---
layout: docs
page_title: 'Drivers: Docker'
sidebar_title: Docker
description: The Docker task driver is used to run Docker based tasks.
---

# Docker Driver

Name: `docker`

The `docker` driver provides a first-class Docker workflow on Nomad. The Docker
driver handles downloading containers, mapping ports, and starting, watching,
and cleaning up after containers.

## Task Configuration

```hcl
task "webservice" {
  driver = "docker"

  config {
    image = "redis:3.2"
    labels {
      group = "webservice-cache"
    }
  }
}
```

The `docker` driver supports the following configuration in the job spec. Only
`image` is required.

- `image` - The Docker image to run. The image may include a tag or custom URL
  and should include `https://` if required. By default it will be fetched from
  Docker Hub. If the tag is omitted or equal to `latest` the driver will always
  try to pull the image. If the image to be pulled exists in a registry that
  requires authentication credentials must be provided to Nomad. Please see the
  [Authentication section](#authentication).

  ```hcl
  config {
    image = "https://hub.docker.internal/redis:3.2"
  }
  ```

- `args` - (Optional) A list of arguments to the optional `command`. If no
  `command` is specified, the arguments are passed directly to the container.
  References to environment variables or any [interpretable Nomad
  variables](/docs/runtime/interpolation) will be interpreted before
  launching the task. For example:

  ```hcl
  config {
    args = [
      "-bind", "${NOMAD_PORT_http}",
      "${nomad.datacenter}",
      "${MY_ENV}",
      "${meta.foo}",
    ]
  }
  ```

- `auth` - (Optional) Provide authentication for a private registry (see below).

- `auth_soft_fail` `(bool: false)` - Don't fail the task on an auth failure.
  Attempt to continue without auth.

- `command` - (Optional) The command to run when starting the container.

  ```hcl
  config {
    command = "my-command"
  }
  ```

- `dns_search_domains` - (Optional) A list of DNS search domains for the container
  to use.

- `dns_options` - (Optional) A list of DNS options for the container to use.

- `dns_servers` - (Optional) A list of DNS servers for the container to use
  (e.g. ["8.8.8.8", "8.8.4.4"]). Requires Docker v1.10 or greater.

- `entrypoint` - (Optional) A string list overriding the image's entrypoint.

- `extra_hosts` - (Optional) A list of hosts, given as host:IP, to be added to
  `/etc/hosts`.

- `force_pull` - (Optional) `true` or `false` (default). Always pull most recent image
  instead of using existing local image. Should be set to `true` if repository tags
  are mutable. If image's tag is `latest` or omitted, the image will always be pulled
  regardless of this setting.

- `hostname` - (Optional) The hostname to assign to the container. When
  launching more than one of a task (using `count`) with this option set, every
  container the task starts will have the same hostname.

- `interactive` - (Optional) `true` or `false` (default). Keep STDIN open on
  the container.

- `sysctl` - (Optional) A key-value map of sysctl configurations to set to the
  containers on start.

  ```hcl
  config {
    sysctl {
      net.core.somaxconn = "16384"
    }
  }
  ```

- `ulimit` - (Optional) A key-value map of ulimit configurations to set to the
  containers on start.

  ```hcl
  config {
    ulimit {
      nproc = "4242"
      nofile = "2048:4096"
    }
  }
  ```

- `privileged` - (Optional) `true` or `false` (default). Privileged mode gives
  the container access to devices on the host. Note that this also requires the
  nomad agent and docker daemon to be configured to allow privileged
  containers.

- `ipc_mode` - (Optional) The IPC mode to be used for the container. The default
  is `none` for a private IPC namespace. Other values are `host` for sharing
  the host IPC namespace or the name or id of an existing container. Note that
  it is not possible to refer to Docker containers started by Nomad since their
  names are not known in advance. Note that setting this option also requires the
  Nomad agent to be configured to allow privileged containers.

- `ipv4_address` - (Optional) The IPv4 address to be used for the container when
  using user defined networks. Requires Docker 1.13 or greater.

- `ipv6_address` - (Optional) The IPv6 address to be used for the container when
  using user defined networks. Requires Docker 1.13 or greater.

