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MOUNT.CIF...OUNT.SMB3(8) System Manager's Manual MOUNT.CIF...OUNT.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.
username=arg|user=arg
specifies the username to connect as. If this is not given,
then the environment variable USER is used.
Earlier versions of mount.cifs also allowed one to specify
the username in a user%password or workgroup/user or
workgroup/user%password to allow the password and workgroup
to be specified as part of the username. Support for those
alternate username formats is now deprecated and should no
longer be used. Users should use the discrete password= and
domain= to specify those values. While some versions of the
cifs kernel module accept user= as an abbreviation for this
option, its use can confuse the standard mount program into
thinking that this is a non-superuser mount. It is
therefore recommended to use the full username= option
name.
password=arg|pass=arg
specifies the CIFS password. If this option is not given
then the environment variable PASSWD is used. If the
password is not specified directly or indirectly via an
argument to mount, mount.cifs will prompt for a password,
unless the guest option is specified.
Note that a password which contains the delimiter character
(i.e. a comma ',') will fail to be parsed correctly on the
command line. However, the same password defined in the
PASSWD environment variable or via a credentials file or
entered at the password prompt will be read correctly.
password2=arg|pass2=arg
specifies an alternate password to help with password
rotation. If this option is not given, then the environment
variable PASSWD2 is used. If password2 is not specified
directly or indirectly via an argument to mount, mount.cifs
will NOT prompt for password2.
Note that a password2 which contains the delimiter
character (i.e. a comma ',') will fail to be parsed
correctly on the command line. However, the same password2
defined in the PASSWD2 environment variable or via a
credentials file (see below) will be read correctly.
credentials=filename|cred=filename
specifies a file that contains a username and/or password
and optionally an alternate password and/or the name of the
workgroup. The format of the file is:
username=value
password=value
password2=value
domain=value
This is preferred over having passwords in plaintext in a
shared file, such as /etc/fstab . Be sure to protect any
credentials file properly.
uid=arg
sets the uid that will own all files or directories on the
mounted filesystem when the server does not provide
ownership information. It may be specified as either a
username or a numeric uid. When not specified, the default
is uid 0. The mount.cifs helper must be at version 1.10 or
higher to support specifying the uid in non-numeric form.
See the section on FILE AND DIRECTORY OWNERSHIP AND
PERMISSIONS below for more information.
forceuid
instructs the client to ignore any uid provided by the
server for files and directories and to always assign the
owner to be the value of the uid= option. See the section
on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for
more information.
cruid=arg
sets the uid of the owner of the credentials cache. This is
primarily useful with sec=krb5. The default is the real uid
of the process performing the mount. Setting this parameter
directs the upcall to look for a credentials cache owned by
that user.
gid=arg
sets the gid that will own all files or directories on the
mounted filesystem when the server does not provide
ownership information. It may be specified as either a
groupname or a numeric gid. When not specified, the default
is gid 0. The mount.cifs helper must be at version 1.10 or
higher to support specifying the gid in non-numeric form.
See the section on FILE AND DIRECTORY OWNERSHIP AND
PERMISSIONS below for more information.
forcegid
instructs the client to ignore any gid provided by the
server for files and directories and to always assign the
owner to be the value of the gid= option. See the section
on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for
more information.
idsfromsid
Extract uid/gid from special SID instead of mapping it. See
the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS
below for more information.
port=arg
sets the port number on which the client will attempt to
contact the CIFS server. If this value is specified, look
for an existing connection with this port, and use that if
one exists. If one doesn't exist, try to create a new
connection on that port. If that connection fails, return
an error. If this value isn't specified, look for an
existing connection on port 445 or 139. If no such
connection exists, try to connect on port 445 first and
then port 139 if that fails. Return an error if both fail.
netbiosname=arg
When mounting to servers via port 139, specifies the
RFC1001 source name to use to represent the client netbios
machine during the netbios session initialization.
servern=arg
Similar to netbiosname except it specifies the netbios name
of the server instead of the client. Although rarely needed
for mounting to newer servers, this option is needed for
mounting to some older servers (such as OS/2 or Windows 98
and Windows ME) since when connecting over port 139 they,
unlike most newer servers, do not support a default server
name. A server name can be up to 15 characters long and is
usually uppercased.
file_mode=arg
If the server does not support the CIFS Unix extensions
this overrides the default file mode.
