Autossh is a Linux tool used for automatically and persistently maintaining SSH tunnels. SSH tunnels are a secure way of transmitting data between two computers over an unsecured network. By default, an SSH connection may drop or disconnect due to various reasons, such as network instability or server issues. Autossh monitors the SSH connection and automatically reestablishes it if it gets disconnected or dropped. This ensures that the SSH tunnel remains operational and your data transmission remains secure. Autossh is easy to install and use, and it can be configured to automatically start and stop the SSH tunnel, as well as restart it if it crashes.

Install Autossh

Autossh is available in most Linux distributions’ package repositories. To install it, open a terminal and run the following command:

sudo apt-get install autossh 

If you are using a different package manager, you can search for and install the “autossh” package using the appropriate command.

Setting up SSH tunnel

Before we can use Autossh, we need to set up an SSH tunnel that it can manage. In this example, we will create a tunnel that forwards local port 8080 to port 80 on a remote server.

To create the tunnel, run the following command in a terminal:

ssh -L 8080:localhost:80 remote-user@remote-server 

This command will establish an SSH connection to the remote server and create a tunnel that forwards traffic from port 8080 on your local machine to port 80 on the remote server.

Once the SSH tunnel is established, you can test it by opening a web browser and navigating to http://localhost:8080. If everything is set up correctly, you should see the default web page for the remote server.

Use Autossh to manage the SSH tunnel

Now that we have an SSH tunnel set up and working, we can use Autossh to automatically restart the tunnel if it is disconnected or interrupted.

To use Autossh, we need to modify the SSH command used to create the tunnel. Here’s an example:

autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -N -f -L 8080:localhost:80 remote-user@remote-server 

This command is similar to the SSH command we used to create the tunnel earlier, but it includes some additional options that tell Autossh to manage the tunnel.

• The -M 0 option tells Autossh to use a built-in monitoring port to detect if the SSH tunnel has disconnected.
• The -o "ServerAliveInterval 30" and -o "ServerAliveCountMax 3" options tell Autossh to send keepalive packets every 30 seconds, and to attempt to reconnect if three consecutive keepalive packets fail.
• The -N -f options tell SSH to create the tunnel in the background without executing any remote commands.

Test Autossh

To test Autossh, disconnect your internet connection or manually stop the SSH tunnel by pressing `CTRL + C`. Autossh should detect that the tunnel has been disconnected and automatically restart it.

You can verify that the tunnel has been restarted by navigating to http://localhost:8080 in your web browser.