Setting up NodeLocal DNSCache

This page explains how to configure NodeLocal DNSCache on a Google Kubernetes Engine (GKE) cluster.

NodeLocal DNSCache is a GKE add-on that you can run in addition to kube-dns.

NodeLocal DNSCache improves DNS lookup latency, makes DNS lookup times more consistent, and reduces the number of DNS queries to kube-dns by running a DNS cache on each node in a cluster.

For an overview of how service discovery and managed DNS works on GKE, see Service discovery and DNS.

Architecture

GKE implements NodeLocal DNSCache as a DaemonSet that runs a DNS cache on each node in your cluster.

When a Pod makes a DNS request, the request goes to the DNS cache running on the same node as the Pod. If the cache can't resolve the DNS request, the cache forwards the request to one of the following places based on the query destination:

  • kube-dns: all queries for the cluster DNS domain (cluster.local) are forwarded to kube-dns. The node-local-dns Pods use the kube-dns Service to access kube-dns Pods.
  • Custom stub domains or upstream nameservers: queries are forwarded directly from NodeLocal DNSCache Pods.
  • Cloud DNS: all other queries are forwarded to the local metadata server that runs on the same node as the Pod the query originated from. The local metadata server accesses Cloud DNS.

A diagram of the path of a DNS request, as described in the previous paragraph

Benefits of NodeLocal DNSCache

NodeLocal DNSCache provides the following benefits:

  • Reduced average DNS lookup time
  • Connections from Pods to their local cache don't create conntrack table entries. This prevents dropped and rejected connections caused by conntrack table exhaustion and race conditions.
  • You can use NodeLocal DNSCache with Cloud DNS for GKE.
  • DNS queries for external URLs (URLs that don't refer to cluster resources) are forwarded directly to the local Cloud DNS metadata server, bypassing kube-dns.
  • The local DNS caches automatically pick up stub domains and upstream nameservers that are specified in the kube-dns ConfigMap.

Requirements and limitations

  • NodeLocal DNSCache consumes compute resources on each node of your cluster.
  • NodeLocal DNSCache is not supported with Windows Server node pools.
  • NodeLocal DNSCache requires GKE version 1.15 or later.
  • NodeLocal DNSCache accesses upstreamServers using TCP. This means that upstream servers must support DNS resolution using TCP and UDP. UDP is available in GKE versions 1.17.5-gke.6 and 1.18.3-gke.0 and later for custom configurations provided using kube-dns ConfigMap. As a best practice, use a DNS server that is reachable using TCP and UDP.
  • DNS records are cached for the following periods:
    • The record's time to live (TTL), or 30 seconds if the TTL is more than 30 seconds.
    • 5 seconds if the DNS response is NXDOMAIN.
  • NodeLocal DNSCache Pods listen on port 53, 9253, 9353, and 8080 on the nodes. If you run any other hostNetwork Pod or configure a hostPorts with those ports, NodeLocal DNSCache fails and DNS errors occur. NodeLocal DNSCache Pods do not use hostNetwork mode when using GKE Dataplane V2.
  • If your cluster has PodSecurityPolicy enabled, your cluster must be on one of the following versions to use NodeLocal DNSCache:
    • 1.15.12-gke.9 and later
    • 1.16.11-gke.5 and later
    • 1.17.7-gke.15 and later
  • The local DNS cache only runs on node pools running GKE versions 1.15 and later. If you enable NodeLocal DNSCache in a cluster with nodes running earlier versions, Pods on those nodes use kube-dns.

Enable NodeLocal DNSCache

You can enable NodeLocal DNSCache on new or existing clusters using the gcloud command-line tool. You can enable NodeLocal DNSCache in new clusters using the Google Cloud Console.

gcloud

Enable NodeLocal DNSCache in a new cluster

To enable NodeLocal DNSCache in a new cluster, use the --addons flag with the argument NodeLocalDNS:

gcloud container clusters create CLUSTER_NAME \
    --region=COMPUTE_REGION \
    --cluster-version=CLUSTER_VERSION \
    --addons=NodeLocalDNS

Replace the following:

  • CLUSTER_NAME: the name of your new cluster.
  • COMPUTE_REGION: the Compute Engine region for your cluster.
  • CLUSTER_VERSION: the version for your cluster (1.15 or later).

