] Briefly, users make connection attempts to sequences of
] closed ports. The failed connections are logged by the
] server-side packet filtering firewall and detected by a
] dæmon that monitors the firewall log file. When a
] properly formatted knock sequence, playing the role of
] the secret used in the authentication, is received,
] firewall rules are manipulated based on the information
] content of the sequence. This user-based authentication
] system is both robust, being mediated by the kernel
] firewall, and stealthy--it's not possible to detect
] whether a networked machine is listening for port knocks.
] Port knocking does not require any open ports, and it can
] be extended to transmit any type of information encoded
] in a port sequence.

This is so very very cool

Port Knocking