CoreDNS

CoreDNS is a DNS server that chains plugins and provides DNS Services

TL;DR;

$ helm repo add coredns https://coredns.github.io/helm
$ helm --namespace=kube-system install coredns coredns/coredns

Introduction

This chart bootstraps a CoreDNS deployment on a Kubernetes cluster using the Helm package manager. This chart will provide DNS Services and can be deployed in multiple configuration to support various scenarios listed below:

• CoreDNS as a cluster dns service and a drop-in replacement for Kube/SkyDNS. This is the default mode and CoreDNS is deployed as cluster-service in kube-system namespace. This mode is chosen by setting isClusterService to true.
• CoreDNS as an external dns service. In this mode CoreDNS is deployed as any kubernetes app in user specified namespace. The CoreDNS service can be exposed outside the cluster by using using either the NodePort or LoadBalancer type of service. This mode is chosen by setting isClusterService to false.
• CoreDNS as an external dns provider for kubernetes federation. This is a sub case of 'external dns service' which uses etcd plugin for CoreDNS backend. This deployment mode as a dependency on etcd-operator chart, which needs to be pre-installed.

Prerequisites

• Kubernetes 1.10 or later

Installing the Chart

The chart can be installed as follows:

$ helm repo add coredns https://coredns.github.io/helm
$ helm --namespace=kube-system install coredns coredns/coredns

The command deploys CoreDNS on the Kubernetes cluster in the default configuration. The configuration section lists various ways to override default configuration during deployment.

Tip: List all releases using helm list --all-namespaces

Uninstalling the Chart

To uninstall/delete the coredns deployment:

$ helm uninstall coredns

The command removes all the Kubernetes components associated with the chart and deletes the release.

Configuration

See values.yaml for configuration notes. Specify each parameter using the --set key=value[,key=value] argument to helm install. For example,

$ helm install coredns \
  coredns/coredns \
  --set rbac.create=false

The above command disables automatic creation of RBAC rules.

Alternatively, a YAML file that specifies the values for the above parameters can be provided while installing the chart. For example,

$ helm install coredns coredns/coredns -f values.yaml

Tip: You can use the default values.yaml

Caveats

The chart will automatically determine which protocols to listen on based on the protocols you define in your zones. This means that you could potentially use both "TCP" and "UDP" on a single port. Some cloud environments like "GCE" or "Azure container service" cannot create external loadbalancers with both "TCP" and "UDP" protocols. So When deploying CoreDNS with serviceType="LoadBalancer" on such cloud environments, make sure you do not attempt to use both protocols at the same time.

Autoscaling

By setting autoscaler.enabled = true a cluster-proportional-autoscaler will be deployed. This will default to a coredns replica for every 256 cores, or 16 nodes in the cluster. These can be changed with autoscaler.coresPerReplica and autoscaler.nodesPerReplica. When cluster is using large nodes (with more cores), coresPerReplica should dominate. If using small nodes, nodesPerReplica should dominate.

This also creates a ServiceAccount, ClusterRole, and ClusterRoleBinding for the autoscaler deployment.

replicaCount is ignored if this is enabled.

By setting hpa.enabled = true a Horizontal Pod Autoscaler is enabled for Coredns deployment. This can scale number of replicas based on meitrics like CpuUtilization, MemoryUtilization or Custom ones.