Docker Networking Explained: How Containers Talk to Each Other

Question

I have been using Docker for a while and I am comfortable running single containers. But as soon as I try to get two containers talking to each other I get confused.

For example I have a Node.js app and a PostgreSQL container and I cannot figure out how to get the app to connect to the database. I tried using localhost but that does not work inside a container.

Can someone explain how Docker networking actually works and what the right way is to connect multiple containers?

Bobby · Answer

Hi there, The localhost confusion is the classic Docker networking gotcha. Inside a container, localhost refers to the container itself, not your host machine or any other container. That is why it does not work. Docker has a few networking modes but the one you want for multi-container setups is a bridge network, which is what Docker Compose creates automatically. If you want to practice Docker networking commands in your browser before touching your real setup, this interactive simulator is worth trying: Docker terminal simulator For your Node.js and PostgreSQL setup, the cleanest approach is Docker Compose: services: app: image: node:18 depends_on: - db environment: DATABASE_URL: postgres://postgres:secret@db:5432/mydb db: image: postgres:15 environment: POSTGRES_PASSWORD: secret POSTGRES_DB: mydb The key thing to notice is @db:5432 in the connection string. Docker Compose creates a shared bridge network for all services in the same file, and each service is reachable by its service name. So db resolves to the PostgreSQL container’s IP automatically. If you are not using Compose and running containers manually, you need to create a network and attach both containers to it: docker network create mynetwork docker run -d --name db --network mynetwork postgres:15 docker run -d --name app --network mynetwork node:18 Then inside the app container, db resolves to the PostgreSQL container the same way. DigitalOcean has a solid tutorial on Docker Compose if you want to go deeper: https://www.digitalocean.com/community/tutorials/how-to-install-and-use-docker-compose-on-ubuntu-22-04

KFSys · Answer

Heya, Good news, this trips up almost everyone and the fix is simple once it clicks. Inside a container, localhost means that container itself, not the host or your other containers. So your Node app looking for Postgres on localhost is really looking inside its own container, where nothing is listening. That’s why it fails. The right way is to put both containers on the same user-defined bridge network. On that network, Docker gives each container a DNS name equal to its name, so they can find each other by name instead of IP. docker network create myapp-net docker run -d --name postgres --network myapp-net -e POSTGRES_PASSWORD=secret postgres docker run -d --name node-app --network myapp-net my-node-image Now in your Node app’s connection string, use the Postgres container’s name as the host: postgres://postgres:secret@postgres :5432/mydb That postgres hostname resolves to the database container automatically. No IPs to chase. If you use Docker Compose, this is even easier. Compose puts every service on a shared network by default and uses the service name as the hostname: services: db: image: postgres environment: POSTGRES_PASSWORD: secret app: image: my-node-image depends_on: - db Here your app just connects to db:5432. Done. Quick rule of thumb: container-to-container, use the container/service name. Only use localhost when something needs to reach a service inside its own container. Hope that clears it up.

alexdo · Answer

Hi there, the networking explanations above are both correct, bridge network plus service name as hostname is the right mental model. One thing worth flagging before you hit it: depends_on in that Compose example only controls start order, not whether Postgres is actually ready to accept connections yet. Compose starts the db container and immediately starts app right after, but Postgres can take a moment to finish initializing, so your Node app’s first connection attempt can still fail even though the networking is set up correctly. Add a healthcheck and make app wait on it properly: services: db: image: postgres:15 environment: POSTGRES_PASSWORD: secret healthcheck: test: ["CMD-SHELL", "pg_isready -U postgres"] interval: 5s timeout: 5s retries: 5 app: image: node:18 depends_on: db: condition: service_healthy With condition: service_healthy, Compose actually waits for pg_isready to succeed before starting app, rather than just waiting for the Postgres process to start. Same networking as above, just avoids the “works most of the time, fails on a fast restart” flakiness. Regards

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Docker Networking Explained: How Containers Talk to Each Other

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