Table of Contents
1. Preamble
    1.1. Legal stuff
    1.2. Disclaimer
    1.3. Feedback
    1.4. Translation
    1.5. Dedication
2. Introduction
    2.1. What is NFS?
    2.2. What is this HOWTO and what is it not?
    2.3. Knowledge Pre-Requisites
    2.4. Software Pre-Requisites: Kernel Version and nfs-utils
    2.5. Where to get help and further information
3. Setting Up an NFS Server
    3.1. Introduction to the server setup
    3.2. Setting up the Configuration Files
    3.3. Getting the services started
    3.4. Verifying that NFS is running
    3.5. Making changes to /etc/exports later on
4. Setting up an NFS Client
    4.1. Mounting remote directories
    4.2. Getting NFS File Systems to Be Mounted at Boot Time
    4.3. Mount options
5. Optimizing NFS Performance
    5.1. Setting Block Size to Optimize Transfer Speeds
    5.2. Packet Size and Network Drivers
    5.3. Overflow of Fragmented Packets
    5.4. NFS over TCP
    5.5. Timeout and Retransmission Values
    5.6. Number of Instances of the NFSD Server Daemon
    5.7. Memory Limits on the Input Queue
    5.8. Turning Off Autonegotiation of NICs and Hubs
    5.9. Synchronous vs. Asynchronous Behavior in NFS
    5.10. Non-NFS-Related Means of Enhancing Server Performance
6. Security and NFS
    6.1. The portmapper
    6.2. Server security: nfsd and mountd
    6.3. Client Security
    6.4. NFS and firewalls (ipchains and netfilter)
    6.5. Tunneling NFS through SSH
    6.6. Summary
7. Troubleshooting
    7.1. Unable to See Files on a Mounted File System
    7.2. File requests hang or timeout waiting for access to the file.
    7.3. Unable to mount a file system
    7.4. I do not have permission to access files on the mounted volume.
    7.5. When I transfer really big files, NFS takes over all the CPU cycles
        on the server and it screeches to a halt.
    7.6. Strange error or log messages
    7.7. Real permissions don't match what's in /etc/exports.
    7.8. Flaky and unreliable behavior
    7.9. nfsd won't start
    7.10. File Corruption When Using Multiple Clients
8. Using Linux NFS with Other OSes
    8.1. AIX
    8.2. BSD
    8.3. Tru64 Unix
    8.4. HP-UX
    8.5. IRIX
    8.6. Solaris
    8.7. SunOS

1. Preamble

1.1. Legal stuff

Copyright (c) <2002> by Tavis Barr, Nicolai Langfeldt, Seth Vidal, and Tom
McNeal. This material may be distributed only subject to the terms and
conditions set forth in the Open Publication License, v1.0 or later (the
latest version is presently available at [

1.2. Disclaimer

This document is provided without any guarantees, including merchantability
or fitness for a particular use. The maintainers cannot be responsible if
following instructions in this document leads to damaged equipment or data,
angry neighbors, strange habits, divorce, or any other calamity.

1.3. Feedback

This will never be a finished document; we welcome feedback about how it can
be improved. As of February 2002, the Linux NFS home page is being hosted at
[] Check there for
mailing lists, bug fixes, and updates, and also to verify who currently
maintains this document.

1.4. Translation

If you are able to translate this document into another language, we would be
grateful and we will also do our best to assist you. Please notify the

1.5. Dedication

NFS on Linux was made possible by a collaborative effort of many people, but
a few stand out for special recognition. The original version was developed
by Olaf Kirch and Alan Cox. The version 3 server code was solidified by Neil
Brown, based on work from Saadia Khan, James Yarbrough, Allen Morris, H.J.
Lu, and others (including himself). The client code was written by Olaf Kirch
and updated by Trond Myklebust. The version 4 lock manager was developed by
Saadia Khan. Dave Higgen and H.J. Lu both have undertaken the thankless job
of extensive maintenance and bug fixes to get the code to actually work the
way it was supposed to. H.J. has also done extensive development of the
nfs-utils package. Of course this dedication is leaving many people out.

The original version of this document was developed by Nicolai Langfeldt. It
was heavily rewritten in 2000 by Tavis Barr and Seth Vidal to reflect
substantial changes in the workings of NFS for Linux developed between the
2.0 and 2.4 kernels. It was edited again in February 2002, when Tom McNeal
made substantial additions to the performance section. Thomas Emmel, Neil
Brown, Trond Myklebust, Erez Zadok, and Ion Badulescu also provided valuable
comments and contributions.

2. Introduction

2.1. What is NFS?

The Network File System (NFS) was developed to allow machines to mount a disk
partition on a remote machine as if it were on a local hard drive. This
allows for fast, seamless sharing of files across a network.

It also gives the potential for unwanted people to access your hard drive
over the network (and thereby possibly read your email and delete all your
files as well as break into your system) if you set it up incorrectly. So
please read the Security section of this document carefully if you intend to
implement an NFS setup.

There are other systems that provide similar functionality to NFS. Samba
([] provides file services to
Windows clients. The Andrew File System from IBM ([
index.html), recently open-sourced, provides a file sharing mechanism with
some additional security and performance features. The Coda File System
([] is still in
development as of this writing but is designed to work well with disconnected
clients. Many of the features of the Andrew and Coda file systems are slated
for inclusion in the next version of NFS (Version 4) ([] The advantage of NFS today is that it is mature,
standard, well understood, and supported robustly across a variety of

2.2. What is this HOWTO and what is it not?

This HOWTO is intended as a complete, step-by-step guide to setting up NFS
correctly and effectively. Setting up NFS involves two steps, namely
configuring the server and then configuring the client. Each of these steps
is dealt with in order. The document then offers some tips for people with
particular needs and hardware setups, as well as security and troubleshooting

This HOWTO is not a description of the guts and underlying structure of NFS.
For that you may wish to read Linux NFS and Automounter Administration by
Erez Zadok (Sybex, 2001). The classic NFS book, updated and still quite
useful, is Managing NFS and NIS by Hal Stern, published by O'Reilly &
Associates, Inc. A much more advanced technical description of NFS is
available in NFS Illustrated by Brent Callaghan.

This document is also not intended as a complete reference manual, and does
not contain an exhaustive list of the features of Linux NFS. For that, you
can look at the man pages for nfs(5), exports(5), mount(8), fstab(5), nfsd(8)
, lockd(8), statd(8), rquotad(8), and mountd(8).

It will also not cover PC-NFS, which is considered obsolete (users are
encouraged to use Samba to share files with Windows machines) or NFS Version
4, which is still in development.

2.3. Knowledge Pre-Requisites

You should know some basic things about TCP/IP networking before reading this
HOWTO; if you are in doubt, read the Networking- Overview-HOWTO.

2.4. Software Pre-Requisites: Kernel Version and nfs-utils

The difference between Version 2 NFS and version 3 NFS will be explained
later on; for now, you might simply take the suggestion that you will need
NFS Version 3 if you are installing a dedicated or high-volume file server.
NFS Version 2 should be fine for casual use.

NFS Version 2 has been around for quite some time now (at least since the 1.2
kernel series) however you will need a kernel version of at least 2.2.18 if
you wish to do any of the following:

  * Mix Linux NFS with other operating systems' NFS
  * Use file locking reliably over NFS
  * Use NFS Version 3.

There are also patches available for kernel versions above 2.2.14 that
provide the above functionality. Some of them can be downloaded from the
Linux NFS homepage. If your kernel version is 2.2.14- 2.2.17 and you have the
source code on hand, you can tell if these patches have been added because
NFS Version 3 server support will be a configuration option. However, unless
you have some particular reason to use an older kernel, you should upgrade
because many bugs have been fixed along the way. Kernel 2.2.19 contains some
additional locking improvements over 2.2.18.

Version 3 functionality will also require the nfs-utils package of at least
version 0.1.6, and mount version 2.10m or newer. However because nfs-utils
and mount are fully backwards compatible, and because newer versions have
lots of security and bug fixes, there is no good reason not to install the
newest nfs-utils and mount packages if you are beginning an NFS setup.

All 2.4 and higher kernels have full NFS Version 3 functionality.

In all cases, if you are building your own kernel, you will need to select
NFS and NFS Version 3 support at compile time. Most (but not all) standard
distributions come with kernels that support NFS version 3.

Handling files larger than 2 GB will require a 2.4x kernel and a 2.2.x
version of glibc.

All kernels after 2.2.18 support NFS over TCP on the client side. As of this
writing, server-side NFS over TCP only exists in a buggy form as an
experimental option in the post-2.2.18 series; patches for 2.4 and 2.5
kernels have been introduced starting with 2.4.17 and 2.5.6. The patches are
believed to be stable, though as of this writing they are relatively new and
have not seen widespread use or integration into the mainstream 2.4 kernel.

Because so many of the above functionalities were introduced in kernel
version 2.2.18, this document was written to be consistent with kernels above
this version (including 2.4.x). If you have an older kernel, this document
may not describe your NFS system correctly.

As we write this document, NFS version 4 has only recently been finalized as
a protocol, and no implementations are considered production-ready. It will
not be dealt with here.

2.5. Where to get help and further information

As of November 2000, the Linux NFS homepage is at [http://] Please check there for NFS
related mailing lists as well as the latest version of nfs-utils, NFS kernel
patches, and other NFS related packages.

When you encounter a problem or have a question not covered in this manual,
the faq or the man pages, you should send a message to the nfs mailing list
(<>). To best help the developers and other users
help you assess your problem you should include:

  * the version of nfs-utils you are using
  * the version of the kernel and any non-stock applied kernels.
  * the distribution of linux you are using
  * the version(s) of other operating systems involved.

It is also useful to know the networking configuration connecting the hosts.

If your problem involves the inability mount or export shares please also

  * a copy of your /etc/exports file
  * the output of rpcinfo -p localhost run on the server
  * the output of rpcinfo -p servername run on the client

Sending all of this information with a specific question, after reading all
the documentation, is the best way to ensure a helpful response from the

You may also wish to look at the man pages for nfs(5), exports(5), mount(8), 
fstab(5), nfsd(8), lockd(8), statd(8), rquotad(8), and mountd(8).

3. Setting Up an NFS Server

3.1. Introduction to the server setup

It is assumed that you will be setting up both a server and a client. If you
are just setting up a client to work off of somebody else's server (say in
your department), you can skip to Section 4. However, every client that is
set up requires modifications on the server to authorize that client (unless
the server setup is done in a very insecure way), so even if you are not
setting up a server you may wish to read this section to get an idea what
kinds of authorization problems to look out for.

Setting up the server will be done in two steps: Setting up the configuration
files for NFS, and then starting the NFS services.

3.2. Setting up the Configuration Files

There are three main configuration files you will need to edit to set up an
NFS server: /etc/exports, /etc/hosts.allow, and /etc/hosts.deny. Strictly
speaking, you only need to edit /etc/exports to get NFS to work, but you
would be left with an extremely insecure setup. You may also need to edit
your startup scripts; see Section 3.3.3 for more on that.

