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   4 >UNIX Permission Bits and Windows NT Access Control Lists</TITLE
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  15 ><DIV
  16 CLASS="ARTICLE"
  17 ><DIV
  18 CLASS="TITLEPAGE"
  19 ><H1
  20 CLASS="TITLE"
  21 ><A
  22 NAME="UNIX-PERMISSIONS"
  23 >UNIX Permission Bits and Windows NT Access Control Lists</A
  24 ></H1
  25 ><HR></DIV
  26 ><DIV
  27 CLASS="SECT1"
  28 ><H1
  29 CLASS="SECT1"
  30 ><A
  31 NAME="AEN3"
  32 >Viewing and changing UNIX permissions using the NT
  33         security dialogs</A
  34 ></H1
  35 ><P
  36 >New in the Samba 2.0.4 release is the ability for Windows
  37         NT clients to use their native security settings dialog box to
  38         view and modify the underlying UNIX permissions.</P
  39 ><P
  40 >Note that this ability is careful not to compromise
  41         the security of the UNIX host Samba is running on, and
  42         still obeys all the file permission rules that a Samba
  43         administrator can set.</P
  44 ><P
  45 >In Samba 2.0.4 and above the default value of the
  46         parameter <A
  47 HREF="smb.conf.5.html#NTACLSUPPORT"
  48 TARGET="_top"
  49 ><TT
  50 CLASS="PARAMETER"
  51 ><I
  52 >       nt acl support</I
  53 ></TT
  54 ></A
  55 > has been changed from
  56         <TT
  57 CLASS="CONSTANT"
  58 >false</TT
  59 > to <TT
  60 CLASS="CONSTANT"
  61 >true</TT
  62 >, so
  63         manipulation of permissions is turned on by default.</P
  64 ></DIV
  65 ><DIV
  66 CLASS="SECT1"
  67 ><HR><H1
  68 CLASS="SECT1"
  69 ><A
  70 NAME="AEN12"
  71 >How to view file security on a Samba share</A
  72 ></H1
  73 ><P
  74 >From an NT 4.0 client, single-click with the right
  75         mouse button on any file or directory in a Samba mounted
  76         drive letter or UNC path. When the menu pops-up, click
  77         on the <I
  78 CLASS="EMPHASIS"
  79 >Properties</I
  80 > entry at the bottom of
  81         the menu. This brings up the normal file properties dialog
  82         box, but with Samba 2.0.4 this will have a new tab along the top
  83         marked <I
  84 CLASS="EMPHASIS"
  85 >Security</I
  86 >. Click on this tab and you
  87         will see three buttons, <I
  88 CLASS="EMPHASIS"
  89 >Permissions</I
  90 >,
  91         <I
  92 CLASS="EMPHASIS"
  93 >Auditing</I
  94 >, and <I
  95 CLASS="EMPHASIS"
  96 >Ownership</I
  97 >.
  98         The <I
  99 CLASS="EMPHASIS"
 100 >Auditing</I
 101 > button will cause either
 102         an error message <SPAN
 103 CLASS="ERRORNAME"
 104 >A requested privilege is not held
 105         by the client</SPAN
 106 > to appear if the user is not the
 107         NT Administrator, or a dialog which is intended to allow an
 108         Administrator to add auditing requirements to a file if the
 109         user is logged on as the NT Administrator. This dialog is
 110         non-functional with a Samba share at this time, as the only
 111         useful button, the <B
 112 CLASS="COMMAND"
 113 >Add</B
 114 > button will not currently
 115         allow a list of users to be seen.</P
 116 ></DIV
 117 ><DIV
 118 CLASS="SECT1"
 119 ><HR><H1
 120 CLASS="SECT1"
 121 ><A
 122 NAME="AEN23"
 123 >Viewing file ownership</A
 124 ></H1
 125 ><P
 126 >Clicking on the <B
 127 CLASS="COMMAND"
 128 >"Ownership"</B
 129 > button
 130         brings up a dialog box telling you who owns the given file. The
 131         owner name will be of the form :</P
 132 ><P
 133 ><B
 134 CLASS="COMMAND"
 135 >"SERVER\user (Long name)"</B
 136 ></P
 137 ><P
 138 >Where <TT
 139 CLASS="REPLACEABLE"
 140 ><I
 141 >SERVER</I
 142 ></TT
 143 > is the NetBIOS name of
 144         the Samba server, <TT
 145 CLASS="REPLACEABLE"
 146 ><I
 147 >user</I
 148 ></TT
 149 > is the user name of
 150         the UNIX user who owns the file, and <TT
 151 CLASS="REPLACEABLE"
 152 ><I
 153 >(Long name)</I
 154 ></TT
 155 >
