Implement streamlocal-forward for remote => local UDS forwarding

[kanpov]

I left a // NEED HELP comment on places where I didn't fully figure out what to do, so I'd really appreciate it if some maintainers helped me out in those places.

[kanpov]

I seem to have implemented most if not all of the needed behavior (but not tests yet), but this needs thorough review by people who know the library better than me.

[kanpov]

@Eugeny would really appreciate it if you took a look

[kanpov]

I'm pretty sure I handled the reserved field correctly, but not completely sure

[Eugeny]

I think you can go ahead and try it out :) looks mostly fine so far

[kanpov]

I think you can go ahead and try it out :) looks mostly fine so far

I'll try by including the crate by path, hope it works

[kanpov]

Update: it doesn't work :( RequestDenied, gonna try to investigate. Definitely my fault since ssh -L forwards just fine

[Eugeny]

streamlocal-forward is -R :)

[kanpov]

Well -L also works for some reason

[kanpov]

@Eugeny Makes no sense, where's the issue here?

Impl:

            push_packet!(enc.write, {
                enc.write.push(msg::GLOBAL_REQUEST);
                enc.write
                    .extend_ssh_string(b"[email protected]");
                enc.write.push(want_reply as u8);
                enc.write.extend_ssh_string(socket_path.as_bytes());
            });

Spec:

	byte		SSH2_MSG_GLOBAL_REQUEST
	string		"[email protected]"
	boolean		TRUE
	string		socket path

[kanpov]

@Eugeny While testing this, I also tested tcp/ip forwarding with handle.tcpip_forward and even that didn't work, even though a port was returned that port didn't have any connectivity.

[kanpov]

So it seems forwarding generally is completely broken.

[kanpov]

@Eugeny Considering forwarding is broken, I'm pretty sure making it work is not as simple as just sending global messages to enable/cancel forwarding over SSH.

My brain is too smooth to understand openssh's code, but I suppose what they're doing is not just making these global requests (which russh is capable of doing correctly, with some caveats), but also actually binding to these sockets itself and proxying all traffic over.

Unfortunately, there is little reference aside from openssh C code (which I'll still try to dig into sometime) about how exactly this is done, since openssh is one of the only implementations of ssh that actually support forwarding.

[kanpov]

@Eugeny Have you tested whether this actually works after your fixes? Sorry for abandoning the PR

[Eugeny]

Fixed a few issues and now it works:

• Removed duplicate implementation for non-standard client-to-server forwarding channels
• Added the missing @openssh.com in channel types and responses
• The server doesn't return a socket path in the response

[Eugeny]

No worries! It works with simple client & server examples - but I just tested it briefly