Introduction

CoreDNS is a critical component in Amazon Elastic Kubernetes Service (EKS), providing DNS (Domain Name System) services within Kubernetes clusters. It is responsible for translating domain names into IP addresses, allowing pods and services within a Kubernetes cluster to communicate with each other using domain names rather than IPs.

What is CoreDNS?

CoreDNS is a flexible, extensible DNS server that acts as the default DNS provider in Kubernetes. It is used in Amazon EKS (and other Kubernetes platforms) to handle DNS resolution for internal cluster services. In simple terms, CoreDNS ensures that when Kubernetes services or pods try to connect to each other by their service names (like my-service.default.svc.cluster.local), CoreDNS resolves these names into the appropriate IP addresses.

How CoreDNS Works in EKS:

DNS Resolution: CoreDNS listens for DNS queries within the Kubernetes cluster. When a pod or service makes a request to resolve a DNS name, CoreDNS translates that name into an internal cluster IP. For example, if a pod tries to access a service like http://my-service, CoreDNS resolves my-service to the correct IP address within the cluster.
Service Discovery: CoreDNS enables service discovery in Kubernetes. Kubernetes services are exposed using DNS names, and CoreDNS automatically handles name resolution, so pods and services can communicate using these names. This eliminates the need for managing IP addresses manually.
Custom DNS Records: CoreDNS can be extended with custom plugins, allowing you to configure it to resolve DNS queries for external domains or use specific DNS configurations. You can also use it to forward queries to external DNS servers or other Kubernetes clusters.
Configurable: CoreDNS is highly configurable via the Corefile, where you can define rules and behaviors for DNS resolution. For example, you can set up caching, log DNS queries, or forward queries to another DNS server if needed.

Key Features of CoreDNS in EKS:

Automatic DNS Resolution: CoreDNS automatically resolves service names to IPs within the Kubernetes cluster, simplifying communication between services and pods.
Service Discovery: Pods can access services via their DNS names, which is important for dynamic environments like Kubernetes where IP addresses are not static.
Performance and Scalability: CoreDNS is lightweight and designed to be performant in large-scale environments. It uses a plugin-based architecture that can be extended or customized.
Integration with Kubernetes: CoreDNS is tightly integrated into Kubernetes, making it the default DNS service. It can handle DNS for both internal Kubernetes services and external DNS queries.
Customizable and Extensible: You can extend CoreDNS with custom plugins or modify its configuration to suit your specific DNS needs.

CoreDNS in EKS: How it Works

In Amazon EKS, CoreDNS runs as a set of pods within the kube-system namespace. These pods serve as the DNS service for the entire cluster. When you create a Kubernetes service in EKS, CoreDNS automatically adds the necessary DNS records, allowing pods to reach services by name.

Example Use Case

If you have a service named my-api in the default namespace of your EKS cluster, the DNS name for this service would be my-api.default.svc.cluster.local. When a pod needs to access this service, it can simply use this DNS name (e.g., http://my-api.default.svc.cluster.local), and CoreDNS will resolve it to the correct IP address.

Conclusion

CoreDNS in EKS is essential for service discovery and DNS resolution within your Kubernetes cluster. It ensures that pods and services can communicate with each other using easily manageable DNS names instead of hardcoded IPs. CoreDNS is highly configurable, lightweight, and designed for cloud-native environments like EKS, making it a crucial component in any Kubernetes deployment.

Understanding CoreDNS in Amazon EKS

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