Comment by michaelaiello

Many Many Moons ago I implemented an OS version of this for windows. It was taken over a few years back and is released as KnockKnock. https://sourceforge.net/projects/knockknock/

We are now using the same technique in a network security product at AppGate which is used to protect highly sensitive workloads around the world. https://www.appgate.com/blog/dont-be-fooled-by-lowercase-sdp...

Port Knocking is great, another layer of defense.