Sharing Files with NFS

Exporting a File System with NFS

Start with the server system that exports-makes available to the client systems-the contents of a directory. On the server, you must run the NFS service and also designate one or more file systems that are to be exported-made available to the client systems.

To export a file system, you have to add an appropriate entry to the /etc/exports file. For example, suppose that you want to export the /home directory and you want to enable the host named LNBP75 to mount this file system for read and write operations. You can do this by adding the following entry to the /etc/exports file:

/home LNBP75(rw, sync)

If you want to give access to all hosts on a LAN such as 192.168.0.0, you could change this line to:

/home 192.168.0.0/24(rw,sync)

Secret

Each line in the /etc/exports file has this general format:

directory    host1(options)  host2(options)  ...

The first field is the directory being shared via NFS, followed by one or more fields that specify which hosts can mount that directory remotely and a number of options within parentheses. You can specify the hosts with names or IP addresses, including ranges of addresses.

The options within parentheses denote the kind of access each host is granted and how user and group IDs from the server are mapped to IDs the client (for example, if a file is owned by root on the server, what owner should that be on the client). Within the parentheses, the options are separated by commas. For example, if a host is allowed both read and write access and all IDs are to be mapped to the anonymous user (by default this is the user named nobody), then the options would look like this:

(rw,all_squash)

Table 19-1 shows the options you can use in the /etc/exports file. There are two types of options-general options and user ID mapping options.

Table 19-1: Options in /etc/exports

Option	Description
General Options
secure	Allows connections only from ports 1024 or lower (default)
insecure	Allows connections from 1024 or higher
ro	Allows read-only access (default)
rw	Allows both read and write access
sync	Performs write operations (this means writing information to the disk) when requested (default)
async	Performs write operations when the server is ready
no_wdelay	Performs write operations immediately
wdelay	Waits a bit to see if related write requests arrive and then performs then together (default)
hide	Hides an exported directory that's a subdirectory of another exported directory (default)
no_hide	Behaves exactly the opposite of hide
subtree_check	Performs subtree checking, which involves checking parent directories of an exported subdirectory whenever a file is accessed (default)
no_subtree_check	Turns off subtree checking (opposite of subtree_check)
insecure_locks	Allows insecure file locking
User ID Mapping Options
all_squash	Maps all user IDs and group IDs to the anonymous user on the client
no_all_squash	Maps remote user and group IDs to similar IDs on the client (default)
root_squash	Maps remote root user to the anonymous user on the client (default)
no_root_squash	Maps remote root user to the local root user
anonuid=UID	Sets the user ID of anonymous user to be used for the all_squash and root_squash options
anongid=GID	Sets the group ID of anonymous user to be used for the all_squash and root_squash options

After adding the entry in the /etc/exports file, manually export the file system by typing the following command in a terminal window:

exportfs -a

This command exports all file systems defined in the /etc/exports file.

Now, you can start the NFS server processes. To do this, log in as root and type the following command in a terminal window:

service nfs start

chkconfig --level 35 nfs on

service nfs restart

Mounting an NFS File System

To access an exported NFS file system on a client system, you have to mount that file system on a mount point. The mount point is nothing more than a local directory. For example, suppose that you want to access the /home directory exported from the server named LNBP200 at the local directory /mnt/lnbp200 on the client system. To do this, follow these steps:

1. Log in as root, and create the directory with the command

   mkdir /mnt/lnbp200

2. Type the following command to mount the directory from the remote system (LNBP200) on the local directory /mnt/lnbp200:

   mount lnbp200:/home /mnt/lnbp200

After these steps, you can view and access exported files from the local directory /mnt/lnbp200.

To confirm that the NFS file system is indeed mounted, log in as root on the client system and type mount in a terminal window. You should see a line similar to the following one about the NFS file system:

lnbp200:/home/public on /mnt/lnbp200 type nfs (rw,addr=192.168.1.200)