves the scanner is broken\n- c) It lowers the average severity\n\n> **Answer:** a) It spends the reviewer's trust. Every false positive costs attention and teaches people to ignore the next alert. The product is a decision, and a wrong one is a defect, not a rounding error.\n\n**Q3. What makes a finding reproducible?**\n\n- a) A command, a version and a fixture\n- b) A screenshot and a summary\n- c) A severity and an owner\n\n> **Answer:** a) A command, a version and a fixture. Someone else can run the same command on the same version with the same input and get the same result. Without that, it is an anecdote, not a finding.\n\n## Explained\n\n### Question one\n\nWhy does a report need an exit code? Because the exit code is the one machine-readable verdict. 0, 1 and 2 let CI gate without parsing prose, and the number survives every log format and every reader.\n\n### Question two\n\nWhy is a false positive a product bug? Because every false positive spends a reviewer's attention and teaches people to ignore the next alert. The scanner's product is a decision, and a wrong one is a defect, not a rounding error. A report that cries wolf once is a report people stop reading.\n\n### Question three\n\nWhat makes a finding reproducible? A command, a version and a fixture. Someone else can run the same command on the same version with the same input and get the same result. Without that, it is an anecdote, not a finding.\n\n> **The honest caveat:** passing this quiz does not make a report true. The evidence does, and the evidence is the part you have to go and collect.\n\n> **The lesson:** a report is finished when someone can act on it.\n\nThat is the series. Fifty posts, from the first sshd check to a report a pipeline can read, five phases and twenty Python scripts. Thank you for following along.\n\n## In one line\n\n> **Report it, then gate it.** Fifty posts end where the work starts: a report someone can act on.\n\n**Start with one**\n\n```bash\npython3 log_redact.py app.log\n```\n"},{"id":"p572","n":"572","slug":"an-inode-exhaustion-forecaster","title":"An inode exhaustion forecaster","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SysAdmin","SRE","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"inode_watch.py","date":"2030-05-23","files":{"it":"notes/p572-an-inode-exhaustion-forecaster.md","devsecops":"notes/p572-an-inode-exhaustion-forecaster.md"},"md":"# An inode exhaustion forecaster\n\nAt 03:10 a deploy host stops writing logs. `df -h` reports 41% free, every write returns ENOSPC, and the pager insists the disk is healthy. The bytes are there; the inodes are not.\n\n## Quick reference\n\nThe full script is at the end of this note: `inode_watch.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 inode_watch.py /tmp/df_i.txt /tmp/inode_history.json\n```\n\n**Example output**\n\n```text\nFINDING 2 filesystems to watch\nALERT /var 96% 5d left, /srv/data 92% 29d left\n```\n\n## Explained\n\n### What it does\n\n`inode_watch.py` reads `df -i` output, compares each filesystem's IUse% with `--threshold (80` by default), and projects the fill from a growth history JSON: mount point to [epoch, used] samples. A filesystem under the threshold is still flagged when the projection lands inside `--horizon` days.\n\n### What the screen shows\n\nThe fourteen lines read the df path and the history, filter rows at or above 80%, then print FINDING 2 filesystems to watch and ALERT `/var 96% 5d` left, `/srv/data 92% 29d` left.\n\n### How to run it\n\n`df -iP | python3 inode_watch.py` - `/var/lib/s0sta/inode_history.json`, or a captured file. Exit 0 is clean, 1 is a finding, 2 is bad input. It writes nothing, so cron and CI can run it.\n\n### The fix\n\nOn `/var`, find what creates files and never removes them: PHP sessions, mail queues, container layers. `du --inodes -x --max-depth=1 /var | sort -n |` tail -5 names the directory that grew. Clean the producer, not the files.\n\n### Prevention\n\nSample hourly and alert at 80%, not 95%. ext4 defaults to one inode per 16 KB and each inode costs 256 bytes, so a volume sized for large files can still run out of names.\n\n> **The honest caveat:** two samples make a line, not a trend.\n\n> **The lesson:** `df -h` measures bytes, and `df -i` measures the things that hold them.\n\n## In one line\n\n> **Free space is not free inodes.** `df -h` counts bytes. The filesystem also runs out of names.\n\n**Project the fill**\n\n```bash\ndf -iP | python3 inode_watch.py\n```\n\n## Script: inode_watch.py\n\nDownload: [scripts/inode_watch.py](../scripts/inode_watch.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"inode_watch.py - forecast inode exhaustion from df -i output.\n\nReads the text of `df -i` (a file or stdin), flags every filesystem whose\nIUse% is at or above a threshold, and projects when it fills from a growth\nhistory JSON. A filesystem below the threshold is still reported when the\nprojection says it fills inside --horizon days.\n\nThe history file is a JSON object that maps a mount point to a list of\n[epoch_seconds, used_inodes] samples, oldest first:\n\n    {\"/var\": [[1761000000, 1258291], [1761028800, 1267000]]}\n\nWrite it from cron or a metrics agent; this script only reads it. The\nprojection is the average growth between the first and the last sample,\nso a longer history gives a steadier slope than the last two points.\n\nUsage:\n    df -iP | python3 inode_watch.py\n    python3 inode_watch.py /tmp/df_i.txt\n    python3 inode_watch.py /tmp/df_i.txt /tmp/inode_history.json\n    python3 inode_watch.py /tmp/df_i.txt /tmp/inode_history.json --threshold 90\n    python3 inode_watch.py /tmp/df_i.txt /tmp/inode_history.json --horizon 7\n    python3 inode_watch.py --help\n\nCI (block a deploy onto a host near the inode limit):\n    python3 inode_watch.py /var/lib/s0sta/df_i.txt /var/lib/s0sta/history.json || exit 1\n\nCron (hourly capture, one report per run):\n    5 * * * * df -iP > /var/lib/s0sta/df_i.txt && python3 /opt/s0sta/inode_watch.py /var/lib/s0sta/df_i.txt /var/lib/s0sta/history.json\n\nExit codes: 0 no finding, 1 a filesystem is at or above the threshold or\nfills inside --horizon days, 2 usage error or unreadable, empty or\nunparseable input.\n\nRead-only: opens the given files, parses text and prints. It writes\nnothing, changes nothing and makes no network call.\n\nMount points that contain a space are printed by df as `\\\\040`; the report\nkeeps the escaped name. Rows that report `-` for inodes (NFS, some\noverlay mounts) are skipped with a note on stderr.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nDEFAULT_THRESHOLD = 80\nDEFAULT_HORIZON = 14.0\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef read_text(source):\n    \"\"\"Return the whole input as text, or raise InputError.\"\"\"\n    try:\n        if source == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (source, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % source)\n    return text\n\n\ndef parse_df(text):\n    \"\"\"Return a list of row dicts from df -i output, or raise InputError.\"\"\"\n    rows, pending, skipped = [], None, 0\n    for number, raw in enumerate(text.splitlines(), start=1):\n        line = raw.strip()\n        if not line:\n            continue\n        fields = line.split()\n        if fields[0].lower() == \"filesystem\":\n            continue\n        # A long device name is printed on its own line, then the row.\n        if len(fields) == 1 and fields[0].startswith(\"/\"):\n            pending = fields[0]\n            continue\n        if pending and len(fields) == 5 and fields[0].isdigit():\n            fields = [pending] + fields\n            pending = None\n        elif pending:\n            pending = None\n        if len(fields) < 6:\n            raise InputError(\n                \"line %d is not a df -i row: %s\" % (number, line))\n        device, total, used, free, pct = fields[0], fields[1], fields[2], fields[3], fields[4]\n        mount = \" \".join(fields[5:])\n        if pct == \"-\" or not (total.isdigit() and used.isdigit() and free.isdigit()):\n            skipped += 1\n            continue\n        percent = int(pct.rstrip(\"%\"))\n        if int(total) <= 0:\n            skipped += 1\n            continue\n        rows.append({\n            \"device\": device,\n            \"mount\": mount,\n            \"total\": int(total),\n            \"used\": int(used),\n            \"free\": int(free),\n            \"percent\": min(100, percent),\n        })\n    if not rows:\n        raise InputError(\"no df -i row with an inode count\")\n    return rows, skipped\n\n\ndef load_history(path):\n    \"\"\"Read and validate the growth history JSON, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if not isinstance(doc, dict):\n        raise InputError(\"%s must be a JSON object of mount -> samples\" % path)\n    history = {}\n    for mount, samples in doc.items():\n        if not isinstance(samples, list):\n            raise InputError(\"%s: %s must be a list of samples\" % (path, mount))\n        pairs = []\n        for item in samples:\n            if not isinstance(item, (list, tuple)) or len(item) != 2:\n                raise InputError(\n                    \"%s: %s needs [epoch, used] pairs\" % (path, mount))\n            stamp, used = item\n            if isinstance(stamp, bool) or not isinstance(stamp, (int, float)):\n                raise InputError(\n                    \"%s: %s has a non-numeric epoch\" % (path, mount))\n            if isinstance(used, bool) or not isinstance(used, (int, float)):\n                raise InputError(\n                    \"%s: %s has a non-numeric inode count\" % (path, mount))\n            pairs.append((float(stamp), float(used)))\n        pairs.sort()\n        history[mount] = pairs\n    return history\n\n\ndef project(samples, free):\n    \"\"\"Return (inodes_per_hour, days_left) or None when there is no slope.\"\"\"\n    if len(samples) < 2:\n        return None\n    (t0, u0), (t1, u1) = samples[0], samples[-1]\n    hours = (t1 - t0) / 3600.0\n    if hours <= 0 or u1 <= u0:\n        return None\n    rate = (u1 - u0) / hours\n    return rate, free / rate / 24.0\n\n\ndef eta(forecast):\n    \"\"\"Return the short projection used in the one-line alert.\"\"\"\n    return \"no data\" if forecast is None else \"%dd left\" % forecast[1]\n\n\ndef report(rows, history, threshold, horizon):\n    \"\"\"Return (finding, lines) for every filesystem worth acting on.\"\"\"\n    hits = []\n    for row in rows:\n        forecast = project(history.get(row[\"mount\"]) or [], row[\"free\"])\n        days = None if forecast is None else forecast[1]\n        urgent = days is not None and days <= horizon\n        if row[\"percent\"] < threshold and not urgent:\n            continue\n        hits.append((row, forecast))\n    if not hits:\n        return False, [\"OK    no filesystem at or above %d%% inodes, \"\n                       \"nothing fills inside %gd\" % (threshold, horizon)]\n    lines = []\n    for row, forecast in hits:\n        if forecast is None:\n            detail = \"no growth history\"\n        else:\n            detail = \"fills in %dd at %d/h\" % (forecast[1], forecast[0])\n        lines.append(\"WARN  %-22s %3d%% used, %8d free, %s\"\n                     % (row[\"mount\"], row[\"percent\"], row[\"free\"], detail))\n    lines.append(\"FINDING %d filesystem%s to watch\"\n                 % (len(hits), \"\" if len(hits) == 1 else \"s\"))\n    lines.append(\"ALERT \" + \", \".join(\n        \"%s %d%% %s\" % (row[\"mount\"], row[\"percent\"], eta(forecast))\n        for row, forecast in hits))\n    return True, lines\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Forecast inode exhaustion from df -i output (read-only).\")\n    parser.add_argument(\n        \"df\", nargs=\"?\", default=\"-\",\n        help=\"df -i output to read, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"history\", nargs=\"?\", default=None,\n        help=\"growth history JSON: mount -> [[epoch, used], ...]\")\n    parser.add_argument(\n        \"--threshold\", type=int, default=DEFAULT_THRESHOLD, metavar=\"PCT\",\n        help=\"flag a filesystem at or above this inode use level, in percent (default: %d)\"\n             % DEFAULT_THRESHOLD)\n    parser.add_argument(\n        \"--horizon\", type=float, default=DEFAULT_HORIZON, metavar=\"DAYS\",\n        help=\"also flag a fill projected inside this many days (default: %g)\"\n             % DEFAULT_HORIZON)\n    args = parser.parse_args(argv)\n\n    if not 1 <= args.threshold <= 100:\n        parser.error(\"--threshold must be between 1 and 100\")\n    if args.horizon < 0:\n        parser.error(\"--horizon must not be negative\")\n\n    try:\n        rows, skipped = parse_df(read_text(args.df))\n        history = load_history(args.history) if args.history else {}\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if skipped:\n        print(\"NOTE %d row(s) skipped: no inode count\" % skipped, file=sys.stderr)\n    if not history and args.history is None:\n        print(\"NOTE no history file: projection unavailable\", file=sys.stderr)\n\n    finding, lines = report(rows, history, args.threshold, args.horizon)\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p573","n":"573","slug":"an-oom-kill-report","title":"An OOM kill report","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SRE","Kernel","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"oom_report.py","date":"2030-05-26","files":{"it":"notes/p573-an-oom-kill-report.md","devsecops":"notes/p573-an-oom-kill-report.md"},"md":"# An OOM kill report\n\nAt 03:04 the API goes quiet and comes back at 03:06 with no deploy and no traceback. The kernel killed the process, and the only witness is a few hundred lines of dmesg that nobody reads until morning.\n\n## Quick reference\n\nThe full script is at the end of this note: `oom_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 oom_report.py /tmp/dmesg.txt\n```\n\n**Example output**\n\n```text\nFINDING 3 OOM kills\nALERT python3 pid 4242, 3082MB, adj 0\n```\n\n## Explained\n\n### What it does\n\n`oom_report.py` reads dmesg or journal text, finds every Out of memory: Killed process line and pulls the pid, the process name, anon-rss and oom_score_adj out of it. Older kernels print score N on the line before, so that score is attached to the same pid. Victims are ranked by resident memory.\n\n### What the screen shows\n\nThe fourteen lines compile one regex, loop over the file, convert kB to MB, sort by RSS, print the count and name the worst victim. On the dirty fixture: FINDING 3 OOM kills and ALERT python3 pid 4242, 3082MB, adj 0.\n\n### How to run it\n\n`journalctl -k --since 03:00 | python3 oom_report.py` - `--since 1h`, or point it at a saved dmesg. Exit 0 is no kill, 1 is at least one, 2 is unreadable input.\n\n### The fix\n\nThe victim is rarely the problem. Check its resident memory a day earlier, then cap the unit with MemoryMax=2G, so the cgroup kills one service, not the whole box.\n\n### Prevention\n\nAlert on the oom_kill counter, not on the restart. A service killed twice in a week is a leak with a schedule.\n\n> **The honest caveat:** modern kernels log oom_score_adj, the adjustment, not the oom_score they computed.\n\n> **The lesson:** the kernel does not kill the biggest process, it kills the one with the worst score.\n\n## In one line\n\n> **The kernel kept the receipts.** dmesg remembers the victim, the pid and the score.\n\n**Read the kills**\n\n```bash\njournalctl -k | grep -i oom\n```\n\n## Script: oom_report.py\n\nDownload: [scripts/oom_report.py](../scripts/oom_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"oom_report.py - rank the victims of OOM kills from dmesg or journal text.\n\nReads kernel log text (a file or stdin), finds every\n`Out of memory: Killed process` line, and ranks the victims by resident\nmemory, with the kernel's oom_score_adj and, when the log has it, the\n`score N` that older kernels print.\n\n    journalctl -k --since \"03:00\" > /tmp/kernel.txt\n    python3 oom_report.py /tmp/kernel.txt\n    dmesg -T | python3 oom_report.py\n\nThe report prints a table (pid, RSS in MB, adjust, score, process, time)\nand two summary lines. Exit 1 when at least one kill is in the window.\n\n--since accepts a duration (90m, 24h, 7d) or an ISO instant\n(2026-10-21T03:00:00). Lines with no timestamp cannot be filtered: they\nare kept and counted in a note on stderr. `dmesg` without -T has only\nuptime, so use `dmesg -T` or journalctl -k when you need the window.\n\nUsage:\n    python3 oom_report.py /tmp/kernel.txt\n    python3 oom_report.py /tmp/kernel.txt --since 24h --top 3\n    journalctl -k --since \"today\" | python3 oom_report.py - --since 24h\n    python3 oom_report.py --help\n\nCI (fail a soak test that triggered the OOM killer):\n    python3 oom_report.py /var/log/kern.log --since 2h || exit 1\n\nCron (a daily kill report on the on-call channel):\n    0 7 * * * journalctl -k --since yesterday | python3 /opt/s0sta/oom_report.py - --since 24h\n\nExit codes: 0 no kill in the window, 1 at least one kill in the window,\n2 usage error or unreadable, empty input.\n\nRead-only: opens the given file, parses text and prints. It writes\nnothing, changes nothing and makes no network call.\n\nThe kernel logs oom_score_adj, not the oom_score it computed, so on a\nmodern host the adjust column is what you get and RSS decides the order.\nOlder kernels print `Kill process <pid> (... ) score N`, and that score\nis shown when the line is present.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\nfrom datetime import datetime, timedelta\n\nDEFAULT_TOP = 5\n\nKILL = re.compile(r\"Killed process (\\d+)(?: \\(([^)]*)\\))?\")\nDECISION = re.compile(r\"Kill process (\\d+) \\([^)]*\\) score (\\d+)\")\nRSS = re.compile(r\"anon-rss:(\\d+)kB\")\nADJ = re.compile(r\"oom_score_adj:(-?\\d+)\")\n\nSTAMP = (\n    # dmesg -T: [Tue Oct 21 03:12:01 2026]\n    (re.compile(r\"\\[[A-Z][a-z]{2} ([A-Z][a-z]{2} +\\d{1,2} \\d\\d:\\d\\d:\\d\\d \\d{4})\\]\"),\n     \"%b %d %H:%M:%S %Y\"),\n    # journalctl -o short-iso: 2026-10-21T03:12:01+0000\n    (re.compile(r\"(\\d{4}-\\d\\d-\\d\\dT\\d\\d:\\d\\d:\\d\\d(?:[.,]\\d+)?(?:Z|[+-]\\d\\d:?\\d\\d)?)\"),\n     \"iso\"),\n    # app log or journal short-iso with a space\n    (re.compile(r\"^(\\d{4}-\\d\\d-\\d\\d \\d\\d:\\d\\d:\\d\\d)\"), \"iso\"),\n    # journalctl short: Oct 21 03:12:01\n    (re.compile(r\"^([A-Z][a-z]{2} +\\d{1,2} \\d\\d:\\d\\d:\\d\\d)\"), \"syslog\"),\n)\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read.\"\"\"\n\n\ndef read_text(source):\n    \"\"\"Return the whole input as text, or raise InputError.\"\"\"\n    try:\n        if source == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (source, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % source)\n    return text\n\n\ndef parse_stamp(line):\n    \"\"\"Return a naive local datetime for the line, or None.\"\"\"\n    for regex, kind in STAMP:\n        found = regex.search(line)\n        if not found:\n            continue\n        token = found.group(1)\n        try:\n            if kind == \"iso\":\n                value = datetime.fromisoformat(token.replace(\",\", \".\"))\n                if value.tzinfo is not None:\n                    value = value.astimezone().replace(tzinfo=None)\n                return value\n            if kind == \"%b %d %H:%M:%S %Y\":\n                return datetime.strptime(token, kind)\n            value = datetime.strptime(token, \"%b %d %H:%M:%S\")\n            value = value.replace(year=datetime.now().year)\n            if value - datetime.now() > timedelta(days=1):\n                value = value.replace(year=value.year - 1)\n            return value\n        except ValueError:\n            return None\n    return None\n\n\ndef parse_window(text):\n    \"\"\"Return the cutoff datetime for --since, or raise InputError.\"\"\"\n    found = re.fullmatch(r\"(\\d+)([smhd])\", text.strip())\n    if found:\n        unit = {\"s\": 1, \"m\": 60, \"h\": 3600, \"d\": 86400}[found.group(2)]\n        return datetime.now() - timedelta(seconds=int(found.group(1)) * unit)\n    try:\n        value = datetime.fromisoformat(text.strip())\n    except ValueError:\n        raise InputError(\"--since needs 90m, 24h, 7d or an ISO datetime\")\n    if value.tzinfo is not None:\n        value = value.astimezone().replace(tzinfo=None)\n    return value\n\n\ndef parse(text):\n    \"\"\"Return (victims, undated, unreadable) from kernel log text.\"\"\"\n    victims, scores, undated, unreadable = [], {}, 0, 0\n    for raw in text.splitlines():\n        decision = DECISION.search(raw)\n        if decision:\n            scores[int(decision.group(1))] = int(decision.group(2))\n            continue\n        found = KILL.search(raw)\n        if not found:\n            continue\n        pid = int(found.group(1))\n        comm = (found.group(2) or \"?\").strip() or \"?\"\n        rss = RSS.search(raw)\n        adj = ADJ.search(raw)\n        if rss is None:\n            unreadable += 1\n        stamp = parse_stamp(raw)\n        if stamp is None:\n            undated += 1\n        victims.append({\n            \"pid\": pid,\n            \"comm\": comm,\n            \"rss_kb\": int(rss.group(1)) if rss else 0,\n            \"adj\": int(adj.group(1)) if adj else None,\n            \"score\": scores.get(pid),\n            \"stamp\": stamp,\n        })\n    return victims, undated, unreadable\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank the victims of OOM kills from kernel log text \"\n                    \"(read-only).\")\n    parser.add_argument(\n        \"source\", nargs=\"?\", default=\"-\",\n        help=\"dmesg or journal text to read, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"--since\", default=None, metavar=\"WINDOW\",\n        help=\"only kills inside this window: 90m, 24h, 7d or an ISO instant\")\n    parser.add_argument(\n        \"--top\", type=int, default=DEFAULT_TOP, metavar=\"N\",\n        help=\"rows to print, largest resident memory first (default: %d)\"\n             % DEFAULT_TOP)\n    args = parser.parse_args(argv)\n\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n\n    try:\n        text = read_text(args.source)\n        cutoff = parse_window(args.since) if args.since else None\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    victims, undated, unreadable = parse(text)\n    if cutoff is not None:\n        victims = [v for v in victims\n                   if v[\"stamp\"] is None or v[\"stamp\"] >= cutoff]\n    if undated and cutoff is not None:\n        print(\"NOTE %d kill(s) have no timestamp and were kept\" % undated,\n              file=sys.stderr)\n    if unreadable:\n        print(\"NOTE %d kill(s) have no anon-rss value\" % unreadable,\n              file=sys.stderr)\n\n    if not victims:\n        where = \" in the window\" if cutoff is not None else \"\"\n        print(\"OK    no OOM kill%s\" % where)\n        return 0\n\n    victims.sort(key=lambda v: (-v[\"rss_kb\"], -(v[\"score\"] or -1)))\n    print(\"%6s %7s %5s %6s  %-16s %s\"\n          % (\"PID\", \"RSS_MB\", \"ADJ\", \"SCORE\", \"PROCESS\", \"TIME\"))\n    for victim in victims[:args.top]:\n        print(\"%6d %7d %5s %6s  %-16s %s\"\n              % (victim[\"pid\"],\n                 victim[\"rss_kb\"] // 1024,\n                 \"-\" if victim[\"adj\"] is None else victim[\"adj\"],\n                 \"-\" if victim[\"score\"] is None else victim[\"score\"],\n                 victim[\"comm\"][:16],\n                 victim[\"stamp\"].strftime(\"%H:%M:%S\") if victim[\"stamp\"] else \"-\"))\n    worst = victims[0]\n    where = \" in the window\" if cutoff is not None else \"\"\n    print(\"FINDING %d OOM kill%s%s\"\n          % (len(victims), \"\" if len(victims) == 1 else \"s\", where))\n    print(\"ALERT %s pid %d, %dMB, adj %s\"\n          % (worst[\"comm\"], worst[\"pid\"], worst[\"rss_kb\"] // 1024,\n             \"-\" if worst[\"adj\"] is None else worst[\"adj\"]))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p574","n":"574","slug":"a-zombie-process-scan","title":"A zombie process scan","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SysAdmin","Processes","SRE"],"series_name":"PythonFixes","design":"code","script":"zombie_scan.py","date":"2030-05-28","files":{"it":"notes/p574-a-zombie-process-scan.md","devsecops":"notes/p574-a-zombie-process-scan.md"},"md":"# A zombie process scan\n\nA worker stops accepting connections at 02:50. CPU is idle, memory is fine, and ps shows four processes in state Z. Nobody is using the box; four children are waiting to be buried by a parent that never asked.\n\n## Quick reference\n\nThe full script is at the end of this note: `zombie_scan.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 zombie_scan.py /tmp/ps.txt\n```\n\n**Example output**\n\n```text\nFINDING 4 zombies in 2 parents\nALERT parent 412 holds 3 zombies\n```\n\n## Explained\n\n### What it does\n\n`zombie_scan.py` reads `ps -eo` pid,ppid,stat,comm, keeps every row whose state starts with Z, and groups the dead children by parent. A parent at or above `--max-per-parent`, three by default, gets a PARENT line and the nudge to reap.\n\n### What the screen shows\n\nThe fourteen lines split the ps output, filter on the Z state, count by ppid, print one line per zombie and one per parent, then the summary. On the dirty fixture: FINDING 4 zombies in 2 parents and ALERT parent 412 holds 3 zombies.\n\n### How to run it\n\n`python3 zombie_scan.py /tmp/ps.txt`, or pipe ps in. Exit 0 is clean, 1 is a finding, 2 is bad input.\n\n### The fix\n\nThe parent is the bug. It forked and never called wait(), so each finished child keeps its pid. Restart the parent to clear the backlog, then fix the loop: waitpid() after each fork, a SIGCHLD handler, or a double fork.\n\n### Prevention\n\nWatch the Z count per parent over a day. A parent that holds one zombie for an hour will hold a thousand by Friday.\n\n> **The honest caveat:** a zombie under pid 1 is usually transient, because init reaps it on the next pass.\n\n> **The lesson:** a zombie is not a stuck process, it is a parent that never finished the conversation.\n\n## In one line\n\n> **A zombie is a bug in its parent.** The child is dead. The parent is the one still holding it.\n\n**Count the dead**\n\n```bash\nps -eo stat | grep -c Z\n```\n\n## Script: zombie_scan.py\n\nDownload: [scripts/zombie_scan.py](../scripts/zombie_scan.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"zombie_scan.py - find zombie processes and the parents that keep them.\n\nReads the output of `ps -eo pid,ppid,stat,comm` (a file or stdin), finds\nevery process whose state starts with Z, groups the dead children by\nparent, and reports the parents that hold the most.\n\n    ps -eo pid,ppid,stat,comm > /tmp/ps.txt\n    python3 zombie_scan.py /tmp/ps.txt\n    ps -eo pid,ppid,stat,comm | python3 zombie_scan.py\n\nA parent at or above --max-per-parent (default 3) is printed as a PARENT\nfinding with the command that nudges it to reap. Every zombie is a\nfinding, so the exit code is 1 whenever one is present.\n\nUsage:\n    python3 zombie_scan.py /tmp/ps.txt\n    python3 zombie_scan.py /tmp/ps.txt --max-per-parent 1\n    ps -eo pid,ppid,stat,comm | python3 zombie_scan.py -\n    python3 zombie_scan.py --help\n\nCI (fail a soak test that leaks child processes):\n    python3 zombie_scan.py /tmp/ps.txt || exit 1\n\nCron (a five-minute sweep on a busy worker):\n    */5 * * * * ps -eo pid,ppid,stat,comm > /tmp/ps.txt && python3 /opt/s0sta/zombie_scan.py /tmp/ps.txt\n\nExit codes: 0 no zombie process, 1 at least one zombie, 2 usage error or\nunreadable, empty or row-free input.\n\nRead-only: opens the given file, parses text and prints a suggested\ncommand. It never signals a process, writes nothing and makes no network\ncall.\n\nA zombie holds a pid and a process table slot, not memory. Only its\nparent can reap it, so the parent is the finding. A zombie whose parent\nis pid 1 is normally transient: init reaps it on the next pass.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nfrom collections import Counter, defaultdict\n\nDEFAULT_MAX = 3\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read.\"\"\"\n\n\ndef read_text(source):\n    \"\"\"Return the whole input as text, or raise InputError.\"\"\"\n    try:\n        if source == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (source, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % source)\n    return text\n\n\ndef parse(text):\n    \"\"\"Return (rows, zombies) with zombies as (pid, ppid, stat, comm).\"\"\"\n    rows, zombies = {}, []\n    for number, raw in enumerate(text.splitlines(), start=1):\n        fields = raw.split()\n        if not fields or not fields[0].isdigit():\n            continue          # header, banner, or a wrapped line\n        if len(fields) < 4:\n            raise InputError(\n                \"line %d is not a ps row: %s\" % (number, raw.strip()))\n        pid, ppid, stat = int(fields[0]), int(fields[1]), fields[2]\n        comm = \" \".join(fields[3:])\n        rows[pid] = comm\n        if stat[:1].upper() == \"Z\":\n            zombies.append((pid, ppid, stat, comm))\n    if not rows:\n        raise InputError(\"no ps row found: is this ps -eo pid,ppid,stat,comm?\")\n    return rows, zombies\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find zombie processes and the parents that keep them \"\n                    \"(read-only).\")\n    parser.add_argument(\n        \"source\", nargs=\"?\", default=\"-\",\n        help=\"ps -eo pid,ppid,stat,comm output, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"--max-per-parent\", type=int, default=DEFAULT_MAX, metavar=\"N\",\n        help=\"flag a parent with at least this many zombies (default: %d)\"\n             % DEFAULT_MAX)\n    args = parser.parse_args(argv)\n\n    if args.max_per_parent < 1:\n        parser.error(\"--max-per-parent must be at least 1\")\n\n    try:\n        rows, zombies = parse(read_text(args.source))\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not zombies:\n        print(\"OK    no zombie in %d processes\" % len(rows))\n        return 0\n\n    for pid, ppid, stat, comm in zombies:\n        print(\"ZOMBIE pid %d ppid %d %s %s\" % (pid, ppid, stat, comm))\n\n    groups = defaultdict(list)\n    for pid, ppid, stat, comm in zombies:\n        groups[ppid].append(pid)\n    counts = Counter({ppid: len(pids) for ppid, pids in groups.items()})\n    flagged = 0\n    for ppid, count in sorted(counts.items(), key=lambda item: (-item[1], item[0])):\n        if count < args.max_per_parent:\n            continue\n        flagged += 1\n        name = rows.get(ppid)\n        label = \"PARENT %d (%s)\" % (ppid, name) if name else \"PARENT %d\" % ppid\n        print(\"%s holds %d zombies\" % (label, count))\n        print(\"SUGGEST kill -CHLD %d   # or restart it to clear the children\"\n              % ppid)\n\n    orphans = sum(1 for _pid, ppid, _stat, _comm in zombies if ppid == 1)\n    if orphans:\n        print(\"NOTE  %d zombie(s) under pid 1: transient unless they persist\"\n              % orphans, file=sys.stderr)\n    if not flagged:\n        print(\"NOTE  no parent at or above %d zombies\" % args.max_per_parent,\n              file=sys.stderr)\n\n    worst = min(counts, key=lambda ppid: (-counts[ppid], ppid))\n    print(\"FINDING %d zombies in %d parents\" % (len(zombies), len(counts)))\n    print(\"ALERT parent %d holds %d zombies\" % (worst, counts[worst]))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p575","n":"575","slug":"a-log-flood-detector","title":"A log flood detector","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","Logging","SRE","Observability"],"series_name":"PythonFixes","design":"code","script":"log_flood.py","date":"2030-05-30","files":{"it":"notes/p575-a-log-flood-detector.md","devsecops":"notes/p575-a-log-flood-detector.md"},"md":"# A log flood detector\n\nBetween 03:12 and 03:13 the app log gains 1,600 lines, ten times the hour before. logrotate runs at 03:14, and the evidence goes with it. The incident is now a gap in the log.\n\n## Quick reference\n\nThe full script is at the end of this note: `log_flood.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 log_flood.py /tmp/app.log\n```\n\n**Example output**\n\n```text\nFINDING 2 minutes above 200 lines/min\nALERT 03:12 900 lines, top: upstream timeout service=payments\n```\n\n## Explained\n\n### What it does\n\n`log_flood.py` reads a log file or stdin, takes the minute from each line in ISO, syslog or apache format, counts lines per minute and reports every minute above `--rate`. For each one it normalises the message, replacing numbers, hashes and addresses, then names the dominating pattern.\n\n### What the screen shows\n\nThe thirteen lines read the file, define the normaliser, count the minutes, keep the ones over 200, then take the peak and its most common pattern. On the dirty fixture: FINDING 2 minutes above 200 lines/min and ALERT 03:12 900 lines, top: upstream timeout service=payments.\n\n### How to run it\n\n`python3 log_flood.py /var/log/app.log --rate 500`, or pipe journalctl into it. Exit 0 is no flood, 1 is a minute over the rate, 2 is unreadable input.\n\n### The fix\n\nThe pattern names the code path. A retry loop needs a cap and jitter. One slow upstream needs a timeout shorter than the retry interval. Then delete the flood, not the log.\n\n### Prevention\n\nAlert on lines per minute at the source, before the disk does. journald already drops messages above its own rate limit, so a flood becomes a gap in the evidence.\n\n> **The honest caveat:** a shared file hides which stream grew. Run it per file.\n\n> **The lesson:** the minute is the symptom, and the repeated line is the cause.\n\n## In one line\n\n> **A flood hides the first error.** The minute is the symptom; the repeated line is the cause.\n\n**Find the minute**\n\n```bash\npython3 log_flood.py /var/log/app.log\n```\n\n## Script: log_flood.py\n\nDownload: [scripts/log_flood.py](../scripts/log_flood.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"log_flood.py - find the minute that flooded and the pattern behind it.\n\nReads a log file or stdin, buckets the lines per minute, and reports every\nminute whose line count is above --rate. For each of those minutes it also\nshows the dominant message pattern, with numbers, addresses and ids\nnormalised so that a thousand copies of one line collapse into one key.\n\n    python3 log_flood.py /var/log/app.log --rate 200\n    journalctl -u app --since \"03:00\" | python3 log_flood.py -\n    tail -n 200000 /var/log/nginx/access.log | python3 log_flood.py - --rate 500\n\nTimestamps are read from ISO-8601 lines, syslog/journal short lines and\napache-style `[21/Oct/2026:03:12:01` prefixes. Lines without a timestamp\nare counted and reported on stderr, never bucketed.\n\nUsage:\n    python3 log_flood.py /tmp/app.log\n    python3 log_flood.py /tmp/app.log --rate 50 --top 3\n    python3 log_flood.py /tmp/app.log --pattern-len 40\n    cat /tmp/app.log | python3 log_flood.py - --rate 120\n    python3 log_flood.py --help\n\nCI (fail a test that floods the log):\n    python3 log_flood.py /tmp/test.log --rate 500 || exit 1\n\nCron (a five-minute check on the app log):\n    */5 * * * * python3 /opt/s0sta/log_flood.py /var/log/app.log --rate 500 || exit 1\n\nExit codes: 0 no minute above the rate, 1 at least one minute above it,\n2 usage error, unreadable input, empty input or no timestamped line.\n\nRead-only: opens the given file, parses text and prints. It writes\nnothing, changes nothing and makes no network call.\n\nA rate is only meaningful for one source. If several applications write\nto the same file, run this per file or filter first: a shared rate hides\nwhich stream grew.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\nfrom collections import Counter, defaultdict\n\nDEFAULT_RATE = 200\nDEFAULT_TOP = 5\nDEFAULT_LEN = 60\n\n# A timestamp prefix that ends just before the message. Group 1 is the\n# minute key for the access-log format; ISO and syslog build it from two\n# or three groups.\nSTAMP = (\n    (re.compile(r\"^(\\d{4}-\\d\\d-\\d\\d)[T ](\\d\\d:\\d\\d):\\d\\d\"), \"iso\"),\n    (re.compile(r\"^([A-Z][a-z]{2}) +(\\d{1,2}) (\\d\\d:\\d\\d):\\d\\d\"), \"syslog\"),\n    (re.compile(r\"^\\[(\\d\\d/[A-Za-z]{3}/\\d{4}:\\d\\d:\\d\\d):\\d\\d(?: [^\\]]*)?\\]\"),\n     \"access\"),\n)\n\nUUID = re.compile(r\"\\b[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\\b\",\n                  re.IGNORECASE)\nIP = re.compile(r\"\\b\\d{1,3}(?:\\.\\d{1,3}){3}\\b\")\nHASH = re.compile(r\"\\b(?:0x)?[0-9a-f]{12,}\\b\", re.IGNORECASE)\nNUMBER = re.compile(r\"\\d+\")\nSPACE = re.compile(r\"\\s+\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read.\"\"\"\n\n\ndef read_text(source):\n    \"\"\"Return the whole input as text, or raise InputError.\"\"\"\n    try:\n        if source == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (source, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % source)\n    return text\n\n\ndef split_line(line):\n    \"\"\"Return (minute_key, message) with the key None when undated.\"\"\"\n    for regex, kind in STAMP:\n        found = regex.match(line)\n        if not found:\n            continue\n        if kind == \"iso\":\n            return \"%s %s\" % (found.group(1), found.group(2)), line[found.end():]\n        if kind == \"syslog\":\n            return (\"%s %s %s\" % (found.group(1), found.group(2), found.group(3)),\n                    line[found.end():])\n        return found.group(1), line[found.end():]\n    return None, line\n\n\ndef pattern(message, limit):\n    \"\"\"Normalise a message so that copies of one line share one key.\"\"\"\n    text = UUID.sub(\"UUID\", message)\n    text = IP.sub(\"IP\", text)\n    text = HASH.sub(\"H\", text)\n    text = NUMBER.sub(\"N\", text)\n    text = SPACE.sub(\" \", text).strip(\" :-|\")\n    return text[:limit] or \"(no message)\"\n\n\ndef clock(key):\n    \"\"\"Return the short time label for a minute key.\"\"\"\n    return key.split(\" \")[-1] if \" \" in key else key\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find the minute that flooded and the pattern behind it \"\n                    \"(read-only).\")\n    parser.add_argument(\n        \"source\", nargs=\"?\", default=\"-\",\n        help=\"log file to read, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"--rate\", type=int, default=DEFAULT_RATE, metavar=\"N\",\n        help=\"flag a minute with more than this many lines (default: %d)\"\n             % DEFAULT_RATE)\n    parser.add_argument(\n        \"--top\", type=int, default=DEFAULT_TOP, metavar=\"N\",\n        help=\"minutes to list (default: %d)\" % DEFAULT_TOP)\n    parser.add_argument(\n        \"--pattern-len\", type=int, default=DEFAULT_LEN, metavar=\"N\",\n        help=\"characters kept per pattern (default: %d)\" % DEFAULT_LEN)\n    args = parser.parse_args(argv)\n\n    if args.rate < 1:\n        parser.error(\"--rate must be at least 1\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n    if args.pattern_len < 10:\n        parser.error(\"--pattern-len must be at least 10\")\n\n    try:\n        text = read_text(args.source)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    minutes, by_minute, undated = Counter(), defaultdict(Counter), 0\n    for line in text.splitlines():\n        key, message = split_line(line)\n        if key is None:\n            undated += 1\n            continue\n        minutes[key] += 1\n        by_minute[key][pattern(message, args.pattern_len)] += 1\n\n    if not minutes:\n        print(\"ERROR no timestamped line: expected ISO, syslog or \"\n              \"[DD/Mon/YYYY:HH:MM:SS\", file=sys.stderr)\n        return 2\n    if undated:\n        print(\"NOTE %d line(s) have no timestamp and were not bucketed\"\n              % undated, file=sys.stderr)\n\n    floods = [(key, count) for key, count in minutes.items() if count > args.rate]\n    floods.sort(key=lambda item: (-item[1], item[0]))\n    if not floods:\n        peak = max(minutes.items(), key=lambda item: (item[1], item[0]))\n        print(\"OK    no minute above %d lines/min, peak %s %d\"\n              % (args.rate, clock(peak[0]), peak[1]))\n        return 0\n\n    for key, count in floods[:args.top]:\n        print(\"RATE  %s %d lines/min, top: %s\"\n              % (key, count, by_minute[key].most_common(1)[0][0]))\n\n    peak_key, peak_count = floods[0]\n    print(\"FINDING %d minutes above %d lines/min\" % (len(floods), args.rate))\n    print(\"ALERT %s %d lines, top: %s\"\n          % (clock(peak_key), peak_count, by_minute[peak_key].most_common(1)[0][0]))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p576","n":"576","slug":"a-file-descriptor-leak-finder","title":"A file descriptor leak finder","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","SRE","Performance","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"fd_leak.py","date":"2030-06-02","files":{"it":"notes/p576-a-file-descriptor-leak-finder.md","devsecops":"notes/p576-a-file-descriptor-leak-finder.md"},"md":"# A file descriptor leak finder\n\nA worker handles 200 requests a minute, then 20 after two days, then none. CPU and memory look ordinary, the network is fine, and the process is holding 1,012 descriptors against a limit of 1,024.\n\n## Quick reference\n\nThe full script is at the end of this note: `fd_leak.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json\n```\n\n**Example output**\n\n```text\nFINDING 1/4 grew by 10+ fds\nALERT pid 4300 +172 fds, 1024 in 2 min\n```\n\n## Explained\n\n### What it does\n\n`fd_leak.py` reads two JSON snapshots, each a map of pid to fd count, taken `--interval` seconds apart. It ranks the growth, flags any process that gained `--min-growth` or more, and projects when it reaches `--limit`.\n\n### What the screen shows\n\nThe fourteen lines load both snapshots, diff the pids they share, sort by growth, keep the ones over ten, then project the worst against 1024. On the dirty fixture: FINDING 1/4 grew by 10+ fds and ALERT pid 4300 +172 fds, 1024 in 2 min.\n\n### How to run it\n\nTake the samples with `ls /proc/PID/fd | wc -l` in a loop, then `python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json`. Exit 0 is no leak, 1 is a finding, 2 is bad input.\n\n### The fix\n\nFind the unclosed handle: `lsof -p 4300` lists every descriptor the process holds, and the last ones are the ones it forgot. In Python, a with block or try/finally around every open() closes the path that leaks.\n\n### Prevention\n\nAlert on fd growth per process, and set LimitNOFILE on purpose. The default soft limit is often 1024, which is small for a busy server.\n\n> **The honest caveat:** a high count is not a leak. Only growth between two samples is.\n\n> **The lesson:** count the handles, not the CPU. A leak looks like a slow machine until the limit is reached.\n\n## In one line\n\n> **Count the handles, not the CPU.** A leak looks like a slow machine until the limit is reached.\n\n**Count one pid**\n\n```bash\nls /proc/4300/fd | wc -l\n```\n\n## Script: fd_leak.py\n\nDownload: [scripts/fd_leak.py](../scripts/fd_leak.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"fd_leak.py - find the process that is leaking file descriptors.\n\nReads two snapshots of file-descriptor counts, each a JSON object that\nmaps a pid to the number of entries in /proc/<pid>/fd, and ranks the\nprocesses by growth. A process that gained --min-growth or more between\nthe samples is a finding, and the report projects when it reaches the\ndescriptor limit.\n\nTake the two samples --interval seconds apart with\nls /proc/<pid>/fd | wc -l (as root, so every pid is readable) and write\none JSON object per sample:\n\n    python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json --interval 60\n\nUsage:\n    python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json\n    python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json --min-growth 20\n    python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json --limit 65536 --interval 300\n    python3 fd_leak.py --help\n\nCI (fail a soak test that leaks descriptors):\n    python3 fd_leak.py /tmp/fd-a.json /tmp/fd-b.json || exit 1\n\nCron (compare this hour with the one before):\n    5 * * * * python3 /opt/s0sta/fd_leak.py /var/lib/s0sta/fd-1.json /var/lib/s0sta/fd-0.json\n\nExit codes: 0 no process grew by --min-growth, 1 at least one did,\n2 usage error or unreadable, invalid or shapeless input.\n\nRead-only: opens the two JSON files, compares the numbers and prints. It\nnever reads /proc itself, writes nothing and makes no network call.\n\nA high fd count is not a leak; growth between two samples taken at a known\ninterval is. Count the second sample against the same limit the process\nactually runs with: `ulimit -n` in its shell, or LimitNOFILE in the unit.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nDEFAULT_MIN = 10\nDEFAULT_LIMIT = 1024\nDEFAULT_INTERVAL = 60\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read.\"\"\"\n\n\ndef read_snapshot(path):\n    \"\"\"Return {pid: fd_count} from one JSON snapshot, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if not isinstance(doc, dict):\n        raise InputError(\"%s must be a JSON object of pid -> count\" % path)\n    if not doc:\n        raise InputError(\"%s is an empty snapshot\" % path)\n    snapshot = {}\n    for pid, count in doc.items():\n        key = str(pid).strip()\n        if not key.isdigit():\n            raise InputError(\"%s: %r is not a pid\" % (path, pid))\n        if isinstance(count, bool) or not isinstance(count, int):\n            raise InputError(\"%s: pid %s has a non-integer count\" % (path, key))\n        if count < 0:\n            raise InputError(\"%s: pid %s has a negative count\" % (path, key))\n        snapshot[int(key)] = count\n    return snapshot\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank file-descriptor growth between two snapshots \"\n                    \"(read-only).\")\n    parser.add_argument(\"first\", help=\"earlier snapshot: JSON pid -> count\")\n    parser.add_argument(\"second\", help=\"later snapshot: JSON pid -> count\")\n    parser.add_argument(\n        \"--min-growth\", type=int, default=DEFAULT_MIN, metavar=\"N\",\n        help=\"flag a process that gained at least N fds (default: %d)\"\n             % DEFAULT_MIN)\n    parser.add_argument(\n        \"--limit\", type=int, default=DEFAULT_LIMIT, metavar=\"N\",\n        help=\"descriptor limit to project against (default: %d)\" % DEFAULT_LIMIT)\n    parser.add_argument(\n        \"--interval\", type=int, default=DEFAULT_INTERVAL, metavar=\"SECONDS\",\n        help=\"seconds between the two samples (default: %d)\" % DEFAULT_INTERVAL)\n    args = parser.parse_args(argv)\n\n    if args.min_growth < 1:\n        parser.error(\"--min-growth must be at least 1\")\n    if args.limit < 1:\n        parser.error(\"--limit must be at least 1\")\n    if args.interval < 1:\n        parser.error(\"--interval must be at least 1\")\n\n    try:\n        before = read_snapshot(args.first)\n        after = read_snapshot(args.second)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    common = sorted(set(before) & set(after))\n    if not common:\n        print(\"ERROR the two snapshots share no pid\", file=sys.stderr)\n        return 2\n\n    growth = sorted(((after[pid] - before[pid], pid) for pid in common),\n                    key=lambda item: (-item[0], item[1]))\n    fresh = sorted(set(after) - set(before))\n    gone = len(set(before) - set(after))\n\n    for grown, pid in growth[:10]:\n        if grown <= 0:\n            continue\n        print(\"GROWTH %d %d -> %d (%+d in %ds)\"\n              % (pid, before[pid], after[pid], grown, args.interval))\n    if fresh:\n        print(\"NEW    %d process(es) not in the first snapshot\" % len(fresh))\n    if gone:\n        print(\"GONE   %d process(es) not in the second snapshot\" % gone)\n\n    leaks = [(grown, pid) for grown, pid in growth if grown >= args.min_growth]\n    if not leaks:\n        print(\"OK    no process grew by %d or more fds in %ds (%d compared)\"\n              % (args.min_growth, args.interval, len(common)))\n        return 0\n\n    print(\"FINDING %d/%d grew by %d+ fds\"\n          % (len(leaks), len(common), args.min_growth))\n    for grown, pid in leaks[:5]:\n        left = args.limit - after[pid]\n        if left <= 0:\n            print(\"ALERT pid %d +%d fds, %d now\"\n                  % (pid, grown, args.limit))\n            continue\n        per_minute = grown * 60.0 / args.interval\n        minutes = int(left / per_minute)\n        print(\"ALERT pid %d +%d fds, %d in %d min\"\n              % (pid, grown, args.limit, minutes))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p577","n":"577","slug":"the-incident-triage-path","title":"The incident triage path","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["SRE","IncidentResponse","DevOps","OnCall","Linux"],"series_name":"PythonFixes","design":"architecture","script":"","date":"2030-06-04","files":{"it":"notes/p577-the-incident-triage-path.md","devsecops":"notes/p577-the-incident-triage-path.md"},"md":"# The incident triage path\n\nThe page fires at 03:12. By 03:14 four people are in four terminals, and two of them are already restarting things. The host did not fail four times; four people skipped the same path.\n\n## Quick reference\n\n- **Alert:** Pager (PAGE) · Dashboard (DASH)\n- **Diagnose:** Triage script (RUN) · Kernel log (DMESG) · App log (LOG)\n- **Fix:** Rollback (ROLL) · Restart (BOOT)\n\n**Request flow:** Pager → Triage script → Kernel log → Rollback; Triage script → App log → Restart; Dashboard → Triage script\n\n**Around it:** Exit code (EXIT) · Runbook (RUN) · Timeline (TIME)\n\n## Explained\n\n### Alert\n\nThe alert tier is two surfaces, not one. The pager wakes a human, and the dashboard shows whether anything else is on fire. Neither may touch the host. Their only job is to open the runbook and start the clock, because the timestamp is the first fact of the incident.\n\n### Diagnose\n\nThe diagnose tier is one script and two logs. The script is read-only and answers with an exit code: 0 clean, 1 finding, 2 the check itself broke. The kernel log says whether an OOM kill or a disk error came first, and the app log says what the process was doing. Read both before you change either.\n\n### Fix\n\nThe fix tier has two moves. Rollback undoes the last change. Restart clears the state that the change left behind. Pick one, say which one out loud, and write the time down.\n\nUnder the three tiers sit the three things that outlive the incident: the exit code the script returned, the runbook someone will read next time, and the timeline that turns a bad night into a postmortem.\n\n> **The honest caveat:** the order matters more than the speed. A restart at 03:13 destroys the state that would have explained the failure at 03:30.\n\n> **The lesson:** triage is a path, not a conversation.\n\n## In one line\n\n> **Alert, diagnose, then fix.** Three tiers, and only one of them may change the host.\n\n**Open the runbook**\n\n```bash\nless runbooks/host-down.md\n```\n"},{"id":"p578","n":"578","slug":"first-response-commands-for-a-host-incident","title":"First-response commands for a host incident","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SysAdmin","SRE","CommandLine","Troubleshooting"],"series_name":"PythonFixes","design":"cheatsheet","script":"","date":"2030-06-06","files":{"it":"notes/p578-first-response-commands-for-a-host-incident.md","devsecops":"notes/p578-first-response-commands-for-a-host-incident.md"},"md":"# First-response commands for a host incident\n\nThe page says the host is down. You have four minutes before the incident channel fills with theories, and twenty commands that answer the first questions. Four blocks, in this order.\n\n## Quick reference\n\n### Load\n\n```bash\nuptime\ncat /proc/loadavg\nnproc\nvmstat 1 5\nps -eo stat | grep -c R\n```\n\n### Disk\n\n```bash\ndf -h\ndf -i\ndu -xh --max-depth=1 / | sort -h\nlsof +L1 | head\niostat -xz 1 3\n```\n\n### Memory\n\n```bash\nfree -m\nps -eo rss,comm --sort=-rss | head\ndmesg -T | grep -i 'killed process'\ncat /proc/pressure/memory\nslabtop -o | head\n```\n\n### Processes\n\n```bash\nps -eo pid,ppid,stat,comm\nps -eo stat | grep -c Z\nps -eo pid,etime,cmd --sort=-etime\nlsof -p 4242 | wc -l\nss -s\n```\n\n## Explained\n\n### Load\n\n`uptime` gives three load numbers. `cat /proc/loadavg` adds the running and total process counts, and nproc tells you how many cores that load is spread over. `vmstat 1 5` says whether the box is CPU-bound, IO-bound or swapping. `ps -eo stat | grep -c R` counts what is runnable.\n\n### Disk\n\n`df -h` for bytes and `df -i` for inodes, because a filesystem can be full either way. `du -xh --max-depth=1 / | sort -h` finds the directory that grew. `lsof +L1` finds deleted files still held open, which df counts and du cannot see. `iostat -xz 1 3` says whether the device is saturated.\n\n### Memory\n\n`free -m`, then `ps -eo rss,comm --sort=-rss | head` for the top consumers. `dmesg -T | grep -i 'killed process'` answers whether the kernel already acted. `cat /proc/pressure/memory` shows stall time, the number that predicts a page. slabtop -o | head catches kernel memory that no process owns.\n\n### Processes\n\n`ps -eo pid,ppid,stat,comm` names the state and the parent. `ps -eo stat | grep -c Z` counts zombies. `ps -eo pid,etime,cmd --sort=-etime` shows what has run longest. `lsof -p 4242 | wc -l` counts one process's descriptors; `ss -s` summarises every socket.\n\n> **The honest caveat:** a snapshot is not a timeline. Run the same four blocks twice, a minute apart, and compare.\n\n> **The lesson:** read before you restart, because a restart deletes the evidence.\n\n## In one line\n\n> **Look before you restart.** Four minutes of reading beats an hour of guessing.\n\n**Start at load**\n\n```bash\nuptime\n```\n"},{"id":"p579","n":"579","slug":"four-host-failure-modes","title":"Four host failure modes","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SysAdmin","SRE","DevOps","ITInfrastructure"],"series_name":"PythonFixes","design":"explainer","script":"","date":"2030-06-09","files":{"it":"notes/p579-four-host-failure-modes.md","devsecops":"notes/p579-four-host-failure-modes.md"},"md":"# Four host failure modes\n\nAt 03:00 the pager sends the same sentence four times in a month: the host is unhealthy. Four failures look alike from the alert and nothing alike from a shell.\n\n## Quick reference\n\n### Disk full\n\n- `df -h` shows 100%, writes return ENOSPC\n- Logs, images and backups, never rotated\n- `lsof +L1` finds a deleted file still open\n- Fix: rotate, then truncate, not rm\n\n### Inodes full\n\n- `df -h` says 40% free, `df -i` says 100%\n- One inode per file, 256 bytes each\n- Millions of session or cache files\n- Fix: find the directory that grew\n\n### OOM killer\n\n- The kernel kills to keep the box alive\n- dmesg keeps the pid, RSS and score\n- The highest oom_score dies first\n- Fix: cap the unit with MemoryMax\n\n### fd exhaustion\n\n- Every socket, pipe and file takes one slot\n- The default soft limit is often 1024\n- The process logs EMFILE: too many files\n- Fix: close in finally, raise LimitNOFILE\n\n> **Note:** Four ways a healthy host stops working.\n\n## Explained\n\n### Disk full\n\n`df -h` shows 100% and every write returns ENOSPC. The usual cause is a log, an image layer or a backup that nobody rotated. A deleted file still held open keeps its blocks: `lsof +L1` finds space that du cannot see, and restarting the writer returns it.\n\n### Inodes full\n\n`df -h` shows 40% free while `df -i` shows 100%. The filesystem has blocks and no inode to describe a new file, because ext4 allocates one inode per 16 KB and each one costs 256 bytes. Millions of session or cache files do this, and the fix is the directory that grew.\n\n### OOM killer\n\nThe kernel kills a process to keep the box alive. dmesg keeps the pid, the resident memory and oom_score_adj, and the highest score dies first. A cgroup memory limit contains the damage to one service, not the whole host.\n\nfd exhaustion\n\nEvery socket, pipe and open file takes a descriptor. The default soft limit is often 1024, and a leak reaches it in days. The process logs EMFILE: too many open files, which reads like a network error until you count.\n\n> **The honest caveat:** two of these can be true at once. A log flood fills the disk, the writer blocks, and the OOM killer arrives to finish the job.\n\n> **The lesson:** the symptom names the tier, and the tool names the cause.\n\n## In one line\n\n> **Same page, four causes.** The symptom names the tier; the tool names the cause.\n\n**Check both**\n\n```bash\ndf -h; df -i\n```\n"},{"id":"p580","n":"580","slug":"9-steps-of-a-host-incident","title":"9 steps of a host incident","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["SRE","IncidentResponse","Linux","DevOps","OnCall"],"series_name":"PythonFixes","design":"steps","script":"","date":"2030-06-11","files":{"it":"notes/p580-9-steps-of-a-host-incident.md","devsecops":"notes/p580-9-steps-of-a-host-incident.md"},"md":"# 9 steps of a host incident\n\nThe page arrives at 03:12 and the host is still up, which is the worst kind of incident: nothing is obviously broken and everything is slow. Nine steps, three groups, one order.\n\n## Quick reference\n\n### Assess — Name the tier in three commands\n\n```bash\n# Load, memory and bytes in one shot.\nuptime; free -m; df -h\n# Inodes and the kernel's last words.\ndf -i; dmesg -T | tail -40\n# Sockets, states and dead children.\nss -s; ps -eo stat | grep -c Z\n```\n\n### Contain — Stop the damage, not the box\n\n```bash\n# Stop the loop before it fills the disk.\nsystemctl stop api.service\n# Put a memory cap in before the restart.\nsystemctl edit api.service\n# Drop the flood at the door.\niptables -I INPUT -s 10.0.3.17 -j DROP\n```\n\n### Fix — Change one thing, then prove it\n\n```bash\n# Restart with the new limit in place.\nsystemctl restart api.service\n# Read the line before the first kill.\njournalctl -u api --since 03:00\n# Leave a check behind, not a note.\nsystemctl enable --now inode_watch.timer\n```\n\n## Explained\n\n### Assess\n\nThe terminal runs the first three commands before you touch anything. `uptime; free -m; df -h` gives load, memory and bytes in one line. `df -i; dmesg -T | tail -40` adds inodes and the kernel's last words, which is where an OOM kill or a disk error appears first. `ss -s; ps -eo stat | grep -c Z` counts sockets, states and dead children. Under a minute, and you know which tier you are in.\n\n### Contain\n\n`systemctl stop api.service` stops the loop before it fills the disk. `systemctl edit api.service` puts a memory cap in place before the restart, so the next spike is contained by the cgroup rather than the kernel. `iptables -I INPUT -s 10.0.3.17 -j DROP` removes a flood at the door. Containment is temporary by design; it buys the fix, not the week.\n\n### Fix\n\n`systemctl restart api.service` brings the service back with the new limit. `journalctl -u api --since 03:00` reads the line before the first kill, which is the actual cause. `systemctl enable --now inode_watch.timer` leaves a check behind, so the same disk does not surprise the next on-call.\n\n> **The honest caveat:** the restart in the contain step clears the evidence. Capture the logs before you stop anything.\n\n> **The lesson:** assess, contain, fix, in that order, every time.\n\n## In one line\n\n> **Assess, contain, then fix.** Three groups, nine commands, one order that holds.\n\n**Assess first**\n\n```bash\nuptime; df -h; df -i\n```\n"},{"id":"p581","n":"581","slug":"quiz-3-host-incident-questions","title":"Quiz: 3 host incident questions","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SRE","SysAdmin","DevOps","Troubleshooting"],"series_name":"PythonFixes","design":"quiz","script":"","date":"2030-06-13","files":{"it":"notes/p581-quiz-3-host-incident-questions.md","devsecops":"notes/p581-quiz-3-host-incident-questions.md"},"md":"# Quiz: 3 host incident questions\n\nPhase one closes with three questions, and each one comes from a tool earlier in the phase. Answer before the reveal.\n\n## Quick reference\n\n**Q1. `df -h` shows 40% free but every write fails. What ran out?**\n\n- a) The inode table on that filesystem\n- b) The page cache\n- c) The block device queue\n\n> **Answer:** a) The inode table on that filesystem. `df -h` counts bytes, not inodes. `df -i` shows IUse% at 100%: blocks are free and there is no inode for a new file, so open() returns ENOSPC.\n\n**Q2. What does the kernel's oom_score decide?**\n\n- a) Which process the OOM killer picks\n- b) How much swap a process may use\n- c) The priority of the page cache\n\n> **Answer:** a) Which process the OOM killer picks. The score runs 0 to 1000, driven by memory use and shifted by oom_score_adj. The highest score dies first, and -1000 protects a process from the killer.\n\n**Q3. Why does a zombie's parent matter more than the zombie?**\n\n- a) Only the parent can reap it\n- b) The parent owns its memory\n- c) The parent holds its file locks\n\n> **Answer:** a) Only the parent can reap it. A zombie is already dead and holds no memory. It keeps a pid until its parent calls wait(); if the parent never does, the entry stays. Restart the parent.\n\n## Explained\n\n### Question one\n\n`df -h` is clean at 40% and every write on that filesystem fails with ENOSPC. Nothing was deleted, no quota is set, and the application has been running for weeks. The bytes are free, so the question is what a file needs besides bytes, and why deleting one large log would not help. `df -i` is the command that answers it.\n\n### Question two\n\nThe kernel picks a victim and kills it. Two processes hold the same resident memory, one has oom_score_adj at -900 and the other at 0, and the one at 0 dies. What does the score measure, and what does the adjustment do to it? Neither is a priority, a swap quota or a nice value.\n\n### Question three\n\nA child exits and its parent never calls wait(). The zombie holds a pid, a process table slot and nothing else. Why does the parent matter more than the dead child, and what happens to the zombie if the parent is restarted or exits first?\n\n> **The honest caveat:** the answers are the easy part. At 03:00 you have one command, not three, so the habit is to know which one before the page arrives.\n\n> **The lesson:** the tool names the cause, and the cause names the fix.\n\n## In one line\n\n> **Three answers, one pager.** The tool names the cause; the cause names the fix.\n\n**Check your answer**\n\n```bash\ndf -i; dmesg -T | tail\n```\n"},{"id":"p582","n":"582","slug":"a-certificate-bundle-check-in-python","title":"A certificate bundle check in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","TLS","DevSecOps","CyberSecurity","SRE"],"series_name":"PythonFixes","design":"code","script":"cert_bundle_check.py","date":"2030-06-16","files":{"it":"notes/p582-a-certificate-bundle-check-in-python.md","devsecops":"notes/p582-a-certificate-bundle-check-in-python.md"},"md":"# A certificate bundle check in Python\n\nThe leaf certificate had a year left. The bundle still failed on Friday, because the intermediate above it had expired in January.\n\n## Quick reference\n\nThe full script is at the end of this note: `cert_bundle_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cert_bundle_check.py /tmp/fullchain.pem\n```\n\n**Example output**\n\n```text\nOK api.example.com 2027-10-20\nEXPIRED Example Intermediate CA 2026-01-01\n```\n\n## Explained\n\n### What it does\n\nIt reads every CERTIFICATE block in a PEM bundle, decodes the DER, and prints the subject and expiry of each one. The visible lines print OK api.example.com 2027-10-20 and EXPIRED Example Intermediate CA 2026-01-01. The full script adds the chain order and, with `--key`, an ssl check that the key matches the leaf. Exit 1 is expiry, order or a mismatch; 2 is unreadable input.\n\n### How to run it\n\n`python3 cert_bundle_check.py fullchain.pem --key tls.key`. Add `--warn-days 21` to fail three weeks early. CI runs it before the deploy; cron runs it weekly against the file the ingress mounts, not a repository copy.\n\n### The fix\n\nReplace the expired intermediate, order the file leaf first, then intermediate, then root, and deploy the full chain. Re-run until it prints OK. If the key does not match, find the certificate that belongs to the key before you touch DNS.\n\n### Prevention\n\nBuild the bundle in the pipeline and check it there, so what reaches production is what passed. Keep the expiry warning in the deploy job; a calendar reminder is not a check.\n\n> **The honest caveat:** it checks dates and order, not signatures. A forged bundle with the right names passes, so use openssl verify when trust matters.\n\n> **The lesson:** a certificate is valid only inside the bundle that carries it.\n\n## In one line\n\n> **A valid leaf does not save a bundle.** The date is one check; the chain and the key are the other three.\n\n**Check the dates**\n\n```bash\npython3 cert_bundle_check.py fullchain.pem\n```\n\n## Script: cert_bundle_check.py\n\nDownload: [scripts/cert_bundle_check.py](../scripts/cert_bundle_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"cert_bundle_check.py - check a PEM certificate bundle before you deploy it.\n\nReads every CERTIFICATE block in a PEM bundle, decodes the DER with base64\nand the standard library, and reports each certificate's subject, expiry and\nposition in the chain. With --key it asks ssl to load the bundle and the key\ntogether, which proves the private key matches the leaf.\n\nA bundle that deploys cleanly can still fail in a browser: the leaf may be\nexpired, the chain may be out of order, or the key may belong to another\ncertificate. This script answers those three questions with an exit code.\n\nUsage:\n    python3 cert_bundle_check.py fullchain.pem\n    python3 cert_bundle_check.py fullchain.pem --key tls.key\n    python3 cert_bundle_check.py fullchain.pem --warn-days 21\n    cat fullchain.pem | python3 cert_bundle_check.py -\n    python3 cert_bundle_check.py --help\n\nCI (stop a deploy of an expired or misordered bundle):\n    python3 cert_bundle_check.py fullchain.pem --key tls.key || exit 1\n\nCron (weekly, with three weeks of warning):\n    0 7 * * 1 python3 /opt/s0sta/cert_bundle_check.py /etc/tls/fullchain.pem --warn-days 21\n\nExit codes: 0 clean, 1 expired, inside --warn-days, out of order or a key\nmismatch, 2 usage error or unreadable, empty or invalid input.\n\nRead-only: both files are opened for reading. Nothing is written, no service\nis reloaded and no network call is made. A certificate is not a secret and no\nkey material is printed, only whether the key matches the leaf.\n\nIt checks dates and order, not signatures: it does not build a trust path to\na system store, so a forged bundle with the right names passes. Use openssl\nverify for trust and this script for the deploy-time facts.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport base64\nimport binascii\nimport re\nimport ssl\nimport sys\nfrom datetime import datetime, timedelta, timezone\n\nPEM = re.compile(\n    r\"-----BEGIN CERTIFICATE-----\\s*(.*?)\\s*-----END CERTIFICATE-----\",\n    re.S,\n)\nLABELS = {\n    \"2.5.4.3\": \"CN\",\n    \"2.5.4.5\": \"serialNumber\",\n    \"2.5.4.6\": \"C\",\n    \"2.5.4.7\": \"L\",\n    \"2.5.4.8\": \"ST\",\n    \"2.5.4.10\": \"O\",\n    \"2.5.4.11\": \"OU\",\n    \"1.2.840.113549.1.9.1\": \"emailAddress\",\n}\nSTRINGS = {\n    0x0C: \"utf-8\",       # UTF8String\n    0x13: \"ascii\",       # PrintableString\n    0x14: \"latin-1\",     # TeletexString\n    0x16: \"ascii\",       # IA5String\n    0x1E: \"utf-16-be\",   # BMPString\n}\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef tlv(data, pos):\n    \"\"\"Read one DER tag-length-value; return (tag, value, next position).\"\"\"\n    if pos + 2 > len(data):\n        raise InputError(\"truncated DER\")\n    tag = data[pos]\n    size = data[pos + 1]\n    pos += 2\n    if size & 0x80:\n        count = size & 0x7F\n        if not 1 <= count <= 4 or pos + count > len(data):\n            raise InputError(\"bad DER length\")\n        size = int.from_bytes(data[pos:pos + count], \"big\")\n        pos += count\n    end = pos + size\n    if end > len(data):\n        raise InputError(\"truncated DER value\")\n    return tag, data[pos:end], end\n\n\ndef children(data):\n    \"\"\"Yield (tag, value) for each DER value inside data.\"\"\"\n    pos = 0\n    while pos < len(data):\n        tag, value, pos = tlv(data, pos)\n        yield tag, value\n\n\ndef decode_oid(raw):\n    \"\"\"Turn DER OID bytes into dotted form.\"\"\"\n    if not raw:\n        raise InputError(\"empty OID\")\n    parts = [raw[0] // 40, raw[0] % 40]\n    value = 0\n    for byte in raw[1:]:\n        value = (value << 7) | (byte & 0x7F)\n        if not byte & 0x80:\n            parts.append(value)\n            value = 0\n    if value:\n        raise InputError(\"truncated OID\")\n    return \".\".join(str(part) for part in parts)\n\n\ndef decode_string(tag, raw):\n    \"\"\"Decode the string types that appear in an X.509 name.\"\"\"\n    if tag not in STRINGS:\n        raise InputError(\"unsupported name string\")\n    try:\n        return raw.decode(STRINGS[tag])\n    except UnicodeDecodeError:\n        raise InputError(\"undecodable name\")\n\n\ndef read_name(der):\n    \"\"\"Return an X.501 name as [(label, value), ...].\"\"\"\n    parts = []\n    for tag, rdn in children(der):\n        if tag != 0x31:\n            raise InputError(\"bad name: expected a set\")\n        for atag, attribute in children(rdn):\n            if atag != 0x30:\n                raise InputError(\"bad name: expected a sequence\")\n            items = list(children(attribute))\n            if len(items) < 2:\n                raise InputError(\"bad name attribute\")\n            oid = decode_oid(items[0][1])\n            parts.append((LABELS.get(oid, oid),\n                          decode_string(items[1][0], items[1][1])))\n    if not parts:\n        raise InputError(\"empty name\")\n    return parts\n\n\ndef read_time(tag, raw):\n    \"\"\"Parse a UTCTime or GeneralizedTime into an aware UTC datetime.\"\"\"\n    try:\n        text = raw.decode(\"ascii\")\n    except UnicodeDecodeError:\n        raise InputError(\"undecodable time\")\n    zone = \"+0000\"\n    if text.endswith(\"Z\"):\n        text = text[:-1]\n    elif len(text) > 5 and text[-5] in \"+-\":\n        zone = text[-5:]\n        text = text[:-5]\n    if not text.isdigit():\n        raise InputError(\"bad time\")\n    if tag == 0x17:\n        if len(text) == 10:\n            text += \"00\"\n        if len(text) != 12:\n            raise InputError(\"bad UTCTime\")\n        year = int(text[:2])\n        text = str(year + (2000 if year < 50 else 1900)) + text[2:]\n    elif tag == 0x18:\n        if len(text) == 12:\n            text += \"00\"\n        if len(text) != 14:\n            raise InputError(\"bad GeneralizedTime\")\n    else:\n        raise InputError(\"bad time tag\")\n    try:\n        value = datetime(\n            int(text[0:4]), int(text[4:6]), int(text[6:8]),\n            int(text[8:10]), int(text[10:12]), int(text[12:14]),\n            tzinfo=timezone.utc,\n        )\n    except ValueError:\n        raise InputError(\"bad time value\")\n    if zone != \"+0000\":\n        sign = 1 if zone[0] == \"+\" else -1\n        value -= sign * timedelta(\n            hours=int(zone[1:3]), minutes=int(zone[3:5]))\n    return value\n\n\ndef read_cert(der):\n    \"\"\"Return subject, issuer and validity for one DER certificate.\"\"\"\n    tag, outer, end = tlv(der, 0)\n    if tag != 0x30 or end != len(der):\n        raise InputError(\"not a DER certificate\")\n    blocks = list(children(outer))\n    if len(blocks) != 3 or blocks[0][0] != 0x30:\n        raise InputError(\"not a certificate sequence\")\n    fields = list(children(blocks[0][1]))\n    start = 1 if fields and fields[0][0] == 0xA0 else 0\n    if len(fields) < start + 6:\n        raise InputError(\"truncated certificate\")\n    wanted = (0x02, 0x30, 0x30, 0x30, 0x30, 0x30)\n    for offset, expected in enumerate(wanted):\n        if fields[start + offset][0] != expected:\n            raise InputError(\"unexpected certificate field\")\n    validity = list(children(fields[start + 3][1]))\n    if len(validity) != 2:\n        raise InputError(\"bad validity\")\n    return {\n        \"issuer\": read_name(fields[start + 2][1]),\n        \"not_before\": read_time(validity[0][0], validity[0][1]),\n        \"not_after\": read_time(validity[1][0], validity[1][1]),\n        \"subject\": read_name(fields[start + 4][1]),\n    }\n\n\ndef canon(name):\n    \"\"\"Normalise a name for issuer/subject comparison.\"\"\"\n    return tuple((label, \" \".join(value.lower().split()))\n                 for label, value in name)\n\n\ndef show(name):\n    \"\"\"Return CN=... for a name, or the first attribute when there is no CN.\"\"\"\n    for label, value in name:\n        if label == \"CN\":\n            return \"CN=%s\" % value\n    label, value = name[0]\n    return \"%s=%s\" % (label, value)\n\n\ndef read_bundle(path):\n    \"\"\"Return parsed certificates from a PEM file or stdin.\"\"\"\n    try:\n        if path == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not text\" % path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    blocks = PEM.findall(text)\n    if not blocks:\n        raise InputError(\"%s: no CERTIFICATE block found\" % path)\n    certs = []\n    for number, body in enumerate(blocks, start=1):\n        try:\n            der = base64.b64decode(\"\".join(body.split()), validate=True)\n        except (binascii.Error, ValueError):\n            raise InputError(\"certificate %d: invalid base64\" % number)\n        try:\n            certs.append(read_cert(der))\n        except InputError as exc:\n            raise InputError(\"certificate %d: %s\" % (number, exc))\n    return certs\n\n\ndef parse_now(text):\n    \"\"\"Parse --now; a naive value is treated as UTC.\"\"\"\n    raw = text.strip()\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        value = datetime.fromisoformat(raw)\n    except ValueError:\n        raise InputError(\"--now is not ISO 8601: %r\" % text)\n    if value.tzinfo is None:\n        value = value.replace(tzinfo=timezone.utc)\n    return value.astimezone(timezone.utc)\n\n\ndef check_key(bundle, key):\n    \"\"\"Return (matches, reason) for a bundle and key, using ssl.\"\"\"\n    context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)\n    try:\n        context.load_cert_chain(certfile=bundle, keyfile=key)\n    except ssl.SSLError as exc:\n        reason = getattr(exc, \"reason\", \"\") or str(exc)\n        if reason == \"KEY_VALUES_MISMATCH\":\n            return False, \"key values mismatch\"\n        raise InputError(\"cannot load the key: %s\" % reason)\n    except OSError as exc:\n        raise InputError(\"cannot load the key: %s\" % (exc.strerror or exc))\n    return True, \"\"\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check a PEM certificate bundle (read-only).\")\n    parser.add_argument(\"bundle\", nargs=\"?\", default=\"-\",\n                        help=\"PEM bundle file; stdin when omitted\")\n    parser.add_argument(\"--key\", help=\"private key file to match to the leaf\")\n    parser.add_argument(\"--warn-days\", type=int, default=0,\n                        help=\"also fail when a certificate expires inside N days\")\n    parser.add_argument(\"--now\", help=\"ISO 8601 reference time (for testing)\")\n    args = parser.parse_args(argv)\n    if args.warn_days < 0:\n        parser.error(\"--warn-days must not be negative\")\n    if args.bundle == \"-\" and args.key:\n        parser.error(\"--key needs the bundle as a file, not stdin\")\n\n    try:\n        now = parse_now(args.now) if args.now else datetime.now(timezone.utc)\n        certs = read_bundle(args.bundle)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings = []\n    for number, cert in enumerate(certs, start=1):\n        subject = show(cert[\"subject\"])\n        if cert[\"not_after\"] <= now:\n            days = (now - cert[\"not_after\"]).days\n            findings.append(\"cert %d expired\" % number)\n            state = \"EXPIRED %s (%d days ago)\" % (cert[\"not_after\"].date(), days)\n        elif cert[\"not_before\"] > now:\n            findings.append(\"cert %d not yet valid\" % number)\n            state = \"NOT YET VALID %s\" % cert[\"not_before\"].date()\n        else:\n            days = (cert[\"not_after\"] - now).days\n            if args.warn_days and days < args.warn_days:\n                findings.append(\"cert %d expires in %d days\" % (number, days))\n                state = \"EXPIRING %s (%d days)\" % (cert[\"not_after\"].date(), days)\n            else:\n                state = \"expires %s (%d days)\" % (cert[\"not_after\"].date(), days)\n        print(\"CERT %d %s %s\" % (number, subject, state))\n\n    for number in range(len(certs) - 1):\n        below = certs[number]\n        above = certs[number + 1]\n        if canon(below[\"issuer\"]) != canon(above[\"subject\"]):\n            findings.append(\"order %d/%d\" % (number + 1, number + 2))\n            print(\"ORDER cert %d issuer %s does not match cert %d subject %s\"\n                  % (number + 1, show(below[\"issuer\"]),\n                     number + 2, show(above[\"subject\"])))\n    if len(certs) > 1:\n        names = \" -> \".join(show(cert[\"subject\"]) for cert in certs)\n        ordered = not any(item.startswith(\"order \") for item in findings)\n        print(\"ORDER %s%s\" % (\"\" if ordered else \"CHECK \", names))\n    if canon(certs[-1][\"subject\"]) != canon(certs[-1][\"issuer\"]):\n        print(\"NOTE %s is not self-signed; a root may be missing\"\n              % show(certs[-1][\"subject\"]))\n\n    key_state = \"not checked\"\n    if args.key:\n        try:\n            matches, reason = check_key(args.bundle, args.key)\n        except InputError as exc:\n            print(\"ERROR %s\" % exc, file=sys.stderr)\n            return 2\n        if matches:\n            key_state = \"matches the leaf\"\n            print(\"KEY matches the leaf certificate\")\n        else:\n            key_state = \"does not match\"\n            findings.append(\"key mismatch\")\n            print(\"KEY does not match the leaf certificate (%s)\" % reason)\n\n    if findings:\n        print(\"FINDING %d: %s\" % (len(findings), \", \".join(findings)))\n        return 1\n    print(\"OK %d certificate(s), order fine, key %s\"\n          % (len(certs), key_state))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p583","n":"583","slug":"a-secret-rotation-report-in-python","title":"A secret rotation report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["Python","SecretsManagement","DevSecOps","CyberSecurity","Automation"],"series_name":"PythonFixes","design":"code","script":"secret_rotation_due.py","date":"2030-06-18","files":{"devsecops":"notes/p583-a-secret-rotation-report-in-python.md"},"md":"# A secret rotation report in Python\n\nA production token issued in 2023 was still valid this month. Rotation was a calendar reminder, the reminder moved with a meeting, and nobody could say when the secret had last changed.\n\n## Quick reference\n\nThe full script is at the end of this note: `secret_rotation_due.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 secret_rotation_due.py /tmp/secrets.json\n```\n\n**Example output**\n\n```text\nDUE db-password prod 290\nNEVER api-token prod\n```\n\n## Explained\n\n### What it does\n\nIt reads a secret inventory and computes the age of every entry from last_rotated. A secret past its own max_age_days is DUE; a null date is NEVER. The per-record window wins, and `--max-days` supplies the default. On screen the loop prints DUE db-password prod 290 and NEVER api-token prod, then exits 1.\n\n### How to run it\n\n`python3 secret_rotation_due.py secrets.json --max-days 90`. The Monday cron line is in the docstring. In CI, run it against the inventory in the same repository as the service, so a new secret cannot merge without a rotation date.\n\n### The fix\n\nRotate the DUE entries first, oldest first, and write the new date as part of the same change. Add the NEVER entries to the inventory before you rotate them, or the next report will still be blind. The inventory holds names and dates, never values.\n\n### Prevention\n\nKeep the inventory in Git and review it like code. A pull request that adds a secret adds its rotation window in the same diff. The check then fails the pipeline when the window closes, instead of asking a person to remember.\n\n> **The honest caveat:** an inventory is a claim. A secret rotated outside the process is invisible, and a recent date does not prove the value changed.\n\n> **The lesson:** a rotation you cannot prove is a guess.\n\n## In one line\n\n> **A rotation you cannot prove is a guess.** A date in an inventory is the only evidence that rotation happened.\n\n**Run it monday**\n\n```bash\npython3 secret_rotation_due.py secrets.json\n```\n\n## Script: secret_rotation_due.py\n\nDownload: [scripts/secret_rotation_due.py](../scripts/secret_rotation_due.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"secret_rotation_due.py - report secrets past their rotation window.\n\nReads a secret inventory (metadata only, never a secret value) and prints the\nentries that are older than their own max_age_days or were never rotated. The\ninventory is the missing control in most estates: everyone believes rotation\nhappens, nobody can show when it last did.\n\nInventory shape, a JSON object with a \"secrets\" list (a bare list also works):\n\n    {\"secrets\": [\n      {\"name\": \"db-password\", \"env\": \"prod\",\n       \"last_rotated\": \"2026-01-04\", \"max_age_days\": 90},\n      {\"name\": \"api-token\", \"env\": \"prod\",\n       \"last_rotated\": null, \"max_age_days\": 30}\n    ]}\n\nlast_rotated is an ISO 8601 date or timestamp, or null for never rotated.\nmax_age_days is optional; --max-days supplies the default.\n\nUsage:\n    python3 secret_rotation_due.py secrets.json\n    python3 secret_rotation_due.py secrets.json --max-days 60\n    python3 secret_rotation_due.py secrets.json --now 2026-10-21\n    cat secrets.json | python3 secret_rotation_due.py -\n    python3 secret_rotation_due.py --help\n\nCI (fail the job when a secret is past its window):\n    python3 secret_rotation_due.py secrets.json --max-days 90 || exit 1\n\nCron (Monday morning report to the security channel):\n    0 7 * * 1 python3 /opt/s0sta/secret_rotation_due.py /etc/s0sta/secrets.json\n\nExit codes: 0 every secret is inside its window, 1 at least one is due or\nnever rotated, 2 usage error or unreadable, invalid or empty input.\n\nRead-only: the inventory is opened for reading only. Nothing is rotated,\ncreated or revoked and no network call is made. The inventory holds names,\nenvironments and dates; this script never asks for and never prints a secret\nvalue. A rotation performed outside the inventory is invisible to it.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef parse_time(text, field, where):\n    \"\"\"Parse an ISO 8601 date or timestamp as an aware UTC datetime.\"\"\"\n    if not isinstance(text, str) or not text.strip():\n        raise InputError(\"%s: %s is not a timestamp\" % (where, field))\n    raw = text.strip()\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        value = datetime.fromisoformat(raw)\n    except ValueError:\n        raise InputError(\"%s: %s is not ISO 8601: %r\" % (where, field, text))\n    if value.tzinfo is None:\n        value = value.replace(tzinfo=timezone.utc)\n    return value.astimezone(timezone.utc)\n\n\ndef load_inventory(path):\n    \"\"\"Return the list of secret records, or raise InputError.\"\"\"\n    try:\n        if path == \"-\":\n            document = json.load(sys.stdin)\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                document = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except (json.JSONDecodeError, UnicodeError) as exc:\n        raise InputError(\"%s: not valid JSON: %s\" % (path, exc))\n    if isinstance(document, dict):\n        records = document.get(\"secrets\")\n    else:\n        records = document\n    if not isinstance(records, list):\n        raise InputError(\"%s: expected a list or an object with a secrets list\"\n                         % path)\n    return records\n\n\ndef check_record(record, index, default_days):\n    \"\"\"Validate one record; return (where, name, env, last, max_days).\"\"\"\n    where = \"secret %d\" % (index + 1)\n    if not isinstance(record, dict):\n        raise InputError(\"%s is not an object\" % where)\n    for field in (\"name\", \"env\"):\n        if not isinstance(record.get(field), str) or not record[field].strip():\n            raise InputError(\"%s: %s is missing\" % (where, field))\n    last = record.get(\"last_rotated\")\n    if last is not None and not isinstance(last, str):\n        raise InputError(\"%s: last_rotated must be a string or null\" % where)\n    max_days = record.get(\"max_age_days\", default_days)\n    if not isinstance(max_days, int) or isinstance(max_days, bool):\n        raise InputError(\"%s: max_age_days must be an integer\" % where)\n    if max_days < 1:\n        raise InputError(\"%s: max_age_days must be positive\" % where)\n    return where, record[\"name\"].strip(), record[\"env\"].strip(), last, max_days\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Report secrets past their rotation window (read-only).\")\n    parser.add_argument(\"inventory\", nargs=\"?\", default=\"-\",\n                        help=\"inventory JSON file; stdin when omitted\")\n    parser.add_argument(\"--max-days\", type=int, default=90,\n                        help=\"default rotation window in days (default 90)\")\n    parser.add_argument(\"--now\", help=\"ISO 8601 reference time for the report\")\n    args = parser.parse_args(argv)\n    if args.max_days < 1:\n        parser.error(\"--max-days must be a positive integer\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        if args.now:\n            now = parse_time(args.now, \"--now\", \"arguments\")\n        else:\n            now = datetime.now(timezone.utc)\n        records = load_inventory(args.inventory)\n        flags = 0\n        for index, record in enumerate(records):\n            where, name, env, last, max_days = check_record(\n                record, index, args.max_days)\n            if last is None:\n                flags += 1\n                print(\"NEVER %s %s (no rotation record)\" % (name, env))\n                continue\n            rotated = parse_time(last, \"last_rotated\", where)\n            age = (now - rotated).days\n            if age < 0:\n                raise InputError(\"%s: last_rotated is in the future\" % where)\n            if age > max_days:\n                flags += 1\n                print(\"DUE %s %s %dd (max %dd)\" % (name, env, age, max_days))\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    total = len(records) if isinstance(records, list) else 0\n    if flags:\n        noun = \"secret\" if total == 1 else \"secrets\"\n        print(\"FINDING %d of %d %s past their window\" % (flags, total, noun))\n        return 1\n    if total == 1:\n        print(\"OK 1 secret inside its window\")\n    else:\n        print(\"OK %d secrets inside their windows\" % total)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p584","n":"584","slug":"an-offboarding-access-audit-in-python","title":"An offboarding access audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Identity","CyberSecurity","DevSecOps","Automation"],"series_name":"PythonFixes","design":"code","script":"offboarding_audit.py","date":"2030-06-20","files":{"it":"notes/p584-an-offboarding-access-audit-in-python.md","devsecops":"notes/p584-an-offboarding-access-audit-in-python.md"},"md":"# An offboarding access audit in Python\n\nA contractor's last day was 30 September. On 5 October the GitHub account was still active, and the last login was three days after the farewell message.\n\n## Quick reference\n\nThe full script is at the end of this note: `offboarding_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 offboarding_audit.py leavers.json access.json\n```\n\n**Example output**\n\n```text\nUSED ana@example.com github\nUSED sam@example.com vpn\n```\n\n## Explained\n\n### What it does\n\nIt joins an HR leavers export to an access export on the email address. An account whose kind is active or enabled after the last day is ACTIVE. A last_login after that day is USED, even when the account is marked disabled elsewhere. On screen the join prints USED ana@example.com github and USED sam@example.com vpn, then exits 1.\n\n### How to run it\n\n`python3 offboarding_audit.py leavers.json access.json`. `--grace-days 2` allows a handover window. Run it when HR closes a leaver ticket, and again from cron every morning as a safety net for the feed that never arrived.\n\n### The fix\n\nDisable each flagged account, revoke its sessions and tokens, then run the audit again until it prints OK. Check the identity provider as well as the application: disabling the SSO account does not always disable the token that was issued before it.\n\n### Prevention\n\nMake the HR event the trigger, not a quarterly review. The access export must name every system, including the local ones that no SSO log covers. One owner per system is what makes the export complete.\n\n> **The honest caveat:** the audit only sees the systems in the export. A shared password or a local account on a build host can outlive the leaver without appearing in either file.\n\n> **The lesson:** offboarding ends when access ends, not when the ticket closes.\n\n## In one line\n\n> **Offboarding ends when access ends.** The ticket closes on the last day; the account does not.\n\n**Check the join**\n\n```bash\npython3 offboarding_audit.py lv.json ac.json\n```\n\n## Script: offboarding_audit.py\n\nDownload: [scripts/offboarding_audit.py](../scripts/offboarding_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"offboarding_audit.py - find accounts a leaver can still use.\n\nJoins an HR leavers export to an access export and reports accounts that are\nstill active after the person's last day, or were used after it. The join is\nthe part most offboarding checks miss: HR knows the date, the systems know the\naccount, and nobody puts the two files side by side.\n\nLeavers shape (a bare list also works):\n\n    {\"leavers\": [\n      {\"email\": \"ana@example.com\", \"last_day\": \"2026-09-30\"},\n      {\"email\": \"sam@example.com\", \"last_day\": \"2026-10-15\"}\n    ]}\n\nAccess shape (a bare list also works):\n\n    {\"accounts\": [\n      {\"user\": \"ana@example.com\", \"system\": \"github\",\n       \"last_login\": \"2026-10-05T10:00:00Z\", \"kind\": \"active\"},\n      {\"user\": \"sam@example.com\", \"system\": \"vpn\",\n       \"last_login\": null, \"kind\": \"disabled\"}\n    ]}\n\nkind active or enabled counts as open; any other value counts as disabled,\nbut a login after the last day is still reported.\n\nUsage:\n    python3 offboarding_audit.py leavers.json access.json\n    python3 offboarding_audit.py leavers.json access.json --grace-days 2\n    python3 offboarding_audit.py leavers.json access.json --now 2026-10-21\n    python3 offboarding_audit.py --help\n\nCI (run on every leaver event, not once a quarter):\n    python3 offboarding_audit.py leavers.json access.json || exit 1\n\nCron (daily safety net for the HR feed):\n    0 6 * * * python3 /opt/s0sta/offboarding_audit.py /etc/s0sta/leavers.json /etc/s0sta/access.json\n\nExit codes: 0 no account outlives its owner's last day, 1 at least one\nfinding, 2 usage error or unreadable, invalid or empty input.\n\nRead-only: both files are opened for reading. Nothing is disabled, revoked\nor written and no network call is made. Reports name the account and the\nsystem, never a token, password or session value.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date, timedelta\n\nOPEN_KINDS = (\"active\", \"enabled\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef load_list(path, key):\n    \"\"\"Return the list under key (or the document itself), or raise.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            document = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except (json.JSONDecodeError, UnicodeError) as exc:\n        raise InputError(\"%s: not valid JSON: %s\" % (path, exc))\n    if isinstance(document, dict):\n        records = document.get(key)\n    else:\n        records = document\n    if not isinstance(records, list):\n        raise InputError(\"%s: expected a list or an object with a %s list\"\n                         % (path, key))\n    return records\n\n\ndef parse_day(value, where, field):\n    \"\"\"Parse an ISO 8601 date or timestamp's date part.\"\"\"\n    if not isinstance(value, str) or len(value.strip()) < 10:\n        raise InputError(\"%s: %s is not an ISO 8601 date\" % (where, field))\n    try:\n        return date.fromisoformat(value.strip()[:10])\n    except ValueError:\n        raise InputError(\"%s: %s is not an ISO 8601 date: %r\"\n                         % (where, field, value))\n\n\ndef load_leavers(path):\n    \"\"\"Return {email: last_day} and the number of records.\"\"\"\n    records = load_list(path, \"leavers\")\n    leavers = {}\n    for index, record in enumerate(records):\n        where = \"leaver %d\" % (index + 1)\n        if not isinstance(record, dict):\n            raise InputError(\"%s is not an object\" % where)\n        email = record.get(\"email\")\n        if not isinstance(email, str) or not email.strip():\n            raise InputError(\"%s: email is missing\" % where)\n        leavers[email.strip().lower()] = parse_day(\n            record.get(\"last_day\"), where, \"last_day\")\n    return leavers\n\n\ndef load_accounts(path):\n    \"\"\"Return a list of (user, system, last_login date or None, kind).\"\"\"\n    records = load_list(path, \"accounts\")\n    accounts = []\n    for index, record in enumerate(records):\n        where = \"account %d\" % (index + 1)\n        if not isinstance(record, dict):\n            raise InputError(\"%s is not an object\" % where)\n        for field in (\"user\", \"system\"):\n            if not isinstance(record.get(field), str) or not record[field].strip():\n                raise InputError(\"%s: %s is missing\" % (where, field))\n        last = record.get(\"last_login\")\n        if last is not None:\n            last = parse_day(last, where, \"last_login\")\n        kind = record.get(\"kind\", \"active\")\n        if not isinstance(kind, str):\n            raise InputError(\"%s: kind must be a string\" % where)\n        accounts.append((record[\"user\"].strip().lower(),\n                         record[\"system\"].strip(), last, kind.strip().lower()))\n    return accounts\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Join leavers to access and report surviving accounts.\")\n    parser.add_argument(\"leavers\", help=\"HR leavers JSON\")\n    parser.add_argument(\"accounts\", help=\"access export JSON\")\n    parser.add_argument(\"--grace-days\", type=int, default=0,\n                        help=\"days of access allowed after the last day\")\n    parser.add_argument(\"--now\", help=\"ISO 8601 reference date (for testing)\")\n    args = parser.parse_args(argv)\n    if args.grace_days < 0:\n        parser.error(\"--grace-days must not be negative\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        leavers = load_leavers(args.leavers)\n        accounts = load_accounts(args.accounts)\n        if args.now:\n            today = parse_day(args.now, \"arguments\", \"--now\")\n        else:\n            today = date.today()\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    seen = {email: 0 for email in leavers}\n    findings = 0\n    for user, system, last, kind in accounts:\n        if user not in leavers:\n            continue\n        seen[user] += 1\n        cutoff = leavers[user] + timedelta(days=args.grace_days)\n        reasons = []\n        if kind in OPEN_KINDS and today > cutoff:\n            reasons.append(\"ACTIVE\")\n        if last is not None and last > cutoff:\n            reasons.append(\"USED %s\" % last.isoformat())\n        if reasons:\n            findings += 1\n            print(\"%s %s %s (last day %s)\"\n                  % (\", \".join(reasons), user, system, leavers[user].isoformat()))\n\n    for email in sorted(leavers):\n        if seen[email] == 0:\n            print(\"CLEAR %s (no accounts in the export)\" % email)\n\n    if findings:\n        print(\"FINDING %d of %d accounts outlive their last day\"\n              % (findings, len(accounts)))\n        return 1\n    print(\"OK %d leavers, %d accounts, none outlive their last day\"\n          % (len(leavers), len(accounts)))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p585","n":"585","slug":"a-password-age-audit-in-python","title":"A password age audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","CyberSecurity","DevSecOps","SysAdmin"],"series_name":"PythonFixes","design":"code","script":"password_age.py","date":"2030-06-23","files":{"it":"notes/p585-a-password-age-audit-in-python.md","devsecops":"notes/p585-a-password-age-audit-in-python.md"},"md":"# A password age audit in Python\n\nAn audit found a service account whose password had not changed since 2019. The hash was still in shadow, the account expiry field was empty, and the password was on a wiki page.\n\n## Quick reference\n\nThe full script is at the end of this note: `password_age.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 password_age.py /tmp/shadow\n```\n\n**Example output**\n\n```text\nNOEXP,OLD deploy\nEMPTY,NOEXP,OLD svc-backup\n```\n\n## Explained\n\n### What it does\n\nIt reads a colon-separated shadow export and reports four things: an empty password field, an account with no expiry date, a password older than `--max-days`, and a last-change field that cannot be aged. Locked entries are skipped unless `--include-locked`. On screen the flags print NOEXP,OLD deploy and EMPTY,NOEXP,OLD svc-backup, then the exit code is 1.\n\n### How to run it\n\n`python3 password_age.py shadow.txt --max-days 90`. The weekly cron line is in the docstring. A passwd-format export is accepted for the empty-field check, with a note that age and expiry need the shadow file.\n\n### The fix\n\nLock or rotate the account, set a maximum age with `chage -M 90`, give it an end date with `chage -E`, and remove the password from the wiki. Then check what the account can reach: a service password is usually also a sudo rule, a systemd unit or a database grant.\n\n### Prevention\n\nSet the maximum age in the image build and report the exceptions monthly. Treat password age as an input to the access review, not as a separate compliance chore.\n\n> **The honest caveat:** lastchg records the last hash change. A reset through an external identity provider, or a hash copied between hosts, does not show up, and the field can be edited.\n\n> **The lesson:** an old password is an open door with a date on it.\n\n## In one line\n\n> **An old password is an open door.** The hash never leaves the file; the age is the finding.\n\n**Set an end date**\n\n```bash\nchage -E 2027-01-01 deploy\n```\n\n## Script: password_age.py\n\nDownload: [scripts/password_age.py](../scripts/password_age.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"password_age.py - audit a passwd/shadow export without printing hashes.\n\nReads a colon-separated shadow export and reports four things: an empty\npassword field (a login with no password), an account with no expiry date,\na password older than --max-days, and a last-change field that cannot be\naged. A passwd-format line (seven fields) is accepted for the empty-field\ncheck only; its aging data lives in shadow.\n\nShadow fields: name:hash:lastchg:min:max:warn:inactive:expire:reserved.\nlastchg and expire are days since 1 January 1970. Locked entries (a hash\nstarting with ! or *) are skipped unless --include-locked is given, because\na locked system account is not a password risk.\n\nUsage:\n    python3 password_age.py shadow.txt\n    python3 password_age.py shadow.txt --max-days 60\n    python3 password_age.py shadow.txt --include-locked\n    python3 password_age.py shadow.txt --now 2026-10-21\n    python3 password_age.py --help\n\nCI (fail when a privileged account is past the policy):\n    python3 password_age.py /etc/shadow --max-days 90 || exit 1\n\nCron (weekly, before the access review):\n    0 6 * * 1 python3 /opt/s0sta/password_age.py /etc/shadow --max-days 90\n\nExit codes: 0 no finding, 1 at least one finding, 2 usage error or\nunreadable, empty or malformed input.\n\nRead-only: the export is opened for reading only. Nothing is locked, reset\nor written and no network call is made. The password hash is never printed,\nstored or used as anything but a yes/no answer about emptiness and locking;\nthe report names the account and the age, never the hash.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nfrom datetime import date\n\nEPOCH = date(1970, 1, 1)\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef parse_day(text):\n    \"\"\"Parse an ISO 8601 date used by --now.\"\"\"\n    try:\n        return date.fromisoformat(text.strip()[:10])\n    except (AttributeError, ValueError):\n        raise InputError(\"--now is not an ISO 8601 date: %r\" % text)\n\n\ndef read_lines(path):\n    \"\"\"Return the non-blank, non-comment lines of a colon file.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            lines = handle.readlines()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not text\" % path)\n    entries = [(number, line.rstrip(\"\\n\"))\n               for number, line in enumerate(lines, start=1)\n               if line.strip() and not line.lstrip().startswith(\"#\")]\n    if not entries:\n        raise InputError(\"%s: empty input\" % path)\n    return entries\n\n\ndef audit(path, max_days, include_locked, today):\n    \"\"\"Return (findings, accounts, notes) for one colon export.\"\"\"\n    findings = []\n    accounts = 0\n    passwd_lines = 0\n    for number, line in read_lines(path):\n        fields = line.split(\":\")\n        if len(fields) < 7:\n            raise InputError(\"line %d: expected at least 7 fields\" % number)\n        name = fields[0].strip()\n        if not name:\n            raise InputError(\"line %d: empty account name\" % number)\n        accounts += 1\n        if len(fields) == 7:\n            passwd_lines += 1\n            if fields[1] == \"\":\n                findings.append(\"EMPTY %s (no password field)\" % name)\n            continue\n        password = fields[1]\n        if password == \"\":\n            findings.append(\"EMPTY %s (no password field)\" % name)\n        if password.startswith((\"!\", \"*\")) and not include_locked:\n            continue\n        if not fields[7].strip():\n            findings.append(\"NOEXP %s (no account expiry)\" % name)\n        raw = fields[2].strip()\n        if raw == \"0\":\n            findings.append(\"FORCE %s (lastchg 0, change at next login)\" % name)\n        elif not raw.isdigit():\n            findings.append(\"NOAGE %s (no usable last-change field)\" % name)\n        else:\n            age = (today - EPOCH).days - int(raw)\n            if age < 0:\n                findings.append(\"FUTURE %s (last change is in the future)\" % name)\n            elif age > max_days:\n                findings.append(\"OLD %s %dd (max %dd)\" % (name, age, max_days))\n    notes = []\n    if passwd_lines:\n        notes.append(\"%d passwd-format line(s): empty field checked, age and \"\n                     \"expiry need the shadow file\" % passwd_lines)\n    return findings, accounts, notes\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Audit a passwd/shadow export for weak password age.\")\n    parser.add_argument(\"export\", help=\"colon-separated passwd or shadow export\")\n    parser.add_argument(\"--max-days\", type=int, default=90,\n                        help=\"flag passwords older than this (default 90)\")\n    parser.add_argument(\"--include-locked\", action=\"store_true\",\n                        help=\"also check entries whose hash starts with ! or *\")\n    parser.add_argument(\"--now\", help=\"ISO 8601 reference date (for testing)\")\n    args = parser.parse_args(argv)\n    if args.max_days < 1:\n        parser.error(\"--max-days must be a positive integer\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        today = parse_day(args.now) if args.now else date.today()\n        findings, accounts, notes = audit(\n            args.export, args.max_days, args.include_locked, today)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    for note in notes:\n        print(\"NOTE %s\" % note, file=sys.stderr)\n    if findings:\n        for finding in findings:\n            print(finding)\n        print(\"FINDING %d in %d account%s\" % (\n            len(findings), accounts, \"\" if accounts == 1 else \"s\"))\n        return 1\n    print(\"OK %d account%s, no weak password age\" % (\n        accounts, \"\" if accounts == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p586","n":"586","slug":"a-sudo-drift-report-in-python","title":"A sudo drift report in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Linux","Sudo","DevSecOps","CyberSecurity"],"series_name":"PythonFixes","design":"code","script":"sudo_drift.py","date":"2030-06-25","files":{"it":"notes/p586-a-sudo-drift-report-in-python.md","devsecops":"notes/p586-a-sudo-drift-report-in-python.md"},"md":"# A sudo drift report in Python\n\nA one-line sudoers change let the deploy user run every command without a password. The review approved a restart command; the merged line said ALL.\n\n## Quick reference\n\nThe full script is at the end of this note: `sudo_drift.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 sudo_drift.py before after\n```\n\n**Example output**\n\n```text\nWIDEN %deploy ALL\nADDED carol /usr/bin/passwd\n```\n\n## Explained\n\n### What it does\n\nIt parses two sudoers snapshots, expands the aliases, and compares what each user can run. It reports a new user, a new NOPASSWD grant, a command added to an existing rule, a wildcard path and a runas list that became ALL. A change that only removes or narrows grants exits 0. On screen the diff prints WIDEN %deploy ALL and ADDED carol `/usr/bin/passwd`, then exits 1.\n\n### How to run it\n\n`python3 sudo_drift.py before.sudoers after.sudoers`. In CI, render the before file from the main branch and the after file from the pull request, then let the exit code gate the merge. From cron, compare the baseline with the live file.\n\n### The fix\n\nRevert the widening or scope it to one command: NOPASSWD on `/usr/bin/systemctl` restart app is a grant you can defend. Re-run the diff until it exits 0, then run `visudo -c` to prove the file still parses.\n\n### Prevention\n\nKeep sudoers in Git and review every NOPASSWD line as a privilege change. A command added to an existing rule is the quiet case, because the diff looks like a longer line rather than a new permission.\n\n> **The honest caveat:** this is a line parser, not visudo. Includes are reported and not followed, and arguments are compared as text, so a clean diff is not a clean host.\n\n> **The lesson:** sudo drift is a silent grant until someone diffs it.\n\n## In one line\n\n> **Sudo drift is a silent grant.** A longer line in a text file can be full root on the host.\n\n**Diff before merge**\n\n```bash\npython3 sudo_drift.py old.conf new.conf\n```\n\n## Script: sudo_drift.py\n\nDownload: [scripts/sudo_drift.py](../scripts/sudo_drift.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"sudo_drift.py - show what a sudoers change widened.\n\nCompares two sudoers snapshots (before and after) and reports the grants that\ngrew: a new user, a new NOPASSWD command, a command added to an existing rule,\na wildcard path, or a runas list that became ALL. A change that only removes\nor narrows grants exits 0, so the check can gate a pull request.\n\nUsage:\n    python3 sudo_drift.py before.sudoers after.sudoers\n    python3 sudo_drift.py before.sudoers after.sudoers --json\n    python3 sudo_drift.py --help\n\nCI (fail the change that widens sudo):\n    python3 sudo_drift.py before.sudoers after.sudoers || exit 1\n\nCron (weekly diff of the managed snapshot against the live file):\n    0 6 * * 1 python3 /opt/s0sta/sudo_drift.py /etc/s0sta/sudoers.baseline /etc/sudoers\n\nExit codes: 0 no widening, 1 at least one widening, 2 usage error or\nunreadable, empty or rule-free input.\n\nRead-only: both files are opened for reading. Nothing is written, no rule is\ninstalled and no command is executed. Reports name users and command paths,\nnever a command argument, so a password passed on a sudo line is not echoed.\n\nThis is a line parser, not visudo. User_Alias, Runas_Alias and Cmnd_Alias are\nexpanded; #include and #includedir are reported on stderr and not followed; a\ncomma inside a command argument must be escaped, as sudoers itself requires.\nRun visudo -c for syntax and this script for the change in exposure.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport fnmatch\nimport json\nimport re\nimport sys\n\nALIAS = re.compile(\n    r\"^(User_Alias|Runas_Alias|Cmnd_Alias)\\s+([A-Za-z][A-Za-z0-9_]*)\\s*=\\s*(.+)$\",\n    re.IGNORECASE,\n)\nINCLUDE = re.compile(r\"^[#@]include(dir)?\\s+(\\S+)\", re.IGNORECASE)\nRUNAS = re.compile(r\"^\\(([^)]*)\\)\\s*(.*)$\")\nTAG = re.compile(\n    r\"^(?:(?:NOPASSWD|PASSWD|NOEXEC|EXEC|SETENV|NOSETENV|LOG_INPUT|\"\n    r\"NOLOG_INPUT|LOG_OUTPUT|NOLOG_OUTPUT|MAIL|NOMAIL|FOLLOW|NOFOLLOW)\"\n    r\"\\s*:\\s*)+\",\n    re.IGNORECASE,\n)\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return the file text, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except UnicodeDecodeError:\n        raise InputError(\"%s: not UTF-8 text\" % path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    return text\n\n\ndef logical_lines(text):\n    \"\"\"Yield (line number, joined line) with continuations folded.\"\"\"\n    buffered = \"\"\n    start = 1\n    for number, raw in enumerate(text.splitlines(), start=1):\n        line = raw.rstrip()\n        if not buffered:\n            start = number\n        if line.endswith(\"\\\\\"):\n            buffered += line[:-1] + \" \"\n            continue\n        buffered += line\n        yield start, buffered\n        buffered = \"\"\n    if buffered:\n        yield start, buffered\n\n\ndef strip_comment(line):\n    \"\"\"Remove a # comment, keeping an include directive readable.\"\"\"\n    stripped = line.strip()\n    if INCLUDE.match(stripped):\n        return stripped\n    if stripped.startswith((\"#\", \"@\")):\n        return \"\"\n    return stripped.split(\"#\", 1)[0].strip()\n\n\ndef split_escaped(text):\n    \"\"\"Split a comma list, keeping escaped commas inside an argument.\"\"\"\n    parts = []\n    current = \"\"\n    escaped = False\n    for char in text:\n        if escaped:\n            current += char\n            escaped = False\n        elif char == \"\\\\\":\n            escaped = True\n        elif char == \",\":\n            parts.append(current)\n            current = \"\"\n        else:\n            current += char\n    parts.append(current)\n    return [part.strip() for part in parts if part.strip()]\n\n\ndef parse_aliases(lines):\n    \"\"\"Return user, runas and command aliases plus include lines.\"\"\"\n    users = {}\n    runas = {}\n    commands = {}\n    includes = []\n    for number, raw in lines:\n        line = strip_comment(raw)\n        if not line:\n            continue\n        if INCLUDE.match(line):\n            includes.append((number, line))\n            continue\n        found = ALIAS.match(line)\n        if not found:\n            continue\n        kind = found.group(1).lower()\n        name = found.group(2)\n        values = split_escaped(found.group(3))\n        if kind == \"user_alias\":\n            users[name] = values\n        elif kind == \"runas_alias\":\n            runas[name] = values\n        else:\n            commands[name] = values\n    return users, runas, commands, includes\n\n\ndef expand(items, aliases, seen=None):\n    \"\"\"Expand alias names in a list, dropping negated entries.\"\"\"\n    seen = set() if seen is None else set(seen)\n    out = []\n    for item in items:\n        raw = item.strip()\n        if not raw or raw.startswith(\"!\"):\n            continue\n        if raw in aliases and raw not in seen:\n            out.extend(expand(aliases[raw], aliases, seen | {raw}))\n        else:\n            out.append(raw)\n    return out\n\n\ndef parse_runas(text):\n    \"\"\"Return the runas users from the text inside the parentheses.\"\"\"\n    users = []\n    for chunk in text.replace(\":\", \",\").split(\",\"):\n        name = chunk.strip()\n        if name and not name.startswith(\"!\"):\n            users.append(name)\n    return users or [\"root\"]\n\n\ndef parse_grants(text):\n    \"\"\"Return ({user: profile}, rule count, include lines, skipped count).\"\"\"\n    lines = list(logical_lines(text))\n    user_alias, runas_alias, cmd_alias, includes = parse_aliases(lines)\n    profiles = {}\n    rules = 0\n    skipped = 0\n    for number, raw in lines:\n        line = strip_comment(raw)\n        if (not line or INCLUDE.match(line) or ALIAS.match(line)\n                or line.lower().startswith(\"defaults\")):\n            continue\n        if \"=\" not in line:\n            skipped += 1\n            continue\n        left, right = line.split(\"=\", 1)\n        tokens = left.split()\n        if len(tokens) < 2:\n            skipped += 1\n            continue\n        users = expand(split_escaped(\" \".join(tokens[:-1])), user_alias)\n        if not users:\n            skipped += 1\n            continue\n        body = right.strip()\n        runas = [\"root\"]\n        found = RUNAS.match(body)\n        if found:\n            runas = expand(parse_runas(found.group(1)), runas_alias)\n            body = found.group(2).strip()\n        nopasswd = False\n        commands = []\n        for segment in split_escaped(body):\n            tags = TAG.match(segment)\n            if tags:\n                names = re.findall(r\"([A-Z_]+)\\s*:\", tags.group(0))\n                if names:\n                    nopasswd = names[-1].upper() == \"NOPASSWD\"\n                segment = segment[tags.end():].strip()\n            if not segment:\n                continue\n            if segment in cmd_alias:\n                for command in expand([segment], cmd_alias):\n                    commands.append((\" \".join(command.split()), nopasswd))\n            else:\n                commands.append((\" \".join(segment.split()), nopasswd))\n        if not commands:\n            skipped += 1\n            continue\n        rules += 1\n        for user in users:\n            profile = profiles.setdefault(\n                user, {\"cmds\": {}, \"runas\": set(), \"rules\": 0})\n            profile[\"rules\"] += 1\n            profile[\"runas\"].update(runas)\n            for command, flag in commands:\n                profile[\"cmds\"][command] = profile[\"cmds\"].get(command, False) or flag\n    return profiles, rules, includes, skipped\n\n\ndef covered(command, paths):\n    \"\"\"True when a command is already inside one of the granted paths.\"\"\"\n    if \"ALL\" in paths:\n        return True\n    return any(fnmatch.fnmatchcase(command, path)\n               for path in paths if \"*\" in path or \"?\" in path)\n\n\ndef covered_nopasswd(command, profile):\n    \"\"\"True when a command is already granted without a password.\"\"\"\n    for path, flag in profile[\"cmds\"].items():\n        if not flag:\n            continue\n        if path == \"ALL\" or path == command:\n            return True\n        if (\"*\" in path or \"?\" in path) and fnmatch.fnmatchcase(command, path):\n            return True\n    return False\n\n\ndef display(command):\n    \"\"\"Show a command path, never its arguments.\"\"\"\n    return command if command == \"ALL\" else command.split()[0]\n\n\ndef compare(before, after):\n    \"\"\"Return (widenings, removed users, narrowed command count).\"\"\"\n    widenings = []\n    seen = set()\n\n    def note(kind, user, detail):\n        key = (kind, user, detail)\n        if key not in seen:\n            seen.add(key)\n            widenings.append(key)\n\n    removed = sorted(set(before) - set(after))\n    narrowed = 0\n    for user in sorted(after):\n        profile = after[user]\n        if user not in before:\n            note(\"ADDED\", user, \"new user\")\n            continue\n        old = before[user]\n        old_paths = set(old[\"cmds\"])\n        for command in sorted(profile[\"cmds\"]):\n            flag = profile[\"cmds\"][command]\n            if command in old[\"cmds\"]:\n                if flag and not old[\"cmds\"][command]:\n                    note(\"NOPASSWD\", user, display(command))\n            elif not covered(command, old_paths):\n                if flag:\n                    note(\"NOPASSWD\", user, display(command))\n                elif \"*\" in command or \"?\" in command:\n                    note(\"WILDCARD\", user, display(command))\n                else:\n                    note(\"WIDEN\", user, display(command))\n            elif flag and not covered_nopasswd(command, old):\n                note(\"NOPASSWD\", user, display(command))\n        if \"ALL\" in profile[\"runas\"] and \"ALL\" not in old[\"runas\"]:\n            note(\"RUNAS\", user, \"ALL\")\n        for command in old[\"cmds\"]:\n            if command not in profile[\"cmds\"]:\n                narrowed += 1\n    return widenings, removed, narrowed\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Compare two sudoers snapshots for wider privileges.\")\n    parser.add_argument(\"before\", help=\"sudoers snapshot before the change\")\n    parser.add_argument(\"after\", help=\"sudoers snapshot after the change\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print the widenings as a JSON list\")\n    args = parser.parse_args(argv)\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        before_text = read_text(args.before)\n        after_text = read_text(args.after)\n        before, before_rules, before_includes, before_skipped = parse_grants(\n            before_text)\n        after, after_rules, after_includes, after_skipped = parse_grants(\n            after_text)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    for path, includes, skipped in ((args.before, before_includes, before_skipped),\n                                    (args.after, after_includes, after_skipped)):\n        if includes:\n            print(\"NOTE %s: %d include directive(s) not followed\"\n                  % (path, len(includes)), file=sys.stderr)\n        if skipped:\n            print(\"NOTE %s: %d line(s) not understood and skipped\"\n                  % (path, skipped), file=sys.stderr)\n    if not before_rules or not after_rules:\n        print(\"ERROR no sudoers rules found in %s\"\n              % (args.before if not before_rules else args.after),\n              file=sys.stderr)\n        return 2\n\n    widenings, removed, narrowed = compare(before, after)\n    if args.json:\n        print(json.dumps([{\"kind\": kind, \"user\": user, \"detail\": detail}\n                          for kind, user, detail in widenings], indent=2))\n        return 1 if widenings else 0\n    for kind, user, detail in widenings:\n        print(\"%-8s %s %s\" % (kind, user, detail))\n    for user in removed:\n        print(\"REMOVED  %s\" % user)\n    if widenings:\n        print(\"FINDING %d widening(s) in %s\" % (len(widenings), args.after))\n        return 1\n    print(\"OK no widening (%d user(s) removed, %d command(s) narrowed)\"\n          % (len(removed), narrowed))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p587","n":"587","slug":"the-certificate-lifecycle","title":"The certificate lifecycle","sections":["it","devsecops"],"topic":"Security & hardening","tags":["TLS","PKI","DevSecOps","CloudNative","CyberSecurity"],"series_name":"PythonFixes","design":"architecture","script":"","date":"2030-06-27","files":{"it":"notes/p587-the-certificate-lifecycle.md","devsecops":"notes/p587-the-certificate-lifecycle.md"},"md":"# The certificate lifecycle\n\nA certificate is not a file you create once and forget. It is a loop with three tiers, and each tier has its own way to fail.\n\n## Quick reference\n\n- **Issue:** CSR (CSR) · Private CA (CA)\n- **Deploy:** Secret store (K8S) · Ingress (TLS)\n- **Renew:** Expiry monitor (MON) · Rotator (JOB)\n\n**Request flow:** CSR → Private CA → Secret store → Ingress → Expiry monitor → Rotator → CSR\n\n## Explained\n\n### Issue\n\nA service generates a CSR with the names it will serve, and a private CA signs it with a short lifetime. The failure here is a CSR with the wrong subject or a CA that signs for a year because a default was never changed. The diagram starts at the CSR and ends at the CA.\n\n### Deploy\n\nThe signed certificate and its key go into a secret store. The ingress mounts the pair and terminates TLS. The failure here is a secret that holds the key but not the intermediate, or a pod that keeps the old secret after the update. The arrow runs from the CA to the secret, then to the ingress.\n\n### Renew\n\nAn expiry monitor watches the certificate the ingress serves, not the one in the repository. When the date comes inside the window, a rotator requests a new certificate and redeploys the secret. The loop closes from the monitor to the rotator to the CSR.\n\n> **The honest caveat:** automation removes the calendar but adds a new failure. A rotator with the wrong permissions can replace a valid certificate with a broken one, so the deploy still needs the bundle check.\n\n> **The lesson:** issue, deploy, renew: three tiers, one loop, no forgotten date.\n\n## In one line\n\n> **A certificate is a loop, not a file.** Every tier has a date, an owner and a way to go stale.\n\n**Check the dates**\n\n```bash\npython3 cert_bundle_check.py chain.pem\n```\n"},{"id":"p588","n":"588","slug":"the-certificate-and-key-cheat-sheet","title":"The certificate and key cheat sheet","sections":["it","devsecops"],"topic":"Security & hardening","tags":["OpenSSL","TLS","DevSecOps","Linux","CyberSecurity"],"series_name":"PythonFixes","design":"cheatsheet","script":"","date":"2030-06-30","files":{"it":"notes/p588-the-certificate-and-key-cheat-sheet.md","devsecops":"notes/p588-the-certificate-and-key-cheat-sheet.md"},"md":"# The certificate and key cheat sheet\n\nA certificate alert gives you four questions: what is in the file, does it verify, what format is it in, and which key belongs to it.\n\n## Quick reference\n\n### Inspect\n\n```bash\nopenssl x509 -in cert.pem -noout -text\nopenssl x509 -in cert.pem -noout -dates\nopenssl x509 -in cert.pem -noout -subject\nopenssl x509 -in cert.pem -noout -issuer\nopenssl s_client -connect api:443\n```\n\n### Verify\n\n```bash\nopenssl verify -CAfile root.pem leaf.pem\nopenssl verify -untrusted mid.pem leaf.pem\nopenssl verify -purpose sslserver leaf.pem\nopenssl verify -x509_strict leaf.pem\n```\n\n### Convert\n\n```bash\nopenssl x509 -in cert.pem -outform der\nopenssl x509 -inform der -in cert.der\nopenssl pkcs12 -in bundle.p12 -nokeys\nopenssl pkcs12 -in bundle.p12 -nocerts\nopenssl pkcs8 -topk8 -nocrypt -in key.pem\n```\n\n### Keys\n\n```bash\nopenssl genpkey -algorithm RSA\nopenssl req -new -key key.pem -out req.csr\nopenssl req -in req.csr -noout -text\nssh-keygen -t ed25519 -C \"ops@host\"\nssh-keygen -y -f ~/.ssh/id_ed25519\n```\n\n## Explained\n\n### Inspect\n\n`openssl x509 -in cert.pem -noout -text` prints the subject, issuer, dates, SANs and extensions. -dates, -subject and -issuer give one field each when the full text is too much. `openssl s_client -connect api:443` shows what the server sends, often a different file from the one you edited.\n\n### Verify\n\n`openssl verify -CAfile root.pem leaf.pem` builds the path from the trust anchor. -untrusted `mid.pem` supplies the intermediate when it is not in the trust store. -purpose sslserver checks the certificate for that role, and -x509_strict disables the legacy workarounds.\n\n### Convert\n\n`openssl x509 -in cert.pem -outform der` and -inform der -in cert.der move between PEM and DER. The pkcs12 commands extract certificates with -nokeys and keys with -nocerts. `openssl pkcs8 -topk8 -nocrypt -in key.pem` converts a key to the format some services demand.\n\n### Keys\n\n`openssl genpkey -algorithm RSA` creates a key; `openssl req -new -key key.pem -out req.csr` creates the request, and `openssl req -in req.csr -noout -text` reads it back. `ssh-keygen -t ed25519 -C \"ops@host\"` makes an SSH key, and `ssh-keygen -y -f ~/.ssh/id_ed25519` prints its public half.\n\n> **The honest caveat:** -text prints the public key and the signature, never the private key. The command that prints private key material is pkcs12 -nodes, so keep it out of a shared terminal.\n\n> **The lesson:** read the file before you renew it, and read the command before you run it.\n\n## In one line\n\n> **Read the file before you renew it.** Four blocks that turn a certificate alert into a command.\n\n**Verify the chain**\n\n```bash\nopenssl verify -CAfile root.pem leaf.pem\n```\n"},{"id":"p589","n":"589","slug":"four-identity-failures","title":"Four identity failures","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Identity","CyberSecurity","TLS","DevSecOps","AccessControl"],"series_name":"PythonFixes","design":"explainer","script":"","date":"2030-07-02","files":{"it":"notes/p589-four-identity-failures.md","devsecops":"notes/p589-four-identity-failures.md"},"md":"# Four identity failures\n\nFour failures, one shape: a credential that outlived the decision that created it.\n\n## Quick reference\n\n### Expired certificate\n\n- The monitor watched the repository copy\n- Clients fail closed on the date\n- Renew 21 days before notAfter\n\n### Stale key\n\n- Rotation added a key, not removed one\n- The old key still decrypts\n- Test revocation, not just creation\n\n### Leaver access\n\n- HR closed the ticket, not the account\n- A login came after the last day\n- Join leavers to the access export\n\n### Old password\n\n- No expiry and no owner\n- One hash copied across hosts\n- Age is a prompt, not proof\n\n> **Note:** Four failures, one shape: a credential that outlived the decision that created it.\n\n## Explained\n\n### Expired certificate\n\nThe date passed quietly. Clients fail closed, the error names the host, and the first instinct is to blame the client. The monitor was watching a copy in a repository, not the certificate the ingress served. Renew three weeks before the date and check the served file, not the source.\n\n### Stale key\n\nRotation created a second key, and the first one kept working. Two copies, one revocation that was never tested, and a decrypt path nobody owned. A key is not rotated until the old one is deleted and the service proves it cannot use it. Test the revocation, not the creation.\n\n### Leaver access\n\nHR closed the ticket and the account stayed active. The last login came after the last day, and no export joined the two facts. This is not a phishing problem; it is a join that nobody ran. Run the audit on the leaver event, then again from cron.\n\n### Old password\n\nNo expiry, no owner, one hash copied across hosts, and a wiki page that remembers it. Age is a prompt, not proof: a fresh password can be shared and an old one can be strong. Age is still the cheapest signal you have.\n\n> **The honest caveat:** none of these is caught by a vulnerability scanner, because each one is a valid credential used at the wrong time or by the wrong person.\n\n> **The lesson:** identity fails quietly, on a date.\n\n## In one line\n\n> **Nothing is broken; it is only old.** Each failure is a valid credential used at the wrong time.\n\n**Join the exports**\n\n```bash\npython3 offboarding_audit.py\n```\n"},{"id":"p590","n":"590","slug":"9-checks-before-an-access-review","title":"9 checks before an access review","sections":["it","devsecops"],"topic":"Security & hardening","tags":["AccessControl","CyberSecurity","Identity","DevSecOps","Compliance"],"series_name":"PythonFixes","design":"checklist","script":"","date":"2030-07-04","files":{"it":"notes/p590-9-checks-before-an-access-review.md","devsecops":"notes/p590-9-checks-before-an-access-review.md"},"md":"# 9 checks before an access review\n\nAn access review is a meeting with a spreadsheet, and the spreadsheet is usually the weakest part. Nine checks in three groups decide whether the meeting can produce a decision.\n\n## Quick reference\n\n### People\n\n- **Leavers list is current** — HR export, dated\n- **Movers are included** — promotion, transfer\n- **Contractors are listed** — agency and temp\n\n### Systems\n\n- **Every system is named** — SSO, cloud, local\n- **Service accounts owned** — one owner per account\n- **Tokens and keys counted** — not just passwords\n\n### Evidence\n\n- **Last login is real** — system log, not HR\n- **Reviewer signs off** — name and date\n- **Fixes have tickets** — one per revocation\n\n## Explained\n\n### People\n\nThe leavers list is current: an HR export with a date, not a memory of who left. Movers are included, because a promotion changes what an account can reach. Contractors are listed too; an agency account with no end date is a leaver waiting to happen.\n\n### Systems\n\nEvery system is named: SSO, cloud, local, and the admin console nobody put in the inventory. Service accounts have an owner, because an account with no owner has no one to disable it. Tokens and keys are counted alongside passwords; a token that outlives its owner is the same finding with a longer date.\n\n### Evidence\n\nLast login comes from the system, not from HR. The reviewer signs off with a name and a date, so the decision has an author. Every fix has a ticket, one per revocation, or the review becomes a document that nobody closes.\n\n> **The honest caveat:** a review is a snapshot. Between two reviews, a leaver can keep a token that no export shows, which is why the offboarding audit runs on the event and not only here.\n\n> **The lesson:** an access review is only as good as the join behind it.\n\n## In one line\n\n> **Access review starts with a join.** A review without the leaver join is a list of names.\n\n**Run the join**\n\n```bash\npython3 offboarding_audit.py\n```\n"},{"id":"p591","n":"591","slug":"quiz-3-access-questions","title":"Quiz: 3 access questions","sections":["it","devsecops"],"topic":"Security & hardening","tags":["CyberSecurity","Quiz","Identity","DevSecOps","TLS"],"series_name":"PythonFixes","design":"quiz","script":"","date":"2030-07-07","files":{"it":"notes/p591-quiz-3-access-questions.md","devsecops":"notes/p591-quiz-3-access-questions.md"},"md":"# Quiz: 3 access questions\n\nThree questions come back in every access review, and each one is a join or a date rather than a scanner.\n\n## Quick reference\n\n**Q1. Why can a certificate be valid and still fail?**\n\n- a) Its chain is incomplete or out of order\n- b) Its key is shorter than 2048 bits\n- c) The CA has no OCSP responder\n\n> **Answer:** a) Its chain is incomplete or out of order. The leaf date is one check. A missing or misordered intermediate, or a key that does not match, fails the handshake while every date is still in range.\n\n**Q2. What must a leaver audit join?**\n\n- a) HR leavers to access exports\n- b) Password hashes to expiry dates\n- c) Certificates to private keys\n\n> **Answer:** a) HR leavers to access exports. HR knows the last day and the systems know the account. Neither file is a finding on its own; the gap between them is the access that survived.\n\n**Q3. Why is sudo drift a security event?**\n\n- a) Privilege grows without a change request\n- b) sudo becomes slower after edits\n- c) It changes file ownership\n\n> **Answer:** a) Privilege grows without a change request. A new NOPASSWD line or a wildcard turns a scoped grant into full root. The change usually arrives as a small favour and is never reviewed as a privilege change.\n\n## Explained\n\n### Question one\n\nWhy can a certificate be valid and still fail? The leaf date is one check. A missing or misordered intermediate, or a key that does not match, fails the handshake while every date is still in range. The browser error will name the host, not the tier. A bundle can be ordered wrong and still contain a valid leaf.\n\n### Question two\n\nWhat must a leaver audit join? HR knows the last day and the systems know the account. Neither file is a finding on its own; the gap between them is the access that survived. An active flag and a recent login are two different findings. A review that reads one file is a guess.\n\n### Question three\n\nWhy is sudo drift a security event? A new NOPASSWD line or a wildcard turns a scoped grant into full root. The change usually arrives as a small favour and is never reviewed as a privilege change. The diff is the review that catches it.\n\n> **The honest caveat:** a quiz cannot tell you which account is exposed. Run the audit on the real export, take the exit code, and read the lines it prints. A clean exit only means the files it read were clean.\n\n> **The lesson:** the date, the join and the diff are the whole review.\n\n## In one line\n\n> **Valid is not the same as safe.** Three answers that decide whether the review can be trusted.\n\n**Check the exit**\n\n```bash\npython3 cert_bundle_check.py; echo $?\n```\n"},{"id":"p592","n":"592","slug":"why-is-this-pod-pending","title":"Why is this pod Pending?","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","Python","SRE","DevOps","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"k8s_pending.py","date":"2030-07-09","files":{"devsecops":"notes/p592-why-is-this-pod-pending.md"},"md":"# Why is this pod Pending?\n\nIt is 02:40 and checkout has been Pending for eleven minutes. There is no log to read: no container started.\n\n## Quick reference\n\nThe full script is at the end of this note: `k8s_pending.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 k8s_pending.py /tmp/pods.json /tmp/events.json\n```\n\n**Example output**\n\n```text\nPENDING checkout-7f9c2 cpu\nALERT 1 pending\n```\n\n## Explained\n\n### What it does\n\nThe script reads two exports: kubectl get pods -A -o json and kubectl get events -A -o json. For every Pending pod it finds the FailedScheduling event and maps the message to cpu, memory, taint, pvc, affinity, unknown or no event. It prints one PENDING line per pod and a count, and exits 1 while anything is Pending.\n\n### What the screen shows\n\nThe 14 visible lines index the events, collect the Pending pods, pull each scheduler message and test five tokens. On the dirty fixture they print PENDING checkout-7f9c2 cpu, then ALERT 1 pending.\n\n### How to run it\n\n`python3 k8s_pending.py /tmp/pods.json /tmp/events.json`, after saving both exports. Run it every five minutes; 0 is clean, 1 is Pending, 2 is bad input.\n\n### The fix\n\nFor cpu or memory, raise the request or add capacity; do not lower it to fit. For a taint, add a toleration only if the workload belongs there. For pvc, check the StorageClass and access mode. For affinity, read the selector first.\n\n### Prevention\n\nAlert when a pod has been Pending for more than ten minutes. A new pod is not an incident; a stuck one is.\n\n> **The honest caveat:** a quota or a webhook can block a pod before the scheduler, and neither writes FailedScheduling. A missing event is a clue, not a verdict.\n\n> **The lesson:** the scheduler writes the reason; the pod cannot.\n\n## In one line\n\n> **Pending is a schedule problem.** The event names the tier; the pod only reports the symptom.\n\n**Read the event**\n\n```bash\nkubectl describe pod -n prod checkout-7f9c2\n```\n\n## Script: k8s_pending.py\n\nDownload: [scripts/k8s_pending.py](../scripts/k8s_pending.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Explain why pods are Pending from a kubectl JSON dump and its events.\n\nReads two read-only exports:\n\n    kubectl get pods -A -o json > /tmp/pods.json\n    kubectl get events -A -o json > /tmp/events.json\n\nFor every pod whose status.phase is Pending it finds the FailedScheduling\nevent and reports one short reason:\n\n    cpu        the scheduler reports insufficient CPU\n    memory     the scheduler reports insufficient memory\n    taint      a node taint is not tolerated by the pod\n    pvc        a PersistentVolumeClaim is unbound or missing\n    affinity   node affinity, pod affinity or a node selector excludes nodes\n    unknown    a scheduling event exists but names none of the above\n    no event   no FailedScheduling event in the dump (a brand-new pod, or\n               one blocked before the scheduler, such as a quota or an\n               admission webhook)\n\nUsage:\n    python3 k8s_pending.py\n    python3 k8s_pending.py /tmp/pods.json /tmp/events.json\n    kubectl get pods -A -o json | python3 k8s_pending.py - /tmp/events.json\n    python3 k8s_pending.py /tmp/pods.json /tmp/events.json --verbose\n\nExit codes: 0 no Pending pods, 1 at least one Pending pod, 2 usage,\nunreadable or invalid input.\n\nCI (fail the pipeline while anything is stuck):\n    kubectl get pods -A -o json > /tmp/pods.json\n    kubectl get events -A -o json > /tmp/events.json\n    python3 k8s_pending.py /tmp/pods.json /tmp/events.json || exit 1\n\nCron (every five minutes, page on a new stuck pod):\n    */5 * * * * kubectl get events -A -o json > /tmp/events.json && kubectl get pods -A -o json | python3 /opt/s0sta/k8s_pending.py - /tmp/events.json\n\nRead-only: it parses two local files (or stdin) and never calls the API\nserver, so it is safe against production. The reason is only as good as\nthe event dump: a Pending pod can be blocked by a quota or a webhook that\nnever writes FailedScheduling, and an old event can describe a condition\nthat has since changed. Pair the output with kubectl describe pod.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nCPU = (\"insufficient cpu\",)\nMEMORY = (\"insufficient memory\",)\nTAINT = (\"taint\", \"untolerated taint\")\nPVC = (\"persistentvolumeclaim\", \"unbound\", \"volume\", \"pvc\")\nAFFINITY = (\"affinity\", \"selector\", \"didn't match\", \"did not match\")\n\n\nclass PendingError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    \"\"\"Read one JSON document from a path or '-' for stdin.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise PendingError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise PendingError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef as_dict(value, label):\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise PendingError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise PendingError(\"%s must be a list\" % label)\n    return value\n\n\ndef items_of(doc, label):\n    \"\"\"Return the item list from a List, or wrap a single object.\"\"\"\n    if not isinstance(doc, dict):\n        raise PendingError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        return as_list(items, \"%s items\" % label)\n    if \"metadata\" in doc or \"kind\" in doc:\n        return [doc]\n    raise PendingError(\"%s has no items list and no metadata\" % label)\n\n\ndef kind_of(obj):\n    return str(obj.get(\"kind\") or \"\").lower()\n\n\ndef pod_identity(pod, number):\n    metadata = as_dict(pod.get(\"metadata\"), \"pod %d metadata\" % number)\n    name = metadata.get(\"name\")\n    if not isinstance(name, str) or not name:\n        raise PendingError(\"pod %d has no metadata.name\" % number)\n    namespace = metadata.get(\"namespace\") or \"default\"\n    if not isinstance(namespace, str):\n        raise PendingError(\"pod %d has a non-string namespace\" % number)\n    uid = metadata.get(\"uid\")\n    return namespace, name, uid if isinstance(uid, str) else \"\"\n\n\ndef pod_phase(pod, number):\n    status = as_dict(pod.get(\"status\"), \"pod %d status\" % number)\n    phase = status.get(\"phase\") or \"\"\n    if not isinstance(phase, str):\n        raise PendingError(\"pod %d status.phase must be a string\" % number)\n    return phase\n\n\ndef event_pod(event, number):\n    \"\"\"Return (namespace, name, uid, reason, message) for one event.\"\"\"\n    if not isinstance(event, dict):\n        raise PendingError(\"event %d is not an object\" % number)\n    involved = as_dict(event.get(\"involvedObject\"),\n                       \"event %d involvedObject\" % number)\n    if kind_of(involved) not in (\"\", \"pod\"):\n        return None\n    name = involved.get(\"name\")\n    if not isinstance(name, str) or not name:\n        return None\n    namespace = involved.get(\"namespace\") or \"default\"\n    uid = involved.get(\"uid\") if isinstance(involved.get(\"uid\"), str) else \"\"\n    reason = event.get(\"reason\") or \"\"\n    message = event.get(\"message\") or event.get(\"note\") or \"\"\n    if not isinstance(reason, str) or not isinstance(message, str):\n        raise PendingError(\"event %d reason/message must be strings\" % number)\n    return namespace, name, uid, reason, message\n\n\ndef same_pod(event, namespace, name, uid):\n    e_ns, e_name, e_uid, _reason, _message = event\n    if e_ns != namespace or e_name != name:\n        return False\n    if uid and e_uid:\n        return uid == e_uid\n    return True\n\n\ndef classify(message):\n    \"\"\"Map one scheduler message to a short reason label.\"\"\"\n    text = message.lower()\n    if any(token in text for token in CPU):\n        return \"cpu\"\n    if any(token in text for token in MEMORY):\n        return \"memory\"\n    if any(token in text for token in TAINT):\n        return \"taint\"\n    if any(token in text for token in PVC):\n        return \"pvc\"\n    if any(token in text for token in AFFINITY):\n        return \"affinity\"\n    return \"unknown\"\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Explain Pending pods from kubectl pods and events JSON.\",\n        epilog=\"Exit codes: 0 no Pending pods, 1 Pending found, 2 usage or \"\n               \"input error.\")\n    parser.add_argument(\"pods\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get pods -A -o json output, or - for stdin\")\n    parser.add_argument(\"events\", nargs=\"?\", default=None,\n                        help=\"kubectl get events -A -o json output (optional)\")\n    parser.add_argument(\"-v\", \"--verbose\", action=\"store_true\",\n                        help=\"print the scheduler message for each pod\")\n    args = parser.parse_args(argv)\n\n    try:\n        pods = items_of(read_json(args.pods), \"pod dump\")\n        events = []\n        if args.events:\n            events = items_of(read_json(args.events), \"event dump\")\n        parsed_events = [event_pod(e, i) for i, e in enumerate(events, 1)]\n        parsed_events = [e for e in parsed_events if e is not None]\n        parsed_events = [e for e in parsed_events if e[3] == \"FailedScheduling\"]\n\n        pending = []\n        for number, pod in enumerate(pods, 1):\n            if not isinstance(pod, dict):\n                raise PendingError(\"pod %d is not an object\" % number)\n            if pod_phase(pod, number) != \"Pending\":\n                continue\n            namespace, name, uid = pod_identity(pod, number)\n            messages = [e[4] for e in parsed_events\n                        if same_pod(e, namespace, name, uid)]\n            reason = \"no event\"\n            detail = \"\"\n            if messages:\n                labels = [classify(m) for m in messages]\n                reason = next((r for r in labels if r != \"unknown\"), \"unknown\")\n                detail = next((m for m, r in zip(messages, labels)\n                               if r == reason), messages[0])\n            pending.append((namespace, name, reason, detail))\n    except PendingError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not pending:\n        print(\"OK %d pods scheduled\" % len(pods))\n        return 0\n\n    for namespace, name, reason, detail in pending:\n        print(\"PENDING %s/%s %s\" % (namespace, name, reason))\n        if args.verbose and detail:\n            print(\"  %s\" % detail)\n\n    print(\"ALERT %d pending\" % len(pending))\n    if args.events is None:\n        print(\"NOTE no events dump; reasons are unknown\", file=sys.stderr)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p593","n":"593","slug":"an-eviction-report","title":"An eviction report","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","SRE","Python","CloudNative","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"k8s_evictions.py","date":"2030-07-11","files":{"devsecops":"notes/p593-an-eviction-report.md"},"md":"# An eviction report\n\nAt 03:10 six pods on one node are Evicted. The app team sees restarts; the node knows better.\n\n## Quick reference\n\nThe full script is at the end of this note: `k8s_evictions.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 k8s_evictions.py /tmp/events.json\n```\n\n**Example output**\n\n```text\nEVICTED ip-10-0-12-44 memory 8\nALERT 15 evictions\n```\n\n## Explained\n\n### What it does\n\nThe script reads the output of kubectl get events -A -o json. It keeps reason Evicted, takes the node from source.host and classifies the message as memory, disk, pid or other. It groups the count by node and reason, ranks the nodes and exits 1.\n\n### What the screen shows\n\nThe 14 visible lines build a counter, skip every non-Evicted event, read the resource after 'resource: ' and reduce ephemeral-storage to disk. On the fixture they print EVICTED ip-10-0-12-44 memory 8, then ALERT 15 evictions.\n\n### How to run it\n\n`python3 k8s_evictions.py /tmp/events.json`, or pipe kubectl get events -A -o json into it. Exit 0 is clean, 1 is any eviction, 2 is bad input.\n\n### The fix\n\nFor memory, right-size the requests and limits and check for a leak; then reserve enough for the kubelet. For disk, set ephemeral-storage limits, prune images and check the kubelet eviction threshold. For pid, find the fork leak before adding processes.\n\n### Prevention\n\nRun it daily and keep the report. A node that evicts twice in a week is undersized, not unlucky.\n\n> **The honest caveat:** events expire after an hour by default, so a dump from yesterday proves nothing about last night. Collect them on a schedule or send them to a log store.\n\n> **The lesson:** the node ran out before the pod did.\n\n## In one line\n\n> **The node ran out before the pod did.** Evictions are a capacity signal, not a pod bug.\n\n**Rank the nodes**\n\n```bash\npython3 k8s_evictions.py /tmp/events.json\n```\n\n## Script: k8s_evictions.py\n\nDownload: [scripts/k8s_evictions.py](../scripts/k8s_evictions.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank pod evictions by node and by the resource that ran out.\n\nReads one read-only events export:\n\n    kubectl get events -A -o json > /tmp/events.json\n    python3 k8s_evictions.py /tmp/events.json\n\nEvery event with reason Evicted is grouped by the node that evicted the\npod and by the resource named in the message:\n\n    memory     the node was low on memory (the usual 3 a.m. page)\n    disk       ephemeral-storage, nodefs or imagefs was low\n    pid        the node ran out of process IDs\n    other      an eviction whose message names none of the above\n\nNodes are ranked by total evictions, then by reason. A node that evicts\nthree workloads in a week is not unlucky; it is undersized or leaking.\n\nUsage:\n    python3 k8s_evictions.py\n    python3 k8s_evictions.py /tmp/events.json\n    kubectl get events -A -o json | python3 k8s_evictions.py -\n    python3 k8s_evictions.py /tmp/events.json --verbose\n\nExit codes: 0 no evictions, 1 at least one eviction, 2 usage, unreadable\nor invalid input.\n\nCI (fail a weekly capacity review):\n    kubectl get events -A -o json > /tmp/events.json\n    python3 k8s_evictions.py /tmp/events.json || exit 1\n\nCron (daily at 07:10, after the night batch):\n    10 7 * * * kubectl get events -A -o json | python3 /opt/s0sta/k8s_evictions.py -\n\nRead-only: it reads one local file or stdin and never calls the API\nserver. The node name comes from event.source.host, which the kubelet\nsets on eviction events; an event without it is counted under unknown\nrather than guessed. Evictions are only a symptom: check the node's\nMemoryPressure or DiskPressure condition and its allocatable headroom.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom collections import Counter, defaultdict\n\n\nclass EvictionError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise EvictionError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise EvictionError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef as_dict(value, label):\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise EvictionError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise EvictionError(\"%s must be a list\" % label)\n    return value\n\n\ndef items_of(doc, label):\n    if not isinstance(doc, dict):\n        raise EvictionError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        return as_list(items, \"%s items\" % label)\n    if \"metadata\" in doc or \"kind\" in doc:\n        return [doc]\n    raise EvictionError(\"%s has no items list and no metadata\" % label)\n\n\ndef classify(message):\n    text = message.lower()\n    if \"memory\" in text or \"oom\" in text:\n        return \"memory\"\n    if (\"storage\" in text or \"disk\" in text or \"nodefs\" in text\n            or \"imagefs\" in text):\n        return \"disk\"\n    if \"pid\" in text:\n        return \"pid\"\n    return \"other\"\n\n\ndef parse_event(event, number):\n    \"\"\"Return (node, reason, namespace, pod) for an Evicted event.\"\"\"\n    if not isinstance(event, dict):\n        raise EvictionError(\"event %d is not an object\" % number)\n    if event.get(\"reason\") != \"Evicted\":\n        return None\n    source = as_dict(event.get(\"source\"), \"event %d source\" % number)\n    node = source.get(\"host\") or \"unknown\"\n    if not isinstance(node, str):\n        raise EvictionError(\"event %d source.host must be a string\" % number)\n    message = event.get(\"message\") or \"\"\n    if not isinstance(message, str):\n        raise EvictionError(\"event %d message must be a string\" % number)\n    involved = as_dict(event.get(\"involvedObject\"),\n                       \"event %d involvedObject\" % number)\n    pod = involved.get(\"name\") or \"?\"\n    namespace = involved.get(\"namespace\") or \"default\"\n    return node, classify(message), str(namespace), str(pod)\n\n\ndef plural(count, one, many):\n    return one if count == 1 else many\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank pod evictions by node and by memory, disk or pid.\",\n        epilog=\"Exit codes: 0 no evictions, 1 evictions found, 2 usage or \"\n               \"input error.\")\n    parser.add_argument(\"events\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get events -A -o json output, or - for stdin\")\n    parser.add_argument(\"-v\", \"--verbose\", action=\"store_true\",\n                        help=\"list the evicted pods under each row\")\n    args = parser.parse_args(argv)\n\n    try:\n        events = items_of(read_json(args.events), \"event dump\")\n        totals = Counter()\n        pods = defaultdict(list)\n        for number, event in enumerate(events, 1):\n            parsed = parse_event(event, number)\n            if parsed is None:\n                continue\n            node, reason, namespace, pod = parsed\n            totals[(node, reason)] += 1\n            pods[(node, reason)].append(\"%s/%s\" % (namespace, pod))\n    except EvictionError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not totals:\n        print(\"OK no evictions in %d %s\" % (\n            len(events), \"event\" if len(events) == 1 else \"events\"))\n        return 0\n\n    per_node = Counter()\n    for (node, _reason), count in totals.items():\n        per_node[node] += count\n    ordered = sorted(totals.items(),\n                     key=lambda item: (-per_node[item[0][0]], item[0][0],\n                                       -item[1], item[0][1]))\n    for (node, reason), count in ordered:\n        print(\"EVICTED %s %s %d\" % (node, reason, count))\n        if args.verbose:\n            for pod in sorted(pods[(node, reason)]):\n                print(\"  %s\" % pod)\n\n    total = sum(totals.values())\n    print(\"ALERT %d %s on %d %s\" % (\n        total, plural(total, \"eviction\", \"evictions\"),\n        len(per_node), plural(len(per_node), \"node\", \"nodes\")))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p594","n":"594","slug":"an-image-pull-failure-report","title":"An image pull failure report","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","Docker","Python","DevOps","Troubleshooting"],"series_name":"PythonFixes","design":"code","script":"image_pull_errors.py","date":"2030-07-14","files":{"devsecops":"notes/p594-an-image-pull-failure-report.md"},"md":"# An image pull failure report\n\nAt 14:05 a deploy turns every new pod into ImagePullBackOff. The application is fine; it never started.\n\n## Quick reference\n\nThe full script is at the end of this note: `image_pull_errors.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 image_pull_errors.py /tmp/events.json\n```\n\n**Example output**\n\n```text\nPULL ghcr.io/acme/api:v1 auth 6\nALERT 9 failed pulls\n```\n\n## Explained\n\n### What it does\n\nThe script reads events, keeps Failed, ErrImagePull and ImagePullBackOff, pulls the image out of the message and classifies auth, notfound, timeout or other. It groups by image and reason and flags every registry outside Docker Hub.\n\n### What the screen shows\n\nThe 14 visible lines skip unrelated events, lowercase the message, test auth and timeout, extract the image and count the pair. On the fixture they print PULL ghcr.io/acme/api:v1 auth 6, then ALERT 9 failed pulls.\n\n### How to run it\n\n`python3 image_pull_errors.py /tmp/events.json`. Exit 0 is clean, 1 is any pull failure, 2 is bad input.\n\n### The fix\n\nFor auth, create the imagePullSecret in the workload namespace and attach it to the service account. For notfound, confirm the tag or digest exists. For timeout, test the registry from a node and check the proxy and the egress rule.\n\n### Prevention\n\nPull the exact digest from a node network in CI before the rollout, and alert when a pod sits in ImagePullBackOff for two minutes.\n\n> **The honest caveat:** a single timeout can be noise, and an event is evidence of a past failure, not the current registry state. Confirm with kubectl describe pod before editing a manifest.\n\n> **The lesson:** the registry said no; the pod stayed silent.\n\n## In one line\n\n> **The registry said no, the pod stayed silent.** A pull failure is a credential or network problem, not a code problem.\n\n**Check the secret**\n\n```bash\nkubectl get pod -n prod api -o yaml\n```\n\n## Script: image_pull_errors.py\n\nDownload: [scripts/image_pull_errors.py](../scripts/image_pull_errors.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Group image pull failures by image and by the reason in the event.\n\nReads one read-only events export:\n\n    kubectl get events -A -o json > /tmp/events.json\n    python3 image_pull_errors.py /tmp/events.json\n\nIt keeps events with reason Failed, ErrImagePull or ImagePullBackOff,\npulls the image name out of the message and classifies the cause. A\nFailed event is kept only when its message mentions pull or image, so a\nFailedMount or FailedCreate is not counted as a registry problem:\n\n    auth       unauthorized, pull access denied, no basic auth credentials\n    notfound   manifest unknown, repository or image does not exist\n    timeout    i/o timeout, TLS handshake, connection refused, no such host\n    other      a pull failure whose message fits none of the above\n\nImages are counted per reason. The summary also counts registries other\nthan Docker Hub, because that is where a missing imagePullSecret usually\nhides.\n\nUsage:\n    python3 image_pull_errors.py\n    python3 image_pull_errors.py /tmp/events.json\n    kubectl get events -A -o json | python3 image_pull_errors.py -\n    python3 image_pull_errors.py /tmp/events.json --verbose\n\nExit codes: 0 no pull failures, 1 at least one failure, 2 usage,\nunreadable or invalid input.\n\nCI (fail the deploy check when an image cannot be pulled):\n    kubectl get events -A -o json > /tmp/events.json\n    python3 image_pull_errors.py /tmp/events.json || exit 1\n\nCron (daily at 07:20, after the night rollout):\n    20 7 * * * kubectl get events -A -o json | python3 /opt/s0sta/image_pull_errors.py -\n\nRead-only: it reads one local file or stdin and never calls the API\nserver. An event is evidence, not proof: the registry may have recovered,\nand a transient timeout can appear once and never again. Check the pod's\nimagePullSecrets and the registry status before changing a manifest.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\nfrom collections import Counter\n\n\nclass PullError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\nREASONS = (\"Failed\", \"ErrImagePull\", \"ImagePullBackOff\")\nAUTH = (\"unauthorized\", \"authentication required\", \"no basic auth\",\n        \"pull access denied\", \"access denied\", \"denied:\", \"forbidden\")\nNOTFOUND = (\"not found\", \"manifest unknown\", \"does not exist\",\n            \"no such image\", \"repository does not exist\", \"name unknown\",\n            \"invalid reference\")\nTIMEOUT = (\"timeout\", \"timed out\", \"connection refused\", \"no such host\",\n           \"network is unreachable\", \"tls handshake\", \"i/o timeout\",\n           \"context deadline exceeded\", \"dial tcp\")\nPUBLIC = (\"docker.io\", \"index.docker.io\", \"registry-1.docker.io\")\nIMAGE_RE = re.compile(r'image \"([^\"]+)\"')\n\n\ndef read_json(source):\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise PullError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise PullError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef as_dict(value, label):\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise PullError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise PullError(\"%s must be a list\" % label)\n    return value\n\n\ndef items_of(doc, label):\n    if not isinstance(doc, dict):\n        raise PullError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        return as_list(items, \"%s items\" % label)\n    if \"metadata\" in doc or \"kind\" in doc:\n        return [doc]\n    raise PullError(\"%s has no items list and no metadata\" % label)\n\n\ndef classify(message):\n    text = message.lower()\n    if any(token in text for token in AUTH):\n        return \"auth\"\n    if any(token in text for token in NOTFOUND):\n        return \"notfound\"\n    if any(token in text for token in TIMEOUT):\n        return \"timeout\"\n    return \"other\"\n\n\ndef image_of(message):\n    match = IMAGE_RE.search(message)\n    if match:\n        return match.group(1)\n    return \"?\"\n\n\ndef registry_of(image):\n    \"\"\"Return the registry host, or docker.io for a short Docker Hub name.\"\"\"\n    if \"/\" not in image:\n        return \"docker.io\"\n    first = image.split(\"/\", 1)[0]\n    if \".\" in first or \":\" in first or first == \"localhost\":\n        return first\n    return \"docker.io\"\n\n\ndef parse_event(event, number):\n    if not isinstance(event, dict):\n        raise PullError(\"event %d is not an object\" % number)\n    if event.get(\"reason\") not in REASONS:\n        return None\n    message = event.get(\"message\") or \"\"\n    if not isinstance(message, str):\n        raise PullError(\"event %d message must be a string\" % number)\n    text = message.lower()\n    if event.get(\"reason\") == \"Failed\" and \"pull\" not in text \\\n            and \"image\" not in text:\n        return None\n    return image_of(message), classify(message)\n\n\ndef plural(count, one, many):\n    return one if count == 1 else many\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Group image pull failures by image, cause and registry.\",\n        epilog=\"Exit codes: 0 no failures, 1 failures found, 2 usage or \"\n               \"input error.\")\n    parser.add_argument(\"events\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get events -A -o json output, or - for stdin\")\n    parser.add_argument(\"-v\", \"--verbose\", action=\"store_true\",\n                        help=\"print one line per registry as well\")\n    args = parser.parse_args(argv)\n\n    try:\n        events = items_of(read_json(args.events), \"event dump\")\n        counts = Counter()\n        for number, event in enumerate(events, 1):\n            parsed = parse_event(event, number)\n            if parsed is None:\n                continue\n            counts[parsed] += 1\n    except PullError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not counts:\n        print(\"OK no image pull failures in %d %s\" % (\n            len(events), \"event\" if len(events) == 1 else \"events\"))\n        return 0\n\n    registries = Counter()\n    for (image, _reason), count in counts.items():\n        if image != \"?\":\n            registries[registry_of(image)] += count\n    external = {name: count for name, count in registries.items()\n                if name not in PUBLIC}\n\n    for (image, reason), count in sorted(\n            counts.items(), key=lambda item: (-item[1], item[0][0],\n                                              item[0][1])):\n        print(\"PULL %s %s %d\" % (image, reason, count))\n        if args.verbose:\n            print(\"  registry %s\" % registry_of(image))\n    if args.verbose:\n        for name, count in sorted(external.items(),\n                                  key=lambda item: (-item[1], item[0])):\n            print(\"EXTERNAL %s %d\" % (name, count))\n\n    total = sum(counts.values())\n    summary = \"ALERT %d failed %s\" % (total, plural(total, \"pull\", \"pulls\"))\n    if external:\n        summary += \", %d external %s\" % (\n            len(external), plural(len(external), \"registry\", \"registries\"))\n    print(summary)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p595","n":"595","slug":"a-node-pressure-report","title":"A node pressure report","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","SRE","Linux","Python","CloudNative"],"series_name":"PythonFixes","design":"code","script":"node_pressure.py","date":"2030-07-16","files":{"devsecops":"notes/p595-a-node-pressure-report.md"},"md":"# A node pressure report\n\nAt 04:00 the pager fires for three evictions. The dashboard shows healthy nodes; kubectl describe node shows MemoryPressure True.\n\n## Quick reference\n\nThe full script is at the end of this note: `node_pressure.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 node_pressure.py /tmp/nodes.json\n```\n\n**Example output**\n\n```text\nNODE ip-10-0-12-44 MemoryPressure\nALERT 1 of 9 nodes under pressure\n```\n\n## Explained\n\n### What it does\n\nThe script reads the output of kubectl get nodes -o json, builds each condition map and flags MemoryPressure, DiskPressure, PIDPressure and NotReady. For every flagged node it prints allocatable against capacity for cpu and memory, then exits 1.\n\n### What the screen shows\n\nThe 14 visible lines read the conditions, keep the three pressure types and print each flagged node plus the cluster count. On the fixture they print NODE ip-10-0-12-44 MemoryPressure, then ALERT 1 of 9 nodes under pressure.\n\n### How to run it\n\n`python3 node_pressure.py /tmp/nodes.json`. Use `--all` to print healthy nodes too; 0 is clean, 1 is pressure, 2 is bad input.\n\n### The fix\n\nFor memory, find the top consumer and set requests and limits; then add capacity or raise the kubelet reserve. For disk, prune images and set ephemeral-storage limits. For pid, find the process leak.\n\n### Prevention\n\nRun it every ten minutes and alert on a condition transition, not on the next eviction. Keep the last report for the capacity review, because the condition clears before the next page.\n\n> **The honest caveat:** allocatable is a ceiling, not live usage. A node can be 97% allocated and still have free memory, and kubectl top needs metrics-server to show the difference.\n\n> **The lesson:** pressure is a node condition, not a guess.\n\n## In one line\n\n> **Pressure is a node condition, not a guess.** The kubelet says it before the evictions do.\n\n**Check the node**\n\n```bash\nkubectl describe node ip-10-0-12-44\n```\n\n## Script: node_pressure.py\n\nDownload: [scripts/node_pressure.py](../scripts/node_pressure.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report node pressure conditions and allocatable headroom from nodes JSON.\n\nReads one read-only nodes export:\n\n    kubectl get nodes -o json > /tmp/nodes.json\n    python3 node_pressure.py /tmp/nodes.json\n\nFor every node it reads status.conditions and flags:\n\n    MemoryPressure   the kubelet is evicting pods to reclaim memory\n    DiskPressure     ephemeral-storage or imagefs is low\n    PIDPressure      the node is short of process IDs\n    NotReady         the Ready condition is False or Unknown\n\nFor a flagged node it also prints allocatable against capacity for CPU\nand memory. Allocatable is the pod-facing ceiling; the gap to capacity\nis what the kubelet and the system daemons reserve.\n\nUsage:\n    python3 node_pressure.py\n    python3 node_pressure.py /tmp/nodes.json\n    kubectl get nodes -o json | python3 node_pressure.py -\n    python3 node_pressure.py /tmp/nodes.json --all\n\nExit codes: 0 no node under pressure, 1 at least one node under\npressure, 2 usage, unreadable or invalid input.\n\nCI (fail a cluster health check):\n    kubectl get nodes -o json > /tmp/nodes.json\n    python3 node_pressure.py /tmp/nodes.json || exit 1\n\nCron (every ten minutes, alert before the evictions start):\n    */10 * * * * kubectl get nodes -o json | python3 /opt/s0sta/node_pressure.py -\n\nRead-only: it reads one local file or stdin and never calls the API\nserver. Capacity and allocatable are static limits, not live usage: a\nnode can be 97% allocated and still have free memory, or look empty and\nbe one cache away from pressure. Use kubectl top node for utilisation.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass PressureError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\nCPU_SUFFIX = {\"n\": 1e-9, \"u\": 1e-6, \"m\": 1e-3}\nMEM_SUFFIX = {\n    \"Ki\": 1024, \"Mi\": 1024 ** 2, \"Gi\": 1024 ** 3, \"Ti\": 1024 ** 4,\n    \"Pi\": 1024 ** 5, \"K\": 1000, \"M\": 1000 ** 2, \"G\": 1000 ** 3,\n    \"T\": 1000 ** 4, \"P\": 1000 ** 5,\n}\n\n\ndef read_json(source):\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise PressureError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise PressureError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef as_dict(value, label):\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise PressureError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise PressureError(\"%s must be a list\" % label)\n    return value\n\n\ndef items_of(doc, label):\n    if not isinstance(doc, dict):\n        raise PressureError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        return as_list(items, \"%s items\" % label)\n    if \"metadata\" in doc or \"kind\" in doc:\n        return [doc]\n    raise PressureError(\"%s has no items list and no metadata\" % label)\n\n\ndef cpu_cores(text):\n    \"\"\"Parse a Kubernetes CPU quantity into cores, or None.\"\"\"\n    if not isinstance(text, str) or not text:\n        return None\n    for suffix, factor in CPU_SUFFIX.items():\n        if text.endswith(suffix):\n            try:\n                return float(text[:-len(suffix)]) * factor\n            except ValueError:\n                return None\n    try:\n        return float(text)\n    except ValueError:\n        return None\n\n\ndef mem_bytes(text):\n    \"\"\"Parse a Kubernetes memory quantity into bytes, or None.\"\"\"\n    if not isinstance(text, str) or not text:\n        return None\n    for suffix in sorted(MEM_SUFFIX, key=len, reverse=True):\n        if text.endswith(suffix):\n            try:\n                return float(text[:-len(suffix)]) * MEM_SUFFIX[suffix]\n            except ValueError:\n                return None\n    try:\n        return float(text)\n    except ValueError:\n        return None\n\n\ndef fmt_cpu(value):\n    if value is None:\n        return \"n/a\"\n    if value >= 10:\n        return \"%g\" % round(value, 1)\n    return \"%g\" % round(value, 3)\n\n\ndef fmt_mem(value):\n    if value is None:\n        return \"n/a\"\n    for suffix, size in ((\"Ti\", 1024 ** 4), (\"Gi\", 1024 ** 3),\n                         (\"Mi\", 1024 ** 2)):\n        if value >= size:\n            return \"%g%s\" % (round(value / size, 1), suffix)\n    return \"%dB\" % (round(value),)\n\n\ndef ratio(alloc, capacity):\n    if alloc is None or capacity is None or capacity <= 0:\n        return \"\"\n    return \" (%d%%)\" % round(100 * alloc / capacity)\n\n\ndef node_report(node, number, show_all):\n    if not isinstance(node, dict):\n        raise PressureError(\"node %d is not an object\" % number)\n    metadata = as_dict(node.get(\"metadata\"), \"node %d metadata\" % number)\n    name = metadata.get(\"name\")\n    if not isinstance(name, str) or not name:\n        raise PressureError(\"node %d has no metadata.name\" % number)\n    status = as_dict(node.get(\"status\"), \"node %s status\" % name)\n    conditions = as_list(status.get(\"conditions\"), \"node %s conditions\" % name)\n    state = {}\n    for index, cond in enumerate(conditions, 1):\n        if not isinstance(cond, dict):\n            raise PressureError(\"node %s condition %d is not an object\"\n                                % (name, index))\n        kind = cond.get(\"type\")\n        if not isinstance(kind, str):\n            raise PressureError(\"node %s condition %d has no type\"\n                                % (name, index))\n        state[kind] = str(cond.get(\"status\") or \"Unknown\")\n\n    reasons = [kind for kind in (\"MemoryPressure\", \"DiskPressure\", \"PIDPressure\")\n               if state.get(kind) == \"True\"]\n    ready = state.get(\"Ready\")\n    if ready != \"True\":\n        reasons.append(\"NotReady\")\n    if not reasons and not show_all:\n        return name, None\n\n    capacity = as_dict(status.get(\"capacity\"), \"node %s capacity\" % name)\n    allocatable = as_dict(status.get(\"allocatable\"),\n                          \"node %s allocatable\" % name)\n    cpu_cap = cpu_cores(capacity.get(\"cpu\"))\n    cpu_alloc = cpu_cores(allocatable.get(\"cpu\"))\n    mem_cap = mem_bytes(capacity.get(\"memory\"))\n    mem_alloc = mem_bytes(allocatable.get(\"memory\"))\n    headroom = \"ready=%s cpu %s/%s%s mem %s/%s%s\" % (\n        ready or \"Unknown\",\n        fmt_cpu(cpu_alloc), fmt_cpu(cpu_cap), ratio(cpu_alloc, cpu_cap),\n        fmt_mem(mem_alloc), fmt_mem(mem_cap), ratio(mem_alloc, mem_cap))\n    return name, (reasons, headroom)\n\n\ndef plural(count, one, many):\n    return one if count == 1 else many\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report node pressure conditions and allocatable headroom.\",\n        epilog=\"Exit codes: 0 no pressure, 1 pressure found, 2 usage or \"\n               \"input error.\")\n    parser.add_argument(\"nodes\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get nodes -o json output, or - for stdin\")\n    parser.add_argument(\"-a\", \"--all\", action=\"store_true\",\n                        help=\"print every node, including the healthy ones\")\n    args = parser.parse_args(argv)\n\n    try:\n        nodes = items_of(read_json(args.nodes), \"node dump\")\n        reports = [node_report(node, number, args.all)\n                   for number, node in enumerate(nodes, 1)]\n    except PressureError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    flagged = 0\n    for name, report in reports:\n        if report is None:\n            continue\n        reasons, headroom = report\n        if reasons:\n            flagged += 1\n        print(\"NODE %s %s\" % (name, \",\".join(reasons) or \"ok\"))\n        print(\"  %s\" % headroom)\n\n    if not flagged:\n        print(\"OK %d %s, no pressure\" % (\n            len(nodes), plural(len(nodes), \"node\", \"nodes\")))\n        return 0\n\n    print(\"ALERT %d of %d %s under pressure\" % (\n        flagged, len(nodes), plural(len(nodes), \"node\", \"nodes\")))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p596","n":"596","slug":"a-failed-job-report","title":"A failed job report","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","Python","CICD","SRE","DevOps"],"series_name":"PythonFixes","design":"code","script":"job_failures.py","date":"2030-07-18","files":{"devsecops":"notes/p596-a-failed-job-report.md"},"md":"# A failed job report\n\nOn Monday the nightly ETL is missing. The CronJob still schedules, so every dashboard is green. It failed on Friday and nothing paged.\n\n## Quick reference\n\nThe full script is at the end of this note: `job_failures.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 job_failures.py /tmp/jobs.json\n```\n\n**Example output**\n\n```text\nJOB nightly-etl failed 6\nALERT 2 failed, 1 suspended\n```\n\n## Explained\n\n### What it does\n\nThe script reads the output of kubectl get jobs,cronjobs -A -o json. It prints each failed Job with its count and terminal reason, sorts by failure count, lists suspended CronJobs and exits 1.\n\n### What the screen shows\n\nThe 14 visible lines split Jobs from CronJobs, read status.failed and spec.suspend, then print the worst Job and the summary. On the fixture they print JOB nightly-etl failed 6, then ALERT 2 failed, 1 suspended.\n\n### How to run it\n\n`python3 job_failures.py /tmp/jobs.json`. Exit 0 is clean, 1 is a failed Job or suspended CronJob, 2 is bad input.\n\n### The fix\n\nRead the failed pod's log with kubectl logs job/nightly-etl -n batch, fix the cause, then delete the Job so the next run starts clean. Deleting a Job also deletes its pods and their logs, so read them first. Confirm a suspended CronJob with its owner before resuming it.\n\n### Prevention\n\nAlert on a Job condition Failed and on spec.suspend, and check status.lastSuccessfulTime every morning. A schedule that runs is not a job that succeeds.\n\n> **The honest caveat:** a recreated Job loses its history, and an old failed Job may already be fixed. Suspended can be deliberate; confirm before resuming.\n\n> **The lesson:** a failed job is a silent outage.\n\n## In one line\n\n> **A failed job is a silent outage.** The schedule keeps running; the work does not.\n\n**Rank the jobs**\n\n```bash\npython3 job_failures.py /tmp/jobs.json\n```\n\n## Script: job_failures.py\n\nDownload: [scripts/job_failures.py](../scripts/job_failures.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank failed Jobs and list suspended CronJobs from a kubectl JSON dump.\n\nReads one read-only export of both kinds:\n\n    kubectl get jobs,cronjobs -A -o json > /tmp/jobs.json\n    python3 job_failures.py /tmp/jobs.json\n\nFor every Job it reports status.failed and the terminal condition:\n\n    BackoffLimitExceeded   the Job retried spec.backoffLimit times and gave up\n    DeadlineExceeded       activeDeadlineSeconds ran out\n    Error                  another Failed condition, or failures with no reason\n\nJobs are ranked by failure count. A Job whose failed count reached its\nbackoffLimit is marked even when the condition is missing. Suspended\nCronJobs are listed separately: a suspended schedule is a silent outage,\nnot a clean one.\n\nUsage:\n    python3 job_failures.py\n    python3 job_failures.py /tmp/jobs.json\n    kubectl get jobs,cronjobs -A -o json | python3 job_failures.py -\n    python3 job_failures.py /tmp/jobs.json --verbose\n\nExit codes: 0 no failed Job and no suspended CronJob, 1 at least one\nfinding, 2 usage, unreadable or invalid input.\n\nCI (fail the nightly check when a batch Job has failed):\n    kubectl get jobs,cronjobs -A -o json > /tmp/jobs.json\n    python3 job_failures.py /tmp/jobs.json || exit 1\n\nCron (daily at 07:30, before the morning review):\n    30 7 * * * kubectl get jobs,cronjobs -A -o json | python3 /opt/s0sta/job_failures.py -\n\nRead-only: it reads one local file or stdin and never calls the API\nserver. A failed Job is a fact, not a cause: the pod logs and the\ncontainer exit code are still the evidence. A suspended CronJob may be\ndeliberate, so confirm with the owner before resuming it.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass JobError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef read_json(source):\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise JobError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise JobError(\"%s is not JSON: %s\" % (source, exc))\n\n\ndef as_dict(value, label):\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise JobError(\"%s must be an object\" % label)\n    return value\n\n\ndef as_list(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        raise JobError(\"%s must be a list\" % label)\n    return value\n\n\ndef items_of(doc, label):\n    if not isinstance(doc, dict):\n        raise JobError(\"%s must be a JSON object\" % label)\n    items = doc.get(\"items\")\n    if items is not None:\n        return as_list(items, \"%s items\" % label)\n    if \"metadata\" in doc or \"kind\" in doc:\n        return [doc]\n    raise JobError(\"%s has no items list and no metadata\" % label)\n\n\ndef identity(obj, number):\n    if not isinstance(obj, dict):\n        raise JobError(\"object %d is not an object\" % number)\n    metadata = as_dict(obj.get(\"metadata\"), \"object %d metadata\" % number)\n    name = metadata.get(\"name\")\n    if not isinstance(name, str) or not name:\n        raise JobError(\"object %d has no metadata.name\" % number)\n    namespace = metadata.get(\"namespace\") or \"default\"\n    return str(namespace), name\n\n\ndef integer(value, label, default=0):\n    if value is None:\n        return default\n    if isinstance(value, bool) or not isinstance(value, int):\n        raise JobError(\"%s must be an integer\" % label)\n    return value\n\n\ndef condition_of(status, wanted):\n    conditions = as_list(status.get(\"conditions\"), \"status.conditions\")\n    for index, cond in enumerate(conditions, 1):\n        if not isinstance(cond, dict):\n            raise JobError(\"condition %d is not an object\" % index)\n        if cond.get(\"type\") == wanted:\n            return str(cond.get(\"status\") or \"\"), str(cond.get(\"reason\") or \"\"), \\\n                str(cond.get(\"message\") or \"\")\n    return \"\", \"\", \"\"\n\n\ndef job_record(obj, number):\n    namespace, name = identity(obj, number)\n    spec = as_dict(obj.get(\"spec\"), \"Job %s spec\" % name)\n    status = as_dict(obj.get(\"status\"), \"Job %s status\" % name)\n    failed = integer(status.get(\"failed\"), \"Job %s status.failed\" % name)\n    backoff = integer(spec.get(\"backoffLimit\"), \"Job %s spec.backoffLimit\" % name,\n                      6)\n    failed_status, reason, message = condition_of(status, \"Failed\")\n    exhausted = failed > 0 and failed >= backoff\n    if failed_status != \"True\" and not exhausted:\n        return None\n    if not reason and exhausted:\n        reason = \"BackoffLimitExceeded\"\n    return {\n        \"namespace\": namespace,\n        \"name\": name,\n        \"failed\": failed,\n        \"backoff\": backoff,\n        \"reason\": reason,\n        \"message\": message,\n    }\n\n\ndef cron_record(obj, number):\n    namespace, name = identity(obj, number)\n    spec = as_dict(obj.get(\"spec\"), \"CronJob %s spec\" % name)\n    if spec.get(\"suspend\") is not True:\n        return None\n    return {\"namespace\": namespace, \"name\": name}\n\n\ndef plural(count, one, many):\n    return one if count == 1 else many\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank failed Jobs and list suspended CronJobs.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\")\n    parser.add_argument(\"objects\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get jobs,cronjobs -A -o json output, or -\")\n    parser.add_argument(\"-v\", \"--verbose\", action=\"store_true\",\n                        help=\"print the terminal condition message\")\n    args = parser.parse_args(argv)\n\n    try:\n        items = items_of(read_json(args.objects), \"job dump\")\n        jobs = []\n        crons = []\n        for number, obj in enumerate(items, 1):\n            if not isinstance(obj, dict):\n                raise JobError(\"object %d is not an object\" % number)\n            kind = str(obj.get(\"kind\") or \"\")\n            if kind == \"Job\":\n                record = job_record(obj, number)\n                if record is not None:\n                    jobs.append(record)\n            elif kind == \"CronJob\":\n                record = cron_record(obj, number)\n                if record is not None:\n                    crons.append(record)\n    except JobError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not jobs and not crons:\n        print(\"OK no failed jobs or suspended cronjobs in %d objects\"\n              % len(items))\n        return 0\n\n    jobs.sort(key=lambda job: (-job[\"failed\"], job[\"namespace\"], job[\"name\"]))\n    for job in jobs:\n        limit = \"/%d\" % job[\"backoff\"] if job[\"backoff\"] else \"\"\n        suffix = \" %s\" % job[\"reason\"] if job[\"reason\"] else \"\"\n        print(\"JOB %s/%s failed=%d%s%s\" % (\n            job[\"namespace\"], job[\"name\"], job[\"failed\"], limit, suffix))\n        if args.verbose and job[\"message\"]:\n            print(\"  %s\" % job[\"message\"])\n    for cron in sorted(crons, key=lambda c: (c[\"namespace\"], c[\"name\"])):\n        print(\"CRONJOB %s/%s suspended\" % (cron[\"namespace\"], cron[\"name\"]))\n\n    parts = []\n    if jobs:\n        parts.append(\"%d failed %s\" % (\n            len(jobs), plural(len(jobs), \"job\", \"jobs\")))\n    if crons:\n        parts.append(\"%d suspended %s\" % (\n            len(crons), plural(len(crons), \"cronjob\", \"cronjobs\")))\n    print(\"ALERT \" + \", \".join(parts))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p597","n":"597","slug":"the-pod-failure-path-drawn","title":"The pod failure path, drawn","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","SRE","DevOps","CloudNative","Troubleshooting"],"series_name":"PythonFixes","design":"architecture","script":"","date":"2030-07-21","files":{"devsecops":"notes/p597-the-pod-failure-path-drawn.md"},"md":"# The pod failure path, drawn\n\nA pod can fail in three places, and the first failing step tells you which one.\n\n## Quick reference\n\n- **Schedule:** Scheduler (SCHED) · Node capacity (CPU) · Taints and affinity (RULE)\n- **Start:** Image pull (IMG) · Config and secret (ENV)\n- **Run:** Readiness probe (PROBE) · Limits and eviction (OOM)\n\n**Request flow:** Scheduler → Node capacity → Taints and affinity; Node capacity → Image pull → Config and secret → Readiness probe → Limits and eviction\n\n**Around it:** kubectl describe (DESC) · Events (EVT) · Exit code (EXIT)\n\n## Explained\n\n### Schedule\n\nThe first band holds the scheduler, node capacity and the taint rules. A pod that never gets a node stops here: no node fits the request, the selector or the taint. The scheduler writes the FailedScheduling event with the reason.\n\n### Start\n\nThe second band is the image and the configuration. The pod has a node, but the kubelet cannot pull the image or mount the ConfigMap or Secret. The container never starts, so there is no log again.\n\n### Run\n\nThe third band is the probe and the limits. The process starts and then exits, fails its readiness check, or is evicted when the node runs out. This is the only tier that writes application logs. Eviction is a node decision, not a container exit.\n\n### Read it in order\n\nThe poster draws the three bands top to bottom, with arrows from the scheduler to capacity, then to the taints, the image, the config, the probe and the limits. Three chips at the bottom name the tools: kubectl describe, Events and the exit code.\n\n> **The honest caveat:** a pod can fail twice, evicted on one node and then CrashLooping on the next. Fix the first tier that failed; the second may be a symptom.\n\n> **The lesson:** find the tier before the fix.\n\n## In one line\n\n> **Find the tier before the fix.** A CrashLoop is a run failure; a Pending pod never reached run.\n\n**Read the event**\n\n```bash\nkubectl describe pod -n prod api-7d9f\n```\n"},{"id":"p598","n":"598","slug":"the-kubectl-triage-cheat-sheet","title":"The kubectl triage cheat sheet","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","kubectl","SRE","DevOps","CloudNative"],"series_name":"PythonFixes","design":"cheatsheet","script":"","date":"2030-07-23","files":{"devsecops":"notes/p598-the-kubectl-triage-cheat-sheet.md"},"md":"# The kubectl triage cheat sheet\n\nA failing pod produces more output than any human can read. Four blocks keep the order, and the order matters more than the flags.\n\n## Quick reference\n\n### Pods\n\n```bash\nkubectl get pods -A -o wide\nkubectl describe pod -n prod api-7d9f\nkubectl logs -n prod api-7d9f --previous\nkubectl get pod -n prod api-7d9f -o yaml\n```\n\n### Events\n\n```bash\nkubectl get events -A -w\nkubectl get events -A | grep Evicted\nkubectl events --for pod/api-7d9f -n prod\nkubectl get events -A -o wide\n```\n\n### Nodes\n\n```bash\nkubectl get nodes -o wide\nkubectl describe node ip-10-0-12-44\nkubectl top node ip-10-0-12-44\nkubectl cordon ip-10-0-12-44\n```\n\n### Jobs\n\n```bash\nkubectl get jobs -A\nkubectl get cronjobs -A\nkubectl describe job -n batch nightly-etl\nkubectl logs job/nightly-etl -n batch\nkubectl create job y --from=cronjob/x\n```\n\n## Explained\n\n### Pods\n\nStart with the pod: phase, restarts, node and events. `kubectl get pods -A -o wide` gives the shape; kubectl describe pod prints the Events block at the end. A Pending pod has no logs, so describe comes before logs. `kubectl logs --previous` reads the container that died, and -o yaml shows the spec against the status.\n\n### Events\n\nEvents are the cluster's own account. Watch them with `kubectl get events -A -w`; grep Evicted for node pressure; `kubectl events --for` narrows the list to one pod. The newest event is usually the cause; the older ones are the aftermath.\n\n### Nodes\n\nThen the node: conditions, allocatable and current usage. describe node shows MemoryPressure; top node needs metrics-server; cordon stops new pods while you work, but it does not stop the pods already there.\n\n### Jobs\n\nA CronJob that schedules is not a Job that succeeds. `kubectl get jobs -A`, describe job for the backoff reason, logs job/nightly-etl for the last pod, create job `--from=cronjob` to retry by hand. The Job keeps the count; its pods keep the exit codes.\n\n> **The honest caveat:** `kubectl events --for` needs a recent kubectl, and kubectl top needs metrics-server. Without them, describe and the event list still work.\n\n> **The lesson:** read the event, not the dashboard.\n\n## In one line\n\n> **Read the event, not the dashboard.** The cluster writes the reason before you write a theory.\n\n**Describe first**\n\n```bash\nkubectl describe pod -n prod api-7d9f\n```\n"},{"id":"p599","n":"599","slug":"four-pod-failure-reasons","title":"Four pod failure reasons","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","SRE","DevOps","Containers","Troubleshooting"],"series_name":"PythonFixes","design":"explainer","script":"","date":"2030-07-25","files":{"devsecops":"notes/p599-four-pod-failure-reasons.md"},"md":"# Four pod failure reasons\n\nFour reasons cover most failing pods, and each one sends you to a different place. Learn the phase first; the rest is a shorter list.\n\n## Quick reference\n\n### Pending\n\n- The scheduler finds no node that fits\n- cpu, memory, taints or an unbound claim\n- No container started, so there are no logs\n- Read the FailedScheduling event\n\n### ImagePull\n\n- The registry refused or never answered\n- Missing tag, missing secret, blocked egress\n- ImagePullBackOff is a retry, not a fix\n- Check the pull secret and the tag\n\n### CrashLoop\n\n- The container starts, then exits\n- Restart count climbs, the delay doubles\n- Logs `--previous` hold the last words\n- A probe can kill a healthy process\n\n### Evicted\n\n- The node ran out of memory, disk or pid\n- The kubelet kills the pod, not the app\n- The reason names the resource\n- The controller replaces it elsewhere\n\n## Explained\n\n### Pending\n\nPending means the scheduler found no node. The request is too large, a taint is not tolerated, the claim is unbound or the affinity excludes every node. A request larger than every node's allocatable is the common case, and the event names the resource. No container started, so there are no logs.\n\n### ImagePull\n\nImagePull means the node could not get the image. The tag is missing, the pull secret is absent, or the registry is unreachable. A private registry with no imagePullSecret returns the same ErrImagePull symptom as a typo in the tag. ImagePullBackOff is a retry with backoff, not a fix.\n\n### CrashLoop\n\nCrashLoop means the process started and exited. The restart delay doubles from 10 seconds to 5 minutes. Read `kubectl logs --previous`; the container may be healthy and the probe wrong, which is why the previous log matters.\n\n### Evicted\n\nEvicted means the kubelet killed the pod to protect the node. The reason names memory, ephemeral-storage or pid. The app may be the victim, not the cause, so check the node condition before you change the app. A controller recreates it, usually on another node.\n\n> **The honest caveat:** the phase is a symptom, not a diagnosis. A pod can be Evicted and then CrashLoop on another node, and the fix belongs to the tier, not the symptom.\n\n> **The lesson:** the phase is the first clue.\n\n## In one line\n\n> **The phase is the first clue.** Four reasons, four different places to look.\n\n**Read the phase**\n\n```bash\nkubectl get pod -n prod api -o wide\n```\n"},{"id":"p600","n":"600","slug":"9-commands-to-triage-a-failing-pod","title":"9 commands to triage a failing pod","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","kubectl","SRE","DevOps","CommandLine"],"series_name":"PythonFixes","design":"terminal","script":"","date":"2030-07-28","files":{"devsecops":"notes/p600-9-commands-to-triage-a-failing-pod.md"},"md":"# 9 commands to triage a failing pod\n\nAt 03:00 a pod is not serving and the dashboard shows one red cell. Three tabs, nine commands, in the order that avoids guessing.\n\n## Quick reference\n\n### What is the phase?\n\n```bash\n# phase, restarts and node\nkubectl get pod api-7d9f -n prod -o wide\n# the Events block at the end\nkubectl describe pod api-7d9f -n prod\n# spec against status\nkubectl get pod api-7d9f -n prod -o yaml\n```\n\n### What did it say?\n\n```bash\n# only a restarted container has logs\nkubectl logs api-7d9f -n prod --previous\n# one container, thirty lines\nkubectl logs api-7d9f -n prod -c app --tail=30\n# every pod of one label\nkubectl logs -n prod -l app=api --tail=20\n```\n\n### Is the node healthy?\n\n```bash\n# conditions and allocatable\nkubectl describe node ip-10-0-12-44\n# usage against allocatable\nkubectl top node ip-10-0-12-44\n# evictions across the cluster\nkubectl get events -A --field-selector reason=Evicted\n```\n\n## Explained\n\n### Look\n\nStart with the phase. The phase is the first fact because it decides which tab can help. kubectl get pod -o wide adds restarts and the node; describe pod ends with the Events block, which names FailedScheduling, a probe failure or a backoff. kubectl get pod -o yaml shows the spec against the status, where a missing claim or a wrong image appears.\n\n### Logs\n\nLogs only exist once a container has run. `--previous` reads the container that died; -c selects one container; -l app=api merges every pod of a label. If the container never started, the log command returns a waiting error; that is a clue, not a broken command.\n\n### Node\n\nIf the pod looks fine, check the node. describe node lists conditions and allocatable; top node shows usage; get events `--field-selector reason=Evicted` finds the pods that were pushed out. The field selector keeps the list short; -o wide adds the source node, which is the one to inspect. If the node is healthy, the pod is the problem.\n\n> **The honest caveat:** `kubectl logs --previous` fails when the container never started, and kubectl top needs metrics-server. Without metrics-server, describe still shows the conditions.\n\n> **The lesson:** logs are the pod's side of the story.\n\n## In one line\n\n> **Logs are the pod's side of the story.** The pod reports a symptom; the event names the cause.\n\n**Try it**\n\n```bash\nkubectl describe pod api-7d9f -n prod | tail -15\n```\n"},{"id":"p601","n":"601","slug":"quiz-3-kubernetes-failure-questions","title":"Quiz: 3 Kubernetes failure questions","sections":["devsecops"],"topic":"Kubernetes","tags":["Kubernetes","SRE","DevOps","CloudNative","Troubleshooting"],"series_name":"PythonFixes","design":"quiz","script":"","date":"2030-07-30","files":{"devsecops":"notes/p601-quiz-3-kubernetes-failure-questions.md"},"md":"# Quiz: 3 Kubernetes failure questions\n\nThree questions separate a kubectl guess from a diagnosis, and all three come back in a review.\n\n## Quick reference\n\n**Q1. Why does a Pending pod have no logs?**\n\n- a) It never started a container\n- b) The log driver is disabled\n- c) The pod is on an unscheduled node\n\n> **Answer:** a) It never started a container. A Pending pod has not been placed on a node, so no image was pulled and no process ran. The evidence is the FailedScheduling event, not a log.\n\n**Q2. What does an eviction reason tell you?**\n\n- a) Which node resource ran out\n- b) Which container crashed\n- c) Which probe failed\n\n> **Answer:** a) Which node resource ran out. Evicted means the kubelet killed the pod to protect the node. The message names memory, storage or pid, which points at capacity or a leak, not at the app.\n\n**Q3. Why does CrashLoop backoff grow?**\n\n- a) To give the node time to recover\n- b) To slow a failing restart loop\n- c) To wait for the image pull\n\n> **Answer:** b) To slow a failing restart loop. Each restart doubles the wait from 10 seconds up to 5 minutes. It protects the kubelet and the network, and a fast fix still waits for the current backoff.\n\n## Explained\n\n### Question one\n\nWhy does a Pending pod have no logs? Because it never started a container. The scheduler could not place it, so no image was pulled and no process ran. The FailedScheduling event is the evidence; a log search on a Pending pod returns nothing, and that nothing is expected.\n\n### Question two\n\nWhat does an eviction reason tell you? Which node resource ran out. Evicted means the kubelet killed the pod to protect the node, and the message names memory, ephemeral-storage or pid. It is a node-level event, not an application error, so the pod's exit code is not the place to look. The node, not the app, is the first suspect.\n\n### Question three\n\nWhy does CrashLoop backoff grow? To slow a failing restart loop. Each restart doubles the wait from 10 seconds up to 5 minutes. It protects the kubelet and the network, and a fast fix still waits for the current backoff. The delay resets only after a stable run, not after a fix.\n\n> **The honest caveat:** backoff and eviction are both protection mechanisms, one for the restart loop and one for the node. Neither fixes the underlying fault, so the fault is still yours to find.\n\n> **The lesson:** know the phase, skip the guess.\n\n## In one line\n\n> **Know the phase, skip the guess.** Three answers that save a wrong kubectl command.\n\n**Read the phase**\n\n```bash\nkubectl get pod -n prod api -o wide\n```\n"},{"id":"p602","n":"602","slug":"did-the-deploy-actually-land","title":"Did the deploy actually land?","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","CICD","DevOps","Python","Deployment"],"series_name":"PythonFixes","design":"code","script":"deploy_verify.py","date":"2030-08-01","files":{"devsecops":"notes/p602-did-the-deploy-actually-land.md"},"md":"# Did the deploy actually land?\n\nA release goes green at 21:40 and the on-call engineer closes the tab. At 22:05 a customer sees last week's price, because one pod still runs the old image and the Service keeps sending it traffic.\n\n## Quick reference\n\nThe full script is at the end of this note: `deploy_verify.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 deploy_verify.py /tmp/pods.json release.json\n```\n\n**Example output**\n\n```text\nFINDING 2 pod problems\nALERT ready 2 of 3\n```\n\n## Explained\n\n### What it does\n\n`deploy_verify.py` reads kubectl get pods -o json and a release file with image, digest, replicas and max_age_minutes. It strips the tag, compares the digest with each container's imageID, counts pods that are Running with every container Ready, and flags pods older than the window. A stale pod is one the rollout never replaced.\n\n### What the screen shows\n\nThe fourteen lines load pods and the digest, then walk each pod. They add its Ready bit, strip the Z from creationTimestamp, mark stale after 30 minutes and compare the digest. The last lines print FINDING 2 pod problems, then ALERT ready 2 of 3, and exit 1.\n\n### How to run it\n\nkubectl get pods -n prod -o json > `/tmp/pods.json`, then `python3 deploy_verify.py /tmp/pods.json release.json`. Run it after the rollout; exit 1 fails the job.\n\n### The fix\n\nDelete the stale pod and re-apply the digest.\n\n### Prevention\n\nRun the check on a ten-minute cron during the rollout. A deploy is finished when the pods say so.\n\n> **The honest caveat:** creationTimestamp is the pod's age, not the container's, so a restarted container can look stale. Run the check inside the rollout window; a stable release is old by design.\n\n> **The lesson:** verify what runs, not what was sent.\n\n## In one line\n\n> **A green build is not a deploy.** The pod says what is running; the pipeline only says what it sent.\n\n**Check the digest:** crane digest ghcr.io/acme/api:v2\n\n## Script: deploy_verify.py\n\nDownload: [scripts/deploy_verify.py](../scripts/deploy_verify.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Verify that a deployed release is actually running.\n\nReads a pod dump and a small release file, then checks the three things a\ngreen pipeline cannot tell you:\n\n  * the running image is the expected repository, on the expected digest\n  * enough pods are Ready, not merely Running\n  * every pod is younger than the release window, so no stale pod is still\n    serving traffic from the previous ReplicaSet\n\nThe release file is JSON:\n\n    {\"image\": \"ghcr.io/acme/api\",\n     \"digest\": \"sha256:0000000000000000000000000000000000000000000000000000000000000000\",\n     \"replicas\": 3,\n     \"max_age_minutes\": 30,\n     \"namespace\": \"prod\"}\n\nimage is the repository, without a tag. digest is the sha256 the release\nshould be running. replicas is the number of Ready pods expected. namespace\nand max_age_minutes are optional; the defaults are all namespaces and 60\nminutes. The digest is compared against status.containerStatuses[].imageID,\nwhich is what the runtime pulled, not what the manifest asked for.\n\nUsage:\n    kubectl get pods -n prod -o json > /tmp/pods.json\n    python3 deploy_verify.py /tmp/pods.json release.json\n    kubectl get pods -n prod -o json | python3 deploy_verify.py - release.json\n    python3 deploy_verify.py /tmp/pods.json release.json --max-age-minutes 15\n\nExit codes: 0 verified, 1 mismatch or stale pod, 2 usage or unreadable,\nmissing or invalid input.\n\nCI (after a rollout, fail the job when the pods do not match):\n    kubectl get pods -n prod -o json > /tmp/pods.json\n    python3 deploy_verify.py /tmp/pods.json release.json || exit 1\n\nCron (every ten minutes during a rollout):\n    */10 * * * * kubectl get pods -n prod -o json | python3 /opt/s0sta/deploy_verify.py - /opt/s0sta/release.json\n\nThe check is read-only: one file or stdin, no API calls, no writes. It reads\nwhat the kubelet reported. It does not prove the application works; a smoke\ntest with an assertion does that. A container restart keeps the pod's old\ncreationTimestamp, so a restarted pod can look older than the release.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\nfrom datetime import datetime, timedelta, timezone\n\nDIGEST_RE = re.compile(r\"^sha256:[0-9a-f]{64}$\")\n\n\nclass VerifyError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise VerifyError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise VerifyError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef pod_items(doc):\n    \"\"\"Return the pod list from a List, a single Pod or a plain list.\"\"\"\n    if isinstance(doc, list):\n        return doc\n    if not isinstance(doc, dict):\n        raise VerifyError(\"the pods file must be a JSON object or list\")\n    items = doc.get(\"items\")\n    if items is not None:\n        if not isinstance(items, list):\n            raise VerifyError(\"pods.items must be a list\")\n        return items\n    if doc.get(\"kind\") == \"Pod\" or (\"metadata\" in doc and \"status\" in doc):\n        return [doc]\n    raise VerifyError(\"the pods file has no items list and is not a Pod\")\n\n\ndef release_spec(doc):\n    \"\"\"Validate and return (image, digest, replicas, max_age, namespace).\"\"\"\n    if not isinstance(doc, dict):\n        raise VerifyError(\"the release file must be a JSON object\")\n    image = doc.get(\"image\")\n    digest = doc.get(\"digest\")\n    replicas = doc.get(\"replicas\")\n    max_age = doc.get(\"max_age_minutes\", 60)\n    namespace = doc.get(\"namespace\")\n    if not isinstance(image, str) or not image.strip():\n        raise VerifyError(\"release.image must be a non-empty string\")\n    if not isinstance(digest, str) or not DIGEST_RE.match(digest.lower()):\n        raise VerifyError(\"release.digest must be sha256:<64 hex characters>\")\n    if isinstance(replicas, bool) or not isinstance(replicas, int) or replicas < 0:\n        raise VerifyError(\"release.replicas must be a non-negative integer\")\n    if isinstance(max_age, bool) or not isinstance(max_age, int) or max_age <= 0:\n        raise VerifyError(\"release.max_age_minutes must be a positive integer\")\n    if namespace is not None and (not isinstance(namespace, str) or not namespace):\n        raise VerifyError(\"release.namespace must be a non-empty string\")\n    return image.strip(), digest.lower(), replicas, max_age, namespace\n\n\ndef image_repo(ref):\n    \"\"\"Return the repository part of an image reference, without tag or digest.\"\"\"\n    ref = ref.split(\"@\", 1)[0]\n    slash = ref.rfind(\"/\")\n    colon = ref.rfind(\":\")\n    if colon > slash:\n        return ref[:colon]\n    return ref\n\n\ndef parse_time(value, label):\n    \"\"\"Parse a Kubernetes RFC 3339 timestamp into an aware UTC datetime.\"\"\"\n    if not isinstance(value, str) or not value:\n        raise VerifyError(\"%s is missing\" % label)\n    try:\n        parsed = datetime.fromisoformat(value.replace(\"Z\", \"+00:00\"))\n    except ValueError:\n        raise VerifyError(\"%s is not a timestamp: %s\" % (label, value))\n    if parsed.tzinfo is None:\n        parsed = parsed.replace(tzinfo=timezone.utc)\n    return parsed.astimezone(timezone.utc)\n\n\ndef as_dict(value, label):\n    \"\"\"Return a mapping, raising on the wrong shape.\"\"\"\n    if value is None:\n        return {}\n    if not isinstance(value, dict):\n        raise VerifyError(\"%s must be an object\" % label)\n    return value\n\n\ndef pod_report(pod, number, expected_repo, digest, max_age, namespace, now):\n    \"\"\"Return (ready, problems) for one pod, printing one FAIL line each.\"\"\"\n    metadata = as_dict(pod.get(\"metadata\"), \"pod %d metadata\" % number)\n    name = metadata.get(\"name\") or \"pod-%d\" % number\n    if namespace and metadata.get(\"namespace\") != namespace:\n        return None, 0\n    status = as_dict(pod.get(\"status\"), \"pod %s status\" % name)\n    containers = status.get(\"containerStatuses\")\n    if containers is None:\n        containers = []\n    if not isinstance(containers, list):\n        raise VerifyError(\"pod %s containerStatuses must be a list\" % name)\n    created = parse_time(metadata.get(\"creationTimestamp\"),\n                         \"pod %s creationTimestamp\" % name)\n    age = now - created\n    problems = 0\n    for index, container in enumerate(containers, 1):\n        container = as_dict(container, \"pod %s container %d\" % (name, index))\n        image = container.get(\"image\")\n        image_id = container.get(\"imageID\")\n        if not isinstance(image, str) or image_repo(image) != expected_repo:\n            print(\"FAIL pod %s image %s\" % (\n                name, image if isinstance(image, str) else \"missing\"))\n            problems += 1\n        elif not isinstance(image_id, str) or digest not in image_id:\n            print(\"FAIL pod %s digest\" % name)\n            problems += 1\n    if age > timedelta(minutes=max_age):\n        print(\"FAIL pod %s stale %dm\" % (name, int(age.total_seconds() // 60)))\n        problems += 1\n    ready = (status.get(\"phase\") == \"Running\" and bool(containers)\n             and all(c.get(\"ready\") is True for c in containers))\n    return ready, problems\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Verify running pods against an expected image digest, \"\n                    \"replica count and age window.\",\n        epilog=\"Exit codes: 0 verified, 1 mismatch, 2 usage or input error.\")\n    parser.add_argument(\"pods\", help=\"kubectl get pods -o json output, or - for stdin\")\n    parser.add_argument(\"release\", help=\"release JSON with image, digest and replicas\")\n    parser.add_argument(\"--max-age-minutes\", type=int, default=None,\n                        help=\"override release.max_age_minutes (default 60)\")\n    args = parser.parse_args(argv)\n\n    try:\n        image, digest, replicas, max_age, namespace = release_spec(\n            load_json(args.release, \"release file\"))\n        if args.max_age_minutes is not None:\n            if args.max_age_minutes <= 0:\n                raise VerifyError(\"--max-age-minutes must be positive\")\n            max_age = args.max_age_minutes\n        now = datetime.now(timezone.utc)\n        expected_repo = image_repo(image)\n        ready_count = 0\n        problems = 0\n        for number, pod in enumerate(pod_items(load_json(args.pods, \"pods file\")), 1):\n            if not isinstance(pod, dict):\n                raise VerifyError(\"pod %d is not an object\" % number)\n            ready, pod_problems = pod_report(\n                pod, number, expected_repo, digest, max_age, namespace, now)\n            if ready is None:\n                continue\n            if ready:\n                ready_count += 1\n            problems += pod_problems\n    except VerifyError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    short = digest[:19]\n    if problems:\n        print(\"FINDING %d pod problems\" % problems)\n    if ready_count != replicas:\n        print(\"ALERT ready %d of %d\" % (ready_count, replicas))\n        return 1\n    if problems:\n        return 1\n    print(\"OK %d of %d ready on %s\" % (ready_count, replicas, short))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p603","n":"603","slug":"a-missing-artifact-check","title":"A missing artifact check","sections":["devsecops"],"topic":"Python scripts","tags":["CICD","SupplyChain","DevOps","Python","Artifacts"],"series_name":"PythonFixes","design":"code","script":"artifact_missing.py","date":"2030-08-04","files":{"devsecops":"notes/p603-a-missing-artifact-check.md"},"md":"# A missing artifact check\n\nThe upload step is skipped after a retry, the deploy finds a manifest and downloads nothing, and the job stays green for eleven days. Every step that could have caught it was skipped by a condition.\n\n## Quick reference\n\nThe full script is at the end of this note: `artifact_missing.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 artifact_missing.py release.json /srv/releases/1.4.2\n```\n\n**Example output**\n\n```text\nFINDING 2 bad artifacts\nALERT b.bin, c.bin\n```\n\n## Explained\n\n### What it does\n\n`artifact_missing.py` reads a manifest of artifacts: path, size and sha256 for each file. A relative path is resolved against the manifest directory or the root you pass. It checks that each file exists, that the byte count matches and that the hash matches, then exits 1 with the bad names.\n\n### What the screen shows\n\nThe fourteen lines load the list, take the root and walk each entry. A missing file is appended and skipped. The rest are read at once, then size and hash are checked in one condition. The last two lines print FINDING 2 bad artifacts, then ALERT b.bin, c.bin.\n\n### How to run it\n\n`python3 artifact_missing.py release.json /srv/releases/1.4.2`. Exit 0 means every artifact matches, 1 means one is missing or corrupt, 2 means the manifest is wrong.\n\n### The fix\n\nRe-upload the missing file and rebuild the corrupt one. Do not edit the manifest to match the disk, because the manifest is what should exist.\n\n### Prevention\n\nGenerate the manifest from the artifacts at build time and make the deploy step require a clean result.\n\n> **The honest caveat:** a matching hash proves the bytes match the manifest, not that the build is reproducible or signed. A manifest generated from the same corrupt file agrees with itself.\n\n> **The lesson:** a release is a manifest plus the bytes it names.\n\n## In one line\n\n> **No file, no release.** A manifest is a promise; the hash is the proof.\n\n**Check one file**\n\n```bash\nsha256sum -c SHA256SUMS\n```\n\n## Script: artifact_missing.py\n\nDownload: [scripts/artifact_missing.py](../scripts/artifact_missing.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check every artifact in a release manifest: present, sized, hashed.\n\nReads a release manifest, resolves each artifact path, and checks the file\nagainst the manifest before anybody deploys it:\n\n    exists        a missing file is the first failure\n    size          the byte count must match, so a truncated upload fails\n    sha256        the digest must match, so a swapped or corrupt file fails\n\nThe manifest is JSON:\n\n    {\"artifacts\": [\n      {\"path\": \"api-1.4.2.tar.gz\", \"size\": 1048576,\n       \"sha256\": \"9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08\"},\n      {\"path\": \"sbom.cdx.json\", \"size\": 4096,\n       \"sha256\": \"...\"}]}\n\n\"files\" is accepted as an alias for \"artifacts\". A relative path is resolved\nagainst the manifest directory, or against the root argument when one is\ngiven:\n\n    python3 artifact_missing.py release.json\n    python3 artifact_missing.py release.json /srv/releases/1.4.2\n\nUsage:\n    python3 artifact_missing.py release.json /srv/releases/1.4.2\n    python3 artifact_missing.py release.json --help\n\nExit codes: 0 every artifact matches, 1 at least one is missing or corrupt,\n2 usage, or a manifest that is missing, unreadable, invalid or empty.\n\nCI (before the deploy step, fail the job when an artifact is bad):\n    python3 artifact_missing.py release.json /srv/releases/1.4.2 || exit 1\n\nCron (verify the release bucket after a copy):\n    30 3 * * * python3 /opt/s0sta/artifact_missing.py /srv/releases/current/release.json\n\nThe script is read-only. It reads the manifest and each artifact, hashes the\nbytes and writes nothing. A matching hash proves the file matches the\nmanifest; it does not prove the build is reproducible, signed or safe. Pair\nit with a signature check and keep the manifest beside the artifact.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport hashlib\nimport json\nimport os\nimport sys\n\nCHUNK = 1024 * 1024\nHEX = set(\"0123456789abcdef\")\n\n\nclass ManifestError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_manifest(path):\n    \"\"\"Return (manifest_dir, artifact list) or raise ManifestError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise ManifestError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ManifestError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, dict):\n        raise ManifestError(\"the manifest must be a JSON object\")\n    items = doc.get(\"artifacts\")\n    if items is None:\n        items = doc.get(\"files\")\n    if not isinstance(items, list) or not items:\n        raise ManifestError(\"the manifest has no non-empty artifacts list\")\n    return os.path.dirname(os.path.abspath(path)), items\n\n\ndef artifact_fields(item, number):\n    \"\"\"Validate one artifact entry and return (path, size, sha256).\"\"\"\n    if not isinstance(item, dict):\n        raise ManifestError(\"artifact %d is not an object\" % number)\n    path = item.get(\"path\")\n    size = item.get(\"size\")\n    digest = item.get(\"sha256\")\n    if not isinstance(path, str) or not path:\n        raise ManifestError(\"artifact %d has no path\" % number)\n    if isinstance(size, bool) or not isinstance(size, int) or size < 0:\n        raise ManifestError(\"artifact %s has no non-negative size\" % path)\n    if not isinstance(digest, str) or len(digest) != 64 or any(\n            char not in HEX for char in digest.lower()):\n        raise ManifestError(\"artifact %s has no sha256 hex digest\" % path)\n    return path, size, digest.lower()\n\n\ndef sha256_file(path):\n    \"\"\"Return the hex sha256 of a file, streaming it in one MiB chunks.\"\"\"\n    digest = hashlib.sha256()\n    with open(path, \"rb\") as handle:\n        while True:\n            block = handle.read(CHUNK)\n            if not block:\n                break\n            digest.update(block)\n    return digest.hexdigest()\n\n\ndef check_one(path, size, digest):\n    \"\"\"Return a FAIL reason string, or None when the artifact is good.\"\"\"\n    if not os.path.isfile(path):\n        return \"missing\"\n    try:\n        actual_size = os.path.getsize(path)\n        if actual_size != size:\n            return \"size expected %d got %d\" % (size, actual_size)\n        actual_digest = sha256_file(path)\n    except OSError as exc:\n        return \"unreadable (%s)\" % exc\n    if actual_digest != digest:\n        return \"sha256 expected %s got %s\" % (digest[:12], actual_digest[:12])\n    return None\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Verify release artifacts against a JSON manifest of \"\n                    \"path, size and sha256.\",\n        epilog=\"Exit codes: 0 all good, 1 missing or corrupt, 2 usage or input error.\")\n    parser.add_argument(\"manifest\", help=\"release manifest JSON\")\n    parser.add_argument(\"root\", nargs=\"?\",\n                        help=\"directory for relative paths (default: manifest directory)\")\n    args = parser.parse_args(argv)\n\n    try:\n        manifest_dir, items = load_manifest(args.manifest)\n        root = os.path.abspath(args.root) if args.root else manifest_dir\n        if not os.path.isdir(root):\n            raise ManifestError(\"root is not a directory: %s\" % root)\n        bad = []\n        for number, item in enumerate(items, 1):\n            rel, size, digest = artifact_fields(item, number)\n            path = rel if os.path.isabs(rel) else os.path.join(root, rel)\n            reason = check_one(path, size, digest)\n            if reason is not None:\n                if rel not in bad:\n                    bad.append(rel)\n                print(\"FAIL %s %s\" % (rel, reason))\n    except ManifestError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if bad:\n        shown = \", \".join(bad[:5])\n        if len(bad) > 5:\n            shown += \" +%d more\" % (len(bad) - 5)\n        print(\"FINDING %d bad artifacts\" % len(bad))\n        print(\"ALERT %s\" % shown)\n        return 1\n    print(\"OK %d artifacts match\" % len(items))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p604","n":"604","slug":"is-rollback-ready","title":"Is rollback ready?","sections":["devsecops"],"topic":"Python scripts","tags":["Kubernetes","SRE","DevOps","Python","Reliability"],"series_name":"PythonFixes","design":"code","script":"rollback_ready.py","date":"2030-08-06","files":{"devsecops":"notes/p604-is-rollback-ready.md"},"md":"# Is rollback ready?\n\nAt 02:10 the new release returns 500s and the runbook says kubectl rollout undo. It fails with no rollout history found, because revisionHistoryLimit was set to 0 to keep the cluster tidy. The rollback existed on paper only.\n\n## Quick reference\n\nThe full script is at the end of this note: `rollback_ready.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 rollback_ready.py /tmp/deploy.json\n```\n\n**Example output**\n\n```text\nFINDING 2 no rollback\nALERT api, worker\n```\n\n## Explained\n\n### What it does\n\n`rollback_ready.py` reads kubectl get deploy,rs -o json and checks three things per Deployment. revisionHistoryLimit must be greater than 0. The revision annotation is the last applied revision. The ReplicaSet for the previous revision must exist, scaled to zero, and no older set may still run replicas.\n\n### What the screen shows\n\nThe fourteen lines load the objects, filter the sets and define five short lambdas for name, app label, revision, limit and older sets. The bad list keeps each Deployment with a zero limit or no previous set. The last two lines print FINDING 2 no rollback, then ALERT api, worker.\n\n### How to run it\n\nkubectl get deploy,rs -n prod -o json > `/tmp/deploy.json`, then `python3 rollback_ready.py /tmp/deploy.json`. Run it before the release, not after the incident.\n\n### The fix\n\nSet revisionHistoryLimit back to 10, keep the old ReplicaSets and test one undo in staging. A limit of 0 deletes your rollback path.\n\n### Prevention\n\nReject a Deployment with fewer than five revisions in CI, and keep the old images for at least as long as the history.\n\n> **The honest caveat:** a ReplicaSet is necessary, not sufficient. The old image must still be in the registry, and a migration may not be reversible.\n\n> **The lesson:** the undo button is configuration, so test the configuration.\n\n## In one line\n\n> **A rollback needs a target, not hope.** An undo that has nothing to scale up is not a plan.\n\n**Check the history**\n\n```bash\nkubectl rollout history deploy/api\n```\n\n## Script: rollback_ready.py\n\nDownload: [scripts/rollback_ready.py](../scripts/rollback_ready.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check whether every Deployment has a usable rollback target.\n\nReads the output of `kubectl get deploy,rs -n prod -o json` and answers one\nquestion per Deployment: if this release is bad, what does `kubectl rollout\nundo` actually return to?\n\nThree things must be true:\n\n  * revisionHistoryLimit is greater than 0, so old ReplicaSets are kept\n  * the Deployment records a revision (deployment.kubernetes.io/revision)\n  * the ReplicaSet for the immediately previous revision still exists, is\n    scaled to zero, and no older ReplicaSet is still running replicas\n\nA Deployment with no annotation, revision 1, revisionHistoryLimit 0, a gap\nat the previous revision, or a previous ReplicaSet with replicas has no clean\nrollback target. A Deployment with a target prints its current and previous\nrevision.\n\nUsage:\n    kubectl get deploy,rs -n prod -o json > /tmp/deploy.json\n    python3 rollback_ready.py /tmp/deploy.json\n    kubectl get deploy,rs -n prod -o json | python3 rollback_ready.py -\n\nExit codes: 0 every Deployment is rollback ready, 1 at least one is not,\n2 usage or unreadable, missing or invalid input.\n\nCI (before the release step, fail when a Deployment cannot roll back):\n    kubectl get deploy,rs -n prod -o json > /tmp/deploy.json\n    python3 rollback_ready.py /tmp/deploy.json || exit 1\n\nCron (weekly policy check):\n    0 6 * * 1 kubectl get deploy,rs -A -o json | python3 /opt/s0sta/rollback_ready.py -\n\nThe check is read-only: one file or stdin, no API calls, no writes. A target\nReplicaSet is necessary, not sufficient: the old image must still exist in\nthe registry, and a database migration may not be reversible. The check does\nnot prove that rollback restores service.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nREVISION = \"deployment.kubernetes.io/revision\"\n\n\nclass RollbackError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source):\n    \"\"\"Read the kubectl dump from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise RollbackError(\"cannot read %s: %s\" % (source, exc))\n    except ValueError as exc:\n        raise RollbackError(\"%s is not valid JSON: %s\" % (source, exc))\n\n\ndef objects(doc):\n    \"\"\"Return a list of typed objects from a List or a split dump.\"\"\"\n    if isinstance(doc, list):\n        return doc\n    if not isinstance(doc, dict):\n        raise RollbackError(\"the dump must be a JSON object or list\")\n    if isinstance(doc.get(\"items\"), list):\n        return doc[\"items\"]\n    if isinstance(doc.get(\"deployments\"), list) or isinstance(doc.get(\"replicasets\"), list):\n        deployments = doc.get(\"deployments\") or []\n        replicasets = doc.get(\"replicasets\") or []\n        if not isinstance(deployments, list) or not isinstance(replicasets, list):\n            raise RollbackError(\"deployments and replicasets must be lists\")\n        return deployments + replicasets\n    if doc.get(\"kind\") in (\"Deployment\", \"ReplicaSet\"):\n        return [doc]\n    raise RollbackError(\"no items, deployments or replicasets in the dump\")\n\n\ndef metadata_of(obj, label):\n    \"\"\"Return metadata and annotations for one object.\"\"\"\n    metadata = obj.get(\"metadata\")\n    if not isinstance(metadata, dict):\n        raise RollbackError(\"%s has no metadata object\" % label)\n    annotations = metadata.get(\"annotations\")\n    if annotations is None:\n        annotations = {}\n    if not isinstance(annotations, dict):\n        raise RollbackError(\"%s annotations must be an object\" % label)\n    return metadata, annotations\n\n\ndef revision_of(annotations):\n    \"\"\"Return the integer revision annotation, or None when absent or bad.\"\"\"\n    value = annotations.get(REVISION)\n    if isinstance(value, int) and not isinstance(value, bool) and value > 0:\n        return value\n    if isinstance(value, str) and value.isdigit() and int(value) > 0:\n        return int(value)\n    return None\n\n\ndef owned_replicasets(deployment, replicasets):\n    \"\"\"Return (revision, replicas, name) for ReplicaSets owned by deployment.\"\"\"\n    metadata, _ = metadata_of(deployment, \"Deployment\")\n    name = metadata.get(\"name\")\n    namespace = metadata.get(\"namespace\") or \"default\"\n    owned = []\n    for index, replica in enumerate(replicasets, 1):\n        if not isinstance(replica, dict):\n            raise RollbackError(\"ReplicaSet %d is not an object\" % index)\n        meta, annotations = metadata_of(replica, \"ReplicaSet %d\" % index)\n        owners = meta.get(\"ownerReferences\")\n        if owners is None:\n            continue\n        if not isinstance(owners, list):\n            raise RollbackError(\"ReplicaSet %s ownerReferences must be a list\" % meta.get(\"name\"))\n        match = any(\n            isinstance(owner, dict)\n            and owner.get(\"kind\") == \"Deployment\"\n            and owner.get(\"name\") == name\n            for owner in owners)\n        if not match:\n            continue\n        if (meta.get(\"namespace\") or \"default\") != namespace:\n            continue\n        spec = replica.get(\"spec\")\n        if not isinstance(spec, dict):\n            raise RollbackError(\"ReplicaSet %s has no spec object\" % meta.get(\"name\"))\n        replicas = spec.get(\"replicas\", 1)\n        if isinstance(replicas, bool) or not isinstance(replicas, int) or replicas < 0:\n            raise RollbackError(\"ReplicaSet %s spec.replicas must be non-negative\" % meta.get(\"name\"))\n        owned.append((revision_of(annotations),\n                      replicas, meta.get(\"name\") or \"replicaset-%d\" % index))\n    return owned\n\n\ndef check_deployment(deployment, replicasets, number):\n    \"\"\"Return (name, reason or None, detail) for one Deployment.\"\"\"\n    metadata, annotations = metadata_of(deployment, \"Deployment %d\" % number)\n    name = metadata.get(\"name\")\n    if not isinstance(name, str) or not name:\n        raise RollbackError(\"Deployment %d has no name\" % number)\n    spec = deployment.get(\"spec\")\n    if not isinstance(spec, dict):\n        raise RollbackError(\"Deployment %s has no spec object\" % name)\n    limit = spec.get(\"revisionHistoryLimit\", 10)\n    if isinstance(limit, bool) or not isinstance(limit, int) or limit < 0:\n        raise RollbackError(\"Deployment %s revisionHistoryLimit must be non-negative\" % name)\n    current = revision_of(annotations)\n    owned = owned_replicasets(deployment, replicasets)\n    known = [(rev, replicas, rs_name) for rev, replicas, rs_name in owned if rev]\n\n    if current is None:\n        return name, \"no revision annotation\", \"?\"\n    if limit == 0:\n        return name, \"revisionHistoryLimit 0\", str(current)\n    if current < 2:\n        return name, \"revision 1 is the first\", str(current)\n    if any(rev > current for rev, _, _ in known):\n        return name, \"ReplicaSet revision ahead of Deployment\", str(current)\n    running = [(rev, replicas, rs_name) for rev, replicas, rs_name in known\n               if rev < current and replicas > 0]\n    if running:\n        rev, replicas, rs_name = sorted(running)[-1]\n        return name, \"old ReplicaSet %s has %d replicas\" % (rs_name, replicas), str(current)\n    previous = [(rev, replicas, rs_name) for rev, replicas, rs_name in known\n                if rev == current - 1]\n    if not previous:\n        older = [rev for rev, _, _ in known if rev < current]\n        if older:\n            return name, \"no ReplicaSet at revision %d\" % (current - 1), str(current)\n        return name, \"no previous ReplicaSet\", str(current)\n    return name, None, str(current)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Check Deployments for a rollback target: history limit, \"\n                    \"previous ReplicaSet and current revision.\",\n        epilog=\"Exit codes: 0 rollback ready, 1 no target, 2 usage or input error.\")\n    parser.add_argument(\"dump\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get deploy,rs -o json output, or - for stdin\")\n    args = parser.parse_args(argv)\n\n    try:\n        items = objects(load_json(args.dump))\n        deployments = [x for x in items if isinstance(x, dict)\n                       and x.get(\"kind\") == \"Deployment\"]\n        replicasets = [x for x in items if isinstance(x, dict)\n                       and x.get(\"kind\") == \"ReplicaSet\"]\n        if not deployments:\n            raise RollbackError(\"the dump has no Deployment objects\")\n        results = [check_deployment(d, replicasets, i)\n                   for i, d in enumerate(deployments, 1)]\n    except RollbackError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    bad = []\n    for name, reason, current in results:\n        if reason is None:\n            print(\"OK %s revision %s has a target\" % (name, current))\n        else:\n            print(\"FAIL %s %s\" % (name, reason))\n            bad.append(name)\n    if bad:\n        print(\"FINDING %d no rollback\" % len(bad))\n        print(\"ALERT %s\" % \", \".join(bad))\n        return 1\n    word = \"deployment has\" if len(results) == 1 else \"deployments have\"\n    print(\"OK %d %s a rollback target\" % (len(results), word))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p605","n":"605","slug":"a-ci-log-secret-leak-scan","title":"A CI log secret leak scan","sections":["devsecops"],"topic":"Python scripts","tags":["DevSecOps","CICD","Security","Python","Secrets"],"series_name":"PythonFixes","design":"code","script":"ci_log_leak.py","date":"2030-08-08","files":{"devsecops":"notes/p605-a-ci-log-secret-leak-scan.md"},"md":"# A CI log secret leak scan\n\nA debug line prints the deploy role's access key, the job passes, and the log is retained for ninety days where every engineer can read it.\n\n## Quick reference\n\nThe full script is at the end of this note: `ci_log_leak.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ci_log_leak.py /tmp/run.log\n```\n\n**Example output**\n\n```text\nLEAK 3 aws-key ****************\nLEAK 5 bearer ****************\n```\n\n## Explained\n\n### What it does\n\n`ci_log_leak.py` reads one log file or stdin and checks each line for the shapes that leak most often: AWS keys, GitHub and GitLab tokens, Slack tokens, private key headers, Bearer tokens and generic api_key, secret, token or password assignments. Each hit prints the line number, the kind and sixteen asterisks; the run exits 1 without printing the value.\n\n### What the screen shows\n\nThe fourteen lines load the log, define four patterns, then walk the lines and stop at the first match on each. The print line writes LEAK, the line number, the kind and the mask; break keeps one finding per line. On the dirty log the two lines are LEAK 3 aws-key and LEAK 5 bearer, each with the same fixed mask.\n\n### How to run it\n\n`python3 ci_log_leak.py build.log`, or pipe a job log through stdin. Put it before the upload step, so a leak fails the job instead of reaching the archive.\n\n### The fix\n\nRevoke and rotate the credential first, then remove the line and purge the log.\n\n### Prevention\n\nKeep secret masking on, never run `set -x` around a token, and scan every saved log before upload. Masking hides exact strings; a base64 copy can still print.\n\n> **The honest caveat:** pattern matching misses split, encoded and unusual secrets. A clean scan is not proof of a clean log.\n\n> **The lesson:** treat the CI log as a public artefact, because it usually is.\n\n## In one line\n\n> **A green log can still leak.** The build passed; the credential is now in the archive.\n\n**Scan one log**\n\n```bash\npython3 ci_log_leak.py build.log\n```\n\n## Script: ci_log_leak.py\n\nDownload: [scripts/ci_log_leak.py](../scripts/ci_log_leak.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Scan a CI log for likely secrets without printing the secret.\n\nReads one log file (or stdin), checks each line against the shapes that leak\nmost often in build output, and prints one masked line per hit:\n\n    LEAK 12 aws-access-key ****************\n    LEAK 40 bearer ****************\n\nThe value is never printed, not even a prefix: a short prefix is still a\nsecret. The line number and the kind are enough to find the line and rotate\nthe credential.\n\nPatterns: AWS access key IDs (AKIA/ASIA), AWS secret access keys, GitHub and\nGitLab tokens, Slack tokens, private key headers, Bearer tokens and generic\napi_key/secret/token/password assignments. The match is only the trigger; the\nline text is discarded.\n\nUsage:\n    python3 ci_log_leak.py build.log\n    cat build.log | python3 ci_log_leak.py\n    gh run view 123456 --log > /tmp/run.log\n    python3 ci_log_leak.py /tmp/run.log\n\nExit codes: 0 no likely secret, 1 at least one finding, 2 usage or an\nunreadable input.\n\nCI (scan a saved job log before it is uploaded):\n    python3 ci_log_leak.py build.log || exit 1\n\nCron (scan yesterday's deploy log):\n    15 5 * * * python3 /opt/s0sta/ci_log_leak.py /var/log/deploy.log\n\nThe scan is read-only and opens no network socket. It is pattern matching,\nnot proof: a secret split across lines, base64-encoded, or written by a tool\nin another format is missed. GitHub and GitLab mask exact values in the live\nlog, but a transformed value or a debug dump can still print. Revoke first,\nthen remove the line; deleting the log does not un-leak the credential.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport re\nimport sys\n\nMASK = \"*\" * 16\n\nPATTERNS = (\n    (\"aws-key\", re.compile(r\"\\b(?:AKIA|ASIA)[0-9A-Z]{16}\\b\")),\n    (\"aws-secret-key\", re.compile(\n        r\"(?i)aws_?secret_?access_?key\\s*[:=]\\s*[\\\"']?[A-Za-z0-9/+=]{40}\")),\n    (\"private-key\", re.compile(\n        r\"-----BEGIN [A-Z ]*PRIVATE KEY(?: BLOCK)?-----\")),\n    (\"github-token\", re.compile(\n        r\"\\b(?:gh[pousr]_[A-Za-z0-9]{36}|github_pat_[A-Za-z0-9_]{60,})\\b\")),\n    (\"gitlab-token\", re.compile(r\"\\bglpat-[A-Za-z0-9_-]{20}\\b\")),\n    (\"slack-token\", re.compile(r\"\\bxox[baprs]-[A-Za-z0-9-]{10,}\\b\")),\n    (\"bearer\", re.compile(r\"(?i)\\bbearer\\s+[A-Za-z0-9._~+/-]{20,}=*\")),\n    (\"assignment\", re.compile(\n        r\"(?i)\\b(?:api[_-]?key|secret|token|password)\\s*[:=]\\s*\"\n        r\"[\\\"']?[A-Za-z0-9_\\-./+=]{12,}\")),\n)\n\n\ndef scan_line(line):\n    \"\"\"Return the kind of the first matching pattern, or None.\"\"\"\n    for kind, pattern in PATTERNS:\n        if pattern.search(line):\n            return kind\n    return None\n\n\ndef read_lines(source):\n    \"\"\"Return the lines of a log file or stdin, or raise OSError.\"\"\"\n    if source == \"-\":\n        return sys.stdin.readlines()\n    with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n        return handle.readlines()\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find likely secrets in a CI log and print the line \"\n                    \"number with a masked value, never the value.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\")\n    parser.add_argument(\"log\", nargs=\"?\", default=\"-\",\n                        help=\"log file, or - for stdin (default)\")\n    args = parser.parse_args(argv)\n\n    try:\n        lines = read_lines(args.log)\n    except (OSError, UnicodeError) as exc:\n        print(\"ERR cannot read %s: %s\" % (args.log, exc), file=sys.stderr)\n        return 2\n\n    hits = []\n    for number, line in enumerate(lines, 1):\n        kind = scan_line(line)\n        if kind is not None:\n            hits.append((number, kind))\n    if not hits:\n        print(\"OK no likely secrets in %d lines\" % len(lines))\n        return 0\n    print(\"SCAN %d lines, %d findings\" % (len(lines), len(hits)))\n    for number, kind in hits:\n        print(\"LEAK %d %s %s\" % (number, kind, MASK))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p606","n":"606","slug":"a-pipeline-flake-report","title":"A pipeline flake report","sections":["devsecops"],"topic":"Python scripts","tags":["CICD","DevOps","Python","SRE","Automation"],"series_name":"PythonFixes","design":"code","script":"pipeline_flake.py","date":"2030-08-11","files":{"devsecops":"notes/p606-a-pipeline-flake-report.md"},"md":"# A pipeline flake report\n\nThe integration job fails twice a week, someone clicks rerun, and it goes green. By Friday the team calls it flaky with no number behind the word, so nobody knows whether it is getting worse or which job to fix first.\n\n## Quick reference\n\nThe full script is at the end of this note: `pipeline_flake.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 pipeline_flake.py /tmp/runs.json\n```\n\n**Example output**\n\n```text\nFLAKY api-tests 40% failed\nFLAKY e2e 30% failed\n```\n\n## Explained\n\n### What it does\n\n`pipeline_flake.py` reads CI history JSON with one record per job per run: id, job, status and duration. It counts passes and failures per job, keeps the jobs that have both, and ranks them by failures divided by the runs that reached a pass or fail. Only jobs above the threshold, 20 percent by default, are printed.\n\n### What the screen shows\n\nThe fourteen lines load the runs and count each job into a two-item list. The bad list keeps jobs with both counts, filters at 20 percent and sorts. Two ranked lines are printed with the job and the rounded rate. On the dirty history they read FLAKY api-tests 40% failed, then FLAKY e2e 30% failed, and the script exits 1.\n\n### How to run it\n\n`python3 pipeline_flake.py runs.json`, with `--threshold 10` for a new job.\n\n### The fix\n\nQuarantine the top job or fix the failure behind it. A rerun that passes is evidence of a flake, not a fix.\n\n### Prevention\n\nKeep the weekly report and compare it with last week. A rate that doubles is a regression even below the threshold.\n\n> **The honest caveat:** a history without reruns understates the rate, and duration is not used to rank.\n\n> **The lesson:** measure instability before you schedule work around it.\n\n## In one line\n\n> **Flaky is a rate, not a feeling.** Count the reruns before you call a job unstable.\n\n**Export the runs**\n\n```bash\npython3 pipeline_flake.py runs.json\n```\n\n## Script: pipeline_flake.py\n\nDownload: [scripts/pipeline_flake.py](../scripts/pipeline_flake.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Report CI jobs that both passed and failed, ranked by flake rate.\n\nReads a CI run history as JSON. Each record needs a job name and a status;\nrun id and duration may be present; they are not needed for the rate. A job is flaky when\nthe same job has at least one pass and at least one failure in the history.\nIts flake rate is failures divided by the runs that reached a pass or fail:\n\n    {\"runs\": [\n      {\"id\": 101, \"job\": \"api-tests\", \"status\": \"passed\", \"duration\": 82},\n      {\"id\": 102, \"job\": \"api-tests\", \"status\": \"failed\", \"duration\": 79},\n      {\"id\": 103, \"job\": \"lint\", \"status\": \"passed\", \"duration\": 12}]}\n\nA plain list is also accepted. Statuses passed/pass/success/succeeded and\nfailed/fail/failure/error/errored/timed_out are counted; skipped, cancelled\nand queued records are ignored. The list is sorted by rate, then by failure\ncount, then by job name, and only jobs above --threshold (default 20 percent)\nare printed.\n\nUsage:\n    python3 pipeline_flake.py runs.json\n    python3 pipeline_flake.py runs.json --threshold 10\n    export job records from your CI API, one record per job per run\n    python3 pipeline_flake.py runs.json --help\n\nExit codes: 0 no job above the threshold, 1 at least one job above it,\n2 usage or unreadable, missing or invalid input.\n\nCI (a weekly flake gate):\n    python3 pipeline_flake.py runs.json --threshold 20 || exit 1\n\nCron (Mondays at 07:10):\n    10 7 * * 1 python3 /opt/s0sta/pipeline_flake.py /var/lib/ci/runs.json\n\nThe report is read-only. It counts what the history says, so a job that was\nrerun after a failure looks worse here than in the UI; keep reruns in the\nexport or the rate is understated. A flaky job is a symptom: the rate says\nwhich job to fix first, not why it fails.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nPASS = {\"passed\", \"pass\", \"success\", \"succeeded\", \"ok\", \"green\"}\nFAIL = {\"failed\", \"fail\", \"failure\", \"error\", \"errored\", \"timed_out\",\n        \"timedout\", \"red\"}\nIGNORE = {\"skipped\", \"cancelled\", \"canceled\", \"queued\", \"neutral\", \"unknown\"}\n\n\nclass FlakeError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_runs(path):\n    \"\"\"Return the run list from a path, or raise FlakeError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise FlakeError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise FlakeError(\"%s is not valid JSON: %s\" % (path, exc))\n    if isinstance(doc, list):\n        return doc\n    if isinstance(doc, dict) and isinstance(doc.get(\"runs\"), list):\n        return doc[\"runs\"]\n    raise FlakeError(\"expected a runs list or an object with a runs list\")\n\n\ndef count_runs(runs):\n    \"\"\"Return (counts by job, terminal records, ignored records).\"\"\"\n    counts = {}\n    terminal = 0\n    ignored = 0\n    for index, run in enumerate(runs, 1):\n        if not isinstance(run, dict):\n            raise FlakeError(\"run %d is not an object\" % index)\n        job = run.get(\"job\")\n        status = run.get(\"status\")\n        if not isinstance(job, str) or not job.strip():\n            raise FlakeError(\"run %d has no job name\" % index)\n        if not isinstance(status, str):\n            raise FlakeError(\"run %d (%s) has no status\" % (index, job))\n        state = status.strip().lower()\n        if state in IGNORE:\n            ignored += 1\n            continue\n        if state not in PASS and state not in FAIL:\n            raise FlakeError(\"run %d (%s) has unknown status %r\" % (index, job, status))\n        terminal += 1\n        counters = counts.setdefault(job, [0, 0])\n        counters[0 if state in PASS else 1] += 1\n    return counts, terminal, ignored\n\n\ndef flaky_jobs(counts):\n    \"\"\"Return (rate, failures, total, job) for jobs with both outcomes.\"\"\"\n    found = []\n    for job, (passed, failed) in counts.items():\n        if passed and failed:\n            total = passed + failed\n            found.append((failed / total, failed, total, job))\n    found.sort(key=lambda item: (-item[0], -item[1], item[3]))\n    return found\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank CI jobs by flake rate and fail the run above a \"\n                    \"threshold.\",\n        epilog=\"Exit codes: 0 clean, 1 flaky job above threshold, 2 usage or input error.\")\n    parser.add_argument(\"history\", help=\"CI run history JSON\")\n    parser.add_argument(\"--threshold\", type=float, default=20.0, metavar=\"PCT\",\n                        help=\"fail when a flake rate is above this percent (default 20)\")\n    args = parser.parse_args(argv)\n    if not 0 <= args.threshold <= 100:\n        parser.error(\"--threshold must be between 0 and 100\")\n\n    try:\n        counts, terminal, ignored = count_runs(load_runs(args.history))\n    except FlakeError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    found = flaky_jobs(counts)\n    over = [item for item in found if item[0] > args.threshold / 100.0]\n    if not over:\n        print(\"OK no flaky job over %g%% in %d runs\" % (args.threshold, terminal))\n        return 0\n    word = \"job\" if len(over) == 1 else \"jobs\"\n    print(\"SCAN %d runs, %d %s over %g%%\" % (terminal, len(over), word, args.threshold))\n    for rate, _failed, _total, job in over:\n        print(\"FLAKY %s %d%% failed\" % (job, round(rate * 100)))\n    if ignored:\n        print(\"NOTE %d records ignored\" % ignored)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p607","n":"607","slug":"deploy-verification-drawn","title":"Deploy verification, drawn","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","Kubernetes","CICD","Deployment","SRE"],"series_name":"PythonFixes","design":"architecture","script":"","date":"2030-08-13","files":{"devsecops":"notes/p607-deploy-verification-drawn.md"},"md":"# Deploy verification, drawn\n\nA release passes every pipeline stage and still serves the old code, because the stage checked a tag and the cluster pulled a different digest. The drawing splits the release into the three places where that can be caught.\n\n## Quick reference\n\n- **Build:** Artifact (ART) · Image digest (SHA)\n- **Verify:** Manifest check (MAN) · Smoke test (SMK)\n- **Ship:** Traffic shift (TRF) · Watch window (WCH)\n\n**Request flow:** Artifact → Manifest check → Smoke test → Traffic shift → Watch window; Image digest → Manifest check\n\n**Around it:** Release record (REL) · Exit code (EXIT)\n\n## Explained\n\n### Build\n\nThe first tier is the artifact and its digest. The build produces the file, hashes it and records the digest that will be deployed. An artifact without a digest is a name, and a name can point somewhere else tomorrow.\n\n### Verify\n\nThe second tier is the manifest check and the smoke test. The manifest check compares the artifact on disk with path, size and sha256. The smoke test sends one request that only the new release can answer, then asserts the value. A check that runs but asserts nothing is a green light wired to nothing.\n\n### Ship\n\nThe third tier is the traffic shift and the watch window. Move a share of traffic, watch error rate and latency against the baseline you recorded before the shift. A strip below the tiers holds the release record and the exit code.\n\nThe fix is to make each tier produce evidence: a digest, a manifest result, an asserted response, a watched metric. Prevention is refusing to move traffic until the previous tier has written its evidence.\n\n> **The honest caveat:** this is a model, not an implementation. Traffic may move at the ingress, the service mesh or DNS, and a watch window without a baseline is just waiting.\n\n> **The lesson:** a deploy is a chain of evidence, and the last link is the running pod.\n\n## In one line\n\n> **Built, verified, then shipped.** Three tiers, and a check you can fail at each one.\n\n**Check the digest:** crane digest ghcr.io/acme/api:v2\n"},{"id":"p608","n":"608","slug":"deployment-verification-commands","title":"Deployment verification commands","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Kubernetes","DevOps","CICD","SRE","CommandLine"],"series_name":"PythonFixes","design":"cheatsheet","script":"","date":"2030-08-15","files":{"devsecops":"notes/p608-deployment-verification-commands.md"},"md":"# Deployment verification commands\n\nA release is done and someone asks the only question that matters: is the new version actually serving traffic? Four blocks of commands answer it.\n\n## Quick reference\n\n### Image\n\n```bash\nkubectl get deploy api -o yaml\nkubectl rollout history deploy/api\nkubectl get rs -o wide\ndocker inspect api | grep -i digest\naws ecr list-images --repository-name api\n```\n\n### Pods\n\n```bash\nkubectl get pods -l app=api -o wide\nkubectl describe pod api-7d9f\nkubectl top pod -l app=api\nkubectl logs deploy/api --tail=50\n```\n\n### Traffic\n\n```bash\nkubectl get svc api -o wide\nkubectl get endpoints api\nkubectl get ingress api -o yaml\ncurl -sS https://api.example.com/healthz\n```\n\n### Rollback\n\n```bash\nkubectl rollout undo deploy/api\nkubectl rollout status deploy/api\nkubectl rollout restart deploy/api\nkubectl rollout pause deploy/api\n```\n\n## Explained\n\n### Image\n\nStart with the Deployment and its history: the image in the pod template, the revisions the controller kept, and the ReplicaSets still on disk. Docker inspect finds the digest the runtime recorded; the ECR list shows the tags the registry still holds.\n\n### Pods\n\nThe pod list with labels shows age and readiness. Describe one pod for its imageID and events, top for the resources it is using, and logs for the version banner or the request it just served.\n\n### Traffic\n\nThe Service, its endpoints and the Ingress tell you where requests can go. A curl to the health or version path proves the edge reaches a pod, but only if you read the body. A 200 from the old ReplicaSet is still a 200.\n\n### Rollback\n\nThe undo, status, restart and pause commands are the controls. Run rollout history first so you know which revision undo will pick, and run status after the undo so you know it finished rather than started.\n\nUse the blocks in order and save the outputs beside the release record. Prevention is making the same four checks a gate, not a ritual.\n\n> **The honest caveat:** a command that prints a tag or a controller status is not proof of the bytes on the node. Only imageID and the digest answer that.\n\n> **The lesson:** read the running system, then compare it with the release.\n\n## In one line\n\n> **Read the system, not the pipeline.** A command is only a check when you know what a bad answer looks like.\n\n**Check one pod**\n\n```bash\nkubectl get pod api-7d9f -o yaml\n```\n"},{"id":"p609","n":"609","slug":"four-deploy-failure-modes","title":"Four deploy failure modes","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","SRE","CICD","Kubernetes","Reliability"],"series_name":"PythonFixes","design":"explainer","script":"","date":"2030-08-18","files":{"devsecops":"notes/p609-four-deploy-failure-modes.md"},"md":"# Four deploy failure modes\n\nThe deploy passed every stage and the error rate tripled. Four failure modes explain almost every version of that sentence.\n\n## Quick reference\n\n### Wrong artifact\n\n- The build uploads a.tar.gz, the deploy pulls b.tar.gz\n- A tag moved after the release was approved\n- Compare the digest the runtime pulled\n\n### Missing digest\n\n- The manifest names a tag, not a sha256\n- The same tag resolves to new bytes tomorrow\n- Pin image@sha256 in the Deployment\n\n### No rollback\n\n- revisionHistoryLimit is 0, so old sets are gone\n- rollout undo has nothing to scale up\n- Keep ten revisions and test one undo\n\n### Silent smoke test\n\n- curl exits 0 on a 500 without `--fail`\n- A 200 can come from the old pods\n- Assert a value only the new code returns\n\n> **Note:** Four checks; most pipelines run one of them and call it verification.\n\n## Explained\n\n### Wrong artifact\n\nThe build uploads one file and the deploy pulls another. Sometimes a retry leaves both names in the bucket, sometimes a tag moved after approval. The fix is a digest, not a filename: compare the bytes the runtime pulled with the bytes the build recorded.\n\n### Missing digest\n\nThe manifest names a tag, the cluster resolves it, and the next pull gets different bytes because someone pushed the same tag again. A tag is a pointer. Pin image@sha256 in the Deployment and keep the tag for humans.\n\n### No rollback\n\nrevisionHistoryLimit is 0, or the old ReplicaSet was deleted, so rollout undo has nothing to scale up. Keep ten revisions, keep the old images in the registry, and test one undo before you need it.\n\n### Silent smoke test\n\nThe step curls the health endpoint without `--fail`, gets a 500, and exits 0. Or it gets a 200 from the pod that never left. A smoke test needs an assertion: a value only the new release returns, checked before traffic moves.\n\nThe note at the bottom says most pipelines run one of them and call it verification. A wrong artifact often arrives with a missing digest, and a silent smoke test hides both.\n\n> **The honest caveat:** most real incidents combine two of these, and a checklist long enough to catch all four is a pipeline of its own.\n\n> **The lesson:** name the failure mode before adding another green step.\n\n## In one line\n\n> **Trust the check, not the green.** A deploy is done when the running system says so.\n\n**Verify the pod**\n\n```bash\nkubectl get pod api-7d9f -o yaml\n```\n"},{"id":"p610","n":"610","slug":"verify-after-deploy-vs-trust-the-pipeline","title":"Verify after deploy vs trust the pipeline","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","Kubernetes","CICD","SRE","Deployment"],"series_name":"PythonFixes","design":"compare","script":"","date":"2030-08-20","files":{"devsecops":"notes/p610-verify-after-deploy-vs-trust-the-pipeline.md"},"md":"# Verify after deploy vs trust the pipeline\n\nA release turns green and the argument starts: the pipeline already verified it, or someone should look at the pods.\n\n## Quick reference\n\n| | **VERIFY** | **TRUST** |\n|:--|:--|:--|\n| Question | What is running? | What did we ship? |\n| Evidence | Pod digest and age | A green job step |\n| Timing | After traffic moves | Before traffic moves |\n| Catches | Stale pod, wrong digest | A failed command |\n| Misses | A bad smoke assertion | A moved tag |\n| Owner | The on-call engineer | The pipeline author |\n\n**VERIFY** (Check what is running): Reads the pods after traffic moves and compares digest, readiness and age.\n\nExample: `deploy_verify.py - release.json`\n\n**TRUST** (Believe the pipeline): Reads the controller's rollout and trusts that the job did what it said.\n\n```bash\nkubectl get deploy/api -o yaml\n```\n\n- Verify → the pods after the shift.\n- Trust → the pipeline before it.\n\n## Explained\n\n### The check\n\nVerify reads the running system after traffic moves. It asks what image digest each pod is on, how many are Ready, and whether any pod is older than the release window. It can catch a stale endpoint, a wrong digest and a rollout that never replaced a pod.\n\n### The trust\n\nTrust reads the pipeline before traffic moves. It asks whether every job step succeeded, whether the rollout controller reports the new revision, and whether the smoke step exited 0. It catches a failed command or a missing step.\n\n### Where they meet\n\nThe rows line up the trade. Verify is evidence about now; trust is evidence about intent. Verify catches a wrong digest that trust cannot see when the wrong digest was what the pipeline shipped. Trust catches a broken build that verify never reaches because there is no pod to inspect. Verify misses a silent assertion; trust misses a moved tag. Neither replaces the other, and the owner differs: the pipeline author owns the gate, the on-call engineer owns the check.\n\nUse trust to stop bad releases and verify to prove good ones. The short lines put it plainly: verify the pods after the shift, trust the pipeline before it.\n\n> **The honest caveat:** verify is a snapshot, not a monitor. A pod can be replaced by a bad rollout five minutes after the check passes.\n\n> **The lesson:** the pipeline says what it sent; the pod says what arrived.\n\nThe pipeline ships.  \nThe pod proves it.\n"},{"id":"p611","n":"611","slug":"quiz-3-deployment-questions","title":"Quiz: 3 deployment questions","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","Kubernetes","CICD","Python","SRE"],"series_name":"PythonFixes","design":"quiz","script":"","date":"2030-08-22","files":{"devsecops":"notes/p611-quiz-3-deployment-questions.md"},"md":"# Quiz: 3 deployment questions\n\nThree questions come back in every release review, and the answers separate a deploy from a guess.\n\n## Quick reference\n\n**Q1. Why is an image tag not an artifact?**\n\n- a) It can point to a different digest\n- b) Tags are encrypted strings\n- c) Tags are deleted after deploy\n\n> **Answer:** a) It can point to a different digest. A tag is a mutable label. The same tag can resolve to new bytes, so it names a version, not the artifact. Pin the digest.\n\n**Q2. What makes a rollback target?**\n\n- a) A previous ReplicaSet at revision N-1\n- b) A database backup from last night\n- c) The latest image tag\n\n> **Answer:** a) A previous ReplicaSet at revision N-1. A Deployment keeps old ReplicaSets up to revisionHistoryLimit. The target is a previous revision present and scaled to zero; a limit of 0 deletes it.\n\n**Q3. Why does a smoke test need an assertion?**\n\n- a) A 200 can come from the old pods\n- b) Curl already checks the status\n- c) Logs are not kept long enough\n\n> **Answer:** a) A 200 can come from the old pods. A 200 can be served by the old ReplicaSet, a cache or a login page. Assert a value only the new release returns, and fail the job when it is absent.\n\n## Explained\n\n### The tag\n\nQuestion one: why is an image tag not an artifact? A tag is a mutable label. The same tag can resolve to new bytes on the next pull, so it names a version, not the thing that ran. A digest names the bytes. If the release record holds only a tag, nobody can prove which artifact was live at 03:00.\n\n### The target\n\nQuestion two: what makes a rollback target? A Deployment keeps old ReplicaSets up to revisionHistoryLimit, and the target is the previous revision still present and scaled to zero. If the limit is 0, or the old set was deleted, rollout undo has nothing to scale up.\n\n### The assertion\n\nQuestion three: why does a smoke test need an assertion? A request that returns 200 can be served by the old ReplicaSet, a cache or a login page. The status code proves that something answered. An assertion on a value only the new release returns proves that the new code answered, and it fails the job when the value is missing.\n\nThe three connect: a digest pins the artifact, a target makes it reversible, and an assertion proves the new one is live. Remove any one and the other two report success without evidence.\n\n> **The honest caveat:** passing this quiz does not make a release safe; the gate and the check do.\n\n> **The lesson:** green is a colour, not a proof.\n\n## In one line\n\n> **Green is not the same as right.** Three questions to ask before the next release.\n\n**Check one pod**\n\n```bash\nkubectl get pod api-7d9f -o yaml\n```\n"},{"id":"p612","n":"612","slug":"a-restore-drill-in-python","title":"A restore drill in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Backup","DevOps","SRE","DataProtection"],"series_name":"PythonFixes","design":"code","script":"restore_drill.py","date":"2030-08-25","files":{"it":"notes/p612-a-restore-drill-in-python.md","devsecops":"notes/p612-a-restore-drill-in-python.md"},"md":"# A restore drill in Python\n\nThe backup job is green for eleven months, and the first restore takes four hours because nobody checked the archive opens. A drill asks one question: can this file give the data back?\n\n## Quick reference\n\nThe full script is at the end of this note: `restore_drill.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 restore_drill.py backup.tar --expected etc.txt\n```\n\n**Example output**\n\n```text\nFOUND 1 of 2\nDRILL FAIL 1 missing etc/secret.key\n```\n\n## Explained\n\n### What it does\n\nIt opens the tar with tarfile and calls getmembers, so nothing is extracted. It reads the expected paths, normalises the names and checks them. The unpacked total must clear `--min-mb`, and a zero-byte file is a finding.\n\n### What the screen shows\n\nThe 14 lines open the archive, index the names and count what is restorable. On the dirty fixture: FOUND 1 of 2, then DRILL FAIL 1 missing etc/secret.key. The full script adds RESTORE lines and the MISSING, EMPTY and SMALL flags.\n\n### How to run it\n\n`python3 restore_drill.py backup.tar --expected etc.txt`, after the upload or weekly in cron. Exit 0 is clean, 1 is unreadable, missing, empty or small, 2 is a usage error, including an unreadable expected list.\n\n### The fix\n\nA missing path is a backup-job bug: the dump went to the wrong directory or the exclude grew. Re-run the job, then drill the new archive. An unreadable file means the upload finished and the tar never opened.\n\n### Prevention\n\nReview the list when a table or a config file is added, and drill the newest archive, not last quarter's.\n\n> **The honest caveat:** listing proves the archive opens and the names exist; it does not prove the bytes. Only a real restore of one file does.\n\n> **The lesson:** a backup without a restore is a receipt for a promise.\n\n## In one line\n\n> **A backup is a receipt, not a restore.** A drill is the test that turns the receipt into evidence.\n\n**Drill the newest**\n\n```bash\npython3 restore_drill.py backup.tar\n```\n\n## Script: restore_drill.py\n\nDownload: [scripts/restore_drill.py](../scripts/restore_drill.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"restore_drill.py - verify that a tar backup could be restored, without restoring it.\n\nOpens a tar archive read-only and lists its members. Every path in the\nexpected-paths file must be present, every expected entry must hold bytes,\nand the unpacked total must reach --min-mb. Nothing is extracted, so the\ndrill is safe against a production backup and safe in CI.\n\nExpected-paths file: one path per line. Blank lines and lines starting with\n\"#\" are ignored. Paths are matched after normalisation, so \"./etc/app.conf\"\nand \"etc/app.conf\" are the same entry. A directory path matches every member\nunder it, which is how you assert that a whole tree is present.\n\nUsage:\n    python3 restore_drill.py backup.tar --expected etc.txt\n    python3 restore_drill.py backup.tar.gz --expected etc.txt --min-mb 50\n    python3 restore_drill.py --help\n\nCI (fail the job when the drill fails):\n    python3 restore_drill.py /backups/app.tar --expected /etc/s0sta/expected.txt || exit 1\n\nCron (weekly drill against the newest archive):\n    0 3 * * 0 python3 /opt/s0sta/restore_drill.py /backups/latest.tar --expected /etc/s0sta/expected.txt\n\nExit codes: 0 every expected path is present and the size is sane,\n1 the archive will not open, a path is missing or the size is too small,\n2 usage or the expected-paths file cannot be read.\n\nRead-only: the archive is opened with tarfile and only getmembers() is\ncalled; nothing is extracted, written or executed. The restore command\nprinted at the end is a suggestion for a human, never run here.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport sys\nimport tarfile\n\nMIB = 1024.0 * 1024.0\n\n\ndef load_expected(path):\n    \"\"\"Return the expected paths, or raise ValueError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            lines = handle.read().splitlines()\n    except (OSError, UnicodeError) as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    expected = []\n    for line in lines:\n        item = line.strip()\n        if item and not item.startswith(\"#\"):\n            expected.append(item)\n    if not expected:\n        raise ValueError(\"%s: no expected paths\" % path)\n    return expected\n\n\ndef norm(name):\n    \"\"\"Normalise a tar member name for comparison.\"\"\"\n    while name.startswith(\"./\"):\n        name = name[2:]\n    return name.lstrip(\"/\").rstrip(\"/\")\n\n\ndef args_for(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Check that a tar backup holds the expected paths.\")\n    parser.add_argument(\"archive\", help=\"tar file to inspect\")\n    parser.add_argument(\"--expected\", required=True,\n                        help=\"file with one expected path per line\")\n    parser.add_argument(\"--min-mb\", type=int, default=1,\n                        help=\"minimum unpacked size in MB (default 1)\")\n    args = parser.parse_args(argv)\n    if args.min_mb < 0:\n        parser.error(\"--min-mb must be zero or more\")\n    return args\n\n\ndef main(argv=None):\n    args = args_for(argv)\n    try:\n        expected = load_expected(args.expected)\n    except ValueError as exc:\n        print(\"restore_drill: %s\" % exc, file=sys.stderr)\n        return 2\n\n    try:\n        with tarfile.open(args.archive, \"r:*\") as archive:\n            members = archive.getmembers()\n    except (tarfile.TarError, OSError) as exc:\n        print(\"DRILL FAIL %s: %s\" % (args.archive, exc))\n        print(\"SUGGEST check the backup job, re-run it, then drill the new archive\")\n        return 1\n\n    exact = {}\n    dirs = set()\n    for member in members:\n        name = norm(member.name)\n        if member.isdir():\n            dirs.add(name)\n        else:\n            exact[name] = member\n    total = sum(m.size for m in members if m.isfile())\n    mib = total / MIB\n\n    found = []\n    missing = []\n    empty = []\n    for path in expected:\n        key = norm(path)\n        member = exact.get(key)\n        if member is not None:\n            found.append(\"RESTORE %s %d bytes\" % (path, member.size))\n            if member.size == 0:\n                empty.append(path)\n            continue\n        under = [(n, m.size) for n, m in exact.items()\n                 if n.startswith(key + \"/\")]\n        if key in dirs or under:\n            size = sum(s for _, s in under)\n            word = \"file\" if len(under) == 1 else \"files\"\n            found.append(\"RESTORE %s/ %d %s, %d bytes\"\n                         % (key, len(under), word, size))\n            if size == 0:\n                empty.append(path)\n            continue\n        missing.append(path)\n\n    small = total < args.min_mb * 1024 * 1024\n    print(\"ARCHIVE %s %d members %.1f MiB\" % (args.archive, len(members), mib))\n    for line in found:\n        print(line)\n    for path in missing:\n        print(\"MISSING %s\" % path)\n    for path in empty:\n        print(\"EMPTY %s\" % path)\n    if small:\n        print(\"SMALL unpacked %.1f MiB < %d MiB\" % (mib, args.min_mb))\n\n    if missing or empty or small:\n        print(\"DRILL FAIL %d missing, %d empty, %.1f MiB\"\n              % (len(missing), len(empty), mib))\n        print(\"SUGGEST check the backup job, re-run it, then drill the new archive\")\n        return 1\n\n    print(\"DRILL OK %d paths, %.1f MiB\" % (len(expected), mib))\n    print(\"SUGGEST mkdir -p /tmp/restore && tar -xf %s -C /tmp/restore\"\n          % args.archive)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p613","n":"613","slug":"a-table-growth-report-in-python","title":"A table growth report in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Databases","SRE","DevOps","CapacityPlanning"],"series_name":"PythonFixes","design":"code","script":"table_growth.py","date":"2030-08-27","files":{"it":"notes/p613-a-table-growth-report-in-python.md","devsecops":"notes/p613-a-table-growth-report-in-python.md"},"md":"# A table growth report in Python\n\nOrders doubled in a month and nobody noticed until the disk alarm. Two exports and one comparison turn that into a decision.\n\n## Quick reference\n\nThe full script is at the end of this note: `table_growth.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 table_growth.py old.csv new.csv\n```\n\n**Example output**\n\n```text\nFLAG orders 100.0% 12 GB\nFLAGGED 1 of 3\n```\n\n## Explained\n\n### What it does\n\nThe script reads two CSV snapshots with date, table and bytes, takes the older date as the baseline and sums rows that share a table. It reports growth in GiB and per cent, then flags a table above `--percent` or `--max-gb`. A table absent before is NEW, not a percentage from zero.\n\n### What the screen shows\n\nThe 14 lines load both files, compare each table, print a FLAG when a threshold is crossed and end with FLAGGED 1 of 3. Orders goes from 6 to 12 GiB, a hundred per cent against the default 30. The full script adds a TABLE line per table and the total delta.\n\n### How to run it\n\n`python3 table_growth.py old.csv new.csv`, weekly, from the export behind the capacity dashboard. `--percent 30` and `--max-gb 10` are the defaults.\n\n### The fix\n\nA table that doubles is a feature that works or a writer that leaks. Find the writer, add retention or partition the table, and record the decision.\n\n### Prevention\n\nRun it weekly, keep the snapshots, and alert on the second consecutive flag, not the first spike.\n\n> **The honest caveat:** bytes on disk include indexes, bloat and free space, so a table can grow without gaining rows. VACUUM FULL can change the number, and the report cannot tell you which.\n\n> **The lesson:** growth is a number until it crosses a line you chose.\n\n## In one line\n\n> **A table that doubles is a decision.** Growth is a number until it crosses a line you chose.\n\n**Set the threshold**\n\n```bash\n--percent 30 --max-gb 10\n```\n\n## Script: table_growth.py\n\nDownload: [scripts/table_growth.py](../scripts/table_growth.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"table_growth.py - compare two table-size snapshots and flag growth.\n\nReads two CSV snapshots of table sizes and compares them. The older date is\nthe baseline and the newer one is today. A table is flagged when it grew by\nmore than --percent or when its current size is over --max-gb. Rows with the\nsame table are summed, so a partition-level export works without a pivot.\n\nCSV shape (header required, column order free):\n\n    date,table,bytes\n    2026-09-01,orders,6442450944\n    2026-09-01,users,2147483648\n\nEach file must hold one date. Bytes are decimal; thousands separators are\naccepted. A table that is new in the later snapshot is reported as NEW and\nonly the size rule applies, because a percentage from zero is not a number.\n\nUsage:\n    python3 table_growth.py old.csv new.csv\n    python3 table_growth.py old.csv new.csv --percent 50 --max-gb 20\n    python3 table_growth.py --help\n\nCI (fail the job when a table outgrows its budget):\n    python3 table_growth.py /tmp/old.csv /tmp/new.csv --max-gb 50 || exit 1\n\nCron (weekly report from two size exports):\n    0 6 * * 1 python3 /opt/s0sta/table_growth.py /var/lib/s0sta/last.csv /var/lib/s0sta/now.csv\n\nExit codes: 0 no table is over a threshold, 1 at least one flag,\n2 usage, unreadable CSV or a row that does not match the shape.\n\nRead-only: both files are opened for reading only. Nothing is written to a\ndatabase, and no query, vacuum or resize is run.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport sys\nfrom datetime import date\n\nCOLUMNS = {\"date\", \"table\", \"bytes\"}\nGIB = 1024.0 ** 3\n\n\ndef load_snapshot(path):\n    \"\"\"Return (snapshot_date, {table: bytes}); raise ValueError on bad input.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\", newline=\"\") as handle:\n            reader = csv.DictReader(handle)\n            fields = reader.fieldnames or []\n            if len(fields) != 3 or set(fields) != COLUMNS:\n                raise ValueError(\"%s: header must be date,table,bytes\" % path)\n            rows = list(reader)\n    except (OSError, UnicodeError) as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    if not rows:\n        raise ValueError(\"%s: no data rows\" % path)\n\n    sizes = {}\n    found = None\n    for index, row in enumerate(rows):\n        where = \"%s line %d\" % (path, index + 2)\n        raw_date = (row.get(\"date\") or \"\").strip()\n        try:\n            day = date.fromisoformat(raw_date)\n        except ValueError:\n            raise ValueError(\"%s: bad date %r\" % (where, raw_date))\n        if found is None:\n            found = day\n        elif day != found:\n            raise ValueError(\"%s: more than one date in the file\" % where)\n        table = (row.get(\"table\") or \"\").strip()\n        if not table:\n            raise ValueError(\"%s: empty table name\" % where)\n        raw_bytes = (row.get(\"bytes\") or \"\").strip().replace(\",\", \"\")\n        try:\n            value = int(raw_bytes)\n        except ValueError:\n            raise ValueError(\"%s: bad byte count %r\" % (where, raw_bytes))\n        if value < 0:\n            raise ValueError(\"%s: negative byte count\" % where)\n        sizes[table] = sizes.get(table, 0) + value\n    return found, sizes\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Compare two table-size snapshots and flag growth.\")\n    parser.add_argument(\"old\", help=\"earlier snapshot CSV\")\n    parser.add_argument(\"new\", help=\"later snapshot CSV\")\n    parser.add_argument(\"--percent\", type=float, default=30.0,\n                        help=\"flag growth above this percent (default 30)\")\n    parser.add_argument(\"--max-gb\", type=float, default=10.0,\n                        help=\"flag tables larger than this in GB (default 10)\")\n    args = parser.parse_args(argv)\n    if args.percent < 0:\n        parser.error(\"--percent must be zero or more\")\n    if args.max_gb <= 0:\n        parser.error(\"--max-gb must be greater than zero\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        old_date, old = load_snapshot(args.old)\n        new_date, new = load_snapshot(args.new)\n    except ValueError as exc:\n        print(\"table_growth: %s\" % exc, file=sys.stderr)\n        return 2\n    if old_date == new_date:\n        print(\"table_growth: both snapshots are dated %s\" % old_date,\n              file=sys.stderr)\n        return 2\n    if old_date > new_date:\n        old_date, new_date = new_date, old_date\n        old, new = new, old\n\n    max_bytes = args.max_gb * GIB\n    flags = 0\n    print(\"GROWTH %s -> %s\" % (old_date, new_date))\n    for table in sorted(new, key=lambda name: (-new[name], name)):\n        now = new[table]\n        was = old.get(table, 0)\n        gib = now / GIB\n        if was == 0:\n            print(\"NEW %s %.1f GiB\" % (table, gib))\n            if now > max_bytes:\n                flags += 1\n                print(\"FLAG %s %.1f GiB > %.1f GiB\"\n                      % (table, gib, args.max_gb))\n            continue\n        pct = (now - was) * 100.0 / was\n        print(\"TABLE %s %.1f -> %.1f GiB %+.1f%%\"\n              % (table, was / GIB, gib, pct))\n        if pct > args.percent:\n            flags += 1\n            print(\"FLAG %s growth %+.1f%% > %.1f%%\"\n                  % (table, pct, args.percent))\n        if now > max_bytes:\n            flags += 1\n            print(\"FLAG %s %.1f GiB > %.1f GiB\" % (table, gib, args.max_gb))\n\n    delta = (sum(new.values()) - sum(old.values())) / GIB\n    print(\"SUMMARY %d tables, %+.1f GiB, %d flags\"\n          % (len(new), delta, flags))\n    if flags:\n        print(\"SUGGEST review the writer for the flagged table, then archive or partition it\")\n        return 1\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p614","n":"614","slug":"an-s3-cost-hotspot-report-in-python","title":"An S3 cost hotspot report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","S3","FinOps","CloudCost","Python"],"series_name":"PythonFixes","design":"code","script":"s3_cost_hotspots.py","date":"2030-08-29","files":{"devsecops":"notes/p614-an-s3-cost-hotspot-report-in-python.md"},"md":"# An S3 cost hotspot report in Python\n\nThe S3 bill doubled and the data did not. A bucket of cold objects is paying the STANDARD price, and the invoice only says S3.\n\n## Quick reference\n\nThe full script is at the end of this note: `s3_cost_hotspots.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 s3_cost_hotspots.py s3.json --prices p.json\n```\n\n**Example output**\n\n```text\nCOLD media 40960 GiB\nTOTAL $1072.25 budget $1000.00\n```\n\n## Explained\n\n### What it does\n\nThe script reads buckets with a name, a class, bytes and requests, plus a price table of dollars per GiB-month and per 1,000 requests. It prices each bucket, ranks them and flags a STANDARD bucket with at least `--cold-gb` and fewer than `--cold-requests` requests per GiB per month.\n\n### What the screen shows\n\nThe 14 lines price every bucket, total the month and print the cold one: COLD media 40960 GiB, then TOTAL $1072.25 budget $1000.00, exit 1. The full script adds a ranked COST line per bucket, the suggested class and the saving.\n\n### How to run it\n\n`python3 s3_cost_hotspots.py s3.json --prices p.json`, weekly, with a budget that matches the account. The price table lives in the repo because list prices change.\n\n### The fix\n\nA cold bucket needs a lifecycle rule: transition after the access window, expire old versions, and check the retrieval bill for a month.\n\n### Prevention\n\nReport weekly, tag every bucket with an owner, and compare the total with the budget before the invoice arrives.\n\n> **The honest caveat:** requests are one number, so the estimate cannot tell a cheap GET from an expensive PUT, and the saving ignores retrieval and request charges. It is a lead, not a quote.\n\n> **The lesson:** the invoice names a service, so the bucket has to be found.\n\n## In one line\n\n> **Cold bytes pay the hot price.** The invoice names a service, so the bucket has to be found.\n\n**Price the classes**\n\n```bash\n--prices prices.json\n```\n\n## Script: s3_cost_hotspots.py\n\nDownload: [scripts/s3_cost_hotspots.py](../scripts/s3_cost_hotspots.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"s3_cost_hotspots.py - rank S3 buckets by monthly cost and flag cold STANDARD data.\n\nReads an S3 inventory JSON and a price table JSON, estimates one month of\ncost per bucket, ranks the buckets and flags STANDARD buckets whose access\ndensity is cold enough to move to a cheaper storage class.\n\nInventory shape (a bare list of buckets also works):\n\n    {\"buckets\": [\n      {\"name\": \"media\", \"class\": \"STANDARD\",\n       \"bytes\": 43980465111040, \"requests\": 2000},\n      {\"name\": \"archive\", \"class\": \"GLACIER\",\n       \"bytes\": 2199023255552, \"requests\": 1000}\n    ]}\n\nPrice table shape, dollars per GiB-month and per 1,000 requests:\n\n    {\"STANDARD\": {\"gb\": 0.023, \"req\": 0.005},\n     \"STANDARD_IA\": {\"gb\": 0.0125, \"req\": 0.01},\n     \"GLACIER\": {\"gb\": 0.0036, \"req\": 0.03}}\n\nRequests are one number, not split by GET and PUT, so the table's req price\nis an assumption you choose and the estimate is only as good as it. The\ncold rule is deliberately simple: a STANDARD bucket is flagged when it holds\nat least --cold-gb and receives fewer than --cold-requests requests per GiB\nper month. The saving is the storage difference only; retrieval and request\ncharges on the move are not modelled.\n\nUsage:\n    python3 s3_cost_hotspots.py inventory.json --prices prices.json\n    python3 s3_cost_hotspots.py inventory.json --prices prices.json --budget 500\n    python3 s3_cost_hotspots.py --help\n\nCI (fail the job when the month is over budget):\n    python3 s3_cost_hotspots.py /etc/s0sta/s3.json --prices /etc/s0sta/prices.json --budget 2000 || exit 1\n\nCron (weekly cost report to the FinOps channel):\n    0 7 * * 1 python3 /opt/s0sta/s3_cost_hotspots.py /etc/s0sta/s3.json --prices /etc/s0sta/prices.json\n\nExit codes: 0 under budget with no cold bucket, 1 over budget or at least\none cold bucket, 2 usage, unreadable JSON, an unknown storage class or a\nrecord that does not match the shape.\n\nRead-only: both files are opened for reading only. No object is copied,\nmoved or deleted and no AWS API is called; the lifecycle command printed at\nthe end is a suggestion for a human.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nGIB = 1024.0 ** 3\n\n\ndef load_json(path):\n    \"\"\"Read a JSON file, or raise ValueError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except (OSError, UnicodeError, json.JSONDecodeError) as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n\n\ndef number(value, where, field):\n    \"\"\"Return a non-negative whole number, or raise ValueError.\"\"\"\n    if isinstance(value, bool) or not isinstance(value, (int, float)):\n        raise ValueError(\"%s: %s must be a number\" % (where, field))\n    if value < 0 or int(value) != value:\n        raise ValueError(\"%s: %s must be a non-negative whole number\"\n                         % (where, field))\n    return int(value)\n\n\ndef load_prices(path):\n    \"\"\"Return {class: {'gb': float, 'req': float}}, or raise ValueError.\"\"\"\n    document = load_json(path)\n    if not isinstance(document, dict) or not document:\n        raise ValueError(\"%s: expected an object of storage classes\" % path)\n    prices = {}\n    for name, entry in document.items():\n        where = \"%s: class %s\" % (path, name)\n        if not isinstance(entry, dict):\n            raise ValueError(\"%s is not an object\" % where)\n        for key in (\"gb\", \"req\"):\n            value = entry.get(key)\n            if isinstance(value, bool) or not isinstance(value, (int, float)):\n                raise ValueError(\"%s: %s must be a number\" % (where, key))\n            if value < 0:\n                raise ValueError(\"%s: %s must be zero or more\" % (where, key))\n        prices[name] = {\"gb\": float(entry[\"gb\"]), \"req\": float(entry[\"req\"])}\n    return prices\n\n\ndef load_buckets(path, prices):\n    \"\"\"Return the validated bucket list, or raise ValueError.\"\"\"\n    document = load_json(path)\n    if isinstance(document, dict):\n        buckets = document.get(\"buckets\")\n    else:\n        buckets = document\n    if not isinstance(buckets, list):\n        raise ValueError(\"%s: expected a list or an object with a buckets list\"\n                         % path)\n    if not buckets:\n        raise ValueError(\"%s: no buckets\" % path)\n    seen = set()\n    rows = []\n    for index, bucket in enumerate(buckets):\n        where = \"%s bucket %d\" % (path, index + 1)\n        if not isinstance(bucket, dict):\n            raise ValueError(\"%s is not an object\" % where)\n        name = bucket.get(\"name\")\n        if not isinstance(name, str) or not name.strip():\n            raise ValueError(\"%s: name is missing\" % where)\n        name = name.strip()\n        if name in seen:\n            raise ValueError(\"%s: duplicate bucket %s\" % (where, name))\n        seen.add(name)\n        storage = bucket.get(\"class\")\n        if not isinstance(storage, str) or storage not in prices:\n            raise ValueError(\"%s: unknown class %r\" % (where, storage))\n        size = number(bucket.get(\"bytes\"), where, \"bytes\")\n        requests = number(bucket.get(\"requests\"), where, \"requests\")\n        rows.append({\"name\": name, \"class\": storage,\n                     \"bytes\": size, \"requests\": requests})\n    return rows\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Rank S3 buckets by monthly cost and flag cold data.\")\n    parser.add_argument(\"inventory\", help=\"S3 inventory JSON\")\n    parser.add_argument(\"--prices\", required=True,\n                        help=\"price table JSON, dollars per GiB-month\")\n    parser.add_argument(\"--budget\", type=float, default=1000.0,\n                        help=\"monthly budget in dollars (default 1000)\")\n    parser.add_argument(\"--top\", type=int, default=10,\n                        help=\"buckets to rank (default 10)\")\n    parser.add_argument(\"--cold-gb\", type=float, default=100.0,\n                        help=\"minimum size for a cold flag (default 100)\")\n    parser.add_argument(\"--cold-requests\", type=float, default=1.0,\n                        help=\"requests per GiB per month below this is cold \"\n                             \"(default 1.0)\")\n    args = parser.parse_args(argv)\n    if args.budget < 0:\n        parser.error(\"--budget must be zero or more\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n    if args.cold_gb < 0 or args.cold_requests < 0:\n        parser.error(\"--cold-gb and --cold-requests must be zero or more\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        prices = load_prices(args.prices)\n        buckets = load_buckets(args.inventory, prices)\n    except ValueError as exc:\n        print(\"s3_cost_hotspots: %s\" % exc, file=sys.stderr)\n        return 2\n\n    rows = []\n    for bucket in buckets:\n        price = prices[bucket[\"class\"]]\n        gib = bucket[\"bytes\"] / GIB\n        cost = gib * price[\"gb\"] + bucket[\"requests\"] / 1000.0 * price[\"req\"]\n        rows.append((cost, bucket, gib))\n    rows.sort(key=lambda row: (-row[0], row[1][\"name\"]))\n    total = sum(row[0] for row in rows)\n\n    for rank, (cost, bucket, gib) in enumerate(rows[:args.top], 1):\n        print(\"COST %d %s $%.2f/mo %s %.1f GiB\"\n              % (rank, bucket[\"name\"], cost, bucket[\"class\"], gib))\n    if len(rows) > args.top:\n        print(\"SHOWN %d of %d buckets\" % (args.top, len(rows)))\n    print(\"TOTAL $%.2f/mo budget $%.2f\" % (total, args.budget))\n\n    cold = []\n    others = [name for name in prices if name != \"STANDARD\"]\n    cheap = min(others, key=lambda name: prices[name][\"gb\"]) if others else None\n    for _cost, bucket, gib in rows:\n        if bucket[\"class\"] != \"STANDARD\" or gib < args.cold_gb:\n            continue\n        density = bucket[\"requests\"] / gib if gib else 0.0\n        if density >= args.cold_requests:\n            continue\n        cold.append(bucket[\"name\"])\n        line = (\"COLD %s %.2f req/GiB in STANDARD\"\n                % (bucket[\"name\"], density))\n        if cheap and prices[cheap][\"gb\"] < prices[\"STANDARD\"][\"gb\"]:\n            saving = (prices[\"STANDARD\"][\"gb\"] - prices[cheap][\"gb\"]) * gib\n            line += \" -> %s saves $%.2f/mo\" % (cheap, saving)\n        print(line)\n\n    problems = []\n    if total > args.budget:\n        problems.append(\"over budget by $%.2f\" % (total - args.budget))\n    if cold:\n        problems.append(\"%d cold bucket%s\" % (len(cold),\n                                              \"\" if len(cold) == 1 else \"s\"))\n    if problems:\n        print(\"FLAG \" + \", \".join(problems))\n        if cold and cheap:\n            print(\"SUGGEST aws s3api put-bucket-lifecycle-configuration \"\n                  \"--bucket %s --lifecycle-configuration file://lifecycle.json\"\n                  % cold[0])\n        return 1\n    print(\"OK no cold data, under budget\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p615","n":"615","slug":"a-log-retention-audit-in-python","title":"A log retention audit in Python","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Logging","SRE","DevOps","Observability"],"series_name":"PythonFixes","design":"code","script":"log_retention.py","date":"2030-09-01","files":{"it":"notes/p615-a-log-retention-audit-in-python.md","devsecops":"notes/p615-a-log-retention-audit-in-python.md"},"md":"# A log retention audit in Python\n\nAt 03:10 the disk is full and `/var/log` is the reason. The files are old, the directory is fat, and nobody has a number.\n\n## Quick reference\n\nThe full script is at the end of this note: `log_retention.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 log_retention.py logs.csv\n```\n\n**Example output**\n\n```text\nFLAG /var/log/api 12 GB 1 stale\nTOTAL 2 dirs, 1 flagged\n```\n\n## Explained\n\n### What it does\n\nThe script reads a CSV listing of path, bytes and modified, groups it by directory and flags files older than `--max-days` and directories over `--max-gb`. It prints a DIR line per directory, an OVER line for a volume breach, a STALE line per old file and a total.\n\n### What the screen shows\n\nThe 14 lines parse the listing, group by directory, add the bytes and count the files past 30 days: FLAG `/var/log/api 12` GB 1 stale, then TOTAL 2 dirs, 1 flagged. The full script adds the DIR lines, the STALE paths and a find command.\n\n### How to run it\n\n`python3 log_retention.py logs.csv`, daily, from a find or an object-store listing. The defaults are 30 days and 10 GB; `--max-days 14` is a tighter policy.\n\n### The fix\n\nArchive the stale files, delete what the policy allows, and put a logrotate or lifecycle rule in front of the directory. A manual cleanup buys a week; the policy is the fix.\n\n### Prevention\n\nRun it daily, alert on a directory over the limit for two days, and keep the rule with the service.\n\n> **The honest caveat:** mtime is not creation time, so a copied file looks new. A stale log may also be an audit record, which is why the report names files and never deletes them.\n\n> **The lesson:** retention is a budget, not a habit.\n\n## In one line\n\n> **Retention is a budget, not a habit.** Delete by policy, not by df.\n\n**Set the policy**\n\n```bash\n--max-days 30 --max-gb 10\n```\n\n## Script: log_retention.py\n\nDownload: [scripts/log_retention.py](../scripts/log_retention.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"log_retention.py - find stale logs and over-volume directories in a listing.\n\nReads a CSV listing of log files (path, bytes, modified), groups it by\ndirectory, and flags every file older than --max-days and every directory\nwhose total size is over --max-gb. Nothing is rotated, compressed or\ndeleted; the report names what a retention policy would act on.\n\nCSV shape (header required, column order free):\n\n    path,bytes,modified\n    /var/log/nginx/access.log,1048576,2026-10-20T09:00:00Z\n    /var/log/api/app.log,2048,2026-08-01\n\nmodified accepts YYYY-MM-DD or an ISO 8601 timestamp; a trailing Z is\naccepted and a value without a timezone is treated as UTC. A duplicate\npath is an input error, because a listing that counts one file twice would\ndouble the directory volume.\n\nUsage:\n    python3 log_retention.py listing.csv\n    python3 log_retention.py listing.csv --max-days 14 --max-gb 5\n    python3 log_retention.py listing.csv --now 2026-10-21T00:00:00Z\n    python3 log_retention.py --help\n\nCI (fail the job when the volume or the age crosses the policy):\n    python3 log_retention.py /tmp/logs.csv --max-days 30 --max-gb 20 || exit 1\n\nCron (weekly retention report):\n    0 6 * * 1 find /var/log -type f -printf '%p,%s,%TY-%Tm-%Td\\\\n' > /tmp/logs.csv && python3 /opt/s0sta/log_retention.py /tmp/logs.csv\n\nExit codes: 0 no stale file and no directory over the volume, 1 at least\none finding, 2 usage, unreadable CSV or a row that does not match the shape.\n\nRead-only: the listing is opened for reading only. No log is deleted,\ntruncated, compressed or moved; the find command printed at the end is a\nsuggestion for a human.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport os\nimport sys\nfrom datetime import datetime, timedelta, timezone\n\nGIB = 1024.0 ** 3\nMIB = 1024.0 ** 2\nCOLUMNS = {\"path\", \"bytes\", \"modified\"}\n\n\ndef parse_time(text, where):\n    \"\"\"Parse an ISO 8601 timestamp; a naive value is treated as UTC.\"\"\"\n    raw = text.strip()\n    if not raw:\n        raise ValueError(\"%s: modified is empty\" % where)\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        value = datetime.fromisoformat(raw)\n    except ValueError:\n        raise ValueError(\"%s: bad timestamp %r\" % (where, text))\n    if value.tzinfo is None:\n        value = value.replace(tzinfo=timezone.utc)\n    return value.astimezone(timezone.utc)\n\n\ndef load_listing(path):\n    \"\"\"Return the validated file rows, or raise ValueError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\", newline=\"\") as handle:\n            reader = csv.DictReader(handle)\n            fields = reader.fieldnames or []\n            if len(fields) != 3 or set(fields) != COLUMNS:\n                raise ValueError(\"%s: header must be path,bytes,modified\"\n                                 % path)\n            rows = list(reader)\n    except (OSError, UnicodeError) as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n    if not rows:\n        raise ValueError(\"%s: no data rows\" % path)\n\n    seen = set()\n    files = []\n    for index, row in enumerate(rows):\n        where = \"%s line %d\" % (path, index + 2)\n        name = (row.get(\"path\") or \"\").strip()\n        if not name:\n            raise ValueError(\"%s: empty path\" % where)\n        if name in seen:\n            raise ValueError(\"%s: duplicate path %s\" % (where, name))\n        seen.add(name)\n        raw_bytes = (row.get(\"bytes\") or \"\").strip().replace(\",\", \"\")\n        try:\n            size = int(raw_bytes)\n        except ValueError:\n            raise ValueError(\"%s: bad byte count %r\" % (where, raw_bytes))\n        if size < 0:\n            raise ValueError(\"%s: negative byte count\" % where)\n        modified = parse_time(row.get(\"modified\") or \"\", where)\n        files.append({\"path\": name, \"bytes\": size, \"modified\": modified})\n    return files\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Flag stale logs and oversized directories.\")\n    parser.add_argument(\"listing\", help=\"CSV listing of path,bytes,modified\")\n    parser.add_argument(\"--max-days\", type=int, default=30,\n                        help=\"flag files older than this (default 30)\")\n    parser.add_argument(\"--max-gb\", type=float, default=10.0,\n                        help=\"flag directories larger than this (default 10)\")\n    parser.add_argument(\"--now\",\n                        help=\"ISO 8601 reference time for the report\")\n    args = parser.parse_args(argv)\n    if args.max_days < 0:\n        parser.error(\"--max-days must be zero or more\")\n    if args.max_gb <= 0:\n        parser.error(\"--max-gb must be greater than zero\")\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        files = load_listing(args.listing)\n        as_of = parse_time(args.now, \"--now\") if args.now else \\\n            datetime.now(timezone.utc)\n    except ValueError as exc:\n        print(\"log_retention: %s\" % exc, file=sys.stderr)\n        return 2\n\n    cutoff = as_of - timedelta(days=args.max_days)\n    dirs = {}\n    stale = []\n    for item in files:\n        directory = os.path.dirname(item[\"path\"]) or \".\"\n        age = max(0, (as_of - item[\"modified\"]).days)\n        info = dirs.setdefault(directory,\n                               {\"bytes\": 0, \"files\": 0, \"stale\": 0})\n        info[\"bytes\"] += item[\"bytes\"]\n        info[\"files\"] += 1\n        if item[\"modified\"] < cutoff:\n            info[\"stale\"] += 1\n            stale.append((age, item))\n    max_bytes = args.max_gb * GIB\n    over = [name for name in dirs if dirs[name][\"bytes\"] > max_bytes]\n\n    for name in sorted(dirs, key=lambda key: (-dirs[key][\"bytes\"], key)):\n        info = dirs[name]\n        word = \"file\" if info[\"files\"] == 1 else \"files\"\n        print(\"DIR %s %.1f GiB %d %s %d stale\"\n              % (name, info[\"bytes\"] / GIB, info[\"files\"], word,\n                 info[\"stale\"]))\n    for name in sorted(over):\n        print(\"OVER %s %.1f GiB > %.1f GiB\"\n              % (name, dirs[name][\"bytes\"] / GIB, args.max_gb))\n    for age, item in sorted(stale, key=lambda row: (-row[0], row[1][\"path\"])):\n        print(\"STALE %s %dd %.1f MiB\"\n              % (item[\"path\"], age, item[\"bytes\"] / MIB))\n\n    total = sum(info[\"bytes\"] for info in dirs.values()) / GIB\n    print(\"TOTAL %d dirs %.1f GiB, %d stale, %d over\"\n          % (len(dirs), total, len(stale), len(over)))\n    if stale or over:\n        target = max(dirs, key=lambda key: (dirs[key][\"stale\"],\n                                            dirs[key][\"bytes\"]))\n        if stale:\n            print(\"SUGGEST find %s -type f -mtime +%d -print\"\n                  % (target, args.max_days))\n        else:\n            print(\"SUGGEST du -sh %s/* | sort -h | tail\" % target)\n        return 1\n    print(\"OK no stale logs, no directory over %.1f GiB\" % args.max_gb)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p616","n":"616","slug":"an-orphan-resource-report-in-python","title":"An orphan resource report in Python","sections":["devsecops"],"topic":"Python scripts","tags":["AWS","FinOps","CloudCost","Python","CloudNative"],"series_name":"PythonFixes","design":"code","script":"orphan_resources.py","date":"2030-09-03","files":{"devsecops":"notes/p616-an-orphan-resource-report-in-python.md"},"md":"# An orphan resource report in Python\n\nA cost review finds a line for EBS that no team recognises. The volume is unattached, the address is idle, and the snapshot is older than the service it came from.\n\n## Quick reference\n\nThe full script is at the end of this note: `orphan_resources.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 orphan_resources.py cloud.json\n```\n\n**Example output**\n\n```text\nORPHAN {'volume': 1, 'snapshot': 1, 'address': 1} WASTE $21.65/mo\n```\n\n## Explained\n\n### What it does\n\nThe script reads an inventory of volumes, addresses and snapshots, or one flat list with a kind field. It flags a volume in state available, an address with no association and a snapshot older than `--snapshot-days`, then prices each one with an overrideable rate.\n\n### What the screen shows\n\nThe 14 lines classify each row, check the snapshot age against a 90-day cut-off and add the cost: ORPHAN {'volume': 1, 'snapshot': 1, 'address': 1} WASTE $21.65/mo. The full script adds the resource id, size, reason and a line per orphan.\n\n### How to run it\n\n`python3 orphan_resources.py cloud.json`, weekly, from the inventory export. `--snapshot-days 90` is the default; set the prices to your contract if it is not list.\n\n### The fix\n\nTag the resource, find the owner, and decide: keep it with a note, or snapshot and delete it. The tags command is the first step, not the delete.\n\n### Prevention\n\nRequire an owner tag at create time and report orphans weekly. A tag is cheaper than an investigation.\n\n> **The honest caveat:** an unattached volume can be a disaster-recovery copy that must stay, and a snapshot can be the only copy of a deleted database. The report names candidates; a person decides.\n\n> **The lesson:** the invoice is per account, so the orphan has no owner.\n\n## In one line\n\n> **An idle disk still bills.** The invoice is per account, so nobody sees the orphan.\n\n**Check one orphan**\n\n```bash\naws ec2 describe-volumes --volume-ids vol-0a1\n```\n\n## Script: orphan_resources.py\n\nDownload: [scripts/orphan_resources.py](../scripts/orphan_resources.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"orphan_resources.py - report idle cloud resources and their monthly waste.\n\nReads a cloud inventory JSON and flags the resources that bill every month\nwithout serving anything: volumes in state available, elastic IPs with no\nassociation, and snapshots older than --snapshot-days. Each orphan gets an\nestimated monthly cost and the total is printed at the end. Nothing is\ndeleted or changed.\n\nInventory shape, either three lists:\n\n    {\"volumes\": [\n       {\"id\": \"vol-0a1\", \"size_gb\": 100, \"state\": \"available\"}\n     ],\n     \"addresses\": [\n       {\"id\": \"eipalloc-1\", \"public_ip\": \"203.0.113.7\",\n        \"state\": \"unassociated\"}\n     ],\n     \"snapshots\": [\n       {\"id\": \"snap-9\", \"size_gb\": 200,\n        \"created\": \"2025-01-01T00:00:00Z\"}\n     ]}\n\nor one flat list with a kind field (volume, address, snapshot). A volume is\nan orphan only in state available; in-use and attaching volumes are left\nalone. An address is an orphan when it has no association_id and its state\nis not associated. created accepts a date or an ISO timestamp.\n\nPrices are us-east-1 list prices at the time of writing: gp3 EBS at $0.08\nper GiB-month, EBS snapshots at $0.05 per GiB-month, and a public IPv4\naddress at $3.65 per month. Every price can be overridden, because list\nprices change and this is an estimate, not an invoice.\n\nUsage:\n    python3 orphan_resources.py inventory.json\n    python3 orphan_resources.py inventory.json --snapshot-days 180\n    python3 orphan_resources.py inventory.json --volume-price 0.10\n    python3 orphan_resources.py --help\n\nCI (fail the job when an orphan appears):\n    python3 orphan_resources.py /etc/s0sta/cloud.json || exit 1\n\nCron (weekly waste report):\n    0 7 * * 1 python3 /opt/s0sta/orphan_resources.py /etc/s0sta/cloud.json\n\nExit codes: 0 no orphan, 1 at least one orphan, 2 usage, unreadable JSON or\na record that does not match the shape.\n\nRead-only: reads one JSON file. It does not call a cloud API, change a tag\nor delete a resource; the describe-tags command printed at the end is a\nsuggestion for a human.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timedelta, timezone\n\nKINDS = (\"volume\", \"address\", \"snapshot\")\nLISTS = ((\"volume\", \"volumes\"), (\"address\", \"addresses\"),\n         (\"snapshot\", \"snapshots\"))\n\n\ndef load_json(path):\n    \"\"\"Read a JSON file, or raise ValueError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except (OSError, UnicodeError, json.JSONDecodeError) as exc:\n        raise ValueError(\"%s: %s\" % (path, exc))\n\n\ndef whole(value, where, field):\n    \"\"\"Return a non-negative whole number, or raise ValueError.\"\"\"\n    if isinstance(value, bool) or not isinstance(value, (int, float)):\n        raise ValueError(\"%s: %s must be a number\" % (where, field))\n    if value < 0 or int(value) != value:\n        raise ValueError(\"%s: %s must be a non-negative whole number\"\n                         % (where, field))\n    return int(value)\n\n\ndef parse_time(text, where, field=\"created\"):\n    \"\"\"Parse an ISO 8601 timestamp; a naive value is treated as UTC.\"\"\"\n    if not isinstance(text, str) or not text.strip():\n        raise ValueError(\"%s: %s is missing\" % (where, field))\n    raw = text.strip()\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        value = datetime.fromisoformat(raw)\n    except ValueError:\n        raise ValueError(\"%s: bad %s %r\" % (where, field, text))\n    if value.tzinfo is None:\n        value = value.replace(tzinfo=timezone.utc)\n    return value.astimezone(timezone.utc)\n\n\ndef inventory_rows(document, path):\n    \"\"\"Return a flat list of rows with a kind field, or raise ValueError.\"\"\"\n    if isinstance(document, dict) and \"resources\" in document:\n        items = document[\"resources\"]\n        if not isinstance(items, list):\n            raise ValueError(\"%s: resources must be a list\" % path)\n        rows = []\n        for index, item in enumerate(items):\n            if not isinstance(item, dict):\n                raise ValueError(\"%s resource %d is not an object\"\n                                 % (path, index + 1))\n            rows.append(dict(item))\n        return rows\n    if isinstance(document, dict):\n        rows = []\n        known = False\n        for kind, key in LISTS:\n            if key not in document:\n                continue\n            known = True\n            items = document[key]\n            if not isinstance(items, list):\n                raise ValueError(\"%s: %s must be a list\" % (path, key))\n            for index, item in enumerate(items):\n                if not isinstance(item, dict):\n                    raise ValueError(\"%s %s %d is not an object\"\n                                     % (path, key, index + 1))\n                row = dict(item)\n                row[\"kind\"] = kind\n                rows.append(row)\n        if not known:\n            raise ValueError(\"%s: expected resources or volumes, addresses \"\n                             \"and snapshots\" % path)\n        return rows\n    raise ValueError(\"%s: expected a JSON object\" % path)\n\n\ndef validate(rows):\n    \"\"\"Check every row and return the normalised records, or raise ValueError.\"\"\"\n    seen = set()\n    records = []\n    for index, row in enumerate(rows):\n        where = \"resource %d\" % (index + 1)\n        kind = row.get(\"kind\")\n        if kind not in KINDS:\n            raise ValueError(\"%s: kind must be volume, address or snapshot\"\n                             % where)\n        rid = row.get(\"id\")\n        if not isinstance(rid, str) or not rid.strip():\n            raise ValueError(\"%s: id is missing\" % where)\n        rid = rid.strip()\n        if (kind, rid) in seen:\n            raise ValueError(\"%s: duplicate %s %s\" % (where, kind, rid))\n        seen.add((kind, rid))\n        record = {\"kind\": kind, \"id\": rid}\n        if kind == \"volume\":\n            record[\"size_gb\"] = whole(row.get(\"size_gb\"), where, \"size_gb\")\n            state = row.get(\"state\")\n            if not isinstance(state, str) or not state.strip():\n                raise ValueError(\"%s: state is missing\" % where)\n            record[\"state\"] = state.strip()\n        elif kind == \"address\":\n            state = row.get(\"state\")\n            if state is not None and not isinstance(state, str):\n                raise ValueError(\"%s: state must be a string\" % where)\n            assoc = row.get(\"association_id\")\n            if assoc is not None and not isinstance(assoc, str):\n                raise ValueError(\"%s: association_id must be a string\" % where)\n            record[\"state\"] = (state or \"\").strip() or \"unassociated\"\n            record[\"association_id\"] = assoc or \"\"\n            public = row.get(\"public_ip\")\n            record[\"public_ip\"] = public.strip() if isinstance(public, str) else \"\"\n        else:\n            record[\"size_gb\"] = whole(row.get(\"size_gb\"), where, \"size_gb\")\n            record[\"created\"] = parse_time(row.get(\"created\"), where)\n        records.append(record)\n    return records\n\n\ndef parse_args(argv):\n    parser = argparse.ArgumentParser(\n        description=\"Report orphaned cloud resources and monthly waste.\")\n    parser.add_argument(\"inventory\", help=\"cloud inventory JSON\")\n    parser.add_argument(\"--snapshot-days\", type=int, default=90,\n                        help=\"flag snapshots older than this (default 90)\")\n    parser.add_argument(\"--volume-price\", type=float, default=0.08,\n                        help=\"EBS price per GiB-month (default 0.08)\")\n    parser.add_argument(\"--snapshot-price\", type=float, default=0.05,\n                        help=\"snapshot price per GiB-month (default 0.05)\")\n    parser.add_argument(\"--address-price\", type=float, default=3.65,\n                        help=\"idle public IPv4 price per month (default 3.65)\")\n    parser.add_argument(\"--now\",\n                        help=\"ISO 8601 reference time for snapshot ages\")\n    args = parser.parse_args(argv)\n    if args.snapshot_days < 0:\n        parser.error(\"--snapshot-days must be zero or more\")\n    for name in (\"volume_price\", \"snapshot_price\", \"address_price\"):\n        if getattr(args, name) < 0:\n            parser.error(\"--%s must be zero or more\" % name.replace(\"_\", \"-\"))\n    return args\n\n\ndef main(argv=None):\n    args = parse_args(argv)\n    try:\n        document = load_json(args.inventory)\n        rows = inventory_rows(document, args.inventory)\n        records = validate(rows)\n        as_of = parse_time(args.now, \"--now\", \"timestamp\") if args.now else \\\n            datetime.now(timezone.utc)\n    except ValueError as exc:\n        print(\"orphan_resources: %s\" % exc, file=sys.stderr)\n        return 2\n\n    cutoff = as_of - timedelta(days=args.snapshot_days)\n    orphans = []\n    for record in records:\n        kind = record[\"kind\"]\n        if kind == \"volume\":\n            if record[\"state\"] == \"available\":\n                cost = record[\"size_gb\"] * args.volume_price\n                orphans.append((cost, record, \"available\"))\n        elif kind == \"address\":\n            if not record[\"association_id\"] and record[\"state\"] != \"associated\":\n                orphans.append((args.address_price, record, \"unassociated\"))\n        else:\n            if record[\"created\"] < cutoff:\n                age = max(0, (as_of - record[\"created\"]).days)\n                cost = record[\"size_gb\"] * args.snapshot_price\n                orphans.append((cost, record, \"%dd\" % age))\n\n    orphans.sort(key=lambda row: (-row[0], row[1][\"kind\"], row[1][\"id\"]))\n    for cost, record, reason in orphans:\n        if record[\"kind\"] == \"address\":\n            detail = record[\"public_ip\"] or \"-\"\n            print(\"ORPHAN address %s %s %s $%.2f/mo\"\n                  % (record[\"id\"], detail, reason, cost))\n        elif record[\"kind\"] == \"snapshot\":\n            print(\"ORPHAN snapshot %s %d GiB %s $%.2f/mo\"\n                  % (record[\"id\"], record[\"size_gb\"], reason, cost))\n        else:\n            print(\"ORPHAN volume %s %d GiB %s $%.2f/mo\"\n                  % (record[\"id\"], record[\"size_gb\"], reason, cost))\n    if not orphans:\n        print(\"OK no orphaned volumes, addresses or snapshots\")\n        return 0\n\n    waste = sum(cost for cost, _record, _reason in orphans)\n    print(\"WASTE %d resources $%.2f/mo\" % (len(orphans), waste))\n    print(\"SUGGEST aws ec2 describe-tags --filters \"\n          \"Name=resource-id,Values=%s\" % orphans[0][1][\"id\"])\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p617","n":"617","slug":"backup-and-cost-control","title":"Backup and cost control","sections":["it","devsecops"],"topic":"Backup & recovery","tags":["Backup","DevOps","SRE","CloudCost","DisasterRecovery"],"series_name":"PythonFixes","design":"architecture","script":"","date":"2030-09-05","files":{"it":"notes/p617-backup-and-cost-control.md","devsecops":"notes/p617-backup-and-cost-control.md"},"md":"# Backup and cost control\n\nThe backup run is green and the bill is real. Three tiers decide whether the copy is cheap and whether it is true.\n\n## Quick reference\n\n- **Backup:** Volume snapshot (SNAP) · Object archive (ARCH)\n- **Verify:** Restore drill (DRILL) · Checksum (HASH)\n- **Retire:** Retention policy (KEEP) · Delete (DEL)\n\n**Request flow:** Volume snapshot → Restore drill → Retention policy → Delete; Object archive → Checksum → Retention policy\n\n**Around it:** Exit code (EXIT) · Cost report (COST)\n\n## Explained\n\n### Backup\n\nA volume snapshot for the fast copy, and an object archive for the slow one. The snapshot is the one you restore today; the archive is the one you keep for the year. Both are useless until something proves them.\n\n### Verify\n\nA restore drill for the path, and a checksum for the bytes. The drill opens the archive, lists the expected paths and fails on a missing one; the checksum catches a file that changed after it was written. The drawing runs from each backup node to its check, then both checks feed retention.\n\n### Retire\n\nA retention policy and a delete step. The policy says what stays: seven daily, four weekly, twelve monthly. The delete step runs on a schedule, so the archive does not become the largest line on the invoice.\n\n### The flow\n\nSnapshot to drill, archive to checksum, drill and checksum to retention, retention to delete. Two nodes per tier, five arrows, one exit code.\n\n> **The honest caveat:** verification costs time and compute, and a checksum is not a restore. The drill is the only tier that proves the data is readable.\n\n> **The lesson:** a backup is a process with three tiers, not a file in a bucket.\n\n## In one line\n\n> **Take the copy, then test the copy.** The copy that restores is the only one worth paying for.\n\n**Drill the archive**\n\n```bash\npython3 restore_drill.py backup.tar\n```\n"},{"id":"p618","n":"618","slug":"the-backup-verification-cheat-sheet","title":"The backup verification cheat sheet","sections":["it","devsecops"],"topic":"Backup & recovery","tags":["Backup","Linux","DevOps","SRE","CloudStorage"],"series_name":"PythonFixes","design":"cheatsheet","script":"","date":"2030-09-08","files":{"it":"notes/p618-the-backup-verification-cheat-sheet.md","devsecops":"notes/p618-the-backup-verification-cheat-sheet.md"},"md":"# The backup verification cheat sheet\n\nA backup nobody opened is a hope with a schedule. Four blocks of commands turn the hope into evidence.\n\n## Quick reference\n\n### Archive\n\n```bash\ntar -czf /backup/etc.tgz /etc\ntar -tzf /backup/etc.tgz | head\naws s3 cp etc.tgz s3://bkp/etc/\nsha256sum /backup/etc.tgz\n```\n\n### Test\n\n```bash\ntar -xzf /backup/etc.tgz -C /tmp/r\ntar -df /backup/etc.tgz\nsha256sum -c SHA256SUMS\ndiff -r /etc /tmp/r/etc | head\n```\n\n### Retention\n\n```bash\nfind /backup -mtime +30 -type f\nfind /backup -type f -ls\nlogrotate -d /etc/logrotate.d/bkp\naws s3 rm s3://bkp/old/ --dryrun\n```\n\n### Cost\n\n```bash\ndu -sh /backup/*\ndu -h --max-depth=1 /backup\naws s3 ls s3://bkp --recursive --summarize\naws s3 ls s3://bkp --recursive\n```\n\n## Explained\n\n### Archive\n\n`tar -czf /backup/etc.tgz /etc` builds the archive. `tar -tzf /backup/etc.tgz | head` lists it without writing a file. `aws s3 cp etc.tgz s3://bkp/etc/` moves the copy off the host. `sha256sum /backup/etc.tgz` prints the digest, and the digest belongs beside the archive, not in a chat message.\n\n### Test\n\n`tar -xzf /backup/etc.tgz -C /tmp/r` extracts to a scratch directory. `tar -df /backup/etc.tgz` compares the archive with the filesystem. `sha256sum -c SHA256SUMS` verifies the digest. diff -r `/etc /tmp/r/etc |` head shows what differs.\n\n### Retention\n\n`find /backup -mtime +30 -type f` lists what is older than the policy. `find /backup -type f -ls` shows the size and the path. `logrotate -d /etc/logrotate.d/bkp` tests the rule without moving a file. `aws s3 rm s3://bkp/old/ --dryrun` shows what a delete would remove.\n\n### Cost\n\n`du -sh /backup/*` shows the big directories. `du -h --max-depth=1 /backup` shows the split. `aws s3 ls s3://bkp --recursive --summarize` totals the objects. `aws s3 ls s3://bkp --recursive` lists every object and its size.\n\n> **The honest caveat:** a checksum proves the copy matches the digest, not that the archive holds the data you need. Only a restore into a scratch directory proves that.\n\n> **The lesson:** the archive is the easy part; the test is the work.\n\n## In one line\n\n> **A backup you never opened is a hope.** The commands are cheap; the restore is the proof.\n\n**Open one archive**\n\n```bash\ntar -tzf /backup/etc.tgz | head\n```\n"},{"id":"p619","n":"619","slug":"four-data-and-cost-leaks","title":"Four data and cost leaks","sections":["devsecops"],"topic":"Cloud: AWS & Azure","tags":["CloudCost","FinOps","SRE","DevOps","DataProtection"],"series_name":"PythonFixes","design":"explainer","script":"","date":"2030-09-10","files":{"devsecops":"notes/p619-four-data-and-cost-leaks.md"},"md":"# Four data and cost leaks\n\nA cost review finds four leaks in one afternoon, and none of them looks like a mistake on the day it is made.\n\n## Quick reference\n\n### Untested backups\n\n- A copy that never restored is a hypothesis\n- The dashboard is green, the restore is not\n- One drill a month is the cheapest test\n\n### Unbounded logs\n\n- Retention by size, not by hope\n- One chatty service can fill a disk\n- The archive is a second bill\n\n### Cold data in hot storage\n\n- Standard bills every idle byte\n- Requests per GB decides the class\n- Move it, then delete the source\n\n### Orphaned volumes\n\n- An unattached disk bills the same\n- An idle IP bills by the hour\n- A snapshot with no owner is forever\n\n> **Note:** Each leak is invisible until the invoice arrives.\n\n## Explained\n\n### Untested backups\n\nA copy that never restored is a hypothesis. The job writes a file, the dashboard turns green, and the first real test is the incident. The bill is not the problem here; the recovery time is.\n\n### Unbounded logs\n\nRetention by size, not by hope. One chatty service fills a disk, the archive becomes a second bill, and the useful line is buried in a terabyte. A policy with a number is the fix.\n\n### Cold data in hot storage\n\nStandard bills every idle byte. A bucket with a few requests per GiB is paying the hot price for cold data, and the invoice never names the bucket. Move it, then delete the source copy.\n\n### Orphaned volumes\n\nAn unattached disk bills the same as a busy one. An idle address bills by the hour, a snapshot with no owner lives for ever, and nobody sees the line because it belongs to the account, not the team.\n\n> **The honest caveat:** a leak is only a leak if nobody owns it. Some cold data is hot for a week, and some orphans are disaster-recovery copies that must stay.\n\n> **The lesson:** data costs money after it stops being useful, not while it is.\n\n## In one line\n\n> **Nothing leaks faster than storage.** Four leaks that never show up as a line item with an owner.\n\n**Count the orphans**\n\n```bash\npython3 orphan_resources.py inv.json\n```\n"},{"id":"p620","n":"620","slug":"5-backup-mistakes","title":"5 backup mistakes","sections":["it","devsecops"],"topic":"Backup & recovery","tags":["Backup","DisasterRecovery","SRE","DevOps","Security"],"series_name":"PythonFixes","design":"dontdo","script":"","date":"2030-09-12","files":{"it":"notes/p620-5-backup-mistakes.md","devsecops":"notes/p620-5-backup-mistakes.md"},"md":"# 5 backup mistakes\n\nThe region goes down at 04:00 and the backup bucket is in it. Five mistakes turn a backup plan into a story about one.\n\n## Quick reference\n\n**1.** ❌ **Never restore-test** — A backup that never restored is a hope, not a copy.  \n✅ **Drill one restore a month** — One path, off the backup host, timed and logged.\n\n**2.** ❌ **Keep one copy, one region** — The region that holds the bucket can be the region that fails.  \n✅ **Copy to a second region** — Cross-region replication with its own credentials.\n\n**3.** ❌ **No retention policy** — Every backup is kept, so the bill grows and the restore slows.  \n✅ **Retention: 7d, 4w, 12m** — Seven daily, four weekly, twelve monthly, then delete.\n\n**4.** ❌ **Unencrypted backups** — A stolen archive is the production database in a bag.  \n✅ **Encrypt at write, key separate** — The key lives in KMS, not on the backup host.\n\n**5.** ❌ **No owner on the backup** — Nobody knows what is inside, so nobody deletes it.  \n✅ **Owner and purpose tag** — A name and a restore purpose, checked each quarter.\n\n## Explained\n\n### The five\n\n1. **Never restore-test.** A backup that never restored is a hope, not a copy. Drill one restore a month: one path, off the backup host, timed and logged.\n   \\2. Keep one copy, one region. The region that holds the bucket can be the region that fails. Copy to a second region with its own credentials.\n   \\3. No retention policy. Every backup is kept, so the bill grows and the restore slows. Seven daily, four weekly and twelve monthly is a start; delete the rest on a schedule.\n   \\4. Unencrypted backups. A stolen archive is the production database in a bag. Encrypt at write and keep the key in KMS, not on the backup host.\n   \\5. No owner on the backup. Nobody knows what is inside, so nobody deletes it. Put an owner and a restore purpose in the tag, and check it each quarter.\n\n### What they share\n\nEach one is invisible while the job is green. The first restore is where they all arrive at once.\n\n> **The honest caveat:** a backup is a process, not a file. The policy, the key and the owner are the parts people skip, and they are the parts an audit asks for.\n\n> **The lesson:** a backup is a promise, and a restore is the only proof.\n\n## In one line\n\n> **One copy is not a backup.** Five mistakes, each one a restore that will not happen.\n\n**Restore one file**\n\n```bash\ntar -xzf backup.tgz -C /tmp/r\n```\n"},{"id":"p621","n":"621","slug":"quiz-3-data-questions-2","title":"Quiz: 3 data questions","sections":["it","devsecops"],"topic":"Backup & recovery","tags":["Python","DataProtection","FinOps","DevOps","SRE"],"series_name":"PythonFixes","design":"quiz","script":"","date":"2030-09-15","files":{"it":"notes/p621-quiz-3-data-questions-2.md","devsecops":"notes/p621-quiz-3-data-questions-2.md"},"md":"# Quiz: 3 data questions\n\nThree questions close the series, and each one is a decision you make about data you cannot see. Each answer is a test you can run this week.\n\n## Quick reference\n\n**Q1. Why is a backup without a restore test only a hope?**\n\n- a) It proves a copy, not a restore\n- b) The archive cannot be opened twice\n- c) A checksum proves the bytes\n\n> **Answer:** a) It proves a copy, not a restore. The job writes a file; only a restore proves the file can give the data back. A missing path or an unreadable archive stays invisible until the day you need it.\n\n**Q2. What drives an S3 bill the most?**\n\n- a) Stored bytes, class and requests\n- b) The number of buckets\n- c) The length of the bucket name\n\n> **Answer:** a) Stored bytes, class and requests. Bytes times the class price, plus requests and retrieval. Cold data in STANDARD can cost six times its GLACIER price, and the line item never names the bucket.\n\n**Q3. Why do orphaned volumes stay for years?**\n\n- a) No owner sees the line item\n- b) They cannot be deleted\n- c) They are free while unattached\n\n> **Answer:** a) No owner sees the line item. Billing is per account, not per team. An unattached disk has no owner to notice it, and the line item says EBS, not vol-0a1. A tag and a weekly report are the fix.\n\n## Explained\n\n### Question one\n\nWhy is a backup without a restore test only a hope? Because the job proves a copy exists, not that it works. A missing path or an unreadable archive stays invisible until the day you need it, and the first restore is the worst time to find out.\n\n### Question two\n\nWhat drives an S3 bill? Stored bytes times the class price, plus requests and retrieval. A large bucket of cold data in STANDARD can cost six times what it would in GLACIER, and the line item never names the bucket.\n\n### Question three\n\nWhy do orphaned volumes stay for years? Because billing is per account, not per team. An unattached disk has no owner to notice it, and the line item says EBS, not vol-0a1. A tag and a weekly report are the fix.\n\n> **The honest caveat:** passing this quiz does not make a backup restorable or a bill correct. The drill and the inventory do, and both are work you have to run.\n\n> **The lesson:** data costs money until someone proves it is useful.\n\nThat is the series. Fifty posts, five phases and twenty-five Python tools, from the first host incident to this quiz. Thank you for following along.\n\n## In one line\n\n> **Back it up, then prove it.** Fifty posts end with the copy you can actually restore.\n\n**Run the drill**\n\n```bash\npython3 restore_drill.py backup.tar\n```\n"},{"id":"p622","n":"622","slug":"logs-that-never-rotate","title":"Logs that never rotate","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","SysAdmin","DevOps","Automation"],"series_name":"ShellSec","design":"code","script":"log_rotate_check.sh","date":"2030-09-17","files":{"it":"notes/p622-logs-that-never-rotate.md","devsecops":"notes/p622-logs-that-never-rotate.md"},"md":"# Logs that never rotate\n\nA 40 GB log partition fills at 03:40, and every service that writes to it fails. logrotate is installed, its configuration is valid, and the file that grew has no matching stanza.\n\n## Quick reference\n\nThe full script is at the end of this note: `log_rotate_check.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash log_rotate_check.sh /tmp/p622/logs\n```\n\n**Example output**\n\n```text\nFINDING 2 flags in /tmp/p622/logs\nALERT /tmp/p622/logs/app.log 3104MB over 100MB\n```\n\n## Explained\n\n### What it does\n\n`log_rotate_check.sh` walks directories and applies three limits: a `file over --max-mb (100` by default), a file untouched for `--max-days (30`), and a directory over `--dir-budget (1024` MB). It prints a FINDING line, an ALERT for the worst item and one logrotate stanza. It reads metadata only and changes nothing.\n\n### What the screen shows\n\nThe fourteen lines count files over the size limit and older than 30 days, then print the summary and the largest offender. On the fixture: FINDING 2 flags in `/tmp/p622/logs` and ALERT `/tmp/p622/logs/app.log 3104MB` over 100MB.\n\n### How to run it\n\nbash `log_rotate_check.sh --max-mb 200 /var/log`. Exit 0 is clean, 1 a finding, 2 usage or an unreadable directory. The cron line is `0 6 * * * /usr/local/bin/log_rotate_check.sh /var/log`.\n\n### The fix\n\nGive the file a size stanza: size 200M, rotate 30, compress. Give the quiet directory a time policy. Rotate before you delete, because rotation keeps the last copy and a delete keeps nothing. Then cap the producer that writes without a bound.\n\n### Prevention\n\nRun the check daily and alert on exit 1. Keep `--dir-budget` below the filesystem's real capacity, and check inodes with `df -i`.\n\n> **The honest caveat:** the script measures the tree you point it at; a log written to journald or a socket never appears.\n\n> **The lesson:** a full disk is usually a policy failure, not a storage failure.\n\n## In one line\n\n> **The disk fills by policy.** Rotation is a policy you write before the disk is full.\n\n**Check the tree:** bash `log_rotate_check.sh /var/log`\n\n## Script: log_rotate_check.sh\n\nDownload: [scripts/log_rotate_check.sh](../scripts/log_rotate_check.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# log_rotate_check.sh - find the logs that will fill the disk before they do.\n# Fixes: a file over --max-mb, a file untouched for --max-days, and a directory\n#        over --dir-budget. Read-only: prints findings and one logrotate stanza,\n#        changes nothing.\n#\n# Usage:\n#   log_rotate_check.sh [options] DIR [DIR...]\n#   log_rotate_check.sh --max-mb 200 --max-days 14 /var/log\n#\n# Options:\n#   --max-mb N      flag files larger than N MiB (default 100)\n#   --max-days N    flag files older than N days (default 30)\n#   --dir-budget N  flag directories over N MiB (default 1024)\n#   --top N         show at most N details per class (default 5)\n#   -h, --help      show this help and exit 0\n#\n# Exit codes: 0 clean, 1 findings, 2 usage or unreadable input.\n# Needs: GNU findutils, coreutils (find, stat, du, sort), awk. Linux.\n# Cron: 0 6 * * * /usr/local/bin/log_rotate_check.sh /var/log\n# By s0sta.com\n\nusage() {\n    cat <<'EOF'\nlog_rotate_check.sh - find the logs that will fill the disk before they do.\n\nUsage:\n  log_rotate_check.sh [options] DIR [DIR...]\n  log_rotate_check.sh --max-mb 200 --max-days 14 /var/log\n\nOptions:\n  --max-mb N      flag files larger than N MiB (default 100)\n  --max-days N    flag files older than N days (default 30)\n  --dir-budget N  flag directories over N MiB (default 1024)\n  --top N         show at most N details per class (default 5)\n  -h, --help      show this help and exit 0\n\nRead-only. Prints one suggested logrotate stanza per directory.\nExit codes: 0 clean, 1 findings, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n    printf 'ERROR %s\\n' \"$*\" >&2\n    exit 2\n}\n\nis_number() {\n    case \"$1\" in\n        ''|*[!0-9]*) return 1 ;;\n        *) return 0 ;;\n    esac\n}\n\nmax_mb=100\nmax_days=30\nbudget_mb=1024\ntop=5\ndirs=()\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        --max-mb)       [ \"$#\" -ge 2 ] || die \"missing value for $1\"; max_mb=$2; shift 2 ;;\n        --max-days)     [ \"$#\" -ge 2 ] || die \"missing value for $1\"; max_days=$2; shift 2 ;;\n        --dir-budget)   [ \"$#\" -ge 2 ] || die \"missing value for $1\"; budget_mb=$2; shift 2 ;;\n        --top)          [ \"$#\" -ge 2 ] || die \"missing value for $1\"; top=$2; shift 2 ;;\n        -h|--help)      usage; exit 0 ;;\n        --)             shift; while [ \"$#\" -gt 0 ]; do dirs+=(\"$1\"); shift; done ;;\n        -*)             die \"unknown option: $1 (try --help)\" ;;\n        *)              dirs+=(\"$1\"); shift ;;\n    esac\ndone\n\nfor value in \"$max_mb\" \"$max_days\" \"$budget_mb\" \"$top\"; do\n    is_number \"$value\" || die \"not a number: $value\"\ndone\n[ \"${#dirs[@]}\" -gt 0 ] || die \"no directory given (try --help)\"\nfor dir in \"${dirs[@]}\"; do\n    [ -d \"$dir\" ] || die \"not a directory: $dir\"\n    [ -r \"$dir\" ] || die \"unreadable directory: $dir\"\ndone\n\nif stat -c %s / >/dev/null 2>&1; then\n    STAT_STYLE=gnu\nelse\n    STAT_STYLE=bsd\nfi\n\nsize_bytes() {\n    if [ \"$STAT_STYLE\" = gnu ]; then\n        stat -c %s -- \"$1\" 2>/dev/null || printf '0'\n    else\n        stat -f %z -- \"$1\" 2>/dev/null || printf '0'\n    fi\n}\n\nmtime_epoch() {\n    if [ \"$STAT_STYLE\" = gnu ]; then\n        stat -c %Y -- \"$1\" 2>/dev/null || printf '0'\n    else\n        stat -f %m -- \"$1\" 2>/dev/null || printf '0'\n    fi\n}\n\nnow=$(date +%s)\ntotal=0\n\nfor dir in \"${dirs[@]}\"; do\n    big_count=0\n    old_count=0\n    budget_hit=0\n    big_list=()\n    old_list=()\n    biggest=\"\"\n    biggest_size=0\n    oldest=\"\"\n    oldest_days=0\n\n    while IFS= read -r -d '' file; do\n        big_count=$((big_count + 1))\n        bytes=$(size_bytes \"$file\")\n        big_list+=(\"$bytes\"$'\\t'\"$file\")\n        if [ \"$bytes\" -gt \"$biggest_size\" ]; then\n            biggest=$file\n            biggest_size=$bytes\n        fi\n    done < <(find \"$dir\" -type f -size +\"${max_mb}\"M -print0 2>/dev/null)\n\n    while IFS= read -r -d '' file; do\n        old_count=$((old_count + 1))\n        mtime=$(mtime_epoch \"$file\")\n        days=$(( (now - mtime) / 86400 ))\n        old_list+=(\"$days\"$'\\t'\"$file\")\n        if [ \"$days\" -gt \"$oldest_days\" ]; then\n            oldest=$file\n            oldest_days=$days\n        fi\n    done < <(find \"$dir\" -type f -mtime +\"${max_days}\" -print0 2>/dev/null)\n\n    dir_mb=$(du -sm \"$dir\" 2>/dev/null | awk '{print $1}')\n    dir_mb=${dir_mb:-0}\n    if [ \"$dir_mb\" -gt \"$budget_mb\" ]; then\n        budget_hit=1\n    fi\n\n    flags=$((big_count + old_count + budget_hit))\n    total=$((total + flags))\n    if [ \"$flags\" -eq 1 ]; then\n        label=flag\n    else\n        label=flags\n    fi\n    printf 'FINDING %d %s in %s\\n' \"$flags\" \"$label\" \"$dir\"\n\n    alert=\"\"\n    skip_big=\"\"\n    skip_old=\"\"\n    if [ \"$biggest_size\" -gt 0 ]; then\n        alert=big\n        skip_big=$biggest\n    elif [ \"$oldest_days\" -gt 0 ]; then\n        alert=old\n        skip_old=$oldest\n    elif [ \"$budget_hit\" -eq 1 ]; then\n        alert=budget\n    fi\n\n    if [ \"$alert\" = big ]; then\n        biggest_mb=$((biggest_size / 1048576))\n        printf 'ALERT %s %dMB over %dMB\\n' \"$biggest\" \"$biggest_mb\" \"$max_mb\"\n    elif [ \"$alert\" = old ]; then\n        printf 'ALERT %s %dd untouched (max %dd)\\n' \"$oldest\" \"$oldest_days\" \"$max_days\"\n    elif [ \"$alert\" = budget ]; then\n        printf 'ALERT %s %dMB over %dMB\\n' \"$dir\" \"$dir_mb\" \"$budget_mb\"\n    fi\n\n    if [ \"${#big_list[@]}\" -gt 0 ]; then\n        printf '%s\\n' \"${big_list[@]}\" \\\n            | sort -rn \\\n            | awk -F'\\t' -v n=\"$top\" -v skip=\"$skip_big\" \\\n                'NR <= n && $2 != skip { printf \"BIG %s %dMB\\n\", $2, $1 / 1048576 }'\n    fi\n\n    if [ \"${#old_list[@]}\" -gt 0 ]; then\n        printf '%s\\n' \"${old_list[@]}\" \\\n            | sort -rn \\\n            | awk -F'\\t' -v n=\"$top\" -v skip=\"$skip_old\" \\\n                'NR <= n && $2 != skip { printf \"OLD %s %dd\\n\", $2, $1 }'\n    fi\n\n    if [ \"$budget_hit\" -eq 1 ] && [ \"$alert\" != budget ]; then\n        printf 'BUDGET %s %dMB over %dMB\\n' \"$dir\" \"$dir_mb\" \"$budget_mb\"\n    fi\n\n    if [ \"$alert\" = big ]; then\n        printf 'ROTATE %s: size %dM, rotate %d, compress\\n' \"$biggest\" \"$max_mb\" \"$max_days\"\n    elif [ \"$alert\" = old ]; then\n        printf 'ROTATE %s: daily, rotate 14, maxage %d, compress\\n' \"$oldest\" \"$max_days\"\n    fi\ndone\n\nif [ \"$total\" -gt 0 ]; then\n    exit 1\nfi\nexit 0\n```\n"},{"id":"p623","n":"623","slug":"accounts-hiding-in-plain-sight","title":"Accounts hiding in plain sight","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","SysAdmin","DevSecOps"],"series_name":"ShellSec","design":"code","script":"user_audit.sh","date":"2030-09-19","files":{"it":"notes/p623-accounts-hiding-in-plain-sight.md","devsecops":"notes/p623-accounts-hiding-in-plain-sight.md"},"md":"# Accounts hiding in plain sight\n\nThe access review said twelve administrators. `/etc/passwd` had thirteen accounts with UID 0, and the extra one had a home directory nobody recognised.