Image Template Reference#

Templates are YAML files that define what goes into a custom OS image, the target platform, packages, disk layout, users, and build-time customizations. This document is the authoritative field-by-field reference for the template format.

For a conceptual overview of how templates fit into the build pipeline, see Understanding the Build Process.

What Are Templates and How Do They Work?#

Templates are predefined build specifications that serve as a foundation for building operating system images. Here’s what templates empower you to do:

Create standardized baseline configurations.
Impose consistency across multiple images.
Reduce duplication of effort.
Share and reuse common configurations with your team.

The ICT provides default image templates on a per-distribution basis and image type (RAW vs. ISO) that can be used directly to build an operating system from those defaults. You can override these default templates by providing your own template and configure or override the settings and values you want. The tool will internally merge the two to create the final template used for image composition.

image-templates

How Templates Work#

ICT ships default templates for each distribution and image type (raw, ISO, initrd). When you provide a user template, the tool merges it with the matching default; your values override or extend the defaults. The merged result is validated against a JSON schema before the build begins.

image-templates

Default templates live at:

config/osv/<target.os>/<target.dist>/imageconfigs/defaultconfigs/default-<imageType>-<arch>.yml

Note: imageType: img maps to default-initrd-<arch>.yml (there is no default-img- filename).

You do not need to edit the defaults. You can start from one of the examples in image-templates/ and override only what you need.

Quick Start Example#

A minimal user template only needs image, target, and optionally systemConfig with extra packages:

image:
  name: my-edge-device
  version: "1.0.0"

target:
  os: edge-microvisor-toolkit
  dist: emt3
  arch: x86_64
  imageType: raw

systemConfig:
  name: edge
  packages:
    - cloud-init
    - rsyslog

Everything else (disk layout, bootloader, kernel, default packages) comes from the default template for emt3 / raw / x86_64.

Top-Level Structure#

A template file has up to five top-level sections plus an optional metadata block:

metadata:       # Optional - AI-searchable discovery metadata
  ...
image:          # Required - image name and version
  ...
target:         # Required - OS, distribution, architecture, image type
  ...
baseline:       # Optional - "create" (default) or "overlay" an existing image
  ...
overlayPolicy:  # Optional - overlay-mode policy (only with baseline.mode: overlay)
  ...
disk:           # Optional - disk layout, partitions, output artifacts
  ...
packageRepositories:  # Optional - additional package repositories
  - ...
systemConfig:   # Required in merged template - packages, kernel, users, etc.
  ...

Note: User templates require only image and target. The remaining sections are merged from the default template if omitted.

Field Reference#

`metadata`#

Optional block for AI-powered template discovery. Ignored by the build engine.

Field	Type	Description
`description`	string	Human-readable description of the template
`use_cases`	string[]	Use cases this template targets
`keywords`	string[]	Keywords for search and discovery

metadata:
  description: "Edge device image with container runtime"
  use_cases: ["edge computing", "IoT gateway"]
  keywords: [edge, docker, emt3]

`image` (required)#

Image identification. Both fields are required.

Field	Type	Required	Validation	Description
`name`	string	Yes	`^[a-zA-Z0-9]([a-zA-Z0-9\-_]*[a-zA-Z0-9])?$`	Image name (alphanumeric, hyphens, underscores)
`version`	string	Yes	Semver-like: `1.0.0`, `24.04`, `1.0.0+build1`	Version string

image:
  name: my-edge-device
  version: "1.0.0"

`target` (required)#

Target platform. All four fields are required.

Field	Type	Required	Valid Values	Description
`os`	string	Yes	`azure-linux`, `edge-microvisor-toolkit`, `wind-river-elxr`, `ubuntu`, `redhat-compatible-distro`	Target operating system
`dist`	string	Yes	See OS constraints below	Distribution identifier
`arch`	string	Yes	`x86_64`, `aarch64`, `armv7hl`	Target CPU architecture
`imageType`	string	Yes	`raw`, `iso`, `img`, `wsl2`	Output image format

OS → dist constraints:

OS	Valid `dist`
`azure-linux`	`azl3`
`edge-microvisor-toolkit`	`emt3`
`wind-river-elxr`	`elxr12`
`ubuntu`	`ubuntu24`, `ubuntu26`
`redhat-compatible-distro`	Any (e.g., `el10`)

target:
  os: ubuntu
  dist: ubuntu24
  arch: x86_64
  imageType: raw

WSL-Compatible Images#

Set target.imageType: wsl2 to compose a WSL-compatible root filesystem.

target:
  os: ubuntu
  dist: ubuntu24
  arch: x86_64
  imageType: wsl2

For a complete Ubuntu 24 WSL example, see image-templates/ubuntu24/ubuntu24-x86_64-agentic-wsl2.yml.

`baseline`#

Selects how the image is assembled. If omitted, the build defaults to create mode (build the image from scratch, the behavior described everywhere else in this reference). Set mode: overlay to instead layer packages onto an existing baseline disk image without rebuilding it. The baseline may be a RAW image or a qcow2/vhd/vhdx image, which is converted to RAW before the overlay runs.

