mount.cifs

MOUNT.CIFS(8)                                                    MOUNT.CIFS(8)



NAME
       mount.cifs - 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.

       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.

       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 .

       prefixpath=arg
              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=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(8)