Quick HOWTO : Ch29 : Remote Disk Access with NFS

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Contents

Introduction

Samba is usually the solution of choice when you want to share disk space between Linux and Windows machines. The Network File System protocol (NFS) is used when disks need to be shared between Linux servers. Basic configuration is fairly simple, and this chapter will explain all the essential steps.

NFS Operation Overview

Linux data storage disks contain files stored in filesystems with a standardized directory structure. New disks are added by attaching, or mounting, the directories of their filesystems to a directory of an already existing filesystem. This in effect makes the new hard disk transparently appear to be a subdirectory of the filesystem to which it is attached.

NFS was developed to allow a computer system to access directories on remote computers by mounting them on a local filesystem as if they were a local disk. The systems administrator on the NFS server has to define the directories that need to be activated, or exported, for access by the NFS clients, and administrators on the clients need to define both the NFS server and the subset of its exported directories to use.

General NFS Rules

You should follow some general rules when configuring NFS.

  1. Only export directories beneath the / directory.
  2. Do not export a subdirectory of a directory that has already been exported. The exception being when the subdirectory is on a different physical device. Likewise, do not export the parent of a subdirectory unless it is on a separate device.
  3. Only export local filesystems.

Keep in mind that when you mount any filesystem on a directory, the original contents of the directory are ignored, or obscured, in favor of the files in the mounted filesystem. When the filesystem is unmounted, then the original files in the directory reappear unchanged.

Key NFS Concepts

Data access over a network always introduces a variety of challenges, especially if the operation is intended to be transparent to the user, as in the case of NFS. Here are some key NFS background concepts that will help in your overall understanding.

VFS

The virtual filesystem (VFS) interface is the mechanism used by NFS to transparently and automatically redirect all access to NFS-mounted files to the remote server. This is done in such a way that files on the remote NFS server appear to the user to be no different than those on a local disk.

VFS also translates these requests to match the actual filesystem format on the NFS server's disks. This allows the NFS server to be not only a completely different operating system, but also use different naming schemes for files with different file attribute types.

Stateless Operation

Programs that read and write to files on a local filesystem rely on the operating system to track their access location within the file with a pointer. As NFS is a network-based file system, and networks can be unreliable, it was decided that the NFS client daemon would act as a failsafe intermediary between regular programs running on the NFS client and the NFS server.

Normally, when a server fails, file accesses timeout and the file pointers are reset to zero. With NFS, the NFS server doesn't maintain the file pointer information, the NFS client does. This means that if an NFS server suddenly fails, the NFS client can precisely restart the file access once more after patiently waiting until the server returns online.

Caching

NFS clients typically request more data than they need and cache the results in memory locally so that further sequential access of the data can be done locally versus over the network. This is also known as a read ahead cache. Data that's to be written to the NFS server is cached with the data being written to the server when the cache becomes full. Caching therefore helps to reduce the amount of network traffic while simultaneously improving the speed of some types of data access.

The NFS server caches information too, such as the directory information for the most recently accessed files and a read ahead cache for recently read files.

NFS And Symbolic Links

You have to be careful with the use of symbolic links on exported NFS directories. If an absolute link points to a directory on the NFS server that hasn't been exported, then the NFS client won't be able to access it.

Unlike absolute links, relative symbolic links are interpreted relative to the client's filesystem. Consider an example where the /data1 directory on the server is mounted on the /data1 directory. If there is a link to the ../data2 directory on the NFS server and a directory corresponding to ../data2 doesn't exist on the NFS client, then an error will occur.

Also, mounting a filesystem on a symbolic link actually mounts the filesystem on the target of the symbolic link. You'll have to be careful not to obscure the contents of this original directory in the process. Plan carefully before doing this.

NFS Background Mounting

NFS clients use the remote procedure call (RPC) suite of network application helper programs to mount remote filesystems. If the mount cannot occur during the default RPC timeout period, then the client retries the mount process until the NFS number of retires has been exceeded. The default is 10,000 minutes, which is approximately a week. The difficulty here is that if the NFS server is unavailable, the mount command will hang for a week until it returns online. It is possible to use a bg option spawn the retries off as a subprocess so that the main mount command can continue to process other requests.

