mirror of
https://github.com/genuinetools/reg.git
synced 2024-08-18 13:00:24 -04:00
275 lines
13 KiB
Markdown
275 lines
13 KiB
Markdown
# OCI Image Configuration
|
|
|
|
An OCI *Image* is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime.
|
|
This specification outlines the JSON format describing images for use with a container runtime and execution tool and its relationship to filesystem changesets, described in [Layers](layer.md).
|
|
|
|
This section defines the `application/vnd.oci.image.config.v1+json` [media type](media-types.md).
|
|
|
|
## Terminology
|
|
|
|
This specification uses the following terms:
|
|
|
|
### [Layer](layer.md)
|
|
|
|
* Image filesystems are composed of *layers*.
|
|
* Each layer represents a set of filesystem changes in a tar-based [layer format](layer.md), recording files to be added, changed, or deleted relative to its parent layer.
|
|
* Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer.
|
|
* Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if they were one cohesive filesystem.
|
|
|
|
### Image JSON
|
|
|
|
* Each image has an associated JSON structure which describes some basic information about the image such as date created, author, as well as execution/runtime configuration like its entrypoint, default arguments, networking, and volumes.
|
|
* The JSON structure also references a cryptographic hash of each layer used by the image, and provides history information for those layers.
|
|
* This JSON is considered to be immutable, because changing it would change the computed [ImageID](#imageid).
|
|
* Changing it means creating a new derived image, instead of changing the existing image.
|
|
|
|
### Layer DiffID
|
|
|
|
A layer DiffID is the digest over the layer's uncompressed tar archive and serialized in the descriptor digest format, e.g., `sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9`.
|
|
Layers SHOULD be packed and unpacked reproducibly to avoid changing the layer DiffID, for example by using [tar-split][] to save the tar headers.
|
|
|
|
NOTE: Do not confuse DiffIDs with [layer digests](manifest.md#image-manifest-property-descriptions), often referenced in the manifest, which are digests over compressed or uncompressed content.
|
|
|
|
### Layer ChainID
|
|
|
|
For convenience, it is sometimes useful to refer to a stack of layers with a single identifier.
|
|
While a layer's `DiffID` identifies a single changeset, the `ChainID` identifies the subsequent application of those changesets.
|
|
This ensures that we have handles referring to both the layer itself, as well as the result of the application of a series of changesets.
|
|
Use in combination with `rootfs.diff_ids` while applying layers to a root filesystem to uniquely and safely identify the result.
|
|
|
|
#### Definition
|
|
|
|
The `ChainID` of an applied set of layers is defined with the following recursion:
|
|
|
|
```
|
|
ChainID(L₀) = DiffID(L₀)
|
|
ChainID(L₀|...|Lₙ₋₁|Lₙ) = Digest(ChainID(L₀|...|Lₙ₋₁) + " " + DiffID(Lₙ))
|
|
```
|
|
|
|
For this, we define the binary `|` operation to be the result of applying the right operand to the left operand.
|
|
For example, given base layer `A` and a changeset `B`, we refer to the result of applying `B` to `A` as `A|B`.
|
|
|
|
Above, we define the `ChainID` for a single layer (`L₀`) as equivalent to the `DiffID` for that layer.
|
|
Otherwise, the `ChainID` for a set of applied layers (`L₀|...|Lₙ₋₁|Lₙ`) is defined as the recursion `Digest(ChainID(L₀|...|Lₙ₋₁) + " " + DiffID(Lₙ))`.
|
|
|
|
#### Explanation
|
|
|
|
Let's say we have layers A, B, C, ordered from bottom to top, where A is the base and C is the top.
|
|
Defining `|` as a binary application operator, the root filesystem may be `A|B|C`.
|
|
While it is implied that `C` is only useful when applied to `A|B`, the identifier `C` is insufficient to identify this result, as we'd have the equality `C = A|B|C`, which isn't true.
|
|
|
|
The main issue is when we have two definitions of `C`, `C = C` and `C = A|B|C`.
|
|
If this is true (with some handwaving), `C = x|C` where `x = any application`.
|
|
This means that if an attacker can define `x`, relying on `C` provides no guarantee that the layers were applied in any order.
|
|
|
|
The `ChainID` addresses this problem by being defined as a compound hash.
