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diff --git a/docs/howto/AccessControls.xml b/docs/howto/AccessControls.xml new file mode 100644 index 0000000000..13228b7ff0 --- /dev/null +++ b/docs/howto/AccessControls.xml @@ -0,0 +1,1309 @@ +<chapter id="AccessControls"> +<chapterinfo> + &author.jht; + &author.jeremy; + <author>&person.jelmer;<contrib>drawing</contrib></author> + <pubdate>May 10, 2003</pubdate> +</chapterinfo> +<title>File, Directory and Share Access Controls</title> + +<para> +<indexterm><primary>ACLs</primary></indexterm> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +<indexterm><primary>Extended Attributes</primary></indexterm> +<indexterm><primary>ACLs</primary><secondary>POSIX</secondary></indexterm> + +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<sect1> +<title>Features and Benefits</title> + + <para> + Samba offers a lot of flexibility in file system access management. These are the key access control + facilities present in Samba today: + </para> + + <itemizedlist> + <title>Samba Access Control Facilities</title> + <listitem><para> + <indexterm><primary>permissions</primary><secondary>UNIX file and directory</secondary></indexterm> + <emphasis>UNIX File and Directory Permissions</emphasis> + </para> + + <para> + 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 + 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. + </para> + </listitem> + + <listitem><para> + <emphasis>Samba Share Definitions</emphasis> + </para> + + <para> + 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 + to effect behavior that is more like what MS Windows NT users expect + but it is seldom the <emphasis>best</emphasis> way to achieve this. + The basic options and techniques are described herein. + </para> + </listitem> + + <listitem><para> + <emphasis>Samba Share ACLs</emphasis> + <indexterm><primary>ACLs</primary><secondary>share</secondary></indexterm> + </para> + + <para> + Just like it is possible in MS Windows NT to set ACLs on shares + themselves, so it is possible to do this 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. + </para> + </listitem> + + <listitem><para> + <indexterm><primary>ACLs</primary><secondary>POSIX</secondary></indexterm> + <indexterm><primary>ACLs</primary><secondary>Windows</secondary></indexterm> + <emphasis>MS Windows ACLs through UNIX POSIX ACLs</emphasis> + </para> + + <para> + 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 also provide + this. 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. + </para> + </listitem> + </itemizedlist> + +</sect1> + +<sect1> +<title>File System Access Controls</title> + +<para> +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. +</para> + + <sect2> + <title>MS Windows NTFS Comparison with UNIX File Systems</title> + + <para> +<indexterm><primary>NTFS</primary></indexterm> +<indexterm><primary>File System</primary></indexterm> +<indexterm><primary>File System</primary><secondary>UNIX</secondary></indexterm> +<indexterm><primary>File System</primary><secondary>Windows</secondary></indexterm> + + 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 + that in a transparent and consistent manner. + </para> + + <para> + It is good news that Samba does this to a large extent and on top of that provides a high degree + of optional configuration to override the default behavior. We look at some of these over-rides, + but for the greater part we will stay within the bounds of default behavior. Those wishing to explore + the depths of control ability should review the &smb.conf; man page. + </para> + + <para>The following compares file system features for UNIX with those of Microsoft Windows NT/200x: + <indexterm><primary>File System</primary><secondary>feature comparison</secondary></indexterm> + + </para> + + <variablelist> + <varlistentry> + <term>Name Space</term> + <listitem> + <para> + MS Windows NT4/200x/XP files names may be up to 254 characters long, and 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. + </para> + <para> + What MS Windows calls a folder, UNIX calls a directory. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Case Sensitivity</term> + <listitem> + <para> + <indexterm><primary>8.3 file names</primary></indexterm> + <indexterm><primary>File System</primary><secondary>case sensitivity</secondary></indexterm> + MS Windows file names are generally upper case 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. + </para> + + <para> + UNIX file and directory names are case sensitive and case preserving. Samba implements the + MS Windows file name behavior, 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. + </para> + <para> + Consider the following. All are unique UNIX names but one single MS Windows file name: + <screen> + MYFILE.TXT + MyFile.txt + myfile.txt + </screen></para> + + <para> + So clearly, in an MS Windows file name space these three files cannot co-exist, but in UNIX + they can. + </para> + <para> + So what should Samba do if all three are present? That which is lexically first will be + accessible to MS Windows users, the others are invisible and unaccessible &smbmdash; any + other solution would be suicidal. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Directory Separators</term> + <listitem> + + <para> + <indexterm><primary>Directory Separators</primary></indexterm> + MS Windows and DOS uses the backslash <constant>\</constant> as a directory delimiter, and UNIX uses + the forward-slash <constant>/</constant> as its directory delimiter. This is handled transparently by Samba. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Drive Identification</term> + <listitem> + <para> + <indexterm><primary>Drive Identification</primary></indexterm> + MS Windows products support a notion of drive letters, like <command>C:</command> 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 <constant>/</constant> just like the root of a DOS drive is specified as + <constant>C:\</constant>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>File Naming Conventions</term> + <listitem> + <para> + <indexterm><primary>File Naming Conventions</primary></indexterm> + MS Windows generally never experiences file names that begin with a dot (<constant>.</constant>) while in UNIX these + are commonly found in a user's home directory. Files that begin with a dot (<constant>.</constant>) are typically + either start-up files for various UNIX applications, or they may be files that contain + start-up configuration data. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Links and Short-Cuts</term> + <listitem> + <para> + <indexterm><primary>Links</primary><secondary>hard</secondary></indexterm> + <indexterm><primary>Links</primary><secondary>soft</secondary></indexterm> + <indexterm><primary>Short-Cuts</primary></indexterm> + MS Windows make use of <quote>links and short-cuts</quote> 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. + </para> + <para> + 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 <quote>soft links.</quote> 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. + </para> + </listitem> + </varlistentry> + </variablelist> + + <para> + 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. + </para> + + </sect2> + + <sect2> + <title>Managing Directories</title> + + <para> + There are three basic operations for managing directories: <command>create, delete, rename</command>. + <table frame="all"> + <title>Managing Directories with UNIX and Windows</title> + <tgroup align="center" cols="3"> + <thead> + <row><entry>Action</entry><entry>MS Windows Command</entry><entry>UNIX Command</entry></row> + </thead> + + <tbody> + <row><entry>create</entry><entry>md folder</entry><entry>mkdir folder</entry></row> + <row><entry>delete</entry><entry>rd folder</entry><entry>rmdir folder</entry></row> + <row><entry>rename</entry><entry>rename oldname newname</entry><entry>mv oldname newname</entry></row> + </tbody> + </tgroup> + </table> + </para> + + </sect2> + + <sect2> + <title>File and Directory Access Control</title> + + + <para> + <indexterm><primary>ACLs</primary><secondary>File System</secondary></indexterm> + 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). + </para> + + <para> + UNIX/Linux file and directory access permissions involves setting three primary sets of data and one control set. + A UNIX file listing looks as follows: +<screen> +&prompt;<userinput>ls -la</userinput> +total 632 +drwxr-xr-x 13 maryo gnomes 816 2003-05-12 22:56 . +drwxrwxr-x 37 maryo gnomes 3800 2003-05-12 22:29 .. +dr-xr-xr-x 2 maryo gnomes 48 2003-05-12 22:29 muchado02 +drwxrwxrwx 2 maryo gnomes 48 2003-05-12 22:29 muchado03 +drw-rw-rw- 2 maryo gnomes 48 2003-05-12 22:29 muchado04 +d-w--w--w- 2 maryo gnomes 48 2003-05-12 22:29 muchado05 +dr--r--r-- 2 maryo gnomes 48 2003-05-12 22:29 muchado06 +drwsrwsrwx 2 maryo gnomes 48 2003-05-12 22:29 muchado08 +---------- 1 maryo gnomes 1242 2003-05-12 22:31 mydata00.lst +--w--w--w- 1 maryo gnomes 7754 2003-05-12 22:33 mydata02.lst +-r--r--r-- 1 maryo gnomes 21017 2003-05-12 22:32 mydata04.lst +-rw-rw-rw- 1 maryo gnomes 41105 2003-05-12 22:32 mydata06.lst +&prompt; +</screen> + </para> + + <para> + The columns above represent (from left to right): permissions, number of hard links to file, owner, group, size (bytes), access date, access time, file name. + </para> + + <para> + An overview of the permissions field can be found in <link linkend="access1">Overview of UNIX permissions field</link>. + </para> + + <image id="access1" scale="40"><imagedescription>Overview of UNIX permissions field.</imagedescription><imagefile>access1</imagefile></image> + + <para> + Any bit flag may be unset. An unset bit flag is the equivalent of <quote>cannot</quote> and is represented as a <quote>-</quote> character. + + <example> + <title>Example File</title> + <programlisting> + -rwxr-x--- Means: The owner (user) can read, write, execute + the group can read and execute + everyone else cannot do anything with it. + </programlisting> + </example> + + </para> + + <para> + Additional possibilities in the [type] field are: c = character device, b = block device, p = pipe device, s = UNIX Domain Socket. + </para> + + <para> + The letters <constant>rwxXst</constant> 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). + </para> + + <para> + 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 <filename>/tmp</filename>, that are world-writable. + </para> + + <para> + 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 + to be exclusively owned by a user whose primary group is not the group that all such users belong to. + </para> + + <para> + When a directory is set <constant>d-wx--x---</constant> this means that the owner can read and create (write) files in it, but because + 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. + </para> + + </sect2> + +</sect1> + +<sect1> +<title>Share Definition Access Controls</title> + + +<para> +<indexterm><primary>permissions</primary><secondary>share</secondary></indexterm> +The following parameters in the &smb.conf; file sections define a share control or effect access controls. +Before using any of the following options, please refer to the man page for &smb.conf;. +</para> + + <sect2> + <title>User and Group-Based Controls</title> + + <para> + User