Hard and Soft Mounts

The process of continuous retrying, whether in the background or foreground, is called a hard mount. NFS attempts to guarantee the consistency of your data with these constant retries. With soft mounts, repeated RPC failures cause the NFS operation to fail not hang and data consistency is therefore not guaranteed. The advantage is that the operation completes quickly, whether it fails or not. The disadvantage is that the use of the soft option implies that you are using an unreliable NFS server, if this is the case it is best not to place critical data that needs to be updated regularly or executable programs in such a location.

NFS Versions

Three versions of NFS are available currently: versions 2, 3, and 4. Version 1 was a prototype. This chapter focuses on version 2, which:

  • Supports files up to 4GB long
  • Requires an NFS server to successfully write data to its disks before the write request is considered successful
  • Has a limit of 8KB per read or write request.

The main differences with version 3 are that it

  • Supports extremely large file sizes of up to 264 - 1 bytes
  • Supports the NFS server data updates as being successful when the data is written to the server's cache
  • Negotiates the data limit per read or write request between the client and server to a mutually decided optimal value.

Version 4 maintains many of version 3's features, but with the additions that

  • File locking and mounting are integrated in the NFS daemon and operate on a single, well known TCP port, making network security easier
  • File locking is mandatory, whereas before it was optional
  • Support for the bundling of requests from each client provides more efficient processing by the NFS server.

It is important to match the versions of NFS running on clients and server to help ensure the necessary compatibility to get NFS to work predictably.

Important NFS Daemons

NFS isn't a single program, but a suite of interrelated programs that work together to get the job done.

  • rpcbind: (portmap in older versions of Linux) The primary daemon upon which all the others rely, rpcbind manages connections for applications that use the RPC specification. By default, rpcbind listens to TCP port 111 on which an initial connection is made. This is then used to negotiate a range of TCP ports, usually above port 1024, to be used for subsequent data transfers. You need to run rpcbind on both the NFS server and client.
  • nfs: Starts the RPC processes needed to serve shared NFS file systems. The nfs daemon needs to be run on the NFS server only.
  • nfslock: Used to allow NFS clients to lock files on the server via RPC processes. The nfslock daemon needs to be run on both the NFS server and client.
  • netfs: Allows RPC processes run on NFS clients to mount NFS filesystems on the server. The netfs daemon needs to be run on the NFS client only.

Now take a look at how to configure these daemons to create functional NFS client/server peering.

Installing NFS

RedHat Linux installs nfs by default, and also by default nfs is activated when the system boots. You can determine whether you have nfs installed using the RPM command in conjunction with the grep command to search for all installed nfs packages.

[root@bigboy tmp]# rpm -qa | grep nfs
redhat-config-nfs-1.1.3-1
nfs-utils-1.0.1-3.9
[root@bigboy tmp]#

A blank list means that you'll have to install the required packages.

You also need to have the RPC rpcbind package installed, and the rpm command can tell you whether it's on your system already. When you use rpm in conjunction with grep, you can determine all the rpcbind applications installed:

[root@bigboy tmp]# rpm -q rpcbind
rpcbind-4.0-57
[root@bigboy tmp]#

A blank list means that you'll have to install the required packages.

If nfs and rpcbind are not installed, they can be added fairly easily once you find the nfs-utils and rpcbind RPMs. (If you need a refresher, see Chapter 6, "Installing Linux Software".) Remember that RPM filenames usually start with the software's name and a version number, as in nfs-utils-1.1.3-1.i386.rpm.

A Typical NFS Scenario

A small office has an old Linux server that is running out of disk space. The office cannot tolerate any down time, even after hours, because the server is accessed by overseas programmers and clients at nights and local ones by day.

Budgets are tight and the company needs a quick solution until it can get a purchase order approved for a hardware upgrade. Another Linux server on the network has additional disk capacity in its /data partition and the office would like to expand into it as an interim expansion NFS server.

Configuring NFS on The Server

Both the NFS server and NFS client have to have parts of the NFS package installed and running. The server needs rpcbind, nfs, and nfslock operational, as well as a correctly configured /etc/exports file. Here's how to do it.

