exports
EXPORTS(5) EXPORTS(5)
NAME
exports - NFS file systems being exported (for Kernel based NFS)
SYNOPSIS
/etc/exports
DESCRIPTION
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:
single host
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.
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.
wildcards
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 cs.foo.edu.
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 address result in identical sub-
networks with 10 bits of host). Wildcard characters generally
do not work on IP addresses, though they may work by accident
when reverse DNS lookups fail.
RPCSEC_GSS security
To restrict access to an export using rpcsec_gss security, use the
special string "gss/krb5" as the client. It is not possible to
simultaneously require rpcsec_gss and to make requirements on the IP
address of the client.
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 filesys-
tem. 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.
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.
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 with hide.
no_subtree_check
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 nor-
mally exported at the root and may see lots of file renames,
should be exported with subtree checking disabled. A filesys-
tem 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.
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 synony-
mous 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 nominted path must be a mountpoint for the exportpoint to
be exported.
fsid=num
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
exported filesystems.
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
failover.
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.
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 squash-
ing’) 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:
root_squash
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.
no_root_squash
Turn off root squashing. This option is mainly useful for disk-
less 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 set-
ting.
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).
EXAMPLE
# 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)
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.
FILES
/etc/exports
4.2 Berkeley Distribution 28 October 1999 EXPORTS(5)