- `labels` - (Optional) A key-value map of labels to set to the containers on
  start.

  ```hcl
  config {
    labels {
      foo = "bar"
      zip = "zap"
    }
  }
  ```

- `load` - (Optional) Load an image from a `tar` archive file instead of from a
  remote repository. Equivalent to the `docker load -i <filename>` command.

  ```hcl
  artifact {
    source = "http://path.to/redis.tar"
  }
  config {
    load = "redis.tar"
    image = "redis"
  }
  ```

- `logging` - (Optional) A key-value map of Docker logging options.
  Defaults to `json-file` with log rotation (`max-file=2` and `max-size=2m`).

  ```hcl
  config {
    logging {
      type = "fluentd"
      config {
        fluentd-address = "localhost:24224"
        tag = "your_tag"
      }
    }
  }
  ```

- `mac_address` - (Optional) The MAC address for the container to use (e.g.
  "02:68:b3:29:da:98").

- `memory_hard_limit` - (Optional) The maximum allowable amount of memory used
  (megabytes) by the container. If set, the [`memory`](/docs/job-specification/resources#memory)
  parameter of the task resource configuration becomes a soft limit passed to the
  docker driver as [`--memory_reservation`](https://docs.docker.com/config/containers/resource_constraints/#limit-a-containers-access-to-memory),
  and `memory_hard_limit` is passed as the [`--memory`](https://docs.docker.com/config/containers/resource_constraints/#limit-a-containers-access-to-memory)
  hard limit. When the host is under memory pressure, the behavior of soft limit
  activation is governed by the [Kernel](https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt).

- `network_aliases` - (Optional) A list of network-scoped aliases, provide a way for a
  container to be discovered by an alternate name by any other container within
  the scope of a particular network. Network-scoped alias is supported only for
  containers in user defined networks

  ```hcl
  config {
    network_mode = "user-network"
    network_aliases = [
      "${NOMAD_TASK_NAME}",
      "${NOMAD_TASK_NAME}-${NOMAD_ALLOC_INDEX}"
    ]
  }
  ```

- `network_mode` - (Optional) The network mode to be used for the container. In
  order to support userspace networking plugins in Docker 1.9 this accepts any
  value. The default is `bridge` for all operating systems but Windows, which
  defaults to `nat`. Other networking modes may not work without additional
  configuration on the host (which is outside the scope of Nomad). Valid values
  pre-docker 1.9 are `default`, `bridge`, `host`, `none`, or `container:name`.

- `pid_mode` - (Optional) `host` or not set (default). Set to `host` to share
  the PID namespace with the host. Note that this also requires the Nomad agent
  to be configured to allow privileged containers.
  See below for more details.

- `ports` - (Optional) A list of port labels to map into the container (see below).

- `port_map` - (Optional) *Deprecated* A key-value map of port labels (see below).

- `security_opt` - (Optional) A list of string flags to pass directly to
  [`--security-opt`](https://docs.docker.com/engine/reference/run/#security-configuration).
  For example:

  ```hcl
  config {
    security_opt = [
      "credentialspec=file://gmsaUser.json",
    ]
  }
  ```

- `shm_size` - (Optional) The size (bytes) of /dev/shm for the container.

- `storage_opt` - (Optional) A key-value map of storage options set to the containers on start.
  This overrides the [host dockerd configuration](https://docs.docker.com/engine/reference/commandline/dockerd/#options-per-storage-driver).
  For example:

  ```hcl
  config {
    storage_opt = {
      size = "40G"
    }
  }
  ```

- `SSL` - (Optional) If this is set to true, Nomad uses SSL to talk to the
  repository. The default value is `true`. **Deprecated as of 0.5.3**

- `tty` - (Optional) `true` or `false` (default). Allocate a pseudo-TTY for the
  container.

- `uts_mode` - (Optional) `host` or not set (default). Set to `host` to share
  the UTS namespace with the host. Note that this also requires the Nomad agent
  to be configured to allow privileged containers.

- `userns_mode` - (Optional) `host` or not set (default). Set to `host` to use
  the host's user namespace when user namespace remapping is enabled on the
  docker daemon.

