sysctl

What it does

How to write it

sysctl [OPTION]... [NAME[=VALUE]]...
sysctl -p [FILE]
sysctl --system

Most used

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$ run as a normal user# run as root (or with sudo)

1. 1Read a value

   $sysctl net.ipv4.ip_forward
   net.ipv4.ip_forward = 0

   Same as cat /proc/sys/net/ipv4/ip_forward.

2. 2Change it now

   #sysctl -w net.ipv4.ip_forward=1

   Lost at reboot.

3. 3Make it permanent

   #echo "net.ipv4.ip_forward = 1" > /etc/sysctl.d/90-forward.conf
   #sysctl --system

   Files are read in name order; later wins.

4. 4Only the value

   $sysctl -n vm.swappiness
   60

   For scripts.

5. 5Search settings

   $sysctl -a 2>/dev/null | grep -i swappiness

   Or sysctl -a -r swap.

6. 6Lower swapping on a database server

   #echo "vm.swappiness = 10" > /etc/sysctl.d/90-swap.conf; sysctl --system

   The kernel keeps more in RAM.

7. 7Network hardening (security)

   #cat /etc/sysctl.d/90-hardening.conf
   net.ipv4.conf.all.accept_redirects = 0
   net.ipv4.conf.all.send_redirects = 0
   net.ipv4.conf.all.rp_filter = 1
   net.ipv4.tcp_syncookies = 1
   kernel.kptr_restrict = 2
   kernel.dmesg_restrict = 1

   Common CIS benchmark settings.

8. 8Load one file

   #sysctl -p /etc/sysctl.d/90-hardening.conf

   Test a new file.

9. 9More open files for big servers

   #sysctl -w fs.file-max=2097152

   System-wide limit (per-process limits are in limits.conf/systemd).

10. 10Where does a setting come from?

    $grep -r swappiness /etc/sysctl.d/ /usr/lib/sysctl.d/ /etc/sysctl.conf

    tuned can also set values (check the tuned profile).

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Files

/etc/sysctl.d/*.conf

Your permanent settings.

/usr/lib/sysctl.d/

Defaults from packages.

/etc/sysctl.conf

Old main file (still read).

/proc/sys/

The live values.

Exit codes