EXPORTS(5) Linux File Formats Manual EXPORTS(5)
exports - NFS file systems being exported (for Kernel based NFS)
The file /etc/exports serves as the access control list for file sys-
tems which may be exported to NFS clients. It is used by exportfs(8)
to give information to mountd(8) and to the kernel based NFS file
server daemon nfsd(8).
The file format is similar to the SunOS exports file. Each line con-
tains 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
Machine Name Formats
NFS clients may be specified in a number of ways:
This is the most common format. You may specify a host either by
an abbreviated name recognized be the resolver, the fully quali-
fied domain name, or an IP address.
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
Machine names may contain the wildcard characters * and ?. 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
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 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
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 crypto-
graphic 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.
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, this option
was the default. In all subsequence releases, 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 perfor-
mance, 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
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
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
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 hide.
This option is similar to 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 filesys-
tem "B" is mounted on a parent "A", setting crossmnt on "A" has
the same effect as setting "nohide" on B.
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 nor-
mally 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
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 prob-
lems than it is worth. If you genuinely require subtree check-
ing, 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
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 synony-
mous auth_nlm, or secure_locks.
no_acl On some specially patched kernels, and when exporting filesys-
tems that support ACLs, this option tells nfsd not to reveal
ACLs to clients, so they will see only a subset of actual per-
missions 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 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, no_acl is off).
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 filesys-
tem 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
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
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 punctua-
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.
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 dif-
ferent 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.)
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 anony-
mous 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:
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 disk-
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 set-
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 exam-
ple section below, which maps all requests to uid 150 (which is
supposedly that of user joe).
# 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)
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.
exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
Linux 4 March 2005 EXPORTS(5)