Difference between revisions of "Change root"

Revision as of 10:23, 6 February 2013

Chroot is the process of changing of the apparent disk root directory (and the current running process and its children) to another root directory. When you change root to another directory you cannot access files and commands outside that directory. This directory is called a chroot jail. Changing root is commonly done for system maintenance, such as reinstalling the bootloader or resetting a forgotten password.

Requirements

• You'll need to boot from another working Linux environment (e.g. from a LiveCD or USB flash media, or from another installed Linux distribution).

• Root privileges are required in order to chroot.

• Be sure that the architecture of the Linux environment you have booted into matches the architecture of the root directory you wish to enter (i.e. i686, x86_64). You can find the architecture of your current environment with:

# uname -m

• If you need any kernel modules loaded in the chroot environment, load them before chrooting. It may also be useful to initialize your swap (swapon /dev/sdxY) and to establish an internet connection before chrooting.

Mount the partitions

The root partition of the Linux system that you're trying to chroot into needs to be mounted first. To find out the device name assigned by the kernel, run:

# lsblk /dev/sda

You can also run the following to get an idea of your partition layout.

# fdisk -l

Now create a directory where you would like to mount the root partition and mount it:

# mkdir /mnt/arch
# mount /dev/sda3 /mnt/arch

Next, if you have separate partitions for other parts of your system (e.g. /boot, /home, /var, etc), you should mount them, as well:

# mount /dev/sda1 /mnt/arch/boot/
# mount /dev/sdb5 /mnt/arch/home/
# mount ...

While it's possible to mount filesystems after you've chrooted, it is more convenient to do so beforehand. The reasoning for this is that you'll have to unmount the temporary filesystems after you exit the chroot, so this lets you umount all the filesystems with a single command. This also allows for a safer shutdown. Because the external Linux environment knows all mounted partitions, it can safely unmount them during shutdown.

Change root

Mount the temporary filesystems:

# cd /mnt/arch
# mount -t proc proc proc/
# mount -t sysfs sys sys/
# mount -o bind /dev dev/
# mount -t devpts pts dev/pts/

If you established an internet connection and want to use it in the chroot environment, you may have to copy over your DNS servers so that you will be connected to the network:

# cp -L /etc/resolv.conf etc/resolv.conf

Now chroot into your installed system and define your shell:

# chroot . /bin/bash

Optionally, to source your Bash configuration (~/.bashrc and /etc/bash.bashrc), run:

# source ~/.bashrc
# source /etc/profile

Optionally, create a unique prompt to be able to differentiate your chroot environment:

# export PS1="(chroot) $PS1"

Run graphical chrooted applications

If you have X running on your system, you can start graphical applications from the chroot environment.

To allow the connection to your X server, you have to run the following from a terminal:

# xhost +

Then, to direct the applications to your X server, run:

# export DISPLAY=:0.0

Perform system maintenance

At this point you can perform whatever system maintenance you require inside the chroot environment. A few common examples are:

• Reinstall the bootloader.
• Rebuild your initramfs image.
• Upgrade or downgrade packages.
• Reset a forgotten password.

Exit the chroot environment

When you're finished with system maintenance, exit the chroot:

# exit

Then unmount the temporary filesystems and any mounted devices:

# umount {proc,sys,dev,boot,[...],}

Finally, attempt to unmount your root partition:

# cd ..
# umount arch/

After this, you will be able to safely reboot.

Example

This may protect your system from Internet attacks during browsing:

cd
mkdir myroot
sudo pacman -i arch-install-scripts
sudo mount --bind myroot myroot # pacstrap must see myroot as mounted 
sudo pacstrap -i myroot base base-devel
sudo mount -t proc proc myroot/proc/
sudo mount -t sysfs sys myroot/sys/
sudo mount -o bind /dev myroot/dev/
sudo mount -t devpts pts myroot/dev/pts/
sudo cp -i /etc/resolv.conf myroot/etc/
sudo chroot myroot
passwd # set a password 
useradd -m -s /bin/bash user
passwd user # set a password
# in a shell outside the chroot: 
sudo pacman -S xorg-server-xnest
Xnest -ac :1
# continue inside the chroot: 
pacman -S xterm
xterm
pacman -S xorg-server xorg-xinit xorg-server-utils
pacman -S openbox
pacman -S ttf-dejavu
pacman -S firefox
exit
sudo chroot --userspec=user myroot
DISPLAY=:1
openbox &
firefox

See also: Basic Chroot