linux/Documentation/filesystems/nfs/nfsroot.txt
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   1Mounting the root filesystem via NFS (nfsroot)
   2===============================================
   3
   4Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
   5Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
   6Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
   7Updated 2006 by Horms <horms@verge.net.au>
   8
   9
  10
  11In order to use a diskless system, such as an X-terminal or printer server
  12for example, it is necessary for the root filesystem to be present on a
  13non-disk device. This may be an initramfs (see Documentation/filesystems/
  14ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
  15filesystem mounted via NFS. The following text describes on how to use NFS
  16for the root filesystem. For the rest of this text 'client' means the
  17diskless system, and 'server' means the NFS server.
  18
  19
  20
  21
  221.) Enabling nfsroot capabilities
  23    -----------------------------
  24
  25In order to use nfsroot, NFS client support needs to be selected as
  26built-in during configuration. Once this has been selected, the nfsroot
  27option will become available, which should also be selected.
  28
  29In the networking options, kernel level autoconfiguration can be selected,
  30along with the types of autoconfiguration to support. Selecting all of
  31DHCP, BOOTP and RARP is safe.
  32
  33
  34
  35
  362.) Kernel command line
  37    -------------------
  38
  39When the kernel has been loaded by a boot loader (see below) it needs to be
  40told what root fs device to use. And in the case of nfsroot, where to find
  41both the server and the name of the directory on the server to mount as root.
  42This can be established using the following kernel command line parameters:
  43
  44
  45root=/dev/nfs
  46
  47  This is necessary to enable the pseudo-NFS-device. Note that it's not a
  48  real device but just a synonym to tell the kernel to use NFS instead of
  49  a real device.
  50
  51
  52nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
  53
  54  If the `nfsroot' parameter is NOT given on the command line,
  55  the default "/tftpboot/%s" will be used.
  56
  57  <server-ip>   Specifies the IP address of the NFS server.
  58                The default address is determined by the `ip' parameter
  59                (see below). This parameter allows the use of different
  60                servers for IP autoconfiguration and NFS.
  61
  62  <root-dir>    Name of the directory on the server to mount as root.
  63                If there is a "%s" token in the string, it will be
  64                replaced by the ASCII-representation of the client's
  65                IP address.
  66
  67  <nfs-options> Standard NFS options. All options are separated by commas.
  68                The following defaults are used:
  69                        port            = as given by server portmap daemon
  70                        rsize           = 4096
  71                        wsize           = 4096
  72                        timeo           = 7
  73                        retrans         = 3
  74                        acregmin        = 3
  75                        acregmax        = 60
  76                        acdirmin        = 30
  77                        acdirmax        = 60
  78                        flags           = hard, nointr, noposix, cto, ac
  79
  80
  81ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
  82
  83  This parameter tells the kernel how to configure IP addresses of devices
  84  and also how to set up the IP routing table. It was originally called
  85  `nfsaddrs', but now the boot-time IP configuration works independently of
  86  NFS, so it was renamed to `ip' and the old name remained as an alias for
  87  compatibility reasons.
  88
  89  If this parameter is missing from the kernel command line, all fields are
  90  assumed to be empty, and the defaults mentioned below apply. In general
  91  this means that the kernel tries to configure everything using
  92  autoconfiguration.
  93
  94  The <autoconf> parameter can appear alone as the value to the `ip'
  95  parameter (without all the ':' characters before).  If the value is
  96  "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
  97  autoconfiguration will take place.  The most common way to use this
  98  is "ip=dhcp".
  99
 100  <client-ip>   IP address of the client.
 101
 102                Default:  Determined using autoconfiguration.
 103
 104  <server-ip>   IP address of the NFS server. If RARP is used to determine
 105                the client address and this parameter is NOT empty only
 106                replies from the specified server are accepted.
 107
 108                Only required for NFS root. That is autoconfiguration
 109                will not be triggered if it is missing and NFS root is not
 110                in operation.
 111
 112                Default: Determined using autoconfiguration.
 113                         The address of the autoconfiguration server is used.
 114
 115  <gw-ip>       IP address of a gateway if the server is on a different subnet.
 116
 117                Default: Determined using autoconfiguration.
 118
 119  <netmask>     Netmask for local network interface. If unspecified
 120                the netmask is derived from the client IP address assuming
 121                classful addressing.
 122
 123                Default:  Determined using autoconfiguration.
 124
 125  <hostname>    Name of the client. May be supplied by autoconfiguration,
 126                but its absence will not trigger autoconfiguration.
 127
 128                Default: Client IP address is used in ASCII notation.
 129
 130  <device>      Name of network device to use.
 131
 132                Default: If the host only has one device, it is used.
 133                         Otherwise the device is determined using
 134                         autoconfiguration. This is done by sending
 135                         autoconfiguration requests out of all devices,
 136                         and using the device that received the first reply.
