diskimage-builder with a single yaml file⌗

Why diskimage-builder⌗

With the recent move of OpenStack devstack to require/support Ubuntu 24.04 Noble, I needed to make some changes to my local environment. Previously, I was using an unmodified Ubuntu cloud image as a base, supplying a cloud-init ISO (created by cockpit-machines) to do just-in-time user and network configuration.

For environmental reasons mostly irrelevant to this post, that approach doesn’t work for me anymore. So I decided to use diskimage-builder to create an image that’s ready to go from get-go. While I’ve used diskimage-builder in the past – I actually just fixed the Gentoo element for it – I have typically used it to create ramdisks for IPA, and rarely to create my own cloud base images.

Why yaml⌗

One of the biggest weaknesses in typical usage of diskimage-builder is that the most obvious way to configure it is via a complex combination of command lines and environment variables. This works good for ad-hoc or scripted use cases, but I wanted to centralize it into one place so I didn’t have to remember (or alias) a command.

So I dug into the support diskimage-builder has for configuration via yaml file. This support, despite being fairly comprehensive, is not clearly documented. In the simpliest terms possible; you can convert this command::

    # DIB_DEV_USER_USERNAME=jay DIB_DEV_USER_PWDLESS_SUDO=1 \
      DIB_DEV_USER_AUTHORIZED_KEYS=/home/jay/authorized-keys-dib \
      DIB_RELEASE=noble DIB_UBUNTU_KERNEL=linux-image-kvm \
      disk-image-create -a amd64 -o devstack-noble.qcow2 -p kitty-terminfo \
      -p iptables -p iputils-ping -p tmux -p vim-scripts \
      vm ubuntu-minimal block-device-efi devuser openssh-server growroot \
      simple-init

Into this command and configuration file combo::

    # diskimage-builder devstack-noble.yaml
    # cat devstack-noble.yaml
    - imagename: devstack-noble.qcow2
      arch: amd64
      elements:
        - vm
        - ubuntu-minimal
        - block-device-efi
        - devuser
        - openssh-server
        - growroot
        - simple-init
      environment:
        DIB_DEV_USER_USERNAME: jay
        DIB_DEV_USER_PWDLESS_SUDO: "1"
        DIB_DEV_USER_AUTHORIZED_KEYS: /home/jay/authorized-keys-dib
        DIB_RELEASE: noble
        DIB_UBUNTU_KERNEL: linux-image-kvm
      packages:
        - kitty-terminfo
        - iptables
        - iputils-ping
        - tmux
        - vim-scripts

I think the advantages are obvious – it’s much simpler (at least to me) to to manage and version a yaml file than a long, complex command line or script. And feel free to steal my config if you, too, want a simple Ubuntu Noble image useful as a base for devstack :).

More information⌗

If your needs are different, you can do even more with the yaml – just run diskimage-builder --help for a full list of top-level keys in for the yaml.

Please also review the diskimage-builder documentation for full details on all the elements and configurations – it’s extremely extensible and you might be surprised what tricks are already implemented for you in diskimage-builder, if you just know to activate the element.