\n\n## Quick reference\n\nThe full script is at the end of this note: `user_audit.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash user_audit.sh --passwd /tmp/p623/passwd\n```\n\n**Example output**\n\n```text\nFINDING 2 account risks\nALERT uid 0: toor (/bin/bash)\n```\n\n## Explained\n\n### What it does\n\n`user_audit.sh` reads `/etc/passwd` and, when you pass `--shadow`, a shadow export. It flags four things: a second UID 0 account, an empty password field, an expired account or password, and a system account with a login shell. `--today` fixes the date so a test is repeatable.\n\n### What the screen shows\n\nThe fourteen lines in the GIF read the passwd file, catch UID 0 accounts and service accounts with a real shell, then print the count and the first risk. On the fixture: FINDING 2 account risks and ALERT uid 0: toor (`/bin/bash`).\n\n### How to run it\n\nbash `user_audit.sh --passwd /etc/passwd`, then add `--shadow /root/shadow.export`. Exit 0 is clean, 1 is a finding, 2 is usage or an unreadable file. Run it weekly from cron.\n\n### The fix\n\nA second UID 0 account is either a mistake or a backdoor. Lock it, log it, and remove it after the change window. Replace a service account's shell with `/usr/sbin/nologin`. Lock an empty password with `passwd -l`, and expire the account with `chage -E 0`.\n\n### Prevention\n\nKeep a known-good list of UID 0 accounts and login shells in version control, and diff it in the image and on the host.\n\n> **The honest caveat:** a shadow field of ! or \\* means locked, not empty; only a truly empty field is reported.\n\n> **The lesson:** a login shell is a door, and a system account rarely needs one.\n\n## In one line\n\n> **UID 0 is a shared secret.** A login shell on a system account is a door left open.\n\n**Audit the host:** bash `user_audit.sh --passwd /etc/passwd`\n\n## Script: user_audit.sh\n\nDownload: [scripts/user_audit.sh](../scripts/user_audit.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# user_audit.sh - find the accounts that should not be able to log in.\n# Fixes: a second UID 0 account, an empty password field, an expired account,\n#        and a service account with a login shell. Read-only.\n#\n# Usage:\n#   user_audit.sh [options]\n#   user_audit.sh --shadow /root/shadow.export --today 2026-10-21\n#\n# Options:\n#   --passwd FILE   passwd file to read (default /etc/passwd)\n#   --shadow FILE   shadow export to read (default: none; needs root)\n#   --today DATE    treat DATE as today, YYYY-MM-DD (default: system date)\n#   -h, --help      show this help and exit 0\n#\n# Exit codes: 0 clean, 1 findings, 2 usage or unreadable input.\n# Needs: awk, date. Linux; /etc/shadow is usually readable by root only, so\n#        export it first: getent shadow > /root/shadow.export\n# Cron: 15 6 * * 1 /usr/local/bin/user_audit.sh --shadow /root/shadow.export\n# By s0sta.com\n\nusage() {\n    cat <<'EOF'\nuser_audit.sh - find the accounts that should not be able to log in.\n\nUsage:\n  user_audit.sh [options]\n  user_audit.sh --shadow /root/shadow.export --today 2026-10-21\n\nOptions:\n  --passwd FILE   passwd file to read (default /etc/passwd)\n  --shadow FILE   shadow export to read (default: none; needs root)\n  --today DATE    treat DATE as today, YYYY-MM-DD (default: system date)\n  -h, --help      show this help and exit 0\n\nChecks: UID 0 accounts other than root, empty password fields, expired\naccounts (password and account expiry), and system accounts with a login\nshell. Read-only.\nExit codes: 0 clean, 1 findings, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n    printf 'ERROR %s\\n' \"$*\" >&2\n    exit 2\n}\n\nis_number() {\n    case \"$1\" in\n        ''|*[!0-9]*) return 1 ;;\n        *) return 0 ;;\n    esac\n}\n\npasswd_file=/etc/passwd\nshadow_file=\"\"\ntoday=\"\"\ndetails=()\nalert=\"\"\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        --passwd)  [ \"$#\" -ge 2 ] || die \"missing value for $1\"; passwd_file=$2; shift 2 ;;\n        --shadow)  [ \"$#\" -ge 2 ] || die \"missing value for $1\"; shadow_file=$2; shift 2 ;;\n        --today)   [ \"$#\" -ge 2 ] || die \"missing value for $1\"; today=$2; shift 2 ;;\n        -h|--help) usage; exit 0 ;;\n        *)         die \"unknown option: $1 (try --help)\" ;;\n    esac\ndone\n\n[ -r \"$passwd_file\" ] || die \"cannot read passwd file: $passwd_file\"\nif [ -n \"$shadow_file\" ]; then\n    [ -r \"$shadow_file\" ] || die \"cannot read shadow file: $shadow_file\"\nfi\n\nif [ -n \"$today\" ]; then\n    case \"$today\" in\n        [0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]) ;;\n        *) die \"bad --today, want YYYY-MM-DD: $today\" ;;\n    esac\n    if date -d \"$today\" +%s >/dev/null 2>&1; then\n        epoch=$(date -d \"$today\" +%s)\n    elif epoch=$(date -j -f \"%Y-%m-%d\" \"$today\" +%s 2>/dev/null); then\n        :\n    else\n        die \"cannot parse date: $today\"\n    fi\nelse\n    epoch=$(date +%s)\nfi\ntoday_days=$((epoch / 86400))\n\nadd_finding() {\n    details+=(\"$1\")\n    if [ -z \"$alert\" ]; then\n        alert=$2\n    fi\n}\n\nwhile IFS=: read -r name pw uid _ _ home shell; do\n    case \"$name\" in\n        ''|'#'*) continue ;;\n    esac\n\n    if [ \"$uid\" = 0 ] && [ \"$name\" != root ]; then\n        add_finding \"UID0 $name shell=$shell home=$home\" \"uid 0: $name ($shell)\"\n    fi\n\n    if [ -z \"$pw\" ]; then\n        add_finding \"EMPTY-PW $name passwd password field empty\" \"empty password: $name\"\n    fi\n\n    if [ -n \"$shadow_file\" ]; then\n        sline=$(awk -F: -v u=\"$name\" '$1 == u { print; exit }' \"$shadow_file\")\n        if [ -n \"$sline\" ]; then\n            IFS=: read -r _ spw slast _ smax _ _ sexpire _ <<< \"$sline\"\n\n            if [ -z \"$spw\" ]; then\n                add_finding \"EMPTY-PW $name shadow password field empty\" \"empty password: $name\"\n            fi\n\n            if is_number \"$sexpire\" && [ \"$sexpire\" -gt 0 ] && [ \"$sexpire\" -le \"$today_days\" ]; then\n                ago=$((today_days - sexpire))\n                add_finding \"EXPIRED $name account expired ${ago}d ago\" \"expired account: $name\"\n            fi\n\n            if is_number \"$slast\" && is_number \"$smax\" && [ \"$slast\" -gt 0 ] \\\n                && [ \"$smax\" -gt 0 ] && [ \"$smax\" -ne 99999 ]; then\n                pw_end=$((slast + smax))\n                if [ \"$pw_end\" -lt \"$today_days\" ]; then\n                    ago=$((today_days - pw_end))\n                    add_finding \"EXPIRED $name password expired ${ago}d ago\" \"expired account: $name\"\n                fi\n            fi\n        fi\n    fi\n\n    if is_number \"$uid\" && [ \"$uid\" -lt 1000 ] && [ \"$uid\" -ne 0 ]; then\n        case \"$shell\" in\n            ''|*/nologin|*/false|/bin/sync|/sbin/shutdown|/sbin/halt) ;;\n            *) add_finding \"SHELL $name shell=$shell uid=$uid\" \"service shell: $name ($shell)\" ;;\n        esac\n    fi\ndone < \"$passwd_file\"\n\nif [ \"${#details[@]}\" -eq 1 ]; then\n    label=risk\nelse\n    label=risks\nfi\nprintf 'FINDING %d account %s\\n' \"${#details[@]}\" \"$label\"\nif [ -n \"$alert\" ]; then\n    printf 'ALERT %s\\n' \"$alert\"\nfi\nif [ \"${#details[@]}\" -gt 0 ]; then\n    printf '%s\\n' \"${details[@]}\"\n    exit 1\nfi\nexit 0\n```\n"},{"id":"p624","n":"624","slug":"ports-nobody-remembers-opening","title":"Ports nobody remembers opening","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","Networking","CyberSecurity","SRE"],"series_name":"ShellSec","design":"code","script":"open_ports.sh","date":"2030-09-22","files":{"it":"notes/p624-ports-nobody-remembers-opening.md","devsecops":"notes/p624-ports-nobody-remembers-opening.md"},"md":"# Ports nobody remembers opening\n\nA crypto miner ran for eleven days on port 8080. Nobody opened it on purpose; a test harness started on 0.0.0.0 and was never stopped.\n\n## Quick reference\n\nThe full script is at the end of this note: `open_ports.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash open_ports.sh --ss /tmp/p624/ss.txt --allow /tmp/p624/ok\n```\n\n**Example output**\n\n```text\nFINDING 1 listener to review\nALERT 8080/tcp python3 on 0.0.0.0\n```\n\n## Explained\n\n### What it does\n\n`open_ports.sh` compares every listener in `ss -H -lntup` with an allowlist file. A port that is not listed is UNEXPECTED. A port allowed only on localhost but bound to 0.0.0.0 or [::] is WILDCARD. Each finding names the protocol, port, process and address; nothing is changed.\n\n### What the screen shows\n\nThe fourteen lines filter ss output to TCP and UDP, pull the port and address from each line, and compare them with the allowlist. On the fixture: FINDING 1 listener to review and ALERT 8080/tcp python3 on 0.0.0.0.\n\n### How to run it\n\nbash `open_ports.sh --allow /etc/s0sta/allowed_ports`. Pass `--ss capture.txt` for a saved host. Exit 0 is clean, 1 is a finding, 2 is usage or an unreadable file. Run it after every deploy and once a day from cron.\n\n### The fix\n\nStop and disable a listener that is not needed. Bind a local-only one to 127.0.0.1 behind a reverse proxy. Add the port to the allowlist with the right public or local qualifier.\n\n### Prevention\n\nKeep the allowlist in configuration management and review its diff. A new listener should be a pull request, not a discovery at 3 a.m.\n\n> **The honest caveat:** ss sees the network namespace it runs in, so a container host needs the host namespace and root for process names.\n\n> **The lesson:** a listener is a promise, and if nobody remembers making it, it is a finding.\n\n## In one line\n\n> **Every port is a decision.** A socket nobody wrote down is a socket nobody owns.\n\n**List the ports**\n\n```bash\nss -H -lntup\n```\n\n## Script: open_ports.sh\n\nDownload: [scripts/open_ports.sh](../scripts/open_ports.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# open_ports.sh - show the listeners nobody remembers opening.\n# Fixes: a listening port missing from the allowlist, and an allowed service\n#        bound to 0.0.0.0 when the allowlist says localhost only. Read-only.\n#\n# Usage:\n#   open_ports.sh [options]\n#   open_ports.sh --allow /etc/s0sta/allowed_ports\n#   open_ports.sh --ss /tmp/ss.txt --allow ./allowed_ports\n#\n# Options:\n#   --allow FILE   allowlist file (default /etc/s0sta/allowed_ports)\n#   --ss FILE      captured `ss -H -lntup` output (default: run ss)\n#   -h, --help     show this help and exit 0\n#\n# Allowlist format, one listener per line, PORT/PROTO [public|local]:\n#   22/tcp public\n#   443/tcp public\n#   3306/tcp local\n#   \"local\" is the default: a 0.0.0.0 or [::] bind is then a finding.\n#\n# Exit codes: 0 clean, 1 findings, 2 usage or unreadable input.\n# Needs: iproute2 (ss), grep, awk. Linux. Root shows process names.\n# Cron: 30 6 * * * /usr/local/bin/open_ports.sh\n# By s0sta.com\n\nusage() {\n    cat <<'EOF'\nopen_ports.sh - show the listeners nobody remembers opening.\n\nUsage:\n  open_ports.sh [options]\n  open_ports.sh --allow /etc/s0sta/allowed_ports\n  open_ports.sh --ss /tmp/ss.txt --allow ./allowed_ports\n\nOptions:\n  --allow FILE   allowlist file (default /etc/s0sta/allowed_ports)\n  --ss FILE      captured `ss -H -lntup` output (default: run ss)\n  -h, --help     show this help and exit 0\n\nAllowlist format: PORT/PROTO [public|local], one per line.\nExit codes: 0 clean, 1 findings, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n    printf 'ERROR %s\\n' \"$*\" >&2\n    exit 2\n}\n\nallow_file=/etc/s0sta/allowed_ports\nss_file=\"\"\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        --allow)   [ \"$#\" -ge 2 ] || die \"missing value for $1\"; allow_file=$2; shift 2 ;;\n        --ss)      [ \"$#\" -ge 2 ] || die \"missing value for $1\"; ss_file=$2; shift 2 ;;\n        -h|--help) usage; exit 0 ;;\n        *)         die \"unknown option: $1 (try --help)\" ;;\n    esac\ndone\n\n[ -r \"$allow_file\" ] || die \"cannot read allowlist: $allow_file\"\n\nif [ -n \"$ss_file\" ]; then\n    [ -r \"$ss_file\" ] || die \"cannot read ss output: $ss_file\"\n    ss_out=$(cat \"$ss_file\")\nelse\n    command -v ss >/dev/null 2>&1 || die \"ss not found; pass --ss FILE on non-Linux hosts\"\n    ss_out=$(ss -H -lntup) || die \"ss failed\"\nfi\n\nis_wildcard() {\n    case \"$1\" in\n        0.0.0.0|::|'*'|0:0:0:0:0:0:0:0) return 0 ;;\n        *) return 1 ;;\n    esac\n}\n\nfindings=()\n\nwhile IFS=$' \\t' read -r proto _ _ _ local_addr _ rest; do\n    case \"$proto\" in\n        tcp|udp) ;;\n        *) continue ;;\n    esac\n    [ -n \"${local_addr:-}\" ] || continue\n\n    addr=${local_addr%:*}\n    port=${local_addr##*:}\n    addr=${addr%%\\%*}\n    addr=${addr#[}\n    addr=${addr%]}\n    case \"$port\" in\n        ''|'*'|*[!0-9]*) continue ;;\n    esac\n\n    proc=\"?\"\n    if [ -n \"${rest:-}\" ]; then\n        proc=$(printf '%s\\n' \"$rest\" | sed -n 's/.*users:((\"\\([^\"]*\\)\".*/\\1/p')\n        proc=${proc:-?}\n    fi\n\n    key=\"$port/$proto\"\n    qual=$(awk -v k=\"$key\" '$1 == k { print ($2 == \"\" ? \"local\" : $2); found = 1; exit }\n                             END { if (!found) print \"absent\" }' \"$allow_file\")\n\n    kind=\"\"\n    if [ \"$qual\" = absent ]; then\n        kind=UNEXPECTED\n    elif is_wildcard \"$addr\" && [ \"$qual\" != public ]; then\n        kind=WILDCARD\n    fi\n\n    if [ -n \"$kind\" ]; then\n        findings+=(\"$(printf '%05d\\t%s %s %s on %s' \"$port\" \"$kind\" \"$key\" \"$proc\" \"$addr\")\")\n    fi\ndone <<< \"$ss_out\"\n\nsorted=\"\"\nif [ \"${#findings[@]}\" -gt 0 ]; then\n    sorted=$(printf '%s\\n' \"${findings[@]}\" | sort -u)\nfi\ncount=$(printf '%s\\n' \"$sorted\" | grep -c . || true)\n\nif [ \"$count\" -eq 1 ]; then\n    label=listener\nelse\n    label=listeners\nfi\nprintf 'FINDING %d %s to review\\n' \"$count\" \"$label\"\n\nif [ \"$count\" -gt 0 ]; then\n    first=$(printf '%s\\n' \"$sorted\" | sed -n '1p')\n    IFS=$'\\t' read -r _ detail <<< \"$first\"\n    printf 'ALERT %s\\n' \"${detail#* }\"\n    printf '%s\\n' \"$sorted\" | while IFS=$'\\t' read -r _ detail; do\n        printf '%s\\n' \"$detail\"\n    done\nfi\n\nif [ \"$count\" -gt 0 ]; then\n    exit 1\nfi\nexit 0\n```\n"},{"id":"p625","n":"625","slug":"services-that-died-quietly","title":"Services that died quietly","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","Systemd","SRE","DevOps"],"series_name":"ShellSec","design":"code","script":"service_health.sh","date":"2030-09-24","files":{"it":"notes/p625-services-that-died-quietly.md","devsecops":"notes/p625-services-that-died-quietly.md"},"md":"# Services that died quietly\n\nA service failed at 02:00 and systemd restarted it. By 08:00 the restart counter read 412, and every dashboard scrape had caught the unit active. Nothing was green; the samples were.\n\n## Quick reference\n\nThe full script is at the end of this note: `service_health.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash service_health.sh --units /tmp/p625/units.txt\n```\n\n**Example output**\n\n```text\nFINDING 1 service problem\nALERT api.service failed\n```\n\n## Explained\n\n### What it does\n\n`service_health.sh` reads `systemctl list-units --failed` and systemctl show properties: ActiveState, SubState, UnitFileState and NRestarts. It reports three classes: a failed unit, a unit restarted at least `--restarts` times (3 by default), and an enabled unit that is inactive. It never restarts anything.\n\n### What the screen shows\n\nThe thirteen lines in the GIF read the captured failed list, keep the service units, and print the count and the first failed unit. On the fixture: FINDING 1 service problem and ALERT `api.service` failed.\n\n### How to run it\n\nbash `service_health.sh`, or pass `--units` and `--props` captures from a host you cannot reach. Exit 0 is clean, 1 is a finding, 2 is usage or an unreadable file. The cron line is `45 6 * * * /usr/local/bin/service_health.sh`.\n\n### The fix\n\nRead `journalctl -u api.service --since '30` min ago' before you touch it. Fix the cause, then systemctl reset-failed `api.service` so the next failure starts clean. For a restart loop, cap it with StartLimitBurst and StartLimitIntervalSec instead of letting it spin.\n\n### Prevention\n\nAlert on the NRestarts delta and on ActiveState=failed, not only on the unit being inactive. A restart is a recovery attempt, not a recovery.\n\n> **The honest caveat:** NRestarts counts automatic restarts since the last reset, so it measures the recent past, not the year.\n\n> **The lesson:** active is a state, while healthy is a history.\n\n## In one line\n\n> **A restart is not a recovery.** Count restarts, not just the state at the last scrape.\n\n**Read the unit**\n\n```bash\nsystemctl status api --no-pager\n```\n\n## Script: service_health.sh\n\nDownload: [scripts/service_health.sh](../scripts/service_health.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# service_health.sh - find the services that died quietly.\n# Fixes: a failed unit, a unit the manager keeps restarting, and an enabled\n#        unit that is inactive. Read-only: it never starts, stops or restarts.\n#\n# Usage:\n#   service_health.sh [options]\n#   service_health.sh --units /tmp/failed.txt --props /tmp/show.txt\n#\n# Options:\n#   --units FILE    captured `systemctl list-units --failed` output\n#   --props FILE    captured `systemctl show` output (Id, ActiveState,\n#                   SubState, UnitFileState, NRestarts)\n#   --restarts N    flag a unit with N or more restarts (default 3)\n#   -h, --help      show this help and exit 0\n#\n# With no --units and no --props the script reads systemctl directly.\n# Exit codes: 0 clean, 1 findings, 2 usage or unreadable input.\n# Needs: systemd (systemctl), awk, sort. Linux.\n# Cron: 45 6 * * * /usr/local/bin/service_health.sh\n# By s0sta.com\n\nusage() {\n    cat <<'EOF'\nservice_health.sh - find the services that died quietly.\n\nUsage:\n  service_health.sh [options]\n  service_health.sh --units /tmp/failed.txt --props /tmp/show.txt\n\nOptions:\n  --units FILE    captured `systemctl list-units --failed` output\n  --props FILE    captured `systemctl show` output (Id, ActiveState,\n                  SubState, UnitFileState, NRestarts)\n  --restarts N    flag a unit with N or more restarts (default 3)\n  -h, --help      show this help and exit 0\n\nReports failed units, auto-restarting units and enabled-but-inactive units.\nRead-only.\nExit codes: 0 clean, 1 findings, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n    printf 'ERROR %s\\n' \"$*\" >&2\n    exit 2\n}\n\nis_number() {\n    case \"$1\" in\n        ''|*[!0-9]*) return 1 ;;\n        *) return 0 ;;\n    esac\n}\n\nunits_file=\"\"\nprops_file=\"\"\nmin_restarts=3\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        --units)    [ \"$#\" -ge 2 ] || die \"missing value for $1\"; units_file=$2; shift 2 ;;\n        --props)    [ \"$#\" -ge 2 ] || die \"missing value for $1\"; props_file=$2; shift 2 ;;\n        --restarts) [ \"$#\" -ge 2 ] || die \"missing value for $1\"; min_restarts=$2; shift 2 ;;\n        -h|--help)  usage; exit 0 ;;\n        *)          die \"unknown option: $1 (try --help)\" ;;\n    esac\ndone\n\nis_number \"$min_restarts\" || die \"not a number: $min_restarts\"\nif [ -n \"$units_file\" ]; then\n    [ -r \"$units_file\" ] || die \"cannot read units file: $units_file\"\nfi\nif [ -n \"$props_file\" ]; then\n    [ -r \"$props_file\" ] || die \"cannot read props file: $props_file\"\nfi\n\nif [ -z \"$units_file\" ] && [ -z \"$props_file\" ]; then\n    command -v systemctl >/dev/null 2>&1 || die \"systemctl not found; pass --units/--props\"\n    units_out=$(systemctl list-units --failed --no-legend --plain --type=service) \\\n        || die \"systemctl list-units failed\"\n    props_out=$(systemctl show --all --type=service \\\n        -p Id -p ActiveState -p SubState -p UnitFileState -p NRestarts) \\\n        || die \"systemctl show failed\"\nelse\n    units_out=\"\"\n    props_out=\"\"\n    [ -z \"$units_file\" ] || units_out=$(cat \"$units_file\")\n    [ -z \"$props_file\" ] || props_out=$(cat \"$props_file\")\nfi\n\nids=()\nactives=()\nsubs=()\nufiles=()\nrestart_counts=()\n\nadd_unit() {\n    local i\n    i=0\n    while [ \"$i\" -lt \"${#ids[@]}\" ]; do\n        if [ \"${ids[$i]}\" = \"$1\" ]; then\n            actives[i]=$2\n            subs[i]=$3\n            ufiles[i]=$4\n            restart_counts[i]=$5\n            return 0\n        fi\n        i=$((i + 1))\n    done\n    ids+=(\"$1\")\n    actives+=(\"$2\")\n    subs+=(\"$3\")\n    ufiles+=(\"$4\")\n    restart_counts+=(\"$5\")\n    return 0\n}\n\nif [ -n \"$props_out\" ]; then\n    id=\"\"\n    active=\"\"\n    sub=\"\"\n    ufile=\"\"\n    nrestarts=0\n    flush_unit() {\n        if [ -n \"$id\" ]; then\n            add_unit \"$id\" \"${active:-unknown}\" \"${sub:-unknown}\" \"${ufile:-unknown}\" \"${nrestarts:-0}\"\n        fi\n        id=\"\"\n        active=\"\"\n        sub=\"\"\n        ufile=\"\"\n        nrestarts=0\n        return 0\n    }\n    while IFS= read -r line; do\n        case \"$line\" in\n            Id=*)            id=${line#Id=} ;;\n            ActiveState=*)   active=${line#ActiveState=} ;;\n            SubState=*)      sub=${line#SubState=} ;;\n            UnitFileState=*) ufile=${line#UnitFileState=} ;;\n            NRestarts=*)     nrestarts=${line#NRestarts=} ;;\n            '')              flush_unit ;;\n        esac\n    done <<< \"$props_out\"\n    flush_unit\nfi\n\nmark_failed() {\n    local i\n    i=0\n    while [ \"$i\" -lt \"${#ids[@]}\" ]; do\n        if [ \"${ids[$i]}\" = \"$1\" ]; then\n            actives[i]=failed\n            return 0\n        fi\n        i=$((i + 1))\n    done\n    add_unit \"$1\" failed unknown unknown 0\n    return 0\n}\n\nif [ -n \"$units_out\" ]; then\n    while IFS= read -r line; do\n        line=${line#\"${line%%[![:space:]]*}\"}\n        unit=${line%% *}\n        case \"$unit\" in\n            *.service) ;;\n            *) continue ;;\n        esac\n        mark_failed \"$unit\"\n    done <<< \"$units_out\"\nfi\n\nfailed_list=()\nrestart_list=()\ninactive_list=()\n\nif [ \"${#ids[@]}\" -gt 0 ]; then\nfor i in \"${!ids[@]}\"; do\n    id=${ids[$i]}\n    active=${actives[$i]}\n    sub=${subs[$i]}\n    ufile=${ufiles[$i]}\n    nrestarts=${restart_counts[$i]}\n\n    if [ \"$active\" = failed ]; then\n        failed_list+=(\"$id|$active|$sub|$nrestarts\")\n    elif is_number \"$nrestarts\" && [ \"$nrestarts\" -ge \"$min_restarts\" ]; then\n        restart_list+=(\"$id|$nrestarts\")\n    elif [ \"$ufile\" = enabled ] && [ \"$active\" = inactive ]; then\n        inactive_list+=(\"$id\")\n    fi\ndone\nfi\n\ncount=$(( ${#failed_list[@]} + ${#restart_list[@]} + ${#inactive_list[@]} ))\nif [ \"$count\" -eq 1 ]; then\n    label=problem\nelse\n    label=problems\nfi\nprintf 'FINDING %d service %s\\n' \"$count\" \"$label\"\n\nif [ \"$count\" -gt 0 ]; then\n    if [ \"${#failed_list[@]}\" -gt 0 ]; then\n        first=${failed_list[0]}\n        IFS='|' read -r fid _ _ _ <<< \"$first\"\n        printf 'ALERT %s failed\\n' \"$fid\"\n    elif [ \"${#restart_list[@]}\" -gt 0 ]; then\n        first=${restart_list[0]}\n        IFS='|' read -r rid rnum <<< \"$first\"\n        printf 'ALERT %s restarted %s times\\n' \"$rid\" \"$rnum\"\n    else\n        printf 'ALERT %s enabled but inactive\\n' \"${inactive_list[0]}\"\n    fi\nfi\n\nif [ \"${#failed_list[@]}\" -gt 0 ]; then\n    printf '%s\\n' \"${failed_list[@]}\" | while IFS='|' read -r id active sub _; do\n        printf 'FAILED %s (%s/%s)\\n' \"$id\" \"$active\" \"$sub\"\n    done\nfi\nif [ \"${#restart_list[@]}\" -gt 0 ]; then\n    printf '%s\\n' \"${restart_list[@]}\" | while IFS='|' read -r id nrestarts; do\n        printf 'RESTARTING %s restarts=%s\\n' \"$id\" \"$nrestarts\"\n    done\nfi\nif [ \"${#inactive_list[@]}\" -gt 0 ]; then\n    printf '%s\\n' \"${inactive_list[@]}\" | while IFS= read -r id; do\n        printf 'INACTIVE-ENABLED %s enabled but inactive\\n' \"$id\"\n    done\nfi\n\nif [ \"$count\" -gt 0 ]; then\n    exit 1\nfi\nexit 0\n```\n"},{"id":"p626","n":"626","slug":"packages-that-changed-without-a-deploy","title":"Packages that changed without a deploy","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","DevSecOps","SysAdmin"],"series_name":"ShellSec","design":"code","script":"package_integrity.sh","date":"2030-09-26","files":{"it":"notes/p626-packages-that-changed-without-a-deploy.md","devsecops":"notes/p626-packages-that-changed-without-a-deploy.md"},"md":"# Packages that changed without a deploy\n\nThe web shell changed on a Tuesday. No deploy ran, no package upgraded, and `rpm -Va` could have answered in one pass.\n\n## Quick reference\n\nThe full script is at the end of this note: `package_integrity.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash package_integrity.sh --verify /tmp/v --owners /tmp/o\n```\n\n**Example output**\n\n```text\nFINDING files changed: 3, packages: 2\nALERT top package: openssh-server (2 of 3)\n```\n\n## Explained\n\n### What it does\n\n`package_integrity.sh` runs `rpm -Va` on RHEL-family hosts or `dpkg --verify` on Debian hosts. It keeps the changed paths, attributes each to its package, counts both, and exits 1 on a finding. The digest column matters: 5 means the content no longer matches the database.\n\n### What the screen shows\n\nThe fourteen lines in the GIF read the verify output, group the paths by owner and print the totals with the package that changed most. On the fixture: FINDING files changed: 3, packages: 2 and ALERT top package: openssh-server (2 of 3).\n\n### How to run it\n\nbash `package_integrity.sh`, or `--verify capture.txt --owners map.txt` for a saved capture. `--max` limits the changed paths shown. Exit 0 is clean, 1 is a finding, 2 is usage or unreadable input. Run it weekly, and after any incident that touched a shell.\n\n### The fix\n\nReinstall the package, not the file: dnf reinstall openssh-server or `apt install --reinstall openssh-server`. Compare edited configuration with rpmconf or dpkg's conffile prompt first. If the change was not yours, treat the host as compromised and preserve the evidence.\n\n### Prevention\n\nSend exit 1 to the same channel as your alerts, and run the check in the image build too.\n\n> **The honest caveat:** `rpm -Va` also reports configuration files you edited on purpose, so keep a short exception list.\n\n> **The lesson:** if the file changed and the package did not, someone else did.\n\n## In one line\n\n> **Trust the package, verify the file.** A digest that no longer matches is a file that changed hands.\n\n**Verify one file**\n\n```bash\nrpm -Vf /etc/ssh/sshd_config\n```\n\n## Script: package_integrity.sh\n\nDownload: [scripts/package_integrity.sh](../scripts/package_integrity.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# package_integrity.sh - find package files that changed without a deploy.\n# Fixes: a binary, config or library modified after its package was installed.\n# Read-only: runs rpm -Va or dpkg --verify and reads the result.\n#\n# Usage:\n#   package_integrity.sh [options]\n#   package_integrity.sh --verify /tmp/verify.txt --owners /tmp/owners.txt\n#\n# Options:\n#   --verify FILE   captured rpm -Va or dpkg --verify output (default: run it)\n#   --tool NAME     rpm, dpkg or auto (default auto)\n#   --owners FILE   path-to-package map for fixture runs, \"path package\"\n#   --max N         show at most N changed paths (default 10)\n#   -h, --help      show this help and exit 0\n#\n# Exit codes: 0 clean, 1 findings, 2 usage or unreadable input.\n# Needs: rpm (RHEL family) or dpkg (Debian family), awk, sort. Linux.\n# Cron: 0 7 * * * /usr/local/bin/package_integrity.sh\n# By s0sta.com\n\nusage() {\n    cat <<'EOF'\npackage_integrity.sh - find package files that changed without a deploy.\n\nUsage:\n  package_integrity.sh [options]\n  package_integrity.sh --verify /tmp/verify.txt --owners /tmp/owners.txt\n\nOptions:\n  --verify FILE   captured rpm -Va or dpkg --verify output (default: run it)\n  --tool NAME     rpm, dpkg or auto (default auto)\n  --owners FILE   path-to-package map for fixture runs, \"path package\"\n  --max N         show at most N changed paths (default 10)\n  -h, --help      show this help and exit 0\n\nCompares the package database with the files on disk, groups the changes by\npackage and exits 1 when anything differs. Read-only.\nExit codes: 0 clean, 1 findings, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n    printf 'ERROR %s\\n' \"$*\" >&2\n    exit 2\n}\n\nis_number() {\n    case \"$1\" in\n        ''|*[!0-9]*) return 1 ;;\n        *) return 0 ;;\n    esac\n}\n\nverify_file=\"\"\nowners_file=\"\"\ntool=auto\nmax_show=10\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        --verify) [ \"$#\" -ge 2 ] || die \"missing value for $1\"; verify_file=$2; shift 2 ;;\n        --tool)   [ \"$#\" -ge 2 ] || die \"missing value for $1\"; tool=$2; shift 2 ;;\n        --owners) [ \"$#\" -ge 2 ] || die \"missing value for $1\"; owners_file=$2; shift 2 ;;\n        --max)    [ \"$#\" -ge 2 ] || die \"missing value for $1\"; max_show=$2; shift 2 ;;\n        -h|--help) usage; exit 0 ;;\n        *)        die \"unknown option: $1 (try --help)\" ;;\n    esac\ndone\n\nis_number \"$max_show\" || die \"not a number: $max_show\"\ncase \"$tool\" in\n    rpm|dpkg|auto) ;;\n    *) die \"bad --tool, want rpm, dpkg or auto: $tool\" ;;\nesac\n\nif [ -n \"$verify_file\" ]; then\n    [ -r \"$verify_file\" ] || die \"cannot read verify output: $verify_file\"\nfi\nif [ -n \"$owners_file\" ]; then\n    [ -r \"$owners_file\" ] || die \"cannot read owners map: $owners_file\"\nfi\n\nif [ \"$tool\" = auto ]; then\n    if command -v rpm >/dev/null 2>&1; then\n        tool=rpm\n    elif command -v dpkg >/dev/null 2>&1; then\n        tool=dpkg\n    elif [ -z \"$verify_file\" ]; then\n        die \"neither rpm nor dpkg found\"\n    else\n        tool=unknown\n    fi\nfi\n\nif [ -n \"$verify_file\" ]; then\n    verify_out=$(cat \"$verify_file\")\nelse\n    if [ \"$tool\" = rpm ]; then\n        verify_out=$(rpm -Va) || die \"rpm -Va failed\"\n    elif [ \"$tool\" = dpkg ]; then\n        verify_out=$(dpkg --verify) || die \"dpkg --verify failed\"\n    else\n        die \"no package tool available\"\n    fi\nfi\n\nverify_out=$(printf '%s\\n' \"$verify_out\" | sort -u)\n\nattribute() {\n    local f pkg\n    f=$1\n    pkg=\"\"\n    if [ -n \"$owners_file\" ]; then\n        pkg=$(awk -v f=\"$f\" '$1 == f { print $2; exit }' \"$owners_file\" || true)\n    elif [ \"$tool\" = rpm ]; then\n        pkg=$(rpm -qf --qf '%{NAME}\\n' \"$f\" 2>/dev/null | sed -n '1p' || true)\n    elif [ \"$tool\" = dpkg ]; then\n        pkg=$(dpkg -S \"$f\" 2>/dev/null | sed -n '1p' | cut -d: -f1 || true)\n    fi\n    printf '%s\\n' \"${pkg:-unknown}\"\n}\n\nfiles=()\npkgs=()\n\nwhile IFS= read -r line; do\n    [ -n \"$line\" ] || continue\n    path=${line##* }\n    case \"$path\" in\n        /*) ;;\n        *) continue ;;\n    esac\n    files+=(\"$path\")\n    pkgs+=(\"$(attribute \"$path\")\")\ndone <<< \"$verify_out\"\n\nfile_count=${#files[@]}\nif [ \"$file_count\" -gt 0 ]; then\n    pkg_count=$(printf '%s\\n' \"${pkgs[@]}\" | sort -u | grep -c . || true)\nelse\n    pkg_count=0\nfi\n\nprintf 'FINDING files changed: %d, packages: %d\\n' \"$file_count\" \"$pkg_count\"\n\nif [ \"$file_count\" -gt 0 ]; then\n    top=$(printf '%s\\n' \"${pkgs[@]}\" | sort | uniq -c | sort -k1,1nr -k2,2 | sed -n '1p')\n    top=${top#\"${top%%[![:space:]]*}\"}\n    top_count=${top%% *}\n    top_pkg=${top#* }\n    printf 'ALERT top package: %s (%s of %d)\\n' \"$top_pkg\" \"$top_count\" \"$file_count\"\n\n    printf '%s\\n' \"${pkgs[@]}\" | sort | uniq -c | sort -k1,1nr -k2,2 | while IFS=' ' read -r n pkg; do\n        printf 'PACKAGE %s %s\\n' \"$pkg\" \"$n\"\n    done\n\n    if [ \"$max_show\" -gt 0 ]; then\n        printf '%s\\n' \"${files[@]}\" | sort -u | while IFS= read -r path; do\n            printf 'CHANGED %s\\n' \"$path\"\n        done | sed -n \"1,${max_show}p\"\n    fi\nfi\n\nif [ \"$file_count\" -gt 0 ]; then\n    exit 1\nfi\nexit 0\n```\n"},{"id":"p627","n":"627","slug":"the-host-triage-flow","title":"The host triage flow","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","SRE","DevOps","IncidentResponse"],"series_name":"ShellSec","design":"architecture","script":"","date":"2030-09-29","files":{"it":"notes/p627-the-host-triage-flow.md","devsecops":"notes/p627-the-host-triage-flow.md"},"md":"# The host triage flow\n\nAt 03:00 you have an alert, a host and no plan. Triage is a flow with three tiers, and each tier has exactly one job.\n\n## Quick reference\n\n- **Tier 1 · Signal:** Alert fires (ALERT) · Host is slow (HOST)\n- **Tier 2 · Check:** Run the script (CHECK) · Read the log (LOG)\n- **Tier 3 · Decide:** Fix now (FIX) · Escalate (ESC)\n\n**Request flow:** Alert fires → Run the script → Fix now; Alert fires → Read the log → Escalate; Host is slow → Run the script; Host is slow → Read the log\n\n**Around it:** Alertmanager (ALRT) · journald (JRNL) · Runbook (BOOK)\n\n## Explained\n\n### Signal\n\nThe alert tier is what fired: a disk threshold, a failed unit, a listener that should not exist. The host tier is what the box feels like: load, memory, a shell that answers slowly. Both are inputs, not answers. A signal is allowed to be noisy; it is not allowed to be ignored.\n\n### Check\n\nThe script tier is the check you wrote while the system was calm: `log_rotate_check.sh`, `user_audit.sh`, `open_ports.sh`. The log tier is the witness: journalctl, dmesg, the last 200 lines before the restart. A check is repeatable and has an exit code. A log is evidence, and it does not care what you expected.\n\n### Decide\n\nThe fix tier is the change you can explain and reverse. The escalate tier is the one you hand over with the evidence attached. Fix what you understand; escalate what you do not. The boundary is the whole skill.\n\n### The habit\n\nWrite the check before the incident, and give the decision a name. At 3 a.m. you execute a plan, you do not design one. Around the flow sit three tools: Alertmanager routes the signal, journald keeps the log, and the runbook names the decision.\n\n> **The honest caveat:** a noisy signal trains people to skip the check, which is how a real alert gets missed.\n\n> **The lesson:** signals are cheap, checks are scripted, decisions are named.\n\n## In one line\n\n> **Signal, check, then decide.** Three tiers, one flow, and a name for every step.\n\n**Run the check:** bash `log_rotate_check.sh /var/log`\n"},{"id":"p628","n":"628","slug":"bash-one-liners-for-host-triage","title":"Bash one-liners for host triage","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","CommandLine","SysAdmin","SRE"],"series_name":"ShellSec","design":"cheatsheet","script":"","date":"2030-10-01","files":{"it":"notes/p628-bash-one-liners-for-host-triage.md","devsecops":"notes/p628-bash-one-liners-for-host-triage.md"},"md":"# Bash one-liners for host triage\n\nA host checkup needs a handful of one-liners you can type from memory, and the judgement to read them in the right order.\n\n## Quick reference\n\n### Disk\n\n```bash\ndf -hP | awk '$5+0 > 80'\ndf -i | awk '$5+0 > 80'\ndu -xsh /var/* | sort -h | tail\nfind /var/log -type f -size +1G\nlsof +L1 2>/dev/null | head\n```\n\n### Memory\n\n```bash\nfree -m\ngrep MemAvailable /proc/meminfo\nps -eo rss,comm --sort=-rss | head\ndmesg | grep -i 'killed process'\ncat /proc/pressure/memory\n```\n\n### Processes\n\n```bash\nps -eo stat,pid,comm | grep ^Z\nps -eo pcpu,pid,comm --sort=-pcpu | head\nps -eo nlwp,pid,comm --sort=-nlwp | head\nlsof -p <pid> | wc -l\ncat /proc/loadavg\n```\n\n### Network\n\n```bash\nss -lntup\nss -s\nss -tan state established | wc -l\nip -br addr\nss -lntp | grep 0.0.0.0\n```\n\n## Explained\n\n### Disk\n\n`df -hP` shows the filesystem and `df -i` shows the inodes. `du -xsh /var/*` finds the directory that grew, `find -size +1G` lists the biggest logs, and `lsof +L1` finds a deleted file still holding its blocks.\n\n### Memory\n\n`free -m` gives the summary, but available is the number that matters, not free. The RSS sort names the biggest resident process, `/proc/pressure/memory` shows tasks stalled, and `dmesg | grep -i 'killed process'` says what the kernel killed. PSI needs kernel 4.20 or newer.\n\n### Processes\n\n`ps -eo stat,pid,comm | grep ^Z` finds zombies, which are a bug in the parent. The two sort lines find the process eating CPU and the one with the most threads. `lsof -p <pid>` counts one process's descriptors, and `cat /proc/loadavg` gives load without parsing uptime.\n\n### Network\n\n`ss -lntup` lists listeners with their process, and `ss -s` gives the summary. The established count is the load you serve, `ip -br addr` shows the addresses, and grep 0.0.0.0 finds a service that meant to stay local.\n\n### The habit\n\nRun them in that order: disk, memory, processes, network. Each answer changes which question you ask next.\n\n> **The honest caveat:** ss and lsof need privileges to see other users' processes, so a hardened host may show ? instead of a name.\n\n> **The lesson:** know the command before you need it, because at 3 a.m. you will not learn it.\n\n## In one line\n\n> **Twenty commands, one host.** Five per area beats fifty you cannot recall under pressure.\n\n**A first look**\n\n```bash\nuptime; df -h; free -h\n```\n"},{"id":"p629","n":"629","slug":"four-host-signals","title":"Four host signals","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SRE","Observability","SysAdmin","Troubleshooting"],"series_name":"ShellSec","design":"explainer","script":"","date":"2030-10-03","files":{"it":"notes/p629-four-host-signals.md","devsecops":"notes/p629-four-host-signals.md"},"md":"# Four host signals\n\nA slow host rarely has one cause. Four signals explain most of the pages I have answered, and each one is narrower than its name suggests.\n\n## Quick reference\n\n### Disk is full\n\n- `df -h` shows bytes; `df -i` shows inodes\n- A deleted file holds space until its fd closes\n- `du -x --max-depth=1 /var` finds the growth\n- Alert at 80%, not at 100%: at 100% you are down\n\n### Memory pressure\n\n- Available is the number, not free\n- The OOM killer scores; it does not rank RSS\n- PSI shows tasks stalled on memory\n- MemoryMax caps one service, not the host\n\n### Load average\n\n- Load counts runnable plus uninterruptible tasks\n- Compare load with nproc, never with 1.0\n- High load and idle CPU means I/O wait\n- A 4-core box at load 4 is saturated\n\n### File descriptors\n\n- Every socket and open file is a descriptor\n- ulimit -n is per process, not per host\n- `ls /proc/`\\<pid>/fd counts one process\n- Too many open files is a leak, not a limit\n\n> **Note:** Load is a queue length, not a percentage. Read it beside nproc.\n\n## Explained\n\n### Disk\n\n`df -h` measures bytes, `df -i` measures inodes, and a deleted file keeps both until its last descriptor closes. That is why du and df can disagree on a host with no free space. Alert at 80 per cent, because at 100 per cent writes fail and the cleanup is no longer controlled.\n\n### Memory\n\nAvailable is the number, not free. The kernel uses spare memory for cache and gives it back under pressure. The OOM killer scores processes instead of ranking them by RSS, so the largest process is not always the victim. Pressure stall information shows the tasks that are actually waiting.\n\n### Load\n\nLoad counts runnable tasks plus tasks in uninterruptible sleep, usually disk I/O. On four cores, a load of 4 is saturated and a load of 8 is a queue. Read it beside CPU usage: high load with idle CPU means the disks, not the processor.\n\n### File descriptors\n\nEvery socket and open file is a descriptor. ulimit -n is a per-process limit, not a host limit, so one service can hit it while the box looks idle. `ls /proc/`\\<pid>/fd counts one process, and a rising count is a leak long before the error appears.\n\n> **The honest caveat:** these four are the common signals, not the only ones; a host can pass all four and still drop packets.\n\n> **The lesson:** read the signal behind the alarm, not the alarm itself.\n\n## In one line\n\n> **Read the signal, not the alarm.** One number rarely explains a slow host. Four usually do.\n\n**Check all four**\n\n```bash\nuptime; free -m; df -h; df -i\n```\n"},{"id":"p630","n":"630","slug":"9-shell-commands-for-a-host-checkup","title":"9 shell commands for a host checkup","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","CommandLine","SRE","SysAdmin"],"series_name":"ShellSec","design":"terminal","script":"","date":"2030-10-06","files":{"it":"notes/p630-9-shell-commands-for-a-host-checkup.md","devsecops":"notes/p630-9-shell-commands-for-a-host-checkup.md"},"md":"# 9 shell commands for a host checkup\n\nA host checkup is nine commands in three passes. Look at the whole box, measure the one thing that hurts, then act once and write down what you changed.\n\n## Quick reference\n\n### Look at the whole host\n\n```bash\n# 8 cores, load 9.4: saturated\nuptime\n# bytes left before inodes\ndf -h /\n# inodes are the wall here\ndf -i /\n```\n\n### Measure the one thing that hurts\n\n```bash\n# available is what matters\nfree -m\n# one process owns the box\nps -eo pcpu,pid,comm --sort=-pcpu | head -3\n# sockets, not ports\nss -s\n```\n\n### Act once, and note the time\n\n```bash\n# read before the restart\njournalctl -u api --since '15 min ago'\n# rotate first, delete never\nlogrotate -f /etc/logrotate.d/app\n# last resort, one unit\nsystemctl restart api\n```\n\n## Explained\n\n### Look\n\n`uptime` gives the load and the age of the boot. `df -h` gives the bytes, and `df -i` gives the inodes; on a container host the second one is usually the surprise. Stop when one number explains the page.\n\n### Measure\n\n`free -m` separates cache from available. The process sort shows the one process that owns the CPU, not the top ten. `ss -s` counts the sockets, which is the number that often grows before a file descriptor limit does.\n\n### Act\n\nRead the journal before the restart, because the failure you are about to clear is the evidence you will want at noon. Rotate a log before you delete it. Restart one unit, not the host: the restart buys an hour, and the journal buys the fix.\n\n### The habit\n\nLook, measure, act, in that order. The order is the discipline: it stops you from restarting a host because one column in top looked red.\n\n> **The honest caveat:** restart and logrotate change the system, so run them one at a time and note the time in the incident channel.\n\n> **The lesson:** nine commands and one order beat a dashboard you cannot read under pressure.\n\n## In one line\n\n> **Look, measure, then act.** Nine commands, one order, no guessing.\n\n**Try it**\n\n```bash\ndf -h; df -i; free -m\n```\n"},{"id":"p631","n":"631","slug":"quiz-3-host-questions","title":"Quiz: 3 host questions","sections":["it","devsecops"],"topic":"Linux","tags":["Linux","SRE","Troubleshooting","SysAdmin","Quiz"],"series_name":"ShellSec","design":"quiz","script":"","date":"2030-10-08","files":{"it":"notes/p631-quiz-3-host-questions.md","devsecops":"notes/p631-quiz-3-host-questions.md"},"md":"# Quiz: 3 host questions\n\nThree host questions I ask in reviews, because the wrong answer sends you to the wrong tool under pressure.\n\n## Quick reference\n\n**Q1. Why can df and du disagree on a full host?**\n\n- a) du counts sparse files twice\n- b) Deleted files stay open\n- c) df rounds to gigabytes\n\n> **Answer:** b) Deleted files stay open. du walks names; a deleted file keeps its blocks until the last descriptor closes, and df counts blocks, not names.\n\n**Q2. What does load average count that CPU usage misses?**\n\n- a) Uninterruptible I/O wait\n- b) Steal time only\n- c) Idle threads\n\n> **Answer:** a) Uninterruptible I/O wait. Load counts runnable and uninterruptible tasks, so a stalled disk raises load while the CPU sits idle.\n\n**Q3. Why is the disk still full after deleting a log?**\n\n- a) The inode is still open\n- b) The filesystem is read-only\n- c) df caches the old value\n\n> **Answer:** a) The inode is still open. The process still holds the file descriptor. Truncate through `/proc/`\\<pid>/fd or restart the writer.\n\n## Explained\n\n1. Why can df and du disagree on a full host?\n\n> **Answer:** du walks names, and a deleted file keeps its blocks until the last descriptor closes. df counts blocks, so it sees the space as used while du cannot find a file that owns it. Check `lsof +L1` before you delete anything else.\n\n2. What does load average count that CPU usage misses?\n\n> **Answer:** runnable tasks plus tasks in uninterruptible sleep, usually waiting on disk. A stalled volume raises load while the CPU sits idle. Compare load with nproc and read `/proc/pressure/io` before you blame the processor.\n\n3. Why is the disk still full after deleting a log?\n\n> **Answer:** the process still holds the file descriptor. The name is gone and the blocks are not. Truncate through `/proc/`\\<pid>/fd, or restart the writer, then confirm with df and `lsof +L1`.\n\n### The pattern\n\nEach answer replaces a number with a mechanism. That is the whole skill: df, du and load average all measure something narrower than their names suggest.\n\n> **The honest caveat:** a deleted-but-open file explains a gap between df and du, not every gap; reserved blocks and snapshots explain the rest.\n\n> **The lesson:** the number is not the cause.\n\n## In one line\n\n> **The number is not the cause.** df, du and load all measure less than their names suggest.\n\n**Ask the host**\n\n```bash\ndf -h; df -i; lsof +L1\n```\n"},{"id":"p632","n":"632","slug":"config-that-drifted-overnight","title":"Config that drifted overnight","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","DevSecOps","ConfigManagement","SRE"],"series_name":"ShellSec","design":"code","script":"config_drift.sh","date":"2030-10-10","files":{"it":"notes/p632-config-that-drifted-overnight.md","devsecops":"notes/p632-config-that-drifted-overnight.md"},"md":"# Config that drifted overnight\n\nAt 02:40 the deploy key stops working. The sshd config differs from Git by one line, and nobody opened a change request.\n\n## Quick reference\n\nThe full script is at the end of this note: `config_drift.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash config_drift.sh /var/lib/etc-base /etc\n```\n\n**Example output**\n\n```text\nFINDING 2 files drifted\nCHANGED ssh/sshd_config\n```\n\n## Explained\n\n### What it does\n\n`config_drift.sh` hashes every regular file in a live tree and compares it with a baseline copy or tarball. A file is CHANGED when the hash differs, ADDED when only the live tree has it, and REMOVED when only the baseline does.\n\n### What the screen shows\n\nThe fourteen lines hash each live file, write ADDED and CHANGED lines to a temp list, then print the count and the first path. On the fixture: FINDING 2 files drifted and CHANGED ssh/sshd_config. The script also walks the baseline for REMOVED files.\n\n### How to run it\n\nbash `config_drift.sh -b /var/lib/s0sta/etc-baseline -r /etc`. The cron line is 15 3 \\* \\* \\* and the log is `/var/log/s0sta/config-drift.log`. Exit 0 is clean, 1 is drift, 2 is a bad path or unreadable baseline.\n\n### The fix\n\nDiff the named file against the baseline, then Git. If it was a hotfix, commit it and re-baseline; if not, restore it and find the session that wrote it. An ADDED file in sshd_config.d matters most: it changes behaviour without touching the main file.\n\n### Prevention\n\nRe-baseline after every approved change, in the same pull request. A baseline taken once and never refreshed becomes a finding everyone ignores.\n\n> **The honest caveat:** a hash proves the file changed, not who changed it. Pair it with auditd.\n\n> **The lesson:** drift is not a mysterious force. It is a file, a time and a person.\n\n## In one line\n\n> **The file changed. Nobody did it.** A baseline turns an unseen edit into a named file and a line.\n\n**Diff the tree:** `config_drift.sh -b /var/lib/etc-base`\n\n## Script: config_drift.sh\n\nDownload: [scripts/config_drift.sh](../scripts/config_drift.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n#\n# config_drift.sh - report config files that changed against a baseline.\n#\n# Fixes: config that drifted overnight. Every regular file under --root is\n# compared with a baseline directory, or with a tarball made like this:\n#   tar -C /etc -czf /var/lib/s0sta/etc-baseline.tgz .\n# A file is CHANGED when its SHA-256 differs, ADDED when only the live tree\n# has it, and REMOVED when only the baseline has it. Read-only: the target\n# tree is never written to.\n#\n# Usage:\n#   config_drift.sh --baseline /var/lib/s0sta/etc-baseline\n#   config_drift.sh -b /var/lib/s0sta/etc-baseline -r /etc\n#   config_drift.sh -b /var/lib/s0sta/etc-baseline.tgz -r /etc\n#   config_drift.sh --help\n#\n# Cron (03:15 daily; exit 1 means drift):\n#   15 3 * * * /usr/local/sbin/config_drift.sh -b /var/lib/s0sta/etc \\\n#     >>/var/log/s0sta/config-drift.log 2>&1\n#\n# Exit: 0 no drift, 1 drift found, 2 usage or unreadable input.\n# Needs: sha256sum, or shasum -a 256 on macOS/BSD; tar for a tarball baseline.\n# By s0sta.com\n#\nusage() {\n    cat <<'EOF'\nconfig_drift.sh - report config drift against a baseline copy.\n\nUsage:\n  config_drift.sh --baseline DIR|TARBALL [--root DIR]\n  config_drift.sh -b DIR -r /etc\n\n  -b, --baseline PATH  baseline directory (a copy of the tree) or .tar/.tgz\n  -r, --root DIR       live tree to inspect (default: /etc)\n  -h, --help           show this help and exit 0\n\nExit: 0 no drift, 1 drift found, 2 usage or unreadable input.\nRead-only: the live tree is never modified.\nEOF\n}\n\ndie() {\n    printf 'config_drift: %s\\n' \"$1\" >&2\n    exit 2\n}\n\nhash_file() {\n    if command -v sha256sum >/dev/null 2>&1; then\n        sha256sum \"$1\" 2>/dev/null | cut -d' ' -f1\n    else\n        shasum -a 256 \"$1\" 2>/dev/null | cut -d' ' -f1\n    fi\n}\n\nlist_tree() {\n    ( cd \"$1\" && find . -type f -print ) | sed 's|^\\./||' | LC_ALL=C sort\n}\n\nROOT=/etc\nBASE=\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help) usage; exit 0 ;;\n        -b|--baseline)\n            [ \"$#\" -ge 2 ] || die \"--baseline needs a path\"\n            BASE=$2; shift 2 ;;\n        -r|--root)\n            [ \"$#\" -ge 2 ] || die \"--root needs a path\"\n            ROOT=$2; shift 2 ;;\n        *) die \"unknown argument: $1\" ;;\n    esac\ndone\n\n[ -n \"$BASE\" ] || die \"missing --baseline\"\n[ -d \"$ROOT\" ] || die \"not a directory: $ROOT\"\n[ -r \"$ROOT\" ] || die \"cannot read: $ROOT\"\nif ! command -v sha256sum >/dev/null 2>&1 && ! command -v shasum >/dev/null 2>&1; then\n    die \"need sha256sum or shasum\"\nfi\n\nTMP=$(mktemp -d \"${TMPDIR:-/tmp}/config_drift.XXXXXX\")\ntrap 'rm -rf \"$TMP\"' EXIT\n\nif [ -d \"$BASE\" ]; then\n    BDIR=$BASE\nelif [ -f \"$BASE\" ]; then\n    mkdir -p \"$TMP/base\"\n    tar -xf \"$BASE\" -C \"$TMP/base\" 2>/dev/null || die \"cannot unpack: $BASE\"\n    BDIR=$TMP/base\nelse\n    die \"baseline not found: $BASE\"\nfi\n\nlist_tree \"$BDIR\" >\"$TMP/base.list\"\nlist_tree \"$ROOT\" >\"$TMP/live.list\"\ncomm -12 \"$TMP/base.list\" \"$TMP/live.list\" >\"$TMP/common.list\"\ncomm -13 \"$TMP/base.list\" \"$TMP/live.list\" >\"$TMP/added.list\"\ncomm -23 \"$TMP/base.list\" \"$TMP/live.list\" >\"$TMP/removed.list\"\n\n: >\"$TMP/changed.list\"\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    if [ \"$(hash_file \"$BDIR/$rel\")\" != \"$(hash_file \"$ROOT/$rel\")\" ]; then\n        printf '%s\\n' \"$rel\" >>\"$TMP/changed.list\"\n    fi\ndone <\"$TMP/common.list\"\n\nCHANGED=$(wc -l <\"$TMP/changed.list\" || true)\nADDED=$(wc -l <\"$TMP/added.list\" || true)\nREMOVED=$(wc -l <\"$TMP/removed.list\" || true)\nTOTAL=$((CHANGED + ADDED + REMOVED))\n\nif [ \"$TOTAL\" -eq 0 ]; then\n    FILES=$(wc -l <\"$TMP/live.list\" || true)\n    printf 'OK no drift against %s (%d files)\\n' \"$BASE\" \"$FILES\"\n    exit 0\nfi\n\nprintf 'FINDING %d files drifted\\n' \"$TOTAL\"\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'CHANGED %s\\n' \"$rel\"\ndone <\"$TMP/changed.list\"\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'ADDED %s\\n' \"$rel\"\ndone <\"$TMP/added.list\"\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'REMOVED %s\\n' \"$rel\"\ndone <\"$TMP/removed.list\"\n\nexit 1\n```\n"},{"id":"p633","n":"633","slug":"ssh-keys-you-forgot-about","title":"SSH keys you forgot about","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","SSH","CyberSecurity","SysAdmin"],"series_name":"ShellSec","design":"code","script":"ssh_key_audit.sh","date":"2030-10-13","files":{"it":"notes/p633-ssh-keys-you-forgot-about.md","devsecops":"notes/p633-ssh-keys-you-forgot-about.md"},"md":"# SSH keys you forgot about\n\nA contractor left in March. In October their key still opens the jump host, because it sits on line 3 of an authorized_keys file nobody has read since the account was created.\n\n## Quick reference\n\nThe full script is at the end of this note: `ssh_key_audit.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash ssh_key_audit.sh /home/ana\n```\n\n**Example output**\n\n```text\nFINDING 2 issues in 3 keys\nWEAK ssh-rsa .ssh/authorized_keys:2\n```\n\n## Explained\n\n### What it does\n\n`ssh_key_audit.sh` scans `/home/*/.ssh/authorized_keys` and `/root/.ssh`. It checks the mode on .ssh and the file, flags ssh-dss and ssh-rsa keys, keys with no comment, unparsable lines and duplicate public keys. It never prints a key blob.\n\n### What the screen shows\n\nThe fourteen lines read one authorized_keys file, count the keys, write WEAK and NOCOMMENT lines to a temp list, then print the summary and the first hit. On the fixture: FINDING 2 issues in 3 keys and WEAK ssh-rsa .ssh/authorized_keys:2.\n\n### How to run it\n\nbash `ssh_key_audit.sh /home /root`. The Monday cron line is 40 6 \\* \\* 1. Exit 0 is clean, 1 is a finding, 2 is an unreadable path. It writes nothing outside its temp directory.\n\n### The fix\n\nRemove the flagged key, then rotate the account it belonged to. A key removed without a rotation may already have been copied. Replace ssh-rsa and ssh-dss with ed25519, and give every replacement a comment with an owner and a ticket.\n\n### Prevention\n\nReview keys when someone leaves, not once a year. Put the expected keys in configuration management and let the audit diff against that list.\n\n> **The honest caveat:** modes and key types are only part of the story. sshd StrictModes also checks the home directory.\n\n> **The lesson:** an authorized key is a password that never expires.\n\n## In one line\n\n> **A key is a permanent password.** It has no expiry, no lockout and no second factor.\n\n**Count the keys:** `ssh_key_audit.sh /home /root`\n\n## Script: ssh_key_audit.sh\n\nDownload: [scripts/ssh_key_audit.sh](../scripts/ssh_key_audit.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n#\n# ssh_key_audit.sh - audit authorized_keys files without printing key material.\n#\n# Fixes: the SSH keys you forgot about. Scans /home/*/.ssh/authorized_keys\n# and /root/.ssh/authorized_keys for:\n#   LOOSE      .ssh or authorized_keys writable by group or other\n#   WEAK       ssh-dss or ssh-rsa keys (DSA, and RSA restricted to SHA-1)\n#   NOCOMMENT  a key with no comment, so nobody can say whose it is\n#   UNPARSED   a line that is not a key and not a comment\n#   DUP        the same public key installed more than once\n# It reads public keys only. The output never contains a key blob or a\n# private key, and the script writes nothing outside its temporary directory.\n# sshd's StrictModes also checks the home directory; this audits .ssh and\n# authorized_keys, not every parent directory.\n#\n# Usage:\n#   ssh_key_audit.sh                     # /home and /root\n#   ssh_key_audit.sh /home /root         # explicit roots\n#   ssh_key_audit.sh /tmp/fixture/home   # test tree\n#   ssh_key_audit.sh --help\n#\n# Cron (Mondays, 06:40; exit 1 means findings):\n#   40 6 * * 1 /usr/local/sbin/ssh_key_audit.sh /home /root \\\n#     >>/var/log/s0sta/ssh-keys.log 2>&1\n#\n# Exit: 0 no findings, 1 findings, 2 usage or unreadable input.\n# Needs: find, stat (GNU or BSD), awk. No ssh-keygen required.\n# By s0sta.com\n#\nusage() {\n    cat <<'EOF'\nssh_key_audit.sh - audit authorized_keys files.\n\nUsage:\n  ssh_key_audit.sh [ROOT ...]\n  ssh_key_audit.sh --help\n\n  ROOT   a home directory, a tree of homes, or an authorized_keys file\n         (default: /home /root)\n\nExit: 0 no findings, 1 findings, 2 usage or unreadable input.\nRead-only: public keys are read, nothing is changed.\nEOF\n}\n\ndie() {\n    printf 'ssh_key_audit: %s\\n' \"$1\" >&2\n    exit 2\n}\n\nmode_of() {\n    stat -c '%a' \"$1\" 2>/dev/null || stat -f '%Lp' \"$1\" 2>/dev/null || true\n}\n\n# True when group or other has a write bit (2, 3, 6 or 7 in either place).\nis_loose() {\n    local m g o\n    m=$(mode_of \"$1\")\n    [ -n \"$m\" ] || return 1\n    g=${m: -2:1}\n    o=${m: -1:1}\n    case \"${g}${o}\" in\n        *[2367]*) return 0 ;;\n    esac\n    return 1\n}\n\n[ \"$#\" -gt 0 ] || set -- /home /root\nfor arg in \"$@\"; do\n    case \"$arg\" in\n        -h|--help) usage; exit 0 ;;\n        -*) die \"unknown argument: $arg\" ;;\n    esac\ndone\n\nTMP=$(mktemp -d \"${TMPDIR:-/tmp}/ssh_key_audit.XXXXXX\")\ntrap 'rm -rf \"$TMP\"' EXIT\n: >\"$TMP/files\"\nfor root in \"$@\"; do\n    [ -r \"$root\" ] || die \"cannot read: $root\"\n    if [ -f \"$root\" ]; then\n        printf '%s\\n' \"$root\" >>\"$TMP/files\"\n    elif [ -d \"$root\" ]; then\n        find \"$root\" -maxdepth 3 -type f -name authorized_keys -print \\\n            2>/dev/null >>\"$TMP/files\" || true\n    else\n        die \"not a file or directory: $root\"\n    fi\ndone\nsort -u \"$TMP/files\" -o \"$TMP/files\"\n\nif [ ! -s \"$TMP/files\" ]; then\n    printf 'OK no authorized_keys found under: %s\\n' \"$*\"\n    exit 0\nfi\n\n: >\"$TMP/loose\"\n: >\"$TMP/weak\"\n: >\"$TMP/nocomment\"\n: >\"$TMP/unparsed\"\n: >\"$TMP/keys\"\nKEYS=0\nFILES=0\n\nwhile IFS= read -r keyfile; do\n    [ -n \"$keyfile\" ] || continue\n    FILES=$((FILES + 1))\n    sshdir=$(dirname \"$keyfile\")\n    if is_loose \"$sshdir\"; then\n        printf 'LOOSE %s %s\\n' \"$sshdir\" \"$(mode_of \"$sshdir\")\" >>\"$TMP/loose\"\n    fi\n    if is_loose \"$keyfile\"; then\n        printf 'LOOSE %s %s\\n' \"$keyfile\" \"$(mode_of \"$keyfile\")\" >>\"$TMP/loose\"\n    fi\n\n    lineno=0\n    while IFS= read -r line || [ -n \"$line\" ]; do\n        lineno=$((lineno + 1))\n        case \"$line\" in\n            ''|\\#*) continue ;;\n        esac\n        # authorized_keys may start with options; brackets are not valid in\n        # base64, so the first field that looks like a key type wins.\n        IFS=' ' read -r -a fields <<<\"$line\"\n        type=\n        blob=\n        idx=0\n        while [ \"$idx\" -lt \"${#fields[@]}\" ]; do\n            case \"${fields[$idx]}\" in\n                ssh-*|ecdsa-sha2-*|sk-*) type=${fields[$idx]}; break ;;\n            esac\n            idx=$((idx + 1))\n        done\n        if [ -z \"$type\" ] || [ \"$((idx + 1))\" -ge \"${#fields[@]}\" ]; then\n            printf 'UNPARSED %s:%s\\n' \"$keyfile\" \"$lineno\" >>\"$TMP/unparsed\"\n            continue\n        fi\n        blob=${fields[$((idx + 1))]}\n        [ -n \"$blob\" ] || {\n            printf 'UNPARSED %s:%s\\n' \"$keyfile\" \"$lineno\" >>\"$TMP/unparsed\"\n            continue\n        }\n        KEYS=$((KEYS + 1))\n        printf '%s %s:%s\\n' \"$blob\" \"$keyfile\" \"$lineno\" >>\"$TMP/keys\"\n        case \"$type\" in\n            ssh-dss*|ssh-rsa*)\n                printf 'WEAK %s %s:%s\\n' \"$type\" \"$keyfile\" \"$lineno\" >>\"$TMP/weak\" ;;\n        esac\n        comment=\n        j=$((idx + 2))\n        while [ \"$j\" -lt \"${#fields[@]}\" ]; do\n            comment=\"$comment ${fields[$j]}\"\n            j=$((j + 1))\n        done\n        if [ -z \"$comment\" ]; then\n            printf 'NOCOMMENT %s:%s\\n' \"$keyfile\" \"$lineno\" >>\"$TMP/nocomment\"\n        fi\n    done <\"$keyfile\"\ndone <\"$TMP/files\"\n\nawk '{ c[$1]++; loc[$1] = loc[$1] \" \" $2 }\n     END { for (k in c) if (c[k] > 1) printf \"DUP %d%s\\n\", c[k], loc[k] }' \\\n    \"$TMP/keys\" | LC_ALL=C sort >\"$TMP/dup\"\n\nLOOSE=$(wc -l <\"$TMP/loose\" || true)\nWEAK=$(wc -l <\"$TMP/weak\" || true)\nNOCOMMENT=$(wc -l <\"$TMP/nocomment\" || true)\nUNPARSED=$(wc -l <\"$TMP/unparsed\" || true)\nDUP=$(wc -l <\"$TMP/dup\" || true)\nISSUES=$((LOOSE + WEAK + NOCOMMENT + UNPARSED + DUP))\n\nif [ \"$KEYS\" -eq 1 ]; then\n    k=key\nelse\n    k=keys\nfi\nif [ \"$FILES\" -eq 1 ]; then\n    f='file'\nelse\n    f='files'\nfi\nif [ \"$ISSUES\" -eq 1 ]; then\n    i=issue\nelse\n    i=issues\nfi\n\nif [ \"$ISSUES\" -eq 0 ]; then\n    printf 'OK %d %s in %d %s, no issues\\n' \"$KEYS\" \"$k\" \"$FILES\" \"$f\"\n    exit 0\nfi\n\nprintf 'FINDING %d %s in %d %s\\n' \"$ISSUES\" \"$i\" \"$KEYS\" \"$k\"\ncat \"$TMP/loose\"\ncat \"$TMP/weak\"\ncat \"$TMP/nocomment\"\ncat \"$TMP/unparsed\"\ncat \"$TMP/dup\"\n\nexit 1\n```\n"},{"id":"p634","n":"634","slug":"tls-that-is-still-offering-1-0","title":"TLS that is still offering 1.0","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","TLS","CyberSecurity","DevSecOps","Linux"],"series_name":"ShellSec","design":"code","script":"tls_scan.sh","date":"2030-10-15","files":{"it":"notes/p634-tls-that-is-still-offering-1-0.md","devsecops":"notes/p634-tls-that-is-still-offering-1-0.md"},"md":"# TLS that is still offering 1.0\n\nYour compliance scan says TLS 1.0 is disabled. Then one integration fails, someone adds it back, and the next scan is six months away.\n\n## Quick reference\n\nThe full script is at the end of this note: `tls_scan.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash tls_scan.sh api.localhost:8443\n```\n\n**Example output**\n\n```text\nWEAK api.localhost:8443 TLSv1.0\nFINDING 1 weak protocol accepted\n```\n\n## Explained\n\n### What it does\n\n`tls_scan.sh` probes each host:port with openssl s_client for TLS 1.0, TLS 1.1 and three legacy ciphers, and reads the certificate enddate. It exits 1 on a weak protocol, cipher or certificate inside `--warn-days`.\n\n### What the screen shows\n\nThe fourteen lines probe TLS 1.0 and 1.1, print WEAK for each accepted protocol, then count them: WEAK api.localhost:8443 TLSv1.0 and FINDING 1 weak protocol accepted. The script adds CERT and WARN lines.\n\n### How to run it\n\nbash `tls_scan.sh -f hosts.txt --warn-days 21 --timeout 4`. The cron line runs daily at 04:20. Exit 0 is clean, 1 is a finding, 2 is a bad host or list. timeout(1) is optional: a blackholed SYN can stall the scan.\n\n### The fix\n\nDisable TLS 1.0 and 1.1 at the terminator, remove the legacy ciphers, then find the client that fails and give it a date. One integration is not worth every client on the port.\n\n### Prevention\n\nKeep the host list in Git and scan after every edge change. Alert on certificates inside 21 days, not on the morning they expire.\n\n> **The honest caveat:** a local openssl without RC4 cannot prove the server rejects it. The scan is a floor, not a full audit.\n\n> **The lesson:** the protocol you keep for one client is the one you keep for everyone.\n\n## In one line\n\n> **A protocol you cannot disable is a habit.** Old TLS stays because one client still needs it, and nobody wrote that down.\n\n**Probe one host:** `tls_scan.sh api.example.com:443`\n\n## Script: tls_scan.sh\n\nDownload: [scripts/tls_scan.sh](../scripts/tls_scan.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n#\n# tls_scan.sh - find hosts that still accept TLS 1.0, TLS 1.1 or a weak cipher.\n#\n# Fixes: TLS that is still offering 1.0. For every host:port it runs\n# openssl s_client once for the certificate, then probes TLS 1.0, TLS 1.1\n# and three legacy cipher suites. It prints the certificate's days to expiry\n# and exits 1 when a weak protocol or cipher is accepted, or when a\n# certificate is inside --warn-days.\n#\n# Usage:\n#   tls_scan.sh api.example.com:443 mail.example.com:993\n#   tls_scan.sh -f hosts.txt --warn-days 21 --timeout 4\n#   tls_scan.sh --help\n#\n#   hosts.txt holds one host:port per line; blank lines and # comments are\n#   ignored. IPv6 addresses are not supported in this version.\n#\n# Cron (daily, 04:20; exit 1 means a finding):\n#   20 4 * * * /usr/local/sbin/tls_scan.sh -f /etc/s0sta/tls-hosts.txt \\\n#     >>/var/log/s0sta/tls-scan.log 2>&1\n#\n# Exit: 0 clean, 1 weak protocol/cipher or certificate inside --warn-days,\n#       2 usage or unreadable input.\n# Needs: openssl with s_client. timeout(1) is optional but recommended: on a\n# host that blackholes the SYN, s_client can wait for the kernel timeout.\n# By s0sta.com\n#\nusage() {\n    cat <<'EOF'\ntls_scan.sh - probe TLS 1.0, TLS 1.1 and legacy ciphers, and certificate expiry.\n\nUsage:\n  tls_scan.sh [OPTIONS] HOST:PORT ...\n  tls_scan.sh -f FILE [OPTIONS]\n\n  -f, --file FILE      read host:port entries from FILE\n  -w, --warn-days N    warn when a certificate expires in fewer days (default 30)\n  -t, --timeout N      seconds per connection (default 5)\n  -h, --help           show this help and exit 0\n\nExit: 0 clean, 1 finding, 2 usage or unreadable input.\nRead-only: openssl s_client only reads the handshake and the certificate.\nEOF\n}\n\ndie() {\n    printf 'tls_scan: %s\\n' \"$1\" >&2\n    exit 2\n}\n\nWARN_DAYS=30\nTIMEOUT=5\nLIST_FILE=\nTMP=$(mktemp -d \"${TMPDIR:-/tmp}/tls_scan.XXXXXX\")\ntrap 'rm -rf \"$TMP\"' EXIT\n: >\"$TMP/targets\"\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help) usage; exit 0 ;;\n        -f|--file)\n            [ \"$#\" -ge 2 ] || die \"--file needs a path\"\n            LIST_FILE=$2; shift 2 ;;\n        -w|--warn-days)\n            [ \"$#\" -ge 2 ] || die \"--warn-days needs a number\"\n            WARN_DAYS=$2; shift 2 ;;\n        -t|--timeout)\n            [ \"$#\" -ge 2 ] || die \"--timeout needs a number\"\n            TIMEOUT=$2; shift 2 ;;\n        -*) die \"unknown argument: $1\" ;;\n        *) printf '%s\\n' \"$1\" >>\"$TMP/targets\"; shift ;;\n    esac\ndone\n\ncase \"$WARN_DAYS\" in\n    ''|*[!0-9]*) die \"--warn-days must be a whole number\" ;;\nesac\ncase \"$TIMEOUT\" in\n    ''|*[!0-9]*) die \"--timeout must be a whole number\" ;;\nesac\n[ \"$TIMEOUT\" -ge 1 ] || die \"--timeout must be at least 1\"\ncommand -v openssl >/dev/null 2>&1 || die \"openssl not found\"\nHELP=$(openssl s_client -help 2>&1 || true)\ncase \"$HELP\" in\n    *-tls1*) ;;\n    *) die \"this openssl has no -tls1 option\" ;;\nesac\n\nif [ -n \"$LIST_FILE\" ]; then\n    [ -r \"$LIST_FILE\" ] || die \"cannot read: $LIST_FILE\"\n    while IFS= read -r line || [ -n \"$line\" ]; do\n        case \"$line\" in\n            ''|\\#*) continue ;;\n        esac\n        printf '%s\\n' \"$line\" >>\"$TMP/targets\"\n    done <\"$LIST_FILE\"\nfi\n[ -s \"$TMP/targets\" ] || die \"no targets (give HOST:PORT or --file FILE)\"\n\nTMO=\nif command -v timeout >/dev/null 2>&1; then\n    TMO=timeout\nelif command -v gtimeout >/dev/null 2>&1; then\n    TMO=gtimeout\nfi\n\nsclient() {\n    if [ -n \"$TMO\" ]; then\n        \"$TMO\" \"$TIMEOUT\" openssl s_client \"$@\" </dev/null 2>/dev/null || true\n    else\n        openssl s_client \"$@\" </dev/null 2>/dev/null &\n        local pid=$! wd\n        ( sleep \"$TIMEOUT\"; kill -TERM \"$pid\" 2>/dev/null || true ) &\n        wd=$!\n        wait \"$pid\" 2>/dev/null || true\n        kill -TERM \"$wd\" 2>/dev/null || true\n        wait \"$wd\" 2>/dev/null || true\n    fi\n    return 0\n}\n\nto_epoch() {\n    local s\n    s=$(printf '%s' \"$1\" | tr -s ' ')\n    if date -u -d \"$s\" +%s >/dev/null 2>&1; then\n        date -u -d \"$s\" +%s\n    else\n        date -u -j -f \"%b %d %T %Y %Z\" \"$s\" +%s 2>/dev/null || true\n    fi\n}\n\nfmt_date() {\n    if date -u -d \"@$1\" +%F >/dev/null 2>&1; then\n        date -u -d \"@$1\" +%F\n    else\n        date -u -r \"$1\" +%F 2>/dev/null || true\n    fi\n}\n\nNOW=$(date -u +%s)\n: >\"$TMP/weak\"\n: >\"$TMP/certs\"\nHOSTS=0\nWEAK=0\nCERTBAD=0\n\nwhile IFS= read -r target; do\n    [ -n \"$target\" ] || continue\n    case \"$target\" in\n        *:*) ;;\n        *) die \"not host:port: $target\" ;;\n    esac\n    host=${target%:*}\n    port=${target##*:}\n    [ -n \"$host\" ] || die \"empty host in: $target\"\n    case \"$port\" in\n        ''|*[!0-9]*) die \"bad port in: $target\" ;;\n    esac\n    HOSTS=$((HOSTS + 1))\n\n    cert=$(sclient -connect \"$target\" -servername \"$host\")\n    end=$(printf '%s\\n' \"$cert\" | openssl x509 -noout -enddate 2>/dev/null \\\n          | sed 's/^notAfter=//' || true)\n    if [ -z \"$end\" ]; then\n        printf 'ERROR %s no certificate returned\\n' \"$target\" >>\"$TMP/certs\"\n        CERTBAD=$((CERTBAD + 1))\n    else\n        epoch=$(to_epoch \"$end\")\n        if [ -z \"$epoch\" ]; then\n            printf 'ERROR %s cannot parse notAfter: %s\\n' \"$target\" \"$end\" >>\"$TMP/certs\"\n            CERTBAD=$((CERTBAD + 1))\n        else\n            days=$(( (epoch - NOW) / 86400 ))\n            expires=$(fmt_date \"$epoch\")\n            if [ \"$epoch\" -le \"$NOW\" ]; then\n                printf 'EXPIRED %s certificate (%s)\\n' \\\n                    \"$target\" \"$expires\" >>\"$TMP/certs\"\n                CERTBAD=$((CERTBAD + 1))\n            elif [ \"$days\" -lt \"$WARN_DAYS\" ]; then\n                if [ \"$days\" -eq 0 ]; then\n                    printf 'WARN %s certificate under 1 day (%s)\\n' \\\n                        \"$target\" \"$expires\" >>\"$TMP/certs\"\n                else\n                    printf 'WARN %s certificate %d days (%s)\\n' \\\n                        \"$target\" \"$days\" \"$expires\" >>\"$TMP/certs\"\n                fi\n                CERTBAD=$((CERTBAD + 1))\n            else\n                printf 'CERT %s %d days (%s)\\n' \\\n                    \"$target\" \"$days\" \"$expires\" >>\"$TMP/certs\"\n            fi\n        fi\n    fi\n\n    for probe in -tls1 -tls1_1; do\n        out=$(sclient -connect \"$target\" -servername \"$host\" \"$probe\")\n        case \"$out\" in\n            *\"BEGIN CERTIFICATE\"*)\n                case \"$probe\" in\n                    -tls1) label=TLSv1.0 ;;\n                    *) label=TLSv1.1 ;;\n                esac\n                printf 'WEAK %s %s accepted\\n' \"$target\" \"$label\" >>\"$TMP/weak\"\n                WEAK=$((WEAK + 1)) ;;\n        esac\n    done\n\n    for cipher in RC4-SHA DES-CBC3-SHA AES128-SHA; do\n        out=$(sclient -connect \"$target\" -servername \"$host\" \\\n                      -tls1_2 -cipher \"$cipher\")\n        case \"$out\" in\n            *\"BEGIN CERTIFICATE\"*)\n                printf 'WEAK %s cipher %s accepted\\n' \"$target\" \"$cipher\" >>\"$TMP/weak\"\n                WEAK=$((WEAK + 1)) ;;\n        esac\n    done\ndone <\"$TMP/targets\"\n\nif [ \"$WEAK\" -eq 0 ] && [ \"$CERTBAD\" -eq 0 ]; then\n    if [ \"$HOSTS\" -eq 1 ]; then\n        printf 'OK 1 host, no weak protocol or expiring certificate\\n'\n    else\n        printf 'OK %d hosts, no weak protocol or expiring certificate\\n' \"$HOSTS\"\n    fi\n    exit 0\nfi\n\nif [ \"$WEAK\" -eq 0 ]; then\n    printf 'FINDING no weak protocols\\n'\nelif [ \"$WEAK\" -eq 1 ]; then\n    printf 'FINDING 1 weak protocol accepted\\n'\nelse\n    printf 'FINDING %d weak protocols accepted\\n' \"$WEAK\"\nfi\ncat \"$TMP/weak\"\ncat \"$TMP/certs\"\n\nexit 1\n```\n"},{"id":"p635","n":"635","slug":"files-that-changed-under-you","title":"Files that changed under you","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","FileIntegrity","DevSecOps","SRE"],"series_name":"ShellSec","design":"code","script":"file_integrity.sh","date":"2030-10-17","files":{"it":"notes/p635-files-that-changed-under-you.md","devsecops":"notes/p635-files-that-changed-under-you.md"},"md":"# Files that changed under you\n\nA deploy is clean, the tests pass, and three days later a binary in `/srv/app` is 400 bytes larger. mtime says last Tuesday. Nobody deployed on Tuesday.\n\n## Quick reference\n\nThe full script is at the end of this note: `file_integrity.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash file_integrity.sh /var/lib/app.sha /srv/app\n```\n\n**Example output**\n\n```text\nFINDING 2 changes\nCHANGED bin/run.sh\n```\n\n## Explained\n\n### What it does\n\n`file_integrity.sh --init` writes a plain-text baseline: one SHA-256 and one relative path per regular file. `--check re-hashes` the tree, compares each path with the baseline, and reports CHANGED, ADDED and REMOVED files. Any difference exits 1. The baseline must live outside the tree.\n\n### What the screen shows\n\nThe fourteen lines read the baseline, hash each live file, compare it with the stored line, then walk the baseline for REMOVED paths. On the fixture: FINDING 2 changes and CHANGED bin/run.sh.\n\n### How to run it\n\nbash `file_integrity.sh -c -b /var/lib/s0sta/app.sha /srv/app`. The cron line runs at 02:40 daily. Exit 0 is clean, 1 is a change, 2 is a bad path or unreadable baseline.\n\n### The fix\n\nDiff the changed file against the package or the build artefact. sha256sum tells you it changed; `dpkg -V` or `rpm -V` tells you whether the packaged files still match. If the difference is unexplained, take the host out of rotation and redeploy from the signed artefact.\n\n### Prevention\n\nBaseline after every approved deploy, store the baseline off the host, and alert on exit 1 rather than writing to a log nobody opens.\n\n> **The honest caveat:** a root attacker can rewrite the baseline along with the tree. Storing it off-host is what makes the check worth running.\n\n> **The lesson:** a timestamp is a claim. A hash is a measurement.\n\n## In one line\n\n> **Trust the hash, not the timestamp.** mtime lies; a hash is the file's fingerprint.\n\n**Take a baseline:** `file_integrity.sh -i -b app.sha /srv/app`\n\n## Script: file_integrity.sh\n\nDownload: [scripts/file_integrity.sh](../scripts/file_integrity.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n#\n# file_integrity.sh - create or check a SHA-256 baseline of a directory tree.\n#\n# Fixes: files that changed under you. --init writes a plain-text baseline\n# (one \"hash  relative/path\" line per regular file); --check re-hashes the\n# tree, then reports CHANGED, ADDED and REMOVED files and exits 1. The\n# baseline file itself must live outside the tree it describes.\n#\n# Usage:\n#   file_integrity.sh --init  --baseline /var/lib/s0sta/app.sha /srv/app\n#   file_integrity.sh --check --baseline /var/lib/s0sta/app.sha /srv/app\n#   file_integrity.sh -c -b /var/lib/s0sta/app.sha /srv/app\n#   file_integrity.sh --help\n#\n# Cron (daily, 02:40; exit 1 means the tree changed):\n#   40 2 * * * /usr/local/sbin/file_integrity.sh -c \\\n#     -b /var/lib/s0sta/app.sha /srv/app >>/var/log/s0sta/integrity.log 2>&1\n#\n# Exit: 0 clean or baseline written, 1 changes found, 2 usage or unreadable\n#       input.\n# Needs: sha256sum, or shasum -a 256 on macOS/BSD.\n# Read-only on --check: only the baseline is read. --init writes one file.\n# By s0sta.com\n#\nusage() {\n    cat <<'EOF'\nfile_integrity.sh - create or check a SHA-256 baseline of a directory tree.\n\nUsage:\n  file_integrity.sh --init  --baseline FILE DIR\n  file_integrity.sh --check --baseline FILE DIR\n  file_integrity.sh --help\n\n  -i, --init         write a new baseline from DIR\n  -c, --check        compare DIR with the baseline (default mode)\n  -b, --baseline F   baseline file, kept outside DIR\n  -h, --help         show this help and exit 0\n\nExit: 0 clean or baseline written, 1 changes found, 2 usage or unreadable\ninput. Files are compared by SHA-256; symlinks and special files are skipped.\nEOF\n}\n\ndie() {\n    printf 'file_integrity: %s\\n' \"$1\" >&2\n    exit 2\n}\n\nhash_file() {\n    if command -v sha256sum >/dev/null 2>&1; then\n        sha256sum \"$1\" 2>/dev/null | cut -d' ' -f1\n    else\n        shasum -a 256 \"$1\" 2>/dev/null | cut -d' ' -f1\n    fi\n}\n\nMODE=check\nBASE=\nDIR=\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help) usage; exit 0 ;;\n        -i|--init) MODE=init; shift ;;\n        -c|--check) MODE=check; shift ;;\n        -b|--baseline)\n            [ \"$#\" -ge 2 ] || die \"--baseline needs a path\"\n            BASE=$2; shift 2 ;;\n        -*) die \"unknown argument: $1\" ;;\n        *)\n            [ -z \"$DIR\" ] || die \"only one tree may be given\"\n            DIR=$1; shift ;;\n    esac\ndone\n\n[ -n \"$BASE\" ] || die \"missing --baseline\"\n[ -n \"$DIR\" ] || die \"missing DIR\"\n[ -d \"$DIR\" ] || die \"not a directory: $DIR\"\n[ -r \"$DIR\" ] || die \"cannot read: $DIR\"\nif ! command -v sha256sum >/dev/null 2>&1 && ! command -v shasum >/dev/null 2>&1; then\n    die \"need sha256sum or shasum\"\nfi\n\nBASEDIR=$(cd \"$(dirname \"$BASE\")\" 2>/dev/null && pwd) || die \"no such directory: $BASE\"\nBASE_ABS=\"$BASEDIR/$(basename \"$BASE\")\"\nDIR_ABS=$(cd \"$DIR\" && pwd)\ncase \"$BASE_ABS\" in\n    \"$DIR_ABS\"/*) die \"baseline must live outside the tree: $BASE_ABS\" ;;\nesac\n\nif [ \"$MODE\" = init ]; then\n    TMPBASE=$(mktemp \"${TMPDIR:-/tmp}/file_integrity.XXXXXX\")\n    trap 'rm -f \"$TMPBASE\"' EXIT\n    COUNT=0\n    while IFS= read -r -d '' path; do\n        rel=${path#./}\n        hash=$(hash_file \"$DIR/$path\") || hash=\n        [ -n \"$hash\" ] || die \"cannot hash: $path\"\n        printf '%s  %s\\n' \"$hash\" \"$rel\" >>\"$TMPBASE\"\n        COUNT=$((COUNT + 1))\n    done < <(cd \"$DIR\" && find . -type f -print0)\n    LC_ALL=C sort -k2 -o \"$TMPBASE\" \"$TMPBASE\"\n    mv \"$TMPBASE\" \"$BASE_ABS\"\n    printf 'BASELINE %d files written to %s\\n' \"$COUNT\" \"$BASE_ABS\"\n    exit 0\nfi\n\n[ -f \"$BASE_ABS\" ] || die \"baseline not found: $BASE_ABS\"\n[ -r \"$BASE_ABS\" ] || die \"cannot read baseline: $BASE_ABS\"\n\nTMP=$(mktemp -d \"${TMPDIR:-/tmp}/file_integrity.XXXXXX\")\ntrap 'rm -rf \"$TMP\"' EXIT\n: >\"$TMP/changed\"\n: >\"$TMP/added\"\n: >\"$TMP/removed\"\n: >\"$TMP/paths\"\n\n# Pass 1: everything in the baseline must still exist and match.\nwhile IFS= read -r line || [ -n \"$line\" ]; do\n    [ -n \"$line\" ] || continue\n    hash=${line%%  *}\n    rel=${line#*  }\n    if [ \"$hash\" = \"$line\" ] || [ -z \"$rel\" ]; then\n        die \"malformed baseline line: $line\"\n    fi\n    printf '%s\\n' \"$rel\" >>\"$TMP/paths\"\n    target=\"$DIR_ABS/$rel\"\n    if [ ! -e \"$target\" ]; then\n        printf '%s\\n' \"$rel\" >>\"$TMP/removed\"\n        continue\n    fi\n    live=$(hash_file \"$target\") || live=\n    if [ -z \"$live\" ]; then\n        die \"cannot hash: $target\"\n    fi\n    if [ \"$live\" != \"$hash\" ]; then\n        printf '%s\\n' \"$rel\" >>\"$TMP/changed\"\n    fi\ndone <\"$BASE_ABS\"\n\n# Pass 2: everything in the tree must be in the baseline.\nwhile IFS= read -r -d '' path; do\n    rel=${path#./}\n    if ! grep -qxF -e \"$rel\" \"$TMP/paths\"; then\n        printf '%s\\n' \"$rel\" >>\"$TMP/added\"\n    fi\ndone < <(cd \"$DIR_ABS\" && find . -type f -print0)\n\nCHANGED=$(wc -l <\"$TMP/changed\" || true)\nADDED=$(wc -l <\"$TMP/added\" || true)\nREMOVED=$(wc -l <\"$TMP/removed\" || true)\nTOTAL=$((CHANGED + ADDED + REMOVED))\n\nif [ \"$TOTAL\" -eq 0 ]; then\n    printf 'OK no changes in %s\\n' \"$DIR_ABS\"\n    exit 0\nfi\n\nif [ \"$TOTAL\" -eq 1 ]; then\n    printf 'FINDING 1 change\\n'\nelse\n    printf 'FINDING %d changes\\n' \"$TOTAL\"\nfi\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'CHANGED %s\\n' \"$rel\"\ndone < <(LC_ALL=C sort \"$TMP/changed\")\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'ADDED %s\\n' \"$rel\"\ndone < <(LC_ALL=C sort \"$TMP/added\")\nwhile IFS= read -r rel; do\n    [ -n \"$rel\" ] || continue\n    printf 'REMOVED %s\\n' \"$rel\"\ndone < <(LC_ALL=C sort \"$TMP/removed\")\n\nexit 1\n```\n"},{"id":"p636","n":"636","slug":"certificates-before-they-expire","title":"Certificates before they expire","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","TLS","PKI","DevOps","SRE"],"series_name":"ShellSec","design":"code","script":"cert_renew_check.sh","date":"2030-10-20","files":{"it":"notes/p636-certificates-before-they-expire.md","devsecops":"notes/p636-certificates-before-they-expire.md"},"md":"# Certificates before they expire\n\nThe leaf expires at 09:00 on a Saturday. Monitoring says the endpoint is fine, because the load balancer still serves a valid certificate from another node.\n\n## Quick reference\n\nThe full script is at the end of this note: `cert_renew_check.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash cert_renew_check.sh certs.txt 30\n```\n\n**Example output**\n\n```text\nEXPIRED old.example.com.pem -2 days\nFINDING 1 certificate warning\n```\n\n## Explained\n\n### What it does\n\n`cert_renew_check.sh` reads host:port entries and PEM paths. Hosts go through openssl s_client, files through openssl x509. It prints days to expiry, flags anything inside `--warn-days`, and marks an expired certificate with a negative count. Exit 1 means action.\n\n### What the screen shows\n\nThe fourteen lines read the list, load a PEM or open a socket, parse notAfter, subtract now, then print the status and the count: EXPIRED `old.example.com.pem -2` days and FINDING 1 certificate warning.\n\n### How to run it\n\nbash `cert_renew_check.sh -f certs.txt --warn-days 30`. The cron line runs at 05:10 daily. Exit 0 is clean, 1 is a warning or expired certificate, 2 is an unreadable file or a host with no certificate.\n\n### The fix\n\nRenew at 30 days, deploy at 14, and verify the mounted file on every node, not just the one monitoring probes. A certificate renewed in one place and pinned in another is two problems.\n\n### Prevention\n\nKeep one list in Git, add a host when it joins the load balancer, and run the check after every deploy. The list is the inventory; the scan is the proof.\n\n> **The honest caveat:** a bundle is read at its first certificate. Chain order, signatures and key matches need openssl verify, which this script does not run.\n\n> **The lesson:** a certificate does not expire when you remember. It expires when the clock says.\n\n## In one line\n\n> **Renew on the number, not the date.** A calendar reminder is a hope; a day count is a check.\n\n**Check the list:** `cert_renew_check.sh -f certs.txt`\n\n## Script: cert_renew_check.sh\n\nDownload: [scripts/cert_renew_check.sh](../scripts/cert_renew_check.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n#\n# cert_renew_check.sh - days to expiry for host:port endpoints and PEM files.\n#\n# Fixes: certificates before they expire. Reads a list of host:port entries\n# or PEM file paths, prints the days to expiry for each one, flags anything\n# inside --warn-days, and flags an expired certificate as EXPIRED. A negative\n# day count means the certificate expired that many days ago.\n#\n# Usage:\n#   cert_renew_check.sh -f certs.txt\n#   cert_renew_check.sh -f certs.txt --warn-days 21 --timeout 4\n#   cert_renew_check.sh api.example.com:443 /etc/ssl/certs/api.pem\n#   cert_renew_check.sh --help\n#\n#   certs.txt holds one host:port or one PEM path per line; blank lines and\n#   # comments are ignored. A PEM bundle is read at its first certificate.\n#   IPv6 addresses are not supported in this version.\n#\n# Cron (daily, 05:10; exit 1 means a finding):\n#   10 5 * * * /usr/local/sbin/cert_renew_check.sh -f /etc/s0sta/certs.txt \\\n#     --warn-days 30 >>/var/log/s0sta/cert-renew.log 2>&1\n#\n# Exit: 0 clean, 1 certificate inside --warn-days or expired, 2 usage or\n#       unreadable input (including a host that returns no certificate).\n# Needs: openssl with x509 and s_client. timeout(1) is optional but\n# recommended for host entries that blackhole the SYN.\n# Read-only: certificates are read; no key file is opened or printed.\n# By s0sta.com\n#\nusage() {\n    cat <<'EOF'\ncert_renew_check.sh - report certificate expiry for hosts and PEM files.\n\nUsage:\n  cert_renew_check.sh [OPTIONS] [HOST:PORT|FILE ...]\n  cert_renew_check.sh -f FILE [OPTIONS]\n\n  -f, --file FILE      read host:port or PEM paths from FILE\n  -w, --warn-days N    warn inside this many days (default 30)\n  -t, --timeout N      seconds per TLS connection (default 5)\n  -h, --help           show this help and exit 0\n\nExit: 0 clean, 1 warning or expired, 2 usage or unreadable input.\nRead-only: no private key is read or printed.\nEOF\n}\n\ndie() {\n    printf 'cert_renew_check: %s\\n' \"$1\" >&2\n    exit 2\n}\n\nWARN_DAYS=30\nTIMEOUT=5\nLIST_FILE=\nTMP=$(mktemp -d \"${TMPDIR:-/tmp}/cert_renew_check.XXXXXX\")\ntrap 'rm -rf \"$TMP\"' EXIT\n: >\"$TMP/entries\"\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help) usage; exit 0 ;;\n        -f|--file)\n            [ \"$#\" -ge 2 ] || die \"--file needs a path\"\n            LIST_FILE=$2; shift 2 ;;\n        -w|--warn-days)\n            [ \"$#\" -ge 2 ] || die \"--warn-days needs a number\"\n            WARN_DAYS=$2; shift 2 ;;\n        -t|--timeout)\n            [ \"$#\" -ge 2 ] || die \"--timeout needs a number\"\n            TIMEOUT=$2; shift 2 ;;\n        -*) die \"unknown argument: $1\" ;;\n        *) printf '%s\\n' \"$1\" >>\"$TMP/entries\"; shift ;;\n    esac\ndone\n\ncase \"$WARN_DAYS\" in\n    ''|*[!0-9]*) die \"--warn-days must be a whole number\" ;;\nesac\ncase \"$TIMEOUT\" in\n    ''|*[!0-9]*) die \"--timeout must be a whole number\" ;;\nesac\n[ \"$TIMEOUT\" -ge 1 ] || die \"--timeout must be at least 1\"\ncommand -v openssl >/dev/null 2>&1 || die \"openssl not found\"\n\nif [ -n \"$LIST_FILE\" ]; then\n    [ -r \"$LIST_FILE\" ] || die \"cannot read: $LIST_FILE\"\n    while IFS= read -r line || [ -n \"$line\" ]; do\n        case \"$line\" in\n            ''|\\#*) continue ;;\n        esac\n        printf '%s\\n' \"$line\" >>\"$TMP/entries\"\n    done <\"$LIST_FILE\"\nfi\n[ -s \"$TMP/entries\" ] || die \"no entries (give HOST:PORT, FILE or --file FILE)\"\n\nTMO=\nif command -v timeout >/dev/null 2>&1; then\n    TMO=timeout\nelif command -v gtimeout >/dev/null 2>&1; then\n    TMO=gtimeout\nfi\n\nsclient() {\n    if [ -n \"$TMO\" ]; then\n        \"$TMO\" \"$TIMEOUT\" openssl s_client \"$@\" </dev/null 2>/dev/null || true\n    else\n        openssl s_client \"$@\" </dev/null 2>/dev/null &\n        local pid=$! wd\n        ( sleep \"$TIMEOUT\"; kill -TERM \"$pid\" 2>/dev/null || true ) &\n        wd=$!\n        wait \"$pid\" 2>/dev/null || true\n        kill -TERM \"$wd\" 2>/dev/null || true\n        wait \"$wd\" 2>/dev/null || true\n    fi\n    return 0\n}\n\nto_epoch() {\n    local s\n    s=$(printf '%s' \"$1\" | tr -s ' ')\n    if date -u -d \"$s\" +%s >/dev/null 2>&1; then\n        date -u -d \"$s\" +%s\n    else\n        date -u -j -f \"%b %d %T %Y %Z\" \"$s\" +%s 2>/dev/null || true\n    fi\n}\n\nfmt_date() {\n    if date -u -d \"@$1\" +%F >/dev/null 2>&1; then\n        date -u -d \"@$1\" +%F\n    else\n        date -u -r \"$1\" +%F 2>/dev/null || true\n    fi\n}\n\nNOW=$(date -u +%s)\nBAD=0\nRC=0\n\nwhile IFS= read -r entry; do\n    [ -n \"$entry\" ] || continue\n    end=\n    if [ -f \"$entry\" ]; then\n        end=$(openssl x509 -in \"$entry\" -noout -enddate 2>/dev/null \\\n              | sed 's/^notAfter=//' || true)\n    else\n        case \"$entry\" in\n            *:*) ;;\n            *) die \"not a file or host:port: $entry\" ;;\n        esac\n        host=${entry%:*}\n        port=${entry##*:}\n        [ -n \"$host\" ] || die \"empty host in: $entry\"\n        case \"$port\" in\n            ''|*[!0-9]*) die \"bad port in: $entry\" ;;\n        esac\n        cert=$(sclient -connect \"$entry\" -servername \"$host\")\n        end=$(printf '%s\\n' \"$cert\" | openssl x509 -noout -enddate 2>/dev/null \\\n              | sed 's/^notAfter=//' || true)\n    fi\n    if [ -z \"$end\" ]; then\n        printf 'ERROR %s no certificate\\n' \"$entry\"\n        RC=2\n        continue\n    fi\n    epoch=$(to_epoch \"$end\")\n    if [ -z \"$epoch\" ]; then\n        printf 'ERROR %s cannot parse notAfter: %s\\n' \"$entry\" \"$end\"\n        RC=2\n        continue\n    fi\n    days=$(( (epoch - NOW) / 86400 ))\n    expires=$(fmt_date \"$epoch\")\n    if [ \"$epoch\" -le \"$NOW\" ]; then\n        printf 'EXPIRED %s %d days (%s)\\n' \"$entry\" \"$days\" \"$expires\"\n        BAD=$((BAD + 1))\n    elif [ \"$days\" -lt \"$WARN_DAYS\" ]; then\n        printf 'WARN %s %d days (%s)\\n' \"$entry\" \"$days\" \"$expires\"\n        BAD=$((BAD + 1))\n    else\n        printf 'OK %s %d days (%s)\\n' \"$entry\" \"$days\" \"$expires\"\n    fi\ndone <\"$TMP/entries\"\n\nif [ \"$BAD\" -eq 1 ]; then\n    printf 'FINDING 1 certificate warning\\n'\nelif [ \"$BAD\" -gt 1 ]; then\n    printf 'FINDING %d certificate warnings\\n' \"$BAD\"\nelif [ \"$RC\" -eq 0 ]; then\n    printf 'OK no certificate inside %d days\\n' \"$WARN_DAYS\"\nfi\n\n[ \"$RC\" -eq 0 ] || exit 2\n[ \"$BAD\" -eq 0 ] || exit 1\nexit 0\n```\n"},{"id":"p637","n":"637","slug":"the-config-and-crypto-sweep","title":"The config and crypto sweep","sections":["it","devsecops"],"topic":"Security & hardening","tags":["DevSecOps","Linux","Bash","CyberSecurity","SRE"],"series_name":"ShellSec","design":"architecture","script":"","date":"2030-10-22","files":{"it":"notes/p637-the-config-and-crypto-sweep.md","devsecops":"notes/p637-the-config-and-crypto-sweep.md"},"md":"# The config and crypto sweep\n\nA config and crypto sweep is three tiers, and most teams start at the third. They open a ticket, change a file, and never learn what the host looked like before the change.\n\n## Quick reference\n\n- **Baseline:** File hashes (HASH) · Authorised keys (KEY) · Certificate store (CERT)\n- **Compare:** Drift check (DRIFT) · Key audit (AUDIT) · Expiry window (DAYS)\n- **Report:** Fix list (FIX) · Ticket (TKT)\n\n**Request flow:** File hashes → Drift check → Fix list → Ticket; Authorised keys → Key audit → Fix list; Certificate store → Expiry window → Ticket\n\n**Around it:** Exit code (EXIT) · Cron (CRON) · Baseline store (BASE) · Runbook (RUN)\n\n## Explained\n\n### Baseline\n\nThe first tier is a read-only record. File hashes answer what the bytes were. Authorised keys answer who could log in. The certificate store answers what will expire. Take it at a known-good moment, store it off the host, and never let the check write into the tree it watches. A hash without a date is a fact without a context.\n\n### Compare\n\nThe second tier is the diff. Drift compares hashes with the baseline. The key audit compares authorized_keys with policy. The expiry window compares notAfter with today. None of them changes a file, and each returns an exit code: 0 clean, 1 finding, 2 the check itself broke.\n\n### Report\n\nThe third tier is the only one that writes. A fix list names the file, the owner and the command. A ticket carries the same facts into the queue. A report that says \"config drift\" without a path is a rumour with a timestamp.\n\n> **The honest caveat:** a baseline is a claim about a moment. If the host was already compromised when you took it, you have baselined the compromise.\n\n> **The lesson:** hash first, compare second, decide last.\n\n## In one line\n\n> **Hash first, then compare.** A baseline you cannot trust makes every later tier a guess.\n\n**Start at the hash**\n\n```bash\nsha256sum /etc/ssh/sshd_config\n```\n"},{"id":"p638","n":"638","slug":"the-openssl-shell-cheat-sheet","title":"The openssl shell cheat sheet","sections":["it","devsecops"],"topic":"Security & hardening","tags":["OpenSSL","SSH","Linux","DevSecOps","CommandLine"],"series_name":"ShellSec","design":"cheatsheet","script":"","date":"2030-10-24","files":{"it":"notes/p638-the-openssl-shell-cheat-sheet.md","devsecops":"notes/p638-the-openssl-shell-cheat-sheet.md"},"md":"# The openssl shell cheat sheet\n\nMost openssl questions are one of four: what is in this file, do I trust it, how do I convert it, and which key is this. Four blocks, in that order.\n\n## Quick reference\n\n### Inspect\n\n```bash\nopenssl x509 -in cert.pem -noout -subject\nopenssl x509 -in cert.pem -noout -dates\nopenssl x509 -in cert.pem -noout -text\nopenssl s_client -connect host:443\n```\n\n### Verify\n\n```bash\nopenssl verify -CAfile ca.pem cert.pem\nopenssl verify -untrusted int.pem cert\nopenssl x509 -in cert.pem -checkend 0\nopenssl x509 -in cert.pem -checkend 86400\nopenssl crl -in crl.pem -noout -text\n```\n\n### Convert\n\n```bash\nopenssl x509 -inform der -in c.der\nopenssl x509 -in cert.pem -outform der\nopenssl pkcs12 -in b.p12 -out b.pem\nopenssl pkcs12 -in b.p12 -clcerts -nokeys\nopenssl pkey -in k.pem -pubout\n```\n\n### Keys\n\n```bash\nssh-keygen -t ed25519 -C \"you@host\"\nssh-keygen -lf ~/.ssh/id_ed25519.pub\nssh-keygen -y -f ~/.ssh/id_ed25519\nssh-keygen -F host.example.com\nssh-keygen -R host.example.com\nssh -Q key\n```\n\n## Explained\n\n### Inspect\n\n`openssl x509 -in cert.pem -noout -subject` prints the subject, -dates prints notBefore and notAfter, and -text prints everything, including the public key and extensions. `openssl s_client -connect host:443` opens a TLS connection and prints the certificate chain it receives.\n\n### Verify\n\n`openssl verify -CAfile ca.pem cert.pem` checks a leaf against a root. -untrusted `int.pem` supplies the intermediate. `openssl x509 -checkend 0` fails when expired; -checkend 86400 fails within a day. `openssl crl -in crl.pem -noout -text` reads a revocation list.\n\n### Convert\n\n`openssl x509 -inform der -in c.der` prints a DER certificate as PEM. `openssl x509 -in cert.pem -outform der` writes the reverse. `openssl pkcs12 -in b.p12 -out b.pem` exports a PKCS12 bundle; -clcerts -nokeys keeps only the client certificate. `openssl pkey -in k.pem -pubout` derives the public key.\n\n### Keys\n\n`ssh-keygen -t ed25519 -C \"you@host\"` makes a key with a comment you can trace. `ssh-keygen -lf` prints a fingerprint without printing the key. `ssh-keygen -y -f` prints only the public half of a private key. `ssh-keygen -F` and -R search and remove host keys; `ssh -Q key` lists the algorithms this client supports.\n\n> **The honest caveat:** -text prints public data, but a private key in the same directory is still one careless cat away. Never paste a private key into a ticket.\n\n> **The lesson:** read the certificate before you trust the connection.\n\n## In one line\n\n> **Read the cert, then trust it.** Every command here inspects or converts; none of them signs a certificate.\n\n**Inspect first**\n\n```bash\nopenssl x509 -in cert.pem -noout -text\n```\n"},{"id":"p639","n":"639","slug":"four-config-and-crypto-failures","title":"Four config and crypto failures","sections":["it","devsecops"],"topic":"Security & hardening","tags":["DevSecOps","TLS","SSH","Linux","CyberSecurity"],"series_name":"ShellSec","design":"explainer","script":"","date":"2030-10-27","files":{"it":"notes/p639-four-config-and-crypto-failures.md","devsecops":"notes/p639-four-config-and-crypto-failures.md"},"md":"# Four config and crypto failures\n\nFour failures look different in a ticket and identical in a shell. Each one is a comparison that nobody ran.\n\n## Quick reference\n\n### Silent drift\n\n- A file changes and no ticket records it\n- The next deploy quietly reverts it\n- Fix: hash the tree, diff nightly\n- Keep the baseline off the host\n\n### Expired certificate\n\n- The leaf expires while the chain is fine\n- The client reports an error, not a renewal date\n- Fix: warn at 30 days, renew at 14\n- Check the mounted file on every node\n\n### Weak key type\n\n- ssh-rsa and ssh-dss are legacy algorithms\n- A long RSA key can still sign with SHA-1\n- Fix: ed25519 for people, rsa-sha2 for legacy\n- Remove the key, then rotate the account\n\n### Missing baseline\n\n- No baseline means no definition of changed\n- A backup is a copy, not a measurement\n- Fix: store hashes off the host you audit\n- Re-baseline after every approved change\n\n> **Note:** Four failures, one habit: compare with something you trust.\n\n## Explained\n\n### Silent drift\n\nA file changes and no ticket records it. The next deploy reverts it, or worse, keeps it. The fix is a hash of the tree and a nightly diff, with the baseline stored off the host so a local change cannot edit the evidence.\n\n### Expired certificate\n\nThe leaf expires while the chain is fine, and the client reports a certificate error, not a date. Warn at 30 days and renew at 14, then check the file the ingress actually mounts on every node.\n\n### Weak key type\n\nssh-rsa and ssh-dss are legacy algorithms. A long RSA key can still sign with SHA-1, because key length does not choose the signature hash. Use ed25519 for people and rsa-sha2 for legacy systems, then remove the old key and rotate the account.\n\n### Missing baseline\n\nNo baseline means no definition of changed. A backup is a copy, not a measurement. Store hashes off the host you audit and re-baseline after every approved change, or the first finding will be an argument.\n\n> **The honest caveat:** a weak key type is a policy failure, not proof of compromise, and a missing baseline cannot be reconstructed after the fact.\n\n> **The lesson:** every one of these is a comparison, and every comparison needs a reference.\n\n## In one line\n\n> **Name the failure, then hash it.** Each panel is a check you can run before the page.\n\n**Run the four**\n\n```bash\nsha256sum; openssl x509; ssh-keygen -lf\n```\n"},{"id":"p640","n":"640","slug":"9-steps-of-a-config-audit","title":"9 steps of a config audit","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","Linux","DevSecOps","Audit","SRE"],"series_name":"ShellSec","design":"steps","script":"","date":"2030-10-29","files":{"it":"notes/p640-9-steps-of-a-config-audit.md","devsecops":"notes/p640-9-steps-of-a-config-audit.md"},"md":"# 9 steps of a config audit\n\nA config audit is nine commands in three small groups: establish what is true, compare what changed, decide what to fix. Skip the first group and the other six are opinions.\n\n## Quick reference\n\n### Baseline — Write down what is true\n\n```bash\n# One file, one hash, one known-good value.\nsha256sum /etc/ssh/sshd_config\n# A copy you can diff, not a backup.\ntar -C /etc -czf /var/lib/etc.tgz .\n# Fingerprint every key you keep.\nssh-keygen -lf ~/.ssh/id_ed25519.pub\n```\n\n### Compare — Diff the tree, then the crypto\n\n- **config_drift.sh -b /var/lib/etc-base** — Changed, added, removed; exit 1.\n- **file_integrity.sh -c -b app.sha /srv** — The app tree, not just `/etc`.\n- **tls_scan.sh -f hosts.txt --warn-days 21** — Protocols and the expiry window.\n\n### Decide — Fix, then leave a check\n\n- **cert_renew_check.sh -f certs.txt** — Renew what is inside 30 days.\n- `ssh-keygen -R old.example.com` — Remove the host key you distrust.\n- `systemctl reload sshd` — Reload the config, do not restart.\n\n## Explained\n\n### Baseline\n\nThe terminal starts with `sha256sum /etc/ssh/sshd_config`, one file and one known-good hash. Then `tar -C /etc -czf /var/lib/etc.tgz .` gives a copy you can diff, not a backup you hope restores. `ssh-keygen -lf ~/.ssh/id_ed25519.pub` gives every key a fingerprint you can compare later.\n\n### Compare\n\n`config_drift.sh -b /var/lib/etc-base` names changed, added and removed files and exits 1. `file_integrity.sh -c -b app.sha /srv` applies the same idea to the application tree. `tls_scan.sh -f hosts.txt --warn-days 21` checks the protocols and the expiry window together, because an old protocol and a short certificate are the same page.\n\n### Decide\n\n`cert_renew_check.sh -f certs.txt` names what is inside 30 days. `ssh-keygen -R old.example.com` removes a host key you no longer trust. `systemctl reload sshd` applies the config change without dropping the sessions you are trying to protect. Then write the owner and the date in the same ticket.\n\n> **The honest caveat:** a baseline taken after the change freezes the wrong state. Write down the date and the change ticket before you run the first hash.\n\n> **The lesson:** baseline, compare, decide, in that order, with an owner, a date and a check for every fix.\n\n## In one line\n\n> **Baseline, compare, then decide.** Nine commands, three groups, and a written owner for every fix.\n\n**Start the clock**\n\n```bash\nsha256sum /etc/ssh/sshd_config\n```\n"},{"id":"p641","n":"641","slug":"quiz-3-config-questions","title":"Quiz: 3 config questions","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","Linux","DevSecOps","CyberSecurity","TLS"],"series_name":"ShellSec","design":"quiz","script":"","date":"2030-10-31","files":{"it":"notes/p641-quiz-3-config-questions.md","devsecops":"notes/p641-quiz-3-config-questions.md"},"md":"# Quiz: 3 config questions\n\nPhase two closes with three questions about the checks that keep a config and crypto sweep honest. Answer each before the reveal.\n\n## Quick reference\n\n**Q1. Why must the baseline stay read-only while you audit?**\n\n- a) So the tree cannot change under the check\n- b) Because hashes are one-way\n- c) So the baseline file stays small\n\n> **Answer:** a) So the tree cannot change under the check. If the audit writes into the tree, it changes what it measures. A baseline you can edit is not evidence; keep it on another host and mount it read-only.\n\n**Q2. Why does key type matter more than key length alone?**\n\n- a) It fixes the algorithm and default hash\n- b) Long keys are always weak\n- c) It sets the file permissions\n\n> **Answer:** a) It fixes the algorithm and default hash. The type sets the algorithm family and signature hash. An RSA key can still sign with SHA-1; ed25519 cannot. Length raises one cost, not the protocol.\n\n**Q3. What does a certificate chain failure mean for the client?**\n\n- a) It could not build a trusted path to a root\n- b) The leaf private key is missing\n- c) The server is offline\n\n> **Answer:** a) It could not build a trusted path to a root. The client walks leaf to intermediate to a root in its trust store. A missing, expired or untrusted intermediate breaks that path even when the leaf is valid.\n\n## Explained\n\n### Question one\n\nYour audit script writes a report into the tree it is checking. Why is that a problem? Because the next check hashes the report, the baseline no longer describes the system, and a file the script can edit is not evidence. Read-only is not a style choice; it is what makes the comparison meaningful.\n\n### Question two\n\nTwo keys are both 4096 bits. One is ssh-rsa, one is ed25519. Why does the type matter more than the length? The type sets the algorithm family and signature hash. An RSA key can still sign with SHA-1; ed25519 cannot. Length raises the cost of one attack, not the protocol.\n\n### Question three\n\nA client reports a certificate chain failure. The leaf is valid and in date. What failed? The client could not build a trusted path from the leaf through an intermediate to a root in its trust store. A missing, expired or untrusted intermediate breaks that path before the leaf's dates are considered.\n\n> **The honest caveat:** passing this quiz does not audit a host; the commands do.\n\n> **The lesson:** a baseline must be read-only, a key type is a policy, and a chain is a path. A report is not a control.\n\n## In one line\n\n> **Three answers, one checklist.** A baseline, a key type and a chain: three things to check before the page.\n\n**Check the chain**\n\n```bash\nopenssl verify -CAfile ca.pem cert.pem\n```\n"},{"id":"p642","n":"642","slug":"a-deploy-gate-that-says-no","title":"A deploy gate that says no","sections":["devsecops"],"topic":"Shell scripts","tags":["Bash","Kubernetes","CICD","DevOps","SRE"],"series_name":"ShellSec","design":"code","script":"deploy_gate.sh","date":"2030-11-03","files":{"devsecops":"notes/p642-a-deploy-gate-that-says-no.md"},"md":"# A deploy gate that says no\n\nAt 21:40 the release job goes green and the new API is live on staging. The runner's kubeconfig still pointed at the previous job's cluster.\n\n## Quick reference\n\nThe full script is at the end of this note: `deploy_gate.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash Scripts/deploy_gate.sh prod-eu api api\n```\n\n**Example output**\n\n```text\nGATE FAIL context staging, want prod-eu\nGATE 1 of 5 failed\n```\n\n## Explained\n\n### What it does\n\n`deploy_gate.sh` takes a context, a namespace and a deployment, and runs five checks against the cluster it is talking to: the context matches, the namespace exists, the previous rollout is healthy, no incident flag file is waiting, and the image is pinned by digest. It exits 1 if any gate failed.\n\n### What the screen shows\n\nThe fourteen lines compare the current context, check the namespace, wait on the previous rollout, test the alert file, and accept only an @sha256: reference in the YAML. With staging current and prod-eu expected, it prints GATE FAIL context staging, want prod-eu and GATE 1 of 5 failed.\n\n### How to run it\n\nbash Scripts/deploy_gate.sh prod-eu api api, as the first step after checkout. The arguments are context, namespace and deployment. A nightly cron run is a readiness probe only.\n\n### The fix\n\nSwitch context, create the namespace, clear the rollout, remove the incident file, and pin the image in the manifest.\n\n### Prevention\n\nGive each environment its own kubeconfig from a secret, never the runner default, and scope the job to one namespace.\n\n> **The honest caveat:** rollout status reports what the controller knows, so it can pass while the endpoint is broken. That is the smoke test's job.\n\n> **The lesson:** a gate is a check that can stop the pipeline, not a list someone reads.\n\n## In one line\n\n> **A gate that cannot say no is a button.** A pipeline with no veto is a deploy button with a log.\n\n**Check the context**\n\n```bash\nkubectl config current-context\n```\n\n## Script: deploy_gate.sh\n\nDownload: [scripts/deploy_gate.sh](../scripts/deploy_gate.sh)\n\n```bash\n#!/usr/bin/env bash\n# deploy_gate.sh - five checks a deploy must pass before it touches a cluster.\n#\n# Fixes: a pipeline that deploys to whatever context the runner happened to\n# load, into a namespace that does not exist, over a rollout that is already\n# broken, during an open incident, or with a mutable image tag.\n#\n# Usage:\n#   ./deploy_gate.sh CONTEXT NAMESPACE DEPLOYMENT [options]\n#\n# Examples:\n#   ./deploy_gate.sh prod-eu api api\n#   ./deploy_gate.sh prod-eu api api --expect-digest sha256:4f2c1d...\n#   ./deploy_gate.sh prod-eu api api --alert-file /run/s0sta/freeze\n#\n# Options:\n#   --alert-file PATH   block while PATH exists (default /var/run/s0sta/deploy.block)\n#   --expect-digest D   require the running image to carry digest D\n#   --kubeconfig PATH   pass --kubeconfig to every kubectl call\n#   --timeout SECONDS   rollout status timeout (default 60)\n#   -h, --help          print this help and exit 0\n#\n# Cron (nightly readiness probe; the gate itself belongs in CI):\n#   0 3 * * * /usr/local/bin/deploy_gate.sh prod-eu api api >/var/log/deploy_gate.log 2>&1\n#\n# Exit codes:\n#   0  every gate passed\n#   1  at least one gate failed\n#   2  usage error or unreadable input\n#\n# Needs kubectl on PATH. Read-only: it only runs config, get and rollout status.\n# By s0sta.com\nset -euo pipefail\n\nusage() {\n    cat <<'EOF'\ndeploy_gate.sh - five checks a deploy must pass before it touches a cluster.\n\nUsage:\n  ./deploy_gate.sh CONTEXT NAMESPACE DEPLOYMENT [options]\n\nArguments:\n  CONTEXT      the kubectl context production expects, e.g. prod-eu\n  NAMESPACE    the namespace the release belongs in, e.g. api\n  DEPLOYMENT   the Deployment that is already running, e.g. api\n\nOptions:\n  --alert-file PATH   block while PATH exists\n                      (default /var/run/s0sta/deploy.block)\n  --expect-digest D   require the running image to carry digest D\n  --kubeconfig PATH   pass --kubeconfig to every kubectl call\n  --timeout SECONDS   rollout status timeout (default 60)\n  -h, --help          print this help and exit 0\n\nExit codes:\n  0  every gate passed\n  1  at least one gate failed\n  2  usage error or unreadable input\n\nBy s0sta.com\nEOF\n}\n\ncontext=\"\"\nnamespace=\"\"\ndeployment=\"\"\nalert_file=\"/var/run/s0sta/deploy.block\"\nexpect_digest=\"\"\nkubeconfig=\"\"\ntimeout=60\n\nneed_value() {\n    if [ \"$#\" -lt 2 ]; then\n        printf 'deploy_gate: %s needs a value\\n' \"$1\" >&2\n        exit 2\n    fi\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help)\n            usage\n            exit 0\n            ;;\n        --alert-file)\n            need_value \"$@\"\n            alert_file=$2\n            shift 2\n            ;;\n        --expect-digest)\n            need_value \"$@\"\n            expect_digest=$2\n            shift 2\n            ;;\n        --kubeconfig)\n            need_value \"$@\"\n            kubeconfig=$2\n            shift 2\n            ;;\n        --timeout)\n            need_value \"$@\"\n            timeout=$2\n            shift 2\n            ;;\n        -*)\n            printf 'deploy_gate: unknown option %s\\n' \"$1\" >&2\n            usage >&2\n            exit 2\n            ;;\n        *)\n            if [ -z \"$context\" ]; then\n                context=$1\n            elif [ -z \"$namespace\" ]; then\n                namespace=$1\n            elif [ -z \"$deployment\" ]; then\n                deployment=$1\n            else\n                printf 'deploy_gate: unexpected argument %s\\n' \"$1\" >&2\n                exit 2\n            fi\n            shift\n            ;;\n    esac\ndone\n\nif [ -z \"$context\" ] || [ -z \"$namespace\" ] || [ -z \"$deployment\" ]; then\n    printf 'deploy_gate: CONTEXT, NAMESPACE and DEPLOYMENT are required\\n' >&2\n    usage >&2\n    exit 2\nfi\n\ncase \"$timeout\" in\n    ''|*[!0-9]*)\n        printf 'deploy_gate: --timeout must be a number of seconds\\n' >&2\n        exit 2\n        ;;\nesac\n\nif ! command -v kubectl >/dev/null 2>&1; then\n    printf 'deploy_gate: kubectl not found on PATH\\n' >&2\n    exit 2\nfi\n\nKUBECTL=(kubectl)\nif [ -n \"$kubeconfig\" ]; then\n    KUBECTL+=(--kubeconfig \"$kubeconfig\")\nfi\n\npassed=0\nfailed=0\n\nok() {\n    passed=$((passed + 1))\n    printf 'GATE ok   %s\\n' \"$1\"\n}\n\nno() {\n    failed=$((failed + 1))\n    printf 'GATE FAIL %s\\n' \"$1\"\n}\n\n# 1. The context the runner is pointed at. This is first because every later\n#    check would answer for the wrong cluster.\ncurrent_context=$(\"${KUBECTL[@]}\" config current-context 2>/dev/null) || current_context=\"\"\nif [ \"$current_context\" = \"$context\" ]; then\n    ok \"context $current_context\"\nelse\n    no \"context ${current_context:-none}, want $context\"\nfi\n\n# 2. The namespace exists.\nif \"${KUBECTL[@]}\" get \"namespace/$namespace\" -o name >/dev/null 2>&1; then\n    ok \"namespace $namespace\"\nelse\n    no \"namespace $namespace is missing\"\nfi\n\n# 3. The rollout already running is healthy; never stack a release on a bad one.\nif \"${KUBECTL[@]}\" rollout status \"deployment/$deployment\" -n \"$namespace\" \\\n        --timeout=\"${timeout}s\" >/dev/null 2>&1; then\n    ok \"previous rollout $deployment\"\nelse\n    no \"previous rollout $deployment is not healthy\"\nfi\n\n# 4. The incident flag file. Incident automation creates it; the gate honours it.\nif [ -e \"$alert_file\" ]; then\n    no \"alert file $alert_file exists\"\nelse\n    ok \"no alert file\"\nfi\n\n# 5. The image is pinned by digest, not by a tag that can be rebuilt.\nimage=$(\"${KUBECTL[@]}\" get \"deployment/$deployment\" -n \"$namespace\" \\\n    -o jsonpath='{.spec.template.spec.containers[0].image}' 2>/dev/null) || image=\"\"\n\ncase \"$image\" in\n    \"\")\n        no \"image is unreadable\"\n        ;;\n    *@sha256:*)\n        digest=${image##*@}\n        if [ -n \"$expect_digest\" ] && [ \"$digest\" != \"$expect_digest\" ]; then\n            no \"digest $digest, want $expect_digest\"\n        else\n            ok \"image pinned $digest\"\n        fi\n        ;;\n    *)\n        no \"image $image is a mutable tag\"\n        ;;\nesac\n\nif [ \"$failed\" -eq 0 ]; then\n    printf 'GATE %d of 5 passed\\n' \"$passed\"\n    exit 0\nfi\n\nprintf 'GATE %d of 5 failed\\n' \"$failed\"\nexit 1\n```\n"},{"id":"p643","n":"643","slug":"the-rollback-nobody-tested","title":"The rollback nobody tested","sections":["devsecops"],"topic":"Shell scripts","tags":["Bash","Kubernetes","Rollback","SRE","DevOps"],"series_name":"ShellSec","design":"code","script":"rollback.sh","date":"2030-11-05","files":{"devsecops":"notes/p643-the-rollback-nobody-tested.md"},"md":"# The rollback nobody tested\n\nAt 02:10 the release returns 500s and the runbook says roll back. The person on call types kubectl rollout undo for the first time on production, then spends twenty minutes guessing whether it worked.\n\n## Quick reference\n\nThe full script is at the end of this note: `rollback.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash Scripts/rollback.sh api api\n```\n\n**Example output**\n\n```text\nPLAN api/api revision 4 -> 3\nUNDO kubectl rollout undo deployment/api -n api\n```\n\n## Explained\n\n### What it does\n\n`rollback.sh` reads the rollout history, finds the current and previous revisions, and prints the plan: the revision pair, the exact undo command, and the health check to run afterwards. Dry-run is the default. With `--apply` it runs the undo, waits for the rollout, and optionally curls a health URL.\n\n### What the screen shows\n\nThe thirteen lines parse the history, guard against an empty one, subtract one from the current revision, print the plan and the health check, then stop at a dry-run guard. At revision 4 it prints PLAN api/api revision 4 -> 3 and UNDO `kubectl rollout undo deployment/api -n api`.\n\n### How to run it\n\nbash Scripts/rollback.sh api api prints the plan. Add `--apply` when you mean it, and `--to-revision 3` when the previous revision is not the target. A weekly dry-run against staging keeps it warm.\n\n### The fix\n\nRun the dry-run while the cluster is calm. It exposes a zero revisionHistoryLimit, a pruned ReplicaSet or a missing image before an incident does.\n\n### Prevention\n\nKeep revisionHistoryLimit at 10, hold the last five images, and rehearse an undo once a quarter.\n\n> **The honest caveat:** kubectl rollout undo reverses the Deployment, not a database migration. A schema change needs its own plan.\n\n> **The lesson:** a rollback you have never run is a rumour with a runbook entry.\n\n## In one line\n\n> **An undo you have not run is a rumour.** The command is configuration. Test it before the incident does.\n\n**The undo it prints**\n\n```bash\nkubectl rollout undo deployment/api -n api\n```\n\n## Script: rollback.sh\n\nDownload: [scripts/rollback.sh](../scripts/rollback.sh)\n\n```bash\n#!/usr/bin/env bash\n# rollback.sh - rehearse a Deployment rollback, and run it only on purpose.\n#\n# Fixes: the rollback nobody tested. It reads the rollout history, finds the\n# revision before the current one, prints the exact kubectl rollout undo\n# command and the health check to run afterwards. Dry-run is the default;\n# --apply executes the undo and waits for the rollout.\n#\n# Usage:\n#   ./rollback.sh NAMESPACE DEPLOYMENT [options]\n#\n# Examples:\n#   ./rollback.sh api api\n#   ./rollback.sh api api --to-revision 3\n#   ./rollback.sh api api --apply --health-url https://api.example.com/healthz\n#\n# Options:\n#   --to-revision N     undo to revision N instead of the previous one\n#   --apply             run the undo and the health check\n#   --timeout SECONDS   rollout status timeout after the undo (default 120)\n#   --health-url URL    also curl this URL after the rollout is healthy\n#   --kubeconfig PATH   pass --kubeconfig to every kubectl call\n#   -h, --help          print this help and exit 0\n#\n# Cron (weekly dry-run rehearsal against staging):\n#   0 4 * * 1 /usr/local/bin/rollback.sh staging api >/var/log/rollback_drill.log 2>&1\n#\n# Exit codes:\n#   0  plan printed, or the undo ran and the rollout became healthy\n#   1  no rollout history, no previous revision, or the apply failed\n#   2  usage error or unreadable input\n#\n# Needs kubectl on PATH. curl is needed only with --health-url.\n# Read-only unless --apply is passed.\n# By s0sta.com\nset -euo pipefail\n\nusage() {\n    cat <<'EOF'\nrollback.sh - rehearse a Deployment rollback, and run it only on purpose.\n\nUsage:\n  ./rollback.sh NAMESPACE DEPLOYMENT [options]\n\nArguments:\n  NAMESPACE    the namespace the Deployment lives in, e.g. api\n  DEPLOYMENT   the Deployment to roll back, e.g. api\n\nOptions:\n  --to-revision N     undo to revision N instead of the previous one\n  --apply             run the undo and the health check\n  --timeout SECONDS   rollout status timeout after the undo (default 120)\n  --health-url URL    also curl this URL after the rollout is healthy\n  --kubeconfig PATH   pass --kubeconfig to every kubectl call\n  -h, --help          print this help and exit 0\n\nExit codes:\n  0  plan printed, or the undo ran and the rollout became healthy\n  1  no rollout history, no previous revision, or the apply failed\n  2  usage error or unreadable input\n\nBy s0sta.com\nEOF\n}\n\nnamespace=\"\"\ndeployment=\"\"\nto_revision=\"\"\napply=0\ntimeout=120\nhealth_url=\"\"\nkubeconfig=\"\"\n\nneed_value() {\n    if [ \"$#\" -lt 2 ]; then\n        printf 'rollback: %s needs a value\\n' \"$1\" >&2\n        exit 2\n    fi\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help)\n            usage\n            exit 0\n            ;;\n        --to-revision)\n            need_value \"$@\"\n            to_revision=$2\n            shift 2\n            ;;\n        --timeout)\n            need_value \"$@\"\n            timeout=$2\n            shift 2\n            ;;\n        --health-url)\n            need_value \"$@\"\n            health_url=$2\n            shift 2\n            ;;\n        --kubeconfig)\n            need_value \"$@\"\n            kubeconfig=$2\n            shift 2\n            ;;\n        --apply)\n            apply=1\n            shift\n            ;;\n        -*)\n            printf 'rollback: unknown option %s\\n' \"$1\" >&2\n            usage >&2\n            exit 2\n            ;;\n        *)\n            if [ -z \"$namespace\" ]; then\n                namespace=$1\n            elif [ -z \"$deployment\" ]; then\n                deployment=$1\n            else\n                printf 'rollback: unexpected argument %s\\n' \"$1\" >&2\n                exit 2\n            fi\n            shift\n            ;;\n    esac\ndone\n\nif [ -z \"$namespace\" ] || [ -z \"$deployment\" ]; then\n    printf 'rollback: NAMESPACE and DEPLOYMENT are required\\n' >&2\n    usage >&2\n    exit 2\nfi\n\ncase \"$timeout\" in\n    ''|*[!0-9]*)\n        printf 'rollback: --timeout must be a number of seconds\\n' >&2\n        exit 2\n        ;;\nesac\n\nif [ -n \"$to_revision\" ]; then\n    case \"$to_revision\" in\n        ''|*[!0-9]*)\n            printf 'rollback: --to-revision must be a revision number\\n' >&2\n            exit 2\n            ;;\n    esac\nfi\n\nif ! command -v kubectl >/dev/null 2>&1; then\n    printf 'rollback: kubectl not found on PATH\\n' >&2\n    exit 2\nfi\n\nKUBECTL=(kubectl)\nif [ -n \"$kubeconfig\" ]; then\n    KUBECTL+=(--kubeconfig \"$kubeconfig\")\nfi\n\nhistory=$(\"${KUBECTL[@]}\" rollout history \"deployment/$deployment\" \\\n    -n \"$namespace\" 2>/dev/null) || history=\"\"\n\nif [ -z \"$history\" ]; then\n    printf 'ROLLBACK no rollout history for %s/%s\\n' \"$namespace\" \"$deployment\" >&2\n    exit 1\nfi\n\nrevisions=$(printf '%s\\n' \"$history\" \\\n    | awk 'NR > 2 && $1 ~ /^[0-9]+$/ { print $1 }' \\\n    | sort -n)\n\nif [ -z \"$revisions\" ]; then\n    printf 'ROLLBACK no numbered revisions for %s/%s\\n' \"$namespace\" \"$deployment\" >&2\n    exit 1\nfi\n\ncurrent=$(printf '%s\\n' \"$revisions\" | tail -n 1)\n\nif [ -n \"$to_revision\" ]; then\n    if ! printf '%s\\n' \"$revisions\" | grep -qx -- \"$to_revision\"; then\n        printf 'ROLLBACK revision %s is not in the history\\n' \"$to_revision\" >&2\n        exit 2\n    fi\n    target=$to_revision\nelse\n    target=$(printf '%s\\n' \"$revisions\" \\\n        | awk -v c=\"$current\" '$1 + 0 < c + 0 { t = $1 } END { if (t != \"\") print t }')\n    if [ -z \"$target\" ]; then\n        printf 'ROLLBACK no revision before %s for %s/%s\\n' \\\n            \"$current\" \"$namespace\" \"$deployment\" >&2\n        exit 1\n    fi\nfi\n\nif [ \"$target\" = \"$current\" ]; then\n    printf 'ROLLBACK revision %s is already current\\n' \"$target\" >&2\n    exit 2\nfi\n\nkubectl_bin=\"kubectl\"\nif [ -n \"$kubeconfig\" ]; then\n    kubectl_bin=\"kubectl --kubeconfig $kubeconfig\"\nfi\n\nundo=\"$kubectl_bin rollout undo deployment/$deployment -n $namespace\"\nif [ -n \"$to_revision\" ]; then\n    undo=\"$undo --to-revision=$target\"\nfi\nhealth=\"$kubectl_bin rollout status deployment/$deployment -n $namespace --timeout=${timeout}s\"\n\nprintf 'PLAN %s/%s revision %s -> %s\\n' \"$namespace\" \"$deployment\" \"$current\" \"$target\"\nprintf 'UNDO %s\\n' \"$undo\"\nprintf 'HEALTH %s\\n' \"$health\"\nif [ -n \"$health_url\" ]; then\n    printf 'HEALTH curl -fsS %s\\n' \"$health_url\"\nfi\n\nif [ \"$apply\" -eq 0 ]; then\n    printf 'DRY-RUN nothing executed; pass --apply to run\\n'\n    exit 0\nfi\n\napply_args=(rollout undo \"deployment/$deployment\" -n \"$namespace\")\nif [ -n \"$to_revision\" ]; then\n    apply_args+=(--to-revision=\"$target\")\nfi\n\nprintf 'APPLY rolling %s/%s back to revision %s\\n' \"$namespace\" \"$deployment\" \"$target\"\nif ! \"${KUBECTL[@]}\" \"${apply_args[@]}\"; then\n    printf 'ROLLBACK kubectl rollout undo failed\\n' >&2\n    exit 1\nfi\n\nif ! \"${KUBECTL[@]}\" rollout status \"deployment/$deployment\" -n \"$namespace\" \\\n        --timeout=\"${timeout}s\"; then\n    printf 'ROLLBACK undo ran but the rollout is not healthy\\n' >&2\n    exit 1\nfi\n\nif [ -n \"$health_url\" ]; then\n    if ! command -v curl >/dev/null 2>&1; then\n        printf 'rollback: curl not found on PATH, needed for --health-url\\n' >&2\n        exit 2\n    fi\n    if ! curl -fsS --max-time 10 \"$health_url\" >/dev/null; then\n        printf 'ROLLBACK rollout is healthy but %s failed\\n' \"$health_url\" >&2\n        exit 1\n    fi\nfi\n\nprintf 'ROLLBACK done: %s/%s at revision %s\\n' \"$namespace\" \"$deployment\" \"$target\"\nexit 0\n```\n"},{"id":"p644","n":"644","slug":"artifacts-that-did-not-arrive","title":"Artifacts that did not arrive","sections":["devsecops"],"topic":"Shell scripts","tags":["Bash","CICD","SupplyChain","DevOps","Linux"],"series_name":"ShellSec","design":"code","script":"artifact_check.sh","date":"2030-11-07","files":{"devsecops":"notes/p644-artifacts-that-did-not-arrive.md"},"md":"# Artifacts that did not arrive\n\nThe upload step fails once, the retry skips it, and the deploy downloads a release with files missing. The pipeline stays green because nothing compares the manifest with the disk.\n\n## Quick reference\n\nThe full script is at the end of this note: `artifact_check.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash Scripts/artifact_check.sh SHA256SUMS .\n```\n\n**Example output**\n\n```text\nMISSING dist/api.tar.gz\nARTIFACTS 1 bad of 4\n```\n\n## Explained\n\n### What it does\n\n`artifact_check.sh` reads a sha256sum manifest, resolves every path against a base directory, and verifies each file. It prints MISSING for a file that is not there and CORRUPT for one whose hash does not match, then a count. Exit 1 means at least one bad file.\n\n### What the screen shows\n\nThe thirteen lines open the manifest, read each hash and path, test the file, hash it and compare. With one tarball missing it prints MISSING dist/api.tar.gz and ARTIFACTS 1 bad of 4.\n\n### How to run it\n\nbash Scripts/artifact_check.sh SHA256SUMS . from the release directory, or pass a base path instead of the dot. Run it after the download, before anything is applied. A nightly cron catches a bucket that lost an object.\n\n### The fix\n\nRe-upload the missing file and rebuild the corrupt one. Never edit the manifest to match the disk: it states what should exist.\n\n### Prevention\n\nGenerate SHA256SUMS at build time, sign it, and make the deploy step refuse a release without a clean check.\n\n> **The honest caveat:** a matching hash proves the bytes match the manifest, not that the build is reproducible or that the manifest was signed by the right key.\n\n> **The lesson:** a release is a manifest plus the bytes it names, and both have to arrive.\n\n## In one line\n\n> **A manifest is a promise with a hash.** The bytes are the release. The hash is the proof.\n\n**Verify one file**\n\n```bash\nsha256sum -c SHA256SUMS\n```\n\n## Script: artifact_check.sh\n\nDownload: [scripts/artifact_check.sh](../scripts/artifact_check.sh)\n\n```bash\n#!/usr/bin/env bash\n# artifact_check.sh - verify a release directory against a sha256 manifest.\n#\n# Fixes: artifacts that did not arrive. A skipped upload step, a truncated\n# download or a rebuilt file leaves the deploy server with something that is\n# not what the build produced. The manifest says what should be there; this\n# script proves it byte by byte before the release is allowed near a cluster.\n#\n# Usage:\n#   ./artifact_check.sh MANIFEST [BASE]\n#\n# Examples:\n#   ./artifact_check.sh SHA256SUMS\n#   ./artifact_check.sh SHA256SUMS /srv/releases/1.4.2\n#   sha256sum -c SHA256SUMS          # the same check without the summary\n#\n# The manifest is sha256sum output: \"<64 hex>  <path>\", one file per line.\n# Blank lines and lines starting with # are ignored. A relative path is\n# resolved against BASE, which defaults to the manifest's directory. An\n# absolute path is used as written.\n#\n# Cron (check the release directory that is about to be deployed):\n#   0 5 * * * /usr/local/bin/artifact_check.sh /srv/releases/current/SHA256SUMS\n#\n# Exit codes:\n#   0  every file exists and matches its hash\n#   1  at least one file is missing or corrupt\n#   2  usage error, unreadable manifest, or a malformed manifest line\n#\n# Needs sha256sum (GNU coreutils) or shasum -a 256 (macOS/BSD) on PATH.\n# Read-only. IFS is set to newline and tab so paths with spaces survive.\n# By s0sta.com\nset -euo pipefail\nIFS=$'\\n\\t'\n\nusage() {\n    cat <<'EOF'\nartifact_check.sh - verify a release directory against a sha256 manifest.\n\nUsage:\n  ./artifact_check.sh MANIFEST [BASE]\n\nArguments:\n  MANIFEST    sha256sum output: \"<64 hex>  <path>\", one file per line\n  BASE        directory relative paths resolve against\n              (default: the manifest's directory)\n\nExamples:\n  ./artifact_check.sh SHA256SUMS\n  ./artifact_check.sh SHA256SUMS /srv/releases/1.4.2\n\nExit codes:\n  0  every file exists and matches its hash\n  1  at least one file is missing or corrupt\n  2  usage error, unreadable manifest, or a malformed manifest line\n\nBy s0sta.com\nEOF\n}\n\nif [ \"$#\" -eq 0 ]; then\n    usage >&2\n    exit 2\nfi\n\ncase \"$1\" in\n    -h|--help)\n        usage\n        exit 0\n        ;;\n    -*)\n        printf 'artifact_check: unknown option %s\\n' \"$1\" >&2\n        usage >&2\n        exit 2\n        ;;\nesac\n\nif [ \"$#\" -gt 2 ]; then\n    printf 'artifact_check: too many arguments\\n' >&2\n    usage >&2\n    exit 2\nfi\n\nmanifest=$1\nbase=${2:-}\n\nif [ ! -f \"$manifest\" ] || [ ! -r \"$manifest\" ]; then\n    printf 'ARTIFACTS cannot read manifest %s\\n' \"$manifest\" >&2\n    exit 2\nfi\n\nif [ -z \"$base\" ]; then\n    base=$(dirname \"$manifest\")\nfi\n\nif [ ! -d \"$base\" ]; then\n    printf 'ARTIFACTS base %s is not a directory\\n' \"$base\" >&2\n    exit 2\nfi\n\nif command -v sha256sum >/dev/null 2>&1; then\n    sha_of() {\n        sha256sum \"$1\" | cut -d' ' -f1\n    }\nelif command -v shasum >/dev/null 2>&1; then\n    sha_of() {\n        shasum -a 256 \"$1\" | cut -d' ' -f1\n    }\nelse\n    printf 'ARTIFACTS needs sha256sum or shasum on PATH\\n' >&2\n    exit 2\nfi\n\ntotal=0\nbad=0\n\nwhile IFS= read -r line || [ -n \"$line\" ]; do\n    case \"$line\" in\n        ''|'#'*)\n            continue\n            ;;\n    esac\n\n    hash=${line%% *}\n    rest=${line#* }\n    rest=${rest# }\n    file=${rest#\\*}\n\n    if [ -z \"$file\" ] || [ \"$hash\" = \"$line\" ]; then\n        printf 'ARTIFACTS malformed manifest line: %s\\n' \"$line\" >&2\n        exit 2\n    fi\n\n    case \"$hash\" in\n        *[!0-9a-f]*)\n            printf 'ARTIFACTS malformed hash: %s\\n' \"$line\" >&2\n            exit 2\n            ;;\n    esac\n\n    if [ \"${#hash}\" -ne 64 ]; then\n        printf 'ARTIFACTS hash is not 64 hex digits: %s\\n' \"$line\" >&2\n        exit 2\n    fi\n\n    total=$((total + 1))\n\n    path=$file\n    case \"$path\" in\n        /*) ;;\n        *) path=\"$base/$file\" ;;\n    esac\n\n    if [ ! -f \"$path\" ]; then\n        printf 'MISSING %s\\n' \"$file\"\n        bad=$((bad + 1))\n        continue\n    fi\n\n    actual=$(sha_of \"$path\") || {\n        printf 'ARTIFACTS cannot hash %s\\n' \"$file\" >&2\n        exit 2\n    }\n\n    if [ \"$actual\" != \"$hash\" ]; then\n        printf 'CORRUPT %s\\n' \"$file\"\n        bad=$((bad + 1))\n    fi\ndone < \"$manifest\"\n\nif [ \"$total\" -eq 0 ]; then\n    printf 'ARTIFACTS manifest %s has no entries\\n' \"$manifest\" >&2\n    exit 2\nfi\n\nif [ \"$bad\" -eq 0 ]; then\n    printf 'ARTIFACTS %d of %d verified\\n' \"$total\" \"$total\"\n    exit 0\nfi\n\nprintf 'ARTIFACTS %d bad of %d\\n' \"$bad\" \"$total\"\nexit 1\n```\n"},{"id":"p645","n":"645","slug":"a-smoke-test-with-teeth","title":"A smoke test with teeth","sections":["devsecops"],"topic":"Shell scripts","tags":["Bash","CICD","SRE","Testing","DevOps"],"series_name":"ShellSec","design":"code","script":"smoke_test.sh","date":"2030-11-10","files":{"devsecops":"notes/p645-a-smoke-test-with-teeth.md"},"md":"# A smoke test with teeth\n\nThe deploy finishes, the pipeline curls /health, gets 200 from a process four seconds old, and marks the release green. Checkout returns 500 for an hour, because nobody asked the endpoint that matters.\n\n## Quick reference\n\nThe full script is at the end of this note: `smoke_test.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash Scripts/smoke_test.sh checks.txt\n```\n\n**Example output**\n\n```text\nFAIL https://api.test/health 503 want 200\nSMOKE 1 of 3 failed\n```\n\n## Explained\n\n### What it does\n\n`smoke_test.sh` reads url|expected_status|expected_body lines, calls each with curl, and asserts both the status code and a fixed body string. A failed attempt is retried with exponential backoff, three times. It prints only failures, then a summary, and exits 1 if any endpoint fails.\n\n### What the screen shows\n\nThe fourteen lines read the list, loop over three attempts, capture the status with `curl -w`, check status and body, sleep through the backoff, and print the failure. With one endpoint at 503 it prints FAIL https://api.test/health 503 want 200 and SMOKE 1 of 3 failed.\n\n### How to run it\n\nbash Scripts/smoke_test.sh `checks.txt`, after the rollout and before traffic moves. Keep the file next to the manifests. A five-minute cron outside the cluster catches the release that rots after the pipeline ends.\n\n### The fix\n\nPoint the list at business endpoints, not only liveness. Assert a body marker such as a version string, and include one authenticated read.\n\n### Prevention\n\nRun the same list against staging on every merge and after every rollout, so a broken endpoint fails a build instead of a customer.\n\n> **The honest caveat:** retries turn a flapping service into a pass. Log the attempts and alert when an endpoint only passes on the third try.\n\n> **The lesson:** a smoke test that asks whether the port is open is a ping with a budget.\n\n## In one line\n\n> **Status 200 is not a working app.** A port that answers is not an endpoint that works.\n\n**Assert the body**\n\n```bash\ncurl -fsS https://api.example.com/healthz\n```\n\n## Script: smoke_test.sh\n\nDownload: [scripts/smoke_test.sh](../scripts/smoke_test.sh)\n\n```bash\n#!/usr/bin/env bash\n# smoke_test.sh - assert status and body on the endpoints a release touches.\n#\n# Fixes: a smoke test with no teeth. A single curl for 200 on /health proves\n# the process is listening, not that the release works. This script reads a\n# list of url|expected_status|expected_body lines, retries with exponential\n# backoff, asserts both the status code and a body marker, and exits 1 on the\n# first endpoint that never gets there.\n#\n# Usage:\n#   ./smoke_test.sh LIST [options]\n#\n# Examples:\n#   ./smoke_test.sh checks.txt\n#   ./smoke_test.sh checks.txt --attempts 4 --timeout 10 --verbose\n#   ./smoke_test.sh checks.txt --delay 0        # no sleep between attempts\n#\n# List format, one endpoint per line:\n#   https://api.example.com/healthz|200|\"status\":\"ok\"\n#   https://api.example.com/v1/ping|200|pong\n#   https://api.example.com/ready|204|\n# Blank lines and lines starting with # are ignored. The body field is a\n# fixed string (grep -F), not a regex; an empty body field asserts only the\n# status.\n#\n# Options:\n#   --attempts N     tries per endpoint (default 3)\n#   --timeout S      curl max time per try, seconds (default 5)\n#   --delay S        first backoff sleep in seconds, doubled per retry (default 2)\n#   --verbose        print PASS lines as well as failures\n#   -h, --help       print this help and exit 0\n#\n# Cron (every five minutes from a synthetic runner):\n#   */5 * * * * /usr/local/bin/smoke_test.sh /etc/s0sta/checks.txt >/var/log/smoke.log 2>&1\n#\n# Exit codes:\n#   0  every endpoint returned the expected status and body\n#   1  at least one endpoint failed after all attempts\n#   2  usage error, unreadable list, or a malformed line\n#\n# Needs curl on PATH. Read-only apart from a temporary body file.\n# By s0sta.com\nset -euo pipefail\nIFS=$'\\n\\t'\n\nusage() {\n    cat <<'EOF'\nsmoke_test.sh - assert status and body on the endpoints a release touches.\n\nUsage:\n  ./smoke_test.sh LIST [options]\n\nArguments:\n  LIST    one \"url|expected_status|expected_body\" line per endpoint\n          (blank lines and # comments are ignored; empty body = status only)\n\nOptions:\n  --attempts N     tries per endpoint (default 3)\n  --timeout S      curl max time per try, seconds (default 5)\n  --delay S        first backoff sleep in seconds, doubled per retry (default 2)\n  --verbose        print PASS lines as well as failures\n  -h, --help       print this help and exit 0\n\nExit codes:\n  0  every endpoint returned the expected status and body\n  1  at least one endpoint failed after all attempts\n  2  usage error, unreadable list, or a malformed line\n\nBy s0sta.com\nEOF\n}\n\ntrim() {\n    local s=$1\n    s=${s#\"${s%%[![:space:]]*}\"}\n    s=${s%\"${s##*[![:space:]]}\"}\n    printf '%s' \"$s\"\n}\n\nnumeric() {\n    case \"$1\" in\n        ''|*[!0-9]*)\n            return 1\n            ;;\n    esac\n    return 0\n}\n\nlist=\"\"\nattempts=3\ntimeout=5\ndelay=2\nverbose=0\n\nneed_value() {\n    if [ \"$#\" -lt 2 ]; then\n        printf 'smoke_test: %s needs a value\\n' \"$1\" >&2\n        exit 2\n    fi\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help)\n            usage\n            exit 0\n            ;;\n        --attempts)\n            need_value \"$@\"\n            attempts=$2\n            shift 2\n            ;;\n        --timeout)\n            need_value \"$@\"\n            timeout=$2\n            shift 2\n            ;;\n        --delay)\n            need_value \"$@\"\n            delay=$2\n            shift 2\n            ;;\n        --verbose)\n            verbose=1\n            shift\n            ;;\n        -*)\n            printf 'smoke_test: unknown option %s\\n' \"$1\" >&2\n            usage >&2\n            exit 2\n            ;;\n        *)\n            if [ -z \"$list\" ]; then\n                list=$1\n            else\n                printf 'smoke_test: unexpected argument %s\\n' \"$1\" >&2\n                exit 2\n            fi\n            shift\n            ;;\n    esac\ndone\n\nif [ -z \"$list\" ]; then\n    printf 'smoke_test: LIST is required\\n' >&2\n    usage >&2\n    exit 2\nfi\n\nif [ ! -f \"$list\" ] || [ ! -r \"$list\" ]; then\n    printf 'SMOKE cannot read list %s\\n' \"$list\" >&2\n    exit 2\nfi\n\nif ! numeric \"$attempts\" || [ \"$attempts\" -eq 0 ]; then\n    printf 'smoke_test: --attempts must be a positive number\\n' >&2\n    exit 2\nfi\n\nif ! numeric \"$timeout\" || [ \"$timeout\" -eq 0 ]; then\n    printf 'smoke_test: --timeout must be a positive number of seconds\\n' >&2\n    exit 2\nfi\n\nif ! numeric \"$delay\"; then\n    printf 'smoke_test: --delay must be a number of seconds\\n' >&2\n    exit 2\nfi\n\nif ! command -v curl >/dev/null 2>&1; then\n    printf 'smoke_test: curl not found on PATH\\n' >&2\n    exit 2\nfi\n\nbody_file=$(mktemp \"${TMPDIR:-/tmp}/smoke_test.XXXXXX\")\ntrap 'rm -f \"$body_file\"' EXIT\n\ntotal=0\nfailed=0\nlineno=0\n\nwhile IFS= read -r line || [ -n \"$line\" ]; do\n    lineno=$((lineno + 1))\n\n    case \"$line\" in\n        ''|'#'*)\n            continue\n            ;;\n    esac\n\n    IFS='|' read -r url want_status want_body <<< \"$line\"\n    url=$(trim \"${url:-}\")\n    want_status=$(trim \"${want_status:-}\")\n    want_body=$(trim \"${want_body:-}\")\n\n    if [ -z \"$url\" ] || [ -z \"$want_status\" ] || ! numeric \"$want_status\"; then\n        printf 'SMOKE malformed line %d: %s\\n' \"$lineno\" \"$line\" >&2\n        exit 2\n    fi\n\n    total=$((total + 1))\n    ok=0\n    code=\"000\"\n    try=1\n\n    while [ \"$try\" -le \"$attempts\" ]; do\n        code=$(curl -sS -o \"$body_file\" -w '%{http_code}' \\\n            --max-time \"$timeout\" \"$url\" 2>/dev/null) || code=\"000\"\n        ok=1\n\n        if [ \"$code\" != \"$want_status\" ]; then\n            ok=0\n        fi\n\n        if [ \"$ok\" -eq 1 ] && [ -n \"$want_body\" ]; then\n            if ! grep -qF -e \"$want_body\" \"$body_file\"; then\n                ok=0\n            fi\n        fi\n\n        if [ \"$ok\" -eq 1 ]; then\n            break\n        fi\n\n        if [ \"$try\" -lt \"$attempts\" ] && [ \"$delay\" -gt 0 ]; then\n            nap=$((delay * (2 ** (try - 1))))\n            sleep \"$nap\"\n        fi\n\n        try=$((try + 1))\n    done\n\n    if [ \"$ok\" -eq 1 ]; then\n        if [ \"$verbose\" -eq 1 ]; then\n            printf 'PASS %s %s\\n' \"$url\" \"$code\"\n        fi\n    else\n        if [ \"$code\" != \"$want_status\" ]; then\n            printf 'FAIL %s %s want %s\\n' \"$url\" \"$code\" \"$want_status\"\n        else\n            printf 'FAIL %s body lacks %s\\n' \"$url\" \"$want_body\"\n        fi\n        failed=$((failed + 1))\n    fi\ndone < \"$list\"\n\nif [ \"$total\" -eq 0 ]; then\n    printf 'SMOKE list %s has no endpoints\\n' \"$list\" >&2\n    exit 2\nfi\n\nif [ \"$failed\" -eq 0 ]; then\n    printf 'SMOKE %d of %d passed\\n' \"$total\" \"$total\"\n    exit 0\nfi\n\nprintf 'SMOKE %d of %d failed\\n' \"$failed\" \"$total\"\nexit 1\n```\n"},{"id":"p646","n":"646","slug":"ci-caches-that-ate-the-disk","title":"CI caches that ate the disk","sections":["devsecops"],"topic":"Shell scripts","tags":["Bash","CICD","Linux","DevOps","Automation"],"series_name":"ShellSec","design":"code","script":"ci_cache_clean.sh","date":"2030-11-12","files":{"devsecops":"notes/p646-ci-caches-that-ate-the-disk.md"},"md":"# CI caches that ate the disk\n\nThe runner fills its disk on a Tuesday and the build fails unpacking a toolchain. The cause is months of workspaces and caches from jobs that finished long ago, and nothing removes them.\n\n## Quick reference\n\nThe full script is at the end of this note: `ci_cache_clean.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash Scripts/ci_cache_clean.sh /var/ci/workspaces 14 50\n```\n\n**Example output**\n\n```text\nWOULD DELETE 400M workspace-4821\nCACHE 1 dir, 400M reclaimable\n```\n\n## Explained\n\n### What it does\n\n`ci_cache_clean.sh` takes a root, an age in days and a minimum size. It looks at each immediate child, keeps the ones older than the age and at least the minimum size, prints WOULD DELETE with size and path, then a summary. Dry-run is the default. `--apply` deletes, after resolving each path and refusing anything outside the root.\n\n### What the screen shows\n\nThe fourteen lines canonicalise the root, test each child with `find -mtime`, measure with `du -sm`, filter on the minimum, print the candidate and total the size. With one 400M workspace older than 14 days it prints WOULD DELETE 400M workspace-4821 and CACHE 1 dir, 400M reclaimable.\n\n### How to run it\n\nbash Scripts/ci_cache_clean.sh `/var/ci/workspaces 14 50`. Point it at the workspaces root, then at the caches root. The weekly cron adds `--apply` when the dry-run looks right.\n\n### The fix\n\nDelete the old workspaces, then set a retention rule per cache: three days for workspaces, thirty for dependencies, and a size cap per repository.\n\n### Prevention\n\nWatch disk per runner, alert at 80 percent, and treat a cache as a lease with an expiry.\n\n> **The honest caveat:** mtime is not usefulness. A workspace can be old and still needed for a rerun, so exempt those names before you add `--apply`.\n\n> **The lesson:** a cache with no expiry is a disk leak with good intentions.\n\n## In one line\n\n> **A cache with no expiry is a leak.** Disk on a runner fills slowly, then all at once.\n\n**Sweep it weekly:** `ci_cache_clean.sh /var/ci/workspaces`\n\n## Script: ci_cache_clean.sh\n\nDownload: [scripts/ci_cache_clean.sh](../scripts/ci_cache_clean.sh)\n\n```bash\n#!/usr/bin/env bash\n# ci_cache_clean.sh - find old, large CI workspaces and caches, then delete\n# only the ones you approve.\n#\n# Fixes: CI caches that ate the disk. Workspaces and dependency caches are\n# created by every job and removed by none, so the runner fills up and the\n# next build fails while unpacking. The script reports what is old and large\n# enough to remove; dry-run is the default and --apply deletes.\n#\n# Usage:\n#   ./ci_cache_clean.sh ROOT [DAYS] [MIN_MB] [options]\n#\n# Examples:\n#   ./ci_cache_clean.sh /var/ci/workspaces\n#   ./ci_cache_clean.sh /var/ci/workspaces 14 50\n#   ./ci_cache_clean.sh /var/ci/workspaces 14 50 --apply\n#   ./ci_cache_clean.sh /var/ci/caches 30 200 --name 'npm-*' --apply\n#\n# Arguments:\n#   ROOT      directory whose immediate children are workspaces/caches\n#   DAYS      minimum age in days (default 14)\n#   MIN_MB    minimum size in megabytes (default 50)\n#\n# Options:\n#   --name GLOB   only children matching this find -name pattern (default *)\n#   --apply       delete the reported directories\n#   -h, --help    print this help and exit 0\n#\n# Only direct children of ROOT are examined. A child is a candidate when its\n# own mtime is older than DAYS and its size is at least MIN_MB. Paths are\n# printed relative to ROOT, which the cron command names, so log lines stay\n# short.\n#\n# Cron (weekly sweep, Sunday 04:00):\n#   0 4 * * 0 /usr/local/bin/ci_cache_clean.sh /var/ci/workspaces 14 50 --apply\n#\n# Exit codes:\n#   0  nothing to do, or every candidate was deleted\n#   1  candidates were found (dry-run), or at least one delete failed\n#   2  usage error or unreadable input\n#\n# Needs find, du and rm (GNU coreutils or BSD). IFS is set to newline and tab\n# so directory names with spaces survive. Deletion is guarded: every resolved\n# path must stay under ROOT, and ROOT itself is never deleted.\n# By s0sta.com\nset -euo pipefail\nIFS=$'\\n\\t'\n\nusage() {\n    cat <<'EOF'\nci_cache_clean.sh - find old, large CI workspaces and caches, then delete\nonly the ones you approve.\n\nUsage:\n  ./ci_cache_clean.sh ROOT [DAYS] [MIN_MB] [options]\n\nArguments:\n  ROOT      directory whose immediate children are workspaces/caches\n  DAYS      minimum age in days (default 14)\n  MIN_MB    minimum size in megabytes (default 50)\n\nOptions:\n  --name GLOB   only children matching this find -name pattern (default *)\n  --apply       delete the reported directories\n  -h, --help    print this help and exit 0\n\nExit codes:\n  0  nothing to do, or every candidate was deleted\n  1  candidates were found (dry-run), or at least one delete failed\n  2  usage error or unreadable input\n\nBy s0sta.com\nEOF\n}\n\nroot=\"\"\ndays=14\nmin_mb=50\nname=\"*\"\napply=0\npositionals=0\n\nnumeric() {\n    case \"$1\" in\n        ''|*[!0-9]*)\n            return 1\n            ;;\n    esac\n    return 0\n}\n\nneed_value() {\n    if [ \"$#\" -lt 2 ]; then\n        printf 'ci_cache_clean: %s needs a value\\n' \"$1\" >&2\n        exit 2\n    fi\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n    case \"$1\" in\n        -h|--help)\n            usage\n            exit 0\n            ;;\n        --name)\n            need_value \"$@\"\n            name=$2\n            shift 2\n            ;;\n        --apply)\n            apply=1\n            shift\n            ;;\n        -*)\n            printf 'ci_cache_clean: unknown option %s\\n' \"$1\" >&2\n            usage >&2\n            exit 2\n            ;;\n        *)\n            case \"$positionals\" in\n                0)\n                    root=$1\n                    ;;\n                1)\n                    days=$1\n                    ;;\n                2)\n                    min_mb=$1\n                    ;;\n                *)\n                    printf 'ci_cache_clean: unexpected argument %s\\n' \"$1\" >&2\n                    exit 2\n                    ;;\n            esac\n            positionals=$((positionals + 1))\n            shift\n            ;;\n    esac\ndone\n\nif [ -z \"$root\" ]; then\n    printf 'ci_cache_clean: ROOT is required\\n' >&2\n    usage >&2\n    exit 2\nfi\n\nif ! numeric \"$days\" || [ \"$days\" -eq 0 ]; then\n    printf 'ci_cache_clean: DAYS must be a positive number\\n' >&2\n    exit 2\nfi\n\nif ! numeric \"$min_mb\"; then\n    printf 'ci_cache_clean: MIN_MB must be a number\\n' >&2\n    exit 2\nfi\n\nif [ ! -d \"$root\" ]; then\n    printf 'CACHE root %s is not a directory\\n' \"$root\" >&2\n    exit 2\nfi\n\nroot=$(cd \"$root\" && pwd -P)\nif [ \"$root\" = \"/\" ]; then\n    printf 'CACHE refusing to sweep /\\n' >&2\n    exit 2\nfi\n\ncandidates=0\nreclaimed=0\nfreed=0\nfailed=0\n\nwhile IFS= read -r dir; do\n    if [ -z \"$dir\" ]; then\n        continue\n    fi\n\n    old=$(find \"$dir\" -prune -mtime +\"$days\" -print 2>/dev/null) || old=\"\"\n    if [ -z \"$old\" ]; then\n        continue\n    fi\n\n    size=$(du -sm \"$dir\" 2>/dev/null | cut -f1) || size=\"\"\n    if ! numeric \"$size\"; then\n        printf 'SKIP cannot size %s\\n' \"${dir#\"$root\"/}\" >&2\n        continue\n    fi\n\n    if [ \"$size\" -lt \"$min_mb\" ]; then\n        continue\n    fi\n\n    rel=${dir#\"$root\"/}\n    candidates=$((candidates + 1))\n    reclaimed=$((reclaimed + size))\n\n    if [ \"$apply\" -eq 0 ]; then\n        printf 'WOULD DELETE %dM %s\\n' \"$size\" \"$rel\"\n        continue\n    fi\n\n    if ! abs=$(cd \"$dir\" && pwd -P); then\n        printf 'SKIP vanished before delete: %s\\n' \"$rel\" >&2\n        failed=$((failed + 1))\n        continue\n    fi\n\n    if [ \"$abs\" = \"$root\" ]; then\n        printf 'SKIP refusing to delete the root itself\\n' >&2\n        failed=$((failed + 1))\n        continue\n    fi\n\n    case \"$abs\" in\n        \"$root\"/*)\n            ;;\n        *)\n            printf 'SKIP resolved outside root: %s\\n' \"$rel\" >&2\n            failed=$((failed + 1))\n            continue\n            ;;\n    esac\n\n    if rm -rf \"$abs\"; then\n        printf 'DELETED %dM %s\\n' \"$size\" \"$rel\"\n        freed=$((freed + size))\n    else\n        printf 'FAIL could not delete %s\\n' \"$rel\" >&2\n        failed=$((failed + 1))\n    fi\ndone < <(find \"$root\" -mindepth 1 -maxdepth 1 -type d -name \"$name\" 2>/dev/null)\n\nif [ \"$candidates\" -eq 0 ]; then\n    printf 'CACHE clean: nothing older than %sd over %sM in %s\\n' \\\n        \"$days\" \"$min_mb\" \"$root\"\n    exit 0\nfi\n\nunit=dirs\nif [ \"$candidates\" -eq 1 ]; then\n    unit=dir\nfi\n\nif [ \"$apply\" -eq 1 ]; then\n    if [ \"$failed\" -gt 0 ]; then\n        printf 'CACHE %d %s, %dM freed, %d failed\\n' \\\n            \"$candidates\" \"$unit\" \"$freed\" \"$failed\"\n        exit 1\n    fi\n    printf 'CACHE %d %s, %dM freed\\n' \"$candidates\" \"$unit\" \"$freed\"\n    exit 0\nfi\n\nprintf 'CACHE %d %s, %dM reclaimable\\n' \"$candidates\" \"$unit\" \"$reclaimed\"\nexit 1\n```\n"},{"id":"p647","n":"647","slug":"the-deploy-gate-in-three-tiers","title":"The deploy gate, in three tiers","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Bash","DevOps","Kubernetes","CICD","SRE"],"series_name":"ShellSec","design":"architecture","script":"","date":"2030-11-14","files":{"devsecops":"notes/p647-the-deploy-gate-in-three-tiers.md"},"md":"# The deploy gate, in three tiers\n\nEvery deploy passes through the same three tiers, whether or not anyone drew them. The first decides whether the release is allowed, the second turns five answers into one exit code, and the third changes the cluster and watches it.\n\n## Quick reference\n\n- **Tier 1 · Checks:** Context match (CTX) · Namespace exists (NS) · Previous rollout (DEP) · Alert file (ALRT)\n- **Tier 2 · Gate:** Digest pinned (SHA) · Pass or fail (EXIT)\n- **Tier 3 · Ship:** Rollout (SET) · Watch 10 min (OBS)\n\n**Request flow:** Context match → Namespace exists → Previous rollout → Alert file → Digest pinned → Pass or fail → Rollout → Watch 10 min\n\n**Around it:** CI runner (RUN) · Registry (IMG) · Pager (PAGE) · Audit log (LOG)\n\n## Explained\n\n### Checks\n\nFour checks run before anything is applied. The context must match the environment the pipeline claims to deploy to. The namespace must exist. The previous rollout must already be healthy, because a second release on top of a broken one hides both failures. The incident flag file must be absent, which is how on-call freezes deploys without editing the pipeline.\n\n### Gate\n\nThe digest check sits in the gate tier because it is the last thing that can veto the release: an image pinned by tag is a promise someone can rewrite. The verdict is a single exit code. Zero opens the gate; anything else stops the pipeline before kubectl apply runs.\n\n### Ship\n\nOnly then does the rollout start. The gate waits for the Deployment to report healthy, and the watch step holds for ten minutes of error rate and latency, because a rollout that reports success is not the same as a release that works.\n\n> **The honest caveat:** a gate only guards the path that runs it. Anyone with kubectl and permission can go around it, so RBAC still matters.\n\n> **The lesson:** the gate is where a team agrees, in code, what a deploy is allowed to assume.\n\n## In one line\n\n> **Checks decide, the gate enforces.** A gate is the deploy argument, settled in code.\n\n**The first check**\n\n```bash\nkubectl config current-context\n```\n"},{"id":"p648","n":"648","slug":"deployment-shell-commands","title":"Deployment shell commands","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Bash","Kubernetes","DevOps","SRE","CommandLine"],"series_name":"ShellSec","design":"cheatsheet","script":"","date":"2030-11-17","files":{"devsecops":"notes/p648-deployment-shell-commands.md"},"md":"# Deployment shell commands\n\nDeployment work is four blocks of commands: what cluster am I on, what is rolling out, what is it saying, and how do I undo it. The order is not cosmetic. The first block is the one that prevents the expensive mistake.\n\n## Quick reference\n\n### CONTEXT\n\n```bash\nkubectl config current-context\nkubectl config get-contexts\nkubectl config use-context prod-eu\nkubectl get ns api -o name\nkubectl cluster-info\n```\n\n### ROLLOUT\n\n```bash\nkubectl rollout status deploy/api\nkubectl rollout history deploy/api\nkubectl rollout restart deploy/api\nkubectl set image deploy/api api=api:v2\nkubectl get pods -n api -w\n```\n\n### LOGS\n\n```bash\nkubectl logs -f deploy/api -n api\nkubectl logs --tail=100 deploy/api\nkubectl logs -p deploy/api -n api\nkubectl describe pod -n api -l app=api\nkubectl get events -n api\n```\n\n### ROLLBACK\n\n```bash\nkubectl rollout undo deploy/api\nkubectl rollout undo deploy/api -n api\nkubectl rollout status deploy/api -n api\nkubectl get rs -n api\nkubectl get pods -n api\n```\n\n## Explained\n\n### Context\n\n`kubectl config current-context` prints the name the rest of your commands will use. get-contexts lists what is available; use-context switches. get ns proves the namespace exists before you apply into it.\n\n### Rollout\n\nkubectl rollout status blocks until the Deployment is done, and it is the line that should end a pipeline. rollout history lists revisions, restart cycles the pods without a manifest change, set image updates one container, and get pods -w watches the replacements arrive.\n\n### Logs\n\n`kubectl logs -f` follows one Deployment; `--tail=100` gives the last hundred lines without the scroll; -p reads the previous container after a crash. describe pod shows events and exit codes, and get events shows the cluster's own story.\n\n### Rollback\n\nkubectl rollout undo returns to the previous revision; add `--to-revision` N when you know the target. rollout status confirms the undo landed, get rs shows the old ReplicaSets still available, and get pods proves what is running.\n\n> **The honest caveat:** kubectl reads the current context, so every command here is one context switch away from the wrong cluster.\n\n> **The lesson:** type the context check before the command, every time.\n\n## In one line\n\n> **Context first, command second.** Every other block assumes the right cluster.\n\n**Check the context**\n\n```bash\nkubectl config current-context\n```\n"},{"id":"p649","n":"649","slug":"four-deploy-failures","title":"Four deploy failures","sections":["devsecops"],"topic":"Git & CI/CD","tags":["DevOps","SRE","Kubernetes","CICD","Bash"],"series_name":"ShellSec","design":"explainer","script":"","date":"2030-11-19","files":{"devsecops":"notes/p649-four-deploy-failures.md"},"md":"# Four deploy failures\n\nFour failures account for most bad deploys, and none of them is exotic. Each one is a check somebody skipped, and each one is cheaper to catch before the rollout than after it.\n\n## Quick reference\n\n### Wrong context\n\n- The runner kept a kubeconfig from the last job.\n- Production manifests land on a staging cluster.\n- Every health check passes, on the wrong cluster.\n\n### Stale image\n\n- The tag was rebuilt; the name never changed.\n- A node reuses the image it already pulled.\n- The pod is Ready and runs the previous code.\n\n### Failed smoke\n\n- Health returns 200 while checkout returns 500.\n- One retry hides a backend that flaps.\n- The pipeline never calls a paying endpoint.\n\n### No rollback\n\n- revisionHistoryLimit: 0 deleted the undo.\n- The previous image was pruned from the registry.\n- A migration moved the schema forward.\n\n> **Note:** Each one is cheaper to catch before the rollout than after it.\n\n## Explained\n\n### Wrong context\n\nThe runner keeps the kubeconfig from the previous job, or someone runs the prod manifest without switching. The release lands on staging, passes every health check, and production stays on the old version. The context check is first because it decides which cluster answers every later question.\n\n### Stale image\n\nThe tag v2 is rebuilt, the name never changes, and a node reuses the image it already pulled. The pod reports Ready while running the previous code. A digest pin is the only reference that cannot be silently rewritten.\n\n### Failed smoke\n\nHealth returns 200 because the process is listening; checkout returns 500 because a dependency is not configured. One retry hides a backend that flaps. A smoke test has to assert a status and a body on the endpoint that earns money.\n\n### No rollback\n\nrevisionHistoryLimit was set to 0 to keep the cluster tidy, the previous image was pruned from the registry, or a migration moved the schema forward and the old code cannot read it. An undo needs a target, an image and a compatible schema.\n\n> **The honest caveat:** these four checks do not cover a bad migration or a slow leak, which is why the gate is a floor, not a ceiling.\n\n> **The lesson:** deploy failures are check failures that arrived late.\n\n## In one line\n\n> **None of these are surprises.** Each one is a check that did not run.\n\n**The first check**\n\n```bash\nkubectl config current-context\n```\n"},{"id":"p650","n":"650","slug":"9-checks-before-a-deploy","title":"9 checks before a deploy","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Bash","CICD","Kubernetes","DevOps","SRE"],"series_name":"ShellSec","design":"checklist","script":"","date":"2030-11-21","files":{"devsecops":"notes/p650-9-checks-before-a-deploy.md"},"md":"# 9 checks before a deploy\n\nNine checks, three groups, one decision. The gate script runs five of them automatically; the other four belong to the artifact and smoke steps.\n\n## Quick reference\n\n### Context\n\n```bash\n# Context is production\nkubectl config current-context\n# Namespace exists\nkubectl get ns api -o name\n# Kubeconfig is the right one\necho $KUBECONFIG\n```\n\n### Artifact\n\n- **Image is pinned by digest** — image@sha256:...\n- **Manifest verifies** — `sha256sum -c SHA256SUMS`\n- **Every file is present** — `artifact_check.sh`\n\n### Health\n\n- **Previous rollout is green** — `kubectl rollout status`\n- **No incident flag is open** — no deploy.block file\n- **Smoke list is current** — `curl -fsS $URL/healthz`\n\n## Explained\n\n### Context\n\nThe current context is production, not the one the runner had cached. The namespace exists, so the apply does not create a surprise or fail halfway through. The kubeconfig is the one from the environment secret, not the copy on your laptop. If those three disagree, stop and fix the runner before the pipeline starts.\n\n### Artifact\n\nThe image is pinned by digest, so a rebuild cannot change what ships. The manifest verifies against the files, so a skipped upload fails here instead of in the cluster. Every expected file is present, including the migration and the config bundle, and the count matches the manifest.\n\n### Health\n\nThe previous rollout is green; stacking a release on a broken one hides both. No incident flag is open, because a deploy during an incident adds a second variable. The smoke list is current, points at endpoints that matter, and ran against staging in the last hour.\n\n> **The honest caveat:** a checklist is not a gate until something fails the build. If every item is a person ticking a box, it is a poster with good typography.\n\n> **The lesson:** nine answers, one go or no-go, and the pipeline, not a person, enforces it.\n\n## In one line\n\n> **Nine checks, one go or no-go.** The gate runs five of them for you.\n\n**The first check**\n\n```bash\nkubectl config current-context\n```\n"},{"id":"p651","n":"651","slug":"quiz-3-deploy-questions","title":"Quiz: 3 deploy questions","sections":["devsecops"],"topic":"Git & CI/CD","tags":["Bash","Kubernetes","DevOps","SRE","CICD"],"series_name":"ShellSec","design":"quiz","script":"","date":"2030-11-24","files":{"devsecops":"notes/p651-quiz-3-deploy-questions.md"},"md":"# Quiz: 3 deploy questions\n\nThree questions close the deploy phase. Each one is a decision the gate makes for you, and each wrong answer is a failure mode from the previous nine posts in this phase.\n\n## Quick reference\n\n**Q1. Why rehearse a rollback before you need one?**\n\n- a) It proves the undo target exists\n- b) It makes the undo faster at 3 a.m.\n- c) kubectl requires a rehearsal first\n\n> **Answer:** a) It proves the undo target exists. A dry-run reads the rollout history and prints the command while the cluster is calm. It catches a zero revisionHistoryLimit or a pruned ReplicaSet.\n\n**Q2. What must a smoke test assert after a deploy?**\n\n- a) Status code and a body marker\n- b) That the pod restarted\n- c) That the pipeline went green\n\n> **Answer:** a) Status code and a body marker. A 200 from a process that is still warming proves little. Assert the status and a string that only a working release returns, on an endpoint that matters.\n\n**Q3. Why is the context check the first gate?**\n\n- a) Wrong cluster, every check is moot\n- b) It is the fastest kubectl call\n- c) kubectl refuses other contexts\n\n> **Answer:** a) Wrong cluster, every check is moot. The other gates ask the cluster that kubectl is pointed at. If that is staging, a pass means staging is healthy and production stays behind.\n\n## Explained\n\n### Question one\n\nWhy rehearse a rollback before you need it? Because a dry-run reads the rollout history and prints the command while the cluster is calm. It finds a zero revisionHistoryLimit or a pruned ReplicaSet before an incident does. The undo target is configuration, and configuration is testable.\n\n### Question two\n\nWhat must a smoke test assert? The status code and a body marker on an endpoint that matters. A 200 from /health proves the process is listening, not that the release works. A retry can turn a flapping service into a pass, so log the attempts and make a failure fail the build, not open a ticket.\n\n### Question three\n\nWhy is the context check first? Because a wrong cluster makes every later check meaningless. The namespace, rollout and digest checks all ask the cluster that kubectl is pointed at; if that is staging, a pass says staging is healthy. The first check decides which cluster the answer belongs to.\n\n> **The honest caveat:** passing this quiz does not make a deploy safe; the gate does.\n\n> **The lesson:** a gate is only as good as its last no.\n\n## In one line\n\n> **A gate is only as good as its no.** The gate is the last post's, and this is the test.\n\n**Run the gate:** `deploy_gate.sh` prod-eu api api\n"},{"id":"p652","n":"652","slug":"suid-binaries-that-appeared-overnight","title":"SUID binaries that appeared overnight","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","DevSecOps","SysAdmin"],"series_name":"ShellSec","design":"code","script":"suid_watch.sh","date":"2030-11-26","files":{"it":"notes/p652-suid-binaries-that-appeared-overnight.md","devsecops":"notes/p652-suid-binaries-that-appeared-overnight.md"},"md":"# SUID binaries that appeared overnight\n\nAt 04:12 a setuid binary appears in `/usr/local/bin`. It is root-owned, world-executable, and nobody remembers installing it. The file is newer than the last deploy, the only clue the box offers.\n\n## Quick reference\n\nThe full script is at the end of this note: `suid_watch.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nSUID_BASELINE=/tmp/s0f/suid.base bash suid_watch.sh /tmp/s0f/root\n```\n\n**Example output**\n\n```text\nFINDING 1 new SUID/SGID files\nNEW 4755 root:root /tmp/s0f/root/bin/passwd.old\n```\n\n## Explained\n\n### What it does\n\n`suid_watch.sh` sweeps a root with `find -xdev`, keeps every file with -perm -4000 or -2000, and prints mode, owner and path, sorted. `--update` writes that set as the baseline. A check run diffs both sides with comm, reports new and removed, and exits 1 only when something is new.\n\n### What the screen shows\n\nThe fourteen lines read SUID_BASELINE, sweep the root, write the set to a temp file and diff it. On the dirty fixture: FINDING 1 new SUID/SGID files and NEW 4755 root:root `/tmp/s0f/root/bin/passwd.old`.\n\n### How to run it\n\n`sudo suid_watch.sh --update /` once, then run it from cron at 04:10. Exit 0 is clean or only removals, 1 is a new setuid or setgid file, 2 is a missing baseline or bad argument. It writes only the baseline.\n\n### The fix\n\nA new setuid file is an unexpected package or an intruder. Run `dpkg -S` or `rpm -qf` on the path; if nothing owns it, move it aside and keep a copy for forensics. Clear it with chmod u-s or g-s.\n\n### Prevention\n\nKeep the baseline in git and review each `--update`. A setuid bit is a root shell waiting for a bug, so keep it short.\n\n> **The honest caveat:** find skips unreadable subtrees, so run the sweep as root.\n\n> **The lesson:** a baseline turns \"what is this file\" into \"who changed it\".\n\n## In one line\n\n> **A new bit is a new door.** Setuid is a root shell waiting for a bug. Keep the list short.\n\n**Build the baseline**\n\n```bash\nsudo suid_watch.sh --update /\n```\n\n## Script: suid_watch.sh\n\nDownload: [scripts/suid_watch.sh](../scripts/suid_watch.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n# suid_watch.sh - baseline SUID/SGID files and report the delta.\n#\n# Fixes: a root-owned setuid binary appears overnight and nobody notices\n# until it is used. The sweep compares the current setuid/setgid set with\n# an approved baseline, so the question changes from \"what is this file\"\n# to \"who changed the baseline\".\n#\n# Usage:\n#   sudo ./suid_watch.sh --update /            # write the baseline\n#   sudo ./suid_watch.sh /                     # check against it\n#   ./suid_watch.sh -b ./suid.base -r /usr -r /opt\n#\n# Cron (daily, 04:10):\n#   10 4 * * * root /usr/local/sbin/suid_watch.sh / >> /var/log/suid_watch.log 2>&1\n#\n# Exit: 0 clean or only removals, 1 new SUID/SGID file, 2 usage or unreadable\n# input. Requires GNU find (-printf). Unreadable subtrees are skipped, so run\n# as root for a complete sweep. SUID_BASELINE overrides the default baseline\n# path. Check mode writes nothing; --update writes the baseline file only.\n#\n# By s0sta.com\n\nIFS=$'\\n\\t'\n\nBASELINE=${SUID_BASELINE:-/var/lib/s0sta/suid.baseline}\nMODE=check\nROOTS=()\n\nusage() {\n  cat <<'EOF'\nUsage: suid_watch.sh [options] [ROOT...]\n\nBuild or check a baseline of SUID and SGID files.\n\nOptions:\n  -b, --baseline FILE   baseline to read or write\n                        (default: /var/lib/s0sta/suid.baseline)\n  -u, --update          write the current sweep as the new baseline\n  -r, --root PATH       root to scan; repeatable (default: /)\n  -h, --help            show this help and exit\n\nSUID_BASELINE overrides the default baseline path.\nExit: 0 clean or only removals, 1 new SUID/SGID file, 2 usage or unreadable input.\nBy s0sta.com\nEOF\n}\n\ndie() {\n  printf 'ERROR %s\\n' \"$1\" >&2\n  exit 2\n}\n\nscan() {\n  local root\n  for root in \"${ROOTS[@]}\"; do\n    find \"$root\" -xdev -type f \\( -perm -4000 -o -perm -2000 \\) \\\n      -printf '%m %u:%g %p\\n' 2>/dev/null || true\n  done | LC_ALL=C sort -u\n}\n\nemit() {\n  if [ -n \"$1\" ]; then\n    printf '%s\\n' \"$1\"\n  fi\n}\n\ncount_lines() {\n  if [ -z \"$1\" ]; then\n    printf '0\\n'\n  else\n    printf '%s\\n' \"$1\" | awk 'NF { n++ } END { print n + 0 }'\n  fi\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n  case $1 in\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    -b|--baseline)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      BASELINE=$2\n      shift 2\n      ;;\n    -u|--update)\n      MODE=update\n      shift\n      ;;\n    -r|--root)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a path\"\n      ROOTS+=(\"$2\")\n      shift 2\n      ;;\n    --)\n      shift\n      while [ \"$#\" -gt 0 ]; do\n        ROOTS+=(\"$1\")\n        shift\n      done\n      ;;\n    -*)\n      die \"unknown option: $1\"\n      ;;\n    *)\n      ROOTS+=(\"$1\")\n      shift\n      ;;\n  esac\ndone\n\nif [ \"${#ROOTS[@]}\" -eq 0 ]; then\n  ROOTS=(/)\nfi\n\nfor root in \"${ROOTS[@]}\"; do\n  [ -d \"$root\" ] || die \"not a directory: $root\"\ndone\n\nif [ \"$MODE\" = \"update\" ]; then\n  base_dir=${BASELINE%/*}\n  if [ \"$base_dir\" = \"$BASELINE\" ]; then\n    base_dir=.\n  fi\n  if [ ! -d \"$base_dir\" ]; then\n    mkdir -p \"$base_dir\" || die \"cannot create $base_dir\"\n  fi\n  tmp=$(mktemp \"${BASELINE}.XXXXXX\") || die \"cannot create a temp file next to $BASELINE\"\n  trap 'rm -f \"$tmp\"' EXIT\n  scan > \"$tmp\"\n  chmod 0644 \"$tmp\" 2>/dev/null || true\n  mv \"$tmp\" \"$BASELINE\" || die \"cannot write baseline: $BASELINE\"\n  trap - EXIT\n  entries=$(awk 'END { print NR + 0 }' \"$BASELINE\")\n  printf 'BASELINE %s %s entries\\n' \"$BASELINE\" \"$entries\"\n  exit 0\nfi\n\nif [ ! -f \"$BASELINE\" ]; then\n  die \"baseline missing: $BASELINE (run with --update)\"\nfi\n[ -r \"$BASELINE\" ] || die \"cannot read baseline: $BASELINE\"\n\nbase_sorted=$(LC_ALL=C sort -u \"$BASELINE\")\ncurrent=$(scan)\nnew=$(comm -13 <(emit \"$base_sorted\") <(emit \"$current\") || true)\nremoved=$(comm -23 <(emit \"$base_sorted\") <(emit \"$current\") || true)\n\nnew_n=$(count_lines \"$new\")\nrem_n=$(count_lines \"$removed\")\n\nif [ \"$new_n\" -eq 0 ] && [ \"$rem_n\" -eq 0 ]; then\n  entries=$(awk 'END { print NR + 0 }' \"$BASELINE\")\n  printf 'OK baseline matches: %s entries\\n' \"$entries\"\n  exit 0\nfi\n\nif [ \"$new_n\" -gt 0 ]; then\n  printf 'FINDING %d new SUID/SGID files\\n' \"$new_n\"\n  printf '%s\\n' \"$new\" | sed 's/^/NEW /'\nfi\n\nif [ \"$rem_n\" -gt 0 ]; then\n  printf 'REMOVED %d baseline entries\\n' \"$rem_n\"\n  printf '%s\\n' \"$removed\" | sed 's/^/GONE /'\nfi\n\nif [ \"$new_n\" -gt 0 ]; then\n  exit 1\nfi\n\nexit 0\n```\n"},{"id":"p653","n":"653","slug":"secrets-in-crontabs-and-env-files","title":"Secrets in crontabs and env files","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","SecretsManagement","DevSecOps"],"series_name":"ShellSec","design":"code","script":"cron_secret_scan.sh","date":"2030-11-28","files":{"it":"notes/p653-secrets-in-crontabs-and-env-files.md","devsecops":"notes/p653-secrets-in-crontabs-and-env-files.md"},"md":"# Secrets in crontabs and env files\n\nA backup job needs the database password, so someone puts it in `/etc/cron.d/backup`. Six months later it is in a support bundle, a repo mirror and a screenshot. The job never needed it in plain text.\n\n## Quick reference\n\nThe full script is at the end of this note: `cron_secret_scan.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash cron_secret_scan.sh /tmp/s0f/cron.d/db\n```\n\n**Example output**\n\n```text\n/tmp/s0f/cron.d/db:2: DB_PASS=****\nFINDING 1 secret-like line\n```\n\n## Explained\n\n### What it does\n\n`cron_secret_scan.sh` walks the paths you give it, keeps crontabs and env files, and matches assignment-shaped lines: password, pass, secret, token, api key, access key, private key, credential, authorization. Every hit prints the path, the line and the key, then four stars. The value never reaches stdout.\n\n### What the screen shows\n\nThe fourteen lines read each file with awk, lower-case the line for matching, stop at the assignment and print the masked prefix. On the dirty fixture: `/tmp/s0f/cron.d/db`:2: DB_PASS=\\*\\*\\*\\* and FINDING 1 secret-like line.\n\n### How to run it\n\n`sudo cron_secret_scan.sh /etc/cron.d /etc/crontab /var/spool/cron /srv/app`, or one .env file. Exit 0 is clean, 1 is a hit, 2 is a missing or unreadable path. It writes nothing.\n\n### The fix\n\nMasking is for the report, not the fix. Rotate the secret first, then move it to a root-only file with mode 0600, or to a secret manager the job can read at runtime. Remove the old value from backups and CI logs if they are reachable.\n\n### Prevention\n\nA pre-commit hook and a CI grep catch the copy in git; the host sweep catches the copy that survives a clean-up.\n\n> **The honest caveat:** the pattern matches names, not entropy, so a variable named APPLE will pass.\n\n> **The lesson:** a secret in a cron file is a secret every process can read.\n\n## In one line\n\n> **A cron line is not a vault.** The host copy survives the repo clean-up.\n\n**Mask, then rotate:** `cron_secret_scan.sh /etc/cron.d`\n\n## Script: cron_secret_scan.sh\n\nDownload: [scripts/cron_secret_scan.sh](../scripts/cron_secret_scan.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n# cron_secret_scan.sh - find secret-like assignments in crontabs and env files.\n#\n# Fixes: a database password or API token pasted into /etc/cron.d or a .env\n# file, where the service account, a backup and a support bundle can all read\n# it. The scan prints the path, the line, the key and four stars. The value is\n# never written to stdout.\n#\n# Usage:\n#   sudo ./cron_secret_scan.sh /etc/cron.d /etc/crontab /var/spool/cron /srv/app\n#   ./cron_secret_scan.sh /srv/app/.env\n#\n# Cron (daily, 05:30):\n#   30 5 * * * root /usr/local/sbin/cron_secret_scan.sh /etc /srv >> /var/log/cron_secret.log 2>&1\n#\n# Exit: 0 clean, 1 secret-like line found, 2 usage or unreadable path.\n# A path that names a file is always scanned. A directory is walked with GNU\n# find and only crontabs and env files are read (text files under 1 MB).\n# Matching is by key name, not by entropy, so it is deliberately noisy.\n#\n# By s0sta.com\n\nIFS=$'\\n\\t'\n\nDEFAULT_PATHS=(\n  /etc/cron.d\n  /etc/crontab\n  /etc/cron.daily\n  /etc/cron.hourly\n  /etc/cron.weekly\n  /etc/cron.monthly\n  /var/spool/cron\n  /etc/environment\n)\n\nPATHS=()\nFILES=()\n\nusage() {\n  cat <<'EOF'\nUsage: cron_secret_scan.sh [options] [PATH...]\n\nScan crontabs and env files for secret-like assignments and mask the value.\n\nOptions:\n  -h, --help   show this help and exit\n\nWith no PATH, the usual cron locations and /etc/environment are scanned.\nA PATH that is a file is scanned as given; a directory is filtered to\ncrontabs and env files.\n\nExit: 0 clean, 1 secret-like line found, 2 usage or unreadable path.\nBy s0sta.com\nEOF\n}\n\ndie() {\n  printf 'ERROR %s\\n' \"$1\" >&2\n  exit 2\n}\n\nis_target() {\n  local path=$1 base\n  base=${path##*/}\n  case $base in\n    .env|.env.*|*.env|env|environment|crontab) return 0 ;;\n  esac\n  case $path in\n    */cron.d/*|*/cron.daily/*|*/cron.hourly/*|*/cron.weekly/*|*/cron.monthly/*|*/spool/cron/*) return 0 ;;\n  esac\n  return 1\n}\n\ncollect() {\n  local p f\n  for p in \"$@\"; do\n    if [ -f \"$p\" ]; then\n      printf '%s\\n' \"$p\"\n    elif [ -d \"$p\" ]; then\n      while IFS= read -r f; do\n        if is_target \"$f\"; then\n          printf '%s\\n' \"$f\"\n        fi\n      done < <(find \"$p\" -xdev -type f -size -1024k 2>/dev/null)\n    fi\n  done\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n  case $1 in\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      while [ \"$#\" -gt 0 ]; do\n        PATHS+=(\"$1\")\n        shift\n      done\n      ;;\n    -*)\n      die \"unknown option: $1\"\n      ;;\n    *)\n      PATHS+=(\"$1\")\n      shift\n      ;;\n  esac\ndone\n\nif [ \"${#PATHS[@]}\" -eq 0 ]; then\n  for p in \"${DEFAULT_PATHS[@]}\"; do\n    if [ -e \"$p\" ]; then\n      PATHS+=(\"$p\")\n    fi\n  done\nelse\n  for p in \"${PATHS[@]}\"; do\n    [ -e \"$p\" ] || die \"missing path: $p\"\n    [ -r \"$p\" ] || die \"unreadable path: $p\"\n  done\nfi\n\nif [ \"${#PATHS[@]}\" -eq 0 ]; then\n  printf 'SKIP no cron or env paths found\\n'\n  exit 0\nfi\n\nwhile IFS= read -r f; do\n  FILES+=(\"$f\")\ndone < <(collect \"${PATHS[@]}\")\n\nnfiles=${#FILES[@]}\nif [ \"$nfiles\" -eq 0 ]; then\n  printf 'SKIP no cron or env files under %d paths\\n' \"${#PATHS[@]}\"\n  exit 0\nfi\n\nstatus=0\nawk -v files=\"$nfiles\" '\n  {\n    low = tolower($0)\n    if (match(low, /(^|[^a-z])(password|passwd|pass|secret|token|api[_-]?key|access[_-]?key|private[_-]?key|credential|authorization)[a-z0-9_-]*[ \\t]*[=:]/)) {\n      printf \"%s:%d: %s****\\n\", FILENAME, FNR, substr($0, 1, RSTART + RLENGTH - 1)\n      n++\n    }\n  }\n  END {\n    if (n > 0) {\n      printf \"FINDING %d secret-like %s\\n\", n, (n == 1 ? \"line\" : \"lines\")\n    } else {\n      printf \"OK no secret-like lines in %d files\\n\", files\n    }\n    exit (n > 0)\n  }\n' \"${FILES[@]}\" || status=$?\nexit \"$status\"\n```\n"},{"id":"p654","n":"654","slug":"an-image-gate-for-the-registry","title":"An image gate for the registry","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","DevSecOps","Containers","CyberSecurity","CICD"],"series_name":"ShellSec","design":"code","script":"image_scan_gate.sh","date":"2030-12-01","files":{"it":"notes/p654-an-image-gate-for-the-registry.md","devsecops":"notes/p654-an-image-gate-for-the-registry.md"},"md":"# An image gate for the registry\n\nThe registry gate is green because it scans nothing. trivy is not installed, the step is optional, and a CRITICAL base image ships on a Friday.\n\n## Quick reference\n\nThe full script is at the end of this note: `image_scan_gate.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nIMAGE_ALLOWLIST=/tmp/s0f/allow-none.txt bash image_scan_gate.sh one:1.4\n```\n\n**Example output**\n\n```text\nFINDING 1 HIGH/CRITICAL\nCRITICAL CVE-2024-21626 runc\n```\n\n## Explained\n\n### What it does\n\n`image_scan_gate.sh` takes images from arguments or a file, runs trivy, and keeps every HIGH or CRITICAL vulnerability. Each CVE is checked against an allowlist of IDs, where a comment can carry the owner and expiry. Anything left prints severity, CVE, package and image, then exits 1. When trivy is missing it prints SKIP and exits 2, because a gate that cannot scan must not pass.\n\n### What the screen shows\n\nThe fourteen lines check for trivy, scan, select HIGH and CRITICAL with jq, filter the allowlist and report. On the dirty fixture: FINDING 1 HIGH/CRITICAL and CRITICAL CVE-2024-21626 runc. The full script adds the image.\n\n### How to run it\n\nIMAGE_ALLOWLIST=`trivy-allowlist.txt image_scan_gate.sh -f images.txt` in CI, after build and before deploy. Exit 0 is clean or fully allowlisted, 1 is a finding, 2 is a missing tool, a bad image list or invalid JSON.\n\n### The fix\n\nRebuild on a patched base, or pin the fixed package version. If it does not apply, add the ID with an owner and an expiry date for the next review.\n\n### Prevention\n\nRun it on every tag, not only on release. A weekly scan of what is deployed catches the CVE published after the build.\n\n> **The honest caveat:** a trivy JSON schema change breaks the parse, so pin the version.\n\n> **The lesson:** an allowlist without an owner is a permanent exception.\n\n## In one line\n\n> **A gate that cannot fail is not a gate.** A gate that cannot scan must not report green.\n\n**Fail closed**\n\n```bash\ntrivy image -s HIGH,CRITICAL nginx\n```\n\n## Script: image_scan_gate.sh\n\nDownload: [scripts/image_scan_gate.sh](../scripts/image_scan_gate.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n# image_scan_gate.sh - fail a build on HIGH or CRITICAL image CVEs.\n#\n# Fixes: the registry gate is green because it scans nothing. This gate runs\n# trivy on every image, parses the JSON, and fails unless the CVE is in an\n# allowlist where every exception can name an owner and an expiry.\n#\n# Usage:\n#   ./image_scan_gate.sh -f images.txt -a trivy-allowlist.txt\n#   ./image_scan_gate.sh ghcr.io/s0sta/api:1.4\n#\n# CI (before deploy):\n#   image_scan_gate.sh -f images.txt -a .github/trivy-allowlist.txt\n#\n# Weekly drift scan (Monday, 06:00):\n#   0 6 * * 1 root /usr/local/sbin/image_scan_gate.sh -f /etc/s0sta/images.txt -a /etc/s0sta/allow.txt\n#\n# Exit: 0 clean or every finding allowlisted, 1 HIGH/CRITICAL outside the\n# allowlist, 2 usage, missing tool, bad JSON or a failed scan.\n# trivy and jq are required. Without trivy the gate fails closed with a SKIP\n# line rather than passing a build it could not scan. IMAGE_ALLOWLIST is the\n# default allowlist; -a overrides it. Allowlist format, one ID per line,\n# anything after # is a comment:\n#   CVE-2024-21626  # owner: @platform, expires: 2026-12-31\n#\n# By s0sta.com\n\nIFS=$'\\n\\t'\n\nALLOW_FILE=${IMAGE_ALLOWLIST:-}\nALLOW=\"\"\nIMAGES=()\n\nusage() {\n  cat <<'EOF'\nUsage: image_scan_gate.sh [options] [IMAGE...]\n\nScan images with trivy and fail on HIGH or CRITICAL CVEs.\n\nOptions:\n  -f, --file FILE       read image references from FILE, one per line\n  -a, --allowlist FILE  accepted CVE IDs, one per line; # starts a comment\n  -h, --help            show this help and exit\n\nIMAGE_ALLOWLIST names the default allowlist; -a overrides it.\nExit: 0 clean or every finding allowlisted, 1 finding, 2 usage or tool error.\nBy s0sta.com\nEOF\n}\n\ndie() {\n  printf 'ERROR %s\\n' \"$1\" >&2\n  exit 2\n}\n\nis_allowed() {\n  [ -n \"$ALLOW\" ] || return 1\n  printf '%s\\n' \"$ALLOW\" | grep -Fxq -- \"$1\"\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n  case $1 in\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    -f|--file)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      [ -r \"$2\" ] || die \"cannot read image list: $2\"\n      while IFS= read -r line; do\n        line=${line%%#*}\n        line=$(printf '%s' \"$line\" | tr -d '[:space:]')\n        if [ -n \"$line\" ]; then\n          IMAGES+=(\"$line\")\n        fi\n      done < \"$2\"\n      shift 2\n      ;;\n    -a|--allowlist)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      [ -r \"$2\" ] || die \"cannot read allowlist: $2\"\n      ALLOW_FILE=$2\n      shift 2\n      ;;\n    --)\n      shift\n      while [ \"$#\" -gt 0 ]; do\n        IMAGES+=(\"$1\")\n        shift\n      done\n      ;;\n    -*)\n      die \"unknown option: $1\"\n      ;;\n    *)\n      IMAGES+=(\"$1\")\n      shift\n      ;;\n  esac\ndone\n\nif [ \"${#IMAGES[@]}\" -eq 0 ]; then\n  usage >&2\n  die \"no images given: pass IMAGE or -f FILE\"\nfi\n\nif [ -n \"$ALLOW_FILE\" ]; then\n  ALLOW=$(awk '{ sub(/#.*/, \"\"); if (NF > 0) print $1 }' \"$ALLOW_FILE\")\nfi\n\nif ! command -v trivy >/dev/null 2>&1; then\n  printf 'SKIP trivy not installed: cannot scan %d images\\n' \"${#IMAGES[@]}\"\n  printf 'Install trivy, or run this gate in the image-scan job.\\n' >&2\n  exit 2\nfi\n\nif ! command -v jq >/dev/null 2>&1; then\n  die \"jq not installed: cannot parse trivy JSON\"\nfi\n\nreport=\"\"\nn=0\n\nfor img in \"${IMAGES[@]}\"; do\n  if ! json=$(trivy image --format json --quiet \"$img\" 2>/dev/null); then\n    die \"trivy failed for $img\"\n  fi\n  if ! printf '%s' \"$json\" | jq -e . >/dev/null 2>&1; then\n    die \"trivy returned invalid JSON for $img\"\n  fi\n  hits=$(printf '%s' \"$json\" | jq -r '\n    .Results[]?.Vulnerabilities[]?\n    | select(.Severity == \"HIGH\" or .Severity == \"CRITICAL\")\n    | [.Severity, .VulnerabilityID, .PkgName] | @tsv\n  ')\n  if [ -z \"$hits\" ]; then\n    continue\n  fi\n  while IFS=$'\\t' read -r sev cve pkg; do\n    [ -n \"$cve\" ] || continue\n    if is_allowed \"$cve\"; then\n      continue\n    fi\n    report=\"${report}${sev} ${cve} ${pkg} ${img}\"$'\\n'\n    n=$((n + 1))\n  done <<< \"$hits\"\ndone\n\nif [ \"$n\" -gt 0 ]; then\n  printf 'FINDING %d HIGH/CRITICAL not allowlisted\\n' \"$n\"\n  printf '%s' \"$report\"\n  exit 1\nfi\n\nprintf 'OK no HIGH/CRITICAL outside the allowlist in %d images\\n' \"${#IMAGES[@]}\"\nexit 0\n```\n"},{"id":"p655","n":"655","slug":"logins-that-do-not-look-right","title":"Logins that do not look right","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","SecOps","SysAdmin"],"series_name":"ShellSec","design":"code","script":"audit_logins.sh","date":"2030-12-03","files":{"it":"notes/p655-logins-that-do-not-look-right.md","devsecops":"notes/p655-logins-that-do-not-look-right.md"},"md":"# Logins that do not look right\n\nThree failed logins for alice at 09:00, a success at 09:02, and a root session at 02:14. last and lastb hold it all, in a format nobody parses until an incident.\n\n## Quick reference\n\nThe full script is at the end of this note: `audit_logins.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash audit_logins.sh /tmp/s0f/last.txt\n```\n\n**Example output**\n\n```text\nROOT 02:14 root pts/0 10.0.0.5\nFINDING logins: 1\n```\n\n## Explained\n\n### What it does\n\n`audit_logins.sh` normalises `last -F -i` and `lastb -F -i` into user, tty, host and a minute. It reports root logins, successes outside 06:00-23:00, users missing from passwd, and three failures then a success for the same user and host inside 30 minutes. Any hit exits 1.\n\n### What the screen shows\n\nThe fourteen lines find the month, read the hour, print ROOT and ODD lines and exit 1. On the fixture: ROOT 02:14 root pts/0 10.0.0.5 and FINDING logins: 1. The full script adds the pair and unknown-user passes.\n\n### How to run it\n\n`sudo audit_logins.sh`, or pass a saved last file and `--lastb`. Exit 0 is clean, 1 is a finding, 2 is a missing file or bad option.\n\n### The fix\n\nFor root: confirm the ticket, then set PermitRootLogin prohibit-password and give the engineer a named account. For a pair: block the source, force a password reset and review the session. For an unknown user: find the account that should not exist.\n\n### Prevention\n\nAlert on root logins and on failures per source per hour, not a daily count. A brute force that stops at success is invisible in a total.\n\n> **The honest caveat:** last omits the year unless -F is used, and wtmp rotation can hide older sessions.\n\n> **The lesson:** a login is a fact; a pattern of logins is the story.\n\n## In one line\n\n> **Root at 2 a.m. is a question.** The full script adds failed-then-success pairs and unknown users.\n\n**Read both logs**\n\n```bash\nlast -F -i; sudo lastb -F -i\n```\n\n## Script: audit_logins.sh\n\nDownload: [scripts/audit_logins.sh](../scripts/audit_logins.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n# audit_logins.sh - find root, odd-hour, paired and unknown logins.\n#\n# Fixes: three failed logins for alice, a success two minutes later and a root\n# session at 02:14, with nobody reading last or lastb. This script normalises\n# both files and reports the four patterns that matter.\n#\n# Usage:\n#   sudo ./audit_logins.sh\n#   ./audit_logins.sh --last ./last.txt --lastb ./lastb.txt --known ./users.txt\n#\n# Cron (daily, 06:10):\n#   10 6 * * * root /usr/local/sbin/audit_logins.sh >> /var/log/audit_logins.log 2>&1\n#\n# Exit: 0 clean, 1 login anomaly, 2 usage or unreadable input.\n# Reads only. Linux last/lastb from util-linux; -F adds the year, -i shows IPs.\n# lastb needs root; without it the pair check is skipped with a NOTE.\n# Anomalies: a root login, a success outside --start/--end, at least\n# --min-fails failures followed by a success from the same user and host\n# within --window minutes on the same day, and a user missing from passwd.\n#\n# By s0sta.com\n\nIFS=$'\\n\\t'\n\nLAST_FILE=\"\"\nLASTB_FILE=\"\"\nKNOWN_FILE=\"\"\nWINDOW=30\nMIN_FAILS=3\nSTART_HOUR=6\nEND_HOUR=23\n\nusage() {\n  cat <<'EOF'\nUsage: audit_logins.sh [options] [LAST_FILE]\n\nReport root, odd-hour, failed-then-success and unknown-user logins.\n\nOptions:\n  --last FILE       read saved last output instead of running last -F -i\n  --lastb FILE      read saved lastb output instead of running lastb -F -i\n  --known FILE      known users, one per line (default: passwd database)\n  --window N        failed-then-success window in minutes (default: 30)\n  --min-fails N     failures needed before a success counts (default: 3)\n  --start H         first normal hour, 0-23 (default: 6)\n  --end H           last normal hour, 0-23 (default: 23)\n  -h, --help        show this help and exit\n\nLAST_FILE is a saved last output, shorthand for --last FILE.\nExit: 0 clean, 1 login anomaly, 2 usage or unreadable input.\nBy s0sta.com\nEOF\n}\n\ndie() {\n  printf 'ERROR %s\\n' \"$1\" >&2\n  exit 2\n}\n\nis_int() {\n  case $1 in\n    ''|*[!0-9]*) return 1 ;;\n    *) return 0 ;;\n  esac\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n  case $1 in\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --last)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      LAST_FILE=$2\n      shift 2\n      ;;\n    --lastb)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      LASTB_FILE=$2\n      shift 2\n      ;;\n    --known)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a file\"\n      KNOWN_FILE=$2\n      shift 2\n      ;;\n    --window)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a number\"\n      WINDOW=$2\n      shift 2\n      ;;\n    --min-fails)\n      [ \"$#\" -ge 2 ] || die \"$1 needs a number\"\n      MIN_FAILS=$2\n      shift 2\n      ;;\n    --start)\n      [ \"$#\" -ge 2 ] || die \"$1 needs an hour\"\n      START_HOUR=$2\n      shift 2\n      ;;\n    --end)\n      [ \"$#\" -ge 2 ] || die \"$1 needs an hour\"\n      END_HOUR=$2\n      shift 2\n      ;;\n    -*)\n      die \"unknown option: $1\"\n      ;;\n    *)\n      [ -z \"$LAST_FILE\" ] || die \"unexpected argument: $1\"\n      LAST_FILE=$1\n      shift\n      ;;\n  esac\ndone\n\nis_int \"$WINDOW\" && [ \"$WINDOW\" -ge 1 ] && [ \"$WINDOW\" -le 1440 ] \\\n  || die \"--window needs a number from 1 to 1440\"\nis_int \"$MIN_FAILS\" && [ \"$MIN_FAILS\" -ge 1 ] && [ \"$MIN_FAILS\" -le 1000 ] \\\n  || die \"--min-fails needs a number from 1 to 1000\"\nis_int \"$START_HOUR\" && [ \"$START_HOUR\" -ge 0 ] && [ \"$START_HOUR\" -le 23 ] \\\n  || die \"--start needs an hour from 0 to 23\"\nis_int \"$END_HOUR\" && [ \"$END_HOUR\" -ge 0 ] && [ \"$END_HOUR\" -le 23 ] \\\n  || die \"--end needs an hour from 0 to 23\"\n[ \"$START_HOUR\" -lt \"$END_HOUR\" ] || die \"--start must be before --end\"\n\nWORK=$(mktemp -d \"${TMPDIR:-/tmp}/audit_logins.XXXXXX\") || die \"cannot create a work directory\"\ntrap 'rm -rf \"$WORK\"' EXIT\n\nif [ -n \"$LAST_FILE\" ]; then\n  [ -r \"$LAST_FILE\" ] || die \"cannot read --last file: $LAST_FILE\"\n  last_src=$LAST_FILE\nelse\n  command -v last >/dev/null 2>&1 || die \"last not found: pass --last FILE\"\n  if ! LC_ALL=C last -F -i > \"$WORK/last\" 2>/dev/null; then\n    die \"last -F -i failed: pass --last FILE\"\n  fi\n  last_src=$WORK/last\nfi\n\nif [ -n \"$LASTB_FILE\" ]; then\n  [ -r \"$LASTB_FILE\" ] || die \"cannot read --lastb file: $LASTB_FILE\"\n  lastb_src=$LASTB_FILE\nelse\n  : > \"$WORK/lastb\"\n  lastb_src=$WORK/lastb\n  if command -v lastb >/dev/null 2>&1; then\n    if ! LC_ALL=C lastb -F -i > \"$WORK/lastb\" 2>/dev/null; then\n      : > \"$WORK/lastb\"\n      printf 'NOTE lastb unreadable: failed-login pairs skipped\\n' >&2\n    fi\n  else\n    printf 'NOTE lastb not installed: failed-login pairs skipped\\n' >&2\n  fi\nfi\n\nif [ -n \"$KNOWN_FILE\" ]; then\n  [ -r \"$KNOWN_FILE\" ] || die \"cannot read --known file: $KNOWN_FILE\"\n  awk '{ print $1 }' \"$KNOWN_FILE\" > \"$WORK/known\"\nelse\n  : > \"$WORK/known\"\n  if command -v getent >/dev/null 2>&1; then\n    getent passwd 2>/dev/null | cut -d: -f1 > \"$WORK/known\" || true\n  fi\n  if [ ! -s \"$WORK/known\" ]; then\n    cut -d: -f1 /etc/passwd > \"$WORK/known\" 2>/dev/null || true\n  fi\nfi\n\nnormalize() {\n  awk -v default_year=\"$(date +%Y)\" '\n    BEGIN { split(\"Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec\", mon, \" \") }\n    $1 == \"reboot\" || $1 == \"shutdown\" || $1 == \"wtmp\" { next }\n    {\n      mi = 0\n      for (i = 2; i <= NF; i++) {\n        for (k = 1; k <= 12; k++) {\n          if ($i == mon[k]) { mi = i; mo = k; break }\n        }\n        if (mi > 0) break\n      }\n      if (mi == 0) next\n      day = $(mi + 1)\n      tim = $(mi + 2)\n      yr = $(mi + 3)\n      if (day !~ /^[0-9]+$/) next\n      if (tim !~ /^[0-9][0-9]:[0-9][0-9](:[0-9][0-9])?$/) next\n      if (yr !~ /^[0-9][0-9][0-9][0-9]$/) yr = default_year\n      split(tim, t, \":\")\n      host = (mi >= 5) ? $3 : \"-\"\n      printf \"%s %s %s %s %d %d %d %d\\n\", $1, $2, host, yr, mo, day, t[1], t[2]\n    }\n  ' \"$1\"\n}\n\n{\n  normalize \"$last_src\" | sed 's/^/L /'\n  normalize \"$lastb_src\" | sed 's/^/B /'\n} > \"$WORK/records\"\n\nstatus=0\nawk -v window=\"$WINDOW\" -v minfails=\"$MIN_FAILS\" \\\n    -v starth=\"$START_HOUR\" -v endh=\"$END_HOUR\" '\n  FNR == NR { known[$1] = 1; next }\n  {\n    src = $1; user = $2; tty = $3; host = $4\n    yr = $5; mo = $6 + 0; day = $7 + 0; hh = $8 + 0; mm = $9 + 0\n    key = user SUBSEP host SUBSEP yr SUBSEP mo SUBSEP day\n    tod = hh * 60 + mm\n    if (!(user in known)) {\n      unk[++nu] = sprintf(\"UNKNOWN %02d:%02d %s %s %s\", hh, mm, user, tty, host)\n    }\n    if (src == \"L\") {\n      if (user == \"root\") {\n        root[++nr] = sprintf(\"ROOT %02d:%02d %s %s %s\", hh, mm, user, tty, host)\n      } else if (tod < starth * 60 || tod > endh * 60) {\n        odd[++no] = sprintf(\"ODD %02d:%02d %s %s %s\", hh, mm, user, tty, host)\n      }\n      ns++\n      s_key[ns] = key\n      s_tod[ns] = tod\n      s_hm[ns] = sprintf(\"%02d:%02d\", hh, mm)\n      s_user[ns] = user\n      s_host[ns] = host\n    } else {\n      nf = ++failn[key]\n      fail[key, nf] = tod\n    }\n  }\n  END {\n    for (i = 1; i <= ns; i++) {\n      c = 0\n      k = s_key[i]\n      for (j = 1; j <= failn[k]; j++) {\n        t = fail[k, j]\n        if (t <= s_tod[i] && s_tod[i] - t <= window) c++\n      }\n      if (c >= minfails) {\n        pair[++np] = sprintf(\"PAIR %s %s %d fails then success %s\",\n                             s_hm[i], s_user[i], c, s_host[i])\n      }\n    }\n    total = nr + no + np + nu\n    if (total == 0) {\n      print \"OK no login anomalies\"\n      exit 0\n    }\n    printf \"FINDING %d login anomalies\\n\", total\n    for (i = 1; i <= nr; i++) print root[i]\n    for (i = 1; i <= no; i++) print odd[i]\n    for (i = 1; i <= np; i++) print pair[i]\n    for (i = 1; i <= nu; i++) print unk[i]\n    exit 1\n  }\n' \"$WORK/known\" \"$WORK/records\" || status=$?\nexit \"$status\"\n```\n"},{"id":"p656","n":"656","slug":"acls-that-override-your-permissions","title":"ACLs that override your permissions","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","CyberSecurity","AccessControl","SysAdmin"],"series_name":"ShellSec","design":"code","script":"acl_audit.sh","date":"2030-12-05","files":{"it":"notes/p656-acls-that-override-your-permissions.md","devsecops":"notes/p656-acls-that-override-your-permissions.md"},"md":"# ACLs that override your permissions\n\nA deploy user needs to write one directory. Six months later getfacl shows three named groups with rwx, a default ACL on the parent, and a mode that looks harmless in ls.\n\n## Quick reference\n\nThe full script is at the end of this note: `acl_audit.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbash acl_audit.sh /tmp/s0f/acl.dump\n```\n\n**Example output**\n\n```text\nWRITE /srv/app group:contractors:rwx\nFINDING 1 ACL risks\n```\n\n## Explained\n\n### What it does\n\n`acl_audit.sh` reads a `getfacl -R` dump, or runs getfacl on the paths you give it, and walks each file block. It flags named users and groups whose effective permissions include write, other:: with write, any default ACL, and named entries that grant a bit the base class does not have.\n\n### What the screen shows\n\nThe thirteen lines read the dump, print one line per risk and exit 1 when the count is above zero. On the dirty fixture: WRITE `/srv/app group:contractors:rwx` and FINDING 1 ACL risks.\n\n### How to run it\n\n`acl_audit.sh acl.dump` for an offline review, or `acl_audit.sh /srv /etc` for a live sweep. Exit 0 is clean, 1 is a risk, 2 is a missing path or no getfacl.\n\n### The fix\n\nRemove the entry with `setfacl -x g:contractors /srv/app`, or reduce it with `setfacl -m g:contractors:r-x`. A default ACL needs a decision; `setfacl -k` removes it. Then chmod back to the mode you meant, because the mask follows.\n\n### Prevention\n\nReview ACL changes beside the chmod. A weekly dump diff catches the entry added to unblock a deploy and never removed.\n\n> **The honest caveat:** a read-only ACL entry is invisible in `ls -l`, and this script looks for write.\n\n> **The lesson:** the mode is a summary; the ACL is the access list.\n\n## In one line\n\n> **The mode is not the whole story.** A named entry can grant what `ls -l` never shows.\n\n**Dump the acls**\n\n```bash\ngetfacl -R -p /srv > acl.dump\n```\n\n## Script: acl_audit.sh\n\nDownload: [scripts/acl_audit.sh](../scripts/acl_audit.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\n# acl_audit.sh - find risky POSIX ACLs in a getfacl dump or a live tree.\n#\n# Fixes: a deploy user gets rwx on one directory, the ACL is never removed,\n# and ls -l still shows a harmless mode. The script flags named entries with\n# effective write, other:: with write, default ACLs, and named entries that\n# widen the base class.\n#\n# Usage:\n#   ./acl_audit.sh /srv /etc\n#   getfacl -R -p /srv > acl.dump && ./acl_audit.sh acl.dump\n#\n# Cron (weekly, Monday 06:20):\n#   20 6 * * 1 root /usr/local/sbin/acl_audit.sh /srv /etc >> /var/log/acl_audit.log 2>&1\n#\n# Exit: 0 clean, 1 risky ACL, 2 usage or unreadable input.\n# A directory is scanned with getfacl -R -p (GNU acl package); a regular file is\n# parsed as a saved dump, so the check also runs on a host without getfacl.\n# The mask is applied before write is called effective. Reads only.\n#\n# By s0sta.com\n\nIFS=$'\\n\\t'\n\nPATHS=()\nWORK=\"\"\n\nusage() {\n  cat <<'EOF'\nUsage: acl_audit.sh [options] [PATH...]\n\nAudit POSIX ACLs from a live tree or a saved getfacl dump.\n\nOptions:\n  -h, --help   show this help and exit\n\nA directory is scanned with getfacl -R -p; a regular file is read as a saved\ndump. With no PATH, the usual service directories are used.\n\nExit: 0 clean, 1 risky ACL, 2 usage or unreadable input.\nBy s0sta.com\nEOF\n}\n\ndie() {\n  printf 'ERROR %s\\n' \"$1\" >&2\n  exit 2\n}\n\nwhile [ \"$#\" -gt 0 ]; do\n  case $1 in\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      while [ \"$#\" -gt 0 ]; do\n        PATHS+=(\"$1\")\n        shift\n      done\n      ;;\n    -*)\n      die \"unknown option: $1\"\n      ;;\n    *)\n      PATHS+=(\"$1\")\n      shift\n      ;;\n  esac\ndone\n\nif [ \"${#PATHS[@]}\" -eq 0 ]; then\n  for p in /etc /srv /opt /var/www; do\n    if [ -d \"$p\" ]; then\n      PATHS+=(\"$p\")\n    fi\n  done\nfi\n\nif [ \"${#PATHS[@]}\" -eq 0 ]; then\n  printf 'SKIP no paths to audit\\n'\n  exit 0\nfi\n\nWORK=$(mktemp -d \"${TMPDIR:-/tmp}/acl_audit.XXXXXX\") || die \"cannot create a work directory\"\ntrap 'rm -rf \"$WORK\"' EXIT\nDUMP=$WORK/acl.dump\n: > \"$DUMP\"\n\nfor p in \"${PATHS[@]}\"; do\n  if [ -d \"$p\" ]; then\n    command -v getfacl >/dev/null 2>&1 || die \"getfacl not installed: pass a saved dump instead\"\n    if ! getfacl -R -p \"$p\" >> \"$DUMP\" 2>/dev/null; then\n      printf 'NOTE getfacl reported errors on %s\\n' \"$p\" >&2\n    fi\n  elif [ -f \"$p\" ]; then\n    [ -r \"$p\" ] || die \"cannot read $p\"\n    cat \"$p\" >> \"$DUMP\"\n  else\n    die \"not a file or directory: $p\"\n  fi\ndone\n\nif [ ! -s \"$DUMP\" ]; then\n  printf 'SKIP no ACL data\\n'\n  exit 0\nfi\n\nstatus=0\nawk '\n  function eff(p, m,   e, i, c) {\n    e = \"\"\n    for (i = 1; i <= 3; i++) {\n      c = substr(\"rwx\", i, 1)\n      if (index(p, c) && (m == \"\" || index(m, c))) e = e c\n      else e = e \"-\"\n    }\n    return e\n  }\n  function extra(e, b,   x) {\n    x = \"\"\n    if (index(e, \"r\") && !index(b, \"r\")) x = x \"r\"\n    if (index(e, \"w\") && !index(b, \"w\")) x = x \"w\"\n    if (index(e, \"x\") && !index(b, \"x\")) x = x \"x\"\n    return x\n  }\n  function flush(   i, e, x) {\n    if (f == \"\") return\n    if (def != \"\") {\n      printf \"DEFAULT %s %s\\n\", f, def\n      n++\n    }\n    for (i = 1; i <= nu; i++) {\n      e = eff(u_perm[i], mask)\n      if (index(e, \"w\")) {\n        printf \"WRITE %s user:%s=%s\\n\", f, u_name[i], e\n        n++\n      } else {\n        x = extra(e, u_base)\n        if (x != \"\") {\n          printf \"WIDEN %s user:%s=%s base=%s\\n\", f, u_name[i], e, u_base\n          n++\n        }\n      }\n    }\n    for (i = 1; i <= ng; i++) {\n      e = eff(g_perm[i], mask)\n      if (index(e, \"w\")) {\n        printf \"WRITE %s group:%s=%s\\n\", f, g_name[i], e\n        n++\n      } else {\n        x = extra(e, g_base)\n        if (x != \"\") {\n          printf \"WIDEN %s group:%s=%s base=%s\\n\", f, g_name[i], e, g_base\n          n++\n        }\n      }\n    }\n    if (index(o_perm, \"w\")) {\n      printf \"OTHER %s other::%s\\n\", f, o_perm\n      n++\n    }\n    f = \"\"; def = \"\"; u_base = \"\"; g_base = \"\"; o_perm = \"\"; mask = \"\"\n    nu = 0; ng = 0\n  }\n  /^# file: / {\n    flush()\n    f = substr($0, 9)\n    next\n  }\n  /^#/ { next }\n  /^default:/ {\n    def = (def == \"\" ? $0 : def \" | \" $0)\n    next\n  }\n  {\n    line = $0\n    sub(/[ \\t]*#.*/, \"\", line)\n    if (line !~ /^(user|group|mask|other):/) next\n    split(line, p, \":\")\n    who = p[1]\n    name = p[2]\n    perms = p[3]\n    if (who == \"mask\") { mask = perms; next }\n    if (who == \"other\") { o_perm = perms; next }\n    if (name == \"\") {\n      if (who == \"user\") u_base = perms\n      else g_base = perms\n      next\n    }\n    if (who == \"user\") {\n      nu++\n      u_name[nu] = name\n      u_perm[nu] = perms\n    } else {\n      ng++\n      g_name[ng] = name\n      g_perm[ng] = perms\n    }\n  }\n  END {\n    flush()\n    if (n == 0) {\n      print \"OK no risky ACLs\"\n      exit 0\n    }\n    printf \"FINDING %d ACL risks\\n\", n\n    exit 1\n  }\n' \"$DUMP\" || status=$?\nexit \"$status\"\n```\n"},{"id":"p657","n":"657","slug":"the-security-sweep","title":"The security sweep","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","DevSecOps","CyberSecurity","Automation","SRE"],"series_name":"ShellSec","design":"architecture","script":"","date":"2030-12-08","files":{"it":"notes/p657-the-security-sweep.md","devsecops":"notes/p657-the-security-sweep.md"},"md":"# The security sweep\n\nA security sweep is not a scanner. It is a loop: collect the evidence the host already has, match it against something you decided in advance, and act once.\n\n## Quick reference\n\n- **Tier 1 · Collect:** SUID, SGID and ACLs (FILES) · Auth and cron logs (LOGS) · Registry images (IMG)\n- **Tier 2 · Match:** SUID baseline (BASE) · Secret patterns (RULE) · CVE allowlist (ALLOW) · Login policy (LOGIN)\n- **Tier 3 · Act:** Patch or remove (FIX) · Rotate the secret (ROT) · Ticket and owner (TKT)\n\n**Request flow:** SUID, SGID and ACLs → SUID baseline → Patch or remove; Auth and cron logs → Secret patterns → Rotate the secret; Auth and cron logs → Login policy → Patch or remove; Registry images → CVE allowlist → Ticket and owner\n\n**Around it:** Cron or timer (CRON) · CI image gate (CI) · Ticket queue (TKT) · Audit trail (AUDIT)\n\n## Explained\n\n### Collect\n\nThree sources cover most host regressions. The filesystem gives setuid and setgid bits and ACLs, the auth and cron logs give logins and secret-shaped lines, and the registry gives the image scan. Each source has a read-only script in this phase, and each one prints a line you can diff.\n\n### Match\n\nFour baselines turn evidence into a decision: the approved setuid set, the secret-name patterns, the CVE allowlist with owners, and the login policy of hours, root and failure counts. Matching is where a sweep earns its keep. Without a baseline, every finding is an opinion.\n\n### Act\n\nA new setuid file is removed or explained, a leaked secret is rotated, a privileged image is rebuilt or allowlisted with an expiry, and an unexpected login is blocked and reviewed. The ticket carries the owner, because the sweep will find the same thing tomorrow if nobody owns it.\n\nAround the tiers, cron or a systemd timer starts the collect step, CI runs the image gate before deploy, the ticket queue receives the act step, and the audit trail keeps the evidence.\n\n> **The honest caveat:** a sweep finds what you taught it to look for, so a new attack path needs a new check.\n\n> **The lesson:** collect is cheap, match is the work, and act is the only tier that changes the risk.\n\n## In one line\n\n> **Collect, match, act.** Three tiers, one loop, and an owner for every finding.\n\n**Start the sweep:** `suid_watch.sh --update /`\n"},{"id":"p658","n":"658","slug":"security-shell-commands","title":"Security shell commands","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","Linux","CommandLine","CyberSecurity","SysAdmin"],"series_name":"ShellSec","design":"cheatsheet","script":"","date":"2030-12-10","files":{"it":"notes/p658-security-shell-commands.md","devsecops":"notes/p658-security-shell-commands.md"},"md":"# Security shell commands\n\nSecurity work on a Linux host is mostly reading state that is already there. These are the commands I reach for, grouped by the question they answer. All of them are read-only and safe on a production host.\n\n## Quick reference\n\n### Users\n\n```bash\nlast -F -i | head -20\nsudo lastb -F -i | head -20\ngetent passwd | cut -d: -f1\nawk -F: '$3==0{print $1}' /etc/passwd\npasswd -S alice\nid -nG alice\n```\n\n### Files\n\n```bash\nfind / -xdev -perm -4000 -type f\nfind / -xdev -perm -2000 -type f\ngetfacl -R -p /srv\nstat -c '%a %U:%G %n' /etc/shadow\nls -l /etc/cron.d/ /etc/cron.daily/\n```\n\n### Logs\n\n```bash\njournalctl -u sshd --since -24h\ngrep -Ei 'fail|invalid' /var/log/auth.log\nausearch -m USER_LOGIN -ts today\nzgrep -i accepted /var/log/auth.log*\njournalctl _COMM=sudo --since today\n```\n\n### Images\n\n```bash\ntrivy image --severity HIGH,CRITICAL $IMG\ntrivy image --format json $IMG\ndocker scout cves nginx:1.27\ncrane digest nginx:1.27\ncosign verify $IMAGE\n```\n\n## Explained\n\n### Users\n\n`last -F -i` shows sessions with a year, lastb shows failed attempts, getent passwd lists accounts, and the awk one-liner prints uid 0 accounts, which is the list that matters. `passwd -S alice` shows whether a password is locked, and `id -nG alice` lists the groups that decide what the account can touch.\n\n### Files\n\n`find -perm -4000` and -2000 list setuid and setgid binaries, `getfacl -R` shows the ACLs ls hides, and `stat -c` prints mode and owner in one line. The -xdev flag keeps the sweep on one filesystem.\n\n### Logs\n\n`journalctl -u sshd` answers \"who tried\", grep on `auth.log` answers \"what failed\", ausearch answers \"what did the audit daemon see\", and zgrep covers the rotated files. journalctl _COMM=sudo answers who used sudo.\n\n### Images\n\ntrivy image scans a tag, crane digest resolves it to a digest, and cosign verify checks the signature before the gate trusts it. A digest is immutable; a tag is a label that can move.\n\n> **The honest caveat:** every one of these reads what the local box can see, and a compromised host can lie to all of them.\n\n> **The lesson:** know the command for the question, and the investigation is shorter.\n\n## In one line\n\n> **Know the state before you change it.** A short list per question beats fifty you cannot recall.\n\n**A first look**\n\n```bash\nlast -F -i; df -h; free -h\n```\n"},{"id":"p659","n":"659","slug":"four-security-regressions","title":"Four security regressions","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","Linux","CyberSecurity","DevSecOps","SecOps"],"series_name":"ShellSec","design":"explainer","script":"","date":"2030-12-12","files":{"it":"notes/p659-four-security-regressions.md","devsecops":"notes/p659-four-security-regressions.md"},"md":"# Four security regressions\n\nFour regressions cover most of the security tickets I see on Linux hosts, and each one has a shell check that fits in a cron line.\n\n## Quick reference\n\n### New SUID binary\n\n- Mode 4755 and owned by root\n- A package upgrade or an intruder\n- A baseline turns it into a diff\n\n### Leaked secret\n\n- A password in `/etc/cron.d/db`\n- Readable by the service account\n- Mask the value, then rotate it\n\n### Privileged image\n\n- CRITICAL published after the build\n- The base runs as root by default\n- Gate HIGH and CRITICAL in CI\n\n### Unexpected login\n\n- Root at 02:14 from a new address\n- Three failures, then a success\n- last and lastb already know\n\n> **Note:** A regression is a change you did not approve. The sweep is a diff, not a score.\n\n## Explained\n\n### New SUID binary\n\nA package upgrade or an intruder drops a root-owned file with the setuid bit. Often 4755, it runs as root, and nothing in the deploy log mentions it. A baseline of the setuid set turns it into a diff, and the diff names the file.\n\n### Leaked secret\n\nA cron job needs a database password, so it lives in `/etc/cron.d/db` or a .env file next to the app. The file is readable by the service account, copied into a backup, and pasted into a ticket. A scan that masks the value finds the line without repeating the secret.\n\n### Privileged image\n\nThe base image has a CRITICAL CVE that was published after the build, or the Dockerfile runs as root with a writable filesystem. A gate that fails on HIGH and CRITICAL, with an allowlist that has owners and expiry dates, keeps the exception visible.\n\n### Unexpected login\n\nRoot at 02:14 from a new address, or three failures followed by a success. last and lastb already have the evidence; the check is what turns it into a finding.\n\n> **The honest caveat:** a check only catches the regression you taught it, and an attacker who knows the sweep can avoid all four.\n\n> **The lesson:** a regression is a change nobody approved, and a diff is how you see it.\n\n## In one line\n\n> **A change you did not approve.** Four regressions, four checks, one cron line each.\n\n**Check one host:** `audit_logins.sh`\n"},{"id":"p660","n":"660","slug":"bash-script-vs-security-tool","title":"Bash script vs security tool","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","DevSecOps","CyberSecurity","Automation","SecOps"],"series_name":"ShellSec","design":"compare","script":"","date":"2030-12-15","files":{"it":"notes/p660-bash-script-vs-security-tool.md","devsecops":"notes/p660-bash-script-vs-security-tool.md"},"md":"# Bash script vs security tool\n\nEvery security team argues: write the check in bash, or buy the tool. The honest answer is that they answer different questions.\n\n## Quick reference\n\n| | **BASH** | **TOOL** |\n|:--|:--|:--|\n| Install | Already on the box | Agent, licence, rollout |\n| Scope | The check you wrote | The catalogue it ships |\n| Noise | Yours to tune | Rules, not your context |\n| Evidence | stdout and exit code | Dashboard and tickets |\n| Speed | Seconds, no daemon | Minutes, a pipeline |\n| Best for | One host, one question | Fleet, audit, coverage |\n\n**BASH** (The check you wrote): Runs on the host you are on, in the minute you think of the check.\n\n```bash\nfind / -perm -4000 -type f\n```\n\n**TOOL** (The catalogue you buy): Ships hundreds of rules, a fleet view and an audit trail.\n\n```bash\ntrivy image api:1.4\n```\n\n- Bash → the host you are on, now.\n- Tool → the fleet, the audit, the rules.\n\n## Explained\n\n### The script\n\nBash is already on the box. It reads the files and logs the host exposes, applies the rule you wrote, prints a line and exits non-zero. It has no agent, no database and no licence, and it runs in the same minute you think of the check. The cost is that you own the rule, the false positives and the maintenance.\n\n### The tool\n\nA security tool ships a catalogue: hundreds of rules, a parser per format, a fleet view and an audit trail. It knows the checks you did not think of, and it keeps the evidence in one place. The cost is the agent, the licence, the rollout and the tuning, and the rule you need may still be missing.\n\n### Where they meet\n\nThe rows line up the trade. Bash is best at one host and one question, the thing you need at 03:00. A tool is best at fleet coverage and audit, the thing you need at quarter end. Neither replaces the other: the script catches the host-specific regression the tool has no rule for, and the tool catches the class of problem the script was never written to see.\n\n> **The honest caveat:** a script with no owner rots faster than a tool with no licence.\n\n> **The lesson:** use the tool for coverage, and write the script for the question only your host can ask.\n\nCover the fleet.  \nScript the edge.\n"},{"id":"p661","n":"661","slug":"quiz-3-security-questions-2","title":"Quiz: 3 security questions","sections":["it","devsecops"],"topic":"Security & hardening","tags":["Bash","CyberSecurity","DevSecOps","SecOps","Linux"],"series_name":"ShellSec","design":"quiz","script":"","date":"2030-12-17","files":{"it":"notes/p661-quiz-3-security-questions-2.md","devsecops":"notes/p661-quiz-3-security-questions-2.md"},"md":"# Quiz: 3 security questions\n\nPhase four ends with three questions. Each one comes from a script in this phase, and each answer is a design decision, not a command.\n\n## Quick reference\n\n**Q1. Why does a setuid file need a baseline?**\n\n- a) Because the bit is rare\n- b) Because the change is what matters\n- c) Because root owns the file\n\n> **Answer:** b) Because the change is what matters. The bit is common; a new one is the finding. A list of approved files is the only definition of normal, so the sweep can tell an install from an intrusion.\n\n**Q2. Why mask a secret in the scan output?**\n\n- a) Because the value is long\n- b) Because the report travels\n- c) Because the file is binary\n\n> **Answer:** b) Because the report travels. The report goes to a terminal, a CI log, a ticket and a screenshot. Printing the value copies the secret into each of them, so the scanner becomes the leak.\n\n**Q3. Why does an allowlist need an owner?**\n\n- a) Because CVEs expire\n- b) Because an unowned exception never ends\n- c) Because trivy needs a name\n\n> **Answer:** b) Because an unowned exception never ends. An exception without an owner is permanent by default. An owner and an expiry make it a decision that gets reviewed, not a line nobody remembers adding.\n\n## Explained\n\n### Question one\n\nWhy does a setuid file need a baseline? Because the bit is not rare; the change is. A package upgrade adds one, an admin adds one to fix a permission error, and an intruder adds one for persistence. Without a list of what was approved yesterday, the sweep cannot tell today's file from last year's. The baseline is the definition of normal.\n\n### Question two\n\nWhy mask a secret in the scan output? Because the output travels: to a terminal, a CI log, a ticket, a screenshot. A scan that prints the value copies the secret into every one of those places, and the report becomes the leak. Printing the key and four stars is enough to find the line and fix it.\n\n### Question three\n\nWhy does an allowlist need an owner? Because an exception with no owner is permanent. A CVE that did not apply at build time may apply after the next dependency, and only a person who knows the context can decide. The owner and the expiry date are what make the exception a decision instead of a habit.\n\n> **The honest caveat:** knowing the answers does not run the sweep; the cron line does.\n\n> **The lesson:** a security check is a decision written down, and the writing is the hard part.\n\n## In one line\n\n> **Decide once, check forever.** Three answers, and one cron line that keeps them true.\n\n**Run the sweep:** `suid_watch.sh /`\n"},{"id":"p662","n":"662","slug":"retention-that-never-ran","title":"Retention that never ran","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","SysAdmin","Automation","Storage"],"series_name":"ShellSec","design":"code","script":"retention_cleanup.sh","date":"2030-12-19","files":{"it":"notes/p662-retention-that-never-ran.md","devsecops":"notes/p662-retention-that-never-ran.md"},"md":"# Retention that never ran\n\nA retention policy says thirty days. The volume fills, because the policy is a paragraph in a wiki and the deletion was a ticket someone closed in 2024.\n\n## Quick reference\n\nThe full script is at the end of this note: `retention_cleanup.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nretention_cleanup.sh --path /var/log/app --days 30\n```\n\n**Example output**\n\n```text\nMATCH 3 files, 13.8MB reclaimable\nRM app-2026-07.log 80d 12.0MB\n```\n\n## Explained\n\n### What it does\n\n`retention_cleanup.sh` scans one path, keeps regular files older than `--days`, and prints the oldest ten with their age and size, then the total reclaimable. The default is a dry run. `--apply` deletes, and `--max-delete` refuses a plan larger than 10,000 files.\n\n### What the screen shows\n\nThe fourteen lines read the path and the age, collect matching files with find, sort them oldest first, and sum the bytes. On the fixture they print MATCH 3 files, 13.8MB reclaimable, then RM `app-2026-07.log 80d 12.0MB`.\n\n### How to run it\n\n`retention_cleanup.sh --path /var/log/app --days 30`, then the same line with `--apply`. Exit 0 means nothing matched, 1 means a plan or a failed deletion, 2 means bad input. The cron line in the header runs the dry run on Monday and the apply at 03:15.\n\n### The fix\n\nRead the plan before the apply. The oldest file names the producer: a logrotate rule that stopped matching, an application that writes its own dated files, a backup that never expires. Fix the producer, then let the job delete.\n\n### Prevention\n\nAlert when the reclaimable total grows for three nights. A retention job that suddenly has more to delete is a producer that changed, not a disk that grew.\n\n> **The honest caveat:** the scan stays on one filesystem, so a volume mounted under the path is invisible.\n\n> **The lesson:** a retention policy is not a cron job until it runs.\n\n## In one line\n\n> **Retention is a verb.** A policy with no job behind it is a hope with a date.\n\n**Plan before delete:** `retention_cleanup.sh --path /var`\n\n## Script: retention_cleanup.sh\n\nDownload: [scripts/retention_cleanup.sh](../scripts/retention_cleanup.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# retention_cleanup.sh - plan and apply age-based file retention.\n#\n# Fixes: a retention rule that lives in a wiki and never ran, so the volume\n# fills one old log at a time until a write fails at 03:00.\n#\n# The default is a dry run: it prints every file it would delete, the oldest\n# first, and the total reclaimable. Nothing is removed until --apply.\n#\n# Usage:\n#   retention_cleanup.sh --path /var/log/app --days 30\n#   retention_cleanup.sh --path /var/log/app --days 30 --apply\n#   retention_cleanup.sh --path /var/log/app --days 30 --pattern '*.log'\n#\n# Cron (report every Monday at 06:00, apply every night at 03:15):\n#   0 6 * * 1 /usr/local/bin/retention_cleanup.sh --path /var/log/app \\\n#     --days 30 >> /var/log/retention.log 2>&1\n#   15 3 * * * /usr/local/bin/retention_cleanup.sh --path /var/log/app \\\n#     --days 30 --apply >> /var/log/retention.log 2>&1\n#\n# Exit codes:\n#   0  nothing matched (dry run) or every planned file was deleted (apply)\n#   1  files matched (dry run) or at least one deletion failed (apply)\n#   2  usage error, unreadable path, or a failed scan\n#\n# Requires GNU find (find -printf), GNU sort (sort -z) and numfmt\n# (coreutils). Tested on a fixture tree in /tmp: clean exit 0, dirty exit 1,\n# missing args exit 2.\n# By s0sta.com\n\nPROG=${0##*/}\npath=\"\"\ndays=30\npattern=\"*\"\nmax_delete=10000\napply=0\ntmp=\"\"\n\nusage() {\n  cat <<'EOF'\nretention_cleanup.sh - plan and apply age-based file retention\n\nUsage:\n  retention_cleanup.sh --path DIR --days N [options]\n\nOptions:\n  --path DIR        directory to scan (required, not / and not a symlink)\n  --days N          age in whole days; default 30\n  --pattern GLOB    only files whose name matches GLOB; default '*'\n  --max-delete N    refuse to apply more than N deletions; default 10000\n  --apply           delete the files; without it nothing is removed\n  -h, --help        show this help and exit 0\n\nExamples:\n  retention_cleanup.sh --path /var/log/app --days 30\n  retention_cleanup.sh --path /var/log/app --days 30 --pattern '*.log'\n  retention_cleanup.sh --path /var/log/app --days 30 --apply\n\nExit: 0 nothing to do, 1 files matched or a deletion failed, 2 usage.\nEOF\n}\n\ndie() {\n  printf '%s: %s\\n' \"$PROG\" \"$1\" >&2\n  exit 2\n}\n\n# shellcheck disable=SC2329  # called by the EXIT trap\ncleanup() {\n  if [[ -n $tmp ]]; then\n    rm -f -- \"$tmp\"\n  fi\n}\ntrap cleanup EXIT\n\nfmt_bytes() {\n  if (($1 == 0)); then\n    printf '0B'\n    return\n  fi\n  numfmt --to=iec --suffix=B --format='%.1f' -- \"$1\"\n}\n\nwhile (($#)); do\n  case $1 in\n    --path)\n      [[ $# -ge 2 ]] || die \"--path needs a directory\"\n      path=$2\n      shift 2\n      ;;\n    --days)\n      [[ $# -ge 2 ]] || die \"--days needs a number\"\n      days=$2\n      shift 2\n      ;;\n    --pattern)\n      [[ $# -ge 2 ]] || die \"--pattern needs a glob\"\n      pattern=$2\n      shift 2\n      ;;\n    --max-delete)\n      [[ $# -ge 2 ]] || die \"--max-delete needs a number\"\n      max_delete=$2\n      shift 2\n      ;;\n    --apply)\n      apply=1\n      shift\n      ;;\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      break\n      ;;\n    -*)\n      die \"unknown option: $1 (try --help)\"\n      ;;\n    *)\n      die \"unexpected argument: $1 (try --help)\"\n      ;;\n  esac\ndone\n\n[[ -n $path ]] || die \"--path is required (try --help)\"\n[[ $days =~ ^[0-9]+$ ]] || die \"--days must be a whole number\"\n((days >= 1)) || die \"--days must be at least 1\"\n[[ $max_delete =~ ^[0-9]+$ ]] || die \"--max-delete must be a whole number\"\n((max_delete >= 1)) || die \"--max-delete must be at least 1\"\n[[ -d $path ]] || die \"not a directory: $path\"\nif [[ -L $path ]]; then\n  die \"refusing a symlink path: $path\"\nfi\nwhile [[ $path == */ && $path != \"/\" ]]; do\n  path=${path%/}\ndone\n[[ -n $path ]] || die \"refusing to scan /\"\n[[ -r $path ]] || die \"cannot read: $path\"\ncommand -v find >/dev/null 2>&1 || die \"find is not installed\"\ncommand -v sort >/dev/null 2>&1 || die \"sort is not installed\"\ncommand -v numfmt >/dev/null 2>&1 || die \"numfmt is not installed (coreutils)\"\n\ntmp=$(mktemp \"${TMPDIR:-/tmp}/retention.XXXXXX\") || die \"mktemp failed\"\nif ! find \"$path\" -xdev -type f -mtime +\"$days\" -name \"$pattern\" \\\n      -printf '%T@ %s %p\\0' | sort -z -n >\"$tmp\"; then\n  die \"scan failed under: $path\"\nfi\n\nnow=$(date +%s)\ntotal=0\ncount=0\nwhile IFS= read -r -d '' rec; do\n  rest=${rec#* }\n  size=${rest%% *}\n  count=$((count + 1))\n  total=$((total + size))\ndone <\"$tmp\"\n\nprintf 'MODE %s\\n' \"$([[ $apply -eq 1 ]] && printf apply || printf dry-run)\"\nprintf 'PATH %s\\n' \"$path\"\nprintf 'OLDER %d days\\n' \"$days\"\nprintf 'MATCH %d files, %s reclaimable\\n' \"$count\" \"$(fmt_bytes \"$total\")\"\n\nif ((count == 0)); then\n  printf 'OK nothing older than %d days under %s\\n' \"$days\" \"$path\"\n  exit 0\nfi\n\nshown=0\nwhile IFS= read -r -d '' rec; do\n  if ((shown >= 10)); then\n    break\n  fi\n  stamp=${rec%% *}\n  rest=${rec#* }\n  size=${rest%% *}\n  file=${rest#* }\n  age=$(((now - ${stamp%%.*}) / 86400))\n  rel=${file#\"$path\"/}\n  printf 'RM %s %dd %s\\n' \"$rel\" \"$age\" \"$(fmt_bytes \"$size\")\"\n  shown=$((shown + 1))\ndone <\"$tmp\"\n\nif ((count > shown)); then\n  printf 'MORE %d files not listed\\n' \"$((count - shown))\"\nfi\n\nif ((apply == 0)); then\n  printf 'DRY-RUN nothing deleted. Add --apply to delete.\\n'\n  exit 1\nfi\n\nif ((count > max_delete)); then\n  printf 'REFUSE %d files exceeds --max-delete %d; nothing deleted\\n' \\\n    \"$count\" \"$max_delete\"\n  exit 1\nfi\n\nremoved=0\nfailed=0\nfreed=0\nwhile IFS= read -r -d '' rec; do\n  rest=${rec#* }\n  size=${rest%% *}\n  file=${rest#* }\n  if rm -f -- \"$file\"; then\n    removed=$((removed + 1))\n    freed=$((freed + size))\n  else\n    failed=$((failed + 1))\n    printf 'FAIL could not delete %s\\n' \"$file\" >&2\n  fi\ndone <\"$tmp\"\n\nprintf 'DELETED %d files, %s freed\\n' \"$removed\" \"$(fmt_bytes \"$freed\")\"\nif ((failed > 0)); then\n  printf 'FAIL %d deletions failed\\n' \"$failed\"\n  exit 1\nfi\nexit 0\n```\n"},{"id":"p663","n":"663","slug":"a-backup-that-copied-nothing","title":"A backup that copied nothing","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Backup","Linux","DevOps","DataProtection"],"series_name":"ShellSec","design":"code","script":"backup_sync.sh","date":"2030-12-22","files":{"it":"notes/p663-a-backup-that-copied-nothing.md","devsecops":"notes/p663-a-backup-that-copied-nothing.md"},"md":"# A backup that copied nothing\n\nThe backup job exits 0 for eleven nights. The target holds 0 of the 1,842 files, because the job still writes to the volume's old mount point. rsync copies into a directory nobody reads, and nothing pages.\n\n## Quick reference\n\nThe full script is at the end of this note: `backup_sync.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nbackup_sync.sh --source /srv --target /backup\n```\n\n**Example output**\n\n```text\nDRIFT target 0/1842 files, 0B/48.0MB\nPLAN rsync 1842 files, 48.0MB\n```\n\n## Explained\n\n### What it does\n\n`backup_sync.sh` measures both trees, refuses a `source below --min-files`, and runs rsync with `--dry-run` and `--itemize-changes` before it copies anything. It compares file counts and bytes against the source, prints the drift, then waits for `--apply` to sync. One run measures, copies and verifies again.\n\n### What the screen shows\n\nThe fourteen lines count each tree with find, run the rsync dry run, and print the drift before the plan. On the fixture: DRIFT target 0/1842 files, 0B/48.0MB, then PLAN rsync 1842 files, 48.0MB.\n\n### How to run it\n\n`backup_sync.sh --source /srv/config --target /backup/config --min-files 10`. Add `--apply` to copy. Exit 0 means the trees match, 1 means drift or a failed copy, 2 means bad input. The header carries the cron line.\n\n### The fix\n\nPoint the job at the mounted target, run it once, and read the drift. If the counts still differ, extra files need `--delete`, and a permission error leaves the target short.\n\n### Prevention\n\nVerify after the copy, in the same run. Keep `--min-files` in the cron line: an empty source is a missing mount, not a clean backup.\n\n> **The honest caveat:** counts and bytes cover regular files; a symlink or an ownership change needs the itemized output.\n\n> **The lesson:** copied is not checked.\n\n## In one line\n\n> **Copied is not checked.** rsync exit 0 proves the command ran, not that the target matches.\n\n**Count both trees:** `backup_sync.sh --source /srv --target /b`\n\n## Script: backup_sync.sh\n\nDownload: [scripts/backup_sync.sh](../scripts/backup_sync.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# backup_sync.sh - verify a mirror before and after rsync copies it.\n#\n# Fixes: a nightly backup that exits 0 and copies nothing. The job proves\n# rsync ran, not that the target matches the source, so a wrong path or an\n# unmounted volume stays invisible until the day someone restores.\n#\n# The default is a dry run: it measures both trees, runs rsync with\n# --dry-run, and reports the drift. --apply performs the copy and then\n# measures again. --delete makes the target a mirror (use with care).\n#\n# Usage:\n#   backup_sync.sh --source /srv/config --target /backup/config\n#   backup_sync.sh --source /srv/config --target /backup/config --apply\n#   backup_sync.sh --source /srv/config --target /backup/config --apply --delete\n#\n# Cron (one run measures, copies and verifies again at 02:30):\n#   30 2 * * * /usr/local/bin/backup_sync.sh --source /srv/config \\\n#     --target /backup/config --min-files 10 --apply >> /var/log/backup.log 2>&1\n#\n# Exit codes:\n#   0  target matches source (dry run) or the copy verified (apply)\n#   1  drift, source below --min-files, or the copy did not verify\n#   2  usage error, unreadable tree, or rsync missing\n#\n# Requires rsync, GNU find (find -printf) and numfmt (coreutils). The target\n# must already exist, so a dry run never creates a directory. Tested on clean\n# and dirty fixture trees in /tmp; services are not touched.\n# By s0sta.com\n\nPROG=${0##*/}\nsrc=\"\"\ndst=\"\"\nmin_files=1\napply=0\ndelete=0\ndry_out=\"\"\n\nusage() {\n  cat <<'EOF'\nbackup_sync.sh - verify a mirror before and after rsync copies it\n\nUsage:\n  backup_sync.sh --source DIR --target DIR [options]\n\nOptions:\n  --source DIR      directory to copy from (read only)\n  --target DIR      directory to copy to (must exist)\n  --min-files N     fail if the source has fewer than N files; default 1\n  --delete          delete target files missing from the source (mirror)\n  --apply           run rsync; without it the dry run only reports drift\n  -h, --help        show this help and exit 0\n\nExamples:\n  backup_sync.sh --source /srv/config --target /backup/config\n  backup_sync.sh --source /srv/config --target /backup/config --apply\n  backup_sync.sh --source /srv/config --target /backup/config --apply --delete\n\nExit: 0 in sync, 1 drift or failed copy, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n  printf '%s: %s\\n' \"$PROG\" \"$1\" >&2\n  exit 2\n}\n\n# shellcheck disable=SC2329  # called by the EXIT trap\ncleanup() {\n  if [[ -n $dry_out ]]; then\n    rm -f -- \"$dry_out\"\n  fi\n}\ntrap cleanup EXIT\n\nfmt_bytes() {\n  if (($1 == 0)); then\n    printf '0B'\n    return\n  fi\n  numfmt --to=iec --suffix=B --format='%.1f' -- \"$1\"\n}\n\nn_files() {\n  if (($1 == 1)); then\n    printf '1 file'\n  else\n    printf '%d files' \"$1\"\n  fi\n}\n\nmeasure() {\n  local dir=$1 n b\n  n=$(find \"$dir\" -xdev -type f -printf '.' | wc -c) || return 1\n  n=${n// /}\n  b=$(find \"$dir\" -xdev -type f -printf '%s\\n' |\n    awk '{ total += $1 } END { printf \"%.0f\", total + 0 }') || return 1\n  printf '%s %s\\n' \"$n\" \"$b\"\n}\n\nwhile (($#)); do\n  case $1 in\n    --source)\n      [[ $# -ge 2 ]] || die \"--source needs a directory\"\n      src=$2\n      shift 2\n      ;;\n    --target)\n      [[ $# -ge 2 ]] || die \"--target needs a directory\"\n      dst=$2\n      shift 2\n      ;;\n    --min-files)\n      [[ $# -ge 2 ]] || die \"--min-files needs a number\"\n      min_files=$2\n      shift 2\n      ;;\n    --apply)\n      apply=1\n      shift\n      ;;\n    --delete)\n      delete=1\n      shift\n      ;;\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      break\n      ;;\n    -*)\n      die \"unknown option: $1 (try --help)\"\n      ;;\n    *)\n      die \"unexpected argument: $1 (try --help)\"\n      ;;\n  esac\ndone\n\n[[ -n $src ]] || die \"--source is required (try --help)\"\n[[ -n $dst ]] || die \"--target is required (try --help)\"\n[[ $min_files =~ ^[0-9]+$ ]] || die \"--min-files must be a whole number\"\n[[ -d $src ]] || die \"source is not a directory: $src\"\n[[ -d $dst ]] || die \"target is not a directory: $dst\"\n[[ -r $src ]] || die \"cannot read source: $src\"\n[[ -r $dst ]] || die \"cannot read target: $dst\"\ncommand -v rsync >/dev/null 2>&1 || die \"rsync is not installed\"\ncommand -v find >/dev/null 2>&1 || die \"find is not installed\"\ncommand -v numfmt >/dev/null 2>&1 || die \"numfmt is not installed (coreutils)\"\n\nif [[ $src -ef $dst ]]; then\n  die \"source and target are the same directory\"\nfi\n\nsrc_info=$(measure \"$src\") || die \"cannot measure source: $src\"\nsrc_files=${src_info%% *}\nsrc_bytes=${src_info#* }\ndst_info=$(measure \"$dst\") || die \"cannot measure target: $dst\"\ndst_files=${dst_info%% *}\ndst_bytes=${dst_info#* }\n\nprintf 'MODE %s\\n' \"$([[ $apply -eq 1 ]] && printf apply || printf dry-run)\"\nprintf 'SOURCE %s: %d files, %s\\n' \"$src\" \"$src_files\" \"$(fmt_bytes \"$src_bytes\")\"\nprintf 'TARGET %s: %d files, %s\\n' \"$dst\" \"$dst_files\" \"$(fmt_bytes \"$dst_bytes\")\"\n\nif ((src_files < min_files)); then\n  printf 'FAIL source has %s, below --min-files %d\\n' \\\n    \"$(n_files \"$src_files\")\" \"$min_files\"\n  printf 'FAIL refusing to call an empty source a good backup\\n'\n  exit 1\nfi\n\ndry_out=$(mktemp \"${TMPDIR:-/tmp}/backup_sync.XXXXXX\") || die \"mktemp failed\"\n\nrargs=(-a --itemize-changes --stats)\nif ((delete == 1)); then\n  rargs+=(--delete)\nfi\n\nif ! rsync \"${rargs[@]}\" --dry-run -- \"$src\"/ \"$dst\"/ >\"$dry_out\" 2>&1; then\n  printf 'FAIL rsync dry run failed\\n'\n  tail -n 5 \"$dry_out\" >&2\n  exit 1\nfi\n\ntransferred=$(awk -F': *' '/Number of (regular )?files transferred/ {\n  value = $2; gsub(/,/, \"\", value); print value\n}' \"$dry_out\" | tail -n 1)\nif [[ ! $transferred =~ ^[0-9]+$ ]]; then\n  transferred=0\nfi\n\nxfer_bytes=$(awk -F': *' '/Total transferred file size/ {\n  value = $2; sub(/ .*/, \"\", value); gsub(/,/, \"\", value); print value\n}' \"$dry_out\" | tail -n 1)\nif [[ ! $xfer_bytes =~ ^[0-9]+$ ]]; then\n  xfer_bytes=0\nfi\n\nplan_files=$transferred\nif ((plan_files == 0)); then\n  plan_files=$((src_files - dst_files))\n  if ((plan_files < 0)); then\n    plan_files=0\n  fi\nfi\nplan_bytes=$((src_bytes - dst_bytes))\nif ((plan_bytes < 0)); then\n  plan_bytes=0\nfi\nif ((plan_bytes == 0 && xfer_bytes > 0)); then\n  plan_bytes=$xfer_bytes\nfi\n\nif ((src_files == dst_files && src_bytes == dst_bytes && transferred == 0)); then\n  printf 'DRIFT none: target matches source\\n'\n  printf 'OK %d files, %s in sync\\n' \"$src_files\" \"$(fmt_bytes \"$src_bytes\")\"\n  exit 0\nfi\n\nprintf 'DRIFT target %d/%d files, %s/%s\\n' \\\n  \"$dst_files\" \"$src_files\" \"$(fmt_bytes \"$dst_bytes\")\" \"$(fmt_bytes \"$src_bytes\")\"\nprintf 'PLAN rsync %s, %s\\n' \"$(n_files \"$plan_files\")\" \"$(fmt_bytes \"$plan_bytes\")\"\nif ((dst_files > src_files)); then\n  printf 'EXTRA target holds %s missing from source\\n' \\\n    \"$(n_files \"$((dst_files - src_files))\")\"\n  if ((delete == 0)); then\n    printf 'NOTE rerun with --delete to mirror, or leave the extra files\\n'\n  fi\nfi\n\nif ((apply == 0)); then\n  printf 'DRY-RUN nothing copied. Add --apply to sync.\\n'\n  exit 1\nfi\n\nstart=$(date +%s)\nif ! rsync \"${rargs[@]}\" -- \"$src\"/ \"$dst\"/ >\"$dry_out\" 2>&1; then\n  printf 'FAIL rsync copy failed\\n'\n  tail -n 5 \"$dry_out\" >&2\n  exit 1\nfi\nelapsed=$(( $(date +%s) - start ))\n\ndst_info=$(measure \"$dst\") || die \"cannot measure target after copy: $dst\"\ndst_files=${dst_info%% *}\ndst_bytes=${dst_info#* }\n\nprintf 'SYNCED %s in %ds\\n' \"$(n_files \"$plan_files\")\" \"$elapsed\"\nif ((src_files == dst_files && src_bytes == dst_bytes)); then\n  printf 'VERIFY target matches source: %s, %s\\n' \\\n    \"$(n_files \"$dst_files\")\" \"$(fmt_bytes \"$dst_bytes\")\"\n  exit 0\nfi\nprintf 'FAIL target still differs: %d/%d files, %s/%s\\n' \\\n  \"$dst_files\" \"$src_files\" \"$(fmt_bytes \"$dst_bytes\")\" \"$(fmt_bytes \"$src_bytes\")\"\nexit 1\n```\n"},{"id":"p664","n":"664","slug":"the-cron-job-that-overlapped","title":"The cron job that overlapped","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Cron","Linux","SRE","Automation"],"series_name":"ShellSec","design":"code","script":"cron_wrapper.sh","date":"2030-12-24","files":{"it":"notes/p664-the-cron-job-that-overlapped.md","devsecops":"notes/p664-the-cron-job-that-overlapped.md"},"md":"# The cron job that overlapped\n\nThe job takes twelve minutes and the cron line starts it every five. At 03:05 the second run opens the same output directory while the first is writing, and both exit 0 with half a ledger.\n\n## Quick reference\n\nThe full script is at the end of this note: `cron_wrapper.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\ncron_wrapper.sh -- backup & cron_wrapper.sh -- backup\n```\n\n**Example output**\n\n```text\nSKIP backup: lock held, exit 75\nLOCK backup.lock\n```\n\n## Explained\n\n### What it does\n\n`cron_wrapper.sh` takes an exclusive flock before the command starts. A run that cannot get the lock prints the skip and exits 75 without touching data. Inside the lock it stamps a log, runs the command under timeout, and calls `--alert` on a non-zero exit. `--print-cron` emits the line to paste.\n\n### What the screen shows\n\nThe thirteen lines open the lock file, try flock -n, and print the skip when another run holds it. On the fixture: SKIP backup: lock held, exit 75, then LOCK backup.lock.\n\n### How to run it\n\n`cron_wrapper.sh -n` backup `--timeout 900 --alert /usr/local/bin/page.sh` -- `/usr/local/bin/backup.sh`. Use `--print-cron` for the crontab line. Exit 0 is success, the command's code is passed through, 124 is the timeout, 75 is a skipped run, 2 is usage.\n\n### The fix\n\nPut the wrapper between cron and the job, and set the timeout below the interval. A twelve-minute job on a five-minute timer needs a lock, not a faster disk.\n\n### Prevention\n\nWatch exit 75. One skip a week is a warning that the job is growing; a skip every run means the timeout and the interval disagree.\n\n> **The honest caveat:** flock is advisory, so a process that never opens the lock file can still write.\n\n> **The lesson:** cron does not wait for the last run; the wrapper does.\n\n## In one line\n\n> **One lock, one writer.** Cron will start the next run whether or not the last one ended.\n\n**Wrap the job:** `cron_wrapper.sh --name` backup -- job\n\n## Script: cron_wrapper.sh\n\nDownload: [scripts/cron_wrapper.sh](../scripts/cron_wrapper.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# cron_wrapper.sh - run one cron job at a time, with a log, a timeout and an alert.\n#\n# Fixes: the 5-minute job that takes 12 minutes. The next run starts while the\n# first still holds the data, both write, and neither knows. A second failure\n# hides in the same place: the job dies, cron mails stdout to nobody, and the\n# next run looks exactly like the last good one.\n#\n# The wrapper takes an exclusive flock before the command starts, stamps every\n# log line, kills the command after --timeout seconds, and calls --alert on a\n# non-zero exit. A run that cannot get the lock exits 75 without starting work.\n#\n# Usage:\n#   cron_wrapper.sh --name backup --timeout 900 -- /usr/local/bin/backup.sh\n#   cron_wrapper.sh --name backup --print-cron -- /usr/local/bin/backup.sh\n#\n# Cron (the line --print-cron emits, pasted into crontab -e):\n#   */5 * * * * /usr/local/bin/cron_wrapper.sh --name backup \\\n#     --timeout 900 --alert /usr/local/bin/page.sh -- /usr/local/bin/backup.sh\n#\n# Exit codes:\n#   0    the command finished successfully\n#   1-125 the command's own exit code (124 means timeout killed it)\n#   75   another run still holds the lock; nothing was started\n#   2    usage error, missing tool, or an unwritable log/lock path\n#\n# Requires flock (util-linux), timeout (coreutils) and date. Tested with two\n# overlapping runs on a fixture in /tmp; no real service is touched.\n# By s0sta.com\n\nPROG=${0##*/}\nname=\"\"\nlock=\"\"\nlog=\"\"\ntimeout_s=3600\nalert=\"\"\nschedule=\"*/5 * * * *\"\nprint_cron=0\nlog_set=0\nlock_set=0\nrun_tmp=\"\"\n\nusage() {\n  cat <<'EOF'\ncron_wrapper.sh - run one cron job at a time, with a log, a timeout and an alert\n\nUsage:\n  cron_wrapper.sh [options] -- COMMAND [ARGS...]\n\nOptions:\n  -n, --name NAME   label for the log, lock and messages (default: command name)\n  --lock FILE       lock file; default /run/lock/NAME.lock, or $TMPDIR if unwritable\n  --log FILE        log file; default /var/log/NAME.log, or $TMPDIR if unwritable\n  --timeout SECS    kill the command after SECS seconds; default 3600\n  --alert CMD       executable called with one message when the command fails\n  --schedule EXPR   five-field cron expression used by --print-cron\n  --print-cron      print the crontab line and exit 0\n  -h, --help        show this help and exit 0\n\nThe command must come after --. Example:\n  cron_wrapper.sh --name backup --timeout 900 -- /usr/local/bin/backup.sh\n\nExit: 0 success, the command's code on failure, 124 timeout, 75 lock held.\nEOF\n}\n\ndie() {\n  printf '%s: %s\\n' \"$PROG\" \"$1\" >&2\n  exit 2\n}\n\n# shellcheck disable=SC2329  # called by the EXIT trap\ncleanup() {\n  if [[ -n $run_tmp ]]; then\n    rm -f -- \"$run_tmp\"\n  fi\n}\ntrap cleanup EXIT\n\nts() {\n  date -u +%Y-%m-%dT%H:%M:%SZ\n}\n\nlogline() {\n  printf '%s %s\\n' \"$(ts)\" \"$1\" >>\"$log\"\n}\n\ndefault_dir() {\n  if [[ -d $1 && -w $1 ]]; then\n    printf '%s' \"$1\"\n  else\n    printf '%s' \"${TMPDIR:-/tmp}\"\n  fi\n}\n\nwhile (($#)); do\n  case $1 in\n    -n|--name)\n      [[ $# -ge 2 ]] || die \"--name needs a value\"\n      name=$2\n      shift 2\n      ;;\n    --lock)\n      [[ $# -ge 2 ]] || die \"--lock needs a file\"\n      lock=$2\n      lock_set=1\n      shift 2\n      ;;\n    --log)\n      [[ $# -ge 2 ]] || die \"--log needs a file\"\n      log=$2\n      log_set=1\n      shift 2\n      ;;\n    --timeout)\n      [[ $# -ge 2 ]] || die \"--timeout needs a number of seconds\"\n      timeout_s=$2\n      shift 2\n      ;;\n    --alert)\n      [[ $# -ge 2 ]] || die \"--alert needs an executable\"\n      alert=$2\n      shift 2\n      ;;\n    --schedule)\n      [[ $# -ge 2 ]] || die \"--schedule needs five cron fields\"\n      schedule=$2\n      shift 2\n      ;;\n    --print-cron)\n      print_cron=1\n      shift\n      ;;\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      break\n      ;;\n    -*)\n      die \"unknown option: $1 (try --help)\"\n      ;;\n    *)\n      die \"put the command after -- (try --help)\"\n      ;;\n  esac\ndone\n\n(($#)) || die \"no command after -- (try --help)\"\n\n[[ $timeout_s =~ ^[0-9]+$ ]] || die \"--timeout must be a whole number\"\n((timeout_s >= 1)) || die \"--timeout must be at least 1\"\nIFS=' ' read -r -a sched_fields <<<\"$schedule\"\n((${#sched_fields[@]} == 5)) || die \"--schedule needs five fields\"\nif [[ -z $name ]]; then\n  name=${1##*/}\nfi\n[[ -n $name ]] || die \"cannot derive a name; use --name\"\n[[ $name =~ ^[A-Za-z0-9._-]+$ ]] || \\\n  die \"--name may use letters, digits, dot, underscore and dash\"\n\nif [[ -z $lock ]]; then\n  lock=$(default_dir /run/lock)/$name.lock\nfi\nif [[ -z $log ]]; then\n  log=$(default_dir /var/log)/$name.log\nfi\n[[ $lock == /* ]] || die \"--lock must be an absolute path\"\n[[ $log == /* ]] || die \"--log must be an absolute path\"\n\ncmd_display=\"\"\ncron_cmd=\"\"\nfor arg in \"$@\"; do\n  cmd_display+=\"${cmd_display:+ }$arg\"\n  cron_cmd+=\"${cron_cmd:+ }$(printf '%q' \"$arg\")\"\ndone\n\nself=$0\nif [[ $self != /* ]]; then\n  self=$(cd \"$(dirname \"$self\")\" && pwd)/$(basename \"$self\")\nfi\ncron_line=\"$schedule $(printf '%q' \"$self\") --name $(printf '%q' \"$name\")\"\ncron_line+=\" --timeout $timeout_s\"\nif [[ -n $alert ]]; then\n  cron_line+=\" --alert $(printf '%q' \"$alert\")\"\nfi\nif ((log_set == 1)); then\n  cron_line+=\" --log $(printf '%q' \"$log\")\"\nfi\nif ((lock_set == 1)); then\n  cron_line+=\" --lock $(printf '%q' \"$lock\")\"\nfi\ncron_line+=\" -- $cron_cmd\"\n\nif ((print_cron == 1)); then\n  printf '%s\\n' \"$cron_line\"\n  exit 0\nfi\n\ncommand -v flock >/dev/null 2>&1 || die \"flock is not installed (util-linux)\"\ncommand -v timeout >/dev/null 2>&1 || die \"timeout is not installed (coreutils)\"\nif [[ $1 == */* ]]; then\n  [[ -x $1 ]] || die \"command is not executable: $1\"\nelse\n  command -v \"$1\" >/dev/null 2>&1 || die \"command not found: $1\"\nfi\nif [[ -n $alert ]]; then\n  [[ $alert == /* ]] || die \"--alert must be an absolute path\"\n  [[ -x $alert ]] || die \"--alert is not executable: $alert\"\nfi\n\nlog_dir=${log%/*}\n[[ -n $log_dir ]] || log_dir=/\nlock_dir=${lock%/*}\n[[ -n $lock_dir ]] || lock_dir=/\nmkdir -p -- \"$log_dir\" || die \"cannot create log directory: $log_dir\"\nmkdir -p -- \"$lock_dir\" || die \"cannot create lock directory: $lock_dir\"\nif ! { : >>\"$log\"; } 2>/dev/null; then\n  die \"cannot write log file: $log\"\nfi\nif ! { : >>\"$lock\"; } 2>/dev/null; then\n  die \"cannot write lock file: $lock\"\nfi\n\nexec 9>>\"$lock\"\nif ! flock -n 9; then\n  printf 'SKIP %s: lock held, exit 75\\n' \"$name\"\n  printf 'LOCK %s\\n' \"${lock##*/}\"\n  logline \"SKIP $name: another run holds $lock\"\n  exit 75\nfi\n\nlogline \"START $name: $cmd_display\"\nrun_tmp=$(mktemp \"${TMPDIR:-/tmp}/cron_wrapper.XXXXXX\") || die \"mktemp failed\"\nstart=$(date +%s)\n\nset +e\ntimeout --kill-after=10 \"$timeout_s\" \"$@\" >\"$run_tmp\" 2>&1\nrc=$?\nset -e\n\nelapsed=$(( $(date +%s) - start ))\nif [[ -s $run_tmp ]]; then\n  stamped=$(ts)\n  while IFS= read -r line || [[ -n $line ]]; do\n    printf '%s %s\\n' \"$stamped\" \"$line\" >>\"$log\"\n  done <\"$run_tmp\"\nfi\nlogline \"END $name rc=$rc in ${elapsed}s\"\n\nif ((rc != 0)); then\n  if ((rc == 124)); then\n    msg=\"TIMEOUT $name killed after ${timeout_s}s\"\n  else\n    msg=\"ALERT $name exited $rc after ${elapsed}s\"\n  fi\n  logline \"$msg\"\n  if [[ -n $alert ]]; then\n    if ! \"$alert\" \"$msg\" >>\"$log\" 2>&1; then\n      logline \"ALERT hook failed: $alert\"\n    fi\n  fi\n  printf '%s\\n' \"$msg\" >&2\n  exit \"$rc\"\nfi\n\nlogline \"OK $name in ${elapsed}s\"\nexit 0\n```\n"},{"id":"p665","n":"665","slug":"renewed-certificate-old-process","title":"Renewed certificate, old process","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","TLS","Linux","DevOps","Security"],"series_name":"ShellSec","design":"code","script":"ssl_renew_hook.sh","date":"2030-12-26","files":{"it":"notes/p665-renewed-certificate-old-process.md","devsecops":"notes/p665-renewed-certificate-old-process.md"},"md":"# Renewed certificate, old process\n\ncertbot renews at 02:00 and exits 0. At 09:00 the browser shows the certificate that expires in twelve days, because nginx read the old file at boot and nobody reloaded it.\n\n## Quick reference\n\nThe full script is at the end of this note: `ssl_renew_hook.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\nssl_renew_hook.sh --cert new.pem --domain app.test --apply\n```\n\n**Example output**\n\n```text\nFAIL served cert unchanged\nOLD expires 2026-11-01, 12d left\n```\n\n## Explained\n\n### What it does\n\n`ssl_renew_hook.sh` validates the new certificate: not expired, enough days left, and the domain in the SAN. It records the served fingerprint, reloads nginx or a systemd unit, waits, fetches the certificate again, and compares it with the new one. It is a dry run until `--apply`.\n\n### What the screen shows\n\nThe fourteen lines check the SAN, compare both fingerprints, and print the age of the certificate still live. On the fixture: FAIL served cert unchanged, then OLD expires 2026-11-01, 12d left.\n\n### How to run it\n\n`ssl_renew_hook.sh --cert /etc/letsencrypt/live/app/fullchain.pem --domain app.example.com`, then the same with `--apply`. Wire it into certbot as the deploy hook. Exit 0 is valid or verified, 1 is invalid or unchanged, 2 is bad input.\n\n### The fix\n\nReload after the renewal, then verify. If the served certificate is still old, check the reload command, the nginx config path, and whether TLS terminates somewhere else.\n\n### Prevention\n\nMake the hook the only path from renewal to reload, and alert when the served fingerprint is not the new one. A renewal that cannot prove the reload is a file copy.\n\n> **The honest caveat:** the check proves this host serves the new cert; a CDN or load balancer in front needs its own check.\n\n> **The lesson:** the file on disk is a promise, and the served certificate is the proof.\n\n## In one line\n\n> **The file is not the process.** A renewed certificate changes nothing until something reloads it.\n\n**Compare the served cert:** `ssl_renew_hook.sh --cert` ... `--apply`\n\n## Script: ssl_renew_hook.sh\n\nDownload: [scripts/ssl_renew_hook.sh](../scripts/ssl_renew_hook.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# ssl_renew_hook.sh - prove a renewed certificate is the one being served.\n#\n# Fixes: certbot renews at 02:00 and exits 0, nginx keeps the old certificate\n# in memory, and clients see the expiry date two weeks after the renewal was\n# reported as successful. The file changed; the process did not.\n#\n# The hook validates the new certificate (not expired, enough days left, SAN\n# contains the domain), records the fingerprint the service is currently\n# serving, reloads nginx or a systemd unit, waits, fetches the served\n# certificate again and compares it with the new one. Dry run by default.\n#\n# Usage:\n#   ssl_renew_hook.sh --cert /etc/letsencrypt/live/app/fullchain.pem \\\n#     --domain app.example.com\n#   ssl_renew_hook.sh --cert ... --domain app.example.com --apply\n#   ssl_renew_hook.sh --cert new.pem --domain app.example.com \\\n#     --served-cert /tmp/served.pem --reload-cmd /tmp/mock-reload --apply\n#\n# Certbot deploy hook:\n#   --deploy-hook \"/usr/local/bin/ssl_renew_hook.sh --cert \\\n#     /etc/letsencrypt/live/app/fullchain.pem --domain app.example.com --apply\"\n#\n# Exit codes:\n#   0  certificate valid (dry run) or served certificate verified (apply)\n#   1  invalid certificate, bad SAN, failed reload, or unchanged served cert\n#   2  usage error, unreadable input, or a missing tool\n#\n# Requires openssl 1.1.1+ and GNU date (date -d). nginx or systemctl is\n# needed only for a real reload. --served-cert and --reload-cmd exist for\n# tests and offline checks; no real service is touched in the fixture tests.\n# By s0sta.com\n\nPROG=${0##*/}\ncert=\"\"\ndomain=\"\"\nservice=\"nginx\"\nreload_cmd=\"\"\nhost=\"\"\nport=443\nserved_cert=\"\"\nwait_s=2\nmin_days=30\napply=0\nserved_tmp=\"\"\nafter_tmp=\"\"\n\nusage() {\n  cat <<'EOF'\nssl_renew_hook.sh - prove a renewed certificate is the one being served\n\nUsage:\n  ssl_renew_hook.sh --cert FILE --domain NAME [options]\n\nOptions:\n  --cert FILE        new certificate in PEM form (required)\n  --domain NAME      name that must appear in the certificate SAN (required)\n  --service NAME     systemd unit to reload; default nginx\n  --reload-cmd FILE  executable to run instead of nginx/systemctl (tests)\n  --host NAME        host to fetch the served cert from; default the domain\n  --port N           port for the served cert; default 443\n  --served-cert FILE read the served cert from a file instead of the network\n  --wait SECS        seconds to wait after reload before fetching; default 2\n  --min-days N       require at least N days left; default 30\n  --apply            reload the service and verify the served certificate\n  -h, --help         show this help and exit 0\n\nExamples:\n  ssl_renew_hook.sh --cert /etc/letsencrypt/live/app/fullchain.pem \\\n    --domain app.example.com\n  ssl_renew_hook.sh --cert new.pem --domain app.example.com \\\n    --served-cert served.pem --reload-cmd ./mock-reload --apply\n\nExit: 0 valid or verified, 1 invalid or unchanged, 2 usage or unreadable input.\nEOF\n}\n\ndie() {\n  printf '%s: %s\\n' \"$PROG\" \"$1\" >&2\n  exit 2\n}\n\n# shellcheck disable=SC2329  # called by the EXIT trap\ncleanup() {\n  if [[ -n $served_tmp ]]; then\n    rm -f -- \"$served_tmp\"\n  fi\n  if [[ -n $after_tmp ]]; then\n    rm -f -- \"$after_tmp\"\n  fi\n}\ntrap cleanup EXIT\n\nfail() {\n  printf 'FAIL %s\\n' \"$1\"\n  exit 1\n}\n\nend_date() {\n  openssl x509 -in \"$1\" -noout -enddate 2>/dev/null | cut -d= -f2\n}\n\ndays_left() {\n  local end epoch\n  end=$(end_date \"$1\") || return 1\n  [[ -n $end ]] || return 1\n  epoch=$(date -d \"$end\" +%s 2>/dev/null) || return 1\n  printf '%d' $(((epoch - $(date +%s) + 86399) / 86400))\n}\n\nfingerprint() {\n  openssl x509 -in \"$1\" -noout -fingerprint -sha256 2>/dev/null | cut -d= -f2\n}\n\nshort_fp() {\n  local fp=$1\n  fp=${fp//:/}\n  printf '%s' \"${fp:0:16}\"\n}\n\nfetch_served() {\n  local dest=$1\n  if [[ -n $served_cert ]]; then\n    cp -- \"$served_cert\" \"$dest\" || return 1\n    return 0\n  fi\n  openssl s_client -connect \"$host:$port\" -servername \"$domain\" -showcerts \\\n    </dev/null 2>/dev/null | openssl x509 -outform PEM >\"$dest\" || return 1\n}\n\nreload_service() {\n  if [[ -n $reload_cmd ]]; then\n    \"$reload_cmd\"\n    return\n  fi\n  if [[ $service == nginx ]] && command -v nginx >/dev/null 2>&1; then\n    nginx -t && nginx -s reload\n    return\n  fi\n  if command -v systemctl >/dev/null 2>&1; then\n    systemctl reload \"$service\"\n    return\n  fi\n  return 127\n}\n\nreload_label() {\n  if [[ -n $reload_cmd ]]; then\n    printf '%s' \"$reload_cmd\"\n  elif [[ $service == nginx ]] && command -v nginx >/dev/null 2>&1; then\n    printf 'nginx -s reload'\n  else\n    printf 'systemctl reload %s' \"$service\"\n  fi\n}\n\nwhile (($#)); do\n  case $1 in\n    --cert)\n      [[ $# -ge 2 ]] || die \"--cert needs a file\"\n      cert=$2\n      shift 2\n      ;;\n    --domain)\n      [[ $# -ge 2 ]] || die \"--domain needs a name\"\n      domain=$2\n      shift 2\n      ;;\n    --service)\n      [[ $# -ge 2 ]] || die \"--service needs a unit name\"\n      service=$2\n      shift 2\n      ;;\n    --reload-cmd)\n      [[ $# -ge 2 ]] || die \"--reload-cmd needs an executable\"\n      reload_cmd=$2\n      shift 2\n      ;;\n    --host)\n      [[ $# -ge 2 ]] || die \"--host needs a name\"\n      host=$2\n      shift 2\n      ;;\n    --port)\n      [[ $# -ge 2 ]] || die \"--port needs a number\"\n      port=$2\n      shift 2\n      ;;\n    --served-cert)\n      [[ $# -ge 2 ]] || die \"--served-cert needs a file\"\n      served_cert=$2\n      shift 2\n      ;;\n    --wait)\n      [[ $# -ge 2 ]] || die \"--wait needs a number of seconds\"\n      wait_s=$2\n      shift 2\n      ;;\n    --min-days)\n      [[ $# -ge 2 ]] || die \"--min-days needs a number\"\n      min_days=$2\n      shift 2\n      ;;\n    --apply)\n      apply=1\n      shift\n      ;;\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      break\n      ;;\n    -*)\n      die \"unknown option: $1 (try --help)\"\n      ;;\n    *)\n      die \"unexpected argument: $1 (try --help)\"\n      ;;\n  esac\ndone\n\n[[ -n $cert ]] || die \"--cert is required (try --help)\"\n[[ -n $domain ]] || die \"--domain is required (try --help)\"\n[[ -r $cert ]] || die \"cannot read certificate: $cert\"\n[[ $wait_s =~ ^[0-9]+$ ]] || die \"--wait must be a whole number\"\n[[ $min_days =~ ^[0-9]+$ ]] || die \"--min-days must be a whole number\"\n[[ $port =~ ^[0-9]+$ ]] || die \"--port must be a whole number\"\n((port >= 1 && port <= 65535)) || die \"--port must be 1-65535\"\nif [[ -n $host ]]; then\n  [[ $host != *[[:space:]]* ]] || die \"--host must not contain spaces\"\nelse\n  host=$domain\nfi\nif [[ -n $reload_cmd ]]; then\n  [[ -x $reload_cmd ]] || die \"--reload-cmd is not executable: $reload_cmd\"\nfi\nif [[ -n $served_cert ]]; then\n  [[ -r $served_cert ]] || die \"cannot read --served-cert: $served_cert\"\nfi\n\ncommand -v openssl >/dev/null 2>&1 || die \"openssl is not installed\"\ncommand -v date >/dev/null 2>&1 || die \"date is not installed\"\n\nopenssl x509 -in \"$cert\" -noout >/dev/null 2>&1 ||\n  fail \"not a PEM certificate: $cert\"\n\nif ! openssl x509 -in \"$cert\" -noout -checkend 0 >/dev/null 2>&1; then\n  fail \"certificate has expired: $cert\"\nfi\nif ((min_days > 0)); then\n  if ! openssl x509 -in \"$cert\" -noout \\\n      -checkend $((min_days * 86400)) >/dev/null 2>&1; then\n    fail \"certificate expires in less than --min-days $min_days\"\n  fi\nfi\n\nsan_line=$(openssl x509 -in \"$cert\" -noout -text 2>/dev/null |\n  awk '/X509v3 Subject Alternative Name/ { getline; print; exit }') || true\nif [[ -z $san_line ]]; then\n  fail \"certificate has no subjectAltName\"\nfi\ndomain_lc=$(printf '%s' \"$domain\" | tr '[:upper:]' '[:lower:]')\nsan_ok=0\nwhile IFS= read -r entry; do\n  entry=$(printf '%s' \"$entry\" | sed 's/^[[:space:]]*//; s/[[:space:]]*$//')\n  entry_lc=$(printf '%s' \"$entry\" | tr '[:upper:]' '[:lower:]')\n  if [[ $entry_lc == \"dns:$domain_lc\" ]]; then\n    san_ok=1\n  fi\ndone < <(printf '%s\\n' \"$san_line\" | tr ',' '\\n')\nif ((san_ok == 0)); then\n  fail \"SAN does not contain DNS:$domain\"\nfi\n\nnew_fp=$(fingerprint \"$cert\")\n[[ -n $new_fp ]] || fail \"cannot read the new fingerprint\"\nexp_days=$(days_left \"$cert\") || exp_days=\"?\"\nexp_end=$(end_date \"$cert\")\nexp_day=$(date -u -d \"$exp_end\" +%F 2>/dev/null || printf '%s' \"$exp_end\")\n\nprintf 'CERT %s\\n' \"$cert\"\nprintf 'EXPIRES %s (%sd left)\\n' \"$exp_day\" \"$exp_days\"\nprintf 'SAN DNS:%s present\\n' \"$domain\"\n\nserved_tmp=$(mktemp \"${TMPDIR:-/tmp}/ssl_renew_hook.XXXXXX\") ||\n  die \"mktemp failed\"\nif ! fetch_served \"$served_tmp\"; then\n  fail \"cannot fetch the served certificate from $host:$port\"\nfi\nold_fp=$(fingerprint \"$served_tmp\")\n[[ -n $old_fp ]] || fail \"cannot read the served fingerprint\"\nold_days=$(days_left \"$served_tmp\") || old_days=\"?\"\nold_end=$(end_date \"$served_tmp\")\nold_day=$(date -u -d \"$old_end\" +%F 2>/dev/null || printf '%s' \"$old_end\")\n\nprintf 'SERVED %s (%s, %sd left)\\n' \\\n  \"$(short_fp \"$old_fp\")\" \"$old_day\" \"$old_days\"\n\nif [[ $old_fp == \"$new_fp\" ]]; then\n  printf 'OK already serving the new certificate\\n'\n  exit 0\nfi\n\nif ((apply == 0)); then\n  printf 'PLAN reload %s, wait %ss, fetch the served cert again\\n' \\\n    \"$(reload_label)\" \"$wait_s\"\n  printf 'DRY-RUN not reloaded. Add --apply.\\n'\n  exit 0\nfi\n\nprintf 'RELOAD %s\\n' \"$(reload_label)\"\nif ! reload_service; then\n  fail \"reload failed: $(reload_label)\"\nfi\nsleep \"$wait_s\"\n\nafter_tmp=$(mktemp \"${TMPDIR:-/tmp}/ssl_renew_hook.XXXXXX\") ||\n  die \"mktemp failed\"\nif ! fetch_served \"$after_tmp\"; then\n  fail \"cannot fetch the served certificate after reload\"\nfi\nafter_fp=$(fingerprint \"$after_tmp\")\n[[ -n $after_fp ]] || fail \"cannot read the served fingerprint after reload\"\n\nif [[ $after_fp == \"$new_fp\" ]]; then\n  printf 'OK served certificate is the new one\\n'\n  exit 0\nfi\nif [[ $after_fp == \"$old_fp\" ]]; then\n  printf 'FAIL served cert unchanged\\n'\n  printf 'OLD expires %s, %sd left\\n' \"$old_day\" \"$old_days\"\n  exit 1\nfi\nprintf 'FAIL served certificate is neither old nor new\\n'\nexit 1\n```\n"},{"id":"p666","n":"666","slug":"the-disk-nobody-will-clean","title":"The disk nobody will clean","sections":["it","devsecops"],"topic":"Shell scripts","tags":["Bash","Linux","Storage","SysAdmin","DevOps"],"series_name":"ShellSec","design":"code","script":"disk_cleanup_plan.sh","date":"2030-12-29","files":{"it":"notes/p666-the-disk-nobody-will-clean.md","devsecops":"notes/p666-the-disk-nobody-will-clean.md"},"md":"# The disk nobody will clean\n\nThe disk is at 96%. Three people ran du, and nobody has deleted anything, because every directory might be the one that matters. The cleanup waits for a name, not a number.\n\n## Quick reference\n\nThe full script is at the end of this note: `disk_cleanup_plan.sh` (plain Bash and standard Linux tools).\n\n**Run it**\n\n```bash\ndisk_cleanup_plan.sh --path /var\n```\n\n**Example output**\n\n```text\nDIR 96.0MB docker\nRISK images in use: prune, never rm -rf\n```\n\n## Explained\n\n### What it does\n\n`disk_cleanup_plan.sh` ranks directories with du, ranks files by age above a size floor with find, and prints a risk note for each path. It never deletes. The note says what the path is and how to treat it: rotate logs, prune images, back up a database, confirm owner data.\n\n### What the screen shows\n\nThe fourteen lines take the largest directory under the path, read its size and name, and print the risk. On the demo: DIR 96.0MB docker, then RISK images in use: prune, never `rm -rf`.\n\n### How to run it\n\n`disk_cleanup_plan.sh --path /var --min-size 500M --older-than 60`. Exit 0 means nothing is over the filter, 1 means the plan has items, 2 means bad input. The weekly cron line is in the header.\n\n### The fix\n\nTurn each line into an owner. The largest directory gets a name and a date, and its risk note names the safe command. Deleting is a decision after the plan, not the plan itself.\n\n### Prevention\n\nRun it monthly and keep the output. A directory that grows in two reports is the one to fix; one that stays flat is just old.\n\n> **The honest caveat:** du counts allocated blocks, so a sparse 40GB file can own far less disk than its apparent size.\n\n> **The lesson:** a cleanup plan is an ownership list, not a delete list.\n\n## In one line\n\n> **A plan is not a deletion.** Nobody cleans a disk when every directory looks dangerous.\n\n**Rank, then read:** `disk_cleanup_plan.sh --path /var`\n\n## Script: disk_cleanup_plan.sh\n\nDownload: [scripts/disk_cleanup_plan.sh](../scripts/disk_cleanup_plan.sh)\n\n```bash\n#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'\n\n# disk_cleanup_plan.sh - rank what owns the disk, with a risk note per item.\n#\n# Fixes: the disk nobody will clean. Every candidate looks dangerous, so the\n# cleanup never happens, and the volume fills one directory at a time until a\n# write fails. This tool never deletes. It ranks the biggest directories and\n# the oldest large files, and prints what each one is and how to treat it.\n#\n# Usage:\n#   disk_cleanup_plan.sh --path /var\n#   disk_cleanup_plan.sh --path /var --top 15 --min-size 500M --older-than 60\n#\n# Cron (weekly report, Monday 06:00):\n#   0 6 * * 1 /usr/local/bin/disk_cleanup_plan.sh --path /var \\\n#     >> /var/log/disk_cleanup_plan.log 2>&1\n#\n# Exit codes:\n#   0  nothing above the size and age filters\n#   1  at least one directory or file is worth reviewing\n#   2  usage error or unreadable path\n#\n# Requires GNU du (du -0 -B1 -d), GNU find (-printf), GNU sort (-z),\n# GNU head (-z) and numfmt (coreutils). Read-only: it never deletes,\n# truncates or moves anything. Tested on clean and dirty fixture trees in\n# /tmp; no service is touched.\n# By s0sta.com\n\nPROG=${0##*/}\npath=\"/var\"\ntop=10\nmin_raw=\"100M\"\nmin_bytes=0\nolder=30\ntmp_dirs=\"\"\ntmp_files=\"\"\ntmp_err=\"\"\n\nusage() {\n  cat <<'EOF'\ndisk_cleanup_plan.sh - rank what owns the disk, with a risk note per item\n\nUsage:\n  disk_cleanup_plan.sh [options]\n\nOptions:\n  --path DIR        directory to scan; default /var\n  --top N           rows per list; default 10\n  --min-size SIZE   directory or file size floor, K/M/G/T; default 100M\n  --older-than N    file age floor in days; default 30\n  -h, --help        show this help and exit 0\n\nExamples:\n  disk_cleanup_plan.sh --path /var\n  disk_cleanup_plan.sh --path /srv --min-size 1G --older-than 90 --top 20\n\nRead-only: the plan is printed, nothing is deleted.\nExit: 0 nothing to review, 1 plan items found, 2 usage or unreadable path.\nEOF\n}\n\ndie() {\n  printf '%s: %s\\n' \"$PROG\" \"$1\" >&2\n  exit 2\n}\n\n# shellcheck disable=SC2329  # called by the EXIT trap\ncleanup() {\n  local f\n  for f in \"$tmp_dirs\" \"$tmp_files\" \"$tmp_err\"; do\n    if [[ -n $f ]]; then\n      rm -f -- \"$f\"\n    fi\n  done\n}\ntrap cleanup EXIT\n\nfmt_bytes() {\n  if (($1 == 0)); then\n    printf '0B'\n    return\n  fi\n  numfmt --to=iec --suffix=B --format='%.1f' -- \"$1\"\n}\n\nto_bytes() {\n  local raw=$1 suffix mult=1 number\n  suffix=${raw: -1}\n  case $suffix in\n    k|K) mult=1024 ; raw=${raw%?} ;;\n    m|M) mult=$((1024 * 1024)) ; raw=${raw%?} ;;\n    g|G) mult=$((1024 * 1024 * 1024)) ; raw=${raw%?} ;;\n    t|T) mult=$((1024 * 1024 * 1024 * 1024)) ; raw=${raw%?} ;;\n  esac\n  number=$raw\n  [[ $number =~ ^[0-9]+$ ]] || return 1\n  printf '%d' $((number * mult))\n}\n\nrisk_note() {\n  case $1 in\n    */log|*/log/*)\n      printf 'logs: rotate or vacuum, never rm the live file'\n      ;;\n    *docker*|*containerd*)\n      printf 'images in use: prune, never rm -rf'\n      ;;\n    *mysql*|*postgres*|*pgsql*)\n      printf 'database files: back up, never delete by hand'\n      ;;\n    */cache|*/cache/*)\n      printf 'cache: safe to clear, services refill it'\n      ;;\n    */home/*|*/srv/*)\n      printf 'owner data: confirm with the owner first'\n      ;;\n    *)\n      printf 'unclassified: inspect before touching'\n      ;;\n  esac\n}\n\nwhile (($#)); do\n  case $1 in\n    --path)\n      [[ $# -ge 2 ]] || die \"--path needs a directory\"\n      path=$2\n      shift 2\n      ;;\n    --top)\n      [[ $# -ge 2 ]] || die \"--top needs a number\"\n      top=$2\n      shift 2\n      ;;\n    --min-size)\n      [[ $# -ge 2 ]] || die \"--min-size needs a size\"\n      min_raw=$2\n      shift 2\n      ;;\n    --older-than)\n      [[ $# -ge 2 ]] || die \"--older-than needs a number of days\"\n      older=$2\n      shift 2\n      ;;\n    -h|--help)\n      usage\n      exit 0\n      ;;\n    --)\n      shift\n      break\n      ;;\n    -*)\n      die \"unknown option: $1 (try --help)\"\n      ;;\n    *)\n      die \"unexpected argument: $1 (try --help)\"\n      ;;\n  esac\ndone\n\n[[ $top =~ ^[0-9]+$ ]] || die \"--top must be a whole number\"\n((top >= 1)) || die \"--top must be at least 1\"\n[[ $older =~ ^[0-9]+$ ]] || die \"--older-than must be a whole number\"\n((older >= 1)) || die \"--older-than must be at least 1\"\nmin_bytes=$(to_bytes \"$min_raw\") ||\n  die \"--min-size must look like 500M, 2G or a byte count\"\n((min_bytes >= 1)) || die \"--min-size must be at least 1 byte\"\n[[ -d $path ]] || die \"not a directory: $path\"\nif [[ -L $path ]]; then\n  die \"refusing a symlink path: $path\"\nfi\nwhile [[ $path == */ && $path != \"/\" ]]; do\n  path=${path%/}\ndone\n[[ -n $path ]] || die \"refusing to scan /\"\n[[ -r $path ]] || die \"cannot read: $path\"\n\ncommand -v du >/dev/null 2>&1 || die \"du is not installed\"\ncommand -v find >/dev/null 2>&1 || die \"find is not installed\"\ncommand -v numfmt >/dev/null 2>&1 || die \"numfmt is not installed (coreutils)\"\n\ntmp_dirs=$(mktemp \"${TMPDIR:-/tmp}/disk_plan_dirs.XXXXXX\") || die \"mktemp failed\"\ntmp_files=$(mktemp \"${TMPDIR:-/tmp}/disk_plan_files.XXXXXX\") || die \"mktemp failed\"\ntmp_err=$(mktemp \"${TMPDIR:-/tmp}/disk_plan_err.XXXXXX\") || die \"mktemp failed\"\n\nprintf 'PATH %s\\n' \"$path\"\nprintf 'FILTER size %s, age %sd\\n' \"$min_raw\" \"$older\"\n\ndu -0 -x -B1 -d 1 \"$path\" 2>\"$tmp_err\" |\n  sort -z -rn | head -z -n \"$((top + 1))\" >\"$tmp_dirs\" || true\nerrs=$(wc -l <\"$tmp_err\" | tr -d ' ')\nif ((errs > 0)); then\n  printf 'NOTE %d paths unreadable during the directory scan\\n' \"$errs\"\nfi\n\ndir_count=0\nwhile IFS= read -r -d '' rec; do\n  size=${rec%%$'\\t'*}\n  dir=${rec#*$'\\t'}\n  if [[ $dir == \"$path\" ]]; then\n    continue\n  fi\n  if ((size < min_bytes)); then\n    continue\n  fi\n  printf 'DIR %s %s\\n' \"$(fmt_bytes \"$size\")\" \"$dir\"\n  printf '  RISK %s\\n' \"$(risk_note \"$dir\")\"\n  dir_count=$((dir_count + 1))\n  if ((dir_count >= top)); then\n    break\n  fi\ndone <\"$tmp_dirs\"\n\n: >\"$tmp_err\"\nfind \"$path\" -xdev -type f -size +\"$min_bytes\"c -mtime +\"$older\" \\\n  -printf '%T@ %s %p\\0' 2>\"$tmp_err\" |\n  sort -z -n | head -z -n \"$top\" >\"$tmp_files\" || true\nerrs=$(wc -l <\"$tmp_err\" | tr -d ' ')\nif ((errs > 0)); then\n  printf 'NOTE %d paths unreadable during the file scan\\n' \"$errs\"\nfi\n\nnow=$(date +%s)\nfile_count=0\nwhile IFS= read -r -d '' rec; do\n  stamp=${rec%% *}\n  rest=${rec#* }\n  size=${rest%% *}\n  file=${rest#* }\n  age=$(((now - ${stamp%%.*}) / 86400))\n  printf 'FILE %s %dd %s\\n' \"$(fmt_bytes \"$size\")\" \"$age\" \"$file\"\n  printf '  RISK %s\\n' \"$(risk_note \"$file\")\"\n  file_count=$((file_count + 1))\ndone <\"$tmp_files\"\n\ndir_word=\"directories\"\nif ((dir_count == 1)); then\n  dir_word=\"directory\"\nfi\nfile_word=\"files\"\nif ((file_count == 1)); then\n  file_word=\"file\"\nfi\n\nprintf 'PLAN %d %s, %d %s to review\\n' \\\n  \"$dir_count\" \"$dir_word\" \"$file_count\" \"$file_word\"\nprintf 'READ-ONLY nothing was deleted.\\n'\n\nif ((dir_count == 0 && file_count == 0)); then\n  printf 'OK nothing over the filter under %s\\n' \"$path\"\n  exit 0\nfi\nexit 1\n```\n"},{"id":"p667","n":"667","slug":"the-automation-pipeline","title":"The automation pipeline","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["Bash","Automation","DevOps","SRE","Linux"],"series_name":"ShellSec","design":"architecture","script":"","date":"2030-12-31","files":{"it":"notes/p667-the-automation-pipeline.md","devsecops":"notes/p667-the-automation-pipeline.md"},"md":"# The automation pipeline\n\nThe backup runs every night. Nobody knows whether it started, whether it finished, or whether the file it wrote can be restored. The cron line is only the first of three tiers.\n\n## Quick reference\n\n- **Trigger:** Cron (CRON) · Systemd timer (TIMER) · Deploy hook (HOOK)\n- **Run:** Wrapper (WRAP) · Lock file (LOCK) · Timeout (TIME) · Run log (LOG)\n- **Verify:** Health check (CHECK) · Served cert (CERT) · Pager (PAGE)\n\n**Request flow:** Cron → Wrapper → Lock file; Wrapper → Timeout; Wrapper → Run log; Wrapper → Health check → Pager; Systemd timer → Wrapper; Deploy hook → Wrapper; Served cert → Pager\n\n**Around it:** Exit code (EXIT) · Runbook (RUN) · Cron mail (MAIL) · Pager (PAGE)\n\n## Explained\n\n### Trigger\n\nThe trigger tier starts the work. Cron is a clock; a systemd timer is a scheduler with a calendar; a deploy hook is an event. Each one can fire twice, so the work behind it must be idempotent. The trigger never owns data.\n\n### Run\n\nThe run tier is one wrapper around one job. The wrapper takes the lock, sets the timeout, writes the log and passes the exit code on. The job does one thing and does not schedule itself. If the run cannot start because the lock is held, it exits 75 and says so.\n\n### Verify\n\nThe verify tier decides whether the run counted. A health check reads the output, a certificate check compares the served cert with the new one, and the alert hook pages only after a failed check. This is the only tier allowed to wake a human.\n\nThe three tiers hand off through one exit code and one log line, which is what makes the path debuggable at 03:00. A trigger with no verify is a job that runs; a run with no lock is a job that overlaps itself.\n\n> **The honest caveat:** a check that runs with the same credentials as the job can verify the wrong thing; keep the last check independent.\n\n> **The lesson:** automation is a path with three tiers, and only the last one may page.\n\n## In one line\n\n> **Trigger, run, then verify.** Three tiers, and only the last one is allowed to page.\n\n**Start at the trigger**\n\n```bash\ncrontab -l\n```\n"},{"id":"p668","n":"668","slug":"cron-and-systemd-automation-commands","title":"Cron and systemd automation commands","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Cron","Systemd","Linux","Automation"],"series_name":"ShellSec","design":"cheatsheet","script":"","date":"2031-01-02","files":{"it":"notes/p668-cron-and-systemd-automation-commands.md","devsecops":"notes/p668-cron-and-systemd-automation-commands.md"},"md":"# Cron and systemd automation commands\n\nThe crontab has one line and it has been there since 2023. These are the commands that tell you whether it still runs, and what to do when it does not.\n\n## Quick reference\n\n### Cron\n\n```bash\ncrontab -l\ncrontab -e\ncrontab -u deploy -l\nsystemctl status cron\ngrep CRON /var/log/syslog\n```\n\n### Timers\n\n```bash\nsystemctl list-timers --all\nsystemctl start backup.timer\nsystemctl enable --now backup.timer\nsystemctl cat backup.timer\nsystemd-analyze calendar '*-*-* 03:00'\n```\n\n### Logs\n\n```bash\njournalctl -u backup -n 50\njournalctl -u backup --since today\njournalctl -u backup -o cat\njournalctl --disk-usage\njournalctl --vacuum-time=14d\n```\n\n### Locking\n\n```bash\nflock -n /run/lock/backup.lock cmd\nflock -w 30 /run/lock/x.lock cmd\nexec 9>/run/lock/backup.lock\nflock -n 9\nlsof /run/lock/backup.lock\n```\n\n## Explained\n\n### Cron\n\n`crontab -l` lists your crontab, and `crontab -u deploy -l` reads another user's crontab with the right privileges. `systemctl status cron` says whether the daemon is alive; on RHEL the unit is crond and the log is `/var/log/cron`. `grep CRON /var/log/syslog` shows the last starts and the mail failures.\n\n### Timers\n\n`systemctl list-timers --all` is the schedule you can actually read: next run, last run, and the unit. `systemctl cat backup.timer` shows the OnCalendar line, and `systemd-analyze calendar '*-*-* 03:00'` explains it. `systemctl enable --now backup.timer` starts the schedule and the service.\n\n### Logs\n\n`journalctl -u backup -n 50` reads the last fifty lines; `--since` today narrows the window; -o cat drops the metadata. `journalctl --disk-usage` says how much the journal owns, and `--vacuum-time=14d` gives the space back.\n\n### Locking\n\nflock -n `/run/lock/backup.lock` cmd takes the lock or fails at once. flock -w 30 waits half a minute. The two-line form, exec 9>`/run/lock/backup.lock` and flock -n 9, holds the lock for a whole script. lsof and fuser name the process that owns it.\n\n> **The honest caveat:** cron has no seconds and no overlap protection; a five-minute line starts the next run whether the last one ended or not.\n\n> **The lesson:** a schedule you cannot read is a schedule you cannot trust.\n\n## In one line\n\n> **Automate the check, not the hope.** Twenty commands, four blocks, one unattended night.\n\n**Read the timer**\n\n```bash\nsystemctl list-timers --all\n```\n"},{"id":"p669","n":"669","slug":"four-automation-failures","title":"Four automation failures","sections":["it","devsecops"],"topic":"IT operations & incidents","tags":["Bash","Automation","DevOps","SRE","Linux"],"series_name":"ShellSec","design":"explainer","script":"","date":"2031-01-05","files":{"it":"notes/p669-four-automation-failures.md","devsecops":"notes/p669-four-automation-failures.md"},"md":"# Four automation failures\n\nFour automation failures look like success from the outside. Each one is a missing check, not a missing tool.\n\n## Quick reference\n\n### Overlap\n\n- Two runs share the same output directory\n- The second reads a half-written file\n- Cron starts both on schedule\n- Fix: flock -n before the work\n\n### Silent failure\n\n- Cron mails output to a mailbox nobody reads\n- Exit 0 hides the failed copy\n- No metric, no alert, no page\n- Fix: exit code plus an alert hook\n\n### Stale process\n\n- Certificate renewed on disk at 02:00\n- The process still serves the old one\n- Clients see the expiry date\n- Fix: reload, then verify the served cert\n\n### No retention\n\n- Disk fills at 3% a week\n- Deletion was a manual habit\n- Nobody owns the cleanup\n- Fix: a dry-run plan on a timer\n\n> **Note:** Four failures that all look like success from the outside.\n\n## Explained\n\n### Overlap\n\nTwo runs share one output directory because the schedule is shorter than the work. The second reads a half-written file, both exit 0, and the record is wrong in a way neither reports. A lock is the fix: the second run exits 75 instead of colliding.\n\n### Silent failure\n\nCron mails stdout to a mailbox nobody reads. The copy fails, the script exits 0 because the failure sat on the last line of a loop, and the next run looks like the last good one. An exit code and an alert hook are the fix. Silence is not success.\n\n### Stale process\n\nThe certificate is renewed on disk at 02:00. The process read the old file at boot and keeps it in memory, so clients see the old date for weeks. Reload, then fetch the served certificate and compare it with the new one.\n\n### No retention\n\nThe disk fills at 3% a week. Deletion was a manual habit, the person who owned it left, and no job owns the cleanup. A dry-run plan on a timer, with a risk note per path, turns habit into policy.\n\nThey combine: overlap plus silence gives a corrupted backup that reports success; no retention gives it a deadline. The verify tier separates them.\n\n> **The honest caveat:** a lock and an alert do not make the job correct; they make its failures visible.\n\n> **The lesson:** every automation failure is a check that was never run.\n\n## In one line\n\n> **Four failures, one silent disk.** Each one looks like success from the outside.\n\n**Add the verify tier:** `cron_wrapper.sh --alert page.sh`\n"},{"id":"p670","n":"670","slug":"5-shell-scripting-mistakes","title":"5 shell scripting mistakes","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","Scripting","DevOps","CyberSecurity"],"series_name":"ShellSec","design":"dontdo","script":"","date":"2031-01-07","files":{"it":"notes/p670-5-shell-scripting-mistakes.md","devsecops":"notes/p670-5-shell-scripting-mistakes.md"},"md":"# 5 shell scripting mistakes\n\nA shell script is a promise that the next line runs. These five habits break it.\n\n## Quick reference\n\n**1.** ❌ **for f in $(ls \\*.log)** — Word splitting breaks on spaces and ls output is not a list.  \n✅ **for f in \\*.log** — The shell expands the glob, sorted and safely quoted.\n\n**2. Don't**\n\n```bash\nrm -rf $DIR/\n```\n\nAn empty DIR becomes `rm -rf /` and spaces split the path.\n\n**Do instead**\n\n```bash\nrm -rf -- \"${DIR:?}/\"\n```\n\n`${DIR:?}` aborts if unset or empty; quotes keep one argument.\n\n**3. Don't**\n\nNo `set -e` in the script\n\nA failed cd or curl is ignored, and the next line runs.\n\n**Do instead**\n\n```bash\nset -euo pipefail at the top\n```\n\nAn unhandled failure stops the script with a code.\n\n**4.** ❌ **Cron job with no lock** — A slow run still holds the files when the next starts.  \n✅ **flock -n 9 around the work** — The second run exits 75 instead of colliding.\n\n**5.** ❌ **Failure sent to `/dev/null`** — A silent failure is found weeks later, in the bill.  \n✅ **Alert on a non-zero exit** — The wrapper pages with the code it saw.\n\n## Explained\n\n### The five\n\n1. Parsing ls: for f in $(ls \\*.log) splits on spaces and trusts a listing that was never a list. Use for f in \\*.log; the shell expands the glob and keeps each name whole.\n   \\2. Unquoted variables. `rm -rf $DIR/` turns an empty DIR into `rm -rf /`, and a space in the value into two arguments. Use `rm -rf -- \"${DIR:?}/\"`: the :? aborts when unset, and the quotes keep one argument.\n   \\3. No `set -e`. Without it, a failed cd or curl is ignored and the next line runs against the wrong directory. Start every script with `set -euo pipefail`.\n   \\4. No lock. A slow cron run still holds the files when the next one starts. flock -n 9 around the work makes the second run exit 75 instead of colliding.\n   \\5. Silent failure. Sending the error to `/dev/null` hides the one line that would explain the outage. Let the exit code leave the script, and give cron an alert hook.\n\n### What they share\n\nEach habit saves a line today and costs an incident later. None of them looks wrong in a code review, because the script does run.\n\n> **The honest caveat:** `set -e` does not catch every failure; a command in a condition and a pipeline without pipefail both keep going.\n\n> **The lesson:** the shell does exactly what you wrote, not what you meant.\n\n## In one line\n\n> **Quote it or lose it.** Five habits that turn a working script into a 03:00 page.\n\n**Start with set -e**\n\n```bash\nset -euo pipefail\n```\n"},{"id":"p671","n":"671","slug":"quiz-3-automation-questions-2","title":"Quiz: 3 automation questions","sections":["it","devsecops"],"topic":"Linux","tags":["Bash","Linux","DevOps","Automation","DevSecOps"],"series_name":"ShellSec","design":"quiz","script":"","date":"2031-01-09","files":{"it":"notes/p671-quiz-3-automation-questions-2.md","devsecops":"notes/p671-quiz-3-automation-questions-2.md"},"md":"# Quiz: 3 automation questions\n\nThree questions close the series, and each one is a decision about work you cannot see. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why take a lock before the work starts, not after it finishes?**\n\n- a) So the second run exits instead of colliding\n- b) To make the script run faster\n- c) To keep the log file small\n\n> **Answer:** a) So the second run exits instead of colliding. Cron starts the next run on schedule, not when the last one ends. flock -n makes the second run exit 75 before it opens the data, so two writers never meet.\n\n**Q2. Why is a silent cron failure worse than a crash?**\n\n- a) A crash is noticed; silence looks normal\n- b) A crash uses more CPU than silence\n- c) Silent jobs simply run less often\n\n> **Answer:** a) A crash is noticed; silence looks normal. A crash writes an exit code and usually a page. A silent failure leaves a target that looks like the last good backup, and the gap is found at restore time.\n\n**Q3. Why verify the served certificate after a renew hook reloads?**\n\n- a) The file changes; the process may not reload\n- b) TLS caches the private key on disk\n- c) Clients pin the certificate serial number\n\n> **Answer:** a) The file changes; the process may not reload. nginx reads the certificate at start or reload. The file on disk proves the renewal, and only a fresh connection proves which certificate the process is serving.\n\n## Explained\n\n### Question one\n\nThe backup takes twelve minutes and the cron line starts it every five. What does flock -n change? The second run cannot start, so it exits 75 and leaves the data alone. Without the lock, both runs write the same files and both report success.\n\n### Question two\n\nWhy is a silent cron failure worse than a crash? A crash leaves an exit code and usually a page. Silence leaves a target that looks like the last good backup, and the gap is found weeks later, when someone needs the restore. The wrapper's alert hook is the difference.\n\n### Question three\n\nWhy must a renew hook verify the served certificate? Because the file changing is not the process reloading. nginx reads the certificate at start or reload, and only a fresh connection proves which one is live. The fingerprint comparison is the proof.\n\n> **The honest caveat:** passing this quiz does not make the job safe; the wrapper, the timer and the alert do.\n\n> **The lesson:** automation is only as good as the check after it.\n\nThat is the series. Fifty posts, five phases and twenty-five shell scripts, from the first host triage to this quiz. Thank you for following along.\n\n## In one line\n\n> **Automate it, then verify it.** Fifty posts, five phases, twenty-five scripts: the last three questions.\n\n**Check the lock:** flock -n 9\n"},{"id":"p672","n":"672","slug":"an-alert-dedupe-report","title":"An alert dedupe report","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","SRE","DevOps","OnCall"],"series_name":"IR","design":"code","script":"alert_dedupe.py","date":"2031-01-12","files":{"it":"notes/p672-an-alert-dedupe-report.md","devsecops":"notes/p672-an-alert-dedupe-report.md"},"md":"# An alert dedupe report\n\nThe pager fires at 03:02 for a disk at 91%. By 03:22 it has fired three more times for the same disk and once for another host. Eleven alerts, five incidents, one tired human.\n\n## Quick reference\n\nThe full script is at the end of this note: `alert_dedupe.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 alert_dedupe.py /tmp/s15/a.json --max-fires 3\n```\n\n**Example output**\n\n```text\nDEDUPE 11 alerts -> 5 incidents (6 duplicates)\nNOISE rule DiskFull 5 fires, worst critical\n```\n\n## Explained\n\n### What it does\n\n`alert_dedupe.py` reads the alert webhook JSON, groups events by fingerprint, and keeps the first and last time each group fired. It ranks rules by fire count, counts the incidents behind them, and prints the deduplicated incident total. A rule over `--max-fires` is a finding.\n\n### What the screen shows\n\nThe panel reads the payload, counts rules and fingerprints, then prints DEDUPE 11 alerts -> 5 incidents (6 duplicates), then NOISE rule DiskFull 5 fires, worst critical.\n\n### How to run it\n\n`python3 alert_dedupe.py /tmp/s15/a.json --max-fires 3`, or pipe the webhook body in with -. Exit 0 means no rule is over budget, 1 means at least one is, and 2 means the JSON is unusable.\n\n### The fix\n\nMake the fingerprint stable: rule, host and symptom, not the alert id. Group before you page, and give each rule a budget. Then fix the disk that generated five of the eleven alerts. The pager is not the problem.\n\n### Prevention\n\nAlert on distinct fingerprints per rule, not raw webhook count. A rule that fires five times with two fingerprints has one noisy source, not five incidents.\n\n> **The honest caveat:** a shared fingerprint can merge two real incidents. Include the host in the key, and review the groups after the storm.\n\n> **The lesson:** an alert is a symptom, and a fingerprint is the incident behind it.\n\n## In one line\n\n> **One alert is a symptom.** A fingerprint is the incident behind the alert.\n\n**Count incidents**\n\n```bash\npython3 alert_dedupe.py alerts.json\n```\n\n## Script: alert_dedupe.py\n\nDownload: [scripts/alert_dedupe.py](../scripts/alert_dedupe.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"alert_dedupe.py - group duplicate alerts into incidents and rank noisy rules.\n\nIncident-response teams do not drown in incidents. They drown in pages: the\nsame failing disk sends a new webhook every five minutes, with a new alert id\nand the same fingerprint. This script reads the raw alert payloads, groups\nthem by fingerprint, and reports the incidents behind the alerts.\n\nThe input is a JSON list, or an object with an \"alerts\" list. Each alert is:\n\n    {\"fingerprint\": \"fp-disk-var\",\n     \"rule\": \"DiskFull\",\n     \"severity\": \"critical\",\n     \"started_at\": \"2026-10-20T03:02:10Z\",\n     \"status\": \"firing\"}\n\nstatus is one of firing, acknowledged, resolved or suppressed. Severity is\none of critical, high, medium or low. The fingerprint is the dedupe key and\nmust be stable across repeats; rule plus host plus symptom is a good start.\n\nThe report is a list of incidents, a ranking of the noisiest rules, and one\nline naming the rules over --max-fires. Exit 1 is the finding: a rule is\npaging more often than the budget allows.\n\nUsage:\n    python3 alert_dedupe.py alerts.json\n    python3 alert_dedupe.py alerts.json --max-fires 3\n    cat alerts.json | python3 alert_dedupe.py -\n    python3 alert_dedupe.py --help\n\nCron or CI (hourly, fail the job when a rule is over budget):\n    5 * * * * python3 /opt/s0sta/alert_dedupe.py /var/lib/s0sta/alerts.json --max-fires 3\n\nExit codes: 0 no rule over the threshold, 1 at least one rule over the\nthreshold, 2 usage error or unreadable, empty or malformed input.\n\nRead-only: opens the given file or stdin, parses JSON and prints. It writes\nnothing, changes nothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nSEVERITY_RANK = {\"critical\": 0, \"high\": 1, \"medium\": 2, \"low\": 3}\nSTATUSES = (\"firing\", \"acknowledged\", \"resolved\", \"suppressed\")\nDEFAULT_MAX_FIRES = 3\nDEFAULT_TOP = 5\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as alert data.\"\"\"\n\n\ndef read_json(path):\n    \"\"\"Return the decoded JSON document, or raise InputError.\"\"\"\n    try:\n        if path == \"-\":\n            text = sys.stdin.read()\n        else:\n            with open(path, \"r\", encoding=\"utf-8\") as handle:\n                text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    try:\n        return json.loads(text)\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n\n\ndef parse_time(value, where):\n    \"\"\"Return an aware UTC datetime, or raise InputError.\"\"\"\n    if not isinstance(value, str) or not value.strip():\n        raise InputError(\"%s: started_at must be an ISO timestamp\" % where)\n    text = value.strip()\n    if text.endswith(\"Z\"):\n        text = text[:-1] + \"+00:00\"\n    try:\n        stamp = datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: bad started_at %r\" % (where, value))\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=timezone.utc)\n    return stamp.astimezone(timezone.utc)\n\n\ndef load_alerts(path):\n    \"\"\"Return a validated list of alert dicts, or raise InputError.\"\"\"\n    doc = read_json(path)\n    if isinstance(doc, dict):\n        if \"alerts\" not in doc:\n            raise InputError(\"%s: object needs an \\\"alerts\\\" list\" % path)\n        doc = doc[\"alerts\"]\n    if not isinstance(doc, list):\n        raise InputError(\"%s: expected a list of alerts\" % path)\n    if not doc:\n        raise InputError(\"%s: no alerts in input\" % path)\n    alerts = []\n    for index, raw in enumerate(doc, start=1):\n        where = \"alert %d\" % index\n        if not isinstance(raw, dict):\n            raise InputError(\"%s: expected an object\" % where)\n        fingerprint = raw.get(\"fingerprint\")\n        rule = raw.get(\"rule\")\n        severity = raw.get(\"severity\")\n        status = raw.get(\"status\")\n        if not isinstance(fingerprint, str) or not fingerprint.strip():\n            raise InputError(\"%s: missing fingerprint\" % where)\n        if not isinstance(rule, str) or not rule.strip():\n            raise InputError(\"%s: missing rule\" % where)\n        if not isinstance(severity, str) or severity.lower() not in SEVERITY_RANK:\n            raise InputError(\"%s: severity must be critical, high, medium or low\"\n                             % where)\n        if not isinstance(status, str) or status.lower() not in STATUSES:\n            raise InputError(\"%s: status must be one of %s\"\n                             % (where, \", \".join(STATUSES)))\n        alerts.append({\n            \"fingerprint\": fingerprint.strip(),\n            \"rule\": rule.strip(),\n            \"severity\": severity.lower(),\n            \"status\": status.lower(),\n            \"started\": parse_time(raw.get(\"started_at\"), where),\n        })\n    return alerts\n\n\ndef group_alerts(alerts):\n    \"\"\"Return incidents keyed by fingerprint, earliest first.\"\"\"\n    groups = {}\n    for alert in alerts:\n        key = alert[\"fingerprint\"]\n        group = groups.get(key)\n        if group is None:\n            group = {\n                \"fingerprint\": key,\n                \"rule\": alert[\"rule\"],\n                \"severity\": alert[\"severity\"],\n                \"first\": alert[\"started\"],\n                \"last\": alert[\"started\"],\n                \"count\": 0,\n                \"statuses\": set(),\n            }\n            groups[key] = group\n        group[\"count\"] += 1\n        group[\"statuses\"].add(alert[\"status\"])\n        if alert[\"started\"] < group[\"first\"]:\n            group[\"first\"] = alert[\"started\"]\n        if alert[\"started\"] > group[\"last\"]:\n            group[\"last\"] = alert[\"started\"]\n        if SEVERITY_RANK[alert[\"severity\"]] < SEVERITY_RANK[group[\"severity\"]]:\n            group[\"severity\"] = alert[\"severity\"]\n            group[\"rule\"] = alert[\"rule\"]\n    return sorted(groups.values(), key=lambda g: (g[\"first\"], g[\"fingerprint\"]))\n\n\ndef rank_rules(alerts):\n    \"\"\"Return per-rule counts, incidents and worst severity, noisiest first.\"\"\"\n    rules = {}\n    for alert in alerts:\n        entry = rules.setdefault(alert[\"rule\"], {\n            \"fires\": 0,\n            \"incidents\": set(),\n            \"worst\": alert[\"severity\"],\n        })\n        entry[\"fires\"] += 1\n        entry[\"incidents\"].add(alert[\"fingerprint\"])\n        if SEVERITY_RANK[alert[\"severity\"]] < SEVERITY_RANK[entry[\"worst\"]]:\n            entry[\"worst\"] = alert[\"severity\"]\n    ordered = sorted(\n        rules.items(),\n        key=lambda item: (-item[1][\"fires\"], SEVERITY_RANK[item[1][\"worst\"]],\n                          item[0]),\n    )\n    return ordered\n\n\ndef report(alerts, max_fires, top):\n    \"\"\"Return (finding, lines) for the alert payload.\"\"\"\n    groups = group_alerts(alerts)\n    ranked = rank_rules(alerts)\n    duplicates = len(alerts) - len(groups)\n    lines = [\"DEDUPE %d alert%s -> %d incident%s (%d duplicate%s)\"\n             % (len(alerts), \"\" if len(alerts) == 1 else \"s\",\n                len(groups), \"\" if len(groups) == 1 else \"s\",\n                duplicates, \"\" if duplicates == 1 else \"s\")]\n    for group in groups:\n        lines.append(\n            \"GROUP %-22s %-12s %d alert%s %s -> %s\"\n            % (group[\"fingerprint\"], group[\"rule\"], group[\"count\"],\n               \"\" if group[\"count\"] == 1 else \"s\",\n               group[\"first\"].strftime(\"%H:%M:%S\"),\n               group[\"last\"].strftime(\"%H:%M:%S\")))\n    for place, (rule, entry) in enumerate(ranked[:top], start=1):\n        incidents = len(entry[\"incidents\"])\n        lines.append(\n            \"RANK %d %-16s %d fire%s, %d incident%s, worst %s\"\n            % (place, rule, entry[\"fires\"],\n               \"\" if entry[\"fires\"] == 1 else \"s\",\n               incidents, \"\" if incidents == 1 else \"s\", entry[\"worst\"]))\n    noisy = [(rule, entry) for rule, entry in ranked\n             if entry[\"fires\"] > max_fires]\n    if not noisy:\n        lines.append(\"NOISE none over %d fires\" % max_fires)\n    elif len(noisy) == 1:\n        rule, entry = noisy[0]\n        lines.append(\"NOISE rule %s %d fires, worst %s\"\n                     % (rule, entry[\"fires\"], entry[\"worst\"]))\n    else:\n        lines.append(\"NOISE %d rules over %d fires: %s\"\n                     % (len(noisy), max_fires,\n                        \", \".join(\"%s (%d)\" % (rule, entry[\"fires\"])\n                                  for rule, entry in noisy)))\n    return bool(noisy), lines\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Group duplicate alerts by fingerprint (read-only).\")\n    parser.add_argument(\n        \"alerts\", nargs=\"?\", default=\"-\",\n        help=\"alert payload JSON, or - for stdin (default: -)\")\n    parser.add_argument(\n        \"--max-fires\", type=int, default=DEFAULT_MAX_FIRES, metavar=\"N\",\n        help=\"exit 1 when a rule fires more than N times (default: %d)\"\n             % DEFAULT_MAX_FIRES)\n    parser.add_argument(\n        \"--top\", type=int, default=DEFAULT_TOP, metavar=\"N\",\n        help=\"rank at most N rules (default: %d)\" % DEFAULT_TOP)\n    args = parser.parse_args(argv)\n\n    if args.max_fires < 0:\n        parser.error(\"--max-fires must not be negative\")\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n\n    try:\n        alerts = load_alerts(args.alerts)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    finding, lines = report(alerts, args.max_fires, args.top)\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p673","n":"673","slug":"a-merged-incident-timeline","title":"A merged incident timeline","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","Observability","SRE","OnCall"],"series_name":"IR","design":"code","script":"incident_timeline.py","date":"2031-01-14","files":{"it":"notes/p673-a-merged-incident-timeline.md","devsecops":"notes/p673-a-merged-incident-timeline.md"},"md":"# A merged incident timeline\n\nThe incident channel has forty messages and no order. The deploy note is in one thread, the first 500 is in another, and the on-call's timeline starts eight minutes after the alert.\n\n## Quick reference\n\nThe full script is at the end of this note: `incident_timeline.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 incident_timeline.py /tmp/s15/tl/*.json --gap-min 5\n```\n\n**Example output**\n\n```text\nTIMELINE 9 events, 03:01:30 -> 03:38:00\nGAP 22 min, 03:14:00 -> 03:36:00, 1 undated\n```\n\n## Explained\n\n### What it does\n\n`incident_timeline.py` reads any number of JSON or text sources, takes the timestamp from every event, converts each to UTC, and sorts the dated ones into one timeline. It reports gaps longer than `--gap-min` and lists every event whose timestamp is missing, because an undated event cannot be placed.\n\n### What the screen shows\n\nThe panel loads four sources, sorts the nine dated events, finds the longest gap, and prints TIMELINE 9 events, 03:01:30 -> 03:38:00, then GAP 22 min, 03:14:00 -> 03:36:00, 1 undated.\n\n### How to run it\n\n`python3 incident_timeline.py /tmp/s15/tl/*.json --gap-min 5`. Point it at exported alerts, deploy records, chat and logs. Exit 0 is a clean timeline, 1 is a gap or an undated event, 2 is bad input.\n\n### The fix\n\nWrite the timestamp at the source, in UTC, before storage. Then merge. The twenty-two minute gap is not empty: it is where the rollback happened, or where nobody looked. Fill it from the deploy log, not from memory.\n\n### Prevention\n\nMake a timestamp mandatory in every exporter, and merge the four clocks on a schedule. A missing timestamp is a data bug, not a formatting preference.\n\n> **The honest caveat:** text logs with stack traces produce many undated lines. Treat each one as a lead, not as noise.\n\n> **The lesson:** an incident without one clock has no sequence, and no sequence means no cause.\n\n## In one line\n\n> **A gap is a finding.** An undated event cannot be placed in the sequence.\n\n**Open one timeline**\n\n```bash\npython3 incident_timeline.py *.json\n```\n\n## Script: incident_timeline.py\n\nDownload: [scripts/incident_timeline.py](../scripts/incident_timeline.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"incident_timeline.py - merge timestamped events from any sources into one clock.\n\nEvery incident has four clocks: the alerting system, the deploy system, the\nchat channel and the application log. The postmortem needs one. This script\nreads all four, sorts them into a single UTC timeline, and flags the places\nwhere the record goes quiet.\n\nEach source is a JSON file (a list, or an object with an \"events\" list) or a\nplain text file. A JSON event is an object with a timestamp and a message:\n\n    {\"ts\": \"2026-10-20T03:02:10Z\", \"msg\": \"API 5xx rate 12%\"}\n\nAccepted timestamp keys are ts, timestamp, time and at. Accepted message keys\nare msg, message, event and text. An event may carry \"source\", otherwise the\nfile name is used. An empty timestamp makes the event undated and it is\nreported separately, because it cannot be placed in the timeline.\n\nA text line starts with an ISO timestamp:\n\n    2026-10-20T03:12:30Z ERROR upstream timeout\n\nA non-empty line without that prefix is undated. Blank lines and lines\nstarting with # are ignored.\n\nUsage:\n    python3 incident_timeline.py alerts.json deploys.json chat.json app.log\n    python3 incident_timeline.py /var/lib/s0sta/timeline/*.json --gap-min 5\n    python3 incident_timeline.py --help\n\nCron or CI (export the incident sources, then merge them every five minutes):\n    */5 * * * * python3 /opt/s0sta/incident_timeline.py /var/lib/s0sta/incident/*.json\n\nExit codes: 0 no gap over --gap-min and no undated event, 1 at least one gap\nor undated event, 2 usage error or unreadable, empty, malformed or untimed\ninput.\n\nRead-only: opens the given files, parses them and prints. It writes nothing,\nchanges nothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport os\nimport re\nimport sys\nfrom datetime import datetime, timezone\n\nISO = re.compile(\n    r\"^(\\d{4}-\\d{2}-\\d{2}[T ]\\d{2}:\\d{2}:\\d{2}(?:\\.\\d+)?\"\n    r\"(?:Z|[+-]\\d{2}:?\\d{2})?)\\s+(.+)$\")\nTIME_KEYS = (\"ts\", \"timestamp\", \"time\", \"at\")\nMESSAGE_KEYS = (\"msg\", \"message\", \"event\", \"text\")\nDEFAULT_GAP_MIN = 5\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as timeline data.\"\"\"\n\n\ndef parse_stamp(value, where):\n    \"\"\"Return an aware UTC datetime, or raise InputError.\"\"\"\n    if not isinstance(value, str) or not value.strip():\n        raise InputError(\"%s: timestamp must be a non-empty ISO string\" % where)\n    text = value.strip()\n    if text.endswith(\"Z\"):\n        text = text[:-1] + \"+00:00\"\n    try:\n        stamp = datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: bad timestamp %r\" % (where, value))\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=timezone.utc)\n    return stamp.astimezone(timezone.utc)\n\n\ndef first_key(record, keys):\n    \"\"\"Return the first present key from record, or None.\"\"\"\n    for key in keys:\n        if key in record:\n            return record[key]\n    return None\n\n\ndef source_label(path):\n    \"\"\"Return the file stem used when an event has no source field.\"\"\"\n    stem = os.path.splitext(os.path.basename(path))[0]\n    for prefix in (\"timeline_\", \"tl_\"):\n        if stem.startswith(prefix):\n            stem = stem[len(prefix):]\n    return stem or \"source\"\n\n\ndef load_json(path, text):\n    \"\"\"Return events from a JSON source, or raise InputError.\"\"\"\n    try:\n        doc = json.loads(text)\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if isinstance(doc, dict):\n        if \"events\" in doc:\n            doc = doc[\"events\"]\n        elif \"items\" in doc:\n            doc = doc[\"items\"]\n        else:\n            raise InputError(\"%s: object needs an \\\"events\\\" list\" % path)\n    if not isinstance(doc, list):\n        raise InputError(\"%s: expected a list of events\" % path)\n    label = source_label(path)\n    events, undated = [], []\n    for index, raw in enumerate(doc, start=1):\n        where = \"%s event %d\" % (path, index)\n        if not isinstance(raw, dict):\n            raise InputError(\"%s: expected an object\" % where)\n        message = first_key(raw, MESSAGE_KEYS)\n        if not isinstance(message, str) or not message.strip():\n            raise InputError(\"%s: missing message\" % where)\n        source = raw.get(\"source\") or label\n        if not isinstance(source, str) or not source.strip():\n            raise InputError(\"%s: source must be a string\" % where)\n        stamp_value = first_key(raw, TIME_KEYS)\n        if stamp_value is None or (isinstance(stamp_value, str)\n                                   and not stamp_value.strip()):\n            undated.append((source.strip(), message.strip()))\n            continue\n        events.append({\n            \"stamp\": parse_stamp(stamp_value, where),\n            \"source\": source.strip(),\n            \"message\": message.strip(),\n        })\n    return events, undated\n\n\ndef load_text(path, text):\n    \"\"\"Return events from a text source, or raise InputError.\"\"\"\n    label = source_label(path)\n    events, undated = [], []\n    for number, raw in enumerate(text.splitlines(), start=1):\n        line = raw.strip()\n        if not line or line.startswith(\"#\"):\n            continue\n        match = ISO.match(line)\n        if not match:\n            undated.append((label, line))\n            continue\n        stamp, message = match.groups()\n        events.append({\n            \"stamp\": parse_stamp(stamp, \"%s line %d\" % (path, number)),\n            \"source\": label,\n            \"message\": message.strip(),\n        })\n    return events, undated\n\n\ndef load_source(path):\n    \"\"\"Return (events, undated) for one source, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n            text = handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    if path.lower().endswith(\".json\"):\n        return load_json(path, text)\n    return load_text(path, text)\n\n\ndef merge(paths):\n    \"\"\"Return sorted events and undated items from every source.\"\"\"\n    events, undated = [], []\n    for path in paths:\n        found, missing = load_source(path)\n        events.extend(found)\n        undated.extend(missing)\n    if not events:\n        raise InputError(\"no timestamped events in %d source(s)\" % len(paths))\n    events.sort(key=lambda event: (event[\"stamp\"], event[\"source\"],\n                                   event[\"message\"]))\n    return events, undated\n\n\ndef duration(seconds):\n    \"\"\"Return a compact duration for the gap line.\"\"\"\n    if seconds >= 3600:\n        return \"%dh%02dm\" % (seconds // 3600, (seconds % 3600) // 60)\n    if seconds >= 60:\n        return \"%d min\" % (seconds // 60)\n    return \"%d s\" % seconds\n\n\ndef report(events, undated, gap_min):\n    \"\"\"Return (finding, lines) for the merged timeline.\"\"\"\n    first, last = events[0][\"stamp\"], events[-1][\"stamp\"]\n    if first.date() == last.date():\n        span = \"%s -> %s\" % (first.strftime(\"%H:%M:%S\"),\n                             last.strftime(\"%H:%M:%S\"))\n    else:\n        span = \"%s -> %s\" % (first.strftime(\"%Y-%m-%d %H:%M\"),\n                             last.strftime(\"%Y-%m-%d %H:%M\"))\n    lines = [\"TIMELINE %d event%s, %s\"\n             % (len(events), \"\" if len(events) == 1 else \"s\", span)]\n    for event in events:\n        lines.append(\"%s  %-10s %s\"\n                     % (event[\"stamp\"].strftime(\"%H:%M:%S\"),\n                        event[\"source\"], event[\"message\"]))\n    gaps = 0\n    for before, after in zip(events, events[1:]):\n        seconds = int((after[\"stamp\"] - before[\"stamp\"]).total_seconds())\n        if seconds > gap_min * 60:\n            gaps += 1\n            lines.append(\"GAP %s, %s -> %s\"\n                         % (duration(seconds),\n                            before[\"stamp\"].strftime(\"%H:%M\"),\n                            after[\"stamp\"].strftime(\"%H:%M\")))\n    for source, message in undated:\n        lines.append(\"UNDATED %-10s %s\" % (source, message))\n    if gaps or undated:\n        lines.append(\"FINDING %d gap%s over %d min, %d undated event%s\"\n                     % (gaps, \"\" if gaps == 1 else \"s\", gap_min,\n                        len(undated), \"\" if len(undated) == 1 else \"s\"))\n    else:\n        lines.append(\"OK no gap over %d min, 0 undated\" % gap_min)\n    return bool(gaps or undated), lines\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Merge timestamped incident events into one timeline (read-only).\")\n    parser.add_argument(\n        \"sources\", nargs=\"+\", metavar=\"SOURCE\",\n        help=\"JSON or text files with timestamped events\")\n    parser.add_argument(\n        \"--gap-min\", type=int, default=DEFAULT_GAP_MIN, metavar=\"MIN\",\n        help=\"flag a gap longer than this many minutes (default: %d)\"\n             % DEFAULT_GAP_MIN)\n    args = parser.parse_args(argv)\n\n    if args.gap_min < 0:\n        parser.error(\"--gap-min must not be negative\")\n\n    try:\n        events, undated = merge(args.sources)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    finding, lines = report(events, undated, args.gap_min)\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p674","n":"674","slug":"a-service-blast-radius-report","title":"A service blast radius report","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","Microservices","SRE","DevOps"],"series_name":"IR","design":"code","script":"blast_radius.py","date":"2031-01-16","files":{"it":"notes/p674-a-service-blast-radius-report.md","devsecops":"notes/p674-a-service-blast-radius-report.md"},"md":"# A service blast radius report\n\nA database fails at 03:10. The team restarts it, and by 03:40 five unrelated services are down because nobody mapped who called whom. The failure was one service; the incident was the graph.\n\n## Quick reference\n\nThe full script is at the end of this note: `blast_radius.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 blast_radius.py /tmp/s15/bg.json payments-db --max 3\n```\n\n**Example output**\n\n```text\nBLAST 5 services depend on payments-db\nPATH web -> checkout -> payments-api -> payments-db\n```\n\n## Explained\n\n### What it does\n\n`blast_radius.py` reads a service dependency graph and a failing service, reverses the edges, and walks every transitive dependent. It prints the count, the hop distance for each service, and the longest dependency chains. When the count is over `--max`, it exits 1, so a change that widens the radius fails the check.\n\n### What the screen shows\n\nThe panel loads the graph, propagates depth from payments-db, then prints BLAST 5 services depend on payments-db and PATH web -> checkout -> payments-api -> payments-db.\n\n### How to run it\n\n`python3 blast_radius.py /tmp/s15/bg.json payments-db --max 3`. A flat object, a services object or an edge list all work. Exit 0 is within `--max`, 1 is over it, 2 is a bad graph or an unknown service. It only reads.\n\n### The fix\n\nKeep the dependency graph in the repository and refresh it from traces. Before you restart a shared service, run the reverse closure and warn the five dependents. Restarting is not containment; the dependents are still failing.\n\n### Prevention\n\nReview the graph when a service is added. An unlisted dependency is an unknown blast radius.\n\n> **The honest caveat:** a static graph misses runtime calls through queues, DNS and shared databases. Treat the count as a floor, not a ceiling.\n\n> **The lesson:** impact is a count of dependents, not a feeling about the failing service.\n\n## In one line\n\n> **Impact is a count, not a feeling.** Count the dependents before you call it contained.\n\n**Map the dependents**\n\n```bash\npython3 blast_radius.py graph.json api\n```\n\n## Script: blast_radius.py\n\nDownload: [scripts/blast_radius.py](../scripts/blast_radius.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"blast_radius.py - count the services that depend on a failing one.\n\nWhen a database or an API fails, the interesting number is not its own health\ncheck. It is how many services cannot work without it. This script reads a\nservice dependency graph, reverses it, and walks the dependents transitively.\nIt prints the count, the hop distance for each service, and the longest\ndependency chains, which are the paths an incident commander has to warn.\n\nThe graph is JSON. A flat object maps a service to the services it calls:\n\n    {\"checkout\": [\"payments-api\"], \"payments-api\": [\"payments-db\"]}\n\nAn object with a \"services\" key is accepted too, as is an edge list:\n\n    {\"edges\": [[\"checkout\", \"payments-api\"], [\"payments-api\", \"payments-db\"]]}\n\nEdge a -> b means a depends on b. The failing service must appear in the graph.\nA cycle is reported on stderr and the paths fall back to the shortest chain;\nthe dependent count is still exact.\n\nUsage:\n    python3 blast_radius.py graph.json payments-db\n    python3 blast_radius.py graph.json payments-db --max 3\n    python3 blast_radius.py graph.json payments-db --paths 5\n    python3 blast_radius.py --help\n\nCI (fail a change that widens the blast radius of a critical service):\n    python3 blast_radius.py graph.json payments-db --max 3 || exit 1\n\nExit codes: 0 impact at or below --max, 1 impact above --max, 2 usage error or\nunreadable, empty, malformed or unknown-service input.\n\nRead-only: opens the graph, computes in memory and prints. It writes nothing,\nchanges nothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom collections import deque\n\nDEFAULT_MAX = 5\nDEFAULT_PATHS = 3\n\n\nclass InputError(Exception):\n    \"\"\"A graph that cannot be read.\"\"\"\n\n\ndef load_graph(path):\n    \"\"\"Return a dependency map from JSON, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if not isinstance(doc, dict):\n        raise InputError(\"%s: expected a JSON object\" % path)\n\n    if \"edges\" in doc:\n        raw_edges = doc[\"edges\"]\n        if not isinstance(raw_edges, list):\n            raise InputError(\"%s: edges must be a list of pairs\" % path)\n        graph = {}\n        for index, edge in enumerate(raw_edges, start=1):\n            if (not isinstance(edge, (list, tuple)) or len(edge) != 2\n                    or not all(isinstance(part, str) and part.strip()\n                               for part in edge)):\n                raise InputError(\"%s: edge %d must be [service, service]\"\n                                 % (path, index))\n            dependent, dependency = edge[0].strip(), edge[1].strip()\n            graph.setdefault(dependent, [])\n            if dependency not in graph[dependent]:\n                graph[dependent].append(dependency)\n            graph.setdefault(dependency, [])\n        if not graph:\n            raise InputError(\"%s: no edges in graph\" % path)\n        return graph\n\n    if \"services\" in doc:\n        doc = doc[\"services\"]\n    if not isinstance(doc, dict) or not doc:\n        raise InputError(\"%s: expected a non-empty service map\" % path)\n    graph = {}\n    for name, deps in doc.items():\n        if not isinstance(name, str) or not name.strip():\n            raise InputError(\"%s: service names must be strings\" % path)\n        if not isinstance(deps, list):\n            raise InputError(\"%s: %s must map to a list\" % (path, name))\n        clean = []\n        for dep in deps:\n            if not isinstance(dep, str) or not dep.strip():\n                raise InputError(\"%s: %s has a non-string dependency\"\n                                 % (path, name))\n            if dep.strip() not in clean:\n                clean.append(dep.strip())\n        graph[name.strip()] = clean\n    for deps in list(graph.values()):\n        for dep in deps:\n            graph.setdefault(dep, [])\n    if not graph:\n        raise InputError(\"%s: no services in graph\" % path)\n    return graph\n\n\ndef reverse_graph(graph):\n    \"\"\"Return dependency -> dependents.\"\"\"\n    reverse = {name: [] for name in graph}\n    for name, deps in graph.items():\n        for dep in deps:\n            reverse.setdefault(dep, []).append(name)\n    return reverse\n\n\ndef reachable(start, reverse):\n    \"\"\"Return {service: hops} for every transitive dependent of start.\"\"\"\n    seen = {}\n    queue = deque([(start, 0)])\n    while queue:\n        node, hops = queue.popleft()\n        for nxt in reverse.get(node, []):\n            if nxt in seen or nxt == start:\n                continue\n            seen[nxt] = hops + 1\n            queue.append((nxt, hops + 1))\n    return seen\n\n\ndef longest_paths(start, nodes, reverse):\n    \"\"\"Return (paths, cycle) for the longest chain to every dependent.\n\n    paths maps a dependent to the chain that starts at it and ends at start.\n    A cycle in the graph is reported instead of guessed at.\n    \"\"\"\n    indegree = {node: 0 for node in nodes}\n    for node in nodes:\n        for nxt in reverse.get(node, []):\n            if nxt in nodes:\n                indegree[nxt] += 1\n    queue = deque([start])\n    distance = {start: 0}\n    parent = {start: None}\n    processed = 0\n    while queue:\n        node = queue.popleft()\n        processed += 1\n        for nxt in reverse.get(node, []):\n            if nxt not in nodes:\n                continue\n            if distance.get(node, 0) + 1 > distance.get(nxt, 0):\n                distance[nxt] = distance[node] + 1\n                parent[nxt] = node\n            indegree[nxt] -= 1\n            if indegree[nxt] == 0:\n                queue.append(nxt)\n    cycle = processed != len(nodes)\n    if cycle:\n        # Fall back to the BFS parent so a cycle still yields a usable chain.\n        parent = {start: None}\n        queue = deque([(start, 0)])\n        while queue:\n            node, hops = queue.popleft()\n            for nxt in reverse.get(node, []):\n                if nxt not in parent:\n                    parent[nxt] = node\n                    queue.append((nxt, hops + 1))\n\n    paths = {}\n    for node in nodes:\n        if node == start:\n            continue\n        chain, seen, cursor = [], set(), node\n        while cursor is not None and cursor not in seen:\n            chain.append(cursor)\n            seen.add(cursor)\n            cursor = parent.get(cursor)\n        if chain and chain[-1] == start:\n            paths[node] = chain\n    return paths, cycle\n\n\ndef report(start, reverse, max_impact, max_paths):\n    \"\"\"Return (finding, paths, lines, cycle) for one failing service.\"\"\"\n    dependents = reachable(start, reverse)\n    count = len(dependents)\n    lines = [\"BLAST %d service%s depend%s on %s\"\n             % (count, \"\" if count == 1 else \"s\",\n                \"s\" if count == 1 else \"\", start)]\n    if not count:\n        lines.append(\"OK no downstream dependents\")\n        return False, [], lines, False\n\n    for name, hops in sorted(dependents.items(), key=lambda item: (item[1], item[0])):\n        lines.append(\"DEPENDENT %-20s %d hop%s via %s\"\n                     % (name, hops, \"\" if hops == 1 else \"s\",\n                        parent_name(name, start, reverse, dependents)))\n    paths, cycle = longest_paths(start, [start] + list(dependents), reverse)\n    ordered = sorted(paths.items(), key=lambda item: -len(item[1]))\n    for name, chain in ordered[:max_paths]:\n        lines.append(\"PATH \" + \" -> \".join(chain))\n    finding = count > max_impact\n    if finding:\n        lines.append(\"FINDING %d dependent%s %s --max %d\"\n                     % (count, \"\" if count == 1 else \"s\",\n                        \"exceeds\" if count == 1 else \"exceed\", max_impact))\n    else:\n        lines.append(\"OK %d dependent%s within --max %d\"\n                     % (count, \"\" if count == 1 else \"s\", max_impact))\n    return finding, ordered, lines, cycle\n\n\ndef parent_name(node, start, reverse, dependents):\n    \"\"\"Return a one-hop closer service for the DEPENDENT line.\"\"\"\n    for candidate, hops in dependents.items():\n        if node in reverse.get(candidate, []):\n            return candidate\n    return start\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Count transitive dependents of a failing service (read-only).\")\n    parser.add_argument(\"graph\", help=\"dependency graph JSON\")\n    parser.add_argument(\"service\", help=\"the failing service\")\n    parser.add_argument(\n        \"--max\", type=int, default=DEFAULT_MAX, metavar=\"N\",\n        help=\"exit 1 when more than N services depend on it (default: %d)\"\n             % DEFAULT_MAX)\n    parser.add_argument(\n        \"--paths\", type=int, default=DEFAULT_PATHS, metavar=\"N\",\n        help=\"print at most N critical paths (default: %d)\" % DEFAULT_PATHS)\n    args = parser.parse_args(argv)\n\n    if args.max < 0:\n        parser.error(\"--max must not be negative\")\n    if args.paths < 1:\n        parser.error(\"--paths must be at least 1\")\n\n    try:\n        graph = load_graph(args.graph)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    if args.service not in graph:\n        print(\"ERROR %s is not in the graph\" % args.service, file=sys.stderr)\n        return 2\n\n    reverse = reverse_graph(graph)\n    finding, _paths, lines, cycle = report(\n        args.service, reverse, args.max, args.paths)\n    if cycle:\n        print(\"NOTE cycle in graph: paths are shortest chains\", file=sys.stderr)\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p675","n":"675","slug":"a-severity-policy-check","title":"A severity policy check","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","SRE","DevOps","OnCall"],"series_name":"IR","design":"code","script":"severity_classify.py","date":"2031-01-19","files":{"it":"notes/p675-a-severity-policy-check.md","devsecops":"notes/p675-a-severity-policy-check.md"},"md":"# A severity policy check\n\nThe incident is declared SEV2 because the last one was SEV2. Six minutes later, customer data is in the logs and the status page is still quiet. The declared number was a feeling; the policy was never consulted.\n\n## Quick reference\n\nThe full script is at the end of this note: `severity_classify.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 severity_classify.py /tmp/s15/si.json /tmp/s15/sp.json\n```\n\n**Example output**\n\n```text\nSUMMARY 4 incidents, 1 mismatch\nMISMATCH INC-1042 declared SEV2, actual SEV1\n```\n\n## Explained\n\n### What it does\n\n`severity_classify.py` reads incidents and a written policy with three ascending thresholds for users, revenue and duration, plus a ranked data class. It takes the highest level any dimension reaches, turns that into SEV1 to SEV4, and flags every incident whose declared severity disagrees with the policy.\n\n### What the screen shows\n\nThe panel loads the incidents and policy, scores four incidents, and prints SUMMARY 4 incidents, 1 mismatch, then MISMATCH INC-1042 declared SEV2, actual SEV1.\n\n### How to run it\n\n`python3 severity_classify.py /tmp/s15/si.json /tmp/s15/sp.json`. Exit 0 means every declaration matches, 1 means at least one mismatch, 2 means the policy or incident file is malformed. It writes nothing.\n\n### The fix\n\nClassify before you declare, then revise the declaration as facts arrive. A mismatch is not a failure by itself; an unexplained mismatch is. Write the reason on the incident: users, data, revenue or duration, and the threshold it crossed.\n\n### Prevention\n\nReview mismatches weekly. If SEV2 always means the previous incident, the policy is decoration and the status page is guesswork.\n\n> **The honest caveat:** the policy is only as good as the numbers you feed it. Users affected is the hardest field to measure during an incident.\n\n> **The lesson:** severity is a decision from four inputs, not a memory of the last outage.\n\n## In one line\n\n> **Declare the number you can defend.** A mismatch is a fact, not an argument.\n\n**Classify, then declare**\n\n```bash\npython3 severity_classify.py inc.json\n```\n\n## Script: severity_classify.py\n\nDownload: [scripts/severity_classify.py](../scripts/severity_classify.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"severity_classify.py - turn incident facts into a declared severity.\n\nSeverity is usually declared in the first two minutes, from a feeling and the\nlast incident. Then the facts arrive. This script reads the incident record\nand a written policy, classifies each incident from users affected, data\nclass, revenue per hour and duration, and prints every place where the\ndeclared severity and the policy disagree.\n\nThe policy JSON holds three ascending thresholds per numeric dimension: the\nfirst is the SEV3 line, the second SEV2, the third SEV1. Data maps each\nclass to a level, from 0 (none) to 3 (regulated).\n\n    {\"users\": [500, 5000, 50000],\n     \"revenue\": [500, 5000, 50000],\n     \"duration\": [30, 60, 120],\n     \"data\": {\"none\": 0, \"internal\": 1, \"customer\": 2, \"regulated\": 3}}\n\nThe incidents file is a JSON list, or an object with an \"incidents\" list:\n\n    {\"id\": \"INC-1042\", \"title\": \"payments API 5xx\", \"declared\": \"SEV2\",\n     \"users\": 60000, \"revenue\": 12000, \"duration\": 130,\n     \"data\": \"customer\"}\n\nAn incident takes the highest level any dimension reaches, so one SEV1 input\nmakes it a SEV1. \"none\" is the no-data value; every class must appear in the policy.\n\nUsage:\n    python3 severity_classify.py incidents.json policy.json\n    python3 severity_classify.py incidents.json policy.json --only-mismatch\n    python3 severity_classify.py --help\n\nCI (fail a release review that leaves a declared severity unexplained):\n    python3 severity_classify.py incidents.json policy.json || exit 1\n\nExit codes: 0 every declared severity matches the policy, 1 at least one\nmismatch, 2 usage error or unreadable, empty or malformed input.\n\nRead-only: opens the two files, compares them and prints. It writes nothing,\nchanges nothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nSEVERITIES = (\"SEV1\", \"SEV2\", \"SEV3\", \"SEV4\")\nDIMENSIONS = (\"users\", \"revenue\", \"duration\")\nPOLICY_KEYS = (\"users\", \"revenue\", \"duration\", \"data\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be classified.\"\"\"\n\n\ndef read_json(path):\n    \"\"\"Return a decoded JSON document, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n\n\ndef load_policy(path):\n    \"\"\"Return a validated policy, or raise InputError.\"\"\"\n    doc = read_json(path)\n    if not isinstance(doc, dict):\n        raise InputError(\"%s: expected a JSON object\" % path)\n    for key in POLICY_KEYS:\n        if key not in doc:\n            raise InputError(\"%s: missing %s\" % (path, key))\n    policy = {}\n    for key in DIMENSIONS:\n        values = doc[key]\n        if not isinstance(values, list) or len(values) != 3:\n            raise InputError(\"%s: %s needs three thresholds\" % (path, key))\n        if not all(isinstance(value, (int, float)) and not isinstance(value, bool)\n                   for value in values):\n            raise InputError(\"%s: %s thresholds must be numbers\" % (path, key))\n        if any(values[i] > values[i + 1] for i in range(2)):\n            raise InputError(\"%s: %s thresholds must ascend\" % (path, key))\n        policy[key] = [float(value) for value in values]\n    data = doc[\"data\"]\n    if not isinstance(data, dict) or not data:\n        raise InputError(\"%s: data must map class names to levels\" % path)\n    classes = {}\n    for name, rank in data.items():\n        if not isinstance(name, str) or not name.strip():\n            raise InputError(\"%s: data class names must be strings\" % path)\n        if (not isinstance(rank, int) or isinstance(rank, bool)\n                or not 0 <= rank <= 3):\n            raise InputError(\"%s: data level for %s must be 0 to 3\"\n                             % (path, name))\n        classes[name.strip()] = rank\n    if len(set(classes.values())) != len(classes):\n        raise InputError(\"%s: data levels must be unique\" % path)\n    if 0 not in classes.values():\n        raise InputError(\"%s: data needs a level 0 class\" % path)\n    policy[\"data\"] = classes\n    return policy\n\n\ndef load_incidents(path, policy):\n    \"\"\"Return validated incidents, or raise InputError.\"\"\"\n    doc = read_json(path)\n    if isinstance(doc, dict):\n        if \"incidents\" not in doc:\n            raise InputError(\"%s: object needs an \\\"incidents\\\" list\" % path)\n        doc = doc[\"incidents\"]\n    if not isinstance(doc, list):\n        raise InputError(\"%s: expected a list of incidents\" % path)\n    if not doc:\n        raise InputError(\"%s: no incidents in input\" % path)\n    incidents = []\n    for index, raw in enumerate(doc, start=1):\n        where = \"incident %d\" % index\n        if not isinstance(raw, dict):\n            raise InputError(\"%s: expected an object\" % where)\n        incident_id = raw.get(\"id\")\n        declared = raw.get(\"declared\")\n        data = raw.get(\"data\")\n        title = raw.get(\"title\", \"\")\n        if not isinstance(incident_id, str) or not incident_id.strip():\n            raise InputError(\"%s: missing id\" % where)\n        if declared not in SEVERITIES:\n            raise InputError(\"%s: declared must be SEV1 to SEV4\" % where)\n        for key in DIMENSIONS:\n            value = raw.get(key)\n            if (not isinstance(value, (int, float)) or isinstance(value, bool)\n                    or value < 0):\n                raise InputError(\"%s: %s must be a non-negative number\"\n                                 % (where, key))\n        if not isinstance(data, str) or not data.strip():\n            raise InputError(\"%s: missing data class\" % where)\n        data = data.strip()\n        if data not in policy[\"data\"]:\n            raise InputError(\"%s: unknown data class %r\" % (where, data))\n        if not isinstance(title, str):\n            raise InputError(\"%s: title must be a string\" % where)\n        incidents.append({\n            \"id\": incident_id.strip(),\n            \"title\": title.strip(),\n            \"declared\": declared,\n            \"users\": float(raw[\"users\"]),\n            \"revenue\": float(raw[\"revenue\"]),\n            \"duration\": float(raw[\"duration\"]),\n            \"data\": data,\n        })\n    return incidents\n\n\ndef level(value, thresholds):\n    \"\"\"Return 0 to 3: how many ascending thresholds the value reaches.\"\"\"\n    return sum(1 for threshold in thresholds if value >= threshold)\n\n\ndef classify(incident, policy):\n    \"\"\"Return (actual severity, level, reason).