dockerd

Estimated reading time: 48 minutes

daemon

Usage:	dockerd COMMAND

A self-sufficient runtime for containers.

Options:
      --add-runtime runtime                   Register an additional OCI compatible runtime (default [])
      --allow-nondistributable-artifacts list Push nondistributable artifacts to specified registries (default [])
      --api-cors-header string                Set CORS headers in the Engine API
      --authorization-plugin list             Authorization plugins to load (default [])
      --bip string                            Specify network bridge IP
  -b, --bridge string                         Attach containers to a network bridge
      --cgroup-parent string                  Set parent cgroup for all containers
      --cluster-advertise string              Address or interface name to advertise
      --cluster-store string                  URL of the distributed storage backend
      --cluster-store-opt map                 Set cluster store options (default map[])
      --config-file string                    Daemon configuration file (default "/etc/docker/daemon.json")
      --containerd string                     Path to containerd socket
      --cpu-rt-period int                     Limit the CPU real-time period in microseconds
      --cpu-rt-runtime int                    Limit the CPU real-time runtime in microseconds
      --data-root string                      Root directory of persistent Docker state (default "/var/lib/docker")
  -D, --debug                                 Enable debug mode
      --default-gateway ip                    Container default gateway IPv4 address
      --default-gateway-v6 ip                 Container default gateway IPv6 address
      --default-runtime string                Default OCI runtime for containers (default "runc")
      --default-ulimit ulimit                 Default ulimits for containers (default [])
      --disable-legacy-registry               Disable contacting legacy registries (default true)
      --dns list                              DNS server to use (default [])
      --dns-opt list                          DNS options to use (default [])
      --dns-search list                       DNS search domains to use (default [])
      --exec-opt list                         Runtime execution options (default [])
      --exec-root string                      Root directory for execution state files (default "/var/run/docker")
      --experimental                          Enable experimental features
      --fixed-cidr string                     IPv4 subnet for fixed IPs
      --fixed-cidr-v6 string                  IPv6 subnet for fixed IPs
  -G, --group string                          Group for the unix socket (default "docker")
      --help                                  Print usage
  -H, --host list                             Daemon socket(s) to connect to (default [])
      --icc                                   Enable inter-container communication (default true)
      --init                                  Run an init in the container to forward signals and reap processes
      --init-path string                      Path to the docker-init binary
      --insecure-registry list                Enable insecure registry communication (default [])
      --ip ip                                 Default IP when binding container ports (default 0.0.0.0)
      --ip-forward                            Enable net.ipv4.ip_forward (default true)
      --ip-masq                               Enable IP masquerading (default true)
      --iptables                              Enable addition of iptables rules (default true)
      --ipv6                                  Enable IPv6 networking
      --label list                            Set key=value labels to the daemon (default [])
      --live-restore                          Enable live restore of docker when containers are still running
      --log-driver string                     Default driver for container logs (default "json-file")
  -l, --log-level string                      Set the logging level ("debug", "info", "warn", "error", "fatal") (default "info")
      --log-opt map                           Default log driver options for containers (default map[])
      --max-concurrent-downloads int          Set the max concurrent downloads for each pull (default 3)
      --max-concurrent-uploads int            Set the max concurrent uploads for each push (default 5)
      --metrics-addr string                   Set default address and port to serve the metrics api on
      --mtu int                               Set the containers network MTU
      --no-new-privileges                     Set no-new-privileges by default for new containers
      --oom-score-adjust int                  Set the oom_score_adj for the daemon (default -500)
  -p, --pidfile string                        Path to use for daemon PID file (default "/var/run/docker.pid")
      --raw-logs                              Full timestamps without ANSI coloring
      --registry-mirror list                  Preferred Docker registry mirror (default [])
      --seccomp-profile string                Path to seccomp profile
      --selinux-enabled                       Enable selinux support
      --shutdown-timeout int                  Set the default shutdown timeout (default 15)
  -s, --storage-driver string                 Storage driver to use
      --storage-opt list                      Storage driver options (default [])
      --swarm-default-advertise-addr string   Set default address or interface for swarm advertised address
      --tls                                   Use TLS; implied by --tlsverify
      --tlscacert string                      Trust certs signed only by this CA (default "~/.docker/ca.pem")
      --tlscert string                        Path to TLS certificate file (default "~/.docker/cert.pem")
      --tlskey string                         Path to TLS key file (default ~/.docker/key.pem")
      --tlsverify                             Use TLS and verify the remote
      --userland-proxy                        Use userland proxy for loopback traffic (default true)
      --userland-proxy-path string            Path to the userland proxy binary
      --userns-remap string                   User/Group setting for user namespaces
  -v, --version                               Print version information and quit

Options with [] may be specified multiple times.

Description

dockerd is the persistent process that manages containers. Docker uses different binaries for the daemon and client. To run the daemon you type dockerd.

To run the daemon with debug output, use dockerd -D or add debug: true to the daemon.json file.

