nfsroot.txt

嵌入式系统设计与实验教材二源码linux内核移植与编译

Mounting the root filesystem via NFS (nfsroot)
===============================================

Written 1996 by Gero Kuhlmann <[e-mail]>
Updated 1997 by Martin Mares <[e-mail]>



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:


root=/dev/nfs

  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.


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.

  <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
		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:
			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


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.

  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
	documentation.

3.3) 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.

3.4) 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.




4.) Credits
    -------

  The nfsroot code in the kernel and the RARP support have been written
  by Gero Kuhlmann <[e-mail]>.

  The rest of the IP layer autoconfiguration code has been written
  by Martin Mares <[e-mail]>.

  In order to write the initial version of nfsroot I would like to thank
  Jens-Uwe Mager <[e-mail]> for his help.