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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>

  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 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.

                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




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
        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/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 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.