XForwarding suddenly stops working

Question

I use Cygwin/X to ssh into a VirtualBox hosted Ubuntu Server which has had the various X bits and pieces installed:

ssh -X user@IPADDRESS

Once in, I can run gnome-terminal and the terminal pops up. Once in there I can open new tabs, run geany (a text editor like GEdit) and so-on, and it all works, the windows open and everything is fine.

Then, at some point, something happens and I am unable to open any new applications that make use of X. Existing applications function fine, but anything new will complain that it can't open the display. eg:

user@SERVER:/mnt/smb$ geany
Geany: cannot open display

I think this starts happening after I have used sudo to execute a command such as:

sudo apt-get install XXXX

After the problem starts, the DISPLAY environment variable is the same as it was before.

The only other piece of information that may be of use is that when I first log in to the machine, the following appears immediately after authentication:

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

Any ideas?

Answer 1

Some ideas:

1. Try to invoke ssh as ssh -Y, which is to say that you trust the machine you are ssh-ing to. This might have as effect only the removal of the first warning, but is worth a try.
2. Ensure that no communication is blocked by a firewall
3. Ensure that the remote server doesn't have the XC-SECURITY extension enabled (SECURITY shows up in the output of xdpyinfo).
4. Use ssh -vvv for more info.