CoreDNS serves as the DNS server in Kubernetes clusters, providing service discovery and name resolution. This deep dive explores its architecture, configuration, and advanced features.

Component Architecture

plaintext
Pod -> kubelet -> CoreDNS -> Service Discovery
                         -> External DNS
                         -> Custom DNS

Key Components

  1. CoreDNS Server

    • DNS Service
    • Plugin Chain
    • Caching Layer

  2. Service Discovery

    • Service Records
    • Pod Records
    • External Services

  3. DNS Resolution

    • Internal Resolution
    • Forward Resolution
    • Custom Domains

CoreDNS Configuration

1. Basic Corefile

corefile
.: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
}

2. Custom DNS Configuration

corefile
example.com:53 {
    file /etc/coredns/example.com.db
    prometheus
    errors
    log
}

.:53 {
    kubernetes cluster.local {
        pods insecure
        upstream
        fallthrough in-addr.arpa ip6.arpa
    }
    forward . 8.8.8.8 8.8.4.4
    cache 30
    loop
    reload
    loadbalance
}

Service Discovery

1. DNS Records

yaml
# Service DNS Records
<service-name>.<namespace>.svc.cluster.local
# Example: my-service.default.svc.cluster.local

# Pod DNS Records
<pod-ip>.<namespace>.pod.cluster.local
# Example: 10-244-1-10.default.pod.cluster.local

2. Custom DNS Entries

yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  example.server: |
    example.org {
        forward . 8.8.8.8
    }

Advanced Features

1. DNS Policies

yaml
apiVersion: v1
kind: Pod
metadata:
  name: custom-dns
spec:
  dnsPolicy: "None"
  dnsConfig:
    nameservers:
      - 1.1.1.1
    searches:
      - ns1.svc.cluster.local
      - my.dns.search.suffix
    options:
      - name: ndots
        value: "2"
      - name: edns0

2. Auto Scaling

yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
spec:
  replicas: 2
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
  template:
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: k8s-app
                operator: In
                values:
                - kube-dns
            topologyKey: kubernetes.io/hostname

Performance Tuning

1. Cache Configuration

corefile
.:53 {
    cache {
        success 10000
        denial 5000
        prefetch 10 10m 10%
    }
}

2. Resource Management

yaml
apiVersion: v1
kind: Pod
metadata:
  name: coredns
spec:
  containers:
  - name: coredns
    resources:
      requests:
        memory: 70Mi
        cpu: 100m
      limits:
        memory: 170Mi
        cpu: 200m

Monitoring and Metrics

1. Prometheus Metrics

yaml
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: coredns
  namespace: monitoring
spec:
  endpoints:
  - bearerTokenFile: /var/run/secrets/kubernetes.io/serviceaccount/token
    interval: 15s
    port: metrics
  selector:
    matchLabels:
      k8s-app: kube-dns

2. Important Metrics

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# Key metrics to monitor
coredns_dns_requests_total
coredns_dns_responses_total
coredns_cache_hits_total
coredns_cache_misses_total

Troubleshooting

Common Issues

  1. DNS Resolution Problems
bash
# Check CoreDNS pods
kubectl get pods -n kube-system -l k8s-app=kube-dns

# View CoreDNS logs
kubectl logs -n kube-system -l k8s-app=kube-dns

# Test DNS resolution
kubectl run dnsutils --image=gcr.io/kubernetes-e2e-test-images/dnsutils:1.3 \
  -- sleep infinity
kubectl exec -it dnsutils -- nslookup kubernetes.default
  1. Cache Issues
bash
# Clear CoreDNS cache
kubectl delete pod -n kube-system -l k8s-app=kube-dns

# Monitor cache metrics
curl http://localhost:9153/metrics | grep coredns_cache
  1. Performance Problems
bash
# Check resource usage
kubectl top pod -n kube-system -l k8s-app=kube-dns

# Monitor DNS latency
coredns_dns_request_duration_seconds_bucket

Best Practices

  1. High Availability

    • Run multiple replicas
    • Use pod anti-affinity
    • Configure proper health checks
    • Implement proper monitoring

  2. Performance

    • Optimize cache settings
    • Configure proper TTLs
    • Monitor resource usage
    • Use autoscaling

  3. Security

    • Restrict external queries
    • Implement DNSSEC
    • Use proper RBAC
    • Monitor DNS traffic

Advanced Configuration

1. Custom Plugins

corefile
.:53 {
    errors
    health
    kubernetes cluster.local {
        pods verified
        fallthrough in-addr.arpa ip6.arpa
    }
    hosts custom.hosts {
        10.0.0.1 my.custom.domain
        fallthrough
    }
    prometheus :9153
    forward . 8.8.8.8 {
        max_concurrent 1000
    }
    cache 30
    reload
}

2. Split DNS

corefile
internal:53 {
    kubernetes cluster.local {
        pods insecure
    }
    cache 30
}

external:53 {
    forward . 8.8.8.8
    cache 30
}

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