1 <chapter id="AccessControls">
5 <pubdate>May 10, 2003</pubdate>
7 <title>File, Directory and Share Access Controls</title>
10 Advanced MS Windows users are frequently perplexed when file, directory and share manipulation of
11 resources shared via Samba do not behave in the manner they might expect. MS Windows network
12 adminstrators are often confused regarding network access controls and what is the best way to
13 provide users with the type of access they need while protecting resources from the consequences
14 of untoward access capabilities.
18 Unix administrators frequently are not familiar with the MS Windows environment and in particular
19 have difficulty in visualizing what the MS Windows user wishes to achieve in attempts to set file
20 and directory access permissions.
24 The problem lies in the differences in how file and directory permissions and controls work
25 between the two environments. This difference is one that Samba can not completely hide, even
26 though it does try to make the chasm transparent.
30 POSIX Access Control List technology has been available (along with Extended Attributes)
31 for Unix for many years, yet there is little evidence today of any significant use. This
32 explains to some extent the slow adoption of ACLs into commercial Linux products. MS Windows
33 administrators are astounded at this given that ACLs were a foundational capability of the now
34 decade old MS Windows NT operating system.
38 The purpose of this chapter is to present each of the points of control that are possible with
39 Samba-3 in the hope that this will help the network administrator to find the optimum method
40 for delivering the best environment for MS Windows desktop users.
44 This is an opportune point to mention that it should be borne in mind that Samba was created to
45 provide a means of interoperability and interchange of data between two operating environments
46 that are quite different. It was never the intent to make Unix/Linux like MS Windows NT. Instead
47 the purpose was an is to provide a sufficient level of exchange of data between the two environments.
48 What is available today extends well beyond early plans and expections, yet the gap continues to
53 <title>Features and Benefits</title>
56 Samba offers a lot of flexibility in file system access management. These are the key access control
57 facilities present in Samba today:
61 <title>Samba Access Control Facilities</title>
63 Unix file and directory permissions
67 Samba Share Definitions
75 MS Windows ACLs through Unix POSIX ACLs
82 <title>File System Access Controls</title>
85 Explain here how Unix file and permissions work
91 <title>Share Definition Access Controls</title>
94 Explain here about the smb.conf [share] parameters
100 <title>Access Controls on Shares</title>
103 This section deals with how to configure Samba per share access control restrictions.
104 By default samba sets no restrictions on the share itself. Restrictions on the share itself
105 can be set on MS Windows NT4/200x/XP shares. This can be a very effective way to limit who can
106 connect to a share. In the absence of specific restrictions the default setting is to allow
107 the global user <emphasis>Everyone</emphasis> Full Control (ie: Full control, Change and Read).
111 At this time Samba does NOT provide a tool for configuring access control setting on the Share
112 itself. Samba does have the capacity to store and act on access control settings, but the only
113 way to create those settings is to use either the NT4 Server Manager or the Windows 200x MMC for
118 Samba stores the per share access control settings in a file called <filename>share_info.tdb</filename>.
119 The location of this file on your system will depend on how samba was compiled. The default location
120 for samba's tdb files is under <filename>/usr/local/samba/var</filename>. If the <filename>tdbdump</filename>
121 utility has been compiled and installed on your system then you can examine the contents of this file
122 by: <userinput>tdbdump share_info.tdb</userinput>.
126 <title>Share Permissions Management</title>
129 The best tool for the task is platform dependant. Choose the best tool for your environmemt.
133 <title>Windows NT4 Workstation/Server</title>
135 The tool you need to use to manage share permissions on a Samba server is the NT Server Manager.
136 Server Manager is shipped with Windows NT4 Server products but not with Windows NT4 Workstation.
137 You can obtain the NT Server Manager for MS Windows NT4 Workstation from Microsoft - see details below.
141 <title>Instructions</title>
143 Launch the NT4 Server Manager, click on the Samba server you want to administer, then from the menu
144 select Computer, then click on the Shared Directories entry.
148 Now click on the share that you wish to manage, then click on the Properties tab, next click on
149 the Permissions tab. Now you can Add or change access control settings as you wish.
156 <title>Windows 200x/XP</title>
159 On MS Windows NT4/200x/XP system access control lists on the share itself are set using native
160 tools, usually from filemanager. For example, in Windows 200x: right click on the shared folder,
161 then select 'Sharing', then click on 'Permissions'. The default Windows NT4/200x permission allows
162 <emphasis>Everyone</emphasis> Full Control on the Share.
166 MS Windows 200x and later all comes with a tool called the 'Computer Management' snap-in for the
167 Microsoft Management Console (MMC). This tool is located by clicking on <filename>Control Panel ->
168 Administrative Tools -> Computer Management</filename>.
172 <title>Instructions</title>
174 After launching the MMC with the Computer Management snap-in, click on the menu item 'Action',
175 select 'Connect to another computer'. If you are not logged onto a domain you will be prompted
176 to enter a domain login user identifier and a password. This will authenticate you to the domain.
