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NFS-Root mini-HOWTO
not maintained
V9, 20 September 2002
This mini-HOWTO tries explains how to set up a ``diskless'' Linux
workstation, which mounts its root filesystems via NFS. The newest
version of this mini-HOWTO can always be found at
http://www.tldp.org/HOWTO/mini/NFS-Root.html
<http://www.tldp.org/HOWTO/mini/NFS-Root.html> or a Linux Documenta­
tion Project mirror NEAR YOU.
______________________________________________________________________
Table of Contents
1. Copyright
1.1 Contributors
2. General Overview
3. Setup on the server
3.1 Compiling the kernels
3.2 Creation of the root filesystem
3.2.1 Copying the filesystem
3.2.2 Changes to the root filesystem
3.2.3 Exporting the filesystem
3.2.4 RARP setup
3.2.5 BOOTP setup
3.2.6 DHCP setup
3.2.7 Finding out hardware addresses
4. Booting the workstation
4.1 Using a boot ROM
4.2 Using a raw kernel disk
4.3 Using a bootloader &
4.4 Using a bootloader without
4.5 Sample kernel command lines
5. Known problems
5.1 /sbin/init doesn't start.
5.2 /dev troubles.
6. Other resources
______________________________________________________________________
1. Copyright
(c) 1996 Andreas Kostyrka (e9207884@student.tuwien.ac.at or
andreas@ag.or.at)
Unless otherwise stated, Linux HOWTO documents are copyrighted by
their respective authors. Linux HOWTO documents may be reproduced and
distributed in whole or in part, in any medium physical or electronic,
as long as this copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the author would
like to be notified of any such distributions.
All translations, derivative works, or aggregate works incorporating
any Linux HOWTO documents must be covered under this copyright notice.
That is, you may not produce a derivative work from a HOWTO and impose
additional restrictions on its distribution. Exceptions to these rules
may be granted under certain conditions; please contact the Linux
HOWTO coordinator at the address given below.
In short, we wish to promote dissemination of this information through
as many channels as possible. However, we do wish to retain copyright
on the HOWTO documents, and would like to be notified of any plans to
redistribute the HOWTOs.
If you have questions, please contact Andreas Kostyrka
<mailto:andreas@ag.or.at>, the author of this mini-HOWTO, or the Linux
HOWTO coordinator, at <mailto:linux-howto@sunsite.unc.edu> via email.
1.1. Contributors
· Avery Pennarun <apenwarr @ foxnet.net> (how to boot without LILO)
· Ofer Maor <ofer @ hadar.co.il> (a better mini-HOWTO about setting
up diskless workstations)
· Christian Leutloff <leutloff @ sundancer.tng.oche.de> (info about
netboot)
· Greg Roelofs <newt @ pobox.com> (2.2/2.4 updates, DHCP info, NFS-
export info)
2. General Overview
An NFS-mounted root filesystem is typically most useful in two
situations:
· A system administrator may wish to aggregate storage for multiple
workstations in order to simplify maintenance, improve security and
reliability, and/or make more economical use of limited storage
capacity. In this scenario, a single, large server may host a
dozen or more workstations; all of the systems can be regularly
backed up from a central location, and individual clients are less
prone to damage by unsophisticated users or attack by malicious
parties with physical access. (Of course, if the server itself is
compromised, then so are all of the clients.)
· An embedded system may not have a disk, an IDE interface, or even a
PCI bus. Even if it does, during development it may be too
unstable to use the disk, and a ramdisk may be too small to include
all of the necessary utilities or too large (as a part of the
kernel image) to allow for rapid turnaround during testing and
development. An NFS root allows quick kernel downloads, helps
ensure filesystem integrity (since the server is basically
impervious to crashes by the client), and provides virtually
infinite storage.
(In this document we'll use the terms client and workstation
interchangeably.)
However, there are two small problems from the client's perspective:
· It must find out its own IP address and possibly also the rest of
the ethernet configuration (gateway, netmask, name servers, etc.).
· It must know or discover both the IP address of the NFS server and
the mount path (on the server) to the exported root filesystem.
The current implementation of NFSROOT in the Linux kernel (as of
2.4.x) allows for several approaches, including:
· The complete ethernet configuration, including the NFS-path to be
mounted, may be passed as parameters to the kernel via LILO,
LOADLIN, or a hard-coded string within
linux/arch/i386/kernel/setup.c (or its equivalent for other
architectures).
· The IP address may be discovered by RARP and the NFS-path passed
via kernel parameters.
