EXPORTS

NAME
SYNOPSIS
DESCRIPTION
EXAMPLE
FILES

NAME

exports − NFS file systems being exported (for Kernel based NFS)

SYNOPSIS

/etc/exports

DESCRIPTION

The file /etc/exports serves as the access control list for file systems which may be exported to NFS clients. It is used by exportfs(8) to give information to mountd(8) and to the kernel based NFS file server daemon nfsd(8).

The file format is similar to the SunOS exports file, except that several additional options are permitted. Each line contains an export point and a list of machine or netgroup names allowed to mount the file system at that point. An optional parenthesized list of export parameters may follow each machine name. Blank lines are ignored, and a # introduces a comment to the end of the line. Entries may be continued across newlines using a backslash.

Machine Name Formats

NFS clients may be specified in a number of ways:

single host

This is the most common format. You may specify a host either by an abbreviated name recognized be the resolver, the fully qualified domain name, or an IP address.

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.

wildcards

Machine names may contain the wildcard characters * and ?. 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. However, these wildcard characters do not match the dots in a domain name, so the above pattern does not include hosts such as a.b.cs.foo.edu.

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 address result in identical subnetworks with 10 bits of host).

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.

sync

This option requests that all file writes be committed to disc before the write request completes. This is required for complete safety of data in the face of a server crash, but incurs a performance hit. The default is to allow the server to write the data out whenever it is ready. This can be explicitly requested with the async option.

no_wdelay

This option only has effect if sync 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.

no_subtree_check

This option disables subtree checking, which has mild security implications, but can improve reliability is 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 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.

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.

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 -2 (i.e. 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 that might be equally sensitive, such as user bin.

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).

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)

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.

FILES

/etc/exports