177 If you where already logged in with administrative privilidge this step is not offered.
181 If the Samba server is not shown in the Select Computer box, then type in the name of the target
182 Samba server in the field 'Name:'. Now click on the [+] next to 'System Tools', then on the [+]
183 next to 'Shared Folders' in the left panel.
187 Now in the right panel, double-click on the share you wish to set access control permissions on.
188 Then click on the tab 'Share Permissions'. It is now possible to add access control entities
189 to the shared folder. Do NOT forget to set what type of access (full control, change, read) you
190 wish to assign for each entry.
196 Be careful. If you take away all permissions from the Everyone user without removing this user
197 then effectively no user will be able to access the share. This is a result of what is known as
198 ACL precidence. ie: Everyone with NO ACCESS means that MaryK who is part of the group Everyone
199 will have no access even if this user is given explicit full control access.
209 <title>MS Windows Access Control Lists and Unix Interoperability</title>
212 <title>Viewing and changing UNIX permissions using the NT
213 security dialogs</title>
215 <para>Windows NT clients can use their native security settings
216 dialog box to view and modify the underlying UNIX permissions.</para>
218 <para>Note that this ability is careful not to compromise
219 the security of the UNIX host Samba is running on, and
220 still obeys all the file permission rules that a Samba
221 administrator can set.</para>
225 All access to Unix/Linux system file via Samba is controlled at
226 the operating system file access control level. When trying to
227 figure out file access problems it is vitally important to identify
228 the identity of the Windows user as it is presented by Samba at
229 the point of file access. This can best be determined from the
236 <title>How to view file security on a Samba share</title>
238 <para>From an NT4/2000/XP client, single-click with the right
239 mouse button on any file or directory in a Samba mounted
240 drive letter or UNC path. When the menu pops-up, click
241 on the <emphasis>Properties</emphasis> entry at the bottom of
242 the menu. This brings up the file properties dialog
243 box. Click on the tab <emphasis>Security</emphasis> and you
244 will see three buttons, <emphasis>Permissions</emphasis>,
245 <emphasis>Auditing</emphasis>, and <emphasis>Ownership</emphasis>.
246 The <emphasis>Auditing</emphasis> button will cause either
247 an error message <errorname>A requested privilege is not held
248 by the client</errorname> to appear if the user is not the
249 NT Administrator, or a dialog which is intended to allow an
250 Administrator to add auditing requirements to a file if the
251 user is logged on as the NT Administrator. This dialog is
252 non-functional with a Samba share at this time, as the only
253 useful button, the <command>Add</command> button will not currently
254 allow a list of users to be seen.</para>
259 <title>Viewing file ownership</title>
261 <para>Clicking on the <command>"Ownership"</command> button
262 brings up a dialog box telling you who owns the given file. The
263 owner name will be of the form :</para>
265 <para><command>"SERVER\user (Long name)"</command></para>
267 <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
268 the Samba server, <replaceable>user</replaceable> is the user name of
269 the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
270 is the descriptive string identifying the user (normally found in the
271 GECOS field of the UNIX password database). Click on the <command>Close
272 </command> button to remove this dialog.</para>
274 <para>If the parameter <parameter>nt acl support</parameter>
275 is set to <constant>false</constant> then the file owner will
276 be shown as the NT user <command>"Everyone"</command>.</para>
278 <para>The <command>Take Ownership</command> button will not allow
279 you to change the ownership of this file to yourself (clicking on
280 it will display a dialog box complaining that the user you are
281 currently logged onto the NT client cannot be found). The reason
282 for this is that changing the ownership of a file is a privileged
283 operation in UNIX, available only to the <emphasis>root</emphasis>
284 user. As clicking on this button causes NT to attempt to change
285 the ownership of a file to the current user logged into the NT
286 client this will not work with Samba at this time.</para>
288 <para>There is an NT chown command that will work with Samba
289 and allow a user with Administrator privilege connected
290 to a Samba server as root to change the ownership of
291 files on both a local NTFS filesystem or remote mounted NTFS
292 or Samba drive. This is available as part of the <emphasis>Seclib
293 </emphasis> NT security library written by Jeremy Allison of
294 the Samba Team, available from the main Samba ftp site.</para>
299 <title>Viewing file or directory permissions</title>
301 <para>The third button is the <command>"Permissions"</command>
302 button. Clicking on this brings up a dialog box that shows both
303 the permissions and the UNIX owner of the file or directory.
