Networking in Compose
Estimated reading time: 5 minutesThis page applies to Compose file formats version 2 and higher. Networking features are not supported for Compose file version 1 (legacy).
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: "3"
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 networkmyapp_default
under the nameweb
. - A container is created using
db
’s configuration. It joins the networkmyapp_default
under the namedb
.
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 tofalse
.
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.
Networks can also be given a custom name (since version 3.5):
version: "3.5"
networks:
frontend:
name: custom_frontend
driver: custom-driver-1
For full details of the network configuration options available, see the following references:
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.