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+<chapter id="unix-permissions">
+<chapterinfo>
+ &author.jeremy;
+ <pubdate>12 Apr 1999</pubdate>
+</chapterinfo>
+
+<title>UNIX Permission Bits and Windows NT Access Control Lists</title>
+
+<sect1>
+ <title>Viewing and changing UNIX permissions using the NT
+ security dialogs</title>
+
+ <para>Windows NT clients can use their native security settings
+ dialog box to view and modify the underlying UNIX permissions.</para>
+
+ <para>Note that this ability is careful not to compromise
+ the security of the UNIX host Samba is running on, and
+ still obeys all the file permission rules that a Samba
+ administrator can set.</para>
+
+ <note>
+ <para>
+ All access to Unix/Linux system file via Samba is controlled at
+ the operating system file access control level. When trying to
+ figure out file access problems it is vitally important to identify
+ the identity of the Windows user as it is presented by Samba at
+ the point of file access. This can best be determined from the
+ Samba log files.
+ </para>
+ </note>
+</sect1>
+
+<sect1>
+ <title>How to view file security on a Samba share</title>
+
+ <para>From an NT4/2000/XP client, single-click with the right
+ mouse button on any file or directory in a Samba mounted
+ drive letter or UNC path. When the menu pops-up, click
+ on the <emphasis>Properties</emphasis> entry at the bottom of
+ the menu. This brings up the file properties dialog
+ box. Click on the tab <emphasis>Security</emphasis> and you
+ will see three buttons, <emphasis>Permissions</emphasis>,
+ <emphasis>Auditing</emphasis>, and <emphasis>Ownership</emphasis>.
+ The <emphasis>Auditing</emphasis> button will cause either
+ an error message <errorname>A requested privilege is not held
+ by the client</errorname> to appear if the user is not the
+ NT Administrator, or a dialog which is intended to allow an
+ Administrator to add auditing requirements to a file if the
+ user is logged on as the NT Administrator. This dialog is
+ non-functional with a Samba share at this time, as the only
+ useful button, the <command>Add</command> button will not currently
+ allow a list of users to be seen.</para>
+
+</sect1>
+
+<sect1>
+ <title>Viewing file ownership</title>
+
+ <para>Clicking on the <command>"Ownership"</command> button
+ brings up a dialog box telling you who owns the given file. The
+ owner name will be of the form :</para>
+
+ <para><command>"SERVER\user (Long name)"</command></para>
+
+ <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
+ the Samba server, <replaceable>user</replaceable> is the user name of
+ the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
+ is the descriptive string identifying the user (normally found in the
+ GECOS field of the UNIX password database). Click on the <command>Close
+ </command> button to remove this dialog.</para>
+
+ <para>If the parameter <parameter>nt acl support</parameter>
+ is set to <constant>false</constant> then the file owner will
+ be shown as the NT user <command>"Everyone"</command>.</para>
+
+ <para>The <command>Take Ownership</command> button will not allow
+ you to change the ownership of this file to yourself (clicking on
+ it will display a dialog box complaining that the user you are
+ currently logged onto the NT client cannot be found). The reason
+ for this is that changing the ownership of a file is a privileged
+ operation in UNIX, available only to the <emphasis>root</emphasis>
+ user. As clicking on this button causes NT to attempt to change
+ the ownership of a file to the current user logged into the NT
+ client this will not work with Samba at this time.</para>
+
+ <para>There is an NT chown command that will work with Samba
+ and allow a user with Administrator privilege connected
+ to a Samba server as root to change the ownership of
+ files on both a local NTFS filesystem or remote mounted NTFS
+ or Samba drive. This is available as part of the <emphasis>Seclib
+ </emphasis> NT security library written by Jeremy Allison of
+ the Samba Team, available from the main Samba ftp site.</para>
+
+</sect1>
+
+<sect1>
+ <title>Viewing file or directory permissions</title>
+
+ <para>The third button is the <command>"Permissions"</command>
+ button. Clicking on this brings up a dialog box that shows both
+ the permissions and the UNIX owner of the file or directory.
+ The owner is displayed in the form :</para>
+
+ <para><command>"SERVER\user (Long name)"</command></para>
+
+ <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of
+ the Samba server, <replaceable>user</replaceable> is the user name of
+ the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
+ is the descriptive string identifying the user (normally found in the
+ GECOS field of the UNIX password database).</para>
+
+ <para>If the parameter <parameter>nt acl support</parameter>
+ is set to <constant>false</constant> then the file owner will
+ be shown as the NT user <command>"Everyone"</command> and the
+ permissions will be shown as NT "Full Control".</para>
+
+
+ <para>The permissions field is displayed differently for files
+ and directories, so I'll describe the way file permissions
+ are displayed first.</para>
+
+ <sect2>
+ <title>File Permissions</title>
+
+ <para>The standard UNIX user/group/world triple and
+ the corresponding "read", "write", "execute" permissions
+ triples are mapped by Samba into a three element NT ACL
+ with the 'r', 'w', and 'x' bits mapped into the corresponding
+ NT permissions. The UNIX world permissions are mapped into
+ the global NT group <command>Everyone</command>, followed
+ by the list of permissions allowed for UNIX world. The UNIX
+ owner and group permissions are displayed as an NT
+ <command>user</command> icon and an NT <command>local
+ group</command> icon respectively followed by the list
+ of permissions allowed for the UNIX user and group.</para>
+
+ <para>As many UNIX permission sets don't map into common
+ NT names such as <command>"read"</command>, <command>
+ "change"</command> or <command>"full control"</command> then
+ usually the permissions will be prefixed by the words <command>
+ "Special Access"</command> in the NT display list.</para>
+
+ <para>But what happens if the file has no permissions allowed
+ for a particular UNIX user group or world component ? In order
+ to allow "no permissions" to be seen and modified then Samba
+ overloads the NT <command>"Take Ownership"</command> ACL attribute
+ (which has no meaning in UNIX) and reports a component with
+ no permissions as having the NT <command>"O"</command> bit set.
+ This was chosen of course to make it look like a zero, meaning
+ zero permissions. More details on the decision behind this will
+ be given below.</para>
+ </sect2>
+
+ <sect2>
+ <title>Directory Permissions</title>
+
+ <para>Directories on an NT NTFS file system have two
+ different sets of permissions. The first set of permissions
+ is the ACL set on the directory itself, this is usually displayed
+ in the first set of parentheses in the normal <command>"RW"</command>
+ NT style. This first set of permissions is created by Samba in
+ exactly the same way as normal file permissions are, described
+ above, and is displayed in the same way.</para>
+
+ <para>The second set of directory permissions has no real meaning
+ in the UNIX permissions world and represents the <command>
+ "inherited"</command> permissions that any file created within
+ this directory would inherit.</para>
+
+ <para>Samba synthesises these inherited permissions for NT by
+ returning as an NT ACL the UNIX permission mode that a new file
+ created by Samba on this share would receive.</para>
+ </sect2>
+</sect1>
+
+<sect1>
+ <title>Modifying file or directory permissions</title>
+
+ <para>Modifying file and directory permissions is as simple
+ as changing the displayed permissions in the dialog box, and
+ clicking the <command>OK</command> button. However, there are
+ limitations that a user needs to be aware of, and also interactions
+ with the standard Samba permission masks and mapping of DOS
+ attributes that need to also be taken into account.</para>
+
+ <para>If the parameter <parameter>nt acl support</parameter>
+ is set to <constant>false</constant> then any attempt to set
+ security permissions will fail with an <command>"Access Denied"
+ </command> message.</para>
+
+ <para>The first thing to note is that the <command>"Add"</command>
+ button will not return a list of users in Samba (it will give
+ an error message of <command>"The remote procedure call failed
+ and did not execute"</command>). This means that you can only
+ manipulate the current user/group/world permissions listed in
+ the dialog box. This actually works quite well as these are the
+ only permissions that UNIX actually has.</para>
+
+ <para>If a permission triple (either user, group, or world)
+ is removed from the list of permissions in the NT dialog box,
+ then when the <command>"OK"</command> button is pressed it will
+ be applied as "no permissions" on the UNIX side. If you then
+ view the permissions again the "no permissions" entry will appear
+ as the NT <command>"O"</command> flag, as described above. This
+ allows you to add permissions back to a file or directory once
+ you have removed them from a triple component.</para>
+
+ <para>As UNIX supports only the "r", "w" and "x" bits of
+ an NT ACL then if other NT security attributes such as "Delete
+ access" are selected then they will be ignored when applied on
+ the Samba server.</para>
+
+ <para>When setting permissions on a directory the second
+ set of permissions (in the second set of parentheses) is
+ by default applied to all files within that directory. If this
+ is not what you want you must uncheck the <command>"Replace
+ permissions on existing files"</command> checkbox in the NT
+ dialog before clicking <command>"OK"</command>.</para>
+
+ <para>If you wish to remove all permissions from a
+ user/group/world component then you may either highlight the
+ component and click the <command>"Remove"</command> button,
+ or set the component to only have the special <command>"Take
+ Ownership"</command> permission (displayed as <command>"O"
+ </command>) highlighted.</para>
+</sect1>
+
+<sect1>
+ <title>Interaction with the standard Samba create mask
+ parameters</title>
+
+ <para>There are four parameters
+ to control interaction with the standard Samba create mask parameters.
+ These are :</para>
+
+ <para><parameter>security mask</parameter></para>
+ <para><parameter>force security mode</parameter></para>
+ <para><parameter>directory security mask</parameter></para>
+ <para><parameter>force directory security mode</parameter></para>
+
+ <para>Once a user clicks <command>"OK"</command> to apply the
+ permissions Samba maps the given permissions into a user/group/world
+ r/w/x triple set, and then will check the changed permissions for a
+ file against the bits set in the <ulink url="smb.conf.5.html#SECURITYMASK">
+ <parameter>security mask</parameter></ulink> parameter. Any bits that
+ were changed that are not set to '1' in this parameter are left alone
+ in the file permissions.</para>
+
+ <para>Essentially, zero bits in the <parameter>security mask</parameter>
+ mask may be treated as a set of bits the user is <emphasis>not</emphasis>
+ allowed to change, and one bits are those the user is allowed to change.
+ </para>
+
+ <para>If not set explicitly this parameter is set to the same value as
+ the <ulink url="smb.conf.5.html#CREATEMASK"><parameter>create mask
+ </parameter></ulink> parameter. To allow a user to modify all the
+ user/group/world permissions on a file, set this parameter
+ to 0777.</para>
+
+ <para>Next Samba checks the changed permissions for a file against
+ the bits set in the <ulink url="smb.conf.5.html#FORCESECURITYMODE">
+ <parameter>force security mode</parameter></ulink> parameter. Any bits
+ that were changed that correspond to bits set to '1' in this parameter
+ are forced to be set.</para>
+
+ <para>Essentially, bits set in the <parameter>force security mode
+ </parameter> parameter may be treated as a set of bits that, when
+ modifying security on a file, the user has always set to be 'on'.</para>
+
+ <para>If not set explicitly this parameter is set to the same value
+ as the <ulink url="smb.conf.5.html#FORCECREATEMODE"><parameter>force
+ create mode</parameter></ulink> parameter.
+ To allow a user to modify all the user/group/world permissions on a file
+ with no restrictions set this parameter to 000.</para>
+
+ <para>The <parameter>security mask</parameter> and <parameter>force
+ security mode</parameter> parameters are applied to the change
+ request in that order.</para>
+
+ <para>For a directory Samba will perform the same operations as
+ described above for a file except using the parameter <parameter>
+ directory security mask</parameter> instead of <parameter>security
+ mask</parameter>, and <parameter>force directory security mode
+ </parameter> parameter instead of <parameter>force security mode
+ </parameter>.</para>
+
+ <para>The <parameter>directory security mask</parameter> parameter
+ by default is set to the same value as the <parameter>directory mask
+ </parameter> parameter and the <parameter>force directory security
+ mode</parameter> parameter by default is set to the same value as
+ the <parameter>force directory mode</parameter> parameter. </para>
+
+ <para>In this way Samba enforces the permission restrictions that
+ an administrator can set on a Samba share, whilst still allowing users
+ to modify the permission bits within that restriction.</para>
+
+ <para>If you want to set up a share that allows users full control
+ in modifying the permission bits on their files and directories and
+ doesn't force any particular bits to be set 'on', then set the following
+ parameters in the &smb.conf; file in that share specific section :</para>
+
+ <para><parameter>security mask = 0777</parameter></para>
+ <para><parameter>force security mode = 0</parameter></para>
+ <para><parameter>directory security mask = 0777</parameter></para>
+ <para><parameter>force directory security mode = 0</parameter></para>
+</sect1>
+
+<sect1>
+ <title>Interaction with the standard Samba file attribute
+ mapping</title>
+
+ <para>Samba maps some of the DOS attribute bits (such as "read
+ only") into the UNIX permissions of a file. This means there can
+ be a conflict between the permission bits set via the security
+ dialog and the permission bits set by the file attribute mapping.
+ </para>
+
+ <para>One way this can show up is if a file has no UNIX read access
+ for the owner it will show up as "read only" in the standard
+ file attributes tabbed dialog. Unfortunately this dialog is
+ the same one that contains the security info in another tab.</para>
+
+ <para>What this can mean is that if the owner changes the permissions
+ to allow themselves read access using the security dialog, clicks
+ <command>"OK"</command> to get back to the standard attributes tab
+ dialog, and then clicks <command>"OK"</command> on that dialog, then
+ NT will set the file permissions back to read-only (as that is what
+ the attributes still say in the dialog). This means that after setting
+ permissions and clicking <command>"OK"</command> to get back to the
+ attributes dialog you should always hit <command>"Cancel"</command>
+ rather than <command>"OK"</command> to ensure that your changes
+ are not overridden.</para>
+</sect1>
+
+</chapter>