Samba operates on top of the Unix file system. This means it is subject to Unix file system conventions and permissions. It also means that if the MS Windows networking environment requires file system behaviour that differs from unix file system behaviour then somehow Samba is responsible for emulating that in a transparent and consistent manner.

It is good news that Samba does this to a very large extent and on top of that provides a high degree of optional configuration to over-ride the default behaviour. We will look at some of these over-rides, but for the greater part we will stay within the bounds of default behaviour. Those wishing to explore to depths of control ability should review the smb.conf man page.

There are many other subtle differences that may cause the MS Windows administrator some temporary discomfort in the process of becoming familiar with Unix/Linux. These are best left for a text that is dedicated to the purpose of Unix/Linux training/education.

The network administrator is strongly advised to read foundational training manuals and reference materials regarding file and directory permissions maintenance. Much can be achieved with the basic Unix permissions without having to resort to more complex facilities like POSIX Access Control Lists (ACLs) or Extended Attributes (EAs).

	jht@frodo:~/stuff> ls -la
	total 632
	drwxr-xr-x   13 jht   users      816 2003-05-12 22:56 .
	drwxr-xr-x   37 jht   users     3800 2003-05-12 22:29 ..
	d---------    2 jht   users       48 2003-05-12 22:29 muchado00
	d--x--x--x    2 jht   users       48 2003-05-12 22:29 muchado01
	dr-xr-xr-x    2 jht   users       48 2003-05-12 22:29 muchado02
	drwxrwxrwx    2 jht   users       48 2003-05-12 22:29 muchado03
	drw-rw-rw-    2 jht   users       48 2003-05-12 22:29 muchado04
	d-w--w--w-    2 jht   users       48 2003-05-12 22:29 muchado05
	dr--r--r--    2 jht   users       48 2003-05-12 22:29 muchado06
	drwxrwxrwt    2 jht   users       48 2003-05-12 22:29 muchado07
	drwsrwsrwx    2 jht   users       48 2003-05-12 22:29 muchado08
	----------    1 jht   users     1242 2003-05-12 22:31 mydata00.lst
	---x--x--x    1 jht   users     1674 2003-05-12 22:33 mydata01.lst
	--w--w--w-    1 jht   users     7754 2003-05-12 22:33 mydata02.lst
	--wx-wx-wx    1 jht   users   260179 2003-05-12 22:33 mydata03.lst
	-r--r--r--    1 jht   users    21017 2003-05-12 22:32 mydata04.lst
	-r-xr-xr-x    1 jht   users   206339 2003-05-12 22:32 mydata05.lst
	-rw-rw-rw-    1 jht   users    41105 2003-05-12 22:32 mydata06.lst
	-rwxrwxrwx    1 jht   users    19312 2003-05-12 22:32 mydata07.lst
	jht@frodo:~/stuff>
	

The columns above represent (from left to right): permissions, no blocks used, owner, group, size (bytes), access date, access time, file name.

	 JRV: Put this into a diagram of some sort
	[ type  ] [ users ] [ group ] [ others ]   [File, Directory Permissions]
	[ d | l ] [ r w x ] [ r w x ] [ r w x  ]
	  |   |     | | |     | | |     | | |
	  |   |     | | |     | | |     | | |-----> Can Execute, List files
	  |   |     | | |     | | |     | |-------> Can Write,   Create files
	  |   |     | | |     | | |     |---------> Can Read,    Read files
	  |   |     | | |     | | |---------------> Can Execute, List files
	  |   |     | | |     | |-----------------> Can Write,   Create files
	  |   |     | | |     |-------------------> Can Read,    Read files
	  |   |     | | |-------------------------> Can Execute, List files
	  |   |     | |---------------------------> Can Write,   Create files
	  |   |     |-----------------------------> Can Read,    Read files
	  |   |-----------------------------------> Is a symbolic Link
	  |---------------------------------------> Is a directory
	

		-rwxr-x---   Means: The owner (user) can read, write, execute
		                    the group can read and execute
		                    everyone else can NOT do anything with it
		

Additional possibilities in the [type] field are: c = character device, b = block device, p = pipe device, s = Unix Domain Socket.

The letters `rwxXst' set permissions for the user, group and others as: read (r), write (w), execute (or access for directories) (x), execute only if the file is a directory or already has execute permission for some user (X), set user or group ID on execution (s), sticky (t).

When the sticky bit is set on a directory, files in that directory may be unlinked (deleted) or renamed only by root or their owner. Without the sticky bit, anyone able to write to the directory can delete or rename files. The sticky bit is commonly found on directories, such as /tmp, that are world-writable.

When the set user or group ID bit (s) is set on a directory, then all files created within it will be owned by the user and/or group whose 'set user or group' bit is set. This can be very helpful in setting up directories that for which it is desired that all users who are in a group should be able to write to and read from a file, particularly when it is undesirable for that file to be exclusively owned by a user who's primary group is not the group that all such users belong to.

When a directory is set drw-r----- this means that the owner can read and create (write) files in it, but because the (x) execute flags are not set files can not be listed (seen) in the directory by anyone. The group can read files in the directory but can NOT create new files. NOTE: If files in the directory are set to be readable and writable for the group, then group members will be able to write to (or delete) them.

User and group based controls can prove very useful. In some situations it is distinctly desirable to affect all file system operations as if a single user is doing this, the use of the force user and force group behaviour will achieve this. In other situations it may be necessary to affect a paranoia level of control to ensure that only particular authorised persons will be able to access a share or it's contents, here the use of the valid users or the invalid users may be most useful.

As always, it is highly advisable to use the least difficult to maintain and the least ambiguous method for controlling access. Remember, that when you leave the scene someone else will need to provide assistance and if that person finds too great a mess, or if they do not understand what you have done then there is risk of Samba being removed and an alternative solution being adopted.

The following file and directory permission based controls, if misused, can result in considerable difficulty to diagnose the cause of mis-configuration. Use them sparingly and carefully. By gradually introducing each one by one undesirable side-effects may be detected. In the event of a problem, always comment all of them out and then gradually re-introduce them in a controlled fashion.

The following are documented because of the prevalence of administrators creating inadvertant barriers to file access by not understanding the full implications of smb.conf file settings.

The best tool for the task is platform dependant. Choose the best tool for your environment.

Windows NT clients can 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.

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 Properties entry at the bottom of the menu. This brings up the file properties dialog box. Click on the tab Security 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.

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 descriptive 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 privileged 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 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 Seclib NT security library written by Jeremy Allison of the Samba Team, available from the main Samba ftp site.

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 descriptive 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.

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 (it will give an error message of The remote procedure 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 triplet (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 triplet 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 (displayed as "O" ) highlighted.

Once a user clicks OK to apply the permissions Samba maps the given permissions into a user/group/world r/w/x triplet 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 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 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 force directory mode parameter.

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 file in that share specific section :

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.

“ We are facing some troubles with file / directory permissions. I can log on the domain as admin user(root), and there's a public share, on which everyone needs to have permission to create / modify files, but only root can change the file, no one else can. We need to constantly go to server to chgrp -R users * and chown -R nobody * to allow others users to change the file. ”

There are many ways to solve this problem, here are a few hints:

When you have a user in 'admin users', Samba will always do file operations for this user as root, even if force user has been set.