all: flesh out, fix some bugs in plan9 port

[bradfitz]

See https://tailscale.com/plan9

Updates #5794

[review-ai-agent]

Pull Request Revisions

Revision	Description
r3	Tailscale now supports Plan 9 This comprehensive changeset introduces Plan 9-specific implementations for various Tailscale components, including routing, DNS management, SSH, networking, and userspace support. Key additions include Plan 9-specific handlers for network interfaces, system interactions, and OS-specific configurations.
r2	Go module dependency configuration updated Removed direct dependency on 9fans.net/go and changed it to an indirect requirement in go.mod
r1	Plan 9 platform support added Comprehensive changes to add Plan 9 operating system support across multiple Tailscale components, including new Plan 9-specific implementations for SSH, routing, DNS management, and various system interactions.

✅ AI review completed for r3

[funkaoshi]

Amazing.

[Uramekus]

Hey brad, as a plan9 user still and a extensive Tailscale user, i'm actually so glad this works!!

[fmarier]

Should it be added to https://tailscale.com/install.sh?

[JJJollyjim]

@fmarier plan 9 doesn't use posix-compatible sh, but I guess an sh+rc polyglot might be possible…

[JJJollyjim]

Am I right in understanding that the LocalAPI is completely unauth+world-accessible? E.g. it's not listening on localhost in a special namespaced /net/tcp but the main systemwide one?

[bradfitz]

Am I right in understanding that the LocalAPI is completely unauth+world-accessible? E.g. it's not listening on localhost in a special namespaced /net/tcp but the main systemwide one?

"world" accessible as in localhost, correct.

That's covered a bit more in tomorrow's blog post. It was a short term hack for development since the old implementation didn't work.