Network inspection and debugging in Docker - Mini Project: Build & Apply

Network inspection and debugging

📖 Scenario: You are working on a small Docker project where multiple containers communicate over a custom network. You want to inspect the network and debug connectivity issues between containers.

🎯 Goal: Learn how to create a Docker network, run containers attached to it, inspect the network details, and check container connectivity using Docker commands.

📋 What You'll Learn

Create a custom Docker network named my_custom_net

Run two containers named web and db attached to my_custom_net

Inspect the network my_custom_net to see connected containers

Use Docker commands to check connectivity between web and db

💡 Why This Matters

🌍 Real World

Docker networks help isolate and connect containers in real projects, making sure services can talk to each other securely and efficiently.

💼 Career

Understanding Docker networking and debugging is essential for DevOps roles to troubleshoot container communication issues and maintain healthy deployments.

1

Create a custom Docker network

Create a Docker network called my_custom_net using the docker network create command.

Docker

# Create a Docker network named my_custom_net
# Your code here

Hint

Use the command docker network create my_custom_net to create the network.

2

Run two containers attached to the custom network

Run two containers named web and db using the alpine image. Attach both containers to the my_custom_net network. Use the docker run command with --name and --network options. Run the containers in detached mode with -d.

Docker

docker network create my_custom_net
# Run container named web attached to my_custom_net
# Your code here

Hint

Use docker run -d --name web --network my_custom_net alpine sleep 1000 and similarly for db.

3

Inspect the Docker network to see connected containers

Use the docker network inspect my_custom_net command to view details of the network and verify that both web and db containers are connected.

Docker

docker network create my_custom_net
docker run -d --name web --network my_custom_net alpine sleep 1000
docker run -d --name db --network my_custom_net alpine sleep 1000
# Inspect the network my_custom_net
# Your code here

Hint

Use docker network inspect my_custom_net to see the network details.

4

Check connectivity between containers

Use the docker exec command to run a ping from the web container to the db container by its container name. Use docker exec web ping -c 3 db to send 3 ping packets.

Docker

docker network create my_custom_net
docker run -d --name web --network my_custom_net alpine sleep 1000
docker run -d --name db --network my_custom_net alpine sleep 1000
docker network inspect my_custom_net
# Ping db from web container
# Your code here

Hint

Use docker exec web ping -c 3 db to test connectivity.

Practice

1. What does the Docker command docker network ls do?

easy

A. Creates a new Docker network

B. Shows running containers

C. Deletes a Docker network

D. Lists all Docker networks on the host

Solution

1. Step 1: Understand the command purpose

   The command docker network ls is used to list networks, not containers or create/delete networks.

2. Step 2: Match command to action

   Since it lists networks, it shows all Docker networks on the host machine.

3. Final Answer:

   Lists all Docker networks on the host -> Option D

4. Quick Check:

   docker network ls = list networks [OK]

Hint: Remember: 'ls' means list, so it lists networks [OK]

Common Mistakes:

• Confusing network listing with container listing
• Thinking it creates or deletes networks
• Mixing up 'docker network ls' with 'docker ps'

2. Which of the following is the correct syntax to inspect details of a Docker network named my_network?

easy

A. docker network show my_network

B. docker inspect network my_network

C. docker network inspect my_network

D. docker network details my_network

Solution

1. Step 1: Recall Docker network inspection syntax

   The correct command to inspect a network is docker network inspect <network_name>.

2. Step 2: Verify options

   Only docker network inspect my_network matches the correct syntax exactly.

3. Final Answer:

   docker network inspect my_network -> Option C

4. Quick Check:

   docker network inspect = inspect network [OK]

Hint: Use 'docker network inspect' to see network details [OK]

Common Mistakes:

• Swapping order of 'inspect' and 'network'
• Using non-existent commands like 'show' or 'details'
• Confusing with container inspect syntax

3. You run docker exec container1 ping -c 2 container2 to test connectivity. What output indicates successful communication?

medium

A. ping: unknown host container2

B. 2 packets transmitted, 2 received, 0% packet loss

C. 0 packets transmitted, 0 received, 100% packet loss

D. Error: container2 not found

Solution

1. Step 1: Understand ping output

   Successful ping shows packets transmitted and received with 0% packet loss.

2. Step 2: Match output to success

   2 packets transmitted, 2 received, 0% packet loss shows 2 packets sent and received with no loss, indicating success.

3. Final Answer:

   2 packets transmitted, 2 received, 0% packet loss -> Option B

4. Quick Check:

   Ping success = packets received with 0% loss [OK]

Hint: Look for 0% packet loss to confirm connectivity [OK]

Common Mistakes:

• Confusing error messages with success
• Ignoring packet loss percentage
• Assuming any ping output means success

4. You try docker network inspect mynet but get an error: Error: No such network: mynet. What is the most likely cause?

medium

A. The network 'mynet' does not exist on the host

B. You need to run the command inside a container

C. Docker daemon is not running

D. You must use 'docker inspect network mynet' instead

Solution

1. Step 1: Analyze error message

   The error says 'No such network', meaning Docker cannot find a network named 'mynet'.

2. Step 2: Identify cause

   This usually means the network was not created or was removed.

3. Final Answer:

   The network 'mynet' does not exist on the host -> Option A

4. Quick Check:

   Error 'No such network' = network missing [OK]

Hint: Check network list before inspecting to avoid missing network errors [OK]

Common Mistakes:

• Thinking command must run inside container
• Assuming Docker daemon is stopped without checking
• Using wrong command syntax

5. You have two containers, webapp and db, connected to a user-defined network appnet. You want to debug why webapp cannot reach db. Which sequence of commands helps you inspect and test the network connectivity?

hard

A. docker network inspect appnet; docker exec webapp ping -c 3 db

B. docker ps; docker logs webapp; docker network ls

C. docker network create appnet; docker exec db ping -c 3 webapp

D. docker exec webapp ifconfig; docker network rm appnet

Solution

1. Step 1: Inspect the network configuration

   Use docker network inspect appnet to check if both containers are connected properly.

2. Step 2: Test connectivity from webapp to db

   Run docker exec webapp ping -c 3 db to verify if webapp can reach db by name.

3. Final Answer:

   docker network inspect appnet; docker exec webapp ping -c 3 db -> Option A

4. Quick Check:

   Inspect network then ping from source to target [OK]

Hint: Inspect network first, then ping from source container [OK]

Common Mistakes:

• Trying to ping from db to webapp instead of webapp to db
• Creating network again instead of inspecting existing
• Removing network before debugging