2 .TH NFS 5 "2 November 2007"
4 nfs \- fstab format and options for the
12 NFS is an Internet Standard protocol
13 created by Sun Microsystems in 1984. NFS was developed
14 to allow file sharing between systems residing
15 on a local area network.
16 The Linux NFS client supports three versions
18 NFS version 2 [RFC1094],
19 NFS version 3 [RFC1813],
20 and NFS version 4 [RFC3530].
24 command attaches a file system to the system's
25 name space hierarchy at a given mount point.
30 should assemble a system's file name hierarchy
31 from various independent file systems
32 (including file systems exported by NFS servers).
35 file describes a single file system, its mount point,
36 and a set of default mount options for that mount point.
38 For NFS file system mounts, a line in the
40 file specifies the server name,
41 the path name of the exported server directory to mount,
42 the local directory that is the mount point,
43 the type of file system that is being mounted,
44 and a list of mount options that control
45 the way the filesystem is mounted and
46 how the NFS client behaves when accessing
47 files on this mount point.
48 The fifth and sixth fields on each line are not used
49 by NFS, thus conventionally each contain the digit zero. For example:
53 .TA 2.5i +0.75i +0.75i +1.0i
54 server:path /mountpoint fstype option,option,... 0 0
57 The server's hostname and export pathname
58 are separated by a colon, while
59 the mount options are separated by commas. The remaining fields
60 are separated by blanks or tabs.
61 The server's hostname can be an unqualified hostname,
62 a fully qualified domain name,
63 or a dotted quad IPv4 address.
66 field contains either "nfs" (for version 2 or version 3 NFS mounts)
67 or "nfs4" (for NFS version 4 mounts).
72 file system types share similar mount options,
73 which are described below.
77 for a description of generic mount options
78 available for all file systems. If you do not need to
79 specify any mount options, use the generic option
85 .SS "Valid options for either the nfs or nfs4 file system type"
86 These options are valid to use when mounting either
91 They imply the same behavior
92 and have the same default for both file system types.
95 Determines the recovery behavior of the NFS client
96 after an NFS request times out.
97 If neither option is specified (or if the
99 option is specified), NFS requests are retried indefinitely.
102 option is specified, then the NFS client fails an NFS request
105 retransmissions have been sent,
106 causing the NFS client to return an error
107 to the calling application.
110 A so-called "soft" timeout can cause
111 silent data corruption in certain cases. As such, use the
113 option only when client responsiveness
114 is more important than data integrity.
115 Using NFS over TCP or increasing the value of the
117 option may mitigate some of the risks of using the
122 The time (in tenths of a second) the NFS client waits for a
123 response before it retries an NFS request. If this
124 option is not specified, requests are retried every
125 60 seconds for NFS over TCP.
126 The NFS client does not perform any kind of timeout backoff
129 However, for NFS over UDP, the client uses an adaptive
130 algorithm to estimate an appropriate timeout value for frequently used
131 request types (such as READ and WRITE requests), but uses the
133 setting for infrequently used request types (such as FSINFO requests).
136 option is not specified,
137 infrequently used request types are retried after 1.1 seconds.
138 After each retransmission, the NFS client doubles the timeout for
140 up to a maximum timeout length of 60 seconds.
143 The number of times the NFS client retries a request before
144 it attempts further recovery action. If the
146 option is not specified, the NFS client tries each request
149 The NFS client generates a "server not responding" message
152 retries, then attempts further recovery (depending on whether the
154 mount option is in effect).
157 The maximum number of bytes in each network READ request
158 that the NFS client can receive when reading data from a file
160 The actual data payload size of each NFS READ request is equal to
163 setting. The largest read payload supported by the Linux NFS client
164 is 1,048,576 bytes (one megabyte).
168 value is a positive integral multiple of 1024.
171 values lower than 1024 are replaced with 4096; values larger than
172 1048576 are replaced with 1048576. If a specified value is within the supported
173 range but not a multiple of 1024, it is rounded down to the nearest
178 value is not specified, or if the specified
180 value is larger than the maximum that either client or server can support,
181 the client and server negotiate the largest
183 value that they can both support.
