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CentOS 7.0 - man page for mount.cifs (centos section 8)

MOUNT.CIFS(8)			   System Administration tools			    MOUNT.CIFS(8)

NAME
       mount.cifs - mount using the Common Internet File System (CIFS)

SYNOPSIS
       mount.cifs {service} {mount-point} [-o options]

DESCRIPTION
       This tool is part of the cifs-utils suite.

       mount.cifs mounts a Linux CIFS filesystem. It is usually invoked indirectly by the
       mount(8) command when using the "-t cifs" option. This command only works in Linux, and
       the kernel must support the cifs filesystem. The CIFS protocol is the successor to the SMB
       protocol and is supported by most Windows servers and many other commercial servers and
       Network Attached Storage appliances as well as by the popular Open Source server Samba.

       The mount.cifs utility attaches the UNC name (exported network resource) specified as
       service (using //server/share syntax, where "server" is the server name or IP address and
       "share" is the name of the share) to the local directory mount-point.

       Options to mount.cifs are specified as a comma-separated list of key=value pairs. It is
       possible to send options other than those listed here, assuming that the cifs filesystem
       kernel module (cifs.ko) supports them. Unrecognized cifs mount options passed to the cifs
       vfs kernel code will be logged to the kernel log.

       mount.cifs causes the cifs vfs to launch a thread named cifsd. After mounting it keeps
       running until the mounted resource is unmounted (usually via the umount utility).

       mount.cifs -V command displays the version of cifs mount helper.

       modinfo cifs command displays the version of cifs module.

OPTIONS
       username=arg
	   specifies the username to connect as. If this is not given, then the environment
	   variable USER is used.

	   Earlier versions of mount.cifs also allowed one to specify the username in a
	   "user%password" or "workgroup/user" or "workgroup/user%password" to allow the password
	   and workgroup to be specified as part of the username. Support for those alternate
	   username formats is now deprecated and should no longer be used. Users should use the
	   discrete "password=" and "domain=" to specify those values. While some versions of the
	   cifs kernel module accept "user=" as an abbreviation for this option, its use can
	   confuse the standard mount program into thinking that this is a non-superuser mount.
	   It is therefore recommended to use the full "username=" option name.

       password=arg
	   specifies the CIFS password. If this option is not given then the environment variable
	   PASSWD is used. If the password is not specified directly or indirectly via an
	   argument to mount, mount.cifs will prompt for a password, unless the guest option is
	   specified.

	   Note that a password which contains the delimiter character (i.e. a comma ',') will
	   fail to be parsed correctly on the command line. However, the same password defined in
	   the PASSWD environment variable or via a credentials file (see below) or entered at
	   the password prompt will be read correctly.

       credentials=filename
	   specifies a file that contains a username and/or password and optionally the name of
	   the workgroup. The format of the file is:

			 username=value
			 password=value
			 domain=value

	   This is preferred over having passwords in plaintext in a shared file, such as
	   /etc/fstab. Be sure to protect any credentials file properly.

       uid=arg
	   sets the uid that will own all files or directories on the mounted filesystem when the
	   server does not provide ownership information. It may be specified as either a
	   username or a numeric uid. When not specified, the default is uid 0. The mount.cifs
	   helper must be at version 1.10 or higher to support specifying the uid in non-numeric
	   form. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more
	   information.

       forceuid
	   instructs the client to ignore any uid provided by the server for files and
	   directories and to always assign the owner to be the value of the uid= option. See the
	   section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more information.

       cruid=arg
	   sets the uid of the owner of the credentials cache. This is primarily useful with
	   sec=krb5. The default is the real uid of the process performing the mount. Setting
	   this parameter directs the upcall to look for a credentials cache owned by that user.

       gid=arg
	   sets the gid that will own all files or directories on the mounted filesystem when the
	   server does not provide ownership information. It may be specified as either a
	   groupname or a numeric gid. When not specified, the default is gid 0. The mount.cifs
	   helper must be at version 1.10 or higher to support specifying the gid in non-numeric
	   form. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more
	   information.

       forcegid
	   instructs the client to ignore any gid provided by the server for files and
	   directories and to always assign the owner to be the value of the gid= option. See the
	   section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more information.

