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.

When mapping a UNIX/Linux group to a Windows group account all conflict can be avoided by assuring that the Windows domain group name does not overlap with any user account name.

This functionality is known as nested groups and was first added to Samba-3.0.3.

All Microsoft Windows products since the release of Windows NT 3.10 support the use of nested groups. Many Windows network administrators depend on this capability becasue 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.

Unravelling 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 builtin 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 adminitrative 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 does it. 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 DC 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 UNIX/Linux group demo.

To enable the use of nested groups, winbindd must be used together 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 priviliges. Adding and removing group members can be done via the addmem and delmem subcommands of net rpc group command. For example addition of “DOM\Domain Users” to the local group demo would be 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.

Administrative rights are necessary in two specific forms:

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 that domain administration tasks such as adding/deleting/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 Chapter on 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) account.

Administrative tasks on a Windows Domain Member workstation, can be done by anyone who is a member of the Domain Admins group. This group can be mapped to any convenient UNIX group.

When first installed, Microsoft Windows NT4/200x/XP are pre-configured with certain User, Group, and Alias entities. Each has a well-known Relative Identifier (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 administrators' 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 table.

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 its default RID. 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.

You can list the various groups in the mapping database by executing net groupmap list. Here is an example:

root#  net groupmap list
Domain 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.

A script to create complying group names for use by the Samba group interfaces is provided in smbgrpadd.sh. This script will add a temporary entry in the /etc/group file and then rename it to to the desired name. This is an example of a method to get around operating system maintenance tool limititations such as that present in some version of the groupadd tool.

#!/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

# Now return the GID as would normally happen.
echo $thegid
exit 0

The smb.conf entry for the above script would be something like that in the following example.

In our example we have created a UNIX/Linux group called ntadmin. Our script will create the additional groups Orks, Elves, and Gnomes. It is a good idea to save this shell script for later re-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 initGroups.sh. This script is given in intGroups.sh.

#!/bin/bash

net groupmap modify ntgroup="Domain Admins" unixgroup=ntadmin
net groupmap modify ntgroup="Domain Users" unixgroup=users
net groupmap modify ntgroup="Domain Guests" unixgroup=nobody

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.

This is a common problem when the groupadd is called directly by the Samba interface script for the add group script in the smb.conf file.

The most common cause of failure is an attempt to add an MS Windows group account that has either an upper case character and/or a space character in it.

There are three possible work-arounds. 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: