The `exportfs` Command

Every file system being exported to remote users via NFS, as well as the access level for those file systems, are listed in the /etc/exports file. When the nfs service starts, the /usr/sbin/exportfs command launches and reads this file, passes control to rpc.mountd (if NFSv2 or NFSv3) for the actual mounting process, then to rpc.nfsd where the file systems are then available to remote users.

When issued manually, the /usr/sbin/exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When given the proper options, the /usr/sbin/exportfs command writes the exported file systems to /var/lib/nfs/xtab. Since rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to the list of exported file systems take effect immediately.

The following is a list of commonly used options available for /usr/sbin/exportfs:

-r - Causes all directories listed in /etc/exports to be exported by constructing a new export list in /etc/lib/nfs/xtab. This option effectively refreshes the export list with any changes that have been made to /etc/exports.

-a - Causes all directories to be exported or unexported, depending on what other options are passed to /usr/sbin/exportfs. If no other options are specified, /usr/sbin/exportfs exports all file systems specified in /etc/exports.
-o file-systems - Specifies directories to be exported that are not listed in /etc/exports. Replace file-systems with additional file systems to be exported. These file systems must be formatted in the same way they are specified in /etc/exports. Refer to "The /etc/exports Configuration File" for more information on /etc/exports syntax. This option is often used to test an exported file system before adding it permanently to the list of file systems to be exported.
-i - Ignores /etc/exports; only options given from the command line are used to define exported file systems.
-u - Unexports all shared directories. The command /usr/sbin/exportfs -ua suspends NFS file sharing while keeping all NFS daemons up. To re-enable NFS sharing, type exportfs -r.
-v - Verbose operation, where the file systems being exported or unexported are displayed in greater detail when the exportfs command is executed.

If no options are passed to the /usr/sbin/exportfs command, it displays a list of currently exported file systems.

For more information about the /usr/sbin/exportfs command, refer to the exportfs man page.

Using `exportfs` with NFSv4

The exportfs command is used in maintaining the NFS table of exported file systems. When typed in a terminal with no arguments, the exportfs command shows all the exported directories.

Since NFSv4 no longer utilizes the MOUNT protocol, which was used with the NFSv2 and NFSv3 protocols, the mounting of file systems has changed.

An NFSv4 client now has the ability to see all of the exports served by the NFSv4 server as a single file system, called the NFSv4 pseudo-file system. On Community Enterprise Linux, the pseudo-file system is identified as a single, real file system, identified at export with the fsid=0 option.

For example, the following commands could be executed on an NFSv4 server:

mkdir /exports
mkdir /exports/opt
mkdir /exports/etc
mount --bind /usr/local/opt /exports/opt
mount --bind /usr/local/etc /exports/etc
exportfs -o fsid=0,insecure,no_subtree_check gss/krb5p:/exports
exportfs -o rw,nohide,insecure,no_subtree_check gss/krb5p:/exports/opt
exportfs -o rw,nohide,insecure,no_subtree_check gss/krb5p:/exports/etc

In this example, clients are provided with multiple file systems to mount, by using the --bind option which creates unbreakable links.

Because of the pseudo-file systems feature, NFS version 2, 3 and 4 export configurations are not always compatible. For example, given the following directory tree:

/home
/home/sam
/home/john
/home/joe

and the export:

/home *(rw,fsid=0,sync)

Using NFS version 2,3 and 4 the following would work:

mount server:/home /mnt/home
ls /mnt/home/joe

Using v4 the following would work:

mount -t nfs4 server:/ /mnt/home
ls /mnt/home/joe

The difference being "server:/home" and "server:/". To make the exports configurations compatible for all version, one needs to export (read only) the root filesystem with an fsid=0. The fsid=0 signals the NFS server that this export is the root.

/ *(ro,fsid=0)
/home *(rw,sync,nohide)

Now with these exports, both "mount server:/home /mnt/home" and "mount -t nfs server:/home /mnt/home" will work as expected.

Using exportfs with NFSv4

Using `exportfs` with NFSv4