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.
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.
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
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 might improve performance with version 2 only,
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).
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 enables subtree checking, which does add another
level of security, but can be unreliability in some circum-
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
This type of subtree checking is disabled by default.
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
nominted path must be a mountpoint for the exportpoint to be
This option forces the filesystem identification portion of the
file handle and file attributes used on the wire to be num
instead of a number derived from the major and minor number of
the block device on which the filesystem is mounted. Any 32 bit
number can be used, but it must be unique amongst all the
This can be useful for NFS failover, to ensure that both servers
of the failover pair use the same NFS file handles for the
shared filesystem thus avoiding stale file handles after
Some Linux filesystems are not mounted on a block device;
exporting these via NFS requires the use of the fsid option
(although that may still not be enough).
The value 0 has a special meaning when use with NFSv4. NFSv4
has a concept of a root of the overall exported filesystem. The
export point exported with fsid=0 will be used as this root.
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 currently needs to have a filesystem
mounted here, generally using mount --bind, although it is not
The sec option, followed by a colon-delimited list of security
flavors, restricts the export to clients using those flavors.
Available security flavors include:
none (no cryptographic security)
sys (no cryptographic security)
krb5 (authentication only)
krb5i (integrity protection)
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 depend-
ing 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.
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 that might be equally sen-
sitive, such as user bin.
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)
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.
exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
Linux 4 March 2005 EXPORTS(5)