Setting up NodeLocal DNSCache

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

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

Overview

NodeLocal DNSCache is an optional GKE add-on that you can run in addition to kube-dns. NodeLocal DNSCache is implemented 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:

  • Cloud DNS for external hostname queries. These queries are forwarded to Cloud DNS by the local MetaData Server running on the same node as the Pod the query originated from.
  • kube-dns for all other DNS queries.

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

Pods do not need to be modified to use NodeLocal DNSCache. NodeLocal DNSCache consumes compute resources on each node of your cluster.

Benefits of NodeLocal DNSCache

  • 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.

Details

  • NodeLocal DNSCache requires GKE version 1.15 or higher.
  • Connections between the local DNS cache and kube-dns use TCP instead of UDP for improved reliability.
  • 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.
  • DNS records are cached for:
    • The record's TTL, or 30 seconds if the TTL is more than 30 seconds.
    • 5 seconds if the DNS response is NXDOMAIN.

Enabling NodeLocal DNSCache

You can enable NodeLocal DNSCache in an existing cluster or when creating a new cluster. Enabling NodeLocal DNSCache in an existing cluster is a disruptive process, all cluster nodes running GKE 1.15 and above are recreated.

gcloud

First, enable access to beta features in gcloud tool:

gcloud components install beta

Enabling NodeLocal DNSCache in a new cluster

Use the --addons NodeLocalDNS option to enable local DNS caching:

gcloud beta container clusters create "my-cluster" \
  --zone "us-central1-c"                           \
  --cluster-version "1.15"                         \
  --addons NodeLocalDNS

Enabling NodeLocal DNSCache in an existing cluster

Use the --update-addons=NodeLocalDNS=ENABLED option to enable local DNS caching:

gcloud beta container clusters update "my-cluster" \
  --update-addons=NodeLocalDNS=ENABLED

Verifying that NodeLocal DNSCache is enabled

You can verify that NodeLocal DNSCache is running by listing the node-local-dns Pods. There should be a node-local-dns Pod running on each node running GKE version 1.15 or higher.

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

Disabling NodeLocal DNSCache

NodeLocal DNSCache can be disabled using gcloud:

gcloud beta container clusters update "my-cluster" \
  --update-addons=NodeLocalDNS=DISABLED

Troubleshooting NodeLocal DNSCache

See Debugging DNS Resolution for general information about diagnosing Kubernetes DNS issues.

Validating Pod configuration

To verify that a Pod is using NodeLocal DNSCache, check /etc/resolv.conf on the Pod to see if the Pod is configured to use the correct nameserver:

kubectl exec -it <podname> -- cat /etc/resolv.conf | grep nameserver

The nameserver IP should match the IP address output by:

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

If the nameserver IP address configured in /etc/resolv.conf doesn't match, you need to modify the configuration to use the correct nameserver IP address.

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