To a first approximation, Tailscale/Headscale don’t route and traffic.
Ah, well damn. Is there a way to achieve this while using Tailscale as well, or is that even recommended?
Is there a way to achieve what? Force tailscale to route all traffic through the DERP servers? I don’t know, and I don’t know why you’d want to. When my laptop is at home on the same network as my file-server, I certainly don’t want tailscale sending filserver traffic out to my Headscale server on the Internet just to download it back to my laptop on the same network it came from. I want NAT traversal to allow my laptop and file-server to negotiate the most efficient network path that works for them… whether that’s within my home lab when I’m there, across the internet when I’m traveling, or routing through the DERP server when no other option works.
OpenVPN or vanilla Wireguard are commonly setup with simple hub-and-spoke routing topologies that send all VPN traffic through “the VPN server”, but this is generally slower path than a direct connection. It might be imperceptibly slower over the Internet, but it will be MUCH slower than the local network unless you do some split-dns shenanigans to special-case the local-network scenario. With Tailscale, it all more or less works the same wherever you are which is a big benefit. Of course excepting if you have a true multigigabit network at home and the encryption overhead slows you down… Wireguard is pretty fast though and not a problematic throughout limiter for the vast majority of cases.
Tailscale just partnered with Mullvad so this works out of the box for that setup: https://tailscale.com/blog/mullvad-integration/
For others, it’s a “yes on paper” situation. It will probably often not work out of the box, but it seems likely to be possible as an advanced configuration. At the end of the line of possibilities, it would definitely be possible to set up a couple of docker containers as one-armed routers, one with your VPN and one with Tailscale as an exit node. Then they can each have their own networking stack and you can set up your own routes and DNS delegating only the necessary bits to each one. That’s a pretty advanced setup and you may not have the knowhow for it, but it demonstrates what’s possible.