The iptables and ipchains commands are used to adjust and inspect the network packet filter rules in a running Linux kernel. The ipchains command was used for 2.2.x versions of the kernel, and although it can still be used with 2.4.x kernels, it has been superseded by iptables.

The packet filter rules can be set up to do both firewall and router activities. You can inspect your current rules with the -L option to iptables:

# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

This is an example of a wide-open system, with no routing or firewalling enabled.