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
or a Linux Documentation Project mirror NEAR YOU.
(c) 1996 Andreas Kostyrka (firstname.lastname@example.org or email@example.com)
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- Avery Pennarun
<apenwarr @ foxnet.net> (how to boot without LILO)
<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)
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
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
- 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
- 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
This may or may not be a consideration.
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
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 (
- Ethernet (10 or 100Mbit) (
- 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
For dynamically assigned IP numbers, you'll also need to select one or more
of these kernel options:
- IP: kernel level autoconfiguration (
- RARP support (
- BOOTP support (
- DHCP support (
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
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
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
After having decided where to place the root tree, create it with
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.
Changes to the root filesystem
Now trim the unneeded files, and check the /etc/rc.d scripts. Some
- 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
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:
For example, a DHCP client receiving an IP address on a class C subnet would
need an exports entry similar to this:
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.
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.
killall -HUP rpc.portmap, however!)
You may also need to edit
/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
To enable the client to use the server's portmapper, add a corresponding
There is no need to restart anything in this case. You can check by
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/syslog for errors (for example, run
/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
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
/sbin/rarp -s <ip-addr> <hardware-addr>
is the IP address of the workstation, and
is the ethernet address of the network card of
/sbin/rarp -s 22.214.171.124 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)
For BOOTP setup you need to edit
consult the bootpd(8) and bootptab(5) man pages.
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.
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:
As I have not used such a beast myself yet, I can give you only the
following tips (courtesy of Christian Leutloff
- 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.
available from the local Linux mirror, or as a Debian package
- 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.
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
(this value can be changed when compiling the kernel.)
Give the kernel all needed parameters when booting, and add
where server-ip-addr is the IP address of your NFS-server, and
/path/to/mount is the path to the root directory.
- 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
nfsroot kernel parameters (which can be hardcoded
into the kernel, interactively entered at some bootloader prompts, or
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
Documentation/nfsroot.txt, which is included in
the kernel sources (usually found under
the format for a machine with a static (pre-assigned) IP address:
DHCP is much simpler:
Here's an example of a complete kernel command line such as you might
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
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
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
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.
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.
- In the Documentation directory of kernel source there is a file
documenting NFS-Root systems (
- There are quite a few related HOWTOs:
- Diskless-HOWTO (specifically, the Network Booting section)
- PXE-Server-HOWTO ("Pre-boot eXecution Environment") < coming >
- There is a BOOTP client:
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/ in the