and group-based controls can prove quite useful. In some situations it is distinctly desirable to affect all + file system operations as if a single user were doing so. The use of the <smbconfoption><name>force user</name></smbconfoption> and + <smbconfoption><name>force group</name></smbconfoption> behavior will achieve this. In other situations it may be necessary to effect a + paranoia level of control to ensure that only particular authorized persons will be able to access a share or + its contents. Here the use of the <smbconfoption><name>valid users</name></smbconfoption> or the + <smbconfoption><name>invalid users</name></smbconfoption> may be most useful. + </para> + + <para> + As always, it is highly advisable to use the least difficult to maintain and the least ambiguous method for + controlling access. Remember, when you leave the scene someone else will need to provide assistance and + if he finds too great a mess or does not understand what you have done, there is risk of + Samba being removed and an alternative solution being adopted. + </para> + + <para> + <link linkend="ugbc">Following table</link> enumerates these controls. + </para> + + <table frame='all' pgwide='0' id="ugbc"><title>User and Group Based Controls</title> + <tgroup cols='2'> + <colspec align="left"/> + <colspec align="justify" colwidth="1*"/> + <thead> + <row> + <entry align="center">Control Parameter</entry> + <entry align="center">Description - Action - Notes</entry> + </row> + </thead> + <tbody> + <row> + <entry><smbconfoption><name>admin users</name></smbconfoption></entry> + <entry><para> + 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. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force group</name></smbconfoption></entry> + <entry><para> + Specifies a UNIX group name that will be assigned as the default primary group + for all users connecting to this service. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force user</name></smbconfoption></entry> + <entry><para> + 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. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>guest ok</name></smbconfoption></entry> + <entry><para> + 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. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>invalid users</name></smbconfoption></entry> + <entry><para> + List of users that should not be allowed to login to this service. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>only user</name></smbconfoption></entry> + <entry><para> + Controls whether connections with usernames not in the user list will be allowed. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>read list</name></smbconfoption></entry> + <entry><para> + 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. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>username</name></smbconfoption></entry> + <entry><para> + Refer to the &smb.conf; man page for more information -- this is a complex and potentially misused parameter. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>valid users</name></smbconfoption></entry> + <entry><para> + List of users that should be allowed to login to this service. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>write list</name></smbconfoption></entry> + <entry><para> + List of users that are given read-write access to a service. + </para></entry> + </row> + </tbody> + </tgroup> + </table> + + </sect2> + + <sect2> + <title>File and Directory Permissions-Based Controls</title> + + <para> + The following file and directory permission-based controls, if misused, can result in considerable difficulty to + diagnose causes of misconfiguration. 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 + reintroduce them in a controlled way. + </para> + + <para> + Refer to <link linkend="fdpbc">the following table</link> for information regarding the parameters that may be used to affect file and + directory permission-based access controls. + </para> + + <table frame='all' id="fdpbc"><title>File and Directory Permission Based Controls</title> + <tgroup cols='2'> + <colspec align="left"/> + <colspec align="justify" colwidth="1*"/> + <thead> + <row> + <entry align="center">Control Parameter</entry> + <entry align="center">Description - Action - Notes</entry> + </row> + </thead> + <tbody> + <row> + <entry><smbconfoption><name>create mask</name></smbconfoption></entry> + <entry><para> + Refer to the &smb.conf; man page. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>directory mask</name></smbconfoption></entry> + <entry><para> + The octal modes used when converting DOS modes to UNIX modes when creating UNIX directories. + See also: directory security mask. + </para></entry></row> + <row> + <entry><smbconfoption><name>dos filemode</name></smbconfoption></entry> + <entry><para> + Enabling this parameter allows a user who has write access to the file to modify the permissions on it. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force create mode</name></smbconfoption></entry> + <entry><para> + This parameter specifies a set of UNIX mode bit permissions that will always be set on a file created by Samba. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force directory mode</name></smbconfoption></entry> + <entry><para> + This parameter specifies a set of UNIX mode bit permissions that will always be set on a directory created by Samba. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force directory security mode</name></smbconfoption></entry> + <entry><para> + Controls UNIX permission bits modified when a Windows NT client is manipulating UNIX permissions on a directory. