How to create Ubuntu Core PXE Image

Introduction

It's not as easy as it should be to get a simple linux PXE image booting. Ubuntu Core is a step in the right direction, but we need to do a little work first.

How it works

The standard ubuntu initrd.img starts it's init process, does some init-y things, and hopes by the time it get's to the end of it's processes that something has mounted to /root so it can boot fully to that. Generally grub or something similar will pass boot options that tell it to mount a particular disk.

Unfortunately, there's no option to tell init to grab a rootfs from tftp, so we have to get a little crazy. The busybox included in the initrd doesn't have tftp built in, and even if did, busybox tftp is much slower compared to PXELINUX. It's easier just to build the root.img straight into the initrd.img and copy it over to a RAM disk as part of init.

In the end you end up with an initrd.img that weighs about 100MB. The majority of that lives in /lib/modules, so if you know what you need, you can slim down the image a lot.

Building the Image

These aren't copy and paste commands. Instead it's more of an outline to put everything together.

# get resources
mkdir /opt/ubuntu_core
cd /opt/ubuntu_core
mkdir root
wget <core.tgz>
wget <kernel.deb>
cp <chroot.sh> ./
cp <umount.sh> ./
cp <clean.sh> ./
cp <build.sh> ./

#create optional deploy.sh to be run after building with build.sh
vim deploy.sh
chmod +x deploy.sh

# extract filesystem
tar xzvfp <core.tgz> -C root

# add dns settings
vim root/etc/resolv.conf

# put kernel in filesystem
cp <kernel.deb> root/

# chroot
# if you encounter errors you may need to mount /proc /sys and /dev into root
# see chroot.sh code
./chroot.sh

# install kernel
dpkg -i <kernel.deb>

# back out of chroot
exit

# get kernel and initrd
cp root/boot/<vmlinuz> vmlinuz
cp root/boot/<initrd.img> initrd.img

# extract initrd
cd initrd
zcat ../initrd.img|cpio -i

# make zcat easily available
cd bin
ln -s busybox zcat

# go back to initrd directory
cd ..

# get mount to ram script
cp <mount_to_ram.sh> ./

# add "/mount_to_ram.sh" on line before "# Export relevant variables"
vim init

# go back to main directory
cd ..

# do anything else you want to do.. install packages etc
./chroot.sh

# clean up unneeded files
./clean.sh

# build initrd with root.img included
./build.sh

PXE booting

This assumes you already know how PXELINUX works. Just use these two lines in your menu entry:

kernel /path/to/vmlinuz
initrd /path/to/initrd.img

Networking

From my research, adding the following line to your menu entry should make you client get DHCP on boot:

append ip=dhcp

In actuality, that only works sometimes. It's not too hard to get it working all the time.

# get to working directory
cd /opt/ubuntu_core

#chroot
./chroot.sh

# install dhcp client
apt-get update
apt-get install isc-dhcp-client

# Tell the networking system to give you DHCP
# Put the following two lines in /etc/network/interfaces
# auto eth0
# iface eth0 inet dhcp
vim root/etc/network/interfaces

# get out of chroot
exit

# clean up unneeded files
./clean.sh

# build initrd with root.img included
./build.sh

Troubleshooting

• Don't run apt-get upgrade or risk killing your rootfs with packages not meant to be installed in Ubuntu Core.
• Having trouble installing packages with apt-get giving you weird messages? Make sure /dev, /sys, and /proc are mounted. Look at the comments in chroot.sh for help.
• mount_to_ram.sh creates a RAM disk of 512MB to copy the rootfs into. If you need more or less just edit that script and rebuild.

Copyright

All this is straight-up Public Domain.