MOUNT.CIFS, MOUNT.SMB3(8) System Manager's Manual MOUNT.CIFS, MOUNT.SMB3(8)
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" op-
tion. 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 stor-
age), Macs and many other commercial servers and Network Attached Stor-
age appliances as well as by the popular Open Source server Samba.
mount.smb3 mounts only SMB3 filesystem. It is usually invoked indi-
rectly 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 re-
source) 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) sup-
ports them. Unrecognized cifs mount options passed to the cifs vfs ker-
nel 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 (usu-
ally 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 work-
group/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 ac-
cept user= as an abbreviation for this option, its use can con-
fuse 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 com-
mand line. However, the same password defined in the PASSWD en-
vironment variable or via a credentials file (see below) or en-
tered 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 nu-
meric 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 informa-
tion.
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 DIREC-
TORY OWNERSHIP AND PERMISSIONS below for more information.
cruid=arg
sets the uid of the owner of the credentials cache. This is pri-
marily 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 nu-
meric 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 informa-
tion.
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 DIREC-
TORY 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 exist-
ing 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 dur-
ing 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 automati-
cally 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 automat-
ically 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 lo-
cal process on newly created files, directories, and devices
(create, mkdir, mknod). If the CIFS Unix Extensions are not ne-
gotiated, for newly created files and directories instead of us-
ing 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 re-
mounts 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 ne-
gotiated then the uid and gid for new files will appear to be
the uid (gid) of the mounter or the uid (gid) parameter speci-
fied 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 sys-
tem do not match closely enough to allow access by the user do-
ing 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 de-
tails. 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 redun-
dant. 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 im-
provement. 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 con-
tain 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 accessi-
ble 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 er-
rors 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 re-
quires 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 speci-
fying noacl on mount.
cifsacl
This option is used to map CIFS/NTFS ACLs to/from Linux permis-
sion bits, map SIDs to/from UIDs and GIDs, and get and set Secu-
rity Descriptors.
See section on CIFS/NTFS ACL, SID/UID/GID MAPPING, SECURITY DE-
SCRIPTORS 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 argu-
ment, 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 en-
abled automatically if it's enabled in /proc/fs/cifs/Securi-
tyFlags.
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 start-
ing from v4.19.
nobrl Do not send byte range lock requests to the server. This is nec-
essary 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, at-
tempt 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 mod-
ule.
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 op-
tion. Minshall+French symlinks are used even if the server sup-
ports 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 re-
quired 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 in-
ode numbers on the client. Although server inode numbers make it
easier to spot hardlinked files (as they will have the same in-
ode numbers) and inode numbers may be persistent (which is use-
ful for some software), the server does not guarantee that the
inode numbers are unique if multiple server side mounts are ex-
ported 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 necessar-
ily 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 ac-
tual 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 re-
trieving 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 tar-
get 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_DONT-
WAIT 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 typi-
cally 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 re-
quest in bytes. Maximum size that servers will accept is typi-
cally 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 (re-
quires 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 situa-
tion/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_FS-
CACHE. 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 permis-
sions enforcement, so this option also implies noperm . Further-
more, 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 in-
creased number of calls to the server. Longer timeouts mean a
reduced number of calls to the server but looser cache co-
herency. 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.
upcall_target=arg
Determines the namespace in which upcalls from the SMB filesys-
tem 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 re-
solved in the application's namespace, ensuring the correct cre-
dentials are used. mount allows resolution in the host name-
space, which may be necessary when credentials or configurations
are managed outside the container.
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 path-
name 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 sup-
ported 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 nego-
tiate the highest possible version greater than or equal to 2.1.
In kernels prior to v4.13, the default was 1.0. For kernels be-
tween 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 compo-
nents on Windows machines and the client can convert them to back-
slashes (\) unconditionally. Conversely, backslash characters are al-
lowed by POSIX to be part of a path component, and can't be automati-
cally 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 pro-
vided 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 num-
ber. 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 ac-
cesses go to the server directly. This is performance prohibitive how-
ever, 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 co-
herency. 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 oc-
cur before a write system call returns.
In the case of a read without holding an oplock, the client will at-
tempt to periodically check the attributes of the file in order to as-
certain 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 co-
herency, 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 cor-
ruption 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 al-
ways 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 De-
scriptors and CIFS/NTFS ACL instead of UID, GID, file permission bits,
and POSIX ACL as user authentication model. This is the most common au-
thentication 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 re-
quest-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 win-
bindd(8) for more information.
Security descriptors for a file object can be retrieved and set di-
rectly using extended attribute named system.cifs_acl. The security de-
scriptors 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 addi-
tional 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 ac-
cess 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 desir-
able 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 direc-
tories 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 re-
flect 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 re-
ceive what appear to be proper permissions. These permissions are not
stored on the server however and can disappear at any time in the fu-
ture (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 alto-
gether via the noperm option. Server-side permission checks cannot be
overridden. The permission checks done by the server will always corre-
spond 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 user-
name 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 in-
stalled 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 configura-
tion 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 sta-
tistics. 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 mod-
info 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 configur-
ing 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 mod-
probe 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 lead-
ing 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 ker-
nel 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)
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