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>force security mode</name></smbconfoption></entry> + <entry><para> + Controls UNIX permission bits modified when a Windows NT client manipulates UNIX permissions. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>hide unreadable</name></smbconfoption></entry> + <entry><para> + Prevents clients from seeing the existence of files that cannot be read. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>hide unwriteable files</name></smbconfoption></entry> + <entry><para> + Prevents clients from seeing the existence of files that cannot be written to. Unwriteable directories are shown as usual. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>nt acl support</name></smbconfoption></entry> + <entry><para> + This parameter controls whether smbd will attempt to map UNIX permissions into Windows NT access control lists. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>security mask</name></smbconfoption></entry> + <entry><para> + Controls UNIX permission bits modified when a Windows NT client is manipulating the UNIX permissions on a file. + </para></entry> + </row> + </tbody> + </tgroup> + </table> + + </sect2> + + <sect2> + <title>Miscellaneous Controls</title> + + <para> + The following are documented because of the prevalence of administrators creating inadvertent barriers to file + access by not understanding the full implications of &smb.conf; file settings. See <link linkend="mcoc">following table</link>. + </para> + + <table frame='all' id="mcoc"><title>Other Controls</title> + <tgroup cols='2'> + <colspec align="justify" colwidth="1*"/> + <colspec align="justify" colwidth="1*"/> + <thead> + <row> + <entry align="center">Control Parameter</entry> + <entry align="center">Description - Action - Notes</entry> + </row> + </thead> + <tbody> + <row> + <entry><smbconfoption><name>case sensitive</name></smbconfoption>, <smbconfoption><name>default case</name></smbconfoption>, <smbconfoption><name>short preserve case</name></smbconfoption></entry> + <entry><para> + This means that all file name lookup will be done in a case sensitive manner. + Files will be created with the precise file name Samba received from the MS Windows client. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>csc policy</name></smbconfoption></entry> + <entry><para> + Client Side Caching Policy - parallels MS Windows client side file caching capabilities. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>dont descend</name></smbconfoption></entry> + <entry><para> + Allows specifying a comma-delimited list of directories that the server should always show as empty. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>dos filetime resolution</name></smbconfoption></entry> + <entry><para> + This option is mainly used as a compatibility option for Visual C++ when used against Samba shares. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>dos filetimes</name></smbconfoption></entry> + <entry><para> + DOS and Windows allow users to change file time stamps if they can write to the file. POSIX semantics prevent this. + This option allows DOS and Windows behavior. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>fake oplocks</name></smbconfoption></entry> + <entry><para> + Oplocks are the way that SMB clients get permission from a server to locally cache file operations. If a server grants an + oplock, the client is free to assume that it is the only one accessing the file and it will aggressively cache file data. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>hide dot files</name></smbconfoption>, <smbconfoption><name>hide files</name></smbconfoption>, <smbconfoption><name>veto files</name></smbconfoption></entry> + <entry><para> + Note: MS Windows Explorer allows over-ride of files marked as hidden so they will still be visible. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>read only</name></smbconfoption></entry> + <entry><para> + If this parameter is yes, then users of a service may not create or modify files in the service's directory. + </para></entry> + </row> + <row> + <entry><smbconfoption><name>veto files</name></smbconfoption></entry> + <entry><para> + List of files and directories that are neither visible nor accessible. + </para></entry> + </row> + </tbody> + </tgroup> + </table> + + </sect2> + +</sect1> + +<sect1> +<title>Access Controls on Shares</title> + + + <para> +<indexterm><primary>permissions</primary><secondary>share ACLs</secondary></indexterm> + 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 <constant>Everyone - Full Control</constant> (full control, change and read). + </para> + + <para> + 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. + </para> + + <para> + Samba stores the per share access control settings in a file called <filename>share_info.tdb</filename>. + 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 <filename>/usr/local/samba/var</filename>. If the <filename>tdbdump</filename> + utility has been compiled and installed on your system, then you can examine the contents of this file + by executing: <command>tdbdump share_info.tdb</command> in the directory containing the tdb files. + </para> + + <sect2> + <title>Share Permissions Management</title> + + <para> + The best tool for the task is platform dependant. Choose the best tool for your environment. + </para> + + <sect3> + <title>Windows NT4 Workstation/Server</title> + <para> + 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 &smbmdash; see details below. + </para> + + <?latex \newpage ?