why we need kube-proxy in Kubernetes (k8s)

In a Kubernetes cluster, kube-proxy is a critical component responsible for managing network communication to and from Pods. It operates at the network layer and performs several essential functions that ensure connectivity and service discovery within the cluster. Here’s why kube-proxy is needed on each node:

Key Responsibilities of Kube-proxy:

1. Service Discovery and Load Balancing:

   • kube-proxy maintains network rules (iptables or IPVS rules) on each node to implement Kubernetes Services.
   • It ensures that clients can access a Service regardless of which node in the cluster the Pod is running on.
   • Implements round-robin load balancing across Pods of a Service to distribute traffic evenly.

2. Network Address Translation (NAT):

   • When external clients communicate with a Service, kube-proxy performs Source Network Address Translation (SNAT) for the packets sent from the client to ensure responses return to the correct source.

3. Pod-to-Pod Communication:

   • kube-proxy sets up network routes and rules to allow Pods to communicate with each other across nodes.
   • It handles the network isolation and segmentation between Pods running on the same node or different nodes.

4. Endpoint Slices:

   • With the introduction of Endpoint Slices in newer Kubernetes versions, kube-proxy manages these slices to efficiently represent large numbers of endpoints (Pod IPs) for a Service.
   • It reduces the overhead of managing large iptables or IPVS rules directly and improves scalability.

5. Session Affinity:

   • Optionally, kube-proxy supports session affinity (sticky sessions) for Services, ensuring that subsequent client requests from the same source IP are routed to the same Pod.

Example Scenario:

Consider a scenario where a client accesses a Kubernetes Service:

1. Service Access:

   • The client sends a request to access a Service within the Kubernetes cluster.

2. Service Resolution:

   • kube-proxy on the node intercepts the request and determines the destination Pods associated with the Service.

3. Load Balancing:

   • If multiple Pods are serving the Service, kube-proxy uses load-balancing techniques (e.g., round-robin) to distribute incoming requests among the Pods.

4. Network Translation:

   • kube-proxy performs SNAT if necessary, ensuring that responses from Pods are correctly routed back to the client.

5. Monitoring and Scaling:

   • kube-proxy continuously monitors the health of Service endpoints (Pods). If a Pod becomes unhealthy or is scaled down, it adjusts its routing rules accordingly.