Enable NodeLocal DNSCache in an existing cluster

To enable NodeLocal DNSCache in an existing cluster, use the --update-addons flag with the argument NodeLocalDNS=ENABLED:

gcloud container clusters update CLUSTER_NAME \
    --update-addons=NodeLocalDNS=ENABLED

Replace the following:

  • CLUSTER_NAME: the name of your cluster.

Console

To enable NodeLocal DNSCache on a new cluster, use the following steps:

  1. Go to the Google Kubernetes Engine page in Cloud Console.

    Go to Google Kubernetes Engine

  2. Next to Standard, click Configure.

  3. Configure your cluster how you want.

  4. From the navigation pane, click Networking.

  5. In the Advanced networking options section, select the Enable NodeLocal DNSCache checkbox.

  6. Click Create.

Verify that NodeLocal DNSCache is enabled

You can verify that NodeLocal DNSCache is running by listing the node-local-dns Pods:

kubectl get pods -n kube-system -o wide | grep node-local-dns

The output is similar to the following:

node-local-dns-869mt    1/1   Running   0   6m24s   10.128.0.35   gke-test-pool-69efb6b8-5d7m   <none>   <none>
node-local-dns-htx4w    1/1   Running   0   6m24s   10.128.0.36   gke-test-pool-69efb6b8-wssk   <none>   <none>
node-local-dns-v5njk    1/1   Running   0   6m24s   10.128.0.33   gke-test-pool-69efb6b8-bhz3   <none>   <none>

The output shows a node-local-dns Pod for each node that is running GKE version 1.15 or later.

Disable NodeLocal DNSCache

You can disable NodeLocal DNSCache using the following command:

gcloud container clusters update CLUSTER_NAME \
    --update-addons=NodeLocalDNS=DISABLED

Replace the following:

  • CLUSTER_NAME: the name of the cluster to disable.

Troubleshoot NodeLocal DNSCache

For general information about diagnosing Kubernetes DNS issues, see Debugging DNS Resolution.

NodeLocal DNSCache with Cloud DNS

If you use NodeLocal DNSCache with Cloud DNS, the cluster uses the nameserver IP address 169.254.20.10, as shown in the following diagram:

NodeLocal DNSCache with Cloud DNS architecture

You can view the IP address of the Cluster IP by using the following command:

kubectl get svc -n kube-system kube-dns -o jsonpath="{.spec.clusterIP}"

The output is similar to the following:

169.254.20.10

Network policy with NodeLocal DNSCache

If you use network policy with NodeLocal DNSCache and you are not using Cloud DNS or GKE Dataplane V2, you must configure rules to permit node-local-dns Pods to send and receive DNS queries.

Use an ipBlock rule in your manifest to allow communication between node-local-dns Pods and kube-dns.

The following manifest describes a network policy that uses an ipBlock rule:

spec:
  egress:
  - ports:
    - port: 53
      protocol: TCP
    - port: 53
      protocol: UDP
    to:
    - ipBlock:
        cidr: KUBE_DNS_CLUSTER_IP/32
  podSelector: {}
  policyTypes:
    - Ingress
    - Egress

Replace KUBE_DNS_CLUSTER_IP with the IP address of the kube-dns service. You can get the IP address of the kube-dns service using the following command:

kubectl get svc -n kube-system kube-dns -o jsonpath="{.spec.clusterIP}"

Known issues

DNS timeout in ClusterFirstWithHostNet dnsPolicy when using NodeLocal DNSCache and GKE Dataplane V2

On clusters using GKE Dataplane V2 and NodeLocal DNSCache, pods with hostNetwork set to true and dnsPolicy set to ClusterFirstWithHostNet cannot reach cluster DNS backends. DNS logs might contain entries similar to the following:

nslookup: write to 'a.b.c.d': Operation not permitted

;; connection timed out; no servers could be reached

The output indicates that the DNS requests cannot reach the backend servers.

