Hello,
unfortunately for some reason the sudo command doesn't work anymore on my ubuntu (7.04) machine. It's not displaying any output no matter which command I try to run. Also, the root account which I enabled a couple of weeks ago doesn't let me log in. It just says "Authentication failure". I tried resetting it from the Live-CD - no change. And finally, when I try to change some systems settings from the gui menus, after I enter my password it first says "Wrong password" and then after the second try "the communication with su failed" (or rather the german equivalent to that).
So I pretty much haven't got any control over my system right now which is rather annoying.
Any ideas?

Cheers,
Chris

You can reset the root password by booting with a rescue cd then mounting the root partition via chroot; then run the passwd command.

I had the same problem one time and never did find a fix for it. I did a re install of Ubuntu.

For sudo to work on Ubuntu:
1) make sure your user belongs to the 'sudo' group (cat /etc/group)
2) make sure your user is configured correctly in /etc/sudoers Sometimes it's configured so that users in a specific group, like wheel, are entitled to sudo and therefore you would have to add yourself there (I don't think this is how it goes on Ubuntu). Belonging to group 'sudo' entitles to use the command, but unless sudoers is configured, there are no permissions to run anything with it.

After those two I can't think of much else. Root password is an odd thing. I'd search for rootkits or something, just in case, to make sure the system is not compromised. Ubuntu is not too well secured after installation, unless it's hardened a bit - any other distribution isn't either, for that matter.

To reset root password:
1) reboot your computer, and when GRUB loads, get to the menu (usually pressing ESC is what you're asked to do, to display GRUB menu, unless it isn't by default)
2) select (with arrow keys) the kernel you usually boot, if it's not selected (highlighted)
3) Press button 'e' to edit commands before booting, select the 'kernel' line and press 'e' again (information at the bottom tells this)
4) add the word 'single' to the kernel line; this modification tells to boot into single-user mode (maintenance)
5) boot that kernel by pressing 'b'
6) once booted, you should be logged in as root (THIS is why you should set a bootloader password, get it?)
7) run 'passwd' and set some password for root
8) reboot your machine the usual way

thanks b0uncer, I've solved the problem now. Somehow I had managed to kick myself out of the "admin" group, so I couldn't sudo anymore.