Endlessh: an SSH tarpit

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 at time. The purpose is to put your real SSH server on another port and then let the script kiddies get themselves stuck in this tarpit instead of bothering a real server.

Since the tarpit is 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.

Usage

Usage information is printed with -h.

Usage: endlessh [-vh] [-d MSECS] [-m LIMIT] [-p PORT]
  -d INT    Message millisecond delay [10000]
  -h        Print this help message and exit
  -m INT    Maximum number of clients [4096]
  -p INT    Listening port [2222]
  -v        Print diagnostics to standard output (repeatable)

By default no log messages are produced. The first -v enables basic logging and a second -v enables debug logging (noisy). All log messages are sent to standard output.

endlessh -v >endlessh.log 2>endlessh.err

The purpose of limiting the number of clients (-m) is to avoid tying up too many system resources with the tarpit. Clients beyond this limit are left in the accept queue, not rejected instantly.

A SIGTERM signal will gracefully shut down the daemon, allowing it to write a complete, consistent log.