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 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 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 subnetworks 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 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).
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.
crossmnt
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
filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
the same effect as setting "nohide" on B.
subtree_check
This option enables subtree checking, which does add
another level of security, but can be unreliability
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 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.
This type of subtree checking is disabled by default.
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 synonymous
auth_nlm,
or
secure_locks.
no_acl
On some specially patched kernels, and when exporting filesystems that
support ACLs, this option tells nfsd not to reveal ACLs to clients, so
they will see only a subset of actual permissions 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).
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.
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 actually exported.)
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 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 diskless 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 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).
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.
