How Do You Configure CoreDNS in Kubernetes?

Detailed Answer

CoreDNS is the default DNS server in Kubernetes since version 1.13. Its configuration is stored in a ConfigMap and uses a plugin-based architecture defined in a Corefile.

Viewing the Default Configuration

kubectl get configmap coredns -n kube-system -o yaml

Default 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 {
        max_concurrent 1000
    }
    cache 30
    loop
    reload
    loadbalance
}

Plugin Breakdown

| Plugin | Purpose | |---|---| | errors | Logs errors to stdout | | health | Provides health check endpoint at :8080/health | | ready | Provides readiness endpoint at :8181/ready | | kubernetes | Serves DNS for Kubernetes Services and Pods | | prometheus | Exposes metrics at :9153/metrics | | forward | Forwards non-cluster queries to upstream DNS | | cache | Caches DNS responses (30 seconds default) | | loop | Detects and stops forwarding loops | | reload | Auto-reloads Corefile on ConfigMap changes | | loadbalance | Randomizes DNS response order for round-robin |

Customizing CoreDNS

Forward Specific Domains to Custom DNS

Route corporate domain lookups to an internal DNS server:

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

# Custom domain forwarding
internal.corp:53 {
    errors
    cache 30
    forward . 10.0.0.53 10.0.0.54
}

staging.mycompany.com:53 {
    errors
    cache 30
    forward . 10.10.0.53
}

Add Static DNS Entries

Use the hosts plugin for static entries:

.:53 {
    errors
    health
    ready
    hosts {
        10.0.0.100 legacy-app.internal
        10.0.0.101 legacy-db.internal
        fallthrough
    }
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
        ttl 30
    }
    forward . /etc/resolv.conf
    cache 30
    loop
    reload
    loadbalance
}

The fallthrough directive is important — it allows queries not matched by the hosts plugin to continue to the next plugin.

Enable DNS Logging

Add the log plugin for query logging (useful for debugging but adds overhead):

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

Applying Changes

Edit the ConfigMap:

kubectl edit configmap coredns -n kube-system

The reload plugin automatically detects changes and reloads the configuration. You can verify by checking CoreDNS logs:

kubectl logs -n kube-system -l k8s-app=kube-dns -f
# [INFO] Reloading
# [INFO] plugin/reload: Running configuration SHA changed

Scaling CoreDNS

For large clusters, CoreDNS may need scaling:

# Scale manually
kubectl scale deployment coredns -n kube-system --replicas=4

# Or use the dns-autoscaler addon
kubectl get deployment dns-autoscaler -n kube-system

The DNS autoscaler adjusts CoreDNS replicas based on the number of nodes and cores in the cluster.

CoreDNS Performance Tuning

Increase Cache TTL

cache 60    # Cache for 60 seconds instead of 30

Prefetch Popular Records

cache 30 {
    prefetch 10 60s 10%    # Prefetch when a record is requested 10+ times
}

Node-Local DNS Cache

For large clusters, deploy NodeLocal DNSCache to reduce CoreDNS load:

kubectl apply -f https://raw.githubusercontent.com/kubernetes/kubernetes/master/cluster/addons/dns/nodelocaldns/nodelocaldns.yaml

NodeLocal DNSCache runs a DaemonSet that caches DNS queries on each node, reducing latency and CoreDNS load.

Monitoring CoreDNS

# Check CoreDNS Pod health
kubectl get pods -n kube-system -l k8s-app=kube-dns

# View metrics
kubectl port-forward -n kube-system svc/kube-dns 9153:9153
curl http://localhost:9153/metrics

# Key metrics to monitor:
# coredns_dns_requests_total - Total DNS requests
# coredns_dns_responses_total - Total responses by rcode
# coredns_dns_request_duration_seconds - Latency histogram
# coredns_cache_hits_total - Cache hit rate