CoreDNS is a DNS resolver for Kubernetes clusters. CoreDNS can resolve custom internal domain names and external domain names. CoreDNS provides a variety of plug-ins. You can use these plug-ins to configure custom DNS settings, and customize host records, Canonical Name (CNAME) records, and rewrite rules for Kubernetes clusters. The default CoreDNS deployment in ACK clusters may be unable to handle DNS queries efficiently when the DNS queries per second (QPS) increases. In this case, some DNS queries may fail. This topic describes how to optimize DNS resolution for an ACK cluster.

Background information

For more information about CoreDNS, see CoreDNS.

Adjust the number of CoreDNS pods

Setting an appropriate ratio of CoreDNS pods to nodes in a cluster improves the performance of service discovery for the cluster. We recommend that you set the ratio to 1:8.

  • If you do not want to scale your cluster on a large scale, you can run the following command to adjust the number of pods: 
    kubectl scale --replicas={target} deployment/coredns -n kube-system
    Note Replace {target} with the required value.
  • If you want to scale your cluster on a large scale, you can use the following YAML template to create a Deployment and use cluster-proportional-autoscaler to dynamically adjust the number of pods. We recommend that you do not use Horizontal Pod Autoscaler (HPA) to scale the number of pods based on the QPS, CPU utilization, or memory usage of pods. HPA cannot meet the requirements in actual tests. 
    ---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: dns-autoscaler
  namespace: kube-system
  labels:
    k8s-app: dns-autoscaler
spec:
  selector:
    matchLabels:
      k8s-app: dns-autoscaler
  template:
    metadata:
      labels:
        k8s-app: dns-autoscaler
    spec:
      serviceAccountName: admin
      containers:
      - name: autoscaler
        image: registry.cn-hangzhou.aliyuncs.com/acs/cluster-proportional-autoscaler:1.8.4
        resources:
          requests:
            cpu: "200m"
            memory: "150Mi"
        command:
        - /cluster-proportional-autoscaler
        - --namespace=kube-system
        - --configmap=dns-autoscaler
        - --nodelabels=type!=virtual-kubelet
        - --target=Deployment/coredns
        - --default-params={"linear":{"coresPerReplica":64,"nodesPerReplica":8,"min":2,"max":100,"preventSinglePointFailure":true}}
        - --logtostderr=true
        - --v=9
    Note In the preceding example, a linear scaling policy is used. The number of CoreDNS pods is calculated based on the following formula: Replicas (pods) = Max (Ceil (Cores × 1/coresPerReplica), Ceil (Nodes × 1/nodesPerReplica)). The number of CoreDNS pods is subject to the values of max and min in the linear scaling policy.

    The following code block shows the parameters of the linear scaling policy: 

    {
      "coresPerReplica": 64,
      "nodesPerReplica": 8,
      "min": 2,
      "max": 100,
      "preventSinglePointFailure": true
}

Deploy NodeLocal DNSCache on cluster nodes

ACK allows you to deploy NodeLocal DNSCache to improve the stability and performance of service discovery. NodeLocal DNSCache is implemented as a DaemonSet and runs a DNS caching agent on cluster nodes to improve the DNS performance.

For more information about NodeLocal DNSCache and how to deploy NodeLocal DNSCache in ACK clusters, see Use NodeLocal DNSCache in an ACK cluster.