· The IP address may be discovered by RARP, with the NFS-path derived
from the RARP server and the just-granted IP address (loosely
speaking, ``mount -t nfs <RARP-server>:/tftpboot/<IP-address-of-
client>/dev/nfs'').
· The client configuration may be discovered by BOOTP.
· The client configuration may be discovered by DHCP.
Since the most common dynamic-address protocol these days is DHCP, its
addition as an option in kernels 2.2.19 and 2.4.x (3 < x <= 14) is
particularly welcome.
Before starting to set up a diskless environment, you should decide if
you will be booting via LILO, LOADLIN, or a custom, embedded
bootloader. The advantage of using something like LILO is flexibility;
the disadvantage is speed--booting a Linux kernel without LILO is
faster. This may or may not be a consideration.
3. Setup on the server
3.1. Compiling the kernels
On the server side, if you don't plan to use the old, user-mode NFS
daemon, you'll need to compile NFS server support into the kernel
(``NFS server support,'' a.k.a. knfsd or CONFIG_NFSD). If you plan to
use the older RARP protocol to assign the client an IP address, RARP
support in the kernel of the server is probably a good idea. (You must
have it if you will boot via RARP without kernel parameters.) On the
other hand, it doesn't help you if the client isn't on the same subnet
as the server.
The kernel for the workstation needs the following settings, as a
minimum:
· NFS filesystem support (CONFIG_NFS_FS). Note that there is no need
for ext2 support.
· Root file system on NFS (CONFIG_ROOT_NFS).
· Ethernet (10 or 100Mbit) (CONFIG_NET_ETHERNET).
· The ethernet driver for the workstation's network card (or onboard
ethernet chip, if it's built into the motherboard or chipset).
Where there is an option to compile something in as a module, do
not do so; modules only work after the kernel is booted, and these
things are needed during boot.
For dynamically assigned IP numbers, you'll also need to select one or
more of these kernel options:
· IP: kernel level autoconfiguration (CONFIG_IP_PNP)
· RARP support (CONFIG_IP_PNP_RARP)
· BOOTP support (CONFIG_IP_PNP_BOOTP)
· DHCP support (CONFIG_IP_PNP_DHCP)
If the workstation will be booted without kernel parameters, you need
also to set the root device to 0:255. Do this by creating a dummy
device file with mknod /dev/nfsroot b 0 255. After having created such
a device file, you can set root device of the kernel image with rdev
<kernel-image> /dev/nfsroot. [NOTE: Modern kernels recognize
root=/dev/nfs as a command-line argument; for consistency and/or
compatibility, it may be better to use /dev/nfs as the device name
instead of /dev/nfsroot.]
3.2. Creation of the root filesystem
3.2.1. Copying the filesystem
Warning: while these instruction might work for you, they are by no
means sensefull in a production environment. For a better way to set
up a root filesystem for the clients, see the NFS-Root-Client mini-
HOWTO by Ofer Maor <ofer@hadar.co.il>.
After having decided where to place the root tree, create it with
(e.g.) mkdir -p <directory> and tar cClf / - | tar xpCf <directory> -.
If you boot your kernel without LILO, then the rootdir has to be
/tftpboot/<IP-address>. If you don't like it, you can change it in the
top Makefile in the kernel sources, look for a line like: NFS_ROOT =
-DNFS_ROOT="\"/tftpboot/%s\"" If you change this, you have to
recompile the kernel.
3.2.2. Changes to the root filesystem
Now trim the unneeded files, and check the /etc/rc.d scripts. Some
important points:
· One important thing is eth0 setup. The workstation comes up with
eth0 set up, at least partially. Setting the IP address of the
workstation to the the IP address of the server is not considered a
clever thing to do. (As it happened to the original author on one
of his early attempts.)
· Another point is the /etc/fstab of the workstation. It should be
set up for NFS filesystems. <NOTE: this is not true in 2.4
kernels; the NFS mount is implicit and may actually cause mount(1)
error messages if it's explicitly listed in /etc/fstab. It is not
clear when this changed.>
· WARNING: Don't confuse the server root filesystem and the
workstation root filesystem. (I've already patched up a rc.inet1 on
the server, and wondered why the workstation still didn't work.)
