An RPC server written in Python designed for controlling the firewall rules of a captive portal system.
A page describing how to use IP tables on a Linux machine to set up a captive portal that also does traffic shaping to maximize the amount of bandwidth available. With a tarball of downloadable scripts.
Oskar Andreasson's Ultimate IPtables Tutorial for Linux. Considered one of the best ways to learn how to set up and use IP tables. Comes in multiple formats and several languages, so there should be something for everybody there.
A (theoretically) simple set of IP tables rules for building a captive portal on a network.
A useful blog post about using IPtables' 'raw' table and TRACE target to debug IPtables rules by tracing activity. The TRACE target is used to log every packet that hits netfilter and what happens to it. The post tells you what to enable in the kernel to provide this functionality.