3.2.1. /etc/exports

This file contains a list of entries; each entry indicates a volume that is
shared and how it is shared. Check the man pages (man exports) for a complete
description of all the setup options for the file, although the description
here will probably satistfy most people's needs.

An entry in /etc/exports will typically look like this:
 directory machine1(option11,option12) machine2(option21,option22)           


    the directory that you want to share. It may be an entire volume though
    it need not be. If you share a directory, then all directories under it
    within the same file system will be shared as well.
machine1 and machine2
    client machines that will have access to the directory. The machines may
    be listed by their DNS address or their IP address (e.g., or Using IP addresses is more reliable
    and more secure. If you need to use DNS addresses, and they do not seem
    to be resolving to the right machine, see Section 7.3.
    the option listing for each machine will describe what kind of access
    that machine will have. Important options are:
      + ro: The directory is shared read only; the client machine will not be
        able to write to it. This is the default.
      + rw: The client machine will have read and write access to the
      + no_root_squash: By default, any file request made by user root on the
        client machine is treated as if it is made by user nobody on the
        server. (Excatly which UID the request is mapped to depends on the
        UID of user "nobody" on the server, not the client.) If 
        no_root_squash is selected, then root on the client machine will have
        the same level of access to the files on the system as root on the
        server. This can have serious security implications, although it may
        be necessary if you want to perform any administrative work on the
        client machine that involves the exported directories. You should not
        specify this option without a good reason.
      + no_subtree_check: If only part of a volume is exported, a routine
        called subtree checking verifies that a file that is requested from
        the client is in the appropriate part of the volume. If the entire
        volume is exported, disabling this check will speed up transfers.
      + sync: By default, all but the most recent version (version 1.11) of
        the exportfs command will use async behavior, telling a client
        machine that a file write is complete - that is, has been written to
        stable storage - when NFS has finished handing the write over to the
        filesysytem. This behavior may cause data corruption if the server
        reboots, and the sync option prevents this. See Section 5.9 for a
        complete discussion of sync and async behavior.

Suppose we have two client machines, slave1 and slave2, that have IP
addresses and, respectively. We wish to share our
software binaries and home directories with these machines. A typical setup
for /etc/exports might look like this:
|  /usr/local                             |
|  /home                             |
|                                                                           |

Here we are sharing /usr/local read-only to slave1 and slave2, because it
probably contains our software and there may not be benefits to allowing
slave1 and slave2 to write to it that outweigh security concerns. On the
other hand, home directories need to be exported read-write if users are to
save work on them.

If you have a large installation, you may find that you have a bunch of
computers all on the same local network that require access to your server.
There are a few ways of simplifying references to large numbers of machines.
First, you can give access to a range of machines at once by specifying a
network and a netmask. For example, if you wanted to allow access to all the
machines with IP addresses between and then you
could have the entries:
|  /usr/local                                 |
|  /home                                 |
|                                                                           |

See the []
Networking-Overview HOWTO for further information about how netmasks work,
and you may also wish to look at the man pages for init and hosts.allow.

Second, you can use NIS netgroups in your entry. To specify a netgroup in
your exports file, simply prepend the name of the netgroup with an "@". See
the [] NIS HOWTO for details on
how netgroups work.

Third, you can use wildcards such as * or 192.168. instead of
hostnames. There were problems with wildcard implementation in the 2.2 kernel
series that were fixed in kernel 2.2.19.

However, you should keep in mind that any of these simplifications could
cause a security risk if there are machines in your netgroup or local network
that you do not trust completely.

A few cautions are in order about what cannot (or should not) be exported.
First, if a directory is exported, its parent and child directories cannot be
exported if they are in the same filesystem. However, exporting both should
not be necessary because listing the parent directory in the /etc/exports
file will cause all underlying directories within that file system to be

Second, it is a poor idea to export a FAT or VFAT (i.e., MS-DOS or Windows 95
/98) filesystem with NFS. FAT is not designed for use on a multi-user
machine, and as a result, operations that depend on permissions will not work
well. Moreover, some of the underlying filesystem design is reported to work
poorly with NFS's expectations.

Third, device or other special files may not export correctly to non-Linux
clients. See Section 8 for details on particular operating systems.

3.2.2. /etc/hosts.allow and /etc/hosts.deny

These two files specify which computers on the network can use services on
your machine. Each line of the file contains a single entry listing a service
and a set of machines. When the server gets a request from a machine, it does
the following:

  * It first checks hosts.allow to see if the machine matches a description
    listed in there. If it does, then the machine is allowed access.
  * If the machine does not match an entry in hosts.allow, the server then
    checks hosts.deny to see if the client matches a listing in there. If it
    does then the machine is denied access.
  * If the client matches no listings in either file, then it is allowed

In addition to controlling access to services handled by inetd (such as
telnet and FTP), this file can also control access to NFS by restricting
connections to the daemons that provide NFS services. Restrictions are done
on a per-service basis.

The first daemon to restrict access to is the portmapper. This daemon
essentially just tells requesting clients how to find all the NFS services on
the system. Restricting access to the portmapper is the best defense against
someone breaking into your system through NFS because completely unauthorized
clients won't know where to find the NFS daemons. However, there are two
things to watch out for. First, restricting portmapper isn't enough if the
intruder already knows for some reason how to find those daemons. And second,
if you are running NIS, restricting portmapper will also restrict requests to
NIS. That should usually be harmless since you usually want to restrict NFS
and NIS in a similar way, but just be cautioned. (Running NIS is generally a
good idea if you are running NFS, because the client machines need a way of
knowing who owns what files on the exported volumes. Of course there are
other ways of doing this such as syncing password files. See the [http://] NIS HOWTO for information on setting
up NIS.)

In general it is a good idea with NFS (as with most internet services) to
explicitly deny access to IP addresses that you don't need to allow access

The first step in doing this is to add the followng entry to /etc/hosts.deny:

|   portmap:ALL                                                             |
|                                                                           |

Starting with nfs-utils 0.2.0, you can be a bit more careful by controlling
access to individual daemons. It's a good precaution since an intruder will
often be able to weasel around the portmapper. If you have a newer version of
nfs-utils, add entries for each of the NFS daemons (see the next section to
find out what these daemons are; for now just put entries for them in

|    lockd:ALL                                                              |
|    mountd:ALL                                                             |
|    rquotad:ALL                                                            |
|    statd:ALL                                                              |
|                                                                           |

Even if you have an older version of nfs-utils, adding these entries is at
worst harmless (since they will just be ignored) and at best will save you
some trouble when you upgrade. Some sys admins choose to put the entry ALL:
ALL in the file /etc/hosts.deny, which causes any service that looks at these
files to deny access to all hosts unless it is explicitly allowed. While this
is more secure behavior, it may also get you in trouble when you are
installing new services, you forget you put it there, and you can't figure
out for the life of you why they won't work.

Next, we need to add an entry to hosts.allow to give any hosts access that we
want to have access. (If we just leave the above lines in hosts.deny then
nobody will have access to NFS.) Entries in hosts.allow follow the format

|    service: host [or network/netmask] , host [or network/netmask]         |
|                                                                           |

Here, host is IP address of a potential client; it may be possible in some
versions to use the DNS name of the host, but it is strongly discouraged.

Suppose we have the setup above and we just want to allow access to and, and suppose that the IP addresses of these
machines are and, respectively. We could add the
following entry to /etc/hosts.allow:

|   portmap: ,                                      |
|                                                                           |

For recent nfs-utils versions, we would also add the following (again, these
entries are harmless even if they are not supported):

|    lockd: ,                                       |
|    rquotad: ,                                     |
|    mountd: ,                                      |
|    statd: ,                                       |
|                                                                           |

If you intend to run NFS on a large number of machines in a local network, /
etc/hosts.allow also allows for network/netmask style entries in the same
manner as /etc/exports above.

3.3. Getting the services started

3.3.1. Pre-requisites

The NFS server should now be configured and we can start it running. First,
you will need to have the appropriate packages installed. This consists
mainly of a new enough kernel and a new enough version of the nfs-utils
package. See Section 2.4 if you are in doubt.

Next, before you can start NFS, you will need to have TCP/IP networking
functioning correctly on your machine. If you can use telnet, FTP, and so on,
then chances are your TCP networking is fine.

That said, with most recent Linux distributions you may be able to get NFS up
and running simply by rebooting your machine, and the startup scripts should
detect that you have set up your /etc/exports file and will start up NFS
correctly. If you try this, see Section 3.4 Verifying that NFS is running. If
this does not work, or if you are not in a position to reboot your machine,
then the following section will tell you which daemons need to be started in
order to run NFS services. If for some reason nfsd was already running when
you edited your configuration files above, you will have to flush your
configuration; see Section 3.5 for details.

3.3.2. Starting the Portmapper

NFS depends on the portmapper daemon, either called portmap or rpc.portmap.
It will need to be started first. It should be located in /sbin but is
sometimes in /usr/sbin. Most recent Linux distributions start this daemon in
the boot scripts, but it is worth making sure that it is running before you
begin working with NFS (just type ps aux | grep portmap).

3.3.3. The Daemons

NFS serving is taken care of by five daemons: rpc.nfsd, which does most of
the work; rpc.lockd and rpc.statd, which handle file locking; rpc.mountd,
which handles the initial mount requests, and rpc.rquotad, which handles user
file quotas on exported volumes. Starting with 2.2.18, lockd is called by 
nfsd upon demand, so you do not need to worry about starting it yourself. 
statd will need to be started separately. Most recent Linux distributions
will have startup scripts for these daemons.

The daemons are all part of the nfs-utils package, and may be either in the /
sbin directory or the /usr/sbin directory.

If your distribution does not include them in the startup scripts, then then
you should add them, configured to start in the following order:

rpc.mountd, rpc.nfsd                                
rpc.statd, rpc.lockd (if necessary), and rpc.rquotad

The nfs-utils package has sample startup scripts for RedHat and Debian. If
you are using a different distribution, in general you can just copy the
RedHat script, but you will probably have to take out the line that says:
|    . ../init.d/functions                                                  |
|                                                                           |
to avoid getting error messages.

3.4. Verifying that NFS is running

To do this, query the portmapper with the command rpcinfo -p to find out what
services it is providing. You should get something like this:
|    program vers proto   port                                              |
|    100000    2   tcp    111  portmapper                                   |
|    100000    2   udp    111  portmapper                                   |
|    100011    1   udp    749  rquotad                                      |
|    100011    2   udp    749  rquotad                                      |
|    100005    1   udp    759  mountd                                       |
|    100005    1   tcp    761  mountd                                       |
|    100005    2   udp    764  mountd                                       |
|    100005    2   tcp    766  mountd                                       |
|    100005    3   udp    769  mountd                                       |
|    100005    3   tcp    771  mountd                                       |
|    100003    2   udp   2049  nfs                                          |
|    100003    3   udp   2049  nfs                                          |
|    300019    1   tcp    830  amd                                          |
|    300019    1   udp    831  amd                                          |
|    100024    1   udp    944  status                                       |
|    100024    1   tcp    946  status                                       |
|    100021    1   udp   1042  nlockmgr                                     |
|    100021    3   udp   1042  nlockmgr                                     |
|    100021    4   udp   1042  nlockmgr                                     |
|    100021    1   tcp   1629  nlockmgr                                     |
|    100021    3   tcp   1629  nlockmgr                                     |
|    100021    4   tcp   1629  nlockmgr                                     |
|                                                                           |

This says that we have NFS versions 2 and 3, rpc.statd version 1, network
lock manager (the service name for rpc.lockd) versions 1, 3, and 4. There are
also different service listings depending on whether NFS is travelling over
TCP or UDP. Linux systems use UDP by default unless TCP is explicitly
requested; however other OSes such as Solaris default to TCP.

