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Assuming an Active Directory domain with FQDN "domain.local" and NetBIOS name "DOMAIN", there are two standard formats to identify user accounts:

DOMAIN\username
username@domain.local

(The second one is called the "UserPrincipalName", or "UPN").

They need not have the same username part, so you can also have

DOMAIN\somename
someothername@domain.local

Also, the UPN can have a different suffix from the actual domain FQDN:

DOMAIN\username
username@something.else

Last but not least, you can even mix both:

DOMAIN\somename
someothername@something.else

All fine and good and well known.

However, several times I've seen someone using the intermediate format domain.local\username. Which AFAIK is not defined or documented anywhere, and yet often works, but sometimes fails; I've not done extensive research on it, but I think it fails in one or more of the last three examples, i.e. when non-standard mappings between username, UPN and domain FQDN are involved.

My question: is this kind of login name actually defined or documented anywhere?
If yes, how is it called and what are its requirements to work?
If not, why does it even work at all?

Massimo
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  • I found a reference to "long DNS credentials" when answering this question of my own, and they were indeed the solution in this case: https://serverfault.com/questions/676867/error-adding-child-active-directory-domain-to-existing-forest. However, even then they just seemed to come out of nowhere. – Massimo Apr 14 '20 at 22:26

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