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[PATCH] nfs: Update Documentation/nfsroot.txt to include dhcp, syslin…
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…ux and isolinux

* Document the ip command a little differently to make the
  interaction between defaults and autoconfiguration a little clearer
  (I hope)

* Update autoconfiguration the current set of options, including DHCP

* Update the boot methods to add syslinux and isolinux, and remove
  dd of=/dev/fd0 which is no longer supported by linux

* Add a referance to initramfs along side initrd.
  Should the latter and its document be removed some time soon?

* Various cleanups to put the text consistently into the thrid person

* Reformated a bit to fit into 80 columns a bit more nicely

* Should the bootloaders documentation be removed or split
  into a separate documentation, it seems somewhat out of scope

Signed-off-by: Horms <horms@verge.net.au>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Horms authored and Linus Torvalds committed Jul 10, 2006
1 parent 73ce593 commit 64552a5
Showing 1 changed file with 160 additions and 115 deletions.
275 changes: 160 additions & 115 deletions Documentation/nfsroot.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -4,15 +4,16 @@ Mounting the root filesystem via NFS (nfsroot)
Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
Updated 2006 by Horms <horms@verge.net.au>



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
In order to use a diskless system, such as an X-terminal or printer server
for example, it is necessary for the root filesystem to be present on a
non-disk device. This may be an initramfs (see Documentation/filesystems/
ramfs-rootfs-initramfs.txt), a ramdisk (see Documenation/initrd.txt) 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.


Expand All @@ -21,23 +22,24 @@ 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.
In order to use nfsroot, NFS client support needs to be selected as
built-in during configuration. Once this has been selected, the nfsroot
option will become available, which should also be selected.

In the networking options, kernel level autoconfiguration can be selected,
along with the types of autoconfiguration to support. Selecting all of
DHCP, 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:
When the kernel has been loaded by a boot loader (see below) it needs to be
told what root fs device to use. And in the case of nfsroot, where to find
both the server and the name of the directory on the server to mount as root.
This can be established using the following kernel command line parameters:


root=/dev/nfs
Expand All @@ -49,23 +51,21 @@ root=/dev/nfs

nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]

If the `nfsroot' parameter is NOT given on the command line, the default
"/tftpboot/%s" will be used.
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.
<server-ip> Specifies the IP address of the NFS server.
The default address is determined by the `ip' parameter
(see below). This parameter allows the use of different
servers for IP autoconfiguration and NFS.

<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
address.
<root-dir> Name of the directory on the server to mount as root.
If there is a "%s" token in the string, it will be
replaced by the ASCII-representation of the client's
IP address.

<nfs-options> Standard NFS options. All options are separated by commas.
If the options field is not given, the following defaults
will be used:
The following defaults are used:
port = as given by server portmap daemon
rsize = 1024
wsize = 1024
Expand All @@ -81,129 +81,174 @@ nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>

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
reasons.
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 reasons.

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).
this means that the kernel tries to configure everything using
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.

<client-ip> IP address of the client.

<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.
Default: Determined using autoconfiguration.

<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,
replies from the specified server are accepted.

Only required for for NFS root. That is autoconfiguration
will not be triggered if it is missing and NFS root is not
in operation.

Default: Determined using autoconfiguration.
The address of the autoconfiguration server is used.

<gw-ip> IP address of a gateway if the server is on a different subnet.

Default: Determined using autoconfiguration.

<netmask> Netmask for local network interface. If unspecified
the netmask is derived from the client IP address assuming
classful addressing, unless overridden in BOOTP reply.
classful addressing.

<hostname> Name of the client. If empty, the client IP address is
used in ASCII notation, or the value received by BOOTP.
Default: Determined using autoconfiguration.

<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.
<hostname> Name of the client. May be supplied by autoconfiguration,
but its absence will not trigger autoconfiguration.

<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.
Default: Client IP address is used in ASCII notation.

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.
<device> Name of network device to use.

Default: If the host only has one device, it is used.
Otherwise the device is determined using
autoconfiguration. This is done by sending
autoconfiguration requests out of all devices,
and using the device that received the first reply.

