Expose Docker services with port binding

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Let’s look at a simple Node.js application that responds with Hello World to every request and starts a web server on port 8000:

// Load the http module to create an http server.
var http = require('http');

// Configure our HTTP server to respond with Hello World to all requests.
var server = http.createServer(function (request, response) {
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.end("Hello World\n");
});

// Listen on port 8000, IP defaults to 127.0.0.1
server.listen(8000);

// Put a friendly message on the terminal
console.log("Server running at http://127.0.0.1:8000/");

The Dockerfile copies the application into the image and starts Node.js:

FROM mhart/alpine-node
COPY index.js .
EXPOSE 8000
CMD node index.js

Build the image and name it foo:

docker build -t foo .

Start a container from the image:

docker run -d foo

The application is running inside the container, but Docker locks down container ports by default. If you try to access port 8000 from the host, the connection is refused:

curl localhost:8000

Use the -p option with docker run to publish a container port on the host. The host port comes first, followed by the container port:

docker run -d -p 8000:8000 foo

Now the host exposes port 8000 and forwards requests to port 8000 inside the container:

curl localhost:8000

The application responds with:

Hello World

You can map the container port to a different host port. This is useful when multiple containers listen on the same port internally:

docker run -d -p 8001:8000 foo

The application still listens on port 8000 inside the container, but the host exposes it on port 8001:

curl localhost:8001

When you use Docker Compose, define the same mapping with the ports option. The host port still comes first, followed by the container port:

version: '3'
services:
  app:
    build: .
    ports:
      - 8010:8000

Start the application with Compose:

docker-compose up -d

Then test the published host port:

curl localhost:8010

Docker forwards the request from port 8010 on the host to port 8000 in the container, so the response is:

Hello World