Docker Engine managed plugin system

• Installing and using a plugin
• Developing a plugin
• Debugging plugins

Docker Engine's plugin system lets you install, start, stop, and remove plugins using Docker Engine.

For information about legacy (non-managed) plugins, refer to Understand legacy Docker Engine plugins.

Note

Docker Engine managed plugins are currently not supported on Windows daemons.

Installing and using a plugin

Plugins are distributed as Docker images and can be hosted on Docker Hub or on a private registry.

To install a plugin, use thedocker plugin installcommand, which pulls the plugin from Docker Hub or your private registry, prompts you to grant permissions or capabilities if necessary, and enables the plugin.

To check the status of installed plugins, use thedocker plugin lscommand. Plugins that start successfully are listed as enabled in the output.

After a plugin is installed, you can use it as an option for another Docker operation, such as creating a volume.

In the following example, you install thesshfsplugin, verify that it is enabled, and use it to create a volume.

Note

This example is intended for instructional purposes only. Once the volume is created, your SSH password to the remote host is exposed as plaintext when inspecting the volume. Delete the volume as soon as you are done with the example.

1. Install thesshfsplugin.

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker plugin install vieux/sshfs
   
   Plugin "vieux/sshfs" is requesting the following privileges:
   - network: [host]
   - capabilities: [CAP_SYS_ADMIN]
   Do you grant the above permissions? [y/N] y
   
   vieux/sshfs
   

   The plugin requests 2 privileges:

   • It needs access to thehostnetwork.
   • It needs theCAP_SYS_ADMINcapability, which allows the plugin to run themountcommand.

2. Check that the plugin is enabled in the output ofdocker plugin ls.

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker plugin ls
   
   ID NAME TAG DESCRIPTION ENABLED
   69553ca1d789 vieux/sshfs latest the `sshfs` plugin true
   

3. Create a volume using the plugin. This example mounts the/remotedirectory on host1.2.3.4into a volume namedsshvolume.

   This volume can now be mounted into containers.

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker volume create\
   -d vieux/sshfs \
   --name sshvolume \
   -o [email protected]:/remote \
   -o password=$(cat file_containing_password_for_remote_host)
   
   sshvolume
   

4. Verify that the volume was created successfully.

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker volume ls
   
   DRIVER NAME
   vieux/sshfs sshvolume
   

5. Start a container that uses the volumesshvolume.

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker run --rm -v sshvolume:/data busybox ls /data
   
   <content of /remote on machine 1.2.3.4>
   

6. Remove the volumesshvolume

   ]\s+/gm, '')); copying = true; setTimeout(() => copying = false, 2000); ">

   $docker volume rm sshvolume
   
   sshvolume
   

To disable a plugin, use thedocker plugin disablecommand. To completely remove it, use thedocker plugin removecommand. For other available commands and options, see the command line reference.

Developing a plugin

The rootfs directory

Therootfsdirectory represents the root filesystem of the plugin. In this example, it was created from a Dockerfile:

Note

The/run/docker/pluginsdirectory is mandatory inside of the plugin's filesystem for Docker to communicate with the plugin.

$git clone https://github /vieux/docker-volume-sshfs
$cddocker-volume-sshfs
$docker build -t rootfsimage.
$id=$(docker create rootfsimagetrue)# id was cd851ce43a403 when the image was created
$sudo mkdir -p myplugin/rootfs
$sudo dockerexport"$id"|sudo tar -x -C myplugin/rootfs
$docker rm -vf"$id"
$docker rmi rootfsimage

The config.json file

Theconfig.jsonfile describes the plugin. See the plugins config reference.

Consider the followingconfig.jsonfile.

{
"description":"sshFS plugin for Docker",
"documentation":"https://docs.docker /engine/extend/plugins/",
"entrypoint":["/docker-volume-sshfs"],
"network":{
"type":"host"
},
"interface":{
"types":["docker.volumedriver/1.0"],
"socket":"sshfs.sock"
},
"linux":{
"capabilities":["CAP_SYS_ADMIN"]
}
}

This plugin is a volume driver. It requires ahostnetwork and the CAP_SYS_ADMINcapability. It depends upon the/docker-volume-sshfs entrypoint and uses the/run/docker/plugins/sshfs.socksocket to communicate with Docker Engine. This plugin has no runtime parameters.

