I'm wondering whether there's a way to let users run X apps with "su - <user>" (or even "su <user>") after connecting via ssh.

If I ssh into a network machine (RHEL4, OpenSSH 3.9p1, OpenSSL 0.9.7a) as root, and I can run X apps. I did an "xhost +" as root to authorize users. Now, if I ssh directly to the machine as the user, I can display X. But if I ssh as root and then "su" to the user (which is what I would like to do) and try an X app, it returns:
X11 connection rejected because of wrong authentication.

I believe this has to do with ssh X-forwarding. I wonder whether it's possible to enable root to switch to this user and to run X apps (despite the security implications)?

hey,
Try to do xhost+ from that user or search in the xorg.conf may u can find something has to do with ur case.

Thanks. Tried that as the user (su - <user>), and I get:
$ xhost +
X11 connection rejected because of wrong authentication.
X connection to localhost:10.0 broken (explicit kill or server shutdown).

Try passing the -Y option to your SSH command instead of -X. This is slightly less secure, but slightly more convenient. YMMV. I use it all the time between computers I control.

I thought that might help, but passing -Y as root, su - <user>, and then xhost + gives the same result as before, except the display shows a different number (localhost:12.0 instead of localhost:10.0).