> + + <procedure> + <title>Instructions</title> + <step><para> + Launch the <application>NT4 Server Manager</application>, click on the Samba server you want to administer. From the menu + select <guimenu>Computer</guimenu>, then click on <guimenuitem>Shared Directories</guimenuitem>. + </para></step> + + <step><para> + Click on the share that you wish to manage, then click the <guilabel>Properties</guilabel> tab. then click + the <guilabel>Permissions</guilabel> tab. Now you can add or change access control settings as you wish. + </para></step> + </procedure> + + </sect3> + + <sect3> + <title>Windows 200x/XP</title> + + <para> + On <application>MS Windows NT4/200x/XP</application> system access control lists on the share itself are set using native + tools, usually from File Manager. For example, in Windows 200x, right click on the shared folder, + then select <guimenuitem>Sharing</guimenuitem>, then click on <guilabel>Permissions</guilabel>. The default + Windows NT4/200x permission allows <quote>Everyone</quote> full control on the share. + </para> + + <para> + MS Windows 200x and later versions come with a tool called the <application>Computer Management</application> snap-in for the + Microsoft Management Console (MMC). This tool is located by clicking on <guimenu>Control Panel -> + Administrative Tools -> Computer Management</guimenu>. + </para> + + <procedure> + <title>Instructions</title> + <step><para> + After launching the MMC with the Computer Management snap-in, click the menu item <guimenuitem>Action</guimenuitem>, + and select <guilabel>Connect to another computer</guilabel>. 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 are already logged in with administrative privilege, this step is not offered. + </para></step> + + <step><para> + If the Samba server is not shown in the <guilabel>Select Computer</guilabel> box, type in the name of the target + Samba server in the field <guilabel>Name:</guilabel>. Now click the on <guibutton>[+]</guibutton> next to + <guilabel>System Tools</guilabel>, then on the <guibutton>[+]</guibutton> next to <guilabel>Shared Folders</guilabel> in the + left panel. + </para></step> + + <step><para> + In the right panel, double-click on the share on which you wish to set access control permissions. + Then click the tab <guilabel>Share Permissions</guilabel>. 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 + wish to assign for each entry. + </para></step> + </procedure> + + <warning> + <para> + Be careful. If you take away all permissions from the <constant>Everyone</constant> user without removing this user, + effectively no user will be able to access the share. This is a result of what is known as + ACL precedence. Everyone with <emphasis>no access</emphasis> means that <constant>MaryK</constant> who is part of the group + <constant>Everyone</constant> will have no access even if she is given explicit full control access. + </para> + </warning> + + </sect3> + </sect2> + +</sect1> + +<sect1> +<title>MS Windows Access Control Lists and UNIX Interoperability</title> + + <sect2> + <title>Managing UNIX Permissions Using NT Security Dialogs</title> + + + <para> +<indexterm><primary>permissions</primary><secondary>file/directory ACLs</secondary></indexterm> + Windows NT clients can use their native security settings dialog box to view and modify the + underlying UNIX permissions. + </para> + + <para> + This ability is careful not to compromise the security of the UNIX host on which Samba is running, and + still obeys all the file permission rules that a Samba administrator can set. + </para> + + <para> + Samba does not attempt to go beyond POSIX ACLs, so the various finer-grained access control + options provided in Windows are actually ignored. + </para> + + <note> + <para> + All access to UNIX/Linux system files via Samba is controlled by the operating system file access controls. + 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. + </para> + </note> + </sect2> + + <sect2> + <title>Viewing File Security on a Samba Share</title> + + <para> + From an NT4/2000/XP client, right click on any file or directory in a Samba-mounted drive letter + or UNC path. When the menu pops up, click on the <guilabel>Properties</guilabel> entry at the bottom + of the menu. This brings up the file <constant>Properties</constant> dialog box. Click on the + <guilabel>Security</guilabel> tab and you will see three buttons: <guibutton>Permissions</guibutton>, + <guibutton>Auditing</guibutton>, and <guibutton>Ownership</guibutton>. The <guibutton>Auditing</guibutton> + button will cause either an error message <errorname>`A requested privilege is not held by the client'</errorname> + 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 <guibutton>Add</guibutton> + button, will not currently allow a list of users to be seen. + </para> + + </sect2> + + <sect2> + <title>Viewing File Ownership</title> + + <para> + Clicking on the <guibutton>Ownership</guibutton> button brings up a dialog box telling you who owns + the given file. The owner name will be displayed like this: + </para> + + <para> + <command><quote>SERVER\user (Long name)</quote></command> + </para> + + <para> + <replaceable>SERVER</replaceable> is the NetBIOS name of the Samba server, <replaceable>user</replaceable> + is the user name of the UNIX user who owns the file, and <replaceable>(Long name)</replaceable> is the + descriptive string identifying the user (normally found in the GECOS field of the UNIX password database). + Click on the <guibutton>Close </guibutton> button to remove this dialog. + </para> + + <para> + If the parameter <smbconfoption><name>nt acl support</name></smbconfoption> is set to <constant>false</constant>, + the file owner will be shown as the NT user <emphasis>Everyone</emphasis>. + </para> + + <para> + The <guibutton>Take Ownership</guibutton> button will not allow you to change the ownership of this file to + yourself (clicking 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 <emphasis>root</emphasis> 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.