\"\"\"\n    levels = {\n        \"users\": level(incident[\"users\"], policy[\"users\"]),\n        \"revenue\": level(incident[\"revenue\"], policy[\"revenue\"]),\n        \"duration\": level(incident[\"duration\"], policy[\"duration\"]),\n        \"data\": policy[\"data\"][incident[\"data\"]],\n    }\n    top = max(levels.values())\n    actual = \"SEV%d\" % (4 - top)\n    if top == 0:\n        return actual, top, \"no threshold met\"\n    if levels[\"users\"] == top:\n        reason = \"users %g >= %g\" % (incident[\"users\"],\n                                     policy[\"users\"][top - 1])\n    elif levels[\"revenue\"] == top:\n        reason = \"revenue %g/h >= %g\" % (incident[\"revenue\"],\n                                         policy[\"revenue\"][top - 1])\n    elif levels[\"duration\"] == top:\n        reason = \"duration %g min >= %g\" % (incident[\"duration\"],\n                                            policy[\"duration\"][top - 1])\n    else:\n        reason = \"data %s\" % incident[\"data\"]\n    return actual, top, reason\n\n\ndef report(incidents, policy, only_mismatch):\n    \"\"\"Return (finding, lines) for the incident set.\"\"\"\n    classified, mismatches = [], []\n    for incident in incidents:\n        actual, _top, reason = classify(incident, policy)\n        classified.append((incident, actual, reason))\n        if actual != incident[\"declared\"]:\n            mismatches.append((incident, actual, reason))\n    lines = []\n    for incident, actual, _reason in classified:\n        if only_mismatch and actual == incident[\"declared\"]:\n            continue\n        detail = (\"%s %s\" % (incident[\"id\"],\n                             (\"(%s) \" % incident[\"title\"]) if incident[\"title\"] else \"\"))\n        lines.append(\"CLASSIFY %-34s %s (declared %s) users %g, %g/h, %g min, data %s\"\n                     % (detail, actual, incident[\"declared\"],\n                        incident[\"users\"], incident[\"revenue\"],\n                        incident[\"duration\"], incident[\"data\"]))\n    lines.append(\"SUMMARY %d incident%s, %d mismatch%s\"\n                 % (len(classified), \"\" if len(classified) == 1 else \"s\",\n                    len(mismatches), \"\" if len(mismatches) == 1 else \"es\"))\n    for incident, actual, _reason in mismatches:\n        lines.append(\"MISMATCH %s declared %s, actual %s\"\n                     % (incident[\"id\"], incident[\"declared\"], actual))\n    for incident, actual, reason in mismatches:\n        lines.append(\"WHY %s %s (%s)\" % (incident[\"id\"], reason, actual))\n    if not mismatches:\n        lines.append(\"OK all declared severities match\")\n    return bool(mismatches), lines\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Classify incident severity from a written policy (read-only).\")\n    parser.add_argument(\"incidents\", help=\"incidents JSON\")\n    parser.add_argument(\"policy\", help=\"severity policy JSON\")\n    parser.add_argument(\n        \"--only-mismatch\", action=\"store_true\",\n        help=\"print only the incidents whose declared severity is wrong\")\n    args = parser.parse_args(argv)\n\n    try:\n        policy = load_policy(args.policy)\n        incidents = load_incidents(args.incidents, policy)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    finding, lines = report(incidents, policy, args.only_mismatch)\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p676","n":"676","slug":"an-on-call-handoff-brief","title":"An on-call handoff brief","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","OnCall","SRE","DevOps"],"series_name":"IR","design":"code","script":"handoff_brief.py","date":"2031-01-21","files":{"it":"notes/p676-an-on-call-handoff-brief.md","devsecops":"notes/p676-an-on-call-handoff-brief.md"},"md":"# An on-call handoff brief\n\nAt 07:00 the night on-call writes a paragraph in the channel and goes to bed. The day on-call reads it twice and still misses the unacknowledged SEV1 at the bottom. The handoff was a summary; the incident was in the detail.\n\n## Quick reference\n\nThe full script is at the end of this note: `handoff_brief.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 handoff_brief.py /tmp/s15/hi.json /tmp/s15/hr.json --now 2026-10-20T07:00:00Z\n```\n\n**Example output**\n\n```text\nHANDOFF ana: 3 open, 2 unacked, 3 actions\nALERT INC-1042 SEV1 unacked 41m\n```\n\n## Explained\n\n### What it does\n\n`handoff_brief.py` reads the open incidents and the on-call rota, sorts by severity and age, and builds the brief: top items, unacknowledged pages, pending actions and inherited risks. A critical unacknowledged item makes the script exit 1, so the handoff cannot quietly pass it on.\n\n### What the screen shows\n\nThe panel filters the open incidents, counts the unacknowledged pages and pending actions, and prints HANDOFF ana: 3 open, 2 unacked, 3 actions, then ALERT INC-1042 SEV1 unacked 41m.\n\n### How to run it\n\n`python3 handoff_brief.py /tmp/s15/hi.json /tmp/s15/hr.json --now 2026-10-20T07:00:00Z`. Exit 0 is a clean handoff, 1 is a critical item unacknowledged, 2 is bad input. It reads the tracker and rota.\n\n### The fix\n\nPut the unacknowledged page at the top, not in a paragraph. Then acknowledge it, assign an owner, or escalate before the shift ends. A handoff that lists everything and ranks nothing is a transcript.\n\n### Prevention\n\nMake an unacknowledged critical item block the handoff, and record the acknowledgement in the tracker. The brief is generated; the decision is still human.\n\n> **The honest caveat:** the brief is only as current as the incident tracker. If the tracker is not updated during the shift, the next person inherits the same silence.\n\n> **The lesson:** a handoff is a ranked list of what the next person must not miss.\n\n## In one line\n\n> **Hand off the unacked page.** A ranked brief is a transfer of responsibility.\n\n**Hand off a brief**\n\n```bash\npython3 handoff_brief.py inc rota\n```\n\n## Script: handoff_brief.py\n\nDownload: [scripts/handoff_brief.py](../scripts/handoff_brief.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"handoff_brief.py - build the change-of-shift brief from incidents and the rota.\n\nA handoff is not a channel summary. It is a short, ordered brief that says what\nis still open, which pages nobody acknowledged, which actions are still\npending and which risks the next person inherits. This script reads the open\nincidents and the on-call rota and prints that brief. It exits 1 when a\ncritical incident is still unacknowledged, because that is the item a handoff\nmust never bury.\n\nThe incidents file is a JSON list, or an object with an \"incidents\" list:\n\n    {\"id\": \"INC-1042\", \"title\": \"payments API 5xx\", \"severity\": \"SEV1\",\n     \"status\": \"paged\", \"opened_at\": \"2026-10-20T02:19:00Z\",\n     \"owner\": null, \"actions\": [\"capture heap dump\"]}\n\nStatus is paged, open, acknowledged, mitigated, resolved or closed. Paged and\nopen count as unacknowledged; resolved and closed leave the brief. Severity is\nSEV1 to SEV4. Critical means SEV1 or SEV2.\n\nThe rota file names the primary and, optionally, the secondary and shift end:\n\n    {\"primary\": \"ana\", \"secondary\": \"sam\", \"shift_end\": \"2026-10-20T07:00:00Z\"}\n\nUsage:\n    python3 handoff_brief.py incidents.json rota.json\n    python3 handoff_brief.py incidents.json rota.json --now 2026-10-20T03:00:00Z\n    python3 handoff_brief.py --help\n\nCron (post the brief to the on-call channel fifteen minutes before the shift):\n    45 6,14,22 * * * python3 /opt/s0sta/handoff_brief.py /var/lib/s0sta/incidents.json /etc/s0sta/rota.json\n\nExit codes: 0 no critical item is unacknowledged, 1 a critical item is\nunacknowledged, 2 usage error or unreadable, empty or malformed input.\n\nRead-only: opens the two files, compares them and prints. It writes nothing,\nchanges nothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nSEVERITY_RANK = {\"SEV1\": 0, \"SEV2\": 1, \"SEV3\": 2, \"SEV4\": 3}\nUNACKED = (\"paged\", \"open\")\nOPEN = (\"paged\", \"open\", \"acknowledged\", \"mitigated\")\nCRITICAL = (\"SEV1\", \"SEV2\")\nDEFAULT_TOP = 3\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot become a brief.\"\"\"\n\n\ndef read_json(path):\n    \"\"\"Return a decoded JSON document, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n\n\ndef parse_stamp(value, where):\n    \"\"\"Return an aware UTC datetime, or raise InputError.\"\"\"\n    if not isinstance(value, str) or not value.strip():\n        raise InputError(\"%s: expected an ISO timestamp\" % where)\n    text = value.strip()\n    if text.endswith(\"Z\"):\n        text = text[:-1] + \"+00:00\"\n    try:\n        stamp = datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: bad timestamp %r\" % (where, value))\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=timezone.utc)\n    return stamp.astimezone(timezone.utc)\n\n\ndef load_incidents(path):\n    \"\"\"Return validated incidents, or raise InputError.\"\"\"\n    doc = read_json(path)\n    if isinstance(doc, dict):\n        if \"incidents\" not in doc:\n            raise InputError(\"%s: object needs an \\\"incidents\\\" list\" % path)\n        doc = doc[\"incidents\"]\n    if not isinstance(doc, list):\n        raise InputError(\"%s: expected a list of incidents\" % path)\n    if not doc:\n        raise InputError(\"%s: no incidents in input\" % path)\n    incidents = []\n    for index, raw in enumerate(doc, start=1):\n        where = \"incident %d\" % index\n        if not isinstance(raw, dict):\n            raise InputError(\"%s: expected an object\" % where)\n        incident_id = raw.get(\"id\")\n        title = raw.get(\"title\")\n        severity = raw.get(\"severity\")\n        status = raw.get(\"status\")\n        owner = raw.get(\"owner\")\n        actions = raw.get(\"actions\", [])\n        if not isinstance(incident_id, str) or not incident_id.strip():\n            raise InputError(\"%s: missing id\" % where)\n        if not isinstance(title, str) or not title.strip():\n            raise InputError(\"%s: missing title\" % where)\n        if severity not in SEVERITY_RANK:\n            raise InputError(\"%s: severity must be SEV1 to SEV4\" % where)\n        if status not in OPEN + (\"resolved\", \"closed\"):\n            raise InputError(\"%s: unknown status %r\" % (where, status))\n        if owner is not None and (not isinstance(owner, str)\n                                  or not owner.strip()):\n            raise InputError(\"%s: owner must be a name or null\" % where)\n        if not isinstance(actions, list):\n            raise InputError(\"%s: actions must be a list\" % where)\n        clean_actions = []\n        for action in actions:\n            if not isinstance(action, str) or not action.strip():\n                raise InputError(\"%s: actions must be non-empty strings\" % where)\n            clean_actions.append(action.strip())\n        incidents.append({\n            \"id\": incident_id.strip(),\n            \"title\": title.strip(),\n            \"severity\": severity,\n            \"status\": status,\n            \"opened\": parse_stamp(raw.get(\"opened_at\"),\n                                  \"%s opened_at\" % where),\n            \"owner\": owner.strip() if isinstance(owner, str) else None,\n            \"actions\": clean_actions,\n        })\n    return incidents\n\n\ndef load_rota(path):\n    \"\"\"Return a validated rota, or raise InputError.\"\"\"\n    doc = read_json(path)\n    if not isinstance(doc, dict):\n        raise InputError(\"%s: expected a JSON object\" % path)\n    primary = doc.get(\"primary\")\n    secondary = doc.get(\"secondary\")\n    if not isinstance(primary, str) or not primary.strip():\n        raise InputError(\"%s: missing primary\" % path)\n    if secondary is not None and (not isinstance(secondary, str)\n                                  or not secondary.strip()):\n        raise InputError(\"%s: secondary must be a name or null\" % path)\n    shift_end = doc.get(\"shift_end\")\n    if shift_end is not None:\n        shift_end = parse_stamp(shift_end, \"%s shift_end\" % path)\n    return {\n        \"primary\": primary.strip(),\n        \"secondary\": secondary.strip() if isinstance(secondary, str) else None,\n        \"shift_end\": shift_end,\n    }\n\n\ndef minutes_between(start, end):\n    \"\"\"Return whole minutes from start to end, or raise InputError.\"\"\"\n    seconds = int((end - start).total_seconds())\n    if seconds < 0:\n        raise InputError(\"opened_at is after --now\")\n    return seconds // 60\n\n\ndef report(incidents, rota, now, top):\n    \"\"\"Return (finding, lines) for the handoff brief.\"\"\"\n    open_items = [item for item in incidents if item[\"status\"] in OPEN]\n    open_items.sort(key=lambda item: (SEVERITY_RANK[item[\"severity\"]],\n                                     item[\"opened\"], item[\"id\"]))\n    unacked = [item for item in open_items if item[\"status\"] in UNACKED]\n    critical = [item for item in unacked if item[\"severity\"] in CRITICAL]\n    actions = sum(len(item[\"actions\"]) for item in open_items)\n    lines = [\"HANDOFF %s: %d open, %d unacked, %d action%s\"\n             % (rota[\"primary\"], len(open_items), len(unacked), actions,\n                \"\" if actions == 1 else \"s\")]\n    if rota[\"secondary\"]:\n        if rota[\"shift_end\"]:\n            lines.append(\"ROTA %s -> %s, shift ends %sZ\"\n                         % (rota[\"primary\"], rota[\"secondary\"],\n                            rota[\"shift_end\"].strftime(\"%H:%M\")))\n        else:\n            lines.append(\"ROTA %s -> %s\" % (rota[\"primary\"],\n                                            rota[\"secondary\"]))\n    else:\n        lines.append(\"ROTA %s, no secondary\" % rota[\"primary\"])\n    for item in open_items[:top]:\n        lines.append(\"TOP %s %s %s, %s %d min\"\n                     % (item[\"id\"], item[\"severity\"], item[\"title\"],\n                        item[\"status\"], minutes_between(item[\"opened\"], now)))\n    if unacked:\n        lines.append(\"UNACKED \" + \", \".join(\n            \"%s %s\" % (item[\"id\"], item[\"severity\"]) for item in unacked))\n    else:\n        lines.append(\"UNACKED none\")\n    if actions:\n        parts = [\"%s x%d\" % (item[\"id\"], len(item[\"actions\"]))\n                 for item in open_items if item[\"actions\"]]\n        lines.append(\"ACTIONS %d pending: %s\" % (actions, \", \".join(parts)))\n    else:\n        lines.append(\"ACTIONS none pending\")\n\n    risks = []\n    for item in critical:\n        risks.append(\"critical unacknowledged: %s\" % item[\"id\"])\n    for item in open_items:\n        if item[\"severity\"] in CRITICAL and not item[\"owner\"]:\n            risks.append(\"no owner: %s\" % item[\"id\"])\n    if not rota[\"secondary\"]:\n        risks.append(\"no secondary on the rota\")\n    if risks:\n        for risk in risks:\n            lines.append(\"RISK \" + risk)\n    else:\n        lines.append(\"RISK none at handoff\")\n\n    if critical:\n        worst = critical[0]\n        lines.append(\"ALERT %s %s unacked %d min\"\n                     % (worst[\"id\"], worst[\"severity\"],\n                        minutes_between(worst[\"opened\"], now)))\n        lines.append(\"FINDING %d critical item%s unacknowledged\"\n                     % (len(critical), \"\" if len(critical) == 1 else \"s\"))\n    else:\n        lines.append(\"OK no critical item unacknowledged\")\n    return bool(critical), lines\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Build the on-call handoff brief (read-only).\")\n    parser.add_argument(\"incidents\", help=\"open incidents JSON\")\n    parser.add_argument(\"rota\", help=\"on-call rota JSON\")\n    parser.add_argument(\n        \"--now\", default=None, metavar=\"ISO\",\n        help=\"evaluate ages at this UTC timestamp (default: now)\")\n    parser.add_argument(\n        \"--top\", type=int, default=DEFAULT_TOP, metavar=\"N\",\n        help=\"show at most N top items (default: %d)\" % DEFAULT_TOP)\n    args = parser.parse_args(argv)\n\n    if args.top < 1:\n        parser.error(\"--top must be at least 1\")\n\n    try:\n        incidents = load_incidents(args.incidents)\n        rota = load_rota(args.rota)\n        now = (parse_stamp(args.now, \"--now\") if args.now\n               else datetime.now(timezone.utc))\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    try:\n        finding, lines = report(incidents, rota, now, args.top)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    for line in lines:\n        print(line)\n    return 1 if finding else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p677","n":"677","slug":"the-incident-triage-pipeline","title":"The incident triage pipeline","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","DevOps","OnCall","CyberSecurity"],"series_name":"IR","design":"architecture","script":"","date":"2031-01-23","files":{"it":"notes/p677-the-incident-triage-pipeline.md","devsecops":"notes/p677-the-incident-triage-pipeline.md"},"md":"# The incident triage pipeline\n\nA page is a signal, not an incident. Between the two sits a pipeline that decides whether anyone should be woken, what the incident is called, and who owns it.\n\n## Quick reference\n\n- **Signal:** Alert rule (RULE) · On-call page (PAGE)\n- **Triage:** Dedupe (DEDUP) · Severity policy (SEV)\n- **Decide:** Declare incident (DECL) · Assign owner (OWNER)\n\n**Request flow:** Alert rule → Dedupe → Severity policy → Declare incident → Assign owner; On-call page → Dedupe\n\n**Around it:** Runbook (RUN) · Timeline (TIME) · Status page (STAT)\n\n## Explained\n\n### Signal\n\nTwo surfaces arrive first: the alert rule and the on-call page. They carry a fingerprint, a rule, a severity and a start time. Neither may declare an incident. Their job is to start the clock and open the runbook, because the first fact of any incident is when it began.\n\n### Triage\n\nTriage is where the noise dies. Dedupe groups repeated fingerprints, so eleven alerts become five incidents. The severity policy turns users, data, revenue and duration into one number, and shows where the declared number disagrees. The output is a short list of real incidents, ranked.\n\n### Decide\n\nDecide is a human step with two moves: declare the incident and assign one owner. The declaration names the severity, the incident id and the channel. The owner is the person who will run the timeline, not the person who happened to be in the channel first.\n\nThe three tiers are deliberately narrow. Signal cannot decide, triage cannot act, and decide cannot investigate. A pipeline that lets one tier do all three is a hero culture with better diagrams.\n\n> **The honest caveat:** a pipeline that nobody can skip becomes a queue. Keep a break-glass path for the obvious SEV1, and record why it was used.\n\n> **The lesson:** a page becomes an incident only when someone decides it is one.\n\n## In one line\n\n> **Signal, triage, then decide.** Three tiers, and only the last one may declare.\n\n**Declare one owner:** pd incident:create -S api -t API\n"},{"id":"p678","n":"678","slug":"triage-commands-for-the-first-ten-minutes","title":"Triage commands for the first ten minutes","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","OnCall","DevOps","CommandLine"],"series_name":"IR","design":"cheatsheet","script":"","date":"2031-01-26","files":{"it":"notes/p678-triage-commands-for-the-first-ten-minutes.md","devsecops":"notes/p678-triage-commands-for-the-first-ten-minutes.md"},"md":"# Triage commands for the first ten minutes\n\nThe first ten minutes of an incident are a reading exercise. These are the commands that answer what is firing, when it started, what it touches and what the next person must know. Four blocks, one order.\n\n## Quick reference\n\n### Alerts\n\n```bash\namtool alert query\namtool silence add alertname=DiskFull\ncurl -s localhost:9093/api/v2/alerts\npromtool check rules rules.yml\njq '.[] | .fingerprint' alerts.json\n```\n\n### Timeline\n\n```bash\ndate -u +%Y-%m-%dT%H:%M:%SZ\njournalctl -u api --since 03:00\ngit log --since=03:00 --oneline\nkubectl rollout history deploy/api\njq -r '.[].started_at' alerts.json\n```\n\n### Scope\n\n```bash\nkubectl get pods -A -o wide\nkubectl get endpoints payments\nss -tnp | grep :5432\ndig +short payments.internal\ncurl -s localhost:8080/healthz\n```\n\n### Handoff\n\n```bash\npd incident:list -s open\npd incident:ack -i INC-1042\npd incident:notes -i INC-1042 -n handoff\njq '.primary,.secondary' rota.json\ndate -u +%H:%M\n```\n\n## Explained\n\n### Alerts\n\n`amtool alert query` lists what is firing now, not what emailed at 03:00. amtool silence add alertname=\"DiskFull\" stops a known repeat at the source. `curl -s localhost:9093/api/v2/alerts` returns the raw payload, and `promtool check rules rules.yml` proves the rule file still parses.\n\n### Timeline\n\n`date -u` gives the clock in UTC, which is the only clock a postmortem can use. `journalctl -u api --since 03:00` reads the service log from the incident window. `git log --since=03:00 --oneline` shows the change before the alert, and `kubectl rollout history deploy/api` shows the last revision.\n\n### Scope\n\n`kubectl get pods -A -o wide` shows what is running and where. `ss -tnp | grep :5432` finds live database connections. `kubectl get endpoints payments` shows whether the service has any healthy backends left, and `dig +short payments.internal` answers whether the name still resolves.\n\n### Handoff\n\npd incident:list -s open lists the open incidents. pd incident:ack -i INC-1042 acknowledges the page, and pd incident:notes -i INC-1042 -n handoff writes the handoff line into the incident. `jq '.primary,.secondary' rota.json` names the next shift.\n\n> **The honest caveat:** the first three blocks only read. The handoff block adds an acknowledgement and a note, so say who did it and when.\n\n> **The lesson:** read the signal, the clock, the scope and the rota, then act once.\n\n## In one line\n\n> **Read, then command.** Four blocks, in the order the first ten minutes need.\n\n**Start with alerts**\n\n```bash\namtool alert query\n```\n"},{"id":"p679","n":"679","slug":"four-triage-mistakes","title":"Four triage mistakes","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","DevOps","OnCall","CyberSecurity"],"series_name":"IR","design":"explainer","script":"","date":"2031-01-28","files":{"it":"notes/p679-four-triage-mistakes.md","devsecops":"notes/p679-four-triage-mistakes.md"},"md":"# Four triage mistakes\n\nTriage fails in the same four places every time, and none of them is a tooling problem. The tools are fine; the order is wrong.\n\n## Quick reference\n\n### Alert storms\n\n- One disk fires every five minutes, a new thread each time\n- Ten versions of one incident hide the second failure\n- A stable fingerprint turns eleven alerts into five\n- A rule budget stops the repeat before the pager\n\n### No timeline\n\n- Alerts, deploys, chat and logs keep four clocks\n- Nobody can say what happened first\n- Merge the clocks and the gap becomes visible\n- Without it, the postmortem is memory\n\n### Unknown scope\n\n- The failing service is healthy again, so it closes\n- Then checkout, mobile and admin fail with it\n- Reverse the graph and count the dependents\n- Declare recovery only after the closure\n\n### Silent handoff\n\n- The night shift writes a paragraph, not a brief\n- The day shift reads the first line and stops\n- The unacked SEV1 stays unacked all day\n- Rank the open items and exit 1 on a critical page\n\n> **Note:** Four ways triage loses the incident.\n\n## Explained\n\n### Alert storms\n\nOne failing disk fires every five minutes, and every webhook opens a new thread. The channel fills with ten versions of one incident, and the real second failure arrives unseen. A stable fingerprint and a rule budget turn eleven alerts into five incidents before anyone is paged.\n\n### No timeline\n\nThe alert is in one system, the deploy in another, the chat in a third and the log on the host. Nobody can say what happened first. Merge the clocks and the gap becomes visible; without it, the postmortem is a story about memory.\n\n### Unknown scope\n\nThe failing service is healthy again, so the incident is closed. Then checkout, mobile and admin fail, because they depended on it. Reverse the graph and count the dependents before you declare recovery, not after.\n\n### Silent handoff\n\nThe night shift writes a paragraph, the day shift reads the first line, and the unacknowledged SEV1 stays unacknowledged. A brief that ranks the open items and exits 1 on a critical unacked page is not bureaucracy; it is the only part of the handoff that cannot be skipped.\n\n> **The honest caveat:** fixing all four at once is a programme, not a sprint. Start with the one that cost you the last incident.\n\n> **The lesson:** triage fails in four places, and all four are decisions, not tools.\n\n## In one line\n\n> **Triage fails in four places.** The tools are fine; the order is wrong.\n\n**Check the order**\n\n```bash\npython3 alert_dedupe.py alerts.json\n```\n"},{"id":"p680","n":"680","slug":"9-steps-from-alert-to-incident","title":"9 steps from alert to incident","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","DevOps","OnCall","Python"],"series_name":"IR","design":"steps","script":"","date":"2031-01-30","files":{"it":"notes/p680-9-steps-from-alert-to-incident.md","devsecops":"notes/p680-9-steps-from-alert-to-incident.md"},"md":"# 9 steps from alert to incident\n\nThe alert arrives, and nine small decisions follow. Most teams make seven of them and skip the two that matter: the dedupe and the handoff.\n\n## Quick reference\n\n### Detect — Name the signal in two commands\n\n```bash\n# List what is firing now, not what emailed.\namtool alert query --active\n# Start the clock once, in UTC, before you act.\ndate -u +%Y-%m-%dT%H:%M:%SZ\n# Count incidents; exit 1 means too much noise.\npython3 alert_dedupe.py alerts.json\n```\n\n### Triage — Turn facts into a severity and a scope\n\n```bash\n# Turn users, data and revenue into SEV1 to SEV4.\npython3 severity_classify.py inc.json\n# Count the transitive dependents before restart.\npython3 blast_radius.py graph.json api\n# Merge every source into one ordered clock.\npython3 incident_timeline.py *.json\n```\n\n### Declare — Give the incident an owner\n\n- **pd incident:create -S api -t \"API 5xx\"** — One id, one channel, one severity on the record.\n- **pd incident:ack -i INC-1042** — Acknowledge the page before the investigation.\n- `python3 handoff_brief.py inc rota` — Hand over a ranked brief, not a paragraph.\n\n## Explained\n\n### Detect\n\n`amtool alert query --active` lists what is firing right now, not what the inbox remembers. `date -u +%Y-%m-%dT%H:%M:%SZ` starts the incident clock in UTC, and every later timestamp is compared to it. `python3 alert_dedupe.py alerts.json` turns the pages into incidents; its exit code 1 is the noise finding, not a failure.\n\n### Triage\n\n`python3 severity_classify.py inc.json policy.json` turns users, data, revenue and duration into a declared severity. `python3 blast_radius.py graph.json api` counts who depends on the failing service, transitively. `python3 incident_timeline.py` merges alerts, deploys, chat and logs into one order.\n\n### Declare\n\npd incident:create -S api -t \"API 5xx\" gives the incident one id and one channel. pd incident:ack -i INC-1042 acknowledges the page before the investigation starts. `python3 handoff_brief.py inc.json rota.json` hands over a ranked brief, with the unacknowledged critical item on top.\n\nThe nine steps are not a checklist for its own sake. Each one produces a fact the next one needs: a count, a severity, a scope, an owner.\n\n> **The honest caveat:** the ninth step is not the end. The brief is the first input to the next shift, and it is only as good as the tracker behind it.\n\n> **The lesson:** detect, triage, declare, in that order, with one fact from each step.\n\n## In one line\n\n> **Detect, triage, declare.** Three groups, nine commands, one order that holds.\n\n**Declare after triage:** pd incident:create -S api -t API\n"},{"id":"p681","n":"681","slug":"quiz-3-triage-questions","title":"Quiz: 3 triage questions","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","OnCall","DevOps","CyberSecurity"],"series_name":"IR","design":"quiz","script":"","date":"2031-02-02","files":{"it":"notes/p681-quiz-3-triage-questions.md","devsecops":"notes/p681-quiz-3-triage-questions.md"},"md":"# Quiz: 3 triage questions\n\nPhase one closes with three questions. Each one comes from a tool in this block, and each one is a decision you make before the incident is declared.\n\n## Quick reference\n\n**Q1. Why dedupe before paging?**\n\n- a) A page is noise; an incident is a fact\n- b) Every alert deserves its own page\n- c) Dedupe makes the dashboard faster\n\n> **Answer:** a) A page is noise; an incident is a fact. Eleven alerts with five fingerprints are five incidents. The fingerprint gives the count first, so the pager carries an incident id, not eleven alert ids.\n\n**Q2. What does blast radius measure?**\n\n- a) The transitive services that depend on it\n- b) The failing service's own health check\n- c) The number of pods in the namespace\n\n> **Answer:** a) The transitive services that depend on it. The dependents, not the failing service. A green database can still take down checkout, mobile and admin; reverse the graph before calling it contained.\n\n**Q3. Why does a handoff need unacked items?**\n\n- a) An unacked page has no owner yet\n- b) Unacked pages are always SEV1\n- c) The night shift cannot write notes\n\n> **Answer:** a) An unacked page has no owner yet. An unacknowledged page has no owner, and an incident without an owner is not being worked. Put it on top and exit 1 when it is critical.\n\n## Explained\n\n### Question one\n\nWhy dedupe before paging? Because a page is an interruption and an incident is a fact. Eleven alerts with five fingerprints are five incidents, and paging on all eleven wakes the same person five times. The fingerprint gives you the incident count first, then the pager can carry the incident id rather than the alert id.\n\n### Question two\n\nWhat does blast radius measure? The services that transitively depend on the failing one, not the failing service's own health. A database with a green health check can still take down checkout, mobile and admin. Reverse the graph and count the dependents before you call the incident contained.\n\n### Question three\n\nWhy does a handoff need unacked items? Because an unacknowledged page has no owner, and an incident without an owner is not being worked. The brief must put that item on top and exit non-zero when it is critical, so the next shift cannot mistake a written summary for a transfer of responsibility.\n\n> **The honest caveat:** the answers are easy. At 03:00 you get one command and one minute, so the habit is knowing which answer applies.\n\n> **The lesson:** dedupe the noise, measure the reach, and hand off the unacked page.\n\n## In one line\n\n> **Three answers, one pager.** The tool names the cause; the cause names the fix.\n\n**Check your answer**\n\n```bash\npython3 alert_dedupe.py alerts.json\n```\n"},{"id":"p682","n":"682","slug":"a-host-quarantine-plan-that-keeps-the-console","title":"A host quarantine plan that keeps the console","sections":["it","devsecops"],"topic":"Python scripts","tags":["IncidentResponse","Python","Linux","SRE","CyberSecurity"],"series_name":"IR","design":"code","script":"quarantine_plan.py","date":"2031-02-04","files":{"it":"notes/p682-a-host-quarantine-plan-that-keeps-the-console.md","devsecops":"notes/p682-a-host-quarantine-plan-that-keeps-the-console.md"},"md":"# A host quarantine plan that keeps the console\n\nAt 02:40 you cut the host off the network. At 02:41 the SSH session you are fixing it from freezes, and the box becomes a black hole.\n\n## Quick reference\n\nThe full script is at the end of this note: `quarantine_plan.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 quarantine_plan.py inv.json pol.json --host web-01\n```\n\n**Example output**\n\n```text\nKEEP mgmt 10.20.0.0/24 tcp/22\nREADY 7 rules, management path first\n```\n\n## Explained\n\n### What it does\n\n`quarantine_plan.py` reads a host inventory and an isolation policy and builds the ruleset that cuts east-west traffic while keeping the management path open. The management accept is emitted before every drop, and the policy is drop. A policy with no management source is refused: exit 1. Bad JSON, an unknown host or a bad CIDR is exit 2.\n\n### What the screen shows\n\nThe panel checks the management source, then builds the accept rules before the cut. It prints KEEP mgmt 10.20.0.0/24 tcp/22 and READY 7 rules, management path first.\n\n### How to run it\n\n`python3 quarantine_plan.py inventory.json policy.json --host web-01`. Add `--iptables` for a host without nft. `--apply` writes the ruleset with mode 0600 and prints `nft -f <file>`; it never loads it. The cron line is in the docstring.\n\n### The fix\n\nPut the console first. The plan allows the management subnet on tcp/22, then drops the private ranges both ways. If the console is serial or IPMI, add it as a management source.\n\n### Prevention\n\nKeep the policy beside the inventory, and review it with the on-call console subnet in the diff. Check the rendered ruleset with `nft -c -f` before it reaches a host.\n\n> **The honest caveat:** it knows the paths in the policy, not the out-of-band console you forgot to list.\n\n> **The lesson:** a quarantine that locks you out is an outage with a story.\n\n## In one line\n\n> **Seal the host, keep the door.** A quarantine that locks you out is an outage with a story.\n\n**Plan first**\n\n```bash\npython3 quarantine_plan.py inv.json pol.json\n```\n\n## Script: quarantine_plan.py\n\nDownload: [scripts/quarantine_plan.py](../scripts/quarantine_plan.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"quarantine_plan.py - build a host quarantine ruleset that keeps the console.\n\nReads a host inventory and an isolation policy, then prints the nftables (or\niptables) rules that cut east-west traffic while keeping the management path\nopen. The management accept rule is emitted first and the chain default is\ndrop, so the printed order is the whole argument.\n\nThe default is a dry run: it prints the plan and writes nothing. --apply\nwrites the ruleset to a file with mode 0600 and prints the command that loads\nit (nft -f or sh). It never runs a firewall command itself.\n\nUsage:\n  ./quarantine_plan.py inventory.json policy.json --host web-01\n  ./quarantine_plan.py inventory.json policy.json --host web-01 --iptables\n  ./quarantine_plan.py inventory.json policy.json --host web-01 \\\n      --apply --out /root/quarantine.nft\n\nCron / CI (render the plan on every inventory change, never apply):\n  30 * * * * /usr/local/bin/quarantine_plan.py /etc/ir/inventory.json \\\n    /etc/ir/policy.json --host web-01 >> /var/log/quarantine_plan.log 2>&1\n\nExit codes:\n  0  plan built and the management rule is present and first\n  1  refused: no management source, so the plan would cut the console\n  2  usage error, unreadable JSON, unknown host, bad address or port\n\nThe ruleset is IPv4. An IPv6 cut is a usage error, not a rule that silently\ndoes nothing.\n\nInventory: {\"hosts\": [{\"name\": \"web-01\", \"ip\": \"10.20.4.11\"}, ...]}\nPolicy: {\"management\": {\"sources\": [\"10.20.0.0/24\"], \"port\": 22,\n         \"proto\": \"tcp\"},\n         \"keep_egress\": [{\"to\": \"10.20.0.10\", \"port\": 53, \"proto\": \"udp\"}],\n         \"cut\": [\"10.0.0.0/8\", \"172.16.0.0/12\", \"192.168.0.0/16\"]}\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport json\nimport os\nimport re\nimport sys\n\nDEFAULT_CUT = [\"10.0.0.0/8\", \"172.16.0.0/12\", \"192.168.0.0/16\",\n               \"169.254.0.0/16\"]\n\n\ndef die(msg):\n    print(\"ERROR \" + msg, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as fh:\n            return json.load(fh)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef net(value, label):\n    try:\n        return ipaddress.ip_network(str(value), strict=False)\n    except ValueError:\n        die(\"%s: not a CIDR: %s\" % (label, value))\n\n\ndef net4(value, label):\n    n = net(value, label)\n    if n.version != 4:\n        die(\"%s: IPv6 is not supported in this IPv4 ruleset: %s\" % (label, value))\n    return n\n\n\ndef port(value, label):\n    try:\n        n = int(value)\n    except (TypeError, ValueError):\n        die(\"%s: not a port: %s\" % (label, value))\n    if not 1 <= n <= 65535:\n        die(\"%s: port out of range: %s\" % (label, value))\n    return n\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Print the nftables or iptables rules that quarantine a \"\n                    \"host and keep its management path.\")\n    ap.add_argument(\"inventory\", help=\"host inventory JSON\")\n    ap.add_argument(\"policy\", help=\"isolation policy JSON\")\n    ap.add_argument(\"--host\", required=True, help=\"host name in the inventory\")\n    ap.add_argument(\"--iptables\", action=\"store_true\",\n                    help=\"emit iptables commands instead of an nft ruleset\")\n    ap.add_argument(\"--table\", default=\"ir_quarantine\",\n                    help=\"nftables table name (default: ir_quarantine)\")\n    ap.add_argument(\"--apply\", action=\"store_true\",\n                    help=\"write the ruleset file; nothing is loaded\")\n    ap.add_argument(\"--out\", default=None,\n                    help=\"ruleset path for --apply (default /tmp/quarantine-<host>)\")\n    ap.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = ap.parse_args()\n\n    table = args.table.strip()\n    if not re.match(r\"^[A-Za-z0-9_]+$\", table):\n        die(\"--table must be letters, digits or underscore: %s\" % args.table)\n\n    inv = load(args.inventory)\n    pol = load(args.policy)\n    hosts = inv.get(\"hosts\", inv) if isinstance(inv, dict) else inv\n    if not isinstance(hosts, list):\n        die(\"inventory: expected a list under 'hosts'\")\n    match = None\n    for h in hosts:\n        if isinstance(h, dict) and str(h.get(\"name\", \"\")) == args.host:\n            match = h\n            break\n    if match is None:\n        die(\"host %s not in %s\" % (args.host, args.inventory))\n    ip = str(match.get(\"ip\") or match.get(\"address\") or \"unknown\")\n    if ip != \"unknown\":\n        net4(ip, \"inventory host ip\")\n\n    if not isinstance(pol, dict):\n        die(\"policy: expected a JSON object\")\n    mgmt = pol.get(\"management\") or {}\n    if not isinstance(mgmt, dict):\n        die(\"policy: management must be an object\")\n    sources = mgmt.get(\"sources\") or []\n    if not isinstance(sources, list):\n        die(\"policy: management.sources must be a list\")\n    mport = port(mgmt.get(\"port\", 22), \"management.port\")\n    mproto = str(mgmt.get(\"proto\", \"tcp\")).lower()\n    if mproto not in (\"tcp\", \"udp\"):\n        die(\"management.proto must be tcp or udp: %s\" % mproto)\n\n    cut_raw = pol.get(\"cut\") or DEFAULT_CUT\n    if not isinstance(cut_raw, list):\n        die(\"policy: cut must be a list of CIDRs\")\n    cut = [net4(c, \"cut\") for c in cut_raw]\n    if not cut:\n        die(\"policy: cut is empty, nothing to isolate\")\n\n    keep = pol.get(\"keep_egress\") or []\n    if not isinstance(keep, list):\n        die(\"policy: keep_egress must be a list\")\n    egress = []\n    for item in keep:\n        if not isinstance(item, dict):\n            die(\"keep_egress entries must be objects\")\n        proto = str(item.get(\"proto\", \"tcp\")).lower()\n        if proto not in (\"tcp\", \"udp\"):\n            die(\"keep_egress.proto must be tcp or udp: %s\" % proto)\n        egress.append((net4(item.get(\"to\"), \"keep_egress.to\"), proto,\n                       port(item.get(\"port\"), \"keep_egress.port\")))\n\n    if not sources:\n        print(\"REFUSED no management.sources in the policy\")\n        print(\"FINDING a ruleset without a management accept cuts the console\")\n        sys.exit(1)\n\n    net_sources = [net4(src, \"management.sources\") for src in sources]\n    print(\"PLAN quarantine %s %s%s\" % (args.host, ip,\n                                       \" (iptables)\" if args.iptables else \"\"))\n    for src in net_sources:\n        print(\"KEEP mgmt %s %s/%d\" % (src, mproto, mport))\n    for to, proto, p in egress:\n        print(\"KEEP egress %s %s/%d\" % (to, proto, p))\n    print(\"CUT %s\" % \" \".join(str(c) for c in cut))\n\n    if args.iptables:\n        rules = [\n            \"iptables -I INPUT 1 -i lo -j ACCEPT\",\n            \"iptables -I INPUT 2 -m conntrack \"\n            \"--ctstate ESTABLISHED,RELATED -j ACCEPT\",\n        ]\n        for i, src in enumerate(net_sources, start=3):\n            rules.append(\"iptables -I INPUT %d -s %s -p %s --dport %d \"\n                         \"-j ACCEPT\" % (i, src, mproto, mport))\n        pos = len(rules) + 1\n        for c in cut:\n            rules.append(\"iptables -I INPUT %d -s %s -j DROP\" % (pos, c))\n            pos += 1\n        rules.append(\"iptables -P INPUT DROP\")\n        rules.append(\"iptables -I OUTPUT 1 -o lo -j ACCEPT\")\n        rules.append(\"iptables -I OUTPUT 2 -m conntrack \"\n                     \"--ctstate ESTABLISHED,RELATED -j ACCEPT\")\n        opos = 3\n        for to, proto, p in egress:\n            rules.append(\"iptables -I OUTPUT %d -d %s -p %s --dport %d \"\n                         \"-j ACCEPT\" % (opos, to, proto, p))\n            opos += 1\n        for c in cut:\n            rules.append(\"iptables -I OUTPUT %d -d %s -j DROP\" % (opos, c))\n            opos += 1\n        rules.append(\"iptables -P OUTPUT DROP\")\n        text = \"#!/bin/sh\\n# quarantine %s, generated by quarantine_plan.py\\n\" \\\n               \"set -eu\\n%s\\n\" % (args.host, \"\\n\".join(rules))\n        loader = \"sh\"\n        suffix = \".sh\"\n    else:\n        in_rules = [\"iif lo accept\", \"ct state established,related accept\"]\n        for src in net_sources:\n            in_rules.append(\"ip saddr %s %s dport %d accept\"\n                            % (src, mproto, mport))\n        in_rules.append(\"ip saddr { %s } drop\"\n                        % \", \".join(str(c) for c in cut))\n        out_rules = [\"oif lo accept\", \"ct state established,related accept\"]\n        for to, proto, p in egress:\n            out_rules.append(\"ip daddr %s %s dport %d accept\" % (to, proto, p))\n        out_rules.append(\"ip daddr { %s } drop\"\n                         % \", \".join(str(c) for c in cut))\n        rules = in_rules + out_rules\n        text = (\n            \"table inet %s {\\n\"\n            \"    chain input {\\n\"\n            \"        type filter hook input priority 0; policy drop;\\n\"\n            \"        %s\\n\"\n            \"    }\\n\"\n            \"    chain output {\\n\"\n            \"        type filter hook output priority 0; policy drop;\\n\"\n            \"        %s\\n\"\n            \"    }\\n\"\n            \"}\\n\" % (table, \"\\n        \".join(in_rules),\n                     \"\\n        \".join(out_rules)))\n        loader = \"nft -f\"\n        suffix = \".nft\"\n\n    for i, rule in enumerate(rules, start=1):\n        print(\"RULE %d %s\" % (i, rule))\n    print(\"READY %d rules, management path first\" % len(rules))\n\n    if not args.apply:\n        print(\"DRY-RUN nothing written. Add --apply to write the ruleset.\")\n        return 0\n\n    safe = re.sub(r\"[^A-Za-z0-9_.-]\", \"_\", args.host)\n    out = args.out or (\"/tmp/quarantine-%s%s\" % (safe, suffix))\n    try:\n        fd = os.open(out, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)\n        with os.fdopen(fd, \"w\", encoding=\"utf-8\") as fh:\n            fh.write(text)\n        os.chmod(out, 0o600)\n    except OSError as exc:\n        die(\"%s: %s\" % (out, exc.strerror or exc))\n    print(\"WROTE %s mode 0600\" % out)\n    print(\"APPLY %s %s\" % (loader, out))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p683","n":"683","slug":"an-ordered-account-lockout-plan","title":"An ordered account lockout plan","sections":["it","devsecops"],"topic":"Python scripts","tags":["IncidentResponse","Python","IAM","SRE","CyberSecurity"],"series_name":"IR","design":"code","script":"account_lockout.py","date":"2031-02-06","files":{"it":"notes/p683-an-ordered-account-lockout-plan.md","devsecops":"notes/p683-an-ordered-account-lockout-plan.md"},"md":"# An ordered account lockout plan\n\nThe identity is compromised. The first instinct is to disable every account carrying the email, including the shared build account four teams deploy through.\n\n## Quick reference\n\nThe full script is at the end of this note: `account_lockout.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 account_lockout.py identity.json\n```\n\n**Example output**\n\n```text\nLOCK 3 systems in order: okta, aws, github\nHOLD jenkins build shared, owner unknown\n```\n\n## Explained\n\n### What it does\n\n`account_lockout.py` reads an identity export: systems with a tier and a kind, accounts with a login, a shared flag and a break-glass flag. It matches the identity, sorts accounts by tier, and prints the plan in dependency order. The identity provider comes first because it ends the sessions; cloud, source control and CI follow.\n\n### What the screen shows\n\nThe panel sorts four accounts and prints LOCK 3 systems in order: okta, aws, github, then HOLD jenkins build shared, owner unknown. Exit 1 means a human must decide.\n\n### How to run it\n\n`python3 account_lockout.py identity.json --identity ana@example.com`, or let the export name the identity. `--min-tier 2` skips the identity provider when the sessions are gone. Pipe it into the ticket; it writes nothing.\n\n### The fix\n\nDisable the IdP account first, so the next sign-in fails, then revoke cloud keys, then block source control and CI. Hold the shared account, rotate the secret it uses, and tell its owner. A break-glass account goes to the incident commander, not to the script.\n\n### Prevention\n\nGive every account an owner field, keep a break-glass register, and audit both quarterly. An export with no owner column cannot produce this plan.\n\n> **The honest caveat:** the export is a snapshot, and a token issued after it was taken is invisible.\n\n> **The lesson:** the account is the access; the person is the witness.\n\n## In one line\n\n> **End the access, not the human.** A shared account is a team's access: hold it, rotate what it reaches.\n\n**Order the locks**\n\n```bash\npython3 account_lockout.py identity.json\n```\n\n## Script: account_lockout.py\n\nDownload: [scripts/account_lockout.py](../scripts/account_lockout.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"account_lockout.py - plan a compromised identity lockout in dependency order.\n\nReads an identity export and a compromised identity, then prints the\ndisable/revoke plan in the order the systems depend on each other: the identity\nprovider first, because it ends the sessions, then cloud roles, source control\nand CI, then everything else. A shared or break-glass account is held for a\nhuman decision, never disabled by the plan.\n\nRead-only: it prints the plan, writes nothing and calls no provider API. The\nprinted lines are the deliverable for the incident ticket.\n\nUsage:\n  ./account_lockout.py identity.json --identity ana@example.com\n  ./account_lockout.py identity.json --identity ana@example.com --min-tier 2\n  ./account_lockout.py identity.json          # identity from the export\n\nCron / CI (incident job, not a schedule):\n  ./account_lockout.py /etc/ir/identity.json \\\n    --identity \"$(cat /tmp/ir/compromised)\" | tee /var/log/ir/lockout.log\n\nExit codes:\n  0  plan complete, no shared or break-glass account in scope\n  1  held: a shared or break-glass account needs a human decision first\n  2  usage error, unreadable JSON, identity missing or no matching accounts\n\nIdentity export:\n  {\"identity\": \"ana@example.com\",\n   \"systems\": [{\"name\": \"okta\", \"tier\": 1, \"kind\": \"idp\"}, ...],\n   \"accounts\": [{\"system\": \"okta\", \"id\": \"00u1x\",\n                 \"login\": \"ana@example.com\", \"shared\": false,\n                 \"break_glass\": false}, ...]}\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nKINDS = {\n    \"idp\": (\"DISABLE\", \"deactivate, ends the sessions\"),\n    \"cloud\": (\"REVOKE\", \"keys, roles and console access\"),\n    \"scm\": (\"BLOCK\", \"tokens and login\"),\n    \"ci\": (\"DISABLE\", \"the build account\"),\n    \"other\": (\"DISABLE\", \"the account\"),\n}\n\n\ndef die(msg):\n    print(\"ERROR \" + msg, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as fh:\n            return json.load(fh)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Print a compromised identity lockout plan in dependency \"\n                    \"order, holding shared and break-glass accounts.\")\n    ap.add_argument(\"export\", help=\"identity export JSON\")\n    ap.add_argument(\"--identity\", default=None,\n                    help=\"compromised identity (default: the export's identity)\")\n    ap.add_argument(\"--min-tier\", type=int, default=0,\n                    help=\"skip systems below this tier (default: 0, all)\")\n    ap.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = ap.parse_args()\n\n    doc = load(args.export)\n    if not isinstance(doc, dict):\n        die(\"export: expected a JSON object\")\n    identity = (args.identity or doc.get(\"identity\") or \"\").strip()\n    if not identity:\n        die(\"no identity: pass --identity or set 'identity' in the export\")\n    if args.min_tier < 0:\n        die(\"--min-tier must be zero or more\")\n\n    systems = {}\n    raw_systems = doc.get(\"systems\") or []\n    if not isinstance(raw_systems, list):\n        die(\"export: systems must be a list\")\n    for s in raw_systems:\n        if not isinstance(s, dict):\n            die(\"export: every system must be an object\")\n        name = str(s.get(\"name\", \"\")).strip()\n        if not name:\n            die(\"export: a system has no name\")\n        if name in systems:\n            die(\"export: duplicate system %s\" % name)\n        try:\n            tier = int(s.get(\"tier\", 99))\n        except (TypeError, ValueError):\n            die(\"system %s: tier is not an integer\" % name)\n        kind = str(s.get(\"kind\", \"other\")).lower()\n        if kind not in KINDS:\n            die(\"system %s: unknown kind %s\" % (name, kind))\n        systems[name] = (tier, kind)\n\n    accounts = doc.get(\"accounts\") or []\n    if not isinstance(accounts, list):\n        die(\"export: accounts must be a list\")\n    hits = []\n    for a in accounts:\n        if not isinstance(a, dict):\n            die(\"export: every account must be an object\")\n        login = str(a.get(\"login\") or a.get(\"user\") or \"\").strip()\n        if login.lower() != identity.lower():\n            continue\n        system = str(a.get(\"system\", \"\")).strip()\n        if system not in systems:\n            die(\"account %s references unknown system %s\"\n                % (a.get(\"id\", \"?\"), system or \"(none)\"))\n        if not a.get(\"id\"):\n            die(\"account in %s has no id\" % system)\n        tier, kind = systems[system]\n        if tier < args.min_tier:\n            continue\n        hits.append({\n            \"system\": system,\n            \"id\": str(a[\"id\"]),\n            \"tier\": tier,\n            \"kind\": kind,\n            \"shared\": bool(a.get(\"shared\")),\n            \"break_glass\": bool(a.get(\"break_glass\")),\n            \"owner\": str(a.get(\"owner\") or \"\").strip(),\n        })\n\n    if not hits:\n        die(\"no accounts for %s in %s\" % (identity, args.export))\n\n    hits.sort(key=lambda x: (x[\"tier\"], x[\"system\"], x[\"id\"]))\n    print(\"PLAN lock %s across %d system%s\"\n          % (identity, len(hits), \"\" if len(hits) == 1 else \"s\"))\n    order = []\n    for h in hits:\n        if h[\"system\"] not in order:\n            order.append(h[\"system\"])\n    print(\"ORDER %s\" % \", \".join(\"%s(%d)\" % (s, systems[s][0]) for s in order))\n\n    held = 0\n    for h in hits:\n        if h[\"break_glass\"]:\n            print(\"HOLD %s %s break-glass, confirm before disable\"\n                  % (h[\"system\"], h[\"id\"]))\n            held += 1\n            continue\n        if h[\"shared\"]:\n            owner = h[\"owner\"] or \"unknown\"\n            print(\"HOLD %s %s shared, owner %s\"\n                  % (h[\"system\"], h[\"id\"], owner))\n            held += 1\n            continue\n        verb, why = KINDS[h[\"kind\"]]\n        print(\"%s %s %s %s\" % (verb, h[\"system\"], h[\"id\"], why))\n\n    if held:\n        print(\"FINDING %d held, resolve before disabling\" % held)\n        sys.exit(1)\n    print(\"READY %d accounts in dependency order\" % len(hits))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p684","n":"684","slug":"a-token-revocation-list-oldest-first","title":"A token revocation list, oldest first","sections":["it","devsecops"],"topic":"Python scripts","tags":["IncidentResponse","Python","SecretsManagement","CyberSecurity","SRE"],"series_name":"IR","design":"code","script":"token_revoke.py","date":"2031-02-09","files":{"it":"notes/p684-a-token-revocation-list-oldest-first.md","devsecops":"notes/p684-a-token-revocation-list-oldest-first.md"},"md":"# A token revocation list, oldest first\n\nThe laptop is rebuilt, the password is changed, and a token issued in 2023 still has access to the repository. Nobody inventoried it.\n\n## Quick reference\n\nThe full script is at the end of this note: `token_revoke.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 token_revoke.py tokens.json --user ana@example.com\n```\n\n**Example output**\n\n```text\nREVOKE 3 tokens for ana@example.com\nOLDEST tok-11c 1269d, last used 215d\n```\n\n## Explained\n\n### What it does\n\n`token_revoke.py` reads a token inventory: id, system, kind, owner, scopes, created, last_used. It selects tokens for one user or service, computes both ages, sorts oldest first, and prints a REVOKE line plus a command where the provider has one. AWS keys use aws iam delete-access-key; Vault tokens use vault token revoke -accessor. A value or secret in the export is never printed.\n\n### What the screen shows\n\nThe panel computes the ages and prints REVOKE 3 tokens for ana@example.com, then OLDEST tok-11c 1269d, last used 215d. The full tool adds the scopes and the command.\n\n### How to run it\n\n`python3 token_revoke.py tokens.json --user ana@example.com`. Use `--service ci-runner` for a service, and `--stale-days 90` for tokens unused this quarter.\n\n### The fix\n\nRevoke the oldest first, then every token with a write scope. Delete the AWS key, revoke the Vault accessor, and work the console list for the rest. Change the password after the tokens: a live token ignores the reset.\n\n### Prevention\n\nInventory a token when it is issued, with an owner and a date. Run the stale report weekly; a token with no last_used is a candidate for the next incident.\n\n> **The honest caveat:** last_used is provider-reported and often coarse, so a background job can look stale while it is working.\n\n> **The lesson:** a revocation without a list is a guess with a shell.\n\n## In one line\n\n> **Inventory first, then revoke.** A revocation without a list is a guess with a shell.\n\n**Revoke by id**\n\n```bash\nvault token revoke -accessor tok-11c\n```\n\n## Script: token_revoke.py\n\nDownload: [scripts/token_revoke.py](../scripts/token_revoke.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"token_revoke.py - find every token an identity owns and print the revokes.\n\nReads a token inventory, selects the tokens for one user or one service, and\nprints a revocation command for each one, oldest first. The inventory carries\nid, owner, scopes, created and last_used; the script prints ids, ages and\nscopes and never prints a value, secret or hash, even when the export contains\none.\n\nRead-only: it prints the commands. It does not call a provider, and it does not\nneed the token value to revoke an AWS key or a Vault accessor.\n\nUsage:\n  ./token_revoke.py tokens.json --user ana@example.com\n  ./token_revoke.py tokens.json --service ci-runner --stale-days 90\n\nCron / CI (weekly drift report, exits 1 while tokens exist):\n  0 7 * * 1 /usr/local/bin/token_revoke.py /etc/ir/tokens.json \\\n    --user ana@example.com >> /var/log/ir/tokens.log 2>&1\n\nExit codes:\n  0  no token matches\n  1  at least one token matches and needs revocation\n  2  usage error, unreadable JSON, bad date, or neither --user nor --service\n\nInventory:\n  {\"tokens\": [{\"id\": \"tok-11c\", \"system\": \"vault\", \"kind\": \"vault-token\",\n               \"owner\": \"ana@example.com\", \"service\": \"ci-runner\",\n               \"scopes\": [\"repo:read\"], \"created\": \"2024-05-01\",\n               \"last_used\": \"2026-03-20\"}, ...]}\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date\n\n\ndef die(msg):\n    print(\"ERROR \" + msg, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as fh:\n            return json.load(fh)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef day(value, label):\n    if value in (None, \"\"):\n        return None\n    try:\n        return date.fromisoformat(str(value)[:10])\n    except ValueError:\n        die(\"%s: not an ISO date: %s\" % (label, value))\n\n\ndef age(d):\n    if d is None:\n        return None\n    return max(0, (date.today() - d).days)\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Print a revocation command for every token a user or \"\n                    \"service owns. Values are never printed.\")\n    ap.add_argument(\"inventory\", help=\"token inventory JSON\")\n    who = ap.add_mutually_exclusive_group(required=True)\n    who.add_argument(\"--user\", help=\"owner to match\")\n    who.add_argument(\"--service\", help=\"service to match\")\n    ap.add_argument(\"--stale-days\", type=int, default=None,\n                    help=\"only tokens unused for at least this many days\")\n    ap.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = ap.parse_args()\n\n    if args.stale_days is not None and args.stale_days < 0:\n        die(\"--stale-days must be zero or more\")\n\n    doc = load(args.inventory)\n    rows = doc.get(\"tokens\", doc) if isinstance(doc, dict) else doc\n    if not isinstance(rows, list):\n        die(\"inventory: expected a list under 'tokens'\")\n\n    want = (args.user or args.service or \"\").strip().lower()\n    field = \"owner\" if args.user else \"service\"\n    picked = []\n    for t in rows:\n        if not isinstance(t, dict):\n            die(\"inventory: every token must be an object\")\n        if not t.get(\"id\"):\n            die(\"inventory: a token has no id\")\n        if str(t.get(field, \"\")).strip().lower() != want:\n            continue\n        created = day(t.get(\"created\"), \"created\")\n        used = day(t.get(\"last_used\"), \"last_used\")\n        if created is None:\n            die(\"token %s: created is required\" % t[\"id\"])\n        c_age = age(created)\n        u_age = age(used)\n        if args.stale_days is not None:\n            if u_age is not None and u_age < args.stale_days:\n                continue\n        picked.append({\n            \"id\": str(t[\"id\"]),\n            \"system\": str(t.get(\"system\", \"unknown\")),\n            \"kind\": str(t.get(\"kind\", \"other\")),\n            \"owner\": str(t.get(\"owner\", \"\")),\n            \"scopes\": [str(x) for x in (t.get(\"scopes\") or [])],\n            \"created_age\": c_age,\n            \"last_age\": u_age,\n            \"principal\": str(t.get(\"principal\", \"\")),\n        })\n\n    picked.sort(key=lambda x: (-x[\"created_age\"], x[\"id\"]))\n    print(\"PLAN revoke tokens for %s\" % (args.user or args.service))\n    if not picked:\n        print(\"OK no tokens for %s\" % (args.user or args.service))\n        return 0\n\n    for t in picked:\n        last = \"never\" if t[\"last_age\"] is None else \"%dd\" % t[\"last_age\"]\n        scopes = \",\".join(t[\"scopes\"]) or \"-\"\n        print(\"REVOKE %s %s created=%dd last=%s scopes=%s\"\n              % (t[\"id\"], t[\"system\"], t[\"created_age\"], last, scopes))\n        if t[\"kind\"] == \"aws-key\":\n            print(\"CMD aws iam delete-access-key --user-name %s \"\n                  \"--access-key-id %s\" % (t[\"principal\"] or t[\"owner\"], t[\"id\"]))\n        elif t[\"kind\"] == \"vault-token\":\n            print(\"CMD vault token revoke -accessor %s\" % t[\"id\"])\n        else:\n            print(\"CMD no template for %s, revoke %s in the admin console\"\n                  % (t[\"kind\"], t[\"id\"]))\n\n    print(\"REVOKE %d token%s for %s\"\n          % (len(picked), \"\" if len(picked) == 1 else \"s\",\n             args.user or args.service))\n    oldest = picked[0]\n    last = \"never\" if oldest[\"last_age\"] is None else \"%dd\" % oldest[\"last_age\"]\n    print(\"OLDEST %s %dd, last used %s\"\n          % (oldest[\"id\"], oldest[\"created_age\"], last))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p685","n":"685","slug":"an-edge-block-that-checks-the-allow-list","title":"An edge block that checks the allow list","sections":["it","devsecops"],"topic":"Python scripts","tags":["IncidentResponse","Python","WAF","CyberSecurity","Networking"],"series_name":"IR","design":"code","script":"edge_block.py","date":"2031-02-11","files":{"it":"notes/p685-an-edge-block-that-checks-the-allow-list.md","devsecops":"notes/p685-an-edge-block-that-checks-the-allow-list.md"},"md":"# An edge block that checks the allow list\n\nThe block list has one address. It belongs to a CDN range the marketing site uses, and the partner allow covers the same range.\n\n## Quick reference\n\nThe full script is at the end of this note: `edge_block.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 edge_block.py rules.json 203.0.113.0/25\n```\n\n**Example output**\n\n```text\nCONFLICT 203.0.113.0/25 inside allow partner-api\nFINDING 1 conflict in 2 targets\n```\n\n## Explained\n\n### What it does\n\n`edge_block.py` reads the rule list and the addresses to block. Rules are first-match, so a new block goes to the top, which is why every allow rule and protected range is checked first. A block that contains an allow shadows it; one inside an allow never fires. It prints BLOCK, DUP or CONFLICT, exits 1 on conflict and writes nothing.\n\n### What the screen shows\n\nThe panel checks a /25 against the partner /24: CONFLICT 203.0.113.0/25 inside allow partner-api, then FINDING 1 conflict in 2 targets. The clean target becomes a rule; the conflict does not.\n\n### How to run it\n\n`python3 edge_block.py rules.json 203.0.113.0/25 198.51.100.7`. Add `--protect` for ranges that must never be blocked, such as your edge. `--apply` writes the merged list with mode 0600, or refuses on conflict.\n\n### The fix\n\nNarrow the block to the host, or move the control to the path or session: a rate limit on /login, a token revocation, a WAF rule on the user agent. If the range is hostile, remove the allow rule, with the partner's ticket open.\n\n### Prevention\n\nEvery block request goes through the overlap check. Keep an id and owner on each allow rule, so the next person sees what the block breaks.\n\n> **The honest caveat:** it reasons about CIDR overlap, not ASN or hosting, and a /24 can still be shared.\n\n> **The lesson:** a block is a promise to check the allow list.\n\n## In one line\n\n> **Block the address, not the customer.** Every allow rule the block overlaps is a ticket you write in advance.\n\n**Find the overlap**\n\n```bash\npython3 edge_block.py rules.json 1.1.1.1\n```\n\n## Script: edge_block.py\n\nDownload: [scripts/edge_block.py](../scripts/edge_block.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"edge_block.py - add block rules at the edge without breaking an allow.\n\nReads the edge/WAF rule list and one or more IPs or CIDRs, then builds the\nblock rules that would be prepended to the list. Every request is checked\nagainst the existing allow rules and against a protected list, because a block\nplaced first wins: a target that overlaps an allow silently disables it.\n\nThe default is a dry run: it prints the plan and writes nothing. --apply writes\nthe new rule list to a file; it refuses to write when any target conflicts, and\nit never calls an edge API.\n\nUsage:\n  ./edge_block.py rules.json 198.51.100.7\n  ./edge_block.py rules.json 203.0.113.0/25 198.51.100.7 --apply\n  ./edge_block.py rules.json 198.51.100.7 --protect 10.0.0.0/8\n\nCron / CI (check a block request before the change window):\n  ./edge_block.py /etc/edge/rules.json \"$(cat /tmp/ir/block-list)\" \\\n    | tee /var/log/ir/edge-block.log\n\nExit codes:\n  0  no conflict; every target is new, duplicate or already blocked\n  1  at least one conflict with an allow or a protected range\n  2  usage error, unreadable JSON, bad CIDR, unknown action or duplicate id\n\nRules: {\"rules\": [{\"id\": \"allow-partner\", \"action\": \"allow\",\n                   \"cidr\": \"203.0.113.0/24\"}, ...],\n        \"protect\": [\"198.51.100.10/32\"]}\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport ipaddress\nimport json\nimport os\nimport sys\n\n\ndef die(msg):\n    print(\"ERROR \" + msg, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as fh:\n            return json.load(fh)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef cidr(value, label):\n    try:\n        return ipaddress.ip_network(str(value), strict=False)\n    except ValueError:\n        die(\"%s: not an IP or CIDR: %s\" % (label, value))\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Build edge block rules and refuse targets that overlap \"\n                    \"an allow or a protected range.\")\n    ap.add_argument(\"rules\", help=\"edge/WAF rules JSON\")\n    ap.add_argument(\"targets\", nargs=\"+\", help=\"IPs or CIDRs to block\")\n    ap.add_argument(\"--protect\", action=\"append\", default=[],\n                    help=\"address that must never be blocked (repeatable)\")\n    ap.add_argument(\"--apply\", action=\"store_true\",\n                    help=\"write the new rule list; nothing is deployed\")\n    ap.add_argument(\"--out\", default=None,\n                    help=\"output path for --apply (default /tmp/edge-rules.json)\")\n    ap.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = ap.parse_args()\n\n    doc = load(args.rules)\n    if not isinstance(doc, dict):\n        die(\"rules: expected a JSON object\")\n    rules = doc.get(\"rules\") or []\n    if not isinstance(rules, list):\n        die(\"rules: 'rules' must be a list\")\n    seen_ids = set()\n    allows, blocks = [], []\n    for r in rules:\n        if not isinstance(r, dict):\n            die(\"rules: every rule must be an object\")\n        rid = str(r.get(\"id\", \"\")).strip()\n        if not rid:\n            die(\"rules: a rule has no id\")\n        if rid in seen_ids:\n            die(\"rules: duplicate id %s\" % rid)\n        seen_ids.add(rid)\n        action = str(r.get(\"action\", \"\")).lower()\n        if action not in (\"allow\", \"block\"):\n            die(\"rule %s: action must be allow or block\" % rid)\n        entry = (rid, cidr(r.get(\"cidr\"), \"rule %s cidr\" % rid))\n        (allows if action == \"allow\" else blocks).append(entry)\n\n    protect = [cidr(p, \"--protect\") for p in args.protect]\n    doc_protect = doc.get(\"protect\") or []\n    if not isinstance(doc_protect, list):\n        die(\"rules: protect must be a list of CIDRs\")\n    protect += [cidr(p, \"protect\") for p in doc_protect]\n\n    targets = []\n    for t in args.targets:\n        n = cidr(t, \"target\")\n        if n not in targets:\n            targets.append(n)\n    targets.sort(key=lambda n: (int(n.network_address), n.prefixlen))\n\n    print(\"PLAN block %d target%s before %d rules\"\n          % (len(targets), \"\" if len(targets) == 1 else \"s\", len(rules)))\n    new_rules, conflicts, dups = [], 0, 0\n    for t in targets:\n        block_hit = None\n        for rid, a in allows:\n            if not t.overlaps(a):\n                continue\n            conflicts += 1\n            if t.supernet_of(a) or t == a:\n                print(\"CONFLICT %s shadows allow %s\" % (t, rid))\n            else:\n                print(\"CONFLICT %s inside allow %s\" % (t, rid))\n            block_hit = rid\n        for p in protect:\n            if t.overlaps(p):\n                conflicts += 1\n                print(\"CONFLICT %s overlaps protected %s\" % (t, p))\n                block_hit = block_hit or \"protect\"\n        covered = [rid for rid, b in blocks if t.subnet_of(b)]\n        if covered and not block_hit:\n            dups += 1\n            print(\"DUP %s already blocked by %s\" % (t, covered[0]))\n            continue\n        if block_hit:\n            continue\n        n = len(new_rules) + 1\n        while \"ir-block-%d\" % n in seen_ids:\n            n += 1\n        nid = \"ir-block-%d\" % n\n        seen_ids.add(nid)\n        new_rules.append({\"id\": nid, \"action\": \"block\", \"cidr\": str(t),\n                          \"source\": \"edge_block\"})\n        print(\"BLOCK %s (new rule %s)\" % (t, nid))\n\n    if conflicts:\n        print(\"FINDING %d conflict%s in %d target%s\"\n              % (conflicts, \"\" if conflicts == 1 else \"s\",\n                 len(targets), \"\" if len(targets) == 1 else \"s\"))\n        if args.apply:\n            print(\"REFUSED --apply declined, no rule list written\")\n        sys.exit(1)\n    print(\"READY %d new rule%s, %d duplicate%s, no conflict\"\n          % (len(new_rules), \"\" if len(new_rules) == 1 else \"s\",\n             dups, \"\" if dups == 1 else \"s\"))\n\n    if not args.apply:\n        print(\"DRY-RUN nothing written. Add --apply to write the rule list.\")\n        return 0\n\n    out = args.out or \"/tmp/edge-rules.json\"\n    merged = {\"rules\": new_rules + rules}\n    if protect:\n        merged[\"protect\"] = [str(p) for p in protect]\n    try:\n        fd = os.open(out, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)\n        with os.fdopen(fd, \"w\", encoding=\"utf-8\") as fh:\n            json.dump(merged, fh, indent=2, sort_keys=True)\n            fh.write(\"\\n\")\n        os.chmod(out, 0o600)\n    except OSError as exc:\n        die(\"%s: %s\" % (out, exc.strerror or exc))\n    print(\"WROTE %s mode 0600, %d rules\" % (out, len(merged[\"rules\"])))\n    print(\"APPLY load %s into the edge rule set, then reload\" % out)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p686","n":"686","slug":"a-kill-switch-audit-ranked-by-blast-radius","title":"A kill-switch audit ranked by blast radius","sections":["it","devsecops"],"topic":"Python scripts","tags":["IncidentResponse","Python","FeatureFlags","SRE","DevOps"],"series_name":"IR","design":"code","script":"kill_switch_audit.py","date":"2031-02-13","files":{"it":"notes/p686-a-kill-switch-audit-ranked-by-blast-radius.md","devsecops":"notes/p686-a-kill-switch-audit-ranked-by-blast-radius.md"},"md":"# A kill-switch audit ranked by blast radius\n\nThe rollback is approved and someone asks which flag to pull. The export lists forty flags for checkout, and three of them have no owner.\n\n## Quick reference\n\nThe full script is at the end of this note: `kill_switch_audit.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 kill_switch_audit.py flags.json --service checkout\n```\n\n**Example output**\n\n```text\nFLAG checkout.payments-v2 blast 70 default=off\nNOOWNER checkout.legacy blast 6 default=on\n```\n\n## Explained\n\n### What it does\n\n`kill_switch_audit.py` reads a feature-flag export and keeps the flags that gate the named service. For each it checks the two things a kill switch needs at 03:00: an owner and a boolean default. It ranks by blast radius, the traffic share multiplied by the number of services that read the flag, minimum one.\n\n### What the screen shows\n\nThe panel ranks two flags and prints FLAG checkout.payments-v2 blast 70 default=off, then NOOWNER checkout.legacy blast 6 default=on. The first line is the one to pull first.\n\n### How to run it\n\n`python3 kill_switch_audit.py flags.json --service checkout`. Add `--env prod` for production only, and `--min-blast 10` for the long tail. Exit 1 is a missing owner or default.\n\n### The fix\n\nPull the highest-blast flag with an owner, verify the default is the safe state, and watch the error rate for one deploy cycle before the next. A flag with no owner stops the rollback until someone claims it.\n\n### Prevention\n\nThe flag template requires an owner and a default. Run the audit weekly and retire flags after rollout; a flag on for a year is configuration with a nicer name.\n\n> **The honest caveat:** blast radius is a model from traffic share and declared dependents, not a measurement, and the dependency list goes stale.\n\n> **The lesson:** a flag with no owner is a rumour with a dashboard.\n\n## In one line\n\n> **No owner, no switch.** A flag nobody owns is a change nobody can approve at 03:00.\n\n**Rank the flags**\n\n```bash\npython3 kill_switch_audit.py flags.json\n```\n\n## Script: kill_switch_audit.py\n\nDownload: [scripts/kill_switch_audit.py](../scripts/kill_switch_audit.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"kill_switch_audit.py - rank the flags that gate one service before a rollback.\n\nReads a feature-flag export, finds every flag that gates the named service, and\nchecks the two things a kill switch needs at 03:00: an owner and a default. The\nflags are ranked by blast radius, the traffic share multiplied by the number of\nservices that read the flag, so the first line is the one to pull first.\n\nRead-only: it prints the audit and writes nothing. A flag without an owner is a\nfinding, not a warning: nobody can approve pulling it.\n\nUsage:\n  ./kill_switch_audit.py flags.json --service checkout\n  ./kill_switch_audit.py flags.json --service checkout --env prod\n  ./kill_switch_audit.py flags.json --service checkout --min-blast 10\n\nCron / CI (weekly, fails while a flag has no owner):\n  0 6 * * 1 /usr/local/bin/kill_switch_audit.py /etc/ir/flags.json \\\n    --service checkout >> /var/log/ir/flags.log 2>&1\n\nExit codes:\n  0  every flag in scope has an owner and a default\n  1  at least one flag in scope has no owner or no default\n  2  usage error, unreadable JSON, bad traffic value, or no flags for service\n\nFlags: {\"service\": \"checkout\",\n        \"flags\": [{\"key\": \"checkout.payments-v2\", \"owner\": \"payments\",\n                   \"default\": false, \"traffic\": 35,\n                   \"dependents\": [\"cart\", \"orders\"]}, ...]}\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef die(msg):\n    print(\"ERROR \" + msg, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as fh:\n            return json.load(fh)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef blast(traffic, dependents):\n    return traffic * max(1, len(dependents))\n\n\ndef main():\n    ap = argparse.ArgumentParser(\n        description=\"Audit the feature flags that gate a service: owner, \"\n                    \"default and blast radius.\")\n    ap.add_argument(\"flags\", help=\"feature-flag export JSON\")\n    ap.add_argument(\"--service\", required=True, help=\"service to audit\")\n    ap.add_argument(\"--env\", default=None,\n                    help=\"only flags that apply to this environment\")\n    ap.add_argument(\"--min-blast\", type=int, default=0,\n                    help=\"only flags with a blast radius of at least this\")\n    ap.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = ap.parse_args()\n\n    if args.min_blast < 0:\n        die(\"--min-blast must be zero or more\")\n\n    doc = load(args.flags)\n    if not isinstance(doc, dict):\n        die(\"export: expected a JSON object\")\n    flags = doc.get(\"flags\") or []\n    if not isinstance(flags, list):\n        die(\"export: 'flags' must be a list\")\n    default_service = str(doc.get(\"service\", \"\")).strip().lower()\n    want = args.service.strip().lower()\n\n    rows = []\n    in_service = 0\n    for f in flags:\n        if not isinstance(f, dict):\n            die(\"export: every flag must be an object\")\n        key = str(f.get(\"key\", \"\")).strip()\n        if not key:\n            die(\"export: a flag has no key\")\n        service = str(f.get(\"service\") or default_service).strip().lower()\n        if service != want:\n            continue\n        in_service += 1\n        raw_envs = f.get(\"environments\") or []\n        if not isinstance(raw_envs, list):\n            die(\"flag %s: environments must be a list\" % key)\n        envs = [str(e).lower() for e in raw_envs]\n        if args.env and args.env.lower() not in envs:\n            continue\n        owner = str(f.get(\"owner\") or \"\").strip()\n        has_default = isinstance(f.get(\"default\"), bool)\n        if f.get(\"default\") is not None and \"default\" in f and not has_default:\n            die(\"flag %s: default must be true or false\" % key)\n        try:\n            traffic = float(f.get(\"traffic\", 0))\n        except (TypeError, ValueError):\n            die(\"flag %s: traffic is not a number\" % key)\n        if not 0 <= traffic <= 100:\n            die(\"flag %s: traffic must be 0-100\" % key)\n        deps = f.get(\"dependents\") or []\n        if not isinstance(deps, list):\n            die(\"flag %s: dependents must be a list\" % key)\n        b = blast(traffic, deps)\n        if b < args.min_blast:\n            continue\n        rows.append({\"key\": key, \"owner\": owner, \"default\": f.get(\"default\"),\n                     \"has_default\": has_default, \"blast\": int(b)})\n\n    if not in_service:\n        die(\"no flags gate service %s\" % args.service)\n    if not rows:\n        print(\"OK no flags for %s match the filters\" % args.service)\n        return 0\n\n    rows.sort(key=lambda r: (-r[\"blast\"], r[\"key\"]))\n    print(\"PLAN kill-switch audit for %s, %d flag%s\"\n          % (args.service, len(rows), \"\" if len(rows) == 1 else \"s\"))\n    no_owner = no_default = 0\n    for r in rows:\n        if not r[\"owner\"]:\n            no_owner += 1\n            if not r[\"has_default\"]:\n                no_default += 1\n            d = (\"on\" if r[\"default\"] else \"off\") if r[\"has_default\"] else \"missing\"\n            print(\"NOOWNER %s blast %d default=%s\" % (r[\"key\"], r[\"blast\"], d))\n        elif not r[\"has_default\"]:\n            no_default += 1\n            print(\"NODEFAULT %s blast %d owner=%s\"\n                  % (r[\"key\"], r[\"blast\"], r[\"owner\"]))\n        else:\n            print(\"FLAG %s blast %d default=%s\"\n                  % (r[\"key\"], r[\"blast\"],\n                     \"on\" if r[\"default\"] else \"off\"))\n\n    if no_owner or no_default:\n        print(\"FINDING %d flag%s without an owner, %d without a default\"\n              % (no_owner, \"\" if no_owner == 1 else \"s\", no_default))\n        sys.exit(1)\n    print(\"READY %d flags, every one owned and defaulted\" % len(rows))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p687","n":"687","slug":"the-containment-tree","title":"The containment tree","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","SRE","DevOps","Containment"],"series_name":"IR","design":"architecture","script":"","date":"2031-02-16","files":{"it":"notes/p687-the-containment-tree.md","devsecops":"notes/p687-the-containment-tree.md"},"md":"# The containment tree\n\nThe first hour of containment is a tree, not a checklist. You scope what you know, cut the paths you can see, and verify that the cut did what you meant.\n\n## Quick reference\n\n- **Tier 1 · Scope:** Host (HOST) · Identity (IDENT)\n- **Tier 2 · Cut:** Network path (NET) · Access path (ACL)\n- **Tier 3 · Verify:** Service health (SVC) · Evidence (EVID)\n\n**Request flow:** Host → Network path → Service health; Network path → Evidence; Identity → Access path → Service health; Access path → Evidence\n\n## Explained\n\n### Scope\n\nScope is the host and the identity. The host is an address, a name, a role and a set of open ports. The identity is a login, a session and the tokens it can mint. You cannot contain what you cannot name, and you cannot name what nobody inventoried.\n\n### Cut\n\nCut is the network and the access. The network cut drops east-west traffic first and keeps the management path. The access cut ends the sessions at the identity provider, then revokes the keys and tokens that outlive a password. Both cuts are paths, not machines: a host with no route is contained even when it is still running.\n\n### Verify\n\nVerify is the service and the evidence. The service check proves the safe part still answers, so you are not trading an incident for a silent outage. The evidence step captures the process list, the connections and the auth log before they age out.\n\n> **The honest caveat:** a missing arrow is the failure. If the tree does not show that the build account reads the flag, the cut will find it for you.\n\n> **The lesson:** scope it, cut it, then prove it, in that order, every time.\n\n## In one line\n\n> **Scope it, cut it, then prove it.** Containment is a tree: the root is what you know, the leaves what you can prove.\n\n**Scope before cut**\n\n```bash\nss -tlnp; last -n 20\n```\n"},{"id":"p688","n":"688","slug":"containment-commands","title":"Containment commands","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","Linux","CommandLine","CyberSecurity","SRE"],"series_name":"IR","design":"cheatsheet","script":"","date":"2031-02-18","files":{"it":"notes/p688-containment-commands.md","devsecops":"notes/p688-containment-commands.md"},"md":"# Containment commands\n\nThe containment step is not a mood. It is a short list of commands, run in an order you can defend afterwards.\n\n## Quick reference\n\n### Host\n\n```bash\nss -tlnp\nps aux --sort=-%cpu | head\nls -l /proc/$PID/fd | wc -l\nsystemctl isolate rescue.target\nhistory -a; tail -20 ~/.bash_history\n```\n\n### Identity\n\n```bash\npasswd -l ana\nchage -E 0 ana\npkill -KILL -u ana\nloginctl terminate-user ana\ncrontab -r -u ana\n```\n\n### Edge\n\n```bash\niptables -I INPUT -s 1.2.3.4 -j DROP\nufw deny from 1.2.3.4\nnft list ruleset\nconntrack -L | grep 1.2.3.4\ndig +short api.example.com\n```\n\n### Flags\n\n```bash\njq '.flags[]|select(.owner==null)'\njq -r '.flags[] | .key' flags.json\ncurl -s $FLAG_API/flags | jq .\ncurl -s $FLAG_API/flags | jq '.on'\njq '.flags[] | select(.on)' flags.json\n```\n\n## Explained\n\n### Host\n\n`ss -tlnp` shows what still answers. `ps aux --sort=-%cpu | head` finds the process that appeared at 03:04. `ls -l /proc/$PID/fd | wc -l` counts its open files. history -a; tail -20 ~/.bash_history shows what the last shell did. `systemctl isolate rescue.target` drops to a single-user shell.\n\n### Identity\n\n`passwd -l ana` locks the password, `chage -E 0 ana` expires the account, and `pkill -KILL -u ana` ends the processes that are already running. `loginctl terminate-user ana` closes the session, and `crontab -r -u ana` removes the persistence nobody looks at.\n\n### Edge\n\n`iptables -I INPUT -s 1.2.3.4 -j DROP` is still the fastest block on a host. `ufw deny from 1.2.3.4` is the same rule with a nicer face. conntrack -L | grep 1.2.3.4 shows the live connections, and `nft list ruleset` proves the rule is where you think it is.\n\n### Flags\n\n`jq '.flags[]|select(.owner==null)'` finds the flags nobody owns. `curl -s $FLAG_API/flags | jq .` reads the export without a dashboard. Pulling a flag is a change: record the key, the old value and the time.\n\n> **The honest caveat:** these are host and file commands; the control plane, the cloud IAM and the SaaS admin need their own runbook.\n\n> **The lesson:** a command is not containment; the order is.\n\n## In one line\n\n> **Contain the path, not the story.** Four families, one order: host, identity, edge, flags.\n\n**Cut, then check**\n\n```bash\nss -tlnp\n```\n"},{"id":"p689","n":"689","slug":"four-containment-failures","title":"Four containment failures","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","SRE","DevOps","Containment"],"series_name":"IR","design":"explainer","script":"","date":"2031-02-20","files":{"it":"notes/p689-four-containment-failures.md","devsecops":"notes/p689-four-containment-failures.md"},"md":"# Four containment failures\n\nContainment fails in four predictable ways, and each one turns a response into a second incident.\n\n## Quick reference\n\n### Cutting your own access\n\n- You deny 0.0.0.0/0 and lose SSH mid-edit\n- Keep a console outside the rules you change\n- Test the rule from the management subnet\n- Out-of-band access is the real control\n\n### Locking a shared account\n\n- The build account serves four teams\n- Disabling it stops every pipeline at once\n- Disable the person, rotate the shared secret\n- Hold the shared account for its owner\n\n### Blocking a CDN IP\n\n- One edge address serves a thousand customers\n- A /32 block is a shared outage\n- Check the ASN and the allow rules first\n- Block the path or the session, not the CDN\n\n### Killing the wrong flag\n\n- Two flags gate the same checkout path\n- The kill switch is not the rollback\n- Pull the flag with an owner and a default\n- Rehearse the off state in a drill\n\n> **Note:** A containment step fails when it removes the path you need to finish it.\n\n## Explained\n\n### Cutting your own access\n\nYou paste a deny rule for 0.0.0.0/0, the SSH session dies mid-edit, and the host is a black hole with a pager. The fix is a management path outside the rules you are changing: a console, a jump host on another subnet, or an out-of-band card.\n\n### Locking a shared account\n\nThe build account carries four teams' pipelines. Disabling it stops every deploy at once, and the incident becomes a delivery outage. Disable the person's account, rotate the shared secret, and hold the shared account for its owner.\n\n### Blocking a CDN IP\n\nOne edge address serves a thousand customers. A /32 block looks precise and takes a slice of the internet with it. Check the ASN, the allow rules and the hostnames behind the address before the block.\n\n### Killing the wrong flag\n\nTwo flags gate the same checkout path, and the friendly name is not the one that controls the failure. Pull the flag with an owner and a default, and rehearse the off state in staging.\n\n> **The honest caveat:** every one of these is cheap to discover in a drill and expensive to discover at 03:00.\n\n> **The lesson:** containment removes a path, and the path you remove is sometimes your own.\n\n## In one line\n\n> **Contain the blast, not the response.** Every failed containment is a control that was never tried in a drill.\n\n**Try it in a drill**\n\n```bash\nnft -c -f quarantine.nft\n```\n"},{"id":"p690","n":"690","slug":"9-commands-to-contain-a-host","title":"9 commands to contain a host","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","Linux","SRE","CyberSecurity","CommandLine"],"series_name":"IR","design":"terminal","script":"","date":"2031-02-23","files":{"it":"notes/p690-9-commands-to-contain-a-host.md","devsecops":"notes/p690-9-commands-to-contain-a-host.md"},"md":"# 9 commands to contain a host\n\nA host is contained when three things are true: the paths are cut, the process is understood, and the console still answers.\n\n## Quick reference\n\n### Cut the paths\n\n```bash\n# what still answers on the host\nss -tlnp\n# database subnet, console kept\niptables -I INPUT -s 10.20.5.0/24 -j DROP\n# stop the app, not the kernel\nsystemctl stop app.service\n```\n\n### Check the process\n\n```bash\n# parent chain, reparented to 1\nps -eo pid,ppid,stat,cmd | head\n# open files: one number\nlsof -p 412 | wc -l\n# the last ten minutes\njournalctl -u app --since -10m\n```\n\n### Keep the console\n\n```bash\n# prove the console first\nssh -o ConnectTimeout=5 ops@10.20.4.11\n# management rule is first\nnft list chain inet fw input\n# timestamp the containment\ndate -u\n```\n\n## Explained\n\n### Cut\n\n`ss -tlnp` shows what is still listening; a service you did not expect is the first finding, and a listener you cannot name is a listener you have not contained. `iptables -I INPUT -s 10.20.5.0/24 -j DROP` drops traffic from the database subnet while the management subnet stays out of the rule. `systemctl stop app.service` stops the application without stopping the kernel, which keeps the host and its logs up.\n\n### Check\n\n`ps -eo pid,ppid,stat,cmd | head` shows the parent chain, and a process reparented to 1 has outlived its supervisor. `lsof -p 412 | wc -l` gives one number for open files; a jump from 200 to 2,000 is a leak or a download. Write the pid down; it is the handle for everything else. `journalctl -u app --since -10m` is the last ten minutes, which is usually the whole incident.\n\n### Keep\n\n`ssh -o ConnectTimeout=5 ops@10.20.4.11` proves the console, and you run it before the first cut. `nft list chain inet fw input` shows the management accept sitting above the drop. `date -u` timestamps the containment for the timeline you will write later.\n\n> **The honest caveat:** rescue.target stops everything, including the agent you need; use it on a host you can reach through a console.\n\n> **The lesson:** cut the path, keep the console, and write down the time.\n\n## In one line\n\n> **Cut the path, keep the console.** Nine commands, one rule: the cut never includes the way in.\n\n**Try it**\n\n```bash\nssh -o ConnectTimeout=5 ops@10.20.4.11\n```\n"},{"id":"p691","n":"691","slug":"quiz-3-containment-questions","title":"Quiz: 3 containment questions","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","SRE","DevOps","Quiz"],"series_name":"IR","design":"quiz","script":"","date":"2031-02-25","files":{"it":"notes/p691-quiz-3-containment-questions.md","devsecops":"notes/p691-quiz-3-containment-questions.md"},"md":"# Quiz: 3 containment questions\n\nPhase two of this series is containment, and it ends with three questions. Nine posts of tools and posters came before it. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why keep a management path open when you quarantine a host?**\n\n- a) So you can finish the fix without travelling\n- b) Because firewalls need a default allow\n- c) To keep the metrics agent reporting\n\n> **Answer:** a) So you can finish the fix without travelling. The host you cannot reach is the host you cannot fix. The console is the last rule you remove, not the first one you cut.\n\n**Q2. Why does dependency order matter when disabling an identity?**\n\n- a) Cloud roles are slower to revoke than logins\n- b) Alphabetical order is easier to audit\n- c) The IdP session outlives a downstream lock\n\n> **Answer:** c) The IdP session outlives a downstream lock. Kill the IdP session first, or the user signs back into every downstream system. Disable in tier order: IdP, cloud, source control, CI.\n\n**Q3. Why does an allowlist beat a blocklist at the edge?**\n\n- a) It is cheaper to store in the WAF\n- b) It fails closed when a new address appears\n- c) It blocks more traffic by default\n\n> **Answer:** b) It fails closed when a new address appears. A blocklist only stops what you already know. An allowlist denies the address nobody has seen yet, which is the one the attacker uses next.\n\n## Explained\n\n### Question one\n\nWhy keep a management path open when you quarantine a host? Because the host you cannot reach is the host you cannot fix. The console is the last rule you remove, not the first one you cut. A management rule buried under a drop never fires. A quarantine that locks you out is an outage with a story.\n\n### Question two\n\nWhy does dependency order matter when disabling an identity? Because the identity provider session outlives a downstream lock. Disable the IdP first, then cloud, source control and CI. Do it alphabetically and the user signs back into everything you just locked. A break-glass account is held, never disabled by a script.\n\n### Question three\n\nWhy does an allowlist beat a blocklist at the edge? Because a blocklist only stops what you already know. An allowlist denies the address nobody has seen yet, which is the one the attacker will use next. The review cost is the control; the benefit is that failure is closed.\n\n> **The honest caveat:** an allowlist is only as good as its review, and a stale allow is a blocklist with better marketing.\n\nPhase two is done: five tools, ten posts, one order. Scope, cut, verify. If you missed one, reread P682 before the next incident.\n\n## In one line\n\n> **Contain by rule, not by memory.** Three answers, one habit: prove the path before you cut it.\n\n**Replay the phase:** P682 to P690\n"},{"id":"p692","n":"692","slug":"a-credential-rotation-planner","title":"A credential rotation planner","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","CyberSecurity","SRE","SecretsManagement"],"series_name":"IR","design":"code","script":"credential_rotate.py","date":"2031-02-27","files":{"it":"notes/p692-a-credential-rotation-planner.md","devsecops":"notes/p692-a-credential-rotation-planner.md"},"md":"# A credential rotation planner\n\nAt 02:40 the old database password still works on two workers while the new one is live in the API. Rotate out of order and the next hour is authentication errors.\n\n## Quick reference\n\nThe full script is at the end of this note: `credential_rotate.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 credential_rotate.py /tmp/rot.json\n```\n\n**Example output**\n\n```text\nORDER db-root -> api-db -> jwt-key\nOK 3 secrets in order, 3 verify steps\n```\n\n## Explained\n\n### What it does\n\n`credential_rotate.py` reads a rotation registry. Each secret has an id, an owner, the systems that consume it, and depends_on. The tool sorts the registry so a dependency rotates first, then prints a STEP and a VERIFY line per secret. A cycle, an unknown dependency or a secret with no owner exits 1.\n\n### What the screen shows\n\nThe fourteen lines emit ready secrets in sorted waves; a cycle breaks the loop and prints FAIL. The clean fixture ends on ORDER db-root -> api-db -> jwt-key and OK 3 secrets in order, 3 verify steps.\n\n### How to run it\n\n`python3 credential_rotate.py rot.json`. Exit 0 is a clean order, 1 is a finding, 2 is bad input. It reads metadata only.\n\n### The fix\n\nRotate db-root, then api-db, then jwt-key. After each step perform its VERIFY check: the consumer accepts the new value and the old one fails. Do not move on until both are true. A failed step names the systems still on the old value.\n\n### Prevention\n\nPut owner and depends_on in the registry before the next incident. A secret with no owner is a secret nobody will rotate at 3 a.m.\n\n> **The honest caveat:** the order is only as good as depends_on; a missing edge produces a wrong plan.\n\n> **The lesson:** a rotation is a sequence with proof between steps.\n\n## In one line\n\n> **Order is the safety rail.** A rotation is a sequence, not a single change.\n\n**Prove each step**\n\n```bash\npython3 credential_rotate.py rot.json\n```\n\n## Script: credential_rotate.py\n\nDownload: [scripts/credential_rotate.py](../scripts/credential_rotate.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"credential_rotate.py - plan a credential rotation in dependency order.\n\nReads a rotation registry and prints the order the secrets must be rotated in,\none STEP and one VERIFY line per secret. A secret's depends_on list names the\nsecrets that must be rotated before it, so a database root credential moves\nbefore the application credential built on it.\n\nRegistry (metadata only; a value/token/key field is ignored and never read):\n\n    {\"secrets\": [\n      {\"id\": \"db-root\", \"owner\": \"dba\", \"systems\": [\"db-01\"],\n       \"depends_on\": []},\n      {\"id\": \"api-db\", \"owner\": \"platform\", \"systems\": [\"api\", \"worker\"],\n       \"depends_on\": [\"db-root\"]},\n      {\"id\": \"jwt-key\", \"owner\": \"app\", \"systems\": [\"api\"],\n       \"depends_on\": [\"api-db\"]}\n    ]}\n\nA bare list of secrets also works. A cycle, an unknown dependency or a secret\nwith no owner is a finding. The script prints the plan; it does not rotate,\nwrite or call a vault. The printed plan is the deliverable.\n\nUsage:\n    python3 credential_rotate.py rot.json\n    python3 credential_rotate.py rot.json --json\n    cat rot.json | python3 credential_rotate.py -\n\nCron (Monday morning, before the change window):\n    0 6 * * 1 python3 /opt/s0sta/credential_rotate.py /etc/ir/rot.json \\\n      >> /var/log/ir/rotation.log 2>&1\n\nCI (fail the job when the registry cannot be ordered):\n    python3 credential_rotate.py /etc/ir/rot.json || exit 1\n\nExit codes: 0 the registry orders cleanly, 1 a cycle, an unknown dependency or\na secret with no owner, 2 usage error or unreadable, invalid or empty input.\n\nRead-only: the registry is opened for reading only. No secret value is read or\nprinted, and no vault, cloud or API call is made. The registry holds ids,\nowners, systems and dependencies; nothing here can rotate a credential. The\norder is only as good as depends_on.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef die(message):\n    print(\"ERROR \" + message, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        if path == \"-\":\n            return json.load(sys.stdin)\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except (ValueError, UnicodeError) as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef records(document):\n    rows = document.get(\"secrets\") if isinstance(document, dict) else document\n    if not isinstance(rows, list) or not rows:\n        die(\"expected a non-empty list under 'secrets'\")\n    return rows\n\n\ndef text(value):\n    return value.strip() if isinstance(value, str) else \"\"\n\n\ndef names(value, label):\n    if value is None:\n        return []\n    if not isinstance(value, list):\n        die(\"%s must be a list\" % label)\n    out = []\n    for item in value:\n        name = text(item)\n        if not name:\n            die(\"%s must contain non-empty names\" % label)\n        out.append(name)\n    return out\n\n\ndef parse(rows):\n    secrets = {}\n    for index, row in enumerate(rows, 1):\n        if not isinstance(row, dict):\n            die(\"secret %d is not an object\" % index)\n        sid = text(row.get(\"id\")) or text(row.get(\"secret\"))\n        if not sid:\n            die(\"secret %d has no id\" % index)\n        if sid in secrets:\n            die(\"duplicate secret id %s\" % sid)\n        owner = text(row.get(\"owner\"))\n        secrets[sid] = {\n            \"id\": sid,\n            \"owner\": owner or None,\n            \"systems\": names(row.get(\"systems\"), \"%s.systems\" % sid),\n            \"deps\": names(row.get(\"depends_on\"), \"%s.depends_on\" % sid),\n        }\n    return secrets\n\n\ndef find_cycle(secrets, remaining):\n    \"\"\"Return one cycle as a list with the start repeated at the end.\"\"\"\n    node = sorted(remaining)[0]\n    path, seen = [], {}\n    while node not in seen:\n        seen[node] = len(path)\n        path.append(node)\n        next_nodes = sorted(set(secrets[node][\"deps\"]) & remaining)\n        node = next_nodes[0] if next_nodes else path[0]\n    return path[seen[node]:] + [node]\n\n\ndef order(secrets):\n    remaining = set(secrets)\n    result = []\n    while remaining:\n        ready = sorted(sid for sid in remaining\n                       if not (set(secrets[sid][\"deps\"]) & remaining))\n        if not ready:\n            return result, find_cycle(secrets, remaining)\n        result.extend(ready)\n        remaining.difference_update(ready)\n    return result, None\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        description=\"Plan a credential rotation in dependency order. Prints \"\n                    \"the plan; it does not rotate anything.\")\n    parser.add_argument(\"registry\", nargs=\"?\", default=\"-\",\n                        help=\"rotation registry JSON; stdin when omitted\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print the ordered plan as JSON\")\n    parser.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = parser.parse_args()\n\n    secrets = parse(records(load(args.registry)))\n\n    findings = []\n    for sid in sorted(secrets):\n        for dep in secrets[sid][\"deps\"]:\n            if dep not in secrets:\n                findings.append(\"%s depends on unknown secret %s\" % (sid, dep))\n\n    if findings:\n        for finding in findings:\n            print(\"FINDING \" + finding)\n        print(\"PLAN blocked: fix the registry, then re-run\")\n        print(\"FAIL %d registry finding%s\"\n              % (len(findings), \"\" if len(findings) == 1 else \"s\"))\n        return 1\n\n    ordered, cycle = order(secrets)\n    if cycle:\n        print(\"FINDING cycle \" + \" -> \".join(cycle))\n        print(\"PLAN blocked: break the cycle by hand, then re-run\")\n        print(\"FAIL no safe rotation order\")\n        return 1\n\n    if args.json:\n        ownerless = [sid for sid in ordered if not secrets[sid][\"owner\"]]\n        print(json.dumps({\n            \"order\": ordered,\n            \"steps\": [{\"id\": sid, \"owner\": secrets[sid][\"owner\"],\n                       \"systems\": secrets[sid][\"systems\"]} for sid in ordered],\n            \"ownerless\": ownerless,\n        }, indent=2))\n        return 1 if ownerless else 0\n\n    print(\"PLAN %d secret%s, %d step%s\"\n          % (len(ordered), \"\" if len(ordered) == 1 else \"s\",\n             len(ordered), \"\" if len(ordered) == 1 else \"s\"))\n    for number, sid in enumerate(ordered, 1):\n        item = secrets[sid]\n        owner = item[\"owner\"] or \"-\"\n        systems = \",\".join(item[\"systems\"]) or \"-\"\n        print(\"STEP %d %s owner=%s systems=%s\" % (number, sid, owner, systems))\n        print(\"VERIFY %s on %s: new value accepted, old value rejected\"\n              % (sid, systems))\n\n    print(\"ORDER \" + \" -> \".join(ordered))\n    ownerless = [sid for sid in ordered if not secrets[sid][\"owner\"]]\n    if ownerless:\n        for sid in ownerless:\n            print(\"FINDING %s has no owner\" % sid)\n        print(\"FAIL %d secret%s %s an owner\"\n              % (len(ownerless), \"\" if len(ownerless) == 1 else \"s\",\n                 \"needs\" if len(ownerless) == 1 else \"need\"))\n        return 1\n    print(\"OK %d secret%s in order, %d verify step%s\"\n          % (len(ordered), \"\" if len(ordered) == 1 else \"s\",\n             len(ordered), \"\" if len(ordered) == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p693","n":"693","slug":"a-config-rollback-planner","title":"A config rollback planner","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","Linux","SRE","ConfigurationManagement"],"series_name":"IR","design":"code","script":"config_rollback.py","date":"2031-03-02","files":{"it":"notes/p693-a-config-rollback-planner.md","devsecops":"notes/p693-a-config-rollback-planner.md"},"md":"# A config rollback planner\n\nThe 01:55 deploy changed one line in `nginx.conf` and the edge started returning 502s. Git has the change; the last-known-good directory has the version that worked. The hard part is proving which files differ.\n\n## Quick reference\n\nThe full script is at the end of this note: `config_rollback.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 config_rollback.py /tmp/c /tmp/l -r /etc\n```\n\n**Example output**\n\n```text\nROLLBACK web-01: cp -p /tmp/l/web-01/nginx.conf /etc/nginx.conf\nFAIL drift on web-01: 1 file\n```\n\n## Explained\n\n### What it does\n\n`config_rollback.py` walks a current tree and a last-known-good tree, host by host, and hashes every regular file with SHA-256. Equal files are OK, changed files are DRIFT, files only in the LKG are MISSING, and files only in current are EXTRA. For every drifted or missing file it prints the exact `cp -p` command, and exits 1 on any difference.\n\n### What the screen shows\n\nThe fourteen lines hash both sides, collect the drift, then print the command and summary. The dirty fixture ends on ROLLBACK web-01: `cp -p /tmp/l/web-01/nginx.conf /etc/nginx.conf` and FAIL drift on web-01: 1 file.\n\n### How to run it\n\n`python3 config_rollback.py current/ lkg/ --target-root /etc`. Add `--host web-01` to plan one host. Exit 0 is a clean tree, 1 is drift, 2 is bad layout.\n\n### The fix\n\nCopy the named files, then reload the service that owns them. A config file on disk is not a config in memory: nginx, systemd and the app all cache. Compare the running config too, with `nginx -T` or the app's endpoint.\n\n### Prevention\n\nSnapshot the LKG tree before every deploy, outside the release path. A last-known-good directory inside the release directory is deleted by the next release.\n\n> **The honest caveat:** SHA-256 proves the bytes, not that the file parses. Run `nginx -t` after the copy.\n\n> **The lesson:** rollback is a file-level plan, not a feeling.\n\n## In one line\n\n> **The old file is the rollback.** A hash match is the only rollback you can prove.\n\n**Plan first**\n\n```bash\npython3 config_rollback.py cur lkg\n```\n\n## Script: config_rollback.py\n\nDownload: [scripts/config_rollback.py](../scripts/config_rollback.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"config_rollback.py - print the per-host rollback plan for drifted config.\n\nCompares a current config tree with a last-known-good (LKG) tree, host by\nhost, using SHA-256. Equal files are OK; a file that changed is DRIFT; a file\nthat exists only in the LKG is MISSING; a file that exists only in current is\nEXTRA. Drift, missing and extra all make the run exit 1.\n\nBoth trees must use the same shape: <root>/<host>/<relative path>, so\ncurrent/web-01/etc/nginx/nginx.conf is compared with\nlkg/web-01/etc/nginx/nginx.conf. The plan prints the exact cp -p command for\nevery drifted or missing file, with --target-root as the live root.\n\nThe comparison is read-only: it hashes files and prints commands. It does not\ncopy, reload or restart anything. A copied file is not a loaded config, so\nreload the service after the plan and run its own config test.\n\nUsage:\n    python3 config_rollback.py current/ lkg/\n    python3 config_rollback.py current/ lkg/ --target-root /srv/live\n    python3 config_rollback.py current/ lkg/ --host web-01\n    python3 config_rollback.py --help\n\nCron (hourly drift report, exit 1 on any drift):\n    0 * * * * python3 /opt/s0sta/config_rollback.py /srv/current \\\n      /srv/lkg >> /var/log/ir/config_drift.log 2>&1\n\nCI (before a deploy, prove the tree still matches the last known good):\n    python3 config_rollback.py current/ lkg/ || exit 1\n\nExit codes: 0 every file matches the last known good, 1 at least one file\ndrifted, is missing or is extra, 2 usage error, unreadable tree, bad layout or\nunreadable file.\n\nRead-only: both trees are opened for reading only and nothing is written.\nSymlinks are skipped, not followed. The SHA-256 proves the bytes, not that the\nconfig parses; run nginx -t (or the application's own check) after the copy.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport hashlib\nimport os\nimport shlex\nimport sys\n\n\ndef die(message):\n    print(\"ERROR \" + message, file=sys.stderr)\n    sys.exit(2)\n\n\ndef digest(path):\n    value = hashlib.sha256()\n    try:\n        with open(path, \"rb\") as handle:\n            for chunk in iter(lambda: handle.read(1024 * 1024), b\"\"):\n                value.update(chunk)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    return value.hexdigest()\n\n\ndef walk(root, label):\n    if not os.path.isdir(root):\n        die(\"%s is not a directory: %s\" % (label, root))\n    files = {}\n    for base, dirs, names in os.walk(root):\n        dirs[:] = sorted(d for d in dirs\n                         if not os.path.islink(os.path.join(base, d)))\n        for name in sorted(names):\n            path = os.path.join(base, name)\n            if os.path.islink(path) or not os.path.isfile(path):\n                continue\n            files[os.path.relpath(path, root)] = path\n    if not files:\n        die(\"%s has no regular files: %s\" % (label, root))\n    return files\n\n\ndef host_names(root, label):\n    names = []\n    for name in sorted(os.listdir(root)):\n        path = os.path.join(root, name)\n        if os.path.isdir(path) and not os.path.islink(path):\n            names.append(name)\n        elif os.path.isfile(path):\n            die(\"%s: top-level file %s; expected <root>/<host>/...\"\n                % (label, name))\n    if not names:\n        die(\"%s: expected <root>/<host>/...; no host directories in %s\"\n            % (label, root))\n    return names\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        description=\"Compare current config with a last-known-good tree and \"\n                    \"print the exact per-host rollback commands.\")\n    parser.add_argument(\"current\", help=\"current config tree (<root>/<host>/...)\")\n    parser.add_argument(\"lkg\", help=\"last-known-good tree, same shape\")\n    parser.add_argument(\"--host\", action=\"append\", default=None,\n                        help=\"only this host; repeat for more than one\")\n    parser.add_argument(\"-r\", \"--target-root\", default=\"/\",\n                        help=\"live root the relative paths are restored under \"\n                             \"(default /)\")\n    parser.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = parser.parse_args()\n\n    current = walk(args.current, \"current\")\n    lkg = walk(args.lkg, \"lkg\")\n    target_root = os.path.abspath(args.target_root)\n\n    hosts = sorted(set(host_names(args.current, \"current\"))\n                   | set(host_names(args.lkg, \"lkg\")))\n    if args.host:\n        missing = [h for h in args.host if h not in hosts]\n        if missing:\n            die(\"unknown host%s: %s\"\n                % (\"\" if len(missing) == 1 else \"s\", \", \".join(missing)))\n        hosts = [h for h in hosts if h in args.host]\n\n    findings = 0\n    plans = []\n    summary = []\n    for host in hosts:\n        prefix = host + os.sep\n        left = {rel: path for rel, path in current.items()\n                if rel.startswith(prefix)}\n        right = {rel: path for rel, path in lkg.items()\n                 if rel.startswith(prefix)}\n        rows = []\n        drift = 0\n        for rel in sorted(set(left) | set(right)):\n            short = rel[len(prefix):]\n            if rel in left and rel in right:\n                first, second = digest(left[rel]), digest(right[rel])\n                if first == second:\n                    rows.append((\"OK\", short, first, second))\n                else:\n                    drift += 1\n                    rows.append((\"DRIFT\", short, first, second))\n            elif rel in right:\n                drift += 1\n                rows.append((\"MISSING\", short, \"-\", digest(right[rel])))\n            else:\n                drift += 1\n                rows.append((\"EXTRA\", short, digest(left[rel]), \"-\"))\n\n        if drift == 0:\n            print(\"HOST %s %d file%s match\" %\n                  (host, len(rows), \"\" if len(rows) == 1 else \"s\"))\n        else:\n            print(\"HOST %s %d file%s, %d difference%s\"\n                  % (host, len(rows), \"\" if len(rows) == 1 else \"s\",\n                     drift, \"\" if drift == 1 else \"s\"))\n        for kind, short, first, second in rows:\n            if kind == \"OK\":\n                print(\"OK %s\" % short)\n            elif kind == \"DRIFT\":\n                print(\"DRIFT %s lkg=%s current=%s\" % (short, second[:8], first[:8]))\n            elif kind == \"MISSING\":\n                print(\"MISSING %s (only in the last known good)\" % short)\n            else:\n                print(\"EXTRA %s (not in the last known good, review it)\" % short)\n            if kind in (\"DRIFT\", \"MISSING\"):\n                source = right[host + os.sep + short]\n                target = os.path.join(target_root, short)\n                plans.append(\"ROLLBACK %s: cp -p %s %s\"\n                             % (host, shlex.quote(source), shlex.quote(target)))\n        findings += drift\n        summary.append((host, len(rows), drift))\n\n    total_files = sum(item[1] for item in summary)\n    print(\"CHECK %d host%s, %d file%s, %d difference%s\"\n          % (len(summary), \"\" if len(summary) == 1 else \"s\",\n             total_files, \"\" if total_files == 1 else \"s\",\n             findings, \"\" if findings == 1 else \"s\"))\n    if findings == 0:\n        print(\"OK every file matches the last known good\")\n        return 0\n\n    print(\"PLAN %d rollback command%s\" % (len(plans),\n                                          \"\" if len(plans) == 1 else \"s\"))\n    for plan in plans:\n        print(plan)\n    bad_hosts = [host for host, _, drift in summary if drift]\n    print(\"FAIL drift on %s: %d file%s\"\n          % (\", \".join(bad_hosts), findings,\n             \"\" if findings == 1 else \"s\"))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p694","n":"694","slug":"a-certificate-replacement-planner","title":"A certificate replacement planner","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","TLS","Certificates","IncidentResponse","SRE"],"series_name":"IR","design":"code","script":"cert_replace.py","date":"2031-03-04","files":{"it":"notes/p694-a-certificate-replacement-planner.md","devsecops":"notes/p694-a-certificate-replacement-planner.md"},"md":"# A certificate replacement planner\n\nThe certificate on the load balancer expires at noon. The same serial is also installed on two web hosts and a mail gateway. Replace it on the balancer, reload, and the other three still serve yesterday's certificate.\n\n## Quick reference\n\nThe full script is at the end of this note: `cert_replace.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 cert_replace.py certs.json --today 2026-10-21\n```\n\n**Example output**\n\n```text\nSHARED 5C:91 on 3 hosts: lb-01, web-01, web-02\nFAIL 1 shared certificate, replace as one group\n```\n\n## Explained\n\n### What it does\n\n`cert_replace.py` reads a certificate inventory: host, path, serial, expiry and shared_with. It sorts by expiry, prints an ORDER line per certificate, and expands shared_with so the plan names every host that must receive the same serial. It exits 1 for a shared, expired or duplicate certificate.\n\n### What the screen shows\n\nThe fourteen lines load and sort the inventory, then print the check, the order, the shared group and the failure. The shared fixture ends on SHARED 5C:91 on 3 hosts: lb-01, web-01, web-02 and FAIL 1 shared certificate, replace as one group.\n\n### How to run it\n\n`python3 cert_replace.py certs.json --today 2026-10-21`. Exit 0 is a clean inventory, 1 is a finding, 2 is bad input. It never reads a private key or opens a socket.\n\n### The fix\n\nReplace the file on every host in the group before reloading any listener. Reload one host at a time, then verify the serial with `openssl x509 -noout -serial -in /etc/tls/site.pem`. A reload before the last copy is how one host serves a certificate the others do not have.\n\n### Prevention\n\nInventory by serial, not by filename. The same path on ten hosts can hold ten different certificates.\n\n> **The honest caveat:** the tool trusts the expiry and shared_with fields; a stale inventory produces a confident, wrong window.\n\n> **The lesson:** a shared certificate is one change with several blast radii.\n\n## In one line\n\n> **Replace all, then reload.** A shared serial needs one window, not four reloads.\n\n**Check the serial**\n\n```bash\nopenssl x509 -noout -serial\n```\n\n## Script: cert_replace.py\n\nDownload: [scripts/cert_replace.py](../scripts/cert_replace.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"cert_replace.py - plan a certificate replacement without breaking a peer.\n\nReads a certificate inventory and prints the replacement order, the hosts that\nmust receive the same certificate, and the serial check that proves each host\ndid. A certificate with shared_with is used by more than one host: every copy\nmust be in place before any listener reloads, or one host serves a serial the\nother hosts are not serving.\n\nInventory (metadata only; private keys are never read):\n\n    {\"certificates\": [\n      {\"host\": \"lb-01\", \"path\": \"/etc/tls/site.pem\", \"serial\": \"5C:91\",\n       \"expiry\": \"2026-11-04\", \"shared_with\": [\"web-01\", \"web-02\"]},\n      {\"host\": \"db-01\", \"path\": \"/etc/tls/db.pem\", \"serial\": \"91:02\",\n       \"expiry\": \"2027-02-01\", \"shared_with\": []}\n    ]}\n\nA shared certificate, an expired certificate and a serial listed twice are\nfindings. The plan is ordered by expiry, soonest first. The script prints the\nplan; it does not copy a file, reload a listener or connect to a host.\n\nUsage:\n    python3 cert_replace.py certs.json\n    python3 cert_replace.py certs.json --today 2026-10-21\n    cat certs.json | python3 cert_replace.py -\n\nCron (weekly, two weeks before the first expiry):\n    0 7 * * 1 python3 /opt/s0sta/cert_replace.py /etc/ir/certs.json \\\n      >> /var/log/ir/certs.log 2>&1\n\nCI (fail when the inventory needs a coordinated window):\n    python3 cert_replace.py /etc/ir/certs.json || exit 1\n\nExit codes: 0 no shared, expired or duplicate certificate, 1 at least one of\nthose findings, 2 usage error or unreadable, invalid or empty input.\n\nRead-only: the inventory is opened for reading only. A certificate value or\nprivate key in the file is ignored and never printed. The serial check is\nopenssl on the host; this script never runs it and never opens a socket.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date\n\n\ndef die(message):\n    print(\"ERROR \" + message, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        if path == \"-\":\n            return json.load(sys.stdin)\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except (ValueError, UnicodeError) as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef rows_of(document):\n    rows = document.get(\"certificates\") if isinstance(document, dict) else document\n    if not isinstance(rows, list) or not rows:\n        die(\"expected a non-empty list under 'certificates'\")\n    return rows\n\n\ndef word(value):\n    return value.strip() if isinstance(value, str) else \"\"\n\n\ndef day(value, label):\n    if not isinstance(value, str) or not value.strip():\n        die(\"%s is not an ISO date\" % label)\n    try:\n        return date.fromisoformat(value.strip())\n    except ValueError:\n        die(\"%s is not an ISO date: %s\" % (label, value))\n\n\ndef parse(rows):\n    certs = []\n    seen = set()\n    for index, row in enumerate(rows, 1):\n        if not isinstance(row, dict):\n            die(\"certificate %d is not an object\" % index)\n        host = word(row.get(\"host\"))\n        path = word(row.get(\"path\"))\n        serial = word(row.get(\"serial\"))\n        if not host:\n            die(\"certificate %d has no host\" % index)\n        if not path:\n            die(\"certificate %d has no path\" % index)\n        if not serial:\n            die(\"certificate %d has no serial\" % index)\n        if (host, path) in seen:\n            die(\"duplicate inventory entry for %s:%s\" % (host, path))\n        seen.add((host, path))\n        expiry = day(row.get(\"expiry\"), \"%s.expiry\" % host)\n        shared = row.get(\"shared_with\")\n        if shared is None:\n            shared = []\n        if not isinstance(shared, list):\n            die(\"%s.shared_with must be a list\" % host)\n        clean = []\n        for item in shared:\n            name = word(item)\n            if not name:\n                die(\"%s.shared_with must contain host names\" % host)\n            if name == host:\n                die(\"%s.shared_with cannot list itself\" % host)\n            clean.append(name)\n        certs.append({\"host\": host, \"path\": path, \"serial\": serial,\n                      \"expiry\": expiry, \"shared\": clean})\n    return certs\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        description=\"Plan a certificate replacement in expiry order and name \"\n                    \"every host that shares a serial.\")\n    parser.add_argument(\"inventory\", nargs=\"?\", default=\"-\",\n                        help=\"certificate inventory JSON; stdin when omitted\")\n    parser.add_argument(\"--today\", help=\"ISO date to compare expiry against\")\n    parser.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = parser.parse_args()\n\n    today = day(args.today, \"--today\") if args.today else date.today()\n    certs = parse(rows_of(load(args.inventory)))\n    certs.sort(key=lambda c: (c[\"expiry\"], c[\"host\"], c[\"path\"]))\n\n    serials = {}\n    for cert in certs:\n        serials.setdefault(cert[\"serial\"], []).append(cert[\"host\"])\n\n    for number, cert in enumerate(certs, 1):\n        hosts = [cert[\"host\"]] + cert[\"shared\"]\n        print(\"ORDER %d %s %s serial %s expires %s\"\n              % (number, cert[\"host\"], cert[\"path\"], cert[\"serial\"],\n                 cert[\"expiry\"].isoformat()))\n        if cert[\"shared\"]:\n            print(\"  REPLACE on %s before any reload\" % \", \".join(hosts))\n        else:\n            print(\"  REPLACE on %s, then reload the listener\" % cert[\"host\"])\n        where = \"every host\" if cert[\"shared\"] else \"the host\"\n        print(\"  VERIFY openssl x509 -noout -serial -in %s on %s\"\n              % (cert[\"path\"], where))\n\n    expired = [c for c in certs if c[\"expiry\"] < today]\n    for cert in expired:\n        print(\"EXPIRED %s %s serial %s on %s\"\n              % (cert[\"host\"], cert[\"path\"], cert[\"serial\"],\n                 cert[\"expiry\"].isoformat()))\n\n    shared = [c for c in certs if c[\"shared\"]]\n    for cert in shared:\n        hosts = [cert[\"host\"]] + cert[\"shared\"]\n        print(\"SHARED %s on %d hosts: %s\"\n              % (cert[\"serial\"], len(hosts), \", \".join(hosts)))\n\n    duplicates = {serial: hosts for serial, hosts in serials.items()\n                  if len(hosts) > 1}\n    for serial in sorted(duplicates):\n        print(\"FINDING serial %s listed twice: %s\"\n              % (serial, \", \".join(duplicates[serial])))\n\n    if not expired and not shared and not duplicates:\n        print(\"OK %d certificate%s, 0 shared\"\n              % (len(certs), \"\" if len(certs) == 1 else \"s\"))\n        return 0\n\n    reasons = []\n    if expired:\n        reasons.append(\"%d expired\" % len(expired))\n    if shared:\n        reasons.append(\"%d shared certificate%s\"\n                       % (len(shared), \"\" if len(shared) == 1 else \"s\"))\n    if duplicates:\n        reasons.append(\"%d duplicate serial%s\"\n                       % (len(duplicates), \"\" if len(duplicates) == 1 else \"s\"))\n    end = \", replace as one group\" if shared else \"\"\n    print(\"FAIL \" + \", \".join(reasons) + end)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p695","n":"695","slug":"a-restore-point-picker","title":"A restore point picker","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Backup","IncidentResponse","SRE","DataProtection"],"series_name":"IR","design":"code","script":"restore_point.py","date":"2031-03-06","files":{"it":"notes/p695-a-restore-point-picker.md","devsecops":"notes/p695-a-restore-point-picker.md"},"md":"# A restore point picker\n\nThe incident started at 23:40. The newest snapshot was taken at 23:10 and still says creating. The one before it is complete, four hours old, and never restored.\n\n## Quick reference\n\nThe full script is at the end of this note: `restore_point.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 restore_point.py cat.json -p gcp -i 2026-10-20T23:40Z\n```\n\n**Example output**\n\n```text\nRESTORE snap-2210 412GiB taken 2026-10-20T22:00:00Z\nCOMMAND gcloud compute disks create restore --source-snapshot snap-2210\n```\n\n## Explained\n\n### What it does\n\n`restore_point.py` reads a backup catalog: id, taken_at, status, size_gb. It keeps snapshots taken before the incident and inside `--window` hours, drops anything not restorable, and drops zero-size entries. It ranks what remains newest first and prints the provider's restore command for the best candidate. It exits 1 when none qualify.\n\n### What the screen shows\n\nThe fourteen lines parse the timestamps, keep pre-incident points, sort by time, and print the command. The clean fixture ends on RESTORE snap-2210 412GiB taken 2026-10-20T22:00:00Z and COMMAND gcloud compute disks create restore `--source-snapshot snap-2210`.\n\n### How to run it\n\n`python3 restore_point.py catalog.json --incident 2026-10-20T23:40:00Z --window 6 --provider` gcp. Exit 0 is a candidate, 1 is none, 2 is bad input. It never calls the provider.\n\n### The fix\n\nRestore the newest point that is complete and before the incident. Then prove it: row counts, the last known good totals, and the application's health check. A restore that boots is not a restore that is correct.\n\n### Prevention\n\nRun a restore drill monthly and record the time and row counts in the catalog. A status field written by the backup job is a claim. A drilled restore is evidence.\n\n> **The honest caveat:** the catalog status is the backup system's claim; the tool cannot verify the bytes without restoring them.\n\n> **The lesson:** a restore point is a promise the backup job made, and only a restore keeps it.\n\n## In one line\n\n> **Restore what you can prove.** The newest snapshot is not the same as a restorable one.\n\n**Pick the point**\n\n```bash\npython3 restore_point.py cat.json\n```\n\n## Script: restore_point.py\n\nDownload: [scripts/restore_point.py](../scripts/restore_point.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"restore_point.py - pick a restorable point before an incident.\n\nReads a backup catalog and answers one question: which snapshot can I restore\nto, and with what command? A candidate must have been taken before the\nincident (restoring to a later point restores the damage), inside --window\nhours, with a restorable status and a non-zero size. Candidates are ranked\nnewest first and the newest one wins.\n\nCatalog (metadata only; size is in GiB):\n\n    {\"snapshots\": [\n      {\"id\": \"snap-2210\", \"taken_at\": \"2026-10-20T22:00:00Z\",\n       \"status\": \"completed\", \"size_gb\": 412},\n      {\"id\": \"snap-2310\", \"taken_at\": \"2026-10-20T23:10:00Z\",\n       \"status\": \"creating\", \"size_gb\": 0}\n    ]}\n\nRestorable status: available, complete, completed, ok or restorable. Anything else is skipped and named, so a creating snapshot cannot\nwin by being the newest. The script prints the plan and the provider command;\nit never calls the provider and never restores anything.\n\nUsage:\n    python3 restore_point.py catalog.json --incident 2026-10-20T23:40:00Z\n    python3 restore_point.py catalog.json --incident 2026-10-20T23:40:00Z \\\n        --window 12 --provider gcp\n    cat catalog.json | python3 restore_point.py - --incident 2026-10-20T23:40:00Z\n\nCron (daily, report the restore point for yesterday's incident window):\n    0 6 * * * python3 /opt/s0sta/restore_point.py /etc/ir/catalog.json \\\n      --incident \"$(date -u -d yesterday +%FT%TZ)\" >> /var/log/ir/restore.log 2>&1\n\nCI (fail when a window has no restorable point):\n    python3 restore_point.py catalog.json --incident \"$INCIDENT\" || exit 1\n\nExit codes: 0 at least one candidate qualifies, 1 none qualifies, 2 usage\nerror or unreadable, invalid or empty input.\n\nRead-only: the catalog is opened for reading only. The printed command is a\nsuggestion for a human; it is never executed. The status field is the backup\nsystem's claim, not a restore test; only a real restore proves the bytes.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import datetime, timezone\n\nRESTORABLE = {\"available\", \"complete\", \"completed\", \"ok\", \"restorable\"}\nKNOWN_BAD = {\"creating\", \"pending\", \"in_progress\", \"running\", \"recovering\",\n             \"failed\", \"error\", \"deleted\", \"expired\", \"aborted\"}\n\n\ndef die(message):\n    print(\"ERROR \" + message, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        if path == \"-\":\n            return json.load(sys.stdin)\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except (ValueError, UnicodeError) as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef rows_of(document):\n    rows = document.get(\"snapshots\") if isinstance(document, dict) else document\n    if not isinstance(rows, list) or not rows:\n        die(\"expected a non-empty list under 'snapshots'\")\n    return rows\n\n\ndef word(value):\n    return value.strip() if isinstance(value, str) else \"\"\n\n\ndef stamp(value, label):\n    if not isinstance(value, str) or not value.strip():\n        die(\"%s is not an ISO timestamp\" % label)\n    raw = value.strip()\n    if raw.endswith((\"Z\", \"z\")):\n        raw = raw[:-1] + \"+00:00\"\n    try:\n        parsed = datetime.fromisoformat(raw)\n    except ValueError:\n        die(\"%s is not an ISO timestamp: %s\" % (label, value))\n    if parsed.tzinfo is None:\n        parsed = parsed.replace(tzinfo=timezone.utc)\n    return parsed.astimezone(timezone.utc)\n\n\ndef size_of(row, label):\n    value = row.get(\"size_gb\", row.get(\"size\"))\n    if isinstance(value, bool) or not isinstance(value, (int, float)):\n        die(\"%s.size_gb must be a number of GiB\" % label)\n    if value < 0:\n        die(\"%s.size_gb cannot be negative\" % label)\n    return float(value)\n\n\ndef parse(rows):\n    out = []\n    for index, row in enumerate(rows, 1):\n        if not isinstance(row, dict):\n            die(\"snapshot %d is not an object\" % index)\n        sid = word(row.get(\"id\"))\n        if not sid:\n            die(\"snapshot %d has no id\" % index)\n        status = word(row.get(\"status\")).lower()\n        if not status:\n            die(\"%s has no status\" % sid)\n        out.append({\"id\": sid,\n                    \"taken\": stamp(row.get(\"taken_at\"), \"%s.taken_at\" % sid),\n                    \"status\": status,\n                    \"size\": size_of(row, sid)})\n    return out\n\n\ndef human_age(minutes):\n    if minutes < 60:\n        return \"%dm\" % minutes\n    return \"%dh%02dm\" % (minutes // 60, minutes % 60)\n\n\ndef command_for(args, snapshot):\n    if args.provider == \"aws\":\n        return (\"aws ec2 create-volume --snapshot-id %s \"\n                \"--availability-zone %s --volume-type gp3\"\n                % (snapshot[\"id\"], args.az))\n    if args.provider == \"gcp\":\n        return (\"gcloud compute disks create %s --source-snapshot %s\"\n                % (args.disk_name, snapshot[\"id\"]))\n    return (\"az disk create -n %s --source %s\"\n            % (args.disk_name, snapshot[\"id\"]))\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        description=\"Pick the newest restorable snapshot before an incident \"\n                    \"and print the provider's restore command.\")\n    parser.add_argument(\"catalog\", nargs=\"?\", default=\"-\",\n                        help=\"backup catalog JSON; stdin when omitted\")\n    parser.add_argument(\"-i\", \"--incident\", required=True,\n                        help=\"ISO timestamp of the incident\")\n    parser.add_argument(\"--window\", type=float, default=6.0,\n                        help=\"hours before the incident to search (default 6)\")\n    parser.add_argument(\"-p\", \"--provider\", choices=(\"aws\", \"gcp\", \"az\"),\n                        default=\"aws\", help=\"restore command family (default aws)\")\n    parser.add_argument(\"--az\", default=\"ZONE\",\n                        help=\"availability zone for the aws command\")\n    parser.add_argument(\"--disk-name\", default=\"restore\",\n                        help=\"disk name for the gcp and az commands\")\n    parser.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = parser.parse_args()\n\n    if args.window <= 0:\n        die(\"--window must be greater than zero\")\n    if not args.disk_name.strip():\n        die(\"--disk-name cannot be empty\")\n\n    incident = stamp(args.incident, \"--incident\")\n    snapshots = parse(rows_of(load(args.catalog)))\n    snapshots.sort(key=lambda s: s[\"taken\"], reverse=True)\n    print(\"INCIDENT %s, window %gh\"\n          % (incident.strftime(\"%Y-%m-%dT%H:%M:%SZ\"), args.window))\n\n    candidates = []\n    for snapshot in snapshots:\n        age = (incident - snapshot[\"taken\"]).total_seconds()\n        if age < 0:\n            print(\"SKIP %s taken after the incident\" % snapshot[\"id\"])\n            continue\n        if age > args.window * 3600:\n            continue\n        if snapshot[\"status\"] not in RESTORABLE:\n            reason = (\"status=\" + snapshot[\"status\"]\n                      if snapshot[\"status\"] in KNOWN_BAD\n                      else \"unknown status=\" + snapshot[\"status\"])\n            print(\"SKIP %s %s\" % (snapshot[\"id\"], reason))\n            continue\n        if snapshot[\"size\"] <= 0:\n            print(\"SKIP %s size=0\" % snapshot[\"id\"])\n            continue\n        candidates.append(snapshot)\n\n    if not candidates:\n        print(\"CHECK 0 candidates in the %gh before the incident\" % args.window)\n        print(\"FAIL no restorable point; widen --window or check the backup job\")\n        return 1\n\n    for snapshot in candidates:\n        minutes = int((incident - snapshot[\"taken\"]).total_seconds() // 60)\n        print(\"CANDIDATE %s %s %s %.0fGiB, %s before\"\n              % (snapshot[\"id\"],\n                 snapshot[\"taken\"].strftime(\"%Y-%m-%dT%H:%M:%SZ\"),\n                 snapshot[\"status\"], snapshot[\"size\"], human_age(minutes)))\n\n    best = candidates[0]\n    minutes = int((incident - best[\"taken\"]).total_seconds() // 60)\n    print(\"OK newest restorable %s, %s before the incident\"\n          % (best[\"id\"], human_age(minutes)))\n    print(\"RESTORE %s %.0fGiB taken %s\"\n          % (best[\"id\"], best[\"size\"],\n             best[\"taken\"].strftime(\"%Y-%m-%dT%H:%M:%SZ\")))\n    print(\"COMMAND \" + command_for(args, best))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p696","n":"696","slug":"a-node-drain-simulator","title":"A node drain simulator","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Kubernetes","IncidentResponse","SRE","Reliability"],"series_name":"IR","design":"code","script":"node_drain.py","date":"2031-03-09","files":{"it":"notes/p696-a-node-drain-simulator.md","devsecops":"notes/p696-a-node-drain-simulator.md"},"md":"# A node drain simulator\n\nThe node is hot and the fix is a drain. One Deployment has no PDB headroom; another has two pods on the node. kubectl drain moves what it can and waits on the rest.\n\n## Quick reference\n\nThe full script is at the end of this note: `node_drain.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 node_drain.py /tmp/drain.json -n node-1 -m 3\n```\n\n**Example output**\n\n```text\nORDER web-1, web-2\nFAIL 1 pod would block the drain\n```\n\n## Explained\n\n### What it does\n\n`node_drain.py` reads kubectl get pods,nodes,pdb -A -o json, matches each pod on the target node to its PDBs, and simulates evictions one at a time. It prints EVICT for each movable pod and BLOCK for each pod a zero PDB stops. DaemonSets are skipped, emptyDir is flagged, and capacity is checked against `--min-free`. Exit 1 means a block or capacity below the floor.\n\n### What the screen shows\n\nThe fourteen lines map each PDB to the app label and walk the pods. The blocked fixture ends on ORDER web-1, web-2 and FAIL 1 pod would block the drain.\n\n### How to run it\n\nkubectl get pods,nodes,pdb -A -o json > `/tmp/drain.json`, then `python3 node_drain.py /tmp/drain.json --node node-1 --min-free 3`. It only reads the dump; the printed command carries `--dry-run=server`.\n\n### The fix\n\nDrain in the printed order. If a PDB blocks, scale up, wait for the new pod to become Ready, then evict. Raise `--min-free` for a small cluster; two nodes is not a capacity plan.\n\n### Prevention\n\nSet PDBs from the real replica count. minAvailable 2 on three replicas leaves one eviction; on two replicas it blocks every drain.\n\n> **The honest caveat:** the simulation trusts the PDB status in the dump; a controller that has not reconciled can make the numbers stale.\n\n> **The lesson:** a drain is a capacity change with a queue, so respect the queue.\n\n## In one line\n\n> **Drain one pod, not the app.** Capacity is a constraint, not a result.\n\n**Count the headroom**\n\n```bash\npython3 node_drain.py drain.json\n```\n\n## Script: node_drain.py\n\nDownload: [scripts/node_drain.py](../scripts/node_drain.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"node_drain.py - simulate a node drain before you run it.\n\nReads the output of `kubectl get pods,nodes,pdb -A -o json`, matches every pod\non the target node to the PodDisruptionBudgets that select it, and simulates\nthe eviction order one pod at a time. It prints the pods that would block, the\ncapacity left after the node goes, and the exact kubectl drain command with\n--dry-run=server.\n\nA drain blocks when a matching PDB has no disruptions left, when a pod has no\nowner (kubectl needs --force), and when a pod uses emptyDir (kubectl needs\n--delete-emptydir-data and the data is lost). DaemonSet and mirror pods are\nskipped with --ignore-daemonsets. A drain fails the run when a pod would block\nor when the ready schedulable node count falls below --min-free.\n\nUsage:\n    kubectl get pods,nodes,pdb -A -o json > /tmp/drain.json\n    python3 node_drain.py /tmp/drain.json --node node-1\n    python3 node_drain.py /tmp/drain.json --node node-1 --min-free 3\n    kubectl get pods,nodes,pdb -A -o json | python3 node_drain.py - --node node-1\n\nCron (before a maintenance window, report only):\n    0 5 * * 1 python3 /opt/s0sta/node_drain.py /etc/ir/drain.json \\\n      --node node-1 --min-free 3 >> /var/log/ir/drain.log 2>&1\n\nCI (refuse a drain that would break the PDB or the capacity floor):\n    python3 node_drain.py /tmp/drain.json --node node-1 --min-free 3 || exit 1\n\nExit codes: 0 the drain is clear, 1 a pod would block or capacity would fall\nbelow --min-free, 2 usage error, unreadable JSON, bad object or unknown node.\n\nRead-only: the JSON is opened for reading only. Nothing is cordoned, evicted\nor drained and no kubectl command is run. The printed command carries\n--dry-run=server so the API server can answer without changing the cluster;\nremove that flag only after reading this plan. The status fields are a snapshot\nand can lag the controllers.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef die(message):\n    print(\"ERROR \" + message, file=sys.stderr)\n    sys.exit(2)\n\n\ndef load(path):\n    try:\n        if path == \"-\":\n            return json.load(sys.stdin)\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        die(\"%s: %s\" % (path, exc.strerror or exc))\n    except (ValueError, UnicodeError) as exc:\n        die(\"%s: not valid JSON (%s)\" % (path, exc))\n\n\ndef items_of(document):\n    if isinstance(document, list):\n        items = document\n    elif isinstance(document, dict):\n        if isinstance(document.get(\"items\"), list):\n            items = document[\"items\"]\n        else:\n            items = []\n            for key in (\"pods\", \"nodes\", \"pdbs\", \"poddisruptionbudgets\"):\n                value = document.get(key)\n                if value is None:\n                    continue\n                if not isinstance(value, list):\n                    die(\"%s must be a list\" % key)\n                items.extend(value)\n    else:\n        die(\"the dump must be a JSON object or list\")\n    if not items:\n        die(\"no Kubernetes objects in the dump\")\n    for index, item in enumerate(items, 1):\n        if not isinstance(item, dict):\n            die(\"object %d is not an object\" % index)\n        if not isinstance(item.get(\"kind\"), str):\n            die(\"object %d has no kind\" % index)\n    return items\n\n\ndef meta_of(item, label):\n    metadata = item.get(\"metadata\")\n    if not isinstance(metadata, dict):\n        die(\"%s has no metadata object\" % label)\n    name = metadata.get(\"name\")\n    if not isinstance(name, str) or not name.strip():\n        die(\"%s has no name\" % label)\n    namespace = metadata.get(\"namespace\") or \"default\"\n    if not isinstance(namespace, str):\n        die(\"%s has a bad namespace\" % name)\n    labels = metadata.get(\"labels\") or {}\n    if not isinstance(labels, dict):\n        die(\"%s has bad labels\" % name)\n    return name.strip(), namespace, labels, metadata\n\n\ndef ready_of(node):\n    status = node.get(\"status\") or {}\n    conditions = status.get(\"conditions\") or []\n    if not isinstance(conditions, list):\n        die(\"%s has bad status.conditions\" % node.get(\"metadata\", {}).get(\"name\"))\n    for condition in conditions:\n        if isinstance(condition, dict) and condition.get(\"type\") == \"Ready\":\n            return condition.get(\"status\") == \"True\"\n    return False\n\n\ndef number(value, label):\n    if isinstance(value, bool):\n        die(\"%s must be a number\" % label)\n    if isinstance(value, int):\n        return value\n    if isinstance(value, str) and value.isdigit():\n        return int(value)\n    die(\"%s must be a number\" % label)\n\n\ndef bound(value, total, label):\n    \"\"\"Return an integer bound; percentages round up like Kubernetes.\"\"\"\n    if isinstance(value, bool):\n        die(\"%s must be an integer or a percentage\" % label)\n    if isinstance(value, int):\n        return value\n    if isinstance(value, str) and value.endswith(\"%\"):\n        head = value[:-1]\n        if head.isdigit() and total is not None:\n            return (total * int(head) + 99) // 100\n    die(\"%s must be an integer or a percentage\" % label)\n\n\ndef allowed_of(pdb, name):\n    status = pdb.get(\"status\") or {}\n    if \"disruptionsAllowed\" in status:\n        value = status[\"disruptionsAllowed\"]\n        if isinstance(value, int) and not isinstance(value, bool):\n            return max(0, value)\n    spec = pdb.get(\"spec\") or {}\n    expected = status.get(\"expectedPods\")\n    current = status.get(\"currentHealthy\")\n    if expected is not None:\n        expected = number(expected, \"%s.status.expectedPods\" % name)\n    if current is not None:\n        current = number(current, \"%s.status.currentHealthy\" % name)\n    if \"minAvailable\" in spec:\n        limit = bound(spec[\"minAvailable\"], expected if expected is not None\n                      else current, \"%s.spec.minAvailable\" % name)\n        if current is None:\n            return 0\n        return max(0, current - limit)\n    if \"maxUnavailable\" in spec:\n        limit = bound(spec[\"maxUnavailable\"], expected if expected is not None\n                      else current, \"%s.spec.maxUnavailable\" % name)\n        if expected is None or current is None:\n            return 0\n        return max(0, limit - (expected - current))\n    return 0\n\n\ndef matches(selector, labels):\n    if not isinstance(selector, dict):\n        return False\n    match_labels = selector.get(\"matchLabels\") or {}\n    expressions = selector.get(\"matchExpressions\") or []\n    if not isinstance(match_labels, dict) or not isinstance(expressions, list):\n        die(\"a PDB selector is malformed\")\n    if not match_labels and not expressions:\n        return False\n    for key, value in match_labels.items():\n        if labels.get(key) != value:\n            return False\n    for expression in expressions:\n        if not isinstance(expression, dict):\n            die(\"a PDB matchExpressions entry is not an object\")\n        key = expression.get(\"key\")\n        operator = expression.get(\"operator\")\n        values = expression.get(\"values\") or []\n        if operator == \"In\" and labels.get(key) not in values:\n            return False\n        elif operator == \"NotIn\" and labels.get(key) in values:\n            return False\n        elif operator == \"Exists\" and key not in labels:\n            return False\n        elif operator == \"DoesNotExist\" and key in labels:\n            return False\n        elif operator not in (\"In\", \"NotIn\", \"Exists\", \"DoesNotExist\"):\n            die(\"unsupported PDB operator: %r\" % operator)\n    return True\n\n\ndef main():\n    parser = argparse.ArgumentParser(\n        description=\"Simulate a node drain against PodDisruptionBudgets and \"\n                    \"print the safe order and the drain command.\")\n    parser.add_argument(\"dump\", nargs=\"?\", default=\"-\",\n                        help=\"kubectl get pods,nodes,pdb -o json; stdin when omitted\")\n    parser.add_argument(\"-n\", \"--node\", required=True, help=\"node to drain\")\n    parser.add_argument(\"-m\", \"--min-free\", type=int, default=2,\n                        help=\"ready schedulable nodes that must remain (default 2)\")\n    parser.add_argument(\"--version\", action=\"version\", version=\"%(prog)s 1.0\")\n    args = parser.parse_args()\n\n    if args.min_free < 0:\n        die(\"--min-free must be zero or more\")\n\n    items = items_of(load(args.dump))\n    nodes, pods, pdbs = [], [], []\n    for item in items:\n        kind = item[\"kind\"]\n        if kind == \"Node\":\n            name, _, _, _ = meta_of(item, \"Node\")\n            spec = item.get(\"spec\") or {}\n            if not isinstance(spec, dict):\n                die(\"%s has a bad spec\" % name)\n            nodes.append({\"name\": name, \"ready\": ready_of(item),\n                          \"unschedulable\": bool(spec.get(\"unschedulable\"))})\n        elif kind == \"Pod\":\n            name, namespace, labels, metadata = meta_of(item, \"Pod\")\n            spec = item.get(\"spec\") or {}\n            if not isinstance(spec, dict):\n                die(\"%s has a bad spec\" % name)\n            node = spec.get(\"nodeName\")\n            owners = metadata.get(\"ownerReferences\") or []\n            if not isinstance(owners, list):\n                die(\"%s has bad ownerReferences\" % name)\n            kinds = [owner.get(\"kind\") for owner in owners\n                     if isinstance(owner, dict)]\n            volumes = spec.get(\"volumes\") or []\n            if not isinstance(volumes, list):\n                die(\"%s has bad volumes\" % name)\n            annotations = metadata.get(\"annotations\") or {}\n            if not isinstance(annotations, dict):\n                die(\"%s has bad annotations\" % name)\n            pods.append({\n                \"name\": name, \"namespace\": namespace, \"labels\": labels,\n                \"node\": node, \"phase\": (item.get(\"status\") or {}).get(\"phase\"),\n                \"deleting\": bool(metadata.get(\"deletionTimestamp\")),\n                \"daemonset\": \"DaemonSet\" in kinds,\n                \"mirror\": \"kubernetes.io/config.mirror\" in annotations,\n                \"bare\": not owners,\n                \"empty_dir\": any(isinstance(v, dict) and v.get(\"emptyDir\") is not None\n                                 for v in volumes),\n            })\n        elif kind == \"PodDisruptionBudget\":\n            name, namespace, _, _ = meta_of(item, \"PDB\")\n            spec = item.get(\"spec\") or {}\n            if not isinstance(spec, dict):\n                die(\"%s has a bad spec\" % name)\n            pdbs.append({\n                \"name\": name, \"namespace\": namespace,\n                \"selector\": spec.get(\"selector\") or {},\n                \"allowed\": allowed_of(item, name),\n            })\n        else:\n            continue\n\n    if not nodes:\n        die(\"the dump has no Node objects\")\n    target = next((node for node in nodes if node[\"name\"] == args.node), None)\n    if target is None:\n        die(\"node %s is not in the dump\" % args.node)\n\n    on_node = [p for p in pods\n               if p[\"node\"] == args.node and p[\"phase\"] not in (\"Succeeded\", \"Failed\")\n               and not p[\"deleting\"]]\n    total = 0\n    order = []\n    blocked = 0\n    any_daemonset = False\n    any_empty = False\n    for pod in sorted(on_node, key=lambda p: p[\"name\"]):\n        if pod[\"daemonset\"]:\n            any_daemonset = True\n            print(\"SKIP %s daemonset (--ignore-daemonsets)\" % pod[\"name\"])\n            continue\n        if pod[\"mirror\"]:\n            print(\"SKIP %s mirror pod\" % pod[\"name\"])\n            continue\n        pod[\"pdbs\"] = [pdb for pdb in pdbs\n                       if pdb[\"namespace\"] == pod[\"namespace\"]\n                       and matches(pdb[\"selector\"], pod[\"labels\"])]\n        pod[\"headroom\"] = min([pdb[\"allowed\"] for pdb in pod[\"pdbs\"]]\n                              or [999])\n        total += 1\n\n    candidates = [p for p in on_node if not p[\"daemonset\"] and not p[\"mirror\"]]\n    candidates.sort(key=lambda p: (p[\"bare\"], -p[\"headroom\"], p[\"name\"]))\n\n    for pod in candidates:\n        if pod[\"bare\"]:\n            print(\"BLOCK %s bare pod, move it or add --force\" % pod[\"name\"])\n            blocked += 1\n            continue\n        zero = [pdb for pdb in pod[\"pdbs\"] if pdb[\"allowed\"] <= 0]\n        if zero:\n            print(\"BLOCK %s pdb %s no disruptions allowed\"\n                  % (pod[\"name\"], \",\".join(pdb[\"name\"] for pdb in zero)))\n            blocked += 1\n            continue\n        before = [(pdb[\"name\"], pdb[\"allowed\"]) for pdb in pod[\"pdbs\"]]\n        for pdb in pod[\"pdbs\"]:\n            pdb[\"allowed\"] -= 1\n        order.append(pod[\"name\"])\n        if pod[\"pdbs\"]:\n            detail = \", \".join(\"%s:%d->%d\"\n                               % (name, old, pdb[\"allowed\"])\n                               for (name, old), pdb in zip(before, pod[\"pdbs\"]))\n            print(\"EVICT %d/%d %s pdb %s\"\n                  % (len(order), total, pod[\"name\"], detail))\n        else:\n            print(\"EVICT %d/%d %s no pdb\"\n                  % (len(order), total, pod[\"name\"]))\n\n    for pod in candidates:\n        if pod[\"empty_dir\"]:\n            any_empty = True\n            print(\"WARN %s emptyDir, --delete-emptydir-data loses data\"\n                  % pod[\"name\"])\n\n    schedulable = [node for node in nodes\n                   if node[\"ready\"] and not node[\"unschedulable\"]]\n    remaining = len([node for node in schedulable if node[\"name\"] != args.node])\n    print(\"CAPACITY %d schedulable -> %d after the drain, min-free %d\"\n          % (len(schedulable), remaining, args.min_free))\n\n    flags = []\n    if any_daemonset:\n        flags.append(\"--ignore-daemonsets\")\n    if any_empty:\n        flags.append(\"--delete-emptydir-data\")\n    flags.append(\"--dry-run=server\")\n    print(\"COMMAND kubectl drain %s %s\" % (args.node, \" \".join(flags)))\n    print(\"ORDER \" + (\", \".join(order) if order else \"-\"))\n\n    low = remaining < args.min_free\n    if low or blocked:\n        reasons = []\n        if low:\n            reasons.append(\"capacity %d < min-free %d\"\n                           % (remaining, args.min_free))\n        if blocked:\n            reasons.append(\"%d pod%s would block the drain\"\n                           % (blocked, \"\" if blocked == 1 else \"s\"))\n        print(\"FAIL \" + \", \".join(reasons))\n        return 1\n    print(\"OK drain %s leaves %d schedulable nodes, %d eviction%s\"\n          % (args.node, remaining, len(order),\n             \"\" if len(order) == 1 else \"s\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p697","n":"697","slug":"the-recovery-path-drawn","title":"The recovery path, drawn","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","SRE","DevSecOps","Recovery"],"series_name":"IR","design":"architecture","script":"","date":"2031-03-11","files":{"it":"notes/p697-the-recovery-path-drawn.md","devsecops":"notes/p697-the-recovery-path-drawn.md"},"md":"# The recovery path, drawn\n\nThe incident is contained. The attacker's session is dead, the bad deploy is stopped, and the database is three hours behind. The order you recover in decides whether the next hour is a fix or a second incident.\n\n## Quick reference\n\n- **Tier 1 · Rotate:** Secrets (ROT) · Certificates (REPL) · Tokens (REVOK)\n- **Tier 2 · Rollback:** Config files (HASH) · Container image (DIG) · Feature flags (FLAG)\n- **Tier 3 · Restore:** Data snapshot (RSTR) · Traffic (SHFT) · Verification (PROVE)\n\n**Request flow:** Secrets → Config files → Data snapshot → Traffic → Verification; Certificates → Config files; Tokens → Config files; Container image → Data snapshot\n\n**Around it:** Audit log (LOG) · Status page (PAGE) · Evidence store (EVID)\n\n## Explained\n\n### Rotate\n\nAccess comes first. Rotate every secret the attacker could have read: the database password, the API token, the signing key. Replace every certificate they could have copied, including the shared ones. This is the tier most often skipped. Verification is not \"the new value is in the vault\"; it is \"the old value fails\".\n\n### Roll back\n\nConfiguration comes second. Roll the config files back to the last known good hash, then the container image to the digest that passed the last check, then the feature flags. Roll back to a known good, not to \"the previous version\"; the previous version may be the one that broke.\n\n### Restore\n\nData comes last. Pick a snapshot before the incident, prove it is complete, and restore into a scratch environment before you point traffic at it. Move traffic only after verification. Restoring data first can overwrite the evidence you need and re-infect the system through a credential you have not rotated.\n\nAround the three tiers sit the audit log, the status page and the evidence store, because a recovery without a record is just a story.\n\n> **The honest caveat:** the order is a default, not a law. A data-destroying incident may force restore earlier, but access still comes first.\n\n> **The lesson:** recovery is a sequence with dependencies, not a checklist of independent fixes.\n\n## In one line\n\n> **Access first, data last.** The order is a dependency chain, not a menu.\n\n**Start with access**\n\n```bash\npython3 credential_rotate.py rot.json\n```\n"},{"id":"p698","n":"698","slug":"recovery-command-cheat-sheet","title":"Recovery command cheat sheet","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","Linux","SRE","CommandLine"],"series_name":"IR","design":"cheatsheet","script":"","date":"2031-03-13","files":{"it":"notes/p698-recovery-command-cheat-sheet.md","devsecops":"notes/p698-recovery-command-cheat-sheet.md"},"md":"# Recovery command cheat sheet\n\nRecovery is a set of small proofs. Did the old credential stop working? Does the serial match? Is the config the hash that worked? Is the snapshot restorable? Each block answers one of those.\n\n## Quick reference\n\n### Secrets\n\n```bash\nvault kv metadata get secret/db\naws secretsmanager list-secrets\ngcloud secrets versions list db\nvault token lookup\nsystemctl status vault-agent\n```\n\n### Certs\n\n```bash\nopenssl x509 -noout -serial -in cert.pem\nopenssl x509 -noout -dates -in cert.pem\nopenssl s_client -connect host:443\nsystemctl reload nginx\nkubectl rollout restart deploy/api\n```\n\n### Config\n\n```bash\nsha256sum /etc/nginx/nginx.conf\ndiff -u lkg/nginx.conf /etc/nginx.conf\ncp -p lkg/nginx.conf /etc/nginx.conf\nnginx -t && systemctl reload nginx\nkubectl rollout undo deploy/api\n```\n\n### Data\n\n```bash\naws rds describe-db-snapshots\npg_restore -l /backup/app.dump\nrestic snapshots --last 5\nvelero restore create --from-backup b\nkubectl get pvc -n prod\n```\n\n## Explained\n\n### Secrets\n\nRead metadata, never the value. `vault kv metadata get secret/db` shows versions and timestamps; `aws secretsmanager list-secrets` shows rotation dates; `gcloud secrets versions list db` lists versions; `vault token lookup` shows policies and TTL; `systemctl status vault-agent` proves the agent is running. A command that prints a secret is not a recovery command.\n\n### Certs\n\n`openssl x509 -noout -serial -in cert.pem` reads the serial; -dates reads the validity window; `openssl s_client -connect host:443` proves what the listener serves. `systemctl reload nginx` applies a new file, and `kubectl rollout restart deploy/api` does the same for a pod. Reload after every copy, never before.\n\n### Config\n\nsha256sum proves the file; diff shows what changed; `cp -p` restores it; `nginx -t` proves it parses; `nginx -t && systemctl reload nginx` applies it. `kubectl rollout undo deploy/api` returns the Deployment to the previous ReplicaSet. A hash match is not a config test.\n\n### Data\n\n`aws rds describe-db-snapshots` lists the RDS snapshots; `pg_restore -l` reads a dump without restoring it; `restic snapshots --last 5` lists the restic ones; velero restore create `--from-backup b` starts a cluster restore; `kubectl get pvc -n prod` shows what is bound. Prove before you restore.\n\n> **The honest caveat:** some commands prove; some change. None of them proves the restored data is correct.\n\n> **The lesson:** recovery commands should prove state before they change it.\n\n## In one line\n\n> **Prove the state, then change it.** Every block answers a question the incident asked.\n\n**Prove the serial**\n\n```bash\nopenssl x509 -noout -serial\n```\n"},{"id":"p699","n":"699","slug":"four-recovery-failures-explained","title":"Four recovery failures, explained","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","CyberSecurity","Kubernetes","Reliability"],"series_name":"IR","design":"explainer","script":"","date":"2031-03-16","files":{"it":"notes/p699-four-recovery-failures-explained.md","devsecops":"notes/p699-four-recovery-failures-explained.md"},"md":"# Four recovery failures, explained\n\nMost recoveries fail in the same four places, and none of them is a tool failure. They are all a step taken in the wrong order, or a check that was never run.\n\n## Quick reference\n\n### Rotation out of order\n\n- Dependency rotates after its consumers\n- Two credentials live at once\n- Auth errors look like a new incident\n\n### One certificate, many hosts\n\n- One reload, several listeners\n- The edge serves a serial the backends do not\n- Replace everywhere before any reload\n\n### The restore point is missing\n\n- Newest snapshot still creating\n- Catalog says complete, nobody restored it\n- No complete point inside the window\n\n### Draining too fast\n\n- PDB allows one, node holds two replicas\n- Second eviction blocks the drain\n- Force it and capacity drops below the floor\n\n> **Note:** All four are a step taken too early, not a tool that failed.\n\n## Explained\n\n### Rotation out of order\n\nRotate a dependency after its consumers and both credentials are live at once. The API keeps the old password in a pool, the worker reconnects with it, and every auth error looks like a new incident. Rotate the dependency first, verify the consumer, then retire the old value.\n\n### One certificate, many hosts\n\nA certificate shared by four hosts is one change with four reloads. Reload the load balancer before the web hosts have the file and the edge serves a serial the backends do not. Replace everywhere, then reload one host at a time.\n\n### The restore point is missing\n\nThe newest snapshot is still creating, the one before it failed, and the one that works is outside the window. The catalog said \"backup complete\"; nobody restored it. A restore point must be complete, before the incident, and drilled.\n\n### Draining too fast\n\nThe PDB allows one disruption and the node holds two of the three replicas. Drain it and the second eviction blocks; force it and the service drops below its floor. Capacity is part of the plan, not an afterthought.\n\n> **The honest caveat:** these four are the common failures, not the only ones.\n\n> **The lesson:** recovery fails in the gaps between steps, where nobody checked.\n\n## In one line\n\n> **Recovery fails in the gaps.** The plan is the check between the steps.\n\n**Check the order**\n\n```bash\npython3 node_drain.py drain.json\n```\n"},{"id":"p700","n":"700","slug":"9-checks-before-you-call-it-recovered","title":"9 checks before you call it recovered","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","CyberSecurity","DevSecOps","Checklist"],"series_name":"IR","design":"checklist","script":"","date":"2031-03-18","files":{"it":"notes/p700-9-checks-before-you-call-it-recovered.md","devsecops":"notes/p700-9-checks-before-you-call-it-recovered.md"},"md":"# 9 checks before you call it recovered\n\nThe pager is quiet and the dashboard is green. That is the moment to slow down: a service can be up and still be compromised, misconfigured or running on data from the wrong hour.\n\n## Quick reference\n\n### Access\n\n```bash\n# Old credentials fail\ncurl -H 'Authorization: old'\n# Certificate serial matches\nopenssl x509 -noout -serial\n# Sessions are revoked\nSELECT count(*) FROM sessions\n```\n\n### Config\n\n- **Config hash matches LKG** — `sha256sum -c manifest`\n- **Image digest is pinned** — `kubectl get pod -o yaml`\n- **Feature flags are reverted** — flag audit log\n\n### Data\n\n```bash\n# Restore point is drilled\npg_restore -l dump\n# Row counts match the LKG\nSELECT count(*) FROM orders\n# Replication is caught up\npg_last_wal_replay_lsn()\n```\n\n## Explained\n\n### Access\n\nOld credentials fail. Run the old token against the API and watch it return 401; a vault entry that says \"rotated\" is not proof.  \nThe certificate serial matches. `openssl x509 -noout -serial -in cert.pem` on each host, then `openssl s_client -connect host:443` from outside; a reload that never happened is invisible from the file.  \nSessions are revoked. Count the active sessions for the affected users, then count them again after the revoke. A stolen cookie does not care that the password changed.\n\n### Config\n\nThe config hash matches the LKG. sha256sum every drifted file against the manifest before the service starts.  \nThe image digest is the one that passed. `kubectl get pod -o yaml` shows the digest; a tag can move, a digest cannot.  \nFeature flags are back to the known good. The flag audit log names who changed what, and the kill switch is off.\n\n### Data\n\nThe restore point is drilled. `pg_restore -l` reads the dump; a row count in a scratch schema is better.  \nThe row counts match the last known good. Compare orders, users and the ledger, not just the table count.  \nReplication is caught up. pg_last_wal_replay_lsn on the replica should match the primary before you trust a read.\n\n> **The honest caveat:** a green checklist is evidence of checks, not proof of safety.\n\n> **The lesson:** recovery ends when an independent check passes, not when the pager stops.\n\n## In one line\n\n> **Recovered is a test result.** A quiet pager is not a passing check.\n\n**The first check**\n\n```bash\nopenssl x509 -noout -serial\n```\n"},{"id":"p701","n":"701","slug":"quiz-3-recovery-questions","title":"Quiz: 3 recovery questions","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","CyberSecurity","SRE","Backup","Quiz"],"series_name":"IR","design":"quiz","script":"","date":"2031-03-20","files":{"it":"notes/p701-quiz-3-recovery-questions.md","devsecops":"notes/p701-quiz-3-recovery-questions.md"},"md":"# Quiz: 3 recovery questions\n\nThree questions about the last mile of an incident: the order you rotate in, the certificate you share, and the restore point you bet the recovery on. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why does a credential rotation have an order?**\n\n- a) Alphabetical order is easier to audit\n- b) Dependents move after their dependencies\n- c) The vault requires a fixed order\n\n> **Answer:** b) Dependents move after their dependencies. A consumer that still holds the old value fails when its dependency rotates. Rotate the dependency, verify each consumer, then retire the old value.\n\n**Q2. Why does a shared certificate change the replacement plan?**\n\n- a) It always expires sooner\n- b) Every host holds the file before any reload\n- c) It needs a new serial number\n\n> **Answer:** b) Every host holds the file before any reload. One reload can serve the new serial while the other hosts still serve the old. Replace everywhere, then reload one host at a time and check the serial from outside.\n\n**Q3. What must a restore point prove before you use it?**\n\n- a) That it is the newest snapshot\n- b) That it is complete, pre-incident and restored\n- c) That it is stored in another region\n\n> **Answer:** b) That it is complete, pre-incident and restored. A catalog entry is a claim. The point must be complete, taken before the incident, and drilled at least once; an untested backup is a gamble.\n\n## Explained\n\n### Question one\n\nWhy does a credential rotation have an order? Because the registry is alphabetical? No. A consumer that still holds the old value fails the moment its dependency rotates. The dependency goes first, the consumer is verified against the new value, and only then is the old value retired. Skip that order and the outage looks like a new incident.\n\n### Question two\n\nWhy does a shared certificate change the replacement plan? Because it expires sooner? No. One reload can start serving the new serial while the other hosts still serve the old. Replace the file everywhere, then reload one host at a time and check the serial from outside. The second host is not a backup; it is a second place the same change must be verified.\n\n### Question three\n\nWhat must a restore point prove before you use it? That it is the newest? No. It must be complete, taken before the incident, and restored at least once. A catalog entry is a claim; a drilled restore is evidence. A snapshot nobody has restored is a backup, not a recovery plan.\n\n> **The honest caveat:** passing this quiz tests the plan, not your incident.\n\n> **The lesson:** recovery is a sequence, not a checklist. If you got these, you are already treating it that way.\n\n## In one line\n\n> **Order, group, prove.** Three answers that keep a recovery in sequence.\n\n**Run the plan**\n\n```bash\npython3 restore_point.py cat.json\n```\n"},{"id":"p702","n":"702","slug":"an-evidence-bundle","title":"An evidence bundle","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","CyberSecurity","Forensics","DFIR"],"series_name":"IR","design":"code","script":"evidence_collect.py","date":"2031-03-23","files":{"it":"notes/p702-an-evidence-bundle.md","devsecops":"notes/p702-an-evidence-bundle.md"},"md":"# An evidence bundle\n\nAt 04:20 the incident is over and someone asks the question nobody prepared for: prove the log you read was the log on the host.\n\n## Quick reference\n\nThe full script is at the end of this note: `evidence_collect.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 evidence_collect.py case case/ps.txt case/ss.txt\n```\n\n**Example output**\n\n```text\nMANIFEST 2 files, 2 exports\nBUNDLE 4bf106cf015e6cdc3127b8ce7c904bac60213d1a8f7b09ae8aaaef741580e21a\n```\n\n## Explained\n\n### What it does\n\n`evidence_collect.py` walks a case directory, hashes every regular file with SHA-256, and records its size and path. It counts rows in a process export and sockets in a network export, hashes those too, and prints a manifest with a bundle hash over every artefact in order. It never prints contents and never follows a symlink, so a planted link cannot pull in a file from outside the case.\n\n### What the screen shows\n\nThe fourteen lines build the artefact list, skip the exports, concatenate every digest and print MANIFEST 2 files, 2 exports and the bundle hash.\n\n### How to run it\n\n`python3 evidence_collect.py /cases/inc-214 --ps ps.txt --net ss.txt > manifest.txt`. Exit 0 is a complete manifest, 1 an artefact it could not read, 2 a bad path. In CI, upload that `manifest.txt` as the build artefact.\n\n### The fix\n\nSeal the bundle while you still have the machine, and keep the manifest beside it in the ticket: an edit to one file then changes one digest and the bundle.\n\n### Prevention\n\nCollect before you investigate. The first command on a suspect host should be a capture, the second its hash.\n\n> **The honest caveat:** a digest proves the bytes have not changed since it was taken; it says nothing about who could reach them before that.\n\n> **The lesson:** the manifest is the evidence, and the bundle hash is the seal.\n\n## In one line\n\n> **A hash is a promise the bytes keep.** A copy is a claim; a digest is the proof beside it.\n\n**Seal the bundle**\n\n```bash\nsha256sum auth.log > auth.log.sha256\n```\n\n## Script: evidence_collect.py\n\nDownload: [scripts/evidence_collect.py](../scripts/evidence_collect.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"evidence_collect.py - hash a case directory into one defensible manifest.\n\nWalks a path, hashes every regular file with SHA-256, counts the lines in a\nprocess export and the sockets in a network export, and prints one manifest\nto stdout: a line per artefact, the totals, and a bundle hash over all the\nartefacts together. Capture the manifest beside the bundle, not inside it.\n\nThe manifest is the evidence. It proves that the bytes you kept are the\nbytes you collected, and the bundle hash proves that no line was removed\nfrom the list afterwards.\n\nUsage:\n    python3 evidence_collect.py /cases/inc-214\n    python3 evidence_collect.py /cases/inc-214 --ps ps.txt --net ss.txt\n    python3 evidence_collect.py /cases/inc-214 ps.txt ss.txt\n    python3 evidence_collect.py /cases/inc-214 --ps ps.txt --net ss.txt --json\n    python3 evidence_collect.py /cases/inc-214 --ps ps.txt --net ss.txt > manifest.txt\n    python3 evidence_collect.py --help\n\nCI (seal the bundle and fail the job when an artefact cannot be read):\n    python3 evidence_collect.py /cases/inc-214 --ps ps.txt --net ss.txt > manifest.txt\n\nCron (one manifest per case, kept next to the bundle):\n    17 * * * * python3 /opt/s0sta/evidence_collect.py /cases/inc-214 --ps /cases/inc-214/ps.txt --net /cases/inc-214/ss.txt > /cases/inc-214/manifest.txt\n\nExit codes: 0 manifest complete, 1 an artefact could not be read or the path\nholds no regular file, 2 usage error or a missing path.\n\nRead-only: opens the given files for reading, hashes the bytes and prints.\nIt writes nothing, changes nothing and makes no network call. It never\nprints file contents, only names, sizes and digests.\n\nSymbolic links are recorded by target and not followed, so a link planted\ninside the case directory cannot pull a file from outside it into the\nbundle. Directories are not hashed; an empty directory is a finding, not a\nclean run.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport hashlib\nimport json\nimport os\nimport stat\nimport sys\n\nCHUNK = 1024 * 1024\nPS_SKIP = (\"pid\", \"user\", \"uid\", \"owner\")\nNET_SKIP = (\"state\", \"proto\", \"netid\", \"recv-q\", \"active\", \"local\")\nNET_STATES = (\"estab\", \"listen\", \"time-wait\", \"close-wait\", \"syn-sent\",\n              \"fin-wait-1\", \"fin-wait-2\", \"last-ack\", \"closing\", \"closed\",\n              \"unconn\", \"syn-recv\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be audited.\"\"\"\n\n\ndef sha256_file(path):\n    \"\"\"Return the hex digest of a file, reading it in chunks.\"\"\"\n    digest = hashlib.sha256()\n    with open(path, \"rb\") as handle:\n        for block in iter(lambda: handle.read(CHUNK), b\"\"):\n            digest.update(block)\n    return digest.hexdigest()\n\n\ndef read_text(source):\n    \"\"\"Return the whole input as text, or raise InputError.\"\"\"\n    try:\n        if source == \"-\":\n            return sys.stdin.read()\n        with open(source, \"r\", encoding=\"utf-8\", errors=\"replace\") as handle:\n            return handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (source, exc.strerror or exc))\n\n\ndef count_ps(text):\n    \"\"\"Count process rows in a ps export, skipping any header and callbacks.\"\"\"\n    rows = 0\n    for line in text.splitlines():\n        fields = line.split()\n        if not fields or fields[0].lower() in PS_SKIP:\n            continue\n        if line.startswith((\"UID\", \"PID\", \"USER\")):\n            continue\n        rows += 1\n    return rows\n\n\ndef count_net(text):\n    \"\"\"Count socket rows in an ss or netstat export.\"\"\"\n    rows = 0\n    for line in text.splitlines():\n        fields = line.split()\n        if not fields or fields[0].lower() in NET_SKIP:\n            continue\n        if fields[0].lower() in NET_STATES or fields[0].startswith((\"tcp\", \"udp\", \"raw\")):\n            rows += 1\n    return rows\n\n\ndef walk(root, exclude=()):\n    \"\"\"Return (files, links, skipped) for a directory, without following links.\"\"\"\n    files, links, skipped = [], [], []\n    for dirpath, dirnames, filenames in os.walk(root, followlinks=False):\n        dirnames.sort()\n        for name in sorted(filenames):\n            full = os.path.join(dirpath, name)\n            rel = os.path.relpath(full, root)\n            if os.path.realpath(full) in exclude:\n                continue\n            try:\n                info = os.lstat(full)\n            except OSError as exc:\n                skipped.append((rel, exc.strerror or str(exc)))\n                continue\n            if stat.S_ISLNK(info.st_mode):\n                try:\n                    links.append((rel, os.readlink(full)))\n                except OSError as exc:\n                    skipped.append((rel, exc.strerror or str(exc)))\n                continue\n            if not stat.S_ISREG(info.st_mode):\n                skipped.append((rel, \"not a regular file\"))\n                continue\n            try:\n                digest = sha256_file(full)\n            except OSError as exc:\n                skipped.append((rel, exc.strerror or str(exc)))\n                continue\n            files.append({\"path\": rel, \"size\": info.st_size, \"sha256\": digest})\n    files.sort(key=lambda row: row[\"path\"])\n    links.sort(key=lambda row: row[0])\n    skipped.sort(key=lambda row: row[0])\n    return files, links, skipped\n\n\ndef bundle_hash(files, exports):\n    \"\"\"Hash the artefact digests in manifest order: files first, then exports.\n\n    Each digest is 64 hex characters, so the concatenation is unambiguous.\n    Removing, adding or reordering one artefact changes the bundle hash.\n    \"\"\"\n    digest = hashlib.sha256()\n    for row in files:\n        digest.update(row[\"sha256\"].encode())\n    for row in exports:\n        digest.update(row[\"sha256\"].encode())\n    return digest.hexdigest()\n\n\ndef build_manifest(root, ps_path, net_path):\n    \"\"\"Return the manifest as a dict, or raise InputError.\"\"\"\n    if not os.path.exists(root):\n        raise InputError(\"%s: no such path\" % root)\n    exclude = set(os.path.realpath(p) for p in (ps_path, net_path) if p and p != \"-\")\n    if os.path.isdir(root):\n        files, links, skipped = walk(root, exclude)\n    else:\n        info = os.lstat(root)\n        if not stat.S_ISREG(info.st_mode):\n            raise InputError(\"%s: not a regular file or directory\" % root)\n        files = [{\"path\": os.path.basename(root), \"size\": info.st_size,\n                  \"sha256\": sha256_file(root)}]\n        links, skipped = [], []\n\n    exports = []\n    if ps_path:\n        text = read_text(ps_path)\n        exports.append({\"kind\": \"ps\", \"path\": ps_path, \"count\": count_ps(text),\n                        \"unit\": \"lines\", \"sha256\": sha256_file(ps_path)\n                        if ps_path != \"-\" else hashlib.sha256(text.encode()).hexdigest()})\n    if net_path:\n        text = read_text(net_path)\n        exports.append({\"kind\": \"net\", \"path\": net_path, \"count\": count_net(text),\n                        \"unit\": \"sockets\", \"sha256\": sha256_file(net_path)\n                        if net_path != \"-\" else hashlib.sha256(text.encode()).hexdigest()})\n\n    manifest = {\n        \"root\": root,\n        \"files\": files,\n        \"links\": [{\"path\": p, \"target\": t} for p, t in links],\n        \"skipped\": [{\"path\": p, \"reason\": r} for p, r in skipped],\n        \"exports\": exports,\n        \"bytes\": sum(row[\"size\"] for row in files),\n        \"bundle\": bundle_hash(files, exports),\n    }\n    return manifest\n\n\ndef print_manifest(man):\n    \"\"\"Print the human manifest, one line per artefact.\"\"\"\n    print(\"MANIFEST %d files, %d exports\" % (len(man[\"files\"]), len(man[\"exports\"])))\n    for row in man[\"files\"]:\n        print(\"FILE %s %d %s\" % (row[\"sha256\"], row[\"size\"], row[\"path\"]))\n    for row in man[\"links\"]:\n        print(\"LINK %s -> %s\" % (row[\"path\"], row[\"target\"]))\n    for row in man[\"exports\"]:\n        print(\"EXPORT %s %s %d %s\" % (row[\"kind\"], row[\"sha256\"], row[\"count\"], row[\"unit\"]))\n    for row in man[\"skipped\"]:\n        print(\"SKIP %s: %s\" % (row[\"path\"], row[\"reason\"]))\n    if man[\"skipped\"]:\n        print(\"INCOMPLETE %d artefact(s) not hashed\" % len(man[\"skipped\"]))\n    print(\"TOTAL %d files, %d bytes\" % (len(man[\"files\"]), man[\"bytes\"]))\n    print(\"BUNDLE %s\" % man[\"bundle\"])\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Hash a case directory into an evidence manifest (read-only).\")\n    parser.add_argument(\"path\", help=\"directory or single file to hash\")\n    parser.add_argument(\"exports\", nargs=\"*\", metavar=\"EXPORT\",\n                        help=\"optional process and network exports, in that order\")\n    parser.add_argument(\"--ps\", metavar=\"FILE\",\n                        help=\"text export of the process list (ps -eo pid,user,args)\")\n    parser.add_argument(\"--net\", metavar=\"FILE\",\n                        help=\"text export of the socket table (ss -tanp)\")\n    parser.add_argument(\"--json\", action=\"store_true\",\n                        help=\"print the manifest as JSON instead of lines\")\n    args = parser.parse_args(argv)\n\n    if len(args.exports) > 2:\n        print(\"ERROR at most two positional exports (ps, then net)\", file=sys.stderr)\n        return 2\n    if args.exports:\n        args.ps = args.ps or args.exports[0]\n    if len(args.exports) > 1:\n        args.net = args.net or args.exports[1]\n\n    try:\n        man = build_manifest(args.path, args.ps, args.net)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n    except OSError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not man[\"files\"] and not man[\"skipped\"]:\n        print(\"MANIFEST 0 files, 0 exports\")\n        print(\"EMPTY no regular file under %s\" % man[\"root\"])\n        print(\"BUNDLE %s\" % man[\"bundle\"])\n        return 1\n\n    if args.json:\n        print(json.dumps(man, indent=2, sort_keys=True))\n    else:\n        print_manifest(man)\n    return 1 if man[\"skipped\"] else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p703","n":"703","slug":"a-status-page-draft","title":"A status page draft","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","SRE","StatusPage","CyberSecurity"],"series_name":"IR","design":"code","script":"status_update.py","date":"2031-03-25","files":{"it":"notes/p703-a-status-page-draft.md","devsecops":"notes/p703-a-status-page-draft.md"},"md":"# A status page draft\n\nAt 03:40 the incident channel has two hundred messages and the status page still carries the 03:10 entry: we are investigating. Nobody has written the next one.\n\n## Quick reference\n\nThe full script is at the end of this note: `status_update.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 status_update.py status/partial.json identified\n```\n\n**Example output**\n\n```text\nSTATUS identified INC-214 Checkout errors\nMISSING next_update\n```\n\n## Explained\n\n### What it does\n\n`status_update.py` reads the incident JSON and the phase, then prints the fields a status page needs: what happened, the impact, the action, the owner and the next update time. Each phase has its own required set. Investigating may say the impact is still being measured; identified and later must state it, so unknown and assessing are rejected as placeholders. A missing field is printed as MISSING and the exit code is 1.\n\n### What the screen shows\n\nThe thirteen lines load the incident, hold the phase table, collect the missing fields and print STATUS identified INC-214 Checkout errors, then MISSING next_update.\n\n### How to run it\n\n`python3 status_update.py incident.json` monitoring `--at 2026-10-20T03:16:00Z`. Exit 0 is a complete draft, 1 a missing field, 2 an unknown phase or unreadable JSON. In cron, draft the update five minutes before it is due.\n\n### The fix\n\nThe draft is the checklist. If the next update time is missing, you do not have an update, you have a sentence. Publish the time, then keep it.\n\n### Prevention\n\nPut the required fields in the template, not in the reviewer's memory, and let the phase decide which are mandatory.\n\n> **The honest caveat:** the tool can check that an impact figure exists, not that it is true.\n\n> **The lesson:** a status page is a promise with a timestamp.\n\n## In one line\n\n> **An update without a next time is a rumour.** The page needs a time it will be true again, not just a mood.\n\n**Draft the update**\n\n```bash\npython3 status_update.py inc.json identified\n```\n\n## Script: status_update.py\n\nDownload: [scripts/status_update.py](../scripts/status_update.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"status_update.py - draft the status page update from an incident file.\n\nReads an incident JSON and a phase (investigating, identified, monitoring or\nresolved), then prints the update a status page needs: what happened, the\nimpact, the action, the owner and the next update time. Every field the\nphase requires is checked, and a missing impact or next-update time is\nprinted as MISSING, which is the difference between an update and a\nsentence.\n\nThe incident file is a JSON object. Only id and title are required:\n\n    {\"id\": \"INC-214\", \"title\": \"Checkout errors\",\n     \"impact\": \"4.1% of card payments failing since 03:04Z\",\n     \"action\": \"Roll back release 2026.10.20-3 and replay the queue\",\n     \"owner\": \"payments on-call\",\n     \"next_update\": \"2026-10-20T03:34:00Z\"}\n\nUsage:\n    python3 status_update.py incident.json investigating\n    python3 status_update.py incident.json identified --at 2026-10-20T03:16:00Z\n    python3 status_update.py incident.json resolved\n    python3 status_update.py --help\n\nCI (refuse to publish an update that names no impact and no next time):\n    python3 status_update.py incident.json monitoring --at \"$(date -u +%FT%TZ)\" || exit 1\n\nCron (draft the next update before the promised time):\n    */5 * * * * python3 /opt/s0sta/status_update.py /cases/inc-214.json monitoring > /tmp/update.txt\n\nExit codes: 0 the draft is complete, 1 a required field is missing or holds\na placeholder, 2 usage error, unreadable JSON or an unknown phase.\n\nRead-only: opens the incident file, prints, and writes nothing.\n\nThe phase is not decoration. Investigating needs the impact and a next update\ntime, and accepts a placeholder while the impact is being measured, because\na number at 03:12 would be a guess. Identified and later also need the action,\nand a placeholder impact is rejected there. Every phase needs a next update\ntime, because a page that will never speak again is a page nobody follows.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\nPHASES = (\"investigating\", \"identified\", \"monitoring\", \"resolved\")\nREQUIRED = {\n    \"investigating\": (\"impact\", \"next_update\"),\n    \"identified\": (\"impact\", \"action\", \"next_update\"),\n    \"monitoring\": (\"impact\", \"action\", \"next_update\"),\n    \"resolved\": (\"impact\", \"action\", \"next_update\"),\n}\nPLACEHOLDERS = {\"unknown\", \"tbd\", \"tba\", \"n/a\", \"na\", \"none\", \"-\", \"?\", \"assessing\", \"\"}\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be used.\"\"\"\n\n\ndef parse_at(raw):\n    \"\"\"Return an aware UTC datetime from ISO 8601 text, or raise InputError.\"\"\"\n    text = raw.strip().replace(\"Z\", \"+00:00\").replace(\"z\", \"+00:00\")\n    try:\n        stamp = datetime.datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: not an ISO 8601 time\" % raw)\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=datetime.timezone.utc)\n    return stamp.astimezone(datetime.timezone.utc)\n\n\ndef load_json(path):\n    \"\"\"Return the parsed JSON, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s: not JSON (%s)\" % (path, exc))\n    return data\n\n\ndef normalise(data):\n    \"\"\"Return the incident as a dict of strings, or raise InputError.\"\"\"\n    if not isinstance(data, dict):\n        raise InputError(\"incident: expected a JSON object\")\n    incident = {}\n    for key, value in data.items():\n        if isinstance(value, (dict, list)):\n            continue\n        incident[str(key)] = str(value).strip()\n    for key in (\"id\", \"title\"):\n        if not incident.get(key):\n            raise InputError(\"incident: %s is required\" % key)\n    return incident\n\n\ndef relative(stamp, at):\n    \"\"\"Return a short description of a time relative to --at.\"\"\"\n    minutes = int(round((stamp - at).total_seconds() / 60.0))\n    if minutes == 0:\n        return \"now\"\n    if minutes > 0:\n        return \"in %d min\" % minutes\n    return \"%d min late\" % -minutes\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Draft a status page update from an incident file (read-only).\")\n    parser.add_argument(\"incident\", help=\"incident JSON file\")\n    parser.add_argument(\"phase\", help=\"one of: \" + \", \".join(PHASES))\n    parser.add_argument(\"--at\", metavar=\"ISO\",\n                        help=\"treat this time as now (default: the clock in UTC)\")\n    args = parser.parse_args(argv)\n\n    if args.phase not in PHASES:\n        print(\"ERROR unknown phase %r, expected one of: %s\"\n              % (args.phase, \", \".join(PHASES)), file=sys.stderr)\n        return 2\n\n    try:\n        incident = normalise(load_json(args.incident))\n        at = parse_at(args.at) if args.at else datetime.datetime.now(datetime.timezone.utc)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    required = REQUIRED[args.phase]\n    missing = []\n    print(\"STATUS %s %s %s\" % (args.phase, incident[\"id\"], incident[\"title\"]))\n    for field in (\"impact\", \"action\", \"owner\"):\n        value = incident.get(field, \"\")\n        if field not in required:\n            if value:\n                print(\"%s %s\" % (field.upper(), value))\n            continue\n        if not value:\n            missing.append(field)\n            continue\n        if field == \"impact\" and args.phase != \"investigating\" and value.lower() in PLACEHOLDERS:\n            missing.append(field)\n            continue\n        print(\"%s %s\" % (field.upper(), value))\n\n    if \"next_update\" in required:\n        raw = incident.get(\"next_update\", \"\")\n        if not raw:\n            missing.append(\"next_update\")\n        else:\n            try:\n                stamp = parse_at(raw)\n                print(\"NEXT UPDATE %s (%s)\" % (raw, relative(stamp, at)))\n            except InputError:\n                missing.append(\"next_update\")\n    elif incident.get(\"next_update\"):\n        print(\"NEXT UPDATE %s\" % incident[\"next_update\"])\n    for field in missing:\n        print(\"MISSING %s\" % field)\n    return 1 if missing else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p704","n":"704","slug":"a-timeline-export","title":"A timeline export","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","SRE","Postmortem","DFIR"],"series_name":"IR","design":"code","script":"timeline_export.py","date":"2031-03-27","files":{"it":"notes/p704-a-timeline-export.md","devsecops":"notes/p704-a-timeline-export.md"},"md":"# A timeline export\n\nThe postmortem starts and three people remember three different orders for the same forty minutes. The log is complete; the story is not.\n\n## Quick reference\n\nThe full script is at the end of this note: `timeline_export.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 timeline_export.py timeline/dirty.json\n```\n\n**Example output**\n\n```text\n| 03:04Z | Checkout error rate 4.1% | alertmanager | mon-prod | ev-001 |\nFLAGS 2, no timestamp 1, no source 1\n```\n\n## Explained\n\n### What it does\n\n`timeline_export.py` reads an events JSON file and writes a markdown table with the time, the event, the actor, the source and the evidence reference for each row. It sorts by time, puts undated events in their own section, and flags every event with no timestamp and every event with no source. A flagged timeline exits 1, so it cannot be published by accident.\n\n### What the screen shows\n\nThe fourteen lines sort the events, format the first row and count the flags. On the dirty fixture they print the 03:04Z row with its actor, source and evidence id, then FLAGS 2, no timestamp 1, no source 1.\n\n### How to run it\n\n`python3 timeline_export.py events.json --out timeline.md --title \"INC-214` checkout errors\". Exit 0 is a fully sourced timeline, 1 is at least one flag, 2 is unreadable JSON. CI blocks on that code.\n\n### The fix\n\nWrite the row when the event happens, not when the review starts. The actor, the source and the evidence id are cheap at 03:10 and expensive at 11:00.\n\n### Prevention\n\nLet alerting, deploys and paging append rows automatically, then review the flags during the incident, not after it.\n\n> **The honest caveat:** the table is the export; keep the JSON beside it and hash both, or an edited table becomes a story.\n\n> **The lesson:** a timeline without a source is a memory with a table around it.\n\n## In one line\n\n> **A timeline without a source is a memory.** The row is only evidence when it names who saw it and where.\n\n**Export the rows**\n\n```bash\npython3 timeline_export.py events.json\n```\n\n## Script: timeline_export.py\n\nDownload: [scripts/timeline_export.py](../scripts/timeline_export.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"timeline_export.py - export incident events as a markdown timeline.\n\nReads an events JSON file and writes a markdown table with the time, the\nevent, the actor, the source and the evidence reference for each row. It\nsorts the rows by time, keeps events that carry no timestamp in their own\nsection at the end, and flags every event with no timestamp or no source,\nbecause an unsourced row is a memory, not evidence.\n\nThe events file is a JSON list. Only \"event\" is required:\n\n    [{\"at\": \"2026-10-20T03:04:00Z\", \"event\": \"Checkout error rate 4.1%\",\n      \"actor\": \"alertmanager\", \"source\": \"mon-prod\", \"evidence\": \"ev-001\"},\n     {\"at\": \"2026-10-20T03:19:00Z\", \"event\": \"Deploy 2026.10.20-3 finished\",\n      \"actor\": \"cd\", \"source\": \"cd-prod\", \"evidence\": \"ev-004\"}]\n\nUsage:\n    python3 timeline_export.py events.json\n    python3 timeline_export.py events.json --out timeline.md\n    python3 timeline_export.py events.json --title \"INC-214 checkout errors\"\n    python3 timeline_export.py --help\n\nCI (refuse to publish a timeline with an unsourced event):\n    python3 timeline_export.py events.json --out timeline.md || exit 1\n\nCron (refresh the public timeline every five minutes):\n    */5 * * * * python3 /opt/s0sta/timeline_export.py /cases/inc-214/events.json --out /srv/status/timeline.md\n\nExit codes: 0 every event has a timestamp and a source, 1 at least one event\nis missing a timestamp or a source, 2 usage error or unreadable JSON.\n\nRead-only: reads the events file and prints or writes the markdown file named\nby --out, and nothing else.\n\nThe table is the export, not the record. Keep the JSON beside it and hash\nboth; a markdown table that was edited after the incident is a story, not a\ntimeline.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\nCOLUMNS = (\"Time\", \"Event\", \"Actor\", \"Source\", \"Evidence\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be exported.\"\"\"\n\n\ndef parse_at(raw):\n    \"\"\"Return a naive UTC datetime from loose ISO text, or None.\"\"\"\n    text = str(raw).strip()\n    if not text:\n        return None\n    text = text.replace(\"Z\", \"+00:00\").replace(\"z\", \"+00:00\")\n    try:\n        stamp = datetime.datetime.fromisoformat(text)\n    except ValueError:\n        return None\n    if stamp.tzinfo is not None:\n        stamp = stamp.astimezone(datetime.timezone.utc).replace(tzinfo=None)\n    return stamp\n\n\ndef load_events(path):\n    \"\"\"Return a list of event dicts, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s: not JSON (%s)\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"events\")\n    if not isinstance(data, list):\n        raise InputError(\"%s: expected a list of events\" % path)\n    events = []\n    for index, row in enumerate(data, start=1):\n        if not isinstance(row, dict):\n            raise InputError(\"%s: event %d is not an object\" % (path, index))\n        event = {key: str(value).strip() for key, value in row.items()\n                 if not isinstance(value, (dict, list)) and value is not None}\n        event[\"event\"] = event.get(\"event\", \"\").strip()\n        if not event[\"event\"]:\n            raise InputError(\"%s: event %d has no event text\" % (path, index))\n        events.append(event)\n    if not events:\n        raise InputError(\"%s: no events\" % path)\n    return events\n\n\ndef cell(value):\n    \"\"\"Return a markdown table cell, escaping pipes and collapsing newlines.\"\"\"\n    text = \" \".join(str(value or \"\").split())\n    return text.replace(\"|\", \"\\\\|\") if text else \"-\"\n\n\ndef sort_key(event):\n    stamp = parse_at(event.get(\"at\", \"\"))\n    return (stamp is None, stamp or datetime.datetime.max)\n\n\ndef build(events, title):\n    \"\"\"Return (lines, flags) for the timeline.\"\"\"\n    flags = []\n    no_time, no_source = 0, 0\n    dated, undated = [], []\n    for event in events:\n        if parse_at(event.get(\"at\", \"\")):\n            dated.append(event)\n        else:\n            undated.append(event)\n            no_time += 1\n            flags.append((event[\"event\"], \"TIMESTAMP\"))\n        if not event.get(\"source\"):\n            no_source += 1\n            flags.append((event[\"event\"], \"SOURCE\"))\n    dated.sort(key=sort_key)\n    stamps = [parse_at(event.get(\"at\", \"\")) for event in dated]\n    days = sorted({stamp.date() for stamp in stamps})\n    multi = len(days) > 1\n\n    heading = \"# Timeline - %s\" % title\n    if len(days) == 1:\n        heading += \" (%s)\" % days[0].isoformat()\n    lines = [heading, \"\"]\n    lines.append(\"| \" + \" | \".join(COLUMNS) + \" |\")\n    lines.append(\"| \" + \" | \".join(\"---\" for _ in COLUMNS) + \" |\")\n    for stamp, event in zip(stamps, dated):\n        shown = stamp.strftime(\"%m-%d %H:%MZ\" if multi else \"%H:%MZ\")\n        lines.append(\"| %s | %s | %s | %s | %s |\" % (\n            cell(shown), cell(event[\"event\"]), cell(event.get(\"actor\")),\n            cell(event.get(\"source\")), cell(event.get(\"evidence\"))))\n    if undated:\n        lines.extend([\"\", \"## No timestamp\", \"\"])\n        for event in undated:\n            lines.append(\"- %s%s\" % (\n                cell(event[\"event\"]),\n                \" (actor %s)\" % cell(event[\"actor\"]) if event.get(\"actor\") else \"\"))\n    if flags:\n        lines.extend([\"\", \"## Flags\", \"\"])\n        for text, reason in flags:\n            lines.append(\"NO %s %s\" % (reason, text))\n    if flags:\n        lines.append(\"\")\n        lines.append(\"FLAGS %d, no timestamp %d, no source %d\"\n                     % (len(flags), no_time, no_source))\n    return lines, flags\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Export incident events as a markdown timeline (read-only).\")\n    parser.add_argument(\"events\", help=\"events JSON file\")\n    parser.add_argument(\"--out\", metavar=\"FILE\", help=\"write the markdown here instead of stdout\")\n    parser.add_argument(\"--title\", default=\"\", help=\"incident title for the heading\")\n    args = parser.parse_args(argv)\n\n    try:\n        events = load_events(args.events)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    title = args.title.strip() or \"incident\"\n    lines, flags = build(events, title)\n    body = \"\\n\".join(lines) + \"\\n\"\n    if args.out:\n        try:\n            with open(args.out, \"w\", encoding=\"utf-8\") as handle:\n                handle.write(body)\n        except OSError as exc:\n            print(\"ERROR %s: %s\" % (args.out, exc.strerror or exc), file=sys.stderr)\n            return 2\n        print(\"WROTE %s, %d events, %d flags\" % (args.out, len(events), len(flags)))\n    else:\n        sys.stdout.write(body)\n    return 1 if flags else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p705","n":"705","slug":"an-action-item-tracker","title":"An action item tracker","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Postmortem","SRE","IncidentResponse","DevOps"],"series_name":"IR","design":"code","script":"action_items.py","date":"2031-03-30","files":{"it":"notes/p705-an-action-item-tracker.md","devsecops":"notes/p705-an-action-item-tracker.md"},"md":"# An action item tracker\n\nThe postmortem wrote twelve actions, assigned nine, dated six, and the next incident finds the same three still open.\n\n## Quick reference\n\nThe full script is at the end of this note: `action_items.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 action_items.py actions/dirty.json --now 2026-10-21\n```\n\n**Example output**\n\n```text\nACTIONS 7 items, 6 open, 3 overdue, 1 unowned\nRATE 50% overdue of 6 open, limit 20%\n```\n\n## Explained\n\n### What it does\n\n`action_items.py` reads an action list and computes the overdue rate: overdue open items divided by open items. It names the items with no owner and the ones that duplicate another, and exits 1 when the rate passes `--max-overdue`, 20 per cent by default. Done, closed, dropped and cancelled all count as closed.\n\n### What the screen shows\n\nThe fourteen lines keep the open items, compare each due date with the date you pass, count the unowned ones and print ACTIONS 7 items, 6 open, 3 overdue, 1 unowned, then RATE 50% overdue of 6 open, limit 20%.\n\n### How to run it\n\n`python3 action_items.py actions.json --now 2026-10-21`, or in CI with `--now \"$(date -u +%F)\"`. Exit 0 is at or below the threshold, 1 is above it, 2 is bad input; `--strict` also fails on an unowned item or an open duplicate.\n\n### The fix\n\nTreat the tracker as a check, not a document. A weekly job that fails is the only thing that makes an action list close.\n\n### Prevention\n\nNo action without an owner and a date when it is written, and a duplicate points at its original instead of living beside it.\n\n> **The honest caveat:** an item can be on time and still be useless; this measures dates, not decisions.\n\n> **The lesson:** an action item with no owner is a wish with a ticket number.\n\n## In one line\n\n> **An action item with no owner is a wish.** The tracker closes because a job fails, not because a meeting did.\n\n**Count the open**\n\n```bash\npython3 action_items.py actions.json\n```\n\n## Script: action_items.py\n\nDownload: [scripts/action_items.py](../scripts/action_items.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"action_items.py - measure the action items a postmortem promised.\n\nReads a postmortem action list and reports the three reasons an item never\ncloses: it is past its due date, it has no owner, or it duplicates another\nitem. The overdue rate is the number of overdue open items divided by the\nnumber of open items; when that rate passes --max-overdue the tool exits 1,\nso a tracker that is quietly rotting fails the build like any other check.\n\nThe items file is a JSON list. Only \"id\" is required:\n\n    [{\"id\": \"A-1\", \"owner\": \"payments\", \"due\": \"2026-10-24\",\n      \"status\": \"open\", \"duplicate_of\": null},\n     {\"id\": \"A-2\", \"owner\": \"\", \"due\": \"2026-10-01\", \"status\": \"open\"},\n     {\"id\": \"A-3\", \"owner\": \"sre\", \"due\": \"2026-10-12\",\n      \"status\": \"open\", \"duplicate_of\": \"A-2\"}]\n\nUsage:\n    python3 action_items.py items.json\n    python3 action_items.py items.json --now 2026-10-21 --max-overdue 10\n    python3 action_items.py items.json --strict\n    python3 action_items.py --help\n\nCI (fail the week the action list stops closing):\n    python3 action_items.py postmortem/actions.json --now \"$(date -u +%F)\" || exit 1\n\nCron (Monday morning report, mailed by the CI job):\n    0 7 * * 1 python3 /opt/s0sta/action_items.py /srv/postmortems/actions.json --now \"$(date -u +%F)\"\n\nExit codes: 0 the overdue rate is at or below --max-overdue, 1 it is above,\n2 usage error or unreadable JSON. With --strict, 1 also covers any open item\nwithout an owner and any duplicate that is still open.\n\nRead-only: opens the items file, prints, and writes nothing. A \"due\" date is\nread as the end of that day in UTC, so an item due today is not overdue yet.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\nCLOSED = {\"done\", \"closed\", \"dropped\", \"cancelled\", \"wontfix\", \"won't fix\"}\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be measured.\"\"\"\n\n\ndef parse_day(raw):\n    \"\"\"Return a UTC datetime for an ISO date or datetime, or raise InputError.\"\"\"\n    text = str(raw).strip().replace(\"Z\", \"+00:00\")\n    try:\n        stamp = datetime.datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: not an ISO date\" % raw)\n    if stamp.tzinfo is not None:\n        stamp = stamp.astimezone(datetime.timezone.utc).replace(tzinfo=None)\n    if len(text) <= 10:\n        stamp = stamp.replace(hour=23, minute=59, second=59)\n    return stamp\n\n\ndef load_items(path, now):\n    \"\"\"Return a list of normalised item dicts, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s: not JSON (%s)\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"items\")\n    if not isinstance(data, list) or not data:\n        raise InputError(\"%s: expected a non-empty list of items\" % path)\n\n    items = []\n    for index, row in enumerate(data, start=1):\n        if not isinstance(row, dict):\n            raise InputError(\"%s: item %d is not an object\" % (path, index))\n        item = {key: str(value).strip() for key, value in row.items()\n                if not isinstance(value, (dict, list)) and value is not None}\n        if not item.get(\"id\"):\n            raise InputError(\"%s: item %d has no id\" % (path, index))\n        status = item.get(\"status\", \"open\").lower()\n        item[\"open\"] = status not in CLOSED\n        item[\"overdue\"] = False\n        if item[\"open\"] and item.get(\"due\"):\n            item[\"overdue\"] = parse_day(item[\"due\"]) < now\n        items.append(item)\n    return items\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report the action items a postmortem never closed (read-only).\")\n    parser.add_argument(\"items\", help=\"action items JSON file\")\n    parser.add_argument(\"--now\", metavar=\"DATE\",\n                        help=\"treat this date as today (default: the clock in UTC)\")\n    parser.add_argument(\"--max-overdue\", type=float, default=20.0, metavar=\"PCT\",\n                        help=\"fail when the overdue rate is above this percentage (default 20)\")\n    parser.add_argument(\"--strict\", action=\"store_true\",\n                        help=\"also fail on an unowned open item or an open duplicate\")\n    args = parser.parse_args(argv)\n\n    if args.max_overdue < 0 or args.max_overdue > 100:\n        print(\"ERROR --max-overdue must be between 0 and 100\", file=sys.stderr)\n        return 2\n\n    try:\n        now = parse_day(args.now) if args.now else datetime.datetime.utcnow().replace(\n            microsecond=0)\n        if args.now and len(str(args.now).strip()) <= 10:\n            now = now.replace(hour=12, minute=0, second=0)\n        items = load_items(args.items, now)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    open_items = [item for item in items if item[\"open\"]]\n    overdue = [item for item in open_items if item[\"overdue\"]]\n    unowned = [item for item in open_items if not item.get(\"owner\")]\n    duplicates = [item for item in items if item.get(\"duplicate_of\")]\n    rate = (100.0 * len(overdue) / len(open_items)) if open_items else 0.0\n\n    print(\"ACTIONS %d items, %d open, %d overdue, %d unowned\"\n          % (len(items), len(open_items), len(overdue), len(unowned)))\n    for item in overdue:\n        print(\"OVERDUE %s due %s\" % (item[\"id\"], item.get(\"due\", \"-\")))\n    for item in unowned:\n        print(\"UNOWNED %s\" % item[\"id\"])\n    for item in duplicates:\n        print(\"DUPLICATE %s -> %s\" % (item[\"id\"], item[\"duplicate_of\"]))\n    print(\"RATE %.0f%% overdue of %d open, limit %.0f%%\"\n          % (rate, len(open_items), args.max_overdue))\n\n    if rate > args.max_overdue:\n        return 1\n    if args.strict and (unowned or any(item[\"open\"] for item in duplicates)):\n        return 1\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p706","n":"706","slug":"a-notification-log","title":"A notification log","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","IncidentResponse","SRE","Communication","CyberSecurity"],"series_name":"IR","design":"code","script":"comms_log.py","date":"2031-04-01","files":{"it":"notes/p706-a-notification-log.md","devsecops":"notes/p706-a-notification-log.md"},"md":"# A notification log\n\nThe incident closes and legal asks a simple question: when were we told? The channel holds the answer somewhere, and nobody can produce it in a minute.\n\n## Quick reference\n\nThe full script is at the end of this note: `comms_log.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 comms_log.py log.json people.json\n```\n\n**Example output**\n\n```text\nCOMMS 3 updates to 2 of 4 stakeholders\nMISSED legal, finance\n```\n\n## Explained\n\n### What it does\n\n`comms_log.py` reads a notification log and a stakeholder list. It treats the first notification per person as their latency, names every required stakeholder who was never told, and measures the gaps between updates, including the one between the start and the first update. A gap over `--target`, 30 minutes by default, or a missed stakeholder exits 1.\n\n### What the screen shows\n\nThe fourteen lines join the two files, compute the first notification per person, the lag from the start and the missing names, then print COMMS 3 updates to 2 of 4 stakeholders and MISSED legal, finance.\n\n### How to run it\n\n`python3 comms_log.py log.json stakeholders.json --target 15`. Exit 0 is everyone told inside the target, 1 is a missed person or a long gap, 2 is bad input. CI runs it with `--target 15`, tighter than the incident used.\n\n### The fix\n\nWrite the log at notification time. A log reconstructed the next morning records who should have been told, not who was.\n\n### Prevention\n\nPut the stakeholder list in the runbook and the target in the review template, so the gap is measured against a number somebody chose in advance.\n\n> **The honest caveat:** the log records who was told and when; it cannot tell you whether they read it.\n\n> **The lesson:** the gap before the first update is where the damage usually happens.\n\n## In one line\n\n> **The gap before the first update.** Latency is an incident metric, not a communications chore.\n\n**Count the missed**\n\n```bash\npython3 comms_log.py log.json people.json\n```\n\n## Script: comms_log.py\n\nDownload: [scripts/comms_log.py](../scripts/comms_log.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"comms_log.py - who was told, when, and who was missed.\n\nReads a notification log and a stakeholder list, then reports the two\nnumbers every incident review asks for: the notification latency (how long\nafter the start each stakeholder heard) and the gaps between updates. It\nnames every required stakeholder with no notification at all, and flags the\ngap between the start and the first update, which is where most of the\ndamage is done.\n\nThe log is a JSON object:\n\n    {\"incident\": \"INC-214\", \"started\": \"2026-10-20T03:04:00Z\",\n     \"notifications\": [\n       {\"to\": \"cto\", \"at\": \"2026-10-20T03:38:00Z\", \"channel\": \"phone\"},\n       {\"to\": \"support\", \"at\": \"2026-10-20T03:41:00Z\", \"channel\": \"slack\"}]}\n\nThe stakeholders file is a list of names, or a list of objects with \"name\"\nand an optional \"required\" flag (default true):\n\n    [\"cto\", \"legal\", \"support\"]\n    [{\"name\": \"cto\", \"team\": \"exec\"}, {\"name\": \"legal\", \"required\": false}]\n\nUsage:\n    python3 comms_log.py log.json stakeholders.json\n    python3 comms_log.py log.json stakeholders.json --start 2026-10-20T03:04:00Z\n    python3 comms_log.py log.json stakeholders.json --target 15\n    python3 comms_log.py --help\n\nCI (fail the review when a required stakeholder was never told):\n    python3 comms_log.py log.json stakeholders.json --target 15 || exit 1\n\nCron (draft the comms section of the review every hour):\n    7 * * * * python3 /opt/s0sta/comms_log.py /cases/inc-214/log.json /cases/inc-214/stakeholders.json\n\nExit codes: 0 every required stakeholder was told and no gap is over\n--target minutes, 1 someone was missed or a gap is too long, 2 usage error or\nunreadable JSON.\n\nRead-only: reads the two files, prints, and writes nothing. The log records\nwho was told, never what they were told, so the tool cannot leak the content\nof a notification.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\nDEFAULT_TARGET = 30\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be measured.\"\"\"\n\n\ndef parse_at(raw):\n    \"\"\"Return an aware UTC datetime from ISO text, or raise InputError.\"\"\"\n    text = str(raw).strip().replace(\"Z\", \"+00:00\").replace(\"z\", \"+00:00\")\n    try:\n        stamp = datetime.datetime.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s: not an ISO 8601 time\" % raw)\n    if stamp.tzinfo is None:\n        stamp = stamp.replace(tzinfo=datetime.timezone.utc)\n    return stamp.astimezone(datetime.timezone.utc)\n\n\ndef load_json(path):\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    except ValueError as exc:\n        raise InputError(\"%s: not JSON (%s)\" % (path, exc))\n\n\ndef load_log(path):\n    \"\"\"Return (incident name, start, notifications) or raise InputError.\"\"\"\n    data = load_json(path)\n    if not isinstance(data, dict):\n        raise InputError(\"%s: expected a JSON object\" % path)\n    if not data.get(\"started\"):\n        raise InputError(\"%s: started is required\" % path)\n    started = parse_at(data[\"started\"])\n    rows = data.get(\"notifications\")\n    if not isinstance(rows, list):\n        raise InputError(\"%s: notifications must be a list\" % path)\n    notes = []\n    for index, row in enumerate(rows, start=1):\n        if not isinstance(row, dict) or not row.get(\"to\") or not row.get(\"at\"):\n            raise InputError(\"%s: notification %d needs to and at\" % (path, index))\n        notes.append({\"to\": str(row[\"to\"]).strip(), \"at\": parse_at(row[\"at\"]),\n                      \"channel\": str(row.get(\"channel\", \"\")).strip(),\n                      \"update\": row.get(\"update\")})\n    notes.sort(key=lambda note: note[\"at\"])\n    return str(data.get(\"incident\", \"incident\")), started, notes\n\n\ndef load_stakeholders(path):\n    \"\"\"Return a list of (name, required) or raise InputError.\"\"\"\n    data = load_json(path)\n    if isinstance(data, dict):\n        data = data.get(\"stakeholders\")\n    if not isinstance(data, list) or not data:\n        raise InputError(\"%s: expected a list of stakeholders\" % path)\n    people = []\n    for index, row in enumerate(data, start=1):\n        if isinstance(row, str):\n            people.append((row.strip(), True))\n            continue\n        if not isinstance(row, dict) or not row.get(\"name\"):\n            raise InputError(\"%s: stakeholder %d has no name\" % (path, index))\n        people.append((str(row[\"name\"]).strip(), bool(row.get(\"required\", True))))\n    return people\n\n\ndef minutes(delta):\n    return int(round(delta.total_seconds() / 60.0))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report notification latency and missed stakeholders (read-only).\")\n    parser.add_argument(\"log\", help=\"notification log JSON file\")\n    parser.add_argument(\"stakeholders\", help=\"stakeholder list JSON file\")\n    parser.add_argument(\"--start\", metavar=\"ISO\",\n                        help=\"override the incident start time in the log\")\n    parser.add_argument(\"--target\", type=int, default=DEFAULT_TARGET, metavar=\"MIN\",\n                        help=\"maximum acceptable gap in minutes (default %d)\" % DEFAULT_TARGET)\n    args = parser.parse_args(argv)\n\n    if args.target <= 0:\n        print(\"ERROR --target must be a positive number of minutes\", file=sys.stderr)\n        return 2\n\n    try:\n        name, started, notes = load_log(args.log)\n        if args.start:\n            started = parse_at(args.start)\n        people = load_stakeholders(args.stakeholders)\n    except InputError as exc:\n        print(\"ERROR %s\" % exc, file=sys.stderr)\n        return 2\n\n    first = {}\n    for note in notes:\n        first.setdefault(note[\"to\"], note[\"at\"])\n    missed = [who for who, required in people if required and who not in first]\n    told = sum(1 for who, _ in people if who in first)\n    latencies = sorted((minutes(stamp - started), who) for who, stamp in first.items())\n    gaps = []\n    if notes:\n        gaps.append({\"min\": minutes(notes[0][\"at\"] - started), \"from\": started,\n                     \"to\": notes[0][\"at\"], \"start\": True})\n    for earlier, later in zip(notes, notes[1:]):\n        if later[\"at\"] > earlier[\"at\"]:\n            gaps.append({\"min\": minutes(later[\"at\"] - earlier[\"at\"]),\n                         \"from\": earlier[\"at\"], \"to\": later[\"at\"], \"start\": False})\n    worst = max(gaps, key=lambda gap: gap[\"min\"]) if gaps else {\n        \"min\": 0, \"from\": started, \"to\": started, \"start\": False}\n\n    print(\"COMMS %d updates to %d of %d stakeholders\" % (len(notes), told, len(people)))\n    if missed:\n        print(\"MISSED %s\" % \", \".join(missed))\n    if latencies:\n        slow = max(latencies)\n        print(\"LATENCY first %d min, slowest %d min (%s)\"\n              % (latencies[0][0], slow[0], slow[1]))\n    else:\n        print(\"LATENCY none, no stakeholder was told\")\n    if not gaps:\n        print(\"GAP none, target %d\" % args.target)\n    else:\n        if worst[\"start\"]:\n            window = \"before the first update\"\n        else:\n            window = \"%s to %s\" % (worst[\"from\"].strftime(\"%H:%MZ\"),\n                                   worst[\"to\"].strftime(\"%H:%MZ\"))\n        print(\"GAP %d min %s, target %d\" % (worst[\"min\"], window, args.target))\n    print(\"INCIDENT %s started %s\" % (name, started.strftime(\"%Y-%m-%dT%H:%M:%SZ\")))\n\n    if missed or worst[\"min\"] > args.target:\n        return 1\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p707","n":"707","slug":"the-evidence-path","title":"The evidence path","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","DFIR","CyberSecurity","SRE","Forensics"],"series_name":"IR","design":"architecture","script":"","date":"2031-04-03","files":{"it":"notes/p707-the-evidence-path.md","devsecops":"notes/p707-the-evidence-path.md"},"md":"# The evidence path\n\nAn incident review asks for the evidence and gets a folder of files with no order, no digests and no list. The drawn path is the order: collect, preserve, present.\n\n## Quick reference\n\n- **Collect:** Case directory (CASE) · Process export (PS) · Network export (SS) · Logs and config (LOG)\n- **Preserve:** SHA-256 per file (HASH) · Manifest (MAN) · Sealed bundle (STORE)\n- **Present:** Timeline (TIME) · Report (RPT) · Review (REV)\n\n**Request flow:** Case directory → SHA-256 per file → Manifest → Sealed bundle → Timeline → Review; Sealed bundle → Report → Review; Process export → SHA-256 per file; Network export → SHA-256 per file; Logs and config → SHA-256 per file\n\n**Around it:** sha256sum (SHA) · ps -eo (PS) · ss -tanp (SS) · evidence_collect.py (PY)\n\n## Explained\n\n### Collect\n\nThe first tier holds the case directory, the process export, the network export and the logs. Collection is a capture, not an investigation: take the files, the running processes and the sockets first, because processes and connections disappear on reboot and a copied file can wait.\n\n### Preserve\n\nThe second tier hashes every artefact, writes the manifest and seals the bundle. One digest per file, one bundle hash over all of them, and the manifest kept with the bundle. This is the tier that makes the rest defensible, because it is the only one that can prove nothing changed.\n\n### Present\n\nThe third tier is the timeline, the report and the review. It reads from the preserved bundle, never from the live host, and every row points back to an evidence id.\n\n### Read it in order\n\nThe poster draws the three bands top to bottom, with arrows from each collected artefact to the hash, then to the manifest and the sealed store, then to the timeline and the report. Four chips at the bottom name the tools: sha256sum, ps, ss and the collector.\n\n> **The honest caveat:** no diagram survives a host that was rebooted before collection; the first tier is the one you cannot redo.\n\n> **The lesson:** collect, then hash, then say it.\n\n## In one line\n\n> **Collect, then hash, then say it.** Nothing after the reboot can rebuild the first tier.\n\n**Seal the bundle**\n\n```bash\npython3 evidence_collect.py /cases\n```\n"},{"id":"p708","n":"708","slug":"the-evidence-command-sheet","title":"The evidence command sheet","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","DFIR","Linux","CyberSecurity","CommandLine"],"series_name":"IR","design":"cheatsheet","script":"","date":"2031-04-06","files":{"it":"notes/p708-the-evidence-command-sheet.md","devsecops":"notes/p708-the-evidence-command-sheet.md"},"md":"# The evidence command sheet\n\nFour blocks of commands for the hour when the evidence still exists: hash it, copy it, and describe what was running and connected.\n\n## Quick reference\n\n### Hash\n\n```bash\nsha256sum auth.log\nsha256sum -c manifest.sha256\nfind . -type f -exec sha256sum {} +\nsha256sum auth.log > auth.log.sha256\nsha256sum /dev/sda | tee disk.sha256\n```\n\n### Copy\n\n```bash\ncp -a /var/log/auth.log /evidence/\ncp --preserve=all auth.log evidence/\nrsync -aHAX /var/log/ /evidence/logs/\ntar --acls --xattrs -czf logs.tgz logs/\ndd if=/dev/sda of=/evidence/sda.img bs=1M\n```\n\n### Process\n\n```bash\nps -eo pid,ppid,user,args\nps -eo pid,rss,comm --sort=-rss\nls -l /proc/412/cwd\nlsof -p 412\ncat /proc/412/status\n```\n\n### Network\n\n```bash\nss -tanp\nss -tulpn\nip neigh show\nconntrack -L\ntcpdump -i any -w capture.pcap\n```\n\n## Explained\n\n### Hash\n\nsha256sum prints a digest for one file, and the -c form checks a saved list. The find one-liner hashes a whole tree, while a redirect keeps the digest beside the name it belongs to. Hash the copy and compare it with the source, or hash once and never let the file move.\n\n### Copy\n\nCopy with metadata: `cp -a` keeps times and modes, `--preserve=all` adds the rest, `rsync -aHAX` keeps ACLs and extended attributes, and dd takes a block-device image. tar with `--acls` and `--xattrs` packs a directory without dropping what a plain copy drops.\n\n### Process\n\nps with an explicit field list is the only version you can diff later. The sort by resident memory names the biggest process, the cwd link shows where it was started, lsof shows what it held open, and `/proc/PID/status` carries the credential and namespace facts.\n\n### Network\n\n`ss -tanp` lists sockets with the owning process, and -tulpn adds the listeners. ip neigh shows the layer-two neighbours, conntrack -L the flows, and tcpdump writes a capture you can hash like any other file.\n\n> **The honest caveat:** a command that prints a secret into a terminal has copied it into your scrollback; redirect to a file with mode 600.\n\n> **The lesson:** hash what you copy, then copy the hash.\n\n## In one line\n\n> **Hash what you copy, then copy the hash.** The digest belongs beside the file, not in a chat message.\n\n**Seal the log**\n\n```bash\nsha256sum auth.log > auth.log.sha256\n```\n"},{"id":"p709","n":"709","slug":"four-evidence-failures","title":"Four evidence failures","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","DFIR","CyberSecurity","SRE","Postmortem"],"series_name":"IR","design":"explainer","script":"","date":"2031-04-08","files":{"it":"notes/p709-four-evidence-failures.md","devsecops":"notes/p709-four-evidence-failures.md"},"md":"# Four evidence failures\n\nFour evidence failures turn up in reviews, and none of them is a missing tool. Every one is a step somebody skipped while the incident was still moving.\n\n## Quick reference\n\n### No hash\n\n- A copy nobody hashed is a claim, not evidence\n- The bytes may be right; you cannot show it\n- Hash at collection, on the host, not later\n\n### No chain\n\n- Who held the disk, and on which day\n- A bundle with an unknown middle is unproven\n- Storage, handover, hash, repeat\n\n### No timeline\n\n- Memory orders events by who is speaking\n- A row with no source is a memory\n- Export it while the incident still runs\n\n### No owner\n\n- An unowned action never closes\n- A name and a date, at write time\n- A duplicate points at its original\n\n> **Note:** The tools were fine. The process left a gap a reviewer could walk through.\n\n## Explained\n\n### No hash\n\nA copy that nobody hashed is a claim. The bytes may be right; you cannot show it. Hash at collection, on the host, and keep the digest beside the file name where a reviewer can find it.\n\n### No chain\n\nWho held the disk, and on which day? A bundle that moves between laptops and shared drives with no record has an unknown middle. An unbroken list of storage, handover, hash, repeat beats a good story every time.\n\n### No timeline\n\nMemory orders events by who is speaking loudest. A row with no source is a memory, and a timeline assembled during the review is a narrative. Export it while the incident runs, from the systems that saw it happen.\n\n### No owner\n\nAn unowned action never closes. The tracker needs a name and a date at the moment the action is written, and a duplicate points at its original instead of living beside it.\n\n### The pattern\n\nEach failure is cheap to prevent at the time and impossible to repair afterwards. The review does not need better tools; it needs these four steps to happen in order, while the evidence is still warm.\n\n> **The honest caveat:** a perfect bundle does not prove the incident was handled well, only that it can be examined.\n\n> **The lesson:** evidence fails in the gaps between steps, not in the tools.\n\n## In one line\n\n> **Evidence fails in the gaps.** Four steps skipped, four questions you cannot answer.\n\n**Hash the copy**\n\n```bash\nsha256sum auth.log > auth.log.sha256\n```\n"},{"id":"p710","n":"710","slug":"status-update-by-phase","title":"Status update by phase","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","SRE","StatusPage","CyberSecurity","DevOps"],"series_name":"IR","design":"matrix","script":"","date":"2031-04-10","files":{"it":"notes/p710-status-update-by-phase.md","devsecops":"notes/p710-status-update-by-phase.md"},"md":"# Status update by phase\n\nThree status updates, three different jobs. The phase decides what the page must contain, and a template that asks for the same six fields every time makes the difference visible.\n\n## Quick reference\n\n| | **Investigating** | **Identified** | **Resolved** |\n|:--|:--|:--|:--|\n| What happened | An alert, scope unknown | A named failure | The fix, verified |\n| Impact | Being measured | Numbers and scope | Final count |\n| Action | Looking, nothing changed | Contained, fixing | Rolled back, watched |\n| ETA | None promised | A time you will hold | When we close |\n| Next update | Inside 15 minutes | At the promised time | Before the close |\n| Owner | Incident lead | Incident lead | Service owner |\n\n**How to pick**\n\n- **Investigating** → name the symptom and the next update time.\n- **Identified** → put numbers on the impact and hold the ETA.\n- **Resolved** → say what changed and what is still watched.\n\n## Explained\n\nWhat happened. Investigating names the symptom, not the cause: checkout errors, scope unknown. Identified names the failure: a bad release, one service, 4.1 per cent of card payments. Resolved names what changed, and what is still being watched.\n\nImpact. Investigating measures it, identified quantifies it with a scope, and resolved closes the count. A page that keeps saying significant impact after the fix is a page nobody trusts.\n\nAction. Investigating is a verb with no object yet. Identified is containment and a fix in progress. Resolved is the rollback or the patch, plus the monitoring that follows it.\n\nETA. Investigating promises no time for the fix, only a time for the next update. Identified holds a time you can defend. Resolved needs none, because the next communication is the close.\n\nNext update. Fifteen minutes while investigating, the promised time once identified, and one final update before close.\n\nOwner. The incident lead owns the first two columns; the service owner owns the third, because the follow-up outlives the incident.\n\nThe grid lays the three phases side by side, so a missing cell is a blank you can see before you publish.\n\n> **The honest caveat:** phases overlap in a real incident, and a page that lags the room is worse than one that admits it.\n\n> **The lesson:** the phase is a contract about what the update must contain.\n\n## In one line\n\n> **The phase decides what the update says.** Three columns, one template, six fields that must not be blank.\n\n**Draft the update**\n\n```bash\npython3 status_update.py inc.json identified\n```\n"},{"id":"p711","n":"711","slug":"quiz-3-evidence-questions","title":"Quiz: 3 evidence questions","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["IncidentResponse","DFIR","CyberSecurity","SRE","Postmortem"],"series_name":"IR","design":"quiz","script":"","date":"2031-04-13","files":{"it":"notes/p711-quiz-3-evidence-questions.md","devsecops":"notes/p711-quiz-3-evidence-questions.md"},"md":"# Quiz: 3 evidence questions\n\nThe last post of the evidence phase, and the right place to stop: the hour after the fix, when the review asks what you can prove. Three questions. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why does a SHA-256 hash matter more than a copy of the file?**\n\n- a) It compresses the evidence\n- b) It proves the bytes have not changed\n- c) It encrypts the contents\n\n> **Answer:** b) It proves the bytes have not changed. A copy can be edited and still look identical. The digest changes with one byte, so it is the only cheap proof that what you kept is what you collected.\n\n**Q2. What must every status update contain, whatever the phase?**\n\n- a) A root cause and a fix date\n- b) An impact and a next update time\n- c) A named engineer and a ticket\n\n> **Answer:** b) An impact and a next update time. The impact says who is affected; the next time says when you speak again. A root cause and a fix date are guesses early on, and a page built on guesses ages fast.\n\n**Q3. Why is notification latency an incident metric?**\n\n- a) It measures how fast the pager fires\n- b) It measures time exposed without knowing\n- c) It measures the size of the on-call rota\n\n> **Answer:** b) It measures time exposed without knowing. Every minute before a stakeholder is told is a minute they cannot act: legal cannot issue the statement, support cannot answer the queue.\n\n## Explained\n\n### Question one\n\nWhy does a hash matter more than a copy? Because a copy can be edited and still look identical, while one changed byte changes the digest. SHA-256 is the cheap proof that the file you kept is the file you collected, and it is the first thing a reviewer asks for. A copy without a digest is a claim.\n\n### Question two\n\nWhat must every status update contain, whatever the phase? An impact and a next update time. The impact says who is affected; the next time says when you will speak again. A root cause and a fix date are guesses in the first hour, and a page built on guesses ages badly, which is how a status page loses its audience.\n\n### Question three\n\nWhy is notification latency an incident metric? Because every minute before a stakeholder is told is a minute they cannot act. Legal cannot hold the statement, support cannot answer the queue, and the first version of the story comes from someone outside the incident. The gap is part of the damage.\n\n> **The honest caveat:** knowing these three does not produce the evidence; somebody has to run the commands while the host is still up.\n\nIf you answered all three, you already treat communication as part of the incident.\n\n## In one line\n\n> **Evidence is a habit, not a tool.** Five tools, one rule: collect, hash, say it, and keep the list.\n\n**Start the habit**\n\n```bash\npython3 evidence_collect.py /cases/inc-214\n```\n"},{"id":"p712","n":"712","slug":"the-detection-that-was-never-written","title":"The detection that was never written","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","DetectionEngineering","SOC","ThreatHunting","CyberSecurity"],"series_name":"IR","design":"code","script":"detection_gap.py","date":"2031-04-15","files":{"it":"notes/p712-the-detection-that-was-never-written.md","devsecops":"notes/p712-the-detection-that-was-never-written.md"},"md":"# The detection that was never written\n\nThe technique was in the threat report in January. In October the same actor used it again. The SIEM had no rule, so nothing could fire.\n\n## Quick reference\n\nThe full script is at the end of this note: `detection_gap.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 detection_gap.py tech.json det.json\n```\n\n**Example output**\n\n```text\nCOVERAGE 4/8 techniques, 2 high gaps\nGAP T1059, T1566 high relevance\n```\n\n## Explained\n\n### What it does\n\n`detection_gap.py` reads two lists. The list gives each technique a name, a relevance and its actors. The detection inventory gives each rule a technique and an enabled flag. A technique is covered only when at least one enabled rule carries its exact id. A disabled rule is a gap with a file: the rule exists, the detection does not.\n\n### How to run it\n\n`python3 detection_gap.py techniques.json detections.json`, then the same command in CI with || exit 1. It exits 0 when every technique at or above `--fail-on` is covered, 1 when one is not, and 2 on bad input.\n\n### What the screen shows\n\nThe fourteen lines load both files, build the enabled set, compare it with the list, and print the coverage and high-relevance ids. On the dirty fixture: COVERAGE 4/8 techniques, 2 high gaps and GAP T1059, T1566 high relevance.\n\n### The fix\n\nTake the high gaps to the backlog with the actor attached. One rule, one owner, one date. A gap on a slide for a quarter is not a plan.\n\n### Prevention\n\nRun the check weekly from the list the threat report uses. When the list changes, the gaps change.\n\n> **The honest caveat:** exact id matching does not roll sub-techniques up, and an existing rule has not been tested.\n\n> **The lesson:** a detection you never wrote cannot fire.\n\n## In one line\n\n> **A gap you can name is a gap you can close.** A technique with no enabled rule is a gap, even when the rule exists.\n\n**Map the gaps**\n\n```bash\npython3 detection_gap.py t.json d.json\n```\n\n## Script: detection_gap.py\n\nDownload: [scripts/detection_gap.py](../scripts/detection_gap.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Find techniques with no enabled detection and rank the gaps.\n\nReads a technique list and a detection inventory, then answers one question\nper technique: is there at least one enabled rule that covers it?\n\nThe technique list is JSON, one object per technique:\n\n    [{\"id\": \"T1059\", \"name\": \"Command and Scripting Interpreter\",\n      \"relevance\": \"high\", \"actors\": [\"APT29\", \"FIN7\"]},\n     {\"id\": \"T1566\", \"name\": \"Phishing\", \"relevance\": \"high\",\n      \"actors\": [\"APT29\"]}]\n\nThe detection inventory is JSON, one object per rule:\n\n    [{\"technique\": \"T1059\", \"rule\": \"powershell-encoded\", \"enabled\": true},\n     {\"technique\": \"T1566\", \"rule\": \"mail-forwarding-rule\", \"enabled\": false}]\n\nA technique is covered when at least one detection for the exact technique id\nis enabled. A disabled rule does not count, because a rule that is off is a\ndetection that was never written. Matching is by exact id: a rule for\nT1059.001 does not cover T1059, and the summary does not roll sub-techniques\nup into their parent.\n\nUsage:\n    python3 detection_gap.py techniques.json detections.json\n    python3 detection_gap.py techniques.json detections.json --fail-on medium\n\nThe default fail level is high: only a high-relevance gap exits 1. Use\n--fail-on medium or --fail-on low to be stricter. An empty detection list is\nvalid and means every technique is a gap.\n\nExit codes: 0 no gap at or above the fail level, 1 at least one, 2 usage, or\na file that is missing, unreadable, invalid or empty.\n\nCI (weekly, fail the job when a high-relevance technique is uncovered):\n    python3 detection_gap.py techniques.json detections.json || exit 1\n\nCron (Monday 06:00, mail the gap report):\n    0 6 * * 1 python3 /opt/s0sta/detection_gap.py /etc/s0sta/techniques.json \\\n        /etc/s0sta/detections.json\n\nThe script is read-only: it reads both files, prints the coverage and writes\nnothing. It tells you where the list has no rule; it does not tell you whether\nthe rule that exists would fire. Run a purple-team test for that.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nRANK = {\"high\": 0, \"medium\": 1, \"low\": 2}\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_list(path, label, allow_empty=False):\n    \"\"\"Return a list from a JSON file, or raise InputError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s has no %s list\" % (path, label))\n    if not doc and not allow_empty:\n        raise InputError(\"%s has an empty %s list\" % (path, label))\n    return doc\n\n\ndef technique_fields(item, number):\n    \"\"\"Validate one technique entry and return (id, name, relevance, actors).\"\"\"\n    if not isinstance(item, dict):\n        raise InputError(\"technique %d is not an object\" % number)\n    tech_id = item.get(\"id\")\n    name = item.get(\"name\")\n    relevance = item.get(\"relevance\")\n    actors = item.get(\"actors\", [])\n    if not isinstance(tech_id, str) or not tech_id:\n        raise InputError(\"technique %d has no id\" % number)\n    if not isinstance(name, str) or not name:\n        raise InputError(\"technique %s has no name\" % tech_id)\n    if relevance not in RANK:\n        raise InputError(\"technique %s has no high, medium or low relevance\" % tech_id)\n    if not isinstance(actors, list) or any(not isinstance(a, str) for a in actors):\n        raise InputError(\"technique %s actors must be a list of names\" % tech_id)\n    return tech_id, name, relevance, actors\n\n\ndef detection_fields(item, number):\n    \"\"\"Validate one detection entry and return (technique, rule, enabled).\"\"\"\n    if not isinstance(item, dict):\n        raise InputError(\"detection %d is not an object\" % number)\n    technique = item.get(\"technique\")\n    rule = item.get(\"rule\")\n    enabled = item.get(\"enabled\")\n    if not isinstance(technique, str) or not technique:\n        raise InputError(\"detection %d has no technique\" % number)\n    if not isinstance(rule, str) or not rule:\n        raise InputError(\"detection %d has no rule\" % number)\n    if not isinstance(enabled, bool):\n        raise InputError(\"detection %s has no boolean enabled\" % rule)\n    return technique, rule, enabled\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Map detection coverage against a technique list and \"\n                    \"report every technique with no enabled rule.\",\n        epilog=\"Exit codes: 0 covered, 1 gap at or above --fail-on, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"techniques\", help=\"JSON list of techniques\")\n    parser.add_argument(\"detections\", help=\"JSON list of detection rules\")\n    parser.add_argument(\"--fail-on\", choices=(\"high\", \"medium\", \"low\"),\n                        default=\"high\",\n                        help=\"lowest relevance that exits 1 (default: high)\")\n    args = parser.parse_args(argv)\n\n    try:\n        techniques = load_list(args.techniques, \"techniques\")\n        detections = load_list(args.detections, \"detections\", allow_empty=True)\n\n        seen = set()\n        techs = []\n        for number, item in enumerate(techniques, 1):\n            tech_id, name, relevance, actors = technique_fields(item, number)\n            if tech_id in seen:\n                raise InputError(\"duplicate technique id %s\" % tech_id)\n            seen.add(tech_id)\n            techs.append((tech_id, name, relevance, actors))\n\n        enabled = {}\n        disabled = {}\n        for number, item in enumerate(detections, 1):\n            technique, rule, on = detection_fields(item, number)\n            if technique not in seen:\n                print(\"WARN detection %s covers unknown technique %s\"\n                      % (rule, technique), file=sys.stderr)\n                continue\n            if on:\n                enabled[technique] = enabled.get(technique, 0) + 1\n            else:\n                disabled[technique] = disabled.get(technique, 0) + 1\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    gaps = []\n    for tech_id, name, relevance, actors in techs:\n        if tech_id not in enabled:\n            gaps.append((tech_id, name, relevance, actors,\n                         disabled.get(tech_id, 0)))\n    gaps.sort(key=lambda g: (RANK[g[2]], g[0]))\n\n    covered = len(techs) - len(gaps)\n    high = sum(1 for g in gaps if g[2] == \"high\")\n    print(\"COVERAGE %d/%d techniques, %d high gaps\"\n          % (covered, len(techs), high))\n    for tech_id, name, relevance, actors, off in gaps:\n        who = \", \".join(actors) if actors else \"no actor listed\"\n        extra = \", %d disabled\" % off if off else \"\"\n        print(\"GAP %s %s %s (%s%s)\"\n              % (tech_id, relevance, name, who, extra))\n\n    limit = RANK[args.fail_on]\n    failing = [g for g in gaps if RANK[g[2]] <= limit]\n    if failing:\n        return 1\n    if not gaps:\n        print(\"OK every technique has an enabled detection\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p713","n":"713","slug":"runbooks-that-rotted","title":"Runbooks that rotted","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","SRE","IncidentResponse","Documentation","DevOps"],"series_name":"IR","design":"code","script":"runbook_freshness.py","date":"2031-04-17","files":{"it":"notes/p713-runbooks-that-rotted.md","devsecops":"notes/p713-runbooks-that-rotted.md"},"md":"# Runbooks that rotted\n\nThe runbook was written by the person who knew the system. It still says to restart a service replaced last spring. The on-call follows it at 03:00 because it is the only page with the steps.\n\n## Quick reference\n\nThe full script is at the end of this note: `runbook_freshness.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 runbook_freshness.py docs/runbooks\n```\n\n**Example output**\n\n```text\nFINDING 2 stale, 1 unowned, 1 broken\n```\n\n## Explained\n\n### What it does\n\n`runbook_freshness.py` walks a directory of markdown files and reads each file's first forty lines. It looks for a last reviewed date and an owner, then resolves every relative link against the runbook's directory. A missing or old date, a placeholder owner or a dead relative link is a finding.\n\n### How to run it\n\n`python3 runbook_freshness.py docs/runbooks --days 90`. Exit 0 is clean, 1 is a finding, 2 is a path with no markdown. `--today` makes the age reproducible in a review.\n\n### What the screen shows\n\nThe fourteen lines glob the directory, parse the date, test the owner and check each local link, then print three counts. On the dirty fixture: FINDING 2 stale, 1 unowned, 1 broken.\n\n### The fix\n\nGive every runbook an owner and a real review date, and fix the links in the same commit. If a runbook has no owner, find one or delete the file: an unowned runbook is a rumour with formatting.\n\n### Prevention\n\nPut the check in the docs pipeline and pair it with the restore drill. The drill proves the steps work; the check proves somebody still owns them.\n\n> **The honest caveat:** a fresh date proves somebody opened the file, not that the commands in it still run.\n\n> **The lesson:** a runbook is a promise, and a promise needs a review date.\n\n## In one line\n\n> **A runbook nobody reviews is a rumour.** A fresh date is a promise; a drill is the proof.\n\n**Check the shelf**\n\n```bash\npython3 runbook_freshness.py docs\n```\n\n## Script: runbook_freshness.py\n\nDownload: [scripts/runbook_freshness.py](../scripts/runbook_freshness.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Flag runbooks that are stale, unowned or pointing at a missing file.\n\nScans a directory of markdown runbooks and checks three things a runbook\nneeds before anyone trusts it at 03:00:\n\n    reviewed    a \"Last reviewed: YYYY-MM-DD\" line in the first 40 lines,\n                no older than --days (default 90)\n    owner       an \"Owner: name\" line that is not a placeholder\n    links       every relative markdown link resolves to a real file\n\nMetadata is read from the first 40 lines and may be plain, bold or a list\nitem:\n\n    Last reviewed: 2026-07-14\n    **Owner:** payments-oncall\n\nAccepted date formats are YYYY-MM-DD, YYYY/MM/DD and DD/MM/YYYY. A missing\ndate counts as stale; an unreadable one is reported as BADDATE. External\nlinks, anchors and absolute paths are ignored. A relative link is resolved\nagainst the directory of the runbook that contains it.\n\nUsage:\n    python3 runbook_freshness.py docs/runbooks\n    python3 runbook_freshness.py docs/runbooks --days 120\n    python3 runbook_freshness.py docs/runbooks --today 2026-10-21\n\n--today makes the age calculation reproducible in a test or during a review.\n\nExit codes: 0 every runbook is fresh, owned and linked, 1 at least one\nfinding, 2 usage or an input directory with no markdown files.\n\nCI (documentation gate, fail the build when a runbook rots):\n    python3 runbook_freshness.py docs/runbooks --days 90 || exit 1\n\nCron (Monday 07:00, mail the owners):\n    0 7 * * 1 python3 /opt/s0sta/runbook_freshness.py /srv/docs/runbooks\n\nThe script is read-only. It reads the markdown and checks the filesystem;\nit never edits a runbook or touches a link. A fresh date proves somebody\nopened the file, not that the commands in it still work. Rehearse the drill.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport os\nimport re\nimport sys\nfrom urllib.parse import unquote\n\nDATE_FORMATS = (\"%Y-%m-%d\", \"%Y/%m/%d\", \"%d/%m/%Y\")\nMETA = re.compile(r\"^(last reviewed|reviewed|owner)\\s*:\\s*(.+)$\", re.I)\nLINK = re.compile(r\"\\]\\(([^)]+)\\)\")\nPLACEHOLDERS = {\"\", \"-\", \"?\", \"n/a\", \"na\", \"none\", \"tbd\", \"todo\", \"unassigned\"}\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef markdown_files(root):\n    \"\"\"Return sorted markdown paths under root, skipping dot directories.\"\"\"\n    found = []\n    for dirpath, dirnames, filenames in os.walk(root):\n        dirnames[:] = sorted(d for d in dirnames if not d.startswith(\".\"))\n        for name in sorted(filenames):\n            if name.lower().endswith(\".md\"):\n                found.append(os.path.join(dirpath, name))\n    return found\n\n\ndef metadata(text):\n    \"\"\"Return {'last reviewed': value, 'owner': value} from the first 40 lines.\"\"\"\n    found = {}\n    for line in text.splitlines()[:40]:\n        clean = re.sub(r\"^[#>\\-+*]+\\s+\", \"\", line.strip())\n        clean = clean.replace(\"**\", \"\").replace(\"__\", \"\")\n        match = META.match(clean.strip())\n        if not match:\n            continue\n        key = match.group(1).lower()\n        if key == \"reviewed\":\n            key = \"last reviewed\"\n        if key not in found:\n            found[key] = match.group(2).strip()\n    return found\n\n\ndef review_date(value):\n    \"\"\"Parse the first token of a review date, or return None.\"\"\"\n    token = value.split()[0] if value.split() else \"\"\n    token = token.strip(\".,;:()[]\")\n    for fmt in DATE_FORMATS:\n        try:\n            return datetime.datetime.strptime(token, fmt).date()\n        except ValueError:\n            continue\n    return None\n\n\ndef relative_links(text):\n    \"\"\"Return [(raw, path)] for relative markdown links worth checking.\"\"\"\n    links = []\n    for raw in LINK.findall(text):\n        target = raw.strip()\n        if target.startswith(\"<\") and target.endswith(\">\"):\n            target = target[1:-1]\n        target = target.split()[0] if target.split() else \"\"\n        if not target:\n            continue\n        low = target.lower()\n        if low.startswith((\"#\", \"/\", \"http://\", \"https://\", \"mailto:\", \"tel:\", \"data:\")):\n            continue\n        if \"{{\" in target or \"}}\" in target:\n            continue\n        path = unquote(target.split(\"#\")[0].split(\"?\")[0])\n        if path:\n            links.append((target, path))\n    return links\n\n\ndef check_runbook(path, root, today, days):\n    \"\"\"Return a list of (kind, detail) findings for one runbook.\"\"\"\n    rel = os.path.relpath(path, root)\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            text = handle.read()\n    except OSError as exc:\n        return [(\"UNREADABLE\", \"%s: %s\" % (rel, exc))]\n\n    findings = []\n    meta = metadata(text)\n\n    if \"last reviewed\" not in meta:\n        findings.append((\"NODATE\", \"%s has no last reviewed line\" % rel))\n    else:\n        when = review_date(meta[\"last reviewed\"])\n        if when is None:\n            findings.append((\"BADDATE\", \"%s has an unreadable review date: %s\"\n                             % (rel, meta[\"last reviewed\"])))\n        elif when > today:\n            findings.append((\"FUTURE\", \"%s claims a future review: %s\" % (rel, when)))\n        else:\n            age = (today - when).days\n            if age > days:\n                findings.append((\"STALE\", \"%s last reviewed %s, %d days ago\"\n                                 % (rel, when, age)))\n\n    owner = meta.get(\"owner\", \"\").strip().lower()\n    if owner in PLACEHOLDERS:\n        findings.append((\"UNOWNED\", \"%s has no owner\" % rel))\n\n    base = os.path.dirname(path)\n    for raw, target in relative_links(text):\n        resolved = os.path.normpath(os.path.join(base, target))\n        if not os.path.exists(resolved):\n            findings.append((\"BROKEN\", \"%s -> %s\" % (rel, raw)))\n    return findings\n\n\ndef positive(value):\n    \"\"\"argparse type for a non-negative integer.\"\"\"\n    try:\n        number = int(value)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"%s is not an integer\" % value)\n    if number < 0:\n        raise argparse.ArgumentTypeError(\"%s is negative\" % value)\n    return number\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find runbooks that are stale, unowned or link to a \"\n                    \"file that no longer exists.\",\n        epilog=\"Exit codes: 0 clean, 1 findings, 2 usage or input error.\")\n    parser.add_argument(\"root\", help=\"directory of markdown runbooks\")\n    parser.add_argument(\"--days\", type=positive, default=90,\n                        help=\"maximum age in days (default: 90)\")\n    parser.add_argument(\"--today\", help=\"date to measure age from (YYYY-MM-DD)\")\n    args = parser.parse_args(argv)\n\n    try:\n        root = os.path.abspath(args.root)\n        if not os.path.isdir(root):\n            raise InputError(\"not a directory: %s\" % root)\n        if args.today:\n            today = review_date(args.today)\n            if today is None:\n                raise InputError(\"--today is not a date: %s\" % args.today)\n        else:\n            today = datetime.date.today()\n        paths = markdown_files(root)\n        if not paths:\n            raise InputError(\"no markdown files under %s\" % root)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    findings = []\n    for path in paths:\n        findings.extend(check_runbook(path, root, today, args.days))\n\n    stale = sum(1 for kind, _ in findings\n                if kind in (\"STALE\", \"NODATE\", \"BADDATE\", \"FUTURE\"))\n    unowned = sum(1 for kind, _ in findings if kind == \"UNOWNED\")\n    broken = sum(1 for kind, _ in findings if kind == \"BROKEN\")\n\n    if not findings:\n        print(\"OK %d runbooks fresh, owned and linked\" % len(paths))\n        return 0\n\n    print(\"FINDING %d stale, %d unowned, %d broken\" % (stale, unowned, broken))\n    for kind, detail in findings:\n        print(\"%s %s\" % (kind, detail))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p714","n":"714","slug":"alerts-nobody-acts-on","title":"Alerts nobody acts on","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","SRE","Alerting","Observability","DevOps"],"series_name":"IR","design":"code","script":"alert_fatigue.py","date":"2031-04-20","files":{"it":"notes/p714-alerts-nobody-acts-on.md","devsecops":"notes/p714-alerts-nobody-acts-on.md"},"md":"# Alerts nobody acts on\n\nThe pager fires eleven times before lunch. The login-failed rule has fired fifty times this month and been actioned twice. Everyone clicks acknowledge, nobody looks, and the one real alert waits behind the noise.\n\n## Quick reference\n\nThe full script is at the end of this note: `alert_fatigue.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 alert_fatigue.py alerts.json\n```\n\n**Example output**\n\n```text\nFINDING 3 noisy rules of 8\nNOISE login-failed ratio 0.04\n```\n\n## Explained\n\n### What it does\n\n`alert_fatigue.py` reads the alert history and computes the signal ratio for each rule: actioned alerts divided by alerts fired. Rules with fewer than `--min-fired` fires are ignored. A rule below `--threshold` is ranked as noise. The list is sorted from the lowest ratio up, so the worst rule is first.\n\n### How to run it\n\n`python3 alert_fatigue.py alerts.json --days 30`. Exit 0 means every judged rule clears the threshold, 1 means at least one does not, 2 means bad input. It never edits or silences a rule.\n\n### What the screen shows\n\nThe fourteen lines count fires and actions per rule, compute the ratios, filter by the minimum and the threshold, then print the count and the worst rule. On the dirty fixture: FINDING 3 noisy rules of 8 and NOISE login-failed ratio 0.04.\n\n### The fix\n\nDelete the rule, or rewrite it until it names a symptom a human can act on. A noisy rule does not just waste pages; it teaches the team that the pager is optional.\n\n### Prevention\n\nReview the ratios monthly with the on-call lead. Every page should have an owner, a runbook and a remembered action.\n\n> **The honest caveat:** a low ratio can mean the alert is right and the response is broken. Read the actions before you delete the rule.\n\n> **The lesson:** attention is the budget, and noise spends it.\n\n## In one line\n\n> **Noise is not an alert.** A page nobody acts on trains the team to ignore the next one.\n\n**Count the signal**\n\n```bash\npython3 alert_fatigue.py alerts.json\n```\n\n## Script: alert_fatigue.py\n\nDownload: [scripts/alert_fatigue.py](../scripts/alert_fatigue.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank alert rules by how rarely an alert leads to an action.\n\nReads an alert history and computes one number per rule:\n\n    signal ratio = actioned alerts / alerts fired\n\nA rule that fires fifty times and is actioned twice has a ratio of 0.04. It\nis not an alert, it is a habit. The tool ranks rules from the noisiest up and\nexits 1 when any rule with enough fires falls below --threshold.\n\nThe history is JSON, one object per alert:\n\n    [{\"rule\": \"login-failed\", \"fired_at\": \"2026-10-20T03:14:00Z\",\n      \"actioned\": false},\n     {\"rule\": \"checkout-latency\", \"fired_at\": \"2026-10-20T09:02:00Z\",\n      \"actioned\": true}]\n\n\"actioned\" means a human did something because of the alert: acknowledged\nand fixed, or acknowledged and closed as expected. An alert that was seen and\nignored is not actioned.\n\nA rule needs --min-fired alerts (default 5) before it is judged. The window\nis all history by default; --days N looks at the last N days. The threshold\nis a minimum acceptable signal ratio, 0.10 by default.\n\nUsage:\n    python3 alert_fatigue.py alerts.json\n    python3 alert_fatigue.py alerts.json --threshold 0.20 --days 30\n\nExit codes: 0 no rule below the threshold, 1 at least one noisy rule, 2\nusage, or a history that is missing, unreadable, invalid or empty.\n\nCI (monthly review, fail when the pager is mostly noise):\n    python3 alert_fatigue.py alerts.json --days 30 || exit 1\n\nCron (Monday 08:00, send the noise ranking to the on-call lead):\n    0 8 * * 1 python3 /opt/s0sta/alert_fatigue.py /var/lib/s0sta/alerts.json\n\nThe script is read-only. It reads the export and prints a ranking; it never\nsilences, edits or deletes a rule. A low ratio can also mean the alert is\ncorrect but the runbook is bad: read the actions before you delete the rule.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_history(path):\n    \"\"\"Return the list of alert events, or raise InputError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list) or not doc:\n        raise InputError(\"%s has no non-empty alert list\" % path)\n    return doc\n\n\ndef event_fields(item, number):\n    \"\"\"Validate one alert event and return (rule, date, actioned).\"\"\"\n    if not isinstance(item, dict):\n        raise InputError(\"event %d is not an object\" % number)\n    rule = item.get(\"rule\")\n    fired_at = item.get(\"fired_at\")\n    actioned = item.get(\"actioned\")\n    if not isinstance(rule, str) or not rule:\n        raise InputError(\"event %d has no rule\" % number)\n    if not isinstance(fired_at, str) or len(fired_at) < 10:\n        raise InputError(\"event %s has no fired_at timestamp\" % rule)\n    try:\n        when = datetime.date.fromisoformat(fired_at[:10])\n    except ValueError:\n        raise InputError(\"event %s has an unreadable fired_at: %s\"\n                         % (rule, fired_at))\n    if not isinstance(actioned, bool):\n        raise InputError(\"event %s has no boolean actioned\" % rule)\n    return rule, when, actioned\n\n\ndef ratio(value):\n    \"\"\"argparse type for a signal ratio between 0 and 1.\"\"\"\n    try:\n        number = float(value)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"%s is not a number\" % value)\n    if not 0.0 <= number <= 1.0:\n        raise argparse.ArgumentTypeError(\"%s is outside 0..1\" % value)\n    return number\n\n\ndef positive(value):\n    \"\"\"argparse type for a positive integer.\"\"\"\n    try:\n        number = int(value)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"%s is not an integer\" % value)\n    if number < 1:\n        raise argparse.ArgumentTypeError(\"%s is less than 1\" % value)\n    return number\n\n\ndef nonneg(value):\n    \"\"\"argparse type for a non-negative integer.\"\"\"\n    try:\n        number = int(value)\n    except ValueError:\n        raise argparse.ArgumentTypeError(\"%s is not an integer\" % value)\n    if number < 0:\n        raise argparse.ArgumentTypeError(\"%s is negative\" % value)\n    return number\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank alert rules by signal ratio and exit 1 when the \"\n                    \"pager is mostly noise.\",\n        epilog=\"Exit codes: 0 quiet enough, 1 noisy rule found, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"history\", help=\"JSON list of alert events\")\n    parser.add_argument(\"--threshold\", type=ratio, default=0.10,\n                        help=\"minimum signal ratio (default: 0.10)\")\n    parser.add_argument(\"--min-fired\", type=positive, default=5,\n                        help=\"fires before a rule is judged (default: 5)\")\n    parser.add_argument(\"--days\", type=nonneg, default=0,\n                        help=\"only the last N days (default: all history)\")\n    parser.add_argument(\"--top\", type=positive, default=20,\n                        help=\"maximum noisy rules to print (default: 20)\")\n    args = parser.parse_args(argv)\n\n    try:\n        history = load_history(args.history)\n        cutoff = None\n        if args.days:\n            cutoff = datetime.date.today() - datetime.timedelta(days=args.days)\n        counts = {}\n        in_window = 0\n        for number, item in enumerate(history, 1):\n            rule, when, actioned = event_fields(item, number)\n            if cutoff is not None and when < cutoff:\n                continue\n            in_window += 1\n            fired, acted = counts.get(rule, (0, 0))\n            counts[rule] = (fired + 1, acted + int(actioned))\n        if not in_window:\n            raise InputError(\"no alert events in the last %d days\" % args.days)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    noisy = []\n    for rule, (fired, acted) in counts.items():\n        if fired < args.min_fired:\n            continue\n        signal = acted / fired\n        if signal < args.threshold:\n            noisy.append((signal, -fired, rule, fired, acted))\n    noisy.sort()\n\n    if noisy:\n        print(\"FINDING %d noisy rules of %d\" % (len(noisy), len(counts)))\n        for signal, _, rule, fired, acted in noisy[:args.top]:\n            print(\"NOISE %s ratio %.2f fired %d actioned %d\"\n                  % (rule, signal, fired, acted))\n        return 1\n    print(\"OK every judged rule is at or above %.2f signal\" % args.threshold)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p715","n":"715","slug":"the-incident-that-keeps-coming-back","title":"The incident that keeps coming back","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","SRE","IncidentResponse","Postmortem","DevOps"],"series_name":"IR","design":"code","script":"incident_recurrence.py","date":"2031-04-22","files":{"it":"notes/p715-the-incident-that-keeps-coming-back.md","devsecops":"notes/p715-the-incident-that-keeps-coming-back.md"},"md":"# The incident that keeps coming back\n\nThe third database pool incident in three months opens with the same graph and the same tag. The postmortem said the fix shipped in July. The incident says it did not hold.\n\n## Quick reference\n\nThe full script is at the end of this note: `incident_recurrence.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 incident_recurrence.py incidents.json\n```\n\n**Example output**\n\n```text\nFINDING 2 recurring causes\nREPEAT db_pool_exhausted 3 incidents\n```\n\n## Explained\n\n### What it does\n\n`incident_recurrence.py` groups incidents by root-cause tag, then splits each group into windows of `--window` days (90 by default). A window with two or more incidents is a recurrence. Causes are ranked by how often they return, and services are flagged when they appear in two or more incidents inside a recurring window.\n\n### How to run it\n\n`python3 incident_recurrence.py incidents.json --window 90`. Exit 0 is no repeat, 1 is a recurrence, 2 is bad input. It reads the export and changes nothing.\n\n### What the screen shows\n\nThe fourteen lines group incidents by tag, keep the groups that repeat inside the window, pick the largest cause and print it. On the dirty fixture: FINDING 2 recurring causes and REPEAT db_pool_exhausted 3 incidents.\n\n### The fix\n\nReopen the original postmortem and find the action that never shipped, or the fix that treated a symptom. A repeat is a failed fix, not a new incident, and it deserves the old action with a new date.\n\n### Prevention\n\nReview recurrence monthly with the service owners. Tag causes with a small, shared list so the counts mean the same thing across teams.\n\n> **The honest caveat:** clustering by tag inherits every mistake in the taxonomy. A vague tag hides a repeat.\n\n> **The lesson:** the same cause twice is a process problem, not bad luck.\n\n## In one line\n\n> **Same cause, same page.** A repeat is a fix that did not hold, not a new incident.\n\n**Read the repeats**\n\n```bash\npython3 incident_recurrence.py inc.json\n```\n\n## Script: incident_recurrence.py\n\nDownload: [scripts/incident_recurrence.py](../scripts/incident_recurrence.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Find root causes that keep coming back, and the services they revisit.\n\nReads an incident history and groups incidents that share a root-cause tag and\nfall inside the same window. A tag that appears twice within --window days is\na recurrence, not bad luck. The report ranks causes by how often they return\nand names the services that carry the repeats.\n\nThe history is JSON, one object per incident:\n\n    [{\"id\": \"INC-1042\", \"opened_at\": \"2026-07-02\",\n      \"root_cause_tags\": [\"db_pool_exhausted\"], \"service\": \"checkout\"},\n     {\"id\": \"INC-1088\", \"opened_at\": \"2026-08-19\",\n      \"root_cause_tags\": [\"db_pool_exhausted\"], \"service\": \"orders\"}]\n\nTags are compared case-insensitively and spaces are collapsed. A window\nstarts at the first incident in a group and stays open for --window days\n(default 90); the next incident after that starts a new window. Two incidents\nin the same window make a repeat. The same tag can recur in several windows,\nand the report adds them together.\n\nUsage:\n    python3 incident_recurrence.py incidents.json\n    python3 incident_recurrence.py incidents.json --window 30\n\nExit codes: 0 no repeated cause, 1 at least one recurrence, 2 usage, or a\nhistory that is missing, unreadable, invalid or empty.\n\nCI (monthly, fail the review when a cause repeats):\n    python3 incident_recurrence.py incidents.json --window 90 || exit 1\n\nCron (first Monday of the month, 08:00, mail the review owner):\n    0 8 1-7 * 1 python3 /opt/s0sta/incident_recurrence.py /var/lib/s0sta/incidents.json\n\nThe script is read-only. It reads the export and prints the ranking; it does\nnot merge, close or reopen an incident. A repeat is evidence that the fix did\nnot hold, not proof of the cause: read the postmortems before you merge them.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime\nimport json\nimport sys\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_incidents(path):\n    \"\"\"Return a non-empty list of incidents, or raise InputError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list) or not doc:\n        raise InputError(\"%s has no non-empty incident list\" % path)\n    return doc\n\n\ndef tags_of(item, incident_id):\n    \"\"\"Return normalised, de-duplicated tags for one incident.\"\"\"\n    raw = item.get(\"root_cause_tags\")\n    if not isinstance(raw, list) or not raw:\n        raise InputError(\"incident %s has no root_cause_tags\" % incident_id)\n    tags = []\n    for tag in raw:\n        if not isinstance(tag, str) or not tag.strip():\n            raise InputError(\"incident %s has an empty root_cause_tag\"\n                             % incident_id)\n        clean = \" \".join(tag.lower().split())\n        if clean not in tags:\n            tags.append(clean)\n    return tags\n\n\ndef incident_fields(item, number):\n    \"\"\"Validate one incident and return (id, date, tags, service).\"\"\"\n    if not isinstance(item, dict):\n        raise InputError(\"incident %d is not an object\" % number)\n    incident_id = item.get(\"id\")\n    opened_at = item.get(\"opened_at\")\n    service = item.get(\"service\")\n    if not isinstance(incident_id, str) or not incident_id:\n        raise InputError(\"incident %d has no id\" % number)\n    if not isinstance(opened_at, str) or len(opened_at) < 10:\n        raise InputError(\"incident %s has no opened_at date\" % incident_id)\n    try:\n        when = datetime.date.fromisoformat(opened_at[:10])\n    except ValueError:\n        raise InputError(\"incident %s has an unreadable opened_at: %s\"\n                         % (incident_id, opened_at))\n    if not isinstance(service, str) or not service:\n        raise InputError(\"incident %s has no service\" % incident_id)\n    return incident_id, when, tags_of(item, incident_id), service\n\n\ndef clusters(events, window):\n    \"\"\"Group (date, incident) pairs into anchored windows.\"\"\"\n    groups = []\n    current = []\n    start = None\n    for when, incident in events:\n        if start is None or (when - start).days <= window:\n            current.append((when, incident))\n            if start is None:\n                start = when\n        else:\n            groups.append(current)\n            current = [(when, incident)]\n            start = when\n    if current:\n        groups.append(current)\n    return groups\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank root causes that repeat inside a window and flag \"\n                    \"the services that keep meeting them.\",\n        epilog=\"Exit codes: 0 no recurrence, 1 recurrence found, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"incidents\", help=\"JSON list of incidents\")\n    parser.add_argument(\"--window\", type=int, default=90,\n                        help=\"window in days (default: 90)\")\n    args = parser.parse_args(argv)\n\n    if args.window < 1:\n        print(\"ERR --window must be at least 1 day\", file=sys.stderr)\n        return 2\n\n    try:\n        events = load_incidents(args.incidents)\n        seen = set()\n        by_tag = {}\n        services = {}\n        for number, item in enumerate(events, 1):\n            incident_id, when, tags, service = incident_fields(item, number)\n            if incident_id in seen:\n                raise InputError(\"duplicate incident id %s\" % incident_id)\n            seen.add(incident_id)\n            services[incident_id] = service\n            for tag in tags:\n                by_tag.setdefault(tag, []).append((when, incident_id))\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    causes = {}\n    for tag, entries in by_tag.items():\n        entries.sort()\n        for group in clusters(entries, args.window):\n            if len(group) < 2:\n                continue\n            count, repeats = causes.get(tag, (0, 0))\n            causes[tag] = (count + len(group), repeats + len(group) - 1)\n\n    ranked = sorted(causes.items(), key=lambda kv: (-kv[1][1], -kv[1][0], kv[0]))\n    if ranked:\n        print(\"FINDING %d recurring causes\" % len(ranked))\n        for tag, (count, _) in ranked:\n            print(\"REPEAT %s %d incidents\" % (tag, count))\n\n        # A service is flagged when it appears in at least two incidents that\n        # sit inside a recurring window. A set per service prevents\n        # double-counting an incident that carries two recurring tags.\n        hits = {}\n        for tag, entries in by_tag.items():\n            for group in clusters(sorted(entries), args.window):\n                if len(group) < 2:\n                    continue\n                for _, incident_id in group:\n                    hits.setdefault(services[incident_id], set()).add(incident_id)\n        flagged = sorted((len(ids), name) for name, ids in hits.items()\n                         if len(ids) >= 2)\n        for count, name in sorted(flagged, key=lambda x: (-x[0], x[1])):\n            print(\"SERVICE %s %d repeat incidents\" % (name, count))\n        return 1\n    print(\"OK no root cause repeated inside %d days\" % args.window)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p716","n":"716","slug":"the-control-you-think-is-on","title":"The control you think is on","sections":["it","devsecops"],"topic":"Python scripts","tags":["Python","Compliance","CloudSecurity","DevSecOps","Audit"],"series_name":"IR","design":"code","script":"control_verify.py","date":"2031-04-24","files":{"it":"notes/p716-the-control-you-think-is-on.md","devsecops":"notes/p716-the-control-you-think-is-on.md"},"md":"# The control you think is on\n\nThe policy says MFA is enforced. The quarterly review says verified. The export says false, and it has said false since June, because nobody opened the export.\n\n## Quick reference\n\nThe full script is at the end of this note: `control_verify.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 control_verify.py controls.json exports\n```\n\n**Example output**\n\n```text\nFINDING 2 disabled, 1 unverified\nDISABLED admin_mfa, audit_logging\n```\n\n## Explained\n\n### What it does\n\n`control_verify.py` joins a control inventory with the config exports it names. Each control has an expected value and an evidence file. The tool loads the export, reads the control's key, and compares the value with a type-strict check: true does not match \"true\". A mismatch is DISABLED. A missing file, invalid JSON or missing key is UNVERIFIED, which is not the same as passing.\n\n### How to run it\n\n`python3 control_verify.py controls.json exports/` after the export job. Exit 0 is every control verified, 1 is disabled or unverified, 2 is bad input. It only reads.\n\n### What the screen shows\n\nThe fourteen lines read the inventory and each export, catch the unverifiable cases, compare the rest and print the two counts. On the dirty fixture: FINDING 2 disabled, 1 unverified and DISABLED admin_mfa, audit_logging.\n\n### The fix\n\nTurn the control back on, or fix the export that says it is off, then record the new evidence. A control you cannot evidence is a control you cannot claim.\n\n### Prevention\n\nRun the check daily and keep the export timestamp beside the evidence. Add the check to the release gate so a disabled control stops the deploy.\n\n> **The honest caveat:** passing proves the export matches the expected value; it does not prove the export is current.\n\n> **The lesson:** a policy states intent, evidence states fact.\n\n## In one line\n\n> **Trust the export, not the policy.** A control you cannot evidence is a control you do not have.\n\n**Check the state**\n\n```bash\npython3 control_verify.py ctl.json exports\n```\n\n## Script: control_verify.py\n\nDownload: [scripts/control_verify.py](../scripts/control_verify.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Check that every control in an inventory is still in its expected state.\n\nA control inventory says what should be on. A config export says what is on.\nThis tool joins the two and reports the controls that are disabled, or that\ncannot be verified at all. A control you cannot prove is a control you do not\nhave.\n\nThe inventory is JSON, one object per control:\n\n    [{\"control\": \"admin_mfa\", \"expected\": true, \"evidence\": \"iam.json\"},\n     {\"control\": \"audit_logging\", \"expected\": true, \"evidence\": \"cloudtrail.json\"},\n     {\"control\": \"encryption_at_rest\", \"expected\": true, \"evidence\": \"storage.json\"}]\n\nEach evidence file is a JSON object keyed by control id:\n\n    {\"admin_mfa\": false, \"audit_logging\": true}\n\nThe evidence path is relative to the export directory and may not escape it.\nComparison is type-strict: true does not match \"true\", and 1 does not match\ntrue. A missing file, invalid JSON, missing key or non-object export makes the\ncontrol UNVERIFIED; a value that is present but different makes it DISABLED.\n\nUsage:\n    python3 control_verify.py controls.json exports/\n    python3 control_verify.py controls.json exports/ --quiet\n\nExit codes: 0 every control verified, 1 at least one disabled or unverified,\n2 usage or an inventory that is missing, unreadable, invalid or empty.\n\nCI (before a release, fail when a control is off):\n    python3 control_verify.py controls.json exports/ || exit 1\n\nCron (daily 06:30, export first, then verify):\n    30 6 * * * /opt/s0sta/export-config.sh && python3 /opt/s0sta/control_verify.py /etc/s0sta/controls.json /var/lib/s0sta/exports\n\nThe script is read-only. It reads the inventory and the exports and writes\nnothing. Passing this check proves the setting matches the expected value in\nthe export you gave it; it does not prove the export is current. Put the\nexport timestamp beside the evidence and check that too.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport os\nimport sys\n\nSCALARS = (bool, int, float, str)\n\n\nclass InputError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_inventory(path):\n    \"\"\"Return a non-empty list of controls, or raise InputError.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list) or not doc:\n        raise InputError(\"%s has no non-empty control list\" % path)\n    return doc\n\n\ndef control_fields(item, number):\n    \"\"\"Validate one control entry and return (control, expected, evidence).\"\"\"\n    if not isinstance(item, dict):\n        raise InputError(\"control %d is not an object\" % number)\n    control = item.get(\"control\")\n    expected = item.get(\"expected\")\n    evidence = item.get(\"evidence\")\n    if not isinstance(control, str) or not control:\n        raise InputError(\"control %d has no name\" % number)\n    if not isinstance(expected, SCALARS):\n        raise InputError(\"control %s has no boolean, number or string expected\"\n                         % control)\n    if not isinstance(evidence, str) or not evidence:\n        raise InputError(\"control %s has no evidence path\" % control)\n    if os.path.isabs(evidence):\n        raise InputError(\"control %s evidence must be relative\" % control)\n    return control, expected, evidence\n\n\ndef same(expected, actual):\n    \"\"\"Return True only when both the type and the value match.\"\"\"\n    return type(expected) is type(actual) and expected == actual\n\n\ndef read_export(path, cache):\n    \"\"\"Return (document, error string). Cached by path.\"\"\"\n    if path not in cache:\n        try:\n            with open(path, encoding=\"utf-8\") as handle:\n                cache[path] = (json.load(handle), None)\n        except OSError as exc:\n            cache[path] = (None, \"cannot read %s: %s\" % (os.path.basename(path), exc))\n        except ValueError:\n            cache[path] = (None, \"invalid JSON in %s\" % os.path.basename(path))\n    return cache[path]\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Verify an inventory of controls against JSON config \"\n                    \"exports and report disabled or unverifiable controls.\",\n        epilog=\"Exit codes: 0 all verified, 1 disabled or unverified, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"inventory\", help=\"JSON list of controls\")\n    parser.add_argument(\"exports\", help=\"directory of JSON config exports\")\n    parser.add_argument(\"--quiet\", action=\"store_true\",\n                        help=\"print the summary only\")\n    args = parser.parse_args(argv)\n\n    try:\n        root = os.path.realpath(args.exports)\n        if not os.path.isdir(root):\n            raise InputError(\"not a directory: %s\" % root)\n        inventory = load_inventory(args.inventory)\n\n        controls = []\n        seen = set()\n        for number, item in enumerate(inventory, 1):\n            control, expected, evidence = control_fields(item, number)\n            if control in seen:\n                raise InputError(\"duplicate control %s\" % control)\n            seen.add(control)\n            controls.append((control, expected, evidence))\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    cache = {}\n    disabled = []\n    unverified = []\n    for control, expected, evidence in controls:\n        path = os.path.realpath(os.path.join(root, evidence))\n        if path != root and not path.startswith(root + os.sep):\n            print(\"ERR control %s evidence escapes the export directory\" % control,\n                  file=sys.stderr)\n            return 2\n        doc, error = read_export(path, cache)\n        if error:\n            unverified.append((control, error))\n            continue\n        if not isinstance(doc, dict):\n            unverified.append((control, \"%s is not an object\"\n                               % os.path.basename(path)))\n            continue\n        if control not in doc:\n            unverified.append((control, \"missing key in %s\"\n                               % os.path.basename(path)))\n            continue\n        actual = doc[control]\n        if not same(expected, actual):\n            disabled.append((control, expected, actual))\n\n    if not disabled and not unverified:\n        print(\"OK %d controls verified\" % len(controls))\n        return 0\n\n    print(\"FINDING %d disabled, %d unverified\"\n          % (len(disabled), len(unverified)))\n    if disabled:\n        print(\"DISABLED \" + \", \".join(control for control, _, _ in disabled))\n    if not args.quiet:\n        for control, expected, actual in disabled:\n            print(\"  %s: expected %s, got %s\"\n                  % (control, json.dumps(expected), json.dumps(actual)))\n        for control, reason in unverified:\n            print(\"UNVERIFIED %s: %s\" % (control, reason))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p717","n":"717","slug":"the-learning-loop","title":"The learning loop","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["Python","SRE","IncidentResponse","DevOps","Security"],"series_name":"IR","design":"architecture","script":"","date":"2031-04-27","files":{"it":"notes/p717-the-learning-loop.md","devsecops":"notes/p717-the-learning-loop.md"},"md":"# The learning loop\n\nEvery postmortem ends with lessons learned, and most of them end there. The action list closes, the document is filed, and the next incident finds the same gap.\n\n## Quick reference\n\n- **Detect:** Coverage map (COV) · Noise ranking (NOISE)\n- **Fix:** Action list (ACT) · Named owner (OWN)\n- **Verify:** Control check (CTRL) · Drill calendar (DRILL)\n\n**Request flow:** Noise ranking → Named owner → Drill calendar → Coverage map → Action list → Control check\n\n**Around it:** Postmortem (PM) · Risk register (RISK) · Change log (CHG)\n\n## Explained\n\n### What it does\n\nThe learning loop has three tiers. Detect holds the coverage map and the noise ranking: which techniques have an enabled rule, and which alerts earn their page. Fix holds the actions, each with an owner and a date. Verify holds the control checks and the drills that prove the fix still works.\n\n### How to use it\n\nRead it as a loop, not a queue. A coverage gap becomes an action with an owner. An action becomes a control that can be checked. A control check that fails becomes next week's fix; a drill that fails becomes next week's detection work. The arrows down are the handoffs; the arrow back is the reason the loop is a loop.\n\n### The fix\n\nGive each tier one weekly output: the coverage diff, the open actions with owners and dates, and the control evidence with the last drill. Three artefacts, one owner each, reviewed on the same day.\n\n### Prevention\n\nPut the loop on the postmortem template. A finding that cannot name its tier, its owner and its evidence is not an action yet.\n\n> **The honest caveat:** a loop with no owner is a diagram. The drawing is the easy part.\n\n> **The lesson:** learning is a system with inputs, owners and checks, not a meeting.\n\n## In one line\n\n> **A loop that closes is a system.** Three tiers, one owner each, reviewed on the same day.\n\n**Close the loop**\n\n```bash\npython3 control_verify.py ctl.json exports\n```\n"},{"id":"p718","n":"718","slug":"post-incident-commands","title":"Post-incident commands","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["Python","IncidentResponse","SRE","CommandLine","DevOps"],"series_name":"IR","design":"cheatsheet","script":"","date":"2031-04-29","files":{"it":"notes/p718-post-incident-commands.md","devsecops":"notes/p718-post-incident-commands.md"},"md":"# Post-incident commands\n\nThe incident is closed, the tab is still open, and the commands that would have shortened it are spread across four notes. A sheet is cheaper than a search.\n\n## Quick reference\n\n### Coverage\n\n```bash\npython3 detection_gap.py t.json d.json\ngrep -rl 'T1059' detection/\njq '.[] | select(.enabled)' det.json\ngit log -1 --format=%cs detection/\n```\n\n### Runbooks\n\n```bash\npython3 runbook_freshness.py docs/runbooks\nfind docs/runbooks -name '*.md' -mtime +90\ngrep -L '^Owner:' docs/runbooks/*.md\ngit log -1 --format=%cs docs/runbooks/*.md\n```\n\n### Alerts\n\n```bash\npython3 alert_fatigue.py alerts.json\njq '[.[]|select(.actioned)]|length'\ngrep -c '\"actioned\": false' alerts.json\npython3 alert_fatigue.py a.json --days 30\n```\n\n### Controls\n\n```bash\npython3 control_verify.py ctl.json exports\njq '.mfa.enforced' exports/iam.json\naws iam get-account-summary\nterraform plan -detailed-exitcode\n```\n\n## Explained\n\n### Coverage\n\n`detection_gap.py` compares the technique list with the rules. The grep and jq lines find a technique and show which rules are enabled. git log tells you when the detection directory last changed.\n\n### Runbooks\n\n`runbook_freshness.py` checks dates, owners and links. find checks the file's modification time and grep finds a missing owner line, which are proxies when the tool is not installed. The last line shows the last commit that touched the runbooks.\n\n### Alerts\n\n`alert_fatigue.py` ranks rules by signal. The jq and grep lines count actioned and unactioned alerts in an export. `--days 30` keeps the review to the current month.\n\n### Controls\n\n`control_verify.py` joins the inventory with the exports. The jq line reads one key, the AWS CLI reads the account summary, and `terraform plan -detailed-exitcode` shows whether the infrastructure still matches the code.\n\n### The fix\n\nPaste the sheet into the incident channel at the start of the review. Answer each question with one command, and write down the command that answered it.\n\n### Prevention\n\nKeep the sheet beside the runbooks and update it when a postmortem finds a better command. A sheet nobody edits is a museum.\n\n> **The honest caveat:** the file names are placeholders. Substitute your paths before you trust the output.\n\n> **The lesson:** after the incident, the useful work is the check you can run again.\n\n## In one line\n\n> **Close the incident, open the loop.** The review is four questions, and each one has a command.\n\n**Run the sheet**\n\n```bash\npython3 detection_gap.py t.json d.json\n```\n"},{"id":"p719","n":"719","slug":"four-learning-failures","title":"Four learning failures","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["Python","SRE","Postmortem","DevOps","CyberSecurity"],"series_name":"IR","design":"explainer","script":"","date":"2031-05-01","files":{"it":"notes/p719-four-learning-failures.md","devsecops":"notes/p719-four-learning-failures.md"},"md":"# Four learning failures\n\nFour postmortems, four different failures, and one shape: nobody checked the thing that was supposed to prevent the repeat. The incidents looked unrelated until the checks were written down.\n\n## Quick reference\n\n### The detection gap\n\n- A technique in the actor report\n- No enabled rule carries its id\n- Invisible, not missed\n- Fix: one rule, one owner, one date\n\n### The stale runbook\n\n- Steps for a service that changed\n- Last reviewed two releases ago\n- The on-call follows them anyway\n- Fix: a review date and a drill\n\n### The noisy alert\n\n- Fifty fires, two actions\n- The team learns to ignore the pager\n- The real alert waits behind the noise\n- Fix: a signal ratio and a review\n\n### The unverified control\n\n- The policy says MFA is on\n- The export says false\n- Nobody opened the export\n- Fix: dated evidence and a check\n\n> **Note:** Four failures, one shape: the check that would have caught it was never run.\n\n## Explained\n\n### The detection gap\n\nA technique appears in the actor report, but no enabled rule covers it. The activity was not missed by the analyst; it was invisible to the tooling.\n\n### The stale runbook\n\nThe steps describe a service that changed shape two releases ago. The on-call follows them, makes the outage longer, and loses trust in the page.\n\n### The noisy alert\n\nThe rule fires fifty times a month and is actioned twice. The team learns to ignore the pager, and the one real alert waits behind a month of expected ones.\n\n### The unverified control\n\nThe policy says MFA is on, the export says false, and nobody looked. The control had been off long enough to become the normal state.\n\n### What they share\n\nEach failure has a measurement: a coverage list, a review date, a signal ratio, an evidence file. None of them needs a new tool; each needs a check that runs on a schedule and has an owner. A postmortem that names the check, the owner and the date is already a control.\n\n> **The honest caveat:** a check that runs once is a screenshot. The schedule is the control.\n\n> **The lesson:** learning is a check, not a meeting.\n\n## In one line\n\n> **What nobody checks comes back.** A control you never test is a control you borrowed.\n\n**Measure the loop**\n\n```bash\npython3 runbook_freshness.py docs\n```\n"},{"id":"p720","n":"720","slug":"5-postmortem-mistakes-2","title":"5 postmortem mistakes","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["Python","SRE","Postmortem","IncidentResponse","DevOps"],"series_name":"IR","design":"dontdo","script":"","date":"2031-05-04","files":{"it":"notes/p720-5-postmortem-mistakes-2.md","devsecops":"notes/p720-5-postmortem-mistakes-2.md"},"md":"# 5 postmortem mistakes\n\nThe postmortem is over, the document is published, and six weeks later the same incident is back. The five mistakes are predictable, and each one leaves the system unchanged.\n\n## Quick reference\n\n**1.** ❌ **Name the engineer** — Blame ends the questions early.  \n✅ **Name the control that failed** — A missing guard can be fixed; a person cannot.\n\n**2.** ❌ **Action with no owner** — Everyone's action is nobody's action.  \n✅ **One name and one date** — Ownership and a deadline make it real.\n\n**3.** ❌ **No deadline on the fix** — Work without a date never competes.  \n✅ **Date it in the tracker** — A date turns intent into a commitment.\n\n**4.** ❌ **Mark it fixed and move on** — An assertion is not a test.  \n✅ **Re-run the control check** — Evidence closes the item; a claim does not.\n\n**5.** ❌ **Keep the lessons internal** — The next team repeats the incident.  \n✅ **Publish the write-up** — One page saves the next on-call a night.\n\n## Explained\n\n### The five\n\n1. **Blame the engineer.** The person who clicked is the end of the story, not the start. Name the control that should have stopped the click, and the missing guard becomes the action.\n   \\2. Leave the action unowned. An action without a name belongs to everyone, which means it belongs to nobody. One name and one date per action.\n   \\3. Set no deadline. A fix with no date never competes with the next feature. Put the date in the tracker and review it like a release.\n   \\4. Never verify the fix. \"Marked fixed\" is an assertion; a re-run control check is evidence. Close the action only when the check passes.\n   \\5. Keep the lessons internal. The next team meets the same failure with the same surprise. Publish the write-up where the on-call can find it.\n\n### What they share\n\nEvery one postpones the proof. The meeting ends, the document is filed, and the next on-call starts from zero.\n\n> **The honest caveat:** blameless does not mean consequence-free. Accountability belongs to the action and the system, not to the person who happened to be on call.\n\n> **The lesson:** a postmortem is a to-do list, and the list is the point.\n\n## In one line\n\n> **A postmortem is a to-do list.** If nothing changed on Monday, it was a meeting.\n\n**Close one item:** assign one action, set one date\n"},{"id":"p721","n":"721","slug":"quiz-3-learning-questions","title":"Quiz: 3 learning questions","sections":["it","devsecops"],"topic":"Incident response & detection","tags":["Python","IncidentResponse","SRE","DevOps","CyberSecurity"],"series_name":"IR","design":"quiz","script":"","date":"2031-05-06","files":{"it":"notes/p721-quiz-3-learning-questions.md","devsecops":"notes/p721-quiz-3-learning-questions.md"},"md":"# Quiz: 3 learning questions\n\nThree questions close the series, and each one is about proof you cannot see from a dashboard. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why is detection coverage a list, not a feeling?**\n\n- a) Because auditors ask for one\n- b) Because every technique is covered or not\n- c) Because feelings are hard to document\n\n> **Answer:** b) Because every technique is covered or not. A list names each technique and its rule. Gaps are rows with no enabled rule, and each can be counted, assigned and checked. A feeling cannot be reviewed.\n\n**Q2. What makes an alert actionable?**\n\n- a) A named owner, a runbook and a signal\n- b) A high severity label\n- c) A dashboard that updates quickly\n\n> **Answer:** a) A named owner, a runbook and a signal. An owner, a runbook and a signal ratio worth acting on. Severity is a label; action is a person with a next step. Fifty fires, two actions: the team ignores it.\n\n**Q3. Why does a control need evidence?**\n\n- a) Because the vendor says so\n- b) Because the policy and the state can differ\n- c) Because screenshots look professional\n\n> **Answer:** b) Because the policy and the state can differ. A policy states intent; an export states fact. Without a file and a key, enabled is a belief, and a control can stay off for months. Evidence is the difference.\n\n## Explained\n\n### Question one\n\nWhy is detection coverage a list, not a feeling? Because a list names every technique and the rule that covers it. The gaps are rows with no enabled rule, and each row can be counted, assigned and checked again next week. A feeling cannot be reviewed.\n\n### Question two\n\nWhat makes an alert actionable? A named owner, a runbook and a signal ratio that says the page is not noise. Severity is a label; action is a person with a next step. A rule that fires fifty times and is actioned twice is training the team to ignore the pager.\n\n### Question three\n\nWhy does a control need evidence? Because a policy states intent and an export states fact. Without a dated file and a key, \"enabled\" is a belief, and a disabled control can survive a quarter of reviews. The evidence is the difference between claiming and checking.\n\n> **The honest caveat:** passing this quiz does not make the loop work; the schedule, the owners and the evidence do.\n\n> **The lesson:** prevention is a check you can show.\n\nThat is the series. Fifty posts, five phases and twenty-five Python scripts, from the first alert to this quiz. Thank you for following along.\n\n## In one line\n\n> **Prevent it, then prove it.** Fifty posts, five phases and twenty-five Python scripts.\n\n**Keep the loop:** schedule the four checks\n"},{"id":"p722","n":"722","slug":"subdomains-you-never-registered","title":"Subdomains you never registered","sections":["devsecops"],"topic":"Python scripts","tags":["Python","CyberSecurity","AttackSurface","Recon","DevOps"],"series_name":"ASM","design":"code","script":"ct_inventory.py","date":"2031-05-08","files":{"devsecops":"notes/p722-subdomains-you-never-registered.md"},"md":"# Subdomains you never registered\n\nAt 09:40 a staging API nobody remembers deploying answers on 443 with a valid certificate. It is not in the inventory, not the CMDB, and not the last four pentest scopes.\n\n## Quick reference\n\nThe full script is at the end of this note: `ct_inventory.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 ct_inventory.py ct.json --scope acme.com\n```\n\n**Example output**\n\n```text\nCT 8 in scope, 3 new\nNEW admin.acme.com, api.staging.acme.com, vpn.acme.com\n```\n\n## Explained\n\n### What it does\n\n`ct_inventory.py` reads a `crt.sh` JSON export, takes name_value and common_name from every certificate, lowercases each name and strips the trailing dot. Wildcards are dropped, because \\*.acme.com is a policy, not a host. Anything outside `--scope` is dropped too. The unique hostnames that remain are diffed against a known list.\n\n### What the screen shows\n\nThe fourteen lines normalise the export, drop wildcards and out-of-scope hosts, then diff against the known file. On the dirty fixture: CT 8 in scope, 3 new and NEW admin.acme.com, api.staging.acme.com, vpn.acme.com.\n\n### How to run it\n\n`python3 ct_inventory.py ct.json --scope acme.com --known known.txt`. Exit 0 is clean, 1 is a new host, 2 is bad input. Run it weekly, or in CI before a scope is signed.\n\n### The fix\n\napi.staging is a live staging API, admin a panel on a marketing subdomain, vpn a decommissioned appliance whose A record still answers. Add the first two to the CMDB with an owner, and remove the third at the DNS provider.\n\n### Prevention\n\nDiff on every certificate issuance, not once a year. A host becomes findable the day its certificate is issued.\n\n> **The honest caveat:** CT logs certificates, not services. A host with a certificate can be dead, and a host without one can still be alive.\n\n> **The lesson:** the certificate log is the inventory you did not write.\n\n## In one line\n\n> **Your next asset is already public.** The log is append-only, and it was never yours to edit.\n\n**Diff the log**\n\n```bash\npython3 ct_inventory.py ct.json\n```\n\n## Script: ct_inventory.py\n\nDownload: [scripts/ct_inventory.py](../scripts/ct_inventory.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"ct_inventory.py - turn a certificate transparency export into a host diff.\n\nEvery certificate a public CA issues is published to the certificate\ntransparency logs, and crt.sh will hand you the result as JSON. The log is\nappend-only and public, so a hostname becomes discoverable the day its\ncertificate is issued, whether or not anyone remembered to put it in the\ninventory.\n\nThis script reads a crt.sh JSON export (or the same shape from another\naggregator), takes name_value and common_name from every certificate,\nnormalises each name to a lowercase hostname, drops wildcards, drops anything\noutside the given scope, and reports the hosts that are not in the known list.\n\nThe input is a JSON list of certificate objects, each with a name_value field:\n\n    [{\"name_value\": \"www.example.com\\\\n*.example.com\"},\n     {\"name_value\": \"api.example.com\", \"common_name\": \"api.example.com\"}]\n\nThe known list is one hostname per line, lowercased on read. Comments start\nwith #. Without --known, every in-scope host counts as new, which is the\nuseful first run: it prints the surface you never wrote down.\n\nUsage:\n    python3 ct_inventory.py crt.json --scope example.com --known known.txt\n    python3 ct_inventory.py crt.json --scope example.com --scope example.net\n    python3 ct_inventory.py crt.json --scope example.com --known known.txt --max-list 20\n    python3 ct_inventory.py --help\n\nCron or CI (weekly, fail the job when a new host appears):\n    0 6 * * 1 python3 /opt/s0sta/ct_inventory.py /var/lib/s0sta/crt.json --scope example.com --known /opt/s0sta/known.txt\n\nExit codes: 0 no new hosts, 1 at least one new host, 2 usage error,\nunreadable or malformed input, or no in-scope names at all.\n\nRead-only: opens the given files or stdin, parses JSON and prints. It writes\nnothing, changes nothing and makes no network call. Fetch the export first,\nthen run this against it.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nDEFAULT_MAX_LIST = 50\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be used as certificate or scope data.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return the file or stdin as text, or raise InputError.\"\"\"\n    try:\n        if path == \"-\":\n            return sys.stdin.read()\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n\n\ndef load_certs(path):\n    \"\"\"Return the list of certificate objects, or raise InputError.\"\"\"\n    text = read_text(path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    try:\n        data = json.loads(text)\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if isinstance(data, dict):\n        for key in (\"certificates\", \"results\", \"data\"):\n            if isinstance(data.get(key), list):\n                data = data[key]\n                break\n        else:\n            raise InputError(\"%s: no certificate list in the JSON\" % path)\n    if not isinstance(data, list):\n        raise InputError(\"%s: expected a JSON list of certificates\" % path)\n    certs = [item for item in data if isinstance(item, dict)]\n    if not certs:\n        raise InputError(\"%s: no certificate objects in the JSON\" % path)\n    return certs\n\n\ndef cert_names(cert):\n    \"\"\"Yield the raw names in one certificate object.\"\"\"\n    for key in (\"name_value\", \"common_name\"):\n        value = cert.get(key)\n        if isinstance(value, str):\n            for line in value.splitlines():\n                line = line.strip()\n                if line:\n                    yield line\n\n\ndef normalise_scope(value):\n    \"\"\"Return a scope domain, or raise InputError.\"\"\"\n    scope = value.strip().lower().rstrip(\".\")\n    if not scope or \"*\" in scope or \"/\" in scope or \" \" in scope:\n        raise InputError(\"bad --scope value: %r\" % value)\n    return scope\n\n\ndef in_scope(host, scopes):\n    \"\"\"True when host equals a scope domain or sits under one.\"\"\"\n    for scope in scopes:\n        if host == scope or host.endswith(\".\" + scope):\n            return True\n    return False\n\n\ndef load_known(path):\n    \"\"\"Return the known hostnames, or raise InputError.\"\"\"\n    if not path:\n        return set()\n    text = read_text(path)\n    hosts = set()\n    for line in text.splitlines():\n        line = line.strip().lower().rstrip(\".\")\n        if not line or line.startswith(\"#\"):\n            continue\n        hosts.add(line)\n    return hosts\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Diff in-scope hosts from a certificate transparency export \"\n                    \"against a known list.\")\n    parser.add_argument(\"crt\", help=\"crt.sh JSON export, or - for stdin\")\n    parser.add_argument(\"--scope\", action=\"append\", required=True,\n                        metavar=\"DOMAIN\",\n                        help=\"in-scope domain; repeat for more than one\")\n    parser.add_argument(\"--known\", metavar=\"FILE\",\n                        help=\"known hostnames, one per line; comments start with #\")\n    parser.add_argument(\"--max-list\", type=int, default=DEFAULT_MAX_LIST,\n                        metavar=\"N\", help=\"print at most N new hosts (default 50)\")\n    args = parser.parse_args(argv)\n    if args.max_list < 1:\n        print(\"ct_inventory: --max-list must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        scopes = [normalise_scope(value) for value in args.scope]\n        certs = load_certs(args.crt)\n        known = load_known(args.known)\n    except InputError as exc:\n        print(\"ct_inventory: %s\" % exc, file=sys.stderr)\n        return 2\n\n    names = 0\n    wildcards = 0\n    outside = 0\n    malformed = 0\n    seen = set()\n\n    for cert in certs:\n        for raw in cert_names(cert):\n            names += 1\n            name = raw.strip().lower().rstrip(\".\")\n            if not name:\n                continue\n            if \"*\" in name:\n                wildcards += 1\n                continue\n            if (\" \" in name or \"/\" in name or \"@\" in name\n                    or \":\" in name or name.startswith(\".\") or \"..\" in name):\n                malformed += 1\n                continue\n            if not in_scope(name, scopes):\n                outside += 1\n                continue\n            seen.add(name)\n\n    if not seen:\n        print(\"ct_inventory: no in-scope names in %s\" % args.crt,\n              file=sys.stderr)\n        print(\"ct_inventory: scope was %s\" % \", \".join(scopes),\n              file=sys.stderr)\n        return 2\n\n    new_hosts = sorted(seen - known)\n    print(\"CT %d certificates, %d names, %d in scope, %d wildcards, \"\n          \"%d out of scope, %d malformed\"\n          % (len(certs), names, len(seen), wildcards, outside, malformed))\n    if not new_hosts:\n        print(\"OK no new hosts against %s\" % (args.known or \"an empty known list\"))\n        return 0\n\n    print(\"NEW %d hosts not in %s\"\n          % (len(new_hosts), args.known or \"an empty known list\"))\n    for host in new_hosts[:args.max_list]:\n        print(\"  + %s\" % host)\n    if len(new_hosts) > args.max_list:\n        print(\"  ... %d more\" % (len(new_hosts) - args.max_list))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p723","n":"723","slug":"the-dns-record-that-points-at-nothing","title":"The DNS record that points at nothing","sections":["devsecops"],"topic":"Python scripts","tags":["Python","DNS","AttackSurface","Networking","CyberSecurity"],"series_name":"ASM","design":"code","script":"dns_inventory.py","date":"2031-05-11","files":{"devsecops":"notes/p723-the-dns-record-that-points-at-nothing.md"},"md":"# The DNS record that points at nothing\n\nA CNAME points at a host deleted when the SaaS trial ended. The record still resolves, the page still says the name is available, and the subdomain is still yours.\n\n## Quick reference\n\nThe full script is at the end of this note: `dns_inventory.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 dns_inventory.py dns.csv --scope acme.com\n```\n\n**Example output**\n\n```text\nDNS 2 dangling, 2 no address, 1 outside scope\nDANGLING blog.acme.com: old.acme.io is NXDOMAIN\n```\n\n## Explained\n\n### What it does\n\n`dns_inventory.py` reads a dig-style CSV export with name, type, value and status. It follows every CNAME chain, flags the ones that end at NXDOMAIN, lists names with no A or AAAA record, and reports CNAME, MX or NS targets that leave `--scope`.\n\n### What the screen shows\n\nThe thirteen lines collect NXDOMAIN names, filter CNAME rows, count names without an address and compare targets with the zone. On the dirty fixture: DNS 2 dangling, 2 no address, 1 outside scope and DANGLING blog.acme.com: old.acme.io is NXDOMAIN.\n\n### How to run it\n\n`python3 dns_inventory.py dns.csv --scope acme.com`. Exit 0 is clean, 1 is a finding, 2 is bad input. Put it in the same weekly job as the CT diff.\n\n### The fix\n\nDelete blog.acme.com and shop.acme.com at the provider. legacy.acme.com has only a TXT record, and vpn.acme.com has an empty A record: neither serves traffic, so confirm they are needed or remove them.\n\n### Prevention\n\nDiff yesterday's export against today's and alert on status changes, not only new names. A record that moves to NXDOMAIN is the cheapest takeover lead you will get.\n\n> **The honest caveat:** NXDOMAIN is a moment, not a fact, and a TXT-only name may be deliberate. A resolver hiccup can return NXDOMAIN for a name that is healthy a minute later.\n\n> **The lesson:** a record can outlive the service it was created for.\n\n## In one line\n\n> **A record can outlive the service.** The name still resolves. The thing behind it does not.\n\n**Follow the cname**\n\n```bash\ndig +short blog.acme.com CNAME\n```\n\n## Script: dns_inventory.py\n\nDownload: [scripts/dns_inventory.py](../scripts/dns_inventory.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"dns_inventory.py - read a dig export and find the records that point at nothing.\n\nA DNS zone accumulates records faster than it loses them. A trial ends, an\nappliance is returned, a SaaS account is closed, and the CNAME or the A record\nstays behind, still answering or still pointing at a name that no longer\nexists. This script reads a plain export of the records and reports three\nkinds of finding:\n\n    dangling   a CNAME whose chain ends at a name with status NXDOMAIN\n    missing    a name with no A or AAAA record and no CNAME to resolve through\n    outside    a CNAME, MX or NS target that leaves the given scope\n\nThe input is a CSV export with a header row. Required columns are name and\ntype; value and status are read when present. The export looks like:\n\n    name,type,value,status\n    www.example.com,A,10.0.0.5,NOERROR\n    blog.example.com,CNAME,old.example.io,NOERROR\n    old.example.io,A,,NXDOMAIN\n    example.com,MX,10 mail.example.com,NOERROR\n\nStatus is any resolver verdict, usually NOERROR or NXDOMAIN. An empty status\nmeans the export did not record one; it is never read as NXDOMAIN.\n\nUsage:\n    python3 dns_inventory.py dns.csv --scope example.com\n    python3 dns_inventory.py dns.csv --scope example.com --scope example.net\n    python3 dns_inventory.py dns.csv --scope example.com --max-list 10\n    python3 dns_inventory.py --help\n\nCron or CI (daily, fail the job when a record points at nothing):\n    15 5 * * * python3 /opt/s0sta/dns_inventory.py /var/lib/s0sta/dns.csv --scope example.com\n\nExit codes: 0 no finding, 1 at least one finding, 2 usage error, unreadable\ninput, missing columns or no usable rows.\n\nRead-only: opens the given CSV and prints. It writes nothing, changes nothing\nand makes no DNS query; the export is the input.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport sys\n\nDEFAULT_MAX_LIST = 50\nNAMESERVER_TYPES = (\"CNAME\", \"MX\", \"NS\")\nADDRESS_TYPES = (\"A\", \"AAAA\")\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as a DNS export.\"\"\"\n\n\ndef read_rows(path):\n    \"\"\"Return the CSV rows as dicts with lowercase keys, or raise InputError.\"\"\"\n    try:\n        handle = sys.stdin if path == \"-\" else open(path, \"r\", encoding=\"utf-8\",\n                                                    newline=\"\")\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    try:\n        reader = csv.DictReader(handle)\n        if not reader.fieldnames:\n            raise InputError(\"%s: no header row\" % path)\n        fields = [name.strip().lower() for name in reader.fieldnames]\n        if \"name\" not in fields or \"type\" not in fields:\n            raise InputError(\"%s: header needs name and type columns\" % path)\n        rows = []\n        for raw in reader:\n            row = {}\n            for key, value in raw.items():\n                if key is None:\n                    continue\n                row[key.strip().lower()] = (value or \"\").strip()\n            if not row.get(\"name\"):\n                continue\n            rows.append(row)\n    finally:\n        if handle is not sys.stdin:\n            handle.close()\n    if not rows:\n        raise InputError(\"%s: no usable rows\" % path)\n    return rows\n\n\ndef normalise_host(value):\n    \"\"\"Return a lowercase hostname without the trailing dot.\"\"\"\n    return value.strip().lower().rstrip(\".\")\n\n\ndef target_host(record_type, value):\n    \"\"\"Return the hostname a record points at, or None for address records.\"\"\"\n    value = value.strip()\n    if not value:\n        return None\n    if record_type == \"MX\":\n        parts = value.split()\n        value = parts[-1]\n    if record_type in NAMESERVER_TYPES:\n        return normalise_host(value)\n    return None\n\n\ndef in_scope(host, scopes):\n    \"\"\"True when host equals a scope domain or sits under one.\"\"\"\n    for scope in scopes:\n        if host == scope or host.endswith(\".\" + scope):\n            return True\n    return False\n\n\ndef resolve_chain(start, cname_map):\n    \"\"\"Follow CNAMEs from start. Return (final_name, path, looped).\"\"\"\n    path = []\n    current = start\n    seen = {start}\n    for _ in range(12):\n        nxt = cname_map.get(current)\n        if not nxt:\n            return current, path, False\n        if nxt in seen:\n            return nxt, path, True\n        path.append(nxt)\n        seen.add(nxt)\n        current = nxt\n    return current, path, True\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find dangling CNAMEs, names without addresses and \"\n                    \"out-of-scope targets in a dig export.\")\n    parser.add_argument(\"records\", help=\"CSV export of DNS records, or - for stdin\")\n    parser.add_argument(\"--scope\", action=\"append\", required=True,\n                        metavar=\"DOMAIN\", help=\"in-scope domain; repeat as needed\")\n    parser.add_argument(\"--max-list\", type=int, default=DEFAULT_MAX_LIST,\n                        metavar=\"N\", help=\"print at most N lines per finding \"\n                                          \"(default 50)\")\n    args = parser.parse_args(argv)\n    if args.max_list < 1:\n        print(\"dns_inventory: --max-list must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        scopes = []\n        for value in args.scope:\n            scope = normalise_host(value)\n            if not scope or \"*\" in scope or \"/\" in scope or \" \" in scope:\n                raise InputError(\"bad --scope value: %r\" % value)\n            scopes.append(scope)\n        rows = read_rows(args.records)\n    except InputError as exc:\n        print(\"dns_inventory: %s\" % exc, file=sys.stderr)\n        return 2\n\n    names = []\n    cname_map = {}\n    nxdomain = set()\n    for row in rows:\n        name = normalise_host(row[\"name\"])\n        record_type = row.get(\"type\", \"\").upper()\n        value = row.get(\"value\", \"\")\n        status = row.get(\"status\", \"\").upper()\n        if record_type not in (\"A\", \"AAAA\", \"CNAME\", \"MX\", \"NS\", \"TXT\", \"SRV\"):\n            continue\n        names.append((name, record_type, value, status))\n        if record_type == \"CNAME\" and value:\n            cname_map.setdefault(name, normalise_host(value))\n        if status == \"NXDOMAIN\":\n            nxdomain.add(name)\n\n    if not names:\n        print(\"dns_inventory: no usable records in %s\" % args.records,\n              file=sys.stderr)\n        return 2\n\n    unique_names = {row[0] for row in names}\n    owners = {row[0] for row in names}\n\n    dangling = []\n    seen_dangling = set()\n    for name, record_type, value, _status in names:\n        if record_type != \"CNAME\" or not value:\n            continue\n        target = normalise_host(value)\n        final, path, looped = resolve_chain(target, cname_map)\n        if final in nxdomain:\n            route = \" -> \".join([target] + path)\n            key = (name, target)\n            if key not in seen_dangling:\n                seen_dangling.add(key)\n                dangling.append((name, route, looped))\n\n    cname_owners = set(cname_map)\n    missing = []\n    addressed = set()\n    for name, record_type, value, status in names:\n        if record_type in ADDRESS_TYPES and value and status != \"NXDOMAIN\":\n            addressed.add(name)\n    for name in sorted(unique_names):\n        if name in nxdomain or name in cname_owners or name in addressed:\n            continue\n        if name in owners:\n            missing.append(name)\n\n    outside = []\n    seen_outside = set()\n    for name, record_type, value, status in names:\n        if record_type not in NAMESERVER_TYPES or status == \"NXDOMAIN\":\n            continue\n        host = target_host(record_type, value)\n        if not host or host in nxdomain:\n            continue\n        if not in_scope(host, scopes):\n            key = (name, record_type, host)\n            if key not in seen_outside:\n                seen_outside.add(key)\n                outside.append((name, record_type, host))\n\n    print(\"DNS %d records, %d names, %d dangling, %d without A/AAAA, \"\n          \"%d outside scope\"\n          % (len(names), len(unique_names), len(dangling), len(missing),\n             len(outside)))\n\n    for name, route, looped in dangling[:args.max_list]:\n        suffix = \" (loop)\" if looped else \" (NXDOMAIN)\"\n        print(\"DANGLING %s -> %s%s\" % (name, route, suffix))\n    for name in missing[:args.max_list]:\n        print(\"MISSING %s has no A or AAAA record\" % name)\n    for name, record_type, host in outside[:args.max_list]:\n        print(\"OUTSIDE %s -> %s %s\" % (name, record_type, host))\n\n    return 1 if (dangling or missing or outside) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p724","n":"724","slug":"takeover-before-someone-else-does","title":"Takeover before someone else does","sections":["devsecops"],"topic":"Python scripts","tags":["Python","CyberSecurity","DNS","AttackSurface","BugBounty"],"series_name":"ASM","design":"code","script":"takeover_check.py","date":"2031-05-13","files":{"devsecops":"notes/p724-takeover-before-someone-else-does.md"},"md":"# Takeover before someone else does\n\nblog.acme.com points at acme.github.io. The vendor account was deleted in March, and the GitHub Pages 404 still answers with the sentence that says the name is unclaimed.\n\n## Quick reference\n\nThe full script is at the end of this note: `takeover_check.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 takeover_check.py dns.csv --fingerprints fp.json\n```\n\n**Example output**\n\n```text\nTAKEOVER 5 CNAMEs, 3 matched, 1 confirmed\nVULN blog.acme.com -> acme.github.io (GitHub Pages)\n```\n\n## Explained\n\n### What it does\n\n`takeover_check.py` matches CNAME targets against a JSON fingerprint list: a service, a CNAME suffix and the text the service serves on an unclaimed name. With `--snapshot` it reads a saved HTTP body and looks for the marker. A suffix match is a candidate; the marker makes it a finding.\n\n### What the screen shows\n\nThe fourteen lines load the fingerprints and snapshot, walk each CNAME, match the suffix, test the body and print TAKEOVER 5 CNAMEs, 3 matched, 1 confirmed and VULN blog.acme.com -> acme.github.io (GitHub Pages).\n\n### How to run it\n\n`python3 takeover_check.py dns.csv --fingerprints fingerprints.json --snapshot bodies.json`. Exit 0 is nothing matched or everything cleared, 1 is a confirmed or unverified candidate, 2 is bad input.\n\n### The fix\n\nRemove the CNAME, or claim the name yourself. Removing it is cheaper: the service behind it was retired in March. If a product still needs the hostname, point it at a name you control.\n\n### Prevention\n\nFingerprint every SaaS CNAME when the asset enters the inventory, and run the check on every DNS export. A CNAME to a vendor has a renewal date you do not control.\n\n> **The honest caveat:** a suffix match is not a takeover. The provider may hold the name, or serve a different page, so verify before you page anyone.\n\n> **The lesson:** a CNAME is a promise that a vendor can revoke.\n\n## In one line\n\n> **A CNAME is a promise a vendor can revoke.** The record is yours. The name at the other end is not.\n\n**Check the marker**\n\n```bash\ncurl -sI https://blog.acme.com\n```\n\n## Script: takeover_check.py\n\nDownload: [scripts/takeover_check.py](../scripts/takeover_check.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"takeover_check.py - match CNAME targets against known takeover fingerprints.\n\nA subdomain takeover needs three things: a DNS record that points at a\nthird-party service, a name at that service that nobody has claimed, and a\nprovider that lets the next person claim it. This script checks the first two\nfrom an export, and the third from a saved HTTP body when you have one.\n\nThe DNS input is a CSV export with a header row. Either a two-column export:\n\n    name,target\n    blog.example.com,acme.github.io\n\nor a dig-style export with name, type, value and status, where CNAME rows are\nused and the other rows are ignored.\n\nThe fingerprint file is JSON, a list of objects or an object with a\n\"fingerprints\" list. Each entry is:\n\n    {\"service\": \"GitHub Pages\",\n     \"cname\": \"github.io\",\n     \"marker\": \"There isn't a GitHub Pages site here.\"}\n\ncname is the suffix a target must end with (a leading *. is ignored). marker\nis the text the provider serves on an unclaimed name. A suffix match alone is\na candidate, not a takeover; the marker is the evidence.\n\n--snapshot takes a JSON object mapping hostname to the HTTP body you saved\nfrom that host. With a body, the marker confirms the finding. Without one, a\ncandidate is reported as UNVERIFIED, because it cannot be cleared from an\nexport alone. The script never fetches anything.\n\nUsage:\n    python3 takeover_check.py dns.csv --fingerprints fingerprints.json\n    python3 takeover_check.py dns.csv --fingerprints fingerprints.json --snapshot bodies.json\n    python3 takeover_check.py --help\n\nCron or CI (daily, fail the job on a confirmed or unverified candidate):\n    30 6 * * * python3 /opt/s0sta/takeover_check.py /var/lib/s0sta/dns.csv --fingerprints /opt/s0sta/takeover.json --snapshot /var/lib/s0sta/bodies.json\n\nExit codes: 0 nothing matched or every match was cleared by a snapshot,\n1 at least one confirmed or unverified match, 2 usage error or bad input.\n\nRead-only: opens the given files and prints. It writes nothing, changes\nnothing and makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport json\nimport sys\n\nDEFAULT_MAX_LIST = 50\n# Services that serve a claim page to anyone who registers the name. The\n# marker strings are the ones those services return today; verify them before\n# you trust a VULN line.\nBUILTIN = [\n    {\"service\": \"GitHub Pages\", \"cname\": \"github.io\",\n     \"marker\": \"There isn't a GitHub Pages site here.\"},\n    {\"service\": \"Amazon S3\", \"cname\": \"s3.amazonaws.com\",\n     \"marker\": \"NoSuchBucket\"},\n    {\"service\": \"Heroku\", \"cname\": \"herokuapp.com\",\n     \"marker\": \"No such app\"},\n    {\"service\": \"Azure\", \"cname\": \"azurewebsites.net\",\n     \"marker\": \"404 Web Site not found\"},\n    {\"service\": \"Fastly\", \"cname\": \"fastly.net\",\n     \"marker\": \"Fastly error: unknown domain\"},\n]\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as a CNAME or fingerprint export.\"\"\"\n\n\ndef read_text(path):\n    \"\"\"Return the file as text, or raise InputError.\"\"\"\n    try:\n        with open(path, \"r\", encoding=\"utf-8\") as handle:\n            return handle.read()\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n\n\ndef normalise_host(value):\n    \"\"\"Return a lowercase hostname without the trailing dot.\"\"\"\n    return value.strip().lower().rstrip(\".\")\n\n\ndef normalise_suffix(value):\n    \"\"\"Return a fingerprint suffix without a leading wildcard or dot.\"\"\"\n    suffix = normalise_host(value)\n    if suffix.startswith(\"*.\"):\n        suffix = suffix[2:]\n    return suffix.lstrip(\".\")\n\n\ndef load_cnames(path):\n    \"\"\"Return (name, target) pairs from a CSV export, or raise InputError.\"\"\"\n    try:\n        handle = sys.stdin if path == \"-\" else open(path, \"r\", encoding=\"utf-8\",\n                                                    newline=\"\")\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    try:\n        reader = csv.DictReader(handle)\n        if not reader.fieldnames:\n            raise InputError(\"%s: no header row\" % path)\n        fields = [name.strip().lower() for name in reader.fieldnames]\n        if \"name\" not in fields:\n            raise InputError(\"%s: header needs a name column\" % path)\n        direct = \"target\" in fields or \"cname\" in fields\n        if not direct and not (\"type\" in fields and \"value\" in fields):\n            raise InputError(\"%s: header needs target or type and value\" % path)\n        pairs = []\n        for raw in reader:\n            row = {}\n            for key, value in raw.items():\n                if key is None:\n                    continue\n                row[key.strip().lower()] = (value or \"\").strip()\n            name = normalise_host(row.get(\"name\", \"\"))\n            if not name:\n                continue\n            if direct:\n                target = row.get(\"target\") or row.get(\"cname\") or \"\"\n            else:\n                if (row.get(\"type\", \"\").upper() != \"CNAME\"):\n                    continue\n                target = row.get(\"value\", \"\")\n            target = normalise_host(target)\n            if target:\n                pairs.append((name, target))\n    finally:\n        if handle is not sys.stdin:\n            handle.close()\n    if not pairs:\n        raise InputError(\"%s: no CNAME records\" % path)\n    return pairs\n\n\ndef load_fingerprints(path):\n    \"\"\"Return the fingerprint list, or raise InputError.\"\"\"\n    if not path:\n        return [dict(item) for item in BUILTIN]\n    text = read_text(path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    try:\n        data = json.loads(text)\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if isinstance(data, dict) and isinstance(data.get(\"fingerprints\"), list):\n        data = data[\"fingerprints\"]\n    if not isinstance(data, list):\n        raise InputError(\"%s: expected a JSON list of fingerprints\" % path)\n    items = []\n    for entry in data:\n        if not isinstance(entry, dict):\n            continue\n        service = str(entry.get(\"service\", \"\")).strip()\n        cname = normalise_suffix(str(entry.get(\"cname\", \"\")))\n        marker = str(entry.get(\"marker\", \"\"))\n        if not service or not cname:\n            continue\n        items.append({\"service\": service, \"cname\": cname, \"marker\": marker})\n    if not items:\n        raise InputError(\"%s: no fingerprint has service and cname\" % path)\n    return items\n\n\ndef load_snapshot(path):\n    \"\"\"Return a host to body map, or raise InputError.\"\"\"\n    if not path:\n        return None\n    text = read_text(path)\n    if not text.strip():\n        raise InputError(\"%s: empty input\" % path)\n    try:\n        data = json.loads(text)\n    except ValueError as exc:\n        raise InputError(\"%s is not JSON: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise InputError(\"%s: expected a JSON object of host to body\" % path)\n    bodies = {}\n    for host, body in data.items():\n        if isinstance(body, str):\n            bodies[normalise_host(host)] = body\n    if not bodies:\n        raise InputError(\"%s: no host bodies\" % path)\n    return bodies\n\n\ndef matches(target, suffix):\n    \"\"\"True when target is the suffix or a name under it.\"\"\"\n    return target == suffix or target.endswith(\".\" + suffix)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Match CNAME targets against takeover fingerprints and \"\n                    \"confirm with a saved HTTP body.\")\n    parser.add_argument(\"dns\", help=\"CSV export with name,target or dig columns\")\n    parser.add_argument(\"--fingerprints\", metavar=\"JSON\",\n                        help=\"fingerprint JSON; without it a small built-in list is used\")\n    parser.add_argument(\"--snapshot\", metavar=\"JSON\",\n                        help=\"JSON object mapping host to a saved HTTP body\")\n    parser.add_argument(\"--max-list\", type=int, default=DEFAULT_MAX_LIST,\n                        metavar=\"N\", help=\"print at most N lines per state \"\n                                          \"(default 50)\")\n    args = parser.parse_args(argv)\n    if args.max_list < 1:\n        print(\"takeover_check: --max-list must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        pairs = load_cnames(args.dns)\n        fingerprints = load_fingerprints(args.fingerprints)\n        snapshot = load_snapshot(args.snapshot)\n    except InputError as exc:\n        print(\"takeover_check: %s\" % exc, file=sys.stderr)\n        return 2\n\n    confirmed = []\n    unverified = []\n    cleared = []\n    for name, target in pairs:\n        for fingerprint in fingerprints:\n            if not matches(target, fingerprint[\"cname\"]):\n                continue\n            body = snapshot.get(name) if snapshot is not None else None\n            if body is None:\n                unverified.append((name, target, fingerprint[\"service\"]))\n            elif fingerprint[\"marker\"] and fingerprint[\"marker\"] in body:\n                confirmed.append((name, target, fingerprint[\"service\"]))\n            else:\n                cleared.append((name, target, fingerprint[\"service\"]))\n            break\n\n    matched = len(confirmed) + len(unverified) + len(cleared)\n    print(\"TAKEOVER %d CNAMEs, %d matched, %d confirmed\"\n          % (len(pairs), matched, len(confirmed)))\n\n    if not matched:\n        print(\"OK no CNAME matches a known fingerprint\")\n        return 0\n\n    for name, target, service in confirmed[:args.max_list]:\n        print(\"VULN %s -> %s (%s)\" % (name, target, service))\n    for name, target, service in unverified[:args.max_list]:\n        print(\"UNVERIFIED %s -> %s (%s, no snapshot)\"\n              % (name, target, service))\n    for name, target, service in cleared[:args.max_list]:\n        print(\"CLEARED %s -> %s (%s, marker absent)\"\n              % (name, target, service))\n\n    return 1 if (confirmed or unverified) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p725","n":"725","slug":"the-asset-nobody-owns","title":"The asset nobody owns","sections":["devsecops"],"topic":"Python scripts","tags":["Python","CyberSecurity","ITAM","AttackSurface","DevOps"],"series_name":"ASM","design":"code","script":"owner_map.py","date":"2031-05-15","files":{"devsecops":"notes/p725-the-asset-nobody-owns.md"},"md":"# The asset nobody owns\n\nThe scanner finds a host called blog.acme.com. Three people in the room recognise it. None of them will say they own it, and the CMDB row has a blank owner.\n\n## Quick reference\n\nThe full script is at the end of this note: `owner_map.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 owner_map.py assets.csv cmdb.csv\n```\n\n**Example output**\n\n```text\nOWNER 5 assets, 1 unowned, 1 in conflict\nCONFLICT api.acme.com tag bob, cmdb erin\n```\n\n## Explained\n\n### What it does\n\n`owner_map.py` joins an asset inventory to a CMDB export on the asset name. The inventory carries tags, and owner:alice is the one that matters. An asset is unowned when there is no row or the owner is blank. It is in conflict when the CMDB lists two owners, or when the tag and the CMDB disagree.\n\n### What the screen shows\n\nThe fourteen lines read both CSVs, build the owner and tag maps, compare them and print OWNER 5 assets, 1 unowned, 1 in conflict and CONFLICT api.acme.com tag bob, cmdb erin.\n\n### How to run it\n\n`python3 owner_map.py assets.csv cmdb.csv`. Exit 0 is one agreed owner per asset, 1 is a finding, 2 is bad input. Run it weekly, and before any access review.\n\n### The fix\n\nFor blog.acme.com, pick a person and a cost centre, then write both in the CMDB. For api.acme.com, decide whether bob or erin owns it: the tag came from the builder, the CMDB from the payer.\n\n### Prevention\n\nMake owner a required field at asset creation, and generate the tag from the CMDB instead of asking teams to type it twice.\n\n> **The honest caveat:** the owner field is not accountability. It only means something when the person agrees to patch, pay for or delete the asset.\n\n> **The lesson:** discovery finds the asset, and ownership is a human decision.\n\n## In one line\n\n> **Discovery finds it. Someone must own it.** An asset with no owner has no patch window and no budget.\n\n**Ask who owns it**\n\n```bash\npython3 owner_map.py assets.csv cmdb.csv\n```\n\n## Script: owner_map.py\n\nDownload: [scripts/owner_map.py](../scripts/owner_map.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"owner_map.py - join an asset inventory to a CMDB and find the assets nobody owns.