- `volumes` - (Optional) A list of `host_path:container_path` strings to bind
  host paths to container paths. Mounting host paths outside of the allocation
  directory can be disabled on clients by setting the `docker.volumes.enabled`
  option set to false. This will limit volumes to directories that exist inside
  the allocation directory. We recommend using [`mounts`](#mounts) if you wish
  to have more control over volume definitions.

  ```hcl
  config {
    volumes = [
      # Use absolute paths to mount arbitrary paths on the host
      "/path/on/host:/path/in/container",

      # Use relative paths to rebind paths already in the allocation dir
      "relative/to/task:/also/in/container"
    ]
  }
  ```

- `volume_driver` - (Optional) The name of the volume driver used to mount
  volumes. Must be used along with `volumes`. If `volume_driver` is omitted,
  then relative paths will be mounted from inside the allocation dir. If a
  `"local"` or other driver is used, then they may be named volumes instead.
  If `docker.volumes.enabled` is false then volume drivers and paths outside the
  allocation directory are disallowed.

  ```hcl
  config {
    volumes = [
      # Use named volume created outside nomad.
      "name-of-the-volume:/path/in/container"
    ]
    # Name of the Docker Volume Driver used by the container
    volume_driver = "pxd"
  }
  ```

- `work_dir` - (Optional) The working directory inside the container.

- `mounts` - (Optional) A list of
  [mounts](https://docs.docker.com/engine/reference/commandline/service_create/#add-bind-mounts-or-volumes)
  to be mounted into the container. Volume, bind, and tmpfs type mounts are supported.

  ```hcl
  config {
    mounts = [
      # sample volume mount
      {
        type = "volume"
        target = "/path/in/container"
        source = "name-of-volume"
        readonly = false
        volume_options {
          no_copy = false
          labels {
            foo = "bar"
          }
          driver_config {
            name = "pxd"
            options = {
              foo = "bar"
            }
          }
        }
      },
      # sample bind mount
      {
        type = "bind"
        target = "/path/in/container"
        source = "/path/in/host"
        readonly = false
        bind_options {
          propagation = "rshared"
        }
      },
      # sample tmpfs mount
      {
        type = "tmpfs"
        target = "/path/in/container"
        readonly = false
        tmpfs_options {
          size = 100000 # size in bytes
        }
      }
    ]
  }
  ```

- `devices` - (Optional) A list of
  [devices](https://docs.docker.com/engine/reference/commandline/run/#add-host-device-to-container-device)
  to be exposed the container. `host_path` is the only required field. By default, the container will be able to
  `read`, `write` and `mknod` these devices. Use the optional `cgroup_permissions` field to restrict permissions.

  ```hcl
  config {
    devices = [
      {
        host_path = "/dev/sda1"
        container_path = "/dev/xvdc"
        cgroup_permissions = "r"
      },
      {
        host_path = "/dev/sda2"
        container_path = "/dev/xvdd"
      }
    ]
  }
  ```

- `cap_add` - (Optional) A list of Linux capabilities as strings to pass directly to
  [`--cap-add`](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities).
  Effective capabilities (computed from `cap_add` and `cap_drop`) have to match the configured whitelist.
  The whitelist can be customized using the [`allow_caps`](#plugin_caps) plugin option key in the client node's configuration.
  For example:

  ```hcl
  config {
    cap_add = [
      "SYS_TIME",
    ]
  }
  ```

- `cap_drop` - (Optional) A list of Linux capabilities as strings to pass directly to
  [`--cap-drop`](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities).
  Effective capabilities (computed from `cap_add` and `cap_drop`) have to match the configured whitelist.
  The whitelist can be customized using the [`allow_caps`](#plugin_caps) plugin option key in the client node's configuration.
  For example:

  ```hcl
  config {
    cap_drop = [
      "MKNOD",
    ]
  }
  ```

- `cpu_hard_limit` - (Optional) `true` or `false` (default). Use hard CPU
  limiting instead of soft limiting. By default this is `false` which means
  soft limiting is used and containers are able to burst above their CPU limit
  when there is idle capacity.

- `cpu_cfs_period` - (Optional) An integer value that specifies the duration in microseconds of the period
  during which the CPU usage quota is measured. The default is 100000 (0.1 second) and the maximum allowed
  value is 1000000 (1 second). See [here](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/resource_management_guide/sec-cpu#sect-cfs)
  for more details.