187 mount option as specified on the
189 command line appears in the
191 file. However, the effective
193 value negotiated by the client and server is reported in the
198 The maximum number of bytes per network WRITE request
199 that the NFS client can send when writing data to a file
200 on an NFS server. The actual data payload size of each
201 NFS WRITE request is equal to
204 setting. The largest write payload supported by the Linux NFS client
205 is 1,048,576 bytes (one megabyte).
211 value is a positive integral multiple of 1024.
214 values lower than 1024 are replaced with 4096; values larger than
215 1048576 are replaced with 1048576. If a specified value is within the supported
216 range but not a multiple of 1024, it is rounded down to the nearest
221 value is not specified, or if the specified
223 value is larger than the maximum that either client or server can support,
224 the client and server negotiate the largest
226 value that they can both support.
230 mount option as specified on the
232 command line appears in the
234 file. However, the effective
236 value negotiated by the client and server is reported in the
241 Selects whether the client may cache file attributes. If neither
242 option is specified (or if
244 is specified), the client caches file
247 To improve performance, NFS clients cache file
248 attributes. Every few seconds, an NFS client checks the server's version of each
249 file's attributes for updates. Changes that occur on the server in
250 those small intervals remain undetected until the client checks the
253 option prevents clients from caching file
254 attributes so that applications can more quickly detect file changes
257 In addition to preventing the client from caching file attributes,
260 option forces application writes to become synchronous so
261 that local changes to a file become visible on the server
262 immediately. That way, other clients can quickly detect recent
263 writes when they check the file's attributes.
267 option provides greater cache coherence among NFS clients
268 accessing the same files,
269 but it extracts a significant performance penalty.
270 As such, judicious use of file locking is encouraged instead.
271 The DATA AND METADATA COHERENCE section contains a detailed discussion
275 The minimum time (in seconds) that the NFS client caches
276 attributes of a regular file before it requests
277 fresh attribute information from a server.
278 If this option is not specified, the NFS client uses
282 The maximum time (in seconds) that the NFS client caches
283 attributes of a regular file before it requests
284 fresh attribute information from a server.
285 If this option is not specified, the NFS client uses
289 The minimum time (in seconds) that the NFS client caches
290 attributes of a directory before it requests
291 fresh attribute information from a server.
292 If this option is not specified, the NFS client uses
296 The maximum time (in seconds) that the NFS client caches
297 attributes of a directory before it requests
298 fresh attribute information from a server.
299 If this option is not specified, the NFS client uses
312 If this option is not specified, the NFS client uses
313 the defaults for each of these options listed above.
318 command behaves if an attempt to mount an export fails.
323 to exit with an error status if any part of the mount request
324 times out or fails outright.
325 This is called a "foreground" mount,
326 and is the default behavior if neither the
330 mount option is specified.
334 option is specified, a timeout or failure causes the
336 command to fork a child which continues to attempt
338 The parent immediately returns with a zero exit code.
339 This is known as a "background" mount.
341 If the local mount point directory is missing, the
343 command acts as if the mount request timed out.
344 This permits nested NFS mounts specified in
346 to proceed in any order during system initialization,
347 even if some NFS servers are not yet available.
348 Alternatively these issues can be addressed
349 using an automounter (refer to
354 The number of minutes that the
356 command retries an NFS mount operation
357 in the foreground or background before giving up.
358 If this option is not specified, the default value for foreground mounts
359 is 2 minutes, and the default value for background mounts is 10000 minutes (80 minutes shy of one week).
362 The RPCGSS security flavor to use for accessing files on this mount point.
365 option is not specified, or if
367 is specified, the NFS client uses the AUTH_SYS security flavor
368 for all NFS requests on this mount point.