Field	Type	Required	Valid Values	Description
`mode`	string	No	`create` (default), `overlay`	Assembly mode
`source`	object	Yes when `mode: overlay` (must be absent for `create`)	—	The baseline image to overlay

`baseline.source`#

Identifies the baseline image. Exactly one of path or url must be set. The source is copied into the build workspace first and is never modified in place.

Field	Type	Required	Description
`path`	string	one of `path`/`url`	Local filesystem path to the baseline image (no URI scheme)
`url`	string	one of `path`/`url`	`https://` URL of the baseline image; downloaded over TLS before the overlay runs (plain `http` is rejected)
`format`	string	No	Baseline image format: `raw`, `qcow2`, `vhd` or `vhdx` (default `raw`). Non-raw formats are converted to RAW before the overlay runs
`sbomPath`	string	No	Local filesystem path (no URI scheme) to an externally-supplied SPDX SBOM (JSON) describing the baseline image. Defaults to unset. See SBOM generation below

baseline:
  mode: overlay
  source:
    path: /path/to/ubuntu-24.04-base.img
    format: raw
    # Optional: combine this base-image SBOM with the overlay delta to emit a
    # full SBOM. Omit it to use the SBOM embedded in the baseline image, if any.
    sbomPath: /path/to/ubuntu-24.04-base.spdx.json

SBOM generation (overlay mode)#

An overlay build emits SPDX SBOM sidecar files into the build output directory, alongside the emitted image (<image-name>-<version>.raw):

Artifact	Name	Contents
Delta SBOM	`<image-name>-<version>.delta.spdx.json`	Always emitted. Only the overlay-induced package changes — the packages the overlay added and, in `additive-and-upgrade` mode, upgraded.
Complete SBOM	`<image-name>-<version>.complete.spdx.json`	Emitted only when a base SBOM is available (see below). The full final image inventory — the baseline packages plus the overlay result. When no base SBOM exists the complete document would be identical to the delta, so this sidecar is skipped.

Both are SPDX 2.3 JSON, the same format the standalone SBOM/inspect tooling uses. The complete SBOM is also embedded inside the image at /usr/share/sbom (replacing the SBOM the image inherited from the baseline, so the in-image manifest reflects the full final inventory).

The complete SBOM is the union of a base SBOM and the overlay delta. Which base is used follows this order:

baseline.source.sbomPath set and valid — that externally-supplied base SBOM is combined with the delta.
sbomPath unset — the SBOM the baseline image itself carries at /usr/share/sbom is used as the base and combined with the delta.
No base SBOM available — sbomPath is unset or the file is absent/unreadable/malformed and the baseline image embeds no SBOM — then there is no full inventory to build, so the complete sidecar is skipped (it would be identical to the delta) while the in-image /usr/share/sbom manifest and the delta sidecar are still written. This is never an error: the build succeeds.

A missing or malformed sbomPath falls back to the baseline-embedded SBOM (then to delta-only); it does not fail the build. Omitting sbomPath entirely preserves the pre-existing base-resolution behavior. This applies to every overlay-capable provider (Ubuntu and Debian).

To diff these artifacts against the baseline — the overlay RAW vs the baseline RAW, or the complete SBOM vs the baseline SBOM — see Comparing Overlay Outputs.

Note: Overlay mode is currently wired end-to-end for the Ubuntu and Debian providers. Targeting a provider without overlay support fails the build immediately with a clear message rather than silently falling back to a create-mode build. The overlay build is additive by default: packages (and their transitive dependencies not already present in the baseline) are installed into the baseline root, the initramfs is regenerated for the added packages, the template’s systemConfig.configurations commands and systemConfig.additionalFiles are applied, and an optional grow-only resize can enlarge the image to a larger disk.size. Existing baseline packages are not removed unless overlayPolicy.allowPackageRemoval is enabled, which permits a conflict-driven removal of a baseline package a to-install package conflicts with. The installed bootloader binary, the ESP, and the bootable kernel are never modified regardless of policy.

OS defaults do not apply. Unlike a create-mode build, an overlay template is not merged with the target’s create-mode OS default configuration (default-raw-*.yml). Those defaults describe how to build an image from scratch — disk size and partition table, bootloader, kernel, and the base OS package set — all of which the baseline image already provides. The overlay pipeline reads disk/bootloader/kernel geometry from the detected baseline, not from the template, so the effective overlay template is exactly what you declare (folded through any extends: chain) with nothing inherited from the OS default. In particular, an overlay template that omits disk.size keeps the baseline’s size — it does not pick up the default’s disk.size, which against a larger baseline would otherwise be rejected as a shrink.

Unsupported systemConfig sections. Because overlay mode does not re-run the boot/system-provisioning stages, the following systemConfig sections cannot be applied to an overlay build: hostname, network, initramfs, kernel, immutability, fde, and bootloader. Previously these were silently ignored; now setting any of them in an overlay template fails the build up front with a message naming every offending section. Configure them in the baseline image (a create-mode build) instead. The overlay-supported systemConfig inputs are packages, users, configurations, and additionalFiles.

