The Doorman is a port-knocking listener daemon which helps users secure private servers. It allows a Unix server to run invisibly, with all TCP ports closed.
You can download it from the following link: https://packetstormsecurity.com/files/download/39878/doorman-0.81.tgz
Source: https://packetstormsecurity.com/files/39878/doorman-0.81.tgz.html