NFS HOWTO
Nicolai Langfeldt
[email protected]
v0.7, 3 November 1997
HOWTO set up NFS clients and servers.
1. Preamble
1.1. Legal stuff
(C)opyright 1997 Nicolai Langfeldt. Do not modify without amending
copyright, distribute freely but retain this paragraph. The FAQ
section is based on a NFS FAQ compiled by Alan Cox. The Checklist
section is based on a mount problem checklist compiled by the IBM
Corporation.
1.2. Other stuff
This will never be a finished document, please send me mail about your
problems and successes, it can make this a better HOWTO. Please send
money, comments and/or questions to
[email protected]. If you send E-
mail please make sure that the return address is correct and working,
I get a lot of E-mail and figuring out your e-mail address can be a
lot of work. Please.
If you want to translate this HOWTO please notify me so I can keep
track of what languages I have been published in :-).
Curses and Thanks to Olaf Kirch who got me to write this and then gave
good suggestions for it :-)
This HOWTO covers NFS in the 2.0 versions of the kernel. There are
significant enhancements, and changes, of NFS in the 2.1 versions of
the kernel.
1.3. Dedication
This HOWTO is dedicated to Anne Line Norheim Langfeldt. Though she
will probably never read it since she's not that kind of girl.
2. README.first
NFS, the Network File System has three important characteristics:
� It makes sharing of files over a network possible.
� It mostly works well enough.
� It opens a can of security risks that are well understood by
crackers, and easily exploited to get access (read, write and
delete) to all your files.
I'll say something on both issues in this HOWTO. Please make sure you
read the security section of this HOWTO, and you will be vulnerable to
fewer silly security risks. The passages about security will at times
be pretty technical and require some knowledge about IP networking and
the terms used. If you don't recognize the terms you can either go
back and check the networking HOWTO, wing it, or get a book about
TCP/IP network administration to familiarize yourself with TCP/IP.
That's a good idea anyway if you're administrating UNIX/Linux
machines. A very good book on the subject is TCP/IP Network
Administration by Craig Hunt, published by O'Reilly & Associates, Inc.
And after you've read it and understood it you'll have higher value on
the job market, you can't loose ;-)
There are two sections to help you troubleshoot NFS, called Mount
Checklist and FAQs. Please refer to them if something dosn't work as
advertized.
3. Setting up a NFS server
3.1. Prerequisites
Before you continue reading this HOWTO you will need to be able to
telnet back and forth between the machine you're using as server and
the client. If that does not work you need to check the
networking/NET-2 HOWTO and set up networking properly.
3.2. First step
Before we can do anything else we need a NFS server set up. If you're
part of a department or university network there are likely numerous
NFS servers already set up. If they will let you get access to them,
or indeed, if you're reading this HOWTO to get access to one of them
you obviously don't need to read this section and can just skip ahead
to the section on ``setting up a NFS client''
If you need to set up a non-Linux box as server you will have to read
the system manual(s) to discover how to enable NFS serving and export
of file systems through NFS. There is a separate section in this
HOWTO on how to do it on many different systems. After you have
figured all that out you can continue reading the next section of this
HOWTO. Or read more of this section since some of the things I will
say are relevant no matter what kind of machine you use as server.
Those of you still reading will need to set up a number of programs.
3.3. The portmapper
The portmapper on Linux is called either portmap or rpc.portmap. The
man page on my system says it is a "DARPA port to RPC program number
mapper". It is the first security holes you'll open reading this
HOWTO. Description of how to close one of the holes is in the
``security section''. Which I, again, urge you to read.
Start the portmapper. It's either called portmap or rpc.portmap and
it should live in the /usr/sbin directory (on some machines it's
called rpcbind). You can start it by hand now, but it will need to be
started every time you boot your machine so you need to make/edit the
rc scripts. Your rc scripts are explained more closely in the init
man page, they usually reside in /etc/rc.d, /etc/init.d or
/etc/rc.d/init.d. If there is a script called something like inet
it's probably the right script to edit. But, what to write or do is
outside the scope of this HOWTO. Start portmap, and check that it
lives by running ps aux. It does? Good.
3.4. Mountd and nfsd
The next programs we need running are mountd and nfsd. But first
we'll edit another file. /etc/exports this time. Say I want the file
system /mn/eris/local which lives on the machine eris to be available
to the machine called apollon. Then I'd put this in /etc/exports on
eris:
______________________________________________________________________
/mn/eris/local apollon(rw)
______________________________________________________________________
The above line gives apollon read/write access to /mn/eris/local.
