Docker Engine managed plugin system

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

Docker Engine’s plugin system allows you to install, start, stop, and remove plugins using Docker Engine.

For information about the legacy plugin system available in Docker Engine 1.12 and earlier, see 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 the docker plugin install command, 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 the docker plugin ls command. 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 the sshfs plugin, 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 will be exposed as plaintext when inspecting the volume. You should delete the volume as soon as you are done with the example.

1. Install the sshfs plugin.

   $ 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 the host network.
   • It needs the CAP_SYS_ADMIN capability, which allows the plugin to run the mount command.

2. Check that the plugin is enabled in the output of docker plugin ls.

   $ 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 /remote directory on host 1.2.3.4 into a volume named sshvolume.

   This volume can now be mounted into containers.

   $ docker volume create \
     -d vieux/sshfs \
     --name sshvolume \
     -o sshcmd=user@1.2.3.4:/remote \
     -o password=$(cat file_containing_password_for_remote_host)
   
   sshvolume
   

4. Verify that the volume was created successfully.

   $ docker volume ls
   
   DRIVER              NAME
   vieux/sshfs         sshvolume
   

5. Start a container that uses the volume sshvolume.

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

6. Remove the volume sshvolume

   docker volume rm sshvolume
   
   sshvolume
   

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

Developing a plugin

The rootfs directory

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

Note: The /run/docker/plugins directory is mandatory inside of the plugin’s filesystem for docker to communicate with the plugin.

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

The config.json file

The config.json file describes the plugin. See the plugins config reference.

Consider the following config.json file.

{
	"description": "sshFS plugin for Docker",
	"documentation": "https://docs.docker.com/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 a host network and the CAP_SYS_ADMIN capability. It depends upon the /docker-volume-sshfs entrypoint and uses the /run/docker/plugins/sshfs.sock socket 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/data where the plugin data contains a plugin configuration file config.json and a root filesystem in subdirectory rootfs.

After that the plugin <plugin-name> will show up in docker 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 f52a3df433b9aceee436eaada0752f5797aab1de47e5485f1690a073b860ff62 and 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 docker-runc to obtain logfiles and shell into the plugin.

docker-runc, the default docker container runtime can be used for debugging plugins. This is specifically useful to collect plugin logs if they are redirected to a file.

$ sudo docker-runc --root /var/run/docker/plugins/runtime-root/moby-plugins 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 docker-runc --root /var/run/docker/plugins/runtime-root/moby-plugins exec 93f1e7dbfe11c938782c2993628c895cf28e2274072c4a346a6002446c949b25 cat /var/log/plugin.log

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

$ sudo docker-runc --root /var/run/docker/plugins/runtime-root/moby-plugins exec -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, where hostname is the valid hostname where the plugin is installed and APICall is the call to the plugin API.

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