The /etc/exports File

The /etc/exports file is the main NFS configuration file, and it consists of two columns. The first column lists the directories you want to make available to the network. The second column has two parts. The first part lists the networks or DNS domains that can get access to the directory, and the second part lists NFS options in brackets.

For the scenario you need:

  • Read-only access to the /data/files directory to all networks
  • Read/write access to the /home directory from all servers on the 192.168.1.0 /24 network, which is all addresses from 192.168.1.0 to 192.168.1.255
  • Read/write access to the /data/test directory from servers in the my-site.com DNS domain
  • Read/write access to the /data/database directory from a single server 192.168.1.203.

In all cases, use the sync option to ensure that file data cached in memory is automatically written to the disk after the completion of any disk data copying operation.

#/etc/exports
/data/files           *(ro,sync)
/home                 192.168.1.0/24(rw,sync)
/data/test            *.my-site.com(rw,sync)
/data/database        192.168.1.203/32(rw,sync)

After configuring your /etc/exports file, you need to activate the settings, but first make sure that NFS is running correctly.

Starting NFS on the Server

Configuring an NFS server is straightforward:

1) Use the chkconfig command to configure the required nfs and RPC rpcbind daemons to start at boot. You also should activate NFS file locking to reduce the risk of corrupted data.

[root@bigboy tmp]# chkconfig --level 35 nfs on
[root@bigboy tmp]# chkconfig --level 35 nfslock on 
[root@bigboy tmp]# chkconfig --level 35 rpcbind on

2) Use the init scripts in the /etc/init.d directory to start the nfs and RPC rpcbind daemons. The examples use the start option, but when needed, you can also stop and restart the processes with the stop and restart options.

[root@bigboy tmp]# service rpcbind start
[root@bigboy tmp]# service nfs start
[root@bigboy tmp]# service nfslock start

3) Test whether NFS is running correctly with the rpcinfo command. You should get a listing of running RPC programs that must include mountd, portmapper, nfs, and nlockmgr.

[root@bigboy tmp]# rpcinfo -p localhost
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100021    1   udp   1024  nlockmgr
    100021    3   udp   1024  nlockmgr
    100021    4   udp   1024  nlockmgr
    100005    1   udp   1042  mountd
    100005    1   tcp   2342  mountd
    100005    2   udp   1042  mountd
    100005    2   tcp   2342  mountd
    100005    3   udp   1042  mountd
    100005    3   tcp   2342  mountd
[root@bigboy tmp]#

Configuring NFS on The Client

NFS configuration on the client requires you to start the NFS application; create a directory on which to mount the NFS server's directories that you exported via the /etc/exports file, and finally to mount the NFS server's directory on your local directory, or mount point. Here's how to do it all.

Starting NFS on the Client

Three more steps easily configure NFS on the client.

1) Use the chkconfig command to configure the required nfs and RPC rpcbind daemons to start at boot. Activate nfslock to lock the files and reduce the risk of corrupted data.

[root@smallfry tmp]# chkconfig --level 35 netfs on
[root@smallfry tmp]# chkconfig --level 35 nfslock on
[root@smallfry tmp]# chkconfig --level 35 rpcbind on

2) Use the init scripts in the /etc/init.d directory to start the nfs and RPC rpcbind daemons. As on the server, the examples use the start option, but you can also stop and restart the processes with the stop and restart options.

[root@smallfry tmp]# service rpcbind start
[root@smallfry tmp]# service netfs start
[root@smallfry tmp]# service nfslock start

3) Test whether NFS is running correctly with the rpcinfo command. The listing of running RPC programs you get must include status, portmapper, and nlockmgr.

[root@smallfry root]# rpcinfo -p
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100024    1   udp  32768  status
    100024    1   tcp  32768  status
    100021    1   udp  32769  nlockmgr
    100021    3   udp  32769  nlockmgr
    100021    4   udp  32769  nlockmgr
    100021    1   tcp  32769  nlockmgr
    100021    3   tcp  32769  nlockmgr
    100021    4   tcp  32769  nlockmgr
    391002    2   tcp  32770  sgi_fam
[root@smallfry root]#

NFS And DNS

The NFS client must have a matching pair of forward and reverse DNS entries on the DNS server used by the NFS server. In other words, a DNS lookup on the NFS server for the IP address of the NFS client must return a server name that will map back to the original IP address when a DNS lookup is done on that same server name.

