Editing Reverse SSH Tunnel

Without the ability to port forward a network under a NAT, your server is useless. In the case that you are incapable of allowing incoming traffic to certain ports into your network, you can use a reverse tunnel to give tunnel necessary services through another server elsewhere.

Edit <code>/etc/ssh/sshd_config</code>.

<pre>$ sudo nano /etc/ssh/sshd_config</pre>

If it doesn't exist already, add the following line:
<pre>GatewayPorts clientspecified</pre>

Now restart the SSH daemon. Note: non Debian/Ubuntu systems use <code>sudo systemctl restart sshd</code>
 
<pre>$ sudo systemctl restart ssh</pre>

Finally, you can create the tunnel. The format is <code>ssh -N -R :<local port>:localhost:<remote port> <username>@<remote ip></code>

<pre>
$ ssh -N -R :9000:localhost:22 user@example.org
</pre>

== Creating a persistent tunnel ==
Using the command above does not keep the connection alive if the connection is lost. To make it a persistent connection, we can use <code>autossh</code>.

<pre>
sudo apt install autossh
</pre>

or

<pre>
sudo yum install autossh
</pre>

Then, to start it run:
<pre>
autossh -M 20110 -o ServerAliveInterval=20 -R :9000:localhost:22 user@example.org & >/dev/null 2>&1
</pre>

== Resources ==
* [https://linuxhostsupport.com/blog/how-to-setup-reverse-ssh-tunnel-on-linux/ LinuxHostSupport- How to setup a reverse SSH tunnel on Linux ]
* [https://superuser.com/questions/37738/how-to-reliably-keep-an-ssh-tunnel-open superuser - How to reliably keep an SSH tunnel open]