NAME¶
exports - NFS server export table
DESCRIPTION¶
The file
/etc/exports contains a table of local physical file systems on
an NFS server that are accessible to NFS clients. The contents of the file are
maintained by the server's system administrator.
Each file system in this table has a list of options and an access control list.
The table is used by
exportfs(8) to give information to
mountd(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.
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 only.
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 server.
NFS clients may be specified in a number of ways:
- single host
- You may specify a host either by an abbreviated name recognized be the
resolver, the fully qualified domain name, an IPv4 address, or an IPv6
address. IPv6 addresses must not be inside square brackets in /etc/exports
lest they be confused with character-class wildcard matches.
- 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 IPv4 address
results in identical subnetworks with 10 bits of host. IPv6 addresses must
use a contiguous mask length and must not be inside square brackets to
avoid confusion with character-class wildcards. Wildcard characters
generally do not work on IP addresses, though they may work by accident
when reverse DNS lookups fail.
- wildcards
- Machine names may contain the wildcard characters * and ?,
or may contain character class lists within [square brackets]. 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.
- 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.
- anonymous
- This is specified by a single * character (not to be confused with
the wildcard entry above) and will match all clients.
If a client matches more than one of the specifications above, then the first
match from the above list order takes precedence - regardless of the order
they appear on the export line. However, if a client matches more than one of
the same type of specification (e.g. two netgroups), then the first match from
the order they appear on the export line takes precedence.
RPCSEC_GSS security¶
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 cryptographic 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.
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 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, the async option
was the default. In all releases after 1.0.0, 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.
- 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.
- no_subtree_check
- 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 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 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 problems
than it is worth. If you genuinely require subtree checking, you should
explicitly put that option in the exports file. If you put neither
option, exportfs will warn you that the change is pending.
- 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.
- 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 nominated path must be a mountpoint for the exportpoint to be
exported.
- fsid=num|root|uuid
- 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 device).
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 fsid= option.
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 punctuation.
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.
- refer=path@host[+host][:path@host[+host]]
- 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 different filesystem is not
required; so, for example, mount --bind /path /path is
sufficient.)
- replicas=path@host[+host][:path@host[+host]]
- 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 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 or gids that might be equally sensitive, such
as user bin or group staff.
- 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).
After reading
/etc/exports exportfs reads files under
/etc/exports.d. directory as extra export tables.
exportfs
regards only a file which name is ended with
.exports and not started
with
. as an extra export file. A file which name is not met this
condition is just ignored. The format for extra export tables is the same as
/etc/exports
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)
/srv/www -sync,rw server @trusted @external(ro)
/foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
/build buildhost[0-9].local.domain(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. 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. The seventh line exports a directory to both an IPv6
and an IPv4 subnet. The eighth line demonstrates a character class wildcard
match.
FILES¶
/etc/exports /etc/exports.d
SEE ALSO¶
exportfs(8),
netgroup(5),
mountd(8),
nfsd(8),
showmount(8).