369 Valid security flavors are
382 Refer to the SECURITY CONSIDERATIONS section for details.
384 .BR sharecache " / " nosharecache
385 Determines how the client's data cache and attribute cache are shared
386 when mounting the same export more than once concurrently. Using the
387 same cache reduces memory requirements on the client and presents
388 identical file contents to applications when the same remote file is
389 accessed via different mount points.
391 If neither option is specified, or if the
394 specified, then a single cache is used for all mount points that
395 access the same export. If the
398 then that mount point gets a unique cache. Note that when data and
399 attribute caches are shared, the mount options from the first mount
400 point take effect for subsequent concurrent mounts of the same export.
402 As of kernel 2.6.18, the behavior specified by
404 is legacy caching behavior. This
405 is considered a data risk since multiple cached copies
406 of the same file on the same client can become out of sync
407 following a local update of one of the copies.
408 .SS "Valid options for the nfs file system type"
409 Use these options, along with the options in the above subsection,
415 The transport protocol used by the NFS client
416 to transmit requests to the NFS server for this mount point.
422 Each transport protocol uses different default
426 settings; refer to the description of these two mount options for details.
428 In addition to controlling how the NFS client transmits requests to
429 the server, this mount option also controls how the
431 command communicates with the server's rpcbind and mountd services.
434 forces all traffic from the
436 command and the NFS client to use TCP.
439 forces all traffic types to use UDP.
443 mount option is not specified, the
445 command discovers which protocols the server supports
446 and chooses an appropriate transport for each service.
447 Refer to the TRANSPORT METHODS section for more details.
452 option is an alternative to specifying
454 It is included for compatibility with other operating systems.
459 option is an alternative to specifying
461 It is included for compatibility with other operating systems.
464 The numeric value of the server's NFS service port.
465 If the server's NFS service is not available on the specified port,
466 the mount request fails.
468 If this option is not specified, or if the specified port value is 0,
469 then the NFS client uses the NFS service port number
470 advertised by the server's rpcbind service.
471 The mount request fails if the server's rpcbind service is not available,
472 the server's NFS service is not registered with its rpcbind service,
473 or the server's NFS service is not available on the advertised port.
476 The numeric value of the server's mountd port.
477 If the server's mountd service is not available on the specified port,
478 the mount request fails.
480 If this option is not specified,
481 or if the specified port value is 0, then the
483 command uses the mountd service port number
484 advertised by the server's rpcbind service.
485 The mount request fails if the server's rpcbind service is not available,
486 the server's mountd service is not registered with its rpcbind service,
487 or the server's mountd service is not available on the advertised port.
489 This option can be used when mounting an NFS server
490 through a firewall that blocks the rpcbind protocol.
492 .BI mountproto= netid
493 The transport protocol used by the NFS client
494 to transmit requests to the NFS server's mountd service when performing
495 this mount request, and when later unmounting this mount point.
502 This option can be used when mounting an NFS server
503 through a firewall that blocks a particular transport protocol.
504 When used in combination with the
506 option, different transports for mountd requests and NFS requests
508 If the server's mountd service is not available via the specified
509 transport, the mount request fails.
512 The hostname of the host running mountd.
513 If this option is not specified, the
515 command assumes that the mountd service runs
516 on the same host as the NFS service.
519 The RPC version number used to contact the server's mountd.
520 If this option is not specified, the client uses a version number
521 appropriate to the requested NFS version.
522 This option is useful when multiple NFS services
523 are running on the same remote server host.
526 The maximum length of a pathname component on this mount.
527 If this option is not specified, the maximum length is negotiated
528 with the server. In most cases, this maximum length is 255 characters.
530 Some early versions of NFS did not support this negotiation.
531 Using this option ensures that
533 reports the proper maximum component length to applications
537 The NFS protocol version number used to contact the server's NFS service.
538 The Linux client supports version 2 and version 3 of the NFS protocol
539 when using the file system type
541 If the server does not support the requested version,
542 the mount request fails.
543 If this option is not specified, the client attempts to use version 3,
544 but negotiates the NFS version with the server if version 3 support
548 This option is an alternative to the
551 It is included for compatibility with other operating systems.
553 .BR lock " / " nolock
554 Selects whether to use the NLM sideband protocol to lock files on the server.
555 If neither option is specified (or if
557 is specified), NLM locking is used for this mount point.
