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CoreDNS

A fast, flexible DNS server written in Go with a plugin-based architecture — the default DNS server in Kubernetes and a CNCF graduated project.

Overview

Plugin-Based DNS for Cloud Native

CoreDNS is built around a plugin chain — every DNS function is implemented as a plugin, and you compose a server by listing plugins in a Corefile. This makes CoreDNS extremely flexible: the same binary can act as an authoritative server, a forwarder, a caching resolver, or all three simultaneously on different ports.

Kubernetes uses CoreDNS as its in-cluster DNS service. Every Pod's DNS queries go through CoreDNS, which resolves Kubernetes Service names (service.namespace.svc.cluster.local) and forwards external queries upstream. Understanding CoreDNS configuration is essential for Kubernetes DNS troubleshooting.

Single Go binary — easy to deploy in containers, no external dependencies
Plugin chain: cache, forward, hosts, file, kubernetes, etcd, and 30+ more
Kubernetes plugin reads Service and Pod records directly from the API server
Prometheus metrics plugin for monitoring DNS query rates and latency
Health check endpoints (health plugin) for load balancer integration
DNS-over-TLS and DNS-over-HTTPS via tls plugin
Reload plugin for zero-downtime config changes

When to Choose CoreDNS

You are running Kubernetes (it's already there)
You need a lightweight containerized DNS server
You want plugin composability over monolithic config
You need etcd-backed dynamic DNS records

CNCF

Graduated project

K8s

Default Kubernetes DNS

Configuration

Corefile Examples

Default Kubernetes Corefile

# ConfigMap: coredns in kube-system
.: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
}

Forwarder + caching server

# Forward all queries to upstream
# with local caching
.:53 {
    # Cache responses for 30s minimum
    cache 30

    # Forward to Cloudflare over TLS
    forward . tls://1.1.1.1 tls://1.0.0.1 {
        tls_servername cloudflare-dns.com
        health_check 5s
    }

    # Prometheus metrics
    prometheus 0.0.0.0:9153

    # Log queries
    log

    errors
}

Split-horizon with file plugin

# Internal zone served from file
corp.internal:53 {
    file /etc/coredns/db.corp.internal
    log
    errors
}

# Everything else forwarded upstream
.:53 {
    forward . 8.8.8.8 8.8.4.4
    cache 300
    errors
}

# db.corp.internal zone file
# (standard BIND zone format)

Troubleshoot K8s DNS

# Check CoreDNS pods are running
kubectl -n kube-system get pods -l k8s-app=kube-dns

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

# Test DNS from a pod
kubectl run dns-test --image=busybox:1.28 --rm -it \
  --restart=Never -- nslookup kubernetes.default

# Edit CoreDNS config
kubectl -n kube-system edit configmap coredns

# View current Corefile
kubectl -n kube-system get configmap coredns -o yaml

CoreDNS

Plugin-Based DNS for Cloud Native

Corefile Examples

Default Kubernetes Corefile

Forwarder + caching server

Split-horizon with file plugin

Troubleshoot K8s DNS

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