\n\nDiscovery tells you a host exists. It does not tell you who patches it, who\npays for it, or who agrees to the risk. That answer lives in the CMDB, and the\njoin between the two is where asset management usually fails: a host with no\nrow, a row with a blank owner, a tag that names one person while the CMDB\nnames another.\n\nThis script reads two CSV exports and joins them on the asset column.\n\nAssets (what discovery found), with a tags column of semicolon-separated\nkey:value pairs; owner:alice is the one that matters here:\n\n    asset,tags\n    www.example.com,owner:alice;env:prod;cc:CC-104\n    blog.example.com,env:prod\n\nCMDB (what the organisation thinks it owns):\n\n    asset,owner,team,cost_centre\n    www.example.com,alice,web,CC-104\n    api.example.com,erin,api,CC-207\n\nFindings are:\n\n    unowned    an asset with no CMDB row, or a row whose owner is blank or\n               one of the placeholders -, unknown, none, tbd or n/a\n    conflict   two different non-empty owners in the CMDB, or a tag owner\n               that disagrees with the single CMDB owner\n\nThe cost centre is printed with each conflict so the finding has a budget\nattached. Entries that exist only in the CMDB are out of scope here; that is\nthe stale side of the comparison, and shadow_it.py reports it.\n\nUsage:\n    python3 owner_map.py assets.csv cmdb.csv\n    python3 owner_map.py assets.csv cmdb.csv --max-list 10\n    python3 owner_map.py --help\n\nCron or CI (weekly, fail the job when an asset has no owner):\n    0 7 * * 1 python3 /opt/s0sta/owner_map.py /var/lib/s0sta/assets.csv /var/lib/s0sta/cmdb.csv\n\nExit codes: 0 every asset has one agreed owner, 1 at least one unowned asset\nor conflict, 2 usage error, unreadable input or missing columns.\n\nRead-only: opens the two CSVs and prints. It writes nothing, changes nothing\nand makes no network call.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport sys\n\nDEFAULT_MAX_LIST = 50\nPLACEHOLDERS = {\"\", \"-\", \"unknown\", \"none\", \"tbd\", \"n/a\", \"na\", \"null\"}\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as an inventory or CMDB export.\"\"\"\n\n\ndef read_csv(path, required):\n    \"\"\"Return rows as lowercase-key dicts, or raise InputError.\"\"\"\n    try:\n        handle = sys.stdin if path == \"-\" else open(path, \"r\", encoding=\"utf-8\",\n                                                    newline=\"\")\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    try:\n        reader = csv.DictReader(handle)\n        if not reader.fieldnames:\n            raise InputError(\"%s: no header row\" % path)\n        fields = [name.strip().lower() for name in reader.fieldnames]\n        for column in required:\n            if column not in fields:\n                raise InputError(\"%s: header needs a %s column\" % (path, column))\n        rows = []\n        for raw in reader:\n            row = {}\n            for key, value in raw.items():\n                if key is None:\n                    continue\n                row[key.strip().lower()] = (value or \"\").strip()\n            if row.get(\"asset\"):\n                rows.append(row)\n    finally:\n        if handle is not sys.stdin:\n            handle.close()\n    if not rows:\n        raise InputError(\"%s: no usable rows\" % path)\n    return rows\n\n\ndef tag_owners(tags):\n    \"\"\"Return the owner values in a semicolon-separated tag string.\"\"\"\n    owners = []\n    for tag in tags.split(\";\"):\n        tag = tag.strip()\n        if not tag:\n            continue\n        key, _, value = tag.partition(\":\")\n        if key.strip().lower() == \"owner\" and value.strip():\n            owners.append(value.strip())\n    return owners\n\n\ndef real_owner(value):\n    \"\"\"Return the owner, or an empty string for a placeholder.\"\"\"\n    value = (value or \"\").strip()\n    return \"\" if value.lower() in PLACEHOLDERS else value\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Join an asset inventory to a CMDB and report unowned \"\n                    \"assets and ownership conflicts.\")\n    parser.add_argument(\"assets\", help=\"inventory CSV with asset and tags columns\")\n    parser.add_argument(\"cmdb\", help=\"CMDB CSV with asset, owner, team, cost_centre\")\n    parser.add_argument(\"--max-list\", type=int, default=DEFAULT_MAX_LIST,\n                        metavar=\"N\", help=\"print at most N lines per finding \"\n                                          \"(default 50)\")\n    args = parser.parse_args(argv)\n    if args.max_list < 1:\n        print(\"owner_map: --max-list must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        asset_rows = read_csv(args.assets, (\"asset\", \"tags\"))\n        cmdb_rows = read_csv(args.cmdb, (\"asset\", \"owner\"))\n    except InputError as exc:\n        print(\"owner_map: %s\" % exc, file=sys.stderr)\n        return 2\n\n    inventory = []\n    tags_by_asset = {}\n    for row in asset_rows:\n        asset = row[\"asset\"].strip().lower()\n        if not asset:\n            continue\n        if asset not in tags_by_asset:\n            inventory.append(asset)\n            tags_by_asset[asset] = row.get(\"tags\", \"\")\n        else:\n            tags_by_asset[asset] += \";\" + row.get(\"tags\", \"\")\n\n    cmdb = {}\n    for row in cmdb_rows:\n        asset = row[\"asset\"].strip().lower()\n        owner = real_owner(row.get(\"owner\"))\n        cost = row.get(\"cost_centre\", \"\") or \"-\"\n        cmdb.setdefault(asset, []).append((owner, cost))\n\n    unowned = []\n    conflicts = []\n    for asset in inventory:\n        rows = cmdb.get(asset, [])\n        owners = {owner for owner, _cost in rows if owner}\n        cost = rows[0][1] if rows else \"-\"\n        shares = tag_owners(tags_by_asset.get(asset, \"\"))\n        tag = shares[0] if shares else \"\"\n\n        if not rows:\n            unowned.append((asset, \"not in CMDB\"))\n            continue\n        if not owners:\n            unowned.append((asset, \"no owner in CMDB\"))\n            continue\n        if len(owners) > 1:\n            conflicts.append((asset, \"cmdb \" + \", \".join(sorted(owners)), cost))\n            continue\n        only = sorted(owners)[0]\n        if len(set(shares)) > 1:\n            conflicts.append((asset, \"tag \" + \", \".join(sorted(set(shares))),\n                              cost))\n        elif tag and tag.lower() != only.lower():\n            conflicts.append((asset, \"tag %s, cmdb %s\" % (tag, only), cost))\n\n    print(\"OWNER %d assets, %d unowned, %d in conflict\"\n          % (len(inventory), len(unowned), len(conflicts)))\n\n    for asset, why in unowned[:args.max_list]:\n        print(\"UNOWNED %s (%s)\" % (asset, why))\n    for asset, why, cost in conflicts[:args.max_list]:\n        print(\"CONFLICT %s %s (%s)\" % (asset, why, cost))\n\n    return 1 if (unowned or conflicts) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p726","n":"726","slug":"shadow-it-in-your-own-account","title":"Shadow IT in your own account","sections":["devsecops"],"topic":"Python scripts","tags":["Python","CloudSecurity","ITAM","ShadowIT","CyberSecurity"],"series_name":"ASM","design":"code","script":"shadow_it.py","date":"2031-05-18","files":{"devsecops":"notes/p726-shadow-it-in-your-own-account.md"},"md":"# Shadow IT in your own account\n\nOn Friday a developer creates a database in the shared cloud account. On Monday it holds customer data and appears in no CMDB.\n\n## Quick reference\n\nThe full script is at the end of this note: `shadow_it.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 shadow_it.py found.csv cmdb.csv --today 2026-10-21\n```\n\n**Example output**\n\n```text\nSHADOW 5 discovered, 2 unknown, 2 stale\nUNKNOWN api-lab.acme.com first seen 2026-10-14 (7d)\n```\n\n## Explained\n\n### What it does\n\n`shadow_it.py` compares two exports in both directions. Discovered assets carry first_seen; CMDB entries carry added. An asset in discovery but not the CMDB is unknown, and its age is the exposure clock. An asset in the CMDB but not discovery is stale, and its age is how long the record has been wrong.\n\n### What the screen shows\n\nThe fourteen lines build the two sets, age each gap from a pinned date and print SHADOW 5 discovered, 2 unknown, 2 stale and UNKNOWN api-lab.acme.com first seen 2026-10-14 (7d).\n\n### How to run it\n\n`python3 shadow_it.py discovered.csv cmdb.csv --today 2026-10-21`. Exit 0 is agreement, 1 is a gap, 2 is a bad date. Run it daily: the age of an unknown asset should drive the ticket.\n\n### The fix\n\napi-lab is seven days old and needs an owner today, not at the next audit. old.acme.com has been in the CMDB for 963 days and answers nothing, so delete the record.\n\n### Prevention\n\nReconcile both ways on a schedule and make the CMDB the place a resource is created, not a spreadsheet updated afterwards. Discovery always runs ahead of registration; the gap is the risk.\n\n> **The honest caveat:** discovery only sees what it can reach. An asset the scanner cannot route to will look stale, and deleting its record makes the gap worse.\n\n> **The lesson:** the CMDB is a claim, and discovery is the evidence.\n\n## In one line\n\n> **The CMDB is a claim. Discovery is evidence.** One side records intent. The other records what answers.\n\n**Reconcile weekly**\n\n```bash\npython3 shadow_it.py found.csv cmdb.csv\n```\n\n## Script: shadow_it.py\n\nDownload: [scripts/shadow_it.py](../scripts/shadow_it.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"shadow_it.py - compare discovered assets with the CMDB in both directions.\n\nA scanner finds what answers. A CMDB records what the organisation believes it\nowns. The gap between them has two sides, and both are findings:\n\n    unknown    an asset that answers but has no CMDB entry; first_seen is the\n               day discovery first saw it, so the age is the exposure clock\n    stale      a CMDB entry that no longer answers; added is the day it was\n               registered, so the age is how long the record has been wrong\n\nThis script reads two CSV exports and reports both sides with an age in days.\n\nDiscovered:\n\n    asset,first_seen\n    www.example.com,2026-01-04\n    api-lab.example.com,2026-10-14\n\nCMDB:\n\n    asset,owner,added\n    www.example.com,alice,2025-06-01\n    old.example.com,frank,2024-03-02\n\nDates are ISO dates or ISO timestamps; only the first ten characters are\nused. --today pins the clock so a report is reproducible. A future date is\nclamped to zero days rather than printed as a negative age.\n\nUsage:\n    python3 shadow_it.py discovered.csv cmdb.csv\n    python3 shadow_it.py discovered.csv cmdb.csv --today 2026-10-21\n    python3 shadow_it.py discovered.csv cmdb.csv --max-list 10\n    python3 shadow_it.py --help\n\nCron or CI (daily, fail the job on either side of the gap):\n    45 6 * * * python3 /opt/s0sta/shadow_it.py /var/lib/s0sta/discovered.csv /var/lib/s0sta/cmdb.csv --today \"$(date +%F)\"\n\nExit codes: 0 the two lists agree, 1 at least one unknown or stale asset,\n2 usage error, unreadable input, missing columns or a bad date.\n\nRead-only: opens the two CSVs and prints. It writes nothing, changes nothing\nand makes no network call. Pairs with owner_map.py: this finds the gap, that\none finds who should close it.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport sys\nfrom datetime import date\n\nDEFAULT_MAX_LIST = 50\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be read as an asset export.\"\"\"\n\n\ndef read_csv(path, required):\n    \"\"\"Return rows as lowercase-key dicts, or raise InputError.\"\"\"\n    try:\n        handle = sys.stdin if path == \"-\" else open(path, \"r\", encoding=\"utf-8\",\n                                                    newline=\"\")\n    except OSError as exc:\n        raise InputError(\"%s: %s\" % (path, exc.strerror or exc))\n    try:\n        reader = csv.DictReader(handle)\n        if not reader.fieldnames:\n            raise InputError(\"%s: no header row\" % path)\n        fields = [name.strip().lower() for name in reader.fieldnames]\n        for column in required:\n            if column not in fields:\n                raise InputError(\"%s: header needs a %s column\" % (path, column))\n        rows = []\n        for raw in reader:\n            row = {}\n            for key, value in raw.items():\n                if key is None:\n                    continue\n                row[key.strip().lower()] = (value or \"\").strip()\n            if row.get(\"asset\"):\n                rows.append(row)\n    finally:\n        if handle is not sys.stdin:\n            handle.close()\n    if not rows:\n        raise InputError(\"%s: no usable rows\" % path)\n    return rows\n\n\ndef parse_day(value, where):\n    \"\"\"Return a date from an ISO date or timestamp, or raise InputError.\"\"\"\n    text = (value or \"\").strip()\n    if not text:\n        raise InputError(\"%s: missing date\" % where)\n    try:\n        return date.fromisoformat(text[:10])\n    except ValueError:\n        raise InputError(\"%s: bad date %r\" % (where, value))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Compare discovered assets with the CMDB and report \"\n                    \"unknown assets and stale entries with their age.\")\n    parser.add_argument(\"discovered\", help=\"CSV with asset and first_seen columns\")\n    parser.add_argument(\"cmdb\", help=\"CSV with asset, owner and added columns\")\n    parser.add_argument(\"--today\", metavar=\"YYYY-MM-DD\",\n                        help=\"pin the report date (default: today, UTC)\")\n    parser.add_argument(\"--max-list\", type=int, default=DEFAULT_MAX_LIST,\n                        metavar=\"N\", help=\"print at most N lines per side \"\n                                          \"(default 50)\")\n    args = parser.parse_args(argv)\n    if args.max_list < 1:\n        print(\"shadow_it: --max-list must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        discovered_rows = read_csv(args.discovered, (\"asset\", \"first_seen\"))\n        cmdb_rows = read_csv(args.cmdb, (\"asset\", \"added\"))\n        today = parse_day(args.today, \"--today\") if args.today else date.today()\n\n        discovered = {}\n        for row in discovered_rows:\n            asset = row[\"asset\"].strip().lower()\n            seen = parse_day(row.get(\"first_seen\"), \"%s first_seen\" % asset)\n            if asset not in discovered or seen < discovered[asset]:\n                discovered[asset] = seen\n\n        cmdb = {}\n        for row in cmdb_rows:\n            asset = row[\"asset\"].strip().lower()\n            added = parse_day(row.get(\"added\"), \"%s added\" % asset)\n            if asset not in cmdb or added < cmdb[asset]:\n                cmdb[asset] = added\n    except InputError as exc:\n        print(\"shadow_it: %s\" % exc, file=sys.stderr)\n        return 2\n\n    unknown = []\n    for asset in sorted(discovered):\n        if asset in cmdb:\n            continue\n        age = (today - discovered[asset]).days\n        unknown.append((asset, discovered[asset].isoformat(), max(0, age)))\n\n    stale = []\n    for asset in sorted(cmdb):\n        if asset in discovered:\n            continue\n        age = (today - cmdb[asset]).days\n        stale.append((asset, cmdb[asset].isoformat(), max(0, age)))\n    stale.sort(key=lambda item: (-item[2], item[0]))\n\n    print(\"SHADOW %d discovered, %d unknown, %d stale\"\n          % (len(discovered), len(unknown), len(stale)))\n\n    for asset, seen, age in unknown[:args.max_list]:\n        print(\"UNKNOWN %s first seen %s (%dd)\" % (asset, seen, age))\n    for asset, added, age in stale[:args.max_list]:\n        print(\"STALE %s in CMDB since %s (%dd)\" % (asset, added, age))\n\n    return 1 if (unknown or stale) else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p727","n":"727","slug":"the-surface-map","title":"The surface map","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","ASM","DevSecOps","CloudSecurity"],"series_name":"ASM","design":"architecture","script":"","date":"2031-05-20","files":{"devsecops":"notes/p727-the-surface-map.md"},"md":"# The surface map\n\nEvery attack surface programme starts with the same argument: the scanner says one thing, the CMDB another, and the spreadsheet a third. The map that ends it has three tiers, each answering a different question.\n\n## Quick reference\n\n- **Discover:** Certificate log (CT) · DNS export (DNS) · Port scan (PORT)\n- **Inventory:** CMDB (CMDB) · Owner map (OWN) · Cost centre (COST)\n- **Score:** Exposure (EXPO) · Risk (RISK)\n\n**Request flow:** Certificate log → CMDB → Owner map → Exposure → Risk; CMDB → Cost centre → Risk; DNS export → CMDB; Port scan → CMDB\n\n**Around it:** crt.sh export (JSON) · dig export (CSV) · CMDB export (CSV) · Owner tag (TAG)\n\n## Explained\n\n### Discover\n\nThe bottom tier is evidence. Certificate transparency gives you every name a CA has ever issued for, including the staging host that never reached DNS. The DNS export gives you the records that exist now, with their status. A port scan gives you what answers on the ranges you own. No source trusts another, which is the point: three independent views of the same estate.\n\n### Inventory\n\nThe middle tier turns names into assets. The CMDB says what the organisation believes it owns, the owner map says who agreed to it, and the cost centre says who pays. Most programmes stall here, because a host with no owner cannot be prioritised, patched or deleted.\n\n### Score\n\nThe top tier is a decision. Exposure asks what the internet can reach, and risk asks what happens if it is reached. Both need the two tiers below: exposure without inventory is a port list, and risk without an owner is a number nobody accepts.\n\nThe tiers are a loop, not a stack. A score nobody owns goes back to inventory, and an unowned asset goes back to discovery until someone claims it.\n\n> **The honest caveat:** the map is only as good as the exports. A tier that reads a stale CSV produces a confident answer about last month.\n\n> **The lesson:** discover, attribute, score, in that order, and never skip the middle.\n\n## In one line\n\n> **Discover, attribute, then score.** Three tiers, one loop, and no tier trusts the one below it.\n\n**Start at discover**\n\n```bash\npython3 ct_inventory.py ct.json\n```\n"},{"id":"p728","n":"728","slug":"surface-discovery-commands","title":"Surface discovery commands","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","Linux","AWS","CommandLine","AttackSurface"],"series_name":"ASM","design":"cheatsheet","script":"","date":"2031-05-22","files":{"devsecops":"notes/p728-surface-discovery-commands.md"},"md":"# Surface discovery commands\n\nSurface discovery is four command families, and most teams are good at one. The DNS team knows dig, the cloud team knows its CLI, the pentester knows nmap, and nobody owns the certificate log.\n\n## Quick reference\n\n### DNS\n\n```bash\ndig +short acme.com A\ndig +short acme.com MX\ndig +short acme.com NS\ndig +trace acme.com\ndig +short acme.com TXT\n```\n\n### CT\n\n```bash\ncurl -s 'https://crt.sh/?q=acme.com'\njq -r '.[].name_value' ct.json\njq -r '.[].name_value' ct.json | sort -u\nsort -u ct_hosts.txt\ngrep -v '^\\*' ct_hosts.txt\n```\n\n### Ports\n\n```bash\nnmap -sn 10.0.0.0/24\nnmap -sV --top-ports 100 10.0.0.5\nnc -zv api.acme.com 443\nss -tulpn | grep LISTEN\nmasscan -p443 10.0.0.0/24 --rate 1000\n```\n\n### Cloud\n\n```bash\naws s3api list-buckets\naws ec2 describe-instances --output table\naws route53 list-hosted-zones\ngcloud compute instances list\naz vm list -d -o table\n```\n\n## Explained\n\n### DNS\n\n`dig +short acme.com A` gives addresses, and the MX and NS queries name the mail and DNS providers. `dig +trace` follows the delegation from the root, which is how you find the zone nobody mentioned. TXT is where SPF, DMARC and verification tokens live, and each token names a service.\n\n### CT\n\n`curl -s 'https://crt.sh/?q=acme.com'` returns the certificate log; add output=json to the query and pipe it to `jq -r '.[].name_value'` for hostnames. `sort -u` collapses the duplicates, and `grep -v '^\\*'` removes the wildcards. The result is every name a CA has issued for, including ones that never resolved.\n\n### Ports\n\n`nmap -sn` finds hosts, and `nmap -sV --top-ports 100` names the services on one. `nc -zv` checks a single port in a script. `ss -tulpn` shows what the host listens on, often more than the firewall allows.\n\n### Cloud\n\n`aws s3api list-buckets`, `aws ec2 describe-instances --output table` and `aws route53 list-hosted-zones` are read-only and need only the permissions you already have. `gcloud compute instances list` and `az vm list -d -o table` do the same elsewhere. The cloud inventory is usually complete and usually unread.\n\n> **The honest caveat:** a command that returns nothing is not proof of absence. It is proof that this credential, from this network, saw nothing.\n\n> **The lesson:** four families, one map, and the gap between them is the finding.\n\n## In one line\n\n> **Four families, one map.** The gap between the four answers is the finding.\n\n**Read the log**\n\n```bash\ncurl -s https://crt.sh/?q=acme.com\n```\n"},{"id":"p729","n":"729","slug":"four-surface-blind-spots","title":"Four surface blind spots","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","ASM","CloudSecurity","ITAM"],"series_name":"ASM","design":"explainer","script":"","date":"2031-05-25","files":{"devsecops":"notes/p729-four-surface-blind-spots.md"},"md":"# Four surface blind spots\n\nFour assets stay invisible after a scan, and none of them is a scanner problem. They are process problems that leave a record behind, and each has a different fix.\n\n## Quick reference\n\n### Forgotten subdomain\n\n- A staging host from a 2023 pilot, still resolving\n- It never reached the CMDB or the pentest scope\n- CT shows it the day its certificate is issued\n- Fix: delete the record, or give it an owner\n\n### Dangling CNAME\n\n- The record points at a SaaS name nobody claimed\n- The provider serves a claim page to anyone\n- A suffix match is a lead; the marker is proof\n- Fix: remove the CNAME, or claim the name first\n\n### Unowned asset\n\n- The scanner sees it; no team will admit to it\n- No owner means no patch window and no budget\n- A cost centre is not an owner, it is a chargeback\n- Fix: name a person, not a mailbox\n\n### Shadow account\n\n- A shared cloud account with no CMDB entry\n- Discovered assets run ahead of registered ones\n- Age since first seen is the exposure clock\n- Fix: reconcile weekly, and reconcile both ways\n\n> **Note:** Four ways an asset hides after you have already scanned it.\n\n## Explained\n\n### Forgotten subdomain\n\nA staging host from a 2023 pilot still resolves and accepts logins. It never reached the CMDB because the pilot had no ticket, and never reached a pentest scope because the scope came from the CMDB. Certificate transparency finds it the day the certificate is issued.\n\n### Dangling CNAME\n\nThe record points at a SaaS name nobody claimed. The provider serves a claim page to anyone who asks, and the subdomain comes with it. A suffix match is a lead, not a finding: the marker on the claim page is the evidence.\n\n### Unowned asset\n\nThe scanner sees it, and no team will admit to it. No owner means no patch window, no budget and no one to accept the risk. A cost centre is a chargeback code, not an owner.\n\n### Shadow account\n\nA shared cloud account with no CMDB entry, where discovery runs ahead of registration. The age since first seen is the exposure clock, and an unreviewed account is where a test database becomes customer data.\n\n> **The honest caveat:** two of these can be true at once. The forgotten subdomain is usually the dangling CNAME, because the service behind it was retired first.\n\n> **The lesson:** the scan finds the asset, and the process decides whether anyone owns it.\n\n## In one line\n\n> **Scanned is not the same as seen.** A scan finds the asset; a process decides who owns it.\n\n**Delete or own**\n\n```bash\ndig +short old.acme.com\n```\n"},{"id":"p730","n":"730","slug":"9-steps-to-map-your-attack-surface","title":"9 steps to map your attack surface","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","ASM","Python","CloudSecurity"],"series_name":"ASM","design":"steps","script":"","date":"2031-05-27","files":{"devsecops":"notes/p730-9-steps-to-map-your-attack-surface.md"},"md":"# 9 steps to map your attack surface\n\nMapping an attack surface looks like a scanning problem and behaves like a bookkeeping problem. Nine commands, three groups, one order: find what answers, give it a name and an owner, then prove the finding before you close it.\n\n## Quick reference\n\n### Discover — Find what answers first\n\n```bash\n# Start at the zone, not the scanner.\ndig +short acme.com NS\n# Every certificate issued for the name.\ncurl -s 'https://crt.sh/?q=acme.com'\n# Hosts that answer on your own ranges.\nnmap -sn 10.0.0.0/24 -oG - | grep Up\n```\n\n### Attribute — Give every asset a person\n\n```bash\n# Tag owner against CMDB owner.\npython3 owner_map.py assets.csv cmdb.csv\n# Unknown assets, stale CMDB rows.\npython3 shadow_it.py found.csv cmdb.csv\n# What the cloud says it owns.\naws resourcegroupstaggingapi get-resources\n```\n\n### Verify — Prove it, then close it\n\n```bash\n# Suffix match, then the marker.\npython3 takeover_check.py dns.csv fp.json\n# The record that should not exist.\ndig +short blog.acme.com CNAME\n# What the target answers today.\ncurl -sI https://blog.acme.com | head -1\n```\n\n## Explained\n\n### Discover\n\nThe terminal starts at the zone. `dig +short acme.com NS` names the DNS provider, which is where the records actually live. `curl -s 'https://crt.sh/?q=acme.com'` returns the certificate log, the only inventory that includes names that never resolved. `nmap -sn 10.0.0.0/24 -oG - | grep Up` lists the hosts that answer on your own ranges, which is the part the cloud console forgets.\n\n### Attribute\n\n`python3 owner_map.py assets.csv cmdb.csv` compares the tag owner with the CMDB owner and names the assets with neither. `python3 shadow_it.py found.csv cmdb.csv` reports unknown assets and stale rows with an age, so the ticket has a number. `aws resourcegroupstaggingapi get-resources` is what the cloud says it owns, which is often a shorter list than the bill.\n\n### Verify\n\n`python3 takeover_check.py dns.csv fp.json` matches CNAMEs against fingerprints and confirms the claim page marker. `dig +short blog.acme.com CNAME` is the single record that should not exist. `curl -sI https://blog.acme.com | head -1` is what the target answers today, and that answer is the evidence.\n\n> **The honest caveat:** the loop is weekly, not annual. A surface map older than a quarter describes a company that no longer exists.\n\n> **The lesson:** discover, attribute, verify, and let the exit code decide when the work is done.\n\n## In one line\n\n> **Discover, attribute, then verify.** Three groups, nine commands, one surface map.\n\n**Map it weekly**\n\n```bash\npython3 ct_inventory.py ct.json\n```\n"},{"id":"p731","n":"731","slug":"quiz-3-surface-questions","title":"Quiz: 3 surface questions","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","ASM","DNS","Python"],"series_name":"ASM","design":"quiz","script":"","date":"2031-05-29","files":{"devsecops":"notes/p731-quiz-3-surface-questions.md"},"md":"# Quiz: 3 surface questions\n\nThree questions close phase one. Each one comes from a tool earlier in the phase, and each answer is the mechanism behind the tool.\n\n## Quick reference\n\n**Q1. Why does certificate transparency find hosts that DNS brute force misses?**\n\n- a) It records names at issuance\n- b) It resolves wildcards before DNS does\n- c) Brute force uses the wrong wordlist\n\n> **Answer:** a) It records names at issuance. A cert is issued for a name before, or without, a public A record, so CT keeps staging names a wordlist never guesses. It only sees names that have a cert.\n\n**Q2. What makes a CNAME takeover possible?**\n\n- a) The target name is unclaimed at the provider\n- b) The CNAME exceeds 255 characters\n- c) The zone uses DNSSEC without NSEC3\n\n> **Answer:** a) The target name is unclaimed at the provider. The record points at a third-party name nobody claimed, and the provider lets the next person claim it. If the provider reserves the name, there is no takeover.\n\n**Q3. Why is ownership the hard part of asset management?**\n\n- a) Scanners cannot see most assets\n- b) An owner is a person who accepts the risk\n- c) CMDB exports are hard to parse\n\n> **Answer:** b) An owner is a person who accepts the risk. Discovery is a scan and produces facts. Ownership is a commitment: a person agrees to patch, pay for or delete the asset. A tag can be added without agreement.\n\n## Explained\n\n### Question one\n\nWhy does certificate transparency find hosts that DNS brute force misses? A certificate is issued for a name before, or without, a public A record. CT records the name at issuance, so a staging host that never resolved is still in the log. A wordlist only finds names someone guessed; the log finds names a CA was asked to sign.\n\n### Question two\n\nWhat makes a CNAME takeover possible? The record points at a third-party name that nobody has claimed, and the provider lets the next person claim it. If the provider reserves the name, the record is stale but not takeoverable. That is why the marker matters more than the suffix: it proves the claim page is being served.\n\n### Question three\n\nWhy is ownership the hard part? Discovery is a scan, and a scan produces facts. Ownership is a commitment, and a commitment produces a person who agrees to patch, pay for or delete the asset. A tag or a cost centre can be filled in without anyone agreeing, which is why an asset can be fully documented and still unowned.\n\n> **The honest caveat:** no single tool in this phase proves the surface is complete. Three independent views, CT, DNS and the cloud API, give you a defensible map, not a perfect one.\n\n> **The lesson:** the map is a set of questions, and the answers are people.\n\n## In one line\n\n> **Three questions, one surface.** The map is only as good as the people who answer for it.\n\n**Check your answer**\n\n```bash\ndig +short blog.acme.com CNAME\n```\n"},{"id":"p732","n":"732","slug":"the-admin-panel-on-the-internet","title":"The admin panel on the internet","sections":["devsecops"],"topic":"Python scripts","tags":["CyberSecurity","AttackSurface","Python","AppSec","CloudSecurity"],"series_name":"ASM","design":"code","script":"exposed_admin.py","date":"2031-06-01","files":{"devsecops":"notes/p732-the-admin-panel-on-the-internet.md"},"md":"# The admin panel on the internet\n\nThe quarterly probe lists 4,812 URLs. Line 3,104 says 200 on /admin, and has since February.\n\n## Quick reference\n\nThe full script is at the end of this note: `exposed_admin.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 exposed_admin.py /tmp/probe.json\n```\n\n**Example output**\n\n```text\nFINDING 4 panels reachable without auth\nALERT admin 60 https://app.example.com/admin\n```\n\n## Explained\n\n### What it does\n\n`exposed_admin.py` reads that export, JSON or CSV, with url, status, title and headers. It classifies each row as admin, jenkins, grafana, phpmyadmin or login, drops anything that is not a 200, and drops any 200 that carries a WWW-Authenticate challenge. What remains is ranked by product weight plus 20 for an open response: admin 40, jenkins 38, grafana 36, phpmyadmin 34, login 20. The weights are a policy choice; the attached tool prints them as [40+20]. The fourteen lines print FINDING 4 panels reachable without auth and ALERT admin 60 https://app.example.com/admin.\n\n### How to run it\n\n`python3 exposed_admin.py probe.json --top 5`. In cron: `0 6 * * * python3 exposed_admin.py /srv/exports/http-probe.json`. Exit 0 is clean, 1 is a finding, 2 is bad input. It writes nothing.\n\n### The fix\n\nOpen the URL in a clean browser, not with your admin cookie. If the dashboard loads, put it behind SSO or a VPN, or remove the DNS record. If only the login form loads, the finding is reachability, not a bypass; record it and close the exposure anyway.\n\n### Prevention\n\nKeep the panel list in the CMDB, probe weekly and alert on any new 200. The one that appears between quarters is the one nobody owns.\n\n> **The honest caveat:** 200 proves the panel answered, not that auth is missing. A login form can be the only guard, and a 403 can be a WAF.\n\n> **The lesson:** reachability is the first fact and authorisation is the second.\n\n## In one line\n\n> **Reachable first, authorised second.** A 200 proves the panel answered. It does not prove the door is unlocked.\n\n**Probe the panel**\n\n```bash\ncurl -sI https://app.example.com/admin\n```\n\n## Script: exposed_admin.py\n\nDownload: [scripts/exposed_admin.py](../scripts/exposed_admin.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank admin and login panels that an HTTP probe saw reachable.\n\nUsage:\n    python3 exposed_admin.py probe.json\n    python3 exposed_admin.py probe.csv --top 5\n\nInput (an export, never a live request):\n    JSON: [{\"url\": \"...\", \"status\": 200, \"title\": \"Jenkins\",\n            \"headers\": {\"server\": \"nginx\"}}, ...]\n          or {\"probes\": [...]} / {\"results\": [...]}\n    CSV:  url,status,title,headers\n          headers may be a JSON object or \"Key: Value; Key2: Value2\".\n\nReachable means status 200 and no WWW-Authenticate challenge. The score is\nthe product weight plus 20 for an open response, and the two factors are\nprinted so the number is traceable:\n    admin 40, jenkins 38, grafana 36, phpmyadmin 34, login 20.\n\nCron/CI:\n    0 6 * * * python3 exposed_admin.py /srv/exports/http-probe.json\n\nExit codes:\n    0 no reachable admin or login panel\n    1 at least one reachable panel\n    2 bad input (unreadable file, bad JSON/CSV, missing url or status)\n\nRead-only: it makes no network calls and writes nothing. By s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport io\nimport json\nimport sys\n\nBASE = {\"admin\": 40, \"jenkins\": 38, \"grafana\": 36, \"phpmyadmin\": 34, \"login\": 20}\nOPEN_BONUS = 20\nADMIN_HINTS = (\"/admin\", \"admin\", \"administrator\", \"wp-admin\", \"manager/html\",\n               \"adminer\", \"console\")\nLOGIN_HINTS = (\"login\", \"log in\", \"signin\", \"sign-in\", \"sign in\", \"auth\")\nPRODUCTS = (\"jenkins\", \"grafana\", \"phpmyadmin\")\n\n\ndef fail(message):\n    \"\"\"Bad input is exit 2, never a silent pass.\"\"\"\n    print(\"ERROR %s\" % message, file=sys.stderr)\n    return 2\n\n\ndef norm_keys(row):\n    return {str(k).strip().lower(): v for k, v in row.items() if k is not None}\n\n\ndef parse_status(value):\n    text = str(value).strip()\n    if not text:\n        raise ValueError(\"empty status\")\n    head = text.split()[0]\n    if not head.isdigit():\n        raise ValueError(\"status is not a number: %r\" % text)\n    return int(head)\n\n\ndef parse_headers(value):\n    \"\"\"Return a lower-case header map. Unparseable headers are not fatal.\"\"\"\n    if isinstance(value, dict):\n        return {str(k).lower(): str(v) for k, v in value.items()}\n    text = str(value or \"\").strip()\n    if not text:\n        return {}\n    try:\n        data = json.loads(text)\n    except ValueError:\n        data = None\n    if isinstance(data, dict):\n        return {str(k).lower(): str(v) for k, v in data.items()}\n    out = {}\n    for part in text.replace(\"\\n\", \";\").split(\";\"):\n        if \":\" in part:\n            key, val = part.split(\":\", 1)\n            out[key.strip().lower()] = val.strip()\n    return out\n\n\ndef load(path):\n    \"\"\"Read JSON or CSV and return a list of normalised probe rows.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            raw = handle.read()\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    if not raw.strip():\n        raise ValueError(\"empty input: %s\" % path)\n\n    if raw.lstrip().startswith((\"[\", \"{\")):\n        try:\n            data = json.loads(raw)\n        except ValueError as exc:\n            raise ValueError(\"bad JSON in %s: %s\" % (path, exc))\n        if isinstance(data, dict):\n            for key in (\"probes\", \"results\", \"items\"):\n                if isinstance(data.get(key), list):\n                    data = data[key]\n                    break\n            else:\n                raise ValueError(\"JSON object has no probes/results list\")\n        if not isinstance(data, list):\n            raise ValueError(\"JSON must be a list or an object with a probes list\")\n        rows = []\n        for i, row in enumerate(data, 1):\n            if not isinstance(row, dict):\n                raise ValueError(\"probe %d is not an object\" % i)\n            rows.append(normalise(norm_keys(row), i))\n        return rows\n\n    reader = csv.DictReader(io.StringIO(raw))\n    fields = [str(f).strip().lower() for f in (reader.fieldnames or [])]\n    if \"url\" not in fields or \"status\" not in fields:\n        raise ValueError(\"CSV needs url and status columns, got %s\" % fields)\n    return [normalise(norm_keys(row), i + 2) for i, row in enumerate(reader)]\n\n\ndef normalise(row, line_no):\n    url = str(row.get(\"url\") or \"\").strip()\n    if not url:\n        raise ValueError(\"row %d has no url\" % line_no)\n    try:\n        status = parse_status(row.get(\"status\", row.get(\"http_status\", \"\")))\n    except ValueError as exc:\n        raise ValueError(\"row %d (%s): %s\" % (line_no, url, exc))\n    title = str(row.get(\"title\") or \"\").strip()\n    headers = parse_headers(row.get(\"headers\", \"\"))\n    return {\"url\": url, \"status\": status, \"title\": title, \"headers\": headers}\n\n\ndef product(url, title):\n    \"\"\"Classify a panel. Specific products win before the generic admin match.\"\"\"\n    hay = (url + \" \" + title).lower()\n    for name in PRODUCTS:\n        if name in hay:\n            return name\n    if any(hint in hay for hint in ADMIN_HINTS):\n        return \"admin\"\n    if any(hint in hay for hint in LOGIN_HINTS):\n        return \"login\"\n    return \"\"\n\n\ndef reachable(row):\n    \"\"\"200 alone is not enough: a Basic challenge means the panel is gated.\"\"\"\n    return row[\"status\"] == 200 and \"www-authenticate\" not in row[\"headers\"]\n\n\ndef score(kind):\n    return BASE[kind] + OPEN_BONUS\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Rank admin and login panels reachable without auth.\",\n        epilog=\"Reads an HTTP probe export (JSON or CSV). Exit 1 on a finding.\")\n    parser.add_argument(\"probe\", help=\"HTTP probe export: JSON or CSV\")\n    parser.add_argument(\"--top\", type=int, default=10,\n                        help=\"show at most N panels (default: 10)\")\n    args = parser.parse_args(argv)\n    if args.top < 1:\n        return fail(\"--top must be at least 1\")\n    try:\n        rows = load(args.probe)\n    except ValueError as exc:\n        return fail(str(exc))\n\n    found = []\n    for row in rows:\n        kind = product(row[\"url\"], row[\"title\"])\n        if kind and reachable(row):\n            found.append((score(kind), kind, row))\n    found.sort(key=lambda item: (-item[0], item[2][\"url\"]))\n\n    word = \"probe\" if len(rows) == 1 else \"probes\"\n    if not found:\n        print(\"OK no reachable admin or login panel in %d %s\" % (len(rows), word))\n        return 0\n\n    print(\"FINDING %d reachable panels of %d %s\" % (len(found), len(rows), word))\n    for total, kind, row in found[:args.top]:\n        print(\"ALERT %d %s %d %s [%d+%d]\"\n              % (total, kind, row[\"status\"], row[\"url\"], BASE[kind], OPEN_BONUS))\n    if len(found) > args.top:\n        print(\"NOTE %d more panels not shown\" % (len(found) - args.top))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p733","n":"733","slug":"api-keys-in-the-javascript","title":"API keys in the JavaScript","sections":["devsecops"],"topic":"Python scripts","tags":["AppSec","JavaScript","Python","SecretsManagement","DevSecOps"],"series_name":"ASM","design":"code","script":"js_secret_scan.py","date":"2031-06-03","files":{"devsecops":"notes/p733-api-keys-in-the-javascript.md"},"md":"# API keys in the JavaScript\n\nThe bundle shipped with a live Stripe key. It worked for nine days, because the build cached it and the CDN kept serving it.\n\n## Quick reference\n\nThe full script is at the end of this note: `js_secret_scan.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 js_secret_scan.py /tmp/js.json\n```\n\n**Example output**\n\n```text\nFINDING 5 secrets in 2 bundles\nALERT dist/app.js:1 aws AKIA...\n```\n\n## Explained\n\n### What it does\n\n`js_secret_scan.py` reads an export of JavaScript bundles, each with a file name and its content. It searches five patterns: AWS access key id, Google API key, Stripe live secret, bearer token and private key header. For each match it prints file:line, the pattern name and a masked value: the first four characters, never the secret. The fourteen lines print FINDING 5 secrets in 2 bundles and ALERT dist/app.js:1 aws AKIA.... The attached tool adds the value length.\n\n### How to run it\n\n`python3 js_secret_scan.py dist/bundles.json`. In CI, run it after the build and before the upload, and let the exit code fail the job. Exit 0 is clean, 1 is a hit, 2 is bad input. `--types` aws,stripe narrows the scan.\n\n### The fix\n\nRevoke the key first. Then remove it from the source, rebuild, and purge the CDN and browser caches; the old bundle is still downloadable. Moving the call server-side is the second step. Deleting the line without rotating leaves a live key in every cache.\n\n### Prevention\n\nScan the built artefact, not just the repository. Add the same five patterns to pre-commit and to the upload job.\n\n> **The honest caveat:** publishable keys are meant to ship (Stripe pk_live_, Google browser keys restricted by referrer), and this scanner only matches server-side patterns. A hit is a candidate; the owner confirms scope.\n\n> **The lesson:** a key in a bundle is a key in every browser that loaded it.\n\n## In one line\n\n> **Rotate first, clean second.** Five patterns, a masked value, and an exit code a build can read.\n\n**Scan the bundle**\n\n```bash\npython3 js_secret_scan.py dist/\n```\n\n## Script: js_secret_scan.py\n\nDownload: [scripts/js_secret_scan.py](../scripts/js_secret_scan.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Find credential patterns in exported JavaScript and print a masked hit.\n\nUsage:\n    python3 js_secret_scan.py bundles.json\n    python3 js_secret_scan.py bundles.json --types aws,stripe --max 20\n\nInput (an export, never a live fetch):\n    JSON: [{\"file\": \"dist/app.js\", \"content\": \"...\"}] or {\"bundles\": [...]}\n\nPatterns: AWS access key id, Google API key, Stripe live secret, bearer\ntoken, private key header. The output is file:line, the pattern name and a\nmasked value: the first four characters and the length, never the secret.\nA hit exits 1 so a build can fail on it.\n\nCron/CI:\n    python3 js_secret_scan.py dist/bundles.json || exit 1\n\nExit codes:\n    0 no match\n    1 at least one match\n    2 bad input (unreadable file, bad JSON, missing file/content, bad --types)\n\nRead-only: it makes no network calls and writes nothing. By s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\n# (name, compiled pattern, match group holding the value)\nPATTERNS = [\n    (\"aws\", re.compile(r\"\\b(?:AKIA|ASIA)[0-9A-Z]{16}\\b\"), 0),\n    (\"google\", re.compile(r\"\\bAIza[0-9A-Za-z_-]{35}(?![0-9A-Za-z_-])\"), 0),\n    (\"stripe\", re.compile(r\"\\b(?:sk|rk)_live_[0-9A-Za-z]{16,}\\b\"), 0),\n    (\"bearer\", re.compile(r\"(?i)\\bbearer[ \\t]+([A-Za-z0-9._~+/-]{16,}=*)\"), 1),\n    (\"key\", re.compile(r\"-----BEGIN (?:RSA |EC |DSA |OPENSSH )?PRIVATE KEY-----\"), 0),\n]\nNAMES = tuple(name for name, _rx, _grp in PATTERNS)\n\n\ndef fail(message):\n    print(\"ERROR %s\" % message, file=sys.stderr)\n    return 2\n\n\ndef mask(value):\n    \"\"\"Show the four-character prefix and the length only, never the value.\"\"\"\n    value = value.strip()\n    if len(value) <= 8:\n        return \"*\" * len(value)\n    return \"%s... (%d chars)\" % (value[:4], len(value))\n\n\ndef load(path):\n    \"\"\"Read the bundle export and return [(file, content), ...].\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            raw = handle.read()\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    if not raw.strip():\n        raise ValueError(\"empty input: %s\" % path)\n    try:\n        data = json.loads(raw)\n    except ValueError as exc:\n        raise ValueError(\"bad JSON in %s: %s\" % (path, exc))\n\n    if isinstance(data, dict):\n        for key in (\"bundles\", \"files\", \"items\"):\n            if isinstance(data.get(key), list):\n                data = data[key]\n                break\n        else:\n            raise ValueError(\"JSON object has no bundles list\")\n    if not isinstance(data, list):\n        raise ValueError(\"JSON must be a list or an object with a bundles list\")\n\n    out = []\n    for i, item in enumerate(data, 1):\n        if not isinstance(item, dict):\n            raise ValueError(\"bundle %d is not an object\" % i)\n        name = item.get(\"file\", item.get(\"name\", \"\"))\n        if \"content\" in item:\n            content = item[\"content\"]\n        elif \"source\" in item:\n            content = item[\"source\"]\n        else:\n            content = None\n        if not isinstance(name, str) or not name.strip():\n            raise ValueError(\"bundle %d has no file name\" % i)\n        if not isinstance(content, str):\n            raise ValueError(\"bundle %d (%s) has no string content\" % (i, name))\n        out.append((name.strip(), content))\n    return out\n\n\ndef scan(bundles, enabled):\n    \"\"\"Return deduplicated findings as (file, line, name, masked).\"\"\"\n    hits = []\n    seen = set()\n    order = {name: i for i, name in enumerate(NAMES)}\n    for name, content in bundles:\n        for lineno, line in enumerate(content.splitlines(), 1):\n            for kind, rx, group in PATTERNS:\n                if kind not in enabled:\n                    continue\n                for match in rx.finditer(line):\n                    value = match.group(group) or \"\"\n                    key = (name, lineno, kind, value)\n                    if not value or key in seen:\n                        continue\n                    seen.add(key)\n                    hits.append((name, lineno, kind, mask(value)))\n    hits.sort(key=lambda hit: (hit[0], hit[1], order[hit[2]]))\n    return hits\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Find credential patterns in exported JavaScript bundles.\",\n        epilog=\"Prints file:line and a masked value. Exit 1 on any hit.\")\n    parser.add_argument(\"bundles\", help=\"bundle export: JSON with file and content\")\n    parser.add_argument(\"--types\", default=\",\".join(NAMES),\n                        help=\"comma list of patterns: %s\" % \", \".join(NAMES))\n    parser.add_argument(\"--max\", type=int, default=100,\n                        help=\"show at most N hits (default: 100)\")\n    args = parser.parse_args(argv)\n\n    enabled = [part.strip().lower() for part in args.types.split(\",\") if part.strip()]\n    unknown = [part for part in enabled if part not in NAMES]\n    if unknown:\n        return fail(\"unknown type(s): %s; choose from %s\"\n                    % (\", \".join(unknown), \", \".join(NAMES)))\n    if not enabled:\n        return fail(\"--types needs at least one pattern\")\n    if args.max < 1:\n        return fail(\"--max must be at least 1\")\n\n    try:\n        bundles = load(args.bundles)\n    except ValueError as exc:\n        return fail(str(exc))\n\n    hits = scan(bundles, set(enabled))\n    word = \"bundle\" if len(bundles) == 1 else \"bundles\"\n    if not hits:\n        print(\"OK no credential pattern in %d %s\" % (len(bundles), word))\n        return 0\n\n    print(\"FINDING %d secrets in %d %s\" % (len(hits), len(bundles), word))\n    for name, lineno, kind, shown in hits[:args.max]:\n        print(\"ALERT %s:%d %s %s\" % (name, lineno, kind, shown))\n    if len(hits) > args.max:\n        print(\"NOTE %d more hits not shown\" % (len(hits) - args.max))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p734","n":"734","slug":"the-bucket-that-lists-itself","title":"The bucket that lists itself","sections":["devsecops"],"topic":"Python scripts","tags":["CloudSecurity","AWS","Azure","GCP","Python"],"series_name":"ASM","design":"code","script":"bucket_exposure.py","date":"2031-06-05","files":{"devsecops":"notes/p734-the-bucket-that-lists-itself.md"},"md":"# The bucket that lists itself\n\nThe bucket policy said private. The ACL said public-read-write, and the account block was off.\n\n## Quick reference\n\nThe full script is at the end of this note: `bucket_exposure.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 bucket_exposure.py /tmp/buckets.json\n```\n\n**Example output**\n\n```text\nFINDING 3 exposed buckets of 5\nALERT acme-logs public,anon-list,no-enc\n```\n\n## Explained\n\n### What it does\n\n`bucket_exposure.py` reads a multi-cloud inventory: provider, bucket, acl, public_access_block, policy and encryption. It resolves grants per provider: an S3 public ACL or Principal star, a GCP allUsers or allAuthenticatedUsers binding, an Azure container or blob ACL. If the account-level public access block is on, the grant is reported as blocked, not exposed. A public ACL on a bucket also grants listing, so the flags are public and anon-list. Missing encryption adds no-enc. The fourteen lines print FINDING 3 exposed buckets of 5 and ALERT acme-logs public,anon-list,no-enc.\n\n### How to run it\n\n`python3 bucket_exposure.py inventory.csv`. Run it weekly from cron, or in CI after a storage change. Exit 0 is clean, 1 is a finding, 2 is bad input.\n\n### The fix\n\nTurn on Block Public Access at the account or organisation level first, then remove the AllUsers grant and the public ACL. Enable default encryption, then test from outside with an unauthenticated request; the console's private badge is not evidence.\n\n### Prevention\n\nPolicy-as-code for every bucket, a weekly inventory, and an alert on any new public grant. The block is the seatbelt; the grant removal is the repair.\n\n> **The honest caveat:** S3 encrypts new objects by default since January 2023, so a blank encryption field can mean not exported rather than unencrypted. Check the account default before calling it a finding.\n\n> **The lesson:** an ACL and a policy are two doors to the same room; test both from outside.\n\n## In one line\n\n> **Private is a claim. Prove it anonymously.** The account block can neutralise a legacy grant. The encryption field cannot.\n\n**Test anonymously**\n\n```bash\naws s3 ls s3://acme --no-sign-request\n```\n\n## Script: bucket_exposure.py\n\nDownload: [scripts/bucket_exposure.py](../scripts/bucket_exposure.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Flag public, anonymously listable and unencrypted storage buckets.\n\nUsage:\n    python3 bucket_exposure.py inventory.csv\n    python3 bucket_exposure.py inventory.json --max 20\n\nInput (an export, never a cloud API call):\n    JSON: [{\"provider\": \"aws\", \"bucket\": \"acme-logs\", \"acl\": \"private\",\n            \"public_access_block\": true, \"policy\": \"...\", \"encryption\": \"AES256\"}]\n          or {\"buckets\": [...]}\n    CSV:  provider,bucket,acl,public_access_block,policy,encryption\n\nProviders: aws (s3), gcp (gcs), azure (blob). public_access_block true means\nthe account-level public-access prevention control is on, so a legacy public\ngrant is reported as blocked rather than exposed.\n\nScores are the sum of the flags, printed with the factors:\n    public 40, anon-list 30, no-enc 20.\n\nCron/CI:\n    0 7 * * 1 python3 bucket_exposure.py /srv/exports/storage.csv\n\nExit codes:\n    0 no exposure\n    1 at least one exposed bucket\n    2 bad input (unreadable file, bad JSON/CSV, unknown provider, bad boolean)\n\nRead-only: it makes no network calls and writes nothing. By s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport io\nimport json\nimport re\nimport sys\n\nWEIGHT = {\"public\": 40, \"anon-list\": 30, \"no-enc\": 20}\nPROVIDERS = {\n    \"aws\": (\"aws\", \"s3\", \"amazon\"),\n    \"gcp\": (\"gcp\", \"gcs\", \"gs\", \"google\"),\n    \"azure\": (\"azure\", \"az\", \"blob\", \"microsoft\"),\n}\nPUBLIC_PRINCIPAL = re.compile(r'\"principal\"\\s*:\\s*\"\\*\"')\nPUBLIC_PRINCIPAL_OBJECT = re.compile(r'\"principal\"\\s*:\\s*\\{[^{}]*\"\\*\"')\n\n\ndef fail(message):\n    print(\"ERROR %s\" % message, file=sys.stderr)\n    return 2\n\n\ndef norm_keys(row):\n    return {str(k).strip().lower(): v for k, v in row.items() if k is not None}\n\n\ndef as_bool(value):\n    \"\"\"True/False, or None when the export left the field blank.\"\"\"\n    if isinstance(value, bool):\n        return value\n    text = str(value).strip().lower()\n    if not text:\n        return None\n    if text in (\"true\", \"1\", \"yes\", \"on\", \"enabled\", \"enforce\", \"enforced\"):\n        return True\n    if text in (\"false\", \"0\", \"no\", \"off\", \"disabled\", \"unblocked\"):\n        return False\n    raise ValueError(\"not a boolean: %r\" % str(value))\n\n\ndef as_provider(value):\n    text = str(value or \"\").strip().lower()\n    for canonical, aliases in PROVIDERS.items():\n        if text in aliases:\n            return canonical\n    raise ValueError(\"unknown provider: %r\" % text)\n\n\ndef encrypted(value):\n    text = str(value or \"\").strip().lower()\n    return text not in (\"\", \"none\", \"false\", \"off\", \"no\", \"unencrypted\", \"-\")\n\n\ndef policy_text(value):\n    if isinstance(value, dict):\n        return json.dumps(value)\n    return str(value or \"\")\n\n\ndef load(path):\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            raw = handle.read()\n    except OSError as exc:\n        raise ValueError(\"cannot read %s: %s\" % (path, exc))\n    if not raw.strip():\n        raise ValueError(\"empty input: %s\" % path)\n\n    if raw.lstrip().startswith((\"[\", \"{\")):\n        try:\n            data = json.loads(raw)\n        except ValueError as exc:\n            raise ValueError(\"bad JSON in %s: %s\" % (path, exc))\n        if isinstance(data, dict):\n            for key in (\"buckets\", \"storage\", \"items\"):\n                if isinstance(data.get(key), list):\n                    data = data[key]\n                    break\n            else:\n                raise ValueError(\"JSON object has no buckets list\")\n        if not isinstance(data, list):\n            raise ValueError(\"JSON must be a list or an object with a buckets list\")\n        rows = []\n        for i, row in enumerate(data, 1):\n            if not isinstance(row, dict):\n                raise ValueError(\"bucket %d is not an object\" % i)\n            rows.append(normalise(norm_keys(row), i))\n        return rows\n\n    reader = csv.DictReader(io.StringIO(raw))\n    fields = [str(f).strip().lower() for f in (reader.fieldnames or [])]\n    if \"provider\" not in fields or \"bucket\" not in fields:\n        raise ValueError(\"CSV needs provider and bucket columns, got %s\" % fields)\n    return [normalise(norm_keys(row), i + 2) for i, row in enumerate(reader)]\n\n\ndef normalise(row, line_no):\n    name = str(row.get(\"bucket\") or \"\").strip()\n    try:\n        provider = as_provider(row.get(\"provider\"))\n        block = as_bool(row.get(\"public_access_block\"))\n    except ValueError as exc:\n        raise ValueError(\"row %d (%s): %s\" % (line_no, name or \"?\", exc))\n    if not name:\n        raise ValueError(\"row %d has no bucket\" % line_no)\n    return {\n        \"provider\": provider,\n        \"bucket\": name,\n        \"acl\": str(row.get(\"acl\") or \"\").strip().lower(),\n        \"block\": block,\n        \"policy\": policy_text(row.get(\"policy\")),\n        \"encrypted\": encrypted(row.get(\"encryption\")),\n    }\n\n\ndef grants(row):\n    \"\"\"Return (public, anonymous-list) before the account block is applied.\"\"\"\n    provider, acl = row[\"provider\"], row[\"acl\"]\n    policy = row[\"policy\"].lower()\n    public = False\n    listed = False\n\n    if provider == \"aws\":\n        if acl in (\"public-read\", \"public-read-write\"):\n            public = True\n            listed = True  # bucket ACL READ grants ListBucket\n        elif acl == \"authenticated-read\":\n            public = True  # any AWS account, not just this one\n        if PUBLIC_PRINCIPAL.search(policy) or PUBLIC_PRINCIPAL_OBJECT.search(policy):\n            public = True\n            if \"listbucket\" in policy:\n                listed = True\n    elif provider == \"gcp\":\n        anonymous = (\"allusers\" in policy or \"allusers\" in acl\n                     or \"public\" in acl)\n        if anonymous or \"allauthenticatedusers\" in policy:\n            public = True\n        if anonymous and any(token in policy for token in\n                             (\"objectviewer\", \"objectlist\", \"objects.list\",\n                              \"legacyobjectreader\")):\n            listed = True\n    else:  # azure\n        if acl == \"container\":\n            public = True\n            listed = True\n        elif acl == \"blob\":\n            public = True\n\n    if row[\"block\"] is True:\n        return False, False, public or listed\n    return public, listed, False\n\n\ndef evaluate(row):\n    public, listed, blocked = grants(row)\n    flags = []\n    if public:\n        flags.append(\"public\")\n    if listed:\n        flags.append(\"anon-list\")\n    if not row[\"encrypted\"]:\n        flags.append(\"no-enc\")\n    return flags, blocked\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Flag public, anonymously listable and unencrypted buckets.\",\n        epilog=\"Reads a multi-cloud storage inventory (JSON or CSV).\")\n    parser.add_argument(\"inventory\", help=\"storage inventory: JSON or CSV\")\n    parser.add_argument(\"--max\", type=int, default=50,\n                        help=\"show at most N buckets (default: 50)\")\n    parser.add_argument(\"--skip-encryption\", action=\"store_true\",\n                        help=\"do not treat a missing encryption field as a finding\")\n    args = parser.parse_args(argv)\n    if args.max < 1:\n        return fail(\"--max must be at least 1\")\n\n    try:\n        rows = load(args.inventory)\n    except ValueError as exc:\n        return fail(str(exc))\n\n    found = []\n    blocked_count = 0\n    for row in rows:\n        flags, blocked = evaluate(row)\n        if args.skip_encryption:\n            flags = [flag for flag in flags if flag != \"no-enc\"]\n        if blocked:\n            blocked_count += 1\n        if flags:\n            total = sum(WEIGHT[flag] for flag in flags)\n            found.append((total, row[\"bucket\"], flags))\n    found.sort(key=lambda item: (-item[0], item[1]))\n\n    if not found:\n        print(\"OK no exposed bucket in %d buckets\" % len(rows))\n        if blocked_count:\n            print(\"NOTE %d public grant(s) blocked by public access prevention\"\n                  % blocked_count)\n        return 0\n\n    print(\"FINDING %d exposed buckets of %d\" % (len(found), len(rows)))\n    for total, name, flags in found[:args.max]:\n        factors = \"+\".join(str(WEIGHT[flag]) for flag in flags)\n        print(\"ALERT %d %s %s [%s]\" % (total, name, \",\".join(flags), factors))\n    if len(found) > args.max:\n        print(\"NOTE %d more buckets not shown\" % (len(found) - args.max))\n    if blocked_count:\n        print(\"NOTE %d public grant(s) blocked by public access prevention\"\n              % blocked_count)\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p735","n":"735","slug":"ports-that-should-never-face-the-internet","title":"Ports that should never face the internet","sections":["devsecops"],"topic":"Python scripts","tags":["NetworkSecurity","CloudSecurity","Python","Firewalls","DevSecOps"],"series_name":"ASM","design":"code","script":"port_policy.py","date":"2031-06-08","files":{"devsecops":"notes/p735-ports-that-should-never-face-the-internet.md"},"md":"# Ports that should never face the internet\n\nPort 3306 was opened for a migration that finished in March. In November it was still answering from the internet.\n\n## Quick reference\n\nThe full script is at the end of this note: `port_policy.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 port_policy.py /tmp/scan.csv /tmp/policy.json\n```\n\n**Example output**\n\n```text\nFINDING 5 port-policy violations\nALERT 203.0.113.10:22 ssh admin open to 0.0.0.0/0\n```\n\n## Explained\n\n### What it does\n\n`port_policy.py` reads an external scan export (host, port, service, banner) and an allowed-exposure policy JSON. The scan is external, so every open port is treated as reachable from 0.0.0.0/0. A port not in the allow list is an admin port, a database port, an unknown service, or a known service on the wrong port. A blank service is inferred from the banner when possible. The fourteen lines print FINDING 5 port-policy violations and ALERT 203.0.113.10:22 ssh admin open to 0.0.0.0/0.\n\n### How to run it\n\n`python3 port_policy.py scan.csv policy.json`. Run it weekly from cron, or in CI after a security-group change. Exit 0 is clean, 1 is a finding, 2 is bad input.\n\n### The fix\n\nClose the rule in the security group, not only the host firewall. For the database, remove the public address or use a bastion. For the unknown service, identify it before you close it.\n\n### Prevention\n\nGenerate the policy from the architecture, diff the scan against it every week, and alert on any new admin or database port. The diff is the control; the scan is only the sensor.\n\n> **The honest caveat:** an external scan sees what the network allows, not what the application does. A port can be open on the host and filtered upstream, and banner inference is a guess when the service field is blank.\n\n> **The lesson:** the internet does not need your database port.\n\n## In one line\n\n> **Close the rule, not the service.** Every open port is reachable from 0.0.0.0/0 until the rule says otherwise.\n\n**Check from outside**\n\n```bash\nnmap -Pn -p 22,3306 203.0.113.10\n```\n\n## Script: port_policy.py\n\nDownload: [scripts/port_policy.py](../scripts/port_policy.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Compare an external port scan with the exposure policy you meant to have.\n\nUsage:\n    python3 port_policy.py scan.csv policy.json\n    python3 port_policy.py scan.json policy.json --max 20\n\nInput (an export, never a live scan):\n    scan:    CSV or JSON with host, port, service, banner\n    policy:  JSON:\n             {\n               \"allow\": [{\"port\": 80, \"service\": \"http\"},\n                         {\"port\": 443, \"service\": \"https\"}],\n               \"tiers\": {\"ssh\": \"admin\", \"mysql\": \"database\", ...},\n               \"known\": [\"http\", \"https\", \"ssh\", \"smtp\", ...]\n             }\n\nThe scan is external, so every open port is treated as reachable from\n0.0.0.0/0. A port that the policy does not allow is reported as:\n    admin     admin service open to the internet\n    database  database service open to the internet\n    unknown   service not in the policy's known list\n    unlisted  known service, but not allowed on that port\nA blank or \"unknown\" service is inferred from the banner when possible.\n\nCron/CI:\n    0 5 * * 1 python3 port_policy.py /srv/exports/scan.csv /etc/exposure.json\n\nExit codes:\n    0 every open port is allowed\n    1 at least one violation\n    2 bad input (unreadable file, bad JSON/CSV/policy, bad port)\n\nRead-only: it makes no network calls and writes nothing. By s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport io\nimport json\nimport re\nimport sys\n\nALIASES = {\n    \"https\": \"https\", \"ssl/https\": \"https\", \"ssl\": \"https\", \"tls\": \"https\",\n    \"http\": \"http\", \"http-alt\": \"http\", \"www\": \"http\",\n    \"ssh\": \"ssh\", \"openssh\": \"ssh\",\n    \"domain\": \"dns\", \"dns\": \"dns\",\n    \"mysql\": \"mysql\", \"mariadb\": \"mysql\",\n    \"postgres\": \"postgresql\", \"postgresql\": \"postgresql\",\n    \"mssql\": \"mssql\", \"ms-sql-s\": \"mssql\",\n    \"redis\": \"redis\", \"mongodb\": \"mongodb\", \"mongo\": \"mongodb\",\n    \"elasticsearch\": \"elasticsearch\", \"elastic\": \"elasticsearch\",\n    \"rdp\": \"rdp\", \"ms-wbt-server\": \"rdp\",\n    \"smtp\": \"smtp\", \"smtps\": \"smtp\",\n    \"imap\": \"imap\", \"imaps\": \"imap\",\n    \"pop3\": \"pop3\", \"pop3s\": \"pop3\",\n    \"ftp\": \"ftp\", \"telnet\": \"telnet\", \"vnc\": \"vnc\",\n}\nBLANK = (\"\", \"unknown\", \"tcpwrapped\", \"-\", \"none\")\nRANK = {\"admin\": 4, \"database\": 3, \"unknown\": 2, \"unlisted\": 1}\nREASONS = {\n    \"admin\": \"admin port open to 0.0.0.0/0\",\n    \"database\": \"database port open to 0.0.0.0/0\",\n    \"unknown\": \"unknown service, not in policy\",\n    \"unlisted\": \"known service, not allowed\",\n}\n\n\ndef fail(message):\n    print(\"ERROR %s\" % message, file=sys.stderr)\n    return 2\n\n\ndef norm_keys(row):\n    return {str(k).strip().lower(): v for k, v in row.items() if k is not None}\n\n\ndef canon_service(value):\n    text = str(value or \"\").strip().lower().replace(\"_\", \"-\")\n    if text in ALIASES:\n        return ALIASES[text]\n    token = re.split(r\"[\\s/]\", text)[0]\n    return ALIASES.get(token, token)\n\n\ndef parse_port(value):\n    text = str(value).strip()\n    if not text.isdigit():\n        raise ValueError(\"port is not a number: %r\" % text)\n    port = int(text)\n    if not 1 <= port <= 65535:\n        raise ValueError(\"port out of range: %d\" % port)\n    return port\n\n\ndef infer_service(banner):\n    \"\"\"Best effort from a banner. A guess is labelled as the service, not proof.\"\"\"\n    text = str(banner or \"\").strip()\n    low = text.lower()\n    if text.startswith(\"SSH-\"):\n        return \"ssh\"\n    if text.startswith(\"HTTP/\") or \"server:\" in low:\n        return \"http\"\n    if \"mysql\" in low or \"mariadb\" in low:\n        return \"mysql\"\n    if \"mongodb\" in low:\n        return \"mongodb\"\n    if \"redis\" in low or text.startswith(\"-NOAUTH\"):\n        return \"redis\"\n    if text.startswith(\"220\") and \"smtp\" in low:\n        return \"smtp\"\n    if text.startswith(\"+OK\"):\n        return \"pop3\"\n    if text.startswith(\"* OK\"):\n        return \"imap\"\n    if \"postgres\" in low:\n        return \"postgresql\"\n    return \"unknown\"\n\n\ndef load_policy(path):\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read policy %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ValueError(\"bad JSON in policy %s: %s\" % (path, exc))\n    if not isinstance(data, dict):\n        raise ValueError(\"policy must be a JSON object\")\n    allow = data.get(\"allow\")\n    if not isinstance(allow, list) or not allow:\n        raise ValueError(\"policy needs a non-empty allow list\")\n    rules = []\n    for i, item in enumerate(allow, 1):\n        if not isinstance(item, dict):\n            raise ValueError(\"allow entry %d is not an object\" % i)\n        port = item.get(\"port\")\n        service = item.get(\"service\")\n        if port is None and service is None:\n            raise ValueError(\"allow entry %d needs a port or a service\" % i)\n        try:\n            port = parse_port(port) if port is not None else None\n        except ValueError as exc:\n            raise ValueError(\"allow entry %d: %s\" % (i, exc))\n        service = canon_service(service) if service not in (None, \"*\") else \"*\"\n        rules.append((port, service))\n\n    tiers = data.get(\"tiers\") or {}\n    if not isinstance(tiers, dict):\n        raise ValueError(\"policy tiers must be an object\")\n    tiers = {canon_service(k): str(v).lower() for k, v in tiers.items()}\n    for key in (\"admin_services\", \"database_services\", \"known\"):\n        if key in data and not isinstance(data[key], list):\n            raise ValueError(\"policy %s must be a list\" % key)\n    for service in data.get(\"admin_services\", []):\n        tiers.setdefault(canon_service(service), \"admin\")\n    for service in data.get(\"database_services\", []):\n        tiers.setdefault(canon_service(service), \"database\")\n    bad = [s for s, tier in tiers.items() if tier not in (\"admin\", \"database\")]\n    if bad:\n        raise ValueError(\"tiers must be admin or database, not %s\" % \", \".join(bad))\n\n    known = {canon_service(s) for s in data.get(\"known\", [])}\n    if not known:\n        known = set(tiers)\n        known.update(service for _port, service in rules if service != \"*\")\n    return rules, tiers, known\n\n\ndef load_scan(path):\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            raw = handle.read()\n    except OSError as exc:\n        raise ValueError(\"cannot read scan %s: %s\" % (path, exc))\n    if not raw.strip():\n        raise ValueError(\"empty input: %s\" % path)\n\n    if raw.lstrip().startswith((\"[\", \"{\")):\n        try:\n            data = json.loads(raw)\n        except ValueError as exc:\n            raise ValueError(\"bad JSON in scan %s: %s\" % (path, exc))\n        if isinstance(data, dict):\n            for key in (\"hosts\", \"scan\", \"results\", \"items\"):\n                if isinstance(data.get(key), list):\n                    data = data[key]\n                    break\n            else:\n                raise ValueError(\"JSON object has no hosts/scan/results list\")\n        if not isinstance(data, list):\n            raise ValueError(\"scan must be a list or an object with a hosts list\")\n        rows = []\n        for i, row in enumerate(data, 1):\n            if not isinstance(row, dict):\n                raise ValueError(\"host %d is not an object\" % i)\n            rows.append(normalise(norm_keys(row), i))\n        return rows\n\n    reader = csv.DictReader(io.StringIO(raw))\n    fields = [str(f).strip().lower() for f in (reader.fieldnames or [])]\n    if \"host\" not in fields or \"port\" not in fields:\n        raise ValueError(\"CSV needs host and port columns, got %s\" % fields)\n    return [normalise(norm_keys(row), i + 2) for i, row in enumerate(reader)]\n\n\ndef normalise(row, line_no):\n    host = str(row.get(\"host\") or \"\").strip()\n    if not host:\n        raise ValueError(\"row %d has no host\" % line_no)\n    try:\n        port = parse_port(row.get(\"port\"))\n    except ValueError as exc:\n        raise ValueError(\"row %d (%s): %s\" % (line_no, host, exc))\n    service = canon_service(row.get(\"service\"))\n    banner = str(row.get(\"banner\") or \"\").strip()\n    if service in BLANK:\n        service = infer_service(banner)\n    return {\"host\": host, \"port\": port, \"service\": service, \"banner\": banner}\n\n\ndef allowed(port, service, rules):\n    for rule_port, rule_service in rules:\n        if rule_port is not None and rule_port != port:\n            continue\n        if rule_service not in (None, \"*\") and rule_service != service:\n            continue\n        return True\n    return False\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Compare an external port scan with an exposure policy.\",\n        epilog=\"Every open port is treated as reachable from 0.0.0.0/0.\")\n    parser.add_argument(\"scan\", help=\"scan export: CSV or JSON\")\n    parser.add_argument(\"policy\", help=\"allowed-exposure policy: JSON\")\n    parser.add_argument(\"--max\", type=int, default=100,\n                        help=\"show at most N violations (default: 100)\")\n    args = parser.parse_args(argv)\n    if args.max < 1:\n        return fail(\"--max must be at least 1\")\n\n    try:\n        rules, tiers, known = load_policy(args.policy)\n        rows = load_scan(args.scan)\n    except ValueError as exc:\n        return fail(str(exc))\n\n    found = []\n    for row in rows:\n        if allowed(row[\"port\"], row[\"service\"], rules):\n            continue\n        tier = tiers.get(row[\"service\"])\n        if tier in (\"admin\", \"database\"):\n            kind = tier\n        elif row[\"service\"] == \"unknown\" or row[\"service\"] not in known:\n            kind = \"unknown\"\n        else:\n            kind = \"unlisted\"\n        found.append((RANK[kind], row[\"host\"], row[\"port\"], row[\"service\"], kind))\n    found.sort(key=lambda item: (-item[0], item[1], item[2]))\n\n    if not found:\n        hosts = len({row[\"host\"] for row in rows})\n        word = \"host\" if hosts == 1 else \"hosts\"\n        print(\"OK all %d open ports are allowed on %d %s\" % (len(rows), hosts, word))\n        return 0\n\n    hosts = len({item[1] for item in found})\n    print(\"FINDING %d port-policy violations on %d hosts\" % (len(found), hosts))\n    for _rank, host, port, service, kind in found[:args.max]:\n        print(\"ALERT %s:%d %s %s\" % (host, port, service, REASONS[kind]))\n    if len(found) > args.max:\n        print(\"NOTE %d more violations not shown\" % (len(found) - args.max))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p736","n":"736","slug":"the-api-you-forgot-to-authenticate","title":"The API you forgot to authenticate","sections":["devsecops"],"topic":"Python scripts","tags":["APISecurity","OpenAPI","Python","AppSec","DevSecOps"],"series_name":"ASM","design":"code","script":"api_discovery.py","date":"2031-06-10","files":{"devsecops":"notes/p736-the-api-you-forgot-to-authenticate.md"},"md":"# The API you forgot to authenticate\n\nThe spec declared bearer auth. The gateway still routed the deprecated path, and the admin route answered without a token.\n\n## Quick reference\n\nThe full script is at the end of this note: `api_discovery.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 api_discovery.py /tmp/openapi.json /tmp/routes.csv\n```\n\n**Example output**\n\n```text\nFINDING 4 exposed endpoints of 6\nALERT GET /v1/admin/users admin\n```\n\n## Explained\n\n### What it does\n\n`api_discovery.py` reads one or more OpenAPI 2/3 documents and an endpoint inventory (method, path, optional public). It matches templated paths, resolves global and operation security, and flags four things: an admin-looking path with no security, any route with no security declaration, a deprecated operation still routed, and a route the gateway serves that no spec declares. A route marked public is exempt from the auth flags. The thirteen lines print FINDING 4 exposed endpoints of 6 and ALERT GET /v1/admin/users admin. The attached tool adds the no-security flag and the spec warning.\n\n### How to run it\n\n`python3 api_discovery.py openapi.json routes.csv`. Export the route list from the gateway, not from the spec. Exit 0 is clean, 1 is a finding, 2 is bad input.\n\n### The fix\n\nAdd security at the operation or document level for the missing routes, remove the deprecated paths from the gateway, and move admin routes to a separate listener behind SSO. Adding security to the spec does not enforce it; the gateway does.\n\n### Prevention\n\nGenerate the route inventory from the gateway config on every deploy, diff it with the spec, and give every deprecation a removal date and an owner.\n\n> **The honest caveat:** the spec is a declaration, not enforcement. A gateway route can bypass the global security block, and an operation can use auth the spec does not describe.\n\n> **The lesson:** every route the gateway serves is an API, documented or not.\n\n## In one line\n\n> **The gateway routes. The spec hopes.** The gateway routes what the spec forgot, and the spec cannot enforce what it declares.\n\n**Diff the routes:** `api_discovery.py openapi.json routes.csv`\n\n## Script: api_discovery.py\n\nDownload: [scripts/api_discovery.py](../scripts/api_discovery.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Diff an OpenAPI spec against the routes a gateway actually serves.\n\nUsage:\n    python3 api_discovery.py openapi.json routes.csv\n    python3 api_discovery.py specs.json routes.json --max 20\n\nInput (exports, never a live gateway call):\n    spec:    one OpenAPI 2/3 document, a list of documents, or {\"specs\": [...]}\n    routes:  CSV or JSON with method, path and optional public (true/false)\n\nFlags:\n    admin         admin-looking path with no security requirement\n    no-security   no global or operation security declared\n    undocumented  routed, but absent from every spec\n    deprecated    deprecated in the spec and still routed\n\nA route marked public in the inventory is exempt from the auth flags: a\nhealth check is meant to be public. A deprecated route is flagged whether\nor not it has auth. When two specs declare the same method and path, the\nfirst spec in the file wins, so put the authoritative one first.\n\nCron/CI:\n    0 6 * * * python3 api_discovery.py openapi.json /srv/exports/routes.csv\n\nExit codes:\n    0 no exposure\n    1 at least one exposed endpoint\n    2 bad input (unreadable file, bad JSON/CSV, missing method/path, bad public)\n\nRead-only: it makes no network calls and writes nothing. By s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport io\nimport json\nimport re\nimport sys\nfrom urllib.parse import urlparse\n\nMETHODS = (\"get\", \"put\", \"post\", \"delete\", \"options\", \"head\", \"patch\", \"trace\")\nADMIN_SEGMENTS = {\"admin\", \"administrator\", \"internal\", \"manage\", \"management\",\n                  \"debug\", \"actuator\", \"console\", \"private\", \"staff\", \"system\"}\nRANK = {\"admin\": 4, \"no-security\": 3, \"undocumented\": 2, \"deprecated\": 1}\n\n\ndef fail(message):\n    print(\"ERROR %s\" % message, file=sys.stderr)\n    return 2\n\n\ndef norm_keys(row):\n    return {str(k).strip().lower(): v for k, v in row.items() if k is not None}\n\n\ndef as_bool(value):\n    if isinstance(value, bool):\n        return value\n    text = str(value or \"\").strip().lower()\n    if text in (\"\", \"false\", \"0\", \"no\", \"off\"):\n        return False\n    if text in (\"true\", \"1\", \"yes\", \"on\"):\n        return True\n    raise ValueError(\"not a boolean: %r\" % str(value))\n\n\ndef norm_path(value):\n    text = str(value or \"\").strip()\n    text = text.split(\"?\")[0].split(\"#\")[0]\n    if not text.startswith(\"/\"):\n        text = \"/\" + text\n    if len(text) > 1:\n        text = text.rstrip(\"/\")\n    return text\n\n\ndef path_regex(path):\n    parts = re.split(r\"(\\{[^}]+\\})\", path)\n    body = \"\".join(\"[^/]+\" if part.startswith(\"{\") else re.escape(part)\n                   for part in parts)\n    return re.compile(\"^\" + body + \"$\")\n\n\ndef spec_name(doc, index):\n    info = doc.get(\"info\") if isinstance(doc.get(\"info\"), dict) else {}\n    title = str(info.get(\"title\") or \"\").strip()\n    return title or \"spec %d\" % index\n\n\ndef base_path(doc):\n    servers = doc.get(\"servers\")\n    if isinstance(servers, list) and servers and isinstance(servers[0], dict):\n        url = str(servers[0].get(\"url\") or \"\")\n        return urlparse(url).path.rstrip(\"/\")\n    return str(doc.get(\"basePath\") or \"\").rstrip(\"/\")\n\n\ndef load_specs(path):\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            data = json.load(handle)\n    except OSError as exc:\n        raise ValueError(\"cannot read spec %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise ValueError(\"bad JSON in spec %s: %s\" % (path, exc))\n\n    if isinstance(data, dict) and isinstance(data.get(\"specs\"), list):\n        data = data[\"specs\"]\n    elif isinstance(data, dict):\n        data = [data]\n    if not isinstance(data, list) or not data:\n        raise ValueError(\"spec must be an object, a list, or {specs: [...]}\")\n    specs = []\n    for i, doc in enumerate(data, 1):\n        if not isinstance(doc, dict) or not isinstance(doc.get(\"paths\"), dict):\n            raise ValueError(\"spec %d has no paths object\" % i)\n        specs.append((spec_name(doc, i), doc))\n    return specs\n\n\ndef build_routes(specs):\n    routes = []\n    seen = set()\n    for name, doc in specs:\n        base = base_path(doc)\n        for template, item in doc[\"paths\"].items():\n            if not isinstance(item, dict):\n                continue\n            for method in METHODS:\n                op = item.get(method)\n                if not isinstance(op, dict):\n                    continue\n                for prefix in (base, \"\"):\n                    full = norm_path(str(prefix) + str(template))\n                    key = (method, full)\n                    if key in seen:\n                        continue\n                    seen.add(key)\n                    routes.append({\n                        \"method\": method,\n                        \"path\": full,\n                        \"regex\": path_regex(full),\n                        \"op\": op,\n                        \"doc\": doc,\n                        \"spec\": name,\n                    })\n    return routes\n\n\ndef load_routes(path):\n    try:\n        with open(path, encoding=\"utf-8\", errors=\"replace\") as handle:\n            raw = handle.read()\n    except OSError as exc:\n        raise ValueError(\"cannot read routes %s: %s\" % (path, exc))\n    if not raw.strip():\n        raise ValueError(\"empty input: %s\" % path)\n\n    if raw.lstrip().startswith((\"[\", \"{\")):\n        try:\n            data = json.loads(raw)\n        except ValueError as exc:\n            raise ValueError(\"bad JSON in routes %s: %s\" % (path, exc))\n        if isinstance(data, dict):\n            for key in (\"routes\", \"endpoints\", \"items\"):\n                if isinstance(data.get(key), list):\n                    data = data[key]\n                    break\n            else:\n                raise ValueError(\"JSON object has no routes/endpoints list\")\n        if not isinstance(data, list):\n            raise ValueError(\"routes must be a list or an object with a routes list\")\n        rows = []\n        for i, row in enumerate(data, 1):\n            if not isinstance(row, dict):\n                raise ValueError(\"route %d is not an object\" % i)\n            rows.append(normalise(norm_keys(row), i))\n        return rows\n\n    reader = csv.DictReader(io.StringIO(raw))\n    fields = [str(f).strip().lower() for f in (reader.fieldnames or [])]\n    if \"method\" not in fields or \"path\" not in fields:\n        raise ValueError(\"CSV needs method and path columns, got %s\" % fields)\n    return [normalise(norm_keys(row), i + 2) for i, row in enumerate(reader)]\n\n\ndef normalise(row, line_no):\n    method = str(row.get(\"method\") or \"\").strip()\n    path = str(row.get(\"path\") or \"\").strip()\n    if not method:\n        raise ValueError(\"row %d has no method\" % line_no)\n    if not path:\n        raise ValueError(\"row %d has no path\" % line_no)\n    try:\n        public = as_bool(row.get(\"public\"))\n    except ValueError as exc:\n        raise ValueError(\"row %d (%s %s): %s\" % (line_no, method, path, exc))\n    return {\"method\": method.upper(), \"path\": norm_path(path), \"public\": public}\n\n\ndef has_security(op, doc):\n    \"\"\"[] is explicit public; [{}] is an anonymous alternative, so also public.\"\"\"\n    sec = op[\"security\"] if \"security\" in op else doc.get(\"security\")\n    if not isinstance(sec, list) or not sec:\n        return False\n    return all(isinstance(req, dict) and bool(req) for req in sec)\n\n\ndef schemes(doc):\n    components = doc.get(\"components\")\n    if isinstance(components, dict) and isinstance(components.get(\"securitySchemes\"), dict):\n        return components[\"securitySchemes\"]\n    definitions = doc.get(\"securityDefinitions\")\n    return definitions if isinstance(definitions, dict) else {}\n\n\ndef is_admin(path):\n    return any(segment in ADMIN_SEGMENTS for segment in path.lower().split(\"/\"))\n\n\ndef classify(route, public, path):\n    admin = is_admin(path)\n    if route is None:\n        return ([\"admin\"] if admin else []) + [\"undocumented\"]\n    op, doc = route[\"op\"], route[\"doc\"]\n    secured = has_security(op, doc)\n    flags = []\n    if admin and not secured and not public:\n        flags.append(\"admin\")\n    if not secured and not public:\n        flags.append(\"no-security\")\n    if op.get(\"deprecated\") is True:\n        flags.append(\"deprecated\")\n    return flags\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Diff an OpenAPI spec against a gateway route inventory.\",\n        epilog=\"Flags auth gaps, admin routes, undocumented routes and routed deprecations.\")\n    parser.add_argument(\"spec\", help=\"OpenAPI spec: JSON\")\n    parser.add_argument(\"routes\", help=\"endpoint inventory: CSV or JSON\")\n    parser.add_argument(\"--max\", type=int, default=100,\n                        help=\"show at most N endpoints (default: 100)\")\n    args = parser.parse_args(argv)\n    if args.max < 1:\n        return fail(\"--max must be at least 1\")\n\n    try:\n        specs = load_specs(args.spec)\n        inventory = load_routes(args.routes)\n    except ValueError as exc:\n        return fail(str(exc))\n\n    for name, doc in specs:\n        if not schemes(doc):\n            print(\"WARN spec %r defines no securitySchemes\" % name)\n\n    routes = build_routes(specs)\n    found = []\n    for endpoint in inventory:\n        match = None\n        for route in routes:\n            if route[\"method\"] == endpoint[\"method\"].lower() and \\\n                    route[\"regex\"].match(endpoint[\"path\"]):\n                match = route\n                break\n        if match is None:\n            flags = classify(None, endpoint[\"public\"], endpoint[\"path\"])\n        else:\n            flags = classify(match, endpoint[\"public\"], endpoint[\"path\"])\n        if flags:\n            rank = max(RANK[flag] for flag in flags)\n            found.append((rank, endpoint[\"method\"], endpoint[\"path\"], flags))\n    found.sort(key=lambda item: (-item[0], item[1], item[2]))\n\n    if not found:\n        print(\"OK no exposed endpoint in %d routed\" % len(inventory))\n        return 0\n\n    print(\"FINDING %d exposed endpoints of %d routed\" % (len(found), len(inventory)))\n    for _rank, method, path, flags in found[:args.max]:\n        print(\"ALERT %s %s %s\" % (method, path, \",\".join(flags)))\n    if len(found) > args.max:\n        print(\"NOTE %d more endpoints not shown\" % (len(found) - args.max))\n    return 1\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p737","n":"737","slug":"the-exposure-pipeline","title":"The exposure pipeline","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["AttackSurface","CyberSecurity","DevSecOps","CloudSecurity","VulnerabilityManagement"],"series_name":"ASM","design":"architecture","script":"","date":"2031-06-12","files":{"devsecops":"notes/p737-the-exposure-pipeline.md"},"md":"# The exposure pipeline\n\nThe scan found 312 exposures. The review found 40 that mattered, because 272 had no owner and no ticket.\n\n## Quick reference\n\n- **Probe:** HTTP probe (HTTP) · Port scan (PORT)\n- **Classify:** Admin panel (ADMIN) · Storage (BUCK) · API (ROUTE)\n- **Own:** Team (OWNER) · Ticket (TICK)\n\n**Request flow:** HTTP probe → Admin panel → Team; HTTP probe → API → Team; API → Ticket; Port scan → Admin panel; Port scan → Storage → Ticket\n\n**Around it:** Owner field (OWN) · SLA (SLA) · Evidence (EV)\n\n## Explained\n\n### Probe\n\nThe first tier is the export: an HTTP probe for panels and a port scan for services. It answers one question, what is reachable from outside. It does not decide severity, and it does not open a ticket.\n\n### Classify\n\nThe second tier assigns a type: admin panel, storage, API. Each type carries a rule and a score. An admin panel at 200 outranks a login page at 200; a public bucket with a listing grant outranks a private bucket with no encryption. Classification is where the export becomes a finding.\n\n### Own\n\nThe third tier assigns a team and a ticket. A finding without an owner is not a finding, it is a screenshot. The owner gets the evidence, the type and the deadline.\n\nThe fix is the ticket: close the panel, remove the grant, rotate the key. The pipeline's job ends when the ticket exists; the team's job ends when the rescan is clean.\n\n### Prevention\n\nRun the pipeline weekly, compare the new list with the previous one, and report by owner. A number that never reaches a team is a dashboard, not a programme.\n\n> **The honest caveat:** the pipeline sees only what the probes saw. A missing probe is not a clean result; coverage is a separate metric.\n\n> **The lesson:** an exposure without an owner is a permanent exposure.\n\n## In one line\n\n> **Probe, classify, then assign.** Probe sees surface, classify names it, own makes it someone's job.\n\n**Start with probe**\n\n```bash\ncurl -sI https://app.example.com\n```\n"},{"id":"p738","n":"738","slug":"exposure-hunting-commands","title":"Exposure hunting commands","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","CommandLine","CloudSecurity","DevSecOps"],"series_name":"ASM","design":"cheatsheet","script":"","date":"2031-06-15","files":{"devsecops":"notes/p738-exposure-hunting-commands.md"},"md":"# Exposure hunting commands\n\nFour blocks of commands answer the four questions an exposure review asks: what answers, what listens, what lists, what leaked.\n\n## Quick reference\n\n### HTTP\n\n```bash\ncurl -sI https://app.example.com/admin\nhttpx -u app.example.com -title -sc\nnuclei -u app.example.com -tags admin\nwhatweb -a 3 https://app.example.com\ncurl -s https://example.com/robots.txt\n```\n\n### Ports\n\n```bash\nnmap -Pn -p 22,3306,6379 203.0.113.10\nnmap -sV -p 22,3306,6379 203.0.113.10\nnaabu -host 203.0.113.10 -top-ports 1000\nnc -zv 203.0.113.10 22\nmasscan -p22,3306 203.0.113.10 --rate 1000\n```\n\n### Storage\n\n```bash\naws s3api get-bucket-acl --bucket acme\naws s3 ls s3://acme --no-sign-request\naws s3api get-bucket-policy --bucket acme\ngsutil iam get gs://acme-backups\naz storage container show-permission -n c1\n```\n\n### JavaScript\n\n```bash\nrg -n 'AKIA[0-9A-Z]{16}' dist/\nrg -n 'sk_live_' dist/*.js\ngitleaks detect --no-git -s dist/\nsemgrep --config p/secrets dist/\ntrufflehog filesystem dist/\n```\n\n## Explained\n\n### HTTP\n\n`curl -sI https://app.example.com/admin` returns the status of a panel. httpx -u app.example.com -title -sc adds the title and status, and nuclei -u app.example.com -tags admin runs the templates. whatweb -a 3 https://app.example.com names the stack, and `curl -s https://example.com/robots.txt` finds the path a site admits. First pass, not a verdict.\n\n### Ports\n\n`nmap -Pn -p 22,3306,6379 203.0.113.10` names the admin and database ports. `nmap -sV` adds the service, naabu -host 203.0.113.10 -top-ports 1000 is the fast sweep, `nc -zv` proves one port answers, and masscan -p22,3306 203.0.113.10 `--rate 1000` covers a range. Run it from outside.\n\n### Storage\n\n`aws s3api get-bucket-acl --bucket acme` shows the grants, and `aws s3 ls s3://acme --no-sign-request` proves anonymous listing. `aws s3api get-bucket-policy --bucket acme` shows the policy; gsutil iam get gs://acme-backups and `az storage container show-permission -n c1` cover the other clouds.\n\n### JavaScript\n\n`rg -n 'AKIA[0-9A-Z]{16}' dist/` finds AWS key ids, `rg -n 'sk_live_' dist/*.js` finds Stripe secrets, and `gitleaks detect --no-git -s dist/` runs a wider set. `semgrep --config p/secrets dist/` adds rules, and trufflehog filesystem dist/ verifies what it finds. Mask the value.\n\n### The fix\n\nThe output is a queue: one line per exposure, with an owner and a date.\n\n### Prevention\n\nRun the same four blocks weekly and diff the output.\n\n> **The honest caveat:** a filtered port looks closed, not absent, and a 200 can be a login page. The commands find surface; confirmation is manual.\n\n> **The lesson:** exposure hunting is four habits, not forty tools.\n\n## In one line\n\n> **Ask four questions. Keep four answers.** Four blocks, one loop: run, diff, assign, rescan.\n\n**Ask the first**\n\n```bash\ncurl -sI https://app.example.com\n```\n"},{"id":"p739","n":"739","slug":"four-exposures-that-persist","title":"Four exposures that persist","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","AppSec","CloudSecurity","RiskManagement"],"series_name":"ASM","design":"explainer","script":"","date":"2031-06-17","files":{"devsecops":"notes/p739-four-exposures-that-persist.md"},"md":"# Four exposures that persist\n\nEvery remediation sprint closes the same tickets and leaves the same four exposures. Each one sits outside a different team's patch window.\n\n## Quick reference\n\n### Admin panel\n\n- 200 at /admin, no SSO, no owner in the CMDB\n- Survives because nobody probes it from outside\n- Fix: SSO or VPN, then remove the old DNS record\n- A 200 is reachability, not a login bypass\n\n### Leaked key\n\n- AWS, Google, Stripe or bearer, shipped in a bundle\n- Still valid because rotation breaks a build\n- Fix: revoke, rotate, then purge the CDN cache\n- Deleting the line leaves the key in every cache\n\n### Public bucket\n\n- Public ACL, private policy, account block off\n- ListBucket returns keys, sizes and dates\n- Fix: block public access, then remove the grant\n- Test with an unauthenticated request, not the console\n\n### Legacy API\n\n- Deprecated in the spec, still routed by the gateway\n- No security scheme, no owner, no removal date\n- Fix: remove the route, not just the documentation\n- Every gateway route is an API, documented or not\n\n> **Note:** The four that survive a sprint because each has no owner.\n\n## Explained\n\n### Admin panel\n\nThe panel answers 200, has no SSO, and is owned by a team that calls it internal. It survives because nobody probes it from outside and the DNS record is old.\n\n### Leaked key\n\nThe key is in a bundle, still valid, and shared by three services. It survives because rotation breaks a build and the build has no owner.\n\n### Public bucket\n\nThe ACL says public and the policy says private. It survives because the console shows a private badge and nobody tests with an unauthenticated request.\n\n### Legacy API\n\nThe route is deprecated, still routed, and documented in a wiki that nobody updates. It survives because removal needs a date, a team and a consumer list.\n\n### The fix\n\nAssign each one an owner and a removal step: SSO, rotation, block plus ACL cleanup, gateway removal. A date without an owner is a wish.\n\n### Prevention\n\nReview the same four every month, with the same four questions. The list should get shorter; if it does not, the process is the exposure.\n\n> **The honest caveat:** these four persist because each is a decision nobody made, not because the fix is hard. The technical work is an afternoon; the ownership is the delay.\n\n> **The lesson:** persistence is an ownership failure, not a technical one.\n\n## In one line\n\n> **Four doors, four owners.** Admin panel, leaked key, public bucket, legacy API: four owners, not four tools.\n\n**Find the owner**\n\n```bash\ngrep -c owner findings.csv\n```\n"},{"id":"p740","n":"740","slug":"9-commands-to-check-your-exposure","title":"9 commands to check your exposure","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","CommandLine","CloudSecurity","AttackSurface","DevSecOps"],"series_name":"ASM","design":"terminal","script":"","date":"2031-06-19","files":{"devsecops":"notes/p740-9-commands-to-check-your-exposure.md"},"md":"# 9 commands to check your exposure\n\nNine commands, three passes: probe from outside, confirm what answered, scope how far it reaches.\n\n## Quick reference\n\n### What answers?\n\n```bash\n# an admin path answers 200\ncurl -sI https://app.example.com/admin\n# admin and database ports\nnmap -Pn -p 22,3306 203.0.113.10\n# the panel names itself\ncurl -s https://example.com/robots.txt\n```\n\n### Is it real?\n\n```bash\n# 200, not a 302 to /login\ncurl -sI https://app.example.com/admin/\n# a public grant\naws s3api get-bucket-acl --bucket acme\n# admin route still declared\njq '.paths|keys' openapi.json\n```\n\n### How far?\n\n```bash\n# key in the shipped bundle\nrg -l 'AKIA[0-9A-Z]{16}' dist/\n# anonymous listing works\naws s3 ls s3://acme --no-sign-request\n# route added 41 days ago\ngit log -S 'admin' --oneline -- src/\n```\n\n## Explained\n\n### Probe\n\nThe first pass asks what is there. `curl -sI` on the admin path, nmap on the admin and database ports, `robots.txt` for the paths the site admits. It is noisy and cheap; do it from outside your network.\n\n### Confirm\n\nThe second pass removes the false positives. Does /admin return 200 or redirect to /login? Does the bucket ACL really grant AllUsers? Does the OpenAPI document still declare the admin route? A 403 can be a WAF; a 200 can be a login page.\n\n### Scope\n\nThe third pass asks what the exposure touches. rg for a key pattern in the bundle, an anonymous S3 list, git log for when the admin route appeared. Scope decides whether this is a ticket or an incident.\n\n### The fix\n\nClose the port, remove the grant, rotate the key. The nine commands produce evidence, not a fix.\n\n### Prevention\n\nRun the nine weekly and alert on a new 200, a new AllUsers grant or a new key pattern. The loop is the control.\n\n> **The honest caveat:** reachability is cheap to test and impact is expensive to prove. The nine commands tell you where to look, not what an attacker got.\n\n> **The lesson:** a 200 is a door, not a finding.\n\n## In one line\n\n> **A 200 is a door, not a finding.** Reachability is the evidence; the owner is the fix.\n\n**Try it**\n\n```bash\ncurl -sI https://app.example.com/admin\n```\n"},{"id":"p741","n":"741","slug":"quiz-3-exposure-questions","title":"Quiz: 3 exposure questions","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["CyberSecurity","AttackSurface","AppSec","IncidentResponse","CloudSecurity"],"series_name":"ASM","design":"quiz","script":"","date":"2031-06-22","files":{"devsecops":"notes/p741-quiz-3-exposure-questions.md"},"md":"# Quiz: 3 exposure questions\n\nThree questions from this phase, each one a decision you make while the clock is running.\n\n## Quick reference\n\n**Q1. Why is a 200 on a login page worse than a 403?**\n\n- a) The request reached the panel\n- b) The login is already bypassed\n- c) A 403 means the page is cached\n\n> **Answer:** a) The request reached the panel. A 403 is a refusal, often from an edge or ACL. A 200 means the request reached the panel, so the exposure is real. It still does not prove a bypass.\n\n**Q2. What does an anonymous bucket listing reveal?**\n\n- a) Only the bucket name\n- b) The account's IAM policy\n- c) Object names, sizes and dates\n\n> **Answer:** c) Object names, sizes and dates. ListBucket returns keys, sizes and last-modified dates. Names leak customer IDs, backups and internal paths, and every key is a URL if the object is also public.\n\n**Q3. Why is a leaked key an incident, not a finding?**\n\n- a) It costs money to rotate\n- b) It may already be in use\n- c) It expires automatically\n\n> **Answer:** b) It may already be in use. A finding is a weakness to schedule; a live credential is access that may already be exercised. You cannot tell from the repo, so rotate first and read the logs.\n\n## Explained\n\n### Question one\n\nA login page returns 200; the same path behind a WAF returns 403. Which is worse? The 200 means the request reached the panel, so the exposure is real. The 403 is a refusal, often from something in front. Neither proves the login is bypassed, but the 200 is the one you can test.\n\n### Question two\n\nA bucket lists anonymously. What did you just leak? Object names, sizes and last-modified dates. Names carry customer IDs, backup schedules and internal paths, and every listed object is a URL if it is also public. The listing is the map; the objects are the data.\n\n### Question three\n\nA key is found in a bundle. Why is that an incident and not a finding? A finding is a weakness you can schedule. A live credential is access that may already be in use. You cannot tell from the repository whether it was exercised, so you rotate first and read the logs second.\n\n> **The honest caveat:** the first two answers are about exposure, not impact. A reachable panel with MFA and a listing with no sensitive keys are still findings, but the response is a ticket, not a page.\n\n> **The lesson:** a finding is a weakness; an incident is access.\n\n## In one line\n\n> **A key is access, not a finding.** Exposure is what answered; impact is what it can reach.\n\n**Check the key**\n\n```bash\naws iam list-access-keys --user-name app\n```\n"},{"id":"p742","n":"742","slug":"rank-findings-by-exploit-evidence","title":"Rank findings by exploit evidence","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","EPSS","CISAKEV","CyberSecurity","DevSecOps"],"series_name":"ASM","design":"code","script":"epss_rank.py","date":"2031-06-24","files":{"devsecops":"notes/p742-rank-findings-by-exploit-evidence.md"},"md":"# Rank findings by exploit evidence\n\nA quarterly scan returns 2,041 findings. The team fixes eighteen, and next quarter brings 2,041 back.\n\n## Quick reference\n\nThe full script is at the end of this note: `epss_rank.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 epss_rank.py findings.json epss.csv kev.csv\n```\n\n**Example output**\n\n```text\nRANK 3 findings, 1 in KEV, 1 unranked\nTOP CVE-2024-3400 cvss 10.0 epss 0.94 x1.5\n```\n\n## Explained\n\n### What it does\n\n`epss_rank.py` reads a findings export (cve, cvss, package, asset), the daily EPSS scores export and the KEV catalog. A CVE in KEV ranks first, because exploitation is a fact, not a forecast. Everything else ranks by EPSS times an exposure multiplier you supply per asset. Every factor is printed beside it.\n\n### What the screen shows\n\nThe panel sorts three rows on the KEV flag, then on EPSS times multiplier, and prints RANK 3 findings, 1 in KEV, 1 unranked, then TOP CVE-2024-3400 cvss 10.0 epss 0.94 x1.5. The script does the same on yours.\n\n### How to run it\n\n`python3 epss_rank.py findings.json epss_scores.csv kev.csv --top 20`, nightly, after the EPSS export refreshes. The scores are republished daily. Exit 0 means every finding above the CVSS floor was ranked, 1 means one had neither a score nor a KEV entry, 2 means bad input.\n\n### The fix\n\nWork the top twenty on Monday, not position 400. A KEV hit gets an owner and a date the same day.\n\n### Prevention\n\nRank at ingest, not in a spreadsheet at quarter end. A critical finding without an EPSS score should fail the pipeline, not sink to the bottom.\n\n> **The honest caveat:** EPSS is a probability for the CVE in general, not a statement about your asset; the multiplier and the CVSS floor are policy.\n\n> **The lesson:** severity says how bad it would be, and exploitation evidence says whether anyone is trying.\n\n## In one line\n\n> **Rank the queue, not the fear.** Severity says how bad. Exploitation evidence says how soon.\n\n**Sort by signal**\n\n```bash\npython3 epss_rank.py f.json e.csv\n```\n\n## Script: epss_rank.py\n\nDownload: [scripts/epss_rank.py](../scripts/epss_rank.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Rank findings by exploit evidence, then by severity.\n\nJoins a findings export (JSON) with the FIRST EPSS scores export (CSV or CSV.GZ)\nand the CISA KEV catalog (CSV, or one CVE per line). A CVE in KEV ranks first.\nEverything else ranks by EPSS score times an exposure multiplier for the asset.\nEvery number that produced a rank is printed beside it.\n\n    python3 epss_rank.py findings.json epss_scores.csv kev.csv --top 20\n    python3 epss_rank.py findings.json epss.csv kev.csv --exposure ex.json\n\nNightly, after the EPSS export is refreshed:\n\n    0 4 * * * python3 epss_rank.py findings.json epss.csv kev.csv --top 20\n\nThe default --cvss-floor of 7.0 is a policy threshold, not a standard: it is\nthe line above which a finding with no EPSS score is called unrankable.\n\nExit codes: 0 every high-CVSS finding was ranked, 1 at least one high-CVSS\nfinding has no EPSS score and is not in KEV, 2 bad input.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport gzip\nimport json\nimport sys\n\n\ndef die(msg):\n    sys.stderr.write(\"epss_rank: %s\\n\" % msg)\n    raise SystemExit(2)\n\n\ndef open_text(path):\n    if path.endswith(\".gz\"):\n        return gzip.open(path, \"rt\", encoding=\"utf-8\", errors=\"replace\")\n    return open(path, encoding=\"utf-8\", errors=\"replace\")\n\n\ndef load_findings(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read findings %s: %s\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"findings\", data.get(\"results\", []))\n    if not isinstance(data, list):\n        die(\"findings must be a JSON list, got %s\" % type(data).__name__)\n    out = []\n    for i, f in enumerate(data):\n        if not isinstance(f, dict):\n            die(\"finding %d is not an object\" % i)\n        cve = str(f.get(\"cve\", \"\")).strip().upper()\n        if not cve.startswith(\"CVE-\"):\n            die(\"finding %d has no cve field\" % i)\n        if f.get(\"cvss\") is None:\n            die(\"finding %d (%s) has no cvss field\" % (i, cve))\n        try:\n            cvss = float(f[\"cvss\"])\n        except (TypeError, ValueError):\n            die(\"finding %d (%s) has a non-numeric cvss\" % (i, cve))\n        if not 0.0 <= cvss <= 10.0:\n            die(\"finding %d (%s) cvss out of range\" % (i, cve))\n        out.append({\n            \"cve\": cve,\n            \"cvss\": cvss,\n            \"package\": str(f.get(\"package\", \"\") or \"-\"),\n            \"asset\": str(f.get(\"asset\", \"\") or \"-\"),\n        })\n    return out\n\n\ndef load_epss(path):\n    \"\"\"FIRST export: optional # comment line, then cve,epss,percentile.\"\"\"\n    scores = {}\n    try:\n        fh = open_text(path)\n    except OSError as exc:\n        die(\"cannot read EPSS export %s: %s\" % (path, exc))\n    with fh:\n        for row in csv.reader(fh):\n            if not row:\n                continue\n            first = row[0].strip()\n            if not first or first.startswith(\"#\"):\n                continue\n            if first.lower() in (\"cve\", \"cveid\", \"cve_id\"):\n                continue\n            if len(row) < 2 or not first.upper().startswith(\"CVE-\"):\n                continue\n            try:\n                score = float(row[1])\n            except ValueError:\n                continue\n            if not 0.0 <= score <= 1.0:\n                continue\n            pct = None\n            if len(row) > 2:\n                try:\n                    pct = float(row[2])\n                except ValueError:\n                    pct = None\n            scores[first.upper()] = (score, pct)\n    if not scores:\n        die(\"no EPSS scores in %s\" % path)\n    return scores\n\n\ndef load_kev(path):\n    \"\"\"CISA catalog CSV with a CVE column, or a plain list, one CVE per line.\"\"\"\n    kev = set()\n    try:\n        fh = open_text(path)\n    except OSError as exc:\n        die(\"cannot read KEV list %s: %s\" % (path, exc))\n    with fh:\n        rows = csv.reader(fh)\n        idx = 0\n        for row in rows:\n            if not row:\n                continue\n            first = row[0].strip()\n            if not first or first.startswith(\"#\"):\n                continue\n            names = [c.strip().lower() for c in row]\n            if any(name in (\"cve\", \"cveid\", \"cve_id\") for name in names):\n                for i, name in enumerate(names):\n                    if name in (\"cve\", \"cveid\", \"cve_id\"):\n                        idx = i\n                        break\n                continue\n            if len(row) > idx:\n                cve = row[idx].strip().upper()\n                if cve.startswith(\"CVE-\"):\n                    kev.add(cve)\n    if not kev:\n        die(\"no CVE entries in KEV list %s\" % path)\n    return kev\n\n\ndef load_exposure(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read exposure map %s: %s\" % (path, exc))\n    if isinstance(data, dict) and \"assets\" in data:\n        data = data[\"assets\"]\n    if not isinstance(data, dict):\n        die(\"exposure map must be a JSON object of asset -> multiplier\")\n    out = {}\n    for asset, value in data.items():\n        try:\n            mult = float(value)\n        except (TypeError, ValueError):\n            die(\"exposure for %s is not a number\" % asset)\n        if mult <= 0:\n            die(\"exposure for %s must be greater than zero\" % asset)\n        out[str(asset)] = mult\n    return out\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Rank findings by KEV, then EPSS x exposure.\")\n    ap.add_argument(\"findings\", help=\"findings export (JSON)\")\n    ap.add_argument(\"epss\", help=\"EPSS scores export (CSV or CSV.GZ)\")\n    ap.add_argument(\"kev\", help=\"KEV catalog (CSV) or one CVE per line\")\n    ap.add_argument(\"--top\", type=int, default=20,\n                    help=\"how many ranked findings to print (default 20)\")\n    ap.add_argument(\"--cvss-floor\", type=float, default=7.0,\n                    help=\"policy line for high CVSS (default 7.0)\")\n    ap.add_argument(\"--exposure\", help=\"JSON object: asset -> multiplier\")\n    ap.add_argument(\"--json\", action=\"store_true\", help=\"print JSON, not text\")\n    args = ap.parse_args(argv)\n\n    if args.top < 1:\n        die(\"--top must be at least 1\")\n    if not 0.0 <= args.cvss_floor <= 10.0:\n        die(\"--cvss-floor must be between 0 and 10\")\n\n    findings = load_findings(args.findings)\n    epss = load_epss(args.epss)\n    kev = load_kev(args.kev)\n    exposure = load_exposure(args.exposure) if args.exposure else {}\n\n    rows = []\n    for f in findings:\n        score, pct = epss.get(f[\"cve\"], (None, None))\n        mult = exposure.get(f[\"asset\"], 1.0)\n        rows.append({\n            \"cve\": f[\"cve\"],\n            \"cvss\": f[\"cvss\"],\n            \"package\": f[\"package\"],\n            \"asset\": f[\"asset\"],\n            \"kev\": f[\"cve\"] in kev,\n            \"epss\": score,\n            \"percentile\": pct,\n            \"exposure\": mult,\n            \"rank\": None if score is None else round(score * mult, 6),\n        })\n\n    def order(row):\n        rank = -1.0 if row[\"rank\"] is None else row[\"rank\"]\n        return (0 if row[\"kev\"] else 1, -rank, -row[\"cvss\"], row[\"cve\"])\n\n    ranked = sorted(rows, key=order)\n    shown = ranked[:args.top]\n    unranked = [r for r in ranked\n                if r[\"rank\"] is None and not r[\"kev\"]\n                and r[\"cvss\"] >= args.cvss_floor]\n    kev_hits = [r for r in ranked if r[\"kev\"]]\n\n    if args.json:\n        print(json.dumps({\n            \"counts\": {\"findings\": len(rows), \"kev\": len(kev_hits),\n                       \"shown\": len(shown), \"unranked\": len(unranked)},\n            \"policy\": {\"cvss_floor\": args.cvss_floor,\n                       \"exposure_default\": 1.0},\n            \"top\": shown,\n            \"unranked\": unranked,\n        }, indent=2))\n        return 1 if unranked else 0\n\n    print(\"POLICY cvss-floor %.1f, exposure default 1.0\" % args.cvss_floor)\n    print(\"RANK %d shown of %d findings (%d in KEV)\"\n          % (len(shown), len(rows), len(kev_hits)))\n    for i, r in enumerate(shown, 1):\n        epss_txt = \"-\" if r[\"epss\"] is None else \"%.3f\" % r[\"epss\"]\n        rank_txt = \"-\" if r[\"rank\"] is None else \"%.3f\" % r[\"rank\"]\n        print(\"%3d %-3s %-16s cvss %4.1f epss %5s x%.2f = %-6s %s %s\"\n              % (i, \"KEV\" if r[\"kev\"] else \"\", r[\"cve\"], r[\"cvss\"],\n                 epss_txt, r[\"exposure\"], rank_txt, r[\"asset\"], r[\"package\"]))\n    for r in unranked:\n        print(\"UNRANKED %-16s cvss %4.1f %s %s no EPSS score\"\n              % (r[\"cve\"], r[\"cvss\"], r[\"asset\"], r[\"package\"]))\n    print(\"SUMMARY %d finding%s, %d in KEV, %d unranked high CVSS\"\n          % (len(rows), \"\" if len(rows) == 1 else \"s\",\n             len(kev_hits), len(unranked)))\n    return 1 if unranked else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p743","n":"743","slug":"match-findings-against-the-kev-catalog","title":"Match findings against the KEV catalog","sections":["devsecops"],"topic":"Python scripts","tags":["CISAKEV","VulnerabilityManagement","ThreatIntel","PatchManagement","CyberSecurity"],"series_name":"ASM","design":"code","script":"kev_match.py","date":"2031-06-26","files":{"devsecops":"notes/p743-match-findings-against-the-kev-catalog.md"},"md":"# Match findings against the KEV catalog\n\nThe finding sat in the medium pile for five months. CVSS said 6.5; KEV said exploited in the wild since March, with a due date already passed.\n\n## Quick reference\n\nThe full script is at the end of this note: `kev_match.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 kev_match.py findings.json kev.csv\n```\n\n**Example output**\n\n```text\nHIT CVE-2024-3400 edge-01 due 2024-05-03 900d overdue\nSUMMARY 2 findings, 1 KEV, 1 overdue\n```\n\n## Explained\n\n### What it does\n\n`kev_match.py` joins a findings export with the CISA KEV catalog. It matches on CVE, pulls vendor, product and the date the entry was added, turns the due date into days remaining, and prints each hit with its deadline. The real CSV needs no editing: the loader reads cveID, vendorProject, product and dueDate.\n\n### What the screen shows\n\nThe panel walks two findings and prints HIT CVE-2024-3400 edge-01 due 2024-05-03 900d overdue, then SUMMARY 2 findings, 1 KEV, 1 overdue. A real export prints one HIT line per finding, most overdue first.\n\n### How to run it\n\n`python3 kev_match.py findings.json known_exploited_vulnerabilities.csv --today 2026-10-20`, every weekday before standup. Exit 0 is no match, 1 is at least one, 2 is bad input.\n\n### The fix\n\nA KEV hit is not a debate. Give it an owner, a patch window and a date today; if it cannot be patched, the compensating control needs the same date.\n\n### Prevention\n\nCheck KEV at ingest and again daily. Absence is not safety: the catalogue holds exploitation with public evidence, not every exploitation.\n\n> **The honest caveat:** KEV is a floor. Real attacks never get an entry, and the due dates are written for US federal agencies.\n\n> **The lesson:** a severity score is an opinion about impact, and KEV is a record of someone trying.\n\n## In one line\n\n> **Exploited beats scored.** A severity score is an opinion. KEV is a record.\n\n**Check the catalog**\n\n```bash\npython3 kev_match.py f.json kev.csv\n```\n\n## Script: kev_match.py\n\nDownload: [scripts/kev_match.py](../scripts/kev_match.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Match findings against the CISA KEV catalog and show the due dates.\n\nReads a findings export (JSON) and a KEV catalog export (CSV). Prints every\nfinding whose CVE is listed, with vendor, product, the date it was added, the\nremediation due date, and whether that date has passed.\n\n    python3 kev_match.py findings.json known_exploited_vulnerabilities.csv\n    python3 kev_match.py findings.json kev.csv --today 2026-10-20\n\nThe findings export is a JSON list of objects with a cve field, plus asset,\ncvss and package where you have them.\n\nDaily, before the standup:\n\n    30 7 * * 1-5 python3 kev_match.py findings.json kev.csv > kev_today.txt\n\nKEV is a catalog of vulnerabilities with evidence of exploitation in the wild,\npublished by CISA. Each entry carries a due date for US federal civilian\nagencies; for everyone else it is still the best public deadline to copy.\n\nExit codes: 0 no finding matches KEV, 1 at least one matches, 2 bad input.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport csv\nimport datetime\nimport json\nimport sys\n\nCVE_COLS = (\"cve\", \"cveid\", \"cve_id\")\nVENDOR_COLS = (\"vendor\", \"vendorproject\", \"vendor_project\")\nPRODUCT_COLS = (\"product\",)\nADDED_COLS = (\"added\", \"dateadded\", \"date_added\")\nDUE_COLS = (\"due\", \"duedate\", \"due_date\")\nRANSOM_COLS = (\"ransomware\", \"knownransomwarecampaignuse\")\n\n\ndef die(msg):\n    sys.stderr.write(\"kev_match: %s\\n\" % msg)\n    raise SystemExit(2)\n\n\ndef pick(names, aliases, default):\n    for i, name in enumerate(names):\n        if name in aliases:\n            return i\n    return default\n\n\ndef parse_day(text):\n    text = (text or \"\").strip()\n    if not text:\n        return None\n    try:\n        return datetime.date.fromisoformat(text)\n    except ValueError:\n        return None\n\n\ndef load_findings(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read findings %s: %s\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"findings\", data.get(\"results\", []))\n    if not isinstance(data, list):\n        die(\"findings must be a JSON list, got %s\" % type(data).__name__)\n    out = []\n    for i, f in enumerate(data):\n        if not isinstance(f, dict):\n            die(\"finding %d is not an object\" % i)\n        cve = str(f.get(\"cve\", \"\")).strip().upper()\n        if not cve.startswith(\"CVE-\"):\n            die(\"finding %d has no cve field\" % i)\n        try:\n            cvss = float(f.get(\"cvss\", 0.0))\n        except (TypeError, ValueError):\n            die(\"finding %d (%s) has a non-numeric cvss\" % (i, cve))\n        out.append({\n            \"cve\": cve,\n            \"cvss\": cvss,\n            \"asset\": str(f.get(\"asset\", \"\") or \"-\"),\n            \"package\": str(f.get(\"package\", \"\") or \"-\"),\n        })\n    return out\n\n\ndef load_kev(path):\n    catalog = {}\n    try:\n        fh = open(path, encoding=\"utf-8\", errors=\"replace\")\n    except OSError as exc:\n        die(\"cannot read KEV catalog %s: %s\" % (path, exc))\n    with fh:\n        rows = csv.reader(fh)\n        header = None\n        for row in rows:\n            if not row or not any(c.strip() for c in row):\n                continue\n            if row[0].strip().startswith(\"#\"):\n                continue\n            names = [c.strip().lower() for c in row]\n            if header is None:\n                if any(n in CVE_COLS for n in names):\n                    header = names\n                    continue\n                header = []\n            if header:\n                ic = pick(header, CVE_COLS, 0)\n                iv = pick(header, VENDOR_COLS, -1)\n                ip = pick(header, PRODUCT_COLS, -1)\n                ia = pick(header, ADDED_COLS, -1)\n                idu = pick(header, DUE_COLS, -1)\n                ir = pick(header, RANSOM_COLS, -1)\n            else:\n                ic, iv, ip, ia, idu, ir = 0, -1, -1, -1, -1, -1\n            if len(row) <= ic:\n                continue\n            cve = row[ic].strip().upper()\n            if not cve.startswith(\"CVE-\"):\n                continue\n\n            def cell(i):\n                return row[i].strip() if 0 <= i < len(row) else \"\"\n\n            catalog[cve] = {\n                \"vendor\": cell(iv),\n                \"product\": cell(ip),\n                \"added\": parse_day(cell(ia)),\n                \"due\": parse_day(cell(idu)),\n                \"ransomware\": cell(ir),\n            }\n    if not catalog:\n        die(\"no CVE entries in KEV catalog %s\" % path)\n    return catalog\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Match findings against the CISA KEV catalog.\")\n    ap.add_argument(\"findings\", help=\"findings export (JSON)\")\n    ap.add_argument(\"kev\", help=\"KEV catalog export (CSV)\")\n    ap.add_argument(\"--today\", help=\"ISO date to compare due dates with\")\n    ap.add_argument(\"--json\", action=\"store_true\", help=\"print JSON, not text\")\n    args = ap.parse_args(argv)\n\n    today = datetime.date.today()\n    if args.today:\n        today = parse_day(args.today)\n        if today is None:\n            die(\"--today must be an ISO date such as 2026-10-20\")\n\n    findings = load_findings(args.findings)\n    catalog = load_kev(args.kev)\n\n    hits = []\n    for f in findings:\n        entry = catalog.get(f[\"cve\"])\n        if not entry:\n            continue\n        row = dict(f)\n        row.update({\n            \"vendor\": entry[\"vendor\"],\n            \"product\": entry[\"product\"],\n            \"added\": entry[\"added\"].isoformat() if entry[\"added\"] else \"\",\n            \"due\": entry[\"due\"].isoformat() if entry[\"due\"] else \"\",\n            \"ransomware\": entry[\"ransomware\"],\n        })\n        if entry[\"due\"] is None:\n            row[\"days\"] = None\n        else:\n            row[\"days\"] = (entry[\"due\"] - today).days\n        hits.append(row)\n\n    # Most overdue first, then the nearest due date, then unknown dates.\n    hits.sort(key=lambda r: (r[\"days\"] is None,\n                             r[\"days\"] if r[\"days\"] is not None else 0,\n                             r[\"cve\"]))\n    overdue = [h for h in hits if h[\"days\"] is not None and h[\"days\"] < 0]\n\n    if args.json:\n        print(json.dumps({\n            \"today\": today.isoformat(),\n            \"counts\": {\"findings\": len(findings), \"kev_hits\": len(hits),\n                       \"overdue\": len(overdue)},\n            \"hits\": hits,\n        }, indent=2, default=str))\n        return 1 if hits else 0\n\n    for h in hits:\n        name = \" \".join(x for x in (h[\"vendor\"], h[\"product\"]) if x) or \"-\"\n        if h[\"days\"] is None:\n            when = \"due unknown\"\n        elif h[\"days\"] < 0:\n            when = \"due %s %dd overdue\" % (h[\"due\"], abs(h[\"days\"]))\n        elif h[\"days\"] == 0:\n            when = \"due %s today\" % h[\"due\"]\n        else:\n            when = \"due %s in %dd\" % (h[\"due\"], h[\"days\"])\n        print(\"HIT %-16s %-10s %-28s added %-10s %s\"\n              % (h[\"cve\"], h[\"asset\"], name[:28],\n                 h[\"added\"] or \"-\", when))\n    print(\"SUMMARY %d finding%s, %d KEV hit%s, %d overdue as of %s\"\n          % (len(findings), \"\" if len(findings) == 1 else \"s\",\n             len(hits), \"\" if len(hits) == 1 else \"s\",\n             len(overdue), today.isoformat()))\n    return 1 if hits else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p744","n":"744","slug":"map-vulnerable-packages-onto-live-import-paths","title":"Map vulnerable packages onto live import paths","sections":["devsecops"],"topic":"Python scripts","tags":["AppSec","SCA","VulnerabilityManagement","Python","CyberSecurity"],"series_name":"ASM","design":"code","script":"reachability_map.py","date":"2031-06-29","files":{"devsecops":"notes/p744-map-vulnerable-packages-onto-live-import-paths.md"},"md":"# Map vulnerable packages onto live import paths\n\nA CVSS 9.8 in a PDF library led the report for three sprints. No live module imported it: the service that used it was deleted a year earlier.\n\n## Quick reference\n\nThe full script is at the end of this note: `reachability_map.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 reachability_map.py graph.json vuln.txt --entry app.main\n```\n\n**Example output**\n\n```text\nREACHABLE requests via app.api\nUNREACHABLE paramiko\n```\n\n## Explained\n\n### What it does\n\n`reachability_map.py` reads an import graph export, a JSON object mapping each module to what it imports, plus a list of vulnerable import names. It walks from your entry points, marks every module it can reach, then reports each package as reachable with its path, or unreachable with the reason.\n\n### What the screen shows\n\nThe panel walks from app.main and prints REACHABLE requests via app.api, then UNREACHABLE paramiko. The script adds the full path, the importer count and a summary; `--entry` names your roots.\n\n### How to run it\n\n`python3 reachability_map.py graph.json vuln.txt --entry app.main --entry` wsgi. Export the graph from your build tool or modulefinder and regenerate it on every merge. Exit 0 means nothing vulnerable is reachable, 1 means something is, 2 means bad input.\n\n### The fix\n\nMove the unreachable ones out of the weekly queue and keep the evidence line. Then delete the dead module, so the next graph needs no footnote.\n\n### Prevention\n\nTreat the graph as a build artefact: generate it in CI, diff it in review, and fail when a vulnerable package becomes reachable.\n\n> **The honest caveat:** static imports miss importlib, plugins, template loaders and deserialisation, and a reachable import is not proof the vulnerable function runs.\n\n> **The lesson:** reachability does not make a finding safe; it makes it later.\n\n## In one line\n\n> **Reachable is a maybe, not a no.** It moves a finding down the queue, not into the bin.\n\n**Walk the imports**\n\n```bash\npython3 reachability_map.py g.json v.txt\n```\n\n## Script: reachability_map.py\n\nDownload: [scripts/reachability_map.py](../scripts/reachability_map.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Mark vulnerable packages that a live code path can actually import.\n\nReads an import graph export (JSON: module -> list of modules it imports) and a\nlist of vulnerable packages, walks the graph from the entry points, and reports\nwhich packages are imported by a reachable module and which are not. For a\nreachable package it prints the shortest path from an entry point.\n\n    python3 reachability_map.py graph.json vuln_packages.txt\n    python3 reachability_map.py graph.json vuln.txt --entry app.main --entry wsgi\n\nEntry points default to modules that no other module imports. CI step, after\nthe graph export is regenerated (the build fails on exit 1):\n\n    python3 reachability_map.py build/import_graph.json build/vuln.txt\n\nThe package list is import names, not distribution names: pyyaml is imported as\nyaml, and python-dateutil as dateutil. A graph is static analysis. It cannot see\nimportlib, entry-point plugins, template loaders or code that is never executed.\n\nExit codes: 0 no vulnerable package is reachable, 1 at least one is, 2 bad\ninput.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\ndef die(msg):\n    sys.stderr.write(\"reachability_map: %s\\n\" % msg)\n    raise SystemExit(2)\n\n\ndef norm(name):\n    return name.strip().lower().replace(\"-\", \"_\")\n\n\ndef top(name):\n    return norm(name).split(\".\")[0]\n\n\ndef imports_package(imported, wanted):\n    name = norm(imported)\n    return name == wanted or top(name) == wanted\n\n\ndef load_graph(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read graph %s: %s\" % (path, exc))\n    if isinstance(data, dict) and \"modules\" in data:\n        data = data[\"modules\"]\n    if not isinstance(data, dict):\n        die(\"graph must be a JSON object of module -> imports\")\n    graph = {}\n    for module, imports in data.items():\n        if not isinstance(imports, list):\n            die(\"imports for %s must be a list\" % module)\n        clean = []\n        for item in imports:\n            if not isinstance(item, str):\n                die(\"import for %s is not a string\" % module)\n            if item.strip():\n                clean.append(item.strip())\n        graph[str(module).strip()] = clean\n    if not graph:\n        die(\"graph %s has no modules\" % path)\n    return graph\n\n\ndef load_packages(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            text = fh.read()\n    except OSError as exc:\n        die(\"cannot read package list %s: %s\" % (path, exc))\n    names = []\n    if text.lstrip().startswith(\"[\"):\n        try:\n            data = json.loads(text)\n        except ValueError as exc:\n            die(\"package list looks like JSON but does not parse: %s\" % exc)\n        if not isinstance(data, list):\n            die(\"JSON package list must be an array\")\n        for item in data:\n            if isinstance(item, str):\n                names.append(item)\n            elif isinstance(item, dict) and item.get(\"package\"):\n                names.append(str(item[\"package\"]))\n    else:\n        for line in text.splitlines():\n            line = line.split(\"#\", 1)[0].strip()\n            if line:\n                names.append(line)\n    out = []\n    for name in names:\n        for sep in (\"==\", \">=\", \"<=\", \"~=\", \">\", \"<\", \"=\"):\n            if sep in name:\n                name = name.split(sep, 1)[0]\n        if name.strip():\n            out.append(name.strip())\n    if not out:\n        die(\"no package names in %s\" % path)\n    seen, unique = set(), []\n    for name in out:\n        if norm(name) not in seen:\n            seen.add(norm(name))\n            unique.append(name)\n    return unique\n\n\ndef walk(graph, entries):\n    parent = {}\n    queue = []\n    for entry in entries:\n        if entry not in graph:\n            die(\"entry point %s is not in the graph\" % entry)\n        if entry not in parent:\n            parent[entry] = None\n            queue.append(entry)\n    i = 0\n    while i < len(queue):\n        module = queue[i]\n        i += 1\n        for imported in graph.get(module, []):\n            if imported not in graph:\n                continue\n            if imported not in parent:\n                parent[imported] = module\n                queue.append(imported)\n    return parent\n\n\ndef path_to(parent, module):\n    path = []\n    while module is not None:\n        path.append(module)\n        module = parent[module]\n    path.reverse()\n    return \" -> \".join(path)\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Report vulnerable packages on a live import path.\")\n    ap.add_argument(\"graph\", help=\"import graph export (JSON)\")\n    ap.add_argument(\"packages\", help=\"vulnerable package list (txt or JSON)\")\n    ap.add_argument(\"--entry\", action=\"append\", default=[],\n                    help=\"entry point module; repeatable\")\n    ap.add_argument(\"--json\", action=\"store_true\", help=\"print JSON, not text\")\n    args = ap.parse_args(argv)\n\n    graph = load_graph(args.graph)\n    packages = load_packages(args.packages)\n\n    imported_by = {}\n    for module, imports in graph.items():\n        for imported in imports:\n            imported_by.setdefault(imported, set()).add(module)\n\n    if args.entry:\n        entries = args.entry\n        root = False\n    else:\n        entries = sorted(m for m in graph if m not in imported_by)\n        root = True\n        if not entries:\n            # Every module is imported by another: a cycle. Assume the worst\n            # rather than report real code as unreachable.\n            entries = sorted(graph)\n            print(\"NOTE no root modules; treating all %d as entry points\"\n                  % len(entries), file=sys.stderr)\n\n    parent = walk(graph, entries)\n    live = set(parent)\n\n    rows = []\n    for package in packages:\n        wanted = norm(package)\n        live_hits = sorted(m for m in live\n                           if any(imports_package(i, wanted)\n                                  for i in graph.get(m, [])))\n        dead_hits = sorted(m for m in graph\n                           if m not in live\n                           and any(imports_package(i, wanted)\n                                   for i in graph.get(m, [])))\n        if live_hits:\n            rows.append({\"package\": package, \"state\": \"REACHABLE\",\n                         \"path\": path_to(parent, live_hits[0])\n                                 + \" -> \" + norm(package),\n                         \"importers\": live_hits})\n        elif dead_hits:\n            rows.append({\"package\": package, \"state\": \"UNREACHABLE\",\n                         \"path\": \"\", \"importers\": dead_hits,\n                         \"reason\": \"imported only by modules off the live path\"})\n        else:\n            rows.append({\"package\": package, \"state\": \"UNREACHABLE\",\n                         \"path\": \"\", \"importers\": [],\n                         \"reason\": \"no import edge in the graph\"})\n\n    reachable = [r for r in rows if r[\"state\"] == \"REACHABLE\"]\n\n    if args.json:\n        print(json.dumps({\n            \"entries\": entries,\n            \"counts\": {\"modules\": len(graph), \"reachable_modules\": len(live),\n                       \"packages\": len(rows), \"reachable_packages\": len(reachable)},\n            \"packages\": rows,\n        }, indent=2))\n        return 1 if reachable else 0\n\n    if root:\n        print(\"ENTRY %d roots: %s\"\n              % (len(entries), \", \".join(entries[:6])\n                 + (\" ...\" if len(entries) > 6 else \"\")))\n        print(\"NOTE no --entry given: every root counts as live, including \"\n              \"scripts and tests\")\n    else:\n        print(\"ENTRY %d given: %s\" % (len(entries), \", \".join(entries)))\n    for r in rows:\n        if r[\"state\"] == \"REACHABLE\":\n            print(\"REACHABLE %-14s path %s (%d live importer%s)\"\n                  % (r[\"package\"], r[\"path\"], len(r[\"importers\"]),\n                     \"\" if len(r[\"importers\"]) == 1 else \"s\"))\n        elif r[\"importers\"]:\n            print(\"UNREACHABLE %-12s %s: %s\"\n                  % (r[\"package\"], r[\"reason\"], \", \".join(r[\"importers\"])))\n        else:\n            print(\"UNREACHABLE %-12s %s\" % (r[\"package\"], r[\"reason\"]))\n    print(\"SUMMARY %d module%s, %d reachable, %d of %d vulnerable package%s \"\n          \"reachable\"\n          % (len(graph), \"\" if len(graph) == 1 else \"s\", len(live),\n             len(reachable), len(rows), \"\" if len(rows) == 1 else \"s\"))\n    return 1 if reachable else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p745","n":"745","slug":"score-the-asset-then-rank-the-finding","title":"Score the asset, then rank the finding","sections":["devsecops"],"topic":"Python scripts","tags":["RiskManagement","VulnerabilityManagement","AssetManagement","CyberSecurity","DevSecOps"],"series_name":"ASM","design":"code","script":"exposure_score.py","date":"2031-07-01","files":{"devsecops":"notes/p745-score-the-asset-then-rank-the-finding.md"},"md":"# Score the asset, then rank the finding\n\nThe same CVE sits on an admin console behind SSO and on a public checkout. The queue ranked them identically, because the only input was CVSS.\n\n## Quick reference\n\nThe full script is at the end of this note: `exposure_score.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 exposure_score.py assets.json findings.json\n```\n\n**Example output**\n\n```text\nASSET edge-01 score 100 exposure +40 data +30 complexity +30\nRANK 1000.0 CVE-2024-3400 cvss 10.0 edge-01 score 100\n```\n\n## Explained\n\n### What it does\n\n`exposure_score.py` scores every asset from four factors: internet-facing, authentication required, data class and access complexity. It prints the score with every contributor, then ranks findings by asset score times CVSS. The weights are a policy choice, stated at the top of the file.\n\n### What the screen shows\n\nThe panel scores edge-01 and prints ASSET edge-01 score 100 exposure +40 data +30 complexity +30, then RANK 1000.0 CVE-2024-3400 cvss 10.0 edge-01 score 100. The script does that for every asset, then every finding.\n\n### How to run it\n\n`python3 exposure_score.py assets.json findings.json --min-score 60`, weekly, against a CMDB export. The score is only as good as the inventory. Exit 0 means no high finding on a high-score asset, 1 means at least one, 2 means bad input.\n\n### The fix\n\nFix the checkout before the console, and put the score and its factors in the ticket so the owner sees the reasoning.\n\n### Prevention\n\nReview the weights quarterly with the asset owners. A score nobody can explain gets argued with, then ignored.\n\n> **The honest caveat:** two teams will score the same host differently, and both can be right; the point is consistency inside one estate.\n\n> **The lesson:** a CVE describes code, and the asset decides what that code is worth.\n\n## In one line\n\n> **The asset is the context.** A CVSS score cannot see where the code runs.\n\n**Score the asset**\n\n```bash\npython3 exposure_score.py a.json f.json\n```\n\n## Script: exposure_score.py\n\nDownload: [scripts/exposure_score.py](../scripts/exposure_score.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Score each asset, then rank findings by asset score x CVSS.\n\nReads an asset export (JSON) and a findings export (JSON). Scores every asset\nfrom four factors, prints the factors next to the score, then ranks the\nfindings by asset score times CVSS base score.\n\n    python3 exposure_score.py assets.json findings.json\n    python3 exposure_score.py cmdb.json findings.json --min-score 70\n\nThe asset export is an object keyed by asset name, each with internet_facing\nand auth_required booleans, data (high, medium, low) and complexity (low,\nmedium, high). A list of objects with an asset field also works.\n\nWeekly, after the CMDB export:\n\n    0 6 * * 1 python3 exposure_score.py cmdb.json findings.json --min-score 60\n\nThe weights below are a policy choice, not an industry standard. Change them\nin one place, write down why, and review them quarterly. The --min-score and\n--cvss-floor defaults are policy lines too.\n\nExit codes: 0 no finding at or above the CVSS floor sits on an asset at or\nabove the score line, 1 at least one does, 2 bad input.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nWEIGHTS = {\n    \"internet_facing\": 40,\n    \"internal\": 10,\n    \"auth_required\": -15,\n    \"data_high\": 30,\n    \"data_medium\": 18,\n    \"data_low\": 6,\n    \"complexity_low\": 30,\n    \"complexity_medium\": 15,\n    \"complexity_high\": 5,\n}\nDATA = (\"high\", \"medium\", \"low\")\nCOMPLEXITY = (\"low\", \"medium\", \"high\")\n\n\ndef die(msg):\n    sys.stderr.write(\"exposure_score: %s\\n\" % msg)\n    raise SystemExit(2)\n\n\ndef as_bool(value, field, name):\n    if isinstance(value, bool):\n        return value\n    if isinstance(value, str) and value.strip().lower() in (\"true\", \"yes\", \"1\"):\n        return True\n    if isinstance(value, str) and value.strip().lower() in (\"false\", \"no\", \"0\"):\n        return False\n    die(\"%s: %s must be true or false\" % (name, field))\n\n\ndef load_assets(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read assets %s: %s\" % (path, exc))\n    if isinstance(data, dict) and \"assets\" in data:\n        data = data[\"assets\"]\n    items = []\n    if isinstance(data, dict):\n        for name, spec in data.items():\n            if not isinstance(spec, dict):\n                die(\"asset %s must be an object\" % name)\n            row = dict(spec)\n            row[\"asset\"] = str(name)\n            items.append(row)\n    elif isinstance(data, list):\n        for i, spec in enumerate(data):\n            if not isinstance(spec, dict):\n                die(\"asset %d is not an object\" % i)\n            row = dict(spec)\n            row[\"asset\"] = str(row.get(\"asset\") or row.get(\"name\") or \"\")\n            items.append(row)\n    else:\n        die(\"assets must be a JSON object or list\")\n    out = {}\n    for row in items:\n        name = row[\"asset\"].strip()\n        if not name:\n            die(\"an asset has no name\")\n        data_class = str(row.get(\"data\", \"\")).strip().lower()\n        complexity = str(row.get(\"complexity\", \"\")).strip().lower()\n        if data_class not in DATA:\n            die(\"%s: data must be one of %s\" % (name, \", \".join(DATA)))\n        if complexity not in COMPLEXITY:\n            die(\"%s: complexity must be one of %s\"\n                % (name, \", \".join(COMPLEXITY)))\n        out[name] = {\n            \"asset\": name,\n            \"internet_facing\": as_bool(row.get(\"internet_facing\"), \"internet_facing\", name),\n            \"auth_required\": as_bool(row.get(\"auth_required\"), \"auth_required\", name),\n            \"data\": data_class,\n            \"complexity\": complexity,\n        }\n    if not out:\n        die(\"no assets in %s\" % path)\n    return out\n\n\ndef load_findings(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read findings %s: %s\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"findings\", data.get(\"results\", []))\n    if not isinstance(data, list):\n        die(\"findings must be a JSON list\")\n    out = []\n    for i, f in enumerate(data):\n        if not isinstance(f, dict):\n            die(\"finding %d is not an object\" % i)\n        cve = str(f.get(\"cve\", \"\")).strip().upper()\n        if not cve.startswith(\"CVE-\"):\n            die(\"finding %d has no cve field\" % i)\n        try:\n            cvss = float(f.get(\"cvss\", 0.0))\n        except (TypeError, ValueError):\n            die(\"finding %d (%s) has a non-numeric cvss\" % (i, cve))\n        out.append({\n            \"cve\": cve,\n            \"cvss\": cvss,\n            \"asset\": str(f.get(\"asset\", \"\") or \"\").strip(),\n            \"package\": str(f.get(\"package\", \"\") or \"-\"),\n        })\n    return out\n\n\ndef score_asset(spec):\n    exposure = WEIGHTS[\"internet_facing\"] if spec[\"internet_facing\"] \\\n        else WEIGHTS[\"internal\"]\n    auth = WEIGHTS[\"auth_required\"] if spec[\"auth_required\"] else 0\n    exposure = max(0, exposure + auth)\n    data = WEIGHTS[\"data_\" + spec[\"data\"]]\n    complexity = WEIGHTS[\"complexity_\" + spec[\"complexity\"]]\n    total = max(0, min(100, exposure + data + complexity))\n    return total, {\"exposure\": exposure, \"data\": data,\n                   \"complexity\": complexity}\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Score assets and rank findings by score x CVSS.\")\n    ap.add_argument(\"assets\", help=\"asset export (JSON)\")\n    ap.add_argument(\"findings\", help=\"findings export (JSON)\")\n    ap.add_argument(\"--min-score\", type=int, default=60,\n                    help=\"policy line for a high-score asset (default 60)\")\n    ap.add_argument(\"--cvss-floor\", type=float, default=7.0,\n                    help=\"policy line for a high CVSS (default 7.0)\")\n    ap.add_argument(\"--json\", action=\"store_true\", help=\"print JSON, not text\")\n    args = ap.parse_args(argv)\n\n    if not 0 <= args.min_score <= 100:\n        die(\"--min-score must be between 0 and 100\")\n    if not 0.0 <= args.cvss_floor <= 10.0:\n        die(\"--cvss-floor must be between 0 and 10\")\n\n    assets = load_assets(args.assets)\n    findings = load_findings(args.findings)\n\n    scored = {}\n    unknown = set()\n    for name, spec in assets.items():\n        total, parts = score_asset(spec)\n        scored[name] = {\"spec\": spec, \"score\": total, \"parts\": parts}\n\n    rows = []\n    for f in findings:\n        hit = scored.get(f[\"asset\"])\n        if hit is None:\n            unknown.add(f[\"asset\"] or \"-\")\n            rows.append({\"cve\": f[\"cve\"], \"asset\": f[\"asset\"], \"cvss\": f[\"cvss\"],\n                         \"package\": f[\"package\"], \"score\": 0, \"rank\": 0.0,\n                         \"known\": False})\n            continue\n        rows.append({\"cve\": f[\"cve\"], \"asset\": f[\"asset\"], \"cvss\": f[\"cvss\"],\n                     \"package\": f[\"package\"], \"score\": hit[\"score\"],\n                     \"rank\": round(hit[\"score\"] * f[\"cvss\"], 1),\n                     \"known\": True})\n    rows.sort(key=lambda r: (-r[\"rank\"], -r[\"cvss\"], r[\"cve\"]))\n\n    flagged = [r for r in rows if r[\"known\"] and r[\"cvss\"] >= args.cvss_floor\n               and r[\"score\"] >= args.min_score]\n\n    if args.json:\n        print(json.dumps({\n            \"policy\": {\"min_score\": args.min_score,\n                       \"cvss_floor\": args.cvss_floor},\n            \"weights\": WEIGHTS,\n            \"assets\": [{\"asset\": n, \"score\": s[\"score\"], \"parts\": s[\"parts\"],\n                        \"spec\": s[\"spec\"]} for n, s in sorted(scored.items())],\n            \"ranked\": rows,\n            \"flagged\": len(flagged),\n            \"unknown_assets\": sorted(unknown),\n        }, indent=2))\n        return 1 if flagged else 0\n\n    for name, s in sorted(scored.items(), key=lambda kv: -kv[1][\"score\"]):\n        spec, parts = s[\"spec\"], s[\"parts\"]\n        print(\"ASSET %-10s score %3d  exposure %+d (internet %s, auth %s)  \"\n              \"data %+d (%s)  complexity %+d (%s)\"\n              % (name, s[\"score\"], parts[\"exposure\"],\n                 \"yes\" if spec[\"internet_facing\"] else \"no\",\n                 \"yes\" if spec[\"auth_required\"] else \"no\",\n                 parts[\"data\"], spec[\"data\"], parts[\"complexity\"],\n                 spec[\"complexity\"]))\n    print(\"POLICY min-score %d, cvss-floor %.1f\"\n          % (args.min_score, args.cvss_floor))\n    print(\"RANK %d findings by asset score x CVSS\" % len(rows))\n    for r in rows:\n        print(\"RANK %6.1f  %-16s cvss %4.1f  %-10s score %3d  %s\"\n              % (r[\"rank\"], r[\"cve\"], r[\"cvss\"], r[\"asset\"], r[\"score\"],\n                 r[\"package\"]))\n    for name in sorted(unknown):\n        print(\"NOTE asset %s is not in the asset export; scored 0\" % name)\n    print(\"SUMMARY %d assets, %d findings, %d flagged high on a high-score asset\"\n          % (len(scored), len(rows), len(flagged)))\n    return 1 if flagged else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p746","n":"746","slug":"merge-the-same-finding-from-five-scanners","title":"Merge the same finding from five scanners","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","AppSec","DevSecOps","Automation","CyberSecurity"],"series_name":"ASM","design":"code","script":"finding_dedupe.py","date":"2031-07-03","files":{"devsecops":"notes/p746-merge-the-same-finding-from-five-scanners.md"},"md":"# Merge the same finding from five scanners\n\nFive scanners, one estate, 2,041 findings. After a merge on CVE and asset there were 712; the report had said 2,041 for a year.\n\n## Quick reference\n\nThe full script is at the end of this note: `finding_dedupe.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 finding_dedupe.py s1.json s2.json\n```\n\n**Example output**\n\n```text\nMERGED 3 reports into 1 finding, cvss 10.0\nweb-01 duplicates 2, scanners nessus,snyk,trivy\n```\n\n## Explained\n\n### What it does\n\n`finding_dedupe.py` reads two or more scanner exports and merges records with the same asset and CVE, or the same asset and a normalised title when there is no CVE. The merged record keeps the strongest evidence: highest CVSS, every scanner, every package name. Then it counts reports and findings per asset.\n\n### What the screen shows\n\nThe panel merges three reports into one and prints MERGED 3 reports into 1 finding, cvss 10.0, then web-01 duplicates 2, scanners nessus,snyk,trivy. The script prints one line per merged finding, then one per asset.\n\n### How to run it\n\n`python3 finding_dedupe.py exports/*.json --max-dupes 2`, after the nightly exports and before anyone writes a report. Exit 0 means no asset is over the limit, 1 means at least one, 2 means bad input.\n\n### The fix\n\nDedupe at ingest: one ticket per logical finding, with the scanners attached. A ticket that says five tools agree is easier to schedule than five lookalikes.\n\n### Prevention\n\nAgree the key first: CVE plus asset id plus package. Keep the scanner name as provenance, never as identity.\n\n> **The honest caveat:** merging can hide two vulnerable copies on one asset, in two containers or virtualenvs; check the package set before you close.\n\n> **The lesson:** five tools agreeing is one finding with five receipts, not five findings.\n\n## In one line\n\n> **One finding, five receipts.** Five tools agreeing is corroboration, not five problems.\n\n**Merge the noise**\n\n```bash\npython3 finding_dedupe.py s1.json s2.json\n```\n\n## Script: finding_dedupe.py\n\nDownload: [scripts/finding_dedupe.py](../scripts/finding_dedupe.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Merge the same finding reported by several scanners into one record.\n\nReads two or more findings exports (JSON) and merges records that describe the\nsame thing: same asset and same CVE, or same asset and same normalised title\nwhen there is no CVE. The merged record keeps the strongest evidence: the\nhighest CVSS, every scanner that saw it, and every package name reported. The\nreport then counts, per asset, how many reports arrived and how many findings\nthey actually were.\n\n    python3 finding_dedupe.py nessus.json snyk.json trivy.json\n    python3 finding_dedupe.py scan_*.json --max-dupes 2 --show 30\n\nNightly, after the scanner exports land:\n\n    15 5 * * * python3 finding_dedupe.py exports/*.json --max-dupes 2\n\n--max-dupes is a policy threshold, not a standard: it is how many duplicate\nreports on a single asset you are willing to accept before the job fails.\n\nExit codes: 0 no asset is over the duplicate threshold, 1 at least one is,\n2 bad input.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport re\nimport sys\n\nWORD = re.compile(r\"[^a-z0-9]+\")\n\n\ndef die(msg):\n    sys.stderr.write(\"finding_dedupe: %s\\n\" % msg)\n    raise SystemExit(2)\n\n\ndef norm_title(text):\n    return WORD.sub(\" \", (text or \"\").lower()).strip()\n\n\ndef load(path):\n    try:\n        with open(path, encoding=\"utf-8\") as fh:\n            data = json.load(fh)\n    except (OSError, ValueError) as exc:\n        die(\"cannot read %s: %s\" % (path, exc))\n    if isinstance(data, dict):\n        data = data.get(\"findings\", data.get(\"results\", []))\n    if not isinstance(data, list):\n        die(\"%s must contain a JSON list of findings\" % path)\n    out = []\n    for i, f in enumerate(data):\n        if not isinstance(f, dict):\n            die(\"%s: finding %d is not an object\" % (path, i))\n        asset = str(f.get(\"asset\", \"\") or \"\").strip()\n        if not asset:\n            die(\"%s: finding %d has no asset\" % (path, i))\n        cve = str(f.get(\"cve\", \"\") or \"\").strip().upper()\n        title = str(f.get(\"title\", \"\") or \"\").strip()\n        if not cve and not title:\n            die(\"%s: finding %d has neither cve nor title\" % (path, i))\n        try:\n            cvss = float(f.get(\"cvss\", 0.0) or 0.0)\n        except (TypeError, ValueError):\n            die(\"%s: finding %d has a non-numeric cvss\" % (path, i))\n        out.append({\n            \"asset\": asset,\n            \"cve\": cve,\n            \"title\": title,\n            \"scanner\": str(f.get(\"scanner\", \"\") or \"unknown\").strip(),\n            \"package\": str(f.get(\"package\", \"\") or \"\").strip(),\n            \"cvss\": cvss,\n            \"source\": path,\n        })\n    return out\n\n\ndef main(argv=None):\n    ap = argparse.ArgumentParser(\n        description=\"Merge duplicate scanner findings into one record.\")\n    ap.add_argument(\"files\", nargs=\"+\", help=\"findings exports (JSON)\")\n    ap.add_argument(\"--max-dupes\", type=int, default=2,\n                    help=\"policy: duplicates per asset allowed (default 2)\")\n    ap.add_argument(\"--show\", type=int, default=20,\n                    help=\"merged records to print (default 20)\")\n    ap.add_argument(\"--json\", action=\"store_true\", help=\"print JSON, not text\")\n    args = ap.parse_args(argv)\n\n    if args.max_dupes < 0:\n        die(\"--max-dupes cannot be negative\")\n    if args.show < 1:\n        die(\"--show must be at least 1\")\n\n    reports = []\n    for path in args.files:\n        reports.extend(load(path))\n    if not reports:\n        die(\"no findings in %d file(s)\" % len(args.files))\n\n    groups = {}\n    for r in reports:\n        if r[\"cve\"]:\n            key = (r[\"asset\"].lower(), r[\"cve\"])\n        else:\n            key = (r[\"asset\"].lower(), \"title:\" + norm_title(r[\"title\"]))\n        if key not in groups:\n            groups[key] = {\"asset\": r[\"asset\"], \"cve\": r[\"cve\"],\n                           \"title\": r[\"title\"], \"cvss\": r[\"cvss\"],\n                           \"scanners\": set(), \"packages\": set(),\n                           \"sources\": set(), \"reports\": 0}\n        g = groups[key]\n        g[\"reports\"] += 1\n        g[\"scanners\"].add(r[\"scanner\"])\n        g[\"sources\"].add(r[\"source\"])\n        if r[\"package\"]:\n            g[\"packages\"].add(r[\"package\"])\n        if r[\"cvss\"] > g[\"cvss\"]:\n            g[\"cvss\"] = r[\"cvss\"]\n        if not g[\"title\"] and r[\"title\"]:\n            g[\"title\"] = r[\"title\"]\n\n    per_asset = {}\n    for g in groups.values():\n        a = per_asset.setdefault(g[\"asset\"], {\"reported\": 0, \"unique\": 0})\n        a[\"reported\"] += g[\"reports\"]\n        a[\"unique\"] += 1\n\n    duplicate_total = sum(a[\"reported\"] - a[\"unique\"] for a in per_asset.values())\n    over = sorted(n for n, a in per_asset.items()\n                  if a[\"reported\"] - a[\"unique\"] > args.max_dupes)\n\n    merged = sorted(groups.values(),\n                    key=lambda g: (-g[\"reports\"], -g[\"cvss\"], g[\"asset\"],\n                                   g[\"cve\"]))\n\n    if args.json:\n        print(json.dumps({\n            \"counts\": {\"files\": len(args.files), \"reports\": len(reports),\n                       \"unique\": len(groups), \"duplicates\": duplicate_total},\n            \"policy\": {\"max_dupes\": args.max_dupes},\n            \"over_threshold\": [\n                {\"asset\": n,\n                 \"duplicates\": per_asset[n][\"reported\"] - per_asset[n][\"unique\"]}\n                for n in over],\n            \"assets\": [{\"asset\": n, \"reported\": a[\"reported\"],\n                        \"unique\": a[\"unique\"],\n                        \"duplicates\": a[\"reported\"] - a[\"unique\"]}\n                       for n, a in sorted(per_asset.items())],\n            \"merged\": [dict(g, scanners=sorted(g[\"scanners\"]),\n                            packages=sorted(g[\"packages\"]),\n                            sources=sorted(g[\"sources\"]))\n                       for g in merged[:args.show]],\n        }, indent=2))\n        return 1 if over else 0\n\n    print(\"MERGED %d report%s from %d file%s into %d finding%s\"\n          % (len(reports), \"\" if len(reports) == 1 else \"s\",\n             len(args.files), \"\" if len(args.files) == 1 else \"s\",\n             len(groups), \"\" if len(groups) == 1 else \"s\"))\n    for g in merged[:args.show]:\n        label = g[\"cve\"] or g[\"title\"][:40]\n        packages = \",\".join(sorted(g[\"packages\"])) or \"-\"\n        print(\"%-10s %-16s %d report%s  scanners %-24s cvss %4.1f  %s\"\n              % (g[\"asset\"], label, g[\"reports\"],\n                 \"\" if g[\"reports\"] == 1 else \"s\",\n                 \",\".join(sorted(g[\"scanners\"]))[:24], g[\"cvss\"], packages))\n    for name, a in sorted(per_asset.items(), key=lambda kv: -kv[1][\"reported\"]):\n        dupes = a[\"reported\"] - a[\"unique\"]\n        print(\"ASSET %-10s %3d reported, %3d unique, %3d duplicates\"\n              % (name, a[\"reported\"], a[\"unique\"], dupes))\n    print(\"SUMMARY %d file%s, %d report%s, %d unique, %d duplicate%s, \"\n          \"%d asset(s) over the policy limit of %d\"\n          % (len(args.files), \"\" if len(args.files) == 1 else \"s\",\n             len(reports), \"\" if len(reports) == 1 else \"s\",\n             len(groups), duplicate_total,\n             \"\" if duplicate_total == 1 else \"s\",\n             len(over), args.max_dupes))\n    return 1 if over else 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p747","n":"747","slug":"the-prioritisation-ladder-signal-context-decision","title":"The prioritisation ladder: signal, context, decision","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","RiskManagement","CyberSecurity","EPSS","CISAKEV"],"series_name":"ASM","design":"architecture","script":"","date":"2031-07-06","files":{"devsecops":"notes/p747-the-prioritisation-ladder-signal-context-decision.md"},"md":"# The prioritisation ladder: signal, context, decision\n\nA queue sorted by CVSS is sorted by the least relevant number in the room. Prioritisation is a ladder, and the rungs are not interchangeable: each one removes an assumption the rung above could not check.\n\n## Quick reference\n\n- **Tier 1 · Signal:** CVE record (CVE) · CVSS base (CVSS) · EPSS score (EPSS)\n- **Tier 2 · Context:** KEV catalog (KEV) · Asset exposure (ASSET) · Reachability (REACH)\n- **Tier 3 · Decision:** Ranked queue (RANK) · Owner and date (OWNER) · Exception log (EXC)\n\n**Request flow:** CVE record → CVSS base; CVE record → EPSS score → Ranked queue → Owner and date; Ranked queue → Exception log; KEV catalog → Ranked queue; Asset exposure → Ranked queue; Reachability → Ranked queue\n\n**Around it:** Scanner exports (SCAN) · Asset CMDB (CMDB) · EPSS daily feed (FEED) · KEV feed (KEV)\n\n## Explained\n\n### Signal\n\nSignal is what the record says on its own. The CVE identifies the flaw, CVSS scores the severity of the code, and EPSS estimates the probability of exploitation in the next 30 days. None of the three knows where the code runs, so none can tell you whether it matters here.\n\n### Context\n\nContext is what your estate adds. KEV says exploitation has been observed, exposure says whether the asset is reachable from the internet, and reachability says whether a live import path can execute the vulnerable package. This is the rung where a 9.8 can drop and a 6.5 can climb, and the only one where you learn something a stranger could not.\n\n### Decision\n\nDecision is a person, a date and a record. A ranked queue without an owner is a reading list; an owner without a date is a wish. The exception log is what keeps the ladder honest when the answer is not now.\n\nUnder the rungs sit the inputs: scanner exports, the asset CMDB, the daily EPSS feed and the KEV feed.\n\n> **The honest caveat:** a ladder still needs someone to decide what the bottom rung is worth.\n\n> **The lesson:** every step down the ladder removes an assumption, and the decision belongs at the bottom.\n\n## In one line\n\n> **Signal is not a decision.** Every rung down the ladder removes an assumption.\n\n**Start at signal**\n\n```bash\ngrep -c '^CVE-' kev.csv\n```\n"},{"id":"p748","n":"748","slug":"prioritisation-commands-epss-kev-reachability-dedupe","title":"Prioritisation commands: EPSS, KEV, reachability, dedupe","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","EPSS","CISAKEV","CommandLine","CyberSecurity"],"series_name":"ASM","design":"cheatsheet","script":"","date":"2031-07-08","files":{"devsecops":"notes/p748-prioritisation-commands-epss-kev-reachability-dedupe.md"},"md":"# Prioritisation commands: EPSS, KEV, reachability, dedupe\n\nThe prioritisation argument is usually an argument about which tool to open. Four blocks answer the four questions in order, and none of them needs a dashboard.\n\n## Quick reference\n\n### EPSS\n\n```bash\nzcat epss_scores-current.csv.gz | head\nsort -t, -k2 -gr epss.csv | head -20\nawk -F, '$2>=0.5' epss.csv\npython3 epss_rank.py f.json e.csv --top 5\n```\n\n### KEV\n\n```bash\ngrep -c '^CVE-' kev.csv\ngrep CVE-2024-3400 kev.csv\ncsvcut -c cveID,dueDate kev.csv | head\ncsvsort -c dueDate kev.csv | head\npython3 kev_match.py f.json kev.csv\n```\n\n### Reachability\n\n```bash\npython3 -m modulefinder app.py > graph.txt\ngrep -i requests graph.txt\ngrep -rn \"import requests\" .\npython3 -X importtime app.py 2>&1 | tail\npython3 reachability_map.py g.json v.txt\n```\n\n### Dedupe\n\n```bash\njq -s 'add|unique_by(.cve)' *.json\njq -s 'add|length' *.json\njq -r '.[]|.cve' s1.json | sort | uniq -c\njq -s 'add|group_by(.cve)|length' *.json\npython3 finding_dedupe.py s1.json s2.json\n```\n\n## Explained\n\n### EPSS\n\n`zcat epss_scores-current.csv.gz | head` shows the export format: a comment line, then cve, epss and percentile. `sort -t, -k2 -gr epss.csv | head -20` ranks your local file by score, and `awk -F, '$2>=0.5' epss.csv` keeps the ones above a half. The last line ranks your own findings.\n\n### KEV\n\n`grep -c '^CVE-' kev.csv` counts CVE entries, `grep CVE-2024-3400 kev.csv` checks one. The catalog quotes any field containing a comma, so cut and `awk -F`, misread column eight; csvcut -c cveID,dueDate kev.csv | head keeps the quoting honest, and csvsort -c dueDate kev.csv | head sorts the catalog by deadline.\n\n### Reachability\n\n`python3 -m modulefinder app.py > graph.txt` writes what a script imports, and `grep -i requests graph.txt` reads it back; `grep -rn \"import requests\" .` finds direct importers, and `python3 -X importtime app.py 2>&1 | tail` shows what actually loaded at run time. Then the map script turns the graph into reachable or not.\n\n### Dedupe\n\n`jq -s 'add|unique_by(.cve)' *.json` merges by CVE, `jq -s 'add|length' *.json` counts reports before and after, and `jq -r '.[]|.cve' s1.json | sort | uniq -c` shows the worst repeat offenders.\n\n> **The honest caveat:** none of these commands proves exploitation, and the thresholds in them are yours to defend.\n\n> **The lesson:** the queue is a pipeline, and every block is one filter you can check.\n\n## In one line\n\n> **Know the score before the queue.** A number you cannot reproduce is a rumour.\n\n**Start with kev**\n\n```bash\ngrep -c '^CVE-' kev.csv\n```\n"},{"id":"p749","n":"749","slug":"four-false-priorities-in-vulnerability-management","title":"Four false priorities in vulnerability management","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","RiskManagement","CyberSecurity","EPSS","AppSec"],"series_name":"ASM","design":"explainer","script":"","date":"2031-07-10","files":{"devsecops":"notes/p749-four-false-priorities-in-vulnerability-management.md"},"md":"# Four false priorities in vulnerability management\n\nFour findings looked urgent last quarter and were not, and one that looked routine was being exploited. Each false priority had a real signal behind it; none had context.\n\n## Quick reference\n\n### A CVSS 10 nobody can reach\n\n- The scanner scores the library, not the call path\n- No live module imports it; the graph says so\n- Unreachable today is not unreachable for ever\n- Keep it, date it, stop calling it critical\n\n### The KEV entry left open\n\n- Exploitation is observed, not forecast\n- The due date was public when the entry was added\n- It ranks below a 9.8 with no exploit evidence\n- That inversion is the whole problem\n\n### The same bug, five times\n\n- Five scanners, one vulnerable package\n- The count inflates the backlog, not the risk\n- Merge on asset and CVE before you count\n- Keep scanners as evidence, not as findings\n\n### Asset-blind ranking\n\n- The same CVE on a laptop and a checkout\n- CVSS cannot see either host\n- Exposure and data class change the order\n- Rank the asset with the finding\n\n> **Note:** Each false priority is a real signal used without context.\n\n## Explained\n\n### A CVSS 10 nobody can reach\n\nThe scanner scores the library. The call path decides whether the score is reachable, and here no live module imported it. Unreachable today is not unreachable for ever, so keep the finding, date it, and stop calling it critical.\n\n### The KEV entry left open\n\nThis one inverted the queue. Exploitation was observed, the due date was public when the entry was added, and the finding sat below a 9.8 with no exploit evidence at all. That is not a close call; it is a ranking that ignores the only fact that matters.\n\n### The same bug, five times\n\nFive scanners reported one vulnerable package, and the report counted five problems. The count inflated the backlog, not the risk. Merge on asset and CVE before counting, and keep the scanners as evidence.\n\n### Asset-blind ranking\n\nThe same CVE sat on a laptop and on a checkout. CVSS cannot see either host, so exposure and data class never entered the arithmetic. Rank the asset with the finding.\n\n> **The honest caveat:** context can be wrong too. An import graph misses dynamic loading, and an asset inventory misses the host someone built on Friday.\n\n> **The lesson:** a signal without context is a number that feels like a decision.\n\n## In one line\n\n> **A score is not a priority.** The number is an input. The decision is a person.\n\n**Name the false rank**\n\n```bash\npython3 kev_match.py f.json kev.csv\n```\n"},{"id":"p750","n":"750","slug":"9-checks-before-you-call-a-finding-critical","title":"9 checks before you call a finding critical","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","RiskManagement","CyberSecurity","AppSec","DevSecOps"],"series_name":"ASM","design":"checklist","script":"","date":"2031-07-13","files":{"devsecops":"notes/p750-9-checks-before-you-call-a-finding-critical.md"},"md":"# 9 checks before you call a finding critical\n\nA finding gets called critical in a meeting, and the word does three jobs at once: it describes the bug, the asset and the urgency. Nine checks separate them, in three groups.\n\n## Quick reference\n\n### Severity\n\n- **CVSS is 9.0 or higher** — base score, v3.1 or v4.0\n- **EPSS is above the line** — policy, not truth\n- **It is in KEV, or it is not** — check today's catalogue\n\n### Reach\n\n- **Package is imported live** — walk from entry points\n- **The function is called** — grep the call site\n- **No dynamic loading** — importlib, plugins\n\n### Exposure\n\n- **Asset is internet-facing** — CMDB, not guesswork\n- **No auth in front of it** — SSO still counts\n- **Data class is high** — PII, payments, keys\n\n## Explained\n\n### Severity\n\nCVSS is 9.0 or higher, on the base score, and you know whether it is v3.1 or v4.0, because the two do not compare cleanly. That is a judgement about the code, and the only group a scanner can answer alone. EPSS is above the line you agreed, and the line is policy, not truth. And you have checked today's KEV catalog, so you know whether this is exploited or merely severe.\n\n### Reach\n\nThe package is imported on a live path, walked from a real entry point. The vulnerable function is actually called, which a grep of the call site usually settles. And nothing loads the module dynamically: importlib, plugins and template loaders all defeat a static graph.\n\n### Exposure\n\nThe asset is internet-facing, according to the CMDB rather than a guess. No authentication sits in front of it, and SSO counts as authentication even when it is one password away. The data class is high: personal data, payments or keys.\n\n> **The honest caveat:** a checklist makes the decision explainable, not correct. Someone still has to own the date.\n\n> **The lesson:** critical is a claim about consequence, and consequence needs an asset.\n\n## In one line\n\n> **Critical is a decision, not a score.** Nine questions turn a severity into a sentence someone can act on.\n\n**The first check**\n\n```bash\ngrep CVE-2024-3400 kev.csv\n```\n"},{"id":"p751","n":"751","slug":"quiz-3-prioritisation-questions","title":"Quiz: 3 prioritisation questions","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","EPSS","CISAKEV","CyberSecurity","AppSec"],"series_name":"ASM","design":"quiz","script":"","date":"2031-07-15","files":{"devsecops":"notes/p751-quiz-3-prioritisation-questions.md"},"md":"# Quiz: 3 prioritisation questions\n\nThree questions separate a number from a decision. Answer before the reveal.\n\n## Quick reference\n\n**Q1. EPSS reports 0.71 for a CVE. What does that number mean?**\n\n- a) 71% chance of exploitation in 30 days\n- b) A severity score of 7.1 out of 10\n- c) 71% of your assets are exposed\n\n> **Answer:** a) 71% chance of exploitation in 30 days. EPSS is a probability between 0 and 1, estimated for the next 30 days. It says nothing about severity or about your estate, and 0.71 is not 7.1.\n\n**Q2. What does a KEV listing add that EPSS cannot?**\n\n- a) A probability of exploitation\n- b) Evidence it was exploited in the wild\n- c) A CVSS base score\n\n> **Answer:** b) Evidence it was exploited in the wild. KEV records vulnerabilities with evidence of exploitation, plus a due date. It is a fact about the past, not a forecast, which is why it outranks a higher score.\n\n**Q3. Why can reachability change the order of a queue?**\n\n- a) It lowers the CVSS base score\n- b) Unreachable code cannot be exploited now\n- c) It proves the CVE is a false positive\n\n> **Answer:** b) Unreachable code cannot be exploited now. A vulnerable package that no live path imports cannot run today. That is a reason to defer, not to close: the call path can appear in the next release.\n\n## Explained\n\n### Question one\n\nEPSS reports 0.71 for a CVE. What does that mean? A 71% chance of exploitation in the next 30 days, not a severity of 7.1. EPSS is a probability between 0 and 1, estimated daily for the next 30 days from observed exploitation signals. It says nothing about impact, and nothing about your asset. Reading 0.71 as 7.1 is the most common mistake in the whole queue.\n\n### Question two\n\nWhat does a KEV listing add that EPSS cannot? Evidence. KEV records vulnerabilities with public evidence of exploitation in the wild, plus a remediation due date. EPSS is a forecast; KEV is a fact about the past. That is why a 6.5 in KEV outranks a 9.8 that has only a probability.\n\n### Question three\n\nWhy can reachability change the order? Because unreachable code cannot run today. A package that no live path imports is a reason to defer, not to close: the call path can reappear with the next release. Static analysis gives you the argument, not a proof.\n\n> **The honest caveat:** none of the three decides alone, and all three can be wrong on a given Tuesday.\n\n> **The lesson:** a probability, a record and a path are three different claims, and a priority is what you do when they disagree.\n\n## In one line\n\n> **Know what each number means.** A probability, a record and a path are three different claims.\n\n**Check your answer**\n\n```bash\npython3 epss_rank.py f.json e.csv\n```\n"},{"id":"p752","n":"752","slug":"patch-in-waves-not-in-panic","title":"Patch in waves, not in panic","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","PatchManagement","DevOps","Python","CyberSecurity"],"series_name":"ASM","design":"code","script":"patch_waves.py","date":"2031-07-17","files":{"devsecops":"notes/p752-patch-in-waves-not-in-panic.md"},"md":"# Patch in waves, not in panic\n\nThe scanner returns 2,000 findings, someone sorts by CVSS, and the board is asked for 247 patches in one weekend. The board refuses, and nothing ships.\n\n## Quick reference\n\nThe full script is at the end of this note: `patch_waves.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 patch_waves.py findings.json\n```\n\n**Example output**\n\n```text\nBLOCKED 2 findings: 1 no fix version, 1 no owner\nFAIL wave now: 9 items over max 8\n```\n\n## Explained\n\n### What it does\n\n`patch_waves.py` reads a findings export. Critical findings go to the now wave, high to this sprint, the rest later. Inside a wave it groups rows by owner, service and fix version, so one ticket covers every host on the same build, and prints the blast radius: findings, assets and services. A row with no fix version or owner is blocked and counted separately.\n\n### What the screen shows\n\nThe fourteen lines count the blocked rows by reason and the critical rows that can be scheduled. The last two read BLOCKED 2 findings: 1 no fix version, 1 no owner, then FAIL wave now: 9 items over max 8, and exit 1.\n\n### How to run it\n\n`python3 patch_waves.py findings.json`, with `--max-items 5` for a tighter window. CI fails the plan when a wave is too big; a Monday cron rebuilds it.\n\n### The fix\n\nSplit the now wave by service, or move a group to the next window. Then chase the blocked rows: the vendor for the fix, the owner for the name.\n\n### Prevention\n\nCap every wave and keep the blocked queue beside it. A wave that grows every week is a plan nobody reads.\n\n> **The honest caveat:** severity is the scanner's label, so a critical with no fix version is a vendor conversation. `--max-items` is a policy choice.\n\n> **The lesson:** a patch plan is a queue with a size.\n\n## In one line\n\n> **Patch in waves, not in panic.** A wave is a change window, not a wish list.\n\n**Rebuild the plan**\n\n```bash\npython3 patch_waves.py findings.json\n```\n\n## Script: patch_waves.py\n\nDownload: [scripts/patch_waves.py](../scripts/patch_waves.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Turn a findings export into patch waves a change window can carry.\n\nReads a vulnerability-management export as JSON:\n\n    {\"findings\": [\n        {\"id\": \"F-1001\", \"asset\": \"api-01\", \"service\": \"api\",\n         \"owner\": \"platform\", \"severity\": \"critical\",\n         \"fix_version\": \"1.24.0\"},\n        ...]}\n\nEvery finding is grouped by owner, service and fix version, then placed in\none of three waves:\n\n    now      critical\n    sprint   high\n    later    medium and low\n\nEach wave reports its blast radius: how many findings, assets and services\nit touches, plus the largest owner/service/version group in it. A wave larger\nthan --max-items is a change window nobody can review, so the run exits 1 and\nthe wave has to be split.\n\nA finding with no owner or no fix version is not put in a wave. It is counted\nas blocked, because neither a change ticket nor a deployment can be opened for\nit. A finding with both problems is counted under no fix version: without a\nfix, even a named owner cannot schedule the change. Blocked findings do not\nfail the run on their own; they are a separate queue with a different owner.\n\nUsage:\n    scanner-export > /tmp/findings.json\n    python3 patch_waves.py /tmp/findings.json\n    python3 patch_waves.py /tmp/findings.json --max-items 5\n    cat /tmp/findings.json | python3 patch_waves.py -\n\nExit codes: 0 every wave fits, 1 a wave exceeds --max-items, 2 usage or\ninvalid input.\n\nCI (fail the plan when a wave cannot be reviewed):\n    python3 patch_waves.py findings.json --max-items 8\n\nCron (Monday morning, rebuild the plan from the weekly export):\n    0 7 * * 1 python3 /opt/s0sta/patch_waves.py /exports/findings.json --max-items 8\n\nThe script is read-only: one JSON file or stdin, no scanner, no API calls, no\nwrites. It does not decide whether a finding is real or how urgent it is; it\nonly decides what fits in a window. --max-items is a policy choice, not a\ntechnical limit: set it to what the change board can review in one sitting.\nSeverity is taken from the export, so a mislabelled finding lands in the wrong\nwave.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom collections import Counter\n\nWAVE_OF = {\"critical\": \"now\", \"high\": \"sprint\", \"medium\": \"later\",\n           \"low\": \"later\"}\nWAVE_ORDER = (\"now\", \"sprint\", \"later\")\nMISSING_FIX = (\"\", \"none\", \"n/a\", \"na\", \"unknown\", \"-\", \"null\")\n\n\nclass WaveError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise WaveError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise WaveError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef text_field(item, key, label):\n    \"\"\"Return a stripped string field, treating null as empty.\"\"\"\n    value = item.get(key)\n    if value is None:\n        return \"\"\n    if not isinstance(value, str):\n        raise WaveError(\"%s must be a string\" % label)\n    return value.strip()\n\n\ndef read_findings(doc):\n    \"\"\"Validate the export and return one row per finding.\"\"\"\n    if isinstance(doc, dict):\n        doc = doc.get(\"findings\")\n    if not isinstance(doc, list):\n        raise WaveError(\"the export must be a list or an object with findings\")\n    rows = []\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise WaveError(\"finding %d is not an object\" % number)\n        fid = item.get(\"id\") or \"finding %d\" % number\n        if not isinstance(fid, str):\n            raise WaveError(\"finding %d id must be a string\" % number)\n        fid = fid.strip()\n        asset = text_field(item, \"asset\", \"%s asset\" % fid)\n        service = text_field(item, \"service\", \"%s service\" % fid)\n        owner = text_field(item, \"owner\", \"%s owner\" % fid)\n        fix = text_field(item, \"fix_version\", \"%s fix_version\" % fid)\n        severity = text_field(item, \"severity\", \"%s severity\" % fid).lower()\n        if not asset or not service:\n            raise WaveError(\"%s needs an asset and a service\" % fid)\n        if severity not in WAVE_OF:\n            raise WaveError(\"%s has unknown severity %r\" % (fid, severity))\n        rows.append({\"id\": fid, \"asset\": asset, \"service\": service,\n                     \"owner\": owner, \"fix\": fix, \"severity\": severity})\n    return rows\n\n\ndef build_plan(rows):\n    \"\"\"Return the three waves and the blocked list.\"\"\"\n    waves = {wave: [] for wave in WAVE_ORDER}\n    blocked = []\n    for row in rows:\n        if row[\"fix\"].lower() in MISSING_FIX:\n            blocked.append(\"no fix version\")\n            continue\n        if not row[\"owner\"]:\n            blocked.append(\"no owner\")\n            continue\n        waves[WAVE_OF[row[\"severity\"]]].append(row)\n    return waves, blocked\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Group findings into patch waves by owner, service and \"\n                    \"fix version, and fail when a wave is too large.\",\n        epilog=\"Exit codes: 0 every wave fits, 1 a wave exceeds --max-items, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"findings\", help=\"findings JSON export, or - for stdin\")\n    parser.add_argument(\"--max-items\", type=int, default=8,\n                        help=\"most findings one wave may carry (default 8); \"\n                             \"this is a change-window policy, not a limit\")\n    args = parser.parse_args(argv)\n    if args.max_items < 1:\n        print(\"ERR --max-items must be at least 1\", file=sys.stderr)\n        return 2\n\n    try:\n        rows = read_findings(load_json(args.findings, \"findings file\"))\n    except WaveError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not rows:\n        print(\"OK 0 findings, nothing to schedule\")\n        return 0\n\n    waves, blocked = build_plan(rows)\n    print(\"FINDINGS %d in export\" % len(rows))\n    over = []\n    for wave in WAVE_ORDER:\n        items = waves[wave]\n        if not items:\n            continue\n        assets = len({row[\"asset\"] for row in items})\n        services = len({row[\"service\"] for row in items})\n        print(\"WAVE %-6s %2d items, %2d assets, %2d services\"\n              % (wave, len(items), assets, services))\n        groups = Counter((row[\"owner\"], row[\"service\"], row[\"fix\"])\n                         for row in items)\n        (owner, service, fix), count = groups.most_common(1)[0]\n        print(\"  top %s/%s %s x%d\" % (owner, service, fix, count))\n        if len(items) > args.max_items:\n            over.append((wave, len(items)))\n\n    if blocked:\n        reasons = Counter(blocked)\n        print(\"BLOCKED %d %s: %d no fix version, %d no owner\"\n              % (len(blocked), \"finding\" if len(blocked) == 1 else \"findings\",\n                 reasons[\"no fix version\"], reasons[\"no owner\"]))\n\n    for wave, count in over:\n        print(\"FAIL wave %s: %d items over max %d\"\n              % (wave, count, args.max_items))\n    if over:\n        return 1\n\n    scheduled = len(rows) - len(blocked)\n    used = len([wave for wave in WAVE_ORDER if waves[wave]])\n    print(\"OK %d waves, %d scheduled, max %d per wave\"\n          % (used, scheduled, args.max_items))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p753","n":"753","slug":"the-exception-that-never-expires","title":"The exception that never expires","sections":["devsecops"],"topic":"Python scripts","tags":["RiskManagement","GRC","CyberSecurity","Python","VulnerabilityManagement"],"series_name":"ASM","design":"code","script":"exception_register.py","date":"2031-07-20","files":{"devsecops":"notes/p753-the-exception-that-never-expires.md"},"md":"# The exception that never expires\n\nThe exception was approved for thirty days in March. It is still open in October, the owner has left, and the reason field says temporary.\n\n## Quick reference\n\nThe full script is at the end of this note: `exception_register.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 exception_register.py register.json --today 2026-10-21\n```\n\n**Example output**\n\n```text\nEXPIRED 2: EXC-101 50d past, EXC-102 no expiry\nFAIL 2 expired, 1 missing owner\n```\n\n## Explained\n\n### What it does\n\n`exception_register.py` reads a register JSON: finding, owner, reason and expires. Expired means the date passed, or there is no date at all, because an exception with no end date is a permanent acceptance. It reports the ones expiring inside `--soon` days, thirty by default, and the ones with no owner. An expired row exits 1.\n\n### What the screen shows\n\nThe fourteen lines fix the as-of date, walk every row and collect the expired list. On the dirty register it ends with EXPIRED 2: EXC-101 50d past, EXC-102 no expiry and FAIL 2 expired, 1 missing owner, then exits 1.\n\n### How to run it\n\n`python3 exception_register.py register.json --today 2026-10-21`. Run it on a Monday cron and send the expired list to the risk owner, not the ticket queue.\n\n### The fix\n\nClose the exception with a patch, or renew it with a new date, a reason and an owner. Delete the rows nobody can defend.\n\n### Prevention\n\nMake the expiry mandatory at approval, cap it at ninety days, and require the risk owner to re-approve. A register that renews itself is a list of things you decided not to fix.\n\n> **The honest caveat:** a register only knows what somebody wrote down; an exception agreed in chat is invisible, and treating no date as expired is a policy choice.\n\n> **The lesson:** an exception is a promise to revisit.\n\n## In one line\n\n> **Every exception needs an end date.** An approval with no expiry is a permanent exception.\n\n**Audit the register**\n\n```bash\npython3 exception_register.py reg.json\n```\n\n## Script: exception_register.py\n\nDownload: [scripts/exception_register.py](../scripts/exception_register.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Find the exceptions that expired, the ones about to, and the ones with no owner.\n\nReads a risk-exception register as JSON:\n\n    {\"exceptions\": [\n        {\"id\": \"EXC-101\", \"finding\": \"CVE-2024-3094\", \"owner\": \"platform\",\n         \"reason\": \"vendor patch due in Q2\", \"expires\": \"2026-09-30\"},\n        ...]}\n\nThree rules decide the report:\n\n  * expires before --today, or no expires value at all  -> expired\n  * expires within --soon days                         -> expiring soon\n  * owner empty or missing                             -> missing owner\n\nA missing expiry is treated as expired. An exception with no end date is a\npermanent acceptance, which is the failure this script exists to catch. A\nregister row that expires today is expiring soon, not expired: the owner still\nhas the day.\n\nAn exception can appear under two headings, for example expired and missing\nowner. The counts are per category, not a partition, and the last line repeats\nthe two that matter.\n\nUsage:\n    python3 exception_register.py register.json\n    python3 exception_register.py register.json --today 2026-10-21\n    python3 exception_register.py register.json --soon 14\n    cat register.json | python3 exception_register.py -\n\nExit codes: 0 nothing expired, 1 at least one expired, 2 usage or invalid\ninput.\n\nCI (block the release when an exception has run out):\n    python3 exception_register.py risk-register.json --today \"$(date +%F)\"\n\nCron (Mondays, mail the expired and soon lists to the risk owner):\n    0 6 * * 1 python3 /opt/s0sta/exception_register.py /exports/register.json --soon 30\n\nThe script is read-only: one JSON file or stdin, no API calls, no writes.\n--today exists so a report can be reproduced; a live run uses the real date.\nThe register is only as good as its entries: an approved exception that was\nnever written down is invisible here.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date\n\nDEFAULT_SOON_DAYS = 30\n\n\nclass RegisterError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise RegisterError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise RegisterError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef parse_day(value, label):\n    \"\"\"Parse a YYYY-MM-DD date, raising RegisterError on anything else.\"\"\"\n    if not isinstance(value, str):\n        raise RegisterError(\"%s must be a YYYY-MM-DD string\" % label)\n    try:\n        return date.fromisoformat(value.strip())\n    except ValueError:\n        raise RegisterError(\"%s is not a YYYY-MM-DD date: %r\" % (label, value))\n\n\ndef read_register(doc):\n    \"\"\"Validate the register and return one row per exception.\"\"\"\n    if isinstance(doc, dict):\n        doc = doc.get(\"exceptions\")\n    if not isinstance(doc, list):\n        raise RegisterError(\n            \"the register must be a list or an object with exceptions\")\n    rows = []\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise RegisterError(\"exception %d is not an object\" % number)\n        eid = item.get(\"id\") or \"row %d\" % number\n        if not isinstance(eid, str):\n            raise RegisterError(\"exception %d id must be a string\" % number)\n        eid = eid.strip()\n        finding = item.get(\"finding\")\n        if not isinstance(finding, str) or not finding.strip():\n            raise RegisterError(\"%s needs a finding\" % eid)\n        owner = item.get(\"owner\")\n        if owner is None:\n            owner = \"\"\n        elif not isinstance(owner, str):\n            raise RegisterError(\"%s owner must be a string\" % eid)\n        expires = item.get(\"expires\")\n        if expires is None or expires == \"\":\n            day = None\n        else:\n            day = parse_day(expires, \"%s expires\" % eid)\n        rows.append({\"id\": eid, \"finding\": finding.strip(),\n                     \"owner\": owner.strip(), \"expires\": day})\n    return rows\n\n\ndef classify(rows, today, soon_days):\n    \"\"\"Return expired, expiring-soon and missing-owner lists.\"\"\"\n    expired, upcoming, no_owner = [], [], []\n    for row in rows:\n        if row[\"expires\"] is None:\n            expired.append((row, None))\n        else:\n            left = (row[\"expires\"] - today).days\n            if left < 0:\n                expired.append((row, -left))\n            elif left <= soon_days:\n                upcoming.append((row, left))\n        if not row[\"owner\"]:\n            no_owner.append(row)\n    upcoming.sort(key=lambda pair: (pair[1], pair[0][\"id\"]))\n    expired.sort(key=lambda pair: pair[0][\"id\"])\n    return expired, upcoming, no_owner\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Report expired, expiring and unowned risk exceptions \"\n                    \"from a register JSON file.\",\n        epilog=\"Exit codes: 0 nothing expired, 1 at least one expired, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"register\", help=\"exception register JSON, or - for stdin\")\n    parser.add_argument(\"--today\", default=None,\n                        help=\"as-of date, YYYY-MM-DD (default today)\")\n    parser.add_argument(\"--soon\", type=int, default=DEFAULT_SOON_DAYS,\n                        help=\"days that count as expiring soon (default 30)\")\n    args = parser.parse_args(argv)\n\n    if args.soon < 0:\n        print(\"ERR --soon cannot be negative\", file=sys.stderr)\n        return 2\n    if args.today is None:\n        today = date.today()\n    else:\n        try:\n            today = date.fromisoformat(args.today)\n        except ValueError:\n            print(\"ERR --today is not a YYYY-MM-DD date: %r\" % args.today,\n                  file=sys.stderr)\n            return 2\n\n    try:\n        rows = read_register(load_json(args.register, \"register file\"))\n    except RegisterError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    if not rows:\n        print(\"OK 0 exceptions in register\")\n        return 0\n\n    expired, upcoming, no_owner = classify(rows, today, args.soon)\n    print(\"REGISTER %d exceptions, as of %s\"\n          % (len(rows), today.isoformat()))\n\n    if upcoming:\n        parts = []\n        for row, left in upcoming:\n            parts.append(\"%s %dd\" % (row[\"id\"], left))\n        print(\"SOON %d: %s\" % (len(upcoming), \", \".join(parts)))\n\n    if no_owner:\n        print(\"NO OWNER %d: %s\"\n              % (len(no_owner), \", \".join(row[\"id\"] for row in no_owner)))\n\n    if expired:\n        parts = []\n        for row, days in expired:\n            if days is None:\n                parts.append(\"%s no expiry\" % row[\"id\"])\n            else:\n                parts.append(\"%s %dd past\" % (row[\"id\"], days))\n        print(\"EXPIRED %d: %s\" % (len(expired), \", \".join(parts)))\n        print(\"FAIL %d expired, %d missing owner\"\n              % (len(expired), len(no_owner)))\n        return 1\n\n    if no_owner:\n        print(\"WARN 0 expired, %d missing owner\" % len(no_owner))\n        return 0\n\n    active = len(rows) - len(upcoming)\n    print(\"OK %d active, %d due within %d days\"\n          % (active, len(upcoming), args.soon))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p754","n":"754","slug":"did-the-fix-actually-fix-it","title":"Did the fix actually fix it?","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","AppSec","Python","Security","DevOps"],"series_name":"ASM","design":"code","script":"rescan_diff.py","date":"2031-07-22","files":{"devsecops":"notes/p754-did-the-fix-actually-fix-it.md"},"md":"# Did the fix actually fix it?\n\nThe dashboard says 34 findings closed. The rescan ran eleven checks instead of twelve, and the missing check was the one that had found the closed finding.\n\n## Quick reference\n\nThe full script is at the end of this note: `rescan_diff.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 rescan_diff.py before.json after.json\n```\n\n**Example output**\n\n```text\nHIDDEN 1: api-02 CIS-3.4.1 (no check after)\nFAIL 1 new finding, 1 hidden fix\n```\n\n## Explained\n\n### What it does\n\n`rescan_diff.py` reads a before and an after export. A finding is identified by asset and check, because scanner ids change between runs. It reports closed, still open, new, and hidden: a finding that closed while its check stopped running. The after export must list its checks, because a closure cannot be proved without knowing which checks ran.\n\n### What the screen shows\n\nThe fourteen lines build the two sets, collect the closed pairs whose check is missing from the after list, and print HIDDEN 1: api-02 CIS-3.4.1 (no check after), then FAIL 1 new finding, 1 hidden fix. It exits 1.\n\n### How to run it\n\n`python3 rescan_diff.py before.json after.json`. Run it after every patch window, with the before export kept from the last scan. `--allow-new` exists for a scan that widened its scope.\n\n### The fix\n\nRe-enable the check, rescan, and only then close the finding. Treat a missing check as an open finding with no evidence.\n\n### Prevention\n\nPin the check list in the scan profile, diff the check list as well as the findings, and keep the before export until the closure is reviewed.\n\n> **The honest caveat:** an asset dropped from the after export also looks closed, and no check list can tell you the asset was dropped.\n\n> **The lesson:** closure is a claim until a second scan with the same checks agrees.\n\n## In one line\n\n> **Closed is a claim. A rescan is proof.** The dashboard closes tickets; the second scan closes findings.\n\n**Diff two exports**\n\n```bash\npython3 rescan_diff.py b.json a.json\n```\n\n## Script: rescan_diff.py\n\nDownload: [scripts/rescan_diff.py](../scripts/rescan_diff.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Compare two scan exports and show what the fix actually changed.\n\nReads a before export and an after export:\n\n    {\"tool\": \"scanner\",\n     \"checks\": [\"CIS-1.1.0\", \"CIS-2.1.0\", \"CIS-3.4.1\"],\n     \"findings\": [\n         {\"asset\": \"api-01\", \"check\": \"CIS-1.1.0\",\n          \"title\": \"SSH root login\", \"severity\": \"high\"},\n         ...]}\n\nA finding is identified by (asset, check), not by the scanner's finding id:\nids change between runs, while an asset and the check that failed do not. The\ndiff then reports:\n\n  * closed  in before, not in after\n  * still   in both\n  * new     in after, not in before\n  * hidden  closed, but the check no longer appears in after.checks\n\nThe hidden list is the point of the script. A finding can disappear because\nsomebody fixed it, or because somebody stopped running the check. Both look\nlike a closed finding in a dashboard; only the check list tells them apart. An\nafter export without a checks list is rejected, because a closure cannot be\nproven when nobody recorded which checks ran.\n\nUsage:\n    python3 rescan_diff.py before.json after.json\n    python3 rescan_diff.py before.json after.json --allow-new\n\nExit codes: 0 no new finding and no hidden fix, 1 a new finding or a hidden\nfix, 2 usage or invalid input.\n\nCI (run after the patch window, fail the job on a regression):\n    python3 rescan_diff.py before.json after.json\n\nCron (daily, after the overnight scan lands):\n    30 5 * * * python3 /opt/s0sta/rescan_diff.py /exports/yesterday.json /exports/today.json\n\nThe script is read-only: two JSON files, no scanner, no API calls, no writes.\n--allow-new exists for a scan that legitimately widens its scope; it does not\nsuppress hidden fixes. The diff can only see what both exports cover: if an\nasset left the after export entirely, its findings count as closed, and no\ncheck list can tell you that the asset was dropped.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\n\nclass DiffError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise DiffError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise DiffError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef read_export(doc, label, need_checks):\n    \"\"\"Return (findings set, checks set) for one export.\"\"\"\n    if not isinstance(doc, dict):\n        raise DiffError(\"%s must be a JSON object\" % label)\n    findings = doc.get(\"findings\")\n    checks = doc.get(\"checks\")\n    if not isinstance(findings, list):\n        raise DiffError(\"%s needs a findings list\" % label)\n    if need_checks and not isinstance(checks, list):\n        raise DiffError(\"%s needs a checks list to prove which checks ran\"\n                        % label)\n    if checks is None:\n        checks = []\n    if not isinstance(checks, list) or not all(isinstance(c, str) for c in checks):\n        raise DiffError(\"%s checks must be a list of strings\" % label)\n    pairs = set()\n    for number, item in enumerate(findings, 1):\n        if not isinstance(item, dict):\n            raise DiffError(\"%s finding %d is not an object\" % (label, number))\n        asset = item.get(\"asset\")\n        check = item.get(\"check\")\n        if not isinstance(asset, str) or not asset.strip():\n            raise DiffError(\"%s finding %d needs an asset\" % (label, number))\n        if not isinstance(check, str) or not check.strip():\n            raise DiffError(\"%s finding %d needs a check\" % (label, number))\n        pairs.add((asset.strip(), check.strip()))\n    return pairs, {check.strip() for check in checks if check.strip()}\n\n\ndef count(number, one, many):\n    \"\"\"Return '1 finding' or '2 findings'.\"\"\"\n    return \"%d %s\" % (number, one if number == 1 else many)\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Diff two scan exports: closed, still open, new, and \"\n                    \"closures caused by a check that stopped running.\",\n        epilog=\"Exit codes: 0 clean, 1 new finding or hidden fix, 2 usage or \"\n               \"input error.\")\n    parser.add_argument(\"before\", help=\"scan export from before the fix\")\n    parser.add_argument(\"after\", help=\"scan export from after the fix\")\n    parser.add_argument(\"--allow-new\", action=\"store_true\",\n                        help=\"do not fail on new findings (hidden fixes still fail)\")\n    args = parser.parse_args(argv)\n\n    try:\n        before, before_checks = read_export(\n            load_json(args.before, \"before export\"), \"before\", False)\n        after, after_checks = read_export(\n            load_json(args.after, \"after export\"), \"after\", True)\n    except DiffError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    closed = sorted(before - after)\n    still = sorted(before & after)\n    new = sorted(after - before)\n    hidden = [pair for pair in closed if pair[1] not in after_checks]\n\n    per_asset = {}\n    for asset, _check in before:\n        per_asset[asset] = per_asset.get(asset, 0) - 1\n    for asset, _check in after:\n        per_asset[asset] = per_asset.get(asset, 0) + 1\n    moved = sorted((asset, delta) for asset, delta in per_asset.items() if delta)\n\n    print(\"BEFORE %d findings, %d checks, AFTER %d findings, %d checks\"\n          % (len(before), len(before_checks), len(after), len(after_checks)))\n\n    def pair_list(pairs):\n        return \", \".join(\"%s %s\" % pair for pair in pairs)\n\n    if closed:\n        print(\"CLOSED %d: %s\" % (len(closed), pair_list(closed)))\n    if still:\n        print(\"STILL %d: %s\" % (len(still), pair_list(still)))\n    if new:\n        print(\"NEW %d: %s\" % (len(new), pair_list(new)))\n    if moved:\n        print(\"PER ASSET %s\" % \", \".join(\n            \"%s %+d\" % (asset, delta) for asset, delta in moved))\n    if hidden:\n        print(\"HIDDEN %d: %s\" % (len(hidden), \", \".join(\n            \"%s %s (no check after)\" % pair for pair in hidden)))\n\n    failed = bool(hidden) or (bool(new) and not args.allow_new)\n    if failed:\n        parts = []\n        if new and not args.allow_new:\n            parts.append(count(len(new), \"new finding\", \"new findings\"))\n        if hidden:\n            parts.append(count(len(hidden), \"hidden fix\", \"hidden fixes\"))\n        print(\"FAIL %s\" % \", \".join(parts))\n        return 1\n    if new:\n        print(\"OK %d closed, %d still open, %d new allowed\"\n              % (len(closed), len(still), len(new)))\n    else:\n        print(\"OK %d closed, %d still open, nothing new\"\n              % (len(closed), len(still)))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p755","n":"755","slug":"give-the-owner-a-packet-not-a-dashboard","title":"Give the owner a packet, not a dashboard","sections":["devsecops"],"topic":"Python scripts","tags":["VulnerabilityManagement","SRE","Python","Automation","CyberSecurity"],"series_name":"ASM","design":"code","script":"owner_brief.py","date":"2031-07-24","files":{"devsecops":"notes/p755-give-the-owner-a-packet-not-a-dashboard.md"},"md":"# Give the owner a packet, not a dashboard\n\nThe owner opens the dashboard, filters to their service, and gets 84 rows with no order, no fix version and no due date. They close the tab, and the findings stay open.\n\n## Quick reference\n\nThe full script is at the end of this note: `owner_brief.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 owner_brief.py findings.json --owner platform --today 2026-10-21\n```\n\n**Example output**\n\n```text\nVERIFY python3 rescan_diff.py before.json after.json\nFAIL 1 overdue, 2 due this week\n```\n\n## Explained\n\n### What it does\n\n`owner_brief.py` reads a findings export and one owner name. It keeps that owner's open rows, sorts them by severity, then due date, then id, and prints a packet: the counts, the overdue rows, the top five with asset, severity, fix version and due date, a title and evidence link each, and one command that proves the fix. It exits 1 when anything is overdue.\n\n### What the screen shows\n\nThe thirteen lines filter by owner, turn the due dates into overdue and due-this-week counts, and end with VERIFY `python3 rescan_diff.py before.json after.json` and FAIL 1 overdue, 2 due this week.\n\n### How to run it\n\n`python3 owner_brief.py findings.json --owner platform --today 2026-10-21 > brief.txt`. Run it per owner every Monday and paste it into the ticket.\n\n### The fix\n\nSend the packet. The owner replies with a date or an exception, and both beat a dashboard nobody opens.\n\n### Prevention\n\nKeep one template, one owner per packet, and the verify command at the end. A packet that fits in a message gets read.\n\n> **The honest caveat:** the brief repeats the export, so a wrong due date or severity is wrong in the packet too.\n\n> **The lesson:** an owner needs a short list they can finish.\n\n## In one line\n\n> **A packet, not a dashboard.** One owner, five items, one command that proves them.\n\n**Build a brief**\n\n```bash\npython3 owner_brief.py f.json --owner api\n```\n\n## Script: owner_brief.py\n\nDownload: [scripts/owner_brief.py](../scripts/owner_brief.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Build a remediation brief for one owner from a findings export.\n\nReads a findings export as JSON:\n\n    {\"findings\": [\n        {\"id\": \"F-2001\", \"asset\": \"api-01\", \"service\": \"api\",\n         \"owner\": \"platform\", \"severity\": \"critical\", \"title\": \"request smuggling\",\n         \"cvss\": 9.8, \"fix_version\": \"1.24.0\", \"due\": \"2026-10-24\",\n         \"evidence\": \"https://scanner.local/f/F-2001\"},\n        ...]}\n\nThe output is a packet, not a dashboard: one owner, the counts that matter,\nthe overdue items, the top items with a fix version, a due date and an\nevidence link each, and one command that proves the fix afterwards. It fits in\na chat message, a ticket or the body of an email.\n\nItems are ordered by severity, then due date, then id. An item with no due\ndate sorts last and prints as due none. --top limits the list; the counts and\nthe overdue line still cover every open finding for the owner.\n\nUsage:\n    python3 owner_brief.py findings.json --owner platform\n    python3 owner_brief.py findings.json --owner platform --top 3\n    python3 owner_brief.py findings.json --owner platform --today 2026-10-21\n    python3 owner_brief.py findings.json --owner platform > brief.txt\n\nExit codes: 0 no item overdue, 1 at least one item overdue, 2 usage or\ninvalid input.\n\nCI (post the brief to the owner's channel every Monday):\n    python3 owner_brief.py findings.json --owner platform --top 5\n\nCron (daily at 07:00, one brief per owner):\n    0 7 * * * python3 /opt/s0sta/owner_brief.py /exports/findings.json --owner platform\n\nThe script is read-only: one JSON file, no scanner, no API calls, no writes.\nRedirect the output if you want a file. The brief repeats the export: it does\nnot re-score a finding, and a wrong due date in the export is a wrong due date\nin the packet. The verify command at the end is the rescan diff from this\nseries; run it with the same two exports the finding came from.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date\n\nRANK = {\"critical\": 0, \"high\": 1, \"medium\": 2, \"low\": 3}\nORDER = (\"critical\", \"high\", \"medium\", \"low\")\nDUE_SOON_DAYS = 7\n\n\nclass BriefError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise BriefError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise BriefError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef text_field(item, key, label):\n    \"\"\"Return a stripped string field, treating null as empty.\"\"\"\n    value = item.get(key)\n    if value is None:\n        return \"\"\n    if not isinstance(value, str):\n        raise BriefError(\"%s must be a string\" % label)\n    return value.strip()\n\n\ndef parse_day(value, label):\n    \"\"\"Parse a YYYY-MM-DD date or return None when it is absent.\"\"\"\n    if value is None or value == \"\":\n        return None\n    if not isinstance(value, str):\n        raise BriefError(\"%s must be a YYYY-MM-DD string\" % label)\n    try:\n        return date.fromisoformat(value.strip())\n    except ValueError:\n        raise BriefError(\"%s is not a YYYY-MM-DD date: %r\" % (label, value))\n\n\ndef read_findings(doc):\n    \"\"\"Validate the export and return one row per finding.\"\"\"\n    if isinstance(doc, dict):\n        doc = doc.get(\"findings\")\n    if not isinstance(doc, list):\n        raise BriefError(\"the export must be a list or an object with findings\")\n    rows = []\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise BriefError(\"finding %d is not an object\" % number)\n        fid = item.get(\"id\") or \"finding %d\" % number\n        if not isinstance(fid, str):\n            raise BriefError(\"finding %d id must be a string\" % number)\n        fid = fid.strip()\n        asset = text_field(item, \"asset\", \"%s asset\" % fid)\n        owner = text_field(item, \"owner\", \"%s owner\" % fid)\n        severity = text_field(item, \"severity\", \"%s severity\" % fid).lower()\n        if not asset:\n            raise BriefError(\"%s needs an asset\" % fid)\n        if severity not in RANK:\n            raise BriefError(\"%s has unknown severity %r\" % (fid, severity))\n        cvss = item.get(\"cvss\")\n        if cvss is not None and (isinstance(cvss, bool)\n                                 or not isinstance(cvss, (int, float))):\n            raise BriefError(\"%s cvss must be a number\" % fid)\n        rows.append({\n            \"id\": fid,\n            \"asset\": asset,\n            \"service\": text_field(item, \"service\", \"%s service\" % fid),\n            \"owner\": owner,\n            \"severity\": severity,\n            \"cvss\": cvss,\n            \"title\": text_field(item, \"title\", \"%s title\" % fid),\n            \"fix\": text_field(item, \"fix_version\", \"%s fix_version\" % fid),\n            \"evidence\": text_field(item, \"evidence\", \"%s evidence\" % fid),\n            \"due\": parse_day(item.get(\"due\"), \"%s due\" % fid),\n        })\n    return rows\n\n\ndef sort_key(row):\n    \"\"\"Severity first, then due date, then id; no due date sorts last.\"\"\"\n    return (RANK[row[\"severity\"]],\n            row[\"due\"] is None,\n            row[\"due\"] or date.max,\n            row[\"id\"])\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Build a short, per-owner remediation brief with top \"\n                    \"items, evidence, fix versions, due dates and a verify command.\",\n        epilog=\"Exit codes: 0 nothing overdue, 1 an item is overdue, \"\n               \"2 usage or input error.\")\n    parser.add_argument(\"findings\", help=\"findings JSON export, or - for stdin\")\n    parser.add_argument(\"--owner\", required=True, help=\"owner name to brief\")\n    parser.add_argument(\"--top\", type=int, default=5,\n                        help=\"how many top items to list (default 5)\")\n    parser.add_argument(\"--today\", default=None,\n                        help=\"as-of date, YYYY-MM-DD (default today)\")\n    args = parser.parse_args(argv)\n\n    if args.top < 1:\n        print(\"ERR --top must be at least 1\", file=sys.stderr)\n        return 2\n    if args.today is None:\n        today = date.today()\n    else:\n        try:\n            today = date.fromisoformat(args.today)\n        except ValueError:\n            print(\"ERR --today is not a YYYY-MM-DD date: %r\" % args.today,\n                  file=sys.stderr)\n            return 2\n\n    try:\n        rows = read_findings(load_json(args.findings, \"findings file\"))\n    except BriefError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    owner = args.owner.strip().lower()\n    mine = sorted((row for row in rows if row[\"owner\"].lower() == owner),\n                  key=sort_key)\n    if not mine:\n        print(\"OK no open findings for owner %s\" % args.owner.strip())\n        return 0\n\n    counts = {name: 0 for name in ORDER}\n    for row in mine:\n        counts[row[\"severity\"]] += 1\n    summary = \", \".join(\"%d %s\" % (counts[name], name)\n                        for name in ORDER if counts[name])\n\n    overdue = []\n    due_soon = 0\n    for row in mine:\n        if row[\"due\"] is None:\n            continue\n        days = (today - row[\"due\"]).days\n        if days > 0:\n            overdue.append((row, days))\n        elif -days <= DUE_SOON_DAYS:\n            due_soon += 1\n\n    print(\"BRIEF %s, as of %s\" % (args.owner.strip(), today.isoformat()))\n    print(\"%s: %s\" % (\"1 open finding\" if len(mine) == 1\n                      else \"%d open findings\" % len(mine), summary))\n    if overdue:\n        print(\"OVERDUE %d: %s\" % (len(overdue), \", \".join(\n            \"%s %dd\" % (row[\"id\"], days) for row, days in overdue)))\n\n    for place, row in enumerate(mine[:args.top], 1):\n        score = \"\" if row[\"cvss\"] is None else \" cvss %.1f\" % row[\"cvss\"]\n        print(\"TOP %d %s %s %s%s fix %s due %s\"\n              % (place, row[\"id\"], row[\"asset\"], row[\"severity\"], score,\n                 row[\"fix\"] or \"none\",\n                 row[\"due\"].isoformat() if row[\"due\"] else \"none\"))\n        line = row[\"title\"] or \"no title\"\n        if row[\"evidence\"]:\n            line += \"; \" + row[\"evidence\"]\n        print(\"      \" + line)\n\n    print(\"VERIFY python3 rescan_diff.py before.json after.json\")\n    if overdue:\n        print(\"FAIL %d overdue, %d due this week\" % (len(overdue), due_soon))\n        return 1\n    print(\"OK %d findings, none overdue, %d due this week\"\n          % (len(mine), due_soon))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p756","n":"756","slug":"escalate-on-evidence-not-on-mood","title":"Escalate on evidence, not on mood","sections":["devsecops"],"topic":"Python scripts","tags":["RiskManagement","CyberSecurity","Python","VulnerabilityManagement","Leadership"],"series_name":"ASM","design":"code","script":"escalation.py","date":"2031-07-27","files":{"devsecops":"notes/p756-escalate-on-evidence-not-on-mood.md"},"md":"# Escalate on evidence, not on mood\n\nThe security lead escalates a finding because it feels old. The owner asks how old. Nobody knows, because the ticket has no due date.\n\n## Quick reference\n\nThe full script is at the end of this note: `escalation.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 escalation.py findings.json owners.json --today 2026-10-21\n```\n\n**Example output**\n\n```text\ncommittee: app (Omar Hani) 1 item, oldest 22 days\nFAIL 1 committee, 2 manager\n```\n\n## Explained\n\n### What it does\n\n`escalation.py` reads the findings export and an owner-to-manager map. It sorts each row into a tier: reminder when the due date is up to seven days away, manager when it is one to fourteen days overdue, committee when it is past fourteen days or critical and three days late. Every tier prints its rule and counts, every escalated owner prints with the manager who receives it, and the run exits 1 at the manager tier.\n\n### What the screen shows\n\nThe thirteen lines compute the days left, separate committee rows from manager rows and print the committee line with the manager name. The last two read committee: app (Omar Hani) 1 item, oldest 22 days, then FAIL 1 committee, 2 manager.\n\n### How to run it\n\n`python3 escalation.py findings.json owners.json --today 2026-10-21`. Run it daily, after the owner has had the reminder week.\n\n### The fix\n\nSend the committee the rule that moved the item and the date it was due. Send the manager the two items and the owner they sit with.\n\n### Prevention\n\nPut a due date on every finding at creation, keep the thresholds fixed for a quarter, and write the rules into the ticket template.\n\n> **The honest caveat:** the thresholds are policy, not law, and a finding with no due date cannot be escalated at all.\n\n> **The lesson:** an escalation without a rule is an argument with a date attached.\n\n## In one line\n\n> **Escalate on evidence, not on mood.** A tier is a rule with a date, not a feeling.\n\n**Name the tier**\n\n```bash\npython3 escalation.py f.json o.json\n```\n\n## Script: escalation.py\n\nDownload: [scripts/escalation.py](../scripts/escalation.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Turn overdue findings into escalation tiers, with a reason for each move.\n\nReads a findings export and an owner-to-manager map:\n\n    findings.json\n    {\"findings\": [\n        {\"id\": \"E-3001\", \"owner\": \"platform\", \"severity\": \"critical\",\n         \"due\": \"2026-10-22\"},\n        ...]}\n\n    owners.json\n    {\"platform\": \"Omar Hani\", \"infra\": \"Sara Ali\", \"app\": \"Omar Hani\"}\n\nThe tiers are rules, not opinions:\n\n    reminder    due in 0 to --remind-days days (default 7)\n    manager     overdue 1 to --manager-days days (default 14)\n    committee   overdue more than --manager-days days, or critical and\n                overdue at least --critical-days days (default 3)\n\nEvery tier prints its rule and its counts, and every owner in the manager or\ncommittee tier prints with the manager who receives it. The last line repeats\nthe two counts that matter. A finding with no due date cannot be escalated and\nis listed as NO DUE; an owner with no entry in the map is listed as NO MANAGER,\nbecause an escalation with nowhere to go is not an escalation.\n\nUsage:\n    python3 escalation.py findings.json owners.json\n    python3 escalation.py findings.json owners.json --today 2026-10-21\n    python3 escalation.py findings.json owners.json --remind-days 3\n    cat findings.json | python3 escalation.py - owners.json\n\nExit codes: 0 nothing past the reminder tier, 1 at least one manager or\ncommittee item, 2 usage or invalid input.\n\nCI (weekly, fail the job when anything reaches the committee):\n    python3 escalation.py findings.json owners.json --today \"$(date +%F)\"\n\nCron (daily at 07:30, after the owners have had a week to act):\n    30 7 * * * python3 /opt/s0sta/escalation.py /exports/findings.json /opt/s0sta/owners.json\n\nThe script is read-only: two JSON files or stdin, no API calls, no writes.\nThe thresholds are policy choices; change them to match your SLA, then keep\nthem fixed for a quarter so the trend means something. The script counts\nfindings, not risk: a critical reminder can matter more than a low committee\nitem, and the tier line says which rule put it there.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\nfrom datetime import date\n\nRANK = {\"critical\": 0, \"high\": 1, \"medium\": 2, \"low\": 3}\n\n\nclass EscalationError(Exception):\n    \"\"\"Anything that should end the run with exit code 2.\"\"\"\n\n\ndef load_json(source, label):\n    \"\"\"Read JSON from a path, or stdin when source is '-'.\"\"\"\n    try:\n        if source == \"-\":\n            return json.load(sys.stdin)\n        with open(source, encoding=\"utf-8\") as handle:\n            return json.load(handle)\n    except OSError as exc:\n        raise EscalationError(\"cannot read %s: %s\" % (label, exc))\n    except ValueError as exc:\n        raise EscalationError(\"%s is not valid JSON: %s\" % (label, exc))\n\n\ndef parse_day(value, label):\n    \"\"\"Parse a YYYY-MM-DD date, returning None when it is absent.\"\"\"\n    if value is None or value == \"\":\n        return None\n    if not isinstance(value, str):\n        raise EscalationError(\"%s must be a YYYY-MM-DD string\" % label)\n    try:\n        return date.fromisoformat(value.strip())\n    except ValueError:\n        raise EscalationError(\"%s is not a YYYY-MM-DD date: %r\" % (label, value))\n\n\ndef read_findings(doc):\n    \"\"\"Validate the export and return one row per finding.\"\"\"\n    if isinstance(doc, dict):\n        doc = doc.get(\"findings\")\n    if not isinstance(doc, list):\n        raise EscalationError(\n            \"the export must be a list or an object with findings\")\n    rows = []\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise EscalationError(\"finding %d is not an object\" % number)\n        fid = item.get(\"id\") or \"finding %d\" % number\n        if not isinstance(fid, str):\n            raise EscalationError(\"finding %d id must be a string\" % number)\n        owner = item.get(\"owner\")\n        if not isinstance(owner, str) or not owner.strip():\n            raise EscalationError(\"%s needs an owner\" % fid.strip())\n        severity = item.get(\"severity\")\n        if not isinstance(severity, str) or severity.strip().lower() not in RANK:\n            raise EscalationError(\"%s has unknown severity %r\"\n                                  % (fid.strip(), severity))\n        rows.append({\n            \"id\": fid.strip(),\n            \"owner\": owner.strip(),\n            \"severity\": severity.strip().lower(),\n            \"due\": parse_day(item.get(\"due\"), \"%s due\" % fid.strip()),\n        })\n    return rows\n\n\ndef read_owners(doc):\n    \"\"\"Validate the owner-to-manager map.\"\"\"\n    if not isinstance(doc, dict):\n        raise EscalationError(\"the owners file must be a JSON object\")\n    mapping = {}\n    for owner, manager in doc.items():\n        if not isinstance(owner, str) or not owner.strip():\n            raise EscalationError(\"the owners file has an empty owner name\")\n        if not isinstance(manager, str) or not manager.strip():\n            raise EscalationError(\"owner %r needs a manager name\" % owner)\n        mapping[owner.strip().lower()] = manager.strip()\n    return mapping\n\n\ndef count(number, one, many):\n    \"\"\"Return '1 finding' or '2 findings'.\"\"\"\n    return \"%d %s\" % (number, one if number == 1 else many)\n\n\ndef tier_line(name, rows, rule):\n    \"\"\"Print one tier summary line.\"\"\"\n    owners = len({row[\"owner\"] for row in rows})\n    print(\"%s %s, %s, %s\"\n          % (name, count(len(rows), \"finding\", \"findings\"),\n             count(owners, \"owner\", \"owners\"), rule))\n\n\ndef oldest_days(rows, today):\n    \"\"\"Return the age of the oldest item in a group.\"\"\"\n    return max((today - row[\"due\"]).days for row in rows)\n\n\ndef owner_lines(name, rows, managers, today):\n    \"\"\"Print one line per owner in a tier, with the manager and the oldest age.\"\"\"\n    grouped = {}\n    for row in rows:\n        grouped.setdefault(row[\"owner\"], []).append(row)\n    ordered = sorted(grouped, key=lambda owner: (-oldest_days(grouped[owner], today),\n                                                 owner))\n    parts = []\n    for owner in ordered:\n        items = grouped[owner]\n        manager = managers.get(owner.lower(), \"no manager\")\n        parts.append(\"%s (%s) %s, oldest %s\"\n                     % (owner, manager,\n                        count(len(items), \"item\", \"items\"),\n                        count(oldest_days(items, today), \"day\", \"days\")))\n    print(\"%s: %s\" % (name.lower(), \"; \".join(parts)))\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Sort overdue findings into reminder, manager and \"\n                    \"committee tiers with the rule that moved each one.\",\n        epilog=\"Exit codes: 0 nothing past the reminder tier, 1 manager or \"\n               \"committee items, 2 usage or input error.\")\n    parser.add_argument(\"findings\", help=\"findings JSON export, or - for stdin\")\n    parser.add_argument(\"owners\", help=\"owner-to-manager map, JSON object\")\n    parser.add_argument(\"--today\", default=None,\n                        help=\"as-of date, YYYY-MM-DD (default today)\")\n    parser.add_argument(\"--remind-days\", type=int, default=7,\n                        help=\"days before the due date that count as a reminder \"\n                             \"(default 7)\")\n    parser.add_argument(\"--manager-days\", type=int, default=14,\n                        help=\"days overdue before the committee tier (default 14)\")\n    parser.add_argument(\"--critical-days\", type=int, default=3,\n                        help=\"days a critical finding may be overdue before the \"\n                             \"committee tier (default 3)\")\n    args = parser.parse_args(argv)\n\n    for name, value in ((\"--remind-days\", args.remind_days),\n                        (\"--manager-days\", args.manager_days),\n                        (\"--critical-days\", args.critical_days)):\n        if value < 0:\n            print(\"ERR %s cannot be negative\" % name, file=sys.stderr)\n            return 2\n    if args.today is None:\n        today = date.today()\n    else:\n        try:\n            today = date.fromisoformat(args.today)\n        except ValueError:\n            print(\"ERR --today is not a YYYY-MM-DD date: %r\" % args.today,\n                  file=sys.stderr)\n            return 2\n\n    try:\n        rows = read_findings(load_json(args.findings, \"findings file\"))\n        managers = read_owners(load_json(args.owners, \"owners file\"))\n    except EscalationError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    reminder, manager, committee, no_due = [], [], [], []\n    for row in rows:\n        if row[\"due\"] is None:\n            no_due.append(row[\"id\"])\n            continue\n        left = (row[\"due\"] - today).days\n        if left >= 0 and left <= args.remind_days:\n            reminder.append(row)\n        elif left < 0:\n            overdue = -left\n            if (overdue > args.manager_days\n                    or (row[\"severity\"] == \"critical\"\n                        and overdue >= args.critical_days)):\n                committee.append(row)\n            else:\n                manager.append(row)\n\n    print(\"FINDINGS %d, as of %s\" % (len(rows), today.isoformat()))\n    if reminder:\n        tier_line(\"REMINDER\", reminder,\n                  \"due in 0-%d days\" % args.remind_days)\n    if manager:\n        tier_line(\"MANAGER\", manager,\n                  \"overdue 1-%d days\" % args.manager_days)\n        owner_lines(\"MANAGER\", manager, managers, today)\n    if committee:\n        tier_line(\"COMMITTEE\", committee,\n                  \"overdue %d+ days or critical %d+ days\"\n                  % (args.manager_days + 1, args.critical_days))\n        owner_lines(\"COMMITTEE\", committee, managers, today)\n    if no_due:\n        print(\"NO DUE %d: %s\" % (len(no_due), \", \".join(no_due)))\n    missing = sorted({row[\"owner\"] for row in manager + committee\n                      if row[\"owner\"].lower() not in managers})\n    if missing:\n        print(\"NO MANAGER %d: %s\" % (len(missing), \", \".join(missing)))\n\n    if committee or manager:\n        print(\"FAIL %d committee, %d manager\"\n              % (len(committee), len(manager)))\n        return 1\n    print(\"OK %s, nothing escalated\"\n          % count(len(reminder), \"reminder\", \"reminders\"))\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p757","n":"757","slug":"the-remediation-loop","title":"The remediation loop","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","DevOps","CyberSecurity","SRE","Automation"],"series_name":"ASM","design":"architecture","script":"","date":"2031-07-29","files":{"devsecops":"notes/p757-the-remediation-loop.md"},"md":"# The remediation loop\n\nThe remediation loop has three tiers, and every finding travels through all of them or it does not close.\n\n## Quick reference\n\n- **Plan:** Wave plan (WAVE) · Owner map (OWN) · Due dates (DUE)\n- **Execute:** Patch window (PATCH) · Exception (EXC) · Change record (CHG)\n- **Verify:** Rescan diff (DIFF) · Closure (CLOSE) · Escalation (ESC)\n\n**Request flow:** Wave plan → Patch window → Rescan diff → Closure; Rescan diff → Escalation; Owner map → Patch window; Due dates → Escalation; Exception → Rescan diff\n\n**Around it:** patch_waves.py (PY) · exception_register.py (PY) · rescan_diff.py (PY) · owner_brief.py (PY)\n\n## Explained\n\n### Plan\n\nThe first tier is the wave plan, the owner map and the due dates. A finding becomes work when it has an owner, a fix version and a due date. The plan groups by owner, service and fix version, and caps each wave so a change window can review it.\n\n### Execute\n\nThe second tier is the patch window, the exception and the change record. The exception is the one row allowed to leave the loop, with an owner, a reason and an expiry. The change record links the ticket to the build that shipped.\n\n### Verify\n\nThe third tier is the rescan diff, the closure and the escalation. The rescan runs the same checks as before and reports closed, still open, new and hidden. A closure is written only after the rescan, never after the patch command returns. Escalation catches the wave that missed its date.\n\n### Read it in order\n\nThe poster draws the three bands top to bottom, with arrows from the wave plan and the owner map into the patch window, then to the rescan and the closure. The due dates and the rescan feed the escalation node, the loop's exit for a finding that will not close. Four chips at the bottom name the scripts.\n\n> **The honest caveat:** the loop is only as fast as the scan, and a weekly scan makes a daily fix wait a week for proof.\n\n> **The lesson:** plan, execute, verify, close, then plan again.\n\n## In one line\n\n> **Plan, execute, verify, close.** Three tiers, and a finding leaves only through the last one.\n\n**Check the loop**\n\n```bash\npython3 rescan_diff.py b.json a.json\n```\n"},{"id":"p758","n":"758","slug":"remediation-commands","title":"Remediation commands","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","CommandLine","CyberSecurity","Linux","DevOps"],"series_name":"ASM","design":"cheatsheet","script":"","date":"2031-07-31","files":{"devsecops":"notes/p758-remediation-commands.md"},"md":"# Remediation commands\n\nThe fix phase has four command blocks worth knowing: patch a host, prove the fix, record the exception, and escalate on a date.\n\n## Quick reference\n\n### Patch\n\n```bash\napt-get install --only-upgrade nginx\ndnf update --security\nkubectl rollout restart deploy/api\nsystemctl restart nginx\npip install -U 'django>=4.2.15'\n```\n\n### Verify\n\n```bash\npython3 rescan_diff.py b.json a.json\ncurl -sS https://api-01/healthz\ndpkg -l | grep nginx\ntrivy image --quiet api:2.1\nnmap -sV -p 443 api-01\n```\n\n### Exception\n\n```bash\npython3 exception_register.py reg.json\njq '.exceptions[].expires' reg.json\ngrep -c '\"expires\": \"\"' reg.json\ndate -d '+90 days' +%F\n```\n\n### Escalate\n\n```bash\npython3 escalation.py f.json o.json\nmail -s \"overdue\" owner@example.com\ngrep -c committee escalation.txt\nsort -k2 -n overdue.txt | tail -5\n```\n\n## Explained\n\n### Patch\n\nThe patch block upgrades one package without moving the rest: apt-get with `--only-upgrade`, dnf with `--security`, kubectl rollout restart for a new image, systemctl for a config change, pip with a pinned floor. Run one, then read the version back.\n\n### Verify\n\nThe verify block proves the change. rescan_diff compares two exports with the same checks, curl reads the version endpoint, dpkg and trivy name what is installed, and nmap shows what still answers on the port.\n\n### Exception\n\nThe exception block keeps an acceptance honest: the register script reports what expired, jq lists the dates, grep counts the blank ones, and date prints the ninety-day limit. An exception without a date is a permanent one.\n\n### Escalate\n\nThe escalation block moves a finding to a person: run escalation with the findings and the owner map, mail the result, count the committee tier, sort the overdue list by age.\n\n### Read them in order\n\nThe sheet draws four rows, each with a monogram tile, the block name and the commands to the right, in the order a fix travels from the package to the person.\n\n> **The honest caveat:** a command that prints a version is not proof that the vulnerable path is gone.\n\n> **The lesson:** read the command, then the result.\n\n## In one line\n\n> **Read the command, then the result.** A version string proves an install, not a fix.\n\n**Patch one host**\n\n```bash\napt-get install --only-upgrade nginx\n```\n"},{"id":"p759","n":"759","slug":"four-fixes-that-fail","title":"Four fixes that fail","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","CyberSecurity","SRE","DevOps","RiskManagement"],"series_name":"ASM","design":"explainer","script":"","date":"2031-08-03","files":{"devsecops":"notes/p759-four-fixes-that-fail.md"},"md":"# Four fixes that fail\n\nFour fixes fail the same way: the finding goes away and the risk does not.\n\n## Quick reference\n\n### Hidden finding\n\n- The check that found it was disabled after the scan\n- The finding leaves the report, not the host\n- Pin the check list and diff it with the findings\n\n### Exception rot\n\n- Approved for a quarter, renewed four times\n- The reason field still says temporary\n- The date moves whenever the review arrives\n\n### Wave too big\n\n- Forty patches in one window, one rollback\n- The board approves without reading the list\n- Cap the wave and split it by service\n\n### No verification\n\n- The patch command returned 0 and the ticket closed\n- The service was never restarted\n- Close only after a rescan with the same checks\n\n> **Note:** Most failed fixes are two of these at once, in the gap between the patch and the proof.\n\n## Explained\n\n### Hidden finding\n\nThe rescan runs fewer checks than the scan before. The finding disappears because the check that found it was disabled, not because the host changed. Pin the check list and compare it with the findings.\n\n### Exception rot\n\nThe exception was approved for a quarter and renewed four times. Nobody remembers the reason, the owner has changed team, and the date keeps moving. Give it an owner, a reason and an expiry that only the risk owner can extend.\n\n### Wave too big\n\nThe window holds forty patches because the team wants the backlog gone. The first failure rolls back the batch, and the next window is smaller. A wave is a size, not a mood: cap it and split it by service.\n\n### No verification\n\nThe patch command returned 0, the ticket is closed, and the scan still shows the finding open: the package went to the wrong host, or nobody restarted the service. Closure needs a second scan with the same checks.\n\nThe poster draws four panels in a 2x2 grid, each with a drawing and three bullets, in the order a fix goes wrong. The note says most failed fixes are two of these at once.\n\n> **The honest caveat:** a rescan with the same checks is evidence, not proof; an asset that left the scope looks fixed.\n\n> **The lesson:** the fix fails in the gap between patch and proof.\n\n## In one line\n\n> **The fix fails in the gaps.** Four ways the finding goes away and the risk does not.\n\n**Rescan before close**\n\n```bash\npython3 rescan_diff.py b.json a.json\n```\n"},{"id":"p760","n":"760","slug":"fix-now-vs-fix-later","title":"Fix now vs fix later","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","RiskManagement","CyberSecurity","SRE","DevOps"],"series_name":"ASM","design":"matrix","script":"","date":"2031-08-05","files":{"devsecops":"notes/p760-fix-now-vs-fix-later.md"},"md":"# Fix now vs fix later\n\nThree columns, six rows, one question: does this finding get fixed now, this sprint, or in the backlog?\n\n## Quick reference\n\n| | **Now** | **This sprint** | **Backlog** |\n|:--|:--|:--|:--|\n| Severity | Critical | High | Medium or low |\n| Exposure | Internet-facing | Internal, reachable | Isolated |\n| KEV | On the KEV list | Not listed, EPSS high | No exploit signal |\n| Effort | Under an hour | Under a day | A project |\n| Owner | Named owner | Team queue | No owner yet |\n| Verification | Rescan same check | Rescan at close | Next scan cycle |\n\n**How to pick**\n\n- **Now** → fix inside the window, rescan the same check.\n- **This sprint** → schedule it, then rescan at close.\n- **Backlog** → review it at the next scan cycle.\n\n## Explained\n\n### The rows\n\nSeverity, exposure, KEV, effort, owner and verification. The matrix puts them side by side, so a critical finding with no owner and a week of work does not become a now item by accident. The columns are the answer; the rows are the evidence behind it.\n\n### Now\n\nCritical, internet-facing, on the KEV list, under an hour of work, a named owner and a rescan with the same check. Five of six is not a now item; it is a conversation about what is missing.\n\n### This sprint\n\nHigh, reachable from the internal network, no KEV hit but a high EPSS score, under a day of work, a team queue, a rescan before the close.\n\n### Backlog\n\nMedium or low, isolated, no exploit signal, a project-sized effort, no owner yet, verified at the next scan cycle. The backlog is where a finding waits for review, not where it is forgotten.\n\n### The grid\n\nThe poster draws three columns and six rows, with the row labels down the left, a coloured header for each column and a pick box at the bottom.\n\n> **The honest caveat:** KEV means exploitation has been observed, not that it is coming for you; EPSS estimates the chance and is not a verdict. The thresholds are policy choices, so write them down.\n\n> **The lesson:** now is a decision you can defend, not a feeling you had on Monday.\n\n## In one line\n\n> **Now is a decision, not a default.** A deadline is a set of facts, not a mood on Monday.\n\n**Date the fix**\n\n```bash\npython3 owner_brief.py f.json --owner api\n```\n"},{"id":"p761","n":"761","slug":"quiz-3-remediation-questions","title":"Quiz: 3 remediation questions","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","CyberSecurity","Python","DevOps","SRE"],"series_name":"ASM","design":"quiz","script":"","date":"2031-08-07","files":{"devsecops":"notes/p761-quiz-3-remediation-questions.md"},"md":"# Quiz: 3 remediation questions\n\nThree questions come back in every remediation review. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why must a patch wave be small?**\n\n- a) A change window can review only so much\n- b) Small waves fix more findings\n- c) Large waves need more scanners\n\n> **Answer:** a) A change window can review only so much. A wave is a change request. If the board cannot read every item, it approves the plan without reading it, and one bad patch rolls back the whole batch.\n\n**Q2. What must a risk exception carry?**\n\n- a) An owner, a reason and an expiry\n- b) A CVSS score and a ticket\n- c) A vendor contact and a date\n\n> **Answer:** a) An owner, a reason and an expiry. Without an owner nobody acts, without a reason nobody can judge it, and without an expiry it never returns. The date turns an acceptance into a decision.\n\n**Q3. Why is a rescan the only closure proof?**\n\n- a) It runs the same checks and shows the delta\n- b) It proves the patch command exited 0\n- c) It lists the findings that were fixed\n\n> **Answer:** a) It runs the same checks and shows the delta. A patch command proves an install ran, not that the finding is gone. A rescan with the same checks shows it absent, and catches the check someone disabled.\n\n## Explained\n\n### Question one\n\nWhy must a patch wave be small? Because a wave is a change request, and a board that cannot read every item approves the plan without reading it. Nine critical patches in one window is not urgency, it is a batch rollback waiting to happen. Split by service, keep the owner attached, and let the next window carry the rest.\n\n### Question two\n\nWhat must a risk exception carry? An owner, a reason and an expiry. Without an owner nobody acts, without a reason nobody can judge the risk, and without an expiry it never comes back. The date turns an acceptance into a decision, and the register script fails the run when the date passes.\n\n### Question three\n\nWhy is a rescan the only closure proof? Because a patch command proves that an install ran, not that the finding is gone. A rescan with the same checks shows the finding absent and no new one, and catches the check that someone disabled. Without the check list, a closed finding is a claim.\n\nThe three connect: a small wave ships, an exception with a date expires, and a rescan closes. Remove any one and the other two report progress without evidence.\n\n> **The honest caveat:** passing this quiz does not fix a host; the window and the rescan do.\n\n> **The lesson:** remediation is a loop, and the last step is the one that counts.\n\n## In one line\n\n> **Closing needs a second scan.** Three questions before the next closure.\n\n**Diff the rescan**\n\n```bash\npython3 rescan_diff.py b.json a.json\n```\n"},{"id":"p762","n":"762","slug":"one-page-for-the-board","title":"One page for the board","sections":["devsecops"],"topic":"Python scripts","tags":["Python","RiskManagement","CyberSecurity","CISO","GRC"],"series_name":"ASM","design":"code","script":"risk_report.py","date":"2031-08-10","files":{"devsecops":"notes/p762-one-page-for-the-board.md"},"md":"# One page for the board\n\nThe board asks one question: are we safer than last quarter? The security team sends a forty-page export and three colours, and the meeting moves on.\n\n## Quick reference\n\nThe full script is at the end of this note: `risk_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 risk_report.py findings.json --previous prev.json\n```\n\n**Example output**\n\n```text\nRISK 76 up 14\n2 critical, 2 KEV open\n```\n\n## Explained\n\n### What it does\n\n`risk_report.py` reads the current findings JSON and the export behind the last report. It prints open findings by severity, the known-exploited (KEV) count, mean and oldest age, the three assets with the most open findings, and a weighted score with every factor on the line. It exits 1 on an open KEV or a critical past 30 days, a policy line.\n\n### On the screen\n\nRISK 76 up 14, then 2 critical, 2 KEV open. The report prints the working: 10\\*2 + 5\\*5 + 2\\*9 + 1\\*3 + 5\\*2 KEV.\n\n### How to run it\n\n`python3 risk_report.py findings.json --previous prev.json --today 2026-10-20`, before the pack. Exit 0 is clean, 1 is a board-level finding, 2 is a bad export.\n\n### The fix\n\nSend the page, not the export. The trend and the changed line carry the argument: a score that rose because two KEVs arrived is a sprint decision, not a colour.\n\n### Prevention\n\nKeep each month's export, same schema, in one directory. A report with no previous file is a snapshot pretending to be a trend.\n\n> **The honest caveat:** the weights are a policy choice, not a standard. Change them and the trend changes, so freeze them for a year and write the choice down.\n\n> **The lesson:** the board needs the delta and the reason, not more findings.\n\n## In one line\n\n> **A number the board can act on.** One page, one trend, and the factors behind the score.\n\n**Send one page:** `risk_report.py f.json --previous p.json`\n\n## Script: risk_report.py\n\nDownload: [scripts/risk_report.py](../scripts/risk_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"risk_report.py - one page for the board, built from a findings export.\n\nReads a findings JSON export and, optionally, the export used for the previous\nboard report. It prints the open count by severity, the known-exploited (KEV)\ncount, the mean and oldest age, the three assets with the most open findings,\na weighted risk score with its factors, the trend against the previous report\nand what changed since it.\n\nFindings JSON is a list of objects:\n\n    [{\"id\": \"CVE-2026-1001\", \"severity\": \"critical\", \"asset\": \"web-01\",\n      \"first_seen\": \"2026-07-15\", \"status\": \"open\", \"kev\": true}]\n\nseverity is critical, high, medium or low. status is open or fixed. kev is\noptional and defaults to false. Ages are counted to --today. A finding whose\nfirst_seen is in the future is a bad export, not a negative age.\n\nThe risk score is a policy choice, not a standard: critical 10, high 5,\nmedium 2, low 1, plus 5 for each open KEV. The line prints every factor, so a\nreader can reproduce the number without trusting the tool.\n\nAccepted risk stays in the open count. A finding that is open in the export is\nopen on the page, whatever the exception register says.\n\nUsage:\n    python3 risk_report.py findings.json --previous last-month.json\n    python3 risk_report.py findings.json --previous last-month.json --today 2026-10-20\n    python3 risk_report.py findings.json --sla-days 30\n\nCron (Monday 06:00, before the board pack):\n    0 6 * * 1 python3 /opt/s0sta/risk_report.py /var/lib/s0sta/findings.json --previous /var/lib/s0sta/findings.last.json\n\nExit codes: 0 no open KEV and no critical older than --sla-days, 1 either is\npresent, 2 usage or a findings export that is missing, unreadable, invalid,\nduplicated or inconsistent.\n\nRead-only: reads the exports and prints. It writes nothing and makes no\nnetwork call.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport json\nimport sys\nfrom collections import Counter\n\nSEVERITIES = (\"critical\", \"high\", \"medium\", \"low\")\nWEIGHTS = {\"critical\": 10, \"high\": 5, \"medium\": 2, \"low\": 1}\nKEV_POINTS = 5\n\n\nclass InputError(Exception):\n    \"\"\"An export that cannot be turned into a board page.\"\"\"\n\n\ndef parse_day(text, label):\n    \"\"\"Return a date from an ISO string, or raise InputError.\"\"\"\n    if not isinstance(text, str):\n        raise InputError(\"%s is not an ISO date\" % label)\n    try:\n        return dt.date.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s is not an ISO date: %s\" % (label, text))\n\n\ndef load_findings(path, today):\n    \"\"\"Read and validate one findings export. Return normalised records.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s must contain a JSON list of findings\" % path)\n\n    rows = []\n    seen = set()\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: finding %d is not an object\" % (path, number))\n        finding_id = item.get(\"id\")\n        if not isinstance(finding_id, str) or not finding_id.strip():\n            raise InputError(\"%s: finding %d has no id\" % (path, number))\n        finding_id = finding_id.strip()\n        if finding_id in seen:\n            raise InputError(\"%s: duplicate finding id %s\" % (path, finding_id))\n        seen.add(finding_id)\n        severity = item.get(\"severity\")\n        if severity not in WEIGHTS:\n            raise InputError(\"%s: %s has an unknown severity %r\"\n                             % (path, finding_id, severity))\n        asset = item.get(\"asset\")\n        if not isinstance(asset, str) or not asset.strip():\n            raise InputError(\"%s: %s has no asset\" % (path, finding_id))\n        status = item.get(\"status\")\n        if status not in (\"open\", \"fixed\"):\n            raise InputError(\"%s: %s has an unknown status %r\"\n                             % (path, finding_id, status))\n        kev = item.get(\"kev\", False)\n        if not isinstance(kev, bool):\n            raise InputError(\"%s: %s kev must be true or false\"\n                             % (path, finding_id))\n        first_seen = parse_day(item.get(\"first_seen\"),\n                               \"%s: %s first_seen\" % (path, finding_id))\n        if first_seen > today:\n            raise InputError(\"%s: %s starts in the future\" % (path, finding_id))\n        rows.append({\n            \"id\": finding_id,\n            \"severity\": severity,\n            \"asset\": asset.strip(),\n            \"status\": status,\n            \"kev\": kev,\n            \"first_seen\": first_seen,\n        })\n    return rows\n\n\ndef summarise(rows, today):\n    \"\"\"Return the board numbers for one export.\"\"\"\n    open_rows = [row for row in rows if row[\"status\"] == \"open\"]\n    counts = Counter(row[\"severity\"] for row in open_rows)\n    ages = [(today - row[\"first_seen\"]).days for row in open_rows]\n    kev_open = sum(row[\"kev\"] for row in open_rows)\n    kev_all = sum(row[\"kev\"] for row in rows)\n    score = sum(WEIGHTS[s] * counts[s] for s in SEVERITIES) + KEV_POINTS * kev_open\n    factors = [\"%d*%d\" % (WEIGHTS[s], counts[s]) for s in SEVERITIES]\n    factors.append(\"%d*%d KEV\" % (KEV_POINTS, kev_open))\n    top = Counter(row[\"asset\"] for row in open_rows)\n    return {\n        \"open\": open_rows,\n        \"counts\": counts,\n        \"total\": len(open_rows),\n        \"kev_open\": kev_open,\n        \"kev_all\": kev_all,\n        \"mean_age\": int(round(sum(ages) / len(ages))) if ages else 0,\n        \"oldest\": max(ages) if ages else 0,\n        \"score\": score,\n        \"factors\": \" + \".join(factors),\n        \"top\": sorted(top.items(), key=lambda item: (-item[1], item[0]))[:3],\n    }\n\n\ndef trend_line(now, before):\n    \"\"\"Return the trend sentence for two summaries.\"\"\"\n    if before is None:\n        return \"TREND none, no previous report\"\n    delta = now[\"score\"] - before[\"score\"]\n    word = \"up\" if delta > 0 else \"down\" if delta < 0 else \"flat\"\n    return \"TREND %s %d since last report\" % (word, abs(delta))\n\n\ndef changed_line(now, before):\n    \"\"\"Return the change sentence for two summaries.\"\"\"\n    if before is None:\n        return \"CHANGED none, no previous report\"\n    parts = []\n    for severity in SEVERITIES:\n        delta = now[\"counts\"][severity] - before[\"counts\"][severity]\n        if delta:\n            parts.append(\"%+d %s\" % (delta, severity))\n    now_ids = {row[\"id\"] for row in now[\"open\"]}\n    before_ids = {row[\"id\"] for row in before[\"open\"]}\n    new_ids = now_ids - before_ids\n    closed_ids = before_ids - now_ids\n    if new_ids:\n        parts.append(\"new %d\" % len(new_ids))\n    if closed_ids:\n        parts.append(\"closed %d\" % len(closed_ids))\n    return \"CHANGED \" + (\", \".join(parts) if parts else \"none\")\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Build a one-page board summary from a findings export.\")\n    parser.add_argument(\"findings\", help=\"current findings JSON export\")\n    parser.add_argument(\"--previous\", help=\"findings export used for the last report\")\n    parser.add_argument(\"--today\", help=\"ISO date for the report, default today\")\n    parser.add_argument(\"--sla-days\", type=int, default=30,\n                        help=\"days a critical finding may stay open (default 30)\")\n    args = parser.parse_args(argv)\n    if args.sla_days < 0:\n        parser.error(\"--sla-days must be zero or more\")\n\n    try:\n        today = parse_day(args.today, \"--today\") if args.today else dt.date.today()\n        rows = load_findings(args.findings, today)\n        previous = load_findings(args.previous, today) if args.previous else None\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    now = summarise(rows, today)\n    before = summarise(previous, today) if previous is not None else None\n\n    print(\"OPEN %d total: %s\" % (\n        now[\"total\"], \", \".join(\"%d %s\" % (now[\"counts\"][s], s) for s in SEVERITIES)))\n    print(\"KEV %d open of %d known\" % (now[\"kev_open\"], now[\"kev_all\"]))\n    print(\"AGE mean %d days, oldest %d days\" % (now[\"mean_age\"], now[\"oldest\"]))\n    print(\"TOP \" + (\", \".join(\"%s %d\" % item for item in now[\"top\"]) or \"none\"))\n    print(\"RISK %d = %s\" % (now[\"score\"], now[\"factors\"]))\n    print(trend_line(now, before))\n    print(changed_line(now, before))\n\n    old_critical = [row for row in now[\"open\"]\n                    if row[\"severity\"] == \"critical\"\n                    and (today - row[\"first_seen\"]).days > args.sla_days]\n    if now[\"kev_open\"] or old_critical:\n        print(\"FINDING %d KEV open, %d critical past %d days\"\n              % (now[\"kev_open\"], len(old_critical), args.sla_days))\n        return 1\n    print(\"OK no open KEV, no critical past %d days\" % args.sla_days)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p763","n":"763","slug":"is-it-getting-better","title":"Is it getting better?","sections":["devsecops"],"topic":"Python scripts","tags":["Python","VulnerabilityManagement","Metrics","CyberSecurity","DevOps"],"series_name":"ASM","design":"code","script":"trend_report.py","date":"2031-08-12","files":{"devsecops":"notes/p763-is-it-getting-better.md"},"md":"# Is it getting better?\n\nThe monthly slide says 1,842 findings closed. It does not say 2,100 opened, so the backlog grew and the room applauds.\n\n## Quick reference\n\nThe full script is at the end of this note: `trend_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 trend_report.py history.json\n```\n\n**Example output**\n\n```text\nNET +11, closure 22%\nWORSENING two periods up\n```\n\n## Explained\n\n### What it does\n\n`trend_report.py` reads a JSON history of scan snapshots. Each period prints the open count, net flow (opened minus closed), and the closure rate with its denominator: closed divided by the findings in play, which is the open count at the start plus everything opened. The start count comes from mass balance; an inconsistent history is rejected. It also prints time-to-close, weighted by closures, and flags a closure drop of five points, a backlog that grew two periods running, or critical plus high rising.\n\n### On the screen\n\nNET +11, closure 22%, then WORSENING two periods up. Behind it: 33 days mean, 186 closures.\n\n### How to run it\n\n`python3 trend_report.py history.json`, after the scans land. Exit 0 is no signal, 1 is at least one, 2 is a broken history.\n\n### The fix\n\nPut net flow and the denominator beside the closures on the same slide. When the backlog has grown for two periods, stop adding scanners and close the oldest severe findings.\n\n### Prevention\n\nAppend one snapshot after every scan cycle, same fields. A missing month is a gap; a hand-edited month is worse.\n\n> **The honest caveat:** these are counts, not cohorts; the five-point line is policy, not a standard. A finding opened in March and closed in September counts twice, and the mean hides the slow tail.\n\n> **The lesson:** a closure count is a numerator; a programme is a denominator.\n\n## In one line\n\n> **Direction beats a single number.** Net flow and a denominator turn a count into a direction.\n\n**Check the flow**\n\n```bash\npython3 trend_report.py history.json\n```\n\n## Script: trend_report.py\n\nDownload: [scripts/trend_report.py](../scripts/trend_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"trend_report.py - is the vulnerability programme getting better?\n\nReads a history of scan snapshots and prints, per period, the open count, the\nnet flow (opened minus closed) and the closure rate with the denominator that\nproduced it. It then prints the window totals, the time-to-close weighted by\nclosures, and flags a worsening trend.\n\nSnapshots JSON is a list in date order:\n\n    [{\"date\": \"2026-07-31\", \"critical\": 12, \"high\": 44, \"medium\": 74,\n      \"low\": 22, \"opened\": 55, \"closed\": 43, \"mean_days_to_close\": 32}]\n\nThe four severity counts are the findings open at the end of the period.\nopened and closed are the flows during it. The open count at the start is\nreconstructed by mass balance:\n\n    open_start = open_end - opened + closed\n\nThe closure rate is closed / (open_start + opened): the share of everything\nin play during the period that was closed. A rate without that denominator is\na number about the team, not about the backlog.\n\nThe trend is a finding when any of these is true:\n  - the latest closure rate is --drop points below the previous period;\n  - net flow is positive in each of the last two periods;\n  - critical plus high is higher than in the previous period.\nThose thresholds are a policy choice. The printed factors let you argue with\nthem.\n\nUsage:\n    python3 trend_report.py history.json\n    python3 trend_report.py history.json --drop 3\n\nCron (daily 07:00, after the scan exports land):\n    0 7 * * * python3 /opt/s0sta/trend_report.py /var/lib/s0sta/history.json\n\nCI (fail the report job when the trend worsens):\n    python3 trend_report.py history.json || exit 1\n\nExit codes: 0 no worsening signal, 1 at least one signal, 2 usage or a history\nthat is missing, unreadable, invalid, out of order or inconsistent.\n\nRead-only: reads the history and prints. It writes nothing and makes no\nnetwork call.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport json\nimport sys\n\nSEVERITIES = (\"critical\", \"high\", \"medium\", \"low\")\n\n\nclass InputError(Exception):\n    \"\"\"A history that cannot be turned into a trend.\"\"\"\n\n\ndef parse_day(text, label):\n    \"\"\"Return a date from an ISO string, or raise InputError.\"\"\"\n    if not isinstance(text, str):\n        raise InputError(\"%s is not an ISO date\" % label)\n    try:\n        return dt.date.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s is not an ISO date: %s\" % (label, text))\n\n\ndef count_field(snapshot, key, number):\n    \"\"\"Return a non-negative integer field, or raise InputError.\"\"\"\n    value = snapshot.get(key)\n    if isinstance(value, bool) or not isinstance(value, int) or value < 0:\n        raise InputError(\"snapshot %d: %s must be a non-negative integer\"\n                         % (number, key))\n    return value\n\n\ndef load_history(path):\n    \"\"\"Read and validate the snapshots. Return a list of metric dicts.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s must contain a JSON list of snapshots\" % path)\n    if len(doc) < 2:\n        raise InputError(\"%s needs at least two snapshots to show a trend\" % path)\n\n    rows = []\n    previous_day = None\n    for number, snapshot in enumerate(doc, 1):\n        if not isinstance(snapshot, dict):\n            raise InputError(\"snapshot %d is not an object\" % number)\n        day = parse_day(snapshot.get(\"date\"), \"snapshot %d date\" % number)\n        if previous_day is not None and day <= previous_day:\n            raise InputError(\"snapshot %d is not after the one before it\" % number)\n        previous_day = day\n        opened = count_field(snapshot, \"opened\", number)\n        closed = count_field(snapshot, \"closed\", number)\n        mean = snapshot.get(\"mean_days_to_close\")\n        if isinstance(mean, bool) or not isinstance(mean, (int, float)) or mean < 0:\n            raise InputError(\"snapshot %d: mean_days_to_close must be a \"\n                             \"non-negative number\" % number)\n        counts = {key: count_field(snapshot, key, number) for key in SEVERITIES}\n        open_end = sum(counts.values())\n        open_start = open_end - opened + closed\n        if open_start < 0:\n            raise InputError(\"snapshot %d: opened and closed do not fit the \"\n                             \"open counts\" % number)\n        denominator = open_start + opened\n        rows.append({\n            \"date\": day,\n            \"counts\": counts,\n            \"open_end\": open_end,\n            \"open_start\": open_start,\n            \"opened\": opened,\n            \"closed\": closed,\n            \"net\": opened - closed,\n            \"denominator\": denominator,\n            \"rate\": (closed / denominator) if denominator else None,\n            \"mean\": float(mean),\n            \"severe\": counts[\"critical\"] + counts[\"high\"],\n        })\n    return rows\n\n\ndef rate_text(rate):\n    \"\"\"Format a closure rate, or n/a when nothing was in play.\"\"\"\n    return \"n/a\" if rate is None else \"%d%%\" % round(100 * rate)\n\n\ndef worsening(rows, drop):\n    \"\"\"Return a list of reasons the latest period is worse.\"\"\"\n    latest, previous = rows[-1], rows[-2]\n    reasons = []\n    if latest[\"rate\"] is not None and previous[\"rate\"] is not None:\n        points = round(100 * (previous[\"rate\"] - latest[\"rate\"]))\n        if points >= drop:\n            reasons.append(\"closure down %d pts\" % points)\n    if rows[-1][\"net\"] > 0 and rows[-2][\"net\"] > 0:\n        reasons.append(\"backlog up 2 periods\")\n    if latest[\"severe\"] > previous[\"severe\"]:\n        reasons.append(\"severe up %d\" % (latest[\"severe\"] - previous[\"severe\"]))\n    return reasons\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Show the vulnerability trend and flag a worsening period.\")\n    parser.add_argument(\"history\", help=\"JSON list of scan snapshots in date order\")\n    parser.add_argument(\"--drop\", type=int, default=5,\n                        help=\"closure-rate drop in points that counts as worse \"\n                             \"(default 5)\")\n    args = parser.parse_args(argv)\n    if not 0 <= args.drop <= 100:\n        parser.error(\"--drop must be between 0 and 100\")\n\n    try:\n        rows = load_history(args.history)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    for row in rows:\n        print(\"%s open %d, net %+d, closed %d/%d (%s)\" % (\n            row[\"date\"], row[\"open_end\"], row[\"net\"], row[\"closed\"],\n            row[\"denominator\"], rate_text(row[\"rate\"])))\n\n    total_closed = sum(row[\"closed\"] for row in rows)\n    total_denominator = sum(row[\"denominator\"] for row in rows)\n    window_rate = (total_closed / total_denominator) if total_denominator else None\n    weighted_mean = (sum(row[\"closed\"] * row[\"mean\"] for row in rows)\n                     / total_closed) if total_closed else None\n    print(\"WINDOW %s to %s, net %+d, open %d -> %d\" % (\n        rows[0][\"date\"], rows[-1][\"date\"],\n        sum(row[\"net\"] for row in rows), rows[0][\"open_end\"], rows[-1][\"open_end\"]))\n    mean_text = (\"n/a\" if weighted_mean is None\n                 else \"%d days\" % round(weighted_mean))\n    print(\"CLOSE %s latest, %s window, mean %s over %d closures\" % (\n        rate_text(rows[-1][\"rate\"]), rate_text(window_rate),\n        mean_text, total_closed))\n\n    reasons = worsening(rows, args.drop)\n    if reasons:\n        print(\"WORSENING \" + \", \".join(reasons))\n        return 1\n    print(\"OK closure stable or improving, backlog not growing\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p764","n":"764","slug":"what-the-scanner-never-saw","title":"What the scanner never saw","sections":["devsecops"],"topic":"Python scripts","tags":["Python","AttackSurface","VulnerabilityManagement","CyberSecurity","DevSecOps"],"series_name":"ASM","design":"code","script":"coverage_report.py","date":"2031-08-14","files":{"devsecops":"notes/p764-what-the-scanner-never-saw.md"},"md":"# What the scanner never saw\n\nThe coverage slide says 94%. The number is scans sent divided by the assets the scanner already knew about, and the database nobody added to the inventory is invisible.\n\n## Quick reference\n\nThe full script is at the end of this note: `coverage_report.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 coverage_report.py inv.json scans.json\n```\n\n**Example output**\n\n```text\nGAP 2 assets with a tool gap\nCRITICAL db-02\n```\n\n## Explained\n\n### What it does\n\n`coverage_report.py` joins the asset inventory with the scan records. For each required tool it looks for a successful scan inside the window; a failed scan is an attempt, not a result. It prints scanned and tool-complete counts, lists the gaps, and names scanned assets that are not in the inventory at all. Exit 1 when a gap reaches `--fail-on`, critical by default; that line is policy.\n\n### On the screen\n\nGAP 2 assets with a tool gap, then CRITICAL db-02. Behind it, 9 of 10 assets were scanned and 8 had every tool; lab-07 was scanned but unknown to the inventory.\n\n### How to run it\n\n`python3 coverage_report.py inventory.json scans.json --tools` trivy,osquery `--days 30`. Exit 0 is no gap at the line, 1 is at least one, 2 is a bad export.\n\n### The fix\n\nAdd db-02 to the inventory and the scan scope, then rerun. Send lab-07 to the CMDB owner: an asset you cannot name is not covered.\n\n### Prevention\n\nRun the join weekly and treat a missing critical asset as an incident. The inventory is the denominator, so keep it current.\n\n> **The honest caveat:** coverage proves a scan ran, not that it was configured to find anything. A scanner with stale plugins covers the asset and misses the CVE.\n\n> **The lesson:** a scanner can only report on what it was pointed at.\n\n## In one line\n\n> **Unscanned is not clean.** A clean report over half the estate is not a clean report.\n\n**Find the blind**\n\n```bash\npython3 coverage_report.py inv scans\n```\n\n## Script: coverage_report.py\n\nDownload: [scripts/coverage_report.py](../scripts/coverage_report.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"coverage_report.py - what the scanner never saw.\n\nCompares the asset inventory with the scan records and reports the assets that\nwere never scanned, the assets missing one of the required tools, and the\nscanned assets that are not in the inventory at all. A clean report over half\nthe estate is not a clean report.\n\nInventory JSON is a list of assets:\n\n    [{\"asset\": \"web-01\", \"criticality\": \"critical\"},\n     {\"asset\": \"lab-01\", \"criticality\": \"low\"}]\n\nScans JSON is a list of scan records:\n\n    [{\"asset\": \"web-01\", \"tool\": \"trivy\", \"date\": \"2026-10-12\",\n      \"status\": \"ok\"},\n     {\"asset\": \"web-01\", \"tool\": \"osquery\", \"date\": \"2026-10-12\",\n      \"status\": \"ok\"}]\n\nstatus is ok or failed. Only an ok scan inside the --days window counts as\ncoverage; a failed scan is an attempt, not a result. The required tools are a\ncomma-separated list: --tools trivy,osquery by default.\n\nThe exit code is a policy decision. --fail-on critical (the default) fails\nonly when a critical asset has a gap; --fail-on high tightens it. Gaps below\nthat line are still printed, because a high asset nobody scanned is not a\ndetail.\n\nUsage:\n    python3 coverage_report.py inventory.json scans.json\n    python3 coverage_report.py inventory.json scans.json --today 2026-10-20\n    python3 coverage_report.py inventory.json scans.json --days 7 --fail-on high\n\nCron (daily 06:15, before the morning stand-up):\n    15 6 * * * python3 /opt/s0sta/coverage_report.py /etc/s0sta/assets.json /var/lib/s0sta/scans.json\n\nCI (block a release when a critical asset is blind):\n    python3 coverage_report.py assets.json scans.json || exit 1\n\nExit codes: 0 no gap at or above --fail-on, 1 at least one, 2 usage or an\ninventory or scan export that is missing, unreadable, invalid or inconsistent.\n\nRead-only: reads the two exports and prints. It writes nothing and makes no\nnetwork call.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport json\nimport sys\n\nRANK = {\"low\": 0, \"medium\": 1, \"high\": 2, \"critical\": 3}\nDEFAULT_TOOLS = \"trivy,osquery\"\n\n\nclass InputError(Exception):\n    \"\"\"An export that cannot be compared with the other one.\"\"\"\n\n\ndef parse_day(text, label):\n    \"\"\"Return a date from an ISO string, or raise InputError.\"\"\"\n    if not isinstance(text, str):\n        raise InputError(\"%s is not an ISO date\" % label)\n    try:\n        return dt.date.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s is not an ISO date: %s\" % (label, text))\n\n\ndef load_inventory(path):\n    \"\"\"Read the inventory. Return a list of (asset, criticality).\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list) or not doc:\n        raise InputError(\"%s must contain a non-empty JSON list of assets\" % path)\n\n    assets = []\n    seen = set()\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: asset %d is not an object\" % (path, number))\n        name = item.get(\"asset\")\n        if not isinstance(name, str) or not name.strip():\n            raise InputError(\"%s: asset %d has no name\" % (path, number))\n        name = name.strip()\n        if name in seen:\n            raise InputError(\"%s: duplicate asset %s\" % (path, name))\n        seen.add(name)\n        criticality = item.get(\"criticality\")\n        if criticality not in RANK:\n            raise InputError(\"%s: %s has an unknown criticality %r\"\n                             % (path, name, criticality))\n        assets.append((name, criticality))\n    return assets\n\n\ndef load_scans(path, today, inventory):\n    \"\"\"Read the scan records. Return (ok scans, unknown assets).\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s must contain a JSON list of scan records\" % path)\n\n    scans = []\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: scan %d is not an object\" % (path, number))\n        asset = item.get(\"asset\")\n        if not isinstance(asset, str) or not asset.strip():\n            raise InputError(\"%s: scan %d has no asset\" % (path, number))\n        tool = item.get(\"tool\")\n        if not isinstance(tool, str) or not tool.strip():\n            raise InputError(\"%s: scan %d has no tool\" % (path, number))\n        status = item.get(\"status\")\n        if status not in (\"ok\", \"failed\"):\n            raise InputError(\"%s: scan %d has an unknown status %r\"\n                             % (path, number, status))\n        day = parse_day(item.get(\"date\"), \"%s: scan %d date\" % (path, number))\n        if day > today:\n            raise InputError(\"%s: scan %d is dated in the future\"\n                             % (path, number))\n        scans.append({\n            \"asset\": asset.strip(),\n            \"tool\": tool.strip(),\n            \"status\": status,\n            \"date\": day,\n        })\n\n    unknown = sorted({scan[\"asset\"] for scan in scans\n                      if scan[\"asset\"] not in {a for a, _ in inventory}})\n    return scans, unknown\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Compare an asset inventory with scan coverage.\")\n    parser.add_argument(\"inventory\", help=\"JSON list of assets\")\n    parser.add_argument(\"scans\", help=\"JSON list of scan records\")\n    parser.add_argument(\"--tools\", default=DEFAULT_TOOLS,\n                        help=\"required tools, comma separated (default %s)\"\n                             % DEFAULT_TOOLS)\n    parser.add_argument(\"--days\", type=int, default=30,\n                        help=\"coverage window in days (default 30)\")\n    parser.add_argument(\"--today\", help=\"ISO date for the window, default today\")\n    parser.add_argument(\"--fail-on\", choices=(\"critical\", \"high\", \"medium\", \"low\"),\n                        default=\"critical\",\n                        help=\"criticality at which a gap fails the job \"\n                             \"(default critical)\")\n    args = parser.parse_args(argv)\n    if args.days < 1:\n        parser.error(\"--days must be at least 1\")\n\n    tools = []\n    for tool in args.tools.split(\",\"):\n        tool = tool.strip()\n        if tool and tool not in tools:\n            tools.append(tool)\n    if not tools:\n        parser.error(\"--tools needs at least one tool name\")\n\n    try:\n        today = parse_day(args.today, \"--today\") if args.today else dt.date.today()\n        inventory = load_inventory(args.inventory)\n        scans, unknown = load_scans(args.scans, today, inventory)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    cutoff = today - dt.timedelta(days=args.days)\n    covered = {asset: set() for asset, _ in inventory}\n    for scan in scans:\n        if (scan[\"status\"] == \"ok\" and scan[\"date\"] >= cutoff\n                and scan[\"asset\"] in covered):\n            covered[scan[\"asset\"]].add(scan[\"tool\"])\n\n    scanned = sum(1 for asset, _ in inventory if covered[asset])\n    complete = sum(1 for asset, _ in inventory\n                   if covered[asset] >= set(tools))\n    print(\"COVERAGE %d/%d assets scanned, %d/%d with every tool (%d days: %s)\"\n          % (scanned, len(inventory), complete, len(inventory), args.days,\n             \", \".join(tools)))\n\n    gaps = []\n    for asset, criticality in inventory:\n        missing = [tool for tool in tools if tool not in covered[asset]]\n        if missing:\n            gaps.append((asset, criticality, missing))\n\n    print(\"GAP %d assets with a tool gap\" % len(gaps))\n    for asset, criticality, _missing in gaps:\n        if RANK[criticality] >= RANK[args.fail_on]:\n            print(\"%s %s\" % (criticality.upper(), asset))\n    for asset, criticality, missing in gaps:\n        if RANK[criticality] < RANK[args.fail_on]:\n            print(\"TOOL %s: %s\" % (asset, \", \".join(missing)))\n    if unknown:\n        print(\"NOTE %s scanned but not in inventory\" % \", \".join(unknown))\n\n    failing = [gap for gap in gaps if RANK[gap[1]] >= RANK[args.fail_on]]\n    if failing:\n        print(\"FAIL %d gap%s at or above %s\"\n              % (len(failing), \"\" if len(failing) == 1 else \"s\", args.fail_on))\n        return 1\n    if gaps:\n        print(\"OK no gap at or above %s (%d below the line)\"\n              % (args.fail_on, len(gaps)))\n    else:\n        print(\"OK every asset has every required tool in the window\")\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p765","n":"765","slug":"map-the-finding-to-the-control","title":"Map the finding to the control","sections":["devsecops"],"topic":"Python scripts","tags":["Python","GRC","SOC2","Compliance","CyberSecurity"],"series_name":"ASM","design":"code","script":"control_map.py","date":"2031-08-17","files":{"devsecops":"notes/p765-map-the-finding-to-the-control.md"},"md":"# Map the finding to the control\n\nThe auditor asks which control covers the backup finding. The answer is a category name and a hope, and the finding goes into the pack without an id.\n\n## Quick reference\n\nThe full script is at the end of this note: `control_map.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 control_map.py findings.json soc2.json\n```\n\n**Example output**\n\n```text\nCONTROL MAP 7/9 categories (SOC 2)\nUNMAPPED 5 findings, 2 high\n```\n\n## Explained\n\n### What it does\n\n`control_map.py` reads the findings and a framework file that maps each category to SOC 2 or ISO control ids. It prints the categories covered, the control ids in the framework, and every finding whose category has no control. Exit 1 when an unmapped finding is at or above `--fail-on`, high by default.\n\n### On the screen\n\nCONTROL MAP 7/9 categories (SOC 2), then UNMAPPED 5 findings, 2 high. The two categories are saas_review and backup_policy; both high findings have no control id.\n\n### How to run it\n\n`python3 control_map.py findings.json soc2.json`, weekly before the audit sync. Exit 0 is every high finding mapped, 1 is at least one without a control, 2 is a bad file; the high line is policy.\n\n### The fix\n\nMap every scanner category once, in a versioned file reviewed with the control owner. Every finding then inherits a control, and the audit query becomes a join instead of a search.\n\n### Prevention\n\nFail the weekly job on a high unmapped finding. A new scanner category cannot ship without a mapping, and the mapping lives in git beside the policy.\n\n> **The honest caveat:** a mapped control is not a working control. The map says where the finding belongs; the evidence is a separate packet.\n\n> **The lesson:** a finding without a control is a story the auditor cannot file.\n\n## In one line\n\n> **A finding without a control is a story.** A control id is what turns a finding into an audit row.\n\n**Map it first**\n\n```bash\npython3 control_map.py f.json soc2.json\n```\n\n## Script: control_map.py\n\nDownload: [scripts/control_map.py](../scripts/control_map.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"control_map.py - map every finding category to a control id.\n\nReads a findings export and a control framework file and answers two audit\nquestions: how much of the finding set has a control, and which findings have\nnone. A finding with no control cannot be reported against SOC 2 or ISO 27001\nwithout a story, and a story is not evidence.\n\nFindings JSON is a list of objects:\n\n    [{\"id\": \"F-101\", \"category\": \"saas_review\", \"severity\": \"high\",\n      \"asset\": \"notion\"}]\n\nFramework JSON maps each finding category to one or more control ids:\n\n    {\"framework\": \"SOC 2\", \"version\": \"2026\",\n     \"controls\": {\"patch_management\": [\"CC7.1\"],\n                  \"access_review\": [\"CC6.1\", \"CC6.2\"]}}\n\nCategories are the vocabulary your scanners emit. Keep them stable: a renamed\ncategory silently becomes an unmapped finding. The exit code fails only for\nfindings at or above --fail-on, because an unmapped low finding is a backlog\nitem, while an unmapped high finding is an audit gap.\n\nUsage:\n    python3 control_map.py findings.json soc2.json\n    python3 control_map.py findings.json soc2.json --fail-on critical\n    python3 control_map.py findings.json soc2.json --show 20\n\nCron (weekly, Monday 06:30, before the audit sync):\n    30 6 * * 1 python3 /opt/s0sta/control_map.py /var/lib/s0sta/findings.json /etc/s0sta/soc2.json\n\nCI (fail when a high finding has no control):\n    python3 control_map.py findings.json soc2.json || exit 1\n\nExit codes: 0 no unmapped finding at or above --fail-on, 1 at least one,\n2 usage or a findings or framework file that is missing, unreadable, invalid\nor inconsistent.\n\nRead-only: reads the two files and prints. It writes nothing and makes no\nnetwork call.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport json\nimport sys\n\nRANK = {\"low\": 0, \"medium\": 1, \"high\": 2, \"critical\": 3}\n\n\nclass InputError(Exception):\n    \"\"\"A file that cannot be used for the mapping.\"\"\"\n\n\ndef load_findings(path):\n    \"\"\"Read findings. Return a list of (id, category, severity).\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s must contain a JSON list of findings\" % path)\n\n    rows = []\n    seen = set()\n    for number, item in enumerate(doc, 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: finding %d is not an object\" % (path, number))\n        finding_id = item.get(\"id\")\n        if not isinstance(finding_id, str) or not finding_id.strip():\n            raise InputError(\"%s: finding %d has no id\" % (path, number))\n        finding_id = finding_id.strip()\n        if finding_id in seen:\n            raise InputError(\"%s: duplicate finding id %s\" % (path, finding_id))\n        seen.add(finding_id)\n        category = item.get(\"category\")\n        if not isinstance(category, str) or not category.strip():\n            raise InputError(\"%s: %s has no category\" % (path, finding_id))\n        severity = item.get(\"severity\")\n        if severity not in RANK:\n            raise InputError(\"%s: %s has an unknown severity %r\"\n                             % (path, finding_id, severity))\n        rows.append((finding_id, category.strip(), severity))\n    return rows\n\n\ndef load_framework(path):\n    \"\"\"Read the framework. Return (name, {category: [control ids]}).\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, dict):\n        raise InputError(\"%s must contain a JSON object\" % path)\n    name = doc.get(\"framework\")\n    if not isinstance(name, str) or not name.strip():\n        raise InputError(\"%s has no framework name\" % path)\n    controls = doc.get(\"controls\")\n    if not isinstance(controls, dict):\n        raise InputError(\"%s has no controls object\" % path)\n\n    mapping = {}\n    for category, ids in controls.items():\n        if not isinstance(category, str) or not category.strip():\n            raise InputError(\"%s has an empty control category\" % path)\n        if not isinstance(ids, list) or not ids:\n            raise InputError(\"%s: category %s has no control ids\"\n                             % (path, category))\n        clean = []\n        for control in ids:\n            if not isinstance(control, str) or not control.strip():\n                raise InputError(\"%s: category %s has an empty control id\"\n                                 % (path, category))\n            control = control.strip()\n            if control not in clean:\n                clean.append(control)\n        mapping[category.strip()] = clean\n    return name.strip(), mapping\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Map finding categories to control framework ids.\")\n    parser.add_argument(\"findings\", help=\"JSON list of findings\")\n    parser.add_argument(\"framework\", help=\"JSON framework with category to control ids\")\n    parser.add_argument(\"--fail-on\", choices=(\"critical\", \"high\", \"medium\", \"low\"),\n                        default=\"high\",\n                        help=\"severity at which an unmapped finding fails the \"\n                             \"job (default high)\")\n    parser.add_argument(\"--show\", type=int, default=10,\n                        help=\"unmapped findings to list (default 10)\")\n    args = parser.parse_args(argv)\n    if args.show < 0:\n        parser.error(\"--show must be zero or more\")\n\n    try:\n        findings = load_findings(args.findings)\n        framework, mapping = load_framework(args.framework)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    categories = {category for _, category, _ in findings}\n    mapped = categories & set(mapping)\n    controls = {control for ids in mapping.values() for control in ids}\n    unmapped = [row for row in findings if row[1] not in mapping]\n    high_unmapped = [row for row in unmapped\n                     if RANK[row[2]] >= RANK[args.fail_on]]\n\n    print(\"CONTROL MAP %d/%d categories (%s)\"\n          % (len(mapped), len(categories), framework))\n    print(\"UNMAPPED %d findings, %d %s\"\n          % (len(unmapped), len(high_unmapped), args.fail_on))\n    if unmapped:\n        print(\"CATEGORIES \" + \", \".join(sorted({row[1] for row in unmapped})))\n        for finding_id, category, severity in unmapped[:args.show]:\n            print(\"  %s %s %s\" % (finding_id, category, severity))\n        if len(unmapped) > args.show:\n            print(\"  ... %d more\" % (len(unmapped) - args.show))\n    print(\"CONTROLS %d ids across %d categories\" % (len(controls), len(mapping)))\n\n    if high_unmapped:\n        print(\"FAIL %d finding%s at or above %s without a control\"\n              % (len(high_unmapped), \"\" if len(high_unmapped) == 1 else \"s\",\n                 args.fail_on))\n        return 1\n    print(\"OK every finding at or above %s has a control\" % args.fail_on)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p766","n":"766","slug":"the-audit-packet","title":"The audit packet","sections":["devsecops"],"topic":"Python scripts","tags":["Python","Audit","Compliance","GRC","CyberSecurity"],"series_name":"ASM","design":"code","script":"audit_packet.py","date":"2031-08-19","files":{"devsecops":"notes/p766-the-audit-packet.md"},"md":"# The audit packet\n\nThe auditor asks for evidence that the patch control operated in Q3. The team sends a folder, a screenshot and a promise, and the review waits.\n\n## Quick reference\n\nThe full script is at the end of this note: `audit_packet.py` (standard library only, no `pip install`).\n\n**Run it**\n\n```bash\npython3 audit_packet.py evidence/ -f f.json -s s.json -r r.json\n```\n\n**Example output**\n\n```text\nPACKET 8 files, 7 referenced\nGAPS 1 missing, 2 high open\n```\n\n## Explained\n\n### What it does\n\n`audit_packet.py` reads an evidence directory plus the findings, scan and remediation records. It prints a manifest with a SHA-256 and size per file, one bundle hash over the files, a per-control evidence index, and a gaps section: missing files, controls with no evidence, orphan files, and high findings open without evidence. It prints, never writes.\n\n### On the screen\n\nPACKET 8 files, 7 referenced, then GAPS 1 missing, 2 high open. The missing file is a critical patch note; the two open findings have no usable remediation evidence.\n\n### How to run it\n\n`python3 audit_packet.py evidence/ -f findings.json -s scans.json -r remediation.json`, monthly before the control review. Redirect stdout to keep the packet. Exit 0 is complete, 1 is a gap, 2 is a bad input; `--fail-on` sets the line, high by default.\n\n### The fix\n\nBuild the packet from records, not memory. Every scan and remediation record names its evidence file, so the index is a join and a missing file fails before the auditor finds it.\n\n### Prevention\n\nMake evidence a required field on the remediation ticket. The packet then assembles itself; sign the printed manifest and store it with the packet.\n\n> **The honest caveat:** the hash proves the bytes match, not that the scan was complete or the evidence current. Read the dates too.\n\n> **The lesson:** an audit is a join between a control and a dated file.\n\n## In one line\n\n> **Evidence, or it did not happen.** A control without a dated file is a belief with a policy number.\n\n**Build the packet**\n\n```bash\npython3 audit_packet.py ev -f f -s s -r r\n```\n\n## Script: audit_packet.py\n\nDownload: [scripts/audit_packet.py](../scripts/audit_packet.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"audit_packet.py - assemble a read-only audit evidence packet.\n\nReads a directory of evidence files plus the findings, scans and remediation\nrecords that reference them, and prints a packet to stdout:\n\n  - a manifest with the size and SHA-256 of every evidence file, plus one\n    bundle hash over the sorted manifest lines;\n  - a per-control evidence index, built from successful scans and from\n    remediation records;\n  - a gaps section: referenced files that are missing, controls with no\n    evidence, orphan files nobody references, and high findings still open\n    without evidence.\n\nFindings JSON:\n\n    [{\"id\": \"F-1\", \"control\": \"CC7.1\", \"severity\": \"high\", \"status\": \"open\"}]\n\nScans JSON:\n\n    [{\"date\": \"2026-10-12\", \"tool\": \"trivy\", \"target\": \"web-01\",\n      \"control\": \"CC7.1\", \"status\": \"ok\", \"evidence\": \"trivy-web-01.json\"}]\n\nRemediation JSON:\n\n    [{\"finding\": \"F-3\", \"control\": \"CC6.1\", \"closed\": \"2026-10-14\",\n      \"evidence\": \"patch-web-03.txt\"}]\n\nEvidence names are file names inside the evidence directory, never paths. A\nsymlink is not followed: it is reported as a gap, because evidence that can\npoint outside the packet is not evidence. Only a successful scan is indexed;\na failed scan is an attempt.\n\nThe packet is printed, never written. Redirect stdout if you want a file, then\nsign or archive it. The script opens the evidence files read-only.\n\nUsage:\n    python3 audit_packet.py evidence/ -f findings.json -s scans.json -r remediation.json\n    python3 audit_packet.py evidence/ -f findings.json -s scans.json -r remediation.json --fail-on critical\n\nCron (monthly, on the first, before the control review):\n    0 5 1 * * python3 /opt/s0sta/audit_packet.py /var/lib/s0sta/evidence -f /var/lib/s0sta/findings.json -s /var/lib/s0sta/scans.json -r /var/lib/s0sta/remediation.json\n\nExit codes: 0 the packet is complete for every record, 1 a referenced file is\nmissing or unreadable, a symlink or non-file sits in the evidence directory,\nor a finding at or above --fail-on is open without evidence, 2 usage or an\ninput that is missing, unreadable, invalid or inconsistent.\n\nStandard library only, Python 3.8+.\n\nBy s0sta.com\n\"\"\"\nimport argparse\nimport datetime as dt\nimport hashlib\nimport json\nimport os\nimport stat\nimport sys\n\nRANK = {\"low\": 0, \"medium\": 1, \"high\": 2, \"critical\": 3}\nCHUNK = 1024 * 1024\n\n\nclass InputError(Exception):\n    \"\"\"An input that cannot be assembled into a packet.\"\"\"\n\n\ndef parse_day(text, label):\n    \"\"\"Return a date from an ISO string, or raise InputError.\"\"\"\n    if not isinstance(text, str):\n        raise InputError(\"%s is not an ISO date\" % label)\n    try:\n        return dt.date.fromisoformat(text)\n    except ValueError:\n        raise InputError(\"%s is not an ISO date: %s\" % (label, text))\n\n\ndef check_evidence_name(name, label):\n    \"\"\"Accept only a plain file name inside the evidence directory.\"\"\"\n    if not isinstance(name, str) or not name.strip():\n        raise InputError(\"%s has no evidence file name\" % label)\n    name = name.strip()\n    if (name in (\".\", \"..\") or os.path.isabs(name)\n            or os.path.basename(name) != name\n            or (os.altsep and os.altsep in name)):\n        raise InputError(\"%s evidence must be a file name, not a path: %s\"\n                         % (label, name))\n    return name\n\n\ndef load_json(path, kind):\n    \"\"\"Read one JSON document with a clear error.\"\"\"\n    try:\n        with open(path, encoding=\"utf-8\") as handle:\n            doc = json.load(handle)\n    except OSError as exc:\n        raise InputError(\"cannot read %s: %s\" % (path, exc))\n    except ValueError as exc:\n        raise InputError(\"%s is not valid JSON: %s\" % (path, exc))\n    if not isinstance(doc, list):\n        raise InputError(\"%s must contain a JSON list of %s\" % (path, kind))\n    return doc\n\n\ndef load_findings(path):\n    \"\"\"Return validated findings records.\"\"\"\n    rows = []\n    seen = set()\n    for number, item in enumerate(load_json(path, \"findings\"), 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: finding %d is not an object\" % (path, number))\n        finding_id = item.get(\"id\")\n        if not isinstance(finding_id, str) or not finding_id.strip():\n            raise InputError(\"%s: finding %d has no id\" % (path, number))\n        finding_id = finding_id.strip()\n        if finding_id in seen:\n            raise InputError(\"%s: duplicate finding id %s\" % (path, finding_id))\n        seen.add(finding_id)\n        control = item.get(\"control\")\n        if not isinstance(control, str) or not control.strip():\n            raise InputError(\"%s: %s has no control\" % (path, finding_id))\n        severity = item.get(\"severity\")\n        if severity not in RANK:\n            raise InputError(\"%s: %s has an unknown severity %r\"\n                             % (path, finding_id, severity))\n        status = item.get(\"status\")\n        if status not in (\"open\", \"fixed\"):\n            raise InputError(\"%s: %s has an unknown status %r\"\n                             % (path, finding_id, status))\n        rows.append({\"id\": finding_id, \"control\": control.strip(),\n                     \"severity\": severity, \"status\": status})\n    return rows\n\n\ndef load_scans(path):\n    \"\"\"Return validated scan records.\"\"\"\n    rows = []\n    for number, item in enumerate(load_json(path, \"scans\"), 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: scan %d is not an object\" % (path, number))\n        for key in (\"tool\", \"target\", \"control\"):\n            if not isinstance(item.get(key), str) or not item[key].strip():\n                raise InputError(\"%s: scan %d has no %s\" % (path, number, key))\n        parse_day(item.get(\"date\"), \"%s: scan %d date\" % (path, number))\n        status = item.get(\"status\")\n        if status not in (\"ok\", \"failed\"):\n            raise InputError(\"%s: scan %d has an unknown status %r\"\n                             % (path, number, status))\n        evidence = None\n        if item.get(\"evidence\") is not None:\n            evidence = check_evidence_name(item[\"evidence\"],\n                                           \"%s: scan %d\" % (path, number))\n        rows.append({\"tool\": item[\"tool\"].strip(), \"target\": item[\"target\"].strip(),\n                     \"control\": item[\"control\"].strip(), \"status\": status,\n                     \"evidence\": evidence})\n    return rows\n\n\ndef load_remediation(path):\n    \"\"\"Return validated remediation records.\"\"\"\n    rows = []\n    seen = set()\n    for number, item in enumerate(load_json(path, \"remediation records\"), 1):\n        if not isinstance(item, dict):\n            raise InputError(\"%s: record %d is not an object\" % (path, number))\n        for key in (\"finding\", \"control\"):\n            if not isinstance(item.get(key), str) or not item[key].strip():\n                raise InputError(\"%s: record %d has no %s\" % (path, number, key))\n        finding = item[\"finding\"].strip()\n        if finding in seen:\n            raise InputError(\"%s: duplicate remediation for %s\" % (path, finding))\n        seen.add(finding)\n        parse_day(item.get(\"closed\"), \"%s: record %d closed\" % (path, number))\n        evidence = None\n        if item.get(\"evidence\") is not None:\n            evidence = check_evidence_name(item[\"evidence\"],\n                                           \"%s: record %d\" % (path, number))\n        rows.append({\"finding\": finding, \"control\": item[\"control\"].strip(),\n                     \"evidence\": evidence})\n    return rows\n\n\ndef sha256_file(path):\n    \"\"\"Return the hex digest and size of one file.\"\"\"\n    digest = hashlib.sha256()\n    size = 0\n    with open(path, \"rb\") as handle:\n        while True:\n            chunk = handle.read(CHUNK)\n            if not chunk:\n                break\n            digest.update(chunk)\n            size += len(chunk)\n    return digest.hexdigest(), size\n\n\ndef read_evidence(root):\n    \"\"\"Hash the directory. Return (files dict, problems list).\"\"\"\n    files = {}\n    problems = []\n    try:\n        names = sorted(os.listdir(root))\n    except OSError as exc:\n        raise InputError(\"cannot list %s: %s\" % (root, exc))\n    for name in names:\n        path = os.path.join(root, name)\n        try:\n            info = os.lstat(path)\n        except OSError as exc:\n            problems.append(\"%s: %s\" % (name, exc))\n            continue\n        if stat.S_ISLNK(info.st_mode):\n            problems.append(\"%s: symlink not followed\" % name)\n            continue\n        if not stat.S_ISREG(info.st_mode):\n            problems.append(\"%s: not a regular file\" % name)\n            continue\n        try:\n            digest, size = sha256_file(path)\n        except OSError as exc:\n            problems.append(\"%s: %s\" % (name, exc))\n            continue\n        files[name] = {\"sha256\": digest, \"size\": size}\n    return files, problems\n\n\ndef main(argv=None):\n    parser = argparse.ArgumentParser(\n        description=\"Assemble a read-only audit evidence packet from findings, \"\n                    \"scans and remediation records.\")\n    parser.add_argument(\"evidence\", help=\"directory of evidence files\")\n    parser.add_argument(\"-f\", \"--findings\", required=True,\n                        help=\"findings JSON export\")\n    parser.add_argument(\"-s\", \"--scans\", required=True,\n                        help=\"scan records JSON export\")\n    parser.add_argument(\"-r\", \"--remediation\", required=True,\n                        help=\"remediation records JSON export\")\n    parser.add_argument(\"--fail-on\", choices=(\"critical\", \"high\", \"medium\", \"low\"),\n                        default=\"high\",\n                        help=\"severity at which an open finding without \"\n                             \"evidence fails the packet (default high)\")\n    args = parser.parse_args(argv)\n\n    try:\n        root = os.path.realpath(args.evidence)\n        if not os.path.isdir(root):\n            raise InputError(\"not a directory: %s\" % args.evidence)\n        findings = load_findings(args.findings)\n        scans = load_scans(args.scans)\n        remediation = load_remediation(args.remediation)\n        files, problems = read_evidence(root)\n    except InputError as exc:\n        print(\"ERR %s\" % exc, file=sys.stderr)\n        return 2\n\n    referenced = set()\n    for scan in scans:\n        if scan[\"evidence\"]:\n            referenced.add(scan[\"evidence\"])\n    for record in remediation:\n        if record[\"evidence\"]:\n            referenced.add(record[\"evidence\"])\n    present = set(files)\n    missing = sorted(referenced - present)\n    orphans = sorted(present - referenced)\n\n    print(\"PACKET %d files, %d referenced\" % (len(files), len(referenced & present)))\n    bundle = hashlib.sha256()\n    for name in sorted(files):\n        bundle.update((\"%s  %s\\n\" % (files[name][\"sha256\"], name)).encode(\"utf-8\"))\n    print(\"MANIFEST %d files, %d bytes, bundle %s\" % (\n        len(files), sum(row[\"size\"] for row in files.values()),\n        bundle.hexdigest() if files else \"-\"))\n    for name in sorted(files):\n        print(\"FILE %s %9d %s\" % (files[name][\"sha256\"],\n                                  files[name][\"size\"], name))\n\n    controls = sorted({finding[\"control\"] for finding in findings})\n    for control in controls:\n        names = set()\n        for scan in scans:\n            if (scan[\"control\"] == control and scan[\"status\"] == \"ok\"\n                    and scan[\"evidence\"]):\n                names.add(scan[\"evidence\"])\n        for record in remediation:\n            if record[\"control\"] == control and record[\"evidence\"]:\n                names.add(record[\"evidence\"])\n        if not names:\n            print(\"EVIDENCE %s: none\" % control)\n            continue\n        shown = \", \".join(\n            name + (\"\" if name in present else \" (missing)\")\n            for name in sorted(names))\n        print(\"EVIDENCE %s: %s\" % (control, shown))\n\n    open_high = [finding for finding in findings\n                 if finding[\"status\"] == \"open\"\n                 and RANK[finding[\"severity\"]] >= RANK[args.fail_on]]\n    with_evidence = {record[\"finding\"] for record in remediation\n                     if record[\"evidence\"] in present}\n    open_gap = [finding for finding in open_high\n                if finding[\"id\"] not in with_evidence]\n    no_evidence = []\n    for control in controls:\n        covered = any(scan[\"control\"] == control and scan[\"status\"] == \"ok\"\n                      and scan[\"evidence\"] in present for scan in scans)\n        covered = covered or any(record[\"control\"] == control\n                                 and record[\"evidence\"] in present\n                                 for record in remediation)\n        if not covered:\n            no_evidence.append(control)\n\n    print(\"GAPS %d missing, %d %s open\"\n          % (len(missing), len(open_gap), args.fail_on))\n    for name in missing:\n        print(\"MISSING %s\" % name)\n    if no_evidence:\n        print(\"NO EVIDENCE \" + \", \".join(no_evidence))\n    if orphans:\n        print(\"ORPHAN \" + \", \".join(orphans))\n    for problem in problems:\n        print(\"PROBLEM %s\" % problem)\n\n    if not files:\n        print(\"FAIL packet has no evidence files\")\n        return 1\n    if missing or open_gap or problems:\n        parts = []\n        if missing:\n            parts.append(\"%d missing file%s\"\n                         % (len(missing), \"\" if len(missing) == 1 else \"s\"))\n        if open_gap:\n            parts.append(\"%d finding%s without evidence\"\n                         % (len(open_gap), \"\" if len(open_gap) == 1 else \"s\"))\n        if problems:\n            parts.append(\"%d evidence problem%s\"\n                         % (len(problems), \"\" if len(problems) == 1 else \"s\"))\n        print(\"FAIL \" + \", \".join(parts))\n        return 1\n    print(\"OK packet complete, every %s finding has evidence\" % args.fail_on)\n    return 0\n\n\nif __name__ == \"__main__\":\n    sys.exit(main())\n```\n"},{"id":"p767","n":"767","slug":"the-proof-chain","title":"The proof chain","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","CyberSecurity","GRC","DevSecOps","CISO"],"series_name":"ASM","design":"architecture","script":"","date":"2031-08-21","files":{"devsecops":"notes/p767-the-proof-chain.md"},"md":"# The proof chain\n\nEvery quarter the team presents a clean dashboard, and every quarter the auditor asks the same question: show me the proof, dated and owned.\n\n## Quick reference\n\n- **Measure:** Trend report (TREND) · Coverage report (COV)\n- **Map:** Control map (CTL) · Owner register (OWN)\n- **Attest:** Audit packet (PACK) · Sign-off record (SIGN)\n\n**Request flow:** Coverage report → Owner register → Sign-off record → Trend report → Control map → Audit packet\n\n**Around it:** Findings export (FIND) · Scan history (SCAN) · Evidence store (EVID)\n\n## Explained\n\n### The three tiers\n\nMeasure holds the trend report and the coverage report: is the backlog shrinking, and was every asset scanned by every required tool. Map holds the control map and the owner register: which control each finding belongs to, and who closes it. Attest holds the audit packet and the sign-off: the dated evidence, indexed by control, and a named person who accepts it.\n\n### The arrows\n\nTrend and coverage feed the map, because a gap with no owner is not actionable. The map feeds the packet, because evidence without a control id cannot be filed. The signed packet returns to measure as next quarter's baseline; that loop is what makes it a chain rather than three reports. Below the tiers sit the inputs: the findings export, the scan history and the evidence store.\n\n### How to use it\n\nGive each tier one weekly output and one owner, reviewed on the same day, in the same hour. If a link is missing, stop and fix it: an attestation over an unscanned estate is a signature on a guess.\n\n> **The honest caveat:** the chain is only as strong as the weakest link, and a signed packet cannot repair a blind asset or an unowned gap.\n\n> **The lesson:** proof is a chain, not a slide.\n\n## In one line\n\n> **Proof is a chain, not a slide.** A signed packet over an unscanned estate is a signature on a guess.\n\n**Follow the chain**\n\n```bash\npython3 trend_report.py history.json\n```\n"},{"id":"p768","n":"768","slug":"reporting-commands","title":"Reporting commands","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["Python","VulnerabilityManagement","CommandLine","CyberSecurity","DevOps"],"series_name":"ASM","design":"cheatsheet","script":"","date":"2031-08-24","files":{"devsecops":"notes/p768-reporting-commands.md"},"md":"# Reporting commands\n\nThe board pack is due and the numbers live in four files. The commands are short enough to put in a Monday job.\n\n## Quick reference\n\n### Trend\n\n```bash\npython3 trend_report.py history.json\njq '.[].opened - .closed' history.json\njq -r '.[-1].date' history.json\npython3 trend_report.py h.json --drop 3\n```\n\n### Coverage\n\n```bash\npython3 coverage_report.py inv scans\njq -r '.[].asset' inv.json | sort -u\njq -r '.[].asset' scans.json | sort -u\njq -r '.[].tool' scans.json | sort -u\n```\n\n### Controls\n\n```bash\npython3 control_map.py f.json soc2.json\njq -r '.controls | keys[]' soc2.json\njq -r '.[].category' f.json | sort -u\njq -r '.controls[][]' soc2.json | sort -u\n```\n\n### Evidence\n\n```bash\nsha256sum evidence/* > manifest.txt\ngrep -c '^FILE ' packet.txt\ntar --sort=name -cf packet.tar evidence/\ngpg --verify packet.tar.sig packet.tar\n```\n\n## Explained\n\n### Trend\n\n`trend_report.py` prints net flow and the closure rate with its denominator. The jq lines pull the flows and the latest date, and `--drop` tightens the line.\n\n### Coverage\n\n`coverage_report.py` joins inventory to scans; the jq lines list assets and tools. A gap list is only useful when the inventory is the population.\n\n### Controls\n\n`control_map.py` maps categories to SOC 2 ids; jq lists the categories and the ids. A finding with no id is a finding the auditor cannot file.\n\n### Evidence\n\n`audit_packet.py` prints the manifest and the per-control index. sha256sum and tar seal the files, grep counts the manifest lines, and gpg verifies the signature. The packet is printed, so redirect it.\n\n### The fix\n\nRun the sheet at the start of the review and paste the output into the pack. Each command answers one question with a number.\n\n### Prevention\n\nKeep the sheet beside the report job and update it when a column changes. A sheet nobody edits becomes a museum of last year's paths.\n\n> **The honest caveat:** the file names are placeholders, and the commands assume jq, GNU tar and gpg are installed. Substitute your paths before you trust the output.\n\n> **The lesson:** a report you can rerun is a control; one you rebuild by hand is a screenshot.\n\n## In one line\n\n> **Run the report, not the meeting.** A command you can rerun beats a number you rebuilt by hand.\n\n**Run the sheet**\n\n```bash\npython3 coverage_report.py inv scans\n```\n"},{"id":"p769","n":"769","slug":"four-reports-that-lie","title":"Four reports that lie","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["Metrics","CyberSecurity","RiskManagement","VulnerabilityManagement","CISO"],"series_name":"ASM","design":"explainer","script":"","date":"2031-08-26","files":{"devsecops":"notes/p769-four-reports-that-lie.md"},"md":"# Four reports that lie\n\nNone of these four reports is false. Each is true in a way that hides the thing it claims to show.\n\n## Quick reference\n\n### The vanity metric\n\n- 1,842 findings closed this quarter\n- 2,100 opened, so the backlog grew\n- 48% closure with no denominator\n- Fix: net flow and a denominator\n\n### The unscanned asset\n\n- Coverage on scans the scanner sent\n- The database was never in the count\n- A clean report over part of the estate\n- Fix: join the inventory to the scans\n\n### The unmapped finding\n\n- High finding, no control id\n- It cannot appear in the audit\n- The framework looks fully covered\n- Fix: map the category, then the finding\n\n### The stale evidence\n\n- The export is dated January\n- The control changed in March\n- The packet still passes review\n- Fix: date every artefact you send\n\n> **Note:** Each report is true on its own. Together they hide four different gaps.\n\n## Explained\n\n### The vanity metric\n\n1,842 findings closed this quarter sounds like progress. Opened were 2,100, so the backlog grew and the closure rate of 48% has no denominator worth the name.\n\n### The unscanned asset\n\nCoverage is measured on scans sent, divided by the assets the scanner knew about. The database that was never added to the inventory is not a gap in the report; it is outside the population.\n\n### The unmapped finding\n\nA high finding with no control id cannot appear in the audit. The framework looks fully covered because the finding was never in the count.\n\n### The stale evidence\n\nThe export is from January, the control changed in March, and the packet still passes review. The date is on the file; nobody reads it until the audit, and then it is too late.\n\n### What they share\n\nEach one measures what is easy to count, then presents it as the whole. The fix is the same in all four: name the population, the denominator and the date on the same line as the number.\n\n> **The honest caveat:** a simple metric is not a lie. It becomes one when it is presented as the whole picture.\n\n> **The lesson:** a report is a claim; make it show its working.\n\n## In one line\n\n> **A report can be true and useless.** A number without its population is a claim without its evidence.\n\n**Check the claim**\n\n```bash\npython3 trend_report.py h.json\n```\n"},{"id":"p770","n":"770","slug":"5-vulnerability-programme-mistakes","title":"5 vulnerability programme mistakes","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["VulnerabilityManagement","CyberSecurity","RiskManagement","DevSecOps","CISO"],"series_name":"ASM","design":"dontdo","script":"","date":"2031-08-28","files":{"devsecops":"notes/p770-5-vulnerability-programme-mistakes.md"},"md":"# 5 vulnerability programme mistakes\n\nFive mistakes turn a vulnerability programme into a reporting exercise with a green dashboard. None of them looks wrong on the day it is chosen, and each one can survive for years.\n\n## Quick reference\n\n**1.** ❌ **Count findings, not risk** — A thousand mediums is a queue, not a priority.  \n✅ **Rank by KEV and exposure** — One exploited critical outranks all of them.\n\n**2.** ❌ **Leave the finding unowned** — An ownerless finding belongs to nobody.  \n✅ **One owner and one date** — Ownership and a date make it real.\n\n**3.** ❌ **Accept it and forget it** — Silence turns a temporary risk into a permanent one.  \n✅ **Exception with an expiry** — An expiry puts the decision back on the calendar.\n\n**4.** ❌ **Fix without a rescan** — A closed ticket is a claim, not evidence.  \n✅ **Rescan proves the fix** — The finding must vanish from the export.\n\n**5.** ❌ **Board report with no trend** — A total with no direction cannot support a decision.  \n✅ **Trend, denominator, coverage** — Three numbers that show whether it is working.\n\n## Explained\n\n### The five\n\n1. **Counting findings instead of risk.** A thousand mediums is a queue; one exploited critical is an incident. Rank by KEV, exposure and reachability, then count by asset.\n   \\2. Leaving the finding unowned. An ownerless finding belongs to everyone, which means nobody. One name and one date per finding, in the tracker.\n   \\3. Accepting risk without an expiry. An exception with no end date is a permanent control failure with a signature. Give it an expiry and a named reviewer.\n   \\4. Fixing without a rescan. A closed ticket is a claim; a rescan is the evidence. Close only when the finding disappears from the export.\n   \\5. Sending the board a report with no trend. A total without direction cannot support a decision. Send the delta, the denominator and the coverage behind it.\n\n### What they share\n\nEach one moves the proof one step further from the work. The programme looks busy, the slides look green, and the risk stays where it was. The work moved; the risk did not.\n\n> **The honest caveat:** none of these is fixed by a tool. The tool makes the gap visible; an owner still has to act on it.\n\n> **The lesson:** count less, prove more.\n\n## In one line\n\n> **Count less. Prove more.** Five mistakes, one habit: move the proof closer to the work.\n\n**Prove one fix:** rescan, then hash the evidence\n"},{"id":"p771","n":"771","slug":"quiz-3-proof-questions","title":"Quiz: 3 proof questions","sections":["devsecops"],"topic":"Vulnerability management & attack surface","tags":["Python","VulnerabilityManagement","CyberSecurity","GRC","DevSecOps"],"series_name":"ASM","design":"quiz","script":"","date":"2031-08-31","files":{"devsecops":"notes/p771-quiz-3-proof-questions.md"},"md":"# Quiz: 3 proof questions\n\nThe series ends with three questions, and each one is about proof that survives a review. Answer before the reveal.\n\n## Quick reference\n\n**Q1. Why does a closure rate need a denominator?**\n\n- a) Because closed counts are always wrong\n- b) Because a rate is a ratio of two counts\n- c) Because auditors ask for a percentage\n\n> **Answer:** b) Because a rate is a ratio of two counts. Closed divided by findings in play, open at the start plus opened, gives a rate you can compare. Without it, a team closes more while the backlog grows.\n\n**Q2. What must a coverage report include to be true?**\n\n- a) Inventory, required tools and a time window\n- b) The number of scans the scanner sent\n- c) Every asset the scanner already knows\n\n> **Answer:** a) Inventory, required tools and a time window. Coverage is a join against the inventory, tool by tool, in a window. Scans over known assets measure the scanner, and an asset outside the inventory is invisible.\n\n**Q3. Why does a control need dated evidence?**\n\n- a) Evidence makes the report longer\n- b) The framework requires a screenshot\n- c) A policy is intent; a file is fact\n\n> **Answer:** c) A policy is intent; a file is fact. A dated file and a hash let someone who was not there check the control. Without them, enabled is a belief, and a disabled control can survive a quarter of reviews.\n\n## Explained\n\n### Question one\n\nWhy does a closure rate need a denominator? Because a rate is a ratio, and 48% means nothing until you name the bottom. Closed divided by the findings in play, open at the start plus opened during the period, gives a number you can compare across months. Without it, a team can close more findings every quarter while the backlog grows.\n\n### Question two\n\nWhat must coverage include? The inventory, the required tools and the time window. Scans sent divided by assets the scanner knew about is not coverage; an asset missing from the inventory is invisible. A clean report that omits a critical database is a report about the scanner, not the estate.\n\n### Question three\n\nWhy does a control need evidence? Because a policy states intent and an export states fact. A dated file and a hash make the control checkable by someone who was not there. Without them, \"enabled\" is a belief, and a disabled control can survive a quarter of reviews.\n\n> **The honest caveat:** passing this quiz does not make the programme provable; the schedule, the owners and the evidence do.\n\n> **The lesson:** measure it, map it, prove it.\n\nThat is the series. Fifty posts, five phases and twenty-five Python scripts, from the first certificate to this packet. Thank you for following along.\n\n## In one line\n\n> **Measure it, map it, prove it.** Fifty posts, five phases, twenty-five scripts, one proof chain.\n\n**Keep the proof**\n\n```bash\npython3 audit_packet.py ev -f f -s s -r r\n```\n"},{"id":"sum-install","n":1001,"slug":"install","title":"Chapter 1: Install Red Hat Enterprise Linux","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"After this chapter you know which RHEL version to use for practice, how to get it for free, and how to install it the right way for the exam: with a good hostname, an admin user and the software you need.\n\n## Lessons in this chapter\n\n- Know what RHEL 10 is and where to get it\n- Understand where software comes from\n- Plan your practice machines\n- Install RHEL step by step\n- Check the new system\n- Register the system and get updates\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 1.1\n","page":"summary/rhcsa/install/","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-what-is-rhel","n":1002,"slug":"what-is-rhel","title":"Know what RHEL 10 is and where to get it","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** RHEL is the Linux of Red Hat for companies. You can use it for free with a developer subscription, or practice on a clone that works almost the same.\n\n- **Red Hat Enterprise Linux (RHEL) 10** is the version of the RHCSA exam (EX200). Red Hat supports it for about 10 years.\n- **CentOS Stream 10** is the version just before the next RHEL update. It is very close to RHEL.\n- **AlmaLinux 10** and **Rocky Linux 10** are free rebuilds of RHEL. For the exam topics they work the same.\n- **Fedora** is the fast-moving community version where new ideas are tested first.\n\nYou get RHEL for free with the **Red Hat Developer Subscription for Individuals** (developers.redhat.com). It allows up to 16 systems and gives access to the official repositories.\n\n| Option | Updates from Red Hat | Good for |\n|---|---|---|\n| RHEL + developer subscription | Yes | The real exam system |\n| CentOS Stream 10 | From CentOS | Practice, newest packages |\n| AlmaLinux / Rocky 10 | From their own mirrors | Practice, servers without a subscription |\n\n> **Remember:** Exam = RHEL 10. Practice on RHEL if you can; a clone is fine for most tasks.\n\n> **Exam tip:** The exam does not use a subscription. Software comes from repositories that the exam gives you (often on a local or internal server). Chapter 9 shows how to use them.\n","page":"summary/rhcsa/install/#what-is-rhel","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-repositories","n":1003,"slug":"repositories","title":"Understand where software comes from","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Software on RHEL comes from **repositories** (online or local). Without a repository, `dnf install` cannot find anything.\n\n- A registered RHEL system gets two main repositories from Red Hat: **BaseOS** (the base system) and **AppStream** (applications, languages, databases).\n- Without registration you can still use the installation DVD (ISO) as a local repository. Chapter 9 shows how.\n- Clones (CentOS Stream, AlmaLinux, Rocky) have their repositories ready after the install.\n\n```bash\ndnf repolist\n```\n```text\nrepo id                             repo name\nrhel-10-for-x86_64-appstream-rpms   Red Hat Enterprise Linux 10 for x86_64 - AppStream (RPMs)\nrhel-10-for-x86_64-baseos-rpms      Red Hat Enterprise Linux 10 for x86_64 - BaseOS (RPMs)\n```\n\n> **Remember:** BaseOS = the base system, AppStream = the applications.\n","page":"summary/rhcsa/install/#repositories","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-lab-setup","n":1004,"slug":"lab-setup","title":"Plan your practice machines","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Build a small lab: two virtual machines on one network. Most labs in these summaries use **server1** and **server2**.\n\n| Machine | Use | Minimum |\n|---|---|---|\n| server1 | Main practice system (with a graphical desktop) | 2 CPUs, 4 GB RAM, 20 GB disk |\n| server2 | Second system: SSH, NFS, network tests | 1 CPU, 2 GB RAM, 20 GB disk |\n\n1. Install a hypervisor: KVM/virt-manager on Linux, VirtualBox, VMware or Hyper-V.\n2. Put both machines on the same virtual network (NAT network or host-only), so they can reach each other.\n3. Add an **extra empty disk** of 10 GB to server1. You need it for the storage and LVM chapters (14 and 15).\n4. Download the DVD image (ISO), not the boot-only image: the DVD has all packages, so you can install without internet.\n\n> **Work tip:** Take a **snapshot** of each machine right after the install. When a lab breaks the system, go back in one minute.\n","page":"summary/rhcsa/install/#lab-setup","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-install","n":1005,"slug":"install","title":"Install RHEL step by step","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The installer (Anaconda) shows one summary screen. Every item with a warning sign must be fixed before you can start.\n\n1. Boot the machine from the ISO and choose **Install Red Hat Enterprise Linux 10**.\n2. Choose the language for the installation.\n3. On the **Installation Summary** screen, set each item:\n   - **Keyboard** and **Language support**: add the layouts you type with.\n   - **Time & Date**: choose your time zone and turn on network time.\n   - **Software Selection**: *Server with GUI* for server1, *Minimal Install* or *Server* for server2.\n   - **Installation Destination**: select the main disk (not the extra disk) and keep *Automatic* partitioning. This makes an LVM layout.\n   - **Network & Host Name**: switch the network card **On** and type the hostname, like `server1.example.com`.\n   - **Root Password**: set a password. Allow root SSH login with password only on a practice system.\n   - **User Creation**: create your own user and tick **Make this user administrator** (this puts the user in the group `wheel`, so it can use `sudo`).\n4. Click **Begin Installation**. When it is done, click **Reboot System**.\n5. Log in as your user. On a graphical system, accept the first-start questions.\n\n> **Watch out:** If you do not switch the network card on during the install, it does not start at boot. Fix it later with `nmcli con mod NAME connection.autoconnect yes`.\n\n> **Exam tip:** You do not install RHEL in the exam, but you must know what the installer does: LVM layout, the wheel group, the hostname and the network.\n","page":"summary/rhcsa/install/#install","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-first-checks","n":1006,"slug":"first-checks","title":"Check the new system","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** After the first boot, check the version, the name, the network and the disks. It takes one minute and finds most install mistakes.\n\n```bash\ncat /etc/redhat-release\n```\n```text\nRed Hat Enterprise Linux release 10.0 (Coughlan)\n```\n```bash\nhostnamectl\nuname -r\nip -br a\nlsblk\ndf -h\nid\n```\n\n- `cat /etc/redhat-release` shows the exact RHEL version.\n- `hostnamectl` shows the hostname, the operating system and the kernel.\n- `ip -br a` shows the addresses: is the network up?\n- `lsblk` shows the disks and the LVM volumes; your extra disk has no partitions yet.\n- `id` shows your groups: `wheel` must be in the list for sudo.\n\n> **Check:** Can your user work as administrator?\n\n```bash\nsudo -l\nsudo whoami\n```\n```text\nroot\n```\n","page":"summary/rhcsa/install/#first-checks","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-register","n":1007,"slug":"register","title":"Register the system and get updates","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A registered RHEL system gets updates and the official repositories. Register it once, then update everything.\n\n1. Register with your Red Hat account. The command asks for the user name and password.\n```bash\nsubscription-manager register\n```\n2. Check the status and the repositories.\n```bash\nsubscription-manager status\ndnf repolist\n```\n3. Install all updates and reboot when a new kernel was installed.\n```bash\ndnf upgrade -y\nsystemctl reboot\n```\n\n> **Remember:** register → repolist → upgrade.\n\n> **Work tip:** In a company, systems are often registered with an **activation key** instead of a password, for example in a provisioning script: `subscription-manager register --org=ID --activationkey=KEY`.\n","page":"summary/rhcsa/install/#register","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-main-points","n":1008,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- The RHCSA exam (EX200) uses **RHEL 10**. CentOS Stream 10, AlmaLinux 10 and Rocky Linux 10 are good for practice.\n- A free **Red Hat Developer Subscription** gives you real RHEL with updates for up to 16 systems.\n- Software comes from **repositories**: BaseOS (the core system) and AppStream (applications and tools).\n- For practice, build **two virtual machines** (server1 and server2) with snapshots, so you can break things and go back.\n- In the installer, set the **hostname**, the **network**, the **root password** (or no root login) and an **admin user** in the wheel group.\n- The default disk layout is **XFS** on **LVM**: a small /boot partition, and logical volumes for / and swap.\n- After the install, check the version (`cat /etc/redhat-release`), the network (`ip a`) and the time.\n- Register with `rhc connect` or `subscription-manager register` to get updates from Red Hat.\n","page":"summary/rhcsa/install/#main-points","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-key-terms","n":1009,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Distribution | A Linux kernel plus tools and software, packed and tested by one group (for example RHEL). |\n| RHEL | Red Hat Enterprise Linux: the Linux for companies, with about 10 years of support. |\n| CentOS Stream | The version that shows the next RHEL update before it comes out. |\n| Repository | A server or folder with software packages that `dnf` can install. |\n| BaseOS / AppStream | The two main RHEL repositories: core system / extra applications. |\n| Subscription | The right to get RHEL software and updates from Red Hat. |\n| ISO image | One file with the full installation DVD. |\n| Anaconda | The RHEL installer program. |\n| Kickstart | A text file that answers all installer questions, for an automatic install. |\n| Snapshot | A saved state of a virtual machine that you can go back to. |\n","page":"summary/rhcsa/install/#key-terms","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-install-review","n":1010,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-1"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which RHEL version does the current RHCSA exam use?**\nA: RHEL 10.\n\n\n**Q: Which two repositories hold most of the RHEL software?**\nA: BaseOS (the core operating system) and AppStream (applications, languages and tools).\n\n\n**Q: How can you get real RHEL for free for practice?**\nA: With the Red Hat Developer Subscription for Individuals (up to 16 systems).\n\n\n**Q: Which file system and disk layout does the installer use by default?**\nA: XFS on LVM, with a separate /boot partition.\n\n\n**Q: Why should you take a snapshot of your practice VM?**\nA: So you can go back to a working system after a lab breaks it.\n\n\n**Q: Which command shows the exact RHEL version?**\nA: `cat /etc/redhat-release` (or `cat /etc/os-release`).\n\n\n**Q: What is a Kickstart file?**\nA: A text file with the answers for the installer, so an install runs without questions.\n\n\n**Q: Which group gives a normal user admin rights with sudo?**\nA: The `wheel` group.\n","page":"summary/rhcsa/install/#review","label":"Ch 1","where":"RHCSA 10 · Chapter 1 · Install Red Hat Enterprise Linux"},{"id":"sum-shell","n":1011,"slug":"shell","title":"Chapter 2: Work in the shell","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"The shell (Bash) is where you do almost all the work on a Linux server. This chapter shows how commands are built, how to send output to files and other commands, how to repeat old commands, how to edit files with vim or nano, and how to find help without internet.\n\n## Lessons in this chapter\n\n- Know the three kinds of commands\n- Type commands with options and arguments\n- Send output to a file (redirection)\n- Connect commands with pipes\n- Use the command history\n- Save typing with Tab completion\n- Edit a file with vim\n- Edit a file with nano\n- Use variables\n- Know the files that set up the shell\n- Show a message at login\n- Find help with --help and man\n- Ask the AI assistant (RHEL Lightspeed)\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 2.1\n","page":"summary/rhcsa/shell/","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-command-types","n":1012,"slug":"command-types","title":"Know the three kinds of commands","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A command can be an **alias**, a **built-in** of the shell, or a **program** on the disk. `type` tells you which one it is.\n\n- An **alias** is a short name for a longer command, like `ll` for `ls -l --color=auto`.\n- A **built-in** is part of Bash itself, like `cd`, `echo` or `history`.\n- A **program** (external command) is a file on the disk, found through the folders in `$PATH`.\n\n```bash\ntype ll\n```\n```text\nll is aliased to `ls -l --color=auto'\n```\n```bash\ntype cd\n```\n```text\ncd is a shell builtin\n```\n```bash\ntype passwd\n```\n```text\npasswd is /usr/bin/passwd\n```\n```bash\nwhich passwd\n```\n```text\n/usr/bin/passwd\n```\n```bash\necho $PATH\n```\n```text\n/home/student/.local/bin:/home/student/bin:/usr/local/bin:/usr/bin\n```\n\nWhen you type a name, Bash looks in this order: alias → built-in → the folders of `$PATH` (left to right). The current folder is **not** in the path, so start your own script with `./`:\n\n```bash\n./backup.sh\n```\n\n> **Remember:** `type NAME` answers \"what will run?\". `which NAME` only searches `$PATH`.\n\n> **Watch out:** An alias with the same name as a program wins. To run the real program once, start it with a backslash: `\\ls`.\n","page":"summary/rhcsa/shell/#command-types","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-run-commands","n":1013,"slug":"run-commands","title":"Type commands with options and arguments","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A command line is: the command, then **options** (they change how it works), then **arguments** (what it works on).\n\n```bash\nls -l -a /etc\nls -la /etc\nls --all --format=long /etc\n```\n\n- Short options have one dash and one letter (`-l`). You can join them: `-la`.\n- Long options have two dashes and a word (`--all`). They are easier to read in scripts.\n- Arguments are names of files, folders, users …\n\nJoin commands on one line:\n\n| Sign | Meaning | Example |\n|---|---|---|\n| `;` | Run one after the other | `cd /tmp; ls` |\n| `&&` | Run the next one only if the first one worked | `dnf install -y httpd && systemctl start httpd` |\n| `\\|\\|` | Run the next one only if the first one failed | `ping -c1 server2 \\|\\| echo \"server2 is down\"` |\n\n> **Remember:** `&&` = \"and then if OK\", `||` = \"or else\".\n","page":"summary/rhcsa/shell/#run-commands","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-redirect","n":1014,"slug":"redirect","title":"Send output to a file (redirection)","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Every command has 3 channels: **0 = input**, **1 = output**, **2 = errors**. Redirection sends a channel to a file instead of the screen.\n\n| Sign | What it does |\n|---|---|\n| `>` | Output to a file. The file is emptied first. |\n| `>>` | Output added to the end of a file. |\n| `2>` | Errors to a file. |\n| `2>&1` | Errors go to the same place as the output. |\n| `&>` | Output and errors to one file. |\n| `<` | Read the input from a file. |\n| `/dev/null` | A \"black hole\": everything sent there is gone. |\n\n```bash\nls /etc > etc-list.txt\ndate >> etc-list.txt\nfind / -name \"*.conf\" 2> /dev/null\nfind / -name \"*.conf\" &> find-all.txt\nmail -s \"report\" root < report.txt\n```\n\n> **Remember:** `>` = new, `>>` = add, `2>` = errors.\n\n> **Watch out:** `>` on an important file empties it at once. Use `>>` when you are not sure.\n","page":"summary/rhcsa/shell/#redirect","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-pipes","n":1015,"slug":"pipes","title":"Connect commands with pipes","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A pipe `|` sends the output of one command as input to the next one. Small tools together do big jobs.\n\n```bash\nps aux | grep sshd\nls -l /etc | less\ncat /var/log/messages | grep -i error | wc -l\ndnf list installed | tee installed.txt | wc -l\n```\n\n`tee` writes the data to a file **and** passes it on, so you can save a step in the middle of a pipe.\n\n> **Work tip:** Build a pipe step by step. Run the first command, look at the output, then add the next `|`.\n","page":"summary/rhcsa/shell/#pipes","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-history","n":1016,"slug":"history","title":"Use the command history","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Bash remembers your commands in `~/.bash_history`. Repeat or search them instead of typing again.\n\n```bash\nhistory\nhistory 10\n!42\n!!\n!dnf\nsudo !!\n```\n\n| Keys / command | What it does |\n|---|---|\n| ↑ and ↓ | Go through the last commands |\n| `Ctrl+R` then text | Search back in the history; press Ctrl+R again for the next match |\n| `!42` | Run command number 42 again |\n| `!!` | Run the last command again (`sudo !!` = last command with sudo) |\n| `!dnf` | Run the last command that started with `dnf` |\n| `history -c` | Clear the history of this shell |\n| `history -w` | Write the current history to the file now |\n\nThe variable `HISTSIZE` sets how many commands Bash keeps in memory, `HISTFILESIZE` how many are kept in the file.\n\n> **Watch out:** A password typed on the command line ends up in the history file. Remove it with `history -d NUMBER`, then `history -w`.\n","page":"summary/rhcsa/shell/#history","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-completion","n":1017,"slug":"completion","title":"Save typing with Tab completion","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Press **Tab** to complete a command, a file name or an option. Press **Tab twice** to see all possible choices.\n\n```bash\nsystemctl sta<Tab>\nsystemctl status ssh<Tab>\nnmcli con mod enp1s0 ipv4.<Tab><Tab>\n```\n\nCompletion for options and sub-commands comes from the package `bash-completion`. It is installed on most systems.\n\n> **Exam tip:** Tab completion is the fastest way to type long names without typing mistakes. Use it all the time in the exam.\n","page":"summary/rhcsa/shell/#completion","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-vim","n":1018,"slug":"vim","title":"Edit a file with vim","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** vim has two main modes. In **command mode** keys are commands. In **insert mode** keys are text. `Esc` always takes you back to command mode.\n\n```bash\nvim /etc/hosts\n```\n\n| Key | What it does |\n|---|---|\n| `i` / `a` / `o` | Insert before the cursor / after the cursor / on a new line |\n| `Esc` | Back to command mode |\n| `:wq` or `ZZ` | Save and quit |\n| `:q!