macOS: tailscale serve proxy not connectable from host

[KevinLiang10]

What is the issue?

The tailscale serve proxy is no longer connectable from the host.

Steps to reproduce

On latest MacOS build, run tailscale serve --bg text:test_serve and curl the output url, would just hang.

Are there any recent changes that introduced the issue?

#17823 is the first broken commit

OS

macOS

OS version

No response

Tailscale version

1.93.82 (not the most accurate, that would be for the commit, but is the one I tested on)

Other software

No response

Bug report

No response

[barnstar]

netmon's timing isn't compatible with Serve which bootstraps before netmon's initial state (and therefore, the tailscale interface name/index tuple) is set/available. There's a similar issue in the peerAPI listener setup where we'll bind to interface index - which is incorrect - though it will retry and self-heal on interface changes there so it likely went unnoticed.

The proper fix here is to decouple the tailscale interface details from netmon's state and store it in its own static structure. This property was only used only on sandboxed darwin clients to properly bind the serve and peerAPI listeners and dialers to the tunnel interface. Moving this out of netmon means it can also be used to simplify tailscale interface lookups in netns in Windows.