       port=arg
	   sets the port number on which the client will attempt to contact the CIFS server. If
	   this value is specified, look for an existing connection with this port, and use that
	   if one exists. If one doesn't exist, try to create a new connection on that port. If
	   that connection fails, return an error. If this value isn't specified, look for an
	   existing connection on port 445 or 139. If no such connection exists, try to connect
	   on port 445 first and then port 139 if that fails. Return an error if both fail.

       servernetbiosname=arg
	   Specify the server netbios name (RFC1001 name) to use when attempting to setup a
	   session to the server. Although rarely needed for mounting to newer servers, this
	   option is needed for mounting to some older servers (such as OS/2 or Windows 98 and
	   Windows ME) since when connecting over port 139 they, unlike most newer servers, do
	   not support a default server name. A server name can be up to 15 characters long and
	   is usually uppercased.

       servern=arg
	   Synonym for servernetbiosname.

       netbiosname=arg
	   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.

       file_mode=arg
	   If the server does not support the CIFS Unix extensions this overrides the default
	   file mode.

       dir_mode=arg
	   If the server does not support the CIFS Unix extensions this overrides the default
	   mode for directories.

       ip=arg
	   sets the destination IP address. This option is set automatically if the server name
	   portion of the requested UNC name can be resolved so rarely needs to be specified by
	   the user.

       domain=arg
	   sets the domain (workgroup) of the user

       guest
	   don't prompt for a password

       iocharset
	   Charset 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.

       ro
	   mount read-only

       rw
	   mount read-write

       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). If the CIFS Unix Extensions are not
	   negotiated, for newly created files and directories instead of using the default uid
	   and gid specified on the the mount, cache the new file's uid and gid locally which
	   means that the uid for the file can change when the inode is reloaded (or the user
	   remounts the share).

       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.If the CIFS Unix Extensions are not negotiated then the uid and gid for new
	   files will appear to be the uid (gid) of the mounter or the uid (gid) parameter
	   specified on the mount.

       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).

       dynperm
	   Instructs the server to maintain ownership and permissions in memory that can't be
	   stored on the server. This information can disappear at any time (whenever the inode
	   is flushed from the cache), so while this may help make some applications work, it's
	   behavior is somewhat unreliable. See the section below on FILE AND DIRECTORY OWNERSHIP
	   AND PERMISSIONS for more information.

       cache=
	   Cache mode. See the section below on CACHE COHERENCY for details. Allowed values are:

	   o   none: do not cache file data at all

	   o   strict: follow the CIFS/SMB2 protocol strictly

	   o   loose: allow loose caching semantics

	   The default in kernels prior to 3.7 was "loose". As of kernel 3.7 the default is
	   "strict".

       directio
	   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. On some kernels this requires the cifs.ko module to be built with
	   the CIFS_EXPERIMENTAL configure option.

	   This option is will be deprecated in 3.7. Users should use cache=none instead on more
	   recent kernels.

       strictcache
	   Use for switching on strict cache mode. In this mode the client reads from the cache
	   all the time it has Oplock Level II, otherwise - read from the server. As for write -
	   the client stores a data in the cache in Exclusive Oplock case, otherwise - write
	   directly to the server.

	   This option is will be deprecated in 3.7. Users should use cache=strict instead on
	   more recent kernels.

       rwpidforward
	   Forward pid of a process who opened a file to any read or write operation on that
	   file. This prevent applications like WINE from failing on read and write if we use
	   mandatory brlock style.

       mapchars
	   Translate six of the seven reserved characters (not backslash, but including the
	   colon, question mark, pipe, asterik, greater than and less than characters) 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. Please note that the files created with mapchars
	   mount option may not be accessible if the share is mounted without that option.

       nomapchars
	   Do not translate any of these seven characters (default)

       intr
	   currently unimplemented

       nointr
	   (default) currently unimplemented

       hard
	   The program accessing a file on the cifs mounted file system will hang when the server
	   crashes.

       soft
	   (default) The program accessing a file on the cifs mounted file system will not hang
	   when the server crashes and will return errors to the user application.

       noacl
	   Do not allow POSIX ACL operations even if server would support them.

