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docs: update the runtime configuration section
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Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>
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dvdksn committed Jun 22, 2023
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258 changes: 228 additions & 30 deletions docs/reference/commandline/dockerd.md
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Expand Up @@ -396,61 +396,256 @@ Defaults to 20G.
C:\> dockerd --storage-opt size=40G
```

### Docker runtime execution options
### Runtime options

The Docker daemon relies on a
[OCI](https://github.com/opencontainers/runtime-spec) compliant runtime
(invoked via the `containerd` daemon) as its interface to the Linux
kernel `namespaces`, `cgroups`, and `SELinux`.

By default, the Docker daemon automatically starts `containerd`. If you want to
control `containerd` startup, manually start `containerd` and pass the path to
the `containerd` socket using the `--containerd` flag. For example:
#### Configure container runtimes

By default, the Docker daemon uses runc as a container runtime.
You can configure the daemon to add additional runtimes.

containerd shims installed on `PATH` can be used directly, without the need
to edit the daemon's configuration. For example, if you install the Kata
Containers shim (`containerd-shim-kata-v2`) on `PATH`, then you can select that
runtime with `docker run` without having to edit the daemon's configuration:

```console
$ sudo dockerd --containerd /var/run/dev/docker-containerd.sock
$ docker run --runtime io.containerd.kata.v2
```

Runtimes can be registered with the daemon either via the
configuration file or using the `--add-runtime` command line argument.
Container runtimes that don't implement containerd shims, or containerd shims
installed outside of `PATH`, must be registered with the daemon, either via the
configuration file or using the `--add-runtime` command line flag.

For examples on how to use other container runtimes, see
[Alternative container runtimes](https://docs.docker.com/engine/alternative-container-runtimes)

##### Configure runtimes using `daemon.json`

The following is an example adding 2 runtimes via the configuration:
To register and configure container runtimes using the daemon's configuration
file, add the runtimes as entries under `runtimes`:

```json
{
"default-runtime": "runc",
"runtimes": {
"custom": {
"path": "/usr/local/bin/my-runc-replacement",
"runtimeArgs": [
"--debug"
]
"<runtime>": {}
}
}
```

The key of the entry (`<runtime>` in the previous example) represents the name
of the runtime. This is the name that you reference when you run a container,
using `docker run --runtime <runtime>`.

The runtime entry contains an object specifying the configuration for your
runtime. The properties of the object depends on what kind of runtime you're
looking to register:

- If the runtime implements its own containerd shim, the object shall contain
a `runtimeType` field and an optional `options` field.

```json
{
"runtimes": {
"<runtime>": {
"runtimeType": "<name-or-path>",
"options": {}
}
}
}
```

See [Configure shims](#configure-containerd-shims).

- If the runtime is designed to be a drop-in replacement for runc,
the object contains a `path` field, and an optional `runtimeArgs` field.

```json
{
"runtimes": {
"<runtime>": {
"path": "/path/to/bin",
"runtimeArgs": ["...args"]
}
}
}
```

See [Configure runc drop-in replacements](#configure-runc-drop-in-replacements).

After changing the runtimes configuration in the configuration file,
you must reload or restart the daemon for changes to take effect:

```console
$ sudo systemctl reload dockerd
```

##### Configure containerd shims

If the runtime that you want to register implements a containerd shim,
or if you want to register a runtime which uses the runc shim,
use the following format for the runtime entry:

```json
{
"runtimes": {
"<runtime>": {
"runtimeType": "<name-or-path>",
"options": {}
}
}
}
```

`runtimeType` refers to either:

- A fully qualified name of a containerd shim.

The fully qualified name of a shim is the same as the `runtime_type` used to
register the runtime in containerd's CRI configuration.
For example, `io.containerd.runsc.v1`.

- The path of a containerd shim binary.

This option is useful if you installed the containerd shim binary outside of
`PATH`.

`options` is optional. It lets you specify the runtime configuration that you
want to use for the shim. The configuration parameters that you can specify in
`options` depends on the runtime you're registering. For most shims,
the supported configuration options are `TypeUrl` and `ConfigPath`.
For example:

```json
{
"runtimes": {
"gvisor": {
"runtimeType": "io.containerd.runsc.v1",
"options": {
"TypeUrl": "io.containerd.runsc.v1.options",
"ConfigPath": "/etc/containerd/runsc.toml",
}
}
}
}
```

You can configure multiple runtimes using the same runtimeType. For example:

