What's the purpose of the default kubernetes service?

Question

When you run: kubectl get svc -n default, you will have a kubernetes service with Type as ClusterIP already there.

What is the purpose of this service? Any references appreciated.

I'm running in Minikube

xyz:Kubernetes _$ kubectl describe svc/kubernetes
Name:              kubernetes
Namespace:         default
Labels:            component=apiserver
               provider=kubernetes
Annotations:       <none>
Selector:          <none>
Type:              ClusterIP
IP:                10.0.0.1
Port:              https  443/TCP
TargetPort:        8443/TCP
Endpoints:         10.0.2.15:8443
Session Affinity:  ClientIP
Events:            <none>

xyz:Kubernetes _$ kubectl cluster-info
Kubernetes master is running at https://192.168.99.100:8443

Answer 1

AFAIK the kubernetes service in the default namespace is a service which forwards requests to the Kubernetes master ( Typically kubernetes API server).

So all the requests to the kubernetes.default service from the cluster will be routed to the configured Endpoint IP. In this scenario its the kubernetes master IP

For example

Lets checkout the output of kubectl describe svc kubernetes and look at the the Endpoint IP.

Now lets check our cluster info

kubectl cluster-info

Please note that the kubernetes master is running at the same IP as the Endpoints IP of kubernetes.default service.

Hope it helps.

Answer 2

It is so that every Pod within your cluster can make API requests of the Kubernetes master without having to hard-code the API URL therein. Your ~/.kube/config may very well have the "external" address of your Kubernetes master, but it makes very little sense for API traffic to leave the cluster and then re-enter the cluster for a Pod that could be co-located on the same Node. Pods are able to use the Service Account credentials injected by kubernetes, unless that Service Account feature is disabled per-Pod.

Your application is free to make use of that functionality, too, if it wishes -- for example -- to discover any annotations on its Pod, or how many other replicas there are in its Deployment, and so forth.

I guess the tl;dr is that for 90% of the Pods it doesn't matter, and for the remaining 10% it is super convenient.