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dockerd

Table of contents

daemon

Usage: dockerd [OPTIONS]
A self-sufficient runtime for containers.
Options:
      --add-runtime runtime                   Register an additional OCI compatible runtime (default [])
      --allow-nondistributable-artifacts list Allow push of nondistributable artifacts to registry
      --api-cors-header string                Set CORS headers in the Engine API
      --authorization-plugin list             Authorization plugins to load
      --bip string                            Specify network bridge IP
  -b, --bridge string                         Attach containers to a network bridge
      --cdi-spec-dir list                     CDI specification directories to use
      --cgroup-parent string                  Set parent cgroup for all containers
      --config-file string                    Daemon configuration file (default "/etc/docker/daemon.json")
      --containerd string                     containerd grpc address
      --containerd-namespace string           Containerd namespace to use (default "moby")
      --containerd-plugins-namespace string   Containerd namespace to use for plugins (default "plugins.moby")
      --cpu-rt-period int                     Limit the CPU real-time period in microseconds for the
                                              parent cgroup for all containers (not supported with cgroups v2)
      --cpu-rt-runtime int                    Limit the CPU real-time runtime in microseconds for the
                                              parent cgroup for all containers (not supported with cgroups v2)
      --cri-containerd                        start containerd with cri
      --data-root string                      Root directory of persistent Docker state (default "/var/lib/docker")
  -D, --debug                                 Enable debug mode
      --default-address-pool pool-options     Default address pools for node specific local networks
      --default-cgroupns-mode string          Default mode for containers cgroup namespace ("host" | "private") (default "private")
      --default-gateway ip                    Container default gateway IPv4 address
      --default-gateway-v6 ip                 Container default gateway IPv6 address
      --default-ipc-mode string               Default mode for containers ipc ("shareable" | "private") (default "private")
      --default-network-opt mapmap            Default network options (default map[])
      --default-runtime string                Default OCI runtime for containers (default "runc")
      --default-shm-size bytes                Default shm size for containers (default 64MiB)
      --default-ulimit ulimit                 Default ulimits for containers (default [])
      --dns list                              DNS server to use
      --dns-opt list                          DNS options to use
      --dns-search list                       DNS search domains to use
      --exec-opt list                         Runtime execution options
      --exec-root string                      Root directory for execution state files (default "/var/run/docker")
      --experimental                          Enable experimental features
      --feature map                           Enable feature in the daemon
      --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
      --host-gateway-ip ip                    IP address that the special 'host-gateway' string in --add-host resolves to.
                                              Defaults to the IP address of the default bridge
      --http-proxy string                     HTTP proxy URL to use for outgoing traffic
      --https-proxy string                    HTTPS proxy URL to use for outgoing traffic
      --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
      --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)
      --ip6tables                             Enable addition of ip6tables rules (experimental)
      --iptables                              Enable addition of iptables rules (default true)
      --ipv6                                  Enable IPv6 networking
      --label list                            Set key=value labels to the daemon
      --live-restore                          Enable live restore of docker when containers are still running
      --log-driver string                     Default driver for container logs (default "json-file")
      --log-format string                     Set the logging format ("text"|"json") (default "text")
  -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 (default 3)
      --max-concurrent-uploads int            Set the max concurrent uploads (default 5)
      --max-download-attempts int             Set the max download attempts for each pull (default 5)
      --metrics-addr string                   Set default address and port to serve the metrics api on
      --mtu int                               Set the containers network MTU (default 1500)
      --network-control-plane-mtu int         Network Control plane MTU (default 1500)
      --no-new-privileges                     Set no-new-privileges by default for new containers
      --no-proxy string                       Comma-separated list of hosts or IP addresses for which the proxy is skipped
      --node-generic-resource list            Advertise user-defined resource
      --oom-score-adjust int                  Set the oom_score_adj for the daemon
  -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 registry mirror
      --rootless                              Enable rootless mode; typically used with RootlessKit
      --seccomp-profile string                Path to seccomp profile. Use "unconfined" to disable the default seccomp profile (default "builtin")
      --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
      --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
      --validate                              Validate daemon configuration and exit
  -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 --debug or add "debug": true the daemon.json file .

