Understanding Container Network Interface (CNI) in Kubernetes: A Comprehensive Overview with Examples

Boyan Kirov
Boyan Kirov
DevOps & Cloud Engineer
Reading time: 4 mins.
Last Updated: 20.09.2023

Table of Contents

Container Network Interface (CNI) plays a crucial role in managing networking capabilities within Kubernetes clusters. As an essential component of the Kubernetes ecosystem, CNI enables seamless communication and connectivity between containers and external networks. Moreover, throughout this article, we’ll dig deeper into CNI and discuss its role in Kubernetes, and offer real-world examples to show how it might be used.

Understanding CNI

Network plugin interactions with container runtimes are outlined in the open standard known as the Container Network Interface (CNI). It serves as a bridge between the container runtime and the network plugins, allowing for the dynamic configuration of networking for Kubernetes pods. CNI plugins handle tasks such as assigning IP addresses, creating network interfaces, and setting up network routes for containers.

CNI Architecture

CNI operates on a simple plugin-based architecture. When a pod is created in Kubernetes, the container runtime (such as Docker) calls the CNI plugins to set up the network environment. The plugins can be written in various programming languages and communicate with the container runtime via standard input and output. They leverage the Linux networking stack to configure networking for containers.

How to implement CNI?

To better understand CNI, let’s consider an example of a Kubernetes cluster running multiple pods. Let’s say we wish to facilitate communication between two pods, A and B.

Container Runtime Requests Network Setup: When pod A is created, the container runtime invokes the configured CNI plugin to set up networking for pod A. The CNI plugin receives information about the pod’s network requirements and assigns an IP address to pod A’s container.

CNI Plugin Sets up Network Environment: The CNI plugin configures a network interface in pod A’s container with the assigned IP address. It also sets up necessary routing rules and network policies.

Pod B Communication: Similarly, when pod B is created, the container runtime calls the CNI plugin, which assigns an IP address to pod B’s container and sets up the required network environment.

Network Connectivity: With the network interfaces and IP addresses assigned by the CNI plugins, pods A and B can communicate using their respective IP addresses. This communication can occur within the cluster or with external networks, depending on the network configuration.

CNI Plugins: A wide range of CNI plugins is available to meet different networking requirements. Some popular examples include Calico, Flannel, Weave, and Canal. These plugins offer features like network isolation, security policies, load balancing, and integration with external network resources.

CNI in Action: For example, when implementing CNI in a Kubernetes cluster using the Calico plugin you will need to:

  • Install Calico Plugin: The first step is to install the Calico CNI plugin in your Kubernetes cluster. This can be achieved by applying the appropriate manifest files or using package managers like Helm.
  • Configure Calico Networking: Configure Calico to meet your networking requirements after installation. This involves defining network policies, IP pool allocations, and any necessary security configurations.
  • Create Pods: Now, create pods within your cluster. The CNI plugin integrated with Calico will automatically assign IP addresses, set up network interfaces, and establish connectivity between the pods.
  • Verify Connectivity: You can verify the network connectivity by communicating between the pods using their assigned IP addresses. Additionally, you can test connectivity with external networks if configured.

 A key part of Kubernetes, is that the Container Network Interface (CNI), enables seamless networking for containers within a cluster. With its plugin-based architecture, CNI provides a flexible and extensible approach to managing networking requirements. 

Installing Calico CNI in a Kubernetes Cluster

Installing Calico CNI in a Kubernetes cluster is a straightforward process. The most crucial steps when installing Calico are: 

  1. Establish a Kubernetes cluster: Make sure your Kubernetes cluster is operational and running.
  1. Download Calico YAML Manifests: Calico provides YAML manifests that define the necessary resources for installation.
  1. Modify Configuration (Optional): If needed, you can modify the configuration in the Calico YAML manifest to suit your networking requirements. Open the calico.yaml file using a text editor and make the necessary changes.
  1. Apply Calico Manifests: Apply the Calico YAML manifests to your Kubernetes cluster using the following command:
kubectl apply -f calico.yaml

This command deploys the Calico components, including the required DaemonSets, ConfigMaps, and RBAC roles.

  1. Verify Installation: To ensure that Calico is installed correctly, use the following command to check the status of the Calico pods: 
kubectl get pods --namespace=kube-system --selector=k8s-app=calico-node

You should see Calico pods running and ready.

  1. Verify Cluster Networking: Confirm that cluster networking is functioning properly. Check the cluster networking status by running the following command:
kubectl get clusterinfo

This command displays the cluster IP address range, pod subnet, and other network-related details.

  1. Test Network Connectivity: Create and deploy a sample pod in your cluster and test the network connectivity between pods. You can use the following command to create a sample pod:
kubectl run test-pod --image=nginx --restart=Never --rm -it – sh

Once inside the pod, you can use tools like ping or curl to verify connectivity with other pods.

That’s it! You have successfully installed Calico CNI in your Kubernetes cluster. Calico will now handle networking and provide the necessary network policies and connectivity between your pods.

To Sum Up

In addition, CNI offers a flexible and adaptable solution when it comes to managing networking requirements. Moreover, the plugins help with handling activities such as, assigning IP addresses, and configuring network routes for containers. However, in order to interact with the container runtime and create seamless connection with external networks, you need to take into account some necessary rules and requirements.

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