-.TH EXPORTS 5 "28 October 1999"
-.UC 5
+.TH EXPORTS 5 "4 March 2005" "Linux" "Linux File Formats Manual"
.SH NAME
exports \- NFS file systems being exported (for Kernel based NFS)
.SH SYNOPSIS
The file
.I /etc/exports
serves as the access control list for file systems which may be
-exported to NFS clients. It it used by
+exported to NFS clients. It is used by
.IR exportfs (8)
to give information to
.IR mountd (8)
.PP
The file format is similar to the SunOS
.I exports
-file, except that several additional options are permitted. Each line
-contains an export point and a list of machine or netgroup names allowed
-to mount the file system at that point. An optional parenthesized list
-of export parameters may follow each machine name. Blank lines are
-ignored, and a # introduces a comment to the end of the line. Entries may
-be continued across newlines using a backslash.
+file. Each line contains an export point and a whitespace-separated list
+of clients allowed to mount the file system at that point. Each listed
+client may be immediately followed by a parenthesized, comma-separated
+list of export options for that client. No whitespace is permitted
+between a client and its option list.
+.PP
+Also, each line may have one or more specifications for default options
+after the path name, in the form of a dash ("\-") followed by an option
+list. The option list is used for all subsequent exports on that line
+only.
+.PP
+Blank lines are ignored. A pound sign ("#") introduces a comment to the
+end of the line. Entries may be continued across newlines using a
+backslash. If an export name contains spaces it should be quoted using
+double quotes. You can also specify spaces or other unusual character in
+the export name using a backslash followed by the character code as three
+octal digits.
+.PP
+To apply changes to this file, run exportfs \-ra or restart the NFS server.
.PP
.SS Machine Name Formats
NFS clients may be specified in a number of ways:
.IP "wildcards
Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
This can be used to make the \fIexports\fR file more compact; for instance,
-\fI*.cs.foo.edu\fR matches all hosts in the domain \fIcs.foo.edu\fR. However,
-these wildcard characters do not match the dots in a domain name, so the
-above pattern does not include hosts such as \fIa.b.cs.foo.edu\fR.
+\fI*.cs.foo.edu\fR matches all hosts in the domain
+\fIcs.foo.edu\fR. As these characters also match the dots in a domain
+name, the given pattern will also match all hosts within any subdomain
+of \fIcs.foo.edu\fR.
.IP "IP networks
You can also export directories to all hosts on an IP (sub-) network
simultaneously. This is done by specifying an IP address and netmask pair
as
-.IR address/netmask .
-.TP
+.IR address/netmask
+where the netmask can be specified in dotted-decimal format, or as a
+contiguous mask length (for example, either `/255.255.252.0' or `/22' appended
+to the network base address result in identical subnetworks with 10 bits of
+host). Wildcard characters generally do not work on IP addresses, though they
+may work by accident when reverse DNS lookups fail.
+'''.TP
'''.B =public
'''This is a special ``hostname'' that identifies the given directory name
'''as the public root directory (see the section on WebNFS in
'''.B \-\-public\-root
'''option. Multiple specifications of a public root will be ignored.
.PP
+.SS RPCSEC_GSS security
+You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
+to restrict access to clients using rpcsec_gss security. However, this
+syntax is deprecated; on linux kernels since 2.6.23, you should instead
+use the "sec=" export option:
+.TP
+.IR sec=
+The sec= option, followed by a colon-delimited list of security flavors,
+restricts the export to clients using those flavors. Available security
+flavors include sys (the default--no cryptographic security), krb5
+(authentication only), krb5i (integrity protection), and krb5p (privacy
+protection). For the purposes of security flavor negotiation, order
+counts: preferred flavors should be listed first. The order of the sec=
+option with respect to the other options does not matter, unless you
+want some options to be enforced differently depending on flavor.
+In that case you may include multiple sec= options, and following options
+will be enforced only for access using flavors listed in the immediately
+preceding sec= option. The only options that are permitted to vary in
+this way are ro, rw, no_root_squash, root_squash, and all_squash.
+.PP
.SS General Options
.IR exportfs
understands the following export options:
.TP
.IR secure "\*d
-This option requires that requests originate on an internet port less
+This option requires that requests originate on an Internet port less
than IPPORT_RESERVED (1024). This option is on by default. To turn it
off, specify
.IR insecure .
