Add `gateway-gateway` source to create DNS records from Gateway Listener hostnames

[kahirokunn]

What would you like to be added:

Add a new --source=gateway-gateway option to external-dns that creates DNS records based on the hostname field defined in Gateway API Gateway Listeners. This source would read listeners[].hostname from Gateway resources and automatically manage corresponding DNS records.

The implementation should:

• Parse Gateway resources and extract hostname values from each Listener
• Support wildcard hostnames (e.g., *.example.com)
• Create/update/delete DNS records based on Gateway lifecycle and status
• Work alongside existing sources like --source=service and --source=ingress

Why is this needed:

Currently, external-dns lacks a "gateway-gateway source" that directly uses Gateway API's Gateway resources. This creates an operational gap for L4-only configurations (using TLS termination and TCPRoute without HTTPRoute), where there's no L7 route resource with hostnames that external-dns can use.

In Gateway API, Listeners can declare hostnames for TLS termination:

https://gateway-api.sigs.k8s.io/guides/tls/#wildcard-tls-listeners

  listeners:
  - name: foo-tls
    protocol: TLS
    port: 443
    hostname: foo.example.com
    tls:
      mode: Terminate
      certificateRefs:
      - kind: Secret
        group: ""
        name: foo-example-com-cert
  - name: wildcard-tls
    protocol: TLS
    port: 443
    hostname: "*.example.com"
    tls:
      mode: Terminate
      certificateRefs:
      - kind: Secret
        group: ""
        name: wildcard-example-com-cert
  - name: wildcard-tls-alt
    protocol: TLS
    port: 6443
    hostname: "*.example.com"
    tls:
      mode: Terminate
      certificateRefs:
      - kind: Secret
        group: ""
        name: wildcard-example-com-cert

For L4-only Gateway configurations:

• HTTPRoute source (--source=gateway-httproute) cannot be used since no HTTPRoutes exist
• Service/Ingress sources don't integrate with Gateway API
• Operators must manually manage DNS records, breaking the automation flow

This feature would enable more DNS automation for L4 Gateway configurations.

[kxs-jnadeau]

It might be a good idea to look at supporting this ListernerSet resource as well: kubernetes-sigs/gateway-api#1713. This will allow a better migration path for Ingress users using host based routing with TLS to Gateway API without the need for wildcard certs. Similar work is in the way in cert-manager.

[maxbrunet]

👍

Connecting dots... Previous related issue #4402