docs/docbook/projdoc/NT_Security.sgml

   1 <chapter>
   2
   3 <chapterinfo>
   4         <author>
   5                 <firstname>Jeremy</firstname><surname>Allison</surname>
   6                 <affiliation>
   7                         <orgname>Samba Team</orgname>
   8                         <address>
   9                                 <email>samba@samba.org</email>
  10                         </address>
  11                 </affiliation>
  12         </author>
  13
  14
  15         <pubdate>12 Apr 1999</pubdate>
  16 </chapterinfo>
  17
  18
  19 <title>UNIX Permission Bits and WIndows NT Access Control Lists</title>
  20
  21 <sect1>
  22         <title>Viewing and changing UNIX permissions using the NT
  23         security dialogs</title>
  24
  25
  26         <para>New in the Samba 2.0.4 release is the ability for Windows
  27         NT clients to use their native security settings dialog box to
  28         view and modify the underlying UNIX permissions.</para>
  29
  30         <para>Note that this ability is careful not to compromise
  31         the security of the UNIX host Samba is running on, and
  32         still obeys all the file permission rules that a Samba
  33         administrator can set.</para>
  34
  35         <para>In Samba 2.0.4 and above the default value of the
  36         parameter <ulink url="smb.conf.5.html#NTACLSUPPOR"><parameter>
  37         nt acl support</parameter></ulink> has been changed from
  38         <constant>false</constant> to <constant>true</constant>, so
  39         manipulation of permissions is turned on by default.</para>
  40 </sect1>
  41
  42 <sect1>
  43         <title>How to view file security on a Samba share</title>
  44
  45         <para>From an NT 4.0 client, single-click with the right
  46         mouse button on any file or directory in a Samba mounted
  47         drive letter or UNC path. When the menu pops-up, click
  48         on the <emphasis>Properties</emphasis> entry at the bottom of
  49         the menu. This brings up the normal file properties dialog
  50         box, but with Samba 2.0.4 this will have a new tab along the top
  51         marked <emphasis>Security</emphasis>. Click on this tab and you
  52         will see three buttons, <emphasis>Permissions</emphasis>,
  53         <emphasis>Auditing</emphasis>, and <emphasis>Ownership</emphasis>.
  54         The <emphasis>Auditing</emphasis> button will cause either
  55         an error message <errorname>A requested privilege is not held
  56         by the client</errorname> to appear if the user is not the
  57         NT Administrator, or a dialog which is intended to allow an
  58         Administrator to add auditing requirements to a file if the
  59         user is logged on as the NT Administrator. This dialog is
  60         non-functional with a Samba share at this time, as the only
  61         useful button, the <command>Add</command> button will not currently
  62         allow a list of users to be seen.</para>
  63
  64 </sect1>
  65
  66 <sect1>
  67         <title>Viewing file ownership</title>
  68
  69         <para>Clicking on the <command>"Ownership"</command> button
  70         brings up a dialog box telling you who owns the given file. The
  71         owner name will be of the form :</para>
  72
  73         <para><command>"SERVER\user (Long name)"</command></para>
  74
  75         <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
  76         the Samba server, <replaceable>user</replaceable> is the user name of
  77         the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
  78         is the discriptive string identifying the user (normally found in the
  79         GECOS field of the UNIX password database). Click on the <command>Close
  80         </command> button to remove this dialog.</para>
  81
  82         <para>If the parameter <parameter>nt acl support</parameter>
  83         is set to <constant>false</constant> then the file owner will
  84         be shown as the NT user <command>"Everyone"</command>.</para>
  85
  86         <para>The <command>Take Ownership</command> button will not allow
  87         you to change the ownership of this file to yourself (clicking on
  88         it will display a dialog box complaining that the user you are
  89         currently logged onto the NT client cannot be found). The reason
  90         for this is that changing the ownership of a file is a privilaged
  91         operation in UNIX, available only to the <emphasis>root</emphasis>
  92         user. As clicking on this button causes NT to attempt to change
  93         the ownership of a file to the current user logged into the NT
  94         client this will not work with Samba at this time.</para>
  95
  96         <para>There is an NT chown command that will work with Samba
  97         and allow a user with Administrator privillage connected
  98         to a Samba 2.0.4 server as root to change the ownership of
  99         files on both a local NTFS filesystem or remote mounted NTFS
 100         or Samba drive. This is available as part of the <emphasis>Seclib
 101         </emphasis> NT security library written by Jeremy Allison of
 102         the Samba Team, available from the main Samba ftp site.</para>
 103
 104 </sect1>
 105
 106 <sect1>
 107         <title>Viewing file or directory permissions</title>
 108
 109         <para>The third button is the <command>"Permissions"</command>
 110         button. Clicking on this brings up a dialog box that shows both
 111         the permissions and the UNIX owner of the file or directory.