560 option, applications can lock files,
561 but such locks provide exclusion only against other applications
562 running on the same client.
563 Remote applications are not affected by these locks.
565 NLM locking must be disabled with the
567 option when using NFS to mount
571 contains files used by the NLM implementation on Linux.
574 option is also required when mounting exports on NFS servers
575 that do not support the NLM protocol.
577 .BR intr " / " nointr
578 Selects whether to allow signals to interrupt file operations
579 on this mount point. If neither option
583 signals do not interrupt NFS file operations. If
585 is specified, system calls return EINTR if an in-progress NFS operation is interrupted by
590 option is preferred to using the
592 option because it is significantly less likely to result in data corruption.
595 Selects whether to use close-to-open cache coherence semantics.
596 If neither option is specified (or if
598 is specified), the client uses close-to-open
599 cache coherence semantics. If the
601 option is specified, the client uses a non-standard heuristic to determine when
602 files on the server have changed.
606 option may improve performance for read-only mounts,
607 but should be used only if the data on the server changes only occasionally.
608 The DATA AND METADATA COHERENCE section discusses the behavior
609 of this option in more detail.
612 Selects whether to use the NFSACL sideband protocol on this mount point.
613 The NFSACL sideband protocol is a proprietary protocol
614 implemented in Solaris that manages Access Control Lists. NFSACL was never
615 made a standard part of the NFS protocol specification.
622 the NFS client negotiates with the server
623 to see if the NFSACL protocol is supported,
624 and uses it if the server supports it.
625 Disabling the NFSACL sideband protocol may be necessary
626 if the negotiation causes problems on the client or server.
627 Refer to the SECURITY CONSIDERATIONS section for more details.
629 .BR rdirplus " / " nordirplus
630 Selects whether to use NFS version 3 READDIRPLUS requests.
631 If this option is not specified, the NFS client uses READDIRPLUS requests
632 on NFS version 3 mounts to read small directories.
633 Some applications perform better if the client uses only READDIR requests
635 .SS "Valid options for the nfs4 file system type"
636 Use these options, along with the options in the first subsection above,
642 The transport protocol used by the NFS client
643 to transmit requests to the NFS server for this mount point.
649 All NFS version 4 servers are required to support TCP,
650 so if this mount option is not specified, the NFS version 4 client
651 uses the TCP transport protocol.
652 Refer to the TRANSPORT METHODS section for more details.
655 The numeric value of the server's NFS service port.
656 If the server's NFS service is not available on the specified port,
657 the mount request fails.
659 If this mount option is not specified,
660 the NFS client uses the standard NFS port number of 2049
661 without first checking the server's rpcbind service.
662 This allows an NFS version 4 client to contact an NFS version 4
663 server through a firewall that may block rpcbind requests.
665 If the specified port value is 0,
666 then the NFS client uses the NFS service port number
667 advertised by the server's rpcbind service.
668 The mount request fails if the server's rpcbind service is not available,
669 the server's NFS service is not registered with its rpcbind service,
670 or the server's NFS service is not available on the advertised port.
672 .BR intr " / " nointr
673 Selects whether to allow signals to interrupt file operations
674 on this mount point. If neither option is specified (or if
676 is specified), system calls return EINTR if an in-progress NFS operation
677 is interrupted by a signal. If
679 is specified, signals do not
680 interrupt NFS operations.
684 option is preferred to using the
686 option because it is significantly less likely to result in data corruption.
689 Selects whether to use close-to-open cache coherence semantics
690 for NFS directories on this mount point.
696 the default is to use close-to-open cache coherence
697 semantics for directories.
699 File data caching behavior is not affected by this option.
700 The DATA AND METADATA COHERENCE section discusses
701 the behavior of this option in more detail.
