Chapter 12. Group Mapping MS Windows and UNIX

Chapter 12. Group Mapping MS Windows and UNIX
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John H. Terpstra

Samba Team

<jht@samba.org>

Jean François Micouleau

Gerald (Jerry) Carter

Samba Team

<jerry@samba.org>

Table of Contents

Features and Benefits

Discussion

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 MS Windows Groups to MS Windows 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. 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 + and . The net groupmap is + used to establish UNIX group to NT SID mappings as shown in . +

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? +

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

Default Users, Groups and Relative Identifiers

+ + + When first installed, Microsoft Windows NT4/200x/XP are preconfigured 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-kown RID. The default Users, Groups, + Aliases, and RIDs are shown in . +

Note

+ When the passdb backend uses LDAP (ldapsam) it is the + admininstrators' 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 . +

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 . +

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 concenience we elect to save this script as a file called initGroups.sh. + This script is given in . +

+ +

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: +

+ Click Start -> Control Panel -> Users and Passwords. +
+ Click the Advanced tab. +
+ Click the Advanced button. +
+ Click Groups. +
+ Double click Power Users. This will launch the panel to add users or groups + to the local machine Power Uses group. +
+ Click the Add button. +
+ Select the domain from which the Domain Users group is to be added. +
+ Double click the Domain Users group. +
+ 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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