exports(5) File Formats Manual exports(5)
exports - NFS server export table
The file /etc/exports contains a table of local physical file systems
on an NFS server that are accessible to NFS clients. The contents of
the file are maintained by the server's system administrator.
Each file system in this table has a list of options and an access
control list. The table is used by exportfs(8) to give information
to mountd(8).
The file format is similar to the SunOS 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 only.
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.
Machine Name Formats
NFS clients may be specified in a number of ways:
single host
You may specify a host either by an abbreviated name
recognized be the resolver, the fully qualified domain name,
an IPv4 address, or an IPv6 address. IPv6 addresses must not
be inside square brackets in /etc/exports lest they be
confused with character-class wildcard matches.
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 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 IPv4 address results in identical
subnetworks with 10 bits of host. IPv6 addresses must use a
contiguous mask length and must not be inside square brackets
to avoid confusion with character-class wildcards. Wildcard
characters generally do not work on IP addresses, though they
may work by accident when reverse DNS lookups fail.
wildcards
Machine names may contain the wildcard characters * and ?, or
may contain character class lists within [square brackets].
This can be used to make the exports file more compact; for
instance, *.cs.foo.edu matches all hosts in the domain
cs.foo.edu. As these characters also match the dots in a
domain name, the given pattern will also match all hosts
within any subdomain of cs.foo.edu.
netgroups
NIS netgroups may be given as @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.
anonymous
This is specified by a single * character (not to be confused
with the wildcard entry above) and will match all clients.
If a client matches more than one of the specifications above, then
the first match from the above list order takes precedence -
regardless of the order they appear on the export line. However, if a
client matches more than one of the same type of specification (e.g.
two netgroups), then the first match from the order they appear on
the export line takes precedence.
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:
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.
General Options
exportfs understands the following export options:
secure 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 insecure.
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 the ro
option.
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.
sync Reply to requests only after the changes have been committed
to stable storage (see async above).
In releases of nfs-utils up to and including 1.0.0, the async
option was the default. In all releases after 1.0.0, sync is
the default, and async must be explicitly requested if needed.
To help make system administrators aware of this change,
exportfs will issue a warning if neither sync nor async is
specified.
no_wdelay
This option has no effect if 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 no_wdelay is available to turn it off. The
default can be explicitly requested with the wdelay option.
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 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.
The nohide option is currently only effective on 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 for NFSv2 and NFSv3 with
hide.
This option is not relevant when NFSv4 is use. NFSv4 never
hides subordinate filesystems. Any filesystem that is
exported will be visible where expected when using NFSv4.
crossmnt
This option is similar to nohide but it makes it possible for
clients to access all filesystems mounted on a filesystem
marked with crossmnt. Thus when a child filesystem "B" is
mounted on a parent "A", setting crossmnt on "A" has a similar
effect to setting "nohide" on B.
With nohide the child filesystem needs to be explicitly
exported. With crossmnt it need not. If a child of a
crossmnt file is not explicitly exported, then it will be
implicitly exported with the same export options as the
parent, except for fsid=. This makes it impossible to not
export a child of a crossmnt filesystem. If some but not all
subordinate filesystems of a parent are to be exported, then
they must be explicitly exported and the parent should not
have crossmnt set.
The nocrossmnt option can explictly disable crossmnt if it was
previously set. This is rarely useful.
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
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 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 subtree_check.
From release 1.1.0 of nfs-utils onwards, the default will be
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 exports
file. If you put neither option, exportfs will warn you that
the change is pending.
insecure_locks
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 auth_nlm, or secure_locks.
mountpoint=path
mp This option makes it possible to only export a directory if it
has successfully been mounted. If no path is given (e.g.
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. mountpoint=/path or mp=/path) then
the nominated path must be a mountpoint for the exportpoint to
be exported.
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 fsid= option.
For NFSv4, there is a distinguished filesystem which is the
root of all exported filesystem. This is specified with
fsid=root or 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.
nordirplus
This option will disable READDIRPLUS request handling. When
set, READDIRPLUS requests from NFS clients return
NFS3ERR_NOTSUPP, and clients fall back on READDIR. This
option affects only NFSv3 clients.
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, mount --bind /path /path is sufficient.)
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.)
pnfs This option allows enables the use of pNFS extension if
protocol level is NFSv4.1 or higher, and the filesystem
supports pNFS exports. With pNFS clients can bypass the
server and perform I/O directly to storage devices. The
default can be explicitly requested with the no_pnfs option.
security_label
With this option set, clients using NFSv4.2 or higher will be
able to set and retrieve security labels (such as those used
by SELinux). This will only work if all clients use a
consistent security policy. Note that early kernels did not
support this export option, and instead enabled security
labels by default.
User ID Mapping
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 nobody uid. This mode of operation (called
`root squashing') is the default, and can be turned off with
no_root_squash.
By default, exportfs chooses a uid and gid of 65534 for squashed
access. These values can also be overridden by the anonuid and
anongid options. Finally, you can map all user requests to the
anonymous uid by specifying the all_squash option.
Here's the complete list of mapping options:
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 user bin or group staff.
no_root_squash
Turn off root squashing. This option is mainly useful for
diskless clients.
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 is no_all_squash, which is the default
setting.
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 /home/joe in the
example section below, which maps all requests to uid 150
(which is supposedly that of user joe).
Extra Export Tables
After reading /etc/exports exportfs reads files in the /etc/exports.d
directory as extra export tables. Only files ending in .exports are
considered. Files beginning with a dot are ignored. The format for
extra export tables is the same as /etc/exports
# 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)
/foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
/build buildhost[0-9].local.domain(rw)
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 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 seventh line exports a directory
to both an IPv6 and an IPv4 subnet. The eighth line demonstrates a
character class wildcard match.
/etc/exports /etc/exports.d
exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a bug
report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
⟨http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary⟩ on
2017-07-05. If you discover any rendering problems in this HTML ver‐
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31 December 2009 exports(5)
Pages that refer to this page: nfs(5), nfsd(7), exportfs(8), mountd(8), nfsd(8)