Chapter 17.

Network File System (NFS)

NFS (Network File System) exists to allow hosts to mount partitions on a remote system and use them

as though they were local file systems. This allows files to be organized in a central location, while

providing the functionality of allowing authorized users continuous access to them.

Two versions of NFS are currently in use. NFS version 2 (NFSv2), which has been around for several

years, is widely supported by various operating systems. NFS version 3 (NFSv3) has several more

features, including a variable file handle size and better error reporting. Red Hat Linux supports both

NFSv2 and NFSv3, and uses NFSv3 by default when connecting with a server that supports it.

This chapter will focus on NFS version 2, though many of the concepts discussed also apply to version

3. Additionally, only fundamental NFS concepts and supplemental information will be provided. For

specific instructions regarding the configuration and operation of NFS on client or server machines,

see the chapter titled Network File System (NFS) in the Official Red Hat Linux Customization Guide.

17.1. Methodology

Linux uses a combination of kernel level support and continuously running daemon processes to

provide NFS file sharing, however, NFS support must be enabled in the Linux kernel to function. NFS

uses Remote Procedure Calls (RPC) to route requests between clients and servers, meaning that the

portmap

service must be enabled and active at the proper runlevels for NFS communication to occur.

Working with

portmap

, various other processes ensure that a particular NFS connection is allowed

and may proceed without error:

  rpc.mountd

  The running process that receives the mount request from an NFS client and

checks to see if it matches with a currently exported file system.

  rpc.nfsd

  The process that implements the user level part of the NFS service. It works with

the Linux kernel to meet the dynamic demands of NFS clients, such as providing additional server

threads for NFS clients to uses.

  rpc.lockd

  A daemon that is not necessary with modern kernels. NFS file locking is now done

by the kernel. It is included with the

nfs utils

package for users of older kernels that do not

include this functionality by default.

  rpc.statd

  Implements the Network Status Monitor (NSM) RPC protocol. This provides reboot

notification when an NFS server is restarted without being gracefully brought down.

  rpc.rquotad

  An RPC server that provides user quota information for remote users.

Not all of these programs are required for NFS service. The only services that must be enabled are

rpc.mountd

,

rpc.nfsd

, and

portmap

. The other daemons provide additional functionality and

should only be used if your server environment requires them.

NFS version 2 uses the User Datagram Protocol (UDP) to provide a stateless network connection

between the client and server. NFS version 3 can use UDP or TCP running over an IP. The stateless

UDP connection minimizes network traffic, as the NFS server sends the client a cookie after the client

is authorized to access the shared volume. This cookie is a random value stored on the server's side

and is passed with along with RPC requests from the client. The NFS server can be restarted without

affecting the clients and the cookie will remain intact.

NFS only performs authentication when a client system attempts to mount a remote file

system. To limit access, the NFS server first employs TCP wrappers. TCP wrappers reads the

/etc/hosts.allow

and

/etc/hosts.deny

files to determine if a particular client should be