View source for Linux/Xen/DomU/Ubuntu/Ubuntu 13.10

Start this process on a existing Ubuntu system. Does not have to be a 13.10 system. You will need to have debootstrap installed and may need to update debootstrap so it has the correct script for 13.10.

Create a empty image file and format it with ext3.

 dd if=/dev/zero of=ubuntu_saucy.img bs=1M count=1 seek=1024
 mkfs.ext3 ubuntu_saucy.img

Create a directory to mount the image on. Using '/mnt/img' for this example.

 mkdir /mnt/img
 mount -oloop ubuntu_saucy.img /mnt/img

Start the debootstrap process.

 debootstrap saucy /mnt/img/

Once that completes, copy your existing /etc/apt/sources.list to the new image so we can run updates.

 cp /etc/apt/sources.list /mnt/img/etc/apt/

Mount the necessary system files so that we can enter the new root filesystem with chroot. 

 mount --bind /dev /mnt/img/dev
 mount --bind /dev/pts /mnt/img/dev/pts
 mount -t proc proc /mnt/img/proc
 mount -t sysfs sys /mnt/img/sys
 chroot /mnt/img

Run updates and install a language pack.

 apt-get update
 apt-get install language-pack-en-base
 apt-get upgrade

Configure the system timezone.

 dpkg-reconfigure tzdata

Install the kernel image, SSH server and the full version of vim
 
 apt-get install linux-image
 apt-get install openssh-server
 apt-get install vim

To use the Xen console 'xm console (domU)' you need to setup a tty on /dev/hvc0. Create the file '/etc/init/hvc0.conf' with this content.

<syntaxhighlight lang="bash">
# This service maintains a getty on hvc0 from the point the system is
# started until it is shut down again.

 start on stopped rc RUNLEVEL=[2345] and (
            not-container or
            container CONTAINER=lxc or
            container CONTAINER=lxc-libvirt)
 
 stop on runlevel [!2345]

 respawn
exec /sbin/getty -8 38400 hvc0
</syntaxhighlight>

The default udev rules for 13.10 ignore Xen generated MAC addresses so you won't get a '/etc/udev/rules.d/70-persistent-net.rules' This causes your network interfaces not to come up.

Edit '/lib/udev/rules.d/75-persistent-net-generator.rules' and comment these lines. Here's the changes in patch/diff format.

<syntaxhighlight lang="diff">
@@ -21,7 +21,7 @@ KERNEL!="eth*|ath*|wlan*[0-9]|msh*|ra*|s
                                        GOTO="persistent_net_generator_end"

 # ignore Xen virtual interfaces
-SUBSYSTEMS=="xen",                     GOTO="persistent_net_generator_end"
+#SUBSYSTEMS=="xen",                    GOTO="persistent_net_generator_end"

 # ignore UML virtual interfaces
 DRIVERS=="uml-netdev",                 GOTO="persistent_net_generator_end"
@@ -75,7 +75,7 @@ ENV{MATCHADDR}=="00:0c:29:*|00:50:56:*|0
 ENV{MATCHADDR}=="00:15:5d:*",          ENV{MATCHADDR}=""
 ENV{MATCHADDR}=="52:54:00:*|54:52:00:*", ENV{MATCHADDR}=""
 ENV{MATCHADDR}=="08:00:27:*",          ENV{MATCHADDR}=""
-ENV{MATCHADDR}=="00:16:3e:*",          ENV{MATCHADDR}=""
+#ENV{MATCHADDR}=="00:16:3e:*",         ENV{MATCHADDR}=""

 # ignore Windows Azure Hyper-V virtual interfaces
 ENV{MATCHADDR}=="00:03:ff:*", ENV{MATCHADDR}=""
</syntaxhighlight>

Configure your '/etc/network/interfaces' for DHCP

<syntaxhighlight lang="bash">
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto eth0
iface eth0 inet dhcp
</syntaxhighlight>

Configure a basic fstab

<syntaxhighlight lang="bash">
# Begin /etc/fstab
# <file system> <mount-point>   <type>   <options>                      <dump> <pass>
/dev/sda1          /             ext3      defaults,errors=remount-ro    0     0
proc               /proc         proc      defaults                      0     0

# End /etc/fstab
</syntaxhighlight>

====SSH host keys fix====

Since this image will get cloned and used to create new virtual machines we don't want to re-use the same keys for every virtual machine. Ubuntu won't regenerate SSH host keys if you delete them from /etc/ssh. So we need a script to check the host keys and regenerate them if needed.

Create /lib/init/ssh_gen_key and paste this in,

<syntaxhighlight lang="bash">
#!/bin/sh

# Some variables to make things more readable 
KEYGEN=/usr/bin/ssh-keygen
RSA1_KEY=/etc/ssh/ssh_host_key
RSA_KEY=/etc/ssh/ssh_host_rsa_key
DSA_KEY=/etc/ssh/ssh_host_dsa_key
ECDSA_KEY=/etc/ssh/ssh_host_ecdsa_key


do_rsa_keygen() {
        if [ ! -s $RSA_KEY ]; then
                echo -n "Generating SSH2 RSA host key: "
                rm -f $RSA_KEY
                if test ! -f $RSA_KEY && $KEYGEN -q -t rsa -f $RSA_KEY -C '' -N '' >/dev/null; then
                        chmod 600 $RSA_KEY
                        chmod 644 $RSA_KEY.pub
			echo "OK"
                else
			echo "FAIL"
                        exit 1
                fi
        fi
}

do_dsa_keygen() {
        if [ ! -s $DSA_KEY ]; then
                echo -n "Generating SSH2 DSA host key: "
                rm -f $DSA_KEY
                if test ! -f $DSA_KEY && $KEYGEN -q -t dsa -f $DSA_KEY -C '' -N '' >/dev/null; then
                        chmod 600 $DSA_KEY
                        chmod 644 $DSA_KEY.pub
			echo "OK"
                else
			echo "FAIL"
                        exit 1
                fi
        fi
}

do_ecdsa_keygen() {
        if [ ! -s $ECDSA_KEY ]; then
                echo -n "Generating SSH2 ECDSA host key: "
                rm -f $ECDSA_KEY
                if test ! -f $ECDSA_KEY && $KEYGEN -q -t ecdsa -f $ECDSA_KEY -C '' -N '' >/dev/null; then
                        chmod 600 $ECDSA_KEY
                        chmod 644 $ECDSA_KEY.pub
			echo "OK"
                else
			echo "FAIL"
                        exit 1
                fi
        fi
}

do_rsa_keygen
do_dsa_keygen
do_ecdsa_keygen
</syntaxhighlight>

 chmod 755 /lib/init/ssh_gen_keys

Edit /etc/init/ssh.conf

<syntaxhighlight lang="bash">
@@ -20,6 +20,7 @@ pre-start script
     test -c /dev/null || { stop; exit 0; }

     mkdir -p -m0755 /var/run/sshd
+    /lib/init/ssh_gen_keys
 end script

 # if you used to set SSHD_OPTS in /etc/default/ssh, you can change the
</syntaxhighlight>

====Firewall====

<syntaxhighlight lang="bash">
apt-get install iptables
apt-get install ufw

ufw allow OpenSSH
ufw enable
</syntaxhighlight>

====Final Cleanup====

Remove /etc/udev/rules.d/70-persistent-net.rules so that network comes up clean when the image is booted for the first time.

Remove SSH host keys so that new ones get generated on first boot.

<syntaxhighlight lang="bash">
rm /etc/udev/rules.d/70-persistent-net.rules
rm /etc/ssh/ssh_host_*
</syntaxhighlight>