🌐 Custom domain names using Kubernetes CoreDNS

✨ Introduction

🎯 CoreDNS is the DNS service discovery plugin for Kubernetes. It replaces the older kube-dns and is pre-installed in the kube-system namespace.

💡 The objective of this guide is to use CoreDNS to provide custom domain names inside the cluster. For example, replacing the service name nginx.default.svc.cluster.local with something like nginx.default.devops.com.

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🔍 Getting Started

📜 Check the current CoreDNS setup:

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

CoreDNS configuration is saved into a configmap.

kubectl describe configmap coredns -n kube-system  

To provide a custom confiuration, we can use the coredns custom confgmap.

🛠️ Customizing CoreDNS

1) Save the following as create custome-configmap.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
  labels:
    k8s-app: kube-dns
data:
  internal-custom.override: |
    rewrite stop {
      name regex (.*)\.aks\.com\.$ {1}.svc.cluster.local.
      answer name (.*)\.svc\.cluster\.local\.$ {1}.devops.com.
    }

Apply the custom ConfigMap:

kubectl apply -f custom-configmap.yaml

2) Import the Custom ConfigMap

Edit the default CoreDNS ConfigMap to include the custom configuration: 📸 Reference screenshot:

kubectl edit configmap coredns -n kube-system

import custom/*.override

3) ⚙️ Update CoreDNS Deployment To enable the custom configuration, add a volume and volume mount to the CoreDNS deployment: Modify the deployment YAML:

volumes:
  - configMap:
      defaultMode: 420
      items:
        - key: internal-custom.override
          path: internal-custom.override
      name: coredns-custom

volumeMounts:
  - mountPath: /etc/coredns/custom
    name: custom-config
    readOnly: true

📸 Reference screenshot:

4) 📦 Now let’s start deploying some services & expose them. Create Services in Different Namespaces

🚢 Create deployments and expose services in the default and kong namespaces:

kubectl create deployment nginx --image=nginx --replicas=1 -n default
kubectl expose deployment nginx --name nginx --port=80 -n default
kubectl get deploy,svc

kubectl create deployment apache2 --image=httpd --replicas=1 -n kong
kubectl expose deployment apache2 --name apache2 --port=80 -n kong
kubectl get deploy,svc -n kong

🧩 Explanation:

🔄 Incoming domain apache2.kong.devops.com will be rewritten as apache2.kong.svc.cluster.local.

🔄 Incoming domain nginx.default.devops.com will be rewritten as nginx.default.svc.cluster.local.

🧪 Testing:

Let us try resolving with .devops.com . Andunderstand how that works.

✅ nginx.default.devops.com → should resolve to nginx.default.svc.cluster.local.

✅ apache2.kong.devops.com → should resolve to apache2.kong.svc.cluster.local.

kubectl exec -it deploy/apache2 -n kong -- bash
apt update && apt install dnsutils curl -y

nslookup nginx.default.devops.com
nslookup apache2.kong.devops.com

curl nginx.default.devops.com
curl apach2.kong.devops.com

📸 Reference screenshot of final output: