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Notes on reverse SSH tunnel: Connect to servers behind NATs and firewalls through publicly available middle server - Computational Nonsense
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<h1>Notes on reverse SSH tunnel: Connect to servers behind NATs and firewalls through publicly available middle server</h1>
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<span class="date">2018/1/7 2:36 AM</span>
<span>posted in </span>
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<p>In some cases, the servers you use in office/school may be behind some NAT or firewall so that you can not access these servers from the external internet. This can be inconvenient sometimes. A simple walkaround of this problem is reverse SSH tunneling. For all Unix like systems(e.g. Linux, MacOS), this just works fine.</p>
<span id="more"></span><!-- more -->
<h2 id="toc_0">How to establish reverse SSH tunnel</h2>
<p>The whole connection route will look like this:</p>
<p><strong>Destination Server <- |NAT| <- Public Server <- Your PC</strong></p>
<p>In order to establish a reverse SSH tunnel, first you need to forward an unused port(e.g. 19999) from the destination server to the public server:</p>
<pre><code class="language-bash">ssh -R 19999:localhost:22 user@pubic_server
</code></pre>
<p>Then from your PC, SSH to the public server first:</p>
<pre><code class="language-bash">ssh user@public_server
</code></pre>
<p>Then from the public server, do the following to SSH to the destination server:</p>
<pre><code class="language-bash">ssh localhost -p 19999
</code></pre>
<p>Now you have successfully gain access to the destination server from your PC(from external Internet)!</p>
<h2 id="toc_1">Make the reverse SSH tunnel stable</h2>
<p>This tunnel can be unstable for long terms(some times the port forwarding can fail). In order to make it more reliable, we make use of linux system's cron utility.</p>
<p>First we make a configuration for the public server by adding the following to the "~/.ssh/config" file:</p>
<pre><code class="language-bash">Host public_server
ServerAliveInterval 60
ExitOnForwardFailure yes
TCPKeepAlive no
</code></pre>
<p>Then restart sshd service to make the configuration effective:</p>
<pre><code class="language-bash">sudo service sshd restart
</code></pre>
<p>Now make a script file like this(make sure you can SSH to public_server without password using ssh keys):</p>
<pre><code class="language-bash"># reverse_ssh_tunnel.sh
#!/bin/sh
COMMAND="ssh -N -f -R 19999:localhost:22 user@public_server"
pgrep -f -x "$COMMAND" > /dev/null 2>&1 || $COMMAND
</code></pre>
<p>Then add a new crontab by the following command:</p>
<pre><code class="language-bash">crontab -e
</code></pre>
<p>Add the following:</p>
<pre><code class="language-bash">*/1 * * * * /bin/sh [path to your script file]
</code></pre>
<p>This will check if the forwarding is alive or not every 1 minute. If it is not,cron will rerun the SSH command.</p>
<h2 id="toc_2">Using SFTP and etc. through reverse SSH tunnel</h2>
<p>You might want to be able to use SFTP or access the jupyter notebook on the destination server as well. This can be achieved by adding a regular tunnel from any unused port on your PC to port 19999 on public_server:</p>
<pre><code class="language-bash">ssh -L 19998:localhost:19999 user@public_server
</code></pre>
<p>Now you can access the destination server by using the 19998 port on your PC(localhost). Works like a magic!</p>
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