CoreDNS External-DNS Plugin

A CoreDNS plugin that serves DNS records from external-dns DNSEndpoint CRDs.

How It Works

1. Watches Kubernetes Resources:
   • DNSEndpoint CRDs for generic DNS record definitions
   • Services (Type LoadBalancer) for automatic service discovery
   • Ingresses for HTTP/HTTPS routing discovery
2. Caches records in memory for fast DNS query responses
3. Serves DNS queries directly from the cache
4. Auto-generates PTR records when enabled via annotation

Quick Start

# Deploy with Helm
helm install coredns-externaldns oci://ghcr.io/ionos-cloud/coredns-externaldns/charts/coredns-externaldns

# Or build and run standalone
git clone https://github.com/ionos-cloud/coredns-externaldns
cd coredns-externaldns
make build
./coredns-externaldns -conf Corefile.standalone

Configuration

Corefile Example

.:53 {
    externaldns {
        namespace default              # Optional: watch specific namespace
        ttl 300                       # Optional: default TTL (seconds)
        configmap_name zone-serials   # Optional: ConfigMap for serial persistence
        soa_ns ns1.example.com        # Optional: SOA nameserver for AXFR
        soa_mbox admin.example.com    # Optional: SOA mailbox for AXFR
        authoritative_zones example.com,test.com  # Optional: explicit zone list
    }
    forward . 8.8.8.8
    log
    errors
}

Authoritative Zones

Configure authoritative_zones to define which DNS zones this plugin serves. This enables proper zone boundaries and DNS NOTIFY functionality.

By default, the plugin uses the zones from the Corefile where it's loaded (e.g., example.com:53). Setting authoritative_zones overrides this with an explicit zone list.

Format: Comma-separated zone list

authoritative_zones example.com,internal.local,test.net

With DNS NOTIFY:

transfer { to * }
externaldns {
    authoritative_zones example.com
    soa_ns ns1.example.com
}

DNSEndpoint Example

apiVersion: externaldns.k8s.io/v1alpha1
kind: DNSEndpoint
metadata:
  name: example
  annotations:
    coredns-externaldns.ionos.cloud/create-ptr: "true"  # Enable PTR records
spec:
  endpoints:
  - dnsName: app.example.com
    recordType: A
    targets: ["192.168.1.100"]
    recordTTL: 300

Service Support

The plugin automatically creates DNS records for Services of type LoadBalancer.

Requirements

• Service must be of type LoadBalancer
• Must have external-dns.alpha.kubernetes.io/hostname annotation

Example

apiVersion: v1
kind: Service
metadata:
  name: nginx
  annotations:
    external-dns.alpha.kubernetes.io/hostname: nginx.example.com
    external-dns.alpha.kubernetes.io/ttl: "60"
    coredns-externaldns.ionos.cloud/create-ptr: "true"
spec:
  type: LoadBalancer
  ports:
  - port: 80
  selector:
    app: nginx

Ingress Support

The plugin automatically creates DNS records for Ingress resources.

Configuration

Hostnames are discovered from the following sources (can be combined):

1. Explicit Hostname: Using external-dns.alpha.kubernetes.io/hostname annotation
2. From Rules: Using coredns-externaldns.ionos.cloud/ingress-from-rules: "true" annotation to use hosts defined in spec.rules

If both are present, records will be created for all unique hostnames found.

Example (Explicit Hostname)

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: nginx
  annotations:
    external-dns.alpha.kubernetes.io/hostname: nginx.example.com
    coredns-externaldns.ionos.cloud/create-ptr: "true"
spec:
  rules:
  - host: nginx.internal
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: nginx
            port:
              number: 80

Example (From Rules)

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: nginx
  annotations:
    coredns-externaldns.ionos.cloud/ingress-from-rules: "true"
spec:
  rules:
  - host: app.example.com
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: app
            port:
              number: 80

PTR Record Feature

The plugin can automatically create reverse DNS (PTR) records for A and AAAA records.

How to Enable

Add this annotation to your DNSEndpoint, Service, or Ingress:

metadata:
  annotations:
    coredns-externaldns.ionos.cloud/create-ptr: "true"

How It Works

1. Forward Record: app.example.com A 192.168.1.100
2. Auto-Generated PTR: 100.1.168.192.in-addr.arpa PTR app.example.com

When you query 192.168.1.100 for reverse DNS, it returns app.example.com.

Supported: Works with both A (IPv4) and AAAA (IPv6) records.

RBAC Requirements

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: coredns-externaldns
rules:
- apiGroups: ["externaldns.k8s.io"]
  resources: ["dnsendpoints"]
  verbs: ["get", "list", "watch"]
- apiGroups: [""]
  resources: ["services", "configmaps"]
  verbs: ["get", "list", "watch", "create", "update"]
- apiGroups: ["networking.k8s.io"]
  resources: ["ingresses"]
  verbs: ["get", "list", "watch"]

Supported Record Types

A, AAAA, CNAME, MX, TXT, SRV, PTR, NS, SOA

License

Apache 2.0