Introduction to Kubernetes CoreDNS

CoreDNS is a DNS server that runs inside Kubernetes and is responsible for service discovery — i.e., translating service names (like my-service.default.svc.cluster.local) into IP addresses.

What CoreDNS Does

In a Kubernetes cluster:

• Every Pod and Service gets its own DNS name.
• CoreDNS listens for DNS queries from Pods (via /etc/resolv.conf).
• It looks up the name in the cluster’s internal DNS records and returns the correct ClusterIP or Pod IP.

So if a Pod tries to reach mysql.default.svc.cluster.local, CoreDNS will resolve it to the IP of the mysql service.

How It Works

Runs as a Deployment in the kube-system namespace.
Has a Service called kube-dns (for backward compatibility).
Uses a ConfigMap (coredns) to define how DNS queries are processed.
Listens on port 53 (UDP/TCP), the standard DNS port.

Example CoreDNS ConfigMap snippet:

apiVersion: v1

kind: ConfigMap

metadata:

  name: coredns

  namespace: kube-system

data:

  Corefile: |

    .: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

    }

Key Plugins

CoreDNS is modular — it uses plugins for specific functionality:

• kubernetes: handles DNS for cluster Services/Pods.
• forward: forwards queries to upstream resolvers for external domains.
• cache: caches responses for faster resolution.
• prometheus: exposes metrics for monitoring.
• health: adds a health endpoint.

Why It Matters

Without CoreDNS, Pods can’t resolve service names.

It’s essential for communication between microservices.

It’s a critical cluster component — if it breaks, DNS resolution (and often your workloads) fail.

Common Commands

Check CoreDNS pods:

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

View CoreDNS logs:

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

Edit CoreDNS config:

kubectl -n kube-system edit configmap coredns

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