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Kubernetes v1.15 documentation is no longer actively maintained. The version you are currently viewing is a static snapshot. For up-to-date documentation, see the latest version.

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Using NodeLocal DNSCache in Kubernetes clusters

This page provides an overview of NodeLocal DNSCache feature in Kubernetes.

Before you begin
Introduction
Motivation
Architecture Diagram
Configuration

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

Introduction

NodeLocal DNSCache improves Cluster DNS performance by running a dns caching agent on cluster nodes as a DaemonSet. In today’s architecture, Pods in ClusterFirst DNS mode reach out to a kube-dns serviceIP for DNS queries. This is translated to a kube-dns/CoreDNS endpoint via iptables rules added by kube-proxy. With this new architecture, Pods will reach out to the dns caching agent running on the same node, thereby avoiding iptables DNAT rules and connection tracking. The local caching agent will query kube-dns service for cache misses of cluster hostnames(cluster.local suffix by default).

Motivation

With the current DNS architecture, it is possible that Pods with the highest DNS QPS have to reach out to a different node, if there is no local kube-dns/CoreDNS instance.
Having a local cache will help improve the latency in such scenarios.
Skipping iptables DNAT and connection tracking will help reduce conntrack races and avoid UDP DNS entries filling up conntrack table.
Connections from local caching agent to kube-dns servie can be upgraded to TCP. TCP conntrack entries will be removed on connection close in contrast with UDP entries that have to timeout (default nf_conntrack_udp_timeout is 30 seconds)
Upgrading DNS queries from UDP to TCP would reduce tail latency attributed to dropped UDP packets and DNS timeouts usually up to 30s (3 retries + 10s timeout). Since the nodelocal cache listens for UDP DNS queries, applications don’t need to be changed.
Metrics & visibility into dns requests at a node level.
Negative caching can be re-enabled, thereby reducing number of queries to kube-dns service.

Architecture Diagram

This is the path followed by DNS Queries after NodeLocal DNSCache is enabled:

NodeLocal DNSCache flow — Nodelocal DNSCache flow
This image shows how NodeLocal DNSCahe handles DNS queries.

Configuration

This feature can be enabled using the command:

KUBE_ENABLE_NODELOCAL_DNS=true go run hack/e2e.go -v --up

This works for e2e clusters created on GCE. On all other environments, the following steps will setup NodeLocal DNSCache:

A yaml similar to this can be applied using kubectl create -f command.
–cluster-dns flag to kubelet needs to be modified to use the LOCAL_DNS IP that NodeLocal DNSCache is listening on (169.254.20.10 by default)

Once enabled, node-local-dns Pods will run in the kube-system namespace on each of the cluster nodes. This Pod runs CoreDNS in cache mode, so all CoreDNS metrics exposed by the different plugins will be available on a per-node basis.

Feature availability

The addon can be applied using the yaml specified above in any k8s version. The feature support is as described:

k8s version	Feature support
1.15	Beta(Not enabled by default)
1.13	Alpha(Not enabled by default)

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