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

<chapter id="AccessControls">
<chapterinfo>
	&author.jht;
	&author.jeremy;
	<pubdate>May 10, 2003</pubdate>
</chapterinfo>
<title>File, Directory and Share Access Controls</title>

<para>
Advanced MS Windows users are frequently perplexed when file, directory and share manipulation of
resources shared via Samba do not behave in the manner they might expect. MS Windows network
adminstrators are often confused regarding network access controls and what is the best way to
provide users with the type of access they need while protecting resources from the consequences
of untoward access capabilities.
</para>

<para>
Unix administrators frequently are not familiar with the MS Windows environment and in particular
have difficulty in visualizing what the MS Windows user wishes to achieve in attempts to set file
and directory access permissions. 
</para>

<para>
The problem lies in the differences in how file and directory permissions and controls work
between the two environments. This difference is one that Samba can not completely hide, even
though it does try to make the chasm transparent.
</para>

<para>
POSIX Access Control List technology has been available (along with Extended Attributes)
for Unix for many years, yet there is little evidence today of any significant use. This
explains to some extent the slow adoption of ACLs into commercial Linux products. MS Windows
administrators are astounded at this given that ACLs were a foundational capability of the now
decade old MS Windows NT operating system.
</para>

<para>
The purpose of this chapter is to present each of the points of control that are possible with
Samba-3 in the hope that this will help the network administrator to find the optimum method
for delivering the best environment for MS Windows desktop users.
</para>

<para>
This is an opportune point to mention that it should be borne in mind that Samba was created to
provide a means of interoperability and interchange of data between two operating environments
that are quite different. It was never the intent to make Unix/Linux like MS Windows NT. Instead
the purpose was an is to provide a sufficient level of exchange of data between the two environments.
What is available today extends well beyond early plans and expections, yet the gap continues to
shrink.
</para>

<sect1>
<title>Features and Benefits</title>

	<para>
	Samba offers a lot of flexibility in file system access management. These are the key access control
	facilities present in Samba today:
	</para>

	<itemizedlist>
	<title>Samba Access Control Facilities</title>
		<listitem><para>
		Unix file and directory permissions
		</para></listitem>

		<listitem><para>
		Samba Share Definitions
		</para></listitem>

		<listitem><para>
		Samba Share ACLs
		</para></listitem>

		<listitem><para>
		MS Windows ACLs through Unix POSIX ACLs
		</para></listitem>
	</itemizedlist>

</sect1>

<sect1>
<title>File System Access Controls</title>

<para>
Explain here how Unix file and permissions work
</para>

</sect1>

<sect1>
<title>Share Definition Access Controls</title>

<para>
Explain here about the smb.conf [share] parameters
</para>

</sect1>

<sect1>
<title>Access Controls on Shares</title>

	<para>
	This section deals with how to configure Samba per share access control restrictions.
	By default samba sets no restrictions on the share itself. Restrictions on the share itself
	can be set on MS Windows NT4/200x/XP shares. This can be a very effective way to limit who can
	connect to a share. In the absence of specific restrictions the default setting is to allow
	the global user <emphasis>Everyone</emphasis> Full Control (ie: Full control, Change and Read).
	</para>

	<para>
	At this time Samba does NOT provide a tool for configuring access control setting on the Share
	itself. Samba does have the capacity to store and act on access control settings, but  the only
	way to create those settings is to use either the NT4 Server Manager or the Windows 200x MMC for
	Computer Management.
	</para>

	<para>
	Samba stores the per share access control settings in a file called <filename>share_info.tdb</filename>.
	The location of this file on your system will depend on how samba was compiled. The default location
	for samba's tdb files is under <filename>/usr/local/samba/var</filename>. If the <filename>tdbdump</filename>
	utility has been compiled and installed on your system then you can examine the contents of this file
	by: <userinput>tdbdump share_info.tdb</userinput>.
	</para>

	<sect2>
	<title>Share Permissions Management</title>

		<para>
		The best tool for the task is platform dependant. Choose the best tool for your environmemt.
		</para>

			<sect3>
			<title>Windows NT4 Workstation/Server</title>
			<para>
			The tool you need to use to manage share permissions on a Samba server is the NT Server Manager.
			Server Manager is shipped with Windows NT4 Server products but not with Windows NT4 Workstation.
			You can obtain the NT Server Manager for MS Windows NT4 Workstation from Microsoft - see details below.
			</para>

			<procedure>
			<title>Instructions</title>
			<step><para>
			Launch the NT4 Server Manager, click on the Samba server you want to administer, then from the menu
			select Computer, then click on the Shared Directories entry.
			</para></step>

			<step><para>
				Now click on the share that you wish to manage, then click on the Properties tab, next click on
				the Permissions tab. Now you can Add or change access control settings as you wish.
			</para></step>
			</procedure>

			</sect3>

			<sect3>
			<title>Windows 200x/XP</title>

			<para>
			On MS Windows NT4/200x/XP system access control lists on the share itself are set using native
			tools, usually from filemanager. For example, in Windows 200x: right click on the shared folder,
			then select 'Sharing', then click on 'Permissions'. The default Windows NT4/200x permission allows
			<emphasis>Everyone</emphasis> Full Control on the Share.
			</para>

			<para>
			MS Windows 200x and later all comes with a tool called the 'Computer Management' snap-in for the
			Microsoft Management Console (MMC). This tool is located by clicking on <filename>Control Panel ->
			Administrative Tools -> Computer Management</filename>.
			</para>

			<procedure>
			<title>Instructions</title>
			<step><para>
				After launching the MMC with the Computer Management snap-in, click on the menu item 'Action',
				select 'Connect to another computer'. If you are not logged onto a domain you will be prompted
				to enter a domain login user identifier and a password. This will authenticate you to the domain.
				If you where already logged in with administrative privilidge this step is not offered.
			</para></step>

			<step><para>
			If the Samba server is not shown in the Select Computer box, then type in the name of the target
			Samba server in the field 'Name:'. Now click on the [+] next to 'System Tools', then on the [+]
			next to 'Shared Folders' in the left panel.
			</para></step>

			<step><para>
			Now in the right panel, double-click on the share you wish to set access control permissions on.
			Then click on the tab 'Share Permissions'. It is now possible to add access control entities
			to the shared folder. Do NOT forget to set what type of access (full control, change, read) you
			wish to assign for each entry.
			</para></step>
			</procedure>

			<warning>
			<para>
			Be careful. If you take away all permissions from the Everyone user without removing this user
			then effectively no user will be able to access the share. This is a result of what is known as
			ACL precidence. ie: Everyone with NO ACCESS means that MaryK who is part of the group Everyone
			will have no access even if this user is given explicit full control access.
			</para>
			</warning>

			</sect3>
		</sect2>

</sect1>

<sect1>
<title>MS Windows Access Control Lists and Unix Interoperability</title>

	<sect2>
		<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>
	</sect2>

	<sect2>
		<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>

	</sect2>

	<sect2>
		<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>

	</sect2>

	<sect2>
		<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>

		<sect3>
			<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>
		</sect3>
		
		<sect3>
			<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>
		</sect3>
	</sect2>
		
	<sect2>
		<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>
	</sect2>

	<sect2>
		<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>
	</sect2>

	<sect2>
		<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>
	</sect2>
</sect1>

<sect1>
<title>Common Errors</title>

<para>
Stuff here
</para>

</sect1>

</chapter>