703 .BI clientaddr= n.n.n.n
704 Specifies a single IPv4 address (in dotted-quad form)
705 that the NFS client advertises to allow servers
706 to perform NFS version 4 callback requests against
707 files on this mount point. If the server is unable to
708 establish callback connections to clients, performance
709 may degrade, or accesses to files may temporarily hang.
711 If this option is not specified, the
713 command attempts to discover an appropriate callback address automatically.
714 The automatic discovery process is not perfect, however.
715 In the presence of multiple client network interfaces,
716 special routing policies,
717 or atypical network topologies,
718 the exact address to use for callbacks may be nontrivial to determine.
720 To mount an export using NFS version 2,
723 file system type and specify the
726 To mount using NFS version 3,
729 file system type and specify the
732 To mount using NFS version 4,
738 mount option is not supported for the
742 The following example from an
744 file causes the mount command to negotiate
745 reasonable defaults for NFS behavior.
748 .TA 2.5i +0.7i +0.7i +.7i
749 server:/export /mnt nfs defaults 0 0
752 Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.
755 .TA 2.5i +0.7i +0.7i +.7i
756 server:/export /mnt nfs nfsvers=2,proto=udp 0 0
759 Try this example to mount using NFS version 4 over TCP
760 with Kerberos 5 mutual authentication.
763 .TA 2.5i +0.7i +0.7i +.7i
764 server:/export /mnt nfs4 sec=krb5 0 0
767 This example can be used to mount /usr over NFS.
770 .TA 2.5i +0.7i +0.7i +.7i
771 server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0
773 .SH "TRANSPORT METHODS"
774 NFS clients send requests to NFS servers via
775 Remote Procedure Calls, or
777 The RPC client discovers remote service endpoints automatically,
778 handles per-request authentication,
779 adjusts request parameters for different byte endianness on client and server,
780 and retransmits requests that may have been lost by the network or server.
781 RPC requests and replies flow over a network transport.
785 command, NFS client, and NFS server
786 can automatically negotiate proper transport
787 and data transfer size settings for a mount point.
788 In some cases, however, it pays to specify
789 these settings explicitly using mount options.
791 Traditionally, NFS clients used the UDP transport exclusively for
792 transmitting requests to servers. Though its implementation is
793 simple, NFS over UDP has many limitations that prevent smooth
794 operation and good performance in some common deployment
795 environments. Even an insignificant packet loss rate results in the
796 loss of whole NFS requests; as such, retransmit timeouts are usually
797 in the subsecond range to allow clients to recover quickly from
798 dropped requests, but this can result in extraneous network traffic
801 However, UDP can be quite effective in specialized settings where
802 the network’s MTU is large relative to NFS’s data transfer size (such
803 as network environments that enable jumbo Ethernet frames). In such
804 environments, trimming the
808 settings so that each
809 NFS read or write request fits in just a few network frames (or even
810 in a single frame) is advised. This reduces the probability that
811 the loss of a single MTU-sized network frame results in the loss of
812 an entire large read or write request.
814 TCP is the default transport protocol used for all modern NFS
815 implementations. It performs well in almost every conceivable
816 network environment and provides excellent guarantees against data
817 corruption caused by network unreliability. TCP is often a
818 requirement for mounting a server through a network firewall.
820 Under normal circumstances, networks drop packets much more
821 frequently than NFS servers drop requests. As such, an aggressive
822 retransmit timeout setting for NFS over TCP is unnecessary. Typical
823 timeout settings for NFS over TCP are between one and ten minutes.
824 After the client exhausts its retransmits (the value of the
826 mount option), it assumes a network partition has occurred,
827 and attempts to reconnect to the server on a fresh socket. Since
828 TCP itself makes network data transfer reliable,
832 can safely be allowed to default to the largest values supported by
833 both client and server, independent of the network's MTU size.
834 .SH "DATA AND METADATA COHERENCE"
835 Some modern cluster file systems provide
836 perfect cache coherence among their clients.
837 Perfect cache coherence among disparate NFS clients
838 is expensive to achieve, especially on wide area networks.
