openssh bugs - Sep 2017 - [Bug 2780] New: Agent fowarding on slave silently fails when not enabled on master

https://bugzilla.mindrot.org/show_bug.cgi?id=2780

            Bug ID: 2780
           Summary: Agent fowarding on slave silently fails when not
                    enabled on master
           Product: Portable OpenSSH
           Version: 7.5p1
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: minor
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: matthijs at stdin.nl

When a ControlMaster is started with -A, then passing -A to subsequent
slave connections works. But when the master connection does not have
forwarding, passing -A to a slave connection silently fails.

matthijs at grubby:~$ ssh tika-login -o ControlMaster=yes -o
ControlPath=/tmp/foo

And in another terminal:

matthijs at grubby:~$ ssh -o ControlPath=/tmp/foo tika-login -A
matthijs at login:~$ echo $SSH_AUTH_SOCK

matthijs at login:~$


This is somewhat documented in ssh_config:
> X11 and ssh-agent(1) forwarding is supported over these multiplexed
> connections, however the display and agent forwarded will be the one
> belonging to the master connection i.e. it is not possible to forward
> multiple displays or agents.

However, that's easy to miss and I was quite surprised at this
behaviour. It would be useful if the slave SSH connection would print a
warning (or, I guess when ExitOnForwardFailure is set, bail out) when
-A
is passed when no forwarding is active on the master.

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