 156         is the descriptive string identifying the user (normally found in the
 157         GECOS field of the UNIX password database). Click on the <B
 158 CLASS="COMMAND"
 159 >Close
 160         </B
 161 > button to remove this dialog.</P
 162 ><P
 163 >If the parameter <TT
 164 CLASS="PARAMETER"
 165 ><I
 166 >nt acl support</I
 167 ></TT
 168 >
 169         is set to <TT
 170 CLASS="CONSTANT"
 171 >false</TT
 172 > then the file owner will
 173         be shown as the NT user <B
 174 CLASS="COMMAND"
 175 >"Everyone"</B
 176 >.</P
 177 ><P
 178 >The <B
 179 CLASS="COMMAND"
 180 >Take Ownership</B
 181 > button will not allow
 182         you to change the ownership of this file to yourself (clicking on
 183         it will display a dialog box complaining that the user you are
 184         currently logged onto the NT client cannot be found). The reason
 185         for this is that changing the ownership of a file is a privileged
 186         operation in UNIX, available only to the <I
 187 CLASS="EMPHASIS"
 188 >root</I
 189 >
 190         user. As clicking on this button causes NT to attempt to change
 191         the ownership of a file to the current user logged into the NT
 192         client this will not work with Samba at this time.</P
 193 ><P
 194 >There is an NT chown command that will work with Samba
 195         and allow a user with Administrator privilege connected
 196         to a Samba 2.0.4 server as root to change the ownership of
 197         files on both a local NTFS filesystem or remote mounted NTFS
 198         or Samba drive. This is available as part of the <I
 199 CLASS="EMPHASIS"
 200 >Seclib
 201         </I
 202 > NT security library written by Jeremy Allison of
 203         the Samba Team, available from the main Samba ftp site.</P
 204 ></DIV
 205 ><DIV
 206 CLASS="SECT1"
 207 ><HR><H1
 208 CLASS="SECT1"
 209 ><A
 210 NAME="AEN43"
 211 >Viewing file or directory permissions</A
 212 ></H1
 213 ><P
 214 >The third button is the <B
 215 CLASS="COMMAND"
 216 >"Permissions"</B
 217 >
 218         button. Clicking on this brings up a dialog box that shows both
 219         the permissions and the UNIX owner of the file or directory.
 220         The owner is displayed in the form :</P
 221 ><P
 222 ><B
 223 CLASS="COMMAND"
 224 >"SERVER\user (Long name)"</B
 225 ></P
 226 ><P
 227 >Where <TT
 228 CLASS="REPLACEABLE"
 229 ><I
 230 >SERVER</I
 231 ></TT
 232 > is the NetBIOS name of
 233         the Samba server, <TT
 234 CLASS="REPLACEABLE"
 235 ><I
 236 >user</I
 237 ></TT
 238 > is the user name of
 239         the UNIX user who owns the file, and <TT
 240 CLASS="REPLACEABLE"
 241 ><I
 242 >(Long name)</I
 243 ></TT
 244 >
 245         is the descriptive string identifying the user (normally found in the
 246         GECOS field of the UNIX password database).</P
 247 ><P
 248 >If the parameter <TT
 249 CLASS="PARAMETER"
 250 ><I
 251 >nt acl support</I
 252 ></TT
 253 >
 254         is set to <TT
 255 CLASS="CONSTANT"
 256 >false</TT
 257 > then the file owner will
 258         be shown as the NT user <B
 259 CLASS="COMMAND"
 260 >"Everyone"</B
 261 > and the
 262         permissions will be shown as NT "Full Control".</P
 263 ><P
 264 >The permissions field is displayed differently for files
 265         and directories, so I'll describe the way file permissions
 266         are displayed first.</P
 267 ><DIV
 268 CLASS="SECT2"
 269 ><HR><H2
 270 CLASS="SECT2"
 271 ><A
 272 NAME="AEN58"
 273 >File Permissions</A
 274 ></H2
 275 ><P
 276 >The standard UNIX user/group/world triple and
 277                 the corresponding "read", "write", "execute" permissions
 278                 triples are mapped by Samba into a three element NT ACL
 279                 with the 'r', 'w', and 'x' bits mapped into the corresponding
 280                 NT permissions. The UNIX world permissions are mapped into
 281                 the global NT group <B
 282 CLASS="COMMAND"