If you do not at least see a line that says portmapper, a line that says nfs,
and a line that says mountd then you will need to backtrack and try again to
start up the daemons (see Section 7, Troubleshooting, if this still doesn't

If you do see these services listed, then you should be ready to set up NFS
clients to access files from your server.

3.5. Making changes to /etc/exports later on

If you come back and change your /etc/exports file, the changes you make may
not take effect immediately. You should run the command exportfs -ra to force
nfsd to re-read the /etc/exports   file. If you can't find the exportfs
command, then you can kill nfsd with the -HUP flag (see the man pages for
kill for details).

If that still doesn't work, don't forget to check hosts.allow to make sure
you haven't forgotten to list any new client machines there. Also check the
host listings on any firewalls you may have set up (see Section 7 and Section
6 for more details on firewalls and NFS).

4. Setting up an NFS Client

4.1. Mounting remote directories

Before beginning, you should double-check to make sure your mount program is
new enough (version 2.10m if you want to use Version 3 NFS), and that the
client machine supports NFS mounting, though most standard distributions do.
If you are using a 2.2 or later kernel with the /proc filesystem you can
check the latter by reading the file /proc/filesystems and making sure there
is a line containing nfs. If not, typing insmod nfs may make it magically
appear if NFS has been compiled as a module; otherwise, you will need to
build (or download) a kernel that has NFS support built in. In general,
kernels that do not have NFS compiled in will give a very specific error when
the mount command below is run.

To begin using machine as an NFS client, you will need the portmapper running
on that machine, and to use NFS file locking, you will also need rpc.statd
and rpc.lockd running on both the client and the server. Most recent
distributions start those services by default at boot time; if yours doesn't,
see Section 3.2 for information on how to start them up.

With portmap, lockd, and statd running, you should now be able to mount the
remote directory from your server just the way you mount a local hard drive,
with the mount command. Continuing our example from the previous section,
suppose our server above is called,and we want to mount the /
home directory on Then, all we have to do, from the root
prompt on, is type:
|   # mount /mnt/home                                  |
|                                                                           |
and the directory /home on master will appear as the directory /mnt/home on 
slave1. (Note that this assumes we have created the directory /mnt/home as an
empty mount point beforehand.)

If this does not work, see the Troubleshooting section (Section 7).

You can get rid of the file system by typing
|   # umount /mnt/home                                                      |
|                                                                           |
just like you would for a local file system.

4.2. Getting NFS File Systems to Be Mounted at Boot Time

NFS file systems can be added to your /etc/fstab file the same way local file
systems can, so that they mount when your system starts up. The only
difference is that the file system type will be set to nfs and the dump and
fsck order (the last two entries) will have to be set to zero. So for our
example above, the entry in /etc/fstab would look like:
   # device       mountpoint     fs-type     options      dump fsckorder     
   ...                                                                   /mnt    nfs          rw            0    0           

See the man pages for fstab if you are unfamiliar with the syntax of this
file. If you are using an automounter such as amd or autofs, the options in
the corresponding fields of your mount listings should look very similar if
not identical.

At this point you should have NFS working, though a few tweaks may still be
necessary to get it to work well. You should also read Section 6 to be sure
your setup is reasonably secure.

4.3. Mount options

4.3.1. Soft vs. Hard Mounting

There are some options you should consider adding at once. They govern the
way the NFS client handles a server crash or network outage. One of the cool
things about NFS is that it can handle this gracefully. If you set up the
clients right. There are two distinct failure modes:

    If a file request fails, the NFS client will report an error to the
    process on the client machine requesting the file access. Some programs
    can handle this with composure, most won't. We do not recommend using
    this setting; it is a recipe for corrupted files and lost data. You
    should especially not use this for mail disks --- if you value your mail,
    that is.
    The program accessing a file on a NFS mounted file system will hang when
    the server crashes. The process cannot be interrupted or killed (except
    by a "sure kill") unless you also specify intr. When the NFS server is
    back online the program will continue undisturbed from where it was. We
    recommend using hard,intr on all NFS mounted file systems.

Picking up the from previous example, the fstab entry would now look like:
   # device             mountpoint  fs-type    options    dump fsckord       
   ...                                                                   /mnt/home   nfs      rw,hard,intr  0     0          

4.3.2. Setting Block Size to Optimize Transfer Speeds

The rsize and wsize mount options specify the size of the chunks of data that
the client and server pass back and forth to each other.

The defaults may be too big or to small; there is no size that works well on
all or most setups. On the one hand, some combinations of Linux kernels and
network cards (largely on older machines) cannot handle blocks that large. On
the other hand, if they can handle larger blocks, a bigger size might be

Getting the block size right is an important factor in performance and is a
must if you are planning to use the NFS server in a production environment.
See Section 5 for details.

5. Optimizing NFS Performance

Careful analysis of your environment, both from the client and from the
server point of view, is the first step necessary for optimal NFS
performance. The first sections will address issues that are generally
important to the client. Later (Section 5.3 and beyond), server side issues
will be discussed. In both cases, these issues will not be limited
exclusively to one side or the other, but it is useful to separate the two in
order to get a clearer picture of cause and effect.

Aside from the general network configuration - appropriate network capacity,
faster NICs, full duplex settings in order to reduce collisions, agreement in
network speed among the switches and hubs, etc. - one of the most important
client optimization settings are the NFS data transfer buffer sizes,
specified by the mount command options rsize and wsize.

5.1. Setting Block Size to Optimize Transfer Speeds

The mount command options rsize and wsize specify the size of the chunks of
data that the client and server pass back and forth to each other. If no 
rsize and wsize options are specified, the default varies by which version of
NFS we are using. The most common default is 4K (4096 bytes), although for
TCP-based mounts in 2.2 kernels, and for all mounts beginning with 2.4
kernels, the server specifies the default block size.

The theoretical limit for the NFS V2 protocol is 8K. For the V3 protocol, the
limit is specific to the server. On the Linux server, the maximum block size
is defined by the value of the kernel constant NFSSVC_MAXBLKSIZE, found in
the Linux kernel source file ./include/linux/nfsd/const.h. The current
maximum block size for the kernel, as of 2.4.17, is 8K (8192 bytes), but the
patch set implementing NFS over TCP/IP transport in the 2.4 series, as of
this writing, uses a value of 32K (defined in the patch as 32*1024) for the
maximum block size.

All 2.4 clients currently support up to 32K block transfer sizes, allowing
the standard 32K block transfers across NFS mounts from other servers, such
as Solaris, without client modification.

The defaults may be too big or too small, depending on the specific
combination of hardware and kernels. On the one hand, some combinations of
Linux kernels and network cards (largely on older machines) cannot handle
blocks that large. On the other hand, if they can handle larger blocks, a
bigger size might be faster.

You will want to experiment and find an rsize and wsize that works and is as
fast as possible. You can test the speed of your options with some simple
commands, if your network environment is not heavily used. Note that your
results may vary widely unless you resort to using more complex benchmarks,
such as Bonnie, Bonnie++, or IOzone.

The first of these commands transfers 16384 blocks of 16k each from the
special file /dev/zero (which if you read it just spits out zeros really
fast) to the mounted partition. We will time it to see how long it takes. So,
from the client machine, type:
    # time dd if=/dev/zero of=/mnt/home/testfile bs=16k count=16384          

This creates a 256Mb file of zeroed bytes. In general, you should create a
file that's at least twice as large as the system RAM on the server, but make
sure you have enough disk space! Then read back the file into the great black
hole on the client machine (/dev/null) by typing the following:
    # time dd if=/mnt/home/testfile of=/dev/null bs=16k                      

Repeat this a few times and average how long it takes. Be sure to unmount and
remount the filesystem each time (both on the client and, if you are zealous,
locally on the server as well), which should clear out any caches.

Then unmount, and mount again with a larger and smaller block size. They
should be multiples of 1024, and not larger than the maximum block size
allowed by your system. Note that NFS Version 2 is limited to a maximum of
8K, regardless of the maximum block size defined by NFSSVC_MAXBLKSIZE;
Version 3 will support up to 64K, if permitted. The block size should be a
power of two since most of the parameters that would constrain it (such as
file system block sizes and network packet size) are also powers of two.
However, some users have reported better successes with block sizes that are
not powers of two but are still multiples of the file system block size and
the network packet size.

Directly after mounting with a larger size, cd into the mounted file system
and do things like ls, explore the filesystem a bit to make sure everything
is as it should. If the rsize/wsize is too large the symptoms are very odd
and not 100% obvious. A typical symptom is incomplete file lists when doing 
ls, and no error messages, or reading files failing mysteriously with no
error messages. After establishing that the given rsize/ wsize works you can
do the speed tests again. Different server platforms are likely to have
different optimal sizes.

Remember to edit /etc/fstab to reflect the rsize/wsize you found to be the
most desirable.

If your results seem inconsistent, or doubtful, you may need to analyze your
network more extensively while varying the rsize and wsize values. In that
case, here are several pointers to benchmarks that may prove useful:

  * Bonnie []
  * Bonnie++ []
  * IOzone file system benchmark [] http://
  * The official NFS benchmark, SPECsfs97 []

The easiest benchmark with the widest coverage, including an extensive spread
of file sizes, and of IO types - reads, & writes, rereads & rewrites, random
access, etc. - seems to be IOzone. A recommended invocation of IOzone (for
which you must have root privileges) includes unmounting and remounting the
directory under test, in order to clear out the caches between tests, and
including the file close time in the measurements. Assuming you've already
exported /tmp to everyone from the server foo, and that you've installed
IOzone in the local directory, this should work:
    # echo "foo:/tmp /mnt/foo nfs rw,hard,intr,rsize=8192,wsize=8192 0 0"    
    >> /etc/fstab                                                            
    # mkdir /mnt/foo                                                         
    # mount /mnt/foo                                                         
    # ./iozone -a -R -c -U /mnt/foo -f /mnt/foo/testfile > logfile           

The benchmark should take 2-3 hours at most, but of course you will need to
run it for each value of rsize and wsize that is of interest. The web site
gives full documentation of the parameters, but the specific options used
above are:

  * -a Full automatic mode, which tests file sizes of 64K to 512M, using
    record sizes of 4K to 16M
  * -R Generate report in excel spreadsheet form (The "surface plot" option
    for graphs is best)
  * -c Include the file close time in the tests, which will pick up the NFS
    version 3 commit time
  * -U Use the given mount point to unmount and remount between tests; it
    clears out caches
  * -f When using unmount, you have to locate the test file in the mounted
    file system

5.2. Packet Size and Network Drivers

While many Linux network card drivers are excellent, some are quite shoddy,
including a few drivers for some fairly standard cards. It is worth
experimenting with your network card directly to find out how it can best
handle traffic.