</para> + + <para> + There is an NT <command>chown</command> 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 <application>Seclib</application> NT security library written + by Jeremy Allison of the Samba Team, and is available from the main Samba FTP site.</para> + + </sect2> + + <sect2> + <title>Viewing File or Directory Permissions</title> + + <para> + The third button is the <guibutton>Permissions</guibutton> 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 like this: + </para> + + <para><command><replaceable>SERVER</replaceable>\ + <replaceable>user</replaceable> + <replaceable>(Long name)</replaceable></command></para> + + <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of the Samba server, + <replaceable>user</replaceable> is the user name of the UNIX user who owns the file, and + <replaceable>(Long name)</replaceable> is the descriptive string identifying the user (normally found in the + GECOS field of the UNIX password database).</para> + + <para> + If the parameter <smbconfoption><name>nt acl support</name></smbconfoption> is set to <constant>false</constant>, + the file owner will be shown as the NT user <constant>Everyone</constant> and the permissions will be + shown as NT <quote>Full Control</quote>. + </para> + + + <para> + The permissions field is displayed differently for files and directories, so I'll describe the way file permissions + are displayed first. + </para> + + <sect3> + <title>File Permissions</title> + + <para>The standard UNIX user/group/world triplet and the corresponding <constant>read, write, execute</constant> permissions + triplets are mapped by Samba into a three element NT ACL with the <quote>r</quote>, <quote>w</quote> and <quote>x</quote> bits mapped into the corresponding + NT permissions. The UNIX world permissions are mapped into the global NT group <constant>Everyone</constant>, followed + by the list of permissions allowed for UNIX world. The UNIX owner and group permissions are displayed as an NT + <guiicon>user</guiicon> icon and an NT <guiicon>local group</guiicon> icon, respectively, followed by the list + of permissions allowed for the UNIX user and group.</para> + + <para>Because many UNIX permission sets do not map into common NT names such as <constant>read</constant>, + <constant>change</constant> or <constant>full control</constant>, usually the permissions will be prefixed + by the words <constant>Special Access</constant> in the NT display list.</para> + + <para>But what happens if the file has no permissions allowed for a particular UNIX user group or world component? In order + to allow <quote>no permissions</quote> to be seen and modified Samba then overloads the NT <constant>Take Ownership</constant> ACL attribute + (which has no meaning in UNIX) and reports a component with no permissions as having the NT <command>O</command> bit set. + This was chosen, of course, to make it look like a zero, meaning zero permissions. More details on the decision behind this is + given below.</para> + </sect3> + + <sect3> + <title>Directory Permissions</title> + + <para>Directories on an NT NTFS file system have two different sets of permissions. The first set is the ACL set on the + directory itself, which is usually displayed in the first set of parentheses in the normal <constant>RW</constant> + 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.</para> + + <para>The second set of directory permissions has no real meaning in the UNIX permissions world and represents the <constant> + inherited</constant> permissions that any file created within this directory would inherit.</para> + + <para>Samba synthesizes 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.</para> + </sect3> + </sect2> + + <sect2> + <title>Modifying File or Directory Permissions</title> + + <para>Modifying file and directory permissions is as simple + as changing the displayed permissions in the dialog box, and + clicking on <guibutton>OK</guibutton>. 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.</para> + + <para>If the parameter <smbconfoption><name>nt acl support</name></smbconfoption> + is set to <constant>false</constant>, any attempt to set + security permissions will fail with an <errorname>`Access Denied' + </errorname> message.</para> + + <para>The first thing to note is that the <guibutton>Add</guibutton> + button will not return a list of users in Samba (it will give + an error message saying <errorname>`The remote procedure call failed + and did not execute'</errorname>). 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.</para> + + <para>If a permission triplet (either user, group, or world) + is removed from the list of permissions in the NT dialog box, + then when the <guibutton>OK</guibutton> button is pressed it will + be applied as <quote>no permissions</quote> on the UNIX side. If you then + view the permissions again, the <quote>no permissions</quote> entry will appear + as the NT <command>O</command> 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.</para> + + <para>As UNIX supports only the <quote>r</quote>, <quote>w</quote> and <quote>x</quote> bits of + an NT ACL, if other NT security attributes such as <constant>Delete Access</constant> are + selected they will be ignored when applied on the Samba server.