Chapter 12. Group Mapping MS Windows and UNIX
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## Chapter 12. Group Mapping MS Windows and UNIX

### John H. Terpstra

Samba Team

`<jht@samba.org>`

### Jean François Micouleau

### Gerald (Jerry) Carter

Samba Team

`<jerry@samba.org>`

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 12.1. IDMAP: group SID to GID resolution.

Figure 12.2. 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 12.3. 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.

### 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.

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 wither by logging onto the Domain as the user `root`, or by permitting particular users to use a UNIX account that is a member of the UNIX group that has a GID=0 as the primary group 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.

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.

### 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 12.1. Well-Known User Default RIDs

Well-Known Entity | RID | Type | Essential
--- | --- | --- | ---
Domain Administrator | 500 | User | No
Domain Guest | 501 | User | No
Domain KRBTGT | 502 | User | No
Domain Admins | 512 | Group | Yes
Domain Users | 513 | Group | Yes
Domain Guests | 514 | Group | Yes
Domain Computers | 515 | Group | No
Domain Controllers | 516 | Group | No
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 Admins | 544 | Alias | No
Builtin users | 545 | Alias | No
Builtin Guests | 546 | Alias | 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 Replicator | 552 | Alias | No
Builtin RAS Servers | 553 | Alias | No

### 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.

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

    # 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 12.2. 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 12.3. 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 MS Windows Groups to MS Windows Groups Fails

Samba-3 does not support nested groups from the MS Windows control environment.

### 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`.

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