Users in overlay. systemConfig.users is provisioned onto the baseline. A requested user that already exists in the baseline image fails the build up front (an overlay cannot redefine a baseline account); the check is re-run immediately before creation, so a name that a package’s maintainer script adds during install also fails rather than being silently modified. A user’s startupScript must reference a path already present when users are created — i.e. shipped by the baseline or installed by an overlay packages entry — not a file delivered via additionalFiles, which are copied later in the overlay pipeline.

The following systemConfig.users fields are not currently applied (an inherited create-mode limitation, in both create and overlay builds): home, shell, and passwordMaxAge. The login shell is always set to /bin/bash, so a service account cannot yet be pinned to /usr/sbin/nologin via the template.

Sizing: Adding packages does not auto-grow the image, and the overlay preserves the baseline disk layout by default. Growing the image is opt-in: it requires both a disk.size larger than the baseline and overlayPolicy.allowDiskResize: true. When disk.size is larger but allowDiskResize is not set, the build fails early with a clear message rather than silently resizing the baseline. The resize is grow-only and keyed solely on disk.size (compared to the baseline’s current size), not on how much space the added packages need. If the baseline root is near-full, set disk.size larger than the baseline image and enable allowDiskResize to make room; otherwise the package install step fails with a “no space left on device” error (the failure message points back here).

Resize constraints. The grow-only resize extends the last partition on the disk and its filesystem in place. It is rejected (before any disk mutation, with an actionable error) when the root is not the last partition, when the root sits on LVM, and when the root is LUKS-encrypted or dm-verity protected. ext4/ext3/ext2 roots are the supported and CI-covered target; xfs roots use the same code path (xfs_growfs) but are best-effort: the grow sequence has unit-test coverage, but no shipping baseline exercises it against a real xfs filesystem in CI/e2e, so treat xfs resize as unverified end-to-end. The resize shells out to growpart (cloud-guest-utils), sgdisk (gdisk, GPT only), resize2fs (e2fsprogs) or xfs_growfs (xfsprogs), and losetup/partx (util-linux); these must be present on the build host, and the build fails early with a clear message if any is missing. Resize also reads partition start sectors via lsblk -o PATH,START,TYPE, which requires util-linux >= 2.38; Ubuntu 22.04’s stock lsblk/resize2fs are too old and not upgradable via apt — see the util-linux/e2fsprogs build-from-source instructions.

`overlayPolicy`#

Optional policy controls for overlay-mode preflight and install. It is a top-level peer of baseline and may only be set when baseline.mode is overlay. If omitted, the defaults below apply.

Field	Type	Required	Valid Values	Description
`packageOperation`	string	No	`additive-only` (default), `additive-and-upgrade`	Permitted package operations. `additive-only`: packages may only be added, never removed or downgraded. `additive-and-upgrade`: also permits upgrading a package already present in the baseline to a newer version. Downgrades and removals remain blocked in both modes (see note below)
`conflictPolicy`	string	No	`fail` (default), `allow-explicit`	How a package conflict detected during preflight is handled. `fail` aborts the build; `allow-explicit` permits a conflict only when the conflicting package was explicitly requested
`kernelCmdline`	string	No	—	Optional kernel command-line override applied to the overlaid image
`allowDiskResize`	boolean	No	`false` (default), `true`	Permit growing the baseline image to satisfy a larger `disk.size`. Overlay mode preserves the baseline disk layout by default; when `false`, a `disk.size` larger than the baseline is rejected with an error. Resize is always grow-only and never shrinks the image
`allowPackageRemoval`	boolean	No	`false` (default), `true`	Permit removing a baseline package that a to-install package conflicts with (e.g. installing `dracut`, which conflicts with `initramfs-tools`). When `false` (the default) such a conflict fails the build; when `true`, the conflicting baseline package is removed before install. Only valid with `packageOperation: additive-and-upgrade` — removal is more invasive than an in-place upgrade, so it is rejected under the default `additive-only`. Bootloader and bootable-kernel packages are never removed regardless of this flag

additive-and-upgrade scope. Upgrades apply only to the package set: a package already installed in the baseline may be replaced by a newer version when the resolved overlay closure requires it. Downgrades are still rejected at preflight, and the baseline kernel and bootloader remain immutable — an overlay never replaces the kernel or reinstalls the bootloader, regardless of packageOperation. Choose additive-only (the default) to fail the build on any version bump to a baseline package.

