Kubernetes – Port, Targetport and NodePort

When working with Kubernetes Service, you will come across some of the following terminologies:

• Port: Port is the port number which makes a service visible to other services running within the same K8s cluster.  In other words, in case a service wants to invoke another service running within the same Kubernetes cluster, it will be able to do so using port specified against “port” in the service spec file.
• Target Port: Target port is the port on the POD where the service is running.
• Nodeport: Node port is the port on which the service can be accessed from external users using Kube-Proxy.

Take a look at following spec defining a sample service:

apiVersion: v1
kind: Service
metadata:
  name: order-service
spec:
  ports:
  - port: 8080
    targetPort: 8170
    nodePort: 32222
    protocol: TCP 
  selector:
    component: order-service-app

Pay attention to some of the following in above spec:

• The port is 8080 which represents that order-service can be accessed by other services in the cluster at port 8080.
• The targetPort is 8170 which represents the order-service is actually running on port 8170 on pods
• The nodePort is 32222 which represents that order-service can be accessed via kube-proxy on port 32222.

This is ipTables in Kubernetes which does the magic. It maintains the mapping of nodePort vs targetPort. K8s Kube-Proxy uses the ipTables to resolve the requests coming on a specific nodePort and redirect them to appropriate pods.