dockerd (original) (raw)

Options with [] may be specified multiple times.

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": truetothe daemon.json file.

Note

Enabling experimental features

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

Environment variables

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 toEnvironment variablesto learn about environment variables supported by the docker CLI.

Proxy configuration

Note

Refer to theDocker Desktop manualif you are runningDocker 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:

1. Using environment variables (HTTP_PROXY, HTTPS_PROXY, and NO_PROXY).
2. Using the http-proxy, https-proxy, and no-proxy fields in thedaemon configuration file (Docker Engine version 23.0 or later).
3. 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 tocontrol and configure Docker with systemdto set these environment variables on a host using systemd.

Daemon socket option

The Docker daemon can listen forDocker Engine APIrequests 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 thebuilt 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 viasystemd 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 theDocker 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:

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

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:

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

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

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 upssh-agent.

Bind Docker to another host/port or a Unix socket

Warning

Changing the default docker daemon binding to a TCP port or Unix dockeruser 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.sockon Linux, and tcp://127.0.0.1:2376 on Windows.

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

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:

Download an ubuntu image:

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

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 inSelect 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 forbtrfs 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

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 thesize can't be smaller than btrfs.min_space.

Example

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

Windowsfilter options

size

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

Example

Runtime options

The Docker daemon relies on aOCI 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:

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, seeAlternative 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.
  SeeConfigure 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.
  SeeConfigure 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:

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 ofPATH.

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 inoptions 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 inCRI 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:

Defining runtime arguments via the command line is not supported.

For an example configuration for a runc drop-in replacment, seeAlternative 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:

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:

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 specifysystemd and it is not available, the system errors out. If you omit thenative.cgroupdriver option, cgroupfs is used on cgroup v1 hosts, systemdis used on cgroup v2 hosts with systemd available.

This example sets the cgroupdriver to 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:

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:

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

Insecure registries

In this section, "registry" refers to a private registry, and myregistry:5000is 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:

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 only adds the proxy and authentication to the Docker daemon's requests. To use the proxy when building images and running containers, seeConfigure 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, seedocker run reference.

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.

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 theauthorization plugin section.

Daemon user namespace options

The Linux kerneluser namespace supportprovides 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 theroot 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, seeIsolate containers with a user namespace.

Configure host gateway IP

The Docker daemon supports a special host-gateway value for the --add-hostflag for the docker run and docker build commands. This value resolves to addresses on the host, so that containers can connect to services running on the host.

By default, host-gateway resolves to the IPv4 address of the default bridge, and its IPv6 address if it has one.

You can configure this to resolve to a different IP using the --host-gateway-ipflag for the dockerd command line interface, or the host-gateway-ip key in the daemon configuration file.

To supply both IPv4 and IPv6 addresses on the command line, use two--host-gateway-ip options.

To supply addresses in the daemon configuration file, use "host-gateway-ips"with a JSON array, as shown below. For compatibility with older versions of the daemon, a single IP address can also be specified as a JSON string in option"host-gateway-ip".

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 totrue in the daemon.json configuration file.

Container Device Interface (CDI) is astandardizedmechanism 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 thecdi-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 thedaemon configuration filelets 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;

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:

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=foobarcreates 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.slicemeans 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-parentoption 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 theprometheus format.

Port 9323 is thedefault port associated with Docker metricsto 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, seeCollect 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 thedaemon.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-snapshotterfeatures enabled. The cdi option is provided with a value;

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.

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:

Note

You can't 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 can't use the hosts key in daemon.json to add listening addresses. Seecustom Docker daemon optionsfor an example on how to configure the daemon using systemd drop-in files.

On Windows

The default location of the configuration file on Windows is%programdata%\docker\config\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 Windows:

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, seecontainerd 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:

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 adaemon configuration file.

The following daemon options must be configured for each daemon:

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:

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 thebridge network driver pagefor 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.

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: