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.
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.
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. To help make system administrators aware of this change, exportfs will issue a warning if neither sync nor async is specified.
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.
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.
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.
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.
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.
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.
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:
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.
# 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.