Documentation/admin-guide/nfs/nfsroot.rst - third_party/kernel - Git at Google

 ===============================================
 Mounting the root filesystem via NFS (nfsroot)
 ===============================================

 :Authors:
 	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>

 	Updated 2018 by Chris Novakovic <chris@chrisn.me.uk>


 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.rst), a ramdisk (see
 Documentation/admin-guide/initrd.rst) 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.


 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.


 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>:<dns0-ip>:<dns1-ip>:<ntp0-ip>
   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.

 		Value is exported to /proc/net/pnp with the prefix "bootserver "
 		(see below).

 		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.
 		If a '.' character is present, anything
 		before the first '.' is used as the client's hostname, and anything
 		after it is used as its NIS domain name. May be supplied by
 		autoconfiguration, but its absence will not trigger autoconfiguration.
 		If specified and DHCP is used, the user-provided hostname (and NIS
 		domain name, if present) will be carried in the DHCP request; this
 		may cause a DNS record to be created or updated for the client.

   		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)

 		if dhcp is used, the client identifier can be used by following
 		format "ip=dhcp,client-id-type,client-id-value"

                 Default: any

   <dns0-ip>	IP address of primary nameserver.
 		Value is exported to /proc/net/pnp with the prefix "nameserver "
 		(see below).

 		Default: None if not using autoconfiguration; determined
 		automatically if using autoconfiguration.

   <dns1-ip>	IP address of secondary nameserver.
 		See <dns0-ip>.

   <ntp0-ip>	IP address of a Network Time Protocol (NTP) server.
 		Value is exported to /proc/net/ipconfig/ntp_servers, but is
 		otherwise unused (see below).

 		Default: None if not using autoconfiguration; determined
 		automatically if using autoconfiguration.

   After configuration (whether manual or automatic) is complete, two files
   are created in the following format; lines are omitted if their respective
   value is empty following configuration:

   - /proc/net/pnp:

 	#PROTO: <DHCP|BOOTP|RARP|MANUAL>	(depending on configuration method)
 	domain <dns-domain>			(if autoconfigured, the DNS domain)
 	nameserver <dns0-ip>			(primary name server IP)
 	nameserver <dns1-ip>			(secondary name server IP)
 	nameserver <dns2-ip>			(tertiary name server IP)
 	bootserver <server-ip>			(NFS server IP)

   - /proc/net/ipconfig/ntp_servers:

 	<ntp0-ip>				(NTP server IP)
 	<ntp1-ip>				(NTP server IP)
 	<ntp2-ip>				(NTP server IP)

   <dns-domain> and <dns2-ip> (in /proc/net/pnp) and <ntp1-ip> and <ntp2-ip>
   (in /proc/net/ipconfig/ntp_servers) are requested during autoconfiguration;
   they cannot be specified as part of the "ip=" kernel command line parameter.

   Because the "domain" and "nameserver" options are recognised by DNS
   resolvers, /etc/resolv.conf is often linked to /proc/net/pnp on systems
   that use an NFS root filesystem.

   Note that the kernel will not synchronise the system time with any NTP
   servers it discovers; this is the responsibility of a user space process
   (e.g. an initrd/initramfs script that passes the IP addresses listed in
   /proc/net/ipconfig/ntp_servers to an NTP client before mounting the real
   root filesystem if it is on NFS).


 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/driver-api/early-userspace/early_userspace_support.rst


 Boot Loader
 ===========

 To get the kernel into memory different approaches can be used.
 They depend on various facilities being available:


 - 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 https://syslinux.zytor.com/

 	.. note::
 		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.

 - 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 https://syslinux.zytor.com/

 - 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.

 	e.g::

 	  mknod /dev/boot255 c 0 255

 	For information on configuring LILO, please refer to its documentation.

 - Using GRUB

 	When using GRUB, kernel parameter are simply appended after the kernel
 	specification: kernel <kernel> <parameters>

 - 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.

 - 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.

 - 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/admin-guide/serial-console.rst for more information.

