Kubernetes Networking Basics: Services, Ingress, and CNI Plugins

Kubernetes Networking Basics (Services, Ingress, CNI Plugins)

Kubernetes is designed to support scalable and dynamic workloads, but its networking model can seem complex at first. In Kubernetes, networking involves connecting Pods and services both inside and outside the cluster. To achieve this, Kubernetes relies on several key components: Services, Ingress, and CNI (Container Network Interface) plugins. These components help ensure that your applications are able to communicate effectively, both internally and externally.

In this article, we’ll explore the core concepts behind Kubernetes networking, the role of Services, Ingress controllers, and CNI plugins, and how they all fit into the broader Kubernetes networking model.

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Kubernetes Networking Overview

In Kubernetes, every Pod gets its own unique IP address, and containers inside the Pod share this IP address. This means Pods can communicate directly with each other via their IP addresses. However, when you need to expose Pods and services to the outside world, or manage traffic routing inside the cluster, Kubernetes relies on Services, Ingress, and CNI plugins.

Key points in Kubernetes networking:

• Pod-to-Pod Communication: Every Pod can talk to every other Pod in the cluster, regardless of where it is located, thanks to the flat networking model.
• Service Discovery: Services abstract Pod IPs and provide stable DNS names for accessing Pods.
• Routing and Load Balancing: For managing traffic entering and leaving the cluster, Ingress controllers and LoadBalancer services are used.

Let’s dive deeper into the key components: Services, Ingress, and CNI plugins.

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1. Kubernetes Services

A Service in Kubernetes is a resource that defines how to access a set of Pods. The most common use case for a Service is to expose an application running on multiple Pods within a cluster. It provides a stable endpoint, so clients don’t have to worry about the changing IP addresses of Pods.

Types of Services:

• ClusterIP (default): Exposes the service only within the cluster.
• NodePort: Exposes the service on each node at a static port, allowing external access.
• LoadBalancer: Provisions an external load balancer (usually in cloud environments) to access the service from outside the cluster.
• ExternalName: Maps the service to an external DNS name.

Example: Service Definition

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: myapp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 8080
  type: ClusterIP

Use Case:

• ClusterIP is commonly used for internal services, like a backend database or API that is accessed by other Pods in the cluster.
• LoadBalancer is used when you need to expose a service to external traffic, with automatic traffic distribution across Pods.

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2. Kubernetes Ingress

Ingress is a Kubernetes API object that manages external access to services within the cluster, typically HTTP. It provides HTTP and HTTPS routing, SSL termination, and virtual hosting for multiple services. An Ingress Controller is responsible for processing Ingress resources and managing traffic routing.

Key Features of Ingress:

• HTTP/HTTPS Routing: Routes traffic to services based on hostnames or URLs.
• SSL Termination: Handles SSL/TLS termination to secure communication.
• Path-based Routing: Allows routing traffic to different services based on URL paths.
• Load Balancing: Provides built-in load balancing across multiple backends.

Example: Ingress Resource

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: my-ingress
spec:
  rules:
    - host: myapp.example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: my-service
                port:
                  number: 80

Use Case:

Ingress is often used when you need to expose multiple services under a single domain (e.g., myapp.example.com) or handle complex HTTP routing and SSL management, such as with web applications or APIs.

Ingress Controller:

• Ingress Controllers are the actual implementations of the Ingress resource. Popular Ingress controllers include NGINX, HAProxy, and Traefik.

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3. CNI (Container Network Interface) Plugins

Kubernetes allows for flexibility in networking by using the CNI (Container Network Interface) model. CNI plugins are used to configure networking for Pods. Each CNI plugin implements a standard interface that allows Kubernetes to interact with various network providers, such as Calico, Flannel, Weave, and more.

CNI Plugin Features:

• Pod Networking: CNI plugins configure the networking for Pods, ensuring they can communicate with each other.
• Network Policies: Many CNI plugins support network policies to control which Pods can communicate with each other.
• Cross-Cluster Communication: Some CNI plugins also support network connectivity across clusters, enabling multi-cluster setups.

Common CNI Plugins:

• Calico: A highly scalable networking plugin with support for network policies.
• Flannel: A simple CNI plugin for networking with Kubernetes that uses overlay networks.
• Weave: A CNI plugin that provides network isolation and encryption.
• Cilium: A CNI plugin that leverages eBPF (extended Berkeley Packet Filter) for high-performance networking and security.

Example: Installing a CNI Plugin (Flannel)

kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

Use Case:

CNI plugins are essential for setting up network connectivity between Pods. When deploying Kubernetes in an environment, the choice of CNI plugin impacts the network topology, performance, and security features available.

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Kubernetes Networking Model

Kubernetes uses a flat networking model, where each Pod has its own unique IP address, and Pods can communicate with each other without Network Address Translation (NAT). The networking model ensures that Pods can be easily addressed and discovered, and that applications running inside the cluster can communicate seamlessly.

• Pod Networking: Each Pod gets a unique IP address, which makes it possible for Pods to communicate with each other directly. This is achieved through the CNI plugin.
• Service Networking: Kubernetes Services provide a stable DNS name and IP address, abstracting away the underlying Pods' IPs. Services expose Pods to other services or external traffic.
• Ingress: Ingress provides HTTP/S routing, SSL termination, and centralized access control for multiple services in a Kubernetes cluster.

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Kubernetes Networking Flow (Diagram)

Here’s a high-level flow of how networking works in Kubernetes:

1. Pod-to-Pod Communication: Each Pod has a unique IP address, and Pods can communicate directly with one another via these IPs.
2. Service Discovery: Services act as proxies for Pods, allowing Pods to be accessed by stable DNS names or IP addresses.
3. Ingress Traffic: External HTTP/S traffic enters through an Ingress controller, which routes the traffic to the appropriate service inside the cluster.
4. CNI Plugin: The CNI plugin configures the networking for Pods, ensuring they can connect with each other and access external resources as needed.

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Comparison of Kubernetes Networking Components

Component	Description	Use Case	Key Features
Services	Provides stable access to a set of Pods in the cluster	Internal or external service access	Load balancing, service discovery, routing
Ingress	Manages external access to services, typically HTTP(S)	Exposing HTTP services to the outside world	HTTP routing, SSL termination, path-based routing
CNI Plugins	Provides networking capabilities for Pods and defines network policies	Pod-to-Pod communication, network policies	Overlay networks, network security, multi-cluster communication

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When to Use Each Kubernetes Networking Component

• Use Services when you need to expose Pods internally or externally with stable endpoints.
• Use Ingress when you need advanced HTTP routing capabilities, such as path-based routing, virtual hosting, or SSL termination.
• Use CNI Plugins to provide networking for Pods, control network policies, and enable cross-cluster communication.

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Conclusion

Kubernetes networking is an essential aspect of managing and scaling containerized applications. The combination of Services, Ingress, and CNI plugins allows Kubernetes to provide robust, scalable, and secure networking between Pods, services, and external resources. Understanding these key components will help you design efficient, highly available applications and services within your Kubernetes clusters.

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