Manpages

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:

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:

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:

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.

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.

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

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.)

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).