Scaling CoreDNS on AWS EKS: How Startups Can Avoid DNS Pitfalls at Scale

Introduction

Imagine this scenario: you're scaling rapidly, traffic surges, and suddenly services start failing, not due to application bugs but DNS timeouts. Your AWS EKS infrastructure might scale effortlessly, but if your DNS setup isn't scaling with it, you're in trouble. Let's explore how to avoid these pitfalls.

What is CoreDNS?

CoreDNS is a flexible, Kubernetes-native DNS server responsible for resolving domain names within your EKS clusters. It maps services to IPs, enabling reliable internal communication and service discovery crucial for smooth operations.

Why Scaling CoreDNS Matters for Startups

Many fast-growing startups prioritize rapid feature delivery and scaling applications, sometimes overlooking underlying DNS infrastructure. However, DNS scalability directly impacts:

Service reliability and latency
User experience
Infrastructure resilience during high traffic or growth

CoreDNS problems are subtle yet catastrophic. High latency, request throttling, or even minor misconfigurations can ripple across your entire stack, bringing down critical user-facing services. Ensuring CoreDNS scalability early on can save hours of troubleshooting later.

The CoreDNS Scaling Framework: A Step-by-Step Approach

Step 1: Understand AWS VPC DNS Limits

AWS’s built-in DNS resolver has hard limits:

Each network interface: 1,024 DNS packets/sec
Route 53 resolver endpoints: 10,000 queries/sec per endpoint

Exceed these, and queries fail silently or introduce latency. Monitor AWS CloudWatch Resolver metrics regularly.

Step 2: Configure CoreDNS with Proper Forwarding and Cache

Set your forward plugin carefully. Use caching wisely:

forward . /etc/resolv.conf {
  max_concurrent 1000
  policy round_robin
  health_check 5s
}
cache {
  success 20000 600
  denial 20000 600
  prefetch 5
}

Step 3: Implement NodeLocal DNSCache

Deploy NodeLocal DNSCache to reduce queries hitting AWS resolvers directly:

Reduces AWS DNS throttling risk
Improves latency and service responsiveness

Step 4: Spread CoreDNS Pods Across Zones

Use Kubernetes features like topologySpreadConstraints and Pod anti-affinity to ensure your DNS infrastructure doesn't become a single point of failure.

Common Misunderstandings

"DNS isn't a scaling bottleneck."

Misconception: DNS services don’t require special handling or scaling. Reality: DNS scales differently—issues are subtle, pervasive, and impactful.

"Default Kubernetes DNS settings are good enough."

Defaults are fine initially but can break at scale. Customized CoreDNS setups become necessary for stable scaling.

"Defaults break at scale. CoreDNS is no exception."

Practical Benefits of Scaling CoreDNS Early

Reduced downtime and fewer mysterious network issues
Improved response times and overall user experience
Easier identification and resolution of infrastructure bottlenecks
Increased infrastructure robustness and reliability during scaling events

Success Tips & Best Practices

Monitor aggressively: Track CoreDNS metrics (coredns_forward_request_duration_seconds) and AWS resolver limits.
Use horizontal autoscaling: Match CoreDNS replica counts to cluster size and workload intensity.
Minimize external dependencies: Cache aggressively, reduce reliance on AWS’s VPC DNS, and use NodeLocal caches.

Conclusion

Scaling CoreDNS correctly isn’t just "nice-to-have"; it's essential for resilient, high-performing cloud infrastructure. Fast-growing startups often underestimate DNS impacts, paying a heavy price later. Proactively scaling your CoreDNS setup on AWS EKS ensures smooth infrastructure transitions and service reliability.

FAQ