reverse ssh tunnel listens on wrong interface

Question

I'm working with a server that is behind a firewall. I have established an ssh tunnel to an intermediate server in the internet like this:

remoteuser@behind_fw$ ssh -N -f -R 10002:localhost:22 middleuser@middle

But I can't connect directly throgh this server, this doesn't work:

user@local$ ssh remoteuser@middle -p 10002

I have to connect in two steps:

user@local$ ssh middleuser@middle
middleuser@middle$ ssh remoteuser@localhost -p 10002

Output of netstat -l on middle:

tcp        0      0 localhost:10002         *:*                     LISTEN

but it should be something like this:

tcp        0      0 *:10002                 *:*                     LISTEN

how can I achieve this?

Answer 1

This being a tunnel opened at a remote server, that server needs to have GatewayPorts set to yes in its /etc/ssh/sshd_config.

Depending on what kind of users that server have you might want to use the Match option to limit that capability to your user.

Match User middleuser
  GatewayPorts yes

Do note that you probably want to add this Match block in the end of your sshd_config, since a Match block goes on until another one begins, or the file ends.

That being said, how about instead trying what I'd consider a slightly cleaner solution?

user@local$ ssh -N -f -L 10002:behind_fw:22 middleuser@middle
user@local$ ssh remoteuser@localhost -p 10002

Answer 2

Assuming you are using OpenSSH you need to specify the bind address. Try something like:

ssh -N -f -R *:10002:localhost:22 middleuser@middle

or

ssh -N -f -R :10002:localhost:22 middleuser@middle

The signature for -R is:

-R [bind_address:]port:host:hostport

on OpenSSH. For security reasons it defaults to localhost if you don't specify it. You might have to change GatewayPorts to get it to work depending on your config. See SSH(1) for more info.