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.TH EXPORTS 5 "4 March 2005" "Linux" "Linux File Formats Manual"
exports \- NFS file systems being exported (for Kernel based NFS)
.B /etc/exports
The file
.I /etc/exports
serves as the access control list for file systems which may be
exported to NFS clients. It is used by
.IR exportfs (8)
to give information to
.IR mountd (8)
and to the kernel based NFS file server daemon
.IR nfsd (8).
The file format is similar to the SunOS
.I exports
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.
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
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.
To apply changes to this file, run exportfs \-ra or restart the NFS server.
.SS Machine Name Formats
NFS clients may be specified in a number of ways:
.IP "single host
This is the most common format. You may specify a host either by an
abbreviated name recognized be the resolver, the fully qualified domain
name, or an IP address.
.IP "netgroups
NIS netgroups may be given as
.IR @group .
Only the host part of each
netgroup members is consider in checking for membership. Empty host
parts or those containing a single dash (\-) are ignored.
.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*\fR matches all hosts in the domain
\\fR. As these characters also match the dots in a domain
name, the given pattern will also match all hosts within any subdomain
of \\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
.IR address/netmask
where the netmask can be specified in dotted-decimal format, or as a
contiguous mask length (for example, either `/' 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.
'''.B =public
'''This is a special ``hostname'' that identifies the given directory name
'''as the public root directory (see the section on WebNFS in
'''.BR nfsd (8)
'''for a discussion of WebNFS and the public root handle). When using this
'''.B =public
'''must be the only entry on this line, and must have no export options
'''associated with it. Note that this does
'''.I not
'''actually export the named directory; you still have to set the exports
'''options in a separate entry.
'''The public root path can also be specified by invoking
'''.I nfsd
'''with the
'''.B \-\-public\-root
'''option. Multiple specifications of a public root will be ignored.
.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:
.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.
.SS General Options
.IR exportfs
understands the following export options:
.IR secure "\*d
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 .
.IR rw
Allow both read and write requests on this NFS volume. The
default is to disallow any request which changes the filesystem.
This can also be made explicit by using
.IR ro " option.
.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
.IR sync
Reply to requests only after the changes have been committed to stable
storage (see
.IR async
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
.I async
is specified.
.IR no_wdelay
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
be committed to disc with the one operation which can improve
performance. If an NFS server received mainly small unrelated
requests, this behaviour could actually reduce performance, so
.IR no_wdelay
is available to turn it off.
The default can be explicitly requested with the
.IR wdelay " option.
.IR nohide
This option is based on the option of the same name provided in IRIX
NFS. Normally, if a server exports two filesystems one of which is
mounted on the other, then the client will have to mount both
filesystems explicitly to get access to them. If it just mounts the
parent, it will see an empty directory at the place where the other
filesystem is mounted. That filesystem is "hidden".
Setting the
.I nohide
option on a filesystem causes it not to be hidden, and an
appropriately authorised client will be able to move from the parent to
that filesystem without noticing the change.
However, some NFS clients do not cope well with this situation as, for
instance, it is then possible for two files in the one apparent
filesystem to have the same inode number.
.I nohide
option is currently only effective on
.I "single host
exports. It does not work reliably with netgroup, subnet, or wildcard
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 .
.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.
.IR no_subtree_check
This option disables subtree checking, which has mild security
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
check not only that the accessed file is in the appropriate filesystem
(which is easy) but also that it is in the exported tree (which is
harder). This check is called the
.IR subtree_check .
In order to perform this check, the server must include some
information about the location of the file in the "filehandle" that is
given to the client. This can cause problems with accessing files that
are renamed while a client has them open (though in many simple cases
it will still work).
subtree checking is also used to make sure that files inside
directories to which only root has access can only be accessed if the
filesystem is exported with
.I no_root_squash
(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
exported with subtree checking disabled. A filesystem which is mostly
readonly, and at least doesn't see many file renames (e.g. /usr or
/var) and for which subdirectories may be exported, should probably be
exported with subtree checks enabled.
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.
.IR insecure_locks
.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 ,
.IR secure_locks .
.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).
