ethernet interface, looking for special "knock" sequences ofport-hits. A client makes these port-hits by sending a TCP (or UDP)packet to a port on the server. This port need not be open -- sinceknockd listens at the link-layer level, it sees all traffic even ifit's destined for a closed port. When the server detects a specificsequence of port-hits, it runs a command defined in its configurationfile. This can be used to open up holes in a firewall for quickaccess.