Networking.md

📑 Networking

Networking is what lets containers talk to each other and to the outside world. This section shows you the simple concepts you need to set up clean, reliable connections.

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✳️ The Problem Docker Networking Solves

Before Docker networking:

• Containers couldn't easily communicate with each other
• Complex manual network configuration required
• Services needed to know exact IP addresses
• No isolation between different applications

After Docker networking:

• Containers can communicate using service names
• Automatic DNS resolution between containers
• Network isolation between applications
• Easy service discovery

✳️ Network Types

Bridge Network (Default)

# Create a custom bridge network
docker network create my-network

# Run containers on the network
docker run -d --name app --network my-network myapp
docker run -d --name db --network my-network postgres:18.1

Containers on the same bridge network can communicate using container names as hostnames.

Host Network

# Run container with host networking
docker run -d --network host nginx:latest

• Container uses the host's network directly. No isolation, but maximum performance.
• Performance-critical applications or when you need to bind to specific host network interfaces.
• Less isolation and portability.

None Network

# Run container with no networking
docker run -d --network none myapp

• Complete network isolation. Container has no network access.
• Security-sensitive processing of local data.

✳️ Network Commands

# List networks
docker network ls

# Create network
docker network create my-network

# Create network with custom subnet
docker network create --subnet=172.18.0.0/16 my-network

# Inspect network
docker network inspect my-network

# Connect running container to network
docker network connect my-network my-container

# Disconnect container from network
docker network disconnect my-network my-container

# Remove network
docker network rm my-network

# Remove unused networks
docker network prune

✳️ Container Communication Examples

🚥 Example 1: Web app connecting to database

# Create network
docker network create app-network

# Start database
docker run -d \
  --name database \
  --network app-network \
  -e POSTGRES_PASSWORD=secret \
  postgres:18.1

# Start application
docker run -d \
  --name app \
  --network app-network \
  -e DB_HOST=database \
  -e DB_PORT=5432 \
  -p 3000:3000 \
  myapp

# The app connects to database using:
# postgresql://database:5432/mydb
# "database" resolves to the database container's IP

🚥 Example 2: Multi-network isolation

# docker-compose.yml
version: '3.8'

services:
  frontend:
    image: myapp-frontend
    networks:
      - frontend-net
    ports:
      - "3000:3000"
  
  backend:
    image: myapp-backend
    networks:
      - frontend-net
      - backend-net
  
  database:
    image: postgres:18.1
    networks:
      - backend-net
    environment:
      - POSTGRES_PASSWORD=password

networks:
  frontend-net:
  backend-net:

Network topology:

• Frontend can talk to Backend (both on frontend-net)
• Backend can talk to Database (both on backend-net)
• Frontend CANNOT talk to Database directly (no shared network)

✳️ DNS Resolution

Docker provides automatic DNS resolution for container names:

# In a container on the same network, you can:
curl http://api:3000/users

ping database
# Works! DNS resolves "database" to the container's IP

✳️ Port Publishing

# Publish to specific host port
docker run -p 8080:80 nginx

# Publish to random host port
docker run -P nginx

# Publish to specific interface
docker run -p 127.0.0.1:8080:80 nginx

# Publish multiple ports
docker run -p 8080:80 -p 8443:443 nginx

Port mapping

• -p 8080:80 means: Host port 8080 → Container port 80
• Traffic to localhost:8080 is forwarded to port 80 inside the container

✳️ Network Aliases

# Container can be reached by multiple names
docker run -d \
  --network my-network \
  --network-alias api \
  --network-alias api-v2 \
  --name api-container \
  myapi

Now other containers can reach this container using:

• api-container (container name)
• api (alias)
• api-v2 (alias)

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📂 Browse Other Concepts:

Introduction | Basic Commands | Containers | Docker Compose | Dockerfile | Networking | Volumes | Best Practice

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Maintained with ❤️ by Reajul Hasan Raju