9.7.2. The exportfs Command

Every file system being exported to remote users with 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/etab. Since rpc.mountd refers to the etab 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:
Causes all directories listed in /etc/exports to be exported by constructing a new export list in /etc/lib/nfs/etab. This option effectively refreshes the export list with any changes made to /etc/exports.
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. This option is often used to test an exported file system before adding it permanently to the list of file systems to be exported. Refer to Section 9.7.1, “The /etc/exports Configuration File” for more information on /etc/exports syntax.
Ignores /etc/exports; only options given from the command line are used to define exported file systems.
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, use exportfs -r.
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 exportfs command, it displays a list of currently exported file systems. For more information about the exportfs command, refer to man exportfs. Using exportfs with NFSv4

In Red Hat Enterprise Linux 6, no extra steps are required to configure NFSv4 exports as any filesystems mentioned are automatically available to NFSv2, NFSv3, and NFSv4 clients using the same path. This was not the case in previous versions.
To prevent clients from using NFSv4, turn it off by sellecting RPCNFSDARGS= -N 4 in /etc/sysconfig/nfs.