path: root/docs/docbook/projdoc/NT_Security.xml

<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>