Instead of rw it could say ro which means read only (if you put
nothing it defaults to read only). There are other options you can
give it, and I will discuss some security related ones later. They
are all enumerated in the exports man page which you should have read
at least once in your life. There are also better ways than listing
all the hosts in the exports file. You can for example use net groups
if you are running NIS (or NYS) (NIS was known as YP), and always
specify domain wild cards and IP-subnets as hosts that are allowed to
mount something. But you should consider who can get access to the
server in unauthorized ways if you use such blanket authorizations.
Note: This exports file is not the same syntax that other Unixes use.
There is a separate section in this HOWTO about other Unixes exports
files.
Now we're set to start mountd (or maybe it's called rpc.mountd and
then nfsd (which could be called rpc.nfsd). They will both read the
exports file.
If you edit /etc/exports you will have to make sure nfsd and mountd
knows that the files have changed. The traditonal way is to run
exportfs. Many Linux distributions lack a exportfs program. If
you're exportfs-less you can install this script on your machine:
______________________________________________________________________
#!/bin/sh
killall -HUP /usr/sbin/rpc.mountd
killall -HUP /usr/sbin/rpc.nfsd
echo re-exported file systems
______________________________________________________________________
Save it in, say, /usr/sbin/exportfs, and don't forget to chmod a+rx
it. Now, whenever you change your exports file, you run exportfs
after, as root.
Now you should check that mountd and nfsd are running properly. First
with rpcinfo -p. It should show something like this:
______________________________________________________________________
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100005 1 udp 745 mountd
100005 1 tcp 747 mountd
100003 2 udp 2049 nfs
100003 2 tcp 2049 nfs
______________________________________________________________________
As you see the portmapper has announced it's services, and so has
mountd and nfsd.
If you get rpcinfo: can't contact portmapper: RPC: Remote system error
- Connection refused or something similar instead then the portmapper
isn't running. Fix it. If you get No remote programs registered.
then either the portmapper doesn't want to talk to you, or something
is broken. Kill nfsd, mountd, and the portmapper and try the ignition
sequence again.
After checking that the portmapper reports the services you can check
with ps too. The portmapper will continue to report the services even
after the programs that extend them have crashed. So a ps check can
be smart if something seems broken.
Of course, you will need to modify your system rc files to start
mountd and nfsd as well as the portmapper when you boot. It is very
likely that the scripts already exist on your machine, you just have
to uncomment the critical section or activate it for the correct init
run levels.
Man pages you should be familiar with now: portmap, mountd, nfsd, and
exports.
Well, if you did everything exactly like I said you should you're all
set to start on the NFS client.
4. Setting up a NFS client
First you will need a kernel with the NFS file system either compiled
in or available as a module. This is configured before you compile
the kernel. If you have never compiled a kernel before you might need
to check the kernel HOWTO and figure it out. If you're using a very
cool distribution (like Red Hat) and you've never fiddled with the
kernel or modules on it (and thus ruined it ;-), nfs is likely
automagicaly available to you.
You can now, at a root prompt, enter a appropriate mount command and
the file system will appear. Continuing the example in the previous
section we want to mount /mn/eris/local from eris. This is done with
this command:
______________________________________________________________________
mount -o rsize=1024,wsize=1024 eris:/mn/eris/local /mnt
______________________________________________________________________
(We'll get back to the rsize and wsize options.) The file system is
now available under /mnt and you can cd there, and ls in it, and look
at the individual files. You will notice that it's not as fast as a
local file system, but a lot more convenient than ftp. If, instead of
mounting the file system, mount produces a error message like mount:
eris:/mn/eris/local failed, reason given by server: Permission denied
then the exports file is wrong, or you forgot to run exportfs after
editing the exports file. If it says mount clntudp_create: RPC:
Program not registered it means that nfsd or mountd is not running on
the server.
To get rid of the file system you can say
______________________________________________________________________
umount /mnt
______________________________________________________________________
To make the system mount a nfs file system upon boot you edit
/etc/fstab in the normal manner. For our example a line such as this
is required:
______________________________________________________________________
# device mountpoint fs-type options dump fsckorder
eris:/mn/eris/local /mnt nfs rsize=1024,wsize=1024 0 0
______________________________________________________________________
That's all there is too it, almost. Read on please.
