Creating a CentOS Xen virtual machine image

In this article we will create a CentOS 5.2 virtual machine image that can be used by Xen. We need to bring three pieces together:

1. The kernel (vmlinuz)
2. The ramdisk (initrd)
3. The filesystem

Kernel

The easiest way to create a suitable kernel, ramdisk and filesystem is to use a system with CentOS 5.2 on it with xen enabled:

yum install xen

Change the default kernel to the new kernel with xen support by editing /boot/grub/menu.lst:

default=0

and reboot

Ramdisk

Create the ramdisk by running mkinitrd:

mkinitrd --omit-scsi-modules --with=xennet --with=xenblk --preload=xenblk /root/xen-image/centos-ramdisk.img

Filesystem

The hardest job is getting a filesystem. Create a directory where we will hold the filesystem image:

mkdir /root/xen-image
cd /root/xen-image

Create an image file and make a filesystem on this file:

dd if=/dev/zero of=centos-root.img bs=1M count=1999
mkfs.ext3 centos-root.img

Create a directory where we can mount the newly created image:

mkdir rootdisk
mount -o loop centos-root.img /root/xen-image/rootdisk/
cd rootdisk

Create the /etc directory:

mkdir etc

and edit the file /etc/fstab:

/dev/sda1   /          ext3     defaults         1 1
none        /dev/pts   devpts   gid=5,mode=620   0 0
none        /dev/shm   tmpfs    defaults         0 0
none        /proc      proc     defaults         0 0
none        /sys       sysfs    defaults         0 0

Create some necessary device files:

mkdir dev
for i in console null zero; do /sbin/MAKEDEV -d /root/xen-image/rootdisk/dev -x $i; done

Copy the kernel modules:

mkdir -p lib/modules
cp -a /lib/modules/2.6.18-128.1.6.el5xen/ /root/xen-image/rootdisk/lib/modules/

Create the directory for network scripts:

mkdir -p etc/sysconfig/network-scripts

Edit the file that describes the first network interface, /etc/sysconfig/network-scripts/ifcfg-eth0:

DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes

Do the same for the second network interface, /etc/sysconfig/network-scripts/ifcfg-eth1:

DEVICE=eth1
BOOTPROTO=dhcp
ONBOOT=yes

And the last network script, etc/sysconfig/network:

NETWORKING=yes
HOSTNAME=centos52
GATEWAY=x.x.x.x

Create the RPM lock directory:

mkdir -p var/lock/rpm

We need to create a specific configuration file for yum, /root/xen-image/yum-xen.conf:

[main]
cachedir=/var/cache/yum
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
distroverpkg=redhat-release
tolerant=1
exactarch=1
obsoletes=1
gpgcheck=0
plugins=1
metadata_expire=1h

[base]
name=CentOS-5.2 - Base
baseurl=http://mirror.centos.org/centos/5.2/os/i386/
enabled=1

[extras]
name=CentOS-5.2 - Extras
baseurl=http://mirror.centos.org/centos/5.2/extras/i386/
enabled=1

[updates]
name=CentOS-5.2 - Updates
baseurl=http://mirror.centos.org/centos/5.2/updates/i386/
enabled=1

[addons]
name=CentOS-5.2 - Addons
baseurl=http://mirror.centos.org/centos/5.2/addons/i386/
enabled=1

Use the configuration file we just edited to install the base system and add the openssh server:

yum -c /root/xen-image/yum-xen.conf --installroot=/root/xen-image/rootdisk -y groupinstall base
yum -c /root/xen-image/yum-xen.conf --installroot=/root/xen-image/rootdisk -y install openssh openssh-server

To set an initial password for root, we chroot into the rootdisk we created:

chroot /root/xen-image/rootdisk

Edit the /etc/passwd file inside the chroot and change the ‘*’ on the first line with an ‘x’:

root:x:0:0:root:/root:/bin/bash

Run pwconv to enable shadow passwords and set the root password:

pwconv
passwd root

Disable TLS:

mv /lib/tls /lib/tls.disabled

Exit from the chroot:

exit

Unmount the rootdisk:

cd /root
umount /root/xen-image/rootdisk

Putting it all together

Create a new configuration file for Xen specific to this image, /etc/xen/centos52:

kernel = "/boot/vmlinuz-2.6.18-128.1.6.el5xen"
ramdisk = "/root/xen-image/centos-ramdisk.img"
name = "centos52"
memory = "256"
disk = [ 'file:/root/xen-image/centos-root.img,sda1,w' ]
root = '/dev/sda1 ro'
vif = [ 'bridge=xenbr0', '']
vcpus=1
on_reboot = 'destroy'
on_crash = 'destroy'

And finally, starting the Xen VM image:

/usr/sbin/xm create -c centos52