	   The CIFS client can get and set POSIX ACLs (getfacl, setfacl) to Samba servers version
	   3.0.10 and later. Setting POSIX ACLs requires enabling both CIFS_XATTR and then
	   CIFS_POSIX support in the CIFS configuration options when building the cifs module.
	   POSIX ACL support can be disabled on a per mount basis by specifying "noacl" on mount.

       cifsacl
	   This option is used to map CIFS/NTFS ACLs to/from Linux permission bits, map SIDs
	   to/from UIDs and GIDs, and get and set Security Descriptors.

	   See sections on CIFS/NTFS ACL, SID/UID/GID MAPPING, SECURITY DESCRIPTORS for more
	   information.

       backupuid=arg
	   Restrict access to files with the backup intent to a user. Either a name or an id must
	   be provided as an argument, there are no default values.

	   See section ACCESSING FILES WITH BACKUP INTENT for more details

       backupgid=arg
	   Restrict access to files with the backup intent to a group. Either a name or an id
	   must be provided as an argument, there are no default values.

	   See section ACCESSING FILES WITH BACKUP INTENT for more details

       nocase
	   Request case insensitive path name matching (case sensitive is the default if the
	   server suports it).

       ignorecase
	   Synonym for nocase.

       sec=
	   Security mode. Allowed values are:

	   o   none - attempt to connection as a null user (no name)

	   o   krb5 - Use Kerberos version 5 authentication

	   o   krb5i - Use Kerberos authentication and forcibly enable packet signing

	   o   ntlm - Use NTLM password hashing

	   o   ntlmi - Use NTLM password hashing and force packet signing

	   o   ntlmv2 - Use NTLMv2 password hashing

	   o   ntlmv2i - Use NTLMv2 password hashing and force packet signing

	   o   ntlmssp - Use NTLMv2 password hashing encapsulated in Raw NTLMSSP message

	   o   ntlmsspi - Use NTLMv2 password hashing encapsulated in Raw NTLMSSP message, and
	       force packet signing

	   The default in mainline kernel versions prior to v3.8 was sec=ntlm. In v3.8, the
	   default was changed to sec=ntlmssp.

	   If the server requires signing during protocol negotiation, then it may be enabled
	   automatically. Packet signing may also be enabled automatically if it's enabled in
	   /proc/fs/cifs/SecurityFlags.

       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).

       sfu
	   When the CIFS Unix Extensions are not negotiated, attempt to create device files and
	   fifos in a format compatible with Services for Unix (SFU). In addition retrieve bits
	   10-12 of the mode via the SETFILEBITS extended attribute (as SFU does). In the future
	   the bottom 9 bits of the mode mode also will be emulated using queries of the security
	   descriptor (ACL). [NB: requires version 1.39 or later of the CIFS VFS. To recognize
	   symlinks and be able to create symlinks in an SFU interoperable form requires version
	   1.40 or later of the CIFS VFS kernel module.

       serverino
	   Use inode numbers (unique persistent file identifiers) returned by the server instead
	   of automatically generating temporary inode numbers on the client. Although server
	   inode numbers make it easier to spot hardlinked files (as they will have the same
	   inode numbers) and inode numbers may be persistent (which is userful for some
	   sofware), 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 not all servers support returning server inode
	   numbers, although those that support the CIFS Unix Extensions, and Windows 2000 and
	   later servers typically do support this (although not necessarily on every local
	   server filesystem). Parameter has no effect if the server lacks support for returning
	   inode numbers or equivalent. This behavior is enabled by default.

       noserverino
	   Client generates inode numbers itself rather than using the actual ones from the
	   server.

	   See section INODE NUMBERS for more information.

       nounix
	   Disable the CIFS Unix Extensions for this mount. This can be useful in order to turn
	   off multiple settings at once. This includes POSIX acls, POSIX locks, POSIX paths,
	   symlink support and retrieving uids/gids/mode from the server. This can also be useful
	   to work around a bug in a server that supports Unix Extensions.

