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Mounting the root filesystem via NFS (nfsroot)
Written 1996 by Gero Kuhlmann <>
Updated 1997 by Martin Mares <>
Updated 2006 by Nico Schottelius <>
If you want to use a diskless system, as an X-terminal or printer
server for example, you have to put your root filesystem onto a
non-disk device. This can either be a ramdisk (see initrd.txt in
this directory for further information) or a filesystem mounted
via NFS. The following text describes on how to use NFS for the
root filesystem. For the rest of this text 'client' means the
diskless system, and 'server' means the NFS server.
1.) Enabling nfsroot capabilities
In order to use nfsroot you have to select support for NFS during
kernel configuration. Note that NFS cannot be loaded as a module
in this case. The configuration script will then ask you whether
you want to use nfsroot, and if yes what kind of auto configuration
system you want to use. Selecting both BOOTP and RARP is safe.
2.) Kernel command line
When the kernel has been loaded by a boot loader (either by loadlin,
LILO or a network boot program) it has to be told what root fs device
to use, and where to find the server and the name of the directory
on the server to mount as root. This can be established by a couple
of kernel command line parameters:
This is necessary to enable the pseudo-NFS-device. Note that it's not a
real device but just a synonym to tell the kernel to use NFS instead of
a real device.
If the `nfsroot' parameter is NOT given on the command line, the default
"/tftpboot/%s" will be used.
<server-ip> Specifies the IP address of the NFS server. If this field
is not given, the default address as determined by the
`ip' variable (see below) is used. One use of this
parameter is for example to allow using different servers
for RARP and NFS. Usually you can leave this blank.
<root-dir> Name of the directory on the server to mount as root. If
there is a "%s" token in the string, the token will be
replaced by the ASCII-representation of the client's IP
<nfs-options> Standard NFS options. All options are separated by commas.
If the options field is not given, the following defaults
will be used:
port = as given by server portmap daemon
rsize = 1024
wsize = 1024
timeo = 7
retrans = 3
acregmin = 3
acregmax = 60
acdirmin = 30
acdirmax = 60
flags = hard, nointr, noposix, cto, ac
This parameter tells the kernel how to configure IP addresses of devices
and also how to set up the IP routing table. It was originally called `nfsaddrs',
but now the boot-time IP configuration works independently of NFS, so it
was renamed to `ip' and the old name remained as an alias for compatibility
If this parameter is missing from the kernel command line, all fields are
assumed to be empty, and the defaults mentioned below apply. In general
this means that the kernel tries to configure everything using both
RARP and BOOTP (depending on what has been enabled during kernel confi-
guration, and if both what protocol answer got in first).
<client-ip> IP address of the client. If empty, the address will either
be determined by RARP or BOOTP. What protocol is used de-
pends on what has been enabled during kernel configuration
and on the <autoconf> parameter. If this parameter is not
empty, neither RARP nor BOOTP will be used.
<server-ip> IP address of the NFS server. If RARP is used to determine
the client address and this parameter is NOT empty only
replies from the specified server are accepted. To use
different RARP and NFS server, specify your RARP server
here (or leave it blank), and specify your NFS server in
the `nfsroot' parameter (see above). If this entry is blank
the address of the server is used which answered the RARP
or BOOTP request.
<gw-ip> IP address of a gateway if the server is on a different
subnet. If this entry is empty no gateway is used and the
server is assumed to be on the local network, unless a
value has been received by BOOTP.
<netmask> Netmask for local network interface. If this is empty,
the netmask is derived from the client IP address assuming
classful addressing, unless overridden in BOOTP reply.
<hostname> Name of the client. If empty, the client IP address is
used in ASCII notation, or the value received by BOOTP.
<device> Name of network device to use. If this is empty, all
devices are used for RARP and BOOTP requests, and the
first one we receive a reply on is configured. If you have
only one device, you can safely leave this blank.
<autoconf> Method to use for autoconfiguration. If this is either
'rarp' or 'bootp', the specified protocol is used.
If the value is 'both' or empty, both protocols are used
so far as they have been enabled during kernel configura-
tion. 'off' means no autoconfiguration.
The <autoconf> parameter can appear alone as the value to the `ip'
parameter (without all the ':' characters before) in which case auto-
configuration is used.
3.) Kernel loader
To get the kernel into memory different approaches can be used. They
depend on what facilities are available:
3.1) Writing the kernel onto a floppy using dd:
As always you can just write the kernel onto a floppy using dd,
but then it's not possible to use kernel command lines at all.
To substitute the 'root=' parameter, create a dummy device on any
linux system with major number 0 and minor number 255 using mknod:
mknod /dev/boot255 c 0 255
Then copy the kernel zImage file onto a floppy using dd:
dd if=/usr/src/linux/arch/i386/boot/zImage of=/dev/fd0
And finally use rdev to set the root device:
rdev /dev/fd0 /dev/boot255
You can then remove the dummy device /dev/boot255 again. There
is no real device available for it.
The other two kernel command line parameters cannot be substi-
tuted with rdev. Therefore, using this method the kernel will
by default use RARP and/or BOOTP, and if it gets an answer via
RARP will mount the directory /tftpboot/<client-ip>/ as its
root. If it got a BOOTP answer the directory name in that answer
is used.
3.2) Using LILO
When using LILO you can specify all necessary command line
parameters with the 'append=' command in the LILO configuration
file. However, to use the 'root=' command you also need to
set up a dummy device as described in 3.1 above. For how to use
LILO and its 'append=' command please refer to the LILO
3.3) Using GRUB
When you use GRUB, you simply append the parameters after the kernel
specification: "kernel <kernel> <parameters>" (without the quotes).
3.4) Using loadlin
When you want to boot Linux from a DOS command prompt without
having a local hard disk to mount as root, you can use loadlin.
I was told that it works, but haven't used it myself yet. In
general you should be able to create a kernel command line simi-
lar to how LILO is doing it. Please refer to the loadlin docu-
mentation for further information.
3.5) Using a boot ROM
This is probably the most elegant way of booting a diskless
client. With a boot ROM the kernel gets loaded using the TFTP
protocol. As far as I know, no commercial boot ROMs yet
support booting Linux over the network, but there are two
free implementations of a boot ROM available on
and its mirrors. They are called 'netboot-nfs' and 'etherboot'.
Both contain everything you need to boot a diskless Linux client.
3.6) Using pxelinux
Using pxelinux you specify the kernel you built with
"kernel <relative-path-below /tftpboot>". The nfsroot parameters
are passed to the kernel by adding them to the "append" line.
You may perhaps also want to fine tune the console output,
see Documentation/serial-console.txt for serial console help.
4.) Credits
The nfsroot code in the kernel and the RARP support have been written
by Gero Kuhlmann <>.
The rest of the IP layer autoconfiguration code has been written
by Martin Mares <>.
In order to write the initial version of nfsroot I would like to thank
Jens-Uwe Mager <> for his help.