3.2.3. Exporting the filesystem
Export the root dir to the workstation. The basic idea is to edit
/etc/exports to include a line similar to one of the following:
· /path/on/server/to/nfs_root <client-IP-
number>(rw,no_root_squash,no_all_squash) <2nd-client-IP-
number>(rw,no_root_squash,no_all_squash)
· /path/on/server/to/nfs_root <client-IP-network>/<client-IP-
netmask>(rw,no_root_squash,no_all_squash)
For example, a DHCP client receiving an IP address on a class C subnet
would need an exports entry similar to this:
· /path/on/server/to/nfs_root
192.168.263.0/255.255.255.0(rw,no_root_squash,no_all_squash)
The no_root_squash parameter allows the superuser (root) to be treated
as such by the NFS server; otherwise root will be remapped to nobody
and will generally be unable to do anything useful with the
filesystem. The no_all_squash parameter is similar but applies to
non-root users. See the exports(5) man page for details.
You will have to notify the NFS server after making any changes to the
exports file. Under Red Hat this can easily be done by typing
/etc/rc.d/init.d/nfs stop; /etc/rc.d/init.d/nfs start. On other
systems, a simple /etc/rc.d/init.d/nfs restart or even exportfs -a may
suffice, while on older machines running the user-mode NFS daemon you
may actually need to killall -HUP rpc.mountd; killall -HUP rpc.nfsd.
(Do not killall -HUP rpc.portmap, however!)
You may also need to edit /etc/hosts.allow and/or /etc/hosts.deny if
tcp_wrappers are installed. In particular, if the remote system
(client) gets RPC: connection refused errors, /etc/hosts.deny probably
contains portmap: ALL or ALL: ALL. To enable the client to use the
server's portmapper, add a corresponding line to /etc/hosts.allow:
portmap: <client-IP-number>
portmap: <2nd-client-IP-number>
portmap: <client-IP-network>/<client-IP-netmask>
There is no need to restart anything in this case. You can check by
running rpcinfo -p on the NFS server and rpcinfo -p NFS-server on a
Linux client within the allowed range; the RPC services listed by both
should match.
In case of problems, check /var/log/messages and /var/log/syslog for
errors (for example, run tail -f /var/log/messages /var/log/syslog and
then try booting the client), and check your man pages (exports,
exportfs, portmap, etc.). As a last resort, a reboot of the NFS
server may help, but that's a borderline Microsoftism...
3.2.4. RARP setup
Set up the RARP somewhere on the net. If you boot without a nfsroot
parameter, the RARP server has to be the NFS server. Usually this will
be the NFS server. To do this, you will need to run a kernel with RARP
support.
To do this, execute (and install it somewhere in /etc/rc.d of the
server!):
/sbin/rarp -s <ip-addr> <hardware-addr>
where
ip-addr
is the IP address of the workstation, and
hardware-addr
is the ethernet address of the network card of the workstation.
example: /sbin/rarp -s 131.131.90.200 00:00:c0:47:10:12
You can also use a symbolic name instead of the IP address, as long
the server is able to find out the IP address. (/etc/hosts or DNS
lookups)
3.2.5. BOOTP setup
For BOOTP setup you need to edit /etc/bootptab. Please consult the
bootpd(8) and bootptab(5) man pages.
3.2.6. DHCP setup
There is no need for the DHCP server to be the same as the NFS server,
and in most cases, a DHCP server will already be set up. If one is
not, however, consult the DHCP mini-HOWTO for further help.
3.2.7. Finding out hardware addresses
I don't know the hardware address! How can I find it out?
· Boot the kernel disk you made, and watch for the line where the
network card is recognized. It usually contains 6 hex bytes, that
should be the address of the card.
· Boot the workstation with some OS with TCP/IP networking enabled.
Then ping the workstation from the server. Look in the ARP-cache by
executing: /sbin/arp -a
4. Booting the workstation
4.1. Using a boot ROM
As I have not used such a beast myself yet, I can give you only the
following tips (courtesy of Christian Leutloff
<leutloff@sundancer.tng.oche.de>):
· You can't use ``normal'' boot ROMs.
· There is a netboot packet by Gero Kuhlmann, that provides for boot
ROMs for Linux, and further information. netboot is available from
the local Linux mirror, or as a Debian package (netboot-0.4).
· Read the documentation coming with your boot ROM carefully.
· You probably will have to enable the tftpd on the server, but this
depends upon your boot ROM's way of loading the kernel.
· Any information on boot-ROM vendors of these Linux variety,
mentioned above, as not everybody has access to PROM burner :(
(especially in Europe, as I'm located there.) welcome, I'll include
them then here.
4.2. Using a raw kernel disk
If you have exported the root filesystem with the correct name for the
default naming and your NFS server is also the RARP server (which
implies that the boxes are on the same subnet.), than you can just
boot the kernel by cating it to a disk. (You have to set the root
device in the kernel to 0:255.) This assumes, that the root directory
on the server is /tftpboot/IP Address (this value can be changed when
compiling the kernel.)
