Add pod source [kops compatibility]

[olemarkus]

Description

The pod source will implement the logic described for pods here: https://github.com/kubernetes-sigs/external-dns/blob/master/docs/legacy/kops-dns-controller.md

This PR just reuses the usual external-dns annotations. A follow-up will add support for the dns-controller annotations, possibly using the --compatibility flag as suggested on the page linked above.

The pod source is a key feature of kOps' DNS Controller. Among other things, i is used for etcd and API discovery.

Relates to #221 and #1997

Checklist

• Unit tests updated
• End user documentation updated

With this PR and the follow-up on the dns-controller annotation compat support, I am hoping it can be used as a drop-in replacement for 70% of kOps users (gossip provider being another major feature needed for 100%). The end-user documentation should be around using external-dns as a drop-in for dns-controller, I think, so would be nice if we could wait a little bit with that one.

[k8s-ci-robot]

Welcome @olemarkus!

It looks like this is your first PR to kubernetes-sigs/external-dns 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes-sigs/external-dns has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[njuettner]

Nice one 👍🏻 , I would suggest adding in the readme some documentation around kops compatibility (what are we covering with this PR). Do you mind adding something there?

I'm happy to merge your PR 🥳 and thanks again.

[olemarkus]

Great. Thanks. As mentioned, I was thinking of adding to the docs as part of the followup. There is a checklist in the README right now that mention kOps as a drop-in replacement and it would make sense to tick that one off at least partially. If you can point me to where in the readme I should drop a mention, I would be happy to add one as part of this PR as well.

[njuettner]

So what we could do is adding additional checkmarks in the readme for https://github.com/kubernetes-sigs/external-dns#v10 under "Ability to replace Kops' DNS Controller")

[olemarkus]

Sounds good. Have a look now.

[njuettner]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: njuettner, olemarkus

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [njuettner]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment