docs/docbook/projdoc/NT_Security.sgml

   1 <chapter id="unix-permissions">
   2 <chapterinfo>
   3         &author.jeremy;
   4         <pubdate>12 Apr 1999</pubdate>
   5 </chapterinfo>
   6
   7 <title>UNIX Permission Bits and Windows NT Access Control Lists</title>
   8
   9 <sect1>
  10         <title>Viewing and changing UNIX permissions using the NT
  11         security dialogs</title>
  12
  13         <para>Windows NT clients can use their native security settings
  14         dialog box to view and modify the underlying UNIX permissions.</para>
  15
  16         <para>Note that this ability is careful not to compromise
  17         the security of the UNIX host Samba is running on, and
  18         still obeys all the file permission rules that a Samba
  19         administrator can set.</para>
  20
  21         <note>
  22         <para>
  23         All access to Unix/Linux system file via Samba is controlled at
  24         the operating system file access control level. When trying to
  25         figure out file access problems it is vitally important to identify
  26         the identity of the Windows user as it is presented by Samba at
  27         the point of file access. This can best be determined from the
  28         Samba log files.
  29         </para>
  30         </note>
  31 </sect1>
  32
  33 <sect1>
  34         <title>How to view file security on a Samba share</title>
  35
  36         <para>From an NT4/2000/XP client, single-click with the right
  37         mouse button on any file or directory in a Samba mounted
  38         drive letter or UNC path. When the menu pops-up, click
  39         on the <emphasis>Properties</emphasis> entry at the bottom of
  40         the menu. This brings up the file properties dialog
  41         box. Click on the tab <emphasis>Security</emphasis> and you
  42         will see three buttons, <emphasis>Permissions</emphasis>,
  43         <emphasis>Auditing</emphasis>, and <emphasis>Ownership</emphasis>.
  44         The <emphasis>Auditing</emphasis> button will cause either
  45         an error message <errorname>A requested privilege is not held
  46         by the client</errorname> to appear if the user is not the
  47         NT Administrator, or a dialog which is intended to allow an
  48         Administrator to add auditing requirements to a file if the
  49         user is logged on as the NT Administrator. This dialog is
  50         non-functional with a Samba share at this time, as the only
  51         useful button, the <command>Add</command> button will not currently
  52         allow a list of users to be seen.</para>
  53
  54 </sect1>
  55
  56 <sect1>
  57         <title>Viewing file ownership</title>
  58
  59         <para>Clicking on the <command>"Ownership"</command> button
  60         brings up a dialog box telling you who owns the given file. The
  61         owner name will be of the form :</para>
  62
  63         <para><command>"SERVER\user (Long name)"</command></para>
  64
  65         <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
  66         the Samba server, <replaceable>user</replaceable> is the user name of
  67         the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
  68         is the descriptive string identifying the user (normally found in the
  69         GECOS field of the UNIX password database). Click on the <command>Close
  70         </command> button to remove this dialog.</para>
  71
  72         <para>If the parameter <parameter>nt acl support</parameter>
  73         is set to <constant>false</constant> then the file owner will
  74         be shown as the NT user <command>"Everyone"</command>.</para>
  75
  76         <para>The <command>Take Ownership</command> button will not allow
  77         you to change the ownership of this file to yourself (clicking on
  78         it will display a dialog box complaining that the user you are
  79         currently logged onto the NT client cannot be found). The reason
  80         for this is that changing the ownership of a file is a privileged
  81         operation in UNIX, available only to the <emphasis>root</emphasis>
  82         user. As clicking on this button causes NT to attempt to change
  83         the ownership of a file to the current user logged into the NT
  84         client this will not work with Samba at this time.</para>
  85
  86         <para>There is an NT chown command that will work with Samba
  87         and allow a user with Administrator privilege connected
  88         to a Samba server as root to change the ownership of
  89         files on both a local NTFS filesystem or remote mounted NTFS
  90         or Samba drive. This is available as part of the <emphasis>Seclib
  91         </emphasis> NT security library written by Jeremy Allison of
  92         the Samba Team, available from the main Samba ftp site.</para>
  93
  94 </sect1>
  95
  96 <sect1>
  97         <title>Viewing file or directory permissions</title>
  98
  99         <para>The third button is the <command>"Permissions"</command>
 100         button. Clicking on this brings up a dialog box that shows both
 101         the permissions and the UNIX owner of the file or directory.
