Ubuntu Server 20.04 image with Packer and Subiquity for Proxmox

For Ubuntu Server 22.04, another post covers this topic.

Starting with 20.04, Ubuntu decided to update the live server installer to implement the autoinstall specification to be able to fully automate the install process using only Subiquity. Subiquity is the new server installer (aka “ubiquity for servers”) and aims to replace the previous classic system based on debian-installer.

This article illustrates how to generate an Ubuntu Server 20.04 image template using Packer and only Subiquity on Proxmox.


Subiquity is only available in the live version of the image file (for instance ubuntu-20.04.4-live-server-amd64.iso). The previous system based on debian-installer (preseed files) has been discontinued and won’t work anymore.

It’s still possible to use debian-installer by downloading the legacy server image that can be found in a dedicated folder ubuntu-legacy-server on the official Ubuntu images repository.


This new live system is based on cloud-init and uses a YAML file to fully automate the installation process. It differs on several points from the previous system.

  • The syntax is much easier to understand (YAML vs debconf-set-selections format).
  • It’s possible to have hybrid situations where some sections can be interactive and others answered automatically from the configuration.

Subiquity setup in Ubuntu Server 20.04


Packer requires a user account to perform actions on the Proxmox API. The following commands will create a new user account packer@pve with restricted permissions.

$ pveum useradd packer@pve
$ pveum passwd packer@pve
Enter new password: ****************
Retype new password: ****************
$ pveum roleadd Packer -privs "VM.Config.Disk VM.Config.CPU VM.Config.Memory Datastore.AllocateSpace Sys.Modify VM.Config.Options VM.Allocate VM.Audit VM.Console VM.Config.CDROM VM.Config.Network VM.PowerMgmt VM.Config.HWType VM.Monitor"
$ pveum aclmod / -user packer@pve -role Packer
Proxmox pveum is available through an SSH connection or from the web shell accessible under the node parameters on the UI.

Download the Ubuntu Server 20.04 ISO from the images repository. At the time of writing, the latest version available was ubuntu-20.04.4-live-server-amd64.iso. Put the ISO inside the local storage under the ISO image category.

Local storage in Proxmox


Builder configuration

Packer will execute a workflow to create a new template that can be used later on to bootstrap new VMs quickly with a pre-based configuration already applied.

Technically, Packer will start a VM in Proxmox, launch the installer from the boot command with a reference to the config file and convert the VM into a template at the end when the setup has been fully completed.

  "builders": [{
    "type": "proxmox",
    "proxmox_url": "https://proxmox.madalynn.xyz/api2/json",
    "username": "{{ user `proxmox_username` }}",
    "password": "{{ user `proxmox_password` }}",
    "node": "proxmox",
    "network_adapters": [{
      "bridge": "vmbr0"
    "disks": [{
      "type": "scsi",
      "disk_size": "20G",
      "storage_pool": "local-lvm",
      "storage_pool_type": "lvm"
    "iso_file": "local:iso/ubuntu-20.04.4-live-server-amd64.iso",
    "unmount_iso": true,
    "boot_wait": "5s",
    "memory": 1024,
    "template_name": "ubuntu-20.04",
    "http_directory": "http",
    "boot_command": [
      "autoinstall ds=nocloud-net;s=http://{{ .HTTPIP }}:{{ .HTTPPort }}/ ",
      "--- <enter>"
    "ssh_username": "madalynn",
    "ssh_password": "madalynn",
    "ssh_timeout": "20m"

The majority of the parameters are pretty straightforward to understand. The ssh_timeout will give time to the installer to download the latest security updates during the setup.

Launch Packer with the following command.

$ packer build -var-file=secrets.json ubuntu.json

The var-file parameter gives the flexibility to extract secrets (like credentials) and dynamic parameters to use the workflow to build several Ubuntu images. The minimum required should include Proxmox credentials from the user created previously.

  "proxmox_username": "packer@pve",
  "proxmox_password": "fQk9f5Wd22aBgv"

Packer will start a HTTP server from the content of the http directory (with the http_directory parameter). This will allow Subiquity to fetch the cloud-init files remotely.


The live installer Subiquity uses more memory than debian-installer. The default value from Packer (512M) is not enough and will lead to weird kernel panic. Use 1G as a minimum.

---[ end Kernel panic - not syncing: No working init found.  Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance. ]---

Ubuntu kernel panic

The boot command tells cloud-init to start and uses the nocloud-net data source to be able to load the user-data and meta-data files from a remote HTTP endpoint. The additional autoinstall parameter will force Subiquity to perform destructive actions without asking confirmation from the user.

  "boot_command": [
    "autoinstall ds=nocloud-net;s=http://{{ .HTTPIP }}:{{ .HTTPPort }}/ ",
    "--- <enter>"

Provisioner configuration

Cloud-init will take care of everything else. However, Packer will assume the provisioning is complete as soon as it is able to connect to the virtual machine via SSH. But at this time, the setup process won’t be fully done. Packer should be told to wait until cloud-init is fully done.

Technically speaking, the easiest solution is to wait until the /var/lib/cloud/instance/boot-finished file is present. Creating this file is the last thing cloud-init does. A bash script with a simple while will do the trick.

  "provisioners": [{
    "type": "shell",
    "inline": [
      "while [ ! -f /var/lib/cloud/instance/boot-finished ]; do echo 'Waiting for cloud-init...'; sleep 1; done"

Provisioners will be executed directly on the VM once the SSH connection is available. Packer supports a lot of provisioners. For instance, it’s possible in this step to launch an Ansible playbook or to configure the Chef client.


