Booting into Rescue Mode

The official Red Hat CD-ROM (disk 1) can serve as a bootable emergency disk for most modern PCs with BIOS that can boot from CD-ROMs. It includes file system tools for emergency system administration. Using this disc you can attempt to recover from file system-related crashes.

When your system crashes due to a power failure or any other dire reasons, you may find your file system to be corrupt, and Red Hat Linux might refuse to boot as normal. In such a case do the following:

Insert the official Red Hat CD-ROM (disc 1) in your CD-ROM and boot your computer.

Rescue mode provides the ability to boot a small Red Hat Linux environment entirely from a diskette, CD-ROM, or some other boot method instead of the system's hard drive.

As the name implies, rescue mode is provided to rescue you from something. During normal operation, your Red Hat Linux system uses files located on your system's hard drive to do everything — run programs, store your files, and more.

However, there may be times when you are unable to get Red Hat Linux running completely enough to access files on your system's hard drive. Using rescue mode, you can access the files stored on your system's hard drive, even if you cannot actually run Red Hat Linux from that hard drive.

To boot into rescue mode, you must be able to boot the system using one of the following methods:

• By booting the system from an installation boot diskette made from the bootdisk.img image. [1]
• By booting the system from an installation boot CD-ROM. [2]
• By booting the system from the Red Hat Linux CD-ROM #1.

Once you have booted using one of the described methods, enter the following command at the installation boot prompt:

linux rescue

You are prompted to answer a few basic questions, including which language to use. It also prompts you to select where a valid rescue image is located. Select from Local CD-ROM, Hard Drive, NFS image, FTP, or HTTP. The location selected must contain a valid installation tree, and the installation tree must be for the same version of Red Hat Linux as the Red Hat Linux CD-ROM #1 from which you booted. If you used a boot CD-ROM or diskette to start rescue mode, the installation tree must be from the same tree from which the media was created. For more information about how to setup an installation tree on a hard drive, NFS server, FTP server, or HTTP server, refer to the Red Hat Linux Installation Guide.

If you select a rescue image that does not require a network connect, you are asked whether or not you want to establish a network connection. A network connection is useful if you need to backup files to a different computer or install some RPM packages from a shared network location, for example.

You will also see the following message:

The rescue environment will now attempt to find your Red Hat
Linux installation and mount it under the directory
/mnt/sysimage.  You can then make any changes required to your
system.  If you want to proceed with this step choose
'Continue'. You can also choose to mount your file systems
read-only instead of read-write by choosing 'Read-only'.
If for some reason this process fails you can choose 'Skip' 
and this step will be skipped and you will go directly to a
command shell.

If you select Continue, it will attempt to mount your file system under the directory /mnt/sysimage. If it fails to mount a partition, it will notify you. If you select Read-Only, it will attempt to mount your file system under the directory /mnt/sysimage, but in read-only mode. If you select Skip, your file system will not be mounted. Choose Skip if you think your file system is corrupted.

Once you have your system in rescue mode, a prompt appears on VC (virtual console) 1 and VC 2 (use the [Ctrl]-[Alt]-[F1] key combination to access VC 1 and [Ctrl]-[Alt]-[F2] to access VC 2):

-/bin/sh-2.05b#

If you selected Continue to mount your partitions automatically and they were mounted successfully, you are in single-user mode.

Even if your file system is mounted, the default root partition while in rescue mode is a temporary root partition, not the root partition of the file system used during normal user mode (runlevel 3 or 5). If you selected to mount your file system and it mounted successfully, you can change the root partition of the rescue mode environment to the root partition of your file system by executing the following command:

chroot /mnt/sysimage

This is useful if you need to run commands such as rpm that require your root partition to be mounted as /. To exit the chroot environment, type exit, and you will return to the prompt.

If you selected Skip, you can still try to mount a partition manually inside rescue mode by creating a directory such as /foo, and typing the following command:

mount -t ext3 /dev/hda5 /foo

In the above command, /foo is a directory that you have created and /dev/hda5 is the partition you want to mount. If the partition is of type ext2, replace ext3 with ext2.

If you do not know the names of your partitions, use the following command to list them:

fdisk -l

From the prompt, you can run many useful commands such as

• list-harddrives to list the hard drives in the system
• ssh,scp, and ping if the network is started
• dump and restore for users with tape drives
• parted and fdisk for managing partitions
• rpm for installing or upgrading software
• joe for editing configuration files (If you try to start other popular editors such as emacs, pico, or vi, the joe editor will be started.)

Notes

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In Grub :

• Type 'e' to edit the default kernel line
• Then 'e' again on the line that starts with 'kernel'
• Add 'init=/bin/bash' to the end of the 'kernel' line
• Press <ENTER>
• Type 'b' to boot it

Once you're at a /bin/bash prompt...

Remount the filesystem read/write (will be ro when bin/bash'ing):

# mount -o remount,rw /

Then change the passwd: # passwd root

Remount the filesystem back to read/only (keep things clean):

# mount -o remount,ro /

Then CTR-ALT-DELETE (though this will result in a kernel panic most likely). After rebooting the system and you should be good to go.

Lost Root Password Linux

First, try single user. If you don't see either a LILO or GRUB boot screen, try hitting CTRL-X to get one. If it's LILO, just type "linux single" and that should do it (assuming that "linux" is the lilo label). If GRUB, hit 'e", then select the "kernel" line, hit "e" again, and add " single" (or just " 1") to the end of the line. Press ENTER, and then "b" to boot.

You should get a fairly normal looking boot sequence except that it terminates a little early at a bash prompt. If you get a "Give root password for system maintenance", this isn't going to work, so see the "init" version below.

If you do get the prompt, the / filesystem may not be mounted rw (although "mount" may say it is). Do

mount -o remount,rw /

If that doesn't work (it might not), just type "mount" to find out where "/" is mounted. Let's say it is on /dev/sda2. You'd then type:

mount -o remount,rw /dev/sda2

If you can do this, just type "passwd" once you are in and change it to whatever you like. Or just edit /etc/shadow to remove the password field: move to just beyond the first ":" and remove everything up to the next ":". With vi, that would be "/:" to move to the first ":", space bar once, then "d/:" and ENTER. You'll get a warning about changing a read-only file; that's normal. Before you do this, /etc/shadow might look like:

root:$1$8NFmV6tr$rT.INHxDBWn1VvU5gjGzi/:12209:0:99999:7:-1:-1:1074970543
bin:*:12187:0:99999:7:::
daemon:*:12187:0:99999:7:::
adm:*:12187:0:99999:7:::

and after, the first few lines should be:

root::12209:0:99999:7:-1:-1:1074970543
bin:*:12187:0:99999:7:::
daemon:*:12187:0:99999:7:::
adm:*:12187:0:99999:7:::

You'll need to force the write: with vi, ":wq!". (If that still doesn't work, you needed to do the -o remount,rw, see above).

Another trick is to add "init=/bin/bash" (LILO "linux init=/bin/bash" or add it to the Grub "kernel" line). This will dump you to a bash prompt much earlier than single user mode, and a lot less has been initialized, mounted, etc. You'll definitely need the "-o remount,rw" here. Also note that other filesystems aren't mounted at all, so you may need to mount them manually if you need them. Look in /etc/fstab for the device names.

See also
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http://aplawrence.com/Bofcusm/873.html

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Last modified: March 12, 2019