knock is a server/client set of tools that implements the idea known as port-knocking. Port-knocking is a method of accessing a backdoor to your firewall through a special sequence of port hits. This can be useful for opening up temporary holes in a restrictive firewall for SSH access or similar.
You can download it from the following link: https://packetstormsecurity.com/files/download/38357/knock-0.5.tar.gz
Source: https://packetstormsecurity.com/files/38357/knock-0.5.tar.gz.html