Container DNS and service discovery in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a container with a custom network for DNS-based service discovery.

Docker

docker network create my_network

docker run --rm -d --name webapp --network [1] nginx

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

Abridge

Bnone

Chost

Dmy_network

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'bridge' network which does not enable container name DNS resolution.

Using 'host' network which bypasses Docker networking.

Forgetting to create the custom network before running the container.

Using --network my_network connects the container to the custom network my_network, enabling DNS-based service discovery within that network.

2fill in blank

medium

Complete the command to run a second container that can reach the first container by its name using Docker DNS.

Docker

docker run --rm -it --network my_network busybox ping [1]

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

Alocalhost

B127.0.0.1

Cwebapp

Dgoogle.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'localhost' which refers to the current container.

Using an external domain like 'google.com' which is unrelated to Docker DNS.

Using the container name webapp allows the second container to resolve and ping the first container via Docker's internal DNS on the same network.

3fill in blank

hard

Fix the error in the Docker Compose service definition to enable service discovery by service name.

Docker

version: '3'
services:
  app:
    image: myapp
    networks:
      - [1]
networks:
  mynet:
    driver: bridge

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

Amynet

Bbridge

Cdefault

Dhost

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'bridge' which is a driver, not the network name.

Using 'host' which disables Docker network isolation.

The service must be connected to the defined network mynet to enable DNS-based service discovery within that network.

4fill in blank

hard

Fill both blanks to create a Docker Compose file that defines two services communicating via service names on a custom network.

Docker

version: '3'
services:
  frontend:
    image: frontend_image
    networks:
      - [1]
  backend:
    image: backend_image
    networks:
      - [2]
networks:
  appnet:
    driver: bridge

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

Aappnet

Bbridge

Cdefault

Dhost

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning different networks to services prevents DNS-based communication.

Using 'default' network name which is not defined explicitly.

Both services must be connected to the same custom network appnet to communicate using service names.

5fill in blank

hard

Fill all three blanks to create a Docker Compose snippet that uses environment variables to configure the backend service URL for the frontend service.

Docker

version: '3'
services:
  frontend:
    image: frontend_image
    environment:
      - BACKEND_URL=http://[1]:[2]
    networks:
      - [3]
  backend:
    image: backend_image
    networks:
      - appnet
networks:
  appnet:
    driver: bridge

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

Abackend

B8080

Cappnet

Dfrontend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'frontend' as hostname instead of 'backend'.

Using wrong port number or network name.

The frontend uses the service name backend and port 8080 to reach the backend. Both services are on the appnet network for DNS resolution.

Practice

1. What does Docker's built-in DNS service provide for containers on the same user-defined network?

easy

A. It assigns static IP addresses to containers.

B. It allows containers to find each other by their container names.

C. It disables network communication between containers.

D. It automatically updates container images.

Solution

1. Step 1: Understand Docker user-defined networks

   User-defined networks enable containers to communicate using DNS names instead of IP addresses.

2. Step 2: Role of Docker DNS service

   Docker's DNS service resolves container names to their current IP addresses on the same network.

3. Final Answer:

   It allows containers to find each other by their container names. -> Option B

4. Quick Check:

   Docker DNS = container name resolution [OK]

Hint: Containers use names, not IPs, to find each other on user networks [OK]

Common Mistakes:

• Thinking Docker assigns static IPs by default
• Believing DNS disables container communication
• Confusing DNS with image updates

2. Which Docker command correctly creates a user-defined network named my-net to enable container DNS service discovery?

easy

A. docker network create my-net

B. docker create network my-net

C. docker network new my-net

D. docker network build my-net

Solution

1. Step 1: Identify correct Docker network creation syntax

   The correct command to create a network is docker network create <name>.

2. Step 2: Verify options

   The options using 'docker network new', 'docker create network', and 'docker network build' have incorrect syntax; only docker network create my-net matches Docker syntax.

3. Final Answer:

   docker network create my-net -> Option A

4. Quick Check:

   Correct command = docker network create [OK]

Hint: Use 'docker network create' to make user-defined networks [OK]

Common Mistakes:

• Using 'docker create network' instead of 'docker network create'
• Confusing 'new' or 'build' with network creation
• Omitting the network name

3. Given two containers web and db on the same user-defined network, what will be the output of docker exec web ping -c 1 db if db is running?

medium

A. Ping succeeds showing db IP address and response time.

B. Ping fails with 'unknown host db' error.

C. Ping succeeds but shows IP of web container.

D. Ping command is not available inside containers.

Solution

1. Step 1: Understand container DNS on user-defined networks

   Containers on the same user-defined network can resolve each other's names to IP addresses.

2. Step 2: Effect of ping command

   Running ping from web to db will resolve db to its IP and send ICMP packets successfully if db is running.

3. Final Answer:

   Ping succeeds showing db IP address and response time. -> Option A

4. Quick Check:

   Container DNS + ping = success if target running [OK]

Hint: Ping container name on same network to test DNS and connectivity [OK]

Common Mistakes:

• Assuming ping fails due to IP changes
• Confusing container names with hostnames outside network
• Believing ping is unavailable inside containers

4. You created two containers on a user-defined network, but web cannot resolve db by name. What is the most likely cause?

medium

A. The web container has no network interface.

B. The db container is stopped.

C. Docker daemon is not running.

D. Containers are on different Docker networks.

Solution

1. Step 1: Check network configuration

   Containers must be on the same user-defined network for DNS name resolution to work.

2. Step 2: Analyze failure cause

   If web cannot resolve db, they are likely on different networks, so DNS does not link them.

3. Final Answer:

   Containers are on different Docker networks. -> Option D

4. Quick Check:

   Different networks block DNS resolution [OK]

Hint: Ensure containers share the same user-defined network for DNS [OK]

Common Mistakes:

• Assuming stopped container causes DNS failure (it causes ping failure)
• Thinking Docker daemon status affects container DNS directly
• Believing container can lack network interface if running

5. You want to deploy a multi-container app with a web server and database. You want the web server to always connect to the database by name, even if the database container restarts and gets a new IP. Which Docker setup ensures this?

hard

A. Run containers on separate networks and link them with --link option.

B. Use the default bridge network and hardcode the database container's IP address in the web server config.

C. Create a user-defined network and run both containers attached to it; use the database container's name in the web server config.

D. Assign static IPs manually to both containers on the default network.

Solution

1. Step 1: Understand service discovery with user-defined networks

   User-defined networks provide automatic DNS resolution by container name, even if IPs change.

2. Step 2: Evaluate options for reliable name-based connection

   Create a user-defined network and run both containers attached to it; use the database container's name in the web server config. uses user-defined network and container names, which is the recommended way for service discovery.

3. Step 3: Why other options fail

   Default bridge network lacks automatic DNS; hardcoding IPs breaks if IP changes; --link is deprecated; static IPs are complex and error-prone.

4. Final Answer:

   Create a user-defined network and run both containers attached to it; use the database container's name in the web server config. -> Option C

5. Quick Check:

   User-defined network + container names = reliable service discovery [OK]

Hint: Use user-defined network and container names for stable service discovery [OK]

Common Mistakes:

• Hardcoding IP addresses instead of using container names
• Using default bridge network without DNS support
• Relying on deprecated --link option
• Manually assigning static IPs unnecessarily