	   See section INODE NUMBERS for more information.

       nouser_xattr
	   Do not allow getfattr/setfattr to get/set xattrs, even if server would support it
	   otherwise. The default is for xattr support to be enabled.

       rsize=bytes
	   Maximum amount of data that the kernel will request in a read request in bytes. Prior
	   to kernel 3.2.0, the default was 16k, and the maximum size was limited by the
	   CIFSMaxBufSize module parameter. As of kernel 3.2.0, the behavior varies according to
	   whether POSIX extensions are enabled on the mount and the server supports large POSIX
	   reads. If they are, then the default is 1M, and the maxmimum is 16M. If they are not
	   supported by the server, then the default is 60k and the maximum is around 127k. The
	   reason for the 60k is because it's the maximum size read that windows servers can
	   fill. Note that this value is a maximum, and the client may settle on a smaller size
	   to accomodate what the server supports. In kernels prior to 3.2.0, no negotiation is
	   performed.

       wsize=bytes
	   Maximum amount of data that the kernel will send in a write request in bytes. Prior to
	   kernel 3.0.0, the default and maximum was 57344 (14 * 4096 pages). As of 3.0.0, the
	   default depends on whether the client and server negotiate large writes via POSIX
	   extensions. If they do, then the default is 1M, and the maximum allowed is 16M. If
	   they do not, then the default is 65536 and the maximum allowed is 131007.

	   Note that this value is just a starting point for negotiation in 3.0.0 and up. The
	   client and server may negotiate this size downward according to the server's
	   capabilities. In kernels prior to 3.0.0, no negotiation is performed. It can end up
	   with an existing superblock if this value isn't specified or it's greater or equal
	   than the existing one.

       fsc
	   Enable local disk caching using FS-Cache for CIFS. This option could be useful to
	   improve performance on a slow link, heavily loaded server and/or network where reading
	   from the disk is faster than reading from the server (over the network). This could
	   also impact the scalability positively as the number of calls to the server are
	   reduced. But, be warned that local caching is not suitable for all workloads, for
	   e.g., read-once type workloads. So, you need to consider carefully the
	   situation/workload before using this option. Currently, local disk caching is enabled
	   for CIFS files opened as read-only.

	   NOTE: This feature is available only in the recent kernels that have been built with
	   the kernel config option CONFIG_CIFS_FSCACHE. You also need to have cachefilesd daemon
	   installed and running to make the cache operational.

       multiuser
	   Map user accesses to individual credentials when accessing the server. By default,
	   CIFS mounts only use a single set of user credentials (the mount credentials) when
	   accessing a share. With this option, the client instead creates a new session with the
	   server using the user's credentials whenever a new user accesses the mount. Further
	   accesses by that user will also use those credentials. Because the kernel cannot
	   prompt for passwords, multiuser mounts are limited to mounts using sec= options that
	   don't require passwords.

	   With this change, it's feasible for the server to handle permissions enforcement, so
	   this option also implies "noperm". Furthermore, when unix extensions aren't in use and
	   the administrator has not overriden ownership using the uid= or gid= options,
	   ownership of files is presented as the current user accessing the share.

       actimeo=arg
	   The time (in seconds) that the CIFS client caches attributes of a file or directory
	   before it requests attribute information from a server. During this period the changes
	   that occur on the server remain undetected until the client checks the server again.

	   By default, the attribute cache timeout is set to 1 second. This means more frequent
	   on-the-wire calls to the server to check whether attributes have changed which could
	   impact performance. With this option users can make a tradeoff between performance and
	   cache metadata correctness, depending on workload needs. Shorter timeouts mean better
	   cache coherency, but frequent increased number of calls to the server. Longer timeouts
	   mean a reduced number of calls to the server but looser cache coherency. The actimeo
	   value is a positive integer that can hold values between 0 and a maximum value of 2^30
	   * HZ (frequency of timer interrupt) setting.

       noposixpaths
	   If unix extensions are enabled on a share, then the client will typically allow
	   filenames to include any character besides '/' in a pathname component, and will use
	   forward slashes as a pathname delimiter. This option prevents the client from
	   attempting to negotiate the use of posix-style pathnames to the server.

       posixpaths
	   Inverse of noposixpaths.