Note: In Docker 1.13 and higher, enable experimental features by starting dockerd with the --experimental flag or adding experimental: true to the daemon.json file. In earlier Docker versions, a different build was required to enable experimental features.

Examples

Daemon socket option

The Docker daemon can listen for Docker Engine API requests via three different types of Socket: unix, tcp, and fd.

By default, a unix domain socket (or IPC socket) is created at /var/run/docker.sock, requiring either root permission, or docker group membership.

If you need to access the Docker daemon remotely, you need to enable the tcp Socket. Beware that the default setup provides un-encrypted and un-authenticated direct access to the Docker daemon - and should be secured either using the built in HTTPS encrypted socket, or by putting a secure web proxy in front of it. You can listen on port 2375 on all network interfaces with -H tcp://0.0.0.0:2375, or on a particular network interface using its IP address: -H tcp://192.168.59.103:2375. It is conventional to use port 2375 for un-encrypted, and port 2376 for encrypted communication with the daemon.

Note: If you’re using an HTTPS encrypted socket, keep in mind that only TLS1.0 and greater are supported. Protocols SSLv3 and under are not supported anymore for security reasons.

On Systemd based systems, you can communicate with the daemon via Systemd socket activation, use dockerd -H fd://. Using fd:// will work perfectly for most setups but you can also specify individual sockets: dockerd -H fd://3. If the specified socket activated files aren’t found, then Docker will exit. You can find examples of using Systemd socket activation with Docker and Systemd in the Docker source tree.

You can configure the Docker daemon to listen to multiple sockets at the same time using multiple -H options:

# listen using the default unix socket, and on 2 specific IP addresses on this host.

$ sudo dockerd -H unix:///var/run/docker.sock -H tcp://192.168.59.106 -H tcp://10.10.10.2

The Docker client will honor the DOCKER_HOST environment variable to set the -H flag for the client. Use one of the following commands:

$ docker -H tcp://0.0.0.0:2375 ps
$ export DOCKER_HOST="tcp://0.0.0.0:2375"

$ docker ps

Setting the DOCKER_TLS_VERIFY environment variable to any value other than the empty string is equivalent to setting the --tlsverify flag. The following are equivalent:

$ docker --tlsverify ps
# or
$ export DOCKER_TLS_VERIFY=1
$ docker ps

The Docker client will honor the HTTP_PROXY, HTTPS_PROXY, and NO_PROXY environment variables (or the lowercase versions thereof). HTTPS_PROXY takes precedence over HTTP_PROXY.

Bind Docker to another host/port or a Unix socket

Warning: Changing the default docker daemon binding to a TCP port or Unix docker user group will increase your security risks by allowing non-root users to gain root access on the host. Make sure you control access to docker. If you are binding to a TCP port, anyone with access to that port has full Docker access; so it is not advisable on an open network.

With -H it is possible to make the Docker daemon to listen on a specific IP and port. By default, it will listen on unix:///var/run/docker.sock to allow only local connections by the root user. You could set it to 0.0.0.0:2375 or a specific host IP to give access to everybody, but that is not recommended because then it is trivial for someone to gain root access to the host where the daemon is running.

Similarly, the Docker client can use -H to connect to a custom port. The Docker client will default to connecting to unix:///var/run/docker.sock on Linux, and tcp://127.0.0.1:2376 on Windows.

-H accepts host and port assignment in the following format:

tcp://[host]:[port][path] or unix://path

For example:

  • tcp:// -> TCP connection to 127.0.0.1 on either port 2376 when TLS encryption is on, or port 2375 when communication is in plain text.
  • tcp://host:2375 -> TCP connection on host:2375
  • tcp://host:2375/path -> TCP connection on host:2375 and prepend path to all requests
  • unix://path/to/socket -> Unix socket located at path/to/socket

-H, when empty, will default to the same value as when no -H was passed in.

-H also accepts short form for TCP bindings: host: or host:port or :port

Run Docker in daemon mode:

$ sudo <path to>/dockerd -H 0.0.0.0:5555 &

Download an ubuntu image:

$ docker -H :5555 pull ubuntu

You can use multiple -H, for example, if you want to listen on both TCP and a Unix socket

# Run docker in daemon mode
$ sudo <path to>/dockerd -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock &
# Download an ubuntu image, use default Unix socket
$ docker pull ubuntu
# OR use the TCP port
$ docker -H tcp://127.0.0.1:2375 pull ubuntu

Daemon storage-driver

The Docker daemon has support for several different image layer storage drivers: aufs, devicemapper, btrfs, zfs, overlay and overlay2.

The aufs driver is the oldest, but is based on a Linux kernel patch-set that is unlikely to be merged into the main kernel. These are also known to cause some serious kernel crashes. However aufs allows containers to share executable and shared library memory, so is a useful choice when running thousands of containers with the same program or libraries.

