[Feature] Support multiple/more specific bind addresses in listen_addr

[leegenthmi]

Use case

Allow dual-stack/multi-IP address server deployment on machines with multiple IP addresses where some of those addresses already have port 443 bound (without the use of a reverse proxy).

Description

Hello, I have a machine with one single public IPv4 address and many public/global IPv6 addreses. I have various other services listening on the various IPv6 addresses on ports 443.

I would like to bind Headscale to one single specific (well, the only) IPv4 address and one single specific IPv6 address. Currently we can use 0.0.0.0:443 which binds to all addresses (and therefore fails because 443 is in use on some irrelevant IPv6 address) or [x:y:z::]:443 which binds to a specific IPv6 address but no IPv4 address. listen_addr: may not be defined multiple times with differing values.

Allowing the software to be able to parse an array of listen-addr:s would enable easier deployment on multi-IP hosts.

Contribution

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

How can it be implemented?

Modify the config file parser to allow an array of listen_addrs to be to specified and bind to each in turn.