Chapter 13. File, Directory and Share Access Controls

+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 +administrators 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. +

+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. +

+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. +

+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. +

+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. +

+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 expectations, yet the gap continues to +shrink. +

Features and Benefits

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

Samba Access Control Facilities

+ Unix File and Directory Permissions +
+ Samba honours and implements Unix file system access controls. Users + who access a Samba server will do so as a particular MS Windows user. + This information is passed to the Samba server as part of the logon or + connection setup process. Samba uses this user identity to validate + whether or not the user should be given access to file system resources + (files and directories). This chapter provides an overview for those + to whom the Unix permissions and controls are a little strange or unknown. +
+ Samba Share Definitions +
+ In configuring share settings and controls in the smb.conf file + the network administrator can exercise over-rides to native file + system permissions and behaviours. This can be handy and convenient + to affect behaviour that is more like what MS Windows NT users expect + but it is seldom the best way to achieve this. + The basic options and techniques are described herein. +
+ Samba Share ACLs +
+ Just like it is possible in MS Windows NT to set ACLs on shares + themselves, so it is possible to do this in Samba. + Very few people make use of this facility, yet it remains on of the + easiest ways to affect access controls (restrictions) and can often + do so with minimum invasiveness compared with other methods. +
+ MS Windows ACLs through Unix POSIX ACLs +
+ The use of POSIX ACLs on Unix/Linux is possible ONLY if the underlying + operating system supports them. If not, then this option will not be + available to you. Current Unix technology platforms have native support + for POSIX ACLs. There are patches for the Linux kernel that provide + this also. Sadly, few Linux platforms ship today with native ACLs and + Extended Attributes enabled. This chapter has pertinent information + for users of platforms that support them. +

File System Access Controls

+Perhaps the most important recognition to be made is the simple fact that MS Windows NT4 / 200x / XP +implement a totally divergent file system technology from what is provided in the Unix operating system +environment. Firstly we should consider what the most significant differences are, then we shall look +at how Samba helps to bridge the differences. +

MS Windows NTFS Comparison with Unix File Systems

+ 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. +

File System Feature Comparison

Name Space

+ MS Windows NT4 / 200x/ XP files names may be up to 254 characters long, Unix file names + may be 1023 characters long. In MS Windows file extensions indicate particular file types, + in Unix this is not so rigorously observed as all names are considered arbitrary. +

+ What MS Windows calls a Folder, Unix calls a directory, +

Case Sensitivity

+ MS Windows file names are generally Upper Case if made up of 8.3 (ie: 8 character file name + and 3 character extension. If longer than 8.3 file names are Case Preserving, and Case + Insensitive. +

+ Unix file and directory names are Case Sensitive and Case Preserving. Samba implements the + MS Windows file name behaviour, but it does so as a user application. The Unix file system + provides no mechanism to perform case insensitive file name lookups. MS Windows does this + by default. This means that Samba has to carry the processing overhead to provide features + that are NOT native to the Unix operating system environment. +

+ Consider the following, all are unique Unix names but one single MS Windows file name: + + MYFILE.TXT + MyFile.txt + myfile.txt + + So clearly, In an MS Windows file name space these three files CAN NOT co-exist! But in Unix + they can. So what should Samba do if all three are present? Answer, the one that is lexically + first will be accessible to MS Windows users, the others are invisible and unaccessible - any + other solution would be suicidal. +

Directory Separators

+ MS Windows and DOS uses the back-slash '\' as a directory delimiter, Unix uses the forward-slash '/' + as it's directory delimiter. This is transparently handled by Samba. +

Drive Identification

+ MS Windows products support a notion of drive letters, like C: to represent + disk partitions. Unix has NO concept if separate identifiers for file partitions since each + such file system is mounted to become part of the over-all directory tree. + The Unix directory tree begins at '/', just like the root of a DOS drive is specified like + C:\. +

File Naming Conventions

+ MS Windows generally never experiences file names that begin with a '.', while in Unix these + are commonly found in a user's home directory. Files that begin with a '.' are typically + either start up files for various Unix applications, or they may be files that contain + start-up configuration data. +

