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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
><TITLE
>UNIX Permission Bits and Windows NT Access Control Lists</TITLE
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="UNIX-PERMISSIONS"
></A
>Chapter 10. UNIX Permission Bits and Windows NT Access Control Lists</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1647"
>10.1. Viewing and changing UNIX permissions using the NT
security dialogs</A
></H1
><P
>New in the Samba 2.0.4 release is the ability for Windows
NT clients to use their native security settings dialog box to
view and modify the underlying UNIX permissions.</P
><P
>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.</P
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1651"
>10.2. How to view file security on a Samba share</A
></H1
><P
>From an NT 4.0 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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Properties</I
></SPAN
> entry at the bottom of
the menu. This brings up the normal file properties dialog
box, but with Samba 2.0.4 this will have a new tab along the top
marked <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Security</I
></SPAN
>. Click on this tab and you
will see three buttons, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Permissions</I
></SPAN
>,
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Auditing</I
></SPAN
>, and <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Ownership</I
></SPAN
>.
The <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Auditing</I
></SPAN
> button will cause either
an error message <SPAN
CLASS="ERRORNAME"
>A requested privilege is not held
by the client</SPAN
> 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 <B
CLASS="COMMAND"
>Add</B
> button will not currently
allow a list of users to be seen.</P
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1662"
>10.3. Viewing file ownership</A
></H1
><P
>Clicking on the <B
CLASS="COMMAND"
>"Ownership"</B
> button
brings up a dialog box telling you who owns the given file. The
owner name will be of the form :</P
><P
><B
CLASS="COMMAND"
>"SERVER\user (Long name)"</B
></P
><P
>Where <VAR
CLASS="REPLACEABLE"
>SERVER</VAR
> is the NetBIOS name of
the Samba server, <VAR
CLASS="REPLACEABLE"
>user</VAR
> is the user name of
the UNIX user who owns the file, and <VAR
CLASS="REPLACEABLE"
>(Long name)</VAR
>
is the descriptive string identifying the user (normally found in the
GECOS field of the UNIX password database). Click on the <B
CLASS="COMMAND"
>Close
</B
> button to remove this dialog.</P
><P
>If the parameter <VAR
CLASS="PARAMETER"
>nt acl support</VAR
>
is set to <CODE
CLASS="CONSTANT"
>false</CODE
> then the file owner will
be shown as the NT user <B
CLASS="COMMAND"
>"Everyone"</B
>.</P
><P
>The <B
CLASS="COMMAND"
>Take Ownership</B
> 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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>root</I
></SPAN
>
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.</P
><P
>There is an NT chown command that will work with Samba
and allow a user with Administrator privilege connected
to a Samba 2.0.4 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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Seclib
</I
></SPAN
> NT security library written by Jeremy Allison of
the Samba Team, available from the main Samba ftp site.</P
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1682"
>10.4. Viewing file or directory permissions</A
></H1
><P
>The third button is the <B
CLASS="COMMAND"
>"Permissions"</B
>
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 :</P
><P
><B
CLASS="COMMAND"
>"SERVER\user (Long name)"</B
></P
><P
>Where <VAR
CLASS="REPLACEABLE"
>SERVER</VAR
> is the NetBIOS name of
the Samba server, <VAR
CLASS="REPLACEABLE"
>user</VAR
> is the user name of
the UNIX user who owns the file, and <VAR
CLASS="REPLACEABLE"
>(Long name)</VAR
>
is the descriptive string identifying the user (normally found in the
GECOS field of the UNIX password database).</P
><P
>If the parameter <VAR
CLASS="PARAMETER"
>nt acl support</VAR
>
is set to <CODE
CLASS="CONSTANT"
>false</CODE
> then the file owner will
be shown as the NT user <B
CLASS="COMMAND"
>"Everyone"</B
> and the
permissions will be shown as NT "Full Control".</P
><P
>The permissions field is displayed differently for files
and directories, so I'll describe the way file permissions
are displayed first.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1697"
