DDI output and metadata¶

A build produces three artifacts in the output directory, sharing a <name>_<version>_<arch> basename:

<name>_<version>_<arch>.raw     ← DDI (GPT image)
<name>_<version>_<arch>.json    ← package metadata
<name>_<version>_<arch>.zip     ← transport zip (only with --compress)

DDI (`.raw`)¶

A GPT image with a protective MBR and exactly three partitions:

#	Name	Type	Contents
1	`root`	arch-specific DPS root UUID	squashfs filesystem (zstd, 128 KiB block, all entries `root:root`)
2	`root-verity`	arch-specific DPS verity UUID	dm-verity superblock + Merkle hash tree over partition 1 (SHA-256, 4 KiB blocks, 32-byte salt)
3	`root-verity-sig`	arch-specific DPS verity-sig UUID	JSON `{rootHash, signature, certificateFingerprint}` (sector-padded)

The partition GUIDs of partitions 1 and 2 are derived from the roothash: root GUID = first 16 bytes, verity GUID = last 16 bytes. systemd requires this binding; mismatched GUIDs cause attach to fail.

Signature partition contents¶

{
  "rootHash": "<32-byte hex>",
  "signature": "<base64 DER PKCS7>",
  "certificateFingerprint": "<sha256 hex of DER cert>"
}

The PKCS7 signature covers rootHash (the hex string, not raw bytes; this is what systemd reconstructs at verify time). SHA-256 is set explicitly — go.mozilla.org/pkcs7 defaults to SHA-1, which OpenSSL 3.x rejects.

The verity salt is randomly generated per build. The same rootfs built twice produces different roothashes; this is expected.

Single-block roothash¶

When the squashfs fits in a single dm-verity data block, the kernel uses zero hash-tree levels and the roothash equals SHA256(salt + data_block). buildctl's vendored encoder handles this without padding.

Architecture UUIDs¶

Arch	DPS root partition type UUID
`arm64`	`B921B045-1DF0-41C3-AF44-4C6F280D3FAE`
`amd64`	`4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709`
`armv7`	`69DAD710-2CE4-4E3C-B16C-21A1D49ABED3`
`armv6`	`737A3E5E-7802-4B9E-9E51-9E1E3C60C6C0`
`i386`	`44479540-F297-41B2-9AF7-D131D5F0458A`
`riscv64`	`72EC70A6-CF74-40E6-BD49-4BDA08E8F224`
`ppc64le`	`C31C45E6-3F39-478E-8098-46DC49F7BD2A`
`s390x`	`08A7B11A-BAEF-4B5D-9B6D-6F4E4A4A3F2C`

Verity and verity-sig partitions have their own arch-specific type UUIDs per the Discoverable Partitions Specification.

Metadata JSON (`.json`)¶

Carries every field from package.yaml (except build-only fields backend, backend_arguments, strip), plus fields computed at build time:

Field	Source
`format_version`	Constant `"1.0"`.
`ddi_artifact`	`<name>_<version>_<arch>.raw` (the DDI filename).
`ddi_sha256`	SHA-256 of the `.raw` file, lowercase hex.
`ddi_size`	Size of the `.raw` file in bytes.
`created_at`	Build timestamp, RFC 3339, UTC.
`signing_key_id`	SHA-256 of the DER encoding of the signing certificate, lowercase hex. Matches `certificateFingerprint` in the signature partition.

command is included only if set; network only when runtime: nspawn. uid/gid are not included; user allocation is appctl's responsibility at install time.

For the full schema, see package-metadata.schema.json.

Transport zip (`.zip`)¶

--compress (or buildctl compress) writes a zip of the .raw and .json files using Store (no compression) — the squashfs is already zstd-compressed.