|
|
__We differentiate the changeset `C`, from the order-dependent application `A|B|C` by saying that the resulting rootfs is identified by ChainID(A|B|C), which can be calculated by `ImageConfig.rootfs`.__
|
|
|
|
Let's expand the definition of `ChainID(A|B|C)` to explore its internal structure:
|
|
|
|
```
|
|
ChainID(A) = DiffID(A)
|
|
ChainID(A|B) = Digest(ChainID(A) + " " + DiffID(B))
|
|
ChainID(A|B|C) = Digest(ChainID(A|B) + " " + DiffID(C))
|
|
```
|
|
|
|
We can replace each definition and reduce to a single equality:
|
|
|
|
```
|
|
ChainID(A|B|C) = Digest(Digest(DiffID(A) + " " + DiffID(B)) + " " + DiffID(C))
|
|
```
|
|
|
|
Hopefully, the above is illustrative of the _actual_ contents of the `ChainID`.
|
|
Most importantly, we can easily see that `ChainID(C) != ChainID(A|B|C)`, otherwise, `ChainID(C) = DiffID(C)`, which is the base case, could not be true.
|
|
|
|
### ImageID
|
|
|
|
Each image's ID is given by the SHA256 hash of its [configuration JSON](#image-json).
|
|
It is represented as a hexadecimal encoding of 256 bits, e.g., `sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9`.
|
|
Since the [configuration JSON](#image-json) that gets hashed references hashes of each layer in the image, this formulation of the ImageID makes images content-addressable.
|
|
|
|
## Properties
|
|
|
|
Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being absent.
|
|
|
|
- **created** *string*, OPTIONAL
|
|
|
|
An combined date and time at which the image was created, formatted as defined by [RFC 3339, section 5.6][rfc3339-s5.6].
|
|
|
|
- **author** *string*, OPTIONAL
|
|
|
|
Gives the name and/or email address of the person or entity which created and is responsible for maintaining the image.
|
|
|
|
- **architecture** *string*, REQUIRED
|
|
|
|
The CPU architecture which the binaries in this image are built to run on.
|
|
Configurations SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOARCH`][go-environment].
|
|
|
|
- **os** *string*, REQUIRED
|
|
|
|
The name of the operating system which the image is built to run on.
|
|
Configurations SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOOS`][go-environment].
|
|
|
|
- **config** *object*, OPTIONAL
|
|
|
|
The execution parameters which SHOULD be used as a base when running a container using the image.
|
|
This field can be `null`, in which case any execution parameters should be specified at creation of the container.
|
|
|
|
- **User** *string*, OPTIONAL
|
|
|
|
The username or UID which is a platform-specific structure that allows specific control over which user the process run as.
|
|
This acts as a default value to use when the value is not specified when creating a container.
|
|
For Linux based systems, all of the following are valid: `user`, `uid`, `user:group`, `uid:gid`, `uid:group`, `user:gid`.
|
|
If `group`/`gid` is not specified, the default group and supplementary groups of the given `user`/`uid` in `/etc/passwd` from the container are applied.
|
|
|
|
- **ExposedPorts** *object*, OPTIONAL
|
|
|
|
A set of ports to expose from a container running this image.
|
|
Its keys can be in the format of:
|
|
`port/tcp`, `port/udp`, `port` with the default protocol being `tcp` if not specified.
|
|
These values act as defaults and are merged with any specified when creating a container.
|
|
**NOTE:** This JSON structure value is unusual because it is a direct JSON serialization of the Go type `map[string]struct{}` and is represented in JSON as an object mapping its keys to an empty object.
|
|
|
|
- **Env** *array of strings*, OPTIONAL
|
|
|
|
Entries are in the format of `VARNAME=VARVALUE`.
|
|
These values act as defaults and are merged with any specified when creating a container.
|
|
|
|
- **Entrypoint** *array of strings*, OPTIONAL
|
|
|
|
A list of arguments to use as the command to execute when the container starts.
|
|
These values act as defaults and may be replaced by an entrypoint specified when creating a container.
|
|
|
|
- **Cmd** *array of strings*, OPTIONAL
|
|
|
|
Default arguments to the entrypoint of the container.
|
|
These values act as defaults and may be replaced by any specified when creating a container.
|
|
If an `Entrypoint` value is not specified, then the first entry of the `Cmd` array SHOULD be interpreted as the executable to run.
|
|
|
|
- **Volumes** *object*, OPTIONAL
|
|
|
|
A set of directories describing where the process is likely write data specific to a container instance.
|
|
**NOTE:** This JSON structure value is unusual because it is a direct JSON serialization of the Go type `map[string]struct{}` and is represented in JSON as an object mapping its keys to an empty object.
|
|
|
|
- **WorkingDir** *string*, OPTIONAL
|
|
|
|
Sets the current working directory of the entrypoint process in the container.
