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Debian GNU/Linux Reference Guide
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8.1.1 "I forgot the root password!" (1)

It is possible to boot a system and log on to the root account without knowing the root password as long as one has access to the console keyboard. (This assumes there are no password requests from the BIOS or from a boot loader such as lilo that would prevent one from booting the system.)

This is a procedure which requires no external boot disks and no change in BIOS boot settings. Here, "Linux" is the label for booting the Linux kernel in the default Debian install.

At the lilo boot screen, as soon as boot: appears (you must press a shift key at this point on some systems to prevent automatic booting and when lilo uses the framebuffer you have to press TAB to see the options you type), enter:

     boot: Linux init=/bin/sh

This causes the system to boot the kernel and run /bin/sh instead of its standard init. Now you have gained root privileges and a root shell. Since / is currently mounted read-only and many disk partitions have not been mounted yet, you must do the following to have a reasonably functioning system.

     init-2.03# mount -n -o remount,rw /
     init-2.03# mount -avt nonfs,noproc,nosmbfs
     init-2.03# cd /etc
     init-2.03# vi passwd
     init-2.03# vi shadow

(If the second data field in /etc/passwd is "x" for every username, your system uses shadow passwords, and you must edit /etc/shadow.) To disable the root password, edit the second data field in the password file so that it is empty. Now the system can be rebooted and you can log on as root without a password. When booting into runlevel 1, Debian (at least after Potato) requires a password, which some older distributions did not.

It is a good idea to have a minimal editor in /bin/ in case /usr/ is not accessible (see Rescue editors, Section 11.2).

Also consider installing the sash package. When the system becomes unbootable, execute:

     boot: Linux init=/bin/sash

sash serves as an interactive substitute for sh even when /bin/sh is unusable. It's statically linked, and includes many standard utilities as built-ins (type "help" at the prompt for a reference list).

Debian GNU/Linux Reference Guide
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  Published under the terms of the GNU General Public License Design by Interspire