The devicemapper driver uses thin provisioning and Copy on Write (CoW) snapshots. For each devicemapper graph location – typically /var/lib/docker/devicemapper – a thin pool is created based on two block devices, one for data and one for metadata. By default, these block devices are created automatically by using loopback mounts of automatically created sparse files. Refer to Storage driver options below for a way how to customize this setup. ~jpetazzo/Resizing Docker containers with the Device Mapper plugin article explains how to tune your existing setup without the use of options.

The btrfs driver is very fast for docker build - but like devicemapper does not share executable memory between devices. Use dockerd -s btrfs -g /mnt/btrfs_partition.

The zfs driver is probably not as fast as btrfs but has a longer track record on stability. Thanks to Single Copy ARC shared blocks between clones will be cached only once. Use dockerd -s zfs. To select a different zfs filesystem set zfs.fsname option as described in Storage driver options.

The overlay is a very fast union filesystem. It is now merged in the main Linux kernel as of 3.18.0. overlay also supports page cache sharing, this means multiple containers accessing the same file can share a single page cache entry (or entries), it makes overlay as efficient with memory as aufs driver. Call dockerd -s overlay to use it.

Note: As promising as overlay is, the feature is still quite young and should not be used in production. Most notably, using overlay can cause excessive inode consumption (especially as the number of images grows), as well as > being incompatible with the use of RPMs.

The overlay2 uses the same fast union filesystem but takes advantage of additional features added in Linux kernel 4.0 to avoid excessive inode consumption. Call dockerd -s overlay2 to use it.

Note: Both overlay and overlay2 are currently unsupported on btrfs or any Copy on Write filesystem and should only be used over ext4 partitions.

Options per storage driver

Particular storage-driver can be configured with options specified with --storage-opt flags. Options for devicemapper are prefixed with dm, options for zfs start with zfs and options for btrfs start with btrfs.

Devicemapper options

This is an example of the configuration file for devicemapper on Linux:

{
  "storage-driver": "devicemapper",
  "storage-opts": [
    "dm.thinpooldev=/dev/mapper/thin-pool",
    "dm.use_deferred_deletion=true",
    "dm.use_deferred_removal=true"
  ]
}
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 exclusively create snapshot volumes needed for images and containers.

Managing the thin-pool outside of Engine makes for the most feature-rich method of having Docker utilize device mapper thin provisioning as the backing storage for Docker 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.

As a fallback if no thin pool is provided, loopback files are created. Loopback is very slow, but can be used without any pre-configuration of storage. It is strongly recommended that you do not use loopback in production. Ensure your Engine daemon has a --storage-opt dm.thinpooldev argument provided.

Example:
$ sudo dockerd --storage-opt dm.thinpooldev=/dev/mapper/thin-pool
dm.directlvm_device

As an alternative to providing a thin pool as above, Docker can setup a block device for you.

Example:
$ sudo dockerd --storage-opt dm.directlvm_device=/dev/xvdf
dm.thinp_percent

Sets the percentage of passed in block device to use for storage.

Example:
$ sudo dockerd --storage-opt dm.thinp_percent=95
dm.thinp_metapercent

Sets the percentage of the passed in block device to use for metadata storage.

Example:
$ sudo dockerd --storage-opt dm.thinp_metapercent=1
dm.thinp_autoextend_threshold

Sets the value of the percentage of space used before lvm attempts to autoextend the available space [100 = disabled]

Example:
$ sudo dockerd --storage-opt dm.thinp_autoextend_threshold=80
dm.thinp_autoextend_percent

Sets the value percentage value to increase the thin pool by when when lvm attempts to autoextend the available space [100 = disabled]

Example:
$ sudo dockerd --storage-opt dm.thinp_autoextend_percent=20
dm.basesize

Specifies the size to use when creating the base device, which limits the size of images and containers. The default value is 10G. Note, thin devices are inherently “sparse”, so a 10G device which is mostly empty doesn’t use 10 GB of space on the pool. However, the filesystem will use more space for the empty case the larger the device is.

The base device size can be increased at daemon restart which will allow all future images and containers (based on those new images) to be of the new base device size.

Examples
$ sudo dockerd --storage-opt dm.basesize=50G

This will increase the base device size to 50G. The Docker daemon will throw an error if existing base device size is larger than 50G. A user can use this option to expand the base device size however shrinking is not permitted.

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
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
$ sudo dockerd --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
$ sudo dockerd --storage-opt dm.loopmetadatasize=4G
dm.fs

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

Example
$ sudo dockerd --storage-opt dm.fs=ext4
dm.mkfsarg

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

Example
$ sudo dockerd --storage-opt "dm.mkfsarg=-O ^has_journal"
dm.mountopt

Specifies extra mount options used when mounting the thin devices.

Example
$ sudo dockerd --storage-opt dm.mountopt=nodiscard
dm.datadev

(Deprecated, use dm.thinpooldev)

Specifies a custom blockdevice to use for data for the thin pool.

If using a block device for device mapper storage, ideally both datadev and metadatadev should be specified to completely avoid using the loopback device.