 283 >Everyone</B
 284 >, followed
 285                 by the list of permissions allowed for UNIX world. The UNIX
 286                 owner and group permissions are displayed as an NT
 287                 <B
 288 CLASS="COMMAND"
 289 >user</B
 290 > icon and an NT <B
 291 CLASS="COMMAND"
 292 >local
 293                 group</B
 294 > icon respectively followed by the list
 295                 of permissions allowed for the UNIX user and group.</P
 296 ><P
 297 >As many UNIX permission sets don't map into common
 298                 NT names such as <B
 299 CLASS="COMMAND"
 300 >"read"</B
 301 >, <B
 302 CLASS="COMMAND"
 303 >               "change"</B
 304 > or <B
 305 CLASS="COMMAND"
 306 >"full control"</B
 307 > then
 308                 usually the permissions will be prefixed by the words <B
 309 CLASS="COMMAND"
 310 >               "Special Access"</B
 311 > in the NT display list.</P
 312 ><P
 313 >But what happens if the file has no permissions allowed
 314                 for a particular UNIX user group or world component ? In order
 315                 to  allow "no permissions" to be seen and modified then Samba
 316                 overloads the NT <B
 317 CLASS="COMMAND"
 318 >"Take Ownership"</B
 319 > ACL attribute
 320                 (which has no meaning in UNIX) and reports a component with
 321                 no permissions as having the NT <B
 322 CLASS="COMMAND"
 323 >"O"</B
 324 > bit set.
 325                 This was chosen of course to make it look like a zero, meaning
 326                 zero permissions. More details on the decision behind this will
 327                 be given below.</P
 328 ></DIV
 329 ><DIV
 330 CLASS="SECT2"
 331 ><HR><H2
 332 CLASS="SECT2"
 333 ><A
 334 NAME="AEN72"
 335 >Directory Permissions</A
 336 ></H2
 337 ><P
 338 >Directories on an NT NTFS file system have two
 339                 different sets of permissions. The first set of permissions
 340                 is the ACL set on the directory itself, this is usually displayed
 341                 in the first set of parentheses in the normal <B
 342 CLASS="COMMAND"
 343 >"RW"</B
 344 >
 345                 NT style. This first set of permissions is created by Samba in
 346                 exactly the same way as normal file permissions are, described
 347                 above, and is displayed in the same way.</P
 348 ><P
 349 >The second set of directory permissions has no real meaning
 350                 in the UNIX permissions world and represents the <B
 351 CLASS="COMMAND"
 352 >               "inherited"</B
 353 > permissions that any file created within
 354                 this directory would inherit.</P
 355 ><P
 356 >Samba synthesises these inherited permissions for NT by
 357                 returning as an NT ACL the UNIX permission mode that a new file
 358                 created by Samba on this share would receive.</P
 359 ></DIV
 360 ></DIV
 361 ><DIV
 362 CLASS="SECT1"
 363 ><HR><H1
 364 CLASS="SECT1"
 365 ><A
 366 NAME="AEN79"
 367 >Modifying file or directory permissions</A
 368 ></H1
 369 ><P
 370 >Modifying file and directory permissions is as simple
 371         as changing the displayed permissions in the dialog box, and
 372         clicking the <B
 373 CLASS="COMMAND"
 374 >OK</B
 375 > button. However, there are
 376         limitations that a user needs to be aware of, and also interactions
 377         with the standard Samba permission masks and mapping of DOS
 378         attributes that need to also be taken into account.</P
 379 ><P
 380 >If the parameter <TT
 381 CLASS="PARAMETER"
 382 ><I
 383 >nt acl support</I
 384 ></TT
 385 >
 386         is set to <TT
 387 CLASS="CONSTANT"
 388 >false</TT
 389 > then any attempt to set
 390         security permissions will fail with an <B
 391 CLASS="COMMAND"
 392 >"Access Denied"
 393         </B
 394 > message.</P
 395 ><P
 396 >The first thing to note is that the <B
 397 CLASS="COMMAND"
