User-mode Linux

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User-mode-linux (UML) is a method to run linux inside linux as a normal process. Please check [1] for in detail information what uml is and how it works.

Why use UML?

Running UML is a safe way to run multiple instances of (arch-)linux at the same time. The single processes are seperated from each other, which makes it secure to run for example a testing instance and a productive one on the same machine. If something goes wrong inside the testing instance, it does not interfere with the host linux or the productive instance.


There are two methods:

  • Use hostfs with vde2 network (all UMLs use same file system)
  • Use rootfs with tap network (require build image)

Both methods are workable on linux 3.4.4-3(Jul 3 2012)

setup by hostfs + vde2

Hostfs means use host's file system in read-only mode.

required packages

  • AUR package: linux-usermode
  • Utility packages: vde2 screen start-stop-daemon rsyslog

launch script

  • download script as 2vm.bash
  • use normal user to launch 2vm.bash (there are two VM named as 'C1' & 'C2')
  • modify the 2vm.bash to fit your requirement

Setup by rootfs + tap

required packages

  • AUR package: linux-usermode
  • Utility packages: vde2 uml_utilities

build rootfs image

1.) First you have to create a single, big file into which you will install Arch Linux. This command creates a single 1 GB file, only containing zeros - should be enough for a basic Arch Linux installation.

dd if=/dev/zero of=rootfs bs=1MB count=1000

2.) After the build process you have to format the root filesystem image:

mke2fs -F rootfs

3.) After formatting the file you have to mount it. Executing the following command as root does the job (you have also to load the loop module with modprobe):

mount -o loop rootfs /mnt

4.) Now the installation of the basic system may start:

mkdir -p /mnt/var/lib/pacman
pacman -Sy base -r /mnt
cd /mnt/dev
mknod --mode=660 ubd0 b 98 0
chown root:disk ubd0

5.) Before the system can be booted with user-mode-linux, some files inside the arch basic system have to be customised. Add this line to /mnt/etc/fstab:

/dev/ubd0 / ext2 defaults 0 0

To avoid long boot time you are advised to disable hotplugin in /mnt/etc/rc.conf:

DAEMONS=(.. !hotplugin ..)

6.) Now unmount the filesystem. NOTE: If you change anything inside your mounted filesystem (e.g. /mnt) while it is running it may lead to significant filesystem corruption inside your virtual machine and kill it!

umount /mnt

7.) Next step is to set up networking. Therefore you create a so called tun device (Please visit the uml howto for further information about tun/tap), and give it an IP address. The following lines load the necessary tun module, create a tun/tap device that is readable by the users group and set it up with the given ip address. For security you should consider to create a certain uml group with read permissions for the network device.

modprobe tun
tunctl -g users
chown root.users /dev/net/tun
ip addr add dev tap0

8.) Now you can boot the image. To use the network you have to announce the proper device to the uml kernel. (Mind that the user running the uml command needs enough rights to access the tun device!)

vmlinux ubd0=rootfs eth0=tuntap,,,

The ",,," mean:

eth0=transport,tuntap device,MAC address,ip



Headless example:

 vmlinux ubd0=rootfs eth0=tuntap,,, mem=128M con=pty