nfsroot.txt 9.4 KB

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  1. Mounting the root filesystem via NFS (nfsroot)
  2. ===============================================
  3. Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
  4. Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
  5. Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
  6. Updated 2006 by Horms <horms@verge.net.au>
  7. In order to use a diskless system, such as an X-terminal or printer server
  8. for example, it is necessary for the root filesystem to be present on a
  9. non-disk device. This may be an initramfs (see Documentation/filesystems/
  10. ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
  11. filesystem mounted via NFS. The following text describes on how to use NFS
  12. for the root filesystem. For the rest of this text 'client' means the
  13. diskless system, and 'server' means the NFS server.
  14. 1.) Enabling nfsroot capabilities
  15. -----------------------------
  16. In order to use nfsroot, NFS client support needs to be selected as
  17. built-in during configuration. Once this has been selected, the nfsroot
  18. option will become available, which should also be selected.
  19. In the networking options, kernel level autoconfiguration can be selected,
  20. along with the types of autoconfiguration to support. Selecting all of
  21. DHCP, BOOTP and RARP is safe.
  22. 2.) Kernel command line
  23. -------------------
  24. When the kernel has been loaded by a boot loader (see below) it needs to be
  25. told what root fs device to use. And in the case of nfsroot, where to find
  26. both the server and the name of the directory on the server to mount as root.
  27. This can be established using the following kernel command line parameters:
  28. root=/dev/nfs
  29. This is necessary to enable the pseudo-NFS-device. Note that it's not a
  30. real device but just a synonym to tell the kernel to use NFS instead of
  31. a real device.
  32. nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
  33. If the `nfsroot' parameter is NOT given on the command line,
  34. the default "/tftpboot/%s" will be used.
  35. <server-ip> Specifies the IP address of the NFS server.
  36. The default address is determined by the `ip' parameter
  37. (see below). This parameter allows the use of different
  38. servers for IP autoconfiguration and NFS.
  39. <root-dir> Name of the directory on the server to mount as root.
  40. If there is a "%s" token in the string, it will be
  41. replaced by the ASCII-representation of the client's
  42. IP address.
  43. <nfs-options> Standard NFS options. All options are separated by commas.
  44. The following defaults are used:
  45. port = as given by server portmap daemon
  46. rsize = 4096
  47. wsize = 4096
  48. timeo = 7
  49. retrans = 3
  50. acregmin = 3
  51. acregmax = 60
  52. acdirmin = 30
  53. acdirmax = 60
  54. flags = hard, nointr, noposix, cto, ac
  55. ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
  56. This parameter tells the kernel how to configure IP addresses of devices
  57. and also how to set up the IP routing table. It was originally called
  58. `nfsaddrs', but now the boot-time IP configuration works independently of
  59. NFS, so it was renamed to `ip' and the old name remained as an alias for
  60. compatibility reasons.
  61. If this parameter is missing from the kernel command line, all fields are
  62. assumed to be empty, and the defaults mentioned below apply. In general
  63. this means that the kernel tries to configure everything using
  64. autoconfiguration.
  65. The <autoconf> parameter can appear alone as the value to the `ip'
  66. parameter (without all the ':' characters before) in which case auto-
  67. configuration is used.
  68. <client-ip> IP address of the client.
  69. Default: Determined using autoconfiguration.
  70. <server-ip> IP address of the NFS server. If RARP is used to determine
  71. the client address and this parameter is NOT empty only
  72. replies from the specified server are accepted.
  73. Only required for for NFS root. That is autoconfiguration
  74. will not be triggered if it is missing and NFS root is not
  75. in operation.
  76. Default: Determined using autoconfiguration.
  77. The address of the autoconfiguration server is used.
  78. <gw-ip> IP address of a gateway if the server is on a different subnet.
  79. Default: Determined using autoconfiguration.
  80. <netmask> Netmask for local network interface. If unspecified
  81. the netmask is derived from the client IP address assuming
  82. classful addressing.
  83. Default: Determined using autoconfiguration.
  84. <hostname> Name of the client. May be supplied by autoconfiguration,
  85. but its absence will not trigger autoconfiguration.
  86. Default: Client IP address is used in ASCII notation.
  87. <device> Name of network device to use.
  88. Default: If the host only has one device, it is used.
  89. Otherwise the device is determined using
  90. autoconfiguration. This is done by sending
  91. autoconfiguration requests out of all devices,
  92. and using the device that received the first reply.
  93. <autoconf> Method to use for autoconfiguration. In the case of options
  94. which specify multiple autoconfiguration protocols,
  95. requests are sent using all protocols, and the first one
  96. to reply is used.
