Thank you. I tried the two methods and restarted my computer, I'm still getting this error message: "Gtk-Wraning **: Cannot open display:". Any other idea?

The xhost command I gave won't survive a reboot, you have to issue it new with every session. In theory, HappyTux's method should survive a reboot but you might want to check root's .bashrc to make sure.

Thanks. I issued 'xhost localhost && sudo gnome-ppp' but I still get the error message...

Thats odd. Try just issuing xhost localhost on its own and post any output/errors you get back. Also, if you use su to become root in a console, can you then run gnome-ppp? I know that isn't the solution you're after, I'm just trying to get a better handle on where this is going off course.

Hello, "xhost localhost" on its own doesn't report any error message. If I become root using "su -" and then launch "gnome-ppp" it's working fine. But not sudo. Any idea? Thanks.

It should report something. At least on my rig when I run it I get a "localhost being added to access control list" to tell me that it is doing something.

Do you still have the xauth entry in your root's .bashrc the way HappyTux suggested? Since the - loads root's environment, that might explain why this works but sudo doesn't.

There are a couple of syntax differences for xhost that also might work. Try these and see if one works for sudo:

xhost +localhost
xhost +local:

Hello, I removed /root/.bashrc, I restarted and I tried again. When I run "xhost localhost" or one of your suggestions, I get: "localhost being added to access control list", but when I run "sudo gnome-ppp", I get the same error message

I found here: http://www.gnomefiles.org/comment.php?soft_id=41 that you can use "PAM" (6th comment). I tried it but it didn't work either

Removing .bashrc was certainly not needed. All you had to do was comment out the line you added. I also notice you do a lot of restarting. In general Linux doesn't need to do that and in this particular case it may be causing trouble. I know that restarting is the first thing to try in Windows when things don't work, but this isn't Windows.

Lets check on something. Try running (as your normal user) echo $DISPLAY. It should return :0.0, but if it doesn't it might be a clue.

Also have a read through this thread, particularly the last couple of posts. They seem to have solved the same problem you're having.

Thanks! Your link solved finally my problem, I just had to add to /etc/sudoers these 2 lines:

Damn, Linux is so difficult, there's so much stuff to learn and so little free time to dedicate to it

Great, I'm glad we finally figured it out.

To be honest, I don't know why this was so difficult. 99.9% of the time the first advice that HappyTux and I gave you works.