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Troubleshooting CoreDNS Performance Issues: An Essential Guide for Startups on AWS EKS

Introduction

You're experiencing intermittent downtime, sluggish applications, and confused engineers. Infrastructure seems fine, applications pass tests, yet something feels wrong. Often overlooked, DNS—and specifically CoreDNS—might be quietly sabotaging your startup’s scaling ambitions.

Understanding CoreDNS Performance Issues

CoreDNS resolves internal domain names in Kubernetes. Performance issues here can ripple through your entire application stack, manifesting as latency spikes, service timeouts, or unexpected outages, significantly impacting user experience and overall service reliability.

Why CoreDNS Performance Matters for Growing Startups

Fast-growing startups scale rapidly, but underlying infrastructure components like CoreDNS often get overlooked. Reliable DNS resolution is critical as Kubernetes heavily relies on it for internal service discovery. Slow DNS resolution means slower applications, poor user experience, and potentially lost revenue.

Key Impacts:

  • Application latency increases dramatically

  • Service discovery reliability diminishes

  • Kubernetes control-plane instability and troubleshooting difficulty

A Framework for Troubleshooting CoreDNS Issues
Step 1: Recognize Symptoms

Early symptoms often include increased latency, intermittent SERVFAIL responses, slow deployments, and mysterious application performance degradation.

Step 2: Check CoreDNS Logs & Metrics

Quickly verify CoreDNS health:

  • Use kubectl logs on CoreDNS pods to find critical errors:

    kubectl logs deployment/coredns -n kube-system

  • Inspect latency metrics with Prometheus and Grafana:

    • coredns_forward_request_duration_seconds

    • coredns_cache_hits_total

Step 3: Validate AWS DNS Limits

AWS imposes limits on VPC DNS queries (1,024 DNS packets/sec per ENI). Exceeding this causes throttling and dropped requests. Check AWS CloudWatch metrics:

  • AWS Route 53 Resolver metrics

  • VPC DNS throttling events

Step 4: Examine CoreDNS Configuration

Misconfigurations are common. Confirm your setup:

  • Proper forward block

  • Appropriate caching settings

  • Anti-affinity and topology spread constraints

Common Mistakes Startups Make

  • Ignoring DNS Metrics: DNS metrics should be as crucial as CPU or memory monitoring.

  • Relying Solely on Defaults: Defaults work initially but break at scale. Tailor CoreDNS configuration for your specific workload.

  • No Node-Level Caching: NodeLocal DNSCache significantly reduces upstream DNS queries and latency.

"Never underestimate DNS. Invisible until it's not—then everything breaks."

Practical Benefits of Scaling CoreDNS Early
Addressing CoreDNS performance quickly yields:

  • Significantly improved application responsiveness

  • Reduced downtime and increased system reliability

  • Enhanced scalability and predictability

Startups experiencing rapid growth gain the most by proactively addressing these issues, avoiding costly outages.

Success Tips & Best Practices

  • Enable NodeLocal DNSCache: This drastically reduces upstream resolver load.

  • Optimize CoreDNS Caching: Balance cache size and TTL based on usage.

  • Implement Pod Spreading and Anti-Affinity: Ensure CoreDNS pods are resilient and evenly distributed across your infrastructure.

“Investing early in DNS troubleshooting and monitoring saved our startup countless hours during critical scaling moments.” — DevOps Lead, SaaS Startup

Conclusion

CoreDNS performance issues are often the hidden culprit behind scaling challenges. A methodical approach to troubleshooting and proactive optimization can protect your startup from subtle yet destructive downtime. Act early to ensure smooth and reliable growth.

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