Firewall Programing

[blacksadness]

Hey,
I'm doing a little research about tools i need to build a firewall.
It's a university project, and i was looking for actually a way to write a basic firewall, tasks are blocking port, blocking intruder IPs and Intrusion detection, what I'm currently looking for is a way to intercept network packets, and check them for any intrusion or any rules i specify, i found some tools that let me detect raw packets transmitted (libcap) but what's next, how do i manipulate these, how do i get the application to send/receive these packets through my firewall, and am i on the right track or are there other tools to use.

[bhaslinux]

There is a program (used long back) called portsentry. This does intrusion detection , etc., etc., and you can take a look at the source for help !