Introduction

Kubernetes, an open-source container orchestration platform, has revolutionized how applications are deployed and managed. One of the critical components that enable the seamless operation of Kubernetes is its networking model. This blog will take a deep dive into Kubernetes Container Network Interface (CNI) networking, exploring its architecture, components, and popular CNI plugins.

What is CNI?

The Container Network Interface (CNI) is a specification and library for writing plugins that configure network interfaces in Linux containers. Initially developed by CoreOS, CNI is now a project under the Cloud Native Computing Foundation (CNCF). It aims to provide a common interface between the network configuration plugins and container runtimes like Kubernetes.

Kubernetes Networking Model

Kubernetes networking follows a flat network structure where every pod can communicate with every other pod without NAT (Network Address Translation). This model is based on four key requirements:

  1. Pod-to-Pod Communication: Pods in a cluster should be able to communicate with each other directly.
  2. Pod-to-Service Communication: Pods should be able to communicate with services, which may be backed by multiple pods.
  3. External-to-Internal Communication: External clients should be able to communicate with services within the cluster.
  4. Internal-to-External Communication: Pods should be able to communicate with external services outside the cluster.

CNI in Kubernetes

Kubernetes uses CNI to manage network resources for pods. When a pod is created, Kubernetes delegates the network setup to a CNI plugin, which is responsible for configuring the pod’s network interface. The CNI plugin ensures that the pod gets a unique IP address and can communicate with other pods and services within the cluster.

Key Components

  • CNI Plugin: A binary that conforms to the CNI specification. It is responsible for setting up and tearing down network interfaces in containers.
  • CNI Configuration File: A JSON file that specifies which CNI plugin to use and any necessary parameters for the plugin.
  • CNI IPAM (IP Address Management) Plugin: Handles IP address allocation and deallocation for pod interfaces.

Several CNI plugins are available, each offering different features and capabilities. Here are some of the most popular ones:

1. Flannel

Flannel is a simple and easy-to-configure CNI plugin designed for Kubernetes networking. It uses a flat network model with a unique subnet assigned to each host. Flannel supports multiple backend implementations, such as VXLAN, host-gw, and AWS VPC.

Pros:

  • Simple to set up and use.
  • Supports multiple backends.

Cons:

  • Limited features compared to other plugins.

2. Calico

Calico is a powerful CNI plugin that provides networking and network security solutions for Kubernetes. It uses BGP (Border Gateway Protocol) for routing and supports network policies for fine-grained control over pod communication.

Pros:

  • High-performance and scalable.
  • Advanced network policy support.
  • Supports BGP and IP-in-IP encapsulation.

Cons:

  • More complex to configure than Flannel.

3. Weave Net

Weave Net is another popular CNI plugin that provides a simple and flexible networking solution for Kubernetes. It creates a mesh network of all the nodes in the cluster and supports encryption, network segmentation, and network policy enforcement.

Pros:

  • Easy to set up and use.
  • Supports encrypted communication.
  • Network policy enforcement.

Cons:

  • Performance may not be as high as Calico in large clusters.

4. Cilium

Cilium is an advanced CNI plugin that uses eBPF (extended Berkeley Packet Filter) for high-performance networking and security. It provides L3/L4 and L7 network policies and integrates well with service meshes like Istio.

Pros:

  • High-performance due to eBPF.
  • Advanced network and security policies.
  • Integration with service meshes.

Cons:

  • More complex to set up and manage.

How CNI Works in Kubernetes

When a pod is scheduled to run on a node, the following steps occur:

  1. Pod Creation: The Kubernetes API server creates a new pod object.
  2. Node Assignment: The Kubernetes scheduler assigns the pod to a node.
  3. CNI Plugin Invocation: The kubelet on the assigned node invokes the CNI plugin to set up the pod’s network interface.
  4. Network Interface Configuration: The CNI plugin configures the network interface, assigns an IP address, and sets up the necessary routing.
  5. Pod Communication: The pod can now communicate with other pods and services in the cluster.

Configuring CNI in Kubernetes

To configure a CNI plugin in Kubernetes, you need to:

  1. Install the CNI Plugin: Follow the installation instructions for the chosen CNI plugin.
  2. Create the CNI Configuration File: This file typically resides in /etc/cni/net.d and specifies the CNI plugin to use and its parameters.
  3. Configure the Kubernetes Cluster: Ensure the kubelet is configured to use the CNI plugin by setting the --network-plugin=cni flag.

Example: Configuring Flannel

  1. Install Flannel: 
    kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml
  2. Create the CNI Configuration File:
{
  "name": "flannel",
  "type": "flannel",
  "delegate": {
    "isDefaultGateway": true
  }
}
  1. Configure the Kubernetes Cluster: Ensure the kubelet is configured to use CNI:
KUBELET_NETWORK_ARGS="--network-plugin=cni --cni-conf-dir=/etc/cni/net.d --cni-bin-dir=/opt/cni/bin"

Conclusion

Kubernetes CNI networking is a crucial aspect of running a Kubernetes cluster. Understanding how CNI works and the various plugins available can help you choose the right networking solution for your needs. Whether you need a simple setup with Flannel, advanced network policies with Calico, or high-performance networking with Cilium, the CNI ecosystem provides a range of options to suit different use cases.

By configuring and managing CNI plugins effectively, you can ensure robust and scalable network connectivity for your Kubernetes applications.

References