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, and passes to rpc.mountd and rpc.nfsd the file systems 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 passed 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 the same way they are specified in /etc/exports. Refer to Section 9.3.1 NFS Server 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 reenable 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.

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There are three different ways to configure an Network File System (NFS) server in Linux: using the NFS Server Configuration Tool  (redhat-config-nfs), manually you can editing its configuration file /etc/exports as well as using the /usr/sbin/exportfs command.

In this thread we are configuring Network File System server by using last two option which is file /etc/exports and /usr/sbin/exportfs.

The /etc/exports file defines which file systems are exported to remote hosts/servers and specifies options.Blank lines are ignored and comments can be made by starting a line with the hash mark (#), and long lines can be wrapped with a backslash (\). Each exported file system should be on its own individual line, and any lists of authorized hosts placed after an exported file system must be separated by space characters. Options for each of the hosts/servers must be placed in parentheses directly after the host identifier, without any spaces separating the host and the first parenthesis.

A line for an exported file system has the following structure:

1. <export> <host1>(<options>) <hostN>(<options>)…
2. <export> <host2>(<options>) <hostN>(<options>)…
3. <export> <host3>(<options>) <hostN>(<options>)…

In this structure, replace <export>  with the directory being exported, replace <host1> with the host/server or network to which the export is being shared, and replace (<options> with the options for that host/server or network. Additional hosts can be specified in a space separated list.

To specify host names refer NFS Server Configuration part 2

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