Networking in Kubernetes Nodes – An Introduction

Kubernetes is a distributed system, meaning applications often run across multiple worker nodes. But how do these nodes communicate efficiently? How do pods running on different nodes exchange data seamlessly?

The answer lies in Kubernetes Node Networking—the fundamental layer that enables communication between pods, services, and external systems.

Understanding how networking works within Kubernetes nodes is crucial for troubleshooting connectivity issues, optimizing performance, and ensuring seamless service communication.

In this article, we will explore:

• What node networking is and why it is essential in Kubernetes
• Key networking components inside a Kubernetes node
• Two real-world scenarios demonstrating node-level networking
• Step-by-step breakdowns of how traffic flows inside a Kubernetes node

What is Kubernetes Node Networking?

Each worker node in Kubernetes has a networking stack that allows:

? Pods to communicate with each other (on the same node and across nodes).

? Services to expose applications using ClusterIP, NodePort, and LoadBalancer.

? Ingress controllers to manage external access to services.

? Network policies to control traffic flow for security.

Kubernetes does not handle networking natively—instead, it relies on a Container Network Interface (CNI) to manage IP allocation and routing.

Key Components of Kubernetes Node Networking

Component Description Pod Network Provides each pod with a unique IP address. CNI Plugins Handles network configuration (Calico, Flannel, Cilium). Bridge (cbr0) Connects pod networks within a node. Kube-Proxy Routes traffic between services and pods. Network Namespace Isolates container networking.

Now, let’s explore two real-world scenarios demonstrating how Kubernetes node networking works.

Scenario 1: How Pods Communicate Inside a Node

(Understanding Pod-to-Pod Networking in Kubernetes)

Imagine you have two pods running on the same node:

Node Pod Name Pod IP Node-1 frontend-pod 10.244.1.2 Node-1 backend-pod 10.244.1.3

The frontend-pod needs to communicate with the backend-pod using its Pod IP.

Step 1: Pod Network Allocation by CNI

• When the pods are created, Kubernetes assigns them unique IP addresses within the node’s pod network (10.244.1.0/24).
• The CNI plugin (Flannel, Calico, etc.) ensures both pods can talk to each other.

Step 2: Packets Travel Through the Pod Network

• When frontend-pod sends a request to 10.244.1.3, the packet travels: Frontend Pod → Pod Network Bridge (cbr0) → Backend Pod

Step 3: Communication Without NAT

• Since both pods are on the same node, no NAT (Network Address Translation) is needed.
• This ensures low-latency, direct communication.

You can verify pod networking using:

kubectl exec frontend-pod -- ping 10.244.1.3        

If successful, it means pods can communicate within the node.

Scenario 2: How Pods Communicate Across Nodes

(Understanding Inter-Node Networking in Kubernetes)

Now, let’s say frontend-pod is on Node-1, but backend-pod is on Node-2:

Node Pod Name Pod IP Node-1 frontend-pod 10.244.1.2 Node-2 backend-pod 10.244.2.3

Since these pods are on different nodes, networking is more complex.

Step 1: Kubernetes Assigns Pod IPs Across Nodes

• Each node has its own pod subnet (e.g., Node-1: 10.244.1.0/24, Node-2: 10.244.2.0/24).
• The CNI plugin ensures pods from different nodes can communicate.

Step 2: Packet Travels Across Nodes

• When frontend-pod tries to reach backend-pod, the packet follows this route: Frontend Pod → Node-1’s Network Bridge (cbr0) → CNI Overlay Network → Node-2’s Network Bridge → Backend Pod
• The CNI plugin encapsulates packets using VXLAN, IP-in-IP, or BGP (depending on the plugin).

Step 3: Routing Traffic Between Nodes

• The node’s routing table determines how to forward packets.
• The packet is delivered to backend-pod on Node-2.

You can check pod routes with:

kubectl exec frontend-pod -- ip route        

This command will show routes configured for inter-node communication.

Step 4: How Kube-Proxy Helps in Multi-Node Communication

• If frontend-pod communicates via a Kubernetes Service, Kube-Proxy ensures traffic reaches a healthy backend-pod, regardless of which node it runs on.
• This is done via iptables or IPVS rules.

Key Takeaways

• Kubernetes Node Networking enables pod-to-pod, service-to-service, and external communication.
• Scenario 1 demonstrated same-node communication, where packets travel directly without NAT.
• Scenario 2 explained inter-node communication, which involves routing via a CNI plugin.
• Kube-Proxy plays a key role in ensuring service connectivity across nodes.
• Troubleshooting Kubernetes networking starts with understanding how nodes, CNI, and Kube-Proxy work together.

If you’re working with Kubernetes, mastering node networking is essential for performance optimization and debugging connectivity issues.

Let’s Discuss

Have you faced networking issues in Kubernetes? Which CNI plugin do you use in your cluster? Share your experiences in the comments.

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