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Before adding a Docker container, check which network it will join, which services it can reach, and whether any published ports are exposed beyond the host. These six checks focus on Docker bridge networks and Compose; details can vary with your Docker Engine version, operating system, host firewall, and network driver.
1. Choose the network deliberately
Containers started without another network use Docker’s default bridge. A user-defined bridge is configurable and provides automatic DNS resolution between attached containers. Compose normally creates a project network and makes services discoverable by service name. Declare the intended relationships rather than letting network membership happen by default.
These options are not interchangeable:
| Network option | Container-to-container reachability | Service-name DNS | Host port publishing | How membership is configured | Shares host network stack? |
|---|---|---|---|---|---|
| Default bridge | Containers on the bridge can communicate | Does not provide the same name-based discovery described for user-defined bridges | Use publishing for access from outside the host or across networks | Used by containers not assigned another network | No |
| User-defined bridge | Attached containers can reach one another’s ports | Yes, automatic DNS resolution | Use publishing for access from outside the host or across networks | Create the bridge and attach the intended containers | No |
| Compose project network | Services on the same network can communicate | Yes, by service name | Declare published ports when host access is needed | Compose creates a project network by default; services can be assigned to selected networks | No |
| Host mode | Uses the host’s network stack | Compose service-name DNS does not work | Port mapping is not supported | Set the service’s network mode to host | Yes |
Docker’s bridge network documentation and Compose networking documentation describe these bridge and Compose behaviors. Confirm the applicable behavior for your Engine version and platform before changing configuration.
2. Limit which services share a network
Containers on the same user-defined bridge can reach one another on all ports. That makes network membership useful for expressing which services need a connection, but it is not a security boundary between services that share the network.
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In Compose, define separate front and back networks when that matches the application’s relationships. For example, a public-facing service can join the front network, while an application service joins both front and back networks and a database joins only the back network. This limits which services share a network; it does not prevent same-network services from reaching one another’s ports.
Compose lets each service join selected networks; see the Compose networking guide for network configuration details.
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3. Use service names, not fixed container IPs
On a user-defined bridge, Docker provides automatic DNS resolution among attached containers. Compose services are likewise reachable by service name on their shared network. Configure an application to connect to a service name, such as db, rather than a container’s current IP address.
When Compose replaces a container after a configuration change, the replacement may receive a different IP address while keeping the service name. Service-name discovery is therefore the more durable reference; it does not mean the service is healthy or ready to accept connections.
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4. Review every published port and host binding
Docker Docs states: “Publishing container ports is insecure by default.” When a port is published without specifying a host IP, Docker makes it available on host addresses by default. If a service should be accessible only from the host, bind it to 127.0.0.1 or ::1 as appropriate for your setup.
There is a version caveat: before Docker Engine 28.0.0, hosts on the same layer-2 network segment could reach ports published to localhost. Check your Engine version and surrounding firewall rules rather than assuming a localhost binding has identical exposure on every installation. See Docker’s port publishing and mapping documentation.
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- List each published port and identify whether it needs to be reachable from outside the host.
- When host-only access is intended, specify the loopback host IP instead of publishing on all host addresses.
- Check firewall policy, operating system, Docker version, and the relevant network driver before relying on a binding as an access control.
5. Question special network modes
Host mode
In Compose, host mode shares the host network stack. Port mapping is not supported in this mode, and Compose service-name DNS does not work. Use it only when the application genuinely requires host networking and you have accounted for the consequences.
None mode
The none network mode turns off container networking. Do not choose it for a service that needs to communicate over a network. Docker documents these Compose network modes in its networking guide.
6. Inspect the configuration that is actually running
Configuration files show what you intend to run; inspection commands show aspects of the current setup. Use these checks after the container or Compose project is running:
- Check network membership: run
docker network inspect <network-name>to inspect a network and its attached containers. - Check a published mapping: run
docker compose port <service> <container-port>to see the host mapping for a Compose service port. - Test from a running container: use
docker compose exec <service> <command>to run a command inside a service container and investigate connectivity from that container’s perspective.
These checks help confirm current membership, mappings, and connectivity conditions. They do not, by themselves, establish that an application is healthy or that its network configuration is secure. Docker documents these commands in the Compose networking guide.
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