Nginx can't find upstream node app when running via Docker-compose

Question

I have a super simple Node app and an Nginx config which acts as a reverse proxy to the Node app. Everything works fine if I run Nginx (via homebrew) and the Node app locally. If I visit the server defined by the Nginx config at port 8080 I get the output from the node app, which is running on port 3000.

I've been trying to convert this simple setup to use Docker and have written the following Docker-compose file:

version: '3.0'
services:
  web:
    build: .
    ports:
      - 3000:3000
  nginx:
    build:
      context: .
      dockerfile: Dockerfile.nginx
    ports:
      - 8080:8080

On running docker-compose up the images are built and there are no error messages in the console. On visiting localhost:3000 I get the response from the Node app but on visiting localhost:8080 I get an an Nginx 502 error page and the following error in the terminal:

connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:3000/", host: "localhost:8080"

My Dockerfile for the node app looks like so:

FROM node:carbon

WORKDIR /app

ADD . /app

RUN npm install

CMD ["node", "."]

EXPOSE 3000

and the Dockerfile.nginx looks like so:

FROM nginx
COPY nginx.conf /etc/nginx/nginx.conf

and the nginx.conf looks like so:

events {
  worker_connections  1024;
}

http {

  upstream node_app {
    server 127.0.0.1:3000;
  }

  server_tokens off;

  # Define the MIME types for files.
  include       mime.types;
  default_type  application/octet-stream;

  # Speed up file transfers by using sendfile()
  # TODO: Read up on this
  sendfile on;

  server {
    listen 8080;
    server_name localhost;

    location / {
      proxy_pass http://node_app;
      proxy_http_version 1.1;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
    }
  }
}

On starting Docker up I can see that Nginx is running on port 8080 (because I see the 502 Nginx page) and I can see that the node app is running (because I can visit it at localhost:3000). I can't work out why I get the 502 from nginx.

I've tried using various different things like using links to link the containers and depends_on but nothing seems to make any difference. I'm also using docker-compose up --build to make sure I'm not caching previous builds each time I make a change.

EDIT: Something that seems to make it work is to add a container_name property in to the docker-compose:

  web:
    container_name: nodeapp
    build:
      context: .
      dockerfile: Dockerfile.node
    ports:
      - 3000:3000

and then using that container name in the upstream node_app config in nginx.conf:

  upstream node_app {
    server nodeapp:3000;
  }

which makes no sense to me?!

Check out the networking docs for docker compose docs.docker.com/compose/networking, specifically the first paragraph that talks about containers being discoverable by their name, which is why it works with the container name in the nginx conf file. Docker has several networking modes. You can read up on those here docs.docker.com/engine/userguide/networking/#default-networks to better understand how docker networking works. — Jeff Storey
– Jeff Storey, Commented Dec 26, 2017 at 17:34
Thanks for the links and explaining that containers are discoverable by name. Having re-read those docs makes a lot more sense now — Mike Rifgin
– Mike Rifgin, Commented Dec 27, 2017 at 9:39

yeiniel · Accepted Answer · 2017-12-26 17:45:43Z

The problem is that in your Nginx configuration you are referencing the IP of the Web service as 127.0.0.1 which is the loopback address of the Host machine running the docker container. This may work depending on your setup (OS, firewall) or may not.

The correct way would be to make the nginx service depends on the web service in your docker-compose.yml file and update the Nginx config to reference the Web service by name (web) instead of by IP address. Here you can find more info related to docker compose depends on capability.

The updated docker-compose.yml file would be:

version: '3.0'
services:
  web:
    build: .
  nginx:
    build:
      context: .
      dockerfile: Dockerfile.nginx
    ports:
      - 8080:8080
    depends_on:
      - web

Notice that I have stop exposing the port of the web service. May be you need to keep it to monitor the Web service but is not required for the nginx service.

With this update to the docker-compose.yml file the Nginx config will be as follows:

events {
  worker_connections  1024;
}

http {

  upstream node_app {
    server web:3000;
  }

  server_tokens off;

  # Define the MIME types for files.
  include       mime.types;
  default_type  application/octet-stream;

  # Speed up file transfers by using sendfile()
  # TODO: Read up on this
  sendfile on;

  server {
    listen 8080;
    server_name localhost;

    location / {
      proxy_pass http://node_app;
      proxy_http_version 1.1;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
    }
  }
}

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