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-Mounting the root filesystem via NFS (nfsroot)
-===============================================
-
-Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
-Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
-Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
-Updated 2006 by Horms <horms@verge.net.au>
-
-
-
-In order to use a diskless system, such as an X-terminal or printer server
-for example, it is necessary for the root filesystem to be present on a
-non-disk device. This may be an initramfs (see Documentation/filesystems/
-ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
-filesystem mounted via NFS. The following text describes on how to use NFS
-for the root filesystem. For the rest of this text 'client' means the
-diskless system, and 'server' means the NFS server.
-
-
-
-
-1.) Enabling nfsroot capabilities
- -----------------------------
-
-In order to use nfsroot, NFS client support needs to be selected as
-built-in during configuration. Once this has been selected, the nfsroot
-option will become available, which should also be selected.
-
-In the networking options, kernel level autoconfiguration can be selected,
-along with the types of autoconfiguration to support. Selecting all of
-DHCP, BOOTP and RARP is safe.
-
-
-
-
-2.) Kernel command line
- -------------------
-
-When the kernel has been loaded by a boot loader (see below) it needs to be
-told what root fs device to use. And in the case of nfsroot, where to find
-both the server and the name of the directory on the server to mount as root.
-This can be established using the following kernel command line parameters:
-
-
-root=/dev/nfs
-
- This is necessary to enable the pseudo-NFS-device. Note that it's not a
- real device but just a synonym to tell the kernel to use NFS instead of
- a real device.
-
-
-nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
-
- If the `nfsroot' parameter is NOT given on the command line,
- the default "/tftpboot/%s" will be used.
-
- <server-ip> Specifies the IP address of the NFS server.
- The default address is determined by the `ip' parameter
- (see below). This parameter allows the use of different
- servers for IP autoconfiguration and NFS.
-
- <root-dir> Name of the directory on the server to mount as root.
- If there is a "%s" token in the string, it will be
- replaced by the ASCII-representation of the client's
- IP address.
-
- <nfs-options> Standard NFS options. All options are separated by commas.
- The following defaults are used:
- port = as given by server portmap daemon
- rsize = 4096
- wsize = 4096
- timeo = 7
- retrans = 3
- acregmin = 3
- acregmax = 60
- acdirmin = 30
- acdirmax = 60
- flags = hard, nointr, noposix, cto, ac
-
-
-ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
-
- This parameter tells the kernel how to configure IP addresses of devices
- and also how to set up the IP routing table. It was originally called
- `nfsaddrs', but now the boot-time IP configuration works independently of
- NFS, so it was renamed to `ip' and the old name remained as an alias for
- compatibility reasons.
-
- If this parameter is missing from the kernel command line, all fields are
- assumed to be empty, and the defaults mentioned below apply. In general
- this means that the kernel tries to configure everything using
- autoconfiguration.
-
- The <autoconf> parameter can appear alone as the value to the `ip'
- parameter (without all the ':' characters before). If the value is
- "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
- autoconfiguration will take place. The most common way to use this
- is "ip=dhcp".
-
- <client-ip> IP address of the client.
-
- Default: Determined using autoconfiguration.
-
- <server-ip> IP address of the NFS server. If RARP is used to determine
- the client address and this parameter is NOT empty only
- replies from the specified server are accepted.
-
- Only required for NFS root. That is autoconfiguration
- will not be triggered if it is missing and NFS root is not
- in operation.
-
- Default: Determined using autoconfiguration.
- The address of the autoconfiguration server is used.
-
- <gw-ip> IP address of a gateway if the server is on a different subnet.
-
- Default: Determined using autoconfiguration.
-
- <netmask> Netmask for local network interface. If unspecified
- the netmask is derived from the client IP address assuming
- classful addressing.
-
- Default: Determined using autoconfiguration.
-
- <hostname> Name of the client. May be supplied by autoconfiguration,
- but its absence will not trigger autoconfiguration.
- If specified and DHCP is used, the user provided hostname will
- be carried in the DHCP request to hopefully update DNS record.
-
- Default: Client IP address is used in ASCII notation.
-
- <device> Name of network device to use.
-
- Default: If the host only has one device, it is used.
- Otherwise the device is determined using
- autoconfiguration. This is done by sending
- autoconfiguration requests out of all devices,
- and using the device that received the first reply.
-
- <autoconf> Method to use for autoconfiguration. In the case of options
- which specify multiple autoconfiguration protocols,
- requests are sent using all protocols, and the first one
- to reply is used.
-
- Only autoconfiguration protocols that have been compiled
- into the kernel will be used, regardless of the value of
- this option.
