fix: CNAME resolution when External DNS strips trailing dots

[lukasmetzner]

External DNS normalizes CNAME targets by removing the trailing dot, which causes CNAME records pointing to a different domain to resolve incorrectly (e.g. foo.mytarget.com.myzone.com).

External DNS is normalizing CNAMES in multiple locations of their code:

• https://github.com/kubernetes-sigs/external-dns/blob/910dc785d86fbe97c32dabbf4eb0c1d27b84bb4e/source/annotations/processors.go#L103-L118
• https://github.com/kubernetes-sigs/external-dns/blob/910dc785d86fbe97c32dabbf4eb0c1d27b84bb4e/endpoint/endpoint.go#L270

For this reason, other providers implement a check to ensure the trailing dot is present according to the needs of their DNS system:

• Google: https://github.com/kubernetes-sigs/external-dns/blob/910dc785d86fbe97c32dabbf4eb0c1d27b84bb4e/provider/google/google.go#L427-L429
• DigitalOcean: https://github.com/kubernetes-sigs/external-dns/blob/910dc785d86fbe97c32dabbf4eb0c1d27b84bb4e/provider/digitalocean/digital_ocean.go#L292-L294
• OVH: https://github.com/kubernetes-sigs/external-dns/blob/910dc785d86fbe97c32dabbf4eb0c1d27b84bb4e/provider/ovh/ovh.go#L735-L749

The exact reasoning for this normalization is not documented by External DNS.

Fixes #73

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/provider/provider.go	50.00%	2 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[jooola]

Could you add links about your findings in the description please? And why this is not a bug in external dns?

[lukasmetzner]

Could you add links about your findings in the description please? And why this is not a bug in external dns?

I updated the description. Not sure if this explanation is sufficient.

[lukasmetzner]

Decision: We will ask at external-dns why this behavior is necessary and if this could be documented.

[jooola]

There was a patch to external-dns that allowed trailing dots: kubernetes-sigs/external-dns#6218 https://github.com/kubernetes-sigs/external-dns/releases/tag/v0.21.0

So we can probably move forward, and think again if we really want to implement this PR.

[lukasmetzner]

There was a patch to external-dns that allowed trailing dots: kubernetes-sigs/external-dns#6218 https://github.com/kubernetes-sigs/external-dns/releases/tag/v0.21.0

So we can probably move forward, and think again if we really want to implement this PR.

AFAICT this patch only affects the custom resource definition from external-dns and not the Service or Ingress objects.

We could still move forward with this, as the CNAME issue our user ran into still exists.