Understanding Kubernetes Networking and Network Policies

Understanding Kubernetes Networking and Network Policies

Kubernetes networking follows a flat network model where every pod receives a unique IP address and can communicate with any other pod by default. While this simplifies application deployment, it creates security challenges when workloads have different trust levels or sensitivity. Network Policies address this challenge by providing a declarative way to control traffic flow between pods, namespaces, and external endpoints.

Network Policies function as distributed firewalls within Kubernetes clusters. They use label selectors to identify pods and define allowed ingress and egress traffic. When Network Policies apply to a pod, default deny rules activate, blocking all traffic except explicitly allowed connections. This whitelisting approach aligns with zero-trust security principles, requiring explicit permission for all network communications.

The implementation of Network Policies depends on the Container Network Interface (CNI) plugin. Not all CNI plugins support Network Policies, making plugin selection a crucial architectural decision. Popular options like Calico, Weave Net, and Cilium provide Network Policy support with varying features. Some plugins offer enhanced capabilities beyond the Kubernetes specification, such as layer 7 filtering or DNS-based policies.