       prefixpath=
	   It's possible to mount a subdirectory of a share. The preferred way to do this is to
	   append the path to the UNC when mounting. However, it's also possible to do the same
	   by setting this option and providing the path there.

       vers=
	   SMB protocol version. Allowed values are:

	   o   1.0 - The classic CIFS/SMBv1 protocol. This is the default.

	   o   2.0 - The SMBv2.002 protocol. This was initially introduced in Windows Vista
	       Service Pack 1, and Windows Server 2008. Note that the initial release version of
	       Windows Vista spoke a slightly different dialect (2.000) that is not supported.

	   o   2.1 - The SMBv2.1 protocol that was introduced in Microsoft Windows 7 and Windows
	       Server 2008R2.

	   o   3.0 - The SMBv3.0 protocol that was introduced in Microsoft Windows 8 and Windows
	       Server 2012.

	   Note too that while this option governs the protocol version used, not all features of
	   each version are available.

       --verbose
	   Print additional debugging information for the mount. Note that this parameter must be
	   specified before the -o. For example:

	   mount -t cifs //server/share /mnt --verbose -o user=username

SERVICE FORMATTING AND DELIMITERS
       It's generally preferred to use forward slashes (/) as a delimiter in service names. They
       are considered to be the "universal delimiter" since they are generally not allowed to be
       embedded within path components on Windows machines and the client can convert them to
       blackslashes (\) unconditionally. Conversely, backslash characters are allowed by POSIX to
       be part of a path component, and can't be automatically converted in the same way.

       mount.cifs will attempt to convert backslashes to forward slashes where it's able to do
       so, but it cannot do so in any path component following the sharename.

INODE NUMBERS
       When Unix Extensions are enabled, we use the actual inode number provided by the server in
       response to the POSIX calls as an inode number.

       When Unix Extensions are disabled and "serverino" mount option is enabled there is no way
       to get the server inode number. The client typically maps the server-assigned "UniqueID"
       onto an inode number.

       Note that the UniqueID is a different value from the server inode number. The UniqueID
       value is unique over the scope of the entire server and is often greater than 2 power 32.
       This value often makes programs that are not compiled with LFS (Large File Support), to
       trigger a glibc EOVERFLOW error as this won't fit in the target structure field. It is
       strongly recommended to compile your programs with LFS support (i.e. with
       -D_FILE_OFFSET_BITS=64) to prevent this problem. You can also use "noserverino" mount
       option to generate inode numbers smaller than 2 power 32 on the client. But you may not be
       able to detect hardlinks properly.

CACHE COHERENCY
       With a network filesystem such as CIFS or NFS, the client must contend with the fact that
       activity on other clients or the server could change the contents or attributes of a file
       without the client being aware of it. One way to deal with such a problem is to mandate
       that all file accesses go to the server directly. This is performance prohibitive however,
       so most protocols have some mechanism to allow the client to cache data locally.

       The CIFS protocol mandates (in effect) that the client should not cache file data unless
       it holds an opportunistic lock (aka oplock) or a lease. Both of these entities allow the
       client to guarantee certain types of exclusive access to a file so that it can access its
       contents without needing to continually interact with the server. The server will call
       back the client when it needs to revoke either of them and allow the client a certain
       amount of time to flush any cached data.

       The cifs client uses the kernel's pagecache to cache file data. Any I/O that's done
       through the pagecache is generally page-aligned. This can be problematic when combined
       with byte-range locks as Windows' locking is mandatory and can block reads and writes from
       occurring.

       cache=none means that the client never utilizes the cache for normal reads and writes. It
       always accesses the server directly to satisfy a read or write request.

       cache=strict means that the client will attempt to follow the CIFS/SMB2 protocol strictly.
       That is, the cache is only trusted when the client holds an oplock. When the client does
       not hold an oplock, then the client bypasses the cache and accesses the server directly to
       satisfy a read or write request. By doing this, the client avoids problems with byte range
       locks. Additionally, byte range locks are cached on the client when it holds an oplock and
       are "pushed" to the server when that oplock is recalled.

       cache=loose allows the client to use looser protocol semantics which can sometimes provide
       better performance at the expense of cache coherency. File access always involves the
       pagecache. When an oplock or lease is not held, then the client will attempt to flush the
       cache soon after a write to a file. Note that that flush does not necessarily occur before
       a write system call returns.

