add support for wildcard domains

[szuecs]

related: #1810

What would you like to be added:

I think we do not support wildcard domains, because ingress spec did not allow them in v1beta1 and IIRC also not in v1.

People are now working on ingress v2 (aka gateway resource), see also http://sigs.k8s.io/service-apis.

For CRDs we can easily add on our side wildcard and external-dns source CRD has no restriction to the string parameters so wildcard seems to be possible. In case of source CRD, all controllers would need to maintain the external-dns CRD on create/update/delete operations.
For example ALB: when status loadbalancer changes, the controller creating the ALB would also need to update external-dns source CRD, which is quite an overhead if the community have to do it for all controllers (e.g. sig-aws controller, kube-ingress-controller, google, istio, gloo, nginx, .... ).

I think it makes sense to support wildcard hosts like *.example.org, which would need to match [a-z0-9-]+[.].example[.]org, so only one dot supported as in DNS.

Why is this needed:
ingress v2 is in progress and there are other CRDs that might support wildcards or want to support wildcards

[jgrumboe]

@szuecs
I think you would also need to add "-" (dash) as a valid character for the regex matching the wildcard '*'.

[szuecs]

@jgrumboe updated, thanks

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[zcapper]

Came across this after wondering why the annotation on my AWS ingress wasn't working for my wildcard domain:

"external-dns.alpha.kubernetes.io/hostname": "*.foo.com"
"external-dns.alpha.kubernetes.io/ingress-hostname-source": "annotation-only"

Would be great to support this for cases where the ALB gets traffic from many/any domain(s).

---

EDIT: Turns out the correct format is

"external-dns.alpha.kubernetes.io/hostname": "\\*.foo.com"
"external-dns.alpha.kubernetes.io/ingress-hostname-source": "annotation-only"

[mattn]

"external-dns.alpha.kubernetes.io/hostname": "\\*.foo.com"
"external-dns.alpha.kubernetes.io/ingress-hostname-source": "annotation-only"

I'm trying this on cloudflare.

ERRO[0002] failed to create record: DNS Validation Error (1004)  action=CREATE record="a-\\*.foo.com" ttl=1 type=TXT zone=XXXXXXXXXXXXXXXXXXXXXXXXX
FATA[0002] Failed to do run once: failed to submit all changes for the following zones: [XXXXXXXXXXXXXXXXXXXXXXXXX] 

external-dns output this error and then forcefully terminate always.