<para>Among the many different file systems that FreeBSD supports

    <para>&os; supports the Network File System

      is the Network File System, also known as <acronym role="Network

      (<acronym role="Network File System">NFS</acronym>).

      File System">NFS</acronym>.  <acronym role="Network File

      <acronym role="Network File System">NFS</acronym>

      System">NFS</acronym> allows a system to share directories and

      allows a server to share directories and

      files with others over a network.  By using <acronym

      files with clients over a network.  By using

      role="Network File System">NFS</acronym>, users and programs can

      <acronym role="Network File System">NFS</acronym>,

      access files on remote systems almost as if they were local

      users and programs can access files on remote systems

      files.</para>

      as if they were stored locally.</para>

    <para>Some of the most notable benefits that

    <para>The most notable benefits that

      <acronym>NFS</acronym> can provide are:</para>

      <acronym>NFS</acronym> provides are:</para>

	<para>Local workstations use less disk space because commonly

	<para>Data that would otherwise be duplicated on each

	  used data can be stored on a single machine and still remain

	  client can be kept in single location and accessed 

	  accessible to others over the network.</para>

	  by clients on the network.</para>

	<para>There is no need for users to have separate home

	<para>Users home directories can be stored in one location

	  directories on every network machine.  Home directories

	  and accessed by their owners over the network.  This also

	  could be set up on the <acronym>NFS</acronym> server and

	  has the benefit of simplified administration.  For example,

	  made available throughout the network.</para>

	  backing up one location instead of several, security

	  policies on one file system, etc.</para>

	<para>Storage devices such as floppy disks, CDROM drives, and

	<para>Storage devices such as floppy disks or

	  &iomegazip; drives can be used by other machines on the

	  <acronym>CD/DVD-ROM</acronym> drives can be used

	  network.  This may reduce the number of removable media

	  by other machines on the network.  This would reduce

	  drives throughout the network.</para>

	  the number of removable media drives throughout

	  the network and provide a centralized location to

	  manage the usage and security of such devices.</para>

      <para>The server has to be running the following daemons:</para>

      <para>The server must be running the following daemons:</para>

	improves performance, but is not required for normal and

	improves performance but is not required for normal

	correct operation.  See the &man.nfsiod.8; manual page for

	operation.  See the &man.nfsiod.8; manual page for

      <para><acronym>NFS</acronym> configuration is a relatively

      <para>Enabling the <acronym>NFS</acronym> server

	straightforward process.  The processes that need to be

	is straightforward.  The processes that need

	running can all start at boot time with a few modifications

	to be running can setup to be started at boot

	to <filename>/etc/rc.conf</filename>.</para>

	time by adding these options to

	<filename>/etc/rc.conf</filename>:</para>

      <para>On the client, make sure this option is present in

      <para>Likewise, to enable the client, make sure this option

	<filename>/etc/rc.conf</filename>:</para>

	is defined in <filename>/etc/rc.conf</filename>:</para>

	file systems <acronym>NFS</acronym> should export (sometimes

	file systems the <acronym>NFS</acronym> server will

	referred to as <quote>share</quote>).  Each line in

	export (sometimes referred to as <quote>share</quote>).  Each

	<filename>/etc/exports</filename> specifies a file system to

	line in <filename>/etc/exports</filename> specifies a

	be exported and which machines have access to that file

	file system to be exported and which clients have access

	system.  Along with what machines have access to that file

	to that file system, as well as, any access options that may

	system, access options may also be specified.  There are many

	be specified.  There are many such options that can be

	such options that can be used in this file but only a few will

	used in this file but only a few will be mentioned here.