304 The owner is displayed in the form :</para>
306 <para><command>"SERVER\user (Long name)"</command></para>
308 <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
309 the Samba server, <replaceable>user</replaceable> is the user name of
310 the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
311 is the descriptive string identifying the user (normally found in the
312 GECOS field of the UNIX password database).</para>
314 <para>If the parameter <parameter>nt acl support</parameter>
315 is set to <constant>false</constant> then the file owner will
316 be shown as the NT user <command>"Everyone"</command> and the
317 permissions will be shown as NT "Full Control".</para>
320 <para>The permissions field is displayed differently for files
321 and directories, so I'll describe the way file permissions
322 are displayed first.</para>
325 <title>File Permissions</title>
327 <para>The standard UNIX user/group/world triple and
328 the corresponding "read", "write", "execute" permissions
329 triples are mapped by Samba into a three element NT ACL
330 with the 'r', 'w', and 'x' bits mapped into the corresponding
331 NT permissions. The UNIX world permissions are mapped into
332 the global NT group <command>Everyone</command>, followed
333 by the list of permissions allowed for UNIX world. The UNIX
334 owner and group permissions are displayed as an NT
335 <command>user</command> icon and an NT <command>local
336 group</command> icon respectively followed by the list
337 of permissions allowed for the UNIX user and group.</para>
339 <para>As many UNIX permission sets don't map into common
340 NT names such as <command>"read"</command>, <command>
341 "change"</command> or <command>"full control"</command> then
342 usually the permissions will be prefixed by the words <command>
343 "Special Access"</command> in the NT display list.</para>
345 <para>But what happens if the file has no permissions allowed
346 for a particular UNIX user group or world component ? In order
347 to allow "no permissions" to be seen and modified then Samba
348 overloads the NT <command>"Take Ownership"</command> ACL attribute
349 (which has no meaning in UNIX) and reports a component with
350 no permissions as having the NT <command>"O"</command> bit set.
351 This was chosen of course to make it look like a zero, meaning
352 zero permissions. More details on the decision behind this will
353 be given below.</para>
357 <title>Directory Permissions</title>
359 <para>Directories on an NT NTFS file system have two
360 different sets of permissions. The first set of permissions
361 is the ACL set on the directory itself, this is usually displayed
362 in the first set of parentheses in the normal <command>"RW"</command>
363 NT style. This first set of permissions is created by Samba in
364 exactly the same way as normal file permissions are, described
365 above, and is displayed in the same way.</para>
367 <para>The second set of directory permissions has no real meaning
368 in the UNIX permissions world and represents the <command>
369 "inherited"</command> permissions that any file created within
370 this directory would inherit.</para>
372 <para>Samba synthesises these inherited permissions for NT by
373 returning as an NT ACL the UNIX permission mode that a new file
374 created by Samba on this share would receive.</para>
379 <title>Modifying file or directory permissions</title>
381 <para>Modifying file and directory permissions is as simple
382 as changing the displayed permissions in the dialog box, and
383 clicking the <command>OK</command> button. However, there are
384 limitations that a user needs to be aware of, and also interactions
385 with the standard Samba permission masks and mapping of DOS
386 attributes that need to also be taken into account.</para>
388 <para>If the parameter <parameter>nt acl support</parameter>
389 is set to <constant>false</constant> then any attempt to set
390 security permissions will fail with an <command>"Access Denied"
391 </command> message.</para>
393 <para>The first thing to note is that the <command>"Add"</command>
394 button will not return a list of users in Samba (it will give
395 an error message of <command>"The remote procedure call failed
396 and did not execute"</command>). This means that you can only
397 manipulate the current user/group/world permissions listed in
398 the dialog box. This actually works quite well as these are the
399 only permissions that UNIX actually has.</para>
401 <para>If a permission triple (either user, group, or world)
402 is removed from the list of permissions in the NT dialog box,
403 then when the <command>"OK"</command> button is pressed it will
404 be applied as "no permissions" on the UNIX side. If you then
405 view the permissions again the "no permissions" entry will appear
406 as the NT <command>"O"</command> flag, as described above. This
407 allows you to add permissions back to a file or directory once
408 you have removed them from a triple component.</para>
410 <para>As UNIX supports only the "r", "w" and "x" bits of
411 an NT ACL then if other NT security attributes such as "Delete
412 access" are selected then they will be ignored when applied on
413 the Samba server.</para>
415 <para>When setting permissions on a directory the second
416 set of permissions (in the second set of parentheses) is
417 by default applied to all files within that directory. If this
418 is not what you want you must uncheck the <command>"Replace
419 permissions on existing files"</command> checkbox in the NT
420 dialog before clicking <command>"OK"</command>.</para>
422 <para>If you wish to remove all permissions from a
423 user/group/world component then you may either highlight the
424 component and click the <command>"Remove"</command> button,
425 or set the component to only have the special <command>"Take
426 Ownership"</command> permission (displayed as <command>"O"
427 </command>) highlighted.</para>
431 <title>Interaction with the standard Samba create mask
434 <para>There are four parameters
435 to control interaction with the standard Samba create mask parameters.
