vnet: Introduce interface for vnet.Net

[stv0g]

Summary

I propose to add an interface for vnet.Net which dependent packages of vnet.Net should use (e.g. pion/ice in my use case).

Motivation

cunicu is user-space daemon to establish Wireguard peer-to-peer connection via ICE.

cunicu needs to provide custom implementations of net.{Dial,Listen}{UDP,Packet} in order to attach eBPF filters to the created sockets which filter STUN and let all other traffic pass to a kernel tunnel interface.

See: https://github.com/stv0g/cunicu/blob/master/pkg/proxy/ebpf.go#L43

This allows cunicu to limit itself to handling ICE and stay out of the data path which remains fully in the kernel.

By introducing an interface for vnet.Net users could provide custom implementations for most of the functions in the net package.

Describe alternatives you've considered

We are currently providing a custom implementation of ice.UDPMux which is already an interface to do the same.

Additional context

A ice.UDPMux is currently only used for host candidates. We cant use this method for relay or reflexive candidates.
See: pion/ice#400

Also:

• Share one UDP socket with all UDP candidates ice#76
• vnet: Support TCP #27

[Sean-Der]

Yes this would great! I have seen other use cases that would be unblocked by this also.

[stv0g]

Also related: #27

[stv0g]

@Sean-Der Thats great to hear :)

Ideally, I would name that interface identically to the currently existing struct.

However this would break the existing API and hence requires a new major version.
Would this be acceptable?