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   Note that "ip=off" is not the same thing as "ip=::::::off", because in
 101   the latter autoconfiguration will take place if any of DHCP, BOOTP or RARP
 102   are compiled in the kernel.
 103
 104   <client-ip>   IP address of the client.
 105
 106                 Default:  Determined using autoconfiguration.
 107
 108   <server-ip>   IP address of the NFS server. If RARP is used to determine
 109                 the client address and this parameter is NOT empty only
 110                 replies from the specified server are accepted.
 111
 112                 Only required for for NFS root. That is autoconfiguration
 113                 will not be triggered if it is missing and NFS root is not
 114                 in operation.
 115
 116                 Default: Determined using autoconfiguration.
 117                          The address of the autoconfiguration server is used.
 118
 119   <gw-ip>       IP address of a gateway if the server is on a different subnet.
 120
 121                 Default: Determined using autoconfiguration.
 122
 123   <netmask>     Netmask for local network interface. If unspecified
 124                 the netmask is derived from the client IP address assuming
 125                 classful addressing.
 126
 127                 Default:  Determined using autoconfiguration.
 128
 129   <hostname>    Name of the client. May be supplied by autoconfiguration,
 130                 but its absence will not trigger autoconfiguration.
 131
 132                 Default: Client IP address is used in ASCII notation.
 133
 134   <device>      Name of network device to use.
 135
 136                 Default: If the host only has one device, it is used.
 137                          Otherwise the device is determined using
 138                          autoconfiguration. This is done by sending
 139                          autoconfiguration requests out of all devices,
 140                          and using the device that received the first reply.
 141
 142   <autoconf>    Method to use for autoconfiguration. In the case of options
 143                 which specify multiple autoconfiguration protocols,
 144                 requests are sent using all protocols, and the first one
 145                 to reply is used.
 146
 147                 Only autoconfiguration protocols that have been compiled
 148                 into the kernel will be used, regardless of the value of
 149                 this option.
 150
 151                   off or none: don't use autoconfiguration
 152                   on or any:   use any protocol available in the kernel
 153                   dhcp:        use DHCP
 154                   bootp:       use BOOTP
 155                   rarp:        use RARP
 156                   both:        use both BOOTP and RARP but not DHCP
 157                                (old option kept for backwards compatibility)
 158
 159                 Default: any
 160
 161
 162
 163
 164 3.) Boot Loader
 165     ----------
 166
 167 To get the kernel into memory different approaches can be used.
 168 They depend on various facilities being available:
 169
 170
 171 3.1)  Booting from a floppy using syslinux
 172
 173         When building kernels, an easy way to create a boot floppy that uses
 174         syslinux is to use the zdisk or bzdisk make targets which use
 175         and bzimage images respectively. Both targets accept the
 176         FDARGS parameter which can be used to set the kernel command line.
 177
 178         e.g.
 179            make bzdisk FDARGS="root=/dev/nfs"
 180
 181         Note that the user running this command will need to have
 182         access to the floppy drive device, /dev/fd0
 183
 184         For more information on syslinux, including how to create bootdisks
 185         for prebuilt kernels, see http://syslinux.zytor.com/
 186
 187         N.B: Previously it was possible to write a kernel directly to
 188              a floppy using dd, configure the boot device using rdev, and
 189              boot using the resulting floppy. Linux no longer supports this
 190              method of booting.
 191
 192 3.2) Booting from a cdrom using isolinux
 193
 194         When building kernels, an easy way to create a bootable cdrom that
 195         uses isolinux is to use the isoimage target which uses a bzimage
 196         image. Like zdisk and bzdisk, this target accepts the FDARGS
 197         parameter which can be used to set the kernel command line.
 198
 199         e.g.
 200           make isoimage FDARGS="root=/dev/nfs"
 201
 202         The resulting iso image will be arch/<ARCH>/boot/image.iso
 203         This can be written to a cdrom using a variety of tools including
 204         cdrecord.
 205
 206         e.g.
 207           cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
 208
 209         For more information on isolinux, including how to create bootdisks
 210         for prebuilt kernels, see http://syslinux.zytor.com/
 211
 212 3.2) Using LILO
 213         When using LILO all the necessary command line parameters may be
 214         specified using the 'append=' directive in the LILO configuration
 215         file.
 216
 217         However, to use the 'root=' directive you also need to create
 218         a dummy root device, which may be removed after LILO is run.
 219
 220         mknod /dev/boot255 c 0 255
 221
 222         For information on configuring LILO, please refer to its documentation.
 223
 224 3.3) Using GRUB
 225         When using GRUB, kernel parameter are simply appended after the kernel
 226         specification: kernel <kernel> <parameters>
 227
 228 3.4) Using loadlin
 229         loadlin may be used to boot Linux from a DOS command prompt without
 230         requiring a local hard disk to mount as root. This has not been
 231         thoroughly tested by the authors of this document, but in general
 232         it should be possible configure the kernel command line similarly
 233         to the configuration of LILO.
 234
 235         Please refer to the loadlin documentation for further information.
 236
 237 3.5) Using a boot ROM
 238         This is probably the most elegant way of booting a diskless client.
 239         With a boot ROM the kernel is loaded using the TFTP protocol. The
 240         authors of this document are not aware of any no commercial boot
 241         ROMs that support booting Linux over the network. However, there
 242         are two free implementations of a boot ROM, netboot-nfs and
 243         etherboot, both of which are available on sunsite.unc.edu, and both
 244         of which contain everything you need to boot a diskless Linux client.
 245
 246 3.6) Using pxelinux
 247         Pxelinux may be used to boot linux using the PXE boot loader
 248         which is present on many modern network cards.
 249
 250         When using pxelinux, the kernel image is specified using
 251         "kernel <relative-path-below /tftpboot>". The nfsroot parameters
 252         are passed to the kernel by adding them to the "append" line.
 253         It is common to use serial console in conjunction with pxeliunx,
 254         see Documentation/serial-console.txt for more information.
 255
 256         For more information on isolinux, including how to create bootdisks
 257         for prebuilt kernels, see http://syslinux.zytor.com/
 258
 259
 260
 261
 262 4.) Credits
 263     -------
 264
 265   The nfsroot code in the kernel and the RARP support have been written
 266   by Gero Kuhlmann <gero@gkminix.han.de>.
 267
 268   The rest of the IP layer autoconfiguration code has been written
 269   by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
 270
 271   In order to write the initial version of nfsroot I would like to thank
 272   Jens-Uwe Mager <jum@anubis.han.de> for his help.