http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/stronger-firewall-examples.html#RC.FIREWALL-2.4.X-STRONGER
http://www.siliconvalleyccie.com/linux-hn/iptables-intro.htm
http://iptables-tutorial.frozentux.net/iptables-tutorial.html
http://lists.netfilter.org/pipermail/netfilter/2002-September/038504.html
http://www.sns.ias.edu/~jns/security/iptables/iptables_conntrack.html