[Samba] Get last uidNumber

Jonathon Reinhart jonathon.reinhart at gmail.com
Mon Feb 15 03:27:10 UTC 2021

On Sun, Feb 14, 2021 at 6:48 PM Andrew Bartlett <abartlet at samba.org> wrote:
> On Tue, 2020-11-03 at 11:38 -0500, Jonathon Reinhart via samba wrote:
> > See this post where Rowland suggested using these attributes to store
> > the maximum value in LDAP. (These are not automatically calculated,
> > your script would need to keep them updated.)
> >
> > - msSFU30MaxUidNumber
> > - msSFU30MaxGidNumber
> Just be aware that these are not multi-master update safe to update.
> If your domain can get out of sync, just be aware that if your tool is
> pointed at an 'older' DC, it could allocate the same UID twice.

Thanks for the heads up. Am I correct that this is problematic even
without the atomic update problem you mention below?

Let's assume that msSFU30MaxUidNumber is being updated atomically
(from the perspective of the LDAP server serving adman's requests). If
I understand your concern correctly, the following sequence could

- msSFU30MaxUidNumber = 1000
- Adman connects to DC1, assigns UID=1000 to a user, and atomically
increments msSFU30MaxUidNumber to 1001
- Before the replication completes, Adman connects to DC2, sees the
old value of msSFU30MaxUidNumber = 1000, and assigns a duplicate
UID=1000 to a different user
- msSFU30MaxUidNumber = 1001

I have a cron job that runs Adman every minute. Since Adman looks up
the LDAP server via the SRV record [1] (and doesn't sort them), it's
possible that it could choose a different DC the next time. And I
guess if replication was being pokey, this scenario could happen.

I'm not sure how to best resolve this. Perhaps one mitigating method
would be to always target the DC with the PDC emulator FSMO role.
I've opened issue adman#23 [2] to track this.

> > https://lists.samba.org/archive/samba/2019-June/223499.html
> >
> > But also, check out my project ADMan, which does several things, one
> > of which is assign uidNumber / gidNumber attributes to new users:
> >
> > https://gitlab.com/JonathonReinhart/adman/
> Cool.
> One thing I would love is to get the uidNumber/gidNumber assignment
> into Samba.  I would use something like the algorithm from idmap_sssd
> to pick a 'pretty much likely to be unique UID' based on the RID and
> then store it forever in the uidNumber.

One mistake I would love to go back in time to rectify is the
suggested UID/GID ranges. I first asked about this here [3]. I'm
currently using 100000-200000 for both uidNumber and gidNumber. While
this is fine for most use cases, it does preclude the use of SSSD's
"automatic user private groups" feature which, if enabled
(auto_private_groups = true), makes it such that "every AD user has a
GID which is identical to the UID." [4] [5] For that reason, I wish I
had gone with disjoint ranges.

> In the meantime I would suggest to try to do an LDAP atomic update of
> the msSFU30MaxUidNumber regardless.
> See https://ldapwiki.com/wiki/LDIF%20Atomic%20Operations for what I
> mean.

Cool. I had briefly considered the atomicity problems, but kind of
ignored them, as Adman is the only agent in my network updating these
fields. But this would be a great improvement. I actually identified
another related problem while looking at this. I opened adman#24 [6]
to track.

> Sadly Samba as an ideal POSIX directory remains on my 'todo' list.
> Some ideas are here:
> https://wiki.samba.org/index.php/Better_Posix_AD

Yep. If people are running Samba AD DCs, they're almost certainly
running POSIX clients/servers too. If Samba managed some/all of this
automatically, it would save a lot of start-up hassle.

Not having to know intimate details (like Rowland's favorite: Don't
assign gidNumber to Domain Admins b/c it owns things in SYSVOL) would
save a lot of mailing list traffic.

Thanks for taking a look and offering insight!


[1]: https://gitlab.com/JonathonReinhart/adman/-/blob/v0.5.0/adman/locate.py
[2]: https://gitlab.com/JonathonReinhart/adman/-/issues/23
[3]: https://lists.samba.org/archive/samba/2019-June/223497.html
[4]: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/windows_integration_guide/gid-for-ad-users
[5]: https://sssd.io/docs/design_pages/auto_private_groups.html
[6]: https://gitlab.com/JonathonReinhart/adman/-/issues/24

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