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 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.

Usage

Usage information is printed with -h.

Usage: endlessh [-vh] [-d MS] [-f CONFIG] [-l LEN] [-m LIMIT] [-p PORT]
  -d INT    Message millisecond delay [10000]
  -f        Set and load config file [/etc/endlessh/config]
  -h        Print this help message and exit
  -l INT    Maximum banner line length (3-255) [32]
  -m INT    Maximum number of clients [4096]
  -p INT    Listening port [2222]
  -v        Print diagnostics to standard output (repeatable)

Argument order matters. The configuration file is loaded when the -f argument is processed, so only the options that follow will override the configuration file.

By default no log messages are produced. The first -v enables basic logging and a second -v enables debugging 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.

A SIGHUP signal requests a reload of the configuration file (-f).

Sample Configuration File

The configuration file has similar syntax to OpenSSH.

# This is a comment
Port 22
Delay 30000
MaxLineLength 8
MaxClients 512