dir_mode=arg
If the server does not support the CIFS Unix extensions
this overrides the default mode for directories.
ip=arg|addr=arg
sets the destination IP address. This option is set
automatically if the server name portion of the requested
UNC name can be resolved so rarely needs to be specified by
the user.
domain=arg|dom=arg|workgroup=arg
Sets the domain (workgroup) of the user. If no domains are
given, the empty domain will be used. Use domainauto to
automatically guess the domain of the server you are
connecting to.
domainauto
When using NTLM authentication and not providing a domain
via domain, guess the domain from the server NTLM
challenge. This behavior used to be the default on kernels
older than 2.6.36.
guest don't prompt for a password.
iocharset
Charset used to convert local path names to and from
Unicode. Unicode is used by default for network path names
if the server supports it. If iocharset is not specified
then the nls_default specified during the local client
kernel build will be used. If server does not support
Unicode, this parameter is unused.
ro mount read-only.
rw mount read-write.
setuids
If the CIFS Unix extensions are negotiated with the server
the client will attempt to set the effective uid and gid of
the local process on newly created files, directories, and
devices (create, mkdir, mknod). If the CIFS Unix Extensions
are not negotiated, for newly created files and directories
instead of using the default uid and gid specified on the
the mount, cache the new file's uid and gid locally which
means that the uid for the file can change when the inode
is reloaded (or the user remounts the share).
nosetuids
The client will not attempt to set the uid and gid on on
newly created files, directories, and devices (create,
mkdir, mknod) which will result in the server setting the
uid and gid to the default (usually the server uid of the
user who mounted the share). Letting the server (rather
than the client) set the uid and gid is the default. If the
CIFS Unix Extensions are not negotiated then the uid and
gid for new files will appear to be the uid (gid) of the
mounter or the uid (gid) parameter specified on the mount.
perm Client does permission checks (vfs_permission check of uid
and gid of the file against the mode and desired
operation), Note that this is in addition to the normal ACL
check on the target machine done by the server software.
Client permission checking is enabled by default.
noperm Client does not do permission checks. This can expose files
on this mount to access by other users on the local client
system. It is typically only needed when the server
supports the CIFS Unix Extensions but the UIDs/GIDs on the
client and server system do not match closely enough to
allow access by the user doing the mount. Note that this
does not affect the normal ACL check on the target machine
done by the server software (of the server ACL against the
user name provided at mount time).
dynperm
Instructs the server to maintain ownership and permissions
in memory that can't be stored on the server. This
information can disappear at any time (whenever the inode
is flushed from the cache), so while this may help make
some applications work, it's behavior is somewhat
unreliable. See the section below on FILE AND DIRECTORY
OWNERSHIP AND PERMISSIONS for more information.
cache=arg
Cache mode. See the section below on CACHE COHERENCY for
details. Allowed values are:
• none - do not cache file data at all
• strict - follow the CIFS/SMB2 protocol strictly
• loose - allow loose caching semantics
The default in kernels prior to 3.7 was loose. As of kernel
3.7 the default is strict.
nostrictsync
Do not ask the server to flush on fsync(). Some servers
perform non-buffered writes by default in which case
flushing is redundant. In workloads where a client is
performing a lot of small write + fsync combinations and
where network latency is much higher than the server
latency, this brings a 2x performance improvement. This
option is also a good candidate in scenarios where we want
performance over consistency.
handlecache
(default) In SMB2 and above, the client often has to open
the root of the share (empty path) in various places during
mount, path revalidation and the statfs(2) system call.
This option cuts redundant round trip traffic (opens and
closes) by simply keeping the directory handle for the root
around once opened.
nohandlecache
Disable caching of the share root directory handle.
max_cached_dirs=arg
The maximum number of cached directories per share.
Directories are cached locally when a lease is granted by
the server, which improves performance by reducing network
traffic.