438 <para><parameter>security mask</parameter></para>
439 <para><parameter>force security mode</parameter></para>
440 <para><parameter>directory security mask</parameter></para>
441 <para><parameter>force directory security mode</parameter></para>
443 <para>Once a user clicks <command>"OK"</command> to apply the
444 permissions Samba maps the given permissions into a user/group/world
445 r/w/x triple set, and then will check the changed permissions for a
446 file against the bits set in the <ulink url="smb.conf.5.html#SECURITYMASK">
447 <parameter>security mask</parameter></ulink> parameter. Any bits that
448 were changed that are not set to '1' in this parameter are left alone
449 in the file permissions.</para>
451 <para>Essentially, zero bits in the <parameter>security mask</parameter>
452 mask may be treated as a set of bits the user is <emphasis>not</emphasis>
453 allowed to change, and one bits are those the user is allowed to change.
456 <para>If not set explicitly this parameter is set to the same value as
457 the <ulink url="smb.conf.5.html#CREATEMASK"><parameter>create mask
458 </parameter></ulink> parameter. To allow a user to modify all the
459 user/group/world permissions on a file, set this parameter
462 <para>Next Samba checks the changed permissions for a file against
463 the bits set in the <ulink url="smb.conf.5.html#FORCESECURITYMODE">
464 <parameter>force security mode</parameter></ulink> parameter. Any bits
465 that were changed that correspond to bits set to '1' in this parameter
466 are forced to be set.</para>
468 <para>Essentially, bits set in the <parameter>force security mode
469 </parameter> parameter may be treated as a set of bits that, when
470 modifying security on a file, the user has always set to be 'on'.</para>
472 <para>If not set explicitly this parameter is set to the same value
473 as the <ulink url="smb.conf.5.html#FORCECREATEMODE"><parameter>force
474 create mode</parameter></ulink> parameter.
475 To allow a user to modify all the user/group/world permissions on a file
476 with no restrictions set this parameter to 000.</para>
478 <para>The <parameter>security mask</parameter> and <parameter>force
479 security mode</parameter> parameters are applied to the change
480 request in that order.</para>
482 <para>For a directory Samba will perform the same operations as
483 described above for a file except using the parameter <parameter>
484 directory security mask</parameter> instead of <parameter>security
485 mask</parameter>, and <parameter>force directory security mode
486 </parameter> parameter instead of <parameter>force security mode
489 <para>The <parameter>directory security mask</parameter> parameter
490 by default is set to the same value as the <parameter>directory mask
491 </parameter> parameter and the <parameter>force directory security
492 mode</parameter> parameter by default is set to the same value as
493 the <parameter>force directory mode</parameter> parameter. </para>
495 <para>In this way Samba enforces the permission restrictions that
496 an administrator can set on a Samba share, whilst still allowing users
497 to modify the permission bits within that restriction.</para>
499 <para>If you want to set up a share that allows users full control
500 in modifying the permission bits on their files and directories and
501 doesn't force any particular bits to be set 'on', then set the following
502 parameters in the &smb.conf; file in that share specific section :</para>
504 <para><parameter>security mask = 0777</parameter></para>
505 <para><parameter>force security mode = 0</parameter></para>
506 <para><parameter>directory security mask = 0777</parameter></para>
507 <para><parameter>force directory security mode = 0</parameter></para>
511 <title>Interaction with the standard Samba file attribute
514 <para>Samba maps some of the DOS attribute bits (such as "read
515 only") into the UNIX permissions of a file. This means there can
516 be a conflict between the permission bits set via the security
517 dialog and the permission bits set by the file attribute mapping.
520 <para>One way this can show up is if a file has no UNIX read access
521 for the owner it will show up as "read only" in the standard
522 file attributes tabbed dialog. Unfortunately this dialog is
523 the same one that contains the security info in another tab.</para>
525 <para>What this can mean is that if the owner changes the permissions
526 to allow themselves read access using the security dialog, clicks
527 <command>"OK"</command> to get back to the standard attributes tab
528 dialog, and then clicks <command>"OK"</command> on that dialog, then
529 NT will set the file permissions back to read-only (as that is what
530 the attributes still say in the dialog). This means that after setting
531 permissions and clicking <command>"OK"</command> to get back to the
532 attributes dialog you should always hit <command>"Cancel"</command>
533 rather than <command>"OK"</command> to ensure that your changes
534 are not overridden.</para>
539 <title>Common Errors</title>
git.samba.org / sfrench / samba-autobuild / .git / blob