Optional random node suffixes.

[shanna]

The random node suffixes introduced in 0.16.0 mean machines onboarded with preauth keys end up on random MagicDNS hostnames. My use case is I generate preauth keys and unique hostnames for devices during provisioning programatically and I'd like for my software to be able to resolve the hostnames later within the namespace's subdomain.

I've got a branch here that implements the change though I thought I better create a ticket first before submitting a PR since this may generate some discussion.

• I think suffixes should be off by default but YMMV. It does revert the new default prefixes introduced in 0.16.0 and I have no idea what Tailscale does by default. 🤷
• The other ways this could be solved is by setting a node name in advance when you create a preauth key via the API or by honouring the --hostname tailscale cli flag? No idea how difficult these options would be to implement but I'm happy to have a crack if you'd rather not introduce another configuration variable.

[kradalby]

This change was introduced as we had behaviour that would put the MagicDNS in a conflicting state, two or more machines in could try to have the same DNS, while it might not be the optimal solution, it is needed to be implemented over a broken configuration.

It isn't sufficient to have an option to revert the behaviour as it will allow users to end up in a state with unexpected and indeterministic behaviour. The core argument for this is support load, and potential other weirdness.

I think improving the behaviour would be the way to go.
My initial thought would be that when ever a machine is registered/updated/renamed, the database is checked for the machine + namespace combination to verify it is unique, if it is unique, the suffix is not applied, effectively restoring the behaviour in most of the cases.

[shanna]

OK. I've created a new PR that only applies a suffix if the given name collides with another machine in the same namespace.