	be mentioned here.  Other options are discussed in

	Other options are discussed in the &man.exports.5;

	the &man.exports.5; manual page.</para>

	manual page.</para>

	the environment and network configuration.  For instance, to

	the environment and network configuration.</para>

	export the <filename>/cdrom</filename> directory to three

	example machines that have the same domain name as the server

	(hence the lack of a domain name for each) or have entries in

	the <filename>/etc/hosts</filename> file.  The

	<option>-ro</option> flag makes the exported file system

	read-only.  With this flag, the remote system will not be able

	to write any changes to the exported file system.</para>

      <para>The following line exports <filename>/home</filename> to

      <para>The <literal>-ro</literal> flag makes the exported file

	three hosts by IP address.  This is a useful setup on

	system read-only, preventing the clients from making any

	a private network without a <acronym>DNS</acronym> server

	changes to the exported file system.</para>

      <para>The next example exports <filename>/home</filename> to

	three clients by IP address.  This is a useful setup for

	networks that may not have a <acronym>DNS</acronym> server

	<option>-alldirs</option> flag allows the subdirectories to be

	<literal>-alldirs</literal> flag allows the subdirectories

	mount points.  In other words, it will not mount the

	to be mount points.  In other words, it will not mount the

      <para>The following line exports <filename>/a</filename> so that

      <para>This next line exports <filename>/a</filename> so that

	then even if a user has <username>root</username> access on

	the client's <username>root</username> user will be mapped

	the remote system, he will not be able to modify files on

	to the server's <username>nobody</username> account and will

	the exported file system.</para>

	be subject to the access limitations set for the

	<username>nobody</username> user.</para>

      <para>In order for a client to access an exported file system,

      <para>For a client to have access to an exported file system,

	the client must have permission to do so.  Make sure the

	make sure it is listed in

	client is listed in <filename>/etc/exports</filename>.</para>

	<filename>/etc/exports</filename>.</para>

      <para>In <filename>/etc/exports</filename>, each line represents

      <para>In <filename>/etc/exports</filename>, each line defines

	the export information for one file system to one host.  A

	the export information for one file system to one or more

	remote host can only be specified once per file system, and

	clients.  A remote host can only be specified once per file

	may only have one default entry.  For example, assume that

	system, and may only have one default entry.  For example,

	<filename>/usr</filename> is a single file system.  The

	assume that <filename>/usr</filename> is a single file

	following <filename>/etc/exports</filename> would be

	system.  The following <filename>/etc/exports</filename>

	invalid:</para>

	would be invalid:</para>

      <para>One file system, <filename>/usr</filename>, has two lines

      <para>The <filename>/usr</filename> file system has two lines

      <para>The properties of one file system exported to a given host

      <para>The properties of one file system, exported to a given

	must all occur on one line.  Lines without a client specified

	host, must within a single line.  Lines without a client

	are treated as a single host.  This limits how file systems

	specified are treated as a single host.  This limits how

	may be exported; however, for most environments, this is not

	file systems may be exported; however, for most environments,

	an issue.</para>

	this is not an issue.</para>

      <para>The <application>mountd</application> daemon must be

      <para>The <application>mountd</application> daemon only reads

	forced to recheck the <filename>/etc/exports</filename> file

	<filename>/etc/exports</filename> at start up.  To make

	whenever it has been modified, so the changes can take effect.

	<acronym>NFS</acronym> server changes immediately, it must be

	This can be accomplished either by sending a HUP signal to the

	forced to recheck the <filename>/etc/exports</filename> file.

