linux/fs/cifs

The CIFS VFS support for Linux supports many advanced network filesystem 
features such as heirarchical dfs like namespace, hardlinks, locking and more.  
It was designed to comply with the SNIA CIFS Technical Reference (which 
supersedes the 1992 X/Open SMB Standard) as well as to perform best practice 
practical interoperability with Windows 2000, Windows XP, Samba and equivalent 
servers.  

For questions or bug reports please contact:
    sfrench@samba.org (sfrench@us.ibm.com) 

Build instructions:
==================
For Linux 2.4:
1) Get the kernel source (e.g.from http://www.kernel.org)
and download the cifs vfs source (see the project page
at http://us1.samba.org/samba/Linux_CIFS_client.html)
and change directory into the top of the kernel directory
then patch the kernel (e.g. "patch -p1 < cifs_24.patch") 
to add the cifs vfs to your kernel configure options if
it has not already been added (e.g. current SuSE and UL
users do not need to apply the cifs_24.patch since the cifs vfs is
already in the kernel configure menu) and then
mkdir linux/fs/cifs and then copy the current cifs vfs files from
the cifs download to your kernel build directory e.g.

	cp <cifs_download_dir>/fs/cifs/* to <kernel_download_dir>/fs/cifs
	
2) make menuconfig (or make xconfig)
3) select cifs from within the network filesystem choices
4) save and exit
5) make dep
6) make modules (or "make" if CIFS VFS not to be built as a module)

For Linux 2.6:
1) Download the kernel (e.g. from http://www.kernel.org)
and change directory into the top of the kernel directory tree
(e.g. /usr/src/linux-2.5.73)
2) make menuconfig (or make xconfig)
3) select cifs from within the network filesystem choices
4) save and exit
5) make


Installation instructions:
=========================
If you have built the CIFS vfs as module (successfully) simply
type "make modules_install" (or if you prefer, manually copy the file to
the modules directory e.g. /lib/modules/2.4.10-4GB/kernel/fs/cifs/cifs.o).

If you have built the CIFS vfs into the kernel itself, follow the instructions
for your distribution on how to install a new kernel (usually you
would simply type "make install").

If you do not have the utility mount.cifs (in the Samba 3.0 source tree and on 
the CIFS VFS web site) copy it to the same directory in which mount.smbfs and 
similar files reside (usually /sbin).  Although the helper software is not  
required, mount.cifs is recommended.  Eventually the Samba 3.0 utility program 
"net" may also be helpful since it may someday provide easier mount syntax for
users who are used to Windows e.g.  net use <mount point> <UNC name or cifs URL>
Note that running the Winbind pam/nss module (logon service) on all of your
Linux clients is useful in mapping Uids and Gids consistently across the
domain to the proper network user.  The mount.cifs mount helper can be
trivially built from Samba 3.0 or later source e.g. by executing:

	gcc samba/source/client/mount.cifs.c -o mount.cifs

If cifs is built as a module, then the size and number of network buffers
and maximum number of simultaneous requests to one server can be configured.
Changing these from their defaults is not recommended. By executing modinfo
	modinfo kernel/fs/cifs/cifs.ko
on kernel/fs/cifs/cifs.ko the list of configuration changes that can be made
at module initialization time (by running insmod cifs.ko) can be seen.

Allowing User Mounts
====================
To permit users to mount and unmount over directories they own is possible
with the cifs vfs.  A way to enable such mounting is to mark the mount.cifs
utility as suid (e.g. "chmod +s /sbin/mount.cifs). To enable users to 
umount shares they mount requires
1) mount.cifs version 1.4 or later
2) an entry for the share in /etc/fstab indicating that a user may
unmount it e.g.
//server/usersharename  /mnt/username cifs user 0 0

