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http://www.cyberciti.biz/faq/iptables-block-port/
http://stackoverflow.com/questions/19776681/allow-ssh-incoming-outgoing-and-blocking-all-outgoing-besides-specific-ports
http://www.cyberciti.biz/faq/linux-unix-open-ports/
https://wiki.centos.org/HowTos/Network/IPTables
http://www.thegeekstuff.com/2011/01/iptables-fundamentals/