CNI plugins are the foundation of Kubernetes networking. They implement the Container Network Interface specification, which is a standard way for container runtimes to configure network interfaces in Linux containers. In a Kubernetes context, CNI plugins are responsible for:
Allocating IP addresses to Pods
Setting up routes between Pods across different nodes
Implementing network policies for security
Ensuring that every Pod can communicate with every other Pod without NAT
When a Pod is scheduled on a node, the kubelet calls the configured CNI plugin to set up the network for that Pod. The plugin creates the necessary network interfaces and routes so that the Pod can communicate with other Pods, regardless of which node they're running on.
Popular CNI plugins include Calico, Flannel, Cilium, and Weave Net. Each has different features and performance characteristics, but they all ensure that Pods have unique IP addresses and can communicate directly with each other.
Kubernetes networking is specifically designed to avoid NAT between Pods. The Kubernetes networking model requires that Pods can communicate directly with each other using their assigned IP addresses, without address translation. NAT would introduce additional complexity and potential performance issues.