Samba4: ID mapping is hard

steve steve at
Fri Mar 23 16:54:58 MDT 2012

On 23/03/12 23:03, Andrew Bartlett wrote:
> On Sun, 2012-03-18 at 08:19 +0100, steve wrote:
>> Hi
>> There seems to be a discrepancy in the s4 schema concerning security groups.
>> Domain Users comes with gidNumber: 100. This is however contrary to what
>> the schema allows. You can show this as follows:
> Steve,
> Domain Users does not hold that attribute.  There is an idmapping in
> idmap.ldb for this value, but it is not placed in the directory by
> default.
> As you mention here, you could add it if you want to:
>> Create a new group. samba-tool group add mygroup.
>> Use phpldapadmin to add the gidNumber attribute.
>> There is an error because gidNumber is provided by the posixGroup class
>> and that objectclass is not present by default.
>> No problem. We add objectClass: posixGroup and then we can add
>> gidNumber: xxx just fine.
>> This however throws up another error in that mygroup is now not a
>> security group but a posix group and the ability to view and manipulate
>> group members is not available in Active Directory Computers and Users
>> (ADCU). We made the folllowing observations:
>> 1. The members tabs are missing from mygroup properties in ADCU
>> 2. you can still use samba-tool group addmembers to manipulate the groups
>> 3. you can still select and change primary group for a user in ADCU
>> 4. you can add users to the group under phpldapadmin but the users who
>> are already members are not displayed. An error is however correctly
>> displayed if you try to add a user who is already a member.
>> 5. You can still manipulate the posixGroup as if it were a security
>> group, set acl's and permissions etc from the security tab of a file or
>> folder.
>> 6. You can use a big hammer to add attributes that you should not be
>> able to add. e.g. you can add gidNumber without the objectClass (which
>> supplies gidNumber) being present using ldapmodify or ldbmodify.
>> 7. posixAccount and its associated attributes work exactly as advertised
>> in the schema.
>> Conclusion:
>> This is simply an inconvenience. Everything works as expected except
>> being able to view the members that are in a group either in ADCU or
>> phpldapadmin _after_ you have added objectClass: posixGroup to it.
>> Why does adding the posixGroup Class knock out the ability to be able to
>> view group membership? Is this an error in the posixGroup schema?  Is it
>> an aim that s4 be an _exact_ replacement for m$ AD?
>> Is this the schema that is used?
> We use the exact schema Microsoft uses, provided to us by Microsoft as
> part of the WSPP documentation.
>> from: MS-AD_Schema_2K8_R2_Classes, under
>> /usr/local/samba/share/setup/ad-schema
>> cn: PosixAccount
>> ldapDisplayName: posixAccount
> ...
>> There are full details of what we have tried with screenshots in the
>> latter part of this bugzilla:
>> Please let us know if there is anything we can test.
>> Cheers,
>> Steve
>> (Could someone fwd to samba-tecnical?)
> Why can't you raise this on samba-technical yourself?
> If our behaviour differs from Microsoft's behaviour, then please raise
> this as a bug.  I haven't seen any reference to a difference in
> behaviour that we could address.
> Finally,
> I know our idmapping situation in Samba4 sucks.  It really, really
> sucks.  The reason it hasn't been addressed properly is two things:  id
> mapping is hard, and doing it right is difficult.  Honouring uidNumber
> and gidNumber attributes set in the directory seems reasonable, but we
> cannot sensibly automatically assign those values, so what should we do
> between creation of the user and the administrator choosing a uidNumber?
We'd probably suggest that Domain Admins don't have  a uid or gid. What 
they administer is irrelevant on Linux anyway so there's no need for 
them to log onto it. I'm talking about giving domain users a uid and 
gid, the gid being that of a Security group which has the posixGroup 
object class added. In tests here, those users can then log onto both 
Linux and windows and have both Security group properties for windows 
and Linux group properties on Linux honoured perfectly. posix acl's seem 
to play quite nice with windows acls too. The only one we can't mimic 
exactly at file permission level is the windows 'modify'.
> Using idmap_rid also seems quite reasonable, but to really be like AD we
> should honour the trusted domain posixOffset parameter in doing that,
> but we don't yet auto-allocate that posixOffset (handled on the RID
> master).
> There is also the issue that for proper ACL compatibility, uidNumber and
> gidNumber actually causes problems - groups (domain administrators in
> particular) need to own files - but if they only have a gidNumber, how
> would we do that?
We've found that what you do with acls on windows (like not being able 
to access the control panel) don't have any relevance in Linux. At file 
ownership level posix to windows agreement is very good. We are setting 
primaryGroupID for our Linux users and there is a m$ technet somewhere 
where they suggest that unix gid could be mapped to primaryGroupID. ADUC 
has a note about that when changing the group id. One thing we have 
successfully setup on s4 is a group rw share. ACL's work identically for 
our posixified group members. File permissions and ownership are 
identical on both Lin and win sides in the share.
> Andrew Bartlett
What is working well for us in tests is giving Domain Users a uid, gid, 
setting their primaryGroupID to that of a posix-ified security group and 
storing these attributes in their entry in sam.ldb. The only problem I 
have with this is that adding the posixGroup objectClass to a security 
group removes the ability to be able to list its members in ADUC and it 
is really unfortunate that I can't test this against a windows server. 
Because I don't have one. This is merely an inconvenience as the 
posix-ified security group behaves exactly as if it were a normal domain 
group. If we want point and click we can use phpldapadmin.

So, uid gid mapping and the interoperability of domain and posix groups 
like this is really simple. What we fear may happen is that when an 
official s4 mapping method comes along, it will make changes to either 
the schema or sam.ldb which will disallow our storing our attributes in 
the directory.

Are we wasting time proceding with this or does it make at least a 
little sense? Our aim is simply to have a single sign on linux/windows. 
As s4 does not provide an official mechanism for this at the moment we 
invented this.

Thanks for your time,

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