Chapter 16. 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 how to provide users with the access they need while protecting resources from unauthorized access.

- - + + Many UNIX administrators are unfamiliar 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 cannot completely hide, even though it does try to bridge the chasm to a degree.

- - - - + + + + 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 Samba was created to provide a means of interoperability and interchange of data between differing operating environments. Samba has no intent to change UNIX/Linux into a platform like MS Windows. Instead the purpose was and 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 much 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 honors 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 @@ -64,7 +64,7 @@ beyond early plans and expectations, yet the gap continues to shrink.
Samba Share Definitions
- + In configuring share settings and controls in the smb.conf file, the network administrator can exercise overrides to native file system permissions and behaviors. This can be handy and convenient @@ -73,20 +73,20 @@ beyond early plans and expectations, yet the gap continues to shrink. The basic options and techniques are described herein.
Samba Share ACLs - +
- + Just as it is possible in MS Windows NT to set ACLs on shares themselves, so it is possible to do in Samba. Few people make use of this facility, yet it remains one 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 @@ -94,16 +94,16 @@ beyond early plans and expectations, yet the gap continues to shrink. this support. 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. First we consider what the most significant differences are, then we look at how Samba helps to bridge the differences. -

MS Windows NTFS Comparison with UNIX File Systems

- - - - +

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 behavior, that differs from UNIX file system behavior then somehow Samba is responsible for emulating @@ -114,7 +114,7 @@ at how Samba helps to bridge the differences. but for the greater part we stay within the bounds of default behavior. Those wishing to explore the depths of control ability should review the smb.conf man page.

The following compares file system features for UNIX with those of MS Windows NT/200x: - +

Name Space

MS Windows NT4/200x/XP file names may be up to 254 characters long, and UNIX file names @@ -123,8 +123,8 @@ at how Samba helps to bridge the differences.

What MS Windows calls a folder, UNIX calls a directory.

Case Sensitivity

- - + + MS Windows file names are generally uppercase if made up of 8.3 (8-character file name and 3 character extension. File names that are longer than 8.3 are case preserving and case insensitive. @@ -151,26 +151,26 @@ at how Samba helps to bridge the differences. event that the UNIX directory contains multiple files that would match a case insensitive file listing.

Directory Separators

- + MS Windows and DOS use the backslash \ as a directory delimiter, and UNIX uses the forward-slash / as its directory delimiter. This is handled transparently by Samba.

Drive Identification

- + MS Windows products support a notion of drive letters, like C:, to represent disk partitions. UNIX has no concept of separate identifiers for file partitions; each such file system is mounted to become part of the overall directory tree. The UNIX directory tree begins at / just as the root of a DOS drive is specified as C:\.

File Naming Conventions

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

Links and Short-Cuts

- - - + + + MS Windows make use of links and shortcuts 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. @@ -183,17 +183,17 @@ at how Samba helps to bridge the differences. 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 and education. -

Managing Directories

- - - +

Managing Directories

+ + + There are three basic operations for managing directories: create, delete, rename. Managing Directories with UNIX and Windows compares the commands in Windows and UNIX that implement these operations. -

Table 16.1. 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

- - - +

Table 16.1. 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 basic UNIX 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 ACLs or extended attributes (EAs). @@ -226,47 +226,47 @@ drwsrwsrwx 2 maryo gnomes 48 2003-05-12 22:29 muchado08

Figure 16.1. Overview of UNIX permissions field.

Any bit flag may be unset. An unset bit flag is the equivalent of "cannot" and is represented as a “-” character (see “Example File”) - - - - - - + + + + + +

Example 16.1. Example File

 -rwxr-x---   Means: 
  ^^^                The owner (user) can read, write, execute
     ^^^             the group can read and execute
        ^^^          everyone else cannot 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 (SUID) or group ID (SGID) 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 helpful in setting up directories 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 @@ -276,11 +276,11 @@ drwsrwsrwx 2 maryo gnomes 48 2003-05-12 22:29 muchado08 the (r) read flags are not set, files cannot be listed (seen) in the directory by anyone. The group can read files in the directory but cannot create new files. 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. -

Protecting Directories and Files from Deletion

- - - - +

Protecting Directories and Files from Deletion

+ + + + People have asked on the Samba mailing list how is it possible to protect files or directories from deletion by users. For example, Windows NT/2K/XP provides the capacity to set access controls on a directory into which people can write files but not delete them. It is possible to set an ACL on a Windows file that permits the file to be written to @@ -288,27 +288,27 @@ drwsrwsrwx 2 maryo gnomes 48 2003-05-12 22:29 muchado08 anyone who has the ability to create a file can write to it. Anyone who has write permission on the directory that contains a file and has write permission for it has the capability to delete it.