- `advertise_ipv6_address` - (Optional) `true` or `false` (default). Use the container's
  IPv6 address (GlobalIPv6Address in Docker) when registering services and checks.
  See [IPv6 Docker containers](/docs/job-specification/service#ipv6-docker-containers) for details.

- `readonly_rootfs` - (Optional) `true` or `false` (default). Mount
  the container's filesystem as read only.

- `runtime` - (Optional) A string representing a configured runtime to pass to docker.
  This is equivalent to the `--runtime` argument in the docker CLI
  For example, to use gVisor:

  ```hcl
  config {
    # gVisor runtime is runsc
    runtime = "runsc"
  }
  ```

- `pids_limit` - (Optional) An integer value that specifies the pid limit for
  the container. Defaults to unlimited.

Additionally, the docker driver supports customization of the container's user through the task's [`user` option](/docs/job-specification/task#user).

### Container Name

Nomad creates a container after pulling an image. Containers are named
`{taskName}-{allocId}`. This is necessary in order to place more than one
container from the same task on a host (e.g. with count > 1). This also means
that each container's name is unique across the cluster.

This is not configurable.

### Authentication

If you want to pull from a private repo (for example on dockerhub or quay.io),
you will need to specify credentials in your job via:

- the `auth` option in the task config.

- by storing explicit repository credentials or by specifying Docker
  `credHelpers` in a file and setting the auth [config](#plugin_auth_file)
  value on the client in the plugin options.

- by specifying an auth [helper](#plugin_auth_helper) on the client in the
  plugin options.

The `auth` object supports the following keys:

- `username` - (Optional) The account username.

- `password` - (Optional) The account password.

- `email` - (Optional) The account email.

- `server_address` - (Optional) The server domain/IP without the protocol.
  Docker Hub is used by default.

Example task-config:

```hcl
task "example" {
  driver = "docker"

  config {
    image = "secret/service"

    auth {
      username = "dockerhub_user"
      password = "dockerhub_password"
    }
  }
}
```

Example docker-config, using two helper scripts in \$PATH,
"docker-credential-ecr" and "docker-credential-vault":

```json
{
  "auths": {
    "internal.repo": {
      "auth": "`echo -n '<username>:<password>' | base64 -w0`"
    }
  },
  "credHelpers": {
    "<XYZ>.dkr.ecr.<region>.amazonaws.com": "ecr-login"
  },
  "credsStore": "secretservice"
}
```

Example agent configuration, using a helper script "docker-credential-ecr" in
\$PATH

```hcl
client {
  enabled = true
}

plugin "docker" {
  config {
    auth {
      # Nomad will prepend "docker-credential-" to the helper value and call
      # that script name.
      helper = "ecr"
    }
  }
}
```

!> **Be Careful!** At this time these credentials are stored in Nomad in plain
text. Secrets management will be added in a later release.

## Networking

Docker supports a variety of networking configurations, including using host
interfaces, SDNs, etc. Nomad uses `bridged` networking by default, like Docker.

You can specify other networking options, including custom networking plugins
in Docker 1.9. **You may need to perform additional configuration on the host
in order to make these work.** This additional configuration is outside the
scope of Nomad.

### Allocating Ports

You can allocate ports to your task using the port syntax described on the
[networking page](/docs/job-specification/network). Here is a recap:

```hcl
group {
  network {
    port "http" {}
    port "https {}
  }
  task "example" {
    driver = "docker"
    config {
      ports = ["http", "https"]
    }
  }
}
```

### Forwarding and Exposing Ports

A Docker container typically specifies which port a service will listen on by
specifying the `EXPOSE` directive in the `Dockerfile`.

Because dynamic ports will not match the ports exposed in your Dockerfile,
Nomad will automatically expose any ports specified in the `ports` field.

These ports will be identified via environment variables. For example:

```hcl
group {
  network {
    port "http" {}
  }
  task "api" {
    driver = "docker"
    config {
      ports = ["http"]
    }
  }
}
```

If Nomad allocates port `23332` to your api task for `http`, `23332` will be
automatically exposed and forwarded to your container, and the driver will set
an environment variable `NOMAD_PORT_http` with the value `23332` that you can
read inside your container.