Try pinging back and forth between the two machines with large packets using
the -f and -s options with ping (see ping(8) for more details) and see if a
lot of packets get dropped, or if they take a long time for a reply. If so,
you may have a problem with the performance of your network card.

For a more extensive analysis of NFS behavior in particular, use the nfsstat
command to look at nfs transactions, client and server statistics, network
statistics, and so forth. The "-o net" option will show you the number of
dropped packets in relation to the total number of transactions. In UDP
transactions, the most important statistic is the number of retransmissions,
due to dropped packets, socket buffer overflows, general server congestion,
timeouts, etc. This will have a tremendously important effect on NFS
performance, and should be carefully monitored. Note that nfsstat does not
yet implement the -z option, which would zero out all counters, so you must
look at the current nfsstat counter values prior to running the benchmarks.

To correct network problems, you may wish to reconfigure the packet size that
your network card uses. Very often there is a constraint somewhere else in
the network (such as a router) that causes a smaller maximum packet size
between two machines than what the network cards on the machines are actually
capable of. TCP should autodiscover the appropriate packet size for a
network, but UDP will simply stay at a default value. So determining the
appropriate packet size is especially important if you are using NFS over

You can test for the network packet size using the tracepath command: From
the client machine, just type tracepath server 2049 and the path MTU should
be reported at the bottom. You can then set the MTU on your network card
equal to the path MTU, by using the MTU option to ifconfig, and see if fewer
packets get dropped. See the ifconfig man pages for details on how to reset
the MTU.

In addition, netstat -s will give the statistics collected for traffic across
all supported protocols. You may also look at /proc/net/snmp for information
about current network behavior; see the next section for more details.

5.3. Overflow of Fragmented Packets

Using an rsize or wsize larger than your network's MTU (often set to 1500, in
many networks) will cause IP packet fragmentation when using NFS over UDP. IP
packet fragmentation and reassembly require a significant amount of CPU
resource at both ends of a network connection. In addition, packet
fragmentation also exposes your network traffic to greater unreliability,
since a complete RPC request must be retransmitted if a UDP packet fragment
is dropped for any reason. Any increase of RPC retransmissions, along with
the possibility of increased timeouts, are the single worst impediment to
performance for NFS over UDP.

Packets may be dropped for many reasons. If your network topography is
complex, fragment routes may differ, and may not all arrive at the Server for
reassembly. NFS Server capacity may also be an issue, since the kernel has a
limit of how many fragments it can buffer before it starts throwing away
packets. With kernels that support the /proc filesystem, you can monitor the
files /proc/sys/net/ipv4/ipfrag_high_thresh and /proc/sys/net/ipv4/
ipfrag_low_thresh. Once the number of unprocessed, fragmented packets reaches
the number specified by ipfrag_high_thresh (in bytes), the kernel will simply
start throwing away fragmented packets until the number of incomplete packets
reaches the number specified by ipfrag_low_thresh.

Another counter to monitor is IP: ReasmFails in the file /proc/net/snmp; this
is the number of fragment reassembly failures. if it goes up too quickly
during heavy file activity, you may have problem.

5.4. NFS over TCP

A new feature, available for both 2.4 and 2.5 kernels but not yet integrated
into the mainstream kernel at the time of this writing, is NFS over TCP.
Using TCP has a distinct advantage and a distinct disadvantage over UDP. The
advantage is that it works far better than UDP on lossy networks. When using
TCP, a single dropped packet can be retransmitted, without the retransmission
of the entire RPC request, resulting in better performance on lossy networks.
In addition, TCP will handle network speed differences better than UDP, due
to the underlying flow control at the network level.

The disadvantage of using TCP is that it is not a stateless protocol like
UDP. If your server crashes in the middle of a packet transmission, the
client will hang and any shares will need to be unmounted and remounted.

The overhead incurred by the TCP protocol will result in somewhat slower
performance than UDP under ideal network conditions, but the cost is not
severe, and is often not noticable without careful measurement. If you are
using gigabit ethernet from end to end, you might also investigate the usage
of jumbo frames, since the high speed network may allow the larger frame
sizes without encountering increased collision rates, particularly if you
have set the network to full duplex.

5.5. Timeout and Retransmission Values

Two mount command options, timeo and retrans, control the behavior of UDP
requests when encountering client timeouts due to dropped packets, network
congestion, and so forth. The -o timeo option allows designation of the
length of time, in tenths of seconds, that the client will wait until it
decides it will not get a reply from the server, and must try to send the
request again. The default value is 7 tenths of a second. The -o retrans
option allows designation of the number of timeouts allowed before the client
gives up, and displays the Server not responding message. The default value
is 3 attempts. Once the client displays this message, it will continue to try
to send the request, but only once before displaying the error message if
another timeout occurs. When the client reestablishes contact, it will fall
back to using the correct retrans value, and will display the Server OK

If you are already encountering excessive retransmissions (see the output of
the nfsstat command), or want to increase the block transfer size without
encountering timeouts and retransmissions, you may want to adjust these
values. The specific adjustment will depend upon your environment, and in
most cases, the current defaults are appropriate.

5.6. Number of Instances of the NFSD Server Daemon

Most startup scripts, Linux and otherwise, start 8 instances of nfsd. In the
early days of NFS, Sun decided on this number as a rule of thumb, and
everyone else copied. There are no good measures of how many instances are
optimal, but a more heavily-trafficked server may require more. You should
use at the very least one daemon per processor, but four to eight per
processor may be a better rule of thumb. If you are using a 2.4 or higher
kernel and you want to see how heavily each nfsd thread is being used, you
can look at the file /proc/net/rpc/nfsd. The last ten numbers on the th line
in that file indicate the number of seconds that the thread usage was at that
percentage of the maximum allowable. If you have a large number in the top
three deciles, you may wish to increase the number of nfsd instances. This is
done upon starting nfsd using the number of instances as the command line
option, and is specified in the NFS startup script (/etc/rc.d/init.d/nfs on
Red Hat) as RPCNFSDCOUNT. See the nfsd(8) man page for more information.

5.7. Memory Limits on the Input Queue

On 2.2 and 2.4 kernels, the socket input queue, where requests sit while they
are currently being processed, has a small default size limit (rmem_default)
of 64k. This queue is important for clients with heavy read loads, and
servers with heavy write loads. As an example, if you are running 8 instances
of nfsd on the server, each will only have 8k to store write requests while
it processes them. In addition, the socket output queue - important for
clients with heavy write loads and servers with heavy read loads - also has a
small default size (wmem_default).

Several published runs of the NFS benchmark []
SPECsfs specify usage of a much higher value for both the read and write
value sets, [rw]mem_default and [rw]mem_max. You might consider increasing
these values to at least 256k. The read and write limits are set in the proc
file system using (for example) the files /proc/sys/net/core/rmem_default and
/proc/sys/net/core/rmem_max. The rmem_default value can be increased in three
steps; the following method is a bit of a hack but should work and should not
cause any problems:

  * Increase the size listed in the file:
         # echo 262144 > /proc/sys/net/core/rmem_default             
         # echo 262144 > /proc/sys/net/core/rmem_max                 
  * Restart NFS. For example, on Red Hat systems,
         # /etc/rc.d/init.d/nfs restart                              
  * You might return the size limits to their normal size in case other
    kernel systems depend on it:
         # echo 65536 > /proc/sys/net/core/rmem_default              
         # echo 65536 > /proc/sys/net/core/rmem_max                  

This last step may be necessary because machines have been reported to crash
if these values are left changed for long periods of time.

5.8. Turning Off Autonegotiation of NICs and Hubs

If network cards auto-negotiate badly with hubs and switches, and ports run
at different speeds, or with different duplex configurations, performance
will be severely impacted due to excessive collisions, dropped packets, etc.
If you see excessive numbers of dropped packets in the nfsstat output, or
poor network performance in general, try playing around with the network
speed and duplex settings. If possible, concentrate on establishing a
100BaseT full duplex subnet; the virtual elimination of collisions in full
duplex will remove the most severe performance inhibitor for NFS over UDP. Be
careful when turning off autonegotiation on a card: The hub or switch that
the card is attached to will then resort to other mechanisms (such as
parallel detection) to determine the duplex settings, and some cards default
to half duplex because it is more likely to be supported by an old hub. The
best solution, if the driver supports it, is to force the card to negotiate
100BaseT full duplex.

5.9. Synchronous vs. Asynchronous Behavior in NFS

The default export behavior for both NFS Version 2 and Version 3 protocols,
used by exportfs in nfs-utils versions prior to Version 1.11 (the latter is
in the CVS tree, but not yet released in a package, as of January, 2002) is
"asynchronous". This default permits the server to reply to client requests
as soon as it has processed the request and handed it off to the local file
system, without waiting for the data to be written to stable storage. This is
indicated by the async option denoted in the server's export list. It yields
better performance at the cost of possible data corruption if the server
reboots while still holding unwritten data and/or metadata in its caches.
This possible data corruption is not detectable at the time of occurrence,
since the async option instructs the server to lie to the client, telling the
client that all data has indeed been written to the stable storage,
regardless of the protocol used.

In order to conform with "synchronous" behavior, used as the default for most
proprietary systems supporting NFS (Solaris, HP-UX, RS/6000, etc.), and now
used as the default in the latest version of exportfs, the Linux Server's
file system must be exported with the sync option. Note that specifying
synchronous exports will result in no option being seen in the server's
export list:

  * Export a couple file systems to everyone, using slightly different
        # /usr/sbin/exportfs -o rw,sync *:/usr/local                 
        # /usr/sbin/exportfs -o rw *:/tmp                            
  * Now we can see what the exported file system parameters look like:
        # /usr/sbin/exportfs -v                                      
        /usr/local *(rw)                                             
        /tmp *(rw,async)                                             

If your kernel is compiled with the /proc filesystem, then the file /proc/fs/
nfs/exports will also show the full list of export options.

When synchronous behavior is specified, the server will not complete (that
is, reply to the client) an NFS version 2 protocol request until the local
file system has written all data/metadata to the disk. The server will
complete a synchronous NFS version 3 request without this delay, and will
return the status of the data in order to inform the client as to what data
should be maintained in its caches, and what data is safe to discard. There
are 3 possible status values, defined an enumerated type, nfs3_stable_how, in
include/linux/nfs.h. The values, along with the subsequent actions taken due
to these results, are as follows:

  * NFS_UNSTABLE - Data/Metadata was not committed to stable storage on the
    server, and must be cached on the client until a subsequent client commit
    request assures that the server does send data to stable storage.
  * NFS_DATA_SYNC - Metadata was not sent to stable storage, and must be
    cached on the client. A subsequent commit is necessary, as is required
  * NFS_FILE_SYNC - No data/metadata need be cached, and a subsequent commit
    need not be sent for the range covered by this request.

