Restore support for "Override local DNS"

[nblock]

Tailscale allows to override the local DNS settings of a node via "Override local DNS" [1]. Restore this flag with the same config setting name dns.override_local_dns but disable it by default to align it with Tailscale's default behaviour.

Tested with Tailscale 1.80.2 and systemd-resolved on Debian 12.

With dns.override_local_dns: false:

Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: -DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa [snip]

With dns.override_local_dns: true:

Link 12 (tailscale0)
Current Scopes: DNS
     Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 100.100.100.100
    DNS Domain: tn.example.com ~.

[1] https://tailscale.com/kb/1054/dns#override-local-dns

Fixes: #2256

• have read the CONTRIBUTING.md file
• 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

[ghost]

Pull Request Revisions

Revision	Description
r2	Added DNS configuration test cases Expanded test cases for DNS configuration in config_test.go, adding new YAML test files and additional test scenarios for DNS override and configuration handling
r1	Restore DNS override local settings Added override_local_dns configuration option to control DNS resolver behavior, with validation and default settings

✅ AI review completed for r2

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[SuperSandro2000]

I think https://github.com/juanfont/headscale/pull/2438/files#r2046953908 is correct. I also just ran into this and needed to turn local dns override off.

[fallingcats]

This being on by default now immediately breaks old configs that didn't specify the option and don't have any global nameserver set. Was that intentional?

[nblock]

This being on by default now immediately breaks old configs that didn't specify the option and don't have any global nameserver set. Was that intentional?

This was enabled (and enforced) for 0.23.x, 0.24.x and 0.25.x and an upgrade without adjusting the configuration will still have this option enabled. But using "Override local DNS" without specifying global nameservers is an error which is now checked for.

[z3ntu]

To add my 2 cents, I think this has also broken the config for me, on Linux I couldn't resolve the hostnames anymore that are set up using extra_records. I believe for some reason the Android client still was able to resolve it, but maybe that was just a DNS cache, I didn't check much more.
With the headscale config updated to override_local_dns: true and restarting everything appears to make everything work again as expected.