Greetings, fellow technophile. Let’s take a moment to appreciate the marvel that is Tailscale. In the grand tradition of Unix philosophy, it does one thing and does it well: it establishes point-to-point WireGuard tunnels with a level of finesse in UDP hole punching that is truly commendable.

However, as we all know, no software is without its quirks. Tailscale has been grappling with a particular issue since 2021.

In essence, Tailscale attempts to establish connections using all available network interfaces on your device. While this might seem like a good idea on the surface, it can lead to some unintended consequences. For instance, if you’re like me (and it's likely you are, since you've landed here), you might already have a VPN interface (in my case, OpenVPN) connecting you to devices that you also want to interconnect with Tailscale. Unfortunately, Tailscale sometimes routes traffic through the VPN interface instead of the open internet, which can hamper speed optimization.

Fear not, for this project offers a solution as elegant as it is simple. It provides two systemd units: tailgate-allow@ and tailgate-deny@. These allow you to operate in either whitelist or blacklist mode, depending on your preference. With these units, you can control Tailscale’s routing behavior, either preventing it from routing traffic through certain network interfaces on your device, or forcing it to use only the interfaces you select, discarding all others.

The power, as they say, is in your hands. Happy hacking!

The installation process for this project is refreshingly straightforward. All you need to do is copy one or both of the systemd units to the /etc/systemd/system directory on your Linux device. Here’s how you can do it:

sudo cp [email protected] /etc/systemd/system/
sudo cp [email protected] /etc/systemd/system/

Once you’ve done that, you’ll need to reload the systemd daemon to ensure it recognizes the new units. You can do this with the following command:

sudo systemctl daemon-reload

And that’s all there is to it!

In the realm of network routing, precision is key. It’s crucial to understand that using both the tailgate-allow@ and tailgate-deny@ units simultaneously can lead to unexpected behavior. This might seem obvious, as no program in the world operates with both whitelists and blacklists at the same time, but it bears repeating.
It’s like trying to use a joystick and a paddle at the same time on your Atari - it just doesn’t work. So, pick one.

Personally, I’d go with the whitelist mode. Why, you ask? Well, most devices have a limited number of interfaces that connect to the internet (ethernet and WiFi, at most), while the number of virtual interfaces can change faster than you can say “Moore’s Law”.

Now, these systemd units are not your run-of-the-mill units. They don’t have an [Install] section and they’re not meant to boot with your system. They’re special. They’re designed to be linked to the tailscaled service. So, instead of fiddling with the default unit provided by Tailscale, you can just run:

sudo systemctl edit tailscaled.service

Then, add your chosen network interfaces to the Wants= option (and don't forget to add the default network-pre.target), like so:

[Unit]
Wants=network-pre.target [email protected] [email protected]

Once you’ve saved your changes, a new file will be created at /etc/systemd/system/tailscaled.service.d/override.conf

Don't forget to reload the systemd daemon:

sudo systemctl daemon-reload

Now, whenever tailscaled starts, stops, or restarts, our systemd units will follow suit, controlling Tailscale traffic through the desired network interfaces. No further user interaction required. It’s as simple as PEEK and POKE in BASIC!