	running daemon:</para>

	The two recommended methods for executing immediate change

	are:</para>

      <para>NFS services can now be started by running the following

      <para>On a new server being configured with

	command, on the <acronym>NFS</acronym> server, as

	<acronym>NFS</acronym> services, the server can be started by

	running the following command as

      <para>Now everything should be ready to actually mount a remote

      <para>The client now has everything it needs to mount a remote

	file system.  In these examples the server's name will be

	file system.  In these examples, the server's name will be

	a remote file system execute a command like this as

	a remote file system execute the

	<application>mount</application> command as

      <para>This will mount the <filename>/home</filename> directory

      <para>This will mount the server's <filename>/home</filename>

	on the server at <filename>/mnt</filename> on the client.  If

	file system at <filename>/mnt</filename> on the client.  If

	visible and available in the <filename>/mnt</filename>

	visible and available in the client's

	directory.</para>

	<filename>/mnt</filename> directory.</para>

	computer boots, add the file system to the

	client boots, add the remote file system to the

	<filename>/etc/fstab</filename> file.  Here is an

	<filename>/etc/fstab</filename> file:</para>

	example:</para>

	can be used for file locking.  To enable it, add the following

	can be used for file locking.  To enable it, add this line

	and server (it is assumed that the <acronym>NFS</acronym>

	and server:</para>

	client and server are configured already):</para>

      <para>Start the application by using:</para>

      <para>Please note that this assumes that both

	<acronym>NFS</acronym> client and server are already

	configured.</para>

      <para>If real locking between the <acronym>NFS</acronym> clients

      <para>If locking is not required, the <acronym>NFS</acronym>

	and <acronym>NFS</acronym> server is not required, it is

	client can be configured to lock locally by passing

	possible to let the <acronym>NFS</acronym> client do locking

	<option>-L</option> to &man.mount.nfs.8;.  Refer to the

	locally by passing <option>-L</option> to &man.mount.nfs.8;.

	&man.mount.nfs.8; manual page for further details.</para>

	Refer to the &man.mount.nfs.8; manual page for further

	details.</para>

	the more common ones are listed below:</para>

	the more common uses:</para>

	  <para>Set several machines to share a CDROM or other media

	  <para>Share a <acronym>CD/DVD-ROM</acronym> or other media

	    among them.  This is cheaper and often a more convenient

	    to any number of clients.  This is cheaper and often a

	    method to install software on multiple machines.</para>

	    more convenient method to install software on multiple

	    machines.</para>

	  <para>On large networks, it might be more convenient to

	  <para>On large networks, it is more convenient to

	    configure a central <acronym>NFS</acronym> server in which

	    configure a central <acronym>NFS</acronym> server on which

	    to store all the user home directories.  These home

	    all user home directories are stored.  The exported

	    directories can then be exported to the network so that

	    home directories can then be accessed by authorized

	    users would always have the same home directory,

	    clients, allowing authenticated users access to

	    regardless of which workstation they log in to.</para>

	    their data.</para>

	  <para>Several machines could have a common

	  <para>Several clients could have a need for the

	    downloading them on each machine.</para>

	    having to download them to each client.</para>

	mounts a remote file system whenever a file or directory

	mounts or unmounts a remote file system.  It performs a

	within that file system is accessed.  Filesystems that are

	mount whenever a file or directory within a managed, remote

	inactive for a period of time will also be automatically

	file system is accessed.  Likewise,

	unmounted by <application>amd</application>.  Using

	<application>amd</application> unmounts a remote file

	system if it has been inactive for a period of time.  Using

	to permanent mounts, as permanent mounts are usually listed in

	to permanent mounts which are permanently listed in

      <para><application>amd</application> operates by attaching

      <para>By default, <application>amd</application> operates

	itself as an NFS server to the <filename>/host</filename> and

	by attaching itself as an NFS server to the

	<filename>/net</filename> directories.  When a file is

	<filename>/host</filename> and <filename>/net</filename>

	accessed within one of these directories,

	directories.  When a file is accessed within one of these

	<application>amd</application> looks up the corresponding

	directories, <application>amd</application> looks up the

	remote mount and automatically mounts it.

	corresponding remote mount and automatically mounts it.

      <para>An access to a file within

      <para>For instance, an attempt to access a file within

	<application>amd</application> to attempt to mount the

	<application>amd</application> to mount the

	<para>The <command>showmount</command> command shows the

	<para>The <command>showmount</command>, with the

	  available mounts on a remote host.  For example, to view the

	  <literal>-e</literal> flag, shows the exported file systems

	  mounts of a host named <hostid>foobar</hostid>:</para>

	  available for mount from the remote <acronym>NFS</acronym>

	  server:</para>

      <para>As seen in the example, the <command>showmount</command>

      <para>The output from <command>showmount</command>

	<application>amd</application> attempts to resolve the

	<application>amd</application> intercepts the request and

	hostname <hostid>foobar</hostid> and automatically mount the

	attempts to resolve the hostname <hostid>foobar</hostid>.

