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

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

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

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

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

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

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

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

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

       modinfo cifs command displays the version of cifs module.

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

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

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

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

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

			 username=value
			 password=value
			 domain=value

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

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

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

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

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

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

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

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

       servern=arg
	   Synonym for servernetbiosname.

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

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

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

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

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

       guest
	   don´t prompt for a password

       iocharset
	   Charset used to convert local path names to and from Unicode.
	   Unicode is used by default for network path names if the server
	   supports it. If iocharset is not specified then the nls_default
	   specified during the local client kernel build will be used. If
	   server does not support Unicode, this parameter is unused.

       ro
	   mount read-only

       rw
	   mount read-write

       setuids
	   If the CIFS Unix extensions are negotiated with the server the
	   client will attempt to set the effective uid and gid of the local
	   process on newly created files, directories, and devices (create,
	   mkdir, mknod). If the CIFS Unix Extensions are not negotiated, for
	   newly created files and directories instead of using the default
	   uid and gid specified on the the mount, cache the new file´s uid
	   and gid locally which means that the uid for the file can change
	   when the inode is reloaded (or the user remounts the share).

       nosetuids
	   The client will not attempt to set the uid and gid on on newly
	   created files, directories, and devices (create, mkdir, mknod)
	   which will result in the server setting the uid and gid to the
	   default (usually the server uid of the user who mounted the share).
	   Letting the server (rather than the client) set the uid and gid is
	   the default.If the CIFS Unix Extensions are not negotiated then the
	   uid and gid for new files will appear to be the uid (gid) of the
	   mounter or the uid (gid) parameter specified on the mount.

       perm
	   Client does permission checks (vfs_permission check of uid and gid
	   of the file against the mode and desired operation), Note that this
	   is in addition to the normal ACL check on the target machine done
	   by the server software. Client permission checking is enabled by
	   default.

       noperm
	   Client does not do permission checks. This can expose files on this
	   mount to access by other users on the local client system. It is
	   typically only needed when the server supports the CIFS Unix
	   Extensions but the UIDs/GIDs on the client and server system do not
	   match closely enough to allow access by the user doing the mount.
	   Note that this does not affect the normal ACL check on the target
	   machine done by the server software (of the server ACL against the
	   user name provided at mount time).

       dynperm
	   Instructs the server to maintain ownership and permissions in
	   memory that can´t be stored on the server. This information can
	   disappear at any time (whenever the inode is flushed from the
	   cache), so while this may help make some applications work, it´s
	   behavior is somewhat unreliable. See the section below on FILE AND
	   DIRECTORY OWNERSHIP AND PERMISSIONS for more information.

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

	   ·   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".

       directio
	   Do not do inode data caching on files opened on this mount. This
	   precludes mmaping files on this mount. In some cases with fast
	   networks and little or no caching benefits on the client (e.g. when
	   the application is doing large sequential reads bigger than page
	   size without rereading the same data) this can provide better
	   performance than the default behavior which caches reads
	   (readahead) and writes (writebehind) through the local Linux client
	   pagecache if oplock (caching token) is granted and held. Note that
	   direct allows write operations larger than page size to be sent to
	   the server. On some kernels this requires the cifs.ko module to be
	   built with the CIFS_EXPERIMENTAL configure option.

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

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

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

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

       mapchars
	   Translate six of the seven reserved characters (not backslash, but
	   including the colon, question mark, pipe, asterik, greater than and
	   less than characters) to the remap range (above 0xF000), which also
	   allows the CIFS client to recognize files created with such
	   characters by Windows´s POSIX emulation. This can also be useful
	   when mounting to most versions of Samba (which also forbids
	   creating and opening files whose names contain any of these seven
	   characters). This has no effect if the server does not support
	   Unicode on the wire. Please note that the files created with
	   mapchars mount option may not be accessible if the share is mounted
	   without that option.

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

       intr
	   currently unimplemented

       nointr
	   (default) currently unimplemented

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

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

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

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

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

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

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

	   See section ACCESSING FILES WITH BACKUP INTENT for more details

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

	   See section ACCESSING FILES WITH BACKUP INTENT for more details

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

       ignorecase
	   Synonym for nocase.

       sec=
	   Security mode. Allowed values are:

	   ·   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.

       nobrl
	   Do not send byte range lock requests to the server. This is
	   necessary for certain applications that break with cifs style
	   mandatory byte range locks (and most cifs servers do not yet
	   support requesting advisory byte range locks).

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

       serverino
	   Use inode numbers (unique persistent file identifiers) returned by
	   the server instead of automatically generating temporary inode
	   numbers on the client. Although server inode numbers make it easier
	   to spot hardlinked files (as they will have the same inode numbers)
	   and inode numbers may be persistent (which is userful for some
	   sofware), the server does not guarantee that the inode numbers are
	   unique if multiple server side mounts are exported under a single
	   share (since inode numbers on the servers might not be unique if
	   multiple filesystems are mounted under the same shared higher level
	   directory). Note that not all servers support returning server
	   inode numbers, although those that support the CIFS Unix
	   Extensions, and Windows 2000 and later servers typically do support
	   this (although not necessarily on every local server filesystem).
	   Parameter has no effect if the server lacks support for returning
	   inode numbers or equivalent. This behavior is enabled by default.

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

	   See section INODE NUMBERS for more information.

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

	   See section INODE NUMBERS for more information.

       nouser_xattr
	   (default) Do not allow getfattr/setfattr to get/set xattrs, even if
	   server would support it otherwise.

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

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

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

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

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

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

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

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

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

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

       posixpaths
	   Inverse of noposixpaths.

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

       vers=
	   SMB protocol version. Allowed values are:

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

	   ·   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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

AUTHOR
       Steve French

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

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

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