MOUNT.CIFS, MOUNT.SMB3(8) System Manager's Manual MOUNT.CIFS, MOUNT.SMB3(8)

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

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.

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.

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.

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.

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.
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.
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.
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.
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.
Extract uid/gid from special SID instead of mapping it. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more information.
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.
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.
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.
If the server does not support the CIFS Unix extensions this overrides the default file mode.
If the server does not support the CIFS Unix extensions this overrides the default mode for directories.
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.
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.
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.
don't prompt for a password.
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.
mount read-only.
mount read-write.
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).
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.
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.
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).
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 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.

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.
(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.
Disable caching of the share root directory handle.
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).
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.
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.
(default) Do not translate any of these seven characters.
Translate reserved characters similarly to mapchars but use the mapping from Microsoft "Services For Unix".
currently unimplemented.
(default) currently unimplemented.
The program accessing a file on the cifs mounted file system will hang when the server crashes.
(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.
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.

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.

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.

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.

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

Request encryption at the SMB layer. The encryption algorithm used is AES-128-CCM. Requires SMB3 or above (see vers).
Connect directly to the server using SMB Direct via a RDMA adapter. Requires SMB3 or above (see vers).
Enable resilient handles. If the server supports it, keep opened files across reconnections. Requires SMB2.1 (see vers).
(default) Disable resilient handles.
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).
(default) Disable persistent handles.
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.
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).
Do not use POSIX locks even when available via unix extensions. Always use cifs style mandatory locks.
Check cached leases locally instead of querying the server.
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).
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.
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.
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.
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.
Client generates inode numbers itself rather than using the actual ones from the server.

See section INODE NUMBERS for more information.

(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.
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.

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.
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.
Use fixed size for kernel recv/send socket buffers.
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.
Send data on the socket using non blocking operations (MSG_DONTWAIT flag).
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.
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.
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.
Maximum credits the SMB2 client can have. Default is 32000. Must be set to a number between 20 and 60000.
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.

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.

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.

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.
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.
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.
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.
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.

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.
Inverse of noposixpaths .
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.
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.

Allows the system to ignore any unrecognized mount options that follow this option instead of failing to mount altogether.
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

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.

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.

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.

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.

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.

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.

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.

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.

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/.

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.

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.

This man page is correct for version 2.18 of the cifs vfs filesystem (roughly Linux kernel 5.0).

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.

Steve French

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