I have some users who have some problems that are specific to their machine. In order to test, I have to open a GUI on their machine as their user, but if I ssh as root (with -X option) to their machine so I can su to them, I can no longer launch Xwindows... I get the message:

X11 connection rejected because of wrong authentication.
Connection lost to X server `localhost:10.0'

I thought that meant I just had to set the DISPLAY env var, but that either isn't the problem, or I don't know the value to set. I was trying my_machine:0.0 as the DISPLAY var - but that gives a new message about Cannot connect to X server on my_machine:0.0" - maybe 0.0 is not the right number?

For the record, I can open X windows after sshing, just not after sshing then suing.

ideas?

Did you try the -Y option instead of the -X one? Since you're "root," the -X restrictions might not apply, but after the su you're no longer "root," eh?

Just a guess - I never use ssh on my one-person laptop.

thanks, but no, -Y does not help at all.

The default display for X-forwarding, as per sshd_config is :10.0, so that part of the error message is correct.

I'm puzzled as to why you don't/can't ssh as the user, rather than as root and then su'ing. I'm thinking that this may be the issue, since from the user's machine's standpoint, the :10.0 display is 'owned' by root, not by the user, thus the authentication issue.

These are just thoughts, and by no means an attempt at expertise....

cheers,

I can't ssh as the user because I don't know their password & don't want to reset it every time an issue like this comes up.

Again, I'm guessing here, but I just tried this:

Opened a terminal window in my current X session.
Did a su - to start a "root" login shell
Did a su -m Peter to go back to my user environment (without a password prompt)
Did a startx -- :1 to open a second X server for the user login.
Checked that I could switch between the :0 and :1 sessions with <ctrl><alt><F7> and <ctrl><alt><F8>
Logged out of the :1 session and cleaned up.

All that worked.

So, try starting a "root" X-server (which you said worked), do your su to the user, and then a startx -- :1 to start a second X-server for the user. (Perhaps you'll need a startx -- 10:1 per the comments above.)

Found this thread:

which has a number of similarities to what you're attempting to accomplish, with some thoughts about merging magic cookies, etc...

cheers,

I use ssh with the -Y option to log in as a normal user.
To use a GUI program with root privileges, I do 'kdesu <program name>'.
This works fine if you have kde installed as kdesu takes care of all the display stuff.