rkt configuration

rkt reads configuration from two or three directories - a system directory, a local directory and, if provided, a user directory. The system directory defaults to /usr/lib/rkt, the local directory to /etc/rkt, and the user directory to an empty string. These locations can be changed with command line flags described below.

The system directory should contain a configuration created by a vendor (e.g. distribution). The contents of this directory should not be modified - it is meant to be read only.

The local directory keeps configuration local to the machine. It can be modified by the admin.

The user directory may hold some user specific configuration. It may be useful for specifying credentials used for fetching images without spilling them to some directory readable by everyone.

rkt looks for configuration files with the .json file name extension in subdirectories beneath the system and local directories. rkt does not recurse down the directory tree to search for these files. Users may therefore put additional appropriate files (e.g., documentation) alongside rkt configuration in these directories, provided such files are not named with the .json extension.

Every configuration file has two common fields: rktKind and rktVersion. Both fields' values are strings, and the subsequent fields are specified by this pair. The currently supported kinds and versions are described below. These fields must be specified and cannot be empty.

rktKind describes the type of the configuration. This is to avoid putting unrelated values into a single monolithic file.

rktVersion allows configuration versioning for each kind of configuration. A new version should be introduced when doing some backward-incompatible changes: for example, when removing a field or incompatibly changing its semantics. When a new field is added, a default value should be specified for it, documented, and used when the field is absent in any configuration file. This way, an older version of rkt can work with newer-but-compatible versions of configuration files, and newer versions of rkt can still work with older versions of configuration files.

Configuration values in the system directory are superseded by the value of the same field if it exists in the local directory. The same relationship exists between the local directory and the user directory if the user directory is provided. The semantics of overriding configuration in this manner are specific to the kind and version of the configuration, and are described below. File names are not examined in determining local overrides. Only the fields inside configuration files need to match.

Command line flags

To change the system configuration directory, use --system-config flag. To change the local configuration directory, use --local-config flag. To change the user configuration directory, use --user-config flag.

Configuration kinds

rktKind: auth

The auth configuration kind is used to set up necessary credentials when downloading images and signatures. The configuration files should be placed inside the auth.d subdirectory (e.g., in the case of the default system/local directories, in /usr/lib/rkt/auth.d and/or /etc/rkt/auth.d).

rktVersion: v1

Description and examples

This version of the auth configuration specifies three additional fields: domains, type and credentials.

The domains field is an array of strings describing hosts for which the following credentials should be used. Each entry must consist of a host/port combination in a URL as specified by RFC 3986. This field must be specified and cannot be empty.

The type field describes the type of credentials to be sent. This field must be specified and cannot be empty.

The credentials field is defined by the type field. It should hold all the data that are needed for successful authentication with the given hosts.

This version of auth configuration supports three methods - basic HTTP authentication, OAuth Bearer Token, and AWS v4 authentication.

Basic HTTP authentication requires two things - a user and a password. To use this type, define type as basic and the credentials field as a map with two keys - user and password. These fields must be specified and cannot be empty. For example:

/etc/rkt/auth.d/coreos-basic.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["coreos.com", "tectonic.com"],
	"type": "basic",
	"credentials": {
		"user": "foo",
		"password": "bar"
	}
}

OAuth Bearer Token authentication requires only a token. To use this type, define type as oauth and the credentials field as a map with only one key - token. This field must be specified and cannot be empty. For example:

/etc/rkt/auth.d/coreos-oauth.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["coreos.com", "tectonic.com"],
	"type": "oauth",
	"credentials": {
		"token": "sometoken"
	}
}

AWS v4 authentication requires three things - an access key ID, a secret access key and an AWS region. If the region is left empty, it will be determined automatically from the URL/domain. To use this type, define type as aws and the credentials field as a map with two or three keys - accessKeyID and secretAccessKey are mandatory, whilst awsRegion is optional and can be left empty. For example:

/etc/rkt/auth.d/coreos-aws.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["my-s3-bucket.s3.amazonaws.com"],
	"type": "aws",
	"credentials": {
		"accessKeyID": "foo",
		"secretAccessKey": "bar",
		"awsRegion": "us-east-1"
	}
}
Override semantics

Overriding is done for each domain. That means that the user can override authentication type and/or credentials used for each domain. As an example, consider this system configuration:

/usr/lib/rkt/auth.d/coreos.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["coreos.com", "tectonic.com", "kubernetes.io"],
	"type": "oauth",
	"credentials": {
		"token": "common-token"
	}
}

If only this configuration file is provided, then when downloading data from either coreos.com, tectonic.com or kubernetes.io, rkt would send an HTTP header of: Authorization: Bearer common-token.