[root@bigboy tmp]# host 192.168.1.102
201.1.168.192.in-addr.arpa domain name pointer 192-168-1-102.my-site.com.
[root@bigboy tmp]# host 192-168-1-102.my-site.com
192-168-1-102.my-site.com has address 192.168.1.102
[root@bigboy tmp]#

This is a security precaution added into the nfs package that lessens the likelihood of unauthorized servers from gaining access to files on the NFS server. Failure to correctly register your server IPs in DNS can result in "fake hostname" errors:

Nov  7 19:14:40 bigboy rpc.mountd: Fake hostname smallfry.my-site.com for 192.168.1.1 - forward lookup doesn't exist

Accessing NFS Server Directories from the Client

In most cases, users want their NFS directories to be permanently mounted. This requires an entry in the /etc/fstab file in addition to the creation of the mount point directory.

The /etc/fstab File

The /etc/fstab file lists all the partitions that need to be auto-mounted when the system boots. Therefore, you need to edit the /etc/fstab file if you need the NFS directory to be made permanently available to users on the NFS. For the example, mount the /data/files directory on server bigboy (IP address 192.16801.100) as an NFS-type filesystem using the local /mnt/nfs mount point directory.

#/etc/fstab
#Directory                   Mount Point    Type   Options       Dump   FSCK
192.168.1.100:/data/files   /mnt/nfs        nfs    soft,nfsvers=2  0      0

This example used the soft and nfsvers options; Table 29.1 outlines these and other useful NFS mounting options you may want to use. See the NFS man pages for more details.

Table 29.1 Possible NFS Mount Options

Option Description
bg Retry mounting in the background if mounting initially fails
fg Mount in the foreground
soft Use soft mounting
hard Use hard mounting
rsize=n The amount of data NFS will attempt to access per read operation. The default is dependent on the kernel. For NFS version 2, set it to 8192 to assure maximum throughput.
wsize=n The amount of data NFS will attempt to access per write operation. The default is dependent on the kernel. For NFS version 2, set it to 8192 to assure maximum throughput.
nfsvers=n The version of NFS the mount command should attempt to use
tcp Attempt to mount the filesystem using TCP packets: the default is UDP.
intr If the filesystem is hard mounted and the mount times out, allow for the process to be aborted using the usual methods such as CTRL-C and the kill command.


The steps to mount the directory are fairly simple, as you'll see.

Permanently Mounting The NFS Directory

You'll now create a mount point directory, /mnt/nfs, on which to mount the remote NFS directory and then use the mount -a command activate the mount. Notice how before mounting there were no files visible in the /mnt/nfs directory, this changes after the mounting is completed:

[root@smallfry tmp]# mkdir /mnt/nfs
[root@smallfry tmp]# ls /mnt/nfs
[root@smallfry tmp]# mount -a
[root@smallfry tmp]# ls /mnt/nfs
ISO  ISO-RedHat  kickstart  RedHat
[root@smallfry tmp]#

Each time your system boots, it reads the /etc/fstab file and executes the mount -a command, thereby making this a permanent NFS mount.

Note: There are multiple versions of NFS, the most popular of which is version 2, which most NFS clients use. Newer NFS servers may also be able to handle NFS version 4. To be safe, it is best to force the NFS server to export directories as version 2 using the nfsvers=2 option in the /etc/fstab file as shown in the example. Failure to do so may result in an error message.

[root@probe-001 tmp]# mount -a
mount to NFS server '192.168.1.100' failed: server is down.
[root@probe-001 tmp]#

Manually Mounting NFS File Systems

If you don't want a permanent NFS mount, then you can use the mount command without the /etc/fstab entry to gain access only when necessary. This is a manual process; for an automated process, seen in the section "The NFS automounter."

In this case, you're mounting the /data/files directory as an NFS-type filesystem on the /mnt/nfs mount point. The NFS server is bigboy whose IP address is 192.168.1.100.

