mount.cifs

MOUNT.CIFS, MOUNT.SMB3(8)                              MOUNT.CIFS, MOUNT.SMB3(8)



NAME
       mount.cifs, mount.smb3 - mount using the Common Internet File System
       (CIFS)

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

       This tool is part of the cifs-utils suite.

       mount.cifs mounts a CIFS or SMB3 filesystem from Linux. 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 SMB3 protocol is the successor to the CIFS (SMB)
       protocol and is supported by most Windows servers, Azure (cloud storage),
       Macs and many other commercial servers and Network Attached Storage
       appliances as well as by the popular Open Source server Samba.

       mount.smb3 mounts only SMB3 filesystem. It is usually invoked indirectly
       by the mount(8) command when using the "-t smb3" option.  The smb3
       filesystem type was added in kernel-4.18 and above.  It works in a
       similar fashion as mount.cifs except it passes filesystem type as smb3.

       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|user=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|pass=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|cred=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.

       idsfromsid
              Extract uid/gid from special SID instead of mapping it. 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.

       netbiosname=arg
              When mounting to servers via port 139, specifies the RFC1001
              source name to use to represent the client netbios machine during
              the netbios session initialization.

       servern=arg
              Similar to netbiosname except it specifies the netbios name of the
              server instead of the client. 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.

       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|addr=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|dom=arg|workgroup=arg
              Sets the domain (workgroup) of the user. If no domains are given,
              the empty domain will be used. Use domainauto to automatically
              guess the domain of the server you are connecting to.

       domainauto
              When using NTLM authentication and not providing a domain via
              domain, guess the domain from the server NTLM challenge.  This
              behavior used to be the default on kernels older than 2.6.36.

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

              • none - do not cache file data at all

              • strict - follow the CIFS/SMB2 protocol strictly

              • loose - allow loose caching semantics

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

       nostrictsync
              Do not ask the server to flush on fsync().  Some servers perform
              non-buffered writes by default in which case flushing is
              redundant. In workloads where a client is performing a lot of
              small write + fsync combinations and where network latency is much
              higher than the server latency, this brings a 2x performance
              improvement.  This option is also a good candidate in scenarios
              where we want performance over consistency.

       handlecache
              (default) In SMB2 and above, the client often has to open the root
              of the share (empty path) in various places during mount, path
              revalidation and the statfs(2) system call. This option cuts
              redundant round trip traffic (opens and closes) by simply keeping
              the directory handle for the root around once opened.

       nohandlecache
              Disable caching of the share root directory handle.

       handletimeout=arg
              The time (in milliseconds) for which the server should reserve the
              handle after a failover waiting for the client to reconnect.  When
              mounting with resilienthandles or persistenthandles mount option,
              or when their use is requested by the server (continuous
              availability shares) then this parameter overrides the server
              default handle timeout (which for most servers is 120 seconds).

       rwpidforward
              Forward pid of a process who opened a file to any read or write
              operation on that file. This prevent applications like wine(1)
              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 Services for Mac. 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
              (default) Do not translate any of these seven characters.

       mapposix
              Translate reserved characters similarly to mapchars but use the
              mapping from Microsoft "Services For Unix".

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

       backupuid=arg
              File access by this user shall be done with the backup intent flag
              set. 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
              File access by users who are members of this group shall be done
              with the backup intent flag set. 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 supports it).

       ignorecase
              Synonym for nocase.

       sec=arg
              Security mode. Allowed values are:

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

              • krb5 - Use Kerberos version 5 authentication

              • krb5i - Use Kerberos authentication and forcibly enable packet
                signing

              • ntlm - Use NTLM password hashing

              • ntlmi - Use NTLM password hashing and force packet signing

              • ntlmv2 - Use NTLMv2 password hashing

              • ntlmv2i - Use NTLMv2 password hashing and force packet signing

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

              • 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.

       seal   Request encryption at the SMB layer. The encryption algorithm used
              is AES-128-CCM. Requires SMB3 or above (see vers).

       rdma   Connect directly to the server using SMB Direct via a RDMA
              adapter. Requires SMB3 or above (see vers).

       resilienthandles
              Enable resilient handles. If the server supports it, keep opened
              files across reconnections. Requires SMB2.1 (see vers).

       noresilienthandles
              (default) Disable resilient handles.

       persistenthandles
              Enable persistent handles. If the server supports it, keep opened
              files across reconnections. Persistent handles are also valid
              across servers in a cluster and have stronger guarantees than
              resilient handles. Requires SMB3 or above (see vers).

       nopersistenthandles
              (default) Disable persistent handles.

       snapshot=time
              Mount a specific snapshot of the remote share. time must be a
              positive integer identifying the snapshot requested (in
              100-nanosecond units that have elapsed since January 1, 1601, or
              alternatively it can be specified in GMT format e.g.
              @GMT-2019.03.27-20.52.19). Supported in the Linux kernel starting
              from v4.19.