Package removal (allowPackageRemoval). Opt-in, and permitted only under packageOperation: additive-and-upgrade — removal is more invasive than an in-place upgrade, so pairing it with the default additive-only is rejected at validation. By default an overlay never removes a baseline package: a to-install package that Conflicts:/Breaks: a present baseline package fails the build. Set allowPackageRemoval: true (with additive-and-upgrade) to let the overlay remove the conflicting baseline package before installing its replacement — the case that makes dracut (which conflicts with initramfs-tools) installable on a stock baseline. Bootloader and bootable-kernel packages are still never removed, so the flag cannot break the boot path.

baseline:
  mode: overlay
  source:
    path: /path/to/ubuntu-24.04-base.img

overlayPolicy:
  # Removal requires additive-and-upgrade (not the default additive-only).
  packageOperation: additive-and-upgrade
  conflictPolicy: fail

  # Opt in to removing a baseline package a new package conflicts with
  # (e.g. remove initramfs-tools so dracut can install). Default: false.
  allowPackageRemoval: true

A complete additive-only starter template (the default policy, without package removal) lives at image-templates/ubuntu24/ubuntu24-x86_64-overlay-raw.yml; to enable removal, add the overlayPolicy block shown above (additive-and-upgrade plus allowPackageRemoval: true) to it.

`disk`#

Disk layout, partition scheme, and output artifact formats. If omitted, the default template provides sensible values (typically 4–6 GiB GPT disk with EFI boot and ext4 root partitions).

Field	Type	Required	Description
`name`	string	Yes (schema)	Disk configuration name (e.g., `"Default_Raw"`)
`path`	string	No	Disk device path (used by live installer, e.g., `/dev/sda`)
`size`	string	No	Disk size. Accepts: `"4GiB"`, `"8GB"`, `"4096 MiB"`
`partitionTableType`	string	No	`gpt` or `mbr`
`artifacts`	artifact[]	No	Output formats and optional compression
`partitions`	partition[]	No	Partition layout definitions

`disk.artifacts[]`#

Each entry defines one output format:

Field	Type	Required	Valid Values	Description
`type`	string	Yes	`raw`, `qcow2`, `vhd`, `vhdx`, `vmdk`, `vdi`, `tar`	Output image format
`compression`	string	No	`gz`, `gzip`, `xz`, `zstd`, `bz2`	Compression to apply

`disk.partitions[]`#

Each entry defines one partition:

Field	Type	Description
`id`	string	Partition identifier (e.g., `boot`, `rootfs`, `roothashmap`, `userdata`)
`name`	string	Partition label
`type`	string	Partition type (e.g., `esp`, `linux-root-amd64`, `linux`)
`typeUUID`	string	GPT type GUID (e.g., `8300`)
`fsType`	string	Filesystem type: `ext4`, `fat32`, `xfs`, etc.
`fsLabel`	string	Filesystem label
`start`	string	Start offset (e.g., `1MiB`, `513MiB`)
`end`	string	End offset (`0` means rest of disk)
`mountPoint`	string	Mount point (e.g., `/boot/efi`, `/`, `none`)
`mountOptions`	string	Mount options (e.g., `defaults`, `umask=0077`)
`flags`	string[]	Partition flags (e.g., `boot`, `esp`, `hidden`)

Example - raw disk with two partitions and two output formats:

disk:
  name: Edge_Raw
  size: 4GiB
  partitionTableType: gpt
  artifacts:
    - type: raw
      compression: gz
    - type: vhdx
  partitions:
    - id: boot
      type: esp
      flags: [esp, boot]
      start: 1MiB
      end: 513MiB
      fsType: fat32
      mountPoint: /boot/efi
      mountOptions: umask=0077
    - id: rootfs
      type: linux-root-amd64
      start: 513MiB
      end: "0"
      fsType: ext4
      mountPoint: /
      mountOptions: defaults

`packageRepositories`#

Optional list of additional package repositories beyond the OS base repos.

Field	Type	Required	Description
`codename`	string	Yes	Repository identifier (e.g., `company-internal`)
`url`	string	Yes	Repository base URL (must be a valid URI)
`pkey`	string	Yes	GPG key URL, absolute file path, or `[trusted=yes]` to skip verification
`component`	string	No	Repository component (e.g., `main`, `restricted`)
`priority`	int	No	Priority from `-9999` to `9999` (default: `0`, higher = preferred)
`AllowPackages`	string[]	No	Specific packages to include from this repo (package pinning)

packageRepositories:
  - codename: "company-internal"
    url: "https://packages.example.com/repo"
    pkey: "https://packages.example.com/gpg.key"
    component: "main"
    priority: 100
  - codename: "dev-tools"
    url: "https://dev.example.com/repo"
    pkey: "[trusted=yes]"

See Multiple Package Repository Support for detailed configuration guidance.

`systemConfig`#

System configuration - packages, kernel, users, bootloader, build-time commands, and more. Required in the final merged template, but optional in user templates (as defaults already provide a complete base).

Field	Type	Required	Description
`name`	string	No	Configuration name
`description`	string	No	Human-readable description
`hostname`	string	No	System hostname
`packages`	string[]	No	Packages to install (additive with defaults)
`kernel`	object	No	Kernel configuration
`bootloader`	object	No	Bootloader configuration
`immutability`	object	No	dm-verity / Secure Boot configuration
`users`	user[]	No	User account definitions
`initramfs`	object	No	Initramfs config (ISO/initrd builds)
`additionalFiles`	file[]	No	Extra files to copy into the image
`configurations`	cmd[]	No	Shell commands to run during build

Package names must match: ^[A-Za-z0-9](?:[A-Za-z0-9+_.:~-]*[A-Za-z0-9+])?$ and must be unique within the list.