'''.I noaccess
'''This makes everything below the directory inaccessible for the named
'''client. This is useful when you want to export a directory hierarchy to
'''a client, but exclude certain subdirectories. The client's view of a
'''directory flagged with noaccess is very limited; it is allowed to read
'''its attributes, and lookup `.' and `..'. These are also the only entries
'''returned by a readdir.
'''.IR link_relative
'''Convert absolute symbolic links (where the link contents start with a
'''slash) into relative links by prepending the necessary number of ../'s
'''to get from the directory containing the link to the root on the
'''server. This has subtle, perhaps questionable, semantics when the file
'''hierarchy is not mounted at its root.
'''.IR link_absolute
'''Leave all symbolic link as they are. This is the default operation.
.IR mountpoint= path
.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
.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=
For NFSv4, there is a distinguished filesystem which is the root of
all exported filesystem. This is specified with
.I fsid=root
.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.
.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.)
.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.)
.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.)
.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
.I nfsd
bases its access control to files on the server machine on the uid and
gid provided in each NFS RPC request. The normal behavior a user would
expect is that she can access her files on the server just as she would
on a normal file system. This requires that the same uids and gids are
used on the client and the server machine. This is not always true, nor
is it always desirable.
Very often, it is not desirable that the root user on a client machine
is also treated as root when accessing files on the NFS server. To this
end, uid 0 is normally mapped to a different id: the so-called
anonymous or
.I nobody
uid. This mode of operation (called `root squashing') is the default,
and can be turned off with
.IR no_root_squash .
By default,
'''.I nfsd
'''tries to obtain the anonymous uid and gid by looking up user
'''.I nobody
'''in the password file at startup time. If it isn't found, a uid and gid
.I exportfs
chooses a uid and gid
of 65534 for squashed access. These values can also be overridden by
.IR anonuid " and " anongid
'''In addition to this,
'''.I nfsd
'''lets you specify arbitrary uids and gids that should be mapped to user
'''nobody as well.
Finally, you can map all user requests to the
anonymous uid by specifying the
.IR all_squash " option.
Here's the complete list of mapping options:
.IR root_squash
Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
not apply to any other uids or gids that might be equally sensitive, such as
.IR bin
or group
.IR staff .
.IR no_root_squash
Turn off root squashing. This option is mainly useful for diskless clients.
.IR all_squash
Map all uids and gids to the anonymous user. Useful for NFS-exported
public FTP directories, news spool directories, etc. The opposite option
.IR no_all_squash ,
which is the default setting.
.IR anonuid " and " anongid
These options explicitly set the uid and gid of the anonymous account.
This option is primarily useful for PC/NFS clients, where you might want
all requests appear to be from one user. As an example, consider the
export entry for
.B /home/joe
in the example section below, which maps all requests to uid 150 (which
is supposedly that of user joe).
.ta +3i
# sample /etc/exports file
/ master(rw) trusty(rw,no_root_squash)
/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)
/srv/www \-sync,rw server @trusted @external(ro)
'''/pub/private (noaccess)
The first line exports the entire filesystem to machines master and trusty.
In addition to write access, all uid squashing is turned off for host
trusty. The second and third entry show examples for wildcard hostnames
and netgroups (this is the entry `@trusted'). The fourth line shows the
entry for the PC/NFS client discussed above. Line 5 exports the
public FTP directory to every host in the world, executing all requests
under the nobody account. The
.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.
'''Unlike other NFS server implementations, this
'''.I nfsd
'''allows you to export both a directory and a subdirectory thereof to
'''the same host, for instance
'''.IR /usr " and " /usr/X11R6 .
'''In this case, the mount options of the most specific entry apply. For
'''instance, when a user on the client host accesses a file in
'''.IR /usr/X11R6 ,
'''the mount options given in the
'''.I /usr/X11R6
'''entry apply. This is also true when the latter is a wildcard or netgroup
.BR exportfs (8),
.BR netgroup (5),
.BR mountd (8),
.BR nfsd (8),
.BR showmount (8).
'''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
'''host is reported at that time, but often not all hosts are not yet known
'''to named(8) at boot time, thus as hosts are found they are reported
'''with the same syslogd(8) parameters.