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Here’s a little tip on how to tunnel ssh through another machine with the -L option. While not terribly difficult, I did spend some time figuring this out… Maybe this will save someone else some time ;-)

The network setup (simplified):

  [ Workstation ]
         |
         |
   [ Firewall ]
         |
         |
  ~ The Internet ~
         |
         |
[ Public webserver ]

The problem is connecting to public webserver from my workstation, first I had to ssh or sftp to the Linux firewall, and from that machine I could connect to the webserver.

There has to be an easier way… And a look at the SSH manpage provided the answer: The -L option.

Excerpt from ssh(1):

-L [bind_address:]port:host:hostport
	Specifies that the given port on the local (client) host is to be
	forwarded to the given host and port on the remote side.  This
	works by allocating a socket to listen to port on the local side,
	optionally bound to the specified bind_address.

Example on how to create the tunnel:

$ ssh -f -N -p 22 username@firewall -L 2844/webserver.example.com/22

To briefly explain what the other options mean:

You can now connect with ssh, sftp, or scp through localhost:2844

$ ssh -p 2844 myusername@localhost
$ scp -P 2844 file.tar.gz myusername@localhost:file.tar.gz

Note that ssh(1) requires -p and scp(1) -P.

Testing

For debugging, don’t forget you can specify -v up to three times to get more information about what’s going on. In addition, it’s probably best to test with telnet since this excludes things like authentication problems.

$ telnet localhost 2844
Trying ::1...
Connected to localhost.
Escape character is '^]'.
SSH-2.0-OpenSSH_5.1p1 FreeBSD-20080901

If you don’t see the last line, something is wrong.

Bonus tip

As a free complimentary bonus tip, it’s also very easy to setup a convenient shortcut in ~/.ssh/config

Host webserver
	Hostname localhost
	Port 2844
	User myusername

Further reading

Responses

Over at the FreeBSD Forums, Freddie pointed out a clever way to accomplish the same thing using netcat and the ProxyCommand option

Feedback

You can mail me at martin@arp242.net for feedback, questions, etc.

Copyright © 2010-2017 Martin Tournoij <martin@arp242.net>
This document is licensed under a cc-by 4.0 license.