Note that when the mount.cifs utility is run suid (allowing user mounts), 
in order to reduce risks, the "nosuid" mount flag is passed in on mount to
disallow execution of an suid program mounted on the remote target.
When mount is executed as root, nosuid is not passed in by default,
and execution of suid programs on the remote target would be enabled
by default. This can be changed, as with nfs and other filesystems, 
by simply specifying "nosuid" among the mount options. For user mounts 
though to be able to pass the suid flag to mount requires rebuilding 
mount.cifs with the following flag: 
 
        gcc samba/source/client/mount.cifs.c -DCIFS_ALLOW_USR_SUID -o mount.cifs

There is a corresponding manual page for cifs mounting in the Samba 3.0 and
later source tree in docs/manpages/mount.cifs.8 

Allowing User Unmounts
======================
To permit users to ummount directories that they have user mounted (see above),
the utility umount.cifs may be used.  It may be invoked directly, or if 
umount.cifs is placed in /sbin, umount can invoke the cifs umount helper
(at least for most versions of the umount utility) for umount of cifs
mounts, unless umount is invoked with -i (which will avoid invoking a umount
helper). As with mount.cifs, to enable user unmounts umount.cifs must be marked
as suid (e.g. "chmod +s /sbin/umount.cifs") or equivalent (some distributions
allow adding entries to a file to the /etc/permissions file to achieve the
equivalent suid effect).  For this utility to succeed the target path
must be a cifs mount, and the uid of the current user must match the uid
of the user who mounted the resource.

Also note that the customary way of allowing user mounts and unmounts is 
(instead of using mount.cifs and unmount.cifs as suid) to add a line
to the file /etc/fstab for each //server/share you wish to mount, but
this can become unwieldy when potential mount targets include many
or  unpredictable UNC names.

Samba Considerations 
==================== 
To get the maximum benefit from the CIFS VFS, we recommend using a server that 
supports the SNIA CIFS Unix Extensions standard (e.g.  Samba 2.2.5 or later or 
Samba 3.0) but the CIFS vfs works fine with a wide variety of CIFS servers.  
Note that uid, gid and file permissions will display default values if you do 
not have a server that supports the Unix extensions for CIFS (such as Samba 
2.2.5 or later).  To enable the Unix CIFS Extensions in the Samba server, add 
the line: 

	unix extensions = yes
	
to your smb.conf file on the server.  Note that the following smb.conf settings 
are also useful (on the Samba server) when the majority of clients are Unix or 
Linux: 

	case sensitive = yes
	delete readonly = yes 
	ea support = yes

Note that server ea support is required for supporting xattrs from the Linux
cifs client, and that EA support is present in later versions of Samba (e.g. 
3.0.6 and later (also EA support works in all versions of Windows, at least to
shares on NTFS filesystems).  Extended Attribute (xattr) support is an optional
feature of most Linux filesystems which may require enabling via
make menuconfig. Client support for extended attributes (user xattr) can be
disabled on a per-mount basis by specifying "nouser_xattr" on mount.

The CIFS client can get and set POSIX ACLs (getfacl, setfacl) to Samba servers
version 3.10 and later.  Setting POSIX ACLs requires enabling both XATTR and 
then POSIX support in the CIFS configuration options when building the cifs
module.  POSIX ACL support can be disabled on a per mount basic by specifying
"noacl" on mount.
 
Some administrators may want to change Samba's smb.conf "map archive" and 
"create mask" parameters from the default.  Unless the create mask is changed
newly created files can end up with an unnecessarily restrictive default mode,
which may not be what you want, although if the CIFS Unix extensions are
enabled on the server and client, subsequent setattr calls (e.g. chmod) can
fix the mode.  Note that creating special devices (mknod) remotely 
may require specifying a mkdev function to Samba if you are not using 
Samba 3.0.6 or later.  For more information on these see the manual pages
("man smb.conf") on the Samba server system.  Note that the cifs vfs,
unlike the smbfs vfs, does not read the smb.conf on the client system 
(the few optional settings are passed in on mount via -o parameters instead).  
Note that Samba 2.2.7 or later includes a fix that allows the CIFS VFS to delete
open files (required for strict POSIX compliance).  Windows Servers already 
supported this feature. Samba server does not allow symlinks that refer to files
outside of the share, so in Samba versions prior to 3.0.6, most symlinks to
files with absolute paths (ie beginning with slash) such as:
	 ln -s /mnt/foo bar
would be forbidden. Samba 3.0.6 server or later includes the ability to create 
such symlinks safely by converting unsafe symlinks (ie symlinks to server 
files that are outside of the share) to a samba specific format on the server
that is ignored by local server applications and non-cifs clients and that will
not be traversed by the Samba server).  This is opaque to the Linux client
application using the cifs vfs. Absolute symlinks will work to Samba 3.0.5 or
later, but only for remote clients using the CIFS Unix extensions, and will
be invisbile to Windows clients and typically will not affect local
applications running on the same server as Samba.  

