Netfilter

Netfilter is the standard firewall software shipped with Linux.

Applying filters to RHEL 3
One can create the rules with the iptables command and then writes the configuration to a file with iptables-save(8). When this file happens to be /etc/sysconfig/iptables and the rc scripts have iptables enabled then this file will be used to enable the rules as you have created them.