Provided by: cifs-utils_7.0-2.1_amd64 
NAME
mount.cifs, mount.smb3 - mount using the Common Internet File System (CIFS)
SYNOPSIS
mount.cifs {service} {mount-point} [-o options]
This tool is part of the cifs-utils suite.
mount.cifs mounts a CIFS or SMB3 filesystem from Linux. It is usually invoked indirectly by the mount(8)
command when using the "-t cifs" option. This command only works in Linux, and the kernel must support
the cifs filesystem. The SMB3 protocol is the successor to the CIFS (SMB) protocol and is supported by
most Windows servers, Azure (cloud storage), Macs and many other commercial servers and Network Attached
Storage appliances as well as by the popular Open Source server Samba.
mount.smb3 mounts only SMB3 filesystem. It is usually invoked indirectly by the mount(8) command when
using the "-t smb3" option. The smb3 filesystem type was added in kernel-4.18 and above. It works in a
similar fashion as mount.cifs except it passes filesystem type as smb3.
The mount.cifs utility attaches the UNC name (exported network resource) specified as service (using
//server/share syntax, where "server" is the server name or IP address and "share" is the name of the
share) to the local directory mount-point.
Options to mount.cifs are specified as a comma-separated list of key=value pairs. It is possible to send
options other than those listed here, assuming that the cifs filesystem kernel module (cifs.ko) supports
them. Unrecognized cifs mount options passed to the cifs vfs kernel code will be logged to the kernel
log.
mount.cifs causes the cifs vfs to launch a thread named cifsd. After mounting it keeps running until the
mounted resource is unmounted (usually via the umount utility).
mount.cifs -V command displays the version of cifs mount helper.
modinfo cifs command displays the version of cifs module.
OPTIONS
username=arg|user=arg
specifies the username to connect as. If this is not given, then the environment variable USER is
used.
Earlier versions of mount.cifs also allowed one to specify the username in a user%password or
workgroup/user or workgroup/user%password to allow the password and workgroup to be specified as
part of the username. Support for those alternate username formats is now deprecated and should no
longer be used. Users should use the discrete password= and domain= to specify those values. While
some versions of the cifs kernel module accept user= as an abbreviation for this option, its use
can confuse the standard mount program into thinking that this is a non-superuser mount. It is
therefore recommended to use the full username= option name.
password=arg|pass=arg
specifies the CIFS password. If this option is not given then the environment variable PASSWD is
used. If the password is not specified directly or indirectly via an argument to mount, mount.cifs
will prompt for a password, unless the guest option is specified.
Note that a password which contains the delimiter character (i.e. a comma ',') will fail to be
parsed correctly on the command line. However, the same password defined in the PASSWD environment
variable or via a credentials file (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.
nolease
Do not request lease/oplock when openning a file on the server. This turns off local caching of
IO, byte-range lock and read metadata operations (see actimeo for more details about metadata
caching). Requires SMB2 and above (see vers).
sfu When the CIFS or SMB3 Unix Extensions are not negotiated, attempt to create device files and fifos
in a format compatible with Services for Unix (SFU). In addition retrieve bits 10-12 of the mode
via the SETFILEBITS extended attribute (as SFU does). In the future the bottom 9 bits of the mode
mode also will be emulated using queries of the security descriptor (ACL). [NB: requires version
1.39 or later of the CIFS VFS. To recognize symlinks and be able to create symlinks in an SFU
interoperable form requires version 1.40 or later of the CIFS VFS kernel module.
mfsymlinks
Enable support for Minshall+French symlinks (see
http://wiki.samba.org/index.php/UNIX_Extensions#Minshall.2BFrench_symlinks). This option is
ignored when specified together with the sfu option. Minshall+French symlinks are used even if the
server supports the CIFS Unix Extensions.
echo_interval=n
sets the interval at which echo requests are sent to the server on an idling connection. This
setting also affects the time required for a connection to an unresponsive server to timeout. Here
n is the echo interval in seconds. The reconnection happens at twice the value of the
echo_interval set for an unresponsive server. If this option is not given then the default value
of 60 seconds is used. The minimum tunable value is 1 second and maximum can go up to 600
seconds.
serverino
Use inode numbers (unique persistent file identifiers) returned by the server instead of
automatically generating temporary inode numbers on the client. Although server inode numbers make
it easier to spot hardlinked files (as they will have the same inode numbers) and inode numbers
may be persistent (which is useful for some software), the server does not guarantee that the
inode numbers are unique if multiple server side mounts are exported under a single share (since
inode numbers on the servers might not be unique if multiple filesystems are mounted under the
same shared higher level directory). Note that not all servers support returning server inode
numbers, although those that support the CIFS Unix Extensions, and Windows 2000 and later servers
typically do support this (although not necessarily on every local server filesystem). Parameter
has no effect if the server lacks support for returning inode numbers or equivalent. This behavior
is enabled by default.
noserverino
Client generates inode numbers itself rather than using the actual ones from the server.
