% DOCKER(1) Docker User Manuals
% Shishir Mahajan
% SEPTEMBER 2015
# NAME
docker-daemon - Enable daemon mode

# SYNOPSIS
**docker daemon**
[**--api-cors-header**=[=*API-CORS-HEADER*]]
[**-b**|**--bridge**[=*BRIDGE*]]
[**--bip**[=*BIP*]]
[**-D**|**--debug**[=*false*]]
[**--default-gateway**[=*DEFAULT-GATEWAY*]]
[**--default-gateway-v6**[=*DEFAULT-GATEWAY-V6*]]
[**--default-ulimit**[=*[]*]]
[**--disable-legacy-registry**[=*false*]]
[**--dns**[=*[]*]]
[**--dns-opt**[=*[]*]]
[**--dns-search**[=*[]*]]
[**-e**|**--exec-driver**[=*native*]]
[**--exec-opt**[=*[]*]]
[**--exec-root**[=*/var/run/docker*]]
[**--fixed-cidr**[=*FIXED-CIDR*]]
[**--fixed-cidr-v6**[=*FIXED-CIDR-V6*]]
[**-G**|**--group**[=*docker*]]
[**-g**|**--graph**[=*/var/lib/docker*]]
[**-H**|**--host**[=*[]*]]
[**--help**]
[**--icc**[=*true*]]
[**--insecure-registry**[=*[]*]]
[**--ip**[=*0.0.0.0*]]
[**--ip-forward**[=*true*]]
[**--ip-masq**[=*true*]]
[**--iptables**[=*true*]]
[**--ipv6**[=*false*]]
[**-l**|**--log-level**[=*info*]]
[**--label**[=*[]*]]
[**--log-driver**[=*json-file*]]
[**--log-opt**[=*map[]*]]
[**--mtu**[=*0*]]
[**-p**|**--pidfile**[=*/var/run/docker.pid*]]
[**--registry-mirror**[=*[]*]]
[**-s**|**--storage-driver**[=*STORAGE-DRIVER*]]
[**--selinux-enabled**[=*false*]]
[**--storage-opt**[=*[]*]]
[**--tls**[=*false*]]
[**--tlscacert**[=*~/.docker/ca.pem*]]
[**--tlscert**[=*~/.docker/cert.pem*]]
[**--tlskey**[=*~/.docker/key.pem*]]
[**--tlsverify**[=*false*]]
[**--userland-proxy**[=*true*]]

# DESCRIPTION
**docker** has two distinct functions. It is used for starting the Docker
daemon and to run the CLI (i.e., to command the daemon to manage images,
containers etc.) So **docker** is both a server, as a daemon, and a client
to the daemon, through the CLI.

To run the Docker daemon you can specify **docker daemon**.
You can check the daemon options using **docker daemon --help**.
Daemon options should be specified after the **daemon** keyword in the following
format.

**docker daemon [OPTIONS]**

# OPTIONS

**--api-cors-header**=""
  Set CORS headers in the remote API. Default is cors disabled. Give urls like "http://foo, http://bar, ...". Give "*" to allow all.

**-b**, **--bridge**=""
  Attach containers to a pre\-existing network bridge; use 'none' to disable container networking

**--bip**=""
  Use the provided CIDR notation address for the dynamically created bridge (docker0); Mutually exclusive of \-b

**-D**, **--debug**=*true*|*false*
  Enable debug mode. Default is false.

**--default-gateway**=""
  IPv4 address of the container default gateway; this address must be part of the bridge subnet (which is defined by \-b or \--bip)

**--default-gateway-v6**=""
  IPv6 address of the container default gateway

**--default-ulimit**=[]
  Set default ulimits for containers.

**--disable-legacy-registry**=*true*|*false*
  Do not contact legacy registries

**--dns**=""
  Force Docker to use specific DNS servers

**--dns-opt**=""
  DNS options to use.

**--dns-search**=[]
  DNS search domains to use.

**-e**, **--exec-driver**=""
  Force Docker to use specific exec driver. Default is `native`.

**--exec-opt**=[]
  Set exec driver options. See EXEC DRIVER OPTIONS.

**--exec-root**=""
  Path to use as the root of the Docker exec driver. Default is `/var/run/docker`.

**--fixed-cidr**=""
  IPv4 subnet for fixed IPs (e.g., 10.20.0.0/16); this subnet must be nested in the bridge subnet (which is defined by \-b or \-\-bip)

