iptable原理

http://www.cnblogs.com/littlehann/p/3708222.html
http://seanlook.com/2014/02/23/iptables-understand/
http://fishcried.com/2016-02-19/iptables/
http://seanlook.com/2014/02/23/iptables-understand/

iptables -t nat -I OUTPUT --dest 93.184.220.20 -p tcp --dport 80 -j DNAT --to-dest 93.184.220.21