</para> + + <para>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 un-check the <guilabel>Replace + permissions on existing files</guilabel> check-box in the NT + dialog before clicking on <guibutton>OK</guibutton>.</para> + + <para>If you wish to remove all permissions from a + user/group/world component, you may either highlight the + component and click on the <guibutton>Remove</guibutton> button, + or set the component to only have the special <constant>Take + Ownership</constant> permission (displayed as <command>O + </command>) highlighted.</para> + </sect2> + + <sect2> + <title>Interaction with the Standard Samba <quote>create mask</quote> Parameters</title> + + <para>There are four parameters that control interaction with the standard Samba <parameter>create mask</parameter> parameters. + These are: + + <itemizedlist> + <listitem><smbconfoption><name>security mask</name></smbconfoption></listitem> + <listitem><smbconfoption><name>force security mode</name></smbconfoption></listitem> + <listitem><smbconfoption><name>directory security mask</name></smbconfoption></listitem> + <listitem><smbconfoption><name>force directory security mode</name></smbconfoption></listitem> + </itemizedlist> + + </para> + + <para>Once a user clicks on <guibutton>OK</guibutton> to apply the + permissions, Samba maps the given permissions into a user/group/world + r/w/x triplet set, and then checks the changed permissions for a + file against the bits set in the + <smbconfoption><name>security mask</name></smbconfoption> parameter. Any bits that + were changed that are not set to <quote>1</quote> in this parameter are left alone + in the file permissions.</para> + + <para>Essentially, zero bits in the <smbconfoption><name>security mask</name></smbconfoption> + may be treated as a set of bits the user is <emphasis>not</emphasis> + allowed to change, and one bits are those the user is allowed to change. + </para> + + <para>If not explicitly set, this parameter defaults to the same value as + the <smbconfoption><name>create mask</name></smbconfoption> parameter. To allow a user to modify all the + user/group/world permissions on a file, set this parameter to 0777. + </para> + + <para>Next Samba checks the changed permissions for a file against the bits set in the + <smbconfoption><name>force security mode</name></smbconfoption> parameter. Any bits + that were changed that correspond to bits set to <quote>1</quote> in this parameter + are forced to be set.</para> + + <para>Essentially, bits set in the <parameter>force security mode</parameter> parameter + may be treated as a set of bits that, when modifying security on a file, the user has always set to be <quote>on</quote>.</para> + + <para>If not explicitly set, this parameter defaults to the same value + as the <smbconfoption><name>force create mode</name></smbconfoption> 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 + <smbconfoption><name>security mask</name></smbconfoption> and <parameter>force + security mode</parameter> parameters are applied to the change + request in that order.</para> + + <para>For a directory, Samba will perform the same operations as + described above for a file except it uses the parameter <parameter> + directory security mask</parameter> instead of <parameter>security + mask</parameter>, and <parameter>force directory security mode + </parameter> parameter instead of <parameter>force security mode + </parameter>.</para> + + <para>The <smbconfoption><name>directory security mask</name></smbconfoption> parameter + by default is set to the same value as the <parameter>directory mask + </parameter> parameter and the <parameter>force directory security + mode</parameter> parameter by default is set to the same value as + the <smbconfoption><name>force directory mode</name></smbconfoption> parameter. + In this way Samba enforces the permission restrictions that + an administrator can set on a Samba share, while still allowing users + to modify the permission bits within that restriction.</para> + + <para>If you want to set up a share that allows users full control + in modifying the permission bits on their files and directories and + does not force any particular bits to be set <quote>on</quote>, then set the following + parameters in the &smb.conf; file in that share-specific section: + </para> + + <smbconfblock> + <smbconfoption><name>security mask</name><value>0777</value></smbconfoption> + <smbconfoption><name>force security mode</name><value>0</value></smbconfoption> + <smbconfoption><name>directory security mask</name><value>0777</value></smbconfoption> + <smbconfoption><name>force directory security mode</name><value>0</value></smbconfoption> + </smbconfblock> + </sect2> + + <sect2> + <title>Interaction with the Standard Samba File Attribute Mapping</title> + + <note> + <para>Samba maps some of the DOS attribute bits (such as <quote>read + only</quote>) 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. + </para> + </note> + + <para>If a file has no UNIX read access for the owner, it will show up + as <quote>read only</quote> in the standard file attributes tabbed dialog. + Unfortunately, this dialog is the same one that contains the security information + in another tab.</para> + + <para>What this can mean is that if the owner changes the permissions + to allow himself read access using the security dialog, clicks on + <guibutton>OK</guibutton> to get back to the standard attributes tab + dialog, and clicks on <guibutton>OK</guibutton> 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 on <guibutton>OK</guibutton> to get back to the + attributes dialog, you should always press <guibutton>Cancel</guibutton> + rather than <guibutton>OK</guibutton> to ensure that your changes + are not overridden.