4.1. Mount options
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:
soft
The NFS client will report and error to the process accessing a
file on a NFS mounted file system. Some programs can handle
this with composure, most won't. I cannot recommend using this
setting.
hard
The program accessing a file on a NFS mounted file system will
hang when the server crashes. The process cannot be interrupted
or killed unless you also specify intr. When the NFS server is
back online the program will continue undisturbed from where it
were. This is probably what you want. I recommend using
hard,intr on all NFS mounted file systems.
Picking up the previous example, this is now your fstab entry:
______________________________________________________________________
# device mountpoint fs-type options dump fsckorder
eris:/mn/eris/local /mnt nfs rsize=1024,wsize=1024,hard,intr 0 0
______________________________________________________________________
4.2. Optimizing NFS
Normally, if no rsize and wsize options are specified NFS will read
and write in chunks of 4096 or 8192 bytes. Some combinations of Linux
kernels and network cards cannot handle that large blocks, and it
might not be optimal, anyway. So we'll want to experiment and find a
rsize and wsize that works and is as fast as possible. You can test
the speed of your options with some simple commands. Given the mount
command above and that you have write access to the disk you can do
this to test the sequential write performance:
______________________________________________________________________
time dd if=/dev/zero of=/mnt/testfile bs=16k count=4096
______________________________________________________________________
This creates a 64Mb file of zeroed bytes (which should be large enough
that caching is no significant part of any performance perceived, use
a larger file if you have a lot of memory). Do it a couple (5-10?)
of times and average the times. It is the `elapsed' or `wall clock'
time that's most interesting in this connection. Then you can test
the read performance by reading back the file:
______________________________________________________________________
time dd if=/mnt/testfile of=/dev/null bs=16k
______________________________________________________________________
do that a couple of times and average. Then umount, and mount again
with a larger rsize and wsize. They should probably be multiples of
1024, and not larger than 16384 bytes since that's the maximum size in
NFS version 2. Directly after mounting with a larger size cd into the
mounted file system and do things like ls, explore the fs 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. SunOS and Solaris is reputedly a lot faster with 4096
byte blocks than with anything else.
Newer Linux kernels (since 1.3 sometime) perform read-ahead for rsizes
larger or equal to the machine page size. On Intel CPUs the page size
is 4096 bytes. Read ahead will significantly increase the NFS read
performance. So on a Intel machine you will want 4096 byte rsize if
at all possible.
Remember to edit /etc/fstab to reflect the rsize/wsize you found.
A trick to increase NFS write performance is to disable synchronous
writes on the server. The NFS specification states that NFS write
requests shall not be considered finished before the data written is
on a non-volatile medium (normally the disk). This restricts the
write performance somewhat, asynchronous writes will speed NFS writes
up. The Linux nfsd has never done synchronous writes since the Linux
file system implementation does not lend itself to this, but on non-
Linux servers you can increase the performance this way with this in
your exports file:
______________________________________________________________________
/dir -async,access=linuxbox
______________________________________________________________________
or something similar. Please refer to the exports man page on the
machine in question. Please note that this increases the risk of data
loss.
5. NFS over slow lines
Slow lines include Modems, ISDN and quite possibly other long distance
connections.
This section is based on knowledge about the used protocols but no
actual experiments. My home computer has been down for 6 months (bad
HD, low on cash) and so I have had no modem connection to test this
with. Please let me hear from you if try this :-)
The first thing to remember is that NFS is a slow protocol. It has
high overhead. Using NFS is almost like using kermit to transfer
files. It's slow. Almost anything is faster than NFS. FTP is
faster. HTTP is faster. rcp is faster. ssh is faster.
Still determined to try it out? Ok.
NFS' default parameters are for quite fast, low latency, lines. If
you use these default parameters over high latency lines it can cause
NFS to report errors, abort operations, pretend that files are shorter
than they really are, and act mysteriously in other ways.
The first thing to do is not to use the soft mount option. This will
cause timeouts to return errors to the software, which will, most
likely not handle the situation at all well. This is a good way to
get for mysterious failures. Instead use the hard mount option. When
hard is active timeouts causes infinite retries instead of aborting
whatever it was the software wanted to do. This is what you want.
Really.
The next thing to do is to tweak the timeo and retrans mount options.
They are described in the nfs(5) man page, but here is a copy:
______________________________________________________________________
timeo=n The value in tenths of a second before
sending the first retransmission after an
RPC timeout. The default value is 7 tenths
of a second. After the first timeout, the
timeout is doubled after each successive
timeout until a maximum timeout of 60 sec-
onds is reached or the enough retransmis-
sions have occured to cause a major time-
out. Then, if the filesystem is hard
mounted, each new timeout cascade restarts
at twice the initial value of the previous
cascade, again doubling at each retransmis-
sion. The maximum timeout is always 60
seconds. Better overall performance may be
achieved by increasing the timeout when
mounting on a busy network, to a slow
server, or through several routers or gate-
ways.
retrans=n The number of minor timeouts and retrans-
missions that must occur before a major
timeout occurs. The default is 3 timeouts.