 398 >"Add"</B
 399 >
 400         button will not return a list of users in Samba 2.0.4 (it will give
 401         an error message of <B
 402 CLASS="COMMAND"
 403 >"The remote procedure call failed
 404         and did not execute"</B
 405 >). This means that you can only
 406         manipulate the current user/group/world permissions listed in
 407         the dialog box. This actually works quite well as these are the
 408         only permissions that UNIX actually has.</P
 409 ><P
 410 >If a permission triple (either user, group, or world)
 411         is removed from the list of permissions in the NT dialog box,
 412         then when the <B
 413 CLASS="COMMAND"
 414 >"OK"</B
 415 > button is pressed it will
 416         be applied as "no permissions" on the UNIX side. If you then
 417         view the permissions again the "no permissions" entry will appear
 418         as the NT <B
 419 CLASS="COMMAND"
 420 >"O"</B
 421 > flag, as described above. This
 422         allows you to add permissions back to a file or directory once
 423         you have removed them from a triple component.</P
 424 ><P
 425 >As UNIX supports only the "r", "w" and "x" bits of
 426         an NT ACL then if other NT security attributes such as "Delete
 427         access" are selected then they will be ignored when applied on
 428         the Samba server.</P
 429 ><P
 430 >When setting permissions on a directory the second
 431         set of permissions (in the second set of parentheses) is
 432         by default applied to all files within that directory. If this
 433         is not what you want you must uncheck the <B
 434 CLASS="COMMAND"
 435 >"Replace
 436         permissions on existing files"</B
 437 > checkbox in the NT
 438         dialog before clicking <B
 439 CLASS="COMMAND"
 440 >"OK"</B
 441 >.</P
 442 ><P
 443 >If you wish to remove all permissions from a
 444         user/group/world  component then you may either highlight the
 445         component and click the <B
 446 CLASS="COMMAND"
 447 >"Remove"</B
 448 > button,
 449         or set the component to only have the special <B
 450 CLASS="COMMAND"
 451 >"Take
 452         Ownership"</B
 453 > permission (displayed as <B
 454 CLASS="COMMAND"
 455 >"O"
 456         </B
 457 >) highlighted.</P
 458 ></DIV
 459 ><DIV
 460 CLASS="SECT1"
 461 ><HR><H1
 462 CLASS="SECT1"
 463 ><A
 464 NAME="AEN101"
 465 >Interaction with the standard Samba create mask
 466         parameters</A
 467 ></H1
 468 ><P
 469 >Note that with Samba 2.0.5 there are four new parameters
 470         to control this interaction.  These are :</P
 471 ><P
 472 ><TT
 473 CLASS="PARAMETER"
 474 ><I
 475 >security mask</I
 476 ></TT
 477 ></P
 478 ><P
 479 ><TT
 480 CLASS="PARAMETER"
 481 ><I
 482 >force security mode</I
 483 ></TT
 484 ></P
 485 ><P
 486 ><TT
 487 CLASS="PARAMETER"
 488 ><I
 489 >directory security mask</I
 490 ></TT
 491 ></P
 492 ><P
 493 ><TT
 494 CLASS="PARAMETER"
 495 ><I
 496 >force directory security mode</I
 497 ></TT
 498 ></P
 499 ><P
 500 >Once a user clicks <B
 501 CLASS="COMMAND"
 502 >"OK"</B
 503 > to apply the
 504         permissions Samba maps the given permissions into a user/group/world
 505         r/w/x triple set, and then will check the changed permissions for a
 506         file against the bits set in the <A
 507 HREF="smb.conf.5.html#SECURITYMASK"
 508 TARGET="_top"
 509 >
 510         <TT
 511 CLASS="PARAMETER"
 512 ><I
 513 >security mask</I
 514 ></TT
 515 ></A
 516 > parameter. Any bits that
 517         were changed that are not set to '1' in this parameter are left alone
 518         in the file permissions.</P
 519 ><P
 520 >Essentially, zero bits in the <TT
 521 CLASS="PARAMETER"
 522 ><I
 523 >security mask</I
 524 ></TT
 525 >
 526         mask may be treated as a set of bits the user is <I
 527 CLASS="EMPHASIS"
 528 >not</I
 529 >
 530         allowed to change, and one bits are those the user is allowed to change.
