Add support for "target" annotation on ingress service

[Deltachaos]

What would you like to be added:

external-dns should use the external-dns.alpha.kubernetes.io/target annotation of the Ingress Service for Ingress objects if defined.

Why is this needed:

If you run your Ingress as a ClusterIP Service and the individual Pods with hostNetwork: true, the real external IP for the ingress controller cannot be determined. In this case, you are able to specify external-dns.alpha.kubernetes.io/target on each Ingress object you create. This information is redundant. external-dns should not use the IP from the ClusterIP Service but instead should use the overwritten IP from the external-dns.alpha.kubernetes.io/target annotation of the ingress controller's ClusterIP Service.

[AayushVinayDev]

Hi @Deltachaos :D , I'm new to this codebase so from my very limited understanding. Is this the change you are suggesting?

case v1.ServiceTypeClusterIP:
            if svc.Spec.ClusterIP == v1.ClusterIPNone {
                endpoints = append(endpoints, sc.extractHeadlessEndpoints(svc, hostname, ttl)...)
            } else if useClusterIP || sc.publishInternal {
                targets = extractServiceIps(svc)
            }
            // Check if the service has the external-dns.alpha.kubernetes.io/target annotation
            if target, ok := svc.Annotations["external-dns.alpha.kubernetes.io/target"]; ok {
                targets = endpoint.Targets{target}
            }

if ClusterIP is true, use the external-dns.alpha.kubernetes.io/target annotation from ingress controller's ClusterIP Service.

Please let me know of my shortcomings! Thanks

[Deltachaos]

I am probably the wrong person to ask, as im not into the code of this project as well, and im currently working on getting some Go skills, but: From my understanding this seems to go in the right direction.

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[rvaidya]

I have a similar use case - MetalLB is reporting the internal NAT IP as the IP for the load balancer, which is then being published as an A record by external-dns.

I'd like to override this with a CNAME record that points to a dns record that automatically gets updated by a dynamic DNS client, since public IP changes on home internet.

This PR seems tangentially related, but has been closed: https://github.com/kubernetes-sigs/external-dns/pull/1077

[fiskhest]

I have a similar use case - MetalLB is reporting the internal NAT IP as the IP for the load balancer, which is then being published as an A record by external-dns.

I'd like to override this with a CNAME record that points to a dns record that automatically gets updated by a dynamic DNS client, since public IP changes on home internet.

This PR seems tangentially related, but has been closed: #1077

I have the exact same use case. For the time being, I've worked around it by disabling external-dns from picking up ingresses by commenting --source=ingress, then create a ExternalName service for each ingress:

kind: Service
apiVersion: v1
metadata:
  name: service-external-dns
  annotations:
    external-dns.alpha.kubernetes.io/hostname: foo.domain.com
spec:
  type: ExternalName
  externalName: ddns-record.domain.com

The ddns-record is managed by a separate ddns-client.

This stops external-dns from trying to publish an A record with the (NATed) IP that is bound to the ingress and instead publish a CNAME record for value of external-dns.alpha.kubernetes.io/hostname pointing to externalName. https://github.com/kubernetes-sigs/external-dns/blob/master/docs/tutorials/externalname.md

Preferably, external-dns would read external-dns.alpha.kubernetes.io/target on an ingress object and understand that the host specified in the ingress should be a CNAME to the target, instead of an A record to the IP.