>10.4.1. File Permissions</A
></H2
><P
>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 <B
CLASS="COMMAND"
>Everyone</B
>, followed
by the list of permissions allowed for UNIX world. The UNIX
owner and group permissions are displayed as an NT
<B
CLASS="COMMAND"
>user</B
> icon and an NT <B
CLASS="COMMAND"
>local
group</B
> icon respectively followed by the list
of permissions allowed for the UNIX user and group.</P
><P
>As many UNIX permission sets don't map into common
NT names such as <B
CLASS="COMMAND"
>"read"</B
>, <B
CLASS="COMMAND"
> "change"</B
> or <B
CLASS="COMMAND"
>"full control"</B
> then
usually the permissions will be prefixed by the words <B
CLASS="COMMAND"
> "Special Access"</B
> in the NT display list.</P
><P
>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 <B
CLASS="COMMAND"
>"Take Ownership"</B
> ACL attribute
(which has no meaning in UNIX) and reports a component with
no permissions as having the NT <B
CLASS="COMMAND"
>"O"</B
> 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.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1711"
>10.4.2. Directory Permissions</A
></H2
><P
>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 <B
CLASS="COMMAND"
>"RW"</B
>
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.</P
><P
>The second set of directory permissions has no real meaning
in the UNIX permissions world and represents the <B
CLASS="COMMAND"
> "inherited"</B
> permissions that any file created within
this directory would inherit.</P
><P
>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.</P
></DIV
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1718"
>10.5. Modifying file or directory permissions</A
></H1
><P
>Modifying file and directory permissions is as simple
as changing the displayed permissions in the dialog box, and
clicking the <B
CLASS="COMMAND"
>OK</B
> 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.</P
><P
>If the parameter <VAR
CLASS="PARAMETER"
>nt acl support</VAR
>
is set to <CODE
CLASS="CONSTANT"
>false</CODE
> then any attempt to set
security permissions will fail with an <B
CLASS="COMMAND"
>"Access Denied"
</B
> message.</P
><P
>The first thing to note is that the <B
CLASS="COMMAND"
>"Add"</B
>
button will not return a list of users in Samba 2.0.4 (it will give
an error message of <B
CLASS="COMMAND"
>"The remote procedure call failed
and did not execute"</B
>). 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.</P
><P
>If a permission triple (either user, group, or world)
is removed from the list of permissions in the NT dialog box,
then when the <B
CLASS="COMMAND"
>"OK"</B
> 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 <B
CLASS="COMMAND"
>"O"</B
> 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.</P
><P
>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.</P
><P
>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 <B
CLASS="COMMAND"
>"Replace
permissions on existing files"</B
> checkbox in the NT
dialog before clicking <B
CLASS="COMMAND"
>"OK"</B
>.</P
><P
>If you wish to remove all permissions from a
user/group/world component then you may either highlight the
component and click the <B
CLASS="COMMAND"
>"Remove"</B
> button,
or set the component to only have the special <B
CLASS="COMMAND"
>"Take
Ownership"</B
> permission (displayed as <B
CLASS="COMMAND"
>"O"
</B
>) highlighted.</P
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1740"
>10.6. Interaction with the standard Samba create mask
parameters</A
></H1
><P
>Note that with Samba 2.0.5 there are four new parameters
to control this interaction. These are :</P
><P
><VAR
CLASS="PARAMETER"
>security mask</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force security mode</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>directory security mask</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force directory security mode</VAR
></P
><P
>Once a user clicks <B
CLASS="COMMAND"
>"OK"</B
> 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 <A
HREF="smb.conf.5.html#SECURITYMASK"
TARGET="_top"
>
<VAR
CLASS="PARAMETER"
>security mask</VAR
></A
> parameter. Any bits that
were changed that are not set to '1' in this parameter are left alone
in the file permissions.</P
><P
>Essentially, zero bits in the <VAR
CLASS="PARAMETER"
>security mask</VAR
>
mask may be treated as a set of bits the user is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
allowed to change, and one bits are those the user is allowed to change.
