Port knocking is a method of establishing a connection to a networked computer that has no open ports.
Before a connection is established, ports are opened using a port knock sequence, which is a series of connection attempts to closed ports. A remote host generates and sends an authentic knock sequence in order to manipulate the server's firewall rules to open one or more specific ports. These manipulations are mediated by a port knock daemon, running on the server, which monitors the firewall log file for connection attempts which can be translated into authentic knock sequences.
Once the desired ports are opened, the remote host can establish a connection and begin a session. Another knock sequence may used to trigger the closing of the port.
Read more about it at PortKnocking.org and give it a whirl.