Note

Enabling experimental features

Enable experimental features by starting dockerd with the --experimental flag or adding "experimental": true to the daemon.json file.

The following list of environment variables are supported by the dockerd daemon. Some of these environment variables are supported both by the Docker Daemon and the docker CLI. Refer to Environment variables to learn about environment variables supported by the docker CLI.

Variable	Description
`DOCKER_CERT_PATH`	Location of your authentication keys. This variable is used both by the `docker` CLI and the `dockerd` daemon.
`DOCKER_DRIVER`	The storage driver to use.
`DOCKER_RAMDISK`	If set this disables `pivot_root` .
`DOCKER_TLS_VERIFY`	When set Docker uses TLS and verifies the remote. This variable is used both by the `docker` CLI and the `dockerd` daemon.
`DOCKER_TMPDIR`	Location for temporary files created by the daemon.
`HTTP_PROXY`	Proxy URL for HTTP requests unless overridden by NoProxy. See the Go specification for details.
`HTTPS_PROXY`	Proxy URL for HTTPS requests unless overridden by NoProxy. See the Go specification for details.
`MOBY_DISABLE_PIGZ`	Disables the use of `unpigz` to decompress layers in parallel when pulling images, even if it is installed.
`NO_PROXY`	Comma-separated values specifying hosts that should be excluded from proxying. See the Go specification for details.

Examples

Proxy configuration

Note

Refer to the Docker Desktop manual if you are running Docker Desktop .

If you are behind an HTTP proxy server, for example in corporate settings, you may have to configure the Docker daemon to use the proxy server for operations such as pulling and pushing images. The daemon can be configured in three ways:

Using environment variables ( HTTP_PROXY , HTTPS_PROXY , and NO_PROXY ).
Using the http-proxy , https-proxy , and no-proxy fields in the daemon configuration file (Docker Engine version 23.0 or later).
Using the --http-proxy , --https-proxy , and --no-proxy command-line options. (Docker Engine version 23.0 or later).

The command-line and configuration file options take precedence over environment variables. Refer to control and configure Docker with systemd to set these environment variables on a host using systemd .

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. When using a TCP socket, the Docker daemon provides un-encrypted and un-authenticated direct access to the Docker daemon by default. You should secure the daemon 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 TLS version 1.0 and higher is supported. Protocols SSLv3 and below are not supported for security reasons.

On systemd based systems, you can communicate with the daemon via systemd socket activation , with dockerd -H fd:// . Using fd:// works for most setups, but you can also specify individual sockets: dockerd -H fd://3 . If the specified socket activated files aren't found, the daemon exits. 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:

The example below runs the daemon listening on 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 honors 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
$ export DOCKER_TLS_VERIFY=1
$ docker ps

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

The Docker client supports connecting to a remote daemon via SSH:

$ docker -H ssh://[email protected]:22/var/run/docker.sock ps
$ docker -H ssh://[email protected]:22 ps
$ docker -H ssh://[email protected] ps
$ docker -H ssh://example.com ps

To use SSH connection, you need to set up ssh so that it can reach the remote host with public key authentication. Password authentication is not supported. If your key is protected with passphrase, you need to set up ssh-agent .

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 introduces security risks, as it may allow 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's not advisable on an open network.

With -H it's possible to make the Docker daemon to listen on a specific IP and port. By default, it listens 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 isn't recommended because someone could 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 defaults 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, defaults 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

$ sudo 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

On Linux, the Docker daemon has support for several different image layer storage drivers: overlay2 , fuse-overlayfs , btrfs , and zfs .

overlay2 is the preferred storage driver for all currently supported Linux distributions, and is selected by default. Unless users have a strong reason to prefer another storage driver, overlay2 should be used.

You can find out more about storage drivers and how to select one in Select a storage driver .

On Windows, the Docker daemon only supports the windowsfilter storage driver.

Options per storage driver

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

ZFS options

`zfs.fsname`

Specifies the ZFS filesystem that the daemon should use to create its datasets. By default, the ZFS filesystem in /var/lib/docker is used.

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 can't be smaller than btrfs.min_space .

Example

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

Overlay2 options

`overlay2.size`

Sets the default max size of the container. It is supported only when the backing filesystem is xfs and mounted with pquota mount option. Under these conditions the user can pass any size less than the backing filesystem size.

