Networking

When using dynamic port mapping or multiple services, it’s not always obvious which host port maps to which container port. docker compose port tells you exactly. Basic usage # Which host port maps to container port 80 on the web service? docker compose port web 80 # Output: 0.0.0.0:8080 Why dynamic ports matter When you let Docker assign ports automatically, the host port changes on every docker compose up: services: web: image: nginx ports: - "80" # Dynamic host port, container port 80 docker compose port web 80 # Output: 0.0.0.0:55432 (assigned dynamically) This is common in CI or when running multiple instances of the same project. ...

Need custom DNS resolution in containers? Use extra_hosts to add hostname mappings without touching system files! Basic extra_hosts usage Add custom host entries to containers: services: app: image: myapp extra_hosts: - "api.local:192.168.1.100" - "db.local:192.168.1.101" - "cache.local:192.168.1.102" Inside the container: docker compose exec app cat /etc/hosts # 127.0.0.1 localhost # 192.168.1.100 api.local # 192.168.1.101 db.local # 192.168.1.102 cache.local Dynamic host resolution Use host machine’s IP dynamically: services: app: image: myapp extra_hosts: - "host.docker.internal:host-gateway" # Magic value! This maps to: ...

Secure your application architecture by isolating services in separate networks. Not every service needs to talk to every other service! Default behavior: All connected By default, all services share the same network: # All services can communicate services: web: image: nginx api: image: myapi database: image: postgres Problem: web can directly access database - potential security risk! Network isolation pattern Create separate networks for different tiers: ...

Choose the right networking mode for your containers. Understand when isolation matters and when performance is key. Bridge mode (default) The default and most secure option - containers get their own network namespace: services: web: image: nginx ports: - "8080:80" # Port mapping required networks: - app_network db: image: postgres:15 networks: - app_network networks: app_network: driver: bridge Containers can communicate using service names (web, db) within the network. ...

Need your frontend project to talk to a backend in another Compose project? External networks let you connect containers across different stacks. The problem Two separate Compose projects need to communicate: frontend/compose.yml - React app backend/compose.yml - API service By default, each creates its own isolated network. The solution Create a shared external network: # Create the network once docker network create shared-network Then reference it in both projects: backend/compose.yml: ...