This provides an easy way to use the `host` networking option for better
performance.

### Using the Port Map

If you prefer to use the traditional port-mapping method, you can specify the
the `to` field in the port configuration. It looks like this:

```hcl
group "example" {
  network {
    port "redis" { to = 6379 }
  }
  task "example" {
    driver = "docker"

    config {
      image = "redis"
      ports = ["redis"]
    }
  }
}
```

If Nomad allocates port `23332` to your allocation, the Docker driver will
automatically setup the port mapping from `23332` on the host to `6379` in your
container, so it will just work!

Note that by default this only works with `bridged` networking mode. It may
also work with custom networking plugins which implement the same API for
expose and port forwarding.

#### Deprecated `port_map` Syntax

Up until Nomad 0.12, ports could be specified in a task's resource stanza and set using the docker
`port_map` field. As more features have been added to the group network resource allocation, task based
network resources are deprecated. With it the `port_map` field is also deprecated and can only be used
with task network resources.

Users should migrate their jobs to define ports in the group network stanza and specified which ports
a task maps with the `ports` field.

### Advertising Container IPs

_New in Nomad 0.6._

When using network plugins like `weave` that assign containers a routable IP
address, that address will automatically be used in any `service`
advertisements for the task. You may override what address is advertised by
using the `address_mode` parameter on a `service`. See
[service](/docs/job-specification/service) for details.

### Networking Protocols

The Docker driver configures ports on both the `tcp` and `udp` protocols.

This is not configurable.

### Other Networking Modes

Some networking modes like `container` or `none` will require coordination
outside of Nomad. First-class support for these options may be improved later
through Nomad plugins or dynamic job configuration.

## Capabilities

The `docker` driver implements the following [capabilities](/docs/internals/plugins/task-drivers#capabilities-capabilities-error).

| Feature | Implementation |
| --- | --- |
| `nomad alloc signal` | true |
| `nomad alloc exec` | true |
| filesystem isolation | image |
| network isolation | host, group, task |
| volume mounting | all |

## Client Requirements

Nomad requires Docker to be installed and running on the host alongside the
Nomad agent. Nomad was developed against Docker `1.8.2` and `1.9`.

By default Nomad communicates with the Docker daemon using the daemon's Unix
socket. Nomad will need to be able to read/write to this socket. If you do not
run Nomad as root, make sure you add the Nomad user to the Docker group so
Nomad can communicate with the Docker daemon.

For example, on Ubuntu you can use the `usermod` command to add the `nomad`
user to the `docker` group so you can run Nomad without root:

```shell-session
$ sudo usermod -G docker -a nomad
```

For the best performance and security features you should use recent versions
of the Linux Kernel and Docker daemon.

If you would like to change any of the options related to the `docker` driver on
a Nomad client, you can modify them with the [plugin stanza][plugin-stanza] syntax. Below is an example of a configuration (many of the values are the default). See the next section for more information on the options.

```hcl
plugin "docker" {
  config {
    endpoint = "unix:///var/run/docker.sock"

    auth {
      config = "/etc/docker-auth.json"
      helper = "docker-credential-aws"
    }

    tls {
      cert = "/etc/nomad/nomad.pub"
      key  = "/etc/nomad/nomad.pem"
      ca   = "/etc/nomad/nomad.cert"
    }

    gc {
      image       = true
      image_delay = "3m"
      container   = true

      dangling_containers {
        enabled        = true
        dry_run        = false
        period         = "5m"
        creation_grace = "5m"
      }
    }

    volumes {
      enabled      = true
      selinuxlabel = "z"
    }

    allow_privileged = false
    allow_caps       = ["CHOWN", "NET_RAW"]

    # allow_caps can also be set to "ALL"
    # allow_caps = ["ALL"]
  }
}
```

## Plugin Options

- `endpoint` - If using a non-standard socket, HTTP or another location, or if
  TLS is being used, docker.endpoint must be set. If unset, Nomad will attempt
  to instantiate a Docker client using the `DOCKER_HOST` environment variable and
  then fall back to the default listen address for the given operating system.
  Defaults to `unix:///var/run/docker.sock` on Unix platforms and
  `npipe:////./pipe/docker_engine` for Windows.

