Debian UML step-by-step

Setting up the host
Kernel Options
TUN/TAP
bridging
Skas Mode
tmpfs
Host configuration
Running UML
Get a root fs
starting it up
sans devfs
/etc/init.d/uml
Configuring Guest
configuring guest's network
basic stuff
Issues
HOSTFS
Links

How to set up several User Mode Linux instances running on one box each with a real external ip and access to the internet.

host: the real machine which will host the virtual user mode linuxes. In our example the real machine is ibis.riseup.net (216.162.217.194)

guest: the user mode linux (uml) running on the host. In our example, the uml is stork.riseup.net (216.162.217.193).
Setting up the host
Kernel Options
Assumes you run "make menuconfig"

TUN/TAP
choose "Universal TUN/TAP device driver support" under "Network Device Support"

This is needed to get virtual network between host and uml. there are other ways, but this is easiest. If run as module, then insmod on startup. Also, this works best with devfs installed too (actually, I can't get it to work otherwise, need to mknod something something.).

bridging
Choose "802.1d Ethernet Bridging" (in "Networking options")

Skas Mode
Skas mode is much faster than normal tt mode, but requires a patch in the host's kernel. If the host has the patch installed, then the UML will go into skas mode automatically.

The patch can be downloaded from the UML website's download section.

tmpfs
User Mode Linux createas a tmp file as big as the RAM you allocate the UML instance. It's placed in your environment's TMPDIR path. One useful strategy is to use the tempfs filesystem for your TMPDIR.

For example:

> mkdir /tmp/uml
> mount -t tmpfs -o mode=1777,size=512M none /tmp/uml
> TMPDIR=/tmp/uml linux mem=512M ...


or

/etc/fstab

tmpfs /tmp/uml tmpfs defaults,size=7168M 0 0


UML unlinks the tmp file right after creating it. So, an ls -al /tmp/uml is going to show an empty directory. Use "lsof" to check if it is used.

Host configuration
host> apt-get install bridge-utils uml-utilities user-mode-linux devfsd
host> useradd -m uml
host> chown root:uml /dev/net/tun
host> chmod 660 /dev/net/tun


note: do you need to add user 'uml' to group 'uml-net'?

create file host:/etc/init.d/bridge:

#!/bin/sh

# configuration

HOST_IP="216.162.217.194"
HOST_NETMASK="255.255.255.0"
HOST_GATEWAY="216.162.217.1"
HOST_IFACE="eth0"

case "$1" in
start)
echo -n "Setting up fancy dancy bridge networking"

ifconfig $HOST_IFACE 0.0.0.0 promisc up
brctl addbr umlbridge
brctl setfd umlbridge 0
brctl sethello umlbridge 0
brctl stp umlbridge off

ifconfig umlbridge $HOST_IP netmask $HOST_NETMASK up
route add default gw $HOST_GATEWAY
brctl addif umlbridge $HOST_IFACE

# first UML instance
tunctl -u uml -t tap0
ifconfig tap0 0.0.0.0 promisc up
brctl addif umlbridge tap0
# second UML instance
# tunctl -u uml -t tap1
# ifconfig tap1 0.0.0.0 promisc up
# brctl addif umlbridge tap1
# and so on...
echo "."
;;
stop)
echo -n "Stopping networking bridge"
# add more tab1,tab2, etc as needed
ifconfig umlbridge down
brctl delif umlbridge $HOST_IFACE
brctl delif umlbridge tab0
brctl delbr umlbridge
tunctl -d tab0
ifconfig $HOST_IFACE $HOST_IP netmask $HOST_NETMASK up
echo "."
;;
esac

exit 0


Do not put any networking stuff in host:/etc/network/interfaces. Instead, run this at startup (e.g. run rcconf to add this script to the boot sequence).

