Interesting..... http://portknocking.org/
In one instance, port knocking refers to a method of communication between two computers (arbitrarily named here client and server) in which information is encoded, and possibly encrypted, into a sequence of port numbers. This sequence is termed the knock. The server initially presents no open ports to a public network and is monitoring all connection attempts. The client initiates connection attempts to the server by sending SYN packets to the ports specified in the knock. This process of knocking is what gives port knocking its name. The server offers no response to the client during the knocking phase, as it "silently" processes the port sequence. When the server decodes a valid knock it triggers a server-side process.