- `allow_privileged` - Defaults to `false`. Changing this to true will allow
  containers to use privileged mode, which gives the containers full access to
  the host's devices. Note that you must set a similar setting on the Docker
  daemon for this to work.

- `pull_activity_timeout` - Defaults to `2m`. If Nomad receives no communication
  from the Docker engine during an image pull within this timeframe, Nomad will
  timeout the request that initiated the pull command. (Minimum of `1m`)

- `allow_caps`<a id="plugin_caps"></a> - A list of allowed Linux capabilities.
  Defaults to
  `CHOWN,DAC_OVERRIDE,FSETID,FOWNER,MKNOD,NET_RAW,SETGID,SETUID,SETFCAP,SETPCAP,
  NET_BIND_SERVICE,SYS_CHROOT,KILL,AUDIT_WRITE` which is the list of
  capabilities allowed by docker by default, as defined here. Allows the
  operator to control which capabilities can be obtained by tasks using cap_add
  and cap_drop options. Supports the value "ALL" as a shortcut for whitelisting
  all capabilities.

- `allow_runtimes` - defaults to `["runc", "nvidia"]` - A list of the allowed
  docker runtimes a task may use.

- `auth` stanza:

  - `config`<a id="plugin_auth_file"></a> - Allows an operator to specify a
    JSON file which is in the dockercfg format containing authentication
    information for a private registry, from either (in order) `auths`,
    `credHelpers` or `credsStore`.
  - `helper`<a id="plugin_auth_helper"></a> - Allows an operator to specify a
    [credsStore](https://docs.docker.com/engine/reference/commandline/login/#credential-helper-protocol)
    like script on `$PATH` to lookup authentication information from external
    sources. The script's name must begin with `docker-credential-` and this
    option should include only the basename of the script, not the path.

- `tls` stanza:

  - `cert` - Path to the server's certificate file (`.pem`). Specify this
    along with `key` and `ca` to use a TLS client to connect to the docker
    daemon. `endpoint` must also be specified or this setting will be ignored.
  - `key` - Path to the client's private key (`.pem`). Specify this along with
    `cert` and `ca` to use a TLS client to connect to the docker daemon.
    `endpoint` must also be specified or this setting will be ignored.
  - `ca` - Path to the server's CA file (`.pem`). Specify this along with
    `cert` and `key` to use a TLS client to connect to the docker daemon.
    `endpoint` must also be specified or this setting will be ignored.

- `disable_log_collection` - Defaults to `false`. Setting this to true will
  disable Nomad logs collection of Docker tasks. If you don't rely on nomad log
  capabilities and exclusively use host based log aggregation, you may consider
  this option to disable nomad log collection overhead.

- `gc` stanza:

  - `image` - Defaults to `true`. Changing this to `false` will prevent Nomad
    from removing images from stopped tasks.
  - `image_delay` - A time duration, as [defined
    here](https://golang.org/pkg/time/#ParseDuration), that defaults to `3m`.
    The delay controls how long Nomad will wait between an image being unused
    and deleting it. If a tasks is received that uses the same image within
    the delay, the image will be reused.
  - `container` - Defaults to `true`. This option can be used to disable Nomad
    from removing a container when the task exits. Under a name conflict,
    Nomad may still remove the dead container.
  - `dangling_containers` stanza for controlling dangling container detection
    and cleanup:

    - `enabled` - Defaults to `true`. Enables dangling container handling.
    - `dry_run` - Defaults to `false`. Only log dangling containers without
      cleaning them up.
    - `period` - Defaults to `"5m"`. A time duration that controls interval
      between Nomad scans for dangling containers.
    - `creation_grace` - Defaults to `"5m"`. Grace period after a container is
      created during which the GC ignores it. Only used to prevent the GC from
      removing newly created containers before they are registered with the
      GC. Should not need adjusting higher but may be adjusted lower to GC
      more aggressively.

- `volumes` stanza:

  - `enabled` - Defaults to `false`. Allows tasks to bind host paths
    (`volumes`) inside their container and use volume drivers
    (`volume_driver`). Binding relative paths is always allowed and will be
    resolved relative to the allocation's directory.
  - `selinuxlabel` - Allows the operator to set a SELinux label to the
    allocation and task local bind-mounts to containers. If used with
    `docker.volumes.enabled` set to false, the labels will still be applied to
    the standard binds in the container.

