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