External-dns: Support for Services/Ingresses with externally managed endpoints (e.g. type=NodePort/HostPort)

[whereisaaron]

Services and Ingresses with type=NodePort and sometimes type=HostPort are often externally managed endpoints providing NAT, reverse proxy, load balancers, CDNs, or combinations of these that eventually route to the Service/Ingress. external-dns can't be expected to work out the external IP addresses or domain name that will get traffic to the NodePort/HostPort service.

I propose external-dns support annotations that let users supply the correct external IP address list or domain name that should be used at the target of the DNS record. E.g. specify either and target-hostname or target-ips annotation.

e.g. create myapp.example.com CNAME mycdn.domain.name for Service

apiVersion: v1
kind: Service
metadata:
  name: myapp
  annotations:
    external-dns.alpha.kubernetes.io/hostname: myapp.example.com.
    external-dns.alpha.kubernetes.io/target-hostname: mycdn.domain.name.
...

e.g. create myapp.example.com A 10.20.30.40 and myapp.example.com A 10.20.30.41 for Service

apiVersion: v1
kind: Service
metadata:
  name: myapp
  annotations:
    external-dns.alpha.kubernetes.io/hostname: myapp.example.com.
    external-dns.alpha.kubernetes.io/target-ips: "10.20.30.40, 10.20.30.41"
...

e.g. create myapp.example.com CNAME mycdn.domain.name for Ingress

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: myapp
  annotations:
    kubernetes.io/ingress.class: "nginx"
    external-dns.alpha.kubernetes.io/target-hostname: mycdn.domain.name.
spec:
  rules:
  - host: myapp.example.com
...

e.g. create myapp.example.com CNAME mycdn.domain.name and
myapp-alias.example.com CNAME mycdn.domain.name for Ingress

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: myapp
  annotations:
    kubernetes.io/ingress.class: "nginx"
    external-dns.alpha.kubernetes.io/target-hostname: mycdn.domain.name.
spec:
  rules:
  - host: myapp.example.com
  - host: myapp-alias.example.com
...

e.g. create
myapp.example.com A 10.20.30.40
myapp.example.com A 10.20.30.41
myapp-alias.example.com A 10.20.30.40
myapp-alias.example.com A 10.20.30.41
for Ingress

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: myapp
  annotations:
    kubernetes.io/ingress.class: "nginx"
    external-dns.alpha.kubernetes.io/target-ips: "10.20.30.40, 10.20.30.41"
spec:
  rules:
  - host: myapp.example.com
  - host: myapp-alias.example.com
...

[montyz]

I could really use this. In order to support IPv6 on AWS I need to use an ALB, which is not supported as a type LoadBalancer. So I set the ALB up manually and expose ingress as a NodePort. It would be great if I could specify my ALB name in an annotation and have external-dns create the CNAME pointing from the ingress hostname to my ALB name.

[Raffo]

@whereisaaron I'm not sure about this, it seems to be pushing what ExternalDNS was designed to do a bit over. While I understand why you want to do this in general, I'm not sure if it should be ExternalDNS to do that (i.e. Route53 can be managed via CloudFormation on AWS).
That said, if we decide to move forward with that, I wouldn't use annotations, but rather a CRD (to define?) to make very clear what is happening.

[whereisaaron]

Thanks @Raffo. I agree it extends the scope of external-dns, but I think it still fits the mission. And otherwise we'll end up with two different DNS controllers for different use cases for provisioning external DNS records for ingress. I'd rather that external-dns was the definitive solution for external DNS for ingress (e.g. paired with cert-manager for certificates and optionally nginx-ingress), than fragmented point solutions.

This is the missing link for us for k8s deployments. Everything else for ingress, like load balancers, ingress controller configuration, or TLS certificates, can be configured from k8s resources. Right now, after doing helm install and have to follow up with a second action against e.g. the Route53 API. We'd like the domain names to be a first class component of the deployment (small 'd') configuration.

Yes CRDs would be the best, stable, long-term approach. I suggested annotations to be consistent with the other annotations I saw used for external-dns. And because annotations can be handy still, since it is easier to inject them into existing Helm charts.

[Raffo]

I see, but I'd still go with a CRD as it seems hacky to adopt annotations on Services/Ingresses for this particular use case. I don't have a strong opinion though, WDYT @linki @ideahitme @njuettner ?

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[whereisaaron]

Ref #21 #555 #892

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