By default, max_cached_dirs is set to 16 and can hold
values between 0 and a maximum value of 2^32 - 1.
handletimeout=arg
The time (in milliseconds) for which the server should
reserve the handle after a failover waiting for the client
to reconnect. When mounting with resilienthandles or
persistenthandles mount option, or when their use is
requested by the server (continuous availability shares)
then this parameter overrides the server default handle
timeout (which for most servers is 120 seconds).
rwpidforward
Forward pid of a process who opened a file to any read or
write operation on that file. This prevent applications
like wine(1) from failing on read and write if we use
mandatory brlock style.
mapchars
Translate six of the seven reserved characters (not
backslash, but including the colon, question mark, pipe,
asterik, greater than and less than characters) to the
remap range (above 0xF000), which also allows the CIFS
client to recognize files created with such characters by
Windows's Services for Mac. This can also be useful when
mounting to most versions of Samba (which also forbids
creating and opening files whose names contain any of these
seven characters). This has no effect if the server does
not support Unicode on the wire. Please note that the files
created with mapchars mount option may not be accessible if
the share is mounted without that option.
nomapchars
(default) Do not translate any of these seven characters.
mapposix
Translate reserved characters similarly to mapchars but use
the mapping from Microsoft "Services For Unix".
intr currently unimplemented.
nointr (default) currently unimplemented.
hard The program accessing a file on the cifs mounted file
system will hang when the server crashes.
soft (default) The program accessing a file on the cifs mounted
file system will not hang when the server crashes and will
return errors to the user application.
noacl Do not allow POSIX ACL operations even if server would
support them.
The CIFS client can get and set POSIX ACLs (getfacl,
setfacl) to Samba servers version 3.0.10 and later. Setting
POSIX ACLs requires enabling both CONFIG_CIFS_XATTR and
then CONFIG_CIFS_POSIX support in the CIFS configuration
options when building the cifs module. POSIX ACL support
can be disabled on a per mount basis by specifying noacl on
mount.
cifsacl
This option is used to map CIFS/NTFS ACLs to/from Linux
permission bits, map SIDs to/from UIDs and GIDs, and get
and set Security Descriptors.
See section on CIFS/NTFS ACL, SID/UID/GID MAPPING, SECURITY
DESCRIPTORS for more information.
backupuid=arg
File access by this user shall be done with the backup
intent flag set. Either a name or an id must be provided as
an argument, there are no default values.
See section ACCESSING FILES WITH BACKUP INTENT for more
details.
backupgid=arg
File access by users who are members of this group shall be
done with the backup intent flag set. Either a name or an
id must be provided as an argument, there are no default
values.
See section ACCESSING FILES WITH BACKUP INTENT for more
details.
nocase Request case insensitive path name matching (case sensitive
is the default if the server supports it).
ignorecase
Synonym for nocase.
sec=arg
Security mode. Allowed values are:
• none - attempt to connection as a null user (no name)
• krb5 - Use Kerberos version 5 authentication
• krb5i - Use Kerberos authentication and forcibly enable
packet signing
• ntlm - Use NTLM password hashing
• ntlmi - Use NTLM password hashing and force packet
signing
• ntlmv2 - Use NTLMv2 password hashing
• ntlmv2i - Use NTLMv2 password hashing and force packet
signing
• ntlmssp - Use NTLMv2 password hashing encapsulated in Raw
NTLMSSP message
• ntlmsspi - Use NTLMv2 password hashing encapsulated in
Raw NTLMSSP message, and force packet signing
The default in mainline kernel versions prior to v3.8 was
sec=ntlm. In v3.8, the default was changed to sec=ntlmssp.
If the server requires signing during protocol negotiation,
then it may be enabled automatically. Packet signing may
also be enabled automatically if it's enabled in
/proc/fs/cifs/SecurityFlags.
seal Request encryption at the SMB layer. The encryption
algorithm used is AES-128-CCM. Requires SMB3 or above (see
vers).
esize=arg
The minimum size (in bytes) of an encrypted read response
at which the client will offload decryption to a separate
worker thread. This will work when the number of in-flight
requests are greater than 1.
Decryption of large encrypted read responses can be
computationally expensive and, when multiple reads are in
flight, offloading the work can improve performance.