 137
 138  <autoconf>    Method to use for autoconfiguration. In the case of options
 139                which specify multiple autoconfiguration protocols,
 140                requests are sent using all protocols, and the first one
 141                to reply is used.
 142
 143                Only autoconfiguration protocols that have been compiled
 144                into the kernel will be used, regardless of the value of
 145                this option.
 146
 147                  off or none: don't use autoconfiguration
 148                                (do static IP assignment instead)
 149                  on or any:   use any protocol available in the kernel
 150                               (default)
 151                  dhcp:        use DHCP
 152                  bootp:       use BOOTP
 153                  rarp:        use RARP
 154                  both:        use both BOOTP and RARP but not DHCP
 155                               (old option kept for backwards compatibility)
 156
 157                Default: any
 158
 159
 160
 161
 1623.) Boot Loader
 163    ----------
 164
 165To get the kernel into memory different approaches can be used.
 166They depend on various facilities being available:
 167
 168
 1693.1)  Booting from a floppy using syslinux
 170
 171        When building kernels, an easy way to create a boot floppy that uses
 172        syslinux is to use the zdisk or bzdisk make targets which use zimage
 173        and bzimage images respectively. Both targets accept the
 174        FDARGS parameter which can be used to set the kernel command line.
 175
 176        e.g.
 177           make bzdisk FDARGS="root=/dev/nfs"
 178
 179        Note that the user running this command will need to have
 180        access to the floppy drive device, /dev/fd0
 181
 182        For more information on syslinux, including how to create bootdisks
 183        for prebuilt kernels, see http://syslinux.zytor.com/
 184
 185        N.B: Previously it was possible to write a kernel directly to
 186             a floppy using dd, configure the boot device using rdev, and
 187             boot using the resulting floppy. Linux no longer supports this
 188             method of booting.
 189
 1903.2) Booting from a cdrom using isolinux
 191
 192        When building kernels, an easy way to create a bootable cdrom that
 193        uses isolinux is to use the isoimage target which uses a bzimage
 194        image. Like zdisk and bzdisk, this target accepts the FDARGS
 195        parameter which can be used to set the kernel command line.
 196
 197        e.g.
 198          make isoimage FDARGS="root=/dev/nfs"
 199
 200        The resulting iso image will be arch/<ARCH>/boot/image.iso
 201        This can be written to a cdrom using a variety of tools including
 202        cdrecord.
 203
 204        e.g.
 205          cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
 206
 207        For more information on isolinux, including how to create bootdisks
 208        for prebuilt kernels, see http://syslinux.zytor.com/
 209
 2103.2) Using LILO
 211        When using LILO all the necessary command line parameters may be
 212        specified using the 'append=' directive in the LILO configuration
 213        file.
 214
 215        However, to use the 'root=' directive you also need to create
 216        a dummy root device, which may be removed after LILO is run.
 217
 218        mknod /dev/boot255 c 0 255
 219
 220        For information on configuring LILO, please refer to its documentation.
 221
 2223.3) Using GRUB
 223        When using GRUB, kernel parameter are simply appended after the kernel
 224        specification: kernel <kernel> <parameters>
 225
 2263.4) Using loadlin
 227        loadlin may be used to boot Linux from a DOS command prompt without
 228        requiring a local hard disk to mount as root. This has not been
 229        thoroughly tested by the authors of this document, but in general
 230        it should be possible configure the kernel command line similarly
 231        to the configuration of LILO.
 232
 233        Please refer to the loadlin documentation for further information.
 234
 2353.5) Using a boot ROM
 236        This is probably the most elegant way of booting a diskless client.
 237        With a boot ROM the kernel is loaded using the TFTP protocol. The
 238        authors of this document are not aware of any no commercial boot
 239        ROMs that support booting Linux over the network. However, there
 240        are two free implementations of a boot ROM, netboot-nfs and
 241        etherboot, both of which are available on sunsite.unc.edu, and both
 242        of which contain everything you need to boot a diskless Linux client.
 243
 2443.6) Using pxelinux
 245        Pxelinux may be used to boot linux using the PXE boot loader
 246        which is present on many modern network cards.
 247
 248        When using pxelinux, the kernel image is specified using
 249        "kernel <relative-path-below /tftpboot>". The nfsroot parameters
 250        are passed to the kernel by adding them to the "append" line.
 251        It is common to use serial console in conjunction with pxeliunx,
 252        see Documentation/serial-console.txt for more information.
 253
 254        For more information on isolinux, including how to create bootdisks
 255        for prebuilt kernels, see http://syslinux.zytor.com/
 256
 257
 258
 259
 2604.) Credits
 261    -------
 262
 263  The nfsroot code in the kernel and the RARP support have been written
 264  by Gero Kuhlmann <gero@gkminix.han.de>.
 265
 266  The rest of the IP layer autoconfiguration code has been written
 267  by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
 268
 269  In order to write the initial version of nfsroot I would like to thank
 270  Jens-Uwe Mager <jum@anubis.han.de> for his help.
 271
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