path: root/docs/textdocs/NT_Security.txt

TITLE INFORMATION: Viewing and changing UNIX permissions using the NT security dialogs in Samba 2.0.4 
AUTHOR INFORMATION: Jeremy Allison, Samba Team 
DATE INFORMATION: 12th April 1999 

Table of Contents 

Viewing and changing UNIX permissions using the NT security dialogs 

------------------------------------------------------------------- 

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.

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.

In Samba 2.0.4 and above the default value of the parameter
"nt acl support" has been
changed from "false" to "true", so manipulation of permissions is
turned on by default.

How to view file security on a Samba share

------------------------------------------

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 Properties 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 Security. Click on this tab and you will see three buttons,
Permissions, Auditing, and Ownership. The Auditing
button will cause either an error message "A requested privilege is
not held by the client" 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 Add button
will not currently allow a list of users to be seen.

Viewing file ownership

----------------------

Clicking on the "Ownership" button brings up a dialog box telling
you who owns the given file. The owner name will be of the form :

"SERVER\user (Long name)"

Where SERVER is the NetBIOS name of the Samba server, user
is the user name of the UNIX user who owns the file, and (Long name)
is the discriptive string identifying the user (normally found in the
GECOS field of the UNIX password database). Click on the Close
button to remove this dialog.

If the parameter "nt acl support"
is set to "false" then the file owner will be shown as the NT user
"Everyone".

The Take Ownership 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 privilaged operation in UNIX, available
only to the root 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.

There is an NT chown command that will work with Samba and allow
a user with Administrator privillage 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 Seclib NT security library written by Jeremy
Allison of the Samba Team, available from the main Samba ftp site.

Viewing file or directory permissions

-------------------------------------

The third button is the "Permissions" 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 :

"SERVER\user (Long name)"

Where SERVER is the NetBIOS name of the Samba server, user
is the user name of the UNIX user who owns the file, and (Long name)
is the discriptive string identifying the user (normally found in the
GECOS field of the UNIX password database).

If the parameter "nt acl support"
is set to "false" then the file owner will be shown as the NT user
"Everyone" and the permissions will be shown as NT "Full Control".

The permissions field is displayed differently for files and directories,
so I'll describe the way file permissions are displayed first.

File Permissions

----------------

The standard UNIX user/group/world triple and the correspinding
"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 Everyone, followed by the list of permissions
allowed for UNIX world. The UNIX owner and group permissions
are displayed as an NT user icon and an NT local group icon
respectively followed by the list of permissions allowed for the
UNIX user and group.

As many UNIX permission sets don't map into common NT names such as
"read", "change" or "full control" then usually the permissions
will be prefixed by the words "Special Access" in the NT display 
list.

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 "Take Ownership" ACL attribute (which has no meaning in
UNIX) and reports a component with no permissions as having the NT
"O" 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.

Directory Permissions

---------------------

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 "RW" 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.

The second set of directory permissions has no real meaning in the
UNIX permissions world and represents the "inherited" permissions
that any file created within this directory would inherit.

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.

Modifying file or directory permissions

---------------------------------------

Modifying file and directory permissions is as simple as changing
the displayed permissions in the dialog box, and clicking the OK
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.

If the parameter "nt acl support"
is set to "false" then any attempt to set security permissions will
fail with an "Access Denied" message.

The first thing to note is that the "Add" button will not return
a list of users in Samba 2.0.4 (it will give an error message of
"The remote proceedure call failed and did not execute"). 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.

If a permission triple (either user, group, or world) is removed from
the list of permissions in the NT dialog box, then when the "OK"
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 "O" 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.

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.

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 "Replace permissions on existing files" checkbox
in the NT dialog before clicking "OK".

If you wish to remove all permissions from a user/group/world 
component then you may either highlight the component and click
the "Remove" button, or set the component to only have the special
"Take Ownership" permission (dsplayed as "O") highlighted.

Interaction with the standard Samba create mask parameters

----------------------------------------------------------

Note that with Samba 2.0.5 there are four new parameters to
control this interaction.

These are :

security mask
force security mode
directory security mask
force directory security mode

Once a user clicks "OK" 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 "security mask"
parameter. Any bits that were changed that are not set to '1'
in this parameter are left alone in the file permissions.

Essentially, zero bits in the "security mask"
mask may be treated as a set of bits the user is not allowed to change,
and one bits are those the user is allowed to change.

If not set explicitly this parameter is set to the same value as the
"create mask" 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.

Next Samba checks the changed permissions for a file against the
bits set in the "force security mode"
parameter. Any bits that were changed that correspond to bits set
to '1' in this parameter are forced to be set.

Essentially, bits set in the "force security mode"
parameter may be treated as a set of bits that, when modifying security on a file, the
user has always set to be 'on'.

If not set explicitly this parameter is set to the same value as the
"force create mode" 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.

The "security mask" and
"force security mode" parameters
are applied to the change request in that order.

For a directory Samba will perform the same operations as described above
for a file except using the parameter "directory security mask"
instead of "security mask", and 
"force directory security mode" parameter instead
of "force security mode".

The "directory security mask"
parameter by default is set to the same value as the "directory mask"
parameter and the "force directory security mode"
parameter by default is set to the same value as the 
iurl("force directory mode")(smb.conf.5.html#forcedirectorymode) parameter
to provide compatibility with Samba 2.0.4 where the permission change facility was introduced.

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.

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.5 file in
that share specific section :

security mask = 0777
force security mode = 0
directory security mask = 0777
force directory security mode = 0

As described, in Samba 2.0.4 the parameters :

create mask
force create mode
directory mask
force directory mode

were used instead of the parameters discussed here.

Interaction with the standard Samba file attribute mapping

----------------------------------------------------------

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.

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.

What this can mean is that if the owner changes the permissions
to allow themselves read access using the security dialog, clicks
"OK" to get back to the standard attributes tab dialog, and
then clicks "OK" 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 "OK" to get back to the attributes dialog you
should always hit "Cancel" rather than "OK" to ensure
that your changes are not overridden.