Endlessh is an SSH tarpit that very slowly sends an endless, random SSH banner.
It keeps SSH clients locked up for hours or even days at a time.
The purpose is to put your real SSH server on another port and then let the script kiddies get stuck in this tarpit instead of bothering a real server.
Since the tarpit is in the banner before any cryptographic exchange occurs, this program doesn't depend on any cryptographic libraries.
It's a simple, single-threaded, standalone C program.
It uses poll() to trap multiple clients at a time.