Network inspection and debugging in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Inspect Docker container network details

You run the command docker network inspect bridge. What key information does this command provide about the Docker bridge network?

A It displays the CPU and memory usage of all running containers.

B It shows the containers connected to the bridge network, their IP addresses, and network settings.

C It lists all Docker images available on the host machine.

D It removes the bridge network and disconnects all containers.

💡 Hint

Think about what 'inspect' means in Docker networking context.

The docker network inspect command shows detailed information about a specific network, including connected containers and their IP addresses.

💻 Command Output

intermediate

Diagnose container connectivity with ping

You have two containers on the same user-defined Docker network. You run docker exec container1 ping -c 3 container2. What output do you expect if the network is correctly configured?

A 3 packets transmitted, 3 received, 0% packet loss, time=XYZ ms

B ping: unknown host container2

C Permission denied

D connect: Network is unreachable

💡 Hint

Successful ping means packets reach the target container.

If containers are on the same network and reachable, ping will show all packets received with no loss.

❓ Troubleshoot

advanced

Troubleshoot Docker container cannot reach external internet

A container cannot access the internet, but other containers on the same host can. Which Docker network setting is most likely causing this?

A The container is connected to a user-defined bridge network without proper DNS or gateway settings.

B The container is running with the <code>--network host</code> option.

C The container has <code>--restart always</code> set.

D The container is using a volume mount for /etc/hosts.

💡 Hint

Check network isolation and DNS configuration.

User-defined bridge networks require proper DNS and gateway settings for internet access. Missing these can block external connectivity.

✅ Best Practice

advanced

Best practice for inspecting live network traffic of a container

You want to capture and analyze live network traffic inside a running Docker container. Which approach is best?

A Modify the container's Dockerfile to add network capture tools and rebuild.

B Restart the container with <code>--net=host</code> to capture traffic on the host.

C Use <code>docker logs</code> to view network packets.

D Use <code>docker exec</code> to run <code>tcpdump</code> inside the container if installed.

💡 Hint

Think about capturing traffic without restarting or rebuilding.

Running tcpdump inside the container via docker exec allows live capture without downtime.

🔀 Workflow

expert

Order the steps to debug a Docker container network issue

Arrange these steps in the correct order to debug why a Docker container cannot connect to another container on the same network.

A 1,3,2,4

B 2,1,3,4

C 1,2,3,4

D 3,1,2,4

💡 Hint

Start with network configuration, then test connectivity, then check host settings, then logs.

First confirm network setup, then test connectivity, then check host firewall, finally check logs for app errors.

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