</para> + </sect2> +</sect1> + +<sect1> +<title>Common Errors</title> + +<para> +File, directory and share access problems are common on the mailing list. The following +are examples taken from the mailing list in recent times. +</para> + + + <sect2> + <title>Users Cannot Write to a Public Share</title> + + <para> + <quote> + 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 the server to + <userinput>chgrp -R users *</userinput> and <userinput>chown -R nobody *</userinput> to allow others users to change the file. + </quote> + </para> + + <para> + There are many ways to solve this problem and here are a few hints: + </para> + + <procedure> + <step> + <para> + Go to the top of the directory that is shared. + </para> + </step> + + <step> + <para> + Set the ownership to what ever public owner and group you want +<screen> +&prompt;find 'directory_name' -type d -exec chown user.group {}\; +&prompt;find 'directory_name' -type d -exec chmod 1775 'directory_name' +&prompt;find 'directory_name' -type f -exec chmod 0775 {} \; +&prompt;find 'directory_name' -type f -exec chown user.group {}\; +</screen> + </para> + + <note><para> + The above will set the <constant>sticky bit</constant> 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. + </para></note> + </step> + <step> + <para> + + Directory is: <replaceable>/foodbar</replaceable> +<screen> +&prompt;<userinput>chown jack.engr /foodbar</userinput> +</screen> + </para> + + <note><para> + <para>This is the same as doing:</para> +<screen> +&prompt;<userinput>chown jack /foodbar</userinput> +&prompt;<userinput>chgrp engr /foodbar</userinput> +</screen> + </para></note> + </step> + <step> + <para>Now type: + +<screen> +&prompt;<userinput>chmod 6775 /foodbar</userinput> +&prompt;<userinput>ls -al /foodbar/..</userinput> +</screen> + + </para> + + <para>You should see: +<screen> +drwsrwsr-x 2 jack engr 48 2003-02-04 09:55 foodbar +</screen> + </para> + </step> + <step> + + <para>Now type: +<screen> +&prompt;<userinput>su - jill</userinput> +&prompt;<userinput>cd /foodbar</userinput> +&prompt;<userinput>touch Afile</userinput> +&prompt;<userinput>ls -al</userinput> +</screen> + </para> + + <para> + You should see that the file <filename>Afile</filename> created by Jill will have ownership + and permissions of Jack, as follows: +<screen> +-rw-r--r-- 1 jack engr 0 2003-02-04 09:57 Afile +</screen> + </para> + </step> + + <step> + <para> + Now in your &smb.conf; for the share add: + <smbconfblock> +<smbconfoption><name>force create mode</name><value>0775</value></smbconfoption> +<smbconfoption><name>force directory mode</name><value>6775</value></smbconfoption> + </smbconfblock> + </para> + + <note><para> + These procedures are needed only if your users are not members of the group + you have used. That is if within the OS do not have write permission on the directory. + </para> + </note> + + <para> + An alternative is to set in the &smb.conf; entry for the share: + <smbconfblock> +<smbconfoption><name>force user</name><value>jack</value></smbconfoption> +<smbconfoption><name>force group</name><value>engr</value></smbconfoption> + </smbconfblock> + </para> + </step> + </procedure> + </sect2> + + + <sect2> + <title>File Operations Done as <emphasis>root</emphasis> with <emphasis>force user</emphasis> Set</title> + + <para> + When you have a user in <smbconfoption><name>admin users</name></smbconfoption>, Samba will always do file operations for + this user as <emphasis>root</emphasis>, even if <smbconfoption><name>force user</name></smbconfoption> has been set. + </para> + </sect2> + + <sect2> + <title>MS Word with Samba Changes Owner of File</title> + + <para> + <emphasis>Question:</emphasis> <quote>When user B saves a word document that is owned by user A the updated file is now owned by user B. + Why is Samba doing this? How do I fix this?</quote> + </para> + + <para> + <emphasis>Answer:</emphasis> Word does the following when you modify/change a Word document: MS Word creates a NEW document with + a temporary name, Word then closes the old document and deletes it, Word then renames the new document to the original document name. + There is no mechanism by which Samba can in any way know that the new document really should be owned by the owners + of the original file. Samba has no way of knowing that the file will be renamed by MS Word. As far as Samba is able + to tell, the file that gets created is a NEW file, not one that the application (Word) is updating. + </para> + + <para> + There is a work-around to solve the permissions problem. That work-around involves understanding how you can manage file + system behavior from within the &smb.conf; file, as well as understanding how UNIX file systems work. Set on the directory + in which you are changing Word documents: <command>chmod g+s `directory_name'</command> This ensures that all files will + be created with the group that owns the directory. In &smb.conf; share declaration section set: + </para> + + <para> + <smbconfblock> + <smbconfoption><name>force create mode</name><value>0660</value></smbconfoption> + <smbconfoption><name>force directory mode</name><value>0770</value></smbconfoption> + </smbconfblock> + </para> + + <para> + These two settings will ensure that all directories and files that get created in the share will be read/writable by the + owner and group set on the directory itself. + </para> + + </sect2> + +</sect1> + +</chapter> |