Stop 16 · Docker & ContainersTopics · 6/9

Networking in Docker

In short

Your API container can't find the database, or your laptop can't reach the API. Almost every Docker networking problem comes down to two ideas: shared networks and published ports.

How containers communicate with each other and the outside world — Docker's default bridge networks let containers reach each other by service name within the same Compose setup.

The blob picks up a wall intercom handset in a building lobby while the main gate stays closed.

Think of it like this

The apartment intercom

Inside your building, you press 'B-204' on the intercom and reach your neighbour directly, no outside phone number needed. Visitors from the street can only reach flats the gate lets them through to. Containers on a network call each other by name; -p is the gate.

Watch it happen

Who can reach whom

Your laptop

api

Docker DNS

where is db?

1/5Both containers are on the user-defined network shopnet. The api asks Docker's built-in DNS for db.

The big ideas

What you need to know

User-defined networks

Containers on the same user-defined network find each other by container name. The default bridge doesn't do name lookup.

-p publishes a port

-p 8080:3000 means laptop port 8080 forwards to container port 3000. Left is host, right is container.

localhost is per container

Inside a container, localhost means that container. To reach another, use its name.

Publish only what you must

Keep databases unpublished in production. Only the API or proxy should face the outside.

See it in code

Walk through the code

bash

1docker network create shopnet2 3docker run -d --name db --network shopnet -e POSTGRES_PASSWORD=secret postgres:174docker run -d --name api --network shopnet -p 3000:3000 \5  -e DATABASE_URL=postgres://postgres:secret@db:5432/postgres my-api6 7# Inside the network, containers find each other by name8docker run --rm --network shopnet alpine ping -c 1 db

1/3Create a user-defined bridge network. It comes with DNS for container names.

Compose creates a network like shopnet for every project automatically. That's why service names just work there.

Save yourself the pain

Common mistakes

Oops

Publishing the database with -p 5432:5432 on a public server.

Do this instead

That exposes it to the internet. Keep it on a private network and let only the api reach it.

Oops

Mixing up the order in -p 3000:8080.

Do this instead

It's host:container. Check which port your app listens on inside the container.

Your mission

Debug a broken connection

~25 min

Quick check

You run docker run -p 8080:3000 my-api. Where does your browser go?

Go deeper

1Confirm two containers in the same Compose file can reach each other by service name.
2Learn how port mapping (`-p 3000:3000`) exposes a container's port to your host machine.
3Explore Docker's network types: bridge, host, none.