Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION, Руководство по развертыванию

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Chapter 19.
239
Network File System (NFS)
A
Network File System
(
NFS
) allows remote hosts to mount file systems over a network and interact
with those file systems as though they are mounted locally. This enables system administrators to
consolidate resources onto centralized servers on the network.
This chapter focuses on fundamental NFS concepts and supplemental information.
19.1. How It Works
Currently, there are three versions of NFS. NFS version 2 (NFSv2) is older and is widely supported.
NFS version 3 (NFSv3) has more features, including 64bit file handles, Safe Async writes and more
robust error handling. NFS version 4 (NFSv4) works through firewalls and on the Internet, no longer
requires portmapper, supports ACLs, and utilizes stateful operations. Red Hat Enterprise Linux
supports NFSv2, NFSv3, and NFSv4 clients, and when mounting a file system via NFS, Red Hat
Enterprise Linux uses NFSv3 by default, if the server supports it.
All versions of NFS can use
Transmission Control Protocol
(
TCP
) running over an IP network, with
NFSv4 requiring it. NFSv2 and NFSv3 can use the
User Datagram Protocol
(
UDP
) running over an IP
network to provide a stateless network connection between the client and server.
When using NFSv2 or NFSv3 with UDP, the stateless UDP connection under normal conditions has
less Protocol overhead than TCP which can translate into better performance on very clean, non-
congested networks. The NFS server sends the client a file handle after the client is authorized to
access the shared volume. This file handle is an opaque object stored on the server's side and is
passed along with RPC requests from the client. The NFS server can be restarted without affecting
the clients and the cookie remains intact. However, because UDP is stateless, if the server goes
down unexpectedly, UDP clients continue to saturate the network with requests for the server. For this
reason, TCP is the preferred protocol when connecting to an NFS server.
Because protocol support has been incorporated into the v4 protocol, NFSv4 has no interaction with
the
portmap
,
rpc.lockd
, and
rpc.statd
daemons. NFSv4 listens on the well-known TCP port
2049, which eliminates the need for
portmap
interaction. The mounting and locking protocols have
been incorporated into the V4 protocol which eliminates the need for interaction with
rpc.lockd
and
rpc.statd
. The
rpc.mountd
daemon is still required on the server, but is not involved in any over-
the-wire operations.
Note
TCP is the default transport protocol for NFS under Red Hat Enterprise Linux. UDP can be
used for compatibility purposes as needed, but is not recommended for wide usage.
All the RPC/NFS daemon have a
'-p'
command line option that can set the port, making
firewall configuration easier.
After the client is granted access by TCP wrappers, the NFS server refers to its configuration file,
/
etc/exports
, to determine whether the client is allowed to access any of the exported file systems.
Once access is granted, all file and directory operations are available to the user.