(c) 1996 Andreas Kostyrka (email@example.com or firstname.lastname@example.org)
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<apenwarr @ foxnet.net> (how to boot without LILO)
<ofer @ hadar.co.il> (a better mini-HOWTO about setting up diskless workstations)
<leutloff @ sundancer.tng.oche.de> (info about netboot)
<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:
However, there are two small problems from the client's perspective:
The current implementation of NFSROOT in the Linux kernel (as of 2.4.x) allows for several approaches, including:
linux/arch/i386/kernel/setup.c (or its equivalent for other
mount -t nfs
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.
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:
CONFIG_NFS_FS). Note that
there is no need for ext2 support.
For dynamically assigned IP numbers, you'll also need to select one or more of these kernel options:
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
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.
Now trim the unneeded files, and check the /etc/rc.d scripts. Some important points:
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 the workstation.
/sbin/rarp -s 188.8.131.52 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.
I don't know the hardware address! How can I find it out?
As I have not used such a beast myself yet, I can give you only the
following tips (courtesy of Christian Leutloff
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
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.
lock'' feature: Simply
type in once all the correct parameters and add
lock''. Next time when booting let LILO timeout.
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 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.
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.
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.
You can probably get it from http://www.linuxhq.com/ in the unofficial-patches section.