installing Xen domU on Debian Etch

I have just been installing a Xen domU on Debian Etch. I'll blog about installing dom0 later when I have a test system that I can re-install on (my production Xen machines have the dom0 set up already). The following documents a basic Xen domU (virtual machine) installation that has an IP address in the 10.0.0.0/8 private network address space and masquerades outbound network data. It is as general as possible.

lvcreate -n xen1 -L 2G /dev/vg

Firstly use the above command to create a block device for the domU, this can be a regular file but a LVM block device gives better performance. The above command is for a LV named xen1 on an LVM Volume Group named vg.

mke2fs -j /dev/vg/xen1

Then create the filesystem with the above command.

mount /dev/vg/xen1 /mnt/tmp
mount -o loop /tmp/debian-testing-i386-netinst.iso /mnt/cd
cd /mnt/tmp
debootstrap etch . file:///mnt/cd/
chroot . bin/bash
vi /etc/apt/sources.list /etc/hosts /etc/hostname
apt-get update
apt-get install libc6-xen linux-image-xen-686 openssh-server
apt-get dist-upgrade

Then perform the basic Debian install with the above commands. Make sure that you change to the correct directory before running the debootstrap command. The /etc/hosts and /etc/hostname files need to be edited to have the correct contents for the Xen image (the default is an empty /etc/hosts and /etc/hostname has the name of the parent machine). The file /etc/apt/sources.list needs to have the appropriate configuration for the version of Debian you use and for your preferred mirror. libc6-xen is needed to stop a large number of kernel warning messages on boot. It's a little bit of work before you get the virtual machine working on the network so it's best to do these commands (and other package installs) before the following steps. After the above type exit to leave the chroot and run umount /mnt/tmp.

lvcreate -n xen1-swap -L 128M /dev/vg
mkswap /dev/vg/xen1-swap

Create a swap device with the above commands.

auto xenbr0
iface xenbr0 inet static
pre-up brctl addbr xenbr0
post-down brctl delbr xenbr0
post-up iptables -t nat -F
post-up iptables -t nat -A POSTROUTING -o eth0 -s 10.1.0.0/24 -j MASQUERADE
address 10.1.0.1
netmask 255.255.255.0
bridge_fd 0
bridge_hello 0
bridge_stp off

Add the above to /etc/network/interfaces and use the command ifup xenbr0 to enable it. Note that this masquerades all outbound data from the machine that has a source address in the 10.1.0.0/24 range.

net.ipv4.conf.default.forwarding=1

Put the above in /etc/sysctl.conf, run sysctl -p and echo 1 > /proc/sys/net/ipv4/conf/all/forwarding to enable it.

cp /boot/initrd.img-2.6.18-3-xen-686 /boot/xen-initrd-18-3.gz

Set up an initial initrd (actually initramfs) for the domU with a command such as the above. Once the Xen domU is working you can create the initrd from within it which gives a smaller image.

kernel = "/boot/vmlinuz-2.6.18-3-xen-686"
ramdisk = "/boot/xen-initrd-18-3.gz"
memory = 64
name = "xen1"
vif = [ '' ]
disk = [ 'phy:/dev/vg/xen1,hda,w', 'phy:/dev/vg/xen1-swap,hdb,w' ]
root = "/dev/hda ro"
extra = "2 selinux=1 enforcing=0"

The above is a sample Xen config file that can go in /etc/xen/xen1. Note that this will discover an appropriate bridge device by default, if you only plan to have one bridge then it's quite safe, if you want multiple bridges then things will be a little more complex. Also note that there are two block devices created as /dev/hda and /dev/hdb, obviously if we wanted to have a dozen block devices then we would want to make them separate partitions with a virtual partition table. But in most cases a domU will be a simple install and won't need more than two block devices.

xm create -c xen1

Now start the Xen domU with the above command. The -c option means to take the Xen console (use ^] to detach). After that you can login as root at the Xen console with no password, now is a good time to set the password.

Run the command apt-get install udev, this could not be done in the chroot before as it might mess up the dom0 environment. Edit /etc/inittab and disable gettys on tty2 to tty6, I don't know if it's possible to use them (the default and only option for xen console commands is tty1) and in any case you would not want 6, saving a few getty processes will save some memory.

Now you should have a basically functional Xen domU. Of course a pre-requisite for this is having a machine with a working dom0 installation. But the dom0 part is easier (and I will document it in a future blog post).