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.

http://www.boynux.com/how-to-enable-port-knocking-in-linux/?goback=.gde_102999_member_269001685

https://en.wikipedia.org/wiki/Port_knocking

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/