78 lines
3.1 KiB
Markdown
78 lines
3.1 KiB
Markdown
---
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description: describes get by digest pitfall
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keywords: registry, manifest, images, tags, repository, distribution, digest
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title: Registry compatibility
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---
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## Synopsis
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If a manifest is pulled by _digest_ from a registry 2.3 with Docker Engine 1.9
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and older, and the manifest was pushed with Docker Engine 1.10, a security check
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causes the Engine to receive a manifest it cannot use and the pull fails.
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## Registry manifest support
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Historically, the registry has supported a [single manifest type](./spec/manifest-v2-1.md)
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known as _Schema 1_.
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With the move toward multiple architecture images, the distribution project
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introduced two new manifest types: Schema 2 manifests and manifest lists. Registry
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2.3 supports all three manifest types and sometimes performs an on-the-fly
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transformation of a manifest before serving the JSON in the response, to
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preserve compatibility with older versions of Docker Engine.
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This conversion has some implications for pulling manifests by digest and this
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document enumerates these implications.
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## Content Addressable Storage (CAS)
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Manifests are stored and retrieved in the registry by keying off a digest
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representing a hash of the contents. One of the advantages provided by CAS is
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security: if the contents are changed, then the digest no longer matches.
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This prevents any modification of the manifest by a MITM attack or an untrusted
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third party.
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When a manifest is stored by the registry, this digest is returned in the HTTP
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response headers and, if events are configured, delivered within the event. The
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manifest can either be retrieved by the tag, or this digest.
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For registry versions 2.2.1 and below, the registry always stores and
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serves _Schema 1_ manifests. Engine 1.10 first
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attempts to send a _Schema 2_ manifest, falling back to sending a
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Schema 1 type manifest when it detects that the registry does not
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support the new version.
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## Registry v2.3
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### Manifest push with Docker 1.10
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The Engine constructs a _Schema 2_ manifest which the
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registry persists to disk.
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When the manifest is pulled by digest or tag with Docker Engine 1.10, a
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_Schema 2_ manifest is returned. Docker Engine 1.10
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understands the new manifest format.
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When the manifest is pulled by *tag* with Docker Engine 1.9 and older, the
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manifest is converted on-the-fly to _Schema 1_ and sent in the
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response. The Docker Engine 1.9 is compatible with this older format.
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When the manifest is pulled by _digest_ with Docker Engine 1.9 and older, the
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same rewriting process does not happen in the registry. If it did,
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the digest would no longer match the hash of the manifest and would violate the
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constraints of CAS.
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For this reason if a manifest is pulled by _digest_ from a registry 2.3 with Docker
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Engine 1.9 and older, and the manifest was pushed with Docker Engine 1.10, a
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security check causes the Engine to receive a manifest it cannot use and the
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pull fails.
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### Manifest push with Docker 1.9 and older
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The Docker Engine constructs a _Schema 1_ manifest which the
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registry persists to disk.
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When the manifest is pulled by digest or tag with any Docker version, a
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_Schema 1_ manifest is returned.
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