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Registry Configuration - Introduction

New and additional registry hosts config support has been implemented in containerd v1.5 for the ctr client (the containerd tool for admins/developers), containerd image service clients, and CRI clients such as kubectl and crictl.

Configuring registries, for these clients, will be done by specifying (optionally) a hosts.toml file for each desired registry host in a configuration directory. Note: Updates under this directory do not require restarting the containerd daemon.

Registry API Support

All configured registry hosts are expected to comply with the OCI Distribution Specification. Registries which are non-compliant or implement non-standard behavior are not guaranteed to be supported and may break unexpectedly between releases.

Currently supported OCI Distribution version: v1.0.0

Specifying the Configuration Directory

Using Host Namespace Configs with CTR

When pulling a container image via ctr using the --hosts-dir option tells ctr to find and use the host configuration files located in the specified path:

ctr images pull --hosts-dir "/etc/containerd/certs.d" myregistry.io:5000/image_name:tag

CRI

The old CRI config pattern for specifying registry.mirrors and registry.configs has been DEPRECATED. You should now point your registry config_path to the path where your hosts.toml files are located.

Modify your config.toml (default location: /etc/containerd/config.toml) as follows:

version = 2

[plugins."io.containerd.grpc.v1.cri".registry]
   config_path = "/etc/containerd/certs.d"

Support for Docker's Certificate File Pattern

If no hosts.toml configuration exists in the host directory, it will fallback to check certificate files based on Docker's certificate file pattern (".crt" files for CA certificates and ".cert"/".key" files for client certificates).

Registry Host Namespace

A registry host is the location where container images and artifacts are sourced. These registry hosts may be local or remote and are typically accessed via http/https using the OCI distribution specification. A registry mirror is not a registry host but these mirrors can also be used to pull content. Registry hosts are typically referred to by their internet domain names, aka. registry host names. For example, docker.io, quay.io, gcr.io, and ghcr.io.

A registry host namespace is, for the purpose of containerd registry configuration, a path to the hosts.toml file specified by the registry host name, or ip address, and an optional port identifier. When making a pull request for an image the format is typically as follows:

pull [registry_host_name|IP address][:port][/v2][/org_path]<image_name>[:tag|@DIGEST]

The registry host namespace portion is [registry_host_name|IP address][:port]. Example tree for docker.io:

$ tree /etc/containerd/certs.d
/etc/containerd/certs.d
└── docker.io
    └── hosts.toml

The /v2 portion of the pull request format shown above refers to the version of the distribution api. If not included in the pull request, /v2 is added by default for all clients compliant to the distribution specification linked above.

For example when pulling image_name:tag from a private registry named myregistry.io over port 5000:

pull myregistry.io:5000/image_name:tag

The pull will resolve to https://myregistry.io:5000/v2/image_name:tag

Specifying Registry Credentials

CTR

When performing image operations via ctr use the --help option to get a list of options you can set for specifying credentials:

ctr i pull --help
...
OPTIONS:
   --skip-verify, -k                 skip SSL certificate validation
   --plain-http                      allow connections using plain HTTP
   --user value, -u value            user[:password] Registry user and password
   --refresh value                   refresh token for authorization server
   --hosts-dir value                 Custom hosts configuration directory
   --tlscacert value                 path to TLS root CA
   --tlscert value                   path to TLS client certificate
   --tlskey value                    path to TLS client key
   --http-dump                       dump all HTTP request/responses when interacting with container registry
   --http-trace                      enable HTTP tracing for registry interactions
   --snapshotter value               snapshotter name. Empty value stands for the default value. [$CONTAINERD_SNAPSHOTTER]
   --label value                     labels to attach to the image
   --platform value                  Pull content from a specific platform
   --all-platforms                   pull content and metadata from all platforms
   --all-metadata                    Pull metadata for all platforms
   --print-chainid                   Print the resulting image's chain ID
   --max-concurrent-downloads value  Set the max concurrent downloads for each pull (default: 0)

CRI

Although we have deprecated the old CRI config pattern for specifying registry.mirrors and registry.configs you can still specify your credentials via CRI config.

Additionally, the containerd CRI plugin implements/supports the authentication parameters passed in through CRI pull image service requests. For example, when containerd is the container runtime implementation for Kubernetes, the containerd CRI plugin receives authentication credentials from kubelet as retrieved from Kubernetes Image Pull Secrets

Registry Configuration - Examples

Simple (default) Host Config for Docker

Here is a simple example for a default registry hosts configuration. Set config_path = "/etc/containerd/certs.d" in your config.toml for containerd. Make a directory tree at the config path that includes docker.io as a directory representing the host namespace to be configured. Then add a hosts.toml file in the docker.io to configure the host namespace. It should look like this:

$ tree /etc/containerd/certs.d
/etc/containerd/certs.d
└── docker.io
    └── hosts.toml

$ cat /etc/containerd/certs.d/docker.io/hosts.toml
server = "https://docker.io"

[host."https://registry-1.docker.io"]
  capabilities = ["pull", "resolve"]

