Remember when you used to only open the door when a certain pattern of knocks was heard? You can do the same with ports - the so-called “Port Knocking”. It provides another layer of security to your system as ports are not opened until the correct knocking pattern is received.

Haven’t installed or tried this myself, but may do over the weekend.

www.boynux.com/how-to-en…

en.wikipedia.org/wiki/Port…

http://www.microhowto.info/howto/implement_port_knocking_using_iptables.html

https://wiki.archlinux.org/index.php/Port_Knocking

http://www.debian-administration.org/articles/268

http://www.zeroflux.org/projects/knock/