 531         </P
 532 ><P
 533 >If not set explicitly this parameter is set to the same value as
 534         the <A
 535 HREF="smb.conf.5.html#CREATEMASK"
 536 TARGET="_top"
 537 ><TT
 538 CLASS="PARAMETER"
 539 ><I
 540 >create mask
 541         </I
 542 ></TT
 543 ></A
 544 > parameter to provide compatibility with Samba 2.0.4
 545         where this permission change facility was introduced. To allow a user to
 546         modify all the user/group/world permissions on a file, set this parameter
 547         to 0777.</P
 548 ><P
 549 >Next Samba checks the changed permissions for a file against
 550         the bits set in the <A
 551 HREF="smb.conf.5.html#FORCESECURITYMODE"
 552 TARGET="_top"
 553 >       <TT
 554 CLASS="PARAMETER"
 555 ><I
 556 >force security mode</I
 557 ></TT
 558 ></A
 559 > parameter. Any bits
 560         that were changed that correspond to bits set to '1' in this parameter
 561         are forced to be set.</P
 562 ><P
 563 >Essentially, bits set in the <TT
 564 CLASS="PARAMETER"
 565 ><I
 566 >force security mode
 567         </I
 568 ></TT
 569 > parameter may be treated as a set of bits that, when
 570         modifying security on a file, the user has always set to be 'on'.</P
 571 ><P
 572 >If not set explicitly this parameter is set to the same value
 573         as the <A
 574 HREF="smb.conf.5.html#FORCECREATEMODE"
 575 TARGET="_top"
 576 ><TT
 577 CLASS="PARAMETER"
 578 ><I
 579 >force
 580         create mode</I
 581 ></TT
 582 ></A
 583 > parameter to provide compatibility
 584         with Samba 2.0.4 where the permission change facility was introduced.
 585         To allow a user to modify all the user/group/world permissions on a file
 586         with no restrictions set this parameter to 000.</P
 587 ><P
 588 >The <TT
 589 CLASS="PARAMETER"
 590 ><I
 591 >security mask</I
 592 ></TT
 593 > and <TT
 594 CLASS="PARAMETER"
 595 ><I
 596 >force
 597         security mode</I
 598 ></TT
 599 > parameters are applied to the change
 600         request in that order.</P
 601 ><P
 602 >For a directory Samba will perform the same operations as
 603         described above for a file except using the parameter <TT
 604 CLASS="PARAMETER"
 605 ><I
 606 >       directory security mask</I
 607 ></TT
 608 > instead of <TT
 609 CLASS="PARAMETER"
 610 ><I
 611 >security
 612         mask</I
 613 ></TT
 614 >, and <TT
 615 CLASS="PARAMETER"
 616 ><I
 617 >force directory security mode
 618         </I
 619 ></TT
 620 > parameter instead of <TT
 621 CLASS="PARAMETER"
 622 ><I
 623 >force security mode
 624         </I
 625 ></TT
 626 >.</P
 627 ><P
 628 >The <TT
 629 CLASS="PARAMETER"
 630 ><I
 631 >directory security mask</I
 632 ></TT
 633 > parameter
 634         by default is set to the same value as the <TT
 635 CLASS="PARAMETER"
 636 ><I
 637 >directory mask
 638         </I
 639 ></TT
 640 > parameter and the <TT
 641 CLASS="PARAMETER"
 642 ><I
 643 >force directory security
 644         mode</I
 645 ></TT
 646 > parameter by default is set to the same value as
 647         the <TT
 648 CLASS="PARAMETER"
 649 ><I
 650 >force directory mode</I
 651 ></TT
 652 > parameter to provide
 653         compatibility with Samba 2.0.4 where the permission change facility
 654         was introduced.</P
 655 ><P
 656 >In this way Samba enforces the permission restrictions that
