Table of Contents
Starting with Samba-3, new group mapping functionality is available to create associations between Windows group SIDs and UNIX group GIDs. The groupmap subcommand included with the net tool can be used to manage these associations.
The new facility for mapping NT groups to UNIX system groups allows the administrator to decide
which NT domain groups are to be exposed to MS Windows clients. Only those NT groups that map
to a UNIX group that has a value other than the default (
-1) will be exposed
in group selection lists in tools that access domain users and groups.
domain admin group parameter has been removed in Samba-3 and should no longer
be specified in
smb.conf. In Samba-2.2.x, this parameter was used to give the listed users membership in the
Domain Admins Windows group, which gave local admin rights on their workstations
(in default configurations).
Samba allows the administrator to create MS Windows NT4/200x group accounts and to arbitrarily associate them with UNIX/Linux group accounts.
Group accounts can be managed using the MS Windows NT4 or MS Windows 200x/XP Professional MMC tools.
Appropriate interface scripts should be provided in
smb.conf if it is desired that UNIX/Linux system
accounts should be automatically created when these tools are used. In the absence of these scripts, and
so long as winbindd is running, Samba group accounts that are created using these
tools will be allocated UNIX UIDs and GIDs from the ID range specified by the
idmap uid/idmap gid
parameters in the
In both cases, when winbindd is not running, only locally resolvable groups can be recognized. Please refer to IDMAP: Group SID-to-GID Resolution and IDMAP: GID Resolution to Matching SID. The net groupmap is used to establish UNIX group to NT SID mappings as shown in IDMAP: storing group mappings.
Administrators should be aware that where
smb.conf group interface scripts make
direct calls to the UNIX/Linux system tools (the shadow utilities, groupadd,
groupdel, and groupmod), the resulting UNIX/Linux group names will be subject
to any limits imposed by these tools. If the tool does not allow uppercase characters
or space characters, then the creation of an MS Windows NT4/200x-style group of
Engineering Managers will attempt to create an identically named
UNIX/Linux group, an attempt that will of course fail.
There are several possible workarounds for the operating system tools limitation. One method is to use a script that generates a name for the UNIX/Linux system group that fits the operating system limits and that then just passes the UNIX/Linux group ID (GID) back to the calling Samba interface. This will provide a dynamic workaround solution.
Another workaround is to manually create a UNIX/Linux group, then manually create the MS Windows NT4/200x group on the Samba server, and then use the net groupmap tool to connect the two to each other.
When you install MS Windows NT4/200x on a computer, the installation
program creates default users and groups, notably the
and gives that group privileges necessary to perform essential system tasks,
such as the ability to change the date and time or to kill (or close) any process running on the
Administrator user is a member of the
Administrators group, and thus inherits
Administrators group privileges. If a
joe user is created to be a member of the
joe has exactly the same rights as the user
When an MS Windows NT4/200x/XP machine is made a domain member, the “Domain Admins” group of the
PDC is added to the local
Administrators group of the workstation. Every member of the
Domain Admins group inherits the rights of the local
Administrators group when
logging on the workstation.
Create a UNIX group (usually in
/etc/group); let's call it
Map this domadm group to the “Domain Admins” group by executing the command:
net groupmap add ntgroup="Domain Admins" unixgroup=domadm rid=512 type=d
joe, john, and
mary are domain administrators.
It is possible to map any arbitrary UNIX group to any Windows NT4/200x group as well as to make any UNIX group a Windows domain group. For example, if you wanted to include a UNIX group (e.g., acct) in an ACL on a local file or printer on a Domain Member machine, you would flag that group as a domain group by running the following on the Samba PDC:
net groupmap add rid=1000 ntgroup="Accounting" unixgroup=acct type=d
ntgroup value must be in quotes if it contains space characters to prevent
the space from being interpreted as a command delimiter.
Be aware that the RID parameter is an unsigned 32-bit integer that should normally start at 1000. However, this RID must not overlap with any RID assigned to a user. Verification for this is done differently depending on the passdb backend you are using. Future versions of the tools may perform the verification automatically, but for now the burden is on you.
Windows does not permit user and group accounts to have the same name. This has serious implications for all sites that use private group accounts. A private group account is an administrative practice whereby users are each given their own group account. Red Hat Linux, as well as several free distributions of Linux, by default create private groups.
All MS Windows products since the release of Windows NT 3.10 support the use of nested groups. Many Windows network administrators depend on this capability because it greatly simplifies security administration.
The nested group architecture was designed with the premise that day-to-day user and group membership management should be performed on the domain security database. The application of group security should be implemented on domain member servers using only local groups. On the domain member server, all file system security controls are then limited to use of the local groups, which will contain domain global groups and domain global users.
