Copyright (c) 2004, Judd Vinet jvinet@zeroflux.org
This is a port-knocking server/client. Port-knocking is a method where a server can sniff one of its interfaces for a special "knock" sequence of port-hits. When detected, it will run a specified event bound to that port knock sequence. These port-hits need not be on open ports, since we use libpcap to sniff the raw interface traffic.
To build knockd, make sure you have libpcap and the autoconf tools installed. Then run the following:
$ autoreconf -fi
$ ./configure --prefix=/usr/local
$ make
$ sudo make install
The example below could be used to run a strict (DENY policy) firewall that can only be accessed after a successful knock sequence.
The accompanying knock client is very basic. If you want to do more advanced knocks (eg, setting specific tcp flags) then you should take look at more powerful clients.
Here are some other implementations of port-knocking: