View source for Netcat tunnel

If your sshd config disallows listening to remote ports, you can accomplish about the same thing using nc and ssh.  

Typical error message when remote listening is disallowed:
<geshi lang="bash">
$ ssh -R 18080:thirdhost:8080 remotehost cat -
Warning: remote port forwarding failed for listen port 18080
</geshi>

To work around this, you can use netcat and mkfifo to create a tunnel.

<geshi lang="bash">
mkfifo x
while true; do ssh remotehost nc -l -p 8081 -c <x | nc -c thirdhost 8081 >x; done
</geshi>

Then on the remote host, you can access the third host via the local tunnel.  

Unfortunately, this approach is limited to a single connection at a time, and also, there is a slight delay after each socket is closed. Another issue is that it's easy to leave orphan processes on the remote host.

[[Category:Unix]]
[[Category:Bash]]