       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).

       forcemandatorylock
              Do not use POSIX locks even when available via unix extensions.
              Always use cifs style mandatory locks.

       locallease
              Check cached leases locally instead of querying the server.

       nolease
              Do not request lease/oplock when openning a file on the server.
              This turns off local caching of IO, byte-range lock and read
              metadata operations (see actimeo for more details about metadata
              caching). Requires SMB2 and above (see vers).

       sfu    When the CIFS or SMB3 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.

       mfsymlinks
              Enable support for Minshall+French symlinks (see
              http://wiki.samba.org/index.php/UNIX_Extensions#Minshall.2BFrench_symlinks).
              This option is ignored when specified together with the sfu
              option. Minshall+French symlinks are used even if the server
              supports the CIFS Unix Extensions.

       echo_interval=n
              sets the interval at which echo requests are sent to the server on
              an idling connection. This setting also affects the time required
              for a connection to an unresponsive server to timeout. Here n is
              the echo interval in seconds. The reconnection happens at twice
              the value of the echo_interval set for an unresponsive server.  If
              this option is not given then the default value of 60 seconds is
              used.  The minimum tunable value is 1 second and maximum can go up
              to 600 seconds.

       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 useful for
              some software), 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.

       posix|unix|linux
              (default) Enable Unix Extensions for this mount. Requires CIFS
              (vers=1.0) or SMB3.1.1 (vers=3.1.1) and a server supporting them.

       noposix|nounix|nolinux
              Disable the 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.

       nodfs  Do not follow Distributed FileSystem referrals. IO on a file not
              stored on the server will fail instead of connecting to the target
              server transparently.

       noautotune
              Use fixed size for kernel recv/send socket buffers.

       nosharesock
              Do not try to reuse sockets if the system is already connected to
              the server via an existing mount point. This will make the client
              always make a new connection to the server no matter what he is
              already connected to. This can be useful in simulating multiple
              clients connecting to the same server, as each mount point will
              use a different TCP socket.

       noblocksend
              Send data on the socket using non blocking operations
              (MSG_DONTWAIT flag).

       rsize=bytes
              Maximum amount of data that the kernel will request in a read
              request in bytes. Maximum size that servers will accept is
              typically 8MB for SMB3 or later dialects. Default requested during
              mount is 4MB. Prior to the 4.20 kernel the default requested was
              1MB. Prior to the SMB2.1 dialect the maximum was usually 64K.

       wsize=bytes
              Maximum amount of data that the kernel will send in a write
              request in bytes. Maximum size that servers will accept is
              typically 8MB for SMB3 or later dialects. Default requested during
              mount is 4MB. Prior to the 4.20 kernel the default requested was
              1MB. Prior to the SMB2.1 dialect the maximum was usually 64K.

       bsize=bytes
              Override the default blocksize (1MB) reported on SMB3 files
              (requires kernel version of 5.1 or later). Prior to kernel version
              5.1, the blocksize was always reported as 16K instead of 1MB (and
              was not configurable) which can hurt the performance of tools like
              cp and scp (especially for uncached I/O) which decide on the read
              and write size to use for file copies based on the inode
              blocksize. bsize may not be less than 16K or greater than 16M.

       max_credits=n
              Maximum credits the SMB2 client can have. Default is 32000. Must
              be set to a number between 20 and 60000.

       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 overridden 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 .

       vers=arg
              SMB protocol version. Allowed values are:

              • 1.0 - The classic CIFS/SMBv1 protocol.

              • 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.

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

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

              • 3.02 or 3.0.2 - The SMBv3.0.2 protocol that was introduced in
                Microsoft Windows 8.1 and Windows Server 2012R2.

              • 3.1.1 or 3.11 - The SMBv3.1.1 protocol that was introduced in
                Microsoft Windows 10 and Windows Server 2016.

              • 3 - The SMBv3.0 protocol version and above.

              • default - Tries to negotiate the highest SMB2+ version supported
                by both the client and server.

              If no dialect is specified on mount vers=default is used.  To
              check Dialect refer to /proc/fs/cifs/DebugData

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

              The default since v4.13.5 is for the client and server to
              negotiate the highest possible version greater than or equal to
              2.1. In kernels prior to v4.13, the default was 1.0. For kernels
              between v4.13 and v4.13.5 the default is 3.0.

       --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 backslashes (\)
       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:

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

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

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

       • 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:

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

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

       • 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 ownership 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 Backup Operators group,
       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. Only users matching either
       backupuid or backupgid shall attempt to access files with backup intent.
       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
       overridden. 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 noexec 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 and performance statistics.
       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. When configuring dynamic
       tracing (trace-cmd) note that the list of SMB3 events which can be
       enabled can be seen at: /sys/kernel/debug/tracing/events/cifs/.

SECURITY
       The use of SMB2.1 or later (including the latest dialect SMB3.1.1) is
       recommended for improved security and SMB1 is no longer requested by
       default at mount time. Old dialects such as CIFS (SMB1, ie vers=1.0) have
       much weaker security. Use of CIFS (SMB1) can be disabled by modprobe cifs
       disable_legacy_dialects=y.

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 2.18 of the cifs vfs filesystem
       (roughly Linux kernel 5.0).

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 maintainer of the Linux cifs vfs is Steve French. The maintainer of
       the cifs-utils suite of user space tools is Pavel Shilovsky. The Linux
       CIFS Mailing list is the preferred place to ask questions regarding these
       programs.



                                                       MOUNT.CIFS, MOUNT.SMB3(8)