Hack 66 Reset Linux Passwords

Use Knoppix to reset forgotten Linux passwords.

If you have forgotten a user's password under Linux and you have root access, you don't usually need a rescue disk because you can reset the password with the root account. Even if you have forgotten the root password, most Linux distributions let you log in to single-user mode (add the single or init=1 argument to the kernel at the boot prompt) and change the password from the root account.

A paranoid system administrator might password-protect or disable single-user mode altogether in /etc/inittab, but you can still get to a root prompt to reset the password if you pass init=/bin/sh to the kernel when you boot. However, if the boot loader itself is password-protected and you forgot the password, you must have some sort of rescue disk to reset the password both in the boot loader (reference [Hack #2] or [Hack #53] for steps to reconfigure the boot loader) and in the /etc/passwd file.

It is simple to reset a password in Linux, and to reset a password with Knoppix requires just one extra step. Boot the Knoppix CD and mount your Linux system with read/write permissions:

knoppix@ttyp0[knoppix]$ sudo mount -o rw /dev/hda1

 /mnt/hda1

Replace hda1 with your root partition. Now run passwd under chroot, so that the passwd command changes the root password for the mounted system, and not Knoppix:

knoppix@ttyp0[knoppix]$ sudo chroot /mnt/hda1 passwd

Enter new UNIX password:

Retype new UNIX password:

passwd: password updated successfully

knoppix@ttyp0[knoppix]$

To change the password for a user instead or root, invoke the same command but add the name of the user to the end:

knoppix@ttyp0[knoppix]$ sudo chroot /mnt/hda1 passwd username

Enter new UNIX password:

Retype new UNIX password:

passwd: password updated successfully

knoppix@ttyp0[knoppix]$

With the password set to a new value, reboot and get back into your system.