See section INODE NUMBERS for more information.
posix|unix|linux
(default) Enable Unix Extensions for this mount. Requires CIFS (vers=1.0) or SMB3.1.1 (vers=3.1.1)
and a server supporting them.
noposix|nounix|nolinux
Disable the Unix Extensions for this mount. This can be useful in order to turn off multiple
settings at once. This includes POSIX acls, POSIX locks, POSIX paths, symlink support and
retrieving uids/gids/mode from the server. This can also be useful to work around a bug in a
server that supports Unix Extensions.
See section INODE NUMBERS for more information.
nouser_xattr
Do not allow getfattr/setfattr to get/set xattrs, even if server would support it otherwise. The
default is for xattr support to be enabled.
nodfs Do not follow Distributed FileSystem referrals. IO on a file not stored on the server will fail
instead of connecting to the target server transparently.
noautotune
Use fixed size for kernel recv/send socket buffers.
nosharesock
Do not try to reuse sockets if the system is already connected to the server via an existing mount
point. This will make the client always make a new connection to the server no matter what he is
already connected to. This can be useful in simulating multiple clients connecting to the same
server, as each mount point will use a different TCP socket.
noblocksend
Send data on the socket using non blocking operations (MSG_DONTWAIT flag).
rsize=bytes
Maximum amount of data that the kernel will request in a read request in bytes. Maximum size that
servers will accept is typically 8MB for SMB3 or later dialects. Default requested during mount is
4MB. Prior to the 4.20 kernel the default requested was 1MB. Prior to the SMB2.1 dialect the
maximum was usually 64K.
wsize=bytes
Maximum amount of data that the kernel will send in a write request in bytes. Maximum size that
servers will accept is typically 8MB for SMB3 or later dialects. Default requested during mount is
4MB. Prior to the 4.20 kernel the default requested was 1MB. Prior to the SMB2.1 dialect the
maximum was usually 64K.
bsize=bytes
Override the default blocksize (1MB) reported on SMB3 files (requires kernel version of 5.1 or
later). Prior to kernel version 5.1, the blocksize was always reported as 16K instead of 1MB (and
was not configurable) which can hurt the performance of tools like cp and scp (especially for
uncached I/O) which decide on the read and write size to use for file copies based on the inode
blocksize. bsize may not be less than 16K or greater than 16M.
max_credits=n
Maximum credits the SMB2 client can have. Default is 32000. Must be set to a number between 20 and
60000.
fsc Enable local disk caching using FS-Cache for CIFS. This option could be useful to improve
performance on a slow link, heavily loaded server and/or network where reading from the disk is
faster than reading from the server (over the network). This could also impact the scalability
positively as the number of calls to the server are reduced. But, be warned that local caching is
not suitable for all workloads, for e.g., read-once type workloads. So, you need to consider
carefully the situation/workload before using this option. Currently, local disk caching is
enabled for CIFS files opened as read-only.
NOTE: This feature is available only in the recent kernels that have been built with the kernel
config option CONFIG_CIFS_FSCACHE. You also need to have cachefilesd daemon installed and running
to make the cache operational.
multiuser
Map user accesses to individual credentials when accessing the server. By default, CIFS mounts
only use a single set of user credentials (the mount credentials) when accessing a share. With
this option, the client instead creates a new session with the server using the user's credentials
whenever a new user accesses the mount. Further accesses by that user will also use those
credentials. Because the kernel cannot prompt for passwords, multiuser mounts are limited to
mounts using sec= options that don't require passwords.
With this change, it's feasible for the server to handle permissions enforcement, so this option
also implies noperm . Furthermore, when unix extensions aren't in use and the administrator has
not overridden ownership using the uid= or gid= options, ownership of files is presented as the
current user accessing the share.
actimeo=arg
The time (in seconds) that the CIFS client caches attributes of a file or directory before it
requests attribute information from a server. During this period the changes that occur on the
server remain undetected until the client checks the server again.