As Subitiquy uses cloud-init, the configuration should be present in two files, user-data and meta-data. user-data is the main config file that Subitiquity and cloud-init will use for the provisioning. meta-data is an addition file that can host some additional metadata in the EC2 metadata service format.

The meta-data file can be empty (and will be for Proxmox) but must be present, otherwise cloud-init will not start correctly.
├── meta-data
└── user-data

Cloud-init supports multiple formats for config files. YAML is the easiest to understand and will be used for the following snippets. Subiquity adds a new module autoinstall which hosts all the configuration necessary for the installation.

Unlike a classic cloud-init file, everything must be under the autoinstall key. The rest will be ignored.

The official documentation lists all available parameters that can be configured. The scope is reduced compared to what was possible with debian-installer but Subiquity gives the ability to use all other cloud-init modules to compensate.

Under the hood, Subiquity will be able to handle some actions by itself (like partitioning) and will generate a cloud-init config file that will be executed after a reboot for the rest.

All “native” cloud-init modules must be under the user-data key. For instance, to use the write_files cloud-init module, the following configuration can be used.

      - path: /etc/crontab
        content: 15 * * * * root ship_logs
        append: true

Autoinstall configuration

Autoinstall is responsible for answering all questions asked during setup (keyboard layout, additional packages, …). The scope is limited and additional workflows should be managed with cloud-init modules (see above).

  version: 1
  locale: en_US
    layout: fr
    install-server: true
    allow-pw: true
    - qemu-guest-agent

The qemu-guest-agent package is needed for Packer to detect the IP address of the VM to perform the SSH connection. This will also enable Proxmox to display VM resources directly in the user interface.

Proxmox VM meta-data

The VM will be configured in English with a French keyboard. The mapping keys correspond to settings in /etc/default/keyboard. See its manual page for more details.

The SSH server is needed for remote connections from Packer. By default, it will try to connect using only a username and a password. This requires enabling the allow-pw parameter.

Without allow-pw, the SSH server will only accept connections using certificates. Packer must be configured to do so by using the ssh_keypair_name section.


In addition to the previous parameters, Subiquity is also able to create a user account during the provisioning with the identity section.

    hostname: ubuntu
    username: madalynn
    password: $6$xyz$1D0kz5pThgRWqxWw6JaZy.6FdkUCSRndc/PMtDr7hMK5mSw7ysChRdlbhkX83PBbNBpqXqef3sBkqGw3Rahs..

This section is also responsible for setting the hostname. As this VM only serves as the basis for the template, this has no importance and should be set during the final provisioning.

The previous block will create an user madalynn with madalynn as a password.


It’s possible to generate a unix encrypted password with the following command.

$ openssl passwd -6 -salt xyz madalynn

To have more flexibility over how the account is created, it’s possible to use the cloud-init module users instead.

      - name: madalynn
        passwd: $6$xyz$1D0kz5pThgRWqxWw6JaZy.6FdkUCSRndc/PMtDr7hMK5mSw7ysChRdlbhkX83PBbNBpqXqef3sBkqGw3Rahs..
        groups: [adm, cdrom, dip, plugdev, lxd, sudo]
        lock-passwd: false
        sudo: ALL=(ALL) NOPASSWD:ALL
        shell: /bin/bash
          - ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJEXrziuUOCpWPvwOsGuF4K+aq1ufToGMi4ra/1omOZb

This module gives more parameters to configure: groups, shell binary, SSH authorized keys… The sudo parameter allows the usage of the sudo command without entering any password. This will be useful later to use, for instance, Ansible to complete the installation from the template.

It’s not possible to use the cloud-init module users and the autoinstall identity block at the same time. Subiquity will discard the users module configuration if identity is present.


Subiquity allows the usage of two pre-configured layouts, lvm and direct. By default, Subiquity will use lvm with a logical volume of 4G. The installer will not extend the partition to use the full capability of the volume group. It’s also possible to configure the size of the swapfile on the filesystem (0 to disable).

      name: direct
      size: 0

If direct is used, a single partition /dev/sda2 will be created using the full disk.

Filesystem                           Size  Used Avail Use% Mounted on
udev                                 1.9G     0  1.9G   0% /dev
tmpfs                                394M  696K  393M   1% /run
/dev/sda2                             20G  3.6G   15G  20% /
tmpfs                                2.0G     0  2.0G   0% /dev/shm
tmpfs                                5.0M     0  5.0M   0% /run/lock
tmpfs                                2.0G     0  2.0G   0% /sys/fs/cgroup

It’s also possible to use a more complex configuration based on curtin under the config key. This will be a requirement if the VM has more than one disk (pre-configured layouts won’t work in this situation).

      - type: disk
        id: root-disk
        size: largest
      - type: partition
        id: boot-partition
        device: root-disk
        size: 10%
      - type: partition
        id: root-partition
        size: 20G
      - type: partition
        id: data-partition
        device: root-disk
        size: -1

The previous configuration will create three partitions on the largest drive.

  • 10% for the boot partition.
  • 20G for the root partition.
  • The rest for the data partition.

These are the first steps to a custom layout. However, it’s not enough and will require other steps (format, mount, …).

If a pre-configured layout is used, the custom config will be ignored.


The full configuration is available on GitHub in the aerialls/madalynn-packer repository.

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