% CONTAINERS-REGISTRIES.CONF 5 System-wide registry configuration file % Brent Baude % Aug 2017
containers-registries.conf - Syntax of System Registry Configuration File
The CONTAINERS-REGISTRIES configuration file is a system-wide configuration file for container image registries. The file format is TOML.
Container engines will use the $HOME/.config/containers/registries.conf
if it exists, otherwise they will use /etc/containers/registries.conf
unqualified-search-registries
: An array of host[:
port] registries to try when pulling an unqualified image, in order.
credential-helpers
: An array of default credential helpers used as external credential stores. Note that "containers-auth.json" is a reserved value to use auth files as specified in containers-auth.json(5). The credential helpers are set to ["containers-auth.json"]
if none are specified.
additional-layer-store-auth-helper
: A string containing the helper binary name. This enables passing registry credentials to an
Additional Layer Store every time an image is read using the docker://
transport so that it can access private registries. See the 'Enabling Additional Layer Store to access to private registries' section below for
more details.
The bulk of the configuration is represented as an array of [[registry]]
TOML tables; the settings may therefore differ among different registries
as well as among different namespaces/repositories within a registry.
Given an image name, a single [[registry]]
TOML table is chosen based on its prefix
field.
prefix
: A prefix of the user-specified image name, i.e. using one of the following formats:
- host[
:
port] - host[
:
port]/
namespace[/
namespace…] - host[
:
port]/
namespace[/
namespace…]/
repo - host[
:
port]/
namespace[/
namespace…]/
repo(:
_tag|@
digest) - [
*.
]host
The user-specified image name must start with the specified prefix
(and continue
with the appropriate separator) for a particular [[registry]]
TOML table to be
considered; (only) the TOML table with the longest match is used. It can
also include wildcarded subdomains in the format *.example.com
.
The wildcard should only be present at the beginning as shown in the formats
above. Other cases will not work. For example, *.example.com
is valid but
example.*.com
, *.example.com/foo
and *.example.com:5000/foo/bar:baz
are not.
Note that *
matches an arbitrary number of subdomains. *.example.com
will hence
match bar.example.com
, foo.bar.example.com
and so on.
As a special case, the prefix
field can be missing; if so, it defaults to the value
of the location
field (described below).
insecure
: true
or false
.
By default, container runtimes require TLS when retrieving images from a registry.
If insecure
is set to true
, unencrypted HTTP as well as TLS connections with untrusted
certificates are allowed.
blocked
: true
or false
.
If true
, pulling images with matching names is forbidden.
The user-specified image reference is, primarily, a "logical" image name, always used for naming
the image. By default, the image reference also directly specifies the registry and repository
to use, but the following options can be used to redirect the underlying accesses
to different registry servers or locations (e.g. to support configurations with no access to the
internet without having to change Dockerfile
s, or to add redundancy).
location
: Accepts the same format as the prefix
field, and specifies the physical location
of the prefix
-rooted namespace.
By default, this is equal to prefix
(in which case prefix
can be omitted and the
[[registry]]
TOML table can only specify location
).
Example: Given
prefix = "example.com/foo"
location = "internal-registry-for-example.com/bar"
requests for the image example.com/foo/myimage:latest
will actually work with the
internal-registry-for-example.com/bar/myimage:latest
image.
With a prefix
containing a wildcard in the format: "*.example.com" for subdomain matching,
the location can be empty. In such a case,
prefix matching will occur, but no reference rewrite will occur. The
original requested image string will be used as-is. But other settings like
insecure
/ blocked
/ mirrors
will be applied to matching images.
Example: Given
prefix = "*.example.com"
requests for the image blah.example.com/foo/myimage:latest
will be used
as-is. But other settings like insecure/blocked/mirrors will be applied to matching images
mirror
: An array of TOML tables specifying (possibly-partial) mirrors for the
prefix
-rooted namespace (i.e., the current [[registry]]
TOML table).
The mirrors are attempted in the specified order; the first one that can be
contacted and contains the image will be used (and if none of the mirrors contains the image,
the primary location specified by the registry.location
field, or using the unmodified
user-specified reference, is tried last).