Use instructions:
================
Once the CIFS VFS support is built into the kernel or installed as a module 
(cifs.o), you can use mount syntax like the following to access Samba or Windows 
servers: 

  mount -t cifs //9.53.216.11/e$ /mnt -o user=myname,pass=mypassword

Before -o the option -v may be specified to make the mount.cifs
mount helper display the mount steps more verbosely.  
After -o the following commonly used cifs vfs specific options
are supported:

  user=<username>
  pass=<password>
  domain=<domain name>
  
Other cifs mount options are described below.  Use of TCP names (in addition to
ip addresses) is available if the mount helper (mount.cifs) is installed. If
you do not trust the server to which are mounted, or if you do not have
cifs signing enabled (and the physical network is insecure), consider use
of the standard mount options "noexec" and "nosuid" to reduce the risk of 
running an altered binary on your local system (downloaded from a hostile server
or altered by a hostile router).

Although mounting using format corresponding to the CIFS URL specification is
not possible in mount.cifs yet, it is possible to use an alternate format
for the server and sharename (which is somewhat similar to NFS style mount
syntax) instead of the more widely used UNC format (i.e. \\server\share):
  mount -t cifs tcp_name_of_server:share_name /mnt -o user=myname,pass=mypasswd

When using the mount helper mount.cifs, passwords may be specified via alternate
mechanisms, instead of specifying it after -o using the normal "pass=" syntax
on the command line:
1) By including it in a credential file. Specify credentials=filename as one
of the mount options. Credential files contain two lines
        username=someuser
        password=your_password
2) By specifying the password in the PASSWD environment variable (similarly
the user name can be taken from the USER environment variable).
3) By specifying the password in a file by name via PASSWD_FILE
4) By specifying the password in a file by file descriptor via PASSWD_FD

If no password is provided, mount.cifs will prompt for password entry

Restrictions
============
Servers must support the NTLM SMB dialect (which is the most recent, supported 
by Samba and Windows NT version 4, 2000 and XP and many other SMB/CIFS servers) 
Servers must support either "pure-TCP" (port 445 TCP/IP CIFS connections) or RFC 
1001/1002 support for "Netbios-Over-TCP/IP." Neither of these is likely to be a 
problem as most servers support this.  IPv6 support is planned for the future,
and is almost complete.

Valid filenames differ between Windows and Linux.  Windows typically restricts
filenames which contain certain reserved characters (e.g.the character : 
which is used to delimit the beginning of a stream name by Windows), while
Linux allows a slightly wider set of valid characters in filenames. Windows
servers can remap such characters when an explicit mapping is specified in
the Server's registry.  Samba starting with version 3.10 will allow such 
filenames (ie those which contain valid Linux characters, which normally
would be forbidden for Windows/CIFS semantics) as long as the server is
configured for Unix Extensions (and the client has not disabled
/proc/fs/cifs/LinuxExtensionsEnabled).
  