**--fixed-cidr-v6**=""
  IPv6 subnet for global IPv6 addresses (e.g., 2a00:1450::/64)

**-G**, **--group**=""
  Group to assign the unix socket specified by -H when running in daemon mode.
  use '' (the empty string) to disable setting of a group. Default is `docker`.

**-g**, **--graph**=""
  Path to use as the root of the Docker runtime. Default is `/var/lib/docker`.

**-H**, **--host**=[unix:///var/run/docker.sock]: tcp://[host:port] to bind or
unix://[/path/to/socket] to use.
  The socket(s) to bind to in daemon mode specified using one or more
  tcp://host:port, unix:///path/to/socket, fd://* or fd://socketfd.

**--help**
  Print usage statement

**--icc**=*true*|*false*
  Allow unrestricted inter\-container and Docker daemon host communication. If disabled, containers can still be linked together using the **--link** option (see **docker-run(1)**). Default is true.

**--insecure-registry**=[]
  Enable insecure registry communication, i.e., enable un-encrypted and/or untrusted communication.

  List of insecure registries can contain an element with CIDR notation to specify a whole subnet. Insecure registries accept HTTP and/or accept HTTPS with certificates from unknown CAs.

  Enabling `--insecure-registry` is useful when running a local registry.  However, because its use creates security vulnerabilities it should ONLY be enabled for testing purposes.  For increased security, users should add their CA to their system's list of trusted CAs instead of using `--insecure-registry`.

**--ip**=""
  Default IP address to use when binding container ports. Default is `0.0.0.0`.

**--ip-forward**=*true*|*false*
  Enables IP forwarding on the Docker host. The default is `true`. This flag interacts with the IP forwarding setting on your host system's kernel. If your system has IP forwarding disabled, this setting enables it. If your system has IP forwarding enabled, setting this flag to `--ip-forward=false` has no effect.

  This setting will also enable IPv6 forwarding if you have both `--ip-forward=true` and `--fixed-cidr-v6` set. Note that this may reject Router Advertisements and interfere with the host's existing IPv6 configuration. For more information, please consult the documentation about "Advanced Networking - IPv6".

**--ip-masq**=*true*|*false*
  Enable IP masquerading for bridge's IP range. Default is true.

**--iptables**=*true*|*false*
  Enable Docker's addition of iptables rules. Default is true.

**--ipv6**=*true*|*false*
  Enable IPv6 support. Default is false. Docker will create an IPv6-enabled bridge with address fe80::1 which will allow you to create IPv6-enabled containers. Use together with `--fixed-cidr-v6` to provide globally routable IPv6 addresses. IPv6 forwarding will be enabled if not used with `--ip-forward=false`. This may collide with your host's current IPv6 settings. For more information please consult the documentation about "Advanced Networking - IPv6".

**-l**, **--log-level**="*debug*|*info*|*warn*|*error*|*fatal*""
  Set the logging level. Default is `info`.

**--label**="[]"
  Set key=value labels to the daemon (displayed in `docker info`)

**--log-driver**="*json-file*|*syslog*|*journald*|*gelf*|*fluentd*|*awslogs*|*none*"
  Default driver for container logs. Default is `json-file`.
  **Warning**: `docker logs` command works only for `json-file` logging driver.

**--log-opt**=[]
  Logging driver specific options.

**--mtu**=VALUE
  Set the containers network mtu. Default is `0`.

**-p**, **--pidfile**=""
  Path to use for daemon PID file. Default is `/var/run/docker.pid`

**--registry-mirror**=<scheme>://<host>
  Prepend a registry mirror to be used for image pulls. May be specified multiple times.

**-s**, **--storage-driver**=""
  Force the Docker runtime to use a specific storage driver.

**--selinux-enabled**=*true*|*false*
  Enable selinux support. Default is false. SELinux does not presently support the BTRFS storage driver.

