Stealfly is proof of concept perl code that illustrates the usage of port knocking. The server sits on a defined interface and listens for a certain amount of UDP packets to be sent from a chosen ip that will use a range of source ports for each packet. If the rules are satisfied then server will open it’s port or connect back using netcat.
You can download it from the following link: https://packetstormsecurity.com/files/download/42685/Stealfly.tar.gz
Source: https://packetstormsecurity.com/files/42685/Stealfly.tar.gz.html