Chapter 15. User Rights and Privileges
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## Chapter 15. User Rights and Privileges

### Gerald (Jerry) Carter

Samba Team

`<jerry@samba.org>`

### John H. Terpstra

Samba Team

`<jht@samba.org>`

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 and 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 normal user accounts 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 machine account is used only to authenticate domain member machines during start-up. This security measure is designed to block man-in-the-middle attempts to violate network integrity.

### Note

Machine (computer) accounts are used in the Windows NT OS family to store security credentials for domain member servers and workstations. When the domain member starts up, it goes through a validation process that includes an exchange of credentials with a domain controller. If the domain member fails to authenticate using the credentials known for it by domain controllers, the machine will be refused all access by domain users. The computer account is essential to the way that MS Windows secures authentication.

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 user.

All versions of Samba call system interface scripts that permit CIFS function calls that are used to manage users, groups, and machine accounts 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 requirement 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 possess `root`-level access to the UNIX host system.

## Rights Management Capabilities

Samba 3.0.11 introduced 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 “Current Privilege Capabilities”. The remainder of this chapter explains how to manage and use these privileges on Samba servers.

Table 15.1. Current Privilege Capabilities

Privilege | Description
--- | ---
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
SeTakeOwnershipPrivilege | Take ownership of files or other objects

### 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 three new subcommands:

list [name|accounts]

  When called with no arguments, `net rpc list` simply lists 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` returns 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 domain controller, 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

  The following syntax has the same result:

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

  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 backslash 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`. Its 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. There are no default rights and privileges, except the ability for a member of the Domain Admins group to assign them. This means that all administrative rights and privileges (other than the ability to assign them) must be explicitly assigned, even for the Domain Admins group.

By default, no privileges are initially assigned to any account because 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, `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 as a housekeeping matter, so 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 that possess 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

  This right controls whether or not the user can 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.

SeTakeOwnershipPrivilege

  This right permits users to take ownership of files and directories.

### Privileges Suppored by Windows 2000 Domain Controllers

For reference purposes, a Windows NT4 Primary Domain Controller reports support for the following privileges:

             SeCreateTokenPrivilege  Create a token object
      SeAssignPrimaryTokenPrivilege  Replace a process level token
              SeLockMemoryPrivilege  Lock pages in memory
           SeIncreaseQuotaPrivilege  Increase quotas
          SeMachineAccountPrivilege  Add workstations to domain
                     SeTcbPrivilege  Act as part of the operating system
                SeSecurityPrivilege  Manage auditing and security log
           SeTakeOwnershipPrivilege  Take ownership of files or other objects
              SeLoadDriverPrivilege  Load and unload device drivers
           SeSystemProfilePrivilege  Profile system performance
              SeSystemtimePrivilege  Change the system time
    SeProfileSingleProcessPrivilege  Profile single process
    SeIncreaseBasePriorityPrivilege  Increase scheduling priority
          SeCreatePagefilePrivilege  Create a pagefile
         SeCreatePermanentPrivilege  Create permanent shared objects
                  SeBackupPrivilege  Back up files and directories
                 SeRestorePrivilege  Restore files and directories
                SeShutdownPrivilege  Shut down the system
                   SeDebugPrivilege  Debug programs
                   SeAuditPrivilege  Generate security audits
       SeSystemEnvironmentPrivilege  Modify firmware environment values
            SeChangeNotifyPrivilege  Bypass traverse checking
          SeRemoteShutdownPrivilege  Force shutdown from a remote system

And Windows 200x/XP Domain Controllers and workstations reports to support the following privileges:

             SeCreateTokenPrivilege  Create a token object
      SeAssignPrimaryTokenPrivilege  Replace a process level token
              SeLockMemoryPrivilege  Lock pages in memory
           SeIncreaseQuotaPrivilege  Increase quotas
          SeMachineAccountPrivilege  Add workstations to domain
                     SeTcbPrivilege  Act as part of the operating system
                SeSecurityPrivilege  Manage auditing and security log
           SeTakeOwnershipPrivilege  Take ownership of files or other objects
              SeLoadDriverPrivilege  Load and unload device drivers
           SeSystemProfilePrivilege  Profile system performance
              SeSystemtimePrivilege  Change the system time
    SeProfileSingleProcessPrivilege  Profile single process
    SeIncreaseBasePriorityPrivilege  Increase scheduling priority
          SeCreatePagefilePrivilege  Create a pagefile
         SeCreatePermanentPrivilege  Create permanent shared objects
                  SeBackupPrivilege  Back up files and directories
                 SeRestorePrivilege  Restore files and directories
                SeShutdownPrivilege  Shut down the system
                   SeDebugPrivilege  Debug programs
                   SeAuditPrivilege  Generate security audits
       SeSystemEnvironmentPrivilege  Modify firmware environment values
            SeChangeNotifyPrivilege  Bypass traverse checking
          SeRemoteShutdownPrivilege  Force shutdown from a remote system
                  SeUndockPrivilege  Remove computer from docking station
               SeSyncAgentPrivilege  Synchronize directory service data
        SeEnableDelegationPrivilege  Enable computer and user accounts to
                                     be trusted for delegation
            SeManageVolumePrivilege  Perform volume maintenance tasks
             SeImpersonatePrivilege  Impersonate a client after authentication
            SeCreateGlobalPrivilege  Create global objects

The Samba Team is implementing only those privileges that are logical and useful in the UNIX/Linux environment. Many of the Windows 200X/XP privileges have no direct equivalence in UNIX.

## The Administrator Domain SID

Please note that every Windows NT4 and later server requires a domain Administrator account. Samba versions commencing with 3.0.11 permit Administrative duties to be performed via assigned rights and privileges (see User Rights and Privileges). An account in the server's passdb backend can be set to the well-known RID of the default administrator account. To obtain the domain SID on a Samba domain controller, 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

### Note

The RID 500 is the well known standard value of the default Administrator account. It is the RID that confers the rights and privileges that the Administrator account has on a Windows machine or domain. Under UNIX/Linux the equivalent is UID=0 (the root account).

Releases of Samba version 3.0.11 and later make it possible to operate without an Administrator account provided equivalent rights and privileges have been established for a Windows user or a Windows group account.

## Common Errors

### What Rights and Privileges Will Permit Windows Client Administration?

When a Windows NT4 (or later) client joins a domain, the domain global `Domain Admins` group is added to the membership of the local `Administrators` group on the client. Any user who is a member of the domain global `Domain Admins` group will have administrative rights on the Windows client.

This is often not the most desirable solution because it means that the user will have administrative rights and privileges on domain servers also. The `Power Users` group on Windows client workstations permits local administration of the workstation alone. Any domain global user or domain global group can be added to the membership of the local workstation group `Power Users`.

See Nested Group Support for an example of how to add domain users and groups to a local group that is on a Windows workstation. The use of the `net` command permits this to be done from the Samba server.

Another way this can be done is to log onto the Windows workstation as the user `Administrator`, then open a `cmd` shell, then execute:

    C:\>  net localgroup administrators /add domain_name\entity

where `entity` is either a domain user or a domain group account name.

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