Add Customization Options to DERP Map entry of integrated DERP server

[alexhalbi]

This Pull request adds a draft for the possibility to add the IPv4 and IPv6 address to the DERP map for the integrated DERP server.
This enables the DERP server to be reachable by clients, if they are using an Exit-Node via the DERP server and loose DNS connection.

This happens for example when restarting Headscale and the Exit-Node on the same time. It is also possible that this happens if Headscale is restarted and one of the nodes, does not reconnect successfully to the integrated DERP server and tries to communicate DNS through it.

If you take a look into the default derp map, all of them have their IPs in the list, the integrated DERP server does never have it's IP in the list.
See https://pkg.go.dev/tailscale.com/tailcfg#DERPNode for reference.

Furthermore this request adds the possibility to disable the automatic creation of the integrated DERP map entry completely and adds the possibility to use the integrated DERP server with a completely custom etry in the derp.paths.

This enables even more customization, by fully leveraging the official DERPRegion and DERPNode configuration file, which is currently overwriten.
This should also implement #1514.

https://tailscale.com/kb/1118/custom-derp-servers/#step-2-adding-derp-servers-to-your-tailnet

• read the CONTRIBUTING guidelines
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

[alexhalbi]

Hi @kradalby,
Please have a look, if the new variant including the changes in 46d2890 could be merged.
Thanks,
Alex

[kradalby]

@alexhalbi could you rebase and try again fixing the lint issues? The tests should be retrying a bit now and should pass.

[kradalby]

oh and a changelog entry.

[alexhalbi]

@alexhalbi could you rebase and try again fixing the lint issues? The tests should be retrying a bit now and should pass.
The changes I made exclusively impact the following files: config.go, config-example.yaml, derp_server.go, and app.go. Despite running the "make lint" command and encountering numerous errors, it's important to note that none of these errors pertain to the modifications I made. Therefore, please ignore the lint job failures.

I fixed lint errors in config-example.yaml which I created

oh and a changelog entry.

I added an entry, I hope it is okay for you.

[kradalby]

@alexhalbi Looks good now, I rewrote some of the git history as it ended up with some massive commits in the middle proposing this as a 4000 LOC change.

Please double check that I didnt miss anything in the rebasing.

[kradalby]

actually, can you move this under 0.23.0?

[alexhalbi]

Hi,
I moved the changelog entry and we checked everything. It should be all in the branch thanks.