By default, the attribute cache timeout is set to 1 second. This means more frequent on-the-wire
calls to the server to check whether attributes have changed which could impact performance. With
this option users can make a tradeoff between performance and cache metadata correctness,
depending on workload needs. Shorter timeouts mean better cache coherency, but frequent increased
number of calls to the server. Longer timeouts mean a reduced number of calls to the server but
looser cache coherency. The actimeo value is a positive integer that can hold values between 0 and
a maximum value of 2^30 * HZ (frequency of timer interrupt) setting.
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 .
vers=arg
SMB protocol version. Allowed values are:
• 1.0 - The classic CIFS/SMBv1 protocol.
• 2.0 - The SMBv2.002 protocol. This was initially introduced in Windows Vista Service Pack 1, and
Windows Server 2008. Note that the initial release version of Windows Vista spoke a slightly
different dialect (2.000) that is not supported.
• 2.1 - The SMBv2.1 protocol that was introduced in Microsoft Windows 7 and Windows Server 2008R2.
• 3.0 - The SMBv3.0 protocol that was introduced in Microsoft Windows 8 and Windows Server 2012.
• 3.02 or 3.0.2 - The SMBv3.0.2 protocol that was introduced in Microsoft Windows 8.1 and Windows
Server 2012R2.
• 3.1.1 or 3.11 - The SMBv3.1.1 protocol that was introduced in Microsoft Windows 10 and Windows
Server 2016.
• 3 - The SMBv3.0 protocol version and above.
• default - Tries to negotiate the highest SMB2+ version supported by both the client and server.
If no dialect is specified on mount vers=default is used. To check Dialect refer to
/proc/fs/cifs/DebugData
Note too that while this option governs the protocol version used, not all features of each
version are available.
The default since v4.13.5 is for the client and server to negotiate the highest possible version
greater than or equal to 2.1. In kernels prior to v4.13, the default was 1.0. For kernels between
v4.13 and v4.13.5 the default is 3.0.
--verbose
Print additional debugging information for the mount. Note that this parameter must be specified
before the -o . For example:
mount -t cifs //server/share /mnt --verbose -o user=username
SERVICE FORMATTING AND DELIMITERS
It's generally preferred to use forward slashes (/) as a delimiter in service names. They are considered
to be the "universal delimiter" since they are generally not allowed to be embedded within path
components on Windows machines and the client can convert them to backslashes (\) unconditionally.
Conversely, backslash characters are allowed by POSIX to be part of a path component, and can't be
automatically converted in the same way.
mount.cifs will attempt to convert backslashes to forward slashes where it's able to do so, but it cannot
do so in any path component following the sharename.
INODE NUMBERS
When Unix Extensions are enabled, we use the actual inode number provided by the server in response to
the POSIX calls as an inode number.
When Unix Extensions are disabled and serverino mount option is enabled there is no way to get the server
inode number. The client typically maps the server-assigned UniqueID onto an inode number.
Note that the UniqueID is a different value from the server inode number. The UniqueID value is unique
over the scope of the entire server and is often greater than 2 power 32. This value often makes programs
that are not compiled with LFS (Large File Support), to trigger a glibc EOVERFLOW error as this won't fit
in the target structure field. It is strongly recommended to compile your programs with LFS support (i.e.
with -D_FILE_OFFSET_BITS=64) to prevent this problem. You can also use noserverino mount option to
generate inode numbers smaller than 2 power 32 on the client. But you may not be able to detect hardlinks
properly.
CACHE COHERENCY
With a network filesystem such as CIFS or NFS, the client must contend with the fact that activity on
other clients or the server could change the contents or attributes of a file without the client being
aware of it. One way to deal with such a problem is to mandate that all file accesses go to the server
directly. This is performance prohibitive however, so most protocols have some mechanism to allow the
client to cache data locally.
The CIFS protocol mandates (in effect) that the client should not cache file data unless it holds an
opportunistic lock (aka oplock) or a lease. Both of these entities allow the client to guarantee certain
types of exclusive access to a file so that it can access its contents without needing to continually
interact with the server. The server will call back the client when it needs to revoke either of them and
allow the client a certain amount of time to flush any cached data.