This can also be made explicit by using
the
.IR ro " option.
+.TP
+.IR async
+This option allows the NFS server to violate the NFS protocol and
+reply to requests before any changes made by that request have been
+committed to stable storage (e.g. disc drive).
+
+Using this option usually improves performance, but at the cost that
+an unclean server restart (i.e. a crash) can cause data to be lost or
+corrupted.
+
.TP
.IR sync
-This option requests that all file writes be committed to disc before
-the write request completes. This is required for complete safety of
-data in the face of a server crash, but incurs a performance hit.
-The default is to allow the server to write the data out whenever it
-is ready. This can be explicitly requested with the
-.IR async " option.
+Reply to requests only after the changes have been committed to stable
+storage (see
+.IR async
+above).
+
+In releases of nfs-utils up to and including 1.0.0, this option was the
+default. In all releases after 1.0.0,
+.I sync
+is the default, and
+.I async
+must be explicitly requested if needed.
+To help make system administrators aware of this change, 'exportfs'
+will issue a warning if neither
+.I sync
+nor
+.I async
+is specified.
.TP
.IR no_wdelay
-This option only has effect if
-.I sync
+This option has no effect if
+.I async
is also set. The NFS server will normally delay committing a write request
to disc slightly if it suspects that another related write request may be in
progress or may arrive soon. This allows multiple write requests to
instance, it is then possible for two files in the one apparent
filesystem to have the same inode number.
+The
+.I nohide
+option is currently only effective on
+.I "single host
+exports. It does not work reliably with netgroup, subnet, or wildcard
+exports.
+
This option can be very useful in some situations, but it should be
used with due care, and only after confirming that the client system
copes with the situation effectively.
The option can be explicitly disabled with
.IR hide .
.TP
+.IR crossmnt
+This option is similar to
+.I nohide
+but it makes it possible for clients to move from the filesystem marked
+with crossmnt to exported filesystems mounted on it. Thus when a child
+filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
+the same effect as setting "nohide" on B.
+.TP
.IR no_subtree_check
This option disables subtree checking, which has mild security
-implications, but can improve reliability is some circumstances.
+implications, but can improve reliability in some circumstances.
If a subdirectory of a filesystem is exported, but the whole
filesystem isn't then whenever a NFS request arrives, the server must
directories to which only root has access can only be accessed if the
filesystem is exported with
.I no_root_squash
-(see below), even the file itself allows more general access.
+(see below), even if the file itself allows more general access.
As a general guide, a home directory filesystem, which is normally
exported at the root and may see lots of file renames, should be
The default of having subtree checks enabled, can be explicitly
requested with
.IR subtree_check .
+
+From release 1.1.0 of nfs-utils onwards, the default will be
+.I no_subtree_check
+as subtree_checking tends to cause more problems than it is worth.
+If you genuinely require subtree checking, you should explicitly put
+that option in the
+.B exports
+file. If you put neither option,
+.I exportfs
+will warn you that the change is pending.
+
+.TP
+.IR insecure_locks
+.TP
+.IR no_auth_nlm
+This option (the two names are synonymous) tells the NFS server not to require authentication of
+locking requests (i.e. requests which use the NLM protocol). Normally
+the NFS server will require a lock request to hold a credential for a
+user who has read access to the file. With this flag no access checks
+will be performed.
+
+Early NFS client implementations did not send credentials with lock
+requests, and many current NFS clients still exist which are based on
+the old implementations. Use this flag if you find that you can only
+lock files which are world readable.
+
+The default behaviour of requiring authentication for NLM requests can
+be explicitly requested with either of the synonymous
+.IR auth_nlm ,
+or
+.IR secure_locks .
+.TP
+.IR no_acl
+On some specially patched kernels, and when exporting filesystems that
+support ACLs, this option tells nfsd not to reveal ACLs to clients, so
+they will see only a subset of actual permissions on the given file
+system. This option is safe for filesystems used by NFSv2 clients and
+old NFSv3 clients that perform access decisions locally. Current
+NFSv3 clients use the ACCESS RPC to perform all access decisions on
+the server. Note that the
+.I no_acl
+option only has effect on kernels specially patched to support it, and
+when exporting filesystems with ACL support. The default is to export
+with ACL support (i.e. by default,
+.I no_acl
+is off).
