Endlessh: an SSH tarpit with PROXY protocol support

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.

Usage

Usage information is printed with -h.

Usage: endlessh [-vhs] [-d MS] [-f CONFIG] [-l LEN] [-m LIMIT] [-p PORT]
  -4        Bind to IPv4 only
  -6        Bind to IPv6 only
  -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]
  -P        Enable PROXY protocol support
  -s        Print diagnostics to syslog instead of standard output
  -v        Print diagnostics (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 by default. -s causes them to be sent to syslog.

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

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).

A SIGUSR1 signal will print connections stats to the log.

Sample Configuration File

The configuration file has similar syntax to OpenSSH.

# The port on which to listen for new SSH connections.
Port 2222

# The endless banner is sent one line at a time. This is the delay
# in milliseconds between individual lines.
Delay 10000

# The length of each line is randomized. This controls the maximum
# length of each line. Shorter lines may keep clients on for longer if
# they give up after a certain number of bytes.
MaxLineLength 32

# Maximum number of connections to accept at a time. Connections beyond
# this are not immediately rejected, but will wait in the queue.
MaxClients 4096

# Set the detail level for the log.
#   0 = Quiet
#   1 = Standard, useful log messages
#   2 = Very noisy debugging information
LogLevel 0

# Set the family of the listening socket
#   0 = Use IPv4 Mapped IPv6 (Both v4 and v6, default)
#   4 = Use IPv4 only
#   6 = Use IPv6 only
BindFamily 0

# Set PROXY protocol support
#   0 = No PROXY protocol support
#   1 = PROXY protocol support
ProxySupport 1

Sample Haproxy configuration

frontend ssh_frontend
    bind *:2223
    mode tcp
    default_backend endlessh_backend

backend endlessh_backend
    mode tcp
    server endlessh_server  127.0.0.1:2222 send-proxy
    server endlessh_server2 127.0.0.1:2222 send-proxy-v2

Build issues

Some more esoteric systems require extra configuration when building.

RHEL 6 / CentOS 6

This system uses a version of glibc older than 2.17 (December 2012), and clock_gettime(2) is still in librt. For these systems you will need to link against librt:

make LDLIBS=-lrt

Solaris / illumos

These systems don't include all the necessary functionality in libc and the linker requires some extra libraries:

make CC=gcc LDLIBS='-lnsl -lrt -lsocket'

If you're not using GCC or Clang, also override CFLAGS and LDFLAGS to remove GCC-specific options. For example, on Solaris:

make CFLAGS=-fast LDFLAGS= LDLIBS='-lnsl -lrt -lsocket'

The feature test macros on these systems isn't reliable, so you may also need to use -D__EXTENSIONS__ in CFLAGS.

OpenBSD

The man page needs to go into a different path for OpenBSD's man command:

diff --git a/Makefile b/Makefile
index 119347a..dedf69d 100644
--- a/Makefile
+++ b/Makefile
@@ -14,8 +14,8 @@ endlessh: endlessh.c
 install: endlessh
        install -d $(DESTDIR)$(PREFIX)/bin
        install -m 755 endlessh $(DESTDIR)$(PREFIX)/bin/
-       install -d $(DESTDIR)$(PREFIX)/share/man/man1
-       install -m 644 endlessh.1 $(DESTDIR)$(PREFIX)/share/man/man1/
+       install -d $(DESTDIR)$(PREFIX)/man/man1
+       install -m 644 endlessh.1 $(DESTDIR)$(PREFIX)/man/man1/

 clean:
        rm -rf endlessh