**--storage-opt**=[]
  Set storage driver options. See STORAGE DRIVER OPTIONS.

**--tls**=*true*|*false*
  Use TLS; implied by --tlsverify. Default is false.

**--tlscacert**=~/.docker/ca.pem
  Trust certs signed only by this CA.

**--tlscert**=~/.docker/cert.pem
  Path to TLS certificate file.

**--tlskey**=~/.docker/key.pem
  Path to TLS key file.

**--tlsverify**=*true*|*false*
  Use TLS and verify the remote (daemon: verify client, client: verify daemon).
  Default is false.

**--userland-proxy**=*true*|*false*
    Rely on a userland proxy implementation for inter-container and outside-to-container loopback communications. Default is true.

# STORAGE DRIVER OPTIONS

Docker uses storage backends (known as "graphdrivers" in the Docker
internals) to create writable containers from images.  Many of these
backends use operating system level technologies and can be
configured.

Specify options to the storage backend with **--storage-opt** flags. The only
backend that currently takes options is *devicemapper*. Therefore use these
flags with **-s=**devicemapper.

Specifically for devicemapper, the default is a "loopback" model which
requires no pre-configuration, but is extremely inefficient.  Do not
use it in production.

To make the best use of Docker with the devicemapper backend, you must
have a recent version of LVM.  Use `lvm` to create a thin pool; for
more information see `man lvmthin`.  Then, use `--storage-opt
dm.thinpooldev` to tell the Docker engine to use that pool for
allocating images and container snapshots.

Here is the list of *devicemapper* options:

#### dm.thinpooldev

Specifies a custom block storage device to use for the thin pool.

If using a block device for device mapper storage, it is best to use
`lvm` to create and manage the thin-pool volume. This volume is then
handed to Docker to create snapshot volumes needed for images and
containers.

Managing the thin-pool outside of Docker makes for the most feature-rich method
of having Docker utilize device mapper thin provisioning as the backing storage
for Docker's containers. The highlights of the LVM-based thin-pool management
feature include: automatic or interactive thin-pool resize support, dynamically
changing thin-pool features, automatic thinp metadata checking when lvm activates
the thin-pool, etc.

Example use: `docker daemon --storage-opt dm.thinpooldev=/dev/mapper/thin-pool`

#### dm.basesize

Specifies the size to use when creating the base device, which limits
the size of images and containers. The default value is 100G. Note,
thin devices are inherently "sparse", so a 100G device which is mostly
empty doesn't use 100 GB of space on the pool. However, the filesystem
will use more space for base images the larger the device
is.

This value affects the system-wide "base" empty filesystem that may already
be initialized and inherited by pulled images. Typically, a change to this
value requires additional steps to take effect:

        $ sudo service docker stop
        $ sudo rm -rf /var/lib/docker
        $ sudo service docker start

Example use: `docker daemon --storage-opt dm.basesize=20G`

#### dm.fs

Specifies the filesystem type to use for the base device. The
supported options are `ext4` and `xfs`. The default is `ext4`.

Example use: `docker daemon --storage-opt dm.fs=xfs`

#### dm.mkfsarg

Specifies extra mkfs arguments to be used when creating the base device.

Example use: `docker daemon --storage-opt "dm.mkfsarg=-O ^has_journal"`

#### dm.mountopt

Specifies extra mount options used when mounting the thin devices.

Example use: `docker daemon --storage-opt dm.mountopt=nodiscard`

#### dm.use_deferred_removal

Enables use of deferred device removal if `libdm` and the kernel driver
support the mechanism.

Deferred device removal means that if device is busy when devices are
being removed/deactivated, then a deferred removal is scheduled on
device. And devices automatically go away when last user of the device
exits.

For example, when a container exits, its associated thin device is removed. If
that device has leaked into some other mount namespace and can't be removed,
the container exit still succeeds and this option causes the system to schedule
the device for deferred removal. It does not wait in a loop trying to remove a busy
device.