</P
><P
>If not set explicitly this parameter is set to the same value as
the <A
HREF="smb.conf.5.html#CREATEMASK"
TARGET="_top"
><VAR
CLASS="PARAMETER"
>create mask
</VAR
></A
> parameter to provide compatibility with Samba 2.0.4
where this permission change facility was introduced. To allow a user to
modify all the user/group/world permissions on a file, set this parameter
to 0777.</P
><P
>Next Samba checks the changed permissions for a file against
the bits set in the <A
HREF="smb.conf.5.html#FORCESECURITYMODE"
TARGET="_top"
> <VAR
CLASS="PARAMETER"
>force security mode</VAR
></A
> parameter. Any bits
that were changed that correspond to bits set to '1' in this parameter
are forced to be set.</P
><P
>Essentially, bits set in the <VAR
CLASS="PARAMETER"
>force security mode
</VAR
> parameter may be treated as a set of bits that, when
modifying security on a file, the user has always set to be 'on'.</P
><P
>If not set explicitly this parameter is set to the same value
as the <A
HREF="smb.conf.5.html#FORCECREATEMODE"
TARGET="_top"
><VAR
CLASS="PARAMETER"
>force
create mode</VAR
></A
> parameter to provide compatibility
with Samba 2.0.4 where the permission change facility was introduced.
To allow a user to modify all the user/group/world permissions on a file
with no restrictions set this parameter to 000.</P
><P
>The <VAR
CLASS="PARAMETER"
>security mask</VAR
> and <VAR
CLASS="PARAMETER"
>force
security mode</VAR
> parameters are applied to the change
request in that order.</P
><P
>For a directory Samba will perform the same operations as
described above for a file except using the parameter <VAR
CLASS="PARAMETER"
> directory security mask</VAR
> instead of <VAR
CLASS="PARAMETER"
>security
mask</VAR
>, and <VAR
CLASS="PARAMETER"
>force directory security mode
</VAR
> parameter instead of <VAR
CLASS="PARAMETER"
>force security mode
</VAR
>.</P
><P
>The <VAR
CLASS="PARAMETER"
>directory security mask</VAR
> parameter
by default is set to the same value as the <VAR
CLASS="PARAMETER"
>directory mask
</VAR
> parameter and the <VAR
CLASS="PARAMETER"
>force directory security
mode</VAR
> parameter by default is set to the same value as
the <VAR
CLASS="PARAMETER"
>force directory mode</VAR
> parameter to provide
compatibility with Samba 2.0.4 where the permission change facility
was introduced.</P
><P
>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.</P
><P
>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 <A
HREF="smb.conf.5.html"
TARGET="_top"
><TT
CLASS="FILENAME"
>smb.conf(5)
</TT
></A
> file in that share specific section :</P
><P
><VAR
CLASS="PARAMETER"
>security mask = 0777</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force security mode = 0</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>directory security mask = 0777</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force directory security mode = 0</VAR
></P
><P
>As described, in Samba 2.0.4 the parameters :</P
><P
><VAR
CLASS="PARAMETER"
>create mask</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force create mode</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>directory mask</VAR
></P
><P
><VAR
CLASS="PARAMETER"
>force directory mode</VAR
></P
><P
>were used instead of the parameters discussed here.</P
></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1804"
>10.7. Interaction with the standard Samba file attribute
mapping</A
></H1
><P
>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.
</P
><P
>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.</P
><P
>What this can mean is that if the owner changes the permissions
to allow themselves read access using the security dialog, clicks
<B
CLASS="COMMAND"
>"OK"</B
> to get back to the standard attributes tab
dialog, and then clicks <B
CLASS="COMMAND"
>"OK"</B
> 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 <B
CLASS="COMMAND"
>"OK"</B
> to get back to the
attributes dialog you should always hit <B
CLASS="COMMAND"
>"Cancel"</B
>
rather than <B
CLASS="COMMAND"
>"OK"</B
> to ensure that your changes
are not overridden.</P
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