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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>
+Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
+Updated 2006 by Horms <horms@verge.net.au>
+
+
+
+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 Documentation/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.
+
+
+
+
+1.) Enabling nfsroot capabilities
+ -----------------------------
+
+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 (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
+
+ 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.
+ 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, 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.
+ The following defaults are used:
+ port = as given by server portmap daemon
+ rsize = 4096
+ wsize = 4096
+ 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>:
+ <dns0-ip>:<dns1-ip>
+
+ 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
+ autoconfiguration.
+
+ The <autoconf> parameter can appear alone as the value to the `ip'
+ parameter (without all the ':' characters before). If the value is
+ "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
+ autoconfiguration will take place. The most common way to use this
+ is "ip=dhcp".
+
+ <client-ip> IP address of the client.
+
+ 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.
+
+ Only required 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.
+
+ Default: Determined using autoconfiguration.
+
+ <hostname> Name of the client. May be supplied by autoconfiguration,
+ but its absence will not trigger autoconfiguration.
+ If specified and DHCP is used, the user provided hostname will
+ be carried in the DHCP request to hopefully update DNS record.
+
+ Default: Client IP address is used in ASCII notation.
+
+ <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
+ (do static IP assignment instead)
+ on or any: use any protocol available in the kernel
+ (default)
+ dhcp: use DHCP
+ bootp: use BOOTP
+ rarp: use RARP
+ both: use both BOOTP and RARP but not DHCP
+ (old option kept for backwards compatibility)
+
+ Default: any
+
+ <dns0-ip> IP address of first nameserver.
+ Value gets exported by /proc/net/pnp which is often linked
+ on embedded systems by /etc/resolv.conf.
+
+ <dns1-ip> IP address of secound nameserver.
+ Same as above.
+
+
+nfsrootdebug
+
+ This parameter enables debugging messages to appear in the kernel
+ log at boot time so that administrators can verify that the correct
+ NFS mount options, server address, and root path are passed to the
+ NFS client.
+
+
+rdinit=<executable file>
+
+ To specify which file contains the program that starts system
+ initialization, administrators can use this command line parameter.
+ The default value of this parameter is "/init". If the specified
+ file exists and the kernel can execute it, root filesystem related
+ kernel command line parameters, including `nfsroot=', are ignored.
+
+ A description of the process of mounting the root file system can be
+ found in:
+
+ Documentation/early-userspace/README
+
+
+
+
+3.) Boot Loader
+ ----------
+
+To get the kernel into memory different approaches can be used.
+They depend on various facilities being available:
+
+
+3.1) Booting from a floppy using syslinux
+
+ 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 zimage
+ and bzimage images respectively. Both targets accept the
+ FDARGS parameter which can be used to set the kernel command line.
+
+ 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/x86/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 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 using GRUB, kernel parameter are simply appended after the kernel
+ specification: kernel <kernel> <parameters>
+
+3.4) Using loadlin
+ 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 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
+ 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.
+ 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/
+
+
+
+
+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.