       In the case of a read without holding an oplock, the client will attempt to periodically
       check the attributes of the file in order to ascertain whether it has changed and the
       cache might no longer be valid. This mechanism is much like the one that NFSv2/3 use for
       cache coherency, but it particularly problematic with CIFS. Windows is quite "lazy" with
       respect to updating the "LastWriteTime" field that the client uses to verify this. The
       effect is that cache=loose can cause data corruption when multiple readers and writers are
       working on the same files.

       Because of this, when multiple clients are accessing the same set of files, then
       cache=strict is recommended. That helps eliminate problems with cache coherency by
       following the CIFS/SMB2 protocols more strictly.

       Note too that no matter what caching model is used, the client will always use the
       pagecache to handle mmap'ed files. Writes to mmap'ed files are only guaranteed to be
       flushed to the server when msync() is called, or on close().

       The default in kernels prior to 3.7 was "loose". As of 3.7, the default is "strict".

CIFS/NTFS ACL, SID/UID/GID MAPPING, SECURITY DESCRIPTORS
       This option is used to work with file objects which posses Security Descriptors and
       CIFS/NTFS ACL instead of UID, GID, file permission bits, and POSIX ACL as user
       authentication model. This is the most common authentication model for CIFS servers and is
       the one used by Windows.

       Support for this requires both CIFS_XATTR and CIFS_ACL support in the CIFS configuration
       options when building the cifs module.

       A CIFS/NTFS ACL is mapped to file permission bits using an algorithm specified in the
       following Microsoft TechNet document:

       o   http://technet.microsoft.com/en-us/library/bb463216.aspx

       In order to map SIDs to/from UIDs and GIDs, the following is required:

       o   a kernel upcall to the cifs.idmap utility set up via request-key.conf(5)

       o   winbind support configured via nsswitch.conf(5) and smb.conf(5)

       Please refer to the respective manpages of cifs.idmap(8) and winbindd(8) for more
       information.

       Security descriptors for a file object can be retrieved and set directly using extended
       attribute named system.cifs_acl. The security descriptors presented via this interface are
       "raw" blobs of data and need a userspace utility to either parse and format or to assemble
       it such as getcifsacl(1) and setcifsacl(1) respectively.

       Some of the things to consider while using this mount option:

       o   There may be an increased latency when handling metadata due to additional requests to
	   get and set security descriptors.

       o   The mapping between a CIFS/NTFS ACL and POSIX file permission bits is imperfect and
	   some ACL information may be lost in the translation.

       o   If either upcall to cifs.idmap is not setup correctly or winbind is not configured and
	   running, ID mapping will fail. In that case uid and gid will default to either to
	   those values of the share or to the values of uid and/or gid mount options if
	   specified.

ACCESSING FILES WITH BACKUP INTENT
       For an user on the server, desired access to a file is determined by the permissions and
       rights associated with that file.  This is typically accomplished using owenrship and ACL.
       For a user who does not have access rights to a file, it is still possible to access that
       file for a specific or a targeted purpose by granting special rights.  One of the specific
       purposes is to access a file with the intent to either backup or restore i.e. backup
       intent.	The right to access a file with the backup intent can typically be granted by
       making that user a part of the built-in group Backup Operators.	Thus, when this user
       attempts to open a file with the backup intent, open request is sent by setting the bit
       FILE_OPEN_FOR_BACKUP_INTENT as one of the CreateOptions.

       As an example, on a Windows server, a user named testuser, cannot open this file with such
       a security descriptor.

       REVISION:0x1
       CONTROL:0x9404
       OWNER:Administrator
       GROUP:Domain Users
       ACL:Administrator:ALLOWED/0x0/FULL

       But the user testuser, if it becomes part of the group Backup Operators, can open the file
       with the backup intent.

       Any user on the client side who can authenticate as such a user on the server, can access
       the files with the backup intent. But it is desirable and preferable for security reasons
       amongst many, to restrict this special right.