Links and Short-Cuts

+ MS Windows make use of "links and Short-Cuts" that are actually special types of files that will + redirect an attempt to execute the file to the real location of the file. Unix knows of file and directory + links, but they are entirely different from what MS Windows users are used to. +

+ Symbolic links are files in Unix that contain the actual location of the data (file OR directory). An + operation (like read or write) will operate directly on the file referenced. Symbolic links are also + referred to as 'soft links'. A hard link is something that MS Windows is NOT familiar with. It allows + one physical file to be known simultaneously by more than one file name. +

+ 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. +

Managing Directories

+ There are three basic operations for managing directories, create, delete, rename. +

Table 13.1. Managing directories with unix and windows

Action	MS Windows Command	Unix Command
create	md folder	mkdir folder
delete	rd folder	rmdir folder
rename	rename oldname newname	mv oldname newname

File and Directory Access Control

+ 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). +

+ Unix/Linux file and directory access permissions involves setting three (3) primary sets of data and one (1) control set. + A Unix file listing looks as follows:- + +

+	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. +

+ The permissions field is made up of: + +

+	 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
+

+ Any bit flag may be unset. An unset bit flag is the equivalent of 'Can NOT' and is represented as a '-' character. + +

Example 13.1. Example File

+		-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. +

Share Definition Access Controls

+The following parameters in the smb.conf file sections that define a share control or affect access controls. +Before using any of the following options please refer to the man page for smb.conf. +

User and Group Based Controls

+ 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. +

Table 13.2. User and Group Based Controls

Control Parameter	Description - Action - Notes
admin users	+ List of users who will be granted administrative privileges on the share. + They will do all file operations as the super-user (root). + Any user in this list will be able to do anything they like on the share, + irrespective of file permissions. +
force group	+ Specifies a UNIX group name that will be assigned as the default primary group + for all users connecting to this service. +
force user	+ Specifies a UNIX user name that will be assigned as the default user for all users connecting to this service. + This is useful for sharing files. Incorrect use can cause security problems. +
guest ok	+ If this parameter is set for a service, then no password is required to connect to the service. Privileges will be + those of the guest account. +
invalid users	+ List of users that should not be allowed to login to this service. +
only user	+ Controls whether connections with usernames not in the user list will be allowed. +
read list	+ List of users that are given read-only access to a service. Users in this list + will not be given write access, no matter what the read only option is set to. +
username	+ Refer to the `smb.conf` man page for more information - this is a complex and potentially misused parameter. +
valid users	+ List of users that should be allowed to login to this service. +
write list	+ List of users that are given read-write access to a service. +

File and Directory Permissions Based Controls

+ 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. +

Table 13.3. File and Directory Permission Based Controls

Control Parameter	Description - Action - Notes
create mask	+ Refer to the `smb.conf` man page. +
directory mask	+ The octal modes used when converting DOS modes to UNIX modes when creating UNIX directories. + See also: directory security mask. +
dos filemode	+ Enabling this parameter allows a user who has write access to the file to modify the permissions on it. +
force create mode	+ This parameter specifies a set of UNIX mode bit permissions that will always be set on a file created by Samba. +
force directory mode	+ This parameter specifies a set of UNIX mode bit permissions that will always be set on a directory created by Samba. +
force directory security mode	+ Controls UNIX permission bits modified when a Windows NT client is manipulating UNIX permissions on a directory +
force security mode	+ Controls UNIX permission bits modified when a Windows NT client manipulates UNIX permissions. +
hide unreadable	+ Prevents clients from seeing the existence of files that cannot be read. +
hide unwriteable files	+ Prevents clients from seeing the existence of files that cannot be written to. Unwriteable directories are shown as usual. +
nt acl support	+ This parameter controls whether smbd will attempt to map UNIX permissions into Windows NT access control lists. +
security mask	+ Controls UNIX permission bits modified when a Windows NT client is manipulating the UNIX permissions on a file. +

