Protect network services with firewalld and SELinux

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المهارات في هذا الفصل

فتح الخدمات في firewalld وتسمية المنافذ في SELinux

Know firewalld and zones

firewalld manages the firewall rules of the kernel (nftables). Every network card belongs to a zone; the zone says which services are allowed in.

Zone	Meaning
`public`	The default: only selected services (SSH, cockpit, DHCPv6 client)
`internal`, `home`, `work`	More trust: more services open
`trusted`	Everything allowed
`drop`	Everything dropped without an answer
`block`	Everything refused with a "rejected" answer
`dmz`	For servers in a DMZ: only selected services

#systemctl status firewalld
$firewall-cmd --state
running
#firewall-cmd --get-default-zone
public
#firewall-cmd --get-active-zones
public
  interfaces: enp1s0
#firewall-cmd --list-all
public (default, active)
  target: default
  interfaces: enp1s0
  services: cockpit dhcpv6-client ssh
  ports:
  rich rules:

14.2

Open a service

A service is a name for one or more ports (http = 80/tcp). Add it permanently, then reload.

#firewall-cmd --get-services | tr ' ' '\n' | grep -E '^(http|https|nfs|mysql)$'
#firewall-cmd --info-service=https
#firewall-cmd --add-service=http --permanent
#firewall-cmd --add-service={https,nfs} --permanent
#firewall-cmd --reload
#firewall-cmd --list-services
cockpit dhcpv6-client http https nfs ssh
#firewall-cmd --remove-service=cockpit --permanent
#firewall-cmd --reload

Kind of change	Works now	Kept after reload / reboot
`firewall-cmd --add-…`	Yes	No (runtime only)
`firewall-cmd --add-… --permanent`	No (after `--reload`)	Yes
`firewall-cmd --runtime-to-permanent`	—	Saves the current runtime rules

نصيحة للامتحان

Always use --permanent and --reload. Then check with firewall-cmd --list-all.

14.3

Open a port

When there is no service name, open the port with its protocol.

#firewall-cmd --add-port=8443/tcp --permanent
#firewall-cmd --add-port=5000-5010/udp --permanent
#firewall-cmd --reload
#firewall-cmd --list-ports
8443/tcp 5000-5010/udp
#firewall-cmd --remove-port=8443/tcp --permanent

14.4

Use zones and sources

Traffic from certain source addresses can use another zone — for example, the admin network may reach more services than the rest.

#firewall-cmd --zone=internal --add-source=192.168.50.0/24 --permanent
#firewall-cmd --zone=internal --add-service=mysql --permanent
#firewall-cmd --zone=internal --change-interface=enp2s0 --permanent
#firewall-cmd --set-default-zone=dmz
#firewall-cmd --reload
#firewall-cmd --zone=internal --list-all

14.5

Write rich rules

A rich rule is a rule with more conditions: a service or port for one address, logging, or a reject.

#firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.122.20" service name="mysql" accept' --permanent
#firewall-cmd --add-rich-rule='rule family="ipv4" source address="203.0.113.0/24" reject' --permanent
#firewall-cmd --add-rich-rule='rule service name="ssh" log prefix="SSH: " level="info" limit value="5/m" accept' --permanent
#firewall-cmd --reload
#firewall-cmd --list-rich-rules

14.6

Allow a service on a non-standard port

SELinux allows each service only its labeled ports. Apache may use ports labeled http_port_t. To run it on port 82, add that label to port 82.

#semanage port -l | grep -w http_port_t
http_port_t                    tcp      80, 81, 443, 488, 8008, 8009, 8443, 9000
#vim /etc/httpd/conf/httpd.conf
#systemctl restart httpd
Job for httpd.service failed because the control process exited with error code.
#ausearch -m AVC -ts recent | grep name_bind
type=AVC msg=audit(...): avc:  denied  { name_bind } for  pid=3102 comm="httpd" src=82 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:reserved_port_t:s0 tclass=tcp_socket permissive=0
#semanage port -a -t http_port_t -p tcp 82
#systemctl restart httpd
#firewall-cmd --add-port=82/tcp --permanent
#firewall-cmd --reload
$curl -I http://server1:82/

Command	What it does
`semanage port -l`	List all port labels
`semanage port -a -t TYPE -p tcp PORT`	Add a label to a free port
`semanage port -m -t TYPE -p tcp PORT`	Change the label of a port that already has one
`semanage port -d -t TYPE -p tcp PORT`	Remove your label
`semanage port -l -C`	Only your own changes

انتبه

Port 8080 already has the label http_cache_port_t. To use a labeled port for another type, use -m (modify), not -a.

تذكّر

Service on a new port = three steps: the service config, semanage port -a, firewall-cmd --add-port.

14.7

Test from outside

Test from another machine. A test from the server itself does not go through its own firewall.

#ss -tlpn | grep httpd
$curl -I http://server1/
$curl -I http://server1:82/
$nc -zv server1 82
Ncat: Connected to 192.168.122.10:82.

Error from the client	Most likely cause
`Connection refused`	Nothing listens on the port (service down, wrong port), or a firewall reject rule
`No route to host` / timeout	The firewall drops it, or the network is wrong
Works on the server, not from outside	Firewall, or the service listens only on `127.0.0.1`

More: the RHCSA firewalld chapter.