       The mount option backupuid is used to restrict this special right to a user which is
       specified by either a name or an id. The mount option backupgid is used to restrict this
       special right to the users in a group which is specified by either a name or an id. These
       two mount options can be used together.

FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS
       The core CIFS protocol does not provide unix ownership information or mode for files and
       directories. Because of this, files and directories will generally appear to be owned by
       whatever values the uid= or gid= options are set, and will have permissions set to the
       default file_mode and dir_mode for the mount. Attempting to change these values via
       chmod/chown will return success but have no effect.

       When the client and server negotiate unix extensions, files and directories will be
       assigned the uid, gid, and mode provided by the server. Because CIFS mounts are generally
       single-user, and the same credentials are used no matter what user accesses the mount,
       newly created files and directories will generally be given ownership corresponding to
       whatever credentials were used to mount the share.

       If the uid's and gid's being used do not match on the client and server, the forceuid and
       forcegid options may be helpful. Note however, that there is no corresponding option to
       override the mode. Permissions assigned to a file when forceuid or forcegid are in effect
       may not reflect the the real permissions.

       When unix extensions are not negotiated, it's also possible to emulate them locally on the
       server using the "dynperm" mount option. When this mount option is in effect, newly
       created files and directories will receive what appear to be proper permissions. These
       permissions are not stored on the server however and can disappear at any time in the
       future (subject to the whims of the kernel flushing out the inode cache). In general, this
       mount option is discouraged.

       It's also possible to override permission checking on the client altogether via the noperm
       option. Server-side permission checks cannot be overriden. The permission checks done by
       the server will always correspond to the credentials used to mount the share, and not
       necessarily to the user who is accessing the share.

ENVIRONMENT VARIABLES
       The variable USER may contain the username of the person to be used to authenticate to the
       server. The variable can be used to set both username and password by using the format
       username%password.

       The variable PASSWD may contain the password of the person using the client.

       The variable PASSWD_FILE may contain the pathname of a file to read the password from. A
       single line of input is read and used as the password.

NOTES
       This command may be used only by root, unless installed setuid, in which case the noeexec
       and nosuid mount flags are enabled. When installed as a setuid program, the program
       follows the conventions set forth by the mount program for user mounts, with the added
       restriction that users must be able to chdir() into the mountpoint prior to the mount in
       order to be able to mount onto it.

       Some samba client tools like smbclient(8) honour client-side configuration parameters
       present in smb.conf. Unlike those client tools, mount.cifs ignores smb.conf completely.

CONFIGURATION
       The primary mechanism for making configuration changes and for reading debug information
       for the cifs vfs is via the Linux /proc filesystem. In the directory /proc/fs/cifs are
       various configuration files and pseudo files which can display debug information. There
       are additional startup options such as maximum buffer size and number of buffers which
       only may be set when the kernel cifs vfs (cifs.ko module) is loaded. These can be seen by
       running the modinfo utility against the file cifs.ko which will list the options that may
       be passed to cifs during module installation (device driver load). For more information
       see the kernel file fs/cifs/README.

BUGS
       Mounting using the CIFS URL specification is currently not supported.

       The credentials file does not handle usernames or passwords with leading space.

       Note that the typical response to a bug report is a suggestion to try the latest version
       first. So please try doing that first, and always include which versions you use of
       relevant software when reporting bugs (minimum: mount.cifs (try mount.cifs -V), kernel
       (see /proc/version) and server type you are trying to contact.

VERSION
       This man page is correct for version 1.74 of the cifs vfs filesystem (roughly Linux kernel
       3.0).

SEE ALSO
       cifs.upcall(8), getcifsacl(1), setcifsacl(1)

       Documentation/filesystems/cifs.txt and fs/cifs/README in the linux kernel source tree may
       contain additional options and information.

AUTHOR
       Steve French

       The syntax and manpage were loosely based on that of smbmount. It was converted to
       Docbook/XML by Jelmer Vernooij.

       The maintainer of the Linux cifs vfs and the userspace tool mount.cifs is Steve French.
       The Linux CIFS Mailing list is the preferred place to ask questions regarding these
       programs.

cifs-utils				    02/07/2010				    MOUNT.CIFS(8)


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