# mount.cifs

## 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 or
                  entered at the password prompt will be read correctly.
    
           **password2=arg|pass2=arg**
                  specifies an alternate password to help with password
                  rotation. If this option is not given, then the environment
                  variable PASSWD2 is used.  If password2 is not specified
                  directly or indirectly via an argument to mount, mount.cifs
                  will NOT prompt for password2.
    
                  Note that a password2 which contains the delimiter
                  character (i.e. a comma ',') will fail to be parsed
                  correctly on the command line. However, the same password2
                  defined in the PASSWD2 environment variable or via a
                  credentials file (see below) will be read correctly.
    
           **credentials=filename|cred=filename**
                  specifies a file that contains a username and/or password
                  and optionally an alternate password and/or the name of the
                  workgroup.  The format of the file is:
    
                     username=value
                     password=value
                     password2=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.
    
           **max_cached_dirs=arg**
                  The maximum number of cached directories per share.
                  Directories are cached locally when a lease is granted by
                  the server, which improves performance by reducing network
                  traffic.
    
                  By default, **max_cached_dirs** is set to 16 and can hold
                  values between 0 and a maximum value of 2^32 - 1.
    
           **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 **CONFIG_CIFS_XATTR** and
                  then **CONFIG_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**).
    
           **esize=arg**
                  The minimum size (in bytes) of an encrypted read response
                  at which the client will offload decryption to a separate
                  worker thread. This will work when the number of in-flight
                  requests are greater than 1.
    
                  Decryption of large encrypted read responses can be
                  computationally expensive and, when multiple reads are in
                  flight, offloading the work can improve performance.
    
                  The default is **esize=0** , which means that all encrypted read
                  responses are decrypted without offload and can be set up
                  to the maximum buffer size, which can range from 8192 to
                  130048 bytes, and defaults to 16384 bytes.
    
           **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. Setting it to 0 disables caching.
    
           **acregmax=arg**
                  The maximum time (in seconds) that the CIFS client caches
                  attributes of a regular file before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, then acregmax value will be set to **actimeo**
                  value, see **actimeo** for more details.
    
           **acdirmax=arg**
                  The maximum time (in seconds) that the CIFS client caches
                  attributes of a directory before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, then acdirmax value will be set to **actimeo**
                  value, see **actimeo** for more details.
    
           **upcall_target=arg**
                  Determines the namespace in which upcalls from the SMB
                  filesystem should be handled.  Allowed values are: - **mount**
                  - Resolve upcalls to the host namespace.  - **app** - Resolve
                  upcalls in the namespace of the calling thread
                  (application).  Default value is **app**.  This option is
                  useful in environments like Kubernetes, where the mount may
                  be performed by a driver pod on behalf of an application
                  running in a separate container. It ensures that Kerberos
                  credentials and other user-specific data are accessed in
                  the correct namespace.  By specifying **app** , upcalls can be
                  resolved in the application's namespace, ensuring the
                  correct credentials are used. **mount** allows resolution in
                  the host namespace, which may be necessary when credentials
                  or configurations are managed outside the container.
    
           **multichannel**
                  This option enables multichannel feature. Multichannel is
                  an SMB3 protocol feature that allows client to establish
                  multiple transport connections to an SMB server and bind
                  them into a single authenticated SMB session. This feature
                  enhances fault tolerance and increases throughput by
                  distributing traffic across several connections. With this
                  mount option default is to use two channels if the server
                  supports multichannel. The **max_channels** parameter can be
                  specified if you desire to use more than two channels.
    
           **max_channels=arg**
                  This option is applicable while using **multichannel** feature.
                  max_channels option allows the user to specify the number
                  of transport connections that should be establised between
                  client and server up to a limit of 16. Using this option
                  implicitly enables the **multichannel** feature.  If
                  max_channels option not specified, **multichannel** feature
                  defaults to using 2 connections.
    
           **closetimeo=arg**
                  The maximum time (in seconds) that the CIFS client defers
                  sending the final SMB3 close when the client has a handle
                  lease on the file.
    
                  By default, **closetimeo** is set to 1 second and can hold
                  values between 0 and a maximum value of 2^30 * HZ.
    
           **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**.
    
           **compress**
                  **EXPERIMENTAL FEATURE** Enables over-the-wire message
                  compression for SMB 3.1.1 or higher mounts. Mount fails
                  when compress is on and **vers** is set to a version lower than
                  3.1.1.
    
           **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**.
    
           **sloppy** Allows the system to ignore any unrecognized mount options
                  that follow this option instead of failing to mount
                  altogether.
    
           **--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 cifs kernel module built with both
           **CONFIG_CIFS_XATTR** and **CONFIG_CIFS_ACL** options enabled.  Since
           Linux 5.3, **CONFIG_CIFS_ACL** option no longer exists as CIFS/NTFS
           ACL support is always built into cifs kernel module.
    
           Most distribution kernels will already have those options enabled
           by default, but you can still check if they are enabled with:
    
              cat /lib/modules/$(uname -r)/build/.config
    
           Alternatively, if kernel is configured with **CONFIG_IKCONFIG_PROC** :
    
              zcat /proc/config.gz
    
           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 or sssd support configured via nsswitch.conf(5)
    
           Please refer to the respective manpages of cifs.idmap(8),
           winbindd(8) and sssd(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
             or sssd 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 variables **PASSWD** and **PASSWD2** may contain the password and the
           alternate password of the person using the client, respectively.
    
           The variables **PASSWD_FILE** and **PASSWD2_FILE** may contain the
           pathname of the file to read password or password2 from,
           respectively.  A single line of input is read and used as the
           password in each case.
    


## 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).
    


* * *