<autoconf> Method to use for autoconfiguration. In the case of options
which specify multiple autoconfiguration protocols,
requests are sent using all protocols, and the first one
to reply is used.

Only autoconfiguration protocols that have been compiled
into the kernel will be used, regardless of the value of
this option.

off or none: don't use autoconfiguration (default)
on or any: use any protocol available in the kernel
dhcp: use DHCP
bootp: use BOOTP
rarp: use RARP
both: use both BOOTP and RARP but not DHCP
(old option kept for backwards compatibility)

3.) Kernel loader
-------------
Default: any

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
3.) Boot Loader
----------

Then copy the kernel zImage file onto a floppy using dd:
To get the kernel into memory different approaches can be used.
They depend on various facilities being available:

dd if=/usr/src/linux/arch/i386/boot/zImage of=/dev/fd0

And finally use rdev to set the root device:
3.1) Booting from a floppy using syslinux

rdev /dev/fd0 /dev/boot255
When building kernels, an easy way to create a boot floppy that uses
syslinux is to use the zdisk or bzdisk make targets which use
and bzimage images respectively. Both targets accept the
FDARGS parameter which can be used to set the kernel command line.

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.
e.g.
make bzdisk FDARGS="root=/dev/nfs"

Note that the user running this command will need to have
access to the floppy drive device, /dev/fd0

For more information on syslinux, including how to create bootdisks
for prebuilt kernels, see http://syslinux.zytor.com/

N.B: Previously it was possible to write a kernel directly to
a floppy using dd, configure the boot device using rdev, and
boot using the resulting floppy. Linux no longer supports this
method of booting.

3.2) Booting from a cdrom using isolinux

When building kernels, an easy way to create a bootable cdrom that
uses isolinux is to use the isoimage target which uses a bzimage
image. Like zdisk and bzdisk, this target accepts the FDARGS
parameter which can be used to set the kernel command line.

e.g.
make isoimage FDARGS="root=/dev/nfs"

The resulting iso image will be arch/<ARCH>/boot/image.iso
This can be written to a cdrom using a variety of tools including
cdrecord.

e.g.
cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso

For more information on isolinux, including how to create bootdisks
for prebuilt kernels, see http://syslinux.zytor.com/

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
documentation.
When using LILO all the necessary command line parameters may be
specified using the 'append=' directive in the LILO configuration
file.

However, to use the 'root=' directive you also need to create
a dummy root device, which may be removed after LILO is run.

mknod /dev/boot255 c 0 255

For information on configuring LILO, please refer to its documentation.

3.3) Using GRUB
When you use GRUB, you simply append the parameters after the kernel
specification: "kernel <kernel> <parameters>" (without the quotes).
When using GRUB, kernel parameter are simply appended after the kernel
specification: kernel <kernel> <parameters>

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.
loadlin may be used to boot Linux from a DOS command prompt without
requiring a local hard disk to mount as root. This has not been
thoroughly tested by the authors of this document, but in general
it should be possible configure the kernel command line similarly
to the configuration of LILO.

Please refer to the loadlin documentation 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 sunsite.unc.edu
and its mirrors. They are called 'netboot-nfs' and 'etherboot'.
Both contain everything you need to boot a diskless Linux client.
This is probably the most elegant way of booting a diskless client.
With a boot ROM the kernel is loaded using the TFTP protocol. The
authors of this document are not aware of any no commercial boot
ROMs that support booting Linux over the network. However, there
are two free implementations of a boot ROM, netboot-nfs and
etherboot, both of which are available on sunsite.unc.edu, and both
of which contain everything you need to boot a diskless Linux client.

3.6) Using pxelinux
Using pxelinux you specify the kernel you built with
Pxelinux may be used to boot linux using the PXE boot loader
which is present on many modern network cards.

When using pxelinux, the kernel image is specified using
"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.
It is common to use serial console in conjunction with pxeliunx,
see Documentation/serial-console.txt for more information.

For more information on isolinux, including how to create bootdisks
for prebuilt kernels, see http://syslinux.zytor.com/



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