[Samba] id mapping

[Nick Couchman]

On Sun, Sep 19, 2021 at 2:52 PM Rowland Penny via samba <
samba at lists.samba.org> wrote:

> On Sun, 2021-09-19 at 13:16 -0500, Patrick Goetz via samba wrote:
> > Hi -
> >
> > This question is with reference to:
> >   https://wiki.samba.org/index.php/Idmap_config_ad
> >
> > I think I know how this works, but there are still points of
> > confusion.
> > Given the example smb.conf file provided on the page referenced
> > above:
> >
> > [global] section of smb.conf:
> > -------------------------------------
> >    security = ADS
> >    workgroup = SAMDOM
> >    realm = SAMDOM.EXAMPLE.COM
> >
> >    log file = /var/log/samba/%m.log
> >    log level = 1
> >
> >    # Default ID mapping configuration for local BUILTIN accounts
> >    # and groups on a domain member. The default (*) domain:
> >    # - must not overlap with any domain ID mapping configuration!
> >    # - must use a read-write-enabled back end, such as tdb.
> >    idmap config * : backend = tdb
> >    idmap config * : range = 3000-7999
> >    # - You must set a DOMAIN backend configuration
> >    # idmap config for the SAMDOM domain
> >    idmap config SAMDOM:backend = ad
> >    idmap config SAMDOM:schema_mode = rfc2307
> >    idmap config SAMDOM:range = 10000-999999
> >    idmap config SAMDOM:unix_nss_info = yes
> >
> >    vfs objects = acl_xattr
> >    map acl inherit = yes
> >    store dos attributes = yes
> > -------------------------------------
> >
> > I believe "Default domain" is a bit of a misnomer referring to
> > accounts
> > that are identified by nss before it gets to winbind or sssd;
>
> You cannot use sssd with Samba.
>

This may be a side note/topic, or some nuance, but I've seen this stated
multiple times on the list in absolute terms like this, and it isn't
strictly true, and very much depends on what you mean by "use sssd with
Samba." Do the two work together and talk to each other? No. But can they
be used side-by-side on the same system? Yes. I run them both on the same
system in my environment in a couple of places, and it works perfectly
fine. Do I recommend it? Absolutely not. I think in the vast majority of
places - 7x9s - it makes more sense to just run winbind with Samba, and
adding sssd provides nothing but more headaches - another configuration to
maintain, another set of problems to debug, etc. But I've run into
situations where I needed sssd on the same system as Samba, and it can be
done.

>  I"m converting a small NT domain
> > to Active Directory (by hand).  The environment has several linux
> > machines with local UIDs assigned in the 1001-2000 range (but with
> > the
> > UIDs the same across the linux hosts).  Since I don't plan to bind
> > most
> > of the linux machines to the domain (there is a vague user-driven
> > business case for this),
>
> I would rethink this, it is always better to join machines to the
> domain.
>
>
Not sure I would agree, but okay. Maybe for Patrick's benefit you could
provide some additional detail and reasoning on this?


> > I would like the authorization to work the same
> > for Samba shares to AD bound Windows machines and the standalone
> > linux
> > workstations,
>
> Good luck with that, it is probably impossible to get the same numeric
> ID's on unjoined machines.
>

Actually, it's definitely possible - I can think of at least two ways this
can be accomplished:
* Use sssd for NSS info instead of Winbind. Again, I'm not saying you
_should_, I'm saying you can, and sssd seems to be deterministic in its ID
mapping algorithm (similar to Winbind's idmap_autorid backend).
* Use sssd's LDAP backend, and have the uid, gid, and shell attributes
present for users in your AD schema. Yes, this adds further management and
complication that may be undesirable, but it is doable, and is the most
deterministic method :-).

Again, I know sssd comes with its own challenges, but I see all these
absolute statements of "you cannot do this" and "that is impossible" when
my real-world experience says it is possible. Whatever I'm doing wrong is
working in my environment.


> >  since these systems mount the same remote filesystems via
> > either SMB or NFS in the case of the linux systems. So my thought is
> > to
> > do something like this:
> >
> > portion of [global] section of smb.conf
> > -------------------------------------
> > idmap config * : backend = tdb
> > idmap config * : range = 2000-2999
> > # - You must set a DOMAIN backend configuration
> > # idmap config for the SAMDOM domain
> > idmap config SAMDOM:backend = ad
> > idmap config SAMDOM:schema_mode = rfc2307
> > idmap config SAMDOM:range = 1001-1999
>
> That will allow you ONE local Unix user and 1998 AD Unix users
> I wouldn't recommended it.
>

Why one local UNIX user? This doesn't make any sense.


> > -------------------------------------
> >
> > Again, very unclear why I'm configuring a tdb database for local
> > accounts, if my understanding of how this works is correct.
>
You aren't and, unfortunately, it appears you do not understand how it
> works.
>
>
tdb is the default backend that the Samba winbind config recommends or
defaults to, but it isn't the only one. You can look at winbind's rid or
autorid backends if you prefer something more deterministic and less random
(tdb isn't really random, just first-come, first-served on a per-system
basis).


> >  This would
> > reserve the UIDs 2000-2999 for potential local use, while creating an
> > AD
> > UID mapping that seamlessly works with the existing linux systems.
> > Then
> > if I do end up binding some of these linux machines to the domain,
> > everything just works with no acl mapping, or anything like this.
>
> If you are using the winbind 'ad' backend, then there is no acl
> mapping.
>
> >
> > Any thoughts?  Am I confused about how this works?  My understanding
> > of
> > how the default domain works is based on this RHEL article:
> > https://access.redhat.com/solutions/1984483
> >
>
> Try reading our documentation:
> https://wiki.samba.org/index.php/Setting_up_Samba_as_a_Domain_Member
> https://wiki.samba.org/index.php/Idmap_config_ad
>
> If you still do not understand something, please ask.
>
> I personally would ignore what you have now and set up a new AD domain
> and wouldn't use standalone servers, I would join everything to the
> domain.
>
>
Why? You've stated this multiple times, and you haven't really provided any
clear reasoning for the original user as to why you think joining to the
domain is the best option. Maybe it is, but could you help him understand?

-Nick