In addition to the above definition of synchronous behavior, the client may
explicitly insist on total synchronous behavior, regardless of the protocol,
by opening all files with the O_SYNC option. In this case, all replies to
client requests will wait until the data has hit the server's disk,
regardless of the protocol used (meaning that, in NFS version 3, all requests
will be NFS_FILE_SYNC requests, and will require that the Server returns this
status). In that case, the performance of NFS Version 2 and NFS Version 3
will be virtually identical.

If, however, the old default async behavior is used, the O_SYNC option has no
effect at all in either version of NFS, since the server will reply to the
client without waiting for the write to complete. In that case the
performance differences between versions will also disappear.

Finally, note that, for NFS version 3 protocol requests, a subsequent commit
request from the NFS client at file close time, or at fsync() time, will
force the server to write any previously unwritten data/metadata to the disk,
and the server will not reply to the client until this has been completed, as
long as sync behavior is followed. If async is used, the commit is
essentially a no-op, since the server once again lies to the client, telling
the client that the data has been sent to stable storage. This again exposes
the client and server to data corruption, since cached data may be discarded
on the client due to its belief that the server now has the data maintained
in stable storage.

5.10. Non-NFS-Related Means of Enhancing Server Performance

In general, server performance and server disk access speed will have an
important effect on NFS performance. Offering general guidelines for setting
up a well-functioning file server is outside the scope of this document, but
a few hints may be worth mentioning:

  * If you have access to RAID arrays, use RAID 1/0 for both write speed and
    redundancy; RAID 5 gives you good read speeds but lousy write speeds.
  * A journalling filesystem will drastically reduce your reboot time in the
    event of a system crash. Currently, [
    /fs/jfs/] ext3 will work correctly with NFS version 3. In addition,
    Reiserfs version 3.6 will work with NFS version 3 on 2.4.7 or later
    kernels (patches are available for previous kernels). Earlier versions of
    Reiserfs did not include room for generation numbers in the inode,
    exposing the possibility of undetected data corruption during a server
  * Additionally, journalled file systems can be configured to maximize
    performance by taking advantage of the fact that journal updates are all
    that is necessary for data protection. One example is using ext3 with 
    data=journal so that all updates go first to the journal, and later to
    the main file system. Once the journal has been updated, the NFS server
    can safely issue the reply to the clients, and the main file system
    update can occur at the server's leisure.
    The journal in a journalling file system may also reside on a separate
    device such as a flash memory card so that journal updates normally
    require no seeking. With only rotational delay imposing a cost, this
    gives reasonably good synchronous IO performance. Note that ext3
    currently supports journal relocation, and ReiserFS will (officially)
    support it soon. The Reiserfs tool package found at [ftp://] ftp:// contains
    the reiserfstune tool, which will allow journal relocation. It does,
    however, require a kernel patch which has not yet been officially
    released as of January, 2002.
  * Using an automounter (such as autofs or amd) may prevent hangs if you
    cross-mount files on your machines (whether on purpose or by oversight)
    and one of those machines goes down. See the [
    HOWTO/mini/Automount.html] Automount Mini-HOWTO for details.
  * Some manufacturers (Network Appliance, Hewlett Packard, and others)
    provide NFS accelerators in the form of Non-Volatile RAM. NVRAM will
    boost access speed to stable storage up to the equivalent of async

6. Security and NFS

This list of security tips and explanations will not make your site
completely secure. NOTHING will make your site completely secure. Reading
this section may help you get an idea of the security problems with NFS. This
is not a comprehensive guide and it will always be undergoing changes. If you
have any tips or hints to give us please send them to the HOWTO maintainer.

If you are on a network with no access to the outside world (not even a
modem) and you trust all the internal machines and all your users then this
section will be of no use to you. However, its our belief that there are
relatively few networks in this situation so we would suggest reading this
section thoroughly for anyone setting up NFS.

With NFS, there are two steps required for a client to gain access to a file
contained in a remote directory on the server. The first step is mount
access. Mount access is achieved by the client machine attempting to attach
to the server. The security for this is provided by the /etc/exports file.
This file lists the names or IP addresses for machines that are allowed to
access a share point. If the client's ip address matches one of the entries
in the access list then it will be allowed to mount. This is not terribly
secure. If someone is capable of spoofing or taking over a trusted address
then they can access your mount points. To give a real-world example of this
type of "authentication": This is equivalent to someone introducing
themselves to you and you believing they are who they claim to be because
they are wearing a sticker that says "Hello, My Name is ...." Once the
machine has mounted a volume, its operating system will have access to all
files on the volume (with the possible exception of those owned by root; see
below) and write access to those files as well, if the volume was exported
with the rw option.

The second step is file access. This is a function of normal file system
access controls on the client and not a specialized function of NFS. Once the
drive is mounted the user and group permissions on the files determine access

An example: bob on the server maps to the UserID 9999. Bob makes a file on
the server that is only accessible the user (the equivalent to typing chmod
600 filename). A client is allowed to mount the drive where the file is
stored. On the client mary maps to UserID 9999. This means that the client
user mary can access bob's file that is marked as only accessible by him. It
gets worse: If someone has become superuser on the client machine they can su
- username and become any user. NFS will be none the wiser.

Its not all terrible. There are a few measures you can take on the server to
offset the danger of the clients. We will cover those shortly.

If you don't think the security measures apply to you, you're probably wrong.
In Section 6.1 we'll cover securing the portmapper, server and client
security in Section 6.2 and Section 6.3 respectively. Finally, in Section 6.4
we'll briefly talk about proper firewalling for your nfs server.

Finally, it is critical that all of your nfs daemons and client programs are
current. If you think that a flaw is too recently announced for it to be a
problem for you, then you've probably already been compromised.

A good way to keep up to date on security alerts is to subscribe to the
bugtraq mailinglists. You can read up on how to subscribe and various other
information about bugtraq here: [

Additionally searching for NFS at []'s search engine will show you all security reports
pertaining to NFS.

You should also regularly check CERT advisories. See the CERT web page at

6.1. The portmapper

The portmapper keeps a list of what services are running on what ports. This
list is used by a connecting machine to see what ports it wants to talk to
access certain services.

The portmapper is not in as bad a shape as a few years ago but it is still a
point of worry for many sys admins. The portmapper, like NFS and NIS, should
not really have connections made to it outside of a trusted local area
network. If you have to expose them to the outside world - be careful and
keep up diligent monitoring of those systems.

Not all Linux distributions were created equal. Some seemingly up-to-date
distributions do not include a securable portmapper. The easy way to check if
your portmapper is good or not is to run strings(1) and see if it reads the
relevant files, /etc/hosts.deny and /etc/hosts.allow. Assuming your
portmapper is /sbin/portmap you can check it with this command:
     strings /sbin/portmap | grep hosts.                                     

On a securable machine it comes up something like this:
|   /etc/hosts.allow                                                        |
|   /etc/hosts.deny                                                         |
|   @(#) hosts_ctl.c 1.4 94/12/28 17:42:27                                  |
|   @(#) hosts_access.c 1.21 97/02/12 02:13:22                              |
|                                                                           |

First we edit /etc/hosts.deny. It should contain the line

|   portmap: ALL                                                            |
|                                                                           |

which will deny access to everyone. While it is closed run:
|   rpcinfo -p                                                              |
|                                                                           |
just to check that your portmapper really reads and obeys this file. Rpcinfo
should give no output, or possibly an error message. The files /etc/
hosts.allow and /etc/hosts.deny take effect immediately after you save them.
No daemon needs to be restarted.

Closing the portmapper for everyone is a bit drastic, so we open it again by
editing /etc/hosts.allow. But first we need to figure out what to put in it.
It should basically list all machines that should have access to your
portmapper. On a run of the mill Linux system there are very few machines
that need any access for any reason. The portmapper administers nfsd, mountd,
ypbind/ypserv, rquotad, lockd (which shows up as nlockmgr), statd (which
shows up as status) and 'r' services like ruptime and rusers. Of these only 
nfsd, mountd, ypbind/ypserv and perhaps rquotad,lockd and statd are of any
consequence. All machines that need to access services on your machine should
be allowed to do that. Let's say that your machine's address is
and that it lives on the subnet, and that all machines on the
subnet should have access to it (for an overview of those terms see the the
Networking-Overview-HOWTO). Then we write:
|   portmap:                                      |
|                                                                           |
in /etc/hosts.allow. If you are not sure what your network or netmask are,
you can use the ifconfig command to determine the netmask and the netstat
command to determine the network. For, example, for the device eth0 on the
above machine ifconfig should show:

|   ...                                                                     |
|   eth0   Link encap:Ethernet  HWaddr 00:60:8C:96:D5:56                    |
|          inet addr:  Bcast: Mask:  |
|          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1               |
|          RX packets:360315 errors:0 dropped:0 overruns:0                  |
|          TX packets:179274 errors:0 dropped:0 overruns:0                  |
|          Interrupt:10 Base address:0x320                                  |
|   ...                                                                     |
|                                                                           |
and netstat -rn should show:
|   Kernel routing table                                                          |
|   Destination     Gateway         Genmask         Flags Metric Ref Use    Iface |
|   ...                                                                           |
|   U     0      0   174412 eth0  |
|   ...                                                                           |
|                                                                                 |
(The network address is in the first column).

The /etc/hosts.deny and /etc/hosts.allow files are described in the manual
pages of the same names.

IMPORTANT: Do not put anything but IP NUMBERS in the portmap lines of these
files. Host name lookups can indirectly cause portmap activity which will
trigger host name lookups which can indirectly cause portmap activity which
will trigger...

Versions 0.2.0 and higher of the nfs-utils package also use the hosts.allow
and hosts.deny files, so you should put in entries for lockd, statd, mountd,
and rquotad in these files too. For a complete example, see Section 3.2.2.

The above things should make your server tighter. The only remaining problem
is if someone gains administrative access to one of your trusted client
machines and is able to send bogus NFS requests. The next section deals with
safeguards against this problem.

6.2. Server security: nfsd and mountd

On the server we can decide that we don't want to trust any requests made as
root on the client. We can do that by using the root_squash option in /etc/
   /home slave1(rw,root_squash)                                              

This is, in fact, the default. It should always be turned on unless you have
a very good reason to turn it off. To turn it off use the no_root_squash

Now, if a user with UID 0 (i.e., root's user ID number) on the client
attempts to access (read, write, delete) the file system, the server
substitutes the UID of the server's 'nobody' account. Which means that the
root user on the client can't access or change files that only root on the
server can access or change. That's good, and you should probably use 
root_squash on all the file systems you export. "But the root user on the
client can still use su to become any other user and access and change that
users files!" say you. To which the answer is: Yes, and that's the way it is,
and has to be with Unix and NFS. This has one important implication: All
important binaries and files should be owned by root, and not bin or other
non-root account, since the only account the clients root user cannot access
is the servers root account. In the exports(5) man page there are several
other squash options listed so that you can decide to mistrust whomever you
(don't) like on the clients.

