With all the recent activity happening with Linux being ported to things, people started running out of hardware to port linux to. So one smart cookie decided to port Linux to uh, Linux! And thus the UserModeLinux project was born.

User mode linux is basically a program that lets you run linux as a normal process under linux.

To get it running under debian sid I did:

apt-get install user-mode-linux

you then want to add yourself to the uml-net group. I didn't find this documented anywhere, but if you don't do this, you'll get weird random messages when you try and use networking under user-mode-linux. People in this group can do nasty things to your network so only give it to people you trust yadda yadda yadda. Remember to logout/login again after making this change

Then, I created a 512 mb "hard disk"

dd if=/dev/zero of=root_fs bs=1M count=512

and formatted it with ext2

mke2fs ./root_fs

then mounted it as a loop back device

mkdir uml && mount -o loop ./root_fs uml

then built my disto on it

debootstrap woody uml /mirrors/debian

you can then boot uml

linux single rw devfs=mount eth0=tuntap,,,the-ip-of-your-host-machine

it'll boot up into single user mode so you can configure and finalise the installation

ifconfig eth0 the-ip-of-your-uml-machine netmask the-netmask
route add -net 0 gw the-ip-of-your-gateway

If you get the error SIOCSIFFLAGS: Operation not permitted when trying to bring up eth0, ensure that the user you are running UML as on the host has read/write permissions to /dev/net/tun. See the note about the uml-net group above.

apt-get update
apt-get upgrade

This failed on my system, but it passed after creating a /etc/apt/apt.conf file with the following content

APT::Default-Release "woody";
APT::Cache-Limit 10000000;

You don't need lilo, and it gets in the way, so lets get rid of it.

apt-get remove lilo

Now lets configure all the packages

dpkg-reconfigure -a

now before we reboot we need to create some files that the installer usually creates

echo /dev/ubd/0      /       ext2    defaults,errors=remount-ro      0 1 >/etc/fstab
echo none            /proc   proc    defaults                        0 0 >>/etc/fstab

echo hostname-of-your-uml-machine >/etc/hostname

and lets set up networking on boot

echo 'auto lo' >/etc/network/interfaces
echo 'iface lo inet loopback' >>/etc/network/interfaces
echo 'auto eth0' >>/etc/network/interfaces
echo 'iface eth0 inet static' >>/etc/network/interfaces
echo ' address address-of-your-uml-box' >>/etc/network/interfaces
echo ' netmask netmask' >>/etc/network/interfaces
echo ' network network' >>/etc/network/interfaces
echo ' broadcast broadcast' >>/etc/network/interfaces
echo ' gateway gateway' >>/etc/network/interfaces

and setting the hardware clock will always fail in uml

rm /etc/rcS.d/hwclock

now, lets configure /etc/inittab correctly, replace tty1 with vc/0, tt1 with vc/1 in the file.

and now lets copy the modules over

mkdir -p /mnt/host
mount none /mnt/host -o / -t hostfs
mkdir -p /lib/modules/`uname -r`/kernel
cp -r /mnt/host/usr/lib/uml/modules/`uname -r`/kernel/* /lib/modules/`uname -r`/kernel
depmod -a

for me it complains about the scsi devices, since this is a virtual kernel, I figure i don't need them so I deleted them then rerun depmod

and now your box should be ready to boot again, so shutdown -h now the box then

linux devfs=mount eth0=tuntap,,, ro

voila, it should boot up, pop up some windows and ask you to login!

Or you can do it the easy way: copy /etc/rootstrap/rootstrap.conf into the current working directory, edit the networking config (don't meddle with too many of the options, many of them don't work if you change them, for instance installing sid doesn't work...) and run (in the same directory)

./rootstrap root_fs

wait a significant time. you now have a uml image installed. you can start it with

./linux th0=tuntap,,,ip-of-your-host-machine ro

you're done.

If you want to reduce the size that your root_fs take up, try the following:

  • defrag the volume (DebianLinux: apt-get install defrag)
  • zero the free space (eg. using zerofree)
  • make the file sparse

    • either use sparsify from the above link or
    • cp --sparse=always <orig fs> <sparse fs>

Traps for young players

If you receive the following error when trying to start your uml process with networking:

Exec of 'uml_net' failed - errno = 2
tuntap_open_tramp failed - errno = 22
SIOCSIFFLAGS: Invalid argument

I tried EVERYTHING to get rid of this. Finally the solution was VERY simple. Run as a standard user, who is in the uml-net group, and not as root. Note: This is a "works for me". I still don't know why this solved the problem, only that it did. Also, another oddity is that AFTER I had run it as a user, I could then run it as root with no problems. This smacks of tap0 being created with incorrect permissions or some oddity. Anyway, it works now.

Mounting one of the pre-built Root FSes (ON THE HOST!)

It took a while for me to work out how to mount a root FS for RH9 so I could change it before booting UML. The FS had two partitions and a partition table, so mount the_fs /mnt -o loop didn't work. It turns out you need to specify an offset to mount to skip past the partition table. You can find out how many bytes to skip by doing this:

# fdisk redhat9.image.server-minimal.disk
Command (m for help): u
Changing display/entry units to sectors

Command (m for help): p

Disk redhat9.image.server-minimal.disk: 0 MB, 0 bytes
128 heads, 32 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes

                           Device Boot    Start       End    Blocks   Id  System
redhat9.image.server-minimal.disk1   *        32   3756031   1878000   83  Linux
redhat9.image.server-minimal.disk2       3756032   4194303    219136   82  Linux swap

Command (m for help): q

# bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
# mount redhat9.image.server-minimal.disk /mnt -o loop,offset=16384 -t ext3


Compiling your own kernel

This is a fairly trivial task if you are used to compiling kernels. The main point to remember is to add


to each command (or export it as a shell variable).

In brief:

make defconfig ARCH=um
make ARCH=um
mount -o loop ./root_fs /mnt/uml
make modules_install INSTALL_MOD_PATH=/mnt/uml ARCH=um
umount /mnt/uml

If you're compiling on 64 bit Linux you will probably also need to add SUBARCH=i386 as well.