Networking in Kubernetes

Lets demystify Kubernetes networking in this article and explore the key components and how they interact to enable seamless communication within and outside the cluster. 

Fundamental principal of Kubernetes networking:
Kubernetes networking adheres to a few core principles:

• Flat Network: All Pods within a cluster exist within a single, flat network space. They can communicate with each other directly using their IP addresses, regardless of the Node they reside on.
• Unique IP per Pod: Each Pod receives a unique IP address within the cluster. This address remains consistent even if the Pod is rescheduled to a different Node.
• Service Abstraction: Services provide a stable endpoint for accessing a group of Pods, enabling load balancing and service discovery.

What are the key components of Kubernetes networking?

• Kubelet: This agent runs on each Node and manages network configurations for Pods, ensuring they have the necessary network interfaces and routing rules
• Kube-proxy: This network proxy runs on each Node and implements Kubernetes Service definitions. It intercepts traffic destined for Services and forwards it to the appropriate Pods.
• Container Network Interface (CNI): This plugin interface allows for flexibility in choosing and integrating different networking solutions.
  Popular CNI plugins include Calico, Flannel, and Weave Net.
• DNS: Kubernetes provides a built-in DNS service that resolves Service names to their corresponding cluster IP addresses, simplifying inter-service communication.

The Architecture in Action:
Lets breakdown how these components interacts with each other;

1. When a Pod is created, the Kubelet uses the configured CNI plugin to assign it an IP address and connect it to the cluster network.
2. When a Service is created, Kube-proxy on each Node updates its iptables rules to direct traffic destined for the Service's IP and port to the corresponding Pods.
3. When a Pod needs to communicate with a Service, it uses the Service name, which is resolved by the Kubernetes DNS to the Service's cluster IP. Kube-proxy then intercepts this traffic and forwards it to a healthy Pod backing the Service.
   
   This architecture enables seamless communication between Pods and Services, even as Pods are dynamically created, scaled, or moved across Nodes.

How's the external access works?
Kubernetes provides several ways to expose Services to the outside world:

• NodePort: Exposes the Service on a static port on each Node's IP address
• LoadBalancer: Provisions a cloud provider load balancer to distribute traffic to the Service
• Ingress: Provides a more sophisticated way to manage external access, enabling features like HTTP routing, TLS termination, and load balancing.