Hardcoding IP addresses in your containers? Docker Compose provides automatic DNS-based service discovery. Each service can reach another using just the service name.

How it works

Docker Compose creates a default network and registers each container with an internal DNS server. The DNS name matches the service name in your compose.yml.

services:
  web:
    image: nginx
    environment:
      # Just use the service name!
      API_URL: http://api:3000
      DB_HOST: postgres

  api:
    image: myapi
    environment:
      DATABASE_URL: postgres://user:pass@postgres:5432/mydb

  postgres:
    image: postgres:15

No configuration needed. The web service connects to api using http://api:3000, and api connects to postgres using the hostname postgres.

Verify DNS resolution

Check what’s actually happening:

# See what IP 'postgres' resolves to
docker compose exec web nslookup postgres

Output:

Server:    127.0.0.11
Address:   127.0.0.11#53

Name:      postgres
Address:   172.18.0.3

Test connectivity:

docker compose exec web ping -c 2 api

Multiple instances

When scaling, DNS returns all container IPs:

services:
  worker:
    image: myworker
    deploy:
      replicas: 3

docker compose up -d --scale worker=3
docker compose exec web nslookup worker
# Returns 3 IP addresses for round-robin

Common gotchas

Using container names instead of service names:

services:
  web:
    container_name: my-web-container  # Don't use this name for connections!
    # ...

Always use the service name (web), not the container name.

Services on different networks: Services must be on the same network to resolve each other. By default, Compose creates one network for all services.

Real production example

Here’s how we use this at Docker:

services:
  api:
    image: docker/api:latest
    environment:
      # Service names for all connections
      CACHE_URL: redis://cache:6379
      SEARCH_URL: http://search:9200
      METRICS_URL: http://metrics:9090

  cache:
    image: redis:7-alpine

  search:
    image: elasticsearch:8.11

  metrics:
    image: prom/prometheus

Each service finds the others by name. When containers restart and get new IPs, the DNS automatically updates.

Pro tip

For debugging network issues between services:

# Run a debug container on the same network
docker compose run --rm alpine sh
# Inside the container:
apk add curl bind-tools
nslookup api
curl http://api:3000/health

Service discovery through DNS eliminates configuration complexity. No more managing IP addresses or host files - Compose handles it all automatically.

Further reading