Scenario

You are deploying a distributed database named dns-app in the dev namespace. This application requires that each Pod be addressable by a predictable, unchanging hostname (e.g., dns-app-0.dns-app) so the nodes can find each other for data replication. Standard Services (which load balance) are not suitable for this.

Task

Create a Headless Service named dns-app in the dev namespace. Create a StatefulSet named dns-app with 3 replicas image nginx:alpine. Ensure the StatefulSet uses the dns-app service for network identity. Verify that pods have stable DNS names like dns-app-0.dns-app
Use kubectl exec -it netshoot -n dev -- nslookup dns-app-0.dns-app to validate resolution.

Example

Failing Check (Standard Service):

$ kubectl get svc dns-app -n dev
NAME      TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)
dns-app   ClusterIP   10.43.20.155    <none>        80/TCP

Success Check (Headless Service):

$ kubectl get svc dns-app -n dev
NAME      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)
dns-app   ClusterIP   None         <none>        80/TCP

Step 1: Create the Headless Service
The key is clusterIP: None. This tells Kubernetes not to load balance, but to return the specific IP of each Pod backing the service.

cat <<EOF | kubectl apply -f -
apiVersion: v1
kind: Service
metadata:
  name: dns-app
  namespace: dev
  labels:
    app: dns-app
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None   # <--- Critical for Headless DNS
  selector:
    app: dns-app
EOF

Step 2: Create the StatefulSet
Define the StatefulSet referencing the service name.

cat <<EOF | kubectl apply -f -
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: dns-app
  namespace: dev
spec:
  serviceName: "dns-app" # Must match Headless Service name
  replicas: 3
  selector:
    matchLabels:
      app: dns-app
  template:
    metadata:
      labels:
        app: dns-app
    spec:
      containers:
      - name: nginx
        image: nginx:alpine
EOF

Step 3: Verify DNS Resolution
Use the helper pod to confirm the DNS records are created.

# Wait for pods to be ready
kubectl rollout status statefulset/dns-app -n dev

# Perform a lookup for the first pod's specific hostname
kubectl exec -it netshoot -n dev -- nslookup dns-app-0.dns-app
# You should see an "Address" returned, NOT "NXDOMAIN".