 102         The owner is displayed in the form :</para>
 103
 104         <para><command>"SERVER\user (Long name)"</command></para>
 105
 106         <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
 107         the Samba server, <replaceable>user</replaceable> is the user name of
 108         the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
 109         is the descriptive string identifying the user (normally found in the
 110         GECOS field of the UNIX password database).</para>
 111
 112         <para>If the parameter <parameter>nt acl support</parameter>
 113         is set to <constant>false</constant> then the file owner will
 114         be shown as the NT user <command>"Everyone"</command> and the
 115         permissions will be shown as NT "Full Control".</para>
 116
 117
 118         <para>The permissions field is displayed differently for files
 119         and directories, so I'll describe the way file permissions
 120         are displayed first.</para>
 121
 122         <sect2>
 123                 <title>File Permissions</title>
 124
 125                 <para>The standard UNIX user/group/world triple and
 126                 the corresponding "read", "write", "execute" permissions
 127                 triples are mapped by Samba into a three element NT ACL
 128                 with the 'r', 'w', and 'x' bits mapped into the corresponding
 129                 NT permissions. The UNIX world permissions are mapped into
 130                 the global NT group <command>Everyone</command>, followed
 131                 by the list of permissions allowed for UNIX world. The UNIX
 132                 owner and group permissions are displayed as an NT
 133                 <command>user</command> icon and an NT <command>local
 134                 group</command> icon respectively followed by the list
 135                 of permissions allowed for the UNIX user and group.</para>
 136
 137                 <para>As many UNIX permission sets don't map into common
 138                 NT names such as <command>"read"</command>, <command>
 139                 "change"</command> or <command>"full control"</command> then
 140                 usually the permissions will be prefixed by the words <command>
 141                 "Special Access"</command> in the NT display list.</para>
 142
 143                 <para>But what happens if the file has no permissions allowed
 144                 for a particular UNIX user group or world component ? In order
 145                 to  allow "no permissions" to be seen and modified then Samba
 146                 overloads the NT <command>"Take Ownership"</command> ACL attribute
 147                 (which has no meaning in UNIX) and reports a component with
 148                 no permissions as having the NT <command>"O"</command> bit set.
 149                 This was chosen of course to make it look like a zero, meaning
 150                 zero permissions. More details on the decision behind this will
 151                 be given below.</para>
 152         </sect2>
 153
 154         <sect2>
 155                 <title>Directory Permissions</title>
 156
 157                 <para>Directories on an NT NTFS file system have two
 158                 different sets of permissions. The first set of permissions
 159                 is the ACL set on the directory itself, this is usually displayed
 160                 in the first set of parentheses in the normal <command>"RW"</command>
 161                 NT style. This first set of permissions is created by Samba in
 162                 exactly the same way as normal file permissions are, described
 163                 above, and is displayed in the same way.</para>
 164
 165                 <para>The second set of directory permissions has no real meaning
 166                 in the UNIX permissions world and represents the <command>
 167                 "inherited"</command> permissions that any file created within
 168                 this directory would inherit.</para>
 169
 170                 <para>Samba synthesises these inherited permissions for NT by
 171                 returning as an NT ACL the UNIX permission mode that a new file
 172                 created by Samba on this share would receive.</para>
 173         </sect2>
 174 </sect1>
 175
 176 <sect1>
 177         <title>Modifying file or directory permissions</title>
 178
 179         <para>Modifying file and directory permissions is as simple
 180         as changing the displayed permissions in the dialog box, and
 181         clicking the <command>OK</command> button. However, there are
 182         limitations that a user needs to be aware of, and also interactions
 183         with the standard Samba permission masks and mapping of DOS
 184         attributes that need to also be taken into account.</para>
 185
 186         <para>If the parameter <parameter>nt acl support</parameter>
 187         is set to <constant>false</constant> then any attempt to set
 188         security permissions will fail with an <command>"Access Denied"
 189         </command> message.</para>
 190
 191         <para>The first thing to note is that the <command>"Add"</command>
 192         button will not return a list of users in Samba (it will give
 193         an error message of <command>"The remote procedure call failed
 194         and did not execute"</command>). This means that you can only
 195         manipulate the current user/group/world permissions listed in
 196         the dialog box. This actually works quite well as these are the
 197         only permissions that UNIX actually has.</para>
 198
 199         <para>If a permission triple (either user, group, or world)
 200         is removed from the list of permissions in the NT dialog box,
 201         then when the <command>"OK"</command> button is pressed it will
 202         be applied as "no permissions" on the UNIX side. If you then
 203         view the permissions again the "no permissions" entry will appear
 204         as the NT <command>"O"</command> flag, as described above. This
 205         allows you to add permissions back to a file or directory once
 206         you have removed them from a triple component.</para>
 207
 208         <para>As UNIX supports only the "r", "w" and "x" bits of
 209         an NT ACL then if other NT security attributes such as "Delete
 210         access" are selected then they will be ignored when applied on
 211         the Samba server.</para>
 212
 213         <para>When setting permissions on a directory the second
 214         set of permissions (in the second set of parentheses) is
 215         by default applied to all files within that directory. If this
 216         is not what you want you must uncheck the <command>"Replace
 217         permissions on existing files"</command> checkbox in the NT
 218         dialog before clicking <command>"OK"</command>.</para>
 219
 220         <para>If you wish to remove all permissions from a
 221         user/group/world  component then you may either highlight the
 222         component and click the <command>"Remove"</command> button,
 223         or set the component to only have the special <command>"Take
 224         Ownership"</command> permission (displayed as <command>"O"
 225         </command>) highlighted.</para>
 226 </sect1>
 227
 228 <sect1>
 229         <title>Interaction with the standard Samba create mask
 230         parameters</title>
 231
 232         <para>There are four parameters
 233         to control interaction with the standard Samba create mask parameters.
