Provided by: cifs-utils_7.0-2build1_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)