STABForge

Software Tech. Accelerator Blog

CoreDNS in Kubernetes: 5 Powerful Tips for Reliable DNS Setup

Posted on by Monika
coredns

Introduction

When deploying applications on Kubernetes, one of the most essential components that often works silently in the background is DNS (Domain Name System). clusters use this dns plugin as the default DNS service for service discovery and internal communication.

 What is CoreDNS?

It is a flexible and extensible DNS server written in Go. In  it acts as a DNS service that allows pods to discover each other using DNS names rather than IP addresses.

It was introduced as the default DNS server in v1.13, replacing kube-dns due to its performance and flexibility.

 Key Features:

  • Lightweight and fast

  • Modular (plugin-based architecture)

  • Natively integrates with system

  • Supports service discovery

 Why DNS is Important in Kubernetes?

In  services are dynamic — pods come and go, IPs change. But service names remain consistent.

  • Translating service names to IPs

  • Enabling pod-to-pod and pod-to-service communication

  • Load balancing DNS queries

How CoreDNS Works

It runs as a Deployment in the kube-system namespace. It listens to DNS requests made by pods and resolves them based on the current  service and pod metadata.

Here’s the general flow:

  1. A pod makes a DNS query to resolve my-service.default.svc.cluster.local.

  2. It receives this query on port 53 (UDP/TCP).

  3. It checks  API to resolve the service name.

  4. Responds with the ClusterIP of the service.

coredns in kubernetes

 Configuration File (Corefile)

The CoreDNS configuration is stored in a file named Corefile, usually located in the ConfigMap:

kubectl -n kube-system get configmap coredns -o yaml

 Example :

.:53 {
errors
health
kubernetes cluster.local in-addr.arpa ip6.arpa {
pods insecure
fallthrough in-addr.arpa ip6.arpa
}
prometheus :9153
forward . /etc/resolv.conf
cache 30
loop
reload
loadbalance
}

 Explanation:

  • .53: CoreDNS listens on port 53

  • kubernetes cluster.local: Enable Kubernetes service discovery

  • forward: Forward unresolved queries to upstream DNS (like Google)

  • cache: Improves performance by caching

  • prometheus: Metrics endpoint

 Installing/Checking CoreDNS

CoreDNS is installed by default in most  setups via kubeadm or managed services like EKS, GKE, AKS.

To check the deployment:

kubectl get pods -n kube-system -l k8s-app=kube-dns

To restart CoreDNS:

kubectl rollout restart deployment coredns -n kube-system

Use Cases

Use Case Description
 Service Discovery Pods find other services using DNS
External Name Services Mapping to external DNS names
 Monitoring Expose metrics via Prometheus
Load Balancing Round-robin DNS load balancing
 Custom DNS Plugins Add custom behaviors like rewrites, filtering
apiVersion: v1
kind: Pod
metadata:
name: dnsutils
namespace: default
spec:
containers:
- name: dnsutils
image: busybox
command:
- sleep
- "3600"
restartPolicy: Always

Use it to test DNS queries:

kubectl exec -it dnsutils -- nslookup .default

 Debugging  Issues

1. Check Logs:

kubectl logs -n kube-system -l k8s-app=kube-dns

2. Check Corefile syntax:

Ensure there are no errors in the configmap

3. Test DNS resolution in a pod:

nslookup kubernetes.default
dig kube-dns.kube-system.svc.cluster.local

  Using  in Production

  1. Enable caching – Reduces DNS lookup time

  2. Use health checks – Use the health plugin for monitoring

  3. Monitor with Prometheus – Expose metrics for observability

  4. Limit DNS recursion – Prevent external abuse

  5. Split DNS for internal/external – Secure private resolution

  6. Audit logs – Track abnormal query patterns

Real-World Use Case

Imagine a microservices architecture where multiple services (e.g., frontend, backend, auth, database) need to talk to each other inside a  cluster.

Conclusion

It is important because it handles DNS for your Kubernetes cluster.It is a vital yet often overlooked part of networking. From resolving internal service names to enabling reliable pod communication, it keeps the cluster connected and operational.

Mastering for dns not only improves your troubleshooting skills but also makes your cluster more observable and secure.

Final Thoughts

“It’s installed by default and runs as a deployment in the cluster.”DNS is a critical part of every distributed system, and  ensures that service discovery happens seamlessly. Whether you’re deploying microservices or running multi-cluster environments, understanding and optimizing it will give you a major edge in managing  networking efficiently.

For official documentation,  [Kubernetes CoreDNS docs]      click here