Debugging network issues in Docker - Mini Project: Build & Apply

Debugging network issues

📖 Scenario: You are working with Docker containers that need to communicate with each other on a custom network. Sometimes, containers cannot reach each other due to network misconfiguration. You want to set up a simple Docker network and containers, then check their connectivity to debug network issues.

🎯 Goal: Build a Docker setup with a custom network and two containers. Then, use Docker commands to check if the containers can ping each other, helping you understand and debug network connectivity problems.

📋 What You'll Learn

Create a custom Docker network named my_network

Run two containers named container1 and container2 attached to my_network

Use the alpine image for both containers

Check connectivity by pinging container2 from container1

Print the ping command output to verify connectivity

💡 Why This Matters

🌍 Real World

Docker containers often need to communicate in microservices or multi-container apps. Debugging network issues ensures services can talk to each other reliably.

💼 Career

DevOps engineers and developers use Docker networking daily to deploy and troubleshoot containerized applications in production and development.

1

Create a custom Docker network

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

Docker

# Your code here

Hint

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

2

Run two containers on the custom network

Run two containers named container1 and container2 using the alpine image. Attach both containers to the my_network network. Use the docker run command with --network my_network and --name options.

Docker

docker network create my_network
# Your code here

Hint

Use docker run -dit --name container1 --network my_network alpine sh and similarly for container2.

3

Ping container2 from container1 to test connectivity

Use docker exec to run the ping -c 3 container2 command inside container1 to check if it can reach container2.

Docker

docker network create my_network
docker run -dit --name container1 --network my_network alpine sh
docker run -dit --name container2 --network my_network alpine sh
# Your code here

Hint

Run docker exec container1 ping -c 3 container2 to test connectivity.

4

Display the ping output

Run the command docker exec container1 ping -c 3 container2 and print its output to verify if the ping was successful.

Docker

docker network create my_network
docker run -dit --name container1 --network my_network alpine sh
docker run -dit --name container2 --network my_network alpine sh
docker exec container1 ping -c 3 container2
# Your code here

Hint

The output should show lines starting with PING container2 and replies if successful.

Practice

1. What is the primary Docker command to list all networks and check their status?

easy

A. docker ps -a

B. docker container ls

C. docker network ls

D. docker inspect network

Solution

1. Step 1: Identify the command to list networks

   The command docker network ls lists all Docker networks and their status.

2. Step 2: Differentiate from container commands

   docker container ls and docker ps -a list containers, not networks. docker inspect network is incomplete and incorrect syntax.

3. Final Answer:

   docker network ls -> Option C

4. Quick Check:

   List networks = docker network ls [OK]

Hint: Use 'docker network ls' to see all networks quickly [OK]

Common Mistakes:

• Confusing container list commands with network commands
• Using incomplete or wrong inspect syntax
• Trying to check networks with 'docker ps'

2. Which command syntax correctly runs a ping test inside a running container named webapp?

easy

A. docker exec webapp ping -c 4 google.com

B. docker run webapp ping google.com -c 4

C. docker exec -it ping webapp google.com

D. docker ping exec webapp google.com

Solution

1. Step 1: Understand the purpose of docker exec

   docker exec runs commands inside an existing container. The syntax is docker exec [container] [command].

2. Step 2: Check the correct ping command format

   ping -c 4 google.com sends 4 ping packets. docker exec webapp ping -c 4 google.com uses correct syntax. Options A, B, and C misuse docker run or command order.

3. Final Answer:

   docker exec webapp ping -c 4 google.com -> Option A

4. Quick Check:

   Run ping inside container = docker exec [container] ping [options] [OK]

Hint: Use 'docker exec <container> ping -c 4 <host>' to test connectivity [OK]

Common Mistakes:

• Using 'docker run' instead of 'docker exec' for existing containers
• Incorrect order of command arguments
• Missing '-c' option for ping count

3. You run docker network inspect bridge and see containers connected with IPs. What does this output tell you?

medium

A. It shows which containers are connected to the default bridge network and their IP addresses.

B. It lists all running containers and their CPU usage.

C. It displays the Docker daemon logs for network errors.

D. It shows the firewall rules blocking container traffic.

Solution

1. Step 1: Understand 'docker network inspect bridge'

   This command shows detailed info about the 'bridge' network, including connected containers and their IPs.

2. Step 2: Differentiate from other outputs

   It does not show container CPU usage, daemon logs, or firewall rules. Those require other commands.

3. Final Answer:

   It shows which containers are connected to the default bridge network and their IP addresses. -> Option A

4. Quick Check:

   Network inspect = container connections + IPs [OK]

Hint: Inspect bridge network to see container IPs and connections [OK]

Common Mistakes:

• Confusing network inspect with container stats
• Expecting logs or firewall info from network inspect
• Misreading IP info as CPU or memory data

4. A container cannot reach the internet. You try docker exec app ping 8.8.8.8 but get 'ping: command not found'. What is the best next step?

medium

A. Run docker network disconnect bridge app to reset network.

B. Restart the Docker daemon with systemctl restart docker immediately.

C. Remove and recreate the container without any changes.

D. Install ping inside the container or use docker exec app apt-get update && apt-get install iputils-ping.

Solution

1. Step 1: Understand the error 'ping: command not found'

   This means the ping tool is missing inside the container's OS.

2. Step 2: Fix by installing ping inside the container

   Use package manager commands inside the container to install ping, e.g., apt-get install iputils-ping.

3. Final Answer:

   Install ping inside the container or use docker exec app apt-get update && apt-get install iputils-ping. -> Option D

4. Quick Check:

   Missing ping tool = install it inside container [OK]

Hint: If ping missing, install it inside container first [OK]

Common Mistakes:

• Restarting Docker daemon without checking container tools
• Removing container without diagnosing issue
• Disconnecting network without confirming problem source

5. You have two containers, frontend and backend, on a user-defined Docker network mynetwork. The frontend cannot connect to backend by container name. What is the best way to fix this?

hard

A. Use the default bridge network instead of a user-defined network for better name resolution.

B. Ensure both containers are connected to the same user-defined network mynetwork and use container names for communication.

C. Add --link backend option when starting the frontend container to enable name resolution.

D. Restart Docker daemon to refresh network settings.

Solution

1. Step 1: Understand Docker network name resolution

   Containers on the same user-defined network can resolve each other by container name automatically.

2. Step 2: Verify both containers are on the same network

   If frontend cannot reach backend by name, likely they are not on the same network. Connect both to mynetwork.

3. Step 3: Avoid deprecated or less flexible methods

   Using --link is deprecated and default bridge network does not support automatic name resolution.

4. Final Answer:

   Ensure both containers are connected to the same user-defined network mynetwork and use container names for communication. -> Option B

5. Quick Check:

   Same user network = container name resolution works [OK]

Hint: Connect containers to same user network for name resolution [OK]

Common Mistakes:

• Using default bridge network expecting name resolution
• Relying on deprecated --link option
• Restarting Docker daemon without checking network setup