CoreDNS in EKS

CoreDNS provides in-cluster DNS so pods can find each other by service name.

CoreDNS is the cluster DNS server. It lets a pod reach a service by name — for example web.default.svc.cluster.local — instead of a hard-coded IP. EKS installs and manages it as an add-on.

How service discovery works

  • Every Service gets a stable DNS name in the form <service>.<namespace>.svc.cluster.local.
  • Pods query CoreDNS (running in kube-system) for these names.
  • CoreDNS returns the service's cluster IP, which forwards to healthy pods.

Operating it

CoreDNS runs as a Deployment. On busy clusters you may scale its replicas up so DNS lookups don't become a bottleneck. Because it's a managed add-on, you can update it with a single command.

Example

Example · bash
# CoreDNS runs as a Deployment in kube-system
kubectl get deployment coredns -n kube-system

# Update the managed CoreDNS add-on to a specific version
aws eks update-addon --cluster-name demo \
  --addon-name coredns --addon-version v1.11.1-eksbuild.9

When to use it

  • A developer uses the Service DNS name my-service.default.svc.cluster.local to connect microservices without hardcoding IP addresses.
  • A platform team scales CoreDNS from 2 to 4 replicas to handle the increased DNS query load after a 3x cluster growth.
  • An SRE uses kubectl exec to run nslookup inside a pod against CoreDNS to diagnose why a service cannot be resolved by name.

More examples

Check CoreDNS deployment status

Verifies that CoreDNS pods are running and ready, which is required for all in-cluster service discovery.

Example · bash
kubectl get deployment coredns -n kube-system
kubectl get pods -n kube-system -l k8s-app=kube-dns

Test DNS resolution from a pod

Launches a temporary pod to test DNS resolution of the Kubernetes API service, confirming CoreDNS is working correctly.

Example · bash
kubectl run dns-test --image=busybox:1.36 --rm -it --restart=Never -- \
  nslookup kubernetes.default.svc.cluster.local

Scale CoreDNS for high load

Increases CoreDNS replica count to reduce DNS query latency under heavy in-cluster service discovery load.

Example · bash
kubectl scale deployment coredns \
  --replicas=4 \
  -n kube-system
kubectl rollout status deployment/coredns -n kube-system

Related

eks addonseks vpc cni

Discussion

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