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exports(5) File Formats Manual exports(5)
exports - NFS server export table
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.
Machine Name Formats
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.
Transport layer security
The Linux NFS server allows the use of RPC-with-TLS (RFC 9289) to
protect RPC traffic between itself and its clients. Alternately,
administrators can secure NFS traffic using a VPN, or an ssh
tunnel or similar mechanism, in a way that is transparent to the
server.
To enable the use of RPC-with-TLS, the server's administrator
must install and configure tlshd to handle transport layer
security handshake requests from the local kernel. Clients can
then choose to use RPC-with-TLS or they may continue operating
without it.
Administrators may require the use of RPC-with-TLS to protect
access to individual exports. This is particularly useful when
using non-cryptographic security flavors such as sec=sys. The
xprtsec= option, followed by an unordered colon-delimited list of
security policies, can restrict access to the export to only
clients that have negotiated transport-layer security. Currently
supported transport layer security policies include:
none The server permits clients to access the export without
the use of transport layer security.
tls The server permits clients that have negotiated an RPC-
with-TLS session without peer authentication
(confidentiality only) to access the export. Clients are
not required to offer an x.509 certificate when
establishing a transport layer security session.
mtls The server permits clients that have negotiated an RPC-
with-TLS session with peer authentication to access the
export. The server requires clients to offer an x.509
certificate when establishing a transport layer security
session.
If RPC-with-TLS is configured and enabled and the xprtsec= option
is not specified, the default setting for an export is
xprtsec=none:tls:mtls. With this setting, the server permits
clients to use any transport layer security mechanism or none at
all to access the export.
General Options
exportfs understands the following export options:
secure This option requires that requests not using gss originate
on an Internet port less than IPPORT_RESERVED (1024). This
option is on by default. To turn it off, specify
insecure. (NOTE: older kernels (before upstream kernel
version 4.17) enforced this requirement on gss requests as
well.)
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.
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 for NFSv2 and NFSv3
with hide.
This option is not relevant when NFSv4 is use. NFSv4
never hides subordinate filesystems. Any filesystem that
is exported will be visible where expected when using
NFSv4.
crossmnt
This option is similar to nohide but it makes it possible
for clients to access all filesystems mounted on a
filesystem marked with crossmnt. Thus when a child
filesystem "B" is mounted on a parent "A", setting
crossmnt on "A" has a similar effect to setting "nohide"
on B.
With nohide the child filesystem needs to be explicitly
exported. With crossmnt it need not. If a child of a
crossmnt file is not explicitly exported, then it will be
implicitly exported with the same export options as the
parent, except for fsid=. This makes it impossible to not
export a child of a crossmnt filesystem. If some but not
all subordinate filesystems of a parent are to be
exported, then they must be explicitly exported and the
parent should not have crossmnt set.
The nocrossmnt option can explicitly disable crossmnt if
it was previously set. This is rarely useful.
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.
nordirplus
This option will disable READDIRPLUS request handling.
When set, READDIRPLUS requests from NFS clients return
NFS3ERR_NOTSUPP, and clients fall back on READDIR. This
option affects only NFSv3 clients.
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.)
This option affects only NFSv4 clients. Other clients will
ignore all "refer=" parts.
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.)
pnfs This option enables the use of the pNFS extension if the
protocol level is NFSv4.1 or higher, and the filesystem
supports pNFS exports. With pNFS clients can bypass the
server and perform I/O directly to storage devices. The
default can be explicitly requested with the no_pnfs
option.
security_label
With this option set, clients using NFSv4.2 or higher will
be able to set and retrieve security labels (such as those
used by SELinux). This will only work if all clients use
a consistent security policy. Note that early kernels did
not support this export option, and instead enabled
security labels by default.
reexport=auto-fsidnum|predefined-fsidnum
This option helps when a NFS share is re-exported. Since
the NFS server needs a unique identifier for each exported
filesystem and a NFS share cannot provide such, usually a
manual fsid is needed. As soon crossmnt is used manually
assigning fsid won't work anymore. This is where this
option becomes handy. It will automatically assign a
numerical fsid to exported NFS shares. The fsid and path
relations are stored in a SQLite database. If auto-fsidnum
is selected, the fsid is also autmatically allocated.
predefined-fsidnum assumes pre-allocated fsid numbers and
will just look them up. This option depends also on the
kernel, you will need at least kernel version 5.19. Since
reexport= can automatically allocate and assign numerical
fsids, it is no longer possible to have numerical fsids in
other exports as soon this option is used in at least one
export entry.
The association between fsid numbers and paths is stored
in a SQLite database. Don't edit or remove the database
unless you know exactly what you're doing. predefined-
fsidnum is useful when you have used auto-fsidnum before
and don't want further entries stored.
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).
Subdirectory Exports
Normally you should only export only the root of a filesystem.
The NFS server will also allow you to export a subdirectory of a
filesystem, however, this has drawbacks:
First, it may be possible for a malicious user to access files on
the filesystem outside of the exported subdirectory, by guessing
filehandles for those other files. The only way to prevent this
is by using the no_subtree_check option, which can cause other
problems.
Second, export options may not be enforced in the way that you
would expect. For example, the security_label option will not
work on subdirectory exports, and if nested subdirectory exports
change the security_label or sec= options, NFSv4 clients will
normally see only the options on the parent export. Also, where
security options differ, a malicious client may use filehandle-
guessing attacks to access the files from one subdirectory using
the options from another.
Extra Export Tables
After reading /etc/exports exportfs reads files in the
/etc/exports.d directory as extra export tables. Only files
ending in .exports are considered. Files beginning with a dot
are ignored. The format for extra export tables is the same as
/etc/exports
# 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.
/etc/exports /etc/exports.d
exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8),
tlshd(8).
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a
bug report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
⟨git://git.linux-nfs.org/projects/steved/nfs-utils.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-10.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
31 December 2009 exports(5)
Pages that refer to this page: nfs(5), nfsd(7), exportd(8), exportfs(8), mountd(8), nfsd(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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