==========

mount.cifs

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mount using the Common Internet File System (CIFS)

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:Manual section: 8

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SYNOPSIS

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  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 <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 <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

*****