- `infra_image` - This is the Docker image to use when creating the parent
  container necessary when sharing network namespaces between tasks. Defaults
  to "gcr.io/google_containers/pause-amd64:3.0".

## Client Configuration

~> Note: client configuration options will soon be deprecated. Please use
[plugin options][plugin-options] instead. See the [plugin stanza][plugin-stanza]
documentation for more information.

The `docker` driver has the following [client configuration
options](/docs/configuration/client#options):

- `docker.endpoint` - If using a non-standard socket, HTTP or another location,
  or if TLS is being used, `docker.endpoint` must be set. If unset, Nomad will
  attempt to instantiate a Docker client using the `DOCKER_HOST` environment
  variable and then fall back to the default listen address for the given
  operating system. Defaults to `unix:///var/run/docker.sock` on Unix platforms
  and `npipe:////./pipe/docker_engine` for Windows.

- `docker.auth.config` <a id="auth_file"></a>- Allows an operator to specify a
  JSON file which is in the dockercfg format containing authentication
  information for a private registry, from either (in order) `auths`,
  `credHelpers` or `credsStore`.

- `docker.auth.helper` <a id="auth_helper"></a>- Allows an operator to specify a
  [credsStore](https://docs.docker.com/engine/reference/commandline/login/#credential-helper-protocol)
  -like script on \$PATH to lookup authentication information from external
  sources. The script's name must begin with `docker-credential-` and this
  option should include only the basename of the script, not the path.

- `docker.tls.cert` - Path to the server's certificate file (`.pem`). Specify
  this along with `docker.tls.key` and `docker.tls.ca` to use a TLS client to
  connect to the docker daemon. `docker.endpoint` must also be specified or this
  setting will be ignored.

- `docker.tls.key` - Path to the client's private key (`.pem`). Specify this
  along with `docker.tls.cert` and `docker.tls.ca` to use a TLS client to
  connect to the docker daemon. `docker.endpoint` must also be specified or this
  setting will be ignored.

- `docker.tls.ca` - Path to the server's CA file (`.pem`). Specify this along
  with `docker.tls.cert` and `docker.tls.key` to use a TLS client to connect to
  the docker daemon. `docker.endpoint` must also be specified or this setting
  will be ignored.

- `docker.cleanup.image` Defaults to `true`. Changing this to `false` will
  prevent Nomad from removing images from stopped tasks.

- `docker.cleanup.image.delay` A time duration, as [defined
  here](https://golang.org/pkg/time/#ParseDuration), that defaults to `3m`. The
  delay controls how long Nomad will wait between an image being unused and
  deleting it. If a tasks is received that uses the same image within the delay,
  the image will be reused.

- `docker.volumes.enabled`: Defaults to `false`. Allows tasks to bind host paths
  (`volumes`) inside their container and use volume drivers (`volume_driver`).
  Binding relative paths is always allowed and will be resolved relative to the
  allocation's directory.

- `docker.volumes.selinuxlabel`: Allows the operator to set a SELinux label to
  the allocation and task local bind-mounts to containers. If used with
  `docker.volumes.enabled` set to false, the labels will still be applied to the
  standard binds in the container.

- `docker.privileged.enabled` Defaults to `false`. Changing this to `true` will
  allow containers to use `privileged` mode, which gives the containers full
  access to the host's devices. Note that you must set a similar setting on the
  Docker daemon for this to work.

- `docker.caps.whitelist`: A list of allowed Linux capabilities. Defaults to
  `"CHOWN,DAC_OVERRIDE,FSETID,FOWNER,MKNOD,NET_RAW,SETGID,SETUID,SETFCAP, SETPCAP,NET_BIND_SERVICE,SYS_CHROOT,KILL,AUDIT_WRITE"`, which is the list of
  capabilities allowed by docker by default, as [defined
  here](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities).
  Allows the operator to control which capabilities can be obtained by tasks
  using `cap_add` and `cap_drop` options. Supports the value `"ALL"` as a
  shortcut for whitelisting all capabilities.

