Prototype of external-dns + coredns based configuration

[ytsarev]

• Use external-dns to populate coreDNS conf with etcd backend
  Based on https://github.com/kubernetes-sigs/external-dns/blob/master/docs/tutorials/coredns.md
• Use --annotation-filter to figure out health status

Visible limitaitons:

• coreDNS etcd plugin can support only single authoritative zone
• Annotation status is global to Ingress, so all host entries affected
  It might imply single host entry per Gslb requirement

Runtime example:

• External-dns logs

time="2019-12-23T14:50:29Z" level=debug msg="Endpoints generated from ingress: default/example-gslb: [app.cloud.absa.internal 0 IN A 172.17.0.2 [] app1.cloud.absa.external 0 IN A 172.17.0.2 [] app2.cloud.absa.external 0 IN A 172.17.0.2 []

• Query test

dnstools# dig @gslb-coredns-coredns app1.cloud.absa.external +short
172.17.0.2

dnstools# dig @gslb-coredns-coredns SOA app1.cloud.absa.external +short
ns.dns.absa.external. hostmaster.absa.external. 1577113239 7200 1800 86400 30

So coredns etcd plugin(skyDNS) can serve single authoritative zone

Apart from above mentioned limitations this setup has advantages of
being very dynamic and decoupled from ohmyglb controller implementation.

On ohmyglb side we can control dns record population through annotations
instead of direct writes to coreDNS configmap

[ytsarev]

Most probably I will redo the external-dns source to be the CRD - https://github.com/kubernetes-sigs/external-dns/blob/master/docs/contributing/crd-source.md . Looks very flexible

[donovanmuller]

Annotation status is global to Ingress, so all host entries affected
It might imply single host entry per Gslb requirement

A single host entry per Gslb resource IMHO is fine for the initial implementation.

[donovanmuller]

@ytsarev Given #14 (comment), I'll leave this review as is and wait for the refactored PR?

[ytsarev]

@donovanmuller yes, exactly, going to amend the PR soon

[ytsarev]

 kubectl describe dnsendpoints.externaldns.k8s.io
Name:         examplednsrecord
Namespace:    default
Labels:       <none>
Annotations:  kubectl.kubernetes.io/last-applied-configuration:
                {"apiVersion":"externaldns.k8s.io/v1alpha1","kind":"DNSEndpoint","metadata":{"annotations":{},"name":"examplednsrecord","namespace":"defau...
API Version:  externaldns.k8s.io/v1alpha1
Kind:         DNSEndpoint
Metadata:
  Creation Timestamp:  2019-12-27T12:58:59Z
  Generation:          2
  Resource Version:    24976
  Self Link:           /apis/externaldns.k8s.io/v1alpha1/namespaces/default/dnsendpoints/examplednsrecord
  UID:                 fa9c716a-4b0a-4232-adae-41d36df84359
Spec:
  Endpoints:
    Dns Name:     foo.absa.external
    Record TTL:   180
    Record Type:  A
    Targets:
      10.1.1.1
Status:
  Observed Generation:  2
Events:                 <none>

dnstools# dig @gslb-coredns-coredns foo.absa.external +short
10.1.1.1

@donovanmuller PR amended, CRD stuff works! Let's merge it in and I will continue on ohmyglb operator control code update