Resolving DNS Resolution Failures in Kubernetes

Troubleshoot CoreDNS performance issues and service discovery failures by inspecting cluster-wide DNS query logs and cache configurations.

In this lesson, you will learn how to diagnose and resolve persistent DNS resolution failures by inspecting CoreDNS logs and fine-tuning internal caching mechanisms.

When applications in your cluster report intermittent NXDOMAIN errors or timeouts, the bottleneck often lies within CoreDNS's request processing or local cache exhaustion. As a cluster operator, your first step is to enable granular visibility into the traffic hitting your DNS servers.

Inspecting CoreDNS Traffic

You can gain immediate insight into resolution patterns by enabling the log plugin in your Corefile. Edit your coredns ConfigMap to include the log directive:

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        log
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
        }
        forward . /etc/resolv.conf
        cache 30
    }

Once applied, CoreDNS will log every query to stdout. Use kubectl logs -n kube-system -l k8s-app=kube-dns to observe whether high volumes of requests are failing or if specific subdomains are repeatedly triggering upstream forwarded queries instead of being served from the local cache.

Optimizing the Cache

If logs reveal that your CoreDNS pods are struggling under high load—indicated by high latency or CPU saturation—it is often due to a "cache miss" storm. CoreDNS’s cache plugin (set to 30 seconds in the snippet above) determines how long successful responses are kept in memory. If your application creates many ephemeral connections to external services, you may be hitting the upstream DNS server too frequently.

You can improve performance by increasing the cache TTL or adding a prefetch directive to the cache block: cache 120 { prefetch }

This allows CoreDNS to refresh records before they expire, ensuring that your application traffic doesn't experience the latency hit of a synchronous upstream lookup. Always balance this against the risk of serving stale data if your external environment changes frequently.

Key Takeaway: To resolve DNS failures, enable the CoreDNS log plugin to identify query patterns and tune the cache plugin with prefetch to minimize latency and offload upstream infrastructure.