exports - NFS file systems being exported (for Kernel based NFS)
The file /etc/exports serves as the access control list for file systems 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 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
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 qualified 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 ignored.
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. However, these wildcard characters do not match the dots in
a domain name, so the above pattern does not include hosts such as a.b.cs.foo.edu.
You can also export directories to all hosts on an IP (sub-) network simultane-
ously. 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 char-
acters generally do not work on IP addresses, though they may work by accident when
reverse DNS lookups fail.
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 inse-
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.
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.
In releases of nfs-utils upto and including 1.0.0, this option was the default. In
this and future releases, sync is the default, and async must be explicit requested
if needed. To help make system adminstrators 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 perfor-
mance. 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. Nor-
mally, 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 appro-
priately 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
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
The option can be explicitly disabled with hide.
This option disables subtree checking, which has mild security implications, but
can improve reliability is 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 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 sub-
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 per-
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 explic-
itly requested with either of the synonymous auth_nlm, or secure_locks.
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 -2 (i.e. 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 sensitive, such as user bin.
Turn off root squashing. This option is mainly useful for diskless clients.
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).
# 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.
4.2 Berkeley Distribution 28 October 1999 EXPORTS(5)