fix: internal ipv6 being used as external

[TroyKomodo]

Description

This change undoes a check for internal IPv6 addresses to be used when External IP has been requested.

Closes #4807

Checklist

• [ ] Unit tests updated
• [ ] End user documentation updated

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The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: TroyKomodo / name: Troy (c735e76033dc84357b10193e038c7ee00aa7e745)

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[mloiseleur]

@TroyKomodo This subject has already been discussed see https://github.com/kubernetes-sigs/external-dns/pull/4574#issuecomment-2286040002:

We are discussing external-dns change in https://kubernetes.slack.com/archives/C771MKDKQ/p1723531094274339 and the InternalIP/ExternalIP topic was discussed in https://kubernetes.slack.com/archives/C09QYUH5W/p1723197933110369 . sig-network tech-leads said that node InternalIP/ExternalIP has no real meaning and different cloud providers might use it differently.

FTM, we concluded it should be provider specific. Wdyt ? Does it makes sense to you ? Do you think you can rework this PR accordingly ?

[TroyKomodo]

I think it doesn't make sense to force everyone to use a specific bug-fix related to AWS not assigning Node IPs correctly. If anything the default behaviour should likely be to not include the Internal IPv6 addresses as external at all unless an annotation is specified. I also think its a fringe use case in AWS since typically there you would make use of the ELBs rather than expose a port on a host.

I think, at a minimum it should check if the IPv6 address is even routable (not in a restricted range) before attempting to use it as an external address. However IMO the best approach is to not include Internal IPv6 addresses in the external addresses unless an annotation is specified to do so.

Am willing to rework the PR to be whatever is decided the best approach is.

[TroyKomodo]

@mloiseleur any updates here?

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/lifecycle stale

[ivankatliarchuk]

/assign

Maintaining provider-specific behavior offers limited value. However, given the critical nature of this issue, evidenced by multiple discussions, pull requests, and reports highlighting that exposing private IPv6 publicly it as a bug and a potential security leak, I still ask to add of a temporary flag to disable this behavior. This flag would provide a crucial safety net for users who may be negatively impacted by the immediate removal of ipv6-exposing-specific configurations. And we will deprecate/flip/revert this behaviour somehow in the follow-up.

[ivankatliarchuk]

/lifecycle frozen

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[TroyKomodo]

Am willing to rework the PR to be whatever is decided the best approach is.

I was willing to rework this PR 3 months ago when I opened it. I am no longer willing. I am not sure how this issue has taken so long to be discussed. In my opinion it's a very simple to grasp. Don't force everyone to use a "bug-fix" that is specific to AWS. I agree with @jonasbadstuebner that this should be a breaking change because frankly it was a stupid change to merge in to begin with. There shouldn't be a flag to opt out of this behaviour. The other way around makes significantly more sense, that being a flag to opt in to this bug-fix for the 10s of people who rely on this feature in AWS. In fact it shouldn't even be a "flag" it should be an annotation to opt in.

[ivankatliarchuk]

I do understand that, thanks for reply. Please close the pull request then, and you or someone could re-open at some point if you decide so.

Hope you understand that I do not pocess decision-making authority or approve access in this project.

For anyone who may decide to take an ownership of deprecationg/chaning this behavior

To avoid breaking existing users, we should follow a simple deprecation policy for behaviours that a slated to be removed. Add a flag to disable this logic, and add a log.WARN("DEPRECATED will be removed in next minor release") or similar here https://github.com/kubernetes-sigs/external-dns/blob/cd624b6f550c75a5accab84b1354f851e232f384/source/compatibility.go#L163.

That's quite a common practices, the feature either to evolve or need to be removed.