The following methods can be used for resetting the root password if the root password is unknown.

If you use GRUB for booting, select the system to be booted, and add 1 to the end of the kernel boot command. If you're not presented with an edit “box” to add boot parameters, try using GRUB's edit command (the letter e). The 1 tells the kernel to boot to single-user mode.

The system now should boot to a root prompt. At this point, simply use the passwd command to change the root password.

Another option is to boot a rescue CD or an installation CD that lets you get to the command line. Once you're at a command prompt, mount the system's root directory if it's not already mounted:

$ mkdir /mnt/system
$ mount /dev/sda1 /mnt/system

Now, do a chroot and reset the password:

$ chroot /mnt/system
$ passwd