Short syntax uses plain strings in a list. Values must set hostname and IP address for additional hosts in the form of `HOSTNAME=IP`.

```yml
extra_hosts:
  - "somehost=162.242.195.82"
  - "otherhost=50.31.209.229"
  - "myhostv6=::1"
```

IPv6 addresses can be enclosed in square brackets, for example:

```yml
extra_hosts:
  - "myhostv6=[::1]"
```

The separator `=` is preferred, but `:` can also be used. Introduced in Docker Compose version [2.24.1](/manuals/compose/releases/release-notes.md#2241). For example:

```yml
extra_hosts:
  - "somehost:162.242.195.82"
  - "myhostv6:::1"
```

#### Long syntax

Alternatively, `extra_hosts` can be set as a mapping between hostname(s) and IP(s)

```yml
extra_hosts:
  somehost: "162.242.195.82"
  otherhost: "50.31.209.229"
  myhostv6: "::1"
```

Compose creates a matching entry with the IP address and hostname in the container's network
configuration, which means for Linux `/etc/hosts` get extra lines:

```console
162.242.195.82  somehost
50.31.209.229   otherhost
::1             myhostv6
```

### `gpus`

{{< summary-bar feature_name="Compose gpus" >}}

`gpus` specifies GPU devices to be allocated for container usage. This is equivalent to a [device request](deploy.md#devices) with
an implicit `gpu` capability.

```yaml
services:
  model:
    gpus: 
      - driver: 3dfx
        count: 2
```

`gpus` also can be set as string `all` to allocate all available GPU devices to the container.

```yaml
services:
  model:
    gpus: all
```

### `group_add`

`group_add` specifies additional groups, by name or number, which the user inside the container must be a member of.

An example of where this is useful is when multiple containers (running as different users) need to all read or write
the same file on a shared volume. That file can be owned by a group shared by all the containers, and specified in
`group_add`.

```yml
services:
  myservice:
    image: alpine
    group_add:
      - mail
```

Running `id` inside the created container must show that the user belongs to the `mail` group, which would not have
been the case if `group_add` were not declared.

### `healthcheck`

{{% include "compose/services-healthcheck.md" %}}

For more information on `HEALTHCHECK`, see the [Dockerfile reference](/reference/dockerfile.md#healthcheck).

```yml
healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 1m30s
  timeout: 10s
  retries: 3
  start_period: 40s
  start_interval: 5s
```

`interval`, `timeout`, `start_period`, and `start_interval` are [specified as durations](extension.md#specifying-durations). Introduced in Docker Compose version [2.20.2](/manuals/compose/releases/release-notes.md#2202)

`test` defines the command Compose runs to check container health. It can be
either a string or a list. If it's a list, the first item must be either `NONE`, `CMD` or `CMD-SHELL`.
If it's a string, it's equivalent to specifying `CMD-SHELL` followed by that string.

```yml
# Hit the local web app
test: ["CMD", "curl", "-f", "http://localhost"]
```

Using `CMD-SHELL` runs the command configured as a string using the container's default shell
(`/bin/sh` for Linux). Both of the following forms are equivalent:

```yml
test: ["CMD-SHELL", "curl -f http://localhost || exit 1"]
```

```yml
test: curl -f https://localhost || exit 1
```

`NONE` disables the healthcheck, and is mostly useful to disable the Healthcheck Dockerfile instruction set by the service's Docker image. Alternatively,
the healthcheck set by the image can be disabled by setting `disable: true`:

```yml
healthcheck:
  disable: true
```

### `hostname`

`hostname` declares a custom host name to use for the service container. It must be a valid RFC 1123 hostname.

### `image`

`image` specifies the image to start the container from. `image` must follow the Open Container Specification
[addressable image format](https://github.com/opencontainers/org/blob/master/docs/docs/introduction/digests.md),
as `[<registry>/][<project>/]<image>[:<tag>|@<digest>]`.