A workaround is to set the dnsPolicy and dnsConfig for hostNetwork pods:

spec:
 dnsPolicy: "None"
 dnsConfig:
   nameservers:
     - KUBE_DNS_UPSTREAM
   searches:
     - cluster.local
     - svc.cluster.local
     - NAMESPACE.svc.cluster.local
     - c.PROJECT_ID.internal
     - google.internal
   options:
     - name: ndots
       value: "5"

Replace the following:

  • NAMESPACE: the namespace of the hostNetwork pod.
  • PROJECT_ID: the ID of your Google Cloud project.
  • KUBE_DNS_UPSTREAM: the ClusterIP of the upstream kube-dns service. You can get this value using the following command:

    kubectl get svc -n kube-system kube-dns-upstream -o jsonpath="{.spec.clusterIP}"
    

DNS requests from the Pod can now reach kube-dns and bypass NodeLocal DNSCache.

NodeLocal DNSCache timeout errors

On clusters with NodeLocal DNSCache enabled, the logs might contain entries similar to the following:

[ERROR] plugin/errors: 2 <hostname> A: read tcp <node IP: port>-><kubedns IP>:53: i/o timeout

The output includes the IP address of the kube-dns-upstream Cluster IP Service. In this example, the response to a DNS request was not received from kube-dns in 2 seconds. This could be due to one of the following reasons:

  • An underlying network connectivity problem.
  • A known issue with dnsmasq handling TCP connections.

node-local-dns Pods access kube-dns using TCP for improved reliability. When handling connections from multiple source IPs, dnsmasq prioritizes connections from existing connections over new connections. As a result, on a cluster with high DNS queries per second (QPS), node-local-dns Pods on newly created nodes can experience higher DNS latency. This can also occur on clusters with cluster autoscaler enabled because cluster autoscaler dynamically changes the number of nodes.

This issue is resolved in the following GKE versions:

  • 1.19.7-gke.1500 and later
  • 1.18.16-gke.1200 and later
  • 1.17.17-gke.5400 and later

One workaround is to increase the number of kube-dns replicas by tuning the autoscaling parameters.

Scaling up kube-dns

You can use a lower value for nodesPerReplica to ensure that more kube-dns Pods are created as cluster nodes scale up. We highly recommend setting an explicit max value to ensure that the GKE control plane virtual machine (VM) is not overwhelmed due to large number of kube-dns pods watching the Kubernetes API.

You can set max to the number of nodes in the cluster. If the cluster has more than 500 nodes, set max to 500.

For Standard clusters, you can modify the number of kube-dns replicas by editing the kube-dns-autoscaler ConfigMap. This configuration is not supported in Autopilot clusters.

kubectl edit configmap kube-dns-autoscaler --namespace=kube-system

The output is similar to the following:

linear: '{"coresPerReplica":256, "nodesPerReplica":16,"preventSinglePointFailure":true}'

The number of kube-dns replicas is calculated using the following formula:

replicas = max( ceil( cores × 1/coresPerReplica ) , ceil( nodes × 1/nodesPerReplica ), maxValue )

To scale up, change nodesPerReplica to a smaller value and include a max value.

linear: '{"coresPerReplica":256, "nodesPerReplica":8,"max": 15,"preventSinglePointFailure":true}'

The config creates 1 kube-dns pod for every 8 nodes in the cluster. A 24-node cluster will have 3 replicas and a 40-node cluster will have 5 replicas. If the cluster grows beyond 120 nodes, the number of kube-dns replicas does not grow beyond 15, the max value.

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