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Task: Creating and Testing Docker Networks

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This task provides a detailed guide to creating and testing Docker networks. Docker networks enable containers to communicate with each other securely and efficiently.

Prerequisites

  • Docker installed and running
  • Basic understanding of Docker containers
  • Terminal access

Task Overview

In this task, you will:

  • List existing Docker networks
  • Create custom networks with different configurations
  • Run containers connected to networks
  • Test connectivity between containers
  • Manage network connections
  • Clean up resources
Solution

Step-by-Step Solution

01: List Existing Networks

First, let’s see what networks are already available on your Docker host.

docker network ls

This command will show the default networks:

  • bridge: Default network for containers
  • host: Uses the host’s network stack
  • none: No networking

02: Create a Custom Network

Create a custom bridge network for better isolation and control.

docker network create --driver bridge my-custom-network
  • --driver bridge: Specifies the bridge driver (default)
  • my-custom-network: Name of the network

Verify the network was created:

docker network ls

You should see my-custom-network in the list.

03: Inspect the Network

Get detailed information about the network.

docker network inspect my-custom-network

This will show:

  • Network ID
  • Driver
  • Subnet and gateway
  • Connected containers (initially empty)

04: Run Containers on the Network

Launch two containers connected to the custom network.

# Run a web server container
docker run -d --name web-server --network my-custom-network -p 8080:80 nginx

# Run a client container for testing
docker run -d --name test-client --network my-custom-network alpine sleep 3600
  • -d: Run in detached mode
  • --name: Assign container names
  • --network: Connect to the custom network
  • -p 8080:80: Port mapping for the web server

05: Verify Container Connectivity

Check that containers are connected to the network.

docker network inspect my-custom-network

You should now see the two containers listed under “Containers”.

06: Test Network Communication

Test communication between containers on the same network.

# Get the IP address of the web server
docker inspect web-server | grep -A 10 "Networks"

# Or use container names for DNS resolution
docker exec test-client ping -c 4 web-server

Since containers on the same network can resolve each other by name, you can ping using the container name.

07: Test External Access

Test access to the web server from the host.

curl http://localhost:8080

You should see the default Nginx welcome page.

08: Connect Existing Container to Network

Demonstrate connecting a running container to the network.

# Run another container without specifying network
docker run -d --name standalone-container alpine sleep 3600

# Connect it to the custom network
docker network connect my-custom-network standalone-container

# Verify connection
docker network inspect my-custom-network

09: Disconnect Container from Network

Remove a container from the network.

docker network disconnect my-custom-network standalone-container

Verify the container is no longer connected.

10: Create Network with Custom Subnet

Create a network with a specific subnet.

docker network create --driver bridge --subnet 192.168.10.0/24 --gateway 192.168.10.1 custom-subnet-network

Inspect to verify the custom configuration.

11: Clean Up

Remove containers and networks.

# Stop and remove containers
docker stop web-server test-client standalone-container
docker rm web-server test-client standalone-container

# Remove networks
docker network rm my-custom-network custom-subnet-network

Advanced Testing

Test with Docker Compose

Create a docker-compose.yml file:

version: '3.8'
services:
  web:
    image: nginx
    networks:
      - my-network
  client:
    image: alpine
    command: sleep 3600
    networks:
      - my-network
networks:
  my-network:
    driver: bridge

Run with:

docker-compose up -d

Test Network Isolation

Create two separate networks and verify containers can’t communicate across them.

# Create two networks
docker network create network-a
docker network create network-b

# Run containers on each
docker run -d --name container-a --network network-a alpine sleep 3600
docker run -d --name container-b --network network-b alpine sleep 3600

# Try to ping across networks (should fail)
docker exec container-a ping -c 4 container-b  # This will fail

Troubleshooting

Common Issues

  1. Port already in use: Change the port mapping
  2. Network not found: Ensure the network name is correct
  3. Container can’t resolve names: Check if both containers are on the same network

Useful Commands

  • docker network prune: Remove unused networks
  • docker network connect/disconnect: Manage container network connections
  • docker inspect <container>: Get detailed container information including networks

Explanation

  • docker network ls: Lists all networks on the Docker host
  • docker network create: Creates a new network with specified driver and options
  • docker network inspect: Shows detailed network configuration and connected containers
  • docker run –network: Connects a container to a specific network at startup
  • docker network connect/disconnect: Dynamically connects or disconnects running containers from networks
  • Container name resolution: Docker provides built-in DNS for containers on the same network
  • Network isolation: Containers on different networks cannot communicate unless explicitly connected