CentOS 7

On CentOS 7, firewall-cmd is used to maintain the firewall.

List All Rules

firewall-cmd --list-all

Allow a port

This example allows a TCP port through the firewall, for all network interfaces in the "public" zone:

firewall-cmd --zone=public --add-port=3306/tcp --permanent

firewall-cmd --zone=public --add-port=20202/udp --permanent

firewall-cmd --zone=public --remove-port=3306/tcp --permanent

Reload the firewall

This makes the port changes active:

firewall-cmd --reload

If port changes are specified without the --permanent flag, they will be active, but not permanent. The following command makes them permanent:

firewall-cmd --runtime-to-permanent

CentOS 6

On CentOS 6, iptables is used to maintain the firewall.

Caution: iptables is cryptic and sometimes rather stupid. Especially: it will add multiple rules for the same port, and does not check if the port is already open.

List All Rules

iptables -L -n

The -n option causes the ports to be listed numerically.

Alternatively:

iptables-save > iptables_rules
less iptables_rules

This method is particularly useful for cleaning up duplicated rules.

Allow a port

This example allows a TCP port through the firewall:

iptables -I INPUT 4 -p tcp -m tcp --dport 3306 -j ACCEPT
service iptables save

iptables -I INPUT 4 -p tcp -m udp --dport 20202 -j ACCEPT
service iptables save

Reload the firewall

This makes the port changes permanent:

service iptables save

Delete a rule

The simplest way to delete rules in iptables, is to save and restore the configuration:

First, save the rules:

iptables-save > iptables_rules

Now edit the rules:

vi iptables_rules

Now restore the rules, with the changes made, and make them permanent:

iptables-restore < iptables_rules
service iptables save