Fwknop and single packet authorization

Sets a server up to open selected ports following a specific request from an external host.

As defined by Wikipedia, port knocking is "a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port(s)."

Single packet authorization distills the essence of the port knocking concept down to a single packet. Rather than sending a series of packets to predefined ports, single packet authorization encodes the "knock" within the payload of one packet.

fwknop also features a Windows GUI.

Update 2008/06/24: There's a How-To on the Ubuntu Forums.