Example

$ sudo dockerd -s overlay2 --storage-opt overlay2.size=1G

Windowsfilter options

`size`

Specifies the size to use when creating the sandbox which is used for containers. Defaults to 20G.

Example

C:\> dockerd --storage-opt size=40G

Runtime 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 .

Configure container runtimes

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

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

$ docker run --runtime io.containerd.kata.v2

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

For examples on how to use other container runtimes, see Alternative container runtimes

Configure runtimes using `daemon.json`

To register and configure container runtimes using the daemon's configuration file, add the runtimes as entries under runtimes :

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

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

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

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

See Configure runc drop-in replacements .

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

$ sudo systemctl reload dockerd

Configure containerd shims

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

runtimeType refers to either:

A fully qualified name of a containerd shim.

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

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

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

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

The options field takes a special set of configuration parameters when used with "runtimeType": "io.containerd.runc.v2" . For more information about runc parameters, refer to the runc configuration section in CRI Plugin Config Guide .

Configure runc drop-in replacements

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

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

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

And runtimeArgs lets you optionally pass additional arguments to the runtime. Entries with this format use the containerd runc shim to invoke a custom runtime binary.

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

$ sudo dockerd --add-runtime <runtime>=<path>

Defining runtime arguments via the command line is not supported.

For an example configuration for a runc drop-in replacment, see Alternative container runtimes > youki

Configure the default container runtime

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

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

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

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

Run containerd standalone

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

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

Configure cgroup driver

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

You can only specify cgroupfs or systemd . If you specify systemd and it is not available, the system errors out. If you omit the native.cgroupdriver option, cgroupfs is used on cgroup v1 hosts, systemd is used on cgroup v2 hosts with systemd available.

This example sets the cgroupdriver to systemd :

$ sudo dockerd --exec-opt native.cgroupdriver=systemd

Setting this option applies to all containers the daemon launches.

Configure container isolation technology (Windows)

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

The following example makes hyperv the default isolation technology:

> dockerd --exec-opt isolation=hyperv

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 non-distributable 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 non-distributable artifacts to myregistry:5000.
--allow-nondistributable-artifacts 10.1.0.0/16 tells the Docker daemon to push non-distributable 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 non-distributable artifacts to a registry on an air-gapped network so hosts on that network can pull the images without connecting to another server.

Warning

Non-distributable 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 non-distributable artifacts.

Insecure registries

In this section, "registry" refers to a private registry, and myregistry:5000 is a placeholder example of a private registry.

Docker considers a private registry either secure or insecure. 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 wasn't found under /etc/docker/certs.d/myregistry:5000/ , or if the certificate verification failed (i.e., wrong CA).

By default, Docker assumes all registries to be secure, except for local registries. Communicating with an insecure registry isn't 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 isn't marked as insecure, docker pull , docker push , and docker search result in error messages, 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 isn't 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 no longer supported. Specifically, the daemon doesn't attempt to push, pull or sign in to v1 registries. The exception to this is search which can still be performed on v1 registries.

Running a Docker daemon behind an HTTPS_PROXY

When running inside a LAN that uses an HTTPS proxy, the proxy's certificates replace Docker Hub's certificates. These certificates must be added to your Docker host's configuration:

Install the ca-certificates package for your distribution
Ask your network admin for the proxy's CA certificate and append them to /etc/pki/tls/certs/ca-bundle.crt
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 only adds the proxy and authentication to the Docker daemon's requests. To use the proxy when building images and running containers, see Configure Docker to use a proxy server

Default `ulimit` settings

The --default-ulimit flag lets you set the default ulimit options to use for all containers. It takes the same options as --ulimit for docker run . If these defaults aren't set, ulimit settings are inherited from the Docker daemon. Any --ulimit options passed to docker run override the daemon 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, see docker run reference .

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, refer to the authorization plugin section.

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. One of the most important security improvements is that, by default, container processes running as the root user have expected administrative privileges it expects (with some restrictions) inside the container, but are effectively 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 .

Configure host gateway IP

The Docker daemon supports a special host-gateway value for the --add-host flag for the docker run and docker build commands. This value resolves to the host's gateway IP and lets containers connect to services running on the host.