 234         These are :</para>
 235
 236         <para><parameter>security mask</parameter></para>
 237         <para><parameter>force security mode</parameter></para>
 238         <para><parameter>directory security mask</parameter></para>
 239         <para><parameter>force directory security mode</parameter></para>
 240
 241         <para>Once a user clicks <command>"OK"</command> to apply the
 242         permissions Samba maps the given permissions into a user/group/world
 243         r/w/x triple set, and then will check the changed permissions for a
 244         file against the bits set in the <ulink url="smb.conf.5.html#SECURITYMASK">
 245         <parameter>security mask</parameter></ulink> parameter. Any bits that
 246         were changed that are not set to '1' in this parameter are left alone
 247         in the file permissions.</para>
 248
 249         <para>Essentially, zero bits in the <parameter>security mask</parameter>
 250         mask may be treated as a set of bits the user is <emphasis>not</emphasis>
 251         allowed to change, and one bits are those the user is allowed to change.
 252         </para>
 253
 254         <para>If not set explicitly this parameter is set to the same value as
 255         the <ulink url="smb.conf.5.html#CREATEMASK"><parameter>create mask
 256         </parameter></ulink> parameter. To allow a user to modify all the
 257         user/group/world permissions on a file, set this parameter
 258         to 0777.</para>
 259
 260         <para>Next Samba checks the changed permissions for a file against
 261         the bits set in the <ulink url="smb.conf.5.html#FORCESECURITYMODE">
 262         <parameter>force security mode</parameter></ulink> parameter. Any bits
 263         that were changed that correspond to bits set to '1' in this parameter
 264         are forced to be set.</para>
 265
 266         <para>Essentially, bits set in the <parameter>force security mode
 267         </parameter> parameter may be treated as a set of bits that, when
 268         modifying security on a file, the user has always set to be 'on'.</para>
 269
 270         <para>If not set explicitly this parameter is set to the same value
 271         as the <ulink url="smb.conf.5.html#FORCECREATEMODE"><parameter>force
 272         create mode</parameter></ulink> parameter.
 273         To allow a user to modify all the user/group/world permissions on a file
 274         with no restrictions set this parameter to 000.</para>
 275
 276         <para>The <parameter>security mask</parameter> and <parameter>force
 277         security mode</parameter> parameters are applied to the change
 278         request in that order.</para>
 279
 280         <para>For a directory Samba will perform the same operations as
 281         described above for a file except using the parameter <parameter>
 282         directory security mask</parameter> instead of <parameter>security
 283         mask</parameter>, and <parameter>force directory security mode
 284         </parameter> parameter instead of <parameter>force security mode
 285         </parameter>.</para>
 286
 287         <para>The <parameter>directory security mask</parameter> parameter
 288         by default is set to the same value as the <parameter>directory mask
 289         </parameter> parameter and the <parameter>force directory security
 290         mode</parameter> parameter by default is set to the same value as
 291         the <parameter>force directory mode</parameter> parameter. </para>
 292
 293         <para>In this way Samba enforces the permission restrictions that
 294         an administrator can set on a Samba share, whilst still allowing users
 295         to modify the permission bits within that restriction.</para>
 296
 297         <para>If you want to set up a share that allows users full control
 298         in modifying the permission bits on their files and directories and
 299         doesn't force any particular bits to be set 'on', then set the following
 300         parameters in the &smb.conf; file in that share specific section :</para>
 301
 302         <para><parameter>security mask = 0777</parameter></para>
 303         <para><parameter>force security mode = 0</parameter></para>
 304         <para><parameter>directory security mask = 0777</parameter></para>
 305         <para><parameter>force directory security mode = 0</parameter></para>
 306 </sect1>
 307
 308 <sect1>
 309         <title>Interaction with the standard Samba file attribute
 310         mapping</title>
 311
 312         <para>Samba maps some of the DOS attribute bits (such as "read
 313         only") into the UNIX permissions of a file. This means there can
 314         be a conflict between the permission bits set via the security
 315         dialog and the permission bits set by the file attribute mapping.
 316         </para>
 317
 318         <para>One way this can show up is if a file has no UNIX read access
 319         for the owner it will show up as "read only" in the standard
 320         file attributes tabbed dialog. Unfortunately this dialog is
 321         the same one that contains the security info in another tab.</para>
 322
 323         <para>What this can mean is that if the owner changes the permissions
 324         to allow themselves read access using the security dialog, clicks
 325         <command>"OK"</command> to get back to the standard attributes tab
 326         dialog, and then clicks <command>"OK"</command> on that dialog, then
 327         NT will set the file permissions back to read-only (as that is what
 328         the attributes still say in the dialog). This means that after setting
 329         permissions and clicking <command>"OK"</command> to get back to the
 330         attributes dialog you should always hit <command>"Cancel"</command>
 331         rather than <command>"OK"</command> to ensure that your changes
 332         are not overridden.</para>
 333 </sect1>
 334
 335 </chapter>