The cifs client uses the kernel's pagecache to cache file data. Any I/O that's done through the pagecache
is generally page-aligned. This can be problematic when combined with byte-range locks as Windows'
locking is mandatory and can block reads and writes from occurring.
cache=none means that the client never utilizes the cache for normal reads and writes. It always accesses
the server directly to satisfy a read or write request.
cache=strict means that the client will attempt to follow the CIFS/SMB2 protocol strictly. That is, the
cache is only trusted when the client holds an oplock. When the client does not hold an oplock, then the
client bypasses the cache and accesses the server directly to satisfy a read or write request. By doing
this, the client avoids problems with byte range locks. Additionally, byte range locks are cached on the
client when it holds an oplock and are "pushed" to the server when that oplock is recalled.
cache=loose allows the client to use looser protocol semantics which can sometimes provide better
performance at the expense of cache coherency. File access always involves the pagecache. When an oplock
or lease is not held, then the client will attempt to flush the cache soon after a write to a file. Note
that that flush does not necessarily occur before a write system call returns.
In the case of a read without holding an oplock, the client will attempt to periodically check the
attributes of the file in order to ascertain whether it has changed and the cache might no longer be
valid. This mechanism is much like the one that NFSv2/3 use for cache coherency, but it particularly
problematic with CIFS. Windows is quite "lazy" with respect to updating the LastWriteTime field that the
client uses to verify this. The effect is that cache=loose can cause data corruption when multiple
readers and writers are working on the same files.
Because of this, when multiple clients are accessing the same set of files, then cache=strict is
recommended. That helps eliminate problems with cache coherency by following the CIFS/SMB2 protocols more
strictly.
Note too that no matter what caching model is used, the client will always use the pagecache to handle
mmap'ed files. Writes to mmap'ed files are only guaranteed to be flushed to the server when msync() is
called, or on close().
The default in kernels prior to 3.7 was loose. As of 3.7, the default is strict.
CIFS/NTFS ACL, SID/UID/GID MAPPING, SECURITY DESCRIPTORS
This option is used to work with file objects which posses Security Descriptors and CIFS/NTFS ACL instead
of UID, GID, file permission bits, and POSIX ACL as user authentication model. This is the most common
authentication model for CIFS servers and is the one used by Windows.
Support for this requires both CIFS_XATTR and CIFS_ACL support in the CIFS configuration options when
building the cifs module.
A CIFS/NTFS ACL is mapped to file permission bits using an algorithm specified in the following Microsoft
TechNet document:
http://technet.microsoft.com/en-us/library/bb463216.aspx
In order to map SIDs to/from UIDs and GIDs, the following is required:
• a kernel upcall to the cifs.idmap utility set up via request-key.conf(5)
• winbind support configured via nsswitch.conf(5) and smb.conf(5)
Please refer to the respective manpages of cifs.idmap(8) and winbindd(8) for more information.
Security descriptors for a file object can be retrieved and set directly using extended attribute named
system.cifs_acl. The security descriptors presented via this interface are "raw" blobs of data and need a
userspace utility to either parse and format or to assemble it such as getcifsacl(1) and setcifsacl(1)
respectively.
Some of the things to consider while using this mount option:
• There may be an increased latency when handling metadata due to additional requests to get and set
security descriptors.
• The mapping between a CIFS/NTFS ACL and POSIX file permission bits is imperfect and some ACL
information may be lost in the translation.
• If either upcall to cifs.idmap is not setup correctly or winbind is not configured and running, ID
mapping will fail. In that case uid and gid will default to either to those values of the share or to
the values of uid and/or gid mount options if specified.
ACCESSING FILES WITH BACKUP INTENT
For an user on the server, desired access to a file is determined by the permissions and rights
associated with that file. This is typically accomplished using ownership and ACL. For a user who does
not have access rights to a file, it is still possible to access that file for a specific or a targeted
purpose by granting special rights. One of the specific purposes is to access a file with the intent to
either backup or restore i.e. backup intent. The right to access a file with the backup intent can
typically be granted by making that user a part of the built-in group Backup Operators. Thus, when this
user attempts to open a file with the backup intent, open request is sent by setting the bit
FILE_OPEN_FOR_BACKUP_INTENT as one of the CreateOptions.
As an example, on a Windows server, a user named testuser, cannot open this file with such a security
descriptor:
REVISION:0x1
CONTROL:0x9404
OWNER:Administrator
GROUP:Domain Users
ACL:Administrator:ALLOWED/0x0/FULL
But the user testuser, if it becomes part of the Backup Operators group, can open the file with the
backup intent.
Any user on the client side who can authenticate as such a user on the server, can access the files with
the backup intent. But it is desirable and preferable for security reasons amongst many, to restrict this
special right.