 657         an administrator can set on a Samba share, whilst still allowing users
 658         to modify the permission bits within that restriction.</P
 659 ><P
 660 >If you want to set up a share that allows users full control
 661         in modifying the permission bits on their files and directories and
 662         doesn't force any particular bits to be set 'on', then set the following
 663         parameters in the <A
 664 HREF="smb.conf.5.html"
 665 TARGET="_top"
 666 ><TT
 667 CLASS="FILENAME"
 668 >smb.conf(5)
 669         </TT
 670 ></A
 671 > file in that share specific section :</P
 672 ><P
 673 ><TT
 674 CLASS="PARAMETER"
 675 ><I
 676 >security mask = 0777</I
 677 ></TT
 678 ></P
 679 ><P
 680 ><TT
 681 CLASS="PARAMETER"
 682 ><I
 683 >force security mode = 0</I
 684 ></TT
 685 ></P
 686 ><P
 687 ><TT
 688 CLASS="PARAMETER"
 689 ><I
 690 >directory security mask = 0777</I
 691 ></TT
 692 ></P
 693 ><P
 694 ><TT
 695 CLASS="PARAMETER"
 696 ><I
 697 >force directory security mode = 0</I
 698 ></TT
 699 ></P
 700 ><P
 701 >As described, in Samba 2.0.4 the parameters :</P
 702 ><P
 703 ><TT
 704 CLASS="PARAMETER"
 705 ><I
 706 >create mask</I
 707 ></TT
 708 ></P
 709 ><P
 710 ><TT
 711 CLASS="PARAMETER"
 712 ><I
 713 >force create mode</I
 714 ></TT
 715 ></P
 716 ><P
 717 ><TT
 718 CLASS="PARAMETER"
 719 ><I
 720 >directory mask</I
 721 ></TT
 722 ></P
 723 ><P
 724 ><TT
 725 CLASS="PARAMETER"
 726 ><I
 727 >force directory mode</I
 728 ></TT
 729 ></P
 730 ><P
 731 >were used instead of the parameters discussed here.</P
 732 ></DIV
 733 ><DIV
 734 CLASS="SECT1"
 735 ><HR><H1
 736 CLASS="SECT1"
 737 ><A
 738 NAME="AEN165"
 739 >Interaction with the standard Samba file attribute
 740         mapping</A
 741 ></H1
 742 ><P
 743 >Samba maps some of the DOS attribute bits (such as "read
 744         only") into the UNIX permissions of a file. This means there can
 745         be a conflict between the permission bits set via the security
 746         dialog and the permission bits set by the file attribute mapping.
 747         </P
 748 ><P
 749 >One way this can show up is if a file has no UNIX read access
 750         for the owner it will show up as "read only" in the standard
 751         file attributes tabbed dialog. Unfortunately this dialog is
 752         the same one that contains the security info in another tab.</P
 753 ><P
 754 >What this can mean is that if the owner changes the permissions
 755         to allow themselves read access using the security dialog, clicks
 756         <B
 757 CLASS="COMMAND"
 758 >"OK"</B
 759 > to get back to the standard attributes tab
 760         dialog, and then clicks <B
 761 CLASS="COMMAND"
 762 >"OK"</B
 763 > on that dialog, then
 764         NT will set the file permissions back to read-only (as that is what
 765         the attributes still say in the dialog). This means that after setting
 766         permissions and clicking <B
 767 CLASS="COMMAND"
 768 >"OK"</B
 769 > to get back to the
 770         attributes dialog you should always hit <B
 771 CLASS="COMMAND"
 772 >"Cancel"</B
 773 >
 774         rather than <B
 775 CLASS="COMMAND"
 776 >"OK"</B
 777 > to ensure that your changes
 778         are not overridden.</P
 779 ></DIV
 780 ></DIV
 781 ></BODY
 782 ></HTML
 783 >