Provided by: nfs-kernel-server_2.6.4-3ubuntu5_amd64 
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.
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 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 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.)
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
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), tlshd(8).
31 December 2009 exports(5)