It's awkward that it has been partially implemented for 7 months.

Well, it was "partially implemented" because we needed the part we implemented (and now use) for the LetsEncrypt support.

Exposing more of it for this has a ton of product & security considerations and interacts often surprisingly with various DNS stacks on different platforms, which will increase our support burden if we ship something that seems to work sometimes but not on all platforms in all configs.

Example of problems:

if you add a foo.net record, we need to make sure your OS queries the 100.100.100.100 resolver for that record. But some OSes only let us route *.foo.net to us, which means what do we do with bar.foo.net? We'd have to forward that on upstream, but some OSes don't make it easy or possible for us to figure out what the system's underlying/native DNS servers are. So then we'd require you specify an upstream DNS server in your Tailnet config, which we're trying to move away from so we can enable MagicDNS on by default.
if you add a foo.tailnet-name.ts.net record, what if foo already exists as a machine name on the tailnet? what if foo comes into existence later? does that reserve foo for you? do we permit ACME challenges for foo? what if its IP is changed later? which node can then do the ACME challenge?
if you add a record pointing at the IP of a node and share that node, does the extra record get shared to other tailnets? (not for now, but that's a possible use case people might want later)

We're leaning towards shipping this soon but with a bunch of limitations to start like:

no *.ts.net
no *.tailscale.net
must have DNS servers set with force override (unfortunate for now, but makes things consistent)

Then we probably won't box ourselves into a corner and have to remove functionality later and can add more of this when other parts are ready.

The real problem here is that this feature means a number of things to different people and we have to make sure we don't try to do all of it at once, poorly.

FR: Support custom records in MagicDNS #1543

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