839 As such, NFS settles for weaker cache coherence that
840 satisfies the requirements of most file sharing types. Normally,
841 file sharing is completely sequential:
842 first client A opens a file, writes something to it, then closes it;
843 then client B opens the same file, and reads the changes.
845 .SS "Close-to-open cache consistency"
846 When an application opens a file stored on an NFS server,
847 the NFS client checks that it still exists on the server
848 and is permitted to the opener by sending a GETATTR or ACCESS request.
849 When the application closes the file,
850 the NFS client writes back any pending changes
851 to the file so that the next opener can view the changes.
852 This also gives the NFS client an opportunity to report
853 any server write errors to the application
854 via the return code from
856 The behavior of checking at open time and flushing at close time
857 is referred to as close-to-open cache consistency.
858 .SS "Weak cache consistency"
859 There are still opportunities for a client's data cache
860 to contain stale data.
861 The NFS version 3 protocol introduced "weak cache consistency"
862 (also known as WCC) which provides a way of efficiently checking
863 a file's attributes before and after a single request.
864 This allows a client to help identify changes
865 that could have been made by other clients.
867 When a client is using many concurrent operations
868 that update the same file at the same time
869 (for example, during asynchronous write behind),
870 it is still difficult to tell whether it was
871 that client's updates or some other client's updates
872 that altered the file.
873 .SS "Attribute caching"
876 mount option to achieve attribute cache coherence
877 among multiple clients.
878 Almost every file system operation checks
879 file attribute information.
880 The client keeps this information cached
881 for a period of time to reduce network and server load.
884 is in effect, a client's file attribute cache is disabled,
885 so each operation that needs to check a file's attributes
886 is forced to go back to the server.
887 This permits a client to see changes to a file very quickly,
888 at the cost of many extra network operations.
890 Be careful not to confuse the
892 option with "no data caching."
895 mount option prevents the client from caching file metadata,
896 but there are still races that may result in data cache incoherence
897 between client and server.
899 The NFS protocol is not designed to support
900 true cluster file system cache coherence
901 without some type of application serialization.
902 If absolute cache coherence among clients is required,
903 applications should use file locking. Alternatively, applications
904 can also open their files with the O_DIRECT flag
905 to disable data caching entirely.
906 .SS "The sync mount option"
907 The NFS client treats the
909 mount option differently than some other file systems
912 for a description of the generic
921 is specified (or if the
923 option is specified),
924 the NFS client delays sending application
926 until any of these events occur:
928 Memory pressure forces reclamation of system memory resources.
930 An application flushes file data explicitly with
936 An application closes a file with
939 The file is locked/unlocked via
942 In other words, under normal circumstances,
943 data written by an application may not immediately appear
944 on the server that hosts the file.
948 option is specified on a mount point,
949 any system call that writes data to files on that mount point
950 causes that data to be flushed to the server
951 before the system call returns control to user space.
952 This provides greater data cache coherence among clients,
953 but at a significant performance cost.
955 Applications can use the O_SYNC open flag to force application
956 writes to individual files to go to the server immediately without
960 .SS "Using file locks with NFS"
961 The Network Lock Manager protocol is a separate sideband protocol
962 used to manage file locks in NFS version 2 and version 3.
963 To support lock recovery after a client or server reboot,
964 a second sideband protocol --
965 known as the Network Status Manager protocol --
968 file locking is supported directly in the main NFS protocol,
969 and the NLM and NSM sideband protocols are not used.
971 In most cases, NLM and NSM services are started automatically,
972 and no extra configuration is required.
973 Configure all NFS clients with fully-qualified domain names
974 to ensure that NFS servers can find clients to notify them of server reboots.
976 NLM supports advisory file locks only.
977 To lock NFS files, use
979 with the F_GETLK and F_SETLK commands.
980 The NFS client converts file locks obtained via
984 When mounting servers that do not support the NLM protocol,
985 or when mounting an NFS server through a firewall
986 that blocks the NLM service port,
989 mount option. NLM locking must be disabled with the
991 option when using NFS to mount
995 contains files used by the NLM implementation on Linux.
