x/crypto/ssh: TCP/IP port forwarding expects IP addresses

[tt]

(*ssh.Client).ListenTCP expects an IP address (via *net.TCPAddr) and therefore (*ssh.Client).Listen attempts to resolve addresses.

However, section 7.1 of RFC 4254 states:

The 'address to bind' and 'port number to bind' specify the IP
address (or domain name) and port on which connections for forwarding
are to be accepted. Some strings used for 'address to bind' have
special-case semantics.

• "" means that connections are to be accepted on all protocol
  families supported by the SSH implementation.

• "0.0.0.0" means to listen on all IPv4 addresses.

• "::" means to listen on all IPv6 addresses.

• "localhost" means to listen on all protocol families supported by
  the SSH implementation on loopback addresses only ([RFC3330] and
  [RFC3513]).

• "127.0.0.1" and "::1" indicate listening on the loopback
  interfaces for IPv4 and IPv6, respectively.

There are two consequences of the current interface:

1. You can only provide resolvable names. This prohibits two of the strings with special-case semantics from working ("", reported in #33227, and "::").

2. Resolution happens client side. This changes the meaning of the string "localhost" from being "all protocol families supported by the SSH implementation on loopback addresses only" to being only one of those and may provide a different result for other names (AWS hostnames resolving to internal addresses inside a data center comes to mind).

Outside of defining a new public interface, I think the least breaking change would be to extract an unexported listenTCP function taking a string address and call this from Listen which can then drop resolution but of course if you're relying on that behavior, it will still be surprising.

I'm happy to submit a pull request but I'd appreciate some thoughts on how to best evolve the interface into something that both supports the scope of the RFC and doesn't disregard current users.

[tt]

I should mention that it's of course possible to treat "", "::" and "localhost" separately with even less potential breakage but this doesn't solve the problem of client side resolution generally which was my intention for filing this issue.

[toothrot]

/cc @hanwen @FiloSottile