The mount option backupuid is used to restrict this special right to a user which is specified by either
a name or an id. The mount option backupgid is used to restrict this special right to the users in a
group which is specified by either a name or an id. Only users matching either backupuid or backupgid
shall attempt to access files with backup intent. These two mount options can be used together.
FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS
The core CIFS protocol does not provide unix ownership information or mode for files and directories.
Because of this, files and directories will generally appear to be owned by whatever values the uid= or
gid= options are set, and will have permissions set to the default file_mode and dir_mode for the mount.
Attempting to change these values via chmod/chown will return success but have no effect.
When the client and server negotiate unix extensions, files and directories will be assigned the uid,
gid, and mode provided by the server. Because CIFS mounts are generally single-user, and the same
credentials are used no matter what user accesses the mount, newly created files and directories will
generally be given ownership corresponding to whatever credentials were used to mount the share.
If the uid's and gid's being used do not match on the client and server, the forceuid and forcegid
options may be helpful. Note however, that there is no corresponding option to override the mode.
Permissions assigned to a file when forceuid or forcegid are in effect may not reflect the the real
permissions.
When unix extensions are not negotiated, it's also possible to emulate them locally on the server using
the dynperm mount option. When this mount option is in effect, newly created files and directories will
receive what appear to be proper permissions. These permissions are not stored on the server however and
can disappear at any time in the future (subject to the whims of the kernel flushing out the inode
cache). In general, this mount option is discouraged.
It's also possible to override permission checking on the client altogether via the noperm option.
Server-side permission checks cannot be overridden. The permission checks done by the server will always
correspond to the credentials used to mount the share, and not necessarily to the user who is accessing
the share.
ENVIRONMENT VARIABLES
The variable USER may contain the username of the person to be used to authenticate to the server. The
variable can be used to set both username and password by using the format username%password.
The variable PASSWD may contain the password of the person using the client.
The variable PASSWD_FILE may contain the pathname of a file to read the password from. A single line of
input is read and used as the password.
NOTES
This command may be used only by root, unless installed setuid, in which case the noexec and nosuid mount
flags are enabled. When installed as a setuid program, the program follows the conventions set forth by
the mount program for user mounts, with the added restriction that users must be able to chdir() into the
mountpoint prior to the mount in order to be able to mount onto it.
Some samba client tools like smbclient(8) honour client-side configuration parameters present in
smb.conf. Unlike those client tools, mount.cifs ignores smb.conf completely.
CONFIGURATION
The primary mechanism for making configuration changes and for reading debug information for the cifs vfs
is via the Linux /proc filesystem. In the directory /proc/fs/cifs are various configuration files and
pseudo files which can display debug information and performance statistics. There are additional startup
options such as maximum buffer size and number of buffers which only may be set when the kernel cifs vfs
(cifs.ko module) is loaded. These can be seen by running the modinfo utility against the file cifs.ko
which will list the options that may be passed to cifs during module installation (device driver load).
For more information see the kernel file fs/cifs/README. When configuring dynamic tracing (trace-cmd)
note that the list of SMB3 events which can be enabled can be seen at:
/sys/kernel/debug/tracing/events/cifs/.
SECURITY
The use of SMB2.1 or later (including the latest dialect SMB3.1.1) is recommended for improved security
and SMB1 is no longer requested by default at mount time. Old dialects such as CIFS (SMB1, ie vers=1.0)
have much weaker security. Use of CIFS (SMB1) can be disabled by modprobe cifs disable_legacy_dialects=y.
BUGS
Mounting using the CIFS URL specification is currently not supported.
The credentials file does not handle usernames or passwords with leading space.
Note that the typical response to a bug report is a suggestion to try the latest version first. So please
try doing that first, and always include which versions you use of relevant software when reporting bugs
(minimum: mount.cifs (try mount.cifs -V), kernel (see /proc/version) and server type you are trying to
contact.
VERSION
This man page is correct for version 2.18 of the cifs vfs filesystem (roughly Linux kernel 5.0).
SEE ALSO
cifs.upcall(8), getcifsacl(1), setcifsacl(1)
Documentation/filesystems/cifs.txt and fs/cifs/README in the Linux kernel source tree may contain
additional options and information.
AUTHOR
Steve French
The maintainer of the Linux cifs vfs is Steve French. The maintainer of the cifs-utils suite of user
space tools is Pavel Shilovsky. The Linux CIFS Mailing list is the preferred place to ask questions
regarding these programs.
MOUNT.CIFS, MOUNT.SMB3(8)