Sitios web:
http://www.pettingers.org/code/firewall.html
http://www.cyberciti.biz/faq/linux-demilitarized-zone-howto/
http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables
http://piggledy.org/wiki/iptablesopt