&author.jht; &author.jeremy; May 10, 2003 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 adminstrators 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 expections, yet the gap continues to shrink. Samba offers a lot of flexibility in file system access management. These are the key access control facilities present in Samba today: 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 orr 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 paltforms ship today with native ACLs and Extended Attributes enabled. This chapter has pertinent information for users of platforms that support them. 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. 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 withing the bounds of default behaviour. Those wishing to explore to depths of control ability should review the &smb.conf; man page. 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 simulataneously 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. There are three basic operations for managing directories, create, delete, rename. Action MS Windows Command Unix Command ------ ------------------ ------------ create md folder mkdir folder delete rd folder rmdir folder rename rename oldname newname mv oldname newname 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 invloves 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: [ 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. -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 posibilities 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),r 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. Explain here about the smb.conf [share] Access Control parameters, Mode and Mask parameters, force user/group, valid/invalid users, etc. 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. The best tool for the task is platform dependant. Choose the best tool for your environmemt. 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. 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. 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. 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 privilidge 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. 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 precidence. 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. 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. 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. 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. 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 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. 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 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 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 (displayed as "O" ) highlighted. 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 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 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 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. Stuff from mailing lists here