+
'''.TP
'''.I noaccess
'''This makes everything below the directory inaccessible for the named
'''.TP
'''.IR link_absolute
'''Leave all symbolic link as they are. This is the default operation.
+
+.TP
+.IR mountpoint= path
+.TP
+.I mp
+This option makes it possible to only export a directory if it has
+successfully been mounted.
+If no path is given (e.g.
+.IR mountpoint " or " mp )
+then the export point must also be a mount point. If it isn't then
+the export point is not exported. This allows you to be sure that the
+directory underneath a mountpoint will never be exported by accident
+if, for example, the filesystem failed to mount due to a disc error.
+
+If a path is given (e.g.
+.IR mountpoint= "/path or " mp= /path)
+then the nominated path must be a mountpoint for the exportpoint to be
+exported.
+
+.TP
+.IR fsid= num|root|uuid
+NFS needs to be able to identify each filesystem that it exports.
+Normally it will use a UUID for the filesystem (if the filesystem has
+such a thing) or the device number of the device holding the
+filesystem (if the filesystem is stored on the device).
+
+As not all filesystems are stored on devices, and not all filesystems
+have UUIDs, it is sometimes necessary to explicitly tell NFS how to
+identify a filesystem. This is done with the
+.I fsid=
+option.
+
+For NFSv4, there is a distinguished filesystem which is the root of
+all exported filesystem. This is specified with
+.I fsid=root
+or
+.I fsid=0
+both of which mean exactly the same thing.
+
+Other filesystems can be identified with a small integer, or a UUID
+which should contain 32 hex digits and arbitrary punctuation.
+
+Linux kernels version 2.6.20 and earlier do not understand the UUID
+setting so a small integer must be used if an fsid option needs to be
+set for such kernels. Setting both a small number and a UUID is
+supported so the same configuration can be made to work on old and new
+kernels alike.
+
+.TP
+.IR refer= path@host[+host][:path@host[+host]]
+A client referencing the export point will be directed to choose from
+the given list an alternative location for the filesystem.
+(Note that the server must have a mountpoint here, though a different
+filesystem is not required; so, for example,
+.IR "mount --bind" " /path /path"
+is sufficient.)
+.TP
+.IR replicas= path@host[+host][:path@host[+host]]
+If the client asks for alternative locations for the export point, it
+will be given this list of alternatives. (Note that actual replication
+of the filesystem must be handled elsewhere.)
+
+.TP
+.IR refer= path@host[+host][:path@host[+host]]
+A client referencing the export point will be directed to choose from
+the given list an alternative location for the filesystem.
+(Note that the server must have a mountpoint here, though a different
+filesystem is not required; so, for example,
+.IR "mount --bind" " /path /path"
+is sufficient.)
+.TP
+.IR replicas= path@host[+host][:path@host[+host]]
+If the client asks for alternative locations for the export point, it
+will be given this list of alternatives. (Note that actual replication
+of the filesystem must be handled elsewhere.)
+
.SS User ID Mapping
.PP
.I nfsd
'''in the password file at startup time. If it isn't found, a uid and gid
.I exportfs
chooses a uid and gid
-of -2 (i.e. 65534) for squashed access. These values can also be overridden by
+of 65534 for squashed access. These values can also be overridden by
the
.IR anonuid " and " anongid
options.
Finally, you can map all user requests to the
anonymous uid by specifying the
.IR all_squash " option.
-'''.PP
-'''For the benefit of installations where uids differ between different
-'''machines,
-'''.I nfsd
-'''provides several mechanism to dynamically map server uids to client
-'''uids and vice versa: static mapping files, NIS-based mapping, and
-'''.IR ugidd -based
-'''mapping.
-'''.PP
-'''.IR ugidd -based
-'''mapping is enabled with the
-'''.I map_daemon
-'''option, and uses the UGID RPC protocol. For this to work, you have to run
-'''the
-'''.IR ugidd (8)
-'''mapping daemon on the client host. It is the least secure of the three methods,
-'''because by running
-'''.IR ugidd ,
-'''everybody can query the client host for a list of valid user names. You
-'''can protect yourself by restricting access to
-'''.I ugidd
-'''to valid hosts only. This can be done by entering the list of valid
-'''hosts into the
-'''.I hosts.allow
-'''or
-'''.I hosts.deny
-'''file. The service name is
-'''.IR ugidd .
-'''For a description of the file's syntax, please read
-'''.IR hosts_access (5).