999 option may also be advised to improve the performance
1000 of a proprietary application which runs on a single client
1001 and uses file locks extensively.
1002 .SS "NFS version 4 caching features"
1003 The data and metadata caching behavior of NFS version 4
1004 clients is similar to that of earlier versions.
1005 However, NFS version 4 adds two features that improve
1007 .I change attributes
1009 .IR "file delegation" .
1013 is a new part of NFS file and directory metadata
1014 which tracks data changes.
1015 It replaces the use of a file's modification
1016 and change time stamps
1017 as a way for clients to validate the content
1019 Change attributes are independent of the time stamp
1020 resolution on either the server or client, however.
1024 is a contract between an NFS version 4 client
1025 and server that allows the client to treat a file temporarily
1026 as if no other client is accessing it.
1027 The server promises to notify the client (via a callback request) if another client
1028 attempts to access that file.
1029 Once a file has been delegated to a client, the client can
1030 cache that file's data and metadata aggressively without
1031 contacting the server.
1033 File delegations come in two flavors:
1039 delegation means that the server notifies the client
1040 about any other clients that want to write to the file.
1043 delegation means that the client gets notified about
1044 either read or write accessors.
1046 Servers grant file delegations when a file is opened,
1047 and can recall delegations at any time when another
1048 client wants access to the file that conflicts with
1049 any delegations already granted.
1050 Delegations on directories are not supported.
1052 In order to support delegation callback, the server
1053 checks the network return path to the client during
1054 the client's initial contact with the server.
1055 If contact with the client cannot be established,
1056 the server simply does not grant any delegations to
1058 .SH "SECURITY CONSIDERATIONS"
1059 NFS servers control access to file data,
1060 but they depend on their RPC implementation
1061 to provide authentication of NFS requests.
1062 Traditional NFS access control mimics
1063 the standard mode bit access control provided in local file systems.
1064 Traditional RPC authentication uses a number
1065 to represent each user
1066 (usually the user's own uid),
1067 a number to represent the user's group (the user's gid),
1068 and a set of up to 16 auxiliary group numbers
1069 to represent other groups of which the user may be a member.
1071 Typically, file data and user ID values appear unencrypted
1072 (i.e. "in the clear") on the network.
1073 Moreover, NFS versions 2 and 3 use
1074 separate sideband protocols for mounting,
1075 locking and unlocking files,
1076 and reporting system status of clients and servers.
1077 These auxiliary protocols use no authentication.
1079 In addition to combining these sideband protocols with the main NFS protocol,
1080 NFS version 4 introduces more advanced forms of access control,
1081 authentication, and in-transit data protection.
1082 The NFS version 4 specification mandates NFSv4 ACLs,
1083 RPCGSS authentication, and RPCGSS security flavors
1084 that provide per-RPC integrity checking and encryption.
1085 Because NFS version 4 combines the
1086 function of the sideband protocols into the main NFS protocol,
1087 the new security features apply to all NFS version 4 operations
1088 including mounting, file locking, and so on.
1089 RPCGSS authentication can also be used with NFS versions 2 and 3,
1090 but does not protect their sideband protocols.
1094 mount option specifies the RPCGSS security mode
1095 that is in effect on a given NFS mount point.
1098 provides cryptographic proof of a user's identity in each RPC request.
1099 This provides strong verification of the identity of users
1100 accessing data on the server.
1101 Note that additional configuration besides adding this mount option
1102 is required in order to enable Kerberos security.
1105 man page for details.
1107 Two additional flavors of Kerberos security are supported:
1113 security flavor provides a cryptographically strong guarantee
1114 that the data in each RPC request has not been tampered with.
1117 security flavor encrypts every RPC request
1118 to prevent data exposure during network transit; however,
1119 expect some performance impact
1120 when using integrity checking or encryption.
1121 Similar support for other forms of cryptographic security (such as lipkey and SPKM3)
1124 The NFS version 4 protocol allows
1125 clients and servers to negotiate among multiple security flavors
1126 during mount processing.
