User-mode-Linux (UML) is a method to run Linux inside Linux as a normal process. Please check  for in-depth 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 separated from each other, which makes it secure to run, for example, a testing instance and a production instance 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 three methods:
- Use hostfs with vde2 network (all UMLs use same file system)
- Use rootfs with tap network (require build image)
- Use Marionnet
Both methods are workable on Linux 3.4.4-3 (Jul 3 2012)
Setup by hostfs + vde2
Hostfs means use the host's file system in read-only mode.
- Utility packages: AUR AUR
- download http://pastebin.com/qDE0D7Lk script as 2vm.bash
- use normal user to launch 2vm.bash (there are two VMs named as 'C1' & 'C2')
- modify the 2vm.bash to fit your requirements
Setup by rootfs + tap
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 GiB file, only containing zeros, which should be enough for a basic Arch Linux installation.
dd if=/dev/zero of=rootfs bs=1M count=1024
fallocate -l 1GiB ./rootfs
2.) After the build process you have to format the root file system 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
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
/dev/ubd0 / ext2 defaults 0 0
6.) Now unmount the file system.
/mnt) while it is running, it may lead to significant file system corruption inside your virtual machine and kill it!
7.) Next step is to set up networking. Therefore, you create a so called tun device (Please visit the UML how-to 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 sets it up with the given IP address. For security, you should consider creating a certain UML group with read permissions for the network device.
modprobe tun ip tuntap add tap0 mode tap group users chown root.users /dev/net/tun ip addr add 192.168.0.100/24 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,,,192.168.0.100
eth0=transport,tuntap device,MAC address,ip
vmlinux ubd0=rootfs eth0=tuntap,,,192.168.0.100 mem=128M con=pty