Miscellaneous Controls

+ 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. +

Table 13.4. Other Controls

Control Parameter	Description - Action - Notes
case sensitive, default case, short preserve case	+ This means that all file name lookup will be done in a case sensitive manner. + Files will be created with the precise filename Samba received from the MS Windows client. +
csc policy	+ Client Side Caching Policy - parallels MS Windows client side file caching capabilities. +
dont descend	+ Allows to specify a comma-delimited list of directories that the server should always show as empty. +
dos filetime resolution	+ This option is mainly used as a compatibility option for Visual C++ when used against Samba shares. +
dos filetimes	+ DOS and Windows allows users to change file time stamps if they can write to the file. POSIX semantics prevent this. + This options allows DOS and Windows behaviour. +
fake oplocks	+ Oplocks are the way that SMB clients get permission from a server to locally cache file operations. If a server grants an + oplock then the client is free to assume that it is the only one accessing the file and it will aggressively cache file data. +
hide dot files, hide files, veto files	+ Note: MS Windows Explorer allows over-ride of files marked as hidden so they will still be visible. +
read only	+ If this parameter is yes, then users of a service may not create or modify files in the service's directory. +
veto files	+ List of files and directories that are neither visible nor accessible. +

Access Controls on Shares

+ 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 Everyone Full Control (ie: Full control, Change and Read). +

+ 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. +

+ Samba stores the per share access control settings in a file called share_info.tdb. + 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 /usr/local/samba/var. If the tdbdump + utility has been compiled and installed on your system, then you can examine the contents of this file + by: tdbdump share_info.tdb. +

Share Permissions Management

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

Windows NT4 Workstation/Server

+ 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. +

Procedure 13.1. Instructions

+ 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. +
+ 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. +

Windows 200x/XP

+ 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 Everyone Full Control on the Share. +

+ 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 Control Panel -> + Administrative Tools -> Computer Management. +

Procedure 13.2. Instructions

+ 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 privilege this step is not offered. +
+ 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. +
+ 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. +

Warning

+ 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 precedence. 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. +

MS Windows Access Control Lists and Unix Interoperability

Managing UNIX permissions Using NT Security Dialogs

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.

Note

+ 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. +

Viewing File Security on a Samba Share

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.

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 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.

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)"

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 triplet and + the corresponding "read", "write", "execute" permissions + triplets 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 (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.

Interaction with the standard Samba create mask + parameters

There are four parameters + to control interaction with the standard Samba create mask parameters. + 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 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 : +

security mask = 0777

force security mode = 0

directory security mask = 0777

force directory security mode = 0

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.

Common Errors

+File, Directory and Share access problems are very common on the mailing list. The following +are examples taken from the mailing list in recent times. +

Users can not write to a public share

+ “ + 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: +

Procedure 13.3. Example Solution:

+ Go to the top of the directory that is shared +
+ Set the ownership to what ever public owner and group you want +
```
+			find 'directory_name' -type d -exec chown user.group {}\;
+			find 'directory_name' -type d -exec chmod 6775 'directory_name'
+			find 'directory_name' -type f -exec chmod 0775 {} \;
+			find 'directory_name' -type f -exec chown user.group {}\;
+			
```
+
Note
+ The above will set the 'sticky bit' on all directories. Read your + Unix/Linux man page on what that does. It causes the OS to assign + to all files created in the directories the ownership of the + directory. +

+ + Directory is: /foodbar +

+				$ chown jack.engr /foodbar
+

Note

This is the same as doing:

+					$ chown jack /foodbar
+					$ chgrp engr /foodbar
+

Now do: + +

+				$ chmod 6775 /foodbar
+				$ ls -al /foodbar/..
+

+ +

You should see: +

+				drwsrwsr-x  2 jack  engr    48 2003-02-04 09:55 foodbar
+

Now do: +

+				$ su - jill
+				$ cd /foodbar
+				$ touch Afile
+				$ ls -al
+

+ You should see that the file Afile created by Jill will have ownership + and permissions of Jack, as follows: +

+		-rw-r--r--  1 jack  engr     0 2003-02-04 09:57 Afile
+

+ Now in your smb.conf for the share add: +
```
+		force create mode = 0775
+		force directory mode = 6775
+		
```
+
Note
+ The above are only needed if your users are not members of the group + you have used. ie: Within the OS do not have write permission on the directory. +
+ An alternative is to set in the smb.conf entry for the share: +
```
+		force user = jack
+		force group = engr
+		
```
+

I have set force user and Samba still makes root the owner of all the files + I touch!

+ 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. +

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