SSH/HTTP(S) multiplexer. Run a webserver and a sshd on the same port w/o changes.
C++ Shell Makefile

sshttp - hiding SSH servers behind HTTP


0. Intro

In case your FW policy forbids SSH access to the DMZ or internal network from outside, but you still want to use ssh on machines which only have one open port, e.g. HTTP, you can use sshttpd.

sshttpd can multiplex the following protocol pairs:

  • SSH/SMTP (without SMTP multiline banners)

1. Build

Be sure you run recent Linux kernel and install nf-conntrack as well as libcap and libcap-devel if you want to use the capability feature.

$ make

There is a new splice branch inside the git. git checkout splice before make, if you want to test this new branch. It implements zero-copy in terms of the splice(2) system call which has a performance benefit since it avoids copying the network data between user and kernel land back and forth (read()/write()), which could also just be spliced kernel-internally at the "extra cost" of two additional pipe descriptors per connection.

2. Setup for single host

sshttpd is an easy to use OSI-Layer5 switching daemon. It runs transparently on HTTP port (-L switch, default 80) and decides on incoming connections whether this is SSH or HTTP traffic. If its HTTP traffic it switches the traffic to the HTTP_PORT (-H, default 8080) and if its SSH traffic to SSH_PORT (-S, default 22) respectively.

You might need to edit nf-setup script to match your ports (22, 80 and 8080 are just fine) and run it to install the proxy rules. Your sshd has to run on $SSH_PORT and your webserver on $HTTP_PORT. Thats basically it. Go ahead and run sshttpd (as root) and it will layer5-switch your traffic destinated to TCP port 80.

If you want to mux SMTP with sshttpd, just give 25 as -L parameter, 2525 as -H parameter, and setup your smtp daemon to listen on 2525. Then edit the nf-setup script to match these ports. In the Makefile, change the SMTP_DOMAIN and SSH_BANNER to your needs (SSH_BANNER must match exactly yours of the running sshd). SMTP/SSH muxing was tested with OpenSSH client and Postfix client and server.

When muxing IPv6 connections, the setup is basically the same; just use the nf6-setup script and invoke sshttpd with -6.

Do not forget to modprobe nf_conntrack_ipv4 or modprobe nf_conntrack_ipv6.

3. Transparent proxy setup

You can run sshttpd also on your gateway machine and transparently proxy/mux all of your HTTP/SSH traffic to your internal LAN. To do so, run sshttpd with -T and use nf-tproxy rather than nf-setup. Before you do so, carefully read nf-tproxy so you dont lock yourself out of the network.

4. Misc

You dont need to patch any of your ssh/web/smtp client or server software. It works as is. sshttpd runs only on Linux and needs IP_TRANSPARENT support. It would work without, but by using IP_TRANSPARENT it is possible to even have unmodified syslogs, e.g. the original source IP/port of incoming connections is passed as-is to the SSH/HTTP/SMTP servers.

Make sure the nf_conntrack and nf_conntrack_ipv4 modules are loaded. sshttpd is also a tricky anti-SSH0day (if ever:) and anti SSH-scanning/bruteforcing measurement. sshttpd has small footprint and was optimized for speed so it also runs on heavily loaded web servers.

Since version 0.24, sshttpd also supports multiple CPU cores. Unless -n 1 is used as switch, sshttpd binds one thread per CPU core, to better exploit the hardware if running on heavily used web servers. It still runs this fixed number of threads no matter how many 1000s connection it handles at the same time. sshttpd runs as nobody user inside a chroot() (configurable via -U and -R switch) if compiled with USE_CAPS. It can also distinguish between SSH and SSL sessions, you just have to use an LOCAL_PORT (-L) of 443 or 4433 and change the HTTP_PORT in the nf-setup script to match your webservers HTTPS port. You cannot mix HTTP/SSH and HTTPS/SSH in one sshttpd instance but you can run two sshttpd's to reach that goal: one on LOCAL_PORT 80 and one on LOCAL_PORT 443.

Hints/bug reports beyond RTFM to sebastian.krahmer [at] gmail com.