Notice how before mounting there were no files visible in the /mnt/nfs directory, this changes after the mounting is complete:

[root@smallfry tmp]# mkdir /mnt/nfs
[root@smallfry tmp]# ls /mnt/nfs
[root@smallfry tmp]# mount -t nfs 192.168.1.100:/data/files /mnt/nfs
[root@smallfry tmp]# ls /mnt/nfs
ISO  ISO-RedHat  kickstart  RedHat
[root@smallfry tmp]#

Congratulations! You've made your first steps towards being an NFS administrator.

Activating Modifications To The /etc/exports File

You can force your system to re-read the /etc/exports file by restarting NFS. In a nonproduction environment, this may cause disruptions when an exported directory suddenly disappears without prior notification to users. Here are some methods you can use to update and activate the file with the least amount of inconvenience to others.

New Exports File

When no directories have yet been exported to NFS, use the exportfs -a command.

[root@bigboy tmp]# exportfs -a

Adding A Shared Directory To An Existing Exports File

When adding a shared directory, you can use the exportfs -r command to export only the new entries.

[root@bigboy tmp]# exportfs -r

Deleting, Moving Or Modifying A Share

Removing an exported directory from the /etc/exports file requires work on both the NFS client and server. The steps are:

1) Unexport the mount point directory on the NFS client using the umount command. In this case, you're unmounting the /mnt/nfs mount point.

[root@smallfry tmp]# umount /mnt/nfs

Note: You may also need to edit the /etc/fstab file of any entries related to the mount point if you want to make the change permanent even after rebooting.

2) Comment out the corresponding entry in the NFS server's /etc/exports file and reload the modified file.

[root@bigboy tmp]# exportfs -ua
[root@bigboy tmp]# exportfs -a

You have now completed a seamless removal of the exported directory with much less chance of having critical errors.

The NFS Automounter

The permanent mounting of filesystems has its disadvantages. For example, the /etc/fstab file is unique per Linux server and has to be individually edited on each. NFS client management, therefore, becomes more difficult. Also, the mount is permanent, tying up system resources even when the NFS server isn't being accessed.

NFS uses an automounter feature that overcomes these shortcomings by allowing you to bypass the /etc/fstab file for NFS mounts, instead using an NFS-specific map file that can be distributed to multiple clients. In addition, you can use the file to specify the expected duration of the NFS mount, after which time it is unmounted automatically. However, automounter continues to report to the operating system kernel that the mount is still active. When the kernel makes an NFS file request, automounter intercepts it and mounts the remote directory on the mount point defined in the map file. The mount point directory is dynamically created by the automounter when needed, after the timeout period the remote directory is unmounted and the mount point is deleted.

Automounter Map Files

The master map file of automounter has a simple format that defines the name of the mount point directory in the first column and the subsidiary map file that controls its mounting in the second. You can add mounting options to a third column.

In the example, the /home directory needs to be automounted on an NFS server and the configuration information is defined in the /etc/auto.home file. Finally, the mount will only last for five minutes (300 seconds), and this value will act as a default for all the entries in the /etc/auto.home file.

Irregular entries that don't match /home are placed in the /etc/auto.direct file.

#
# File: /etc/auto.master
#
/home   /etc/auto.home --timeout=300
/-      /etc/auto.direct

Direct Maps

Direct maps are used to define NFS filesystems that are mounted on different servers or that all don't start with the same prefix.

Indirect Maps

Indirect maps define directories that can be mounted under the same mount point. A good example would be all the users' directories under /home.

Note: Based on preliminary testing, an early release of Fedora Core 3 doesn't appear to work correctly with automounter. You have to have one indirect map defined to avoid startup errors, and after doing so, the maps don't appear to be activated. No errors occur in the logs either.

The Structure Of Direct And Indirect Map Files

The format of these map files is similar to that of the /etc/auto.master file, except that columns two and three have been switched.

Column one lists all the directory keys that will activate the automounter feature. It is also the name of the mount point under the directory listed in the /etc/auto.master file. The second column provides all the NFS options to be used, and the third column lists the NFS servers and the filesystems that map to the keys.

