1. Introduction

In this tutorial, we aim to provide a step-by-step guide to troubleshoot and resolve network and connectivity issues in Docker containers. Docker is an open-source platform used to automate the deployment, scaling, and management of applications. Docker containers can be thought of as lightweight virtual environments that run applications and their dependencies in isolation.

By the end of this tutorial, you will learn how to:

• Check the status of Docker containers
• Debug network issues within Docker
• Resolve connectivity problems between Docker containers

Prerequisites:

• Basic understanding of Docker and Docker containers
• Docker installed on your machine

2. Step-by-Step Guide

Understanding Docker Networking

Docker uses network drivers to enable communication between containers. By default, Docker creates three networks on installation: bridge, none, and host. The bridge network is the default and most common network that Docker containers use.

Troubleshooting Network Issues

1. Check the status of Docker containers: Use the command docker ps -a to list all containers and their statuses. If a container is not running, you can start it using docker start <container_id>.

2. Inspect the Docker network: Use docker network inspect <network_name> to display detailed information about a network. This command will give you information about connected containers and their IP addresses.

3. Ping between containers: You can use the docker exec command to execute the ping command between containers. If ping fails, there is a network issue.

Best Practices and Tips

• Always keep your Docker software up to date to avoid bugs and security vulnerabilities.
• Use user-defined networks instead of the default.

3. Code Examples

Example 1. Checking the status of Docker containers

# List all containers (running and stopped)
docker ps -a

Example 2. Inspecting the Docker network

# Inspect the default bridge network
docker network inspect bridge

Example 3. Pinging between containers

# Ping container2 from container1
docker exec container1 ping container2

4. Summary

In this tutorial, we learned how to troubleshoot and resolve network and connectivity issues within Docker containers. We learned how to check the status of Docker containers, inspect Docker networks, and ping between containers to ensure network connectivity.

For further learning, consider exploring more advanced Docker networking concepts like network isolation and custom network drivers.

5. Practice Exercises

Exercise 1: Create a new Docker network and connect two containers to this network. Check if they can communicate with each other.

Exercise 2: Simulate a network failure and try to identify and resolve it.

Solutions:

Exercise 1:

1. Create a new network: docker network create my_network
2. Run two containers on this network: docker run --network=my_network -d --name container1 nginx and docker run --network=my_network -d --name container2 nginx
3. Check connectivity: docker exec container1 ping container2

Exercise 2:

1. Disconnect a container from network: docker network disconnect my_network container1
2. Check connectivity: docker exec container1 ping container2. This should fail.
3. Resolve the issue: docker network connect my_network container1
4. Check connectivity again: docker exec container1 ping container2. This should succeed.