The default is esize=0, which means that all encrypted read
responses are decrypted without offload and can be set up
to the maximum buffer size, which can range from 8192 to
130048 bytes, and defaults to 16384 bytes.
rdma Connect directly to the server using SMB Direct via a RDMA
adapter. Requires SMB3 or above (see vers).
resilienthandles
Enable resilient handles. If the server supports it, keep
opened files across reconnections. Requires SMB2.1 (see
vers).
noresilienthandles
(default) Disable resilient handles.
persistenthandles
Enable persistent handles. If the server supports it, keep
opened files across reconnections. Persistent handles are
also valid across servers in a cluster and have stronger
guarantees than resilient handles. Requires SMB3 or above
(see vers).
nopersistenthandles
(default) Disable persistent handles.
snapshot=time
Mount a specific snapshot of the remote share. time must be
a positive integer identifying the snapshot requested (in
100-nanosecond units that have elapsed since January 1,
1601, or alternatively it can be specified in GMT format
e.g. @GMT-2019.03.27-20.52.19). Supported in the Linux
kernel starting from v4.19.
nobrl Do not send byte range lock requests to the server. This is
necessary for certain applications that break with cifs
style mandatory byte range locks (and most cifs servers do
not yet support requesting advisory byte range locks).
forcemandatorylock
Do not use POSIX locks even when available via unix
extensions. Always use cifs style mandatory locks.
locallease
Check cached leases locally instead of querying the server.
nolease
Do not request lease/oplock when openning a file on the
server. This turns off local caching of IO, byte-range lock
and read metadata operations (see actimeo for more details
about metadata caching). Requires SMB2 and above (see
vers).
sfu When the CIFS or SMB3 Unix Extensions are not negotiated,
attempt to create device files and fifos in a format
compatible with Services for Unix (SFU). In addition
retrieve bits 10-12 of the mode via the SETFILEBITS
extended attribute (as SFU does). In the future the bottom
9 bits of the mode mode also will be emulated using queries
of the security descriptor (ACL). [NB: requires version
1.39 or later of the CIFS VFS. To recognize symlinks and be
able to create symlinks in an SFU interoperable form
requires version 1.40 or later of the CIFS VFS kernel
module.
mfsymlinks
Enable support for Minshall+French symlinks (see
<http://wiki.samba.org/index.php/UNIX_Extensions#Minshall.2BFrench_symlinks>
). This option is ignored when specified together with the
sfu option. Minshall+French symlinks are used even if the
server supports the CIFS Unix Extensions.
echo_interval=n
sets the interval at which echo requests are sent to the
server on an idling connection. This setting also affects
the time required for a connection to an unresponsive
server to timeout. Here n is the echo interval in seconds.
The reconnection happens at twice the value of the
echo_interval set for an unresponsive server. If this
option is not given then the default value of 60 seconds is
used. The minimum tunable value is 1 second and maximum
can go up to 600 seconds.
serverino
Use inode numbers (unique persistent file identifiers)
returned by the server instead of automatically generating
temporary inode numbers on the client. Although server
inode numbers make it easier to spot hardlinked files (as
they will have the same inode numbers) and inode numbers
may be persistent (which is useful for some software), the
server does not guarantee that the inode numbers are unique
if multiple server side mounts are exported under a single
share (since inode numbers on the servers might not be
unique if multiple filesystems are mounted under the same
shared higher level directory). Note that not all servers
support returning server inode numbers, although those that
support the CIFS Unix Extensions, and Windows 2000 and
later servers typically do support this (although not
necessarily on every local server filesystem). Parameter
has no effect if the server lacks support for returning
inode numbers or equivalent. This behavior is enabled by
default.
noserverino
Client generates inode numbers itself rather than using the
actual ones from the server.
See section INODE NUMBERS for more information.
posix|unix|linux
(default) Enable Unix Extensions for this mount. Requires
CIFS (vers=1.0) or SMB3.1.1 (vers=3.1.1) and a server
supporting them.
noposix|nounix|nolinux
Disable the Unix Extensions for this mount. This can be
useful in order to turn off multiple settings at once. This
includes POSIX acls, POSIX locks, POSIX paths, symlink
support and retrieving uids/gids/mode from the server. This
can also be useful to work around a bug in a server that
supports Unix Extensions.
