How to avoid conflicts between the Docker Compose virtual network and the host network?

Networking in Compose

By default Docker Compose sets up a single network for your app. Each container for a service joins the default network and is both reachable by other containers on that network, and discoverable by them at a hostname identical to the container name.

For example, suppose your app is in a directory called myapp, and your docker-compose.yml looks like this:

services:
  web:
    build: .
    ports:
      - "8000:8000"
  db:
    image: postgres
    ports:
      - "8001:5432"

When you run docker compose up, the following happens:

1. A network called myapp_default is created.
2. A container is created using web’s configuration. It joins the network myapp_default under the name web.
3. A container is created using db’s configuration. It joins the network myapp_default under the name db.

The network uses the subnet 198.18.0.0/16, which is reserved for performance testing according to RFC 3330 (fully documented in RFC 2544) and is considered safe for use in a virtual network. It is quite possible that the automatically created network is conflicing company or home’s private network.

To avoid conflicts between the Docker virtual network and the host network, we could specify the subnet IP range through:

1. Use pre-exisitng external Docker netowrk.
2. Configure the default network.
3. Change Docker daemon configuration.

Use Pre-exisitng External Docker Network

The default bridge network created upon Docker installation is docker0. However, this network doesn’t support name resolution, so we need to create a new bridge network. Here’s the command to create a bridge network with the name docker1 and the specified subnet and gateway:

$ docker network create \
    --driver=bridge \
    --subnet=198.18.0.0/16 \
    --gateway=198.18.0.1 \
    -o com.docker.network.bridge.name=docker1 \
    docker1

In docker-compose.yml, we can configure the service to use the docker1 network like this:

services:
  xxx:
    image: xxx/xxx:latest
    networks:
      - docker1

networks:
  docker1:
    external: true

Configure the default network/Per-File Docker Network

If you prefer to use default networks, you can specify the subnet like this:

networks:
  default:
    driver: bridge
    ipam:
      driver: default
      config:
        - subnet: 198.18.0.0/16

Changing Docker Daemon Setup

To change the Docker daemon setup, edit the file /etc/docker/daemon.json and add the following JSON configuration:

{
  "bip": "198.18.251.1/24",
  "default-address-pools": [
    {
      "base": "198.18.252.0/22",
      "size": 26
    }
  ]
}

This will set the Docker daemon to use the specified IP address pool for assigning IPs to containers on the Docker network.