When the NFS client accesses a file, it refers to the keys in the /etc/auto.master file to see whether any fall within the realm of the automounter's responsibility. If one does, then automounter checks the subsidiary map file for subdirectory mount point key. If it finds one, then automounter mounts the files for the system.

Indirect Map File Example

In the previous example, the /etc/auto.master file redirected all references to the /home directory to the /etc/auto.home file. This second file has entries for peter, bob, and bunny; these directories are actually mount points for directories on servers bigboy, ochorios, and waitabit.

#
# File: /etc/auto.home
#
peter   bigboy:/home/peter
bob     ochorios:/home/bob
bunny   waitabit:/home/bunny

Direct Map File Example

The second entry in the /etc/auto.master file was specifically created to handle all references to one of a kind directory prefixes. In the example the /data/sales and /sql/database are the mount points for directories on servers bigboy and waitabit.

#
# File: /etc/auto.direct
#
/data/sales          -rw           bigboy:/disk1/data/sales
/sql/database        -ro,soft       waitabit:/var/mysql/database

Note: The automounter treats direct mounts as if they were files in a directory, not as individual directories. This means all direct mount points in the same directory are mounted simultaneously even if only one of them is being accessed. This can cause excessive mounting activity that can slow response times. There are tricks you can use to avoid this, perhaps the simplest is just to place direct mount points in different directories.

Note: Direct map entries in the /etc/auto.master file must all begin with /-, and you can use absolute path names with direct map files only, if you don't then you'll get an error like this in your /var/log/messages file:

Nov  7 19:24:12 smallfry automount[31801]: bad map format: found indirect, expected direct exiting

Wildcards In Map Files

You can use two types of wildcards in a map file. The asterisk (*), which means all, and the ampersand (&), which instructs automounter to substitute the value of the key for the & character.

Using the Ampersand Wildcard

In the example below, the key is peter, so the ampersand wildcard is interpreted to mean peter too. This means you'll be mounting the bigboy:/home/peter directory.

#
# File: /etc/auto.home
#
peter   bigboy:/home/&

Using the Asterisk Wildcard

In the example below, the key is *, meaning that automounter will attempt to mount any attempt to enter the /home directory. But what's the value of the ampersand? It is actually assigned the value of the key that triggered the access to the /etc/auto.home file. If the access was for /home/peter, then the ampersand is interpreted to mean peter, and bigboy:/home/peter is mounted. If access was for /home/bob, then bigboy:/home/bob would be mounted.

#
# File: /etc/auto.home
#
*   bigboy:/home/&

Starting Automounter

Fedora Linux installs the automounter RPM, called autofs, by default . Here are some quick steps to get automounter started.

1) Use the chkconfig command to configure the automounter daemons to start at boot. Activate NFS file locking to reduce the risk of corrupted data.

[root@bigboy tmp]# chkconfig autofs on

2) Use the init scripts in the /etc/init.d directory to start the automounter daemons. The example uses the start option, but you can also stop and restart the process with the stop and restart options.

[root@bigboy tmp]# service autofs start

3) Use the pgrep command to determine whether automounter is running. If it is, the command will return the process ID of the automount daemon.

[root@bigboy tmp]# pgrep automount
32261
[root@bigboy tmp]#

As you can see, managing the startup of automounter is very similar to that of other Linux applications and should be easy to remember.

Automounter Examples

Now that you understand NFS automounter, you may benefit from an example. Chapter 30, "Configuring NIS", contains a full scenario in which a school computer laboratory uses automounter to centrally house all the home directories of its students. Additional centralization is also achieved by using NIS for login authentication, access, and accounting control.

Troubleshooting NFS

A basic NFS configuration usually works without problems when the client and server are on the same network. The most common problems are caused by forgetting to start NFS, to edit the /etc/fstab file, or to export the /etc/exports file. Another common cause of failure is the iptables firewall daemon running on either the server or client without the administrator realizing it.

When the client and server are on different networks, these checks still apply, but you'll also have to make sure basic connectivity has been taken care of as outlined in Chapter 4, "Simple Network Troubleshooting". Sometimes a firewall being present on the path between the client and server can cause difficulties.

