[Samba] Linux member joined to AD domain: No login with domain user possible, getent not working

[Rowland Penny]

On Fri, 23 May 2025 20:42:23 +0200
Paul Leiber via samba <samba at lists.samba.org> wrote:
> 
> However, here is a new angle: I have the suspicion that a temporarily 
> missing DC has to do with my issue. I installed a second DC (for 
> redundancy) on a Raspberry Pi some time ago. I had problems with the 
> setup of the Raspberry Pi, therefore this DC2 was inactive for some
> time (several months). I was working under the assumption that a
> missing DC doesn't cause problems as long as another DC is available,
> therefore I didn't think of it much.
> 
> The first observation that brought me to my suspicion was the
> following: When using getent -u on the machine that has the original
> issues (no AD login possible), I could see in TCP traffic that DC1
> was trying to contact DC2 (the missing one). I could also see that
> the output to getent -u takes some time after showing the local users
> until the AD users appeared. This looked like some timeout to me,
> which could be caused by waiting for the missing DC2.
> 
> The second observation came yesterday after updating various Debian 
> packages (among others: Samba 4.22.1), including a reboot, on DC1. I 
> suddenly could not access my shares anymore. (I want to make clear
> that I think this is a new error and not directly connected to the
> login issue.) The corresponding error in the samba log was
> "check_account: Failed to convert SID [SID] to a UID
> (dom_user:[user])". I also noticed again the unsuccessful contacts to
> DC2 from DC1. So I fixed the issue with the Raspberry Pi and spun up
> DC2 to test if this would resolve the issue with the share access,
> and it did.
> 
> Then I also checked if re-adding DC2 solves the login problems, but
> they still exist. However, the timeout between showing local users
> and AD users mentioned above is gone, that's why I think the login
> problem could also have to do with the missing DC.
> 
> Does that suspicion ring a bell with someone, and how could a missing
> DC be related to my login problems?
> 
> On a more general note: Is it really such a bad idea to have a DC
> which is not connected to the AD network for a longer period of time?
> 

It is a very bad idea to shutdown a DC for any length of time, a couple
of hours for maintenance is okay, but anything longer than this isn't
good. Every DC replicates to all other DC and there are dns records
required for each DC, DCs and clients use these dns records to find a
DC, but if the DC isn't there ??? There is also the possibility of
deleted records (that still exist on the turned off DC) coming back
when the turned off DC is turned on again.

If you are going to turn off a DC for any length of time, I suggest you
demote it.

The missing DC could well be your problem.

Rowland