See section INODE NUMBERS for more information.
nouser_xattr
Do not allow getfattr/setfattr to get/set xattrs, even if
server would support it otherwise. The default is for xattr
support to be enabled.
nodfs Do not follow Distributed FileSystem referrals. IO on a
file not stored on the server will fail instead of
connecting to the target server transparently.
noautotune
Use fixed size for kernel recv/send socket buffers.
nosharesock
Do not try to reuse sockets if the system is already
connected to the server via an existing mount point. This
will make the client always make a new connection to the
server no matter what he is already connected to. This can
be useful in simulating multiple clients connecting to the
same server, as each mount point will use a different TCP
socket.
noblocksend
Send data on the socket using non blocking operations
(MSG_DONTWAIT flag).
rsize=bytes
Maximum amount of data that the kernel will request in a
read request in bytes. Maximum size that servers will
accept is typically 8MB for SMB3 or later dialects. Default
requested during mount is 4MB. Prior to the 4.20 kernel the
default requested was 1MB. Prior to the SMB2.1 dialect the
maximum was usually 64K.
wsize=bytes
Maximum amount of data that the kernel will send in a write
request in bytes. Maximum size that servers will accept is
typically 8MB for SMB3 or later dialects. Default requested
during mount is 4MB. Prior to the 4.20 kernel the default
requested was 1MB. Prior to the SMB2.1 dialect the maximum
was usually 64K.
bsize=bytes
Override the default blocksize (1MB) reported on SMB3 files
(requires kernel version of 5.1 or later). Prior to kernel
version 5.1, the blocksize was always reported as 16K
instead of 1MB (and was not configurable) which can hurt
the performance of tools like cp and scp (especially for
uncached I/O) which decide on the read and write size to
use for file copies based on the inode blocksize. bsize may
not be less than 16K or greater than 16M.
max_credits=n
Maximum credits the SMB2 client can have. Default is 32000.
Must be set to a number between 20 and 60000.
fsc Enable local disk caching using FS-Cache for CIFS. This
option could be useful to improve performance on a slow
link, heavily loaded server and/or network where reading
from the disk is faster than reading from the server (over
the network). This could also impact the scalability
positively as the number of calls to the server are
reduced. But, be warned that local caching is not suitable
for all workloads, for e.g., read-once type workloads. So,
you need to consider carefully the situation/workload
before using this option. Currently, local disk caching is
enabled for CIFS files opened as read-only.
NOTE: This feature is available only in the recent kernels
that have been built with the kernel config option
CONFIG_CIFS_FSCACHE. You also need to have cachefilesd
daemon installed and running to make the cache operational.
multiuser
Map user accesses to individual credentials when accessing
the server. By default, CIFS mounts only use a single set
of user credentials (the mount credentials) when accessing
a share. With this option, the client instead creates a new
session with the server using the user's credentials
whenever a new user accesses the mount. Further accesses
by that user will also use those credentials. Because the
kernel cannot prompt for passwords, multiuser mounts are
limited to mounts using sec= options that don't require
passwords.
With this change, it's feasible for the server to handle
permissions enforcement, so this option also implies noperm
. Furthermore, when unix extensions aren't in use and the
administrator has not overridden ownership using the uid=
or gid= options, ownership of files is presented as the
current user accessing the share.
actimeo=arg
The time (in seconds) that the CIFS client caches
attributes of a file or directory before it requests
attribute information from a server. During this period the
changes that occur on the server remain undetected until
the client checks the server again.
By default, the attribute cache timeout is set to 1 second.
This means more frequent on-the-wire calls to the server to
check whether attributes have changed which could impact
performance. With this option users can make a tradeoff
between performance and cache metadata correctness,
depending on workload needs. Shorter timeouts mean better
cache coherency, but frequent increased number of calls to
the server. Longer timeouts mean a reduced number of calls
to the server but looser cache coherency. The actimeo value
is a positive integer that can hold values between 0 and a
maximum value of 2^30 * HZ (frequency of timer interrupt)
setting. Setting it to 0 disables caching.
acregmax=arg
The maximum time (in seconds) that the CIFS client caches
attributes of a regular file before it requests fresh
attribute information from a server. If this option is not
specified, then acregmax value will be set to actimeo
value, see actimeo for more details.
acdirmax=arg
The maximum time (in seconds) that the CIFS client caches
attributes of a directory before it requests fresh
attribute information from a server. If this option is not
specified, then acdirmax value will be set to actimeo
value, see actimeo for more details.
upcall_target=arg
Determines the namespace in which upcalls from the SMB
filesystem should be handled. Allowed values are: - mount
- Resolve upcalls to the host namespace. - app - Resolve
upcalls in the namespace of the calling thread
(application). Default value is app. This option is
useful in environments like Kubernetes, where the mount may
be performed by a driver pod on behalf of an application
running in a separate container. It ensures that Kerberos
credentials and other user-specific data are accessed in
the correct namespace. By specifying app, upcalls can be
resolved in the application's namespace, ensuring the
correct credentials are used. mount allows resolution in
the host namespace, which may be necessary when credentials
or configurations are managed outside the container.
