Documentation/nfsroot.txt

   1 Mounting the root filesystem via NFS (nfsroot)
   2 ===============================================
   3
   4 Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
   5 Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
   6 Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
   7 Updated 2006 by Horms <horms@verge.net.au>
   8
   9
  10
  11 In order to use a diskless system, such as an X-terminal or printer server
  12 for example, it is necessary for the root filesystem to be present on a
  13 non-disk device. This may be an initramfs (see Documentation/filesystems/
  14 ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
  15 filesystem mounted via NFS. The following text describes on how to use NFS
  16 for the root filesystem. For the rest of this text 'client' means the
  17 diskless system, and 'server' means the NFS server.
  18
  19
  20
  21
  22 1.) Enabling nfsroot capabilities
  23     -----------------------------
  24
  25 In order to use nfsroot, NFS client support needs to be selected as
  26 built-in during configuration. Once this has been selected, the nfsroot
  27 option will become available, which should also be selected.
  28
  29 In the networking options, kernel level autoconfiguration can be selected,
  30 along with the types of autoconfiguration to support. Selecting all of
  31 DHCP, BOOTP and RARP is safe.
  32
  33
  34
  35
  36 2.) Kernel command line
  37     -------------------
  38
  39 When the kernel has been loaded by a boot loader (see below) it needs to be
  40 told what root fs device to use. And in the case of nfsroot, where to find
  41 both the server and the name of the directory on the server to mount as root.
  42 This can be established using the following kernel command line parameters:
  43
  44
  45 root=/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
  52 nfsroot=[<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
  81 ip=<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 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
 162 3.) Boot Loader
 163     ----------
 164
 165 To get the kernel into memory different approaches can be used.
 166 They depend on various facilities being available:
 167
 168
 169 3.1)  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
 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
 190 3.2) 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
 210 3.2) 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
 222 3.3) Using GRUB
 223         When using GRUB, kernel parameter are simply appended after the kernel
 224         specification: kernel <kernel> <parameters>
 225
 226 3.4) 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
 235 3.5) 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
 244 3.6) 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
 260 4.) 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.