Knockd and knock are a port-knock server and client, respectively. Knockd
listens to all traffic on an ethernet (or PPP) interface, looking for
special "knock" sequences of port-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 -- since knockd listens at the link-layer level, it
sees all traffic even if it's destined for a closed port. When the
server detects a specific sequence of port-hits, it runs a command
defined in its configuration file. This can be used to open up holes in
a firewall for quick access.
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