here are two i found usefull.
http://www.laser.dist.unige.it/Repos...esTutorial.pdf
http://iptables-tutorial.frozentux.net/

and here you have a desent script (text is in swedish but script works) these two links are from http://www.lysator.liu.se/~kjell-e/tekla/linux/

http://www.lysator.liu.se/~kjell-e/t.../iptables.html
http://www.lysator.liu.se/~kjell-e/t...askering2.html