The TCP ports 1-1024 are reserved for root's use (and therefore sometimes
referred to as "secure ports") A non-root user cannot bind these ports.
Adding the secure option to an /etc/exports means that it will only listed to
requests coming from ports 1-1024 on the client, so that a malicious non-root
user on the client cannot come along and open up a spoofed NFS dialogue on a
non-reserved port. This option is set by default.

6.3. Client Security

6.3.1. The nosuid mount option

On the client we can decide that we don't want to trust the server too much a
couple of ways with options to mount. For example we can forbid suid programs
to work off the NFS file system with the nosuid option. Some unix programs,
such as passwd, are called "suid" programs: They set the id of the person
running them to whomever is the owner of the file. If a file is owned by root
and is suid, then the program will execute as root, so that they can perform
operations (such as writing to the password file) that only root is allowed
to do. Using the nosuid option is a good idea and you should consider using
this with all NFS mounted disks. It means that the server's root user cannot
make a suid-root program on the file system, log in to the client as a normal
user and then use the suid-root program to become root on the client too. One
could also forbid execution of files on the mounted file system altogether
with the noexec option. But this is more likely to be impractical than nosuid
since a file system is likely to at least contain some scripts or programs
that need to be executed.

6.3.2. The broken_suid mount option

Some older programs (xterm being one of them) used to rely on the idea that
root can write everywhere. This is will break under new kernels on NFS
mounts. The security implications are that programs that do this type of suid
action can potentially be used to change your apparent uid on nfs servers
doing uid mapping. So the default has been to disable this broken_suid in the
linux kernel.

The long and short of it is this: If you're using an old linux distribution,
some sort of old suid program or an older unix of some type you might have to
mount from your clients with the broken_suid option to mount. However, most
recent unixes and linux distros have xterm and such programs just as a normal
executable with no suid status, they call programs to do their setuid work.

You enter the above options in the options column, with the rsize and wsize,
separated by commas.

6.3.3. Securing portmapper, rpc.statd, and rpc.lockd on the client

In the current (2.2.18+) implementation of NFS, full file locking is
supported. This means that rpc.statd and rpc.lockd must be running on the
client in order for locks to function correctly. These services require the
portmapper to be running. So, most of the problems you will find with nfs on
the server you may also be plagued with on the client. Read through the
portmapper section above for information on securing the portmapper.

6.4. NFS and firewalls (ipchains and netfilter)

IPchains (under the 2.2.X kernels) and netfilter (under the 2.4.x kernels)
allow a good level of security - instead of relying on the daemon (or perhaps
its TCP wrapper) to determine which machines can connect, the connection
attempt is allowed or disallowed at a lower level. In this case, you can stop
the connection much earlier and more globally, which can protect you from all
sorts of attacks.

Describing how to set up a Linux firewall is well beyond the scope of this
document. Interested readers may wish to read the [
HOWTO/Firewall-HOWTO.html] Firewall-HOWTO or the [
HOWTO/IPCHAINS-HOWTO.HTML] IPCHAINS-HOWTO. For users of kernel 2.4 and above
you might want to visit the netfilter webpage at: [http://] If you are
already familiar with the workings of ipchains or netfilter this section will
give you a few tips on how to better setup your NFS daemons to more easily
firewall and protect them.

A good rule to follow for your firewall configuration is to deny all, and
allow only some - this helps to keep you from accidentally allowing more than
you intended.

In order to understand how to firewall the NFS daemons, it will help to
breifly review how they bind to ports.

When a daemon starts up, it requests a free port from the portmapper. The
portmapper gets the port for the daemon and keeps track of the port currently
used by that daemon. When other hosts or processes need to communicate with
the daemon, they request the port number from the portmapper in order to find
the daemon. So the ports will perpetually float because different ports may
be free at different times and so the portmapper will allocate them
differently each time. This is a pain for setting up a firewall. If you never
know where the daemons are going to be then you don't know precisely which
ports to allow access to. This might not be a big deal for many people
running on a protected or isolated LAN. For those people on a public network,
though, this is horrible.

In kernels 2.4.13 and later with nfs-utils 0.3.3 or later you no longer have
to worry about the floating of ports in the portmapper. Now all of the
daemons pertaining to nfs can be "pinned" to a port. Most of them nicely take
a -p option when they are started; those daemons that are started by the
kernel take some kernel arguments or module options. They are described

Some of the daemons involved in sharing data via nfs are already bound to a
port. portmap is always on port 111 tcp and udp. nfsd is always on port 2049
TCP and UDP (however, as of kernel 2.4.17, NFS over TCP is considered
experimental and is not for use on production machines).

The other daemons, statd, mountd, lockd, and rquotad, will normally move
around to the first available port they are informed of by the portmapper.

To force statd to bind to a particular port, use the -p portnum option. To
force statd to respond on a particular port, additionally use the -o portnum
option when starting it.

To force mountd to bind to a particular port use the -p portnum option.

For example, to have statd broadcast of port 32765 and listen on port 32766,
and mountd listen on port 32767, you would type:
# statd -p 32765 -o 32766                                                    
# mountd -p 32767                                                            

lockd is started by the kernel when it is needed. Therefore you need to pass
module options (if you have it built as a module) or kernel options to force 
lockd to listen and respond only on certain ports.

If you are using loadable modules and you would like to specify these options
in your /etc/modules.conf file add a line like this to the file:
options lockd nlm_udpport=32768 nlm_tcpport=32768                            

The above line would specify the udp and tcp port for lockd to be 32768.

If you are not using loadable modules or if you have compiled lockd into the
kernel instead of building it as a module then you will need to pass it an
option on the kernel boot line.

It should look something like this:
 vmlinuz 3 root=/dev/hda1 lockd.udpport=32768 lockd.tcpport=32768            

The port numbers do not have to match but it would simply add unnecessary
confusion if they didn't.

If you are using quotas and using rpc.quotad to make these quotas viewable
over nfs you will need to also take it into account when setting up your
firewall. There are two rpc.rquotad source trees. One of those is maintained
in the nfs-utils tree. The other in the quota-tools tree. They do not operate
identically. The one provided with nfs-utils supports binding the daemon to a
port with the -p directive. The one in quota-tools does not. Consult your
distribution's documentation to determine if yours does.

For the sake of this discussion lets describe a network and setup a firewall
to protect our nfs server. Our nfs server is our client is only. As in the example above, statd has been started so that it
only binds to port 32765 for incoming requests and it must answer on port
32766. mountd is forced to bind to port 32767. lockd's module parameters have
been set to bind to 32768. nfsd is, of course, on port 2049 and the
portmapper is on port 111.

We are not using quotas.

Using IPCHAINS, a simple firewall might look something like this:
ipchains -A input -f -j ACCEPT -s                               
ipchains -A input -s -d 0/0 32765:32768 -p 6 -j ACCEPT          
ipchains -A input -s -d 0/0 32765:32768 -p 17 -j ACCEPT         
ipchains -A input -s -d 0/0 2049 -p 17 -j ACCEPT                
ipchains -A input -s -d 0/0 2049 -p 6 -j ACCEPT                 
ipchains -A input -s -d 0/0 111 -p 6 -j ACCEPT                  
ipchains -A input -s -d 0/0 111 -p 17 -j ACCEPT                 
ipchains -A input -s 0/0 -d 0/0 -p 6 -j DENY -y -l                           
ipchains -A input -s 0/0 -d 0/0 -p 17 -j DENY -l                             

The equivalent set of commands in netfilter is:
iptables -A INPUT -f -j ACCEPT -s                               
iptables -A INPUT -s -d 0/0 32765:32768 -p 6 -j ACCEPT          
iptables -A INPUT -s -d 0/0 32765:32768 -p 17 -j ACCEPT         
iptables -A INPUT -s -d 0/0 2049 -p 17 -j ACCEPT                
iptables -A INPUT -s -d 0/0 2049 -p 6 -j ACCEPT                 
iptables -A INPUT -s -d 0/0 111 -p 6 -j ACCEPT                  
iptables -A INPUT -s -d 0/0 111 -p 17 -j ACCEPT                 
iptables -A INPUT -s 0/0 -d 0/0 -p 6 -j DENY --syn --log-level 5             
iptables -A INPUT -s 0/0 -d 0/0 -p 17 -j DENY --log-level 5                  

The first line says to accept all packet fragments (except the first packet
fragment which will be treated as a normal packet). In theory no packet will
pass through until it is reassembled, and it won't be reassembled unless the
first packet fragment is passed. Of course there are attacks that can be
generated by overloading a machine with packet fragments. But NFS won't work
correctly unless you let fragments through. See Section 7.8 for details.

The other lines allow specific connections from any port on our client host
to the specific ports we have made available on our server. This means that
if, say, attempts to contact the NFS server it will not be able
to mount or see what mounts are available.

With the new port pinning capabilities it is obviously much easier to control
what hosts are allowed to mount your NFS shares. It is worth mentioning that
NFS is not an encrypted protocol and anyone on the same physical network
could sniff the traffic and reassemble the information being passed back and

6.5. Tunneling NFS through SSH

One method of encrypting NFS traffic over a network is to use the
port-forwarding capabilities of ssh. However, as we shall see, doing so has a
serious drawback if you do not utterly and completely trust the local users
on your server.

The first step will be to export files to the localhost. For example, to
export the /home partition, enter the following into /etc/exports:

The next step is to use ssh to forward ports. For example, ssh can tell the
server to forward to any port on any machine from a port on the client. Let
us assume, as in the previous section, that our server is, and
that we have pinned mountd to port 32767 using the argument -p 32767. Then,
on the client, we'll type:
     # ssh root@ -L 250:localhost:2049  -f sleep 60m             
     # ssh root@ -L 251:localhost:32767 -f sleep 60m             

The above command causes ssh on the client to take any request directed at
the client's port 250 and forward it, first through sshd on the server, and
then on to the server's port 2049. The second line causes a similar type of
forwarding between requests to port 251 on the client and port 32767 on the
server. The localhost is relative to the server; that is, the forwarding will
be done to the server itself. The port could otherwise have been made to
forward to any other machine, and the requests would look to the outside
world as if they were coming from the server. Thus, the requests will appear
to NFSD on the server as if they are coming from the server itself. Note that
in order to bind to a port below 1024 on the client, we have to run this
command as root on the client. Doing this will be necessary if we have
exported our filesystem with the default secure option.