Example
$ sudo dockerd \
      --storage-opt dm.datadev=/dev/sdb1 \
      --storage-opt dm.metadatadev=/dev/sdc1
dm.metadatadev

(Deprecated, use dm.thinpooldev)

Specifies a custom blockdevice to use for metadata for the thin pool.

For best performance the metadata should be on a different spindle than the data, or even better on an SSD.

If setting up a new metadata pool it is required to be valid. This can be achieved by zeroing the first 4k to indicate empty metadata, like this:

$ dd if=/dev/zero of=$metadata_dev bs=4096 count=1
Example
$ sudo dockerd \
      --storage-opt dm.datadev=/dev/sdb1 \
      --storage-opt dm.metadatadev=/dev/sdc1
dm.blocksize

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

Example
$ sudo dockerd --storage-opt dm.blocksize=512K
dm.blkdiscard

Enables or disables the use of blkdiscard when removing devicemapper devices. This is enabled by default (only) if using loopback devices and is required to resparsify the loopback file on image/container removal.

Disabling this on loopback can lead to much faster container removal times, but will make the space used in /var/lib/docker directory not be returned to the system for other use when containers are removed.

Examples
$ sudo dockerd --storage-opt dm.blkdiscard=false
dm.override_udev_sync_check

Overrides the udev synchronization checks between devicemapper and udev. udev is the device manager for the Linux kernel.

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 thedevicemapper and udev during create and cleanup. The race condition results in errors and failures. (For information on these failures, see docker#4036)

To allow the docker daemon to start, regardless of udev sync not being supported, set dm.override_udev_sync_check to true:

$ sudo dockerd --storage-opt dm.override_udev_sync_check=true

When this value is true, the devicemapper 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. Otherwise, set this flag for migrating existing Docker daemons to a daemon with a supported environment.

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
$ sudo dockerd --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.

$ sudo dockerd \
      --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.min_free_space

Specifies the min free space percent in a thin pool require for new device creation to succeed. This check applies to both free data space as well as free metadata space. Valid values are from 0% - 99%. Value 0% disables free space checking logic. If user does not specify a value for this option, the Engine uses a default value of 10%.

Whenever a new a thin pool device is created (during docker pull or during container creation), the Engine checks if the minimum free space is available. If sufficient space is unavailable, then device creation fails and any relevant docker operation fails.

To recover from this error, you must create more free space in the thin pool to recover from the error. You can create free space by deleting some images and containers from the thin pool. You can also add more storage to the thin pool.

To add more space to a LVM (logical volume management) thin pool, just add more storage to the volume group container thin pool; this should automatically resolve any errors. If your configuration uses loop devices, then stop the Engine daemon, grow the size of loop files and restart the daemon to resolve the issue.

Example
$ sudo dockerd --storage-opt dm.min_free_space=10%
dm.xfs_nospace_max_retries

Specifies the maximum number of retries XFS should attempt to complete IO when ENOSPC (no space) error is returned by underlying storage device.

By default XFS retries infinitely for IO to finish and this can result in unkillable process. To change this behavior one can set xfs_nospace_max_retries to say 0 and XFS will not retry IO after getting ENOSPC and will shutdown filesystem.

Example
$ sudo dockerd --storage-opt dm.xfs_nospace_max_retries=0

ZFS options

zfs.fsname

Set zfs filesystem under which docker will create its own datasets. By default docker will pick up the zfs filesystem where docker graph (/var/lib/docker) is located.

Example
$ sudo dockerd -s zfs --storage-opt zfs.fsname=zroot/docker

Btrfs options

btrfs.min_space

Specifies the minimum size to use when creating the subvolume which is used for containers. If user uses disk quota for btrfs when creating or running a container with –storage-opt size option, docker should ensure the size cannot be smaller than btrfs.min_space.

Example
$ sudo dockerd -s btrfs --storage-opt btrfs.min_space=10G

Overlay2 options

overlay2.override_kernel_check

Overrides the Linux kernel version check allowing overlay2. Support for specifying multiple lower directories needed by overlay2 was added to the Linux kernel in 4.0.0. However, some older kernel versions may be patched to add multiple lower directory support for OverlayFS. This option should only be used after verifying this support exists in the kernel. Applying this option on a kernel without this support will cause failures on mount.

Docker runtime execution options

The Docker daemon relies on a OCI 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:

$ sudo dockerd --containerd /var/run/dev/docker-containerd.sock

Runtimes can be registered with the daemon either via the configuration file or using the --add-runtime command line argument.

The following is an example adding 2 runtimes via the configuration:

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

This is the same example via the command line:

$ sudo dockerd --add-runtime runc=runc --add-runtime custom=/usr/local/bin/my-runc-replacement

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

Options for the runtime

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.

The native.cgroupdriver option specifies the management of the container’s cgroups. 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.

This example sets the cgroupdriver to systemd:

$ 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:

> 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.