You may ask, What are the benefits of this arrangement? The answer is obvious to those who have plumbed the dark depths of Windows networking architecture. Consider for a moment a server on which are stored 200,000 files, each with individual domain user and domain group settings. The company that owns the file server is bought by another company, resulting in the server being moved to another location, and then it is made a member of a different domain. Who would you think now owns all the files and directories? Answer: Account Unknown.
Unraveling the file ownership mess is an unenviable administrative task that can be avoided simply
by using local groups to control all file and directory access control. In this case, only the members
of the local groups will have been lost. The files and directories in the storage subsystem will still
be owned by the local groups. The same goes for all ACLs on them. It is administratively much simpler
to delete the
Account Unknown membership entries inside local groups with appropriate
entries for domain global groups in the new domain that the server has been made a member of.
Another prominent example of the use of nested groups involves implementation of administrative privileges
on domain member workstations and servers. Administrative privileges are given to all members of the
built-in local group
Administrators on each domain member machine. To ensure that all domain
administrators have full rights on the member server or workstation, on joining the domain, the
Domain Admins group is added to the local Administrators group. Thus everyone who is
logged into the domain as a member of the Domain Admins group is also granted local administrative
privileges on each domain member.
UNIX/Linux has no concept of support for nested groups, and thus Samba has for a long time not supported
them either. The problem is that you would have to enter UNIX groups as auxiliary members of a group in
/etc/group. This does not work because it was not a design requirement at the time
the UNIX file system security model was implemented. Since Samba-2.2, the winbind daemon can provide
/etc/group entries on demand by obtaining user and group information from the domain
controller that the Samba server is a member of.
In effect, Samba supplements the
/etc/group data via the dynamic
libnss_winbind mechanism. Beginning with Samba-3.0.3, this facility is used to provide
local groups in the same manner as Windows. It works by expanding the local groups on the
fly as they are accessed. For example, the
Domain Users group of the domain is made
a member of the local group
demo. Whenever Samba needs to resolve membership of the
demo local (alias) group, winbind asks the domain controller for demo members of the Domain Users
group. By definition, it can only contain user objects, which can then be faked to be member of the
To enable the use of nested groups, winbindd must be used with NSS winbind.
Creation and administration of the local groups is done best via the Windows Domain User Manager or its
Samba equivalent, the utility net rpc group. Creating the local group
demo is achieved by executing:
root#net rpc group add demo -L -Uroot%not24get
Here the -L switch means that you want to create a local group. It may be necessary to add -S and -U
switches for accessing the correct host with appropriate user or root privileges. Adding and removing
group members can be done via the
delmem subcommands of
net rpc group command. For example, addition of “DOM\Domain Users” to the
demo is done by executing:
net rpc group addmem demo "DOM\Domain Users"
Having completed these two steps, the execution of getent group demo will show demo
members of the global
Domain Users group as members of the group
demo. This also works with any local or domain user. In case the domain DOM trusts
another domain, it is also possible to add global users and groups of the trusted domain as members of
demo. The users from the foreign domain who are members of the group that has been
added to the
demo group now have the same local access permissions as local domain
Administrative rights are necessary in two specific forms:
For Samba-3 domain controllers and domain member servers/clients.
To manage domain member Windows workstations.
Versions of Samba up to and including 3.0.10 do not provide a means for assigning rights and privileges that are necessary for system administration tasks from a Windows domain member client machine, so domain administration tasks such as adding, deleting, and changing user and group account information, and managing workstation domain membership accounts, can be handled by any account other than root.
Samba-3.0.11 introduced a new privilege management interface (see User Rights and Privileges) that permits these tasks to be delegated to non-root (i.e., accounts other than the equivalent of the MS Windows Administrator) accounts.
Administrative tasks on UNIX/Linux systems, such as adding users or groups, requires
root-level privilege. The addition of a Windows client to a Samba domain involves the
addition of a user account for the Windows client.
Many UNIX administrators continue to request that the Samba Team make it possible to add Windows workstations, or
the ability to add, delete, or modify user accounts, without requiring
Such a request violates every understanding of basic UNIX system security.
There is no safe way to provide access on a UNIX/Linux system without providing
root-level privileges. Provision of
root privileges can be done
either by logging on to the Domain as the user
root or by permitting particular users to
use a UNIX account that has a UID=0 in the
/etc/passwd database. Users of such accounts
can use tools like the NT4 Domain User Manager and the NT4 Domain Server Manager to manage user and group
accounts as well as domain member server and client accounts. This level of privilege is also needed to manage
When first installed, Windows NT4/200x/XP are preconfigured with certain user, group, and
alias entities. Each has a well-known RID. These must be preserved for continued
integrity of operation. Samba must be provisioned with certain essential domain groups that require
the appropriate RID value. When Samba-3 is configured to use
tdbsam, the essential
domain groups are automatically created. It is the LDAP administrator's responsibility to create
(provision) the default NT groups.
