Registry compatibilityEstimated reading time: 2 minutes
If a manifest is pulled by _digest_ from a registry 2.3 with Docker Engine 1.9 and older, and the manifest was pushed with Docker Engine 1.10, a security check will cause the Engine to receive a manifest it cannot use and the pull will fail.
Registry Manifest Support
Historically, the registry has supported a single manifest type known as Schema 1.
With the move toward multiple architecture images the distribution project introduced two new manifest types: Schema 2 manifests and manifest lists. The registry 2.3 supports all three manifest types and in order to be compatible with older Docker engines will, in certain cases, do an on-the-fly transformation of a manifest before serving the JSON in the response.
This conversion has some implications for pulling manifests by digest and this document enumerate these implications.
Content Addressable Storage (CAS)
Manifests are stored and retrieved in the registry by keying off a digest representing a hash of the contents. One of the advantages provided by CAS is security: if the contents are changed, then the digest will no longer match. This prevents any modification of the manifest by a MITM attack or an untrusted third party.
When a manifest is stored by the registry, this digest is returned in the HTTP response headers and, if events are configured, delivered within the event. The manifest can either be retrieved by the tag, or this digest.
For registry versions 2.2.1 and below, the registry will always store and serve Schema 1 manifests. The Docker Engine 1.10 will first attempt to send a Schema 2 manifest, falling back to sending a Schema 1 type manifest when it detects that the registry does not support the new version.
Manifest Push with Docker 1.9 and Older
The Docker Engine will construct a Schema 1 manifest which the registry will persist to disk.
When the manifest is pulled by digest or tag with any docker version, a Schema 1 manifest will be returned.
Manifest Push with Docker 1.10
The docker engine will construct a Schema 2 manifest which the registry will persist to disk.
When the manifest is pulled by digest or tag with Docker Engine 1.10, a Schema 2 manifest will be returned. The Docker Engine 1.10 understands the new manifest format.
When the manifest is pulled by tag with Docker Engine 1.9 and older, the manifest is converted on-the-fly to Schema 1 and sent in the response. The Docker Engine 1.9 is compatible with this older format.
When the manifest is pulled by _digest_ with Docker Engine 1.9 and older, the same rewriting process will not happen in the registry. If this were to happen the digest would no longer match the hash of the manifest and would violate the constraints of CAS.
For this reason if a manifest is pulled by digest from a registry 2.3 with Docker Engine 1.9 and older, and the manifest was pushed with Docker Engine 1.10, a security check will cause the Engine to receive a manifest it cannot use and the pull will fail.