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A Single Docker CLI is powerful enough to manage multiple Docker nodes. Before Docker v19.03 release, managing the multiple Docker hosts from a single node was a cumbersome task. The only option available was to use DOCKER_HOST variable to identify a remote host to connect to.

With the release of Docker v19.03, a new docker context CLI was introduced for the first time. The docker context command makes it easy to export and import contexts on different machines with the Docker client installed. You can use the docker context export command to export an existing context to a file. This file can later be imported on another machine that has the docker client installed.

$ docker context --help

Usage:  docker context COMMAND

Manage contexts

Commands:
  create      Create a context
  export      Export a context to a tar or kubeconfig file
  import      Import a context from a tar or zip file
  inspect     Display detailed information on one or more contexts
  ls          List contexts
  rm          Remove one or more contexts
  update      Update a context
  use         Set the current docker context

Run 'docker context COMMAND --help' for more information on a command.

In this blog tutorial, you will see how one can use docker context command to access the remote Docker hosts seamlessly. Before that, let us look how can one achieve this without even using docker context command

Pre-requisite:

Install Docker 19.03+ on Node #1 ( 192.168.0.7)
Install Docker 19.03+ on Node #2 (192.168.0.8)

Listing the current context values

$ docker context ls
NAME        DESCRIPTION                               DOCKER ENDPOINT               KUBERNETES ENDPOINT   ORCHESTRATOR
default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                         swarm

We should see the default context, pointing to the socket file /var/run/docker.sock.

Run a new Docker container on Node 2

Open Node 2 and run the Docker container

$ docker container run -d --name node2 ajeetraina/hellowhale tail -f /dev/null

Listing the container

$ docker container ls
CONTAINER ID   IMAGE                   COMMAND               CREATED              STATUS              PORTS     NAMES
1311ecf00b84   ajeetraina/hellowhale   "tail -f /dev/null"   About a minute ago   Up About a minute   80/tcp    node2
[node2] (local) root@192.168.0.7 ~
$

Setting the Environment Variable

You’ll need to copy down the IP address of the target node(i.e. Node 2) and set it on the first terminal.

export TARGET_HOST=192.168.0.8

Verify you configured variable

$ printenv TARGET_HOST
192.168.0.8

Configure SSH to trust the host

ssh-keyscan -H $TARGET_HOST >~/.ssh/known_hosts

Connecting to Nodes with DOCKER_HOST

Docker will use the DOCKER_HOST variable to identify a remote host to connect to. Let’s compare what happens when listing containers locally and on a remote host.

DOCKER_HOST=ssh://$TGT_HOST docker container ls

This will list the containers running on the target node.

Using Docker Context

Unset the environment variable

unset DOCKER_HOST

With the release of 19.03, docker now supports managing the context within the CLI. Note that this context is overridden by the DOCKER_HOST variable, so to use it, you’ll need to be sure that isn’t set(as shown above).

Create a new context

$ docker context create node2 --description "Node 2" --docker "host=ssh://$TARGET_HOST"
node2
Successfully created context "node2"

Listing the Context

$ docker context ls
NAME        DESCRIPTION                               DOCKER ENDPOINT               KUBERNETES ENDPOINT   ORCHESTRATOR
default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                         swarm
node2       Node 2                                    ssh://192.168.0.8

Switching to the new context

docker context use node2

Listing the overall context

$ docker context ls
NAME      DESCRIPTION                               DOCKER ENDPOINT               KUBERNETES ENDPOINT   ORCHESTRATOR
default   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                         swarm
node2 *   Node-2                                    ssh://192.168.0.7

Listing the remote containers

$ docker container ls
CONTAINER ID   IMAGE                   COMMAND               CREATED          STATUS          PORTS     NAMES
d9ee527be530   ajeetraina/hellowhale   "tail -f /dev/null"   27 seconds ago   Up 27 seconds   80/tcp    node2

How to Connect to Remote Docker using docker context CLI