	desired export.</para>

	If successful, <application>amd</application> automatically

	mounts the desired export.</para>

      <para><application>amd</application> can be started by the

      <para><application>amd</application> is enabled by placing

	startup scripts by placing the following lines in

	the following lines <filename>/etc/rc.conf</filename>:</para>

	<filename>/etc/rc.conf</filename>:</para>

      <para>Additionally, custom flags can be passed to

      <para>It can then be started using the &os; &man.rc.8; scripts

	or by using the &man.service.8; command.</para>

      <para>Custom flags can be passed to

	<varname>amd_flags</varname> option.  By default,

	<varname>amd_flags</varname> environment variable.  By

	<varname>amd_flags</varname> is set to:</para>

	default, <varname>amd_flags</varname> is set to:</para>

      <para>Certain Ethernet adapters for ISA PC systems have

      <para>Certain Ethernet adapters, for ISA bus-type PC

	limitations which can lead to serious network problems,

	systems, have limitations which can lead to serious network

	particularly with NFS.  This difficulty is not specific to

	problems, particularly with NFS.  While this is not

	FreeBSD, but FreeBSD systems are affected by it.</para>

	specific to &os;, &os; systems built on this type

	of hardware are affected.</para>

      <para>The problem nearly always occurs when (FreeBSD) PC systems

      <para>The problem occurs when these systems are networked

	are networked with high-performance workstations, such as

	with high-performance workstations, such as those made by

	those made by Silicon Graphics, Inc., and Sun Microsystems,

	Silicon Graphics, Inc., and Sun Microsystems, Inc.  The

	Inc.  The NFS mount will work fine, and some operations may

	NFS mount will work fine, and initial operations may

	succeed, but suddenly the server will seem to become

	succeed.  Eventually the server will become unresponsive

	unresponsive to the client, even though requests to and from

	to the client, even though requests to and from

	other systems continue to be processed.  This happens to the

	other systems continue unaffected.  Once this has occurred,

	client system, whether the client is the FreeBSD system or the

	there is no way to shut the client down gracefully.  The

	workstation.  On many systems, there is no way to shut down

	only solution is to reboot the client.</para>

	the client gracefully once this problem has manifested itself.

	The only solution is often to reset the client, because the

	NFS situation cannot be resolved.</para>

	FreeBSD system, there is a simple workaround that will allow

	&os; system, there is a simple workaround.  If the

	satisfactory operation.  If the FreeBSD system is the

	&os; system is the <emphasis>server</emphasis>, include

	<emphasis>server</emphasis>, include the option

	the option <literal>-w=1024</literal> in the client's

	<option>-w=1024</option> on the mount from the client.  If the

	<command>mount</command> command.  If the &os; system is

	FreeBSD system is the <emphasis>client</emphasis>, then mount

	the <emphasis>client</emphasis>, then mount the NFS file

	the NFS file system with the option <option>-r=1024</option>.

	system with the option <option>-r=1024</option>.  These

	These options may be specified using the fourth field of the

	options may be specified using the fourth field of the

	<filename>fstab</filename> entry on the client for automatic

	<filename>/etc/fstab</filename> entry on the client for

	mounts, or by using the <option>-o</option> parameter of the

	automatic mounts, or by using the <option>-o</option>

	&man.mount.8; command for manual mounts.</para>

	parameter of the &man.mount.8; command for manual

	mounts.</para>

      <para>It should be noted that there is a different problem,

      <para>It should be noted that there is a similar problem

	sometimes mistaken for this one, when the NFS servers and

	requiring a different workaround.  It often manifests

	clients are on different networks.  If that is the case, make

	itself when the NFS servers and clients are on different

	networks.  If that is the case, make

	necessary <acronym>UDP</acronym> information.</para>

	necessary <acronym>UDP</acronym> information.  In other

	words, a <quote>helper</quote> may need to be enabled for

	the subnets and/or VLANs in question and may require

	consultation with Network Administrators for proper

	configuration.</para>

      <para>In the following examples, <hostid>fastws</hostid> is the

      <para>In these examples, <hostid>fastws</hostid> is the

	FreeBSD system with a lower-performance Ethernet adapter.

