Optional ability to use the host name defined on an ingress's annotations *instead* of its hosts stanza

[dansimone]

There are situations where it would be useful to have external-dns create entries based on the "external-dns.alpha.kubernetes.io/hostname" annotation on an ingress, but not on the hosts defined in the hosts stanza. The current external-dns behavior is to create both entries.

Some examples where this would be useful:

• This open issue: Ingress: external-dns.alpha.kubernetes.io/hostname: force usage #1053
• A similar situation where one defines a wildcard DNS entry via annotation for multiple ingresses whose host names (hosts stanza) are covered by the wildcard entry. In this case, only the wildcard entry is desired in DNS. The hosts stanza on the ingress, in this case, is only used by an ingress-controller for its routing.

Possible approaches to implement this behavior:

1. Globally-scoped parameter: Introduce a new command line parameter: "prefer-ingress-hostname-annotation". If not set, no functional changes occur. If set, then for all ingresses, when processing the ingress host names, if a host name annotation is specified, it is used instead of those from the hosts stanza.
2. Ingress-scoped annotation: Introduce a new optional annotation that can be placed on specific ingresses. If not specified, then no functional changes occur. If set on an ingress, then the same behavior as Document design: features and annotations #1 will occur for records related to that ingress.
3. Both 1 and 2.

This current PR goes with approach 2 (originally proposed by @cryptk) - IMO that is the smallest footprint way of implementing this. 1 and 3 involve a new command line parameter, some function argument changes, etc. It's easily doable though, if desired. I'll defer to feedback from the maintainers on whether to go with 1 or 3.

The current PR adds the annotation described by this included update to the FAQ:

1. For ingress objects ExternalDNS will create a DNS record based on the hosts specified for the ingress object, as well as the `external-dns.alpha.kubernetes.io/hostname` annotation.  For services ExternalDNS will look for the annotation `external-dns.alpha.kubernetes.io/hostname` on the service and use the corresponding value.
    - For ingresses, you can optionally force ExternalDNS to create records based on _either_ the hosts specified or the `external-dns.alpha.kubernetes.io/hostname` annotation.  This behavior is controlled by
      setting the `external-dns.alpha.kubernetes.io/ingress-hostname-source` annotation on that ingress to either `defined-hosts-only` or `annotation-only`. 

Unit tests added to verify the behavior.

[dansimone]

@andrecastro and @cryptk FYI on this PR - I was forced to reopen the previous PR with a fresh PR.

[dansimone]

/assign @njuettner for review and feedback on the approaches mentioned in the description.

Also, I may need help understanding the code coverage failure. The 17 lines of code I added to ingress.go are covered by a new unit test. But it appears that ingress.go (as a whole) is being reported as having 0 lines covered for some reason, which looks like a general problem with the code coverage job to me.

[Dirrk]

/retest

[k8s-ci-robot]

@Dirrk: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest

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[seanmalloy]

/kind feature

[maxenglander]

Thanks for this PR @dansimone! I need this :)

A use case I have is that I have some Ingress objects which I want external-dns to create Route 53 entries for, and other Ingress objects that have DNS records managed outside of external-dns. I don't want external-dns to touch that second set of Ingress objects at all.

Right now (correct me if I'm wrong) I think I would have to update all of my Ingress objects to have a value like none, before I can safely enable ingress source. That's not the end of the world of course, but ideally, I think I'd like the option to specify a --default-ingress-hostname-source command flag and set it to none, or some other value indicating that I want the default behavior to be "leave these ingresses alone!"

Don't have to make those changes in this PR of course, just expressing my interest in this PR in general, and my particular use case for the current or a future PR.

[cubic3d]

All my ingresses define a root domain(example.com) and wildcard domain (*.example.com) in the tls section. Additionally the actual subdomain for the host. External-dns tries always to update all 3 records - I see no other setting to avoid the generation from the tls section.

[dansimone]

@cubic3d - yep, your case is similar to mine.

Side note - I have another PR out to get external-dns's support for wildcard DNS entries working generically, since it doesn't work currently except with some specific providers : #1695).

[dansimone]

@maxenglander - for your case, to get external-dns to not even look at your ingress at all, you could omit all of the external-dns.alpha.kubernetes.io annotations, like "external-dns.alpha.kubernetes.io/target". I confirmed that just now in my environment (although I am running with quite a few patches on external-dns, so take that with a grain of salt!)

This MR here would be for a case where you do want external-dns to act on your ingress, but not to create DNS entries for both the annotation section and the hosts stanza.

[dansimone]

@hjacobs, @njuettner - whenever you guys get a chance to look at this, it's freshly synced with master and passing CI.

[dansimone]

/assign @njuettner

[maxenglander]

Thanks for the reply @dansimone. I found that when I have an Ingress with a host stanza external-dns will try to create a DNS record, even in the absence of any external-dns annotation.

It's possible there was something else going on or some other misconfiguration I had to produce that result, but it does seem to be expected based on these docs.

I'll take another look though.

[dansimone]

It looks like a PR in the same ballpark as this got merged a few days ago: #1645 However that change makes a global setting, and this PR allows the flexibility per-ingress (via a new annotation). I've synced up my branch with master, and both changes should play nice together.

[artem-nefedov]

This feature is exactly what we need.
Hope to see this merged soon.

[dansimone]

@hjacobs, @njuettner please let me know when you get a chance to look at this.

[njuettner]

LGTM, some minor nits. If addressed I'm happy to merge it 👍

[dansimone]

Thanks for the feedback, @njuettner. Let me know how the latest looks to you.

[dansimone]

I just resolved the above conversations, @njuettner, let me know if this looks alright.

[tuusberg]

@dansimone thanks for implementing it. Would this feature allow ignoring certain Kubernetes resources?
e.g. If I don't want to create DNS records for every Ingress host?

[danopia]

@tuusberg

ignoring certain Kubernetes resources

Sounds like you might want an annotation filter there, which has been available and is pretty common to use. I use an annotation filter that overlaps with the ingressclass annotation but you can filter on whatever custom annotation you want

[dansimone]

Ok, a new (minor doc) conflict just popped up. Resolved. @njuettner, please review.

[jlamillan]

I've tested this out and it works well.

[vsetka]

Can we merge this, please?

[dansimone]

@njuettner I just went ahead and closed this comment: #1696 (comment) But I'm looking for input on the pros and cons that I pointed out above, although I went with your suggestion for now.

[arobie1992]

Is there any expected inclusion date for this feature?

[njuettner]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

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[marko-espi]

in which version this is going to be released?

[danopia]

Github's UI indicates that this is present in v0.9.0.