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) in which case auto-
  96   configuration is used.
  97
  98   <client-ip>   IP address of the client.
  99
 100                 Default:  Determined using autoconfiguration.
 101
 102   <server-ip>   IP address of the NFS server. If RARP is used to determine
 103                 the client address and this parameter is NOT empty only
 104                 replies from the specified server are accepted.
 105
 106                 Only required for for NFS root. That is autoconfiguration
 107                 will not be triggered if it is missing and NFS root is not
 108                 in operation.
 109
 110                 Default: Determined using autoconfiguration.
 111                          The address of the autoconfiguration server is used.
 112
 113   <gw-ip>       IP address of a gateway if the server is on a different subnet.
 114
 115                 Default: Determined using autoconfiguration.
 116
 117   <netmask>     Netmask for local network interface. If unspecified
 118                 the netmask is derived from the client IP address assuming
 119                 classful addressing.
 120
 121                 Default:  Determined using autoconfiguration.
 122
 123   <hostname>    Name of the client. May be supplied by autoconfiguration,
 124                 but its absence will not trigger autoconfiguration.
 125
 126                 Default: Client IP address is used in ASCII notation.
 127
 128   <device>      Name of network device to use.
 129
 130                 Default: If the host only has one device, it is used.
 131                          Otherwise the device is determined using
 132                          autoconfiguration. This is done by sending
 133                          autoconfiguration requests out of all devices,
 134                          and using the device that received the first reply.
 135
 136   <autoconf>    Method to use for autoconfiguration. In the case of options
 137                 which specify multiple autoconfiguration protocols,
 138                 requests are sent using all protocols, and the first one
 139                 to reply is used.
 140
 141                 Only autoconfiguration protocols that have been compiled
 142                 into the kernel will be used, regardless of the value of
 143                 this option.
 144
 145                   off or none: don't use autoconfiguration (default)
 146                   on or any:   use any protocol available in the kernel
 147                   dhcp:        use DHCP
 148                   bootp:       use BOOTP
 149                   rarp:        use RARP
 150                   both:        use both BOOTP and RARP but not DHCP
 151                                (old option kept for backwards compatibility)
 152
 153                 Default: any
 154
 155
 156
 157
 158 3.) Boot Loader
 159     ----------
 160
 161 To get the kernel into memory different approaches can be used.
 162 They depend on various facilities being available:
 163
 164
 165 3.1)  Booting from a floppy using syslinux
 166
 167         When building kernels, an easy way to create a boot floppy that uses
 168         syslinux is to use the zdisk or bzdisk make targets which use
 169         and bzimage images respectively. Both targets accept the
 170         FDARGS parameter which can be used to set the kernel command line.
 171
 172         e.g.
 173            make bzdisk FDARGS="root=/dev/nfs"
 174
 175         Note that the user running this command will need to have
 176         access to the floppy drive device, /dev/fd0
 177
 178         For more information on syslinux, including how to create bootdisks
 179         for prebuilt kernels, see http://syslinux.zytor.com/
 180
 181         N.B: Previously it was possible to write a kernel directly to
 182              a floppy using dd, configure the boot device using rdev, and
 183              boot using the resulting floppy. Linux no longer supports this
 184              method of booting.
 185
 186 3.2) Booting from a cdrom using isolinux
 187
 188         When building kernels, an easy way to create a bootable cdrom that
 189         uses isolinux is to use the isoimage target which uses a bzimage
 190         image. Like zdisk and bzdisk, this target accepts the FDARGS
 191         parameter which can be used to set the kernel command line.
 192
 193         e.g.
 194           make isoimage FDARGS="root=/dev/nfs"
 195
 196         The resulting iso image will be arch/<ARCH>/boot/image.iso
 197         This can be written to a cdrom using a variety of tools including
 198         cdrecord.
 199
 200         e.g.
 201           cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
 202
 203         For more information on isolinux, including how to create bootdisks
 204         for prebuilt kernels, see http://syslinux.zytor.com/
 205
 206 3.2) Using LILO
 207         When using LILO all the necessary command line parameters may be
 208         specified using the 'append=' directive in the LILO configuration
 209         file.
 210
 211         However, to use the 'root=' directive you also need to create
 212         a dummy root device, which may be removed after LILO is run.
 213
 214         mknod /dev/boot255 c 0 255
 215
 216         For information on configuring LILO, please refer to its documentation.
 217
 218 3.3) Using GRUB
 219         When using GRUB, kernel parameter are simply appended after the kernel
 220         specification: kernel <kernel> <parameters>
 221
 222 3.4) Using loadlin
 223         loadlin may be used to boot Linux from a DOS command prompt without
 224         requiring a local hard disk to mount as root. This has not been
 225         thoroughly tested by the authors of this document, but in general
 226         it should be possible configure the kernel command line similarly
 227         to the configuration of LILO.
 228
 229         Please refer to the loadlin documentation for further information.
 230
 231 3.5) Using a boot ROM
 232         This is probably the most elegant way of booting a diskless client.
 233         With a boot ROM the kernel is loaded using the TFTP protocol. The
 234         authors of this document are not aware of any no commercial boot
 235         ROMs that support booting Linux over the network. However, there
 236         are two free implementations of a boot ROM, netboot-nfs and
 237         etherboot, both of which are available on sunsite.unc.edu, and both
 238         of which contain everything you need to boot a diskless Linux client.
 239
 240 3.6) Using pxelinux
 241         Pxelinux may be used to boot linux using the PXE boot loader
 242         which is present on many modern network cards.
 243
 244         When using pxelinux, the kernel image is specified using
 245         "kernel <relative-path-below /tftpboot>". The nfsroot parameters
 246         are passed to the kernel by adding them to the "append" line.
 247         It is common to use serial console in conjunction with pxeliunx,
 248         see Documentation/serial-console.txt for more information.
 249
 250         For more information on isolinux, including how to create bootdisks
 251         for prebuilt kernels, see http://syslinux.zytor.com/
 252
 253
 254
 255
 256 4.) Credits
 257     -------
 258
 259   The nfsroot code in the kernel and the RARP support have been written
 260   by Gero Kuhlmann <gero@gkminix.han.de>.
 261
 262   The rest of the IP layer autoconfiguration code has been written
 263   by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
 264
 265   In order to write the initial version of nfsroot I would like to thank
 266   Jens-Uwe Mager <jum@anubis.han.de> for his help.