12/15/2009 -- Imagine the unthinkable has happened: You can't obtain root access on one of your Linux servers!

You scratch your head and wonder whether it was changed by the junior administrator on the day you fired him, if it's a value you mistyped (twice) when last changing it, or if there's simply some sort of corruption keeping it from successfully logging in.

Regardless of the cause, this is a situation that must be rectified immediately; it's necessary to know the root password to perform many system administration tasks.

But keep calm. While your worry is justified, the problem can be solved fairly easily by rebooting the PC and resetting the root password by following these steps:

1. Reboot the PC (select Reboot as you log out of the GUI screen) or power up as usual. Soon, you'll see the graphical GRUB boot loader screen that shows the names of the operating systems you can boot. If your system runs the LILO boot loader, press Ctrl+X and at the boot prompt, type "linux single" and press Enter. Then proceed to Step 4.

2.  If you have more than one operating system installed, use the arrow key to select Linux as your operating system. Then press the A key. GRUB prompts you for commands to add to its default boot command.

3. Press the spacebar, type the following, and press Enter:

single

sh-3.00#

4. Type the passwd command to change the root password as follows:

sh-3.00# passwd
Changing password for user root.
New password:

5. Type the new root password that you want to use (it doesn't appear on-screen) and then press Enter. Linux asks for the password again, like this:

Retype new password:

6. Type the password again and press Enter. If you enter the same password both times, the passwd command changes the root password.

7. Now type "reboot" to reboot the PC.

After Linux starts, it displays the familiar log-in screen. Now you can log in as root with the new password.

NOTE: If you're running SUSE Linux, then at Step 3, type "single init=/bin/sh" (instead of "single") and, before proceeding to Step 4, type "mount / -n -0 remount,rw." After that, perform steps 4 through 6 to change the root password. After changing the password, type "mount / -n -o remount,ro." Then continue to Step 7 and reboot the PC.

As an aside, make sure that your Linux PC is physicallysecure. As these steps show, anyone who can physically access your Linux PC can simply reboot, set a new root password and do whatever he or she wants with the system.

Another way to protect against resetting the password is to set a GRUB password, which causes GRUB to require a valid password before it boots Linux (of course, you must then remember to enter the GRUB password every time you boot your system).

A great How-to on setting a GRUB password can be found on the Ubuntu site here.