` | Quit without saving |\n| `dd` / `5dd` | Delete (cut) one line / 5 lines |\n| `yy` / `p` | Copy a line / paste it under the cursor |\n| `u` / `Ctrl+R` | Undo / redo |\n| `/text` then `n` | Search for text, then go to the next match |\n| `gg` / `G` | Go to the first line / to the last line |\n| `:%s/old/new/g` | Replace every \"old\" with \"new\" in the whole file |\n| `:set number` | Show line numbers |\n\n> **Remember:** Lost? Press `Esc` twice, then type `:q!` to leave without changes.\n\n> **Work tip:** Run `vimtutor` once. It is a 30-minute lesson inside the terminal.\n","page":"summary/rhcsa/shell/#vim","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-nano","n":1019,"slug":"nano","title":"Edit a file with nano","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** nano shows its keys at the bottom of the screen. `^` means the Ctrl key.\n\n```bash\nnano notes.txt\n```\n\n| Keys | What it does |\n|---|---|\n| `Ctrl+O` then Enter | Save |\n| `Ctrl+X` | Quit (it asks to save) |\n| `Ctrl+W` | Search |\n| `Ctrl+K` / `Ctrl+U` | Cut a line / paste it |\n| `Alt+U` | Undo |\n\n> **Exam tip:** nano is fine in the exam. Use the editor you know best; speed matters.\n","page":"summary/rhcsa/shell/#nano","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-variables","n":1020,"slug":"variables","title":"Use variables","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A variable keeps a value under a name. **Environment variables** are passed on to programs you start from the shell.\n\n```bash\nMYDIR=/var/www/html\necho $MYDIR\nexport MYDIR\nenv | grep MYDIR\nunset MYDIR\n```\n\n- `NAME=value` makes a shell variable (no spaces around `=`).\n- `export NAME` makes it an environment variable: programs started from this shell see it.\n- `env` (or `printenv`) shows the environment variables; `set` shows all variables and functions.\n\nImportant variables: `PATH` (where to find programs), `HOME` (your home folder), `USER`, `LANG` (language), `PS1` (the prompt), `EDITOR` (the editor that other tools open).\n\n> **Remember:** No spaces around `=`. `export` = give it to child programs.\n","page":"summary/rhcsa/shell/#variables","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-config-files","n":1021,"slug":"config-files","title":"Know the files that set up the shell","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** When you log in, Bash reads a few files that set variables and aliases. Some are for all users, some only for you.\n\n| File | For whom | When |\n|---|---|---|\n| `/etc/profile` and `/etc/profile.d/*.sh` | All users | At login |\n| `/etc/bashrc` | All users | Every new shell |\n| `~/.bash_profile` | One user | At login |\n| `~/.bashrc` | One user | Every new shell |\n\n1. Put your own settings in `~/.bashrc`, for example an alias.\n```bash\necho \"alias ll='ls -l --color=auto'\" >> ~/.bashrc\n```\n2. Load the file in the current shell without logging out.\n```bash\nsource ~/.bashrc\n```\n\nFor all users, do not change `/etc/profile`: add a small file in `/etc/profile.d/` instead. Updates of the system do not overwrite it.\n\n> **Remember:** login = profile files, every shell = bashrc files. `source` reads a file now.\n","page":"summary/rhcsa/shell/#config-files","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-motd-issue","n":1022,"slug":"motd-issue","title":"Show a message at login","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `/etc/motd` is shown **after** a user logs in. `/etc/issue` is shown **before** login on a text console.\n\n```bash\necho \"Maintenance on Saturday 22:00-23:00. Save your work.\" > /etc/motd\necho \"Authorized users only. All activity is logged.\" > /etc/issue\n```\n\n> **Work tip:** Many security policies want a warning text before login. For SSH, put it in a file and set `Banner /etc/issue.net` in `/etc/ssh/sshd_config`.\n","page":"summary/rhcsa/shell/#motd-issue","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-help","n":1023,"slug":"help","title":"Find help with --help and man","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Help is on the system: `--help` for a quick list of options, `man` for the full manual, `man -k` to find the right command.\n\n1. Quick help:\n```bash\nuseradd --help\n```\n2. The full manual. Search with `/word`, `n` for the next match, `q` to quit.\n```bash\nman useradd\nman 5 passwd\n```\n3. Do not know the command? Search the short descriptions of all manuals:\n```bash\nman -k partition\napropos partition | grep 8\n```\n4. If `man -k` says \"nothing appropriate\", build the search database:\n```bash\nmandb\n```\n5. More documentation, often with examples:\n```bash\nls /usr/share/doc/\ninfo coreutils\n```\n\n| Section | Contains |\n|---|---|\n| 1 | Commands for all users |\n| 5 | Configuration files (like `passwd`, `fstab`) |\n| 8 | Commands for root (system administration) |\n\n> **Remember:** 1 = user commands, 5 = files, 8 = admin commands.\n\n> **Exam tip:** There is no internet in the exam. Practice finding answers with `man -k`, `man`, and the EXAMPLES section at the end of many man pages.\n","page":"summary/rhcsa/shell/#help","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-lightspeed","n":1024,"slug":"lightspeed","title":"Ask the AI assistant (RHEL Lightspeed)","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** RHEL 10 has a command-line assistant: the `c` command sends a question to Red Hat's AI service and shows an answer.\n\n```bash\ndnf install command-line-assistant\nc \"how do I make a user an administrator\"\n```\n\nIt needs a registered system and an internet connection. Do not type secrets or personal data in a question.\n\n> **Exam tip:** The exam has no internet, so the assistant does not work there. Use `man` and `--help`.\n","page":"summary/rhcsa/shell/#lightspeed","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-main-points","n":1025,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- A command has three parts: the **command**, its **options** (`-l`, `--all`) and its **arguments** (files, users …).\n- The shell finds commands as **aliases**, **built-ins** and **programs in $PATH**; `type NAME` shows which one runs.\n- **Redirect** output with `>` (overwrite), `>>` (add), errors with `2>`, both with `&>`; read input with `<`.\n- A **pipe** `|` sends the output of one command into the next command.\n- `history`, `!number`, `!!` and **Ctrl+R** repeat old commands; **Tab** completes names.\n- **vim** has modes: Esc for command mode, `i` to type, `:wq` to save and quit, `:q!` to quit without saving.\n- **Variables** hold values; `export` makes them visible to child programs. Login settings live in `/etc/profile`, `~/.bash_profile` and `~/.bashrc`.\n- Help comes from `man`, `man -k` (`apropos`), `--help`, `info` and `/usr/share/doc`.\n","page":"summary/rhcsa/shell/#main-points","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-key-terms","n":1026,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Shell | The program that reads your commands and starts them (Bash on RHEL). |\n| Internal command | A command built into the shell, like `cd` or `echo`. |\n| External command | A program file on disk, found through `$PATH`. |\n| $PATH | The list of folders where the shell looks for programs. |\n| STDIN / STDOUT / STDERR | Input (0), normal output (1) and error output (2) of a program. |\n| Redirection | Sending input or output to or from a file. |\n| Pipe | Connecting the output of one command to the input of the next. |\n| Environment variable | A name with a value that programs can read, like `LANG` or `HOME`. |\n| Alias | A short name for a longer command. |\n| man page | The manual of a command, in sections (1 = user commands, 5 = file formats, 8 = admin commands). |\n","page":"summary/rhcsa/shell/#key-terms","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-shell-review","n":1027,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-2"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you send only the error messages of a command to a file?**\nA: With `2>`, for example `find / -name x 2> errors.txt`.\n\n\n**Q: What is the difference between `>` and `>>`?**\nA: `>` replaces the file; `>>` adds to the end of the file.\n\n\n**Q: How do you find which man page talks about passwords if you do not know the command?**\nA: `man -k password` (the same as `apropos password`). Run `mandb` first if the list is empty.\n\n\n**Q: How do you make a variable available to programs started from the shell?**\nA: With `export NAME=value`.\n\n\n**Q: Which file do you change to give one user an alias at every login?**\nA: `~/.bashrc` of that user.\n\n\n**Q: In vim, how do you save the file and quit?**\nA: Press Esc, then type `:wq` and Enter.\n\n\n**Q: How do you search the command history for an old command?**\nA: Press Ctrl+R and type part of the command.\n\n\n**Q: How do you see if `ls` is an alias, a built-in or a program?**\nA: `type ls`.\n","page":"summary/rhcsa/shell/#review","label":"Ch 2","where":"RHCSA 10 · Chapter 2 · Work in the shell"},{"id":"sum-files","n":1028,"slug":"files","title":"Chapter 3: Manage files and folders","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"This chapter is about the daily work with files: know where things are in Linux, list, copy, move and delete files, make links, and pack files into compressed archives. You need all of it in the exam and every day at work.\n\n## Lessons in this chapter\n\n- Know the main folders of Linux\n- See what is mounted where\n- Use wildcards to work on many files\n- Make, enter and remove folders\n- Use absolute and relative paths\n- List files with ls\n- Copy files and folders\n- Move and rename files\n- Delete files and folders\n- Make hard links and soft links\n- Make and read tar archives\n- Compress single files\n- Find files with find\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 3.1\n","page":"summary/rhcsa/files/","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-fhs","n":1029,"slug":"fhs","title":"Know the main folders of Linux","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Linux has one tree that starts at `/`. Every folder has a job; the **File System Hierarchy Standard (FHS)** says which.\n\n| Folder | What is in it |\n|---|---|\n| `/` | The root of the tree. Everything starts here. |\n| `/boot` | The kernel and the files to start the system. |\n| `/dev` | Device files: disks (`/dev/sda`), terminals, `/dev/null`. |\n| `/etc` | Configuration files of the system and services. |\n| `/home` | The home folders of the users. |\n| `/root` | The home folder of root. |\n| `/usr` | Programs (`/usr/bin`), libraries, documentation. Mostly read-only. |\n| `/var` | Data that changes: logs (`/var/log`), web sites, mail, caches. |\n| `/tmp` | Temporary files for everybody. Cleaned regularly. |\n| `/run` | Runtime data (process IDs, sockets). Empty after a reboot. |\n| `/opt` | Extra software from other vendors. |\n| `/srv` | Data that this server offers (FTP, web). |\n| `/mnt`, `/media` | Places to mount file systems by hand / removable media. |\n| `/proc`, `/sys` | Virtual folders: live information from the kernel. |\n\n> **Remember:** settings in **/etc**, logs in **/var/log**, programs in **/usr/bin**, users in **/home**.\n\n> **Work tip:** When a disk is full, look first in `/var` (logs, caches, container images) and `/tmp`.\n","page":"summary/rhcsa/files/#fhs","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-mounts","n":1030,"slug":"mounts","title":"See what is mounted where","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A disk or partition becomes part of the tree when it is **mounted** on a folder. One tree can be made of many file systems.\n\n```bash\ndf -hT\n```\n```text\nFilesystem            Type   Size  Used Avail Use% Mounted on\n/dev/mapper/rhel-root xfs     17G  4.1G   13G  25% /\n/dev/vda1             xfs    960M  310M  650M  33% /boot\ntmpfs                 tmpfs  1.9G     0  1.9G   0% /dev/shm\n```\n```bash\nfindmnt\nlsblk\nmount | grep xfs\n```\n\n- `df -hT` shows each mounted file system, its type and how full it is.\n- `findmnt` shows the mounts as a tree.\n- `lsblk` shows the disks and partitions and where they are mounted.\n\nSeparate file systems for `/boot`, `/var` or `/home` protect the system: a full `/var/log` cannot fill `/`.\n\n> **Remember:** `df -h` = how full, `findmnt` = the mount tree, `lsblk` = the disks.\n","page":"summary/rhcsa/files/#mounts","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-wildcards","n":1031,"slug":"wildcards","title":"Use wildcards to work on many files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The shell expands wildcards to matching file names **before** the command runs.\n\n| Wildcard | Matches | Example |\n|---|---|---|\n| `*` | Any text (also nothing) | `ls *.conf` |\n| `?` | Exactly one character | `ls file?.txt` |\n| `[abc]` | One of these characters | `ls [abc]*` |\n| `[a-z]` | One character in this range | `ls host[0-9]` |\n| `[!abc]` | One character not in the list | `ls [!a]*` |\n| `{a,b}` | Each word in the list (brace expansion) | `mkdir {dev,test,prod}` |\n\n```bash\nls /etc/*.conf\nls -d /etc/[a-c]*\ncp /var/log/messages{,.bak}\n```\n\nThe last line copies `messages` to `messages.bak`: `{,.bak}` makes two words.\n\n> **Watch out:** Test a wildcard with `ls` or `echo` before you use it with `rm`.\n","page":"summary/rhcsa/files/#wildcards","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-folders","n":1032,"slug":"folders","title":"Make, enter and remove folders","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `mkdir` makes a folder, `cd` goes into it, `pwd` shows where you are, `rmdir` removes an empty folder.\n\n1. Make a folder, also with the parent folders that do not exist yet.\n```bash\nmkdir /tmp/project\nmkdir -p /tmp/project/app/logs\n```\n2. Move around.\n```bash\ncd /tmp/project/app\npwd\n```\n```text\n/tmp/project/app\n```\n```bash\ncd ..\ncd -\ncd\n```\n3. Remove folders: `rmdir` only when empty, `rm -r` with all content.\n```bash\nrmdir /tmp/project/app/logs\nrm -r /tmp/project\n```\n\n`cd ..` = one folder up, `cd -` = back to the last folder, `cd` alone = your home folder.\n","page":"summary/rhcsa/files/#folders","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-paths","n":1033,"slug":"paths","title":"Use absolute and relative paths","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An **absolute** path starts at `/` and works from everywhere. A **relative** path starts in the current folder.\n\n| Path | Meaning |\n|---|---|\n| `/etc/ssh/sshd_config` | Absolute: the same file from any folder |\n| `ssh/sshd_config` | Relative: only works when you are in `/etc` |\n| `.` | The current folder |\n| `..` | The folder above |\n| `~` | Your home folder (`~student` = the home of student) |\n\n```bash\ncd /etc\nls ssh/sshd_config\ncp ../etc/hosts ~/hosts.copy\n```\n\n> **Exam tip:** Use absolute paths in scripts, cron jobs and config files. They always work.\n","page":"summary/rhcsa/files/#paths","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-list","n":1034,"slug":"list","title":"List files with ls","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `ls` shows the content of a folder. With options it shows owners, sizes, times and hidden files.\n\n```bash\nls -l\nls -la\nls -lh /var/log\nls -lrt /var/log\nls -ld /etc\nls -R /etc/ssh\n```\n\n| Option | What it shows |\n|---|---|\n| `-l` | Long list: type, permissions, links, owner, group, size, date |\n| `-a` | Also hidden files (names that start with `.`) |\n| `-h` | Sizes like 4.0K, 12M |\n| `-t` / `-r` | Sort by time / reverse the order (`-lrt` = newest at the bottom) |\n| `-d` | The folder itself, not its content |\n| `-R` | Also all sub-folders |\n| `-i` | The inode number |\n| `-Z` | The SELinux context |\n\n> **Remember:** `ls -lrt` = \"what changed last?\" (look at the bottom).\n","page":"summary/rhcsa/files/#list","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-copy","n":1035,"slug":"copy","title":"Copy files and folders","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `cp SOURCE TARGET` copies. Folders need `-r`. Use `-a` to keep owners, permissions and times.\n\n```bash\ncp /etc/hosts /tmp/\ncp /etc/hosts /tmp/hosts.bak\ncp -r /etc/ssh /tmp/ssh-backup\ncp -a /var/www/html /backup/html\ncp -a /etc/skel/. ~/test/\n```\n\n- When the target is a folder, the file keeps its name inside it.\n- `-a` (archive) = recursive + keep permissions, owner, times, links and SELinux context. Best for backups.\n- `/etc/skel/.` copies the **content** of the folder, hidden files included.\n\n> **Watch out:** `cp` overwrites the target without asking (unless you use `-i`; root has an alias `cp -i` on RHEL).\n","page":"summary/rhcsa/files/#copy","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-move","n":1036,"slug":"move","title":"Move and rename files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `mv` moves a file to another place, or gives it a new name. It is the same action.\n\n```bash\nmv report.txt /tmp/\nmv report.txt report-2026.txt\nmv /tmp/old-logs /srv/archive/\n```\n\nMoving inside the same file system is instant (only the name changes). Between file systems, `mv` copies and then deletes.\n","page":"summary/rhcsa/files/#move","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-delete","n":1037,"slug":"delete","title":"Delete files and folders","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `rm` deletes files; with `-r` it deletes folders and all their content. There is no trash can.\n\n```bash\nrm notes.txt\nrm -i *.log\nrm -r /tmp/build\nrm -rf /tmp/build\n```\n\n| Option | Meaning |\n|---|---|\n| `-r` | Recursive: folders and everything inside |\n| `-f` | Force: no questions, no error for missing files |\n| `-i` | Ask before each file |\n\n> **Watch out:** `rm -rf` with a wrong path or a space in the wrong place (`rm -rf / tmp/x`) can destroy the system. Read the line twice before you press Enter.\n","page":"summary/rhcsa/files/#delete","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-links","n":1038,"slug":"links","title":"Make hard links and soft links","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **hard link** is a second name for the same data (same inode). A **soft link** (symbolic link) is a small file that points to a path.\n\nEvery file has an **inode** with the data and the permissions. A file name is just a pointer to an inode. `ls -i` shows the inode number, and the second column of `ls -l` shows how many names point to it.\n\n1. Make a hard link and a soft link.\n```bash\nln -s /etc/hosts ~/hosts-soft\necho test > data.txt\nln data.txt data-hard.txt\n```\n2. Look at them.\n```bash\nls -li data.txt data-hard.txt ~/hosts-soft\n```\n3. Remove the original and see what happens.\n```bash\nrm data.txt\ncat data-hard.txt\n```\n```text\ntest\n```\n\n| | Hard link | Soft link |\n|---|---|---|\n| Points to | The inode (the data) | A path (a name) |\n| Other file system | Not possible | Possible |\n| Link to a folder | Not possible | Possible |\n| Original deleted | Data still there | Link is broken |\n\nA hard link only works inside one file system. Also, a normal user may not make a hard link to a file of another user (a safety rule of the kernel).\n\n> **Remember:** soft link = shortcut to a name. Hard link = another name for the same data.\n\n> **Exam tip:** `ln -s TARGET LINKNAME`: the target comes first, like `cp`.\n","page":"summary/rhcsa/files/#links","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-tar","n":1039,"slug":"tar","title":"Make and read tar archives","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `tar` packs many files and folders into one archive file, and unpacks it again. With `-z`, `-j` or `-J` it also compresses.\n\n| Option | Meaning |\n|---|---|\n| `-c` | Create an archive |\n| `-x` | Extract an archive |\n| `-t` | List the content |\n| `-v` | Show the file names while working |\n| `-f FILE` | The archive file (always the last letter before the name) |\n| `-z` / `-j` / `-J` | Compress with gzip / bzip2 / xz |\n| `-C DIR` | Extract into this folder |\n\n1. Create a compressed archive of a folder.\n```bash\ntar -czvf /root/etc-backup.tar.gz /etc\n```\n2. List what is inside without extracting.\n```bash\ntar -tvf /root/etc-backup.tar.gz\n```\n3. Extract it into another folder.\n```bash\nmkdir /tmp/restore\ntar -xzvf /root/etc-backup.tar.gz -C /tmp/restore\n```\n4. Extract only one file.\n```bash\ntar -xzvf /root/etc-backup.tar.gz -C /tmp/restore etc/hosts\n```\n\ntar removes the leading `/` from the names, so extracting never overwrites the real `/etc` by accident (unless you extract in `/`).\n\n> **Remember:** **c**reate, e**x**tract, lis**t** + **v**erbose + **f**ile. Compression: **z** = gz, **j** = bz2, **J** = xz.\n\n> **Exam tip:** The exam objective names tar, gzip and bzip2. Know `tar -czf`, `tar -cjf` and `tar -xf` (extract detects the compression by itself).\n","page":"summary/rhcsa/files/#tar","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-compress","n":1040,"slug":"compress","title":"Compress single files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `gzip`, `bzip2` and `xz` compress one file and replace it with a smaller file. The `un` versions bring it back.\n\n| Tool | Compress | Decompress | Ending |\n|---|---|---|---|\n| gzip | `gzip file` | `gunzip file.gz` | `.gz` (fast) |\n| bzip2 | `bzip2 file` | `bunzip2 file.bz2` | `.bz2` (smaller) |\n| xz | `xz file` | `unxz file.xz` | `.xz` (smallest, slow) |\n| zip | `zip arch.zip files` | `unzip arch.zip` | `.zip` (also for Windows) |\n\n```bash\ngzip big.log\nls big.log*\n```\n```text\nbig.log.gz\n```\n```bash\ngunzip big.log.gz\ngzip -k big.log\nzcat /var/log/messages-*.gz | grep -i error\n```\n\n`-k` keeps the original file. `zcat`, `bzcat` and `xzcat` read a compressed file without unpacking it.\n\n> **Work tip:** Old logs in `/var/log` are compressed by logrotate. Search them with `zcat` or `zgrep` instead of unpacking.\n","page":"summary/rhcsa/files/#compress","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-find","n":1041,"slug":"find","title":"Find files with find","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `find` walks through a folder tree and lists every file that matches your tests (name, owner, size, time …). It can also run a command on each file.\n\n```bash\nfind /etc -name \"*.conf\"\nfind / -user student\nfind / -size +100M\nfind /var/log -mtime -1\nfind /home -type d -name \".ssh\"\n```\n\nCopy all files of a user to another folder:\n\n```bash\nmkdir /root/student-files\nfind / -user student -type f -exec cp --parents {} /root/student-files/ \\; 2>/dev/null\n```\n\n| Test | Meaning |\n|---|---|\n| `-name \"x*\"` | Name (with wildcards; quote them) |\n| `-iname` | Name, ignore upper/lower case |\n| `-type f` / `-type d` / `-type l` | File / folder / link |\n| `-user NAME` / `-group NAME` | Owner / group |\n| `-size +10M` | Bigger than 10 MiB (`-` = smaller) |\n| `-mtime -7` | Changed in the last 7 days |\n| `-perm -4000` | Has the SUID bit |\n| `-exec CMD {} \\;` | Run CMD on each file (`{}` = the file) |\n\n> **Exam tip:** Tasks like \"find all files of user X and copy them to /root/found\" are common. Use `-type f`, `-user`, and `-exec cp`.\n\n> **Work tip:** `find / -perm -4000 -type f 2>/dev/null` lists SUID programs: a classic check in a security audit.\n","page":"summary/rhcsa/files/#find","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-main-points","n":1042,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- The **FHS** places files by purpose: /etc (settings), /var (changing data, logs), /home (users), /usr (programs), /tmp (temporary files).\n- An **absolute path** starts at `/`; a **relative path** starts at the current folder.\n- Wildcards: `*` (any text), `?` (one character), `[abc]` (one of these characters).\n- `cp -a` keeps owner, permissions and time stamps; `mv` moves or renames; `rm -r` deletes folders.\n- A **hard link** is a second name for the same inode (same file system only); a **symbolic link** points to a path and can cross file systems.\n- `tar -czf` makes a gzip archive, `tar -xf` unpacks, `tar -tf` lists; `-C` picks the target folder.\n- `find` searches by name, size, owner, time and type, and can run a command on every file with `-exec`.\n","page":"summary/rhcsa/files/#main-points","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-key-terms","n":1043,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| FHS | Filesystem Hierarchy Standard: the rules for which folder holds what. |\n| Mount | Connecting a file system (disk, partition, network share) to a folder. |\n| Absolute path | A path from the root folder, like `/etc/hosts`. |\n| Relative path | A path from the current folder, like `../docs`. |\n| Wildcard (globbing) | Special characters (`*`, `?`, `[ ]`) the shell turns into matching file names. |\n| Inode | The record that holds the data and properties of a file; a name points to it. |\n| Hard link | One more name for the same inode. |\n| Symbolic link | A small file that points to another path. |\n| Archive | One file that holds many files and folders (tar). |\n| Compression | Making files smaller with gzip, bzip2 or xz. |\n","page":"summary/rhcsa/files/#key-terms","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-files-review","n":1044,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-3"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which folder holds most configuration files?**\nA: `/etc`.\n\n\n**Q: How do you copy a folder with all its files and keep the owners and permissions?**\nA: `cp -a SOURCE TARGET`.\n\n\n**Q: What happens to a symbolic link when you delete the original file?**\nA: It breaks: it points to a path that does not exist any more.\n\n\n**Q: Why can a hard link not point to a file on another disk?**\nA: A hard link is a name for an inode, and inode numbers only mean something inside one file system.\n\n\n**Q: How do you make a compressed archive of /etc in /root?**\nA: `tar -czf /root/etc.tar.gz /etc` (or `-cJf … .tar.xz` for xz).\n\n\n**Q: How do you unpack an archive into /tmp/restore?**\nA: `tar -xf archive.tar.gz -C /tmp/restore`.\n\n\n**Q: How do you find all files bigger than 100 MiB under /var?**\nA: `find /var -type f -size +100M`.\n\n\n**Q: How do you show the files in a folder with hidden files and details?**\nA: `ls -la`.\n","page":"summary/rhcsa/files/#review","label":"Ch 3","where":"RHCSA 10 · Chapter 3 · Manage files and folders"},{"id":"sum-text","n":1045,"slug":"text","title":"Chapter 4: Read, search and change text files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Linux keeps almost everything in text files: settings, logs, user lists. This chapter shows how to read big files, pick lines and columns, sort and count, search with grep and regular expressions, and change text with sed and awk.\n\n## Lessons in this chapter\n\n- Read long files with less\n- Show whole files with cat\n- Show the start or the end of a file\n- Take columns with cut\n- Sort lines and remove doubles\n- Count lines, words and characters\n- Understand regular expressions\n- Search text with grep\n- Print and calculate columns with awk\n- Change text with sed\n- Create text files quickly\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 4.1\n","page":"summary/rhcsa/text/","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-less","n":1046,"slug":"less","title":"Read long files with less","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `less` shows a file one screen at a time. You can scroll up and down and search, and it never changes the file.\n\n```bash\nless /var/log/messages\nless -N /etc/ssh/sshd_config\njournalctl -u sshd | less\n```\n\n| Key | What it does |\n|---|---|\n| `Space` / `b` | One page down / one page up |\n| `↓` `↑` or `j` `k` | One line down / up |\n| `g` / `G` | First line / last line |\n| `/text` | Search forward; `n` = next match, `N` = previous |\n| `?text` | Search backward |\n| `F` | Follow the file like `tail -f` (Ctrl+C to stop) |\n| `q` | Quit |\n\n> **Remember:** `less` = read, never write. `/` searches, `q` quits.\n\n> **Exam tip:** `man` uses less to show pages, so the same keys work there.\n","page":"summary/rhcsa/text/#less","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-cat","n":1047,"slug":"cat","title":"Show whole files with cat","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `cat` prints the whole file at once. Good for short files; for long ones use `less`.\n\n```bash\ncat /etc/hostname\ncat -n /etc/fstab\ncat -A script.sh\ntac /var/log/dnf.log | head\n```\n\n- `-n` numbers the lines.\n- `-A` shows hidden characters: tabs as `^I`, line ends as `$`, Windows ends as `^M$`.\n- `tac` is `cat` backwards: the last line first.\n\n> **Work tip:** `cat -A` finds the invisible characters that break YAML files and shell scripts.\n","page":"summary/rhcsa/text/#cat","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-head-tail","n":1048,"slug":"head-tail","title":"Show the start or the end of a file","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `head` shows the first lines, `tail` the last lines. `tail -f` keeps showing new lines while a log grows.\n\n```bash\nhead /etc/passwd\nhead -n 3 /etc/passwd\ntail -n 20 /var/log/secure\ntail -f /var/log/messages\nhead -n 10 /etc/passwd | tail -n 1\n```\n\nThe last line prints only line 10: the first 10 lines, then the last of them.\n\n> **Remember:** head = top, tail = bottom, `tail -f` = watch live.\n","page":"summary/rhcsa/text/#head-tail","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-cut","n":1049,"slug":"cut","title":"Take columns with cut","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `cut` takes fields (columns) from each line. You give the separator with `-d` and the field numbers with `-f`.\n\n```bash\ncut -d: -f1 /etc/passwd\ncut -d: -f1,7 /etc/passwd\ncut -c1-8 /etc/passwd\n```\n\nIn `/etc/passwd` the fields are separated by `:`. Field 1 is the user name, field 7 the shell.\n\n> **Watch out:** `cut` uses exactly one separator character. For columns separated by several spaces, use `awk`.\n","page":"summary/rhcsa/text/#cut","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-sort","n":1050,"slug":"sort","title":"Sort lines and remove doubles","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `sort` puts lines in order; `uniq` removes repeated lines that are next to each other. Together they make quick reports.\n\n```bash\nsort /etc/passwd\nsort -t: -k3 -n /etc/passwd\ndu -sh /var/* 2>/dev/null | sort -h\ncut -d: -f7 /etc/passwd | sort | uniq -c | sort -rn\n```\n\n| Option | Meaning |\n|---|---|\n| `-n` | Sort as numbers (10 after 9) |\n| `-h` | Sort human sizes (2K, 1M, 3G) |\n| `-r` | Reverse order |\n| `-k3` | Sort on field 3 |\n| `-t:` | Fields are separated by `:` |\n| `-u` | Only unique lines |\n\nThe last example counts how many users use each shell: sort first, then `uniq -c` counts, then sort the counts.\n\n> **Remember:** `sort | uniq -c | sort -rn` = \"top list\" of anything.\n","page":"summary/rhcsa/text/#sort","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-wc","n":1051,"slug":"wc","title":"Count lines, words and characters","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `wc` counts. `wc -l` (lines) is the one you use most.\n\n```bash\nwc /etc/passwd\n```\n```text\n  45   95 2512 /etc/passwd\n```\n```bash\nwc -l /etc/passwd\ngrep -c bash /etc/passwd\nls /etc | wc -l\n```\n\nThe numbers are lines, words, bytes.\n","page":"summary/rhcsa/text/#wc","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-regex","n":1052,"slug":"regex","title":"Understand regular expressions","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A regular expression (regex) is a pattern that describes text. `grep`, `sed`, `awk` and vim all use them.\n\n| Pattern | Matches |\n|---|---|\n| `^text` | text at the start of a line |\n| `text$` | text at the end of a line |\n| `.` | any one character |\n| `*` | the character before, 0 or more times |\n| `+` | the character before, 1 or more times (extended) |\n| `?` | the character before, 0 or 1 time (extended) |\n| `{2,4}` | the character before, 2 to 4 times (extended) |\n| `[abc]` / `[0-9]` | one of these characters / one digit |\n| `[^abc]` | one character that is not a, b or c |\n| `\\.` | a real dot (the backslash escapes it) |\n| `\\b` | a word border |\n| `(a\\|b)` | a or b (extended) |\n\n```bash\ngrep '^root' /etc/passwd\ngrep 'nologin$' /etc/passwd\ngrep -E '^[a-z]+[0-9]{2}$' users.txt\ngrep -v '^#' /etc/ssh/sshd_config | grep -v '^$'\n```\n\nBasic regex (BRE) is the default; **extended** regex (ERE) with `grep -E` lets you use `+ ? {} | ()` without backslashes.\n\n> **Watch out:** Put a regex in single quotes, so the shell does not change `*`, `$` or `\\` before grep sees them.\n\n> **Remember:** `^` start, `$` end, `.` any, `*` repeat, `[]` choice, `\\` escape.\n","page":"summary/rhcsa/text/#regex","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-grep","n":1053,"slug":"grep","title":"Search text with grep","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `grep` prints the lines that match a pattern. It is the most used search tool on Linux.\n\n```bash\ngrep student /etc/passwd\ngrep -i error /var/log/messages\ngrep -rn \"PermitRootLogin\" /etc/ssh/\ngrep -v '^#' /etc/fstab\ngrep -c \"Failed password\" /var/log/secure\ngrep -E 'error|fail' /var/log/messages\ngrep -A2 -B2 \"Out of memory\" /var/log/messages\n```\n\n| Option | Meaning |\n|---|---|\n| `-i` | Ignore upper/lower case |\n| `-v` | Show lines that do **not** match |\n| `-r` | Search all files in folders |\n| `-n` / `-c` / `-l` | Show line numbers / count / only file names |\n| `-w` | Whole words only |\n| `-E` | Extended regex |\n| `-A N` `-B N` `-C N` | N lines after / before / around a match |\n| `-o` | Print only the matching part |\n\n> **Exam tip:** \"Find all lines that contain X in file Y and write them to file Z\" is a typical task: `grep X Y > Z`.\n\n> **Work tip:** `grep -v '^#' FILE | grep -v '^$'` shows the active settings of a config file without comments and empty lines.\n","page":"summary/rhcsa/text/#grep","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-awk","n":1054,"slug":"awk","title":"Print and calculate columns with awk","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `awk` reads each line, splits it into fields (`$1`, `$2` …) and runs a small program on it. Good for columns separated by spaces.\n\n```bash\nawk -F: '{print $1, $7}' /etc/passwd\nawk -F: '$3 >= 1000 {print $1}' /etc/passwd\ndf -h | awk 'NR>1 {print $5, $6}'\nps aux | awk '{sum += $6} END {print sum/1024 \" MiB\"}'\nawk '/Failed password/ {print $(NF-3)}' /var/log/secure | sort | uniq -c | sort -rn\n```\n\n- `-F:` sets the field separator (default: spaces and tabs).\n- `$0` is the whole line, `NF` the number of fields, `NR` the line number.\n- A pattern before `{ }` selects lines: `$3 >= 1000` = normal users.\n\nThe last example counts failed SSH logins per IP address.\n","page":"summary/rhcsa/text/#awk","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-sed","n":1055,"slug":"sed","title":"Change text with sed","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `sed` (stream editor) changes text as it flows by: replace words, delete lines, print some lines. With `-i` it changes the file itself.\n\n```bash\nsed -n '5,10p' /etc/passwd\nsed 's/enforcing/permissive/' /etc/selinux/config\nsed -i 's/^#PermitRootLogin.*/PermitRootLogin no/' /etc/ssh/sshd_config\nsed '/^#/d; /^$/d' /etc/chrony.conf\nsed -i.bak 's/8080/9090/g' /etc/myapp.conf\n```\n\n| Command | Meaning |\n|---|---|\n| `s/old/new/` | Replace the first \"old\" in each line |\n| `s/old/new/g` | Replace every \"old\" |\n| `/pattern/d` | Delete lines that match |\n| `-n '5p'` | Print only line 5 |\n| `-i` | Change the file itself (`-i.bak` keeps a backup) |\n\n> **Watch out:** Test without `-i` first; look at the output; then add `-i` (or `-i.bak`).\n","page":"summary/rhcsa/text/#sed","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-create-files","n":1056,"slug":"create-files","title":"Create text files quickly","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** For short files you do not need an editor: `echo` writes one line, and `cat` with a **here-document** writes many lines.\n\n```bash\necho \"Hello\" > hello.txt\ncat > ~/notes.txt <<'EOF'\nserver1 192.168.122.10\nserver2 192.168.122.20\nEOF\ncat >> /etc/hosts <<'EOF'\n192.168.122.30 server3.example.com server3\nEOF\n```\n\n`<<'EOF'` reads lines until a line with only `EOF`. With quotes around EOF, variables like `$HOME` stay as they are.\n\n> **Exam tip:** The objective \"create and edit text files\" can be done with vim, nano, echo or a here-document. Check the file with `cat` after.\n","page":"summary/rhcsa/text/#create-files","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-main-points","n":1057,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- `less` pages through a file (`/word` to search, `q` to quit); `cat` prints a whole file; `head` and `tail` show the start and end.\n- `tail -f` follows a file live, for example a log file.\n- `cut` takes columns, `sort` orders lines, `uniq` removes repeated lines, `wc` counts lines, words and bytes.\n- A **regular expression** describes text: `^` start, `$` end, `.` any character, `*` repeat, `[ ]` a set.\n- `grep` finds lines: `-i` ignore case, `-v` invert, `-r` recursive, `-E` extended regex, `-n` line numbers.\n- `awk` works with fields (`awk -F: '{print $1}'`); `sed` changes text (`sed -i 's/old/new/g' file`).\n- Quote regular expressions with single quotes, so the shell does not change them.\n","page":"summary/rhcsa/text/#main-points","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-key-terms","n":1058,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Pager | A program that shows text one screen at a time (`less`). |\n| Regular expression | A pattern that describes text, used by grep, sed and awk. |\n| Anchor | `^` (line start) or `$` (line end) in a regular expression. |\n| Field | One column of a line, split by a separator like `:` or a space. |\n| Delimiter | The character between fields (`-d` in cut, `-F` in awk). |\n| Stream editor | `sed`: it changes text line by line as it passes through. |\n| Extended regex | A richer pattern syntax (`grep -E`) with `+`, `?`, `{n}` and `|`. |\n| Here document | Text given to a command inside a script, between `<<EOF` and `EOF`. |\n","page":"summary/rhcsa/text/#key-terms","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-text-review","n":1059,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-4"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you show the last 20 lines of /var/log/messages and keep watching new lines?**\nA: `tail -n 20 -f /var/log/messages`.\n\n\n**Q: How do you show only the user names from /etc/passwd?**\nA: `cut -d: -f1 /etc/passwd` (or `awk -F: '{print $1}' /etc/passwd`).\n\n\n**Q: How do you find lines that start with \"root\" in /etc/passwd?**\nA: `grep '^root' /etc/passwd`.\n\n\n**Q: How do you show all lines that do NOT contain a comment sign?**\nA: `grep -v '^#' FILE`.\n\n\n**Q: How do you count the lines of a file?**\nA: `wc -l FILE`.\n\n\n**Q: How do you replace every \"http\" with \"https\" inside a file?**\nA: `sed -i 's/http/https/g' FILE`.\n\n\n**Q: How do you search all files under /etc for the word \"PermitRootLogin\"?**\nA: `grep -r PermitRootLogin /etc`.\n\n\n**Q: Why do you put a regular expression in single quotes?**\nA: So the shell does not expand `*`, `$` or other special characters before grep gets them.\n","page":"summary/rhcsa/text/#review","label":"Ch 4","where":"RHCSA 10 · Chapter 4 · Read, search and change text files"},{"id":"sum-connect","n":1060,"slug":"connect","title":"Chapter 5: Log in locally and remotely","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"You work on servers on the local console, in terminal windows and — most of the time — over SSH. This chapter shows how to switch consoles, reboot and shut down the right way, log in with SSH and keys, copy files safely between systems, and use the Cockpit web console.\n\n## Lessons in this chapter\n\n- Log in on the local console\n- Switch between terminals\n- Understand pseudo terminals\n- Reboot and shut down the right way\n- Log in to another server with SSH\n- Copy files securely with scp\n- Transfer files with sftp\n- Synchronize folders with rsync\n- Log in with SSH keys\n- Use a passphrase and ssh-agent\n- Switch to another user in a session\n- Manage the server in a browser with Cockpit\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 5.1\n- Lab 5.2\n","page":"summary/rhcsa/connect/","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-console-login","n":1061,"slug":"console-login","title":"Log in on the local console","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The console is the screen and keyboard of the machine itself (or the console window of a virtual machine). You log in there with a user name and password.\n\n- On a server **without** a graphical desktop you get a text login prompt: `server1 login:`.\n- On a server **with** a desktop you log in on the graphical login screen; a terminal window then gives you a shell.\n- The text in `/etc/issue` is shown before the login prompt.\n\nAfter login, check who you are and where:\n\n```bash\nwhoami\ntty\n```\n```text\n/dev/tty3\n```\n```bash\nhostname\n```\n\n> **Exam tip:** In the exam you work on virtual machines through a console in the browser. Keep it open: you need it when the network or SSH is broken.\n","page":"summary/rhcsa/connect/#console-login","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-switch-terminals","n":1062,"slug":"switch-terminals","title":"Switch between terminals","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Linux has several virtual terminals on the same screen. `Ctrl+Alt+F1` … `F6` switch between them.\n\n| Keys | What you get (RHEL with desktop) |\n|---|---|\n| `Ctrl+Alt+F1` | The graphical login screen |\n| `Ctrl+Alt+F2` | Your graphical session |\n| `Ctrl+Alt+F3` … `F6` | Text consoles (tty3–tty6) |\n\nOn a server without a desktop, `Alt+F1` … `Alt+F6` switch between text consoles. As root you can also use `chvt N`.\n\n```bash\nchvt 3\n```\n\n> **Work tip:** When the desktop hangs, switch to a text console (`Ctrl+Alt+F3`), log in and fix it (for example kill the hanging program).\n","page":"summary/rhcsa/connect/#switch-terminals","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-pseudo-terminals","n":1063,"slug":"pseudo-terminals","title":"Understand pseudo terminals","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Terminal windows and SSH sessions do not use a real console. They get a **pseudo terminal**: `/dev/pts/0`, `/dev/pts/1` …\n\n```bash\ntty\n```\n```text\n/dev/pts/0\n```\n```bash\nwho\n```\n```text\nstudent  tty2         2026-10-25 08:10 (tty2)\nstudent  pts/0        2026-10-25 09:02 (192.168.122.1)\n```\n```bash\nw\n```\n\n- `tty` shows your own terminal.\n- `who` lists who is logged in, on which terminal and from where.\n- `w` shows the same plus what each user is doing now and the load of the system.\n\n> **Work tip:** `who` and `w` show active SSH sessions. An unknown user or IP address is a reason to investigate.\n","page":"summary/rhcsa/connect/#pseudo-terminals","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-reboot-shutdown","n":1064,"slug":"reboot-shutdown","title":"Reboot and shut down the right way","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Always stop a system through systemd, so services can stop cleanly and file systems are written. Never just pull the power.\n\n```bash\nsystemctl reboot\nsystemctl poweroff\nshutdown -r +5 \"Reboot for kernel update in 5 minutes\"\nshutdown -c\nshutdown -h now\n```\n\n- `systemctl reboot` / `systemctl poweroff` = restart / switch off now. `reboot` and `poweroff` are short forms.\n- `shutdown` can wait (`+5` = in 5 minutes, `22:00` = at a time) and sends a message to logged-in users. `shutdown -c` cancels it.\n- `systemctl halt` stops the system but does not switch off the power.\n\n> **Watch out:** A hard reset (power button, `echo b > /proc/sysrq-trigger`) can damage open files. Use it only when the system does not react at all.\n\n> **Exam tip:** Reboot your exam systems at the end and check that everything still works. Work that does not survive a reboot does not count.\n","page":"summary/rhcsa/connect/#reboot-shutdown","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-ssh-login","n":1065,"slug":"ssh-login","title":"Log in to another server with SSH","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** SSH (Secure Shell) gives you an encrypted shell on another system. Everything you type and see is protected.\n\n```bash\nssh student@server2\nssh server2\nssh -p 2222 student@server2\nssh root@server2 'df -h /'\nexit\n```\n\n- `ssh user@host` logs in as that user. Without `user@`, your own user name is used.\n- The first time, SSH shows the **fingerprint** of the server and asks you to trust it. Answer `yes` only when you are sure it is the right server. It is then saved in `~/.ssh/known_hosts`.\n- A command after the host runs on the remote system and SSH ends at once.\n- `-X` (or `-Y`) lets you start graphical programs from the remote server on your screen.\n\n> **Watch out:** \"REMOTE HOST IDENTIFICATION HAS CHANGED\" means the server key is different. It can be a reinstall — or an attack. Check before you remove the old key with `ssh-keygen -R server2`.\n","page":"summary/rhcsa/connect/#ssh-login","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-scp","n":1066,"slug":"scp","title":"Copy files securely with scp","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `scp` copies files over SSH. The remote side is written as `user@host:path`.\n\n```bash\nscp /etc/hosts student@server2:/tmp/\nscp student@server2:/var/log/messages ./server2-messages\nscp -r ~/project server2:/home/student/\nscp -P 2222 backup.tar.gz server2:/srv/\n```\n\n- `-r` copies folders.\n- `-P` (capital) sets the port. (In ssh it is a small `-p`.)\n- `-p` (small) keeps times and permissions.\n\n> **Remember:** scp = cp over SSH: `scp SOURCE TARGET`, remote side = `user@host:path`.\n","page":"summary/rhcsa/connect/#scp","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-sftp","n":1067,"slug":"sftp","title":"Transfer files with sftp","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `sftp` opens an interactive file transfer session over SSH, like an FTP client but encrypted.\n\n```bash\nsftp student@server2\n```\n\n| Command | What it does |\n|---|---|\n| `ls` / `lls` | List remote / local files |\n| `cd` / `lcd` | Change remote / local folder |\n| `pwd` / `lpwd` | Show remote / local folder |\n| `get file` | Download a file |\n| `put file` | Upload a file |\n| `mget *.log` / `mput *.txt` | Download / upload many files |\n| `mkdir` / `rm` | Make a remote folder / remove a remote file |\n| `exit` | Leave |\n\nThe `l` in front means \"local\". Everything else works on the remote side.\n","page":"summary/rhcsa/connect/#sftp","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-rsync","n":1068,"slug":"rsync","title":"Synchronize folders with rsync","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `rsync` copies only what is different. The second run of a big copy takes seconds. It works locally and over SSH.\n\n```bash\nrsync -av ~/project/ server2:/srv/project/\nrsync -av --delete /var/www/html/ /backup/html/\nrsync -avn --delete /var/www/html/ /backup/html/\nrsync -avz -e \"ssh -p 2222\" /data/ server2:/data/\n```\n\n| Option | Meaning |\n|---|---|\n| `-a` | Archive: recursive, keep permissions, owners, times, links |\n| `-v` | Show what is copied |\n| `-z` | Compress during the transfer |\n| `--delete` | Remove files in the target that are gone in the source |\n| `-n` | Dry run: show what would happen, change nothing |\n| `-P` | Show progress and keep half-done files |\n\n> **Watch out:** A slash at the end of the source matters. `src/` copies the **content** of src; `src` copies the folder itself into the target.\n\n> **Work tip:** Always run `rsync -avn --delete` first. Read the list. Then run it without `-n`.\n","page":"summary/rhcsa/connect/#rsync","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-ssh-keys","n":1069,"slug":"ssh-keys","title":"Log in with SSH keys","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A key pair replaces the password: the **private key** stays on your computer, the **public key** goes to the server in `~/.ssh/authorized_keys`.\n\n1. Make a key pair on the client (your computer or server1).\n```bash\nssh-keygen -t ed25519\n```\n2. Copy the public key to the server.\n```bash\nssh-copy-id student@server2\n```\n3. Log in: no password is asked any more (only the passphrase of the key, if you set one).\n```bash\nssh student@server2\n```\n\n> **Check:** The permissions on the server must be tight, or SSH ignores the key.\n\n```bash\nssh server2 'ls -ld ~/.ssh; ls -l ~/.ssh/authorized_keys'\n```\n\n| File | Where | Permissions |\n|---|---|---|\n| `~/.ssh/id_ed25519` | Client | 600 — never share it |\n| `~/.ssh/id_ed25519.pub` | Client | 644 — this is the one you copy |\n| `~/.ssh/authorized_keys` | Server | 600 |\n| `~/.ssh/` | Both | 700 |\n\n> **Remember:** private key stays home, public key travels.\n\n> **Exam tip:** The exam objective \"Configure key-based authentication for SSH\" = `ssh-keygen` + `ssh-copy-id` + test the login. Chapter 20 adds the server settings.\n","page":"summary/rhcsa/connect/#ssh-keys","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-passphrase","n":1070,"slug":"passphrase","title":"Use a passphrase and ssh-agent","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **passphrase** encrypts the private key, so a stolen key file is useless. `ssh-agent` keeps the unlocked key in memory, so you type the passphrase only once per session.\n\n```bash\nssh-keygen -t ed25519 -C \"student@server1\"\neval \"$(ssh-agent)\"\nssh-add\nssh-add -l\n```\n\n- With a passphrase: safer, but you must unlock the key.\n- Without a passphrase: needed for automatic jobs (cron, scripts), but protect the key file well and limit what it can do on the server.\n\n> **Work tip:** For automation, use a separate key without passphrase per job, and limit it in `authorized_keys` (for example `from=\"10.0.0.5\",command=\"/usr/local/bin/backup.sh\"`).\n","page":"summary/rhcsa/connect/#passphrase","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-switch-user","n":1071,"slug":"switch-user","title":"Switch to another user in a session","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** You do not need to log out to work as another user. `su - USER` opens a login shell as that user; `exit` takes you back.\n\n```bash\nsu - root\nexit\nsudo -i\nexit\n```\n\nChapter 6 explains `su` and `sudo` in detail.\n","page":"summary/rhcsa/connect/#switch-user","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-cockpit","n":1072,"slug":"cockpit","title":"Manage the server in a browser with Cockpit","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Cockpit is a web console for RHEL: logs, services, storage, network, users and a terminal in the browser, on port 9090.\n\n1. Install it and start the socket.\n```bash\ndnf install -y cockpit\nsystemctl enable --now cockpit.socket\n```\n2. Open the firewall (often already open on RHEL).\n```bash\nfirewall-cmd --add-service=cockpit --permanent\nfirewall-cmd --reload\n```\n3. Open `https://server1:9090` in a browser and log in with a normal user. Choose \"administrative access\" to make changes.\n\n> **Work tip:** Cockpit is great to show a junior colleague what is going on. In production, allow port 9090 only from the admin network.\n","page":"summary/rhcsa/connect/#cockpit","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-main-points","n":1073,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- Local logins use **virtual consoles** (Ctrl+Alt+F1…F6, or `chvt`); remote shells use **pseudo terminals** (`/dev/pts/N`).\n- Reboot with `systemctl reboot` and stop with `systemctl poweroff`; never just pull the power.\n- `ssh user@host` opens a remote shell; the first connection asks you to trust the host key (saved in `~/.ssh/known_hosts`).\n- `scp` and `sftp` copy files over SSH; `rsync` copies only what changed and can keep permissions (`-a`).\n- **Key-based login**: `ssh-keygen` makes a key pair, `ssh-copy-id` puts the public key in `~/.ssh/authorized_keys` on the server.\n- A passphrase protects the private key; `ssh-agent` and `ssh-add` keep it unlocked for a session.\n- `su -` switches to another user with a full login environment.\n- Cockpit is the web console (port 9090) for many admin tasks.\n","page":"summary/rhcsa/connect/#main-points","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-key-terms","n":1074,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Console | The screen and keyboard of the computer itself. |\n| Virtual terminal | One of several text logins on the local console (tty1–tty6). |\n| Pseudo terminal | A terminal made by software, for example for SSH (`/dev/pts/0`). |\n| SSH | Secure Shell: encrypted remote login and file copy. |\n| Host key | The key that proves the identity of an SSH server. |\n| Key pair | A private key (secret) and a public key (shared) for login without a password. |\n| authorized_keys | The file on the server with the public keys that may log in. |\n| Passphrase | The password that protects a private key. |\n| Cockpit | A web interface to manage a RHEL server. |\n","page":"summary/rhcsa/connect/#key-terms","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-connect-review","n":1075,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-5"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you switch to the third virtual console?**\nA: Ctrl+Alt+F3 (or `chvt 3` as root).\n\n\n**Q: Which command shows who is logged in and on which terminal?**\nA: `w` or `who`.\n\n\n**Q: Where does SSH store the host keys of servers you trust?**\nA: In `~/.ssh/known_hosts`.\n\n\n**Q: How do you set up login from server1 to server2 without a password?**\nA: On server1: `ssh-keygen`, then `ssh-copy-id user@server2`.\n\n\n**Q: Which file permission must the private key have?**\nA: Only the owner may read it, for example `600` (`chmod 600 ~/.ssh/id_ed25519`).\n\n\n**Q: How do you copy only changed files from /data to server2, keeping permissions?**\nA: `rsync -av /data/ server2:/data/`.\n\n\n**Q: What is the difference between `su user` and `su - user`?**\nA: `su -` starts a full login shell with the environment of that user; plain `su` keeps most of your old environment.\n\n\n**Q: Which port does the Cockpit web console use?**\nA: 9090 (`systemctl enable --now cockpit.socket`).\n","page":"summary/rhcsa/connect/#review","label":"Ch 5","where":"RHCSA 10 · Chapter 5 · Log in locally and remotely"},{"id":"sum-users","n":1076,"slug":"users","title":"Chapter 6: Manage users, groups and admin rights","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Every person and every service on Linux works as a user. This chapter shows how to create, change and delete users and groups, how to set password rules, and how to give admin rights in a safe way with sudo.\n\n## Lessons in this chapter\n\n- Know the kinds of users\n- Work as root only when you need it\n- Switch user with su\n- Give admin rights with sudo\n- Know PolicyKit\n- Read /etc/passwd and /etc/shadow\n- Create users\n- Set the defaults for new users\n- Change and delete users\n- Set password aging\n- Give users a working environment\n- Understand groups\n- Create and change groups\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 6.1\n- Lab 6.2\n","page":"summary/rhcsa/users/","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-user-types","n":1077,"slug":"user-types","title":"Know the kinds of users","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Linux knows three kinds of accounts. The **UID** (user ID number) tells which one it is.\n\n| Kind | UID | Example | Use |\n|---|---|---|---|\n| root | 0 | `root` | Full control of the system |\n| System accounts | 1–999 | `apache`, `chrony`, `sshd` | Run services; nobody logs in with them |\n| Normal users | 1000+ | `student` | People |\n\n```bash\nid\n```\n```text\nuid=1000(student) gid=1000(student) groups=1000(student),10(wheel)\n```\n```bash\nid apache\ngetent passwd root\n```\n\n> **Remember:** UID 0 = root, under 1000 = services, 1000 and up = people.\n","page":"summary/rhcsa/users/#user-types","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-work-as-root","n":1078,"slug":"work-as-root","title":"Work as root only when you need it","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** root can do anything — also destroy the system with one typo. Work as a normal user and switch to root only for admin tasks.\n\n- The prompt shows it: `$` for a normal user, `#` for root.\n- Use `sudo COMMAND` for one admin command, or `sudo -i` for a root shell when you have many tasks.\n- Leave the root shell with `exit` as soon as you are done.\n\n> **Work tip:** Every `sudo` command is logged in `/var/log/secure` (and the journal) with the real user name. A shared root password has no such trace.\n","page":"summary/rhcsa/users/#work-as-root","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-su","n":1079,"slug":"su","title":"Switch user with su","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `su` (substitute user) starts a shell as another user. You type **that user's** password.\n\n```bash\nsu -\nexit\nsu - lisa\nsu -c 'tail /var/log/messages' -\n```\n\n- `su -` (with the dash) gives a full login shell: the new user's environment and home folder. Always use the dash.\n- `su` without a user name means root.\n- root can switch to any user without a password.\n\n> **Watch out:** `su` without `-` keeps your old environment (PATH, variables). Programs can behave strangely or run from the wrong folder.\n","page":"summary/rhcsa/users/#su","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-sudo","n":1080,"slug":"sudo","title":"Give admin rights with sudo","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `sudo` lets allowed users run commands as root (or another user) with **their own** password. The rules are in `/etc/sudoers` and in files in `/etc/sudoers.d/`.\n\nOn RHEL, members of the group **wheel** may run everything:\n\n**/etc/sudoers (part)**\n```text\n%wheel  ALL=(ALL)       ALL\n```\n\n1. Give a user full admin rights by adding the user to wheel.\n```bash\nusermod -aG wheel lisa\n```\n2. Or give one user only some commands, in a separate file. Always edit with `visudo`, which checks the syntax before saving.\n```bash\nvisudo -f /etc/sudoers.d/webadmins\n```\n**/etc/sudoers.d/webadmins**\n```text\n%webadmins  ALL=(root)  /usr/bin/systemctl restart httpd, /usr/bin/systemctl status httpd\n```\n3. Test it as that user.\n```bash\nsudo -l\nsudo systemctl restart httpd\n```\n\nThe rule format is: `WHO  WHERE=(AS_WHOM)  COMMANDS`. A `%` before the name means a group. `NOPASSWD:` before the commands means no password is asked.\n\n> **Exam tip:** The objective \"configure privileged access\" = sudo rules. Use a file in `/etc/sudoers.d/` and `visudo`; never break `/etc/sudoers`.\n\n> **Watch out:** A syntax error in a sudoers file can lock everybody out of sudo. `visudo` stops you before you save a broken file.\n","page":"summary/rhcsa/users/#sudo","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-polkit","n":1081,"slug":"polkit","title":"Know PolicyKit","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** PolicyKit (polkit) decides which **actions** of system services a user may do — for example change the network, shut down, or mount a disk — mostly from graphical tools.\n\n- When a user without rights tries such an action, polkit asks for an admin password.\n- Members of `wheel` count as administrators for polkit.\n- `pkexec COMMAND` runs a command as root through polkit (like sudo, but with polkit rules).\n- Rules are in `/usr/share/polkit-1/` (do not change) and `/etc/polkit-1/rules.d/` (your own).\n\n```bash\nnmcli general permissions\npkexec systemctl restart chronyd\n```\n","page":"summary/rhcsa/users/#polkit","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-passwd-shadow","n":1082,"slug":"passwd-shadow","title":"Read /etc/passwd and /etc/shadow","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** User accounts are lines in text files. `/etc/passwd` has the account data (readable by all); `/etc/shadow` has the password hashes and password ages (only root).\n\n**/etc/passwd**\n```text\nlisa:x:1001:1001:Lisa Hassan:/home/lisa:/bin/bash\n```\n\n| Field | Meaning |\n|---|---|\n| 1 `lisa` | User name |\n| 2 `x` | The password is in /etc/shadow |\n| 3 `1001` | UID |\n| 4 `1001` | GID of the primary group |\n| 5 `Lisa Hassan` | Comment (GECOS): full name |\n| 6 `/home/lisa` | Home folder |\n| 7 `/bin/bash` | Login shell |\n\n**/etc/shadow**\n```text\nlisa:$6$aBc…:20385:0:90:7:::\n```\n\nFields of shadow: user, password hash (`!` or `!!` = locked or no password), last change (days since 1970), min days, max days, warning days, inactive days, expire date.\n\n> **Watch out:** Do not edit these files with a normal editor while users change passwords. Use the commands (useradd, usermod, passwd) or `vipw`/`vigr`, which lock the files.\n","page":"summary/rhcsa/users/#passwd-shadow","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-useradd","n":1083,"slug":"useradd","title":"Create users","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `useradd` makes the account; `passwd` sets the password. The new home folder gets a copy of `/etc/skel`.\n\n1. Create a user with a full name.\n```bash\nuseradd -c \"Lisa Hassan\" lisa\n```\n2. Set the password.\n```bash\npasswd lisa\n```\n3. Create a user with more settings in one go.\n```bash\nuseradd -u 1500 -G wheel,webadmins -s /bin/bash -c \"Omar Ali\" omar\n```\n4. Create an account for a service that nobody logs in to.\n```bash\nuseradd -r -s /sbin/nologin -d /var/lib/myapp myapp\n```\n\n| Option | Meaning |\n|---|---|\n| `-c` | Comment (full name) |\n| `-u` | A chosen UID |\n| `-g` / `-G` | Primary group / extra groups |\n| `-s` | Login shell |\n| `-d` | Home folder |\n| `-m` / `-M` | Make / do not make the home folder |\n| `-e` | Expire date (YYYY-MM-DD) |\n| `-r` | System account (UID under 1000, no aging) |\n\n> **Check:** The user exists and has the right groups.\n\n```bash\nid omar\ngetent passwd omar\n```\n","page":"summary/rhcsa/users/#useradd","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-defaults","n":1084,"slug":"defaults","title":"Set the defaults for new users","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `useradd` takes its defaults from `/etc/login.defs` and `/etc/default/useradd`. Change them once, and every new user gets them.\n\n```bash\nuseradd -D\nuseradd -D -s /bin/bash\ngrep -E '^(UID_MIN|PASS_MAX_DAYS|PASS_MIN_DAYS|PASS_WARN_AGE|CREATE_HOME|HOME_MODE)' /etc/login.defs\n```\n\n| File | Sets |\n|---|---|\n| `/etc/login.defs` | UID/GID ranges, password aging defaults, create home, home permissions, umask |\n| `/etc/default/useradd` | Default shell, home base folder, skel folder, group, expire |\n| `/etc/skel/` | Files every new home folder gets (like `.bashrc`) |\n\n1. Make every new user get a file with company rules.\n```bash\necho \"Read the IT policy: https://intranet.example.com/policy\" > /etc/skel/README.txt\n```\n2. Make new passwords expire after 90 days by default.\n```bash\nsed -i 's/^PASS_MAX_DAYS.*/PASS_MAX_DAYS   90/' /etc/login.defs\n```\n\n> **Remember:** login.defs = numbers and rules, default/useradd = shell and home, skel = starter files. They only affect **new** users.\n","page":"summary/rhcsa/users/#defaults","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-usermod","n":1085,"slug":"usermod","title":"Change and delete users","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `usermod` changes an existing account; `userdel` removes it.\n\n```bash\nusermod -aG webadmins lisa\nusermod -s /sbin/nologin olduser\nusermod -L lisa\nusermod -U lisa\nusermod -e 2026-12-31 contractor1\nusermod -l lisa.hassan lisa\nuserdel -r contractor1\n```\n\n- `-aG GROUP` adds an extra group. Without `-a`, `-G` **replaces** all extra groups.\n- `-L` / `-U` lock / unlock the password.\n- `-l NEW OLD` renames the login (the home folder keeps its name unless you add `-d NEWHOME -m`).\n- `userdel -r` also removes the home folder and the mail spool.\n\n> **Watch out:** `usermod -G wheel lisa` (without -a) removes lisa from every other extra group. Always use `-aG` to add.\n\n> **Work tip:** When someone leaves, first lock the account (`usermod -L`, `chage -E 0`), keep the files for a while, then delete. Find their files with `find / -user NAME`.\n","page":"summary/rhcsa/users/#usermod","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-password-aging","n":1086,"slug":"password-aging","title":"Set password aging","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Password aging forces users to change passwords and can expire accounts. `chage` sets it per user; `passwd` has some short forms.\n\n```bash\nchage -l lisa\nchage -M 90 -m 1 -W 7 lisa\nchage -E 2026-12-31 contractor1\nchage -d 0 lisa\npasswd -e lisa\n```\n\n| Option | Meaning |\n|---|---|\n| `-M 90` | Maximum: change the password every 90 days |\n| `-m 1` | Minimum: wait 1 day before the next change |\n| `-W 7` | Warn 7 days before it expires |\n| `-I 14` | Lock the account 14 days after the password expired |\n| `-E DATE` | The whole account expires on this date (`-E -1` = never) |\n| `-d 0` | Force a new password at the next login |\n\n> **Check:** Look at the result.\n\n```bash\nchage -l lisa\n```\n> **Exam tip:** Tasks like \"user X must change the password every 30 days\" or \"the account expires on …\" use `chage`. For all new users, set `PASS_MAX_DAYS` in `/etc/login.defs`.\n","page":"summary/rhcsa/users/#password-aging","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-user-env","n":1087,"slug":"user-env","title":"Give users a working environment","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A new home folder starts as a copy of `/etc/skel`. The shell reads the profile files at login, so users get the same start settings.\n\n- Put files for all new users in `/etc/skel` (like `.bashrc` with company aliases).\n- Settings for all users at every login: a file in `/etc/profile.d/`.\n- Accounts that must not log in get the shell `/sbin/nologin`.\n\n```bash\nls -la /etc/skel\nusermod -s /sbin/nologin backup\nsu - backup\n```\n```text\nThis account is currently not available.\n```\n","page":"summary/rhcsa/users/#user-env","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-groups","n":1088,"slug":"groups","title":"Understand groups","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A group gives the same rights to many users. Every user has one **primary group** and can be in many **secondary (extra) groups**.\n\n- The primary group is the group of new files a user makes. On RHEL each user gets a private group with the same name.\n- Secondary groups give access to shared files and folders.\n- Groups are in `/etc/group` (name, password placeholder, GID, members) and `/etc/gshadow`.\n\n```bash\nid lisa\n```\n```text\nuid=1001(lisa) gid=1001(lisa) groups=1001(lisa),1100(webadmins)\n```\n```bash\ngroups lisa\ngetent group webadmins\n```\n```text\nwebadmins:x:1100:lisa,omar\n```\n\n> **Watch out:** A new group membership is active only after the user logs in again (or runs `newgrp GROUP`).\n","page":"summary/rhcsa/users/#groups","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-groupadd","n":1089,"slug":"groupadd","title":"Create and change groups","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `groupadd` makes a group, `groupmod` changes it, `groupdel` removes it. Add members with `usermod -aG` or `gpasswd -a`.\n\n```bash\ngroupadd webadmins\ngroupadd -g 2000 dbadmins\ngroupmod -n web-admins webadmins\ngpasswd -a lisa web-admins\ngpasswd -d lisa web-admins\ngroupdel dbadmins\n```\n\nTo edit the files by hand safely, use `vigr` (for /etc/group) and `vipw` (for /etc/passwd). They lock the files while you edit.\n\n> **Check:** The members of a group.\n\n```bash\ngetent group web-admins\n```\n> **Remember:** add member = `usermod -aG GROUP USER` or `gpasswd -a USER GROUP`. Remove member = `gpasswd -d USER GROUP`.\n","page":"summary/rhcsa/users/#groupadd","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-main-points","n":1090,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- **root** (UID 0) can do everything; system accounts run services; normal users start at UID 1000.\n- Use **sudo** for admin work: members of the `wheel` group may run all commands; own rules go in `/etc/sudoers.d/` with `visudo`.\n- User data is in `/etc/passwd`, passwords and aging in `/etc/shadow`, groups in `/etc/group`.\n- `useradd`, `usermod`, `userdel` manage users; defaults come from `/etc/login.defs` and `/etc/default/useradd`; new home folders get the files of `/etc/skel`.\n- `passwd` sets a password; `chage` sets password aging (`-M` max days, `-m` min days, `-W` warn, `-E` expire date).\n- Every user has one **primary group** and can have **secondary groups** (`usermod -aG group user`).\n- `groupadd`, `groupmod`, `groupdel` and `gpasswd` manage groups.\n","page":"summary/rhcsa/users/#main-points","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-key-terms","n":1091,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| UID / GID | The number of a user / a group. |\n| root | The super user with UID 0. |\n| System account | An account for a service, not for a person (UID below 1000). |\n| sudo | Run one command as root (or another user), with rules in /etc/sudoers. |\n| wheel | The group whose members may use sudo on RHEL. |\n| PolicyKit (polkit) | Rules that let normal users do some admin actions in graphical tools. |\n| /etc/shadow | The file with the password hashes and aging data. |\n| Password aging | Rules for how long a password is valid. |\n| Primary group | The group a new file of the user gets. |\n| Secondary group | An extra group membership for more access. |\n| /etc/skel | Files copied into every new home folder. |\n","page":"summary/rhcsa/users/#key-terms","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-users-review","n":1092,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-6"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you create user lisa with a home folder and the shell /bin/bash?**\nA: `useradd -s /bin/bash lisa` (RHEL makes the home folder by default; `-m` makes sure).\n\n\n**Q: How do you add user lisa to the group sales without removing her other groups?**\nA: `usermod -aG sales lisa`.\n\n\n**Q: How do you force lisa to change her password every 90 days?**\nA: `chage -M 90 lisa`.\n\n\n**Q: How do you give group helpdesk the right to restart sshd as root and nothing else?**\nA: `visudo -f /etc/sudoers.d/helpdesk` with the line `%helpdesk ALL=(root) /usr/bin/systemctl restart sshd`.\n\n\n**Q: Where do you put a file that every new user must get in the home folder?**\nA: In `/etc/skel`.\n\n\n**Q: Which file has the password hashes?**\nA: `/etc/shadow` (only root can read it).\n\n\n**Q: How do you lock an account so the user cannot log in with a password?**\nA: `usermod -L user` or `passwd -l user`.\n\n\n**Q: Which command shows the UID, GID and groups of a user?**\nA: `id USER`.\n","page":"summary/rhcsa/users/#review","label":"Ch 6","where":"RHCSA 10 · Chapter 6 · Manage users, groups and admin rights"},{"id":"sum-permissions","n":1093,"slug":"permissions","title":"Chapter 7: Control who can read, change and run files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Every file has an owner, a group and permissions. This chapter shows how to read them, change them, set good defaults, build shared team folders, and fix \"Permission denied\" problems.\n\n## Lessons in this chapter\n\n- Read owner, group and permissions\n- Know what r, w and x do\n- Change the owner and group\n- Set permissions with numbers\n- Set permissions with letters\n- Set default permissions with umask\n- Use the special permissions\n- Build a shared team directory\n- Give one more user access with an ACL\n- Protect files with attributes\n- Fix \"Permission denied\"\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 7.1\n- Lab 7.2\n","page":"summary/rhcsa/permissions/","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-read-perms","n":1094,"slug":"read-perms","title":"Read owner, group and permissions","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `ls -l` shows who owns a file and what the owner (u), the group (g) and all others (o) may do.\n\n```bash\nls -l /etc/passwd /etc/shadow\n```\n```text\n-rw-r--r--. 1 root root 2401 Oct 25 09:10 /etc/passwd\n----------. 1 root root 1290 Oct 25 09:10 /etc/shadow\n```\n```bash\nls -ld /tmp\n```\n```text\ndrwxrwxrwt. 18 root root 4096 Oct 25 10:00 /tmp\n```\n\n| Part | Example | Meaning |\n|---|---|---|\n| 1st sign | `-` / `d` / `l` | File / directory / symbolic link |\n| 2–4 | `rw-` | Permissions of the owner (user) |\n| 5–7 | `r--` | Permissions of the group |\n| 8–10 | `r--` | Permissions of all others |\n| `.` at the end | `.` | The file has an SELinux context (a `+` = an ACL) |\n| owner, group | `root root` | Who owns the file, which group |\n\nLinux checks in this order and stops at the first match: owner? → group member? → others.\n\n> **Watch out:** If you are the owner, only the owner bits count — even if the group or others have more rights.\n","page":"summary/rhcsa/permissions/#read-perms","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-rwx","n":1095,"slug":"rwx","title":"Know what r, w and x do","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The same letters mean different things for a file and for a directory.\n\n| | File | Directory |\n|---|---|---|\n| `r` (4) | Read the content | List the names in it (`ls`) |\n| `w` (2) | Change the content | Create, delete and rename files in it |\n| `x` (1) | Run it as a program | Enter it (`cd`) and reach files in it |\n\n> **Remember:** To delete a file you need `w` on the **directory**, not on the file. To read a file in a directory you need `x` on every directory in the path.\n\n\n```bash\nls -ld /home/lisa\n```\n```text\ndrwx------. 4 lisa lisa 112 Oct 25 09:20 /home/lisa\n```\n```bash\ncat /home/lisa/notes.txt\n```\n```text\ncat: /home/lisa/notes.txt: Permission denied\n```\n","page":"summary/rhcsa/permissions/#rwx","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-chown","n":1096,"slug":"chown","title":"Change the owner and group","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `chown` changes the owner (and group); `chgrp` changes only the group. Only root can give a file to another user.\n\n```bash\nchown lisa report.txt\nchown lisa:sales report.txt\nchown :sales report.txt\nchgrp sales report.txt\nchown -R apache:apache /var/www/html/app\n```\n\n- `-R` changes a whole directory tree.\n- A normal user can change the group of an own file, but only to a group he is a member of.\n\n> **Check:** The new owner and group.\n\n```bash\nls -l report.txt\n```\n```text\n-rw-r--r--. 1 lisa sales 0 Oct 25 10:20 report.txt\n```\n","page":"summary/rhcsa/permissions/#chown","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-chmod-numbers","n":1097,"slug":"chmod-numbers","title":"Set permissions with numbers","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Add the values: r = 4, w = 2, x = 1. Three digits: owner, group, others.\n\n| Number | Letters | Common use |\n|---|---|---|\n| `644` | `rw-r--r--` | Normal files |\n| `600` | `rw-------` | Private files, SSH keys |\n| `755` | `rwxr-xr-x` | Programs and normal directories |\n| `750` | `rwxr-x---` | Team directories (no access for others) |\n| `700` | `rwx------` | Private directories |\n| `770` | `rwxrwx---` | Shared team directories |\n\n```bash\nchmod 644 index.html\nchmod 600 ~/.ssh/id_ed25519\nchmod 750 /srv/project\n```\n","page":"summary/rhcsa/permissions/#chmod-numbers","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-chmod-letters","n":1098,"slug":"chmod-letters","title":"Set permissions with letters","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The letter form changes only what you name: who (`u g o a`), how (`+ - =`), what (`r w x`).\n\n```bash\nchmod u+x backup.sh\nchmod g+w,o-rwx shared.txt\nchmod a+r info.txt\nchmod o= secret.txt\nchmod -R g+rwX /srv/sales\n```\n\n- `+` adds, `-` removes, `=` sets exactly.\n- Capital `X` adds `x` only to directories (and files that are already executable). Good with `-R`.\n\n> **Exam tip:** Read the task carefully: \"others may not read\" → `o-r`; \"only the owner\" → `chmod 600` or `chmod go=`.\n","page":"summary/rhcsa/permissions/#chmod-letters","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-umask","n":1099,"slug":"umask","title":"Set default permissions with umask","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** New files start at `666` and new directories at `777`. The **umask** takes bits away.\n\n| umask | New file | New directory | Who |\n|---|---|---|---|\n| `0022` | `644` | `755` | RHEL default for root and normal users |\n| `0027` | `640` | `750` | Hardened servers: others get nothing |\n| `0077` | `600` | `700` | Very private |\n\n```bash\numask\n```\n```text\n0022\n```\n```bash\numask 027\ntouch a; mkdir b; ls -ld a b\n```\n```text\n-rw-r-----. 1 student student 0 Oct 25 10:30 a\ndrwxr-x---. 2 student student 6 Oct 25 10:30 b\n```\n\n1. A `umask` command in the shell is valid only for that shell.\n2. For one user, put it in `~/.bashrc`.\n```bash\necho \"umask 027\" >> ~/.bashrc\n```\n3. For all users, put it in a file in `/etc/profile.d/`.\n```bash\necho \"umask 027\" > /etc/profile.d/umask.sh\n```\n\n> **Remember:** The umask never gives `x` to new files. A file is executable only after `chmod +x`.\n","page":"summary/rhcsa/permissions/#umask","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-special","n":1100,"slug":"special","title":"Use the special permissions","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Three extra bits: SUID, SGID and sticky. They are the fourth (first) digit of the number.\n\n| Bit | Value | On a file | On a directory |\n|---|---|---|---|\n| SUID `u+s` | 4 | The program runs as the **owner** (like `passwd`) | — |\n| SGID `g+s` | 2 | The program runs as the group | New files get the **group of the directory** |\n| sticky `o+t` | 1 | — | Users may delete only their **own** files (like `/tmp`) |\n\n```bash\nls -l /usr/bin/passwd\n```\n```text\n-rwsr-xr-x. 1 root root 32656 Oct 21 12:00 /usr/bin/passwd\n```\n```bash\nchmod g+s /srv/sales\nchmod o+t /srv/sales\nchmod 3770 /srv/sales\n```\n\nIn `ls -l` you see `s` or `t` instead of `x`. A capital `S` or `T` means the bit is set but `x` is not there (often a mistake).\n\n> **Watch out:** SUID programs are a favourite target of attackers. Find them now and then: `find / -perm -4000 -type f 2>/dev/null`.\n","page":"summary/rhcsa/permissions/#special","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-shared-dir","n":1101,"slug":"shared-dir","title":"Build a shared team directory","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The classic exam task: a directory where a team works together, new files belong to the team group, and people cannot delete files of others.\n\n1. Create the group and add the members.\n```bash\ngroupadd sales\nusermod -aG sales anna; usermod -aG sales bob\n```\n2. Create the directory and give it to the group.\n```bash\nmkdir -p /srv/sales\nchown root:sales /srv/sales\n```\n3. Give the group full rights, others nothing, plus SGID and sticky.\n```bash\nchmod 3770 /srv/sales\n```\n\n> **Check:** A file made by anna gets the group sales, and bob cannot delete it.\n\n```bash\nls -ld /srv/sales\n```\n```text\ndrwxrws--T. 2 root sales 6 Oct 25 10:40 /srv/sales\n```\n```bash\nsu - anna -c 'touch /srv/sales/plan.txt; ls -l /srv/sales'\nsu - bob -c 'rm -f /srv/sales/plan.txt'\n```\n```text\nrm: cannot remove '/srv/sales/plan.txt': Operation not permitted\n```\n\n> **Note:** The `T` is capital because others have no `x`. This is correct here.\n","page":"summary/rhcsa/permissions/#shared-dir","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-acl","n":1102,"slug":"acl","title":"Give one more user access with an ACL","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Normal permissions know one owner and one group. An **ACL** (access control list) adds rights for more users or groups.\n\n```bash\nsetfacl -m u:carol:rx /srv/sales\nsetfacl -m g:account:rwx /srv/sales\ngetfacl /srv/sales\nsetfacl -d -m g:account:rwX /srv/sales\nsetfacl -x u:carol /srv/sales\nsetfacl -b /srv/sales\n```\n\n- `-m` adds or changes an entry, `-x` removes one, `-b` removes all.\n- `-d` sets a **default** ACL: new files in the directory get it.\n- `ls -l` shows a `+` after the permissions when a file has an ACL.\n\n> **Watch out:** The ACL **mask** limits the rights of all named entries and the group. `chmod g-w` on a file with an ACL changes the mask.\n","page":"summary/rhcsa/permissions/#acl","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-attributes","n":1103,"slug":"attributes","title":"Protect files with attributes","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** File attributes work below the permissions. Even root cannot change an **immutable** file until the attribute is removed.\n\n```bash\nchattr +i /etc/resolv.conf\nlsattr /etc/resolv.conf\n```\n```text\n----i----------------- /etc/resolv.conf\n```\n```bash\nrm /etc/resolv.conf\n```\n```text\nrm: cannot remove '/etc/resolv.conf': Operation not permitted\n```\n```bash\nchattr -i /etc/resolv.conf\nchattr +a /var/log/app/audit.log\n```\n\n| Attribute | Meaning |\n|---|---|\n| `i` | Immutable: no change, delete, rename or link |\n| `a` | Append only: data can only be added (good for logs) |\n\n> **Note:** XFS (the RHEL default) supports `i`, `a` and a few more; ext4 supports more attributes.\n","page":"summary/rhcsa/permissions/#attributes","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-troubleshoot","n":1104,"slug":"troubleshoot","title":"Fix \"Permission denied\"","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Check the layers one by one: path, owner and mode, ACL, attributes, SELinux.\n\n1. Check every directory in the path (`x` needed on each).\n```bash\nnamei -l /srv/sales/plan.txt\n```\n2. Check owner, group, mode and your groups.\n```bash\nls -l /srv/sales/plan.txt; id\n```\n3. Check ACLs and attributes.\n```bash\ngetfacl /srv/sales/plan.txt; lsattr /srv/sales/plan.txt\n```\n4. If everything looks right, check SELinux (see the SELinux chapter).\n```bash\nausearch -m avc -ts recent\n```\n\n> **Work tip:** Do not \"fix\" a problem with `chmod 777`. It opens the file for everybody and often hides the real cause (SELinux, a wrong group, a missing `x` on a directory).\n","page":"summary/rhcsa/permissions/#troubleshoot","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-main-points","n":1105,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- Every file has an **owner**, a **group** and permissions for **user, group and others** (`ugo`).\n- **r** read, **w** write, **x** execute. On a folder: r = list names, w = create and delete files, x = enter the folder.\n- Numbers: r = 4, w = 2, x = 1, so `750` = rwx r-x ---. Letters: `chmod g+w,o-rwx file`.\n- `chown user:group file` changes owner and group; `chgrp group file` changes only the group.\n- **umask** removes permissions from new files: umask 022 gives files 644 and folders 755.\n- Special permissions: **SUID** (4, run as the owner), **SGID** (2, on a folder new files get the folder's group), **sticky bit** (1, only the owner can delete own files).\n- **ACLs** give extra users or groups access: `setfacl -m u:lisa:rx file`, read them with `getfacl`; default ACLs (`-d`) apply to new files.\n- Attributes like **immutable** (`chattr +i`) protect files even from root.\n","page":"summary/rhcsa/permissions/#main-points","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-key-terms","n":1106,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Ownership | Which user and which group a file belongs to. |\n| Permission mode | The rwx rights for user, group and others. |\n| Octal notation | Permissions as numbers (`640`). |\n| Symbolic notation | Permissions as letters (`u+x`, `g-w`). |\n| umask | The permissions that new files and folders do NOT get. |\n| SUID | A program runs with the rights of its owner (like `passwd`). |\n| SGID | On a folder: new files get the group of the folder. |\n| Sticky bit | In a shared folder, users can delete only their own files (like /tmp). |\n| ACL | Access Control List: extra permissions for named users and groups. |\n| Default ACL | An ACL on a folder that new files inherit. |\n| Attribute | An extra file flag like immutable (`i`) or append only (`a`). |\n","page":"summary/rhcsa/permissions/#key-terms","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-permissions-review","n":1107,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-7"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which number gives the owner all rights, the group read and execute, and others nothing?**\nA: `750`.\n\n\n**Q: What do you need on a folder to open a file in it?**\nA: The x (execute) permission on the folder.\n\n\n**Q: How do you make sure new files in /data/sales get the group sales?**\nA: `chgrp sales /data/sales` and `chmod g+s /data/sales` (SGID).\n\n\n**Q: How do you stop users from deleting each other's files in a shared folder?**\nA: Set the sticky bit: `chmod +t FOLDER`.\n\n\n**Q: How do you give user carol read access to one file without changing owner or group?**\nA: `setfacl -m u:carol:r FILE`.\n\n\n**Q: Which permissions does a new file get with umask 027?**\nA: 640 (rw- r-- ---).\n\n\n**Q: How do you see the ACLs of a file?**\nA: `getfacl FILE` (a `+` after the mode in `ls -l` means the file has ACLs).\n\n\n**Q: How do you make a file impossible to change, even for root, until you remove the flag?**\nA: `chattr +i FILE` (remove with `chattr -i`).\n","page":"summary/rhcsa/permissions/#review","label":"Ch 7","where":"RHCSA 10 · Chapter 7 · Control who can read, change and run files"},{"id":"sum-network","n":1108,"slug":"network","title":"Chapter 8: Set up the network","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"After this chapter you can read IP addresses, check that the network works, give a server a fixed address with `nmcli`, change the hostname and set up name resolution. These are some of the most common tasks in the exam and at work.\n\n## Lessons in this chapter\n\n- Read an IPv4 address and its mask\n- Read an IPv6 address\n- Know MAC addresses, protocols and ports\n- See your addresses and network cards\n- Check the route and the default gateway\n- Check that a port or service answers\n- Know who may change the network\n- Change the IPv4 address with nmcli\n- Add an IPv6 address or a second IP\n- Make a new connection profile\n- Change the network with nmtui (menus)\n- Read and edit the profile files\n- Change the hostname\n- Set up DNS and /etc/hosts\n- Start network services at boot\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 8.1\n","page":"summary/rhcsa/network/","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-ipv4-basics","n":1109,"slug":"ipv4-basics","title":"Read an IPv4 address and its mask","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An IPv4 address is 4 numbers from 0 to 255. The mask (or prefix) tells which part is the network and which part is the host.\n\nAn address like `192.168.122.10/24` has two parts:\n\n- `192.168.122.10` is the address of this computer.\n- `/24` is the **prefix**. The first 24 bits (the first 3 numbers) are the network. The last number is the host.\n\nThe same mask can be written in two ways. Linux tools use the prefix form.\n\n| Prefix | Mask | Usable hosts |\n|---|---|---|\n| `/8` | 255.0.0.0 | 16,777,214 |\n| `/16` | 255.255.0.0 | 65,534 |\n| `/24` | 255.255.255.0 | 254 |\n| `/25` | 255.255.255.128 | 126 |\n| `/26` | 255.255.255.192 | 62 |\n| `/30` | 255.255.255.252 | 2 |\n\nEach number is 8 bits. In binary, 255 is `11111111` and 192 is `11000000`. The prefix counts the `1` bits of the mask from the left. The number of hosts is 2 to the power of (32 − prefix), minus 2: one address is for the network itself and one is for broadcast.\n\nPrivate ranges are only used inside networks and are not routed on the internet: `10.0.0.0/8`, `172.16.0.0/12` and `192.168.0.0/16`. The address `127.0.0.1` is the computer itself (loopback).\n\n> **Remember:** /24 = 255.255.255.0 = 254 hosts. Each step up (/25, /26 …) halves the hosts.\n\n> **Exam tip:** The exam gives the address in prefix form, like 192.168.4.10/24. Type it in that form in `nmcli`.\n","page":"summary/rhcsa/network/#ipv4-basics","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-ipv6-basics","n":1110,"slug":"ipv6-basics","title":"Read an IPv6 address","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An IPv6 address is 128 bits, written as 8 groups of 4 hexadecimal digits. You can shorten it.\n\nExample: `2001:0db8:0000:0000:0000:0000:0000:0010` becomes `2001:db8::10`.\n\n1. Remove the zeros at the start of each group: `0db8` becomes `db8`.\n2. Replace one run of groups that are all zero with `::`. You can do this only once in an address.\n\nMost networks use a `/64` prefix: the first 64 bits are the network, the last 64 bits are the host.\n\n| Address | Meaning |\n|---|---|\n| `::1` | The computer itself (loopback) |\n| `fe80::/10` | Link-local. Every card has one. It works only on the local network. |\n| `2000::/3` | Global addresses, used on the internet |\n| `fd00::/8` | Unique local addresses, for private networks |\n\n> **Remember:** `::` can appear only once. A link-local address (fe80::) is normal: it does not mean IPv6 is set up.\n","page":"summary/rhcsa/network/#ipv6-basics","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-mac-and-ports","n":1111,"slug":"mac-and-ports","title":"Know MAC addresses, protocols and ports","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The MAC address finds a card on the local network. The IP address finds a computer across networks. The port finds the program on that computer.\n\n- A **MAC address** is 6 bytes in hexadecimal, like `52:54:00:3a:7c:01`. It is set by the maker of the network card. You see it with `ip link`.\n- **TCP** makes a connection and checks that all data arrives (SSH, web, mail). **UDP** sends without a connection (DNS questions, NTP time).\n- A **port** is a number from 1 to 65535. Ports under 1024 are for well-known services.\n\n| Port | Service |\n|---|---|\n| 22/tcp | SSH |\n| 53/udp and tcp | DNS |\n| 80/tcp, 443/tcp | HTTP, HTTPS |\n| 123/udp | NTP (time) |\n| 2049/tcp | NFS |\n\nThe file `/etc/services` lists the well-known port numbers and their names.\n\n```bash\ngrep -w 22 /etc/services\n```\n```text\nssh             22/tcp\nssh             22/udp\n```\n\n> **Work tip:** In a security review, first list the ports that listen (next lessons). Every open port must have an owner and a reason.\n","page":"summary/rhcsa/network/#mac-and-ports","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-show-addresses","n":1112,"slug":"show-addresses","title":"See your addresses and network cards","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Use `ip` to look at the network. It shows the real settings that the kernel uses right now.\n\n1. Show all addresses. `ip a` is short for `ip address show`.\n```bash\nip a\nip -br a\n```\n```text\nlo               UNKNOWN        127.0.0.1/8 ::1/128\nenp1s0           UP             192.168.122.10/24 fe80::5054:ff:fe3a:7c01/64\n```\n2. Show only one card and only IPv4.\n```bash\nip -4 addr show dev enp1s0\n```\n3. Show the cards (links), their MAC address and state. `-s` adds counters for errors and dropped packets.\n```bash\nip link show\nip -s link show enp1s0\n```\n\nNetwork card names tell where the card sits: `en` = Ethernet, `o1` = on board, `s1` = slot, `p1s0` = PCI bus 1, slot 0. So `eno1`, `ens160` and `enp1s0` are all normal names.\n\n> **Watch out:** Changes with `ip addr add` or `ip link set` are lost after a reboot. Use `nmcli` for settings that must stay.\n","page":"summary/rhcsa/network/#show-addresses","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-check-routes","n":1113,"slug":"check-routes","title":"Check the route and the default gateway","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The routing table says where packets go. The default route (default gateway) is used for every address that has no better route.\n\n```bash\nip route show\n```\n```text\ndefault via 192.168.122.1 dev enp1s0 proto static metric 100\n192.168.122.0/24 dev enp1s0 proto kernel scope link src 192.168.122.10 metric 100\n```\n\nThe first line is the default gateway. The second line says: the local network is reached directly on `enp1s0`.\n\nTo see which way the system will use for one address:\n\n```bash\nip route get 8.8.8.8\n```\n```text\n8.8.8.8 via 192.168.122.1 dev enp1s0 src 192.168.122.10 uid 1000\n```\n\nThen test the way, step by step:\n\n1. Can you reach the gateway? `ping` it 3 times.\n```bash\nping -c 3 192.168.122.1\n```\n2. Can you reach an address on the internet?\n```bash\nping -c 3 8.8.8.8\n```\n3. Where does the path stop? `tracepath` shows every router on the way.\n```bash\ntracepath -n 8.8.8.8\n```\n\n> **Remember:** Gateway ping works but internet ping fails = a problem after your gateway. Internet ping by number works but by name fails = a DNS problem.\n","page":"summary/rhcsa/network/#check-routes","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-check-ports","n":1114,"slug":"check-ports","title":"Check that a port or service answers","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `ss` shows which ports listen on this system and which connections are open.\n\n```bash\nss -tuln\nss -tulpn\n```\n\nThe letters: `t` = TCP, `u` = UDP, `l` = only listening ports, `n` = numbers (no names), `p` = show the program. You need root to see the programs of other users.\n\n```bash\nss -tlpn sport = :22\n```\n```text\nState  Recv-Q Send-Q Local Address:Port  Peer Address:Port Process\nLISTEN 0      128          0.0.0.0:22         0.0.0.0:*     users:((\"sshd\",pid=1021,fd=3))\n```\n\nA service can listen but still be blocked by the firewall. So also check the firewall (chapter 23) and test from another computer.\n\n```bash\ncurl -I http://server1.example.com\n```\n\n> **Check:** Is the service running, and does it listen on the right address and port?\n\n```bash\nsystemctl status sshd\nss -tlpn\n```\n> **Work tip:** `0.0.0.0:22` means \"all IPv4 addresses\". `127.0.0.1:5432` means \"only this computer\" — a database that should not be reachable from outside.\n","page":"summary/rhcsa/network/#check-ports","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-who-can-change","n":1115,"slug":"who-can-change","title":"Know who may change the network","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** root can change everything. A normal user may change the network only when they are logged in on the local console.\n\nNetworkManager asks PolicyKit who may do what. Check your own rights:\n\n```bash\nnmcli general permissions\n```\n\nA user who logs in with SSH gets `auth` or `no` for most actions. Use `sudo` or work as root to change the network over SSH.\n\n> **Watch out:** When you work over SSH, a wrong change can cut your own connection. Check the new settings twice before `nmcli con up`. On a virtual machine, keep the console open.\n","page":"summary/rhcsa/network/#who-can-change","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-set-ipv4","n":1116,"slug":"set-ipv4","title":"Change the IPv4 address with nmcli","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** NetworkManager keeps the settings in a **connection profile**. Change the profile with `nmcli connection modify`, then activate it with `nmcli connection up`.\n\n1. Find the profile name and the device.\n```bash\nnmcli connection show\n```\n```text\nNAME    UUID                                  TYPE      DEVICE\nenp1s0  3f0c3a8e-6c35-4c5a-9a0e-7f1a2b3c4d5e  ethernet  enp1s0\n```\n2. Set a fixed address, the gateway and a DNS server. `ipv4.method manual` turns off DHCP.\n```bash\nnmcli connection modify enp1s0 ipv4.method manual ipv4.addresses 192.168.122.10/24 \\\n    ipv4.gateway 192.168.122.1 ipv4.dns 192.168.122.1\n```\n3. Activate the profile again so the change is used now.\n```bash\nnmcli connection up enp1s0\n```\n\n> **Check:** The address, the route and DNS must be right.\n\n```bash\nip -br a show enp1s0\nip route show default\ncat /etc/resolv.conf\n```\n\nYou can use short words: `con` for `connection`, `mod` for `modify`, `ipv4.addresses` can also be written as `ip4`. Tab completion works for nmcli: press Tab twice to see the next possible words.\n\nTo go back to DHCP:\n\n```bash\nnmcli con mod enp1s0 ipv4.method auto ipv4.addresses \"\" ipv4.gateway \"\" ipv4.dns \"\"\nnmcli con up enp1s0\n```\n\n> **Remember:** modify, then up. Without `up`, the change waits in the profile.\n\n> **Exam tip:** Reboot at the end of the exam and check the address again. A setting that is only active (and not saved in the profile) is lost after a reboot.\n","page":"summary/rhcsa/network/#set-ipv4","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-set-ipv6","n":1117,"slug":"set-ipv6","title":"Add an IPv6 address or a second IP","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A profile can have many addresses. A `+` before the setting adds a value, a `-` removes one.\n\n1. Add a second IPv4 address to the same card.\n```bash\nnmcli con mod enp1s0 +ipv4.addresses 192.168.122.110/24\n```\n2. Add a fixed IPv6 address and gateway.\n```bash\nnmcli con mod enp1s0 ipv6.method manual ipv6.addresses 2001:db8:10::10/64 ipv6.gateway 2001:db8:10::1\n```\n3. Activate the changes.\n```bash\nnmcli con up enp1s0\n```\n\n> **Check:** \n\n```bash\nip -br a show enp1s0\nping -6 -c 2 2001:db8:10::1\n```\n\nTo remove one address again:\n\n```bash\nnmcli con mod enp1s0 -ipv4.addresses 192.168.122.110/24\n```\n\n> **Remember:** `+` adds, `-` removes, no sign replaces all values.\n","page":"summary/rhcsa/network/#set-ipv6","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-new-profile","n":1118,"slug":"new-profile","title":"Make a new connection profile","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** One device can have many profiles, but only one is active at a time. A new profile is useful when you want to switch between setups, or for a new network card.\n\n1. Make the profile. `con-name` is the profile name, `ifname` is the device.\n```bash\nnmcli con add con-name lab-static type ethernet ifname enp1s0 \\\n    ipv4.method manual ipv4.addresses 192.168.122.10/24 ipv4.gateway 192.168.122.1 ipv4.dns 192.168.122.1\n```\n2. Switch to the new profile, and back.\n```bash\nnmcli con up lab-static\nnmcli con up enp1s0\n```\n3. Delete a profile you do not need.\n```bash\nnmcli con delete lab-static\n```\n\n> **Watch out:** When two profiles for one device both have `autoconnect` on, you do not know which one starts after a reboot. Turn off the one you do not want: `nmcli con mod enp1s0 connection.autoconnect no`.\n","page":"summary/rhcsa/network/#new-profile","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-nmtui","n":1119,"slug":"nmtui","title":"Change the network with nmtui (menus)","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `nmtui` is a text menu for the same settings. It is easy when you do not remember the nmcli words.\n\n1. Start it as root.\n```bash\nnmtui\n```\n2. Choose **Edit a connection**, pick the profile, change **IPv4 CONFIGURATION** from Automatic to Manual and fill in the address with its prefix, the gateway and the DNS server.\n3. Choose **OK**, then **Back**, then **Activate a connection**. Deactivate and activate the profile so the change is used.\n\nUse the arrow keys and Tab to move, Space to tick a box, Enter to choose.\n\n> **Exam tip:** nmtui is fast and safe for one address. Use nmcli when you must type many settings or work in a script.\n","page":"summary/rhcsa/network/#nmtui","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-profile-files","n":1120,"slug":"profile-files","title":"Read and edit the profile files","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Each profile is saved as a text file (keyfile) in `/etc/NetworkManager/system-connections/`. You can read it, and edit it if you reload NetworkManager after that.\n\n```bash\nls /etc/NetworkManager/system-connections/\n```\n```text\nenp1s0.nmconnection\n```\n```bash\ncat /etc/NetworkManager/system-connections/enp1s0.nmconnection\n```\n\n**enp1s0.nmconnection**\n```text\n[connection]\nid=enp1s0\ntype=ethernet\ninterface-name=enp1s0\n\n[ipv4]\nmethod=manual\naddress1=192.168.122.10/24,192.168.122.1\ndns=192.168.122.1;\n```\n\nAfter a change by hand, tell NetworkManager to read the files again and activate the profile:\n\n```bash\nnmcli con reload\nnmcli con up enp1s0\n```\n\n> **Watch out:** The file must belong to root and have mode 600 (`-rw-------`), or NetworkManager will not use it. In RHEL 10 the old `ifcfg` files in `/etc/sysconfig/network-scripts/` are no longer used.\n","page":"summary/rhcsa/network/#profile-files","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-hostname","n":1121,"slug":"hostname","title":"Change the hostname","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `hostnamectl` changes the name of the system now and after a reboot. It writes the name to `/etc/hostname`.\n\n1. Show the current name.\n```bash\nhostnamectl\n```\n2. Set the new name. Use the full name (FQDN).\n```bash\nhostnamectl set-hostname server1.example.com\n```\n3. Check it. A new shell shows the new name in the prompt.\n```bash\nhostname\n```\n```text\nserver1.example.com\n```\n\n`nmcli general hostname server1.example.com` does the same job.\n\n> **Remember:** hostnamectl set-hostname = now + after reboot. The `hostname` command alone only changes the name until the next reboot.\n","page":"summary/rhcsa/network/#hostname","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-name-resolution","n":1122,"slug":"name-resolution","title":"Set up DNS and /etc/hosts","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Linux first looks in `/etc/hosts`, then asks the DNS servers. The order is set in `/etc/nsswitch.conf`.\n\n1. Add a name to `/etc/hosts` for a computer that has no DNS name. Format: address, full name, short names.\n**/etc/hosts**\n```text\n127.0.0.1       localhost localhost.localdomain\n::1             localhost localhost.localdomain\n192.168.122.20  server2.example.com server2\n```\n2. Set the DNS servers in the profile. NetworkManager then writes `/etc/resolv.conf` for you. Do not edit that file by hand: your change is lost on the next update.\n```bash\nnmcli con mod enp1s0 ipv4.dns \"192.168.122.1 1.1.1.1\" ipv4.dns-search example.com\nnmcli con up enp1s0\n```\n3. When DHCP also sends DNS servers and you only want yours:\n```bash\nnmcli con mod enp1s0 ipv4.ignore-auto-dns yes\n```\n\n> **Check:** Test the names. `getent` follows the same order as the system (hosts file and DNS). `dig`, `host` and `nslookup` ask DNS only.\n\n```bash\ngetent hosts server2\n```\n```text\n192.168.122.20  server2.example.com server2\n```\n```bash\ndig +short www.redhat.com\nnslookup www.redhat.com\n```\n\n**/etc/nsswitch.conf (part)**\n```text\nhosts:      files dns myhostname\n```\n\n`dig`, `host` and `nslookup` are in the package `bind-utils`.\n\n> **Remember:** hosts file first, then DNS. `getent hosts` tests both, `dig` tests only DNS.\n\n> **Work tip:** When an app cannot reach a name but `dig` works, compare with `getent hosts`. An old line in `/etc/hosts` often wins over DNS.\n","page":"summary/rhcsa/network/#name-resolution","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-services-at-boot","n":1123,"slug":"services-at-boot","title":"Start network services at boot","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The network itself starts at boot when NetworkManager is enabled and the profile has `autoconnect` on. Other network services (like SSH or a web server) start at boot when you `enable` them with systemd.\n\n1. Make sure NetworkManager runs now and at boot.\n```bash\nsystemctl enable --now NetworkManager\n```\n2. Make sure the profile starts by itself.\n```bash\nnmcli con mod enp1s0 connection.autoconnect yes\n```\n3. Start, stop and check a network service, and make it start at boot.\n```bash\nsystemctl status sshd\nsystemctl restart sshd\nsystemctl enable --now sshd\n```\n\n> **Check:** After a reboot, the profile and the service must be active.\n\n```bash\nnmcli -f NAME,DEVICE,STATE con show --active\nsystemctl is-enabled sshd\nsystemctl is-active sshd\n```\n\n> **Remember:** `enable` = start at boot. `--now` = also start it right now.\n","page":"summary/rhcsa/network/#services-at-boot","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-main-points","n":1124,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- An IPv4 address has 4 numbers; the **prefix** (/24) tells which part is the network. IPv6 addresses are 128 bits, written in hex.\n- `ip a` shows addresses, `ip route` the routes, `ss -tulpn` the listening ports.\n- **NetworkManager** keeps settings in **connection profiles**; change them with `nmcli` or `nmtui`. Changes made with `ip` are lost at reboot.\n- Static IPv4: `nmcli con mod NAME ipv4.method manual ipv4.addresses IP/PREFIX ipv4.gateway GW ipv4.dns DNS`, then `nmcli con up NAME`.\n- RHEL 10 saves profiles as keyfiles in `/etc/NetworkManager/system-connections/`.\n- `hostnamectl set-hostname NAME` sets the hostname for good.\n- Name lookup uses `/etc/hosts` first, then DNS (see `/etc/nsswitch.conf`); test with `getent hosts NAME`.\n- Root (or a user allowed by polkit) can change the network.\n","page":"summary/rhcsa/network/#main-points","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-key-terms","n":1125,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| IP address | The address of a network card in a network. |\n| Prefix / netmask | Which part of the address is the network (`/24` = 255.255.255.0). |\n| Gateway | The router that sends traffic to other networks. |\n| DNS | The service that turns names into IP addresses. |\n| MAC address | The hardware address of a network card. |\n| Port | A number that points to one service on a host (22 = SSH, 80 = HTTP). |\n| NetworkManager | The service that manages the network on RHEL. |\n| Connection profile | A saved set of network settings for a device. |\n| Device | A network card or other network interface, like `enp1s0`. |\n| Hostname | The name of the system. |\n","page":"summary/rhcsa/network/#key-terms","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-network-review","n":1126,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Basic system tasks","tags":["rhcsa","chapter-8"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How many usable host addresses has a /24 network?**\nA: 254.\n\n\n**Q: Which command shows all IP addresses in a short form?**\nA: `ip -br a`.\n\n\n**Q: How do you see which services listen on TCP and UDP ports?**\nA: `ss -tulpn`.\n\n\n**Q: Why is a change made with `ip addr add` not good for the exam?**\nA: It is gone after a reboot. Use `nmcli` so it is saved in the profile.\n\n\n**Q: How do you add a second IP address to a profile?**\nA: `nmcli con mod NAME +ipv4.addresses IP/PREFIX`, then `nmcli con up NAME`.\n\n\n**Q: How do you change the hostname for good?**\nA: `hostnamectl set-hostname NAME`.\n\n\n**Q: How can server1 find server2 by name without DNS?**\nA: Add a line for server2 in `/etc/hosts`.\n\n\n**Q: Which command tests name lookup the same way programs do?**\nA: `getent hosts NAME`.\n","page":"summary/rhcsa/network/#review","label":"Ch 8","where":"RHCSA 10 · Chapter 8 · Set up the network"},{"id":"sum-software","n":1127,"slug":"software","title":"Chapter 9: Install, update and remove software","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"RHEL installs software as RPM packages from repositories. This chapter shows how to connect to repositories (also your own), find, install, update and remove packages with dnf, look inside packages with rpm, and install desktop apps with Flatpak.\n\n## Lessons in this chapter\n\n- Understand repositories\n- Register the system with Red Hat\n- Connect to a repository\n- Keep packages trusted\n- Make a repository from the installation ISO\n- Find software\n- Install and remove packages\n- List packages\n- Update the system\n- Install package groups\n- Undo changes with dnf history\n- Read RPM file names\n- Query installed packages with rpm\n- Look inside a package file before installing\n- Install desktop apps with Flatpak\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 9.1\n","page":"summary/rhcsa/software/","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-repositories","n":1128,"slug":"repositories","title":"Understand repositories","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **repository** is a place (web server, folder, ISO) with packages and an index. `dnf` reads the index, solves the dependencies and installs everything that is needed.\n\n- A RHEL system gets the Red Hat repositories only after it is **registered** (subscription).\n- The two main repositories are **BaseOS** (the core OS) and **AppStream** (applications, languages, databases).\n- Repositories are described in files `/etc/yum.repos.d/*.repo`. Red Hat's own file is `redhat.repo`, managed by subscription-manager.\n\n```bash\ndnf repolist\ndnf repolist all\nls /etc/yum.repos.d/\n```\n\n> **Exam tip:** On the exam the system is normally **not** registered. You must create a `.repo` file for the repository URL they give you before you can install anything.\n","page":"summary/rhcsa/software/#repositories","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-register","n":1129,"slug":"register","title":"Register the system with Red Hat","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Registration connects the system to your Red Hat account and enables the Red Hat repositories.\n\n```bash\nsubscription-manager register --username your_rh_user\nsubscription-manager status\nsubscription-manager repos --list-enabled\nsubscription-manager repos --enable codeready-builder-for-rhel-10-x86_64-rpms\n```\n\n- A free **Red Hat Developer** subscription allows up to 16 systems for learning.\n- `subscription-manager unregister` removes the system again (do it before you delete a VM).\n","page":"summary/rhcsa/software/#register","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-repo-file","n":1130,"slug":"repo-file","title":"Connect to a repository","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A `.repo` file needs only three things: a label in `[ ]`, a `name` and a `baseurl`. Add GPG settings to check the packages.\n\n1. Create the file for each repository.\n```bash\nvim /etc/yum.repos.d/exam.repo\n```\n**/etc/yum.repos.d/exam.repo**\n```text\n[BaseOS]\nname=BaseOS\nbaseurl=http://repo.example.com/rhel10/BaseOS\ngpgcheck=0\n\n[AppStream]\nname=AppStream\nbaseurl=http://repo.example.com/rhel10/AppStream\ngpgcheck=0\n```\n2. Or let dnf write the file for you.\n```bash\ndnf config-manager --add-repo http://repo.example.com/rhel10/BaseOS\n```\n3. Check that dnf sees the repositories.\n```bash\ndnf repolist\n```\n\n| Key | Meaning |\n|---|---|\n| `baseurl` | URL of the repo: `http://`, `https://`, `ftp://` or `file:///path` |\n| `enabled` | `1` = use it (default), `0` = do not use it |\n| `gpgcheck` | `1` = check the package signatures |\n| `gpgkey` | Where the public key is, like `file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release` |\n\n> **Watch out:** `dnf config-manager --add-repo` does not add a `gpgcheck` line. With the default `gpgcheck=1` from `/etc/dnf/dnf.conf`, install fails if the key is unknown. Add `gpgcheck=0` (lab only) or the right `gpgkey`.\n","page":"summary/rhcsa/software/#repo-file","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-repo-security","n":1131,"slug":"repo-security","title":"Keep packages trusted","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Packages are signed with a GPG key. dnf checks the signature, so nobody can give you a changed package.\n\n```bash\nrpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release\nrpm -q gpg-pubkey --qf '%{NAME}-%{VERSION}-%{RELEASE}\\t%{SUMMARY}\\n'\nrpm -K package.rpm\n```\n\n> **Work tip:** In production keep `gpgcheck=1` and `repo_gpgcheck` where the repository supports it. `gpgcheck=0` is for labs and the exam only.\n","page":"summary/rhcsa/software/#repo-security","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-own-repo","n":1132,"slug":"own-repo","title":"Make a repository from the installation ISO","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The RHEL installation ISO contains BaseOS and AppStream with their indexes. Mount it and point a `.repo` file to it — a full repository without internet.\n\n1. Mount the ISO for good.\n```bash\nmkdir /repo\necho \"/root/rhel-10.0-x86_64-dvd.iso /repo iso9660 defaults,loop 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a\n```\n2. Point dnf to both folders on the ISO.\n**/etc/yum.repos.d/local.repo**\n```text\n[local-BaseOS]\nname=Local BaseOS\nbaseurl=file:///repo/BaseOS\ngpgcheck=0\n\n[local-AppStream]\nname=Local AppStream\nbaseurl=file:///repo/AppStream\ngpgcheck=0\n```\n3. For your own RPM files, create the index with createrepo.\n```bash\ndnf install -y createrepo_c\ncreaterepo /srv/myrepo\n```\n\n> **Check:** The local repositories are listed.\n\n```bash\ndnf repolist\n```\n","page":"summary/rhcsa/software/#own-repo","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-find","n":1133,"slug":"find","title":"Find software","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Search by name or description, or find the package that gives a file or command.\n\n```bash\ndnf search nmap\ndnf search all \"web server\"\ndnf provides '*/semanage'\ndnf provides /usr/bin/dig\ndnf info httpd\n```\n\n> **Remember:** \"Which package has the command X?\" → `dnf provides */X`.\n","page":"summary/rhcsa/software/#find","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-install","n":1134,"slug":"install","title":"Install and remove packages","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `dnf install` adds packages with all dependencies; `dnf remove` removes them (and dependencies that nothing else needs).\n\n```bash\ndnf install -y httpd\ndnf install -y ./tool-1.2-1.el10.x86_64.rpm\ndnf reinstall -y openssh-server\ndnf remove -y httpd\ndnf autoremove\n```\n\n- `-y` answers yes to every question.\n- A local `.rpm` file with `dnf install` (not `rpm -i`) also installs the dependencies from the repositories.\n","page":"summary/rhcsa/software/#install","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-list","n":1135,"slug":"list","title":"List packages","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `dnf list` shows installed and available packages; use patterns to narrow it down.\n\n```bash\ndnf list installed\ndnf list available 'python3*'\ndnf list --upgrades\ndnf list 'selinux*'\nrpm -qa | wc -l\n```\n","page":"summary/rhcsa/software/#list","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-update","n":1136,"slug":"update","title":"Update the system","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `dnf upgrade` (or `dnf update`) installs newer versions. A new kernel is **added** next to the old one, so you can boot the old one if the new one fails.\n\n```bash\ndnf check-update\ndnf upgrade -y\ndnf upgrade -y openssh-server\ndnf upgrade --security -y\nrpm -q kernel\n```\n\n> **Work tip:** Check `dnf needs-restarting -r` after updates: it tells you if a reboot is needed (kernel, glibc, systemd).\n","page":"summary/rhcsa/software/#update","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-groups","n":1137,"slug":"groups","title":"Install package groups","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **group** is a set of packages for one job, like \"Development Tools\" or \"Server with GUI\".\n\n```bash\ndnf group list\ndnf group list --hidden\ndnf group info \"Development Tools\"\ndnf group install -y \"Development Tools\"\ndnf group remove -y \"Development Tools\"\n```\n","page":"summary/rhcsa/software/#groups","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-history","n":1138,"slug":"history","title":"Undo changes with dnf history","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** dnf remembers every transaction. You can see what changed and undo it.\n\n```bash\ndnf history\ndnf history info 12\ndnf history undo 12\n```\n\n> **Note:** `undo` works only if the old package versions are still in a repository.\n","page":"summary/rhcsa/software/#history","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-rpm-names","n":1139,"slug":"rpm-names","title":"Read RPM file names","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The file name tells name, version, release, OS and architecture.\n\n**example**\n```text\nhttpd-2.4.63-1.el10.x86_64.rpm\n```\n\n| Part | Value |\n|---|---|\n| Name | `httpd` |\n| Version (upstream) | `2.4.63` |\n| Release (Red Hat build) | `1` |\n| OS | `el10` = RHEL 10 |\n| Architecture | `x86_64` (or `noarch` for any) |\n","page":"summary/rhcsa/software/#rpm-names","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-rpm-query","n":1140,"slug":"rpm-query","title":"Query installed packages with rpm","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `rpm -q` asks the database of installed packages: which files, configs and docs a package has, and which package a file belongs to.\n\n```bash\nrpm -qa | grep httpd\nrpm -qi httpd\nrpm -ql httpd\nrpm -qc httpd\nrpm -qd httpd\nrpm -qf /etc/ssh/sshd_config\nrpm -V openssh-server\n```\n\n| Option | Shows |\n|---|---|\n| `-qa` | All installed packages |\n| `-qi` | Information |\n| `-ql` | All files |\n| `-qc` | Config files only |\n| `-qd` | Documentation files |\n| `-qf FILE` | The package of a file |\n| `-V` | Changed files (verify) |\n","page":"summary/rhcsa/software/#rpm-query","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-rpm-file","n":1141,"slug":"rpm-file","title":"Look inside a package file before installing","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Add `-p` to query an `.rpm` file instead of the database. Always check the scripts of a package from the internet.\n\n```bash\nrpm -qpi tool-1.2-1.el10.x86_64.rpm\nrpm -qpl tool-1.2-1.el10.x86_64.rpm\nrpm -qp --scripts tool-1.2-1.el10.x86_64.rpm\nrpm -qpR tool-1.2-1.el10.x86_64.rpm\n```\n\n> **Watch out:** Package scripts run as root during the install. A bad script can do anything.\n","page":"summary/rhcsa/software/#rpm-file","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-flatpak","n":1142,"slug":"flatpak","title":"Install desktop apps with Flatpak","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Flatpak installs mostly graphical apps in a sandbox, with their own runtime, independent from the RPM packages. It is not meant for server software.\n\n1. Install Flatpak and add a remote (a Flatpak repository).\n```bash\ndnf install -y flatpak\nflatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo\nflatpak remotes\n```\n2. Find and install an app.\n```bash\nflatpak search gimp\nflatpak install -y flathub org.gimp.GIMP\nflatpak list --app\n```\n3. Run, update and remove it.\n```bash\nflatpak run org.gimp.GIMP\nflatpak update -y\nflatpak uninstall -y org.gimp.GIMP\n```\n\n- Installed as root = for all users (system). With `--user` = only for that user.\n- Red Hat has its own remote with RHEL apps: `flatpak remote-add rhel https://flatpaks.redhat.io/rhel.flatpakrepo` (needs a Red Hat login).\n","page":"summary/rhcsa/software/#flatpak","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-main-points","n":1143,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- **dnf** installs, updates and removes packages and solves dependencies; `yum` is the same command.\n- Repositories are set in `.repo` files in `/etc/yum.repos.d/` with `baseurl`, `enabled` and `gpgcheck`.\n- GPG keys prove that packages are real; keep `gpgcheck=1` when you can.\n- `dnf search`, `dnf info`, `dnf provides`, `dnf list installed` find packages; `dnf history` shows and undoes transactions.\n- **Package groups** install sets of software: `dnf group list`, `dnf group install \"NAME\"`.\n- `rpm -q` asks the RPM database: `-qa` all, `-qi` info, `-ql` files, `-qf FILE` which package owns a file, `-qc` config files.\n- You can make your own repository from an ISO or folder of RPMs (`createrepo` for a folder).\n- Flatpak installs desktop apps from Flathub in their own sandbox.\n","page":"summary/rhcsa/software/#main-points","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-key-terms","n":1144,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Package | One file with a program, its files and data about it (RPM). |\n| Dependency | Another package that a package needs. |\n| Repository | A collection of packages with an index (metadata). |\n| .repo file | The settings of a repository for dnf. |\n| GPG key | The key that checks the signature of packages. |\n| Package group | A set of packages for one job, like \"Development Tools\". |\n| Module | An older way to offer more versions of software (AppStream). |\n| RPM database | The list of installed packages and their files. |\n| Transaction | One dnf action that installs, updates or removes packages. |\n| Flatpak | A way to install desktop apps with their own runtime. |\n","page":"summary/rhcsa/software/#key-terms","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-software-review","n":1145,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-9"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which three lines does a minimal .repo file need besides the [id] line?**\nA: `name=`, `baseurl=` and `enabled=1` (and `gpgcheck=0` or a `gpgkey=` line).\n\n\n**Q: How do you find which package gives the file /usr/bin/semanage?**\nA: `dnf provides /usr/bin/semanage`.\n\n\n**Q: How do you see which package an installed file belongs to?**\nA: `rpm -qf FILE`.\n\n\n**Q: How do you list the configuration files of the package openssh-server?**\nA: `rpm -qc openssh-server`.\n\n\n**Q: How do you undo the last dnf transaction?**\nA: `dnf history` to find its number, then `dnf history undo NUMBER`.\n\n\n**Q: How do you install a package group?**\nA: `dnf group install \"GROUP NAME\"`.\n\n\n**Q: How do you see all updates that are available?**\nA: `dnf check-update` (or `dnf list updates`).\n\n\n**Q: How do you use a mounted RHEL ISO as a repository?**\nA: Write .repo files with `baseurl=file:///repo/BaseOS` and `baseurl=file:///repo/AppStream` (the folders on the ISO).\n","page":"summary/rhcsa/software/#review","label":"Ch 9","where":"RHCSA 10 · Chapter 9 · Install, update and remove software"},{"id":"sum-processes","n":1146,"slug":"processes","title":"Chapter 10: Find, control and stop processes","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Every running program is a process. This chapter shows how to run jobs in the background, find processes, change their priority, send signals to stop them, watch the system with top, and tune performance with tuned.\n\n## Lessons in this chapter\n\n- Know the kinds of processes\n- Run jobs in the background\n- Understand parent and child processes\n- Show processes with ps\n- Know processes and threads\n- Understand priorities and nice values\n- Change priorities\n- Know slices and cgroups\n- Send signals to processes\n- Deal with zombies\n- Manage processes with top\n- Optimize performance with tuned\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 10.1\n","page":"summary/rhcsa/processes/","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-process-types","n":1147,"slug":"process-types","title":"Know the kinds of processes","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** There are **shell jobs** (started from your terminal) and **daemons** (services started by systemd). You manage them in different ways.\n\n| Kind | Started by | Managed with |\n|---|---|---|\n| Shell job | You, in a terminal | `jobs`, `fg`, `bg`, Ctrl+Z, Ctrl+C |\n| Daemon / service | systemd at boot | `systemctl` (next chapter) |\n| Kernel thread | The kernel | Not managed by you (names in `[ ]` in ps) |\n\nEvery process has a **PID** (process ID) and a **PPID** (the PID of its parent).\n","page":"summary/rhcsa/processes/#process-types","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-background","n":1148,"slug":"background","title":"Run jobs in the background","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A command with `&` at the end runs in the background, so your terminal stays free.\n\n```bash\nsleep 600 &\n```\n```text\n[1] 4123\n```\n```bash\ndd if=/dev/zero of=/dev/null &\njobs\n```\n```text\n[1]-  Running                 sleep 600 &\n[2]+  Running                 dd if=/dev/zero of=/dev/null &\n```\n\n| Key or command | Does |\n|---|---|\n| `COMMAND &` | Start in the background |\n| Ctrl+Z | Pause (stop) the foreground job |\n| `bg` | Continue the paused job in the background |\n| `fg` / `fg %2` | Bring a job to the foreground |\n| Ctrl+C | Stop (end) the foreground job |\n| `jobs` | List the jobs of this shell |\n\n1. Start a long job in the foreground, pause it and send it to the background.\n```bash\nsleep 300\n```\n```text\n^Z\n[1]+  Stopped                 sleep 300\n```\n```bash\nbg\n```\n```text\n[1]+ sleep 300 &\n```\n2. Bring it back and stop it.\n```bash\nfg %1\n```\n```text\n^C\n```\n\n> **Watch out:** Jobs end when you close the terminal (they get a hang-up signal). For long tasks use `nohup COMMAND &`, `tmux`, or a systemd unit.\n","page":"summary/rhcsa/processes/#background","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-parent-child","n":1149,"slug":"parent-child","title":"Understand parent and child processes","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Every process is started by a parent. When the parent ends, the children usually end too — except daemons, which belong to systemd (PID 1).\n\n```bash\nps -ef --forest | grep -A2 sshd\npstree -p | head -20\nsystemd-cgls --no-pager | head -30\n```\n","page":"summary/rhcsa/processes/#parent-child","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-ps","n":1150,"slug":"ps","title":"Show processes with ps","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `ps` takes a snapshot of the processes. Two styles are common: `ps aux` (BSD) and `ps -ef` (UNIX).\n\n```bash\nps aux | head -5\n```\n```text\nUSER   PID %CPU %MEM    VSZ   RSS TTY  STAT START   TIME COMMAND\nroot     1  0.0  0.4 174876 16420 ?    Ss   07:58   0:02 /usr/lib/systemd/systemd --switched-root --system\n```\n```bash\nps -ef | grep httpd\nps -fp 1010\nps -eo pid,ppid,user,%cpu,%mem,cmd --sort=-%mem | head\n```\n\n| Column | Meaning |\n|---|---|\n| `PID` / `PPID` | Process ID / parent ID |\n| `%CPU`, `%MEM` | Use of CPU and memory |\n| `RSS` | Real memory in KiB |\n| `STAT` | State: R running, S sleeping, D waiting for disk, T stopped, Z zombie |\n| `TIME` | Total CPU time used |\n\n> **Remember:** `ps aux --sort=-%cpu | head` = who uses the most CPU; `--sort=-%mem` = the most memory.\n","page":"summary/rhcsa/processes/#ps","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-threads","n":1151,"slug":"threads","title":"Know processes and threads","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A process can have many **threads** that share its memory. The kernel schedules threads; `ps` and `top` can show them.\n\n```bash\nps -eLf | grep chronyd\nps -o nlwp,cmd -p $(pidof chronyd)\ntop -H -p $(pidof chronyd)\n```\n","page":"summary/rhcsa/processes/#threads","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-priorities","n":1152,"slug":"priorities","title":"Understand priorities and nice values","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The **nice** value goes from -20 (highest priority) to 19 (lowest). A process with a higher nice value is \"nicer\" to others: it gets less CPU when the CPU is busy.\n\n- Normal processes start with nice 0.\n- Normal users may only make their processes **nicer** (0 → 19). Only root can lower the value (give more priority).\n- Real-time processes (shown as `rt` in top) always go before normal ones.\n","page":"summary/rhcsa/processes/#priorities","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-nice","n":1153,"slug":"nice","title":"Change priorities","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `nice` starts a command with a nice value; `renice` changes a running process.\n\n```bash\nnice -n 10 tar -czf /tmp/backup.tgz /home\nnice -n -5 important-task\nrenice -n 5 -p 4123\nrenice -n -10 -p $(pidof myapp)\nps -o pid,ni,cmd -p 4123\n```\n\n> **Exam tip:** A task like \"start the process with a lower priority\" = `nice -n 10 COMMAND`. \"Change the priority of a running process\" = `renice -n VALUE -p PID`. In top, press `r` to renice.\n","page":"summary/rhcsa/processes/#nice","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-slices","n":1154,"slug":"slices","title":"Know slices and cgroups","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** systemd puts every process in a **cgroup**. Processes are grouped into **slices**: `system.slice` (services), `user.slice` (logged-in users) and `machine.slice` (VMs, containers). CPU is shared between the slices first.\n\n```bash\nsystemd-cgls --no-pager | head -25\nsystemd-cgtop\nsystemctl status user.slice\n```\n\n> **Note:** So one busy user cannot take all CPU from the services: the slices share it.\n","page":"summary/rhcsa/processes/#slices","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-signals","n":1155,"slug":"signals","title":"Send signals to processes","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `kill` sends a **signal** to a process. The default signal asks the process to stop in a clean way.\n\n| Signal | Number | Meaning |\n|---|---|---|\n| `SIGTERM` | 15 | Please stop (default, clean: saves data) |\n| `SIGKILL` | 9 | Stop now (the kernel ends it; no cleanup) |\n| `SIGHUP` | 1 | Re-read the config (many daemons) / terminal closed |\n| `SIGINT` | 2 | Interrupt (= Ctrl+C) |\n| `SIGSTOP` / `SIGCONT` | 19 / 18 | Pause / continue |\n\n```bash\nkill 4123\nkill -9 4123\nkill -l\nkillall dd\npkill -u lisa\npkill -HUP sshd\npgrep -l -u apache\n```\n\n> **Work tip:** Always try `kill PID` (SIGTERM) first and wait a few seconds. `kill -9` can leave broken files, locks and half-written data.\n","page":"summary/rhcsa/processes/#signals","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-zombies","n":1156,"slug":"zombies","title":"Deal with zombies","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **zombie** (state `Z`) is a process that ended, but its parent did not read the exit status yet. It uses no CPU or memory, only a place in the process table. You cannot kill a zombie — it is already dead.\n\n```bash\nps -eo pid,ppid,stat,cmd | awk '$3 ~ /Z/'\nps -o ppid= -p ZOMBIE_PID\n```\n\n1. Find the parent of the zombie (PPID).\n2. Ask the parent to clean up (`kill -s SIGCHLD PARENT_PID`) or restart the parent.\n3. When the parent ends, systemd (PID 1) takes the zombie and removes it.\n","page":"summary/rhcsa/processes/#zombies","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-top","n":1157,"slug":"top","title":"Manage processes with top","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `top` shows the processes live, sorted by CPU, with the load and memory at the top. You can also kill and renice from it.\n\n```bash\ntop\ntop -u apache\ntop -b -n 1 | head -15\n```\n\n| Key in top | Does |\n|---|---|\n| `k` | Kill: send a signal to a PID |\n| `r` | Renice a PID |\n| `M` / `P` | Sort by memory / by CPU |\n| `1` | Show every CPU core |\n| `f` | Choose the columns |\n| `W` | Save the settings |\n| `q` | Quit |\n\nThe header shows the **load average** (1, 5 and 15 minutes). A load higher than the number of CPU cores means processes are waiting.\n\n```bash\nuptime\n```\n```text\n 10:52:01 up  2:54,  2 users,  load average: 0.15, 0.10, 0.09\n```\n```bash\nnproc\n```\n```text\n2\n```\n```bash\nfree -h\n```\n\n> **Note:** In the `%Cpu(s)` line, `wa` is the time the CPU waits for disks. A high `wa` means slow storage, not a busy CPU.\n","page":"summary/rhcsa/processes/#top","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-tuned","n":1158,"slug":"tuned","title":"Optimize performance with tuned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `tuned` sets many kernel and hardware settings at once with a **profile** for a type of system.\n\n```bash\ndnf install -y tuned\nsystemctl enable --now tuned\ntuned-adm list\ntuned-adm active\ntuned-adm recommend\ntuned-adm profile virtual-guest\ntuned-adm off\n```\n\n| Profile | For |\n|---|---|\n| `balanced` | Default for desktops and laptops |\n| `throughput-performance` | Servers with high load |\n| `virtual-guest` | Virtual machines (default for RHEL VMs) |\n| `virtual-host` | Hosts that run VMs |\n| `powersave` | Save energy |\n| `latency-performance`, `network-latency` | Low latency |\n\n> **Exam tip:** \"Set the recommended tuned profile\" = `tuned-adm recommend`, then `tuned-adm profile NAME`. Check with `tuned-adm active`.\n","page":"summary/rhcsa/processes/#tuned","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-main-points","n":1159,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- A command with `&` runs in the **background**; `jobs`, `fg`, `bg` and Ctrl+Z control shell jobs.\n- Every process has a **PID** and a parent (**PPID**); `ps aux`, `ps -ef` and `pstree` show them.\n- `top` shows live CPU and memory use; press `k` to kill, `r` to renice, `M` to sort by memory.\n- **Nice** values go from -20 (high priority) to 19 (low); `nice -n 10 CMD` starts low, `renice -n 5 -p PID` changes a running process. Only root can raise priority.\n- **Signals**: 15 SIGTERM asks a process to stop, 9 SIGKILL forces it, 1 SIGHUP often means \"read the config again\". Use `kill`, `killall`, `pkill`.\n- systemd puts processes in **slices** (cgroups) to share resources.\n- **tuned** sets performance profiles: `tuned-adm recommend`, `tuned-adm profile NAME`.\n","page":"summary/rhcsa/processes/#main-points","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-key-terms","n":1160,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Process | A running program with its own PID. |\n| Thread | A part of a process that runs at the same time as other parts. |\n| Job | A process started from the current shell. |\n| PID / PPID | Process ID / ID of the parent process. |\n| Nice value | The priority of a process for the CPU (-20 to 19). |\n| Signal | A message to a process, like stop or reload. |\n| Zombie | A finished process whose parent did not read its exit status yet. |\n| Load average | The average number of processes waiting to run (1, 5, 15 minutes). |\n| cgroup / slice | A group of processes that shares limits for CPU and memory. |\n| tuned profile | A set of system settings for one kind of work. |\n","page":"summary/rhcsa/processes/#key-terms","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-processes-review","n":1161,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-10"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you move a running command to the background?**\nA: Press Ctrl+Z to stop it, then type `bg`.\n\n\n**Q: How do you find the 5 processes that use the most memory?**\nA: `ps aux --sort=-%mem | head -6` (or press `M` in top).\n\n\n**Q: How do you start a backup script with the lowest priority?**\nA: `nice -n 19 /root/backup.sh`.\n\n\n**Q: How do you lower the priority of the running process 2345?**\nA: `renice -n 10 -p 2345`.\n\n\n**Q: Which signal do you send first to stop a process nicely?**\nA: SIGTERM (15), the default of `kill`.\n\n\n**Q: How do you stop all processes of user lisa?**\nA: `pkill -u lisa` (or `killall -u lisa`).\n\n\n**Q: How do you choose the tuned profile that fits this system?**\nA: `tuned-adm recommend`, then `tuned-adm profile NAME`; check with `tuned-adm active`.\n\n\n**Q: What does a load average of 4.0 mean on a system with 2 CPUs?**\nA: On average 4 processes want the CPU, so it is overloaded (about 2 wait all the time).\n","page":"summary/rhcsa/processes/#review","label":"Ch 10","where":"RHCSA 10 · Chapter 10 · Find, control and stop processes"},{"id":"sum-services","n":1162,"slug":"services","title":"Chapter 11: Start, stop and configure services with systemd","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"systemd starts everything after the kernel: services, mounts, sockets and targets. This chapter shows how to read units, start and enable services, check their state, change their settings safely and understand dependencies.\n\n## Lessons in this chapter\n\n- Understand systemd and units\n- Find the unit files\n- Read a service unit\n- Know mount and socket units\n- Understand targets\n- Start, stop and enable services\n- Check the state of a service\n- Block a service with mask\n- Understand dependencies\n- Change service settings safely\n- Write a simple service of your own\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 11.1\n","page":"summary/rhcsa/services/","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-units","n":1163,"slug":"units","title":"Understand systemd and units","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** systemd manages **units**. A unit is one thing to manage, described in a small text file. The file name ends with the unit type.\n\n| Type | Example | Manages |\n|---|---|---|\n| `.service` | `sshd.service` | A program that runs (daemon) |\n| `.socket` | `cockpit.socket` | A port or socket; starts the service on the first connection |\n| `.mount` | `home.mount` | A mounted file system |\n| `.target` | `multi-user.target` | A group of units (like a runlevel) |\n| `.timer` | `fstrim.timer` | A time plan that starts a service |\n| `.path`, `.slice`, `.device`, `.swap` | | Other types |\n\n```bash\nsystemctl -t help\nsystemctl list-units -t service\nsystemctl list-unit-files -t service\n```\n","page":"summary/rhcsa/services/#units","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-unit-locations","n":1164,"slug":"unit-locations","title":"Find the unit files","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Unit files live in three places. A file in `/etc` wins over the same name in `/usr/lib`.\n\n| Folder | Who writes it | Priority |\n|---|---|---|\n| `/usr/lib/systemd/system/` | Packages (do not edit) | Lowest |\n| `/run/systemd/system/` | Created at run time | Middle |\n| `/etc/systemd/system/` | You, the admin | Highest |\n\n```bash\nsystemctl cat sshd.service\nsystemctl show sshd.service -p FragmentPath,DropInPaths\n```\n\n> **Watch out:** Never edit files in `/usr/lib/systemd/system/` — a package update overwrites them. Use `systemctl edit` (see below).\n","page":"summary/rhcsa/services/#unit-locations","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-service-unit","n":1165,"slug":"service-unit","title":"Read a service unit","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A service unit has three sections: `[Unit]` (description and order), `[Service]` (how to run it) and `[Install]` (what `enable` does).\n\n**/usr/lib/systemd/system/sshd.service (short)**\n```text\n[Unit]\nDescription=OpenSSH server daemon\nAfter=network.target sshd-keygen.target\nWants=sshd-keygen.target\n\n[Service]\nType=notify\nEnvironmentFile=-/etc/sysconfig/sshd\nExecStart=/usr/sbin/sshd -D $OPTIONS\nExecReload=/bin/kill -HUP $MAINPID\nRestart=on-failure\nRestartSec=42s\n\n[Install]\nWantedBy=multi-user.target\n```\n\n| Line | Meaning |\n|---|---|\n| `ExecStart` | The command that starts the service |\n| `Restart=on-failure` | Start it again when it crashes |\n| `After=` | Start after these units (order only) |\n| `Wants=` / `Requires=` | Also start these units (soft / hard dependency) |\n| `WantedBy=multi-user.target` | `enable` links the service into this target |\n","page":"summary/rhcsa/services/#service-unit","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-mount-socket","n":1166,"slug":"mount-socket","title":"Know mount and socket units","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** systemd turns every line in `/etc/fstab` into a `.mount` unit. A `.socket` unit listens on a port and starts the service only when a client connects.\n\n```bash\nsystemctl list-units -t mount\nsystemctl status home.mount\nsystemctl cat cockpit.socket\nsystemctl list-sockets\n```\n\n> **Note:** The name of a mount unit is the path with `-` instead of `/`: `/data/backup` → `data-backup.mount`. `systemd-escape -p /data/backup` prints it.\n","page":"summary/rhcsa/services/#mount-socket","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-targets","n":1167,"slug":"targets","title":"Understand targets","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **target** is a group of units that brings the system into a state. They replace the old runlevels.\n\n| Target | State | Old runlevel |\n|---|---|---|\n| `graphical.target` | Multi-user with a desktop | 5 |\n| `multi-user.target` | Multi-user, text only (servers) | 3 |\n| `rescue.target` | Minimal system, single user, root login | 1 |\n| `emergency.target` | Even less: root file system read-only | — |\n\n```bash\nsystemctl get-default\nsystemctl list-units -t target\nsystemctl list-dependencies multi-user.target | head\n```\n\nBoot targets are covered in the boot chapter.\n","page":"summary/rhcsa/services/#targets","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-manage","n":1168,"slug":"manage","title":"Start, stop and enable services","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `start`/`stop` change the service **now**; `enable`/`disable` decide if it starts **at boot**. `enable --now` does both.\n\n```bash\nsystemctl start httpd\nsystemctl stop httpd\nsystemctl restart httpd\nsystemctl reload httpd\nsystemctl enable httpd\nsystemctl enable --now httpd\nsystemctl disable --now httpd\n```\n\n| Command | Now | At boot |\n|---|---|---|\n| `start` / `stop` | yes | no |\n| `enable` / `disable` | no | yes |\n| `enable --now` / `disable --now` | yes | yes |\n| `reload` | re-read the config without stopping | — |\n\n> **Exam tip:** Every service you set up on the exam must also run after a reboot. Always use `systemctl enable --now SERVICE`, and check with `systemctl is-enabled SERVICE`.\n","page":"summary/rhcsa/services/#manage","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-status","n":1169,"slug":"status","title":"Check the state of a service","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `systemctl status` shows if a service is loaded, enabled, active, its PID, cgroup and the last log lines.\n\n```bash\nsystemctl status sshd\n```\n```text\n● sshd.service - OpenSSH server daemon\n     Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; preset: enabled)\n     Active: active (running) since Sun 2026-10-25 07:58:30 CET; 2h 54min ago\n   Main PID: 1010 (sshd)\n```\n```bash\nsystemctl is-active sshd\nsystemctl is-enabled sshd\nsystemctl --failed\n```\n\n| Word | Meaning |\n|---|---|\n| `loaded` | The unit file was read |\n| `enabled` / `disabled` | Starts at boot / does not |\n| `static` | Cannot be enabled itself; another unit starts it |\n| `masked` | Blocked: cannot be started at all |\n| `active (running)` | Runs now |\n| `active (exited)` | Ran once and finished (OK for one-shot units) |\n| `inactive (dead)` | Not running |\n| `failed` | Crashed or could not start |\n","page":"summary/rhcsa/services/#status","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-mask","n":1170,"slug":"mask","title":"Block a service with mask","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `mask` links the unit to `/dev/null`, so nobody — not even a dependency — can start it. Use it for services that must never run together, or must never run.\n\n```bash\nsystemctl mask --now iptables\nsystemctl start iptables\n```\n```text\nFailed to start iptables.service: Unit iptables.service is masked.\n```\n```bash\nsystemctl unmask iptables\n```\n","page":"summary/rhcsa/services/#mask","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-dependencies","n":1171,"slug":"dependencies","title":"Understand dependencies","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Units can need other units. **Order** (`After`, `Before`) and **need** (`Wants`, `Requires`) are separate settings.\n\n| Setting | Meaning |\n|---|---|\n| `Requires=B` | Start B too; if B fails or stops, stop me too |\n| `Wants=B` | Start B too; I do not care if it fails |\n| `Requisite=B` | B must already be running, or I fail |\n| `Conflicts=B` | Never run together with B |\n| `After=B` / `Before=B` | Only the order of start and stop |\n\n```bash\nsystemctl list-dependencies httpd\nsystemctl list-dependencies --reverse sshd\n```\n","page":"summary/rhcsa/services/#dependencies","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-edit","n":1172,"slug":"edit","title":"Change service settings safely","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `systemctl edit` creates a **drop-in** file in `/etc/systemd/system/NAME.service.d/override.conf`. Only your changed lines go in it; the rest comes from the original file.\n\n1. Open the override editor and add your settings.\n```bash\nsystemctl edit httpd\n```\n**/etc/systemd/system/httpd.service.d/override.conf**\n```text\n[Service]\nRestart=always\nRestartSec=5s\nMemoryMax=1G\n```\n2. systemctl edit reloads systemd for you. After editing files by hand, reload yourself and restart the service.\n```bash\nsystemctl daemon-reload\nsystemctl restart httpd\n```\n3. Check the result.\n```bash\nsystemctl cat httpd\nsystemctl show httpd -p Restart,MemoryMax\n```\n\n- `systemctl edit --full NAME` copies the whole file to `/etc/systemd/system/` (harder to update later).\n- `systemctl revert NAME` removes your changes.\n\n> **Work tip:** Useful hardening lines for your own services: `NoNewPrivileges=yes`, `ProtectSystem=strict`, `ProtectHome=yes`, `PrivateTmp=yes`. `systemd-analyze security NAME` rates a service.\n","page":"summary/rhcsa/services/#edit","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-own-service","n":1173,"slug":"own-service","title":"Write a simple service of your own","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Any program or script can become a service with a small unit file in `/etc/systemd/system/`.\n\n1. Create the unit file.\n**/etc/systemd/system/hello.service**\n```text\n[Unit]\nDescription=Hello demo service\nAfter=network-online.target\n\n[Service]\nExecStart=/usr/bin/bash -c 'while true; do echo \"hello $(date)\"; sleep 60; done'\nRestart=on-failure\n\n[Install]\nWantedBy=multi-user.target\n```\n2. Load it, start it and enable it.\n```bash\nsystemctl daemon-reload\nsystemctl enable --now hello\n```\n3. See its output in the journal.\n```bash\njournalctl -u hello -f\n```\n","page":"summary/rhcsa/services/#own-service","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-main-points","n":1174,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- systemd starts and watches everything as **units**: services, sockets, mounts, timers, targets and more.\n- Unit files live in `/usr/lib/systemd/system` (from packages) and `/etc/systemd/system` (your changes win).\n- `systemctl start|stop|restart|reload NAME` works now; `enable|disable` works at the next boot; `enable --now` does both.\n- `systemctl status NAME` shows state, the main PID and the last log lines.\n- `mask` makes a unit impossible to start; `unmask` undoes it.\n- Change a unit with `systemctl edit NAME` (a drop-in file); `systemctl daemon-reload` after you edit unit files by hand.\n- Dependencies: `Wants=` and `Requires=` say what else must run; `After=` and `Before=` set the order.\n- A target (like `multi-user.target`) is a group of units for a system state.\n","page":"summary/rhcsa/services/#main-points","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-key-terms","n":1175,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| systemd | The first process (PID 1); it starts and manages all units. |\n| Unit | One thing systemd manages, like a service or a mount. |\n| Unit file | The text file that describes a unit. |\n| Service | A unit that runs a program (a daemon). |\n| Socket unit | A unit that listens on a port and starts a service when needed. |\n| Target | A group of units for one system state. |\n| Enable | Start a unit automatically at boot. |\n| Mask | Block a unit so nothing can start it. |\n| Drop-in file | A small file that changes a part of a unit file. |\n| Daemon | A program that runs in the background as a service. |\n","page":"summary/rhcsa/services/#key-terms","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-services-review","n":1176,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-11"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you start httpd now and at every boot with one command?**\nA: `systemctl enable --now httpd`.\n\n\n**Q: How do you check if a service starts at boot?**\nA: `systemctl is-enabled NAME`.\n\n\n**Q: What is the difference between `disable` and `mask`?**\nA: `disable` only stops the automatic start; `mask` blocks every start, also by hand or by other units.\n\n\n**Q: Where do you put your own changes to a unit so updates do not remove them?**\nA: In a drop-in under `/etc/systemd/system/NAME.service.d/`, made with `systemctl edit NAME`.\n\n\n**Q: What must you run after changing a unit file by hand?**\nA: `systemctl daemon-reload`.\n\n\n**Q: How do you list all services that failed?**\nA: `systemctl --failed` (or `systemctl list-units --state=failed`).\n\n\n**Q: How do you see which units a target pulls in?**\nA: `systemctl list-dependencies TARGET`.\n\n\n**Q: How do you make a service restart by itself when it crashes?**\nA: Add `Restart=on-failure` in a drop-in (`systemctl edit NAME`), then restart it.\n","page":"summary/rhcsa/services/#review","label":"Ch 11","where":"RHCSA 10 · Chapter 11 · Start, stop and configure services with systemd"},{"id":"sum-scheduling","n":1177,"slug":"scheduling","title":"Chapter 12: Run tasks on a schedule","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Some work must run at fixed times (backups, reports) or once later (a reboot tonight). RHEL has three tools for this: systemd timers, cron and at. This chapter shows all three, plus anacron and how to control who may use them.\n\n## Lessons in this chapter\n\n- Choose the right scheduler\n- Use systemd timers\n- Manage the cron service\n- Read the cron time format\n- Create cron jobs\n- Understand anacron\n- Control who may use cron\n- Run a task once with at\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 12.1\n","page":"summary/rhcsa/scheduling/","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-options","n":1178,"slug":"options","title":"Choose the right scheduler","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Pick the tool by the kind of task.\n\n| Tool | For | Example |\n|---|---|---|\n| systemd timer | Repeating tasks, with logs in the journal and dependencies | Clean temp files every day |\n| cron | Repeating tasks, simple one-line format | Backup every night at 02:00 |\n| anacron | Daily/weekly jobs on machines that are not always on | Laptop: run once a day when it is on |\n| at | One task, once, at a later time | Restart a service tonight at 23:00 |\n","page":"summary/rhcsa/scheduling/#options","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-timers","n":1179,"slug":"timers","title":"Use systemd timers","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **timer** unit starts a **service** unit with the same name at the times you set. Logs go to the journal.\n\n```bash\nsystemctl list-timers --all\nsystemctl cat fstrim.timer\nsystemctl cat logrotate.timer\n```\n\n1. Create the service that does the work.\n**/etc/systemd/system/backup-etc.service**\n```text\n[Unit]\nDescription=Back up /etc\n\n[Service]\nType=oneshot\nExecStart=/usr/bin/tar -czf /var/backup/etc-%H.tgz /etc\n```\n2. Create the timer with the same name.\n**/etc/systemd/system/backup-etc.timer**\n```text\n[Unit]\nDescription=Back up /etc every day at 02:30\n\n[Timer]\nOnCalendar=*-*-* 02:30:00\nPersistent=true\n\n[Install]\nWantedBy=timers.target\n```\n3. Enable the **timer** (not the service) and check it.\n```bash\nmkdir -p /var/backup\nsystemctl daemon-reload\nsystemctl enable --now backup-etc.timer\nsystemctl list-timers backup-etc.timer\n```\n\n| Timer setting | Meaning |\n|---|---|\n| `OnCalendar=daily` | Every day at 00:00 (also `hourly`, `weekly`, `Mon..Fri 08:00`) |\n| `OnBootSec=15min` | 15 minutes after boot |\n| `OnUnitActiveSec=1h` | 1 hour after the last run |\n| `Persistent=true` | Run missed jobs after the machine was off |\n| `RandomizedDelaySec=10m` | Start up to 10 minutes later at random |\n\n> **Remember:** Test a time expression with `systemd-analyze calendar 'Mon..Fri 08:00'`.\n","page":"summary/rhcsa/scheduling/#timers","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-crond","n":1180,"slug":"crond","title":"Manage the cron service","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The `crond` daemon checks every minute if a job must run. It is installed and enabled on RHEL by default.\n\n```bash\nsystemctl status crond\nls /etc/cron.d/ /etc/cron.daily/ /var/spool/cron/\n```\n","page":"summary/rhcsa/scheduling/#crond","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-cron-time","n":1181,"slug":"cron-time","title":"Read the cron time format","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A cron line has 5 time fields, then the command.\n\n**crontab line**\n```text\n# min  hour  day-of-month  month  day-of-week   command\n  30   2     *             *      *             /usr/local/bin/backup.sh\n```\n\n| Field | Values |\n|---|---|\n| Minute | 0–59 |\n| Hour | 0–23 |\n| Day of month | 1–31 |\n| Month | 1–12 or jan–dec |\n| Day of week | 0–7 (0 and 7 = Sunday) or sun–sat |\n\n| Example | Runs |\n|---|---|\n| `0 * * * *` | Every hour at minute 0 |\n| `*/15 * * * *` | Every 15 minutes |\n| `0 8-17 * * 1-5` | Every hour from 8 to 17, Monday to Friday |\n| `0 0 1 * *` | The first day of every month at midnight |\n| `30 23 * * 0` | Sunday at 23:30 |\n| `@reboot` | Once at every boot |\n\n> **Watch out:** When you fill in both day of month and day of week, cron runs when **either** matches, not both.\n","page":"summary/rhcsa/scheduling/#cron-time","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-crontab","n":1182,"slug":"crontab","title":"Create cron jobs","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Users edit their own job list with `crontab -e`. root can edit the list of any user. System jobs go in files in `/etc/cron.d/`.\n\n1. Edit your own crontab.\n```bash\ncrontab -e\n```\n2. Add a line, save and leave the editor.\n**crontab -e**\n```text\n*/10 * * * * /usr/bin/df -h / >> /var/tmp/disk.log\n```\n3. List it, and as root manage the jobs of another user.\n```bash\ncrontab -l\ncrontab -e -u lisa\ncrontab -l -u lisa\n```\n4. For system jobs, use a file in /etc/cron.d with an extra **user** field.\n**/etc/cron.d/cleanup**\n```text\n0 3 * * * root find /var/tmp -type f -mtime +7 -delete\n```\n\n| Place | Who writes it | User field |\n|---|---|---|\n| `crontab -e` (saved in `/var/spool/cron/USER`) | Each user | No |\n| `/etc/crontab`, `/etc/cron.d/*` | root, packages | Yes |\n| `/etc/cron.hourly/`, `daily/`, `weekly/`, `monthly/` | Scripts that run at that interval | — |\n\n> **Exam tip:** A task like \"user X must run Y every day at 14:23\" = `crontab -e -u X` and the line `23 14 * * * Y`. Check with `crontab -l -u X`.\n\n\n> **Work tip:** cron jobs run with a very small environment (short PATH, no aliases). Use full paths and send output to a log, or it goes to local mail.\n","page":"summary/rhcsa/scheduling/#crontab","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-anacron","n":1183,"slug":"anacron","title":"Understand anacron","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **anacron** runs the daily, weekly and monthly jobs even when the machine was off at the planned time. On RHEL, cron starts anacron every hour; anacron runs what is due.\n\n```bash\ncat /etc/anacrontab\nls /var/spool/anacron/\n```\n\n**/etc/anacrontab (part)**\n```text\n#period in days   delay in minutes   job-identifier   command\n1                 5                  cron.daily       nice run-parts /etc/cron.daily\n7                 25                 cron.weekly      nice run-parts /etc/cron.weekly\n@monthly          45                 cron.monthly     nice run-parts /etc/cron.monthly\n```\n\n> **Note:** To run a script daily in a simple way, put it (executable, without a dot in the name) in `/etc/cron.daily/`.\n","page":"summary/rhcsa/scheduling/#anacron","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-cron-security","n":1184,"slug":"cron-security","title":"Control who may use cron","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Two files decide which users may use `crontab`: `/etc/cron.allow` and `/etc/cron.deny`.\n\n| Situation | Result |\n|---|---|\n| `cron.allow` exists | Only users listed in it (and root) |\n| Only `cron.deny` exists | Everyone except the users in it (RHEL default: empty deny file) |\n\n```bash\necho lisa >> /etc/cron.deny\ncrontab -e\n```\n```text\nYou (lisa) are not allowed to use this program (crontab)\n```\n\n`at` uses the same idea with `/etc/at.allow` and `/etc/at.deny`.\n","page":"summary/rhcsa/scheduling/#cron-security","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-at","n":1185,"slug":"at","title":"Run a task once with at","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `at TIME` reads commands and runs them once at that time. The `atd` service must run.\n\n1. Make sure atd runs.\n```bash\ndnf install -y at\nsystemctl enable --now atd\n```\n2. Plan a task: type the commands, then press Ctrl+D.\n```bash\nat 23:00\n```\n```text\nat> systemctl restart httpd\nat> <EOT>\njob 3 at Sun Oct 25 23:00:00 2026\n```\n3. Plan with a pipe, list and remove jobs.\n```bash\necho \"tar -czf /tmp/home.tgz /home/lisa\" | at now + 2 hours\natq\nat -c 3\natrm 3\n```\n\n| Time form | Example |\n|---|---|\n| Clock time | `at 16:30`, `at 4pm` |\n| Relative | `at now + 30 minutes`, `at now + 2 days` |\n| Date | `at 09:00 2026-12-31`, `at noon tomorrow` |\n\n`batch` runs a job when the system load is low (under 0.8 by default).\n","page":"summary/rhcsa/scheduling/#at","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-main-points","n":1186,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- RHEL has three ways to plan jobs: **systemd timers**, **cron** (repeating jobs) and **at** (one time).\n- A cron line has 5 time fields: minute, hour, day of month, month, day of week — then the command.\n- `crontab -e` edits your own jobs; `crontab -e -u USER` edits jobs of another user (as root).\n- System cron jobs go in `/etc/crontab`, `/etc/cron.d/` and the `/etc/cron.hourly|daily|weekly|monthly` folders.\n- **anacron** runs daily, weekly and monthly jobs that were missed while the system was off.\n- `/etc/cron.allow` and `/etc/cron.deny` (and the same for at) control who may plan jobs.\n- `at 14:00` runs commands once; `atq` lists and `atrm` removes jobs.\n- A timer unit (`NAME.timer`) starts a service unit with `OnCalendar=` or `OnBootSec=`.\n","page":"summary/rhcsa/scheduling/#main-points","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-key-terms","n":1187,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| cron / crond | The service that runs planned jobs. |\n| crontab | The table of cron jobs of one user. |\n| Cron fields | The 5 time fields of a cron line. |\n| anacron | Runs periodic jobs that were missed. |\n| at | Runs a job one time at a set time. |\n| Timer unit | A systemd unit that starts a service on a schedule. |\n| OnCalendar | The time rule in a timer, like `*-*-* 02:00:00`. |\n| cron.allow / cron.deny | Lists of users who may / may not use cron. |\n","page":"summary/rhcsa/scheduling/#key-terms","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-scheduling-review","n":1188,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-12"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Write the cron time for every weekday at 14:23.**\nA: `23 14 * * 1-5`.\n\n\n**Q: How do you plan a job for user anna as root?**\nA: `crontab -e -u anna`.\n\n\n**Q: Which cron line runs a script every 15 minutes?**\nA: `*/15 * * * * /path/script.sh`.\n\n\n**Q: How do you run a command one time today at 18:00?**\nA: `echo \"COMMAND\" | at 18:00` (or type `at 18:00`, then the command, then Ctrl+D).\n\n\n**Q: How do you see the at jobs that are waiting?**\nA: `atq`.\n\n\n**Q: What does anacron do for a laptop that was off at night?**\nA: It runs the missed daily, weekly or monthly jobs after the system starts.\n\n\n**Q: How do you see all timers and when they run next?**\nA: `systemctl list-timers`.\n\n\n**Q: How do you stop user bob from using cron?**\nA: Put `bob` in `/etc/cron.deny` (or use `/etc/cron.allow` with only the allowed users).\n","page":"summary/rhcsa/scheduling/#review","label":"Ch 12","where":"RHCSA 10 · Chapter 12 · Run tasks on a schedule"},{"id":"sum-logs","n":1189,"slug":"logs","title":"Chapter 13: Read, keep and manage system logs","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Logs tell you what happened and why something failed. RHEL has two log systems that work together: the systemd journal and rsyslog. This chapter shows how to read logs, search the journal, keep it after a reboot, send messages to your own files, rotate logs and write test messages.\n\n## Lessons in this chapter\n\n- Know the two log systems\n- Read the classic log files\n- Understand a log line\n- Watch logs live\n- Search the journal with journalctl\n- Keep the journal after a reboot\n- Configure rsyslog\n- Know facilities and priorities\n- Rotate log files\n- Write test messages with logger\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 13.1\n","page":"summary/rhcsa/logs/","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-log-systems","n":1190,"slug":"log-systems","title":"Know the two log systems","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **systemd-journald** collects all messages (kernel, services, boot) in a binary journal. **rsyslogd** reads them and writes text files in `/var/log/`.\n\n| | journald | rsyslogd |\n|---|---|---|\n| Storage | Binary journal (`/run/log/journal` or `/var/log/journal`) | Text files in `/var/log/` |\n| Read with | `journalctl` | `less`, `tail`, `grep` |\n| Strong at | Search by unit, time, priority, boot | Sending logs to other servers, long-term files |\n\n```bash\nsystemctl status systemd-journald rsyslog\nls /var/log/\n```\n","page":"summary/rhcsa/logs/#log-systems","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-log-files","n":1191,"slug":"log-files","title":"Read the classic log files","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important text logs on RHEL:\n\n| File | Contains |\n|---|---|\n| `/var/log/messages` | Most system messages |\n| `/var/log/secure` | Logins, sudo, su, SSH (security) |\n| `/var/log/audit/audit.log` | Audit and SELinux events (written by auditd) |\n| `/var/log/boot.log` | Messages of services at boot |\n| `/var/log/cron` | cron jobs |\n| `/var/log/maillog` | Mail server |\n| `/var/log/httpd/` | Apache access and error logs |\n| `/var/log/dnf.log` | Package changes |\n\n```bash\nless /var/log/messages\ngrep -i 'failed password' /var/log/secure | tail\n```\n","page":"summary/rhcsa/logs/#log-files","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-log-line","n":1192,"slug":"log-line","title":"Understand a log line","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A syslog line has: time, host, program[PID], message.\n\n**/var/log/secure**\n```text\nOct 25 09:02:11 server1 sshd[2215]: Accepted publickey for admin from 192.168.122.1 port 51322 ssh2: ED25519 SHA256:k9...\n```\n\n| Part | Value |\n|---|---|\n| Time | `Oct 25 09:02:11` |\n| Host | `server1` |\n| Program [PID] | `sshd[2215]` |\n| Message | `Accepted publickey for admin …` |\n","page":"summary/rhcsa/logs/#log-line","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-live","n":1193,"slug":"live","title":"Watch logs live","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Follow a log while you test something — you see the error the moment it happens.\n\n```bash\ntail -f /var/log/messages\njournalctl -f\njournalctl -fu httpd\n```\n\n> **Work tip:** Open a second terminal with `journalctl -f` before you restart a service or change a config. Errors appear right away.\n","page":"summary/rhcsa/logs/#live","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-journalctl","n":1194,"slug":"journalctl","title":"Search the journal with journalctl","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `journalctl` filters by unit, time, priority, boot and more. Filters can be combined.\n\n```bash\njournalctl -u sshd\njournalctl -p err\njournalctl --since today\njournalctl --since \"2026-10-25 08:00\" --until \"2026-10-25 09:00\"\njournalctl -b\njournalctl -b -1 -p warning\njournalctl -k\njournalctl _UID=1001\njournalctl -o verbose -n 1\n```\n\n| Option | Shows |\n|---|---|\n| `-u UNIT` | One service |\n| `-p err` | Priority err and worse (also `warning`, `crit` …) |\n| `--since` / `--until` | A time window (`today`, `yesterday`, `-1h`, a date) |\n| `-b` / `-b -1` | This boot / the boot before |\n| `-k` | Kernel messages |\n| `-n 20` | Last 20 lines |\n| `-x` | Explanations for some messages |\n| `-o verbose` | All fields of an entry |\n| `FIELD=VALUE` | Filter on a field, like `_PID=1010`, `_COMM=sudo` |\n\n> **Remember:** Type `journalctl -u ` and press Tab twice: bash completes unit names. In `-o verbose` you see all fields you can filter on.\n","page":"summary/rhcsa/logs/#journalctl","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-persistent","n":1195,"slug":"persistent","title":"Keep the journal after a reboot","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** By default the journal may live only in memory (`/run/log/journal`) and is lost at reboot. With a folder `/var/log/journal` it is stored on disk.\n\n1. Check where the journal is now.\n```bash\njournalctl --list-boots\nls -d /run/log/journal /var/log/journal\n```\n2. Make it persistent: create a drop-in folder and set `Storage=persistent` in a file there (or simply create `/var/log/journal`).\n```bash\nmkdir -p /etc/systemd/journald.conf.d\n```\n**/etc/systemd/journald.conf.d/persistent.conf**\n```text\n[Journal]\nStorage=persistent\nSystemMaxUse=500M\n```\n3. Restart journald and flush the memory journal to disk.\n```bash\nsystemctl restart systemd-journald\njournalctl --flush\n```\n\n> **Check:** More than one boot is listed after a reboot.\n\n```bash\njournalctl --list-boots\n```\n\n| Storage= | Meaning |\n|---|---|\n| `auto` | Disk if `/var/log/journal` exists, else memory (default) |\n| `persistent` | Disk; creates the folder |\n| `volatile` | Memory only |\n| `none` | Keep nothing (only forward to rsyslog) |\n\n> **Exam tip:** \"The journal must survive a reboot\" = Storage=persistent (or `mkdir /var/log/journal`) and a restart of systemd-journald. Check with `journalctl --list-boots` after a reboot.\n","page":"summary/rhcsa/logs/#persistent","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-rsyslog","n":1196,"slug":"rsyslog","title":"Configure rsyslog","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `/etc/rsyslog.conf` (and files in `/etc/rsyslog.d/`) decide which messages go where. A rule is: `FACILITY.PRIORITY   DESTINATION`.\n\n**/etc/rsyslog.conf (part)**\n```text\n*.info;mail.none;authpriv.none;cron.none    /var/log/messages\nauthpriv.*                                  /var/log/secure\nmail.*                                      -/var/log/maillog\ncron.*                                      /var/log/cron\n*.emerg                                     :omusrmsg:*\n```\n\nThe config has three parts: **modules** (inputs like imjournal), **global directives**, and **rules**.\n","page":"summary/rhcsa/logs/#rsyslog","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-facilities","n":1197,"slug":"facilities","title":"Know facilities and priorities","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Every message has a **facility** (which part of the system) and a **priority** (how important).\n\n| Facilities | |\n|---|---|\n| `auth`, `authpriv` | Login and security |\n| `cron`, `mail`, `daemon`, `kern` | Cron, mail, services, kernel |\n| `user` | User programs (default of logger) |\n| `local0` … `local7` | Free for your own use |\n\n| Priority (high → low) | Number |\n|---|---|\n| `emerg` | 0 |\n| `alert` | 1 |\n| `crit` | 2 |\n| `err` | 3 |\n| `warning` | 4 |\n| `notice` | 5 |\n| `info` | 6 |\n| `debug` | 7 |\n\n`cron.err` means: cron messages with priority err **or higher**. `cron.=err` means only err. `cron.none` means none.\n\n1. Send all messages of facility local5 to their own file.\n**/etc/rsyslog.d/app.conf**\n```text\nlocal5.*    /var/log/app.log\n```\n2. Restart rsyslog and test with logger.\n```bash\nsystemctl restart rsyslog\nlogger -p local5.info \"test message for app.log\"\ntail -1 /var/log/app.log\n```\n\n> **Work tip:** Send logs to a central log server so an attacker cannot delete them on the hacked machine: `*.* @@logserver.example.com:514` (`@@` = TCP, `@` = UDP).\n","page":"summary/rhcsa/logs/#facilities","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-logrotate","n":1198,"slug":"logrotate","title":"Rotate log files","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **logrotate** renames old logs (like `messages-20261025`), compresses them and deletes the oldest, so the disk does not fill up. A systemd timer runs it every day.\n\n```bash\ncat /etc/logrotate.conf\nls /etc/logrotate.d/\nsystemctl list-timers logrotate.timer\n```\n\n**/etc/logrotate.d/app**\n```text\n/var/log/app.log {\n    weekly\n    rotate 8\n    compress\n    missingok\n    notifempty\n    create 0640 root root\n}\n```\n\n```bash\nlogrotate -d /etc/logrotate.d/app\nlogrotate -f /etc/logrotate.d/app\n```\n\n| Setting | Meaning |\n|---|---|\n| `daily` / `weekly` / `monthly` / `size 100M` | When to rotate |\n| `rotate 8` | Keep 8 old files |\n| `compress` | gzip old files |\n| `missingok`, `notifempty` | No error if missing; skip empty files |\n| `create MODE OWNER GROUP` | Make a new empty log with these rights |\n| `postrotate … endscript` | Run commands after the rotation (like a reload) |\n","page":"summary/rhcsa/logs/#logrotate","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-logger","n":1199,"slug":"logger","title":"Write test messages with logger","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `logger` writes a message to the log from the command line or a script — good to test rules and to log what your scripts do.\n\n```bash\nlogger \"backup started\"\nlogger -p user.err -t backup \"backup failed: disk full\"\njournalctl -t backup\ngrep backup /var/log/messages\n```\n","page":"summary/rhcsa/logs/#logger","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-main-points","n":1200,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- RHEL logs with **systemd-journald** (the journal) and **rsyslogd** (text files in /var/log).\n- Important files: `/var/log/messages` (general), `/var/log/secure` (logins and sudo), `/var/log/audit/audit.log` (SELinux and audit).\n- `journalctl` reads the journal: `-u UNIT`, `-f` follow, `-p err` priority, `-b` this boot, `--since \"1 hour ago\"`, `-o verbose`.\n- By default the journal lives in /run and is lost at reboot. Make it **persistent** with `Storage=persistent` in a journald drop-in (or create `/var/log/journal`), then restart journald.\n- rsyslog sends messages by **facility** and **priority** to files: `authpriv.* /var/log/secure`; own rules go in `/etc/rsyslog.d/*.conf`.\n- **logrotate** rotates, compresses and removes old log files (`/etc/logrotate.conf`, `/etc/logrotate.d/`).\n- `logger` writes your own message into the logs, good for testing.\n","page":"summary/rhcsa/logs/#main-points","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-key-terms","n":1201,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| journald | The systemd logging service that keeps the journal. |\n| Journal | The binary log of systemd, read with journalctl. |\n| rsyslogd | The service that writes log messages into text files. |\n| Facility | The kind of program that sent the message (auth, cron, mail, local0 …). |\n| Priority (severity) | How important a message is (debug … emerg). |\n| Persistent journal | A journal that is kept on disk in /var/log/journal. |\n| Log rotation | Moving old logs away and starting new files. |\n| auditd | The service that writes the audit log (also SELinux denials). |\n","page":"summary/rhcsa/logs/#key-terms","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-logs-review","n":1202,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-13"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you see all messages of sshd since the last boot?**\nA: `journalctl -u sshd -b`.\n\n\n**Q: How do you watch new journal messages live?**\nA: `journalctl -f`.\n\n\n**Q: How do you show only errors and worse?**\nA: `journalctl -p err`.\n\n\n**Q: How do you make the journal survive a reboot?**\nA: Set `Storage=persistent` in `/etc/systemd/journald.conf.d/*.conf` (or make `/var/log/journal`) and run `systemctl restart systemd-journald`.\n\n\n**Q: Which file has the failed SSH logins?**\nA: `/var/log/secure`.\n\n\n**Q: How do you send all messages of facility local5 to /var/log/app.log?**\nA: Add `local5.* /var/log/app.log` in a file in `/etc/rsyslog.d/`, then `systemctl restart rsyslog`.\n\n\n**Q: How do you test that rule?**\nA: `logger -p local5.info \"test message\"`, then look in /var/log/app.log.\n\n\n**Q: Which program keeps log files from filling the disk?**\nA: logrotate (run daily by a systemd timer).\n","page":"summary/rhcsa/logs/#review","label":"Ch 13","where":"RHCSA 10 · Chapter 13 · Read, keep and manage system logs"},{"id":"sum-storage","n":1203,"slug":"storage","title":"Chapter 14: Create partitions, file systems and swap, and mount them","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"A new disk is empty space. This chapter shows how to split it into partitions (MBR or GPT), put a file system or swap on them, change file system settings, and mount them — by hand, at boot through /etc/fstab, or with systemd mount units.\n\n## Lessons in this chapter\n\n- Know MBR and GPT\n- See the disks\n- Understand size units\n- Create an MBR partition with fdisk\n- Use extended and logical partitions\n- Create GPT partitions with fdisk\n- Create GPT partitions with parted\n- Create file systems\n- Change ext4 file system settings\n- Change XFS file system settings\n- Add a swap partition\n- Add a swap file\n- Mount file systems by hand\n- Use UUIDs and labels\n- Mount at boot with /etc/fstab\n- Mount with systemd mount units\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 14.1\n","page":"summary/rhcsa/storage/","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-mbr-gpt","n":1204,"slug":"mbr-gpt","title":"Know MBR and GPT","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **partition table** describes the parts of a disk. There are two kinds.\n\n| | MBR (msdos) | GPT |\n|---|---|---|\n| Max. disk size | 2 TiB | 8 ZiB (practically unlimited) |\n| Partitions | 4 primary (or 3 + 1 extended with logical ones) | 128 by default |\n| Backup of the table | No | Yes, at the end of the disk |\n| Boot | BIOS | UEFI (and BIOS with a BIOS boot partition) |\n\n> **Remember:** New systems and disks over 2 TiB use GPT. MBR is for old BIOS systems and small disks.\n","page":"summary/rhcsa/storage/#mbr-gpt","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-lsblk","n":1205,"slug":"lsblk","title":"See the disks","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** First find the right disk. A mistake here destroys data.\n\n```bash\nlsblk\n```\n```text\nNAME          MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS\nvda           252:0    0   20G  0 disk\n├─vda1        252:1    0    1G  0 part /boot\n└─vda2        252:2    0   19G  0 part\n  ├─rhel-root 253:0    0   17G  0 lvm  /\n  └─rhel-swap 253:1    0    2G  0 lvm  [SWAP]\nvdb           252:16   0   10G  0 disk\n```\n```bash\nlsblk -f\nfdisk -l /dev/vdb\n```\n\n| Device names | Disk type |\n|---|---|\n| `/dev/sda`, `/dev/sdb` | SATA, SAS, USB, SCSI disks |\n| `/dev/vda`, `/dev/vdb` | KVM virtual disks (virtio) |\n| `/dev/nvme0n1` (partitions `nvme0n1p1`) | NVMe SSDs |\n","page":"summary/rhcsa/storage/#lsblk","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-units","n":1206,"slug":"units","title":"Understand size units","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Disk sellers use powers of 1000 (GB), Linux tools often use powers of 1024 (GiB). The difference grows with the size.\n\n| Unit | Bytes |\n|---|---|\n| KB / KiB | 1000 / 1024 |\n| MB / MiB | 1000² / 1024² |\n| GB / GiB | 1000³ (1 000 000 000) / 1024³ (1 073 741 824) |\n| TB / TiB | 1000⁴ / 1024⁴ |\n\n> **Note:** fdisk takes `+1G` as 1 GiB. parted takes `1GB` as 1000³ bytes and `1GiB` as 1024³.\n","page":"summary/rhcsa/storage/#units","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-fdisk-mbr","n":1207,"slug":"fdisk-mbr","title":"Create an MBR partition with fdisk","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `fdisk` is a menu tool. Changes are only written when you press `w`.\n\n1. Open the disk and create an MBR table (only on an empty disk).\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): o\n```\n2. Create a primary partition of 1 GiB.\n```text\nCommand (m for help): n\nSelect (default p): p\nPartition number (1-4, default 1): 1\nFirst sector (2048-20971519, default 2048): <Enter>\nLast sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971519, default 20971519): +1G\n```\n3. Show the table, then write it.\n```text\nCommand (m for help): p\nCommand (m for help): w\n```\n4. Make sure the kernel sees the new partition.\n```bash\npartprobe /dev/vdb\nlsblk /dev/vdb\n```\n\n| fdisk key | Does |\n|---|---|\n| `m` | Help |\n| `p` | Print the table |\n| `n` | New partition |\n| `d` | Delete a partition |\n| `t` | Change the type (like 82 swap, 8e LVM on MBR) |\n| `o` / `g` | New empty MBR / GPT table |\n| `w` | Write and quit |\n| `q` | Quit without saving |\n","page":"summary/rhcsa/storage/#fdisk-mbr","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-logical","n":1208,"slug":"logical","title":"Use extended and logical partitions","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** MBR has only 4 slots. To get more than 4 partitions, make the 4th one **extended**: a container for **logical** partitions (numbered from 5).\n\n```text\nCommand (m for help): n\nSelect (default e): e\nCommand (m for help): n\nAll primary partitions are in use.\nAdding logical partition 5\n```\n\n> **Watch out:** An extended partition holds no data itself. Use all its space for logical partitions.\n","page":"summary/rhcsa/storage/#logical","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-fdisk-gpt","n":1209,"slug":"fdisk-gpt","title":"Create GPT partitions with fdisk","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The same `fdisk`, but start with `g` (new GPT table). There is no primary/extended question.\n\n```bash\nfdisk /dev/vdc\n```\n```text\nCommand (m for help): g\nCommand (m for help): n\nPartition number (1-128, default 1): <Enter>\nFirst sector (2048-20971486, default 2048): <Enter>\nLast sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971486, default 20969471): +2G\nCommand (m for help): t\nPartition type or alias (type L to list all): swap\nCommand (m for help): w\n```\n\nGPT type aliases you can type: `linux`, `swap`, `lvm`, `raid`, `uefi`.\n","page":"summary/rhcsa/storage/#fdisk-gpt","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-parted","n":1210,"slug":"parted","title":"Create GPT partitions with parted","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `parted` works in a menu or with one command line — good for scripts. It writes changes **at once** (no `w`).\n\n```bash\nparted /dev/vdc mklabel gpt\nparted /dev/vdc mkpart data xfs 1MiB 1GiB\nparted /dev/vdc mkpart swap linux-swap 1GiB 2GiB\nparted /dev/vdc print\nparted /dev/vdc unit MiB print free\n```\n\n> **Watch out:** `mklabel` on a used disk deletes the partition table at once, without a question in command-line mode.\n","page":"summary/rhcsa/storage/#parted","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-mkfs","n":1211,"slug":"mkfs","title":"Create file systems","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A partition needs a **file system** before it can store files. RHEL uses **XFS** by default; **ext4** is the other common choice; **vfat** is for USB sticks and the EFI partition.\n\n```bash\nmkfs.xfs /dev/vdb1\nmkfs.ext4 /dev/vdb2\nmkfs.vfat /dev/vdb3\nmkfs -t xfs -L data /dev/vdb1\nlsblk -f /dev/vdb\n```\n\n| | XFS | ext4 |\n|---|---|---|\n| Default on RHEL | yes | no |\n| Grow while mounted | yes | yes |\n| Shrink | **no** | yes (only unmounted) |\n| Check/repair | `xfs_repair` | `fsck.ext4` / `e2fsck` |\n\n> **Watch out:** `mkfs` on the wrong device destroys its data. Check with `lsblk -f` first; mkfs.xfs refuses to overwrite a file system unless you add `-f`.\n","page":"summary/rhcsa/storage/#mkfs","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-ext4","n":1212,"slug":"ext4","title":"Change ext4 file system settings","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `tune2fs` changes settings of ext4, `dumpe2fs` shows them all.\n\n```bash\ntune2fs -l /dev/vdb2 | head -20\ndumpe2fs -h /dev/vdb2\ntune2fs -L backup /dev/vdb2\ntune2fs -o acl,user_xattr /dev/vdb2\ntune2fs -m 1 /dev/vdb2\ntune2fs -c 30 /dev/vdb2\n```\n\n| Option | Changes |\n|---|---|\n| `-L NAME` | Label |\n| `-o OPTS` | Default mount options |\n| `-m N` | Reserved blocks for root (default 5 %) |\n| `-c N` / `-i TIME` | Check after N mounts / after a time |\n| `-U UUID` | New UUID |\n","page":"summary/rhcsa/storage/#ext4","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-xfs","n":1213,"slug":"xfs","title":"Change XFS file system settings","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** XFS has `xfs_admin` for label and UUID (only when unmounted) and `xfs_info` to show the settings.\n\n```bash\nxfs_admin -L data /dev/vdb1\nxfs_admin -l /dev/vdb1\nxfs_info /data\nxfs_admin -U generate /dev/vdb1\n```\n","page":"summary/rhcsa/storage/#xfs","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-swap-partition","n":1214,"slug":"swap-partition","title":"Add a swap partition","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **Swap** is disk space the kernel uses when RAM is full. Make a partition with the swap type, format it with `mkswap`, then switch it on.\n\n1. Create the partition (type `swap` in fdisk, or `linux-swap` in parted), then format it.\n```bash\nmkswap /dev/vdb3\n```\n2. Switch it on and check.\n```bash\nswapon /dev/vdb3\nswapon --show\nfree -h\n```\n3. Make it permanent in /etc/fstab.\n**/etc/fstab (line)**\n```text\nUUID=8f1b…   none   swap   defaults   0 0\n```\n```bash\nswapoff /dev/vdb3; swapon -a; swapon --show\n```\n","page":"summary/rhcsa/storage/#swap-partition","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-swap-file","n":1215,"slug":"swap-file","title":"Add a swap file","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Without free disk space for a partition, use a file as swap.\n\n```bash\ndd if=/dev/zero of=/swapfile bs=1M count=1024\nchmod 600 /swapfile\nmkswap /swapfile\nswapon /swapfile\necho \"/swapfile none swap defaults 0 0\" >> /etc/fstab\n```\n\n> **Note:** Use `dd` (not `fallocate`) for swap files on XFS to be safe; the file must not have holes.\n","page":"summary/rhcsa/storage/#swap-file","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-mount","n":1216,"slug":"mount","title":"Mount file systems by hand","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **Mounting** connects a file system to a folder (the mount point). `umount` disconnects it.\n\n```bash\nmkdir /data\nmount /dev/vdb1 /data\nfindmnt /data\numount /data\nmount -o ro /dev/vdb1 /data\nmount -o remount,rw /data\n```\n\n> **Watch out:** `umount` fails with \"target is busy\" when a process uses the folder (also your own shell). Leave the folder and check with `lsof /data` or `fuser -vm /data`.\n","page":"summary/rhcsa/storage/#mount","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-uuid","n":1217,"slug":"uuid","title":"Use UUIDs and labels","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Names like `/dev/vdb1` can change when you add disks. A **UUID** (unique ID) or a **label** stays the same — use them in /etc/fstab.\n\n```bash\nblkid\n```\n```text\n/dev/vdb1: LABEL=\"data\" UUID=\"4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\" TYPE=\"xfs\" PARTUUID=\"…\"\n```\n```bash\nlsblk -f\nmount UUID=\"4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\" /data\nmount LABEL=data /data\n```\n","page":"summary/rhcsa/storage/#uuid","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-fstab","n":1218,"slug":"fstab","title":"Mount at boot with /etc/fstab","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Every line in `/etc/fstab` is a file system that is mounted at boot.\n\n**/etc/fstab**\n```text\nUUID=4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93  /data     xfs   defaults        0 0\nLABEL=backup                               /backup   ext4  defaults,noexec 0 2\nUUID=8f1b…                                 none      swap  defaults        0 0\n```\n\n| Field | Meaning |\n|---|---|\n| 1 | What: UUID=, LABEL=, or a device |\n| 2 | Where: the mount point (`none` for swap) |\n| 3 | File system type |\n| 4 | Mount options: `defaults`, `ro`, `noexec`, `nosuid`, `nodev`, `_netdev`, `nofail` |\n| 5 | dump (backup flag): 0 |\n| 6 | fsck order: 0 = never; 1 = root; 2 = others (XFS: 0) |\n\n1. Add the line, then reload systemd and test **before** you reboot.\n```bash\nvim /etc/fstab\nsystemctl daemon-reload\nmount -a\nfindmnt --verify\n```\n\n> **Exam tip:** A wrong fstab line can stop the boot (emergency mode). Always run `mount -a` and `findmnt --verify` after editing. On the exam, reboot at the end to prove everything mounts.\n\n\n> **Work tip:** `nofail` lets the system boot even when that disk is missing. For a USB or network disk add `nofail` (and `_netdev` for network storage).\n","page":"summary/rhcsa/storage/#fstab","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-mount-units","n":1219,"slug":"mount-units","title":"Mount with systemd mount units","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** systemd creates a `.mount` unit for every fstab line. You can also write a mount unit yourself — the file name must match the mount point.\n\n**/etc/systemd/system/data.mount**\n```text\n[Unit]\nDescription=Data disk\n\n[Mount]\nWhat=/dev/disk/by-uuid/4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\nWhere=/data\nType=xfs\nOptions=defaults\n\n[Install]\nWantedBy=multi-user.target\n```\n\n```bash\nsystemctl daemon-reload\nsystemctl enable --now data.mount\nsystemctl status data.mount\n```\n\n> **Remember:** `/data` → `data.mount`; `/srv/web` → `srv-web.mount`. Check the name with `systemd-escape -p --suffix=mount /srv/web`.\n","page":"summary/rhcsa/storage/#mount-units","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-main-points","n":1220,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- Disks use **MBR** (old, max 4 primary partitions, 2 TiB) or **GPT** (up to 128 partitions, very large disks).\n- `lsblk` shows disks and partitions; `fdisk` and `gdisk` change them interactively; `parted` works with commands too.\n- Make a file system with `mkfs.xfs` (default on RHEL), `mkfs.ext4` or `mkfs.vfat`.\n- XFS can grow but **not shrink**; ext4 can grow and shrink.\n- Swap: make it with `mkswap`, turn it on with `swapon`; a swap file must be made with `dd` or `fallocate` and get permission 600.\n- Mount at boot with a line in `/etc/fstab`: device (best a **UUID** from `blkid`), mount point, type, options, dump, fsck order.\n- Test fstab with `mount -a` (and `findmnt --verify`) before you reboot: a wrong line can stop the boot.\n- systemd can also mount with `.mount` units.\n","page":"summary/rhcsa/storage/#main-points","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-key-terms","n":1221,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Partition | A part of a disk with its own start and end. |\n| MBR | Master Boot Record: old partition table. |\n| GPT | GUID Partition Table: modern partition table. |\n| Extended / logical partition | On MBR, a container partition that holds more partitions. |\n| File system | The structure that stores files on a partition (XFS, ext4, vfat). |\n| Mount point | The folder where a file system appears. |\n| UUID | A unique ID of a file system, safe to use in fstab. |\n| Label | A name you give a file system. |\n| Swap | Disk space used as extra memory. |\n| fstab | The file with the mounts that happen at boot. |\n","page":"summary/rhcsa/storage/#key-terms","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-storage-review","n":1222,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-14"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which partition table do you need for a 4 TiB disk?**\nA: GPT.\n\n\n**Q: How do you make an XFS file system on /dev/vdb1?**\nA: `mkfs.xfs /dev/vdb1`.\n\n\n**Q: How do you find the UUID of /dev/vdb1?**\nA: `blkid /dev/vdb1` (or `lsblk -f`).\n\n\n**Q: Write an fstab line that mounts that XFS file system on /data.**\nA: `UUID=xxxx-xxxx /data xfs defaults 0 0`.\n\n\n**Q: How do you test the new fstab line without a reboot?**\nA: `mkdir -p /data`, then `mount -a` and `findmnt /data`.\n\n\n**Q: How do you add a 1 GiB swap file?**\nA: `dd if=/dev/zero of=/swapfile bs=1M count=1024`, `chmod 600 /swapfile`, `mkswap /swapfile`, `swapon /swapfile`, and an fstab line `/swapfile none swap defaults 0 0`.\n\n\n**Q: Can you make an XFS file system smaller?**\nA: No. XFS can only grow. Use ext4 if you need to shrink.\n\n\n**Q: How do you make the kernel read a changed partition table?**\nA: `partprobe` (or `udevadm settle`).\n","page":"summary/rhcsa/storage/#review","label":"Ch 14","where":"RHCSA 10 · Chapter 14 · Create partitions, file systems and swap, and mount them"},{"id":"sum-lvm","n":1223,"slug":"lvm","title":"Chapter 15: Build and resize logical volumes (LVM)","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"LVM puts a flexible layer between disks and file systems. You can join disks into one pool, cut volumes from it, and make them bigger later without downtime. This chapter shows how to create physical volumes, volume groups and logical volumes, how to name and find them, and how to grow and shrink them.\n\n## Lessons in this chapter\n\n- Understand the LVM layers\n- Know what LVM can do\n- Create physical volumes\n- Create a volume group\n- Create logical volumes and file systems\n- Name and find LVM devices\n- Add space to a volume group\n- Grow a logical volume and its file system\n- Shrink volumes and reduce a volume group\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 15.1\n- Lab 15.2\n","page":"summary/rhcsa/lvm/","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-architecture","n":1224,"slug":"architecture","title":"Understand the LVM layers","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** LVM has three layers: disks become **physical volumes (PV)**, PVs are joined into a **volume group (VG)**, and from the VG you cut **logical volumes (LV)**. The file system goes on the LV.\n\n**LVM layers**\n```text\n file systems:   /data (xfs)        /web (ext4)\n logical vols:   lvdata 5G          lvweb 3G\n                 └────────── vgdata (VG, 12G) ──────────┘\n physical vols:  /dev/vdb1 (PV)  /dev/vdc (PV)\n```\n\n| Term | Meaning |\n|---|---|\n| PV | A disk or partition prepared for LVM |\n| VG | The storage pool made of one or more PVs |\n| LV | A \"partition\" cut from the VG; it gets a file system |\n| PE / extent | The unit of space in a VG (default 4 MiB) |\n","page":"summary/rhcsa/lvm/#architecture","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-features","n":1225,"slug":"features","title":"Know what LVM can do","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The main reasons to use LVM.\n\n- **Resize**: grow (and for ext4 also shrink) volumes, add disks to a VG while it runs.\n- **Join disks**: one LV can use space from many disks.\n- **Snapshots**: a frozen view of an LV for backups.\n- **Move data**: `pvmove` moves data off a disk while it is in use, so you can replace the disk.\n- **Thin provisioning**, RAID and cache LVs (more advanced).\n\n> **Note:** RHEL installs the root and swap on LVM by default (VG `rhel`).\n","page":"summary/rhcsa/lvm/#features","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-pv","n":1226,"slug":"pv","title":"Create physical volumes","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A PV can be a whole disk or a partition. With partitions, set the type to `lvm` first.\n\n1. (Partition way) Create a partition with type lvm.\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): n          (+5G)\nCommand (m for help): t          (lvm)\nCommand (m for help): w\n```\n2. Turn the partition (or a whole disk) into a PV.\n```bash\npvcreate /dev/vdb1\n```\n```text\n  Physical volume \"/dev/vdb1\" successfully created.\n```\n```bash\npvcreate /dev/vdc\n```\n3. Check the PVs.\n```bash\npvs\n```\n```text\n  PV         VG   Fmt  Attr PSize   PFree\n  /dev/vda2  rhel lvm2 a--  <19.00g     0\n  /dev/vdb1       lvm2 ---    5.00g  5.00g\n  /dev/vdc        lvm2 ---   10.00g 10.00g\n```\n```bash\npvdisplay /dev/vdb1\n```\n","page":"summary/rhcsa/lvm/#pv","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-vg","n":1227,"slug":"vg","title":"Create a volume group","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Join one or more PVs into a VG. You can choose the extent size with `-s` (default 4 MiB).\n\n```bash\nvgcreate vgdata /dev/vdb1 /dev/vdc\n```\n```text\n  Volume group \"vgdata\" successfully created\n```\n```bash\nvgs\n```\n```text\n  VG     #PV #LV #SN Attr   VSize   VFree\n  vgdata   2   0   0 wz--n- <14.99g <14.99g\n```\n```bash\nvgdisplay vgdata\nvgcreate -s 8M vgbig /dev/vdd\n```\n\n> **Exam tip:** A task may ask for an extent size (\"extents of 8 MiB\") and an LV size in extents (\"20 extents\"): use `vgcreate -s 8M` and `lvcreate -l 20`. Check with `vgdisplay` (PE Size).\n","page":"summary/rhcsa/lvm/#vg","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-lv","n":1228,"slug":"lv","title":"Create logical volumes and file systems","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `lvcreate` cuts an LV from the VG: with `-L` a size, with `-l` a number of extents or a percentage.\n\n1. Create the LVs.\n```bash\nlvcreate -n lvdata -L 5G vgdata\nlvcreate -n lvweb -l 50%FREE vgdata\nlvcreate -n lvlog -l 100 vgdata\nlvs\n```\n2. Put a file system on them and mount them for good.\n```bash\nmkfs.xfs /dev/vgdata/lvdata\nmkdir /data\necho \"/dev/vgdata/lvdata /data xfs defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a\n```\n\n| lvcreate size | Means |\n|---|---|\n| `-L 5G` | 5 GiB |\n| `-L 500M` | 500 MiB |\n| `-l 100` | 100 extents |\n| `-l 50%FREE` | Half of the free space in the VG |\n| `-l 100%FREE` | All free space |\n| `-l 20%VG` | 20 % of the whole VG |\n","page":"summary/rhcsa/lvm/#lv","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-naming","n":1229,"slug":"naming","title":"Name and find LVM devices","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An LV has several device names. All point to the same device-mapper device (`/dev/dm-N`).\n\n| Name | Example |\n|---|---|\n| `/dev/VG/LV` | `/dev/vgdata/lvdata` |\n| `/dev/mapper/VG-LV` | `/dev/mapper/vgdata-lvdata` |\n| `/dev/dm-N` | `/dev/dm-3` (can change; do not use it) |\n\n```bash\nls -l /dev/vgdata/ /dev/mapper/\nlsblk /dev/vdc\nlvdisplay /dev/vgdata/lvdata\n```\n\n> **Remember:** In fstab, `/dev/VG/LV` or `/dev/mapper/VG-LV` are stable names; you do not need the UUID for LVs.\n","page":"summary/rhcsa/lvm/#naming","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-vgextend","n":1230,"slug":"vgextend","title":"Add space to a volume group","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** When the VG is full, add a new PV with `vgextend`.\n\n```bash\npvcreate /dev/vdd\nvgextend vgdata /dev/vdd\nvgs vgdata\n```\n","page":"summary/rhcsa/lvm/#vgextend","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-lvextend","n":1231,"slug":"lvextend","title":"Grow a logical volume and its file system","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `lvextend -r` grows the LV **and** the file system in one step, while it is mounted.\n\n```bash\nlvextend -r -L +2G /dev/vgdata/lvdata\nlvextend -r -l +100%FREE /dev/vgdata/lvweb\nlvextend -r -L 10G /dev/vgdata/lvlog\ndf -h /data\n```\n\n| | Meaning |\n|---|---|\n| `-L +2G` | Add 2 GiB |\n| `-L 10G` | Make it 10 GiB in total |\n| `-l +100%FREE` | Add all free space of the VG |\n| `-r` | Also resize the file system (xfs_growfs or resize2fs) |\n\n1. Without `-r`, grow the file system yourself.\n```bash\nlvextend -L +2G /dev/vgdata/lvdata\nxfs_growfs /data\nresize2fs /dev/vgdata/lvweb\n```\n\n> **Exam tip:** Forgetting the file system is the classic mistake: the LV is bigger but `df -h` shows the old size. Always use `lvextend -r`.\n","page":"summary/rhcsa/lvm/#lvextend","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-reduce","n":1232,"slug":"reduce","title":"Shrink volumes and reduce a volume group","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** XFS cannot shrink. ext4 can, but only unmounted — `lvreduce -r` does the steps for you. To remove a disk from a VG, first move its data away.\n\n1. Shrink an ext4 LV (with its file system).\n```bash\nlvreduce -r -L 2G /dev/vgdata/lvweb\n```\n2. Move data off a PV and remove it from the VG.\n```bash\npvmove /dev/vdc\nvgreduce vgdata /dev/vdc\npvremove /dev/vdc\n```\n3. Remove an LV and a VG you do not need anymore.\n```bash\numount /data\nlvremove /dev/vgdata/lvdata\nvgremove vgdata\n```\n\n> **Watch out:** Shrinking an LV **without** shrinking the file system first destroys the file system. Always use `-r` (or run resize2fs first). Remove the fstab line before you delete an LV, or the next boot fails.\n","page":"summary/rhcsa/lvm/#reduce","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-main-points","n":1233,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- LVM has three layers: **physical volumes** (PV) → **volume group** (VG) → **logical volumes** (LV).\n- Commands: `pvcreate`, `vgcreate`, `lvcreate`; show with `pvs`, `vgs`, `lvs` (or the `display` versions).\n- The VG is split in **extents** (default 4 MiB); `vgcreate -s 8M` sets another extent size; `lvcreate -l 100` gives 100 extents, `-L 500M` gives a size.\n- LVs appear as `/dev/VG/LV` and `/dev/mapper/VG-LV`.\n- Make a VG bigger with `vgextend VG /dev/new-pv`.\n- Grow an LV and its file system together: `lvextend -r -L +1G /dev/VG/LV` (`-r` runs xfs_growfs or resize2fs).\n- Only ext4 can shrink: `lvreduce -r -L 3G …`. Move data off a PV with `pvmove`, then `vgreduce`.\n","page":"summary/rhcsa/lvm/#main-points","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-key-terms","n":1234,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| LVM | Logical Volume Manager: flexible storage on top of disks. |\n| Physical volume (PV) | A disk or partition prepared for LVM. |\n| Volume group (VG) | A pool of space made from one or more PVs. |\n| Logical volume (LV) | A part of a VG, used like a partition. |\n| Physical extent (PE) | The smallest block of space in a VG. |\n| Device mapper | The kernel part that makes LVs appear as devices. |\n| Snapshot | A frozen copy of an LV at one moment. |\n| Thin provisioning | LVs that can be bigger than the space really used. |\n","page":"summary/rhcsa/lvm/#key-terms","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-lvm-review","n":1235,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Running the system","tags":["rhcsa","chapter-15"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Write the commands to make LV `data` of 500 MiB in a new VG `vgdata` on /dev/vdb.**\nA: `pvcreate /dev/vdb`, `vgcreate vgdata /dev/vdb`, `lvcreate -n data -L 500M vgdata`.\n\n\n**Q: How do you make a VG with extents of 16 MiB?**\nA: `vgcreate -s 16M NAME PV`.\n\n\n**Q: How do you make an LV of exactly 50 extents?**\nA: `lvcreate -n NAME -l 50 VG`.\n\n\n**Q: How do you grow a mounted XFS LV by 1 GiB?**\nA: `lvextend -r -L +1G /dev/VG/LV`.\n\n\n**Q: The VG is full. How do you add the new disk /dev/vdc?**\nA: `pvcreate /dev/vdc` and `vgextend VG /dev/vdc` (vgextend can also make the PV for you).\n\n\n**Q: How do you see the free space in a VG?**\nA: `vgs` (column VFree) or `vgdisplay VG`.\n\n\n**Q: How do you take a disk out of a VG that holds data?**\nA: `pvmove /dev/vdb` to move the data to other PVs, then `vgreduce VG /dev/vdb`.\n\n\n**Q: Which two names can you use in fstab for LV data in VG vgdata?**\nA: `/dev/vgdata/data` or `/dev/mapper/vgdata-data` (or its UUID).\n","page":"summary/rhcsa/lvm/#review","label":"Ch 15","where":"RHCSA 10 · Chapter 15 · Build and resize logical volumes (LVM)"},{"id":"sum-kernel","n":1236,"slug":"kernel","title":"Chapter 16: Look at the kernel, drivers and modules","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"The kernel sits between the hardware and every program. This chapter shows how to see what the kernel does, how drivers are loaded as modules, how to check that hardware has a driver, how to set module and kernel parameters, and how kernel updates work.\n\n## Lessons in this chapter\n\n- Understand the role of the kernel\n- See kernel threads and drivers at work\n- Analyze what the kernel is doing\n- Understand hardware initialization\n- Manage kernel modules\n- Check that hardware has a driver\n- Set module parameters\n- Change kernel settings with sysctl\n- Upgrade the kernel\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 16.1\n","page":"summary/rhcsa/kernel/","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-role","n":1237,"slug":"role","title":"Understand the role of the kernel","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The kernel manages memory, CPU time, devices and file systems. Programs ask the kernel for everything through **system calls**. Most drivers are **kernel modules** that are loaded when needed.\n\n```bash\nuname -r\n```\n```text\n6.12.0-55.9.1.el10_0.x86_64\n```\n```bash\nuname -a\ncat /proc/version\n```\n","page":"summary/rhcsa/kernel/#role","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-threads","n":1238,"slug":"threads","title":"See kernel threads and drivers at work","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The kernel runs its own threads (shown in `[ ]` in ps). Their names often show which driver or job they belong to.\n\n```bash\nps -ef | grep '\\[' | head\nps -o pid,cmd -p 2\n```\n","page":"summary/rhcsa/kernel/#threads","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-analyze","n":1239,"slug":"analyze","title":"Analyze what the kernel is doing","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The kernel writes messages to a ring buffer: hardware found, drivers loaded, errors. Read it with `dmesg` or `journalctl -k`. `/proc` and `/sys` show live kernel data.\n\n```bash\ndmesg | less\ndmesg -T --level=err,warn\njournalctl -k -b\ncat /proc/meminfo | head -5\ncat /proc/cmdline\nls /sys/class/net/\n```\n\n| Place | Shows |\n|---|---|\n| `dmesg`, `journalctl -k` | Kernel messages |\n| `/proc/cpuinfo`, `/proc/meminfo` | CPU and memory details |\n| `/proc/cmdline` | Boot options the kernel got |\n| `/proc/PID/` | Data of each process |\n| `/sys/` | Devices, drivers and their settings |\n\n> **Remember:** `dmesg -T` shows human times. `journalctl -k` also works for earlier boots (`-b -1`) when the journal is persistent.\n","page":"summary/rhcsa/kernel/#analyze","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-hardware","n":1240,"slug":"hardware","title":"Understand hardware initialization","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** At boot the kernel finds hardware and loads drivers. Later, when you plug in a device, **udev** gets an event from the kernel, loads the right module and creates the device file in `/dev`.\n\n```bash\nudevadm monitor\nudevadm info /dev/vdb\nls /usr/lib/udev/rules.d/ /etc/udev/rules.d/\n```\n\n> **Note:** Plug in a USB stick while `udevadm monitor` runs: you see the kernel and udev events one by one.\n","page":"summary/rhcsa/kernel/#hardware","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-modules","n":1241,"slug":"modules","title":"Manage kernel modules","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Modules are driver files (`.ko`) in `/lib/modules/$(uname -r)/`. Load them with `modprobe` — it also loads the modules they depend on.\n\n```bash\nlsmod | head\nlsmod | grep -i xfs\nmodinfo xfs\nmodprobe vfat\nmodprobe -r vfat\n```\n\n| Command | Does |\n|---|---|\n| `lsmod` | List loaded modules |\n| `modinfo NAME` | Information and parameters of a module |\n| `modprobe NAME` | Load a module with its dependencies |\n| `modprobe -r NAME` | Unload a module (and unused dependencies) |\n| `insmod`, `rmmod` | Load / unload one file (no dependencies; rarely used) |\n\n1. Load a module at every boot: put its name in a file in `/etc/modules-load.d/`.\n```bash\necho \"vfat\" > /etc/modules-load.d/vfat.conf\n```\n2. Block a module from loading (for example a protocol you never use).\n**/etc/modprobe.d/blacklist-usb-storage.conf**\n```text\nblacklist usb-storage\ninstall usb-storage /bin/false\n```\n\n> **Work tip:** Blocking `usb-storage` is a common hardening step: USB sticks then do not work on the server.\n","page":"summary/rhcsa/kernel/#modules","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-drivers","n":1242,"slug":"drivers","title":"Check that hardware has a driver","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** List the devices and the driver (\"Kernel driver in use\") each one has.\n\n```bash\nlspci -k\n```\n```text\n00:02.0 VGA compatible controller: Red Hat, Inc. Virtio 1.0 GPU (rev 01)\n\tSubsystem: Red Hat, Inc. Device 1100\n\tKernel driver in use: virtio-pci\n```\n```bash\nlsusb\nlscpu\nlshw -short\n```\n\n> **Note:** A device without \"Kernel driver in use\" has no working driver. Look for the module name in `modinfo` or the hardware vendor documentation.\n","page":"summary/rhcsa/kernel/#drivers","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-module-params","n":1243,"slug":"module-params","title":"Set module parameters","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Many modules have **parameters** (options). Set them when loading, or for good in `/etc/modprobe.d/`.\n\n```bash\nmodinfo -p cdrom\ncat /sys/module/cdrom/parameters/lockdoor\nmodprobe -r cdrom\nmodprobe cdrom lockdoor=0\n```\n\n**/etc/modprobe.d/cdrom.conf**\n```text\noptions cdrom lockdoor=0\n```\n","page":"summary/rhcsa/kernel/#module-params","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-sysctl","n":1244,"slug":"sysctl","title":"Change kernel settings with sysctl","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Many kernel settings live in `/proc/sys/`. `sysctl` reads and changes them; files in `/etc/sysctl.d/` make the changes permanent.\n\n```bash\nsysctl net.ipv4.ip_forward\nsysctl -w net.ipv4.ip_forward=1\necho \"net.ipv4.ip_forward = 1\" > /etc/sysctl.d/90-forward.conf\nsysctl --system\nsysctl -a | grep swappiness\n```\n\n> **Work tip:** Common hardening settings: `kernel.kptr_restrict = 2`, `kernel.dmesg_restrict = 1`, `net.ipv4.conf.all.accept_redirects = 0`, `net.ipv4.conf.all.rp_filter = 1`.\n","page":"summary/rhcsa/kernel/#sysctl","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-upgrade","n":1245,"slug":"upgrade","title":"Upgrade the kernel","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A new kernel is **installed next to** the old ones, not over them. The boot menu lets you start an older kernel if the new one fails. By default RHEL keeps 3 kernels (`installonly_limit=3` in `/etc/dnf/dnf.conf`).\n\n```bash\ndnf upgrade -y kernel\nrpm -q kernel\ngrubby --default-kernel\ngrubby --info=ALL | grep -E '^(index|kernel|title)'\ndnf needs-restarting -r\nsystemctl reboot\n```\n\n> **Exam tip:** \"Install the new kernel and make sure the system boots with it\" = install with dnf (it becomes the default), reboot, check `uname -r`. To boot an older one by default: `grubby --set-default /boot/vmlinuz-VERSION`.\n","page":"summary/rhcsa/kernel/#upgrade","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-main-points","n":1246,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- The **kernel** talks to the hardware and shares CPU, memory and devices between processes.\n- `uname -r` shows the kernel version; `dmesg` (or `journalctl -k`) shows kernel messages.\n- `lspci`, `lsusb`, `lscpu` and `lsblk` show hardware; **udev** makes device files in /dev.\n- Drivers are **kernel modules**: `lsmod` lists, `modinfo` shows details, `modprobe NAME` loads, `modprobe -r NAME` removes.\n- Load a module at boot with a file in `/etc/modules-load.d/`; set module options in `/etc/modprobe.d/*.conf`.\n- Kernel settings live in `/proc/sys`; change them now with `sysctl -w`, and for good in `/etc/sysctl.d/*.conf` (`sysctl --system` to load).\n- A new kernel comes with `dnf update kernel`; the old kernels stay, so you can boot an older one from GRUB.\n","page":"summary/rhcsa/kernel/#main-points","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-key-terms","n":1247,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Kernel | The core of the operating system. |\n| Kernel module | A part of the kernel that can be loaded when needed, often a driver. |\n| Driver | Code that lets the kernel use one kind of hardware. |\n| udev | The service that creates device files and handles new hardware. |\n| /proc | A virtual file system with information from the kernel. |\n| sysctl | A tool to read and set kernel settings in /proc/sys. |\n| Kernel thread | Work the kernel does by itself, shown in [brackets] in ps. |\n| initramfs | A small file system with drivers that the kernel loads first at boot. |\n","page":"summary/rhcsa/kernel/#key-terms","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-kernel-review","n":1248,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-16"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you see which kernel version runs now?**\nA: `uname -r`.\n\n\n**Q: How do you load the module `dummy` and make it load at every boot?**\nA: `modprobe dummy`, and a file `/etc/modules-load.d/dummy.conf` with the word `dummy`.\n\n\n**Q: How do you see the options a module accepts?**\nA: `modinfo NAME` (the \"parm\" lines).\n\n\n**Q: How do you turn on IP forwarding for good?**\nA: A file in `/etc/sysctl.d/` with `net.ipv4.ip_forward = 1`, then `sysctl --system`.\n\n\n**Q: Where do you see the messages of the kernel at boot?**\nA: `dmesg` or `journalctl -k -b`.\n\n\n**Q: Which command lists the PCI devices with their driver?**\nA: `lspci -k`.\n\n\n**Q: What happens to the old kernel when you install a new one?**\nA: It stays installed (a few versions are kept), so you can choose it in the GRUB menu.\n\n\n**Q: Which file system shows the live kernel settings?**\nA: `/proc/sys` (part of /proc).\n","page":"summary/rhcsa/kernel/#review","label":"Ch 16","where":"RHCSA 10 · Chapter 16 · Look at the kernel, drivers and modules"},{"id":"sum-boot","n":1249,"slug":"boot","title":"Chapter 17: Control what the system boots into","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Two things decide how a RHEL system starts: the **GRUB 2** boot loader (which kernel, which kernel options) and the **systemd target** (which services). This chapter shows how to work with targets, set the default target, switch targets, and change the boot options with GRUB 2.\n\n## Lessons in this chapter\n\n- Understand systemd targets\n- Look inside a target\n- Understand wants\n- Switch to another target now\n- Set the default target\n- Understand GRUB 2\n- Know the GRUB 2 configuration files\n- Change the default boot options\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 17.1\n- Lab 17.2\n","page":"summary/rhcsa/boot/","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-targets","n":1250,"slug":"targets","title":"Understand systemd targets","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A target is a group of units that brings the system to a state. At boot, systemd starts the **default target** and everything it needs.\n\n| Target | Use |\n|---|---|\n| `graphical.target` | Desktop login (includes multi-user) |\n| `multi-user.target` | Normal server, text login, network, services |\n| `rescue.target` | Repair: single user, root login, few services, file systems mounted |\n| `emergency.target` | Minimal: root file system read-only, almost nothing started |\n| `poweroff.target`, `reboot.target` | Turn off / restart |\n\n```bash\nsystemctl get-default\nsystemctl list-units -t target\n```\n","page":"summary/rhcsa/boot/#targets","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-target-units","n":1251,"slug":"target-units","title":"Look inside a target","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A target unit file has little content. The real list of services comes from **wants**: links in `TARGET.target.wants/` folders.\n\n```bash\nsystemctl cat multi-user.target\nls /etc/systemd/system/multi-user.target.wants/\nsystemctl list-dependencies multi-user.target | head -20\n```\n\n**/usr/lib/systemd/system/multi-user.target**\n```text\n[Unit]\nDescription=Multi-User System\nDocumentation=man:systemd.special(7)\nRequires=basic.target\nConflicts=rescue.service rescue.target\nAfter=basic.target rescue.service rescue.target\nAllowIsolate=yes\n```\n\n> **Note:** `AllowIsolate=yes` means you can switch to this target with `isolate`. Only targets that are a full system state have it.\n","page":"summary/rhcsa/boot/#target-units","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-wants","n":1252,"slug":"wants","title":"Understand wants","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `systemctl enable` does nothing more than create a link in the `.wants` folder of the target named in the `[Install]` section (`WantedBy=`).\n\n```bash\nsystemctl enable httpd\n```\n```text\nCreated symlink '/etc/systemd/system/multi-user.target.wants/httpd.service' → '/usr/lib/systemd/system/httpd.service'.\n```\n```bash\nsystemctl disable httpd\n```\n```text\nRemoved '/etc/systemd/system/multi-user.target.wants/httpd.service'.\n```\n","page":"summary/rhcsa/boot/#wants","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-isolate","n":1253,"slug":"isolate","title":"Switch to another target now","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `systemctl isolate TARGET` starts that target and **stops** everything that is not part of it. Use it to go to text mode or rescue mode without a reboot.\n\n```bash\nsystemctl isolate multi-user.target\nsystemctl isolate graphical.target\nsystemctl isolate rescue.target\nsystemctl list-units -t target --state=active\n```\n\n> **Watch out:** Isolating `rescue.target` stops the network and SSH. Only do it on the console.\n","page":"summary/rhcsa/boot/#isolate","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-default-target","n":1254,"slug":"default-target","title":"Set the default target","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The default target is a link `/etc/systemd/system/default.target`. `set-default` changes it.\n\n```bash\nsystemctl get-default\n```\n```text\ngraphical.target\n```\n```bash\nsystemctl set-default multi-user.target\n```\n```text\nRemoved '/etc/systemd/system/default.target'.\nCreated symlink '/etc/systemd/system/default.target' → '/usr/lib/systemd/system/multi-user.target'.\n```\n\n> **Exam tip:** \"The system must boot into text mode\" = `systemctl set-default multi-user.target`. Check after a reboot with `systemctl get-default`.\n","page":"summary/rhcsa/boot/#default-target","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-grub2","n":1255,"slug":"grub2","title":"Understand GRUB 2","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** GRUB 2 is the first program after the firmware (BIOS or UEFI). It shows the boot menu, loads the kernel and the initramfs, and passes the **kernel command line**.\n\n- RHEL uses **BLS** (Boot Loader Specification): one small file per kernel in `/boot/loader/entries/`.\n- The main config `grub.cfg` is generated — do not edit it by hand.\n- At the boot menu, press `e` to edit an entry for one boot only, `Ctrl+X` to boot it.\n\n```bash\nls /boot/loader/entries/\ncat /boot/loader/entries/*$(uname -r).conf\ncat /proc/cmdline\n```\n","page":"summary/rhcsa/boot/#grub2","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-grub-files","n":1256,"slug":"grub-files","title":"Know the GRUB 2 configuration files","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** You change settings in `/etc/default/grub` (defaults for all kernels) or with `grubby` (per kernel). `grub2-mkconfig` writes the generated config.\n\n| File | Purpose |\n|---|---|\n| `/etc/default/grub` | Your settings: timeout, default kernel options (`GRUB_CMDLINE_LINUX`) |\n| `/etc/grub.d/` | Scripts that build grub.cfg |\n| `/boot/grub2/grub.cfg` | Generated config (BIOS and UEFI on RHEL 9+/10) |\n| `/boot/loader/entries/*.conf` | One entry per kernel (BLS) |\n| `/boot/efi/EFI/redhat/grub.cfg` | On UEFI: small stub that points to /boot/grub2/grub.cfg |\n\n**/etc/default/grub**\n```text\nGRUB_TIMEOUT=5\nGRUB_DISTRIBUTOR=\"$(sed 's, release .*$,,g' /etc/system-release)\"\nGRUB_DEFAULT=saved\nGRUB_DISABLE_SUBMENU=true\nGRUB_TERMINAL_OUTPUT=\"console\"\nGRUB_CMDLINE_LINUX=\"crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap\"\nGRUB_DISABLE_RECOVERY=\"true\"\nGRUB_ENABLE_BLSCFG=true\n```\n","page":"summary/rhcsa/boot/#grub-files","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-boot-options","n":1257,"slug":"boot-options","title":"Change the default boot options","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Add or remove kernel options for all kernels with `grubby`, or edit `/etc/default/grub` and regenerate the config.\n\n1. Change options with grubby (applies to the BLS entries right away).\n```bash\ngrubby --update-kernel=ALL --args=\"console=ttyS0\"\ngrubby --update-kernel=ALL --remove-args=\"rhgb quiet\"\ngrubby --info=DEFAULT\n```\n2. Or change the timeout or the default command line in `/etc/default/grub` and regenerate. When you changed `GRUB_CMDLINE_LINUX`, add `--update-bls-cmdline`, or the kernel entries keep their old options.\n```bash\nvim /etc/default/grub\ngrub2-mkconfig -o /boot/grub2/grub.cfg\ngrub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline\n```\n3. Choose the default kernel.\n```bash\ngrubby --default-kernel\ngrubby --set-default /boot/vmlinuz-6.12.0-55.el10.x86_64\n```\n\n> **Exam tip:** Tasks like \"remove rhgb and quiet\" or \"set the boot timeout to 10 seconds\" need GRUB changes. After `/etc/default/grub` changes, always run `grub2-mkconfig -o /boot/grub2/grub.cfg` (with `--update-bls-cmdline` for command line changes) — or use `grubby`, which changes the entries directly. Check with `cat /proc/cmdline` after a reboot.\n\n\n> **Work tip:** Protect the boot menu with a password so nobody at the console can edit kernel options: `grub2-setpassword`.\n","page":"summary/rhcsa/boot/#boot-options","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-main-points","n":1258,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- A **target** is a system state: `multi-user.target` (text, network), `graphical.target` (desktop), `rescue.target`, `emergency.target`.\n- `systemctl get-default` shows and `systemctl set-default NAME` sets the target for the next boots.\n- `systemctl isolate NAME` changes to another target now (only targets with `AllowIsolate=yes`).\n- **Wants** in `/etc/systemd/system/TARGET.wants/` decide which units a target starts (made by `systemctl enable`).\n- GRUB 2 starts the kernel; default settings are in `/etc/default/grub`, boot entries are BLS files in `/boot/loader/entries/`.\n- Change kernel options for all kernels with `grubby --update-kernel=ALL --args=\"…\"` (or `--remove-args`), or edit `/etc/default/grub` and run `grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline`.\n- At the GRUB menu press **e** to change the kernel line one time; Ctrl+X boots it.\n","page":"summary/rhcsa/boot/#main-points","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-key-terms","n":1259,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Target | A group of units for one system state. |\n| Default target | The target the system boots into. |\n| Isolate | Stop everything not needed and start a target. |\n| Wants | A soft dependency: start this too if you can. |\n| GRUB 2 | The boot loader of RHEL. |\n| BLS | Boot Loader Specification: one small file per boot entry. |\n| Kernel argument | An option given to the kernel at boot, like `quiet` or `rd.break`. |\n| grubby | A tool to change boot entries and kernel arguments. |\n","page":"summary/rhcsa/boot/#key-terms","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-boot-review","n":1260,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-17"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you make the system always boot into text mode?**\nA: `systemctl set-default multi-user.target`.\n\n\n**Q: How do you switch to the desktop now, without a reboot?**\nA: `systemctl isolate graphical.target`.\n\n\n**Q: Which target is the smallest one that still mounts the file systems?**\nA: `rescue.target` (emergency.target mounts only / read-only).\n\n\n**Q: How do you remove `rhgb quiet` from all kernels?**\nA: `grubby --update-kernel=ALL --remove-args=\"rhgb quiet\"`.\n\n\n**Q: How do you change the GRUB timeout to 10 seconds?**\nA: Set `GRUB_TIMEOUT=10` in `/etc/default/grub`, then `grub2-mkconfig -o /boot/grub2/grub.cfg`.\n\n\n**Q: How do you add a kernel option for only one boot?**\nA: Press `e` at the GRUB menu, add it to the `linux` line, press Ctrl+X.\n\n\n**Q: Which command shows the current default kernel?**\nA: `grubby --default-kernel`.\n\n\n**Q: Where are the boot entries of RHEL 10?**\nA: In `/boot/loader/entries/` (BLS files).\n","page":"summary/rhcsa/boot/#review","label":"Ch 17","where":"RHCSA 10 · Chapter 17 · Control what the system boots into"},{"id":"sum-troubleshoot","n":1261,"slug":"troubleshoot","title":"Chapter 18: Fix a system that does not boot","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"When a system does not start, you need to know which boot step failed and how to get a shell to repair it. This chapter walks through the boot procedure, kernel boot arguments, rescue targets, the rescue disk, and fixes for GRUB, the initramfs, file systems and a lost root password.\n\n## Lessons in this chapter\n\n- Follow the boot procedure\n- Pass kernel boot arguments\n- Start a troubleshooting target\n- Use a rescue disk\n- Reinstall GRUB 2\n- Fix the initramfs\n- Recover from file system problems\n- Reset the root password\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 18.1\n","page":"summary/rhcsa/troubleshoot/","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-boot-steps","n":1262,"slug":"boot-steps","title":"Follow the boot procedure","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The boot is a chain. When it breaks, the step tells you where to fix it.\n\n| Step | What happens | Typical problem → fix |\n|---|---|---|\n| 1. Firmware (BIOS/UEFI) | Hardware test, finds a boot device | Wrong boot order → firmware setup |\n| 2. GRUB 2 | Menu, loads kernel + initramfs, passes arguments | GRUB damaged → reinstall from rescue disk |\n| 3. Kernel | Starts, uses the initramfs drivers | Wrong arguments → edit in GRUB (`e`) |\n| 4. initramfs | systemd in RAM finds and mounts the root file system | Missing driver/LVM → recreate with dracut |\n| 5. switch root | Real root file system becomes `/` | Root FS damaged → fsck / xfs_repair |\n| 6. systemd | Starts the default target and its units | fstab error → emergency mode, fix fstab |\n\n```bash\njournalctl -b -p err\nsystemd-analyze critical-chain\n```\n","page":"summary/rhcsa/troubleshoot/#boot-steps","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-boot-args","n":1263,"slug":"boot-args","title":"Pass kernel boot arguments","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** At the GRUB menu press `e`, go to the line that starts with `linux`, add arguments at the end, and press `Ctrl+X` to boot. The change is for this boot only.\n\n| Argument | Result |\n|---|---|\n| `systemd.unit=rescue.target` | Rescue mode: root login, file systems mounted, no network |\n| `systemd.unit=emergency.target` | Emergency mode: root file system read-only, almost nothing started |\n| `rd.break` | Stop in the initramfs, before the real root is used (root password reset) |\n| `init=/bin/bash` | A root shell instead of systemd (last resort) |\n| `systemd.debug-shell` | A root shell on tty9 during boot (for debugging hangs) |\n| `rd.debug`, `systemd.log_level=debug` | Very detailed boot messages |\n\n> **Remember:** Remove `rhgb quiet` from the line to see the boot messages while you test.\n","page":"summary/rhcsa/troubleshoot/#boot-args","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-rescue-target","n":1264,"slug":"rescue-target","title":"Start a troubleshooting target","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **Rescue** and **emergency** mode give you a root shell with few services running. You need the root password.\n\n1. Boot with `systemd.unit=emergency.target` (or rescue). Type the root password.\n2. In emergency mode the root file system is read-only. Make it writable.\n```bash\nmount -o remount,rw /\n```\n3. Fix the problem, then continue the normal boot or reboot.\n```bash\nsystemctl default\nsystemctl reboot\n```\n\n> **Note:** Emergency mode starts automatically when a file system in `/etc/fstab` cannot be mounted.\n","page":"summary/rhcsa/troubleshoot/#rescue-target","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-rescue-disk","n":1265,"slug":"rescue-disk","title":"Use a rescue disk","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** When GRUB or the root password is broken and nothing else works, boot from the RHEL installation ISO and choose **Troubleshooting → Rescue a Red Hat Enterprise Linux system**. It finds your installation and mounts it on `/mnt/sysroot`.\n\n```bash\nchroot /mnt/sysroot\nexit\n```\n\nAfter `chroot` the commands work on the installed system as if it booted normally.\n","page":"summary/rhcsa/troubleshoot/#rescue-disk","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-grub-reinstall","n":1266,"slug":"grub-reinstall","title":"Reinstall GRUB 2","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** When the boot loader is damaged (\"no boot device\", a GRUB shell), install it again from a rescue shell.\n\n1. BIOS systems: write GRUB to the boot disk and rebuild the config.\n```bash\ngrub2-install /dev/vda\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n2. UEFI systems: reinstall the packages that hold the EFI files.\n```bash\ndnf reinstall -y grub2-efi-x64 shim-x64\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n\n> **Watch out:** Do not run `grub2-install` on UEFI systems; it does not set up Secure Boot correctly. Reinstall the packages instead.\n","page":"summary/rhcsa/troubleshoot/#grub-reinstall","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-initramfs","n":1267,"slug":"initramfs","title":"Fix the initramfs","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The initramfs has the drivers and tools to find the root file system. When it is damaged or misses a driver, the boot stops in the initramfs (\"dracut emergency shell\"). Rebuild it with `dracut`.\n\n```bash\nlsinitrd /boot/initramfs-$(uname -r).img | head\ndracut -f\ndracut -f /boot/initramfs-6.12.0-55.el10.x86_64.img 6.12.0-55.el10.x86_64\n```\n\n> **Note:** From a rescue disk, `uname -r` shows the kernel of the ISO. Use the version from `ls /lib/modules` of the installed system.\n","page":"summary/rhcsa/troubleshoot/#initramfs","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-fs-issues","n":1268,"slug":"fs-issues","title":"Recover from file system problems","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A wrong line in `/etc/fstab` or a damaged file system drops you into **emergency mode**. Fix the line, or repair the file system while it is **not mounted**.\n\n1. Fix a bad fstab line.\n```bash\nmount -o remount,rw /\nvim /etc/fstab\nsystemctl daemon-reload\nmount -a\nsystemctl reboot\n```\n2. Repair a damaged file system (unmounted).\n```bash\numount /data\nxfs_repair /dev/vgdata/lvdata\nfsck.ext4 -f /dev/vgdata/lvweb\n```\n\n| File system | Check / repair |\n|---|---|\n| XFS | `xfs_repair DEVICE` (`-n` = only check) |\n| ext4 | `fsck.ext4 -f DEVICE` or `e2fsck -f DEVICE` |\n\n> **Watch out:** Never repair a mounted file system. For the root file system, repair from a rescue disk or emergency mode with `/` read-only.\n","page":"summary/rhcsa/troubleshoot/#fs-issues","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-root-password","n":1269,"slug":"root-password","title":"Reset the root password","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Without the root password you can stop the boot in the initramfs with `rd.break`, then change the password in the real root file system. SELinux must relabel afterwards.\n\n1. Reboot, press `e` at the GRUB menu, add `rd.break` to the end of the `linux` line, press `Ctrl+X`.\n2. At the `switch_root:/#` prompt, make the real root writable and enter it.\n```bash\nmount -o remount,rw /sysroot\nchroot /sysroot\n```\n3. Set the new password and ask SELinux to relabel at the next boot.\n```bash\npasswd root\ntouch /.autorelabel\n```\n4. Leave the chroot and the initramfs shell. The system continues to boot (the relabel takes some minutes and reboots once).\n```bash\nexit\nexit\n```\n\n> **Exam tip:** The relabel step (`touch /.autorelabel`) is the one people forget. Without it, `/etc/shadow` has a wrong SELinux label and nobody can log in.\n\n\n> **Work tip:** Anybody at the console can do this. Protect the GRUB menu with `grub2-setpassword` and use disk encryption (LUKS) for systems in unsafe places.\n","page":"summary/rhcsa/troubleshoot/#root-password","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-main-points","n":1270,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- Boot steps: firmware (BIOS/UEFI) → GRUB 2 → kernel + initramfs → systemd → default target.\n- Useful kernel arguments at the GRUB menu: `systemd.unit=rescue.target`, `systemd.unit=emergency.target`, `rd.break`.\n- In **emergency** mode / is read-only: remount with `mount -o remount,rw /` to fix files like fstab.\n- **Reset a lost root password**: `rd.break` (or `init=/bin/bash`), `mount -o remount,rw /sysroot`, `chroot /sysroot`, `passwd root`, `touch /.autorelabel`, exit twice.\n- A broken GRUB can be fixed from a rescue disk with `chroot` and `grub2-install` (BIOS) or by reinstalling the GRUB packages (UEFI).\n- A broken initramfs is made again with `dracut -f`.\n- File system problems: run `xfs_repair` (XFS) or `fsck` (ext4) on an unmounted file system.\n","page":"summary/rhcsa/troubleshoot/#main-points","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-key-terms","n":1271,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| UEFI / BIOS | The firmware that starts the computer and the boot loader. |\n| initramfs | A small first file system with the drivers needed to find the real root. |\n| rd.break | A kernel argument that stops in the initramfs before the real root is used. |\n| Rescue mode | A minimal system with root login to repair things. |\n| Emergency mode | An even smaller system: only / is mounted, read-only. |\n| chroot | Run a shell with another folder as the root folder. |\n| Relabel | Set all SELinux labels again (`/.autorelabel`). |\n| dracut | The tool that builds the initramfs. |\n","page":"summary/rhcsa/troubleshoot/#key-terms","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-troubleshoot-review","n":1272,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-18"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Name the 5 big steps of the boot process.**\nA: Firmware, GRUB 2, kernel with initramfs, systemd, the default target.\n\n\n**Q: Why must you run `touch /.autorelabel` after you change the root password from rd.break?**\nA: The new /etc/shadow has no correct SELinux label; the relabel at the next boot fixes it, else nobody can log in.\n\n\n**Q: The system stops in emergency mode after an fstab change. What do you do?**\nA: Log in as root, `mount -o remount,rw /`, fix `/etc/fstab`, test with `mount -a`, reboot.\n\n\n**Q: Which kernel argument starts the system in rescue mode?**\nA: `systemd.unit=rescue.target`.\n\n\n**Q: How do you build a new initramfs for the running kernel?**\nA: `dracut -f`.\n\n\n**Q: How do you check an XFS file system?**\nA: Unmount it, then `xfs_repair /dev/DEVICE`.\n\n\n**Q: Why do you need `chroot /sysroot` after rd.break?**\nA: So commands like `passwd` work on the real system and not on the initramfs.\n\n\n**Q: How do you see why the last boot failed?**\nA: `journalctl -b -1 -p err` (needs a persistent journal).\n","page":"summary/rhcsa/troubleshoot/#review","label":"Ch 18","where":"RHCSA 10 · Chapter 18 · Fix a system that does not boot"},{"id":"sum-scripts","n":1273,"slug":"scripts","title":"Chapter 19: Automate tasks with Bash scripts","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"A shell script is a text file with commands. With variables, input, conditions and loops, a script turns a manual task into one command. This chapter covers the core elements, positional parameters, variables, if/then/else, `&&` and `||`, for, while, until, case, and how to debug a script.\n\n## Lessons in this chapter\n\n- Write your first script\n- Read input and use variables\n- Use positional parameters\n- Use tests and if then else\n- Use && and ||\n- Repeat with for\n- Repeat with while and until\n- Choose with case\n- Debug a script\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 19.1\n","page":"summary/rhcsa/scripts/","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-first-script","n":1274,"slug":"first-script","title":"Write your first script","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A script starts with the **shebang** line (`#!/bin/bash`), which says which program runs it. Make it executable and run it with its path.\n\n1. Create the file.\n**/usr/local/bin/hello.sh**\n```text\n#!/bin/bash\n# hello.sh - first script\necho \"Hello, $USER. Today is $(date +%A).\"\nexit 0\n```\n2. Make it executable and run it.\n```bash\nchmod +x /usr/local/bin/hello.sh\nhello.sh\n./hello.sh\nbash hello.sh\n```\n\n| Part | Why |\n|---|---|\n| `#!/bin/bash` | Use bash (not another shell) |\n| `# comment` | Explain what the script does |\n| `exit 0` | Tell the caller: success (other numbers = an error) |\n\n> **Remember:** Folders in `$PATH` (like `/usr/local/bin` or `~/bin`) let you start the script by name. Elsewhere use `./name.sh`.\n","page":"summary/rhcsa/scripts/#first-script","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-variables","n":1275,"slug":"variables","title":"Read input and use variables","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A variable stores a value: `NAME=value` (no spaces around `=`), and `$NAME` uses it. `read` gets input from the user. `$(command)` stores the output of a command.\n\n**input.sh**\n```text\n#!/bin/bash\nread -p \"Which folder? \" DIR\nCOUNT=$(ls -1 \"$DIR\" | wc -l)\necho \"$DIR has $COUNT entries\"\n```\n\n```bash\n./input.sh\n```\n```text\nWhich folder? /etc\n/etc has 182 entries\n```\n\n- Always put variables in double quotes: `\"$DIR\"`. Without quotes, names with spaces break.\n- `export NAME` makes a variable visible for programs the script starts.\n- `${NAME:-default}` uses `default` when NAME is empty.\n","page":"summary/rhcsa/scripts/#variables","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-parameters","n":1276,"slug":"parameters","title":"Use positional parameters","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Arguments after the script name arrive as `$1`, `$2` … The script can check how many it got.\n\n| Variable | Contains |\n|---|---|\n| `$0` | The script name |\n| `$1` … `$9`, `${10}` | The arguments |\n| `$#` | The number of arguments |\n| `\"$@\"` | All arguments, each one separate |\n| `$?` | Exit status of the last command |\n| `$$` | PID of the script |\n\n**adduser.sh**\n```text\n#!/bin/bash\n# usage: adduser.sh NAME [GROUP]\nif [ $# -lt 1 ]; then\n    echo \"usage: $0 NAME [GROUP]\" >&2\n    exit 1\nfi\nuseradd -G \"${2:-users}\" \"$1\" && echo \"user $1 created\"\n```\n","page":"summary/rhcsa/scripts/#parameters","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-if","n":1277,"slug":"if","title":"Use tests and if then else","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `if` runs commands only when a **test** succeeds (exit status 0). `test` or `[ … ]` checks files, strings and numbers.\n\n**checkfile.sh**\n```text\n#!/bin/bash\nFILE=$1\nif [ -f \"$FILE\" ]; then\n    echo \"$FILE is a file\"\nelif [ -d \"$FILE\" ]; then\n    echo \"$FILE is a folder\"\nelse\n    echo \"$FILE does not exist\"\nfi\n```\n\n| Test | True when |\n|---|---|\n| `-f FILE`, `-d DIR`, `-e PATH` | File / folder / anything exists |\n| `-r`, `-w`, `-x FILE` | Readable / writable / executable |\n| `-z \"$S\"`, `-n \"$S\"` | String empty / not empty |\n| `\"$A\" = \"$B\"`, `\"$A\" != \"$B\"` | Strings equal / not equal |\n| `$X -eq $Y`, `-ne`, `-lt`, `-le`, `-gt`, `-ge` | Numbers: equal, not equal, less, less/equal, greater, greater/equal |\n| `!` | Not |\n\n> **Watch out:** Spaces are needed inside the brackets: `[ -f \"$FILE\" ]`, not `[-f \"$FILE\"]`.\n","page":"summary/rhcsa/scripts/#if","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-and-or","n":1278,"slug":"and-or","title":"Use && and ||","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `A && B` runs B only when A worked. `A || B` runs B only when A failed. Good for short checks.\n\n```bash\nmkdir /tmp/work && cd /tmp/work\nping -c1 -W1 server2 >/dev/null || echo \"server2 is down\"\n[ -d /backup ] || mkdir /backup\ngrep -q lisa /etc/passwd && echo \"lisa exists\" || echo \"lisa missing\"\n```\n","page":"summary/rhcsa/scripts/#and-or","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-for","n":1279,"slug":"for","title":"Repeat with for","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `for` runs the same commands for each item of a list: words, files, numbers or command output.\n\n**loops.sh**\n```text\n#!/bin/bash\nfor USER in anna bob carol; do\n    useradd \"$USER\"\ndone\n\nfor FILE in /var/log/*.log; do\n    echo \"$FILE: $(wc -l < \"$FILE\") lines\"\ndone\n\nfor i in {1..5}; do\n    echo \"round $i\"\ndone\n\nfor HOST in $(cat hosts.txt); do\n    ping -c1 -W1 \"$HOST\" >/dev/null && echo \"$HOST up\"\ndone\n```\n","page":"summary/rhcsa/scripts/#for","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-while","n":1280,"slug":"while","title":"Repeat with while and until","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `while` repeats **while** a test is true; `until` repeats **until** it becomes true. `while read` reads a file line by line.\n\n**wait.sh**\n```text\n#!/bin/bash\n# wait until the web server answers\nuntil curl -fs http://localhost >/dev/null; do\n    echo \"waiting for httpd...\"\n    sleep 2\ndone\necho \"httpd is up\"\n\n# read a list line by line\nwhile read -r NAME GROUP; do\n    useradd -G \"$GROUP\" \"$NAME\"\ndone < users.txt\n```\n","page":"summary/rhcsa/scripts/#while","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-case","n":1281,"slug":"case","title":"Choose with case","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `case` compares one value with patterns — cleaner than many `elif` lines. Common for start/stop/status scripts.\n\n**service.sh**\n```text\n#!/bin/bash\ncase \"$1\" in\n    start)\n        systemctl start httpd ;;\n    stop)\n        systemctl stop httpd ;;\n    status|state)\n        systemctl status httpd ;;\n    *)\n        echo \"usage: $0 {start|stop|status}\" >&2\n        exit 1 ;;\nesac\n```\n","page":"summary/rhcsa/scripts/#case","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-debug","n":1282,"slug":"debug","title":"Debug a script","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `bash -x` prints every command before it runs, with the values filled in. `bash -n` checks the syntax without running anything.\n\n```bash\nbash -n backup.sh\nbash -x backup.sh\n```\n```text\n+ DIR=/etc\n+ tar -czf /backup/etc.tgz /etc\n```\n\n| Setting | Effect |\n|---|---|\n| `set -x` / `set +x` | Trace on / off from this line |\n| `set -e` | Stop at the first command that fails |\n| `set -u` | Error on unset variables |\n| `set -o pipefail` | A pipe fails if any part fails |\n\n> **Work tip:** Start real scripts with `set -euo pipefail` and check them with `shellcheck` (from EPEL): it finds quoting bugs and other common mistakes.\n\n\n> **Exam tip:** The exam can ask for a small script: for example \"write a script that creates users from a list\" or \"prints a message based on an argument\". Test it with good and bad input, and make it executable.\n","page":"summary/rhcsa/scripts/#debug","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-main-points","n":1283,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- A script starts with `#!/bin/bash` and needs execute permission (`chmod +x`).\n- **Variables**: `NAME=value` (no spaces), use them as `\"$NAME\"`; `$(command)` puts the output of a command in a variable.\n- **Arguments**: `$1`, `$2` …, all with `\"$@\"`, the number with `$#`; `$?` is the exit code of the last command.\n- `if [ test ]; then … elif …; else …; fi` checks conditions; `test` / `[ ]` compares numbers (`-eq`, `-gt`), text (`=`, `-z`) and files (`-f`, `-d`, `-e`).\n- `&&` runs the next command only on success, `||` only on failure.\n- Loops: `for x in LIST; do …; done`, `while CONDITION; do …; done`, `until`.\n- `case \"$1\" in start) … ;; stop) … ;; *) … ;; esac` picks one of many choices.\n- Debug with `bash -x script.sh`; end with `exit 0` (ok) or another number (error).\n","page":"summary/rhcsa/scripts/#main-points","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-key-terms","n":1284,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Shebang | The first line `#!/bin/bash`: which program runs the script. |\n| Variable | A name that holds a value. |\n| Positional parameter | An argument of the script: `$1`, `$2` … |\n| Exit code | The number a command returns: 0 = ok, other = error. |\n| Command substitution | `$(cmd)`: the output of a command used as a value. |\n| Condition | A test that is true or false. |\n| Loop | Code that runs many times. |\n| Here document | Text inside the script given to a command with `<<EOF`. |\n","page":"summary/rhcsa/scripts/#key-terms","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-scripts-review","n":1285,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Advanced admin tasks","tags":["rhcsa","chapter-19"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: What must the first line of a Bash script be?**\nA: `#!/bin/bash`.\n\n\n**Q: How do you check in a script that the user gave at least one argument?**\nA: `if [ $# -lt 1 ]; then echo \"usage: $0 NAME\"; exit 1; fi`.\n\n\n**Q: How do you put today's date in a variable?**\nA: `TODAY=$(date +%F)`.\n\n\n**Q: How do you test if /etc/hosts is a normal file?**\nA: `[ -f /etc/hosts ]`.\n\n\n**Q: Write a loop that creates the users anna, bob and carl.**\nA: `for u in anna bob carl; do useradd \"$u\"; done`.\n\n\n**Q: How do you see the exit code of the last command?**\nA: `echo $?`.\n\n\n**Q: How do you see every line a script runs, for debugging?**\nA: `bash -x script.sh` (or `set -x` inside the script).\n\n\n**Q: Why do you write \"$1\" with quotes?**\nA: So a value with spaces stays one argument.\n","page":"summary/rhcsa/scripts/#review","label":"Ch 19","where":"RHCSA 10 · Chapter 19 · Automate tasks with Bash scripts"},{"id":"sum-ssh","n":1286,"slug":"ssh","title":"Chapter 20: Secure and tune the SSH server","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"SSH is the door to every server, so it is also the first thing attackers try. This chapter shows how to harden the SSH server: limit root, change the port (with SELinux and the firewall), allow only some users, tune sessions and keepalives, and use key logins with passphrases.\n\n## Lessons in this chapter\n\n- Know the sshd configuration\n- Limit root access\n- Use another port\n- Allow only some users\n- Use other useful sshd options\n- Keep connections alive\n- Use key logins with passphrases\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 20.1\n","page":"summary/rhcsa/ssh/","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-sshd-config","n":1287,"slug":"sshd-config","title":"Know the sshd configuration","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The SSH server (`sshd`) reads `/etc/ssh/sshd_config`. On RHEL that file first includes `/etc/ssh/sshd_config.d/*.conf`. For each setting, **the first value found wins** — so put your own settings in a drop-in file with a low number.\n\n```bash\ngrep -v '^#' /etc/ssh/sshd_config | grep .\n```\n```text\nInclude /etc/ssh/sshd_config.d/*.conf\n```\n```bash\nls /etc/ssh/sshd_config.d/\nsshd -T | grep -iE 'permitrootlogin|passwordauthentication|port '\n```\n\n1. Write your settings in a drop-in file.\n**/etc/ssh/sshd_config.d/10-hardening.conf**\n```text\nPermitRootLogin no\nMaxAuthTries 3\n```\n2. Test the syntax, then reload (open sessions stay connected).\n```bash\nsshd -t\nsystemctl reload sshd\n```\n\n> **Watch out:** Keep your current SSH session open while you change sshd. Test a new login in a second terminal before you log out. A mistake can lock you out.\n","page":"summary/rhcsa/ssh/#sshd-config","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-root-login","n":1288,"slug":"root-login","title":"Limit root access","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `PermitRootLogin` decides how root may log in over SSH.\n\n| Value | Meaning |\n|---|---|\n| `yes` | Root may log in with a password or a key |\n| `prohibit-password` | Root only with a key (RHEL default) |\n| `no` | Root may not log in at all (best: log in as a user, then sudo) |\n| `forced-commands-only` | Root only with keys that run a fixed command |\n\n**/etc/ssh/sshd_config.d/10-hardening.conf**\n```text\nPermitRootLogin no\n```\n","page":"summary/rhcsa/ssh/#root-login","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-port","n":1289,"slug":"port","title":"Use another port","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A different port does not make SSH safe, but it removes most automated attacks from the logs. SELinux and the firewall must know the new port too.\n\n1. Set the port in sshd.\n**/etc/ssh/sshd_config.d/10-hardening.conf**\n```text\nPort 2022\n```\n2. Tell SELinux that sshd may use this port.\n```bash\nsemanage port -a -t ssh_port_t -p tcp 2022\nsemanage port -l | grep ssh\n```\n3. Open the port in the firewall and reload sshd.\n```bash\nfirewall-cmd --add-port=2022/tcp --permanent\nfirewall-cmd --reload\nsystemctl restart sshd\n```\n4. Test from another machine.\n```bash\nssh -p 2022 student@server1\n```\n\n> **Exam tip:** Three places for a new SSH port: sshd config, `semanage port -a -t ssh_port_t`, `firewall-cmd --add-port`. Forget one and the login fails.\n","page":"summary/rhcsa/ssh/#port","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-limit-users","n":1290,"slug":"limit-users","title":"Allow only some users","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `AllowUsers` and `AllowGroups` list who may log in; everybody else is refused. `DenyUsers` and `DenyGroups` block some.\n\n**/etc/ssh/sshd_config.d/20-users.conf**\n```text\nAllowGroups sshusers wheel\n```\n\n```bash\ngroupadd sshusers\nusermod -aG sshusers lisa\nsystemctl reload sshd\n```\n\n- `AllowUsers lisa@192.168.122.*` allows lisa only from that network.\n- If you use both Allow lines, a user must match both.\n","page":"summary/rhcsa/ssh/#limit-users","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-options","n":1291,"slug":"options","title":"Use other useful sshd options","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** More settings that make SSH safer or friendlier.\n\n| Setting | Effect |\n|---|---|\n| `PasswordAuthentication no` | Keys only (set up keys first!) |\n| `MaxAuthTries 3` | Fewer password guesses per connection |\n| `LoginGraceTime 30` | Seconds to finish the login |\n| `MaxSessions 4` | Sessions per connection |\n| `X11Forwarding no` | No graphical forwarding |\n| `AllowTcpForwarding no` | No tunnels through this server |\n| `Banner /etc/issue.net` | Show a legal text before login |\n| `UseDNS no` | Faster logins (no reverse lookup) |\n","page":"summary/rhcsa/ssh/#options","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-keepalive","n":1292,"slug":"keepalive","title":"Keep connections alive","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Firewalls close idle connections. Keepalive messages keep them open — or close dead ones on purpose.\n\n**/etc/ssh/sshd_config.d/30-keepalive.conf**\n```text\nClientAliveInterval 300\nClientAliveCountMax 2\n```\n\nThe server asks the client every 300 seconds; after 2 missed answers it closes the session (idle sessions end after 10 minutes of no answer). On the client side:\n\n**~/.ssh/config**\n```text\nHost *\n    ServerAliveInterval 60\n```\n","page":"summary/rhcsa/ssh/#keepalive","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-keys","n":1293,"slug":"keys","title":"Use key logins with passphrases","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A key with a **passphrase** is safe even if the private key file is stolen. `ssh-agent` remembers the unlocked key for your session, so you type the passphrase once.\n\n```bash\nssh-keygen -t ed25519 -C \"student@server1\"\nssh-copy-id student@server2\neval \"$(ssh-agent)\"\nssh-add\nssh-add -l\nssh student@server2\n```\n\n1. When all users have keys, turn off password logins.\n**/etc/ssh/sshd_config.d/10-hardening.conf**\n```text\nPasswordAuthentication no\n```\n2. Test and reload.\n```bash\nsshd -t && systemctl reload sshd\n```\n\n> **Work tip:** Check who tries to get in: `journalctl -u sshd | grep -E 'Failed|Invalid'`. Tools like fail2ban (EPEL) block IPs after many failures.\n","page":"summary/rhcsa/ssh/#keys","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-main-points","n":1294,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- The SSH server is **sshd**; its settings are in `/etc/ssh/sshd_config` and drop-in files in `/etc/ssh/sshd_config.d/*.conf`.\n- The drop-in files are read first and the **first value wins**, so put your own settings in a drop-in with a low number.\n- `PermitRootLogin no` (or `prohibit-password`) stops root password logins; RHEL 10 already blocks root password login by default.\n- A new SSH port needs three steps: the `Port` line, an SELinux port label (`semanage port -a -t ssh_port_t -p tcp PORT`) and a firewall rule.\n- `AllowUsers` and `AllowGroups` limit who may log in.\n- `PasswordAuthentication no` allows only key logins; test a key login before you turn passwords off.\n- `ClientAliveInterval` and `ClientAliveCountMax` close dead sessions; on the client `ServerAliveInterval` keeps sessions open.\n- Check the config with `sshd -t` before `systemctl restart sshd`.\n","page":"summary/rhcsa/ssh/#main-points","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-key-terms","n":1295,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| sshd | The SSH server service. |\n| sshd_config | The main settings file of the SSH server. |\n| Drop-in config | An extra settings file in /etc/ssh/sshd_config.d/. |\n| PermitRootLogin | The setting that allows or blocks root logins. |\n| AllowUsers | The list of users who may log in with SSH. |\n| Key-based authentication | Login with a key pair instead of a password. |\n| ssh_port_t | The SELinux type for ports that sshd may use. |\n| Keep-alive | Small messages that check if a connection is still alive. |\n","page":"summary/rhcsa/ssh/#key-terms","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-ssh-review","n":1296,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-20"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Where do you put your own sshd settings on RHEL 10?**\nA: In a file like `/etc/ssh/sshd_config.d/10-local.conf`.\n\n\n**Q: Two files set `Port`. Which value does sshd use?**\nA: The first one it reads (drop-in files come first, in name order).\n\n\n**Q: Which three things do you change to move sshd to port 2022?**\nA: `Port 2022` in a drop-in, `semanage port -a -t ssh_port_t -p tcp 2022`, `firewall-cmd --permanent --add-port=2022/tcp` + `--reload`.\n\n\n**Q: How do you allow only users anna and bob to log in?**\nA: `AllowUsers anna bob`, then restart sshd.\n\n\n**Q: How do you check the sshd config for errors?**\nA: `sshd -t`.\n\n\n**Q: How do you allow only key logins?**\nA: `PasswordAuthentication no` (test a key login first, keep a second session open).\n\n\n**Q: How do you see the settings sshd really uses?**\nA: `sshd -T` (shows the final values).\n\n\n**Q: Which setting closes a session after 3 missed keep-alive checks of 60 seconds?**\nA: `ClientAliveInterval 60` and `ClientAliveCountMax 3`.\n","page":"summary/rhcsa/ssh/#review","label":"Ch 20","where":"RHCSA 10 · Chapter 20 · Secure and tune the SSH server"},{"id":"sum-apache","n":1297,"slug":"apache","title":"Chapter 21: Run a web server with Apache","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Apache (`httpd`) is the web server of RHEL. This chapter shows how to install and start it, read its configuration, publish content, understand the configuration files, and host several websites with virtual hosts — including the firewall and SELinux steps that make it reachable.\n\n## Lessons in this chapter\n\n- Install and start Apache\n- Find the main configuration file\n- Create web content\n- Understand the configuration files\n- Create virtual hosts\n- Serve on another port\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 21.1\n","page":"summary/rhcsa/apache/","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-install","n":1298,"slug":"install","title":"Install and start Apache","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Install the package, start the service for good, open the firewall, and test.\n\n```bash\ndnf install -y httpd\nsystemctl enable --now httpd\nfirewall-cmd --add-service=http --permanent\nfirewall-cmd --add-service=https --permanent\nfirewall-cmd --reload\ncurl -I http://localhost\n```\n\n> **Note:** The group \"Basic Web Server\" (`dnf group install \"Basic Web Server\"`) adds more modules, like mod_ssl for HTTPS.\n","page":"summary/rhcsa/apache/#install","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-main-config","n":1299,"slug":"main-config","title":"Find the main configuration file","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The main file is `/etc/httpd/conf/httpd.conf`. Lines are **directives**; `<Directory>` blocks set rules for folders.\n\n```bash\ngrep -vE '^\\s*(#|$)' /etc/httpd/conf/httpd.conf | head -30\n```\n\n| Directive | Meaning |\n|---|---|\n| `ServerRoot \"/etc/httpd\"` | Base folder for relative paths |\n| `Listen 80` | Port to listen on |\n| `Include conf.modules.d/*.conf` | Load the modules |\n| `User apache`, `Group apache` | Account that serves the pages |\n| `DocumentRoot \"/var/www/html\"` | Folder with the website files |\n| `<Directory \"/var/www/html\">` … `</Directory>` | Rules for a folder (Require, Options, AllowOverride) |\n| `IncludeOptional conf.d/*.conf` | Your own config files |\n| `ErrorLog \"logs/error_log\"` | Errors (in /var/log/httpd/) |\n","page":"summary/rhcsa/apache/#main-config","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-content","n":1300,"slug":"content","title":"Create web content","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Files in the DocumentRoot are the website. `index.html` is the default page.\n\n```bash\necho \"Welcome to server1\" > /var/www/html/index.html\ncurl http://localhost\n```\n```text\nWelcome to server1\n```\n```bash\nls -Z /var/www/html/index.html\n```\n\n> **Watch out:** A file created in your home folder and then **moved** (`mv`) to /var/www/html keeps its old SELinux label (`user_home_t`) and Apache gets \"403 Forbidden\". Use `cp`, or fix it with `restorecon -Rv /var/www/html`.\n","page":"summary/rhcsa/apache/#content","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-config-files","n":1301,"slug":"config-files","title":"Understand the configuration files","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Apache's configuration is split over several folders.\n\n| Path | Contains |\n|---|---|\n| `/etc/httpd/conf/httpd.conf` | Main settings |\n| `/etc/httpd/conf.d/*.conf` | Extra config: vhosts, ssl.conf, welcome page |\n| `/etc/httpd/conf.modules.d/*.conf` | Which modules are loaded |\n| `/var/www/html/` | Default DocumentRoot |\n| `/var/log/httpd/` | access_log and error_log |\n\n```bash\napachectl configtest\n```\n```text\nSyntax OK\n```\n```bash\nhttpd -M | head\nsystemctl reload httpd\n```\n\n> **Remember:** After every config change: `apachectl configtest`, then `systemctl reload httpd`.\n","page":"summary/rhcsa/apache/#config-files","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-vhosts","n":1302,"slug":"vhosts","title":"Create virtual hosts","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** One Apache can serve many websites. A **virtual host** chooses the site by the host name the browser asks for.\n\n1. Create the content folders.\n```bash\nmkdir -p /srv/web/{site1,site2}\necho \"This is site1\" > /srv/web/site1/index.html\necho \"This is site2\" > /srv/web/site2/index.html\n```\n2. Create a config file for both sites.\n**/etc/httpd/conf.d/vhosts.conf**\n```text\n<Directory \"/srv/web\">\n    Require all granted\n    AllowOverride None\n</Directory>\n\n<VirtualHost *:80>\n    ServerName site1.example.com\n    DocumentRoot /srv/web/site1\n    ErrorLog logs/site1_error_log\n    CustomLog logs/site1_access_log combined\n</VirtualHost>\n\n<VirtualHost *:80>\n    ServerName site2.example.com\n    DocumentRoot /srv/web/site2\n</VirtualHost>\n```\n3. Give the new folder the web content SELinux label (see the SELinux chapter).\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/web(/.*)?\"\nrestorecon -Rv /srv/web\n```\n4. Test the syntax, reload, and test with the names (here via /etc/hosts).\n```bash\napachectl configtest && systemctl reload httpd\necho \"192.168.122.10 site1.example.com site2.example.com\" >> /etc/hosts\ncurl http://site1.example.com\ncurl http://site2.example.com\n```\n\n> **Note:** The first VirtualHost is also the default for requests with an unknown name.\n","page":"summary/rhcsa/apache/#vhosts","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-port","n":1303,"slug":"port","title":"Serve on another port","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A non-standard port needs three changes: Apache, SELinux and the firewall.\n\n```bash\necho \"Listen 82\" > /etc/httpd/conf.d/port82.conf\nsemanage port -a -t http_port_t -p tcp 82\nfirewall-cmd --add-port=82/tcp --permanent; firewall-cmd --reload\nsystemctl restart httpd\ncurl http://localhost:82\n```\n\n> **Exam tip:** When the web server \"does not work\", check in this order: `systemctl status httpd`, `apachectl configtest`, `curl localhost` on the server, the firewall, then SELinux (`ls -Z`, `ausearch -m avc -ts recent`).\n\n\n> **Work tip:** Hide the version and add security headers: `ServerTokens Prod`, `ServerSignature Off`, `Header always set X-Content-Type-Options nosniff`, and use HTTPS (mod_ssl) for every real site.\n","page":"summary/rhcsa/apache/#port","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-main-points","n":1304,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- Install with `dnf install httpd`, start with `systemctl enable --now httpd`, open the firewall service `http` (and `https`).\n- The main config is `/etc/httpd/conf/httpd.conf`; extra files go in `/etc/httpd/conf.d/*.conf`.\n- The default web folder is `/var/www/html` (DocumentRoot).\n- **Virtual hosts** serve more websites on one server, chosen by `ServerName`; each has its own DocumentRoot.\n- A new DocumentRoot outside /var/www needs a `<Directory>` block with `Require all granted` and the SELinux type `httpd_sys_content_t`.\n- A new port needs `Listen PORT`, an SELinux port label (`http_port_t`) and a firewall rule.\n- Test the config with `apachectl configtest` (or `httpd -t`); test the site with `curl http://NAME/`.\n","page":"summary/rhcsa/apache/#main-points","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-key-terms","n":1305,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| httpd | The Apache web server program and service. |\n| DocumentRoot | The folder with the files of a website. |\n| Virtual host | One of several websites on the same server. |\n| ServerName | The host name a virtual host answers to. |\n| Directory block | Settings for one folder, like who may read it. |\n| Listen | The port (and address) Apache waits on. |\n| conf.d | The folder for extra Apache config files. |\n| http_port_t | The SELinux type for ports that web servers may use. |\n","page":"summary/rhcsa/apache/#key-terms","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-apache-review","n":1306,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-21"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: Which three commands make a basic web server work and reachable?**\nA: `dnf install httpd`, `systemctl enable --now httpd`, `firewall-cmd --permanent --add-service=http` + `--reload`.\n\n\n**Q: Where do you put a new virtual host file?**\nA: In `/etc/httpd/conf.d/NAME.conf`.\n\n\n**Q: Your site is in /web and Apache shows \"Forbidden\". Which two things do you check?**\nA: A `<Directory \"/web\">` block with `Require all granted`, and the SELinux type `httpd_sys_content_t` (`semanage fcontext` + `restorecon`).\n\n\n**Q: How do you test the Apache config before a restart?**\nA: `apachectl configtest`.\n\n\n**Q: How do you let Apache listen on port 82?**\nA: `Listen 82`, `semanage port -a -t http_port_t -p tcp 82`, firewall rule for 82/tcp, restart httpd.\n\n\n**Q: Why is port 8080 a bad choice for the example?**\nA: It already has another SELinux type (`http_cache_port_t`), so it needs `-m` instead of `-a`.\n\n\n**Q: Which log shows Apache errors?**\nA: `/var/log/httpd/error_log`.\n\n\n**Q: How do you test a virtual host from the server itself?**\nA: Add the name to /etc/hosts and run `curl http://NAME/`.\n","page":"summary/rhcsa/apache/#review","label":"Ch 21","where":"RHCSA 10 · Chapter 21 · Run a web server with Apache"},{"id":"sum-selinux","n":1307,"slug":"selinux","title":"Chapter 22: Protect services with SELinux","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"SELinux adds a second security check on top of normal permissions: every process may only touch files, ports and other objects that the policy allows for its **type**. A hacked web server then still cannot read your home folder. This chapter shows the modes, how to read and set labels, ports and booleans, and how to find and fix SELinux denials — without switching SELinux off.\n\n## Lessons in this chapter\n\n- Understand why SELinux matters\n- Know the SELinux modes\n- Read context labels\n- Set context types the right way\n- Find the context type you need\n- Restore default file contexts\n- Manage port access\n- Use booleans\n- Diagnose and fix SELinux denials\n- Make SELinux analysis easier\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 22.1\n- Lab 22.2\n","page":"summary/rhcsa/selinux/","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-why","n":1308,"slug":"why","title":"Understand why SELinux matters","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Normal permissions check **who** you are (user, group). SELinux checks **what program** you are: the web server (type `httpd_t`) may read web files (`httpd_sys_content_t`), but not `/etc/shadow` (`shadow_t`) — even when it runs as root.\n\n- Everything has a **context** (label): processes, files, ports.\n- The **policy** (rules from Red Hat) says which process types may do what with which object types.\n- What is not allowed is denied and logged as an **AVC** message.\n\n> **Remember:** Most SELinux problems come from a **wrong file label**, a **non-standard port**, or a **boolean** that is off. Fix those — do not disable SELinux.\n","page":"summary/rhcsa/selinux/#why","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-modes","n":1309,"slug":"modes","title":"Know the SELinux modes","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** SELinux has three modes. Only **enforcing** protects the system.\n\n| Mode | Effect |\n|---|---|\n| `enforcing` | Rules are enforced and denials logged (default) |\n| `permissive` | Nothing is blocked, but denials are logged (for troubleshooting) |\n| `disabled` | SELinux is off; files get no labels (avoid!) |\n\n```bash\ngetenforce\n```\n```text\nEnforcing\n```\n```bash\nsestatus\nsetenforce 0\nsetenforce 1\ngrep ^SELINUX= /etc/selinux/config\n```\n```text\nSELINUX=enforcing\n```\n\n1. `setenforce 0|1` switches between permissive and enforcing until the next reboot.\n2. The mode at boot is set in `/etc/selinux/config` (`SELINUX=enforcing|permissive`).\n3. One service can be permissive while the rest stays enforcing.\n```bash\nsemanage permissive -a httpd_t\nsemanage permissive -d httpd_t\n```\n\n> **Exam tip:** SELinux must be **enforcing** at the end of the exam — services must work with it. Use permissive mode only to test if SELinux is the cause, then switch back.\n\n\n> **Watch out:** To disable SELinux fully, RHEL needs the kernel argument `selinux=0`. Coming back from disabled needs a full relabel. Keep it on.\n","page":"summary/rhcsa/selinux/#modes","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-contexts","n":1310,"slug":"contexts","title":"Read context labels","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A context has four parts: `user:role:type:level`. For RHCSA work, the **type** (ending in `_t`) is what counts.\n\n```bash\nls -Z /var/www/html/index.html\n```\n```text\nunconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/index.html\n```\n```bash\nls -Zd /home/student /etc/shadow\nps -Z -C httpd\n```\n```text\nsystem_u:system_r:httpd_t:s0      1420 ?  00:00:00 httpd\n```\n```bash\nid -Z\nss -tlnZ | grep :80\n```\n\n| Part | Example | Meaning |\n|---|---|---|\n| user | `system_u`, `unconfined_u` | SELinux user |\n| role | `object_r`, `system_r` | Role |\n| type | `httpd_sys_content_t` | **The important part** |\n| level | `s0` | MLS/MCS level (used for containers and VMs) |\n","page":"summary/rhcsa/selinux/#contexts","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-set-types","n":1311,"slug":"set-types","title":"Set context types the right way","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Write the rule in the policy with `semanage fcontext`, then apply it with `restorecon`. A label set this way survives a relabel.\n\n1. Add a rule for a folder and everything in it.\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/web(/.*)?\"\n```\n2. Apply the rule to the existing files.\n```bash\nrestorecon -Rv /srv/web\n```\n```text\nRelabeled /srv/web from unconfined_u:object_r:var_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0\n```\n3. Check.\n```bash\nls -Zd /srv/web\nsemanage fcontext -l -C\n```\n\n| Tool | Effect |\n|---|---|\n| `semanage fcontext -a` + `restorecon` | Permanent: rule in the policy + applied |\n| `chcon -t TYPE FILE` | Temporary: lost at the next restorecon or relabel |\n\n> **Watch out:** `chcon` seems to work, but the next `restorecon` or relabel removes it. On the exam use `semanage fcontext` + `restorecon`.\n","page":"summary/rhcsa/selinux/#set-types","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-find-type","n":1312,"slug":"find-type","title":"Find the context type you need","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Copy the type from the default folder of the service, search the policy rules, or read the service's SELinux man page.\n\n```bash\nls -Zd /var/www/html\nsemanage fcontext -l | grep '/var/www'\ndnf install -y selinux-policy-doc\nman -k _selinux | grep httpd\nman httpd_selinux\n```\n\n> **Remember:** `selinux-policy-doc` gives a man page per service (`httpd_selinux`, `ftpd_selinux`, `samba_selinux` …) with all types, booleans and ports.\n","page":"summary/rhcsa/selinux/#find-type","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-restorecon","n":1313,"slug":"restorecon","title":"Restore default file contexts","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `restorecon` sets files back to the label the policy wants. Use it after moving files or when labels look wrong. For the whole system, ask for a relabel at the next boot.\n\n```bash\nrestorecon -Rv /var/www/html\nrestorecon -Rvn /etc\ntouch /.autorelabel\nfixfiles -F onboot\n```\n\n> **Note:** `mv` keeps the old label, `cp` gives the label of the target folder. That is why moved web files often give \"403 Forbidden\".\n","page":"summary/rhcsa/selinux/#restorecon","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-ports","n":1314,"slug":"ports","title":"Manage port access","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Services may only listen on ports with the right type. To use a new port, add it to the type.\n\n```bash\nsemanage port -l | grep -E '^(http_port_t|ssh_port_t)'\n```\n```text\nhttp_port_t                    tcp      80, 81, 443, 488, 8008, 8009, 8443, 9000\nssh_port_t                     tcp      22\n```\n```bash\nsemanage port -a -t http_port_t -p tcp 82\nsemanage port -d -t http_port_t -p tcp 82\nsemanage port -m -t http_port_t -p tcp 8080\n```\n\n> **Note:** When a port is already defined for another type, `-a` fails (\"already defined\"); use `-m` (modify) instead.\n","page":"summary/rhcsa/selinux/#ports","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-booleans","n":1315,"slug":"booleans","title":"Use booleans","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **Booleans** are on/off switches for parts of the policy, like \"may Apache connect to a database\" or \"may Apache serve home folders\".\n\n```bash\ngetsebool -a | grep httpd\nsemanage boolean -l | grep httpd_enable_homedirs\nsetsebool httpd_enable_homedirs on\nsetsebool -P httpd_enable_homedirs on\nsemanage boolean -l -C\n```\n\n- Without `-P` the change is lost at reboot.\n- `semanage boolean -l` shows the current and default state with a description.\n\n> **Exam tip:** Always use `setsebool -P` on the exam, so the boolean survives the reboot.\n","page":"summary/rhcsa/selinux/#booleans","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-denials","n":1316,"slug":"denials","title":"Diagnose and fix SELinux denials","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** When SELinux blocks something, an **AVC** message goes to `/var/log/audit/audit.log`. Find it, read which **source type**, **target type** and **class** are involved, then fix the label, port or boolean.\n\n1. Find recent denials.\n```bash\nausearch -m AVC -ts recent\n```\n```text\ntype=AVC msg=audit(1729850000.123:412): avc:  denied  { read } for  pid=1421 comm=\"httpd\" name=\"index.html\" dev=\"dm-0\" ino=1234 scontext=system_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:user_home_t:s0 tclass=file permissive=0\n```\n```bash\njournalctl -t setroubleshoot --since -1h\n```\n2. Read it: `httpd_t` tried to `read` a `file` with type `user_home_t` — the file has the wrong label.\n3. Fix the label (or port, or boolean) and test again.\n```bash\nrestorecon -Rv /var/www/html\n```\n\n| Part of the AVC | Meaning |\n|---|---|\n| `denied { read }` | The action that was blocked |\n| `comm=\"httpd\"` | The program |\n| `scontext=…httpd_t…` | Source (process) type |\n| `tcontext=…user_home_t…` | Target (file/port) type |\n| `tclass=file` | Kind of object: file, dir, tcp_socket … |\n","page":"summary/rhcsa/selinux/#denials","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-sealert","n":1317,"slug":"sealert","title":"Make SELinux analysis easier","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `setroubleshoot-server` explains each denial in plain text and suggests the fix.\n\n```bash\ndnf install -y setroubleshoot-server\nsealert -a /var/log/audit/audit.log\nausearch -m AVC -ts recent | audit2why\n```\n\n> **Work tip:** `audit2allow` can build a policy module from denials, but it allows exactly what was blocked — also an attack. Prefer labels, ports and booleans; write custom modules only when you understand every rule.\n","page":"summary/rhcsa/selinux/#sealert","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-main-points","n":1318,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- SELinux adds a second check after the normal permissions: a process type may only touch file and port types that the policy allows.\n- Modes: **enforcing** (blocks and logs), **permissive** (only logs), **disabled**. `getenforce` shows, `setenforce 0|1` changes until reboot, `/etc/selinux/config` sets it for good.\n- See labels with `ls -Z`, `ps -Z`, `id -Z`.\n- Set a file type for good: `semanage fcontext -a -t TYPE \"/path(/.*)?\"`, then `restorecon -Rv /path`. `chcon` changes only until the next relabel.\n- Ports: `semanage port -l`, `-a` to add, `-m` to change.\n- **Booleans** switch parts of the policy: `getsebool -a`, `setsebool -P NAME on` (`-P` = for good).\n- Denials are in `/var/log/audit/audit.log`; read them with `ausearch -m avc` or `sealert -a` (setroubleshoot).\n","page":"summary/rhcsa/selinux/#main-points","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-key-terms","n":1319,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| SELinux | Security-Enhanced Linux: rules about what each program may access. |\n| Context (label) | The SELinux name of a file, process or port: user:role:type:level. |\n| Type | The main part of a label used in the rules, like `httpd_t` or `httpd_sys_content_t`. |\n| Policy | All SELinux rules of the system (targeted policy on RHEL). |\n| Enforcing / permissive | Block and log / only log. |\n| restorecon | Sets labels back to the policy default. |\n| File context rule | A rule that says which label a path should have. |\n| Boolean | An on/off switch for a part of the policy. |\n| AVC denial | A log line about something SELinux blocked. |\n","page":"summary/rhcsa/selinux/#key-terms","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-selinux-review","n":1320,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-22"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you see the SELinux mode now?**\nA: `getenforce` (or `sestatus`).\n\n\n**Q: How do you make SELinux enforcing after every boot?**\nA: `SELINUX=enforcing` in `/etc/selinux/config`.\n\n\n**Q: Your web files are in /web. How do you label them correctly for good?**\nA: `semanage fcontext -a -t httpd_sys_content_t \"/web(/.*)?\"`, then `restorecon -Rv /web`.\n\n\n**Q: Why is `chcon` not enough for the exam?**\nA: A relabel or restorecon puts the old label back; only `semanage fcontext` rules stay.\n\n\n**Q: How do you let Apache connect to a database over the network?**\nA: `setsebool -P httpd_can_network_connect_db on`.\n\n\n**Q: How do you find the SELinux denials of the last minutes?**\nA: `ausearch -m avc -ts recent`.\n\n\n**Q: How do you see which port types allow port 80?**\nA: `semanage port -l | grep -w 80`.\n\n\n**Q: How do you see the label of a running process?**\nA: `ps -eZ | grep NAME`.\n","page":"summary/rhcsa/selinux/#review","label":"Ch 22","where":"RHCSA 10 · Chapter 22 · Protect services with SELinux"},{"id":"sum-firewall","n":1321,"slug":"firewall","title":"Chapter 23: Open and close ports with firewalld","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"A firewall decides which network traffic may reach the server. RHEL uses **firewalld** on top of the kernel's netfilter (nftables). This chapter explains zones and services, and shows how to open services and ports, make changes permanent, and check what is open.\n\n## Lessons in this chapter\n\n- Understand Linux firewalling\n- Understand zones\n- Understand services\n- Open services and ports\n- Work with zones and sources\n- Use rich rules and forwarding\n- Test the firewall\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 23.1\n","page":"summary/rhcsa/firewall/","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-background","n":1322,"slug":"background","title":"Understand Linux firewalling","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The kernel part is **netfilter**. Tools write rules into it: first iptables, now **nftables**. On RHEL you normally do not write nftables rules yourself — **firewalld** does it for you with zones and services.\n\n| Tool | Role on RHEL 10 |\n|---|---|\n| netfilter | Kernel framework that filters packets |\n| nftables (`nft`) | Rule engine and low-level tool (firewalld backend) |\n| iptables | Old tool; replaced by nftables |\n| firewalld (`firewall-cmd`) | The service and tool you use |\n\n```bash\nsystemctl status firewalld\nnft list ruleset | head -20\n```\n\n> **Watch out:** Do not mix your own nft or iptables rules with firewalld; firewalld may overwrite them at reload.\n","page":"summary/rhcsa/firewall/#background","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-zones","n":1323,"slug":"zones","title":"Understand zones","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **zone** is a trust level with its own rules. Every network interface (or source address) belongs to one zone. Traffic is checked by the rules of that zone.\n\n| Zone | Default behaviour |\n|---|---|\n| `public` | Default on RHEL: only selected services (ssh, cockpit, dhcpv6-client) |\n| `drop` | Drop everything incoming, no answer |\n| `block` | Reject incoming with an ICMP message |\n| `external`, `dmz`, `work`, `home`, `internal` | Different preset services |\n| `trusted` | Allow everything |\n\n```bash\nfirewall-cmd --get-default-zone\nfirewall-cmd --get-active-zones\nfirewall-cmd --get-zones\nfirewall-cmd --list-all\nfirewall-cmd --list-all --zone=internal\n```\n","page":"summary/rhcsa/firewall/#zones","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-services","n":1324,"slug":"services","title":"Understand services","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A firewalld **service** is a name for one or more ports (and helpers). `http` = 80/tcp, `ssh` = 22/tcp, `nfs` = 2049/tcp … Using names keeps rules readable.\n\n```bash\nfirewall-cmd --get-services | tr ' ' '\\n' | grep -E '^(http|https|ssh|nfs|samba)$'\nfirewall-cmd --info-service=http\nls /usr/lib/firewalld/services/ | head\n```\n\nYou can define your own service in `/etc/firewalld/services/NAME.xml`.\n","page":"summary/rhcsa/firewall/#services","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-open","n":1325,"slug":"open","title":"Open services and ports","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Add a service or port to the zone. Without `--permanent` it is lost at reload or reboot; with `--permanent` it is not active until a reload.\n\n1. Open a service and a port permanently, then reload.\n```bash\nfirewall-cmd --add-service=http --permanent\nfirewall-cmd --add-port=8443/tcp --permanent\nfirewall-cmd --reload\n```\n2. Check.\n```bash\nfirewall-cmd --list-services\nfirewall-cmd --list-ports\n```\n3. Close them again.\n```bash\nfirewall-cmd --remove-service=http --permanent\nfirewall-cmd --remove-port=8443/tcp --permanent\nfirewall-cmd --reload\n```\n\n| Way | Effect |\n|---|---|\n| without `--permanent` | Active now, gone after reload/reboot |\n| `--permanent` + `--reload` | Saved and active |\n| `--runtime-to-permanent` | Save all current runtime rules |\n\n> **Exam tip:** Use `--permanent` and `--reload` for every firewall task, and check with `firewall-cmd --list-all`. Test from another machine.\n","page":"summary/rhcsa/firewall/#open","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-zone-config","n":1326,"slug":"zone-config","title":"Work with zones and sources","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Change the default zone, move an interface to another zone, or give a source network its own zone.\n\n```bash\nfirewall-cmd --set-default-zone=public\nfirewall-cmd --zone=internal --change-interface=enp2s0 --permanent\nfirewall-cmd --zone=trusted --add-source=192.168.122.0/24 --permanent\nfirewall-cmd --reload\nfirewall-cmd --get-active-zones\n```\n\n> **Note:** NetworkManager can store the zone of a connection: `nmcli con mod enp2s0 connection.zone internal`.\n","page":"summary/rhcsa/firewall/#zone-config","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-rich-rules","n":1327,"slug":"rich-rules","title":"Use rich rules and forwarding","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **Rich rules** allow more detail: a service only for one network, logging, rejecting one IP. **Port forwarding** sends traffic to another port or host.\n\n```bash\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"192.168.122.0/24\" service name=\"mysql\" accept'\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"203.0.113.50\" drop'\nfirewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080\nfirewall-cmd --reload\nfirewall-cmd --list-rich-rules\n```\n\n> **Work tip:** Open only what you need, and limit admin services (ssh, cockpit, databases) to admin networks with rich rules or a separate zone.\n","page":"summary/rhcsa/firewall/#rich-rules","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-test","n":1328,"slug":"test","title":"Test the firewall","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Check the rules on the server, then test from outside.\n\n```bash\nfirewall-cmd --list-all\nfirewall-cmd --query-service=http\n```\n```text\nyes\n```\n```bash\ncurl -I http://server1\nnc -zv server1 8443\nfirewall-cmd --set-log-denied=all\njournalctl -k | grep -i 'filter_IN_public_REJECT'\n```\n","page":"summary/rhcsa/firewall/#test","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-main-points","n":1329,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- **firewalld** manages the firewall (nftables underneath); the tool is `firewall-cmd`.\n- **Zones** set trust levels; every interface or source belongs to one zone; the default zone is `public`.\n- Open a **service** (`--add-service=http`) or a **port** (`--add-port=8080/tcp`).\n- Changes without `--permanent` are lost at reload or reboot; with `--permanent` they wait until `--reload`. Do both, or use `--runtime-to-permanent`.\n- `firewall-cmd --list-all` shows the active zone; `--get-services` lists known services.\n- **Rich rules** allow more detail, for example one source network for one service.\n- Test from another host with `curl`, `nc` or `nmap`.\n","page":"summary/rhcsa/firewall/#main-points","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-key-terms","n":1330,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| firewalld | The firewall service of RHEL. |\n| Zone | A set of rules with one trust level. |\n| Default zone | The zone for interfaces that have no other zone. |\n| Service | A name for one or more ports, like `ssh` or `http`. |\n| Runtime config | The rules that are active now. |\n| Permanent config | The rules saved on disk for the next start. |\n| Rich rule | A detailed rule with source, service, port and action. |\n| nftables | The kernel firewall that firewalld uses. |\n","page":"summary/rhcsa/firewall/#key-terms","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-firewall-review","n":1331,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-23"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you open the https service now and for good?**\nA: `firewall-cmd --permanent --add-service=https` and `firewall-cmd --reload`.\n\n\n**Q: How do you open port 2022/tcp for good?**\nA: `firewall-cmd --permanent --add-port=2022/tcp`, then `--reload`.\n\n\n**Q: How do you see all rules of the active zone?**\nA: `firewall-cmd --list-all`.\n\n\n**Q: How do you see which zone is the default?**\nA: `firewall-cmd --get-default-zone`.\n\n\n**Q: How do you put interface enp2s0 in the zone internal?**\nA: `firewall-cmd --permanent --zone=internal --change-interface=enp2s0`, then `--reload` (or set the zone in the NetworkManager profile).\n\n\n**Q: How do you allow SSH only from 192.168.122.0/24?**\nA: A rich rule: `firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=192.168.122.0/24 service name=ssh accept'` and remove the normal ssh service.\n\n\n**Q: You added a rule without --permanent. What happens at reboot?**\nA: It is gone.\n\n\n**Q: Which file holds the ports of a known service?**\nA: The service XML file in `/usr/lib/firewalld/services/` (your own in `/etc/firewalld/services/`).\n","page":"summary/rhcsa/firewall/#review","label":"Ch 23","where":"RHCSA 10 · Chapter 23 · Open and close ports with firewalld"},{"id":"sum-nfs","n":1332,"slug":"nfs","title":"Chapter 24: Share folders with NFS and mount them automatically","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"NFS shares folders over the network between Linux systems. This chapter shows how to set up a simple NFS server, mount a share by hand and through /etc/fstab, and use **autofs** to mount shares automatically when a user opens them — including the wildcard setup for home folders.\n\n## Lessons in this chapter\n\n- Understand NFS and its security\n- Set up an NFS server\n- Mount an NFS share\n- Mount NFS through fstab\n- Understand autofs\n- Define an indirect autofs mount\n- Use wildcards for home folders\n- Use a direct autofs mount\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 24.1\n","page":"summary/rhcsa/nfs/","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-nfs-basics","n":1333,"slug":"nfs-basics","title":"Understand NFS and its security","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An NFS **server** exports folders; **clients** mount them like a local disk. RHEL uses NFSv4 by default (one TCP port: 2049).\n\n- By default (`sec=sys`) the server trusts the UID and GID the client sends. Users need the **same UIDs** on all systems (or central users from IdM/LDAP).\n- **root_squash** (default) maps root on the client to the user `nobody`, so client root is not root on the share.\n- For real security use Kerberos (`sec=krb5p`: authentication and encryption).\n","page":"summary/rhcsa/nfs/#nfs-basics","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-nfs-server","n":1334,"slug":"nfs-server","title":"Set up an NFS server","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Install nfs-utils, write the exports, start the service and open the firewall.\n\n1. Install and create the folder to share.\n```bash\ndnf install -y nfs-utils\nmkdir -p /nfsdata /home/ldap\necho \"shared data\" > /nfsdata/readme.txt\n```\n2. Describe the exports.\n**/etc/exports**\n```text\n/nfsdata    192.168.122.0/24(rw,sync)\n/home/ldap  192.168.122.0/24(rw,sync,no_root_squash)\n```\n3. Start the server and open the firewall.\n```bash\nsystemctl enable --now nfs-server\nfirewall-cmd --add-service={nfs,mountd,rpc-bind} --permanent\nfirewall-cmd --reload\n```\n4. Check the exports.\n```bash\nexportfs -v\nexportfs -ra\n```\n\n| Export option | Meaning |\n|---|---|\n| `rw` / `ro` | Read-write / read-only |\n| `sync` | Write to disk before answering (safe) |\n| `root_squash` | Client root → nobody (default) |\n| `no_root_squash` | Client root stays root (only for trusted clients) |\n| `all_squash` | Every user → nobody |\n\n> **Note:** `mountd` and `rpc-bind` are only needed for NFSv3 and for `showmount`. Pure NFSv4 needs only the `nfs` service.\n","page":"summary/rhcsa/nfs/#nfs-server","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-mount-nfs","n":1335,"slug":"mount-nfs","title":"Mount an NFS share","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** See what the server offers, then mount it like a disk with type `nfs`.\n\n```bash\ndnf install -y nfs-utils\nshowmount -e server1\n```\n```text\nExport list for server1:\n/home/ldap 192.168.122.0/24\n/nfsdata   192.168.122.0/24\n```\n```bash\nmkdir /mnt/data\nmount -t nfs server1:/nfsdata /mnt/data\nfindmnt /mnt/data\numount /mnt/data\n```\n\n> **Remember:** With NFSv4 you can also mount the server root and browse: `mount server1:/ /mnt` shows all exports as folders.\n","page":"summary/rhcsa/nfs/#mount-nfs","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-fstab-nfs","n":1336,"slug":"fstab-nfs","title":"Mount NFS through fstab","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** For a share that must always be there, add it to /etc/fstab with `_netdev`, so it is mounted after the network is up.\n\n**/etc/fstab (line)**\n```text\nserver1:/nfsdata   /mnt/data   nfs   _netdev,defaults   0 0\n```\n\n```bash\nsystemctl daemon-reload\nmount -a\nfindmnt -t nfs4\n```\n\n> **Work tip:** Add `nofail` for shares that are not critical: the system then boots even when the NFS server is down.\n","page":"summary/rhcsa/nfs/#fstab-nfs","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-autofs","n":1337,"slug":"autofs","title":"Understand autofs","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **autofs** mounts a share only when someone opens its folder, and unmounts it after some idle minutes. No boot problems when the server is away, and no need to mount many home folders.\n\n- The **master map** (`/etc/auto.master` or files in `/etc/auto.master.d/`) names a base folder and a map file.\n- The **map file** lists the subfolders (keys) and where they come from.\n\n```bash\ndnf install -y autofs\nsystemctl enable --now autofs\n```\n","page":"summary/rhcsa/nfs/#autofs","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-autofs-indirect","n":1338,"slug":"autofs-indirect","title":"Define an indirect autofs mount","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** An **indirect map** creates subfolders under a base folder: `/shares/data` is mounted when someone opens it.\n\n1. Add a line to the master map.\n**/etc/auto.master.d/shares.autofs**\n```text\n/shares   /etc/auto.shares\n```\n2. Create the map: key, mount options, source.\n**/etc/auto.shares**\n```text\ndata   -rw   server1:/nfsdata\n```\n3. Restart autofs and open the folder.\n```bash\nsystemctl restart autofs\nls /shares\nls /shares/data\n```\n```text\nreadme.txt\n```\n```bash\nfindmnt /shares/data\n```\n\n> **Note:** The folder `/shares/data` appears only when you open it (`ls /shares` can be empty before). Do not create it yourself.\n","page":"summary/rhcsa/nfs/#autofs-indirect","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-autofs-wildcard","n":1339,"slug":"autofs-wildcard","title":"Use wildcards for home folders","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** With `*` as the key and `&` in the source, one line serves every user: `/home/ldap/lisa` mounts `server1:/home/ldap/lisa`.\n\n**/etc/auto.master.d/home.autofs**\n```text\n/home/ldap   /etc/auto.home\n```\n\n**/etc/auto.home**\n```text\n*   -rw   server1:/home/ldap/&\n```\n\n```bash\nsystemctl restart autofs\nsu - ldapuser1\npwd\n```\n```text\n/home/ldap/ldapuser1\n```\n\n> **Exam tip:** The classic task: \"users from the LDAP/NFS server must get their home folder automatically under /home/ldap\" — that is exactly this wildcard map. Check with `su - USER` and `findmnt`.\n","page":"summary/rhcsa/nfs/#autofs-wildcard","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-autofs-direct","n":1340,"slug":"autofs-direct","title":"Use a direct autofs mount","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A **direct map** mounts at a full path anywhere in the file system. In the master map the base is `/-`.\n\n**/etc/auto.master.d/direct.autofs**\n```text\n/-   /etc/auto.direct\n```\n\n**/etc/auto.direct**\n```text\n/srv/projects   -rw,soft   server1:/nfsdata\n```\n\n```bash\nsystemctl restart autofs\nls /srv/projects\n```\n","page":"summary/rhcsa/nfs/#autofs-direct","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-main-points","n":1341,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- **NFS** shares folders over the network. RHEL uses NFSv4 by default (port 2049).\n- Server: install `nfs-utils`, write the share in `/etc/exports`, run `exportfs -r`, `systemctl enable --now nfs-server`, open the services `nfs` (and `mountd`, `rpc-bind` for showmount).\n- Client: `showmount -e SERVER` lists shares; `mount SERVER:/path /mnt` mounts one.\n- At boot: an fstab line with type `nfs` and the option `_netdev`.\n- **autofs** mounts shares only when a user opens the folder and unmounts them later.\n- Master map `/etc/auto.master.d/NAME.autofs`; the map file lists the keys and shares.\n- An **indirect** map mounts under one base folder; a **wildcard** map (`* -rw server:/home/&`) works for all user folders; a **direct** map uses full paths (`/-`).\n","page":"summary/rhcsa/nfs/#main-points","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-key-terms","n":1342,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| NFS | Network File System: share folders between Linux systems. |\n| Export | A folder that the NFS server shares. |\n| /etc/exports | The list of exports and who may use them. |\n| showmount | A tool that lists the exports of a server. |\n| _netdev | An fstab option: wait for the network before mounting. |\n| autofs | A service that mounts shares when they are used. |\n| Master map | The autofs file that points to the map files. |\n| Indirect map | A map with keys under one base folder. |\n| Direct map | A map with full mount paths. |\n| Wildcard map | A map with `*` and `&` that works for many folders. |\n","page":"summary/rhcsa/nfs/#key-terms","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-nfs-review","n":1343,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-24"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you see the shares of server2?**\nA: `showmount -e server2`.\n\n\n**Q: Write an fstab line that mounts server2:/data on /mnt/data at boot.**\nA: `server2:/data /mnt/data nfs defaults,_netdev 0 0`.\n\n\n**Q: Which file shares /data with the network 192.168.122.0/24, read-write?**\nA: `/etc/exports` with the line `/data 192.168.122.0/24(rw)`.\n\n\n**Q: How do you load a changed /etc/exports without a restart?**\nA: `exportfs -r`.\n\n\n**Q: Home folders of LDAP users must appear under /home/remote when they are used. Which autofs lines do you write?**\nA: `/home/remote /etc/auto.remote` in a file in `/etc/auto.master.d/`, and `* -rw server:/home/remote/&` in /etc/auto.remote.\n\n\n**Q: What does `&` mean in an autofs map?**\nA: It is replaced by the key (the folder name the user opened).\n\n\n**Q: Which service must run for autofs?**\nA: `autofs` (`systemctl enable --now autofs`).\n\n\n**Q: Why does `ls /home/remote` show nothing before a user logs in?**\nA: autofs mounts a folder only when it is opened by its name.\n","page":"summary/rhcsa/nfs/#review","label":"Ch 24","where":"RHCSA 10 · Chapter 24 · Share folders with NFS and mount them automatically"},{"id":"sum-time","n":1344,"slug":"time","title":"Chapter 25: Keep the system time right","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Correct time matters: logs from many servers must line up, TLS certificates and Kerberos fail with a wrong clock, and scheduled jobs run at the wrong moment. This chapter explains hardware and system time, time zones, and how to sync the clock with NTP using chrony.\n\n## Lessons in this chapter\n\n- Understand local time\n- Use network time\n- Show and set the time with date\n- Use hwclock\n- Use timedatectl\n- Manage the time zone\n- Configure time service clients\n- Remember the main points\n- Learn the key terms\n- Check what you learned\n\n## Labs\n\n- Lab 25.1\n","page":"summary/rhcsa/time/","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-local-time","n":1345,"slug":"local-time","title":"Understand local time","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A computer has two clocks. The **hardware clock** (RTC) runs on the mainboard battery also when the machine is off. The **system clock** is kept by the kernel while Linux runs; it starts from the hardware clock at boot.\n\n| Clock | Where | Tool |\n|---|---|---|\n| Hardware clock (RTC) | Mainboard, battery | `hwclock` |\n| System clock | Kernel | `date`, `timedatectl` |\n\n- Linux keeps the system clock in **UTC** and shows it in your **time zone**.\n- The hardware clock should also be UTC (`RTC in local TZ: no`).\n","page":"summary/rhcsa/time/#local-time","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-ntp","n":1346,"slug":"ntp","title":"Use network time","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** **NTP** (Network Time Protocol) syncs the clock with time servers. RHEL uses **chrony** (`chronyd`) as NTP client and server. Servers are ordered in **strata**: stratum 1 has a real reference clock (GPS, atomic); stratum 2 syncs from stratum 1, and so on.\n\n```bash\nsystemctl status chronyd\nchronyc sources -v\nchronyc tracking\n```\n","page":"summary/rhcsa/time/#ntp","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-date","n":1347,"slug":"date","title":"Show and set the time with date","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `date` shows the system time in any format, and root can set it (better: let NTP do it).\n\n```bash\ndate\n```\n```text\nSun Oct 25 11:42:10 CET 2026\n```\n```bash\ndate +%F_%H%M\n```\n```text\n2026-10-25_1142\n```\n```bash\ndate -u\ndate -d '+3 days'\ndate -d @1761389000\ndate -s \"2026-10-25 12:00:00\"\n```\n\n| Format | Gives |\n|---|---|\n| `%F` | 2026-10-25 |\n| `%T` | 11:42:10 |\n| `%s` | Seconds since 1970 (epoch) |\n| `%A`, `%B` | Day and month name |\n","page":"summary/rhcsa/time/#date","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-hwclock","n":1348,"slug":"hwclock","title":"Use hwclock","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `hwclock` reads and sets the hardware clock, and copies time between the two clocks.\n\n```bash\nhwclock\nhwclock --systohc\nhwclock --hctosys\n```\n\n> **Note:** chronyd keeps the hardware clock in sync by itself when `rtcsync` is in `/etc/chrony.conf` (RHEL default).\n","page":"summary/rhcsa/time/#hwclock","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-timedatectl","n":1349,"slug":"timedatectl","title":"Use timedatectl","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** `timedatectl` shows all time settings at once and changes time, time zone and NTP use.\n\n```bash\ntimedatectl\n```\n```text\n               Local time: Sun 2026-10-25 11:42:10 CET\n           Universal time: Sun 2026-10-25 10:42:10 UTC\n                 RTC time: Sun 2026-10-25 10:42:10\n                Time zone: Europe/Amsterdam (CET, +0100)\nSystem clock synchronized: yes\n              NTP service: active\n          RTC in local TZ: no\n```\n```bash\ntimedatectl set-ntp true\ntimedatectl set-ntp false\ntimedatectl set-time \"2026-10-25 12:00:00\"\n```\n\n> **Watch out:** `set-time` only works when NTP is off. With NTP on, chrony corrects the clock anyway.\n","page":"summary/rhcsa/time/#timedatectl","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-timezone","n":1350,"slug":"timezone","title":"Manage the time zone","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The time zone is the link `/etc/localtime` → a file in `/usr/share/zoneinfo/`. Change it with `timedatectl set-timezone`.\n\n```bash\ntimedatectl list-timezones | grep -i riyadh\n```\n```text\nAsia/Riyadh\n```\n```bash\ntimedatectl set-timezone Asia/Riyadh\nls -l /etc/localtime\nTZ=America/New_York date\ntzselect\n```\n\n> **Note:** `tzselect` only helps you find the name (with questions); it does not change the system.\n","page":"summary/rhcsa/time/#timezone","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-chrony-client","n":1351,"slug":"chrony-client","title":"Configure time service clients","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Tell chrony which time servers to use in `/etc/chrony.conf`, restart it, and check the sources.\n\n1. Set the server (replace the pool line or add a server line).\n**/etc/chrony.conf (part)**\n```text\n# pool 2.rhel.pool.ntp.org iburst\nserver 192.168.122.1 iburst\ndriftfile /var/lib/chrony/drift\nmakestep 1.0 3\nrtcsync\n```\n2. Restart chrony and open the firewall only if this host is a time **server** for others.\n```bash\nsystemctl restart chronyd\nchronyc sources -v\nchronyc tracking\n```\n3. Make the clock step at once if it is far off.\n```bash\nchronyc makestep\n```\n\n| chronyc output | Meaning |\n|---|---|\n| `^*` | The source chrony uses now |\n| `^+` | A good source that is combined |\n| `^?` / `^x` | Not reachable / falseticker (wrong time) |\n| `Leap status: Normal` (tracking) | Synchronized |\n\n> **Exam tip:** \"Configure the system as an NTP client of classroom.example.com\" = a `server classroom.example.com iburst` line in /etc/chrony.conf, `systemctl restart chronyd`, then check `chronyc sources` shows `^*`.\n\n\n> **Work tip:** To make a server an NTP server for your network, add `allow 192.168.122.0/24` to chrony.conf and open the firewall service `ntp`.\n","page":"summary/rhcsa/time/#chrony-client","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-main-points","n":1352,"slug":"main-points","title":"Remember the main points","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The most important facts of this chapter, in short.\n\n- A system has two clocks: the **hardware clock** (RTC, on the mainboard) and the **system clock** (kernel).\n- Linux keeps the system clock in UTC and shows it in the local **time zone**.\n- `timedatectl` shows everything; `timedatectl set-timezone Area/City` changes the zone; `timedatectl set-ntp true` turns on time sync.\n- `date` shows and formats the time; `hwclock` reads and sets the hardware clock.\n- RHEL syncs time with **chrony** (`chronyd`); servers are set in `/etc/chrony.conf` with `server NAME iburst` or `pool NAME iburst`.\n- After a change: `systemctl restart chronyd`, then check with `chronyc sources -v` (`^*` = the source in use) and `chronyc tracking`.\n","page":"summary/rhcsa/time/#main-points","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-key-terms","n":1353,"slug":"key-terms","title":"Learn the key terms","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** The words of this chapter in simple English.\n\n| Term | Meaning |\n|---|---|\n| Hardware clock (RTC) | The clock on the mainboard that runs when the computer is off. |\n| System clock | The clock of the running kernel. |\n| UTC | Coordinated Universal Time: the base time without time zones. |\n| Time zone | The local time rule of a place, like Asia/Riyadh. |\n| NTP | Network Time Protocol: sync the clock over the network. |\n| chronyd | The NTP service of RHEL. |\n| Stratum | How far a time server is from a real reference clock. |\n| iburst | Send the first requests fast to sync quickly. |\n","page":"summary/rhcsa/time/#key-terms","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-time-review","n":1354,"slug":"review","title":"Check what you learned","sections":["summary"],"topic":"RHCSA · Network services","tags":["rhcsa","chapter-25"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Think about the answer first, then open it.\n\n**Q: How do you set the time zone to Europe/Amsterdam?**\nA: `timedatectl set-timezone Europe/Amsterdam`.\n\n\n**Q: How do you find the exact name of a time zone?**\nA: `timedatectl list-timezones | grep -i NAME`.\n\n\n**Q: Make this system an NTP client of classroom.example.com.**\nA: Add `server classroom.example.com iburst` to `/etc/chrony.conf`, `systemctl restart chronyd`, check `chronyc sources`.\n\n\n**Q: In `chronyc sources`, what does `^*` mean?**\nA: That is the source chrony uses now.\n\n\n**Q: How do you turn on time sync with timedatectl?**\nA: `timedatectl set-ntp true`.\n\n\n**Q: Why can `timedatectl set-time` fail?**\nA: It does not work while NTP sync is on.\n\n\n**Q: How do you show the date as 2026-10-25?**\nA: `date +%F`.\n\n\n**Q: How do you copy the system time to the hardware clock?**\nA: `hwclock --systohc`.\n","page":"summary/rhcsa/time/#review","label":"Ch 25","where":"RHCSA 10 · Chapter 25 · Keep the system time right"},{"id":"sum-exam","n":1355,"slug":"exam","title":"Chapter 26: Prepare for the exam day","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"The RHCSA exam (EX200) is fully practical: you get real systems and a list of tasks, and at the end the systems are checked. This chapter gives tips for the weeks before, the registration, the exam day, and a working method for the exam itself.\n\n## Lessons in this chapter\n\n- Check that you are ready\n- Register for the exam\n- Know what happens on exam day\n- Work smart during the exam\n- Respect the exam rules\n","page":"summary/rhcsa/exam/","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-exam-ready","n":1356,"slug":"ready","title":"Check that you are ready","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Knowing the commands is not enough — you must be fast and exact without help from the internet.\n\n- Do every lab in this summary **without** looking at the solution, on fresh VMs.\n- Do the two practice exams (chapter 27) in the time limit. Score yourself honestly.\n- Practice the hard topics until they are routine: LVM, SELinux, autofs, root password reset, fstab, users and sudo.\n- Learn to use `man`, `--help`, `/usr/share/doc` and `man -k` fast: they are the only help in the exam.\n\n> **Remember:** The official objectives list (on the Red Hat exam page) is the real study plan. Check it again before you book — it can change between RHEL versions.\n","page":"summary/rhcsa/exam/#ready","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-exam-register","n":1357,"slug":"register","title":"Register for the exam","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** You book through Red Hat (or a partner): in a test center, or as a **remote exam** from home with a booted USB live system and a webcam.\n\n- Check the current price, duration and rules on the Red Hat EX200 page.\n- For a remote exam: test your hardware with Red Hat's compatibility tool well before the day (camera, microphone, USB boot, network).\n- Book a time of day when you are fresh.\n","page":"summary/rhcsa/exam/#register","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-exam-exam-day","n":1358,"slug":"exam-day","title":"Know what happens on exam day","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Come early, with a valid photo ID. You cannot bring notes, phones or other devices.\n\n- The proctor checks your ID and your room (remote: with the webcam).\n- You get a desk with the virtual machines and the task list.\n- Read the general instructions first: host names, passwords, repository URLs, which system to work on.\n","page":"summary/rhcsa/exam/#exam-day","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-exam-during","n":1359,"slug":"during","title":"Work smart during the exam","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** A clear method saves time and points.\n\n1. Read **all** tasks first. Note dependencies (for example: network and repositories before software; a user before its cron job).\n2. Do the tasks that \"unlock\" others first: network, host name, repositories, root password if needed.\n3. Make every change **persistent**: `enable --now`, `--permanent`, fstab, `-P` for booleans, files instead of runtime commands.\n4. After each task, **check** it: `systemctl is-enabled`, `findmnt`, `id`, `curl`, `getenforce`.\n5. Do not get stuck: skip a task after a few minutes and come back later.\n6. **Reboot** before the end (with enough time left!) and check that everything still works — the graders check after a reboot.\n\n> **Exam tip:** Typical mistakes that cost points: a service not enabled, a firewall change without `--permanent`, an fstab typo that stops the boot, forgetting `restorecon` or `semanage fcontext`, working on the wrong host, and SELinux left in permissive mode.\n\n\n> **Work tip:** Use tools that help: Tab completion, `history`, `Ctrl+R`, `man -k`, and keep a second terminal open with `journalctl -f` while you configure a service.\n","page":"summary/rhcsa/exam/#during","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-exam-nda","n":1360,"slug":"nda","title":"Respect the exam rules","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-26"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Before the exam you accept a **non-disclosure agreement**: you may not share the exam tasks. Study material that claims to contain \"real exam questions\" breaks these rules — and is usually wrong anyway. Learn the skills, not the questions.\n\n\n","page":"summary/rhcsa/exam/#nda","label":"Ch 26","where":"RHCSA 10 · Chapter 26 · Prepare for the exam day"},{"id":"sum-self-test","n":1361,"slug":"self-test","title":"Chapter 27: Test yourself before the exam","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"chapter","script":"","date":"","files":{},"md":"Before the practical practice exams, check if you know **what** to do for each kind of task. For every question, say the steps and the main commands out loud — then compare with the short answer. Any question you cannot answer shows a chapter to repeat.\n\n## Lessons in this chapter\n\n- Users, groups and permissions\n- Storage\n- Software, services and scheduling\n- Network and security\n- Boot and repair\n\n## Labs\n\n- Lab 27.1\n- Lab 27.2\n","page":"summary/rhcsa/self-test/","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"sum-self-test-q-users","n":1362,"slug":"q-users","title":"Users, groups and permissions","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Questions about accounts, rights and shared folders.\n\n| Task | Short answer |\n|---|---|\n| Create a folder where all members of `sales` share files, new files get the group, and people cannot delete each other's files | `chgrp sales DIR`, `chmod 3770 DIR` (SGID + sticky) — lesson |\n| User `lisa` must change her password every 60 days | `chage -M 60 lisa` — lesson |\n| All new users must get a file in their home folder | Put it in `/etc/skel` — lesson |\n| Group `webadmins` may restart httpd as root, nothing else | File in `/etc/sudoers.d/` with `visudo -f` — lesson |\n| Give user `carol` read access to one file without changing owner or group | `setfacl -m u:carol:r FILE` — lesson |\n","page":"summary/rhcsa/self-test/#q-users","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"sum-self-test-q-storage","n":1363,"slug":"q-storage","title":"Storage","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Partitions, LVM, swap and mounts.\n\n| Task | Short answer |\n|---|---|\n| Create an LV of 500 MiB named `my_lv` with extents of 8 MiB, ext4, mounted on `/data` at boot | `pvcreate`, `vgcreate -s 8M`, `lvcreate -n my_lv -L 500M`, `mkfs.ext4`, fstab line, `mount -a` — lesson |\n| Make an XFS LV 1 GiB bigger while it is mounted | `lvextend -r -L +1G` — lesson |\n| Add 1 GiB of swap that is active after reboot | Partition or LV, `mkswap`, fstab `swap` line, `swapon -a` — lesson |\n| Mount a file system by UUID at boot | `blkid` for the UUID, fstab `UUID=…` line — lesson |\n| Home folders of NFS users must appear under `/home/remote` when used | autofs wildcard map `* -rw server:/path/&` — lesson |\n","page":"summary/rhcsa/self-test/#q-storage","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"sum-self-test-q-services","n":1364,"slug":"q-services","title":"Software, services and scheduling","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Repositories, services, boot and jobs.\n\n| Task | Short answer |\n|---|---|\n| Install software from a repository at a given URL | `.repo` file with `baseurl`, then `dnf install` — lesson |\n| A service must run now and after every reboot | `systemctl enable --now SERVICE` — lesson |\n| The system must boot into text mode | `systemctl set-default multi-user.target` — lesson |\n| User `anna` runs a command every weekday at 14:23 | `crontab -e -u anna`: `23 14 * * 1-5 COMMAND` — lesson |\n| The journal must survive reboots | `Storage=persistent` in a journald drop-in, restart journald — lesson |\n| Set the recommended tuned profile | `tuned-adm recommend`, then `tuned-adm profile NAME` — lesson |\n","page":"summary/rhcsa/self-test/#q-services","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"sum-self-test-q-network","n":1365,"slug":"q-network","title":"Network and security","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Network settings, SSH, firewall, SELinux, time.\n\n| Task | Short answer |\n|---|---|\n| Set a fixed IPv4 address, gateway and DNS persistently | `nmcli con mod … ipv4.method manual …`, `nmcli con up` — lesson |\n| Change the host name persistently | `hostnamectl set-hostname NAME` — lesson |\n| Run Apache with a document root in `/web` | Config + `semanage fcontext -a -t httpd_sys_content_t \"/web(/.*)?\"` + `restorecon -Rv /web` — lesson |\n| Let sshd listen on port 2022 | sshd drop-in, `semanage port -a -t ssh_port_t`, `firewall-cmd --add-port` — lesson |\n| Open the http service in the firewall for good | `firewall-cmd --add-service=http --permanent`, `--reload` — lesson |\n| Sync time from `time.example.com` | `server time.example.com iburst` in chrony.conf, restart chronyd — lesson |\n","page":"summary/rhcsa/self-test/#q-network","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"sum-self-test-q-repair","n":1366,"slug":"q-repair","title":"Boot and repair","sections":["summary"],"topic":"RHCSA · Final preparation","tags":["rhcsa","chapter-27"],"series_name":"RHCSA 10","design":"lesson","script":"","date":"","files":{},"md":"**Idea:** Getting into a broken system.\n\n| Task | Short answer |\n|---|---|\n| Reset the unknown root password | `rd.break`, remount `/sysroot` rw, `chroot`, `passwd`, `touch /.autorelabel` — lesson |\n| The system stops in emergency mode after an fstab change | `mount -o remount,rw /`, fix fstab, `mount -a`, reboot — lesson |\n| Remove `rhgb quiet` from the kernel line | `grubby --update-kernel=ALL --remove-args=\"rhgb quiet\"` — lesson |\n| Make a module load at every boot | File in `/etc/modules-load.d/` — lesson |\n\n> **Remember:** When you can answer every row, do the practice exams below — on clean VMs, in the time limit, and reboot at the end before you check.\n","page":"summary/rhcsa/self-test/#q-repair","label":"Ch 27","where":"RHCSA 10 · Chapter 27 · Test yourself before the exam"},{"id":"cmd-alias","n":1367,"slug":"alias","title":"alias — Make a short name for a longer command (alias), or remove it (unalias)","sections":["commands"],"topic":"Commands · Shell and help","tags":["shortcut","bashrc","shell","command"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Make a short name for a longer command (alias), or remove it (unalias).\n\nSame page for: `unalias`\n\n## About\n\nAn alias replaces the first word of a command with other text. `alias ll='ls -l'` means: when you type `ll`, Bash runs `ls -l`.\n\nAn alias made on the command line lives only in that shell. To keep it, put the `alias` line in `~/.bashrc` (only you) or in a file in `/etc/profile.d/` (all users).\n\nAliases are for interactive work. In scripts, use the full commands or a function.\n\n## Syntax\n\n```text\nalias [-p] [NAME[=VALUE] ...]\nunalias [-a] NAME [NAME ...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p` | With alias: print all aliases in a form you can use again. |\n| `-a` | With unalias: remove all aliases of this shell. |\n\n## Examples\n\n### List all aliases\n\n```bash\nalias\n```\n```text\nalias l.='ls -d .* --color=auto'\nalias ll='ls -l --color=auto'\nalias ls='ls --color=auto'\n```\n\n> RHEL adds a few aliases for every user.\n\n### Make a short name\n\n```bash\nalias ports='ss -tulpn'\nports\n```\n\n> Works until you close the shell.\n\n### Keep the alias after logout\n\n```bash\necho \"alias ports='ss -tulpn'\" >> ~/.bashrc\nsource ~/.bashrc\n```\n\n> `source` loads it now; new shells read `~/.bashrc` by themselves.\n\n### Make a safer rm, cp and mv\n\n```bash\nalias rm='rm -i' cp='cp -i' mv='mv -i'\n```\n\n> `-i` asks before it removes or overwrites a file. root has these by default on RHEL.\n\n### Run the real command once, without the alias\n\n```bash\n\\rm old.log\n```\n\n> A backslash (or `command rm`) skips the alias.\n\n### Remove an alias\n\n```bash\nunalias ports\n```\n\n> Only for this shell. Delete the line in `~/.bashrc` to remove it for good.\n\n### Show what one alias does\n\n```bash\nalias ll\n```\n```text\nalias ll='ls -l --color=auto'\n```\n\n> Good when a short name behaves in a strange way.\n\n### Give all admins the same shortcuts\n\n```bash\ncat > /etc/profile.d/ops-aliases.sh <<'EOF'\nalias jctl='journalctl -xe'\nalias fwl='firewall-cmd --list-all'\nEOF\n```\n\n> Every user gets these at the next login. Updates do not touch your file.\n\n### Shorten a long kubectl or podman command\n\n```bash\nalias k='kubectl'\nalias pps='podman ps --format \"table {{.Names}}\\t{{.Status}}\\t{{.Ports}}\"'\n```\n\n> Typical DevOps shortcuts. Keep them in `~/.bashrc`.\n\n### Check for a dangerous alias in a security review\n\n```bash\nalias | grep -E \"sudo|ssh|su=\"\n```\n\n> An alias that changes `sudo` or `ssh` can steal passwords. There should be none.\n\n### Remove all aliases in this shell\n\n```bash\nunalias -a\n```\n\n> Useful to test a problem without any aliases.\n\n## Watch out\n\n- Aliases are not used in scripts (non-interactive shells), unless the script turns on `shopt -s expand_aliases`.\n- Use single quotes `'…'` so variables are read when the alias runs, not when you define it.\n- An alias can hide a real program with the same name. Check with `type NAME`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.bashrc` | Your own aliases (read by every new shell). |\n| `/etc/profile.d/*.sh` | Aliases and settings for all users. |\n| `/etc/bashrc` | System-wide settings for every shell (do not edit; use profile.d). |\n\n## Tips\n\n- alias = make, unalias = remove, `~/.bashrc` = keep.\n- `\\name` runs the real command once.\n\nFull manual: [man alias](https://linux.die.net/man/1/bash)\n","page":"commands/alias/","label":"alias","where":"Command page · Shell and help","aka":["unalias"]},{"id":"cmd-apachectl","n":1368,"slug":"apachectl","title":"apachectl — Control and test the Apache web server: check the config syntax, list virtual hosts and modules. (Start and stop with systemctl.)","sections":["commands"],"topic":"Commands · Web","tags":["apache","web server","config test","virtual hosts","modules"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Control and test the Apache web server: check the config syntax, list virtual hosts and modules. (Start and stop with systemctl.)\n\nSame page for: `httpd`\n\n## About\n\nOn RHEL, Apache runs as `httpd.service`; use systemctl to start, stop and reload it. `apachectl configtest` (= `httpd -t`) checks the syntax before you reload. `httpd -S` shows the virtual hosts as Apache understands them; `httpd -M` lists the loaded modules.\n\n## Syntax\n\n```text\napachectl configtest | graceful | graceful-stop | start | stop | restart | fullstatus | status\nhttpd [-t] [-S] [-M] [-V] [-v] [-D NAME] [-f FILE] [-C DIRECTIVE]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `configtest` | Check the configuration syntax (same as httpd -t). |\n| `graceful` | Reload without dropping open connections (on RHEL: systemctl reload httpd). |\n| `start, stop, restart` | On RHEL these call systemctl. |\n| `status, fullstatus` | Status (fullstatus needs mod_status). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t` | Test the syntax (httpd). |\n| `-S` | Show the parsed virtual host settings. |\n| `-M` | List loaded modules. |\n| `-l` | List compiled-in modules. |\n| `-L` | List directives. |\n| `-V` | Show version and build settings. |\n| `-v` | Show the version. |\n| `-D NAME` | Define a parameter for <IfDefine>; `-D DUMP_VHOSTS`, `-D DUMP_RUN_CFG`, `-D DUMP_INCLUDES`. |\n| `-f FILE` | Use another main config file. |\n| `-C \"DIRECTIVE\"` | Process a directive before the config. |\n| `-c \"DIRECTIVE\"` | Process a directive after the config. |\n| `-d DIR` | Another ServerRoot. |\n| `-e LEVEL` | Log level at startup. |\n| `-E FILE` | Startup errors to a file. |\n| `-X` | Debug mode (one process, foreground). |\n| `-h` | Show help. |\n\n## Examples\n\n### Test the config\n\n```bash\napachectl configtest\n```\n```text\nSyntax OK\n```\n\n> Always before a reload.\n\n### Reload after a change\n\n```bash\napachectl configtest && systemctl reload httpd\n```\n\n> Only reloads when the syntax is OK.\n\n### Show the virtual hosts\n\n```bash\nhttpd -S\n```\n```text\nVirtualHost configuration:\n*:80                   is a NameVirtualHost\n         default server www1.example.com (/etc/httpd/conf.d/sites.conf:5)\n```\n\n> Which vhost is the default and where it is defined.\n\n### List modules\n\n```bash\nhttpd -M | grep -E 'ssl|rewrite|headers'\n```\n\n> Is mod_ssl loaded?\n\n### Version\n\n```bash\nhttpd -v\n```\n\n> Server version and build date.\n\n### Where are the config files?\n\n```bash\nhttpd -t -D DUMP_INCLUDES\n```\n\n> All included files in order.\n\n### Status and logs\n\n```bash\nsystemctl status httpd\ntail -f /var/log/httpd/error_log\n```\n\n> Errors at start or for requests.\n\n### A 403 Forbidden? Check SELinux\n\n```bash\nausearch -m avc -ts recent | grep httpd\nls -Z /var/www/html\n```\n\n> Wrong labels are the most common reason.\n\n### Hide version information (hardening)\n\n```bash\nprintf 'ServerTokens Prod\\nServerSignature Off\\n' > /etc/httpd/conf.d/security.conf\napachectl configtest && systemctl reload httpd\n```\n\n> Headers show only \"Apache\".\n\n### HTTPS\n\n```bash\ndnf install -y mod_ssl; systemctl restart httpd\n```\n\n> Adds ssl.conf with a test certificate on port 443.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/httpd/conf/httpd.conf` | Main config. |\n| `/etc/httpd/conf.d/` | Extra configs and vhosts. |\n| `/etc/httpd/conf.modules.d/` | Module loading. |\n| `/var/log/httpd/` | Logs. |\n| `/var/www/html/` | Default content. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (syntax OK). |\n| `1` | Error. |\n\nFull manual: [man apachectl](https://linux.die.net/man/8/apachectl)\n","page":"commands/apachectl/","label":"apachectl","where":"Command page · Web","aka":["httpd"]},{"id":"cmd-apropos","n":1369,"slug":"apropos","title":"apropos — Search the short descriptions of the man pages (apropos), or show the description of one page (whatis)","sections":["commands"],"topic":"Commands · Shell and help","tags":["help","manual","search","keyword","description"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Search the short descriptions of the man pages (apropos), or show the description of one page (whatis).\n\nSame page for: `whatis`\n\n## About\n\n- `apropos WORD` (same as `man -k WORD`) lists every man page whose name or short description contains WORD. Use it when you do not know which command does a job.\n- `whatis NAME` (same as `man -f NAME`) shows the one-line description of a page, for every section where it exists.\n\nBoth read the database that `mandb` builds. If they say \"nothing appropriate\", run `mandb` as root.\n\n## Syntax\n\n```text\napropos [OPTIONS] KEYWORD ...\nwhatis [OPTIONS] NAME ...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-e, --exact` | apropos: the keyword must match a whole name or a whole word of the description. |\n| `-a, --and` | apropos: show only pages that match all keywords. |\n| `-r, --regex` | Treat each keyword as a regular expression (the default for apropos). |\n| `-w, --wildcard` | Treat each keyword as a shell wildcard like `pass*`. |\n| `-l, --long` | Do not cut long lines to the screen width. |\n| `-s, --sections=<list>` | Search only these sections, like `-s 8` or `-s 1:8`. |\n| `-m, --systems=<list>` | Search the pages of other systems. |\n| `-M, --manpath=<path>` | Search these folders instead of $MANPATH. |\n| `-L, --locale=<locale>` | Use this language. |\n| `-C, --config-file=<file>` | Use another configuration file. |\n| `-v, --verbose` | Print more messages. |\n| `-d, --debug` | Print debug information. |\n| `-?, --help` | Show help. |\n| `--usage` | Show a short usage message. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Find commands about a topic\n\n```bash\napropos partition\n```\n\n> Same as `man -k partition`.\n\n### Find only admin commands (section 8)\n\n```bash\napropos -s 8 volume\n```\n\n> Shows lvcreate, vgcreate, pvcreate … but not library functions.\n\n### Show what a command is\n\n```bash\nwhatis chage\n```\n```text\nchage (1)            - change user password expiry information\n```\n\n> One line, the fastest reminder.\n\n### Match all words, not one of them\n\n```bash\napropos -a firewall zone\n```\n\n> Only pages that mention both words.\n\n### Match a whole word only\n\n```bash\napropos -e cron\n```\n\n> Avoids matches like \"crontab\" when you want exactly \"cron\".\n\n### Use a wildcard\n\n```bash\napropos -w 'ssh*'\n```\n\n> Pages that start with ssh: ssh, ssh-keygen, ssh-copy-id, sshd_config …\n\n### Find the page of a config file\n\n```bash\napropos -s 5 resolv\n```\n```text\nresolv.conf (5)      - resolver configuration file\n```\n\n> Section 5 has the file formats.\n\n### Check what several commands do at once\n\n```bash\nwhatis ss ip nmcli dig\n```\n\n> A quick reminder table in the terminal.\n\n### Find SELinux pages of a service\n\n```bash\napropos -s 8 _selinux | grep httpd\n```\n\n> Needs the package selinux-policy-doc. Very useful in the SELinux tasks.\n\n### Long lines without cutting\n\n```bash\napropos -l \"security context\"\n```\n\n> Full descriptions, even in a small terminal.\n\n### Fix an empty search\n\n```bash\nmandb\napropos user\n```\n\n> Build the database first when the system is new.\n\n## Watch out\n\n- apropos searches only the short description, not the full text. For full text use `man -K`.\n- The results depend on the installed man pages; minimal systems have fewer pages.\n\n## Tips\n\n- apropos = \"which command for this job?\" whatis = \"what is this command?\"\n\nFull manual: [man apropos](https://linux.die.net/man/1/apropos)\n","page":"commands/apropos/","label":"apropos","where":"Command page · Shell and help","aka":["whatis"]},{"id":"cmd-at","n":1370,"slug":"at","title":"at — Run commands once at a later time (at), list (atq) or remove (atrm) those jobs, or run them when the load is low (batch)","sections":["commands"],"topic":"Commands · Scheduling","tags":["one time job","schedule","later","atd","batch"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run commands once at a later time (at), list (atq) or remove (atrm) those jobs, or run them when the load is low (batch).\n\nSame page for: `atq`, `atrm`, `batch`\n\n## About\n\n`at TIME` reads commands from the keyboard (end with Ctrl+D) or from a pipe, and the `atd` service runs them once at that time — with your user, your current folder and your environment.\n\nTimes can be clock times (`16:30`, `4pm`, `midnight`), relative (`now + 2 hours`) or with a date (`09:00 2026-12-31`, `noon tomorrow`). Access is controlled by `/etc/at.allow` and `/etc/at.deny`.\n\n## Syntax\n\n```text\nat [-V] [-q QUEUE] [-f FILE] [-u USER] [-mMkv] TIME\nat [-t [[CC]YY]MMDDhhmm[.ss]]\nat -c JOB...\natq [-q QUEUE] [-o FORMAT]\natrm JOB...