CoreDNS: AI-Optimized Technical Reference

Technology Overview

Primary Function: Modern DNS server written in Go, default DNS solution for Kubernetes clusters since v1.11

Key Differentiator: Plugin-based architecture where everything is a plugin, replacing the previous three-container kube-dns system with single binary deployment

Critical Context: Replaced kube-dns due to reliability issues with multi-container coordination (kubedns, dnsmasq, sidecar containers)

Configuration Requirements

Production-Ready Basic Configuration

.:53 {
    forward . 8.8.8.8 9.9.9.9
    cache 30
    log
    errors
}

Kubernetes Production Configuration

.:53 {
    errors
    health {
        lameduck 5s
    }
    ready
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
        ttl 30
    }
    prometheus :9153
    forward . /etc/resolv.conf
    cache 30
    loop
    reload
    loadbalance
}

company.internal:53 {
    forward . 10.0.1.10 10.0.1.11
    cache 300
}

Security Configuration with ACL

.:53 {
    acl {
        allow net 10.0.0.0/8
        allow net 192.168.0.0/16
        block net 0.0.0.0/0
    }
    kubernetes cluster.local in-addr.arpa ip6.arpa
    forward . 8.8.8.8
}

Critical Warnings and Failure Modes

Plugin Execution Order

• Critical Issue: Plugins execute in compile-time order from plugin.cfg, NOT config file order
• Failure Impact: Cache plugin may run first even when placed last in Corefile
• Detection: Check plugin.cfg file for actual execution sequence
• Consequence: Debugging DNS issues without understanding can take hours

Configuration Syntax Gotchas

• Silent Failure: Mixed tabs and spaces cause CoreDNS to ignore entire configuration blocks
• Impact: Changes appear ignored, no error messages generated
• Versions Affected: v1.11.1 had specific silent failure bug with mixed indentation
• Debugging Time: Can consume entire weekends troubleshooting "working" configs

Hot Reload Limitations

• Method: kill -SIGUSR1 or reload plugin
• Reliability: Flaky with complex multi-plugin configurations
• Failure Handling: Process restart required when hot reload fails
• Production Impact: Not truly zero-downtime for complex configs

Resource Requirements and Performance

Baseline Resources

• Minimum: 1 CPU, 512MB RAM
• Scaling Factor: Highly dependent on plugin count and DNS query volume
• Memory Risk: Improper cache configuration can consume all available memory

Performance Thresholds

• Cache TTL Impact: Hour-long caching (3600s) causes memory issues, optimal range 30-300 seconds
• GOMAXPROCS Issue: Container runtime may not set correctly, limiting CPU utilization (observed: 2/16 cores used on m5.4xlarge)
• Query Limits: Performance degrades significantly with excessive plugin loading

Kubernetes Scaling

• Default Setup: 2 replicas in kube-system namespace
• Performance Issue: Single replica failure causes cluster-wide DNS slowdown
• High Availability: Multiple replicas with anti-affinity rules across nodes required

Implementation Reality vs Documentation

Kubernetes Integration

• API Changes: Kubernetes 1.28+ breaks CoreDNS v1.10.x due to deprecated API usage
• Error Signature: error retrieving resource lock kube-system/coredns: the server could not find the requested resource
• Resolution: Upgrade CoreDNS version to maintain compatibility

DNS Resolution Failures

• Common Issue: "no such host" errors despite apparent correct configuration
• Debug Process: Test with kubectl exec -it <pod> -- nslookup kubernetes.default.svc.cluster.local
• Root Cause: Often application DNS configuration or incorrect service naming

Memory and CPU Consumption

• Symptom: CoreDNS consuming 100% CPU and crashing nodes
• Root Cause: Repeated queries for non-existent domains causing upstream timeouts
• Log Signature: Thousands of [ERROR] plugin/errors: 2 SERVFAIL: dial tcp 8.8.8.8:53: connect: network is unreachable
• Solution: Enable cache plugin with negative caching for NXDOMAIN responses

Decision Support Matrix

DNS Server Comparison

Server	Use Case	Strengths	Critical Weaknesses
CoreDNS	Kubernetes mandatory, modern deployments	Single binary, plugin architecture, Go performance	Plugin ecosystem gaps, complex debugging
BIND	Legacy enterprise, comprehensive features	Mature, survives nuclear apocalypse	Configuration complexity, human-hostile documentation
Unbound	Recursive DNS specialist	Reliable, focused functionality	Limited authoritative capabilities
PowerDNS	Database-backed DNS	MySQL integration, enterprise features	DBA dependency, schema optimization risks

When CoreDNS Is Appropriate

• Mandatory: Kubernetes deployments (default since v1.11)
• Good Fit: Container-based infrastructure, need for custom plugins
• Poor Fit: Exotic DNS requirements with plugin gaps, legacy integration needs

Migration Strategies

From kube-dns to CoreDNS

• Risk Level: Low - usually automatic in Kubernetes upgrades
• Exception: Custom kube-dns configurations with stub domains require manual translation
• Knowledge Gap: Previous configurations often undocumented, original implementers unavailable

From BIND to CoreDNS

• Approach: Gradual migration, non-critical zones first
• Parallel Operation: Keep BIND running for critical zones during transition
• Zone Sync: Secondary plugin can sync from BIND masters (reliability varies)
• Time Investment: Plan 3x estimated time plus additional week for edge cases

Protocol Support

Supported Protocols

• DNS over UDP/TCP (standard)
• DNS over TLS (DoT) - native support
• DNS over gRPC - native support
• DNS over HTTPS (DoH) - requires external proxy, not direct support

Forward Plugin Capabilities

• Load balancing between multiple upstream servers
• Health checking with automatic failover
• Reliability: More effective than traditional DNS servers for upstream management

Monitoring and Troubleshooting

Essential Monitoring Metrics

• Query rates and response times
• Cache hit ratios
• Error rates and NXDOMAIN frequency
• Memory and CPU utilization patterns

Debugging Tools and Plugins

• Essential: errors plugin (must enable first)
• Development: log plugin for query visibility
• Deep Debug: trace plugin (generates excessive logs, disable in production)
• Metrics: prometheus plugin for monitoring integration

Common Production Issues

1. Plugin ordering conflicts - Check plugin.cfg for compile-time order
2. Cache misconfiguration - Verify TTL settings and cache size limits
3. Upstream failover delays - Monitor forward plugin health checks
4. Silent configuration failures - Validate Corefile syntax carefully

Critical File References

• Plugin Execution Order: plugin.cfg
• Configuration Location: kube-system/coredns ConfigMap (Kubernetes)
• Health Endpoints: ready plugin provides functional health checks

Breaking Points and Limits

• UI Threshold: Kubernetes troubleshooting becomes impossible at 1000+ spans
• Configuration Changes: Syntax errors break DNS for entire cluster
• Memory Growth: Improper cache settings lead to memory exhaustion
• Hot Reload Failure: Complex configurations require process restart instead of signal-based reload

Resource Requirements for Decision Making

• Time Investment: Migration planning requires 3x initial estimates
• Expertise Required: Kubernetes DNS knowledge, plugin architecture understanding
• Support Quality: CNCF Slack #coredns channel provides maintainer access
• Documentation Quality: Official docs adequate but examples often outdated or non-functional