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Mounting the root filesystem via NFS (nfsroot)

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Written 1996 by Gero Kuhlmann 

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=[:][,]

  If the nfsroot parameter is NOT give on the command line, the default

  "/tftpboot/%s" will be used.

        Specifies the IP address of the NFS server. If this field

                is not given, the default address as determined by the

                nfsaddrs 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.

        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.

        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

nfsaddrs=::::::

  If this parameter is missing on the kernel command line, all fields are

  assumed to be empty, and the below mentioned defaults 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).

        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  parameter. If this parameter is not

                empty, neither RARP nor BOOTP will be used.

        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.

        IP address of a gateway if the server in 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 for local network interface. If this is empty,

                the netmask is derived from the client IP address, un-

                less a value has been received by BOOTP.

        Name of the client. If empty, the client IP address is

                used in ASCII notation, or the value received by BOOTP.

        Name of network device to use. If this is empty, all

                devices are used for RARP requests, and the first one

                found for BOOTP. For NFS the device is used on which

                either RARP or BOOTP replies have been received. If

                you only have one device you can safely leave this blank.

        Method to use for autoconfiguration. If this is either

                'rarp' or 'bootp' the specified protocol is being used.

                If the value is 'both' or empty, both protocols are used

                so far as they have been enabled during kernel configura-

                tion. 'none' means no autoconfiguration. In this case you

                have to specify all necessary values in the fields before.

  The  parameter can appear alone as the value to the nfsaddrs

  parameter (without all the ':' characters before) in which case auto-

  configuration is used. However, the 'none' value is not available in

  that case.

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// 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 bootrom

        This is probably the most elegant way of booting a diskless

        client. With a bootrom the kernel gets loaded using the TFTP

        protocol. As far as I know no commercial bootroms already

        support booting Linux over the network, but there are two

        free implementations of a bootrom 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 has been written by me, Gero Kuhlmann

  , with the BOOTP code and a couple of bug fixes

  contributed by Martin Mares . In order to write

  the initial version of nfsroot I would like to thank Jens-Uwe Mager

   for his help.