CIFS VFS Mount Options
======================
A partial list of the supported mount options follows:
  user		The user name to use when trying to establish
		the CIFS session.
  password	The user password.  If the mount helper is
		installed, the user will be prompted for password
		if it is not supplied.
  ip		The ip address of the target server
  unc		The target server Universal Network Name (export) to 
		mount.	
  domain	Set the SMB/CIFS workgroup name prepended to the
		username during CIFS session establishment
  uid		If CIFS Unix extensions are not supported by the server
		this overrides the default uid for inodes. For mounts to
		servers which do support the CIFS Unix extensions, such
		as a properly configured Samba server, the server provides
		the uid, gid and mode.  For servers which do not support
		the Unix extensions, the default uid (and gid) returned on
		lookup of existing files is the uid (gid) of the person
		who executed the mount (root, except when mount.cifs
		is configured setuid for user mounts) unless the "uid=" 
		(gid) mount option is specified.  For the uid (gid) of newly
		created files and directories, ie files created since 
		the last mount of the server share, the expected uid 
		(gid) is cached as as long as the inode remains in 
		memory on the client.   Also note that permission
		checks (authorization checks) on accesses to a file occur
		at the server, but there are cases in which an administrator
		may want to restrict at the client as well.  For those
		servers which do not report a uid/gid owner
		(such as Windows), permissions can also be checked at the
		client, and a crude form of client side permission checking 
		can be enabled by specifying file_mode and dir_mode on 
		the client
  gid		If CIFS Unix extensions are not supported by the server
		this overrides the default gid for inodes.
  file_mode     If CIFS Unix extensions are not supported by the server
		this overrides the default mode for file inodes.
  dir_mode      If CIFS Unix extensions are not supported by the server 
		this overrides the default mode for directory inodes.
  port		attempt to contact the server on this tcp port, before
		trying the usual ports (port 445, then 139).
  iocharset     Codepage used to convert local path names to and from
		Unicode. Unicode is used by default for network path
		names if the server supports it.  If iocharset is
		not specified then the nls_default specified
		during the local client kernel build will be used.
		If server does not support Unicode, this parameter is
		unused.
  rsize		default read size (usually 16K)
  wsize		default write size (usually 16K, 32K is often better over GigE)
		maximum wsize currently allowed by CIFS is 57344 (14 4096 byte
		pages)
  rw		mount the network share read-write (note that the
		server may still consider the share read-only)
  ro		mount network share read-only
  version	used to distinguish different versions of the
		mount helper utility (not typically needed)
  sep		if first mount option (after the -o), overrides
		the comma as the separator between the mount
		parms. e.g.
			-o user=myname,password=mypassword,domain=mydom
		could be passed instead with period as the separator by
			-o sep=.user=myname.password=mypassword.domain=mydom
		this might be useful when comma is contained within username
		or password or domain. This option is less important
		when the cifs mount helper cifs.mount (version 1.1 or later)
		is used.
  nosuid        Do not allow remote executables with the suid bit 
		program to be executed.  This is only meaningful for mounts
		to servers such as Samba which support the CIFS Unix Extensions.
		If you do not trust the servers in your network (your mount
		targets) it is recommended that you specify this option for
		greater security.
  exec		Permit execution of binaries on the mount.
  noexec	Do not permit execution of binaries on the mount.
  dev		Recognize block devices on the remote mount.
  nodev		Do not recognize devices on the remote mount.
  suid          Allow remote files on this mountpoint with suid enabled to 
		be executed (default for mounts when executed as root,
		nosuid is default for user mounts).
  credentials   Although ignored by the cifs kernel component, it is used by 
		the mount helper, mount.cifs. When mount.cifs is installed it
		opens and reads the credential file specified in order  
		to obtain the userid and password arguments which are passed to
		the cifs vfs.
  guest         Although ignored by the kernel component, the mount.cifs
		mount helper will not prompt the user for a password
		if guest is specified on the mount options.  If no
		password is specified a null password will be used.
