Network File System (NFS)A Network File System (NFS) allows remote hosts to mount file systems over a network andinteract with those file systems as though they are mounted locally. This enables systemadministrators to consolidate resources onto centralized servers on the network.This chapter focuses on fundamental NFS concepts and supplemental information. For specificinstructions regarding the configuration and operation of NFS server and client software, refer tothe chapter titled Network File System (NFS) in the Red Hat Enterprise Linux SystemAdministration Guide.1. How It WorksCurrently, there are three versions of NFS. NFS version 2 (NFSv2) is older and is widelysupported. NFS version 3 (NFSv3) has more features, including variable size file handling andbetter error reporting, but is not fully compatible with NFSv2 clients. NFS version 4 (NFSv4)works through firewalls and on the Internet, no longer requires portmapper, supports ACLs, andutilizes stateful operations. Red Hat Enterprise Linux supports NFSv2, NFSv3, and NFSv4clients, and when mounting a file system via NFS, Red Hat Enterprise Linux uses NFSv3 bydefault, 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) runningover 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 conditionsminimizes network traffic, as the NFS server sends the client a cookie after the client isauthorized to access the shared volume. This cookie is a random value stored on the server'sside and is passed along with RPC requests from the client. The NFS server can be restartedwithout 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 withrequests for the server. For this reason, TCP is the preferred protocol when connecting to anNFS server.NFSv4 has no interaction with portmapper, rpc.mountd, rpc.lockd, and rpc.statd, since theyhave been rolled into the kernel. NFSv4 listens on the well known TCP port 2049.NoteTCP is the default transport protocol for NFS under Red Hat Enterprise Linux.Refer to the chapter titled Network File System (NFS) in the Red Hat EnterpriseLinux System Administration Guide for more information about connecting toNFS servers using TCP. UDP can be used for compatibility purposes as needed,but is not recommended for wide usage.Chapter 9.131
