Creating a custom Ubuntu image (snap/2.9/UI)
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MAAS supports deploying custom DD or TGZ images. Canonical provides both lp:maas-image-builder and gh:canonical/packer-maas to support creating custom images however these tools do not currently support Ubuntu. Instead Canonical suggests customising Ubuntu using cloud-init user_data or Curtin preseed data.
Why customised Ubuntu deployments aren’t supported
When the MAAS stream is generated by lp:maas-images it starts by downloading the base SquashFS rootfs from cloud-images.ubuntu.com that is used for all clouds. The SquashFS does not contain a kernel so lp:maas-images mounts the SquashFS with an overlay and chroots in. It then installs a kernel and extra initramfs scripts from the cloud-initramfs-rooturl and cloud-initramfs-copymods packages to allow network booting. Once everything is installed the kernel and newly generated initramfs are pulled out of the overlay and everything is unmounted. lp:maas-images provides the unmodified SquashFS, installed kernel, and generated initramfs as separate files on images.maas.io.
MAAS uses the kernel, initramfs, and SquashFS to perform network booting which allows commissioning, testing, and deployments. When deploying Ubuntu MAAS uses the same version of Ubuntu to network boot into and perform the deployment. This ensures there are no compatibility issues. Other operating systems use the Ubuntu version selected for the ephemeral environment for deployment.
Currently MAAS only allows custom images to be loaded as a single TGZ or DD.GZ, there is no way to upload a kernel,
rootfs as separate files. Even if MAAS was modified to allow the kernel,
rootfs as separate files MAAS requires
cloud-initramfs-rooturl to be included in the initrd. It is difficult for MAAS to detect if these scripts are missing and even harder for users to debug if they are missing.
Warnings on creating a custom Ubuntu image
- Custom images are always deployed with the ephemeral operating system. This can cause hard to debug errors. For example CentOS 6 can only be deployed by Ubuntu Xenial due to advances in the ext4 filesystem.
- MAAS will still install and configure the GA kernel. If your custom image contains a kernel it most likely won’t be used. MAAS will not allow you to select which kernel(GA, HWE, lowlatency, etc) when deploying a custom Ubuntu image.
- All GNU/Linux custom images should be created as a TGZ to enable storage customisation. When a DD.GZ is used MAAS/Curtin simply writes the file to the filesystem.
How to create a custom Ubuntu image for MAAS
Note: LXD may prevent device files from being created when extracting the rootfs, it is suggested to do this on metal or on a VM:
Download the rootfs from images.maas.io
Create a work directory
Extract the rootfs
sudo tar xf ../focal-server-cloudimg-amd64-root.tar.xz
sudo mount -o bind /proc /tmp/work/proc
sudo mount -o bind /dev /tmp/work/dev
sudo mount -o bind /sys /tmp/work/sys
sudo mv /tmp/work/etc/resolv.conf /tmp/work/etc/resolv.conf.bak
sudo cp /etc/resolv.conf /tmp/work/etc/
sudo chroot /tmp/work /bin/bash
apt install emacs-nox jq tree
Exit chroot and unmount binds
sudo umount /tmp/work/proc
sudo umount /tmp/work/dev
sudo umount /tmp/work/sys
sudo mv /tmp/work/etc/resolv.conf.bak /tmp/work/etc/resolv.conf
sudo tar -czf /tmp/focal-custom.tgz -C /tmp/work .
Upload it to MAAS
Note: Ubuntu release names and versions are reserved
maas $PROFILE boot-resources create name='custom/focal-custom' title='Ubuntu 20.04 Custom Image' architecture='amd64/generic' filetype='tgz' content@=focal-custom.tgz
Configure and deploy as normal