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>
-If you want to use a diskless system, as an X-terminal or printer
-server for example, you have to put your root filesystem onto a
-non-disk device. This can either be a ramdisk (see initrd.txt in
-this directory for further information) 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
+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 Documenation/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 you have to select support for NFS during
-kernel configuration. Note that NFS cannot be loaded as a module
-in this case. The configuration script will then ask you whether
-you want to use nfsroot, and if yes what kind of auto configuration
-system you want to use. Selecting both BOOTP and RARP is safe.
+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 (either by loadlin,
-LILO or a network boot program) it has to be told what root fs device
-to use, and where to find the server and the name of the directory
-on the server to mount as root. This can be established by a couple
-of kernel command line parameters:
+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
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.
+ 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. If this field
- is not given, the default address as determined by the
- `ip' variable (see below) is used. One use of this
- parameter is for example to allow using different servers
- for RARP and NFS. Usually you can leave this blank.
+ <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, the token will be
- replaced by the ASCII-representation of the client's IP
- address.
+ <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.
- If the options field is not given, the following defaults
- will be used:
+ The following defaults are used:
port = as given by server portmap daemon
rsize = 1024
wsize = 1024
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.
+ 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 both
- RARP and BOOTP (depending on what has been enabled during kernel confi-
- guration, and if both what protocol answer got in first).
+ 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) in which case auto-
+ configuration is used.
+
+ <client-ip> IP address of the client.
- <client-ip> IP address of the client. If empty, the address will either
- be determined by RARP or BOOTP. What protocol is used de-
- pends on what has been enabled during kernel configuration
- and on the <autoconf> parameter. If this parameter is not
- empty, neither RARP nor BOOTP will be used.
+ 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. To use
- different RARP and NFS server, specify your RARP server
- here (or leave it blank), and specify your NFS server in
- the `nfsroot' parameter (see above). If this entry is blank
- the address of the server is used which answered the RARP
- or BOOTP request.
-
- <gw-ip> IP address of a gateway if the server is on a different
- subnet. If this entry is empty no gateway is used and the
- server is assumed to be on the local network, unless a
- value has been received by BOOTP.
-
- <netmask> Netmask for local network interface. If this is empty,
+ replies from the specified server are accepted.
+
+ Only required for 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, unless overridden in BOOTP reply.
+ classful addressing.
- <hostname> Name of the client. If empty, the client IP address is
- used in ASCII notation, or the value received by BOOTP.
+ Default: Determined using autoconfiguration.
- <device> Name of network device to use. If this is empty, all
- devices are used for RARP and BOOTP requests, and the
- first one we receive a reply on is configured. If you have
- only one device, you can safely leave this blank.
+ <hostname> Name of the client. May be supplied by autoconfiguration,
+ but its absence will not trigger autoconfiguration.
- <autoconf> Method to use for autoconfiguration. If this is either
- 'rarp' or 'bootp', the specified protocol is used.
- If the value is 'both' or empty, both protocols are used
- so far as they have been enabled during kernel configura-
- tion. 'off' means no autoconfiguration.
+ Default: Client IP address is used in ASCII notation.
- The <autoconf> parameter can appear alone as the value to the `ip'
- parameter (without all the ':' characters before) in which case auto-
- configuration is used.
+ <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 (default)
+ on or any: use any protocol available in the kernel
+ dhcp: use DHCP
+ bootp: use BOOTP
+ rarp: use RARP
+ both: use both BOOTP and RARP but not DHCP
+ (old option kept for backwards compatibility)
-3.) Kernel loader
- -------------
+ Default: any
-To get the kernel into memory different approaches can be used. They
-depend on what facilities are available:
-3.1) Writing the kernel onto a floppy using dd:
- As always you can just write the kernel onto a floppy using dd,
- but then it's not possible to use kernel command lines at all.
- To substitute the 'root=' parameter, create a dummy device on any
- linux system with major number 0 and minor number 255 using mknod:
- mknod /dev/boot255 c 0 255
+3.) Boot Loader
+ ----------
- Then copy the kernel zImage file onto a floppy using dd:
+To get the kernel into memory different approaches can be used.
+They depend on various facilities being available:
- dd if=/usr/src/linux/arch/i386/boot/zImage of=/dev/fd0
- And finally use rdev to set the root device:
+3.1) Booting from a floppy using syslinux
- rdev /dev/fd0 /dev/boot255
+ 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
+ and bzimage images respectively. Both targets accept the
+ FDARGS parameter which can be used to set the kernel command line.
- You can then remove the dummy device /dev/boot255 again. There
- is no real device available for it.
- The other two kernel command line parameters cannot be substi-
- tuted with rdev. Therefore, using this method the kernel will
- by default use RARP and/or BOOTP, and if it gets an answer via
- RARP will mount the directory /tftpboot/<client-ip>/ as its
- root. If it got a BOOTP answer the directory name in that answer
- is used.
+ 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/i386/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 you can specify all necessary command line
- parameters with the 'append=' command in the LILO configuration
- file. However, to use the 'root=' command you also need to
- set up a dummy device as described in 3.1 above. For how to use
- LILO and its 'append=' command please refer to the LILO
- documentation.
+ 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 you use GRUB, you simply append the parameters after the kernel
- specification: "kernel <kernel> <parameters>" (without the quotes).
+ When using GRUB, kernel parameter are simply appended after the kernel
+ specification: kernel <kernel> <parameters>
3.4) Using loadlin
- When you want to boot Linux from a DOS command prompt without
- having a local hard disk to mount as root, you can use loadlin.
- I was told that it works, but haven't used it myself yet. In
- general you should be able to create a kernel command line simi-
- lar to how LILO is doing it. Please refer to the loadlin docu-
- mentation for further information.
+ 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 gets loaded using the TFTP
- protocol. As far as I know, no commercial boot ROMs yet
- support booting Linux over the network, but there are two
- free implementations of a boot ROM available on sunsite.unc.edu
- and its mirrors. They are called 'netboot-nfs' and 'etherboot'.
- Both contain everything you need to boot a diskless Linux client.
+ 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
- Using pxelinux you specify the kernel you built with
+ 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.
- You may perhaps also want to fine tune the console output,
- see Documentation/serial-console.txt for serial console help.
+ 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/
projects / linux-2.6 / commitdiff