  97. Only autoconfiguration protocols that have been compiled
  98. into the kernel will be used, regardless of the value of
  99. this option.
  100. off or none: don't use autoconfiguration (default)
  101. on or any: use any protocol available in the kernel
  102. dhcp: use DHCP
  103. bootp: use BOOTP
  104. rarp: use RARP
  105. both: use both BOOTP and RARP but not DHCP
  106. (old option kept for backwards compatibility)
  107. Default: any
  108. 3.) Boot Loader
  109. ----------
  110. To get the kernel into memory different approaches can be used.
  111. They depend on various facilities being available:
  112. 3.1) Booting from a floppy using syslinux
  113. When building kernels, an easy way to create a boot floppy that uses
  114. syslinux is to use the zdisk or bzdisk make targets which use
  115. and bzimage images respectively. Both targets accept the
  116. FDARGS parameter which can be used to set the kernel command line.
  117. e.g.
  118. make bzdisk FDARGS="root=/dev/nfs"
  119. Note that the user running this command will need to have
  120. access to the floppy drive device, /dev/fd0
  121. For more information on syslinux, including how to create bootdisks
  122. for prebuilt kernels, see http://syslinux.zytor.com/
  123. N.B: Previously it was possible to write a kernel directly to
  124. a floppy using dd, configure the boot device using rdev, and
  125. boot using the resulting floppy. Linux no longer supports this
  126. method of booting.
  127. 3.2) Booting from a cdrom using isolinux
  128. When building kernels, an easy way to create a bootable cdrom that
  129. uses isolinux is to use the isoimage target which uses a bzimage
  130. image. Like zdisk and bzdisk, this target accepts the FDARGS
  131. parameter which can be used to set the kernel command line.
  132. e.g.
  133. make isoimage FDARGS="root=/dev/nfs"
  134. The resulting iso image will be arch/<ARCH>/boot/image.iso
  135. This can be written to a cdrom using a variety of tools including
  136. cdrecord.
  137. e.g.
  138. cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
  139. For more information on isolinux, including how to create bootdisks
  140. for prebuilt kernels, see http://syslinux.zytor.com/
  141. 3.2) Using LILO
  142. When using LILO all the necessary command line parameters may be
  143. specified using the 'append=' directive in the LILO configuration
  144. file.
  145. However, to use the 'root=' directive you also need to create
  146. a dummy root device, which may be removed after LILO is run.
  147. mknod /dev/boot255 c 0 255
  148. For information on configuring LILO, please refer to its documentation.
  149. 3.3) Using GRUB
  150. When using GRUB, kernel parameter are simply appended after the kernel
  151. specification: kernel <kernel> <parameters>
  152. 3.4) Using loadlin
  153. loadlin may be used to boot Linux from a DOS command prompt without
  154. requiring a local hard disk to mount as root. This has not been
  155. thoroughly tested by the authors of this document, but in general
  156. it should be possible configure the kernel command line similarly
  157. to the configuration of LILO.
  158. Please refer to the loadlin documentation for further information.
  159. 3.5) Using a boot ROM
  160. This is probably the most elegant way of booting a diskless client.
  161. With a boot ROM the kernel is loaded using the TFTP protocol. The
  162. authors of this document are not aware of any no commercial boot
  163. ROMs that support booting Linux over the network. However, there
  164. are two free implementations of a boot ROM, netboot-nfs and
  165. etherboot, both of which are available on sunsite.unc.edu, and both
  166. of which contain everything you need to boot a diskless Linux client.
  167. 3.6) Using pxelinux
  168. Pxelinux may be used to boot linux using the PXE boot loader
  169. which is present on many modern network cards.
  170. When using pxelinux, the kernel image is specified using
  171. "kernel <relative-path-below /tftpboot>". The nfsroot parameters
  172. are passed to the kernel by adding them to the "append" line.
  173. It is common to use serial console in conjunction with pxeliunx,
  174. see Documentation/serial-console.txt for more information.
  175. For more information on isolinux, including how to create bootdisks
  176. for prebuilt kernels, see http://syslinux.zytor.com/
  177. 4.) Credits
  178. -------
  179. The nfsroot code in the kernel and the RARP support have been written
  180. by Gero Kuhlmann <gero@gkminix.han.de>.
  181. The rest of the IP layer autoconfiguration code has been written
  182. by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
  183. In order to write the initial version of nfsroot I would like to thank
  184. Jens-Uwe Mager <jum@anubis.han.de> for his help.