\nbatch\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-f FILE` | Read the commands from a file. |\n| `-t TIME` | Time in the form [[CC]YY]MMDDhhmm[.ss]. |\n| `-l` | List jobs (same as atq). |\n| `-r, -d` | Remove jobs (same as atrm). |\n| `-c JOB` | Show the full script of a job. |\n| `-b` | Run when the load is low (same as batch). |\n| `-q QUEUE` | Use a queue (a–z; later letters = nicer). |\n| `-m` | Send mail when the job is done, even without output. |\n| `-M` | Never send mail. |\n| `-u USER` | The user for mail (root only). |\n| `-v` | Show the run time before reading the job. |\n| `-k` | Keep the environment of the job unchanged. |\n| `-o FORMAT` | With atq: time format. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Install and start atd\n\n```bash\ndnf install -y at\nsystemctl enable --now atd\n```\n\n> Without atd nothing runs.\n\n### Plan a command\n\n```bash\nat 23:00\n```\n```text\nat> systemctl restart httpd\nat> <EOT>\njob 3 at Sun Oct 25 23:00:00 2026\n```\n\n> Ctrl+D ends the input.\n\n### Plan with a pipe\n\n```bash\necho \"tar -czf /tmp/home.tgz /home/lisa\" | at now + 2 hours\n```\n\n> No typing needed; good in scripts.\n\n### List and inspect jobs\n\n```bash\natq\n```\n```text\n3\tSun Oct 25 23:00:00 2026 a root\n```\n```bash\nat -c 3 | tail -3\n```\n\n> -c shows the environment and the commands.\n\n### Remove a job\n\n```bash\natrm 3\n```\n\n> Use the number from atq.\n\n### Run a script file at a time\n\n```bash\nat -f /root/maintenance.sh 02:00 tomorrow\n```\n\n> The script runs with sh.\n\n### Safety net for remote firewall changes\n\n```bash\necho \"firewall-cmd --reload\" | at now + 10 minutes\n```\n\n> If your new rule locks you out, the reload restores the permanent rules. Remove the job when all is fine.\n\n### Run when the system is quiet\n\n```bash\necho \"/usr/local/bin/reindex.sh\" | batch\n```\n\n> Starts when the load drops under 0.8.\n\n### Plan a reboot for the night\n\n```bash\necho \"systemctl reboot\" | at 03:00\n```\n\n> Same idea as `shutdown -r 03:00`.\n\n### Block a user from at\n\n```bash\necho lisa >> /etc/at.deny\n```\n\n> Like cron.deny.\n\n### Check for unknown at jobs (security)\n\n```bash\natq; ls -l /var/spool/at/\n```\n\n> Attackers can hide one-time jobs here.\n\n## Watch out\n\n- at uses `sh`, not bash. Avoid bash-only syntax.\n- Output is sent by local mail; redirect it to a file if you need it.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/spool/at/` | The queued jobs. |\n| `/etc/at.allow, /etc/at.deny` | Who may use at. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (bad time, not allowed, atd problem). |\n\nFull manual: [man at](https://linux.die.net/man/1/at)\n","page":"commands/at/","label":"at","where":"Command page · Scheduling","aka":["atq","atrm","batch"]},{"id":"cmd-audit2allow","n":1371,"slug":"audit2allow","title":"audit2allow — Explain SELinux denials (audit2why) or build a policy module that allows them (audit2allow) — the last resort","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","policy module","denials","explanation"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Explain SELinux denials (audit2why) or build a policy module that allows them (audit2allow) — the last resort.\n\nSame page for: `audit2why`\n\n## About\n\n`audit2why` reads AVC messages and says why each one happened: missing rule, a boolean that is off, or a wrong label. `audit2allow -M NAME` writes a policy module that allows exactly the denied actions; `semodule -i NAME.pp` installs it.\n\nUse modules only when labels, ports and booleans cannot solve the problem, and read the rules first: allowing a denial blindly may allow an attack.\n\n## Syntax\n\n```text\naudit2allow [OPTION]...\naudit2why [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Read audit.log and messages. |\n| `-b, --boot` | Only messages since the last boot. |\n| `-i, --input FILE` | Read from a file. |\n| `-d, --dmesg` | Read from dmesg. |\n| `-l, --lastreload` | Only since the last policy reload. |\n| `-M NAME` | Create a module package NAME.pp (and NAME.te). |\n| `-m, --module NAME` | Print a module named NAME. |\n| `-o, --output FILE` | Write to a file. |\n| `-w, --why` | Explain (same as audit2why). |\n| `-D, --dontaudit` | Generate dontaudit rules. |\n| `-R, --reference` | Use interface macros from the reference policy. |\n| `-N, --noreference` | Do not use macros. |\n| `-r, --requires` | Add requires lines. |\n| `-x, --xperms` | Extended permission rules. |\n| `-p, --policy FILE` | Use another policy file. |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Why was it denied?\n\n```bash\nausearch -m AVC -ts recent | audit2why\n```\n```text\nWas caused by:\n\tThe boolean httpd_can_network_connect was set incorrectly.\n```\n\n> Then: setsebool -P httpd_can_network_connect on.\n\n### Missing type enforcement rule\n\n```bash\nausearch -m AVC -c myapp -ts today | audit2why\n```\n\n> \"Missing type enforcement (TE) allow rule\" = no label or boolean fix; maybe a module.\n\n### Show the rules that would be needed\n\n```bash\nausearch -m AVC -c myapp -ts today | audit2allow\n```\n```text\nallow myapp_t var_log_t:file { append open };\n```\n\n> Read them carefully.\n\n### Build and install a module\n\n```bash\nausearch -m AVC -c myapp -ts today | audit2allow -M myapp_local\nsemodule -i myapp_local.pp\n```\n\n> Only after checking myapp_local.te.\n\n### List and remove modules\n\n```bash\nsemodule -l | grep local\nsemodule -r myapp_local\n```\n\n> Remove modules you no longer need.\n\n### Explain from a saved log\n\n```bash\naudit2why -i /root/avc.log\n```\n\n> Analyze offline.\n\n### Since the last boot\n\n```bash\naudit2allow -b -w\n```\n\n> Explanations for this boot.\n\n### Do not allow everything (security)\n\n```bash\nausearch -m AVC -ts today | audit2allow | grep -E 'shadow_t|execmem|setenforce'\n```\n\n> Rules like these mean an attack or a broken app — do not allow them.\n\n### Use reference macros\n\n```bash\nausearch -m AVC -c myapp | audit2allow -R\n```\n\n> Nicer rules with policy interfaces.\n\n### Generate dontaudit rules for noisy denials\n\n```bash\nausearch -m AVC -c myapp | audit2allow -D -M myapp_quiet\n```\n\n> Hides harmless denials (they stay blocked).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man audit2allow](https://linux.die.net/man/1/audit2allow)\n","page":"commands/audit2allow/","label":"audit2allow","where":"Command page · SELinux and security","aka":["audit2why"]},{"id":"cmd-ausearch","n":1372,"slug":"ausearch","title":"ausearch — Search the audit log (/var/log/audit/audit.log): SELinux denials, logins, commands and other kernel audit events","sections":["commands"],"topic":"Commands · SELinux and security","tags":["audit log","selinux denials","avc","forensics","events"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Search the audit log (/var/log/audit/audit.log): SELinux denials, logins, commands and other kernel audit events.\n\n## About\n\nThe audit daemon writes security events. `ausearch` filters them by message type (`-m AVC` for SELinux denials), time (`-ts recent` = last 10 minutes, `today`, `boot`), program, user, file or audit key. `-i` turns numbers (UIDs, syscalls, times) into names.\n\n## Syntax\n\n```text\nausearch [OPTION]...\n```\n\n## Options\n\n\n### What\n\n| Option | What it does |\n|---|---|\n| `-m, --message TYPE[,TYPE]` | Message type: AVC, USER_AVC, SELINUX_ERR, USER_LOGIN, USER_AUTH, SYSCALL, EXECVE, MAC_STATUS … |\n| `-c, --comm NAME` | Program name. |\n| `-x, --executable PATH` | Program path. |\n| `-k, --key KEY` | Events of an audit rule with this key. |\n| `-f, --file NAME` | Events about a file. |\n| `-p, --pid PID` | Process ID. |\n| `-pp, --ppid PID` | Parent process ID. |\n| `-sc, --syscall NAME` | System call. |\n| `-sv, --success yes\\|no` | Only successful / failed events. |\n| `-e, --exit CODE` | Exit code or errno. |\n| `-a, --event ID` | One event by ID. |\n\n### Who\n\n| Option | What it does |\n|---|---|\n| `-ua, --uid-all UID` | Any user ID. |\n| `-ui, --uid UID` | User ID. |\n| `-ue, --uid-effective UID` | Effective user ID. |\n| `-ul, --loginuid ID` | The original login user (survives su/sudo). |\n| `-ga, --gid-all, -gi, --gid, -ge, --gid-effective GID` | Group IDs. |\n| `-hn, --host NAME` | Remote host. |\n| `-tm, --terminal TERM` | Terminal. |\n| `--session ID` | Login session. |\n| `-se, --context CONTEXT` | SELinux context (subject or object). |\n| `-su, --subject CONTEXT` | Subject context. |\n| `-o, --object CONTEXT` | Object context. |\n\n### When\n\n| Option | What it does |\n|---|---|\n| `-ts, --start [DATE] [TIME]` | From: recent, now, today, yesterday, this-week, boot … |\n| `-te, --end [DATE] [TIME]` | Until. |\n\n### Output and input\n\n| Option | What it does |\n|---|---|\n| `-i, --interpret` | Translate numbers into names. |\n| `-r, --raw` | Raw log lines (for audit2allow). |\n| `--format raw\\|default\\|interpret\\|csv\\|text` | Output format. |\n| `--extra-keys, --extra-labels, --extra-obj2, --extra-time` | Extra CSV fields. |\n| `-w, --word` | Match whole words only. |\n| `--just-one` | Stop after the first event. |\n| `-l, --line-buffered` | Flush output after each line. |\n| `-if, --input FILE\\|DIR` | Read another log file or folder. |\n| `--input-logs` | Use the log files from auditd.conf. |\n| `--checkpoint FILE` | Continue where the last search stopped. |\n| `--eoe-timeout SECONDS, --escape OPTION, --arch CPU, -n, --node NAME, -vm, -uu` | Special filters. |\n| `--debug` | Debug output. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### Recent SELinux denials\n\n```bash\nausearch -m AVC -ts recent\n```\n\n> Last 10 minutes.\n\n### Denials of one program today\n\n```bash\nausearch -m AVC -c httpd -ts today -i\n```\n\n> -i gives readable names and times.\n\n### Since the last boot\n\n```bash\nausearch -m AVC,USER_AVC,SELINUX_ERR -ts boot\n```\n\n> All SELinux problems of this boot.\n\n### Explain the denials\n\n```bash\nausearch -m AVC -ts recent | audit2why\n```\n\n> Says if a label, boolean or missing rule is the cause.\n\n### Failed logins (security)\n\n```bash\nausearch -m USER_LOGIN -sv no -ts today -i\n```\n\n> Who failed to log in, from where.\n\n### Who really did it (after sudo)\n\n```bash\nausearch -ul 1001 -ts today -i | less\n```\n\n> loginuid stays the original user.\n\n### Watch a file with an audit rule\n\n```bash\nauditctl -w /etc/passwd -p wa -k passwd_changes\nausearch -k passwd_changes -i\n```\n\n> Every write to /etc/passwd with user and program.\n\n### SELinux mode changes\n\n```bash\nausearch -m MAC_STATUS -ts this-week -i\n```\n\n> Who ran setenforce.\n\n### Commands run as root\n\n```bash\nausearch -m EXECVE -ts today -i | grep -i 'a0=' | head\n```\n\n> Needs audit rules for execve.\n\n### Summary reports\n\n```bash\naureport --avc --summary\naureport -au -ts today\n```\n\n> Counts instead of single events.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/log/audit/audit.log` | The audit log. |\n| `/etc/audit/rules.d/` | Audit rules. |\n| `/etc/audit/auditd.conf` | auditd settings. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Events found. |\n| `1` | Nothing found or error. |\n\nFull manual: [man ausearch](https://linux.die.net/man/8/ausearch)\n","page":"commands/ausearch/","label":"ausearch","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-automount","n":1373,"slug":"automount","title":"automount — The autofs daemon: mounts file systems (mostly NFS) when they are used and unmounts them when idle. Managed with systemctl; the command helps debugging","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["autofs","nfs","on demand","maps","home folders"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"The autofs daemon: mounts file systems (mostly NFS) when they are used and unmounts them when idle. Managed with systemctl; the command helps debugging.\n\n## About\n\nautofs runs as `autofs.service`. The master map (`/etc/auto.master` and `/etc/auto.master.d/*.autofs`) lists base folders and map files; map files list keys, options and sources. `*` and `&` make one line work for many folders (home folders). After changing maps, restart or reload autofs. `automount -m` prints all maps as autofs sees them.\n\n## Syntax\n\n```text\nautomount [OPTION]... [MASTER_MAP]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `/base /etc/auto.NAME` | Master map line: indirect mounts under /base. |\n| `/- /etc/auto.direct` | Master map line: direct mounts (full paths in the map). |\n| `key -options server:/path` | Map line (indirect). |\n| `/full/path -options server:/path` | Map line (direct). |\n| `* -rw server:/home/&` | Wildcard: & is the key that was asked for. |\n| `--timeout=N` | In the master map: unmount after N idle seconds. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-m, --dumpmaps [TYPE NAME]` | Print the maps and exit. |\n| `-f, --foreground` | Run in the foreground (debug). |\n| `-d[LEVEL], --debug[=LEVEL]` | Debug output. |\n| `-v, --verbose` | Verbose log. |\n| `-t, --timeout SECONDS` | Default idle timeout (600). |\n| `-n, --negative-timeout SECONDS` | Cache time for failed lookups. |\n| `-M, --master-wait SECONDS` | Wait for the master map at start. |\n| `-D VARIABLE=VALUE, --define VARIABLE=VALUE` | Define a map variable. |\n| `-p, --pid-file FILE` | PID file. |\n| `-r, --random-multimount-selection` | Random server choice for replicated mounts. |\n| `-O, --global-options OPTIONS` | Options added to all mounts. |\n| `-l, --set-log-priority PRIORITY PATH` | Change the log level of a running daemon for a path. |\n| `-S, --systemd-service` | Run as a systemd service. |\n| `-C, --dont-check-daemon` | Do not check for a running daemon. |\n| `-F, --force` | Force unlink of old mount points at start. |\n| `-U, --force-exit` | Force exit, unmount everything. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Install and start\n\n```bash\ndnf install -y autofs\nsystemctl enable --now autofs\n```\n\n> Reads the master map.\n\n### Indirect map\n\n```bash\ncat /etc/auto.master.d/shares.autofs /etc/auto.shares\n```\n```text\n/shares /etc/auto.shares\ndata -rw server2:/exports/data\n```\n\n> /shares/data mounts on first use.\n\n### Home folders with a wildcard\n\n```bash\ncat /etc/auto.rhome\n```\n```text\n* -rw server2:/exports/home/&\n```\n\n> /rhome/lisa → server2:/exports/home/lisa.\n\n### Apply map changes\n\n```bash\nsystemctl restart autofs\n```\n\n> Or `systemctl reload autofs` for map file changes.\n\n### Show what autofs knows\n\n```bash\nautomount -m\n```\n\n> All maps after parsing.\n\n### Trigger and check a mount\n\n```bash\nls /shares/data; findmnt /shares/data\n```\n\n> The folder appears only when used.\n\n### Debug a failing map\n\n```bash\nsystemctl stop autofs; automount -f -d\n```\n\n> Foreground with debug output; then `ls /shares/data` in another terminal.\n\n### Unmount after 60 seconds\n\n```bash\necho \"/shares /etc/auto.shares --timeout=60\" > /etc/auto.master.d/shares.autofs\n```\n\n> Shorter idle time.\n\n### Direct map\n\n```bash\ncat /etc/auto.direct\n```\n```text\n/srv/projects -rw,soft server2:/exports/data\n```\n\n> Master map line: `/- /etc/auto.direct`.\n\n### Logs\n\n```bash\njournalctl -u autofs --since -10min\n```\n\n> Errors like \"mount(nfs): nfs: mount failure\".\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/auto.master, /etc/auto.master.d/*.autofs` | Master map. |\n| `/etc/auto.*` | Map files. |\n| `/etc/autofs.conf` | Daemon settings. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man automount](https://linux.die.net/man/8/automount)\n","page":"commands/automount/","label":"automount","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-awk","n":1374,"slug":"awk","title":"awk — Process text line by line as fields: print columns, filter by conditions, count and sum","sections":["commands"],"topic":"Commands · Text and search","tags":["columns","fields","report","sum","text processing","csv","logs"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Process text line by line as fields: print columns, filter by conditions, count and sum.\n\nSame page for: `gawk`\n\n## About\n\n`awk` reads input line by line and splits each line into **fields**: `$1`, `$2` … (`$0` is the whole line). An awk program is a list of `pattern { action }` pairs: when the pattern matches a line, the action runs.\n\nUseful built-in variables: `NR` (line number), `NF` (number of fields, so `$NF` = last field), `FS` (field separator, set with `-F`), `OFS` (output separator). `BEGIN { }` runs before the first line, `END { }` after the last.\n\nOn RHEL, `awk` is GNU awk (gawk).\n\n## Syntax\n\n```text\nawk [OPTIONS] 'PROGRAM' [FILE...]\nawk [OPTIONS] -f PROGRAM-FILE [FILE...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-F <fs>, --field-separator <fs>` | The field separator, like `-F:` or `-F,` (a regex is allowed). |\n| `-v <var>=<value>, --assign <var>=<value>` | Set a variable before the program starts. |\n| `-f <file>, --file <file>` | Read the program from a file. |\n| `-e <text>, --source <text>` | Program text (can be combined with -f). |\n| `-E <file>, --exec <file>` | Like -f, but the last option (for scripts with #!). |\n| `-i <file>, --include <file>` | Load an awk library file. |\n| `-l <lib>, --load <lib>` | Load a shared library extension. |\n| `-b, --characters-as-bytes` | Treat all input as bytes, not characters. |\n| `-c, --traditional` | Compatibility mode: no GNU extensions. |\n| `-P, --posix` | Strict POSIX mode. |\n| `-r, --re-interval` | Allow {n,m} in regular expressions (default in new versions). |\n| `-n, --non-decimal-data` | Read octal and hex numbers in the input. |\n| `-N, --use-lc-numeric` | Use the local decimal point when reading input. |\n| `-M, --bignum` | Use big numbers with any precision. |\n| `-S, --sandbox` | Disable system(), redirections and reading other files. |\n| `-d[<file>], --dump-variables[=<file>]` | Print all global variables at the end. |\n| `-D[<file>], --debug[=<file>]` | Start the awk debugger. |\n| `-p[<file>], --profile[=<file>]` | Write a profile of the program. |\n| `-o[<file>], --pretty-print[=<file>]` | Write a nicely formatted version of the program. |\n| `-L[<value>], --lint[=<value>]` | Warn about bad or not portable code. |\n| `-t, --lint-old` | Warn about code that old awk versions do not support. |\n| `-O, --optimize` | Turn on optimizations (default). |\n| `-s, --no-optimize` | Turn off optimizations. |\n| `-I, --trace` | Print each instruction as it runs. |\n| `-g, --gen-pot` | Make a .pot file for translations. |\n| `-C, --copyright` | Show the license. |\n| `-V, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Print one column\n\n```bash\nawk '{print $1}' /var/log/httpd/access_log\n```\n\n> Field 1 = the client IP address. Fields are split by spaces.\n\n### Use another separator\n\n```bash\nawk -F: '{print $1, $7}' /etc/passwd\n```\n\n> User name and shell. `-F:` sets the separator.\n\n### Filter by a condition\n\n```bash\nawk -F: '$3 >= 1000 {print $1}' /etc/passwd\n```\n\n> Only normal users (UID 1000 and higher).\n\n### Print lines that match a pattern\n\n```bash\nawk '/sshd/ {print $1, $2, $3, $5}' /var/log/messages\n```\n\n> Like grep, but you choose the columns.\n\n### The last field of each line\n\n```bash\nawk '{print $NF}' file.txt\n```\n\n> `NF` = number of fields, so `$NF` = the last one.\n\n### Sum a column\n\n```bash\nps aux | awk 'NR>1 {sum += $6} END {printf \"%.1f MiB\\n\", sum/1024}'\n```\n\n> Total RSS memory of all processes. `NR>1` skips the header.\n\n### Disk use above 80%\n\n```bash\ndf -P | awk 'NR>1 && $5+0 > 80 {print $6 \" is \" $5 \" full\"}'\n```\n\n> `$5+0` turns \"85%\" into the number 85.\n\n### Count lines per value (like a pivot)\n\n```bash\nawk '{count[$9]++} END {for (c in count) print c, count[c]}' /var/log/httpd/access_log\n```\n\n> Number of requests per HTTP status code.\n\n### Pass a shell variable into awk\n\n```bash\nLIMIT=500; awk -v max=\"$LIMIT\" '$2 > max' usage.txt\n```\n\n> `-v` is the safe way; do not put shell variables inside the awk quotes.\n\n### Change the output separator to make a CSV\n\n```bash\nawk -F: 'BEGIN {OFS=\",\"} {print $1, $3, $6}' /etc/passwd\n```\n\n> user,uid,home for every account.\n\n### Print a range of lines\n\n```bash\nawk 'NR>=10 && NR<=20' /etc/services\n```\n\n> Lines 10 to 20.\n\n### Failed SSH logins per IP\n\n```bash\nawk '/Failed password/ {print $(NF-3)}' /var/log/secure | sort | uniq -c | sort -rn | head\n```\n\n> The IP is the 4th field from the end in these log lines.\n\n### Remove repeated lines and keep the order\n\n```bash\nawk '!seen[$0]++' list.txt\n```\n\n> Prints each line only the first time it appears.\n\n## Watch out\n\n- Put the program in single quotes, so the shell does not change `$1`.\n- Without `-F`, fields are split on runs of spaces and tabs (good for most command output).\n- awk counts fields from 1; `$0` is the whole line.\n\n## Tips\n\n- Pattern { action }. `$1` first field, `$NF` last, `NR` line number, `-F:` separator, `END {}` totals.\n\nFull manual: [man awk](https://linux.die.net/man/1/awk)\n","page":"commands/awk/","label":"awk","where":"Command page · Text and search","aka":["gawk"]},{"id":"cmd-bash","n":1375,"slug":"bash","title":"bash — The Bourne Again Shell: the default command shell of RHEL, and the program that runs shell scripts","sections":["commands"],"topic":"Commands · Shell and help","tags":["shell","scripts","login shell","syntax check","debug","startup files"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"The Bourne Again Shell: the default command shell of RHEL, and the program that runs shell scripts.\n\n## About\n\n`bash` reads commands from the terminal or from a script. A **login shell** reads `/etc/profile`, `/etc/profile.d/*.sh` and `~/.bash_profile`; an interactive shell reads `~/.bashrc` (and `/etc/bashrc`). `bash -n script` checks the syntax; `bash -x script` shows each command as it runs.\n\n## Syntax\n\n```text\nbash [OPTION]... [-c COMMAND | SCRIPT [ARGUMENT]...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-c STRING` | Run the commands in STRING. |\n| `-n` | Read but do not run (syntax check). |\n| `-x` | Print each command before running it (trace). |\n| `-v` | Print each line as it is read. |\n| `-e` | Exit at the first failing command. |\n| `-u` | Error on unset variables. |\n| `-i` | Interactive shell. |\n| `-l, --login` | Act as a login shell (read the profile files). |\n| `-s` | Read commands from standard input. |\n| `-r, --restricted` | Restricted shell (no cd, no PATH changes …). |\n| `-D, --dump-strings` | List translatable strings. |\n| `-O OPTION, +O OPTION` | Set or unset a shopt option. |\n| `-o OPTION` | Set an option like `set -o`. |\n| `--noprofile` | Do not read the profile files. |\n| `--norc` | Do not read ~/.bashrc. |\n| `--rcfile FILE` | Read FILE instead of ~/.bashrc. |\n| `--posix` | POSIX mode. |\n| `--debugger` | Debugger profile. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Run a script\n\n```bash\nbash backup.sh\n```\n\n> Works even without the x permission.\n\n### Check the syntax\n\n```bash\nbash -n backup.sh\n```\n\n> No output = no syntax error.\n\n### Trace a script\n\n```bash\nbash -x backup.sh 2>&1 | less\n```\n\n> Each command with filled-in values (lines start with +).\n\n### Run one command line\n\n```bash\nbash -c 'for i in 1 2 3; do echo $i; done'\n```\n\n> Useful with sudo, ssh and find -exec.\n\n### A clean shell for tests\n\n```bash\nbash --noprofile --norc\n```\n\n> Without your aliases and settings.\n\n### Start a login shell\n\n```bash\nbash -l\n```\n\n> Reads the profile files again (after changes).\n\n### Which shell am I using?\n\n```bash\necho $SHELL; ps -p $$ -o comm=\n```\n```text\n/bin/bash\nbash\n```\n\n> $SHELL is the login shell; ps shows the running one.\n\n### Version (for script features)\n\n```bash\nbash --version | head -1\n```\n\n> RHEL 10 has bash 5.2.\n\n### Run a script with sudo and a heredoc\n\n```bash\nsudo bash -s <<'EOF2'\ndnf -y upgrade\nsystemctl restart httpd\nEOF2\n```\n\n> Several root commands in one go.\n\n### Find scripts with strict settings\n\n```bash\ngrep -l 'set -euo pipefail' /usr/local/bin/*.sh\n```\n\n> Scripts that stop on errors.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/profile, /etc/profile.d/` | Login settings for all users. |\n| `/etc/bashrc` | Interactive settings for all users. |\n| `~/.bash_profile, ~/.bashrc` | Personal settings. |\n| `~/.bash_history` | Command history. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The script or command worked. |\n| `1-255` | The exit status of the last command (2 = wrong use of a builtin, 126 = cannot run, 127 = not found). |\n\nFull manual: [man bash](https://linux.die.net/man/1/bash)\n","page":"commands/bash/","label":"bash","where":"Command page · Shell and help","aka":[]},{"id":"cmd-blkid","n":1376,"slug":"blkid","title":"blkid — Show the UUID, label and type of file systems and swap spaces — the values you need for /etc/fstab","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["uuid","label","file system type","fstab","partuuid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the UUID, label and type of file systems and swap spaces — the values you need for /etc/fstab.\n\n## About\n\n`blkid` reads the signatures on block devices and prints them as `NAME=\"value\"` pairs: UUID, LABEL, TYPE, PARTUUID, PARTLABEL. Without root it may show cached (old) data; run it as root for fresh values.\n\n## Syntax\n\n```text\nblkid [OPTION]... [DEVICE]...\nblkid -L LABEL | -U UUID\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --match-tag TAG` | Show only this tag (UUID, LABEL, TYPE …). |\n| `-o, --output FORMAT` | value, device, export, full, list, udev. |\n| `-L, --label LABEL` | Print the device with this label. |\n| `-U, --uuid UUID` | Print the device with this UUID. |\n| `-t, --match-token NAME=VALUE` | Only devices with this tag value, like TYPE=xfs. |\n| `-n, --match-types LIST` | Only these file system types (with -p). |\n| `-u, --usages LIST` | Only these usages (filesystem, raid, crypto, other). |\n| `-p, --probe` | Low-level probe, no cache. |\n| `-i, --info` | I/O limits of the device. |\n| `-k, --list-filesystems` | List all known types. |\n| `-l, --list-one` | Only the best match (with -t). |\n| `-c, --cache-file FILE` | Use another cache file. |\n| `-g, --garbage-collect` | Clean the cache. |\n| `-d, --no-encoding` | Do not encode special characters. |\n| `-D, --no-part-details` | No partition table details. |\n| `-H, --hint SETTING` | Probing hints. |\n| `-O, --offset OFFSET` | Probe at an offset. |\n| `-S, --size SIZE` | Size of the device to probe. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### All devices\n\n```bash\nblkid\n```\n```text\n/dev/vdb1: LABEL=\"data\" UUID=\"4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\" BLOCK_SIZE=\"512\" TYPE=\"xfs\" PARTUUID=\"2a9b…\"\n```\n\n> UUID for fstab, TYPE for the type field.\n\n### One device\n\n```bash\nblkid /dev/vdb1\n```\n\n> Only that partition.\n\n### Only the UUID value\n\n```bash\nblkid -s UUID -o value /dev/vdb1\n```\n```text\n4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\n```\n\n> Perfect for scripts.\n\n### Add an fstab line without typing the UUID\n\n```bash\necho \"UUID=$(blkid -s UUID -o value /dev/vdb1) /data xfs defaults 0 0\" >> /etc/fstab\n```\n\n> No typing mistakes.\n\n### Find the device of a UUID or label\n\n```bash\nblkid -U 4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\n```\n```text\n/dev/vdb1\n```\n```bash\nblkid -L data\n```\n\n> The reverse lookup.\n\n### All XFS file systems\n\n```bash\nblkid -t TYPE=xfs\n```\n\n> Filter by type.\n\n### As shell variables\n\n```bash\neval $(blkid -o export /dev/vdb1); echo $TYPE $UUID\n```\n\n> Use the values in a script.\n\n### Probe without cache\n\n```bash\nblkid -p /dev/vdb1\n```\n\n> Fresh data, also for partitions without a file system.\n\n### Find duplicate UUIDs (cloned disks)\n\n```bash\nblkid -s UUID -o value | sort | uniq -d\n```\n\n> Duplicates cause mount and LVM problems.\n\n### Find encrypted volumes\n\n```bash\nblkid -t TYPE=crypto_LUKS\n```\n\n> LUKS-encrypted partitions.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `2` | Nothing found. |\n| `4` | Usage error. |\n| `8` | Ambivalent result (more than one signature). |\n\nFull manual: [man blkid](https://linux.die.net/man/8/blkid)\n","page":"commands/blkid/","label":"blkid","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-bzip2","n":1377,"slug":"bzip2","title":"bzip2 — Compress files to .bz2 (bzip2), unpack them (bunzip2), or read them without unpacking (bzcat)","sections":["commands"],"topic":"Commands · Archive and compress","tags":["compress","decompress","bz2","archive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Compress files to .bz2 (bzip2), unpack them (bunzip2), or read them without unpacking (bzcat).\n\nSame page for: `bunzip2`, `bzcat`, `bzgrep`\n\n## About\n\nbzip2 makes smaller files than gzip, but it is slower. It replaces FILE with `FILE.bz2`. The RHCSA objectives name bzip2 together with tar and gzip.\n\nLike gzip, it works on single files. For folders, use `tar -cjf` (the `-j` option calls bzip2).\n\nRead `.bz2` files without unpacking with `bzcat`, `bzless`, `bzgrep`.\n\n## Syntax\n\n```text\nbzip2 [OPTIONS] [FILE...]\nbunzip2 [OPTIONS] [FILE...]\nbzcat [-s] [FILE...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --decompress` | Decompress (same as bunzip2). |\n| `-z, --compress` | Compress (the default for bzip2). |\n| `-k, --keep` | Keep the original file. |\n| `-c, --stdout` | Write to the screen (stdout), keep the files. |\n| `-t, --test` | Test the file: is it complete and not damaged? |\n| `-f, --force` | Overwrite existing output files. |\n| `-q, --quiet` | Hide warnings that are not important. |\n| `-v, --verbose` | Show the ratio of each file (-vv for more). |\n| `-s, --small` | Use less memory (slower), for small machines. |\n| `-1 ... -9` | Block size 100k to 900k. -9 (the default) compresses best. |\n| `--fast` | Same as -1. |\n| `--best` | Same as -9. |\n| `-L, --license, -V, --version` | Show the license and version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Compress a file\n\n```bash\nbzip2 report.csv\n```\n\n> You get report.csv.bz2; the original is removed.\n\n### Decompress\n\n```bash\nbunzip2 report.csv.bz2\n```\n\n> Same as `bzip2 -d report.csv.bz2`.\n\n### Keep the original\n\n```bash\nbzip2 -k big.log\n```\n\n> Both files stay.\n\n### Make a tar archive with bzip2\n\n```bash\ntar -cjf etc.tar.bz2 /etc\n```\n\n> The `-j` option uses bzip2.\n\n### Extract a tar.bz2 archive\n\n```bash\ntar -xjf etc.tar.bz2 -C /tmp\n```\n\n> Plain `-xf` also works; tar detects bzip2.\n\n### Read without unpacking\n\n```bash\nbzcat data.txt.bz2 | head\n```\n\n> Fast look at the first lines.\n\n### Search inside .bz2 files\n\n```bash\nbzgrep \"ERROR\" app-*.log.bz2\n```\n\n> Like grep, but on compressed files.\n\n### Test a file\n\n```bash\nbzip2 -tv backup.sql.bz2\n```\n\n> Shows \"ok\" when the file is good.\n\n### Compare sizes of gzip, bzip2 and xz\n\n```bash\nfor c in gzip bzip2 xz; do $c -c big.log > big.$c; done; ls -lh big.*\n```\n\n> See which one fits your data best.\n\n### Compress a stream\n\n```bash\nmysqldump shop | bzip2 > shop.sql.bz2\n```\n\n> No uncompressed copy on disk.\n\n### Decompress to stdout and keep the archive\n\n```bash\nbzip2 -dc logs.tar.bz2 | tar -t\n```\n\n> List the tar content without writing anything.\n\n### Use less memory on a small VM\n\n```bash\nbzip2 -s huge.dump\n```\n\n> Slower, but it fits in little RAM.\n\n## Watch out\n\n- bzip2 removes the original by default. Use `-k` to keep it.\n- bzip2 is slow for big data. For speed use gzip (or pigz); for the smallest size use xz.\n\n## Tips\n\n- bzip2 / bunzip2 / bzcat. In tar: `-j`.\n\nFull manual: [man bzip2](https://linux.die.net/man/1/bzip2)\n","page":"commands/bzip2/","label":"bzip2","where":"Command page · Archive and compress","aka":["bunzip2","bzcat","bzgrep"]},{"id":"cmd-cat","n":1378,"slug":"cat","title":"cat — Print files to the screen, join files together (cat), or print them last line first (tac)","sections":["commands"],"topic":"Commands · Text and search","tags":["show","print","concatenate","file","here document","hidden characters"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print files to the screen, join files together (cat), or print them last line first (tac).\n\nSame page for: `tac`\n\n## About\n\n`cat` (concatenate) writes the content of one or more files to standard output. With one file you read it; with several files you join them; with a redirect you create a new file.\n\nIt is perfect for short files. For long files, use `less` (scrolling) or `head`/`tail` (parts).\n\n`tac` does the same, but starts with the last line (useful for logs: newest first).\n\n## Syntax\n\n```text\ncat [OPTION]... [FILE]...\ntac [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --number` | Number all lines. |\n| `-b, --number-nonblank` | Number only non-empty lines. |\n| `-A, --show-all` | Show hidden characters: tabs as ^I, line ends as $, others as ^ or M- (same as -vET). |\n| `-E, --show-ends` | Show $ at the end of each line. |\n| `-T, --show-tabs` | Show tabs as ^I. |\n| `-v, --show-nonprinting` | Show non-printable characters with ^ and M- notation. |\n| `-e` | Same as -vE. |\n| `-t` | Same as -vT. |\n| `-s, --squeeze-blank` | Show several empty lines as one. |\n| `-u` | Ignored. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n### tac\n\n| Option | What it does |\n|---|---|\n| `-b, --before` | Put the separator before the record instead of after. |\n| `-r, --regex` | Treat the separator as a regular expression. |\n| `-s, --separator=<string>` | Use this separator instead of a new line. |\n\n## Examples\n\n### Show a short file\n\n```bash\ncat /etc/hostname\n```\n```text\nserver1.example.com\n```\n\n> The fastest way to read a small file.\n\n### Show line numbers\n\n```bash\ncat -n /etc/fstab\n```\n\n> Easy to talk about \"line 7\".\n\n### Find hidden characters (tabs, Windows line ends)\n\n```bash\ncat -A deploy.sh | head\n```\n```text\n#!/bin/bash^M$\n```\n\n> `^M$` means Windows line ends: Bash will fail. `^I` would show tabs (bad in YAML).\n\n### Join several files into one\n\n```bash\ncat part1.csv part2.csv part3.csv > all.csv\n```\n\n> The files are added one after the other.\n\n### Create a file with several lines (here-document)\n\n```bash\ncat > /tmp/test.conf <<'EOF'\nport=8080\nlog=/var/log/app.log\nEOF\n```\n\n> Everything until the line `EOF` goes into the file.\n\n### Add lines to a root-owned file\n\n```bash\ncat <<'EOF' | sudo tee -a /etc/hosts\n192.168.122.20 server2.example.com server2\nEOF\n```\n\n> `sudo tee -a` appends as root.\n\n### Show the newest log lines first\n\n```bash\ntac /var/log/dnf.log | head -20\n```\n\n> The last 20 events, newest at the top.\n\n### Show the OS version\n\n```bash\ncat /etc/os-release\n```\n\n> NAME, VERSION_ID and ID of the distribution.\n\n### Squeeze empty lines while reading\n\n```bash\ncat -s notes.txt\n```\n\n> Several empty lines become one.\n\n### Read kernel information\n\n```bash\ncat /proc/cpuinfo | grep \"model name\" | head -1\n```\n\n> /proc files are created by the kernel when you read them.\n\n### Show a file only if it exists\n\n```bash\n[ -f /etc/motd ] && cat /etc/motd\n```\n\n> No error message in a script when the file is missing.\n\n### Copy a disk image to a USB stick (as a simple dd)\n\n```bash\ncat rhel-10.iso > /dev/sdX\n```\n\n> Works like dd for this job. Check the device name three times: it overwrites the whole disk.\n\n## Watch out\n\n- `cat bigfile` floods the screen; use `less`.\n- `cat file > file` empties the file before reading it. Never read and write the same file in one command.\n- \"Useless use of cat\": `cat file | grep x` works, but `grep x file` is simpler.\n\n## Tips\n\n- cat = show/join, tac = backwards, `cat -A` = invisible characters, `cat > file <<'EOF'` = new file.\n\nFull manual: [man cat](https://linux.die.net/man/1/cat)\n","page":"commands/cat/","label":"cat","where":"Command page · Text and search","aka":["tac"]},{"id":"cmd-cd","n":1379,"slug":"cd","title":"cd — Change the current folder of your shell","sections":["commands"],"topic":"Commands · Files and folders","tags":["folder","directory","navigate","home","path"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the current folder of your shell.\n\n## About\n\n`cd` (change directory) moves your shell to another folder. It is built into the shell, because only the shell itself can change its own current folder.\n\nSpecial targets: `cd` alone or `cd ~` = your home folder, `cd -` = the previous folder, `cd ..` = one level up, `cd /` = the root.\n\nThe variable `CDPATH` can hold a list of base folders that `cd` also searches.\n\n## Syntax\n\n```text\ncd [-L|[-P [-e]]] [DIRECTORY]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `DIRECTORY` | The folder to go to. Without it: your home folder. |\n| `-` | Go back to the previous folder (and print its name). |\n| `-L` | Follow symbolic links logically (the default): `pwd` shows the link path. |\n| `-P` | Use the physical folder: resolve symbolic links. |\n| `-e` | With -P: return an error if the current folder cannot be found. |\n| `-@` | On systems that support it: show extended attributes as a folder. |\n\n## Examples\n\n### Go to a folder\n\n```bash\ncd /var/log\n```\n\n> An absolute path works from anywhere.\n\n### Go back to your home folder\n\n```bash\ncd\n```\n\n> Same as `cd ~` or `cd $HOME`.\n\n### Switch between two folders\n\n```bash\ncd /etc/nginx/conf.d\ncd /var/log/nginx\ncd -\n```\n\n> `cd -` jumps back and forth. Very handy while you edit config and read logs.\n\n### Go up one or two levels\n\n```bash\ncd ..\ncd ../..\n```\n\n> `..` is the parent folder.\n\n### Go to the home folder of another user\n\n```bash\ncd ~alice\n```\n\n> Works for root or when you have the rights to that folder.\n\n### Go to the real folder behind a link\n\n```bash\ncd -P /etc/alternatives/java\npwd\n```\n\n> Shows the real path of the link target.\n\n### Stop a script when cd fails\n\n```bash\ncd /srv/app || exit 1\n```\n\n> Without this check, the next commands (like `rm -rf *`) would run in the wrong folder.\n\n### Use the folder of the script itself\n\n```bash\ncd \"$(dirname \"$0\")\"\n```\n\n> Makes a script work from any place it is called from.\n\n### Use a short base path list\n\n```bash\nexport CDPATH=.:/srv/apps:/etc\ncd ssh\n```\n\n> `cd ssh` now finds `/etc/ssh`. Put CDPATH in `~/.bashrc` if you like it.\n\n### Remember folders on a stack\n\n```bash\npushd /etc/ssh\npushd /var/log\npopd\n```\n\n> `pushd` and `popd` keep a list of folders. `dirs -v` shows it.\n\n### Go to a folder with spaces in its name\n\n```bash\ncd \"/mnt/backup/Project Files\"\n```\n\n> Quotes (or a backslash before the space) keep the name together.\n\n## Watch out\n\n- `cd` in a script changes only the folder of that script, not of your shell. Use `source` if you need that.\n- `sudo cd /root` does not work (`cd` is a built-in). Use `sudo -i` and then `cd`.\n\n## Tips\n\n- `cd` = home, `cd -` = back, `cd ..` = up.\n\nFull manual: [man cd](https://linux.die.net/man/1/bash)\n","page":"commands/cd/","label":"cd","where":"Command page · Files and folders","aka":[]},{"id":"cmd-chage","n":1380,"slug":"chage","title":"chage — Show and change the password aging and the expire date of an account","sections":["commands"],"topic":"Commands · Users and groups","tags":["password aging","expire","account expiration","max days","policy","compliance"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change the password aging and the expire date of an account.\n\n## About\n\n`chage` (change age) sets how long a password is valid, when users get a warning, when an account is locked after the password expired, and when the whole account expires.\n\nAll values are in `/etc/shadow`. Without options (as root) chage asks for every value. Normal users may only list their own data with `chage -l`.\n\nFor new users, the defaults come from `PASS_MAX_DAYS`, `PASS_MIN_DAYS` and `PASS_WARN_AGE` in `/etc/login.defs`.\n\n## Syntax\n\n```text\nchage [OPTION]... LOGIN\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --list` | Show the aging data of the account. |\n| `-M, --maxdays DAYS` | Maximum days a password is valid (-1 = no limit). |\n| `-m, --mindays DAYS` | Minimum days between two changes (0 = any time). |\n| `-W, --warndays DAYS` | Warn this many days before the password expires. |\n| `-I, --inactive DAYS` | Lock the account this many days after the password expired (-1 = never). |\n| `-E, --expiredate DATE` | The account expires on this date (YYYY-MM-DD, or days since 1970; -1 = never; 0 = expired now). |\n| `-d, --lastday DATE` | Set the date of the last change (0 = the user must change the password at the next login). |\n| `-i, --iso8601` | With -l: show dates as YYYY-MM-DD. |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Show the aging data\n\n```bash\nchage -l lisa\n```\n```text\nLast password change                                    : Oct 25, 2026\nPassword expires                                        : Jan 23, 2027\nPassword inactive                                       : never\nAccount expires                                         : never\nMinimum number of days between password change          : 1\nMaximum number of days between password change          : 90\nNumber of days of warning before password expires       : 7\n```\n\n> Users can run `chage -l` for their own account.\n\n### Change every 90 days, warn 7 days before\n\n```bash\nchage -M 90 -W 7 lisa\n```\n\n> A common company rule.\n\n### Let the account expire on a date\n\n```bash\nchage -E 2026-12-31 contractor1\n```\n\n> After this day the user cannot log in.\n\n### Force a new password at the next login\n\n```bash\nchage -d 0 lisa\n```\n\n> Good after you gave a temporary password.\n\n### Block an account at once\n\n```bash\nchage -E 0 olduser\n```\n\n> Stops all logins, also SSH keys (better than passwd -l).\n\n### Remove the expire date\n\n```bash\nchage -E -1 contractor1\n```\n\n> The account never expires.\n\n### Lock the account 14 days after the password expired\n\n```bash\nchage -I 14 lisa\n```\n\n> A forgotten account gets locked by itself.\n\n### Stop a user from changing back at once\n\n```bash\nchage -m 1 lisa\n```\n\n> With a password history rule, users cannot cycle back to the old password.\n\n### Ask for every value\n\n```bash\nchage lisa\n```\n\n> Interactive mode: press Enter to keep a value.\n\n### Show dates in ISO format\n\n```bash\nchage -l -i lisa | head -2\n```\n\n> Easier to read in scripts.\n\n### Find accounts without a max age (audit)\n\n```bash\nawk -F: '$2 ~ /^\\$/ && ($5 == \"\" || $5 == 99999) {print $1}' /etc/shadow\n```\n\n> Users with a password that never expires (check against your policy).\n\n### Apply a rule to all normal users\n\n```bash\nfor u in $(awk -F: '$3 >= 1000 && $3 < 60000 {print $1}' /etc/passwd); do chage -M 90 \"$u\"; done\n```\n\n> login.defs changes only affect new users; this changes the existing ones.\n\n## Watch out\n\n- `-E` is the account expire date; `-M` is about the password. Do not mix them.\n- Changes in /etc/login.defs do not change existing users.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/shadow` | The aging fields (fields 3 to 8). |\n| `/etc/login.defs` | Defaults for new users. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Permission denied. |\n| `2` | Wrong syntax. |\n| `15` | The shadow file is missing. |\n\nFull manual: [man chage](https://linux.die.net/man/1/chage)\n","page":"commands/chage/","label":"chage","where":"Command page · Users and groups","aka":[]},{"id":"cmd-chattr","n":1381,"slug":"chattr","title":"chattr — Set or remove file attributes, like immutable (no change at all) and append only","sections":["commands"],"topic":"Commands · Permissions","tags":["attributes","immutable","append only","protect file","ransomware","logs"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Set or remove file attributes, like immutable (no change at all) and append only.\n\n## About\n\nAttributes work below the normal permissions. A file with `+i` (immutable) cannot be changed, deleted, renamed or linked — not even by root — until the attribute is removed with `chattr -i`. With `+a` you can only add data at the end.\n\nOnly root (with the right capability) can set `i` and `a`. Support depends on the file system: ext4 knows many attributes; XFS supports a smaller set (like i, a, d, S, A, s… only some).\n\n## Syntax\n\n```text\nchattr [-RVf] [-v VERSION] [-p PROJECT] [+-=ATTRIBUTES] FILE...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `+ - =` | Add, remove, set exactly. |\n| `i` | Immutable: no change, delete, rename, link. |\n| `a` | Append only: only add data (good for logs). |\n| `A` | Do not update the access time. |\n| `d` | Skip the file in `dump` backups. |\n| `S` | Write changes to disk at once (synchronous). |\n| `c` | Compress (some file systems). |\n| `C` | No copy-on-write (btrfs). |\n| `e` | The file uses extents (ext4, cannot be removed). |\n| `u, s` | Undeletable / secure delete (not used by ext4). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R` | Change directories and their content recursively. |\n| `-V` | Verbose; also show the version. |\n| `-f` | Do not show most errors. |\n| `-v VERSION` | Set the version/generation number. |\n| `-p PROJECT` | Set the project number (for project quotas). |\n\n## Examples\n\n### Protect a file against every change\n\n```bash\nchattr +i /etc/resolv.conf\n```\n\n> Stops tools (and people) from changing DNS settings.\n\n### Show the attributes\n\n```bash\nlsattr /etc/resolv.conf\n```\n```text\n----i----------------- /etc/resolv.conf\n```\n\n> The `i` is set.\n\n### Even root cannot delete it\n\n```bash\nrm -f /etc/resolv.conf\n```\n```text\nrm: cannot remove '/etc/resolv.conf': Operation not permitted\n```\n\n> Remove the attribute first.\n\n### Remove the protection\n\n```bash\nchattr -i /etc/resolv.conf\n```\n\n> Now normal permissions count again.\n\n### Make a log file append only\n\n```bash\nchattr +a /var/log/app/audit.log\n```\n\n> An attacker cannot delete old lines (but can add new ones).\n\n### Protect important files against ransomware or mistakes\n\n```bash\nchattr -R +i /srv/archive/2025\n```\n\n> Old data that must never change.\n\n### Find immutable files when a change fails\n\n```bash\nlsattr -R /etc 2>/dev/null | grep -- '-i-'\n```\n\n> \"Operation not permitted\" as root often means `i`.\n\n### Check for attacker tricks (security)\n\n```bash\nlsattr /etc/passwd /etc/shadow /root/.ssh/authorized_keys\n```\n\n> Attackers set `+i` on their backdoor files so admins cannot remove them.\n\n### No access time updates for a busy file\n\n```bash\nchattr +A /srv/cache/data.db\n```\n\n> Fewer writes (most systems use relatime anyway).\n\n### Set exactly these attributes\n\n```bash\nchattr =a /var/log/app/audit.log\n```\n\n> Removes all others.\n\n## Watch out\n\n- An immutable file breaks updates: package updates and config tools fail on it. Document where you use it.\n- Not all file systems support all attributes (\"Operation not supported\").\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chattr](https://linux.die.net/man/1/chattr)\n","page":"commands/chattr/","label":"chattr","where":"Command page · Permissions","aka":[]},{"id":"cmd-chcon","n":1382,"slug":"chcon","title":"chcon — Change the SELinux context of files directly — only for a quick test, because restorecon or a relabel undoes it","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","labels","temporary","context"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the SELinux context of files directly — only for a quick test, because restorecon or a relabel undoes it.\n\n## About\n\n`chcon -t TYPE FILE` writes a new label on the file but does not add a rule to the policy. The next `restorecon` or full relabel sets the old label again. For permanent labels use `semanage fcontext` + `restorecon`.\n\n## Syntax\n\n```text\nchcon [OPTION]... CONTEXT FILE...\nchcon [OPTION]... [-u USER] [-r ROLE] [-l RANGE] [-t TYPE] FILE...\nchcon [OPTION]... --reference=RFILE FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t, --type=TYPE` | Set the type. |\n| `-u, --user=USER` | Set the SELinux user. |\n| `-r, --role=ROLE` | Set the role. |\n| `-l, --range=RANGE` | Set the level/range. |\n| `--reference=RFILE` | Copy the context of RFILE. |\n| `-R, --recursive` | Whole trees. |\n| `-v, --verbose` | Report every file. |\n| `-h, --no-dereference` | Change links themselves. |\n| `--dereference` | Change the targets of links (default). |\n| `-H, -L, -P` | Link handling with -R. |\n| `--preserve-root, --no-preserve-root` | Protect / with -R. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Test a type quickly\n\n```bash\nchcon -t httpd_sys_content_t /srv/test/index.html\n```\n\n> Works until the next restorecon.\n\n### Copy the label of another file\n\n```bash\nchcon --reference=/var/www/html /srv/web\n```\n\n> Same context as /var/www/html.\n\n### A whole folder\n\n```bash\nchcon -R -t httpd_sys_content_t /srv/web\n```\n\n> Still temporary.\n\n### Watch it disappear\n\n```bash\nrestorecon -v /srv/test/index.html\n```\n\n> The label goes back to the policy rule.\n\n### The permanent way\n\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/web(/.*)?\"; restorecon -Rv /srv/web\n```\n\n> Use this on the exam.\n\n### Break a label for a lab\n\n```bash\nchcon -t user_home_t /var/www/html/index.html\n```\n\n> Practice finding AVC denials.\n\n### Show the full context\n\n```bash\nls -Z /srv/web\n```\n\n> Check the result.\n\n### Set the full context at once\n\n```bash\nchcon system_u:object_r:httpd_sys_content_t:s0 /srv/web/index.html\n```\n\n> All four parts.\n\n### Container volume label (alternative)\n\n```bash\npodman run -v /srv/data:/data:Z …\n```\n\n> :Z lets podman set the right label instead of chcon.\n\n### Find files with temporary labels\n\n```bash\nrestorecon -Rvn /srv\n```\n\n> Shows labels that differ from the policy (chcon changes too).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chcon](https://linux.die.net/man/1/chcon)\n","page":"commands/chcon/","label":"chcon","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-chgrp","n":1383,"slug":"chgrp","title":"chgrp — Change the group of files and directories","sections":["commands"],"topic":"Commands · Permissions","tags":["group","ownership","shared folder","recursive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the group of files and directories.\n\n## About\n\n`chgrp GROUP FILE` sets the group. root can set any group. A normal user can change the group of an **own** file, but only to a group he is a member of.\n\nIt is the same as `chown :GROUP FILE`.\n\n## Syntax\n\n```text\nchgrp [OPTION]... GROUP FILE...\nchgrp [OPTION]... --reference=RFILE FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R, --recursive` | Change the whole tree. |\n| `-v, --verbose` | Report every file. |\n| `-c, --changes` | Report only changes. |\n| `-f, --silent, --quiet` | Do not show most errors. |\n| `-h, --no-dereference` | Change a link itself. |\n| `--dereference` | Change the target of a link (default). |\n| `--reference=RFILE` | Use the group of RFILE. |\n| `--preserve-root` | Refuse to work recursively on /. |\n| `--no-preserve-root` | Do not treat / special (default). |\n| `-H` | With -R: follow a link given on the command line. |\n| `-L` | With -R: follow every link to a directory. |\n| `-P` | With -R: follow no links (default). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Give a folder to a team\n\n```bash\nchgrp sales /srv/sales\n```\n\n> Combine with chmod 2770 for a shared folder.\n\n### Change a whole tree\n\n```bash\nchgrp -R sales /srv/sales\n```\n\n> All files and folders inside.\n\n### As a normal user, for your own file\n\n```bash\nchgrp sales plan.txt\n```\n\n> Works only if you are in sales.\n\n### Error when you are not a member\n\n```bash\nchgrp wheel plan.txt\n```\n```text\nchgrp: changing group of 'plan.txt': Operation not permitted\n```\n\n> Join the group first.\n\n### Show what changed\n\n```bash\nchgrp -Rc sales /srv/sales\n```\n\n> Lists only files that got a new group.\n\n### Use the group of another file\n\n```bash\nchgrp --reference=/srv/sales /srv/sales/import.csv\n```\n\n> Same group as the folder.\n\n### Fix files after a group GID change\n\n```bash\nfind /srv -gid 3000 -exec chgrp -h sales {} +\n```\n\n> Files with the old number get the group name again.\n\n### Group of the web logs for a log team\n\n```bash\nchgrp -R logreaders /var/log/httpd; chmod -R g+rX /var/log/httpd\n```\n\n> Read access without root (check that the logs rotate with the same group).\n\n### Group for a config read by a service\n\n```bash\nchgrp nginx /etc/pki/tls/private/site.key; chmod 640 /etc/pki/tls/private/site.key\n```\n\n> The service reads the key; others cannot.\n\n### Find files of a group (audit)\n\n```bash\nfind /srv -group sales -ls\n```\n\n> Shows everything the group owns.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chgrp](https://linux.die.net/man/1/chgrp)\n","page":"commands/chgrp/","label":"chgrp","where":"Command page · Permissions","aka":[]},{"id":"cmd-chmod","n":1384,"slug":"chmod","title":"chmod — Change the permissions (mode) of files and directories, with numbers or letters","sections":["commands"],"topic":"Commands · Permissions","tags":["permissions","mode","rwx","octal","suid","sgid","sticky","executable"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the permissions (mode) of files and directories, with numbers or letters.\n\n## About\n\nPermissions are three groups of `r w x`: for the owner (u), the group (g) and others (o). Use numbers (`chmod 750 dir`, r = 4, w = 2, x = 1) or letters (`chmod g+w,o-rwx dir`).\n\nA fourth digit in front sets the special bits: 4 = SUID, 2 = SGID, 1 = sticky. Only the owner of a file (or root) may change its mode.\n\n## Syntax\n\n```text\nchmod [OPTION]... MODE[,MODE]... FILE...\nchmod [OPTION]... OCTAL-MODE FILE...\nchmod [OPTION]... --reference=RFILE FILE...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `u g o a` | Who: user (owner), group, others, all. |\n| `+ - =` | Add, remove, set exactly. |\n| `r w x` | Read, write, execute. |\n| `X` | x only for directories and files that already have an x. |\n| `s` | SUID (with u) or SGID (with g). |\n| `t` | Sticky bit. |\n| `NNN / NNNN` | Numbers: owner, group, others (and the special bits in front). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R, --recursive` | Change the whole tree. |\n| `-v, --verbose` | Report every file. |\n| `-c, --changes` | Report only changes. |\n| `-f, --silent, --quiet` | Do not show most errors. |\n| `--reference=RFILE` | Use the mode of RFILE. |\n| `--preserve-root` | Refuse to work recursively on /. |\n| `--no-preserve-root` | Do not treat / special (default). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Make a script executable\n\n```bash\nchmod +x backup.sh\n./backup.sh\n```\n\n> `+x` = `a+x` (filtered by the umask).\n\n### Normal file and private file\n\n```bash\nchmod 644 index.html\nchmod 600 ~/.ssh/id_ed25519\n```\n\n> SSH refuses private keys that others can read.\n\n### Team folder without access for others\n\n```bash\nchmod 770 /srv/sales\n```\n\n> Owner and group full, others nothing.\n\n### Shared folder with SGID and sticky\n\n```bash\nchmod 3770 /srv/sales\nls -ld /srv/sales\n```\n```text\ndrwxrws--T. 2 root sales 6 Oct 25 10:40 /srv/sales\n```\n\n> New files get the group; users delete only their own files.\n\n### Add and remove in one command\n\n```bash\nchmod g+w,o-rwx shared.txt\n```\n\n> Letters change only what you name.\n\n### Give folders x but not files\n\n```bash\nchmod -R u=rwX,g=rX,o= /srv/project\n```\n\n> Capital X: folders get x, normal files do not.\n\n### Copy the mode of another file\n\n```bash\nchmod --reference=index.html about.html\n```\n\n> Same permissions.\n\n### Remove a dangerous SUID bit\n\n```bash\nchmod u-s /usr/local/bin/oldtool\n```\n\n> SUID programs run as their owner — a big risk if the owner is root.\n\n### Find world-writable files (security)\n\n```bash\nfind / -xdev -type f -perm -0002 -ls 2>/dev/null\n```\n\n> Anybody can change these files.\n\n### Find SUID and SGID programs (security)\n\n```bash\nfind / -xdev -type f \\( -perm -4000 -o -perm -2000 \\) -ls 2>/dev/null\n```\n\n> Compare the list with a known good list.\n\n### Protect a config with secrets\n\n```bash\nchmod 640 /etc/myapp/db.conf; chgrp myapp /etc/myapp/db.conf\n```\n\n> The service group reads it; others cannot.\n\n### See the mode as a number\n\n```bash\nstat -c '%a %A %n' /etc/shadow /tmp\n```\n```text\n0 ---------- /etc/shadow\n1777 drwxrwxrwt /tmp\n```\n\n> `%a` = number, `%A` = letters.\n\n## Watch out\n\n- `chmod -R 777` is never a fix: it gives everybody full access.\n- `chmod -R 644 dir` removes the x from folders: nobody can enter them. Use `X`.\n- `S` or `T` in ls means the special bit without x (often a mistake, except on a folder with no access for others).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chmod](https://linux.die.net/man/1/chmod)\n","page":"commands/chmod/","label":"chmod","where":"Command page · Permissions","aka":[]},{"id":"cmd-chown","n":1385,"slug":"chown","title":"chown — Change the owner and/or the group of files and directories","sections":["commands"],"topic":"Commands · Permissions","tags":["owner","group","ownership","recursive","permissions","web root"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the owner and/or the group of files and directories.\n\n## About\n\n`chown USER FILE` gives a file to another user; `chown USER:GROUP FILE` also changes the group; `chown :GROUP FILE` changes only the group. Only root may change the owner.\n\nWith `-R` it works on a whole tree. Numbers (UID/GID) work as well as names, which helps when the user does not exist on this system (like files from a backup or NFS).\n\n## Syntax\n\n```text\nchown [OPTION]... [OWNER][:[GROUP]] FILE...\nchown [OPTION]... --reference=RFILE FILE...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `USER` | Only the owner changes. |\n| `USER:GROUP` | Owner and group change. |\n| `:GROUP` | Only the group changes (same as chgrp). |\n| `USER:` | Owner changes; group becomes the login group of USER. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R, --recursive` | Change files and directories in the whole tree. |\n| `-v, --verbose` | Report every file. |\n| `-c, --changes` | Report only files that changed. |\n| `-f, --silent, --quiet` | Do not show most errors. |\n| `-h, --no-dereference` | Change a symbolic link itself, not the file it points to. |\n| `--dereference` | Change the file a link points to (default). |\n| `--from=CURRENT_OWNER:CURRENT_GROUP` | Change only files that have this owner/group now. |\n| `--reference=RFILE` | Use the owner and group of RFILE. |\n| `--preserve-root` | Refuse to work recursively on /. |\n| `--no-preserve-root` | Do not treat / special (default). |\n| `-H` | With -R: follow a link given on the command line. |\n| `-L` | With -R: follow every link to a directory. |\n| `-P` | With -R: follow no links (default). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Give a file to a user\n\n```bash\nchown lisa report.txt\n```\n\n> Only root can do this.\n\n### Set owner and group\n\n```bash\nchown lisa:sales report.txt\n```\n\n> Both in one step.\n\n### Change only the group\n\n```bash\nchown :sales report.txt\n```\n\n> Same as `chgrp sales report.txt`.\n\n### Give a web app to the web server user\n\n```bash\nchown -R apache:apache /var/www/html/app/uploads\n```\n\n> Only the upload folder; the code itself should stay owned by root (hardening).\n\n### Show only what changed\n\n```bash\nchown -Rc lisa:lisa /home/lisa\n```\n```text\nchanged ownership of '/home/lisa/old.txt' from root:root to lisa:lisa\n```\n\n> Good after you copied files as root into a home folder.\n\n### Change only files of an old user\n\n```bash\nchown -R --from=olduser newuser /srv/project\n```\n\n> Files of other owners stay as they are.\n\n### Use numbers for files from another server\n\n```bash\nchown -R 1500:3000 /srv/restore\n```\n\n> Works even when the names do not exist here.\n\n### Copy the owner of another file\n\n```bash\nchown --reference=/var/www/html/index.html /var/www/html/new.html\n```\n\n> Same owner and group as the reference.\n\n### Change a symbolic link itself\n\n```bash\nchown -h lisa /home/lisa/link-to-data\n```\n\n> Without -h the target file changes.\n\n### Fix files after a UID change\n\n```bash\nfind / -xdev -uid 1500 -exec chown -h sam {} +\n```\n\n> All files of the old UID go to sam.\n\n### Protect against a typo on /\n\n```bash\nchown -R --preserve-root lisa /\n```\n```text\nchown: it is dangerous to operate recursively on '/'\n```\n\n> Use it in scripts with variables.\n\n## Watch out\n\n- `chown -R user /` (for example from an empty variable `$DIR/`) breaks the system. Check paths in scripts.\n- A normal user cannot give files away.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chown](https://linux.die.net/man/1/chown)\n","page":"commands/chown/","label":"chown","where":"Command page · Permissions","aka":[]},{"id":"cmd-chronyc","n":1386,"slug":"chronyc","title":"chronyc — Talk to the chrony NTP daemon: show time sources, sync state and statistics, or make the clock step","sections":["commands"],"topic":"Commands · Time","tags":["ntp","chrony","time sync","sources","tracking"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Talk to the chrony NTP daemon: show time sources, sync state and statistics, or make the clock step.\n\n## About\n\nchronyd syncs the clock with the servers in `/etc/chrony.conf`. `chronyc sources` lists them (the one with `^*` is used); `chronyc tracking` shows how far the clock is from the reference. Some commands (like makestep) need root.\n\n## Syntax\n\n```text\nchronyc [OPTION]... [COMMAND]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `sources [-v]` | Time sources and their state (-v = with a legend). |\n| `sourcestats [-v]` | Drift and offset statistics per source. |\n| `tracking` | Sync state of the system clock. |\n| `makestep` | Correct the clock at once (step). |\n| `activity` | Number of online/offline sources. |\n| `clients` | NTP clients of this server. |\n| `ntpdata [ADDR]` | Details of the last NTP packets. |\n| `online / offline` | Mark sources online or offline. |\n| `burst N/M` | Send quick measurements. |\n| `add server NAME / delete ADDR` | Change sources at run time. |\n| `reload sources` | Reload sources from sourcedir files. |\n| `serverstats` | Statistics of the NTP server part. |\n| `waitsync [N [MAX [MAXSKEW [INTERVAL]]]]` | Wait until synchronized (scripts). |\n| `rtcdata` | RTC information. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n` | Do not resolve addresses to names. |\n| `-c` | CSV output. |\n| `-4, -6` | Use IPv4 or IPv6 only. |\n| `-N` | Show the original source names. |\n| `-a` | Authenticate (old option). |\n| `-e` | End each answer with a dot line. |\n| `-d` | Debug. |\n| `-m` | Allow several commands on the command line. |\n| `-h HOST` | Connect to chronyd on another host. |\n| `-p PORT` | Use another port. |\n| `-f FILE` | Config file (ignored, for compatibility). |\n| `-v, --version` | Show the version. |\n| `--help` | Show help. |\n\n## Examples\n\n### Which sources?\n\n```bash\nchronyc sources\n```\n```text\nMS Name/IP address         Stratum Poll Reach LastRx Last sample\n===============================================================================\n^* server1                       3   6   377    34   +112us[ +140us] +/-   18ms\n```\n\n> ^* = selected source; Reach 377 = the last 8 polls worked.\n\n### With a legend\n\n```bash\nchronyc sources -v\n```\n\n> Explains every column.\n\n### Is the clock synced?\n\n```bash\nchronyc tracking\n```\n```text\nReference ID    : C0A87A0A (server1)\nStratum         : 4\nSystem time     : 0.000012 seconds fast of NTP time\nLeap status     : Normal\n```\n\n> Leap status Normal = synchronized.\n\n### Correct a big offset now\n\n```bash\nchronyc makestep\n```\n```text\n200 OK\n```\n\n> Normally chrony slews slowly; makestep jumps.\n\n### Wait in a script until synced\n\n```bash\nchronyc waitsync 30 0.1\n```\n\n> Up to 30 tries until the offset is under 0.1 s.\n\n### NTP clients of a time server\n\n```bash\nchronyc clients\n```\n\n> Which hosts sync from this server.\n\n### Statistics\n\n```bash\nchronyc sourcestats -v\n```\n\n> Frequency and offset of each source.\n\n### Add a server at run time\n\n```bash\nchronyc add server 192.168.122.1 iburst\n```\n\n> Not saved; put it in chrony.conf too.\n\n### Activity\n\n```bash\nchronyc activity\n```\n```text\n200 OK\n1 sources online\n```\n\n> Count of reachable sources.\n\n### Check a remote chronyd\n\n```bash\nchronyc -h server1 tracking\n```\n\n> Needs `cmdallow` on the remote side.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/chrony.conf` | Servers, pools, allow lines. |\n| `/var/lib/chrony/drift` | Clock drift data. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man chronyc](https://linux.die.net/man/1/chronyc)\n","page":"commands/chronyc/","label":"chronyc","where":"Command page · Time","aka":[]},{"id":"cmd-chroot","n":1387,"slug":"chroot","title":"chroot — Run a command (default: a shell) with another folder as the root `/` — the key step to repair an installed system from a rescue shell","sections":["commands"],"topic":"Commands · System information","tags":["change root","rescue","repair","initramfs","isolation"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run a command (default: a shell) with another folder as the root `/` — the key step to repair an installed system from a rescue shell.\n\n## About\n\nAfter `chroot /mnt/sysroot`, commands see `/mnt/sysroot` as `/`: they use the installed system's programs, config and package database. That is how you reset a password with `rd.break`, reinstall GRUB or rebuild the initramfs from a rescue disk. For tools like grub2-install or dracut, `/proc`, `/sys`, `/dev` and `/run` should be mounted inside first (the RHEL rescue mode does this for you).\n\nchroot is not a security boundary; use containers for isolation.\n\n## Syntax\n\n```text\nchroot [OPTION]... NEWROOT [COMMAND [ARGUMENT]...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `NEWROOT` | The folder that becomes the new /. |\n| `--userspec=USER:GROUP` | Run as this user and group. |\n| `--groups=G_LIST` | Extra groups. |\n| `--skip-chdir` | Do not change the working folder to /. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Enter the installed system from rd.break\n\n```bash\nmount -o remount,rw /sysroot\nchroot /sysroot\n```\n\n> Now passwd works on the real /etc/shadow.\n\n### From the rescue disk\n\n```bash\nchroot /mnt/sysroot\n```\n\n> The rescue mode mounted the system there.\n\n### Mount the special file systems by hand\n\n```bash\nfor d in proc sys dev run; do mount --rbind /$d /mnt/sysroot/$d; done\nchroot /mnt/sysroot\n```\n\n> Needed when you mounted the root yourself.\n\n### Run one command\n\n```bash\nchroot /mnt/sysroot dracut -f --regenerate-all\n```\n\n> Without an interactive shell.\n\n### Reinstall GRUB from rescue\n\n```bash\nchroot /mnt/sysroot grub2-install /dev/vda\n```\n\n> BIOS systems.\n\n### Check packages of the installed system\n\n```bash\nchroot /mnt/sysroot rpm -qa | grep kernel\n```\n\n> The installed kernels, not those of the ISO.\n\n### As another user\n\n```bash\nchroot --userspec=lisa:lisa /mnt/sysroot /bin/bash\n```\n\n> Test user rights inside.\n\n### Leave the chroot\n\n```bash\nexit\n```\n\n> Back to the rescue shell.\n\n### Is my shell in a chroot?\n\n```bash\nls -di /\n```\n```text\n2 /\n```\n\n> An inode number other than 2 (or 128 on btrfs) often means a chroot.\n\n### Do not use it for security\n\n```bash\npodman run --rm -it registry.access.redhat.com/ubi10/ubi bash\n```\n\n> Containers isolate with namespaces and SELinux; chroot does not.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `125` | chroot itself failed. |\n| `126` | The command could not be run. |\n| `127` | The command was not found. |\n| `0` | Otherwise the exit status of the command. |\n\nFull manual: [man chroot](https://linux.die.net/man/8/chroot)\n","page":"commands/chroot/","label":"chroot","where":"Command page · System information","aka":[]},{"id":"cmd-chvt","n":1388,"slug":"chvt","title":"chvt — Switch the screen of the machine to another virtual terminal (tty1 … tty12)","sections":["commands"],"topic":"Commands · Shell and help","tags":["console","virtual terminal","tty","switch"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Switch the screen of the machine to another virtual terminal (tty1 … tty12).\n\n## About\n\n`chvt N` makes `/dev/ttyN` the active console, like pressing `Ctrl+Alt+FN`. It is useful when you work on a machine through a remote tool that cannot send Ctrl+Alt key combinations, or from a script.\n\nYou must be root (or own the console) to switch.\n\n## Syntax\n\n```text\nchvt [OPTION] N\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `N` | The number of the virtual terminal (1 to 12, or more). |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Go to the third text console\n\n```bash\nchvt 3\n```\n\n> Same as Ctrl+Alt+F3.\n\n### Go back to the graphical login screen\n\n```bash\nchvt 1\n```\n\n> On RHEL with a desktop, tty1 is the login screen.\n\n### Switch to your graphical session\n\n```bash\nchvt 2\n```\n\n> On RHEL your desktop session usually runs on tty2.\n\n### Use it over SSH to show a console to a colleague\n\n```bash\nchvt 4\n```\n\n> The physical screen in the server room changes.\n\n### See which terminal is active\n\n```bash\ncat /sys/class/tty/tty0/active\n```\n```text\ntty2\n```\n\n> Shows the active virtual terminal.\n\n### Switch when the remote console cannot send Ctrl+Alt+F keys\n\n```bash\nchvt 3\n```\n\n> Many web consoles of virtual machines catch these keys.\n\n### Free a virtual terminal that is not used\n\n```bash\ndeallocvt 7\n```\n\n> Removes unused consoles from memory.\n\n### Open a login on another console from a script\n\n```bash\nopenvt -c 5 -- /bin/login\n```\n\n> Starts a login program on tty5.\n\n### Check which consoles have a login prompt\n\n```bash\nsystemctl list-units 'getty@*'\n```\n\n> Each text console with a login prompt is a getty service.\n\n### Switch back after maintenance\n\n```bash\nchvt 1\n```\n\n> Leave the screen at the login prompt.\n\n## Watch out\n\n- chvt changes the physical screen. Users on the console notice it.\n\n## Tips\n\n- chvt N = Ctrl+Alt+FN from the command line.\n\nFull manual: [man chvt](https://linux.die.net/man/1/chvt)\n","page":"commands/chvt/","label":"chvt","where":"Command page · Shell and help","aka":[]},{"id":"cmd-clear","n":1389,"slug":"clear","title":"clear — Clean the terminal screen (and, by default, the scrollback buffer)","sections":["commands"],"topic":"Commands · Shell and help","tags":["terminal","screen","scrollback","reset"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Clean the terminal screen (and, by default, the scrollback buffer).\n\n## About\n\n`clear` removes all text from the terminal window and puts the prompt at the top. It sends the right control codes for your terminal type (from the `TERM` variable).\n\n`Ctrl+L` does almost the same: it clears the visible screen but usually keeps the scrollback. When the terminal shows strange characters after you printed a binary file, `reset` is stronger than `clear`.\n\n## Syntax\n\n```text\nclear [-x] [-T TYPE] [-V]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-x` | Do not clear the scrollback buffer (only the visible screen). |\n| `-T <type>` | Use this terminal type instead of $TERM. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Clear the screen\n\n```bash\nclear\n```\n\n> The prompt moves to the top of the window.\n\n### Clear the screen with the keyboard\n\n```bash\n# press Ctrl+L\n```\n\n> Faster than typing `clear`. Works in Bash and most programs.\n\n### Keep the scrollback, clear only the screen\n\n```bash\nclear -x\n```\n\n> You can still scroll up to see older output.\n\n### Clear the screen before a demo or a screen share\n\n```bash\nclear && history -c\n```\n\n> Clean screen and no old commands with internal names or tokens.\n\n### Fix a terminal that prints strange signs\n\n```bash\nreset\n```\n\n> `reset` sets up the terminal again. Use it after you `cat` a binary file by mistake.\n\n### Clear the screen at the start of a menu script\n\n```bash\nclear; echo \"1) status  2) restart  3) logs\"; read -p \"Choice: \" c\n```\n\n> Each time the menu shows, the old output is gone.\n\n### Refresh a simple status view every 5 seconds\n\n```bash\nwhile true; do clear; date; systemctl is-active httpd sshd; sleep 5; done\n```\n\n> A tiny dashboard. `watch` does this better: `watch -n5 'systemctl is-active httpd sshd'`.\n\n### Clear for another terminal type\n\n```bash\nclear -T xterm\n```\n\n> Rarely needed: when $TERM is wrong for the terminal you use.\n\n### Use the tput way in a portable script\n\n```bash\ntput clear\n```\n\n> Same result; `tput` can also set colors and move the cursor.\n\n### Clear the screen and the scrollback in a jump host session\n\n```bash\nclear && printf '\\e[3J'\n```\n\n> Some terminals keep the scrollback; the extra code removes it too. Good after showing secrets.\n\n### Make an alias that clears and lists\n\n```bash\nalias cls='clear; ls -l'\n```\n\n> Personal shortcut; put it in `~/.bashrc`.\n\n## Watch out\n\n- `clear` does not remove anything from your shell history or from log files. It only cleans the window.\n- Some terminal programs keep the scrollback even after `clear`.\n\n## Tips\n\n- Ctrl+L = fast clear. `reset` = repair a broken terminal.\n\nFull manual: [man clear](https://linux.die.net/man/1/clear)\n","page":"commands/clear/","label":"clear","where":"Command page · Shell and help","aka":[]},{"id":"cmd-cp","n":1390,"slug":"cp","title":"cp — Copy files and folders","sections":["commands"],"topic":"Commands · Files and folders","tags":["copy","backup","archive","recursive","permissions","selinux"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Copy files and folders.\n\n## About\n\n`cp SOURCE TARGET` copies a file. When the target is an existing folder, the copy keeps its name inside that folder. Several sources can go into one folder: `cp a b c /dir/`.\n\nFolders need `-r`. For backups use `-a` (archive): it copies recursively and keeps permissions, owners, times, links and the SELinux context.\n\nOn RHEL, root has the alias `cp='cp -i'`, so it asks before it overwrites a file.\n\n## Syntax\n\n```text\ncp [OPTION]... SOURCE DEST\ncp [OPTION]... SOURCE... DIRECTORY\ncp [OPTION]... -t DIRECTORY SOURCE...\n```\n\n## Options\n\n\n### Most used\n\n| Option | What it does |\n|---|---|\n| `-r, -R, --recursive` | Copy folders with everything in them. |\n| `-a, --archive` | Archive copy: recursive and keep everything (same as -dR --preserve=all). |\n| `-p` | Keep mode, owner and times (same as --preserve=mode,ownership,timestamps). |\n| `-i, --interactive` | Ask before overwriting a file. |\n| `-v, --verbose` | Print each file that is copied. |\n| `-u` | Copy only when the source is newer than the target (or the target is missing). |\n| `-n, --no-clobber` | Never overwrite an existing file. |\n| `-f, --force` | If the target cannot be opened, remove it and try again. |\n\n### Keep or drop attributes\n\n| Option | What it does |\n|---|---|\n| `--preserve[=<list>]` | Keep these attributes: mode, ownership, timestamps, context, links, xattr, all. |\n| `--no-preserve=<list>` | Do not keep these attributes. |\n| `--attributes-only` | Copy only the attributes, not the data. |\n| `-Z` | Give the copy the default SELinux context of the target place. |\n| `--context[=<ctx>]` | Like -Z, or set this exact context. |\n\n### Links\n\n| Option | What it does |\n|---|---|\n| `-d` | Copy symbolic links as links (same as --no-dereference --preserve=links). |\n| `-P, --no-dereference` | Never follow symbolic links in the source. |\n| `-L, --dereference` | Always follow symbolic links in the source (copy the target file). |\n| `-H` | Follow symbolic links given on the command line only. |\n| `-l, --link` | Make hard links instead of copies. |\n| `-s, --symbolic-link` | Make symbolic links instead of copies. |\n\n### Target and backups\n\n| Option | What it does |\n|---|---|\n| `-t, --target-directory=<dir>` | Copy all sources into this folder. |\n| `-T, --no-target-directory` | Treat the target as a normal file, never as a folder. |\n| `--parents` | Use the full source path under the target folder (cp --parents a/b/c /x → /x/a/b/c). |\n| `--backup[=<control>]` | Make a backup of each target that would be overwritten (numbered, existing, simple, none). |\n| `-b` | Like --backup, without an argument. |\n| `-S, --suffix=<suffix>` | The ending of backup files (default ~). |\n| `--remove-destination` | Remove each existing target before copying. |\n| `--update[=<which>]` | Control which existing files are replaced: all, none, none-fail, older. |\n\n### Special cases\n\n| Option | What it does |\n|---|---|\n| `-x, --one-file-system` | Stay on this file system. |\n| `--sparse=<when>` | Keep sparse files sparse: auto, always, never. |\n| `--reflink[=<when>]` | Make a fast clone (copy-on-write) when the file system supports it (XFS, Btrfs). |\n| `--copy-contents` | Copy the content of special files (like devices) when recursive. |\n| `--strip-trailing-slashes` | Remove the slash at the end of each source. |\n| `--debug` | Explain how each file is copied (implies -v). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Copy a file before you change it\n\n```bash\ncp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak\n```\n\n> Always keep a copy of a config file before an edit.\n\n### Copy a folder with all its content\n\n```bash\ncp -r ~/project /tmp/\n```\n\n> Without -r, cp skips folders.\n\n### Make a full backup that keeps owners and permissions\n\n```bash\ncp -a /var/www/html /backup/html-$(date +%F)\n```\n\n> `-a` keeps everything, also SELinux labels and links.\n\n### Copy the content of a folder, hidden files included\n\n```bash\ncp -a /etc/skel/. ~/newhome/\n```\n\n> `/.` at the end means \"the content\", including dotfiles.\n\n### Copy many files into one folder\n\n```bash\ncp *.log /tmp/logs/\n```\n\n> Wildcards pick the files; the last argument is the folder.\n\n### Copy only newer files\n\n```bash\ncp -ru ~/docs/* /mnt/usb/docs/\n```\n\n> Only files that changed are copied again.\n\n### Never overwrite existing files\n\n```bash\ncp -n templates/* /etc/myapp/\n```\n\n> Keeps the files that are already there.\n\n### Keep the folder path of the source\n\n```bash\ncp --parents /etc/ssh/sshd_config /etc/sudoers /root/config-backup/\n```\n\n> Result: /root/config-backup/etc/ssh/sshd_config and /root/config-backup/etc/sudoers.\n\n### Copy all files of a user (common exam task)\n\n```bash\nfind / -user student -type f -exec cp -a --parents {} /root/found/ \\; 2>/dev/null\n```\n\n> `--parents` keeps the paths, so files with the same name do not overwrite each other.\n\n### Copy and fix the SELinux label for the new place\n\n```bash\ncp -Z index.html /var/www/html/\n```\n\n> The copy gets the label of /var/www/html. A `mv` would keep the old, wrong label.\n\n### Keep numbered backups when overwriting\n\n```bash\ncp --backup=numbered nginx.conf /etc/nginx/nginx.conf\n```\n\n> The old file becomes nginx.conf.~1~, then .~2~ and so on.\n\n### Fast copy of a big disk image on XFS\n\n```bash\ncp --reflink=always vm.qcow2 vm-clone.qcow2\n```\n\n> A reflink copy takes almost no time and no extra space until one copy changes.\n\n## Watch out\n\n- `cp` overwrites without a question for normal users. Use `-i` or `-n` when you are not sure.\n- `cp -r dir1 dir2`: if dir2 exists, you get dir2/dir1; if not, dir2 becomes the copy.\n- A copy gets the SELinux label of its new place by default, but `-a` keeps the old label. For web content that is usually wrong: use `restorecon` after.\n\n## Tips\n\n- File: `cp`. Folder: `cp -r`. Backup: `cp -a`.\n- Before an edit: `cp file file.bak`.\n\nFull manual: [man cp](https://linux.die.net/man/1/cp)\n","page":"commands/cp/","label":"cp","where":"Command page · Files and folders","aka":[]},{"id":"cmd-createrepo","n":1391,"slug":"createrepo","title":"createrepo — Create the index (repodata) for a folder of RPM files, so dnf can use it as a repository","sections":["commands"],"topic":"Commands · Software packages","tags":["repository","metadata","local repo","rpm","offline"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create the index (repodata) for a folder of RPM files, so dnf can use it as a repository.\n\nSame page for: `createrepo_c`\n\n## About\n\nA repository is just a folder with `.rpm` files and a `repodata/` folder with the index. `createrepo DIR` (on RHEL the fast C version `createrepo_c`) reads all packages and writes the index. Then point a `.repo` file to the folder (`baseurl=file:///DIR`) or share it with a web server.\n\nAfter you add or remove packages, run it again with `--update` (faster).\n\n## Syntax\n\n```text\ncreaterepo [OPTION]... DIRECTORY\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--update` | Reuse old data; only read new or changed packages. |\n| `-o, --outputdir DIR` | Write repodata to another folder. |\n| `-x, --excludes PATTERN` | Leave out packages matching the pattern. |\n| `-i, --pkglist FILE` | Only the packages listed in the file. |\n| `-n, --includepkg PACKAGE` | Add this package file (more allowed). |\n| `-g, --groupfile FILE` | Add a comps file with package groups. |\n| `-s, --checksum TYPE` | Checksum type (sha256 default). |\n| `--workers N` | Number of worker threads. |\n| `-d, --database` | Also create SQLite databases. |\n| `--no-database` | Do not create SQLite databases. |\n| `--skip-stat` | Do not check file times with --update. |\n| `--retain-old-md N` | Keep this many old metadata files. |\n| `--revision TEXT` | Set a revision string. |\n| `--distro TEXT, --content TEXT, --repo TEXT` | Add tags to the metadata. |\n| `--compress-type TYPE` | Compression of metadata (gz, xz, zstd …). |\n| `--zck` | Also create zchunk metadata. |\n| `--deltas` | Create delta RPMs. |\n| `-q, --quiet` | Quiet. |\n| `-v, --verbose` | Verbose. |\n| `-V, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Install the tool\n\n```bash\ndnf install -y createrepo_c\n```\n\n> Gives the `createrepo` and `createrepo_c` commands.\n\n### Make a repository of a folder\n\n```bash\ncreaterepo /srv/myrepo\n```\n\n> Creates /srv/myrepo/repodata/.\n\n### Use it on this machine\n\n```bash\ncat /etc/yum.repos.d/myrepo.repo\n```\n```text\n[myrepo]\nname=My packages\nbaseurl=file:///srv/myrepo\ngpgcheck=0\n```\n\n> Then `dnf repolist` shows it.\n\n### Update after adding packages\n\n```bash\ncp new-tool-2.0-1.el10.x86_64.rpm /srv/myrepo/; createrepo --update /srv/myrepo\n```\n\n> Faster than a full run.\n\n### Share the repository over HTTP\n\n```bash\ndnf install -y httpd; ln -s /srv/myrepo /var/www/html/myrepo\n```\n\n> Clients use `baseurl=http://server1/myrepo` (check SELinux labels: restorecon or semanage fcontext).\n\n### Build an offline mirror of needed packages\n\n```bash\ndnf download --resolve --destdir=/srv/myrepo ansible-core\ncreaterepo /srv/myrepo\n```\n\n> For servers without internet.\n\n### Add package groups\n\n```bash\ncreaterepo -g comps.xml /srv/myrepo\n```\n\n> Then `dnf group list` shows your groups.\n\n### Leave out test packages\n\n```bash\ncreaterepo -x '*-debuginfo-*' /srv/myrepo\n```\n\n> Smaller index.\n\n### Sign your packages (security)\n\n```bash\nrpm --addsign /srv/myrepo/*.rpm\n```\n\n> Needs a GPG key in ~/.rpmmacros; then clients can use gpgcheck=1.\n\n### Clean the dnf cache after changes\n\n```bash\ndnf clean metadata; dnf makecache\n```\n\n> Clients see the new packages.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man createrepo](https://linux.die.net/man/8/createrepo)\n","page":"commands/createrepo/","label":"createrepo","where":"Command page · Software packages","aka":["createrepo_c"]},{"id":"cmd-crontab","n":1392,"slug":"crontab","title":"crontab — Edit, list or remove the cron job list (crontab) of a user","sections":["commands"],"topic":"Commands · Scheduling","tags":["cron","schedule","repeating jobs","automation","backup"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit, list or remove the cron job list (crontab) of a user.\n\n## About\n\nEach user can have a crontab: lines with 5 time fields (minute, hour, day of month, month, day of week) and a command. `crontab -e` opens it in your editor and installs it when you save; `crond` checks the lists every minute.\n\nroot can manage the crontab of any user with `-u`. The files are stored in `/var/spool/cron/USER` — do not edit them directly. Who may use crontab is set in `/etc/cron.allow` and `/etc/cron.deny`.\n\n## Syntax\n\n```text\ncrontab [-u USER] FILE\ncrontab [-u USER] -e | -l | -r [-i]\ncrontab -T FILE\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `MINUTE` | 0–59. |\n| `HOUR` | 0–23. |\n| `DAY-OF-MONTH` | 1–31. |\n| `MONTH` | 1–12 or jan–dec. |\n| `DAY-OF-WEEK` | 0–7 (0 and 7 = Sunday) or sun–sat. |\n| `* , - /` | Any value, list (1,15), range (1-5), step (*/10). |\n| `@reboot @hourly @daily @weekly @monthly @yearly` | Shortcuts instead of the 5 fields. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-e` | Edit the crontab (creates it if needed). |\n| `-l` | List the crontab. |\n| `-r` | Remove the whole crontab. |\n| `-i` | Ask before removing (with -r). |\n| `-u USER` | Work on the crontab of another user (root). |\n| `FILE` | Install this file as the new crontab (- = standard input). |\n| `-T FILE` | Test the syntax of a crontab file (newer cronie). |\n| `-s` | Add the SELinux security context (MLS systems). |\n| `-n HOST` | Set the host that runs the jobs (cluster mode). |\n| `-c` | Show the cluster host. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Edit your crontab\n\n```bash\ncrontab -e\n```\n\n> Opens vi (or $EDITOR). Save and quit to install it.\n\n### List your jobs\n\n```bash\ncrontab -l\n```\n```text\n*/10 * * * * /usr/bin/df -h / >> /var/tmp/disk.log\n```\n\n> No crontab: \"no crontab for USER\".\n\n### Manage another user's jobs\n\n```bash\ncrontab -e -u lisa\ncrontab -l -u lisa\n```\n\n> The RHCSA way for tasks \"user X must run …\".\n\n### Weekdays at 14:23\n\n```bash\ncrontab -l | grep logger\n```\n```text\n23 14 * * 1-5 logger \"anna was here\"\n```\n\n> Monday to Friday.\n\n### Install a crontab from a file\n\n```bash\ncrontab mycron.txt\n```\n\n> Replaces the whole list. Good for versioned crontabs in git.\n\n### Add one line without an editor (scripts)\n\n```bash\n(crontab -l 2>/dev/null; echo '0 2 * * * /usr/local/bin/backup.sh') | crontab -\n```\n\n> Keeps the existing lines.\n\n### Back up all crontabs\n\n```bash\ntar -czf /root/crontabs.tgz /var/spool/cron /etc/cron.d /etc/crontab\n```\n\n> Before you rebuild a server.\n\n### Remove a crontab with a question\n\n```bash\ncrontab -ri\n```\n```text\ncrontab: really delete student's crontab? y\n```\n\n> Without -i, -r removes everything at once.\n\n### Run a job at every boot\n\n```bash\ncrontab -l | grep reboot\n```\n```text\n@reboot /usr/local/bin/start-tunnel.sh\n```\n\n> A systemd unit is often the better choice.\n\n### Log the output of a job\n\n```bash\ncrontab -l\n```\n```text\n0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1\n```\n\n> Without it, output goes to local mail.\n\n### Find all cron jobs on a system (security)\n\n```bash\nls -l /var/spool/cron/ /etc/cron.d/; cat /etc/crontab\n```\n\n> Attackers like cron for persistence; check for unknown jobs.\n\n### Check the cron log\n\n```bash\ngrep CROND /var/log/cron | tail\n```\n\n> Shows when jobs ran and for which user.\n\n## Watch out\n\n- `crontab -r` is next to `-e` on the keyboard and deletes everything without asking. Use `-ri`.\n- cron uses a small PATH: write full paths.\n- `%` in a crontab line means a newline; write `\\%` (for example in `date +\\%F`).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/spool/cron/USER` | The crontabs of users. |\n| `/etc/crontab, /etc/cron.d/` | System crontabs (with a user field). |\n| `/etc/cron.allow, /etc/cron.deny` | Who may use crontab. |\n| `/var/log/cron` | Log of crond. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (not allowed, syntax error, no crontab). |\n\nFull manual: [man crontab](https://linux.die.net/man/1/crontab)\n","page":"commands/crontab/","label":"crontab","where":"Command page · Scheduling","aka":[]},{"id":"cmd-curl","n":1393,"slug":"curl","title":"curl — Send requests to URLs (HTTP, HTTPS, FTP …): test web servers, call APIs, download files, check TLS","sections":["commands"],"topic":"Commands · Web","tags":["http","https","api","download","headers","tls","rest","test web server"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Send requests to URLs (HTTP, HTTPS, FTP …): test web servers, call APIs, download files, check TLS.\n\n## About\n\n`curl URL` prints the answer of a server. With options you choose the method (`-X`), send headers (`-H`) and data (`-d`), follow redirects (`-L`), save to a file (`-o`), and see headers (`-I`, `-i`) or the full talk with the server (`-v`).\n\nIt is the tool to test a web server on the RHCSA exam (`curl http://localhost`) and the daily tool for REST APIs in DevOps.\n\ncurl has more than 250 options. This page lists the ones admins use most; see all with `curl --help all`.\n\n## Syntax\n\n```text\ncurl [OPTION]... URL...\n```\n\n## Options\n\n\n### Request\n\n| Option | What it does |\n|---|---|\n| `-X, --request METHOD` | The HTTP method: GET, POST, PUT, DELETE, PATCH … |\n| `-H, --header \"NAME: VALUE\"` | Add a header. |\n| `-d, --data DATA` | Send data with POST (`@file` reads a file). |\n| `--data-raw DATA` | Send data; @ has no special meaning. |\n| `--data-binary DATA` | Send data exactly as it is. |\n| `--data-urlencode DATA` | URL-encode the data. |\n| `--json DATA` | Send JSON (sets Content-Type and Accept). |\n| `-F, --form NAME=CONTENT` | Send a form (multipart), `file=@path` uploads a file. |\n| `-G, --get` | Put -d data in the URL as a query string. |\n| `-T, --upload-file FILE` | Upload a file (PUT). |\n| `-A, --user-agent TEXT` | Set the User-Agent. |\n| `-e, --referer URL` | Set the Referer. |\n| `-b, --cookie DATA\\|FILE` | Send cookies. |\n| `-c, --cookie-jar FILE` | Save cookies to a file. |\n| `-u, --user USER[:PASSWORD]` | Basic login (asks the password if not given). |\n| `--oauth2-bearer TOKEN` | Send a bearer token. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-o, --output FILE` | Save to this file. |\n| `-O, --remote-name` | Save with the name from the URL. |\n| `-J, --remote-header-name` | Use the file name the server sends (with -O). |\n| `-i, --include` | Show the response headers with the body. |\n| `-I, --head` | Get only the headers (HEAD request). |\n| `-D, --dump-header FILE` | Save the headers to a file. |\n| `-s, --silent` | No progress bar and no errors. |\n| `-S, --show-error` | Show errors even with -s. |\n| `-v, --verbose` | Show the whole request and response, also TLS. |\n| `-w, --write-out FORMAT` | Print data after the transfer, like `%{http_code}`. |\n| `--trace FILE` | Full trace with data. |\n| `-#, --progress-bar` | Simple progress bar. |\n| `--create-dirs` | Make local folders for -o. |\n\n### Connection\n\n| Option | What it does |\n|---|---|\n| `-L, --location` | Follow redirects. |\n| `--max-redirs N` | Maximum redirects. |\n| `-m, --max-time SECONDS` | Stop after this many seconds. |\n| `--connect-timeout SECONDS` | Maximum time to connect. |\n| `--retry N` | Try again N times on errors. |\n| `-x, --proxy [PROTOCOL://]HOST[:PORT]` | Use a proxy. |\n| `--noproxy LIST` | Hosts that do not use the proxy. |\n| `--resolve HOST:PORT:ADDRESS` | Use this IP for the host (test before a DNS change). |\n| `--connect-to HOST1:PORT1:HOST2:PORT2` | Connect to another host/port. |\n| `-4, --ipv4` | Use IPv4 only. |\n| `-6, --ipv6` | Use IPv6 only. |\n| `--interface NAME` | Use this interface. |\n| `-C, --continue-at OFFSET` | Continue a download (`-C -` = automatic). |\n| `--limit-rate SPEED` | Limit the speed, like 1M. |\n| `-Z, --parallel` | Do transfers in parallel. |\n\n### TLS\n\n| Option | What it does |\n|---|---|\n| `-k, --insecure` | Do not check the certificate (tests only). |\n| `--cacert FILE` | Trust this CA file. |\n| `--cert FILE[:PASSWORD]` | Client certificate. |\n| `--key FILE` | Private key of the client certificate. |\n| `--tlsv1.2, --tlsv1.3` | Use at least this TLS version. |\n| `--http1.1, --http2, --http3` | Use this HTTP version. |\n\n### Errors and others\n\n| Option | What it does |\n|---|---|\n| `-f, --fail` | Exit with an error (22) on HTTP errors, no body. |\n| `--fail-with-body` | Like -f but print the body. |\n| `-n, --netrc` | Read login data from ~/.netrc. |\n| `-K, --config FILE` | Read options from a file. |\n| `-q, --disable` | Do not read ~/.curlrc (must be first). |\n| `-h, --help [CATEGORY]` | Show help (`--help all` for every option). |\n| `-V, --version` | Show the version and protocols. |\n\n## Examples\n\n### Test a web server\n\n```bash\ncurl http://localhost\n```\n\n> RHCSA: check that httpd serves the page.\n\n### Only the headers\n\n```bash\ncurl -I https://s0sta.com\n```\n```text\nHTTP/2 200\ncontent-type: text/html; charset=UTF-8\nstrict-transport-security: max-age=31536000\n```\n\n> Check status code and security headers.\n\n### Only the status code\n\n```bash\ncurl -s -o /dev/null -w '%{http_code}\\n' https://s0sta.com\n```\n```text\n200\n```\n\n> Good for health checks and scripts.\n\n### Follow redirects\n\n```bash\ncurl -sIL http://s0sta.com | grep -iE '^(HTTP|location)'\n```\n\n> Shows the chain http → https.\n\n### Download a file\n\n```bash\ncurl -LO https://example.com/tool.tar.gz\n```\n\n> Saves as tool.tar.gz. Check the checksum afterwards.\n\n### Call a JSON API\n\n```bash\ncurl -s -H \"Authorization: Bearer $TOKEN\" https://api.example.com/v1/servers | jq '.[].name'\n```\n\n> Keep tokens in variables or files, not in the history.\n\n### Send JSON with POST\n\n```bash\ncurl -s -X POST -H 'Content-Type: application/json' -d '{\"name\":\"web1\"}' https://api.example.com/v1/servers\n```\n\n> Or shorter: `--json '{\"name\":\"web1\"}'`.\n\n### Test a site on a new server before the DNS change\n\n```bash\ncurl -sI --resolve s0sta.com:443:203.0.113.20 https://s0sta.com\n```\n\n> Uses the new IP with the real name and certificate.\n\n### See the TLS handshake and certificate\n\n```bash\ncurl -sv https://s0sta.com -o /dev/null 2>&1 | grep -E 'SSL connection|subject:|expire date'\n```\n\n> Check TLS version and expiry.\n\n### Measure response times\n\n```bash\ncurl -s -o /dev/null -w 'dns %{time_namelookup}s connect %{time_connect}s total %{time_total}s\\n' https://s0sta.com\n```\n\n> Find out if DNS, the network or the app is slow.\n\n### Fail in a script on HTTP errors\n\n```bash\ncurl -fsS https://example.com/health || echo \"health check failed\"\n```\n\n> -f makes 404/500 an error (exit code 22).\n\n### Check from inside a container network\n\n```bash\npodman run --rm registry.access.redhat.com/ubi10/ubi curl -sI http://web:8080\n```\n\n> Tests service-to-service access.\n\n## Watch out\n\n- `-k` turns off certificate checks: never in production scripts.\n- `curl URL | bash` runs code from the internet without a check. Download, read, then run.\n- Passwords with `-u user:pass` show up in the process list and history.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.curlrc` | Default options. |\n| `~/.netrc` | Login data for -n. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `6` | Could not resolve the host. |\n| `7` | Could not connect. |\n| `22` | HTTP error with -f. |\n| `28` | Timeout. |\n| `35` | TLS handshake failed. |\n| `60` | The certificate could not be checked. |\n\nFull manual: [man curl](https://linux.die.net/man/1/curl)\n","page":"commands/curl/","label":"curl","where":"Command page · Web","aka":[]},{"id":"cmd-cut","n":1394,"slug":"cut","title":"cut — Take parts of each line: fields split by a separator, or character positions","sections":["commands"],"topic":"Commands · Text and search","tags":["columns","fields","delimiter","csv","passwd","extract"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Take parts of each line: fields split by a separator, or character positions.\n\n## About\n\n`cut` prints selected parts of every line. With `-d` you give the separator (one character) and with `-f` the field numbers. With `-c` you select characters by position.\n\nIt is ideal for files with a fixed separator like `/etc/passwd` (`:`) or CSV (`,`). For columns separated by a varying number of spaces, `awk` is better.\n\n## Syntax\n\n```text\ncut OPTION... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-f, --fields=<list>` | Select these fields, like 1,3 or 2-4 or 3-. |\n| `-d, --delimiter=<char>` | The field separator (default: TAB). |\n| `-c, --characters=<list>` | Select these character positions. |\n| `-b, --bytes=<list>` | Select these byte positions. |\n| `-s, --only-delimited` | Do not print lines that have no separator. |\n| `--complement` | Print everything except the selected parts. |\n| `--output-delimiter=<string>` | Use this separator in the output. |\n| `-z, --zero-terminated` | Lines end with NUL. |\n| `-n` | Ignored. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n### LIST forms\n\n| Option | What it does |\n|---|---|\n| `N` | Only field (or character) N. |\n| `N-M` | From N to M. |\n| `N-` | From N to the end of the line. |\n| `-M` | From the first to M. |\n\n## Examples\n\n### All user names\n\n```bash\ncut -d: -f1 /etc/passwd\n```\n\n> Field 1 of /etc/passwd is the user name.\n\n### User name and shell\n\n```bash\ncut -d: -f1,7 /etc/passwd\n```\n```text\nroot:/bin/bash\n```\n\n> Fields 1 and 7, still joined by `:`.\n\n### The first 8 characters of each line\n\n```bash\ncut -c1-8 /etc/passwd\n```\n\n> Character positions, not fields.\n\n### Columns of a CSV file\n\n```bash\ncut -d, -f2,4 users.csv\n```\n\n> Fields 2 and 4 (note: quoted commas inside fields break this; use a CSV tool then).\n\n### Everything except field 2\n\n```bash\ncut -d: -f2 --complement /etc/passwd\n```\n\n> Removes the password placeholder column.\n\n### Change the output separator\n\n```bash\ncut -d: -f1,6 --output-delimiter=' -> ' /etc/passwd\n```\n```text\nroot -> /root\n```\n\n> Readable output for a report.\n\n### Only the group names\n\n```bash\ncut -d: -f1 /etc/group | sort\n```\n\n> All local groups, sorted.\n\n### Get the IP address part of an address/prefix\n\n```bash\nip -4 -o addr show dev enp1s0 | awk '{print $4}' | cut -d/ -f1\n```\n```text\n192.168.122.10\n```\n\n> `cut -d/ -f1` removes the `/24`.\n\n### Field 3 to the end\n\n```bash\ncut -d' ' -f3- server.log\n```\n\n> Drops the date and time columns of a log line.\n\n### Skip lines without the separator\n\n```bash\ncut -d= -f2 -s app.conf\n```\n\n> Comment lines without `=` are not printed.\n\n### The size column of a TAB separated report\n\n```bash\ncut -f2 report.tsv | sort -n | tail -1\n```\n\n> TAB is the default separator.\n\n## Watch out\n\n- `cut` uses exactly one separator character. Many spaces in a row make empty fields; use `awk` or `tr -s ' '` first.\n- Field numbers start at 1.\n\n## Tips\n\n- `cut -d: -f1` = first column of passwd-style files.\n\nFull manual: [man cut](https://linux.die.net/man/1/cut)\n","page":"commands/cut/","label":"cut","where":"Command page · Text and search","aka":[]},{"id":"cmd-date","n":1395,"slug":"date","title":"date — Show the date and time in any format, calculate other dates, or set the system clock","sections":["commands"],"topic":"Commands · Time","tags":["time","date","format","epoch","timestamp","scripts"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the date and time in any format, calculate other dates, or set the system clock.\n\n## About\n\n`date` prints the current time in your time zone. `+FORMAT` controls the output (`%F` = 2026-10-25), `-d` shows another moment (\"tomorrow\", \"+3 days\", \"@EPOCH\"), `-u` uses UTC. root can set the clock with `-s`, but on a system with NTP chrony will correct it — use timedatectl and chrony instead.\n\n## Syntax\n\n```text\ndate [OPTION]... [+FORMAT]\ndate [-u] -s \"DATE TIME\"\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `%F` | Date: 2026-10-25. |\n| `%T` | Time: 11:42:10. |\n| `%Y %m %d` | Year, month, day. |\n| `%H %M %S` | Hour, minute, second. |\n| `%s` | Seconds since 1970-01-01 UTC (epoch). |\n| `%a %A / %b %B` | Short/long day name / month name. |\n| `%j` | Day of the year. |\n| `%u` | Day of the week (1 = Monday). |\n| `%V` | ISO week number. |\n| `%z %Z` | Time zone offset / name. |\n| `%N` | Nanoseconds. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --date=STRING` | Show this moment instead of now: \"yesterday\", \"+2 hours\", \"next monday\", \"@1761389000\". |\n| `-u, --utc, --universal` | Use UTC. |\n| `-s, --set=STRING` | Set the clock (root). |\n| `-I[FMT], --iso-8601[=FMT]` | ISO 8601 output (date, hours, minutes, seconds, ns). |\n| `--rfc-3339=FMT` | RFC 3339 output (date, seconds, ns). |\n| `-R, --rfc-email` | RFC 5322 (mail) format. |\n| `-r, --reference=FILE` | Show the change time of a file. |\n| `-f, --file=FILE` | Like -d for each line of a file. |\n| `--debug` | Show how the date string was understood. |\n| `--resolution` | Show the time resolution. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Now\n\n```bash\ndate\n```\n```text\nSun Oct 25 11:42:10 +03 2026\n```\n\n> Local time zone.\n\n### A file-name friendly stamp\n\n```bash\ntar -czf /backup/etc-$(date +%F_%H%M).tgz /etc\n```\n\n> etc-2026-10-25_1142.tgz.\n\n### UTC\n\n```bash\ndate -u\n```\n\n> Same moment in UTC (logs from many servers).\n\n### Calculate dates\n\n```bash\ndate -d '+30 days' +%F\ndate -d 'last friday' +%F\n```\n\n> Expiry dates for chage -E.\n\n### Epoch seconds\n\n```bash\ndate +%s\n```\n```text\n1761381730\n```\n```bash\ndate -d @1761381730\n```\n\n> Convert log time stamps.\n\n### ISO format\n\n```bash\ndate -Iseconds\n```\n```text\n2026-10-25T11:42:10+03:00\n```\n\n> For JSON and APIs.\n\n### Time of a file change\n\n```bash\ndate -r /etc/passwd\n```\n\n> When was a user changed?\n\n### Time in another zone\n\n```bash\nTZ=Europe/Amsterdam date\n```\n\n> Without changing the system.\n\n### Set the clock (lab without NTP)\n\n```bash\ntimedatectl set-ntp false; date -s \"2026-10-25 12:00:00\"\n```\n\n> With NTP on, chrony corrects it.\n\n### Days until a certificate expires (security)\n\n```bash\necho $(( ( $(date -d \"$(openssl x509 -enddate -noout -in /etc/pki/tls/certs/site.crt | cut -d= -f2)\" +%s) - $(date +%s) ) / 86400 ))\n```\n\n> Alert when it gets low.\n\n### Measure how long something takes\n\n```bash\nS=$(date +%s); sleep 2; echo \"took $(( $(date +%s) - S )) s\"\n```\n\n> Simple timing in scripts.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man date](https://linux.die.net/man/1/date)\n","page":"commands/date/","label":"date","where":"Command page · Time","aka":[]},{"id":"cmd-dd","n":1396,"slug":"dd","title":"dd — Copy data block by block: make disk images, write ISO files to USB, create files of a fixed size, wipe or test disks","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["copy blocks","disk image","write iso","swap file","wipe","benchmark"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Copy data block by block: make disk images, write ISO files to USB, create files of a fixed size, wipe or test disks.\n\n## About\n\n`dd if=INPUT of=OUTPUT bs=SIZE count=N` reads blocks from the input and writes them to the output. It works on files and on raw devices (whole disks), so it can copy or destroy anything — people call it \"disk destroyer\" for a reason. Always check `of=`.\n\nOptions are written as `name=value`, not with dashes.\n\n## Syntax\n\n```text\ndd [OPERAND]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `if=FILE` | Read from this file or device (default stdin). |\n| `of=FILE` | Write to this file or device (default stdout). |\n| `bs=BYTES` | Block size for read and write (like 1M, 4M). |\n| `count=N` | Copy only N blocks. |\n| `ibs=BYTES` | Read block size. |\n| `obs=BYTES` | Write block size. |\n| `cbs=BYTES` | Conversion block size. |\n| `skip=N, iseek=N` | Skip N input blocks. |\n| `seek=N, oseek=N` | Skip N output blocks. |\n| `conv=LIST` | Conversions: notrunc, noerror, sync, sparse, fsync, fdatasync, excl, nocreat, ucase, lcase … |\n| `iflag=LIST` | Input flags: fullblock, direct, nocache, skip_bytes, count_bytes … |\n| `oflag=LIST` | Output flags: direct, sync, dsync, append, nocache, seek_bytes … |\n| `status=LEVEL` | none, noxfer or progress (show progress). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Create a file of 1 GiB with zeros\n\n```bash\ndd if=/dev/zero of=/swapfile bs=1M count=1024\n```\n\n> Good for swap files (no holes).\n\n### Write an ISO to a USB stick\n\n```bash\ndd if=rhel-10.0-x86_64-dvd.iso of=/dev/sdc bs=4M status=progress oflag=sync\n```\n\n> /dev/sdc is the whole stick (no number). Check with lsblk first!\n\n### Back up a partition to an image\n\n```bash\ndd if=/dev/vdb1 of=/backup/vdb1.img bs=4M status=progress\n```\n\n> Unmount it first for a consistent copy.\n\n### Back up the MBR (first 512 bytes)\n\n```bash\ndd if=/dev/vda of=/root/mbr.bin bs=512 count=1\n```\n\n> Boot code and MBR table.\n\n### Wipe the start of a disk\n\n```bash\ndd if=/dev/zero of=/dev/vdb bs=1M count=10\n```\n\n> Removes partition table and signatures at the start (wipefs is cleaner).\n\n### Test write speed\n\n```bash\ndd if=/dev/zero of=/var/tmp/test bs=1M count=1024 oflag=direct status=progress\n```\n\n> direct skips the cache; delete the file after.\n\n### Test read speed\n\n```bash\ndd if=/var/tmp/test of=/dev/null bs=1M iflag=direct\n```\n\n> Real disk reads.\n\n### A busy CPU process for labs\n\n```bash\ndd if=/dev/zero of=/dev/null &\n```\n\n> Practice top, nice and kill.\n\n### Random data file\n\n```bash\ndd if=/dev/urandom of=random.bin bs=1M count=10\n```\n\n> Test files that do not compress.\n\n### Copy a failing disk and go on after errors\n\n```bash\ndd if=/dev/sdb of=/backup/sdb.img bs=64K conv=noerror,sync status=progress\n```\n\n> Bad blocks become zeros (ddrescue is better for this).\n\n### Restore only the boot code\n\n```bash\ndd if=/root/mbr.bin of=/dev/vda bs=446 count=1\n```\n\n> Restores only the boot code, not the partition table (446 bytes).\n\n## Watch out\n\n- `of=` on the wrong disk destroys it at once. There is no question and no undo.\n- Without status=progress dd prints nothing until it is done; send `kill -USR1 PID` for a status.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man dd](https://linux.die.net/man/1/dd)\n","page":"commands/dd/","label":"dd","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-df","n":1397,"slug":"df","title":"df — Show how much space is used and free on each mounted file system","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["disk","space","free","file system","mount","inodes","full"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show how much space is used and free on each mounted file system.\n\n## About\n\n`df` (disk free) lists the mounted file systems with their size, used space, free space, use percentage and mount point. It is the first command when somebody says \"the disk is full\".\n\nWith `-h` sizes are easy to read; with `-T` you also see the type (xfs, ext4, tmpfs, nfs). With `-i` it shows inodes: a disk can also be \"full\" when all inodes are used (millions of tiny files), even if there is free space.\n\n## Syntax\n\n```text\ndf [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h, --human-readable` | Sizes in powers of 1024, like 950M or 17G. |\n| `-T, --print-type` | Show the file system type. |\n| `-i, --inodes` | Show inode use instead of block use. |\n| `-H, --si` | Sizes in powers of 1000 (like disk makers use). |\n| `-a, --all` | Also show pseudo, duplicate and inaccessible file systems. |\n| `-l, --local` | Only local file systems (no NFS or CIFS). |\n| `-t, --type=<type>` | Only file systems of this type, like xfs. |\n| `-x, --exclude-type=<type>` | Leave out this type, like tmpfs. |\n| `-k` | Sizes in 1 KiB blocks. |\n| `-B, --block-size=<size>` | Sizes in this unit, like M or G. |\n| `--output[=<fields>]` | Choose the columns: source, fstype, itotal, iused, iavail, ipcent, size, used, avail, pcent, file, target. |\n| `--total` | Add a line with the total. |\n| `-P, --portability` | Use the POSIX output format (one line per file system, good for scripts). |\n| `--sync` | Run sync before reading the numbers. |\n| `--no-sync` | Do not run sync first (the default). |\n| `-v` | Ignored. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show all file systems with readable sizes\n\n```bash\ndf -h\n```\n\n> Look at the Use% column. Above 90% needs action.\n\n### Show the types too\n\n```bash\ndf -hT\n```\n\n> You see which ones are xfs, ext4, tmpfs or NFS.\n\n### Which file system holds this folder?\n\n```bash\ndf -h /var/log\n```\n\n> Shows the file system that the folder is on.\n\n### Check inodes when the disk is \"full\" but has space\n\n```bash\ndf -i /var\n```\n\n> 100% IUse% means no new files can be made. Look for folders with millions of small files.\n\n### Only real disks, no tmpfs\n\n```bash\ndf -h -x tmpfs -x devtmpfs\n```\n\n> A cleaner list for reports.\n\n### Only XFS file systems\n\n```bash\ndf -hT -t xfs\n```\n\n> Useful when you check the volumes you created in the LVM chapter.\n\n### Choose the columns for a report\n\n```bash\ndf -h --output=target,size,used,avail,pcent\n```\n\n> Easy to read and to paste in a ticket.\n\n### Alert when a file system is above 85% (monitoring script)\n\n```bash\ndf -P | awk 'NR>1 && $5+0 > 85 {print \"WARN\", $6, $5}'\n```\n\n> `-P` keeps each file system on one line, so awk reads it safely.\n\n### Check a new mount\n\n```bash\nmount /dev/vdb1 /mnt/data && df -h /mnt/data\n```\n\n> Confirms the size and that the mount worked.\n\n### Only local file systems\n\n```bash\ndf -hl\n```\n\n> Skips NFS shares that may hang when the server is down.\n\n### Show the total of all file systems\n\n```bash\ndf -h --total | tail -1\n```\n\n> One number for the whole server.\n\n## Watch out\n\n- `df` and `du` can disagree: a deleted file that a program still has open still uses space. Find it with `lsof +L1`.\n- XFS and ext4 keep some space reserved; \"Avail\" can be smaller than Size minus Used.\n- A hanging NFS server can make `df` hang. Use `df -l` or `df -x nfs4`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/self/mountinfo` | The list of mounts that df reads. |\n\n## Tips\n\n- `df -hT` = which disk, how full, what type. `df -i` = inodes.\n- df = per file system, du = per folder.\n\nFull manual: [man df](https://linux.die.net/man/1/df)\n","page":"commands/df/","label":"df","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-dig","n":1398,"slug":"dig","title":"dig — Ask DNS servers for records (A, AAAA, MX, TXT, NS …) and show the full answer — the main DNS test tool","sections":["commands"],"topic":"Commands · Network","tags":["dns","lookup","records","troubleshoot","mx","txt","dnssec"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Ask DNS servers for records (A, AAAA, MX, TXT, NS …) and show the full answer — the main DNS test tool.\n\n## About\n\n`dig NAME TYPE @SERVER` sends a DNS query and prints the question, the answer, the authority section and statistics (server, time). Without a server it uses the servers in `/etc/resolv.conf`.\n\nOptions with `+` change the query and the output: `+short` gives only the answer, `+trace` follows the delegation from the root servers. `dig` asks DNS only — not `/etc/hosts`; use `getent hosts` to see what programs get.\n\n## Syntax\n\n```text\ndig [@SERVER] [NAME] [TYPE] [CLASS] [OPTION]... [+QUERY-OPTION]...\ndig -x ADDRESS\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `@SERVER` | The DNS server to ask (name or IP). |\n| `NAME` | The name to look up. |\n| `TYPE` | Record type: A, AAAA, MX, NS, TXT, CNAME, SOA, PTR, SRV, CAA, ANY … |\n\n## Options\n\n\n### Options\n\n| Option | What it does |\n|---|---|\n| `-x ADDR` | Reverse lookup (PTR) of an IP address. |\n| `-t TYPE` | Record type. |\n| `-q NAME` | The name (useful when it looks like an option). |\n| `-c CLASS` | Class (IN is default). |\n| `-p PORT` | Use another port. |\n| `-4` | Use IPv4 for the transport. |\n| `-6` | Use IPv6 for the transport. |\n| `-b ADDRESS[#PORT]` | Send from this source address. |\n| `-f FILE` | Read many queries from a file. |\n| `-k KEYFILE` | TSIG key file. |\n| `-y [HMAC:]NAME:SECRET` | TSIG key on the command line. |\n| `-r` | Do not read ~/.digrc. |\n| `-u` | Times in microseconds. |\n| `-m` | Memory debugging. |\n| `-h` | Show help. |\n| `-v` | Show the version. |\n\n### Query options (+)\n\n| Option | What it does |\n|---|---|\n| `+short` | Only the answer data. |\n| `+trace` | Follow the delegation from the root servers down. |\n| `+noall +answer` | Show only the answer section. |\n| `+norecurse` | Ask without recursion (test a server's own data). |\n| `+dnssec` | Ask for DNSSEC records. |\n| `+tcp` | Use TCP instead of UDP. |\n| `+time=T` | Timeout in seconds. |\n| `+tries=N` | Number of tries. |\n| `+retry=N` | Number of retries. |\n| `+search` | Use the search domains from resolv.conf. |\n| `+nssearch` | Ask every authoritative server for the SOA. |\n| `+multiline` | Long records on more lines. |\n| `+stats, +nostats` | Show or hide the statistics. |\n| `+question, +answer, +authority, +additional, +comments` | Show or hide these sections (with +no… to hide). |\n| `+cmd, +nocmd` | Show or hide the first line. |\n| `+ttlunits` | TTL in readable units. |\n| `+nsid` | Ask the server for its ID. |\n| `+subnet=ADDR/LEN` | Send an EDNS client subnet. |\n| `+bufsize=B` | EDNS buffer size. |\n| `+edns=N, +noedns` | EDNS version or no EDNS. |\n| `+cd` | Checking disabled (no DNSSEC validation). |\n| `+ad` | Ask for the authenticated data flag. |\n| `+tls` | DNS over TLS. |\n| `+https` | DNS over HTTPS. |\n| `+yaml` | YAML output. |\n| `+identify` | Show the server for +short answers. |\n\n## Examples\n\n### Look up an address\n\n```bash\ndig www.redhat.com +short\n```\n```text\nwww.redhat.com.edgekey.net.\ne3396.dscx.akamaiedge.net.\n23.40.100.73\n```\n\n> CNAMEs first, then the IP.\n\n### Ask a chosen server\n\n```bash\ndig @192.168.122.1 server2.example.com\n```\n\n> Compare servers when only some clients fail.\n\n### Only the answer section\n\n```bash\ndig example.com MX +noall +answer\n```\n```text\nexample.com.  3600  IN  MX  10 mail.example.com.\n```\n\n> Name, TTL, class, type, data.\n\n### Reverse lookup\n\n```bash\ndig -x 192.168.122.20 +short\n```\n```text\nserver2.example.com.\n```\n\n> Mail servers and SSH logs use reverse DNS.\n\n### Follow the delegation\n\n```bash\ndig +trace s0sta.com\n```\n\n> Shows root → .com → your name servers; finds wrong NS records.\n\n### Check mail security records\n\n```bash\ndig s0sta.com TXT +short; dig _dmarc.s0sta.com TXT +short\n```\n\n> SPF and DMARC protect against spoofed mail.\n\n### Check the name servers and SOA\n\n```bash\ndig s0sta.com NS +short; dig s0sta.com SOA +short\n```\n\n> The SOA serial must grow after every zone change.\n\n### See the TTL before a migration\n\n```bash\ndig www.s0sta.com A +noall +answer +ttlunits\n```\n\n> Lower the TTL a day before you change the IP.\n\n### Check DNSSEC\n\n```bash\ndig s0sta.com A +dnssec +multiline\n```\n\n> RRSIG records show that the zone is signed.\n\n### Test a server's own data\n\n```bash\ndig @ns1.example.com example.com SOA +norecurse\n```\n\n> An authoritative server must answer without recursion.\n\n### Find open resolvers (security, own servers only)\n\n```bash\ndig @203.0.113.10 www.google.com +short\n```\n\n> Your public DNS server should not answer for other domains.\n\n### Many names from a file\n\n```bash\ndig -f names.txt +short\n```\n\n> One name per line.\n\n## Watch out\n\n- dig ignores /etc/hosts. A name can work in a browser and fail in dig (or the other way).\n- The answer may come from a cache: check the TTL.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/resolv.conf` | Default DNS servers and search domains. |\n| `~/.digrc` | Default dig options. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | An answer came (also NXDOMAIN). |\n| `1` | Wrong syntax. |\n| `8` | Could not open the batch file. |\n| `9` | No reply from the server. |\n| `10` | Internal error. |\n\nFull manual: [man dig](https://linux.die.net/man/1/dig)\n","page":"commands/dig/","label":"dig","where":"Command page · Network","aka":[]},{"id":"cmd-dmesg","n":1399,"slug":"dmesg","title":"dmesg — Show the kernel messages (the kernel ring buffer): hardware found, drivers loaded, disk and memory errors","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel messages","ring buffer","hardware","drivers","errors","boot"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the kernel messages (the kernel ring buffer): hardware found, drivers loaded, disk and memory errors.\n\n## About\n\nThe kernel writes its messages into a ring buffer in memory. `dmesg` prints it: from the first boot messages to new events, like a USB stick being plugged in or a disk error. Times are seconds since boot; `-T` makes them readable.\n\nOn RHEL, normal users may be blocked (`kernel.dmesg_restrict = 1`); root can always read it. `journalctl -k` shows the same messages and also those of earlier boots.\n\n## Syntax\n\n```text\ndmesg [OPTION]...\n```\n\n## Options\n\n\n### Filter\n\n| Option | What it does |\n|---|---|\n| `-l, --level LIST` | Only these levels: emerg, alert, crit, err, warn, notice, info, debug. |\n| `-f, --facility LIST` | Only these facilities: kern, user, daemon … |\n| `-k, --kernel` | Only kernel messages. |\n| `-u, --userspace` | Only user-space messages. |\n| `--since TIME, --until TIME` | Only messages in this time window. |\n\n### Time and format\n\n| Option | What it does |\n|---|---|\n| `-T, --ctime` | Human-readable time stamps. |\n| `-H, --human` | Human output with colors and a pager. |\n| `-e, --reltime` | Local time and time difference. |\n| `-d, --show-delta` | Time since the previous message. |\n| `-t, --notime` | No time stamps. |\n| `--time-format FORMAT` | ctime, notime, delta, reltime, iso, raw. |\n| `-x, --decode` | Show facility and level as words. |\n| `-r, --raw` | Raw messages with level prefixes. |\n| `-J, --json` | JSON output. |\n| `-L, --color[=WHEN]` | Colors. |\n| `-p, --force-prefix` | Prefix every line of multi-line messages. |\n| `--noescape` | Do not escape unprintable characters. |\n| `-P, --nopager` | No pager. |\n\n### Follow\n\n| Option | What it does |\n|---|---|\n| `-w, --follow` | Wait for new messages (like tail -f). |\n| `-W, --follow-new` | Only new messages. |\n\n### Buffer\n\n| Option | What it does |\n|---|---|\n| `-c, --read-clear` | Print and clear the buffer (root). |\n| `-C, --clear` | Clear the buffer (root). |\n| `-s, --buffer-size SIZE` | Buffer size to read. |\n| `-S, --syslog` | Use the syslog interface instead of /dev/kmsg. |\n| `-F, --file FILE` | Read from a file. |\n\n### Console\n\n| Option | What it does |\n|---|---|\n| `-n, --console-level LEVEL` | Messages up to this level go to the console. |\n| `-D, --console-off` | Stop messages on the console. |\n| `-E, --console-on` | Messages on the console again. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### All kernel messages\n\n```bash\ndmesg | less\n```\n\n> From the boot until now.\n\n### Readable times\n\n```bash\ndmesg -T | tail\n```\n```text\n[Sun Oct 25 10:31:07 2026] sd 2:0:0:0: [sdc] Attached SCSI removable disk\n```\n\n> When did it happen?\n\n### Errors and warnings only\n\n```bash\ndmesg -T --level=err,warn\n```\n\n> First check for hardware trouble.\n\n### Watch live\n\n```bash\ndmesg -wT\n```\n\n> Plug in a device and see the messages.\n\n### Disk errors (I/O)\n\n```bash\ndmesg -T | grep -iE 'i/o error|blk_update_request|ata[0-9].*error'\n```\n\n> A failing disk or cable.\n\n### Memory killer\n\n```bash\ndmesg -T | grep -i 'out of memory'\n```\n\n> Which process the OOM killer killed.\n\n### Find the device name of a new USB disk\n\n```bash\ndmesg -T | tail -20 | grep -E 'sd[a-z]'\n```\n\n> Then use lsblk.\n\n### Firewall drops logged by the kernel (security)\n\n```bash\ndmesg -T | grep -i 'filter_'\n```\n\n> Shows packets that were logged and dropped.\n\n### Segfaults of programs\n\n```bash\ndmesg -T | grep -i segfault\n```\n\n> Crashes and possible exploit attempts.\n\n### Check if users may read it (hardening)\n\n```bash\nsysctl kernel.dmesg_restrict\n```\n```text\nkernel.dmesg_restrict = 1\n```\n\n> 1 = only root (hides kernel addresses from attackers).\n\n### Earlier boots\n\n```bash\njournalctl -k -b -1\n```\n\n> dmesg only shows the current boot.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man dmesg](https://linux.die.net/man/1/dmesg)\n","page":"commands/dmesg/","label":"dmesg","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-dnf","n":1400,"slug":"dnf","title":"dnf — Install, update, remove and search RPM packages from repositories, with all dependencies","sections":["commands"],"topic":"Commands · Software packages","tags":["packages","install","update","repository","rpm","groups","history","security updates"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Install, update, remove and search RPM packages from repositories, with all dependencies.\n\nSame page for: `yum`\n\n## About\n\n`dnf` is the package manager of RHEL (`yum` is a link to it). It reads the repositories in `/etc/yum.repos.d/`, solves dependencies, checks GPG signatures and keeps a history of every transaction that you can undo.\n\nThe general settings are in `/etc/dnf/dnf.conf` (like `gpgcheck=1`, `installonly_limit=3` = keep 3 kernels). Commands that change the system need root; searching and listing do not.\n\n## Syntax\n\n```text\ndnf [OPTION]... COMMAND [ARGUMENT]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `install PKG...` | Install packages (names, files, URLs, `@group`). |\n| `remove PKG...` | Remove packages and unneeded dependencies. |\n| `reinstall PKG...` | Install the same version again. |\n| `upgrade [PKG...]` | Update packages (all without names). `update` is the same. |\n| `upgrade-minimal` | Only the updates needed for bug and security fixes. |\n| `downgrade PKG` | Go back to an older version. |\n| `distro-sync` | Make all packages match the repository versions. |\n| `check-update` | List available updates (exit code 100 if there are some). |\n| `autoremove` | Remove dependencies nothing needs anymore. |\n| `search WORD...` | Search names and summaries (`search all` = also descriptions). |\n| `provides FILE` | Find the package that gives a file or command. |\n| `info PKG` | Details of a package. |\n| `list [installed\\|available\\|--upgrades]` | List packages. |\n| `repolist [all\\|enabled\\|disabled]` | List repositories. |\n| `repoinfo` | Details of repositories. |\n| `repoquery` | Advanced queries (dependencies, files, who requires). |\n| `deplist PKG` | Dependencies of a package. |\n| `group list\\|info\\|install\\|remove` | Package groups. |\n| `history [list\\|info\\|undo\\|redo\\|rollback]` | Past transactions. |\n| `updateinfo` | Advisories (security, bugfix). |\n| `mark install\\|remove PKG` | Mark packages as user-installed or dependency. |\n| `swap OLD NEW` | Replace one package by another in one step. |\n| `makecache` | Download the repository indexes. |\n| `clean all\\|metadata\\|packages` | Remove cached data. |\n| `config-manager` | Add, enable or disable repositories (plugin). |\n| `needs-restarting` | Show if a reboot or service restart is needed (plugin). |\n| `download PKG` | Download the .rpm without installing (plugin). |\n| `shell` | Interactive dnf shell. |\n\n## Options\n\n\n### Answers and output\n\n| Option | What it does |\n|---|---|\n| `-y, --assumeyes` | Answer yes to all questions. |\n| `--assumeno` | Answer no to all questions. |\n| `-q, --quiet` | Less output. |\n| `-v, --verbose` | More output. |\n| `--color=WHEN` | always, never or auto. |\n| `-d, --debuglevel=LEVEL` | Debug output level. |\n| `-e, --errorlevel=LEVEL` | Error output level. |\n\n### Repositories\n\n| Option | What it does |\n|---|---|\n| `--repo=REPOID` | Use only this repository. |\n| `--enablerepo=REPOID` | Enable a repository for this command. |\n| `--disablerepo=REPOID` | Disable a repository for this command. |\n| `--repofrompath=REPO,PATH` | Add a temporary repository. |\n| `-C, --cacheonly` | Use only the cache; do not download indexes. |\n| `--refresh` | Download fresh indexes first. |\n| `--releasever=VERSION` | Use another release version. |\n| `--setopt=OPTION=VALUE` | Set a config option for this command. |\n| `--nogpgcheck` | Do not check signatures (not recommended). |\n\n### Which packages\n\n| Option | What it does |\n|---|---|\n| `-x, --exclude=PKG` | Leave these packages out. |\n| `--disableexcludes=all\\|main\\|REPOID` | Ignore exclude settings. |\n| `--best` | Use only the newest version, or fail. |\n| `--nobest` | Allow older versions when the newest has problems. |\n| `--allowerasing` | Allow removing packages to solve conflicts. |\n| `--skip-broken` | Skip packages with dependency problems. |\n| `--showduplicates` | Show all versions, not only the newest. |\n| `--obsoletes` | Also handle obsoleted packages. |\n| `--noautoremove` | Do not remove unneeded dependencies with remove. |\n| `--nodocs` | Do not install documentation. |\n| `--forcearch=ARCH` | Use another architecture. |\n\n### Security updates\n\n| Option | What it does |\n|---|---|\n| `--security` | Only security updates. |\n| `--bugfix` | Only bug fix updates. |\n| `--enhancement` | Only enhancement updates. |\n| `--newpackage` | Only new package updates. |\n| `--advisory=ID` | Only this advisory (RHSA-…). |\n| `--cve=CVE` | Only updates for this CVE. |\n| `--bz=ID` | Only updates for this Bugzilla ID. |\n| `--sec-severity=LEVEL` | Only Critical, Important, Moderate or Low. |\n\n### Downloads and others\n\n| Option | What it does |\n|---|---|\n| `--downloadonly` | Download but do not install. |\n| `--downloaddir=DIR, --destdir=DIR` | Where to put downloads. |\n| `--installroot=PATH` | Install into another root folder. |\n| `-c, --config=FILE` | Use another config file. |\n| `--comment=TEXT` | Add a comment to the history. |\n| `--noplugins` | Disable all plugins. |\n| `--disableplugin=NAMES` | Disable some plugins. |\n| `--enableplugin=NAMES` | Enable some plugins. |\n| `-R, --randomwait=MINUTES` | Wait a random time first (for cron). |\n| `-4` | Use IPv4 only. |\n| `-6` | Use IPv6 only. |\n| `--rpmverbosity=NAME` | rpm output level. |\n| `--debugsolver` | Save debug data of the solver. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Install a package\n\n```bash\ndnf install -y httpd\n```\n\n> Dependencies come too.\n\n### Find the package of a command\n\n```bash\ndnf provides '*/semanage'\n```\n```text\npolicycoreutils-python-utils-3.8-1.el10.noarch : SELinux policy core python utilities\n```\n\n> Then install that package.\n\n### Search for software\n\n```bash\ndnf search nmap\n```\n\n> Searches names and summaries.\n\n### Update everything\n\n```bash\ndnf upgrade -y\n```\n\n> New kernels are added next to the old ones.\n\n### Only security updates\n\n```bash\ndnf upgrade --security -y\ndnf updateinfo list --security\n```\n\n> Patch fast with less risk.\n\n### Install a local RPM file with dependencies\n\n```bash\ndnf install -y ./tool-1.2-1.el10.x86_64.rpm\n```\n\n> Better than rpm -i: missing dependencies come from the repositories.\n\n### Show repositories and enable one for a command\n\n```bash\ndnf repolist all\ndnf install -y --enablerepo=codeready-builder-for-rhel-10-x86_64-rpms ninja-build\n```\n\n> The repo is used only for this command.\n\n### Add a repository file\n\n```bash\ndnf config-manager --add-repo http://repo.example.com/rhel10/AppStream\n```\n\n> Writes a .repo file in /etc/yum.repos.d/.\n\n### Install a group\n\n```bash\ndnf group install -y \"Development Tools\"\n```\n\n> Many packages for one job.\n\n### Undo the last change\n\n```bash\ndnf history\ndnf -y history undo last\n```\n\n> Removes what the last transaction installed.\n\n### Show all versions and install an older one\n\n```bash\ndnf list --showduplicates httpd\ndnf install -y httpd-2.4.62-1.el10\n```\n\n> Name-version-release selects the version.\n\n### Keep a package on its version\n\n```bash\necho \"excludepkgs=kernel*\" >> /etc/dnf/dnf.conf\n```\n\n> Or use the versionlock plugin. Remove it later or you miss security fixes.\n\n### Is a reboot needed after updates?\n\n```bash\ndnf needs-restarting -r\n```\n\n> Kernel or glibc updates need a reboot.\n\n### Download packages for an offline server\n\n```bash\ndnf download --resolve --destdir=/tmp/pkgs nmap\n```\n\n> Copy the folder to the server and install with dnf install.\n\n### Clean the cache after repository problems\n\n```bash\ndnf clean all; dnf makecache\n```\n\n> Fixes errors from old indexes.\n\n## Watch out\n\n- The exam system has no Red Hat repositories: first create the .repo file.\n- `--nogpgcheck` and `gpgcheck=0` trust any package. Only in labs.\n- `dnf remove` also removes dependencies; read the list before you say yes.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/dnf/dnf.conf` | General settings. |\n| `/etc/yum.repos.d/*.repo` | Repository definitions. |\n| `/var/cache/dnf/` | Downloaded indexes and packages. |\n| `/var/log/dnf.log` | Log of dnf actions. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (check-update: no updates). |\n| `1` | Error. |\n| `100` | check-update: updates are available. |\n| `200` | Another dnf holds the lock. |\n\nFull manual: [man dnf](https://linux.die.net/man/8/yum)\n","page":"commands/dnf/","label":"dnf","where":"Command page · Software packages","aka":["yum"]},{"id":"cmd-dracut","n":1401,"slug":"dracut","title":"dracut — Create (or rebuild) the initramfs: the small root file system in RAM that finds and mounts the real root at boot","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["initramfs","boot","drivers","rescue","kernel"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create (or rebuild) the initramfs: the small root file system in RAM that finds and mounts the real root at boot.\n\n## About\n\nThe initramfs holds the drivers, LVM/RAID/crypt tools and systemd units needed to reach the root file system. `dracut` builds it for a kernel; by default only what this machine needs (host-only). Kernel package updates call it automatically. Rebuild it after you change storage drivers, LVM names, `/etc/crypttab` or module blacklists that matter at boot.\n\n## Syntax\n\n```text\ndracut [OPTION]... [IMAGE [KERNEL-VERSION]]\n```\n\n## Options\n\n\n### Basic\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Overwrite an existing image. |\n| `--kver VERSION` | The kernel version (default: the running one). |\n| `--regenerate-all` | Rebuild the images of all installed kernels. |\n| `-v, --verbose` | More output. |\n| `-q, --quiet` | Less output. |\n| `--list-modules` | List the available dracut modules. |\n| `--print-cmdline` | Print the kernel arguments for the current setup. |\n\n### Content\n\n| Option | What it does |\n|---|---|\n| `-a, --add LIST` | Add dracut modules (like lvm, crypt, nfs, network). |\n| `--force-add LIST` | Add even if not needed for this host. |\n| `-o, --omit LIST` | Leave out dracut modules. |\n| `-m, --modules LIST` | Use only these dracut modules. |\n| `-d, --drivers LIST` | Use only these kernel modules. |\n| `--add-drivers LIST` | Add kernel modules. |\n| `--force-drivers LIST` | Add and load these kernel modules early. |\n| `--omit-drivers LIST` | Leave out kernel modules. |\n| `--filesystems LIST` | Add file system modules. |\n| `-I, --install LIST` | Add these files. |\n| `--install-optional LIST` | Add files if they exist. |\n| `-i, --include SOURCE TARGET` | Copy a file or folder into the image. |\n| `--kernel-cmdline ARGS` | Built-in kernel arguments. |\n| `--add-fstab FILE, --fstab, --mount \"DEV MNT TYPE OPTS\", --add-device DEV` | Extra mounts and devices. |\n| `--mdadmconf, --nomdadmconf, --lvmconf, --nolvmconf` | Include RAID / LVM config. |\n| `--fscks LIST, --nofscks` | fsck tools to include. |\n| `--early-microcode, --no-early-microcode` | CPU microcode in the image. |\n| `--kernel-only, --no-kernel` | Only kernel modules / no kernel modules. |\n| `--sshkey FILE` | SSH key for network boot unlock. |\n\n### Host-only\n\n| Option | What it does |\n|---|---|\n| `-H, --hostonly` | Only what this host needs (default on RHEL). |\n| `-N, --no-hostonly` | Generic image for any hardware (rescue image). |\n| `--hostonly-mode sloppy\\|strict, --hostonly-cmdline, --no-hostonly-cmdline, --no-hostonly-default-device, --hostonly-i18n, --no-hostonly-i18n, --hostonly-nics LIST, --persistent-policy POLICY` | Host-only details. |\n\n### Build\n\n| Option | What it does |\n|---|---|\n| `--gzip, --bzip2, --lzma, --xz, --lzo, --lz4, --zstd, --compress PROGRAM, --no-compress` | Compression. |\n| `--strip, --nostrip, --aggressive-strip, --hardlink, --nohardlink` | Size tricks. |\n| `-k, --kmoddir DIR, --fwdir DIRS, --libdirs DIRS` | Where to find modules, firmware, libraries. |\n| `-c, --conf FILE, --confdir DIR` | Other config files. |\n| `--tmpdir DIR, -r, --sysroot DIR, --prefix DIR, --noprefix, -l, --local` | Build settings. |\n| `--logfile FILE, --debug` | Logging. |\n| `--uefi, --uefi-stub FILE, --uefi-splash-image FILE` | Build a unified kernel image (UKI). |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Rebuild for the running kernel\n\n```bash\ndracut -f\n```\n\n> Writes /boot/initramfs-$(uname -r).img.\n\n### For a chosen kernel\n\n```bash\ndracut -f /boot/initramfs-6.12.0-55.el10.x86_64.img 6.12.0-55.el10.x86_64\n```\n\n> From a rescue disk, take the version from ls /lib/modules.\n\n### All kernels\n\n```bash\ndracut -f --regenerate-all\n```\n\n> After a change that matters for every kernel.\n\n### Add a storage driver\n\n```bash\ndracut -f --add-drivers \"nvme\"\n```\n\n> When the root disk moved to another controller.\n\n### Generic rescue image\n\n```bash\ndracut -f -N /boot/initramfs-generic.img\n```\n\n> Boots on other hardware (after a P2V/V2V move).\n\n### Make module options permanent in the image\n\n```bash\necho \"options loop max_part=8\" > /etc/modprobe.d/loop.conf; dracut -f\n```\n\n> Early modules read the config from the initramfs.\n\n### Look inside the result\n\n```bash\nlsinitrd | less\n```\n\n> Files and dracut modules in the image.\n\n### Available dracut modules\n\n```bash\ndracut --list-modules | head\n```\n\n> lvm, crypt, nfs, network, multipath …\n\n### Kernel arguments it would need\n\n```bash\ndracut --print-cmdline\n```\n\n> rd.lvm.lv=… and root=… for the current setup.\n\n### Back up before rebuilding\n\n```bash\ncp /boot/initramfs-$(uname -r).img /root/initramfs.bak\n```\n\n> A safe way back.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/dracut.conf, /etc/dracut.conf.d/` | Settings. |\n| `/boot/initramfs-VERSION.img` | The images. |\n| `/usr/lib/dracut/modules.d/` | dracut modules. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man dracut](https://linux.die.net/man/8/dracut)\n","page":"commands/dracut/","label":"dracut","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-du","n":1402,"slug":"du","title":"du — Show how much disk space files and folders use","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["disk","usage","folder","size","space","big files"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show how much disk space files and folders use.\n\n## About\n\n`du` (disk usage) adds up the space that files use, folder by folder. Where `df` tells you that a file system is full, `du` tells you **which folder** fills it.\n\nBy default it shows every sub-folder. Usually you want `-s` (one total per argument), `-h` (readable sizes) and `--max-depth=1` (one level).\n\n## Syntax\n\n```text\ndu [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h, --human-readable` | Sizes like 4.0K, 23M, 1.2G. |\n| `-s, --summarize` | Only one total for each argument. |\n| `-d, --max-depth=<n>` | Show totals only N levels deep. |\n| `-c, --total` | Add a grand total at the end. |\n| `-a, --all` | Show files too, not only folders. |\n| `-x, --one-file-system` | Skip folders on other file systems. |\n| `--exclude=<pattern>` | Leave out files that match. |\n| `-X, --exclude-from=<file>` | Read exclude patterns from a file. |\n| `-t, --threshold=<size>` | Show only entries bigger than SIZE (or smaller if negative). |\n| `--apparent-size` | Show the file length, not the space on disk. |\n| `-b, --bytes` | Apparent size in bytes. |\n| `-k, -m` | Sizes in KiB / MiB. |\n| `-B, --block-size=<size>` | Sizes in this unit. |\n| `--si` | Like -h, with powers of 1000. |\n| `--inodes` | Count inodes (number of files) instead of space. |\n| `-S, --separate-dirs` | Do not include sub-folders in a folder's total. |\n| `-l, --count-links` | Count hard-linked files every time. |\n| `-L, --dereference` | Follow all symbolic links. |\n| `-D, -H, --dereference-args` | Follow links given on the command line only. |\n| `-P, --no-dereference` | Do not follow links (the default). |\n| `--time` | Show the last change time of each folder. |\n| `--time=<word>` | Which time: atime, access, use, ctime, status. |\n| `--time-style=<style>` | Time format: full-iso, long-iso, iso, +FORMAT. |\n| `-0, --null` | End each line with NUL. |\n| `--files0-from=<file>` | Read the names from a file (NUL separated). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Size of one folder\n\n```bash\ndu -sh /var/log\n```\n```text\n2.1G\t/var/log\n```\n\n> One line, readable size.\n\n### Which sub-folder is big?\n\n```bash\ndu -h --max-depth=1 /var | sort -h\n```\n\n> Sorted, the biggest at the bottom. Then go one level deeper.\n\n### The 10 biggest folders on the root file system\n\n```bash\ndu -xh / 2>/dev/null | sort -h | tail -10\n```\n\n> `-x` stays on `/` and does not count other mounts.\n\n### Sizes of all home folders\n\n```bash\ndu -sh /home/*\n```\n\n> Who uses the most space?\n\n### Show files too, bigger than 100 MiB\n\n```bash\ndu -ah /srv | awk '$1 ~ /G|[0-9]{3}M/' | sort -h\n```\n\n> Lists big files and folders. `find -size +100M` is another way.\n\n### Leave out some files\n\n```bash\ndu -sh --exclude='*.iso' ~/Downloads\n```\n\n> Size without the ISO images.\n\n### Count files instead of size\n\n```bash\ndu --inodes -d 1 /var | sort -n | tail\n```\n\n> Finds folders with millions of small files (when df -i is at 100%).\n\n### Grand total of several folders\n\n```bash\ndu -shc /var/log /var/cache /tmp\n```\n\n> The last line is the total.\n\n### Real size vs. apparent size of a VM disk\n\n```bash\ndu -h vm.qcow2; du -h --apparent-size vm.qcow2\n```\n\n> A sparse file can look big but use little space.\n\n### Show when folders changed\n\n```bash\ndu -h --time -d 1 /srv/backups\n```\n\n> Find backup folders that stopped growing.\n\n### Only show folders above 1 GiB\n\n```bash\ndu -h -t 1G -d 2 /var/lib\n```\n\n> Hides all the small stuff.\n\n### Check container storage\n\n```bash\ndu -sh /var/lib/containers/storage\n```\n\n> Container images often fill /var. `podman system prune` cleans unused images.\n\n## Watch out\n\n- du counts only files it can read. Run it as root for system folders.\n- `du` does not see deleted files that are still open; `df` does. That explains differences.\n- On big trees du can take time and a lot of disk reads.\n\n## Tips\n\n- `du -sh *` = sizes here. `du -h -d1 | sort -h` = biggest last.\n\nFull manual: [man du](https://linux.die.net/man/1/du)\n","page":"commands/du/","label":"du","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-dumpe2fs","n":1403,"slug":"dumpe2fs","title":"dumpe2fs — Show the superblock and block group details of an ext2/ext3/ext4 file system","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["ext4","superblock","block groups","backup superblock","file system info"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the superblock and block group details of an ext2/ext3/ext4 file system.\n\n## About\n\n`dumpe2fs -h` prints the superblock (like `tune2fs -l`): label, UUID, features, sizes, mount options. Without `-h` it also lists every block group — including where the **backup superblocks** are, which you need to repair a damaged file system with `e2fsck -b`.\n\n## Syntax\n\n```text\ndumpe2fs [-bfghixV] [-o superblock=N] [-o blocksize=N] DEVICE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h` | Only the superblock information. |\n| `-b` | List the blocks marked as bad. |\n| `-x` | Show block numbers in hex. |\n| `-g` | Group descriptors in a machine-readable form. |\n| `-i` | Read an image file made by e2image. |\n| `-f` | Force output even with unknown features. |\n| `-m` | Check if the device is in use (MMP feature). |\n| `-o superblock=N` | Use this backup superblock. |\n| `-o blocksize=N` | Use this block size (with superblock=). |\n| `-V` | Show the version. |\n\n## Examples\n\n### Superblock only\n\n```bash\ndumpe2fs -h /dev/vdb2\n```\n\n> Label, UUID, features, counts, times.\n\n### Find the backup superblocks\n\n```bash\ndumpe2fs /dev/vdb2 | grep -i 'backup superblock'\n```\n```text\n  Backup superblock at 32768, Group descriptors at 32769-32769\n```\n\n> Write these numbers down.\n\n### Repair with a backup superblock\n\n```bash\ne2fsck -b 32768 /dev/vdb2\n```\n\n> When the main superblock is damaged.\n\n### Default mount options\n\n```bash\ndumpe2fs -h /dev/vdb2 | grep -i 'mount options'\n```\n\n> Set with tune2fs -o.\n\n### Features of the file system\n\n```bash\ndumpe2fs -h /dev/vdb2 | grep -i features\n```\n\n> has_journal, extent, 64bit, metadata_csum …\n\n### Reserved blocks and block size\n\n```bash\ndumpe2fs -h /dev/vdb2 | grep -E 'Reserved block count|Block size|Block count'\n```\n\n> Calculate the percentage.\n\n### When was it created and checked?\n\n```bash\ndumpe2fs -h /dev/vdb2 | grep -E 'created|Last checked|Last mount time'\n```\n\n> History of the file system.\n\n### Bad blocks list\n\n```bash\ndumpe2fs -b /dev/vdb2\n```\n\n> Empty on healthy disks.\n\n### Use a backup superblock to read\n\n```bash\ndumpe2fs -o superblock=32768 -o blocksize=4096 /dev/vdb2 | head\n```\n\n> When the main one cannot be read.\n\n### Check if the device is in use (MMP)\n\n```bash\ndumpe2fs -m /dev/vdb2\n```\n\n> For shared storage with multi-mount protection.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man dumpe2fs](https://linux.die.net/man/8/dumpe2fs)\n","page":"commands/dumpe2fs/","label":"dumpe2fs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-echo","n":1404,"slug":"echo","title":"echo — Print text or the value of variables to the screen (or to a file with redirection)","sections":["commands"],"topic":"Commands · Shell and help","tags":["print","text","variable","output","newline","script"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print text or the value of variables to the screen (or to a file with redirection).\n\n## About\n\n`echo` writes its arguments, separated by spaces, followed by a new line. It is the simplest way to show a variable, write a line to a file, or print a message in a script.\n\nBash has `echo` built in; there is also a program `/usr/bin/echo` from coreutils. They work the same for normal use.\n\nFor exact formatting (numbers, tabs, no surprises with backslashes), `printf` is the safer tool.\n\n## Syntax\n\n```text\necho [-neE] [TEXT ...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n` | Do not print the new line at the end. |\n| `-e` | Read backslash codes like \\n (new line) and \\t (tab). |\n| `-E` | Do not read backslash codes (the default). |\n\n### Backslash codes (with -e)\n\n| Option | What it does |\n|---|---|\n| `\\n` | New line. |\n| `\\t` | Tab. |\n| `\\\\` | A backslash. |\n| `\\a` | Alert (beep). |\n| `\\b` | Backspace. |\n| `\\c` | Stop printing here (nothing more is printed). |\n| `\\e` | Escape character (start of color codes). |\n| `\\r` | Carriage return (go to the start of the line). |\n| `\\0NNN` | The character with octal value NNN. |\n| `\\xHH` | The character with hex value HH. |\n\n## Examples\n\n### Print a message\n\n```bash\necho \"Backup started\"\n```\n```text\nBackup started\n```\n\n> Quotes keep the spaces as they are.\n\n### Show the value of a variable\n\n```bash\necho \"$HOME\"\n```\n```text\n/home/student\n```\n\n> Always put variables in double quotes, so spaces in the value do not break the line.\n\n### Write a line to a file\n\n```bash\necho \"192.168.122.20  server2.example.com server2\" >> /etc/hosts\n```\n\n> `>>` adds the line at the end. With `>` the file would be emptied first.\n\n### Write to a file that needs root, from a normal user\n\n```bash\necho \"net.ipv4.ip_forward = 1\" | sudo tee /etc/sysctl.d/90-forward.conf\n```\n\n> `sudo echo ... > file` does not work, because the redirect runs as your user. `tee` runs with sudo.\n\n### Print several lines with -e\n\n```bash\necho -e \"line one\\nline two\\ttab\"\n```\n\n> `\\n` makes a new line and `\\t` a tab.\n\n### Print without a new line (for a prompt)\n\n```bash\necho -n \"Enter the server name: \"; read SERVER\n```\n\n> The cursor stays on the same line.\n\n### Show the exit code of the last command\n\n```bash\ngrep -q root /etc/passwd; echo $?\n```\n```text\n0\n```\n\n> 0 means success. Any other number means an error.\n\n### Print colored text in a script\n\n```bash\necho -e \"\\e[31mERROR\\e[0m disk is full\"\n```\n\n> `\\e[31m` = red, `\\e[0m` = back to normal. Good for warnings in deploy scripts.\n\n### Make a quick test file\n\n```bash\necho \"hello\" > /tmp/test.txt\n```\n\n> Useful to test permissions, SELinux labels or a web server.\n\n### Show the PATH, one folder per line\n\n```bash\necho \"$PATH\" | tr ':' '\\n'\n```\n\n> Easier to read than one long line.\n\n### Check a variable inside a script while you debug\n\n```bash\necho \"DEBUG: user=$USER host=$(hostname) dir=$PWD\" >&2\n```\n\n> `>&2` sends the message to the error channel, so it does not mix with the normal output.\n\n### Use brace expansion to see what it makes\n\n```bash\necho web{01..03}.example.com\n```\n```text\nweb01.example.com web02.example.com web03.example.com\n```\n\n> Bash expands the braces before echo runs. Good to test a pattern before you use it.\n\n## Watch out\n\n- `sudo echo text > /root/file` fails: the redirect is done by your shell, not by sudo. Use `| sudo tee`.\n- Text that starts with `-n` or `-e` is read as an option. Use `printf '%s\\n' \"$TEXT\"` for unknown text.\n- Without quotes, several spaces become one and `*` is expanded to file names.\n\n## Tips\n\n- `echo $?` = did the last command work?\n- `>` = new file, `>>` = add to the file.\n\nFull manual: [man echo](https://linux.die.net/man/1/echo)\n","page":"commands/echo/","label":"echo","where":"Command page · Shell and help","aka":[]},{"id":"cmd-env","n":1405,"slug":"env","title":"env — Show the environment variables, or run a program with a changed environment","sections":["commands"],"topic":"Commands · Shell and help","tags":["environment","variable","path","shebang","run","clean"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the environment variables, or run a program with a changed environment.\n\n## About\n\nWithout arguments, `env` prints all environment variables, one `NAME=value` per line. These are the variables that programs started from your shell receive.\n\nWith a command after it, `env` runs that command with extra, removed or no variables at all. The main shell does not change.\n\nYou also see `env` in the first line of scripts: `#!/usr/bin/env python3` finds `python3` through `$PATH`, so the script works on systems where the program is in another folder.\n\n## Syntax\n\n```text\nenv [OPTION]... [-] [NAME=VALUE]... [COMMAND [ARG]...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-i, --ignore-environment` | Start with an empty environment (only the variables you give). |\n| `-u, --unset=<name>` | Remove this variable from the environment of the command. |\n| `-C, --chdir=<dir>` | Change to this folder before the command runs. |\n| `-S, --split-string=<text>` | Split TEXT into separate arguments (used in shebang lines with options). |\n| `-0, --null` | End each output line with a NUL byte instead of a new line. |\n| `-v, --debug` | Print what env does, step by step. |\n| `--block-signal[=<sig>]` | Block these signals for the command. |\n| `--default-signal[=<sig>]` | Reset these signals to the default for the command. |\n| `--ignore-signal[=<sig>]` | Make the command ignore these signals. |\n| `--list-signal-handling` | Print the non-default signal handling to stderr. |\n| `-` | Same as -i. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show all environment variables\n\n```bash\nenv\n```\n\n> Sorted output is easier to read: `env | sort`.\n\n### Find one variable\n\n```bash\nenv | grep -i proxy\n```\n\n> Shows HTTP_PROXY, https_proxy and others. Proxy variables often explain why `curl` or `dnf` fails.\n\n### Run one command with an extra variable\n\n```bash\nenv LANG=C df -h\n```\n\n> The command runs in English with standard formats. Good for scripts that read command output.\n\n### Run a command without a variable\n\n```bash\nenv -u HTTP_PROXY -u HTTPS_PROXY curl -I https://intranet.example.com\n```\n\n> Skip the proxy for one internal test without changing your shell.\n\n### Run a program in a clean environment\n\n```bash\nenv -i PATH=/usr/bin:/bin HOME=/tmp bash -c 'env'\n```\n\n> Shows what a program sees with almost no variables. Close to what cron and systemd give a job.\n\n### Use env in the first line of a script\n\n```bash\nhead -1 deploy.py\n```\n```text\n#!/usr/bin/env python3\n```\n\n> The script uses the first `python3` in $PATH, for example from a virtual environment.\n\n### Pass options to the interpreter in a shebang\n\n```bash\nhead -1 tool.sh\n```\n```text\n#!/usr/bin/env -S bash -euo pipefail\n```\n\n> `-S` splits the text into several arguments. Without it, the line does not work on Linux.\n\n### Compare the environment of root and your user\n\n```bash\nenv | sort > /tmp/me.txt; sudo env | sort > /tmp/root.txt; diff /tmp/me.txt /tmp/root.txt\n```\n\n> sudo resets most variables. This shows what changes.\n\n### Check the variables of a running process\n\n```bash\ntr '\\0' '\\n' < /proc/$(pgrep -o sshd)/environ\n```\n\n> Each process keeps the environment it started with. Good to debug a service.\n\n### Run a command in another folder\n\n```bash\nenv -C /var/www/html ls -l\n```\n\n> The shell stays in its own folder.\n\n### Print variables safely for a script (NUL separated)\n\n```bash\nenv -0 | sort -z | tr '\\0' '\\n' | head\n```\n\n> NUL separators are safe even when a value contains a new line.\n\n## Watch out\n\n- `env` shows only **exported** variables. A plain shell variable (`X=1` without export) is not in the list; use `set` for all.\n- Secrets in environment variables can be read by root and by tools that dump the environment. Prefer files with tight permissions.\n- `env NAME=value` changes the variable only for that one command.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/PID/environ` | The environment of a running process (NUL separated). |\n| `/etc/environment` | Variables set for all users at login (simple NAME=value lines). |\n\n## Tips\n\n- `env` = show, `env NAME=val cmd` = one command with a change, `env -i` = clean start.\n\nFull manual: [man env](https://linux.die.net/man/1/env)\n","page":"commands/env/","label":"env","where":"Command page · Shell and help","aka":[]},{"id":"cmd-exit","n":1406,"slug":"exit","title":"exit — End the shell or script with an exit status (0 = success, 1–255 = error); `return` does the same for a function","sections":["commands"],"topic":"Commands · Shell and help","tags":["exit status","scripts","logout","return code"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"End the shell or script with an exit status (0 = success, 1–255 = error); `return` does the same for a function.\n\nSame page for: `return`\n\n## About\n\nEvery command returns an **exit status**: 0 means success, any other number an error. `$?` holds the status of the last command. Scripts should end with `exit 0` on success and a non-zero code for errors, so that other scripts, cron and systemd know what happened. `exit` in an interactive shell logs you out.\n\n## Syntax\n\n```text\nexit [N]\nreturn [N]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `0` | Success. |\n| `1` | General error. |\n| `2` | Wrong use (often from builtins). |\n| `126` | Found, but cannot run. |\n| `127` | Command not found. |\n| `128+N` | Killed by signal N (130 = Ctrl+C, 137 = kill -9). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `N` | The exit status (0–255; default: the status of the last command). |\n\n## Examples\n\n### End a script with success\n\n```bash\ntail -1 backup.sh\n```\n```text\nexit 0\n```\n\n> The caller sees 0.\n\n### Exit with an error\n\n```bash\n[ -d /backup ] || { echo \"no backup folder\" >&2; exit 1; }\n```\n\n> Message to stderr, code 1.\n\n### See the status of the last command\n\n```bash\nls /nothing; echo $?\n```\n```text\nls: cannot access '/nothing': No such file or directory\n2\n```\n\n> ls uses 2 for this error.\n\n### Command not found\n\n```bash\nnosuchcommand; echo $?\n```\n```text\n127\n```\n\n> 127 = not found.\n\n### Killed by Ctrl+C\n\n```bash\nsleep 100\n```\n```text\n^C\n```\n```bash\necho $?\n```\n```text\n130\n```\n\n> 128 + 2 (SIGINT).\n\n### Return from a function\n\n```bash\nisroot() { [ \"$(id -u)\" -eq 0 ] && return 0 || return 1; }; isroot && echo root || echo \"not root\"\n```\n\n> return = exit for functions.\n\n### Use the status in if\n\n```bash\nif grep -q lisa /etc/passwd; then echo found; fi\n```\n\n> if checks the exit status.\n\n### Pass on the status of the last command\n\n```bash\ncp a b; exit\n```\n\n> exit without N uses the status of cp.\n\n### Leave a root shell\n\n```bash\nexit\n```\n\n> Back to your own user.\n\n### Exit codes in systemd\n\n```bash\nsystemctl status myapp | grep -i 'code='\n```\n\n> status=203/EXEC = the program could not be run.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `N` | The given value (or the last command's status). |\n\nFull manual: [man exit](https://linux.die.net/man/1/bash)\n","page":"commands/exit/","label":"exit","where":"Command page · Shell and help","aka":["return"]},{"id":"cmd-export","n":1407,"slug":"export","title":"export — Make a shell variable available to the programs you start (export), or remove a variable (unset)","sections":["commands"],"topic":"Commands · Shell and help","tags":["variable","environment","path","shell","bashrc"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Make a shell variable available to the programs you start (export), or remove a variable (unset).\n\nSame page for: `unset`\n\n## About\n\nA variable you set in Bash (`NAME=value`) is only known to that shell. `export NAME` puts it in the **environment**, so every program and script started from this shell gets a copy.\n\n`unset NAME` removes a variable (or a function with `unset -f`).\n\nTo keep a variable for future logins, write the `export` line in `~/.bashrc` (for you) or in a file in `/etc/profile.d/` (for everybody).\n\n## Syntax\n\n```text\nexport [-fn] [NAME[=VALUE] ...]\nexport -p\nunset [-fv] NAME [NAME ...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `NAME=VALUE` | Set the value and export it in one step. |\n| `-p` | List all exported variables (as export lines). |\n| `-n` | Remove the export mark: the variable stays in the shell but is no longer passed on. |\n| `-f` | The names are shell functions (export a function to child shells). |\n\n### unset\n\n| Option | What it does |\n|---|---|\n| `-v` | The names are variables (the default). |\n| `-f` | The names are functions. |\n\n## Examples\n\n### Set and export a variable in one step\n\n```bash\nexport APP_ENV=staging\nbash -c 'echo $APP_ENV'\n```\n```text\nstaging\n```\n\n> The new shell (a child program) sees the value.\n\n### Add a folder to the PATH\n\n```bash\nexport PATH=\"$PATH:$HOME/bin\"\n```\n\n> Keep the old PATH and add your folder at the end. Put the line in `~/.bashrc` to keep it.\n\n### Keep a variable for every login\n\n```bash\necho 'export EDITOR=vim' >> ~/.bashrc\n```\n\n> Tools like `crontab -e`, `visudo` and `git` use the EDITOR variable.\n\n### Set a variable for all users\n\n```bash\necho 'export HISTTIMEFORMAT=\"%F %T \"' > /etc/profile.d/history-time.sh\n```\n\n> Every user gets times in `history` after the next login.\n\n### Use a proxy for the tools in this shell\n\n```bash\nexport http_proxy=http://proxy.example.com:3128 https_proxy=http://proxy.example.com:3128\n```\n\n> `curl`, `dnf` and `pip` read these variables.\n\n### Remove a variable\n\n```bash\nunset http_proxy https_proxy\n```\n\n> Turns the proxy off for this shell again.\n\n### List the exported variables\n\n```bash\nexport -p | head\n```\n\n> Same information as `env`, but in a form you could paste back into a shell.\n\n### Stop exporting a variable but keep it\n\n```bash\nexport -n APP_ENV\n```\n\n> The value stays in this shell; new programs no longer get it.\n\n### Export a function to a child script\n\n```bash\nlog() { echo \"$(date +%T) $*\"; }\nexport -f log\nbash -c 'log \"hello from child\"'\n```\n\n> Child Bash processes can use the function.\n\n### Pass settings to a container or a tool without editing files\n\n```bash\nexport KUBECONFIG=$HOME/.kube/staging.yaml\nkubectl get nodes\n```\n\n> One variable switches the cluster for this terminal only.\n\n### Remove a secret from the shell after use\n\n```bash\nexport DB_PASSWORD='s3cret'; ./migrate.sh; unset DB_PASSWORD\n```\n\n> Shorter life for the secret in the shell. Better: let the tool read it from a protected file.\n\n## Watch out\n\n- No spaces around `=`: `export A = 1` is wrong.\n- A child can never change the variables of its parent. A script that exports a variable does not change your shell, unless you run it with `source`.\n- `unset PATH` breaks the shell until you set PATH again.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.bashrc` | Your exported variables for every shell. |\n| `/etc/profile.d/*.sh` | Exported variables for all users. |\n\n## Tips\n\n- Set: `NAME=value`. Pass on: `export NAME`. Both: `export NAME=value`. Remove: `unset NAME`.\n\nFull manual: [man export](https://linux.die.net/man/1/bash)\n","page":"commands/export/","label":"export","where":"Command page · Shell and help","aka":["unset"]},{"id":"cmd-exportfs","n":1408,"slug":"exportfs","title":"exportfs — Manage the NFS export table: apply changes of /etc/exports, show the active exports, or export a folder for a moment","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["nfs","exports","server","share"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Manage the NFS export table: apply changes of /etc/exports, show the active exports, or export a folder for a moment.\n\n## About\n\nThe NFS server reads `/etc/exports` and `/etc/exports.d/*.exports` when it starts. After editing, run `exportfs -ra` to apply the changes without a restart. `exportfs -v` shows what is exported now, with all options.\n\n## Syntax\n\n```text\nexportfs [-avfrsu] [-o OPTIONS] [CLIENT:/PATH]\nexportfs -ra | -ua | -v | -s\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-r` | Re-export all: sync with /etc/exports (removes deleted entries). |\n| `-a` | Export (or with -u unexport) all folders. |\n| `-u` | Unexport one or more folders (`-ua` = all). |\n| `-v` | Verbose; with no other option: show the exports with options. |\n| `-s` | Show the exports in /etc/exports format. |\n| `-o OPTIONS` | Options for a folder given on the command line. |\n| `-i` | Ignore /etc/exports; only use the command line. |\n| `-f` | Flush the kernel export table. |\n| `-d, --debug KIND` | Debug output (all, auth, call, general, parse). |\n\n## Examples\n\n### Apply changes of /etc/exports\n\n```bash\nexportfs -ra\n```\n\n> No restart of nfs-server needed.\n\n### Show active exports\n\n```bash\nexportfs -v\n```\n```text\n/exports/data  192.168.122.0/24(sync,wdelay,hide,no_subtree_check,sec=sys,rw,secure,root_squash,no_all_squash)\n```\n\n> All options, also the defaults.\n\n### Example exports file\n\n```bash\ncat /etc/exports\n```\n```text\n/exports/data  192.168.122.0/24(rw,sync)\n/exports/home  192.168.122.0/24(rw,sync)\n```\n\n> Folder, then client(options) — no space before the bracket!\n\n### Export for a moment (not saved)\n\n```bash\nexportfs -o ro,sync 192.168.122.20:/srv/iso\n```\n\n> Gone after a restart.\n\n### Stop exporting one folder\n\n```bash\nexportfs -u 192.168.122.0/24:/exports/data\n```\n\n> Clients lose access.\n\n### Stop all exports\n\n```bash\nexportfs -ua\n```\n\n> For maintenance.\n\n### Files in exports.d\n\n```bash\necho \"/srv/iso *(ro)\" > /etc/exports.d/iso.exports; exportfs -ra\n```\n\n> One file per share is easy to manage.\n\n### The space mistake\n\n```bash\ncat /etc/exports\n```\n```text\n/exports/data 192.168.122.0/24 (rw)\n```\n\n> With a space, (rw) applies to everyone and the network gets read-only: a security hole.\n\n### Read-only for all, write for one host\n\n```bash\necho \"/exports/pub *(ro) 192.168.122.20(rw)\" >> /etc/exports; exportfs -ra\n```\n\n> Different options per client.\n\n### Check the server service\n\n```bash\nsystemctl status nfs-server\n```\n\n> exportfs works only with the server running.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/exports, /etc/exports.d/*.exports` | Export definitions. |\n| `/var/lib/nfs/etab` | The active export table. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man exportfs](https://linux.die.net/man/8/exportfs)\n","page":"commands/exportfs/","label":"exportfs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-fallocate","n":1409,"slug":"fallocate","title":"fallocate — Reserve disk space for a file at once (no data is written), or punch holes into a file","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["preallocate","file size","big file","test file","punch hole"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Reserve disk space for a file at once (no data is written), or punch holes into a file.\n\n## About\n\n`fallocate -l 1G FILE` creates a 1 GiB file in a moment: the file system reserves the blocks without writing zeros. Great for test files and disk-full tests. Other modes remove ranges (punch holes) or find and free zero blocks (`-d`).\n\n## Syntax\n\n```text\nfallocate [-c|-p|-z] [-o OFFSET] -l LENGTH [-n] FILE\nfallocate -d [-o OFFSET] [-l LENGTH] FILE\nfallocate -x [-o OFFSET] -l LENGTH FILE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --length LENGTH` | Size, like 500M or 2G. |\n| `-o, --offset OFFSET` | Start position. |\n| `-n, --keep-size` | Do not change the visible file size. |\n| `-p, --punch-hole` | Free the blocks of a range (the file keeps its size). |\n| `-d, --dig-holes` | Find blocks with only zeros and free them. |\n| `-z, --zero-range` | Make a range zeros. |\n| `-c, --collapse-range` | Remove a range and move the rest up. |\n| `-i, --insert-range` | Insert a hole and move the rest down. |\n| `-x, --posix` | Use the POSIX way (writes zeros; slow). |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Create a 1 GiB file at once\n\n```bash\nfallocate -l 1G /var/tmp/test.img\nls -lh /var/tmp/test.img\n```\n\n> Done in milliseconds.\n\n### Fill a disk for a test\n\n```bash\nfallocate -l 9G /data/fill\n```\n\n> Test alerts and app behaviour on a full disk; delete it after.\n\n### Make a sparse file smaller on disk\n\n```bash\nfallocate -d disk.img\n```\n\n> Zero blocks are freed (VM images, backups).\n\n### Free a range in a log file\n\n```bash\nfallocate -p -o 0 -l 100M big.log\n```\n\n> The file keeps its size, uses less disk.\n\n### Reserve space without changing the size\n\n```bash\nfallocate -n -l 500M db.dat\n```\n\n> Space is reserved for later writes.\n\n### Compare used and visible size\n\n```bash\ndu -h --apparent-size disk.img; du -h disk.img\n```\n\n> After -d the real use is smaller.\n\n### Swap file (careful)\n\n```bash\nfallocate -l 1G /swapfile\n```\n\n> Works on many file systems, but swapon may refuse it; dd is the safe way.\n\n### Test file for upload limits\n\n```bash\nfallocate -l 101M /tmp/too-big.bin\n```\n\n> Test a 100 MB upload limit of a web app.\n\n### Truncate is different\n\n```bash\ntruncate -s 1G sparse.img\n```\n\n> truncate makes a sparse file (no blocks reserved).\n\n### Check quota limits\n\n```bash\nfallocate -l 2G ~/quota-test\n```\n\n> Fails with \"Disk quota exceeded\" when the limit is lower.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man fallocate](https://linux.die.net/man/1/fallocate)\n","page":"commands/fallocate/","label":"fallocate","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-fdisk","n":1410,"slug":"fdisk","title":"fdisk — Create, delete and change partitions on MBR and GPT disks with a menu","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["partitions","mbr","gpt","partition table","disk"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create, delete and change partitions on MBR and GPT disks with a menu.\n\n## About\n\n`fdisk DEVICE` opens a menu. Changes are kept in memory until you press `w` (write); `q` quits without saving. `fdisk -l` lists the partition tables of disks.\n\nIt handles both MBR (DOS) and GPT tables: `o` creates a new MBR table, `g` a new GPT table. After writing, the kernel normally reads the new table; if the disk is in use, run `partprobe`.\n\n## Syntax\n\n```text\nfdisk [OPTION]... DEVICE\nfdisk -l [DEVICE]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `m` | Help. |\n| `p` | Print the partition table. |\n| `n` | New partition. |\n| `d` | Delete a partition. |\n| `t` | Change the partition type (alias like swap, lvm, linux, raid, uefi). |\n| `l` | List known types. |\n| `o / g` | Create a new empty MBR / GPT table. |\n| `a` | Toggle the bootable flag (MBR). |\n| `x` | Expert menu. |\n| `v` | Verify the table. |\n| `i` | Information about a partition. |\n| `F` | Show free space. |\n| `w` | Write the table and quit. |\n| `q` | Quit without saving. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --list` | List the partition tables and exit. |\n| `-x, --list-details` | List with more details. |\n| `-o, --output LIST` | Columns for -l. |\n| `-t, --type TYPE` | Only recognize this table type (dos, gpt …). |\n| `-w, --wipe auto\\|never\\|always` | Wipe old file system signatures from the disk. |\n| `-W, --wipe-partitions auto\\|never\\|always` | Wipe signatures from new partitions. |\n| `-n, --noauto-pt` | Do not create a default table on an empty disk. |\n| `-B, --protect-boot` | Do not wipe the first sector with a new table. |\n| `-b, --sector-size SIZE` | Sector size (512, 1024, 2048, 4096). |\n| `-u, --units[=cylinders\\|sectors]` | Units for the table. |\n| `-c, --compatibility[=dos\\|nondos]` | DOS compatibility mode. |\n| `-C, --cylinders N, -H, --heads N, -S, --sectors N` | Old geometry settings. |\n| `-s, --getsz` | Print the size in 512-byte sectors (old). |\n| `-L, --color[=WHEN]` | Colors. |\n| `--lock[=MODE]` | Lock the device while working. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### List the partition tables\n\n```bash\nfdisk -l /dev/vdb\n```\n```text\nDisk /dev/vdb: 10 GiB, 10737418240 bytes, 20971520 sectors\nDisklabel type: gpt\n```\n\n> Check the disk before changing it.\n\n### Create a GPT table and a partition\n\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): g\nCommand (m for help): n\nPartition number (1-128, default 1): <Enter>\nFirst sector (2048-20971486, default 2048): <Enter>\nLast sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971486, default 20969471): +2G\nCommand (m for help): w\n```\n\n> Sizes like +2G are GiB.\n\n### MBR primary partition\n\n```bash\nfdisk /dev/vdc\n```\n```text\nCommand (m for help): o\nCommand (m for help): n\nSelect (default p): p\nCommand (m for help): w\n```\n\n> MBR: up to 4 primary partitions.\n\n### Change the type to swap or LVM\n\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): t\nPartition number (1-3, default 3): 2\nPartition type or alias (type L to list all): lvm\n```\n\n> Aliases: linux, swap, lvm, raid, uefi.\n\n### Delete a partition\n\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): d\nPartition number (1-3, default 3): 3\n```\n\n> Nothing changes on disk until `w`.\n\n### Show free space\n\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): F\n```\n\n> Where new partitions can go.\n\n### Script a partition (no menu)\n\n```bash\nprintf 'g\\nn\\n\\n\\n+1G\\nw\\n' | fdisk /dev/vdb\n```\n\n> For labs; for real scripts prefer `parted -s` or `sfdisk`.\n\n### Make the kernel read the new table\n\n```bash\npartprobe /dev/vdb; lsblk /dev/vdb\n```\n\n> Needed when the disk was in use.\n\n### Back up and restore a partition table\n\n```bash\nsfdisk -d /dev/vdb > /root/vdb-table.txt\nsfdisk /dev/vdb < /root/vdb-table.txt\n```\n\n> sfdisk is the script companion of fdisk.\n\n### Check details of all disks\n\n```bash\nfdisk -x\n```\n\n> Shows UUIDs and attributes of GPT partitions.\n\n## Watch out\n\n- Double check the device name. `w` on the wrong disk can destroy a system.\n- Deleting a partition does not wipe data, but the file system becomes unreachable.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man fdisk](https://linux.die.net/man/8/fdisk)\n","page":"commands/fdisk/","label":"fdisk","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-file","n":1411,"slug":"file","title":"file — Find out what kind of data a file contains (text, script, program, image, archive …), whatever its name","sections":["commands"],"topic":"Commands · Files and folders","tags":["type","mime","binary","text","identify","forensics"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Find out what kind of data a file contains (text, script, program, image, archive …), whatever its name.\n\n## About\n\nLinux does not need file endings: `report` can be a PDF and `photo.jpg` can be a script. `file` looks at the content (the first bytes, \"magic numbers\") and tells you the real type.\n\nUse it before you open an unknown file, when you investigate a suspicious upload, or to see if a program is 32-bit, 64-bit or for ARM.\n\n## Syntax\n\n```text\nfile [OPTIONS] FILE...\nfile -C [-m MAGICFILES]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-b, --brief` | Print only the type, without the file name. |\n| `-i, --mime` | Print the MIME type and charset, like `text/plain; charset=utf-8`. |\n| `--mime-type, --mime-encoding` | Print only the MIME type / only the charset. |\n| `-z, --uncompress` | Look inside compressed files. |\n| `-Z, --uncompress-noreport` | Look inside compressed files, report only the content. |\n| `-L, --dereference` | Follow symbolic links. |\n| `-h, --no-dereference` | Do not follow links (the default). |\n| `-s, --special-files` | Read block and character devices too (like a disk `/dev/vda1`). |\n| `-k, --keep-going` | Do not stop at the first match: show all matching types. |\n| `-f, --files-from <file>` | Read the file names from a file (one per line). |\n| `-F, --separator <sep>` | Use another separator than `:` after the name. |\n| `-N, --no-pad` | Do not pad the names to line up the output. |\n| `-0, --print0` | Print a NUL after the name (for safe scripts). |\n| `-p, --preserve-date` | Do not change the access time of the file. |\n| `-r, --raw` | Do not translate unprintable characters. |\n| `-E` | On errors (file not found), print an error and stop with a failure code. |\n| `-e, --exclude <test>` | Skip one test: apptype, ascii, encoding, tokens, cdf, compress, csv, elf, json, soft, tar. |\n| `--exclude-quiet` | Like --exclude, but no error for unknown tests. |\n| `--extension` | Print the usual file endings for this type. |\n| `-l, --list` | List the strength of each magic pattern. |\n| `-m, --magic-file <list>` | Use these magic files instead of the default. |\n| `-C, --compile` | Compile a magic file into a .mgc file. |\n| `-c, --checking-printout` | Show a parsed form of the magic file (debug). |\n| `-P, --parameter <name=value>` | Set limits of the parser (like bytes to read). |\n| `-n, --no-buffer` | Flush the output after each file. |\n| `-S, --no-sandbox` | Turn off the system call sandbox. |\n| `-d` | Print debug information. |\n| `--apple` | Print the Apple creator/type. |\n| `-v, --version` | Show the version. |\n| `--help` | Show help. |\n\n## Examples\n\n### What is this file?\n\n```bash\nfile mystery\n```\n```text\nmystery: PDF document, version 1.7\n```\n\n> The name had no ending; the content says PDF.\n\n### Check a program's architecture\n\n```bash\nfile /usr/bin/ls\n```\n```text\n/usr/bin/ls: ELF 64-bit LSB pie executable, x86-64, dynamically linked …\n```\n\n> A download for the wrong CPU (ARM vs x86-64) gives \"Exec format error\"; `file` shows why.\n\n### Get the MIME type for a script\n\n```bash\nfile -b --mime-type upload.jpg\n```\n```text\nimage/jpeg\n```\n\n> Web apps should check the real type of uploads, not the file name.\n\n### Find a disguised script in an upload folder\n\n```bash\nfile /var/www/uploads/* | grep -iv image\n```\n\n> A \"picture\" that is really `PHP script` is a typical web shell.\n\n### Check the text encoding\n\n```bash\nfile -i data.csv\n```\n```text\ndata.csv: text/plain; charset=iso-8859-1\n```\n\n> Convert to UTF-8 with `iconv` before an import.\n\n### Look inside a compressed file\n\n```bash\nfile -z backup.sql.gz\n```\n\n> Shows the type of the data inside the gzip file.\n\n### Check what is on a disk partition\n\n```bash\nfile -s /dev/vdb1\n```\n```text\n/dev/vdb1: SGI XFS filesystem data …\n```\n\n> Find the file system type of a partition (or \"data\" when it is empty).\n\n### Check Windows line endings in a script\n\n```bash\nfile deploy.sh\n```\n```text\ndeploy.sh: Bourne-Again shell script, ASCII text executable, with CRLF line terminators\n```\n\n> CRLF breaks bash scripts (\"bad interpreter\"). Fix it with `sed -i 's/\\r$//' deploy.sh`.\n\n### Check many files from a list\n\n```bash\nfind /tmp -type f -newer /tmp/marker > list.txt; file -f list.txt\n```\n\n> Classify every new file in one run (incident response).\n\n### See if a binary is static or dynamic\n\n```bash\nfile /usr/local/bin/kubectl\n```\n\n> \"statically linked\" binaries do not need system libraries.\n\n### Only the type, no name\n\n```bash\nfile -b /etc/passwd\n```\n```text\nASCII text\n```\n\n> Easy to use in `if` tests in scripts.\n\n## Watch out\n\n- `file` guesses from patterns; very short or special files can be reported wrongly.\n- Running `file` on hostile files is mostly safe (it uses a sandbox), but do not open them in other programs to \"check\".\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/misc/magic.mgc` | The compiled database of file patterns. |\n| `/etc/magic` | Your own extra patterns. |\n\n## Tips\n\n- The name lies, the content does not: use `file`.\n\nFull manual: [man file](https://linux.die.net/man/1/file)\n","page":"commands/file/","label":"file","where":"Command page · Files and folders","aka":[]},{"id":"cmd-find","n":1412,"slug":"find","title":"find — Search a folder tree for files that match tests (name, owner, size, time, permissions …) and act on them","sections":["commands"],"topic":"Commands · Files and folders","tags":["search","files","owner","size","time","permissions","suid","exec","delete"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Search a folder tree for files that match tests (name, owner, size, time, permissions …) and act on them.\n\n## About\n\n`find` walks through every folder below a starting point and checks each file against an **expression** of tests. By default it prints the matching names; with actions it can list, delete or run any command on them.\n\nThe pattern is: `find WHERE TESTS ACTIONS`. Several tests are joined with AND; use `-o` for OR, `!` (or `-not`) for NOT, and `\\( … \\)` for groups.\n\nNumbers in tests: `+N` = more than N, `-N` = less than N, `N` = exactly N. Times are in days (`-mtime`) or minutes (`-mmin`).\n\nIt reads the real file system, so it is always correct (unlike `locate`, which uses a database), but it can be slow on big trees. Errors like \"Permission denied\" go to stderr: add `2>/dev/null`.\n\n## Syntax\n\n```text\nfind [-H] [-L] [-P] [-D DEBUG] [-O LEVEL] [STARTING-POINT...] [EXPRESSION]\n```\n\n## Options\n\n\n### Before the starting point\n\n| Option | What it does |\n|---|---|\n| `-P` | Never follow symbolic links (the default). |\n| `-L` | Follow symbolic links. |\n| `-H` | Follow links only for the starting points. |\n| `-D <opts>` | Print debug information (help, tree, search, stat, rates, opt, exec). |\n| `-O<level>` | Optimize the order of tests (0 to 3). |\n\n### Global options (put them first in the expression)\n\n| Option | What it does |\n|---|---|\n| `-maxdepth <n>` | Go at most N levels deep (1 = only the starting folder). |\n| `-mindepth <n>` | Do not test the first N levels. |\n| `-xdev, -mount` | Stay on the file system where you started. |\n| `-depth, -d` | Handle the content of a folder before the folder itself. |\n| `-daystart` | Measure times from the start of today, not from now. |\n| `-files0-from <file>` | Read the starting points from a file (NUL separated). |\n| `-ignore_readdir_race, -noignore_readdir_race` | Ignore (or report) files that disappear during the search. |\n| `-noleaf` | Do not optimize for Unix file systems (for CD-ROMs, some network file systems). |\n| `-regextype <type>` | Regex style for -regex: emacs, posix-extended, posix-basic … |\n| `-warn, -nowarn` | Turn warnings on or off. |\n| `-follow` | Old form of -L. |\n| `-help, --help` | Show help. |\n| `-version, --version` | Show the version. |\n\n### Tests: name and path\n\n| Option | What it does |\n|---|---|\n| `-name <pattern>` | The file name matches the pattern (wildcards, quote them). |\n| `-iname <pattern>` | Like -name, ignore upper/lower case. |\n| `-path <pattern>, -wholename <pattern>` | The whole path matches the pattern. |\n| `-ipath <pattern>, -iwholename <pattern>` | Like -path, ignore case. |\n| `-regex <pattern>` | The whole path matches the regular expression. |\n| `-iregex <pattern>` | Like -regex, ignore case. |\n| `-lname <pattern>, -ilname <pattern>` | The file is a link whose target matches the pattern. |\n\n### Tests: type, owner, permissions\n\n| Option | What it does |\n|---|---|\n| `-type <c>` | The type: f file, d folder, l link, b block, c character, p pipe, s socket. |\n| `-xtype <c>` | The type of the link target (`-xtype l` = broken link). |\n| `-user <name>` | Owned by this user. |\n| `-group <name>` | Owned by this group. |\n| `-uid <n>, -gid <n>` | Owned by this user/group number. |\n| `-nouser, -nogroup` | The owner/group number has no name (deleted users). |\n| `-perm <mode>` | Exactly these permissions (like 644). |\n| `-perm -<mode>` | At least all these permission bits (like -4000 for SUID). |\n| `-perm /<mode>` | Any of these permission bits. |\n| `-readable, -writable, -executable` | You can read / write / run the file. |\n| `-context <pattern>` | The SELinux context matches the pattern. |\n| `-fstype <type>` | The file is on this file system type (xfs, nfs …). |\n\n### Tests: size and content\n\n| Option | What it does |\n|---|---|\n| `-size <n>[ckMG]` | The size: c bytes, k KiB, M MiB, G GiB (+ bigger, - smaller). |\n| `-empty` | Empty file or empty folder. |\n| `-links <n>` | Has N hard links. |\n| `-inum <n>` | Has this inode number. |\n| `-samefile <name>` | Is a hard link to the same inode as NAME. |\n\n### Tests: time\n\n| Option | What it does |\n|---|---|\n| `-mtime <n>` | Data changed N days ago (+ more, - less). |\n| `-mmin <n>` | Data changed N minutes ago. |\n| `-atime <n>, -amin <n>` | Last read N days / minutes ago. |\n| `-ctime <n>, -cmin <n>` | Status (owner, permissions) changed N days / minutes ago. |\n| `-newer <file>` | Changed more recently than FILE. |\n| `-anewer <file>, -cnewer <file>` | Accessed / status-changed more recently than FILE's modification. |\n| `-newerXY <ref>` | Compare time X of the file with time Y of the reference (a = access, c = change, m = modify, B = birth, t = a date text). |\n| `-used <n>` | Accessed N days after its status changed. |\n\n### Operators\n\n| Option | What it does |\n|---|---|\n| `\\( expr \\)` | Group tests. |\n| `! expr, -not expr` | NOT. |\n| `expr1 expr2, expr1 -a expr2, -and` | AND (the default). |\n| `expr1 -o expr2, -or` | OR. |\n| `expr1 , expr2` | Run both, use the result of the second. |\n| `-true, -false` | Always true / always false. |\n\n### Actions\n\n| Option | What it does |\n|---|---|\n| `-print` | Print the path (the default action). |\n| `-print0` | Print the path with a NUL byte at the end (safe with `xargs -0`). |\n| `-printf <format>` | Print with a format: %p path, %s size, %u owner, %m mode, %t time … |\n| `-ls` | Print details like `ls -dils`. |\n| `-delete` | Delete the matching files (implies -depth). Test with -print first! |\n| `-exec <cmd> {} \\;` | Run CMD for each file (`{}` = the file). |\n| `-exec <cmd> {} +` | Run CMD once with many files (faster). |\n| `-execdir <cmd> {} \\;` | Like -exec, but run in the file's folder (safer). |\n| `-ok <cmd> {} \\;, -okdir <cmd> {} \\;` | Like -exec / -execdir, but ask first. |\n| `-prune` | Do not go into this folder. |\n| `-quit` | Stop at the first match. |\n| `-fprint <file>, -fprint0 <file>, -fprintf <file> <format>, -fls <file>` | Write the output to a file. |\n\n## Examples\n\n### Find files by name\n\n```bash\nfind /etc -name \"*.conf\"\n```\n\n> Quote the pattern, so the shell does not expand it first.\n\n### Find all files of a user (exam classic)\n\n```bash\nfind / -user student -type f 2>/dev/null\n```\n\n> `2>/dev/null` hides the \"Permission denied\" lines.\n\n### Copy all files of a user to a folder\n\n```bash\nmkdir -p /root/found\nfind / -user student -type f -exec cp -a --parents {} /root/found/ \\; 2>/dev/null\n```\n\n> `--parents` keeps the paths, so files with the same name do not overwrite each other.\n\n### Find big files that fill a disk\n\n```bash\nfind / -xdev -type f -size +500M -exec ls -lh {} \\; 2>/dev/null\n```\n\n> `-xdev` stays on one file system, for example only `/`.\n\n### Find files changed in the last 24 hours\n\n```bash\nfind /etc -mtime -1 -type f\n```\n\n> After an incident or an update: what changed?\n\n### Find SUID and SGID programs (security audit)\n\n```bash\nfind / -xdev -type f \\( -perm -4000 -o -perm -2000 \\) -ls 2>/dev/null\n```\n\n> Compare the list with a known-good list. A new SUID file can be a backdoor.\n\n### Find world-writable files\n\n```bash\nfind / -xdev -type f -perm -0002 ! -path \"/proc/*\" 2>/dev/null\n```\n\n> Files everyone can change are a risk, except on purpose in /tmp.\n\n### Find files without an owner\n\n```bash\nfind / -xdev \\( -nouser -o -nogroup \\) 2>/dev/null\n```\n\n> Left over from deleted users. Give them an owner or remove them.\n\n### Delete old logs safely\n\n```bash\nfind /var/log/myapp -name \"*.log\" -mtime +30 -print\nfind /var/log/myapp -name \"*.log\" -mtime +30 -delete\n```\n\n> Run with -print first, look at the list, then use -delete.\n\n### Run a command on many files at once (fast)\n\n```bash\nfind . -name \"*.sh\" -exec chmod 750 {} +\n```\n\n> `+` passes many names in one call.\n\n### Find text inside files\n\n```bash\nfind /etc -type f -name \"*.conf\" -exec grep -l \"PermitRootLogin\" {} + 2>/dev/null\n```\n\n> `grep -l` prints only the names of files that contain the text.\n\n### Handle names with spaces safely\n\n```bash\nfind /srv/uploads -type f -name \"*.tmp\" -print0 | xargs -0 rm -f\n```\n\n> NUL separators work with any file name.\n\n### Search only the top level of a folder\n\n```bash\nfind /var/www -maxdepth 1 -type d\n```\n\n> Lists the sites, not every folder inside them.\n\n### Skip a folder\n\n```bash\nfind /srv -path /srv/backup -prune -o -name \"*.php\" -print\n```\n\n> Everything except what is under /srv/backup.\n\n### Find files with a wrong SELinux label\n\n```bash\nfind /var/www/html ! -context \"*httpd_sys_content_t*\" -ls\n```\n\n> Files that Apache cannot read because of their label.\n\n### Find broken symbolic links\n\n```bash\nfind /etc -xtype l\n```\n\n> Links whose target was removed.\n\n### Print a custom report\n\n```bash\nfind /home -type f -size +100M -printf \"%s\\t%u\\t%p\\n\" | sort -n\n```\n\n> Size, owner and path; sorted by size.\n\n## Watch out\n\n- Order matters: `-delete` before a test deletes everything. Put tests first, the action last.\n- `-delete` and `-exec rm` are permanent. Always run the same command with `-print` first.\n- Quote patterns (`\"*.log\"`), or the shell expands them in the current folder first.\n- `-mtime +1` means \"more than 1 full day ago\", which is at least 48 hours.\n\n## Tips\n\n- find WHERE WHAT DO: `find /var -name \"*.log\" -size +100M -ls`.\n- `-exec … {} \\;` = one call per file, `-exec … {} +` = few calls (faster).\n\nFull manual: [man find](https://linux.die.net/man/1/find)\n","page":"commands/find/","label":"find","where":"Command page · Files and folders","aka":[]},{"id":"cmd-findmnt","n":1413,"slug":"findmnt","title":"findmnt — Show mounted file systems as a tree, find one mount, or check /etc/fstab for errors","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["mounts","fstab","verify","mount options","tree"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show mounted file systems as a tree, find one mount, or check /etc/fstab for errors.\n\n## About\n\n`findmnt` lists all mounts with source, type and options — clearer than `mount`. With a folder or device it shows only that mount. `findmnt --verify` checks every line of `/etc/fstab` (devices, mount points, types) — use it after editing fstab, before a reboot.\n\n## Syntax\n\n```text\nfindmnt [OPTION]... [DEVICE|MOUNTPOINT]\nfindmnt [OPTION]... [--source] DEVICE [--target PATH]\n```\n\n## Options\n\n\n### Select\n\n| Option | What it does |\n|---|---|\n| `-s, --fstab` | Look in /etc/fstab instead of the kernel list. |\n| `-m, --mtab` | Look in /etc/mtab. |\n| `-k, --kernel` | Look in the kernel mount table (default). |\n| `-t, --types LIST` | Only these types (like xfs,ext4; noxfs = not). |\n| `-O, --options LIST` | Only mounts with these options. |\n| `-S, --source SPEC` | Search by source (device, UUID=, LABEL=). |\n| `-T, --target PATH` | Search by mount point (or the mount that contains PATH). |\n| `-M, --mountpoint PATH` | Same as --target. |\n| `-N, --task TID` | Use the mount namespace of this task. |\n| `-R, --submounts` | Also show the sub-mounts. |\n| `-f, --first-only` | Only the first match. |\n| `--real` | Only real file systems. |\n| `--pseudo` | Only pseudo file systems. |\n| `--shadow, --shadowed` | Only mounts hidden by another mount. |\n| `--deleted` | Only mounts of deleted files. |\n| `-d, --direction forward\\|backward` | Search direction. |\n| `-F, --tab-file PATH` | Use another file. |\n\n### Check\n\n| Option | What it does |\n|---|---|\n| `-x, --verify` | Check the fstab file for errors. |\n| `--verbose` | More detail with --verify. |\n| `-p, --poll[=LIST]` | Watch for mount changes (mount, umount, remount, move). |\n| `-w, --timeout MS` | Stop polling after this time. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-D, --df` | Output like df (size, used, avail). |\n| `-o, --output LIST` | Choose columns (SOURCE,TARGET,FSTYPE,OPTIONS,UUID …). |\n| `--output-all` | All columns. |\n| `-n, --noheadings` | No header line. |\n| `-l, --list` | List instead of a tree. |\n| `--tree` | Tree output (default). |\n| `-r, --raw` | Raw output. |\n| `-P, --pairs` | KEY=\"value\" output. |\n| `-J, --json` | JSON output. |\n| `-y, --shell` | Column names usable as shell variables. |\n| `-e, --evaluate` | Turn LABEL=/UUID= into device names. |\n| `-c, --canonicalize` | Canonical paths. |\n| `-C, --nocanonicalize` | Do not resolve paths. |\n| `-v, --nofsroot` | Do not show [/dir] for bind mounts. |\n| `-U, --uniq` | Ignore duplicates. |\n| `-u, --notruncate` | Do not cut text. |\n| `-a, --ascii` | ASCII tree lines. |\n| `-b, --bytes` | Sizes in bytes. |\n| `-i, --invert` | Invert the -t and -O match. |\n| `--vfs-all` | Show all VFS options. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### All mounts as a tree\n\n```bash\nfindmnt\n```\n\n> Source, type and options for every mount.\n\n### One mount point\n\n```bash\nfindmnt /data\n```\n```text\nTARGET SOURCE    FSTYPE OPTIONS\n/data  /dev/vdb1 xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota\n```\n\n> Exit code 1 when nothing is mounted there.\n\n### Check /etc/fstab before reboot\n\n```bash\nfindmnt --verify\n```\n```text\nSuccess, no errors or warnings detected\n```\n\n> Catches wrong UUIDs, missing folders and typos.\n\n### Only real disk file systems\n\n```bash\nfindmnt -t xfs,ext4,vfat\n```\n\n> Without proc, sys, cgroup …\n\n### Like df\n\n```bash\nfindmnt -D\n```\n\n> Size and use per mount.\n\n### Which file system holds a file?\n\n```bash\nfindmnt -T /var/log/messages\n```\n\n> Shows the mount that contains this path.\n\n### What is in fstab?\n\n```bash\nfindmnt -s\n```\n\n> The fstab lines as a table.\n\n### Find mounts without noexec (security)\n\n```bash\nfindmnt -O noexec -i -n -o TARGET -t xfs,ext4,tmpfs\n```\n\n> Folders where users could run programs.\n\n### Get the device of a mount for scripts\n\n```bash\nfindmnt -n -o SOURCE /\n```\n```text\n/dev/mapper/rhel-root\n```\n\n> Clean value.\n\n### Watch mount changes live\n\n```bash\nfindmnt -p\n```\n\n> Shows mounts and unmounts as they happen (Ctrl+C to stop).\n\n### JSON for monitoring\n\n```bash\nfindmnt -J -o TARGET,FSTYPE,SIZE,USE% -t xfs\n```\n\n> Feed a dashboard.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Found (or verify OK). |\n| `1` | Nothing found (or verify found errors). |\n\nFull manual: [man findmnt](https://linux.die.net/man/8/findmnt)\n","page":"commands/findmnt/","label":"findmnt","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-firewall-cmd","n":1414,"slug":"firewall-cmd","title":"firewall-cmd — Configure firewalld: zones, services, ports, sources, rich rules, forwarding — at run time or permanently","sections":["commands"],"topic":"Commands · Firewall","tags":["firewall","firewalld","zones","services","ports","rich rules","nftables"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Configure firewalld: zones, services, ports, sources, rich rules, forwarding — at run time or permanently.\n\n## About\n\n`firewall-cmd` talks to the firewalld service. Changes without `--permanent` work at once but are lost at reload or reboot; with `--permanent` they are saved but only active after `--reload`. Most options follow one pattern: `--add-X`, `--remove-X`, `--query-X`, `--list-X`, optionally with `--zone=ZONE` (default: the default zone).\n\n## Syntax\n\n```text\nfirewall-cmd [--permanent] [--zone=ZONE] OPTION...\nfirewall-cmd --reload | --state | --list-all\n```\n\n## Options\n\n\n### General\n\n| Option | What it does |\n|---|---|\n| `--state` | Is firewalld running? |\n| `--reload` | Load the permanent config (runtime-only changes are lost). |\n| `--complete-reload` | Full reload (drops connections). |\n| `--permanent` | Change the saved config (needs --reload to apply). |\n| `--runtime-to-permanent` | Save the current runtime config. |\n| `--check-config` | Check the saved config for errors. |\n| `--reset-to-defaults` | Back to the default config. |\n| `--get-log-denied, --set-log-denied=all\\|unicast\\|broadcast\\|multicast\\|off` | Log rejected packets. |\n| `--timeout=TIME` | With --add-…: remove the rule again after TIME (like 10m). |\n| `-q, --quiet` | No messages. |\n| `-h, --help, -V, --version` | Help / version. |\n\n### Zones\n\n| Option | What it does |\n|---|---|\n| `--get-default-zone` | Show the default zone. |\n| `--set-default-zone=ZONE` | Change the default zone. |\n| `--get-active-zones` | Zones that have interfaces or sources. |\n| `--get-zones` | All zones. |\n| `--get-zone-of-interface=IF, --get-zone-of-source=SOURCE` | Find the zone. |\n| `--list-all [--zone=ZONE]` | Everything in a zone. |\n| `--list-all-zones` | Everything in all zones. |\n| `--info-zone=ZONE` | Details of a zone. |\n| `--zone=ZONE` | Work on this zone. |\n| `--new-zone=ZONE, --delete-zone=ZONE, --load-zone-defaults=ZONE, --path-zone=ZONE` | Create, delete, reset zones (with --permanent). |\n| `--get-target, --set-target=default\\|ACCEPT\\|DROP\\|REJECT` | Default action of a zone (permanent). |\n| `--set-description, --set-short, --get-priority, --set-priority …` | Zone details (permanent). |\n\n### Zone content\n\n| Option | What it does |\n|---|---|\n| `--add-service=SERVICE, --remove-service=SERVICE, --query-service=SERVICE, --list-services` | Services in the zone. |\n| `--add-port=PORT[-PORT]/PROTO, --remove-port=…, --query-port=…, --list-ports` | Ports, like 8080/tcp. |\n| `--add-source=ADDR[/MASK]\\|ipset:NAME, --remove-source=…, --query-source=…, --list-sources` | Bind source networks to the zone. |\n| `--add-interface=IF, --remove-interface=IF, --change-interface=IF, --query-interface=IF, --list-interfaces` | Interfaces of the zone. |\n| `--add-rich-rule='RULE', --remove-rich-rule='RULE', --query-rich-rule='RULE', --list-rich-rules` | Detailed rules. |\n| `--add-forward-port=port=P:proto=tcp[:toport=P][:toaddr=A], --remove-forward-port=…, --list-forward-ports` | Port forwarding. |\n| `--add-masquerade, --remove-masquerade, --query-masquerade` | NAT for traffic leaving this zone. |\n| `--add-protocol=PROTO, --remove-protocol=…, --list-protocols` | Allow a protocol (like gre). |\n| `--add-source-port=PORT/PROTO, --remove-source-port=…, --list-source-ports` | Allow by source port. |\n| `--add-icmp-block=TYPE, --remove-icmp-block=…, --list-icmp-blocks, --add-icmp-block-inversion` | Block ICMP types. |\n| `--add-forward, --remove-forward, --query-forward` | Intra-zone forwarding. |\n\n### Services, ipsets, helpers, icmptypes\n\n| Option | What it does |\n|---|---|\n| `--get-services` | All known services. |\n| `--info-service=SERVICE` | Ports and details of a service. |\n| `--new-service=NAME, --delete-service=NAME, --service=NAME --add-port=…` | Define your own service (permanent). |\n| `--get-ipsets, --new-ipset=NAME --type=TYPE, --ipset=NAME --add-entry=ADDR, --info-ipset=NAME` | IP sets (lists of addresses). |\n| `--get-helpers, --get-icmptypes, --info-helper=…, --info-icmptype=…` | Helpers and ICMP types. |\n| `--new-policy=NAME, --policy=NAME …, --get-policies` | Policies between zones (newer firewalld). |\n\n### Emergency and lockdown\n\n| Option | What it does |\n|---|---|\n| `--panic-on, --panic-off, --query-panic` | Drop all traffic (also your SSH!). |\n| `--lockdown-on, --lockdown-off, --query-lockdown, --add-lockdown-whitelist-…` | Only allowed programs may change firewalld. |\n| `--direct --add-rule …` | Old direct rules (deprecated). |\n\n## Examples\n\n### What is open?\n\n```bash\nfirewall-cmd --list-all\n```\n```text\npublic (active)\n  interfaces: enp1s0\n  services: cockpit dhcpv6-client ssh\n  ports:\n  rich rules:\n```\n\n> The default zone on RHEL.\n\n### Open a service permanently\n\n```bash\nfirewall-cmd --add-service=http --permanent\nfirewall-cmd --reload\n```\n\n> Survives reboots.\n\n### Open a port\n\n```bash\nfirewall-cmd --add-port=82/tcp --permanent; firewall-cmd --reload\n```\n\n> For non-standard ports (also fix SELinux!).\n\n### Close a service\n\n```bash\nfirewall-cmd --remove-service=cockpit --permanent; firewall-cmd --reload\n```\n\n> Smaller attack surface.\n\n### Test a rule for 5 minutes\n\n```bash\nfirewall-cmd --add-port=9090/tcp --timeout=5m\n```\n\n> Removed automatically.\n\n### Trust an admin network\n\n```bash\nfirewall-cmd --zone=trusted --add-source=192.168.122.0/24 --permanent; firewall-cmd --reload\n```\n\n> Everything from that network is allowed.\n\n### Service only for one network\n\n```bash\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"10.0.0.0/24\" service name=\"ssh\" accept'\n```\n\n> Combine with removing ssh from the zone.\n\n### Block one address\n\n```bash\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"203.0.113.50\" drop'; firewall-cmd --reload\n```\n\n> An attacker from the logs.\n\n### Port forwarding\n\n```bash\nfirewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080; firewall-cmd --reload\n```\n\n> Port 80 goes to 8080 on the same host.\n\n### Save runtime changes\n\n```bash\nfirewall-cmd --runtime-to-permanent\n```\n\n> When you tested without --permanent.\n\n### Which ports does a service open?\n\n```bash\nfirewall-cmd --info-service=samba\n```\n\n> ports: 139/tcp 445/tcp …\n\n### Log denied packets (security)\n\n```bash\nfirewall-cmd --set-log-denied=all\njournalctl -k -f | grep REJECT\n```\n\n> See what is blocked.\n\n## Watch out\n\n- --permanent changes are not active until --reload; runtime changes are lost at reload.\n- --panic-on cuts your SSH session.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/firewalld/` | Your saved config (zones, services). |\n| `/usr/lib/firewalld/` | Default zones and services. |\n| `/etc/firewalld/firewalld.conf` | DefaultZone, backend, logging. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (query: yes). |\n| `1` | Query: no. |\n| `2` | Wrong use. |\n| `252` | firewalld is not running. |\n\nFull manual: [man firewall-cmd](https://linux.die.net/man/1/firewall-cmd)\n","page":"commands/firewall-cmd/","label":"firewall-cmd","where":"Command page · Firewall","aka":[]},{"id":"cmd-flatpak","n":1415,"slug":"flatpak","title":"flatpak — Install, update and run (mostly graphical) applications in a sandbox, from Flatpak remotes like Flathub","sections":["commands"],"topic":"Commands · Software packages","tags":["flatpak","apps","sandbox","desktop","flathub","remote","runtime"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Install, update and run (mostly graphical) applications in a sandbox, from Flatpak remotes like Flathub.\n\n## About\n\nFlatpak apps come with their own runtime, so they do not depend on the RPM packages of the system, and they run in a sandbox with limited access. That is good for desktop apps, not for server software.\n\nApps come from **remotes** (Flatpak repositories). Installs are **system-wide** (as root, `--system`, default) or **per user** (`--user`). App IDs look like `org.gimp.GIMP`.\n\n## Syntax\n\n```text\nflatpak [OPTION]... COMMAND [ARGUMENT]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `remote-add NAME URL` | Add a remote (`--if-not-exists`). |\n| `remote-delete NAME` | Remove a remote. |\n| `remote-modify NAME` | Change a remote (enable, disable, priority). |\n| `remotes` | List the remotes. |\n| `remote-ls [NAME]` | List what a remote offers (`--app`, `--updates`). |\n| `remote-info NAME REF` | Details of a remote app. |\n| `search TEXT` | Search apps in the remotes. |\n| `install [REMOTE] REF` | Install an app or runtime. |\n| `update [REF]` | Update apps and runtimes. |\n| `uninstall REF` | Remove an app (`--unused` = old runtimes). |\n| `list` | Installed apps and runtimes (`--app`, `--runtime`). |\n| `info REF` | Details of an installed app. |\n| `run APP` | Start an app. |\n| `ps` | Running sandboxes. |\n| `kill INSTANCE` | Stop a running app. |\n| `override APP` | Change the sandbox permissions of an app. |\n| `permissions` | Show stored permissions. |\n| `history` | Show past changes. |\n| `repair` | Fix a broken installation. |\n| `mask, pin` | Stop updates of a ref / keep a runtime. |\n| `config` | Show or change settings (like languages). |\n| `make-current, enter, document-*, build*` | Other and developer commands. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--user` | Work on the per-user installation. |\n| `--system` | Work on the system installation (default). |\n| `--installation NAME` | Work on another installation. |\n| `-y, --assumeyes` | Answer yes. |\n| `--noninteractive` | No questions, little output (scripts). |\n| `--if-not-exists` | With remote-add: no error when it exists. |\n| `--app` | With list/remote-ls/update: only apps. |\n| `--runtime` | Only runtimes. |\n| `--columns=LIST` | Choose output columns (like application,version). |\n| `--from FILE` | Install from a .flatpakref file. |\n| `--reinstall` | Reinstall if installed. |\n| `--or-update` | With install: update if installed. |\n| `--no-deps` | Do not install dependencies. |\n| `--unused` | With uninstall: remove runtimes nothing uses. |\n| `--delete-data` | With uninstall: also remove the app data. |\n| `-v, --verbose` | More output. |\n| `--ostree-verbose` | Debug output of OSTree. |\n| `--version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Install Flatpak\n\n```bash\ndnf install -y flatpak\n```\n\n> On a desktop system it is often installed.\n\n### Add the Flathub remote\n\n```bash\nflatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo\n```\n\n> System-wide remote.\n\n### Add the Red Hat remote\n\n```bash\nflatpak remote-add --if-not-exists rhel https://flatpaks.redhat.io/rhel.flatpakrepo\n```\n\n> Apps built by Red Hat (needs a registered system / Red Hat login).\n\n### List remotes and their apps\n\n```bash\nflatpak remotes\nflatpak remote-ls flathub --app | head\n```\n\n> What can be installed.\n\n### Search and install an app\n\n```bash\nflatpak search gimp\nflatpak install -y flathub org.gimp.GIMP\n```\n\n> Runtimes come too.\n\n### Install only for one user\n\n```bash\nflatpak install --user -y flathub org.mozilla.firefox\n```\n\n> No root needed; only this user has it.\n\n### Run, list and update\n\n```bash\nflatpak run org.gimp.GIMP\nflatpak list --app\nflatpak update -y\n```\n\n> Updates all apps and runtimes.\n\n### Remove an app and old runtimes\n\n```bash\nflatpak uninstall -y org.gimp.GIMP\nflatpak uninstall --unused -y\n```\n\n> Frees disk space.\n\n### Check the sandbox permissions (security)\n\n```bash\nflatpak info --show-permissions org.mozilla.firefox\n```\n\n> Shows file system, network and device access.\n\n### Take away home folder access\n\n```bash\nflatpak override --nofilesystem=home org.mozilla.firefox\n```\n\n> Tighter sandbox.\n\n### See running apps\n\n```bash\nflatpak ps\n```\n\n> Instance, PID and app ID.\n\n## Watch out\n\n- Flatpak is for desktop apps. Install server software with dnf.\n- A user installation is not visible for other users.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/lib/flatpak/` | System installation. |\n| `~/.local/share/flatpak/` | User installation. |\n| `~/.var/app/` | App data of the user. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man flatpak](https://linux.die.net/man/1/flatpak)\n","page":"commands/flatpak/","label":"flatpak","where":"Command page · Software packages","aka":[]},{"id":"cmd-free","n":1416,"slug":"free","title":"free — Show the use of memory (RAM) and swap","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["memory","ram","swap","cache","available"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the use of memory (RAM) and swap.\n\n## About\n\n`free -h` prints total, used and free memory, the shared memory, the buffers and cache, and **available**: the memory programs can still get (free + cache the kernel can give back). Look at \"available\", not \"free\" — Linux uses spare RAM as cache on purpose.\n\n## Syntax\n\n```text\nfree [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h, --human` | Readable units (Mi, Gi). |\n| `-b, --bytes` | In bytes. |\n| `-k, --kibi` | In KiB (default). |\n| `-m, --mebi` | In MiB. |\n| `-g, --gibi` | In GiB. |\n| `--tebi, --pebi` | In TiB or PiB. |\n| `--kilo, --mega, --giga, --tera, --peta` | Powers of 1000 (implies --si). |\n| `--si` | Use 1000 instead of 1024. |\n| `-w, --wide` | Buffers and cache in their own columns. |\n| `-t, --total` | Add a total line (RAM + swap). |\n| `-l, --lohi` | Low and high memory details. |\n| `-L, --line` | Everything on one line. |\n| `-v, --committed` | Show committed memory. |\n| `-s, --seconds DELAY` | Repeat every DELAY seconds. |\n| `-c, --count COUNT` | Repeat COUNT times. |\n| `--help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Memory in readable units\n\n```bash\nfree -h\n```\n```text\n               total        used        free      shared  buff/cache   available\nMem:           3.6Gi       812Mi       2.1Gi        12Mi       891Mi       2.6Gi\nSwap:          4.0Gi          0B       4.0Gi\n```\n\n> available = what new programs can still use.\n\n### Wide view\n\n```bash\nfree -hw\n```\n\n> Buffers and cache apart.\n\n### Watch every 2 seconds\n\n```bash\nfree -h -s 2\n```\n\n> Ctrl+C to stop.\n\n### With a total line\n\n```bash\nfree -ht\n```\n\n> RAM + swap together.\n\n### Value for a script\n\n```bash\nfree -m | awk '/^Mem:/ {print $7}'\n```\n```text\n2662\n```\n\n> Available MiB.\n\n### One line\n\n```bash\nfree -hL\n```\n\n> Short for logs.\n\n### Is swap used a lot?\n\n```bash\nfree -h | grep Swap; vmstat 1 5\n```\n\n> High si/so in vmstat = the system swaps actively (too little RAM).\n\n### Drop the cache for a test (not in production)\n\n```bash\nsync; echo 3 > /proc/sys/vm/drop_caches; free -h\n```\n\n> Shows that cache is given back.\n\n### Who uses the memory?\n\n```bash\nps -eo pid,rss,cmd --sort=-rss | head\n```\n\n> free shows totals; ps shows the processes.\n\n### Check OOM killer events (security/stability)\n\n```bash\njournalctl -k | grep -i 'out of memory'\n```\n\n> The kernel killed a process because memory was full.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/meminfo` | The source of the numbers. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man free](https://linux.die.net/man/1/free)\n","page":"commands/free/","label":"free","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-fsck","n":1417,"slug":"fsck","title":"fsck — Check and repair file systems — the front end `fsck`, and `e2fsck` (= `fsck.ext4`) for ext2/3/4","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["file system check","repair","ext4","boot errors"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Check and repair file systems — the front end `fsck`, and `e2fsck` (= `fsck.ext4`) for ext2/3/4.\n\nSame page for: `e2fsck`, `fsck.ext4`\n\n## About\n\n`fsck DEVICE` calls the checker for the file system type (for ext4: e2fsck). Run it on **unmounted** file systems only. At boot, systemd runs fsck for ext4 lines with a non-zero 6th field in fstab. For XFS use `xfs_repair` (fsck.xfs does nothing).\n\n## Syntax\n\n```text\nfsck [-lsAVRTMNP] [-C [FD]] [-t TYPE] [FS-OPTIONS] DEVICE...\ne2fsck [-pacnyrdfkvtDFV] [-b SUPERBLOCK] [-B BLOCKSIZE] DEVICE\n```\n\n## Options\n\n\n### fsck (front end)\n\n| Option | What it does |\n|---|---|\n| `-A` | Check all file systems in /etc/fstab. |\n| `-R` | With -A: skip the root file system. |\n| `-t TYPES` | Only these types. |\n| `-N` | Show what would be done. |\n| `-C [FD]` | Show progress. |\n| `-M` | Skip mounted file systems. |\n| `-P` | With -A: check root in parallel with others. |\n| `-s` | Serial, one at a time. |\n| `-T` | No title. |\n| `-V` | Verbose. |\n| `-l` | Lock the whole disk. |\n| `-r [FD]` | Report statistics. |\n\n### e2fsck / fsck.ext4\n\n| Option | What it does |\n|---|---|\n| `-f` | Force a check even when the file system looks clean. |\n| `-n` | Read-only: answer no to all (check only). |\n| `-y` | Answer yes to all repairs. |\n| `-p, -a` | Repair safe problems automatically (preen). |\n| `-b SUPERBLOCK` | Use a backup superblock (from dumpe2fs or mkfs -n). |\n| `-B BLOCKSIZE` | Block size for -b. |\n| `-c` | Check for bad blocks (twice = read-write test). |\n| `-l FILE, -L FILE` | Add / set the bad block list. |\n| `-k` | Keep existing bad blocks with -c. |\n| `-D` | Optimize directories. |\n| `-E OPTIONS` | Extended options. |\n| `-j JOURNAL` | External journal. |\n| `-C FD` | Progress to a file descriptor (0 = screen). |\n| `-F` | Flush buffers first. |\n| `-t` | Timing statistics. |\n| `-v` | Verbose. |\n| `-d` | Debug. |\n| `-z FILE` | Undo file. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Check an ext4 file system\n\n```bash\numount /backup\nfsck.ext4 -f /dev/vdb3\n```\n\n> -f forces a full check.\n\n### Check only, change nothing\n\n```bash\ne2fsck -fn /dev/vdb3\n```\n\n> See the problems first.\n\n### Repair everything without questions\n\n```bash\ne2fsck -fy /dev/vdb3\n```\n\n> For many errors; check backups first.\n\n### Use a backup superblock\n\n```bash\ndumpe2fs /dev/vdb3 | grep -i 'backup superblock' | head -2\ne2fsck -b 32768 /dev/vdb3\n```\n\n> When the main superblock is broken.\n\n### With progress\n\n```bash\ne2fsck -f -C 0 /dev/vgdata/lvweb\n```\n\n> Progress bar for big disks.\n\n### Dry run of fsck for fstab\n\n```bash\nfsck -A -N\n```\n\n> Shows the checks it would run.\n\n### Before shrinking (required)\n\n```bash\ne2fsck -f /dev/vgfiles/lvdocs; resize2fs /dev/vgfiles/lvdocs 3G\n```\n\n> resize2fs asks for a fresh check.\n\n### XFS? Use xfs_repair\n\n```bash\nfsck.xfs /dev/vdb1\n```\n\n> Does nothing; xfs_repair is the tool.\n\n### Force a check at the next boot\n\n```bash\ntune2fs -C 100 -c 20 /dev/vdb3\n```\n\n> Mount count above the max = check at boot (for fstab entries with pass 2).\n\n### Find bad blocks on an old disk\n\n```bash\ne2fsck -fc /dev/vdb3\n```\n\n> Read test with badblocks.\n\n## Watch out\n\n- Running fsck on a mounted file system can destroy it.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | No errors. |\n| `1` | Errors were corrected. |\n| `2` | Corrected; reboot needed. |\n| `4` | Errors left uncorrected. |\n| `8` | Operational error. |\n| `16` | Usage error. |\n| `32` | Cancelled by the user. |\n| `128` | Shared library error. |\n\nFull manual: [man fsck](https://linux.die.net/man/8/fsck)\n","page":"commands/fsck/","label":"fsck","where":"Command page · Disks and file systems","aka":["e2fsck","fsck.ext4"]},{"id":"cmd-gdisk","n":1418,"slug":"gdisk","title":"gdisk — Edit GPT partition tables with a menu (like fdisk, made for GPT)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["gpt","partitions","convert mbr","guid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit GPT partition tables with a menu (like fdisk, made for GPT).\n\n## About\n\n`gdisk` (GPT fdisk) works like fdisk but only with GPT. It can convert MBR to GPT, repair GPT tables (the backup at the end of the disk) and uses 4-digit type codes (8300 Linux, 8200 swap, 8e00 LVM, ef00 EFI). The `sgdisk` command does the same without a menu. On RHEL it may need `dnf install gdisk`.\n\n## Syntax\n\n```text\ngdisk [-l] DEVICE\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `?` | Help. |\n| `p` | Print the table. |\n| `n` | New partition. |\n| `d` | Delete a partition. |\n| `t` | Change the type code. |\n| `l` | List the type codes. |\n| `c` | Change a partition name. |\n| `i` | Details of a partition. |\n| `o` | New empty GPT table. |\n| `r` | Recovery and transformation menu (rebuild, convert). |\n| `x` | Expert menu. |\n| `v` | Verify. |\n| `w` | Write and quit. |\n| `q` | Quit without saving. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l` | List the partition table and exit. |\n\n## Examples\n\n### Install gdisk\n\n```bash\ndnf install -y gdisk\n```\n\n> In the BaseOS repository.\n\n### List a GPT table\n\n```bash\ngdisk -l /dev/vdb\n```\n\n> Shows the type codes and names.\n\n### Create a Linux LVM partition\n\n```bash\ngdisk /dev/vdb\n```\n```text\nCommand (? for help): n\nHex code or GUID (L to show codes, Enter = 8300): 8e00\nCommand (? for help): w\n```\n\n> 8e00 = Linux LVM.\n\n### Swap partition type\n\n```bash\ngdisk /dev/vdb\n```\n```text\nHex code or GUID (L to show codes, Enter = 8300): 8200\n```\n\n> 8200 = Linux swap.\n\n### Name a partition\n\n```bash\ngdisk /dev/vdb\n```\n```text\nCommand (? for help): c\n```\n\n> GPT partitions can have names (PARTLABEL).\n\n### Verify a disk\n\n```bash\ngdisk /dev/vdb\n```\n```text\nCommand (? for help): v\n```\n\n> Finds problems in the table.\n\n### Repair a damaged main table from the backup\n\n```bash\ngdisk /dev/vdb\n```\n```text\nCommand (? for help): r\nRecovery/transformation command (? for help): b\n```\n\n> Uses the backup header at the end of the disk.\n\n### Convert MBR to GPT (keep data)\n\n```bash\ngdisk /dev/vdc\n```\n\n> gdisk offers the conversion when it finds MBR; write with w. Back up first.\n\n### Script with sgdisk\n\n```bash\nsgdisk -n 1:0:+2G -t 1:8300 -c 1:data /dev/vdb\n```\n\n> One partition of 2 GiB, Linux type, name data.\n\n### Back up the GPT table\n\n```bash\nsgdisk --backup=/root/vdb-gpt.bin /dev/vdb\n```\n\n> Restore with `sgdisk --load-backup=`.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man gdisk](https://linux.die.net/man/8/gdisk)\n","page":"commands/gdisk/","label":"gdisk","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-getenforce","n":1419,"slug":"getenforce","title":"getenforce — Show the SELinux mode (getenforce), or switch between enforcing and permissive until the next reboot (setenforce)","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","mode","enforcing","permissive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the SELinux mode (getenforce), or switch between enforcing and permissive until the next reboot (setenforce).\n\nSame page for: `setenforce`\n\n## About\n\n`getenforce` prints Enforcing, Permissive or Disabled. `setenforce 0` switches to permissive (log only), `setenforce 1` back to enforcing. It does not work when SELinux is disabled, and it does not change the boot mode — that is `SELINUX=` in `/etc/selinux/config`.\n\n## Syntax\n\n```text\ngetenforce\nsetenforce Enforcing|Permissive|1|0\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `1, Enforcing` | Enforce the policy. |\n| `0, Permissive` | Only log denials. |\n\n## Examples\n\n### Show the mode\n\n```bash\ngetenforce\n```\n```text\nEnforcing\n```\n\n> The normal state.\n\n### Permissive for a test\n\n```bash\nsetenforce 0\ngetenforce\n```\n```text\nPermissive\n```\n\n> If the problem is gone now, SELinux was the cause.\n\n### Back to enforcing\n\n```bash\nsetenforce 1\n```\n\n> Never forget this step.\n\n### Mode at boot\n\n```bash\ngrep ^SELINUX= /etc/selinux/config\n```\n```text\nSELINUX=enforcing\n```\n\n> Change here for the next boot.\n\n### Test a service quickly\n\n```bash\nsetenforce 0; systemctl restart httpd; curl -I localhost; setenforce 1\n```\n\n> Compare the result with enforcing.\n\n### Better: only one domain permissive\n\n```bash\nsemanage permissive -a httpd_t\n```\n\n> The rest of the system stays protected.\n\n### Disabled system\n\n```bash\nsetenforce 1\n```\n```text\nsetenforce: SELinux is disabled\n```\n\n> Needs a config change, relabel and reboot.\n\n### In a monitoring check (security)\n\n```bash\n[ \"$(getenforce)\" = Enforcing ] || echo \"ALERT: SELinux not enforcing on $(hostname)\"\n```\n\n> Attackers often switch SELinux off.\n\n### Check the kernel arguments\n\n```bash\ngrep -o 'selinux=[01]\\|enforcing=[01]' /proc/cmdline\n```\n\n> selinux=0 disables SELinux; enforcing=0 boots permissive.\n\n### Logged mode changes\n\n```bash\nausearch -m MAC_STATUS -ts today\n```\n\n> Who changed the mode and when.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (for example SELinux disabled). |\n\nFull manual: [man getenforce](https://linux.die.net/man/8/getenforce)\n","page":"commands/getenforce/","label":"getenforce","where":"Command page · SELinux and security","aka":["setenforce"]},{"id":"cmd-getent","n":1420,"slug":"getent","title":"getent — Look up entries in the system databases (users, groups, hosts, services …) the same way programs do","sections":["commands"],"topic":"Commands · Users and groups","tags":["user lookup","group lookup","hosts","nsswitch","ldap","sssd","dns"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Look up entries in the system databases (users, groups, hosts, services …) the same way programs do.\n\n## About\n\n`getent` asks the name service switch (`/etc/nsswitch.conf`). So it finds users from `/etc/passwd` **and** from LDAP, IdM or Active Directory (through SSSD), and host names from `/etc/hosts` **and** DNS — exactly as the system sees them. `cat /etc/passwd` only shows local users.\n\nCommon databases: passwd, group, shadow, gshadow, hosts, ahosts, ahostsv4, ahostsv6, services, protocols, networks, netgroup, initgroups, aliases, ethers, rpc.\n\n## Syntax\n\n```text\ngetent [OPTION]... DATABASE [KEY]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `passwd` | User accounts. |\n| `group` | Groups and members. |\n| `shadow` | Password data (root only). |\n| `hosts` | Host names (files and DNS, by nsswitch order). |\n| `ahosts` | Host names with getaddrinfo (all addresses, IPv4 and IPv6). |\n| `services` | Port numbers of services (/etc/services). |\n| `initgroups` | All groups of a user, also from LDAP/AD. |\n| `netgroup` | NIS netgroups. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --service SERVICE` | Use this source only (like files or sss). |\n| `-s, --service DATABASE:SERVICE` | Use this source only for one database. |\n| `-i, --no-idn` | Do not use IDN encoding for host lookups. |\n| `-?, --help` | Show help. |\n| `--usage` | Short usage. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Show a user\n\n```bash\ngetent passwd lisa\n```\n```text\nlisa:x:1001:1001:Lisa Hassan:/home/lisa:/bin/bash\n```\n\n> Works for local and directory (LDAP/AD) users.\n\n### Show a group and its members\n\n```bash\ngetent group wheel\n```\n```text\nwheel:x:10:student,devops1\n```\n\n> The last field lists the members.\n\n### Resolve a host name like programs do\n\n```bash\ngetent hosts server2\n```\n```text\n192.168.122.20  server2\n```\n\n> Uses /etc/hosts first, then DNS (nsswitch order). nslookup/dig ask DNS only.\n\n### All addresses of a name\n\n```bash\ngetent ahosts www.redhat.com\n```\n\n> IPv4 and IPv6, like a browser sees them.\n\n### Find the port of a service\n\n```bash\ngetent services ssh https\n```\n```text\nssh                   22/tcp\nhttps                 443/tcp\n```\n\n> Reads /etc/services.\n\n### Check a directory user from AD/IdM\n\n```bash\ngetent passwd anna@corp.example.com\n```\n\n> If it shows nothing, check SSSD (`systemctl status sssd`).\n\n### All groups of a user, also from AD\n\n```bash\ngetent initgroups anna\n```\n\n> Fast check when sudo rules use AD groups.\n\n### Look only in local files\n\n```bash\ngetent -s files passwd lisa\n```\n\n> Tells you if a user is local or from the directory.\n\n### List all normal users\n\n```bash\ngetent passwd | awk -F: '$3 >= 1000 && $3 < 60000 {print $1}'\n```\n\n> nobody (65534) is not in this range.\n\n### Read the password data (root)\n\n```bash\ngetent shadow lisa\n```\n\n> Like the line in /etc/shadow.\n\n### Test if a user exists in a script\n\n```bash\ngetent passwd lisa >/dev/null && echo exists || echo missing\n```\n\n> Exit code 2 = not found.\n\n### Show the order of the sources\n\n```bash\ngrep -E '^(passwd|group|hosts):' /etc/nsswitch.conf\n```\n```text\npasswd:     files sss systemd\ngroup:      files sss systemd\nhosts:      files dns myhostname\n```\n\n> RHEL manages this file with authselect.\n\n## Watch out\n\n- `getent passwd` without a key may not list all directory users (SSSD does not enumerate by default).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/nsswitch.conf` | Which sources each database uses, in which order. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Found. |\n| `1` | Wrong arguments or unknown database. |\n| `2` | One or more keys not found. |\n| `3` | The database cannot list all entries. |\n\nFull manual: [man getent](https://linux.die.net/man/1/getent)\n","page":"commands/getent/","label":"getent","where":"Command page · Users and groups","aka":[]},{"id":"cmd-getfacl","n":1421,"slug":"getfacl","title":"getfacl — Show the access control lists (ACLs) of files and directories","sections":["commands"],"topic":"Commands · Permissions","tags":["acl","access control list","permissions","default acl","backup"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the access control lists (ACLs) of files and directories.\n\n## About\n\n`getfacl` shows the owner, group and normal permissions plus the extra ACL entries: rights for named users and groups, the mask, and default entries that new files inherit. A `+` after the mode in `ls -l` tells you a file has an ACL.\n\nThe output format can be used to restore ACLs with `setfacl --restore`.\n\n## Syntax\n\n```text\ngetfacl [-aceEsRLPtpndvh] FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --access` | Show only the access ACL. |\n| `-d, --default` | Show only the default ACL. |\n| `-c, --omit-header` | Do not show the comment lines (file, owner, group). |\n| `-e, --all-effective` | Show the effective rights of all entries. |\n| `-E, --no-effective` | Do not show effective rights. |\n| `-s, --skip-base` | Skip files that have only the normal permissions. |\n| `-R, --recursive` | List the whole tree. |\n| `-L, --logical` | Follow symbolic links to directories. |\n| `-P, --physical` | Do not follow symbolic links. |\n| `-t, --tabular` | Show a table. |\n| `-p, --absolute-names` | Keep the leading / in names. |\n| `-n, --numeric` | Show UIDs and GIDs as numbers. |\n| `-v, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Show the ACL of a folder\n\n```bash\ngetfacl /srv/sales\n```\n```text\n# file: srv/sales\n# owner: root\n# group: sales\n# flags: -st\nuser::rwx\nuser:carol:r-x\ngroup::rwx\ngroup:account:r-x\nmask::rwx\nother::---\ndefault:user::rwx\ndefault:group::rwx\ndefault:group:account:r-x\ndefault:mask::rwx\ndefault:other::---\n```\n\n> flags -st = SGID and sticky; default lines are for new files.\n\n### See which files have an ACL\n\n```bash\nls -l /srv/sales\n```\n```text\ndrwxrws--T+ 2 root sales 22 Oct 25 11:00 /srv/sales\n```\n\n> The + means an ACL.\n\n### Only the extra entries\n\n```bash\ngetfacl -cs /srv/sales/*\n```\n\n> Skips files without ACL and the header.\n\n### Effective rights after the mask\n\n```bash\ngetfacl /srv/sales/new.txt\n```\n```text\ngroup:account:r-x\t\t#effective:r--\n```\n\n> The mask removes x: account can only read.\n\n### Back up all ACLs of a tree\n\n```bash\ngetfacl -R /srv > /root/srv-acl.txt\n```\n\n> Restore with `setfacl --restore=/root/srv-acl.txt`.\n\n### Only the default ACL\n\n```bash\ngetfacl -d /srv/sales\n```\n\n> What new files will get.\n\n### Copy an ACL to another file\n\n```bash\ngetfacl plan.txt | setfacl --set-file=- report.txt\n```\n\n> Same ACL on report.txt.\n\n### Numbers instead of names\n\n```bash\ngetfacl -n /srv/restore\n```\n\n> Helps when users from another server do not exist here.\n\n### Audit folders with extra access (security)\n\n```bash\ngetfacl -R -s -p /srv 2>/dev/null | grep -E '^(# file|user:|group:)'\n```\n\n> Lists who got extra rights beyond the normal mode.\n\n### Table view\n\n```bash\ngetfacl -t /srv/sales\n```\n\n> Access and default in columns.\n\n## Watch out\n\n- `cp` and `mv` keep ACLs only with `cp -a` (or `--preserve=all`); tar needs `--acls`.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man getfacl](https://linux.die.net/man/1/getfacl)\n","page":"commands/getfacl/","label":"getfacl","where":"Command page · Permissions","aka":[]},{"id":"cmd-getsebool","n":1422,"slug":"getsebool","title":"getsebool — Show SELinux booleans (getsebool) or switch them on and off (setsebool; with -P permanently)","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","booleans","policy switches"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show SELinux booleans (getsebool) or switch them on and off (setsebool; with -P permanently).\n\nSame page for: `setsebool`\n\n## About\n\nBooleans turn parts of the policy on or off without writing rules: `httpd_enable_homedirs`, `httpd_can_network_connect_db`, `samba_export_all_rw` … `setsebool NAME on` changes it now; add **-P** to keep it after reboot. `semanage boolean -l` shows descriptions.\n\n## Syntax\n\n```text\ngetsebool -a | BOOLEAN...\nsetsebool [-PNV] BOOLEAN on|off|1|0 | BOOL1=VAL1 …\n```\n\n## Options\n\n\n### getsebool\n\n| Option | What it does |\n|---|---|\n| `-a` | Show all booleans. |\n\n### setsebool\n\n| Option | What it does |\n|---|---|\n| `-P` | Permanent: also write it to the policy on disk. |\n| `-N` | Do not reload the policy. |\n| `-V` | Verbose errors. |\n\n## Examples\n\n### All web booleans\n\n```bash\ngetsebool -a | grep httpd\n```\n\n> on/off per boolean.\n\n### One boolean\n\n```bash\ngetsebool httpd_enable_homedirs\n```\n```text\nhttpd_enable_homedirs --> off\n```\n\n> Off by default.\n\n### Switch on permanently\n\n```bash\nsetsebool -P httpd_enable_homedirs on\n```\n\n> Survives a reboot.\n\n### Only until reboot\n\n```bash\nsetsebool httpd_can_network_connect on\n```\n\n> For a quick test.\n\n### Several at once\n\n```bash\nsetsebool -P httpd_can_network_connect_db=1 httpd_can_sendmail=1\n```\n\n> Name=value pairs.\n\n### Descriptions\n\n```bash\nsemanage boolean -l | grep httpd_can_network_connect\n```\n\n> What the boolean allows.\n\n### Only your changes\n\n```bash\nsemanage boolean -l -C\n```\n\n> Booleans that differ from the default.\n\n### Find a boolean from the man page\n\n```bash\nman httpd_selinux | grep -A2 httpd_use_nfs\n```\n\n> Needs selinux-policy-doc.\n\n### NFS home folders\n\n```bash\nsetsebool -P use_nfs_home_dirs on\n```\n\n> Users with NFS homes can log in.\n\n### Audit risky booleans (security)\n\n```bash\ngetsebool -a | grep -E 'httpd_execmem|httpd_unified|allow_execstack|selinuxuser_execstack' \n```\n\n> Booleans that weaken protection should stay off unless needed.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (unknown boolean, not allowed). |\n\nFull manual: [man getsebool](https://linux.die.net/man/8/getsebool)\n","page":"commands/getsebool/","label":"getsebool","where":"Command page · SELinux and security","aka":["setsebool"]},{"id":"cmd-gpasswd","n":1423,"slug":"gpasswd","title":"gpasswd — Add or remove members of a group, set group admins, or set a group password","sections":["commands"],"topic":"Commands · Users and groups","tags":["group members","add member","remove member","group admin"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Add or remove members of a group, set group admins, or set a group password.\n\n## About\n\n`gpasswd` changes `/etc/group` and `/etc/gshadow`. Its most useful jobs are `-a` (add a member) and `-d` (remove a member) — removing is not easy with usermod on older systems.\n\nroot can also make users **group administrators** (`-A`): they may then add and remove members of that group themselves, without sudo.\n\nGroup passwords (`gpasswd GROUP`) let non-members join with `newgrp`. They are rarely a good idea.\n\n## Syntax\n\n```text\ngpasswd [OPTION] GROUP\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --add USER` | Add the user to the group. |\n| `-d, --delete USER` | Remove the user from the group. |\n| `-M, --members USER,...` | Set the full member list (replaces it). |\n| `-A, --administrators USER,...` | Set the group administrators. |\n| `-r, --remove-password` | Remove the group password (only members can use newgrp). |\n| `-R, --restrict` | Lock the group: only members can use newgrp. |\n| `-Q, --root DIR` | Work in this chroot folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Add a member\n\n```bash\ngpasswd -a lisa webadmins\n```\n```text\nAdding user lisa to group webadmins\n```\n\n> Same as `usermod -aG webadmins lisa`.\n\n### Remove a member\n\n```bash\ngpasswd -d lisa webadmins\n```\n```text\nRemoving user lisa from group webadmins\n```\n\n> The easy way to take a group away.\n\n### Set the whole member list\n\n```bash\ngpasswd -M anna,bob,carol sales\n```\n\n> Replaces all members; good for a sync from a list.\n\n### Let a team lead manage the group\n\n```bash\ngpasswd -A anna sales\ngpasswd -a dave sales\n```\n\n> anna (as group admin) adds members without sudo.\n\n### Remove all members\n\n```bash\ngpasswd -M '' sales\n```\n\n> An empty list.\n\n### Remove the group password\n\n```bash\ngpasswd -r sales\n```\n\n> Only members can then use `newgrp sales`.\n\n### Check the members and admins\n\n```bash\ngetent group sales; getent gshadow sales\n```\n```text\nsales:x:3000:anna,bob,carol\nsales:!::anna:anna,bob,carol\n```\n\n> gshadow field 3 = admins, field 4 = members.\n\n### Remove a user from all extra groups (offboarding)\n\n```bash\nfor g in $(id -Gn olduser | cut -d' ' -f2-); do gpasswd -d olduser \"$g\"; done\n```\n\n> The first group is the primary group and stays.\n\n### Take away admin rights\n\n```bash\ngpasswd -d lisa wheel\n```\n\n> She keeps sudo until her session ends; check open sessions.\n\n### Audit the members of admin groups\n\n```bash\nfor g in wheel webadmins; do getent group \"$g\"; done\n```\n\n> Review these lists often.\n\n## Watch out\n\n- `-M` replaces the list: users not in it are removed.\n- Membership changes work after the next login of the user.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/group` | Group name, GID and members. |\n| `/etc/gshadow` | Group password, admins and members. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (not allowed, user or group not found). |\n\nFull manual: [man gpasswd](https://linux.die.net/man/1/gpasswd)\n","page":"commands/gpasswd/","label":"gpasswd","where":"Command page · Users and groups","aka":[]},{"id":"cmd-grep","n":1424,"slug":"grep","title":"grep — Search files or command output for lines that match a pattern (text or regular expression)","sections":["commands"],"topic":"Commands · Text and search","tags":["search","regex","pattern","logs","filter","lines"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Search files or command output for lines that match a pattern (text or regular expression).\n\nSame page for: `egrep`, `fgrep`\n\n## About\n\n`grep` prints every line that contains a match. It is the main tool to search logs, config files and command output. The pattern is a regular expression: basic (default), extended (`-E`), fixed text (`-F`) or Perl style (`-P`).\n\n`egrep` and `fgrep` are old names for `grep -E` and `grep -F`.\n\nExit codes: 0 = something matched, 1 = nothing matched, 2 = an error. Scripts use this with `-q`.\n\n## Syntax\n\n```text\ngrep [OPTION...] PATTERNS [FILE...]\ngrep [OPTION...] -e PATTERNS ... [FILE...]\ngrep [OPTION...] -f PATTERN_FILE ... [FILE...]\n```\n\n## Options\n\n\n### Pattern type\n\n| Option | What it does |\n|---|---|\n| `-E, --extended-regexp` | Extended regex: + ? {} \\| () work without backslashes. |\n| `-F, --fixed-strings` | The pattern is plain text, not a regex (fast, no special characters). |\n| `-G, --basic-regexp` | Basic regex (the default). |\n| `-P, --perl-regexp` | Perl regex (like \\d, lookahead). |\n| `-e, --regexp=<pattern>` | A pattern; use it several times for several patterns. |\n| `-f, --file=<file>` | Read patterns from a file, one per line. |\n\n### Matching\n\n| Option | What it does |\n|---|---|\n| `-i, --ignore-case` | Ignore upper/lower case. |\n| `--no-ignore-case` | Do not ignore case (the default). |\n| `-v, --invert-match` | Print lines that do NOT match. |\n| `-w, --word-regexp` | Match whole words only. |\n| `-x, --line-regexp` | Match whole lines only. |\n| `-y` | Old name of -i. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-c, --count` | Print only the number of matching lines. |\n| `-l, --files-with-matches` | Print only the names of files with a match. |\n| `-L, --files-without-match` | Print the names of files without a match. |\n| `-n, --line-number` | Print the line number before each line. |\n| `-o, --only-matching` | Print only the matching part of the line. |\n| `-q, --quiet, --silent` | Print nothing; only the exit code tells the result. |\n| `-s, --no-messages` | Hide error messages about missing or unreadable files. |\n| `-m, --max-count=<num>` | Stop after NUM matching lines. |\n| `-H, --with-filename` | Print the file name with each line. |\n| `-h, --no-filename` | Do not print file names. |\n| `-b, --byte-offset` | Print the byte position of each line. |\n| `--label=<label>` | The name to show for standard input. |\n| `-T, --initial-tab` | Line up the output with a tab. |\n| `-Z, --null` | Print a NUL after file names. |\n| `--color[=<when>], --colour[=<when>]` | Highlight matches: never, always, auto. |\n| `--line-buffered` | Print each line at once (for live pipes with tail -f). |\n\n### Context\n\n| Option | What it does |\n|---|---|\n| `-A, --after-context=<num>` | Also print NUM lines after each match. |\n| `-B, --before-context=<num>` | Also print NUM lines before each match. |\n| `-C, --context=<num>, -<num>` | Print NUM lines before and after. |\n| `--group-separator=<sep>` | Text between context groups (default --). |\n| `--no-group-separator` | No text between context groups. |\n\n### Files and folders\n\n| Option | What it does |\n|---|---|\n| `-r, --recursive` | Search all files in folders. |\n| `-R, --dereference-recursive` | Like -r, and follow all symbolic links. |\n| `--include=<glob>` | Search only files that match, like `--include='*.conf'`. |\n| `--exclude=<glob>` | Skip files that match. |\n| `--exclude-from=<file>` | Skip files that match patterns in a file. |\n| `--exclude-dir=<glob>` | Skip folders that match, like .git. |\n| `-d, --directories=<action>` | What to do with folders: read, skip, recurse. |\n| `-D, --devices=<action>` | What to do with devices, pipes and sockets: read, skip. |\n| `-a, --text` | Search binary files as if they were text. |\n| `-I` | Skip binary files. |\n| `--binary-files=<type>` | binary, text or without-match. |\n| `-U, --binary` | Do not remove CR characters (Windows files). |\n| `-z, --null-data` | Lines end with NUL. |\n| `-V, --version` | Show the version. |\n| `--help` | Show help. |\n\n## Examples\n\n### Find a user in the passwd file\n\n```bash\ngrep student /etc/passwd\n```\n\n> Every line that contains \"student\".\n\n### Search a log, ignore case\n\n```bash\ngrep -i error /var/log/messages\n```\n\n> Finds Error, ERROR and error.\n\n### Search all files in a folder, with line numbers\n\n```bash\ngrep -rn \"PermitRootLogin\" /etc/ssh/\n```\n\n> File name, line number and the line.\n\n### Show the active settings (no comments, no empty lines)\n\n```bash\ngrep -Ev '^\\s*(#|$)' /etc/ssh/sshd_config\n```\n\n> One command instead of two greps.\n\n### Count matches\n\n```bash\ngrep -c \"Failed password\" /var/log/secure\n```\n\n> Number of failed SSH logins in the file.\n\n### Search for several words\n\n```bash\ngrep -E 'error|fail|denied' /var/log/messages\ngrep -e error -e fail /var/log/messages\n```\n\n> Two ways to search for \"this OR that\".\n\n### Show lines around a match\n\n```bash\ngrep -B3 -A5 \"Out of memory\" /var/log/messages\n```\n\n> Context shows what happened before and after.\n\n### Only list the files that contain the text\n\n```bash\ngrep -rl \"192.168.122.10\" /etc 2>/dev/null\n```\n\n> Find every config that still uses an old IP address.\n\n### Print only the matching part (extract IPs)\n\n```bash\ngrep -oE '([0-9]{1,3}\\.){3}[0-9]{1,3}' /var/log/secure | sort | uniq -c | sort -rn | head\n```\n\n> The IP addresses that appear most in the log.\n\n### Whole words only\n\n```bash\ngrep -w root /etc/passwd\n```\n\n> Matches \"root\" but not \"chroot\".\n\n### Use grep in an if test\n\n```bash\nif grep -q '^student:' /etc/passwd; then echo \"user exists\"; fi\n```\n\n> `-q` prints nothing; the exit code decides.\n\n### Search only some file types\n\n```bash\ngrep -rn --include='*.yaml' --exclude-dir=.git \"image:\" .\n```\n\n> All container images used in a repo.\n\n### Find secrets before a commit\n\n```bash\ngrep -rniE '(password|secret|api[_-]?key)\\s*[:=]' --exclude-dir=.git .\n```\n\n> A quick check; real projects use tools like gitleaks.\n\n### Search compressed rotated logs\n\n```bash\nzgrep \"Accepted publickey\" /var/log/secure-*.gz\n```\n\n> Same options as grep, on .gz files.\n\n## Watch out\n\n- Put the pattern in single quotes, or the shell may change `*`, `$`, `\\` or `|` first.\n- `grep pattern file > file` empties the file. Write to a new file.\n- A dot means \"any character\": use `\\.` or `-F` for a real dot (IP addresses!).\n\n## Tips\n\n- i = ignore case, v = invert, r = recursive, n = numbers, c = count, l = list files, w = word, E = extended, o = only match.\n\nFull manual: [man grep](https://linux.die.net/man/1/grep)\n","page":"commands/grep/","label":"grep","where":"Command page · Text and search","aka":["egrep","fgrep"]},{"id":"cmd-groupadd","n":1425,"slug":"groupadd","title":"groupadd — Create a group (groupadd), change it (groupmod) or delete it (groupdel)","sections":["commands"],"topic":"Commands · Users and groups","tags":["create group","rename group","delete group","gid","shared folder"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a group (groupadd), change it (groupmod) or delete it (groupdel).\n\nSame page for: `groupmod`, `groupdel`\n\n## About\n\nGroups give the same rights to many users, for example write access to a shared folder. The groups are in `/etc/group` and `/etc/gshadow`. The GID comes from the range in `/etc/login.defs` (GID_MIN … GID_MAX) unless you give one.\n\n`groupmod` renames a group or changes its GID. `groupdel` removes a group, but not if it is the primary group of a user.\n\nAdd members with `usermod -aG GROUP USER` or `gpasswd -a USER GROUP`.\n\n## Syntax\n\n```text\ngroupadd [OPTION]... GROUP\ngroupmod [OPTION]... GROUP\ngroupdel [OPTION]... GROUP\n```\n\n## Options\n\n\n### groupadd\n\n| Option | What it does |\n|---|---|\n| `-g, --gid GID` | Use this GID. |\n| `-r, --system` | Create a system group (GID under 1000). |\n| `-f, --force` | Exit with success if the group already exists (and pick another GID if -g is used). |\n| `-o, --non-unique` | Allow a GID that is used already (with -g). |\n| `-U, --users USER1[,USER2...]` | Add these users as members right away. |\n| `-K, --key KEY=VALUE` | Override a value of /etc/login.defs (like GID_MIN). |\n| `-p, --password HASH` | Set a hashed group password (not recommended). |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n### groupmod\n\n| Option | What it does |\n|---|---|\n| `-n, --new-name NEW_GROUP` | Rename the group. |\n| `-g, --gid GID` | Change the GID (files keep the old number; fix them with find). |\n| `-a, --append` | With -U: add the users to the members (do not replace). |\n| `-U, --users USER1[,USER2...]` | Set the member list. |\n| `-o, --non-unique` | Allow a GID that is used already. |\n| `-p, --password HASH` | Change the hashed group password. |\n\n### groupdel\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Delete the group even if it is the primary group of a user. |\n\n## Examples\n\n### Create a group\n\n```bash\ngroupadd webadmins\n```\n\n> Gets the next free GID.\n\n### Create a group with a fixed GID\n\n```bash\ngroupadd -g 3000 sales\n```\n\n> Same GID on all servers for NFS shares.\n\n### Create a system group for a service\n\n```bash\ngroupadd -r appsvc\n```\n\n> GID under 1000.\n\n### Create a group with members\n\n```bash\ngroupadd -U anna,bob projectx\n```\n\n> Newer versions only; else use gpasswd -a.\n\n### Rename a group\n\n```bash\ngroupmod -n web-admins webadmins\n```\n\n> Members and files keep working: they use the GID.\n\n### Change the GID and fix the files\n\n```bash\ngroupmod -g 3100 sales\nfind / -xdev -gid 3000 -exec chgrp -h sales {} +\n```\n\n> Files still have the old number until you change them.\n\n### Delete a group\n\n```bash\ngroupdel projectx\n```\n\n> Fails if it is the primary group of a user.\n\n### Set up a shared team folder\n\n```bash\ngroupadd sales\nmkdir /srv/sales\nchgrp sales /srv/sales; chmod 2770 /srv/sales\n```\n\n> 2 = setgid: new files get the group sales (see the permissions chapter).\n\n### Do not fail when the group exists (scripts)\n\n```bash\ngroupadd -f sales\n```\n\n> Safe to run twice, for example in Ansible-like scripts.\n\n### Check the group\n\n```bash\ngetent group sales\n```\n```text\nsales:x:3000:anna,bob\n```\n\n> Name, password placeholder, GID, members.\n\n### Find groups with no members (cleanup)\n\n```bash\nawk -F: '$4 == \"\" && $3 >= 1000 {print $1}' /etc/group\n```\n\n> Private groups of users are listed too; they are normal.\n\n## Watch out\n\n- groupdel does not change files: they then show the old GID number.\n- A user gets new groups only after a new login.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/group, /etc/gshadow` | The group data. |\n| `/etc/login.defs` | GID_MIN, GID_MAX, SYS_GID_MIN, SYS_GID_MAX. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `2` | Wrong syntax. |\n| `3` | Wrong value for an option. |\n| `4` | The GID is used already. |\n| `6` | The group does not exist (groupmod, groupdel). |\n| `8` | Cannot delete the primary group of a user (groupdel). |\n| `9` | The group name is used already. |\n| `10` | Could not update the group file. |\n\nFull manual: [man groupadd](https://linux.die.net/man/8/groupadd)\n","page":"commands/groupadd/","label":"groupadd","where":"Command page · Users and groups","aka":["groupmod","groupdel"]},{"id":"cmd-groups","n":1426,"slug":"groups","title":"groups — Show the names of the groups a user is in","sections":["commands"],"topic":"Commands · Users and groups","tags":["groups","membership","user info"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the names of the groups a user is in.\n\n## About\n\n`groups` prints the group names of the current process, or of the users you name. It is the same as `id -Gn`. The first name is the primary group.\n\n## Syntax\n\n```text\ngroups [OPTION]... [USER]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `USER` | Show the groups of this user (more names allowed). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### My groups\n\n```bash\ngroups\n```\n```text\nstudent wheel\n```\n\n> The first is the primary group.\n\n### Groups of other users\n\n```bash\ngroups lisa omar\n```\n```text\nlisa : lisa webadmins\nomar : omar wheel webadmins\n```\n\n> With names, each line starts with the user.\n\n### After adding a group\n\n```bash\nusermod -aG webadmins student\ngroups; groups student\n```\n```text\nstudent wheel\nstudent : student wheel webadmins\n```\n\n> The shell gets the new group only after a new login.\n\n### Check admin rights quickly\n\n```bash\ngroups | grep -qw wheel && echo \"you can use sudo\"\n```\n\n> wheel = sudo on RHEL.\n\n### Count the groups of a user\n\n```bash\ngroups lisa | wc -w\n```\n\n> The output includes the name and the colon for a named user.\n\n### Use it in a script for many users\n\n```bash\nfor u in anna bob carol; do groups \"$u\"; done\n```\n\n> Fast check after a lab.\n\n### Compare with id\n\n```bash\nid -Gn lisa\n```\n\n> Same names, no \"lisa :\" before them.\n\n### Check a group for a shared folder\n\n```bash\ngroups | grep -w sales\n```\n\n> Needed to write in a folder owned by group sales.\n\n### Check the groups of a service user\n\n```bash\ngroups apache\n```\n```text\napache : apache\n```\n\n> Service accounts should have as few groups as possible.\n\n### Find members of the docker or wheel group (audit)\n\n```bash\ngetent group wheel\n```\n\n> `groups` shows per user; `getent group` shows per group.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | A user does not exist. |\n\nFull manual: [man groups](https://linux.die.net/man/1/groups)\n","page":"commands/groups/","label":"groups","where":"Command page · Users and groups","aka":[]},{"id":"cmd-grub2-install","n":1427,"slug":"grub2-install","title":"grub2-install — Install the GRUB 2 boot loader to a disk (BIOS systems) — used to repair a system that does not find its boot loader","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["grub2","boot loader","mbr","repair","bios"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Install the GRUB 2 boot loader to a disk (BIOS systems) — used to repair a system that does not find its boot loader.\n\n## About\n\nOn BIOS systems, `grub2-install /dev/vda` writes the GRUB boot code to the start of the disk and the modules to `/boot/grub2/`. On **UEFI** systems with Secure Boot, do not use it: reinstall the packages `grub2-efi-x64` and `shim-x64` instead, which put the signed files in `/boot/efi/EFI/redhat/`.\n\nAfter installing, create the config with `grub2-mkconfig -o /boot/grub2/grub.cfg`.\n\n## Syntax\n\n```text\ngrub2-install [OPTION]... DEVICE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--target=TARGET` | Platform: i386-pc (BIOS), x86_64-efi … |\n| `--boot-directory=DIR` | Where /boot is (default /boot). |\n| `--efi-directory=DIR` | The EFI system partition (UEFI). |\n| `--bootloader-id=ID` | Name of the EFI folder and boot entry. |\n| `--removable` | Install to the fallback path (removable media). |\n| `--recheck` | Probe the devices again. |\n| `--force` | Install even with warnings. |\n| `-v, --verbose` | Verbose. |\n| `--modules=MODULES, --install-modules=MODULES` | Modules to preload / install. |\n| `-d, --directory=DIR` | Use images and modules from DIR. |\n| `--no-nvram` | Do not change the UEFI boot entries. |\n| `--no-bootsector` | Do not write the boot sector. |\n| `--uefi-secure-boot, --no-uefi-secure-boot` | Secure Boot layout. |\n| `-s, --skip-fs-probe` | Do not probe the file system. |\n| `--compress=no\\|xz\\|gz\\|lzo, --core-compress=xz\\|none\\|auto` | Compression. |\n| `--fonts=, --locales=, --themes=, --locale-directory=DIR` | Extra files. |\n| `-k, --pubkey=FILE` | Check signatures with this key. |\n| `--disk-module=MODULE, --allow-floppy, --force-file-id, --no-rs-codes, --product-version=STRING, --sbat=FILE, --disable-shim-lock, --dtb=FILE, --label-*, --macppc-directory=DIR` | Special settings. |\n| `-?, --help, --usage` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Reinstall GRUB on a BIOS disk\n\n```bash\ngrub2-install /dev/vda\n```\n```text\nInstalling for i386-pc platform.\nInstallation finished. No error reported.\n```\n\n> The disk, not a partition.\n\n### Then rebuild the config\n\n```bash\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n\n> Menu entries come from BLS files.\n\n### From a rescue disk\n\n```bash\nchroot /mnt/sysroot\ngrub2-install /dev/vda\n```\n\n> Inside the installed system.\n\n### Is this BIOS or UEFI?\n\n```bash\n[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS\n```\n\n> Decides which repair to use.\n\n### UEFI repair (instead of grub2-install)\n\n```bash\ndnf reinstall -y grub2-efi-x64 shim-x64\n```\n\n> Signed files for Secure Boot.\n\n### UEFI boot entries\n\n```bash\nefibootmgr -v\n```\n\n> Check that \"Red Hat Enterprise Linux\" points to shimx64.efi.\n\n### Both disks of a software RAID1\n\n```bash\ngrub2-install /dev/vda; grub2-install /dev/vdb\n```\n\n> The system boots even when one disk fails.\n\n### Verbose run\n\n```bash\ngrub2-install -v /dev/vda 2>&1 | tail\n```\n\n> Find the reason of an error.\n\n### Another boot folder\n\n```bash\ngrub2-install --boot-directory=/mnt/sysroot/boot /dev/vda\n```\n\n> Without chroot.\n\n### Check the boot sector\n\n```bash\ndd if=/dev/vda bs=512 count=1 2>/dev/null | strings | grep -i grub\n```\n```text\nGRUB\n```\n\n> The boot code is there.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man grub2-install](https://linux.die.net/man/8/grub2-install)\n","page":"commands/grub2-install/","label":"grub2-install","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-grub2-mkconfig","n":1428,"slug":"grub2-mkconfig","title":"grub2-mkconfig — Generate the GRUB 2 configuration file from /etc/default/grub and the scripts in /etc/grub.d/","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["grub2","boot loader","grub.cfg","timeout","kernel command line"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Generate the GRUB 2 configuration file from /etc/default/grub and the scripts in /etc/grub.d/.\n\n## About\n\nNever edit `grub.cfg` by hand. Change `/etc/default/grub`, then run `grub2-mkconfig -o /boot/grub2/grub.cfg` (the same path on BIOS and UEFI on current RHEL). On RHEL with BLS, kernel arguments live in the entry files; add `--update-bls-cmdline` when you changed `GRUB_CMDLINE_LINUX` so the entries get the new arguments.\n\n## Syntax\n\n```text\ngrub2-mkconfig [-o FILE] [--update-bls-cmdline]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `GRUB_TIMEOUT` | Seconds the menu waits. |\n| `GRUB_DEFAULT` | Default entry (`saved` = use grubenv). |\n| `GRUB_CMDLINE_LINUX` | Kernel arguments for all kernels. |\n| `GRUB_TERMINAL_OUTPUT` | console, serial, gfxterm. |\n| `GRUB_DISABLE_RECOVERY` | No rescue entries. |\n| `GRUB_ENABLE_BLSCFG` | Use BLS entries (true on RHEL). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-o, --output=FILE` | Write to this file (without: print to the screen). |\n| `--update-bls-cmdline` | Also write GRUB_CMDLINE_LINUX into the BLS entries. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Regenerate after a change\n\n```bash\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n```text\nGenerating grub configuration file ...\ndone\n```\n\n> After every edit of /etc/default/grub.\n\n### Change the timeout\n\n```bash\nsed -i 's/^GRUB_TIMEOUT=.*/GRUB_TIMEOUT=10/' /etc/default/grub\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n\n> Menu waits 10 seconds.\n\n### Change kernel arguments for all kernels\n\n```bash\nvim /etc/default/grub\ngrub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline\n```\n\n> Without the option the existing entries keep their old arguments.\n\n### Preview without writing\n\n```bash\ngrub2-mkconfig | less\n```\n\n> Check before you overwrite.\n\n### Back up the old config\n\n```bash\ncp -a /boot/grub2/grub.cfg /root/grub.cfg.bak\n```\n\n> Easy way back.\n\n### Show the menu on a serial console\n\n```bash\ngrep -E 'GRUB_TERMINAL|GRUB_SERIAL' /etc/default/grub\n```\n\n> For VMs without a screen.\n\n### Password for the GRUB menu (security)\n\n```bash\ngrub2-setpassword\n```\n\n> Nobody can edit boot entries at the console without it (stops rd.break tricks).\n\n### Where is the config on UEFI?\n\n```bash\ncat /boot/efi/EFI/redhat/grub.cfg\n```\n\n> A small stub that loads /boot/grub2/grub.cfg.\n\n### Boot an entry once\n\n```bash\ngrub2-reboot 1; reboot\n```\n\n> Only the next boot uses entry 1.\n\n### Check the BLS entries afterwards\n\n```bash\ngrep options /boot/loader/entries/*.conf\n```\n\n> The kernel arguments per entry.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/default/grub` | Your settings. |\n| `/etc/grub.d/` | Scripts that build the config. |\n| `/boot/grub2/grub.cfg` | Generated config. |\n| `/boot/loader/entries/` | BLS entries. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man grub2-mkconfig](https://linux.die.net/man/8/grub2-mkconfig)\n","page":"commands/grub2-mkconfig/","label":"grub2-mkconfig","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-grubby","n":1429,"slug":"grubby","title":"grubby — Show and change the boot entries of GRUB 2: kernel arguments, the default kernel, and entry details","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["grub2","boot loader","kernel arguments","default kernel","bls"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change the boot entries of GRUB 2: kernel arguments, the default kernel, and entry details.\n\n## About\n\nOn RHEL each kernel has a BLS entry file in `/boot/loader/entries/`. `grubby` reads and edits these entries: add or remove kernel arguments for one or all kernels, choose the default kernel, and show what each entry boots. Changes work at the next boot; no grub2-mkconfig is needed.\n\n## Syntax\n\n```text\ngrubby [OPTION]...\n```\n\n## Options\n\n\n### Show\n\n| Option | What it does |\n|---|---|\n| `--info=KERNEL\\|ALL\\|DEFAULT` | Show entries: index, kernel, args, root, initrd, title. |\n| `--default-kernel` | Show the kernel that boots by default. |\n| `--default-index` | Show the index of the default entry. |\n| `--default-title` | Show the title of the default entry. |\n\n### Change arguments\n\n| Option | What it does |\n|---|---|\n| `--update-kernel=KERNEL\\|ALL\\|DEFAULT\\|TITLE=…` | Which entry to change (with --args / --remove-args). |\n| `--args=ARGS` | Add these kernel arguments. |\n| `--remove-args=ARGS` | Remove these kernel arguments. |\n\n### Default entry\n\n| Option | What it does |\n|---|---|\n| `--set-default=KERNEL` | Make this kernel the default (path like /boot/vmlinuz-VERSION). |\n| `--set-default-index=N` | Make entry N the default. |\n| `--make-default` | With --add-kernel: make the new entry the default. |\n\n### Add and remove entries\n\n| Option | What it does |\n|---|---|\n| `--add-kernel=KERNEL` | Add an entry for this kernel. |\n| `--initrd=FILE` | The initrd for --add-kernel. |\n| `--title=TITLE` | The title for --add-kernel. |\n| `--copy-default` | Copy the arguments of the default entry to the new one. |\n| `--remove-kernel=KERNEL\\|ALL\\|TITLE=…` | Remove entries. |\n| `--add-multiboot, --mbargs, --remove-mbargs` | Multiboot settings (rare). |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `-c, --config-file=FILE` | Use another config file. |\n| `-o, --output-file=FILE` | Write to another file. |\n| `--bls-directory=DIR` | Use another BLS folder. |\n| `--env=FILE` | Use another grubenv file. |\n| `--grub2, --zipl` | Choose the boot loader type. |\n| `--bootloader-probe` | Show the boot loader in use. |\n| `--no-etc-grub-update` | Do not also change /etc/default/grub. |\n| `--version` | Show the version. |\n| `-?, --help` | Show help. |\n\n## Examples\n\n### Show the default kernel\n\n```bash\ngrubby --default-kernel\n```\n```text\n/boot/vmlinuz-6.12.0-55.9.1.el10_0.x86_64\n```\n\n> The newest kernel after an update.\n\n### Show all entries\n\n```bash\ngrubby --info=ALL\n```\n\n> index, kernel, args, root, initrd, title per entry.\n\n### Arguments of the default entry\n\n```bash\ngrubby --info=DEFAULT | grep ^args\n```\n\n> Same as /proc/cmdline after a boot.\n\n### Remove rhgb and quiet\n\n```bash\ngrubby --update-kernel=ALL --remove-args=\"rhgb quiet\"\n```\n\n> Shows the boot messages.\n\n### Add an argument to all kernels\n\n```bash\ngrubby --update-kernel=ALL --args=\"console=ttyS0,115200\"\n```\n\n> Serial console for VMs.\n\n### Boot an older kernel by default\n\n```bash\ngrubby --set-default /boot/vmlinuz-6.12.0-55.el10.x86_64\n```\n\n> When the new kernel has a problem.\n\n### Only for one kernel\n\n```bash\ngrubby --update-kernel=/boot/vmlinuz-$(uname -r) --args=\"systemd.unit=multi-user.target\"\n```\n\n> Other kernels keep their arguments.\n\n### Turn off a mitigation for a test (security trade-off)\n\n```bash\ngrubby --update-kernel=ALL --args=\"mitigations=off\"\n```\n\n> Faster, but open to CPU attacks — only in a lab; remove it again.\n\n### Enable SELinux auditing details or FIPS\n\n```bash\ngrubby --update-kernel=ALL --args=\"fips=1\"\n```\n\n> FIPS mode needs more steps (fips-mode-setup / crypto policies); this is the kernel part.\n\n### Check the result after reboot\n\n```bash\ncat /proc/cmdline\n```\n\n> The live arguments.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/boot/loader/entries/` | BLS entry files. |\n| `/boot/grub2/grubenv` | Saved default entry. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man grubby](https://linux.die.net/man/8/grubby)\n","page":"commands/grubby/","label":"grubby","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-gzip","n":1430,"slug":"gzip","title":"gzip — Compress files to .gz (gzip), unpack them (gunzip), or read them without unpacking (zcat)","sections":["commands"],"topic":"Commands · Archive and compress","tags":["compress","decompress","gz","logs","archive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Compress files to .gz (gzip), unpack them (gunzip), or read them without unpacking (zcat).\n\nSame page for: `gunzip`, `zcat`, `zgrep`, `zless`\n\n## About\n\n`gzip FILE` replaces FILE with a smaller `FILE.gz`. `gunzip FILE.gz` (same as `gzip -d`) brings the original back. gzip is fast and everywhere; it is the default compression for logs and for `.tar.gz` archives.\n\ngzip compresses **one file at a time**. To pack a folder, use `tar -czf`.\n\nTools to work with `.gz` files without unpacking them: `zcat` (show), `zless` (page), `zgrep` (search), `zdiff` (compare).\n\n## Syntax\n\n```text\ngzip [OPTION]... [FILE]...\ngunzip [OPTION]... [FILE]...\nzcat [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --decompress` | Decompress (same as gunzip). |\n| `-k, --keep` | Keep the original file. |\n| `-c, --stdout` | Write to the screen (stdout); keep the files. |\n| `-l, --list` | Show the compressed and uncompressed size and the ratio. |\n| `-r, --recursive` | Compress every file in the folders. |\n| `-t, --test` | Test the compressed file (is it complete and not damaged?). |\n| `-v, --verbose` | Show the name and the ratio of each file. |\n| `-q, --quiet` | No warnings. |\n| `-f, --force` | Overwrite existing files; also compress links and files with more names. |\n| `-1, --fast` | Fastest compression (bigger file). |\n| `-9, --best` | Best compression (slower). The default is -6. |\n| `-n, --no-name` | Do not save (or restore) the original name and time. |\n| `-N, --name` | Save (or restore) the original name and time. |\n| `-S, --suffix=<suf>` | Use another ending instead of .gz. |\n| `--rsyncable` | Make the output friendly for rsync (small changes = small transfers). |\n| `--synchronous` | Write safely to disk (slower). |\n| `-h, --help` | Show help. |\n| `-L, --license` | Show the license. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Compress a file\n\n```bash\ngzip access.log\nls access.log*\n```\n```text\naccess.log.gz\n```\n\n> The original is replaced by the .gz file.\n\n### Decompress\n\n```bash\ngunzip access.log.gz\n```\n\n> Same as `gzip -d access.log.gz`.\n\n### Keep the original\n\n```bash\ngzip -k database.sql\n```\n\n> You get database.sql and database.sql.gz.\n\n### Read a compressed log without unpacking\n\n```bash\nzcat /var/log/messages-20261018.gz | tail\n```\n\n> No temporary file, no extra space used.\n\n### Search in compressed logs\n\n```bash\nzgrep -i \"failed password\" /var/log/secure-*.gz\n```\n\n> Searches old rotated logs directly.\n\n### See the compression ratio\n\n```bash\ngzip -l backup.tar.gz\n```\n\n> Shows compressed size, original size and ratio.\n\n### Test a file after a transfer\n\n```bash\ngzip -t backup.sql.gz && echo OK\n```\n\n> Finds broken downloads or cut transfers.\n\n### Compress a database dump on the fly\n\n```bash\npg_dump appdb | gzip -9 > appdb-$(date +%F).sql.gz\n```\n\n> The dump never touches the disk uncompressed.\n\n### Restore a compressed dump directly\n\n```bash\ngunzip -c appdb-2026-10-25.sql.gz | psql appdb\n```\n\n> `-c` writes to stdout, the file stays.\n\n### Compress all old files in a folder\n\n```bash\nfind /srv/exports -name \"*.csv\" -mtime +7 -exec gzip {} \\;\n```\n\n> Saves space for data you rarely read.\n\n### Compress every file in a tree\n\n```bash\ngzip -r /tmp/old-reports/\n```\n\n> Each file is compressed alone; folders stay.\n\n### Faster compression with all CPU cores\n\n```bash\ntar -cf - /data | pigz > data.tar.gz\n```\n\n> `pigz` is a parallel gzip; the output works with normal gunzip.\n\n## Watch out\n\n- gzip deletes the original by default. Use `-k` or `-c` if you need it.\n- gzip does not pack folders into one file: use tar.\n- `gzip` on a file that is still being written (a live log) can lose data; let logrotate do it.\n\n## Tips\n\n- gzip = pack, gunzip = unpack, zcat/zgrep/zless = read packed files.\n- `-k` keep, `-c` to stdout, `-l` list, `-t` test.\n\nFull manual: [man gzip](https://linux.die.net/man/1/gzip)\n","page":"commands/gzip/","label":"gzip","where":"Command page · Archive and compress","aka":["gunzip","zcat","zgrep","zless"]},{"id":"cmd-head","n":1431,"slug":"head","title":"head — Show the first lines (or bytes) of files or command output","sections":["commands"],"topic":"Commands · Text and search","tags":["first lines","top","file","preview"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the first lines (or bytes) of files or command output.\n\n## About\n\n`head` prints the first 10 lines of each file. With `-n` you choose how many. With a minus number (`-n -5`) it prints everything except the last 5 lines.\n\nIt is perfect for a quick look at a file, the header of a CSV file, or the top of a sorted list.\n\n## Syntax\n\n```text\nhead [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --lines=[-]<num>` | Print the first NUM lines. With a minus: all lines except the last NUM. |\n| `-c, --bytes=[-]<num>` | Print the first NUM bytes (K, M suffixes allowed). With a minus: all but the last NUM bytes. |\n| `-<num>` | Old short form of -n NUM (like `head -5`). |\n| `-q, --quiet, --silent` | Do not print the file name headers. |\n| `-v, --verbose` | Always print the file name headers. |\n| `-z, --zero-terminated` | Lines end with NUL, not a new line. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### The first 10 lines\n\n```bash\nhead /etc/passwd\n```\n\n> Default is 10 lines.\n\n### The first 3 lines\n\n```bash\nhead -n 3 /etc/passwd\n```\n\n> Same as `head -3 /etc/passwd`.\n\n### Top 5 of a sorted list\n\n```bash\ndu -sh /var/* 2>/dev/null | sort -rh | head -5\n```\n\n> The 5 biggest folders in /var.\n\n### Look at the header of a CSV file\n\n```bash\nhead -1 export.csv\n```\n\n> Shows the column names.\n\n### Print line N only\n\n```bash\nhead -n 15 /etc/services | tail -n 1\n```\n\n> Line 15. Same as `sed -n '15p'`.\n\n### Everything except the last 2 lines\n\n```bash\nhead -n -2 report.txt\n```\n\n> Removes a footer.\n\n### The first bytes of a file\n\n```bash\nhead -c 16 /dev/urandom | base64\n```\n\n> 16 random bytes as text: a quick random token.\n\n### Check a file's start for its type\n\n```bash\nhead -c 4 unknown.bin | xxd\n```\n\n> PDF files start with %PDF, ELF programs with .ELF.\n\n### The first lines of several files with names\n\n```bash\nhead -n 2 /etc/*-release\n```\n\n> Each file gets a `==> name <==` header.\n\n### The top processes by memory\n\n```bash\nps aux --sort=-%mem | head -6\n```\n\n> The header plus the 5 biggest.\n\n### Read the start of a huge log quickly\n\n```bash\nhead -n 50 /var/log/huge.log\n```\n\n> head stops reading after 50 lines, so it is fast even on big files.\n\n## Tips\n\n- head = top, tail = bottom. `head -n N | tail -n 1` = line N.\n\nFull manual: [man head](https://linux.die.net/man/1/head)\n","page":"commands/head/","label":"head","where":"Command page · Text and search","aka":[]},{"id":"cmd-history","n":1432,"slug":"history","title":"history — Show, search, repeat and clear the commands you typed before","sections":["commands"],"topic":"Commands · Shell and help","tags":["history","bash","repeat","search","audit","ctrl-r"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show, search, repeat and clear the commands you typed before.\n\n## About\n\nBash keeps the commands of the current shell in memory and writes them to `~/.bash_history` when the shell ends. `history` shows the list with numbers, and the `!` signs repeat old commands.\n\nThe size of the list and what is saved are set by variables like `HISTSIZE`, `HISTFILESIZE`, `HISTCONTROL` and `HISTTIMEFORMAT`.\n\nThe history is personal convenience, not a security log: a user can change or delete it. For real auditing use `auditd` or session recording.\n\n## Syntax\n\n```text\nhistory [N]\nhistory -c\nhistory -d OFFSET\nhistory -a | -n | -r | -w [FILE]\nhistory -p ARG... | -s ARG...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `N` | Show only the last N commands. |\n| `-c` | Clear the history list of this shell. |\n| `-d <offset>` | Delete the entry with this number. |\n| `-a` | Add the new lines of this shell to the history file now. |\n| `-n` | Read lines from the history file that are not read yet (from other shells). |\n| `-r` | Read the history file and add it to the list. |\n| `-w` | Write the whole current list to the history file (replaces the file). |\n| `-p <arg>` | Do history expansion on ARG and print the result, without running it. |\n| `-s <arg>` | Add ARG to the history list as one entry, without running it. |\n\n### Expansion (type these at the prompt)\n\n| Option | What it does |\n|---|---|\n| `!!` | The last command. |\n| `!N` | Command number N. |\n| `!-N` | The command N lines back. |\n| `!text` | The last command that starts with text. |\n| `!?text?` | The last command that contains text. |\n| `!$` | The last word of the last command. |\n| `^old^new` | Run the last command again with old replaced by new. |\n\n### Variables\n\n| Option | What it does |\n|---|---|\n| `HISTSIZE` | How many commands are kept in memory. |\n| `HISTFILESIZE` | How many lines are kept in the history file. |\n| `HISTFILE` | The history file (default ~/.bash_history). |\n| `HISTCONTROL` | ignorespace, ignoredups or ignoreboth: what is not saved. |\n| `HISTIGNORE` | Patterns of commands that are not saved, like \"ls:pwd\". |\n| `HISTTIMEFORMAT` | Save and show the date and time of each command, like \"%F %T \". |\n\n## Examples\n\n### Show the last 10 commands\n\n```bash\nhistory 10\n```\n\n> Each line has a number you can use with `!`.\n\n### Run command number 42 again\n\n```bash\n!42\n```\n\n> Bash prints the command before it runs it.\n\n### Repeat the last command with sudo\n\n```bash\nsudo !!\n```\n\n> The classic fix for \"Permission denied\".\n\n### Search the history interactively\n\n```bash\n# press Ctrl+R and type: nmcli con\n```\n\n> Ctrl+R again goes to the next older match. Enter runs it, the right arrow lets you edit it first.\n\n### Reuse the last argument\n\n```bash\nmkdir -p /srv/app/logs\ncd !$\n```\n\n> `!$` is `/srv/app/logs`. Saves typing long paths.\n\n### Fix a typo in the last command\n\n```bash\nsystemctl restart httdp\n^httdp^httpd\n```\n\n> Runs the last command again with the word replaced.\n\n### Show the date and time of each command\n\n```bash\nexport HISTTIMEFORMAT=\"%F %T \"\nhistory 5\n```\n\n> Put the export line in `~/.bashrc` to keep it. Very helpful when you write an incident timeline.\n\n### Remove a password you typed by mistake\n\n```bash\nhistory -d 1203\nhistory -w\n```\n\n> Delete the line, then write the clean list to the file at once.\n\n### Do not save commands that start with a space\n\n```bash\nexport HISTCONTROL=ignoreboth\n export API_TOKEN=secret123\n```\n\n> With `ignoreboth`, lines that start with a space and repeated lines are not saved.\n\n### Share the history between open terminals\n\n```bash\nexport PROMPT_COMMAND=\"history -a; history -n; $PROMPT_COMMAND\"\n```\n\n> Every prompt writes new lines and reads the lines of the other shells.\n\n### Clear the history of this shell\n\n```bash\nhistory -c\nhistory -w\n```\n\n> `-c` clears the memory; `-w` writes the empty list to the file, so it is gone there too.\n\n### Look at another user's history during an investigation\n\n```bash\ntail -n 50 /home/alice/.bash_history\n```\n\n> A first hint of what happened. Remember: the user could have changed it.\n\n## Watch out\n\n- History is written to the file when the shell **exits**. A shell that crashes or is killed may not save its last commands.\n- Passwords typed as arguments are saved in clear text. Let tools ask for the password instead.\n- `history -c` alone does not change the file; use `history -w` after it.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.bash_history` | The history file of each user. |\n| `~/.bashrc` | The place for HISTSIZE, HISTCONTROL and HISTTIMEFORMAT settings. |\n\n## Tips\n\n- `!!` = last, `!$` = last word, `!text` = last starting with text, Ctrl+R = search.\n- `-c` = clear memory, `-w` = write file, `-d` = delete one line.\n\nFull manual: [man history](https://linux.die.net/man/1/bash)\n","page":"commands/history/","label":"history","where":"Command page · Shell and help","aka":[]},{"id":"cmd-host","n":1433,"slug":"host","title":"host — Simple DNS lookups with short, readable output","sections":["commands"],"topic":"Commands · Network","tags":["dns","lookup","reverse","mx","simple"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Simple DNS lookups with short, readable output.\n\n## About\n\n`host NAME` prints the addresses and mail servers of a name in plain sentences. `host IP` does a reverse lookup. Use `-t` for other record types. For details use `dig`.\n\n## Syntax\n\n```text\nhost [OPTION]... NAME [SERVER]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t TYPE` | Record type: A, AAAA, MX, NS, TXT, SOA, PTR, SRV, CNAME, ANY … |\n| `-a` | All: verbose output with type ANY. |\n| `-A` | Almost all: like -a without RRSIG, NSEC, NSEC3. |\n| `-v, -d` | Verbose (like dig output). |\n| `-C` | Compare the SOA records of all authoritative servers. |\n| `-l` | List a zone with a zone transfer (AXFR). |\n| `-r` | No recursion. |\n| `-s` | Do not try the next server after a SERVFAIL. |\n| `-T` | Use TCP. |\n| `-U` | Use UDP (default). |\n| `-4` | Use IPv4 for the transport. |\n| `-6` | Use IPv6 for the transport. |\n| `-c CLASS` | Query class (IN default). |\n| `-N NDOTS` | Dots needed before the name is tried as is. |\n| `-p PORT` | Use another port. |\n| `-R NUMBER` | Number of UDP retries. |\n| `-W WAIT` | Wait this many seconds for an answer. |\n| `-w` | Wait forever. |\n| `-m FLAG` | Memory debugging (record, trace, usage). |\n| `-V` | Show the version. |\n\n## Examples\n\n### Look up a name\n\n```bash\nhost www.redhat.com\n```\n```text\nwww.redhat.com is an alias for www.redhat.com.edgekey.net.\nwww.redhat.com.edgekey.net is an alias for e3396.dscx.akamaiedge.net.\ne3396.dscx.akamaiedge.net has address 23.40.100.73\n```\n\n> Plain text answer.\n\n### Reverse lookup\n\n```bash\nhost 192.168.122.20\n```\n```text\n20.122.168.192.in-addr.arpa domain name pointer server2.example.com.\n```\n\n> PTR record of the IP.\n\n### Mail servers\n\n```bash\nhost -t MX s0sta.com\n```\n\n> Check after you move mail.\n\n### Ask another DNS server\n\n```bash\nhost server2.example.com 192.168.122.1\n```\n\n> The last argument is the server.\n\n### Text records\n\n```bash\nhost -t TXT s0sta.com\n```\n\n> SPF and site verification records.\n\n### Compare the SOA of all name servers\n\n```bash\nhost -C s0sta.com\n```\n\n> Different serials = a secondary did not get the update.\n\n### All records, verbose\n\n```bash\nhost -a s0sta.com\n```\n\n> Many servers block ANY today.\n\n### Test a zone transfer (security, own zones)\n\n```bash\nhost -l example.com ns1.example.com\n```\n\n> Should be refused for the public; open AXFR leaks all names.\n\n### IPv6 address\n\n```bash\nhost -t AAAA www.google.com\n```\n\n> Check that IPv6 records exist.\n\n### Quick check in a script\n\n```bash\nhost -W 2 server2 >/dev/null && echo \"DNS ok\"\n```\n\n> Exit code 0 when the name was found.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Found. |\n| `1` | Not found or error. |\n\nFull manual: [man host](https://linux.die.net/man/1/host)\n","page":"commands/host/","label":"host","where":"Command page · Network","aka":[]},{"id":"cmd-hostname","n":1434,"slug":"hostname","title":"hostname — Show the host name (and its FQDN, domain or IP addresses), or set it until the next reboot","sections":["commands"],"topic":"Commands · Network","tags":["host name","fqdn","ip address","domain"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the host name (and its FQDN, domain or IP addresses), or set it until the next reboot.\n\nSame page for: `dnsdomainname`, `domainname`\n\n## About\n\n`hostname` prints the current host name. Options show the FQDN, the DNS domain or all IP addresses.\n\nSetting a name with `hostname NAME` lasts only until reboot. To set it for good on RHEL, use `hostnamectl set-hostname NAME` (writes `/etc/hostname`).\n\n## Syntax\n\n```text\nhostname [OPTION]\nhostname [-b] [-F FILE] [NAME]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --short` | Short name (until the first dot). |\n| `-f, --fqdn, --long` | Fully qualified domain name (from DNS or /etc/hosts). |\n| `-d, --domain` | The DNS domain part. |\n| `-i, --ip-address` | Addresses of the host name (by name lookup). |\n| `-I, --all-ip-addresses` | All addresses of all interfaces (no lookup). |\n| `-a, --alias` | Alias names (old, not recommended). |\n| `-A, --all-fqdns` | All FQDNs of the host addresses. |\n| `-y, --yp, --nis` | The NIS domain name. |\n| `-b, --boot` | When setting: use a default name if none is given. |\n| `-F, --file FILE` | Read the name from a file. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Show the host name\n\n```bash\nhostname\n```\n```text\nserver1.example.com\n```\n\n> RHEL often uses the full name as host name.\n\n### Short name and FQDN\n\n```bash\nhostname -s; hostname -f\n```\n```text\nserver1\nserver1.example.com\n```\n\n> -f needs /etc/hosts or DNS to know the full name.\n\n### All IP addresses\n\n```bash\nhostname -I\n```\n```text\n192.168.122.10 fe80::5054:ff:fe12:3456\n```\n\n> Quick way to find the IP of a fresh VM.\n\n### Domain part\n\n```bash\nhostname -d\n```\n```text\nexample.com\n```\n\n> Same as `dnsdomainname`.\n\n### Set a name until reboot\n\n```bash\nhostname test1\n```\n\n> For a quick test only.\n\n### Set the name for good\n\n```bash\nhostnamectl set-hostname server1.example.com\n```\n\n> Writes /etc/hostname (the RHCSA way).\n\n### Use the name in a prompt or file name\n\n```bash\ntar -czf /backup/$(hostname -s)-etc-$(date +%F).tgz /etc\n```\n\n> Each server makes a different file name.\n\n### Check the name for certificates\n\n```bash\nhostname -f; getent hosts $(hostname -f)\n```\n\n> The FQDN must resolve to the server IP before you request a TLS certificate.\n\n### Find wrong FQDN settings\n\n```bash\nhostname -f\n```\n```text\nhostname: Name or service not known\n```\n\n> Add the name to /etc/hosts or DNS.\n\n### Show the domain of NIS\n\n```bash\nhostname -y\n```\n\n> Only for old NIS setups.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/hostname` | The static host name. |\n| `/etc/hosts` | Local name to address list. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man hostname](https://linux.die.net/man/1/hostname)\n","page":"commands/hostname/","label":"hostname","where":"Command page · Network","aka":["dnsdomainname","domainname"]},{"id":"cmd-hostnamectl","n":1435,"slug":"hostnamectl","title":"hostnamectl — Show and change the hostname of the system, and see the OS, kernel and hardware facts","sections":["commands"],"topic":"Commands · System information","tags":["hostname","name","fqdn","system","os","chassis"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change the hostname of the system, and see the OS, kernel and hardware facts.\n\n## About\n\n`hostnamectl` talks to the systemd service that manages the hostname. A change takes effect at once and stays after a reboot, because the name is written to `/etc/hostname`.\n\nWithout a command it shows the status: the hostname, the machine ID, the operating system, the kernel, the architecture and (on newer versions) the hardware vendor and model.\n\nThere are three names:\n\n- **static**: the normal hostname, stored in `/etc/hostname`.\n- **transient**: a temporary name, for example from DHCP; it is used only when there is no static name.\n- **pretty**: a free text name for people, like \"Web server 1 (Riyadh)\".\n\n## Syntax\n\n```text\nhostnamectl [OPTIONS] [COMMAND]\nhostnamectl [status]\nhostnamectl hostname NAME\nhostnamectl set-hostname NAME\nhostnamectl { icon-name | chassis | deployment | location } [VALUE]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `status` | Show the current names and the system facts (the default command). |\n| `hostname [NAME]` | Show the hostname, or set it when you give a NAME. |\n| `set-hostname NAME` | Set the hostname (older name of the same command; it still works). |\n| `icon-name [NAME]` | Show or set the icon name used by graphical tools. |\n| `chassis [TYPE]` | Show or set the chassis type: desktop, laptop, server, vm, container … |\n| `deployment [ENV]` | Show or set the deployment environment, like production or staging. |\n| `location [TEXT]` | Show or set the location text, like \"Rack 4, DC Riyadh\". |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--static` | With hostname or status: only the static name. |\n| `--transient` | Only the transient (temporary) name. |\n| `--pretty` | Only the pretty name. With set: set the pretty name. |\n| `--json={short\\|pretty\\|off}` | Show the status as JSON (on newer systemd versions). |\n| `-j` | Short for --json=pretty when stdout is a terminal, else --json=short. |\n| `-H, --host=[USER@]HOST` | Run the command on a remote host over SSH. |\n| `-M, --machine=NAME` | Run the command in a local container. |\n| `--no-ask-password` | Do not ask for a password for authorization. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show the hostname and system facts\n\n```bash\nhostnamectl\n```\n```text\n Static hostname: server1.example.com\n       Icon name: computer-vm\n         Chassis: vm\nOperating System: Red Hat Enterprise Linux 10.0 (Coughlan)\n          Kernel: Linux 6.12.0-55.el10.x86_64\n    Architecture: x86-64\n```\n\n> One command for the name, the OS version, the kernel and whether this is a VM.\n\n### Set the hostname (stays after reboot)\n\n```bash\nhostnamectl set-hostname server1.example.com\n```\n\n> Use the full name (FQDN). Open a new shell to see the new name in the prompt.\n\n### Set the hostname with the newer command word\n\n```bash\nhostnamectl hostname web01.example.com\n```\n\n> Same result as set-hostname on systemd versions that have the short form.\n\n### Print only the static name for a script\n\n```bash\nhostnamectl --static\n```\n```text\nserver1.example.com\n```\n\n> Clean output, one line. Good in provisioning checks.\n\n### Give the server a pretty name for people\n\n```bash\nhostnamectl set-hostname --pretty \"Web server 1 (Riyadh)\"\n```\n\n> The pretty name is only a label; the real hostname does not change.\n\n### Mark a server as production\n\n```bash\nhostnamectl deployment production\nhostnamectl status | grep -i deployment\n```\n\n> Monitoring and inventory tools can read this field and treat production hosts with more care.\n\n### Store the rack location\n\n```bash\nhostnamectl location \"DC-RUH rack 4 U12\"\n```\n\n> The next engineer finds the physical server faster.\n\n### Check the hostname of a remote server\n\n```bash\nhostnamectl -H student@server2\n```\n\n> Works over SSH. Useful to check many servers from one place.\n\n### Read the status as JSON\n\n```bash\nhostnamectl --json=pretty\n```\n\n> Easy to read with `jq` in automation, for example to collect the OS version of every server.\n\n### Check that the name matches the inventory in a pipeline\n\n```bash\ntest \"$(hostnamectl --static)\" = \"web01.example.com\" || { echo \"wrong hostname\"; exit 1; }\n```\n\n> Stop a deployment when the script runs on the wrong server.\n\n### Check the chassis type\n\n```bash\nhostnamectl chassis\n```\n```text\nvm\n```\n\n> Tells you if you are on a virtual machine, a container or real hardware.\n\n## Watch out\n\n- Run as root (or with sudo) to change a name.\n- The prompt of your current shell keeps the old name until you open a new shell.\n- `hostnamectl` does not add the name to `/etc/hosts` or DNS. Other machines still need a DNS record or a hosts entry.\n- Use only letters, numbers, `-` and `.` in a hostname. No spaces, no `_`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/hostname` | The static hostname (one line). |\n| `/etc/machine-info` | The pretty name, icon name, chassis, deployment and location. |\n\n## Tips\n\n- `hostnamectl set-hostname NAME` = change now **and** after reboot.\n- `hostname NAME` = change only until the next reboot.\n\nFull manual: [man hostnamectl](https://linux.die.net/man/1/hostnamectl)\n","page":"commands/hostnamectl/","label":"hostnamectl","where":"Command page · System information","aka":[]},{"id":"cmd-hwclock","n":1436,"slug":"hwclock","title":"hwclock — Read or set the hardware clock (RTC), and copy the time between the hardware clock and the system clock","sections":["commands"],"topic":"Commands · Time","tags":["hardware clock","rtc","bios clock","time"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read or set the hardware clock (RTC), and copy the time between the hardware clock and the system clock.\n\n## About\n\nThe hardware clock keeps time when the machine is off. At boot the kernel sets the system clock from it. `hwclock` shows it, `--systohc` writes the system time into it, `--hctosys` the other way. On RHEL chronyd keeps the RTC in sync (`rtcsync`), so you rarely need it.\n\n## Syntax\n\n```text\nhwclock [FUNCTION] [OPTION]...\n```\n\n## Options\n\n\n### Functions\n\n| Option | What it does |\n|---|---|\n| `-r, --show` | Show the hardware clock time (default). |\n| `--get` | Show the time with drift correction. |\n| `-w, --systohc` | Set the hardware clock from the system clock. |\n| `-s, --hctosys` | Set the system clock from the hardware clock. |\n| `--set --date=DATE` | Set the hardware clock to DATE. |\n| `--systz` | Reset the system time based on the time zone (boot scripts). |\n| `-a, --adjust` | Correct the hardware clock for drift. |\n| `--predict --date=DATE` | Predict the RTC value at DATE. |\n| `--param-get=PARAM, --param-set=PARAM=VALUE` | Read or set RTC parameters. |\n\n### Options\n\n| Option | What it does |\n|---|---|\n| `-u, --utc` | The hardware clock is in UTC. |\n| `-l, --localtime` | The hardware clock is in local time. |\n| `--date=DATE` | Date for --set and --predict. |\n| `--update-drift` | Recalculate the drift factor. |\n| `--noadjfile` | Do not use /etc/adjtime. |\n| `--adjfile=FILE` | Another adjtime file. |\n| `-f, --rtc=FILE` | Another RTC device (default /dev/rtc0). |\n| `--delay=SECONDS` | Delay for setting the RTC. |\n| `--directisa` | Direct ISA access (old hardware). |\n| `--test` | Do everything except change the clock. |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Show the hardware clock\n\n```bash\nhwclock\n```\n```text\n2026-10-25 11:42:10.123456+03:00\n```\n\n> Shown in local time.\n\n### Write the system time to the RTC\n\n```bash\nhwclock --systohc\n```\n\n> After you set the time by hand.\n\n### Set the system time from the RTC\n\n```bash\nhwclock --hctosys\n```\n\n> Rarely needed (boot does it).\n\n### Is the RTC in UTC?\n\n```bash\ngrep -E 'UTC|LOCAL' /etc/adjtime\n```\n```text\nUTC\n```\n\n> Linux servers should use UTC.\n\n### Compare both clocks\n\n```bash\nhwclock; date\n```\n\n> A big difference = RTC battery or sync problem.\n\n### Set the RTC to a date\n\n```bash\nhwclock --set --date=\"2026-10-25 12:00:00\"\n```\n\n> Test only.\n\n### Dry run\n\n```bash\nhwclock --systohc --test\n```\n\n> Shows what would happen.\n\n### Same data in timedatectl\n\n```bash\ntimedatectl | grep RTC\n```\n\n> RTC time and RTC in local TZ.\n\n### Dual boot with Windows\n\n```bash\ntimedatectl set-local-rtc 1\n```\n\n> Windows uses local time in the RTC; avoid on servers.\n\n### Check chrony keeps the RTC in sync\n\n```bash\ngrep rtcsync /etc/chrony.conf\n```\n```text\nrtcsync\n```\n\n> The kernel copies the time to the RTC every 11 minutes.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/adjtime` | Drift data and UTC/LOCAL setting. |\n| `/dev/rtc0` | The RTC device. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man hwclock](https://linux.die.net/man/8/hwclock)\n","page":"commands/hwclock/","label":"hwclock","where":"Command page · Time","aka":[]},{"id":"cmd-id","n":1437,"slug":"id","title":"id — Show the UID, primary group and all groups of a user (or of yourself)","sections":["commands"],"topic":"Commands · Users and groups","tags":["uid","gid","groups","user info","selinux context","effective user"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the UID, primary group and all groups of a user (or of yourself).\n\n## About\n\nWithout a name `id` shows the data of the current process: the user and the groups that are active **now**. With a name it reads the account data, so it already shows new groups that the logged-in user does not have yet.\n\nWith SELinux on, `id` without a name also shows the security context.\n\n## Syntax\n\n```text\nid [OPTION]... [USER]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-u, --user` | Only the UID. |\n| `-g, --group` | Only the primary GID. |\n| `-G, --groups` | All GIDs. |\n| `-n, --name` | Names instead of numbers (with -u, -g, -G). |\n| `-r, --real` | The real ID instead of the effective ID (with -u, -g, -G). |\n| `-Z, --context` | Only the SELinux context. |\n| `-z, --zero` | Separate items with NUL (not with the default output). |\n| `-a` | Ignored (for other systems). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Your own IDs\n\n```bash\nid\n```\n```text\nuid=1000(student) gid=1000(student) groups=1000(student),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023\n```\n\n> You are in wheel, so you may use sudo.\n\n### Another user\n\n```bash\nid lisa\n```\n```text\nuid=1001(lisa) gid=1001(lisa) groups=1001(lisa),1100(webadmins)\n```\n\n> Check new group memberships here.\n\n### Only the UID\n\n```bash\nid -u\n```\n```text\n1000\n```\n\n> In scripts: `[ \"$(id -u)\" -eq 0 ]` tests for root.\n\n### Only the user name\n\n```bash\nid -un\n```\n```text\nstudent\n```\n\n> Same as whoami.\n\n### All group names\n\n```bash\nid -Gn lisa\n```\n```text\nlisa webadmins\n```\n\n> Easy to grep.\n\n### Check if a user is in a group\n\n```bash\nid -nG lisa | grep -qw wheel && echo \"lisa is admin\"\n```\n\n> -w matches the whole word only.\n\n### See the SELinux context\n\n```bash\nid -Z\n```\n```text\nunconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023\n```\n\n> Shows the SELinux user, role and type of your shell.\n\n### Real and effective user\n\n```bash\nsudo id -u; sudo id -ru\n```\n```text\n0\n0\n```\n\n> sudo sets both to root. Programs with setuid only change the effective ID.\n\n### Compare the active groups with the account\n\n```bash\nid -Gn; id -Gn $USER\n```\n\n> When the second list is longer, log in again (or use newgrp).\n\n### Check the user of a container\n\n```bash\npodman run --rm registry.access.redhat.com/ubi10/ubi id\n```\n```text\nuid=0(root) gid=0(root) groups=0(root)\n```\n\n> Root in a container: check it in a security review.\n\n### Check a service account\n\n```bash\nid apache\n```\n```text\nuid=48(apache) gid=48(apache) groups=48(apache)\n```\n\n> A system account (UID under 1000).\n\n## Tips\n\n- id USER = account data. id = what your shell has now.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | The user does not exist or another error. |\n\nFull manual: [man id](https://linux.die.net/man/1/id)\n","page":"commands/id/","label":"id","where":"Command page · Users and groups","aka":[]},{"id":"cmd-info","n":1438,"slug":"info","title":"info — Read the longer GNU documentation (Texinfo manuals) with chapters and links","sections":["commands"],"topic":"Commands · Shell and help","tags":["help","documentation","manual","gnu","texinfo"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read the longer GNU documentation (Texinfo manuals) with chapters and links.\n\nSame page for: `pinfo`\n\n## About\n\nSome tools, mainly the GNU tools (coreutils, bash, grep, sed, tar), have a full manual in **info** format. It is longer than the man page and split into nodes (chapters) with links.\n\nKeys in `info`: `Space` = next page, `n`/`p` = next/previous node, `u` = up, `Enter` on a link = follow it, `l` = back, `s` = search, `q` = quit. `pinfo` is a friendlier viewer with colors (when it is installed).\n\nThe exam objectives name \"man, info and /usr/share/doc\": know where to look.\n\n## Syntax\n\n```text\ninfo [OPTION]... [MENU-ITEM...]\npinfo [OPTIONS] [INFO-PAGE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-k, --apropos=<text>` | Search all indexes of all manuals for the text. |\n| `-f, --file=<manual>` | Open this info manual (file). |\n| `-n, --node=<name>` | Go to this node of the manual. |\n| `--index-search=<text>` | Go to the index entry that matches the text. |\n| `-a, --all` | Use all matching manuals. |\n| `-o, --output=<file>` | Write the selected nodes to a file (as text). |\n| `--subnodes` | With --output: also write the sub-nodes. |\n| `-d, --directory=<dir>` | Add a folder to the search path. |\n| `-w, --where, --location` | Print where the info file is. |\n| `-v, --variable <var>=<value>` | Set an info setting. |\n| `--vi-keys` | Use vi-like keys. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Open the coreutils manual\n\n```bash\ninfo coreutils\n```\n\n> All basic commands (ls, cp, sort …) in one manual with chapters.\n\n### Open the part about one command\n\n```bash\ninfo coreutils 'ls invocation'\n```\n\n> Goes straight to the node about ls.\n\n### Search all info indexes\n\n```bash\ninfo -k \"time style\"\n```\n\n> Lists the nodes where the topic is explained.\n\n### Read the bash manual\n\n```bash\ninfo bash\n```\n\n> The complete Bash reference: expansions, built-ins, variables.\n\n### Save a chapter as text\n\n```bash\ninfo coreutils 'sort invocation' -o sort.txt\n```\n\n> Read it later or add it to your notes.\n\n### Find where an info manual lives\n\n```bash\ninfo -w grep\n```\n```text\n/usr/share/info/grep.info.gz\n```\n\n> Shows the file and the package documentation.\n\n### Use pinfo for easier navigation\n\n```bash\npinfo tar\n```\n\n> Arrow keys and Enter follow links; `q` quits.\n\n### Open the GNU sed manual for regex details\n\n```bash\ninfo sed 'Regular Expressions'\n```\n\n> The info manual explains BRE and ERE better than the man page.\n\n### Look for extra documentation files\n\n```bash\nls /usr/share/doc/chrony/\n```\n\n> Many packages add examples and README files in /usr/share/doc.\n\n### Find documentation files of a package\n\n```bash\nrpm -qd openssh-server\n```\n\n> Lists the documentation files that the package installed.\n\n### Use info when the man page is short\n\n```bash\nman ls | tail -3\n```\n\n> Many GNU man pages end with \"Full documentation … or available locally via: info '(coreutils) ls invocation'\".\n\n## Watch out\n\n- Not every tool has an info manual. If none exists, `info NAME` shows the man page instead.\n- Minimal installs may miss the `info` package; install it with `dnf install info`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/info/` | The info manuals. |\n| `/usr/share/doc/` | Extra documentation, examples and READMEs per package. |\n\n## Tips\n\n- man = short and exact. info = long and with chapters. /usr/share/doc = examples.\n\nFull manual: [man info](https://linux.die.net/man/1/info)\n","page":"commands/info/","label":"info","where":"Command page · Shell and help","aka":["pinfo"]},{"id":"cmd-insmod","n":1439,"slug":"insmod","title":"insmod — Load one module file into the kernel (insmod) or remove a loaded module (rmmod) — without dependency handling","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel modules","load","unload","low level"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Load one module file into the kernel (insmod) or remove a loaded module (rmmod) — without dependency handling.\n\nSame page for: `rmmod`\n\n## About\n\n`insmod FILE` loads exactly that `.ko` file; it does not load dependencies or read /etc/modprobe.d. `rmmod NAME` removes a module that has no users. In daily work use `modprobe` and `modprobe -r` instead; insmod is for testing your own or special modules.\n\n## Syntax\n\n```text\ninsmod FILE [PARAMETER=VALUE]...\nrmmod [-f] [-s] [-v] MODULE...\n```\n\n## Options\n\n\n### insmod\n\n| Option | What it does |\n|---|---|\n| `FILE` | The path of the module file (.ko, .ko.xz). |\n| `PARAMETER=VALUE` | Module parameters. |\n\n### rmmod\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Remove even if in use (very dangerous; may crash). |\n| `-s, --syslog` | Errors to syslog. |\n| `-v, --verbose` | Verbose. |\n| `-V, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Load a file\n\n```bash\ninsmod /lib/modules/$(uname -r)/kernel/fs/fat/fat.ko.xz\n```\n\n> Dependencies must be loaded first by hand.\n\n### Error because of a missing dependency\n\n```bash\ninsmod /lib/modules/$(uname -r)/kernel/fs/fat/vfat.ko.xz\n```\n```text\ninsmod: ERROR: could not insert module …: Unknown symbol in module\n```\n\n> modprobe vfat would load fat automatically.\n\n### Remove a module\n\n```bash\nrmmod vfat\n```\n\n> Must not be in use.\n\n### Module in use\n\n```bash\nrmmod xfs\n```\n```text\nrmmod: ERROR: Module xfs is in use\n```\n\n> Unmount the file systems first.\n\n### With parameters\n\n```bash\ninsmod ./mydriver.ko debug=1\n```\n\n> Testing your own driver.\n\n### Find the file of a module\n\n```bash\nmodinfo -n vfat\n```\n\n> Then insmod that file.\n\n### Prefer modprobe\n\n```bash\nmodprobe vfat; modprobe -r vfat\n```\n\n> Handles dependencies and config.\n\n### Check signature enforcement (Secure Boot)\n\n```bash\nmokutil --sb-state\n```\n\n> With Secure Boot, unsigned modules are refused.\n\n### Log of module loading\n\n```bash\njournalctl -k | grep -i module\n```\n\n> Load and taint messages.\n\n### Check the kernel taint state\n\n```bash\ncat /proc/sys/kernel/tainted\n```\n```text\n0\n```\n\n> Not 0 = an unsigned, out-of-tree or forced module was loaded.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man insmod](https://linux.die.net/man/8/insmod)\n","page":"commands/insmod/","label":"insmod","where":"Command page · Kernel and hardware","aka":["rmmod"]},{"id":"cmd-ip","n":1440,"slug":"ip","title":"ip — Show and change network cards, addresses, routes and neighbours (the live kernel settings)","sections":["commands"],"topic":"Commands · Network","tags":["network","address","route","link","mac","arp","neighbour","gateway","namespace"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change network cards, addresses, routes and neighbours (the live kernel settings).\n\n## About\n\n`ip` talks directly to the Linux kernel. It shows the network exactly as it works right now: every address, every route and the state of every card. Everybody can read; you need root to change.\n\nChanges made with `ip` are **temporary**. They are gone after a reboot or when NetworkManager sets the card again. Use `ip` to look and to test; use `nmcli` to make settings that stay.\n\n`ip` replaces the old tools `ifconfig`, `route` and `arp`. Each part of the network is an **object**: `address`, `link`, `route`, `neigh` and more. Objects and commands can be shortened: `ip a` = `ip address show`, `ip r` = `ip route show`.\n\n## Syntax\n\n```text\nip [OPTIONS] OBJECT { COMMAND | help }\nip address { show | add | del | flush } [ dev IFNAME ] ...\nip link { show | set | add | del } [ dev IFNAME ] ...\nip route { show | get | add | del | replace | flush } ...\nip neigh { show | add | del | flush } ...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `address (a, addr)` | IPv4 and IPv6 addresses on the cards. show, add, del, flush. |\n| `link (l)` | The network cards themselves: state up or down, MAC address, MTU, type. |\n| `route (r)` | The routing table: the default gateway and the other routes. show, get, add, del. |\n| `neigh (n, neighbour)` | Neighbours: the IP to MAC table (ARP for IPv4, NDP for IPv6). |\n| `rule (ru)` | Policy routing rules: which routing table to use for which traffic. |\n| `netns` | Network namespaces, used by containers. list, add, exec, del. |\n| `monitor` | Watch changes of addresses, links and routes live. |\n| `maddress` | Multicast addresses. |\n| `tunnel, tuntap` | Tunnels (GRE, IPIP) and TUN/TAP devices for VPNs. |\n| `xfrm` | IPsec settings. |\n\n## Options\n\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-br, -brief` | Short output: one line per card with only the state and the addresses. |\n| `-c, -color[=always\\|auto\\|never]` | Colors for names, addresses and states. |\n| `-4` | Only IPv4. Short for `-family inet`. |\n| `-6` | Only IPv6. Short for `-family inet6`. |\n| `-f, -family <family>` | Choose the protocol family: inet, inet6, link, bridge, mpls. |\n| `-0` | Only the link layer (MAC addresses). Short for `-family link`. |\n| `-o, -oneline` | Every record on one line. Easy to use with grep and awk. |\n| `-s, -stats, -statistics` | Add counters: packets, errors, dropped. Use it twice for more. |\n| `-d, -details` | Show more details, like the type of a link (vlan, bond, bridge). |\n| `-h, -human, -human-readable` | Show counters in human units (K, M, G). |\n| `-iec` | Human units with powers of 1024 (Ki, Mi). |\n| `-j, -json` | Output in JSON, for scripts and tools like jq. |\n| `-p, -pretty` | With -json: indented JSON that is easy to read. |\n| `-r, -resolve` | Show DNS names instead of addresses. |\n| `-N, -Numeric` | Show numbers for protocols and scopes, not names. |\n| `-t, -timestamp` | With `ip monitor`: add the time to every event. |\n| `-ts, -tshort` | Like -timestamp, but shorter. |\n\n### Work\n\n| Option | What it does |\n|---|---|\n| `-n, -netns <name>` | Run the command inside a network namespace. |\n| `-a, -all` | Run the command on all objects, for example in all namespaces. |\n| `-b, -batch <file>` | Read many ip commands from a file (or - for stdin). |\n| `-force` | In batch mode, go on after errors. |\n| `-l, -loops <count>` | How many times `ip address flush` tries (default 10, 0 = until all are gone). |\n| `-rc, -rcvbuf <size>` | Size of the netlink receive buffer. |\n| `-echo` | Ask the kernel to send back the change that was made. |\n| `-V, -Version` | Show the version. |\n\n### Words used with address, link and route\n\n| Option | What it does |\n|---|---|\n| `dev <IFNAME>` | The network card, like enp1s0. |\n| `via <ADDR>` | The next router (gateway) of a route. |\n| `default` | The default route (0.0.0.0/0 or ::/0). |\n| `metric <n>` | The cost of a route. The lowest metric wins. |\n| `up, down` | With `ip link set`: switch a card on or off. |\n| `mtu <bytes>` | With `ip link set`: the maximum packet size. |\n| `address <MAC>` | With `ip link set`: change the MAC address. |\n| `scope {global\\|link\\|host}` | Where an address is valid. host = only this computer. |\n| `label <name>` | A name for an extra IPv4 address, like enp1s0:1. |\n\n## Examples\n\n### Show all addresses on all cards\n\n```bash\nip a\nip -br -c a\n```\n```text\nlo               UNKNOWN        127.0.0.1/8 ::1/128\nenp1s0           UP             192.168.122.10/24 fe80::5054:ff:fe3a:7c01/64\n```\n\n> -br shows one line per card. This is the fastest health check on a new server.\n\n### Show only the IPv4 address of one card\n\n```bash\nip -4 -o addr show dev enp1s0 | awk '{print $4}'\n```\n```text\n192.168.122.10/24\n```\n\n> -o puts each address on one line, so awk can take the 4th field. Good in scripts.\n\n### Find the default gateway\n\n```bash\nip route show default\n```\n```text\ndefault via 192.168.122.1 dev enp1s0 proto static metric 100\n```\n\n> No default line means the server cannot reach other networks.\n\n### See which route and source address one destination uses\n\n```bash\nip route get 10.20.5.7\n```\n```text\n10.20.5.7 via 192.168.122.254 dev enp1s0 src 192.168.122.10 uid 1000\n```\n\n> Very useful when a server has two cards or a VPN: you see which card and which address the traffic leaves with.\n\n### Check a card for errors and dropped packets\n\n```bash\nip -s -h link show enp1s0\n```\n\n> Growing errors or drops point to a bad cable, a bad driver, or a full buffer. Compare two runs a minute apart.\n\n### Show the MAC addresses of all cards\n\n```bash\nip -br link\n```\n```text\nlo               UNKNOWN        00:00:00:00:00:00 <LOOPBACK,UP,LOWER_UP>\nenp1s0           UP             52:54:00:3a:7c:01 <BROADCAST,MULTICAST,UP,LOWER_UP>\n```\n\n> The MAC address is needed for DHCP reservations and for some software licenses.\n\n### Add a test address for a few minutes (temporary)\n\n```bash\nip addr add 192.168.122.50/24 dev enp1s0\nip addr del 192.168.122.50/24 dev enp1s0\n```\n\n> Good for a quick test. NetworkManager or a reboot removes it anyway. For a fixed address use nmcli.\n\n### Add a route for a test (temporary)\n\n```bash\nip route add 10.30.0.0/16 via 192.168.122.254 dev enp1s0\nip route del 10.30.0.0/16\n```\n\n> Test first with ip. When it works, save it with `nmcli con mod enp1s0 +ipv4.routes \"10.30.0.0/16 192.168.122.254\"`.\n\n### See the neighbours (ARP table)\n\n```bash\nip neigh show\n```\n```text\n192.168.122.1 dev enp1s0 lladdr 52:54:00:aa:bb:cc REACHABLE\n```\n\n> Two IP addresses with the same MAC, or a gateway MAC that changes, can be a sign of ARP spoofing.\n\n### Clear a wrong neighbour entry\n\n```bash\nip neigh flush dev enp1s0\n```\n\n> After you replace a router or move an IP address to another server, old entries can block traffic for a while.\n\n### Switch a card off and on\n\n```bash\nip link set enp1s0 down\nip link set enp1s0 up\n```\n\n> Do not do this over SSH on the card you are using: you lose the connection.\n\n### Change the MTU for a test\n\n```bash\nip link set dev enp2s0 mtu 9000\nip link show enp2s0 | grep -o 'mtu [0-9]*'\n```\n```text\nmtu 9000\n```\n\n> Jumbo frames help for storage networks. All switches and servers on the path must use the same MTU.\n\n### Get addresses as JSON for a script\n\n```bash\nip -j addr show dev enp1s0 | jq -r '.[0].addr_info[] | select(.family==\"inet\") | .local'\n```\n```text\n192.168.122.10\n```\n\n> JSON output does not change between versions, so scripts do not break.\n\n### Look inside a container network namespace\n\n```bash\nip netns list\nip netns exec blue ip -br a\n```\n\n> Containers and some VPN tools use namespaces. Each one has its own cards and routes.\n\n### Watch network changes live\n\n```bash\nip -t monitor address route\n```\n\n> Shows every address and route change with the time. Press Ctrl+C to stop. Good when a DHCP lease or a VPN changes routes.\n\n## Watch out\n\n- All changes with `ip` are lost at reboot. NetworkManager can also overwrite them earlier.\n- `ip link set DEV down` over SSH on the same card cuts your connection.\n- `ip addr flush dev DEV` removes **all** addresses of the card.\n- Read the mask: `192.168.122.10/24` and `192.168.122.10/32` are very different.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/iproute2/rt_tables` | Names of the routing tables (used by policy routing). |\n| `/proc/net/dev` | Raw counters of all cards (what `ip -s link` shows). |\n| `/sys/class/net/` | One folder per network card with its settings. |\n\n## Tips\n\n- Read the objects as questions: **a** = which addresses? **r** = which way? **l** = which cards? **n** = who is next to me?\n- `ip -br a` and `ip r` are the two first commands on any broken network.\n- Look with `ip`, change with `nmcli`.\n\nFull manual: [man ip](https://linux.die.net/man/8/ip)\n","page":"commands/ip/","label":"ip","where":"Command page · Network","aka":[]},{"id":"cmd-jobs","n":1441,"slug":"jobs","title":"jobs — List the jobs of your shell, and move them between foreground (fg) and background (bg)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["shell jobs","background","foreground","ctrl-z","job control"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List the jobs of your shell, and move them between foreground (fg) and background (bg).\n\nSame page for: `bg`, `fg`, `disown`\n\n## About\n\nA **job** is a command (or pipeline) started from your current shell. `COMMAND &` starts it in the background; Ctrl+Z pauses the foreground job. `jobs` lists them with a number: `%1`, `%2` …; `+` marks the current job, `-` the previous one.\n\n`bg` continues a paused job in the background, `fg` brings a job to the foreground, and `disown` removes a job from the shell, so it keeps running when you log out. These are bash built-ins.\n\n## Syntax\n\n```text\njobs [-lnprs] [JOBSPEC...]\nbg [JOBSPEC...]\nfg [JOBSPEC]\ndisown [-ar] [-h] [JOBSPEC...]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `%N` | Job number N. |\n| `%+ or %%` | The current job. |\n| `%-` | The previous job. |\n| `%STRING` | The job whose command starts with STRING. |\n| `%?STRING` | The job whose command contains STRING. |\n\n## Options\n\n\n### jobs\n\n| Option | What it does |\n|---|---|\n| `-l` | Also show the PIDs. |\n| `-p` | Show only the PIDs (of the process group leaders). |\n| `-r` | Only running jobs. |\n| `-s` | Only stopped jobs. |\n| `-n` | Only jobs that changed state since the last report. |\n\n### disown\n\n| Option | What it does |\n|---|---|\n| `-h` | Keep the job in the list, but do not send it SIGHUP at logout. |\n| `-a` | All jobs. |\n| `-r` | Only running jobs. |\n\n## Examples\n\n### Start a job in the background\n\n```bash\nsleep 600 &\n```\n```text\n[1] 4123\n```\n\n> Job number 1, PID 4123.\n\n### List the jobs\n\n```bash\njobs -l\n```\n```text\n[1]+  4123 Running                 sleep 600 &\n```\n\n> -l adds the PID.\n\n### Pause and continue in the background\n\n```bash\ntar -czf /tmp/home.tgz ~\n```\n```text\n^Z\n[1]+  Stopped                 tar -czf /tmp/home.tgz ~\n```\n```bash\nbg\n```\n\n> Ctrl+Z, then bg: the terminal is free again.\n\n### Bring a job back\n\n```bash\nfg %1\n```\n\n> Now Ctrl+C would stop it.\n\n### Stop a background job\n\n```bash\nkill %1\n```\n\n> Job numbers work with kill.\n\n### Keep a job running after logout\n\n```bash\n./long-task.sh &\ndisown -h %1\n```\n\n> Or start it with nohup.\n\n### Show only stopped jobs\n\n```bash\njobs -s\n```\n\n> Forgotten paused jobs block logout (\"There are stopped jobs\").\n\n### Use the job by name\n\n```bash\nfg %tar\n```\n\n> The job whose command starts with tar.\n\n### PIDs for other commands\n\n```bash\nrenice -n 10 -p $(jobs -p)\n```\n\n> Makes all your jobs nicer.\n\n### Wait for all background jobs in a script\n\n```bash\nfor h in server1 server2; do ssh $h 'dnf -y upgrade' & done; wait\n```\n\n> Runs in parallel; `wait` waits for all.\n\n## Watch out\n\n- Jobs belong to one shell. Another terminal does not see them.\n- A background job that wants input stops (\"Stopped (tty input)\").\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | No such job. |\n\nFull manual: [man jobs](https://linux.die.net/man/1/bash)\n","page":"commands/jobs/","label":"jobs","where":"Command page · Processes and jobs","aka":["bg","fg","disown"]},{"id":"cmd-journalctl","n":1442,"slug":"journalctl","title":"journalctl — Read and search the systemd journal: by service, time, priority, boot, user and any field","sections":["commands"],"topic":"Commands · Logs","tags":["journal","logs","systemd-journald","filter","boot","priority","troubleshoot","audit"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read and search the systemd journal: by service, time, priority, boot, user and any field.\n\n## About\n\njournald stores messages of the kernel, the boot, services and programs with many fields (unit, PID, UID, priority …). `journalctl` shows them, oldest first, in a pager. Filters can be combined: `journalctl -u sshd -p warning --since today`.\n\nNormal users see their own messages; members of `wheel`, `adm` and `systemd-journal` see all. The journal is persistent only if `/var/log/journal` exists (or `Storage=persistent`).\n\n## Syntax\n\n```text\njournalctl [OPTION]... [FIELD=VALUE]...\n```\n\n## Options\n\n\n### Filter\n\n| Option | What it does |\n|---|---|\n| `-u, --unit=UNIT` | Messages of a unit (pattern allowed). |\n| `--user-unit=UNIT` | Messages of a user unit. |\n| `-p, --priority=PRIO` | Priority or range: emerg, alert, crit, err, warning, notice, info, debug (or 0-7, err..warning). |\n| `-S, --since=TIME` | From this time: `today`, `yesterday`, `-1h`, `\"2026-10-25 08:00\"`. |\n| `-U, --until=TIME` | Until this time. |\n| `-b, --boot[=ID\\|±N\\|all]` | This boot, or another (-1 = the boot before). |\n| `-k, --dmesg` | Only kernel messages (implies -b). |\n| `-t, --identifier=ID` | Messages with this syslog tag. |\n| `-g, --grep=PATTERN` | Messages that match a regular expression. |\n| `--case-sensitive[=BOOL]` | Case for --grep. |\n| `--facility=FACILITY` | Syslog facility. |\n| `FIELD=VALUE` | Field filter, like `_PID=1010`, `_UID=1001`, `_COMM=sudo`, `_SYSTEMD_UNIT=sshd.service`. |\n| `-c, --cursor=CURSOR, --after-cursor=CURSOR, --cursor-file=FILE` | Start at a saved position. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-f, --follow` | Show new messages live. |\n| `-n, --lines=N` | The last N lines (default 10). |\n| `-r, --reverse` | Newest first. |\n| `-e, --pager-end` | Jump to the end in the pager. |\n| `-o, --output=MODE` | Format: short, short-iso, short-precise, verbose, json, json-pretty, cat, export … |\n| `--output-fields=LIST` | Fields to show with json/verbose. |\n| `-x, --catalog` | Add explanations from the message catalog. |\n| `-a, --all` | Show all fields, also long or unprintable ones. |\n| `-l, --full, --no-full` | Do not cut / cut long lines. |\n| `--no-hostname` | Hide the host name. |\n| `--utc` | Times in UTC. |\n| `--truncate-newline` | Only the first line of multi-line messages. |\n| `--show-cursor` | Print the cursor at the end. |\n| `-q, --quiet` | No info messages. |\n| `--no-pager` | Do not use a pager. |\n| `--no-tail` | With -f: show all lines first. |\n\n### Sources\n\n| Option | What it does |\n|---|---|\n| `--system, --user` | System or user journal. |\n| `-m, --merge` | Merge all journals (also remote ones). |\n| `-D, --directory=DIR` | Read the journal in this folder. |\n| `--file=GLOB` | Read these journal files. |\n| `--root=ROOT, --image=IMAGE, --image-policy=POLICY` | Read from another root or image. |\n| `--namespace=NAME` | A journal namespace. |\n| `-M, --machine=NAME` | A container's journal. |\n\n### Fields and boots\n\n| Option | What it does |\n|---|---|\n| `-F, --field=FIELD` | List all values of a field (like `_SYSTEMD_UNIT`). |\n| `-N, --fields` | List all field names. |\n| `--list-boots` | List the stored boots. |\n\n### Maintenance\n\n| Option | What it does |\n|---|---|\n| `--disk-usage` | Space used by the journal. |\n| `--vacuum-size=SIZE` | Delete old files until the journal is under this size. |\n| `--vacuum-time=TIME` | Delete entries older than this. |\n| `--vacuum-files=N` | Keep only N files. |\n| `--rotate` | Start new journal files. |\n| `--flush` | Move the memory journal to /var/log/journal. |\n| `--sync` | Write all pending messages to disk. |\n| `--relinquish-var, --smart-relinquish-var` | Stop writing to /var (before unmounting it). |\n| `--verify` | Check the journal files for damage. |\n| `--setup-keys, --verify-key=KEY, --interval=TIME, --force` | Forward Secure Sealing (tamper proof logs). |\n| `--header` | Show the file headers. |\n| `--list-catalog, --dump-catalog, --update-catalog` | Message catalog tools. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Messages of one service\n\n```bash\njournalctl -u sshd\n```\n\n> Everything sshd logged.\n\n### Errors since the boot\n\n```bash\njournalctl -p err -b\n```\n\n> The first look after a problem.\n\n### Follow a service live\n\n```bash\njournalctl -fu httpd\n```\n\n> Leave it open while you test.\n\n### A time window\n\n```bash\njournalctl --since \"2026-10-25 08:00\" --until \"2026-10-25 09:00\"\n```\n\n> Also: `--since -2h`, `--since yesterday`.\n\n### The boot before\n\n```bash\njournalctl --list-boots\njournalctl -b -1 -p warning\n```\n\n> Why did the server crash or reboot?\n\n### Kernel messages\n\n```bash\njournalctl -k -p warning\n```\n\n> Disk, memory and driver problems.\n\n### All sudo commands today (audit)\n\n```bash\njournalctl _COMM=sudo --since today\n```\n\n> User, folder and command of each sudo call.\n\n### Failed SSH logins (security)\n\n```bash\njournalctl -u sshd --since today -g 'Failed password|Invalid user'\n```\n\n> Many lines from one IP = a brute-force attack.\n\n### Messages of one user\n\n```bash\njournalctl _UID=1001 --since today\n```\n\n> What lisa's processes logged.\n\n### All fields of one entry\n\n```bash\njournalctl -u sshd -n 1 -o verbose\n```\n\n> Shows the fields you can filter on.\n\n### JSON for tools\n\n```bash\njournalctl -u myapp -o json --since -10min | jq -r '.MESSAGE'\n```\n\n> Feed a SIEM or script.\n\n### Explain a failure\n\n```bash\njournalctl -xeu httpd\n```\n\n> -x adds help text, -e jumps to the end.\n\n### Disk use and cleanup\n\n```bash\njournalctl --disk-usage\njournalctl --vacuum-time=30d\n```\n\n> Removes entries older than 30 days.\n\n### Which units logged at all?\n\n```bash\njournalctl -F _SYSTEMD_UNIT | sort | head\n```\n\n> Lists every unit name in the journal.\n\n### Check the journal for damage\n\n```bash\njournalctl --verify\n```\n\n> PASS/FAIL per file.\n\n## Watch out\n\n- Without a persistent journal, `-b -1` shows nothing: older boots are gone.\n- Times in --since need quotes when they contain a space.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/run/log/journal/` | Journal in memory (lost at reboot). |\n| `/var/log/journal/` | Journal on disk (persistent). |\n| `/etc/systemd/journald.conf, /etc/systemd/journald.conf.d/` | journald settings. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (or no match with some options). |\n\nFull manual: [man journalctl](https://linux.die.net/man/1/journalctl)\n","page":"commands/journalctl/","label":"journalctl","where":"Command page · Logs","aka":[]},{"id":"cmd-kill","n":1443,"slug":"kill","title":"kill — Send a signal to processes by PID (or job number) — by default SIGTERM, a polite request to stop","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["signal","stop process","sigterm","sigkill","sighup","pid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Send a signal to processes by PID (or job number) — by default SIGTERM, a polite request to stop.\n\n## About\n\n`kill PID` sends SIGTERM (15): the program can save its data and exit. If it does not react, `kill -9 PID` (SIGKILL) makes the kernel end it at once — without cleanup. Other signals do other things: SIGHUP (1) makes many daemons re-read their config, SIGSTOP/SIGCONT pause and continue.\n\nUsers can only signal their own processes; root can signal all. In bash, `kill` is a built-in that also accepts job numbers like `%1`.\n\n## Syntax\n\n```text\nkill [-s SIGNAL | -SIGNAL] PID...\nkill -l [NUMBER] | -L\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `1 HUP` | Hang up; daemons often reload the config. |\n| `2 INT` | Interrupt (Ctrl+C). |\n| `3 QUIT` | Quit with a core dump. |\n| `9 KILL` | Kill now; cannot be caught. |\n| `15 TERM` | Terminate politely (default). |\n| `18 CONT` | Continue a paused process. |\n| `19 STOP` | Pause; cannot be caught. |\n| `20 TSTP` | Pause from the terminal (Ctrl+Z). |\n| `10 USR1, 12 USR2` | Program-specific (like reopen logs). |\n| `-1 (PID)` | All processes you may signal (dangerous). |\n| `-PGID` | A negative PID = the whole process group. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --signal SIGNAL` | The signal (name or number). |\n| `-SIGNAL` | Short form: `-9`, `-KILL`, `-SIGKILL`. |\n| `-l, --list [NUMBER]` | List the signal names (or the name of a number). |\n| `-L, --table` | List names and numbers as a table. |\n| `-q, --queue VALUE` | Send the signal with a value (sigqueue). |\n| `-p, --pid` | Only print the PIDs (util-linux kill, with a name). |\n| `-a, --all` | Also match processes of other users by name (util-linux kill). |\n| `--timeout MS SIGNAL` | Send a second signal if the process still runs after the time (util-linux kill). |\n\n## Examples\n\n### Stop a process politely\n\n```bash\nkill 4123\n```\n\n> SIGTERM: the program can clean up.\n\n### Force it\n\n```bash\nkill -9 4123\n```\n\n> Only when SIGTERM did not work after some seconds.\n\n### Reload a daemon's config\n\n```bash\nkill -HUP $(cat /run/sshd.pid)\n```\n\n> On RHEL `systemctl reload sshd` does the same.\n\n### List the signals\n\n```bash\nkill -l\n```\n\n> Names and numbers.\n\n### Stop a background job\n\n```bash\nkill %2\n```\n\n> Job number from `jobs`.\n\n### Pause and continue a process\n\n```bash\nkill -STOP 4123; kill -CONT 4123\n```\n\n> Pause a heavy job for a while.\n\n### Polite, then hard after a timeout\n\n```bash\n/usr/bin/kill --timeout 5000 KILL -TERM 4123\n```\n\n> util-linux kill: SIGKILL if it still runs after 5 seconds.\n\n### Stop a whole pipeline (process group)\n\n```bash\nkill -TERM -4120\n```\n\n> Negative number = process group.\n\n### Contain a suspicious process (security)\n\n```bash\nkill -STOP 6666\nls -l /proc/6666/exe; cat /proc/6666/cmdline | tr '\\0' ' '\n```\n\n> Pause it, look at it, then decide; a killed process loses evidence in memory.\n\n### Check if a process is still alive\n\n```bash\nkill -0 4123 && echo \"running\"\n```\n\n> Signal 0 sends nothing; it only tests.\n\n### Make a service reopen its log files\n\n```bash\nkill -USR1 $(pidof nginx)\n```\n\n> nginx reopens logs on USR1 (log rotation).\n\n## Watch out\n\n- `kill -9` can leave broken files, lock files and shared memory. Use it last.\n- Processes in state D (disk wait) do not die even with -9 until the I/O ends.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The signal was sent. |\n| `1` | Error (no such process or not allowed). |\n\nFull manual: [man kill](https://linux.die.net/man/1/kill)\n","page":"commands/kill/","label":"kill","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-killall","n":1444,"slug":"killall","title":"killall — Send a signal to all processes with a given name","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["signal","kill by name","stop process"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Send a signal to all processes with a given name.\n\n## About\n\n`killall NAME` sends SIGTERM to every process with exactly that command name. It is handy when a program has many processes. Options select by user, age or a regular expression, and `-i` asks before each one.\n\n## Syntax\n\n```text\nkillall [OPTION]... [-s SIGNAL | -SIGNAL] NAME...\nkillall -l\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --signal SIGNAL, -SIGNAL` | The signal to send (default TERM). |\n| `-u, --user USER` | Only processes of this user. |\n| `-i, --interactive` | Ask before each process. |\n| `-e, --exact` | Exact match for long names (over 15 characters). |\n| `-I, --ignore-case` | Ignore upper and lower case. |\n| `-r, --regexp` | NAME is an extended regular expression. |\n| `-o, --older-than TIME` | Only processes older than TIME (like 30m, 2h). |\n| `-y, --younger-than TIME` | Only processes younger than TIME. |\n| `-w, --wait` | Wait until the processes are gone. |\n| `-g, --process-group` | Signal the process group. |\n| `-q, --quiet` | No message if nothing was killed. |\n| `-v, --verbose` | Report each signal. |\n| `-n, --ns PID` | Only processes in the same PID namespace as PID. |\n| `-Z, --context PATTERN` | Only processes with this SELinux context. |\n| `-l, --list` | List the signal names. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Stop all processes with a name\n\n```bash\nkillall dd\n```\n\n> Every dd of yours.\n\n### Force\n\n```bash\nkillall -9 stuck-app\n```\n\n> When SIGTERM did not help.\n\n### Only one user's processes\n\n```bash\nkillall -u lisa firefox\n```\n\n> Other users' firefox keeps running.\n\n### Ask for each one\n\n```bash\nkillall -i bash\n```\n```text\nKill bash(4301) ? (y/N)\n```\n\n> Safe when you are not sure.\n\n### Wait until they are gone\n\n```bash\nkillall -w myapp && systemctl start myapp\n```\n\n> No race between stop and start.\n\n### Old processes only\n\n```bash\nkillall -o 2h find\n```\n\n> Stops find jobs that run more than 2 hours.\n\n### With a regular expression\n\n```bash\nkillall -r '^python3\\.1[0-9]$'\n```\n\n> Matches python3.10 … python3.19.\n\n### Reload many processes\n\n```bash\nkillall -HUP rsyslogd\n```\n\n> Same as a reload.\n\n### Quiet in scripts\n\n```bash\nkillall -q sleep || true\n```\n\n> No message when none run.\n\n### Verbose report\n\n```bash\nkillall -v dd\n```\n```text\nKilled dd(4123) with signal 15\n```\n\n> Shows what happened.\n\n## Watch out\n\n- On some other UNIX systems `killall` kills **all** processes. Use it only on Linux.\n- Names are cut to 15 characters; use -e for long names.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | At least one process got the signal. |\n| `1` | No process was killed. |\n\nFull manual: [man killall](https://linux.die.net/man/1/killall)\n","page":"commands/killall/","label":"killall","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-less","n":1445,"slug":"less","title":"less — Read a file or command output one screen at a time, with scrolling and searching","sections":["commands"],"topic":"Commands · Text and search","tags":["pager","read","view","log","search","follow"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read a file or command output one screen at a time, with scrolling and searching.\n\n## About\n\n`less` is a **pager**: it shows text page by page and lets you move up and down, search and follow a growing file. It reads only what it needs, so it opens huge log files at once. It never changes the file.\n\n`man`, `journalctl` and `git` use less to show their output. The keys are the same everywhere.\n\n## Syntax\n\n```text\nless [OPTIONS] [FILE...]\nCOMMAND | less [OPTIONS]\n```\n\n## Parts\n\n\n### Keys inside less\n\n| Part | Meaning |\n|---|---|\n| `Space, f, PgDn` | One page down. |\n| `b, PgUp` | One page up. |\n| `Enter, j, ↓` | One line down. |\n| `k, ↑` | One line up. |\n| `d / u` | Half a page down / up. |\n| `g / G` | First line / last line. 50g = line 50. |\n| `/pattern` | Search forward (a regular expression). |\n| `?pattern` | Search backward. |\n| `n / N` | Next / previous match. |\n| `&pattern` | Show only lines that match (filter). & alone = all lines again. |\n| `F` | Follow the end of the file, like tail -f (Ctrl+C stops following). |\n| `m<letter> / '<letter>` | Set a mark / go back to a mark. |\n| `-N (typed in less)` | Turn line numbers on or off while reading. |\n| `:n / :p` | Next / previous file (when you opened several). |\n| `v` | Open the file in your editor ($VISUAL or $EDITOR). |\n| `h` | Help with all keys. |\n| `q` | Quit. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-N, --LINE-NUMBERS` | Show line numbers. |\n| `-i, --ignore-case` | Searches ignore case, unless the pattern has capitals. |\n| `-I, --IGNORE-CASE` | Searches always ignore case. |\n| `+F` | Start in follow mode (like tail -f). |\n| `+<cmd>` | Run a less command at the start, like `+G` (go to the end) or `+/error`. |\n| `-S, --chop-long-lines` | Do not wrap long lines; scroll sideways instead. |\n| `-R, --RAW-CONTROL-CHARS` | Show colors (ANSI codes) correctly. |\n| `-r, --raw-control-chars` | Show all control characters raw. |\n| `-F, --quit-if-one-screen` | Quit at once when the text fits on one screen. |\n| `-X, --no-init` | Do not clear the screen when you quit. |\n| `-p, --pattern=<pattern>` | Start at the first match of the pattern. |\n| `-s, --squeeze-blank-lines` | Show many empty lines as one. |\n| `-x, --tabs=<n>` | Tab stops every N columns. |\n| `-g, --hilite-search` | Highlight only the last match (default: all). |\n| `-G, --HILITE-SEARCH` | Do not highlight matches. |\n| `-J, --status-column` | Show a column that marks lines with matches. |\n| `-m, --long-prompt / -M, --LONG-PROMPT` | Show more info in the prompt (percent, lines). |\n| `-n, --line-numbers` | Do not count line numbers (faster on huge files). |\n| `-e, --quit-at-eof / -E, --QUIT-AT-EOF` | Quit at the second / first time you reach the end. |\n| `-c, --clear-screen` | Repaint from the top instead of scrolling. |\n| `-a, --search-skip-screen` | Start searching after the visible screen. |\n| `-j, --jump-target=<n>` | Show matches on line N of the screen. |\n| `-z, --window=<n>` | Scroll N lines per page. |\n| `-h, --max-back-scroll=<n>, -y, --max-forw-scroll=<n>` | Limit scrolling. |\n| `-b, --buffers=<n>, -B, --auto-buffers` | Control the memory buffers. |\n| `-o, --log-file=<file>` | Copy the input to a file (when reading a pipe). |\n| `-k, --lesskey-file=<file>` | Use a key binding file. |\n| `-L, --no-lessopen` | Ignore the LESSOPEN preprocessor. |\n| `-K, --quit-on-intr` | Quit when you press Ctrl+C. |\n| `-q, --quiet` | No bell sound. |\n| `-w, --hilite-unread` | Highlight the first new line after a page move. |\n| `-u, --underline-special` | Treat backspaces and carriage returns as normal characters. |\n| `-~, --tilde` | Do not show ~ after the end of the file. |\n| `-#, --shift=<n>` | Scroll N columns sideways with ← →. |\n| `--mouse` | Use the mouse wheel to scroll. |\n| `--incsearch` | Search while you type the pattern. |\n| `--use-color` | Use colors in the less interface. |\n| `--follow-name` | In follow mode, reopen the file when it is replaced (log rotation). |\n| `-V, --version` | Show the version. |\n| `-?, --help` | Show help. |\n\n## Examples\n\n### Read a log file\n\n```bash\nless /var/log/messages\n```\n\n> Space = next page, `/` = search, `q` = quit.\n\n### Start at the end of a file\n\n```bash\nless +G /var/log/secure\n```\n\n> The newest lines are at the end of logs.\n\n### Follow a log live\n\n```bash\nless +F /var/log/messages\n```\n\n> Like `tail -f`, but press Ctrl+C to stop following and search back, then F to follow again.\n\n### Read command output page by page\n\n```bash\ndnf list installed | less\n```\n\n> Any long output becomes easy to read.\n\n### Show line numbers\n\n```bash\nless -N /etc/ssh/sshd_config\n```\n\n> Helpful when an error message names a line.\n\n### Long lines without wrapping\n\n```bash\nless -S /var/log/httpd/access_log\n```\n\n> Each log line stays on one screen line; use ← → to scroll.\n\n### Keep colors from a command\n\n```bash\ngrep --color=always -n error app.log | less -R\n```\n\n> `-R` shows the color codes as colors.\n\n### Open at the first match of a word\n\n```bash\nless -p \"Out of memory\" /var/log/messages\n```\n\n> Jumps straight to the first OOM message.\n\n### Search without caring about case\n\n```bash\nless -i /var/log/messages\n```\n```text\n/fail\n```\n\n> Finds fail, Fail and FAIL.\n\n### Show only matching lines inside less\n\n```bash\nless /var/log/secure\n```\n```text\n&Accepted\n```\n\n> `&` filters the view; type `&` and Enter to see all lines again.\n\n### Follow a log that is rotated\n\n```bash\nless --follow-name +F /var/log/myapp/app.log\n```\n\n> Keeps following the new file after logrotate.\n\n### Quit at once when output is short\n\n```bash\ngit log --oneline | less -FX\n```\n\n> Short output is printed and less ends; long output gets a pager.\n\n## Watch out\n\n- In follow mode (F), the keys do not work until you press Ctrl+C.\n- less opens binary files too; if the screen looks broken, quit and use `file` first.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.lesshst` | The history of searches. |\n| `~/.config/lesskey` | Your own key bindings. |\n\n## Tips\n\n- Space down, b up, / search, n next, G end, g start, F follow, q quit.\n- \"less is more\": less can do everything more can, and scroll back.\n\nFull manual: [man less](https://linux.die.net/man/1/less)\n","page":"commands/less/","label":"less","where":"Command page · Text and search","aka":[]},{"id":"cmd-ln","n":1446,"slug":"ln","title":"ln — Make links: hard links (a second name for the same data) or symbolic links (a pointer to a path)","sections":["commands"],"topic":"Commands · Files and folders","tags":["link","symlink","hard link","soft link","inode","shortcut"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Make links: hard links (a second name for the same data) or symbolic links (a pointer to a path).\n\n## About\n\n- A **hard link** is another directory entry for the same inode. Both names are equal; the data stays until the last name is removed. Only for files, and only inside one file system.\n- A **symbolic (soft) link** is a small file that contains a path. It can point to folders and to other file systems. If the target is removed, the link is broken.\n\nRemember the order: `ln -s TARGET LINKNAME` — first what exists, then the new name (like `cp`).\n\n## Syntax\n\n```text\nln [OPTION]... TARGET LINK_NAME\nln [OPTION]... TARGET\nln [OPTION]... TARGET... DIRECTORY\nln [OPTION]... -t DIRECTORY TARGET...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --symbolic` | Make a symbolic link instead of a hard link. |\n| `-f, --force` | Remove an existing target name first. |\n| `-n, --no-dereference` | If LINK_NAME is a link to a folder, treat it as a normal file (use with -f to replace the link). |\n| `-r, --relative` | With -s: make the link path relative to the link's place. |\n| `-i, --interactive` | Ask before removing an existing name. |\n| `-v, --verbose` | Print each link made. |\n| `-t, --target-directory=<dir>` | Make the links in this folder. |\n| `-T, --no-target-directory` | Treat LINK_NAME as a normal file always. |\n| `-L, --logical` | For a hard link to a symlink: link to the file the symlink points to. |\n| `-P, --physical` | For a hard link to a symlink: link to the symlink itself. |\n| `-d, -F, --directory` | Let root try to hard link folders (almost always fails). |\n| `--backup[=<control>]` | Make a backup of each existing name. |\n| `-b` | Like --backup without an argument. |\n| `-S, --suffix=<suffix>` | The ending of backup files. