1) Free software programmed in C 
Linux IP Firewalling Chains, normally called ipchains, is free software to control the packet filter or firewall capabilities in the 2.2 series of Linux kernels. It superseded ipfwadm, but was replaced by iptables in the 2.4 series. Unlike iptables, ipchains is stateless. It is a rewrite of Linux`s previous IPv4 firewall, ipfwadm. This newer ipch....
IP firewall administration, see Section 10.4.4.2.