ssh from within GNU screen does not work properly

Question

I try to start a screen session and connect to three different machines with the same user.

Here is what I have in my .screenrc

screen -t "machine1" 0 ssh user@machine1
screen -t "machine2" 1 ssh user@machine2
screen -t "machine3" 2 ssh user@machine3

My ~/.ssh/config contains

ForwardX11 yes

I get the following error in the terminals of machine2 and machine3.

Warning: No xauth data; using fake authentication data for X11 forwarding
/usr/X11/bin/xauth: error in locking authority file /home/user/.Xauthority

If I remove only one of the machines from my .screenrc it works correctly.

How can I avoid this error and connect to multiple machines from screen.

EDIT: The user home dir is on NFS and locking doesn't work properly with NFS. To circumvent this I tried hard to either have the XAuthority file on a local file system (/tmp) or to have one file per ssh. I didn't manage, somehow my xauth seems to completely ignore the XAUTHORITY environment variable. Still puzzeled...

Answer 1

/usr/X11/bin/xauth: error in locking authority file /home/user/.Xauthority

Do you ever get that same error without screen, after the first SSH session has been established? It's odd that your "If I remove only one of the machines [..] it works correctly" seems to say that 2 machines work, but a 3rd yields problems. If ssh supports multiple X-forwarded connections, then it might just be a matter of doing things more slowly? That can best be tested without screen I guess. And what if you change the order of the machines?

It guess that this .Xauthority file is on the remote machine, but I am no expert. (If /home/user in your question was in fact a more specific directory, then you can probably easily tell if it's local or remote.) So, odd question: are you sure that all 3 machines are really different machines, and that no-one else is using the same account? From man ssh:

ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).

(And couldn't you replace screen with ssh -f -N, or are you using the screen's title to be able to stop things?)