 112         The owner is displayed in the form :</para>
 113
 114         <para><command>"SERVER\user (Long name)"</command></para>
 115
 116         <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
 117         the Samba server, <replaceable>user</replaceable> is the user name of
 118         the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
 119         is the discriptive string identifying the user (normally found in the
 120         GECOS field of the UNIX password database).</para>
 121
 122         <para>If the parameter <parameter>nt acl support</parameter>
 123         is set to <constant>false</constant> then the file owner will
 124         be shown as the NT user <command>"Everyone"</command> and the
 125         permissions will be shown as NT "Full Control".</para>
 126
 127
 128         <para>The permissions field is displayed differently for files
 129         and directories, so I'll describe the way file permissions
 130         are displayed first.</para>
 131
 132         <sect2>
 133                 <title>File Permissions</title>
 134
 135                 <para>The standard UNIX user/group/world triple and
 136                 the correspinding "read", "write", "execute" permissions
 137                 triples are mapped by Samba into a three element NT ACL
 138                 with the 'r', 'w', and 'x' bits mapped into the corresponding
 139                 NT permissions. The UNIX world permissions are mapped into
 140                 the global NT group <command>Everyone</command>, followed
 141                 by the list of permissions allowed for UNIX world. The UNIX
 142                 owner and group permissions are displayed as an NT
 143                 <command>user</command> icon and an NT <command>local
 144                 group</command> icon respectively followed by the list
 145                 of permissions allowed for the UNIX user and group.</para>
 146
 147                 <para>As many UNIX permission sets don't map into common
 148                 NT names such as <command>"read"</command>, <command>
 149                 "change"</command> or <command>"full control"</command> then
 150                 usually the permissions will be prefixed by the words <command>
 151                 "Special Access"</command> in the NT display list.</para>
 152
 153                 <para>But what happens if the file has no permissions allowed
 154                 for a particular UNIX user group or world component ? In order
 155                 to  allow "no permissions" to be seen and modified then Samba
 156                 overloads the NT <command>"Take Ownership"</command> ACL attribute
 157                 (which has no meaning in UNIX) and reports a component with
 158                 no permissions as having the NT <command>"O"</command> bit set.