By default, host-gateway resolves to the IP address of the default bridge. You can configure this to resolve to a different IP using the --host-gateway-ip flag for the dockerd command line interface, or the host-gateway-ip key in the daemon configuration file.

$ cat > /etc/docker/daemon.json
{ "host-gateway-ip": "192.0.2.0" }
$ sudo systemctl restart docker
$ docker run -it --add-host host.docker.internal:host-gateway \
  busybox ping host.docker.internal 
PING host.docker.internal (192.0.2.0): 56 data bytes

Enable CDI devices

Note

This is experimental feature and as such doesn't represent a stable API.

This feature isn't enabled by default. To this feature, set features.cdi to true in the daemon.json configuration file.

Container Device Interface (CDI) is a standardized mechanism for container runtimes to create containers which are able to interact with third party devices.

The Docker daemon supports running containers with CDI devices if the requested device specifications are available on the filesystem of the daemon.

The default specification directors are:

/etc/cdi/ for static CDI Specs
/var/run/cdi for generated CDI Specs

Alternatively, you can set custom locations for CDI specifications using the cdi-spec-dirs option in the daemon.json configuration file, or the --cdi-spec-dir flag for the dockerd CLI.

When CDI is enabled for a daemon, you can view the configured CDI specification directories using the docker info command.

Daemon logging format

The --log-format option or "log-format" option in the daemon configuration file lets you set the format for logs produced by the daemon. The logging format should only be configured either through the --log-format command line option or through the "log-format" field in the configuration file; using both the command-line option and the "log-format" field in the configuration file produces an error. If this option is not set, the default is "text".

The following example configures the daemon through the --log-format command line option to use json formatted logs;

$ dockerd --log-format=json
# ...
{"level":"info","msg":"API listen on /var/run/docker.sock","time":"2024-09-16T11:06:08.558145428Z"}

The following example shows a daemon.json configuration file with the "log-format" set;

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 soft links 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:

$ export DOCKER_TMPDIR=/mnt/disk2/tmp
$ sudo -E dockerd --data-root /var/lib/docker -H unix://

Default cgroup parent

The --cgroup-parent option lets you set the default cgroup parent for containers. If this option isn't set, it defaults to /docker for the cgroupfs driver, and system.slice for the 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:9323 you would specify --metrics-addr 127.0.0.1:9323 , allowing you to make requests on the API at 127.0.0.1:9323/metrics to receive metrics in the prometheus format.

Port 9323 is the default port associated with Docker metrics to avoid collisions with other Prometheus exporters and services.

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, see Collect Docker metrics with Prometheus .

Node generic resources

The --node-generic-resources option takes a list of key-value pair ( key=value ) that allows you to advertise user defined resources in a Swarm cluster.

The current expected use case is to advertise NVIDIA GPUs so that services requesting NVIDIA-GPU=[0-16] can land on a node that has enough GPUs for the task to run.

Example of usage:

Enable feature in the daemon (--feature)

The --feature option lets you enable or disable a feature in the daemon. This option corresponds with the "features" field in the daemon.json configuration file . Features should only be configured either through the --feature command line option or through the "features" field in the configuration file; using both the command-line option and the "features" field in the configuration file produces an error. The feature option can be specified multiple times to configure multiple features. The --feature option accepts a name and optional boolean value. When omitting the value, the default is true .

The following example runs the daemon with the cdi and containerd-snapshotter features enabled. The cdi option is provided with a value;

$ dockerd --feature cdi=true --feature containerd-snapshotter

The following example is the equivalent using the daemon.json configuration file;

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 using flags. The Docker daemon fails to start if an option is duplicated between the file and the flags, regardless of their value. This is intentional, and avoids 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.

The --validate option allows to validate a configuration file without starting the Docker daemon. A non-zero exit code is returned for invalid configuration files.

$ dockerd --validate --config-file=/tmp/valid-config.json
configuration OK
$ echo $?
$ dockerd --validate --config-file /tmp/invalid-config.json
unable to configure the Docker daemon with file /tmp/invalid-config.json: the following directives don't match any configuration option: unknown-option
$ echo $?

On Linux

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

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

The default-runtime option is by default unset, in which case dockerd automatically detects the runtime. This detection is based on if the containerd flag is set.

