[1] google: iptables tutorial

[2] Iptables Tutorial 1.2.0, Oskar Andreasson, http://iptables-tutorial.frozentux.net/iptables-tutorial.html

[3] Secure Linux Firewall & VPN presented by Somphol Boonjing at singapore September, 2006 workshop

[4] http://www.vicomsoft.com/knowledge/reference/firewalls1.html