 159                 This was chosen of course to make it look like a zero, meaning
 160                 zero permissions. More details on the decision behind this will
 161                 be given below.</para>
 162         </sect2>
 163
 164         <sect2>
 165                 <title>Directory Permissions</title>
 166
 167                 <para>Directories on an NT NTFS file system have two
 168                 different sets of permissions. The first set of permissions
 169                 is the ACL set on the directory itself, this is usually displayed
 170                 in the first set of parentheses in the normal <command>"RW"</command>
 171                 NT style. This first set of permissions is created by Samba in
 172                 exactly the same way as normal file permissions are, described
 173                 above, and is displayed in the same way.</para>
 174
 175                 <para>The second set of directory permissions has no real meaning
 176                 in the UNIX permissions world and represents the <command>
 177                 "inherited"</command> permissions that any file created within
 178                 this directory would inherit.</para>
 179
 180                 <para>Samba synthesises these inherited permissions for NT by
 181                 returning as an NT ACL the UNIX permission mode that a new file
 182                 created by Samba on this share would receive.</para>
 183         </sect2>
 184 </sect1>
 185
 186 <sect1>
 187         <title>Modifying file or directory permissions</title>
 188
 189         <para>Modifying file and directory permissions is as simple
 190         as changing the displayed permissions in the dialog box, and
 191         clicking the <command>OK</command> button. However, there are
 192         limitations that a user needs to be aware of, and also interactions
 193         with the standard Samba permission masks and mapping of DOS
 194         attributes that need to also be taken into account.</para>
 195
 196         <para>If the parameter <parameter>nt acl support</parameter>
 197         is set to <constant>false</constant> then any attempt to set
 198         security permissions will fail with an <command>"Access Denied"
 199         </command> message.</para>
 200
 201         <para>The first thing to note is that the <command>"Add"</command>
 202         button will not return a list of users in Samba 2.0.4 (it will give
 203         an error message of <command>"The remote proceedure call failed
 204         and did not execute"</command>). This means that you can only
 205         manipulate the current user/group/world permissions listed in
 206         the dialog box. This actually works quite well as these are the
 207         only permissions that UNIX actually has.</para>
 208
 209         <para>If a permission triple (either user, group, or world)
 210         is removed from the list of permissions in the NT dialog box,
 211         then when the <command>"OK"</command> button is pressed it will
 212         be applied as "no permissions" on the UNIX side. If you then
 213         view the permissions again the "no permissions" entry will appear
 214         as the NT <command>"O"</command> flag, as described above. This
 215         allows you to add permissions back to a file or directory once
 216         you have removed them from a triple component.</para>
 217
 218         <para>As UNIX supports only the "r", "w" and "x" bits of
 219         an NT ACL then if other NT security attributes such as "Delete
 220         access" are selected then they will be ignored when applied on
 221         the Samba server.</para>
 222
 223         <para>When setting permissions on a directory the second
 224         set of permissions (in the second set of parentheses) is
 225         by default applied to all files within that directory. If this
 226         is not what you want you must uncheck the <command>"Replace
 227         permissions on existing files"</command> checkbox in the NT
 228         dialog before clicking <command>"OK"</command>.</para>
 229
 230         <para>If you wish to remove all permissions from a
 231         user/group/world  component then you may either highlight the
 232         component and click the <command>"Remove"</command> button,
 233         or set the component to only have the special <command>"Take
 234         Ownership"</command> permission (dsplayed as <command>"O"
 235         </command>) highlighted.</para>
 236 </sect1>
 237
 238 <sect1>
 239         <title>Interaction with the standard Samba create mask
 240         parameters</title>
 241
 242         <para>Note that with Samba 2.0.5 there are four new parameters
 243         to control this interaction.  These are :</para>
 244
 245         <para><parameter>security mask</parameter></para>
 246         <para><parameter>force security mode</parameter></para>
 247         <para><parameter>directory security mask</parameter></para>
 248         <para><parameter>force directory security mode</parameter></para>
 249
 250         <para>Once a user clicks <command>"OK"</command> to apply the
 251         permissions Samba maps the given permissions into a user/group/world
 252         r/w/x triple set, and then will check the changed permissions for a
 253         file against the bits set in the <ulink url="smb.conf.5.html#SECURITYMASK">
 254         <parameter>security mask</parameter></ulink> parameter. Any bits that
 255         were changed that are not set to '1' in this parameter are left alone
 256         in the file permissions.</para>
 257
 258         <para>Essentially, zero bits in the <parameter>security mask</parameter>
 259         mask may be treated as a set of bits the user is <emphasis>not</emphasis>
 260         allowed to change, and one bits are those the user is allowed to change.