Running UML
Get a root fs
Here set up a folder for our virtual machine 'guest' and a root filesystem.

host> su uml
host> cd /home/uml
host> mkdir guest
host> cd guest
host> wget http://people.debian.org/~mdz/uml/Debian-3.0r0.ext2.bz2
host> bunzip2 Debian-3.0r0.ext2.bz2


make it bigger (512mb):

host> dd if=/dev/zero of=Debian-3.0r0.ext2 bs=1 count=0 seek=536870912
host> /sbin/e2fsck -f Debian-3.0r0.ext2
host> /sbin/resize2fs -p Debian-3.0r0.ext2


starting it up
host> su uml
host> cd /home/uml/guest
host> linux ubd0=Debian-3.0r0.ext2 devfs=mount


If something goes wrong, you can pass boot options to the linux command, like this:

host> linux ubd0=Debian-3.0r0.ext2 devfs=mount single


which will boot in single user mode.

Once that is working, try it with networking:

host> linux ubd0=Debian-3.0r0.ext2 devfs=mount eth0=tuntap,tap0


tap0 is the name of the tap device created in the bridge script by the command "tunctl -u uml -t tap0". If you have multiple umls, then you would have a unique tap device for each one.

Here is an example startup script:

NAME="stork"
TAP="tap0"
MYOPTIONS="mem=128M ubd1=disk2"
OPTIONS="$MYOPTIONS ubd0=root_fs devfs=mount eth0=tuntap,$TAP umid=$NAME"

case $1 in
"-d")
screen -S $NAME -d -m linux $OPTIONS con=null con0=fd:0,fd:1
echo "Starting machine $NAME..."
echo
echo "management console:"
echo " uml> uml_mconsole $NAME"
echo
echo "terminal console:"
echo " uml> screen -r $NAME"
;;
*)
linux $OPTIONS
;;
esac


sans devfs
The debian root filesystem image that is available from the package maintainer of uml uses devfs. If, for whatever reason, you don't want devfs in your uml, you can build your own root filesystem or modify it to remove devfs naming from the config files. To do this, boot your uml in single user mode and then edit guest:/etc/inittab and guest:/etc/fstab to be the non devfs form (ie ttys/0 becomes tty0, /dev/ubd/0 becomes /dev/ubd0, etc).

/etc/init.d/uml
Here is an example uml startup script:

GUESTS='stork egret'
UML_USER='uml'

if [[ ! -z $2 ]]; then
GUESTS=$2
fi

case "$1" in
start)
for host in $GUESTS; do
if [[ ! -e /home/$UML_USER/.uml/$host ]]; then
echo "Starting $host..."
cd /home/$UML_USER/$host
su $UML_USER -c "./start -d" > /dev/null
echo " done"
else
echo "$host is already running"
fi
done
;;
stop)
for host in $GUESTS; do
echo "Stopping $host..."
if [[ -e /home/$UML_USER/.uml/$host/mconsole ]]; then
su $UML_USER -c "uml_mconsole $host sysrq s; uml_mconsole $host sysrq u; uml_mconsole $host sysrq e; uml_mconsole $host halt"
echo " done.";
else
echo " error: file not found /home/$UML_USER/.uml/$host/mconsole"
fi
done
sleep 4 # not sure if the uml_mconsole commands wait until completed before they return.
;;
restart)
echo "not supported";
;;
*)
echo "Usage: $0 {start|stop} {machine}" >&2
exit 1
;;
esac
exit 0


Configuring Guest
When you boot up, things are pretty minimal. Login as root.

Guest sample configuration:

hostname: stork

ip address: 216.162.217.193

configuring guest's network
edit /etc/network/interfaces:

auto lo
iface lo inet loopback

# eth0
auto eth0
iface eth0 inet static
address 216.162.217.193
netmask 255.255.255.0
network 216.162.217.0
broadcast 216.162.217.255
gateway 216.162.217.1


Then bring it up:

guest> ifup eth0


You should now be connected to the internet like any other machine. Note that this is funky, because the guest machine got to pick its own IP address. Supposedly, here is how to prevent this:
ebtables.sourceforge.net/br_fw_ia/br_fw_ia.html

basic stuff
Set a hostname:

guest> echo stork > /etc/hostname
guest> hostname stork


get some packages:

guest> apt-get update
guest> apt-get install ssh less


Issues
HOSTFS
We don't want the UML virtual machine to be able to mount the host's filesystem, so that means we can't compile hostfs support into the UML Kernel. Unfortunately, someone could still compile and install hostfs support as a kernel module, so that means we must build our UML Kernel without module support.

Links
www.theshore.net/~caker/uml/

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