  perm          Client does permission checks (vfs_permission check of uid
		and gid of the file against the mode and desired operation),
		Note that this is in addition to the normal ACL check on the
		target machine done by the server software. 
		Client permission checking is enabled by default.
  noperm        Client does not do permission checks.  This can expose
		files on this mount to access by other users on the local
		client system. It is typically only needed when the server
		supports the CIFS Unix Extensions but the UIDs/GIDs on the
		client and server system do not match closely enough to allow
		access by the user doing the mount.
		Note that this does not affect the normal ACL check on the
		target machine done by the server software (of the server
		ACL against the user name provided at mount time).
  serverino	Use servers inode numbers instead of generating automatically
		incrementing inode numbers on the client.  Although this will
		make it easier to spot hardlinked files (as they will have
		the same inode numbers) and inode numbers may be persistent,
		note that the server does not guarantee that the inode numbers
		are unique if multiple server side mounts are exported under a
		single share (since inode numbers on the servers might not
		be unique if multiple filesystems are mounted under the same
		shared higher level directory).  Note that this requires that
		the server support the CIFS Unix Extensions as other servers
		do not return a unique IndexNumber on SMB FindFirst (most
		servers return zero as the IndexNumber).  Parameter has no
		effect to Windows servers and others which do not support the
		CIFS Unix Extensions.
  noserverino   Client generates inode numbers (rather than using the actual one
		from the server) by default.
  setuids       If the CIFS Unix extensions are negotiated with the server
		the client will attempt to set the effective uid and gid of
		the local process on newly created files, directories, and
		devices (create, mkdir, mknod).
  nosetuids     The client will not attempt to set the uid and gid on
		on newly created files, directories, and devices (create, 
		mkdir, mknod) which will result in the server setting the
		uid and gid to the default (usually the server uid of the
		user who mounted the share).  Letting the server (rather than
		the client) set the uid and gid is the default. This
		parameter has no effect if the CIFS Unix Extensions are not
		negotiated.
  netbiosname   When mounting to servers via port 139, specifies the RFC1001
		source name to use to represent the client netbios machine 
		name when doing the RFC1001 netbios session initialize.
  direct        Do not do inode data caching on files opened on this mount.
		This precludes mmaping files on this mount. In some cases
		with fast networks and little or no caching benefits on the
		client (e.g. when the application is doing large sequential
		reads bigger than page size without rereading the same data) 
		this can provide better performance than the default
		behavior which caches reads (readahead) and writes 
		(writebehind) through the local Linux client pagecache 
		if oplock (caching token) is granted and held. Note that
		direct allows write operations larger than page size
		to be sent to the server.
  acl   	Allow setfacl and getfacl to manage posix ACLs if server
		supports them.  (default)
  noacl 	Do not allow setfacl and getfacl calls on this mount
  user_xattr    Allow getting and setting user xattrs as OS/2 EAs (extended
		attributes) to the server (default) e.g. via setfattr 
		and getfattr utilities. 
  nouser_xattr  Do not allow getfattr/setfattr to get/set xattrs 
  mapchars      Translate six of the seven reserved characters (not backslash)
			*?<>|:
		to the remap range (above 0xF000), which also
		allows the CIFS client to recognize files created with
		such characters by Windows's POSIX emulation. This can
		also be useful when mounting to most versions of Samba
		(which also forbids creating and opening files
		whose names contain any of these seven characters).
		This has no effect if the server does not support
		Unicode on the wire.
 nomapchars     Do not translate any of these seven characters (default).
 nocase         Request case insensitive path name matching (case
		sensitive is the default if the server suports it).
 nobrl          Do not send byte range lock requests to the server.
		This is necessary for certain applications that break
		with cifs style mandatory byte range locks (and most
		cifs servers do not yet support requesting advisory
		byte range locks).
 remount        remount the share (often used to change from ro to rw mounts
	        or vice versa)
		
