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Section: File Formats (5)
Updated: 28 October 1999
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NAMEexports - NFS file systems being exported (for Kernel based NFS)
DESCRIPTIONThe 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. 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.
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.
Machine Name FormatsNFS 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.
- 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.
- 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). Wildcard characters generally do not work on IP addresses, though they may work by accident when reverse DNS lookups fail.
General Optionsexportfs understands the following export options:
- 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.
- 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.
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.
In releases of nfs-utils upto and including 1.0.0, this option was the default. In this and future releases, sync is the default, and async must be explicit requested if needed. To help make system adminstrators aware of this change, 'exportfs' will issue a warning if neither sync nor async is specified.
- 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.
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.
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.
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:
- 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.
- Turn off root squashing. This option is mainly useful for diskless clients.
- 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).
# 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.
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