- - - + + + For the record, in the UNIX environment the ability to delete a file is controlled by the permissions on the directory that the file is in. In other words, a user can delete a file in a directory to which that user has write access, even if that user does not own the file.

- - - - + + + + Of necessity, Samba is subject to the file system semantics of the host operating system. Samba is therefore limited in the file system capabilities that can be made available through Windows ACLs, and therefore performs a "best fit" translation to POSIX ACLs. Some UNIX file systems do, however support, a feature known as extended attributes. Only the Windows concept of inheritance is implemented by Samba through the appropriate extended attribute.

- - - - + + + + The specific semantics of the extended attributes are not consistent across UNIX and UNIX-like systems such as Linux. For example, it is possible on some implementations of the extended attributes to set a flag that prevents the directory or file from being deleted. The extended attribute that may achieve this is called the immutible bit. @@ -322,7 +322,7 @@ CAP_LINUX_IMMUTABLE capability can set or clear this attribute.

A simple test can be done to check if the immutible flag is supported on files in the file system of the Samba host server. -

Procedure 16.1. Test for File Immutibility Support

+
Procedure 16.1. Test for File Immutibility Support
1. Create a file called filename.
2. Login as the root user, then set the immutibile flag on a test file as follows: @@ -340,11 +340,11 @@ mystic:/home/hannibal > rm filename that cannot be deleted. Check the man page on your particular host system to determine whether or not immutable directories are writable. If they are not, then the entire directory and its contents will effectively be protected from writing (file creation also) and deletion. -

Share Definition Access Controls

- +

Share Definition Access Controls

+ The following parameters in the smb.conf file sections 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 quite useful. In some situations it is distinctly desirable to force all file system operations as if a single user were doing so. The use of the force user and force group behavior will achieve this. @@ -385,7 +385,7 @@ mystic:/home/hannibal > rm filename 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

Directory permission-based controls, if misused, can result in considerable difficulty in diagnosing the causes of misconfiguration. Use them sparingly and carefully. By gradually introducing each, one at a time, undesirable side effects may be detected. In the event of a problem, always comment all of them out and then gradually reintroduce @@ -416,7 +416,7 @@ mystic:/home/hannibal > rm filename This parameter controls whether smbd will attempt to map UNIX permissions into Windows NT ACLs.

security mask

Controls UNIX permission bits modified when a Windows NT client is manipulating the UNIX permissions on a file. -

Miscellaneous Controls

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 an 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 (full control, change and read).

- - - + + + At this time Samba does not provide a tool for configuring access control settings on the share itself the only way to create those settings is to use either the NT4 Server Manager or the Windows 200x Microsoft Management Console (MMC) for Computer Management. There are currently no plans to provide this capability in the Samba command-line tool set.

- - - - + + + + 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 executing tdbdump share_info.tdb in the directory containing the tdb files. -

Share Permissions Management

The best tool for share permissions management is platform-dependent. Choose the best tool for your environment. -

Windows NT4 Workstation/Server

- - - - +

Windows NT4 Workstation/Server

+ + + + The tool you need to manage share permissions on a Samba server from a Windows NT4 Workstation or 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 the Microsoft web site support section. -

Procedure 16.2. Instructions

+
Procedure 16.2. Instructions
1. Launch the NT4 Server Manager and click on the Samba server you want to administer. From the menu select Computer, then click on Shared Directories.
2. Click on the share that you wish to manage and click the Properties tab, then click the Permissions tab. Now you can add or change access control settings as you wish. -

Windows 200x/XP

- - - - +

Windows 200x/XP

+ + + + On MS Windows NT4/200x/XP systems, ACLs on the share itself are set using tools like the MS Explorer. For example, in Windows 200x, right-click on the shared folder, then select Sharing, then click on Permissions. The default Windows NT4/200x permissions allow the group "Everyone" full control on the share.