multichannel
This option enables multichannel feature. Multichannel is
an SMB3 protocol feature that allows client to establish
multiple transport connections to an SMB server and bind
them into a single authenticated SMB session. This feature
enhances fault tolerance and increases throughput by
distributing traffic across several connections. With this
mount option default is to use two channels if the server
supports multichannel. The max_channels parameter can be
specified if you desire to use more than two channels.
max_channels=arg
This option is applicable while using multichannel feature.
max_channels option allows the user to specify the number
of transport connections that should be establised between
client and server up to a limit of 16. Using this option
implicitly enables the multichannel feature. If
max_channels option not specified, multichannel feature
defaults to using 2 connections.
closetimeo=arg
The maximum time (in seconds) that the CIFS client defers
sending the final SMB3 close when the client has a handle
lease on the file.
By default, closetimeo is set to 1 second and can hold
values between 0 and a maximum value of 2^30 * HZ.
noposixpaths
If unix extensions are enabled on a share, then the client
will typically allow filenames to include any character
besides '/' in a pathname component, and will use forward
slashes as a pathname delimiter. This option prevents the
client from attempting to negotiate the use of posix-style
pathnames to the server.
posixpaths
Inverse of noposixpaths .
compress
EXPERIMENTAL FEATURE Enables over-the-wire message
compression for SMB 3.1.1 or higher mounts. Mount fails
when compress is on and vers is set to a version lower than
3.1.1.
vers=arg
SMB protocol version. Allowed values are:
• 1.0 - The classic CIFS/SMBv1 protocol.
• 2.0 - The SMBv2.002 protocol. This was initially
introduced in Windows Vista Service Pack 1, and Windows
Server 2008. Note that the initial release version of
Windows Vista spoke a slightly different dialect (2.000)
that is not supported.
• 2.1 - The SMBv2.1 protocol that was introduced in
Microsoft Windows 7 and Windows Server 2008R2.
• 3.0 - The SMBv3.0 protocol that was introduced in
Microsoft Windows 8 and Windows Server 2012.
• 3.02 or 3.0.2 - The SMBv3.0.2 protocol that was
introduced in Microsoft Windows 8.1 and Windows Server
2012R2.
• 3.1.1 or 3.11 - The SMBv3.1.1 protocol that was
introduced in Microsoft Windows 10 and Windows Server
2016.
• 3 - The SMBv3.0 protocol version and above.
• default - Tries to negotiate the highest SMB2+ version
supported by both the client and server.
If no dialect is specified on mount vers=default is used.
To check Dialect refer to /proc/fs/cifs/DebugData
Note too that while this option governs the protocol
version used, not all features of each version are
available.
The default since v4.13.5 is for the client and server to
negotiate the highest possible version greater than or
equal to 2.1. In kernels prior to v4.13, the default was
1.0. For kernels between v4.13 and v4.13.5 the default is
3.0.
sloppy Allows the system to ignore any unrecognized mount options
that follow this option instead of failing to mount
altogether.
--verbose
Print additional debugging information for the mount. Note
that this parameter must be specified before the -o . For
example:
mount -t cifs //server/share /mnt --verbose -o user=username
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 variables PASSWD and PASSWD2 may contain the password and the
alternate password of the person using the client, respectively.
The variables PASSWD_FILE and PASSWD2_FILE may contain the
pathname of the file to read password or password2 from,
respectively. A single line of input is read and used as the
password in each case.
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.
This page is part of the LinuxCIFS utils (network filesystem
mounts from Linux (e.g. to Samba, ksmbd, etc.)) project.
Information about the project can be found at
⟨https://wiki.samba.org/index.php/LinuxCIFS_utils⟩. If you have a
bug report for this manual page, send it to
linux-cifs@vger.kernel.org, samba-technical@lists.samba.org. This
page was obtained from the project's upstream Git repository
⟨git://git.samba.org/cifs-utils.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-06-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
MOUNT.CIF...OUNT.SMB3(8)
Pages that refer to this page: flock(1), getcifsacl(1), homectl(1), pmdacifs(1), setcifsacl(1), smbinfo(1), flock(2), cifs.idmap(8), cifs.upcall(8), mount(8)
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