Networking in Compose
Note: This document only applies if you’re using version 2 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 theCOMPOSE_PROJECT_NAME
environment variable.
For example, suppose your app is in a directory called myapp
, and your docker-compose.yml
looks like this:
version: '2' services: web: build: . ports: - "8000:8000" db: image: postgres
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 networkmyapp_default
under the nameweb
. - A container is created using
db
’s configuration. It joins the networkmyapp_default
under the namedb
.
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.
Because web
explicitly maps a port, it’s also accessible from the outside world via port 8000 on your Docker host’s network interface.
Updating containers
If you make a configuration change to a service and run docker-compose up
to update it, the old container will be removed and the new one will join the network under a different IP address but the same name. Running containers will be able to look up that name and connect to the new address, but the old address will stop working.
If any containers have connections open to the old container, they will be 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: '2' services: web: build: . links: - "db:database" db: image: postgres
See the links reference for more information.
Multi-host networking
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 will use the overlay
driver by default, but you can specify it explicitly if you prefer - see below for how to do this.
Specifying 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: '2' services: proxy: build: ./proxy networks: - front app: build: ./app networks: - front - back db: image: postgres networks: - back networks: front: # Use a custom driver driver: custom-driver-1 back: # 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:
Configuring 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: '2' services: web: build: . ports: - "8000:8000" db: image: postgres networks: default: # Use a custom driver driver: custom-driver-1
Using 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 attemping to create a network called [projectname]_default
, Compose will look for a network called my-pre-existing-network
and connect your app’s containers to it.
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