When a major timeout occurs, the file oper-
ation is either aborted or a "server not
responding" message is printed on the con-
sole.
______________________________________________________________________
In other words: If a reply is not received within the 0.7 second
(700ms) timeout the NFS client will repeat the request and double the
timeout to 1.4 seconds. If the reply does not appear within the 1.4
seconds the request is repeated again and the timeout doubled again,
to 2.8 seconds.
A lines speed can be measured with ping with the same packet size as
your rsize/wsize options.
______________________________________________________________________
$ ping -s 8192 lugulbanda
PING lugulbanda.uio.no (129.240.222.99): 8192 data bytes
8200 bytes from 129.240.222.99: icmp_seq=0 ttl=64 time=15.2 ms
8200 bytes from 129.240.222.99: icmp_seq=1 ttl=64 time=15.9 ms
8200 bytes from 129.240.222.99: icmp_seq=2 ttl=64 time=14.9 ms
8200 bytes from 129.240.222.99: icmp_seq=3 ttl=64 time=14.9 ms
8200 bytes from 129.240.222.99: icmp_seq=4 ttl=64 time=15.0 ms
--- lugulbanda.uio.no ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 14.9/15.1/15.9 ms
______________________________________________________________________
The time here is how long the ping packet took to get back and forth
to lugulbanda. 15ms is quite fast. Over a 28.000 bps line you can
expect something like 4000-5000ms, and if the line is otherwise loaded
this time will be even higher, easily double. When this time is high
we say that there is 'high latency'. Generally, for larger packets
and for more loaded lines the latency will tend to increase. Increase
timeo suitably for your line and load. And since the latency
increases when you use the line for other things: If you ever want to
use FTP and NFS at the same time you should try measuring ping times
while using FTP to transfer files.
6. Security and NFS
I am by no means a computer security expert. But I do have a little
advice for the security conscious. But be warned: This is by no means
a complete list of NFS related problems and if you think you're safe
once you're read and implemented all this I have a bridge I want to
sell you.
This section is probably of no concern if you are on a closed network
where you trust all the users, and no-one you don't trust can get
access to machines on the network. I.e., there should be no way to
dial into the network, and it should in no way be connected to other
networks where you don't trust everyone using it as well as the
security. Do you think I sound paranoid? I'm not at all paranoid.
This is just basic security advice. And remember, the things I say
here is just the start of it. A secure site needs a diligent and
knowledgeable admin that knows where to find information about current
and potential security problems.
NFS has a basic problem in that the client, if not told otherwise,
will trust the NFS server and vice versa. This can be bad. It means
that if the server's root account is broken into it can be quite easy
to break into the client's root account as well. And vice versa.
There are a couple of coping strategies for this, which we'll get back
to.
Something you should read is the CERT advisories on NFS, most of the
text below deals with issues CERT has written advisories about. See
ftp.cert.org/01-README for a up to date list of CERT advisories. Here
are some NFS related advisories:
______________________________________________________________________
CA-91:21.SunOS.NFS.Jumbo.and.fsirand 12/06/91
Vulnerabilities concerning Sun Microsystems, Inc. (Sun) Network
File System (NFS) and the fsirand program. These vulnerabilities
affect SunOS versions 4.1.1, 4.1, and 4.0.3 on all architectures.
Patches are available for SunOS 4.1.1. An initial patch for SunOS
4.1 NFS is also available. Sun will be providing complete patches
for SunOS 4.1 and SunOS 4.0.3 at a later date.
CA-94:15.NFS.Vulnerabilities 12/19/94
This advisory describes security measures to guard against several
vulnerabilities in the Network File System (NFS). The advisory was
prompted by an increase in root compromises by intruders using tools
to exploit the vulnerabilities.
CA-96.08.pcnfsd 04/18/96
This advisory describes a vulnerability in the pcnfsd program (also
known as rpc.pcnfsd). A patch is included.
______________________________________________________________________
6.1. Client Security
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. This 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. We 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 needs to be executed.
You enter these options in the options column, with the rsize and
wsize, separated by commas.