Finally, we are pulling a little trick with the last option, -f sleep 60m.
Normally, when we use ssh, even with the -L option, we will open up a shell
on the remote machine. But instead, we just want the port forwarding to
execute in the background so that we get our shell on the client back. So, we
tell ssh to execute a command in the background on the server to sleep for 60
minutes. This will cause the port to be forwarded for 60 minutes until it
gets a connection; at that point, the port will continue to be forwarded
until the connection dies or until the 60 minutes are up, whichever happens
later. The above command could be put in our startup scripts on the client,
right after the network is started.

Next, we have to mount the filesystem on the client. To do this, we tell the
client to mount a filesystem on the localhost, but at a different port from
the usual 2049. Specifically, an entry in /etc/fstab would look like:
  localhost:/home  /mnt/home  nfs  rw,hard,intr,port=250,mountport=251  0 0  

Having done this, we can see why the above will be incredibly insecure if we
have any ordinary users who are able to log in to the server locally. If they
can, there is nothing preventing them from doing what we did and using ssh to
forward a privileged port on their own client machine (where they are
legitimately root) to ports 2049 and 32767 on the server. Thus, any ordinary
user on the server can mount our filesystems with the same rights as root on
our client.

If you are using an NFS server that does not have a way for ordinary users to
log in, and you wish to use this method, there are two additional caveats:
First, the connection travels from the client to the server via sshd;
therefore you will have to leave port 22 (where sshd listens) open to your
client on the firewall. However you do not need to leave the other ports,
such as 2049 and 32767, open anymore. Second, file locking will no longer
work. It is not possible to ask statd or the locking manager to make requests
to a particular port for a particular mount; therefore, any locking requests
will cause statd to connect to statd on localhost, i.e., itself, and it will
fail with an error. Any attempt to correct this would require a major rewrite
of NFS.

It may also be possible to use IPSec to encrypt network traffic between your
client and your server, without compromising any local security on the
server; this will not be taken up here. See the []
FreeS/WAN home page for details on using IPSec under Linux.

6.6. Summary

If you use the hosts.allow, hosts.deny, root_squash, nosuid and privileged
port features in the portmapper/NFS software, you avoid many of the presently
known bugs in NFS and can almost feel secure about that at least. But still,
after all that: When an intruder has access to your network, s/he can make
strange commands appear in your .forward or read your mail when /home or /var
/mail is NFS exported. For the same reason, you should never access your PGP
private key over NFS. Or at least you should know the risk involved. And now
you know a bit of it.

NFS and the portmapper makes up a complex subsystem and therefore it's not
totally unlikely that new bugs will be discovered, either in the basic design
or the implementation we use. There might even be holes known now, which
someone is abusing. But that's life.

7. Troubleshooting

    This is intended as a step-by-step guide to what to do when things go
    wrong using NFS. Usually trouble first rears its head on the client end,
    so this diagnostic will begin there.
7.1. Unable to See Files on a Mounted File System

First, check to see if the file system is actually mounted. There are several
ways of doing this. The most reliable way is to look at the file /proc/
mounts, which will list all mounted filesystems and give details about them.
If this doesn't work (for example if you don't have the /proc filesystem
compiled into your kernel), you can type mount -f although you get less

If the file system appears to be mounted, then you may have mounted another
file system on top of it (in which case you should unmount and remount both
volumes), or you may have exported the file system on the server before you
mounted it there, in which case NFS is exporting the underlying mount point
(if so then you need to restart NFS on the server).

If the file system is not mounted, then attempt to mount it. If this does not
work, see Symptom 3.

7.2. File requests hang or timeout waiting for access to the file.

This usually means that the client is unable to communicate with the server.
See Symptom 3 letter b.

7.3. Unable to mount a file system

There are two common errors that mount produces when it is unable to mount a
volume. These are:

 a. failed, reason given by server: Permission denied
    This means that the server does not recognize that you have access to the
     i. Check your /etc/exports file and make sure that the volume is
        exported and that your client has the right kind of access to it. For
        example, if a client only has read access then you have to mount the
        volume with the ro option rather than the rw option.
    ii. Make sure that you have told NFS to register any changes you made to
        /etc/exports since starting nfsd by running the exportfs command. Be
        sure to type exportfs -ra to be extra certain that the exports are
        being re-read.
    iii. Check the file /proc/fs/nfs/exports and make sure the volume and
        client are listed correctly. (You can also look at the file /var/lib/
        nfs/xtab for an unabridged list of how all the active export options
        are set.) If they are not, then you have not re-exported properly. If
        they are listed, make sure the server recognizes your client as being
        the machine you think it is. For example, you may have an old listing
        for the client in /etc/hosts that is throwing off the server, or you
        may not have listed the client's complete address and it may be
        resolving to a machine in a different domain. One trick is login to
        the server from the client via ssh or telnet; if you then type who,
        one of the listings should be your login session and the name of your
        client machine as the server sees it. Try using this machine name in
        your /etc/exports entry. Finally, try to ping the client from the
        server, and try to ping the server from the client. If this doesn't
        work, or if there is packet loss, you may have lower-level network
    iv. It is not possible to export both a directory and its child (for
        example both /usr and /usr/local). You should export the parent
        directory with the necessary permissions, and all of its
        subdirectories can then be mounted with those same permissions.
 b. RPC: Program Not Registered: (or another "RPC" error):
    This means that the client does not detect NFS running on the server.
    This could be for several reasons.
     i. First, check that NFS actually is running on the server by typing 
        rpcinfo -p on the server. You should see something like this:
        |   program vers proto   port                                |
        |    100000    2   tcp    111  portmapper                    |
        |    100000    2   udp    111  portmapper                    |
        |    100011    1   udp    749  rquotad                       |
        |    100011    2   udp    749  rquotad                       |
        |    100005    1   udp    759  mountd                        |
        |    100005    1   tcp    761  mountd                        |
        |    100005    2   udp    764  mountd                        |
        |    100005    2   tcp    766  mountd                        |
        |    100005    3   udp    769  mountd                        |
        |    100005    3   tcp    771  mountd                        |
        |    100003    2   udp   2049  nfs                           |
        |    100003    3   udp   2049  nfs                           |
        |    300019    1   tcp    830  amd                           |
        |    300019    1   udp    831  amd                           |
        |    100024    1   udp    944  status                        |
        |    100024    1   tcp    946  status                        |
        |    100021    1   udp   1042  nlockmgr                      |
        |    100021    3   udp   1042  nlockmgr                      |
        |    100021    4   udp   1042  nlockmgr                      |
        |    100021    1   tcp   1629  nlockmgr                      |
        |    100021    3   tcp   1629  nlockmgr                      |
        |    100021    4   tcp   1629  nlockmgr                      |
        |                                                            |
        This says that we have NFS versions 2 and 3, rpc.statd version 1,
        network lock manager (the service name for rpc.lockd) versions 1, 3,
        and 4. There are also different service listings depending on whether
        NFS is travelling over TCP or UDP. UDP is usually (but not always)
        the default unless TCP is explicitly requested.
        If you do not see at least portmapper, nfs, and mountd, then you need
        to restart NFS. If you are not able to restart successfully, proceed
        to Symptom 9.
    ii. Now check to make sure you can see it from the client. On the client,
        type rpcinfo -p server where server is the DNS name or IP address of
        your server.
        If you get a listing, then make sure that the type of mount you are
        trying to perform is supported. For example, if you are trying to
        mount using Version 3 NFS, make sure Version 3 is listed; if you are
        trying to mount using NFS over TCP, make sure that is registered.
        (Some non-Linux clients default to TCP). Type man rpcinfo for more
        details on how to read the output. If the type of mount you are
        trying to perform is not listed, try a different type of mount.
        If you get the error No Remote Programs Registered, then you need to
        check your /etc/hosts.allow and /etc/hosts.deny files on the server
        and make sure your client actually is allowed access. Again, if the
        entries appear correct, check /etc/hosts (or your DNS server) and
        make sure that the machine is listed correctly, and make sure you can
        ping the server from the client. Also check the error logs on the
        system for helpful messages: Authentication errors from bad /etc/
        hosts.allow entries will usually appear in /var/log/messages, but may
        appear somewhere else depending on how your system logs are set up.
        The man pages for syslog can help you figure out how your logs are
        set up. Finally, some older operating systems may behave badly when
        routes between the two machines are asymmetric. Try typing tracepath
        [server] from the client and see if the word "asymmetric" shows up
        anywhere in the output. If it does then this may be causing packet
        loss. However asymmetric routes are not usually a problem on recent
        linux distributions.
        If you get the error Remote system error - No route to host, but you
        can ping the server correctly, then you are the victim of an
        overzealous firewall. Check any firewalls that may be set up, either
        on the server or on any routers in between the client and the server.
        Look at the man pages for ipchains, netfilter, and ipfwadm, as well
        as the []
        IPChains-HOWTO and the [
        Firewall-HOWTO.html] Firewall-HOWTO for help.

7.4. I do not have permission to access files on the mounted volume.

This could be one of two problems.

If it is a write permission problem, check the export options on the server
by looking at /proc/fs/nfs/exports and make sure the filesystem is not
exported read-only. If it is you will need to re-export it read/write (don't
forget to run exportfs -ra after editing /etc/exports). Also, check /proc/
mounts and make sure the volume is mounted read/write (although if it is
mounted read-only you ought to get a more specific error message). If not
then you need to re-mount with the rw option.

The second problem has to do with username mappings, and is different
depending on whether you are trying to do this as root or as a non-root user.

If you are not root, then usernames may not be in sync on the client and the
server. Type id [user] on both the client and the server and make sure they
give the same UID number. If they don't then you are having problems with
NIS, NIS+, rsync, or whatever system you use to sync usernames. Check group
names to make sure that they match as well. Also, make sure you are not
exporting with the all_squash option. If the user names match then the user
has a more general permissions problem unrelated to NFS.

If you are root, then you are probably not exporting with the no_root_squash
option; check /proc/fs/nfs/exports or /var/lib/nfs/xtab on the server and
make sure the option is listed. In general, being able to write to the NFS
server as root is a bad idea unless you have an urgent need -- which is why
Linux NFS prevents it by default. See Section 6 for details.

If you have root squashing, you want to keep it, and you're only trying to
get root to have the same permissions on the file that the user nobody should
have, then remember that it is the server that determines which uid root gets
mapped to. By default, the server uses the UID and GID of nobody in the /etc/
passwd file, but this can also be overridden with the anonuid and anongid
options in the /etc/exports file. Make sure that the client and the server
agree about which UID nobody gets mapped to.

7.5. When I transfer really big files, NFS takes over all the CPU cycles on
the server and it screeches to a halt.

This is a problem with the fsync() function in 2.2 kernels that causes all
sync-to-disk requests to be cumulative, resulting in a write time that is
quadratic in the file size. If you can, upgrading to a 2.4 kernel should
solve the problem. Also, exporting with the no_wdelay option forces the
program to use o_sync() instead, which may prove faster.

