Why DNS in Kubernetes (CoreDNS)? - Purpose & Use Cases

The Big Idea

What if your apps could always find each other without you lifting a finger?

The Scenario

Imagine you have many apps running on different computers, and you want them to talk to each other. Without DNS, you must remember and type each app's exact address every time.

The Problem

This manual way is slow and confusing. If an app moves or changes its address, you must update every place that uses it. This causes mistakes and breaks communication.

The Solution

CoreDNS in Kubernetes automatically handles all app addresses. It gives each app a simple name and keeps track of where it lives. Apps can find each other easily without manual updates.

Before vs After

✗ Before

curl http://10.0.0.15:8080
curl http://10.0.0.20:8080

✓ After

curl http://myapp.default.svc.cluster.local
curl http://otherapp.default.svc.cluster.local

What It Enables

CoreDNS makes app communication smooth and reliable by turning complex addresses into easy names that always work.

Real Life Example

When you deploy a new version of a service, CoreDNS updates the address behind the scenes, so other services keep working without any changes.

Key Takeaways

Manual IP management is slow and error-prone.

CoreDNS automates service discovery with easy names.

This keeps apps connected even when they move or change.

Practice

1. What is the main role of CoreDNS in a Kubernetes cluster?

easy

A. To manage container images in the cluster

B. To provide DNS resolution for services and pods inside the cluster

C. To schedule pods on nodes

D. To monitor cluster resource usage

Solution

1. Step 1: Understand CoreDNS function

   CoreDNS acts as the DNS server inside Kubernetes, resolving names of services and pods.

2. Step 2: Compare with other options

   Managing images, scheduling pods, and monitoring are done by other components like container registries, kube-scheduler, and metrics servers.

3. Final Answer:

   To provide DNS resolution for services and pods inside the cluster -> Option B

4. Quick Check:

   CoreDNS = DNS resolution inside Kubernetes [OK]

Hint: CoreDNS is Kubernetes' internal DNS server [OK]

Common Mistakes:

• Confusing CoreDNS with pod scheduling
• Thinking CoreDNS manages container images
• Assuming CoreDNS monitors resources

2. Which Kubernetes resource holds the CoreDNS configuration?

easy

A. Secret named coredns in default namespace

B. Deployment in kube-system namespace

C. ConfigMap named coredns in kube-system namespace

D. ServiceAccount named coredns in kube-system namespace

Solution

1. Step 1: Identify CoreDNS configuration storage

   The CoreDNS configuration is stored in a ConfigMap named 'coredns' inside the 'kube-system' namespace.

2. Step 2: Eliminate other options

   Deployment runs CoreDNS pods but does not hold config; Secrets store sensitive data; ServiceAccounts manage permissions.

3. Final Answer:

   ConfigMap named coredns in kube-system namespace -> Option C

4. Quick Check:

   CoreDNS config = ConfigMap coredns in kube-system [OK]

Hint: CoreDNS config is a ConfigMap in kube-system [OK]

Common Mistakes:

• Confusing ConfigMap with Secret
• Looking in default namespace instead of kube-system
• Thinking Deployment holds config data

3. Inside a pod, you run nslookup kubernetes.default.svc.cluster.local. What is the expected result?

medium

A. An IP address of the Kubernetes API service

B. An error saying domain not found

C. The pod's own IP address

D. The IP address of the CoreDNS pod

Solution

1. Step 1: Understand the DNS query

   The domain 'kubernetes.default.svc.cluster.local' is the default Kubernetes API service DNS name inside the cluster.

2. Step 2: Determine expected DNS resolution

   CoreDNS resolves this to the IP address of the Kubernetes API service, not the pod or CoreDNS pod IP.

3. Final Answer:

   An IP address of the Kubernetes API service -> Option A

4. Quick Check:

   Default service DNS resolves to API server IP [OK]

Hint: Default service DNS resolves to API server IP [OK]

Common Mistakes:

• Expecting pod IP instead of service IP
• Assuming CoreDNS pod IP is returned
• Thinking query fails inside pod

4. You modified the CoreDNS ConfigMap but DNS queries inside pods still fail. What is the most likely fix?

medium

A. Restart the CoreDNS pods to reload the new config

B. Delete all pods in the default namespace

C. Reinstall Kubernetes cluster

D. Change pod DNSPolicy to Default

Solution

1. Step 1: Recognize CoreDNS config reload behavior

   CoreDNS pods need to reload the ConfigMap changes, which usually requires restarting the pods.

2. Step 2: Evaluate other options

   Deleting unrelated pods or reinstalling the cluster is unnecessary; changing DNSPolicy may break cluster DNS.

3. Final Answer:

   Restart the CoreDNS pods to reload the new config -> Option A

4. Quick Check:

   CoreDNS pods restart reloads ConfigMap changes [OK]

Hint: Restart CoreDNS pods after ConfigMap changes [OK]

Common Mistakes:

• Deleting unrelated pods expecting DNS fix
• Reinstalling cluster unnecessarily
• Changing pod DNSPolicy incorrectly

5. You want to add a custom DNS entry for myapp.local inside your Kubernetes cluster using CoreDNS. Which step is required?

hard

A. Deploy a pod named myapp.local with a static IP

B. Create a new Service named myapp.local in default namespace

C. Modify the kubelet to add myapp.local to /etc/hosts on all nodes

D. Add a hosts plugin entry in the CoreDNS ConfigMap with myapp.local mapping

Solution

1. Step 1: Understand how to add custom DNS entries in CoreDNS

   CoreDNS supports a hosts plugin where you can add static hostname-to-IP mappings in its ConfigMap.

2. Step 2: Evaluate other options

   Creating a Service named myapp.local won't create a DNS entry for that domain; modifying kubelet hosts is manual and not scalable; pod names don't create DNS entries.

3. Final Answer:

   Add a hosts plugin entry in the CoreDNS ConfigMap with myapp.local mapping -> Option D

4. Quick Check:

   Custom DNS via hosts plugin in CoreDNS ConfigMap [OK]

Hint: Use hosts plugin in CoreDNS ConfigMap for custom DNS [OK]

Common Mistakes:

• Thinking Service name creates arbitrary DNS names
• Modifying node hosts files manually
• Assuming pod names create DNS entries