Each TOML table in the mirror
array can contain the following fields:
location
: same semantics as specified in the[[registry]]
TOML tableinsecure
: same semantics as specified in the[[registry]]
TOML tablepull-from-mirror
:all
,digest-only
ortag-only
. If "digest-only", mirrors will only be used for digest pulls. Pulling images by tag can potentially yield different images, depending on which endpoint we pull from. Restricting mirrors to pulls by digest avoids that issue. If "tag-only", mirrors will only be used for tag pulls. For a more up-to-date and expensive mirror that it is less likely to be out of sync if tags move, it should not be unnecessarily used for digest references. Default is "all" (or left empty), mirrors will be used for both digest pulls and tag pulls unless the mirror-by-digest-only is set for the primary registry. Note that this per-mirror setting is allowed only whenmirror-by-digest-only
is not configured for the primary registry.
mirror-by-digest-only
: true
or false
.
If true
, mirrors will only be used during pulling if the image reference includes a digest.
Note that if all mirrors are configured to be digest-only, images referenced by a tag will only use the primary
registry.
If all mirrors are configured to be tag-only, images referenced by a digest will only use the primary
registry.
Referencing an image by digest ensures that the same is always used (whereas referencing an image by a tag may cause different registries to return different images if the tag mapping is out of sync).
Note: Redirection and mirrors are currently processed only when reading a single image, not when pushing to a registry nor when doing any other kind of lookup/search on a on a registry. This may change in the future.
The use of unqualified-search registries entails an ambiguity as it is unclear from which registry a given image, referenced by a short name, may be pulled from.
As mentioned in the note at the end of this man page, using short names is
subject to the risk of hitting squatted registry namespaces. If the
unqualified-search registries are set to ["registry1.com", "registry2.com"]
an attacker may take over a namespace of registry1.com such that an image may
be pulled from registry1.com instead of the intended source registry2.com.
While it is highly recommended to always use fully-qualified image references, existing deployments using short names may not be easily changed. To circumvent the aforementioned ambiguity, so called short-name aliases can be configured that point to a fully-qualified image reference.
Short-name aliases can be configured in the [aliases]
table in the form of
"name"="value"
with the left-hand name
being the short name (e.g., "image")
and the right-hand value
being the fully-qualified image reference (e.g.,
"registry.com/namespace/image"). Note that neither "name" nor "value" can
include a tag or digest. Moreover, "name" must be a short name and hence
cannot include a registry domain or refer to localhost.
When pulling a short name, the configured aliases table will be used for
resolving the short name. If a matching alias is found, it will be used
without further consulting the unqualified-search registries list. If no
matching alias is found, the behavior can be controlled via the
short-name-mode
option as described below.
Note that tags and digests are stripped off a user-specified short name for alias resolution. Hence, "image", "image:tag" and "image@digest" all resolve to the same alias (i.e., "image"). Stripped off tags and digests are later appended to the resolved alias.
Further note that drop-in configuration files (see containers-registries.conf.d(5)) can override aliases in the specific loading order of the files. If the "value" of an alias is empty (i.e., ""), the alias will be erased. However, a given "name" may only be specified once in a single config file.
The short-name-mode
option supports three modes to control the behaviour of
short-name resolution.
-
enforcing
: If only one unqualified-search registry is set, use it as there is no ambiguity. If there is more than one registry and the user program is running in a terminal (i.e., stdout & stdin are a TTY), prompt the user to select one of the specified search registries. If the program is not running in a terminal, the ambiguity cannot be resolved which will lead to an error. -
permissive
: Behaves as enforcing but does not lead to an error if the program is not running in a terminal. Instead, fallback to using all unqualified-search registries. -
disabled
: Use all unqualified-search registries without prompting.
If short-name-mode
is not specified at all or left empty, default to the
permissive
mode. If the user-specified short name was not aliased already,
the enforcing
and permissive
mode if prompted, will record a new alias
after a successful pull. Note that the recorded alias will be written to
/var/cache/containers/short-name-aliases.conf
for root to have a clear
separation between possibly human-edited registries.conf files and the
machine-generated short-name-aliases-conf
. Note that $HOME/.cache
is used
for rootless users. If an alias is specified in a
registries.conf
file and also the machine-generated
short-name-aliases.conf
, the short-name-aliases.conf
file has precedence.
The Docker Hub docker.io
is handled in a special way: every push and pull
operation gets internally normalized with /library
if no other specific
namespace is defined (for example on docker.io/namespace/image
).
(Note that the above-described normalization happens to match the behavior of Docker.)
