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Chapter 9. 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. For specific instructions regarding the configuration and operation of NFS server and client software, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.
9.1. 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 variable size file handling and better 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, 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 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 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.
NFSv4 has no interaction with portmapper,
rpc.statd, since they have been rolled into the kernel. NFSv4 listens on the well known TCP port 2049.
TCP is the default transport protocol for NFS under Red Hat Enterprise Linux. Refer to the chapter titled Network File System (NFS) in the System Administrators Guide for more information about connecting to NFS servers using TCP. UDP can be used for compatibility purposes as needed, but is not recommended for wide usage.
The only time NFS performs authentication is when a client system attempts to mount the shared NFS resource. To limit access to the NFS service, TCP wrappers are used. TCP wrappers read the
/etc/hosts.denyfiles to determine if a particular client or network is permitted or denied access to the NFS service. For more information on configuring access controls with TCP wrappers, refer to Chapter 17, TCP Wrappers and
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.
In order for NFS to work with a default installation of Red Hat Enterprise Linux with a firewall enabled, IPTables with the default TCP port 2049 must be configured. Without an IPTables configuration, NFS does not function properly.
The NFS initialization script and
rpc.nfsdprocess now allow binding to any specified port during system start up. However, this can be error prone if the port is unavailable or conflicts with another daemon.
9.1.1. Required Services
Red Hat Enterprise Linux uses a combination of kernel-level support and daemon processes to provide NFS file sharing. NFSv2 and NFSv3 rely on Remote Procedure Calls (RPC) to encode and decode requests between clients and servers. RPC services under Linux are controlled by the
portmapservice. To share or mount NFS file systems, the following services work together, depending on which version of NFS is implemented:
nfs— Starts the appropriate RPC processes to service requests for shared NFS file systems.
nfslock— An optional service that starts the appropriate RPC processes to allow NFS clients to lock files on the server.
portmap— The RPC service for Linux; it responds to requests for RPC services and sets up connections to the requested RPC service. This is not used with NFSv4.
The following RPC processes facilitate NFS services:
rpc.mountd— This process receives mount requests from NFS clients and verifies the requested file system is currently exported. This process is started automatically by the
nfsservice and does not require user configuration. This is not used with NFSv4.
rpc.nfsd— This process is the NFS server. It works with the Linux kernel to meet the dynamic demands of NFS clients, such as providing server threads each time an NFS client connects. This process corresponds to the
rpc.lockd— An optional process that allows NFS clients to lock files on the server. This process corresponds to the
nfslockservice. This is not used with NFSv4.
rpc.statd— This process implements the Network Status Monitor (NSM) RPC protocol which notifies NFS clients when an NFS server is restarted without being gracefully brought down. This process is started automatically by the
nfslockservice and does not require user configuration. This is not used with NFSv4.
rpc.rquotad— This process provides user quota information for remote users. This process is started automatically by the
nfsservice and does not require user configuration.
rpc.idmapd— This process provides NFSv4 client and server upcalls which map between on-the-wire NFSv4 names (which are strings in the form of user@domain) and local UIDs and GIDs. For
idmapdto function with NFSv4, the
/etc/idmapd.confmust be configured. This service is required for use with NFSv4.
rpc.svcgssd— This process is used by the NFS server to perform user authentication and is started only when
SECURE_NFS=yesis set in the
rpc.gssd— This process is used by the NFS server to perform user authentication and is started only when
SECURE_NFS=yesis set in the
9.1.2. NFS and
The following section only applies to NFSv2 or NFSv3 implementations that require the
portmapservice for backward compatibility.
portmapservice under Linux maps RPC requests to the correct services. RPC processes notify
portmapwhen they start, revealing the port number they are monitoring and the RPC program numbers they expect to serve. The client system then contacts
portmapon the server with a particular RPC program number. The
portmapservice redirects the client to the proper port number so it can communicate with the requested service.
Because RPC-based services rely on
portmapto make all connections with incoming client requests,
portmapmust be available before any of these services start.
portmapservice uses TCP wrappers for access control, and access control rules for
portmapaffect all RPC-based services. Alternatively, it is possible to specify access control rules for each of the NFS RPC daemons. The man pages for
rpc.statdcontain information regarding the precise syntax for these rules.
220.127.116.11. Troubleshooting NFS and
portmapprovides coordination between RPC services and the port numbers used to communicate with them, it is useful to view the status of current RPC services using
portmapwhen troubleshooting. The
rpcinfocommand shows each RPC-based service with port numbers, an RPC program number, a version number, and an IP protocol type (TCP or UDP).
To make sure the proper NFS RPC-based services are enabled for
portmap, issue the following command as root:
The following is sample output from this command:
program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 100021 1 udp 32774 nlockmgr 100021 3 udp 32774 nlockmgr 100021 4 udp 32774 nlockmgr 100021 1 tcp 34437 nlockmgr 100021 3 tcp 34437 nlockmgr 100021 4 tcp 34437 nlockmgr 100011 1 udp 819 rquotad 100011 2 udp 819 rquotad 100011 1 tcp 822 rquotad 100011 2 tcp 822 rquotad 100003 2 udp 2049 nfs 100003 3 udp 2049 nfs 100003 2 tcp 2049 nfs 100003 3 tcp 2049 nfs 100005 1 udp 836 mountd 100005 1 tcp 839 mountd 100005 2 udp 836 mountd 100005 2 tcp 839 mountd 100005 3 udp 836 mountd 100005 3 tcp 839 mountd
The output from this command reveals that the correct NFS services are running. If one of the NFS services does not start up correctly,
portmapis unable to map RPC requests from clients for that service to the correct port. In many cases, if NFS is not present in
rpcinfooutput, restarting NFS causes the service to correctly register with
portmapand begin working. For instructions on starting NFS, refer to Section 9.2, “Starting and Stopping NFS”.
Other useful options are available for the
rpcinfocommand. Refer to the
rpcinfoman page for more information.