Daemon DNS options

To set the DNS server for all Docker containers, use:

$ sudo dockerd --dns 8.8.8.8

To set the DNS search domain for all Docker containers, use:

$ sudo dockerd --dns-search example.com

Allow push of nondistributable artifacts

Some images (e.g., Windows base images) contain artifacts whose distribution is restricted by license. When these images are pushed to a registry, restricted artifacts are not included.

To override this behavior for specific registries, use the --allow-nondistributable-artifacts option in one of the following forms:

  • --allow-nondistributable-artifacts myregistry:5000 tells the Docker daemon to push nondistributable artifacts to myregistry:5000.
  • --allow-nondistributable-artifacts 10.1.0.0/16 tells the Docker daemon to push nondistributable artifacts to all registries whose resolved IP address is within the subnet described by the CIDR syntax.

This option can be used multiple times.

This option is useful when pushing images containing nondistributable artifacts to a registry on an air-gapped network so hosts on that network can pull the images without connecting to another server.

Warning: Nondistributable artifacts typically have restrictions on how and where they can be distributed and shared. Only use this feature to push artifacts to private registries and ensure that you are in compliance with any terms that cover redistributing nondistributable artifacts.

Insecure registries

Docker considers a private registry either secure or insecure. In the rest of this section, registry is used for private registry, and myregistry:5000 is a placeholder example for a private registry.

A secure registry uses TLS and a copy of its CA certificate is placed on the Docker host at /etc/docker/certs.d/myregistry:5000/ca.crt. An insecure registry is either not using TLS (i.e., listening on plain text HTTP), or is using TLS with a CA certificate not known by the Docker daemon. The latter can happen when the certificate was not found under /etc/docker/certs.d/myregistry:5000/, or if the certificate verification failed (i.e., wrong CA).

By default, Docker assumes all, but local (see local registries below), registries are secure. Communicating with an insecure registry is not possible if Docker assumes that registry is secure. In order to communicate with an insecure registry, the Docker daemon requires --insecure-registry in one of the following two forms:

  • --insecure-registry myregistry:5000 tells the Docker daemon that myregistry:5000 should be considered insecure.
  • --insecure-registry 10.1.0.0/16 tells the Docker daemon that all registries whose domain resolve to an IP address is part of the subnet described by the CIDR syntax, should be considered insecure.

The flag can be used multiple times to allow multiple registries to be marked as insecure.

If an insecure registry is not marked as insecure, docker pull, docker push, and docker search will result in an error message prompting the user to either secure or pass the --insecure-registry flag to the Docker daemon as described above.

Local registries, whose IP address falls in the 127.0.0.0/8 range, are automatically marked as insecure as of Docker 1.3.2. It is not recommended to rely on this, as it may change in the future.

Enabling --insecure-registry, i.e., allowing un-encrypted and/or untrusted communication, can be 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 enabling --insecure-registry.

Legacy Registries

Operations against registries supporting only the legacy v1 protocol are disabled by default. Specifically, the daemon will not attempt push, pull and login to v1 registries. The exception to this is search which can still be performed on v1 registries.

Add "disable-legacy-registry":false to the daemon configuration file, or set the --disable-legacy-registry=false flag, if you need to interact with registries that have not yet migrated to the v2 protocol.

Interaction v1 registries will no longer be supported in Docker v17.12, and the disable-legacy-registry configuration option will be removed.

Running a Docker daemon behind an HTTPS_PROXY

When running inside a LAN that uses an HTTPS proxy, the Docker Hub certificates will be replaced by the proxy’s certificates. These certificates need to be added to your Docker host’s configuration:

  1. Install the ca-certificates package for your distribution
  2. Ask your network admin for the proxy’s CA certificate and append them to /etc/pki/tls/certs/ca-bundle.crt
  3. Then start your Docker daemon with HTTPS_PROXY=http://username:password@proxy:port/ dockerd. The username: and password@ are optional - and are only needed if your proxy is set up to require authentication.

This will only add the proxy and authentication to the Docker daemon’s requests - your docker builds and running containers will need extra configuration to use the proxy

Default ulimit settings

--default-ulimit allows you to set the default ulimit options to use for all containers. It takes the same options as --ulimit for docker run. If these defaults are not set, ulimit settings will be inherited, if not set on docker run, from the Docker daemon. Any --ulimit options passed to docker run will overwrite these defaults.

Be careful setting nproc with the ulimit flag as nproc is designed by Linux to set the maximum number of processes available to a user, not to a container. For details please check the run reference.

Node discovery

The --cluster-advertise option specifies the host:port or interface:port combination that this particular daemon instance should use when advertising itself to the cluster. The daemon is reached by remote hosts through this value. If you specify an interface, make sure it includes the IP address of the actual Docker host. For Engine installation created through docker-machine, the interface is typically eth1.

