Resolve Docker Container Networking Issues

Diagnose and fix Docker networking step by step:

Step 1: Understand Docker Networks

# List all Docker networks
docker network ls

# Inspect default bridge network
docker network inspect bridge

# Create custom bridge network
docker network create --driver bridge mynetwork

# Connect container to custom network
docker network connect mynetwork mycontainer

Step 2: Fix Container-to-Container Communication

# docker-compose.yml - Proper network setup
version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    networks:
      - app-network
    environment:
      - API_URL=http://api:8000  # Use service name as hostname
    depends_on:
      - api
      - database

  api:
    build: ./api
    ports:
      - "8000:8000"
    networks:
      - app-network
    environment:
      - DATABASE_URL=postgresql://user:pass@database:5432/mydb
    depends_on:
      - database

  database:
    image: postgres:13
    environment:
      - POSTGRES_DB=mydb
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=pass
    networks:
      - app-network
    volumes:
      - postgres_data:/var/lib/postgresql/data

networks:
  app-network:
    driver: bridge

volumes:
  postgres_data:

Step 3: Debug Network Connectivity

# Test container connectivity
docker exec -it container_name ping api
docker exec -it container_name nslookup api
docker exec -it container_name curl http://api:8000/health

# Check if ports are listening
docker exec -it container_name netstat -tlnp
docker exec -it container_name ss -tlnp

# Test external connectivity
docker exec -it container_name ping google.com
docker exec -it container_name curl https://api.github.com

Step 4: Fix Port Mapping Issues

# ❌ Wrong - conflicting port mappings
docker run -p 3000:3000 app1
docker run -p 3000:3000 app2  # Will fail

# ✅ Correct - use different host ports
docker run -p 3000:3000 app1
docker run -p 3001:3000 app2

# Check what ports are in use
docker ps --format "table {{.Names}}\t{{.Ports}}"
netstat -tlnp | grep :3000

Step 5: Application Configuration for Docker

// Node.js app configuration
const express = require('express');
const app = express();

// ❌ Wrong - hardcoded localhost
const API_URL = 'http://localhost:8000';

// ✅ Correct - use environment variables and service names
const API_URL = process.env.API_URL || 'http://api:8000';

// Bind to all interfaces, not just localhost
const PORT = process.env.PORT || 3000;
app.listen(PORT, '0.0.0.0', () => {
    console.log(`Server running on port ${PORT}`);
});

Step 6: Dockerfile Best Practices

FROM node:16-alpine

# Create app directory
WORKDIR /usr/src/app

# Copy package files
COPY package*.json ./

# Install dependencies
RUN npm ci --only=production

# Create non-root user
RUN addgroup -g 1001 -S nodejs
RUN adduser -S nextjs -u 1001

# Copy app source
COPY --chown=nextjs:nodejs . .

# Switch to non-root user
USER nextjs

# Expose port
EXPOSE 3000

# Health check
HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
    CMD curl -f http://localhost:3000/health || exit 1

# Start application
CMD ["npm", "start"]