Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 239)

Chapter 18.

Network File System (NFS)

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

use them as though they were local ﬁlesystems. 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, which has been around for several years, is

widely supported by various operating systems. NFS version 3 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 which 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 Ofﬁcial Red Hat Linux Customization Guide.

18.1. 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 ﬁlesystem.

• 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 utilize.

• 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 utilizing 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, based

on the particular requirements of your server environment.

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, or random value that is stored on the server’s

side, is passed with any RPC requests from the client to the server. The NFS server can be restarted

without affecting the clients and the cookie remains intact.

Using NFS, authentication only occurs when the client is attempting to mount to a remote ﬁlesystem.

The NFS server uses the /etc/hosts.allow and /etc/hosts.deny ﬁles to determine if a partic-

ular host should be speciﬁcally permitted or prevented access via NFS. Then, the NFS server refers

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 239

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