|
|
This value acts as a default and may be replaced by a working directory specified when creating a container.
|
|
|
|
- **Labels** *object*, OPTIONAL
|
|
|
|
The field contains arbitrary metadata for the container.
|
|
This property MUST use the [annotation rules](annotations.md#rules).
|
|
|
|
- **StopSignal** *string*, OPTIONAL
|
|
|
|
The field contains the system call signal that will be sent to the container to exit. The signal can be a signal name in the format `SIGNAME`, for instance `SIGKILL` or `SIGRTMIN+3`.
|
|
|
|
- **rootfs** *object*, REQUIRED
|
|
|
|
The rootfs key references the layer content addresses used by the image.
|
|
This makes the image config hash depend on the filesystem hash.
|
|
|
|
- **type** *string*, REQUIRED
|
|
|
|
MUST be set to `layers`.
|
|
Implementations MUST generate an error if they encounter a unknown value while verifying or unpacking an image.
|
|
|
|
- **diff_ids** *array of strings*, REQUIRED
|
|
|
|
An array of layer content hashes (`DiffIDs`), in order from first to last.
|
|
|
|
- **history** *array of objects*, OPTIONAL
|
|
|
|
Describes the history of each layer.
|
|
The array is ordered from first to last.
|
|
The object has the following fields:
|
|
|
|
- **created** *string*, OPTIONAL
|
|
|
|
A combined date and time at which the layer was created, formatted as defined by [RFC 3339, section 5.6][rfc3339-s5.6].
|
|
|
|
- **author** *string*, OPTIONAL
|
|
|
|
The author of the build point.
|
|
|
|
- **created_by** *string*, OPTIONAL
|
|
|
|
The command which created the layer.
|
|
|
|
- **comment** *string*, OPTIONAL
|
|
|
|
A custom message set when creating the layer.
|
|
|
|
- **empty_layer** *boolean*, OPTIONAL
|
|
|
|
This field is used to mark if the history item created a filesystem diff.
|
|
It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, Dockerfile's [ENV](https://docs.docker.com/engine/reference/builder/#/env) command results in no change to the filesystem).
|
|
|
|
Any extra fields in the Image JSON struct are considered implementation specific and MUST be ignored by any implementations which are unable to interpret them.
|
|
|
|
Whitespace is OPTIONAL and implementations MAY have compact JSON with no whitespace.
|
|
|
|
## Example
|
|
|
|
Here is an example image configuration JSON document:
|
|
|
|
```json,title=Image%20JSON&mediatype=application/vnd.oci.image.config.v1%2Bjson
|
|
{
|
|
"created": "2015-10-31T22:22:56.015925234Z",
|
|
"author": "Alyssa P. Hacker <alyspdev@example.com>",
|
|
"architecture": "amd64",
|
|
"os": "linux",
|
|
"config": {
|
|
"User": "alice",
|
|
"ExposedPorts": {
|
|
"8080/tcp": {}
|
|
},
|
|
"Env": [
|
|
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
|
|
"FOO=oci_is_a",
|
|
"BAR=well_written_spec"
|
|
],
|
|
"Entrypoint": [
|
|
"/bin/my-app-binary"
|
|
],
|
|
"Cmd": [
|
|
"--foreground",
|
|
"--config",
|
|
"/etc/my-app.d/default.cfg"
|
|
],
|
|
"Volumes": {
|
|
"/var/job-result-data": {},
|
|
"/var/log/my-app-logs": {}
|
|
},
|
|
"WorkingDir": "/home/alice",
|
|
"Labels": {
|
|
"com.example.project.git.url": "https://example.com/project.git",
|
|
"com.example.project.git.commit": "45a939b2999782a3f005621a8d0f29aa387e1d6b"
|
|
}
|
|
},
|
|
"rootfs": {
|
|
"diff_ids": [
|
|
"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
|
|
"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
|
|
],
|
|
"type": "layers"
|
|
},
|
|
"history": [
|
|
{
|
|
"created": "2015-10-31T22:22:54.690851953Z",
|
|
"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
|
|
},
|
|
{
|
|
"created": "2015-10-31T22:22:55.613815829Z",
|
|
"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
|
|
"empty_layer": true
|
|
}
|
|
]
|
|
}
|
|
```
|
|
|
|
[rfc3339-s5.6]: https://tools.ietf.org/html/rfc3339#section-5.6
|
|
[go-environment]: https://golang.org/doc/install/source#environment
|
|
[tar-split]: https://github.com/vbatts/tar-split
|