Debugging network issues in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify the output of a Docker network inspect command

What is the output of the following command when run on a Docker network named my_bridge that has two containers connected?

Docker

docker network inspect my_bridge

A Error: No such network: my_bridge

B [{"Name":"my_bridge","Id":"abc123","Containers":{}}]

C [{"Name":"bridge","Id":"abc123","Containers":{"container1":{"Name":"web","IPv4Address":"172.18.0.2/16"}}}]

D [{"Name":"my_bridge","Id":"abc123","Containers":{"container1":{"Name":"web","IPv4Address":"172.18.0.2/16"},"container2":{"Name":"db","IPv4Address":"172.18.0.3/16"}}}]

💡 Hint

Check the network name and container connections.

The inspect command shows detailed info about the network, including connected containers and their IP addresses.

❓ Troubleshoot

intermediate

Troubleshoot container connectivity failure

A container cannot reach another container by its service name on a user-defined Docker network. Which of the following is the most likely cause?

A The containers have the same IP address assigned manually.

B The containers are on different user-defined networks without a shared network.

C The Docker daemon is not running.

D The container's CPU limit is too low.

💡 Hint

Service name resolution works only within the same user-defined network.

Containers must be on the same user-defined network to resolve each other by service name. Different networks isolate DNS resolution.

❓ Configuration

advanced

Correct Docker Compose network configuration for service communication

Given this Docker Compose snippet, which option correctly configures the network so that app and db services can communicate by service name?

Docker

version: '3.8'
services:
  app:
    image: myapp
    networks:
      - backend
  db:
    image: postgres
    networks:
      - backend
networks:
  backend:
    driver: bridge

A Keep the configuration as is; both services are on the same user-defined network 'backend'.

B Remove the networks section; Docker Compose will use the default network automatically.

C Change the network driver to 'host' to allow communication.

D Assign static IPs to both services on different networks.

💡 Hint

User-defined bridge networks enable service name resolution.

Both services are connected to the same user-defined bridge network 'backend', allowing them to communicate by service name.

✅ Best Practice

advanced

Best practice for isolating container network traffic

Which option is the best practice to isolate network traffic between two groups of containers in Docker?

A Create two separate user-defined bridge networks and connect containers to their respective networks.

B Use the default bridge network and rely on container names for isolation.

C Assign all containers to the host network driver.

D Use static IP addresses on the default network to separate traffic.

💡 Hint

User-defined networks provide better isolation than the default bridge.

Separate user-defined bridge networks isolate container traffic and DNS resolution, improving security and organization.

🔀 Workflow

expert

Order the steps to debug a Docker container network connectivity issue

Put these steps in the correct order to debug why a container cannot reach another container by service name on a user-defined network.

A 2,1,4,3

B 2,3,1,4

C 1,2,3,4

D 1,2,4,3

💡 Hint

Start by verifying network membership before testing connectivity and DNS.

First confirm network membership, then inspect details, test IP connectivity, and finally test DNS resolution.