[Bug 3907] New: Control proxy mode doesn't support TCP or Unix domain socket forwarding

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=3907

            Bug ID: 3907
           Summary: Control proxy mode doesn't support TCP or Unix domain
                    socket forwarding
           Product: Portable OpenSSH
           Version: -current
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: ding at diinngg.com

This is a follow-up to this mailing list thread:
https://marc.info/?t=176634596800001

After connecting a client to a control socket in proxy mode both TCP
and Unix domain socket forwarding do not work.

TCP forwarding looks to be partially implemented, but the control
master doesn't forward the response to the "tcpip-forward" global
request downstream (meaning using port 0 doesn't work), and
"cancel-tcpip-forward" isn't implemented. When running

 ssh -oControlPath=<...> -Oproxy -R<port>:127.0.0.1:<port> -N <host>

the forwarding works correctly, but

 ssh -oControlPath=<...> -Oproxy -R0:127.0.0.1:<port> -N <host>

does not show the assigned remote port in the log. Finding the port
manually on the server and writing to it results in the control master
process logging

 WARNING: Server requests forwarding for unknown listen_port <...>

Unix domain socket forwarding doesn't look to be implemented at all.
Running

 ssh -oControlPath=<...> -Oproxy -R<path>:127.0.0.1:<port> -N <host>

results in the control master process logging

 channel_proxy_downstream: unsupported request
streamlocal-forward at openssh.com

The comment at
https://github.com/openssh/openssh-portable/blob/b652322cdc5e94f059b37a8fb87e44ccb1cdff33/channels.c#L3169
 seems to suggest that both should work, and it would be useful if they
did.

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