Networking in Compose

Note: This document only applies if you’re using version 2 or higher of the Compose file format. Networking features are not supported for version 1 (legacy) Compose files.

By default 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.

Note: Your app’s network is given a name based on the “project name”, which is based on the name of the directory it lives in. You can override the project name with either the --project-name flag or the COMPOSE_PROJECT_NAME environment variable.

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

version: "3"
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.

In v2.1+, overlay networks are always attachable

Starting in Compose file format 2.1, overlay networks are always created as attachable, and this is not configurable. This means that standalone containers can connect to overlay networks.

In Compose file format 3.x, you can optionally set the attachable property to false.

Each container can now look up the hostname web or db and get back the appropriate container’s IP address. For example, web’s application code could connect to the URL postgres://db:5432 and start using the Postgres database.

It is important to note the distinction between HOST_PORT and CONTAINER_PORT. In the above example, for db, the HOST_PORT is 8001 and the container port is 5432 (postgres default). Networked service-to-service communication use the CONTAINER_PORT. When HOST_PORT is defined, the service is accessible outside the swarm as well.

Within the web container, your connection string to db would look like postgres://db:5432, and from the host machine, the connection string would look like postgres://{DOCKER_IP}:8001.

Update containers

If you make a configuration change to a service and run docker-compose up to update it, the old container is removed and the new one joins the network under a different IP address but the same name. Running containers can look up that name and connect to the new address, but the old address stops working.

If any containers have connections open to the old container, they are closed. It is a container’s responsibility to detect this condition, look up the name again and reconnect.

Links

Links allow you to define extra aliases by which a service is reachable from another service. They are not required to enable services to communicate - by default, any service can reach any other service at that service’s name. In the following example, db is reachable from web at the hostnames db and database:

version: "3"
services:

  web:
    build: .
    links:
      - "db:database"
  db:
    image: postgres

See the links reference for more information.

Multi-host networking

Note: The instructions in this section refer to legacy Docker Swarm operations, and only work when targeting a legacy Swarm cluster. For instructions on deploying a compose project to the newer integrated swarm mode, consult the Docker Stacks documentation.

When deploying a Compose application to a Swarm cluster, you can make use of the built-in overlay driver to enable multi-host communication between containers with no changes to your Compose file or application code.

Consult the Getting started with multi-host networking to see how to set up a Swarm cluster. The cluster uses the overlay driver by default, but you can specify it explicitly if you prefer - see below for how to do this.

Specify custom networks

Instead of just using the default app network, you can specify your own networks with the top-level networks key. This lets you create more complex topologies and specify custom network drivers and options. You can also use it to connect services to externally-created networks which aren’t managed by Compose.

Each service can specify what networks to connect to with the service-level networks key, which is a list of names referencing entries under the top-level networks key.

Here’s an example Compose file defining two custom networks. The proxy service is isolated from the db service, because they do not share a network in common - only app can talk to both.

version: "3"
services:

  proxy:
    build: ./proxy
    networks:
      - frontend
  app:
    build: ./app
    networks:
      - frontend
      - backend
  db:
    image: postgres
    networks:
      - backend

networks:
  frontend:
    # Use a custom driver
    driver: custom-driver-1
  backend:
    # Use a custom driver which takes special options
    driver: custom-driver-2
    driver_opts:
      foo: "1"
      bar: "2"

Networks can be configured with static IP addresses by setting the ipv4_address and/or ipv6_address for each attached network.

For full details of the network configuration options available, see the following references:

• Top-level networks key
• Service-level networks key

Configure the default network

Instead of (or as well as) specifying your own networks, you can also change the settings of the app-wide default network by defining an entry under networks named default:

version: "3"
services:

  web:
    build: .
    ports:
      - "8000:8000"
  db:
    image: postgres

networks:
  default:
    # Use a custom driver
    driver: custom-driver-1

Use a pre-existing network

If you want your containers to join a pre-existing network, use the external option:

networks:
  default:
    external:
      name: my-pre-existing-network

Instead of attempting to create a network called [projectname]_default, Compose looks for a network called my-pre-existing-network and connect your app’s containers to it.

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