The mount.cifs mount helper also accepts a few mount options before -o
including:

	-S      take password from stdin (equivalent to setting the environment
		variable "PASSWD_FD=0"
	-V      print mount.cifs version
	-?      display simple usage information

With recent 2.6 kernel versions of modutils, the version of the cifs kernel
module can be displayed via modinfo.

Misc /proc/fs/cifs Flags and Debug Info
=======================================
Informational pseudo-files:
DebugData		Displays information about active CIFS sessions
			and shares, as well as the cifs.ko version.
Stats			Lists summary resource usage information as well as per
			share statistics, if CONFIG_CIFS_STATS in enabled
			in the kernel configuration.

Configuration pseudo-files:
MultiuserMount		If set to one, more than one CIFS session to 
			the same server ip address can be established
			if more than one uid accesses the same mount
			point and if the uids user/password mapping
			information is available. (default is 0)
PacketSigningEnabled	If set to one, cifs packet signing is enabled
			and will be used if the server requires 
			it.  If set to two, cifs packet signing is
			required even if the server considers packet
			signing optional. (default 1)
cifsFYI			If set to one, additional debug information is
			logged to the system error log. (default 0)
ExtendedSecurity	If set to one, SPNEGO session establishment
			is allowed which enables more advanced 
			secure CIFS session establishment (default 0)
NTLMV2Enabled		If set to one, more secure password hashes
			are used when the server supports them and
			when kerberos is not negotiated (default 0)
traceSMB		If set to one, debug information is logged to the
			system error log with the start of smb requests
			and responses (default 0)
LookupCacheEnable	If set to one, inode information is kept cached
			for one second improving performance of lookups
			(default 1)
OplockEnabled		If set to one, safe distributed caching enabled.
			(default 1)
LinuxExtensionsEnabled	If set to one then the client will attempt to
			use the CIFS "UNIX" extensions which are optional
			protocol enhancements that allow CIFS servers
			to return accurate UID/GID information as well
			as support symbolic links. If you use servers
			such as Samba that support the CIFS Unix
			extensions but do not want to use symbolic link
			support and want to map the uid and gid fields 
			to values supplied at mount (rather than the 
			actual values, then set this to zero. (default 1)

These experimental features and tracing can be enabled by changing flags in 
/proc/fs/cifs (after the cifs module has been installed or built into the 
kernel, e.g.  insmod cifs).  To enable a feature set it to 1 e.g.  to enable 
tracing to the kernel message log type: 

	echo 7 > /proc/fs/cifs/cifsFYI
	
cifsFYI functions as a bit mask. Setting it to 1 enables additional kernel
logging of various informational messages.  2 enables logging of non-zero
SMB return codes while 4 enables logging of requests that take longer
than one second to complete (except for byte range lock requests). 
Setting it to 4 requires defining CONFIG_CIFS_STATS2 manually in the
source code (typically by setting it in the beginning of cifsglob.h),
and setting it to seven enables all three.  Finally, tracing
the start of smb requests and responses can be enabled via:

	echo 1 > /proc/fs/cifs/traceSMB

Two other experimental features are under development and to test 
require enabling CONFIG_CIFS_EXPERIMENTAL

	More efficient write operations

	DNOTIFY fcntl: needed for support of directory change 
			    notification and perhaps later for file leases)

Per share (per client mount) statistics are available in /proc/fs/cifs/Stats
if the kernel was configured with cifs statistics enabled.  The statistics
represent the number of successful (ie non-zero return code from the server) 
SMB responses to some of the more common commands (open, delete, mkdir etc.).
Also recorded is the total bytes read and bytes written to the server for
that share.  Note that due to client caching effects this can be less than the
number of bytes read and written by the application running on the client.
The statistics for the number of total SMBs and oplock breaks are different in
that they represent all for that share, not just those for which the server
returned success.
	
Also note that "cat /proc/fs/cifs/DebugData" will display information about 
the active sessions and the shares that are mounted.  Note: NTLMv2 enablement 
will not work since its implementation is not quite complete yet. Do not alter
the ExtendedSecurity configuration value unless you are doing specific testing.
Enabling extended security works to Windows 2000 Workstations and XP but not to 
Windows 2000 server or Samba since it does not usually send "raw NTLMSSP" 
(instead it sends NTLMSSP encapsulated in SPNEGO/GSSAPI, which support is not 
complete in the CIFS VFS yet).