[Feature] MagicDNS base_domain fallback

[SpriteOvO]

Use case

Same issue #1510, but it's closed, so I'm opening a new one. CC @6ixfalls.

• I have a DNS record on Cloudflare ZeroTrust, let's call it via-other-vpn.homelab.lan -> 172.24.1.10;

• I configured this Cloudflare ZeroTrust DNS as nameserver with Unbound in my router;

• I configured Headscale on my server with domain headscale.public-domain.com, which has a public IP, with config:

  server_url: https://headscale.public-domain.com
  dns:
    magic_dns: true
    base_domain: homelab.lan
    override_local_dns: false

• I installed Tailscale in my PC-1 and PC-2 with --accept-dns=true;

So, now I should have the following DNS records:

via-other-vpn.homelab.lan -> 172.24.1.10 (pc -> tailscale -> router -> unbound -> cloudflare)
         pc-1.homelab.lan -> 100.64.0.1  (pc -> tailscale)
         pc-2.homelab.lan -> 100.64.0.2  (pc -> tailscale)
                        * -> *           (pc -> tailscale -> router -> unbound -> cloudflare)

I expected all of this records are resolvable on PC-1 and PC-2.

Actually, via-other-vpn.homelab.lan is no longer resolvable after I started Tailscale.

Description

Contribution

• I can write the design doc for this feature
• I can contribute this feature

How can it be implemented?

Add a option allowing fallback unresolved DNS requests for base_domain to local DNS.