PROXY protocol support

[stevapple]

PROXY protocol, originally developed by HAProxy, extends the TCP protocol to explicitly preserve source (and destination) IPs when it's proxied.

This PR adds bidirectional PROXY protocol support to sshmux, opt-in by the API server and configuration file.

[stevapple]

This PR is almost finished and ready for review. It was marked as draft because I may want to discuss how extra functionalities may be integrated into upstream sshmux.

[iBug]

I prefer having proxy_protocol as a boolean flag so that both modes (non-PROXY and PROXY) can be managed separately by starting two processes. This should also eliminate the need to start two servers in one process.

[stevapple]

Since proxyproto.Listener can accept connections both with and without PROXY protocol headers, another solution might be that we let the API server decide whether the connection should use PROXY protocol.

In this way only one server instance is required, and users can mux standard and proxied downstream with custom strategies.

[iBug]

we let the API server decide whether

This is a much better idea. I'd love to see it implemented this way.

[stevapple]

Note that this PR doesn't have a clean commit history so a squash merge should be preferred.

[stevapple]

@iBug @taoky IMHO 2ce97ce provides a much nicer interface without all the messes caused by running multiple TCP listeners and with fine-grained control over PROXY protocol. Check it out if you have time.

[iBug]

IMHO 2ce97ce provides a much nicer interface

Yep, this is certainly better. At least it no longer runs two TCP listeners in one process. Bring it in if you'd like.

[taoky]

I would suggest add a simple documentation (README?) for current config and HTTP API. It looks like a bit more complex than before.

[stevapple]

I would suggest add a simple documentation (README?) for current config and HTTP API. It looks like a bit more complex than before.

Maybe in a separate PR? Changes in this PR is fully opt-in, and existing users don't need to change anything after upgrading.

[stevapple]

@taoky Is it okay to merge in the current state? I'm working on gardening the whole package (including splitting helpers into different files) which should be suitable of adding documentations.