Broadly, port knocking (PK on wikipedia) is a form of host-to-host communication in which information flows across closed ports. There are various variants of the port knocking method - information may be encoded into a port sequence or a packet-payload. In general, data are transmitted to closed ports and received by a monitoring daemon which intercepts the information without sending a receipt to the sender.
Recently a physical knock detecting device that does to the door what port knock does to your server has been reported. This knock detector is mounted on the inside of a door and listens to ... you guessed it, secret knocks. Once a knock is detected, the device unlocks the door.
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. Initially, the server presents no open ports to the public and is monitoring all connection attempts. The client initiates connection attempts to the se
http://www.portknocking.org/view/about