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Make a symbolic link\n\n```bash\nln -s /etc/httpd/conf/httpd.conf ~/httpd.conf\nls -l ~/httpd.conf\n```\n```text\nlrwxrwxrwx. 1 student student 26 Oct 25 10:00 /home/student/httpd.conf -> /etc/httpd/conf/httpd.conf\n```\n\n> The `l` and the arrow show the link and its target.\n\n### Make a hard link\n\n```bash\nln data.txt data-copy.txt\nls -li data.txt data-copy.txt\n```\n\n> Same inode number, link count 2. Both names are the same file.\n\n### Replace an existing link (switch a release)\n\n```bash\nln -sfn /srv/app/releases/43 /srv/app/current\n```\n\n> `-f` replaces, `-n` treats the old link as a file (not as the folder it points to).\n\n### Make a relative link that survives moving the tree\n\n```bash\nln -sr /srv/app/shared/config.yml /srv/app/current/config.yml\n```\n\n> The link stores a path like ../shared/config.yml.\n\n### Link a program into your PATH\n\n```bash\nln -s /opt/tool/bin/tool ~/.local/bin/tool\n```\n\n> Now you can run `tool` from anywhere.\n\n### Find broken links\n\n```bash\nfind /etc -xtype l\n```\n\n> Links whose target is gone. Clean them after removing software.\n\n### Show where a link points\n\n```bash\nreadlink -f /usr/bin/python3\n```\n\n> `-f` follows all links to the final file.\n\n### Keep log files of an app on a bigger disk\n\n```bash\nmv /var/lib/app/logs /data/app-logs && ln -s /data/app-logs /var/lib/app/logs\n```\n\n> The app still writes to the old path. Check SELinux labels on the new place.\n\n### Make several links in a folder\n\n```bash\nln -s -t /usr/local/bin /opt/tools/bin/*\n```\n\n> One link per program in /usr/local/bin.\n\n### See how many names a file has\n\n```bash\nstat -c '%h %n' /etc/passwd\n```\n\n> `%h` = the number of hard links.\n\n### Show each link that is made\n\n```bash\nln -sv /etc/os-release ~/os\n```\n```text\n'/home/student/os' -> '/etc/os-release'\n```\n\n> Clear output for scripts and logs.\n\n## Watch out\n\n- The order is TARGET then LINK. Swapping them is the most common mistake.\n- A relative target in `ln -s` is relative to the **link's folder**, not to your current folder. Use `-r` or an absolute path.\n- Hard links do not work across file systems (\"Invalid cross-device link\") or to folders.\n- A normal user cannot hard link a file of another user (kernel protection `fs.protected_hardlinks`).\n\n## Tips\n\n- `ln -s` = shortcut (soft). `ln` = second name (hard).\n- `ln -sfn` = replace a link safely.\n\nFull manual: [man ln](https://linux.die.net/man/1/ln)\n","page":"commands/ln/","label":"ln","where":"Command page · Files and folders","aka":[]},{"id":"cmd-logger","n":1447,"slug":"logger","title":"logger — Write a message to the system log (journal and rsyslog) — from the command line or a script","sections":["commands"],"topic":"Commands · Logs","tags":["syslog","write log","test","script logging","facility","priority"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Write a message to the system log (journal and rsyslog) — from the command line or a script.\n\n## About\n\n`logger MESSAGE` sends a message with facility `user` and priority `notice`. With `-p FACILITY.PRIORITY` and `-t TAG` you control where it goes and how it is marked. Use it to test rsyslog rules and to log what your scripts do. It can also send to a remote syslog server.\n\n## Syntax\n\n```text\nlogger [OPTION]... [MESSAGE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --priority FACILITY.LEVEL` | Facility and priority, like `local4.warning` (default user.notice). |\n| `-t, --tag TAG` | Mark the line with this tag (default: your user name). |\n| `-i` | Add the PID of logger. |\n| `--id[=ID]` | Add this ID (or the PID) to the line. |\n| `-f, --file FILE` | Log the content of a file, line by line. |\n| `-e, --skip-empty` | Skip empty lines. |\n| `-s, --stderr` | Also print the message on the screen. |\n| `-n, --server SERVER` | Send to this remote syslog server. |\n| `-P, --port PORT` | Remote port (default 514). |\n| `-T, --tcp` | Use TCP. |\n| `-d, --udp` | Use UDP. |\n| `-u, --socket SOCKET` | Write to this socket instead of the default. |\n| `--socket-errors[=MODE]` | Report socket errors (on, off, auto). |\n| `--journald[=FILE]` | Write directly to the journal, with your own fields from a file. |\n| `--msgid ID` | RFC 5424 message ID. |\n| `--sd-id NAME, --sd-param NAME=VALUE` | RFC 5424 structured data. |\n| `--rfc3164` | Use the old BSD syslog format. |\n| `--rfc5424[=without]` | Use the RFC 5424 format. |\n| `--octet-count` | Use octet counting framing (RFC 6587). |\n| `--prio-prefix` | Read the priority from a <N> prefix in each line. |\n| `-S, --size SIZE` | Maximum message size. |\n| `--no-act` | Do everything except write the message. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Write a simple message\n\n```bash\nlogger \"backup started\"\njournalctl -n 1\n```\n```text\nOct 25 11:05:12 server1 student[4410]: backup started\n```\n\n> The tag is your user name.\n\n### With a tag and priority\n\n```bash\nlogger -t backup -p user.err \"backup failed: disk full\"\n```\n\n> Easy to find with `journalctl -t backup -p err`.\n\n### Test an rsyslog rule\n\n```bash\nlogger -p local4.warning \"test for backup.log\"\ntail -1 /var/log/backup.log\n```\n\n> Proves the rule works.\n\n### Also show it on the screen\n\n```bash\nlogger -s -t deploy \"version 2.4 deployed\"\n```\n```text\n<13>Oct 25 11:06:01 deploy: version 2.4 deployed\n```\n\n> -s prints to stderr too.\n\n### Log a whole file\n\n```bash\nlogger -t import -f /var/tmp/import-report.txt\n```\n\n> One log line per file line.\n\n### Log from a script\n\n```bash\nlogger -t myscript -p user.info \"$(date +%F) cleaned $(find /var/tmp -mtime +7 | wc -l) files\"\n```\n\n> Your script results in the central logs.\n\n### Send to a remote syslog server\n\n```bash\nlogger -n logserver.example.com -T -P 514 -t test \"remote test\"\n```\n\n> Check that the log server receives messages.\n\n### Log the result of a command\n\n```bash\ndnf -y upgrade >/dev/null 2>&1 && logger -t patch \"upgrade OK\" || logger -t patch -p user.err \"upgrade FAILED\"\n```\n\n> Success or failure in the log.\n\n### Pipe output into the log\n\n```bash\ndf -h / | logger -t diskcheck\n```\n\n> Every line becomes a log entry.\n\n### Mark tests in security monitoring\n\n```bash\nlogger -t soc-test \"SIEM detection test 2026-10-25\"\n```\n\n> Test that your SIEM gets and alerts on messages.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man logger](https://linux.die.net/man/1/logger)\n","page":"commands/logger/","label":"logger","where":"Command page · Logs","aka":[]},{"id":"cmd-logname","n":1448,"slug":"logname","title":"logname — Print the name of the user who logged in to this session (also after su or sudo)","sections":["commands"],"topic":"Commands · Users and groups","tags":["login name","original user","sudo","su","audit"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the name of the user who logged in to this session (also after su or sudo).\n\n## About\n\n`logname` reads the login record of the session. After `su -` or `sudo -i`, `whoami` shows the new user, but `logname` still shows the person who logged in.\n\nIt needs a terminal session with a login record; in cron or some containers it prints \"no login name\".\n\n## Syntax\n\n```text\nlogname [OPTION]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show the login name\n\n```bash\nlogname\n```\n```text\nstudent\n```\n\n> The user who logged in.\n\n### After switching to root\n\n```bash\nsudo -i\nwhoami; logname\n```\n```text\nroot\nstudent\n```\n\n> logname still shows the real person.\n\n### Use the real user in a root script\n\n```bash\necho \"$(date) $(logname) changed the firewall\" >> /var/log/admin-changes.log\n```\n\n> Better audit text than \"root\".\n\n### Compare with SUDO_USER\n\n```bash\nsudo sh -c 'echo $SUDO_USER; logname'\n```\n```text\nstudent\nstudent\n```\n\n> SUDO_USER is set only by sudo; logname also works after su.\n\n### Without a terminal\n\n```bash\nssh server2 logname\n```\n```text\nlogname: no login name\n```\n\n> A remote command without a terminal has no login record.\n\n### In cron\n\n```bash\ncrontab -l | grep logname\n```\n\n> Cron jobs have no login name; use `id -un` there.\n\n### Give a file back to the real user\n\n```bash\nchown \"$(logname)\": /tmp/report.txt\n```\n\n> After a root script, the user can open the file.\n\n### Check the login records\n\n```bash\nwho am i\n```\n```text\nstudent  pts/0        2026-10-25 09:02 (192.168.122.1)\n```\n\n> logname reads the same data.\n\n### Script that needs a person\n\n```bash\nlogname >/dev/null 2>&1 || echo \"no interactive login\"\n```\n\n> Exit code 1 when there is no login name.\n\n### Show the home folder of the real user\n\n```bash\ngetent passwd \"$(logname)\" | cut -d: -f6\n```\n```text\n/home/student\n```\n\n> Useful in an install script run with sudo.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | No login name. |\n\nFull manual: [man logname](https://linux.die.net/man/1/logname)\n","page":"commands/logname/","label":"logname","where":"Command page · Users and groups","aka":[]},{"id":"cmd-logrotate","n":1449,"slug":"logrotate","title":"logrotate — Rotate, compress and delete old log files by the rules in /etc/logrotate.conf and /etc/logrotate.d/","sections":["commands"],"topic":"Commands · Logs","tags":["rotate logs","compress","disk space","retention"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Rotate, compress and delete old log files by the rules in /etc/logrotate.conf and /etc/logrotate.d/.\n\n## About\n\nlogrotate renames a log (like `messages` → `messages-20261025`), starts a new empty one, compresses old files and deletes the oldest. On RHEL the `logrotate.timer` runs it once a day.\n\nMain settings are in `/etc/logrotate.conf`; packages put their own rules in `/etc/logrotate.d/`. Test a rule with `-d` (debug, changes nothing) and force a rotation with `-f`.\n\n## Syntax\n\n```text\nlogrotate [OPTION]... CONFIG_FILE...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `daily, weekly, monthly, yearly` | When to rotate. |\n| `size SIZE / minsize / maxsize` | Rotate by size (like 100M). |\n| `rotate N` | Keep N old files. |\n| `compress / delaycompress` | gzip old files / wait one cycle. |\n| `dateext` | Add the date to the name (RHEL default). |\n| `missingok / notifempty` | No error if missing / skip empty logs. |\n| `create MODE OWNER GROUP` | Make the new log with these rights. |\n| `copytruncate` | Copy and empty the file (for programs that keep it open). |\n| `postrotate … endscript` | Commands after the rotation, like a reload. |\n| `sharedscripts` | Run the scripts once for all files of the rule. |\n| `maxage DAYS` | Delete rotated files older than this. |\n| `su USER GROUP` | Rotate as this user (for folders not owned by root). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --debug` | Test mode: show what would happen, change nothing. |\n| `-f, --force` | Rotate now, even if it is not time yet. |\n| `-v, --verbose` | Show details. |\n| `-s, --state FILE` | Use another state file. |\n| `--skip-state-lock` | Do not lock the state file. |\n| `--wait-for-state-lock` | Wait for the state file lock. |\n| `-l, --log FILE` | Write a log of what happens. |\n| `-m, --mail COMMAND` | The mail command for mail settings. |\n| `--usage` | Short usage. |\n| `-?, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show the main settings\n\n```bash\ngrep -v '^#' /etc/logrotate.conf | grep .\n```\n```text\nweekly\nrotate 4\ncreate\ndateext\ninclude /etc/logrotate.d\n```\n\n> Defaults for all rules.\n\n### Rules of a package\n\n```bash\ncat /etc/logrotate.d/httpd\n```\n\n> Includes a postrotate reload of httpd.\n\n### Write a rule for your app\n\n```bash\ncat /etc/logrotate.d/app\n```\n```text\n/var/log/app.log {\n    weekly\n    rotate 8\n    compress\n    missingok\n    notifempty\n    create 0640 root root\n}\n```\n\n> Keeps 8 weeks of compressed logs.\n\n### Test a rule\n\n```bash\nlogrotate -d /etc/logrotate.d/app\n```\n\n> Shows the plan; nothing changes.\n\n### Force a rotation now\n\n```bash\nlogrotate -fv /etc/logrotate.d/app\n```\n\n> Check the result with `ls -l /var/log/app.log*`.\n\n### Rotate by size\n\n```bash\ncat /etc/logrotate.d/bigapp\n```\n```text\n/var/log/bigapp/*.log {\n    size 200M\n    rotate 5\n    compress\n    copytruncate\n}\n```\n\n> For apps that cannot reopen their logs.\n\n### When did it run last?\n\n```bash\nsystemctl list-timers logrotate.timer\ncat /var/lib/logrotate/logrotate.status | head\n```\n\n> The state file stores the last rotation per log.\n\n### Keep logs for compliance\n\n```bash\nsed -i 's/^rotate 4/rotate 52/' /etc/logrotate.conf\n```\n\n> One year of weekly logs (check the disk space; also send logs to a central server).\n\n### Find rules with errors\n\n```bash\nlogrotate -d /etc/logrotate.conf 2>&1 | grep -i error\n```\n\n> Wrong owners or rights stop the rotation.\n\n### Reload a service after rotation\n\n```bash\ngrep -A3 postrotate /etc/logrotate.d/nginx\n```\n\n> Without it, the program writes to the renamed old file.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/logrotate.conf` | Main settings. |\n| `/etc/logrotate.d/` | Rules per program. |\n| `/var/lib/logrotate/logrotate.status` | Last rotation times. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man logrotate](https://linux.die.net/man/8/logrotate)\n","page":"commands/logrotate/","label":"logrotate","where":"Command page · Logs","aka":[]},{"id":"cmd-ls","n":1450,"slug":"ls","title":"ls — List the files in a folder, with details like permissions, owner, size and time","sections":["commands"],"topic":"Commands · Files and folders","tags":["list","files","folder","permissions","size","hidden","sort","selinux"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List the files in a folder, with details like permissions, owner, size and time.\n\n## About\n\n`ls` shows what is in a folder (the current one when you give no name). With `-l` you get the long list:\n\n```text\n-rw-r--r--. 1 root root 158 Jan  6 10:12 /etc/hosts\n```\n\nFrom left to right: file type and permissions (the `.` at the end means an SELinux context exists), number of hard links, owner, group, size in bytes, last change time, name.\n\nThe first letter is the type: `-` file, `d` folder, `l` symbolic link, `b` block device, `c` character device, `p` pipe, `s` socket.\n\nOn RHEL, `ls` is an alias for `ls --color=auto`, and `ll` for `ls -l --color=auto`.\n\n## Syntax\n\n```text\nls [OPTION]... [FILE]...\n```\n\n## Options\n\n\n### What to show\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Also show hidden files (names that start with a dot), and `.` and `..`. |\n| `-A, --almost-all` | Show hidden files, but not `.` and `..`. |\n| `-d, --directory` | Show the folder itself, not its content. |\n| `-R, --recursive` | Also list all sub-folders. |\n| `-B, --ignore-backups` | Do not list backup files that end with ~. |\n| `-I, --ignore=<pattern>` | Do not list names that match the pattern. |\n| `--hide=<pattern>` | Like -I, but -a or -A show them again. |\n| `-L, --dereference` | For links, show the file the link points to. |\n| `-H, --dereference-command-line` | Follow links given on the command line. |\n| `--dereference-command-line-symlink-to-dir` | Follow command-line links that point to folders. |\n\n### Long list and details\n\n| Option | What it does |\n|---|---|\n| `-l` | Long list: permissions, links, owner, group, size, time, name. |\n| `-h, --human-readable` | With -l: sizes like 4.0K, 23M, 1.2G. |\n| `--si` | Like -h, but with powers of 1000 instead of 1024. |\n| `-i, --inode` | Show the inode number of each file. |\n| `-n, --numeric-uid-gid` | Like -l, but show user and group IDs as numbers. |\n| `-g` | Like -l, but without the owner. |\n| `-o` | Like -l, but without the group. |\n| `-G, --no-group` | In a long list, do not show the group. |\n| `-s, --size` | Show the space each file uses on disk (in blocks). |\n| `--block-size=<size>` | Show sizes in this unit, like M or G. |\n| `-k, --kibibytes` | Use 1024-byte blocks for the disk use. |\n| `-Z, --context` | Show the SELinux security context. |\n| `--author` | With -l: show the author of each file. |\n| `--full-time` | Like -l with the full date and time. |\n| `--time-style=<style>` | Time format: full-iso, long-iso, iso, locale or +FORMAT. |\n\n### Sort\n\n| Option | What it does |\n|---|---|\n| `-t` | Sort by time, newest first. |\n| `-r, --reverse` | Reverse the order. With -t: oldest first. |\n| `-S` | Sort by size, biggest first. |\n| `-X` | Sort by the file extension. |\n| `-v` | Natural sort of version numbers (file2 before file10). |\n| `-U` | Do not sort; show in the order of the folder. |\n| `-f` | Do not sort, show all files (like -aU). |\n| `--sort=<word>` | Sort by: none, size, time, version, extension, width. |\n| `--group-directories-first` | Show folders before files. |\n| `-c` | With -lt: sort by and show the change time (ctime) of the file status. |\n| `-u` | With -lt: sort by and show the access time. |\n| `--time=<word>` | Which time to show and sort by: atime, ctime, mtime, birth. |\n\n### Format\n\n| Option | What it does |\n|---|---|\n| `-1` | One name per line. |\n| `-C` | Names in columns (the default on a terminal). |\n| `-x` | Names in rows instead of columns. |\n| `-m` | Names separated by commas. |\n| `--format=<word>` | across, commas, horizontal, long, single-column, verbose, vertical. |\n| `-w, --width=<cols>` | Set the screen width. |\n| `-T, --tabsize=<cols>` | Tab stops every COLS columns. |\n| `-F, --classify[=<when>]` | Add a sign to the name: / folder, * program, @ link, \\| pipe, = socket. |\n| `--file-type` | Like -F, but no * for programs. |\n| `-p, --indicator-style=slash` | Add / to folder names. |\n| `--indicator-style=<word>` | none, slash, file-type or classify. |\n| `--color[=<when>]` | Colors: always, auto, never. |\n| `--hyperlink[=<when>]` | Make the names clickable links in terminals that support it. |\n| `-b, --escape` | Show special characters as C escapes. |\n| `-N, --literal` | Show names exactly as they are. |\n| `-q, --hide-control-chars` | Show control characters as ?. |\n| `--show-control-chars` | Show control characters as they are. |\n| `-Q, --quote-name` | Put names in double quotes. |\n| `--quoting-style=<word>` | How to quote names: literal, shell, shell-always, c, escape … |\n| `-D, --dired` | Output for the Emacs dired mode. |\n| `--zero` | End each line with a NUL byte. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Long list with readable sizes\n\n```bash\nls -lh /var/log\n```\n\n> The basic view to check owners, permissions and sizes.\n\n### Show hidden files\n\n```bash\nls -la ~\n```\n\n> Files like `.bashrc` and `.ssh` are hidden. `-a` shows them.\n\n### What changed last?\n\n```bash\nls -lrt /etc | tail\n```\n\n> Sorted by time, the newest files at the bottom. Great after an update or an incident.\n\n### Show the folder itself, not its content\n\n```bash\nls -ld /tmp /etc\n```\n```text\ndrwxrwxrwt. 18 root root 4096 Oct 25 09:12 /tmp\ndrwxr-xr-x. 140 root root 8192 Oct 25 08:55 /etc\n```\n\n> Check the permissions of a folder. The `t` on /tmp is the sticky bit.\n\n### Show the SELinux context\n\n```bash\nls -Z /var/www/html\n```\n\n> The label must be `httpd_sys_content_t` for Apache. Wrong labels cause \"403 Forbidden\".\n\n### Find the biggest files in a folder\n\n```bash\nls -lS /var/log | head\n```\n\n> Biggest first. For a whole tree use `du` or `find -size`.\n\n### Show inode numbers to see hard links\n\n```bash\nls -li /data/projects/web/hosts.old /data/projects/db/hosts.hard\n```\n\n> The same inode number means the same data.\n\n### List only folders\n\n```bash\nls -d /etc/*/\n```\n\n> The slash at the end of the pattern matches folders only.\n\n### Show user and group as numbers\n\n```bash\nls -ln /home\n```\n\n> Useful after a restore or on NFS, when names do not match on two systems.\n\n### Exact times in ISO format for a report\n\n```bash\nls -l --time-style=long-iso /etc/passwd /etc/shadow\n```\n\n> When was the account database last changed? A good question in a security check.\n\n### Show which files are links and programs\n\n```bash\nls -F /usr/local/bin\n```\n\n> `@` = link, `*` = program, `/` = folder.\n\n### One name per line for a script\n\n```bash\nls -1 /etc/yum.repos.d/\n```\n\n> Better: use a `for f in /etc/yum.repos.d/*` loop or `find` in scripts; never parse `ls -l`.\n\n### Folders first, then files, sorted naturally\n\n```bash\nls -l --group-directories-first -v /srv/releases\n```\n\n> Version numbers like v2, v10 are sorted in the right order.\n\n## Watch out\n\n- Do not parse the output of `ls` in scripts (names with spaces or new lines break it). Use globs or `find -print0`.\n- The size in `ls -l` is the file length, not the space used on disk. `ls -s` or `du` shows the real use.\n- `ls -l` of a link shows the link; add `-L` to see the target file.\n\n## Tips\n\n- `-l` long, `-a` all, `-h` human, `-t` time, `-r` reverse, `-d` the folder itself, `-Z` SELinux.\n- `ls -lrt` = newest at the bottom; `ls -lS` = biggest first.\n\nFull manual: [man ls](https://linux.die.net/man/1/ls)\n","page":"commands/ls/","label":"ls","where":"Command page · Files and folders","aka":[]},{"id":"cmd-lsattr","n":1451,"slug":"lsattr","title":"lsattr — List the file attributes (like i = immutable, a = append only)","sections":["commands"],"topic":"Commands · Permissions","tags":["attributes","immutable","append only","list"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List the file attributes (like i = immutable, a = append only).\n\n## About\n\n`lsattr` shows the extra attributes that `chattr` sets. Each letter in the first column is one attribute; `-` means not set. `ls -l` does not show them, so check `lsattr` when even root gets \"Operation not permitted\".\n\n## Syntax\n\n```text\nlsattr [-RVadlpv] [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R` | List directories and their content recursively. |\n| `-a` | Also list hidden files (names with a dot). |\n| `-d` | List directories like files, not their content. |\n| `-l` | Use long names for the attributes. |\n| `-p` | Show the project number. |\n| `-v` | Show the version/generation number. |\n| `-V` | Show the version of the program. |\n\n## Examples\n\n### Attributes of one file\n\n```bash\nlsattr /etc/resolv.conf\n```\n```text\n---------------------- /etc/resolv.conf\n```\n\n> No attributes set.\n\n### Long names\n\n```bash\nlsattr -l /var/log/app/audit.log\n```\n```text\n/var/log/app/audit.log       Append_Only\n```\n\n> Easier to read.\n\n### A directory itself\n\n```bash\nlsattr -d /srv/archive\n```\n\n> Without -d you see the files inside.\n\n### All files, also hidden ones\n\n```bash\nlsattr -a ~\n```\n\n> Includes .bashrc and others.\n\n### Search a tree for immutable files\n\n```bash\nlsattr -R /etc 2>/dev/null | grep -- '-i-'\n```\n\n> Explains failing updates or config tools.\n\n### Check SSH files for attacker tricks (security)\n\n```bash\nlsattr /root/.ssh/authorized_keys /home/*/.ssh/authorized_keys\n```\n\n> `i` on authorized_keys can hide a backdoor key.\n\n### Check before a package update\n\n```bash\nlsattr /usr/bin/* 2>/dev/null | grep -v -- '^-*e*-* '\n```\n\n> Any attribute other than e (extents) is unusual in /usr/bin.\n\n### Show the project number\n\n```bash\nlsattr -p /srv/projects\n```\n\n> Used with project quotas on ext4/XFS.\n\n### Compare with ls\n\n```bash\nls -l /etc/resolv.conf; lsattr /etc/resolv.conf\n```\n\n> ls shows the mode; lsattr shows the attributes.\n\n### Errors on special files\n\n```bash\nlsattr /dev/null\n```\n```text\nlsattr: Operation not supported While reading flags on /dev/null\n```\n\n> Only normal files and folders have attributes.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lsattr](https://linux.die.net/man/1/lsattr)\n","page":"commands/lsattr/","label":"lsattr","where":"Command page · Permissions","aka":[]},{"id":"cmd-lsblk","n":1452,"slug":"lsblk","title":"lsblk — List disks, partitions, LVM volumes and other block devices as a tree, with size, type and mount point","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["disks","partitions","block devices","mount points","file systems","uuid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List disks, partitions, LVM volumes and other block devices as a tree, with size, type and mount point.\n\n## About\n\n`lsblk` reads `/sys` and shows every block device and what sits on it: partitions, LVM logical volumes, RAID, encrypted volumes. With `-f` it adds the file system type, label, UUID and free space. It is the first command before you touch a disk.\n\n## Syntax\n\n```text\nlsblk [OPTION]... [DEVICE]...\n```\n\n## Options\n\n\n### What to show\n\n| Option | What it does |\n|---|---|\n| `-f, --fs` | File system info: type, label, UUID, free space, mount point. |\n| `-o, --output LIST` | Choose columns, like `NAME,SIZE,FSTYPE,MOUNTPOINTS`. |\n| `-O, --output-all` | All columns. |\n| `-p, --paths` | Full device paths (/dev/vdb1). |\n| `-d, --nodeps` | Only the disks, not the partitions. |\n| `-a, --all` | Also empty devices. |\n| `-A, --noempty` | Hide empty devices. |\n| `-e, --exclude LIST` | Leave out these major numbers (default RAM disks). |\n| `-I, --include LIST` | Only these major numbers. |\n| `-m, --perms` | Owner, group and mode. |\n| `-t, --topology` | Alignment, sector sizes, scheduler. |\n| `-D, --discard` | Discard (TRIM) support. |\n| `-S, --scsi` | Only SCSI devices. |\n| `-N, --nvme` | Only NVMe devices. |\n| `-v, --virtio` | Only virtio devices. |\n| `-z, --zoned` | Zone model. |\n| `-s, --inverse` | Upside-down tree (from the device up). |\n| `-M, --merge` | Group parents of sub-trees (RAID, multipath). |\n| `-E, --dedup COLUMN` | Hide duplicates of a column. |\n\n### Format\n\n| Option | What it does |\n|---|---|\n| `-l, --list` | List, no tree. |\n| `-T, --tree[=COLUMN]` | Tree format. |\n| `-J, --json` | JSON output. |\n| `-P, --pairs` | KEY=\"value\" output. |\n| `-r, --raw` | Raw output. |\n| `-y, --shell` | Column names usable as shell variables. |\n| `-n, --noheadings` | No header line. |\n| `-b, --bytes` | Sizes in bytes. |\n| `-i, --ascii` | ASCII tree lines. |\n| `-x, --sort COLUMN` | Sort by a column. |\n| `-w, --width N` | Output width. |\n| `--sysroot DIR` | Read /sys from another root. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### All disks and partitions\n\n```bash\nlsblk\n```\n```text\nNAME          MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS\nvda           252:0    0   20G  0 disk\n├─vda1        252:1    0    1G  0 part /boot\n└─vda2        252:2    0   19G  0 part\n  ├─rhel-root 253:0    0   17G  0 lvm  /\n  └─rhel-swap 253:1    0    2G  0 lvm  [SWAP]\nvdb           252:16   0   10G  0 disk\n```\n\n> vdb has no partitions: a new disk.\n\n### File systems and UUIDs\n\n```bash\nlsblk -f\n```\n\n> FSTYPE, LABEL, UUID, FSAVAIL, FSUSE%, MOUNTPOINTS.\n\n### Only the disks\n\n```bash\nlsblk -d -o NAME,SIZE,MODEL,TRAN\n```\n\n> Model and connection (sata, nvme, usb).\n\n### One disk with full paths\n\n```bash\nlsblk -p /dev/vdb\n```\n\n> Copy the paths for the next commands.\n\n### Choose columns\n\n```bash\nlsblk -o NAME,SIZE,TYPE,FSTYPE,UUID,MOUNTPOINTS\n```\n\n> Just what you need.\n\n### Find empty disks\n\n```bash\nlsblk -dn -o NAME,TYPE,SIZE | awk '$2==\"disk\"' ; lsblk -f\n```\n\n> A disk with no children and no FSTYPE is unused.\n\n### JSON for scripts\n\n```bash\nlsblk -J -o NAME,SIZE,MOUNTPOINTS | jq '.blockdevices[].name'\n```\n\n> Machine-readable.\n\n### Check SSD TRIM support\n\n```bash\nlsblk -D\n```\n\n> DISC-GRAN not 0 = discard works (fstrim.timer can use it).\n\n### Who owns a device file\n\n```bash\nlsblk -m /dev/vdb\n```\n\n> Owner, group and mode of the device node.\n\n### See the device under a mount point\n\n```bash\nlsblk -s /dev/mapper/rhel-root\n```\n\n> Inverse tree: from the LV down to the disk.\n\n### Find USB disks that are plugged in (security)\n\n```bash\nlsblk -o NAME,TRAN,SIZE,MOUNTPOINTS | grep usb\n```\n\n> Check for unknown removable storage.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n| `32` | No device found. |\n| `64` | Some devices worked, some failed. |\n\nFull manual: [man lsblk](https://linux.die.net/man/8/lsblk)\n","page":"commands/lsblk/","label":"lsblk","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-lscpu","n":1453,"slug":"lscpu","title":"lscpu — Show the CPU: architecture, model, sockets, cores, threads, caches, flags, virtualization and CPU security bugs","sections":["commands"],"topic":"Commands · System information","tags":["cpu","cores","threads","sockets","virtualization","vulnerabilities"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the CPU: architecture, model, sockets, cores, threads, caches, flags, virtualization and CPU security bugs.\n\n## About\n\n`lscpu` collects CPU data from `/proc/cpuinfo` and `/sys`. Useful lines: CPU(s), Thread(s) per core, Core(s) per socket, Socket(s), Model name, Hypervisor vendor, Virtualization, and the Vulnerability lines (Spectre, Meltdown …) with their mitigation status.\n\n## Syntax\n\n```text\nlscpu [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-e, --extended[=LIST]` | One line per CPU (CPU, CORE, SOCKET, NODE, ONLINE, MHZ …). |\n| `-p, --parse[=LIST]` | Parsable output per CPU. |\n| `-C, --caches[=LIST]` | Cache details. |\n| `-J, --json` | JSON output. |\n| `-a, --all` | With -e/-p: also offline CPUs. |\n| `-b, --online` | Only online CPUs. |\n| `-c, --offline` | Only offline CPUs. |\n| `-B, --bytes` | Sizes in bytes. |\n| `-x, --hex` | CPU masks in hex. |\n| `-y, --physical` | Physical IDs instead of logical ones. |\n| `--hierarchic[=WHEN]` | Grouped output. |\n| `--output-all` | All columns. |\n| `-s, --sysroot DIR` | Read from another root. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### CPU overview\n\n```bash\nlscpu\n```\n```text\nArchitecture:             x86_64\nCPU(s):                   2\nModel name:               Intel Xeon Processor (Icelake)\nHypervisor vendor:        KVM\n```\n\n> Plus sockets, cores, caches, flags.\n\n### Cores and threads\n\n```bash\nlscpu | grep -E '^(CPU\\(s\\)|Thread|Core|Socket)'\n```\n\n> CPUs = sockets × cores × threads.\n\n### Is this a VM?\n\n```bash\nlscpu | grep -i hypervisor\n```\n```text\nHypervisor vendor:        KVM\n```\n\n> No line = physical machine.\n\n### Can it run VMs?\n\n```bash\nlscpu | grep -i virtualization\n```\n\n> VT-x or AMD-V needed for KVM.\n\n### CPU security mitigations (security)\n\n```bash\nlscpu | grep -i vulnerab\n```\n```text\nVulnerability Spectre v2:  Mitigation; Enhanced / Automatic IBRS\n```\n\n> \"Vulnerable\" lines need a kernel or microcode update.\n\n### Per CPU table\n\n```bash\nlscpu -e\n```\n\n> Which CPU is on which core and socket.\n\n### Caches\n\n```bash\nlscpu -C\n```\n\n> L1, L2, L3 sizes.\n\n### JSON for inventory\n\n```bash\nlscpu -J | jq -r '.lscpu[] | select(.field==\"Model name:\") | .data'\n```\n\n> Model for an asset list.\n\n### CPU flags (like AES or AVX)\n\n```bash\nlscpu | grep -o -w -E 'aes|avx2|avx512f' | sort -u\n```\n\n> Does the CPU support hardware AES?\n\n### Compare with nproc\n\n```bash\nnproc; lscpu | grep '^CPU(s)'\n```\n\n> nproc = usable now, lscpu = present.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lscpu](https://linux.die.net/man/1/lscpu)\n","page":"commands/lscpu/","label":"lscpu","where":"Command page · System information","aka":[]},{"id":"cmd-lsinitrd","n":1454,"slug":"lsinitrd","title":"lsinitrd — Show the content of an initramfs image: dracut modules, files, kernel modules, and the content of single files","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["initramfs","inspect","boot","drivers"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the content of an initramfs image: dracut modules, files, kernel modules, and the content of single files.\n\n## About\n\n`lsinitrd` without arguments shows the initramfs of the running kernel: the dracut version and arguments, the dracut modules used, and a file list. With `-f FILE` it prints one file from the image — useful to check if a config (like `/etc/fstab` or a modprobe file) is inside.\n\n## Syntax\n\n```text\nlsinitrd [OPTION]... [IMAGE [FILE]...]\nlsinitrd [OPTION]... -k KERNEL-VERSION\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-k, --kver VERSION` | The image of this kernel version. |\n| `-f, --file FILE` | Print the content of this file from the image. |\n| `-m, --mod` | List only the dracut modules. |\n| `-s, --size` | Sort files by size. |\n| `--unpack` | Unpack the image into the current folder. |\n| `--unpackearly` | Unpack only the early part (microcode). |\n| `-v, --verbose` | Verbose (with --unpack). |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Image of the running kernel\n\n```bash\nlsinitrd | less\n```\n\n> Header, dracut modules, file list.\n\n### Only the dracut modules\n\n```bash\nlsinitrd -m\n```\n\n> lvm, crypt, systemd …\n\n### Is a driver inside?\n\n```bash\nlsinitrd | grep -E 'virtio_blk|nvme'\n```\n\n> The root disk driver must be there.\n\n### Show a file from the image\n\n```bash\nlsinitrd -f /etc/modprobe.d/loop.conf\n```\n\n> Check that your config is included.\n\n### Image of another kernel\n\n```bash\nlsinitrd -k 6.12.0-55.el10.x86_64 | head\n```\n\n> Before you boot that kernel.\n\n### Biggest files\n\n```bash\nlsinitrd -s | head\n```\n\n> Why is the image so big?\n\n### Unpack to look around\n\n```bash\nmkdir /tmp/ird && cd /tmp/ird && lsinitrd --unpack /boot/initramfs-$(uname -r).img\n```\n\n> Full file tree.\n\n### Check LVM names inside\n\n```bash\nlsinitrd -f /etc/lvm/lvm.conf | grep -v '^\\s*#' | grep -v '^\\s*$' | head\n```\n\n> After renaming a VG, rebuild with dracut.\n\n### Kernel arguments in the image\n\n```bash\nlsinitrd | grep -A3 'dracut cmdline'\n```\n\n> Built-in arguments.\n\n### Early microcode\n\n```bash\nlsinitrd --unpackearly -v\n```\n\n> CPU microcode updates load from here.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lsinitrd](https://linux.die.net/man/1/lsinitrd)\n","page":"commands/lsinitrd/","label":"lsinitrd","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-lsmod","n":1455,"slug":"lsmod","title":"lsmod — List the kernel modules that are loaded now","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel modules","drivers","loaded modules"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List the kernel modules that are loaded now.\n\n## About\n\n`lsmod` formats `/proc/modules` (it has no options): the module name, its size in memory, how many users it has, and which modules use it. A module with users cannot be unloaded until they are gone.\n\n## Syntax\n\n```text\nlsmod\n```\n\n## Examples\n\n### All loaded modules\n\n```bash\nlsmod | head\n```\n```text\nModule                  Size  Used by\nxfs                  2654208  2\nlibcrc32c              12288  2 xfs,nf_tables\n```\n\n> Used by: count and module names.\n\n### Is a module loaded?\n\n```bash\nlsmod | grep -w vfat\n```\n\n> No output = not loaded.\n\n### Network card driver\n\n```bash\nlsmod | grep -E 'virtio_net|e1000|ixgbe|mlx'\n```\n\n> Which NIC driver runs.\n\n### Count modules\n\n```bash\nlsmod | tail -n +2 | wc -l\n```\n\n> Number of loaded modules.\n\n### Modules with users\n\n```bash\nlsmod | awk '$3 > 0'\n```\n\n> These cannot be removed at once.\n\n### Same data from /proc\n\n```bash\ncat /proc/modules | head -3\n```\n\n> lsmod reads this file.\n\n### Biggest modules\n\n```bash\nlsmod | sort -k2 -n -r | head -5\n```\n\n> Memory use of drivers.\n\n### Check a blocked module stays off (security)\n\n```bash\nlsmod | grep usb_storage || echo \"usb-storage not loaded\"\n```\n\n> Verify your blacklist.\n\n### Find unusual modules (security)\n\n```bash\nlsmod | awk 'NR>1 {print $1}' | while read m; do modinfo -F filename $m >/dev/null 2>&1 || echo \"no file: $m\"; done\n```\n\n> A loaded module without a file on disk can be a rootkit.\n\n### Module details\n\n```bash\nmodinfo $(lsmod | awk 'NR==2 {print $1}')\n```\n\n> Info for the first module in the list.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/modules` | The kernel list of loaded modules. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lsmod](https://linux.die.net/man/8/lsmod)\n","page":"commands/lsmod/","label":"lsmod","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-lspci","n":1456,"slug":"lspci","title":"lspci — List the PCI devices (network cards, disk controllers, GPUs …) and the drivers that handle them","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["pci","hardware","drivers","network card","gpu","devices"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List the PCI devices (network cards, disk controllers, GPUs …) and the drivers that handle them.\n\n## About\n\n`lspci` shows every device on the PCI bus with its vendor and model. `-k` adds the kernel driver in use and the modules that could drive it — the quick check that hardware works. `-nn` shows the numeric vendor:device IDs for searching drivers. Some details need root.\n\n## Syntax\n\n```text\nlspci [OPTION]...\n```\n\n## Options\n\n\n### Display\n\n| Option | What it does |\n|---|---|\n| `-k` | Show the kernel driver in use and possible modules. |\n| `-v, -vv, -vvv` | More and more details. |\n| `-nn` | Names and numeric IDs (vendor:device). |\n| `-n` | Only numeric IDs. |\n| `-t` | Tree view of buses and bridges. |\n| `-m, -mm` | Machine-readable output. |\n| `-b` | Bus-centric view (IRQs and addresses as the card sees them). |\n| `-D` | Always show the PCI domain. |\n| `-P, -PP` | Show the path through bridges. |\n| `-x, -xxx, -xxxx` | Hex dump of the config space (root for the full dump). |\n| `-q, -qq, -Q` | Ask the online PCI ID database for unknown devices. |\n\n### Select\n\n| Option | What it does |\n|---|---|\n| `-s [[[[DOMAIN]:]BUS]:][DEV][.[FUNC]]` | Only this slot, like `-s 00:03.0`. |\n| `-d [VENDOR]:[DEVICE][:CLASS]` | Only this vendor/device/class, like `-d 8086:`. |\n\n### Data sources\n\n| Option | What it does |\n|---|---|\n| `-i FILE` | Use this pci.ids file. |\n| `-p FILE` | Use this modules.pcimap file. |\n| `-M` | Bus mapping mode (full scan). |\n| `-A METHOD` | Access method. |\n| `-O PARAM=VALUE` | Library parameters. |\n| `-H1, -H2` | Direct hardware access (old). |\n| `-F FILE` | Read a hex dump from a file. |\n| `-G` | Library debug. |\n| `--version` | Show the version. |\n\n## Examples\n\n### All PCI devices\n\n```bash\nlspci\n```\n```text\n00:01.0 VGA compatible controller: Red Hat, Inc. Virtio 1.0 GPU (rev 01)\n01:00.0 Ethernet controller: Red Hat, Inc. Virtio 1.0 network device (rev 01)\n```\n\n> Slot, class and model.\n\n### Devices with their drivers\n\n```bash\nlspci -k\n```\n```text\n01:00.0 Ethernet controller: Red Hat, Inc. Virtio 1.0 network device (rev 01)\n\tKernel driver in use: virtio-pci\n```\n\n> No \"driver in use\" = no driver loaded.\n\n### Network cards only\n\n```bash\nlspci | grep -i ethernet\n```\n\n> Then `lspci -k -s SLOT`.\n\n### IDs for driver search\n\n```bash\nlspci -nn | grep -i net\n```\n```text\n01:00.0 Ethernet controller [0200]: Red Hat, Inc. Virtio 1.0 network device [1af4:1041] (rev 01)\n```\n\n> 1af4:1041 = vendor:device.\n\n### One device in detail\n\n```bash\nlspci -vv -s 01:00.0\n```\n\n> Capabilities, link speed, IRQ.\n\n### Tree view\n\n```bash\nlspci -t\n```\n\n> Which device sits behind which bridge.\n\n### Only Intel devices\n\n```bash\nlspci -d 8086:\n```\n\n> Filter by vendor ID.\n\n### Check the PCIe link speed of an NVMe disk\n\n```bash\nlspci -vv -s 03:00.0 | grep -E 'LnkCap|LnkSta'\n```\n\n> A slower LnkSta than LnkCap = wrong slot or bad link.\n\n### Update the ID database\n\n```bash\nupdate-pciids\n```\n\n> Newer names for new hardware (needs internet).\n\n### Hardware inventory for audits\n\n```bash\nlspci -mm > /tmp/pci-$(hostname -s).txt\n```\n\n> Machine-readable list per host.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/hwdata/pci.ids` | Names of vendors and devices. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lspci](https://linux.die.net/man/8/lspci)\n","page":"commands/lspci/","label":"lspci","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-lsusb","n":1457,"slug":"lsusb","title":"lsusb — List USB buses and the USB devices connected to them","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["usb","devices","hardware","security","removable"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List USB buses and the USB devices connected to them.\n\n## About\n\n`lsusb` prints one line per device with bus, device number, vendor:product ID and name. `-t` shows the tree with the driver of each interface; `-v` shows every descriptor (root for all details).\n\n## Syntax\n\n```text\nlsusb [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t, --tree` | Tree with drivers and speeds. |\n| `-v, --verbose` | All details. |\n| `-s [[BUS]:][DEVNUM]` | Only this device, like `-s 001:004`. |\n| `-d [VENDOR]:[PRODUCT]` | Only this vendor/product, like `-d 0781:`. |\n| `-D DEVICE` | Read one device file, like /dev/bus/usb/001/004. |\n| `-V, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### All USB devices\n\n```bash\nlsusb\n```\n```text\nBus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub\nBus 001 Device 004: ID 0781:5583 SanDisk Corp. Ultra Fit\n```\n\n> Root hubs are the USB controllers themselves.\n\n### Tree with drivers\n\n```bash\nlsusb -t\n```\n\n> Driver=usb-storage shows a disk; Driver=usbhid a keyboard or mouse.\n\n### Details of one device\n\n```bash\nlsusb -v -s 001:004 | less\n```\n\n> Speed, power, interfaces.\n\n### Filter by vendor\n\n```bash\nlsusb -d 0781:\n```\n\n> All SanDisk devices.\n\n### Find new devices while plugging in\n\n```bash\nwatch -n1 lsusb\n```\n\n> A new line appears.\n\n### Connect with the block device\n\n```bash\ndmesg -T | tail; lsblk\n```\n\n> A USB disk becomes /dev/sdX.\n\n### Check for unknown devices (security)\n\n```bash\nlsusb | grep -viE 'root hub|keyboard|mouse'\n```\n\n> Unknown USB devices on a server can be keyloggers or rogue network cards.\n\n### Is usb-storage blocked?\n\n```bash\nlsusb -t | grep -i storage\n```\n\n> With the blacklist, storage devices have no driver.\n\n### USB speed\n\n```bash\nlsusb -t | grep -o '[0-9]*M$' | sort | uniq -c\n```\n\n> 480M = USB 2, 5000M = USB 3.\n\n### Inventory\n\n```bash\nlsusb > /tmp/usb-$(hostname -s).txt\n```\n\n> Hardware list per host.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/hwdata/usb.ids` | Names of vendors and products. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man lsusb](https://linux.die.net/man/8/lsusb)\n","page":"commands/lsusb/","label":"lsusb","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-lvcreate","n":1458,"slug":"lvcreate","title":"lvcreate — Create a logical volume from the free space of a volume group (lvcreate), or delete one (lvremove)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","logical volume","lv","extents","snapshot","thin"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a logical volume from the free space of a volume group (lvcreate), or delete one (lvremove).\n\nSame page for: `lvremove`\n\n## About\n\n`lvcreate -n NAME -L SIZE VG` makes an LV; `-l` gives the size in extents or percent (`-l 100%FREE`). The new device is `/dev/VG/NAME`; put a file system on it and mount it.\n\nlvcreate also makes snapshots (`-s`), thin pools and thin volumes (`-T`), striped, mirrored and RAID volumes, cache and VDO volumes (with `--type`).\n\n## Syntax\n\n```text\nlvcreate -n NAME (-L SIZE | -l EXTENTS) [OPTION]... VG [PV...]\nlvcreate -s -n NAME -L SIZE VG/LV\nlvremove [OPTION]... VG/LV...\n```\n\n## Options\n\n\n### Basic\n\n| Option | What it does |\n|---|---|\n| `-n, --name NAME` | Name of the new LV. |\n| `-L, --size SIZE` | Size, like 500M, 5G. |\n| `-l, --extents N[%VG\\|%FREE\\|%PVS\\|%ORIGIN]` | Size in extents or percent, like 50, 100%FREE. |\n| `-a, --activate y\\|n\\|ay` | Activate it (default y). |\n| `-p, --permission rw\\|r` | Read-write or read-only. |\n| `-C, --contiguous y\\|n` | Use one contiguous area. |\n| `--alloc POLICY` | Allocation policy. |\n| `-r, --readahead auto\\|none\\|N` | Read-ahead sectors. |\n| `-Z, --zero y\\|n` | Zero the first KiB (default y). |\n| `-W, --wipesignatures y\\|n` | Wipe old signatures on the new LV. |\n| `--addtag TAG` | Add a tag. |\n| `-k, --setactivationskip y\\|n, -K, --ignoreactivationskip, --setautoactivation y\\|n` | Activation settings. |\n| `-M, --persistent y\\|n, -j, --major N, --minor N` | Fixed device numbers. |\n\n### Types\n\n| Option | What it does |\n|---|---|\n| `--type TYPE` | linear, striped, snapshot, raid0/1/4/5/6/10, mirror, thin, thin-pool, cache, writecache, vdo … |\n| `-s, --snapshot` | Create a snapshot of an LV. |\n| `-T, --thin` | Thin pool or thin volume. |\n| `--thinpool LV` | The thin pool for a thin volume. |\n| `-V, --virtualsize SIZE` | Virtual size of a thin volume. |\n| `-c, --chunksize SIZE` | Chunk size (snapshot, thin, cache). |\n| `--poolmetadatasize SIZE, --poolmetadataspare y\\|n` | Pool metadata. |\n| `--discards passdown\\|nopassdown\\|ignore` | Discard behaviour of thin pools. |\n| `--errorwhenfull y\\|n` | Fail writes when a thin pool is full. |\n| `-i, --stripes N` | Spread over N PVs (striped). |\n| `-I, --stripesize SIZE` | Stripe size. |\n| `-m, --mirrors N` | Number of extra copies (mirror/raid1). |\n| `-R, --regionsize SIZE, --mirrorlog core\\|disk, --nosync` | Mirror settings. |\n| `--raidintegrity y\\|n, --raidintegritymode MODE, --raidintegrityblocksize N, --maxrecoveryrate SIZE, --minrecoveryrate SIZE` | RAID settings. |\n| `-H, --cache, --cachepool LV, --cachevol LV, --cachedevice PV, --cachesize SIZE, --cachemode MODE, --cachepolicy STRING, --cachesettings STRING, --cachemetadataformat auto\\|1\\|2` | Cache settings. |\n| `--vdo, --vdopool LV, --compression y\\|n, --deduplication y\\|n` | VDO (deduplication and compression). |\n| `--monitor y\\|n, --ignoremonitoring` | dmeventd monitoring. |\n| `--noudevsync` | Do not wait for udev. |\n| `--metadataprofile STRING` | Metadata profile. |\n| `--reportformat basic\\|json` | Output format. |\n\n### lvremove\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Remove without asking (also active LVs). |\n| `-S, --select EXPR` | Remove LVs that match. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### LV with a size\n\n```bash\nlvcreate -n lvdata -L 5G vgdata\n```\n```text\n  Logical volume \"lvdata\" created.\n```\n\n> Device: /dev/vgdata/lvdata.\n\n### LV with a number of extents\n\n```bash\nlvcreate -n lvfiles -l 50 vgfiles\n```\n\n> 50 × PE size (see vgdisplay).\n\n### Use all free space\n\n```bash\nlvcreate -n lvweb -l 100%FREE vgdata\n```\n\n> Or 50%FREE for half.\n\n### File system and mount\n\n```bash\nmkfs.xfs /dev/vgdata/lvdata\nmkdir /data; echo \"/dev/vgdata/lvdata /data xfs defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a\n```\n\n> The usual next steps.\n\n### A swap LV\n\n```bash\nlvcreate -n lvswap -L 2G rhel; mkswap /dev/rhel/lvswap\n```\n\n> Add it to fstab as swap.\n\n### Snapshot for a backup\n\n```bash\nlvcreate -s -n lvdata-snap -L 1G /dev/vgdata/lvdata\nmount -o ro,nouuid /dev/vgdata/lvdata-snap /mnt\n```\n\n> Frozen view; back up /mnt, then remove the snapshot. (nouuid is needed for XFS.)\n\n### Thin pool and thin volume\n\n```bash\nlvcreate -T -L 10G vgdata/pool1\nlvcreate -T -V 50G -n lvthin vgdata/pool1\n```\n\n> Over-provisioning: watch the pool usage!\n\n### Place an LV on a chosen PV\n\n```bash\nlvcreate -n lvfast -L 2G vgdata /dev/nvme0n1p3\n```\n\n> For example on the faster disk.\n\n### Remove an LV\n\n```bash\numount /data; lvremove /dev/vgdata/lvdata\n```\n```text\nDo you really want to remove active logical volume vgdata/lvdata? [y/n]: y\n```\n\n> Remove the fstab line too.\n\n### RAID1 mirror LV\n\n```bash\nlvcreate --type raid1 -m 1 -n lvsafe -L 2G vgdata\n```\n\n> Two copies on different PVs.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man lvcreate](https://linux.die.net/man/8/lvcreate)\n","page":"commands/lvcreate/","label":"lvcreate","where":"Command page · LVM","aka":["lvremove"]},{"id":"cmd-lvextend","n":1459,"slug":"lvextend","title":"lvextend — Make a logical volume bigger — with `-r` also its file system, while it stays mounted","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","grow","resize","logical volume","file system","online"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Make a logical volume bigger — with `-r` also its file system, while it stays mounted.\n\nSame page for: `lvresize`\n\n## About\n\n`lvextend -L +SIZE` adds space; `-L SIZE` sets the total size; `-l +N` / `-l +100%FREE` work with extents. The VG needs enough free space (add disks with vgextend).\n\nAlways add `-r` (`--resizefs`): it runs `xfs_growfs` or `resize2fs` after the LV grew. Without it the file system keeps its old size. `lvresize` can grow and shrink with the same options.\n\n## Syntax\n\n```text\nlvextend (-L [+]SIZE | -l [+]EXTENTS) [-r] [OPTION]... VG/LV [PV...]\nlvresize (-L [+|-]SIZE | -l [+|-]EXTENTS) [-r] [OPTION]... VG/LV\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L, --size [+]SIZE` | New size or + added size, like +2G or 10G. |\n| `-l, --extents [+]N[%VG\\|%LV\\|%PVS\\|%FREE\\|%ORIGIN]` | In extents or percent, like +100%FREE. |\n| `-r, --resizefs` | Also grow the file system. |\n| `-n, --nofsck` | Do not check the file system before resizing. |\n| `--fs checksize\\|resize\\|resize_fsadm\\|ignore` | How to handle the file system (newer lvm2). |\n| `--fsmode manage\\|offline\\|nochange` | May LVM mount or unmount for the resize (newer lvm2). |\n| `-i, --stripes N, -I, --stripesize SIZE` | Stripes for the new part. |\n| `-m, --mirrors N` | Mirrors (mirror LVs). |\n| `--alloc POLICY` | Allocation policy. |\n| `--poolmetadatasize [+]SIZE` | Grow the metadata of a thin pool. |\n| `--type TYPE` | Segment type. |\n| `--nosync` | Do not sync new mirror parts. |\n| `--usepolicies` | Use the autoextend policy from lvm.conf (thin pools, snapshots). |\n| `-f, --force` | Do not ask. |\n| `-A, --autobackup y\\|n` | Back up the metadata. |\n| `--noudevsync` | Do not wait for udev. |\n| `--reportformat basic\\|json` | Output format. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Add 2 GiB with the file system\n\n```bash\nlvextend -r -L +2G /dev/vgdata/lvdata\n```\n```text\n  Size of logical volume vgdata/lvdata changed from 5.00 GiB to 7.00 GiB.\n  Logical volume vgdata/lvdata successfully resized.\n```\n\n> Works online for XFS and ext4.\n\n### Set a total size\n\n```bash\nlvextend -r -L 10G /dev/vgdata/lvdata\n```\n\n> 10 GiB in total (no +).\n\n### Use all free space in the VG\n\n```bash\nlvextend -r -l +100%FREE /dev/vgdata/lvweb\n```\n\n> Common after vgextend.\n\n### Add extents\n\n```bash\nlvextend -r -l +25 /dev/vgfiles/lvfiles\n```\n\n> 25 × PE size.\n\n### Check the result\n\n```bash\ndf -h /data; lvs vgdata\n```\n\n> df must show the new size too.\n\n### Forgot -r? Grow the file system later\n\n```bash\nxfs_growfs /data\nresize2fs /dev/vgdata/lvweb\n```\n\n> XFS by mount point, ext4 by device.\n\n### Grow the root file system\n\n```bash\nlvextend -r -L +5G /dev/rhel/root\n```\n\n> Online, no reboot.\n\n### Grow a swap LV\n\n```bash\nswapoff /dev/rhel/swap; lvextend -L +1G /dev/rhel/swap; mkswap /dev/rhel/swap; swapon -a\n```\n\n> Swap must be recreated (check that the UUID in fstab still matches, or use the LV path).\n\n### Only use space of a chosen PV\n\n```bash\nlvextend -r -L +2G /dev/vgdata/lvdata /dev/vdd\n```\n\n> New extents come from vdd.\n\n### Grow a thin pool before it fills\n\n```bash\nlvextend -L +5G vgdata/pool1\n```\n\n> A full thin pool stops all writes in its volumes.\n\n## Watch out\n\n- Without -r: LV bigger, file system not. `df -h` shows the problem.\n- \"Insufficient free space\": add a PV with vgextend first.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man lvextend](https://linux.die.net/man/8/lvextend)\n","page":"commands/lvextend/","label":"lvextend","where":"Command page · LVM","aka":["lvresize"]},{"id":"cmd-lvreduce","n":1460,"slug":"lvreduce","title":"lvreduce — Make a logical volume smaller — with `-r` the file system is shrunk first (ext4 only; XFS cannot shrink)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","shrink","resize","ext4","logical volume"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Make a logical volume smaller — with `-r` the file system is shrunk first (ext4 only; XFS cannot shrink).\n\n## About\n\nShrinking is risky: if the LV gets smaller than the file system, data is lost. `lvreduce -r` checks and shrinks the file system before the LV (it may need to unmount it; newer lvm2 asks or does it with `--fsmode manage`).\n\nXFS file systems cannot be shrunk. To make an XFS volume smaller: back up, recreate smaller, restore.\n\n## Syntax\n\n```text\nlvreduce (-L [-]SIZE | -l [-]EXTENTS) [-r] [OPTION]... VG/LV\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L, --size [-]SIZE` | New size or - removed size, like -1G or 3G. |\n| `-l, --extents [-]N[%VG\\|%LV\\|%FREE\\|%ORIGIN]` | In extents or percent. |\n| `-r, --resizefs` | Shrink the file system first (ext4). |\n| `-n, --nofsck` | Do not check the file system first. |\n| `--fs checksize\\|resize\\|resize_fsadm\\|ignore` | File system handling (newer lvm2). |\n| `--fsmode manage\\|offline\\|nochange` | May LVM unmount/mount for the resize. |\n| `-f, --force` | Do not ask (dangerous). |\n| `-A, --autobackup y\\|n` | Back up the metadata. |\n| `--noudevsync` | Do not wait for udev. |\n| `--reportformat basic\\|json` | Output format. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Shrink an ext4 LV safely\n\n```bash\nlvreduce -r -L 3G /dev/vgfiles/lvdocs\n```\n\n> Runs e2fsck and resize2fs, then shrinks the LV.\n\n### Remove 1 GiB\n\n```bash\nlvreduce -r -L -1G /dev/vgfiles/lvdocs\n```\n\n> Relative size.\n\n### Check the free space first\n\n```bash\ndf -h /docs\n```\n\n> The data must fit into the new size.\n\n### Do it by hand (old way)\n\n```bash\numount /docs\ne2fsck -f /dev/vgfiles/lvdocs\nresize2fs /dev/vgfiles/lvdocs 3G\nlvreduce -L 3G /dev/vgfiles/lvdocs\nmount /docs\n```\n\n> File system first, then LV — never the other way.\n\n### Test only\n\n```bash\nlvreduce -t -r -L 3G /dev/vgfiles/lvdocs\n```\n\n> Shows what would happen.\n\n### XFS cannot shrink\n\n```bash\nlvreduce -r -L 3G /dev/vgfiles/lvfiles\n```\n\n> Fails for XFS: back up, lvremove, lvcreate smaller, mkfs.xfs, restore.\n\n### Shrink a swap LV\n\n```bash\nswapoff /dev/rhel/swap; lvreduce -L 1G /dev/rhel/swap; mkswap /dev/rhel/swap; swapon -a\n```\n\n> Swap holds no data after swapoff.\n\n### Free space for another LV\n\n```bash\nlvreduce -r -L -2G /dev/vgdata/lvweb; lvcreate -n lvnew -L 2G vgdata\n```\n\n> Move space between LVs in one VG.\n\n### Verify the data afterwards\n\n```bash\nls /docs; df -h /docs\n```\n\n> Your files are still there.\n\n### Back up before shrinking\n\n```bash\ntar -czf /root/docs-backup.tgz -C /docs .\n```\n\n> Always, on real systems.\n\n## Watch out\n\n- Never lvreduce without -r on an LV with a file system: the end of the file system is cut off.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man lvreduce](https://linux.die.net/man/8/lvreduce)\n","page":"commands/lvreduce/","label":"lvreduce","where":"Command page · LVM","aka":[]},{"id":"cmd-lvs","n":1461,"slug":"lvs","title":"lvs — Show logical volumes: short table (lvs), full details (lvdisplay), or list all (lvscan)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","logical volume","report","size","snapshot use"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show logical volumes: short table (lvs), full details (lvdisplay), or list all (lvscan).\n\nSame page for: `lvdisplay`, `lvscan`\n\n## About\n\n`lvs` prints one line per LV with its VG, attributes, size, and for snapshots and thin volumes the data use. `lvdisplay` gives the device path, UUID, status and extent count. `lvscan` lists all LVs with their state (ACTIVE/inactive).\n\n## Syntax\n\n```text\nlvs [OPTION]... [VG|VG/LV...]\nlvdisplay [OPTION]... [VG|VG/LV...]\nlvscan [OPTION]...\n```\n\n## Options\n\n\n### Report (lvs)\n\n| Option | What it does |\n|---|---|\n| `-o, --options COLUMNS` | Columns, like lv_name,lv_size,lv_path,devices (+col = add). |\n| `-O, --sort COLUMNS` | Sort. |\n| `-S, --select EXPR` | Filter, like 'lv_size>1g'. |\n| `--units UNIT` | Units: h, b, k, m, g, t, e … |\n| `--nosuffix` | No unit letters. |\n| `--noheadings` | No header line. |\n| `--separator TEXT` | Column separator. |\n| `-a, --all` | Also hidden internal LVs (pool data, RAID images). |\n| `--segments` | One line per segment. |\n| `-H, --history` | Also removed LVs (if history is on). |\n| `--aligned, --rows, --binary, --nameprefixes, --unquoted` | Output styles. |\n| `--reportformat basic\\|json` | JSON output. |\n| `--configreport, --logonly, --foreign, --shared, --readonly, --ignorelockingfailure, --unbuffered` | Special report settings. |\n\n### lvdisplay\n\n| Option | What it does |\n|---|---|\n| `-m, --maps` | Show which PVs and extents the LV uses. |\n| `-C, --columns` | Report like lvs. |\n\n### lvscan\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Also internal LVs. |\n| `-b, --blockdevice` | Show device numbers. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### All LVs\n\n```bash\nlvs\n```\n```text\n  LV     VG     Attr       LSize   Pool Origin Data%  Meta%\n  root   rhel   -wi-ao---- <17.00g\n  swap   rhel   -wi-ao----   2.00g\n  lvdata vgdata -wi-ao----   5.00g\n```\n\n> a = active, o = open (mounted).\n\n### Details with path\n\n```bash\nlvdisplay /dev/vgdata/lvdata\n```\n\n> LV Path, UUID, LV Size, Current LE.\n\n### Which disks does an LV use?\n\n```bash\nlvs -o +devices\n```\n\n> Important before removing a PV.\n\n### Size in MiB or extents\n\n```bash\nlvs --units m; lvs -o lv_name,seg_size_pe\n```\n\n> Check exam tasks with extents.\n\n### Snapshot and thin use\n\n```bash\nlvs -o lv_name,origin,data_percent,metadata_percent\n```\n\n> A full snapshot becomes invalid; a full thin pool stops writes.\n\n### All LVs including hidden ones\n\n```bash\nlvs -a\n```\n\n> Pool data, metadata and RAID images.\n\n### Extent map\n\n```bash\nlvdisplay -m /dev/vgdata/lvdata\n```\n\n> Which PV ranges are used.\n\n### List with state\n\n```bash\nlvscan\n```\n```text\n  ACTIVE            '/dev/rhel/root' [<17.00 GiB] inherit\n```\n\n> Inactive LVs cannot be mounted (vgchange -ay).\n\n### JSON\n\n```bash\nlvs --reportformat json -o lv_name,lv_size,vg_name\n```\n\n> For automation.\n\n### Large LVs only\n\n```bash\nlvs -S 'lv_size>10g'\n```\n\n> Filter the report.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man lvs](https://linux.die.net/man/8/lvs)\n","page":"commands/lvs/","label":"lvs","where":"Command page · LVM","aka":["lvdisplay","lvscan"]},{"id":"cmd-man","n":1462,"slug":"man","title":"man — Read the manual page of a command, a configuration file or a system call","sections":["commands"],"topic":"Commands · Shell and help","tags":["help","manual","documentation","apropos","section"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read the manual page of a command, a configuration file or a system call.\n\n## About\n\nAlmost every command and config file has a manual page (\"man page\"). `man NAME` opens it in a pager (usually `less`). The pages are split into **sections**:\n\n| Section | Contains |\n|---|---|\n| 1 | Commands for all users |\n| 2, 3 | System calls and library functions (for programmers) |\n| 4 | Device files |\n| 5 | File formats and config files |\n| 7 | Overviews and conventions |\n| 8 | Commands for system administration (root) |\n\nWhen a name exists in several sections (like `passwd` = command and file), give the section number: `man 5 passwd`.\n\nInside the pager: `/word` searches, `n` goes to the next match, `G` goes to the end (often to EXAMPLES), `q` quits.\n\n## Syntax\n\n```text\nman [OPTIONS] [SECTION] PAGE ...\nman -k KEYWORD\nman -f PAGE\nman -K TEXT\n```\n\n## Options\n\n\n### Find pages\n\n| Option | What it does |\n|---|---|\n| `-k, --apropos` | Search the short descriptions of all pages (same as `apropos`). |\n| `-f, --whatis` | Show the short description of a page (same as `whatis`). |\n| `-K, --global-apropos` | Search the full text of all pages (slow). |\n| `-a, --all` | Show all pages with this name, one after the other. |\n| `-w, --where, --path, --location` | Print the file of the page instead of showing it. |\n| `-W, --where-cat, --location-cat` | Print the file of the pre-formatted page. |\n| `-l, --local-file` | Show a man page file from the disk (not from the database). |\n| `-S, -s, --sections=<list>` | Search only these sections, like 5:8. |\n| `-e, --extension=<sub>` | Only pages with this sub-extension. |\n| `-i, --ignore-case` | Ignore upper/lower case (the default). |\n| `-I, --match-case` | Match upper/lower case exactly. |\n| `--regex` | Treat the names as regular expressions. |\n| `--wildcard` | Treat the names as shell wildcards. |\n| `--names-only` | With --regex or --wildcard: match only page names. |\n| `-u, --update` | Update the cache before searching. |\n| `--no-subpages` | Do not join names like `git diff` into `git-diff`. |\n\n### Display\n\n| Option | What it does |\n|---|---|\n| `-P, --pager=<pager>` | Use another pager program. |\n| `-r, --prompt=<text>` | The prompt text of less. |\n| `-7, --ascii` | Show pure ASCII. |\n| `-E, --encoding=<enc>` | Output encoding. |\n| `--no-hyphenation, --nh` | No hyphenation. |\n| `--no-justification, --nj` | No text justification. |\n| `-p, --preprocessor=<list>` | Preprocessors to run (like t = tables). |\n| `-t, --troff` | Format the page for printing (PostScript). |\n| `-T, --troff-device[=<device>]` | Format for another device. |\n| `-H, --html[=<browser>]` | Show the page as HTML in a browser. |\n| `-X, --gxditview[=<dpi>]` | Show the page in a graphical window. |\n| `-Z, --ditroff` | Output for ditroff. |\n\n### Settings\n\n| Option | What it does |\n|---|---|\n| `-C, --config-file=<file>` | Use another configuration file. |\n| `-M, --manpath=<path>` | Search pages in these folders. |\n| `-m, --systems=<list>` | Search pages of other systems. |\n| `-L, --locale=<locale>` | Use this language. |\n| `-R, --recode=<enc>` | Convert the page to this encoding. |\n| `-c, --catman` | Used by catman only. |\n| `-d, --debug` | Print debug information. |\n| `-D, --default` | Reset all options to the defaults. |\n| `--warnings[=<list>]` | Show groff warnings. |\n| `-?, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Open the manual of a command\n\n```bash\nman useradd\n```\n\n> Press `/` and type a word to search, `n` for the next match, `q` to quit.\n\n### Open the manual of a config file\n\n```bash\nman 5 fstab\n```\n\n> Section 5 explains the six columns of /etc/fstab. Very useful in the exam.\n\n### Find the right command for a job\n\n```bash\nman -k \"logical volume\"\n```\n\n> Lists every page that mentions it in its short description.\n\n### Find only admin commands about a topic\n\n```bash\nman -k partition | grep '(8)'\n```\n\n> Section 8 = commands for root, like fdisk and parted.\n\n### See the short description only\n\n```bash\nman -f passwd\n```\n```text\npasswd (1)           - change user password\npasswd (5)           - the password file\n```\n\n> Two pages: the command (1) and the file format (5).\n\n### Find where the page file is\n\n```bash\nman -w sshd_config\n```\n```text\n/usr/share/man/man5/sshd_config.5.gz\n```\n\n> Shows which package documentation you are reading.\n\n### Jump straight to the examples\n\n```bash\nman nmcli-examples\n```\n\n> Some tools have a separate examples page. For others, open the page and press `G`, then scroll up to EXAMPLES.\n\n### Search all pages for a word in the text\n\n```bash\nman -K \"PermitRootLogin\"\n```\n\n> Slow, but finds pages that mention a word anywhere.\n\n### Read a man page file that came with a download\n\n```bash\nman -l ./docs/mytool.1\n```\n\n> Works for any page file, also outside the system folders.\n\n### Show all pages with the same name\n\n```bash\nman -a crontab\n```\n\n> First crontab(1) (the command), then crontab(5) (the file format). Press q to go to the next one.\n\n### Save a page as plain text for notes\n\n```bash\nman ss | col -b > ss-manual.txt\n```\n\n> `col -b` removes the bold and underline codes.\n\n### Read the systemd unit options\n\n```bash\nman systemd.service\n```\n\n> The pages systemd.unit, systemd.service and systemd.timer explain every option of unit files.\n\n## Watch out\n\n- \"nothing appropriate\" from `man -k` usually means the search database is old or empty. Run `mandb` as root.\n- Minimal installs can miss man pages. Install the package `man-pages` and the documentation of the tool.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/man/` | The man pages, one folder per section (man1, man5, man8 …). |\n| `/etc/man_db.conf` | Settings of man and mandb. |\n\n## Tips\n\n- 1 = user commands, 5 = files, 8 = admin commands.\n- `man -k` = I do not know the name. `man NAME` = I know the name.\n- In the pager: `/` search, `n` next, `G` end, `q` quit.\n\nFull manual: [man man](https://linux.die.net/man/1/man)\n","page":"commands/man/","label":"man","where":"Command page · Shell and help","aka":[]},{"id":"cmd-mandb","n":1463,"slug":"mandb","title":"mandb — Build or update the search database of the man pages (used by `man -k`, `apropos` and `whatis`)","sections":["commands"],"topic":"Commands · Shell and help","tags":["help","manual","database","apropos","index"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Build or update the search database of the man pages (used by `man -k`, `apropos` and `whatis`).\n\n## About\n\n`man -k` and `apropos` do not read every manual page; they search a small database with the short descriptions. `mandb` creates and updates that database.\n\nOn RHEL a timer updates it every day. But right after an install, or after you install new packages, the database can be empty or old. Then `man -k` says \"nothing appropriate\". Run `mandb` as root to fix it.\n\n## Syntax\n\n```text\nmandb [-dqsucpt?V] [-C FILE] [MANPATH]\nmandb [-dqsut] [-C FILE] -f FILENAME ...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-c, --create` | Build the databases from zero, even if they exist. |\n| `-q, --quiet` | Only print warnings and errors. |\n| `-u, --user-db` | Build only the database of your user, not the system databases. |\n| `-f, --filename <file>` | Update only the entry of this one page file. |\n| `-s, --no-straycats` | Do not look for formatted pages without a source. |\n| `-p, --no-purge` | Do not remove entries of pages that were deleted. |\n| `-t, --test` | Check the pages for problems; do not build a database. |\n| `-d, --debug` | Print debug information. |\n| `-C, --config-file=<file>` | Use another configuration file. |\n| `-?, --help` | Show help. |\n| `--usage` | Show a short usage message. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Update the database\n\n```bash\nmandb\n```\n\n> Takes a few seconds. After this, `man -k` finds new pages.\n\n### Fix \"nothing appropriate\" from man -k\n\n```bash\nman -k partition\n```\n```text\npartition: nothing appropriate.\n```\n```bash\nmandb\nman -k partition\n```\n\n> Typical on a fresh system or in the exam: build the database once.\n\n### Build everything from zero\n\n```bash\nmandb -c\n```\n\n> Use it when the database seems broken.\n\n### Update quietly in a script\n\n```bash\nmandb -q\n```\n\n> Only errors are printed. Good after installing many packages.\n\n### Update after you add your own man page\n\n```bash\ncp mytool.8 /usr/local/share/man/man8/ && mandb -q\nman -k mytool\n```\n\n> Your page appears in the search.\n\n### Update only the entry of one file\n\n```bash\nmandb -f /usr/local/share/man/man8/mytool.8\n```\n\n> Faster than a full run.\n\n### Build a database for your own pages (no root)\n\n```bash\nmandb -u\n```\n\n> For pages in your home folder (for example in ~/.local/share/man).\n\n### Check the pages for errors without changing anything\n\n```bash\nmandb -t\n```\n\n> Reports broken or wrong pages.\n\n### Check the timer that updates the database every day\n\n```bash\nsystemctl list-timers | grep man-db\n```\n\n> If the timer is missing or disabled, the database gets old.\n\n### Update the database after a big dnf install in a template\n\n```bash\ndnf install -y bind-utils nfs-utils && mandb -q\n```\n\n> A ready-to-use VM template should have a fresh man database.\n\n### See which folders mandb indexes\n\n```bash\nmanpath\n```\n\n> Prints the folders that man and mandb use.\n\n## Watch out\n\n- `mandb` does not install man pages. If a page is missing, install the package that has it.\n- Run it as root to update the system databases.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/cache/man/` | The databases (index.db files). |\n| `/etc/man_db.conf` | Which folders are indexed. |\n\n## Tips\n\n- \"nothing appropriate\" → `sudo mandb` → try `man -k` again.\n\nFull manual: [man mandb](https://linux.die.net/man/8/mandb)\n","page":"commands/mandb/","label":"mandb","where":"Command page · Shell and help","aka":[]},{"id":"cmd-mkdir","n":1464,"slug":"mkdir","title":"mkdir — Create new folders (directories), also with all missing parent folders","sections":["commands"],"topic":"Commands · Files and folders","tags":["folder","directory","create","parents","permissions","selinux"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create new folders (directories), also with all missing parent folders.\n\n## About\n\n`mkdir NAME` creates a folder. With `-p` it also creates every missing folder in the path, and gives no error when the folder already exists, which makes it safe in scripts.\n\nNew folders get the permissions 777 minus your umask (usually 755). With `-m` you choose the permissions at once.\n\n## Syntax\n\n```text\nmkdir [OPTION]... DIRECTORY...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --parents` | Make the parent folders as needed; no error if the folder exists. |\n| `-m, --mode=<mode>` | Set the permissions of the new folder, like 750 or u=rwx,g=rx. |\n| `-v, --verbose` | Print a line for each folder made. |\n| `-Z` | Set the default SELinux context for the new folder. |\n| `--context[=<ctx>]` | Like -Z, or set this exact SELinux context. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Make one folder\n\n```bash\nmkdir reports\n```\n\n> In the current folder.\n\n### Make a whole path at once\n\n```bash\nmkdir -p /srv/app/releases/2026-10\n```\n\n> Every missing folder is made. No error if it exists.\n\n### Make several folders with braces\n\n```bash\nmkdir -p /data/{web,db,backup}/{logs,tmp}\n```\n\n> Six folders in one command.\n\n### Make a private folder\n\n```bash\nmkdir -m 700 ~/secrets\n```\n\n> Only you can enter it. No separate chmod needed.\n\n### Make a shared folder for a team\n\n```bash\nmkdir -m 2770 /srv/shared\nchgrp devs /srv/shared\n```\n\n> The 2 sets SGID: new files get the group `devs`.\n\n### Show what is made\n\n```bash\nmkdir -pv /tmp/a/b/c\n```\n```text\nmkdir: created directory '/tmp/a'\nmkdir: created directory '/tmp/a/b'\nmkdir: created directory '/tmp/a/b/c'\n```\n\n> Good in logs of scripts.\n\n### Make a folder with the right SELinux label for a web site\n\n```bash\nmkdir -Z /var/www/shop\nls -dZ /var/www/shop\n```\n\n> The folder gets the default label of its place (httpd_sys_content_t).\n\n### Make a dated backup folder\n\n```bash\nmkdir -p /backup/$(date +%F)\n```\n\n> One folder per day, like /backup/2026-10-25.\n\n### Make a folder only if it does not exist (script)\n\n```bash\n[ -d /var/lib/myapp ] || mkdir -p /var/lib/myapp\n```\n\n> With -p the test is not even needed; it shows the idea.\n\n### Make a mount point for a new disk\n\n```bash\nmkdir -p /mnt/data\n```\n\n> A mount point is just an empty folder.\n\n### Make the folders of a project from a list\n\n```bash\nxargs -a dirs.txt mkdir -p\n```\n\n> One folder name per line in dirs.txt.\n\n## Watch out\n\n- Without `-p`, a missing parent gives \"No such file or directory\".\n- `-m` only applies to the last folder; parents made by `-p` get the normal permissions.\n\n## Tips\n\n- `mkdir -p` = safe in scripts.\n- `{a,b,c}` = many folders at once.\n\nFull manual: [man mkdir](https://linux.die.net/man/1/mkdir)\n","page":"commands/mkdir/","label":"mkdir","where":"Command page · Files and folders","aka":[]},{"id":"cmd-mkfs-ext4","n":1465,"slug":"mkfs.ext4","title":"mkfs.ext4 — Create an ext4 (or ext3/ext2) file system","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["ext4","file system","format","label","journal"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create an ext4 (or ext3/ext2) file system.\n\nSame page for: `mke2fs`, `mkfs.ext3`, `mkfs.ext2`\n\n## About\n\n`mkfs.ext4` is `mke2fs -t ext4`. ext4 can grow (also mounted) and shrink (only unmounted) with `resize2fs`. By default 5 % of the blocks are reserved for root (`-m`), so root can still work when users fill the disk.\n\n## Syntax\n\n```text\nmkfs.ext4 [OPTION]... DEVICE [SIZE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L LABEL` | Set a label (max 16 characters). |\n| `-m PERCENT` | Reserved blocks for root (default 5). |\n| `-t TYPE` | ext2, ext3 or ext4 (with mke2fs). |\n| `-T USAGE` | Usage type from mke2fs.conf: news, largefile, largefile4, small, floppy. |\n| `-b SIZE` | Block size (1024, 2048, 4096). |\n| `-i BYTES` | Bytes per inode (smaller = more inodes). |\n| `-N NUMBER` | Number of inodes. |\n| `-I SIZE` | Inode size. |\n| `-U UUID` | Set the UUID. |\n| `-O FEATURES` | Turn features on or off (^feature = off), like ^has_journal. |\n| `-E OPTIONS` | Extended options: stride=, stripe_width=, lazy_itable_init=, nodiscard … |\n| `-j` | Create a journal (ext3). |\n| `-J OPTIONS` | Journal options (size=, device=). |\n| `-e continue\\|remount-ro\\|panic` | What to do on errors. |\n| `-c` | Check for bad blocks first. |\n| `-l FILE` | Read bad blocks from a file. |\n| `-g N, -G N` | Blocks per group / groups per flex group. |\n| `-C SIZE` | Cluster size (bigalloc). |\n| `-d DIR` | Copy this folder into the new file system. |\n| `-M DIR` | Last mounted directory. |\n| `-o OS` | Creator OS. |\n| `-r REVISION` | File system revision. |\n| `-S` | Write only superblock and group descriptors (rescue, dangerous). |\n| `-D` | Use direct I/O. |\n| `-z FILE` | Save an undo file. |\n| `-n` | Dry run: show what would happen. |\n| `-F` | Force. |\n| `-q` | Quiet. |\n| `-v` | Verbose. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Create ext4\n\n```bash\nmkfs.ext4 /dev/vdb2\n```\n\n> Asks before overwriting an existing file system.\n\n### With a label\n\n```bash\nmkfs.ext4 -L backup /dev/vdb2\n```\n\n> Mount with LABEL=backup.\n\n### Less reserved space on a data disk\n\n```bash\nmkfs.ext4 -m 1 /dev/vdb2\n```\n\n> 1 % instead of 5 % (big disks waste a lot with 5 %).\n\n### Many small files\n\n```bash\nmkfs.ext4 -T small /dev/vdb2\n```\n\n> More inodes (mail or cache folders).\n\n### Large files only\n\n```bash\nmkfs.ext4 -T largefile4 /dev/vdb2\n```\n\n> Fewer inodes, more space for data.\n\n### Dry run\n\n```bash\nmkfs.ext4 -n /dev/vdb2\n```\n\n> Shows the backup superblock positions — useful for repair.\n\n### ext3 or ext2\n\n```bash\nmke2fs -t ext3 /dev/vdb3\n```\n\n> Older formats, for old systems.\n\n### Fast create on big devices\n\n```bash\nmkfs.ext4 -E lazy_itable_init=1,nodiscard /dev/vdb2\n```\n\n> Finishes the inode tables in the background.\n\n### Check the result\n\n```bash\ndumpe2fs -h /dev/vdb2 | grep -E 'volume name|Reserved block|Inode count'\n```\n\n> Settings of the new file system.\n\n### Change settings later\n\n```bash\ntune2fs -m 1 -L data2 /dev/vdb2\n```\n\n> No need to recreate.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/mke2fs.conf` | Defaults and usage types. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man mkfs.ext4](https://linux.die.net/man/8/mke2fs)\n","page":"commands/mkfs.ext4/","label":"mkfs.ext4","where":"Command page · Disks and file systems","aka":["mke2fs","mkfs.ext3","mkfs.ext2"]},{"id":"cmd-mkfs","n":1466,"slug":"mkfs","title":"mkfs — Create a file system on a partition or volume (front end for mkfs.xfs, mkfs.ext4, mkfs.vfat …)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["file system","format","xfs","ext4","vfat"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a file system on a partition or volume (front end for mkfs.xfs, mkfs.ext4, mkfs.vfat …).\n\n## About\n\n`mkfs -t TYPE DEVICE` calls the program `mkfs.TYPE`. On RHEL the main types are `xfs` (default), `ext4` and `vfat`. All options after the type go to that program. Creating a file system deletes everything that was on the device.\n\n## Syntax\n\n```text\nmkfs [-t TYPE] [FS-OPTIONS] DEVICE [SIZE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t, --type TYPE` | The file system type (default ext2 — always give it). |\n| `FS-OPTIONS` | Options for the real mkfs program (like -L label). |\n| `-V, --verbose` | Show what is run. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### XFS file system\n\n```bash\nmkfs -t xfs /dev/vdb1\n```\n\n> Same as `mkfs.xfs /dev/vdb1`.\n\n### ext4 with a label\n\n```bash\nmkfs -t ext4 -L backup /dev/vdb2\n```\n\n> The label goes to mkfs.ext4.\n\n### Which mkfs programs exist?\n\n```bash\nls /usr/sbin/mkfs.*\n```\n\n> The types you can use.\n\n### Check the disk first\n\n```bash\nlsblk -f /dev/vdb\n```\n\n> Make sure the partition is empty or not needed.\n\n### FAT for a USB stick\n\n```bash\nmkfs -t vfat -n USBKEY /dev/sdc1\n```\n\n> Readable on Windows and macOS.\n\n### On an LVM logical volume\n\n```bash\nmkfs -t xfs /dev/vgdata/lvweb\n```\n\n> Same as on a partition.\n\n### See what would be run\n\n```bash\nmkfs -V -t ext4 /dev/vdb2\n```\n\n> Prints the real command.\n\n### Overwrite an old file system\n\n```bash\nmkfs -t xfs -f /dev/vdb1\n```\n\n> mkfs.xfs needs -f when it finds an old file system.\n\n### Remove old signatures first\n\n```bash\nwipefs -a /dev/vdb1; mkfs -t xfs /dev/vdb1\n```\n\n> Clean start.\n\n### Check the result\n\n```bash\nblkid /dev/vdb1\n```\n\n> Shows TYPE and the new UUID.\n\n## Watch out\n\n- Without -t, mkfs creates ext2. Always give the type.\n- The wrong device = lost data. Check with lsblk -f.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man mkfs](https://linux.die.net/man/8/mkfs)\n","page":"commands/mkfs/","label":"mkfs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-mkfs-vfat","n":1467,"slug":"mkfs.vfat","title":"mkfs.vfat — Create a FAT file system (FAT12/16/32) — for USB sticks, SD cards and the EFI system partition","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["fat","vfat","fat32","usb","efi","windows"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a FAT file system (FAT12/16/32) — for USB sticks, SD cards and the EFI system partition.\n\nSame page for: `mkfs.fat`, `mkdosfs`\n\n## About\n\nFAT works on almost every system (Windows, macOS, cameras, firmware). It has no Linux permissions and a 4 GiB limit per file. The EFI system partition (`/boot/efi`) must be FAT. `-F 32` forces FAT32; `-n` sets the volume name.\n\n## Syntax\n\n```text\nmkfs.vfat [OPTION]... DEVICE [BLOCK-COUNT]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n NAME` | Volume name (max 11 characters). |\n| `-F 12\\|16\\|32` | FAT size (default: chosen by size). |\n| `-s N` | Sectors per cluster. |\n| `-S SIZE` | Logical sector size. |\n| `-i ID` | Volume ID (32-bit hex). |\n| `-c` | Check for bad blocks first. |\n| `-l FILE` | Read bad blocks from a file. |\n| `-C` | Create the file given as DEVICE (an image file). |\n| `-I` | Ignore safety checks (whole disk without partitions). |\n| `-a` | Do not align structures. |\n| `-A` | Atari variant. |\n| `-b SECTOR` | Backup boot sector position. |\n| `-D NUMBER` | Drive number. |\n| `-f N` | Number of FATs (default 2). |\n| `-g HEADS/SECTORS` | Disk geometry. |\n| `-h N` | Hidden sectors. |\n| `-M TYPE` | Media type. |\n| `-m FILE` | Boot message file. |\n| `-r N` | Root directory entries (FAT12/16). |\n| `-R N` | Reserved sectors. |\n| `--mbr[=y\\|n\\|a]` | Write an MBR on the device. |\n| `--codepage=PAGE` | Codepage for short names. |\n| `--offset SECTOR` | Write at this offset. |\n| `--variant TYPE` | standard or atari. |\n| `--invariant` | Use fixed values (reproducible images). |\n| `-v` | Verbose. |\n| `--help` | Show help. |\n\n## Examples\n\n### Format a USB stick\n\n```bash\nmkfs.vfat -n USBKEY /dev/sdc1\n```\n\n> Check the device name with lsblk first!\n\n### Force FAT32\n\n```bash\nmkfs.vfat -F 32 -n BACKUP /dev/sdc1\n```\n\n> For bigger sticks.\n\n### EFI system partition\n\n```bash\nmkfs.vfat -F 32 -n EFI /dev/vdd1\n```\n\n> Mounted on /boot/efi.\n\n### Make an image file\n\n```bash\nmkfs.vfat -C /tmp/fat.img 65536\n```\n\n> A 64 MiB FAT image (block count in KiB).\n\n### Mount it with your user as owner\n\n```bash\nmount -o uid=1000,gid=1000,umask=022 /dev/sdc1 /mnt\n```\n\n> FAT has no owners; mount options set them.\n\n### Check a FAT file system\n\n```bash\nfsck.vfat -a /dev/sdc1\n```\n\n> Repairs simple errors.\n\n### Change the label later\n\n```bash\nfatlabel /dev/sdc1 NEWNAME\n```\n\n> Without formatting.\n\n### Wipe a used stick first\n\n```bash\nwipefs -a /dev/sdc\n```\n\n> Removes old tables and signatures.\n\n### Reproducible image (build pipelines)\n\n```bash\nmkfs.vfat --invariant -C /tmp/boot.img 32768\n```\n\n> Same input = same output bytes.\n\n### Check the type\n\n```bash\nlsblk -f /dev/sdc\n```\n\n> FSTYPE vfat, FSVER FAT32.\n\n## Watch out\n\n- FAT has a 4 GiB file size limit and no Linux permissions.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man mkfs.vfat](https://linux.die.net/man/8/mkfs.vfat)\n","page":"commands/mkfs.vfat/","label":"mkfs.vfat","where":"Command page · Disks and file systems","aka":["mkfs.fat","mkdosfs"]},{"id":"cmd-mkfs-xfs","n":1468,"slug":"mkfs.xfs","title":"mkfs.xfs — Create an XFS file system — the default file system of RHEL","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["xfs","file system","format","label"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create an XFS file system — the default file system of RHEL.\n\n## About\n\nXFS is fast with large files and many parallel writers. It can grow while mounted (`xfs_growfs`) but can **not** be shrunk. The defaults are good for nearly all cases; the label (`-L`, max 12 characters) is the option you use most.\n\n## Syntax\n\n```text\nmkfs.xfs [OPTION]... DEVICE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L LABEL` | Set a label (max 12 characters). |\n| `-f` | Overwrite an existing file system. |\n| `-b size=N` | Block size (default 4096). |\n| `-d OPTIONS` | Data section, like su=,sw= for RAID stripes, agcount=N. |\n| `-i OPTIONS` | Inode options (size, maxpct). |\n| `-l OPTIONS` | Log (journal) options, like an external log `logdev=`. |\n| `-m OPTIONS` | Metadata options: crc, reflink, bigtime, finobt, uuid=. |\n| `-n OPTIONS` | Directory naming options. |\n| `-r OPTIONS` | Realtime section. |\n| `-s size=N` | Sector size. |\n| `-p FILE` | Prototype file to fill the file system. |\n| `-c options=FILE` | Read options from a config file. |\n| `-N` | Show the settings but do not create anything. |\n| `-K` | Do not discard (TRIM) blocks at creation. |\n| `-q` | Quiet. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Create XFS\n\n```bash\nmkfs.xfs /dev/vdb1\n```\n\n> Prints the geometry it chose.\n\n### With a label\n\n```bash\nmkfs.xfs -L data /dev/vdb1\n```\n\n> Mount with `LABEL=data`.\n\n### Overwrite an old file system\n\n```bash\nmkfs.xfs -f /dev/vdb1\n```\n\n> Without -f it refuses when it finds a file system.\n\n### On a logical volume\n\n```bash\nmkfs.xfs /dev/vgdata/lvdata\n```\n\n> Grow later with lvextend -r.\n\n### Show settings without creating\n\n```bash\nmkfs.xfs -N /dev/vdb1\n```\n\n> Dry run.\n\n### Fast creation on a thin or SSD device\n\n```bash\nmkfs.xfs -K /dev/vdb1\n```\n\n> Skips the discard step (faster on big devices).\n\n### For a RAID with stripe settings\n\n```bash\nmkfs.xfs -d su=64k,sw=4 /dev/md0\n```\n\n> Aligns with the RAID layout.\n\n### Check the result\n\n```bash\nxfs_info /dev/vdb1\n```\n\n> After mounting, use the mount point.\n\n### Set the label later\n\n```bash\nxfs_admin -L newname /dev/vdb1\n```\n\n> Only when not mounted.\n\n### Remember: no shrinking\n\n```bash\nxfs_growfs /data\n```\n\n> XFS grows only. Plan sizes; use ext4 when you may need to shrink.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man mkfs.xfs](https://linux.die.net/man/8/mkfs.xfs)\n","page":"commands/mkfs.xfs/","label":"mkfs.xfs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-mkswap","n":1469,"slug":"mkswap","title":"mkswap — Prepare a partition, logical volume or file to be used as swap space","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["swap","memory","swap partition","swap file"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Prepare a partition, logical volume or file to be used as swap space.\n\n## About\n\n`mkswap` writes a swap signature (with a UUID and optional label). Then `swapon` switches it on. For a swap file, the file must exist first, be owned by root with mode 600, and have no holes.\n\n## Syntax\n\n```text\nmkswap [OPTION]... DEVICE [SIZE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L, --label LABEL` | Set a label. |\n| `-U, --uuid UUID` | Set the UUID (clear, random, time or a value). |\n| `-c, --check` | Check for bad blocks first. |\n| `-f, --force` | Go on even when the device looks in use or too small. |\n| `-p, --pagesize SIZE` | Page size to use. |\n| `-e, --endianness native\\|little\\|big` | Byte order. |\n| `-v, --swapversion 1` | Swap version (only 1). |\n| `-s, --size SIZE` | Size of a swap file to create (newer util-linux). |\n| `-F, --file` | Create the swap file (with --size; newer util-linux). |\n| `--lock[=MODE]` | Lock the device. |\n| `-q, --quiet` | No warnings. |\n| `--verbose` | More output. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Swap on a partition\n\n```bash\nmkswap /dev/vdb2\n```\n```text\nSetting up swapspace version 1, size = 1024 MiB (1073737728 bytes)\nno label, UUID=8f1b2c3d-…\n```\n\n> Then `swapon /dev/vdb2`.\n\n### With a label\n\n```bash\nmkswap -L swap2 /dev/vdb2\n```\n\n> fstab can use LABEL=swap2.\n\n### Swap on a logical volume\n\n```bash\nlvcreate -n lvswap -L 2G rhel\nmkswap /dev/rhel/lvswap\n```\n\n> Easy to grow later.\n\n### A swap file\n\n```bash\ndd if=/dev/zero of=/swapfile bs=1M count=1024\nchmod 600 /swapfile\nmkswap /swapfile\n```\n\n> 1 GiB file without holes.\n\n### Create the swap file in one step (newer util-linux)\n\n```bash\nmkswap -F -s 1G /swapfile\n```\n\n> Makes the file with the right mode.\n\n### Use it and make it permanent\n\n```bash\nswapon /swapfile\necho '/swapfile none swap defaults 0 0' >> /etc/fstab\n```\n\n> Check with `swapon --show`.\n\n### Get the UUID for fstab\n\n```bash\nblkid -s UUID -o value /dev/vdb2\n```\n\n> Use UUID= in fstab, not the device name.\n\n### Change the swap label later\n\n```bash\nswaplabel -L swap2 /dev/vdb2\n```\n\n> Without recreating.\n\n### Check for bad blocks first\n\n```bash\nmkswap -c /dev/vdb2\n```\n\n> Slower; for old disks.\n\n### Overwrite a device that looks in use\n\n```bash\nmkswap -f /dev/vdb2\n```\n\n> Only when you are sure.\n\n## Watch out\n\n- mkswap deletes the old content of the device.\n- A swap file with mode other than 600 lets users read memory pages: security risk.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man mkswap](https://linux.die.net/man/8/mkswap)\n","page":"commands/mkswap/","label":"mkswap","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-modinfo","n":1470,"slug":"modinfo","title":"modinfo — Show information about a kernel module: file, description, license, dependencies, aliases and parameters","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel modules","parameters","driver info","alias"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show information about a kernel module: file, description, license, dependencies, aliases and parameters.\n\n## About\n\n`modinfo NAME` reads the module file (also when it is not loaded). The `parm:` lines list the parameters you can set with modprobe or in `/etc/modprobe.d/`. The `alias:` lines show which hardware IDs the module supports.\n\n## Syntax\n\n```text\nmodinfo [OPTION]... MODULE|FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --parameters` | Only the parameters. |\n| `-F, --field FIELD` | Only one field: filename, description, author, license, depends, alias, parm, version, vermagic … |\n| `-d, --description` | Only the description. |\n| `-a, --author` | Only the author. |\n| `-l, --license` | Only the license. |\n| `-n, --filename` | Only the file name. |\n| `-k KERNEL` | Use the modules of another kernel version. |\n| `-b, --basedir DIR` | Use another root for /lib/modules. |\n| `-0, --null` | Separate fields with NUL. |\n| `-V, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### All information\n\n```bash\nmodinfo xfs\n```\n```text\nfilename:       /lib/modules/6.12.0-55.9.1.el10_0.x86_64/kernel/fs/xfs/xfs.ko.xz\ndescription:    SGI XFS with ACLs, security attributes, realtime, quota, no debug enabled\nlicense:        GPL\n```\n\n> Plus depends, signature and more.\n\n### Parameters of a module\n\n```bash\nmodinfo -p loop\n```\n```text\nmax_loop:Maximum number of loop devices (int)\nmax_part:Maximum number of partitions per loop device (int)\n```\n\n> Set them with `options loop max_part=8`.\n\n### Only the file\n\n```bash\nmodinfo -n vfat\n```\n\n> Where the .ko file is.\n\n### Dependencies\n\n```bash\nmodinfo -F depends vfat\n```\n```text\nfat\n```\n\n> modprobe loads these first.\n\n### Hardware IDs a driver supports\n\n```bash\nmodinfo -F alias e1000e | head\n```\n\n> Match with `lspci -nn` IDs.\n\n### Is the module signed? (Secure Boot)\n\n```bash\nmodinfo -F signer xfs\n```\n\n> Red Hat signs its modules; unsigned ones do not load with Secure Boot.\n\n### Current value of a parameter\n\n```bash\ncat /sys/module/loop/parameters/max_part\n```\n\n> modinfo shows the options; /sys shows the active value.\n\n### Modules of another kernel\n\n```bash\nmodinfo -k 6.12.0-55.el10.x86_64 xfs | head -2\n```\n\n> Check before you boot an older kernel.\n\n### Description only\n\n```bash\nmodinfo -d usb-storage\n```\n\n> Short text.\n\n### License (tainted kernel check)\n\n```bash\nmodinfo -l nvidia 2>/dev/null\n```\n\n> Non-GPL modules taint the kernel; support may ask.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Module not found or error. |\n\nFull manual: [man modinfo](https://linux.die.net/man/8/modinfo)\n","page":"commands/modinfo/","label":"modinfo","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-modprobe","n":1471,"slug":"modprobe","title":"modprobe — Load or unload kernel modules, with their dependencies and the settings from /etc/modprobe.d/","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel modules","load driver","unload","blacklist","parameters"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Load or unload kernel modules, with their dependencies and the settings from /etc/modprobe.d/.\n\n## About\n\n`modprobe NAME` loads a module and everything it needs; `modprobe -r NAME` removes it and unused dependencies. Parameters can be given on the command line (`modprobe loop max_part=8`) or in `/etc/modprobe.d/*.conf` with `options`. In the same folder `blacklist` and `install … /bin/false` stop modules from loading.\n\nModules that must load at boot go in `/etc/modules-load.d/*.conf`.\n\n## Syntax\n\n```text\nmodprobe [OPTION]... MODULE [PARAMETER=VALUE]...\nmodprobe -r [OPTION]... MODULE...\nmodprobe -c\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-r, --remove` | Unload the module (and unused dependencies). |\n| `-a, --all` | Load all modules named on the command line. |\n| `-n, --dry-run, --show` | Do everything except load/unload. |\n| `-v, --verbose` | Show what is done. |\n| `-q, --quiet` | No error if the module is not found. |\n| `-c, --showconfig` | Show the full configuration (options, blacklists, aliases). |\n| `--show-depends` | List the files that would be loaded. |\n| `-b, --use-blacklist` | Also apply blacklists when loading by name. |\n| `-i, --ignore-install, --ignore-remove` | Ignore install/remove commands from the config. |\n| `-f, --force` | Ignore version checks (dangerous). |\n| `--force-vermagic, --force-modversion` | Ignore one kind of version check. |\n| `--first-time` | Fail if already loaded (or not loaded with -r). |\n| `-R, --resolve-alias` | Print the module name of an alias. |\n| `-C, --config FILE` | Use another config file or folder. |\n| `-d, --dirname DIR` | Root folder for modules. |\n| `-S, --set-version VERSION` | Use modules of another kernel version. |\n| `-w, --wait MS` | With -r: wait for the module to be unused. |\n| `--dump-modversions` | Show the version info of a module file. |\n| `-s, --syslog` | Errors to syslog. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Load a module\n\n```bash\nmodprobe vfat\nlsmod | grep vfat\n```\n\n> fat is loaded too (dependency).\n\n### Unload it\n\n```bash\nmodprobe -r vfat\n```\n\n> Fails if it is in use.\n\n### Load with a parameter\n\n```bash\nmodprobe loop max_part=8\n```\n\n> Only until the module is unloaded or reboot.\n\n### Parameter for good\n\n```bash\necho \"options loop max_part=8\" > /etc/modprobe.d/loop.conf\n```\n\n> Used every time the module loads.\n\n### Load at boot\n\n```bash\necho vfat > /etc/modules-load.d/vfat.conf\n```\n\n> systemd-modules-load reads this folder.\n\n### Block a module (hardening)\n\n```bash\nprintf 'blacklist usb-storage\\ninstall usb-storage /bin/false\\n' > /etc/modprobe.d/no-usb-storage.conf\n```\n\n> blacklist stops auto-loading; install /bin/false stops modprobe too.\n\n### Test what would happen\n\n```bash\nmodprobe -nv vfat\n```\n```text\ninsmod /lib/modules/…/fat.ko.xz\ninsmod /lib/modules/…/vfat.ko.xz\n```\n\n> No change.\n\n### Show the active configuration\n\n```bash\nmodprobe -c | grep -E '^(options|blacklist)'\n```\n\n> All options and blacklists from all files.\n\n### Which module serves an alias\n\n```bash\nmodprobe -R pci:v00008086d000010D3sv*sd*bc*sc*i*\n```\n\n> Hardware ID to module name.\n\n### Rebuild the initramfs after changes for early modules\n\n```bash\ndracut -f\n```\n\n> Blacklists of storage drivers must be in the initramfs too.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/modprobe.d/*.conf` | options, blacklist, install, alias lines. |\n| `/etc/modules-load.d/*.conf` | Modules to load at boot. |\n| `/lib/modules/$(uname -r)/` | The module files. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man modprobe](https://linux.die.net/man/8/modprobe)\n","page":"commands/modprobe/","label":"modprobe","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-more","n":1472,"slug":"more","title":"more — Show text one screen at a time (the simple, older pager)","sections":["commands"],"topic":"Commands · Text and search","tags":["pager","read","view","page"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show text one screen at a time (the simple, older pager).\n\n## About\n\n`more` shows a file page by page. Space goes to the next page, `q` quits. It is simpler than `less` and mostly goes forward only. It exists on every Unix-like system, so you find it in old scripts and documentation.\n\nUse `less` when you can; use `more` when it is the only pager on a minimal system.\n\n## Syntax\n\n```text\nmore [OPTIONS] FILE...\nCOMMAND | more\n```\n\n## Parts\n\n\n### Keys inside more\n\n| Part | Meaning |\n|---|---|\n| `Space` | Next page. |\n| `Enter` | Next line. |\n| `b` | One page back (only for files, not pipes). |\n| `/pattern` | Search forward. n = next match. |\n| `=` | Show the current line number. |\n| `v` | Open the file in your editor at the current line. |\n| `h` | Help. |\n| `q` | Quit. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --silent` | Show \"[Press space to continue, 'q' to quit.]\" instead of a bell. |\n| `-l, --logical` | Do not pause at form feed characters (^L). |\n| `-e, --exit-on-eof` | Exit at the end of the file. |\n| `-f, --no-pause` | Count logical lines, not screen lines (no wrapping). |\n| `-p, --print-over` | Clear the screen before each page (no scrolling). |\n| `-c, --clean-print` | Paint each page from the top, clearing the rest. |\n| `-s, --squeeze` | Show several empty lines as one. |\n| `-u, --plain` | Ignore underlining. |\n| `-n, --lines <n>` | The number of lines per screen. |\n| `-<number>` | Same as --lines. |\n| `+<number>` | Start at this line. |\n| `+/<string>` | Start at the first match of the string. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Read a file page by page\n\n```bash\nmore /etc/services\n```\n\n> Space = next page, q = quit.\n\n### Page through long output\n\n```bash\nps aux | more\n```\n\n> On minimal systems without less.\n\n### Start at line 100\n\n```bash\nmore +100 /var/log/messages\n```\n\n> Skips the first 99 lines.\n\n### Start at the first match of a word\n\n```bash\nmore +/sshd /var/log/messages\n```\n\n> Jumps to the first line with sshd.\n\n### Show friendly prompts instead of a beep\n\n```bash\nmore -d README\n```\n\n> Helpful for people new to the terminal.\n\n### Squeeze empty lines\n\n```bash\nmore -s notes.txt\n```\n\n> Several empty lines become one.\n\n### Show 20 lines per page\n\n```bash\nmore -20 /etc/passwd\n```\n\n> Useful on small screens or in serial consoles.\n\n### Page through the environment (Red Hat cheat sheet style)\n\n```bash\nenv | more\n```\n\n> Classic combination to read a long list.\n\n### Read several files one after the other\n\n```bash\nmore /etc/hosts /etc/resolv.conf\n```\n\n> The file name is shown between them; `:n` goes to the next file.\n\n### Open the editor from more\n\n```bash\nmore /etc/fstab\n```\n```text\n(press v to open the file in vi at the current line)\n```\n\n> Quick way from reading to editing.\n\n### Use more as the pager for man (if less is missing)\n\n```bash\nPAGER=more man ls\n```\n\n> Any program that uses $PAGER can use more.\n\n## Watch out\n\n- Going back with `b` does not work when more reads from a pipe.\n\n## Tips\n\n- more = forward only, simple. less = forward and back, search, follow.\n\nFull manual: [man more](https://linux.die.net/man/1/more)\n","page":"commands/more/","label":"more","where":"Command page · Text and search","aka":[]},{"id":"cmd-mount","n":1473,"slug":"mount","title":"mount — Attach a file system (disk, partition, ISO, network share) to a folder, or show what is mounted","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["mount","file system","fstab","mount options","iso","bind","remount"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Attach a file system (disk, partition, ISO, network share) to a folder, or show what is mounted.\n\n## About\n\n`mount DEVICE FOLDER` makes the file system on DEVICE visible in FOLDER (the mount point). The type is detected automatically. `mount -a` mounts everything in `/etc/fstab` that is not mounted yet — the test after editing fstab. Options (`-o`) control the behaviour: read-only, no programs, no SUID …\n\nWithout arguments it lists all mounts; `findmnt` shows the same in a clearer way.\n\n## Syntax\n\n```text\nmount [OPTION]... DEVICE|UUID=…|LABEL=… DIRECTORY\nmount [OPTION]... DEVICE|DIRECTORY\nmount -a [-t TYPE] [-O OPTIONS]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `defaults` | rw, suid, dev, exec, auto, nouser, async. |\n| `ro / rw` | Read-only / read-write. |\n| `noexec` | Programs cannot run from this file system. |\n| `nosuid` | SUID/SGID bits do not work. |\n| `nodev` | Device files do not work. |\n| `noatime / relatime` | Do not update / update access times sometimes. |\n| `_netdev` | Network file system: mount after the network is up. |\n| `nofail` | Do not fail the boot if the device is missing. |\n| `noauto` | Not mounted by mount -a or at boot. |\n| `user / users` | Normal users may mount it. |\n| `loop` | Mount a file (like an ISO) through a loop device. |\n| `remount` | Change options of a mounted file system. |\n| `bind` | Mount a folder in a second place. |\n| `context=` | SELinux context for the whole file system. |\n| `uid=, gid=, umask=` | Owner and mode for file systems without Linux rights (vfat). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Mount everything in /etc/fstab. |\n| `-t, --types TYPE` | File system type (xfs, ext4, vfat, nfs, iso9660, tmpfs …). |\n| `-o, --options OPTIONS` | Mount options, comma separated. |\n| `-r, --read-only` | Read-only (same as -o ro). |\n| `-w, --rw, --read-write` | Read-write. |\n| `-L, --label LABEL` | Mount by label. |\n| `-U, --uuid UUID` | Mount by UUID. |\n| `-B, --bind` | Bind mount (a folder in another place). |\n| `-R, --rbind` | Bind mount with all sub-mounts. |\n| `-M, --move` | Move a mount to another folder. |\n| `-m, --mkdir[=MODE]` | Create the mount point if it is missing. |\n| `-O, --test-opts OPTIONS` | With -a: only fstab lines with these options. |\n| `-T, --fstab PATH` | Use another fstab. |\n| `-F, --fork` | With -a: mount in parallel. |\n| `-f, --fake` | Do everything except the mount (test). |\n| `-n, --no-mtab` | Do not write /etc/mtab (old systems). |\n| `-l, --show-labels` | Show labels in the list. |\n| `-c, --no-canonicalize` | Do not resolve paths. |\n| `-i, --internal-only` | Do not call /sbin/mount.TYPE helpers. |\n| `-N, --namespace NS` | Mount in another mount namespace. |\n| `--source DEVICE, --target DIR` | Name the source or target clearly. |\n| `--target-prefix DIR` | Prefix for all targets. |\n| `--onlyonce` | Fail if already mounted. |\n| `--options-mode MODE` | How to use fstab options (ignore, append, prepend, replace). |\n| `--options-source SOURCE` | Where to read options (fstab, mtab, disable). |\n| `--options-source-force` | Always read options from the source. |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Mount a partition\n\n```bash\nmkdir /data; mount /dev/vdb1 /data\n```\n\n> The type is detected.\n\n### Mount by UUID or label\n\n```bash\nmount UUID=\"4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\" /data\nmount LABEL=data /data\n```\n\n> Stable names.\n\n### Test /etc/fstab\n\n```bash\nsystemctl daemon-reload; mount -a\n```\n\n> No output = all lines work. Do it before every reboot.\n\n### Mount an ISO file\n\n```bash\nmount -o loop,ro rhel-10.0-x86_64-dvd.iso /repo\n```\n\n> Use it as a local repository.\n\n### Read-only\n\n```bash\nmount -o ro /dev/vdb1 /mnt\n```\n\n> Safe for inspection of a disk.\n\n### Change options without unmount\n\n```bash\nmount -o remount,ro /data\n```\n\n> Also used in rescue mode: `mount -o remount,rw /`.\n\n### Harden /tmp-like folders (security)\n\n```bash\nmount -o remount,noexec,nosuid,nodev /var/tmp\n```\n\n> Attackers cannot run downloaded programs from there. Put it in fstab to keep it.\n\n### Bind mount a folder\n\n```bash\nmount --bind /srv/www /var/www/html\n```\n\n> Same files in two places.\n\n### A RAM disk\n\n```bash\nmount -t tmpfs -o size=512M tmpfs /mnt/ram\n```\n\n> Fast temporary space; gone after unmount.\n\n### NFS share\n\n```bash\nmount -t nfs server2:/exports/share /mnt\n```\n\n> Network file systems are covered in the NFS chapter.\n\n### Create the mount point automatically\n\n```bash\nmount -m /dev/vdb1 /mnt/new\n```\n\n> Newer util-linux.\n\n### List mounts of one type\n\n```bash\nmount -t xfs\n```\n\n> Or better: `findmnt -t xfs`.\n\n## Watch out\n\n- Mounting on a folder hides its old content until you unmount.\n- A mount by hand is gone after reboot; use /etc/fstab or a mount unit.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/fstab` | File systems to mount at boot. |\n| `/proc/self/mountinfo` | The kernel list of mounts. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Wrong use or permissions. |\n| `2` | System error (out of memory, cannot fork …). |\n| `4` | Internal mount bug. |\n| `8` | User interrupt. |\n| `16` | Problems writing or locking mtab. |\n| `32` | Mount failure. |\n| `64` | Some mounts worked (with -a). |\n\nFull manual: [man mount](https://linux.die.net/man/8/mount)\n","page":"commands/mount/","label":"mount","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-mv","n":1474,"slug":"mv","title":"mv — Move files and folders, or rename them","sections":["commands"],"topic":"Commands · Files and folders","tags":["move","rename","file","folder"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Move files and folders, or rename them.\n\n## About\n\n`mv SOURCE TARGET` moves a file or folder. If the target is a new name in the same folder, it is a rename. Inside one file system, `mv` only changes the name, so even huge files move in an instant. Between file systems it copies the data and then deletes the source.\n\nA moved file keeps its permissions, owner and **SELinux label**. That is a classic problem when you move files into `/var/www/html`: run `restorecon` after.\n\n## Syntax\n\n```text\nmv [OPTION]... SOURCE DEST\nmv [OPTION]... SOURCE... DIRECTORY\nmv [OPTION]... -t DIRECTORY SOURCE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-i, --interactive` | Ask before overwriting a file. |\n| `-f, --force` | Do not ask before overwriting. |\n| `-n, --no-clobber` | Never overwrite an existing file. |\n| `-u` | Move only when the source is newer than the target, or the target is missing. |\n| `--update[=<which>]` | Control which existing files are replaced: all, none, none-fail, older. |\n| `-v, --verbose` | Print each move. |\n| `-t, --target-directory=<dir>` | Move all sources into this folder. |\n| `-T, --no-target-directory` | Treat the target as a normal file, never as a folder. |\n| `--backup[=<control>]` | Make a backup of each target that would be overwritten. |\n| `-b` | Like --backup without an argument. |\n| `-S, --suffix=<suffix>` | The ending of backup files (default ~). |\n| `--no-copy` | Do not copy when renaming fails (when the target is on another file system). |\n| `--strip-trailing-slashes` | Remove the slash at the end of each source. |\n| `-Z, --context` | Give the moved file the default SELinux context of its new place. |\n| `--debug` | Explain how each file is moved (implies -v). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Rename a file\n\n```bash\nmv report.txt report-2026.txt\n```\n\n> Same folder, new name.\n\n### Move files into a folder\n\n```bash\nmv *.log /var/log/archive/\n```\n\n> All matching files go into the folder.\n\n### Move and fix the SELinux label at once\n\n```bash\nmv -Z /tmp/index.html /var/www/html/\n```\n\n> Without -Z the file keeps the `user_tmp_t` label and Apache shows \"403 Forbidden\".\n\n### Ask before overwriting\n\n```bash\nmv -i new.conf /etc/myapp/app.conf\n```\n\n> Safe when the target may exist.\n\n### Do not overwrite existing files\n\n```bash\nmv -n uploads/* /srv/files/\n```\n\n> Files that already exist in the target stay as they are.\n\n### Rename a folder\n\n```bash\nmv /srv/app-old /srv/app-2025\n```\n\n> Folders need no -r for mv.\n\n### Replace a config file atomically\n\n```bash\ncp app.conf /etc/myapp/app.conf.new && mv /etc/myapp/app.conf.new /etc/myapp/app.conf\n```\n\n> On the same file system the rename is atomic: a reader sees the old or the new file, never half of it.\n\n### Switch a release with a symlink (zero downtime deploy)\n\n```bash\nln -sfn /srv/app/releases/42 /srv/app/current.tmp && mv -T /srv/app/current.tmp /srv/app/current\n```\n\n> `mv -T` replaces the link in one atomic step.\n\n### Keep a backup of the file you overwrite\n\n```bash\nmv --backup=numbered new.cfg app.cfg\n```\n\n> The old app.cfg becomes app.cfg.~1~.\n\n### Move many files given by find\n\n```bash\nfind /tmp -maxdepth 1 -name \"*.csv\" -mtime +7 -exec mv -t /archive/ {} +\n```\n\n> `-t` makes it easy to use with find and xargs.\n\n### Show each move\n\n```bash\nmv -v *.jpg photos/\n```\n\n> Prints \"renamed 'a.jpg' -> 'photos/a.jpg'\".\n\n## Watch out\n\n- `mv` overwrites without asking (normal users). Use `-i` or `-n`.\n- Moved files keep their old SELinux label. Use `mv -Z` or run `restorecon -Rv` on the new place.\n- A typo in the target can rename instead of move: `mv file /backp` creates a file named /backp.\n\n## Tips\n\n- Same folder = rename. Other folder = move. `-Z` = new place, new SELinux label.\n\nFull manual: [man mv](https://linux.die.net/man/1/mv)\n","page":"commands/mv/","label":"mv","where":"Command page · Files and folders","aka":[]},{"id":"cmd-namei","n":1475,"slug":"namei","title":"namei — Follow a path step by step and show every directory and link on the way, with owner and permissions","sections":["commands"],"topic":"Commands · Permissions","tags":["path","permissions","symbolic links","permission denied","troubleshoot"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Follow a path step by step and show every directory and link on the way, with owner and permissions.\n\n## About\n\n\"Permission denied\" often comes from a directory higher up in the path, not from the file. `namei -l PATH` lists each part of the path with its mode, owner and group, so you see at once where an `x` is missing. It also follows symbolic links and shows where they point.\n\n## Syntax\n\n```text\nnamei [OPTION]... PATHNAME...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --long` | Show mode, owner and group (same as -m -o -v). |\n| `-m, --modes` | Show the mode (permissions) of each part. |\n| `-o, --owners` | Show owner and group of each part. |\n| `-v, --vertical` | Line up the modes in a column. |\n| `-n, --nosymlinks` | Do not follow symbolic links. |\n| `-x, --mountpoints` | Show mount point directories with a D. |\n| `-Z, --context` | Show the SELinux context of each part. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Check the whole path\n\n```bash\nnamei -l /srv/sales/plan.txt\n```\n```text\nf: /srv/sales/plan.txt\ndr-xr-xr-x root root  /\ndrwxr-xr-x root root  srv\ndrwxrws--T root sales sales\n-rw-r--r-- anna sales plan.txt\n```\n\n> Others have no x on sales: they cannot reach plan.txt.\n\n### Find where a link points\n\n```bash\nnamei /etc/localtime\n```\n```text\nf: /etc/localtime\n d /\n d etc\n l localtime -> ../usr/share/zoneinfo/Europe/Amsterdam\n```\n\n> Follows every link to the real file.\n\n### Debug a web server 403\n\n```bash\nnamei -l /var/www/html/app/index.html\n```\n\n> The apache user needs x on every directory and r on the file.\n\n### Show SELinux labels on the way\n\n```bash\nnamei -Z /var/www/html/index.html\n```\n\n> Next step after the modes: the SELinux types.\n\n### Show mount points in the path\n\n```bash\nnamei -x /home/lisa/data\n```\n\n> D marks a directory where another file system is mounted.\n\n### Only modes, lined up\n\n```bash\nnamei -mv /etc/ssh/sshd_config\n```\n\n> Short view.\n\n### Do not follow links\n\n```bash\nnamei -n /usr/bin/python3\n```\n\n> Shows the link itself only.\n\n### Check an NFS home path\n\n```bash\nnamei -l /home/lisa/projects\n```\n\n> A root_squash export or a wrong mode on the mount point shows up here.\n\n### Check a path for another user\n\n```bash\nsu - carol -c 'namei -l /data/sales/bob.txt'\n```\n\n> Run it as the user who has the problem.\n\n### Find a broken link\n\n```bash\nnamei /opt/app/current\n```\n\n> \"No such file or directory\" shows which link target is missing.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (for example a part of the path does not exist). |\n\nFull manual: [man namei](https://linux.die.net/man/1/namei)\n","page":"commands/namei/","label":"namei","where":"Command page · Permissions","aka":[]},{"id":"cmd-nano","n":1476,"slug":"nano","title":"nano — Edit text files with a simple terminal editor that shows its keys on the screen","sections":["commands"],"topic":"Commands · Text and search","tags":["editor","edit","text","config","simple"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit text files with a simple terminal editor that shows its keys on the screen.\n\n## About\n\nnano is the easiest terminal editor: you type directly, and the main keys are listed at the bottom. `^` means **Ctrl** and `M-` means **Alt**.\n\nIt is good for quick changes to config files when you do not want to think about vim modes. In the exam you may use it.\n\nSettings for all users are in `/etc/nanorc`; your own settings in `~/.config/nano/nanorc` or `~/.nanorc`.\n\n## Syntax\n\n```text\nnano [OPTIONS] [[+LINE[,COLUMN]] FILE]...\n```\n\n## Parts\n\n\n### Keys\n\n| Part | Meaning |\n|---|---|\n| `Ctrl+O` | Save (write out). Enter confirms the file name. |\n| `Ctrl+X` | Exit. nano asks to save when there are changes. |\n| `Ctrl+W` | Search. Alt+W = search again. |\n| `Ctrl+\\` | Search and replace. |\n| `Ctrl+K` | Cut the line (or the marked text). |\n| `Ctrl+U` | Paste the cut text. |\n| `Alt+6` | Copy the line (or the marked text). |\n| `Alt+A` | Start or stop marking text. |\n| `Alt+U, Alt+E` | Undo, redo. |\n| `Ctrl+_ (or Alt+G)` | Go to a line number. |\n| `Ctrl+C` | Show the cursor position. |\n| `Ctrl+G` | Help. |\n| `Alt+#` | Show or hide line numbers. |\n| `Ctrl+T` | Run the spell checker or a command. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `+<line>[,<col>]` | Start at this line (and column). |\n| `-l, --linenumbers` | Show line numbers. |\n| `-v, --view` | View only: no changes allowed. |\n| `-B, --backup` | Keep a backup of the old file (file~). |\n| `-C, --backupdir=<dir>` | Folder for backup files. |\n| `-E, --tabstospaces` | Type spaces instead of a tab. |\n| `-T, --tabsize=<n>` | Width of a tab. |\n| `-i, --autoindent` | New lines get the same indent as the line above. |\n| `-m, --mouse` | Use the mouse to move the cursor and mark text. |\n| `-w, --nowrap` | Do not wrap long lines (the default in new versions). |\n| `-b, --breaklonglines` | Wrap long lines while you type. |\n| `-S, --softwrap` | Show long lines on several screen lines. |\n| `-c, --constantshow` | Always show the cursor position. |\n| `-x, --nohelp` | Hide the two help lines at the bottom. |\n| `-Y, --syntax=<name>` | Use this syntax coloring (sh, yaml, python …). |\n| `-R, --restricted` | Restricted mode: only the files on the command line. |\n| `-L, --nonewlines` | Do not add a new line at the end of the file. |\n| `-M, --trimblanks` | Remove spaces at the end of lines when wrapping. |\n| `-N, --noconvert` | Do not convert DOS/Mac line ends. |\n| `-u, --unix` | Save files in Unix format. |\n| `-I, --ignorercfiles` | Do not read nanorc files. |\n| `-f, --rcfile=<file>` | Read only this nanorc file. |\n| `-H, --historylog` | Save search history. |\n| `-P, --positionlog` | Remember the cursor position for each file. |\n| `-F, --multibuffer` | Open more files in buffers. |\n| `-G, --locking` | Use vim-style lock files. |\n| `-k, --cutfromcursor` | Ctrl+K cuts from the cursor to the end of the line. |\n| `-a, --atblanks` | With soft wrap: wrap at spaces, not in the middle of a word. |\n| `-A, --smarthome` | The Home key jumps to the first non-space character first. |\n| `-D, --boldtext` | Use bold text instead of reverse colors for the title and status bars. |\n| `-J, --guidestripe=<n>` | Draw a vertical guide line at column n (for line length). |\n| `-q, --indicator` | Show a scroll bar on the right side. |\n| `-_, --minibar` | Hide the title bar; show a small bar with the file name and position at the bottom. |\n| `-0, --zero` | Use the whole screen for text (hide all bars). |\n| `-g, --showcursor` | Show the cursor in the file browser and help. |\n| `-j, --jumpyscrolling` | Scroll half a screen at a time, not line by line. |\n| `-e, --emptyline` | Keep the line below the title bar empty. |\n| `-O, --bookstyle` | A line that starts with spaces starts a new paragraph (for justify). |\n| `-K, --rawsequences` | Read escape sequences from the terminal directly. |\n| `-W, --wordbounds` | Detect word borders more accurately. |\n| `-X, --wordchars=<chars>` | Extra characters that count as part of a word. |\n| `-Z, --zap` | Delete and Backspace erase the marked text. |\n| `-d, --rebinddelete` | Fix the Backspace/Delete keys when they act strangely. |\n| `-y, --afterends` | Ctrl+Right stops at word ends instead of word starts. |\n| `-o, --operatingdir=<dir>` | Only allow files inside this folder. |\n| `-p, --preserve` | Let Ctrl+S and Ctrl+Q reach the terminal (flow control). |\n| `-n, --noread` | Treat the file name as new: do not read the existing file. |\n| `-r, --fill=<n>` | Wrap and justify lines at column n. |\n| `-s, --speller=<program>` | Use this spell checker. |\n| `-Q, --quotestr=<regex>` | How quoted lines (like email quotes) start, for justify. |\n| `-U, --quickblank` | Clear status messages after one keystroke. |\n| `-%, --stateflags` | Show the state (like auto-indent on) in the title bar. |\n| `-!, --magic` | Use libmagic to guess the syntax of the file. |\n| `-t, --saveonexit` | Save changes on exit without asking. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Edit a file\n\n```bash\nnano /etc/motd\n```\n\n> Type the text, then Ctrl+O, Enter (save), Ctrl+X (exit).\n\n### Open a config file at a line number\n\n```bash\nnano +25 /etc/chrony.conf\n```\n\n> Go straight to the line from an error message.\n\n### Show line numbers while you edit\n\n```bash\nnano -l /etc/ssh/sshd_config\n```\n\n> Good when you follow a guide that names line numbers.\n\n### Look at a file without risk of changing it\n\n```bash\nnano -v /etc/passwd\n```\n\n> View mode blocks all changes.\n\n### Keep a backup of the old version\n\n```bash\nsudo nano -B /etc/hosts\n```\n\n> The old file is kept as `/etc/hosts~`.\n\n### Edit YAML with spaces instead of tabs\n\n```bash\nnano -E -T 2 deploy.yaml\n```\n\n> YAML does not allow tabs. `-E` types spaces for the Tab key.\n\n### Search and replace a value\n\n```bash\nnano /etc/httpd/conf/httpd.conf\n```\n```text\n(Ctrl+\\ , type: Listen 80 , Enter, type: Listen 8080 , Enter, A for all)\n```\n\n> Replaces every match after you confirm.\n\n### Edit as root with sudoedit (safer)\n\n```bash\nsudoedit /etc/chrony.conf\n```\n\n> sudoedit opens a copy with your editor and saves it back as root. Set `EDITOR=nano` to use nano.\n\n### Make nano the editor for crontab and visudo\n\n```bash\nexport EDITOR=nano VISUAL=nano\ncrontab -e\n```\n\n> Tools that open an editor read these variables.\n\n### Use nano settings for every session\n\n```bash\nmkdir -p ~/.config/nano && printf 'set linenumbers\\nset tabstospaces\\nset tabsize 2\\n' > ~/.config/nano/nanorc\n```\n\n> Your settings are used every time you start nano.\n\n### Write a quick script\n\n```bash\nnano backup.sh && chmod +x backup.sh\n```\n\n> Syntax colors make shell scripts easier to read.\n\n## Watch out\n\n- Ctrl+X does not save by itself; answer `Y` when it asks.\n- Some terminals use Ctrl+S to freeze output; in nano, save with Ctrl+O (or Ctrl+S in new versions).\n- Long lines can wrap on screen; that does not change the file.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/nanorc` | Settings for all users. |\n| `~/.config/nano/nanorc` | Your own settings. |\n\n## Tips\n\n- `^O` save, `^X` exit, `^W` search, `^K` cut, `^U` paste.\n\nFull manual: [man nano](https://linux.die.net/man/1/nano)\n","page":"commands/nano/","label":"nano","where":"Command page · Text and search","aka":[]},{"id":"cmd-newgrp","n":1477,"slug":"newgrp","title":"newgrp — Start a new shell with another primary group — useful right after you were added to a group","sections":["commands"],"topic":"Commands · Users and groups","tags":["group","primary group","new group membership","login"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Start a new shell with another primary group — useful right after you were added to a group.\n\n## About\n\nWhen an admin adds you to a group, your running sessions do not have it yet. `newgrp GROUP` starts a new shell in which GROUP is your primary group (new files get this group) and the group is active.\n\nIf you are not a member, newgrp asks for the group password (if one is set). `exit` returns to the old shell.\n\n## Syntax\n\n```text\nnewgrp [-] [GROUP]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `GROUP` | The group to use. Without a name: your primary group from the account. |\n| `-` | Start like a new login (reset the environment). |\n\n## Examples\n\n### Use a new group without a new login\n\n```bash\nnewgrp webadmins\nid -gn\n```\n```text\nwebadmins\n```\n\n> `exit` to go back.\n\n### Create files with a team group\n\n```bash\nnewgrp sales\ntouch report.txt; ls -l report.txt\n```\n```text\n-rw-r--r--. 1 anna sales 0 Oct 25 10:12 report.txt\n```\n\n> New files get the primary group sales.\n\n### Check the active groups before and after\n\n```bash\nid -Gn; newgrp webadmins; id -Gn\n```\n\n> The new shell has the group.\n\n### Go back to the normal group\n\n```bash\nexit\n```\n\n> Leaves the newgrp shell.\n\n### Start as a new login\n\n```bash\nnewgrp - sales\n```\n\n> Clean environment, like a new login.\n\n### Run one command with a group (sg)\n\n```bash\nsg sales -c 'touch /srv/sales/plan.txt'\n```\n\n> sg = newgrp for one command.\n\n### Join with a group password\n\n```bash\nnewgrp projectx\n```\n```text\nPassword:\n```\n\n> Works only if a group password was set (rare).\n\n### In a script after adding a user to docker or podman groups\n\n```bash\nnewgrp libvirt\n```\n\n> For example after `usermod -aG libvirt $USER` to use virsh without a new login.\n\n### Check the group of new files\n\n```bash\numask; newgrp sales; touch x; stat -c '%G' x\n```\n```text\nsales\n```\n\n> Shows which group new files get.\n\n### Best alternative: log in again\n\n```bash\nexit\n```\n\n> A new login gives all groups at once, not only one.\n\n## Watch out\n\n- newgrp starts a new shell; your old variables and history are not in it.\n- A setgid folder (`chmod g+s`) is often a better way to give files a team group.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/group, /etc/gshadow` | Members and group passwords. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (exit code of the shell). |\n| `1` | Not allowed or the group does not exist. |\n\nFull manual: [man newgrp](https://linux.die.net/man/1/newgrp)\n","page":"commands/newgrp/","label":"newgrp","where":"Command page · Users and groups","aka":[]},{"id":"cmd-nft","n":1478,"slug":"nft","title":"nft — Show and manage nftables rules: tables, chains, rules and sets — the kernel firewall below firewalld","sections":["commands"],"topic":"Commands · Firewall","tags":["nftables","netfilter","firewall rules","packet filter"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and manage nftables rules: tables, chains, rules and sets — the kernel firewall below firewalld.\n\n## About\n\nnftables replaced iptables. firewalld writes its rules into nftables (table `inet firewalld`). With `nft list ruleset` you see what the kernel really does. On systems with firewalld, use `firewall-cmd` for changes; write nft rules yourself only on systems without firewalld (then use `nftables.service` and `/etc/sysconfig/nftables.conf`).\n\n## Syntax\n\n```text\nnft [OPTION]... COMMAND\nnft -f FILE\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `list ruleset` | Show all rules. |\n| `list tables / list table FAMILY NAME` | Tables. |\n| `list chain FAMILY TABLE CHAIN` | One chain. |\n| `add table, add chain, add rule` | Create. |\n| `insert rule, replace rule, delete rule handle N` | Change rules. |\n| `add set, add element` | Address or port sets. |\n| `flush ruleset` | Remove all rules (dangerous). |\n| `monitor` | Watch rule changes and traces. |\n| `FAMILY` | ip, ip6, inet (both), arp, bridge, netdev. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --handle` | Show rule handles (needed to delete a rule). |\n| `-f, --file FILE` | Read commands from a file. |\n| `-c, --check` | Check the commands, do not apply. |\n| `-n, --numeric` | Numbers instead of names (addresses, ports). |\n| `-N, --reversedns` | Resolve IPs to names. |\n| `-S, --service` | Port numbers as service names. |\n| `-s, --stateless` | Leave out counters and state. |\n| `-t, --terse` | Leave out set contents. |\n| `-j, --json` | JSON output. |\n| `-e, --echo` | Print what was added. |\n| `-i, --interactive` | Interactive mode. |\n| `-D, --define NAME=VALUE` | Define a variable for files. |\n| `-I, --includepath DIR` | Folder for include files. |\n| `-o, --optimize` | Optimize the rules. |\n| `-y, --numeric-priority, -p, --numeric-protocol, -T, --numeric-time, -u, --guid` | More numeric output. |\n| `-d, --debug LEVEL` | Debug output. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### All rules\n\n```bash\nnft list ruleset | less\n```\n\n> Includes the rules firewalld made.\n\n### Tables\n\n```bash\nnft list tables\n```\n```text\ntable inet firewalld\n```\n\n> firewalld's table.\n\n### firewalld zone rules\n\n```bash\nnft list chain inet firewalld filter_IN_public_allow\n```\n\n> The services of the public zone as rules.\n\n### Rules with handles\n\n```bash\nnft -a list ruleset | grep handle | head\n```\n\n> Handles identify single rules.\n\n### Check a rules file\n\n```bash\nnft -c -f /etc/sysconfig/nftables.conf\n```\n\n> Syntax check before loading.\n\n### A simple firewall without firewalld\n\n```bash\nnft add table inet filter\nnft add chain inet filter input '{ type filter hook input priority 0; policy drop; }'\nnft add rule inet filter input ct state established,related accept\nnft add rule inet filter input iif lo accept\nnft add rule inet filter input tcp dport 22 accept\n```\n\n> Only on systems where firewalld is off.\n\n### Block a list of attackers with a set\n\n```bash\nnft add set inet filter blocklist '{ type ipv4_addr; }'\nnft add element inet filter blocklist '{ 203.0.113.50 }'\nnft add rule inet filter input ip saddr @blocklist drop\n```\n\n> Sets scale to many addresses.\n\n### Save the current rules\n\n```bash\nnft list ruleset > /etc/sysconfig/nftables.conf\n```\n\n> Loaded by nftables.service at boot.\n\n### Watch rule changes\n\n```bash\nnft monitor\n```\n\n> Shows changes live (for example from firewall-cmd).\n\n### JSON for tools\n\n```bash\nnft -j list ruleset | jq '.nftables | length'\n```\n\n> Machine-readable.\n\n## Watch out\n\n- Do not change firewalld's tables by hand; a reload overwrites them.\n- `flush ruleset` with a drop policy can cut your SSH session.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/sysconfig/nftables.conf` | Rules for nftables.service. |\n| `/etc/nftables/` | Example rule files. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man nft](https://linux.die.net/man/8/nft)\n","page":"commands/nft/","label":"nft","where":"Command page · Firewall","aka":[]},{"id":"cmd-nice","n":1479,"slug":"nice","title":"nice — Start a command with a changed CPU priority (nice value from -20 to 19)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["priority","nice value","cpu","scheduling"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Start a command with a changed CPU priority (nice value from -20 to 19).\n\n## About\n\nThe nice value decides how much CPU a process gets when the CPU is busy: -20 = highest priority, 19 = lowest, 0 = normal. `nice -n 10 CMD` starts CMD nicer than normal. Without -n, nice adds 10.\n\nNormal users may only make processes nicer (higher numbers). Negative values need root. To change a running process use `renice`.\n\n## Syntax\n\n```text\nnice [-n ADJUSTMENT] [COMMAND [ARGUMENT]...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --adjustment=N` | Add N to the nice value (default 10). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Run a backup with low priority\n\n```bash\nnice -n 19 tar -czf /tmp/home.tgz /home\n```\n\n> Users and services keep their speed.\n\n### Default adjustment\n\n```bash\nnice sleep 100 &\nps -o ni,cmd -p $!\n```\n```text\n NI CMD\n 10 sleep 100\n```\n\n> Without -n: +10.\n\n### Higher priority (root)\n\n```bash\nnice -n -5 /usr/local/bin/important-job\n```\n\n> Only root may use negative values.\n\n### Normal user tries a negative value\n\n```bash\nnice -n -5 sleep 1\n```\n```text\nnice: cannot set niceness: Permission denied\n```\n\n> The command still runs, with nice 0.\n\n### Show the current nice value\n\n```bash\nnice\n```\n```text\n0\n```\n\n> Without a command: print your niceness.\n\n### Low CPU and low disk priority together\n\n```bash\nnice -n 19 ionice -c3 rsync -a /data/ /backup/data/\n```\n\n> ionice -c3 = only use the disk when nobody else does.\n\n### Compile without slowing the system\n\n```bash\nnice -n 15 make -j\"$(nproc)\"\n```\n\n> Use all cores, but give way to others.\n\n### Check the NI column in top\n\n```bash\ntop\n```\n\n> Column NI shows the nice value; PR the kernel priority.\n\n### Set the nice value of a service\n\n```bash\nsystemctl edit myapp\n```\n\n> Add `[Service]` and `Nice=10` — services are not started with nice.\n\n### Run a cron job nicely\n\n```bash\ncrontab -l | grep nice\n```\n```text\n0 2 * * * nice -n 19 /usr/local/bin/report.sh\n```\n\n> Reports at night with low priority.\n\n## Watch out\n\n- Nice only matters when the CPU is busy. On an idle CPU every process gets what it wants.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The exit status of the command. |\n| `125` | nice itself failed. |\n| `126` | The command could not be run. |\n| `127` | The command was not found. |\n\nFull manual: [man nice](https://linux.die.net/man/1/nice)\n","page":"commands/nice/","label":"nice","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-nmcli","n":1480,"slug":"nmcli","title":"nmcli — Set up, change and check network connections with NetworkManager","sections":["commands"],"topic":"Commands · Network","tags":["network","ipv4","ipv6","dns","gateway","profile","vlan","bond","wifi","hostname"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Set up, change and check network connections with NetworkManager.\n\n## About\n\nNetworkManager is the service that runs the network on Red Hat Enterprise Linux. It keeps your settings in **connection profiles**. A profile holds the IP addresses, the gateway, DNS servers and more. `nmcli` is the command-line tool to read and change these profiles and the devices (network cards).\n\nSettings you make with `nmcli` are saved in the profile, so they stay after a reboot. This is the big difference with the `ip` command, whose changes are lost at reboot.\n\nMost commands can be shortened: `nmcli connection show` = `nmcli con show` = `nmcli c s`. Tab completion helps you find the next word.\n\n## Syntax\n\n```text\nnmcli [OPTIONS] OBJECT { COMMAND | help }\nnmcli connection { show | up | down | modify | add | edit | clone | delete | reload | load | import | export | monitor }\nnmcli device { status | show | set | connect | disconnect | reapply | modify | delete | monitor | wifi | lldp }\nnmcli general { status | hostname | permissions | logging | reload }\nnmcli networking { on | off | connectivity }\nnmcli radio { all | wifi | wwan } [ on | off ]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `general` | Overall status of NetworkManager, the hostname, your permissions and the log level. |\n| `networking` | Turn all networking on or off, or check the internet connectivity. |\n| `radio` | Turn Wi-Fi and mobile broadband radios on or off. |\n| `connection (con, c)` | Work with profiles: show, add, modify, up, down, delete, reload. |\n| `device (dev, d)` | Work with network cards: status, details, connect, disconnect, Wi-Fi. |\n| `monitor` | Watch changes of devices and connections live. |\n| `agent` | Run as a secret or polkit agent (asks for passwords). |\n\n## Options\n\n\n### Global options (before the object)\n\n| Option | What it does |\n|---|---|\n| `-t, --terse` | Short output for scripts. No headers, fields split by \":\". |\n| `-f, --fields <list>` | Choose which columns to show, for example NAME,DEVICE. `all` and `common` also work. |\n| `-g, --get-values <list>` | Print only the values of these fields. Same as -t -f together. Good for scripts. |\n| `-p, --pretty` | Easy-to-read output with headers and lines between groups. |\n| `-m, --mode {tabular\\|multiline}` | Show the output as a table or as one field per line. |\n| `-c, --colors {yes\\|no\\|auto}` | Use colors in the output, or not. |\n| `-e, --escape {yes\\|no}` | Escape \":\" and \"\\\" in terse output (yes is the default). |\n| `-a, --ask` | Ask for missing values, like a Wi-Fi password. |\n| `-s, --show-secrets` | Show passwords and keys in the output. |\n| `-w, --wait <seconds>` | How long to wait for an action to finish (for example `con up`). |\n| `--offline` | Work on profile files without a running NetworkManager (with `connection add/modify`, reading and writing keyfiles on stdin and stdout). |\n| `--complete-args` | Print the possible next words (used by tab completion). |\n| `-h, --help` | Show help. Also works after any object: `nmcli con help`. |\n| `-v, --version` | Show the nmcli version. |\n\n### Options of connection commands\n\n| Option | What it does |\n|---|---|\n| `--active` | With `con show`: list only the active profiles. |\n| `--order <list>` | With `con show`: sort the list, for example `--order +type,-name`. |\n| `--temporary` | With `con modify`: change only the running state, not the saved profile. |\n| `save {yes\\|no}` | With `con add`: `save no` makes a profile that is not written to disk. |\n| `con-name <name>` | With `con add`: the name of the new profile. |\n| `ifname <device>` | With `con add`: the device the profile belongs to. Use `*` for any device. |\n| `type <type>` | With `con add`: the kind of profile: ethernet, wifi, bond, bridge, vlan, team, vpn, loopback … |\n\n### Settings you change often (setting.property)\n\n| Option | What it does |\n|---|---|\n| `ipv4.method {auto\\|manual\\|disabled\\|link-local\\|shared}` | auto = DHCP, manual = fixed address, disabled = no IPv4. |\n| `ipv4.addresses <ip/prefix>` | The fixed IPv4 address(es) with prefix, like 192.168.122.10/24. Use + or - to add or remove one. |\n| `ipv4.gateway <ip>` | The default gateway for IPv4. |\n| `ipv4.dns <list>` | The DNS servers, separated by space or comma. |\n| `ipv4.dns-search <list>` | Domains to try for short names, like example.com. |\n| `ipv4.ignore-auto-dns {yes\\|no}` | Do not use the DNS servers that DHCP sends. |\n| `ipv4.ignore-auto-routes {yes\\|no}` | Do not use routes that DHCP sends. |\n| `ipv4.routes <list>` | Extra routes: \"10.20.0.0/16 192.168.122.254\". Use + to add. |\n| `ipv4.route-metric <n>` | Priority of the routes of this profile. Lower wins. |\n| `ipv4.never-default {yes\\|no}` | Never use this profile for the default route. |\n| `ipv4.may-fail {yes\\|no}` | With \"no\", the profile is only up when IPv4 works. |\n| `ipv6.method {auto\\|manual\\|ignore\\|disabled\\|link-local\\|dhcp\\|shared}` | How IPv6 gets its address. disabled = no IPv6 on this profile. |\n| `ipv6.addresses <ip/prefix>` | The fixed IPv6 address(es), like 2001:db8:10::10/64. |\n| `ipv6.gateway <ip>` | The default gateway for IPv6. |\n| `ipv6.dns <list>` | IPv6 DNS servers. |\n| `connection.id <name>` | The profile name. Change it to rename a profile. |\n| `connection.autoconnect {yes\\|no}` | Start this profile by itself (also at boot). |\n| `connection.autoconnect-priority <n>` | When several profiles can start, the highest number wins. |\n| `connection.interface-name <device>` | Bind the profile to one device. |\n| `connection.zone <zone>` | The firewalld zone for this profile, like internal or public. |\n| `802-3-ethernet.mtu <bytes>` | The MTU of the card (short name: ethernet.mtu). |\n| `802-3-ethernet.cloned-mac-address <mac>` | Use another MAC address (or `random`, `stable`). |\n| `vlan.id <n>` | With type vlan: the VLAN number. |\n| `bond.options <list>` | With type bond: like \"mode=active-backup,miimon=100\". |\n\n## Examples\n\n### Show all profiles and which device uses them\n\n```bash\nnmcli connection show\n```\n```text\nNAME    UUID                                  TYPE      DEVICE\nenp1s0  3f0c3a8e-6c35-4c5a-9a0e-7f1a2b3c4d5e  ethernet  enp1s0\nlo      2b1c5e3a-1d2e-4f5a-8b9c-0d1e2f3a4b5c  loopback  lo\n```\n\n> A profile with an empty DEVICE column is not active now.\n\n### Show the state of every network card\n\n```bash\nnmcli device status\n```\n```text\nDEVICE  TYPE      STATE                   CONNECTION\nenp1s0  ethernet  connected               enp1s0\nlo      loopback  connected (externally)  lo\n```\n\n> \"disconnected\" means the card works but no profile is active on it. \"unmanaged\" means NetworkManager does not control it.\n\n### Give a server a fixed IPv4 address\n\n```bash\nnmcli con mod enp1s0 ipv4.method manual ipv4.addresses 192.168.122.10/24 \\\n    ipv4.gateway 192.168.122.1 ipv4.dns 192.168.122.1\nnmcli con up enp1s0\n```\n\n> The change is saved in the profile. `con up` uses it right now.\n\n### Add a second address without removing the first\n\n```bash\nnmcli con mod enp1s0 +ipv4.addresses 192.168.122.110/24\nnmcli con up enp1s0\n```\n\n> Use a minus to remove one address again: `-ipv4.addresses 192.168.122.110/24`.\n\n### Create a new profile with a fixed address\n\n```bash\nnmcli con add con-name app-net type ethernet ifname enp2s0 \\\n    ipv4.method manual ipv4.addresses 10.10.20.15/24 ipv4.gateway 10.10.20.1 ipv4.dns 10.10.20.53\n```\n\n> Good for a second network card, for example a separate network for backups or databases.\n\n### Use only your own DNS servers (ignore DHCP)\n\n```bash\nnmcli con mod enp1s0 ipv4.dns \"10.0.0.53 10.0.1.53\" ipv4.ignore-auto-dns yes ipv4.dns-search corp.example.com\nnmcli con up enp1s0\ncat /etc/resolv.conf\n```\n\n> NetworkManager writes /etc/resolv.conf for you. A hardened server should use only the internal resolvers that log queries.\n\n### Add a fixed route to another network\n\n```bash\nnmcli con mod enp1s0 +ipv4.routes \"10.20.0.0/16 192.168.122.254\"\nnmcli con up enp1s0\nip route show 10.20.0.0/16\n```\n\n> Typical for a management network or a VPN gateway. The route stays after a reboot, unlike `ip route add`.\n\n### Get one value for a script\n\n```bash\nnmcli -g IP4.ADDRESS device show enp1s0\n```\n```text\n192.168.122.10/24\n```\n```bash\nnmcli -t -f NAME,DEVICE con show --active\n```\n```text\nenp1s0:enp1s0\n```\n\n> -g and -t give clean output without headers. Use it in provisioning scripts and health checks.\n\n### Make a VLAN interface for a separate network\n\n```bash\nnmcli con add type vlan con-name vlan30 dev enp1s0 id 30 \\\n    ipv4.method manual ipv4.addresses 172.16.30.5/24\nnmcli con up vlan30\n```\n\n> VLANs keep traffic apart, for example a DMZ for public servers. The switch port must allow VLAN 30.\n\n### Make a bond of two cards for failover\n\n```bash\nnmcli con add type bond con-name bond0 ifname bond0 bond.options \"mode=active-backup,miimon=100\" \\\n    ipv4.method manual ipv4.addresses 192.168.50.10/24 ipv4.gateway 192.168.50.1\nnmcli con add type ethernet port-type bond controller bond0 con-name bond0-p1 ifname enp3s0\nnmcli con add type ethernet port-type bond controller bond0 con-name bond0-p2 ifname enp4s0\nnmcli con up bond0\n```\n\n> If one cable or switch fails, the other card takes over. Check with `cat /proc/net/bonding/bond0`.\n\n### Put a profile in a firewalld zone\n\n```bash\nnmcli con mod enp2s0 connection.zone internal\nnmcli con up enp2s0\nfirewall-cmd --get-active-zones\n```\n\n> Each network card can have its own firewall rules: public for the internet side, internal for the private side.\n\n### Turn off IPv6 on a profile\n\n```bash\nnmcli con mod enp1s0 ipv6.method disabled\nnmcli con up enp1s0\n```\n\n> Only do this when a policy asks for it. Many services work fine with IPv6.\n\n### Set the hostname\n\n```bash\nnmcli general hostname web01.example.com\nhostnamectl --static\n```\n```text\nweb01.example.com\n```\n\n> Same result as `hostnamectl set-hostname`.\n\n### Check who may change the network\n\n```bash\nnmcli general permissions\n```\n\n> Over SSH a normal user usually gets \"no\" or \"auth\". Use sudo.\n\n### Read a profile edited by hand again\n\n```bash\nvim /etc/NetworkManager/system-connections/enp1s0.nmconnection\nnmcli con reload\nnmcli con up enp1s0\n```\n\n> `reload` only reads the files again. `up` makes the new settings active.\n\n### Watch network changes live\n\n```bash\nnmcli monitor\n```\n\n> Useful while you unplug a cable or restart a switch. Press Ctrl+C to stop.\n\n## Watch out\n\n- `nmcli con mod` only changes the profile. Run `nmcli con up NAME` to use the change now.\n- Over SSH, a wrong address or gateway cuts your own connection. Keep a console open.\n- Without `+`, a setting like `ipv4.addresses` **replaces** all old values.\n- Two profiles with autoconnect on for one device: you cannot know which one starts after a reboot.\n- `ipv4.method manual` needs at least one address in `ipv4.addresses`, or the command fails.\n- Do not edit `/etc/resolv.conf` by hand. NetworkManager writes it again.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/NetworkManager/system-connections/` | The profiles, one keyfile (.nmconnection) per profile. Root only (mode 600). |\n| `/etc/NetworkManager/NetworkManager.conf` | Main settings of NetworkManager. |\n| `/etc/NetworkManager/conf.d/` | Extra settings files, read after the main file. |\n| `/run/NetworkManager/system-connections/` | Profiles that are not saved to disk (they are gone after reboot). |\n| `/etc/resolv.conf` | DNS settings. Written by NetworkManager. |\n\n## Tips\n\n- Remember the order: **show → modify → up → check**.\n- `nmcli con show NAME` lists every setting of a profile. Search it with `| grep ipv4`.\n- Lost in the words? Press Tab twice after any word, or type `nmcli con mod NAME ipv4.` and Tab.\n- `man nmcli-examples` has many ready-to-use examples.\n\nFull manual: [man nmcli](https://linux.die.net/man/1/nmcli)\n","page":"commands/nmcli/","label":"nmcli","where":"Command page · Network","aka":[]},{"id":"cmd-nmtui","n":1481,"slug":"nmtui","title":"nmtui — A menu in the terminal to edit NetworkManager connections, activate them and set the host name","sections":["commands"],"topic":"Commands · Network","tags":["networkmanager","text interface","ip address","connection","hostname","menu"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"A menu in the terminal to edit NetworkManager connections, activate them and set the host name.\n\nSame page for: `nmtui-edit`, `nmtui-connect`, `nmtui-hostname`\n\n## About\n\n`nmtui` (NetworkManager text user interface) gives the same settings as `nmcli`, but with menus: use the arrow keys, Tab, Space and Enter.\n\nIt is a quick and safe way on the exam to set IP addresses, gateway and DNS. After editing a running connection, activate it again (\"Activate a connection\": deactivate and activate) so the new settings are used.\n\nThe package NetworkManager-tui may need to be installed.\n\n## Syntax\n\n```text\nnmtui [edit [CONNECTION] | connect [CONNECTION] | hostname [NAME]]\nnmtui-edit [CONNECTION]\nnmtui-connect [CONNECTION]\nnmtui-hostname [NAME]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `edit, nmtui-edit` | Open the editor (for one connection or the list). |\n| `connect, nmtui-connect` | Activate or deactivate connections. |\n| `hostname, nmtui-hostname` | Set the host name. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### Open the main menu\n\n```bash\nnmtui\n```\n\n> Edit a connection / Activate a connection / Set system hostname.\n\n### Edit one connection directly\n\n```bash\nnmtui edit enp1s0\n```\n\n> Set IPv4 to Manual and add address, gateway and DNS.\n\n### Re-activate after a change\n\n```bash\nnmtui connect enp1s0\n```\n\n> Or: `nmcli con up enp1s0`. Without it the old settings stay active.\n\n### Set the host name\n\n```bash\nnmtui hostname server1.example.com\n```\n\n> Same as hostnamectl set-hostname.\n\n### Install it when it is missing\n\n```bash\ndnf install -y NetworkManager-tui\n```\n\n> Part of the RHEL BaseOS repo.\n\n### Check the result with nmcli\n\n```bash\nnmcli con show enp1s0 | grep ipv4\n```\n\n> nmtui writes the same keyfile that nmcli uses.\n\n### Check the address after activation\n\n```bash\nip -br addr show enp1s0\n```\n```text\nenp1s0  UP  192.168.122.10/24\n```\n\n> The new address is active.\n\n### Work over SSH safely\n\n```bash\nnmtui\n```\n\n> Changing the IP you are connected to ends your SSH session — use a console or plan it.\n\n### Use it on a small terminal\n\n```bash\nTERM=xterm-256color nmtui\n```\n\n> Fixes broken drawing on some consoles.\n\n### Find the keyfile it wrote\n\n```bash\nls /etc/NetworkManager/system-connections/\n```\n```text\nenp1s0.nmconnection\n```\n\n> Settings are saved here.\n\n## Watch out\n\n- Mark \"Automatically connect\" or the connection does not come up after a reboot.\n- An address needs a prefix: 192.168.122.10/24, not only the IP.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/NetworkManager/system-connections/` | The connection files. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man nmtui](https://linux.die.net/man/1/nmtui)\n","page":"commands/nmtui/","label":"nmtui","where":"Command page · Network","aka":["nmtui-edit","nmtui-connect","nmtui-hostname"]},{"id":"cmd-nohup","n":1482,"slug":"nohup","title":"nohup — Run a command that keeps running after you log out (it ignores the hang-up signal)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["hangup","logout","background","long running","sighup"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run a command that keeps running after you log out (it ignores the hang-up signal).\n\n## About\n\nWhen you close a terminal, its processes get SIGHUP and stop. `nohup COMMAND &` makes the command ignore SIGHUP. Output that would go to the terminal goes to `nohup.out` (in the current folder, or in your home folder).\n\nFor real long-running work, a systemd unit (`systemd-run`) or `tmux` is often better.\n\n## Syntax\n\n```text\nnohup COMMAND [ARGUMENT]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Keep a job running after logout\n\n```bash\nnohup ./long-task.sh &\n```\n```text\nnohup: ignoring input and appending output to 'nohup.out'\n```\n\n> Log out and come back later.\n\n### Choose the log file\n\n```bash\nnohup ./long-task.sh > /var/tmp/task.log 2>&1 &\n```\n\n> Better than nohup.out in a random folder.\n\n### Follow the output\n\n```bash\ntail -f nohup.out\n```\n\n> See the progress.\n\n### Long copy over SSH\n\n```bash\nnohup rsync -a /data/ server2:/data/ > rsync.log 2>&1 &\n```\n\n> Survives a broken SSH connection.\n\n### Check that it still runs\n\n```bash\npgrep -af long-task.sh\n```\n\n> After you log in again.\n\n### With low priority\n\n```bash\nnohup nice -n 19 ./report.sh > report.log 2>&1 &\n```\n\n> Long and quiet.\n\n### Stop it later\n\n```bash\npkill -f long-task.sh\n```\n\n> nohup does not protect against SIGTERM.\n\n### Better alternative: a transient systemd unit\n\n```bash\nsystemd-run --user --unit=longtask ./long-task.sh\njournalctl --user -u longtask -f\n```\n\n> Logs in the journal, easy to stop with systemctl.\n\n### Alternative: tmux\n\n```bash\ntmux new -s work\n```\n\n> Detach with Ctrl+B D, attach later with `tmux attach -t work`.\n\n### Mind the lingering setting\n\n```bash\nloginctl enable-linger lisa\n```\n\n> With systemd, user processes may be killed at logout unless linger is on (or KillUserProcesses=no).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The exit status of the command. |\n| `125` | nohup failed. |\n| `126` | The command could not be run. |\n| `127` | The command was not found. |\n\nFull manual: [man nohup](https://linux.die.net/man/1/nohup)\n","page":"commands/nohup/","label":"nohup","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-nproc","n":1483,"slug":"nproc","title":"nproc — Print the number of CPUs (cores/threads) that the current process can use","sections":["commands"],"topic":"Commands · System information","tags":["cpu","cores","processors","parallel"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the number of CPUs (cores/threads) that the current process can use.\n\n## About\n\n`nproc` prints how many processing units are available — limited by CPU affinity and cgroup settings. Use it to compare with the load average, or to choose the number of parallel jobs.\n\n## Syntax\n\n```text\nnproc [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--all` | All installed processors, not only the available ones. |\n| `--ignore=N` | Leave out N processors. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Number of CPUs\n\n```bash\nnproc\n```\n```text\n2\n```\n\n> Usable now.\n\n### All installed CPUs\n\n```bash\nnproc --all\n```\n```text\n4\n```\n\n> More than nproc = some are limited (affinity, cgroup, offline).\n\n### Parallel build\n\n```bash\nmake -j\"$(nproc)\"\n```\n\n> One job per core.\n\n### Keep one core free\n\n```bash\nmake -j\"$(nproc --ignore=1)\"\n```\n\n> The system stays usable.\n\n### Compare with the load\n\n```bash\nuptime; nproc\n```\n\n> Load above nproc = processes wait.\n\n### Parallel compression\n\n```bash\nxz -T\"$(nproc)\" big.tar\n```\n\n> All cores.\n\n### In a container with a CPU limit\n\n```bash\npodman run --rm --cpuset-cpus=0 registry.access.redhat.com/ubi10/ubi nproc\n```\n```text\n1\n```\n\n> Containers see the limit.\n\n### Limit a command to 2 CPUs\n\n```bash\ntaskset -c 0,1 nproc\n```\n```text\n2\n```\n\n> CPU affinity changes the result.\n\n### More details\n\n```bash\nlscpu | grep -E '^(CPU\\(s\\)|Thread|Core|Socket)'\n```\n\n> Sockets, cores and threads.\n\n### Script sizing\n\n```bash\nWORKERS=$(( $(nproc) * 2 )); echo $WORKERS\n```\n\n> A common rule for web workers.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man nproc](https://linux.die.net/man/1/nproc)\n","page":"commands/nproc/","label":"nproc","where":"Command page · System information","aka":[]},{"id":"cmd-nslookup","n":1484,"slug":"nslookup","title":"nslookup — Ask a DNS server for the address of a name, or for other DNS records (mail, text, name servers)","sections":["commands"],"topic":"Commands · Network","tags":["dns","name","lookup","mx","txt","spf","dmarc","ptr","reverse","resolver"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Ask a DNS server for the address of a name, or for other DNS records (mail, text, name servers).\n\n## About\n\n`nslookup` sends a question to a DNS server and shows the answer. It works on Linux, macOS and Windows, so it is good to know when you help other people.\n\nThere are two ways to use it:\n\n- **One question:** `nslookup NAME [SERVER]`. It prints the answer and stops.\n- **Interactive:** type `nslookup` alone. You get a `>` prompt and can ask many questions, change the server or the record type.\n\n`nslookup` asks DNS only. It does not read `/etc/hosts`. To test what the system really uses (hosts file + DNS), use `getent hosts NAME`. For more details and clean output in scripts, `dig` is the better tool.\n\nOn Red Hat Enterprise Linux it is in the package `bind-utils` (`dnf install bind-utils`).\n\n## Syntax\n\n```text\nnslookup [-option ...] [name | -] [server]\nnslookup -type=TYPE name [server]\nnslookup                      (interactive mode)\n```\n\n## Options\n\n\n### Options on the command line (also with \"set\" in interactive mode)\n\n| Option | What it does |\n|---|---|\n| `-type=<TYPE>, -query=<TYPE>, -q=<TYPE>` | The record type to ask for: A, AAAA, MX, TXT, NS, SOA, PTR, CNAME, SRV, CAA, ANY. Default: A and AAAA. |\n| `-port=<n>` | Ask on another port than 53, for example a test DNS server. |\n| `-timeout=<seconds>` | How long to wait for an answer before trying again. |\n| `-retry=<n>` | How many times to try again. |\n| `-debug, -nodebug` | Show the full answer with all sections and the TTL values. |\n| `-d2, -nod2` | Even more debug output: what nslookup sends and gets. |\n| `-domain=<name>` | Set the search domain for short names. |\n| `-search, -nosearch` | Add the search domains to names that have no dot at the end (on by default). |\n| `-recurse, -norecurse` | Ask the server to find the answer for you (on by default). -norecurse shows only what this server knows. |\n| `-vc, -novc` | Use TCP instead of UDP. |\n| `-fail, -nofail` | With -fail, stop when a server answers SERVFAIL. With -nofail (default), try the next server. |\n| `-ndots=<n>` | A name with this many dots is asked as it is first, before the search domains. |\n| `-class=<CLASS>` | The DNS class: IN (internet, default), CH, HS or ANY. |\n| `-all` | In interactive mode (as `set all`): show the current settings. |\n| `-version` | Show the version and stop. |\n\n### Commands in interactive mode\n\n| Option | What it does |\n|---|---|\n| `<name> [server]` | Look up a name (or an IP address for the reverse name). |\n| `server <server>` | Use another DNS server for the next questions. |\n| `lserver <server>` | Like server, but use the first server to find the new server's address. |\n| `set <keyword>[=<value>]` | Change a setting, for example `set type=mx` or `set debug`. |\n| `exit` | Leave interactive mode. Ctrl+D works too. |\n\n## Examples\n\n### Find the address of a name\n\n```bash\nnslookup www.redhat.com\n```\n```text\nServer:         192.168.122.1\nAddress:        192.168.122.1#53\n\nNon-authoritative answer:\nwww.redhat.com  canonical name = ds-www.redhat.com.edgekey.net.\n...\n```\n\n> \"Server\" is the DNS server that answered. \"Non-authoritative\" means it came from a cache, not from the owner of the domain.\n\n### Find the name of an IP address (reverse lookup)\n\n```bash\nnslookup 8.8.8.8\n```\n```text\n8.8.8.8.in-addr.arpa    name = dns.google.\n```\n\n> Useful in incident response: which host is behind an IP address in your logs? Not every address has a reverse name.\n\n### Ask one DNS server directly\n\n```bash\nnslookup app.example.com 10.0.0.53\nnslookup app.example.com 1.1.1.1\n```\n\n> Compare the internal and a public server. Different answers show split DNS or an old record in one cache.\n\n### Find the mail servers of a domain\n\n```bash\nnslookup -type=mx example.com\n```\n```text\nexample.com     mail exchanger = 10 mx1.example.com.\n```\n\n> The lowest number has the highest priority. Check this when mail does not arrive.\n\n### Check the SPF record of a domain (mail security)\n\n```bash\nnslookup -type=txt example.com\n```\n```text\nexample.com     text = \"v=spf1 include:_spf.example.com -all\"\n```\n\n> SPF lists the servers that may send mail for the domain. \"-all\" means: reject all others.\n\n### Check the DMARC record\n\n```bash\nnslookup -type=txt _dmarc.example.com\n```\n\n> No answer means no DMARC policy: other people can more easily send fake mail in the name of the domain.\n\n### Find the name servers of a domain\n\n```bash\nnslookup -type=ns example.com\n```\n\n> Shows which servers are the owner (authoritative) of the domain. Ask them directly to see the real records.\n\n### See the SOA record and the serial number\n\n```bash\nnslookup -type=soa example.com\n```\n\n> After you change a zone, the serial number must go up, or other servers do not take the change.\n\n### Check which certificate authorities may issue certificates\n\n```bash\nnslookup -type=caa example.com\n```\n\n> A CAA record allows only some CAs (for example Let's Encrypt) to make certificates for the domain.\n\n### Find a service with an SRV record (like LDAP or Kerberos)\n\n```bash\nnslookup -type=srv _ldap._tcp.corp.example.com\n```\n\n> Linux clients that join Active Directory or IdM find the domain controllers this way.\n\n### See the full answer with TTL values\n\n```bash\nnslookup -debug app.example.com\n```\n\n> The TTL shows how long servers keep the answer in cache. Lower it before you move a service to a new IP.\n\n### Wait in a script until a new name works\n\n```bash\nuntil nslookup -timeout=2 -retry=1 app.example.com >/dev/null 2>&1; do sleep 10; done; echo \"DNS is ready\"\n```\n\n> Useful in a deployment pipeline: only go on when the new DNS record exists.\n\n### Test name resolution inside a Kubernetes pod\n\n```bash\nkubectl exec -it debug-pod -- nslookup my-service.default.svc.cluster.local\n```\n\n> If this fails, check the cluster DNS (CoreDNS) before you look at your app.\n\n### Ask many questions in interactive mode\n\n```bash\nnslookup\n```\n```text\n> server 1.1.1.1\n> set type=aaaa\n> www.example.com\n> exit\n```\n\n> Interactive mode keeps your settings between questions. Type the words after the > prompt of nslookup.\n\n## Watch out\n\n- `nslookup` does not read `/etc/hosts`. A name can work for `ping` but not for `nslookup`, or the other way around.\n- The type `ANY` is often refused by modern DNS servers. Ask for the type you need.\n- In interactive mode the prompt is `>`. Type `exit` to leave.\n- The exit code is 1 when a question failed and 0 when it worked. Scripts can use this.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/resolv.conf` | The DNS servers and search domains that nslookup uses by default. |\n\n## Tips\n\n- One name = address, one address = name.\n- Add a server at the end to ask that server: `nslookup NAME SERVER`.\n- `-type=` picks the record: **mx** for mail, **txt** for SPF and DMARC, **ns** for the owner, **ptr** for reverse.\n\nFull manual: [man nslookup](https://linux.die.net/man/1/nslookup)\n","page":"commands/nslookup/","label":"nslookup","where":"Command page · Network","aka":[]},{"id":"cmd-parted","n":1485,"slug":"parted","title":"parted — Create, delete, resize and list partitions — in a menu or with one command line (good for scripts)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["partitions","gpt","mbr","script","resize partition"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create, delete, resize and list partitions — in a menu or with one command line (good for scripts).\n\n## About\n\n`parted DEVICE` opens a prompt; `parted DEVICE COMMAND` runs one command. Unlike fdisk, parted writes every change **at once**. Positions are start and end (like `1MiB 1GiB`); use MiB/GiB for exact powers of 1024 and good alignment.\n\n`-s` (script mode) never asks questions — so be careful.\n\n## Syntax\n\n```text\nparted [OPTION]... [DEVICE [COMMAND [ARGUMENT]...]...]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `print [free\\|all]` | Show the table (and free space or all devices). |\n| `mklabel gpt\\|msdos` | New empty partition table (deletes all partitions). |\n| `mkpart [NAME\\|TYPE] [FSTYPE] START END` | New partition (GPT: a name; MBR: primary/extended/logical). |\n| `rm N` | Remove partition N. |\n| `resizepart N END` | Change the end of a partition. |\n| `name N NAME` | Name a GPT partition. |\n| `set N FLAG on\\|off` | Set a flag: boot, esp, lvm, raid, swap, bios_grub, msftdata … |\n| `toggle N FLAG` | Toggle a flag. |\n| `type N TYPE-UUID` | Set the GPT type UUID (newer parted). |\n| `unit UNIT` | Units: s, B, KiB, MiB, GiB, %, compact … |\n| `align-check minimal\\|optimal N` | Check alignment. |\n| `rescue START END` | Find a lost partition. |\n| `disk_set / disk_toggle FLAG` | Flags of the whole disk. |\n| `select DEVICE` | Choose another disk. |\n| `quit` | Leave. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --script` | Never ask; for scripts. |\n| `-l, --list` | List the tables of all disks. |\n| `-a, --align none\\|cylinder\\|minimal\\|optimal` | Alignment of new partitions (default optimal). |\n| `-m, --machine` | Machine-readable output. |\n| `-j, --json` | JSON output. |\n| `-f, --fix` | Answer \"fix\" to questions in script mode. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### Show a disk\n\n```bash\nparted /dev/vdb print\n```\n\n> Table type, partitions, sizes and flags.\n\n### New GPT table\n\n```bash\nparted -s /dev/vdb mklabel gpt\n```\n\n> Deletes all partitions on the disk at once.\n\n### Create partitions in one go\n\n```bash\nparted -s /dev/vdb mkpart data xfs 1MiB 2GiB\nparted -s /dev/vdb mkpart swap linux-swap 2GiB 3GiB\n```\n\n> GPT: the first word is the partition name.\n\n### A partition for LVM\n\n```bash\nparted -s /dev/vdb mkpart pv1 3GiB 100%\nparted -s /dev/vdb set 3 lvm on\n```\n\n> 100% = to the end of the disk.\n\n### Show free space in MiB\n\n```bash\nparted /dev/vdb unit MiB print free\n```\n\n> Where can a new partition go?\n\n### MBR with logical partitions\n\n```bash\nparted -s /dev/vdc mklabel msdos mkpart primary 1MiB 1GiB mkpart extended 1GiB 100% mkpart logical 1GiB 2GiB\n```\n\n> More commands in one line.\n\n### Grow a partition to the end of the disk\n\n```bash\nparted /dev/vdb resizepart 3 100%\n```\n\n> After a VM disk was made bigger; then grow the PV or file system.\n\n### EFI system partition\n\n```bash\nparted -s /dev/vdd mkpart ESP fat32 1MiB 601MiB set 1 esp on\n```\n\n> For a UEFI boot disk.\n\n### List all disks\n\n```bash\nparted -l\n```\n\n> Every disk with its table.\n\n### JSON for automation\n\n```bash\nparted -j /dev/vdb print | jq '.disk.partitions[].size'\n```\n\n> Newer parted versions.\n\n## Watch out\n\n- parted writes at once: there is no \"quit without saving\".\n- `mkpart` does not create a file system; use mkfs afterwards (the FSTYPE word only sets the type).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man parted](https://linux.die.net/man/8/parted)\n","page":"commands/parted/","label":"parted","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-partprobe","n":1486,"slug":"partprobe","title":"partprobe — Tell the kernel to read the partition table of a disk again","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["partition table","kernel","reread","partitions"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Tell the kernel to read the partition table of a disk again.\n\n## About\n\nAfter you change partitions, the kernel must learn the new table. fdisk and parted try this themselves; when the disk is in use, the kernel may keep the old table. `partprobe DEVICE` asks again, so new partitions appear in `/dev` without a reboot.\n\n## Syntax\n\n```text\npartprobe [-d] [-s] [DEVICE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `DEVICE` | The disk to probe (all disks without a name). |\n| `-s, --summary` | Show a summary of the partitions. |\n| `-d, --dry-run` | Do not tell the kernel; only check. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### After fdisk\n\n```bash\npartprobe /dev/vdb\nlsblk /dev/vdb\n```\n\n> New partitions now have device files.\n\n### Show what it found\n\n```bash\npartprobe -s /dev/vdb\n```\n```text\n/dev/vdb: gpt partitions 1 2 3\n```\n\n> Quick check.\n\n### All disks\n\n```bash\npartprobe\n```\n\n> Probes every disk.\n\n### When it fails\n\n```bash\npartprobe /dev/vda\n```\n```text\nError: Partition(s) 2 on /dev/vda have been written, but we have been unable to inform the kernel...\n```\n\n> The disk is busy (system disk). A reboot may be needed, or use `partx`.\n\n### Alternative: partx\n\n```bash\npartx -u /dev/vdb\n```\n\n> Updates only the changed partitions.\n\n### Wait for udev to create the devices\n\n```bash\npartprobe /dev/vdb; udevadm settle\n```\n\n> In scripts, before mkfs.\n\n### Dry run\n\n```bash\npartprobe -d -s /dev/vdb\n```\n\n> Test without changing anything.\n\n### After growing a VM disk\n\n```bash\necho 1 > /sys/class/block/vdb/device/rescan; partprobe /dev/vdb\n```\n\n> The kernel learns the new disk size (for SCSI/virtio with rescan support).\n\n### Check the result with the kernel view\n\n```bash\ncat /proc/partitions\n```\n\n> The kernel's list of partitions.\n\n### In an automated disk setup\n\n```bash\nparted -s /dev/vdb mklabel gpt mkpart data xfs 1MiB 100% && partprobe /dev/vdb && mkfs.xfs /dev/vdb1\n```\n\n> Each step only when the step before worked.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man partprobe](https://linux.die.net/man/8/partprobe)\n","page":"commands/partprobe/","label":"partprobe","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-passwd","n":1487,"slug":"passwd","title":"passwd — Set or change a password, and lock, unlock, expire or show the password of an account","sections":["commands"],"topic":"Commands · Users and groups","tags":["password","change password","lock","expire","status","aging"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Set or change a password, and lock, unlock, expire or show the password of an account.\n\n## About\n\nA normal user runs `passwd` to change the own password (the old one is asked first). root can set the password of any user without knowing the old one, and can lock, unlock, delete or expire passwords.\n\nThe password is stored as a hash in `/etc/shadow`. PAM checks the quality of a new password (length, dictionary words) with the rules in `/etc/security/pwquality.conf`; root gets only a warning.\n\n`passwd` also has short options for password aging (`-x`, `-n`, `-w`, `-i`); `chage` has the same and more.\n\n## Syntax\n\n```text\npasswd [OPTION]... [LOGIN]\n```\n\n## Options\n\n\n### Set and change\n\n| Option | What it does |\n|---|---|\n| `-s, --stdin` | Read the new password from standard input (root, for scripts). |\n| `-k, --keep-tokens` | Change only if the password has expired. |\n| `-q, --quiet` | Quiet mode. |\n| `-r, --repository REPOSITORY` | Change the password in this repository. |\n\n### Lock and status\n\n| Option | What it does |\n|---|---|\n| `-l, --lock` | Lock the password (adds ! before the hash). |\n| `-u, --unlock` | Unlock the password. |\n| `-d, --delete` | Delete the password (empty password). |\n| `-e, --expire` | Expire the password now: a new one is needed at the next login. |\n| `-S, --status` | Show the password status of the account. |\n| `-a, --all` | With -S: show the status of all users. |\n\n### Aging\n\n| Option | What it does |\n|---|---|\n| `-x, --maxdays DAYS` | Maximum number of days a password is valid. |\n| `-n, --mindays DAYS` | Minimum number of days between changes. |\n| `-w, --warndays DAYS` | Warn this many days before the password expires. |\n| `-i, --inactive DAYS` | Lock the account this many days after the password expired. |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Change your own password\n\n```bash\npasswd\n```\n```text\nCurrent password:\nNew password:\nRetype new password:\n```\n\n> The new password must pass the quality check.\n\n### Set the password of another user\n\n```bash\npasswd lisa\n```\n\n> root does not need the old password.\n\n### Set a password from a script\n\n```bash\necho 'S3cure#Pass' | passwd --stdin lisa\n```\n\n> Fast for labs. In real scripts the password can end up in the shell history — use `chpasswd` from a protected file.\n\n### Show the password status\n\n```bash\npasswd -S lisa\n```\n```text\nlisa P 2026-10-25 0 90 7 -1\n```\n\n> P = usable password, L = locked, NP = no password. Then last change, min, max, warn, inactive.\n\n### Lock and unlock\n\n```bash\npasswd -l lisa\npasswd -u lisa\n```\n\n> A lock blocks password logins only, not SSH keys.\n\n### Force a new password at the next login\n\n```bash\npasswd -e lisa\n```\n\n> Use it after you set a first password for a new person.\n\n### Set password aging\n\n```bash\npasswd -x 90 -n 1 -w 7 lisa\n```\n\n> Same as `chage -M 90 -m 1 -W 7 lisa`.\n\n### Check all accounts for weak states (security)\n\n```bash\npasswd -Sa | awk '$2 == \"NP\"'\n```\n\n> Accounts without a password are a big risk.\n\n### Set many passwords at once\n\n```bash\nchpasswd < /root/new-passwords.txt\n```\n\n> File lines are `user:password`. Remove the file afterwards.\n\n### Test the quality rules\n\n```bash\ngrep -E '^(minlen|minclass|dictcheck)' /etc/security/pwquality.conf\n```\n\n> Set `minlen = 12` to ask for longer passwords.\n\n### Reset the root password from a rescue shell\n\n```bash\npasswd root\n```\n\n> In the rescue mode (rd.break or init=/bin/bash), see the boot chapter; then `touch /.autorelabel`.\n\n### Delete a password (do not use on servers)\n\n```bash\npasswd -d testuser\n```\n\n> The account then has an empty password — only for special kiosk tests.\n\n## Watch out\n\n- `passwd -l` does not stop SSH key logins. Use `usermod -e 1` or `chage -E 0` to block the account.\n- On RHEL 9 and older, passwd came from its own package with other long names (`--maximum`, `--minimum`); the short options are the same.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/shadow` | The password hashes and aging data. |\n| `/etc/security/pwquality.conf` | Password quality rules. |\n| `/etc/pam.d/passwd` | PAM rules for passwd. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Permission denied. |\n| `2` | Wrong option. |\n| `3` | Unexpected failure, nothing done. |\n| `4` | The password file is missing. |\n| `5` | The password file is busy, try again. |\n| `6` | Wrong argument. |\n\nFull manual: [man passwd](https://linux.die.net/man/1/passwd)\n","page":"commands/passwd/","label":"passwd","where":"Command page · Users and groups","aka":[]},{"id":"cmd-pidof","n":1488,"slug":"pidof","title":"pidof — Print the PIDs of a running program, by its exact name","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["pid","find process","program name"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the PIDs of a running program, by its exact name.\n\n## About\n\n`pidof NAME` prints the PIDs of all processes of that program, on one line. Unlike pgrep, the name must match exactly (no pattern). It is often used inside other commands: `kill $(pidof myapp)`.\n\n## Syntax\n\n```text\npidof [OPTION]... PROGRAM...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s` | Only one PID. |\n| `-x` | Also scripts (the shell that runs the script). |\n| `-o PID` | Leave out this PID (`%PPID` = the calling shell). |\n| `-c` | Only processes with the same root folder (root only). |\n| `-n` | Do not call stat on network file systems. |\n| `-q` | Quiet: only the exit code. |\n| `-z` | Also zombies and processes in disk wait. |\n| `-d SEP` | Separator between PIDs (default space). |\n| `-S SEP` | Same as -d (procps-ng). |\n| `-t` | Show thread IDs (procps-ng). |\n| `-w` | Also kernel workers (procps-ng). |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### PIDs of a program\n\n```bash\npidof sshd\n```\n```text\n2215 2210 1010\n```\n\n> All sshd processes.\n\n### Only one PID\n\n```bash\npidof -s chronyd\n```\n```text\n812\n```\n\n> For commands that take one PID.\n\n### Use it with kill or renice\n\n```bash\nkill -HUP $(pidof rsyslogd)\n```\n\n> Reload by name.\n\n### Find a running script\n\n```bash\npidof -x backup.sh\n```\n\n> Without -x scripts are not found.\n\n### Is it running? (scripts)\n\n```bash\npidof -q httpd && echo \"httpd runs\"\n```\n\n> Exit code 0 when found.\n\n### Leave out the own shell\n\n```bash\npidof -o %PPID -x myscript.sh\n```\n\n> A script that checks if another copy runs.\n\n### Comma separated for other tools\n\n```bash\npidof -d, httpd\n```\n```text\n1420,1421,1422\n```\n\n> Handy for `top -p`.\n\n### Watch all processes of a program\n\n```bash\ntop -p $(pidof -d, httpd)\n```\n\n> Only the web server.\n\n### Threads of a process\n\n```bash\nps -L -p $(pidof chronyd)\n```\n\n> pidof gives the PID, ps the threads.\n\n### Check that a service is not running twice\n\n```bash\npidof myapp | wc -w\n```\n\n> More than 1 for a single-instance app = a problem.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | At least one process found. |\n| `1` | Nothing found. |\n\nFull manual: [man pidof](https://linux.die.net/man/8/pidof)\n","page":"commands/pidof/","label":"pidof","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-ping","n":1489,"slug":"ping","title":"ping — Send ICMP echo requests to a host to test if it is reachable and how fast it answers","sections":["commands"],"topic":"Commands · Network","tags":["icmp","reachability","latency","packet loss","mtu","network test"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Send ICMP echo requests to a host to test if it is reachable and how fast it answers.\n\n## About\n\n`ping` sends small packets and waits for the answer. It shows the round-trip time per packet and at the end the packet loss and min/avg/max times. On Linux it runs until you press Ctrl+C, unless you give a count with `-c`.\n\nNo answer does not always mean the host is down: firewalls often block ICMP. Test the real service too (like `curl` or `ss`).\n\n## Syntax\n\n```text\nping [OPTION]... DESTINATION\n```\n\n## Options\n\n\n### Count and timing\n\n| Option | What it does |\n|---|---|\n| `-c COUNT` | Stop after COUNT packets. |\n| `-i INTERVAL` | Seconds between packets (default 1; under 0.2 only for root). |\n| `-w DEADLINE` | Stop after this many seconds, whatever happens. |\n| `-W TIMEOUT` | Wait this long for each answer. |\n| `-f` | Flood ping: send as fast as possible (root only; shows dots). |\n| `-l PRELOAD` | Send this many packets without waiting first. |\n\n### Address and route\n\n| Option | What it does |\n|---|---|\n| `-4` | Use IPv4 only. |\n| `-6` | Use IPv6 only. |\n| `-I INTERFACE` | Send from this interface or source address. |\n| `-b` | Allow ping to a broadcast address. |\n| `-r` | Do not use the routing table; send directly on the interface. |\n| `-R` | Record the route (IPv4, most hosts ignore it). |\n| `-m MARK` | Set a firewall mark on the packets. |\n| `-Q TOS` | Set the Type of Service / DSCP bits. |\n| `-t TTL` | Set the time to live. |\n| `-F FLOWLABEL` | Set the IPv6 flow label. |\n| `-N NODEINFO_OPTION` | Send IPv6 node info queries. |\n\n### Packets\n\n| Option | What it does |\n|---|---|\n| `-s SIZE` | Data bytes per packet (default 56 = 84 bytes with headers). |\n| `-M DO\\|WANT\\|DONT\\|PROBE` | Path MTU discovery: `do` = do not fragment. |\n| `-p PATTERN` | Fill the packet with this hex pattern. |\n| `-S SNDBUF` | Socket send buffer. |\n| `-T TIMESTAMP_OPTION` | IP timestamp option (tsonly, tsandaddr, tsprespec). |\n| `-e IDENTIFIER` | The ICMP identifier. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-q` | Quiet: only the summary at the end. |\n| `-n` | Numbers only, no name lookups for the replies. |\n| `-a` | Beep for every answer. |\n| `-A` | Adaptive: next packet as soon as the answer comes. |\n| `-D` | Print a time stamp before each line. |\n| `-O` | Report missing answers before the next packet. |\n| `-U` | Print full user-to-user latency. |\n| `-v` | Verbose. |\n| `-d` | Set SO_DEBUG on the socket. |\n| `-B` | Do not change the source address. |\n| `-C` | Call connect() on the socket. |\n| `-L` | Do not loop multicast packets back. |\n| `-H` | Force DNS name resolution for output. |\n| `-h` | Show help. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Test a host with 3 packets\n\n```bash\nping -c 3 192.168.122.1\n```\n```text\nPING 192.168.122.1 (192.168.122.1) 56(84) bytes of data.\n64 bytes from 192.168.122.1: icmp_seq=1 ttl=64 time=0.412 ms\n64 bytes from 192.168.122.1: icmp_seq=2 ttl=64 time=0.389 ms\n64 bytes from 192.168.122.1: icmp_seq=3 ttl=64 time=0.401 ms\n\n--- 192.168.122.1 ping statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time 2003ms\nrtt min/avg/max/mdev = 0.389/0.400/0.412/0.009 ms\n```\n\n> 0% loss = the gateway answers.\n\n### Test the network step by step\n\n```bash\nping -c1 192.168.122.1 && ping -c1 8.8.8.8 && ping -c1 www.redhat.com\n```\n\n> Gateway → internet → DNS: the first failure shows where the problem is.\n\n### Test IPv6\n\n```bash\nping -6 -c 2 2001:db8:10::1\n```\n\n> Use -6 with names that have both addresses.\n\n### Ping from a chosen interface\n\n```bash\nping -c 3 -I enp2s0 10.0.0.1\n```\n\n> Checks a second network card.\n\n### Find the MTU of the path\n\n```bash\nping -c 2 -M do -s 1472 192.168.122.20\n```\n\n> 1472 + 28 header bytes = 1500. \"Message too long\" means the MTU is smaller (VPN, tunnels).\n\n### Only the summary\n\n```bash\nping -q -c 20 server2\n```\n\n> Good for scripts and reports.\n\n### Stop after a fixed time\n\n```bash\nping -w 5 server2\n```\n\n> Ends after 5 seconds even when no answer comes.\n\n### Wait until a host is back after a reboot\n\n```bash\nuntil ping -c1 -W1 server2 >/dev/null; do sleep 2; done; echo \"server2 is up\"\n```\n\n> Then connect with ssh.\n\n### Log with time stamps\n\n```bash\nping -D -i 10 server2 | tee ping.log\n```\n\n> Proof of short outages for the network team.\n\n### Quick sweep of a small subnet\n\n```bash\nfor i in $(seq 1 20); do ping -c1 -W1 192.168.122.$i >/dev/null && echo \"192.168.122.$i up\"; done\n```\n\n> Only on your own networks; better tools: nmap -sn.\n\n### Load test (root, lab only)\n\n```bash\nping -f -c 1000 server2\n```\n\n> Shows packet loss under load. Never on networks you do not own.\n\n## Watch out\n\n- No answer can be a firewall, not a dead host.\n- A name that does not resolve gives \"Name or service not known\": that is a DNS problem.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | At least one answer. |\n| `1` | No answer (with -c or -w). |\n| `2` | Other error (like an unknown name). |\n\nFull manual: [man ping](https://linux.die.net/man/8/ping)\n","page":"commands/ping/","label":"ping","where":"Command page · Network","aka":[]},{"id":"cmd-pkexec","n":1490,"slug":"pkexec","title":"pkexec — Run a program as another user (by default root) when polkit allows it","sections":["commands"],"topic":"Commands · Users and groups","tags":["polkit","policykit","run as root","privileges","authorization"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run a program as another user (by default root) when polkit allows it.\n\n## About\n\n`pkexec` is the polkit way to run a command with more rights. polkit checks the rules for the action `org.freedesktop.policykit.exec`; normally an administrator (on RHEL: member of wheel) must type the own password.\n\nMost admins use sudo. pkexec is used by desktop tools and when sudo is not set up. polkit also controls actions of system services (network, power, disks) — `systemctl` or `nmcli` ask polkit when you are not root.\n\n## Syntax\n\n```text\npkexec [--user USER] PROGRAM [ARGUMENTS]...\npkexec --disable-internal-agent PROGRAM\npkexec --version | --help\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--user USER` | Run the program as this user (default root). |\n| `--keep-cwd` | Keep the current folder (newer versions). |\n| `--disable-internal-agent` | Do not use the text agent to ask the password. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Run a command as root\n\n```bash\npkexec systemctl restart chronyd\n```\n\n> polkit asks the password of an admin user.\n\n### Run as another user\n\n```bash\npkexec --user apache whoami\n```\n\n> polkit decides if you may.\n\n### See what polkit allows you in NetworkManager\n\n```bash\nnmcli general permissions\n```\n\n> yes / auth / no per action.\n\n### List all polkit actions\n\n```bash\npkaction | head\n```\n\n> Every action a service checks with polkit.\n\n### Show details of one action\n\n```bash\npkaction --verbose --action-id org.freedesktop.policykit.exec\n```\n\n> Shows who may do it (implicit any/inactive/active).\n\n### Check one action for your session\n\n```bash\npkcheck --action-id org.freedesktop.NetworkManager.network-control --process $$ -u\n```\n\n> Tests the rule without doing the action.\n\n### Find the rule that makes wheel admin\n\n```bash\ngrep -r wheel /usr/share/polkit-1/rules.d/\n```\n\n> `polkit.addAdminRule` returns unix-group:wheel.\n\n### Restart a service as a normal user\n\n```bash\nsystemctl restart httpd\n```\n\n> Without sudo, systemctl asks polkit, which asks an admin password.\n\n### Check that the polkit service runs\n\n```bash\nsystemctl status polkit\n```\n\n> Without it, polkit checks fail.\n\n### Check the version after a security fix (PwnKit)\n\n```bash\nrpm -q polkit\n```\n\n> CVE-2021-4034 (PwnKit) was a root hole in old pkexec versions; keep polkit updated.\n\n## Watch out\n\n- pkexec is setuid root. Keep polkit updated.\n- Own rules go in /etc/polkit-1/rules.d/, not in /usr/share/polkit-1/.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/share/polkit-1/actions/` | Actions of the services. |\n| `/usr/share/polkit-1/rules.d/` | Rules from packages. |\n| `/etc/polkit-1/rules.d/` | Your own rules. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The program ran (or its exit code). |\n| `126` | Not authorized, or the dialog was closed. |\n| `127` | polkit not available or the program was not found. |\n\nFull manual: [man pkexec](https://linux.die.net/man/1/pkexec)\n","page":"commands/pkexec/","label":"pkexec","where":"Command page · Users and groups","aka":[]},{"id":"cmd-pkill","n":1491,"slug":"pkill","title":"pkill — Find processes by name or other attributes (pgrep), signal them (pkill), or wait for them to end (pidwait)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["find process","signal by name","user processes","pattern"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Find processes by name or other attributes (pgrep), signal them (pkill), or wait for them to end (pidwait).\n\nSame page for: `pgrep`, `pidwait`\n\n## About\n\n`pgrep PATTERN` prints the PIDs of processes whose name matches the pattern (a regular expression). `pkill PATTERN` sends them a signal (TERM by default). Both select with the same options: by user (`-u`), terminal (`-t`), parent (`-P`), full command line (`-f`) and more.\n\nRun `pgrep` first with the same options to see what pkill would hit.\n\n## Syntax\n\n```text\npgrep [OPTION]... PATTERN\npkill [OPTION]... [-SIGNAL] PATTERN\npidwait [OPTION]... PATTERN\n```\n\n## Options\n\n\n### Signal (pkill)\n\n| Option | What it does |\n|---|---|\n| `-SIGNAL, --signal SIGNAL` | The signal to send (default TERM). |\n| `-e, --echo` | Show what was killed. |\n| `-q, --queue VALUE` | Send a value with the signal. |\n| `-H, --require-handler` | Only processes that handle the signal. |\n\n### Output (pgrep)\n\n| Option | What it does |\n|---|---|\n| `-l, --list-name` | Show the name with the PID. |\n| `-a, --list-full` | Show the full command line. |\n| `-c, --count` | Only the number of matches. |\n| `-d, --delimiter TEXT` | Separator between PIDs. |\n| `-w, --lightweight` | Show thread IDs. |\n\n### Select\n\n| Option | What it does |\n|---|---|\n| `-f, --full` | Match the whole command line, not only the name. |\n| `-u, --euid USERS` | Effective user. |\n| `-U, --uid USERS` | Real user. |\n| `-G, --group GIDS` | Real group. |\n| `-g, --pgroup PGRPS` | Process group. |\n| `-s, --session SIDS` | Session. |\n| `-t, --terminal TERMS` | Terminal, like pts/1. |\n| `-P, --parent PPIDS` | Children of these parents. |\n| `-n, --newest` | Only the newest match. |\n| `-o, --oldest` | Only the oldest match. |\n| `-O, --older SECONDS` | Only processes older than this. |\n| `-x, --exact` | The name must match exactly. |\n| `-i, --ignore-case` | Ignore case. |\n| `-v, --inverse` | Everything that does not match. |\n| `-r, --runstates STATES` | Only these states (D, R, S, Z …). |\n| `-A, --ignore-ancestors` | Leave out the own parents. |\n| `-F, --pidfile FILE` | Read PIDs from a file. |\n| `-L, --logpidfile` | Fail if the PID file is not locked. |\n| `--cgroup NAMES` | Only these cgroups. |\n| `--ns PID` | Same namespaces as PID. |\n| `--nslist NAMES` | Which namespaces to compare. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Find PIDs by name\n\n```bash\npgrep -l sshd\n```\n```text\n1010 sshd\n2210 sshd\n```\n\n> -l adds the name.\n\n### See full command lines\n\n```bash\npgrep -a python3\n```\n\n> Find which script is running.\n\n### Stop all processes of a user\n\n```bash\npkill -u olduser\n```\n\n> Before you delete the account.\n\n### Log out a session\n\n```bash\npkill -KILL -t pts/3\n```\n\n> Ends everything on that terminal.\n\n### Match the full command line\n\n```bash\npgrep -af 'backup.sh'\npkill -f 'backup.sh'\n```\n\n> The process name is bash; -f finds the script.\n\n### Count processes\n\n```bash\npgrep -c httpd\n```\n```text\n6\n```\n\n> Quick health check.\n\n### Wait until a process is gone\n\n```bash\npidwait -f backup.sh; echo \"backup finished\"\n```\n\n> Then start the next step.\n\n### Reload a daemon by name\n\n```bash\npkill -HUP rsyslogd\n```\n\n> Re-reads the config.\n\n### The newest or oldest match\n\n```bash\npgrep -n firefox; pgrep -o httpd\n```\n\n> -o httpd = the parent of all httpd children.\n\n### Hunt processes that run from /tmp (security)\n\n```bash\npgrep -af '/tmp/|/dev/shm/'\n```\n\n> Suspicious locations for running programs.\n\n### Show what pkill did\n\n```bash\npkill -e dd\n```\n```text\ndd killed (pid 4123)\n```\n\n> Proof of the action.\n\n## Watch out\n\n- The pattern is a regular expression and matches part of the name: `pkill ssh` also hits `ssh-agent` and `sshd`. Use `-x` for an exact name.\n- Test with pgrep (same options) before pkill.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | At least one process matched. |\n| `1` | No process matched. |\n| `2` | Syntax error. |\n| `3` | Fatal error. |\n\nFull manual: [man pkill](https://linux.die.net/man/1/pkill)\n","page":"commands/pkill/","label":"pkill","where":"Command page · Processes and jobs","aka":["pgrep","pidwait"]},{"id":"cmd-printenv","n":1492,"slug":"printenv","title":"printenv — Print the value of environment variables (all, or the ones you name)","sections":["commands"],"topic":"Commands · Shell and help","tags":["environment","variable","print","path"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the value of environment variables (all, or the ones you name).\n\n## About\n\n`printenv` without arguments prints every environment variable (like `env`). With names, it prints only the values, one per line, and returns an error code when a variable does not exist. That makes it handy in scripts and checks.\n\nIt only shows **exported** variables. A shell variable that was not exported is not printed.\n\n## Syntax\n\n```text\nprintenv [OPTION]... [VARIABLE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `VARIABLE` | Print only the value of this variable. Several names are allowed. |\n| `-0, --null` | End each output line with a NUL byte instead of a new line. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Print one value\n\n```bash\nprintenv HOME\n```\n```text\n/home/student\n```\n\n> Only the value, no name. Easy to use in scripts.\n\n### Print several values\n\n```bash\nprintenv USER SHELL LANG\n```\n```text\nstudent\n/bin/bash\nen_US.UTF-8\n```\n\n> One line per variable, in the order you asked.\n\n### Print all environment variables\n\n```bash\nprintenv | sort\n```\n\n> Same list as `env`, sorted by name.\n\n### Check if a variable exists\n\n```bash\nprintenv KUBECONFIG >/dev/null || echo \"KUBECONFIG is not set\"\n```\n\n> The exit code is 1 when the variable is missing.\n\n### Make a script stop when a needed variable is missing\n\n```bash\nfor v in DB_HOST DB_USER; do printenv \"$v\" >/dev/null || { echo \"missing $v\"; exit 1; }; done\n```\n\n> Fail early with a clear message, before the script touches anything.\n\n### Check the proxy settings\n\n```bash\nprintenv | grep -i _proxy\n```\n\n> Wrong proxy variables are a common reason why downloads fail on servers.\n\n### See the PATH one folder per line\n\n```bash\nprintenv PATH | tr ':' '\\n'\n```\n\n> Easy to see a wrong or missing folder.\n\n### Compare a variable as your user and with sudo\n\n```bash\nprintenv PATH; sudo printenv PATH\n```\n\n> sudo uses its own safe PATH (from `secure_path` in sudoers).\n\n### Check what a systemd service gets (test a unit)\n\n```bash\nsystemd-run --user --wait --pipe printenv | head\n```\n\n> Runs printenv the way a service would run, so you see its environment.\n\n### Print a variable that a CI job sets\n\n```bash\nprintenv CI_COMMIT_SHA\n```\n\n> CI systems pass build data in environment variables; print them to debug a pipeline step.\n\n### Print values with NUL separators\n\n```bash\nprintenv -0 USER HOME | xargs -0 -n1 echo\n```\n\n> Safe for values that contain spaces or new lines.\n\n## Watch out\n\n- Do not write `$` before the name: `printenv HOME`, not `printenv $HOME`.\n- Not exported = not printed. Use `echo \"$NAME\"` or `set` for shell variables.\n\n## Tips\n\n- `printenv NAME` = value only. `env` = all. `echo $NAME` = also non-exported variables.\n\nFull manual: [man printenv](https://linux.die.net/man/1/printenv)\n","page":"commands/printenv/","label":"printenv","where":"Command page · Shell and help","aka":[]},{"id":"cmd-printf","n":1493,"slug":"printf","title":"printf — Print formatted text: columns, numbers, padding — more exact than echo","sections":["commands"],"topic":"Commands · Shell and help","tags":["format output","scripts","columns","numbers"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print formatted text: columns, numbers, padding — more exact than echo.\n\n## About\n\n`printf FORMAT ARGUMENTS` fills the arguments into the format. `%s` = text, `%d` = whole number, `%f` = decimal number, `\\n` = new line. printf does **not** add a new line by itself. If there are more arguments than placeholders, the format is used again.\n\n## Syntax\n\n```text\nprintf [-v VAR] FORMAT [ARGUMENT]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `%s` | Text. |\n| `%d, %i` | Whole number. |\n| `%f, %.2f` | Decimal number (with 2 decimals). |\n| `%x, %o` | Hex / octal. |\n| `%5s, %-10s` | Width 5 (right aligned) / width 10 left aligned. |\n| `%05d` | Pad with zeros. |\n| `%q` | Quote for the shell (bash). |\n| `%(FORMAT)T` | Date and time (bash), like %(%F)T. |\n| `%%` | A % sign. |\n| `\\n, \\t, \\\\` | New line, tab, backslash. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-v VAR` | Store the result in a variable instead of printing (bash). |\n| `--help, --version` | Help / version (coreutils printf). |\n\n## Examples\n\n### Text with a new line\n\n```bash\nprintf \"Hello %s\\n\" \"$USER\"\n```\n```text\nHello student\n```\n\n> Add \\n yourself.\n\n### Aligned columns\n\n```bash\nprintf \"%-10s %5s\\n\" USER UID root 0 lisa 1001\n```\n```text\nUSER         UID\nroot           0\nlisa        1001\n```\n\n> The format repeats for each pair.\n\n### Numbers with decimals\n\n```bash\nprintf \"%.2f GiB\\n\" 3.14159\n```\n```text\n3.14 GiB\n```\n\n> Rounded.\n\n### Leading zeros\n\n```bash\nfor i in 1 2 10; do printf \"server%02d\\n\" $i; done\n```\n```text\nserver01\nserver02\nserver10\n```\n\n> Names for many hosts.\n\n### Write a file with lines\n\n```bash\nprintf 'line1\\nline2\\n' > /tmp/two-lines.txt\n```\n\n> Safer than echo -e.\n\n### Date without calling date\n\n```bash\nprintf '%(%F %T)T\\n' -1\n```\n\n> -1 = now (bash builtin).\n\n### Store in a variable\n\n```bash\nprintf -v STAMP '%(%Y%m%d)T' -1; echo \"backup-$STAMP.tgz\"\n```\n\n> No subshell needed.\n\n### Hex of a number\n\n```bash\nprintf \"%x\\n\" 255\n```\n```text\nff\n```\n\n> Also %o for octal.\n\n### Quote a value safely for the shell (security)\n\n```bash\nprintf '%q\\n' \"file name; rm -rf /\"\n```\n```text\nfile\\ name\\;\\ rm\\ -rf\\ /\n```\n\n> Safe to paste into a command.\n\n### A simple table from a file\n\n```bash\nwhile IFS=: read -r U _ ID _; do printf \"%-15s %6d\\n\" \"$U\" \"$ID\"; done < /etc/passwd | head -3\n```\n\n> Neat report.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (like a wrong number format). |\n\nFull manual: [man printf](https://linux.die.net/man/1/printf)\n","page":"commands/printf/","label":"printf","where":"Command page · Shell and help","aka":[]},{"id":"cmd-ps","n":1494,"slug":"ps","title":"ps — Show a snapshot of running processes: PID, user, CPU and memory use, state and command","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["processes","pid","cpu","memory","process list","threads","forest"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show a snapshot of running processes: PID, user, CPU and memory use, state and command.\n\n## About\n\n`ps` reads `/proc` once and prints a list. It accepts three option styles: UNIX (with a dash: `ps -ef`), BSD (without a dash: `ps aux`) and GNU long options (`--sort`). The two most used forms are `ps aux` and `ps -ef`.\n\nWith `-o` you choose the columns, with `--sort` the order — for example the processes that use the most memory.\n\n## Syntax\n\n```text\nps [OPTION]...\n```\n\n## Options\n\n\n### Select processes\n\n| Option | What it does |\n|---|---|\n| `-e, -A` | All processes. |\n| `a` | All processes with a terminal (BSD), with x: all. |\n| `x` | Also processes without a terminal (BSD). |\n| `-a` | All with a terminal except session leaders. |\n| `-d` | All except session leaders. |\n| `-N, --deselect` | Everything except the selection. |\n| `r` | Only running processes. |\n| `T` | Processes of this terminal. |\n| `-C CMDLIST` | By command name. |\n| `-p, p, --pid PIDLIST` | By PID. |\n| `--ppid PIDLIST` | By parent PID. |\n| `-u, U, --user USERLIST` | By effective user. |\n| `-U USERLIST` | By real user. |\n| `-g, -G, --group GRPLIST` | By group. |\n| `-s, --sid SESSLIST` | By session. |\n| `-t, t, --tty TTYLIST` | By terminal. |\n| `-q, q, --quick-pid PIDLIST` | Fast mode by PID. |\n\n### Output format\n\n| Option | What it does |\n|---|---|\n| `-f` | Full format (UID, PID, PPID, C, STIME, TTY, TIME, CMD). |\n| `-F` | Extra full format. |\n| `u` | User format (%CPU, %MEM, VSZ, RSS, STAT, START). |\n| `-l, l` | Long format. |\n| `-o, o, --format FORMAT` | Your own columns, like `-o pid,user,%cpu,cmd`. |\n| `-O, O FORMAT` | Default columns plus these. |\n| `-j, j` | Job format. |\n| `s` | Signal format. |\n| `v` | Virtual memory format. |\n| `X` | Register format. |\n| `-y` | With -l: RSS instead of ADDR. |\n| `-c` | Scheduler data with -l. |\n| `-M, Z, --context` | Add the SELinux context. |\n| `-P` | Add the CPU each process last ran on. |\n\n### Display\n\n| Option | What it does |\n|---|---|\n| `--sort SPEC, k SPEC` | Sort, like `--sort=-%mem` (- = high first). |\n| `--forest, f, -H` | Show the process tree. |\n| `-L` | Show threads (LWP, NLWP columns). |\n| `-T` | Show threads (SPID column). |\n| `H, -m, m` | Threads as processes / after processes. |\n| `e` | Show the environment after the command. |\n| `c` | Only the program name. |\n| `-w, w` | Wide output (twice = no limit). |\n| `--cols, --columns, --width N` | Set the width. |\n| `--lines, --rows N` | Set the screen height. |\n| `--no-headers, h` | No header line. |\n| `--headers` | Repeat the header every page. |\n| `n` | Numbers for user and WCHAN. |\n| `S` | Add the CPU time of dead children. |\n| `--cumulative` | Same as S. |\n| `-D, --date-format FORMAT` | Format of time columns. |\n| `--signames` | Signal names in signal format. |\n\n### Help\n\n| Option | What it does |\n|---|---|\n| `L` | List all column names. |\n| `--help SECTION` | Help (simple, list, output, threads, misc, all). |\n| `-V, V, --version` | Show the version. |\n\n## Examples\n\n### All processes, BSD style\n\n```bash\nps aux | head -3\n```\n```text\nUSER   PID %CPU %MEM    VSZ   RSS TTY  STAT START   TIME COMMAND\nroot     1  0.0  0.4 174876 16420 ?    Ss   07:58   0:02 /usr/lib/systemd/systemd --switched-root --system\nroot     2  0.0  0.0      0     0 ?    S    07:58   0:00 [kthreadd]\n```\n\n> Names in [ ] are kernel threads.\n\n### All processes, UNIX style\n\n```bash\nps -ef | grep sshd\n```\n\n> Shows PPID: who started the process.\n\n### Top memory users\n\n```bash\nps -eo pid,user,%mem,rss,cmd --sort=-%mem | head\n```\n\n> First check when a server swaps or the OOM killer acts.\n\n### Top CPU users\n\n```bash\nps aux --sort=-%cpu | head -5\n```\n\n> Snapshot; use top for a live view.\n\n### One process by name\n\n```bash\nps -C httpd -o pid,ppid,user,cmd\n```\n\n> httpd runs a root parent and apache children.\n\n### The process tree\n\n```bash\nps -ef --forest | less\n```\n\n> Shows which child belongs to which parent.\n\n### Processes of one user\n\n```bash\nps -u lisa -o pid,tty,stat,cmd\n```\n\n> What a user runs before you lock the account.\n\n### Find zombies and stuck processes\n\n```bash\nps -eo pid,ppid,stat,cmd | awk '$3 ~ /^[ZD]/'\n```\n\n> Z = zombie, D = waiting for disk (often NFS or a bad disk).\n\n### SELinux contexts of services\n\n```bash\nps -eZ | grep httpd\n```\n```text\nsystem_u:system_r:httpd_t:s0    1420 ?  00:00:00 httpd\n```\n\n> The type httpd_t confines the web server.\n\n### Threads of a process\n\n```bash\nps -L -p $(pidof chronyd)\n```\n\n> One line per thread.\n\n### Start time and run time\n\n```bash\nps -o pid,lstart,etime,cmd -p 1010\n```\n\n> When did the service start, how long does it run?\n\n### Find processes started from /tmp (security)\n\n```bash\nps -eo pid,user,cmd | grep -E '(/tmp|/dev/shm)/'\n```\n\n> Malware often runs from these folders.\n\n## Watch out\n\n- `ps -aux` (with a dash) is not the same as `ps aux`; use the BSD form without a dash.\n- ps is a snapshot. Values like %CPU are averages over the life of the process.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error, or no process matched a selection. |\n\nFull manual: [man ps](https://linux.die.net/man/1/ps)\n","page":"commands/ps/","label":"ps","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-pstree","n":1495,"slug":"pstree","title":"pstree — Show the running processes as a tree: who started whom","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["process tree","parent","child","pid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the running processes as a tree: who started whom.\n\n## About\n\n`pstree` draws the processes from `systemd` (PID 1) down. Identical children are compacted: `httpd───5*[httpd]` means five httpd children. Use `-p` for PIDs and `-a` for the full command lines.\n\n## Syntax\n\n```text\npstree [OPTION]... [PID | USER]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p` | Show PIDs (turns off compaction). |\n| `-a` | Show the command line arguments. |\n| `-u` | Show user changes, like `(apache)`. |\n| `-s` | Show the parents of the given PID. |\n| `-h` | Highlight the current process and its parents. |\n| `-H PID` | Highlight this process. |\n| `-n` | Sort by PID. |\n| `-c` | Do not compact identical subtrees. |\n| `-l` | Long lines (no cut). |\n| `-g` | Show process group IDs. |\n| `-t` | Full names of threads. |\n| `-T` | Hide threads. |\n| `-A` | ASCII lines. |\n| `-G` | VT100 lines. |\n| `-U` | UTF-8 lines. |\n| `-C age` | Color by age. |\n| `-N TYPE` | One tree per namespace (ipc, mnt, net, pid, user, uts, cgroup, time). |\n| `-S` | Show namespace changes. |\n| `-Z` | Show the SELinux context. |\n| `-V` | Show the version. |\n\n## Examples\n\n### The whole tree\n\n```bash\npstree | head\n```\n```text\nsystemd─┬─NetworkManager───3*[{NetworkManager}]\n        ├─chronyd\n        ├─sshd───sshd───sshd───bash───pstree\n```\n\n> {…} are threads.\n\n### With PIDs\n\n```bash\npstree -p | head\n```\n\n> Find the PID of a parent quickly.\n\n### Tree of one process\n\n```bash\npstree -p 1010\n```\n\n> Only the sshd master and its sessions.\n\n### Where does my shell come from?\n\n```bash\npstree -s -p $$\n```\n```text\nsystemd(1)───sshd(1010)───sshd(2210)───sshd(2215)───bash(2216)\n```\n\n> The chain of parents.\n\n### Show the user of each part\n\n```bash\npstree -u\n```\n\n> `(apache)` shows where a process changed to another user.\n\n### Full command lines\n\n```bash\npstree -a | less\n```\n\n> Shows the options of each program.\n\n### Processes of one user\n\n```bash\npstree lisa\n```\n\n> All trees owned by lisa.\n\n### Find odd children of a web server (security)\n\n```bash\npstree -p -a $(pgrep -o httpd)\n```\n\n> A shell (bash, sh) under httpd can mean a web shell attack.\n\n### Do not compact\n\n```bash\npstree -c -p | grep httpd\n```\n\n> Every child on its own line.\n\n### Container namespaces\n\n```bash\npstree -N pid\n```\n\n> Separate trees for each PID namespace (containers).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man pstree](https://linux.die.net/man/1/pstree)\n","page":"commands/pstree/","label":"pstree","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-pvcreate","n":1496,"slug":"pvcreate","title":"pvcreate — Turn a disk or partition into an LVM physical volume (pvcreate), or remove the LVM label again (pvremove)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","physical volume","pv","disk","partition"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Turn a disk or partition into an LVM physical volume (pvcreate), or remove the LVM label again (pvremove).\n\nSame page for: `pvremove`\n\n## About\n\n`pvcreate DEVICE` writes an LVM label and metadata area on the device, so it can join a volume group. It works on whole disks and on partitions (type `lvm` recommended). It refuses devices with an existing file system or partition table signature — clear them with `wipefs -a` first if you are sure.\n\n`pvremove` deletes the LVM label from a PV that is no longer in a VG.\n\n## Syntax\n\n```text\npvcreate [OPTION]... DEVICE...\npvremove [OPTION]... PV...\n```\n\n## Options\n\n\n### pvcreate\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Create even when a signature is found (twice: also over an existing PV). |\n| `-u, --uuid UUID` | Use this UUID (with --restorefile, for recovery). |\n| `--restorefile FILE` | Metadata backup for -u. |\n| `--norestorefile` | Allow -u without a restore file. |\n| `--metadatasize SIZE` | Size of the metadata area. |\n| `--pvmetadatacopies 0\\|1\\|2, --metadatacopies 0\\|1\\|2` | Number of metadata copies on the PV. |\n| `--metadataignore y\\|n` | Ignore the metadata areas on this PV. |\n| `-M, --metadatatype lvm2` | Metadata format. |\n| `--dataalignment SIZE` | Align the start of data. |\n| `--dataalignmentoffset SIZE` | Shift the alignment. |\n| `--bootloaderareasize SIZE` | Reserve space for a boot loader. |\n| `--labelsector N` | Sector of the LVM label. |\n| `--setphysicalvolumesize SIZE` | Override the detected size. |\n| `-Z, --zero y\\|n` | Wipe the first sectors. |\n| `--reportformat basic\\|json` | Output format. |\n\n### pvremove\n\n| Option | What it does |\n|---|---|\n| `-ff` | Remove even if the PV is in a VG (dangerous). |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Create a PV on a partition\n\n```bash\npvcreate /dev/vdb1\n```\n```text\n  Physical volume \"/dev/vdb1\" successfully created.\n```\n\n> Partition type lvm first (fdisk t → lvm).\n\n### Whole disks\n\n```bash\npvcreate /dev/vdc /dev/vdd\n```\n\n> No partition table needed.\n\n### Check the result\n\n```bash\npvs\n```\n\n> New PVs have no VG yet.\n\n### A disk with old signatures\n\n```bash\npvcreate /dev/vde\n```\n```text\n  WARNING: xfs signature detected on /dev/vde at offset 0. Wipe it? [y/n]:\n```\n\n> Answer only if the data is not needed; or run wipefs -a first.\n\n### Test first\n\n```bash\npvcreate -t /dev/vdc\n```\n\n> Shows what would happen.\n\n### Remove the LVM label\n\n```bash\npvremove /dev/vdd\n```\n```text\n  Labels on physical volume \"/dev/vdd\" successfully wiped.\n```\n\n> Only after vgreduce took it out of the VG.\n\n### Grow a PV after the disk or partition grew\n\n```bash\npvresize /dev/vdb1\n```\n\n> The VG gets the new space (for example after a VM disk resize).\n\n### See details of a PV\n\n```bash\npvdisplay /dev/vdb1\n```\n\n> Size, PE count, VG.\n\n### Find devices LVM can use\n\n```bash\nlvmdiskscan\n```\n\n> Lists disks and partitions with sizes.\n\n### Restore a lost PV label (recovery)\n\n```bash\npvcreate --uuid \"Wq3k…\" --restorefile /etc/lvm/backup/vgdata /dev/vdc\n```\n\n> Then vgcfgrestore vgdata. Metadata backups are in /etc/lvm/backup.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/lvm/lvm.conf` | LVM settings. |\n| `/etc/lvm/backup/, /etc/lvm/archive/` | Metadata backups. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man pvcreate](https://linux.die.net/man/8/pvcreate)\n","page":"commands/pvcreate/","label":"pvcreate","where":"Command page · LVM","aka":["pvremove"]},{"id":"cmd-pvmove","n":1497,"slug":"pvmove","title":"pvmove — Move the data of a physical volume to other PVs in the same VG — while the LVs stay in use","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","move data","replace disk","migration","online"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Move the data of a physical volume to other PVs in the same VG — while the LVs stay in use.\n\n## About\n\n`pvmove PV` moves all used extents from that PV to free space on the other PVs of the VG. Then you can remove the PV with `vgreduce` — the way to replace a disk without downtime. With a target PV (`pvmove OLD NEW`) the data goes there. A move can be stopped and continued.\n\n## Syntax\n\n```text\npvmove [OPTION]... SOURCE_PV[:PE...] [DEST_PV...]\npvmove --abort\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --name LV` | Move only the extents of this LV. |\n| `-b, --background` | Run in the background. |\n| `-i, --interval SECONDS` | How often to show progress. |\n| `--abort` | Stop all running moves. |\n| `--atomic` | Move all LVs at once (all or nothing). |\n| `--alloc POLICY` | Allocation policy for the new place. |\n| `-A, --autobackup y\\|n` | Back up the metadata. |\n| `--noudevsync` | Do not wait for udev. |\n| `--reportformat basic\\|json` | Output format. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Empty a PV\n\n```bash\npvmove /dev/vdc\n```\n```text\n  /dev/vdc: Moved: 23.53%\n  /dev/vdc: Moved: 100.00%\n```\n\n> The VG needs enough free space on other PVs.\n\n### Move to a chosen PV\n\n```bash\npvmove /dev/vdc /dev/vde\n```\n\n> New disk first added with vgextend.\n\n### Then remove the PV\n\n```bash\nvgreduce vgdata /dev/vdc; pvremove /dev/vdc\n```\n\n> The disk can now be removed.\n\n### Move only one LV\n\n```bash\npvmove -n lvdata /dev/vdc /dev/nvme0n1p3\n```\n\n> Put a busy LV on a faster disk.\n\n### In the background\n\n```bash\npvmove -b /dev/vdc\nlvs -a -o name,copy_percent\n```\n\n> Watch the progress with lvs.\n\n### Continue after an interruption\n\n```bash\npvmove\n```\n\n> Without arguments it continues unfinished moves.\n\n### Stop a move\n\n```bash\npvmove --abort\n```\n\n> The data stays where it was safe.\n\n### Check that the PV is empty\n\n```bash\npvs -o pv_name,pv_used /dev/vdc\n```\n\n> pv_used 0 = empty.\n\n### Full disk replacement in one go\n\n```bash\nvgextend vgdata /dev/vde && pvmove /dev/vdc /dev/vde && vgreduce vgdata /dev/vdc\n```\n\n> Old storage array to new one.\n\n### Move a range of extents\n\n```bash\npvmove /dev/vdc:0-999 /dev/vde\n```\n\n> Fine control (rarely needed).\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man pvmove](https://linux.die.net/man/8/pvmove)\n","page":"commands/pvmove/","label":"pvmove","where":"Command page · LVM","aka":[]},{"id":"cmd-pvs","n":1498,"slug":"pvs","title":"pvs — Show physical volumes: short table (pvs), full details (pvdisplay), or scan for PVs (pvscan)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","physical volume","report","free space"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show physical volumes: short table (pvs), full details (pvdisplay), or scan for PVs (pvscan).\n\nSame page for: `pvdisplay`, `pvscan`\n\n## About\n\n`pvs` prints one line per PV with its VG, size and free space. `pvdisplay` shows all details (PE size, total and free PEs, UUID). With `-o` you choose report columns and with `-S` you filter. `pvscan` scans devices for PVs and can trigger auto-activation.\n\n## Syntax\n\n```text\npvs [OPTION]... [PV...]\npvdisplay [OPTION]... [PV...]\npvscan [OPTION]...\n```\n\n## Options\n\n\n### Report (pvs, also lvs/vgs)\n\n| Option | What it does |\n|---|---|\n| `-o, --options COLUMNS` | Columns, like pv_name,vg_name,pv_size,pv_free (+col = add). |\n| `-O, --sort COLUMNS` | Sort by columns (-col = descending). |\n| `-S, --select EXPR` | Filter, like 'pv_free>1g'. |\n| `--units UNIT` | Units: h, b, k, m, g, t, s (sectors), e (extents)… capital = powers of 1000. |\n| `--nosuffix` | No unit letters (for scripts). |\n| `--noheadings` | No header line. |\n| `--separator TEXT` | Column separator. |\n| `--aligned` | Align with a separator. |\n| `--nameprefixes, --unquoted` | Output as NAME=value pairs. |\n| `--rows` | Columns as rows. |\n| `--segments` | One line per segment. |\n| `-a, --all` | Also devices that are not PVs. |\n| `--binary` | 0/1 instead of words. |\n| `--reportformat basic\\|json` | JSON output. |\n| `--configreport log\\|vg\\|lv\\|pv\\|pvseg\\|seg, --logonly` | Report details. |\n| `--foreign, --shared, --readonly, --ignorelockingfailure, --unbuffered` | Special report settings. |\n\n### pvdisplay\n\n| Option | What it does |\n|---|---|\n| `-m, --maps` | Show which extents belong to which LV. |\n| `-C, --columns` | Report like pvs. |\n| `-s, --short` | Only the device and size. |\n\n### pvscan\n\n| Option | What it does |\n|---|---|\n| `--cache` | Update the device cache and auto-activation. |\n| `-a, --activate ay` | Activate VGs that become complete. |\n| `-n, --novolumegroup` | Only PVs without a VG. |\n| `-e, --exported` | Only PVs of exported VGs. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### All PVs\n\n```bash\npvs\n```\n```text\n  PV         VG     Fmt  Attr PSize   PFree\n  /dev/vda2  rhel   lvm2 a--  <19.00g      0\n  /dev/vdb1  vgdata lvm2 a--   <5.00g   1.00g\n```\n\n> PFree = free space on that PV.\n\n### Details of one PV\n\n```bash\npvdisplay /dev/vdb1\n```\n\n> PE Size, Total PE, Free PE, Allocated PE, UUID.\n\n### Which LVs use this PV?\n\n```bash\npvdisplay -m /dev/vdb1\n```\n\n> Needed before you remove a disk.\n\n### Own columns\n\n```bash\npvs -o pv_name,vg_name,pv_size,pv_free,dev_size\n```\n\n> dev_size shows the size of the device itself.\n\n### PVs with free space\n\n```bash\npvs -S 'pv_free>0'\n```\n\n> Where can new LVs go?\n\n### Sizes in MiB for scripts\n\n```bash\npvs --units m --nosuffix --noheadings -o pv_free /dev/vdb1\n```\n\n> Just a number.\n\n### JSON\n\n```bash\npvs --reportformat json\n```\n\n> For automation.\n\n### PVs not in any VG\n\n```bash\npvscan -n\n```\n\n> New disks ready to use.\n\n### Scan after adding disks\n\n```bash\npvscan --cache\n```\n\n> Updates LVM's view of devices.\n\n### Segments of a PV\n\n```bash\npvs --segments -o +lv_name\n```\n\n> Which LV uses which extents.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man pvs](https://linux.die.net/man/8/pvs)\n","page":"commands/pvs/","label":"pvs","where":"Command page · LVM","aka":["pvdisplay","pvscan"]},{"id":"cmd-pwd","n":1499,"slug":"pwd","title":"pwd — Print the full path of the folder you are in (print working directory)","sections":["commands"],"topic":"Commands · Files and folders","tags":["folder","directory","path","current","where"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the full path of the folder you are in (print working directory).\n\n## About\n\n`pwd` shows where you are in the tree, from `/` to the current folder. It is built into Bash; there is also a program `/usr/bin/pwd` with the same options.\n\nThe same value is in the variable `$PWD`, and the previous folder is in `$OLDPWD`.\n\n## Syntax\n\n```text\npwd [-LP]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-L, --logical` | Show the path as you entered it, with symbolic links (the default in Bash). |\n| `-P, --physical` | Show the real path, with all symbolic links resolved. |\n| `--help` | (Program version) Show help. |\n| `--version` | (Program version) Show the version. |\n\n## Examples\n\n### Show where you are\n\n```bash\npwd\n```\n```text\n/home/student\n```\n\n> The first command to run when you are lost.\n\n### Show the real folder behind a link\n\n```bash\ncd /etc/alternatives\ncd -P .\npwd -P\n```\n\n> With links, the logical and the physical path can be different.\n\n### Use the current folder in a command\n\n```bash\ntar -czf /tmp/backup-$(basename \"$PWD\").tar.gz .\n```\n\n> `$PWD` is the same as `pwd`; `basename` takes the last part.\n\n### Save the folder and come back later in a script\n\n```bash\nHERE=$(pwd); cd /tmp && do_work; cd \"$HERE\"\n```\n\n> Keeps the script predictable.\n\n### Check you are in the right place before a delete\n\n```bash\npwd && ls\n```\n\n> A two-second check that prevents deleting the wrong files.\n\n### Show the previous folder\n\n```bash\necho \"$OLDPWD\"\n```\n\n> The folder that `cd -` would take you back to.\n\n### Print an absolute path of a file in this folder\n\n```bash\necho \"$(pwd)/config.yaml\"\n```\n\n> Good when a tool needs a full path.\n\n### See the difference between logical and physical\n\n```bash\nln -s /var/log ~/logs; cd ~/logs; pwd; pwd -P\n```\n```text\n/home/student/logs\n/var/log\n```\n\n> The first is the path you used, the second the real place.\n\n### Show the folder of a process\n\n```bash\npwdx $(pgrep -o sshd)\n```\n\n> `pwdx` shows the current folder of another process (from /proc/PID/cwd).\n\n### Use the program instead of the built-in\n\n```bash\n/usr/bin/pwd -P\n```\n\n> Same result; useful in other shells.\n\n### Put the path in the prompt\n\n```bash\nPS1='[\\u@\\h \\w]\\$ '\n```\n\n> `\\w` in the prompt shows the current folder all the time.\n\n## Tips\n\n- `pwd` = where am I? `pwd -P` = where am I really (no links)?\n\nFull manual: [man pwd](https://linux.die.net/man/1/pwd)\n","page":"commands/pwd/","label":"pwd","where":"Command page · Files and folders","aka":[]},{"id":"cmd-read","n":1500,"slug":"read","title":"read — Read one line of input (from the keyboard, a file or a pipe) into variables","sections":["commands"],"topic":"Commands · Shell and help","tags":["input","scripts","variables","prompt","loop over lines","password"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read one line of input (from the keyboard, a file or a pipe) into variables.\n\n## About\n\n`read NAME` waits for a line and stores it in NAME. With more names, the words are split: the first word to the first name, the rest to the last one. With no name the line goes to `$REPLY`. `while read -r LINE; do …; done < FILE` is the standard way to process a file line by line.\n\n## Syntax\n\n```text\nread [-ers] [-a ARRAY] [-d DELIM] [-i TEXT] [-n N] [-N N] [-p PROMPT] [-t TIMEOUT] [-u FD] [NAME]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p PROMPT` | Show a prompt first. |\n| `-r` | Backslashes are normal characters (use it almost always). |\n| `-s` | Silent: do not show the typed text (passwords). |\n| `-t SECONDS` | Give up after this time. |\n| `-n N` | Return after N characters. |\n| `-N N` | Read exactly N characters. |\n| `-a ARRAY` | Store the words in an array. |\n| `-d DELIM` | Read until this character instead of a newline. |\n| `-e` | Use readline (editing keys) in a terminal. |\n| `-i TEXT` | Default text for -e. |\n| `-u FD` | Read from this file descriptor. |\n\n## Examples\n\n### Ask a question\n\n```bash\nread -p \"Your name: \" NAME; echo \"Hello $NAME\"\n```\n\n> Simple input in scripts.\n\n### Ask for a password\n\n```bash\nread -s -p \"Password: \" PASS; echo\n```\n\n> Nothing is shown while typing.\n\n### Process a file line by line\n\n```bash\nwhile read -r LINE; do echo \"line: $LINE\"; done < /etc/hostname\n```\n\n> -r keeps backslashes.\n\n### Split fields\n\n```bash\necho \"anna sales 3000\" | { read -r NAME GROUP GID; echo \"$NAME is in $GROUP ($GID)\"; }\n```\n\n> Words go to the names in order.\n\n### Split /etc/passwd fields\n\n```bash\nwhile IFS=: read -r USER _ UID _; do [ \"$UID\" -ge 1000 ] && echo \"$USER\"; done < /etc/passwd\n```\n\n> IFS=: splits on colons.\n\n### Yes/no with a timeout\n\n```bash\nread -t 10 -n 1 -p \"Continue? [y/N] \" ANSWER; echo; [ \"$ANSWER\" = y ] || exit 1\n```\n\n> Defaults to no after 10 seconds.\n\n### Into an array\n\n```bash\nread -r -a PARTS <<< \"a b c\"; echo \"${PARTS[1]}\"\nb\n> Index starts at 0.\n```\n\n### Read command output (not a pipe into while)\n\n```bash\nwhile read -r PID CMD; do echo \"$PID $CMD\"; done < <(ps -eo pid=,comm= | head -3)\n```\n\n> Process substitution keeps variables after the loop.\n\n### Default value with editing\n\n```bash\nread -e -i \"/backup\" -p \"Target folder: \" DIR\n```\n\n> The user can edit the default.\n\n### Pause a script\n\n```bash\nread -r -p \"Press Enter to continue\"\n```\n\n> Waits for Enter.\n\n## Watch out\n\n- Without -r, backslashes in the input disappear.\n- `cmd | while read …` runs the loop in a subshell: variables set inside are gone afterwards.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | A line was read. |\n| `1` | End of file (or timeout > 128 with -t). |\n\nFull manual: [man read](https://linux.die.net/man/1/bash)\n","page":"commands/read/","label":"read","where":"Command page · Shell and help","aka":[]},{"id":"cmd-renice","n":1501,"slug":"renice","title":"renice — Change the nice value (CPU priority) of running processes, by PID, process group or user","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["priority","nice value","running process","cpu"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the nice value (CPU priority) of running processes, by PID, process group or user.\n\n## About\n\n`renice -n VALUE -p PID` sets the new nice value of a running process. With `-u USER` it changes all processes of a user. Normal users can only make their own processes nicer; root can do anything.\n\n## Syntax\n\n```text\nrenice [-n] PRIORITY [-g|-p|-u] ID...\nrenice --relative N [-g|-p|-u] ID...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --priority PRIORITY` | The new nice value (-20 … 19). |\n| `--relative N` | Add N to the current value. |\n| `-p, --pid` | The IDs are PIDs (default). |\n| `-g, --pgrp` | The IDs are process group IDs. |\n| `-u, --user` | The IDs are user names or UIDs. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Make a process nicer\n\n```bash\nrenice -n 10 -p 4123\n```\n```text\n4123 (process ID) old priority 0, new priority 10\n```\n\n> It now gives way to other processes.\n\n### Give a process more priority (root)\n\n```bash\nrenice -n -10 -p $(pidof myapp)\n```\n\n> Only root may lower the value.\n\n### All processes of a user\n\n```bash\nrenice -n 15 -u lisa\n```\n\n> A user with heavy jobs slows others less.\n\n### Normal user tries to go back\n\n```bash\nrenice -n 0 -p 4123\n```\n```text\nrenice: failed to set priority for 4123 (process ID): Permission denied\n```\n\n> Users cannot raise their own priority again.\n\n### Add to the current value\n\n```bash\nrenice --relative 5 -p 4123\n```\n\n> From 10 to 15.\n\n### Several processes at once\n\n```bash\nrenice -n 5 -p $(pgrep -d ' ' dd)\n```\n\n> All dd processes.\n\n### Change a whole process group\n\n```bash\nrenice -n 10 -g 4120\n```\n\n> A pipeline started together.\n\n### From top\n\n```bash\ntop\n```\n\n> Press r, type the PID and the new value.\n\n### Check the result\n\n```bash\nps -o pid,ni,cmd -p 4123\n```\n\n> Column NI.\n\n### Slow down a crypto miner while you investigate (security)\n\n```bash\nrenice -n 19 -p 6666; kill -STOP 6666\n```\n\n> Stop it first, then collect evidence before you kill it.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man renice](https://linux.die.net/man/1/renice)\n","page":"commands/renice/","label":"renice","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-resize2fs","n":1502,"slug":"resize2fs","title":"resize2fs — Grow or shrink an ext2/ext3/ext4 file system","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["ext4","grow","shrink","resize","lvm"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Grow or shrink an ext2/ext3/ext4 file system.\n\n## About\n\n`resize2fs DEVICE` grows the file system to fill the device (also when mounted). With a size it can also **shrink** — only unmounted and after `e2fsck -f`. When you shrink, shrink the file system before the LV; when you grow, grow the LV first.\n\n`lvextend -r` and `lvreduce -r` call it for you.\n\n## Syntax\n\n```text\nresize2fs [OPTION]... DEVICE [SIZE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `SIZE` | New size with unit (K, M, G, T, s = 512-byte sectors); without: fill the device. |\n| `-f` | Force (skip safety checks). |\n| `-F` | Flush the device buffers first. |\n| `-M` | Shrink to the minimum size. |\n| `-P` | Print the minimum size (estimate). |\n| `-p` | Show progress. |\n| `-d FLAGS` | Debug flags. |\n| `-S STRIDE` | RAID stride. |\n| `-b, -s` | Turn the 64bit feature on / off. |\n| `-z FILE` | Save an undo file. |\n\n## Examples\n\n### Grow after lvextend\n\n```bash\nlvextend -L +2G /dev/vgdata/lvweb\nresize2fs /dev/vgdata/lvweb\n```\n\n> Online grow of a mounted ext4.\n\n### Shrink to 3 GiB\n\n```bash\numount /docs\ne2fsck -f /dev/vgfiles/lvdocs\nresize2fs /dev/vgfiles/lvdocs 3G\nlvreduce -L 3G /dev/vgfiles/lvdocs\nmount /docs\n```\n\n> The order matters: file system first.\n\n### How small can it get?\n\n```bash\nresize2fs -P /dev/vgfiles/lvdocs\n```\n```text\nEstimated minimum size of the filesystem: 140326\n```\n\n> In blocks (usually 4 KiB).\n\n### Shrink to the minimum\n\n```bash\nresize2fs -M /dev/vgfiles/lvdocs\n```\n\n> Before copying to a smaller disk.\n\n### Show progress\n\n```bash\nresize2fs -p /dev/vgdata/lvweb\n```\n\n> For big file systems.\n\n### Grow a partition-based ext4\n\n```bash\ngrowpart /dev/vdb 1; resize2fs /dev/vdb1\n```\n\n> After the disk was made bigger.\n\n### Easier: let LVM do it\n\n```bash\nlvextend -r -L +1G /dev/vgdata/lvweb\n```\n\n> Calls resize2fs.\n\n### Undo file for safety\n\n```bash\nresize2fs -z /root/resize.undo /dev/vgfiles/lvdocs 3G\n```\n\n> Restore with e2undo if something goes wrong.\n\n### Check after resize\n\n```bash\ndf -h /docs\n```\n\n> The new size.\n\n### Mounted shrink fails\n\n```bash\nresize2fs /dev/vgfiles/lvdocs 3G\n```\n```text\nresize2fs: On-line shrinking not supported\n```\n\n> Unmount first.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man resize2fs](https://linux.die.net/man/8/resize2fs)\n","page":"commands/resize2fs/","label":"resize2fs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-restorecon","n":1503,"slug":"restorecon","title":"restorecon — Set the SELinux labels of files back to what the policy (and your semanage fcontext rules) say","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","labels","relabel","fix context"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Set the SELinux labels of files back to what the policy (and your semanage fcontext rules) say.\n\n## About\n\n`restorecon -Rv PATH` compares each file with the policy's file context rules and fixes wrong labels. Use it after `semanage fcontext -a`, after moving files with `mv`, or when a service gets \"Permission denied\" because of a wrong label. `-n` only shows what would change.\n\n## Syntax\n\n```text\nrestorecon [OPTION]... PATH...\nrestorecon -f FILE [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-R, -r` | Recursive: whole folder trees. |\n| `-v` | Show every change (twice = more). |\n| `-n` | Do not change anything; only show (with -v). |\n| `-F` | Force: also reset user, role and level, not only the type. |\n| `-i` | Ignore files that do not exist. |\n| `-p` | Show progress. |\n| `-e DIR` | Exclude a folder. |\n| `-x` | Do not cross file system boundaries. |\n| `-f FILE` | Read the paths from a file (- = stdin). |\n| `-0` | Paths in -f are separated by NUL. |\n| `-m` | Do not read /proc/mounts for non-seclabel mounts. |\n| `-I` | Ignore stored digests (check every file). |\n| `-D` | Store SHA1 digests to skip unchanged trees next time. |\n| `-W` | Warn about rules that match no file. |\n| `-o FILE` | Save the list of wrongly labeled files. |\n| `-T N` | Use N threads. |\n| `-h, -?` | Show help. |\n\n## Examples\n\n### Fix a folder\n\n```bash\nrestorecon -Rv /var/www/html\n```\n```text\nRelabeled /var/www/html/index.html from unconfined_u:object_r:user_home_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0\n```\n\n> Typical after mv from a home folder.\n\n### Apply a new semanage rule\n\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/web(/.*)?\"\nrestorecon -Rv /srv/web\n```\n\n> The rule alone changes nothing on disk.\n\n### Only check\n\n```bash\nrestorecon -Rvn /etc\n```\n\n> Lists files with wrong labels without fixing them.\n\n### Fix one file\n\n```bash\nrestorecon -v /etc/shadow\n```\n\n> For example after editing it with a tool that changed the label.\n\n### Fix SSH keys of a user\n\n```bash\nrestorecon -Rv /home/lisa/.ssh\n```\n\n> Wrong labels on authorized_keys stop key logins.\n\n### Full system relabel at next boot\n\n```bash\ntouch /.autorelabel; reboot\n```\n\n> After SELinux was disabled, or many wrong labels.\n\n### Find wrongly labeled files for a report (security)\n\n```bash\nrestorecon -Rn -o /root/badlabels.txt /var /srv\n```\n\n> Audit without changing anything.\n\n### Force full contexts\n\n```bash\nrestorecon -RF /srv/web\n```\n\n> Also resets the SELinux user part.\n\n### Many files fast\n\n```bash\nrestorecon -R -T 0 -p /data\n```\n\n> All CPU threads, with progress.\n\n### Check the result\n\n```bash\nls -Z /srv/web\n```\n\n> The type column.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `255` | Error. |\n\nFull manual: [man restorecon](https://linux.die.net/man/8/restorecon)\n","page":"commands/restorecon/","label":"restorecon","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-rm","n":1504,"slug":"rm","title":"rm — Delete files, and with -r folders with all their content","sections":["commands"],"topic":"Commands · Files and folders","tags":["delete","remove","file","folder","recursive","force"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Delete files, and with -r folders with all their content.\n\n## About\n\n`rm` removes file names. When the last name of a file is gone (and no program has it open), the space is free. There is **no trash can**: a removed file is gone.\n\n`rm` does not remove folders unless you add `-r`. `-f` removes without questions and without errors for missing files.\n\nOn RHEL, root has the alias `rm='rm -i'`, so it asks for each file. `rm` also refuses to remove `/` itself (`--preserve-root`).\n\n## Syntax\n\n```text\nrm [OPTION]... FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-r, -R, --recursive` | Remove folders and everything in them. |\n| `-f, --force` | Never ask, ignore files that do not exist. |\n| `-i` | Ask before every removal. |\n| `-I` | Ask once before removing more than three files or a whole folder (less noisy than -i). |\n| `--interactive[=<when>]` | When to ask: never, once (like -I) or always (like -i). |\n| `-d, --dir` | Remove empty folders too (like rmdir). |\n| `-v, --verbose` | Print each file removed. |\n| `--one-file-system` | With -r: skip folders on other file systems. |\n| `--preserve-root[=all]` | Do not remove `/` (the default). With \"all\": also refuse mount points on other devices. |\n| `--no-preserve-root` | Allow removing `/`. Never use this. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Remove a file\n\n```bash\nrm notes.txt\n```\n\n> The file is gone; no undo.\n\n### Remove a folder with its content\n\n```bash\nrm -r /tmp/build\n```\n\n> Asks for each file when the rm -i alias is active (root on RHEL).\n\n### Remove without any question\n\n```bash\nrm -rf /tmp/build\n```\n\n> Powerful and dangerous. Check the path twice.\n\n### Ask once for a big removal\n\n```bash\nrm -rI ~/old-projects\n```\n\n> One question instead of one per file.\n\n### Remove files with a name that starts with a dash\n\n```bash\nrm -- -weird-name.txt\n```\n\n> `--` means: the options end here.\n\n### Remove old logs with find (safer than wildcards)\n\n```bash\nfind /var/log/myapp -name \"*.log\" -mtime +30 -print -delete\n```\n\n> Removes logs older than 30 days and prints each name.\n\n### Remove a file with spaces in its name\n\n```bash\nrm \"My Report (final).docx\"\n```\n\n> Quotes keep the name together.\n\n### Test a wildcard before removing\n\n```bash\necho /tmp/cache/*.tmp\nrm /tmp/cache/*.tmp\n```\n\n> `echo` shows exactly what the shell will pass to rm.\n\n### Free space that a deleted log still uses\n\n```bash\nlsof +L1 | grep deleted\nsystemctl restart rsyslog\n```\n\n> A program that keeps a deleted file open still uses the space. Restart it to free the space.\n\n### Stay on one file system\n\n```bash\nrm -rf --one-file-system /mnt/oldroot/tmp\n```\n\n> Protects mounted file systems below the folder from being wiped.\n\n### Remove empty folders and files in one go\n\n```bash\nrm -dv empty-dir file.txt\n```\n\n> `-d` removes the empty folder too.\n\n## Watch out\n\n- No trash, no undo. Make a copy first or use `mv` to a temp folder when you are unsure.\n- Never put a variable alone in rm: `rm -rf \"$DIR/\"*` with an empty DIR deletes from `/`. Use `${DIR:?}`.\n- A space in the wrong place: `rm -rf / tmp/x` deletes everything. Read the line before Enter.\n\n## Tips\n\n- File: `rm`. Folder: `rm -r`. No questions: `-f`. One question: `-I`.\n- In scripts: `rm -rf \"${DIR:?}/\"*` stops when DIR is empty.\n\nFull manual: [man rm](https://linux.die.net/man/1/rm)\n","page":"commands/rm/","label":"rm","where":"Command page · Files and folders","aka":[]},{"id":"cmd-rmdir","n":1505,"slug":"rmdir","title":"rmdir — Remove empty folders (and refuse to remove folders that still have files)","sections":["commands"],"topic":"Commands · Files and folders","tags":["folder","directory","delete","empty","remove"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Remove empty folders (and refuse to remove folders that still have files).\n\n## About\n\n`rmdir` deletes a folder only when it is empty. That makes it a safe tool: it cannot delete files by accident. To delete a folder with content, use `rm -r`.\n\nWith `-p` it also removes the parent folders when they become empty.\n\n## Syntax\n\n```text\nrmdir [OPTION]... DIRECTORY...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --parents` | Remove the folder and then each parent in the path, as long as they are empty. |\n| `-v, --verbose` | Print a line for each folder removed. |\n| `--ignore-fail-on-non-empty` | Do not show an error for folders that are not empty. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Remove an empty folder\n\n```bash\nrmdir old-reports\n```\n\n> Fails with \"Directory not empty\" if there is still something inside.\n\n### Remove a path of empty folders\n\n```bash\nrmdir -p a/b/c\n```\n\n> Removes c, then b, then a (if they are all empty).\n\n### Remove all empty folders in a tree\n\n```bash\nfind /srv/uploads -type d -empty -delete\n```\n\n> rmdir works per folder; `find -empty -delete` cleans a whole tree.\n\n### Try to remove folders, ignore the ones that are not empty\n\n```bash\nrmdir --ignore-fail-on-non-empty /tmp/build/*/\n```\n\n> Only the empty ones are removed, no error messages.\n\n### Show what is removed\n\n```bash\nrmdir -v /tmp/test\n```\n```text\nrmdir: removing directory, '/tmp/test'\n```\n\n> Useful in logs.\n\n### Find out why a folder is not empty\n\n```bash\nrmdir /tmp/x || ls -A /tmp/x\n```\n\n> Hidden files (starting with a dot) often keep a folder \"not empty\".\n\n### Remove an empty mount point after unmount\n\n```bash\numount /mnt/usb && rmdir /mnt/usb\n```\n\n> rmdir also protects you: it fails if the disk is still mounted with files.\n\n### Clean empty release folders in a deploy script\n\n```bash\nrmdir --ignore-fail-on-non-empty /srv/app/releases/* 2>/dev/null\n```\n\n> Old empty folders go away, used ones stay.\n\n### Remove a folder with a strange name\n\n```bash\nrmdir -- -tmp-\n```\n\n> `--` ends the options, so a name that starts with a dash is not read as an option.\n\n### Remove several empty folders at once\n\n```bash\nrmdir logs tmp cache\n```\n\n> Each one must be empty.\n\n### Check if a folder is empty before removing it in a script\n\n```bash\n[ -z \"$(ls -A /var/spool/app)\" ] && rmdir /var/spool/app\n```\n\n> `ls -A` prints nothing when the folder is empty.\n\n## Watch out\n\n- rmdir never deletes files. When it says \"not empty\", look with `ls -A` before you switch to `rm -r`.\n\n## Tips\n\n- rmdir = safe (empty only). `rm -r` = everything inside too.\n\nFull manual: [man rmdir](https://linux.die.net/man/1/rmdir)\n","page":"commands/rmdir/","label":"rmdir","where":"Command page · Files and folders","aka":[]},{"id":"cmd-rpm","n":1506,"slug":"rpm","title":"rpm — Query, verify, install and remove RPM packages directly — mostly used to ask what is installed and which files a package has","sections":["commands"],"topic":"Commands · Software packages","tags":["package database","query","verify","signature","scripts","install rpm file","integrity"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Query, verify, install and remove RPM packages directly — mostly used to ask what is installed and which files a package has.\n\n## About\n\n`rpm` works with single package files and the RPM database (`/var/lib/rpm`). It does **not** solve dependencies or use repositories — that is the job of `dnf`. Use `dnf` to install; use `rpm` to ask questions.\n\nQuery mode is `-q`: add select options (`-a` all, `-f` file, `-p` package file) and query options (`-i` info, `-l` files, `-c` configs, `-d` docs, `--scripts`). `-V` verifies installed files against the database — a simple integrity check.\n\n## Syntax\n\n```text\nrpm -q [SELECT-OPTION] [QUERY-OPTION] [PACKAGE...]\nrpm -V [SELECT-OPTION] [VERIFY-OPTION] [PACKAGE...]\nrpm -i|-U|-F [INSTALL-OPTION] PACKAGE_FILE...\nrpm -e [OPTION] PACKAGE...\nrpm --import KEYFILE | -K PACKAGE_FILE\n```\n\n## Options\n\n\n### Modes\n\n| Option | What it does |\n|---|---|\n| `-q, --query` | Query packages. |\n| `-V, --verify` | Compare installed files with the database. |\n| `-i, --install` | Install a package file (no dependency solving). |\n| `-U, --upgrade` | Install or upgrade a package file. |\n| `-F, --freshen` | Upgrade only if an older version is installed. |\n| `--reinstall` | Install the same version again. |\n| `-e, --erase` | Remove a package. |\n| `--import KEYFILE` | Import a GPG public key. |\n| `-K, --checksig` | Check the digests and signature of a package file. |\n| `--querytags` | List the tags for --queryformat. |\n| `--showrc` | Show the rpm settings and macros. |\n| `--restore` | Restore owner, group and mode of files from the database. |\n| `--rebuilddb` | Rebuild the database. |\n\n### Select packages\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | All installed packages. |\n| `-f, --file FILE` | The package that owns this file. |\n| `-p, --package PACKAGE_FILE` | Query a package file, not the database. |\n| `-g, --group GROUP` | Packages in this RPM group. |\n| `--whatprovides CAPABILITY` | Packages that provide it. |\n| `--whatrequires CAPABILITY` | Packages that need it. |\n| `--whatrecommends, --whatsuggests, --whatsupplements, --whatenhances, --whatobsoletes, --whatconflicts CAPABILITY` | Packages with this weak dependency or relation. |\n| `--triggeredby PACKAGE` | Packages with triggers on it. |\n| `--path PATH` | Packages that own the path (also not installed files). |\n| `--hdrid SHA1, --pkgid MD5, --tid TID, --querybynumber NUM` | Select by internal IDs. |\n\n### Query information\n\n| Option | What it does |\n|---|---|\n| `-i, --info` | Package information (name, version, vendor, summary …). |\n| `-l, --list` | All files. |\n| `-c, --configfiles` | Only config files. |\n| `-d, --docfiles` | Only documentation files. |\n| `-s, --state` | State of each file (normal, replaced …). |\n| `-R, --requires` | What the package needs. |\n| `--provides` | What the package provides. |\n| `--conflicts, --obsoletes, --recommends, --suggests, --supplements, --enhances` | Other dependency data. |\n| `--scripts` | The install and remove scripts. |\n| `--triggers, --triggerscripts, --filetriggers` | Trigger scripts. |\n| `--changelog` | The change log (fixed CVEs are listed here). |\n| `--changes` | The change log with full dates. |\n| `--last` | Sort by install time, newest first. |\n| `--qf, --queryformat FORMAT` | Own output format, like `%{NAME} %{VERSION}\\n`. |\n| `--dump` | Detailed file data. |\n| `--filesbypkg` | Files with the package name. |\n| `--filecaps` | File capabilities. |\n| `--noghost, --noconfig, --noartifact` | Leave out these file types. |\n| `--dupes` | Packages installed more than once. |\n| `--xml` | XML output. |\n\n### Verify options\n\n| Option | What it does |\n|---|---|\n| `--nofiles, --nodeps, --noscripts, --nodigest, --nosignature` | Skip these checks. |\n| `--nosize, --nouser, --nogroup, --nomtime, --nomode, --nordev, --nocaps, --nolinkto, --nofiledigest` | Skip these file checks. |\n\n### Install and remove options\n\n| Option | What it does |\n|---|---|\n| `-h, --hash` | Show # signs as progress. |\n| `-v, --verbose` | More output. |\n| `--test` | Only test; change nothing. |\n| `--nodeps` | Do not check dependencies (dangerous). |\n| `--force` | Same as --replacepkgs --replacefiles --oldpackage. |\n| `--replacepkgs, --replacefiles, --oldpackage` | Allow reinstall, overwrite files, downgrade. |\n| `--noscripts, --notriggers` | Do not run scripts or triggers. |\n| `--excludedocs, --includedocs` | Without or with documentation. |\n| `--justdb` | Change only the database, not the files. |\n| `--nosignature, --nodigest` | Do not check signature or digest. |\n| `--allmatches` | Remove all packages with this name. |\n| `--prefix NEWPATH, --relocate OLD=NEW` | Install a relocatable package in another place. |\n| `--ignorearch, --ignoreos, --ignoresize, --noorder, --noplugins, --nocaps, --noverify, --badreloc, --excludepath, --percent, --allfiles, --nodb` | Special install settings. |\n\n### General\n\n| Option | What it does |\n|---|---|\n| `--root DIR` | Use another root folder. |\n| `--dbpath DIR` | Use another database folder. |\n| `--version` | Show the version. |\n| `-?, --help` | Show help. |\n\n## Examples\n\n### Is a package installed?\n\n```bash\nrpm -q openssh-server\n```\n```text\nopenssh-server-9.9p1-7.el10.x86_64\n```\n\n> \"package … is not installed\" if not.\n\n### Which package owns a file?\n\n```bash\nrpm -qf /etc/ssh/sshd_config\n```\n```text\nopenssh-server-9.9p1-7.el10.x86_64\n```\n\n> Find the package of a config before you change it.\n\n### Config and doc files of a package\n\n```bash\nrpm -qc httpd\nrpm -qd httpd\n```\n\n> Where to look for settings and documentation.\n\n### Search the list of installed packages\n\n```bash\nrpm -qa | grep -i kernel\n```\n\n> All installed kernels.\n\n### Newest installed packages first\n\n```bash\nrpm -qa --last | head\n```\n\n> What changed recently? Useful after a problem appears.\n\n### Check a downloaded package before installing\n\n```bash\nrpm -qpi tool-1.2-1.el10.x86_64.rpm\nrpm -qp --scripts tool-1.2-1.el10.x86_64.rpm\n```\n\n> Read the scripts: they run as root.\n\n### Check the signature of a package file\n\n```bash\nrpm -K tool-1.2-1.el10.x86_64.rpm\n```\n```text\ntool-1.2-1.el10.x86_64.rpm: digests signatures OK\n```\n\n> \"NOT OK\" or \"key ID … not found\" = do not install.\n\n### Find changed system files (integrity)\n\n```bash\nrpm -Va 2>/dev/null | grep -v ' c '\n```\n```text\nS.5....T.    /usr/bin/someprog\n```\n\n> S = size, 5 = checksum, T = time changed. A changed binary can be malware; c = config file (often normal).\n\n### Is a CVE fixed?\n\n```bash\nrpm -q --changelog openssl | grep CVE-2024-\n```\n\n> Red Hat backports fixes; the version number alone does not tell.\n\n### Show imported GPG keys\n\n```bash\nrpm -q gpg-pubkey --qf '%{NAME}-%{VERSION}-%{RELEASE}\\t%{SUMMARY}\\n'\n```\n\n> Every key you trust for packages.\n\n### Own report format\n\n```bash\nrpm -qa --qf '%{NAME}\\t%{VERSION}-%{RELEASE}\\t%{INSTALLTIME:date}\\n' | sort | head\n```\n\n> Inventory for audits.\n\n### Restore permissions of package files\n\n```bash\nrpm --restore openssh-server\n```\n\n> Fixes owners and modes changed by mistake.\n\n### Extract files from a package without installing\n\n```bash\nrpm2cpio tool-1.2-1.el10.x86_64.rpm | cpio -idmv\n```\n\n> Look at files of a package safely.\n\n## Watch out\n\n- `rpm -i` does not install dependencies. Use `dnf install ./file.rpm`.\n- `--nodeps` and `--force` can break the system.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/lib/rpm/` | The RPM database. |\n| `/etc/pki/rpm-gpg/` | Red Hat GPG keys. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (for -q: all packages found). |\n| `1` | Error, or packages not installed / verify found problems. |\n\nFull manual: [man rpm](https://linux.die.net/man/8/rpm)\n","page":"commands/rpm/","label":"rpm","where":"Command page · Software packages","aka":[]},{"id":"cmd-rsync","n":1507,"slug":"rsync","title":"rsync — Copy and synchronize files and folders (locally or over SSH), sending only what changed","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["sync","backup","copy","remote","mirror","delete","incremental","ssh"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Copy and synchronize files and folders (locally or over SSH), sending only what changed.\n\n## About\n\n`rsync` compares the source and the target and transfers only the differences. The first copy is a full copy; the next runs take seconds. It can keep permissions, owners, times, links, ACLs and SELinux contexts, remove files that are gone in the source, limit the bandwidth and resume interrupted transfers.\n\nRemote paths use `user@host:path`, like scp; rsync uses SSH by default.\n\n**The slash rule**: `rsync -a src/ dst/` copies the **content** of src into dst. `rsync -a src dst/` copies the folder itself, so you get `dst/src/`.\n\n## Syntax\n\n```text\nrsync [OPTION...] SRC... [DEST]\nrsync [OPTION...] SRC... [USER@]HOST:DEST\nrsync [OPTION...] [USER@]HOST:SRC... [DEST]\n```\n\n## Options\n\n\n### Most used\n\n| Option | What it does |\n|---|---|\n| `-a, --archive` | Archive mode: recursive and keep links, permissions, times, group, owner, devices (-rlptgoD). |\n| `-v, --verbose` | Show the files that are transferred. |\n| `-z, --compress` | Compress data during the transfer. |\n| `-n, --dry-run` | Show what would happen, change nothing. |\n| `-P` | Same as --partial --progress: show progress and keep half-done files. |\n| `--delete` | Delete files in the target that do not exist in the source. |\n| `-e, --rsh=<command>` | The remote shell, like `-e \"ssh -p 2222 -i key\"`. |\n| `-h, --human-readable` | Readable numbers (K, M, G). |\n| `--progress` | Show the progress of each file. |\n| `--stats` | Print statistics at the end. |\n| `-i, --itemize-changes` | Show a short code for what changes for each file. |\n| `-q, --quiet` | Show only errors. |\n\n### What to copy\n\n| Option | What it does |\n|---|---|\n| `-r, --recursive` | Go into folders. |\n| `-d, --dirs` | Copy folders without their content. |\n| `-R, --relative` | Keep the full source path in the target. |\n| `--files-from=<file>` | Copy only the names in this file. |\n| `--exclude=<pattern>` | Leave out files that match. |\n| `--exclude-from=<file>` | Read exclude patterns from a file. |\n| `--include=<pattern>` | Do not exclude files that match. |\n| `--include-from=<file>` | Read include patterns from a file. |\n| `-f, --filter=<rule>` | A filter rule (advanced include/exclude). |\n| `-F` | Read filter rules from .rsync-filter files. |\n| `-C, --cvs-exclude` | Leave out files that version control tools ignore. |\n| `-x, --one-file-system` | Do not cross into other file systems. |\n| `--existing` | Update only files that already exist in the target. |\n| `--ignore-existing` | Skip files that already exist in the target. |\n| `--max-size=<size>, --min-size=<size>` | Skip files bigger / smaller than SIZE. |\n| `-m, --prune-empty-dirs` | Do not create empty folders in the target. |\n| `-0, --from0` | Names in --files-from are NUL separated. |\n\n### What to keep\n\n| Option | What it does |\n|---|---|\n| `-p, --perms` | Keep permissions. |\n| `-o, --owner` | Keep the owner (root only). |\n| `-g, --group` | Keep the group. |\n| `-t, --times` | Keep modification times. |\n| `-U, --atimes` | Keep access times. |\n| `-N, --crtimes` | Keep creation times (where supported). |\n| `-O, --omit-dir-times` | Do not keep the times of folders. |\n| `-J, --omit-link-times` | Do not keep the times of links. |\n| `-l, --links` | Copy symbolic links as links. |\n| `-L, --copy-links` | Copy the files that links point to. |\n| `--copy-unsafe-links` | Copy the target of links that point outside the tree. |\n| `--safe-links` | Ignore links that point outside the tree. |\n| `--munge-links` | Make links safe on the receiving side. |\n| `-k, --copy-dirlinks` | Treat links to folders as folders. |\n| `-K, --keep-dirlinks` | Keep links to folders in the target. |\n| `-H, --hard-links` | Keep hard links. |\n| `-A, --acls` | Keep ACLs (implies -p). |\n| `-X, --xattrs` | Keep extended attributes (SELinux contexts are kept with -X). |\n| `-E, --executability` | Keep the execute bit only. |\n| `--chmod=<mode>` | Change the permissions of transferred files. |\n| `--chown=<user:group>` | Set owner and group of transferred files. |\n| `--usermap=<map>, --groupmap=<map>` | Map user and group names. |\n| `--numeric-ids` | Keep user and group numbers, do not map by name. |\n| `-D` | Same as --devices --specials. |\n| `--devices, --specials` | Copy device files / special files (pipes, sockets). |\n| `--copy-devices, --write-devices` | Copy the content of devices / write into devices. |\n| `-S, --sparse` | Keep sparse files sparse. |\n| `--preallocate` | Reserve the space before writing. |\n| `--super, --fake-super` | Act as root on the receiver / store privileged attributes as xattrs. |\n\n### Delete and update\n\n| Option | What it does |\n|---|---|\n| `--delete-before, --delete-during (--del), --delete-delay, --delete-after` | When to delete extra files in the target. |\n| `--delete-excluded` | Also delete excluded files in the target. |\n| `--max-delete=<n>` | Do not delete more than N files (safety net). |\n| `--ignore-errors` | Delete even when there are I/O errors. |\n| `--force` | Delete non-empty folders when they are replaced by a file. |\n| `--remove-source-files` | Delete the source files after a successful transfer (move). |\n| `-u, --update` | Skip files that are newer in the target. |\n| `-b, --backup` | Make backups of files that are replaced or deleted. |\n| `--backup-dir=<dir>` | Put the backups in this folder. |\n| `--suffix=<suffix>` | The backup ending (default ~). |\n| `--inplace` | Write updates directly into the target file. |\n| `--append, --append-verify` | Add data to the end of shorter files (and check them). |\n\n### Compare\n\n| Option | What it does |\n|---|---|\n| `-c, --checksum` | Compare by checksum, not by size and time (slower, exact). |\n| `--size-only` | Compare only the size. |\n| `-I, --ignore-times` | Do not skip files with the same size and time. |\n| `-@, --modify-window=<n>` | Times within N seconds count as the same. |\n| `-W, --whole-file` | Send whole files, no delta (faster on fast networks). |\n| `--checksum-choice=<algo>` | The checksum algorithm. |\n| `--compare-dest=<dir>, --copy-dest=<dir>` | Compare with (or copy from) another folder too. |\n| `--link-dest=<dir>` | Make hard links to unchanged files in this folder (snapshot backups). |\n| `-y, --fuzzy` | Use a similar file in the target as a base. |\n\n### Transfer\n\n| Option | What it does |\n|---|---|\n| `--partial` | Keep half-transferred files to resume later. |\n| `--partial-dir=<dir>` | Keep half-done files in this folder. |\n| `--delay-updates` | Put all updated files in place at the end. |\n| `-T, --temp-dir=<dir>` | Folder for temporary files. |\n| `--bwlimit=<rate>` | Limit the bandwidth, like 10M. |\n| `--compress-choice=<algo>, --compress-level=<n>` | Choose the compression and its level. |\n| `--skip-compress=<list>` | Do not compress these file endings. |\n| `--timeout=<sec>, --contimeout=<sec>` | I/O and connect timeouts. |\n| `--stop-after=<mins>, --stop-at=<time>` | Stop after some time / at a time. |\n| `--rsync-path=<program>` | The rsync program on the remote side (like \"sudo rsync\"). |\n| `-M, --remote-option=<opt>` | Send an option only to the remote side. |\n| `-s, --secluded-args` | Send file names safely to the remote side. |\n| `--fsync` | Sync each file to disk. |\n| `--log-file=<file>, --log-file-format=<fmt>` | Write a log file. |\n| `--out-format=<fmt>` | Format of the transfer lines. |\n| `--info=<flags>, --debug=<flags>` | Fine control of messages. |\n| `--list-only` | List the files instead of copying. |\n| `--port=<port>, --address=<addr>, --password-file=<file>` | For rsync daemon (rsync://) connections. |\n| `-4, --ipv4 / -6, --ipv6` | Use only IPv4 / IPv6. |\n| `--iconv=<spec>` | Convert file name character sets. |\n| `--write-batch=<file>, --read-batch=<file>` | Save a transfer to a file / apply it elsewhere. |\n| `-V, --version` | Show the version. |\n| `--help` | Show help. |\n\n## Examples\n\n### Sync a folder to a server\n\n```bash\nrsync -av ~/site/ student@server2:/srv/site/\n```\n\n> The slash after `site/` copies the content. Run it again: only changes are sent.\n\n### Dry run first, then the real run\n\n```bash\nrsync -avn --delete /var/www/html/ /backup/html/\nrsync -av --delete /var/www/html/ /backup/html/\n```\n\n> Read the list of \"deleting\" lines before you run without -n.\n\n### Mirror with deletion\n\n```bash\nrsync -a --delete /etc/ /backup/etc-mirror/\n```\n\n> The target becomes an exact copy; files removed in /etc are removed in the backup.\n\n### Use another SSH port and key\n\n```bash\nrsync -avz -e \"ssh -p 2222 -i ~/.ssh/backup_key\" /data/ backup@nas:/backups/data/\n```\n\n> `-e` passes options to ssh.\n\n### Keep SELinux contexts and ACLs\n\n```bash\nrsync -aAX /var/www/ server2:/var/www/\n```\n\n> `-A` ACLs, `-X` extended attributes (SELinux labels). Needed for a web server migration.\n\n### Show progress for big files\n\n```bash\nrsync -avP rhel-10.iso server2:/srv/iso/\n```\n\n> If the link drops, run the same command again: it resumes.\n\n### Leave out files\n\n```bash\nrsync -av --exclude='*.log' --exclude='.git/' app/ server2:/srv/app/\n```\n\n> Do not ship logs and git history to production.\n\n### Daily snapshot backups that use little space\n\n```bash\nrsync -a --delete --link-dest=/backup/2026-10-24 /home/ /backup/2026-10-25/\n```\n\n> Unchanged files are hard links to yesterday's copy: each day looks full but uses only the changes.\n\n### Limit the bandwidth\n\n```bash\nrsync -av --bwlimit=5M /srv/data/ dr-site:/srv/data/\n```\n\n> About 5 MB/s, so the office network stays usable.\n\n### Pull files from a server\n\n```bash\nrsync -av server2:/var/log/httpd/ ./server2-httpd-logs/\n```\n\n> The remote side can be the source too.\n\n### Use sudo on the remote side to read root files\n\n```bash\nrsync -av --rsync-path=\"sudo rsync\" admin@server2:/etc/ ./server2-etc/\n```\n\n> The remote user needs a sudo rule for rsync.\n\n### Move files (delete the source after copying)\n\n```bash\nrsync -av --remove-source-files /var/spool/export/ archive01:/archive/export/\n```\n\n> Good for files that must leave a server once they are safe elsewhere.\n\n### Compare two folders by checksum\n\n```bash\nrsync -avnc /srv/release/ server2:/srv/release/\n```\n\n> Lists files whose content is different, even if size and time are equal.\n\n### Safety net for deletions\n\n```bash\nrsync -a --delete --max-delete=50 /data/ /backup/data/\n```\n\n> If more than 50 files would be deleted (for example the source is empty by mistake), rsync stops.\n\n## Watch out\n\n- The trailing slash on the source changes the result. Check with `-n` first.\n- `--delete` with the wrong source (an empty or unmounted folder) can wipe the target. Use `-n` and `--max-delete`.\n- Normal users cannot keep owners on the target; run as root (or with `--rsync-path=\"sudo rsync\"`) for full system copies.\n\n## Tips\n\n- `rsync -av src/ dst/` = sync content. `-n` = test, `--delete` = mirror, `-z` = compress, `-P` = progress/resume, `-AX` = ACLs and SELinux.\n\nFull manual: [man rsync](https://linux.die.net/man/1/rsync)\n","page":"commands/rsync/","label":"rsync","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-scp","n":1508,"slug":"scp","title":"scp — Copy files between systems over SSH (secure copy)","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["copy","remote","transfer","secure","ssh"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Copy files between systems over SSH (secure copy).\n\n## About\n\n`scp` works like `cp`, but one side (or both) can be on another system: `user@host:path`. All data is encrypted by SSH, and it uses the same login (password or key) as ssh.\n\nNew OpenSSH versions (also on RHEL 9 and 10) use the SFTP protocol under the hood by default; the command stays the same. For big or repeated copies, `rsync` is better: it copies only what changed.\n\n## Syntax\n\n```text\nscp [OPTIONS] SOURCE... TARGET\nscp [OPTIONS] [USER@]HOST:PATH LOCAL_PATH\nscp [OPTIONS] LOCAL_PATH [USER@]HOST:PATH\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-r` | Copy folders with all their content. |\n| `-P <port>` | The SSH port of the remote host (capital P!). |\n| `-p` | Keep modification times, access times and permissions. |\n| `-i <file>` | Use this private key. |\n| `-J <destination>` | Copy through a jump host. |\n| `-C` | Compress the data (helps on slow links). |\n| `-l <limit>` | Limit the bandwidth in Kbit/s. |\n| `-q` | Quiet: no progress meter. |\n| `-v` | Verbose debug output. |\n| `-3` | Copy between two remote hosts through your computer. |\n| `-R` | Copy between two remote hosts directly (the source host connects to the target). |\n| `-B` | Batch mode: never ask for a password (fail instead). |\n| `-o <option>` | Pass an ssh option, like `-o StrictHostKeyChecking=accept-new`. |\n| `-F <file>` | Use another ssh client config file. |\n| `-c <cipher>` | Choose the cipher. |\n| `-S <program>` | Use this program instead of ssh for the connection. |\n| `-4 / -6` | Use only IPv4 / only IPv6. |\n| `-A` | Forward the ssh-agent. |\n| `-O` | Use the old SCP protocol instead of SFTP (for old servers). |\n| `-D <path>` | The sftp-server program to use (with SFTP mode, for testing). |\n| `-X <sftp-option>` | Options for the SFTP protocol (like buffer size). |\n| `-T` | Turn off the strict file name checks of the old protocol. |\n\n## Examples\n\n### Copy a file to a server\n\n```bash\nscp report.pdf student@server2:/tmp/\n```\n\n> The file keeps its name in /tmp on server2.\n\n### Copy a file from a server\n\n```bash\nscp student@server2:/var/log/dnf.log ./server2-dnf.log\n```\n\n> The remote side comes first now.\n\n### Copy a folder\n\n```bash\nscp -r ~/website student@server2:/var/www/\n```\n\n> `-r` for folders.\n\n### Use another SSH port\n\n```bash\nscp -P 2222 backup.tar.gz admin@backup01:/srv/backups/\n```\n\n> Capital P in scp (small p in ssh).\n\n### Keep times and permissions\n\n```bash\nscp -p config.yaml server2:/etc/myapp/\n```\n\n> The copy has the same time stamps.\n\n### Copy through a bastion host\n\n```bash\nscp -J admin@bastion.example.com app.jar admin@10.10.0.15:/opt/app/\n```\n\n> One command, even when the target is in a private network.\n\n### Copy between two remote servers\n\n```bash\nscp -3 server1:/srv/data.sql server2:/srv/\n```\n\n> Data goes through your computer; the servers do not need keys for each other.\n\n### Limit the bandwidth\n\n```bash\nscp -l 8000 big.iso server2:/tmp/\n```\n\n> About 1 MB/s, so the copy does not fill the office link.\n\n### Use a deploy key in a pipeline\n\n```bash\nscp -i ~/.ssh/ci_deploy -o StrictHostKeyChecking=accept-new dist.tar.gz deploy@web01:/srv/releases/\n```\n\n> Typical step in a CI job.\n\n### Copy many files with a wildcard on the remote side\n\n```bash\nscp 'server2:/var/log/httpd/*.log' ./logs/\n```\n\n> Quotes stop your local shell from expanding the `*`.\n\n### Copy a file with spaces in the name\n\n```bash\nscp \"server2:/srv/share/Q3 report.xlsx\" .\n```\n\n> Quote the remote path.\n\n### Batch mode in a script\n\n```bash\nscp -B -q backup.tgz backup@nas:/backups/ || echo \"copy failed\"\n```\n\n> Never hangs on a password prompt.\n\n## Watch out\n\n- `scp file host:` (with nothing after the colon) copies to the home folder; `scp file host` (no colon) makes a local copy named \"host\".\n- scp overwrites existing target files without asking.\n- For large trees or repeated copies use rsync (it can resume and skips unchanged files).\n\n## Tips\n\n- Remote = `user@host:path`. `-r` folders, `-P` port, `-p` keep times.\n\nFull manual: [man scp](https://linux.die.net/man/1/scp)\n","page":"commands/scp/","label":"scp","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-sealert","n":1509,"slug":"sealert","title":"sealert — Explain SELinux denials in plain text, with the likely cause and the commands that fix it","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","denials","explanation","troubleshoot"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Explain SELinux denials in plain text, with the likely cause and the commands that fix it.\n\n## About\n\nWith `setroubleshoot-server` installed, every AVC denial is analyzed. `sealert -a /var/log/audit/audit.log` prints a report per problem: what was blocked, why, and suggestions (\"you need to change the label…\", \"turn on boolean…\", \"allow the port…\"). Short messages also go to the journal (`journalctl -t setroubleshoot`) with an ID for `sealert -l ID`.\n\n## Syntax\n\n```text\nsealert -a FILE\nsealert -l ID | -l \"*\"\nsealert -b\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --analyze FILE` | Analyze a log file (like audit.log). |\n| `-l, --lookupid ID` | Show the report of one alert ID (`*` = all). |\n| `-b, --browser` | Open the GUI browser. |\n| `-s, --service` | Start as a D-Bus service. |\n| `-S, --noservice` | Do not use the service. |\n| `-u, --user` | User mode. |\n| `-p, --printer` | Print to the terminal. |\n| `-e, --emacs` | Emacs format. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Install it\n\n```bash\ndnf install -y setroubleshoot-server\n```\n\n> Restart auditd or reboot so it gets the events.\n\n### Analyze the whole audit log\n\n```bash\nsealert -a /var/log/audit/audit.log | less\n```\n\n> One block per kind of denial, with fixes.\n\n### Short messages in the journal\n\n```bash\njournalctl -t setroubleshoot --since -1h\n```\n```text\nsetroubleshoot[2310]: SELinux is preventing /usr/sbin/httpd from read access on the file index.html. For complete SELinux messages run: sealert -l 3f6a…\n```\n\n> Copy the ID.\n\n### Full report for one alert\n\n```bash\nsealert -l 3f6a2b1c-…\n```\n\n> Cause and suggestions.\n\n### All alerts\n\n```bash\nsealert -l \"*\"\n```\n\n> Every stored alert.\n\n### Typical suggestion: wrong label\n\n```bash\nsealert -a /var/log/audit/audit.log | grep -A3 'restorecon'\n```\n\n> Run the restorecon it shows.\n\n### Typical suggestion: boolean\n\n```bash\nsealert -a /var/log/audit/audit.log | grep setsebool\n```\n\n> Use the boolean it names (with -P).\n\n### Do not just follow audit2allow advice (security)\n\n```bash\nsealert -a /var/log/audit/audit.log | grep -B2 audit2allow\n```\n\n> The last suggestion is often \"make a module\" — only if nothing else fits.\n\n### Analyze an old log\n\n```bash\nsealert -a /var/log/audit/audit.log.1\n```\n\n> Rotated logs.\n\n### Check the service\n\n```bash\nsystemctl status setroubleshootd\n```\n\n> Runs on demand through D-Bus.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man sealert](https://linux.die.net/man/8/sealert)\n","page":"commands/sealert/","label":"sealert","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-sed","n":1510,"slug":"sed","title":"sed — Change text as it flows through: replace words, delete or print lines — and edit files in place with -i","sections":["commands"],"topic":"Commands · Text and search","tags":["replace","edit","stream","regex","in-place","delete lines","config"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change text as it flows through: replace words, delete or print lines — and edit files in place with -i.\n\n## About\n\n`sed` (stream editor) reads text line by line, runs editing commands on it, and prints the result. With `-i` it writes the result back into the file. It is the standard tool to change config files in scripts and automation.\n\nThe most used command is substitute: `s/OLD/NEW/FLAGS`. Commands can be limited to line numbers (`5`, `5,10`, `$` = last line) or to lines that match a pattern (`/regex/`).\n\n## Syntax\n\n```text\nsed [OPTION]... 'SCRIPT' [FILE...]\nsed [OPTION]... -e SCRIPT -e SCRIPT [FILE...]\nsed [OPTION]... -f SCRIPT-FILE [FILE...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-i[<suffix>], --in-place[=<suffix>]` | Edit the file itself. With a suffix (like -i.bak) keep a backup. |\n| `-n, --quiet, --silent` | Do not print every line; print only with the p command. |\n| `-e <script>, --expression=<script>` | Add a script (use it several times for several commands). |\n| `-E, -r, --regexp-extended` | Use extended regular expressions (+ ? {} \\| () without backslashes). |\n| `-f <file>, --file=<file>` | Read the sed commands from a file. |\n| `-s, --separate` | Treat several files as separate files (not one stream). |\n| `-z, --null-data` | Lines end with NUL. |\n| `-u, --unbuffered` | Flush output often (for live pipes). |\n| `-l <n>, --line-length=<n>` | Line wrap length for the l command. |\n| `--follow-symlinks` | With -i: edit the file that a link points to. |\n| `--posix` | Turn off GNU extensions. |\n| `--sandbox` | Do not allow the e, r and w commands. |\n| `--debug` | Show how sed runs the script. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n### Commands inside the script\n\n| Option | What it does |\n|---|---|\n| `s/old/new/` | Replace the first match in each line. |\n| `s/old/new/g` | Replace all matches in each line. |\n| `s/old/new/I` | Replace, ignoring case (GNU). |\n| `s/old/new/2` | Replace only the 2nd match. |\n| `d` | Delete the line. |\n| `p` | Print the line (with -n). |\n| `a text` | Append a line after. |\n| `i text` | Insert a line before. |\n| `c text` | Change (replace) the whole line. |\n| `y/abc/xyz/` | Change characters one by one (like tr). |\n| `=` | Print the line number. |\n| `q` | Quit. |\n| `r file` | Read a file in after the line. |\n| `w file` | Write the line to a file. |\n\n### Addresses\n\n| Option | What it does |\n|---|---|\n| `5` | Line 5. |\n| `5,10` | Lines 5 to 10. |\n| `$` | The last line. |\n| `/regex/` | Lines that match. |\n| `/start/,/end/` | From a line that matches start to a line that matches end. |\n| `addr!` | All lines that do NOT match the address. |\n\n## Examples\n\n### Replace text in the output\n\n```bash\nsed 's/http:/https:/g' links.txt\n```\n\n> The file does not change; only the output.\n\n### Change a config file in place, with a backup\n\n```bash\nsed -i.bak 's/^SELINUX=enforcing/SELINUX=permissive/' /etc/selinux/config\n```\n\n> The old file is kept as config.bak.\n\n### Turn off root login over SSH\n\n```bash\nsed -i 's/^#\\?PermitRootLogin .*/PermitRootLogin no/' /etc/ssh/sshd_config\n```\n\n> Works if the line is commented out or not. Then `systemctl reload sshd`.\n\n### Print only some lines\n\n```bash\nsed -n '10,20p' /etc/services\n```\n\n> Lines 10 to 20.\n\n### Delete comments and empty lines\n\n```bash\nsed '/^\\s*#/d; /^\\s*$/d' /etc/chrony.conf\n```\n\n> Shows only the active settings.\n\n### Add a line after a match\n\n```bash\nsed -i '/^\\[main\\]/a gpgcheck=1' /etc/dnf/dnf.conf\n```\n\n> Inserts a line under the [main] section.\n\n### Comment out a line\n\n```bash\nsed -i 's/^pool /#pool /' /etc/chrony.conf\n```\n\n> Puts `#` before every line that starts with \"pool \".\n\n### Use another separator when the text has slashes\n\n```bash\nsed 's|/var/www/html|/srv/web|g' site.conf\n```\n\n> `|` instead of `/` keeps it readable.\n\n### Use groups to reorder text\n\n```bash\necho \"2026-10-25\" | sed -E 's/([0-9]+)-([0-9]+)-([0-9]+)/\\3.\\2.\\1/'\n```\n```text\n25.10.2026\n```\n\n> `\\1` `\\2` `\\3` are the parts in brackets.\n\n### Print the line numbers of matches\n\n```bash\nsed -n '/Listen/=' /etc/httpd/conf/httpd.conf\n```\n\n> Like `grep -n`, only the numbers.\n\n### Delete a line by number\n\n```bash\nsed -i '3d' ~/.ssh/known_hosts\n```\n\n> Removes an old host key (line 3), after \"REMOTE HOST IDENTIFICATION HAS CHANGED\".\n\n### Replace a value in many files\n\n```bash\nsed -i 's/old-db.example.com/new-db.example.com/g' /etc/myapp/*.conf\n```\n\n> Check first with `grep -l old-db /etc/myapp/*.conf`.\n\n### Mask passwords in a log before you share it\n\n```bash\nsed -E 's/(password=)[^& ]+/\\1*****/g' app.log > app-masked.log\n```\n\n> Never send real secrets in a support ticket.\n\n## Watch out\n\n- Test without `-i` first. Then use `-i.bak` so you can go back.\n- `sed -i` replaces the file with a new one (new inode): links and some permissions (and SELinux labels) can change. Check with `ls -lZ` on system files.\n- In basic regex, `+ ? ( ) { } |` need a backslash; use `-E` to avoid that.\n\n## Tips\n\n- `s/a/b/g` replace all, `-n 'Np'` print line N, `/x/d` delete lines with x, `-i.bak` edit with backup.\n\nFull manual: [man sed](https://linux.die.net/man/1/sed)\n","page":"commands/sed/","label":"sed","where":"Command page · Text and search","aka":[]},{"id":"cmd-semanage","n":1511,"slug":"semanage","title":"semanage — Change the SELinux policy settings that admins need: file context rules, port types, booleans, permissive domains, user mappings","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","fcontext","port","boolean","permissive","policy","labels"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change the SELinux policy settings that admins need: file context rules, port types, booleans, permissive domains, user mappings.\n\n## About\n\n`semanage` writes **permanent** changes to the local policy. The parts you use most:\n- `semanage fcontext` — which label files in a path should get (then apply with restorecon),\n- `semanage port` — which ports a service type may use,\n- `semanage boolean` — list and set booleans with descriptions,\n- `semanage permissive` — make one process type permissive.\n\nInstall it with `dnf install policycoreutils-python-utils` (`dnf provides '*/semanage'`).\n\n## Syntax\n\n```text\nsemanage fcontext -a|-d|-m -t TYPE [-f FTYPE] \"PATH-REGEX\"\nsemanage port -a|-d|-m -t TYPE -p tcp|udp PORT[-PORT]\nsemanage boolean -l | -m --on|--off BOOLEAN\nsemanage permissive -a|-d|-l TYPE\nsemanage SUBCOMMAND -l [-C] | -E | -D\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `fcontext` | File context rules (paths → types). |\n| `port` | Port types. |\n| `boolean` | Booleans (with descriptions). |\n| `permissive` | Permissive process types. |\n| `login` | Linux users → SELinux users. |\n| `user` | SELinux users (roles, levels). |\n| `module` | Policy modules (list, enable, disable, remove). |\n| `interface, node, ibpkey, ibendport` | Network interface, node and InfiniBand types. |\n| `dontaudit` | Turn dontaudit rules off or on (to see hidden denials). |\n| `import, export` | Save / load all local changes (copy to other servers). |\n\n## Options\n\n\n### Actions\n\n| Option | What it does |\n|---|---|\n| `-a, --add` | Add a rule. |\n| `-d, --delete` | Delete a rule. |\n| `-m, --modify` | Change a rule (for example a port already defined for another type). |\n| `-l, --list` | List rules. |\n| `-C, --locallist` | With -l: only your local changes. |\n| `-E, --extract` | Print local changes as semanage commands. |\n| `-D, --deleteall` | Delete all local changes of this kind. |\n\n### Values\n\n| Option | What it does |\n|---|---|\n| `-t, --type TYPE` | The SELinux type. |\n| `-p, --proto tcp\\|udp` | Protocol (port). |\n| `-f, --ftype a\\|f\\|d\\|c\\|b\\|s\\|l\\|p` | File kind for fcontext (all, file, dir, char, block, socket, link, pipe). |\n| `-e, --equal PATH` | fcontext: label PATH like another path (equivalence), like `-e /var/www /srv/www`. |\n| `-r, --range RANGE` | MLS/MCS range. |\n| `-s, --seuser SEUSER` | SELinux user. |\n| `-1, --on / -0, --off` | boolean: on or off (with -m). |\n\n### General\n\n| Option | What it does |\n|---|---|\n| `-n, --noheading` | No header in lists. |\n| `-N, --noreload` | Do not reload the policy after the change. |\n| `-S, --store STORE` | Another policy store. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Label a new web folder\n\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/web(/.*)?\"\nrestorecon -Rv /srv/web\n```\n\n> The regex covers the folder and everything below it.\n\n### Same labels as a known folder\n\n```bash\nsemanage fcontext -a -e /var/www /srv/www\n```\n\n> /srv/www gets the labels of /var/www.\n\n### List your own file rules\n\n```bash\nsemanage fcontext -l -C\n```\n\n> Only local changes.\n\n### Find the rule for a path\n\n```bash\nsemanage fcontext -l | grep '^/var/www'\n```\n\n> Which types the policy gives there.\n\n### Allow a new port\n\n```bash\nsemanage port -a -t http_port_t -p tcp 82\n```\n\n> Apache may now listen on 82.\n\n### Port already used by another type\n\n```bash\nsemanage port -m -t http_port_t -p tcp 8080\n```\n\n> -m moves it (8080 is http_cache_port_t by default).\n\n### Show port types\n\n```bash\nsemanage port -l | grep -w ssh_port_t\n```\n```text\nssh_port_t                     tcp      22, 2222\n```\n\n> Which ports sshd may use.\n\n### Booleans with descriptions\n\n```bash\nsemanage boolean -l | grep httpd_can_network\n```\n\n> Current state, default, and what it does.\n\n### Make one domain permissive\n\n```bash\nsemanage permissive -a httpd_t\nsemanage permissive -d httpd_t\n```\n\n> Debug one service; the rest stays enforcing.\n\n### Copy local changes to another server\n\n```bash\nsemanage export > /root/selinux-local.txt\nsemanage import < /root/selinux-local.txt\n```\n\n> Same rules on all web servers.\n\n### See hidden denials\n\n```bash\nsemanage dontaudit off\n```\n\n> Shows denials that are normally not logged; turn it on again after debugging.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (like \"already defined\" or a bad regex). |\n\nFull manual: [man semanage](https://linux.die.net/man/8/semanage)\n","page":"commands/semanage/","label":"semanage","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-seq","n":1512,"slug":"seq","title":"seq — Print a list of numbers — for loops and test data","sections":["commands"],"topic":"Commands · Shell and help","tags":["numbers","loops","sequence","scripts"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print a list of numbers — for loops and test data.\n\n## About\n\n`seq LAST` prints 1 to LAST; `seq FIRST LAST` and `seq FIRST STEP LAST` set the start and step. In bash, `{1..10}` does the same without a program, but seq also works with variables and decimals.\n\n## Syntax\n\n```text\nseq [OPTION]... LAST\nseq [OPTION]... FIRST LAST\nseq [OPTION]... FIRST INCREMENT LAST\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-w, --equal-width` | Pad with leading zeros to the same width. |\n| `-s, --separator=STRING` | Separator (default a new line). |\n| `-f, --format=FORMAT` | printf format for decimals, like %.2f. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### 1 to 5\n\n```bash\nseq 5\n```\n\n> One number per line.\n\n### Loop over numbers\n\n```bash\nfor i in $(seq 1 3); do echo \"round $i\"; done\n```\n\n> Also: for i in {1..3}.\n\n### With a variable end\n\n```bash\nN=4; for i in $(seq 1 $N); do echo $i; done\n```\n\n> {1..$N} does not work in bash.\n\n### Steps\n\n```bash\nseq 0 10 50\n```\n```text\n0\n10\n20\n30\n40\n50\n```\n\n> Start, step, end.\n\n### Count down\n\n```bash\nseq 5 -1 1\n```\n\n> Negative step.\n\n### Equal width\n\n```bash\nseq -w 8 11\n```\n```text\n08\n09\n10\n11\n```\n\n> For file names.\n\n### One line with commas\n\n```bash\nseq -s, 1 5\n```\n```text\n1,2,3,4,5\n```\n\n> Lists for other commands.\n\n### Ping a range of hosts\n\n```bash\nfor i in $(seq 10 20); do ping -c1 -W1 192.168.122.$i &>/dev/null && echo \".$i up\"; done\n```\n\n> Quick scan of your own lab network.\n\n### Create test files\n\n```bash\nfor i in $(seq -w 1 20); do touch file$i.txt; done\n```\n\n> file01.txt … file20.txt.\n\n### Decimal steps\n\n```bash\nseq -f \"%.1f\" 0 0.5 2\n```\n\n> 0.0 0.5 1.0 1.5 2.0.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man seq](https://linux.die.net/man/1/seq)\n","page":"commands/seq/","label":"seq","where":"Command page · Shell and help","aka":[]},{"id":"cmd-sestatus","n":1513,"slug":"sestatus","title":"sestatus — Show the full SELinux status: on or off, current and config mode, policy name and version","sections":["commands"],"topic":"Commands · SELinux and security","tags":["selinux","status","policy","mode"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the full SELinux status: on or off, current and config mode, policy name and version.\n\n## About\n\n`sestatus` prints whether SELinux is enabled, the loaded policy (targeted on RHEL), the current mode and the mode in the config file. With `-b` it lists all booleans; with `-v` it shows the contexts of the files and processes named in `/etc/sestatus.conf`.\n\n## Syntax\n\n```text\nsestatus [-v] [-b]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-v` | Also show contexts of files and processes from /etc/sestatus.conf. |\n| `-b` | Also show all booleans and their state. |\n\n## Examples\n\n### Full status\n\n```bash\nsestatus\n```\n```text\nSELinux status:                 enabled\nSELinuxfs mount:                /sys/fs/selinux\nLoaded policy name:             targeted\nCurrent mode:                   enforcing\nMode from config file:          enforcing\n```\n\n> Current and config mode should both be enforcing.\n\n### Mode mismatch\n\n```bash\nsestatus | grep -i mode\n```\n\n> Current permissive but config enforcing = someone ran setenforce 0.\n\n### With booleans\n\n```bash\nsestatus -b | grep httpd\n```\n\n> State of the httpd booleans.\n\n### Contexts of important files and processes\n\n```bash\nsestatus -v\n```\n\n> Shows labels of /etc/passwd, /bin/bash, init and others.\n\n### Check the policy type\n\n```bash\nsestatus | grep 'Loaded policy'\n```\n\n> targeted = only selected services are confined (RHEL default).\n\n### Policy version\n\n```bash\nsestatus | grep 'Max kernel policy version'\n```\n\n> For support cases.\n\n### In a compliance script (security)\n\n```bash\nsestatus | awk -F: '/Current mode/ {gsub(/ /,\"\",$2); print $2}'\n```\n```text\nenforcing\n```\n\n> Clean value for a report.\n\n### Is MLS used?\n\n```bash\nsestatus | grep -i mls\n```\n\n> Policy MLS status: enabled (also in targeted, for MCS labels).\n\n### After disabling, nothing is shown\n\n```bash\nsestatus\n```\n```text\nSELinux status:                 disabled\n```\n\n> Coming back needs a relabel.\n\n### Deny unknown classes\n\n```bash\nsestatus | grep -i deny_unknown\n```\n\n> How the kernel handles unknown permissions.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man sestatus](https://linux.die.net/man/8/sestatus)\n","page":"commands/sestatus/","label":"sestatus","where":"Command page · SELinux and security","aka":[]},{"id":"cmd-set","n":1514,"slug":"set","title":"set — Turn shell options on or off (like -x trace, -e stop on error), set the positional parameters, or list all variables","sections":["commands"],"topic":"Commands · Shell and help","tags":["shell options","debug","strict mode","positional parameters","scripts"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Turn shell options on or off (like -x trace, -e stop on error), set the positional parameters, or list all variables.\n\n## About\n\n`set -OPTION` turns an option on, `set +OPTION` turns it off. In scripts the most used line is `set -euo pipefail` (stop on errors, error on unset variables, fail on broken pipes). `set -- a b c` sets `$1 $2 $3`. Without arguments, set prints all shell variables and functions.\n\n## Syntax\n\n```text\nset [-abefhkmnptuvxBCEHPT] [-o OPTION] [--] [ARGUMENT]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-e, -o errexit` | Exit when a command fails. |\n| `-u, -o nounset` | Error when an unset variable is used. |\n| `-x, -o xtrace` | Print commands before running them (trace). |\n| `-o pipefail` | A pipe fails if any command in it fails. |\n| `-v, -o verbose` | Print lines as they are read. |\n| `-n, -o noexec` | Read commands but do not run them. |\n| `-f, -o noglob` | No wildcard expansion. |\n| `-C, -o noclobber` | `>` does not overwrite files (use >\\| to force). |\n| `-a, -o allexport` | Export all new variables. |\n| `-b, -o notify` | Report finished jobs at once. |\n| `-h, -o hashall` | Remember command locations. |\n| `-k, -o keyword` | All assignments go to the environment. |\n| `-m, -o monitor` | Job control. |\n| `-p, -o privileged` | Privileged mode. |\n| `-t, -o onecmd` | Exit after one command. |\n| `-B, -o braceexpand` | Brace expansion (on by default). |\n| `-E, -o errtrace` | ERR traps in functions. |\n| `-H, -o histexpand` | ! history expansion. |\n| `-P, -o physical` | No symlinks for cd and pwd. |\n| `-T, -o functrace` | DEBUG/RETURN traps in functions. |\n| `-o vi, -o emacs` | Editing mode of the command line. |\n| `-o` | List all options and their state. |\n| `--` | Set the positional parameters (or clear them). |\n\n## Examples\n\n### Strict mode for scripts\n\n```bash\nhead -2 /usr/local/bin/backup.sh\n```\n```text\n#!/bin/bash\nset -euo pipefail\n```\n\n> Many hidden errors become visible.\n\n### Trace part of a script\n\n```bash\nset -x; cp a b; set +x\n```\n```text\n+ cp a b\n```\n\n> Only these lines are traced.\n\n### Stop on errors\n\n```bash\nbash -c 'set -e; false; echo \"not reached\"'\n```\n\n> The script ends at false.\n\n### Unset variable error\n\n```bash\nbash -c 'set -u; echo \"$TYPO\"'\n```\n```text\nbash: line 1: TYPO: unbound variable\n```\n\n> Catches typos like rm -rf \"$DIR/\" with an empty DIR.\n\n### Pipe failures\n\n```bash\nbash -c 'set -o pipefail; false | true; echo $?'\n```\n```text\n1\n```\n\n> Without pipefail the result would be 0.\n\n### Do not overwrite files\n\n```bash\nset -C; echo x > existing.txt\n```\n```text\nbash: existing.txt: cannot overwrite existing file\n```\n\n> Protects against a wrong >.\n\n### vi mode on the command line\n\n```bash\nset -o vi\n```\n\n> Esc, then vi keys to edit.\n\n### Show all options\n\n```bash\nset -o | head\n```\n\n> on/off for each.\n\n### Set positional parameters\n\n```bash\nset -- one two three; echo \"$2 / $#\"\n```\n```text\ntwo / 3\n```\n\n> Useful in tests and functions.\n\n### List variables and functions\n\n```bash\nset | grep ^HIST\n```\n\n> All HIST variables.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Wrong option. |\n\nFull manual: [man set](https://linux.die.net/man/1/bash)\n","page":"commands/set/","label":"set","where":"Command page · Shell and help","aka":[]},{"id":"cmd-setfacl","n":1515,"slug":"setfacl","title":"setfacl — Add, change or remove ACL entries: rights for extra users and groups on files and directories","sections":["commands"],"topic":"Commands · Permissions","tags":["acl","access control list","default acl","mask","shared folder","permissions"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Add, change or remove ACL entries: rights for extra users and groups on files and directories.\n\n## About\n\nNormal permissions have one owner and one group. With `setfacl` you add more: `u:carol:rx` for a user, `g:account:rwx` for a group. A **default** ACL (`-d`) on a directory is copied to new files and directories in it.\n\nThe **mask** is the maximum for all named users and groups and the owning group. setfacl sets it automatically; `chmod g=...` on a file with an ACL changes the mask.\n\nOnly the owner or root may change an ACL. XFS and ext4 on RHEL support ACLs without extra mount options.\n\n## Syntax\n\n```text\nsetfacl [-bkndRLPvh] [{-m|-x} ACL] [{-M|-X} FILE] FILE...\nsetfacl --set=ACL FILE...\nsetfacl --restore=FILE\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `u:USER:PERMS` | Rights for a named user (u::PERMS = the owner). |\n| `g:GROUP:PERMS` | Rights for a named group (g::PERMS = the owning group). |\n| `m::PERMS` | The mask. |\n| `o::PERMS` | Others. |\n| `d:ENTRY` | A default entry (same as -d). |\n| `PERMS` | r, w, x, X (x only for folders) or - ; numbers 0–7 work too. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-m, --modify=ACL` | Add or change entries. |\n| `-M, --modify-file=FILE` | Read the entries to change from a file (- = stdin). |\n| `-x, --remove=ACL` | Remove entries (without the rights part, like u:carol). |\n| `-X, --remove-file=FILE` | Read the entries to remove from a file. |\n| `-b, --remove-all` | Remove all extra entries (normal mode stays). |\n| `-k, --remove-default` | Remove the default ACL. |\n| `-d, --default` | Work on the default ACL. |\n| `-R, --recursive` | Work on the whole tree. |\n| `--set=ACL` | Replace the whole ACL. |\n| `--set-file=FILE` | Replace the whole ACL from a file. |\n| `-n, --no-mask` | Do not recalculate the mask. |\n| `--mask` | Recalculate the mask. |\n| `--restore=FILE` | Restore ACLs from a `getfacl -R` dump. |\n| `--test` | Show the result but change nothing. |\n| `-L, --logical` | Follow links to directories. |\n| `-P, --physical` | Do not follow links. |\n| `-v, --version` | Show the version. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Give one user read access\n\n```bash\nsetfacl -m u:carol:r report.txt\n```\n\n> carol may read; the normal mode stays.\n\n### Give a group full access to a folder\n\n```bash\nsetfacl -m g:account:rwx /srv/sales\n```\n\n> A second team without changing the owning group.\n\n### Make new files inherit the rights\n\n```bash\nsetfacl -d -m g:account:rwX /srv/sales\n```\n\n> Without -d only existing entries change.\n\n### Existing files and new files together\n\n```bash\nsetfacl -R -m g:account:rX /srv/sales\nsetfacl -d -m g:account:rX /srv/sales\n```\n\n> First the tree, then the default for later files.\n\n### Remove one entry\n\n```bash\nsetfacl -x u:carol report.txt\n```\n\n> Only carol's entry goes.\n\n### Remove all ACLs\n\n```bash\nsetfacl -R -b /srv/sales\n```\n\n> Back to the normal permissions.\n\n### Let a service read a log folder\n\n```bash\nsetfacl -R -m u:promtail:rX /var/log/httpd\nsetfacl -d -m u:promtail:rX /var/log/httpd\n```\n\n> A log shipper reads logs without being root or in the group (least privilege).\n\n### Let a user only pass through a folder\n\n```bash\nsetfacl -m u:carol:x /home/lisa\n```\n\n> carol can reach a file inside (if she may read it) but cannot list the folder.\n\n### Test before you change\n\n```bash\nsetfacl --test -m u:carol:rw report.txt\n```\n\n> Shows the new ACL only.\n\n### Restore a backup of ACLs\n\n```bash\nsetfacl --restore=/root/srv-acl.txt\n```\n\n> The dump from `getfacl -R`.\n\n### Fix rights cut by the mask\n\n```bash\nsetfacl -m m::rwx /srv/sales/plan.txt\n```\n\n> The mask limits named entries; set it back to rwx.\n\n## Watch out\n\n- The mask limits named users and groups. Check `#effective:` in getfacl.\n- A default ACL on a folder replaces the umask for new files there.\n- Backups need ACL support: `tar --acls`, `rsync -A`.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man setfacl](https://linux.die.net/man/1/setfacl)\n","page":"commands/setfacl/","label":"setfacl","where":"Command page · Permissions","aka":[]},{"id":"cmd-sftp","n":1516,"slug":"sftp","title":"sftp — Transfer files interactively over SSH, like a secure FTP client","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["transfer","files","remote","ftp","ssh","upload","download"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Transfer files interactively over SSH, like a secure FTP client.\n\n## About\n\n`sftp` opens a session with a prompt `sftp>`. You browse the remote folders, download with `get` and upload with `put`. It uses the SSH login and encryption, so no extra server is needed: the SSH server has an SFTP subsystem.\n\nCommands with an `l` in front work on your local side: `lls`, `lcd`, `lpwd`.\n\n## Syntax\n\n```text\nsftp [OPTIONS] [USER@]HOST[:PATH]\nsftp [OPTIONS] -b BATCHFILE [USER@]HOST\n```\n\n## Parts\n\n\n### Commands at the sftp> prompt\n\n| Part | Meaning |\n|---|---|\n| `ls [path] / lls [path]` | List remote / local files. |\n| `cd path / lcd path` | Change the remote / local folder. |\n| `pwd / lpwd` | Show the remote / local folder. |\n| `get remote [local]` | Download a file. `get -r` downloads a folder. |\n| `put local [remote]` | Upload a file. `put -r` uploads a folder. |\n| `mget pattern / mput pattern` | Download / upload many files (wildcards). |\n| `reget / reput` | Continue an interrupted download / upload. |\n| `mkdir path / rmdir path` | Make / remove a remote folder. |\n| `rm file` | Remove a remote file. |\n| `rename old new` | Rename a remote file. |\n| `chmod mode file / chown uid file / chgrp gid file` | Change remote permissions or owner. |\n| `ln -s target link` | Make a remote link. |\n| `df [-h]` | Show free space on the remote side. |\n| `! command` | Run a local shell command. |\n| `help / ?` | Show the commands. |\n| `bye / exit / quit` | Leave sftp. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-P <port>` | The SSH port of the server. |\n| `-i <file>` | Use this private key. |\n| `-b <file>` | Run the commands of a batch file (no prompt). `-b -` reads them from stdin. |\n| `-r` | Copy folders recursively with get and put. |\n| `-a` | Resume interrupted transfers. |\n| `-p` | Keep times and permissions. |\n| `-J <destination>` | Connect through a jump host. |\n| `-l <limit>` | Limit the bandwidth in Kbit/s. |\n| `-C` | Compress the data. |\n| `-q` | Quiet: no progress meter. |\n| `-N` | Turn off quiet mode (for batch mode). |\n| `-v` | Verbose debug output. |\n| `-f` | Flush files to disk right after the transfer. |\n| `-o <option>` | Pass an ssh option. |\n| `-F <file>` | Use another ssh client config file. |\n| `-c <cipher>` | Choose the cipher. |\n| `-B <size>` | Buffer size for transfers. |\n| `-R <n>` | Number of requests at the same time (speed). |\n| `-S <program>` | Use this program instead of ssh. |\n| `-s <subsystem>` | Use another SFTP subsystem or server program. |\n| `-D <command>` | Connect to a local sftp-server directly (testing). |\n| `-X <option>` | SFTP protocol options. |\n| `-4 / -6` | Use only IPv4 / only IPv6. |\n| `-A` | Forward the ssh-agent. |\n\n## Examples\n\n### Open a session\n\n```bash\nsftp student@server2\n```\n```text\nsftp> ls\nsftp> get /etc/os-release\nsftp> bye\n```\n\n> `get` downloads to your current local folder.\n\n### Upload a file\n\n```bash\nsftp student@server2\n```\n```text\nsftp> cd /tmp\nsftp> put report.pdf\n```\n\n> The file goes into the remote folder you are in.\n\n### Change the local folder first\n\n```bash\nsftp student@server2\n```\n```text\nsftp> lcd ~/Downloads\nsftp> get backup.tar.gz\n```\n\n> `lcd` = local cd.\n\n### Download a whole folder\n\n```bash\nsftp -r student@server2:/var/www/html ./site-copy\n```\n\n> `-r` with a path downloads recursively without a prompt.\n\n### Use another port\n\n```bash\nsftp -P 2222 admin@files.example.com\n```\n\n> Capital P, like scp.\n\n### Run commands from a batch file (automation)\n\n```bash\ncat upload.batch\n```\n```text\ncd /incoming\nput daily-report.csv\nbye\n```\n```bash\nsftp -b upload.batch partner@sftp.partner.com\n```\n\n> Typical for sending files to a partner company every night.\n\n### Resume a big download\n\n```bash\nsftp -a student@server2:/srv/iso/rhel-10.iso .\n```\n\n> Continues where the last try stopped.\n\n### Check the free space on the server\n\n```bash\nsftp student@server2\n```\n```text\nsftp> df -h\n```\n\n> Before you upload a big file.\n\n### Upload many files at once\n\n```bash\nsftp student@server2\n```\n```text\nsftp> mput *.csv\n```\n\n> Wildcards work on your local file names.\n\n### Use a jump host\n\n```bash\nsftp -J admin@bastion.example.com admin@10.10.0.20\n```\n\n> Reach a file server in a private network.\n\n### Run a local command without leaving\n\n```bash\nsftp student@server2\n```\n```text\nsftp> !ls -lh\n```\n\n> `!` runs the command on your computer.\n\n## Watch out\n\n- A server can allow SFTP only (chroot), so `ssh` login does not work but `sftp` does — that is on purpose.\n- `put` and `get` overwrite files with the same name without asking.\n\n## Tips\n\n- get = download, put = upload, l = local (lcd, lls, lpwd), bye = leave.\n\nFull manual: [man sftp](https://linux.die.net/man/1/sftp)\n","page":"commands/sftp/","label":"sftp","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-showmount","n":1517,"slug":"showmount","title":"showmount — Show the exports of an NFS server (and which clients mounted them)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["nfs","exports","share list","client"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the exports of an NFS server (and which clients mounted them).\n\n## About\n\n`showmount -e SERVER` asks the server's mountd for its export list: the folders and who may mount them. It uses the NFSv3 mount protocol, so the server firewall must allow `mountd` and `rpc-bind`. With pure NFSv4 you can instead mount the server root (`mount server:/ /mnt`) and look.\n\n## Syntax\n\n```text\nshowmount [OPTION]... [HOST]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-e, --exports` | Show the export list. |\n| `-a, --all` | Show client host and mounted folder pairs. |\n| `-d, --directories` | Show only the folders mounted by clients. |\n| `--no-headers` | No header line. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n\n## Examples\n\n### List the exports of a server\n\n```bash\nshowmount -e server2\n```\n```text\nExport list for server2:\n/exports/home 192.168.122.0/24\n/exports/data 192.168.122.0/24\n```\n\n> Folder and allowed clients.\n\n### Local server\n\n```bash\nshowmount -e localhost\n```\n\n> Check your own exports.\n\n### Firewall blocks it\n\n```bash\nshowmount -e server2\n```\n```text\nclnt_create: RPC: Unable to receive\n```\n\n> Open mountd and rpc-bind on the server: `firewall-cmd --add-service={mountd,rpc-bind} --permanent`.\n\n### Without showmount (NFSv4)\n\n```bash\nmount server2:/ /mnt; ls /mnt\n```\n\n> The pseudo root shows the exports.\n\n### Clients that mounted shares\n\n```bash\nshowmount -a\n```\n\n> On the server (NFSv3 information only).\n\n### Only folders\n\n```bash\nshowmount -d\n```\n\n> Folders in use.\n\n### For a script\n\n```bash\nshowmount -e --no-headers server2 | awk '{print $1}'\n```\n\n> Only the paths.\n\n### Check before an fstab entry\n\n```bash\nshowmount -e server2 | grep -w /exports/data\n```\n\n> The path must match exactly.\n\n### Find open NFS exports in your network (security)\n\n```bash\nfor h in server1 server2; do showmount -e $h 2>/dev/null; done\n```\n\n> Exports to `*` (everyone) are a data leak risk.\n\n### Check allowed client networks\n\n```bash\nshowmount -e server2 | grep '\\*'\n```\n\n> Exports for any host.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man showmount](https://linux.die.net/man/8/showmount)\n","page":"commands/showmount/","label":"showmount","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-shutdown","n":1518,"slug":"shutdown","title":"shutdown — Stop, restart or switch off the system cleanly — now or at a planned time, with a warning to users","sections":["commands"],"topic":"Commands · Services and boot","tags":["reboot","power off","halt","restart","maintenance","schedule"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Stop, restart or switch off the system cleanly — now or at a planned time, with a warning to users.\n\nSame page for: `reboot`, `poweroff`, `halt`\n\n## About\n\nOn RHEL these commands are part of systemd. They ask systemd to stop all services in the right order, write all data to disk and then reboot or power off.\n\n- `shutdown` can wait (a number of minutes or a clock time), sends a warning to logged-in users, and can be cancelled.\n- `reboot`, `poweroff` and `halt` act at once. They are the same as `systemctl reboot`, `systemctl poweroff` and `systemctl halt`.\n\n`halt` stops the system but does not switch off the power; use `poweroff` to switch off.\n\n## Syntax\n\n```text\nshutdown [OPTIONS] [TIME] [MESSAGE]\nshutdown -c\nreboot [OPTIONS]\npoweroff [OPTIONS]\nhalt [OPTIONS]\n```\n\n## Options\n\n\n### shutdown\n\n| Option | What it does |\n|---|---|\n| `-r, --reboot` | Reboot the system. |\n| `-h` | Power off (the same as -P, unless --halt is also given). |\n| `-P, --poweroff` | Power off the machine (the default). |\n| `-H, --halt` | Halt: stop the system but do not switch off. |\n| `-c` | Cancel a planned shutdown. |\n| `-k` | Do not shut down; only send the warning message. |\n| `--no-wall` | Do not send a message to the users. |\n| `--show` | Show the planned shutdown, if there is one. |\n| `TIME` | `now`, `+M` (in M minutes) or `hh:mm` (a clock time, 24 hours). Default: +1. |\n| `MESSAGE` | The text that users see in their terminals. |\n| `--help` | Show help. |\n\n### reboot, poweroff, halt\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Do not stop services first; twice (-ff) = immediately, like pulling the plug. |\n| `-p, --poweroff` | With halt: power off. |\n| `--reboot` | With halt or poweroff: reboot instead. |\n| `--halt` | With reboot or poweroff: halt instead. |\n| `-w, --wtmp-only` | Only write the wtmp record; do nothing else. |\n| `-d, --no-wtmp` | Do not write the wtmp record. |\n| `-n, --no-sync` | Do not sync the disks before. |\n| `--no-wall` | Do not send a message. |\n\n## Examples\n\n### Reboot now\n\n```bash\nreboot\n```\n\n> The same as `systemctl reboot`.\n\n### Power off now\n\n```bash\npoweroff\n```\n\n> The same as `systemctl poweroff` or `shutdown -h now`.\n\n### Reboot in 10 minutes with a message\n\n```bash\nshutdown -r +10 \"Kernel update. Please save your work.\"\n```\n\n> Logged-in users see the message now and again before the reboot.\n\n### Power off at a clock time\n\n```bash\nshutdown -h 23:30 \"Hardware maintenance tonight\"\n```\n\n> 24-hour time of the server's time zone.\n\n### Cancel a planned shutdown\n\n```bash\nshutdown -c\n```\n\n> Users get a message that it was cancelled.\n\n### Show the planned shutdown\n\n```bash\nshutdown --show\n```\n\n> Check before you plan another one.\n\n### Only warn users, do not shut down\n\n```bash\nshutdown -k +5 \"Test message: maintenance starts soon\"\n```\n\n> Useful to test what users see.\n\n### Reboot a remote server and wait until it is back\n\n```bash\nssh root@server2 reboot; sleep 60; until ssh -o ConnectTimeout=5 root@server2 true; do sleep 10; done; echo \"server2 is back\"\n```\n\n> A simple loop in a patch script.\n\n### Reboot after an update only when the kernel changed\n\n```bash\ndnf needs-restarting -r || systemctl reboot\n```\n\n> `needs-restarting -r` returns 1 when a reboot is needed.\n\n### Force a reboot when services hang\n\n```bash\nsystemctl reboot --force\n```\n\n> Skips the clean stop of services. Use only when a normal reboot hangs.\n\n### See past reboots\n\n```bash\nlast -x reboot shutdown | head\n```\n\n> When did the system reboot, and who shut it down?\n\n## Watch out\n\n- `halt` does not power off. On real hardware the machine stays on with the screen showing \"System halted\".\n- `reboot -ff` is like pulling the plug: open files can be damaged.\n- On a remote server, a `poweroff` cannot be undone without someone at the machine (or a remote console).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/log/wtmp` | Records of reboots and logins (read with `last`). |\n| `/run/nologin` | Made by shutdown 5 minutes before the time: new logins are refused. |\n\n## Tips\n\n- Now: `reboot` / `poweroff`. Later: `shutdown -r +10`. Undo: `shutdown -c`.\n\nFull manual: [man shutdown](https://linux.die.net/man/8/shutdown)\n","page":"commands/shutdown/","label":"shutdown","where":"Command page · Services and boot","aka":["reboot","poweroff","halt"]},{"id":"cmd-sleep","n":1519,"slug":"sleep","title":"sleep — Wait for a given time","sections":["commands"],"topic":"Commands · Shell and help","tags":["wait","delay","script","pause","timer"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Wait for a given time.\n\n## About\n\n`sleep NUMBER[SUFFIX]` pauses: s = seconds (default), m = minutes, h = hours, d = days. Decimals and more numbers (added together) are allowed. It is used in scripts and as a harmless test process in labs.\n\n## Syntax\n\n```text\nsleep NUMBER[SUFFIX]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `NUMBER[smhd]` | The time to wait; more values are added. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Wait 5 seconds\n\n```bash\nsleep 5\n```\n\n> No output; the prompt comes back after 5 seconds.\n\n### Minutes and hours\n\n```bash\nsleep 2m; sleep 1.5h\n```\n\n> Suffixes s, m, h, d.\n\n### Add values\n\n```bash\nsleep 1m 30s\n```\n\n> Waits 90 seconds.\n\n### A test process for labs\n\n```bash\nsleep 3600 &\n```\n\n> Practice jobs, ps, kill and renice.\n\n### Retry in a loop\n\n```bash\nuntil curl -fs http://localhost/health; do sleep 2; done\n```\n\n> Wait until a service answers.\n\n### Pace a loop\n\n```bash\nfor h in server1 server2; do ssh $h 'systemctl restart httpd'; sleep 30; done\n```\n\n> Rolling restart, one server at a time.\n\n### Random delay for cron jobs\n\n```bash\nsleep $((RANDOM % 300)); /usr/local/bin/report.sh\n```\n\n> Not all servers hit the API at the same second.\n\n### Fractions of a second\n\n```bash\nsleep 0.2\n```\n\n> Short pauses in scripts.\n\n### Stop a long sleep\n\n```bash\nkill %1\n```\n\n> sleep reacts to signals like other processes.\n\n### Sleep forever (containers)\n\n```bash\nsleep infinity\n```\n\n> Keeps a test container running.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error or interrupted by a signal. |\n\nFull manual: [man sleep](https://linux.die.net/man/1/sleep)\n","page":"commands/sleep/","label":"sleep","where":"Command page · Shell and help","aka":[]},{"id":"cmd-sort","n":1520,"slug":"sort","title":"sort — Put lines in order: alphabetical, by numbers, by sizes, by versions, by a chosen column","sections":["commands"],"topic":"Commands · Text and search","tags":["sort","order","numbers","unique","top list","columns"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Put lines in order: alphabetical, by numbers, by sizes, by versions, by a chosen column.\n\n## About\n\n`sort` reads all lines and prints them sorted. By default it sorts as text, from A to Z (following the language settings). With options it sorts numbers (`-n`), human sizes (`-h`), versions (`-V`), in reverse (`-r`), or on one column (`-k`).\n\nCombined with `uniq -c` it makes \"top lists\" of anything: the most common IPs, errors, users.\n\n## Syntax\n\n```text\nsort [OPTION]... [FILE]...\n```\n\n## Options\n\n\n### How to compare\n\n| Option | What it does |\n|---|---|\n| `-n, --numeric-sort` | Sort as numbers (2 before 10). |\n| `-h, --human-numeric-sort` | Sort human sizes like 2K, 300M, 1G. |\n| `-V, --version-sort` | Sort version numbers in a natural way (v1.9 before v1.10). |\n| `-g, --general-numeric-sort` | Sort numbers with exponents (1e3). |\n| `-M, --month-sort` | Sort month names (JAN < FEB …). |\n| `-R, --random-sort` | Shuffle the lines (equal lines stay together). |\n| `--random-source=<file>` | Random bytes for -R from this file. |\n| `-f, --ignore-case` | Ignore upper/lower case. |\n| `-d, --dictionary-order` | Look only at letters, digits and blanks. |\n| `-i, --ignore-nonprinting` | Ignore non-printable characters. |\n| `-b, --ignore-leading-blanks` | Ignore spaces at the start. |\n| `--sort=<word>` | Same as the options above: general-numeric, human-numeric, month, numeric, random, version. |\n\n### What to compare\n\n| Option | What it does |\n|---|---|\n| `-k, --key=<keydef>` | Sort on a field: `-k2` = from field 2, `-k2,2` = only field 2, `-k3n` = field 3 as a number. |\n| `-t, --field-separator=<sep>` | The field separator (default: change from non-blank to blank). |\n\n### Order and output\n\n| Option | What it does |\n|---|---|\n| `-r, --reverse` | Reverse the order. |\n| `-u, --unique` | Print only the first of equal lines. |\n| `-s, --stable` | Keep the input order for equal lines. |\n| `-o, --output=<file>` | Write the result to a file (can be the same as the input file). |\n| `-c, --check` | Check if the input is sorted; report the first wrong line. |\n| `-C, --check=quiet` | Check silently (only the exit code). |\n| `-m, --merge` | Merge files that are already sorted. |\n| `-z, --zero-terminated` | Lines end with NUL. |\n\n### Performance\n\n| Option | What it does |\n|---|---|\n| `-S, --buffer-size=<size>` | Memory to use, like 1G. |\n| `-T, --temporary-directory=<dir>` | Folder for temporary files. |\n| `--parallel=<n>` | Sort with N threads. |\n| `--batch-size=<n>` | Merge at most N inputs at once. |\n| `--compress-program=<prog>` | Compress temporary files with this program. |\n| `--files0-from=<file>` | Read input file names from a file (NUL separated). |\n| `--debug` | Show which part of each line is used for sorting. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Sort lines alphabetically\n\n```bash\nsort names.txt\n```\n\n> A to Z.\n\n### Sort numbers the right way\n\n```bash\nsort -n numbers.txt\n```\n\n> Without -n, 10 comes before 9 (text order).\n\n### Biggest folders last (human sizes)\n\n```bash\ndu -sh /var/* 2>/dev/null | sort -h\n```\n\n> `-h` understands K, M and G.\n\n### Sort /etc/passwd by UID\n\n```bash\nsort -t: -k3,3n /etc/passwd\n```\n\n> Field separator `:`, key = field 3 only, as a number.\n\n### Top list of anything\n\n```bash\ncut -d: -f7 /etc/passwd | sort | uniq -c | sort -rn\n```\n\n> Count each value, the most common first.\n\n### The IPs with the most requests in a web log\n\n```bash\nawk '{print $1}' /var/log/httpd/access_log | sort | uniq -c | sort -rn | head\n```\n\n> The first step when a site is slow or attacked.\n\n### Unique values only\n\n```bash\nsort -u hosts-from-many-files.txt\n```\n\n> Same as `sort | uniq`.\n\n### Sort version numbers\n\n```bash\nls /srv/releases | sort -V\n```\n\n> v1.2, v1.10, v2.0 in the right order.\n\n### Sort by the second column, reverse\n\n```bash\nsort -k2,2nr scores.txt\n```\n\n> Highest score first.\n\n### Sort a file in place\n\n```bash\nsort -o users.txt users.txt\n```\n\n> `-o` can write to the same file safely (redirect `>` cannot).\n\n### Check if a file is sorted\n\n```bash\nsort -c list.txt && echo sorted\n```\n\n> Exit code 0 when sorted.\n\n### Sort a huge file faster\n\n```bash\nsort -S 2G --parallel=4 -T /var/tmp big.csv > big-sorted.csv\n```\n\n> More memory and threads; temp files on a disk with space.\n\n## Watch out\n\n- Text order depends on the language (LANG). For a byte-order sort in scripts use `LC_ALL=C sort`.\n- `sort file > file` empties the file first. Use `-o file`.\n- Without `,N` in `-k`, the key goes to the end of the line: use `-k2,2` for only field 2.\n\n## Tips\n\n- n = numbers, h = sizes, V = versions, r = reverse, u = unique, k = column, t = separator.\n\nFull manual: [man sort](https://linux.die.net/man/1/sort)\n","page":"commands/sort/","label":"sort","where":"Command page · Text and search","aka":[]},{"id":"cmd-source","n":1521,"slug":"source","title":"source — Run the commands of a file in the current shell, so its variables, aliases and functions stay","sections":["commands"],"topic":"Commands · Shell and help","tags":["bashrc","script","variables","load","dot","environment"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run the commands of a file in the current shell, so its variables, aliases and functions stay.\n\n## About\n\nA script normally runs in a new child shell: when it ends, its variables are gone. `source FILE` (or the short form `. FILE`) reads the file and runs its lines **in your current shell**. Variables, aliases, functions and `cd` changes stay.\n\nUse it to load `~/.bashrc` after an edit, to load settings from a file (`.env`), or to use a library of shell functions in a script.\n\n## Syntax\n\n```text\nsource FILE [ARGUMENTS]\n. FILE [ARGUMENTS]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `FILE` | The file to read. Without a slash, Bash looks for it in $PATH, then in the current folder. |\n| `ARGUMENTS` | Become $1, $2 … inside the file while it runs. |\n\n## Examples\n\n### Load your .bashrc after an edit\n\n```bash\nsource ~/.bashrc\n```\n\n> New aliases and variables work at once, without logging out.\n\n### The short form with a dot\n\n```bash\n. ~/.bashrc\n```\n\n> Exactly the same as `source`.\n\n### Load variables from a settings file\n\n```bash\ncat app.env\n```\n```text\nAPP_PORT=8080\nAPP_ENV=staging\n```\n```bash\nset -a; source app.env; set +a\n```\n\n> `set -a` exports every variable that is set, so programs started later get them.\n\n### Use shared functions in a script\n\n```bash\ncat /usr/local/lib/ops-lib.sh\n```\n```text\nlog() { echo \"$(date +%F_%T) $*\"; }\n```\n```bash\ncat deploy.sh\n```\n```text\n#!/bin/bash\nsource /usr/local/lib/ops-lib.sh\nlog \"deploy started\"\n```\n\n> One library file, many scripts. Fix a function once.\n\n### Activate a Python virtual environment\n\n```bash\nsource ~/venvs/ansible/bin/activate\n```\n\n> The activate script changes PATH in your shell, so it must be sourced, not run.\n\n### Check what a script changes before you trust it\n\n```bash\nbash -n setup.sh && source setup.sh\n```\n\n> `bash -n` checks the syntax without running anything.\n\n### Load the environment of a service in a test\n\n```bash\nset -a; source /etc/sysconfig/myapp; set +a\n```\n\n> Many services read their options from a file in /etc/sysconfig. Load it to test the same settings by hand.\n\n### Pass arguments to the sourced file\n\n```bash\nsource ./set-cluster.sh production\n```\n\n> Inside the file, `$1` is `production`.\n\n### Make a change of folder stay\n\n```bash\ncat goto-logs.sh\n```\n```text\ncd /var/log/myapp\n```\n```bash\nsource goto-logs.sh; pwd\n```\n```text\n/var/log/myapp\n```\n\n> Run as `./goto-logs.sh`, the `cd` would happen in a child shell and you would not move.\n\n### Reload the shell profile for all users after a change (current shell)\n\n```bash\nsource /etc/profile\n```\n\n> Other users get it at their next login.\n\n## Watch out\n\n- `source` runs the file with **your** rights in **your** shell. Never source a file you do not trust.\n- An `exit` inside a sourced file closes your shell. Use `return` in files that are meant to be sourced.\n- `source` needs Bash (or another shell that has it). In plain `sh` scripts, use `.`.\n\n## Tips\n\n- Run a script = new child shell. `source` = this shell.\n- `source` and `.` are the same.\n\nFull manual: [man source](https://linux.die.net/man/1/bash)\n","page":"commands/source/","label":"source","where":"Command page · Shell and help","aka":[]},{"id":"cmd-ss","n":1522,"slug":"ss","title":"ss — Show network sockets: which ports are listening, which connections are open, and which process owns them","sections":["commands"],"topic":"Commands · Network","tags":["sockets","ports","listening","connections","netstat","processes","security"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show network sockets: which ports are listening, which connections are open, and which process owns them.\n\n## About\n\n`ss` (socket statistics) replaces the old `netstat`. The most used form is `ss -tulpn`: TCP and UDP, listening, with processes, as numbers.\n\nYou can filter by state (`state established`), port (`sport = :22`, `dport = :443`) and address (`dst 192.168.122.0/24`). Process names (`-p`) of other users are only shown to root.\n\n## Syntax\n\n```text\nss [OPTION]... [FILTER]\n```\n\n## Options\n\n\n### What to show\n\n| Option | What it does |\n|---|---|\n| `-t, --tcp` | TCP sockets. |\n| `-u, --udp` | UDP sockets. |\n| `-l, --listening` | Only listening sockets. |\n| `-a, --all` | Listening and other sockets. |\n| `-x, --unix` | Unix domain sockets. |\n| `-w, --raw` | Raw sockets. |\n| `-d, --dccp` | DCCP sockets. |\n| `-S, --sctp` | SCTP sockets. |\n| `-M, --mptcp` | MPTCP sockets. |\n| `-0, --packet` | Packet sockets. |\n| `-4, --ipv4` | Only IPv4. |\n| `-6, --ipv6` | Only IPv6. |\n| `-f, --family=FAMILY` | Only this family: inet, inet6, link, unix, netlink, vsock, tipc, xdp. |\n| `-A, --query=QUERY` | Socket tables to show (like all, inet, tcp, udp, unix). |\n| `-N, --net=NETNS` | Switch to this network namespace. |\n\n### Details\n\n| Option | What it does |\n|---|---|\n| `-n, --numeric` | Numbers for ports and addresses (faster). |\n| `-r, --resolve` | Resolve names. |\n| `-p, --processes` | Show the process that uses the socket. |\n| `-T, --threads` | Show the threads too. |\n| `-e, --extended` | Extra information (uid, inode). |\n| `-o, --options` | Timer information. |\n| `-m, --memory` | Socket memory use. |\n| `-i, --info` | Internal TCP information (rtt, cwnd …). |\n| `-s, --summary` | Totals per type. |\n| `-Z, --context` | SELinux context of the process. |\n| `-z, --contexts` | SELinux context of process and socket. |\n| `--tos` | ToS and priority information. |\n| `--cgroup` | The cgroup of the socket. |\n| `--tipcinfo` | TIPC socket details. |\n| `--inet-sockopt` | Show inet socket options. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-H, --no-header` | No header line. |\n| `-O, --oneline` | One line per socket. |\n| `-E, --events` | Show sockets as they are destroyed (live). |\n| `-D, --diag=FILE` | Dump raw data to a file. |\n| `-F, --filter=FILE` | Read the filter from a file. |\n| `-K, --kill` | Close the matching sockets (root). |\n| `-b, --bpf` | Show the BPF filter of sockets. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Which ports are open?\n\n```bash\nss -tulpn\n```\n```text\nNetid State  Recv-Q Send-Q Local Address:Port  Peer Address:Port Process\nudp   UNCONN 0      0          127.0.0.1:323        0.0.0.0:*     users:((\"chronyd\",pid=812,fd=5))\ntcp   LISTEN 0      128          0.0.0.0:22         0.0.0.0:*     users:((\"sshd\",pid=1010,fd=3))\ntcp   LISTEN 0      511                *:80               *:*     users:((\"httpd\",pid=1420,fd=4))\n```\n\n> The most used form. 0.0.0.0 or * = all addresses; 127.0.0.1 = only local.\n\n### Is my service listening?\n\n```bash\nss -tlnp | grep ':80 '\n```\n\n> Nothing = the service did not start or listens on another port.\n\n### Open SSH connections\n\n```bash\nss -tn state established '( sport = :22 )'\n```\n\n> Who is connected to this server now.\n\n### Connections to a remote port\n\n```bash\nss -tn dst :443\n```\n\n> Outgoing HTTPS connections.\n\n### Summary of all sockets\n\n```bash\nss -s\n```\n\n> Totals; many TIME-WAIT can mean a busy web server.\n\n### Connections from one network\n\n```bash\nss -tn dst 192.168.122.0/24\n```\n\n> Filter by address.\n\n### Find unknown listeners (security)\n\n```bash\nss -tulpn | grep -vE ':(22|80|443|323)\\b'\n```\n\n> Every open port should be known; a strange one can be a backdoor.\n\n### Show the SELinux context of listeners\n\n```bash\nss -tlnZ\n```\n\n> Helps when SELinux blocks a service on a new port.\n\n### Close a hanging connection\n\n```bash\nss -K dst 192.168.122.99 dport = :8080\n```\n\n> Needs a kernel with socket destroy support.\n\n### Watch new connections\n\n```bash\nwatch -n1 'ss -tn state established | wc -l'\n```\n\n> Number of open connections every second.\n\n### Unix sockets of a service\n\n```bash\nss -xlp | grep podman\n```\n\n> Local sockets (like the podman or docker API).\n\n## Watch out\n\n- Without root, `-p` shows only your own processes.\n- A port in LISTEN is not reachable when the firewall blocks it. Check `firewall-cmd --list-all`.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man ss](https://linux.die.net/man/8/ss)\n","page":"commands/ss/","label":"ss","where":"Command page · Network","aka":[]},{"id":"cmd-ssh-agent","n":1523,"slug":"ssh-agent","title":"ssh-agent — Keep unlocked SSH keys in memory (ssh-agent) and load or remove keys there (ssh-add), so you type a passphrase only once","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["keys","passphrase","agent","cache","forwarding"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Keep unlocked SSH keys in memory (ssh-agent) and load or remove keys there (ssh-add), so you type a passphrase only once.\n\nSame page for: `ssh-add`\n\n## About\n\nA key with a passphrase is safe on disk, but typing the passphrase for every connection is slow. `ssh-agent` is a small background program that holds the **unlocked** keys in memory. `ssh` asks the agent to sign the login; the private key never leaves the agent.\n\n`ssh-add` loads keys into the agent (asking for the passphrase once), lists them and removes them. On a desktop, the agent usually starts with your session; in a plain terminal you start it with `eval \"$(ssh-agent)\"`.\n\n## Syntax\n\n```text\nssh-agent [-c | -s] [-Dd] [-a BIND_ADDRESS] [-E HASH] [-O OPTION] [-P PROVIDERS] [-t LIFE] [COMMAND [ARG ...]]\nssh-agent [-c | -s] -k\nssh-add [-cCDdKkLlqvXx] [-E HASH] [-H HOSTKEY_FILE] [-h DEST] [-S PROVIDER] [-t LIFE] [FILE ...]\nssh-add -s PKCS11 / ssh-add -e PKCS11 / ssh-add -T PUBKEY ...\n```\n\n## Options\n\n\n### ssh-agent\n\n| Option | What it does |\n|---|---|\n| `-s` | Print Bourne shell (bash) commands to set the variables (default for bash). |\n| `-c` | Print C-shell commands. |\n| `-k` | Stop the agent of this session (SSH_AGENT_PID). |\n| `-t <life>` | Default lifetime of added keys, like 1h. |\n| `-a <socket>` | Use this socket path. |\n| `-D` | Run in the foreground. |\n| `-d` | Debug mode (foreground with debug output). |\n| `-E <hash>` | Fingerprint hash for logs (sha256 or md5). |\n| `-O <option>` | Extra options (like no-restrict-websafe). |\n| `-P <providers>` | Allowed PKCS#11/FIDO library paths. |\n\n### ssh-add\n\n| Option | What it does |\n|---|---|\n| `(no option)` | Add the default keys (~/.ssh/id_*). |\n| `<file>` | Add this key file. |\n| `-l` | List the fingerprints of the loaded keys. |\n| `-L` | List the public keys of the loaded keys. |\n| `-d` | Remove this key from the agent. |\n| `-D` | Remove all keys from the agent. |\n| `-t <life>` | Keep this key only for this time, like 30m. |\n| `-c` | Ask for a confirmation each time the key is used. |\n| `-x / -X` | Lock / unlock the agent with a password. |\n| `-k` | Load only plain keys, skip certificates. |\n| `-K` | Load resident keys from a FIDO security key. |\n| `-s <pkcs11> / -e <pkcs11>` | Add / remove keys from a smart card library. |\n| `-S <provider>` | The FIDO provider library. |\n| `-T <pubkey>` | Test that the private key for this public key works. |\n| `-h <destination>` | Allow the key only for these hosts (destination constraints). |\n| `-H <file>` | Known hosts file for destination constraints. |\n| `-C` | Also load certificates (with -h). |\n| `-E <hash>` | Fingerprint hash for -l (sha256 or md5). |\n| `-q` | Quiet. |\n| `-v` | Verbose. |\n\n## Examples\n\n### Start an agent in your shell\n\n```bash\neval \"$(ssh-agent -s)\"\n```\n```text\nAgent pid 4321\n```\n\n> Sets SSH_AUTH_SOCK and SSH_AGENT_PID for this shell.\n\n### Load your key (type the passphrase once)\n\n```bash\nssh-add\n```\n\n> Every ssh in this session now uses the key without asking.\n\n### List the keys in the agent\n\n```bash\nssh-add -l\n```\n```text\n256 SHA256:Xh7… student@server1 (ED25519)\n```\n\n> \"The agent has no identities\" = nothing loaded yet.\n\n### Load a key for one hour only\n\n```bash\nssh-add -t 1h ~/.ssh/prod_admin\n```\n\n> After an hour the key is removed automatically. Good for production keys.\n\n### Remove all keys when you leave your desk\n\n```bash\nssh-add -D\n```\n\n> Nobody can use your keys from this session any more.\n\n### Ask before each use of a key\n\n```bash\nssh-add -c ~/.ssh/prod_admin\n```\n\n> A confirmation window appears for each use (needs a graphical askpass program).\n\n### Lock the agent with a password\n\n```bash\nssh-add -x\n```\n\n> Keys stay loaded but cannot be used until `ssh-add -X`.\n\n### Use the agent on a jump host safely (forward only when needed)\n\n```bash\nssh -J admin@bastion admin@10.10.0.15\n```\n\n> With -J the key stays on your computer; prefer it to agent forwarding (-A).\n\n### Start a command with its own agent\n\n```bash\nssh-agent bash -c 'ssh-add ~/.ssh/deploy_key && ./deploy.sh'\n```\n\n> The agent and the key disappear when the command ends.\n\n### Stop the agent\n\n```bash\neval \"$(ssh-agent -k)\"\n```\n\n> Kills the agent and clears the variables.\n\n### Limit a key to some servers\n\n```bash\nssh-add -h server2.example.com ~/.ssh/id_ed25519\n```\n\n> The agent refuses to use this key for any other destination.\n\n### Check that the agent is reachable\n\n```bash\necho \"$SSH_AUTH_SOCK\"\n```\n\n> Empty means no agent is connected to this shell.\n\n## Watch out\n\n- Agent forwarding (`ssh -A`) lets root on the remote server use your keys while you are connected. Avoid it on servers you do not fully trust.\n- An agent started in one terminal is not known in another terminal unless you export SSH_AUTH_SOCK there.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `$SSH_AUTH_SOCK` | The socket where programs talk to the agent. |\n\n## Tips\n\n- `eval \"$(ssh-agent)\"` → `ssh-add` → work → `ssh-add -D`.\n\nFull manual: [man ssh-agent](https://linux.die.net/man/1/ssh-agent)\n","page":"commands/ssh-agent/","label":"ssh-agent","where":"Command page · SSH and remote work","aka":["ssh-add"]},{"id":"cmd-ssh-copy-id","n":1524,"slug":"ssh-copy-id","title":"ssh-copy-id — Install your public SSH key on a server, so you can log in with the key instead of a password","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["keys","authorized_keys","login","passwordless"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Install your public SSH key on a server, so you can log in with the key instead of a password.\n\n## About\n\n`ssh-copy-id` logs in to the server (with your password, one last time), adds your public key to `~/.ssh/authorized_keys` of the remote user, and sets the right permissions on `~/.ssh` and the file. It does not add a key twice.\n\nBy default it uses all your keys (or the ones in ssh-agent). With `-i` you choose one key.\n\n## Syntax\n\n```text\nssh-copy-id [-f] [-n] [-s] [-x] [-i [IDENTITY_FILE]] [-t TARGET_PATH] [-p PORT] [-o SSH_OPTION] [USER@]HOST\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-i <file>` | Install this public key (like ~/.ssh/id_ed25519.pub). |\n| `-p <port>` | The SSH port of the server. |\n| `-o <option>` | Pass an option to ssh (like `-o ProxyJump=bastion`). |\n| `-n` | Dry run: show which keys would be installed, change nothing. |\n| `-f` | Force: install the key even if it seems to be there already, and do not test the login. |\n| `-s` | Use SFTP to install the keys (for servers that allow only SFTP). |\n| `-t <path>` | Install into this file instead of ~/.ssh/authorized_keys. |\n| `-x` | Debug output. |\n| `-h, -?` | Show the usage. |\n\n## Examples\n\n### Install your key on a server\n\n```bash\nssh-copy-id student@server2\n```\n\n> Asks for the password of student on server2. Next logins use the key.\n\n### Install one specific key\n\n```bash\nssh-copy-id -i ~/.ssh/deploy_ed25519.pub deploy@web01\n```\n\n> Only this key is added.\n\n### Use another port\n\n```bash\nssh-copy-id -p 2222 admin@bastion.example.com\n```\n\n> Same -p as ssh.\n\n### See which keys would be copied\n\n```bash\nssh-copy-id -n student@server2\n```\n\n> A dry run.\n\n### Copy a key through a jump host\n\n```bash\nssh-copy-id -o ProxyJump=admin@bastion.example.com admin@10.10.0.15\n```\n\n> Works for servers in a private network.\n\n### Test right after copying\n\n```bash\nssh-copy-id student@server2 && ssh -o BatchMode=yes student@server2 true && echo \"key login works\"\n```\n\n> `BatchMode` fails instead of asking for a password, so the test is honest.\n\n### Install keys for root (when root login with password is still allowed)\n\n```bash\nssh-copy-id root@server2\n```\n\n> Then disable password login for root in sshd_config (chapter 20).\n\n### Copy the key of an automation account\n\n```bash\nsudo -u ansible ssh-copy-id -i /home/ansible/.ssh/id_ed25519.pub ansible@web01\n```\n\n> The key of the ansible user, installed for ansible on web01.\n\n### Install the key into a custom file\n\n```bash\nssh-copy-id -t /home/student/.ssh/authorized_keys2 student@server2\n```\n\n> For servers with a special AuthorizedKeysFile setting.\n\n### Use SFTP mode on a restricted server\n\n```bash\nssh-copy-id -s sftpuser@files.example.com\n```\n\n> When the account has no shell.\n\n### Do it by hand when ssh-copy-id is missing\n\n```bash\ncat ~/.ssh/id_ed25519.pub | ssh student@server2 'mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys'\n```\n\n> What ssh-copy-id does for you.\n\n## Watch out\n\n- The server must allow password login for this first copy (or you add the key another way).\n- Wrong permissions on the remote `~` (group-writable), `~/.ssh` or `authorized_keys` make sshd ignore the key.\n\n## Tips\n\n- One time: `ssh-copy-id user@host`. Forever: `ssh user@host` without password.\n\nFull manual: [man ssh-copy-id](https://linux.die.net/man/1/ssh-copy-id)\n","page":"commands/ssh-copy-id/","label":"ssh-copy-id","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-ssh-keygen","n":1525,"slug":"ssh-keygen","title":"ssh-keygen — Create SSH key pairs, change their passphrase, show fingerprints and clean the known_hosts file","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["keys","ed25519","rsa","fingerprint","known_hosts","passphrase","certificate"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create SSH key pairs, change their passphrase, show fingerprints and clean the known_hosts file.\n\n## About\n\n`ssh-keygen` makes a key pair: a **private key** (keep it secret) and a **public key** (`.pub`, you copy it to servers). The default type today is **ed25519**: small, fast and secure. RSA keys should be at least 3072 bits.\n\nIt also does key housekeeping: show the fingerprint of a key, add or change a passphrase, remove an old host key from `known_hosts`, and sign keys with a certificate authority.\n\n## Syntax\n\n```text\nssh-keygen [-q] [-t TYPE] [-b BITS] [-C COMMENT] [-f FILE] [-N PASSPHRASE]\nssh-keygen -p [-f FILE]\nssh-keygen -l [-v] [-E HASH] -f FILE\nssh-keygen -R HOSTNAME [-f KNOWN_HOSTS]\nssh-keygen -F HOSTNAME [-f KNOWN_HOSTS]\nssh-keygen -s CA_KEY -I ID [-h] [-n PRINCIPALS] [-V VALIDITY] PUBKEY\n```\n\n## Options\n\n\n### Make keys\n\n| Option | What it does |\n|---|---|\n| `-t <type>` | Key type: ed25519 (recommended), rsa, ecdsa, ed25519-sk / ecdsa-sk (hardware keys), dsa (old, do not use). |\n| `-b <bits>` | Key size (for RSA: 3072 or 4096). |\n| `-C <comment>` | A comment in the public key, like \"student@server1\" or an email. |\n| `-f <file>` | The key file name (default ~/.ssh/id_<type>). |\n| `-N <passphrase>` | The new passphrase (`-N \"\"` = no passphrase, for automation). |\n| `-a <rounds>` | KDF rounds that protect the passphrase (higher = slower to crack). |\n| `-A` | Make all missing host keys (used by the system for /etc/ssh). |\n| `-q` | Quiet. |\n| `-Z <cipher>` | The cipher that encrypts the private key file. |\n| `-O <option>` | Options for FIDO keys or certificates (like verify-required). |\n| `-w <provider>` | A FIDO security key provider library. |\n| `-K` | Download resident keys from a FIDO security key. |\n\n### Look at and change keys\n\n| Option | What it does |\n|---|---|\n| `-l` | Show the fingerprint of a key file. |\n| `-v` | Verbose (with -l: also show the random art picture). |\n| `-E <hash>` | The fingerprint hash: sha256 (default) or md5. |\n| `-B` | Show the bubblebabble digest of a key. |\n| `-y` | Print the public key of a private key file. |\n| `-p` | Change (or add) the passphrase of a private key. |\n| `-P <passphrase>` | The old passphrase (with -p). |\n| `-c` | Change the comment of a key. |\n| `-e` | Export a key in another format (like RFC4716 for other SSH tools). |\n| `-i` | Import a key from another format. |\n| `-m <format>` | The format for -e / -i / -p: RFC4716, PKCS8, PEM. |\n\n### known_hosts\n\n| Option | What it does |\n|---|---|\n| `-R <host>` | Remove all keys of a host from known_hosts. |\n| `-F <host>` | Find a host in known_hosts. |\n| `-H` | Hash the host names in known_hosts (hide which servers you use). |\n| `-r <host>` | Print a DNS SSHFP record for the host keys. |\n| `-g` | Use the generic DNS format with -r. |\n\n### Certificates and revocation\n\n| Option | What it does |\n|---|---|\n| `-s <ca_key>` | Sign a public key with a CA key (make a certificate). |\n| `-I <id>` | The certificate identity (with -s). |\n| `-h` | Make a host certificate instead of a user certificate. |\n| `-n <principals>` | The users or hosts the certificate is valid for. |\n| `-V <interval>` | How long the certificate is valid, like +52w. |\n| `-z <serial>` | The serial number of the certificate. |\n| `-L` | Print the content of a certificate. |\n| `-D <pkcs11>` | Get the public keys from a PKCS#11 library (smart card). |\n| `-U` | The CA key is in ssh-agent. |\n| `-k` | Make a Key Revocation List (KRL). |\n| `-u` | Update a KRL. |\n| `-Q` | Test if keys are in a KRL. |\n\n### Signing files and other\n\n| Option | What it does |\n|---|---|\n| `-Y sign, -Y verify, -Y check-novalidate, -Y find-principals, -Y match-principals` | Sign and verify files or git commits with SSH keys. |\n| `-M generate, -M screen` | Make and test moduli for Diffie-Hellman. |\n\n## Examples\n\n### Make a modern key pair\n\n```bash\nssh-keygen -t ed25519 -C \"student@server1\"\n```\n\n> Press Enter for the default file. Type a passphrase (recommended for people).\n\n### Make a key without passphrase for automation\n\n```bash\nssh-keygen -t ed25519 -N \"\" -f ~/.ssh/backup_job -C \"backup job server1\"\n```\n\n> For cron jobs and CI. Protect the file and limit the key on the server.\n\n### Make an RSA key for old systems\n\n```bash\nssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa_legacy\n```\n\n> Only when a device does not support ed25519.\n\n### Show the fingerprint of a key\n\n```bash\nssh-keygen -lf ~/.ssh/id_ed25519.pub\n```\n```text\n256 SHA256:Xh7… student@server1 (ED25519)\n```\n\n> Compare fingerprints when you check which key is allowed on a server.\n\n### Add or change a passphrase\n\n```bash\nssh-keygen -p -f ~/.ssh/id_ed25519\n```\n\n> The key stays the same; only its protection changes. No change needed on servers.\n\n### Remove an old host key after a server reinstall\n\n```bash\nssh-keygen -R server2\n```\n\n> Removes server2 from ~/.ssh/known_hosts. Verify the new key before you connect again.\n\n### Check a server's host key fingerprint on the server itself\n\n```bash\nssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub\n```\n\n> Compare with the fingerprint your client shows at the first connect.\n\n### Get the public key back from a private key\n\n```bash\nssh-keygen -y -f ~/.ssh/id_ed25519 > ~/.ssh/id_ed25519.pub\n```\n\n> When the .pub file is lost.\n\n### Find a host in known_hosts\n\n```bash\nssh-keygen -F server2\n```\n\n> Shows if (and which) key you trust for server2.\n\n### Recreate missing host keys on a server\n\n```bash\nssh-keygen -A\nsystemctl restart sshd\n```\n\n> Needed after a VM template was cleaned of its host keys.\n\n### Sign a user key with your CA (short-lived access)\n\n```bash\nssh-keygen -s ~/ca/user_ca -I alice-oncall -n alice -V +8h alice.pub\n```\n\n> Servers that trust the CA accept alice for 8 hours, without editing authorized_keys.\n\n### Sign a git commit file with an SSH key\n\n```bash\nssh-keygen -Y sign -f ~/.ssh/id_ed25519 -n file release.tar.gz\n```\n\n> Makes release.tar.gz.sig. Others verify it with `-Y verify`.\n\n## Watch out\n\n- Never copy or send the private key (the file without .pub).\n- Private key files must be mode 600; otherwise ssh refuses to use them.\n- DSA keys are disabled in new OpenSSH versions. Use ed25519.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.ssh/id_ed25519, ~/.ssh/id_ed25519.pub` | Default user key pair. |\n| `~/.ssh/known_hosts` | Trusted host keys. |\n| `/etc/ssh/ssh_host_*_key` | The host keys of the server. |\n\n## Tips\n\n- `ssh-keygen -t ed25519` → `ssh-copy-id host` → `ssh host`.\n- `-l` fingerprint, `-p` passphrase, `-R` forget a host.\n\nFull manual: [man ssh-keygen](https://linux.die.net/man/1/ssh-keygen)\n","page":"commands/ssh-keygen/","label":"ssh-keygen","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-ssh","n":1526,"slug":"ssh","title":"ssh — Log in to another system over an encrypted connection, run remote commands, and make secure tunnels","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["remote","login","secure shell","tunnel","port forwarding","jump host","keys"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Log in to another system over an encrypted connection, run remote commands, and make secure tunnels.