But with additional configuration provided in the local configuration directory, this can be overridden. For example, given the above system configuration and the following local configuration:

/etc/rkt/auth.d/specific-coreos.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["coreos.com"],
	"type": "basic",
	"credentials": {
		"user": "foo",
		"password": "bar"
	}
}

/etc/rkt/auth.d/specific-tectonic.json:

{
	"rktKind": "auth",
	"rktVersion": "v1",
	"domains": ["tectonic.com"],
	"type": "oauth",
	"credentials": {
		"token": "tectonic-token"
	}
}

The result is that when downloading data from kubernetes.io, rkt still sends Authorization: Bearer common-token, but when downloading from coreos.com, it sends Authorization: Basic Zm9vOmJhcg== (i.e. foo:bar encoded in base64). For tectonic.com, it will send Authorization: Bearer tectonic-token.

Note that within a particular configuration directory (either system or local), it is a syntax error for the same domain to be defined in multiple files.

Command line flags

There are no command line flags for specifying or overriding the auth configuration.

rktKind: dockerAuth

The dockerAuth configuration kind is used to set up necessary credentials when downloading data from Docker registries. The configuration files should be placed inside auth.d subdirectory (e.g. in /usr/lib/rkt/auth.d or /etc/rkt/auth.d).

rktVersion: v1

Description and examples

This version of dockerAuth configuration specifies two additional fields: registries and credentials.

The registries field is an array of strings describing Docker registries for which the associated credentials should be used. This field must be specified and cannot be empty. A short list of popular Docker registries is given below.

The credentials field holds the necessary data to authenticate against the Docker registry. This field must be specified and cannot be empty.

Currently, Docker registries only support basic HTTP authentication, so credentials has two subfields - user and password. These fields must be specified and cannot be empty.

Some popular Docker registries:

  • registry-1.docker.io (Assumed as the default when no specific registry is named on the rkt command line, as in docker:///redis.)
  • quay.io
  • gcr.io

Example dockerAuth configuration:

/etc/rkt/auth.d/docker.json:

{
	"rktKind": "dockerAuth",
	"rktVersion": "v1",
	"registries": ["registry-1.docker.io", "quay.io"],
	"credentials": {
		"user": "foo",
		"password": "bar"
	}
}
Override semantics

Overriding is done for each registry. That means that the user can override credentials used for each registry. For example, given this system configuration:

/usr/lib/rkt/auth.d/docker.json:

{
	"rktKind": "dockerAuth",
	"rktVersion": "v1",
	"registries": ["registry-1.docker.io", "gcr.io", "quay.io"],
	"credentials": {
		"user": "foo",
		"password": "bar"
	}
}

If only this configuration file is provided, then when downloading images from either registry-1.docker.io, gcr.io, or quay.io, rkt would use user foo and password bar.

But with additional configuration provided in the local configuration directory, this can be overridden. For example, given the above system configuration and the following local configuration:

/etc/rkt/auth.d/specific-quay.json:

{
	"rktKind": "dockerAuth",
	"rktVersion": "v1",
	"registries": ["quay.io"],
	"credentials": {
		"user": "baz",
		"password": "quux"
	}
}

/etc/rkt/auth.d/specific-gcr.json:

{
	"rktKind": "dockerAuth",
	"rktVersion": "v1",
	"registries": ["gcr.io"],
	"credentials": {
		"user": "goo",
		"password": "gle"
	}
}

The result is that when downloading images from registry-1.docker.io, rkt still sends user foo and password bar, but when downloading from quay.io, it uses user baz and password quux; and for gcr.io it will use user goo and password gle.

Note that within a particular configuration directory (either system or local), it is a syntax error for the same Docker registry to be defined in multiple files.

Command line flags

There are no command line flags for specifying or overriding the docker auth configuration.

rktKind: paths

The paths configuration kind is used to customize the various paths that rkt uses. The configuration files should be placed inside the paths.d subdirectory (e.g., in the case of the default system/local directories, in /usr/lib/rkt/paths.d and/or /etc/rkt/paths.d).

rktVersion: v1

Description and examples

This version of the paths configuration specifies two additional fields: data and stage1-images.

The data field is a string that defines where image data and running pods are stored. This field is optional.

The stage1-images field is a string that defines where are the stage1 images are stored, so rkt can search for them when using the --stage1-from-dir flag. This field is optional.