Each essential domain group must be assigned its respective well-known RID. The default users, groups, aliases, and RIDs are shown in Well-Known User Default RIDs.
It is permissible to create any domain group that may be necessary; just make certain that the essential domain groups (well known) have been created and assigned their default RIDs. Other groups you create may be assigned any arbitrary RID you care to use.
Be sure to map each domain group to a UNIX system group. That is the only way to ensure that the group will be available for use as an NT domain group.
Table 12.1. Well-Known User Default RIDs
|Domain Certificate Admins||517||Group||No|
|Domain Schema Admins||518||Group||No|
|Domain Enterprise Admins||519||Group||No|
|Domain Policy Admins||520||Group||No|
|Builtin Power Users||547||Alias||No|
|Builtin Account Operators||548||Alias||No|
|Builtin System Operators||549||Alias||No|
|Builtin Print Operators||550||Alias||No|
|Builtin Backup Operators||551||Alias||No|
|Builtin RAS Servers||553||Alias||No|
net groupmap listDomain Admins (S-1-5-21-2547222302-1596225915-2414751004-512) -> domadmin Domain Users (S-1-5-21-2547222302-1596225915-2414751004-513) -> domuser Domain Guests (S-1-5-21-2547222302-1596225915-2414751004-514) -> domguest
For complete details on net groupmap, refer to the net(8) man page.
Everyone needs tools. Some of us like to create our own, others prefer to use canned tools (i.e., prepared by someone else for general use).
A script to create complying group names for use by the Samba group interfaces
is provided in smbgrpadd.sh. This script
adds a temporary entry in the
/etc/group file and then renames
it to the desired name. This is an example of a method to get around operating
system maintenance tool limitations such as those present in some version of the
Example 12.1. smbgrpadd.sh
#!/bin/bash # Add the group using normal system groupadd tool. groupadd smbtmpgrp00 thegid=`cat /etc/group | grep ^smbtmpgrp00 | cut -d ":" -f3` # Now change the name to what we want for the MS Windows networking end cp /etc/group /etc/group.bak cat /etc/group.bak | sed "s/^smbtmpgrp00/$1/g" > /etc/group rm /etc/group.bak # Now return the GID as would normally happen. echo $thegid exit 0
smb.conf entry for the above script shown in the configuration of
smb.conf for the add group Script demonstrates how it may be used.
Example 12.2. Configuration of
smb.conf for the add group Script
In our example we have created a UNIX/Linux group called
Our script will create the additional groups
It is a good idea to save this shell script for later use just in case you ever need to rebuild your mapping database.
For the sake of convenience we elect to save this script as a file called
This script is given in intGroups.sh.
Example 12.3. Script to Set Group Mapping
#!/bin/bash net groupmap add ntgroup="Domain Admins" unixgroup=ntadmin rid=512 type=d net groupmap add ntgroup="Domain Users" unixgroup=users rid=513 type=d net groupmap add ntgroup="Domain Guests" unixgroup=nobody rid=514 type=d groupadd Orks groupadd Elves groupadd Gnomes net groupmap add ntgroup="Orks" unixgroup=Orks type=d net groupmap add ntgroup="Elves" unixgroup=Elves type=d net groupmap add ntgroup="Gnomes" unixgroup=Gnomes type=d
Of course it is expected that the administrator will modify this to suit local needs. For information regarding the use of the net groupmap tool please refer to the man page.
Versions of Samba-3 prior to 3.0.23 automatically create default group mapping for the
Domain Admins, Domain Users and
Domain Guests Windows
groups, but do not map them to UNIX GIDs. This was a cause of administrative confusion and
trouble. Commencing with Samba-3.0.23 this annomaly has been fixed - thus all Windows groups
must now be manually and explicitly created and mapped to a valid UNIX GID by the Samba
At this time there are many little surprises for the unwary administrator. In a real sense it is imperative that every step of automated control scripts be carefully tested manually before putting it into active service.
There are three possible workarounds. First, use only group names that comply with the limitations of the UNIX/Linux groupadd system tool. Second, it involves the use of the script mentioned earlier in this chapter, and third is the option is to manually create a UNIX/Linux group account that can substitute for the MS Windows group name, then use the procedure listed above to map that group to the MS Windows group.
“ What must I do to add domain users to the Power Users group? ”
The Power Users group is a group that is local to each Windows 200x/XP Professional workstation. You cannot add the Domain Users group to the Power Users group automatically, it must be done on each workstation by logging in as the local workstation administrator and then using the following procedure:
Power Users. This will launch the panel to add users or groups
to the local machine
Power Users group.
Select the domain from which the
Domain Users group is to be added.
Domain Users group.
DOMAIN\UserName, that is, for the
MIDEARTH and the user