1127 However, Linux does not yet implement such negotiation.
1128 The Linux client specifies a single security flavor at mount time
1129 which remains in effect for the lifetime of the mount.
1130 If the server does not support this flavor,
1131 the initial mount request is rejected by the server.
1132 .SS "Mounting through a firewall"
1133 A firewall may reside between an NFS client and server,
1134 or the client or server may block some of its own ports via IP
1136 It is still possible to mount an NFS server through a firewall,
1139 command's automatic service endpoint discovery mechanisms may not work; this
1140 requires you to provide specific endpoint details via NFS mount options.
1142 NFS servers normally run a portmapper or rpcbind daemon to advertise
1143 their service endpoints to clients. Clients use the rpcbind daemon to determine:
1145 What network port each RPC-based service is using
1147 What transport protocols each RPC-based service supports
1149 The rpcbind daemon uses a well-known port number (111) to help clients find a service endpoint.
1150 Although NFS often uses a standard port number (2049),
1151 auxiliary services such as the NLM service can choose
1152 any unused port number at random.
1154 Common firewall configurations block the well-known rpcbind port.
1155 In the absense of an rpcbind service,
1156 the server administrator fixes the port number
1157 of NFS-related services so that the firewall
1158 can allow access to specific NFS service ports.
1159 Client administrators then specify the port number
1160 for the mountd service via the
1165 It may also be necessary to enforce the use of TCP or UDP
1166 if the firewall blocks one of those transports.
1167 .SS "NFS Access Control Lists"
1168 Solaris allows NFS version 3 clients direct access
1169 to POSIX Access Control Lists stored in its local file systems.
1170 This proprietary sideband protocol, known as NFSACL,
1171 provides richer access control than mode bits.
1172 Linux implements this protocol
1173 for compatibility with the Solaris NFS implementation.
1174 The NFSACL protocol never became a standard part
1175 of the NFS version 3 specification, however.
1177 The NFS version 4 specification mandates a new version
1178 of Access Control Lists that are semantically richer than POSIX ACLs.
1179 NFS version 4 ACLs are not fully compatible with POSIX ACLs; as such,
1180 some translation between the two is required
1181 in an environment that mixes POSIX ACLs and NFS version 4.
1189 option is not fully supported.
1190 Generic options, such as
1192 can be modified using the
1195 but NFS-specific options are not all supported.
1196 The underlying transport or NFS version
1197 cannot be changed by a remount, for example.
1198 Performing a remount on an NFS file system mounted with the
1200 option may have unintended consequences.
1203 option is a mixture of a generic option,
1205 and an NFS-specific option
1208 Before 2.4.7, the Linux NFS client did not support NFS over TCP.
1210 Before 2.4.20, the Linux NFS client used a heuristic
1211 to determine whether cached file data was still valid
1212 rather than using the standard close-to-open cache coherency method
1215 Starting with 2.4.22, the Linux NFS client employs
1216 a Van Jacobsen-based RTT estimator to determine retransmit
1217 timeout values when using NFS over UDP.
1219 Before 2.6.0, the Linux NFS client did not support NFS version 4.
1221 Before 2.6.8, the Linux NFS client used only synchronous reads and writes
1223 .BR rsize " and " wsize
1224 settings were smaller than the system's page size.
1226 The Linux NFS client does not yet support
1227 certain optional features of the NFS version 4 protocol,
1228 such as security negotiation, server referrals, and named attributes.
1239 .BR rpc.svcgssd (8),
1242 RFC 768 for the UDP specification.
1244 RFC 793 for the TCP specification.
1246 RFC 1094 for the NFS version 2 specification.
1248 RFC 1813 for the NFS version 3 specification.
1250 RFC 1832 for the XDR specification.
1252 RFC 1833 for the RPC bind specification.
1254 RFC 2203 for the RPCSEC GSS API protocol specification.
1256 RFC 3530 for the NFS version 4 specification.