[Samba] Corruption of winbind cache after converting NT4 to AD domain

Michael Tokarev mjt at tls.msk.ru
Sat Feb 12 08:56:46 UTC 2022

Please note: I'm not arguing here, my intention is the understanding.

12.02.2022 11:17, Rowland Penny via samba wrote:
> This is one of the reasons that I keep saying that if you have run an
> NT4-style domain, then you should forget a lot of what you know, it is
> no longer relevant.

Even without NT4-style domain we'll face the same prob. Eg, we had just
linux machines, and added a bunch of windows workstations and had to
configure AD with samba. Should we convert all linux users to AD
to continue using linux?  This sounds counter-productive and confusing.

>> Roughly
>> speaking, we have local linux users with their passwords and ssh
>> keys,
>> and their windows passwords are stored within AD.
> This is where you are going wrong, you do not local Unix users, you
> make AD users into Unix users.
>>   The uid numbers are
>> the same, the names are the same.
> But the SIDs will be different.

Why? Because of some bug in winbind we're discussing?

I think this is where my misunderstanding comes from.

Linux does not have notion of SID, it uses uid. And the uid will
be the same for sure.  I guess we definitely can make samba always
return the "windows SID" in this case when the user is in both the
AD and locally, and only return "unix SID" (whatever it is) when
this user does not exist in AD.  I think this will solve everything
and make whole set of issues to go away, and it will be one user
again as in days of NT4-style domains.

>>    And I don't understand that it is
>> bad and especially WHY it is bad. Besides bad (to my view: buggy)
>> behavor of winbind (it should either give meaningful error message
>> or should work, but not error out randomly with a very difficult to
>> debug issues).
>> It is not mixed users, - I view it is the same single user whos
>> windows-related attributes are stored in the AD. That's it.
> No, it is two users with the same name, one in /etc/passwd, one in AD.

So make winbind to return "unix SID" only when there's no "windows SID"
(or "AD SID") for this user, and these two users will become one again.
I think it's best of all worlds, no?

>>>> So there are two parts of the question:
>>>> First, how such setup should be done? We really used to linux
>>>> auth
>>>> and linux
>>>> work, it's somewhat unnatural to rely on the AD when dealing with
>>>> local linux
>>>> accounts.  But at the same time, these account should have access
>>>> from windows
>>>> to their files.  And most important, _why_ this setup should be
>>>> done?
> Lets go back to my username:
> rowland at devstation:~$ getent passwd rowland
> rowland:*:10000:10000:Rowland Penny:/home/rowland:/bin/bash
> rowland at devstation:~$ grep -c 'rowland' /etc/passwd
> 0

I do have many users like this on my second test server where I've added
only a few users from our list for testing.  It works for sure (with
the only difference of added TLS\\ prefix in my case).  But in order to
do what you suggest, I have to add networking ways to logins locally too
(eg pam_winbind), but we already have prefectly working /etc/passwd &
/etc/shadow which does not depend on the network and which does not
depend on trusting the AD to not use uidNumber=0 somewhere for example.

> As you can see, I have a Unix ID (and the second '10000' is Domain
> Users), but I am NOT in /etc/passwd. This means that Unix knows who I
> am.
> The 'passwd' line in /etc/nsswitch.conf is this:
> passwd:         compat winbind
> Which means that /etc/passwd is checked first, then AD, can you see why
> you cannot have users in /etc/passwd and AD ?

No, I don't see why.  It's the same here too (except I use files not compat,
but in this context it doesn't matter). And honestly I don't see why I can't
have the same username in both, at least not from this config and this
ordering rules.

If we have the same username/uid in /etc/passwd and in AD.

When we need username to uid conversion, we first look at /etc/passwd, find
it and return it. Done.  Ditto for uid-to-username.

When we need to convert something to SID, we should not use /etc/nsswitch.conf
in the first place, - it knows nothing about SIDs, and no unix tool look them up
too. So ask AD, return SID from there, or create the fake "unix SID" if not.
If that's not enough, make it configurable in smb.conf.

Why I can't have the same user (and for me it is the same user) both locally
and in the AD?

>>> I can assure that 'rowland' isn't in /etc/passwd
>> The main question is why. And my inside is fighting with this idea too:
>> why should we move our lovely local users to some remote location and
>> make our main server dependant from some other machine(s) while it already
>> self-contained?  We do have local accounts on all linux servers (sharing
>> the same uids), - this may be difficult to administer (it's not with a
>> bit of getting used to), but it is 100% reliable.
> Because it gives you one place to administrate everything, as I keep
> saying, forget most things you know.

The "one place" has its own good and bad sides. When something can be done
locally I prefer it to be done locally. It's okay for me to have uid->name
mapping over the network, but I tend to configure auth for users locally
(we don't have many servers) and don't understand why just giving some of
my users access to their files from windows machine forces me to REMOVE
these users from the system and move everything to network. Very confusing.

>>> I do not recommend using 'netbios aliases' use a dns 'CNAME'
>>> instead.
>> Hm.
>> It's actually interesting.
>> I didn't plan to mention this but we faced an issue here too.
>> When I added a CNAME for a host, it didn't work, - neither from
>> windows
>> nor from smbclient, when logging in the server returned "wrong
>> password"
>> when connecting to //cname/foo -U foo, but worked fine when using
>> //mainname/foo -U foo.  And it didn't work until I added the above
>> netbios aliases line and re-joined this server to the domain
>> (net ads leave | join). Only after that clients were able to connect.
>> It took me lots of time to figure it out.  In the AD some AltName
>> attributes appeared after the rejoin (I don't remember exactly).
> AD lives on dns and netbios is going away.

This doesn't explain why it still requires netbios :)

And yes, DNS CNAME alone definitely works from windows machines and from
linux machines too, - I tried with smbclient and it too failed to login.
I'll investigate this further later.

Thank you!


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