"This tutorial will walk you through setting up Shorewall
(Shoreline) 4.0 firewall on CentOS 5.1; this can easily be adapted
to any other Linux distribution out there.
"The Shoreline Firewall, more commonly known as 'Shorewall,' is
a high-level tool for configuring Netfilter. You describe your
firewall/gateway requirements using entries in a set of
configuration files. Shorewall reads those configuration files and
with the help of the iptables utility, Shorewall configures
Netfilter to match your requirements. Shorewall can be used on a
dedicated firewall system, a multi-function gateway/router/server
or on a standalone GNU/Linux system. Shorewall does not use
Netfilter's ipchains compatibility mode and can thus take advantage
of Netfilter's connection state tracking capabilities..."