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

And in another terminal:

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

matthijs@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.