Source IPv6 node IPs as AAAA endpoints

[danopia]

Description

In an IPv6-enabled cluster, it's possible (and legitiment) for a node's .status.addresses list to contain IPv6 addresses. Usually alongside an IPv4 address for dual-stack clusters. Here's a partial example: https://kubernetes.io/docs/tasks/network/validate-dual-stack/#validate-node-addressing

Multiple external-dns sources assume that every IP address is IPv4 and thus has A hardcoded as the record type. This results in invalid DNS endpoints being generated. With the node source, this means the sync loop is broken while such a node is still present.

This PR replaces some of the node.go logic to support creating either an A endpoint, or an AAAA endpoint, or both at the same time, for each node. Some of the logic might be questionable and I welcome the review.

Fixes #1875 but does not actually result in AAAA records being created at this time because external-dns itself does not support AAAA records yet. The planner needs some work to support dual-stack records properly and I'm considering it out of scope of upgrading each source to emit proper endpoints.

So, this PR fixes invalid A endpoints being generated, and replaces them with correct AAAA endpoints that simply aren't actioned on at this point in time.

Checklist

• Unit tests updated
• End user documentation updated

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[hrvolapeter]

It's there any update on the progress? Seems like there weren't any comments from maintainers. This PR seems to mitigate problem with IPv6. Currently external-dns creates A records with IPv6 which fails on cloud provider validation

[danopia]

Correct assessment @hrvolapeter. IPv6 support in general is pretty much lacking until further work is done.

Due to this (and other) painpoints, and especially after #1923 I've invested in my own from-scratch analogue of external-dns: https://github.com/danopia/kubernetes-dns-sync & no longer run external-dns myself. However I still would like to see good IPv6 support across the ecosystem so I'm keeping this PR open to see where it goes.

[seanmalloy]

/kind feature

[danopia]

Rebased; there was a conflict from this new line https://github.com/kubernetes-sigs/external-dns/blame/979c2d611bb7a626dfaaade5fe36f7693949a733/source/node.go#L154 which I moved down into the endpoints loop since this changeset can introduce up to two endpoint records per node.

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@danopia: PR needs rebase.

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/lifecycle stale

[bootc]

/remove-lifecycle stale

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/lifecycle stale

[danopia]

I'll finally accept the staleness of this; as of 2022 I have a pretty low appetite for rebasing on this codebase. Hopefully improved AAAA support will eventually merge from other contributions.