Core DNS

The default DNS provider in Kubernetes is CoreDNS, which runs as pods/containers inside the cluster. CoreDNS retrieves pod/service information from the Kubernetes API to update its DNS records.
CoreDNS is a flexible, extensible DNS server that is used in Kubernetes clusters for service discovery and name resolution. It is the default DNS server for Kubernetes clusters since version 1.11, replacing kube-dns.

Key Functions

1. Service Discovery:

   • CoreDNS helps Kubernetes Pods discover and connect to services within the cluster. It provides DNS-based service discovery by resolving service names to their corresponding cluster IP addresses.

2. DNS Resolution:

   • CoreDNS resolves DNS queries from within the cluster, translating service names into IP addresses. For example, a Pod querying for my-service.default.svc.cluster.local will receive the IP address of the service.

3. Custom DNS Records:

   • CoreDNS supports custom DNS records, allowing administrators to define additional DNS records for applications or services.

Configuration

• CoreDNS ConfigMap:
  • CoreDNS is configured via a ConfigMap in the kube-system namespace. This ConfigMap defines how CoreDNS should handle DNS queries, including upstream DNS servers, caching, and custom DNS rules.
  • Example ConfigMap (simplified):

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

Features

1. Plugin System:

   • CoreDNS uses a modular plugin architecture, allowing you to add or remove functionality through plugins. Examples include kubernetes, forward, cache, and prometheus.

2. Health Checks:

   • CoreDNS supports health checks to ensure that it is operational and serving DNS requests correctly.

3. Metrics:

   • CoreDNS exposes metrics for monitoring via Prometheus, which helps track DNS query performance and server health.

4. Upstream DNS Servers:

   • CoreDNS can forward DNS queries to upstream DNS servers if it cannot resolve them internally.

Deployment

• Kubernetes Deployment:

  • CoreDNS is deployed as a set of Pods in the kube-system namespace and is managed by a Deployment object. Kubernetes Service objects expose CoreDNS to other Pods for DNS resolution.

• Scaling and Updates:

  • CoreDNS can be scaled horizontally by adjusting the number of replicas in the Deployment. Updates to CoreDNS or its configuration are applied by updating the Deployment or ConfigMap.

Summary

• CoreDNS is the default DNS service for Kubernetes clusters, handling service discovery and DNS resolution within the cluster.
• It is configured using a ConfigMap in the kube-system namespace and supports a flexible plugin system for extending its functionality.
• CoreDNS provides features like health checks, metrics, and upstream DNS forwarding, making it a robust solution for DNS management in Kubernetes.

Related Hashtags: #CoreDNS #Kubernetes #DNS #ServiceDiscovery #ClusterNetworking