- `docker.cleanup.container`: Defaults to `true`. This option can be used to
  disable Nomad from removing a container when the task exits. Under a name
  conflict, Nomad may still remove the dead container.

- `docker.nvidia_runtime`: Defaults to `nvidia`. This option allows operators to select the runtime that should be used in order to expose Nvidia GPUs to the container.

Note: When testing or using the `-dev` flag you can use `DOCKER_HOST`,
`DOCKER_TLS_VERIFY`, and `DOCKER_CERT_PATH` to customize Nomad's behavior. If
`docker.endpoint` is set Nomad will **only** read client configuration from the
config file.

An example is given below:

```hcl
client {
  options {
    "docker.cleanup.image" = "false"
  }
}
```

## Client Attributes

The `docker` driver will set the following client attributes:

- `driver.docker` - This will be set to "1", indicating the driver is
  available.
- `driver.docker.bridge_ip` - The IP of the Docker bridge network if one
  exists.
- `driver.docker.version` - This will be set to version of the docker server.

Here is an example of using these properties in a job file:

```hcl
job "docs" {
  # Require docker version higher than 1.2.
  constraint {
    attribute = "${driver.docker.version}"
    operator  = ">"
    version   = "1.2"
  }
}
```

## Resource Isolation

### CPU

Nomad limits containers' CPU based on CPU shares. CPU shares allow containers
to burst past their CPU limits. CPU limits will only be imposed when there is
contention for resources. When the host is under load your process may be
throttled to stabilize QoS depending on how many shares it has. You can see how
many CPU shares are available to your process by reading `NOMAD_CPU_LIMIT`.
1000 shares are approximately equal to 1 GHz.

Please keep the implications of CPU shares in mind when you load test workloads
on Nomad.

### Memory

Nomad limits containers' memory usage based on total virtual memory. This means
that containers scheduled by Nomad cannot use swap. This is to ensure that a
swappy process does not degrade performance for other workloads on the same
host.

Since memory is not an elastic resource, you will need to make sure your
container does not exceed the amount of memory allocated to it, or it will be
terminated or crash when it tries to malloc. A process can inspect its memory
limit by reading `NOMAD_MEMORY_LIMIT`, but will need to track its own memory
usage. Memory limit is expressed in megabytes so 1024 = 1 GB.

### IO

Nomad's Docker integration does not currently provide QoS around network or
filesystem IO. These will be added in a later release.

### Security

Docker provides resource isolation by way of
[cgroups and namespaces](https://docs.docker.com/introduction/understanding-docker/#the-underlying-technology).
Containers essentially have a virtual file system all to themselves. If you
need a higher degree of isolation between processes for security or other
reasons, it is recommended to use full virtualization like
[QEMU](/docs/drivers/qemu).

## Caveats

### Dangling Containers

Nomad 0.10.2 introduces a detector and a reaper for dangling Docker containers,
containers that Nomad starts yet does not manage or track. Though rare, they
lead to unexpectedly running services, potentially with stale versions.

When Docker daemon becomes unavailable as Nomad starts a task, it is possible
for Docker to successfully start the container but return a 500 error code from
the API call. In such cases, Nomad retries and eventually aims to kill such
containers. However, if the Docker Engine remains unhealthy, subsequent retries
and stop attempts may still fail, and the started container becomes a dangling
container that Nomad no longer manages.

The newly added reaper periodically scans for such containers. It only targets
containers with a `com.hashicorp.nomad.allocation_id` label, or match Nomad's
conventions for naming and bind-mounts (i.e. `/alloc`, `/secrets`, `local`).
Containers that don't match Nomad container patterns are left untouched.

Operators can run the reaper in a dry-run mode, where it only logs dangling
container ids without killing them, or disable it by setting the
`gc.dangling_containers` config stanza.

### Docker for Windows

Docker for Windows only supports running Windows containers. Because Docker for
Windows is relatively new and rapidly evolving you may want to consult the
[list of relevant issues on GitHub][winissues].

[winissues]: https://github.com/hashicorp/nomad/issues?q=is%3Aopen+is%3Aissue+label%3Adriver%2Fdocker+label%3Aplatform-windows
[plugin-options]: #plugin-options
[plugin-stanza]: /docs/configuration/plugin
