tumbler is a protocol that enables a client piece of software to securely tell a server process on a remote machine to execute a predetermined command. tumbler is similar to port knocking and is designed so that a remote user can securely and stealthily enable and disable server processes, or open and close firewall holes on a computer connected to the Internet.
You can download it from the following link: https://packetstormsecurity.com/files/download/33117/tumbler.tar.gz
Source: https://packetstormsecurity.com/files/33117/tumbler.tar.gz.html