Rescuing a Lost Root Password

Sometimes you wind up taking over a machine for which the root password has been lost. Here are a couple of solutions.

Reboot (hard reboot by pulling the power cable if rebooting requires the root password), and hit 'e' to edit the boot line when you get to the grub menu. Scroll down to the line that starts with kernel, then hit 'e' again to edit it, add 'single' to the end, and hit Enter to accept. Now 'b' to boot and eventually you'll be dumped into a root shell. From here type passwd to change the root password.

Some systems require the root password to boot into 'single' mode. In this case, try editing the grub boot line to add 'init=/bin/bash' to the end of the kernel line. This will boot you into a very basic system, using the bash shell instead of init. You may have to mount the root partition read/write:

mount -no remount,rw /

Then use passwd to reset the password.

If grub is protected, try booting from a LiveCD or USB stick. Open a root shell, and use fdisk -l to show the available disk partitions. Mount the root partition with

mount -o,rw /dev/hda1 /mnt

Check it's the right directory with ls /mnt, then change into that as your root directory:

chroot /mnt

Now use passwd as before.

NOTE: It's important to bear in mind that all of these are also potential security risks if people have physical access to your machines.