Creating the plugin

A new plugin can be created by running docker plugin create <plugin-name>./path/to/plugin/datawhere the plugin data contains a plugin configuration fileconfig.jsonand a root filesystem in subdirectoryrootfs.

After that the plugin<plugin-name>will show up indocker plugin ls. Plugins can be pushed to remote registries with docker plugin push <plugin-name>.

Debugging plugins

Stdout of a plugin is redirected to dockerd logs. Such entries have a plugin=<ID>suffix. Here are a few examples of commands for pluginID f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62and their corresponding log entries in the docker daemon logs.

$docker plugin install tiborvass/sample-volume-plugin

INFO[0036] Starting... Found 0 volumes on startup plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62

$docker volume create -d tiborvass/sample-volume-plugin samplevol

INFO[0193] Create Called... Ensuring directory /data/samplevol exists on host... plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0193] open /var/lib/docker/plugin-data/local-persist.json: no such file or directory plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0193] Created volume samplevol with mountpoint /data/samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0193] Path Called... Returned path /data/samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62

$docker run -v samplevol:/tmp busybox sh

INFO[0421] Get Called... Found samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0421] Mount Called... Mounted samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0421] Path Called... Returned path /data/samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62
INFO[0421] Unmount Called... Unmounted samplevol plugin=f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62

Using runc to obtain logfiles and shell into the plugin.

Userunc,the default docker container runtime, for debugging plugins by collecting plugin logs redirected to a file.

$sudo runc --root /run/docker/runtime-runc/plugins.moby list

ID PID STATUS BUNDLE CREATED OWNER
93f1e7dbfe11c938782c2993628c895cf28e2274072c4a346a6002446c949b25 15806 running /run/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby-plugins/93f1e7dbfe11c938782c2993628c895cf28e2274072c4a346a6002446c949b25 2018-02-08T21:40:08.621358213Z root
9b4606d84e06b56df84fadf054a21374b247941c94ce405b0a261499d689d9c9 14992 running /run/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby-plugins/9b4606d84e06b56df84fadf054a21374b247941c94ce405b0a261499d689d9c9 2018-02-08T21:35:12.321325872Z root
c5bb4b90941efcaccca999439ed06d6a6affdde7081bb34dc84126b57b3e793d 14984 running /run/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby-plugins/c5bb4b90941efcaccca999439ed06d6a6affdde7081bb34dc84126b57b3e793d 2018-02-08T21:35:12.321288966Z root

$sudo runc --root /run/docker/runtime-runc/plugins.mobyexec93f1e7dbfe11c938782c2993628c895cf28e2274072c4a346a6002446c949b25 cat /var/log/plugin.log

If the plugin has a built-in shell, then exec into the plugin can be done as follows:

$sudo runc --root /run/docker/runtime-runc/plugins.mobyexec-t 93f1e7dbfe11c938782c2993628c895cf28e2274072c4a346a6002446c949b25 sh

Using curl to debug plugin socket issues.

To verify if the plugin API socket that the docker daemon communicates with is responsive, use curl. In this example, we will make API calls from the docker host to volume and network plugins using curl 7.47.0 to ensure that the plugin is listening on the said socket. For a well functioning plugin, these basic requests should work. Note that plugin sockets are available on the host under/var/run/docker/plugins/<pluginID>

$curl -H"Content-Type: application/json"-XPOST -d'{}'--unix-socket /var/run/docker/plugins/e8a37ba56fc879c991f7d7921901723c64df6b42b87e6a0b055771ecf8477a6d/plugin.sock http:/VolumeDriver.List

{ "Mountpoint": "", "Err": "", "Volumes":[{ "Name": "myvol1", "Mountpoint": "/data/myvol1" },{ "Name": "myvol2", "Mountpoint": "/data/myvol2" }], "Volume":null}

$curl -H"Content-Type: application/json"-XPOST -d'{}'--unix-socket /var/run/docker/plugins/45e00a7ce6185d6e365904c8bcf62eb724b1fe307e0d4e7ecc9f6c1eb7bcdb70/plugin.sock http:/NetworkDriver.GetCapabilities

{ "Scope": "local" }

When using curl 7.5 and above, the URL should be of the form http://hostname/APICall,wherehostnameis the valid hostname where the plugin is installed andAPICallis the call to the plugin API.

For example,http://localhost/VolumeDriver.List