Reference:
http://security.maruhn.com/iptables-tutorial/x4634.htm
http://www.faqs.org/docs/iptables/icmpconnections.html
ICMP Echo request, Type 8 Code 0
ICMP Echo reply, Type 0 Code 0
Reference:
http://en.wikipedia.org/wiki/Ping
http://www.linuxtopia.org/Linux_Firewall_iptables/x270.html
