[Samba] SID mapping: Samba and SSSD

Andrew Walker walker.aj325 at gmail.com
Thu Sep 3 20:06:08 UTC 2020

On Thu, Sep 3, 2020 at 3:15 PM Rowland penny via samba <
samba at lists.samba.org> wrote:

> On 03/09/2020 20:01, Andrew Walker wrote:
> > Whoops, fumbled my response a bit. slice size by default is 200,000,
> > and to clarify the SID passed into murmur3() is the domain SID, not
> > SID of individual user. Though, manpage for sssd-ad should be
> > consulted for precise details. My understanding though is as long as
> > everything fits in one slice, then you can just use RID. If you have
> > multiple slices, you're stuck with non-deterministic behavior from
> > SSSD and so can continue to use RID in winbind config. Although, I'm
> > happy for someone to prove me wrong (which is the way things usually
> > happen when you open your mouth). :)
> I don't think this is going to work, from my understanding sssd
> calculates the Unix ID from the SID and the result may not be
> deterministic and there is is certainly no way that you could reproduce
> this with the winbind 'rid' backend.
> Can we please forget that sssd exists ? It doesn't work with Samba any
> more (even red-hat admits that).
> Rowland

I wish things worked that way. Then I wouldn't have to worry about MacOS
compatibility anymore. :))

This is more of an issue with making sure that end-users can make
id-mapping as consistent as possible in an environment. Think of the case
of MS AD domain with multiple Samba file servers (some with SSSD for domain
join and some with winbind).

src/lib/idmap/sss_idmap.c indicates that ids are calculated based on the
RIDs that are covered by a particular slice:
 static bool comp_id(struct idmap_range_params *range_params, long long rid,
                    uint32_t *_id)
    uint32_t id;

    if (rid >= range_params->first_rid
            && ((UINT32_MAX - range_params->min_id) >
               (rid - range_params->first_rid))) {
        id = range_params->min_id + (rid - range_params->first_rid);
        if (id <= range_params->max_id) {
            *_id = id;
            return true;
    return false;
So pretty easy fit with idmap_rid if the domain's highest RID is <
slice_size in SSSD. So it might be helpful for people to at least try "id
DOMAIN\\administrator" on Linux client with SSSD joined to a Windows AD
domain (assuming that this is an issue of trying to make sure that clients
with SSSD and Winbind end up with similar ID mapping), subtract 500 from
from the ID, then set that as the RID low range. It's not perfect, but it
works for most environments.

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