Automatically Run Script on tun0 Interface Up/Down Events

In this article, we will explore how to automatically run a script when the tun0 interface goes up or down. This can be particularly useful in situations where you need to perform certain actions or configurations whenever your VPN connection is established or disconnected.

Understanding tun0 Interface

The tun0 interface is a virtual network interface that is often used by VPN services. When you establish a VPN connection, a tun0 interface is typically created and all your network traffic is routed through this interface.

Running Scripts on Interface Up/Down Events

Ubuntu and many other Linux distributions provide a mechanism to automatically run scripts when a network interface goes up or down. This is achieved through the /etc/network/if-up.d/ and /etc/network/if-down.d/ directories. Any executable script placed in these directories will be run whenever a network interface goes up or down, respectively.

Creating the Script

Let’s create a script that will run when the tun0 interface goes up. Open a terminal and run the following command to create and edit a new script file:

sudo nano /etc/network/if-up.d/tun-up

In this command, sudo is used to run the command with root privileges, nano is a text editor, and /etc/network/if-up.d/tun-up is the path and name of the new script file.

Inside the script, we can check if the interface that went up is tun0 by examining the value of the IFACE environment variable. Here’s an example script that logs the tun0 interface going up:

#!/bin/sh

if [ "$IFACE" = tun0 ]; then
 echo "tun0 up" >> /var/log/tun-up.log
fi

In this script, #!/bin/sh is the shebang line that specifies the script interpreter, if [ "$IFACE" = tun0 ]; then is a conditional statement that checks if the IFACE environment variable equals tun0, and echo "tun0 up" >> /var/log/tun-up.log is a command that appends the message “tun0 up” to the /var/log/tun-up.log file.

Making the Script Executable

After creating the script, we need to make it executable. This can be done with the chmod command:

sudo chmod +x /etc/network/if-up.d/tun-up

In this command, chmod +x is used to make the file executable, and sudo is used to run the command with root privileges.

Testing the Script

Now, whenever the tun0 interface goes up, the script will be invoked, and the message “tun0 up” will be appended to the /var/log/tun-up.log file.

To test if the script is working, you can manually bring up the tun0 interface using the ifup command:

sudo ifup tun0

In this command, ifup is used to bring up the network interface, and sudo is used to run the command with root privileges.

After running this command, you can check the /var/log/tun-up.log file to see if the “tun0 up” message is recorded.

Running Scripts on Interface Down Events

Similarly, you can create a script in the /etc/network/if-down.d/ directory to run when the tun0 interface goes down. The process is the same as for the up script, just replace if-up.d with if-down.d in the commands.

Conclusion

In this article, we explored how to automatically run scripts when the tun0 interface goes up or down. This can be a powerful tool for automating network configurations and actions based on your VPN connection status. However, please note that the specific steps may vary depending on your Linux distribution and configuration. Always make sure to adapt the instructions accordingly and test your scripts thoroughly.