 261         </para>
 262
 263         <para>If not set explicitly this parameter is set to the same value as
 264         the <ulink url="smb.conf.5.html#CREATEMASK"><parameter>create mask
 265         </parameter></ulink> parameter to provide compatibility with Samba 2.0.4
 266         where this permission change facility was introduced. To allow a user to
 267         modify all the user/group/world permissions on a file, set this parameter
 268         to 0777.</para>
 269
 270         <para>Next Samba checks the changed permissions for a file against
 271         the bits set in the <ulink url="smb.conf.5.html#FORCESECURITYMODE">
 272         <parameter>force security mode</parameter></ulink> parameter. Any bits
 273         that were changed that correspond to bits set to '1' in this parameter
 274         are forced to be set.</para>
 275
 276         <para>Essentially, bits set in the <parameter>force security mode
 277         </parameter> parameter may be treated as a set of bits that, when
 278         modifying security on a file, the user has always set to be 'on'.</para>
 279
 280         <para>If not set explicitly this parameter is set to the same value
 281         as the <ulink url="smb.conf.5.html#FORCECREATEMODE"><parameter>force
 282         create mode</parameter></ulink> parameter to provide compatibility
 283         with Samba 2.0.4 where the permission change facility was introduced.
 284         To allow a user to modify all the user/group/world permissions on a file,
 285         with no restrictions set this parameter to 000.</para>
 286
 287         <para>The <parameter>security mask</parameter> and <parameter>force
 288         security mode</parameter> parameters are applied to the change
 289         request in that order.</para>
 290
 291         <para>For a directory Samba will perform the same operations as
 292         described above for a file except using the parameter <parameter>
 293         directory security mask</parameter> instead of <parameter>security
 294         mask</parameter>, and <parameter>force directory security mode
 295         </parameter> parameter instead of <parameter>force security mode
 296         </parameter>.</para>
 297
 298         <para>The <parameter>directory security mask</parameter> parameter
 299         by default is set to the same value as the <parameter>directory mask
 300         </parameter> parameter and the <parameter>force directory security
 301         mode</parameter> parameter by default is set to the same value as
 302         the <parameter>force directory mode</parameter> parameter to provide
 303         compatibility with Samba 2.0.4 where the permission change facility
 304         was introduced.</para>
 305
 306         <para>In this way Samba enforces the permission restrictions that
 307         an administrator can set on a Samba share, whilst still allowing users
 308         to modify the permission bits within that restriction.</para>
 309
 310         <para>If you want to set up a share that allows users full control
 311         in modifying the permission bits on their files and directories and
 312         doesn't force any particular bits to be set 'on', then set the following
 313         parameters in the <ulink url="smb.conf.5.html"><filename>smb.conf(5)
 314         </filename></ulink> file in that share specific section :</para>
 315
 316         <para><parameter>security mask = 0777</parameter></para>
 317         <para><parameter>force security mode = 0</parameter></para>
 318         <para><parameter>directory security mask = 0777</parameter></para>
 319         <para><parameter>force directory security mode = 0</parameter></para>
 320
 321         <para>As described, in Samba 2.0.4 the parameters :</para>
 322
 323         <para><parameter>create mask</parameter></para>
 324         <para><parameter>force create mode</parameter></para>
 325         <para><parameter>directory mask</parameter></para>
 326         <para><parameter>force directory mode</parameter></para>
 327
 328         <para>were used instead of the parameters discussed here.</para>
 329 </sect1>
 330
 331 <sect1>
 332         <title>Interaction with the standard Samba file attribute
 333         mapping</title>
 334
 335         <para>Samba maps some of the DOS attribute bits (such as "read
 336         only") into the UNIX permissions of a file. This means there can
 337         be a conflict between the permission bits set via the security
 338         dialog and the permission bits set by the file attribute mapping.
 339         </para>
 340
 341         <para>One way this can show up is if a file has no UNIX read access
 342         for the owner it will show up as "read only" in the standard
 343         file attributes tabbed dialog. Unfortunately this dialog is
 344         the same one that contains the security info in another tab.</para>
 345
 346         <para>What this can mean is that if the owner changes the permissions
 347         to allow themselves read access using the security dialog, clicks
 348         <command>"OK"</command> to get back to the standard attributes tab
 349         dialog, and then clicks <command>"OK"</command> on that dialog, then
 350         NT will set the file permissions back to read-only (as that is what
 351         the attributes still say in the dialog). This means that after setting
 352         permissions and clicking <command>"OK"</command> to get back to the
 353         attributes dialog you should always hit <command>"Cancel"</command>
 354         rather than <command>"OK"</command> to ensure that your changes
 355         are not overridden.</para>
 356 </sect1>
 357
 358 </chapter>