	&os; system with a lower-performance Ethernet adapter.

	Also, <filename>/sharedfs</filename> will be the exported NFS

	The export is the <filename>/sharedfs</filename> file system

	file system (see &man.exports.5;), and

	(see &man.exports.5;), and <filename>/project</filename>

	<filename>/project</filename> will be the mount point on the

	will be the mount point on the client system.  In all cases,

	client for the exported file system.  In all cases, note that

	note that additional options, such as <literal>hard</literal>

	additional options, such as <option>hard</option> or

	or <literal>soft</literal> and <literal>bg</literal> may be

	<option>soft</option> and <option>bg</option> may be desirable

	desirable in the application.</para>

	in the application.</para>

      <para>Examples for the FreeBSD system (<hostid>freebox</hostid>)

      <para>Example of the &os; client, <hostid>freebox</hostid>

	as the client in <filename>/etc/fstab</filename> on

	configured with a permanent mount in

	<hostid>freebox</hostid>:</para>

	<filename>/etc/fstab</filename>:</para>

      <para>As a manual mount command on

      <para>Now, requesting a manual mount command on &os; client,

      <para>Examples for the FreeBSD system as the server in

      <para>Examples for the &os; system, <hostid>freebox</hostid>

	acting as the <acronym>NFS</acronym> server, with the client

      <para>As a manual mount command on

      <para>As a manual mount command on client,

      <para>For anyone who cares, here is what happens when the

      <para>The symptoms of this failure, and why it is unrecoverable,

	failure occurs, which also explains why it is unrecoverable.

	are explained in the way <acronym>NFS</acronym> works.

	NFS typically works with a <quote>block</quote> size of

	<acronym>NFS</acronym> works with a default

	8 K (though it may do fragments of smaller sizes).  Since

	<quote>block</quote> size of 8&nbsp;K (though it may do

	the maximum Ethernet packet is around 1500 bytes, the NFS

	fragments of smaller sizes).  Since the typical ethernet

	<quote>block</quote> gets split into multiple Ethernet

	packet is around 1500 bytes, the NFS

	packets, even though it is still a single unit to the

	<quote>block</quote> gets split into multiple ethernet

	upper-level code, and must be received, assembled, and

	packets.  These packets then must be re-assembled and

	<emphasis>acknowledged</emphasis> as a unit.  The

	<emphasis>acknowledged</emphasis> wholey, as they are

	high-performance workstations can pump out the packets which

	received by the recipient.  High-performance workstations

	comprise the NFS unit one right after the other, just as close

	can transmit these packets faster than the lower capacity

	together as the standard allows.  On the smaller, lower

	cards can receive them, resulting in a buffer overflow

	capacity cards, the later packets overrun the earlier packets

	condition.  Packets being sent to the client cannot recover,

	of the same unit before they can be transferred to the host

	cannot be reconstructed, and therefore cannot be acknowledged.

	and the unit as a whole cannot be reconstructed or

	Ultimately, the workstation will time out and try again.

	acknowledged.  As a result, the workstation will time out and

	Repeating the request, the server sends the entire 8 K

	try again, but it will try again with the entire 8 K

	block, and the process is repeated.</para>

	unit, and the process will be repeated, ad infinitum.</para>

      <para>By keeping the unit size below the Ethernet packet size

      <para>By keeping the block size below the Ethernet packet size

	received can be acknowledged individually, avoiding the

	received can be acknowledged individually, avoiding this

	deadlock situation.</para>

	buffer overflow situation.</para>

      <para>Overruns may still occur when a high-performance

      <para>Buffer overflows (aka overruns) may still occur

	workstations is slamming data out to a PC system, but with the

	when a high-performance workstation is transmitting data

	better cards, such overruns are not guaranteed on NFS

	to a client, but with better cards, such overruns are less

	<quote>units</quote>.  When an overrun occurs, the units

	likely.  When an overrun does occur, the blocks affected

	affected will be retransmitted, and there will be a fair

	will be retransmitted, and the client is more likely to

	chance that they will be received, assembled, and

	recover on its own.</para>

	acknowledged.</para>