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

Parameter	Description	Default
image.repository	The image repository to pull from	coredns/coredns
image.tag	The image tag to pull from	1.8.7
image.pullPolicy	Image pull policy	IfNotPresent
image.pullSecrets	Specify container image pull secrets	[]
replicaCount	Number of replicas	1
resources.limits.cpu	Container maximum CPU	100m
resources.limits.memory	Container maximum memory	128Mi
resources.requests.cpu	Container requested CPU	100m
resources.requests.memory	Container requested memory	128Mi
serviceType	Kubernetes Service type	ClusterIP
prometheus.service.enabled	Set this to true to create Service for Prometheus metrics	false
prometheus.service.annotations	Annotations to add to the metrics Service	{prometheus.io/scrape: "true", prometheus.io/port: "9153"}
prometheus.monitor.enabled	Set this to true to create ServiceMonitor for Prometheus operator	false
prometheus.monitor.additionalLabels	Additional labels that can be used so ServiceMonitor will be discovered by Prometheus	{}
prometheus.monitor.namespace	Selector to select which namespaces the Endpoints objects are discovered from.	""
prometheus.monitor.interval	Scrape interval for polling the metrics endpoint. (E.g. "30s")	""
service.clusterIP	IP address to assign to service	""
service.loadBalancerIP	IP address to assign to load balancer (if supported)	""
service.externalIPs	External IP addresses	[]
service.externalTrafficPolicy	Enable client source IP preservation	[]
service.annotations	Annotations to add to service	{}
serviceAccount.create	If true, create & use serviceAccount	false
serviceAccount.name	If not set & create is true, use template fullname
rbac.create	If true, create & use RBAC resources	true
rbac.pspEnable	Specifies whether a PodSecurityPolicy should be created.	false
isClusterService	Specifies whether chart should be deployed as cluster-service or normal k8s app.	true
priorityClassName	Name of Priority Class to assign pods	""
servers	Configuration for CoreDNS and plugins	See values.yml
livenessProbe.enabled	Enable/disable the Liveness probe	true
livenessProbe.initialDelaySeconds	Delay before liveness probe is initiated	60
livenessProbe.periodSeconds	How often to perform the probe	10
livenessProbe.timeoutSeconds	When the probe times out	5
livenessProbe.failureThreshold	Minimum consecutive failures for the probe to be considered failed after having succeeded.	5
livenessProbe.successThreshold	Minimum consecutive successes for the probe to be considered successful after having failed.	1
readinessProbe.enabled	Enable/disable the Readiness probe	true
readinessProbe.initialDelaySeconds	Delay before readiness probe is initiated	30
readinessProbe.periodSeconds	How often to perform the probe	10
readinessProbe.timeoutSeconds	When the probe times out	5
readinessProbe.failureThreshold	Minimum consecutive failures for the probe to be considered failed after having succeeded.	5
readinessProbe.successThreshold	Minimum consecutive successes for the probe to be considered successful after having failed.	1
affinity	Affinity settings for pod assignment	{}
nodeSelector	Node labels for pod assignment	{}
tolerations	Tolerations for pod assignment	[]
zoneFiles	Configure custom Zone files	[]
extraVolumes	Optional array of volumes to create	[]
extraVolumeMounts	Optional array of volumes to mount inside the CoreDNS container	[]
extraSecrets	Optional array of secrets to mount inside the CoreDNS container	[]
customLabels	Optional labels for Deployment(s), Pod, Service, ServiceMonitor objects	{}
rollingUpdate.maxUnavailable	Maximum number of unavailable replicas during rolling update	1
rollingUpdate.maxSurge	Maximum number of pods created above desired number of pods	25%
podDisruptionBudget	Optional PodDisruptionBudget	{}
podAnnotations	Optional Pod only Annotations	{}
terminationGracePeriodSeconds	Optional duration in seconds the pod needs to terminate gracefully.	30
preStopSleep	Definition of Kubernetes preStop hook executed before Pod termination	{}
hpa.enabled	Enable Hpa autoscaler instead of proportional one	false
hpa.minReplicas	Hpa minimum number of CoreDNS replicas	1
hpa.maxReplicas	Hpa maximum number of CoreDNS replicas	2
hpa.metrics	Metrics definitions used by Hpa to scale up and down	{}
autoscaler.enabled	Optionally enabled a cluster-proportional-autoscaler for CoreDNS	false
autoscaler.coresPerReplica	Number of cores in the cluster per CoreDNS replica	256
autoscaler.nodesPerReplica	Number of nodes in the cluster per CoreDNS replica	16
autoscaler.min	Min size of replicaCount	0
autoscaler.max	Max size of replicaCount	0 (aka no max)
autoscaler.includeUnschedulableNodes	Should the replicas scale based on the total number or only schedulable nodes	false
autoscaler.preventSinglePointFailure	If true does not allow single points of failure to form	true
autoscaler.customFlags	A list of custom flags to pass into cluster-proportional-autoscaler	(no args)
autoscaler.image.repository	The image repository to pull autoscaler from	k8s.gcr.io/cluster-proportional-autoscaler-amd64
autoscaler.image.tag	The image tag to pull autoscaler from	1.8.1
autoscaler.image.pullPolicy	Image pull policy for the autoscaler	IfNotPresent
autoscaler.image.pullSecrets	Specify container image pull secrets	[]
autoscaler.priorityClassName	Optional priority class for the autoscaler pod. priorityClassName used if not set.	""
autoscaler.affinity	Affinity settings for pod assignment for autoscaler	{}
autoscaler.nodeSelector	Node labels for pod assignment for autoscaler	{}
autoscaler.tolerations	Tolerations for pod assignment for autoscaler	[]
autoscaler.resources.limits.cpu	Container maximum CPU for cluster-proportional-autoscaler	20m
autoscaler.resources.limits.memory	Container maximum memory for cluster-proportional-autoscaler	10Mi
autoscaler.resources.requests.cpu	Container requested CPU for cluster-proportional-autoscaler	20m
autoscaler.resources.requests.memory	Container requested memory for cluster-proportional-autoscaler	10Mi
autoscaler.configmap.annotations	Annotations to add to autoscaler config map. For example to stop CI renaming them	{}
autoscaler.livenessProbe.enabled	Enable/disable the Liveness probe	true
autoscaler.livenessProbe.initialDelaySeconds	Delay before liveness probe is initiated	10
autoscaler.livenessProbe.periodSeconds	How often to perform the probe	5
autoscaler.livenessProbe.timeoutSeconds	When the probe times out	5
autoscaler.livenessProbe.failureThreshold	Minimum consecutive failures for the probe to be considered failed after having succeeded.	3
autoscaler.livenessProbe.successThreshold	Minimum consecutive successes for the probe to be considered successful after having failed.	1
deployment.enabled	Optionally disable the main deployment and its respective resources.	true
deployment.name	Name of the deployment if deployment.enabled is true. Otherwise the name of an existing deployment for the autoscaler or HPA to target.	""

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.