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Chapter 14. User Rights and Privileges

Gerald (Jerry) Carter

Samba Team

John H. Terpstra

Samba Team

Table of Contents

Rights Management Capabilities
Using the “net rpc rights” Utility
Description of Privileges
The Administrator Domain SID

The administration of Windows user, group and machine accounts in the Samba domain controlled network necessitates interfacing between the MS Windows networking environment and the UNIX operating system environment. The right (permission) to add machines to the Windows security domain can be assigned (set) to non-administrative users both in Windows NT4 domains as well as in Active Directory domains.

The addition of Windows NT4/2kX/XPPro machines to the domain necessitates the creation of a machine account for each machine added. The machine account is a necessity that is used to validate that the machine can be trusted to permit user logons.

Machine accounts are analogous to user accounts, and thus in implementing them on a UNIX machine that is hosting Samba (i.e.: On which Samba is running) it is necessary to create a special type of user account. Machine accounts differ from a normal user account in that the account name (login ID) is terminated with a $ sign. An additional difference is that this type of account should not ever be able to log into the UNIX environment as a system user and therefore is set to have a shell of /bin/false and a home directory of /dev/null.

The creation of UNIX system accounts has traditionally been the sole right of the system administrator, better known as the root account. It is possible in the UNIX environment to create multiple users who have the same UID. Any UNIX user who has a UID=0 is inherently the same as the root account.

All versions of Samba call system interface scripts that permit CIFS function calls that are used to manage users, groups and machine accounts to be affected in the UNIX environment. All versions of Samba up to and including version 3.0.10 required the use of a Windows Administrator account that unambiguously maps to the UNIX root account to permit the execution of these interface scripts. The reuqirement to do this has understandably met with some disdain and consternation among Samba administrators, particularly where it became necessary to permit people who should not posses root level access to the UNIX host system.

Rights Management Capabilities

Samba 3.0.11 introduces support for the Windows privilege model. This model allows certain rights to be assigned to a user or group SID. In order to enable this feature, enable privileges = yes must be defined in the global section of the smb.conf file.

Currently, the rights supported in Samba 3 are listed in ???. The remainder of this chapter explains how to manage and use these privileges on Samba servers.

Table 14.1. Current Privilege Capabilities

PrivilegeDescription

SeMachineAccountPrivilege

Add machines to domain

SePrintOperatorPrivilege

Manage printers

SeAddUsersPrivilege

Add users and groups to the domain

SeRemoteShutdownPrivilege

Force shutdown from a remote system

SeDiskOperatorPrivilege

Manage disk share

Using the “net rpc rights” Utility

There are two primary means of managing the rights assigned to users and groups on a Samba server. The NT4 User Manager for Domains may be used from any Windows NT4, 2000 or XP Professional domain member client to connect to a Samba domain controller and view/modify the rights assignments. This application, however, appears to have bugs when run on a client running Windows 2000 or later, therefore Samba provides a command line utility for performing the necessary administrative actions.

The net rpc rights utility in Samba 3.0.11 has 3 new subcommands:

list [name|accounts]

When called with no arguments, net rpc list will simply list the available rights on the server. When passed a specific user or group name, the tool lists the privileges currently assigned to the specified account. When invoked using the special string accounts, net rpc rights list will return a list of all privileged accounts on the server and the assigned rights.

grant <user> <right [right ...]>

When called with no arguments, This function is used to assign a list of rights to a specified user or group. For example, to grant the members of the Domain Admins group on a Samba DC the capability to add client machines to the domain, one would run:

root#  net -S server -U domadmin rpc rights grant \
	 'DOMAIN\Domain Admins' SeMachineAccountPrivilege

More than one privilege can be assigned by specifying a list of rights separated by spaces. The parameter 'Domain\Domain Admins' must be quoted with single ticks or using double-quotes to prevent the back-slash and the space from being interpreted by the system shell.

revoke <user> <right [right ...]>

This command is similar in format to net rpc rights grant. It's effect is to remove an assigned right (or list of rights) from a user or group.

Note

You must be connected as a member of the Domain Admins group to be able to grant or revoke privileges assigned to an account. This capability is inherent to the Domain Admins group and is not configurable.

By default, no privileges are initially assigned to any account. The reason for this is that certain actions will be performed as root once smbd determines that a user has the necessary rights. For example, when joining a client to a Windows domain, the 'add machine script' must be executed with superuser rights in most cases. For this reason, you should be very careful about handing out privileges to accounts.

Access as the root user (UID=0) bypasses all privilege checks.

Description of Privileges

The privileges that have been implemented in Samba-3.0.11 are shown below. It is possible, and likely, that additional privileges may be implemented in later releases of Samba. It is also likely that any privileges currently implemented but not used may be removed from future releases, thus it is important that the successful as well as unsuccessful use of these facilities should be reported on the Samba mailing lists.

SeAddUsersPrivilege

This right determines whether or not smbd will allow the user to create new user or group accounts via such tools as net rpc user add or NT4 User Manager for Domains.

SeDiskOperatorPrivilege

Accounts which posses this right will be able to execute scripts defined by the add/delete/change share command in smb.conf file as root. Such users will also be able to modify the ACL associated with file shares on the Samba server.

SeMachineAccountPrivilege

Controls whether or not the user is able join client machines to a Samba controlled domain.

SePrintOperatorPrivilege

This privilege operates identically to the printer admin option in the smb.conf file (see section 5 man page for smb.conf) except that it is a global right (not on a per printer basis). Eventually the smb.conf option will be deprecated and administrative rights to printers will be controlled exclusively by this right and the security descriptor associated with the printer object in the ntprinters.tdb file.

SeRemoteShutdownPrivilege

Samba provides two hooks for shutting down or rebooting the server and for aborting a previously issued shutdown command. Since this is an operation normally limited by the operating system to the root user, an account must possess this right to be able to execute either of these hooks to have any effect.

The Administrator Domain SID

Please note that when configured as a DC, it is now required that an account in the server's passdb backend be set to the domain SID of the default Administrator account. To obtain the domain SID on a Samba DC, run the following command:

root#  net getlocalsid
SID for domain FOO is: S-1-5-21-4294955119-3368514841-2087710299

You may assign the Domain Administrator rid to an account using the pdbedit command as shown here:

root#  pdbedit -U S-1-5-21-4294955119-3368514841-2087710299-500 -u root -r