\n\n## About\n\n`ssh` (Secure Shell client) connects to the SSH server (`sshd`) of another system. Everything is encrypted: passwords, commands, output and files. You log in with a password or — better — with a key.\n\nThe first time you connect, ssh shows the server's **host key fingerprint**. When you accept it, it is stored in `~/.ssh/known_hosts`; next time ssh checks that the server still has the same key, to stop man-in-the-middle attacks.\n\nYour own settings per host (user, port, key, jump host) go in `~/.ssh/config`; system-wide client settings in `/etc/ssh/ssh_config` and `/etc/ssh/ssh_config.d/`.\n\n## Syntax\n\n```text\nssh [OPTIONS] [USER@]HOST [COMMAND [ARGUMENTS]]\nssh [OPTIONS] ssh://[USER@]HOST[:PORT]\n```\n\n## Options\n\n\n### Connection\n\n| Option | What it does |\n|---|---|\n| `-p <port>` | Connect to this port (default 22). |\n| `-l <user>` | The user to log in as (same as user@host). |\n| `-i <file>` | Use this private key file. |\n| `-J <destination>` | Jump through this host first (bastion / jump host). |\n| `-F <file>` | Use this client configuration file. |\n| `-o <option>` | Set any ssh_config option, like `-o StrictHostKeyChecking=accept-new`. |\n| `-4 / -6` | Use only IPv4 / only IPv6. |\n| `-b <address>` | Use this local address for the connection. |\n| `-B <interface>` | Bind to the address of this interface. |\n| `-C` | Compress all data (helps on slow links). |\n| `-c <ciphers>` | Choose the encryption ciphers. |\n| `-m <macs>` | Choose the MAC (integrity) algorithms. |\n| `-E <file>` | Write debug logs to a file instead of stderr. |\n| `-v, -vv, -vvv` | Verbose debug output (more v = more detail). |\n| `-q` | Quiet: hide most warnings. |\n| `-V` | Show the version. |\n| `-G` | Print the final configuration for this host and exit. |\n| `-Q <query>` | List supported algorithms, like `-Q cipher` or `-Q key`. |\n| `-P <tag>` | A tag to select Match blocks in the config. |\n\n### Commands and terminal\n\n| Option | What it does |\n|---|---|\n| `-t` | Force a terminal (needed to run interactive programs like top or sudo remotely). |\n| `-T` | Do not allocate a terminal. |\n| `-n` | Read stdin from /dev/null (for use in background jobs). |\n| `-N` | Do not run a command (only for port forwarding). |\n| `-f` | Go to the background after login (with -N for tunnels). |\n| `-s` | Ask for a subsystem (like sftp) instead of a command. |\n| `-e <char>` | The escape character (default ~). `~.` ends a hanging session. |\n\n### Forwarding and tunnels\n\n| Option | What it does |\n|---|---|\n| `-L [<addr>:]<port>:<host>:<hostport>` | Local forward: a local port goes to host:hostport through the server. |\n| `-R [<addr>:]<port>:<host>:<hostport>` | Remote forward: a port on the server goes back to host:hostport on your side. |\n| `-D [<addr>:]<port>` | Dynamic forward: a local SOCKS proxy through the server. |\n| `-W <host>:<port>` | Forward stdin/stdout to host:port (used in ProxyCommand). |\n| `-w <local_tun>[:<remote_tun>]` | Make a tunnel device (layer 3 VPN). |\n| `-g` | Let other hosts connect to your local forwarded ports. |\n| `-A` | Forward your ssh-agent to the server (only to servers you trust). |\n| `-a` | Do not forward the agent. |\n| `-X` | Forward X11 (show remote graphical programs on your screen). |\n| `-Y` | Trusted X11 forwarding (fewer security checks). |\n| `-x` | Do not forward X11. |\n\n### Connection sharing and others\n\n| Option | What it does |\n|---|---|\n| `-M` | Master mode for connection sharing (ControlMaster). |\n| `-S <path>` | The control socket for connection sharing. |\n| `-O <cmd>` | Control a master connection: check, forward, cancel, exit, stop. |\n| `-K / -k` | Enable / disable GSSAPI (Kerberos) authentication and delegation. |\n| `-I <pkcs11>` | Use a PKCS#11 library (smart card) for keys. |\n| `-y` | Send log messages to syslog. |\n\n## Examples\n\n### Log in to a server\n\n```bash\nssh student@server2\n```\n\n> Type `exit` (or Ctrl+D) to leave.\n\n### Run one command on a remote server\n\n```bash\nssh server2 'df -h / && uptime'\n```\n\n> SSH runs it and ends. Quote the command so your local shell does not change it.\n\n### Use another port\n\n```bash\nssh -p 2222 admin@bastion.example.com\n```\n\n> Many teams move SSH away from port 22 to reduce noise in the logs.\n\n### Use a specific key\n\n```bash\nssh -i ~/.ssh/deploy_ed25519 deploy@app01\n```\n\n> Good when you have one key per project or environment.\n\n### Jump through a bastion host\n\n```bash\nssh -J admin@bastion.example.com admin@10.10.0.15\n```\n\n> Reach a server in a private network in one step. The bastion only forwards the connection.\n\n### Debug a login problem\n\n```bash\nssh -v student@server2\n```\n\n> Shows which keys are tried and why the server refuses them.\n\n### Open a local tunnel to a database\n\n```bash\nssh -N -L 5433:db.internal:5432 admin@bastion.example.com\n```\n\n> Then connect your tool to localhost:5433. The database port is never open to the internet.\n\n### Make a SOCKS proxy for your browser\n\n```bash\nssh -N -D 1080 admin@bastion.example.com\n```\n\n> Set the browser to SOCKS5 localhost:1080 to reach internal web tools.\n\n### Run an interactive command remotely\n\n```bash\nssh -t server2 'sudo journalctl -f'\n```\n\n> `-t` gives a terminal, so sudo can ask for a password and you can stop with Ctrl+C.\n\n### Accept a new host key automatically, but never a changed one\n\n```bash\nssh -o StrictHostKeyChecking=accept-new ansible@web03 true\n```\n\n> Good for first contact in automation. A changed key still stops the connection.\n\n### Start a graphical program from the server\n\n```bash\nssh -X student@server1 gnome-system-monitor\n```\n\n> The window opens on your desktop.\n\n### Copy a file through a pipe\n\n```bash\ncat id_ed25519.pub | ssh server2 'cat >> ~/.ssh/authorized_keys'\n```\n\n> What ssh-copy-id does for you (ssh-copy-id also fixes permissions).\n\n### Check the final settings for a host\n\n```bash\nssh -G server2 | grep -Ei '^(user|port|identityfile|proxyjump) '\n```\n\n> Shows what ssh will really use after reading all config files.\n\n### Close a frozen session\n\n```bash\nssh student@server2\n```\n```text\n(press Enter, then type ~.)\n```\n\n> The escape sequence ends the session when the network is gone.\n\n## Watch out\n\n- A \"host key changed\" warning can be an attack. Verify with the server owner before `ssh-keygen -R host`.\n- Agent forwarding (`-A`) lets the remote root use your keys while you are logged in. Use `-J` instead where you can.\n- Keep private keys private: permissions 600, a passphrase on keys for people.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.ssh/config` | Your settings per host (Host, HostName, User, Port, IdentityFile, ProxyJump). |\n| `~/.ssh/known_hosts` | The host keys of servers you trust. |\n| `~/.ssh/id_ed25519, ~/.ssh/id_rsa` | Your private keys (default names). |\n| `/etc/ssh/ssh_config, /etc/ssh/ssh_config.d/` | Client settings for all users. |\n\n## Tips\n\n- `ssh user@host 'command'` = remote command. `-p` port, `-i` key, `-J` jump, `-L` tunnel, `-v` debug.\n- Put repeated options in `~/.ssh/config` and type only `ssh name`.\n\nFull manual: [man ssh](https://linux.die.net/man/1/ssh)\n","page":"commands/ssh/","label":"ssh","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-sshd","n":1527,"slug":"sshd","title":"sshd — The OpenSSH server. Mostly used through systemctl; the command itself tests the configuration (-t, -T) and helps debugging","sections":["commands"],"topic":"Commands · SSH and remote work","tags":["ssh server","hardening","config test","port","root login","keys"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"The OpenSSH server. Mostly used through systemctl; the command itself tests the configuration (-t, -T) and helps debugging.\n\n## About\n\n`sshd` listens (default port 22) and handles SSH logins. On RHEL it runs as `sshd.service`. Settings are in `/etc/ssh/sshd_config` and the drop-in folder `/etc/ssh/sshd_config.d/` (first value wins). After a change, test with `sshd -t`, then `systemctl reload sshd`.\n\n`sshd -T` prints the **effective** settings after all files are read — the best way to check what really applies.\n\n## Syntax\n\n```text\nsshd [-46DdeGiqTtV] [-C SPEC] [-c CERT] [-E LOG] [-f CONFIG] [-g TIME] [-h KEY] [-o OPTION] [-p PORT]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `Port N` | Listen port. |\n| `PermitRootLogin yes\\|prohibit-password\\|no\\|forced-commands-only` | How root may log in. |\n| `PasswordAuthentication yes\\|no` | Allow passwords. |\n| `PubkeyAuthentication yes\\|no` | Allow keys. |\n| `AllowUsers, AllowGroups, DenyUsers, DenyGroups` | Who may log in. |\n| `MaxAuthTries N` | Tries per connection. |\n| `ClientAliveInterval S, ClientAliveCountMax N` | Keepalive and idle timeout. |\n| `X11Forwarding, AllowTcpForwarding` | Forwarding. |\n| `Banner FILE` | Text before login. |\n| `Match User\\|Group\\|Address …` | Settings for some users or networks. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t` | Test the config and keys; print errors only. |\n| `-T` | Test and print the effective config. |\n| `-C SPEC` | With -T: settings for a connection, like `user=lisa,addr=10.0.0.5,host=x`. |\n| `-G` | Print the config (without key checks). |\n| `-f FILE` | Use another config file. |\n| `-o OPTION` | Override one setting. |\n| `-p PORT` | Listen on this port. |\n| `-d` | Debug mode: one connection, verbose output, no daemon. |\n| `-D` | Do not detach (systemd runs it like this). |\n| `-e` | Debug logs to stderr. |\n| `-E FILE` | Append logs to a file. |\n| `-h FILE` | Host key file. |\n| `-c FILE` | Host certificate file. |\n| `-g SECONDS` | Login grace time. |\n| `-i` | Run from inetd. |\n| `-q` | Quiet. |\n| `-u LEN` | utmp host field length. |\n| `-4, -6` | IPv4 or IPv6 only. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Test the config\n\n```bash\nsshd -t\n```\n\n> No output = fine. Always run before a reload.\n\n### Show the effective settings\n\n```bash\nsshd -T | grep -iE '^(port|permitrootlogin|passwordauthentication|allowgroups)'\n```\n```text\nport 2022\npermitrootlogin no\n```\n\n> What really applies after all files.\n\n### Settings for one user and address\n\n```bash\nsshd -T -C user=lisa,addr=192.168.122.1,host=client | grep -i allowtcp\n```\n\n> Tests Match blocks.\n\n### Reload after a change\n\n```bash\nsystemctl reload sshd\n```\n\n> Existing sessions stay.\n\n### Debug a failing login\n\n```bash\n/usr/sbin/sshd -d -p 2222\n```\n\n> One test connection on port 2222 with full debug output: `ssh -p 2222 user@host`.\n\n### Status and logs\n\n```bash\nsystemctl status sshd\njournalctl -u sshd --since -1h\n```\n\n> Failed logins and errors.\n\n### New port: SELinux and firewall\n\n```bash\nsemanage port -a -t ssh_port_t -p tcp 2022\nfirewall-cmd --add-port=2022/tcp --permanent; firewall-cmd --reload\n```\n\n> Without these, sshd cannot use or expose the port.\n\n### Different rules for a network\n\n```bash\ncat /etc/ssh/sshd_config.d/40-match.conf\n```\n```text\nMatch Address 192.168.122.0/24\n    PasswordAuthentication yes\n```\n\n> Passwords only from the admin network.\n\n### Host key fingerprints (tell users)\n\n```bash\nfor k in /etc/ssh/ssh_host_*_key.pub; do ssh-keygen -lf \"$k\"; done\n```\n\n> Users compare these at the first connection.\n\n### Audit the SSH config (security)\n\n```bash\nsshd -T | grep -E '^(permitrootlogin|passwordauthentication|x11forwarding|maxauthtries|permitemptypasswords)'\n```\n\n> Quick hardening check against your policy.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/ssh/sshd_config` | Main config. |\n| `/etc/ssh/sshd_config.d/*.conf` | Drop-ins (read first; first value wins). |\n| `/etc/ssh/ssh_host_*_key` | Host keys. |\n| `~/.ssh/authorized_keys` | Allowed keys per user. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Config OK (with -t/-T). |\n| `255` | Error. |\n\nFull manual: [man sshd](https://linux.die.net/man/8/sshd)\n","page":"commands/sshd/","label":"sshd","where":"Command page · SSH and remote work","aka":[]},{"id":"cmd-stat","n":1528,"slug":"stat","title":"stat — Show all details of a file: size, blocks, inode, links, permissions, owner, SELinux context and all its times","sections":["commands"],"topic":"Commands · Files and folders","tags":["inode","timestamps","permissions","size","owner","file system"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show all details of a file: size, blocks, inode, links, permissions, owner, SELinux context and all its times.\n\n## About\n\n`stat` prints the inode information of a file. It shows more than `ls -l`: the permissions as numbers (0644), the inode number, the number of hard links, the device, and four times:\n\n- **Access**: last read.\n- **Modify**: last change of the data.\n- **Change**: last change of the inode (permissions, owner, links). It cannot be set with `touch`.\n- **Birth**: when the file was created (if the file system knows it).\n\nWith `-c FORMAT` you choose exactly what to print, which is great in scripts.\n\n## Syntax\n\n```text\nstat [OPTION]... FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-c, --format=<format>` | Print only these fields (see the format codes below). |\n| `--printf=<format>` | Like -c, but read \\n and \\t, and no new line at the end. |\n| `-L, --dereference` | Follow symbolic links. |\n| `-f, --file-system` | Show the status of the file system instead of the file. |\n| `-t, --terse` | Print everything on one line (for scripts). |\n| `--cached=<mode>` | How to use cached data on network file systems: always, never, default. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n### Format codes (for files)\n\n| Option | What it does |\n|---|---|\n| `%n` | File name. |\n| `%s` | Size in bytes. |\n| `%a` | Permissions as numbers (like 644). With %A: like -rw-r--r--. |\n| `%U / %u` | Owner name / owner number. |\n| `%G / %g` | Group name / group number. |\n| `%i` | Inode number. |\n| `%h` | Number of hard links. |\n| `%F` | File type (regular file, directory …). |\n| `%C` | SELinux context. |\n| `%x %y %z %w` | Access, modify, change and birth time (readable). |\n| `%X %Y %Z %W` | The same times as seconds since 1970. |\n| `%b %B` | Number of blocks, and the block size. |\n| `%d %D` | Device number (decimal / hex). |\n| `%N` | Quoted name, with the target for links. |\n\n### Format codes (with -f)\n\n| Option | What it does |\n|---|---|\n| `%T` | File system type (like xfs). |\n| `%S` | Block size. |\n| `%b / %f / %a` | Total / free / available blocks. |\n| `%c / %d` | Total / free inodes. |\n\n## Examples\n\n### Show all details of a file\n\n```bash\nstat /etc/passwd\n```\n\n> Size, inode, links, permissions (0644/-rw-r--r--), owner, SELinux context and 4 times.\n\n### Get the permissions as a number\n\n```bash\nstat -c '%a %n' /etc/shadow /etc/passwd\n```\n```text\n0 /etc/shadow\n644 /etc/passwd\n```\n\n> Easy to compare with the expected values in a compliance check.\n\n### Print owner and group\n\n```bash\nstat -c '%U:%G' /var/www/html\n```\n\n> A wrong owner explains many \"Permission denied\" errors.\n\n### See when a file really changed (incident response)\n\n```bash\nstat -c 'modify=%y change=%z' /usr/bin/sudo\n```\n\n> If \"change\" is recent but \"modify\" is old, someone may have reset the time with touch.\n\n### Count the hard links of a file\n\n```bash\nstat -c '%h' data.txt\n```\n\n> More than 1 means other names point to the same data.\n\n### Show the SELinux context\n\n```bash\nstat -c '%C' /var/www/html/index.html\n```\n```text\nunconfined_u:object_r:httpd_sys_content_t:s0\n```\n\n> Same information as `ls -Z`.\n\n### File system info: type and free inodes\n\n```bash\nstat -f /var\n```\n\n> Shows the type (xfs), block size, free blocks and free inodes.\n\n### Show the target of a link\n\n```bash\nstat -c '%N' /etc/localtime\n```\n```text\n'/etc/localtime' -> '../usr/share/zoneinfo/Asia/Riyadh'\n```\n\n> Here: the time zone of the system.\n\n### Age of a file in seconds (script)\n\n```bash\necho $(( $(date +%s) - $(stat -c %Y /var/backup/last.ok) ))\n```\n\n> Alert when the last backup flag file is too old.\n\n### Compare permissions of many files\n\n```bash\nstat -c '%a %U:%G %n' /etc/ssh/*\n```\n\n> The private host keys must be 600 (or 640 for group ssh_keys), owned by root.\n\n### One-line output for a script\n\n```bash\nstat -t /etc/hosts\n```\n\n> All fields in a fixed order on one line.\n\n## Watch out\n\n- \"Change\" (ctime) is not the creation time; it is the last change of the file's metadata.\n- Birth time can show \"-\" when the file system or kernel does not provide it.\n\n## Tips\n\n- `stat -c '%a %U:%G %n' FILE` = permissions, owner and name in one short line.\n\nFull manual: [man stat](https://linux.die.net/man/1/stat)\n","page":"commands/stat/","label":"stat","where":"Command page · Files and folders","aka":[]},{"id":"cmd-su","n":1529,"slug":"su","title":"su — Start a shell (or one command) as another user. You type the password of that user","sections":["commands"],"topic":"Commands · Users and groups","tags":["switch user","root shell","login shell","substitute user","environment"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Start a shell (or one command) as another user. You type the password of that user.\n\n## About\n\n`su` means \"substitute user\". Without a name it switches to root. You must know the password of the target user — only root can switch without a password.\n\nAlways use `su -` (or `su -l`): it starts a full login shell with the environment and home folder of the new user. Without the dash you keep your own PATH and variables, which can give strange results.\n\nOn RHEL, `sudo` is the better tool for admin work: it uses your own password and logs every command. `su` is still useful for root itself, to test what another user sees.\n\n## Syntax\n\n```text\nsu [OPTION]... [-] [USER [ARGUMENT]...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-, -l, --login` | Start a login shell: clean environment, home folder and profile of the new user. |\n| `-c, --command=COMMAND` | Run only this command as the user, then return. |\n| `-s, --shell=SHELL` | Use this shell instead of the user's login shell (works only if the shell is in /etc/shells or you are root). |\n| `-m, -p, --preserve-environment` | Keep your current environment (do not set HOME, SHELL, USER, LOGNAME). |\n| `-g, --group=GROUP` | Set the primary group (root only). |\n| `-G, --supp-group=GROUP` | Add a supplementary group (root only). |\n| `-P, --pty` | Create a new pseudo-terminal for the session (better isolation from the calling terminal). |\n| `-w, --whitelist-environment=LIST` | With --login, keep these variables (comma list). |\n| `-f, --fast` | Pass -f to the shell (only csh/tcsh: do not read the start files). |\n| `--session-command=COMMAND` | Like -c but without a new session (not recommended). |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Become root with a full login shell\n\n```bash\nsu -\n```\n```text\nPassword:\n```\n```bash\npwd\n```\n```text\n/root\n```\n\n> Type the root password. `exit` brings you back.\n\n### Switch to another user\n\n```bash\nsu - lisa\n```\n\n> You need lisa's password (root does not).\n\n### Run one command as root\n\n```bash\nsu -c 'dnf -y update' -\n```\n\n> Runs the command and returns to your normal shell.\n\n### Test what a user can see\n\n```bash\nsu - lisa -c 'ls -l /srv/share'\n```\n\n> root can check permissions as the user without knowing the password.\n\n### Use a shell for an account with nologin\n\n```bash\nsu -s /bin/bash -c 'whoami; id' apache\n```\n```text\napache\n```\n\n> Service accounts have /sbin/nologin; -s gives a shell for a test.\n\n### See the difference with and without the dash\n\n```bash\nsu\necho $HOME; pwd\n```\n```text\n/root\n/home/student\n```\n\n> Without `-` you stay in your folder with your environment.\n\n### Check which accounts may not log in\n\n```bash\nsu - carol\n```\n```text\nThis account is currently not available.\n```\n\n> The shell /sbin/nologin refuses the login.\n\n### Keep one variable with a login shell\n\n```bash\nsu - -w HTTPS_PROXY lisa\n```\n\n> Useful when a proxy setting must be kept.\n\n### Start the session in its own terminal (more secure)\n\n```bash\nsu -P - lisa\n```\n\n> Protects against tricks where a program writes into your terminal (TIOCSTI).\n\n### Find failed su attempts (security)\n\n```bash\ngrep 'su:' /var/log/secure | grep -i fail\n```\n\n> Many failures for root can mean somebody guesses passwords.\n\n### Allow su to root only for wheel members\n\n```bash\ngrep pam_wheel /etc/pam.d/su\n```\n```text\n#auth required pam_wheel.so use_uid\n```\n\n> Remove the `#` to allow `su` only for members of wheel (hardening).\n\n## Watch out\n\n- Without `-` the environment is not changed. Use `su -`.\n- On many cloud images root has no password, so `su -` fails. Use `sudo -i`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/pam.d/su` | PAM rules for su (like pam_wheel). |\n| `/var/log/secure` | Log of su and sudo use. |\n\n## Tips\n\n- su = password of the target user; sudo = your own password.\n- `sudo -i` gives the same root login shell as `su -`.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The command ran (or the exit code of the command run with -c). |\n| `1` | Error (for example a wrong password). |\n| `126` | The command could not be run. |\n| `127` | The command was not found. |\n\nFull manual: [man su](https://linux.die.net/man/1/su)\n","page":"commands/su/","label":"su","where":"Command page · Users and groups","aka":[]},{"id":"cmd-subscription-manager","n":1530,"slug":"subscription-manager","title":"subscription-manager — Register a RHEL system with Red Hat, and turn Red Hat repositories on or off","sections":["commands"],"topic":"Commands · Software packages","tags":["register","subscription","repository","rhel","activation key","updates","cdn"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Register a RHEL system with Red Hat, and turn Red Hat repositories on or off.\n\n## About\n\nA RHEL system must be registered to get software and updates from the Red Hat Content Delivery Network (CDN). `subscription-manager` connects the system to your Red Hat account (or your company's Satellite server) and manages the repositories it can use.\n\nToday Red Hat uses **Simple Content Access**: after registration the system can use the repositories at once. The old steps to \"attach\" a subscription are no longer needed.\n\nIn a company, systems are usually registered with an **organization ID and an activation key**, so no personal password is used in scripts.\n\n## Syntax\n\n```text\nsubscription-manager COMMAND [OPTIONS]\nsubscription-manager register [--username=USER] [--password=PASS] | [--org=ORG --activationkey=KEY]\nsubscription-manager repos [--list | --list-enabled | --enable=REPO | --disable=REPO]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `register` | Connect this system to Red Hat (or Satellite). |\n| `unregister` | Remove the system from the account and delete its identity. |\n| `status` | Show if the system is registered and its content access state. |\n| `identity` | Show the system ID and the organization. |\n| `repos` | List, enable and disable repositories. |\n| `refresh` | Get the newest data from the server. |\n| `release` | Pin the system to a minor release (like 10.0) or remove the pin. |\n| `syspurpose` | Set role, usage and service level of the system. |\n| `facts` | Show the hardware and system facts that are sent to the server. |\n| `config` | Show or change the settings in /etc/rhsm/rhsm.conf. |\n| `clean` | Remove all local subscription data (without telling the server). |\n| `version` | Show the versions of the tool and the server. |\n\n## Options\n\n\n### register\n\n| Option | What it does |\n|---|---|\n| `--username=<user>` | The Red Hat account name. |\n| `--password=<pass>` | The password (it is asked when you leave it out). |\n| `--org=<id>` | The organization ID, used with an activation key. |\n| `--activationkey=<key>` | Register with an activation key instead of a password. |\n| `--name=<name>` | The name of the system in the account (default: the hostname). |\n| `--force` | Register again, even if the system is already registered. |\n| `--serverurl=<url>` | The server to register with (for Satellite). |\n| `--baseurl=<url>` | The URL to get content from. |\n| `--release=<ver>` | Pin the system to a minor release while registering. |\n| `--environments=<list>` | Content environments (Satellite). |\n| `--proxy=<host:port>` | Use a proxy server. |\n| `--proxyuser=<user>, --proxypassword=<pass>` | Login for the proxy. |\n| `--insecure` | Do not check the SSL certificate of the server (only for tests). |\n\n### repos\n\n| Option | What it does |\n|---|---|\n| `--list` | List all repositories the system may use. |\n| `--list-enabled` | List only the enabled repositories. |\n| `--list-disabled` | List only the disabled repositories. |\n| `--enable=<repo>` | Enable a repository. Wildcards (*) are allowed. |\n| `--disable=<repo>` | Disable a repository. Wildcards (*) are allowed. |\n\n### release\n\n| Option | What it does |\n|---|---|\n| `--show` | Show the current release pin. |\n| `--list` | List the releases you can pin to. |\n| `--set=<ver>` | Pin to this release, like 10.0. |\n| `--unset` | Remove the pin and follow the newest minor release. |\n\n### general\n\n| Option | What it does |\n|---|---|\n| `-h, --help` | Show help (also after a command: `subscription-manager repos --help`). |\n\n## Examples\n\n### Register with your Red Hat account\n\n```bash\nsubscription-manager register --username your-rh-user\n```\n\n> The password is asked. After this, the BaseOS and AppStream repositories work.\n\n### Register with an activation key (for automation)\n\n```bash\nsubscription-manager register --org 1234567 --activationkey web-servers\n```\n\n> No personal password in scripts. The key decides which repositories and settings the system gets.\n\n### Check if the system is registered\n\n```bash\nsubscription-manager status\n```\n\n> Shows \"Content Access Mode is set to Simple Content Access\" on a working system.\n\n### List the enabled repositories\n\n```bash\nsubscription-manager repos --list-enabled\n```\n\n> Faster than the full list. Compare with `dnf repolist`.\n\n### Enable an extra repository\n\n```bash\nsubscription-manager repos --enable codeready-builder-for-rhel-10-x86_64-rpms\n```\n\n> CodeReady Builder has development packages that some build tools need.\n\n### Disable all repositories, then enable only what you need\n\n```bash\nsubscription-manager repos --disable '*' --enable rhel-10-for-x86_64-baseos-rpms --enable rhel-10-for-x86_64-appstream-rpms\n```\n\n> A small set of repositories means a smaller attack surface and predictable updates.\n\n### Pin a server to one minor release\n\n```bash\nsubscription-manager release --set=10.0\ndnf clean all\n```\n\n> Some certified software supports only one minor release. The pin stops updates to 10.1 until you remove it.\n\n### Show the system ID\n\n```bash\nsubscription-manager identity\n```\n\n> Needed when you open a support case or remove an old system from the account.\n\n### Register again after cloning a virtual machine\n\n```bash\nsubscription-manager clean\nsubscription-manager register --org 1234567 --activationkey web-servers --force\n```\n\n> A cloned VM keeps the identity of the original. Clean it so both systems are not counted as one.\n\n### Remove the system from the account before you delete it\n\n```bash\nsubscription-manager unregister\n```\n\n> Keeps the inventory in your Red Hat account clean.\n\n### Register through a proxy\n\n```bash\nsubscription-manager register --org 1234567 --activationkey web-servers --proxy proxy.example.com:3128\n```\n\n> Servers in a protected network often reach the internet only through a proxy.\n\n### Show which facts are sent to Red Hat\n\n```bash\nsubscription-manager facts --list | head -20\n```\n\n> Good to know for a privacy or security review.\n\n## Watch out\n\n- The exam systems are **not** registered. Software comes from repositories that the exam gives you.\n- `--password` on the command line ends up in the shell history. Leave it out so it is asked, or use an activation key.\n- After `clean`, the server still thinks the system exists. Use `unregister` when the system is still online.\n- Pinning a release stops security updates of newer minor releases. Remove the pin when the reason is gone.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/rhsm/rhsm.conf` | Settings: server, proxy, repository options. |\n| `/etc/pki/consumer/` | The identity certificate of the registered system. |\n| `/etc/yum.repos.d/redhat.repo` | The Red Hat repositories, written by subscription-manager. |\n| `/var/log/rhsm/rhsm.log` | Log file of the subscription tools. |\n\n## Tips\n\n- Order: **register → status → repos → dnf upgrade**.\n- Company servers: org + activation key. Your lab: username.\n\nFull manual: [man subscription-manager](https://linux.die.net/man/8/subscription-manager)\n","page":"commands/subscription-manager/","label":"subscription-manager","where":"Command page · Software packages","aka":[]},{"id":"cmd-sudo","n":1531,"slug":"sudo","title":"sudo — Run a command as root (or another user) with your own password, if the sudo rules allow it","sections":["commands"],"topic":"Commands · Users and groups","tags":["admin rights","root","privileges","wheel","sudoers","least privilege","audit"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run a command as root (or another user) with your own password, if the sudo rules allow it.\n\nSame page for: `sudoedit`\n\n## About\n\n`sudo` checks the rules in `/etc/sudoers` and `/etc/sudoers.d/`. When a rule allows you, it asks for **your** password (not root's) and runs the command. After that it remembers you for some minutes (on RHEL 5 minutes per terminal).\n\nEvery use is logged with your real name (in `/var/log/secure` and the journal). This makes sudo safer than a shared root password, and with small rules you can give people only the commands they need (least privilege).\n\nOn RHEL, members of the group `wheel` may run every command. `sudoedit` (= `sudo -e`) edits a file safely with your own editor.\n\n## Syntax\n\n```text\nsudo [OPTION]... COMMAND [ARGUMENT]...\nsudo -i | -s [COMMAND]\nsudo -l [-U USER] [COMMAND]\nsudo -e FILE...\nsudo -v | -k | -K\n```\n\n## Options\n\n\n### Run commands\n\n| Option | What it does |\n|---|---|\n| `-u, --user=USER` | Run as this user instead of root. |\n| `-g, --group=GROUP` | Run with this primary group. |\n| `-i, --login` | Start a login shell of the target user (like `su -`). |\n| `-s, --shell` | Start the shell from $SHELL (keeps more of your environment). |\n| `-E, --preserve-env` | Keep your environment variables (if the rules allow it). |\n| `--preserve-env=LIST` | Keep only these variables. |\n| `-H, --set-home` | Set HOME to the home folder of the target user (default on RHEL). |\n| `-D, --chdir=DIR` | Run the command in this folder (if allowed). |\n| `-R, --chroot=DIR` | Change the root folder before running (if allowed). |\n| `-b, --background` | Run the command in the background. |\n| `-P, --preserve-groups` | Keep your own group list. |\n| `-C, --close-from=NUM` | Close all open files from number NUM up. |\n| `-T, --command-timeout=TIME` | Stop the command after this time (if allowed). |\n| `-r, --role=ROLE` | SELinux role for the command. |\n| `-t, --type=TYPE` | SELinux type for the command. |\n| `--,` | End of sudo options; the rest belongs to the command. |\n\n### Edit files\n\n| Option | What it does |\n|---|---|\n| `-e, --edit` | Edit files as sudoedit: a temporary copy is opened in your editor, then copied back. |\n\n### Check rules\n\n| Option | What it does |\n|---|---|\n| `-l, --list` | Show what you may run (with a command: check that one). |\n| `-U, --other-user=USER` | With -l: show the rules of another user (root or allowed users). |\n| `-h, --host=HOST` | With -l: show rules for another host. |\n\n### Password and time stamp\n\n| Option | What it does |\n|---|---|\n| `-v, --validate` | Refresh the time stamp (asks the password) without running a command. |\n| `-k, --reset-timestamp` | Forget the password now (alone) or for this command. |\n| `-K, --remove-timestamp` | Remove all time stamps of the user. |\n| `-n, --non-interactive` | Never ask: fail if a password is needed (for scripts). |\n| `-S, --stdin` | Read the password from standard input. |\n| `-A, --askpass` | Use a helper program to ask the password (SUDO_ASKPASS). |\n| `-B, --bell` | Ring the bell at the password prompt. |\n| `-p, --prompt=TEXT` | Your own password prompt (%u = user, %h = host). |\n| `-N, --no-update` | Do not update the time stamp. |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `-V, --version` | Show the version (as root also the settings). |\n| `--help` | Show help. |\n\n## Examples\n\n### Run one admin command\n\n```bash\nsudo dnf install -y httpd\n```\n```text\n[sudo] password for student:\n```\n\n> Your own password. For the next minutes sudo does not ask again.\n\n### Get a root shell for many tasks\n\n```bash\nsudo -i\nwhoami\n```\n```text\nroot\n```\n\n> `exit` when you are done.\n\n### See what you may run\n\n```bash\nsudo -l\n```\n```text\nUser student may run the following commands on server1:\n    (ALL) ALL\n```\n\n> `(ALL) ALL` = full rights (wheel).\n\n### Check the rules of another user\n\n```bash\nsudo -l -U web1\n```\n\n> Test a new rule without logging in as that user.\n\n### Run a command as another user\n\n```bash\nsudo -u postgres psql -c 'select version();'\n```\n\n> Many service tasks must run as the service account.\n\n### Edit a system file safely\n\n```bash\nsudoedit /etc/ssh/sshd_config\n```\n\n> Your editor runs as you; only the save is done as root.\n\n### Write to a root file from a pipe\n\n```bash\necho \"192.168.122.20 server2\" | sudo tee -a /etc/hosts\n```\n\n> `sudo echo ... >> file` fails: the redirect is done by your own shell.\n\n### Use sudo in a script without hanging\n\n```bash\nsudo -n systemctl restart httpd || echo \"no rights or a password is needed\"\n```\n\n> -n never waits for a password (good for cron and CI).\n\n### Forget the password at once\n\n```bash\nsudo -k\n```\n\n> Use it before you leave your desk.\n\n### Audit who used sudo (security)\n\n```bash\njournalctl _COMM=sudo --since today\n```\n\n> Shows user, folder and command of every sudo call.\n\n### Find users with full rights\n\n```bash\ngetent group wheel\n```\n```text\nwheel:x:10:student,devops1\n```\n\n> Review this list often: everybody here is root.\n\n### Run a command with a time limit\n\n```bash\nsudo -T 5m /usr/local/bin/cleanup.sh\n```\n\n> Only works if the rule allows it (`Defaults user_command_timeouts`).\n\n## Watch out\n\n- `sudo cmd > /root/file` fails: use `| sudo tee file`.\n- A rule that allows an editor, `less`, `find` or a shell escape gives full root. Allow exact commands only.\n- With `NOPASSWD:` a stolen session gives root at once.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/sudoers` | The main rules (edit only with visudo). |\n| `/etc/sudoers.d/` | Extra rule files (one per team or app). |\n| `/var/log/secure` | Log of sudo use. |\n\n## Tips\n\n- Give rights to a group (`%webadmins`), not to single users.\n- Use full paths in rules and `visudo -c` after every change.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The command ran fine (or sudo -l/-v worked). |\n| `1` | sudo error (not allowed, wrong password, bad config). Otherwise the exit code of the command. |\n\nFull manual: [man sudo](https://linux.die.net/man/8/sudo)\n","page":"commands/sudo/","label":"sudo","where":"Command page · Users and groups","aka":["sudoedit"]},{"id":"cmd-swapon","n":1532,"slug":"swapon","title":"swapon — Switch swap spaces on (swapon) or off (swapoff), and show which are active","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["swap","memory","enable swap","priority"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Switch swap spaces on (swapon) or off (swapoff), and show which are active.\n\nSame page for: `swapoff`\n\n## About\n\n`swapon DEVICE` activates a swap partition, LV or file; `swapon -a` activates all swap lines from `/etc/fstab`. `swapon --show` lists active swap spaces with size, use and priority. `swapoff` moves the pages back to RAM and turns swap off — it needs enough free memory.\n\n## Syntax\n\n```text\nswapon [OPTION]... [DEVICE]...\nswapon --show\nswapoff [-va] [DEVICE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | All swap entries in /etc/fstab (not noauto). |\n| `--show[=COLUMNS]` | List active swap spaces (NAME, TYPE, SIZE, USED, PRIO). |\n| `-s, --summary` | Old summary format (like /proc/swaps). |\n| `-p, --priority N` | Priority (-1 … 32767); higher is used first. |\n| `-d, --discard[=once\\|pages]` | Use TRIM on SSDs. |\n| `-e, --ifexists` | Skip devices that do not exist (with -a). |\n| `-f, --fixpgsz` | Reinitialize when the page size does not match. |\n| `-L LABEL` | Use the swap space with this label. |\n| `-U UUID` | Use the swap space with this UUID. |\n| `-o, --options OPTS` | Options like in fstab (pri=, discard). |\n| `-T, --fstab PATH` | Use another fstab file. |\n| `--output-all` | All columns. |\n| `--noheadings` | No header line. |\n| `--raw` | Raw output. |\n| `--bytes` | Sizes in bytes. |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Switch on a swap partition\n\n```bash\nswapon /dev/vdb2\n```\n\n> Active until reboot.\n\n### Show active swap\n\n```bash\nswapon --show\n```\n```text\nNAME      TYPE      SIZE USED PRIO\n/dev/dm-1 partition   2G   0B   -2\n/dev/vdb2 partition   1G   0B   -3\n```\n\n> Lower PRIO is used later.\n\n### Switch on everything from fstab\n\n```bash\nswapon -a\n```\n\n> Tests your fstab swap lines.\n\n### Turn swap off\n\n```bash\nswapoff /dev/vdb2\n```\n\n> Needs enough free RAM.\n\n### Turn all swap off and on (empty it)\n\n```bash\nswapoff -a && swapon -a\n```\n\n> Moves swapped pages back to RAM. Check `free -h` first.\n\n### Use a fast disk first\n\n```bash\nswapon -p 10 /dev/nvme0n1p3\n```\n\n> Higher priority = used first.\n\n### Priority in fstab\n\n```bash\ngrep swap /etc/fstab\n```\n```text\nUUID=8f1b… none swap defaults,pri=10 0 0\n```\n\n> Kept after reboot.\n\n### By UUID or label\n\n```bash\nswapon -U 8f1b2c3d-…; swapon -L swap2\n```\n\n> Without the device name.\n\n### Discard on SSD\n\n```bash\nswapon -d /dev/nvme0n1p3\n```\n\n> Frees unused blocks on the SSD.\n\n### Check the kernel view\n\n```bash\ncat /proc/swaps\n```\n\n> Same data as swapon -s.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/fstab` | Swap lines (type swap). |\n| `/proc/swaps` | Active swap spaces. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n| `2` | Some swap spaces failed (with -a). |\n\nFull manual: [man swapon](https://linux.die.net/man/8/swapon)\n","page":"commands/swapon/","label":"swapon","where":"Command page · Disks and file systems","aka":["swapoff"]},{"id":"cmd-sysctl","n":1533,"slug":"sysctl","title":"sysctl — Read and change kernel parameters at run time (the files under /proc/sys), and load them from config files","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["kernel parameters","tuning","hardening","ip forward","swappiness","proc sys"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Read and change kernel parameters at run time (the files under /proc/sys), and load them from config files.\n\n## About\n\nEvery file in `/proc/sys/` is a kernel setting; the dots in the name are slashes: `net.ipv4.ip_forward` = `/proc/sys/net/ipv4/ip_forward`. `sysctl NAME` reads it, `sysctl -w NAME=VALUE` changes it until reboot. For permanent values put `NAME = VALUE` lines in a file in `/etc/sysctl.d/` and run `sysctl --system`.\n\n## Syntax\n\n```text\nsysctl [OPTION]... [NAME[=VALUE]]...\nsysctl -p [FILE]\nsysctl --system\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-w, --write` | Change a value: `-w NAME=VALUE`. |\n| `-a, --all` | Show all settings (-A, -X are aliases). |\n| `-p[FILE], --load[=FILE]` | Load settings from a file (default /etc/sysctl.conf). |\n| `--system` | Load all files from all sysctl.d folders. |\n| `-n, --values` | Only the values. |\n| `-N, --names` | Only the names. |\n| `-r, --pattern REGEX` | Only names that match. |\n| `-e, --ignore` | Ignore unknown names. |\n| `-q, --quiet` | Do not print values when setting. |\n| `-b, --binary` | Values without a newline. |\n| `--deprecated` | Also show deprecated names. |\n| `-f` | Same as -p. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Read a value\n\n```bash\nsysctl net.ipv4.ip_forward\n```\n```text\nnet.ipv4.ip_forward = 0\n```\n\n> Same as `cat /proc/sys/net/ipv4/ip_forward`.\n\n### Change it now\n\n```bash\nsysctl -w net.ipv4.ip_forward=1\n```\n\n> Lost at reboot.\n\n### Make it permanent\n\n```bash\necho \"net.ipv4.ip_forward = 1\" > /etc/sysctl.d/90-forward.conf\nsysctl --system\n```\n\n> Files are read in name order; later wins.\n\n### Only the value\n\n```bash\nsysctl -n vm.swappiness\n```\n```text\n60\n```\n\n> For scripts.\n\n### Search settings\n\n```bash\nsysctl -a 2>/dev/null | grep -i swappiness\n```\n\n> Or `sysctl -a -r swap`.\n\n### Lower swapping on a database server\n\n```bash\necho \"vm.swappiness = 10\" > /etc/sysctl.d/90-swap.conf; sysctl --system\n```\n\n> The kernel keeps more in RAM.\n\n### Network hardening (security)\n\n```bash\ncat /etc/sysctl.d/90-hardening.conf\n```\n```text\nnet.ipv4.conf.all.accept_redirects = 0\nnet.ipv4.conf.all.send_redirects = 0\nnet.ipv4.conf.all.rp_filter = 1\nnet.ipv4.tcp_syncookies = 1\nkernel.kptr_restrict = 2\nkernel.dmesg_restrict = 1\n```\n\n> Common CIS benchmark settings.\n\n### Load one file\n\n```bash\nsysctl -p /etc/sysctl.d/90-hardening.conf\n```\n\n> Test a new file.\n\n### More open files for big servers\n\n```bash\nsysctl -w fs.file-max=2097152\n```\n\n> System-wide limit (per-process limits are in limits.conf/systemd).\n\n### Where does a setting come from?\n\n```bash\ngrep -r swappiness /etc/sysctl.d/ /usr/lib/sysctl.d/ /etc/sysctl.conf\n```\n\n> tuned can also set values (check the tuned profile).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/sysctl.d/*.conf` | Your permanent settings. |\n| `/usr/lib/sysctl.d/` | Defaults from packages. |\n| `/etc/sysctl.conf` | Old main file (still read). |\n| `/proc/sys/` | The live values. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n| `255` | Unknown setting or permission denied. |\n\nFull manual: [man sysctl](https://linux.die.net/man/8/sysctl)\n","page":"commands/sysctl/","label":"sysctl","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-systemctl","n":1534,"slug":"systemctl","title":"systemctl — Control systemd: start, stop, enable and check services and other units, change targets, reboot and power off","sections":["commands"],"topic":"Commands · Services and boot","tags":["services","units","start","enable","status","targets","reboot","systemd","daemon"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Control systemd: start, stop, enable and check services and other units, change targets, reboot and power off.\n\n## About\n\n`systemctl COMMAND UNIT` is the main tool for services on RHEL. `start`/`stop`/`restart` act now; `enable`/`disable` decide what starts at boot; `status` shows the state and the last log lines. Without a type, a name means `.service`.\n\nIt also lists units, shows and edits unit files (`cat`, `edit`), sets the default boot target, and powers the system off or reboots it. With `--user` it manages the services of your own user session.\n\n## Syntax\n\n```text\nsystemctl [OPTION]... COMMAND [UNIT]...\n```\n\n## Parts\n\n\n### Units now\n\n| Part | Meaning |\n|---|---|\n| `start UNIT...` | Start units. |\n| `stop UNIT...` | Stop units. |\n| `restart UNIT...` | Stop and start. |\n| `try-restart UNIT...` | Restart only if running. |\n| `reload UNIT...` | Re-read the config without stopping. |\n| `reload-or-restart UNIT...` | Reload if possible, else restart. |\n| `kill UNIT...` | Send a signal to the processes of a unit. |\n| `clean UNIT...` | Remove cache, logs, state of a unit. |\n| `freeze / thaw UNIT...` | Pause / continue all processes of a unit. |\n| `isolate TARGET` | Start this target and stop all others. |\n\n### Boot\n\n| Part | Meaning |\n|---|---|\n| `enable UNIT...` | Start at boot (`--now` also starts now). |\n| `disable UNIT...` | Do not start at boot. |\n| `reenable UNIT...` | Disable and enable again (new links). |\n| `preset UNIT...` | Set enabled/disabled to the preset of the vendor. |\n| `mask UNIT...` | Block the unit completely (link to /dev/null). |\n| `unmask UNIT...` | Remove the block. |\n| `get-default` | Show the default boot target. |\n| `set-default TARGET` | Set the default boot target. |\n\n### State\n\n| Part | Meaning |\n|---|---|\n| `status [UNIT...\\|PID...]` | State, PID, cgroup and last log lines. |\n| `is-active UNIT...` | Print active/inactive (exit code 0 = active). |\n| `is-enabled UNIT...` | Print enabled/disabled/static/masked. |\n| `is-failed UNIT...` | Check for the failed state. |\n| `is-system-running` | Overall state: running, degraded … |\n| `list-units` | Loaded units (`-t` type, `--state`). |\n| `list-unit-files` | Installed unit files and their enable state. |\n| `list-dependencies UNIT` | Dependency tree. |\n| `list-sockets, list-timers, list-jobs, list-automounts, list-paths` | Lists of these types. |\n| `show UNIT` | All properties (`-p` some). |\n\n### Unit files\n\n| Part | Meaning |\n|---|---|\n| `cat UNIT...` | Show the unit file and drop-ins. |\n| `edit UNIT` | Create a drop-in override (`--full` = whole copy). |\n| `revert UNIT...` | Remove your overrides. |\n| `daemon-reload` | Reload all unit files after manual changes. |\n| `set-property UNIT NAME=VALUE...` | Change resource settings (like MemoryMax) at run time and keep them. |\n| `link FILE, add-wants, add-requires` | Other unit file commands. |\n| `set-environment, unset-environment, show-environment` | Environment of the manager. |\n\n### System\n\n| Part | Meaning |\n|---|---|\n| `reboot` | Reboot. |\n| `poweroff` | Power off. |\n| `halt` | Stop the system without power off. |\n| `suspend, hibernate, hybrid-sleep` | Sleep modes. |\n| `rescue` | Go to rescue mode. |\n| `emergency` | Go to emergency mode. |\n| `default` | Go to the default target. |\n| `soft-reboot` | Restart user space only (newer systemd). |\n| `reset-failed [UNIT...]` | Clear the failed state. |\n\n## Options\n\n\n### Select\n\n| Option | What it does |\n|---|---|\n| `-t, --type=TYPE` | Unit type: service, socket, target, timer, mount … |\n| `--state=STATE` | Only units in this state: running, failed, enabled … |\n| `-a, --all` | Also inactive units and empty properties. |\n| `--failed` | Same as --state=failed. |\n| `-r, --recursive` | Also units of containers. |\n| `--reverse` | With list-dependencies: who depends on this unit. |\n| `--after, --before` | With list-dependencies: order instead of needs. |\n| `--with-dependencies` | Also act on the dependencies. |\n\n### Output\n\n| Option | What it does |\n|---|---|\n| `-p, --property=NAME` | Only these properties with show. |\n| `-P NAME` | Only the value of one property. |\n| `--value` | With show: only the values. |\n| `-l, --full` | Do not cut long lines. |\n| `-n, --lines=N` | Number of log lines in status. |\n| `-o, --output=MODE` | Format of the log lines (short, json, cat …). |\n| `--plain` | Lists without tree lines. |\n| `--no-legend, --legend=BOOL` | Without header and footer. |\n| `--no-pager` | Do not use a pager. |\n| `--timestamp=FORMAT` | Format of time stamps. |\n| `--show-types` | Show the type of sockets. |\n| `-q, --quiet` | Less output. |\n| `--no-warn` | No warnings. |\n\n### Behaviour\n\n| Option | What it does |\n|---|---|\n| `--now` | With enable/disable/mask: also start or stop now. |\n| `--user` | Manage your user services. |\n| `--system` | Manage system services (default). |\n| `--global` | With enable: for all users. |\n| `--runtime` | Change only until the next reboot. |\n| `--no-block` | Do not wait for the job to finish. |\n| `--wait` | Wait until the started units end. |\n| `--job-mode=MODE` | How to handle other queued jobs. |\n| `-T, --show-transaction` | Show all jobs that a start adds. |\n| `--dry-run` | Show what would happen. |\n| `--no-reload` | With enable/disable: no daemon-reload. |\n| `--no-ask-password` | Do not ask for passwords. |\n| `--no-wall` | No message to users at reboot. |\n| `--message=TEXT` | A reason for reboot or poweroff (logged). |\n| `--when=TIME` | Plan a reboot or poweroff at this time (newer systemd). |\n| `-f, --force` | Force (with enable: overwrite links; reboot -ff: now). |\n| `-i, --check-inhibitors=MODE` | Respect or ignore inhibitor locks. |\n| `--fail` | With start: fail if other jobs conflict. |\n| `--kill-whom=main\\|control\\|all` | With kill: which processes. |\n| `-s, --signal=SIGNAL` | With kill: the signal. |\n| `--kill-value=INT` | A value with the signal. |\n| `--what=TYPE` | With clean: which data (cache, logs, state …). |\n| `--preset-mode=MODE` | With preset: full, enable-only, disable-only. |\n| `--firmware-setup` | Reboot into the UEFI setup. |\n| `--boot-loader-menu=TIME` | Show the boot menu at the next boot. |\n| `--boot-loader-entry=ID` | Boot this entry next time. |\n| `--reboot-argument=ARG` | Argument for the reboot. |\n| `--mkdir` | Create folders with bind/start. |\n| `--marked` | Act on units marked for restart. |\n| `--read-only` | Bind mounts read only. |\n| `--drop-in=NAME` | Name of the file for edit. |\n| `--root=PATH, --image=PATH, --image-policy=POLICY` | Work on another root or image. |\n| `-H, --host=HOST` | Control another host over SSH. |\n| `-M, --machine=NAME` | Control a container. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Start a service now and at boot\n\n```bash\nsystemctl enable --now httpd\n```\n```text\nCreated symlink '/etc/systemd/system/multi-user.target.wants/httpd.service' → '/usr/lib/systemd/system/httpd.service'.\n```\n\n> The RHCSA way: running now and after reboot.\n\n### Status of a service\n\n```bash\nsystemctl status sshd\n```\n\n> Loaded/enabled, Active, Main PID and the last 10 log lines.\n\n### Short checks for scripts\n\n```bash\nsystemctl is-active httpd; systemctl is-enabled httpd\n```\n```text\nactive\nenabled\n```\n\n> The exit code is 0 when the answer is yes.\n\n### Restart and reload\n\n```bash\nsystemctl restart httpd\nsystemctl reload httpd\n```\n\n> reload keeps open connections; not every service supports it.\n\n### Find failed services\n\n```bash\nsystemctl --failed\n```\n\n> Start here after a boot with problems.\n\n### List running services\n\n```bash\nsystemctl list-units -t service --state=running\n```\n\n> What runs now.\n\n### Which services start at boot?\n\n```bash\nsystemctl list-unit-files -t service --state=enabled\n```\n\n> Review this list: fewer services = smaller attack surface.\n\n### Override settings safely\n\n```bash\nsystemctl edit httpd\nsystemctl cat httpd\n```\n\n> The drop-in goes to /etc/systemd/system/httpd.service.d/.\n\n### Block a service\n\n```bash\nsystemctl mask --now cups\n```\n\n> Not even a dependency can start it.\n\n### Default boot target\n\n```bash\nsystemctl get-default\nsystemctl set-default multi-user.target\n```\n\n> Text mode at the next boot.\n\n### Show some properties\n\n```bash\nsystemctl show sshd -p MainPID,ActiveState,Restart\n```\n\n> Good for scripts and checks.\n\n### Limit memory of a service at run time\n\n```bash\nsystemctl set-property httpd MemoryMax=1G\n```\n\n> Kept after reboot (use --runtime for a test).\n\n### Rate the security of a service\n\n```bash\nsystemd-analyze security sshd\n```\n\n> Exposure score from 0 (safe) to 10 (unsafe).\n\n### Your own user services\n\n```bash\nsystemctl --user list-units -t service\n```\n\n> Services in your session (like podman containers with Quadlet).\n\n### Reboot with a reason\n\n```bash\nsystemctl reboot --message=\"Kernel update KB-2026-10\"\n```\n\n> The reason goes to the journal.\n\n## Watch out\n\n- `start` alone does not survive a reboot; `enable` alone does not start now.\n- After editing unit files by hand, run `systemctl daemon-reload`.\n- `isolate` stops everything that is not part of the target (also your SSH session in rescue).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/lib/systemd/system/` | Unit files from packages. |\n| `/etc/systemd/system/` | Your units, overrides and enable links. |\n| `/run/systemd/system/` | Run-time units. |\n| `~/.config/systemd/user/` | User units. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (is-active/is-enabled: yes). |\n| `1` | Failed or not active/enabled (also used by status for a dead unit with a pid file). |\n| `3` | status: the unit is not active. |\n| `4` | No such unit. |\n\nFull manual: [man systemctl](https://linux.die.net/man/1/systemctl)\n","page":"commands/systemctl/","label":"systemctl","where":"Command page · Services and boot","aka":[]},{"id":"cmd-systemd-analyze","n":1535,"slug":"systemd-analyze","title":"systemd-analyze — Analyze systemd: boot time, slow units, unit file errors, timer expressions and the security of services","sections":["commands"],"topic":"Commands · Services and boot","tags":["boot time","blame","critical chain","security","unit verify","calendar"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Analyze systemd: boot time, slow units, unit file errors, timer expressions and the security of services.\n\n## About\n\n`systemd-analyze` shows how long the boot took (`time`), which units took the most time (`blame`) and which chain of units was the slowest (`critical-chain`). It also checks unit files for errors (`verify`), tests calendar expressions for timers (`calendar`) and rates the sandboxing of services (`security`).\n\n## Syntax\n\n```text\nsystemd-analyze [OPTION]... [COMMAND] [ARGUMENT]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `time` | Boot time in kernel, initrd and user space. |\n| `blame` | Units sorted by start time. |\n| `critical-chain [UNIT]` | The slowest chain of dependencies. |\n| `plot` | SVG picture of the boot. |\n| `dot [PATTERN]` | Dependency graph in dot format. |\n| `verify FILE...` | Check unit files for errors. |\n| `security [UNIT]` | Rate the security settings of services. |\n| `calendar SPEC...` | Test an OnCalendar= expression. |\n| `timestamp TIME...` | Parse a time stamp. |\n| `timespan SPAN...` | Parse a time span. |\n| `cat-config NAME\\|PATH` | Show a config file with all its drop-ins. |\n| `unit-paths` | Folders where unit files are searched. |\n| `unit-files` | List unit files and aliases. |\n| `condition CONDITION...` | Test Condition=/Assert= lines. |\n| `exit-status [STATUS]` | Exit status names and numbers. |\n| `capability [CAP]` | Capability names and numbers. |\n| `syscall-filter [SET]` | System call sets for SystemCallFilter=. |\n| `filesystems [SET]` | File system sets. |\n| `dump` | Full state of the manager (debug). |\n| `compare-versions V1 [OP] V2` | Compare version strings. |\n| `inspect-elf, malloc, fdstore, image-policy, pcrs, srk` | Special tools. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--system` | The system manager (default). |\n| `--user` | The user manager. |\n| `--global` | Global user settings. |\n| `--order, --require` | With dot: only order or only need links. |\n| `--from-pattern=, --to-pattern=` | With dot: filter the graph. |\n| `--fuzz=TIME` | With critical-chain: also show units within this time. |\n| `--man=no` | With verify: do not check man pages. |\n| `--generators` | With verify: also run generators. |\n| `--recursive-errors=MODE` | With verify: also check dependencies. |\n| `--threshold=N` | With security: fail above this score. |\n| `--security-policy=PATH` | With security: own policy file. |\n| `--offline=BOOL` | With security: check the files, not the running units. |\n| `--profile=PATH` | With security/verify: use a portable profile. |\n| `--iterations=N` | With calendar: show N next times. |\n| `--base-time=TIME` | With calendar: start from this time. |\n| `--unit=UNIT` | With condition: use this unit. |\n| `--json=MODE` | JSON output (pretty, short, off). |\n| `--table` | Table output. |\n| `--no-legend` | No header. |\n| `--root=PATH, --image=PATH, --image-policy=POLICY` | Work on another root or image. |\n| `-H, --host=HOST` | Remote host over SSH. |\n| `-M, --machine=NAME` | A container. |\n| `-q, --quiet` | Less output. |\n| `--tldr` | With cat-config: without comments. |\n| `--no-pager` | Do not use a pager. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### How long did the boot take?\n\n```bash\nsystemd-analyze\n```\n```text\nStartup finished in 1.204s (kernel) + 2.810s (initrd) + 9.512s (userspace) = 13.527s\nmulti-user.target reached after 9.480s in userspace.\n```\n\n> Same as `systemd-analyze time`.\n\n### Which units are slow?\n\n```bash\nsystemd-analyze blame | head\n```\n\n> Units that start in parallel can still look slow here.\n\n### The slowest chain\n\n```bash\nsystemd-analyze critical-chain\n```\n\n> The real reason for a slow boot.\n\n### Check a unit file for errors\n\n```bash\nsystemd-analyze verify /etc/systemd/system/hello.service\n```\n\n> Shows typos and unknown settings before you start it.\n\n### Test a timer expression\n\n```bash\nsystemd-analyze calendar --iterations=3 'Mon..Fri 02:30'\n```\n\n> Shows the next 3 run times.\n\n### Rate service security (DevSecOps)\n\n```bash\nsystemd-analyze security\nsystemd-analyze security httpd.service\n```\n\n> Lists missing hardening settings with a score.\n\n### Fail a CI check on weak settings\n\n```bash\nsystemd-analyze security --offline=true --threshold=5 ./myapp.service\n```\n\n> Exit code not 0 when the score is above 5.\n\n### Picture of the boot\n\n```bash\nsystemd-analyze plot > boot.svg\n```\n\n> Open it in a browser.\n\n### See a config with all drop-ins\n\n```bash\nsystemd-analyze cat-config systemd/journald.conf\n```\n\n> The real settings, from all files.\n\n### Time spans and exit codes\n\n```bash\nsystemd-analyze timespan 1h30m; systemd-analyze exit-status 203\n```\n\n> 203 = EXEC: the program in ExecStart could not be run.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (also verify problems or a score above --threshold). |\n\nFull manual: [man systemd-analyze](https://linux.die.net/man/1/systemd-analyze)\n","page":"commands/systemd-analyze/","label":"systemd-analyze","where":"Command page · Services and boot","aka":[]},{"id":"cmd-systemd-cgls","n":1536,"slug":"systemd-cgls","title":"systemd-cgls — Show the control groups (cgroups) as a tree with their processes (systemd-cgls), or their live resource use (systemd-cgtop)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["cgroups","slices","services","resource use","systemd"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the control groups (cgroups) as a tree with their processes (systemd-cgls), or their live resource use (systemd-cgtop).\n\nSame page for: `systemd-cgtop`\n\n## About\n\nsystemd puts every service, user session and container in its own cgroup, grouped in slices: `system.slice`, `user.slice`, `machine.slice`. `systemd-cgls` shows which process belongs to which service — useful when ps does not tell you where a process comes from. `systemd-cgtop` shows CPU, memory and I/O per cgroup, like top.\n\n## Syntax\n\n```text\nsystemd-cgls [OPTION]... [CGROUP...]\nsystemd-cgls [OPTION]... --unit|--user-unit [UNIT...]\nsystemd-cgtop [OPTION]... [GROUP]\n```\n\n## Options\n\n\n### systemd-cgls\n\n| Option | What it does |\n|---|---|\n| `-u, --unit [UNIT]` | Show the cgroup of these units. |\n| `--user-unit [UNIT]` | Show user units. |\n| `-a, --all` | Also empty cgroups. |\n| `-l, --full` | Do not cut lines. |\n| `-k` | Also kernel threads. |\n| `-x, --xattr=yes\\|no` | Show extended attributes. |\n| `-c, --cgroup-id=yes\\|no` | Show cgroup IDs. |\n| `-M, --machine=NAME` | Show a container or VM. |\n| `--no-pager` | Do not use a pager. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n### systemd-cgtop\n\n| Option | What it does |\n|---|---|\n| `-c, --order=cpu` | Sort by CPU. |\n| `-m, --order=memory` | Sort by memory. |\n| `-i, --order=io` | Sort by I/O. |\n| `-t, --order=tasks` | Sort by number of tasks. |\n| `-p, --order=path` | Sort by path. |\n| `-b, --batch` | Batch mode (for pipes). |\n| `-n, --iterations=N` | Stop after N updates. |\n| `-d, --delay=SECONDS` | Time between updates. |\n| `-1` | One update only. |\n| `-r, --raw` | Raw numbers. |\n| `-P` | Count only user processes. |\n| `-k` | Count all processes, also kernel threads. |\n| `--cpu=percentage\\|time` | Show CPU as percent or time. |\n| `--depth=N` | How deep the tree goes. |\n| `--recursive=yes\\|no` | Include child cgroups in the numbers. |\n\n## Examples\n\n### The whole tree\n\n```bash\nsystemd-cgls --no-pager | head -25\n```\n\n> Slices, services and their processes.\n\n### One service\n\n```bash\nsystemd-cgls -u httpd.service\n```\n\n> All processes of the web server, also CGI children.\n\n### Where does this process come from?\n\n```bash\nsystemctl status 4123\n```\n\n> Shows the unit and cgroup of a PID.\n\n### Your own session\n\n```bash\nsystemd-cgls -u user-1000.slice\n```\n\n> Everything you started.\n\n### Live resource use per service\n\n```bash\nsystemd-cgtop\n```\n\n> Press m, c, i to sort.\n\n### Sorted by memory, one snapshot\n\n```bash\nsystemd-cgtop -m -1 -b | head\n```\n\n> For a report.\n\n### Container processes\n\n```bash\nsystemd-cgls -u machine.slice\n```\n\n> VMs and containers.\n\n### Cgroup of a process directly\n\n```bash\ncat /proc/4123/cgroup\n```\n```text\n0::/system.slice/httpd.service\n```\n\n> cgroup v2 path.\n\n### Limit a service with cgroups\n\n```bash\nsystemctl set-property httpd.service MemoryMax=1G CPUQuota=50%\n```\n\n> Resource limits are set on the cgroup.\n\n### Find hidden processes of a service (security)\n\n```bash\nsystemd-cgls -u sshd.service\n```\n\n> A strange child (like a miner) under a service shows up here.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man systemd-cgls](https://linux.die.net/man/1/systemd-cgls)\n","page":"commands/systemd-cgls/","label":"systemd-cgls","where":"Command page · Processes and jobs","aka":["systemd-cgtop"]},{"id":"cmd-systemd-run","n":1537,"slug":"systemd-run","title":"systemd-run — Run a command as a temporary (transient) systemd service or scope — now, later with a timer, or with resource limits","sections":["commands"],"topic":"Commands · Scheduling","tags":["transient unit","timer","scope","resource limit","background job"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Run a command as a temporary (transient) systemd service or scope — now, later with a timer, or with resource limits.\n\n## About\n\n`systemd-run COMMAND` starts the command as a new service unit (like `run-u123.service`). It runs in the background, is managed by systemd, and its output goes to the journal. No unit file is needed.\n\nWith timer options (`--on-calendar`, `--on-active`) it works like `at` with logging. With `-p` you set unit properties, for example memory or CPU limits. `--scope` runs the command in the foreground in its own cgroup.\n\n## Syntax\n\n```text\nsystemd-run [OPTION]... COMMAND [ARGUMENT]...\n```\n\n## Options\n\n\n### Unit\n\n| Option | What it does |\n|---|---|\n| `-u, --unit=NAME` | The name of the unit. |\n| `-p, --property=NAME=VALUE` | Set a unit property, like MemoryMax=500M. |\n| `--description=TEXT` | Description of the unit. |\n| `--slice=SLICE` | Put the unit in this slice. |\n| `--slice-inherit` | Use the slice of the caller. |\n| `--scope` | Run as a scope in the foreground (not a service). |\n| `--service-type=TYPE` | Type= of the service (simple, oneshot …). |\n| `-r, --remain-after-exit` | Keep the unit after the command ends. |\n| `--send-sighup` | Send SIGHUP after SIGTERM when stopping. |\n| `-G, --collect` | Remove the unit also after a failure. |\n\n### Run as / environment\n\n| Option | What it does |\n|---|---|\n| `--uid=USER, --gid=GROUP` | Run as this user and group. |\n| `--nice=N` | Nice value. |\n| `--working-directory=DIR` | The working folder. |\n| `-d, --same-dir` | Use the current folder. |\n| `-E, --setenv=NAME[=VALUE]` | Set an environment variable. |\n| `--expand-environment=BOOL` | Expand variables in the command. |\n\n### Input and output\n\n| Option | What it does |\n|---|---|\n| `-t, --pty` | Connect to the terminal (interactive). |\n| `-P, --pipe` | Connect stdin/stdout/stderr to the caller. |\n| `-S, --shell` | Start an interactive shell as a service. |\n| `-q, --quiet` | Less output. |\n| `--wait` | Wait until the command ends and show its result. |\n| `--no-block` | Do not wait for the start. |\n\n### Timers and triggers\n\n| Option | What it does |\n|---|---|\n| `--on-active=TIME` | Run this long from now. |\n| `--on-boot=TIME, --on-startup=TIME` | Time after boot / after systemd start. |\n| `--on-unit-active=TIME, --on-unit-inactive=TIME` | Relative to the unit's last activity. |\n| `--on-calendar=SPEC` | Run at calendar times, like `*-*-* 02:00`. |\n| `--on-clock-change, --on-timezone-change` | Run when the clock or time zone changes. |\n| `--timer-property=, --path-property=, --socket-property=` | Properties for the timer, path or socket unit. |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `--user` | Use the user manager. |\n| `--system` | Use the system manager (default). |\n| `--no-ask-password` | No password questions. |\n| `-H, --host=HOST` | Remote host over SSH. |\n| `-M, --machine=NAME` | A container. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Run a command as a background service\n\n```bash\nsystemd-run --unit=reindex /usr/local/bin/reindex.sh\n```\n```text\nRunning as unit: reindex.service\n```\n\n> Survives logout; follow it with `journalctl -u reindex -f`.\n\n### Run once in 30 minutes\n\n```bash\nsystemd-run --on-active=30m /usr/bin/systemctl restart httpd\n```\n\n> Like at, but logged in the journal.\n\n### Run every day at 02:00 (until reboot)\n\n```bash\nsystemd-run --on-calendar='*-*-* 02:00' --unit=nightly /usr/local/bin/report.sh\n```\n\n> A quick timer for a test; write unit files for permanent ones.\n\n### Limit memory and CPU of a command\n\n```bash\nsystemd-run --scope -p MemoryMax=500M -p CPUQuota=50% tar -czf /tmp/big.tgz /data\n```\n\n> The command cannot take the whole server.\n\n### Run as another user\n\n```bash\nsystemd-run --uid=lisa --pty --wait /usr/bin/whoami\n```\n```text\nlisa\n```\n\n> Clean environment, like a service.\n\n### See the result of a command\n\n```bash\nsystemd-run --wait --pipe /usr/bin/false\n```\n\n> Shows the exit code and run time.\n\n### Your own long task without root\n\n```bash\nsystemd-run --user --unit=longtask ./long-task.sh\njournalctl --user -u longtask -f\n```\n\n> Better than nohup.\n\n### Test hardening settings on a command (DevSecOps)\n\n```bash\nsystemd-run --pty -p ProtectSystem=strict -p PrivateTmp=yes /usr/bin/bash\n```\n\n> Try what a service could do with these settings.\n\n### List and stop transient units\n\n```bash\nsystemctl list-units 'run-*'\nsystemctl stop reindex\n```\n\n> They disappear after they end.\n\n### Check running timers made by systemd-run\n\n```bash\nsystemctl list-timers\n```\n\n> Transient timers show up like normal ones.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done (with --wait: the exit status of the command). |\n| `1` | Error. |\n\nFull manual: [man systemd-run](https://linux.die.net/man/1/systemd-run)\n","page":"commands/systemd-run/","label":"systemd-run","where":"Command page · Scheduling","aka":[]},{"id":"cmd-tail","n":1538,"slug":"tail","title":"tail — Show the last lines of files, and follow a log file live while it grows","sections":["commands"],"topic":"Commands · Text and search","tags":["last lines","log","follow","live","monitor"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the last lines of files, and follow a log file live while it grows.\n\n## About\n\n`tail` prints the last 10 lines of a file. With `-n` you choose how many. With `-f` (follow) it keeps the file open and prints new lines as they come: the classic way to watch a log while you test something.\n\n`-F` is like `-f`, but it also keeps working when the log file is rotated (renamed and recreated).\n\n## Syntax\n\n```text\ntail [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n, --lines=[+]<num>` | Print the last NUM lines. With a plus: start at line NUM. |\n| `-f, --follow[={name\\|descriptor}]` | Keep printing new lines as the file grows. |\n| `-F` | Same as --follow=name --retry: keep following after log rotation. |\n| `-c, --bytes=[+]<num>` | Print the last NUM bytes (or from byte NUM with +). |\n| `--retry` | Keep trying to open a file that is not there (yet). |\n| `--pid=<pid>` | With -f: stop when this process ends. |\n| `-s, --sleep-interval=<n>` | With -f: check every N seconds. |\n| `--max-unchanged-stats=<n>` | With --follow=name: reopen the file after N checks without change. |\n| `-q, --quiet, --silent` | Do not print file name headers. |\n| `-v, --verbose` | Always print file name headers. |\n| `-z, --zero-terminated` | Lines end with NUL. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### The last 10 lines\n\n```bash\ntail /var/log/messages\n```\n\n> The newest events of a log.\n\n### The last 50 lines\n\n```bash\ntail -n 50 /var/log/secure\n```\n\n> Recent logins and sudo use.\n\n### Watch a log live\n\n```bash\ntail -f /var/log/messages\n```\n\n> Press Ctrl+C to stop. Open it in a second terminal while you restart a service.\n\n### Follow a log that is rotated\n\n```bash\ntail -F /var/log/httpd/access_log\n```\n\n> Keeps following when logrotate makes a new file.\n\n### Watch several logs at once\n\n```bash\ntail -f /var/log/secure /var/log/messages\n```\n\n> A header shows which file each line comes from.\n\n### Watch only errors\n\n```bash\ntail -f /var/log/messages | grep --line-buffered -i error\n```\n\n> `--line-buffered` makes grep print each line at once.\n\n### Everything from line 100 to the end\n\n```bash\ntail -n +100 big.log\n```\n\n> Skip a header or the first part.\n\n### Remove the header line of a CSV\n\n```bash\ntail -n +2 export.csv > data-only.csv\n```\n\n> Starts at line 2.\n\n### Stop following when a process ends\n\n```bash\ntail -f build.log --pid=$(pgrep -n make)\n```\n\n> Ends by itself when the build is done.\n\n### Follow failed SSH logins live\n\n```bash\ntail -F /var/log/secure | grep --line-buffered \"Failed password\"\n```\n\n> Live view of a brute-force attack.\n\n### Watch a deployment log in a container host\n\n```bash\ntail -f /var/log/containers/web-*.log\n```\n\n> Same idea works for any app that writes to files.\n\n## Watch out\n\n- On RHEL, many services log only to the journal. If a file in /var/log stays empty, use `journalctl -f`.\n- `tail -f` on a rotated file stops showing new lines; use `-F`.\n\n## Tips\n\n- `tail -f` = live. `tail -F` = live, survives rotation. `tail -n +2` = skip the first line.\n\nFull manual: [man tail](https://linux.die.net/man/1/tail)\n","page":"commands/tail/","label":"tail","where":"Command page · Text and search","aka":[]},{"id":"cmd-tar","n":1539,"slug":"tar","title":"tar — Pack files and folders into one archive file (and unpack it), with optional compression","sections":["commands"],"topic":"Commands · Archive and compress","tags":["archive","backup","compress","gzip","bzip2","xz","extract","restore"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Pack files and folders into one archive file (and unpack it), with optional compression.\n\n## About\n\n`tar` (tape archiver) collects many files, with their folders, permissions, owners and times, into one `.tar` file. With a compression option it also compresses the archive: `.tar.gz` (gzip), `.tar.bz2` (bzip2) or `.tar.xz` (xz).\n\nEvery tar command has one **main mode**: `-c` create, `-x` extract, `-t` list (and some others). `-f FILE` names the archive; put it last in a group of letters (`-czf backup.tar.gz`).\n\nGNU tar removes the leading `/` from names when it creates an archive. So extracting in another folder never overwrites the real system files by accident.\n\nOn extract, tar finds the compression by itself: `tar -xf file.tar.xz` works without `-J`.\n\n## Syntax\n\n```text\ntar -c [-f ARCHIVE] [OPTIONS] FILE...\ntar -x [-f ARCHIVE] [OPTIONS] [MEMBER...]\ntar -t [-f ARCHIVE] [OPTIONS] [MEMBER...]\ntar {-r|-u} -f ARCHIVE [OPTIONS] FILE...\n```\n\n## Options\n\n\n### Main modes (pick one)\n\n| Option | What it does |\n|---|---|\n| `-c, --create` | Create a new archive. |\n| `-x, --extract, --get` | Extract files from an archive. |\n| `-t, --list` | List the content of an archive. |\n| `-r, --append` | Add files to the end of an archive (not for compressed archives). |\n| `-u, --update` | Add only files that are newer than the copy in the archive. |\n| `-A, --catenate, --concatenate` | Add other tar files to an archive. |\n| `-d, --diff, --compare` | Compare the archive with the files on disk. |\n| `--delete` | Remove members from an archive (not on tapes, not compressed). |\n| `--test-label` | Test the volume label of the archive and exit. |\n\n### Archive file and compression\n\n| Option | What it does |\n|---|---|\n| `-f, --file=<archive>` | The archive file (or device). `-` = stdin/stdout. |\n| `-z, --gzip` | Compress or decompress with gzip (.tar.gz, .tgz). |\n| `-j, --bzip2` | Compress or decompress with bzip2 (.tar.bz2). |\n| `-J, --xz` | Compress or decompress with xz (.tar.xz). |\n| `--zstd` | Compress with zstd (.tar.zst). |\n| `-a, --auto-compress` | Choose the compression from the archive name's ending. |\n| `-I, --use-compress-program=<prog>` | Use this program for compression, like `-I 'zstd -19'` or `pigz`. |\n| `--lzma, --lzip, --lzop` | Use these other compressors. |\n| `-Z, --compress` | Use the old `compress` program (.tar.Z). |\n| `--no-auto-compress` | Do not guess the compression from the name. |\n\n### Show information\n\n| Option | What it does |\n|---|---|\n| `-v, --verbose` | List the files while working. -vv shows details like ls -l. |\n| `--totals[=<signal>]` | Print the total number of bytes at the end. |\n| `--checkpoint[=<n>]` | Show a progress message every N records. |\n| `--checkpoint-action=<action>` | What to do at each checkpoint (like dot or echo). |\n| `--index-file=<file>` | Write the verbose list to this file. |\n| `--full-time` | Show file times with full resolution. |\n| `--utc` | Show times in UTC. |\n| `-R, --block-number` | Show the block number of each message. |\n| `--show-defaults` | Show the default settings of tar. |\n| `--show-omitted-dirs` | Show folders that are skipped. |\n| `--show-transformed-names, --show-stored-names` | Show names after --transform. |\n| `-w, --interactive, --confirmation` | Ask for each file. |\n| `--warning=<keyword>` | Turn certain warnings on or off. |\n\n### Choose files\n\n| Option | What it does |\n|---|---|\n| `-C, --directory=<dir>` | Change to this folder first (for extract: extract into it). |\n| `--exclude=<pattern>` | Leave out files that match the pattern. |\n| `-X, --exclude-from=<file>` | Read exclude patterns from a file. |\n| `--exclude-vcs` | Leave out .git, .svn and other version control folders. |\n| `--exclude-vcs-ignores` | Use the ignore files of git and others as exclude lists. |\n| `--exclude-backups` | Leave out backup and lock files. |\n| `--exclude-caches, --exclude-caches-all, --exclude-caches-under` | Leave out cache folders that have a CACHEDIR.TAG file. |\n| `--exclude-tag=<file>, --exclude-tag-all=<file>, --exclude-tag-under=<file>` | Leave out folders that contain this file. |\n| `--exclude-ignore=<file>, --exclude-ignore-recursive=<file>` | Read exclude patterns per folder from this file. |\n| `-T, --files-from=<file>` | Read the names to archive or extract from a file. |\n| `--null` | With -T: names are separated by NUL bytes (works with find -print0). |\n| `--no-null` | Turn off --null again. |\n| `--add-file=<file>` | Add this file, even if its name starts with a dash. |\n| `--no-recursion` | Do not go into folders. |\n| `--recursion` | Go into folders (the default). |\n| `--one-file-system` | Stay on the file system where you started. |\n| `-N, --newer, --after-date=<date\\|file>` | Only files newer than this date. |\n| `--newer-mtime=<date>` | Only files whose data changed after this date. |\n| `-K, --starting-file=<member>` | Begin at this member of the archive. |\n| `--occurrence[=<n>]` | Use only the Nth copy of a member. |\n| `-h, --dereference` | Archive the files that links point to, not the links. |\n| `--hard-dereference` | Follow hard links and store the data each time. |\n| `-P, --absolute-names` | Keep the leading `/` in names (dangerous on extract). |\n| `--anchored, --no-anchored` | Patterns match at the start of the name / after any slash. |\n| `--ignore-case, --no-ignore-case` | Ignore case in patterns, or not. |\n| `--wildcards, --no-wildcards` | Use wildcards in patterns, or match exactly. |\n| `--wildcards-match-slash, --no-wildcards-match-slash` | Let wildcards match a slash, or not. |\n| `--strip-components=<n>` | On extract: remove the first N folders from each name. |\n| `--transform, --xform=<sed-expr>` | Change names with a sed expression. |\n\n### Extract behavior\n\n| Option | What it does |\n|---|---|\n| `-k, --keep-old-files` | Do not replace existing files (report an error). |\n| `--skip-old-files` | Do not replace existing files (skip silently). |\n| `--keep-newer-files` | Do not replace files that are newer than the archive copy. |\n| `--overwrite` | Overwrite existing files. |\n| `--overwrite-dir` | Overwrite the metadata of existing folders. |\n| `--no-overwrite-dir` | Keep the metadata of existing folders. |\n| `-U, --unlink-first` | Remove each file before extracting over it. |\n| `--recursive-unlink` | Remove whole folders before extracting. |\n| `--keep-directory-symlink` | Keep existing links to folders. |\n| `--one-top-level[=<dir>]` | Extract everything into one new folder. |\n| `-O, --to-stdout` | Extract files to the screen (stdout), not to disk. |\n| `--to-command=<command>` | Send each extracted file to a command. |\n| `-m, --touch` | Do not restore the modification time. |\n| `--remove-files` | Remove the files from disk after adding them to the archive. |\n\n### Owners, permissions, attributes\n\n| Option | What it does |\n|---|---|\n| `-p, --preserve-permissions, --same-permissions` | Restore the exact permissions (default for root). |\n| `--no-same-permissions` | Apply your umask on extract. |\n| `--same-owner` | Restore the owners (default for root). |\n| `--no-same-owner` | Extract files as yourself (default for normal users). |\n| `--numeric-owner` | Use user and group numbers, not names. |\n| `--owner=<name>, --group=<name>` | Set this owner or group for added files. |\n| `--owner-map=<file>, --group-map=<file>` | Map owners and groups with a file. |\n| `--mode=<changes>` | Set these permissions for added files. |\n| `--mtime=<date\\|file>` | Set this modification time for added files. |\n| `--clamp-mtime` | With --mtime: only change files newer than that time. |\n| `--atime-preserve[=<method>]` | Keep the access times of the archived files. |\n| `--delay-directory-restore, --no-delay-directory-restore` | Set folder times at the end (or not). |\n| `--sort=<order>` | Order of folder entries: none, name, inode. |\n| `-s, --preserve-order, --same-order` | Member list is in the order of the archive. |\n| `--acls, --no-acls` | Store and restore POSIX ACLs (or not). |\n| `--selinux, --no-selinux` | Store and restore SELinux contexts (or not). |\n| `--xattrs, --no-xattrs` | Store and restore extended attributes (or not). |\n| `--xattrs-include=<mask>, --xattrs-exclude=<mask>` | Which extended attributes to include or skip. |\n\n### Format, devices and tapes\n\n| Option | What it does |\n|---|---|\n| `-H, --format=<format>` | Archive format: gnu, oldgnu, pax, posix, ustar, v7. |\n| `--posix` | Same as --format=posix. |\n| `-o, --old-archive, --portability` | Old V7 format (with -c). |\n| `--pax-option=<list>` | Options for the pax format. |\n| `-V, --label=<text>` | Give the archive a volume name. |\n| `-S, --sparse` | Store sparse files (like VM images) efficiently. |\n| `--sparse-version=<ver>` | The sparse format to use. |\n| `--hole-detection=<type>` | How to find holes in sparse files. |\n| `-g, --listed-incremental=<file>` | Make an incremental backup using this snapshot file. |\n| `-G, --incremental` | Old GNU incremental format. |\n| `--level=<n>` | Dump level for listed-incremental archives. |\n| `--check-device, --no-check-device` | Check device numbers for incremental backups (or not). |\n| `--ignore-failed-read` | Do not stop when a file cannot be read. |\n| `-W, --verify` | Verify the archive after writing. |\n| `-i, --ignore-zeros` | Ignore blocks of zeros in the archive (read concatenated archives). |\n| `-B, --read-full-records` | Reblock as you read (for pipes). |\n| `-b, --blocking-factor=<blocks>` | Record size in 512-byte blocks. |\n| `--record-size=<bytes>` | Record size in bytes. |\n| `-M, --multi-volume` | Create, list or extract a multi-volume archive. |\n| `-L, --tape-length=<n>` | Change the tape after N x 1024 bytes. |\n| `-F, --info-script, --new-volume-script=<name>` | Run this script at the end of each tape. |\n| `--volno-file=<file>` | Keep the volume number in a file. |\n| `-n, --seek, --no-seek` | The archive can (or cannot) be seeked. |\n| `--force-local` | The archive is local even if its name has a colon. |\n| `--rmt-command=<cmd>, --rsh-command=<cmd>` | Programs for remote tapes. |\n| `--backup[=<control>]` | Make a backup before removing a file on extract. |\n| `--suffix=<string>` | The backup suffix. |\n| `--quoting-style=<style>, --quote-chars=<chars>, --no-quote-chars=<chars>` | How names are quoted in output. |\n| `--unquote, --no-unquote` | Unquote names in input (or not). |\n| `--verbatim-files-from, --no-verbatim-files-from` | Read -T names literally (or as options). |\n| `--ignore-command-error, --no-ignore-command-error` | Ignore exit codes of child programs (or not). |\n| `--restrict` | Disable some options that could be harmful. |\n| `-?, --help` | Show help. |\n| `--usage` | Show a short usage message. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Back up a folder into a gzip archive\n\n```bash\ntar -czvf /root/etc-$(date +%F).tar.gz /etc\n```\n\n> c = create, z = gzip, v = show files, f = archive name.\n\n### List what is in an archive\n\n```bash\ntar -tvf /root/etc-2026-10-25.tar.gz\n```\n\n> Look before you extract.\n\n### Extract into another folder\n\n```bash\ntar -xzvf /root/etc-2026-10-25.tar.gz -C /tmp/restore\n```\n\n> The folder must exist. The real /etc is not touched.\n\n### Extract one file only\n\n```bash\ntar -xzf backup.tar.gz -C /tmp etc/ssh/sshd_config\n```\n\n> Give the name as it is stored (without the leading slash).\n\n### Create a bzip2 or xz archive\n\n```bash\ntar -cjf docs.tar.bz2 docs/\ntar -cJf docs.tar.xz docs/\n```\n\n> xz makes the smallest file but takes more time.\n\n### Leave out folders you do not need\n\n```bash\ntar -czf app.tar.gz --exclude='*.log' --exclude='node_modules' --exclude-vcs app/\n```\n\n> Smaller backups and no secrets from `.git` folders.\n\n### Keep SELinux labels and ACLs in a system backup\n\n```bash\ntar --selinux --acls --xattrs -czpf /backup/www.tar.gz /var/www\n```\n\n> Without these options the labels are lost, and the restored site can give 403 errors.\n\n### Remove the top folder when you extract\n\n```bash\ntar -xzf terraform-1.9.tar.gz --strip-components=1 -C /opt/terraform\n```\n\n> Archives from the web often have one extra top folder.\n\n### Copy a folder to another server through SSH\n\n```bash\ntar -czf - /srv/data | ssh server2 \"tar -xzf - -C /backup\"\n```\n\n> No temporary file: `-` sends the archive through the pipe.\n\n### Back up only files changed in the last day\n\n```bash\ntar -czf /backup/changed.tar.gz -N \"1 day ago\" /home\n```\n\n> A simple differential backup.\n\n### Make incremental backups with a snapshot file\n\n```bash\ntar -czf /backup/home-full.tar.gz -g /backup/home.snar /home\ntar -czf /backup/home-inc1.tar.gz -g /backup/home.snar /home\n```\n\n> The second run stores only what changed since the first.\n\n### Check a backup against the disk\n\n```bash\ntar -dzf /root/etc-2026-10-25.tar.gz -C /\n```\n\n> Lists files that changed since the backup (useful in incident response).\n\n### Read one file from an archive without extracting\n\n```bash\ntar -xzOf backup.tar.gz etc/hosts | less\n```\n\n> `-O` sends the file content to the screen.\n\n## Watch out\n\n- `-f` takes the next word as the archive name: write `-czf name.tar.gz`, not `-cfz`.\n- Extracting as root restores owners and permissions; extracting in `/` overwrites system files.\n- Never extract an archive from an unknown source as root without listing it first (`-t`): names like `../../etc/passwd` or absolute paths can be dangerous.\n- tar does not keep SELinux contexts and ACLs unless you add `--selinux --acls --xattrs`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/rmt` | The remote tape helper (rarely used). |\n\n## Tips\n\n- c = create, x = extract, t = list, v = verbose, f = file, z/j/J = gz/bz2/xz, C = folder.\n- To remember: \"eXtract Ze Files\" = `-xzf`, \"Create Ze File\" = `-czf`.\n\nFull manual: [man tar](https://linux.die.net/man/1/tar)\n","page":"commands/tar/","label":"tar","where":"Command page · Archive and compress","aka":[]},{"id":"cmd-tee","n":1540,"slug":"tee","title":"tee — Copy the input to the screen (or the next command) and to one or more files at the same time","sections":["commands"],"topic":"Commands · Text and search","tags":["pipe","file","output","sudo","log","append"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Copy the input to the screen (or the next command) and to one or more files at the same time.\n\n## About\n\n`tee` sits in a pipe like a T-piece in a water pipe: data goes through, and a copy goes into files. You see the output **and** keep it.\n\nA very common use: write to a file that only root may change, from a normal user, with `echo ... | sudo tee FILE`. The redirect `>` would run as your user and fail; `tee` runs with sudo.\n\n## Syntax\n\n```text\ntee [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --append` | Add to the end of the files instead of overwriting them. |\n| `-i, --ignore-interrupts` | Ignore Ctrl+C (the interrupt signal). |\n| `-p` | Report errors when writing to pipes (and go on with other outputs). |\n| `--output-error[=MODE]` | What to do when a write fails: warn, warn-nopipe, exit, exit-nopipe. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### See the output and save it\n\n```bash\ndnf check-update | tee updates.txt\n```\n\n> You watch the list and keep a copy for your change ticket.\n\n### Write a root-owned file from a normal user\n\n```bash\necho \"net.ipv4.ip_forward = 1\" | sudo tee /etc/sysctl.d/90-forward.conf\n```\n\n> `sudo echo ... > file` fails because the redirect is not done by sudo.\n\n### Add a line to a root-owned file\n\n```bash\necho \"192.168.122.20 server2\" | sudo tee -a /etc/hosts\n```\n\n> `-a` appends. Without it the file would be replaced.\n\n### Hide tee's copy on the screen\n\n```bash\necho \"export EDITOR=vim\" | sudo tee /etc/profile.d/editor.sh > /dev/null\n```\n\n> Only the file gets the text.\n\n### Write several files at once\n\n```bash\nip -br a | tee /tmp/net-$(hostname).txt /srv/reports/net-latest.txt\n```\n\n> One command, two copies.\n\n### Log a long script and still watch it\n\n```bash\n./upgrade.sh 2>&1 | tee -a /var/tmp/upgrade-$(date +%F).log\n```\n\n> `2>&1` sends errors into the pipe too, so the log is complete.\n\n### Save the middle of a pipe\n\n```bash\njournalctl -u sshd --since today | tee ssh-today.log | grep -c \"Failed password\"\n```\n\n> The full log is saved; the screen shows only the count of failed logins.\n\n### Write a heredoc to a protected file\n\n```bash\nsudo tee /etc/yum.repos.d/local.repo > /dev/null <<'EOF'\n[local-baseos]\nname=Local BaseOS\nbaseurl=file:///repo/BaseOS\ngpgcheck=0\nEOF\n```\n\n> A clean way to create config files in scripts that run as a normal user with sudo.\n\n### Keep running when you press Ctrl+C\n\n```bash\nping server2 | tee -i ping.log\n```\n\n> With -i, tee is not stopped by Ctrl+C, so the log gets the last lines.\n\n### Record a kubectl apply for the change log\n\n```bash\nkubectl apply -f deploy.yaml 2>&1 | tee -a changes.log\n```\n\n> Every change is visible now and stored for later.\n\n### Count and save matches at the same time\n\n```bash\ngrep \"ERROR\" app.log | tee errors.txt | wc -l\n```\n\n> The number on the screen, the lines in the file.\n\n## Watch out\n\n- Without `-a`, tee **overwrites** the file.\n- In `sudo tee`, only tee runs as root; the command before the pipe runs as you.\n\n## Tips\n\n- tee = T-piece: one input, two outputs (screen + file).\n- `| sudo tee FILE` = write as root. `| sudo tee -a FILE` = append as root.\n\nFull manual: [man tee](https://linux.die.net/man/1/tee)\n","page":"commands/tee/","label":"tee","where":"Command page · Text and search","aka":[]},{"id":"cmd-test","n":1541,"slug":"test","title":"test — Check a condition (file exists, strings equal, number greater …) and return 0 (true) or 1 (false). Also written as `[ … ]`","sections":["commands"],"topic":"Commands · Shell and help","tags":["conditions","if","file tests","compare","scripts","brackets"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Check a condition (file exists, strings equal, number greater …) and return 0 (true) or 1 (false). Also written as `[ … ]`.\n\n## About\n\n`test EXPRESSION` and `[ EXPRESSION ]` are the same; the `]` is required and needs spaces around it. They are used with `if`, `while`, `&&` and `||`. Bash also has `[[ … ]]`, which adds pattern matching (`==` with `*`), regular expressions (`=~`) and safer handling of empty variables.\n\n## Syntax\n\n```text\ntest EXPRESSION\n[ EXPRESSION ]\n[[ EXPRESSION ]]\n```\n\n## Parts\n\n\n### Files\n\n| Part | Meaning |\n|---|---|\n| `-e FILE` | Exists (any type). |\n| `-f FILE` | Is a regular file. |\n| `-d FILE` | Is a folder. |\n| `-L FILE, -h FILE` | Is a symbolic link. |\n| `-b FILE, -c FILE` | Block / character device. |\n| `-p FILE, -S FILE` | Named pipe / socket. |\n| `-s FILE` | Exists and is not empty. |\n| `-r FILE, -w FILE, -x FILE` | Readable / writable / executable for you. |\n| `-u FILE, -g FILE, -k FILE` | SUID / SGID / sticky bit set. |\n| `-O FILE, -G FILE` | Owned by you / by your group. |\n| `-N FILE` | Changed since last read. |\n| `FILE1 -nt FILE2, FILE1 -ot FILE2` | Newer / older than. |\n| `FILE1 -ef FILE2` | Same file (hard link). |\n\n### Strings\n\n| Part | Meaning |\n|---|---|\n| `-z STRING` | Empty. |\n| `-n STRING` | Not empty. |\n| `S1 = S2, S1 != S2` | Equal / not equal. |\n| `S1 < S2, S1 > S2` | Sort order (escape in [ ]: `\\<`). |\n| `S =~ REGEX` | Matches a regular expression (only [[ ]]). |\n\n### Numbers\n\n| Part | Meaning |\n|---|---|\n| `N1 -eq N2, -ne` | Equal / not equal. |\n| `N1 -lt N2, -le` | Less / less or equal. |\n| `N1 -gt N2, -ge` | Greater / greater or equal. |\n\n### Logic\n\n| Part | Meaning |\n|---|---|\n| `! EXPR` | Not. |\n| `EXPR1 -a EXPR2, EXPR1 -o EXPR2` | And / or (old; better: `[ A ] && [ B ]`). |\n| `&&, \\|\\|` | And / or inside [[ ]]. |\n| `-v NAME` | The variable is set (bash). |\n\n## Examples\n\n### Does a file exist?\n\n```bash\n[ -f /etc/fstab ] && echo \"yes\"\n```\n```text\nyes\n```\n\n> Spaces inside the brackets are required.\n\n### In an if\n\n```bash\nif [ -d /backup ]; then echo \"backup folder ok\"; else mkdir /backup; fi\n```\n\n> -d = folder.\n\n### Compare strings\n\n```bash\n[ \"$USER\" = \"root\" ] || echo \"run as root\"\n```\n\n> Quote variables: empty values break `[ = root ]`.\n\n### Compare numbers\n\n```bash\n[ $(df --output=pcent / | tail -1 | tr -dc 0-9) -gt 80 ] && echo \"disk almost full\"\n```\n\n> -gt for numbers, not >.\n\n### Check the number of arguments\n\n```bash\nset -- a b; [ $# -eq 2 ] && echo \"two arguments\"\n```\n\n> Common at the start of scripts.\n\n### Empty variable\n\n```bash\nNAME=\"\"; [ -z \"$NAME\" ] && echo \"empty\"\n```\n\n> -n for not empty.\n\n### Pattern match with [[ ]]\n\n```bash\nFILE=report.csv; [[ $FILE == *.csv ]] && echo \"CSV file\"\n```\n\n> Wildcards work only in [[ ]].\n\n### Regular expression\n\n```bash\nIP=192.168.122.10; [[ $IP =~ ^([0-9]{1,3}\\.){3}[0-9]{1,3}$ ]] && echo \"looks like an IPv4\"\n```\n\n> =~ in [[ ]].\n\n### Find world-writable scripts (security)\n\n```bash\nfor f in /usr/local/bin/*; do [ -w \"$f\" ] && [ \"$(stat -c %a \"$f\" | cut -c3)\" -ge 2 ] && echo \"$f is world-writable\"; done\n```\n\n> Anybody could change these scripts.\n\n### Is a file newer than another?\n\n```bash\n[ /etc/nginx/nginx.conf -nt /var/run/nginx.pid ] && echo \"config changed since start\"\n```\n\n> Reload needed.\n\n### Exit status of test\n\n```bash\ntest 5 -gt 3; echo $?\n```\n```text\n0\n```\n\n> 0 = true, 1 = false.\n\n## Watch out\n\n- `[ $VAR = x ]` fails when VAR is empty; write `[ \"$VAR\" = x ]`.\n- `>` inside [ ] is a redirect, not a compare. Use -gt for numbers.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The expression is true. |\n| `1` | The expression is false. |\n| `2` | Error in the expression. |\n\nFull manual: [man test](https://linux.die.net/man/1/test)\n","page":"commands/test/","label":"test","where":"Command page · Shell and help","aka":[]},{"id":"cmd-timedatectl","n":1542,"slug":"timedatectl","title":"timedatectl — Show and change the system time, the time zone, the RTC mode and whether NTP sync is on","sections":["commands"],"topic":"Commands · Time","tags":["time","time zone","ntp","rtc","clock"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change the system time, the time zone, the RTC mode and whether NTP sync is on.\n\n## About\n\nWithout arguments `timedatectl` shows local time, UTC, RTC time, time zone, whether the clock is synchronized and whether the NTP service (chronyd) is active. Subcommands change these settings.\n\n## Syntax\n\n```text\ntimedatectl [OPTION]... [COMMAND]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `status` | Show the settings (default). |\n| `show` | Settings as key=value (for scripts). |\n| `set-time \"YYYY-MM-DD HH:MM:SS\"` | Set the clock (NTP must be off). |\n| `set-timezone ZONE` | Set the time zone. |\n| `list-timezones` | All zone names. |\n| `set-local-rtc 0\\|1` | RTC in UTC (0) or local time (1). |\n| `set-ntp true\\|false` | Turn NTP sync (chronyd) on or off. |\n| `timesync-status, show-timesync, ntp-servers, revert` | For systemd-timesyncd (not used by default on RHEL). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--adjust-system-clock` | With set-local-rtc: also set the system clock from the RTC. |\n| `-p, --property=NAME` | With show: only these properties. |\n| `-a, --all` | With show: all properties. |\n| `--value` | With show: only the values. |\n| `--monitor` | With timesync-status: keep updating. |\n| `--no-ask-password` | No password questions. |\n| `--no-pager` | Do not use a pager. |\n| `-H, --host=HOST` | Remote host over SSH. |\n| `-M, --machine=NAME` | A container. |\n| `-h, --help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Show all settings\n\n```bash\ntimedatectl\n```\n```text\n               Local time: Sun 2026-10-25 11:42:10 +03\n           Universal time: Sun 2026-10-25 08:42:10 UTC\n                 RTC time: Sun 2026-10-25 08:42:10\n                Time zone: Asia/Riyadh (+03, +0300)\nSystem clock synchronized: yes\n              NTP service: active\n          RTC in local TZ: no\n```\n\n> Everything at once.\n\n### Set the time zone\n\n```bash\ntimedatectl set-timezone Asia/Riyadh\n```\n\n> Changes the /etc/localtime link.\n\n### Find a zone name\n\n```bash\ntimedatectl list-timezones | grep -i amsterdam\n```\n```text\nEurope/Amsterdam\n```\n\n> Exact names are needed.\n\n### Turn NTP on\n\n```bash\ntimedatectl set-ntp true\n```\n\n> Starts and enables chronyd.\n\n### Set the time by hand\n\n```bash\ntimedatectl set-ntp false\ntimedatectl set-time \"2026-10-25 12:00:00\"\n```\n\n> Only for labs without a time server.\n\n### RTC in UTC\n\n```bash\ntimedatectl set-local-rtc 0\n```\n\n> Recommended for servers.\n\n### Value for a script\n\n```bash\ntimedatectl show -p NTPSynchronized --value\n```\n```text\nyes\n```\n\n> Monitoring check.\n\n### Check many servers\n\n```bash\nfor h in server1 server2; do ssh $h timedatectl show -p Timezone --value; done\n```\n\n> Same zone everywhere?\n\n### Time zone link\n\n```bash\nls -l /etc/localtime\n```\n```text\nlrwxrwxrwx. 1 root root 33 Oct 25 11:40 /etc/localtime -> ../usr/share/zoneinfo/Asia/Riyadh\n```\n\n> What set-timezone changed.\n\n### Unsynced clock alert (security)\n\n```bash\n[ \"$(timedatectl show -p NTPSynchronized --value)\" = yes ] || echo \"clock not synced\"\n```\n\n> Wrong time breaks TLS, Kerberos and log analysis.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man timedatectl](https://linux.die.net/man/1/timedatectl)\n","page":"commands/timedatectl/","label":"timedatectl","where":"Command page · Time","aka":[]},{"id":"cmd-top","n":1543,"slug":"top","title":"top — Watch the processes live, sorted by CPU, with load, CPU and memory totals — and kill or renice from it","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["processes","live","cpu","memory","load average","monitor","performance"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Watch the processes live, sorted by CPU, with load, CPU and memory totals — and kill or renice from it.\n\n## About\n\nThe top part shows uptime and load average, the number of tasks per state, CPU use (`us` user, `sy` system, `ni` niced, `id` idle, `wa` waiting for I/O, `st` stolen by the hypervisor) and memory and swap. Below is the process list, refreshed every 3 seconds.\n\nKeys work while it runs: `k` kill, `r` renice, `M` sort by memory, `P` by CPU, `1` each CPU, `q` quit. With `-b` it writes plain text for logs and scripts.\n\n## Syntax\n\n```text\ntop [OPTION]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `k` | Kill: send a signal to a PID. |\n| `r` | Renice a PID. |\n| `P / M / T / N` | Sort by CPU / memory / time / PID. |\n| `1` | Show every CPU. |\n| `c` | Show the full command line. |\n| `u` | Show only one user. |\n| `f` | Choose and sort columns. |\n| `H` | Show threads. |\n| `V` | Forest (tree) view. |\n| `E / e` | Change memory units (summary / list). |\n| `W` | Save the settings in ~/.config/procps/toprc. |\n| `h` | Help. |\n| `q` | Quit. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-b, --batch` | Batch mode: plain text output (for logs and pipes). |\n| `-n, --iterations NUMBER` | Stop after NUMBER updates. |\n| `-d, --delay SECONDS` | Seconds between updates. |\n| `-p, --pid PIDLIST` | Only these PIDs. |\n| `-u, --filter-only-euser USER` | Only this effective user. |\n| `-U, --filter-any-user USER` | Any user ID match (real, effective, saved, file). |\n| `-o, --sort-override FIELD` | Sort by this field, like `-o %MEM`. |\n| `-H, --threads-show` | Show threads. |\n| `-c, --cmdline-toggle` | Start with full command lines. |\n| `-i, --idle-toggle` | Hide idle processes. |\n| `-S, --accum-time-toggle` | Cumulative time (with dead children). |\n| `-E, --scale-summary-mem k\\|m\\|g\\|t\\|p\\|e` | Units in the summary. |\n| `-e, --scale-task-mem k\\|m\\|g\\|t\\|p` | Units in the list. |\n| `-O, --list-fields` | List the field names. |\n| `-s, --secure-mode` | Secure mode (no kill or renice). |\n| `-w, --width [COLUMNS]` | Output width in batch mode. |\n| `-1, --single-cpu-toggle` | Start with each CPU shown (or one line). |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Start top\n\n```bash\ntop\n```\n```text\ntop - 10:52:01 up  2:54,  2 users,  load average: 0.15, 0.10, 0.09\nTasks: 182 total,   1 running, 181 sleeping,   0 stopped,   0 zombie\n%Cpu(s):  1.2 us,  0.5 sy,  0.0 ni, 98.1 id,  0.2 wa,  0.0 hi,  0.0 si,  0.0 st\nMiB Mem :   3654.2 total,   2201.7 free,    812.4 used,    891.3 buff/cache\n```\n\n> Press q to leave.\n\n### Sort by memory\n\n```bash\ntop -o %MEM\n```\n\n> Or press M inside top.\n\n### Only one user\n\n```bash\ntop -u apache\n```\n\n> Watch the web server processes.\n\n### One process and its threads\n\n```bash\ntop -H -p $(pidof chronyd)\n```\n\n> Which thread uses the CPU.\n\n### Save one snapshot to a file\n\n```bash\ntop -b -n 1 > /tmp/top-$(date +%H%M).txt\n```\n\n> Evidence for a slow-server ticket.\n\n### Log every 10 seconds for one minute\n\n```bash\ntop -b -d 10 -n 6 | grep -A 8 '^top' > /tmp/top.log\n```\n\n> A short performance log.\n\n### Kill from top\n\n```bash\ntop\n```\n\n> Press k, type the PID, press Enter (15) or type 9.\n\n### Read the CPU line\n\n```bash\ntop -b -n 1 | grep Cpu\n```\n\n> High `wa` = slow disk; high `st` = the hypervisor is overloaded; high `sy` = kernel work.\n\n### Find a crypto miner (security)\n\n```bash\ntop -b -n 1 -o %CPU | head -12\n```\n\n> An unknown process at 100% CPU, often from /tmp or a strange user.\n\n### Secure mode for a shared screen\n\n```bash\ntop -s\n```\n\n> Nobody can kill from this top.\n\n## Watch out\n\n- Load average counts waiting processes, also those waiting for disk. High load with low CPU use often means slow storage or NFS.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.config/procps/toprc` | Saved settings (W key). |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man top](https://linux.die.net/man/1/top)\n","page":"commands/top/","label":"top","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-touch","n":1544,"slug":"touch","title":"touch — Create an empty file, or change the access and modification times of a file","sections":["commands"],"topic":"Commands · Files and folders","tags":["create","empty file","timestamp","date","modification time"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create an empty file, or change the access and modification times of a file.\n\n## About\n\n`touch FILE` creates FILE when it does not exist. When it exists, `touch` sets its times to now, without changing the content.\n\nThis is used to make test files, to create flag files that other programs look for (like `/.autorelabel` for SELinux), and to set exact times for tests of backup or cleanup scripts.\n\n## Syntax\n\n```text\ntouch [OPTION]... FILE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a` | Change only the access time. |\n| `-m` | Change only the modification time. |\n| `-c, --no-create` | Do not create files that do not exist. |\n| `-d, --date=<text>` | Use this date and time instead of now, like \"2026-01-31 14:00\" or \"yesterday\". |\n| `-t <stamp>` | Use this time in the form [[CC]YY]MMDDhhmm[.ss]. |\n| `-r, --reference=<file>` | Use the times of this other file. |\n| `-h, --no-dereference` | Change the link itself, not the file it points to. |\n| `--time=<word>` | Which time to change: access (atime) or modify (mtime). |\n| `-f` | Ignored (kept for compatibility). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Create an empty file\n\n```bash\ntouch notes.txt\n```\n\n> If it already exists, only its time changes.\n\n### Create many files at once\n\n```bash\ntouch file{1..5}.txt\n```\n\n> Makes file1.txt to file5.txt. Good for practice.\n\n### Ask SELinux to relabel the whole system at the next boot\n\n```bash\ntouch /.autorelabel\n```\n\n> Needed after a root password reset with rd.break, or after SELinux was disabled.\n\n### Set a file to an old date to test a cleanup script\n\n```bash\ntouch -d \"40 days ago\" /tmp/test-old.log\nfind /tmp -name \"*.log\" -mtime +30\n```\n\n> The file looks 40 days old, so your `find -mtime +30` test can find it.\n\n### Give a file the same time as another file\n\n```bash\ntouch -r original.conf copy.conf\n```\n\n> Keeps times equal, for example after you fix a copy.\n\n### Set an exact time with -t\n\n```bash\ntouch -t 202601011200 newyear.txt\n```\n\n> 2026-01-01 12:00.\n\n### Change only the access time\n\n```bash\ntouch -a report.pdf\n```\n\n> The modification time stays the same.\n\n### Do not create a missing file\n\n```bash\ntouch -c maybe.txt\n```\n\n> No error and no new file if it does not exist.\n\n### Create a flag file for a maintenance window\n\n```bash\ntouch /etc/nologin\n```\n\n> While this file exists, normal users cannot log in (root still can). Remove it after the work.\n\n### Make an empty log file with the right owner\n\n```bash\ntouch /var/log/myapp.log && chown myapp: /var/log/myapp.log\n```\n\n> The service can write to it at its first start.\n\n### Check if a folder is writable in a script\n\n```bash\ntouch /mnt/nfs/.write-test && rm /mnt/nfs/.write-test && echo \"writable\"\n```\n\n> A simple test before a backup job writes to a share.\n\n## Watch out\n\n- `touch` on an existing file does not empty it. To empty a file use `> file` or `truncate -s 0 file`.\n- Changing times can hide what really happened on a system; in forensics, look at ctime (it cannot be set by touch).\n\n## Tips\n\n- New empty file or \"update the time\" — both are touch.\n- `/.autorelabel` = SELinux relabel at the next boot.\n\nFull manual: [man touch](https://linux.die.net/man/1/touch)\n","page":"commands/touch/","label":"touch","where":"Command page · Files and folders","aka":[]},{"id":"cmd-tr","n":1545,"slug":"tr","title":"tr — Replace, delete or squeeze single characters in a text stream","sections":["commands"],"topic":"Commands · Text and search","tags":["translate","replace","characters","uppercase","delete","squeeze"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Replace, delete or squeeze single characters in a text stream.\n\n## About\n\n`tr` (translate) works on characters, not words. It reads standard input only (no file names), so you use it in a pipe or with `<`.\n\nTypical jobs: change lower case to upper case, change one separator to another, remove characters (like Windows `\\r`), or squeeze repeated spaces into one.\n\n## Syntax\n\n```text\ntr [OPTION]... SET1 [SET2]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d, --delete` | Delete the characters in SET1. |\n| `-s, --squeeze-repeats` | Replace a run of the same character (from the last set) with one. |\n| `-c, -C, --complement` | Use every character that is NOT in SET1. |\n| `-t, --truncate-set1` | Make SET1 as long as SET2 first. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n### Sets\n\n| Option | What it does |\n|---|---|\n| `a-z, A-Z, 0-9` | Ranges of characters. |\n| `[:lower:] [:upper:]` | Lower case / upper case letters. |\n| `[:digit:] [:alpha:] [:alnum:]` | Digits / letters / letters and digits. |\n| `[:space:] [:blank:]` | White space / spaces and tabs. |\n| `[:punct:] [:cntrl:] [:print:]` | Punctuation / control characters / printable characters. |\n| `\\n \\t \\r \\\\` | New line, tab, carriage return, backslash. |\n\n## Examples\n\n### Lower case to upper case\n\n```bash\necho \"server1\" | tr 'a-z' 'A-Z'\n```\n```text\nSERVER1\n```\n\n> Same as `tr '[:lower:]' '[:upper:]'`.\n\n### Remove Windows line endings\n\n```bash\ntr -d '\\r' < script-from-windows.sh > script.sh\n```\n\n> Fixes \"bad interpreter: No such file or directory\".\n\n### Squeeze many spaces into one\n\n```bash\nps aux | tr -s ' ' | cut -d' ' -f2,11\n```\n\n> After squeezing, `cut` can use the space as separator.\n\n### The PATH one folder per line\n\n```bash\necho \"$PATH\" | tr ':' '\\n'\n```\n\n> Easy to read.\n\n### Remove all digits\n\n```bash\necho \"abc123def\" | tr -d '0-9'\n```\n```text\nabcdef\n```\n\n> Delete a set of characters.\n\n### Keep only letters and digits\n\n```bash\necho \"us3r_n@me!\" | tr -cd '[:alnum:]'\n```\n```text\nus3rnme\n```\n\n> `-c` = everything else, `-d` = delete it.\n\n### Make a random password\n\n```bash\ntr -dc 'A-Za-z0-9' < /dev/urandom | head -c 20; echo\n```\n\n> 20 random letters and digits.\n\n### Split words into lines\n\n```bash\necho \"one two three\" | tr ' ' '\\n'\n```\n\n> Then `sort | uniq -c` can count words.\n\n### Convert a CSV to TAB separated\n\n```bash\ntr ',' '\\t' < data.csv > data.tsv\n```\n\n> Simple files only (no quoted commas).\n\n### Change a hostname to a valid label\n\n```bash\necho \"Web Server 01\" | tr '[:upper:] ' '[:lower:]-'\n```\n```text\nweb-server-01\n```\n\n> Spaces become dashes, letters become lower case.\n\n### Read NUL-separated data\n\n```bash\ntr '\\0' '\\n' < /proc/1/cmdline\n```\n\n> /proc files separate words with NUL bytes.\n\n## Watch out\n\n- tr does not read file names: use `tr … < file`.\n- tr changes single characters, not words. For words use `sed`.\n\n## Tips\n\n- `tr -d` delete, `tr -s` squeeze, `tr 'a-z' 'A-Z'` upper case.\n\nFull manual: [man tr](https://linux.die.net/man/1/tr)\n","page":"commands/tr/","label":"tr","where":"Command page · Text and search","aka":[]},{"id":"cmd-tracepath","n":1546,"slug":"tracepath","title":"tracepath — Show the routers (hops) on the way to a host and the MTU of the path. Needs no root","sections":["commands"],"topic":"Commands · Network","tags":["route","hops","mtu","path","network test"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the routers (hops) on the way to a host and the MTU of the path. Needs no root.\n\n## About\n\n`tracepath` sends UDP packets with a growing TTL. Each router that drops a packet answers, so you see the path hop by hop with times. It also finds the path MTU (`pmtu`).\n\nIt works like `traceroute` but needs no special rights and is installed on RHEL by default (iputils). For a live view use `mtr`.\n\n## Syntax\n\n```text\ntracepath [-4] [-6] [-n] [-b] [-l PKTLEN] [-m MAX_HOPS] [-p PORT] DESTINATION\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n` | No name lookups: only IP addresses. |\n| `-b` | Show names and IP addresses. |\n| `-4` | Use IPv4 only. |\n| `-6` | Use IPv6 only. |\n| `-m MAX_HOPS` | Stop after this many hops (default 30). |\n| `-l PKTLEN` | Start packet length (default 65535 for IPv4, 128000 for IPv6). |\n| `-p PORT` | Start UDP port. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Path to a host\n\n```bash\ntracepath -n www.redhat.com\n```\n```text\n 1?: [LOCALHOST]                      pmtu 1500\n 1:  192.168.122.1                                         0.512ms\n 2:  10.0.0.1                                              2.104ms\n 3:  no reply\n...\n```\n\n> \"no reply\" = a router that does not answer (common).\n\n### Names and addresses\n\n```bash\ntracepath -b server2.example.com\n```\n\n> Shows both for each hop.\n\n### Find the path MTU\n\n```bash\ntracepath -n 10.20.0.5 | grep pmtu\n```\n\n> A pmtu under 1500 explains hanging SSH or HTTPS over VPN.\n\n### IPv6 path\n\n```bash\ntracepath -6 -n 2001:db8:10::1\n```\n\n> Check IPv6 routes separately from IPv4.\n\n### Limit the hops\n\n```bash\ntracepath -n -m 5 8.8.8.8\n```\n\n> Only the first part of the path (your own network).\n\n### Compare with the routing table\n\n```bash\nip route get 8.8.8.8\n```\n```text\n8.8.8.8 via 192.168.122.1 dev enp1s0 src 192.168.122.10 uid 1000\n```\n\n> The first hop must be this gateway.\n\n### Check if traffic goes through the VPN\n\n```bash\ntracepath -n 10.50.1.10\n```\n\n> The first hop should be the VPN address, not the home router.\n\n### Save the path for a ticket\n\n```bash\ntracepath -n www.example.com > /tmp/path-$(date +%F).txt\n```\n\n> Give it to the network or hosting team.\n\n### Use a fixed port\n\n```bash\ntracepath -p 33434 -n server2\n```\n\n> Some firewalls only allow certain UDP ports.\n\n### When tracepath stops early\n\n```bash\ntracepath -n 192.168.200.5\n```\n\n> Endless \"no reply\" after hop 1: no route or a firewall after your gateway.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | The destination was reached. |\n| `1` | Not reached or error. |\n\nFull manual: [man tracepath](https://linux.die.net/man/8/tracepath)\n","page":"commands/tracepath/","label":"tracepath","where":"Command page · Network","aka":[]},{"id":"cmd-tty","n":1547,"slug":"tty","title":"tty — Print the name of the terminal device you are using (like /dev/tty3 or /dev/pts/0)","sections":["commands"],"topic":"Commands · Shell and help","tags":["terminal","console","pts","session","interactive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the name of the terminal device you are using (like /dev/tty3 or /dev/pts/0).\n\n## About\n\nEvery session has a terminal device: a real console (`/dev/tty1` … `/dev/tty6`) or a pseudo terminal for terminal windows and SSH (`/dev/pts/0`, `/dev/pts/1` …).\n\n`tty` prints the name of yours. When the input is not a terminal (for example in a cron job or a pipe), it prints \"not a tty\" and returns an error code — scripts use this to know if a person is watching.\n\n## Syntax\n\n```text\ntty [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-s, --silent, --quiet` | Print nothing; only return the exit code (0 = terminal, 1 = not a terminal). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Which terminal am I on?\n\n```bash\ntty\n```\n```text\n/dev/pts/0\n```\n\n> pts = terminal window or SSH session.\n\n### On the local text console\n\n```bash\ntty\n```\n```text\n/dev/tty3\n```\n\n> A real virtual console (Ctrl+Alt+F3).\n\n### Check in a script if a person is watching\n\n```bash\nif tty -s; then echo \"interactive\"; else echo \"batch (cron, pipe)\"; fi\n```\n\n> Ask questions only when a person can answer.\n\n### In a pipe there is no terminal\n\n```bash\necho | tty\n```\n```text\nnot a tty\n```\n\n> stdin comes from the pipe, not from a terminal.\n\n### Send a message to another session\n\n```bash\nwho\nwrite admin pts/1\n```\n\n> `write` and `wall` use the terminal names that `tty` and `who` show.\n\n### End another session\n\n```bash\npkill -KILL -t pts/3\n```\n\n> Logs out the session on pts/3 (find it with `who`).\n\n### Set terminal options for your session\n\n```bash\nstty -a\n```\n\n> Shows the settings of the current tty (like the erase key).\n\n### Log the terminal in a script\n\n```bash\necho \"$(date) started on $(tty) by $USER\" >> /var/tmp/run.log\n```\n\n> Helps to see later how and where a script was run.\n\n### See the owner of your terminal device\n\n```bash\nls -l $(tty)\n```\n```text\ncrw--w----. 1 admin tty 136, 0 Oct 25 09:02 /dev/pts/0\n```\n\n> Your terminal belongs to you; group tty may write messages to it.\n\n### Find processes attached to a terminal\n\n```bash\nps -t pts/0\n```\n\n> All processes of that session.\n\n### Check that SSH gave you a terminal\n\n```bash\nssh server2 tty; ssh -t server2 tty\n```\n\n> Without -t a remote command has no terminal (\"not a tty\").\n\n## Tips\n\n- /dev/ttyN = console, /dev/pts/N = window or SSH. `tty -s` = test in scripts.\n\nFull manual: [man tty](https://linux.die.net/man/1/tty)\n","page":"commands/tty/","label":"tty","where":"Command page · Shell and help","aka":[]},{"id":"cmd-tune2fs","n":1548,"slug":"tune2fs","title":"tune2fs — Show and change the settings of an ext2/ext3/ext4 file system (label, reserved blocks, default mount options, checks, UUID)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["ext4","file system settings","label","reserved blocks","mount options","uuid"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show and change the settings of an ext2/ext3/ext4 file system (label, reserved blocks, default mount options, checks, UUID).\n\n## About\n\n`tune2fs -l DEVICE` lists the superblock: label, UUID, features, mount count, reserved blocks and more. Other options change these values, mostly without unmounting. Some changes (like a new UUID or inode size) need the file system to be unmounted.\n\n## Syntax\n\n```text\ntune2fs [OPTION]... DEVICE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l` | List the superblock contents. |\n| `-L LABEL` | Set the label (max 16 characters). |\n| `-m PERCENT` | Reserved blocks for root in percent. |\n| `-r COUNT` | Reserved blocks as a number. |\n| `-u USER, -g GROUP` | Who may use the reserved blocks. |\n| `-o [^]OPTION[,...]` | Default mount options, like acl, user_xattr, journal_data (^ = remove). |\n| `-O [^]FEATURE[,...]` | Turn features on or off, like ^has_journal, metadata_csum. |\n| `-E OPTIONS` | Extended options: stride=, stripe_width=, mount_opts=, hash_alg= … |\n| `-c N` | Check after N mounts (-1 = never). |\n| `-C N` | Set the current mount count. |\n| `-i N[d\\|m\\|w]` | Check after this time (0 = never). |\n| `-T TIME` | Set the time of the last check. |\n| `-e continue\\|remount-ro\\|panic` | Behaviour on errors. |\n| `-U UUID\\|random\\|time\\|clear` | Set a new UUID (unmounted for most cases). |\n| `-j` | Add a journal (ext2 → ext3). |\n| `-J OPTIONS` | Journal options. |\n| `-I SIZE` | Change the inode size (unmounted). |\n| `-M DIR` | Set the last mounted directory. |\n| `-Q OPTIONS` | Quota options: usrquota, grpquota, prjquota. |\n| `-f` | Force (dangerous). |\n| `-z FILE` | Save an undo file. |\n\n## Examples\n\n### Show all settings\n\n```bash\ntune2fs -l /dev/vdb2 | less\n```\n\n> Label, UUID, features, counts.\n\n### Set a label\n\n```bash\ntune2fs -L backup /dev/vdb2\n```\n\n> Then mount with LABEL=backup.\n\n### Less space reserved for root\n\n```bash\ntune2fs -m 1 /dev/vdb2\n```\n\n> Frees 4 % of a big data disk.\n\n### Default mount options\n\n```bash\ntune2fs -o acl,user_xattr /dev/vdb2\n```\n\n> Used even when fstab says only `defaults`.\n\n### Turn off forced checks\n\n```bash\ntune2fs -c -1 -i 0 /dev/vdb2\n```\n\n> No surprise fsck at boot after N mounts.\n\n### New UUID for a cloned disk\n\n```bash\numount /backup; tune2fs -U random /dev/vdb2\n```\n\n> Two disks with the same UUID confuse fstab and LVM.\n\n### Remount read-only on errors (safer)\n\n```bash\ntune2fs -e remount-ro /dev/vdb2\n```\n\n> Stops more damage when the disk has errors.\n\n### Enable quotas\n\n```bash\ntune2fs -O quota -Q usrquota,grpquota /dev/vdb2\n```\n\n> Needs the file system unmounted.\n\n### Show the reserved blocks\n\n```bash\ntune2fs -l /dev/vdb2 | grep -i 'reserved block count'\n```\n\n> Compare with the block count.\n\n### Check the mount count and last check\n\n```bash\ntune2fs -l /dev/vdb2 | grep -E 'Mount count|Last checked'\n```\n\n> Maintenance information.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man tune2fs](https://linux.die.net/man/8/tune2fs)\n","page":"commands/tune2fs/","label":"tune2fs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-tuned-adm","n":1549,"slug":"tuned-adm","title":"tuned-adm — Choose and check the tuned performance profile (a set of kernel, CPU, disk and power settings)","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["tuned","performance","profile","power","virtual machine"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Choose and check the tuned performance profile (a set of kernel, CPU, disk and power settings).\n\n## About\n\nThe `tuned` service applies a **profile**: many settings for one type of system, like a VM, a busy server or a laptop. `tuned-adm` lists the profiles, shows the active and the recommended one, and switches profiles.\n\nProfiles are in `/usr/lib/tuned/` (from packages) and `/etc/tuned/` (your own; a profile can `include=` another and change a few values).\n\n## Syntax\n\n```text\ntuned-adm list | active | recommend | off | verify | profile_mode | auto_profile\ntuned-adm profile PROFILE...\ntuned-adm profile_info [PROFILE]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `list` | All available profiles and the active one. |\n| `active` | The active profile. |\n| `recommend` | The profile tuned suggests for this system. |\n| `profile NAME...` | Switch to a profile (more names = merged). |\n| `profile_info [NAME]` | Description of a profile. |\n| `off` | Stop tuning (settings go back). |\n| `verify` | Check that the profile settings are really applied. |\n| `auto_profile` | Switch to the recommended profile and automatic mode. |\n| `profile_mode` | Show if the profile was set by hand or automatically. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-i, --ignore-missing` | With verify: ignore settings the system does not support. |\n| `-h, --help` | Show help. |\n| `-v, --version` | Show the version. |\n| `-d, --debug` | Debug output. |\n| `-a, --async` | Do not wait for the change to finish. |\n| `-t, --timeout SECONDS` | Timeout for the change. |\n| `-l, --loglevel LEVEL` | Log level to show. |\n\n## Examples\n\n### Install and start tuned\n\n```bash\ndnf install -y tuned\nsystemctl enable --now tuned\n```\n\n> Often installed already.\n\n### Which profile is active?\n\n```bash\ntuned-adm active\n```\n```text\nCurrent active profile: virtual-guest\n```\n\n> RHEL VMs get virtual-guest.\n\n### Recommended profile\n\n```bash\ntuned-adm recommend\n```\n```text\nvirtual-guest\n```\n\n> Based on the hardware and virtualization.\n\n### Switch profile\n\n```bash\ntuned-adm profile throughput-performance\n```\n\n> For a busy physical server.\n\n### List all profiles\n\n```bash\ntuned-adm list\n```\n\n> Shows names with a short description.\n\n### Learn about a profile\n\n```bash\ntuned-adm profile_info powersave\n```\n\n> What it changes.\n\n### Check that the settings are applied\n\n```bash\ntuned-adm verify\n```\n```text\nVerification succeeded, current system settings match the preset profile.\n```\n\n> If not, see /var/log/tuned/tuned.log.\n\n### Back to automatic mode\n\n```bash\ntuned-adm auto_profile\n```\n\n> tuned picks the recommended profile.\n\n### Make your own profile\n\n```bash\nmkdir /etc/tuned/myweb\nprintf '[main]\\ninclude=throughput-performance\\n\\n[sysctl]\\nvm.swappiness=10\\n' > /etc/tuned/myweb/tuned.conf\ntuned-adm profile myweb\n```\n\n> Your changes on top of a Red Hat profile.\n\n### Turn tuning off\n\n```bash\ntuned-adm off\n```\n\n> For tests: compare with and without.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/lib/tuned/` | Profiles from packages. |\n| `/etc/tuned/` | Your own profiles; active_profile and profile_mode. |\n| `/var/log/tuned/tuned.log` | Log. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man tuned-adm](https://linux.die.net/man/8/tuned-adm)\n","page":"commands/tuned-adm/","label":"tuned-adm","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-type","n":1550,"slug":"type","title":"type — Show what a command name really is: an alias, a shell keyword, a function, a built-in or a program file","sections":["commands"],"topic":"Commands · Shell and help","tags":["builtin","alias","function","path","command","which"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show what a command name really is: an alias, a shell keyword, a function, a built-in or a program file.\n\n## About\n\n`type` is built into Bash. It answers the question \"if I type this name, what will run?\" It knows about aliases, functions and shell built-ins, which a tool like `which` cannot see.\n\nUse it when a command does something you did not expect: maybe an alias with the same name hides the real program, or a script found a different program in `$PATH`.\n\nThe help text is in Bash itself: `help type`.\n\n## Syntax\n\n```text\ntype [-afptP] NAME [NAME ...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-t` | Print only one word: alias, keyword, function, builtin, file, or nothing if not found. |\n| `-a` | Show every place where NAME is found: alias, function, built-in and every file in $PATH. |\n| `-p` | Print the path of the file that would run, but only when NAME is a file (nothing for an alias or built-in). |\n| `-P` | Search $PATH for a file, even when NAME is also an alias, function or built-in. |\n| `-f` | Do not look for shell functions. |\n\n## Examples\n\n### Find out what a name is\n\n```bash\ntype ls\n```\n```text\nls is aliased to `ls --color=auto'\n```\n\n> On RHEL, `ls` is an alias that adds colors.\n\n### Show every match, in the order Bash uses them\n\n```bash\ntype -a ls\n```\n```text\nls is aliased to `ls --color=auto'\nls is /usr/bin/ls\n```\n\n> The first line wins. The program file is only used when there is no alias.\n\n### Get one word for a script\n\n```bash\ntype -t cd\n```\n```text\nbuiltin\n```\n\n> Possible answers: alias, keyword, function, builtin, file.\n\n### Check if a command is installed before you use it\n\n```bash\nif type -P jq >/dev/null; then echo \"jq found\"; else echo \"please install jq\"; fi\n```\n\n> `type -P` returns an error code when the program is not in $PATH. Good for install checks.\n\n### See if a name is a shell keyword\n\n```bash\ntype if\n```\n```text\nif is a shell keyword\n```\n\n> Keywords like `if`, `for` and `[[` are part of the Bash language.\n\n### Find the real program behind an alias\n\n```bash\ntype -P grep\n```\n```text\n/usr/bin/grep\n```\n\n> Shows the file even though `grep` is also an alias.\n\n### Look at a shell function\n\n```bash\ntype my_backup\n```\n\n> If `my_backup` is a function, Bash prints its full code. Useful to debug functions from `~/.bashrc`.\n\n### Check several names at once\n\n```bash\ntype -t python3 docker podman kubectl\n```\n```text\nfile\nfile\n```\n\n> Names that are not found print nothing in -t mode. Here only two of four are installed.\n\n### Find out why an old version of a tool runs\n\n```bash\ntype -a terraform\n```\n```text\nterraform is /usr/local/bin/terraform\nterraform is /usr/bin/terraform\n```\n\n> Two copies are installed. The first one in $PATH wins; remove or rename the old one.\n\n### Detect a hidden alias in a security review\n\n```bash\ntype sudo\n```\n\n> If an attacker added `alias sudo=...` to a `.bashrc`, `type` shows it. Real sudo is `/usr/bin/sudo`.\n\n### Use type in a script to require a tool\n\n```bash\ntype -P rsync >/dev/null || { echo \"rsync is missing\" >&2; exit 1; }\n```\n\n> Stops the script early with a clear message.\n\n## Watch out\n\n- `type` is a Bash built-in. It does not exist as a program file, so `sudo type` does not work.\n- `which` does not know functions and built-ins (it only searches files), so the two can give different answers.\n\n## Tips\n\n- `type` = what runs, `type -a` = everything that could run, `type -P` = only the file.\n- Remember the order Bash uses: alias → keyword → function → built-in → file in $PATH.\n\nFull manual: [man type](https://linux.die.net/man/1/bash)\n","page":"commands/type/","label":"type","where":"Command page · Shell and help","aka":[]},{"id":"cmd-tzselect","n":1551,"slug":"tzselect","title":"tzselect — Find the name of a time zone by answering questions (it does not change the system)","sections":["commands"],"topic":"Commands · Time","tags":["time zone","find zone name"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Find the name of a time zone by answering questions (it does not change the system).\n\n## About\n\n`tzselect` asks for the continent, country and region and prints the zone name (like `Asia/Riyadh`) plus a TZ line you can use. Then set it with `timedatectl set-timezone NAME`.\n\n## Syntax\n\n```text\ntzselect [-c COORD] [-n LIMIT]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-c COORD` | Start from coordinates like +2430+04643 (Riyadh). |\n| `-n LIMIT` | With -c: show this many closest zones. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Find a zone interactively\n\n```bash\ntzselect\n```\n\n> Choose continent, country, region; it prints the name.\n\n### Then set it\n\n```bash\ntimedatectl set-timezone Asia/Riyadh\n```\n\n> tzselect only shows the name.\n\n### By coordinates\n\n```bash\ntzselect -c +2430+04643 -n 3\n```\n\n> The 3 zones closest to Riyadh.\n\n### Alternative: list and grep\n\n```bash\ntimedatectl list-timezones | grep -i cairo\n```\n```text\nAfrica/Cairo\n```\n\n> Faster when you know the city.\n\n### Test a zone first\n\n```bash\nTZ='Africa/Cairo' date\n```\n\n> Shows the local time there.\n\n### Use in a user profile\n\n```bash\necho \"TZ='Europe/Amsterdam'; export TZ\" >> ~/.bash_profile\n```\n\n> Only this user sees that zone.\n\n### Zone files\n\n```bash\nls /usr/share/zoneinfo/Asia | head\n```\n\n> Every zone is a file.\n\n### Current zone\n\n```bash\ntimedatectl show -p Timezone --value\n```\n\n> The system zone.\n\n### Check daylight saving changes\n\n```bash\nzdump -v Europe/Amsterdam | grep 2026\n```\n\n> When the clock changes.\n\n### In containers\n\n```bash\npodman run --rm -e TZ=Asia/Riyadh registry.access.redhat.com/ubi10/ubi date\n```\n\n> Set the zone for one container.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man tzselect](https://linux.die.net/man/8/tzselect)\n","page":"commands/tzselect/","label":"tzselect","where":"Command page · Time","aka":[]},{"id":"cmd-udevadm","n":1552,"slug":"udevadm","title":"udevadm — Control and query udev, the service that creates device files and runs rules when hardware appears","sections":["commands"],"topic":"Commands · Kernel and hardware","tags":["udev","devices","events","rules","hotplug","device properties"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Control and query udev, the service that creates device files and runs rules when hardware appears.\n\n## About\n\nWhen the kernel finds a device it sends an event; `systemd-udevd` applies the rules from `/usr/lib/udev/rules.d/` and `/etc/udev/rules.d/`: create `/dev` names and links (like `/dev/disk/by-uuid/`), set owners, load modules, run programs. `udevadm` shows device properties (`info`), watches events (`monitor`), replays events (`trigger`), waits for them (`settle`) and reloads rules (`control`).\n\n## Syntax\n\n```text\nudevadm [-d] COMMAND [OPTION]...\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `info` | Show the properties of a device or the udev database. |\n| `monitor` | Print kernel and udev events live. |\n| `trigger` | Ask the kernel to send events again (apply new rules). |\n| `settle` | Wait until all events are handled. |\n| `control` | Control the udev daemon (reload rules, log level). |\n| `test` | Simulate an event for a device and show the rules used. |\n| `test-builtin` | Test a built-in command. |\n| `verify` | Check rule files for errors (newer systemd). |\n| `wait` | Wait until a device is ready. |\n| `lock` | Lock a block device during a command. |\n| `cat` | Show rule files (newer systemd). |\n\n## Options\n\n\n### General\n\n| Option | What it does |\n|---|---|\n| `-d, --debug` | Debug output. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n### info\n\n| Option | What it does |\n|---|---|\n| `-q, --query=TYPE` | name, symlink, path, property, all. |\n| `-n, --name=FILE` | The device by /dev name. |\n| `-p, --path=DEVPATH` | The device by /sys path. |\n| `-a, --attribute-walk` | All sysfs attributes up the parent chain (for writing rules). |\n| `-r, --root` | Print /dev paths in full. |\n| `-x, --export` | Output as shell variables. |\n| `-P, --export-prefix=PREFIX` | Prefix for exported names. |\n| `--property=NAME, --value` | Only these properties / only values. |\n| `-e, --export-db` | Dump the whole database. |\n| `-c, --cleanup-db` | Clean the database. |\n| `-t, --tree` | Tree view (newer systemd). |\n\n### monitor\n\n| Option | What it does |\n|---|---|\n| `-k, --kernel` | Only kernel events. |\n| `-u, --udev` | Only udev events. |\n| `-p, --property` | Show the event properties. |\n| `-s, --subsystem-match=SUBSYSTEM[/DEVTYPE]` | Only this subsystem, like block. |\n| `-t, --tag-match=TAG` | Only events with this tag. |\n\n### trigger\n\n| Option | What it does |\n|---|---|\n| `-c, --action=ACTION` | add, change (default), remove … |\n| `-n, --dry-run` | Do not send events. |\n| `-v, --verbose` | Show the devices. |\n| `-t, --type=devices\\|subsystems` | What to trigger. |\n| `-s, --subsystem-match=SUBSYSTEM, -S, --subsystem-nomatch=SUBSYSTEM` | Filter by subsystem. |\n| `-a, --attr-match=ATTR=VALUE, -A, --attr-nomatch=ATTR=VALUE` | Filter by attribute. |\n| `-p, --property-match=KEY=VALUE, -g, --tag-match=TAG, -y, --sysname-match=NAME, --name-match=NAME, -b, --parent-match=SYSPATH` | More filters. |\n| `-w, --settle` | Wait until the events are handled. |\n| `--uuid` | Mark events with a UUID. |\n\n### settle\n\n| Option | What it does |\n|---|---|\n| `-t, --timeout=SECONDS` | Maximum wait (default 120). |\n| `-E, --exit-if-exists=FILE` | Stop waiting when the file exists. |\n\n### control\n\n| Option | What it does |\n|---|---|\n| `-R, --reload` | Reload rules and databases. |\n| `-l, --log-level=LEVEL` | Set the log level of udevd. |\n| `-s, --stop-exec-queue, -S, --start-exec-queue` | Pause / continue handling events. |\n| `-p, --property=KEY=VALUE` | Set a global property. |\n| `-m, --children-max=N` | Maximum parallel workers. |\n| `-e, --exit` | Tell udevd to exit. |\n| `--ping` | Check that udevd answers. |\n| `-t, --timeout=SECONDS` | Timeout for the request. |\n\n## Examples\n\n### Properties of a disk\n\n```bash\nudevadm info /dev/vdb\n```\n\n> ID_SERIAL, ID_PATH, DEVLINKS (by-id, by-path …).\n\n### Watch events while plugging in\n\n```bash\nudevadm monitor -p\n```\n\n> Kernel events (KERNEL) and udev events (UDEV) with all properties.\n\n### Only block devices\n\n```bash\nudevadm monitor -s block\n```\n\n> Disks and partitions.\n\n### Attributes for a rule\n\n```bash\nudevadm info -a -n /dev/sdc | less\n```\n\n> Use ATTRS{serial}==\"…\" in your rule.\n\n### A rule for a fixed name\n\n```bash\necho 'SUBSYSTEM==\"block\", ENV{ID_SERIAL}==\"SanDisk_Ultra_Fit_4C53…\", SYMLINK+=\"backupdisk\"' > /etc/udev/rules.d/90-backup.rules\n```\n\n> /dev/backupdisk appears for that stick.\n\n### Reload and apply rules\n\n```bash\nudevadm control --reload\nudevadm trigger -s block\n```\n\n> New rules work without a reboot.\n\n### Wait for devices in scripts\n\n```bash\npartprobe /dev/vdb; udevadm settle\n```\n\n> Partition devices exist after settle.\n\n### Test which rules apply\n\n```bash\nudevadm test $(udevadm info -q path -n /dev/vdb) 2>&1 | tail\n```\n\n> Debug a rule that does not work.\n\n### Find the stable names of a disk\n\n```bash\nudevadm info -q symlink -n /dev/vdb\n```\n\n> by-id and by-path names for fstab or LVM filters.\n\n### Block rogue USB devices with a rule (security)\n\n```bash\necho 'ACTION==\"add\", SUBSYSTEMS==\"usb\", ATTR{authorized}=\"0\"' > /etc/udev/rules.d/99-usb-deny.rules\n```\n\n> New USB devices are not authorized (keyboards too! use USBGuard in production).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/usr/lib/udev/rules.d/` | Rules from packages. |\n| `/etc/udev/rules.d/` | Your rules (win over the same name). |\n| `/run/udev/data/` | udev database. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man udevadm](https://linux.die.net/man/8/udevadm)\n","page":"commands/udevadm/","label":"udevadm","where":"Command page · Kernel and hardware","aka":[]},{"id":"cmd-umask","n":1553,"slug":"umask","title":"umask — Show or set the mask that decides the permissions of new files and directories","sections":["commands"],"topic":"Commands · Permissions","tags":["default permissions","new files","mask","profile","hardening"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show or set the mask that decides the permissions of new files and directories.\n\n## About\n\nPrograms create new files with `666` (rw for all) and new directories with `777`. The umask removes bits from this: with umask `022`, files get `644` and directories `755`.\n\n`umask` is a shell built-in: it changes only the current shell and its children. Make it permanent in `~/.bashrc` (one user) or a file in `/etc/profile.d/` (all users). Services use the `UMask=` setting in their systemd unit.\n\n## Syntax\n\n```text\numask [-p] [-S] [MODE]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `MODE` | The new mask, as a number (027) or in letters (u=rwx,g=rx,o=). |\n| `-S` | Show the mask in letters (as the permissions that stay). |\n| `-p` | Show it in a form you can use again as a command. |\n\n## Examples\n\n### Show the mask\n\n```bash\numask\n```\n```text\n0022\n```\n\n> The RHEL default.\n\n### Show it in letters\n\n```bash\numask -S\n```\n```text\nu=rwx,g=rx,o=rx\n```\n\n> The rights that new directories get.\n\n### Set a safer mask for this shell\n\n```bash\numask 027\ntouch f; mkdir d; ls -ld f d\n```\n```text\n-rw-r-----. 1 student student 0 Oct 25 10:30 f\ndrwxr-x---. 2 student student 6 Oct 25 10:30 d\n```\n\n> Others get nothing.\n\n### Make it permanent for one user\n\n```bash\necho \"umask 027\" >> ~/.bashrc\n```\n\n> Valid from the next shell.\n\n### Make it permanent for all users\n\n```bash\necho \"umask 027\" > /etc/profile.d/umask.sh\n```\n\n> Read at every login shell.\n\n### Very private files\n\n```bash\numask 077\n```\n\n> Files 600, folders 700.\n\n### Set the umask with letters\n\n```bash\numask u=rwx,g=rx,o=\numask\n```\n```text\n0027\n```\n\n> Same as 027.\n\n### Use a mask for one command only\n\n```bash\n(umask 077; openssl genrsa -out private.key 4096)\n```\n\n> The key file is private from the start (no time window).\n\n### Set the umask of a service\n\n```bash\nsystemctl edit myapp\n```\n\n> Add `[Service]` and `UMask=0027` — shell files do not affect services.\n\n### Check the default of new users\n\n```bash\ngrep -E '^(UMASK|HOME_MODE)' /etc/login.defs\n```\n```text\nUMASK\t\t022\nHOME_MODE\t0700\n```\n\n> login.defs sets the umask for login and the mode of new home folders.\n\n### Why a file never gets x from the umask\n\n```bash\numask 000; touch x; ls -l x\n```\n```text\n-rw-rw-rw-. 1 student student 0 Oct 25 10:35 x\n```\n\n> Files start at 666; add x with chmod.\n\n## Watch out\n\n- The umask is not a chmod: it only affects files created later.\n- A default ACL on a directory replaces the umask for new files in it.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.bashrc` | umask for one user. |\n| `/etc/profile.d/*.sh` | umask for all login shells. |\n| `/etc/login.defs` | UMASK and HOME_MODE. |\n\nFull manual: [man umask](https://linux.die.net/man/1/bash)\n","page":"commands/umask/","label":"umask","where":"Command page · Permissions","aka":[]},{"id":"cmd-umount","n":1554,"slug":"umount","title":"umount — Detach a mounted file system from the folder tree","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["unmount","busy","lazy","eject"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Detach a mounted file system from the folder tree.\n\n## About\n\n`umount FOLDER` (or the device) writes pending data and detaches the file system. It fails with \"target is busy\" when a process has open files there or uses it as its current folder — also your own shell. Find the process with `lsof` or `fuser`, stop it, and try again.\n\n## Syntax\n\n```text\numount [OPTION]... DIRECTORY|DEVICE\numount -a [-t TYPE] [-O OPTIONS]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --lazy` | Detach now, clean up when it is no longer busy. |\n| `-f, --force` | Force (mainly for unreachable NFS). |\n| `-R, --recursive` | Also unmount all sub-mounts. |\n| `-a, --all` | Unmount everything in /proc/self/mountinfo (except proc, sys …). |\n| `-A, --all-targets` | Unmount all mount points of the device. |\n| `-t, --types TYPES` | Only these types (with -a). |\n| `-O, --test-opts OPTIONS` | Only mounts with these options (with -a). |\n| `-r, --read-only` | Remount read-only if unmount fails. |\n| `-d, --detach-loop` | Also free the loop device. |\n| `-c, --no-canonicalize` | Do not resolve paths. |\n| `-i, --internal-only` | Do not call umount.TYPE helpers. |\n| `-n, --no-mtab` | Do not write /etc/mtab. |\n| `-N, --namespace NS` | Work in another mount namespace. |\n| `--fake` | Do everything except the unmount. |\n| `-q, --quiet` | No \"not mounted\" errors. |\n| `-v, --verbose` | Verbose. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Unmount a folder\n\n```bash\numount /data\n```\n\n> Or `umount /dev/vdb1`.\n\n### Target is busy\n\n```bash\numount /data\n```\n```text\numount: /data: target is busy.\n```\n```bash\nlsof /data\nfuser -vm /data\n```\n\n> Shows the processes; often a shell that is still in /data.\n\n### Leave the folder yourself\n\n```bash\ncd /; umount /data\n```\n\n> The most common cause.\n\n### Lazy unmount\n\n```bash\numount -l /data\n```\n\n> Disappears now; really freed when the last user is gone.\n\n### Hanging NFS share\n\n```bash\numount -f -l /mnt/nfs\n```\n\n> When the server is gone.\n\n### Unmount a tree\n\n```bash\numount -R /mnt/chroot\n```\n\n> Also /mnt/chroot/proc, dev, sys …\n\n### Unmount an ISO and free the loop device\n\n```bash\numount -d /repo\n```\n\n> Cleans the loop device too.\n\n### Remount read-only if busy\n\n```bash\numount -r /data\n```\n\n> At least no more writes.\n\n### Unmount all NFS mounts\n\n```bash\numount -a -t nfs,nfs4\n```\n\n> Before a network change.\n\n### Check that it is gone\n\n```bash\nfindmnt /data || echo \"not mounted\"\n```\n\n> findmnt returns 1 when nothing is mounted.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n| `32` | Unmount failure. |\n| `64` | Some worked (with -a). |\n\nFull manual: [man umount](https://linux.die.net/man/8/umount)\n","page":"commands/umount/","label":"umount","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-uname","n":1555,"slug":"uname","title":"uname — Show the name and version of the kernel, the machine type and the hostname","sections":["commands"],"topic":"Commands · System information","tags":["kernel","version","architecture","system","hostname"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the name and version of the kernel, the machine type and the hostname.\n\n## About\n\n`uname` prints facts about the running kernel and the hardware type. It is the quickest way to answer \"which kernel runs here?\" and \"is this a 64-bit Intel/AMD or an ARM machine?\".\n\nWithout options it prints only the kernel name (`Linux`). With `-r` you get the kernel release, which you need when you install kernel modules, read security bulletins or check that a kernel update is really active after a reboot.\n\n## Syntax\n\n```text\nuname [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Print everything below, in this order (unknown fields are left out). |\n| `-s, --kernel-name` | The kernel name (`Linux`). This is the default. |\n| `-n, --nodename` | The network name of the machine (the hostname). |\n| `-r, --kernel-release` | The kernel release, like `6.12.0-55.el10.x86_64`. |\n| `-v, --kernel-version` | The build version and date of the kernel. |\n| `-m, --machine` | The hardware type, like `x86_64` or `aarch64`. |\n| `-p, --processor` | The processor type (often `x86_64` or `unknown`). |\n| `-i, --hardware-platform` | The hardware platform (often `x86_64` or `unknown`). |\n| `-o, --operating-system` | The operating system (`GNU/Linux`). |\n| `--help` | Show help and exit. |\n| `--version` | Show the version and exit. |\n\n## Examples\n\n### Show the running kernel release\n\n```bash\nuname -r\n```\n```text\n6.12.0-55.el10.x86_64\n```\n\n> `el10` means a kernel for RHEL 10. The last part is the CPU architecture.\n\n### Show all information on one line\n\n```bash\nuname -a\n```\n```text\nLinux server1.example.com 6.12.0-55.el10.x86_64 #1 SMP PREEMPT_DYNAMIC ... x86_64 GNU/Linux\n```\n\n> Paste this line in a bug report or a support ticket.\n\n### Check the CPU architecture before you download software\n\n```bash\nuname -m\n```\n```text\nx86_64\n```\n\n> Download the `x86_64` (amd64) build for this machine, or the `aarch64` (arm64) build for an ARM server.\n\n### Compare the running kernel with the installed kernels\n\n```bash\nuname -r\nrpm -q kernel\n```\n\n> If the newest installed kernel is not the one that runs, the server still needs a reboot to use the update.\n\n### Find out if a reboot is needed after updates\n\n```bash\n[ \"$(uname -r)\" = \"$(rpm -q --last kernel | head -1 | awk '{print $1}' | sed 's/^kernel-//')\" ] || echo \"reboot needed\"\n```\n\n> A simple check for a patch script: the running kernel must be the newest installed one.\n\n### Use the kernel release in a path\n\n```bash\nls /lib/modules/$(uname -r)/kernel/drivers/net/\n```\n\n> Kernel modules are stored in a folder named after the kernel release.\n\n### Print only the hostname\n\n```bash\nuname -n\n```\n```text\nserver1.example.com\n```\n\n> Same as the `hostname` command. Useful in log lines of scripts.\n\n### Pick the right binary in an install script\n\n```bash\ncase \"$(uname -m)\" in x86_64) ARCH=amd64 ;; aarch64) ARCH=arm64 ;; *) echo \"unknown arch\"; exit 1 ;; esac\n```\n\n> Many tools (kubectl, terraform, trivy) name their downloads amd64 or arm64.\n\n### Check the kernel version in an inventory report\n\n```bash\necho \"$(hostname),$(uname -r),$(uname -m)\" >> /tmp/inventory.csv\n```\n\n> Collect one line per server, then compare kernel versions across the fleet.\n\n### Show the kernel build date\n\n```bash\nuname -v\n```\n\n> A very old build date on an internet-facing server is a warning sign: patches are missing.\n\n### Check the operating system family in a portable script\n\n```bash\n[ \"$(uname -s)\" = \"Linux\" ] && echo \"running on Linux\"\n```\n\n> On macOS `uname -s` prints `Darwin`. Scripts that run on both can branch on this.\n\n## Watch out\n\n- `uname` shows the **running** kernel, not the newest installed one. After a kernel update, `uname -r` changes only after a reboot.\n- `-p` and `-i` often print `unknown`. Use `-m` for the architecture.\n- The distribution version (RHEL 10.0) is not in `uname`. Read `/etc/redhat-release` or `/etc/os-release`.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/sys/kernel/osrelease` | The kernel release (same as `uname -r`). |\n| `/proc/version` | The full kernel version string. |\n| `/etc/os-release` | Name and version of the Linux distribution. |\n\n## Tips\n\n- **r** = release (the kernel version you check most), **m** = machine (the CPU type), **a** = all.\n- Kernel = `uname -r`; distribution = `cat /etc/redhat-release`.\n\nFull manual: [man uname](https://linux.die.net/man/1/uname)\n","page":"commands/uname/","label":"uname","where":"Command page · System information","aka":[]},{"id":"cmd-uniq","n":1556,"slug":"uniq","title":"uniq — Remove or count repeated lines that are next to each other (use it after sort)","sections":["commands"],"topic":"Commands · Text and search","tags":["duplicates","count","unique","top list"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Remove or count repeated lines that are next to each other (use it after sort).\n\n## About\n\n`uniq` compares each line with the line before. When they are the same, it prints the line only once. It only finds **neighbors**, so you almost always run `sort` first.\n\nWith `-c` it counts how often each line appears; with `-d` it shows only repeated lines; with `-u` only lines that appear once.\n\n## Syntax\n\n```text\nuniq [OPTION]... [INPUT [OUTPUT]]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-c, --count` | Put the number of times in front of each line. |\n| `-d, --repeated` | Print only lines that are repeated (one per group). |\n| `-u, --unique` | Print only lines that are not repeated. |\n| `-D` | Print all repeated lines (every copy). |\n| `--all-repeated[=<method>]` | Like -D, with empty lines between groups (none, prepend, separate). |\n| `--group[=<method>]` | Print all lines, with empty lines between groups (separate, prepend, append, both). |\n| `-i, --ignore-case` | Ignore upper/lower case. |\n| `-f, --skip-fields=<n>` | Ignore the first N fields when comparing. |\n| `-s, --skip-chars=<n>` | Ignore the first N characters when comparing. |\n| `-w, --check-chars=<n>` | Compare only the first N characters. |\n| `-z, --zero-terminated` | Lines end with NUL. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Remove repeated lines\n\n```bash\nsort ips.txt | uniq\n```\n\n> Each IP once.\n\n### Count each value\n\n```bash\nsort ips.txt | uniq -c | sort -rn\n```\n\n> The most common values first.\n\n### Show only values that appear more than once\n\n```bash\ncut -d: -f3 /etc/passwd | sort | uniq -d\n```\n\n> Duplicate UIDs: two accounts with the same UID is a security problem.\n\n### Show values that appear only once\n\n```bash\nsort list.txt | uniq -u\n```\n\n> The unique entries.\n\n### Count failed logins per user\n\n```bash\ngrep \"Failed password for\" /var/log/secure | awk '{print $(NF-5)}' | sort | uniq -c | sort -rn\n```\n\n> Which accounts are attacked most?\n\n### Ignore case while counting\n\n```bash\nsort -f words.txt | uniq -ic\n```\n\n> \"Error\" and \"error\" are counted together.\n\n### Compare lines but skip the date field\n\n```bash\nuniq -f 1 events.log\n```\n\n> Lines that are the same after the first field are shown once.\n\n### Find duplicate file contents by checksum\n\n```bash\nsha256sum * | sort | uniq -w64 -D\n```\n\n> The first 64 characters are the hash; equal hashes = equal files.\n\n### Show groups of repeated lines\n\n```bash\nsort names.txt | uniq --all-repeated=separate\n```\n\n> Each group of copies separated by an empty line.\n\n### Count HTTP status codes in a web log\n\n```bash\nawk '{print $9}' /var/log/httpd/access_log | sort | uniq -c\n```\n\n> How many 200, 404, 500 answers.\n\n### Unique lines without changing the order (alternative)\n\n```bash\nawk '!seen[$0]++' list.txt\n```\n\n> uniq needs sorted input; this awk line keeps the first copy in the original order.\n\n## Watch out\n\n- uniq only removes repeats that are **next to each other**. Without sort, repeats far apart stay.\n\n## Tips\n\n- `sort | uniq -c | sort -rn` = the classic top list.\n\nFull manual: [man uniq](https://linux.die.net/man/1/uniq)\n","page":"commands/uniq/","label":"uniq","where":"Command page · Text and search","aka":[]},{"id":"cmd-uptime","n":1557,"slug":"uptime","title":"uptime — Show how long the system runs, how many users are logged in and the load average","sections":["commands"],"topic":"Commands · Processes and jobs","tags":["load average","uptime","boot time","users"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show how long the system runs, how many users are logged in and the load average.\n\n## About\n\nThe load average is the number of processes that run or wait (for CPU or disk), as an average over 1, 5 and 15 minutes. Compare it with the number of CPU cores (`nproc`): a load of 4 on 2 cores means processes wait.\n\n## Syntax\n\n```text\nuptime [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --pretty` | Only the run time, in words. |\n| `-s, --since` | The date and time of the boot. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Default output\n\n```bash\nuptime\n```\n```text\n 10:52:01 up  2:54,  2 users,  load average: 0.15, 0.10, 0.09\n```\n\n> Time, run time, users, load 1/5/15 minutes.\n\n### Run time in words\n\n```bash\nuptime -p\n```\n```text\nup 2 hours, 54 minutes\n```\n\n> Easy to read.\n\n### Boot time\n\n```bash\nuptime -s\n```\n```text\n2026-10-25 07:58:12\n```\n\n> Did the server reboot in the night?\n\n### Load per core\n\n```bash\necho \"$(cut -d' ' -f1 /proc/loadavg) / $(nproc)\" | bc -l\n```\n\n> Over 1.0 = more work than cores.\n\n### Raw load data\n\n```bash\ncat /proc/loadavg\n```\n```text\n0.15 0.10 0.09 1/182 4410\n```\n\n> Also running/total processes and the last PID.\n\n### Watch the load\n\n```bash\nwatch -n 5 uptime\n```\n\n> The 1-minute value reacts first.\n\n### On many servers\n\n```bash\nfor h in server1 server2; do echo -n \"$h: \"; ssh $h uptime; done\n```\n\n> Quick overview.\n\n### Unexpected reboot? (security)\n\n```bash\nuptime -s; last -x reboot | head -3\n```\n\n> Compare with the change calendar.\n\n### Same line in w and top\n\n```bash\nw | head -1\n```\n\n> w and top show the uptime line first.\n\n### Alert on high load in a script\n\n```bash\nawk -v c=$(nproc) '$1 > c {print \"high load: \" $1}' /proc/loadavg\n```\n\n> Simple check for cron.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/proc/uptime` | Seconds since boot. |\n| `/proc/loadavg` | Load averages. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man uptime](https://linux.die.net/man/1/uptime)\n","page":"commands/uptime/","label":"uptime","where":"Command page · Processes and jobs","aka":[]},{"id":"cmd-useradd","n":1558,"slug":"useradd","title":"useradd — Create a new user account (and by default its private group and home folder)","sections":["commands"],"topic":"Commands · Users and groups","tags":["create user","account","home folder","skel","uid","service account","login.defs"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a new user account (and by default its private group and home folder).\n\n## About\n\n`useradd` writes the new account to `/etc/passwd`, `/etc/shadow`, `/etc/group` and `/etc/gshadow`. On RHEL it also creates a group with the same name and a home folder in `/home`, with a copy of the files in `/etc/skel`.\n\nThe new account has no password yet, so nobody can log in with it. Set one with `passwd`.\n\nValues you do not give come from `/etc/login.defs` (UID range, password aging, home mode) and `/etc/default/useradd` (shell, home base, skel). `useradd -D` shows and changes the second file.\n\n## Syntax\n\n```text\nuseradd [OPTION]... LOGIN\nuseradd -D\nuseradd -D [-b BASE] [-e DATE] [-f DAYS] [-g GROUP] [-s SHELL]\n```\n\n## Options\n\n\n### Account data\n\n| Option | What it does |\n|---|---|\n| `-c, --comment COMMENT` | Comment, mostly the full name (GECOS field). |\n| `-u, --uid UID` | Use this UID. |\n| `-o, --non-unique` | Allow a UID that is already used (with -u). |\n| `-s, --shell SHELL` | Login shell, like /bin/bash or /sbin/nologin. |\n| `-e, --expiredate DATE` | The account expires on this date (YYYY-MM-DD). |\n| `-f, --inactive DAYS` | Lock the account this many days after the password expired (-1 = never). |\n| `-p, --password HASH` | Set an already hashed password (not the clear text; not recommended). |\n| `-r, --system` | Create a system account: UID under 1000, no aging, no home folder by default. |\n| `--badname` | Allow names that do not follow the normal rules. |\n\n### Groups\n\n| Option | What it does |\n|---|---|\n| `-g, --gid GROUP` | The primary group (name or GID; it must exist). |\n| `-G, --groups GROUP1[,GROUP2...]` | Extra (supplementary) groups. |\n| `-U, --user-group` | Create a group with the same name as the user (default on RHEL). |\n| `-N, --no-user-group` | Do not create a private group; use -g or the default GROUP. |\n\n### Home folder\n\n| Option | What it does |\n|---|---|\n| `-d, --home-dir DIR` | Use this home folder. |\n| `-m, --create-home` | Create the home folder (default on RHEL via CREATE_HOME). |\n| `-M, --no-create-home` | Do not create the home folder. |\n| `-b, --base-dir DIR` | Base folder for homes (home = DIR/LOGIN). |\n| `-k, --skel DIR` | Copy the start files from this folder instead of /etc/skel (with -m). |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `-D, --defaults` | Show or change the defaults in /etc/default/useradd. |\n| `-K, --key KEY=VALUE` | Override a value of /etc/login.defs, like `-K UID_MIN=5000`. |\n| `-l, --no-log-init` | Do not add the user to the lastlog and faillog files. |\n| `-F, --add-subids-for-system` | Add subuid/subgid ranges also for system accounts. |\n| `-Z, --selinux-user SEUSER` | The SELinux user for this login (like staff_u). |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Create a user with a full name\n\n```bash\nuseradd -c \"Lisa Hassan\" lisa\npasswd lisa\n```\n\n> Without a password nobody can log in.\n\n### Create a user with extra groups\n\n```bash\nuseradd -G wheel,webadmins omar\nid omar\n```\n```text\nuid=1002(omar) gid=1002(omar) groups=1002(omar),10(wheel),1100(webadmins)\n```\n\n> The groups must exist.\n\n### Choose the UID\n\n```bash\nuseradd -u 1500 sam\n```\n\n> Same UID on all servers keeps NFS file owners right.\n\n### Create a service account\n\n```bash\nuseradd -r -s /sbin/nologin -d /var/lib/myapp -M myapp\n```\n\n> No login shell, no home, UID under 1000 (least privilege for an app).\n\n### Create a temporary account that expires\n\n```bash\nuseradd -e 2026-12-31 -c \"Contractor\" contractor1\n```\n\n> After this date the account cannot log in.\n\n### Show the defaults\n\n```bash\nuseradd -D\n```\n```text\nGROUP=100\nHOME=/home\nINACTIVE=-1\nEXPIRE=\nSHELL=/bin/bash\nSKEL=/etc/skel\n```\n\n> These values come from /etc/default/useradd.\n\n### Change the default shell for new users\n\n```bash\nuseradd -D -s /bin/zsh\n```\n\n> Only new users get it.\n\n### Use another start folder for a team\n\n```bash\nuseradd -m -k /etc/skel-dev dev1\n```\n\n> Developers get their own .bashrc and tools config.\n\n### Map a user to a confined SELinux user\n\n```bash\nuseradd -Z staff_u admin2\n```\n\n> A confined admin cannot do everything even as root (hardening).\n\n### Create many users from a list\n\n```bash\nfor u in anna bob carol; do useradd -G sales \"$u\"; echo password | passwd --stdin \"$u\"; done\n```\n\n> Only for labs: real users need a good password each.\n\n### Check the new account\n\n```bash\ngetent passwd lisa; ls -la /home/lisa\n```\n```text\nlisa:x:1001:1001:Lisa Hassan:/home/lisa:/bin/bash\n```\n\n> The home folder has the files from /etc/skel.\n\n## Watch out\n\n- `useradd` does not set a password. Use `passwd` after it.\n- `-p` wants a hash, not the password text. `useradd -p secret` gives an account that cannot log in.\n- Changes in /etc/skel or login.defs only affect new users.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/passwd, /etc/shadow` | Account and password data. |\n| `/etc/group, /etc/gshadow` | Group data. |\n| `/etc/login.defs` | UID/GID ranges, password aging, home mode, umask. |\n| `/etc/default/useradd` | Defaults for shell, home base and skel. |\n| `/etc/skel/` | Start files for new home folders. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Could not update the password file. |\n| `2` | Wrong syntax. |\n| `3` | Wrong value for an option. |\n| `4` | UID already used. |\n| `6` | The group does not exist. |\n| `9` | The user name is already used. |\n| `12` | Could not create the home folder. |\n| `14` | Could not update the SELinux user mapping. |\n\nFull manual: [man useradd](https://linux.die.net/man/8/useradd)\n","page":"commands/useradd/","label":"useradd","where":"Command page · Users and groups","aka":[]},{"id":"cmd-userdel","n":1559,"slug":"userdel","title":"userdel — Delete a user account, and with -r also its home folder and mail","sections":["commands"],"topic":"Commands · Users and groups","tags":["delete user","remove account","home folder","offboarding"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Delete a user account, and with -r also its home folder and mail.\n\n## About\n\n`userdel` removes the user from the account files, and the private group of the user if no other user has it as primary group.\n\nWithout `-r` the home folder stays. Files of the user in other places always stay: they then show a number instead of a name as owner. A new user with the same UID would own them, so find and clean them first.\n\n## Syntax\n\n```text\nuserdel [OPTION]... LOGIN\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-r, --remove` | Also remove the home folder and the mail spool. |\n| `-f, --force` | Remove even when the user is logged in, and remove the group even if it is the primary group of another user. Dangerous. |\n| `-Z, --selinux-user` | Remove the SELinux user mapping of the login. |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Delete a user and the home folder\n\n```bash\nuserdel -r contractor1\n```\n\n> The usual way for a lab or a test account.\n\n### Delete only the account\n\n```bash\nuserdel olduser\n```\n\n> The files in /home/olduser stay (for a backup).\n\n### Find files of a user before you delete it\n\n```bash\nfind / -xdev -user olduser 2>/dev/null\n```\n\n> Keep or move important files first.\n\n### Find files without an owner after a delete\n\n```bash\nfind / -xdev -nouser 2>/dev/null\n```\n\n> A security check: orphan files can be taken over by a new user with the same UID.\n\n### Check that nobody uses the account\n\n```bash\nwho | grep olduser; pgrep -u olduser\n```\n\n> userdel refuses when the user has processes (exit code 8).\n\n### Stop the processes, then delete\n\n```bash\npkill -KILL -u olduser; userdel -r olduser\n```\n\n> Only when you are sure the work is saved.\n\n### Offboarding: lock first, delete later\n\n```bash\nusermod -L -e 1 olduser\n```\n\n> Wait some weeks, then `userdel -r olduser`.\n\n### Remove the cron jobs of the user\n\n```bash\ncrontab -r -u olduser\n```\n\n> userdel does not always remove them on every system.\n\n### Archive the home folder before the delete\n\n```bash\ntar -czf /root/olduser-home.tgz -C /home olduser && userdel -r olduser\n```\n\n> Keep a copy for the legal hold period.\n\n### Delete a service account\n\n```bash\nuserdel myapp\n```\n\n> Its data in /var/lib/myapp stays; remove it yourself.\n\n### Check that it is gone\n\n```bash\nid olduser\n```\n```text\nid: 'olduser': no such user\n```\n\n> The account is removed.\n\n## Watch out\n\n- Files outside the home folder are not removed.\n- `-f` can remove a group that other users still use as primary group.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/passwd, /etc/shadow, /etc/group, /etc/gshadow` | The account files. |\n| `/etc/login.defs` | USERGROUPS_ENAB decides if the private group is removed. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Could not update the password file. |\n| `2` | Wrong syntax. |\n| `6` | The user does not exist. |\n| `8` | The user is logged in. |\n| `10` | Could not update the group file. |\n| `12` | Could not remove the home folder. |\n\nFull manual: [man userdel](https://linux.die.net/man/8/userdel)\n","page":"commands/userdel/","label":"userdel","where":"Command page · Users and groups","aka":[]},{"id":"cmd-usermod","n":1560,"slug":"usermod","title":"usermod — Change an existing user: groups, shell, home folder, name, UID, expire date, lock","sections":["commands"],"topic":"Commands · Users and groups","tags":["change user","add group","lock account","shell","rename","home folder"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Change an existing user: groups, shell, home folder, name, UID, expire date, lock.\n\n## About\n\n`usermod` changes the account data in the user and group files. The most used job is adding a user to an extra group: `usermod -aG GROUP USER`.\n\nTake care with `-G`: without `-a` it **replaces** all extra groups. A user who is logged in gets new groups only after a new login.\n\n## Syntax\n\n```text\nusermod [OPTION]... LOGIN\n```\n\n## Options\n\n\n### Groups\n\n| Option | What it does |\n|---|---|\n| `-a, --append` | Add the groups of -G to the current ones (do not replace). |\n| `-G, --groups GROUP1[,GROUP2...]` | The extra groups (alone it replaces the list). |\n| `-r, --remove` | Remove the user from the groups of -G (newer versions). |\n| `-g, --gid GROUP` | Change the primary group. |\n\n### Account data\n\n| Option | What it does |\n|---|---|\n| `-c, --comment COMMENT` | Change the comment (full name). |\n| `-s, --shell SHELL` | Change the login shell. |\n| `-l, --login NEW_LOGIN` | Rename the login. |\n| `-u, --uid UID` | Change the UID (files in the home folder get the new owner). |\n| `-o, --non-unique` | Allow a UID that is used already (with -u). |\n| `-e, --expiredate DATE` | The account expires on this date (YYYY-MM-DD; empty = never). |\n| `-f, --inactive DAYS` | Lock the account this many days after the password expired. |\n| `-p, --password HASH` | Set an already hashed password. |\n| `-b, --badname` | Allow names that do not follow the rules. |\n\n### Lock\n\n| Option | What it does |\n|---|---|\n| `-L, --lock` | Lock the password (puts ! before the hash). |\n| `-U, --unlock` | Unlock the password. |\n\n### Home folder\n\n| Option | What it does |\n|---|---|\n| `-d, --home DIR` | Set a new home folder. |\n| `-m, --move-home` | Move the files to the new home folder (with -d). |\n\n### Other\n\n| Option | What it does |\n|---|---|\n| `-v, --add-subuids FIRST-LAST` | Add a range of sub UIDs (for rootless containers). |\n| `-V, --del-subuids FIRST-LAST` | Remove a range of sub UIDs. |\n| `-w, --add-subgids FIRST-LAST` | Add a range of sub GIDs. |\n| `-W, --del-subgids FIRST-LAST` | Remove a range of sub GIDs. |\n| `-Z, --selinux-user SEUSER` | Set the SELinux user of the login. |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-P, --prefix DIR` | Use the files under this prefix folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Add a user to a group\n\n```bash\nusermod -aG webadmins lisa\n```\n\n> lisa keeps all other groups.\n\n### Give a user full sudo rights\n\n```bash\nusermod -aG wheel lisa\n```\n\n> On RHEL wheel = all sudo rights.\n\n### Block logins of a service account\n\n```bash\nusermod -s /sbin/nologin backup1\n```\n\n> The account can still own files and run jobs.\n\n### Lock and unlock a password\n\n```bash\nusermod -L lisa\nusermod -U lisa\n```\n\n> A lock does not stop SSH key logins. Use an expire date too.\n\n### Disable an account fully when someone leaves\n\n```bash\nusermod -L -e 1 olduser\n```\n\n> `-e 1` = expired on 2 Jan 1970: no login at all, also not with keys.\n\n### Move the home folder\n\n```bash\nusermod -d /srv/home/lisa -m lisa\n```\n\n> -m moves the files; without it only the entry changes.\n\n### Rename a user\n\n```bash\nusermod -l lisa.hassan lisa\n```\n\n> The home folder and group keep the old name; change them with -d -m and groupmod -n.\n\n### Change the UID to match other servers\n\n```bash\nusermod -u 1501 sam\nfind / -xdev -uid 1500 -exec chown -h sam {} +\n```\n\n> Files outside the home folder keep the old number; fix them with find.\n\n### Remove one extra group\n\n```bash\nusermod -rG webadmins lisa\n```\n\n> Or use `gpasswd -d lisa webadmins` (works on all versions).\n\n### Set an expire date\n\n```bash\nusermod -e 2026-12-31 contractor1\n```\n\n> Same as `chage -E 2026-12-31 contractor1`.\n\n### Give a user ranges for rootless podman\n\n```bash\nusermod --add-subuids 100000-165535 --add-subgids 100000-165535 dev1\n```\n\n> Needed when the account was created without sub IDs.\n\n### Check the result\n\n```bash\nid lisa\n```\n\n> Shows all groups now.\n\n## Watch out\n\n- `usermod -G group user` without `-a` removes all other extra groups.\n- You cannot change the UID or name of a user who has running processes.\n- New groups work only after a new login (or `newgrp`).\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/passwd, /etc/shadow, /etc/group, /etc/gshadow` | The account files. |\n| `/etc/subuid, /etc/subgid` | Sub ID ranges. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Could not update the password file. |\n| `2` | Wrong syntax. |\n| `3` | Wrong value for an option. |\n| `4` | UID already used. |\n| `6` | The user or group does not exist. |\n| `8` | The user is logged in (for changes that need logout). |\n| `9` | The new name is already used. |\n| `12` | Could not move the home folder. |\n\nFull manual: [man usermod](https://linux.die.net/man/8/usermod)\n","page":"commands/usermod/","label":"usermod","where":"Command page · Users and groups","aka":[]},{"id":"cmd-vgcreate","n":1561,"slug":"vgcreate","title":"vgcreate — Create a volume group from one or more physical volumes (vgcreate), or delete a volume group (vgremove)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","volume group","vg","extent size","pool"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Create a volume group from one or more physical volumes (vgcreate), or delete a volume group (vgremove).\n\nSame page for: `vgremove`\n\n## About\n\n`vgcreate NAME PV...` joins PVs into a storage pool. Devices that are not PVs yet are initialized automatically. The **extent size** (`-s`, default 4 MiB) is the unit in which space is given to LVs; it is fixed for the life of the VG.\n\n`vgremove` deletes a VG (and asks to remove its LVs).\n\n## Syntax\n\n```text\nvgcreate [OPTION]... VG_NAME PV...\nvgremove [OPTION]... VG_NAME...\n```\n\n## Options\n\n\n### vgcreate\n\n| Option | What it does |\n|---|---|\n| `-s, --physicalextentsize SIZE` | Extent size, like 8M or 16M (default 4M). |\n| `-l, --maxlogicalvolumes N` | Maximum number of LVs. |\n| `-p, --maxphysicalvolumes N` | Maximum number of PVs. |\n| `--addtag TAG` | Add a tag. |\n| `--alloc POLICY` | Allocation policy: normal, contiguous, cling, anywhere … |\n| `-A, --autobackup y\\|n` | Back up the metadata after the change (default y). |\n| `--setautoactivation y\\|n` | Activate at boot automatically. |\n| `--vgmetadatacopies all\\|unmanaged\\|N` | Metadata copies in the VG. |\n| `--metadatasize SIZE, --pvmetadatacopies N, --dataalignment SIZE, --dataalignmentoffset SIZE, --labelsector N, -Z, --zero y\\|n` | Settings for PVs that are created on the fly. |\n| `-M, --metadatatype lvm2` | Metadata format. |\n| `--metadataprofile STRING` | Metadata profile. |\n| `--systemid STRING` | Set the system ID (owner host). |\n| `--shared, --locktype sanlock\\|dlm\\|none, -c, --clustered y\\|n` | Shared VGs for clusters. |\n| `-f, --force` | Force. |\n| `--reportformat basic\\|json` | Output format. |\n\n### vgremove\n\n| Option | What it does |\n|---|---|\n| `-f, --force` | Remove the LVs without asking (twice: more force). |\n| `--noudevsync` | Do not wait for udev. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Create a VG\n\n```bash\nvgcreate vgdata /dev/vdb1 /dev/vdc\n```\n```text\n  Volume group \"vgdata\" successfully created\n```\n\n> Two PVs in one pool.\n\n### With an extent size of 8 MiB\n\n```bash\nvgcreate -s 8M vgfiles /dev/vdc1\n```\n\n> Exam tasks often name the extent size.\n\n### Check it\n\n```bash\nvgs vgdata; vgdisplay vgdata | grep -E 'PE Size|Total PE|Free'\n```\n\n> PE Size, total and free extents.\n\n### Directly on a new disk\n\n```bash\nvgcreate vgbackup /dev/vdd\n```\n\n> vgcreate makes the PV for you.\n\n### Test first\n\n```bash\nvgcreate -t vgtest /dev/vde\n```\n\n> Nothing changes.\n\n### Remove a VG\n\n```bash\nvgremove vgtest\n```\n\n> Asks for every active LV. Remove fstab lines first!\n\n### Remove with all LVs at once\n\n```bash\nvgremove -f vgtest\n```\n\n> No questions — be sure.\n\n### Rename a VG\n\n```bash\nvgrename vgdata vgprod\n```\n\n> Update fstab and other configs that use the old name.\n\n### Tag a VG\n\n```bash\nvgcreate --addtag prod vgweb /dev/vdf\n```\n\n> Select VGs by tag in scripts (@prod).\n\n### Metadata backup after changes\n\n```bash\nls /etc/lvm/backup/\n```\n\n> LVM writes a backup of each VG here automatically.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man vgcreate](https://linux.die.net/man/8/vgcreate)\n","page":"commands/vgcreate/","label":"vgcreate","where":"Command page · LVM","aka":["vgremove"]},{"id":"cmd-vgextend","n":1562,"slug":"vgextend","title":"vgextend — Add physical volumes to a volume group (vgextend), or remove them (vgreduce)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","volume group","add disk","remove disk"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Add physical volumes to a volume group (vgextend), or remove them (vgreduce).\n\nSame page for: `vgreduce`\n\n## About\n\n`vgextend VG PV...` adds new space to a VG while everything runs. Devices that are not PVs yet are initialized for you. `vgreduce VG PV` removes a PV — it must be empty first (`pvmove` moves its data to the other PVs).\n\n## Syntax\n\n```text\nvgextend [OPTION]... VG PV...\nvgreduce [OPTION]... VG PV...\nvgreduce --removemissing VG\n```\n\n## Options\n\n\n### vgextend\n\n| Option | What it does |\n|---|---|\n| `--restoremissing` | Add back a PV that was missing. |\n| `-f, --force` | Force. |\n| `-A, --autobackup y\\|n` | Back up the metadata. |\n| `--metadatasize SIZE, --pvmetadatacopies N, --metadataignore y\\|n, --dataalignment SIZE, --dataalignmentoffset SIZE, --labelsector N, -M, --metadatatype lvm2, -Z, --zero y\\|n` | Settings for PVs created on the fly. |\n| `--reportformat basic\\|json` | Output format. |\n\n### vgreduce\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Remove all empty PVs. |\n| `--removemissing` | Remove PVs that are gone (failed disk). |\n| `--mirrorsonly` | With --removemissing: only mirror images. |\n| `-f, --force` | Force (with --removemissing: also remove LVs on the missing PV). |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Add a disk to a VG\n\n```bash\nvgextend vgdata /dev/vdd\n```\n```text\n  Volume group \"vgdata\" successfully extended\n```\n\n> The new space is free for LVs at once.\n\n### Check the new size\n\n```bash\nvgs vgdata\n```\n\n> VFree is bigger.\n\n### Then grow an LV\n\n```bash\nlvextend -r -l +100%FREE /dev/vgdata/lvdata\n```\n\n> Uses all new space.\n\n### Remove a disk from a VG\n\n```bash\npvmove /dev/vdc\nvgreduce vgdata /dev/vdc\n```\n\n> pvmove first; vgreduce refuses PVs in use.\n\n### Remove all empty PVs\n\n```bash\nvgreduce -a vgdata\n```\n\n> Cleans unused disks out of the VG.\n\n### A disk died\n\n```bash\nvgreduce --removemissing vgdata\n```\n\n> Lets the VG work again; LVs on that disk are lost (use --force to drop them).\n\n### Test before\n\n```bash\nvgreduce -t vgdata /dev/vdc\n```\n\n> Shows what would happen.\n\n### Replace an old disk with a new one (no downtime)\n\n```bash\nvgextend vgdata /dev/vde; pvmove /dev/vdc /dev/vde; vgreduce vgdata /dev/vdc\n```\n\n> Classic migration to new storage.\n\n### Wipe the removed disk\n\n```bash\npvremove /dev/vdc\n```\n\n> Removes the LVM label.\n\n### Check which PVs are empty\n\n```bash\npvs -o pv_name,vg_name,pv_used\n```\n\n> pv_used 0 = can be removed.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man vgextend](https://linux.die.net/man/8/vgextend)\n","page":"commands/vgextend/","label":"vgextend","where":"Command page · LVM","aka":["vgreduce"]},{"id":"cmd-vgs","n":1563,"slug":"vgs","title":"vgs — Show volume groups: short table (vgs), full details (vgdisplay), or scan for VGs (vgscan)","sections":["commands"],"topic":"Commands · LVM","tags":["lvm","volume group","report","free space","extent size"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show volume groups: short table (vgs), full details (vgdisplay), or scan for VGs (vgscan).\n\nSame page for: `vgdisplay`, `vgscan`\n\n## About\n\n`vgs` prints one line per VG: number of PVs and LVs, size and free space. `vgdisplay` shows the details, including the **PE Size** and the number of free extents — needed for `lvcreate -l`.\n\n## Syntax\n\n```text\nvgs [OPTION]... [VG...]\nvgdisplay [OPTION]... [VG...]\nvgscan [OPTION]...\n```\n\n## Options\n\n\n### Report (vgs)\n\n| Option | What it does |\n|---|---|\n| `-o, --options COLUMNS` | Columns, like vg_name,vg_size,vg_free,vg_extent_size (+col = add). |\n| `-O, --sort COLUMNS` | Sort. |\n| `-S, --select EXPR` | Filter, like 'vg_free>10g'. |\n| `--units UNIT` | Units: h, b, k, m, g, t, e … |\n| `--nosuffix` | No unit letters. |\n| `--noheadings` | No header line. |\n| `--separator TEXT` | Column separator. |\n| `--aligned, --rows, --binary, --nameprefixes, --unquoted` | Output styles. |\n| `-a, --all` | Also empty VGs and orphans. |\n| `--reportformat basic\\|json` | JSON output. |\n| `--configreport, --logonly, --foreign, --shared, --readonly, --ignorelockingfailure, --unbuffered` | Special report settings. |\n\n### vgdisplay\n\n| Option | What it does |\n|---|---|\n| `-v, --verbose` | Also list the LVs and PVs of the VG. |\n| `-C, --columns` | Report like vgs. |\n| `-s, --short` | Short form. |\n| `-A, --activevolumegroups` | Only active VGs. |\n\n### vgscan\n\n| Option | What it does |\n|---|---|\n| `--mknodes` | Create missing /dev nodes for LVs. |\n| `--notifydbusd y\\|n` | Notify the D-Bus service. |\n\n### Common LVM options\n\n| Option | What it does |\n|---|---|\n| `-t, --test` | Test only; change nothing. |\n| `-v, --verbose` | More output (repeat for more). |\n| `-q, --quiet` | Less output. |\n| `-y, --yes` | Answer yes to questions. |\n| `-d, --debug` | Debug output. |\n| `--config STRING` | Override lvm.conf settings for this command only. |\n| `--commandprofile STRING, --profile STRING` | Use a configuration profile. |\n| `--devices PV, --devicesfile STRING` | Limit the devices LVM looks at. |\n| `--driverloaded y\\|n, --nolocking, --lockopt STRING, --nohints, --journal STRING` | Special and debug settings. |\n| `-h, --help, --longhelp` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### All VGs\n\n```bash\nvgs\n```\n```text\n  VG     #PV #LV #SN Attr   VSize   VFree\n  rhel     1   2   0 wz--n- <19.00g      0\n  vgdata   2   1   0 wz--n- <14.99g <9.99g\n```\n\n> VFree = space for new or bigger LVs.\n\n### Details with PE size\n\n```bash\nvgdisplay vgdata\n```\n\n> PE Size, Total PE, Alloc PE, Free PE / Size.\n\n### Free extents only\n\n```bash\nvgs -o vg_name,vg_extent_size,vg_free_count vgdata\n```\n\n> Exactly what lvcreate -l needs.\n\n### In MiB\n\n```bash\nvgs --units m vgdata\n```\n\n> Exact numbers.\n\n### VG with its LVs and PVs\n\n```bash\nvgdisplay -v vgdata\n```\n\n> All in one view.\n\n### VGs that are almost full (monitoring)\n\n```bash\nvgs -S 'vg_free<1g' -o vg_name,vg_free\n```\n\n> Plan to add disks.\n\n### Script value\n\n```bash\nvgs --noheadings --nosuffix --units g -o vg_free vgdata\n```\n\n> A number for checks.\n\n### JSON\n\n```bash\nvgs --reportformat json\n```\n\n> For automation tools.\n\n### Scan after moving disks between servers\n\n```bash\nvgscan; vgchange -ay\n```\n\n> Finds and activates the moved VGs.\n\n### Check the VG attributes\n\n```bash\nvgs -o +vg_attr,vg_systemid\n```\n\n> w = writable, z = resizable, n = normal allocation.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `5` | Error. |\n\nFull manual: [man vgs](https://linux.die.net/man/8/vgs)\n","page":"commands/vgs/","label":"vgs","where":"Command page · LVM","aka":["vgdisplay","vgscan"]},{"id":"cmd-vim","n":1564,"slug":"vim","title":"vim — Edit text files in the terminal with the vi/vim editor (keys and commands, no mouse needed)","sections":["commands"],"topic":"Commands · Text and search","tags":["editor","edit","text","config","search","replace"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit text files in the terminal with the vi/vim editor (keys and commands, no mouse needed).\n\nSame page for: `vi`, `vimtutor`\n\n## About\n\nvim is on almost every Linux server (`vi` is the small version). It has **modes**:\n\n- **Normal mode** (command mode): keys are commands: move, delete, copy, paste. vim starts here.\n- **Insert mode**: keys type text. Enter with `i`, `a` or `o`; leave with `Esc`.\n- **Command-line mode**: commands after `:` like `:w` (save) or `:q` (quit), and searches after `/`.\n- **Visual mode**: select text with `v` (characters), `V` (lines) or `Ctrl+V` (block).\n\nMost commands can take a number first: `5dd` deletes 5 lines, `3w` moves 3 words.\n\nLearn the basics in 30 minutes with `vimtutor`.\n\n## Syntax\n\n```text\nvim [OPTIONS] [FILE ...]\nvim +NUMBER FILE\nvim -d FILE1 FILE2\n```\n\n## Parts\n\n\n### Move (normal mode)\n\n| Part | Meaning |\n|---|---|\n| `h j k l` | Left, down, up, right (arrow keys also work). |\n| `w b e` | Next word, back one word, end of the word. |\n| `0 ^ $` | Start of the line, first character, end of the line. |\n| `gg G` | First line, last line. 25G (or :25) goes to line 25. |\n| `Ctrl+F Ctrl+B` | One page down, one page up. |\n| `%` | Jump to the matching bracket. |\n\n### Insert\n\n| Part | Meaning |\n|---|---|\n| `i a` | Insert before the cursor / after the cursor. |\n| `I A` | Insert at the start / at the end of the line. |\n| `o O` | New line below / above, in insert mode. |\n| `Esc` | Back to normal mode. |\n\n### Edit\n\n| Part | Meaning |\n|---|---|\n| `x` | Delete the character under the cursor. |\n| `dd` | Delete (cut) the line. 5dd = 5 lines. |\n| `dw d$` | Delete to the next word / to the end of the line. |\n| `cw cc` | Change a word / change the whole line. |\n| `yy yw` | Copy (yank) the line / a word. |\n| `p P` | Paste after / before the cursor. |\n| `u Ctrl+R` | Undo / redo. |\n| `.` | Repeat the last change. |\n| `r` | Replace one character. |\n| `>> <<` | Indent / unindent the line. |\n| `J` | Join the next line to this one. |\n\n### Search and replace\n\n| Part | Meaning |\n|---|---|\n| `/text ?text` | Search forward / backward. |\n| `n N` | Next / previous match. |\n| `*` | Search the word under the cursor. |\n| `:s/old/new/` | Replace the first \"old\" in this line. |\n| `:%s/old/new/g` | Replace every \"old\" in the whole file. |\n| `:%s/old/new/gc` | Replace everywhere, but ask each time. |\n\n### Files and quitting\n\n| Part | Meaning |\n|---|---|\n| `:w` | Save. `:w newname` saves to another file. |\n| `:q` | Quit (only when nothing changed). |\n| `:wq :x ZZ` | Save and quit. |\n| `:q!` | Quit and throw the changes away. |\n| `:w !sudo tee %` | Save a file you opened without root (asks the sudo password). |\n| `:e file` | Open another file. |\n| `:r file` | Read (insert) a file under the cursor. |\n\n### Settings\n\n| Part | Meaning |\n|---|---|\n| `:set number` | Show line numbers (`:set nonumber` to hide). |\n| `:set paste` | Paste text without automatic indent. |\n| `:syntax on` | Colors for code. |\n| `:set list` | Show tabs and line ends (find hidden characters). |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `+<number>` | Open the file at this line. `+` alone = the last line. |\n| `+/<pattern>` | Open the file at the first match of the pattern. |\n| `-R` | Read-only mode (view). Same as the `view` command. |\n| `-d` | Diff mode: show 2 to 8 files side by side with the differences. |\n| `-o[N], -O[N]` | Open the files in N windows, one above the other / side by side. |\n| `-p[N]` | Open the files in N tabs. |\n| `-c <command>` | Run this ex command after the first file is loaded. |\n| `--cmd <command>` | Run this command before any vimrc is read. |\n| `-n` | No swap file (good for very big files or read-only media). |\n| `-r [file]` | Recover a file after a crash, from its swap file. |\n| `-b` | Binary mode. |\n| `-M` | Do not allow changes (stronger than -R). |\n| `-m` | Do not allow writing the file. |\n| `-u <vimrc>` | Use this vimrc instead of the normal one (`-u NONE` = no settings). |\n| `-N` | No compatible mode (normal vim behavior). |\n| `-C` | Compatible mode (act like old vi). |\n| `-E` | Start in improved Ex mode. |\n| `-e` | Start in Ex mode. |\n| `-s` | Silent mode (for scripts, with -e). |\n| `-y` | Easy mode (works like a simple editor). |\n| `-Z` | Restricted mode: no shell commands. |\n| `-x` | Encrypt the file when writing (asks for a key). |\n| `-i <viminfo>` | Use another viminfo file. |\n| `--clean` | Start without your settings and plugins. |\n| `--noplugin` | Do not load plugins. |\n| `-V[N]` | Verbose messages (level N). |\n| `-h, --help` | Show help. |\n| `--version` | Show the version and the built-in features. |\n\n## Examples\n\n### Open a file and save after an edit\n\n```bash\nvim /etc/hosts\n```\n```text\n(press i, type your line, press Esc, then type :wq and Enter)\n```\n\n> The basic cycle: i → type → Esc → :wq.\n\n### Leave without saving when you made a mistake\n\n```bash\nvim /etc/fstab\n```\n```text\n(press Esc, then type :q! and Enter)\n```\n\n> `:q!` throws all changes away. Your safety exit.\n\n### Open a file at a line number from an error message\n\n```bash\nvim +42 /etc/httpd/conf/httpd.conf\n```\n\n> Error messages often say \"line 42\". Jump straight there.\n\n### Replace a word in the whole file\n\n```bash\nvim /etc/ssh/sshd_config\n```\n```text\n:%s/^#PermitRootLogin yes/PermitRootLogin no/\n```\n\n> `^` anchors the match at the line start. Then `:wq`.\n\n### Look at a file without changing it\n\n```bash\nvim -R /var/log/messages\n```\n\n> Read-only: no risk of saving by mistake. Same as `view`.\n\n### Compare two config files side by side\n\n```bash\nvim -d /etc/ssh/sshd_config /etc/ssh/sshd_config.rpmnew\n```\n\n> Differences are colored. `]c` jumps to the next difference, `do` takes the other side's version.\n\n### Save a file you opened without sudo\n\n```bash\nvim /etc/motd\n```\n```text\n:w !sudo tee % > /dev/null\n```\n\n> `%` means the current file. Type the sudo password, then `:q!` to leave.\n\n### Comment out many lines at once\n\n```bash\nvim app.conf\n```\n```text\n(Ctrl+V, move down with j, press I, type #, press Esc)\n```\n\n> Block insert puts `#` before every selected line.\n\n### Show hidden tabs and spaces in a YAML file\n\n```bash\nvim deploy.yaml\n```\n```text\n:set list\n```\n\n> YAML breaks on tabs. `:set list` shows them as `^I`.\n\n### Recover a file after a crash or a lost SSH session\n\n```bash\nvim -r notes.txt\n```\n\n> vim keeps a swap file (`.notes.txt.swp`). Delete the swap file after a good recovery.\n\n### Run vim commands in a script (no screen)\n\n```bash\nvim -Es -c '%s/8080/9090/g' -c 'wq' app.conf\n```\n\n> Batch edit without opening the editor. For simple jobs `sed -i` is easier.\n\n### Learn vim in 30 minutes\n\n```bash\nvimtutor\n```\n\n> A lesson file you edit while you learn. Do it once; it pays back every day.\n\n## Watch out\n\n- In insert mode, `:wq` is just text. Press `Esc` first.\n- \"E37: No write since last change\" means you tried `:q` with unsaved changes. Use `:wq` or `:q!`.\n- \"Found a swap file\" means the file is open elsewhere or vim crashed. Choose (R)ecover or (Q)uit, never edit twice at the same time.\n- Use `visudo` for /etc/sudoers and `crontab -e` for crontabs, not plain vim.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `~/.vimrc` | Your vim settings (like `set number`). |\n| `/etc/vimrc` | System-wide settings. |\n| `~/.viminfo` | History of commands, searches and marks. |\n\n## Tips\n\n- Lost? `Esc Esc :q!`.\n- Remember the grammar: **number + verb + move**: `d3w` = delete 3 words, `y$` = copy to the end of the line.\n- `.` repeats the last change. It saves a lot of typing.\n\nFull manual: [man vim](https://linux.die.net/man/1/vim)\n","page":"commands/vim/","label":"vim","where":"Command page · Text and search","aka":["vi","vimtutor"]},{"id":"cmd-vipw","n":1565,"slug":"vipw","title":"vipw — Edit /etc/passwd (vipw) or /etc/group (vigr) safely by hand, with a lock","sections":["commands"],"topic":"Commands · Users and groups","tags":["edit passwd","edit group","lock file","shadow","safe edit"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit /etc/passwd (vipw) or /etc/group (vigr) safely by hand, with a lock.\n\nSame page for: `vigr`\n\n## About\n\n`vipw` and `vigr` open a copy of the account file in your editor (`$VISUAL`, `$EDITOR` or vi) and lock the file, so `passwd`, `useradd` and others cannot change it at the same time. When you save, the copy replaces the real file.\n\nAfter editing `/etc/passwd` you are asked to edit `/etc/shadow` too (and after `/etc/group` also `/etc/gshadow`), so both stay in sync.\n\nMost work is easier and safer with useradd, usermod and groupmod. Use vipw/vigr for fixes those tools cannot do.\n\n## Syntax\n\n```text\nvipw [OPTION]...\nvigr [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-p, --passwd` | Edit the passwd file (default of vipw). |\n| `-g, --group` | Edit the group file (default of vigr). |\n| `-s, --shadow` | Edit the shadow file (/etc/shadow or /etc/gshadow). |\n| `-q, --quiet` | Quiet mode. |\n| `-R, --root DIR` | Work in this chroot folder. |\n| `-h, --help` | Show help. |\n\n## Examples\n\n### Edit the user file\n\n```bash\nvipw\n```\n\n> Opens /etc/passwd with a lock; afterwards it offers to edit /etc/shadow.\n\n### Edit the group file\n\n```bash\nvigr\n```\n\n> Opens /etc/group with a lock.\n\n### Edit the shadow file directly\n\n```bash\nvipw -s\n```\n\n> For example to fix a wrong aging field.\n\n### Edit the group shadow file\n\n```bash\nvigr -s\n```\n\n> Group passwords and group admins.\n\n### Use nano as the editor\n\n```bash\nEDITOR=nano vipw\n```\n\n> Any editor works.\n\n### Check the files after a manual edit\n\n```bash\npwck -r; grpck -r\n```\n\n> -r = only report, change nothing.\n\n### Fix a wrong login shell by hand\n\n```bash\nvipw\n```\n\n> Change the last field, for example `/bin/bsh` back to `/bin/bash`.\n\n### Find a second root account (security)\n\n```bash\nawk -F: '$3 == 0 {print $1}' /etc/passwd\n```\n```text\nroot\n```\n\n> Only root may have UID 0. Another name here means a backdoor.\n\n### Find accounts without a password hash (security)\n\n```bash\nawk -F: '$2 == \"\" {print $1}' /etc/shadow\n```\n\n> Empty field = login without a password. Fix it at once.\n\n### Make a backup before a manual edit\n\n```bash\ncp -a /etc/passwd /etc/shadow /etc/group /root/\n```\n\n> The tools also keep a copy: /etc/passwd- and /etc/group-.\n\n### See the lock files during an edit\n\n```bash\nls /etc/*.lock\n```\n\n> /etc/passwd.lock exists while vipw is open.\n\n## Watch out\n\n- Never edit /etc/passwd or /etc/shadow with a normal editor while the system runs.\n- A broken line can stop logins for everybody. Keep a root shell open while you test.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/passwd, /etc/shadow, /etc/group, /etc/gshadow` | The files that are edited. |\n| `/etc/passwd-, /etc/group-` | Backup copies from the last change. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error (file busy or the editor failed). |\n\nFull manual: [man vipw](https://linux.die.net/man/8/vipw)\n","page":"commands/vipw/","label":"vipw","where":"Command page · Users and groups","aka":["vigr"]},{"id":"cmd-visudo","n":1566,"slug":"visudo","title":"visudo — Edit the sudo rules safely: visudo locks the file and checks the syntax before it saves","sections":["commands"],"topic":"Commands · Users and groups","tags":["sudoers","sudo rules","syntax check","edit safely","privileges"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Edit the sudo rules safely: visudo locks the file and checks the syntax before it saves.\n\n## About\n\n`visudo` opens `/etc/sudoers` (or another file with `-f`) in an editor (vi by default, or the one in `$EDITOR` / `$SUDO_EDITOR` if allowed). When you save, it checks the syntax. With an error it asks what to do: `e` = edit again, `x` = quit without saving, `Q` = save anyway (do not do this).\n\nA broken sudoers file can stop all sudo use. On a server without a root password that means you are locked out — so always use visudo.\n\nRule format: `WHO  WHERE=(AS_WHOM)  COMMANDS`, for example `%wheel ALL=(ALL) ALL`.\n\n## Syntax\n\n```text\nvisudo [OPTION]... [[-f] FILE]\n```\n\n## Parts\n\n| Part | Meaning |\n|---|---|\n| `%group` | A group name in a rule (users without %). |\n| `ALL` | Any host / any user / any command (by position). |\n| `(root)` | The user the command runs as. |\n| `NOPASSWD:` | Do not ask for a password for the commands after it. |\n| `Defaults` | A setting line, like `Defaults timestamp_timeout=10`. |\n| `User_Alias, Cmnd_Alias` | Names for lists of users or commands. |\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-f, --file=FILE` | Edit this file instead of /etc/sudoers (like /etc/sudoers.d/NAME). |\n| `-c, --check` | Only check the syntax of all sudoers files; change nothing. |\n| `-q, --quiet` | With -c: no \"parsed OK\" messages. |\n| `-s, --strict` | Strict check: also errors for unknown aliases and wrong order. |\n| `-O, --owner` | With -c: also check that the owner is root. |\n| `-P, --perms` | With -c: also check the permissions (0440). |\n| `-I, --no-includes` | Do not edit the included files. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Edit the main file\n\n```bash\nvisudo\n```\n\n> Opens /etc/sudoers with locking and a syntax check.\n\n### Create a rule file for a team\n\n```bash\nvisudo -f /etc/sudoers.d/webadmins\n```\n\n> Put the rule in a separate file: updates of the sudo package do not touch it.\n\n### Allow a group only some commands\n\n```bash\ncat /etc/sudoers.d/webadmins\n```\n```text\n%webadmins ALL=(root) /usr/bin/systemctl restart httpd, /usr/bin/systemctl status httpd\n```\n\n> The content of the file. Use full paths.\n\n### Allow one command without a password\n\n```bash\ncat /etc/sudoers.d/backup1\n```\n```text\nbackup1 ALL=(root) NOPASSWD: /usr/bin/rsync\n```\n\n> For a backup job that runs from cron.\n\n### Check all files after a change\n\n```bash\nvisudo -c\n```\n```text\n/etc/sudoers: parsed OK\n/etc/sudoers.d/webadmins: parsed OK\n```\n\n> Run it after every change, also after a config tool wrote the file.\n\n### Check one file before you install it\n\n```bash\nvisudo -cf /tmp/newrule\n```\n\n> Test a file from Ansible or a colleague before you copy it into sudoers.d.\n\n### Strict check with owner and permissions\n\n```bash\nvisudo -c -s -O -P\n```\n\n> A file with wrong permissions is skipped by sudo.\n\n### Use nano instead of vi\n\n```bash\nEDITOR=nano visudo\n```\n\n> Works when `env_editor` is allowed (default).\n\n### Make the password last longer\n\n```bash\necho 'Defaults timestamp_timeout=15' > /etc/sudoers.d/timeout; visudo -c\n```\n\n> sudo asks again after 15 minutes. `0` = ask every time.\n\n### Log all sudo input and output (audit)\n\n```bash\necho 'Defaults log_output' > /etc/sudoers.d/iolog; visudo -c\n```\n\n> Sessions are saved in /var/log/sudo-io; replay them with `sudoreplay`.\n\n### Use command aliases for clear rules\n\n```bash\ncat /etc/sudoers.d/web\n```\n```text\nCmnd_Alias WEB = /usr/bin/systemctl restart httpd, /usr/bin/systemctl reload httpd\n%webadmins ALL=(root) WEB\n```\n\n> One name for a list of commands.\n\n## Watch out\n\n- Never edit /etc/sudoers with a normal editor.\n- Files in /etc/sudoers.d/ with a dot or `~` in the name are ignored.\n- The last matching rule wins: put special rules after general ones.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/etc/sudoers` | The main file (has `#includedir /etc/sudoers.d` or `@includedir`). |\n| `/etc/sudoers.d/` | Extra rule files. |\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Saved, or the check found no error. |\n| `1` | Error in the file or the check failed. |\n\nFull manual: [man visudo](https://linux.die.net/man/8/visudo)\n","page":"commands/visudo/","label":"visudo","where":"Command page · Users and groups","aka":[]},{"id":"cmd-w","n":1567,"slug":"w","title":"w — Show who is logged in and what each user is doing now, plus the uptime and load of the system","sections":["commands"],"topic":"Commands · Users and groups","tags":["users","logged in","load","uptime","sessions","what"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show who is logged in and what each user is doing now, plus the uptime and load of the system.\n\n## About\n\n`w` combines `uptime` and `who`, and adds the current program of each session. The first line shows the time, how long the system is up, the number of users and the **load average** (1, 5 and 15 minutes).\n\nColumns: USER, TTY, FROM (remote address), LOGIN@ (login time), IDLE, JCPU (CPU of all processes of the terminal), PCPU (CPU of the current process) and WHAT (the current command).\n\n## Syntax\n\n```text\nw [OPTIONS] [USER]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-h, --no-header` | Do not print the header lines. |\n| `-s, --short` | Short format: no login time, JCPU and PCPU. |\n| `-f, --from` | Show or hide the FROM column (the default depends on the build). |\n| `-i, --ip-addr` | Show IP addresses instead of host names in FROM. |\n| `-p, --pids` | Show the process ID of the current process. |\n| `-u, --no-current` | Ignore the user name when finding the current process and CPU times. |\n| `-o, --old-style` | Old output style (blank space for short idle times). |\n| `USER` | Show only this user. |\n| `--help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Who does what?\n\n```bash\nw\n```\n```text\n 09:12:01 up  1:14,  2 users,  load average: 0.08, 0.12, 0.10\nUSER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT\nstudent  tty2     tty2             08:10    1:02m  0.05s  0.03s /usr/libexec/gnome-session-binary\nadmin    pts/0    192.168.122.1    09:02    0.00s  0.04s  0.00s w\n```\n\n> Load average under the number of CPUs is normal.\n\n### Only one user\n\n```bash\nw admin\n```\n\n> Sessions and current commands of admin.\n\n### Without the header, for a script\n\n```bash\nw -h\n```\n\n> One line per session.\n\n### Short form\n\n```bash\nw -s\n```\n\n> Fewer columns, easier to read on a small screen.\n\n### IP addresses instead of names\n\n```bash\nw -i\n```\n\n> Useful when reverse DNS is slow or wrong.\n\n### Show the process IDs of the current commands\n\n```bash\nw -p\n```\n\n> Then you can look at a process with `ps -fp PID` or stop it.\n\n### Find who runs a heavy command\n\n```bash\nw | sort -k6 -r | head\n```\n\n> Sort on JCPU to see the busy sessions.\n\n### Check the load before a big job\n\n```bash\nw | head -1\n```\n\n> If the load is high, start the backup later.\n\n### Find idle sessions to clean up\n\n```bash\nw -h | awk '$4 ~ /days/'\n```\n\n> Sessions idle for days. Ask the users or end them with `pkill -t pts/N`.\n\n### Spot a suspicious session (security)\n\n```bash\nw -i | grep -v '192.168.122.'\n```\n\n> A login from an unknown network with a strange WHAT command needs attention.\n\n### Compare with uptime\n\n```bash\nuptime; w | head -1\n```\n\n> The first line of w is the uptime output.\n\n## Tips\n\n- w = uptime + who + what. Look at load average and WHAT.\n\nFull manual: [man w](https://linux.die.net/man/1/w)\n","page":"commands/w/","label":"w","where":"Command page · Users and groups","aka":[]},{"id":"cmd-wc","n":1568,"slug":"wc","title":"wc — Count lines, words, characters and bytes","sections":["commands"],"topic":"Commands · Text and search","tags":["count","lines","words","bytes","characters"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Count lines, words, characters and bytes.\n\n## About\n\n`wc` (word count) prints, for each file: the number of lines, words and bytes. With options you choose one count. `wc -l` is used all the time to count lines of a file or results of a command.\n\n## Syntax\n\n```text\nwc [OPTION]... [FILE]...\nwc [OPTION]... --files0-from=F\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l, --lines` | Count lines (new line characters). |\n| `-w, --words` | Count words (separated by spaces). |\n| `-c, --bytes` | Count bytes. |\n| `-m, --chars` | Count characters (different from bytes for UTF-8 text). |\n| `-L, --max-line-length` | Print the length of the longest line. |\n| `--total=<when>` | When to print the total line: auto, always, only, never. |\n| `--files0-from=<file>` | Read file names from a file (NUL separated). |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Lines, words and bytes\n\n```bash\nwc /etc/passwd\n```\n```text\n  45   95 2512 /etc/passwd\n```\n\n> Three numbers: lines, words, bytes.\n\n### Count lines only\n\n```bash\nwc -l /etc/passwd\n```\n```text\n45 /etc/passwd\n```\n\n> Each line is one account.\n\n### Count results of a command\n\n```bash\nls /etc | wc -l\n```\n\n> How many entries in /etc.\n\n### Only the number, without the file name\n\n```bash\nwc -l < /etc/passwd\n```\n```text\n45\n```\n\n> Input from a redirect has no name. Good in scripts.\n\n### Count running processes\n\n```bash\nps -e --no-headers | wc -l\n```\n\n> How many processes run now.\n\n### Count failed SSH logins today\n\n```bash\njournalctl -u sshd --since today | grep -c \"Failed password\"\n```\n\n> `grep -c` counts directly; same as `grep … | wc -l`.\n\n### Count lines of all scripts in a folder\n\n```bash\nwc -l *.sh\n```\n\n> Each file plus a total line.\n\n### Check the longest line of a file\n\n```bash\nwc -L config.yaml\n```\n\n> Some tools have line length limits.\n\n### Count characters in UTF-8 text\n\n```bash\necho -n \"سلام\" | wc -m\n```\n```text\n4\n```\n\n> `-c` would count 8 bytes; `-m` counts 4 characters.\n\n### Compare the size of two lists\n\n```bash\nwc -l before.txt after.txt\n```\n\n> Quick check after a cleanup or an import.\n\n### Alert when a log grows too fast\n\n```bash\n[ \"$(wc -l < /var/log/app/error.log)\" -gt 1000 ] && echo \"too many errors\"\n```\n\n> A tiny monitoring check.\n\n## Watch out\n\n- The last line of a file without a new line is not counted by `-l`.\n\n## Tips\n\n- `wc -l` = how many lines. `wc -l < file` = number only.\n\nFull manual: [man wc](https://linux.die.net/man/1/wc)\n","page":"commands/wc/","label":"wc","where":"Command page · Text and search","aka":[]},{"id":"cmd-which","n":1569,"slug":"which","title":"which — Show the full path of the program that runs for a command name","sections":["commands"],"topic":"Commands · Shell and help","tags":["path","find","program","alias","location"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the full path of the program that runs for a command name.\n\n## About\n\n`which` searches the folders in your `$PATH`, from left to right, and prints the first file with that name. It answers \"where is this program on the disk?\".\n\nOn RHEL, `which` for Bash users is an alias that also shows aliases and functions (`alias which='(alias; declare -f) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot'`).\n\nTo know which **package** a program comes from, use `rpm -qf $(which NAME)`.\n\n## Syntax\n\n```text\nwhich [OPTIONS] PROGRAM [PROGRAM ...]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Print all matches in $PATH, not only the first one. |\n| `-i, --read-alias` | Read aliases from standard input and report them too. |\n| `--skip-alias` | Ignore --read-alias (use it to override the RHEL alias). |\n| `--read-functions` | Read shell functions from standard input and report them too. |\n| `--skip-functions` | Ignore --read-functions. |\n| `--skip-dot` | Skip folders in $PATH that start with a dot. |\n| `--skip-tilde` | Skip folders in $PATH that start with ~ and home folders. |\n| `--show-dot` | If a $PATH folder starts with a dot, print ./PROGRAM. |\n| `--show-tilde` | Print ~ instead of the full home folder path. |\n| `--tty-only` | Stop reading options when the output is not a terminal (used by the RHEL alias). |\n| `-v, -V, --version` | Show the version. |\n| `--help` | Show help. |\n\n## Examples\n\n### Find where a program is\n\n```bash\nwhich passwd\n```\n```text\n/usr/bin/passwd\n```\n\n> The program file that runs when you type `passwd`.\n\n### See all copies of a program in $PATH\n\n```bash\nwhich -a python3\n```\n```text\n/usr/bin/python3\n/usr/local/bin/python3\n```\n\n> The first one wins. Two copies often explain \"wrong version\" problems.\n\n### Find the package of a program\n\n```bash\nrpm -qf $(which ss)\n```\n```text\niproute-6.x.x.el10.x86_64\n```\n\n> Now you know which package to update or reinstall.\n\n### Check if a tool exists in a script\n\n```bash\nwhich ansible >/dev/null 2>&1 && echo \"ansible installed\"\n```\n\n> The exit code is 0 when found, 1 when not found.\n\n### Show the alias too (RHEL default)\n\n```bash\nwhich ll\n```\n```text\nalias ll='ls -l --color=auto'\n\t/usr/bin/ls\n```\n\n> Because of the RHEL alias, `which` also shows aliases in an interactive shell.\n\n### Use the plain program without the alias\n\n```bash\n/usr/bin/which --skip-alias ll\n```\n\n> In a script the alias does not exist; this shows what a script would find.\n\n### Check the path of a privileged program in a review\n\n```bash\nwhich -a sudo\n```\n\n> Only `/usr/bin/sudo` should appear. Another `sudo` earlier in $PATH is a red flag.\n\n### Find which shell a user would get\n\n```bash\nwhich bash zsh fish\n```\n```text\n/usr/bin/bash\n```\n\n> Only installed shells are printed; the others give an error message.\n\n### Get the path for a cron job or a systemd unit\n\n```bash\nwhich podman\n```\n```text\n/usr/bin/podman\n```\n\n> cron and systemd use a short $PATH. Write the full path in `ExecStart=` and crontab lines.\n\n### Check many tools at once before a deployment\n\n```bash\nfor t in git curl jq rsync; do which $t >/dev/null || echo \"missing: $t\"; done\n```\n\n> Prints only the tools that are not installed.\n\n### Compare with type\n\n```bash\nwhich cd; type cd\n```\n\n> `which` finds no file for `cd` (it is a built-in); `type` explains what it is.\n\n## Watch out\n\n- `which` only searches files in $PATH. It does not know built-ins like `cd` (use `type`).\n- `sudo which NAME` can give another answer, because root has a different $PATH.\n\n## Tips\n\n- `which` = where is the file? `type` = what will run? `rpm -qf` = which package?\n\nFull manual: [man which](https://linux.die.net/man/1/which)\n","page":"commands/which/","label":"which","where":"Command page · Shell and help","aka":[]},{"id":"cmd-who","n":1570,"slug":"who","title":"who — Show who is logged in now, on which terminal, since when and from where","sections":["commands"],"topic":"Commands · Users and groups","tags":["logged in","users","sessions","tty","boot time","runlevel"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show who is logged in now, on which terminal, since when and from where.\n\n## About\n\n`who` reads the login records (utmp) and lists the current sessions: user, terminal (tty1, pts/0 …), login time and the remote address for SSH logins.\n\nWith options it also shows the last boot time, the runlevel, or only a count. `who am i` (or `who -m`) shows only your own session.\n\n## Syntax\n\n```text\nwho [OPTION]... [FILE | ARG1 ARG2]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Everything (same as -b -d --login -p -r -t -T -u). |\n| `-b, --boot` | The time of the last system boot. |\n| `-H, --heading` | Print column headings. |\n| `-u, --users` | List users with idle time and process ID. |\n| `-q, --count` | Only the names and the number of users. |\n| `-m` | Only your own session (same as `who am i`). |\n| `-r, --runlevel` | The current runlevel (target). |\n| `-s, --short` | Name, terminal and time (the default). |\n| `-T, -w, --mesg, --message, --writable` | Show if users accept messages (+, - or ?). |\n| `-l, --login` | Show system login processes. |\n| `-p, --process` | Show active processes started by init. |\n| `-d, --dead` | Show dead processes. |\n| `-t, --time` | Show the last system clock change. |\n| `--lookup` | Try to turn addresses into DNS names. |\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Who is logged in?\n\n```bash\nwho\n```\n```text\nstudent  tty2         2026-10-25 08:10 (tty2)\nadmin    pts/0        2026-10-25 09:02 (192.168.122.1)\n```\n\n> tty = local console, pts = terminal window or SSH (with the client address).\n\n### With headings and idle times\n\n```bash\nwho -Hu\n```\n\n> The IDLE column shows how long a session has been quiet.\n\n### When did the system boot?\n\n```bash\nwho -b\n```\n```text\n         system boot  2026-10-25 07:58\n```\n\n> Same idea as `uptime -s`.\n\n### Count the users\n\n```bash\nwho -q\n```\n```text\nstudent admin\n# users=2\n```\n\n> Quick number for a status script.\n\n### Show only my own session\n\n```bash\nwho am i\n```\n```text\nadmin    pts/0        2026-10-25 09:02 (192.168.122.1)\n```\n\n> Shows from where you are connected.\n\n### Check for unexpected SSH sessions (security)\n\n```bash\nwho | grep -v '192.168.122.'\n```\n\n> Sessions from outside your admin network deserve a look.\n\n### Who accepts messages\n\n```bash\nwho -T\n```\n\n> `+` = `write` and `wall` messages reach the user.\n\n### Show the current runlevel / target\n\n```bash\nwho -r\n```\n\n> \"run-level 5\" means the graphical target; 3 = multi-user (text).\n\n### Warn everyone before a reboot\n\n```bash\nwho | wc -l; wall \"Reboot in 5 minutes\"\n```\n\n> See how many people are online, then send a message.\n\n### List users with the DNS name of their client\n\n```bash\nwho --lookup\n```\n\n> Readable names instead of IPs (if reverse DNS exists).\n\n### Compare with the history of logins\n\n```bash\nwho; last -n 5\n```\n\n> `who` = now, `last` = history.\n\n## Watch out\n\n- `who` only shows sessions that write login records. Some tools (like `ssh host command`) do not create one.\n\n## Files\n\n| File | Meaning |\n|---|---|\n| `/var/run/utmp` | The current sessions. |\n| `/var/log/wtmp` | The history (used by last). |\n\n## Tips\n\n- who = now, last = before, w = now + what they do.\n\nFull manual: [man who](https://linux.die.net/man/1/who)\n","page":"commands/who/","label":"who","where":"Command page · Users and groups","aka":[]},{"id":"cmd-whoami","n":1571,"slug":"whoami","title":"whoami — Print the user name you are working as right now (the effective user)","sections":["commands"],"topic":"Commands · Users and groups","tags":["user","identity","effective user","sudo","script"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Print the user name you are working as right now (the effective user).\n\n## About\n\n`whoami` prints the name of the **effective** user: after `su` or `sudo`, that is the new user, not the one who logged in. It is the same as `id -un`.\n\nTo see the user who originally logged in, use `logname` or `who am i`.\n\n## Syntax\n\n```text\nwhoami [OPTION]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `--help` | Show help. |\n| `--version` | Show the version. |\n\n## Examples\n\n### Who am I?\n\n```bash\nwhoami\n```\n```text\nstudent\n```\n\n> Your current user.\n\n### After sudo\n\n```bash\nsudo whoami\n```\n```text\nroot\n```\n\n> Proves that sudo works for you.\n\n### After su\n\n```bash\nsu - admin -c whoami\n```\n```text\nadmin\n```\n\n> The command runs as admin.\n\n### Compare with the login name\n\n```bash\nsudo -i\nwhoami; logname\n```\n```text\nroot\nstudent\n```\n\n> whoami = now, logname = who logged in.\n\n### Stop a script that must run as root\n\n```bash\n[ \"$(whoami)\" = \"root\" ] || { echo \"Run this as root\"; exit 1; }\n```\n\n> Many install scripts start with this check. `[ \"$EUID\" -eq 0 ]` works too.\n\n### Stop a script that must NOT run as root\n\n```bash\n[ \"$(whoami)\" != \"root\" ] || { echo \"Do not run as root\"; exit 1; }\n```\n\n> Build tools and package managers for users should not run as root.\n\n### Show the user in a prompt or log\n\n```bash\necho \"$(date) $(whoami)@$(hostname): backup started\" >> /var/tmp/ops.log\n```\n\n> Who started the job, on which host.\n\n### Check the user of a container\n\n```bash\npodman run --rm registry.access.redhat.com/ubi10/ubi whoami\n```\n```text\nroot\n```\n\n> Many images run as root by default; a security scan would flag this.\n\n### Check the user of a cron job\n\n```bash\ncrontab -l | grep whoami\n```\n\n> Put `whoami > /tmp/cron-user.txt` in a test job to see which user cron uses.\n\n### Check the user after a privilege escalation test\n\n```bash\nid; whoami\n```\n\n> In a pentest report, `whoami` output proves the reached privilege level.\n\n### Use it in an SSH check\n\n```bash\nssh server2 whoami\n```\n```text\nstudent\n```\n\n> Confirms the remote login user (useful with ProxyJump and keys).\n\n## Tips\n\n- whoami = effective user (after su/sudo). logname = original login. id = user plus groups.\n\nFull manual: [man whoami](https://linux.die.net/man/1/whoami)\n","page":"commands/whoami/","label":"whoami","where":"Command page · Users and groups","aka":[]},{"id":"cmd-wipefs","n":1572,"slug":"wipefs","title":"wipefs — List or remove file system, RAID, LVM and partition table signatures from a device (fast, without wiping the data)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["signatures","wipe","partition table","reuse disk"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"List or remove file system, RAID, LVM and partition table signatures from a device (fast, without wiping the data).\n\n## About\n\nOld signatures confuse tools: mkfs refuses, LVM sees an old PV, the installer shows old RAID members. `wipefs DEVICE` lists the signatures; `wipefs -a DEVICE` removes them. Only the few bytes of the signatures are erased, so it is very fast — but the old data is not securely deleted.\n\n## Syntax\n\n```text\nwipefs [OPTION]... DEVICE...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-a, --all` | Remove all signatures. |\n| `-n, --no-act` | Do everything except writing (test). |\n| `-b, --backup[=DIR]` | Save each removed signature to a file first. |\n| `-o, --offset OFFSET` | Remove only the signature at this offset. |\n| `-t, --types LIST` | Only these types, like xfs,ext4 (no = exclude). |\n| `-f, --force` | Also on mounted or busy devices (dangerous). |\n| `-q, --quiet` | Less output. |\n| `-i, --noheadings` | No header line. |\n| `-J, --json` | JSON output. |\n| `-O, --output LIST` | Choose columns. |\n| `-p, --parsable` | Parsable output. |\n| `--lock[=MODE]` | Lock the device. |\n| `-h, --help` | Show help. |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### List the signatures\n\n```bash\nwipefs /dev/vdb\n```\n```text\nDEVICE OFFSET        TYPE UUID LABEL\nvdb    0x200         gpt\nvdb    0x27ffffe00   gpt\nvdb    0x1fe         PMBR\n```\n\n> GPT has a backup at the end of the disk.\n\n### Remove all signatures of a disk\n\n```bash\nwipefs -a /dev/vdb\n```\n\n> The disk now looks empty for every tool.\n\n### Test first\n\n```bash\nwipefs -n -a /dev/vdb\n```\n\n> Shows what would be removed.\n\n### Keep a backup of the signatures\n\n```bash\nwipefs -a -b /dev/vdb1\n```\n\n> Files like ~/wipefs-vdb1-0x00000000.bak can restore it with dd.\n\n### Remove only one type\n\n```bash\nwipefs -a -t xfs /dev/vdb1\n```\n\n> Other signatures stay.\n\n### Partitions first, then the disk\n\n```bash\nwipefs -a /dev/vdb[0-9]*; wipefs -a /dev/vdb\n```\n\n> Clean reuse of an old disk.\n\n### Before a new LVM PV\n\n```bash\nwipefs -a /dev/vdc; pvcreate /dev/vdc\n```\n\n> pvcreate refuses disks with old signatures.\n\n### JSON for automation\n\n```bash\nwipefs -J /dev/vdb\n```\n\n> Check disks in scripts.\n\n### After wipefs: really empty?\n\n```bash\nlsblk -f /dev/vdb\n```\n\n> No FSTYPE, no partitions.\n\n### Secure wipe needs more (security)\n\n```bash\nblkdiscard /dev/nvme0n1\n```\n\n> wipefs leaves the data; use blkdiscard (SSD) or a full overwrite to destroy data.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man wipefs](https://linux.die.net/man/8/wipefs)\n","page":"commands/wipefs/","label":"wipefs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-xfs-admin","n":1573,"slug":"xfs_admin","title":"xfs_admin — Show or change the label, UUID and some features of an XFS file system (unmounted)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["xfs","label","uuid","file system settings"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show or change the label, UUID and some features of an XFS file system (unmounted).\n\n## About\n\n`xfs_admin` changes XFS settings that are fixed at creation, mainly the label and the UUID. The file system must be **unmounted** for changes (newer versions can set the label of a mounted file system with `xfs_io -c 'label -s NAME' MOUNTPOINT`).\n\n## Syntax\n\n```text\nxfs_admin [-eflpu] [-O FEATURES] [-c 0|1] [-L LABEL] [-U UUID] DEVICE [LOGDEV]\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-l` | Print the label. |\n| `-L LABEL` | Set the label (max 12 characters; `--` = remove). |\n| `-u` | Print the UUID. |\n| `-U UUID\\|generate\\|nil\\|restore` | Set a new UUID. |\n| `-O FEATURE=STATUS,...` | Change features, like bigtime=1, inobtcount=1. |\n| `-c 0\\|1` | Turn lazy counters off or on. |\n| `-e` | Enable unwritten extents (old file systems). |\n| `-j` | Enable version 2 log format (old file systems). |\n| `-p` | Enable 32-bit project IDs. |\n| `-f` | The DEVICE is an image file. |\n| `-r RTDEV` | The realtime device. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Show the label and UUID\n\n```bash\nxfs_admin -l -u /dev/vdb1\n```\n```text\nlabel = \"data\"\nUUID = 4e5c7a6b-3f1d-4c2e-9a5b-7d2f8e1c0a93\n```\n\n> Same values as blkid.\n\n### Set a label\n\n```bash\numount /data; xfs_admin -L data /dev/vdb1\n```\n\n> Unmount first.\n\n### Remove the label\n\n```bash\nxfs_admin -L -- /dev/vdb1\n```\n\n> Clears it.\n\n### New UUID for a cloned disk\n\n```bash\nxfs_admin -U generate /dev/vdb1\n```\n\n> Two XFS file systems with the same UUID cannot be mounted at the same time.\n\n### Mount a clone once without changing it\n\n```bash\nmount -o nouuid /dev/vdc1 /mnt\n```\n\n> Temporary workaround.\n\n### Label on a mounted file system (newer xfsprogs)\n\n```bash\nxfs_io -c 'label -s data' /data\n```\n\n> No unmount needed.\n\n### Enable timestamps after 2038\n\n```bash\nxfs_admin -O bigtime=1 /dev/vdb1\n```\n\n> New file systems have it by default.\n\n### Check the result\n\n```bash\nlsblk -f /dev/vdb1\n```\n\n> LABEL and UUID columns.\n\n### Fix fstab after a UUID change\n\n```bash\ngrep vdb1 /etc/fstab; blkid -s UUID -o value /dev/vdb1\n```\n\n> Update the UUID in fstab or the system may not boot.\n\n### Info of a mounted XFS\n\n```bash\nxfs_info /data\n```\n\n> xfs_admin needs unmounted devices; xfs_info works on mounted ones.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man xfs_admin](https://linux.die.net/man/8/xfs_admin)\n","page":"commands/xfs_admin/","label":"xfs_admin","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-xfs-growfs","n":1574,"slug":"xfs_growfs","title":"xfs_growfs — Grow a mounted XFS file system to fill its (bigger) device","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["xfs","grow","resize","online","lvm"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Grow a mounted XFS file system to fill its (bigger) device.\n\n## About\n\nAfter the LV or partition below it grew, `xfs_growfs MOUNTPOINT` makes the XFS file system use the new space — online, with the file system mounted. It needs the **mount point**, not the device. XFS cannot be shrunk.\n\n`lvextend -r` calls xfs_growfs for you.\n\n## Syntax\n\n```text\nxfs_growfs [OPTION]... MOUNTPOINT\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-d` | Grow the data section to the maximum (default). |\n| `-D SIZE` | Grow the data section to this size in file system blocks. |\n| `-l` | Grow the log section to the maximum. |\n| `-L SIZE` | Grow the log to this size. |\n| `-r` | Grow the realtime section. |\n| `-R SIZE` | Grow the realtime section to this size. |\n| `-m MAXPCT` | Change the maximum percent of space for inodes. |\n| `-n` | Show the geometry, change nothing. |\n| `-t MTAB` | Use another mount table. |\n| `-x` | Turn an internal log into an external one (old). |\n| `-V` | Show the version. |\n\n## Examples\n\n### Grow after lvextend\n\n```bash\nlvextend -L +2G /dev/vgdata/lvdata\nxfs_growfs /data\n```\n\n> Needed when lvextend ran without -r.\n\n### Check the size\n\n```bash\ndf -h /data\n```\n\n> Shows the new size.\n\n### Show the geometry only\n\n```bash\nxfs_growfs -n /data\n```\n\n> Like xfs_info.\n\n### Grow to a fixed block count\n\n```bash\nxfs_growfs -D 2621440 /data\n```\n\n> 2621440 blocks × 4 KiB = 10 GiB.\n\n### Grow the root file system\n\n```bash\nxfs_growfs /\n```\n\n> After lvextend of rhel/root.\n\n### After a VM disk got bigger (partition)\n\n```bash\ngrowpart /dev/vda 2; pvresize /dev/vda2; lvextend -r -l +100%FREE /dev/rhel/root\n```\n\n> growpart (cloud-utils-growpart) grows the partition first.\n\n### Wrong argument\n\n```bash\nxfs_growfs /dev/vgdata/lvdata\n```\n\n> Newer versions accept a device of a mounted file system; old ones need the mount point.\n\n### It does not shrink\n\n```bash\nxfs_growfs -D 1000 /data\n```\n\n> Fails if smaller than now: XFS can only grow.\n\n### Check before and after\n\n```bash\nxfs_info /data | grep ^data\n```\n\n> The blocks value grows.\n\n### In automation\n\n```bash\nlvextend -r -l +100%FREE /dev/vgdata/lvdata\n```\n\n> One command for LV and file system.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man xfs_growfs](https://linux.die.net/man/8/xfs_growfs)\n","page":"commands/xfs_growfs/","label":"xfs_growfs","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-xfs-info","n":1575,"slug":"xfs_info","title":"xfs_info — Show the geometry and features of an XFS file system (block size, allocation groups, log, features)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["xfs","geometry","block size","file system info"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Show the geometry and features of an XFS file system (block size, allocation groups, log, features).\n\n## About\n\n`xfs_info MOUNTPOINT` prints how the XFS file system was created: block size, number of allocation groups, inode size, log size and features like reflink and bigtime. It works on mounted file systems (and on devices or image files).\n\n## Syntax\n\n```text\nxfs_info [-t MTAB] MOUNTPOINT | DEVICE | IMAGE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `MOUNTPOINT \\| DEVICE` | The file system to show. |\n| `-t MTAB` | Use another mount table file. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Show a mounted XFS\n\n```bash\nxfs_info /\n```\n```text\nmeta-data=/dev/mapper/rhel-root isize=512    agcount=4, agsize=1113856 blks\ndata     =                       bsize=4096   blocks=4455424, imaxpct=25\n```\n\n> isize = inode size, bsize = block size.\n\n### By device\n\n```bash\nxfs_info /dev/vdb1\n```\n\n> Also works unmounted in newer xfsprogs.\n\n### Check reflink support (fast copies)\n\n```bash\nxfs_info /data | grep -o 'reflink=[01]'\n```\n```text\nreflink=1\n```\n\n> `cp --reflink` makes instant copies.\n\n### Check bigtime (dates after 2038)\n\n```bash\nxfs_info /data | grep -o 'bigtime=[01]'\n```\n\n> 1 = ready for 2038.\n\n### Size before and after growing\n\n```bash\nxfs_info /data | grep ^data; xfs_growfs /data; xfs_info /data | grep ^data\n```\n\n> The blocks value grows.\n\n### Log (journal) size\n\n```bash\nxfs_info /data | grep ^log\n```\n\n> Bigger logs help heavy metadata work.\n\n### Compare two servers\n\n```bash\nssh server2 xfs_info / | diff - <(xfs_info /)\n```\n\n> Find different creation settings.\n\n### Is it really XFS?\n\n```bash\nfindmnt -no FSTYPE /data\n```\n```text\nxfs\n```\n\n> xfs_info fails on other types.\n\n### Data for capacity reports\n\n```bash\nxfs_info /data | awk -F'[ =,]+' '/^data/ {print $3*$5/1024/1024/1024 \" GiB\"}'\n```\n\n> blocks × block size.\n\n### Use xfs_spaceman for free space details\n\n```bash\nxfs_spaceman -c 'freesp -s' /data\n```\n\n> Fragmentation of free space.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | Done. |\n| `1` | Error. |\n\nFull manual: [man xfs_info](https://linux.die.net/man/8/xfs_info)\n","page":"commands/xfs_info/","label":"xfs_info","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-xfs-repair","n":1576,"slug":"xfs_repair","title":"xfs_repair — Check and repair a damaged XFS file system (it must be unmounted)","sections":["commands"],"topic":"Commands · Disks and file systems","tags":["xfs","repair","check","file system errors","corruption"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Check and repair a damaged XFS file system (it must be unmounted).\n\n## About\n\nXFS repairs itself after a crash by replaying its log at mount time. When the file system is really damaged (mount fails, kernel shows XFS corruption), run `xfs_repair` on the **unmounted** device. `-n` only checks. `-L` throws away the log — a last resort that can lose recent changes.\n\n`fsck.xfs` exists but does nothing; use xfs_repair.\n\n## Syntax\n\n```text\nxfs_repair [OPTION]... DEVICE\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-n` | No modify: only check and report. |\n| `-L` | Zero the log (when it cannot be replayed; may lose data). |\n| `-v` | Verbose (repeat for more). |\n| `-e` | Exit with 4 if something was repaired. |\n| `-d` | Repair a read-only mounted file system (dangerous; for the root FS in rescue). |\n| `-f` | The device is an image file. |\n| `-l LOGDEV` | External log device. |\n| `-r RTDEV` | Realtime device. |\n| `-m MAXMEM` | Memory limit in MiB. |\n| `-P` | Do not prefetch (slower, for problems). |\n| `-c SUBOPT=VALUE` | Change features during repair (like lazycount). |\n| `-o SUBOPT[=VALUE]` | Advanced options (bhash, ag_stride …). |\n| `-t INTERVAL` | Progress report interval. |\n| `-V` | Show the version. |\n\n## Examples\n\n### Check only\n\n```bash\numount /data\nxfs_repair -n /dev/vgdata/lvdata\n```\n\n> Exit code 1 = problems found.\n\n### Repair\n\n```bash\nxfs_repair /dev/vgdata/lvdata\n```\n\n> Then mount again.\n\n### Dirty log error\n\n```bash\nxfs_repair /dev/vgdata/lvdata\n```\n```text\nERROR: The filesystem has valuable metadata changes in a log which needs to be replayed. Mount the filesystem to replay the log, and unmount it before re-running xfs_repair.\n```\n\n> Mount and unmount first.\n\n### Log cannot be replayed\n\n```bash\nxfs_repair -L /dev/vgdata/lvdata\n```\n\n> Last resort: recent changes in the log are lost.\n\n### Root file system from a rescue disk\n\n```bash\nxfs_repair /dev/mapper/rhel-root\n```\n\n> Do not mount it in the rescue environment first.\n\n### Mounted? It refuses\n\n```bash\nxfs_repair /dev/vdb1\n```\n```text\nxfs_repair: /dev/vdb1 contains a mounted filesystem\n```\n\n> Unmount it.\n\n### Find lost files after a repair\n\n```bash\nls /data/lost+found\n```\n\n> Files without a folder end up here, named by inode number.\n\n### Repair an image file\n\n```bash\nxfs_repair -f /var/tmp/disk.img\n```\n\n> For VM images or copies.\n\n### Find kernel messages about corruption\n\n```bash\ndmesg -T | grep -i xfs\n```\n\n> \"Metadata corruption detected\" means: plan a repair.\n\n### Low memory\n\n```bash\nxfs_repair -m 1024 /dev/vgdata/lvdata\n```\n\n> Limit memory use on small systems.\n\n## Watch out\n\n- Never repair a mounted file system.\n- Make an image copy (dd) of important damaged disks before repairing.\n\n## Exit status\n\n| Code | Meaning |\n|---|---|\n| `0` | No problems (or repaired). |\n| `1` | Problems found (with -n) or errors. |\n| `2` | Dirty log: mount first. |\n| `4` | Something was repaired (with -e). |\n\nFull manual: [man xfs_repair](https://linux.die.net/man/8/xfs_repair)\n","page":"commands/xfs_repair/","label":"xfs_repair","where":"Command page · Disks and file systems","aka":[]},{"id":"cmd-xz","n":1577,"slug":"xz","title":"xz — Compress files to .xz with very good compression (xz), unpack them (unxz), or read them (xzcat)","sections":["commands"],"topic":"Commands · Archive and compress","tags":["compress","decompress","xz","lzma","archive"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Compress files to .xz with very good compression (xz), unpack them (unxz), or read them (xzcat).\n\nSame page for: `unxz`, `xzcat`, `xzgrep`\n\n## About\n\nxz usually makes the smallest files of the classic tools, but it uses more time and memory. It is used for kernel modules, software packages and long-term archives.\n\n`xz FILE` makes `FILE.xz` and removes the original. With tar, use `-J` (`tar -cJf`).\n\n## Syntax\n\n```text\nxz [OPTION]... [FILE]...\nunxz [OPTION]... [FILE]...\nxzcat [OPTION]... [FILE]...\n```\n\n## Options\n\n| Option | What it does |\n|---|---|\n| `-z, --compress` | Compress (the default). |\n| `-d, --decompress, --uncompress` | Decompress (same as unxz). |\n| `-k, --keep` | Keep the original file. |\n| `-c, --stdout` | Write to the screen (stdout), keep the files. |\n| `-l, --list` | Show information about .xz files (sizes, ratio, check type). |\n| `-t, --test` | Test the file. |\n| `-f, --force` | Overwrite existing files and compress links. |\n| `-0 ... -9` | Compression level. Default 6. Higher = smaller and slower. |\n| `-e, --extreme` | Try harder for a smaller file (more CPU time). |\n| `-T, --threads=<n>` | Use N threads. 0 = as many as there are CPU cores. |\n| `-q, --quiet` | Hide warnings (twice: hide errors too). |\n| `-v, --verbose` | Show progress (twice: more). |\n| `-F, --format=<fmt>` | File format: auto, xz, lzma, raw. |\n| `-C, --check=<check>` | Integrity check type: none, crc32, crc64, sha256. |\n| `--memlimit=<limit>` | Limit the memory use. |\n| `-S, --suffix=<suf>` | Use another ending than .xz. |\n| `--single-stream` | Decompress only the first stream. |\n| `--no-sparse` | Do not make sparse files on decompress. |\n| `--block-size=<size>` | Start a new block every SIZE bytes (for multi-thread). |\n| `-h, --help` | Show help (-H for the long help with advanced options). |\n| `-V, --version` | Show the version. |\n\n## Examples\n\n### Compress a file\n\n```bash\nxz big.sql\n```\n\n> You get big.sql.xz.\n\n### Decompress\n\n```bash\nunxz big.sql.xz\n```\n\n> Same as `xz -d big.sql.xz`.\n\n### Compress fast with all CPU cores\n\n```bash\nxz -T0 -k vm-disk.raw\n```\n\n> `-T0` uses all cores; `-k` keeps the original.\n\n### Make a tar.xz archive\n\n```bash\ntar -cJf logs-2026.tar.xz /var/log/archive\n```\n\n> Good for archives you keep for a long time.\n\n### Read without unpacking\n\n```bash\nxzcat app.log.xz | grep -c ERROR\n```\n\n> Count errors in a compressed log.\n\n### Show details of a compressed file\n\n```bash\nxz -l app.log.xz\n```\n\n> Compressed size, uncompressed size, ratio and check type.\n\n### Best possible compression\n\n```bash\nxz -9e release-notes.tar\n```\n\n> Very slow but very small. Good for files you send many times.\n\n### Check a downloaded file\n\n```bash\nxz -t image.xz && echo OK\n```\n\n> Finds broken downloads.\n\n### Look at a compressed kernel module\n\n```bash\nls /lib/modules/$(uname -r)/kernel/drivers/net/ethernet/intel/ | head\n```\n\n> RHEL ships kernel modules as .ko.xz files.\n\n### Unpack a cloud image\n\n```bash\nxz -dk rhel-10-cloud.raw.xz\n```\n\n> Many cloud and SD-card images are delivered as .xz.\n\n### Limit memory on a small machine\n\n```bash\nxz --memlimit=256MiB -d huge.xz\n```\n\n> xz stops with an error instead of using too much RAM.\n\n## Watch out\n\n- xz removes the original by default; use `-k`.\n- High levels (-9, -e) need a lot of RAM and time.\n\n## Tips\n\n- Size: xz < bzip2 < gzip. Speed: gzip > bzip2 > xz. In tar: z = gzip, j = bzip2, J = xz.\n\nFull manual: [man xz](https://linux.die.net/man/1/xz)\n","page":"commands/xz/","label":"xz","where":"Command page · Archive and compress","aka":["unxz","xzcat","xzgrep"]},{"id":"cmd-zip","n":1578,"slug":"zip","title":"zip — Pack files into a .zip archive (zip) and unpack it (unzip) — the format that Windows and macOS open directly","sections":["commands"],"topic":"Commands · Archive and compress","tags":["zip","archive","compress","windows","extract","password"],"series_name":"Command pages","design":"command","script":"","date":"","files":{},"md":"Pack files into a .zip archive (zip) and unpack it (unzip) — the format that Windows and macOS open directly.\n\nSame page for: `unzip`\n\n## About\n\n`zip` makes one archive with many files and folders, compressed. `unzip` extracts it. Use zip when you exchange files with Windows users or upload to tools that want `.zip`.\n\nFor Linux backups, `tar` is better: zip does not keep Linux owners and some permissions the same way.\n\n## Syntax\n\n```text\nzip [OPTIONS] ARCHIVE.zip FILE...\nunzip [OPTIONS] ARCHIVE.zip [FILE...] [-x EXCLUDE...] [-d DIR]\n```\n\n## Options\n\n\n### zip\n\n| Option | What it does |\n|---|---|\n| `-r` | Include folders and everything in them (recursive). |\n| `-q` | Quiet: no messages. |\n| `-v` | Verbose output. |\n| `-0 ... -9` | Compression level: 0 = store only, 9 = best. Default 6. |\n| `-e` | Encrypt the files; zip asks for a password. |\n| `-P <password>` | Give the password on the command line (unsafe: it is in the history). |\n| `-x <patterns>` | Leave out files that match. |\n| `-i <patterns>` | Include only files that match. |\n| `-u` | Update: add new files and replace changed files in the archive. |\n| `-f` | Freshen: replace changed files only (do not add new ones). |\n| `-m` | Move: delete the original files after zipping. |\n| `-d` | Delete files from the archive. |\n| `-j` | Junk paths: store only file names, no folders. |\n| `-y` | Store symbolic links as links (not the files they point to). |\n| `-T` | Test the new archive after writing it. |\n| `-s <size>` | Split the archive into parts of this size (like 100m). |\n| `-l, -ll` | Convert line endings (LF to CRLF / CRLF to LF) for text files. |\n| `-o` | Set the archive time to the newest file's time. |\n| `-z` | Add a comment to the archive. |\n| `-c` | Add a one-line comment for each file. |\n| `-@` | Read the file names from standard input. |\n| `-D` | Do not add separate entries for folders. |\n| `-n <suffixes>` | Do not compress files with these endings (like .jpg:.png). |\n| `-b <path>` | Folder for the temporary file. |\n| `-X` | Do not save extra file attributes (Unix UID/GID, times). |\n| `-F, -FF` | Try to fix a damaged archive. |\n| `-A` | Fix the offsets of a self-extracting archive. |\n| `-g` | Grow (add to) an existing archive. |\n| `-t <date>, -tt <date>` | Only files changed on or after / before this date. |\n| `-S` | Include system and hidden files (on MS-DOS/Windows). |\n| `-h, -h2` | Show help (-h2 = extended help). |\n\n### unzip\n\n| Option | What it does |\n|---|---|\n| `-l` | List the files in the archive (short). |\n| `-v` | List with details (verbose). |\n| `-t` | Test the archive. |\n| `-d <dir>` | Extract into this folder. |\n| `-o` | Overwrite existing files without asking. |\n| `-n` | Never overwrite existing files. |\n| `-q` | Quiet. |\n| `-j` | Junk paths: extract all files into one folder. |\n| `-x <files>` | Do not extract these files. |\n| `-p` | Extract to stdout (pipe) without messages. |\n| `-c` | Extract to the screen with the file names. |\n| `-f` | Freshen: extract only files that already exist and are newer in the archive. |\n| `-u` | Update: extract new files and newer versions of existing files. |\n| `-P <password>` | The password for encrypted archives (unsafe on the command line). |\n| `-a` | Convert text files to the local line endings. |\n| `-L` | Make names lowercase. |\n| `-C` | Match names without upper/lower case. |\n| `-z` | Show only the archive comment. |\n| `-Z` | Work like zipinfo (detailed listing). |\n\n## Examples\n\n### Zip a folder\n\n```bash\nzip -r report.zip report/\n```\n\n> `-r` takes the whole folder.\n\n### Unzip into a folder\n\n```bash\nunzip report.zip -d /tmp/report\n```\n\n> The folder is made if it does not exist.\n\n### List the content before you extract\n\n```bash\nunzip -l upload.zip\n```\n\n> Check for strange paths like `../` before you trust a zip from outside.\n\n### Zip with a password\n\n```bash\nzip -e -r secret.zip contracts/\n```\n\n> zip asks for the password. Note: classic zip encryption is weak; use 7z or gpg for real secrets.\n\n### Leave out files\n\n```bash\nzip -r site.zip public_html -x '*.log' '*/.git/*'\n```\n\n> Smaller archive, no logs, no git data.\n\n### Add or update files in an archive\n\n```bash\nzip -u site.zip public_html/index.html\n```\n\n> Only changed files are replaced.\n\n### Extract only one file\n\n```bash\nunzip site.zip public_html/index.html -d /tmp\n```\n\n> Give the path as it is stored in the archive.\n\n### Test an archive\n\n```bash\nunzip -t backup.zip\n```\n\n> \"No errors detected\" means the file is complete.\n\n### Overwrite without questions in a script\n\n```bash\nunzip -o -q release.zip -d /srv/app\n```\n\n> `-o` overwrites, `-q` keeps the log clean.\n\n### Store files without folders\n\n```bash\nzip -j logs.zip /var/log/httpd/*.log\n```\n\n> All files at the top level of the zip.\n\n### Split a big archive for upload limits\n\n```bash\nzip -r -s 100m dataset.zip dataset/\n```\n\n> Makes dataset.z01, dataset.z02 … and dataset.zip.\n\n### Make a zip from a list of files\n\n```bash\nfind . -name \"*.pdf\" | zip invoices.zip -@\n```\n\n> The names come from find.\n\n## Watch out\n\n- Check the content of an unknown zip with `unzip -l` before you extract (path tricks, very large \"zip bombs\").\n- `-P password` is saved in the shell history; use `-e` so it is asked.\n- zip does not keep Linux owners on extract; use tar for system backups.\n\n## Tips\n\n- Pack: `zip -r name.zip folder`. Unpack: `unzip name.zip -d folder`. Look: `unzip -l`.\n\nFull manual: [man zip](https://linux.die.net/man/1/zip)\n","page":"commands/zip/","label":"zip","where":"Command page · Archive and compress","aka":["unzip"]},{"id":"lab-rhcsa-01-1","n":1579,"slug":"rhcsa-01-1","title":"Lab 1.1: Build your two practice servers","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Install the two machines you use in all the other labs. When you are done, both machines run RHEL 10, have the right names, an admin user, and can reach each other.\n\nLevel: **easy** · about 60 minutes · systems: server1, server2\n\n## Before you start\n\n- A computer with at least 8 GB RAM and 60 GB free disk space.\n- A hypervisor (KVM, VirtualBox, VMware or Hyper-V) and the RHEL 10 DVD image (or CentOS Stream 10 / AlmaLinux 10).\n\n## Tasks\n\n1. Create the virtual machine **server1**: 2 CPUs, 4 GB RAM, a 20 GB system disk and a second empty 10 GB disk.\n2. Install server1 with *Server with GUI*. Use automatic partitioning on the 20 GB disk only. Set the hostname `server1.example.com`.\n3. Create the user `student` and make it an administrator. Set a root password.\n4. Create **server2**: 1 CPU, 2 GB RAM, a 20 GB disk. Install it with *Minimal Install* and the hostname `server2.example.com`, the same user and root password.\n5. Make sure both network cards are on and start at boot.\n6. Take a snapshot of both machines called `clean-install`.\n\n## Check your work\n\nRun on each machine:\n\n```bash\ncat /etc/redhat-release\nhostnamectl --static\nid student\nlsblk\nip -br a\nping -c 2 <address of the other machine>\n```\n\n- The hostname is `server1.example.com` or `server2.example.com`.\n- `id student` shows the group `wheel`.\n- On server1, `lsblk` shows the second disk (often `vdb` or `sdb`) without partitions.\n- Each machine can ping the other one.\n\n## Hints\n\n- \"Make this user administrator\" in the installer adds the user to `wheel`.\n- If a network card is off after the install: `nmcli con up NAME` and `nmcli con mod NAME connection.autoconnect yes`.\n- The hostname can still be changed after the install with `hostnamectl set-hostname`.\n\n## Solution\n\n1. Create the machines in your hypervisor as in tasks 1 and 4. Attach the RHEL DVD image as the boot device.\n2. In the installer: Software Selection → *Server with GUI* (server1) or *Minimal Install* (server2). Installation Destination → only the 20 GB disk, *Automatic*. Network & Host Name → switch the card On, type the hostname, click Apply.\n3. Root Password → set it. User Creation → `student`, tick *Make this user administrator*.\n4. Begin Installation, then Reboot System. Log in and check:\n```bash\nhostnamectl --static\nid student\n```\n5. If the card is not up:\n```bash\nnmcli con up enp1s0\nnmcli con mod enp1s0 connection.autoconnect yes\n```\n6. Shut down both machines and take the snapshot in your hypervisor.\n","page":"labs/rhcsa-01-1/","label":"Lab 1.1","where":"Lab 1.1 · easy · 60 min"},{"id":"lab-rhcsa-02-1","n":1580,"slug":"rhcsa-02-1","title":"Lab 2.1: Work faster in the shell","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Practice redirection, pipes, the history, an alias that stays, an environment setting for all users, and finding help without internet.\n\nLevel: **easy** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Log in on server1 as `student`. Use `sudo` for the tasks that need root.\n\n## Tasks\n\n1. Save the list of all files in `/etc` that end in `.conf` (only the names, no errors) to `~/conf-files.txt`. Send all error messages to `/dev/null`.\n2. Add the current date to the end of `~/conf-files.txt` without removing what is in it.\n3. Count how many lines in `~/conf-files.txt` contain the word `ssh`. Show only the number.\n4. Make an alias `ports` for the command `ss -tuln` that still works after you log out and in again.\n5. For all users, set the variable `EDITOR=vim` at login. Do not change `/etc/profile`.\n6. Find which man page (and which section) explains the format of the file `/etc/shadow`. Write the command you used in `~/help.txt`.\n7. Copy `/etc/services` to `~/services.txt`. In vim, replace every `tcp` with `TCP` in the whole file, and save.\n\n## Check your work\n\n```bash\ntail -1 ~/conf-files.txt\ngrep -c ssh ~/conf-files.txt\nbash -lc 'type ports'\nbash -lc 'echo $EDITOR'\ncat ~/help.txt\ngrep -c TCP ~/services.txt\n```\n\n- The last line of `~/conf-files.txt` is a date.\n- `type ports` says it is an alias for `ss -tuln`.\n- `echo $EDITOR` prints `vim` in a new login shell.\n\n## Hints\n\n- `find` writes its errors to channel 2.\n- `>>` adds to a file.\n- `grep -c` counts matching lines.\n- Personal aliases go in `~/.bashrc`; settings for everybody go in a new file in `/etc/profile.d/`.\n- `man -k shadow` lists the pages about shadow; section 5 is for files.\n\n## Solution\n\n1. List the files, hide the errors.\n```bash\nfind /etc -name \"*.conf\" 2> /dev/null > ~/conf-files.txt\n```\n2. Add the date.\n```bash\ndate >> ~/conf-files.txt\n```\n3. Count the lines with ssh.\n```bash\ngrep -c ssh ~/conf-files.txt\n```\n4. Make the alias permanent and load it.\n```bash\necho \"alias ports='ss -tuln'\" >> ~/.bashrc\nsource ~/.bashrc\n```\n5. Make a profile file for all users.\n```bash\necho 'export EDITOR=vim' | sudo tee /etc/profile.d/editor.sh\n```\n6. Search the manuals and write down the command.\n```bash\nman -k shadow\necho \"man 5 shadow\" > ~/help.txt\n```\n7. Copy the file, then replace in vim with `:%s/tcp/TCP/g` and save with `:wq`.\n```bash\ncp /etc/services ~/services.txt\nvim ~/services.txt\n```\n","page":"labs/rhcsa-02-1/","label":"Lab 2.1","where":"Lab 2.1 · easy · 30 min"},{"id":"lab-rhcsa-03-1","n":1581,"slug":"rhcsa-03-1","title":"Lab 3.1: Organize files, make links and back up a folder","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Make a folder structure, copy and move files, create both kinds of links, find files by owner and size, and make a compressed backup that you can restore.\n\nLevel: **easy** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1. Some tasks need root (`sudo -i`).\n\n## Tasks\n\n1. Create the folders `/data/projects/web`, `/data/projects/db` and `/data/archive` with one command.\n2. Copy all files from `/etc` that start with `host` to `/data/projects/web`. Keep their permissions and times.\n3. Rename `/data/projects/web/hosts` to `/data/projects/web/hosts.old`.\n4. Make a symbolic link `/data/current` that points to `/data/projects/web`. Make a hard link `/data/projects/db/hosts.hard` to `/data/projects/web/hosts.old`.\n5. Find all files in `/usr/share` bigger than 5 MiB and write their names to `/data/archive/big-files.txt`.\n6. Create the archive `/data/archive/etc.tar.bz2` of the folder `/etc` compressed with bzip2.\n7. Extract only the file `etc/hosts` from that archive into `/tmp/restore`.\n\n## Check your work\n\n```bash\nls -lR /data\nls -l /data/current\nls -li /data/projects/web/hosts.old /data/projects/db/hosts.hard\nwc -l /data/archive/big-files.txt\ntar -tjf /data/archive/etc.tar.bz2 | head\ncat /tmp/restore/etc/hosts\n```\n\n- `/data/current` shows `-> /data/projects/web`.\n- The two hard-linked files have the same inode number and a link count of 2.\n- The archive lists files that start with `etc/`.\n\n## Hints\n\n- `mkdir -p` with braces makes many folders: `mkdir -p /data/projects/{web,db}`.\n- `cp -a` keeps permissions and times.\n- `find -size +5M -type f`.\n- `tar -cjf` for bzip2; give the member name to extract one file.\n\n## Solution\n\n1. Make the folders.\n```bash\nmkdir -p /data/projects/{web,db} /data/archive\n```\n2. Copy with the attributes.\n```bash\ncp -a /etc/host* /data/projects/web/\n```\n3. Rename.\n```bash\nmv /data/projects/web/hosts /data/projects/web/hosts.old\n```\n4. Make the links.\n```bash\nln -s /data/projects/web /data/current\nln /data/projects/web/hosts.old /data/projects/db/hosts.hard\n```\n5. Find big files.\n```bash\nfind /usr/share -type f -size +5M > /data/archive/big-files.txt\n```\n6. Create the bzip2 archive.\n```bash\ntar -cjf /data/archive/etc.tar.bz2 /etc\n```\n7. Extract one file.\n```bash\nmkdir -p /tmp/restore\ntar -xjf /data/archive/etc.tar.bz2 -C /tmp/restore etc/hosts\n```\n","page":"labs/rhcsa-03-1/","label":"Lab 3.1","where":"Lab 3.1 · easy · 30 min"},{"id":"lab-rhcsa-04-1","n":1582,"slug":"rhcsa-04-1","title":"Lab 4.1: Get answers from text files and logs","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Use grep, regular expressions, cut, sort, awk and sed to answer questions about users, logs and config files — the way you do it in an incident or an audit.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root (`sudo -i`). Create some failed logins first: from server2, try `ssh wronguser@server1` three times with a wrong password.\n\n## Tasks\n\n1. Write all lines of `/etc/passwd` that end with `nologin` to `/root/nologin-users.txt`.\n2. Write only the user names of all users with a UID of 1000 or higher to `/root/real-users.txt`.\n3. Count how many users use each login shell. Save the result, biggest count first, to `/root/shells.txt`.\n4. Show the active settings of `/etc/ssh/sshd_config`: no comment lines, no empty lines. Save them to `/root/sshd-active.txt`.\n5. Find the IP addresses that had failed SSH logins in `/var/log/secure` and count them per address, highest first.\n6. Make a copy of `/etc/chrony.conf` as `/root/chrony.conf`. In the copy, comment out every line that starts with `pool` or `server` (put `#` in front).\n7. Find all lines in `/etc/services` that describe port 22, 80 or 443 over TCP.\n\n## Check your work\n\n```bash\nwc -l /root/nologin-users.txt\ncat /root/real-users.txt\ncat /root/shells.txt\ngrep -c '^#' /root/sshd-active.txt\ngrep -E '^#(pool|server)' /root/chrony.conf\n```\n\n- `/root/real-users.txt` contains `student` (and your other normal users), but not `root`.\n- `/root/sshd-active.txt` has 0 comment lines.\n- The chrony copy shows the commented pool/server lines.\n\n## Hints\n\n- `grep 'nologin$'` — the `$` anchors at the line end.\n- `awk -F: '$3 >= 1000'` selects by UID; `nfsnobody`-like users with UID 65534 may also appear.\n- `sort | uniq -c | sort -rn` makes a top list.\n- `sed -E 's/^(pool|server)/#\\1/'` puts a `#` before the matched word.\n\n## Solution\n\n1. Lines that end with nologin.\n```bash\ngrep 'nologin$' /etc/passwd > /root/nologin-users.txt\n```\n2. Users with UID 1000 or higher (leave out 65534).\n```bash\nawk -F: '$3 >= 1000 && $3 != 65534 {print $1}' /etc/passwd > /root/real-users.txt\n```\n3. Shell top list.\n```bash\ncut -d: -f7 /etc/passwd | sort | uniq -c | sort -rn > /root/shells.txt\n```\n4. Active SSH settings.\n```bash\ngrep -v '^#' /etc/ssh/sshd_config | grep -v '^$' > /root/sshd-active.txt\n```\n5. Failed logins per IP address.\n```bash\ngrep \"Failed password\" /var/log/secure | awk '{print $(NF-3)}' | sort | uniq -c | sort -rn\n```\n6. Comment out pool and server lines in the copy.\n```bash\ncp /etc/chrony.conf /root/chrony.conf\nsed -i -E 's/^(pool|server)/#\\1/' /root/chrony.conf\n```\n7. Ports 22, 80 and 443 over TCP.\n```bash\ngrep -E '\\s(22|80|443)/tcp' /etc/services\n```\n","page":"labs/rhcsa-04-1/","label":"Lab 4.1","where":"Lab 4.1 · medium · 30 min"},{"id":"lab-rhcsa-05-1","n":1583,"slug":"rhcsa-05-1","title":"Lab 5.1: Log in from server1 to server2 with an SSH key","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set up key-based login so that `student` on server1 can log in to server2 without a password, and run commands there.\n\nLevel: **easy** · about 20 minutes · systems: server1, server2\n\n## Before you start\n\n- Both machines run and can reach each other by name (see lab 8.1, or use the IP addresses).\n- The user `student` exists on both machines with a password.\n\n## Tasks\n\n1. On server1, as `student`, make an Ed25519 key pair. Use an empty passphrase for this lab.\n2. Copy the public key to `student` on server2.\n3. Log in to server2 without a password and show the hostname.\n4. From server1, run the command `uptime` on server2 without opening an interactive shell.\n5. Check that the permissions of `~/.ssh` and `~/.ssh/authorized_keys` on server2 are correct.\n\n## Check your work\n\n```bash\nssh -o BatchMode=yes student@server2 hostname\nssh student@server2 'stat -c \"%a %n\" ~/.ssh ~/.ssh/authorized_keys'\n```\n\n- The first command prints `server2.example.com` and does not ask for a password (`BatchMode` makes it fail instead of asking).\n- The permissions are `700` for `~/.ssh` and `600` for `authorized_keys`.\n\n## Hints\n\n- `ssh-keygen -t ed25519` and press Enter for an empty passphrase.\n- `ssh-copy-id` asks for the password one last time.\n\n## Solution\n\n1. Make the key.\n```bash\nssh-keygen -t ed25519 -N \"\" -f ~/.ssh/id_ed25519\n```\n2. Copy the public key.\n```bash\nssh-copy-id student@server2\n```\n3. Log in and check.\n```bash\nssh student@server2\nhostname\nexit\n```\n4. Run one remote command.\n```bash\nssh student@server2 uptime\n```\n5. Check and, if needed, fix the permissions on server2.\n```bash\nssh student@server2 'chmod 700 ~/.ssh; chmod 600 ~/.ssh/authorized_keys'\n```\n","page":"labs/rhcsa-05-1/","label":"Lab 5.1","where":"Lab 5.1 · easy · 20 min"},{"id":"lab-rhcsa-05-2","n":1584,"slug":"rhcsa-05-2","title":"Lab 5.2: Copy, sync and reboot safely","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Move files between the two servers with scp, sftp and rsync, and plan a reboot with a warning to users.\n\nLevel: **easy** · about 25 minutes · systems: server1, server2\n\n## Before you start\n\n- Finish lab 5.1 first (key login from server1 to server2), or type the password when asked.\n\n## Tasks\n\n1. Copy `/etc/hosts` from server1 to `/tmp/hosts-from-server1` on server2 with scp.\n2. Download `/etc/os-release` from server2 to your home folder on server1 with sftp.\n3. On server1, create the folder `~/site` with three files. Synchronize it to `/home/student/site-backup/` on server2 with rsync.\n4. Delete one file in `~/site` and run the sync again so that the file is also removed on server2. First show what would happen without changing anything.\n5. Plan a reboot of server2 in 10 minutes with the message \"Kernel update\". Then cancel it.\n\n## Check your work\n\n```bash\nssh student@server2 'ls -l /tmp/hosts-from-server1 ~/site-backup/'\nls -l ~/os-release\n```\n\n- The removed file from task 4 is gone in `~/site-backup` on server2.\n- After task 5, `ssh root@server2 'systemctl list-jobs'` shows no pending reboot (or check the cancel message).\n\n## Hints\n\n- The remote part of an scp path is `user@host:path`.\n- In sftp, `get` downloads.\n- rsync: a slash at the end of the source copies the content; `-n` = dry run; `--delete` removes extra files.\n- `shutdown -r +10 \"message\"` and `shutdown -c`.\n\n## Solution\n\n1. scp to server2.\n```bash\nscp /etc/hosts student@server2:/tmp/hosts-from-server1\n```\n2. sftp download.\n```bash\nsftp student@server2\n```\n```text\nsftp> get /etc/os-release\nsftp> exit\n```\n3. Make the files and sync them.\n```bash\nmkdir -p ~/site && touch ~/site/{index,about,contact}.html\nrsync -av ~/site/ student@server2:/home/student/site-backup/\n```\n4. Remove a file, dry run, then sync with delete.\n```bash\nrm ~/site/about.html\nrsync -avn --delete ~/site/ student@server2:/home/student/site-backup/\nrsync -av --delete ~/site/ student@server2:/home/student/site-backup/\n```\n5. Plan and cancel the reboot (as root on server2).\n```bash\nshutdown -r +10 \"Kernel update\"\nshutdown -c\n```\n","page":"labs/rhcsa-05-2/","label":"Lab 5.2","where":"Lab 5.2 · easy · 25 min"},{"id":"lab-rhcsa-06-1","n":1585,"slug":"rhcsa-06-1","title":"Lab 6.1: Create users and groups with password rules","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set the defaults for new accounts, then create a small team with groups, password rules and one service account.\n\nLevel: **easy** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root (`sudo -i`).\n- This is a lab: you may use the password `password` for every user.\n\n## Tasks\n\n1. Make sure that every **new** user must change the password every 90 days and gets a file `WELCOME.txt` in the home folder.\n2. Create the group `sales` with GID 3000 and the group `account`.\n3. Create the users `anna` and `bob`. Both are members of `sales`; `bob` is also a member of `account`. Set a password for both.\n4. Create the user `carol` with the shell `/sbin/nologin` and no extra groups. Set a password.\n5. The account of `bob` must expire on 31 December 2026.\n6. `anna` must choose a new password at her next login.\n7. Lock the password of `carol`.\n8. Add `anna` to the group `account` without removing her from `sales`.\n\n## Check your work\n\n```bash\nid anna; id bob; id carol\ngetent group sales account\nchage -l bob | grep 'Account expires'\nchage -l anna | head -2\npasswd -S carol\nls /home/anna\n```\n\n- `anna` and `bob` are in `sales` and `account`; `carol` is only in her own group.\n- `sales` has GID 3000.\n- `bob`: Account expires Dec 31, 2026. `anna`: \"password must be changed\".\n- `passwd -S carol` shows `L` (locked). `/home/anna` has `WELCOME.txt`.\n- `grep PASS_MAX_DAYS /etc/login.defs` shows 90.\n\n## Hints\n\n- Defaults for new users: `/etc/login.defs` and `/etc/skel/`. Do this **before** you create the users.\n- `groupadd -g` sets the GID. `useradd -G` sets the extra groups.\n- Account expire date: `chage -E` or `usermod -e`. Force a new password: `chage -d 0` or `passwd -e`.\n- Lock: `passwd -l` or `usermod -L`.\n- Add a group and keep the others: `usermod -aG`.\n\n## Solution\n\n1. Set the defaults first.\n```bash\nsed -i 's/^PASS_MAX_DAYS.*/PASS_MAX_DAYS   90/' /etc/login.defs\necho \"Welcome to the team\" > /etc/skel/WELCOME.txt\n```\n2. Create the groups.\n```bash\ngroupadd -g 3000 sales\ngroupadd account\n```\n3. Create anna and bob and set the passwords.\n```bash\nuseradd -G sales anna\nuseradd -G sales,account bob\necho password | passwd --stdin anna\necho password | passwd --stdin bob\n```\n4. Create carol.\n```bash\nuseradd -s /sbin/nologin carol\npasswd carol\n```\n5. Expire date for bob.\n```bash\nchage -E 2026-12-31 bob\n```\n6. Force a new password for anna.\n```bash\nchage -d 0 anna\n```\n7. Lock carol.\n```bash\npasswd -l carol\n```\n8. Add the extra group to anna.\n```bash\nusermod -aG account anna\n```\n","page":"labs/rhcsa-06-1/","label":"Lab 6.1","where":"Lab 6.1 · easy · 30 min"},{"id":"lab-rhcsa-06-2","n":1586,"slug":"rhcsa-06-2","title":"Lab 6.2: Give limited admin rights with sudo","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Give one user full admin rights and give a team only the commands it needs. Test both, and make sure the sudo files have no errors.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root (`sudo -i`).\n- The web server does not need to be installed: sudo checks the rule before the command runs.\n\n## Tasks\n\n1. Create the user `devops1` with full admin rights through sudo.\n2. Create the group `webadmins` and the user `web1` as a member of it.\n3. Members of `webadmins` may run only these commands as root: `systemctl restart httpd`, `systemctl status httpd` and `journalctl -u httpd`. Put the rule in its own file.\n4. Create the user `backup1`. It may run `/usr/bin/rsync` as root **without** a password.\n5. Test as `web1`: list the allowed commands, run one allowed command and try `cat /etc/shadow` with sudo (this must fail).\n6. Check that all sudo files have a correct syntax.\n\n## Check your work\n\n```bash\nsudo -l -U web1\nsudo -l -U backup1\nsu - web1 -c 'sudo -n cat /etc/shadow'\nvisudo -c\n```\n\n- `sudo -l -U web1` lists only the three web commands.\n- `backup1` shows `NOPASSWD: /usr/bin/rsync`.\n- The test with `/etc/shadow` fails (\"not allowed\" or \"a password is required\").\n- `visudo -c` says every file is \"parsed OK\".\n\n## Hints\n\n- Full rights on RHEL: membership of the group `wheel`.\n- Use `visudo -f /etc/sudoers.d/NAME`. A group in a rule starts with `%`.\n- Use full paths in the rules: `/usr/bin/systemctl`, `/usr/bin/journalctl`.\n- `NOPASSWD:` goes right before the command list.\n\n## Solution\n\n1. devops1 in wheel.\n```bash\nuseradd -G wheel devops1\npasswd devops1\n```\n2. Group and user.\n```bash\ngroupadd webadmins\nuseradd -G webadmins web1\npasswd web1\n```\n3. A rule file for the group.\n```bash\nvisudo -f /etc/sudoers.d/webadmins\n```\n**/etc/sudoers.d/webadmins**\n```text\n%webadmins ALL=(root) /usr/bin/systemctl restart httpd, /usr/bin/systemctl status httpd, /usr/bin/journalctl -u httpd\n```\n4. backup1 without a password for rsync.\n```bash\nuseradd backup1\nvisudo -f /etc/sudoers.d/backup1\n```\n**/etc/sudoers.d/backup1**\n```text\nbackup1 ALL=(root) NOPASSWD: /usr/bin/rsync\n```\n5. Test as web1.\n```bash\nsu - web1\nsudo -l\nsudo systemctl status httpd\nsudo cat /etc/shadow\nexit\n```\n6. Check the syntax of all files.\n```bash\nvisudo -c\n```\n","page":"labs/rhcsa-06-2/","label":"Lab 6.2","where":"Lab 6.2 · medium · 30 min"},{"id":"lab-rhcsa-07-1","n":1587,"slug":"rhcsa-07-1","title":"Lab 7.1: Build shared team folders","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Create two team folders where members work together, new files get the team group, and nobody can delete the files of others.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root. You need the users and groups from lab 6.1 (`anna`, `bob` in `sales`; `bob` in `account`). If they do not exist, create them.\n\n## Tasks\n\n1. Create the folders `/data/sales` and `/data/account`.\n2. `/data/sales` belongs to the group `sales`, `/data/account` to the group `account`. The owner of both is root.\n3. Group members may read and write in their folder. Others may not even list the content.\n4. New files in each folder must get the group of the folder.\n5. Users may delete only their own files in these folders.\n6. As `anna`, create `/data/sales/anna.txt`. As `bob`, try to delete it (this must fail) and create `/data/sales/bob.txt`.\n\n## Check your work\n\n```bash\nls -ld /data/sales /data/account\nls -l /data/sales\nsu - bob -c 'rm -f /data/sales/anna.txt'\nsu - carol -c 'ls /data/sales'\n```\n\n- Both folders show `drwxrws--T` with the right group.\n- `anna.txt` and `bob.txt` have the group `sales`.\n- bob gets \"Operation not permitted\"; carol gets \"Permission denied\".\n\n## Hints\n\n- `chown root:GROUP` or `chgrp GROUP`.\n- 770 for the base rights; SGID = 2, sticky = 1, so the first digit is 3.\n\n## Solution\n\n1. Create the folders.\n```bash\nmkdir -p /data/sales /data/account\n```\n2. Set the groups.\n```bash\nchgrp sales /data/sales\nchgrp account /data/account\n```\n3. Set rights, SGID and sticky in one go (tasks 3, 4 and 5).\n```bash\nchmod 3770 /data/sales /data/account\n```\n4. Test as the users.\n```bash\nsu - anna -c 'touch /data/sales/anna.txt'\nsu - bob -c 'rm -f /data/sales/anna.txt'\n```\n```text\nrm: cannot remove '/data/sales/anna.txt': Operation not permitted\n```\n```bash\nsu - bob -c 'touch /data/sales/bob.txt'\nls -l /data/sales\n```\n","page":"labs/rhcsa-07-1/","label":"Lab 7.1","where":"Lab 7.1 · medium · 30 min"},{"id":"lab-rhcsa-07-2","n":1588,"slug":"rhcsa-07-2","title":"Lab 7.2: Defaults, ACLs and fixing access problems","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set safer default permissions, give one extra group access with an ACL, protect a file, and fix a broken access path.\n\nLevel: **medium** · about 35 minutes · systems: server1\n\n## Before you start\n\n- Finish lab 7.1 first. Work on server1 as root unless a task says otherwise.\n\n## Tasks\n\n1. For user `anna` only, new files must get `640` and new folders `750`.\n2. Members of `account` must be able to read (not change) everything in `/data/sales`, also files that are created later.\n3. User `carol` (not in any team) must be able to read `/data/sales/bob.txt` only.\n4. Make `/data/sales/bob.txt` immutable, try to delete it as root, then remove the protection again.\n5. Run `chmod 700 /data` (a \"mistake\"). As `anna`, try to open `/data/sales`. Find the cause with `namei -l` and fix it so that the team folders work again and others still cannot list `/data`.\n\n## Check your work\n\n```bash\nsu - anna -c 'umask; touch /tmp/x1; mkdir /tmp/x2; ls -ld /tmp/x1 /tmp/x2'\ngetfacl /data/sales\nsu - bob -c 'echo test > /data/sales/new.txt'; getfacl /data/sales/new.txt\nsu - carol -c 'cat /data/sales/bob.txt'\nls -ld /data; lsattr /data/sales/bob.txt\n```\n\n- anna: umask `0027`, file `-rw-r-----`, folder `drwxr-x---`.\n- `/data/sales` has `group:account:r-x` and `default:group:account:r-x`; `new.txt` has `group:account:r--` (effective).\n- carol can read `bob.txt`.\n- `/data` is `drwx--x--x` (or `711`); `bob.txt` has no `i` attribute.\n\n## Hints\n\n- One user: `~/.bashrc`. All users: `/etc/profile.d/`.\n- Default ACL: `setfacl -d -m`. Use `rX` so folders get `x` and files do not.\n- carol needs `x` on `/data/sales` to reach the file, and `r` on the file. `setfacl -m u:carol:x` on the folder is enough for the path.\n- `chmod 711 /data` lets everybody pass through without listing.\n\n## Solution\n\n1. umask for anna.\n```bash\necho \"umask 027\" >> /home/anna/.bashrc\n```\n2. Normal and default ACL for the group account.\n```bash\nsetfacl -R -m g:account:rX /data/sales\nsetfacl -d -m g:account:rX /data/sales\n```\n3. Path and file for carol.\n```bash\nsetfacl -m u:carol:x /data/sales\nsetfacl -m u:carol:r /data/sales/bob.txt\n```\n4. Immutable test.\n```bash\nchattr +i /data/sales/bob.txt\nrm -f /data/sales/bob.txt\n```\n```text\nrm: cannot remove '/data/sales/bob.txt': Operation not permitted\n```\n```bash\nchattr -i /data/sales/bob.txt\n```\n5. Find and fix the path problem.\n```bash\nchmod 700 /data\nsu - anna -c 'namei -l /data/sales'\nchmod 711 /data\n```\n","page":"labs/rhcsa-07-2/","label":"Lab 7.2","where":"Lab 7.2 · medium · 35 min"},{"id":"lab-rhcsa-08-1","n":1589,"slug":"rhcsa-08-1","title":"Lab 8.1: Give server1 a fixed address, a name and a hosts entry","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set up the network of server1 by hand, the way the exam asks for it. At the end, every setting must still work after a reboot.\n\nLevel: **easy** · about 25 minutes · systems: server1, server2\n\n## Before you start\n\n- You need two virtual machines with RHEL 10 (or CentOS Stream 10 / AlmaLinux 10): **server1** and **server2**, on the same network.\n- In this lab the network is `192.168.122.0/24` with the gateway `192.168.122.1`. Use the numbers of your own network if they are different.\n- Work on the console of server1, not over SSH: you change the address you are connected to.\n\n## Tasks\n\n1. Find the name of the network card and of its active connection profile on server1.\n2. Give the profile a fixed IPv4 address `192.168.122.10/24`, the gateway `192.168.122.1` and the DNS server `192.168.122.1`. Do not make a new profile.\n3. Add a second IPv4 address `192.168.122.110/24` to the same profile.\n4. Set the hostname of server1 to `server1.example.com`.\n5. Make sure that server1 can reach server2 by the names `server2` and `server2.example.com`, also when no DNS server knows server2. (server2 has the address `192.168.122.20`.)\n6. Make sure the profile starts by itself at boot. Reboot server1 and check everything again.\n\n## Check your work\n\nRun these commands after the reboot. Every line must show the expected value.\n\n```bash\nnmcli -f NAME,DEVICE,STATE con show --active\nip -br -4 a show enp1s0\nip route show default\ncat /etc/resolv.conf\nhostnamectl --static\ngetent hosts server2\nping -c 2 server2\n```\n\n- `ip -br -4 a` shows `192.168.122.10/24` and `192.168.122.110/24`.\n- The default route is `via 192.168.122.1`.\n- `/etc/resolv.conf` has `nameserver 192.168.122.1`.\n- `hostnamectl --static` prints `server1.example.com`.\n- `getent hosts server2` prints `192.168.122.20` with both names.\n\n## Hints\n\n- `nmcli connection show` lists the profiles. The DEVICE column shows the card.\n- `ipv4.method manual` is needed for a fixed address.\n- A `+` before `ipv4.addresses` adds an address instead of replacing it.\n- The hosts file is read before DNS. Put the full name first, then the short name.\n\n## Solution\n\n1. Find the card and the profile. Here both are called `enp1s0`; yours can be different.\n```bash\nnmcli con show\n```\n2. Set the fixed address, the gateway and DNS on the existing profile.\n```bash\nnmcli con mod enp1s0 ipv4.method manual ipv4.addresses 192.168.122.10/24 ipv4.gateway 192.168.122.1 ipv4.dns 192.168.122.1\n```\n3. Add the second address, then activate the profile.\n```bash\nnmcli con mod enp1s0 +ipv4.addresses 192.168.122.110/24\nnmcli con up enp1s0\n```\n4. Set the hostname.\n```bash\nhostnamectl set-hostname server1.example.com\n```\n5. Add server2 to the hosts file.\n```bash\necho \"192.168.122.20  server2.example.com server2\" >> /etc/hosts\n```\n6. Turn on autoconnect, reboot, and run the checks again.\n```bash\nnmcli con mod enp1s0 connection.autoconnect yes\nsystemctl reboot\n```\n","page":"labs/rhcsa-08-1/","label":"Lab 8.1","where":"Lab 8.1 · easy · 25 min"},{"id":"lab-rhcsa-09-1","n":1590,"slug":"rhcsa-09-1","title":"Lab 9.1: Use a local repository and manage packages","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Turn the installation ISO into a repository that survives a reboot, then find, install, inspect and undo packages.\n\nLevel: **medium** · about 40 minutes · systems: server1\n\n## Before you start\n\n- Copy the RHEL 10 installation ISO to `/root/` on server1 (or attach it to the VM as a DVD; then it is `/dev/sr0`).\n- Move any other `.repo` files to `/root/repos-backup/` so you only use the local repository (move them back after the lab if you need them).\n\n## Tasks\n\n1. Mount the ISO on `/repo` in a way that survives a reboot.\n2. Create the repository file `/etc/yum.repos.d/local.repo` with BaseOS and AppStream from the ISO. No GPG check.\n3. Show that both repositories are enabled.\n4. Find out which package provides the command `semanage` and install it.\n5. Install `httpd`, show its configuration files, and find which package owns `/etc/httpd/conf/httpd.conf`.\n6. Install the package group \"Development Tools\", look at the dnf history, and undo the group install.\n7. Show which files of `openssh-server` were changed since it was installed.\n\n## Check your work\n\n```bash\nfindmnt /repo\ndnf repolist\nrpm -q policycoreutils-python-utils httpd\nrpm -qc httpd\ndnf history | head -5\ndnf group list --installed\n```\n\n- `/repo` is mounted (also after `reboot`).\n- `dnf repolist` shows `local-BaseOS` and `local-AppStream`.\n- semanage and httpd are installed; \"Development Tools\" is not installed anymore.\n\n## Hints\n\n- fstab line for an ISO file: type `iso9660`, options `defaults,loop`. For the DVD drive: `/dev/sr0 /repo iso9660 defaults 0 0`.\n- `baseurl=file:///repo/BaseOS` — three slashes.\n- `dnf provides '*/semanage'`; `rpm -qf FILE`; `rpm -V PACKAGE`.\n\n## Solution\n\n1. Mount the ISO for good.\n```bash\nmkdir /repo\necho \"/root/rhel-10.0-x86_64-dvd.iso /repo iso9660 defaults,loop 0 0\" >> /etc/fstab\nsystemctl daemon-reload\nmount -a\n```\n2. The repository file.\n**/etc/yum.repos.d/local.repo**\n```text\n[local-BaseOS]\nname=Local BaseOS\nbaseurl=file:///repo/BaseOS\ngpgcheck=0\n\n[local-AppStream]\nname=Local AppStream\nbaseurl=file:///repo/AppStream\ngpgcheck=0\n```\n3. Check the repositories.\n```bash\ndnf repolist\n```\n4. Find and install semanage.\n```bash\ndnf provides '*/semanage'\ndnf install -y policycoreutils-python-utils\n```\n5. httpd and its files.\n```bash\ndnf install -y httpd\nrpm -qc httpd\nrpm -qf /etc/httpd/conf/httpd.conf\n```\n6. Group install and undo.\n```bash\ndnf group install -y \"Development Tools\"\ndnf history\ndnf -y history undo last\n```\n7. Verify the package files.\n```bash\nrpm -V openssh-server\n```\n","page":"labs/rhcsa-09-1/","label":"Lab 9.1","where":"Lab 9.1 · medium · 40 min"},{"id":"lab-rhcsa-10-1","n":1591,"slug":"rhcsa-10-1","title":"Lab 10.1: Control jobs, priorities and signals","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Start busy processes, move them between foreground and background, change their priority, stop them with signals, and set the right tuned profile.\n\nLevel: **easy** · about 25 minutes · systems: server1\n\n## Before you start\n\n- Work on server1. Tasks 1–5 as a normal user, task 6 as root.\n\n## Tasks\n\n1. Start two copies of `dd if=/dev/zero of=/dev/null` in the background and one `sleep 3600` in the foreground. Pause the sleep and continue it in the background.\n2. List your jobs, then show the three processes with ps, with PID, nice value, CPU use and command.\n3. Make the first dd process nicer: nice value 10. Try to set it back to 0 as the normal user (this fails) and explain why.\n4. Start a new `dd` with nice value 15 directly.\n5. Stop all dd processes with one command, and stop the sleep with SIGTERM.\n6. As root: show the recommended tuned profile and activate it.\n\n## Check your work\n\n```bash\njobs\nps -eo pid,ni,%cpu,cmd | grep -E 'dd|sleep' | grep -v grep\ntuned-adm active\n```\n\n- After task 3, ps shows NI 10 for the first dd.\n- After task 5 no dd or sleep is left.\n- `tuned-adm active` shows the same profile as `tuned-adm recommend`.\n\n## Hints\n\n- Ctrl+Z pauses, `bg` continues in the background, `fg` brings it back.\n- `renice -n 10 -p PID`; normal users may only raise the nice value.\n- `pkill dd` or `killall dd`; `kill %3` works for jobs.\n\n## Solution\n\n1. Start the jobs.\n```bash\ndd if=/dev/zero of=/dev/null &\ndd if=/dev/zero of=/dev/null &\nsleep 3600\n```\n```text\n^Z\n```\n```bash\nbg\n```\n2. Jobs and processes.\n```bash\njobs\nps -o pid,ni,%cpu,cmd -C dd,sleep\n```\n3. Renice the first dd (use its PID from task 2).\n```bash\nrenice -n 10 -p 4123\nrenice -n 0 -p 4123\n```\n```text\nrenice: failed to set priority for 4123 (process ID): Permission denied\n```\n4. A new dd with nice 15.\n```bash\nnice -n 15 dd if=/dev/zero of=/dev/null &\n```\n5. Stop them.\n```bash\npkill dd\nkill %3\n```\n6. tuned as root.\n```bash\ntuned-adm recommend\ntuned-adm profile virtual-guest\ntuned-adm active\n```\n","page":"labs/rhcsa-10-1/","label":"Lab 10.1","where":"Lab 10.1 · easy · 25 min"},{"id":"lab-rhcsa-11-1","n":1592,"slug":"rhcsa-11-1","title":"Lab 11.1: Manage and change services","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Install a web server, make it start at boot, make it restart by itself after a crash, block a service, and add a small service of your own.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root. A repository must be available (lab 9.1).\n\n## Tasks\n\n1. Install `httpd`. Start it now and make sure it also starts at boot.\n2. Show if httpd is active and enabled, with two short commands.\n3. Change httpd (without editing the file from the package) so that it is always restarted 5 seconds after it stops for any reason.\n4. Kill the main httpd process with SIGKILL and show that systemd starts it again.\n5. Make sure that the service `nftables` can never be started by accident, then try to start it.\n6. Create the service `hello` that writes \"hello\" and the date to the journal every minute. It must start at boot.\n\n## Check your work\n\n```bash\nsystemctl is-active httpd; systemctl is-enabled httpd\nsystemctl cat httpd | tail -4\nsystemctl status httpd | grep 'Main PID'\nsystemctl is-enabled nftables\njournalctl -u hello --since -5min\n```\n\n- httpd: `active` and `enabled`; the override shows `Restart=always` and `RestartSec=5`.\n- After the kill, the Main PID is new.\n- nftables: `masked`.\n- hello: lines with \"hello\" in the journal; `systemctl is-enabled hello` = `enabled`.\n\n## Hints\n\n- `systemctl enable --now`.\n- `systemctl edit httpd` writes a drop-in file.\n- `systemctl kill -s KILL httpd` or `kill -9` on the Main PID.\n- `systemctl mask`.\n- A unit file in `/etc/systemd/system/` with `[Install] WantedBy=multi-user.target`.\n\n## Solution\n\n1. Install and enable.\n```bash\ndnf install -y httpd\nsystemctl enable --now httpd\n```\n2. Short checks.\n```bash\nsystemctl is-active httpd; systemctl is-enabled httpd\n```\n3. The override: add these lines in the editor.\n```bash\nsystemctl edit httpd\n```\n**/etc/systemd/system/httpd.service.d/override.conf**\n```text\n[Service]\nRestart=always\nRestartSec=5\n```\n4. Kill and watch it come back.\n```bash\nsystemctl kill -s KILL --kill-whom=main httpd\nsleep 6; systemctl status httpd | grep -E 'Active|Main PID'\n```\n5. Mask nftables.\n```bash\nsystemctl mask --now nftables\nsystemctl start nftables\n```\n```text\nFailed to start nftables.service: Unit nftables.service is masked.\n```\n6. The hello service.\n**/etc/systemd/system/hello.service**\n```text\n[Unit]\nDescription=Hello demo service\n\n[Service]\nExecStart=/usr/bin/bash -c 'while true; do echo \"hello $(date)\"; sleep 60; done'\nRestart=on-failure\n\n[Install]\nWantedBy=multi-user.target\n```\n```bash\nsystemctl daemon-reload\nsystemctl enable --now hello\n```\n","page":"labs/rhcsa-11-1/","label":"Lab 11.1","where":"Lab 11.1 · medium · 30 min"},{"id":"lab-rhcsa-12-1","n":1593,"slug":"rhcsa-12-1","title":"Lab 12.1: Schedule jobs with cron, at and a timer","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Plan repeating and one-time jobs with all three tools, and block one user from cron.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root. You need the users `anna` and `bob` (lab 6.1).\n\n## Tasks\n\n1. User `anna` must run `logger \"anna was here\"` every weekday (Monday to Friday) at 14:23.\n2. As root, write the output of `df -h /` to `/var/tmp/disk.log` every 15 minutes, with a file in `/etc/cron.d/`.\n3. User `bob` may not use cron.\n4. In 5 minutes, write the text \"at works\" to `/tmp/at-test.txt` with at. Then plan a second at job and remove it.\n5. Create a systemd timer `cleanup-tmp.timer` that runs `find /var/tmp -type f -mtime +7 -delete` every day at 03:00, also when the machine was off at that time.\n\n## Check your work\n\n```bash\ncrontab -l -u anna\ncat /etc/cron.d/disk\nsu - bob -c 'crontab -l'\natq\ncat /tmp/at-test.txt\nsystemctl list-timers cleanup-tmp.timer\n```\n\n- anna's crontab: `23 14 * * 1-5 logger \"anna was here\"`.\n- `/var/tmp/disk.log` grows every 15 minutes.\n- bob gets \"not allowed\".\n- After 5 minutes `/tmp/at-test.txt` exists; the second job is gone from atq.\n- The timer shows the next run at 03:00.\n\n## Hints\n\n- `crontab -e -u anna`. Day of week 1-5.\n- Files in /etc/cron.d need the user field.\n- `/etc/cron.deny`.\n- `at now + 5 minutes`; `atq`, `atrm`.\n- A `.service` with `Type=oneshot` plus a `.timer` with `OnCalendar` and `Persistent=true`; enable the timer.\n\n## Solution\n\n1. anna's crontab.\n```bash\ncrontab -e -u anna\n```\n**crontab -e -u anna**\n```text\n23 14 * * 1-5 logger \"anna was here\"\n```\n2. The system cron file.\n```bash\necho '*/15 * * * * root df -h / >> /var/tmp/disk.log' > /etc/cron.d/disk\n```\n3. Deny bob.\n```bash\necho bob >> /etc/cron.deny\n```\n4. at jobs.\n```bash\necho 'echo \"at works\" > /tmp/at-test.txt' | at now + 5 minutes\necho 'reboot' | at 23:59\natq\natrm 2\n```\n5. Service and timer.\n**/etc/systemd/system/cleanup-tmp.service**\n```text\n[Unit]\nDescription=Delete old files in /var/tmp\n\n[Service]\nType=oneshot\nExecStart=/usr/bin/find /var/tmp -type f -mtime +7 -delete\n```\n**/etc/systemd/system/cleanup-tmp.timer**\n```text\n[Unit]\nDescription=Daily cleanup of /var/tmp\n\n[Timer]\nOnCalendar=*-*-* 03:00:00\nPersistent=true\n\n[Install]\nWantedBy=timers.target\n```\n```bash\nsystemctl daemon-reload\nsystemctl enable --now cleanup-tmp.timer\n```\n","page":"labs/rhcsa-12-1/","label":"Lab 12.1","where":"Lab 12.1 · medium · 30 min"},{"id":"lab-rhcsa-13-1","n":1594,"slug":"rhcsa-13-1","title":"Lab 13.1: Keep the journal and route your own logs","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Make the journal survive reboots, find events with journalctl, send one facility to its own file with rotation, and test it with logger.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root.\n\n## Tasks\n\n1. Configure the journal so that it is kept after a reboot. Limit it to 300 MB on disk.\n2. Show all messages of priority `err` or worse since the last boot.\n3. Show the sshd messages of the last hour.\n4. Send all messages of the facility `local4` to the file `/var/log/backup.log`.\n5. Rotate `/var/log/backup.log` every week, keep 4 old files, compress them.\n6. Write a message with facility `local4` and priority `warning` and the tag `backup`. Find it in the file and in the journal.\n7. Reboot and show that messages from the boot before are still there.\n\n## Check your work\n\n```bash\njournalctl --list-boots\ndu -sh /var/log/journal\ntail -2 /var/log/backup.log\njournalctl -t backup\nlogrotate -d /etc/logrotate.d/backup 2>&1 | head\n```\n\n- `--list-boots` shows at least two boots after task 7.\n- `/var/log/backup.log` contains the test message.\n- The logrotate test shows \"rotating pattern: /var/log/backup.log weekly\".\n\n## Hints\n\n- A drop-in in `/etc/systemd/journald.conf.d/` with `Storage=persistent` and `SystemMaxUse=300M`.\n- `journalctl -p err -b`, `journalctl -u sshd --since -1h`.\n- A rule `local4.* /var/log/backup.log` in `/etc/rsyslog.d/`, then restart rsyslog.\n- `logger -p local4.warning -t backup \"…\"`.\n\n## Solution\n\n1. Persistent journal: make the folder, write the file, restart journald.\n```bash\nmkdir -p /etc/systemd/journald.conf.d\n```\n**/etc/systemd/journald.conf.d/persistent.conf**\n```text\n[Journal]\nStorage=persistent\nSystemMaxUse=300M\n```\n```bash\nsystemctl restart systemd-journald\n```\n2. Errors of this boot.\n```bash\njournalctl -p err -b\n```\n3. sshd in the last hour.\n```bash\njournalctl -u sshd --since -1h\n```\n4. The rsyslog rule.\n```bash\necho 'local4.*    /var/log/backup.log' > /etc/rsyslog.d/backup.conf\nsystemctl restart rsyslog\n```\n5. The logrotate file.\n**/etc/logrotate.d/backup**\n```text\n/var/log/backup.log {\n    weekly\n    rotate 4\n    compress\n    missingok\n    notifempty\n}\n```\n6. Test message.\n```bash\nlogger -p local4.warning -t backup \"backup job test\"\ntail -1 /var/log/backup.log\njournalctl -t backup\n```\n7. Reboot and check.\n```bash\nsystemctl reboot\njournalctl --list-boots\njournalctl -b -1 -n 5\n```\n","page":"labs/rhcsa-13-1/","label":"Lab 13.1","where":"Lab 13.1 · medium · 30 min"},{"id":"lab-rhcsa-14-1","n":1595,"slug":"rhcsa-14-1","title":"Lab 14.1: Partitions, file systems, swap and fstab","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Use a second disk to create three partitions with a file system, a swap space, and a mount that works after every reboot.\n\nLevel: **medium** · about 45 minutes · systems: server1\n\n## Before you start\n\n- Add an empty 10 GiB disk to server1 (in this lab: `/dev/vdb`). Check with `lsblk` that it really is empty — use your own device name.\n- Work as root.\n\n## Tasks\n\n1. Create a GPT partition table on the disk with three partitions: 2 GiB, 1 GiB and 2 GiB.\n2. Put an XFS file system with the label `data` on partition 1, and mount it on `/data` at every boot by UUID.\n3. Use partition 2 as extra swap, active at every boot.\n4. Put an ext4 file system on partition 3. It must be mounted on `/backup` at boot with the options `noexec,nosuid`, with a systemd mount unit (not fstab).\n5. Set the reserved blocks of the ext4 file system to 1 %.\n6. Reboot and check that everything is mounted and the swap is active.\n\n## Check your work\n\n```bash\nlsblk -f /dev/vdb\nfindmnt /data; findmnt /backup\nswapon --show\ntune2fs -l /dev/vdb3 | grep -i 'reserved block count'\nsystemctl is-enabled backup.mount\n```\n\n- `/data` is XFS with label data, `/backup` is ext4 with `noexec,nosuid`.\n- swapon shows `/dev/vdb2` next to the system swap.\n- After the reboot all of it is still there.\n\n## Hints\n\n- fdisk: `g`, then three times `n` with `+2G`, `+1G`, `+2G`; type of partition 2: `swap`.\n- `blkid /dev/vdb1` gives the UUID for fstab.\n- The unit for /backup is `/etc/systemd/system/backup.mount`.\n- `tune2fs -m 1`.\n\n## Solution\n\n1. Partitions.\n```bash\nfdisk /dev/vdb\n```\n```text\nCommand (m for help): g\nCommand (m for help): n          (Enter, Enter, +2G)\nCommand (m for help): n          (Enter, Enter, +1G)\nCommand (m for help): n          (Enter, Enter, +2G)\nCommand (m for help): t          (2, swap)\nCommand (m for help): w\n```\n```bash\npartprobe /dev/vdb\n```\n2. XFS on /data by UUID.\n```bash\nmkfs.xfs -L data /dev/vdb1\nmkdir /data\necho \"UUID=$(blkid -s UUID -o value /dev/vdb1) /data xfs defaults 0 0\" >> /etc/fstab\n```\n3. Swap.\n```bash\nmkswap /dev/vdb2\necho \"UUID=$(blkid -s UUID -o value /dev/vdb2) none swap defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a; swapon -a\n```\n4. ext4 with a mount unit.\n```bash\nmkfs.ext4 /dev/vdb3\nmkdir /backup\nblkid -s UUID -o value /dev/vdb3\n```\n**/etc/systemd/system/backup.mount**\n```text\n[Unit]\nDescription=Backup disk\n\n[Mount]\nWhat=/dev/disk/by-uuid/PUT-THE-UUID-HERE\nWhere=/backup\nType=ext4\nOptions=noexec,nosuid\n\n[Install]\nWantedBy=multi-user.target\n```\n```bash\nsystemctl daemon-reload\nsystemctl enable --now backup.mount\n```\n5. Reserved blocks.\n```bash\ntune2fs -m 1 /dev/vdb3\n```\n6. Reboot and check.\n```bash\nsystemctl reboot\nfindmnt /data; findmnt /backup; swapon --show\n```\n","page":"labs/rhcsa-14-1/","label":"Lab 14.1","where":"Lab 14.1 · medium · 45 min"},{"id":"lab-rhcsa-15-1","n":1596,"slug":"rhcsa-15-1","title":"Lab 15.1: Create LVM volumes with extents","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Build a volume group with a fixed extent size and two logical volumes that are mounted at every boot.\n\nLevel: **medium** · about 35 minutes · systems: server1\n\n## Before you start\n\n- You need an empty disk of at least 5 GiB (in this lab `/dev/vdc`; check with `lsblk`). Work as root.\n\n## Tasks\n\n1. Create a partition of 4 GiB of the type LVM on the disk and make it a physical volume.\n2. Create the volume group `vgfiles` with an extent size of 8 MiB.\n3. Create the logical volume `lvfiles` with exactly 50 extents. Format it with XFS and mount it on `/files` at every boot.\n4. Create the logical volume `lvdocs` with the rest of the free space. Format it with ext4 and mount it on `/docs` at every boot.\n5. Show the size of `lvfiles` in MiB and the PE size of the VG.\n\n## Check your work\n\n```bash\npvs; vgs vgfiles; lvs vgfiles\nvgdisplay vgfiles | grep 'PE Size'\nfindmnt /files; findmnt /docs\ndf -h /files /docs\n```\n\n- `vgdisplay` shows PE Size 8.00 MiB.\n- `lvfiles` is 400 MiB (50 × 8 MiB).\n- Both file systems are mounted after a reboot.\n\n## Hints\n\n- fdisk type alias: `lvm`.\n- `vgcreate -s 8M`, `lvcreate -l 50`, `lvcreate -l 100%FREE`.\n- In fstab use `/dev/vgfiles/lvfiles`.\n\n## Solution\n\n1. Partition and PV.\n```bash\nfdisk /dev/vdc\n```\n```text\nCommand (m for help): g\nCommand (m for help): n          (Enter, Enter, +4G)\nCommand (m for help): t          (lvm)\nCommand (m for help): w\n```\n```bash\npvcreate /dev/vdc1\n```\n2. VG with 8 MiB extents.\n```bash\nvgcreate -s 8M vgfiles /dev/vdc1\n```\n3. lvfiles with 50 extents, XFS, fstab.\n```bash\nlvcreate -n lvfiles -l 50 vgfiles\nmkfs.xfs /dev/vgfiles/lvfiles\nmkdir /files\necho \"/dev/vgfiles/lvfiles /files xfs defaults 0 0\" >> /etc/fstab\n```\n4. lvdocs with the rest, ext4, fstab.\n```bash\nlvcreate -n lvdocs -l 100%FREE vgfiles\nmkfs.ext4 /dev/vgfiles/lvdocs\nmkdir /docs\necho \"/dev/vgfiles/lvdocs /docs ext4 defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a\n```\n5. Sizes.\n```bash\nlvs --units m vgfiles\nvgdisplay vgfiles | grep 'PE Size'\n```\n","page":"labs/rhcsa-15-1/","label":"Lab 15.1","where":"Lab 15.1 · medium · 35 min"},{"id":"lab-rhcsa-15-2","n":1597,"slug":"rhcsa-15-2","title":"Lab 15.2: Grow and shrink logical volumes","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Add a disk to a full volume group, grow an XFS and an ext4 volume while they are in use, shrink the ext4 volume, and take one disk out of the VG.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Finish lab 15.1. Add a second empty disk of 5 GiB (here `/dev/vdd`). Work as root.\n\n## Tasks\n\n1. Add the whole disk `/dev/vdd` to the volume group `vgfiles`.\n2. Grow `lvfiles` by 1 GiB while it stays mounted. `df -h /files` must show the new size.\n3. Grow `lvdocs` to a total size of 5 GiB, also while mounted.\n4. Shrink `lvdocs` to 3 GiB without losing its files.\n5. Move all data away from `/dev/vdd` and remove the disk from the VG (if the VG has enough free space; if not, shrink first or skip this task and explain why).\n\n## Check your work\n\n```bash\nvgs vgfiles; pvs\nlvs vgfiles\ndf -h /files /docs\n```\n\n- `/files` is 1 GiB bigger than before.\n- `/docs` is 3 GiB at the end, and your test files are still there.\n- After task 5, `pvs` shows `/dev/vdd` without a VG (or not as a PV).\n\n## Hints\n\n- `pvcreate` + `vgextend`.\n- `lvextend -r -L +1G`, `lvextend -r -L 5G`.\n- `lvreduce -r -L 3G` (ext4 only; it unmounts and checks for you, or asks).\n- `pvmove`, `vgreduce`, `pvremove`.\n\n## Solution\n\n1. Extend the VG.\n```bash\npvcreate /dev/vdd\nvgextend vgfiles /dev/vdd\n```\n2. Grow lvfiles with its XFS.\n```bash\nlvextend -r -L +1G /dev/vgfiles/lvfiles\ndf -h /files\n```\n3. Grow lvdocs to 5 GiB.\n```bash\nlvextend -r -L 5G /dev/vgfiles/lvdocs\n```\n4. Shrink lvdocs (ext4) to 3 GiB.\n```bash\ncp -a /etc/hosts /docs/test.txt\nlvreduce -r -L 3G /dev/vgfiles/lvdocs\ncat /docs/test.txt\n```\n5. Free /dev/vdd.\n```bash\npvmove /dev/vdd\nvgreduce vgfiles /dev/vdd\npvremove /dev/vdd\n```\n","page":"labs/rhcsa-15-2/","label":"Lab 15.2","where":"Lab 15.2 · medium · 30 min"},{"id":"lab-rhcsa-16-1","n":1598,"slug":"rhcsa-16-1","title":"Lab 16.1: Modules, parameters and kernel settings","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Inspect the kernel, load and block modules, set a module parameter for good, and change a kernel setting that survives a reboot.\n\nLevel: **medium** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root.\n\n## Tasks\n\n1. Show the running kernel version, the boot options the kernel got, and all kernel messages of priority warning or worse from this boot.\n2. Show if the module `vfat` is loaded. Load it, show its dependencies, then unload it.\n3. Make sure the module `vfat` is loaded at every boot.\n4. Make sure the module `usb-storage` can never be loaded.\n5. Show the parameters of the module `loop`. Set `max_part=8` for it permanently.\n6. Turn on IP forwarding permanently and check it.\n7. List the installed kernels and show which one boots by default.\n\n## Check your work\n\n```bash\ncat /etc/modules-load.d/vfat.conf\nmodprobe usb-storage; lsmod | grep usb_storage\ncat /etc/modprobe.d/*.conf\nsysctl net.ipv4.ip_forward\ngrubby --default-kernel\n```\n\n- `modprobe usb-storage` fails or loads nothing.\n- After a reboot: `lsmod | grep vfat` shows the module, `cat /sys/module/loop/parameters/max_part` shows 8, `sysctl net.ipv4.ip_forward` = 1.\n\n## Hints\n\n- `uname -r`, `cat /proc/cmdline`, `dmesg --level=warn,err,crit,alert,emerg` or `journalctl -k -p warning`.\n- `/etc/modules-load.d/NAME.conf`, `/etc/modprobe.d/NAME.conf` with `blacklist` + `install … /bin/false` or `options`.\n- `/etc/sysctl.d/` + `sysctl --system`.\n\n## Solution\n\n1. Kernel information.\n```bash\nuname -r\ncat /proc/cmdline\njournalctl -k -b -p warning\n```\n2. vfat by hand.\n```bash\nlsmod | grep vfat\nmodprobe vfat\nlsmod | grep -E 'vfat|fat'\nmodprobe -r vfat\n```\n3. Load at boot.\n```bash\necho vfat > /etc/modules-load.d/vfat.conf\n```\n4. Block usb-storage.\n```bash\nprintf 'blacklist usb-storage\\ninstall usb-storage /bin/false\\n' > /etc/modprobe.d/no-usb-storage.conf\n```\n5. loop parameter.\n```bash\nmodinfo -p loop\necho \"options loop max_part=8\" > /etc/modprobe.d/loop.conf\n```\n6. IP forwarding.\n```bash\necho \"net.ipv4.ip_forward = 1\" > /etc/sysctl.d/90-forward.conf\nsysctl --system\nsysctl net.ipv4.ip_forward\n```\n7. Kernels.\n```bash\nrpm -q kernel\ngrubby --default-kernel\n```\n","page":"labs/rhcsa-16-1/","label":"Lab 16.1","where":"Lab 16.1 · medium · 30 min"},{"id":"lab-rhcsa-17-1","n":1599,"slug":"rhcsa-17-1","title":"Lab 17.1: Work with targets","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Find out what the system boots into, change it, switch targets without a reboot, and see how enable works.\n\nLevel: **easy** · about 20 minutes · systems: server1\n\n## Before you start\n\n- Work on the console of server1 (not over SSH) as root.\n\n## Tasks\n\n1. Show the default target and all active targets.\n2. Make the system boot into text mode (no desktop) by default.\n3. Switch to text mode now, without a reboot. Then go back.\n4. Enable `httpd` and show the link that systemd created. Which target pulls it in?\n5. Reboot and check the default target and that httpd runs.\n\n## Check your work\n\n```bash\nsystemctl get-default\nls -l /etc/systemd/system/default.target\nls /etc/systemd/system/multi-user.target.wants/ | grep httpd\nsystemctl is-active httpd\n```\n\n- `get-default` = `multi-user.target`.\n- default.target is a link to multi-user.target.\n- httpd.service is in `multi-user.target.wants`.\n\n## Hints\n\n- `systemctl set-default`, `systemctl isolate`.\n- `WantedBy=` in the `[Install]` section of the unit.\n\n## Solution\n\n1. Default and active targets.\n```bash\nsystemctl get-default\nsystemctl list-units -t target\n```\n2. Text mode by default.\n```bash\nsystemctl set-default multi-user.target\n```\n3. Switch now and back (on a system with a desktop).\n```bash\nsystemctl isolate multi-user.target\nsystemctl isolate graphical.target\n```\n4. Enable and look at the link.\n```bash\nsystemctl enable --now httpd\nls -l /etc/systemd/system/multi-user.target.wants/httpd.service\ngrep WantedBy /usr/lib/systemd/system/httpd.service\n```\n5. Reboot and check.\n```bash\nsystemctl reboot\nsystemctl get-default; systemctl is-active httpd\n```\n","page":"labs/rhcsa-17-1/","label":"Lab 17.1","where":"Lab 17.1 · easy · 20 min"},{"id":"lab-rhcsa-17-2","n":1600,"slug":"rhcsa-17-2","title":"Lab 17.2: Change GRUB 2 boot options","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Make the boot messages visible, change the menu timeout, and choose the default kernel.\n\nLevel: **medium** · about 25 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root. Take a VM snapshot first: mistakes in GRUB can stop the boot.\n\n## Tasks\n\n1. Show the kernel command line of the running kernel and the BLS entries in /boot.\n2. Remove the options `rhgb` and `quiet` from all kernels, so you see the boot messages.\n3. Set the GRUB menu timeout to 10 seconds.\n4. Show the default kernel. If you have more than one kernel, make the older one the default, check it, and set the newest back.\n5. Reboot. In the GRUB menu, press `e` and look at the `linux` line, then boot with Ctrl+X. Check the new command line.\n\n## Check your work\n\n```bash\ncat /proc/cmdline\ngrubby --info=DEFAULT | grep args\ngrep GRUB_TIMEOUT /etc/default/grub\ngrubby --default-kernel\n```\n\n- `/proc/cmdline` has no `rhgb` and no `quiet`.\n- `GRUB_TIMEOUT=10` and the menu waited 10 seconds.\n\n## Hints\n\n- `grubby --update-kernel=ALL --remove-args=\"…\"`.\n- Timeout: `/etc/default/grub` and `grub2-mkconfig -o /boot/grub2/grub.cfg`.\n- `grubby --set-default`.\n\n## Solution\n\n1. Look at the current state.\n```bash\ncat /proc/cmdline\nls /boot/loader/entries/\n```\n2. Remove rhgb and quiet.\n```bash\ngrubby --update-kernel=ALL --remove-args=\"rhgb quiet\"\n```\n3. Timeout of 10 seconds.\n```bash\nsed -i 's/^GRUB_TIMEOUT=.*/GRUB_TIMEOUT=10/' /etc/default/grub\ngrub2-mkconfig -o /boot/grub2/grub.cfg\n```\n4. Default kernel.\n```bash\ngrubby --default-kernel\ngrubby --info=ALL | grep ^kernel\ngrubby --set-default /boot/vmlinuz-OLDER-VERSION\ngrubby --set-default /boot/vmlinuz-$(uname -r)\n```\n5. Reboot and check.\n```bash\nsystemctl reboot\ncat /proc/cmdline\n```\n","page":"labs/rhcsa-17-2/","label":"Lab 17.2","where":"Lab 17.2 · medium · 25 min"},{"id":"lab-rhcsa-18-1","n":1601,"slug":"rhcsa-18-1","title":"Lab 18.1: Break and repair the boot","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","hard"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Practice the three repairs that matter most: a lost root password, a broken fstab, and a damaged initramfs.\n\nLevel: **hard** · about 45 minutes · systems: server1\n\n## Before you start\n\n- Take a VM snapshot of server1 first. You need console access (not SSH).\n- You need the file system `/data` from lab 14.1 (or any extra fstab line you can break).\n\n## Tasks\n\n1. Reset the root password to `redhat` without knowing the old one.\n2. Add a wrong line to `/etc/fstab` (for example a mount of a UUID that does not exist, without nofail) and reboot. Repair the system from emergency mode so it boots normally.\n3. Boot into rescue mode once with a kernel argument, check which units run, and continue to the normal target.\n4. Make a backup of the initramfs of the running kernel, delete the original, and rebuild it with dracut. Check that the system still boots.\n\n## Check your work\n\n```bash\nsu - root\nfindmnt --verify\nlsinitrd | head -5\nls -l /boot/initramfs-$(uname -r).img\n```\n\n- You can log in as root with `redhat` (and SELinux is enforcing: `getenforce`).\n- The system boots to the default target without errors; `findmnt --verify` is clean.\n- The initramfs exists again and the system boots.\n\n## Hints\n\n- `rd.break` → `mount -o remount,rw /sysroot` → `chroot /sysroot` → `passwd` → `touch /.autorelabel`.\n- In emergency mode: `mount -o remount,rw /`, edit fstab, `systemctl daemon-reload`, `mount -a`.\n- `systemd.unit=rescue.target`; leave with `systemctl default`.\n- `dracut -f`; `lsinitrd`.\n\n## Solution\n\n1. Root password (at the GRUB menu: `e`, add `rd.break` to the linux line, `Ctrl+X`).\n```bash\nmount -o remount,rw /sysroot\nchroot /sysroot\necho redhat | passwd --stdin root\ntouch /.autorelabel\nexit\nexit\n```\n2. Break and fix fstab.\n```bash\necho \"UUID=00000000-1111-2222-3333-444444444444 /broken xfs defaults 0 0\" >> /etc/fstab\nsystemctl reboot\n```\nIn emergency mode (log in as root):\n```bash\nmount -o remount,rw /\nsed -i '/\\/broken/d' /etc/fstab\nsystemctl daemon-reload\nmount -a\nsystemctl reboot\n```\n3. Rescue mode (at GRUB add `systemd.unit=rescue.target`).\n```bash\nsystemctl list-units -t service --state=running\nsystemctl default\n```\n4. Rebuild the initramfs.\n```bash\ncp /boot/initramfs-$(uname -r).img /root/initramfs.bak\nrm /boot/initramfs-$(uname -r).img\ndracut -f\nlsinitrd | head -5\nsystemctl reboot\n```\n","page":"labs/rhcsa-18-1/","label":"Lab 18.1","where":"Lab 18.1 · hard · 45 min"},{"id":"lab-rhcsa-19-1","n":1602,"slug":"rhcsa-19-1","title":"Lab 19.1: Write admin scripts","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Write three small scripts that use arguments, tests, loops and case — the kind of task the exam and daily work ask for.\n\nLevel: **medium** · about 40 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root. Put the scripts in `/usr/local/bin/`.\n\n## Tasks\n\n1. Script `mkusers.sh`: reads a file given as the first argument, with one user name per line, and creates each user if it does not exist yet. Without an argument it prints a usage line and exits with code 1.\n2. Script `diskcheck.sh`: prints \"WARNING: / is NN% full\" when the root file system is more than 80 % full, else \"OK\". It takes the limit as an optional argument (default 80).\n3. Script `web.sh`: with the argument `start`, `stop` or `status` it does that with httpd; with anything else it prints a usage line and exits with 1.\n4. Check the syntax of all three scripts and run one of them with tracing.\n\n## Check your work\n\n```bash\nprintf 'tom\\njerry\\ntom\\n' > /tmp/names.txt\nmkusers.sh /tmp/names.txt; id tom; id jerry\nmkusers.sh; echo $?\ndiskcheck.sh; diskcheck.sh 1\nweb.sh status; web.sh foo; echo $?\n```\n\n- tom and jerry exist; the second \"tom\" gives no error.\n- Without arguments the scripts print a usage line and `$?` is 1.\n- `diskcheck.sh 1` prints a WARNING.\n\n## Hints\n\n- `[ $# -lt 1 ]`, `while read -r NAME`, `id \"$NAME\" &>/dev/null`.\n- `df --output=pcent / | tail -1 | tr -dc '0-9'` gives the number.\n- `case \"$1\" in start|stop|status) … ;; *) … ;; esac`.\n- `bash -n FILE`, `bash -x FILE`.\n\n## Solution\n\n1. mkusers.sh\n**/usr/local/bin/mkusers.sh**\n```text\n#!/bin/bash\nif [ $# -lt 1 ] || [ ! -f \"$1\" ]; then\n    echo \"usage: $0 FILE\" >&2\n    exit 1\nfi\nwhile read -r NAME; do\n    [ -z \"$NAME\" ] && continue\n    if id \"$NAME\" &>/dev/null; then\n        echo \"$NAME exists\"\n    else\n        useradd \"$NAME\" && echo \"$NAME created\"\n    fi\ndone < \"$1\"\n```\n2. diskcheck.sh\n**/usr/local/bin/diskcheck.sh**\n```text\n#!/bin/bash\nLIMIT=${1:-80}\nUSED=$(df --output=pcent / | tail -1 | tr -dc '0-9')\nif [ \"$USED\" -gt \"$LIMIT\" ]; then\n    echo \"WARNING: / is ${USED}% full\"\nelse\n    echo \"OK\"\nfi\n```\n3. web.sh\n**/usr/local/bin/web.sh**\n```text\n#!/bin/bash\ncase \"$1\" in\n    start|stop|status)\n        systemctl \"$1\" httpd ;;\n    *)\n        echo \"usage: $0 {start|stop|status}\" >&2\n        exit 1 ;;\nesac\n```\n4. Make them executable, check and trace.\n```bash\nchmod +x /usr/local/bin/{mkusers,diskcheck,web}.sh\nfor f in /usr/local/bin/{mkusers,diskcheck,web}.sh; do bash -n \"$f\" && echo \"$f OK\"; done\nbash -x /usr/local/bin/diskcheck.sh 50\n```\n","page":"labs/rhcsa-19-1/","label":"Lab 19.1","where":"Lab 19.1 · medium · 40 min"},{"id":"lab-rhcsa-20-1","n":1603,"slug":"rhcsa-20-1","title":"Lab 20.1: Harden the SSH server","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Lock down sshd on server2: no root login, another port, only one group, idle timeout and keys — without locking yourself out.\n\nLevel: **medium** · about 35 minutes · systems: server1, server2\n\n## Before you start\n\n- Keep one root session open on server2 (console or SSH) during the whole lab.\n- You need the user `student` on both servers.\n\n## Tasks\n\n1. On server2, put all your sshd settings in `/etc/ssh/sshd_config.d/10-lab.conf`.\n2. root may not log in over SSH.\n3. sshd listens on port 2022 only. Make SELinux and the firewall accept it.\n4. Only members of the group `sshusers` may log in. Add `student` to it.\n5. Idle sessions are closed after about 10 minutes without an answer from the client.\n6. On server1, create a key with a passphrase for student, copy it to server2, and log in with the help of ssh-agent.\n7. Turn off password logins on server2 and test that the key login still works.\n\n## Check your work\n\n```bash\nsshd -T | grep -E '^(port|permitrootlogin|allowgroups|clientalive|passwordauthentication)'\nsemanage port -l | grep ssh_port_t\nfirewall-cmd --list-ports\nssh -p 2022 student@server2 hostname\nssh -p 2022 root@server2\n```\n\n- sshd -T shows port 2022, permitrootlogin no, allowgroups sshusers, passwordauthentication no.\n- student logs in with the key; root is refused.\n\n## Hints\n\n- First value wins: drop-in files beat the main file.\n- `semanage port -a -t ssh_port_t -p tcp 2022`, `firewall-cmd --add-port=2022/tcp --permanent`.\n- `ClientAliveInterval 300` + `ClientAliveCountMax 2`.\n- `ssh-copy-id -p 2022`, `eval \"$(ssh-agent)\"`, `ssh-add`.\n\n## Solution\n\n1. The drop-in file (tasks 1–5) on server2.\n**/etc/ssh/sshd_config.d/10-lab.conf**\n```text\nPort 2022\nPermitRootLogin no\nAllowGroups sshusers\nClientAliveInterval 300\nClientAliveCountMax 2\n```\n2. Group, SELinux, firewall, restart.\n```bash\ngroupadd sshusers; usermod -aG sshusers student\nsemanage port -a -t ssh_port_t -p tcp 2022\nfirewall-cmd --add-port=2022/tcp --permanent; firewall-cmd --reload\nsshd -t && systemctl restart sshd\n```\n3. Key with passphrase on server1 (as student).\n```bash\nssh-keygen -t ed25519\nssh-copy-id -p 2022 student@server2\neval \"$(ssh-agent)\"; ssh-add\nssh -p 2022 student@server2 hostname\n```\n4. No passwords anymore (on server2).\n```bash\necho \"PasswordAuthentication no\" >> /etc/ssh/sshd_config.d/10-lab.conf\nsshd -t && systemctl reload sshd\n```\n","page":"labs/rhcsa-20-1/","label":"Lab 20.1","where":"Lab 20.1 · medium · 35 min"},{"id":"lab-rhcsa-21-1","n":1604,"slug":"rhcsa-21-1","title":"Lab 21.1: Host two websites","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set up Apache on server1 with two name-based websites, one more site on port 82, and test everything from server2.\n\nLevel: **medium** · about 40 minutes · systems: server1, server2\n\n## Before you start\n\n- Work on server1 as root. A repository must be available.\n- On server2, add `192.168.122.10 www1.example.com www2.example.com` to `/etc/hosts`.\n\n## Tasks\n\n1. Install Apache, make it start at boot, and open HTTP in the firewall.\n2. Create the site `www1.example.com` with the content \"www1\" in `/srv/www1`, and `www2.example.com` with \"www2\" in `/srv/www2`.\n3. Make sure SELinux allows Apache to read both folders (permanently).\n4. Serve the folder `/srv/www2` also on port 82 for any name.\n5. From server2, test both names on port 80 and port 82.\n\n## Check your work\n\n```bash\napachectl configtest\nls -Zd /srv/www1 /srv/www2\nsemanage port -l | grep http_port_t\ncurl http://www1.example.com\ncurl http://www2.example.com\ncurl http://192.168.122.10:82\n```\n\n- The answers are \"www1\", \"www2\" and \"www2\".\n- The folders have the type `httpd_sys_content_t`; port 82 is in `http_port_t`.\n\n## Hints\n\n- `<VirtualHost *:80>` with `ServerName` and `DocumentRoot`; a `<Directory \"/srv\">` block with `Require all granted`.\n- `semanage fcontext -a -t httpd_sys_content_t \"/srv/www12?\"` + `restorecon -Rv /srv`.\n- `Listen 82` + `<VirtualHost *:82>`; SELinux port type `http_port_t`; firewall port.\n\n## Solution\n\n1. Install and open.\n```bash\ndnf install -y httpd\nsystemctl enable --now httpd\nfirewall-cmd --add-service=http --permanent; firewall-cmd --reload\n```\n2. Content.\n```bash\nmkdir -p /srv/www1 /srv/www2\necho www1 > /srv/www1/index.html; echo www2 > /srv/www2/index.html\n```\n3. SELinux labels.\n```bash\nsemanage fcontext -a -t httpd_sys_content_t \"/srv/www[12](/.*)?\"\nrestorecon -Rv /srv\n```\n4. The config (tasks 2 and 4) and the port.\n**/etc/httpd/conf.d/sites.conf**\n```text\n<Directory \"/srv\">\n    Require all granted\n</Directory>\n\n<VirtualHost *:80>\n    ServerName www1.example.com\n    DocumentRoot /srv/www1\n</VirtualHost>\n\n<VirtualHost *:80>\n    ServerName www2.example.com\n    DocumentRoot /srv/www2\n</VirtualHost>\n\nListen 82\n<VirtualHost *:82>\n    DocumentRoot /srv/www2\n</VirtualHost>\n```\n```bash\nsemanage port -a -t http_port_t -p tcp 82\nfirewall-cmd --add-port=82/tcp --permanent; firewall-cmd --reload\napachectl configtest && systemctl restart httpd\n```\n5. Test from server2.\n```bash\ncurl http://www1.example.com; curl http://www2.example.com; curl http://192.168.122.10:82\n```\n","page":"labs/rhcsa-21-1/","label":"Lab 21.1","where":"Lab 21.1 · medium · 40 min"},{"id":"lab-rhcsa-22-1","n":1605,"slug":"rhcsa-22-1","title":"Lab 22.1: Make a web server work with SELinux","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Serve web pages from a non-standard folder and port, and from user home folders — with SELinux enforcing the whole time.\n\nLevel: **medium** · about 40 minutes · systems: server1\n\n## Before you start\n\n- Work on server1 as root with httpd installed (lab 21.1 or `dnf install -y httpd`).\n- SELinux must be enforcing: `getenforce`.\n\n## Tasks\n\n1. Change the DocumentRoot of Apache to `/web` with an `index.html` that says \"selinux lab\". Make it work with the right SELinux label (permanent).\n2. Make Apache also listen on port 1001. Fix SELinux and the firewall.\n3. Create `/home/student/public_html/index.html` with the text \"home page\". Enable `userdir` in Apache (`UserDir public_html` in `/etc/httpd/conf.d/userdir.conf`) and set the SELinux boolean that allows home folders (permanent). Set the permissions so `apache` can read the file.\n4. Move a file from `/root` into `/web` with `mv`. Show that the web page for it fails, then fix the label.\n5. Reboot and check that everything still works in enforcing mode.\n\n## Check your work\n\n```bash\ncurl http://localhost/\ncurl http://localhost:1001/\ncurl http://localhost/~student/\ngetenforce; getsebool httpd_enable_homedirs\nls -Zd /web /web/*\n```\n\n- All three curl commands show the page text.\n- `getenforce` = Enforcing, the boolean is on, `/web` has `httpd_sys_content_t`.\n\n## Hints\n\n- `semanage fcontext -a -t httpd_sys_content_t \"/web(/.*)?\"` + `restorecon -Rv /web`.\n- `semanage port -a -t http_port_t -p tcp 1001`.\n- `setsebool -P httpd_enable_homedirs on`; home folder needs `chmod 711`, public_html `755`.\n- `restorecon` after `mv`.\n\n## Solution\n\n1. New DocumentRoot.\n```bash\nmkdir /web; echo \"selinux lab\" > /web/index.html\nsed -i 's|^DocumentRoot.*|DocumentRoot \"/web\"|; s|<Directory \"/var/www/html\">|<Directory \"/web\">|' /etc/httpd/conf/httpd.conf\nsemanage fcontext -a -t httpd_sys_content_t \"/web(/.*)?\"\nrestorecon -Rv /web\nsystemctl restart httpd\n```\n2. Port 1001.\n```bash\necho \"Listen 1001\" > /etc/httpd/conf.d/port1001.conf\nsemanage port -a -t http_port_t -p tcp 1001\nfirewall-cmd --add-port=1001/tcp --permanent; firewall-cmd --reload\nsystemctl restart httpd\n```\n3. User home pages.\n```bash\nsed -i 's/^\\s*UserDir disabled/#UserDir disabled/; s/^\\s*#UserDir public_html/UserDir public_html/' /etc/httpd/conf.d/userdir.conf\nmkdir /home/student/public_html; echo \"home page\" > /home/student/public_html/index.html\nchown -R student: /home/student/public_html\nchmod 711 /home/student; chmod 755 /home/student/public_html\nsetsebool -P httpd_enable_homedirs on\nsystemctl restart httpd\n```\n4. The mv problem.\n```bash\necho \"moved\" > /root/moved.html; mv /root/moved.html /web/\ncurl -s -o /dev/null -w '%{http_code}\\n' http://localhost/moved.html\n```\n```text\n403\n```\n```bash\nrestorecon -v /web/moved.html\n```\n5. Reboot and test again.\n```bash\nsystemctl reboot\n```\n","page":"labs/rhcsa-22-1/","label":"Lab 22.1","where":"Lab 22.1 · medium · 40 min"},{"id":"lab-rhcsa-22-2","n":1606,"slug":"rhcsa-22-2","title":"Lab 22.2: Find and fix SELinux denials","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","hard"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Break a service on purpose, find the SELinux denial in the logs, understand it, and fix it the right way.\n\nLevel: **hard** · about 30 minutes · systems: server1\n\n## Before you start\n\n- Finish lab 22.1. Work as root.\n\n## Tasks\n\n1. Install `setroubleshoot-server` and `selinux-policy-doc`.\n2. Run `chcon -t user_home_t /web/index.html` and request the page. Find the AVC message, name the source type, target type and class, and fix the problem.\n3. Change the sshd port to 2222 in a drop-in file **without** a SELinux change. Restart sshd, see it fail, find the denial, and fix it. Restore port 22 afterwards (or keep both ports).\n4. Set only `httpd_t` to permissive, check that SELinux is still enforcing for the rest, then undo it.\n5. Use the man page of httpd to find the boolean that allows Apache to connect to a database over the network.\n\n## Check your work\n\n```bash\nausearch -m AVC -ts today | grep -c denied\nls -Z /web/index.html\nsemanage port -l | grep ssh_port_t\nsemanage permissive -l\n```\n\n- The web page works again; `index.html` has `httpd_sys_content_t`.\n- sshd runs; `ssh_port_t` has 2222.\n- `semanage permissive -l` shows no httpd_t at the end.\n- Boolean: `httpd_can_network_connect_db`.\n\n## Hints\n\n- `ausearch -m AVC -ts recent`, `sealert -a /var/log/audit/audit.log`, `journalctl -t setroubleshoot`.\n- The fix for a label is restorecon, for a port semanage port.\n- `semanage permissive -a httpd_t` / `-d`.\n- `man httpd_selinux`, search for \"database\".\n\n## Solution\n\n1. Tools.\n```bash\ndnf install -y setroubleshoot-server selinux-policy-doc\n```\n2. Label problem.\n```bash\nchcon -t user_home_t /web/index.html\ncurl -s -o /dev/null -w '%{http_code}\\n' http://localhost/\n```\n```text\n403\n```\n```bash\nausearch -m AVC -ts recent | grep index.html\nrestorecon -v /web/index.html\n```\n3. Port problem.\n```bash\necho \"Port 2222\" > /etc/ssh/sshd_config.d/05-port.conf\nsystemctl restart sshd\nausearch -m AVC -ts recent -c sshd\nsemanage port -a -t ssh_port_t -p tcp 2222\nsystemctl restart sshd\nrm /etc/ssh/sshd_config.d/05-port.conf; systemctl restart sshd\n```\n4. Permissive domain.\n```bash\nsemanage permissive -a httpd_t\ngetenforce\nsemanage permissive -d httpd_t\n```\n5. Find the boolean.\n```bash\nman httpd_selinux | grep -B2 -A2 -i database\ngetsebool httpd_can_network_connect_db\n```\n","page":"labs/rhcsa-22-2/","label":"Lab 22.2","where":"Lab 22.2 · hard · 30 min"},{"id":"lab-rhcsa-23-1","n":1607,"slug":"rhcsa-23-1","title":"Lab 23.1: Configure firewalld zones and services","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","medium"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Open web services on server1, limit a database port to one network, block one host, and test from server2.\n\nLevel: **medium** · about 30 minutes · systems: server1, server2\n\n## Before you start\n\n- server1 runs httpd (lab 21.1). Work as root on server1.\n\n## Tasks\n\n1. Show the default zone, the active zones and everything open in the default zone.\n2. Open the services http and https permanently.\n3. Open port 5432/tcp only for the network 192.168.122.0/24 (rich rule).\n4. Drop all traffic from the address 192.168.122.99.\n5. Remove the service `cockpit` from the default zone.\n6. Test from server2 that HTTP works and that a closed port is refused.\n\n## Check your work\n\n```bash\nfirewall-cmd --list-all\nfirewall-cmd --list-rich-rules\ncurl -I http://server1\nnc -zv server1 3306\n```\n\n- services: dhcpv6-client http https ssh (no cockpit).\n- Two rich rules: the 5432 accept and the drop for .99.\n- curl works from server2; port 3306 is refused or times out.\n\n## Hints\n\n- `--get-default-zone`, `--get-active-zones`, `--list-all`.\n- Always `--permanent` and then `--reload`.\n- `rule family=\"ipv4\" source address=\"…\" port port=\"5432\" protocol=\"tcp\" accept`.\n\n## Solution\n\n1. Look around.\n```bash\nfirewall-cmd --get-default-zone; firewall-cmd --get-active-zones; firewall-cmd --list-all\n```\n2. Web services.\n```bash\nfirewall-cmd --add-service={http,https} --permanent\n```\n3. Database port for one network.\n```bash\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"192.168.122.0/24\" port port=\"5432\" protocol=\"tcp\" accept'\n```\n4. Block one host.\n```bash\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"192.168.122.99\" drop'\n```\n5. Remove cockpit and reload.\n```bash\nfirewall-cmd --remove-service=cockpit --permanent\nfirewall-cmd --reload\n```\n6. Test from server2.\n```bash\ncurl -I http://server1\nnc -zv server1 3306\n```\n","page":"labs/rhcsa-23-1/","label":"Lab 23.1","where":"Lab 23.1 · medium · 30 min"},{"id":"lab-rhcsa-24-1","n":1608,"slug":"rhcsa-24-1","title":"Lab 24.1: NFS server, fstab and autofs","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","hard"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Make server2 an NFS server, mount one share for good on server1, and mount home folders on demand with autofs.\n\nLevel: **hard** · about 45 minutes · systems: server1, server2\n\n## Before you start\n\n- server2 is the NFS server, server1 the client. Work as root.\n- Create the user `nfsuser` with UID 3001 on **both** servers (same UID).\n\n## Tasks\n\n1. On server2, export `/exports/data` (read-write) and `/exports/home` (read-write) to 192.168.122.0/24. Create `/exports/home/nfsuser` owned by nfsuser.\n2. Open the firewall on server2 for NFS and check the export list from server1.\n3. On server1, mount `server2:/exports/data` on `/mnt/data` at every boot.\n4. On server1, configure autofs so `/shares/data` is mounted on demand from `server2:/exports/data`.\n5. On server1, configure autofs so every user folder under `/exports/home` on server2 appears as `/rhome/USER`. Set the home folder of nfsuser on server1 to `/rhome/nfsuser`.\n6. Log in as nfsuser on server1 and create a file in the home folder. Check on server2 that the file is there with the right owner.\n\n## Check your work\n\n```bash\nshowmount -e server2\nfindmnt /mnt/data\nls /shares/data; findmnt /shares/data\nsu - nfsuser -c 'pwd; touch hello.txt'\nssh server2 ls -ln /exports/home/nfsuser\n```\n\n- `/mnt/data` is mounted after a reboot.\n- `/shares/data` and `/rhome/nfsuser` are mounted when used.\n- The file on server2 is owned by UID 3001.\n\n## Hints\n\n- `/etc/exports`, `exportfs -ra`, `firewall-cmd --add-service={nfs,mountd,rpc-bind}`.\n- fstab with `_netdev`.\n- `/etc/auto.master.d/NAME.autofs` + map file; wildcard `* -rw server2:/exports/home/&`.\n- `usermod -d /rhome/nfsuser nfsuser` (no -m).\n\n## Solution\n\n1. Exports on server2.\n```bash\ndnf install -y nfs-utils\nuseradd -u 3001 nfsuser\nmkdir -p /exports/data /exports/home/nfsuser\nchown nfsuser: /exports/home/nfsuser\necho \"/exports/data 192.168.122.0/24(rw,sync)\" >> /etc/exports\necho \"/exports/home 192.168.122.0/24(rw,sync)\" >> /etc/exports\nsystemctl enable --now nfs-server; exportfs -ra\n```\n2. Firewall on server2 and check from server1.\n```bash\nfirewall-cmd --add-service={nfs,mountd,rpc-bind} --permanent; firewall-cmd --reload\nshowmount -e server2\n```\n3. fstab on server1.\n```bash\ndnf install -y nfs-utils autofs\nmkdir /mnt/data\necho \"server2:/exports/data /mnt/data nfs _netdev,defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a\n```\n4. Indirect map.\n```bash\necho \"/shares /etc/auto.shares\" > /etc/auto.master.d/shares.autofs\necho \"data -rw server2:/exports/data\" > /etc/auto.shares\n```\n5. Wildcard map and the user.\n```bash\necho \"/rhome /etc/auto.rhome\" > /etc/auto.master.d/rhome.autofs\necho \"* -rw server2:/exports/home/&\" > /etc/auto.rhome\nsystemctl enable --now autofs; systemctl restart autofs\nuseradd -u 3001 -d /rhome/nfsuser -M nfsuser\n```\n6. Test.\n```bash\nsu - nfsuser -c 'pwd; touch hello.txt'\nssh server2 ls -ln /exports/home/nfsuser\n```\n","page":"labs/rhcsa-24-1/","label":"Lab 24.1","where":"Lab 24.1 · hard · 45 min"},{"id":"lab-rhcsa-25-1","n":1609,"slug":"rhcsa-25-1","title":"Lab 25.1: Time zone and NTP client","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","easy"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"Set the time zone, make server1 a time server for the lab network, and make server2 sync from server1.\n\nLevel: **easy** · about 25 minutes · systems: server1, server2\n\n## Before you start\n\n- Work as root on both servers.\n\n## Tasks\n\n1. On both servers, show all time settings. Set the time zone to `Asia/Riyadh`.\n2. On server1, allow NTP clients from 192.168.122.0/24 and open the firewall for NTP.\n3. On server2, remove the default pool and use only server1 as time source.\n4. On server2, check that server1 is the selected source and that the clock is synchronized.\n5. On server2, write the system time to the hardware clock and compare both clocks.\n\n## Check your work\n\n```bash\ntimedatectl | grep -E 'Time zone|synchronized|NTP service'\nchronyc sources\nchronyc tracking | grep -E 'Reference|Leap'\nhwclock; date\n```\n\n- Time zone: Asia/Riyadh (+03).\n- On server2, `chronyc sources` shows `^* server1`.\n- `System clock synchronized: yes`.\n\n## Hints\n\n- `timedatectl set-timezone`.\n- server1: `allow 192.168.122.0/24` in /etc/chrony.conf, firewall service `ntp`.\n- server2: comment the `pool` line, add `server server1 iburst`.\n- `hwclock --systohc`.\n\n## Solution\n\n1. Time zone (both servers).\n```bash\ntimedatectl\ntimedatectl set-timezone Asia/Riyadh\n```\n2. server1 as NTP server.\n```bash\necho \"allow 192.168.122.0/24\" >> /etc/chrony.conf\nsystemctl restart chronyd\nfirewall-cmd --add-service=ntp --permanent; firewall-cmd --reload\n```\n3. server2 as client.\n```bash\nsed -i 's/^pool /# pool /' /etc/chrony.conf\necho \"server server1 iburst\" >> /etc/chrony.conf\nsystemctl restart chronyd\n```\n4. Check on server2.\n```bash\nchronyc sources -v\nchronyc tracking\n```\n5. Hardware clock.\n```bash\nhwclock --systohc\nhwclock; date\n```\n","page":"labs/rhcsa-25-1/","label":"Lab 25.1","where":"Lab 25.1 · easy · 25 min"},{"id":"lab-rhcsa-27-1","n":1610,"slug":"rhcsa-27-1","title":"Lab 27.1: Practice exam A","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","hard"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"A full practice exam in the style of the real one. Work without notes (man pages are allowed), stop after 2.5 hours, reboot both servers, then check.\n\nLevel: **hard** · about 150 minutes · systems: server1, server2\n\n## Before you start\n\n- Use fresh VMs: server1 (with an extra 10 GiB disk) and server2. A repository URL for BaseOS and AppStream must be available in your lab (for example the ISO shared over HTTP from server2 or your host).\n- Replace example names (repo URL, disk names) with the values of your lab.\n\n## Tasks\n\n1. **server2**: The root password is unknown. Reset it to `examPass1`.\n2. **server1**: Set the static address 192.168.122.10/24, gateway 192.168.122.1, DNS 192.168.122.1, and the host name `server1.lab.example.com`. All settings must survive a reboot.\n3. **server1**: Configure the repositories `http://repo.lab.example.com/BaseOS` and `http://repo.lab.example.com/AppStream` (no GPG check).\n4. **server1**: Create the group `devops`, the users `maria` and `tariq` (members of devops) and `guest` (no login shell). Password for all: `password`. Members of devops may run every command with sudo without a password.\n5. **server1**: Create `/projects/devops`, owned by group devops. Members can read and write, others have no access, new files get the group devops, and members can only delete their own files.\n6. **server1**: `tariq`'s password must expire every 45 days, and the account `guest` expires on 2026-12-31.\n7. **server1**: On the extra disk create a volume group `vgexam` with extents of 16 MiB and a logical volume `lvexam` with 30 extents, formatted as XFS, mounted on `/exam` at boot.\n8. **server1**: Add a swap partition of 512 MiB on the extra disk, active at every boot, next to the existing swap.\n9. **server1**: Install Apache. It must serve `/web/index.html` (content \"exam A\") as its document root, on port 80, with SELinux enforcing and the firewall open. It starts at boot.\n10. **server1**: User `maria` runs `logger \"exam check\"` every day at 08:15.\n11. **server1**: Find all files owned by `tariq` in `/home` and copy them (with their folders) to `/root/tariq-files`.\n12. **server1**: Find the lines with the word `nologin` in `/etc/passwd` and write them to `/root/nologin.txt`.\n13. **server1**: Sync the time from server2 (make server2 an NTP server for 192.168.122.0/24).\n14. **server1**: Set the recommended tuned profile.\n15. **both**: Reboot and check that everything works.\n\n## Check your work\n\n```bash\nhostnamectl; ip -br a; nmcli con show --active\ndnf repolist\nid maria; id tariq; getent passwd guest; sudo -l -U maria\nls -ld /projects/devops\nchage -l tariq | grep Maximum; chage -l guest | grep 'Account expires'\nvgdisplay vgexam | grep 'PE Size'; lvs vgexam; findmnt /exam\nswapon --show\ncurl http://server1.lab.example.com\ngetenforce; firewall-cmd --list-services\ncrontab -l -u maria\nls -R /root/tariq-files; cat /root/nologin.txt\nchronyc sources\ntuned-adm active\n```\n\nGive yourself 1 point per task that fully works after the reboot. 11 of 14 or more: you are on track.\n\n## Hints\n\n- Task 1: `rd.break`. Task 2: `nmcli con mod … ipv4.method manual`, `hostnamectl`.\n- Task 4: `%devops ALL=(ALL) NOPASSWD: ALL` in `/etc/sudoers.d/devops`.\n- Task 5: `chmod 3770`. Task 7: `vgcreate -s 16M`, `lvcreate -l 30`.\n- Task 9: change DocumentRoot and the Directory block; `semanage fcontext` + `restorecon`.\n- Task 11: `find /home -user tariq -exec cp --parents -a {} /root/tariq-files \\;`.\n\n## Solution\n\n1. Root password on server2 (GRUB: `e`, add `rd.break`, Ctrl+X).\n```bash\nmount -o remount,rw /sysroot; chroot /sysroot\necho examPass1 | passwd --stdin root; touch /.autorelabel\nexit; exit\n```\n2. Network and host name on server1.\n```bash\nnmcli con mod enp1s0 ipv4.method manual ipv4.addresses 192.168.122.10/24 ipv4.gateway 192.168.122.1 ipv4.dns 192.168.122.1\nnmcli con up enp1s0\nhostnamectl set-hostname server1.lab.example.com\n```\n3. Repositories.\n**/etc/yum.repos.d/exam.repo**\n```text\n[BaseOS]\nname=BaseOS\nbaseurl=http://repo.lab.example.com/BaseOS\ngpgcheck=0\n\n[AppStream]\nname=AppStream\nbaseurl=http://repo.lab.example.com/AppStream\ngpgcheck=0\n```\n4. Users, group and sudo.\n```bash\ngroupadd devops\nuseradd -G devops maria; useradd -G devops tariq; useradd -s /sbin/nologin guest\nfor u in maria tariq guest; do echo password | passwd --stdin $u; done\necho '%devops ALL=(ALL) NOPASSWD: ALL' > /etc/sudoers.d/devops; visudo -c\n```\n5. Shared folder.\n```bash\nmkdir -p /projects/devops; chgrp devops /projects/devops; chmod 3770 /projects/devops\n```\n6. Password and account aging.\n```bash\nchage -M 45 tariq; chage -E 2026-12-31 guest\n```\n7. LVM (disk /dev/vdb: first partition 1 GiB for LVM, second 512 MiB swap).\n```bash\nparted -s /dev/vdb mklabel gpt mkpart lvm 1MiB 1GiB set 1 lvm on mkpart swap linux-swap 1GiB 1537MiB\npvcreate /dev/vdb1; vgcreate -s 16M vgexam /dev/vdb1; lvcreate -n lvexam -l 30 vgexam\nmkfs.xfs /dev/vgexam/lvexam; mkdir /exam\necho \"/dev/vgexam/lvexam /exam xfs defaults 0 0\" >> /etc/fstab\n```\n8. Swap.\n```bash\nmkswap /dev/vdb2\necho \"UUID=$(blkid -s UUID -o value /dev/vdb2) none swap defaults 0 0\" >> /etc/fstab\nsystemctl daemon-reload; mount -a; swapon -a\n```\n9. Apache with /web.\n```bash\ndnf install -y httpd; mkdir /web; echo \"exam A\" > /web/index.html\nsed -i 's|^DocumentRoot.*|DocumentRoot \"/web\"|; s|<Directory \"/var/www/html\">|<Directory \"/web\">|' /etc/httpd/conf/httpd.conf\nsemanage fcontext -a -t httpd_sys_content_t \"/web(/.*)?\"; restorecon -Rv /web\nsystemctl enable --now httpd; firewall-cmd --add-service=http --permanent; firewall-cmd --reload\n```\n10. Cron for maria.\n```bash\necho '15 8 * * * logger \"exam check\"' | crontab -u maria -\n```\n11. and 12. Find and grep.\n```bash\nmkdir /root/tariq-files; find /home -user tariq -type f -exec cp --parents -a {} /root/tariq-files \\;\ngrep nologin /etc/passwd > /root/nologin.txt\n```\n13. Time: on server2, then on server1.\n```bash\necho \"allow 192.168.122.0/24\" >> /etc/chrony.conf; systemctl restart chronyd; firewall-cmd --add-service=ntp --permanent; firewall-cmd --reload\nsed -i 's/^pool/# pool/' /etc/chrony.conf; echo \"server server2 iburst\" >> /etc/chrony.conf; systemctl restart chronyd\n```\n14. tuned.\n```bash\ntuned-adm profile $(tuned-adm recommend)\n```\n15. Reboot both and run the checks.\n```bash\nsystemctl reboot\n```\n","page":"labs/rhcsa-27-1/","label":"Lab 27.1","where":"Lab 27.1 · hard · 150 min"},{"id":"lab-rhcsa-27-2","n":1611,"slug":"rhcsa-27-2","title":"Lab 27.2: Practice exam B","sections":["labs"],"topic":"Labs · RHCSA 10","tags":["rhcsa","lab","hard"],"series_name":"RHCSA 10 labs","design":"lab","script":"","date":"","files":{},"md":"The second practice exam, with other topics in front: NFS and autofs, resizing, SSH, boot settings and a script. Same rules: 2.5 hours, reboot, then check.\n\nLevel: **hard** · about 150 minutes · systems: server1, server2\n\n## Before you start\n\n- Fresh VMs or the state after exam A. server1 needs a free 5 GiB disk (here `/dev/vdc`).\n- server2 is the NFS server in this exam.\n\n## Tasks\n\n1. **server2**: Export `/exports/users` (rw) to 192.168.122.0/24. Create the users `nfs1` and `nfs2` with UIDs 4001 and 4002 and home folders `/exports/users/nfs1` and `/exports/users/nfs2`.\n2. **server1**: Create the same users with the same UIDs, without local home folders. Their homes `/netusers/nfs1` and `/netusers/nfs2` must be mounted automatically from server2 when used.\n3. **server1**: Create a VG `vgdata` on `/dev/vdc` with an LV `lvdata` of 1 GiB, ext4, mounted on `/data`. Then grow it to 2 GiB while mounted, and shrink it to 1.5 GiB without losing data.\n4. **server1**: sshd must also listen on port 2222 (next to 22), with SELinux and firewall changes. root may not log in with a password.\n5. **server1**: Allow SSH (both ports) only from 192.168.122.0/24 with firewall rules; remove the general ssh service.\n6. **server1**: Remove `rhgb quiet` from the kernel line of all kernels and set the GRUB timeout to 8 seconds.\n7. **server1**: Turn on IP forwarding permanently.\n8. **server1**: The journal must be persistent, max 200 MB.\n9. **server1**: The service `chronyd` must always be restarted 10 seconds after it stops (drop-in, not the package file).\n10. **server1**: Apache must be able to connect to network databases (SELinux, permanent).\n11. **server1**: Write `/usr/local/bin/mkfiles.sh`: it takes a number N and a folder, creates the folder if needed and creates the files `file1` … `fileN` in it. Without two arguments it prints a usage line and exits with code 2.\n12. **server1**: Set the default target to multi-user and disable the cockpit socket.\n13. **both**: Reboot and check.\n\n## Check your work\n\n```bash\nsu - nfs1 -c 'pwd; touch test'; findmnt /netusers/nfs1\nlvs vgdata; df -h /data\nsshd -T | grep -E '^(port|permitrootlogin)'; semanage port -l | grep ssh_port_t\nfirewall-cmd --list-all\ncat /proc/cmdline; grep GRUB_TIMEOUT /etc/default/grub\nsysctl net.ipv4.ip_forward\njournalctl --list-boots\nsystemctl cat chronyd | tail -3\ngetsebool httpd_can_network_connect_db\nmkfiles.sh 3 /tmp/x; ls /tmp/x; mkfiles.sh; echo $?\nsystemctl get-default; systemctl is-enabled cockpit.socket\n```\n\n## Hints\n\n- Task 2: `useradd -u 4001 -M -d /netusers/nfs1 nfs1` + autofs wildcard.\n- Task 3: `lvextend -r -L 2G`, `lvreduce -r -L 1.5G` (ext4).\n- Task 4: `Port 22` and `Port 2222` in a drop-in; `PermitRootLogin prohibit-password`.\n- Task 5: rich rules with `service name=\"ssh\"` and `port port=\"2222\"`.\n- Task 6: grubby + `/etc/default/grub` + `grub2-mkconfig`.\n- Task 11: `[ $# -ne 2 ] && { echo \"usage: $0 N DIR\" >&2; exit 2; }`, `for i in $(seq 1 \"$1\")`.\n\n## Solution\n\n1. NFS server (server2).\n```bash\ndnf install -y nfs-utils; mkdir -p /exports/users\nuseradd -u 4001 -d /exports/users/nfs1 nfs1; useradd -u 4002 -d /exports/users/nfs2 nfs2\necho \"/exports/users 192.168.122.0/24(rw,sync)\" > /etc/exports.d/users.exports\nsystemctl enable --now nfs-server; exportfs -ra\nfirewall-cmd --add-service={nfs,mountd,rpc-bind} --permanent; firewall-cmd --reload\n```\n2. autofs (server1).\n```bash\ndnf install -y autofs nfs-utils\nuseradd -u 4001 -M -d /netusers/nfs1 nfs1; useradd -u 4002 -M -d /netusers/nfs2 nfs2\necho \"/netusers /etc/auto.netusers\" > /etc/auto.master.d/netusers.autofs\necho \"* -rw server2:/exports/users/&\" > /etc/auto.netusers\nsystemctl enable --now autofs\n```\n3. LVM resize.\n```bash\npvcreate /dev/vdc; vgcreate vgdata /dev/vdc; lvcreate -n lvdata -L 1G vgdata\nmkfs.ext4 /dev/vgdata/lvdata; mkdir /data\necho \"/dev/vgdata/lvdata /data ext4 defaults 0 0\" >> /etc/fstab; systemctl daemon-reload; mount -a\nlvextend -r -L 2G /dev/vgdata/lvdata\nlvreduce -r -L 1.5G /dev/vgdata/lvdata\n```\n4. sshd with two ports.\n```bash\nprintf 'Port 22\\nPort 2222\\nPermitRootLogin prohibit-password\\n' > /etc/ssh/sshd_config.d/10-exam.conf\nsemanage port -a -t ssh_port_t -p tcp 2222\nsshd -t && systemctl restart sshd\n```\n5. Firewall.\n```bash\nfirewall-cmd --permanent --remove-service=ssh\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"192.168.122.0/24\" service name=\"ssh\" accept'\nfirewall-cmd --permanent --add-rich-rule='rule family=\"ipv4\" source address=\"192.168.122.0/24\" port port=\"2222\" protocol=\"tcp\" accept'\nfirewall-cmd --reload\n```\n6. Boot settings.\n```bash\ngrubby --update-kernel=ALL --remove-args=\"rhgb quiet\"\nsed -i 's/^GRUB_TIMEOUT=.*/GRUB_TIMEOUT=8/' /etc/default/grub; grub2-mkconfig -o /boot/grub2/grub.cfg\n```\n7. and 8. sysctl and journal.\n```bash\necho \"net.ipv4.ip_forward = 1\" > /etc/sysctl.d/90-forward.conf; sysctl --system\nmkdir -p /etc/systemd/journald.conf.d\nprintf '[Journal]\\nStorage=persistent\\nSystemMaxUse=200M\\n' > /etc/systemd/journald.conf.d/exam.conf\nsystemctl restart systemd-journald\n```\n9. and 10. Drop-in and boolean.\n```bash\nmkdir -p /etc/systemd/system/chronyd.service.d\nprintf '[Service]\\nRestart=always\\nRestartSec=10\\n' > /etc/systemd/system/chronyd.service.d/override.conf\nsystemctl daemon-reload; systemctl restart chronyd\nsetsebool -P httpd_can_network_connect_db on\n```\n11. The script.\n**/usr/local/bin/mkfiles.sh**\n```text\n#!/bin/bash\nif [ $# -ne 2 ]; then\n    echo \"usage: $0 N DIR\" >&2\n    exit 2\nfi\nmkdir -p \"$2\"\nfor i in $(seq 1 \"$1\"); do\n    touch \"$2/file$i\"\ndone\n```\n```bash\nchmod +x /usr/local/bin/mkfiles.sh\n```\n12. Target and cockpit.\n```bash\nsystemctl set-default multi-user.target; systemctl disable --now cockpit.socket\n```\n13. Reboot both and run the checks.\n```bash\nsystemctl reboot\n```\n","page":"labs/rhcsa-27-2/","label":"Lab 27.2","where":"Lab 27.2 · hard · 150 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