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Networking in Compose

Important

From the end of June 2023 Compose V1 won’t be supported anymore and will be removed from all Docker Desktop versions.

Make sure you switch to Compose V2 with the docker compose CLI plugin or by activating the Use Docker Compose V2 setting in Docker Desktop. For more information, see the Evolution of Compose

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:

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

When you run docker compose up, the following happens:

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

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 uses 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}:5432.

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.

Tip

Reference containers by name, not IP, whenever possible. Otherwise you’ll need to constantly update the IP address you use.

Link containers

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:

services
:

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

See the links reference for more information.

Multi-host networking

When deploying a Compose application on a Docker Engine with Swarm mode enabled,
you can make use of the built-in overlay driver to enable multi-host communication.

Overlay networks are always created as attachable. You can optionally set the attachable property to false.

Consult the Swarm mode section, to see how to set up
a Swarm cluster, and the Getting started with multi-host networking
to learn about multi-host overlay networks.

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.

The following example shows a Compose file which defines 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.

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.

Networks can also be given a custom name:

services
:
  # ...
networks
:
  frontend
:
    name
:
 custom_frontend
    driver
:
 custom-driver-1

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:

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

services
:
  # ...
networks
:
  network1
:
    name
:
 my-pre-existing-network
    external
:
 true

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