DNS resolution between containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does Docker DNS resolution work between containers on the same user-defined network?

When two containers are connected to the same user-defined Docker network, how does one container resolve the hostname of the other container?

A Docker uses the host machine's DNS settings to resolve container names.

B Containers must use external DNS servers like 8.8.8.8 to resolve each other's hostnames.

C Containers cannot resolve each other's hostnames unless you manually edit the /etc/hosts file inside each container.

D Docker automatically creates an internal DNS server that resolves container names to their IP addresses within that network.

💡 Hint

Think about how Docker manages networking internally for containers on the same network.

Docker creates an internal DNS server for each user-defined network. This DNS server automatically resolves container names to their IP addresses, allowing containers to communicate by name.

💻 Command Output

intermediate

What is the output of 'docker network inspect' for container name resolution?

Given two containers web and db connected to the same user-defined network mynetwork, what key in the output of docker network inspect mynetwork shows the container names and their IP addresses?

Docker

docker network inspect mynetwork

A The <code>Containers</code> section lists container IDs, names, and IPv4 addresses.

B The <code>Services</code> section lists container names and IPs.

C The <code>Endpoints</code> section shows container names and IPs.

D The <code>Hosts</code> section shows container names and IPs.

💡 Hint

Look for the section that maps containers to their network details.

The Containers section in the network inspect output shows each container's ID, name, and IP address on that network.

❓ Configuration

advanced

How to enable DNS resolution between containers on different Docker networks?

You have two containers on different user-defined Docker networks. You want them to resolve each other's hostnames. Which configuration allows this?

A Manually add entries to each container's /etc/hosts file.

B Connect both containers to a common user-defined network in addition to their existing networks.

C Use the <code>--link</code> flag to link containers across networks.

D Set the <code>--dns</code> flag to the host's DNS server when running containers.

💡 Hint

Think about how Docker networks isolate containers and how to bridge that isolation.

Containers must share at least one common user-defined network for Docker's internal DNS to resolve their names. Connecting both containers to a shared network enables this.

❓ Troubleshoot

advanced

Why does DNS resolution fail between containers on the default bridge network?

You have two containers on the default bridge network. One container tries to ping the other by container name but fails. Why?

A The containers need to have the <code>--dns</code> flag set to enable DNS resolution.

B The containers must be restarted to refresh DNS entries.

C The default bridge network does not provide automatic DNS resolution for container names.

D Docker requires containers to be on the same host to resolve names.

💡 Hint

Consider the difference between the default bridge network and user-defined networks.

The default bridge network does not support automatic DNS resolution by container name. Only user-defined networks provide this feature.

✅ Best Practice

expert

What is the best practice for reliable DNS resolution between containers in a multi-host Docker Swarm cluster?

In a Docker Swarm cluster with multiple hosts, how should you configure services to ensure DNS resolution between containers works reliably?

A Use an overlay network for the services so Docker's internal DNS resolves service names across hosts.

B Manually configure each host's /etc/hosts file with all container IPs.

C Use the default bridge network and rely on external DNS servers.

D Assign static IP addresses to containers and use those IPs directly.

💡 Hint

Think about how Docker Swarm manages networking across multiple hosts.

Docker Swarm uses overlay networks to connect containers across hosts. The internal DNS resolves service names within these overlay networks, enabling reliable cross-host communication.