Supported record types are a bit of a mess

[andsens]

First of all, thank you for this awesome project! It has been incredibly useful, and it's awesome to see the constant improvements it is getting.

Ideally I would like to reopen #5278
The documentation and the code for which record types are supported are quite inconsistent.

From the help menu:

--managed-record-types=A... ...  
    Record types to manage; specify multiple times to include many; (default: A,AAAA,CNAME)
    (supported records: A, AAAA, CNAME, NS, SRV, TXT) ($EXTERNAL_DNS_MANAGED_RECORD_TYPES)

From the docs:

• https://kubernetes-sigs.github.io/external-dns/latest/docs/sources/mx-record/
• https://kubernetes-sigs.github.io/external-dns/latest/docs/sources/txt-record/
• https://kubernetes-sigs.github.io/external-dns/latest/docs/sources/ns-record/

In the code (master, no SRV or NAPTR in v0.20.0. Also why wasn't TXT added?):

external-dns/registry/mapper/mapper.go

Lines 30 to 38 in d38daef

	supportedRecords = []string{
	endpoint.RecordTypeA,
	endpoint.RecordTypeAAAA,
	endpoint.RecordTypeCNAME,
	endpoint.RecordTypeNS,
	endpoint.RecordTypeMX,
	endpoint.RecordTypeSRV,
	endpoint.RecordTypeNAPTR,
	}

The actual issue with that codes snippet is that you can create whatever types are enabled by --managed-record-types, but changing or deleting them is restricted through getSupportedTypes().
This can result in users assuming everything is working as expected, leaving it running, only to discover at a later date that e.g. their ACME certificates have suddenly expired, because external-dns sees existing TXT records but doesn't acknowledge ownership of them and therefore doesn't delete them.

[ivankatliarchuk]

Indeed, there’s quite a bit of mess. For a long time, the project has grown through ad-hoc PRs across different areas, without much structure or clear standards. There’s no central body - like in some other projects - clear scope, driving a roadmap, planning, or a clearly defined future.

That said, this is an open-source project, and it’s maintained on a best-effort basis.

/kind design

[andsens]

I'd be down to refactor that supportedTypes slice away, though I would like to know first if there is any reason that TXT records were not included. Is it just out of an abundance of caution so that there definitely isn't a clash with registry entries?
We're running a fork where I have it enabled, and it seems to working pretty well.

Obviously quite a few more tests need to be added before a change like that can be added, so it might take a while before I'm ready with something.

[ivankatliarchuk]

We’re not going to support TXT records within the TXT registry. There are too many edge cases, and the current TXT record format doesn’t lend itself well to this use case.

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