Kubernetes Networking

DNS in k8s

• Kubernetes comes with DNS implementation by using CoreDNS
• Kubernetes DNS runs as regular service that is scheduled on the cluster.
• This configures kubelet running on each node so that containers use the DNS service IP’s to resolve DNS Name.
• The service exists with name kube-dns in kube-system namespace
  

Activity: Create a k8s service with 3 nginx pods in deployment

• Overview
  
• Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

• Service yaml

---
apiVersion: v1
kind: Service
metadata:
  name: nginx-svc
spec:
  selector:
    app: nginx
  ports:
    - name: nginx-svc
      protocol: TCP
      port: 80

• get endoints
  
• Get Endpoint slices
  

Endpoints

• Endpoints help identify what pods are running for the service, Endpoints are created and managed by services
  
• Now lets describe endpoint

apiVersion: v1
kind: Endpoints
metadata:
  name: nginx-svc
subsets:
- addresses:
  - ip: 192.168.0.8
    nodeName: controlplane
    targetRef:
      kind: Pod
      name: nginx-deployment-cbdccf466-6q5f9
      namespace: default
      uid: 9e2da211-7213-40de-921f-7e871690fcac
  - ip: 192.168.1.3
    nodeName: node01
    targetRef:
      kind: Pod
      name: nginx-deployment-cbdccf466-kxc47
      namespace: default
      uid: 69ea9d52-94fa-4bb9-8650-cddefeb947e8
  ports:
  - name: nginx-svc
    port: 80
    protocol: TCP

Services in K8s

• A Service in k8s is a load balancing abstraction within a cluster.
• There are four types of services
  • ClusterIP
  • NodePort
  • LoadBalancer
  • ExternalName

Exercises:

• expose the nginx service using type NodePort (kubeadm)

Internet/External to Service Networking

• Here we have two perspectives
  • Routing traffic from Pod to Internet (Egress)
  • Routing traffic to Kubernetes (Ingress)
    • Layer 4: Load Balancer
    • Layer 7: Ingress Controller
• External (Egress)