Docker EXPOSE. Can't get it

Question

this past two day I'm having trouble with docker and i can get it. Following to the docker doc you can expose the ports on which a container will listen for connections with EXPOSE. So far, so good!

If my app listen on port 8080 I should expose my docker container with EXPOSE 8080 and bind it to port 80 of the main host with docker run -p 80:8080.

Here is my Dockerfile:

# DOCKER-VERSION 0.0.1

FROM ubuntu:14.10

# make sure apt is up to date
RUN apt-get update

# install nodejs and npm
RUN apt-get install -y nodejs-legacy npm git git-core

ADD package.json /root/
ADD server.js /root/

# start script
ADD start.sh /root/
RUN chmod +x /root/start.sh

EXPOSE 8080

CMD ./root/start.sh

And my start.sh just runan cd /root/ & npm install & node server.js.

I got a simple express nodejs app:

var express = require('express');

// Constants
var PORT = 8080;

// App
var app = express();
app.get('/', function (req, res) {
  res.send('Hello world\n');
});

app.listen(PORT);
console.log('Running on http://localhost:' + PORT);

Here is how i build my docker image: docker build -t app1 . And how i launch my docker: docker run -it -p 80:8080 --name app1 app1

What is really wired, this is not working. To make it work i have to change EXPOSE 8080 to EXPOSE 80. I don't get it.

Any explanation?

Thanks for reading, Tom

Answer 1

In your nodejs app, you have the instruction app.listen(PORT); which tells nodejs to start a server listening for connections on the loopback interface on port PORT. As a result your app will only by able to see connections originating from localhost (the container itself).

You need to tell your app to listen on all interfaces on port PORT:

app.listen(PORT, "0.0.0.0");

This way it will see the connections originating from outside your Docker container.