The daemon uses libkv to advertise the node within the cluster. Some key-value backends support mutual TLS. To configure the client TLS settings used by the daemon can be configured using the --cluster-store-opt flag, specifying the paths to PEM encoded files. For example:

$ sudo dockerd \
    --cluster-advertise 192.168.1.2:2376 \
    --cluster-store etcd://192.168.1.2:2379 \
    --cluster-store-opt kv.cacertfile=/path/to/ca.pem \
    --cluster-store-opt kv.certfile=/path/to/cert.pem \
    --cluster-store-opt kv.keyfile=/path/to/key.pem

The currently supported cluster store options are:

Option Description
discovery.heartbeat Specifies the heartbeat timer in seconds which is used by the daemon as a keepalive mechanism to make sure discovery module treats the node as alive in the cluster. If not configured, the default value is 20 seconds.
discovery.ttl Specifies the TTL (time-to-live) in seconds which is used by the discovery module to timeout a node if a valid heartbeat is not received within the configured ttl value. If not configured, the default value is 60 seconds.
kv.cacertfile Specifies the path to a local file with PEM encoded CA certificates to trust.
kv.certfile Specifies the path to a local file with a PEM encoded certificate. This certificate is used as the client cert for communication with the Key/Value store.
kv.keyfile Specifies the path to a local file with a PEM encoded private key. This private key is used as the client key for communication with the Key/Value store.
kv.path Specifies the path in the Key/Value store. If not configured, the default value is ‘docker/nodes’.

Access authorization

Docker’s access authorization can be extended by authorization plugins that your organization can purchase or build themselves. You can install one or more authorization plugins when you start the Docker daemon using the --authorization-plugin=PLUGIN_ID option.

$ sudo dockerd --authorization-plugin=plugin1 --authorization-plugin=plugin2,...

The PLUGIN_ID value is either the plugin’s name or a path to its specification file. The plugin’s implementation determines whether you can specify a name or path. Consult with your Docker administrator to get information about the plugins available to you.

Once a plugin is installed, requests made to the daemon through the command line or Docker’s Engine API are allowed or denied by the plugin. If you have multiple plugins installed, each plugin, in order, must allow the request for it to complete.

For information about how to create an authorization plugin, see authorization plugin section in the Docker extend section of this documentation.

Daemon user namespace options

The Linux kernel user namespace support provides additional security by enabling a process, and therefore a container, to have a unique range of user and group IDs which are outside the traditional user and group range utilized by the host system. Potentially the most important security improvement is that, by default, container processes running as the root user will have expected administrative privilege (with some restrictions) inside the container but will effectively be mapped to an unprivileged uid on the host.

For details about how to use this feature, as well as limitations, see Isolate containers with a user namespace.

Miscellaneous options

IP masquerading uses address translation to allow containers without a public IP to talk to other machines on the Internet. This may interfere with some network topologies and can be disabled with --ip-masq=false.

Docker supports softlinks for the Docker data directory (/var/lib/docker) and for /var/lib/docker/tmp. The DOCKER_TMPDIR and the data directory can be set like this:

DOCKER_TMPDIR=/mnt/disk2/tmp /usr/local/bin/dockerd -D -g /var/lib/docker -H unix:// > /var/lib/docker-machine/docker.log 2>&1
# or
export DOCKER_TMPDIR=/mnt/disk2/tmp
/usr/local/bin/dockerd -D -g /var/lib/docker -H unix:// > /var/lib/docker-machine/docker.log 2>&1

Default cgroup parent

The --cgroup-parent option allows you to set the default cgroup parent to use for containers. If this option is not set, it defaults to /docker for fs cgroup driver and system.slice for systemd cgroup driver.

If the cgroup has a leading forward slash (/), the cgroup is created under the root cgroup, otherwise the cgroup is created under the daemon cgroup.

Assuming the daemon is running in cgroup daemoncgroup, --cgroup-parent=/foobar creates a cgroup in /sys/fs/cgroup/memory/foobar, whereas using --cgroup-parent=foobar creates the cgroup in /sys/fs/cgroup/memory/daemoncgroup/foobar

The systemd cgroup driver has different rules for --cgroup-parent. Systemd represents hierarchy by slice and the name of the slice encodes the location in the tree. So --cgroup-parent for systemd cgroups should be a slice name. A name can consist of a dash-separated series of names, which describes the path to the slice from the root slice. For example, --cgroup-parent=user-a-b.slice means the memory cgroup for the container is created in /sys/fs/cgroup/memory/user.slice/user-a.slice/user-a-b.slice/docker-<id>.scope.

This setting can also be set per container, using the --cgroup-parent option on docker create and docker run, and takes precedence over the --cgroup-parent option on the daemon.

Daemon metrics

The --metrics-addr option takes a tcp address to serve the metrics API. This feature is still experimental, therefore, the daemon must be running in experimental mode for this feature to work.

To serve the metrics API on localhost:1337 you would specify --metrics-addr 127.0.0.1:1337 allowing you to make requests on the API at 127.0.0.1:1337/metrics to receive metrics in the prometheus format.

