Essential DNS Tools for Kubernetes CoreDNS and ExternalDNS

Explore the roles of CoreDNS and ExternalDNS in Kubernetes, their functions, and how they work together to manage internal and external DNS.

Introduction

CoreDNS and ExternalDNS are two essential components commonly used in Kubernetes clusters for DNS management. They serve different purposes but can work together to provide a seamless DNS experience in a Kubernetes environment. Here's an overview of each and how they can be used together:

CoreDNS

CoreDNS is the default DNS server for Kubernetes. It is responsible for service discovery within the cluster. CoreDNS translates Kubernetes Service names into IP addresses, allowing pods to communicate with each other using service names.

#### Key Functions of CoreDNS:

Service Discovery: Resolves internal Kubernetes service names to their corresponding ClusterIP.

Pod DNS: Resolves pod names to their IP addresses within the cluster.

Custom DNS Configuration: Can be configured to forward DNS queries to external DNS servers or provide custom DNS zones.

CoreDNS Configuration Example:

The CoreDNS configuration is typically found in the ConfigMap associated with the kube-system namespace. Here's an example of a CoreDNS Corefile:

``bash


apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health {
          lameduck 5s
        }
        ready
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
          ttl 30
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }



ExternalDNS

ExternalDNS is a Kubernetes add-on that synchronizes Kubernetes service and ingress resources with external DNS providers. While CoreDNS handles internal DNS resolution, ExternalDNS manages DNS entries with external DNS providers like AWS Route 53, Google Cloud DNS, or Azure DNS.

Key Functions of ExternalDNS:
Automatic DNS Record Management: Creates, updates, or deletes DNS records in external DNS providers based on the state of Kubernetes resources like services or ingresses.
Supports Multiple DNS Providers: Works with various cloud DNS providers and even traditional DNS servers.
Works with Ingress Controllers: Automatically manages DNS records for ingress resources, which is particularly useful for dynamically exposed services.

CoreDNS and ExternalDNS

In a typical Kubernetes setup, CoreDNS handles internal DNS resolution, while ExternalDNS manages external DNS entries. Here’s how you might set up both in an AKS (Azure Kubernetes Service) cluster:

1. Deploy CoreDNS (Usually Installed by Default in AKS)

In AKS, CoreDNS is installed by default as the cluster’s DNS server. You generally don't need to deploy it separately unless you're using a custom Kubernetes setup.

2. Deploy ExternalDNS in Your Cluster

To set up ExternalDNS in an AKS cluster, follow these steps:

#### Step 1: Install ExternalDNS Using Helm

``bash

helm repo add bitnami http