Firewall can either refer to a machine used to filter (usually IP) packets or the software used on that machine to provide packet filtering.

There are distributions that exist only to provide firewalling; PerryLorier is working on a Firewall-on-a-disc system. You can technically speaking shut a Linux machine down into kernel-only mode and still be running a firewall.

In Linux 2.2, firewalling was provided by ipchains(8). In Linux 2.4, firewalling is provided by iptables(8).

Adding a rule

TODO

Deleting a rule

iptables -D chain rule number? iptables -D chain rule description?

Hint: if you want to delete a rule and you don't want to have to mess around with specifying ports etc, try

iptables -L --line-numbers

Then you can just use iptables -D FORWARD 1 to remove it.

You might want to read HowToIPCHAINSHOWTO?, HowToBridgeFirewall?, HowToBridgeFirewallDSL?, HowToFirewallHOWTO?, HowToFirewallPiercing?, HowToSentryFirewallCDHOWTO? or HowToTermFirewall?. (They're all really, REALLY old.)