-'''.PP
-'''Static mapping is enabled by using the
-'''.I map_static
-'''option, which takes a file name as an argument that describes the mapping.
-'''NIS-based mapping queries the client's NIS server to obtain a mapping from
-'''user and group names on the server host to user and group names on the
-'''client.
.PP
Here's the complete list of mapping options:
.TP
.IR root_squash
Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
-not apply to any other uids that might be equally sensitive, such as user
-.IR bin .
+not apply to any other uids or gids that might be equally sensitive, such as
+user
+.IR bin
+or group
+.IR staff .
.TP
.IR no_root_squash
Turn off root squashing. This option is mainly useful for diskless clients.
-'''.TP
-'''.IR squash_uids " and " squash_gids
-'''This option specifies a list of uids or gids that should be subject to
-'''anonymous mapping. A valid list of ids looks like this:
-'''.IP
-'''.IR squash_uids=0-15,20,25-50
-'''.IP
-'''Usually, your squash lists will look a lot simpler.
.TP
.IR all_squash
Map all uids and gids to the anonymous user. Useful for NFS-exported
is
.IR no_all_squash ,
which is the default setting.
-'''.TP
-'''.IR map_daemon
-'''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
-'''will be translated to the equivalent server uid, and each uid in an
-'''NFS reply will be mapped the other way round. This option requires that
-'''.IR rpc.ugidd (8)
-'''runs on the client host. The default setting is
-'''.IR map_identity ,
-'''which leaves all uids untouched. The normal squash options apply regardless
-'''of whether dynamic mapping is requested or not.
-'''.TP
-'''.IR map_static
-'''This option enables static mapping. It specifies the name of the file
-'''that describes the uid/gid mapping, e.g.
-'''.IP
-'''.IR map_static=/etc/nfs/foobar.map
-'''.IP
-'''The file's format looks like this
-'''.IP
-'''.nf
-'''.ta +3i
-'''# Mapping for client foobar:
-'''# remote local
-'''uid 0-99 - # squash these
-'''uid 100-500 1000 # map 100-500 to 1000-1500
-'''gid 0-49 - # squash these
-'''gid 50-100 700 # map 50-100 to 700-750
-'''.fi
-'''.TP
-'''.IR map_nis
-'''This option enables NIS-based uid/gid mapping. For instance, when
-'''the server encounters the uid 123 on the server, it will obtain the
-'''login name associated with it, and contact the NFS client's NIS server
-'''to obtain the uid the client associates with the name.
-'''.IP
-'''In order to do this, the NFS server must know the client's NIS domain.
-'''This is specified as an argument to the
-'''.I map_nis
-'''options, e.g.
-'''.IP
-'''.I map_nis=foo.com
-'''.IP
-'''Note that it may not be sufficient to simply specify the NIS domain
-'''here; you may have to take additional actions before
-'''.I nfsd
-'''is actually able to contact the server. If your distribution uses
-'''the NYS library, you can specify one or more NIS servers for the
-'''client's domain in
-'''.IR /etc/yp.conf .
-'''If you are using a different NIS library, you may have to obtain a
-'''special
-'''.IR ypbind (8)
-'''daemon that can be configured via
-'''.IR yp.conf .
.TP
.IR anonuid " and " anongid
These options explicitly set the uid and gid of the anonymous account.
/projects proj*.local.domain(rw)
/usr *.local.domain(ro) @trusted(rw)
/home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
-/pub (ro,insecure,all_squash)
+/pub *(ro,insecure,all_squash)
+/srv/www \-sync,rw server @trusted @external(ro)
'''/pub/private (noaccess)
.fi
.PP
.I insecure
option in this entry also allows clients with NFS implementations that
don't use a reserved port for NFS.
+The sixth line exports a directory read-write to the machine 'server'
+as well as the `@trusted' netgroup, and read-only to netgroup `@external',
+all three mounts with the `sync' option enabled.
''' The last line denies all NFS clients
'''access to the private directory.
'''.SH CAVEATS
'''entry.
.SH FILES
/etc/exports
+.SH SEE ALSO
+.BR exportfs (8),
+.BR netgroup (5),
+.BR mountd (8),
+.BR nfsd (8),
+.BR showmount (8).
'''.SH DIAGNOSTICS
'''An error parsing the file is reported using syslogd(8) as level NOTICE from
'''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
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