6.2. Server security: nfsd
On the server we can decide that we don't want to trust the client's
root account. We can do that by using the root_squash option in
exports:
______________________________________________________________________
/mn/eris/local apollon(rw,root_squash)
______________________________________________________________________
Now, if a user with UID 0 on the client attempts to access (read,
write, delete) the file system the server substitutes the UID of the
servers `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 NFSd man page there are several other squash options
listed so that you can decide to mistrust whomever you (don't) like on
the clients. You also have options to squash any UID and GID range
you want to. This is described in the Linux NFSd man page.
root_squash is in fact the default with the Linux NFSd, to grant root
access to a filesystem use no_root_squash.
Another important thing is to ensure that nfsd checks that all it's
requests comes from a privileged port. If it accepts requests from
any old port on the client a user with no special privileges can run a
program that's is easy to obtain over the Internet. It talks nfs
protocol and will claim that the user is anyone the user wants to be.
Spooky. The Linux nfsd does this check by default, on other OSes you
have to enable this check yourself. This should be described in the
nfsd man page for the OS.
Another thing. Never export a file system to 'localhost' or
127.0.0.1. Trust me.
6.3. Server security: the portmapper
The basic portmapper, in combination with nfsd has a design problem
that makes it possible to get to files on NFS servers without any
privileges. Fortunately the portmapper Linux uses is relatively
secure against this attack, and can be made more secure by configuring
up access lists in two files.
First we edit /etc/hosts.deny. It should contain the line
______________________________________________________________________
portmap: ALL
______________________________________________________________________
which will deny access to everyone. That's a bit drastic perhaps, 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 administrates nfsd, mountd, ypbind/ypserv,
pcnfsd, and 'r' services like ruptime and rusers. Of these only nfsd,
mountd, ypbind/ypserv and perhaps pcnfsd are of any consequence. All
machines that needs to access services on your machine should be
allowed to do that. Let's say that your machines address is
129.240.223.254 and that it lives on the subnet 129.240.223.0 should
have access to it (those are terms introduced by the networking HOWTO,
go back and refresh your memory if you need to). Then we write
______________________________________________________________________
portmap: 129.240.223.0/255.255.255.0
______________________________________________________________________
in hosts.allow. This is the same as the network address you give to
route and the subnet mask you give to ifconfig. For the device eth0
on this machine ifconfig should show
______________________________________________________________________
eth0 Link encap:10Mbps Ethernet HWaddr 00:60:8C:96:D5:56
inet addr:129.240.223.254 Bcast:129.240.223.255 Mask:255.255.255.0
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
129.240.223.0 0.0.0.0 255.255.255.0 U 0 0 174412 eth0
______________________________________________________________________
(Network address in first column).
The hosts.deny and 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...
The above things should make your server tighter. The only remaining
problem (Yeah, right!) is someone breaking root (or boot MS-DOS) on a
trusted machine and using that privilege to send requests from a
secure port as any user they want to be.
6.4. NFS and firewalls
It's a very good idea to firewall the nfs and portmap ports in your
router or firewall. The nfsd operates at port 2049, both udp and tcp
protocols. The portmapper at port 111, tcp and udp, and mountd at
port 745 and and 747, tcp and udp. Normally. You should check the
ports with the rpcinfo -p command.
If on the other hand you want NFS to go through a firewall there are
options for newer NFSds and mountds to make them use a specific
(nonstandard) port which can be open in the firewall.
6.5. Summary
If you use the hosts.allow/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 /var/spool/mail
are mounted over NFS. 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. To keep
abreast of things like this you should at least read the newsgroups
comp.os.linux.announce and comp.security.announce at a absolute
minimum.
7. Mount Checklist
This section is based on IBM Corp. NFS mount problem checklist. My
thanks to them for making it available for this HOWTO. If you
experience a problem mounting a NFS filesystem please refer to this
list before posting your problem. Each item describes a failure mode
and the fix.
1. File system not exported, or not exported to the client in
question.
Fix: Export it
2. Name resolution doesn't jibe with the exports list.
e.g.: export list says export to johnmad but johnmad's name is
resolved as johnmad.austin.ibm.com. mount permission is denied.
Fix: Export to both forms of the name.
It can also happen if the client has 2 interfaces with different
names for each of the two adapters and the export only specifies
one.
Fix: export both interfaces.
This can also happen if the server can't do a lookuphostbyname or
lookuphostbyaddr (these are library functions) on the client. Make
sure the client can do host <name>; host <ip_addr>; and that both
shows the same machine.
Fix: straighten out name resolution.
3. The file system was mounted after NFS was started (on that server).
In that case the server is exporting underlying mount point, not
the mounted filesystem.