If you are running a prometheus server you can add this address to your scrape configs to have prometheus collect metrics on Docker. For more information on prometheus you can view the website here.

scrape_configs:
  - job_name: 'docker'
    static_configs:
      - targets: ['127.0.0.1:1337']

Please note that this feature is still marked as experimental as metrics and metric names could change while this feature is still in experimental. Please provide feedback on what you would like to see collected in the API.

Daemon configuration file

The --config-file option allows you to set any configuration option for the daemon in a JSON format. This file uses the same flag names as keys, except for flags that allow several entries, where it uses the plural of the flag name, e.g., labels for the label flag.

The options set in the configuration file must not conflict with options set via flags. The docker daemon fails to start if an option is duplicated between the file and the flags, regardless their value. We do this to avoid silently ignore changes introduced in configuration reloads. For example, the daemon fails to start if you set daemon labels in the configuration file and also set daemon labels via the --label flag. Options that are not present in the file are ignored when the daemon starts.

On Linux

The default location of the configuration file on Linux is /etc/docker/daemon.json. The --config-file flag can be used to specify a non-default location.

This is a full example of the allowed configuration options on Linux:

{
	"authorization-plugins": [],
	"data-root": "",
	"dns": [],
	"dns-opts": [],
	"dns-search": [],
	"exec-opts": [],
	"exec-root": "",
	"experimental": false,
	"storage-driver": "",
	"storage-opts": [],
	"labels": [],
	"live-restore": true,
	"log-driver": "",
	"log-opts": {},
	"mtu": 0,
	"pidfile": "",
	"cluster-store": "",
	"cluster-store-opts": {},
	"cluster-advertise": "",
	"max-concurrent-downloads": 3,
	"max-concurrent-uploads": 5,
	"default-shm-size": "64M",
	"shutdown-timeout": 15,
	"debug": true,
	"hosts": [],
	"log-level": "",
	"tls": true,
	"tlsverify": true,
	"tlscacert": "",
	"tlscert": "",
	"tlskey": "",
	"swarm-default-advertise-addr": "",
	"api-cors-header": "",
	"selinux-enabled": false,
	"userns-remap": "",
	"group": "",
	"cgroup-parent": "",
	"default-ulimits": {},
	"init": false,
	"init-path": "/usr/libexec/docker-init",
	"ipv6": false,
	"iptables": false,
	"ip-forward": false,
	"ip-masq": false,
	"userland-proxy": false,
	"userland-proxy-path": "/usr/libexec/docker-proxy",
	"ip": "0.0.0.0",
	"bridge": "",
	"bip": "",
	"fixed-cidr": "",
	"fixed-cidr-v6": "",
	"default-gateway": "",
	"default-gateway-v6": "",
	"icc": false,
	"raw-logs": false,
	"allow-nondistributable-artifacts": [],
	"registry-mirrors": [],
	"seccomp-profile": "",
	"insecure-registries": [],
	"disable-legacy-registry": false,
	"no-new-privileges": false,
	"default-runtime": "runc",
	"oom-score-adjust": -500,
	"runtimes": {
		"runc": {
			"path": "runc"
		},
		"custom": {
			"path": "/usr/local/bin/my-runc-replacement",
			"runtimeArgs": [
				"--debug"
			]
		}
	}
}

Note: You cannot set options in daemon.json that have already been set on daemon startup as a flag. On systems that use systemd to start the Docker daemon, -H is already set, so you cannot use the hosts key in daemon.json to add listening addresses. See https://docs.docker.com/engine/admin/systemd/#custom-docker-daemon-options for how to accomplish this task with a systemd drop-in file.

On Windows

The default location of the configuration file on Windows is %programdata%\docker\config\daemon.json. The --config-file flag can be used to specify a non-default location.

This is a full example of the allowed configuration options on Windows:

{
    "authorization-plugins": [],
    "data-root": "",
    "dns": [],
    "dns-opts": [],
    "dns-search": [],
    "exec-opts": [],
    "experimental": false,
    "storage-driver": "",
    "storage-opts": [],
    "labels": [],
    "log-driver": "",
    "mtu": 0,
    "pidfile": "",
    "cluster-store": "",
    "cluster-advertise": "",
    "max-concurrent-downloads": 3,
    "max-concurrent-uploads": 5,
    "shutdown-timeout": 15,
    "debug": true,
    "hosts": [],
    "log-level": "",
    "tlsverify": true,
    "tlscacert": "",
    "tlscert": "",
    "tlskey": "",
    "swarm-default-advertise-addr": "",
    "group": "",
    "default-ulimits": {},
    "bridge": "",
    "fixed-cidr": "",
    "raw-logs": false,
    "allow-nondistributable-artifacts": [],
    "registry-mirrors": [],
    "insecure-registries": [],
    "disable-legacy-registry": false
}

Configuration reload behavior

Some options can be reconfigured when the daemon is running without requiring to restart the process. We use the SIGHUP signal in Linux to reload, and a global event in Windows with the key Global\docker-daemon-config-$PID. The options can be modified in the configuration file but still will check for conflicts with the provided flags. The daemon fails to reconfigure itself if there are conflicts, but it won’t stop execution.

The list of currently supported options that can be reconfigured is this:

  • debug: it changes the daemon to debug mode when set to true.
  • cluster-store: it reloads the discovery store with the new address.
  • cluster-store-opts: it uses the new options to reload the discovery store.
  • cluster-advertise: it modifies the address advertised after reloading.
  • labels: it replaces the daemon labels with a new set of labels.
  • live-restore: Enables keeping containers alive during daemon downtime.
  • max-concurrent-downloads: it updates the max concurrent downloads for each pull.
  • max-concurrent-uploads: it updates the max concurrent uploads for each push.
  • default-runtime: it updates the runtime to be used if not is specified at container creation. It defaults to “default” which is the runtime shipped with the official docker packages.
  • runtimes: it updates the list of available OCI runtimes that can be used to run containers
  • authorization-plugin: specifies the authorization plugins to use.
  • allow-nondistributable-artifacts: Replaces the set of registries to which the daemon will push nondistributable artifacts with a new set of registries.
  • insecure-registries: it replaces the daemon insecure registries with a new set of insecure registries. If some existing insecure registries in daemon’s configuration are not in newly reloaded insecure resgitries, these existing ones will be removed from daemon’s config.
  • registry-mirrors: it replaces the daemon registry mirrors with a new set of registry mirrors. If some existing registry mirrors in daemon’s configuration are not in newly reloaded registry mirrors, these existing ones will be removed from daemon’s config.

Updating and reloading the cluster configurations such as --cluster-store, --cluster-advertise and --cluster-store-opts will take effect only if these configurations were not previously configured. If --cluster-store has been provided in flags and cluster-advertise not, cluster-advertise can be added in the configuration file without accompanied by --cluster-store. Configuration reload will log a warning message if it detects a change in previously configured cluster configurations.

Run multiple daemons

Note: Running multiple daemons on a single host is considered as “experimental”. The user should be aware of unsolved problems. This solution may not work properly in some cases. Solutions are currently under development and will be delivered in the near future.

This section describes how to run multiple Docker daemons on a single host. To run multiple daemons, you must configure each daemon so that it does not conflict with other daemons on the same host. You can set these options either by providing them as flags, or by using a daemon configuration file.

The following daemon options must be configured for each daemon:

-b, --bridge=                          Attach containers to a network bridge
--exec-root=/var/run/docker            Root of the Docker execdriver
--data-root=/var/lib/docker            Root of persisted Docker data
-p, --pidfile=/var/run/docker.pid      Path to use for daemon PID file
-H, --host=[]                          Daemon socket(s) to connect to
--iptables=true                        Enable addition of iptables rules
--config-file=/etc/docker/daemon.json  Daemon configuration file
--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

When your daemons use different values for these flags, you can run them on the same host without any problems. It is very important to properly understand the meaning of those options and to use them correctly.

  • The -b, --bridge= flag is set to docker0 as default bridge network. It is created automatically when you install Docker. If you are not using the default, you must create and configure the bridge manually or just set it to ‘none’: --bridge=none
  • --exec-root is the path where the container state is stored. The default value is /var/run/docker. Specify the path for your running daemon here.
  • --data-root is the path where persisted data such as images, volumes, and cluster state are stored. The default value is /var/lib/docker. To avoid any conflict with other daemons, set this parameter separately for each daemon.
  • -p, --pidfile=/var/run/docker.pid is the path where the process ID of the daemon is stored. Specify the path for your pid file here.
  • --host=[] specifies where the Docker daemon will listen for client connections. If unspecified, it defaults to /var/run/docker.sock.
  • --iptables=false prevents the Docker daemon from adding iptables rules. If multiple daemons manage iptables rules, they may overwrite rules set by another daemon. Be aware that disabling this option requires you to manually add iptables rules to expose container ports. If you prevent Docker from adding iptables rules, Docker will also not add IP masquerading rules, even if you set --ip-masq to true. Without IP masquerading rules, Docker containers will not be able to connect to external hosts or the internet when using network other than default bridge.
  • --config-file=/etc/docker/daemon.json is the path where configuration file is stored. You can use it instead of daemon flags. Specify the path for each daemon.
  • --tls* Docker daemon supports --tlsverify mode that enforces encrypted and authenticated remote connections. The --tls* options enable use of specific certificates for individual daemons.

Example script for a separate “bootstrap” instance of the Docker daemon without network:

$ sudo dockerd \
        -H unix:///var/run/docker-bootstrap.sock \
        -p /var/run/docker-bootstrap.pid \
        --iptables=false \
        --ip-masq=false \
        --bridge=none \
        --data-root=/var/lib/docker-bootstrap \
        --exec-root=/var/run/docker-bootstrap
container, daemon, runtime