Docker Image: why page not accessible?

Question

I am trying to create a Docker image for a little API I built with Fastify and Node. I came up with the following Dockerfile:

FROM node:current-alpine3.19

WORKDIR /usr/src/app

RUN chown -R node:node /usr/src/app

COPY src/package.json ./
COPY src/package-lock.json ./

RUN npm install

USER node

COPY --chown=node:node ./src .

CMD [ "npm", "start" ]

The build worked and I started the container with "docker run -p 3000:3000 [my-project]".

The file structure seems to be correct and I get the expected response when trying "wget 127.0.0.1:3000" inside the container.

But I can't open the page from my PC with localhost:3000 or 127.0.0.1:3000. Firefox: "The connection to the server was reset while the page was loading." Curl: "curl: (56) Recv failure: Connection reset by peer"

Fastify runs under port 3000, port 3000 is also used in the "docker run" command and I open the page with port 3000.

What am I doing wrong?

In your actual code, how do you set up the network listener? If you only accept connections on 127.0.0.1, that address is private to the container and you'll see symptoms like what you describe. — David Maze
– David Maze, Commented Mar 9, 2024 at 23:45

datawookie · Accepted Answer · 2024-03-10 06:05:53Z

You need to accept connections from 0.0.0.0 because 127.0.0.1 references the container and not the host.

So, for example, supposing that your project looks like this (very simplified!):

├── Dockerfile
└── src
    ├── index.js
    ├── package.json
    └── package-lock.json

then your index.js might be:

const fastify = require('fastify')({
    logger: true
  });
  
  fastify.get('/', async (request, reply) => {
    return { hello: 'world' }
  });
  
  const start = async () => {
    try {
      await fastify.listen({ port: 3000, host: '0.0.0.0' });
      console.log(`Server is running at ${fastify.server.address().port}`);
    } catch (err) {
      fastify.log.error(err);
      process.exit(1);
    }
  }
  start();

The important bit is specifying host: '0.0.0.0' to fastify.listen(). You also need to share this port with the host when you do docker run by giving -p 3000:3000.

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