Container to container communication in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How do Docker containers communicate by default?

When you start two Docker containers without any special network settings, how do they communicate with each other?

A They cannot communicate unless you explicitly connect them to the same user-defined network.

B They communicate automatically using the host's IP address and ports.

C They share the same network namespace and can use localhost to communicate.

D They communicate through a default bridge network with automatic DNS resolution.

💡 Hint

Think about Docker's default network isolation and how containers are separated.

By default, Docker containers are connected to the default bridge network, but they cannot resolve each other by container name or communicate easily unless connected to a user-defined network.

💻 Command Output

intermediate

Output of Docker network inspect command

What is the output type of the command docker network inspect bridge?

Docker

docker network inspect bridge

A A table showing container IDs and their IP addresses.

B A plain text list of container names connected to the bridge network.

C An error message saying the bridge network does not exist.

D A JSON array describing the bridge network including connected containers and settings.

💡 Hint

Docker inspect commands usually return detailed information in a structured format.

The docker network inspect command returns detailed JSON data about the network, including containers connected and network settings.

❓ Configuration

advanced

Correct Docker Compose network configuration for container communication

Which Docker Compose snippet correctly enables two services to communicate by service name?

Docker

version: '3.8'
services:
  app:
    image: myapp
  db:
    image: mydb

A Add a user-defined network and connect both services to it explicitly.

B Use the default bridge network without any changes.

C Expose ports on both services without defining a network.

D Use host networking mode for both services.

💡 Hint

Think about how Docker Compose handles networks and service name resolution.

Docker Compose creates a default network for all services in the same file, but explicitly defining a user network ensures clear communication and control.

❓ Troubleshoot

advanced

Why can't containers communicate despite being on the same user-defined network?

You created a user-defined network and connected two containers to it, but they still cannot ping each other by container name. What is the most likely cause?

A The containers do not have the correct DNS settings to resolve container names.

B The containers are running on different Docker hosts without overlay network setup.

C Containers are using different network drivers incompatible with each other.

D The containers have firewall rules blocking ICMP packets.

💡 Hint

Consider if the containers are on the same physical or virtual machine.

Containers on different Docker hosts need an overlay network to communicate by name; user-defined bridge networks work only on the same host.

🔀 Workflow

expert

Order the steps to enable container-to-container communication using Docker networks

Arrange the steps in the correct order to allow two containers to communicate by name using Docker networks.

A 2,1,3,4

B 1,3,2,4

C 1,2,3,4

D 2,3,1,4

💡 Hint

Think about creating the network before running containers and then using names.

You must first create the network, then run containers attached to it, and finally use their names to communicate.