Debugging network issues in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to list all Docker networks.

Docker

docker network [1]

Drag options to blanks, or click blank then click option'

Als

Bcreate

Crm

Dinspect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'create' instead of 'ls' will try to make a new network.

'rm' deletes a network, not lists it.

The docker network ls command lists all Docker networks on your system.

2fill in blank

medium

Complete the command to inspect details of a Docker network named 'my_net'.

Docker

docker network [1] my_net

Drag options to blanks, or click blank then click option'

Arm

Binspect

Cls

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rm' deletes the network.

'ls' lists networks but doesn't show details.

The docker network inspect my_net command shows detailed information about the network 'my_net'.

3fill in blank

hard

Fix the error in the command to ping a container named 'webapp' from the 'client' container.

Docker

docker exec [1] client ping webapp

Drag options to blanks, or click blank then click option'

A-d

B-p

C-rm

D-it

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-d' runs in detached mode, not interactive.

'-rm' is not a valid docker exec option.

The docker exec -it client ping webapp command uses the -it option to allow interactive terminal access to run the ping command inside the 'client' container.

4fill in blank

hard

Fill both blanks to create a Docker network named 'test_net' with a specific driver.

Docker

docker network [1] test_net --driver [2]

Drag options to blanks, or click blank then click option'

Acreate

Brm

Cbridge

Doverlay

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rm' deletes networks, not creates.

Choosing 'overlay' is for multi-host networks, not local.

Use docker network create to make a new network, and bridge is a common driver for local networks.

5fill in blank

hard

Fill all three blanks to disconnect a container named 'db' from a network named 'backend_net'.

Docker

docker network [1] [2] [3]

Drag options to blanks, or click blank then click option'

Adisconnect

Bdb

Cbackend_net

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'connect' adds a container to a network, not removes.

Swapping container and network names causes errors.

The command docker network disconnect backend_net db removes the container 'db' from the 'backend_net' network.

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