4.3. Using a bootloader & RARP
Give the kernel all needed parameters when booting, and add
nfsroot=<server-ip-addr>:</path/to/mount> where server-ip-addr is the
IP address of your NFS-server, and /path/to/mount is the path to the
root directory.
Tips:
· When using LILO consider using the ``lock'' feature: Simply type in
once all the correct parameters and add ``lock''. Next time when
booting let LILO timeout.
· When generating a workstation specific boot disk, you can also use
the append= feature in lilo.conf.
4.4. Using a bootloader without RARP
The ip and nfsroot kernel parameters (which can be hardcoded into the
kernel, interactively entered at some bootloader prompts, or included
in lilo.conf via the append= parameter; see the next subsection)
provide all of the information necessary for the client to set up its
ethernet interface and to contact the NFS server, respectively. The
parameters are fully documented in Documentation/nfsroot.txt, which is
included in the kernel sources (usually found under /usr/src/linux).
Here's the format for a machine with a static (pre-assigned) IP
address:
· nfsroot=<NFS-server-IP-number>:/path/on/server/to/nfs_root
ip=<client-IP-number>::<gateway-IP-number>:<netmask>:<client-
hostname>:eth0:off
DHCP is much simpler:
· nfsroot=<NFS-server-IP-number>:/path/on/server/to/nfs_root ip=dhcp
4.5. Sample kernel command lines
Here's an example of a complete kernel command line such as you might
include in lilo.conf or equivalent; only the IP numbers and NFS path
are bogus:
· root=/dev/nfs rw nfsroot=12.345.67.89:/path/on/server/to/nfs_root
ip=dhcp console=ttyS1
That uses DHCP to assign an IP address to the machine and puts its
boot messages (console) on the second serial port. The following is
the corresponding example using a static IP address; it also
explicitly specifies Busybox's (non-standard) location for init:
· root=/dev/nfs rw nfsroot=12.345.67.89:/path/on/server/to/nfs_root
ip=12.345.67.88::12.345.67.1:255.255.255.0:embed-o-matic:eth0:off
console=ttyS1 init=/bin/init
5. Known problems
5.1. /sbin/init doesn't start.
A common problem with /sbin/init is that some distributions (e.g., Red
Hat Linux) come with /sbin/init dynamically linked. So you have to
provide a correct /lib setup to the client. An easy thing one could
try is replacing /sbin/init (for the client) with a statically linked
``Hello World'' program. This way you know if it is something more
basic, or ``just'' a problem with dynamic linking.
Also note that Busybox by default installs its init symlink in /bin
rather than /sbin. You may need to move it or pass an explicit init=
parameter on the kernel command line, as shown in the final example of
the previous section.
5.2. /dev troubles.
If you get some garbled messages about ttys when booting, then you
should run a MAKEDEV from the client in the /dev directory. There are
rumors that this doesn't work with certain server OSes that use 64-bit
device numbers; should you run into this, please consider updating
this section! A potential solution would be to create a small /dev
ram disk early in the boot process and reinstall the device nodes each
time, or simply embed directly into the kernel a suitably initialized
ramdisk.
6. Other resources
· In the Documentation directory of kernel source there is a file
documenting NFS-Root systems (Documentation/nfsroot.txt).
· There are quite a few related HOWTOs:
· Diskless-HOWTO (specifically, the Network Booting section)
· Diskless-root-NFS-HOWTO
· Diskless-root-NFS-other-HOWTO
· Network-boot-HOWTO
· PXE-Server-HOWTO ("Pre-boot eXecution Environment") < coming >
· There is a BOOTP client:
http://ibiblio.org/pub/Linux/system/network/admin/bootpc-0.64.tar.gz
<http://ibiblio.org/pub/Linux/system/network/admin/bootpc-0.64.tar.gz>
With initrd (which is included in Linux 2.0), it could be made to
work for diskless stations quite nicely. initrd is actually always
an advanced option for more customized setups.
· For plain bootpd-based boots this is actually probably not needed
as Linux 2.0 contains also the option to use BOOTP instead of RARP.
(To be more precise, you can compile both in the kernel, and the
faster response wins.)
· There is a patch floating around that allows for swapping over NFS.
It was sent to me (during a private high workload phase), but I
somehow managed to lose the mail.
You can probably get it from http://www.linuxhq.com/
<http://www.linuxhq.com/> in the unofficial-patches section.
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