Example use: `docker daemon --storage-opt dm.use_deferred_removal=true`

#### dm.use_deferred_deletion

Enables use of deferred device deletion for thin pool devices. By default,
thin pool device deletion is synchronous. Before a container is deleted, the
Docker daemon removes any associated devices. If the storage driver can not
remove a device, the container deletion fails and daemon returns.

`Error deleting container: Error response from daemon: Cannot destroy container`

To avoid this failure, enable both deferred device deletion and deferred
device removal on the daemon.

`docker daemon --storage-opt dm.use_deferred_deletion=true --storage-opt dm.use_deferred_removal=true`

With these two options enabled, if a device is busy when the driver is
deleting a container, the driver marks the device as deleted. Later, when the
device isn't in use, the driver deletes it.

In general it should be safe to enable this option by default. It will help
when unintentional leaking of mount point happens across multiple mount
namespaces.

#### dm.loopdatasize

**Note**: This option configures devicemapper loopback, which should not be used in production.

Specifies the size to use when creating the loopback file for the
"data" device which is used for the thin pool. The default size is
100G. The file is sparse, so it will not initially take up
this much space.

Example use: `docker daemon --storage-opt dm.loopdatasize=200G`

#### dm.loopmetadatasize

**Note**: This option configures devicemapper loopback, which should not be used in production.

Specifies the size to use when creating the loopback file for the
"metadata" device which is used for the thin pool. The default size
is 2G. The file is sparse, so it will not initially take up
this much space.

Example use: `docker daemon --storage-opt dm.loopmetadatasize=4G`

#### dm.datadev

(Deprecated, use `dm.thinpooldev`)

Specifies a custom blockdevice to use for data for a
Docker-managed thin pool.  It is better to use `dm.thinpooldev` - see
the documentation for it above for discussion of the advantages.

#### dm.metadatadev

(Deprecated, use `dm.thinpooldev`)

Specifies a custom blockdevice to use for metadata for a
Docker-managed thin pool.  See `dm.datadev` for why this is
deprecated.

#### dm.blocksize

Specifies a custom blocksize to use for the thin pool.  The default
blocksize is 64K.

Example use: `docker daemon --storage-opt dm.blocksize=512K`

#### dm.blkdiscard

Enables or disables the use of `blkdiscard` when removing devicemapper
devices.  This is disabled by default due to the additional latency,
but as a special case with loopback devices it will be enabled, in
order to re-sparsify the loopback file on image/container removal.

Disabling this on loopback can lead to *much* faster container removal
times, but it also prevents the space used in `/var/lib/docker` directory
from being returned to the system for other use when containers are
removed.

Example use: `docker daemon --storage-opt dm.blkdiscard=false`

#### dm.override_udev_sync_check

By default, the devicemapper backend attempts to synchronize with the
`udev` device manager for the Linux kernel.  This option allows
disabling that synchronization, to continue even though the
configuration may be buggy.

To view the `udev` sync support of a Docker daemon that is using the
`devicemapper` driver, run:

        $ docker info
        [...]
         Udev Sync Supported: true
        [...]

When `udev` sync support is `true`, then `devicemapper` and `udev` can
coordinate the activation and deactivation of devices for containers.

When `udev` sync support is `false`, a race condition occurs between
the `devicemapper` and `udev` during create and cleanup. The race
condition results in errors and failures. (For information on these
failures, see
[docker#4036](https://github.com/docker/docker/issues/4036))

To allow the `docker` daemon to start, regardless of whether `udev` sync is
`false`, set `dm.override_udev_sync_check` to true:

        $ docker daemon --storage-opt dm.override_udev_sync_check=true

When this value is `true`, the driver continues and simply warns you
the errors are happening.

**Note**: The ideal is to pursue a `docker` daemon and environment
that does support synchronizing with `udev`. For further discussion on
this topic, see
[docker#4036](https://github.com/docker/docker/issues/4036).
Otherwise, set this flag for migrating existing Docker daemons to a
daemon with a supported environment.

# HISTORY
Sept 2015, Originally compiled by Shishir Mahajan <shishir.mahajan@redhat.com>
based on docker.com source material and internal work.