What does “Warning: untrusted X11 forwarding setup failed: xauth key data not generated” mean when ssh'ing with -X?

Question

When I use ssh -X on my Mac (running OS X 10.6.7) to connect to my Ubuntu (11.04) box, I get the following warning:

Warning: untrusted X11 forwarding setup failed: xauth key data not generated Warning: No xauth data; using fake authentication data for X11 forwarding.

Is there something I can do to make this warning go away? If not, can I safely ignore it?

X11 forwarding seems to work fine, though I do see this message:

Xlib: extension "RANDR" missing on display "localhost:10.0".

Is that related to the warning? (I'm guessing not. If it's not, I'll file a new question about that.)

Answer 1

Any reason you don't want to use the -Y flag instead of the -X flag?

Quite simply, the difference between -X and -Y is that -Y enables trusted X11 forwarding.

Answer 2

I don't have a setup that can exhibit this behavior, so this is a shot in the dark:

The warning might be suppressed if you set ForwardX11Trusted to "no" for hosts that give this warning. You can place this in either ~/.ssh/config or /etc/ssh/ssh_config, and you can make the option specific to a particular host by including Host <hostname> on the line above. the <hostname> component matches what you type on the command line (not the resolved hostname), and it can include wildcards.