Sharing networks between multiple docker compose projects

I run a number of docker containers on my Synology NAS, and I want to is put a reverse proxy in front of them. Since they’re defined in separate docker compose projects, I need to attach the reverse proxy to the network defined by each project.

Here’s a simple example. I’ll put a reverse proxy in one project and nginx in another project, and connect the two.

Reverse proxy

For the reverse proxy, I’ll use HAProxy. The first part of the compose file is this:

# proxy.compose.yaml
services:
  proxy:
    image: haproxy:3.1.5-alpine
    user: root
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg

We use the alpine image, because it’s smaller; we run as root (for port 80); the haproxy.cfg (see below) file is mounted as a volume.

The proxy service is then attached to two separate networks:

    # ...continued from above.
    networks:
      macvlan0:
        ipv4_address: 192.168.28.64
      nginx:

The first network, macvlan0 is the one I defined in this post. The second is the network where the nginx container will be running.

The two networks are defined at the top level of the compose file, as follows:

networks:
  macvlan0:
    name: macvlan0
    external: true
  nginx:
    name: nginx
    external: true

Both networks are defined as external: true, because they’re not defined in this compose file.

The haproxy.cfg file looks like this:

global
    log stdout format raw local0 info

defaults
    log global
    maxconn 20000
    timeout connect 5000ms
    timeout client  50000ms
    timeout server  50000ms

frontend http
    bind *:80
    mode http
    default_backend nginx

backend nginx
   mode http
   server default nginx.nginx:80

The interesting bit in here is the server default nginx.nginx:80, which uses the backend server nginx.nginx. This is the service name (nginx) and the network name (nginx) defined previously.

Web server

For the web server, I’ll use nginx. The compose file for that looks like this:

# nginx.compose.yaml
services:
  nginx:
    image: nginx:alpine
    networks:
      - nginx

networks:
  nginx:
    name: nginx
    external: false

It defines the nginx service and attaches it to the nginx network. The network is defined at the top level of the compose file. This time we use external: false, because the network is defined in this compose file. The name used in the other compose file matches the name defined here.