Setup a Local Mirror for Docker

server = "https://registry-1.docker.io"    # Exclude this to not use upstream

[host."https://public-mirror.example.com"]
  capabilities = ["pull"]                  # Requires less trust, won't resolve tag to digest from this host
[host."https://docker-mirror.internal"]
  capabilities = ["pull", "resolve"]
  ca = "docker-mirror.crt"                 # Or absolute path /etc/containerd/certs.d/docker.io/docker-mirror.crt

Bypass TLS Verification Example

To bypass the TLS verification for a private registry at 192.168.31.250:5000

Create a path and hosts.toml text at the path "/etc/containerd/certs.d/docker.io/hosts.toml" with following or similar contents:

server = "https://registry-1.docker.io"

[host."http://192.168.31.250:5000"]
  capabilities = ["pull", "resolve", "push"]
  skip_verify = true

hosts.toml Content Description - Detail

For each registry host namespace directory in your registry config_path you may include a hosts.toml configuration file. The following root level toml fields apply to the registry host namespace:

Note: All paths specified in the hosts.toml file may be absolute or relative to the hosts.toml file.

server field

server specifies the default server for this registry host namespace. When host(s) are specified, the hosts are tried first in the order listed.

server = "https://docker.io"

capabilities field

capabilities is an optional setting for specifying what operations a host is capable of performing. Include only the values that apply.

capabilities =  ["pull", "resolve", "push"]

capabilities (or Host capabilities) represent the capabilities of the registry host. This also represents the set of operations for which the registry host may be trusted to perform.

For example, pushing is a capability which should only be performed on an upstream source, not a mirror.

Resolving (the process of converting a name into a digest) must be considered a trusted operation and only done by a host which is trusted (or more preferably by secure process which can prove the provenance of the mapping).

A public mirror should never be trusted to do a resolve action.

Registry Type Pull Resolve Push
Public Registry yes yes yes
Private Registry yes yes yes
Public Mirror yes no no
Private Mirror yes yes no

ca field

ca (Certificate Authority Certification) can be set to a path or an array of paths each pointing to a ca file for use in authenticating with the registry namespace.

ca = "/etc/certs/mirror.pem"

or

ca = ["/etc/certs/test-1-ca.pem", "/etc/certs/special.pem"]

client field

client certificates are configured as follows

a path:

client = "/etc/certs/client.pem"

an array of paths:

client = ["/etc/certs/client-1.pem", "/etc/certs/client-2.pem"]

an array of pairs of paths:

client = [["/etc/certs/client.cert", "/etc/certs/client.key"],["/etc/certs/client.pem", ""]]

skip_verify field

skip_verify skips verifications of the registry's certificate chain and host name when set to true. This should only be used for testing or in combination with other method of verifying connections. (Defaults to false)

skip_verify = false

header fields (in the toml table format)

[header] contains some number of keys where each key is to one of a string or

an array of strings as follows:

[header]
  x-custom-1 = "custom header"

or

[header]
  x-custom-1 = ["custom header part a","part b"]

or

[header]
  x-custom-1 = "custom header",
  x-custom-1-2 = "another custom header"

override_path field

override_path is used to indicate the host's API root endpoint is defined in the URL path rather than by the API specification. This may be used with non-compliant OCI registries which are missing the /v2 prefix. (Defaults to false)

override_path = true

host field(s) (in the toml table format)

[host]."https://namespace" and [host].http://namespace entries in the hosts.toml configuration are registry namespaces used in lieu of the default registry host namespace. These hosts are sometimes called mirrors because they may contain a copy of the container images and artifacts you are attempting to retrieve from the default registry. Each host/mirror namespace is also configured in much the same way as the default registry namespace. Notably the server is not specified in the host description because it is specified in the namespace. Here are a few rough examples configuring host mirror namespaces for this registry host namespace:

[host."https://mirror.registry"]
  capabilities = ["pull"]
  ca = "/etc/certs/mirror.pem"
  skip_verify = false
  [host."https://mirror.registry".header]
    x-custom-2 = ["value1", "value2"]

[host."https://mirror-bak.registry/us"]
  capabilities = ["pull"]
  skip_verify = true

[host."http://mirror.registry"]
  capabilities = ["pull"]

[host."https://test-1.registry"]
  capabilities = ["pull", "resolve", "push"]
  ca = ["/etc/certs/test-1-ca.pem", "/etc/certs/special.pem"]
  client = [["/etc/certs/client.cert", "/etc/certs/client.key"],["/etc/certs/client.pem", ""]]

[host."https://test-2.registry"]
  client = "/etc/certs/client.pem"

[host."https://test-3.registry"]
  client = ["/etc/certs/client-1.pem", "/etc/certs/client-2.pem"]

[host."https://non-compliant-mirror.registry/v2/upstream"]
  capabilities = ["pull"]
  override_path = true

Note: Recursion is not supported in the specification of host mirror namespaces in the hosts.toml file. Thus the following is not allowed/supported:

[host."http://mirror.registry"]
  capabilities = ["pull"]
  [host."http://double-mirror.registry"]
    capabilities = ["pull"]