FireAngel is a relatively small script that should easy the pain of setting up a Linux 2.4.x iptables based firewall. FireAngel project can do basic firewalling such as specifying unsafe and safe interfaces, host/subnets and ports.
FireAngel can also set up Masquerading for you (over multiple interfaces if you really want that). There's no fancy documentation, just a quick installation guide (a couple of lines) and a documented configuration script. FireAngel just tries to do its work as good as it can, nothing more, nothing less...
Oh and by the way... This thing comes without guarantees! I do not and will never take responsibility for faults in the script or the configuration file.
Copy the "rc.fireangel" file to your /etc directory and edit options as you want them. If needed, create your own "rc.fireangel.allow" and "rc.fireangel.disallow" files. Copy fireangel script to /etc/init.d (or /sbin/init.d or /etc/rc.d), and make sure it gets started, either by including it in some run level, or by calling it in ip-up/ip-down or ifup/ifdown