7.6. Strange error or log messages

 a. Messages of the following format:
    | Jan 7 09:15:29 server kernel: fh_verify: mail/guest permission failure, acc=4, error=13   |
    | Jan 7 09:23:51 server kernel: fh_verify: ekonomi/test permission failure, acc=4, error=13 |
    |                                                                                           |
    These happen when a NFS setattr operation is attempted on a file you
    don't have write access to. The messages are harmless.
 b. The following messages frequently appear in the logs:
    | kernel: nfs: server not responding, still trying |
    | kernel: nfs: task 10754 can't get a request slot                    |
    | kernel: nfs: server OK                           |
    |                                                                     |
    The "can't get a request slot" message means that the client-side RPC
    code has detected a lot of timeouts (perhaps due to network congestion,
    perhaps due to an overloaded server), and is throttling back the number
    of concurrent outstanding requests in an attempt to lighten the load. The
    cause of these messages is basically sluggish performance. See Section 5
    for details.
 c. After mounting, the following message appears on the client:
    |nfs warning: mount version older than kernel                   |
    |                                                               |
    It means what it says: You should upgrade your mount package and/or
    am-utils. (If for some reason upgrading is a problem, you may be able to
    get away with just recompiling them so that the newer kernel features are
    recognized at compile time).
 d. Errors in startup/shutdown log for lockd
    You may see a message of the following kind in your boot log:
    |nfslock: rpc.lockd startup failed                              |
    |                                                               |
    They are harmless. Older versions of rpc.lockd needed to be started up
    manually, but newer versions are started automatically by nfsd. Many of
    the default startup scripts still try to start up lockd by hand, in case
    it is necessary. You can alter your startup scripts if you want the
    messages to go away.
 e. The following message appears in the logs:
    |kmem_create: forcing size word alignment - nfs_fh              |
    |                                                               |
    This results from the file handle being 16 bits instead of a mulitple of
    32 bits, which makes the kernel grimace. It is harmless.

7.7. Real permissions don't match what's in /etc/exports.

/etc/exports is very sensitive to whitespace - so the following statements
are not the same:
/export/dir hostname(rw,no_root_squash)                                      
/export/dir hostname (rw,no_root_squash)                                     
The first will grant hostname rw access to /export/dir without squashing root
privileges. The second will grant hostname rw privileges with root squash and
it will grant everyone else read/write access, without squashing root
privileges. Nice huh?

7.8. Flaky and unreliable behavior

Simple commands such as ls work, but anything that transfers a large amount
of information causes the mount point to lock.

This could be one of two problems:

 i. It will happen if you have ipchains on at the server and/or the client
    and you are not allowing fragmented packets through the chains. Allow
    fragments from the remote host and you'll be able to function again. See 
    Section 6.4 for details on how to do this.
ii. You may be using a larger rsize and wsize in your mount options than the
    server supports. Try reducing rsize and wsize to 1024 and seeing if the
    problem goes away. If it does, then increase them slowly to a more
    reasonable value.

7.9. nfsd won't start

Check the file /etc/exports and make sure root has read permission. Check the
binaries and make sure they are executable. Make sure your kernel was
compiled with NFS server support. You may need to reinstall your binaries if
none of these ideas helps.

7.10. File Corruption When Using Multiple Clients

If a file has been modified within one second of its previous modification
and left the same size, it will continue to generate the same inode number.
Because of this, constant reads and writes to a file by multiple clients may
cause file corruption. Fixing this bug requires changes deep within the
filesystem layer, and therefore it is a 2.5 item.

8. Using Linux NFS with Other OSes

Every operating system, Linux included, has quirks and deviations in the
behavior of its NFS implementation -- sometimes because the protocols are
vague, sometimes because they leave gaping security holes. Linux will work
properly with all major vendors' NFS implementations, as far as we know.
However, there may be extra steps involved to make sure the two OSes are
communicating clearly with one another. This section details those steps.

In general, it is highly ill-advised to attempt to use a Linux machine with a
kernel before 2.2.18 as an NFS server for non-Linux clients. Implementations
with older kernels may work fine as clients; however if you are using one of
these kernels and get stuck, the first piece of advice we would give is to
upgrade your kernel and see if the problems go away. The user-space NFS
implementations also do not work well with non-Linux clients.

Following is a list of known issues for using Linux together with major
operating systems.

8.1. AIX

8.1.1. Linux Clients and AIX Servers

The format for the /etc/exports file for our example in Section 3 is:

8.1.2. AIX clients and Linux Servers

AIX uses the file /etc/filesystems instead of /etc/fstab. A sample entry,
based on the example in Section 4, looks like this:
        dev             = "/home"                                             
        vfs             = nfs                                                 
        nodename        =                                      
        mount           = true                                                
        options         = bg,hard,intr,rsize=1024,wsize=1024,vers=2,proto=udp 
        account         = false                                               

 i. Version 4.3.2 of AIX, and possibly earlier versions as well, requires
    that file systems be exported with the insecure option, which causes NFS
    to listen to requests from insecure ports (i.e., ports above 1024, to
    which non-root users can bind). Older versions of AIX do not seem to
    require this.
ii. AIX clients will default to mounting version 3 NFS over TCP. If your
    Linux server does not support this, then you may need to specify vers=2
    and/or proto=udp in your mount options.
iii. Using netmasks in /etc/exports seems to sometimes cause clients to lose
    mounts when another client is reset. This can be fixed by listing out
    hosts explicitly.
iv. Apparently automount in AIX 4.3.2 is rather broken.

8.2. BSD

8.2.1. BSD servers and Linux clients

BSD kernels tend to work better with larger block sizes.

8.2.2. Linux servers and BSD clients

Some versions of BSD may make requests to the server from insecure ports, in
which case you will need to export your volumes with the insecure option. See
the man page for exports(5) for more details.

8.3. Tru64 Unix

8.3.1. Tru64 Unix Servers and Linux Clients

In general, Tru64 Unix servers work quite smoothly with Linux clients. The
format for the /etc/exports file for our example in Section 3 is:
/usr \                              \                                 
/home \                                  \                                                                  

(The root option is listed in the last entry for informational purposes only;
its use is not recommended unless necessary.)

Tru64 checks the /etc/exports file every time there is a mount request so you
do not need to run the exportfs command; in fact on many versions of Tru64
Unix the command does not exist.

8.3.2. Linux Servers and Tru64 Unix Clients

There are two issues to watch out for here. First, Tru64 Unix mounts using
Version 3 NFS by default. You will see mount errors if your Linux server does
not support Version 3 NFS. Second, in Tru64 Unix 4.x, NFS locking requests
are made by daemon. You will therefore need to specify the insecure_locks
option on all volumes you export to a Tru64 Unix 4.x client; see the exports
man pages for details.

8.4. HP-UX

8.4.1. HP-UX Servers and Linux Clients

A sample /etc/exports entry on HP-UX looks like this:
/usr -ro,                                
(The root option is listed in the last entry for informational purposes only;
its use is not recommended unless necessary.)

8.4.2. Linux Servers and HP-UX Clients

HP-UX diskless clients will require at least a kernel version 2.2.19 (or
patched 2.2.18) for device files to export correctly. Also, any exports to an
HP-UX client will need to be exported with the insecure_locks option.

8.5. IRIX

8.5.1. IRIX Servers and Linux Clients

A sample /etc/exports entry on IRIX looks like this:
/usr -ro,                                
(The root option is listed in the last entry for informational purposes only;
its use is not recommended unless necessary.)

There are reportedly problems when using the nohide option on exports to
linux 2.2-based systems. This problem is fixed in the 2.4 kernel. As a
workaround, you can export and mount lower-down file systems separately.

As of Kernel 2.4.17, there continue to be several minor interoperability
issues that may require a kernel upgrade. In particular:

  * Make sure that Trond Myklebust's seekdir (or dir) kernel patch is
    applied. The latest version (for 2.4.17) is located at:
  * IRIX servers do not always use the same fsid attribute field across
    reboots, which results in inode number mismatch errors on a Linux client
    if the mounted IRIX server reboots. A patch is available from:
    [] http://
  * Linux kernels v2.4.9 and above have problems reading large directories
    (hundreds of files) from exported IRIX XFS file systems that were made
    with naming version=1. The reason for the problem can be found at:
    [] http://
    The naming version can be found by using (on the IRIX server):
            xfs_growfs -n mount_point                                
    The workaround is to export these file systems using the -32bitclients
    option in the /etc/exports file. The fix is to convert the file system to
    'naming version=2'. Unfortunately the only way to do this is by a backup/
    mkfs_xfs on IRIX 6.5.14 (and above) creates naming version=2 XFS file
    systems by default. On IRIX 6.5.5 to 6.5.13, use:
            mkfs_xfs -n version=2 device                             
    Versions of IRIX prior to 6.5.5 do not support naming version=2 XFS file

8.5.2. IRIX clients and Linux servers

Irix versions up to 6.5.12 have problems mounting file systems exported from
Linux boxes - the mount point "gets lost," e.g.,
        # mount linux:/disk1 /mnt                                            
        # cd /mnt/xyz/abc                                                    
        # pwd                                                                

This is known IRIX bug (SGI bug 815265 - IRIX not liking file handles of less
than 32 bytes), which is fixed in IRIX 6.5.13. If it is not possible to
upgrade to IRIX 6.5.13, then the unofficial workaround is to force the Linux 
nfsd to always use 32 byte file handles.

A number of patches exist - see:

  * [] http://
  * [] http://

8.6. Solaris

8.6.1. Solaris Servers

Solaris has a slightly different format on the server end from other
operating systems. Instead of /etc/exports, the configuration file is /etc/
dfs/dfstab. Entries are of the form of a share command, where the syntax for
the example in Section 3 would look like
share -o rw=slave1,slave2 -d "Master Usr" /usr                               
and instead of running exportfs after editing, you run shareall.

Solaris servers are especially sensitive to packet size. If you are using a
Linux client with a Solaris server, be sure to set rsize and wsize to 32768
at mount time.

Finally, there is an issue with root squashing on Solaris: root gets mapped
to the user noone, which is not the same as the user nobody. If you are
having trouble with file permissions as root on the client machine, be sure
to check that the mapping works as you expect.

8.6.2. Solaris Clients

Solaris clients will regularly produce the following message:
|svc: unknown program 100227 (me 100003)                                    |
|                                                                           |

This happens because Solaris clients, when they mount, try to obtain ACL
information - which Linux obviously does not have. The messages can safely be

There are two known issues with diskless Solaris clients: First, a kernel
version of at least 2.2.19 is needed to get /dev/null to export correctly.
Second, the packet size may need to be set extremely small (i.e., 1024) on
diskless sparc clients because the clients do not know how to assemble
packets in reverse order. This can be done from /etc/bootparams on the

8.7. SunOS

SunOS only has NFS Version 2 over UDP.

8.7.1. SunOS Servers

On the server end, SunOS uses the most traditional format for its /etc/
exports file. The example in Section 3 would look like:

Again, the root option is listed for informational purposes and is not
recommended unless necessary.

8.7.2. SunOS Clients

Be advised that SunOS makes all NFS locking requests as daemon, and therefore
you will need to add the insecure_locks option to any volumes you export to a
SunOS machine. See the exports man page for details.

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