This means that a pull of docker.io/alpine
will be internally translated to
docker.io/library/alpine
. A pull of docker.io/user/alpine
will not be
rewritten because this is already the correct remote path.
Therefore, to remap or mirror the docker.io
images in the (implied) /library
namespace (or that whole namespace), the prefix and location fields in this
configuration file must explicitly include that /library
namespace. For
example prefix = "docker.io/library/alpine"
and not prefix = "docker.io/alpine"
. The latter would match the docker.io/alpine/*
repositories but not the docker.io/[library/]alpine
image).
unqualified-search-registries = ["example.com"]
[[registry]]
prefix = "example.com/foo"
insecure = false
blocked = false
location = "internal-registry-for-example.com/bar"
[[registry.mirror]]
location = "example-mirror-0.local/mirror-for-foo"
[[registry.mirror]]
location = "example-mirror-1.local/mirrors/foo"
insecure = true
[[registry]]
location = "registry.com"
[[registry.mirror]]
location = "mirror.registry.com"
Given the above, a pull of example.com/foo/image:latest
will try:
example-mirror-0.local/mirror-for-foo/image:latest
example-mirror-1.local/mirrors/foo/image:latest
internal-registry-for-example.com/bar/image:latest
in order, and use the first one that exists.
Note that a mirror is associated only with the current [[registry]]
TOML table. If using the example above, pulling the image registry.com/image:latest
will hence only reach out to mirror.registry.com
, and the mirrors associated with example.com/foo
will not be considered.
The additional-layer-store-auth-helper
option enables passing registry
credentials to an Additional Layer Store so that it can access private registries.
When accessing a private registry via an Additional Layer Store, a helper binary needs to be provided. This helper binary is
registered via the additional-layer-store-auth-helper
option. Every time an image
is read using the docker://
transport, the specified helper binary is executed
and receives registry credentials from stdin in the following format.
{
"$image_reference": {
"username": "$username",
"password": "$password",
"identityToken": "$identityToken"
}
}
The format of $image_reference
is $repo{:$tag|@$digest}
.
Additional Layer Stores can use this helper binary to access the private registry.
VERSION 1 format is still supported but it does not support using registry mirrors, longest-prefix matches, or location rewriting.
The TOML format is used to build a simple list of registries under three
categories: registries.search
, registries.insecure
, and registries.block
.
You can list multiple registries using a comma separated list.
Search registries are used when the caller of a container runtime does not fully specify the container image that they want to execute. These registries are prepended onto the front of the specified container image until the named image is found at a registry.
Note that insecure registries can be used for any registry, not just the registries listed under search.
The registries.insecure
and registries.block
lists have the same meaning as the
insecure
and blocked
fields in the current version.
The following example configuration defines two searchable registries, one insecure registry, and two blocked registries.
[registries.search]
registries = ['registry1.com', 'registry2.com']
[registries.insecure]
registries = ['registry3.com']
[registries.block]
registries = ['registry.untrusted.com', 'registry.unsafe.com']
We recommend always using fully qualified image names including the registry
server (full dns name), namespace, image name, and tag
(e.g., registry.redhat.io/ubi8/ubi:latest). When using short names, there is
always an inherent risk that the image being pulled could be spoofed. For
example, a user wants to pull an image named foobar
from a registry and
expects it to come from myregistry.com. If myregistry.com is not first in the
search list, an attacker could place a different foobar
image at a registry
earlier in the search list. The user would accidentally pull and run the
attacker's image and code rather than the intended content. We recommend only
adding registries which are completely trusted, i.e. registries which don't
allow unknown or anonymous users to create accounts with arbitrary names. This
will prevent an image from being spoofed, squatted or otherwise made insecure.
If it is necessary to use one of these registries, it should be added at the
end of the list.
It is recommended to use fully-qualified images for pulling as the destination registry is unambiguous. Pulling by digest (i.e., quay.io/repository/name@digest) further eliminates the ambiguity of tags.
containers-auth.json(5) containers-certs.d(5)
Dec 2019, Warning added for unqualified image names by Tom Sweeney tsweeney@redhat.com
Mar 2019, Added additional configuration format by Sascha Grunert sgrunert@suse.com
Aug 2018, Renamed to containers-registries.conf(5) by Valentin Rothberg vrothberg@suse.com
Jun 2018, Updated by Tom Sweeney tsweeney@redhat.com
Aug 2017, Originally compiled by Brent Baude bbaude@redhat.com