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localhost always means the loopback interface of the network namespace where the process is running. Inside a container, that means the container itself—not your host computer or another container. For two services on the same Docker Compose network, connect by service name and container port, such as db:5432. Publish a port when traffic needs to enter from the Docker host or another network.
Choose the address for the caller
Docker addresses depend on where the client process runs. The correct destination for a host-side app is often wrong for a container-side app.
| Caller | Destination | Address or route |
|---|---|---|
| One Compose service | Another service on the same network | service-name:container-port, such as db:5432. Compose DNS resolves the service name. Docker Docs: Networking in Compose |
| Host machine | A container service | localhost:host-port when working locally, or the host address and published port. The port must be published. Docker Docs: Networking in Compose |
| Container on Docker Desktop | A service running on the host | host.docker.internal:port. Docker Desktop networking |
| Container | A service in a separate, isolated Docker network | Connect both containers to an intentional shared network, or publish a port for a route that should cross the network boundary. Docker Engine networking |
Why localhost fails inside Docker
localhost is a loopback name, not a universal alias for the physical computer. A process running in a container has its own network namespace, so a connection to localhost stays inside that container. It will not find a database in another container or a development server running on the host.
For example, if an app container needs a Compose database service, configure its connection to use db:5432, assuming the service is named db and listens on container port 5432. If a browser or other host process needs to reach a containerized web server, use the host port that Docker publishes instead.
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How Docker bridges and Compose DNS work
Default and user-defined bridges
Docker creates a default bridge network named bridge for containers that do not specify another network. A user-defined bridge is usually preferable for applications: containers on that network can communicate, and Docker provides automatic name resolution between them. Containers on the default bridge generally need IP addresses to reach one another unless legacy links are used. Docker Docs: Bridge network driver
A bridge is a network boundary. Containers on the same bridge can communicate with one another; separate networks are isolated by default. A port does not need to be published merely to let another container on the same user-defined bridge reach the service.
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Compose service names
Compose creates a project network by default and connects the project’s services to it. A peer container can use a service name such as web or db as the hostname, together with the destination’s container port. Docker describes the behavior this way: “Each container can now look up the service name web or db and get back the appropriate container’s IP address.” Docker Docs: Networking in Compose
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For more deliberate separation, define multiple networks in Compose. For example, an application service can join both a frontend and backend network, while a proxy and a database join only the network or networks each requires. This limits which services share a direct network.
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Why service names are safer than container IPs
Container IP addresses are assigned dynamically. When Compose recreates a service, it can give the new container a different IP while retaining the service name. Existing connections to the old container close, so clients should handle connection failures, resolve the service name again, and reconnect rather than keep a stale IP. Docker Docs: Networking in Compose
Container ports and published host ports
A container port is where a service listens inside its container. A host port is the port Docker makes available on the host and forwards to the container. For example, -p 8080:80 maps host TCP port 8080 to container port 80. A service on the same bridge network uses the container port; a host process uses the published host port. Docker Docs: Port publishing
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Without a published port, a bridge-network container’s service is ordinarily reachable from the Docker host and peer containers on its network, but not from unrelated networks or remote machines unless direct routing or another route is configured. Publishing is for access from outside that network, not a general fix for container-to-container DNS. Docker Engine networking Docker Docs: Port publishing
Limit a published port to the host
When you publish without specifying a host IP address, Docker binds the port on all host addresses by default. For host-only development access, specify the loopback address, for example -p 127.0.0.1:8080:80. Docker documents a caveat: before Engine 28.0.0, other machines on the same layer-2 network could in some circumstances reach ports published to localhost. Consider the installed Engine version when relying on this boundary. Docker Docs: Port publishing
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Connect a container to a service on the host
Docker Desktop
For a container that needs to reach a service running on the Docker Desktop host, use host.docker.internal, for example host.docker.internal:8000 if the host service listens on port 8000. Docker Desktop routes container networking through its backend and VM; a Linux-style docker0 interface may not be directly visible on the host. Docker Desktop networking
Linux Docker Engine
On Linux Engine, Compose documents host-gateway as a way to add a host mapping such as host.docker.internal with extra_hosts. The host service must also accept connections arriving by that route: check its bind address and firewall. A service listening only on the host’s loopback interface may not accept connections through the bridge gateway. Docker Docs: Networking in Compose
When host networking is appropriate
In host network mode, a container shares the host network stack. Port publishing is unnecessary and unsupported in this mode, and Compose service-name DNS does not work. Docker presents host networking for specialized cases such as system-level network monitoring; it is not a general solution to a mistaken use of localhost. Compose’s default is a project bridge network. Docker Docs: Networking in Compose
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Troubleshoot a failed Docker connection
- Locate the client. Determine whether the process making the connection runs on the host, in a container, or in a Docker Desktop container.
- For container-to-container traffic, check network membership. Confirm both containers share a network, then use the target service name and its container port. Inspect networks with
docker network inspect. Docker Engine networking - For host-to-container traffic, check publishing. Verify the host-to-container port mapping and the host bind address; use the published host port from the host.
- For container-to-host traffic, use the platform’s host route. On Docker Desktop, try
host.docker.internal. On Linux, configure and verify an appropriate host-gateway name or route, and make sure the host service listens on an address reachable through it. Docker Desktop networking Docker Docs: Networking in Compose - After container recreation, reconnect by name. A changed container IP or a closed connection calls for resolving the service name again and reconnecting, not pinning the former IP.
- Inspect injected host mappings when relevant. Check the container’s
/etc/hostsand network configuration to verify the expected mapping and membership.
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