`systemConfig.kernel`#

Field	Type	Description
`version`	string	Kernel version (e.g., `"6.12"`, `"6.14"`)
`cmdline`	string	Kernel boot command line
`packages`	string[]	Kernel packages (e.g., `["linux-image-generic-hwe-24.04"]`)
`enableExtraModules`	string	Additional kernel modules to load
`uki`	bool	Enable Unified Kernel Image (typically set by defaults)

systemConfig:
  kernel:
    version: "6.14"
    cmdline: "console=ttyS0,115200 console=tty0 loglevel=7"
    packages:
      - linux-image-generic-hwe-24.04

# Optional additional repositories
packageRepositories:
  - codename: emtNext
    url: https://example.com/rpms/next/base
    pkey: https://example.com/RPM-GPG-KEY
    priority: 1001
    allowPackages:
      - kernel-6.17.11
      - kernel-drivers-gpu-6.17.11
      - libva*

  - codename: edgeai
    url: https://example2.com/edgeai/
    pkey: https://example2.com/edgeai/GPG-PUB-KEY.gpg
    priority: 500

`systemConfig.bootloader`#

Field	Type	Valid Values	Description
`bootType`	string	`efi`, `legacy`	Boot firmware type
`provider`	string	`grub`, `grub2`, `systemd-boot`	Bootloader software

Typical defaults: raw images use efi / systemd-boot; ISO images use efi / grub.

`systemConfig.network`#

Declarative network configuration for the installed OS. This is a minimal explicit-interface implementation.

Field	Type	Required	Valid Values	Description
`backend`	string	Yes (when section present)	`systemd-networkd`, `netplan`	Network configuration backend
`interfaces`	object[]	No	See below	List of interface configurations

interfaces[] fields:

Field	Type	Required	Description
`name`	string	Yes	Interface name (for example, `enp1s0`, `ens3`)
`dhcp4`	bool	No	Enable DHCPv4
`dhcp6`	bool	No	Enable DHCPv6
`addresses`	string[]	No	Static addresses in CIDR format
`routes`	object[]	No	Static routes (see below)
`nameservers`	string[]	No	DNS server addresses

routes[] fields:

Field	Type	Required	Description
`to`	string	Yes	Destination (`default` for default gateway, or CIDR)
`via`	string	Yes	Gateway address

Note: Interface names are explicit and user-provided. The current implementation does not auto-discover or auto-select NICs at install time.

When backend is systemd-networkd, the builder enables the systemd-networkd service in the image. When backend is netplan, systemd-networkd is not forcibly enabled — netplan manages its own renderer.

systemConfig:
  network:
    backend: systemd-networkd
    interfaces:
      - name: enp1s0
        dhcp4: true
      - name: enp2s0
        addresses:
          - "10.0.0.100/24"
        routes:
          - to: default
            via: "10.0.0.1"
        nameservers:
          - "8.8.8.8"
          - "8.8.4.4"

systemConfig:
  network:
    backend: netplan
    interfaces:
      - name: enp1s0
        dhcp4: true
      - name: enp2s0
        addresses:
          - "192.168.1.10/24"
        routes:
          - to: default
            via: "192.168.1.1"
        nameservers:
          - "1.1.1.1"

`systemConfig.immutability`#

Configures dm-verity immutable root filesystem and optional UEFI Secure Boot signing.

Field	Type	Required	Description
`enabled`	bool	Yes (when section present)	Enable dm-verity immutable root
`secureBootDBKey`	string	Conditional	Private key file (`.key` or `.pem`)
`secureBootDBCrt`	string	Conditional	Certificate in PEM format (`.crt` or `.pem`)
`secureBootDBCer`	string	Conditional	Certificate in DER format (`.cer`)

Note: If any Secure Boot field is provided, all three must be provided and enabled must be true.

systemConfig:
  immutability:
    enabled: true
    secureBootDBKey: /path/to/db.key
    secureBootDBCrt: /path/to/db.crt
    secureBootDBCer: /path/to/db.cer

`systemConfig.fde`#

Configures LUKS2 full-disk encryption for selected partitions.

Field	Type	Required	Description
`enabled`	bool	Yes	Enable full-disk encryption
`passphraseFile`	string	Yes (when `enabled: true`)	Absolute or template-relative local file path containing passphrase material
`partitions`	string[]	No	Disk partition IDs to encrypt (defaults to the root partition)
`unlock`	string	No	Boot unlock mode: `auto` (default) or `manual`

systemConfig:
  fde:
    enabled: true
    passphraseFile: "/run/secrets/fde-passphrase.txt"
    unlock: auto
    partitions:
      - rootfs

See Configure Full-Disk Encryption for a complete guide, including usage with dm-verity immutability.