Fix: Shut down NFSd and then restart it.
Note: The clients that had the underlying mount point mounted will
get problems accessing it after the restart.
4. The date is wildly off on one or both machines (this can mess up
make)
Fix: Get the date set right.
The HOWTO author recommends using NTP to synchronize clocks. Since
there are export restrictions on NTP in the US you have to get NTP
for debian, redhat or slackware from
ftp://ftp.hacktic.nl/pub/replay/pub/linux or a mirror.
5. The server can not accept a mount from a user that is in more than
8 groups.
Fix: decrease the number of groups the user is in or mount via a
different user.
8. FAQs
This is the FAQ section. Most of it was written by Alan Cox.
1. I get a lot of 'stale nfs handle' errors when using Linux as a nfs
server.
This is caused by a bug in some oldish nfsd versions. It is fixed
in nfs-server2.2beta16 and later.
2. When I try to mount a file system I get
can't register with portmap: system error on send
You are probably using a Caldera system. There is a bug in the rc
scripts. Please contact Caldera to obtain a fix.
3. Why can't I execute a file after copying it to the NFS server?
The reason is that nfsd caches open file handles for performance
reasons (remember, it runs in user space). While nfsd has a file
open (as is the case after writing to it), the kernel won't allow
you to execute it. Nfsds newer than spring 95 release open files
after a few seconds, older ones would cling to them for days.
4. My NFS files are all read only
The Linux NFS server defaults to read only. RTFM the ``exports''
and nfsd manual pages. You will need to alter /etc/exports.
5. I mount from a linux nfs server and while ls works I can't read or
write files.
On older versions of Linux you must mount a NFS servers with
rsize=1024,wsize=1024.
6. I mount from a Linux NFS server with a block size of between
3500-4000 and it crashes the Linux box regularly
Basically don't do it then.
7. Can Linux do NFS over TCP
No, not at present.
8. I get loads of strange errors trying to mount a machine from a
Linux box.
Make sure your users are in 8 groups or less. Older servers require
this.
9. When I reboot my machine it sometimes hangs when trying to unmount
a hung NFS server.
Do not unmount NFS servers when rebooting or halting, just ignore
them, it will not hurt anything if you don't unmount them. The
command is umount -avt nonfs.
10.
Linux NFS clients are very slow when writing to Sun and BSD systems
NFS writes are normally synchronous (you can disable this if you
don't mind risking losing data). Worse still BSD derived kernels
tend to be unable to work in small blocks. Thus when you write 4K
of data from a Linux box in the 1K packets it uses BSD does this
read 4K page
alter 1K
write 4K back to physical disk
read 4K page
alter 1K
write 4K page back to physical disk
etc..
9. Exporting filesystems
The way to export filesytems with NFS is not completely consistent
across platforms of course. In this case Linux and Solaris 2 are the
deviants. This section lists, superficially the way to do it on most
systems. If the kind of system you have is not covered you must check
your OS man-pages. Keywords are: nfsd, system administration tool, rc
scripts, boot scripts, boot sequence, /etc/exports, exportfs. I'll
use one example throughout this section: How to export /mn/eris/local
to apollon read/write.
9.1. IRIX, HP-UX, Digital-UNIX, Ultrix, SunOS 4 (Solaris 1), AIX
These OSes use the traditional Sun export format. In /etc/exports
write:
______________________________________________________________________
/mn/eris/local -rw=apollon
______________________________________________________________________
The complete documentation is in the exports man page. After editing
the file run exportfs -av to export the filesystems.
How strict the exportfs command is about the syntax varies. On some
OSes you will find that previously entered lines reads:
______________________________________________________________________
/mn/eris/local apollon
______________________________________________________________________
or even something degenerate like:
______________________________________________________________________
/mn/eris/local rw=apollon
______________________________________________________________________
I recommend being formal. You risk that the next version of exportfs
if much stricter and then suddenly everything will stop working.
9.2. Solaris 2
Sun completely re-invented the wheel when they did Solaris 2. So this
is completely different from all other OSes. What you do is edit the
file /etc/dfs/dfstab. In it you place share commands as documented in
the share(1M) man page. Like this:
______________________________________________________________________
share -o rw=apollon -d "Eris Local" /mn/eris/local
______________________________________________________________________
After editing run the program shareall to export the filesystems.
10. PC-NFS
You should not run PC-NFS. You should run samba.
Sorry: I don't know anything about PC-NFS. If someone feels like
writing something about it please do and I'll include it here.
