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.mountd, rpc.lockd, and rpc.statd, since they have been rolled into the kernel. NFSv4 listens on the well known TCP port 2049.

Note

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.allow and /etc/hosts.deny files 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 xinetd.
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.

Important

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.nfsd process 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 portmap service. 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 nfs service 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 nfs service.
  • rpc.lockd — An optional process that allows NFS clients to lock files on the server. This process corresponds to the nfslock service. 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 nfslock service 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 nfs service 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 idmapd to function with NFSv4, the /etc/idmapd.conf must 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=yes is set in the /etc/sysconfig/nfs file.
  • rpc.gssd — This process is used by the NFS server to perform user authentication and is started only when SECURE_NFS=yes is set in the /etc/sysconfig/nfs file.

9.1.2. NFS and portmap

Note

The following section only applies to NFSv2 or NFSv3 implementations that require the portmap service for backward compatibility.
The portmap service under Linux maps RPC requests to the correct services. RPC processes notify portmap when they start, revealing the port number they are monitoring and the RPC program numbers they expect to serve. The client system then contacts portmap on the server with a particular RPC program number. The portmap service redirects the client to the proper port number so it can communicate with the requested service.
Because RPC-based services rely on portmap to make all connections with incoming client requests, portmap must be available before any of these services start.
The portmap service uses TCP wrappers for access control, and access control rules for portmap affect 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.mountd and rpc.statd contain information regarding the precise syntax for these rules.

9.1.2.1. Troubleshooting NFS and portmap

Because portmap provides 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 portmap when troubleshooting. The rpcinfo command 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:
rpcinfo -p
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, portmap is unable to map RPC requests from clients for that service to the correct port. In many cases, if NFS is not present in rpcinfo output, restarting NFS causes the service to correctly register with portmap and begin working. For instructions on starting NFS, refer to Section 9.2, “Starting and Stopping NFS”.
Other useful options are available for the rpcinfo command. Refer to the rpcinfo man page for more information.