`systemConfig.users[]`#

Field	Type	Required	Description
`name`	string	Yes	Username
`password`	string	No	Password (plain text or pre-hashed with `$` prefix)
`hash_algo`	string	No	Hash algorithm: `bcrypt`, `sha512`, `sha256`, `md5` (md5 is insecure — avoid in production)
`passwordMaxAge`	int	No	Max password age in days
`startupScript`	string	No	Script to run on login
`groups`	string[]	No	Additional groups
`sudo`	bool	No	Grant sudo permissions
`home`	string	No	Custom home directory
`shell`	string	No	Login shell (e.g., `/bin/bash`)

systemConfig:
  users:
    - name: admin
      password: "changeme"
      sudo: true
      groups: [docker, wheel]
      shell: /bin/bash
      - name: service-account
      shell: /usr/sbin/nologin

`systemConfig.initramfs`#

Used for ISO and initrd builds. Points to the initramfs configuration template.

Field	Type	Required	Description
`template`	string	Yes (when section present)	Path to the initramfs config template file

`systemConfig.additionalFiles[]`#

Copy host files into the image at build time.

Field	Type	Description
`local`	string	Source path on the host (absolute, or relative to template directory)
`final`	string	Destination path inside the image
`stage`	string	Overlay-only. When to copy the file relative to initramfs/boot regeneration: `""` (default) copies at the end of the build, after regeneration; `pre-initramfs` copies before boot/initramfs regeneration so the generator can consume it. Ignored by create-mode builds

systemConfig:
  additionalFiles:
    - local: files/dhcp.network
      final: /etc/systemd/network/dhcp.network
    - local: files/motd
      final: /etc/motd
    # A dracut module must be in place BEFORE the initramfs is (re)built, or the
    # already-built initramfs would ignore it — mark it pre-initramfs.
    - local: files/90-custom.conf
      final: /usr/lib/dracut/dracut.conf.d/90-custom.conf
      stage: pre-initramfs

Overlay mode: additionalFiles are honored in overlay builds, and each entry’s copy timing is controlled by its stage marker:

stage: "" (default) — copied as the last build step, after both initramfs and GRUB regeneration. This is the historical behavior, so existing templates are unaffected. It is deliberate: a prebuilt boot artifact (for example a custom /boot/initrd.img-*) dropped here survives, whereas a file placed before regeneration would be overwritten by update-initramfs.

stage: pre-initramfs — copied before boot/initramfs regeneration, so content the generator consumes (a dracut module under /usr/lib/dracut, an initramfs-tools hook under /etc/initramfs-tools/) is in place when the initramfs is rebuilt. Without this, such a file dropped at the end would be ignored by the already-built initramfs.

A file only needs pre-initramfs when the initramfs build must see it; a file that must instead run a generator itself still belongs in a systemConfig.configurations command.

`systemConfig.configurations[]`#

Shell commands executed inside the chroot during the configuration stage.

Field	Type	Description
`cmd`	string	Shell command to execute

systemConfig:
  configurations:
    - cmd: systemctl enable docker
    - cmd: echo "BuildDate=$(date)" >> /etc/image-info

Package Repositories#

Use packageRepositories to add extra Debian or RPM repositories to a build. Each entry defines where to fetch package metadata and how candidates are selected when the same package exists in multiple repositories.

Repository Fields#

codename: repository identifier.
url: repository base URL.
component: optional Debian component (for example, main, universe) for multi-component repositories.
pkey: GPG key reference; supports http:///https:// URLs, file:// URLs, absolute local paths, or [trusted=yes] for supported Debian flows.
priority: numeric repository preference used in conflict resolution.
allowPackages: optional package white list for metadata filtering.

Priority Behavior#

priority is evaluated during package candidate selection across repositories.

Higher numeric values are preferred.
Debian resolver also supports APT-like behavior:
- < 0: block packages from that repository
- 990: prefer over default repositories
- 1000: install even if lower version
- > 1000: force preference

When candidates have equivalent priority, version constraints and dependency context determine the final package choice.

AllowPackages White List#

allowPackages limits which package names are indexed from a specific repository.

If omitted or empty, all repository packages are eligible.
If present, only matching package names are indexed.
Supported matching modes:
- exact name (for example spice-server)
- prefix/version pin (for example kernel-6.17.11)
- glob patterns (for example libva*, wayland*)

Filtering happens at metadata-parse time, before dependency resolution.

Template Merge Behavior#

When your user template is merged with the default template, different sections follow different strategies:

Section	Strategy
`image.name`, `image.version`	User overrides default if non-empty
`target`	User value used entirely
`disk`	User replaces entire default if non-empty
`systemConfig.packages`	Additive - user packages appended to defaults (deduplicated)
`systemConfig.kernel`	User overrides `version`, `cmdline`, `packages` individually if non-empty
`systemConfig.bootloader`	User overrides individual fields if non-empty
`systemConfig.network`	User overrides `backend` if non-empty; `interfaces` replaced when provided
`systemConfig.users`	Merged by `name` - same-name users merged field-by-field; new users appended
`systemConfig.additionalFiles`	Merged by `final` path - same destination overrides; new files appended
`systemConfig.configurations`	Additive - user commands appended after defaults
`systemConfig.immutability`	Merged only if user explicitly provides the section
`systemConfig.fde`	Merged only if user explicitly provides the section
`packageRepositories`	Merged by `codename` - same codename overrides; new repos appended

Template Extends (Inheritance)#

User templates can inherit from another template with the optional extends: field. This lets you keep minimal delta templates that pick up updates from their parent automatically, while still overriding anything you need to change. Each template inherits from at most one parent (single inheritance), so a chain is always a simple linear sequence root → ... → leaf — no diamond problem.

