#02 - Kubernetes Networking

Kubernetes is a container orchestration platform. It is used to help containers in a cluster communicate between them in an easy way. This is where networking in Kubernetes kicks in. To understand how Kubernetes works, knowing its basic underlying networking concepts is a fundamental necessity.

The basic objects that communicate in the Kubernetes network are

• Nodes: real machines in the cluster
• Pods: containers that are encapsulated
  • They are the atomic unit in Kubernetes
  • A pod may contain multiple containers (even though they usually run only one)

Kubernetes networking is based on a flat network structure and does not require you to map ports between hosts and containers. This means:

• All nodes in the cluster have to be able to talk to each other.
• All pods can communicate without NAT.
• Every pod gets its own IP address.

The following diagram shows a generalized view of how Kubernetes networking is implemented.

Kubernetes Networking Scenarios

There are four common networking scenarios

• Container-to-Container Networking (Handled by Pods)
• Pod-to-Pod Networking (Handled by Pods)
• Pod-to-Service Networking (Handled by Services)
• Internet-to-Service Networking (Handled by Services)

1. Container-to-Container Networking

• This occurs between containers that are within the same pod.
• The containers in a pos share a single IP and port space.

2. Pod-to-Pod Networking

This occurs between-

• Pods in the same node
• Pods across multiple nodes

3. Pod-to-Service Networking (Handled by Services)

Kubernetes allows pods to be created and replaced dynamically. Thus, pods running a particular service will have varying IP addresses over time. To address this issue services are used.

Services abstract pod addresses by assigning a single virtual IP (a cluster IP) to a group of pod IPs. Then, any traffic sent to the virtual IP is distributed to the associated pods.

There are three ways to map services to pods-

• ClusterIP (default)
  • Gets own IP
  • Service accessible only from within the cluster
• Nodeport
  • Gets cluster-wide port
  • service accessible from outside of the cluster
• Loadbalancer
  • Integrates with a public cloud load balancer (e.g. AWS ELB, GCP CLB)

4. Internet-to-Service Networking (Handled by Services)

The final networking situation that is needed for most deployments is between the Internet and services. This connectivity enables end-users to access your services.

They are of two types-

• Ingress: Ingress routes traffic an outside connection to a service in your cluster
• Egress: Egress routes traffic from your node to an outside connection

That was all about the basics of networking in Kubernetes. Head over to the next section about Kubernetes storage.

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