iptables

http://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Netfilter-packet-flow.svg/1450px-Netfilter-packet-flow.svg.png

http://en.wikipedia.org/wiki/Iptables

http://mylinuxbook.com/linux-iptables/

http://www.cnblogs.com/JemBai/archive/2009/03/19/1416364.html