Services and Cluster DNS

Pods come and go, and their IPs change. A Service gives a stable name and virtual IP and balances across pods matched by label.

apiVersion: v1
kind: Service
metadata:
  name: web
spec:
  selector: { app: web }
  ports:
    - port: 80
      targetPort: 80

Type	Reachable from
ClusterIP (default)	Inside the cluster only
NodePort	Every node on a port 30000 to 32767
LoadBalancer	The internet, via the cloud provider’s load balancer

CoreDNS gives every service a name: web within its namespace, or web.default.svc.cluster.local from anywhere.

kubectl run tmp --rm -it --image=busybox:1.33 -- wget -qO- http://web
kubectl get endpoints web

If a service has no endpoints, its selector does not match any ready pod. That is the first thing to check.