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exports(5) File Formats Manual exports(5)
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 con-
trol 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 con-
tains an export point and a whitespace-separated list of clients al-
lowed 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 ad-
dress, or an IPv6 address. IPv6 addresses must not be inside
square brackets in /etc/exports lest they be confused with char-
acter-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 subnet-
works with 10 bits of host. IPv6 addresses must use a contiguous
mask length and must not be inside square brackets to avoid con-
fusion with character-class wildcards. Wildcard characters gen-
erally do not work on IP addresses, though they may work by ac-
cident 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 in-
stance, *.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 ig-
nored.
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 crypto-
graphic 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 re-
spect 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 op-
tions 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 pro-
tect RPC traffic between itself and its clients. Alternately, adminis-
trators 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 in-
stall 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-crypto-
graphic security flavors such as sec=sys. The xprtsec= option, fol-
lowed 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 ses-
sion.
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 es-
tablishing 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 xprt-
sec=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 re-
quirement on gss requests as well.)
rw Allow both read and write requests on this NFS volume. The de-
fault 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 re-
quests to be committed to disc with the one operation which can
improve performance. If an NFS server received mainly small un-
related requests, this behaviour could actually reduce perfor-
mance, 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 ex-
ports. 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 set-
ting "nohide" on B.
With nohide the child filesystem needs to be explicitly ex-
ported. With crossmnt it need not. If a child of a crossmnt
file is not explicitly exported, then it will be implicitly ex-
ported 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 filesys-
tems of a parent are to be exported, then they must be explic-
itly exported and the parent should not have crossmnt set.
The nocrossmnt option can explictly 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 in-
formation about the location of the file in the "filehandle"
that is given to the client. This can cause problems with ac-
cessing 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 di-
rectories 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 nor-
mally 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 re-
names (e.g. /usr or /var) and for which subdirectories may be
exported, should probably be exported with subtree checks en-
abled.
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 prob-
lems than it is worth. If you genuinely require subtree check-
ing, 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 re-
quire 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 re-
quests 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 filesys-
tem 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 ex-
ports. 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 de-
vice).
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 punctua-
tion.
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_NOT-
SUPP, 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 dif-
ferent 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.)
pnfs This option enables the use of the pNFS extension if the proto-
col 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 allo-
cated. 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 reex-
port= 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 anony-
mous 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 ac-
cess. 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 disk-
less clients.
all_squash
Map all uids and gids to the anonymous user. Useful for NFS-ex-
ported 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 exam-
ple 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 filehan-
dles 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 subdi-
rectory exports, and if nested subdirectory exports change the secu-
rity_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
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 re-
quests 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 sev-
enth 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), tlshd(8).
31 December 2009 exports(5)
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