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
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
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
- 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.
- 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
- 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.
- This is specified by a single * character (not to be
confused with the wildcard entry above) and will match all
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.
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:
- 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
understands the following export options:
- 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.
- 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.
- 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
- 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.
- 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
- 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
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.
- 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.
- 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
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
The default of having subtree checks enabled, can be explicitly requested
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.
- 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
- 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
- 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.
- 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.
- 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.)
- 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.)
- This option enables the use of the pNFS extension if the
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
- 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.
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
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
options. Finally, you can map all user requests to the anonymous uid by
specifying the all_squash
Here's the complete list of mapping options:
- 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.
- Turn off root squashing. This option is mainly useful for
- 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
- 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).
After reading /etc/exports exportfs
reads files in the
directory as extra export tables. Only files ending in
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)
/usr *.local.domain(ro) @trusted(rw)
/srv/www -sync,rw server @trusted @external(ro)
/foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(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
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