```json
{
"runtimes": {
"gvisor-foo": {
"runtimeType": "io.containerd.runsc.v1",
"options": {
"TypeUrl": "io.containerd.runsc.v1.options",
"ConfigPath": "/etc/containerd/runsc-foo.toml"
}
},
"gvisor-bar": {
"runtimeType": "io.containerd.runsc.v1",
"options": {
"TypeUrl": "io.containerd.runsc.v1.options",
"ConfigPath": "/etc/containerd/runsc-bar.toml"
}
}
}
}
```

The `options` field takes a special set of configuration parameters when used
with `"runtimeType": "io.containerd.runc.v2"`. For more information about runc
parameters, refer to the runc configuration section in
[CRI Plugin Config Guide](https://github.com/containerd/containerd/blob/v1.7.2/docs/cri/config.md#full-configuration).

##### Configure runc drop-in replacements

If the runtime that you want to register can act as a drop-in replacement for
runc, you can register the runtime either using the daemon configuration file,
or using the `--add-runtime` flag for the `dockerd` cli.

When you use the configuration file, the entry uses the following format:

```json
{
"runtimes": {
"<runtime>": {
"path": "/path/to/binary",
"runtimeArgs": ["...args"]
}
}
}
```

Where `path` is either the absolute path to the runtime executable, or the name
of an executable installed on `PATH`:

```json
{
"runtimes": {
"runc": {
"path": "runc"
}
}
}
```

This is the same example via the command line:
And `runtimeArgs` lets you optionally pass additional arguments to the runtime.
Entries with this format use the containerd runc shim to invoke a custom
runtime binary.

When you use the `--add-runtime` CLI flag, use the following format:

```console
$ sudo dockerd --add-runtime runc=runc --add-runtime custom=/usr/local/bin/my-runc-replacement
$ sudo dockerd --add-runtime <runtime>=<path>
```

> **Note**
>
> Defining runtime arguments via the command line is not supported.
Defining runtime arguments via the command line is not supported.

For an example configuration for a runc drop-in replacment, see
[Alternative container runtimes > youki](https://docs.docker.com/engine/alternative-runtimes/#youki)

##### Configure the default container runtime

You can specify either the name of a fully qualified containerd runtime shim,
or the name of a registered runtime. You can specify the default runtime either
using the daemon configuration file, or using the `--default-runtime` flag for
the `dockerd` cli.

#### Options for the runtime
When you use the configuration file, the entry uses the following format:

You can configure the runtime using options specified
with the `--exec-opt` flag. All the flag's options have the `native` prefix. A
single `native.cgroupdriver` option is available.
```json
{
"default-runtime": "io.containerd.runsc.v1"
}
```

The `native.cgroupdriver` option specifies the management of the container's
cgroups. You can only specify `cgroupfs` or `systemd`. If you specify
When you use the `--default-runtime` CLI flag, use the following format:

```console
$ dockerd --default-runtime io.containerd.runsc.v1
```

#### Run containerd standalone

By default, the Docker daemon automatically starts `containerd`. If you want to
control `containerd` startup, manually start `containerd` and pass the path to
the `containerd` socket using the `--containerd` flag. For example:

```console
$ sudo dockerd --containerd /run/containerd/containerd.sock
```

#### Configure cgroup driver

You can configure how the runtime should manage container cgroups, using the
`--exec-opt native.cgroupdriver` CLI flag.

You can only specify `cgroupfs` or `systemd`. If you specify
`systemd` and it is not available, the system errors out. If you omit the
`native.cgroupdriver` option,` cgroupfs` is used on cgroup v1 hosts, `systemd`
is used on cgroup v2 hosts with systemd available.
Expand All @@ -463,16 +658,19 @@ $ sudo dockerd --exec-opt native.cgroupdriver=systemd

Setting this option applies to all containers the daemon launches.

Also Windows Container makes use of `--exec-opt` for special purpose. Docker user
can specify default container isolation technology with this, for example:
#### Configure container isolation technology (Windows)

For Windows containers, you can specify the default container isolation
technology to use, using the `--exec-opt isolation` flag.

The following example makes `hyperv` the default isolation technology:

```console
> dockerd --exec-opt isolation=hyperv
```

Will make `hyperv` the default isolation technology on Windows. If no isolation
value is specified on daemon start, on Windows client, the default is
`hyperv`, and on Windows server, the default is `process`.
If no isolation value is specified on daemon start, on Windows client,
the default is `hyperv`, and on Windows server, the default is `process`.

### Daemon DNS options

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