Syntax#

extends: is a top-level string field. Its value is a path to a parent template file, resolved relative to the child template’s directory. .yml and .yaml are both accepted; symbolic links are not.

The canonical two-level example ships in the repo at image-templates/ubuntu24/ubuntu24-x86_64-extends-example-raw.yml:

# Inherit everything (disk layout, systemConfig, immutability, ...) from the
# parent template, resolved relative to this file's directory. The parent's
# target must match this template's target. Values below override or extend the
# parent using the standard merge strategies (packages are additive).
extends: "ubuntu24-x86_64-minimal-raw.yml"

image:
  name: ubuntu24-x86_64-extends-example
  version: "24.04"

target:
  os: ubuntu # Must match the parent template's target
  dist: ubuntu24
  arch: x86_64
  imageType: raw

systemConfig:
  name: Extends_Example
  # These packages are merged additively on top of the parent's package list.
  packages:
    - htop
    - curl

Merge Behavior in a Chain#

The same per-section rules that govern the user↔default merge above apply at every level of an extends chain. Because MergeConfigurations(child, parent) is a pure function of two inputs, applying it iteratively as a fold produces the same set of merged fields at any depth: if the merge is deterministic for two layers, it is deterministic for N. Note that the ordering of slice entries merged by key (systemConfig.users by name, and systemConfig.additionalFiles by destination path) is not guaranteed to be stable across runs — the merge implementation uses a map to deduplicate keys, so the emitted slice order for those two fields may vary. Every other field’s ordering is stable. The table below lists the full behavior across a chain:

Section	Rule (chain semantics)
`image.name`, `image.version`	Non-empty child value overrides; last non-empty level in the chain wins
`target`	Replaced wholesale by the child’s value (all four sub-fields together)
`baseline` / `overlayPolicy`	Non-nil child pointer replaces the parent value
`disk`	Wholesale replacement when the child provides a non-empty disk config
`systemConfig.name` / `.description` / `.hostname` / `.initramfs.template`	Non-empty child overrides
`systemConfig.packages`	Additive union, deduplicated; each level’s packages are appended in chain order and de-duplicated
`systemConfig.users`	Merged by `name`: same-name entries are field-level merged; new users included (slice order not guaranteed to be stable across runs)
`systemConfig.additionalFiles`	Merged by `final` destination path: same target overrides; new files included (slice order not guaranteed to be stable across runs)
`systemConfig.configurations`	Additive concat: appended in chain order (root first, leaf last), no deduplication
`systemConfig.kernel`	Per-field override: `version`, `cmdline`, `packages`, `enableExtraModules`
`systemConfig.bootloader`	Per-field override: `bootType`, `provider`
`systemConfig.network`	Per-field: `backend` overrides if non-empty; `interfaces` replaced when the child provides them
`systemConfig.immutability`	Merged only when the child provides some immutability configuration
`packageRepositories`	Merged by `codename`: same codename overrides; new codenames appended
`extends`	Stripped from the final merged output — it is a build-time directive, not part of the built template

The rules quoted above are the exact per-field strategies enforced by MergeConfigurations in internal/config/merge.go.

Overlay mode caveat: when the leaf template uses baseline.mode: overlay, the additive-packages rule is deliberately overridden. The merged package list becomes exactly the user’s declared packages — the baseline already ships the create-mode toolchain, and unioning it back in would drag in bootloader packages whose strict version pins the frozen baseline cannot satisfy.

Chain Resolution#

At load time, the resolver walks the chain from leaf to root and then folds it back in root-to-leaf order so leaf values have the highest precedence. Once the chain is folded, the OS defaults for the target distribution are applied underneath, producing the effective layering:

OS defaults → root template → intermediate levels → leaf template

Each successive level takes precedence over everything below it. The merge is applied pairwise: fold(MergeConfigurations, defaults, [root, level1, ..., leaf]).

The build command logs the resolved chain at info level so you can see the inheritance hierarchy directly in build output:

Extends chain: root.yml -> child.yml -> leaf.yml

Worked example. Consider three templates where the leaf declares packages: [my-custom-app], the middle level declares packages: [prometheus-node-exporter, grafana-agent], the root declares packages: [docker-cli, containerd], and the OS defaults declare packages: [openssh-server]. The final merged template’s package list is the union: openssh-server, docker-cli, containerd, prometheus-node-exporter, grafana-agent, my-custom-app (deduplicated, default order preserved). If any two levels also set kernel.version, the last non-empty value in the chain wins.