 	For more information on isolinux, including how to create bootdisks
 	for prebuilt kernels, see https://syslinux.zytor.com/


 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.
	===============================================
	Mounting the root filesystem via NFS (nfsroot)
	===============================================

	:Authors:
	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>

	Updated 2018 by Chris Novakovic <chris@chrisn.me.uk>



	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.rst), a ramdisk (see
	Documentation/admin-guide/initrd.rst) 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.




	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.




	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>:<dns0-ip>:<dns1-ip>:<ntp0-ip>
	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.

	Value is exported to /proc/net/pnp with the prefix "bootserver "
	(see below).

	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.
	If a '.' character is present, anything
	before the first '.' is used as the client's hostname, and anything
	after it is used as its NIS domain name. May be supplied by
	autoconfiguration, but its absence will not trigger autoconfiguration.
	If specified and DHCP is used, the user-provided hostname (and NIS
	domain name, if present) will be carried in the DHCP request; this
	may cause a DNS record to be created or updated for the client.

	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)

	if dhcp is used, the client identifier can be used by following
	format "ip=dhcp,client-id-type,client-id-value"

	Default: any

	<dns0-ip> IP address of primary nameserver.
	Value is exported to /proc/net/pnp with the prefix "nameserver "
	(see below).

	Default: None if not using autoconfiguration; determined
	automatically if using autoconfiguration.

	<dns1-ip> IP address of secondary nameserver.
	See <dns0-ip>.

	<ntp0-ip> IP address of a Network Time Protocol (NTP) server.
	Value is exported to /proc/net/ipconfig/ntp_servers, but is
	otherwise unused (see below).

	Default: None if not using autoconfiguration; determined
	automatically if using autoconfiguration.

	After configuration (whether manual or automatic) is complete, two files
	are created in the following format; lines are omitted if their respective
	value is empty following configuration:

	- /proc/net/pnp:

	#PROTO: <DHCP\|BOOTP\|RARP\|MANUAL> (depending on configuration method)
	domain <dns-domain> (if autoconfigured, the DNS domain)
	nameserver <dns0-ip> (primary name server IP)
	nameserver <dns1-ip> (secondary name server IP)
	nameserver <dns2-ip> (tertiary name server IP)
	bootserver <server-ip> (NFS server IP)

	- /proc/net/ipconfig/ntp_servers:

	<ntp0-ip> (NTP server IP)
	<ntp1-ip> (NTP server IP)
	<ntp2-ip> (NTP server IP)

	<dns-domain> and <dns2-ip> (in /proc/net/pnp) and <ntp1-ip> and <ntp2-ip>
	(in /proc/net/ipconfig/ntp_servers) are requested during autoconfiguration;
	they cannot be specified as part of the "ip=" kernel command line parameter.

	Because the "domain" and "nameserver" options are recognised by DNS
	resolvers, /etc/resolv.conf is often linked to /proc/net/pnp on systems
	that use an NFS root filesystem.

	Note that the kernel will not synchronise the system time with any NTP
	servers it discovers; this is the responsibility of a user space process
	(e.g. an initrd/initramfs script that passes the IP addresses listed in
	/proc/net/ipconfig/ntp_servers to an NTP client before mounting the real
	root filesystem if it is on NFS).


	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/driver-api/early-userspace/early_userspace_support.rst


	Boot Loader
	===========

	To get the kernel into memory different approaches can be used.
	They depend on various facilities being available:


	- 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 https://syslinux.zytor.com/

	.. note::
	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.

	- 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 https://syslinux.zytor.com/

	- 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.

	e.g::

	mknod /dev/boot255 c 0 255

	For information on configuring LILO, please refer to its documentation.

	- Using GRUB

	When using GRUB, kernel parameter are simply appended after the kernel
	specification: kernel <kernel> <parameters>

	- 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.

	- 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.

	- 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/admin-guide/serial-console.rst for more information.

	For more information on isolinux, including how to create bootdisks
	for prebuilt kernels, see https://syslinux.zytor.com/




	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.