- - - + + + MS Windows 200x and later versions come with a tool called the Computer Management snap-in for the MMC. This tool can be accessed via Control Panel -> Administrative Tools -> Computer Management. -

Procedure 16.3. Instructions

+
Procedure 16.3. Instructions
1. After launching the MMC with the Computer Management snap-in, click the menu item Action and 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. @@ -523,7 +523,7 @@ mystic:/home/hannibal > rm filename System Tools, then on the [+] next to Shared Folders in the left panel.
2. - + In the right panel, double-click on the share on which you wish to set access control permissions. Then click the tab Share Permissions. It is now possible to add access control entities to the shared folder. Remember to set what type of access (full control, change, read) you @@ -534,8 +534,8 @@ mystic:/home/hannibal > rm filename ACL precedence. Everyone with no access means that MaryK who is part of the group Everyone will have no access even if she is given explicit full control access. -

MS Windows Access Control Lists and UNIX Interoperability

Managing UNIX Permissions Using NT Security Dialogs

- +

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.

@@ -549,7 +549,7 @@ mystic:/home/hannibal > rm filename When trying to figure out file access problems, it is vitally important to find 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

`Interaction with the Standard Samba File Attribute Mapping`

`Note`

+

security mask = 0777

force security mode = 0

directory security mask = 0777

force directory security mode = 0

`Interaction with the Standard Samba File Attribute Mapping`

`Note`

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 @@ -740,7 +740,7 @@ mystic:/home/hannibal > rm filename attributes dialog, you should always press Cancel rather than OK to ensure that your changes are not overridden. -

`Windows NT/200X ACLs and POSIX ACLs Limitations`

+

`Windows NT/200X ACLs and POSIX ACLs Limitations`

Windows administrators are familiar with simple ACL controls, and they typically consider that UNIX user/group/other (ugo) permissions are inadequate and not sufficiently fine-grained. @@ -768,7 +768,7 @@ mystic:/home/hannibal > rm filename ACLs as implemented in UNIX file systems. Samba provides support for masks that permit normal ugo and ACLs functionality to be overrided. This further complicates the way in which Windows ACLs must be implemented. -

`UNIX POSIX ACL Overview`

+

`UNIX POSIX ACL Overview`

In examining POSIX ACLs we must consider the manner in which they operate for both files and directories. File ACLs have the following significance:

@@ -797,7 +797,7 @@ default:group::r-x    <-- inherited group perms
 default:mask:rwx      <-- inherited default mask
 default:other:---     <-- inherited permissions for everyone (other)

-

`Mapping of Windows File ACLs to UNIX POSIX ACLs`

+

`Mapping of Windows File ACLs to UNIX POSIX ACLs`

Microsoft Windows NT4/200X ACLs must of necessity be mapped to POSIX ACLs. The mappings for file permissions are shown in How Windows File ACLs Map to UNIX POSIX File ACLs. @@ -816,7 +816,7 @@ default:other:--- <-- inherited permissions for everyone (other) The UNIX administrator can set any directory permission from within the UNIX environment. The Windows administrator is more restricted in that it is not possible from within Windows Explorer to remove read permission for the file owner. -

`Mapping of Windows Directory ACLs to UNIX POSIX ACLs`

+

`Mapping of Windows Directory ACLs to UNIX POSIX ACLs`

Interesting things happen in the mapping of UNIX POSIX directory permissions and UNIX POSIX ACLs to Windows ACEs (Access Control Entries, the discrete components of an ACL) are mapped to Windows directory ACLs. @@ -824,10 +824,10 @@ default:other:--- <-- inherited permissions for everyone (other) Directory permissions function in much the same way as shown for file permissions, but there are some notable exceptions and a few peculiarities that the astute administrator will want to take into account in the setting up of directory permissions. -

`Common Errors`

+