Kube-Proxy and CNI: The Backbone of Kubernetes Networking

Source: Dev.to

Overview

Kubernetes networking appears simple: each Pod receives its own IP address, and Services automatically route traffic to the appropriate Pods. This simplicity is powered by two core components:

• Container Network Interface (CNI)
• kube-proxy

Together they handle IP assignment, network interface setup, routing, and service load‑balancing across the cluster.

kube-proxy

kube-proxy is responsible for making Services functional by translating Service IPs and ports into actual Pod endpoints.

• Service IP stability – kube-proxy ensures that a Service IP remains a stable endpoint even though the underlying Pods are ephemeral.
• Routing rule creation – When a Service or its endpoints are created, kube-proxy installs iptables or IPVS rules on the node to direct traffic from the Service IP to the backing Pods.
• Dynamic updates – If a Pod backing a Service is added or removed, kube-proxy updates the kernel rules accordingly, preserving service availability.
• Load distribution – By leveraging kernel networking (iptables or IPVS), kube-proxy distributes incoming traffic across the set of Pods that implement the Service.
• Without kube-proxy – Service IPs would exist but would not forward traffic to Pods; kube-proxy wires Service IPs and ports to the actual Pod endpoints.

Container Network Interface (CNI)

CNI plugins handle the Pod‑level networking that enables Pods to communicate both within and across nodes.

• IP assignment – Each Pod receives a unique IP address from the cluster’s IP pool.
• Interface creation – CNI creates a veth pair for the Pod, placing one end inside the Pod’s network namespace and the other on the host.
• Routing configuration – CNI sets up routing rules so that Pods can reach other Pods on different nodes, typically by configuring the host’s routing table or overlay network.
• Network isolation – By managing namespaces and interfaces, CNI ensures that Pods are isolated from each other while still being able to communicate as needed.

These two components—kube-proxy for Service routing and CNI for Pod networking—form the backbone of Kubernetes networking, enabling seamless communication across the entire cluster.