DNS resolution between containers in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to create a Docker network named 'mynetwork'.

Docker

docker network create [1]

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

Abridge

Bdefault

Cmynetwork

Dhost

Attempts:

3 left

💡 Hint

Common Mistakes

Using default network names like 'bridge' or 'host' which are predefined.

The command docker network create mynetwork creates a new user-defined network named 'mynetwork'. This network allows containers to communicate by name.

2fill in blank

medium

Complete the command to run a container named 'web' attached to the 'mynetwork' network.

Docker

docker run -d --name web --network [1] nginx

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

Abridge

Bmynetwork

Chost

Dnone

Attempts:

3 left

💡 Hint

Common Mistakes

Using default networks like 'bridge' which may not support container name resolution as expected.

Using --network mynetwork attaches the container to the user-defined network 'mynetwork', enabling DNS resolution between containers on that network.

3fill in blank

hard

Fix the error in the command to ping the 'web' container from another container on the same network.

Docker

docker run --rm --network mynetwork alpine ping -c 3 [1]

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

Aweb

Blocalhost

C127.0.0.1

Dgoogle.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'localhost' or '127.0.0.1' which refer to the current container itself.

Using the container name 'web' allows DNS resolution within the 'mynetwork' network, so the ping command reaches the correct container.

4fill in blank

hard

Fill both blanks to create a Docker Compose service named 'db' using the 'mysql' image and connect it to the 'mynetwork' network.

Docker

services:
  db:
    image: mysql
    networks:
      - [1]
networks:
  [2]:

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

Amynetwork

Bdefault

Dbridge

Attempts:

3 left

💡 Hint

Common Mistakes

Using different network names in the two places causing network mismatch.

Both blanks should be 'mynetwork' to define and use the same custom network in Docker Compose, enabling DNS resolution between services.

5fill in blank

hard

Fill all three blanks to define a Docker Compose service 'app' that depends on 'db', uses the 'mynetwork' network, and sets an environment variable 'DB_HOST' to the 'db' service name.

Docker

services:
  app:
    image: myapp
    depends_on:
      - [1]
    environment:
      - DB_HOST=[2]
    networks:
      - [3]

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

Adb

Cmynetwork

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect service names or default networks that break DNS resolution.

The 'app' service depends on 'db', sets the environment variable 'DB_HOST' to 'db' so it can connect by name, and uses the 'mynetwork' network for DNS resolution.

Practice

1. What allows Docker containers on the same user-defined network to communicate using container names instead of IP addresses?

easy

A. Using container IP addresses only

B. Manual editing of /etc/hosts file

C. Docker's built-in DNS service

D. Exposing ports to the host machine

Solution

1. Step 1: Understand Docker networking basics

   Docker provides a built-in DNS service for containers on the same user-defined network.

2. Step 2: Recognize how containers resolve names

   Containers use this DNS service to resolve other container names to their IP addresses automatically.

3. Final Answer:

   Docker's built-in DNS service -> Option C

4. Quick Check:

   Docker DNS = container name resolution [OK]

Hint: User-defined networks enable DNS by default [OK]

Common Mistakes:

• Assuming default bridge network supports DNS resolution by name
• Thinking manual /etc/hosts editing is needed
• Using IP addresses instead of container names

2. Which Docker command correctly creates a user-defined network named mynetwork?

easy

A. docker network create mynetwork

B. docker create network mynetwork

C. docker network new mynetwork

D. docker network build mynetwork

Solution

1. Step 1: Recall Docker network creation syntax

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

2. Step 2: Match the command with options

   docker network create mynetwork matches the correct syntax exactly.

3. Final Answer:

   docker network create mynetwork -> Option A

4. Quick Check:

   docker network create = create network [OK]

Hint: Use 'docker network create' to make new networks [OK]

Common Mistakes:

• Using 'docker create network' which is invalid syntax
• Confusing 'new' or 'build' with 'create'
• 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: unknown host db

B. PING db (172.18.0.2): 56 data bytes
64 bytes from 172.18.0.2: icmp_seq=0 ttl=64 time=0.123 ms

--- db ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss

C. Error: container 'db' not found

D. Request timed out

Solution

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

   Containers on the same user-defined network resolve each other's names automatically.

2. Step 2: Predict ping command output

   Since db is running and reachable, ping will succeed and show reply details.

3. Final Answer:

   Successful ping output with reply from db's IP -> Option B

4. Quick Check:

   Same network + running container = ping success [OK]

Hint: Ping container name works if on same user network [OK]

Common Mistakes:

• Expecting 'unknown host' on user-defined network
• Confusing default bridge network behavior
• Assuming container must expose ports for ping

4. You created two containers on the default bridge network but ping db from web returns "unknown host db". What is the likely cause?

medium

A. The ping command is not installed in the web container

B. The db container is stopped

C. The web container has no network access

D. Containers are not on a user-defined network, so DNS resolution by name fails

Solution

1. Step 1: Recall default bridge network limitations

   Default bridge network does not support automatic DNS resolution by container name.

2. Step 2: Analyze the error message

   "unknown host db" means DNS lookup failed, likely because containers are not on a user-defined network.

3. Final Answer:

   Containers are not on a user-defined network, so DNS resolution by name fails -> Option D

4. Quick Check:

   Default bridge lacks DNS by name [OK]

Hint: Use user-defined networks for container name DNS [OK]

Common Mistakes:

• Assuming default bridge network supports DNS names
• Ignoring network configuration
• Blaming container stopped state without checking

5. You want two containers, app and cache, to communicate by name. You create a user-defined network mynet. Which sequence of commands correctly sets this up?

hard

A. docker network create mynet
docker run -d --name app --network mynet myappimage
docker run -d --name cache --network mynet mycacheimage

B. docker run -d --name app myappimage
docker run -d --name cache mycacheimage
docker network create mynet

C. docker network create mynet
docker run -d --name app myappimage
docker network connect mynet cache

D. docker run -d --name app --network bridge myappimage
docker run -d --name cache --network bridge mycacheimage

Solution

1. Step 1: Create a user-defined network

   Use docker network create mynet to create the network.

2. Step 2: Run containers attached to the network

   Start both containers with --network mynet to enable DNS resolution by name.

3. Final Answer:

   Create network first, then run containers with --network mynet -> Option A

4. Quick Check:

   User-defined network + containers attached = DNS by name [OK]

Hint: Create network first, then run containers on it [OK]

Common Mistakes:

• Running containers before creating network
• Using default bridge network instead of user-defined
• Not attaching containers to the same network