Container to container communication in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to start a Docker container named 'webapp'.

Docker

docker run --name [1] -d nginx

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

Acache

Bwebapp

Cdatabase

Dbackend

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different container name than 'webapp'.

The --name option assigns the container a name. Here, 'webapp' is the container name.

2fill in blank

medium

Complete the Docker network creation command to allow containers to communicate.

Docker

docker network create [1]

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

Aisolated_net

Bbridge

Chost

Dnone

Attempts:

3 left

💡 Hint

Common Mistakes

Using default network driver names instead of a custom network name.

Creating a custom network named 'isolated_net' allows containers to communicate on that network.

3fill in blank

hard

Fix the error in the command to connect a container named 'db' to the network 'isolated_net'.

Docker

docker network [1] isolated_net db

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

Aconnect

Bjoin

Cattach

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'attach' or 'link' instead of 'connect'.

The correct command to connect a container to a network is docker network connect <network> <container>.

4fill in blank

hard

Fill both blanks to run two containers on the same network 'isolated_net'.

Docker

docker run -d --name webapp --network [1] nginx
docker run -d --name db --network [2] mysql

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

Aisolated_net

Bbridge

Chost

Dnone

Attempts:

3 left

💡 Hint

Common Mistakes

Using different networks for each container.

Both containers must be on the same network 'isolated_net' to communicate.

5fill in blank

hard

Fill all three blanks to create a Docker Compose service for 'webapp' that connects to the 'isolated_net' network.

Docker

services:
  webapp:
    image: nginx
    networks:
      - [1]
networks:
  [2]:
    driver: [3]

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

Aisolated_net

Bbridge

Chost

Dnone

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'host' or 'none' as the driver.

The service uses the 'isolated_net' network, which is defined with the 'bridge' driver in Docker Compose.

Practice

1. What is the simplest way for two Docker containers to communicate on the same host?

easy

A. Use a shared volume for communication

B. Use the host's IP address directly inside containers

C. Run containers with --network=none and use port mapping

D. Connect both containers to the same user-defined Docker network

Solution

1. Step 1: Understand Docker networking basics

   Docker containers on the same user-defined network can resolve each other by container name and communicate directly.

2. Step 2: Compare other options

   Using host IP or port mapping is less direct and --network=none disables networking. Shared volumes are for file sharing, not network communication.

3. Final Answer:

   Connect both containers to the same user-defined Docker network -> Option D

4. Quick Check:

   Same network enables container communication [OK]

Hint: Use a user-defined network for easy container communication [OK]

Common Mistakes:

• Using default bridge network without user-defined network
• Trying to communicate via host IP without port mapping
• Assuming shared volumes enable network communication

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 add 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: Check other options for syntax errors

   Options B, C, and D use incorrect verbs or commands not recognized by Docker.

3. Final Answer:

   docker network create mynetwork -> Option A

4. Quick Check:

   Correct command syntax = docker network create [OK]

Hint: Remember: 'docker network create' is the correct syntax [OK]

Common Mistakes:

• Using 'docker create network' instead of 'docker network create'
• Using 'docker network new' which is invalid
• Trying 'docker network add' which does not exist

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

medium

A. ping: unknown host db

B. PING db (172.18.0.2): 56 data bytes\n64 bytes from 172.18.0.2: icmp_seq=0 ttl=64 time=0.123 ms\n\n--- db ping statistics ---\n1 packets transmitted, 1 packets received, 0% packet loss

C. connect: Network is unreachable

D. Request timed out

Solution

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

   Containers on the same user-defined network can resolve each other by container name, so 'db' resolves to its IP.

2. Step 2: Predict ping command output

   Since 'db' is running and reachable, ping will succeed with a response showing bytes received and 0% packet loss.

3. Final Answer:

   Successful ping output with 1 packet transmitted and received -> Option B

4. Quick Check:

   Same network + running container = ping success [OK]

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

Common Mistakes:

• Assuming default bridge network supports container name resolution
• Expecting ping to fail without port mapping
• Confusing network unreachable with DNS resolution failure

4. You created two containers on a user-defined network but web cannot reach db. Which command helps you debug the network connectivity issue?

medium

A. docker ps -a

B. docker container logs web

C. docker network inspect mynetwork

D. docker volume ls

Solution

1. Step 1: Identify the tool to check network details

   docker network inspect shows connected containers and network settings, useful for debugging connectivity.

2. Step 2: Evaluate other commands

   docker container logsdocker ps -a lists containers; docker volume ls lists volumes, none show network details.

3. Final Answer:

   docker network inspect mynetwork -> Option C

4. Quick Check:

   Inspect network to debug connectivity [OK]

Hint: Use 'docker network inspect' to check container connections [OK]

Common Mistakes:

• Checking container logs instead of network info
• Listing volumes which are unrelated
• Assuming 'docker ps' shows network details

5. You want two containers, app and cache, to communicate securely without exposing ports to the host. Which approach achieves this?

hard

A. Connect both containers to the same user-defined network and communicate via container names

B. Expose ports on both containers and use localhost to connect

C. Use host networking mode for both containers

D. Share a volume and write/read data files for communication

Solution

1. Step 1: Understand secure container communication

   Using a user-defined network allows containers to communicate internally without exposing ports to the host, keeping communication private.

2. Step 2: Evaluate other options for security

   Exposing ports or using host networking exposes services outside the host or risks port conflicts. Sharing volumes is not network communication.

3. Final Answer:

   Connect both containers to the same user-defined network and communicate via container names -> Option A

4. Quick Check:

   User-defined network keeps communication internal and secure [OK]

Hint: Use user-defined network to avoid exposing ports externally [OK]

Common Mistakes:

• Exposing ports unnecessarily to the host
• Using host network mode which is less secure
• Confusing volume sharing with network communication