As always, no troubleshooting plan would be complete without frequent reference to the /var/log/messages file when searching for additional clues. Table 29.2 shows some common NFS errors you'll encounter.

Table 29.2 Some Common NFS Error Messages

Error Description
Too many levels of remote in path Attempting to mount a filesystem that has already been mounted.
Permission denied User is denied access. This could be the client's root user who has unprivileged status on the server due to the root_squash option. Could also be because the user on the client doesn't exist on the server.
No such host Typographical or DNS configuration error in the name of the server.
No such file or directory Typographical error in the name of the file or directory: they don't exist.
NFS server is not responding The server could be overloaded or down.
Stale file handle A file that was previously accessed by the client was deleted on the server before the client closed it.
Fake hostname Forward and reverse DNS entries don't exist for the NFS client.

The showmount Command

When run on the server, the showmount -a command lists all the currently exported directories. It also shows a list of NFS clients accessing the server, in this case one client has an IP address of 192.168.1.102.

[root@bigboy tmp]# showmount -a
All mount points on bigboy:
*:/home
192.168.1.102:*
[root@bigboy tmp]#

The "df" Command

The df command lists the disk usage of a mounted filesystem. Run it on the NFS client to verify that NFS mounting has occurred. In many cases, the root_squash mount option will prevent the root user from doing this, so it's best to try it as an unprivileged user.

[nfsuser@smallfry nfsuser]$ df -F nfs
Filesystem           1K-blocks      Used Available Use% Mounted on
192.168.1.100:/home/nfsuser
                       1032056    346552    633068  36% /home/nfsuser
[nfsuser@smallfry nfsuser]$

The nfsstat Command

The nfsstat command provides useful error statistics. The -s option provides NFS server stats, while the -c option provides them of for clients. Threshold guidelines are provided in Table 29.3.

[root@bigboy tmp]# nfsstat -s
Server rpc stats:
calls      badcalls   badauth    badclnt    xdrcall
1547       0          0          0          0
Server nfs v2:
null       getattr    setattr    root       lookup     readlink
244    100% 0       0% 0       0% 0       0% 0       0% 0       0%
read       wrcache    write      create     remove     rename
0       0% 0       0% 0       0% 0       0% 0       0% 0       0%
link       symlink    mkdir      rmdir      readdir    fsstat
0       0% 0       0% 0       0% 0       0% 0       0% 0       0%
 
Server nfs v3:
null       getattr    setattr    lookup     access     readlink
251    19% 332    25% 0       0% 265    20% 320    24% 0       0%
read       write      create     mkdir      symlink    mknod
39      2% 14      1% 1       0% 1       0% 0       0% 0       0%
remove     rmdir      rename     link       readdir     readdirplus
0       0% 0       0% 0       0% 0       0% 0       0% 31       2%
fsstat     fsinfo     pathconf   commit
1       0% 34      2% 0       0% 14      1%
 
[root@bigboy tmp]#

Table 29.3 Error Thresholds For The "nfsstat" Command

Value Threshold Description
readlink > 10% Excessive numbers of symbolic links slowing performance. Try to replace them with a directory and mount the filesystem directly on this new mount point.
getattr > 50% File attributes, like file data, is cached in NFS. This value tracks the percentage of file attribute reads that are not from cache refresh requests. Usually caused by the NFS "noac" mount option which prevents file attribute caching.
badcalls > 0 Bad RPC requests. Could be due to poorly configured authentication, the root user attempting to access data governed by the "root_squash" directive or having a user in too many groups.
retrans > 5% Percentage of requests for service that the client had to retransmit to the servers. Could be due to slow NFS servers or poor network conditions.
writes > 10% Writes are slow due to poor caching values. Check the "noac" and "wsize" mount options.
read > 10% Reads are slow due to poor caching values. Check the "noac" and "rsize" mount options.

Other NFS Considerations

NFS can be temperamental. An incorrect configuration can cause it to be unresponsive. Its security is relatively weak, and you have to be aware of the file permissions on both the NFS client and server to get it to work correctly. Often these issues can be resolved with some basic guidelines outlined in this section.

