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Chapter 11. Group Mapping MS Windows and UNIX

John H. Terpstra

Samba Team

Jean François Micouleau

Gerald (Jerry) Carter

Samba Team

Table of Contents

Features and Benefits
Discussion
Warning User Private Group Problems
Nested Groups: Adding Windows Domain Groups to Windows Local Groups
Important Administrative Information
Default Users, Groups and Relative Identifiers
Example Configuration
Configuration Scripts
Sample smb.conf Add Group Script
Script to Configure Group Mapping
Common Errors
Adding Groups Fails
Adding Domain Users to the Power Users Group

Starting with Samba-3, new group mapping functionality is available to create associations between Windows group SIDs and UNIX groups. 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.

Warning

The 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).

Features and Benefits

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/GIDs from the ID range specified by the idmap uid/idmap gid parameters in the smb.conf file.

Figure 11.1. IDMAP: group SID to GID resolution.

IDMAP: group SID to GID resolution.

Figure 11.2. IDMAP: GID resolution to matching SID.

IDMAP: GID resolution to matching SID.

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.

Figure 11.3. IDMAP storing group mappings.

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 upper case 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 work-arounds 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 work-around solution.

Another work-around 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.

Discussion

When installing MS Windows NT4/200x on a computer, the installation program creates default users and groups, notably the Administrators group, and gives that group privileges necessary privileges 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 local machine.

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 Administrators group, joe has exactly the same rights as the user, Administrator.

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 Administrators group inherits the rights of the local Administrators group when logging on the workstation.

The following steps describe how to make Samba PDC users members of the Domain Admins group?

  1. Create a UNIX group (usually in /etc/group), let's call it domadm.

  2. Add to this group the users that must be “Administrators”. For example, if you want joe, john and mary to be administrators, your entry in /etc/group will look like this:

    		domadm:x:502:joe,john,mary
    		

  3. Map this domadm group to the “Domain Admins” group by running the command:

    	root# net groupmap add ntgroup="Domain Admins" unixgroup=domadm
    	

    The quotes around “Domain Admins” are necessary due to the space in the group name. Also make sure to leave no white-space surrounding the equal character (=).

Now 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 making 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:

root# net groupmap add rid=1000 ntgroup="Accounting" unixgroup=acct

Be aware that the RID parameter is a 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.

Warning User Private Group Problems

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.

Nested Groups: Adding Windows Domain Groups to Windows Local Groups

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.

Important Administrative Information

Administrative rights are necessary in two specific forms:

  1. For Samba-3 Domain Controllers and Domain Member Servers/Clients.

  2. 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 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.

Applicable Only to Versions Earlier than 3.0.11

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 the Samba Team make it possible to add Windows workstations, or to ability to add/delete or modify user accounts, without requiring root privileges. 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 privilege. Provision of root privileges can be done either by logging onto 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 share level ACLs.

Default Users, Groups and Relative Identifiers

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.

Note

When the passdb backend uses LDAP (ldapsam) it is the administrators' responsibility to create the essential Domain Groups, and to assign each its default RID.

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.

Table 11.1. Well-Known User Default RIDs

Well-Known EntityRIDTypeEssential
Domain Administrator500UserNo
Domain Guest501UserNo
Domain KRBTGT502UserNo
Domain Admins512GroupYes
Domain Users513GroupYes
Domain Guests514GroupYes
Domain Computers515GroupNo
Domain Controllers516GroupNo
Domain Certificate Admins517GroupNo
Domain Schema Admins518GroupNo
Domain Enterprise Admins519GroupNo
Domain Policy Admins520GroupNo
Builtin Admins544AliasNo
Builtin users545AliasNo
Builtin Guests546AliasNo
Builtin Power Users547AliasNo
Builtin Account Operators548AliasNo
Builtin System Operators549AliasNo
Builtin Print Operators550AliasNo
Builtin Backup Operators551AliasNo
Builtin Replicator552AliasNo
Builtin RAS Servers553AliasNo

Example Configuration

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.

Configuration Scripts

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).

Sample smb.conf Add Group Script

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.

Example 11.1. 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

# 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.

Example 11.2. Configuration of smb.conf for the add group script.

[global]
add group script = /path_to_tool/smbgrpadd.sh "%g"

Script to Configure Group Mapping

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.

Example 11.3. Script to Set Group Mapping

#!/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.

Common Errors

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 must be carefully tested manually before putting them into active service.

Adding Groups Fails

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.

Adding Domain Users to the Power Users 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:

  1. Click Start -> Control Panel -> Users and Passwords.

  2. Click the Advanced tab.

  3. Click the Advanced button.

  4. Click Groups.

  5. Double click Power Users. This will launch the panel to add users or groups to the local machine Power Uses group.

  6. Click the Add button.

  7. Select the domain from which the Domain Users group is to be added.

  8. Double click the Domain Users group.

  9. Click the Ok button. If a logon box is presented during this process please remember to enter the connect as DOMAIN\UserName. i.e., For the domain MIDEARTH and the user root enter MIDEARTH\root.