Accepted values:

com.docker.hcsshim.v1 - This is the built-in runtime that Docker has used since Windows supported was first added and uses the v1 HCS API's in Windows.
io.containerd.runhcs.v1 - This is uses the containerd runhcs shim to run the container and uses the v2 HCS API's in Windows.

Feature options

The optional field features in daemon.json lets you enable or disable specific daemon features.

The list of feature options include:

containerd-snapshotter : when set to true , the daemon uses containerd snapshotters instead of the classic storage drivers for storing image and container data. For more information, see containerd storage .
windows-dns-proxy : when set to true , the daemon's internal DNS resolver will forward requests to external servers. Without this, most applications running in the container will still be able to use secondary DNS servers configured in the container itself, but nslookup won't be able to resolve external names. The current default is false , it will change to true in a future release. This option is only allowed on Windows.

Warning

The windows-dns-proxy feature flag will be removed in a future release.

Configuration reload behavior

Some options can be reconfigured when the daemon is running without requiring to restart the process. The daemon uses the SIGHUP signal in Linux to reload, and a global event in Windows with the key Global\docker-daemon-config-$PID . You can modify the options in the configuration file, but the daemon still checks for conflicting settings with the specified CLI 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:

Option	Description
`debug`	Toggles debug mode of the daemon.
`labels`	Replaces the daemon labels with a new set of labels.
`live-restore`	Toggles live restore .
`max-concurrent-downloads`	Configures the max concurrent downloads for each pull.
`max-concurrent-uploads`	Configures the max concurrent uploads for each push.
`max-download-attempts`	Configures the max download attempts for each pull.
`default-runtime`	Configures the runtime to be used if not is specified at container creation.
`runtimes`	Configures the list of available OCI runtimes that can be used to run containers.
`authorization-plugin`	Specifies the authorization plugins to use.
`allow-nondistributable-artifacts`	Specifies a list of registries to which the daemon will push non-distributable artifacts.
`insecure-registries`	Specifies a list of registries that the daemon should consider insecure.
`registry-mirrors`	Specifies a list of registry mirrors.
`shutdown-timeout`	Configures the daemon's existing configuration timeout with a new timeout for shutting down all containers.
`features`	Enables or disables specific features.

Run multiple daemons

Note

Running multiple daemons on a single host is considered experimental. You may encounter unsolved problems, and things may not work as expected in some cases.

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 doesn't 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 important that you understand the meaning of these 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 aren't using the default, you must create and configure the bridge manually, or 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 listens 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 also doesn't add IP masquerading rules, even if you set --ip-masq to true . Without IP masquerading rules, Docker containers can't 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

Default network options

The default-network-opts key in the daemon.json configuration file, and the equivalent --default-network-opt CLI flag, let you specify default values for driver network driver options for new networks.

The following example shows how to configure options for the bridge driver using the daemon.json file.

This example uses the bridge network driver. Refer to the bridge network driver page for an overview of available driver options.

After changing the configuration and restarting the daemon, new networks that you create use these option configurations as defaults.

$ docker network create mynet
$ docker network inspect mynet --format "{{json .Options}}"
{"com.docker.network.bridge.host_binding_ipv4":"127.0.0.1","com.docker.network.driver.mtu":"1234"}

Note that changing this daemon configuration doesn't affect pre-existing networks.

Using the --default-network-opt CLI flag is useful for testing and debugging purposes, but you should prefer using the daemon.json file for persistent daemon configuration. The CLI flag expects a value with the following format: driver=opt=value , for example:

$ sudo dockerd \
  --default-network-opt bridge=com.docker.network.bridge.host_binding_ipv4=127.0.0.1 \
  --default-network-opt bridge=com.docker.network.driver.mtu=1234
Request changes
Table of contents
Description
Environment variables
Examples
Proxy configuration
Daemon socket option
Daemon storage-driver
Options per storage driver
Runtime options
Daemon DNS options
Allow push of non-distributable artifacts
Insecure registries
Running a Docker daemon behind an HTTPS_PROXY
Default ulimit settings
Access authorization
Daemon user namespace options
Configure host gateway IP
Enable CDI devices
Miscellaneous options
Enable feature in the daemon (--feature)
Daemon configuration file
Run multiple daemons
Default network options