Example paths configuration:

/etc/rkt/paths.d/paths.json:

{
	"rktKind": "paths",
	"rktVersion": "v1",
	"data": "/home/me/rkt/data",
	"stage1-images": "/home/me/rkt/stage1-images"
}
Override semantics

Overriding is done for each path. For example, given this system configuration:

/usr/lib/rkt/paths.d/data.json:

{
	"rktKind": "paths",
	"rktVersion": "v1",
	"data": "/opt/rkt-stuff/data"
}

If only this configuration file is provided, then rkt will store images and pods in the /opt/rkt-stuff/data directory. Also, when user passes --stage1-from-dir=stage1.aci to rkt, rkt will search for this file in the directory specified at build time (usually /usr/lib/rkt/stage1-images).

But with additional configuration provided in the local configuration directory, this can be overridden. For example, given the above system configuration and the following local configuration:

/etc/rkt/paths.d/paths.json:

{
	"rktKind": "paths",
	"rktVersion": "v1",
	"data": "/home/me/rkt"
}

Now rkt will store the images and pods in the /home/me/rkt directory. It will not know about any other data directory. Also, rkt will still search for the stage1 images in the directory specified at build time for the --stage1-from-dir flag.

To override the stage1 images directory:

/etc/rkt/paths.d/stage1.json:

{
	"rktKind": "paths",
	"rktVersion": "v1",
	"stage1-images": "/home/me/stage1-images"
}

Now rkt will search in the /home/me/stage1/images directory, not in the directory specified at build time.

Command line flags

The data field can be overridden with the --dir flag. The stage1-images field cannot be overridden with a command line flag.

rktKind: stage1

The stage1 configuration kind is used to set up a default stage1 image. The configuration files should be placed inside the stage1.d subdirectory (e.g., in the case of the default system/local directories, in /usr/lib/rkt/stage1.d and/or /etc/rkt/stage1.d).

rktVersion: v1

Description and examples

This version of the stage1 configuration specifies three additional fields: name, version and location.

The name field is a string specifying a name of a default stage1 image. This field is optional. If specified, the version field must be specified too.

The version field is a string specifying a version of a default stage1 image. This field is optional. If specified, the name field must be specified too.

The location field is a string describing the location of a stage1 image file. This field is optional.

The name and version fields are used by rkt (unless overridden with a run-time flag or left empty) to search for the stage1 image in the image store. If it is not found there then rkt will use a value from the location field (again, unless overridden or empty) to fetch the stage1 image.

If the name, version and location fields are specified then it is expected that the file in location is a stage1 image with the same name and version in manifest as values of the name and version fields, respectively. Note that this is not enforced in any way.

The location field can be:

  • a file:// URL
  • a http:// URL
  • a https:// URL
  • a docker:// URL
  • an absolute path (basically the same as a file:// URL)

An example:

{
	"rktKind": "stage1",
	"rktVersion": "v1",
	"name": "example.com/rkt/stage1",
	"version": "1.2.3",
	"location": "https://example.com/download/stage1-1.2.3.aci"
}
Override semantics

Overriding is done separately for the name-and-version pairs and for the locations. That means that the user can override either both a name and a version or a location. As an example, consider this system configuration:

/usr/lib/rkt/stage1.d/coreos.json:

{
	"rktKind": "stage1",
	"rktVersion": "v1",
	"name": "coreos.com/rkt/stage1-coreos",
	"version": "0.15.0+git",
	"location": "/usr/libexec/rkt/stage1-coreos.aci"
}

If only this configuration file is provided then rkt will check if coreos.com/rkt/stage1-coreos with version 0.15.0+git is in image store. If it is absent then it would fetch it from /usr/libexec/rkt/stage1-coreos.aci.

But with additional configuration provided in the local configuration directory, this can be overridden. For example, given the above system configuration and the following local configurations:

/etc/rkt/stage1.d/specific-coreos.json:

{
	"rktKind": "stage1",
	"rktVersion": "v1",
	"location": "https://example.com/coreos-stage1.aci"
}

The result is that rkt will still look for coreos.com/rkt/stage1-coreos with version 0.15.0+git in the image store, but if it is not found, it will fetch it from https://example.com/coreos-stage1.aci.

To continue the example, we can also override name and version with an additional configuration file like this:

/etc/rkt/stage1.d/other-name-and-version.json:

{
	"rktKind": "stage1",
	"rktVersion": "v1",
	"name": "example.com/rkt/stage1",
	"version": "1.2.3"
}

Now rkt will search for example.com/rkt/stage1 with version 1.2.3 in the image store before trying to fetch the image from https://example.com/coreos-stage1.aci.

Note that within a particular configuration directory (either system or local), it is a syntax error for the name, version or location to be defined in multiple files.

Command line flags

The name, version and location fields are ignored in favor of a value coming from --stage1-url, --stage1-path, --stage1-name, --stage1-hash, or --stage1-from-dir flags.