Limitations and Validation Rules#

Single inheritance. Each template may reference at most one parent via extends:. Multiple inheritance and diamond-shaped graphs are not supported.
Cycle detection. The resolver walks the chain with a visited-set keyed on the symlink-resolved canonical absolute path of each template. Any repeat is rejected with circular extends detected: A -> B -> A. Canonicalizing the path ensures a directory symlink cannot alias two textual paths to the same file and evade the check.
Target match. Every template in the chain must share the same target.os, target.dist, target.arch, and target.imageType. A mismatch at any level is rejected with extends target mismatch at level N: child targets os/dist/arch/imageType but parent targets ....
Depth warning. Chains that exceed 4 levels emit extends chain depth N exceeds recommended maximum of 4 as a warning. This is a soft cap intended to keep hierarchies maintainable — the build still succeeds.
Path containment. The parent path is resolved relative to the child template’s directory. Both a lexical guard and a symlink-resolved guard reject any path that escapes that directory (extends path escapes child template's directory: ...), so a parent cannot pull a template in from an unrelated location on disk.
Symlink rejection. Parent templates that are themselves symbolic links are rejected at load time, matching the same policy the CLI uses for the leaf template.
File extension. Only .yml and .yaml extensions are accepted; other extensions are rejected.
Schema. extends: is defined as a plain optional string in os-image-template.schema.json with no pattern or format constraint — all validation happens in Go at load time so the errors above carry more context than a schema violation would.
Output stripping. The extends: field is stripped from the final merged template because it is a build-time directive, not part of the built image’s declarative state.

Debugging an Extends Chain#

The resolve subcommand renders a template exactly as the build system sees it after the chain is folded:

# Chain-only merge, without OS defaults (useful for verifying inheritance)
image-composer-tool resolve image-templates/ubuntu24/ubuntu24-x86_64-extends-example-raw.yml

# Full build-time view: OS defaults as base, extends chain folded on top (leaf wins)
image-composer-tool resolve image-templates/ubuntu24/ubuntu24-x86_64-extends-example-raw.yml --full

The output includes the merged systemConfig.packages union and every other field as it would be used at build time. Sensitive fields (user passwords, hash_algo values, and secure-boot key/cert paths) are redacted so the output is safe to paste into an issue or a code review. See Resolve Command for the full CLI reference.

Variable Substitution#

Templates support variable substitution using ${variable_name} syntax. You can provide variable values via a separate YAML file or command-line flags at build time.

To learn how variables interact with each build stage, see Build Stages in Detail.

WSL Required Fields#

To compose a WSL-compatible image, set target.imageType: wsl2 and include a WSL-compatible disk artifact definition.

Field	Required for WSL	Requirement
`image.name`	Yes	Standard image identifier
`image.version`	Yes	Standard image version
`target.os`	Yes	Any supported OS/distribution with a WSL2 default template
`target.dist`	Yes	Distribution for the selected OS (for example, `ubuntu24`)
`target.arch`	Yes	Use `x86_64` for current WSL2 templates
`target.imageType`	Yes	Must be `wsl2`
`disk.name`	Yes	Required when `imageType: wsl2`
`disk.artifacts[].type`	Yes	Must be `tar`
`disk.artifacts[].compression`	Yes	Must be `gz`

Additional notes for WSL builds:

The default Ubuntu WSL template seeds the standard Ubuntu apt sources via systemConfig.additionalFiles (for example, ubuntu-noble.list), which is the same mechanism used by the raw and initrd defaults.
disk.partitionTableType and disk.partitions are not used for wsl2 templates.
systemConfig.kernel is not allowed for wsl2 templates.

Example disk block for WSL:

disk:
  name: ubuntu24-x86_64-agentic
  artifacts:
    - type: tar
      compression: gz

See the full end-to-end example at image-templates/ubuntu24/ubuntu24-x86_64-agentic-wsl2.yml.

Best Practices#

Start from examples - copy a template from image-templates/ and modify only the fields you need. Let defaults handle the rest.
Keep templates minimal - override only what differs from the default. Smaller templates are easier to maintain and review.
Use descriptive names - name images and configs after their purpose (e.g., factory-floor-edge, not test-image-3).
Version control your templates - store them in Git alongside your deployment code.
Validate before building - run image-composer-tool validate template.yml to catch errors early.
Prefer additionalFiles over configurations - copying config files is more reproducible than running arbitrary shell commands.

Image Template Reference#

Table of Contents#

What Are Templates and How Do They Work?#

How Templates Work#

Quick Start Example#

Top-Level Structure#

Field Reference#

metadata#

image (required)#

target (required)#

WSL-Compatible Images#

baseline#

baseline.source#

SBOM generation (overlay mode)#

overlayPolicy#

disk#

disk.artifacts[]#

disk.partitions[]#

packageRepositories#

systemConfig#

systemConfig.kernel#

systemConfig.bootloader#

systemConfig.network#

systemConfig.immutability#

systemConfig.fde#

systemConfig.users[]#

systemConfig.initramfs#

systemConfig.additionalFiles[]#

systemConfig.configurations[]#