Forum OpenACS Q&A: Re: basic firewall features on RH 8.0
Linux 2.4 stateful firewall design
Here I learnt how to do portforwarding (NAT) to services within my LAN:
Linux 2.4 NAT HOWTO
Red Hat Linux 6.X as an Internet Gateway for a Home Network
was also helpful, though I used iptables instead of ipchains.