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For a typical Docker Compose application, use the project’s user-defined bridge network rather than Docker’s default bridge: Compose then provides service-name discovery, and network membership determines which containers can communicate. A common proxy/app/database layout puts the proxy and database on separate networks, with the app attached to both. Mark a network internal: true only when its members should not use that network for external connectivity; a service attached to another, regular network may still reach the internet.
Docker’s default bridge and a Compose network are not the same thing
Docker Engine creates a network named bridge and connects containers to it when no network is specified. It is distinct from a user-defined bridge network, even though both use the bridge driver. Docker recommends user-defined bridges for application communication because they scope connectivity to their members and provide automatic name and alias resolution. Docker’s bridge network documentation describes these distinctions.
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| Network choice | Name discovery | Typical use |
|---|---|---|
Docker Engine’s default bridge |
Does not provide the same automatic container-name resolution as a user-defined bridge. | Default attachment when no network is selected; not the recommended basis for a Compose application stack. |
| User-defined bridge | Attached containers can resolve each other by name or alias. | Scoped communication among containers on the same Docker Engine host. |
| Compose project network | Compose registers service names for discovery among services sharing the network. | Default network for a Compose project, or one of multiple networks for a segmented stack. |
Compose’s ordinary project network is a user-defined network by default. By contrast, setting a service’s network_mode: bridge connects it to Docker Engine’s default bridge; Docker’s Compose service reference notes that the default bridge does not provide service-name DNS and recommends a user-defined network for that use. See the Compose service reference.
How Compose service-name DNS works
When no custom networks are declared, Compose creates one project network and attaches the project’s services to it. Docker’s documentation puts it simply: “By default, Compose sets up a single network for your app.” Services on that shared network can address one another by service name. For example, an app can use db as the database hostname if both services share a network. Docker’s Compose networking guide explains the default and custom-network behavior.
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Use the service name in application configuration, not a container IP you expect to remain fixed. The name is the discovery mechanism; it does not promise a permanent IP address. When services do not share a network, they do not have that network path for communicating with one another.
Build a proxy, app, and database topology
A simple least-connectivity layout gives the proxy a frontend network, the database a backend network, and the app both. The proxy and database then have no shared network, while the app can communicate with each. “Proxy network” here means a network in this application topology; it is not a built-in Docker network driver.
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services:
proxy:
image: nginx
networks: [frontend]
app:
image: example-app
networks: [frontend, backend]
db:
image: postgres
networks: [backend]
networks:
frontend: {}
backend: {}With this membership, the app can discover the proxy and database by service name on their respective shared networks. The proxy is not directly connected to the backend network, and the database is not directly connected to the frontend. Docker’s Compose documentation uses this network-membership pattern to illustrate separation. See the Compose network reference.
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What internal: true means
Set internal: true on a Compose network when its members should be externally isolated through that network. Docker describes an internal network as having no default gateway and no connection to host network interfaces; its network reference also describes routes and firewall rules that prevent traffic to or from other networks. Members can still communicate with one another on the internal network. The Compose network reference and the docker network create reference cover this behavior.
For example, a backend network can be declared as:
networks:
backend:
internal: trueDo not interpret this setting as making every attached service wholly offline. If a service is attached to both this internal backend and a regular network, it may still use the regular network for internet access. Assess the paths available through all of a service’s network memberships, not just whether one network is internal. Docker’s Compose networking guide calls out this multi-network behavior.
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Ports, host access, and the boundary of a bridge
Containers on a user-defined bridge can access one another’s ports across that network without publishing those ports to the host. To make a container port reachable from a different network or a non-Docker host, publish it with -p or --publish (or the corresponding Compose port configuration). Publishing is about exposing a port beyond the network peers; it is not required simply for two services on the same user-defined network to communicate. Docker’s bridge-driver documentation explains port reachability.
Bridge networks are local to containers on one Docker Engine host. They are not, by themselves, a cluster-wide network. For communication across hosts, Docker documents OS-level routing or an overlay network as possibilities; the appropriate choice depends on the deployment. The network creation reference discusses network scope.
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When a Compose project uses an existing network
Compose can use a pre-existing network as an external network rather than managing that network as part of the project. This can be useful when multiple projects need to share a network, but the external network must already exist before docker compose up. The project does not create that external network for you. The Compose network reference documents external networks.
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