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+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>
+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
+ 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
+ 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 <gero@gkminix.han.de>.
+ The rest of the IP layer autoconfiguration code has been written
+ by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
+ In order to write the initial version of nfsroot I would like to thank
+ Jens-Uwe Mager <jum@anubis.han.de> for his help.