containerd/docs/hosts.md

# 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](https://github.com/opencontainers/distribution-spec).
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](https://github.com/opencontainers/distribution-spec/tree/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:
```toml
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](https://docs.docker.com/engine/reference/commandline/dockerd/#insecure-registries)
(".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](https://github.com/opencontainers/distribution-spec/blob/main/spec.md).
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
```

Optionally the `_default` registry host namespace can be used as a fallback, if no
other namespace matches.

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](https://github.com/containerd/containerd/blob/main/docs/cri/registry.md#configure-registry-credentials).

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](https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/)

# 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
```

### Setup Default Mirror for All Registries

```
$ tree /etc/containerd/certs.d
/etc/containerd/certs.d
└── _default
    └── hosts.toml

$ cat /etc/containerd/certs.d/_default/hosts.toml
server = "https://registry.example.com"

[host."https://registry.example.com"]
  capabilities = ["pull", "resolve"]
```

### 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:

```toml
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"]
```