Port knocking. Port Knocking
is a clever technique to secure a machine.
No ports are open, but failures on ports are monitored for a specific
sequence of port attempts. If the correct ports are attempted in the
correct sequence, then a port is opened, and the external machine
can get in.
The technique adds a meta-level to normal security measures, and
turns failures into interesting information. [Ned Batchelder's blog]