-
- off or none: don't use autoconfiguration
- (do static IP assignment instead)
- on or any: use any protocol available in the kernel
- (default)
- dhcp: use DHCP
- bootp: use BOOTP
- rarp: use RARP
- both: use both BOOTP and RARP but not DHCP
- (old option kept for backwards compatibility)
-
- Default: any
-
-
-nfsrootdebug
-
- This parameter enables debugging messages to appear in the kernel
- log at boot time so that administrators can verify that the correct
- NFS mount options, server address, and root path are passed to the
- NFS client.
-
-
-rdinit=<executable file>
-
- To specify which file contains the program that starts system
- initialization, administrators can use this command line parameter.
- The default value of this parameter is "/init". If the specified
- file exists and the kernel can execute it, root filesystem related
- kernel command line parameters, including `nfsroot=', are ignored.
-
- A description of the process of mounting the root file system can be
- found in:
-
- Documentation/early-userspace/README
-
-
-
-
-3.) Boot Loader
- ----------
-
-To get the kernel into memory different approaches can be used.
-They depend on various facilities being available:
-
-
-3.1) Booting from a floppy using syslinux
-
- When building kernels, an easy way to create a boot floppy that uses
- syslinux is to use the zdisk or bzdisk make targets which use zimage
- and bzimage images respectively. Both targets accept the
- FDARGS parameter which can be used to set the kernel command line.
-
- e.g.
- make bzdisk FDARGS="root=/dev/nfs"
-
- Note that the user running this command will need to have
- access to the floppy drive device, /dev/fd0
-
- For more information on syslinux, including how to create bootdisks
- for prebuilt kernels, see http://syslinux.zytor.com/
-
- N.B: Previously it was possible to write a kernel directly to
- a floppy using dd, configure the boot device using rdev, and
- boot using the resulting floppy. Linux no longer supports this
- method of booting.
-
-3.2) Booting from a cdrom using isolinux
-
- When building kernels, an easy way to create a bootable cdrom that
- uses isolinux is to use the isoimage target which uses a bzimage
- image. Like zdisk and bzdisk, this target accepts the FDARGS
- parameter which can be used to set the kernel command line.
-
- e.g.
- make isoimage FDARGS="root=/dev/nfs"
-
- The resulting iso image will be arch/<ARCH>/boot/image.iso
- This can be written to a cdrom using a variety of tools including
- cdrecord.
-
- e.g.
- cdrecord dev=ATAPI:1,0,0 arch/x86/boot/image.iso
-
- For more information on isolinux, including how to create bootdisks
- for prebuilt kernels, see http://syslinux.zytor.com/
-
-3.2) Using LILO
- When using LILO all the necessary command line parameters may be
- specified using the 'append=' directive in the LILO configuration
- file.
-
- However, to use the 'root=' directive you also need to create
- a dummy root device, which may be removed after LILO is run.
-
- mknod /dev/boot255 c 0 255
-
- For information on configuring LILO, please refer to its documentation.
-
-3.3) Using GRUB
- When using GRUB, kernel parameter are simply appended after the kernel
- specification: kernel <kernel> <parameters>
-
-3.4) Using loadlin
- loadlin may be used to boot Linux from a DOS command prompt without
- requiring a local hard disk to mount as root. This has not been
- thoroughly tested by the authors of this document, but in general
- it should be possible configure the kernel command line similarly
- to the configuration of LILO.
-
- Please refer to the loadlin documentation for further information.
-
-3.5) Using a boot ROM
- This is probably the most elegant way of booting a diskless client.
- With a boot ROM the kernel is loaded using the TFTP protocol. The
- authors of this document are not aware of any no commercial boot
- ROMs that support booting Linux over the network. However, there
- are two free implementations of a boot ROM, netboot-nfs and
- etherboot, both of which are available on sunsite.unc.edu, and both
- of which contain everything you need to boot a diskless Linux client.
-
-3.6) Using pxelinux
- Pxelinux may be used to boot linux using the PXE boot loader
- which is present on many modern network cards.
-
- When using pxelinux, the kernel image is specified using
- "kernel <relative-path-below /tftpboot>". The nfsroot parameters
- are passed to the kernel by adding them to the "append" line.
- It is common to use serial console in conjunction with pxeliunx,
- see Documentation/serial-console.txt for more information.
-
- For more information on isolinux, including how to create bootdisks
- for prebuilt kernels, see http://syslinux.zytor.com/
-
-
-
-
-4.) Credits
- -------
-
- The nfsroot code in the kernel and the RARP support have been written
- by Gero Kuhlmann <gero@gkminix.han.de>.
-
- The rest of the IP layer autoconfiguration code has been written
- by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
-
- In order to write the initial version of nfsroot I would like to thank
- Jens-Uwe Mager <jum@anubis.han.de> for his help.