Security

NFS and rpcbind have had a number of known security deficiencies in the past. As a result, I don't recommended using NFS over insecure networks. NFS doesn't encrypt data and it is possible for root users on NFS clients to have root access the server's filesystems. You can exercise security-related caution with NFS by following a few guidelines:

  • Restrict its use to secure networks
  • Export only the most needed data
  • Consider using read-only exports whenever data updates aren't necessary.
  • Use the root_squash option in /etc/exports file (default) to reduce the risk of the possibility of a root user on the NFS client having root file permission access on the NFS server. This is normally an undesirable condition, especially if the NFS client and NFS server are being managed by different sets of administrators.
  • Make NFS run on predefined TCP and UDP ports as this makes the management of firewall rules much easier. This can be done by editing your /etc/sysconfig/nfs file. In this example we are using ports 4000 to 4003 for the lockd, mountd, statd and rquotad daemons that are a part of NFS. The other NFS components, rpcbind and nfs, always run on TCP/UDP ports 111 and 2049 respectively.
# File: /etc/sysconfig/nfs file

RQUOTAD_PORT=4003
LOCKD_TCPPORT=4001
LOCKD_UDPPORT=4001
MOUNTD_PORT=4002
STATD_PORT=4000

These points should be the foundation of your NFS security policy, however, the list isn't comprehensive due to the concise scope of this book. I'd suggest that you refer to a dedicated NFS reference for more detailed advice.

NFS Hanging

As stated before, if the NFS server fails, the NFS client waits indefinitely for it to return. This also forces programs relying on the same client server relationship to wait indefinitely too.

For this reason, use the soft option in the NFS client's /etc/fstab file. This causes NFS to report an I/O error to the calling program after a long timeout.

You can reduce the risk of NFS hanging by taking a number of precautions:

  • Run NFS on a reliable network.
  • Avoid having NFS servers that NFS mount each other's filesystems or directories.
  • Always use the sync option whenever possible.
  • Do not have mission-critical computers rely on an NFS server to operate, unless the server's reliability can be guaranteed.
  • Do not include NFS-mounted directories as part of your search path, because a hung NFS connection to a directory in your search path could cause your shell to pause at that point in the search path until the NFS session is regained.

File Locking

NFS allows multiple clients to mount the same directory, but NFS has a history of not handling file locking well, although more recent versions are said to have rectified the problem. Test your network-based applications thoroughly before considering using NFS.

Nesting Exports

NFS doesn't allow you to export directories that are subdirectories of directories that have already been exported unless they are on different partitions.

Limiting root Access

NFS doesn't allow a root user on a NFS client to have root privileges on the NFS server. This can be disabled with the no_root_squash export option in the /etc/exports file. This is normally an undesirable condition, especially if the NFS client and NFS server are being managed by different sets of administrators.

Restricting Access to the NFS server

NFS doesn't provide restrictions on a per-user basis. If a user named nfsuser exists on the NFS client, then they will have access to all the files of a user named nfsuser on the NFS server. It is best, therefore, to use the /etc/exports file to limit access to certain trusted servers or networks.

You may also want to use a firewall to protect access to the NFS server. A main communication control channel is usually created between the client and server on TCP port 111, but the data is frequently transferred on a randomly chosen TCP port negotiated between them. There are ways to limit the TCP ports used, but that is beyond the scope of this book.

You may also want to eliminate any wireless networks between your NFS server and client, and it is not wise to mount an NFS share across the Internet as access could be either slow, intermittent or insecure.

File Permissions

The NFS file permissions on the NFS server are inherited by the client. It can become complicated especially if the users and user groups on the NFS client that are expected to access data on the NFS server don't exist on the NFS server.

For simplicity, make the key users and groups on both systems match and make sure the permissions on the NFS client mount point and the exported directories of the NFS server are in keeping with the your operational objectives.

Conclusion

As you have seen NFS can be a very powerful tool in providing clients with access to large amounts of data, such as a database stored on a centralized server. Many of the new network-attached storage products currently available on the market rely on NFS - a testament to its popularity, increasing stability, and improving security.