Underneath the powerful CLI familiar to any Docker user lies the mysterious Docker daemon. If you’ve used Docker on a Linux machine you may be familiar with the dockerd command which starts the Docker daemon. It turns out this daemon is really just a server that takes HTTP requests.

read more here about the daemon

This docker “server” process offers a RESTful interface, which is the interaction point between the docker software and its users. The familiar Docker CLI sits on top of this REST interface, so it is usually invisible. but it is also possible to communicate directly with the daemon thru standard HTTP calls.

Running docker version will tell you the version of the Server you are running. You can use this version number to communicate with the daemon directly. (just look for the version number under Server when you run docker version)

Let’s see a few examples. (I’m piping them into jq for readability)

–

List all images

Docker CLI command:

docker images

Docker REST API request:

curl --unix-socket /var/run/docker.sock http:/v1.27/images/json | jq

–

List all running containers

Docker CLI command:

docker ps

Docker REST API request:

curl --unix-socket /var/run/docker.sock http:/v1.27/containers/json | jq

–

Create and Run a container

docker run does this from the Docker CLI in a single shot. In HTTP/REST we can create and then start it using 2 different requests

Docker CLI command:

docker run --name my_nginx -p "80:80" nginx

This will start an Nginx server and expose port 80 on the host, mapping it to port 80 on the container.

Docker REST API:

(we pass Image and PortBindings as JSON, and the name of the container as a query param) Check out the full API here

curl --unix-socket /var/run/docker.sock \
    -H "Content-Type: application/json" \
    -d '{"Image": "nginx", "PortBindings": { "80/tcp": [{ "HostPort": "80" }] }}' \
    -X POST http:/v1.27/containers/create?name=my_nginx

Start the container:

curl --unix-socket /var/run/docker.sock -X POST http:/v1.27/containers/my_nginx/start

Now if you visit localhost:80 (or wherever your dockerhost lives) in a browser you will see the standard Nginx welcome message.

Amazing in its simplicity and elegance. I am consistently blown away by the power of Docker, but this time I am impressed by the elegance, simplicity and power of its HTTP interface.

In a future post I will explore the unix-socket / docker.sock part of the commands and see what that business is all about.