Normally this xtab file is initialized with the list of all file systems named in /etc/exports by invoking exportfs -a.
However, administrators can choose to add and delete individual file systems without modifying /etc/exports using exportfs.
exportfs and its partner program mountd work in one of two modes, a legacy mode which applies to 2.4 and earlier versions of the Linux kernel, and a new mode which applies to 2.6 and later versions providing the nfsd virtual filesystem has been mounted at /proc/fs/nfsd or /proc/fs/nfs. If this filesystem is not mounted in 2.6, the legacy mode is used.
In the new mode, exportfs does not give any information to the kernel but only provides it to mountd through the /var/lib/nfs/xtab file. mountd will listen to requests from the kernel and will provide information as needed.
In the legacy mode, any export requests which identify a specific host (rather than a subnet or netgroup etc) are entered directly into the kernels export table as well as being written to /var/lib/nfs/xtab. Further, any mount points listed in /var/lib/nfs/rmtab which match a non host-specific export request will cause an appropriate export entry for the host given in rmtab to be entered into the kernels export table.
|-a||Export or unexport all directories.|
|Specify a list of export options in the same manner as in exports(5) .|
|-i||Ignore the /etc/exports file, so that only default options and options given on the command line are used.|
|-r||Reexport all directories. It synchronizes /var/lib/nfs/xtab with /etc/exports. It removes entries in /var/lib/nfs/xtab which are deleted from /etc/exports, and remove any entries from the kernel export table which are no longer valid.|
|-u||Unexport one or more directories.|
|-f||In new mode, flush everything out of the kernels export table. Any clients that are active will get new entries added by mountd when they make their next request.|
|-v||Be verbose. When exporting or unexporting, show whats going on. When displaying the current export list, also display the list of export options.|
The host:/path argument specifies the directory to export along with the host or hosts to export it to. All formats described in exports(5) are supported; to export a directory to the world, simply specify :/path.
The export options for a particular host/directory pair derive from several sources. There is a set of default options which can be overridden by entries in /etc/exports (unless the -i option is given). In addition, the administrator may overide any options from these sources using the -o argument which takes a comma-separated list of options in the same fashion as one would specify them in exports(5) . Thus, exportfs can also be used to modify the export options of an already exported directory.
Modifications of the kernel export table used by nfsd(8) take place immediately after parsing the command line and updating the xtab file.
The default export options are sync,ro,root_squash,wdelay.
To remove individial export entries, one can specify a host:/path pair. This deletes the specified entry from xtab and removes the corresponding kernel entry (if any).
# exportfs -a
To export the /usr/tmp directory to host django, allowing asynchronous writes, one would do this:
# exportfs -o async django:/usr/tmp
When unexporting a network or domain entry, any current exports to members of this group will be checked against the remaining valid exports and if they themselves are nolonger valid they will be removed.