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
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.txt), 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.
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=[
If the `nfsroot’ parameter is NOT given on the command line,
the default “/tftpboot/%s” will be used.
The default address is determined by the `ip’ parameter
(see below). This parameter allows the use of different
servers for IP autoconfiguration and NFS.
If there is a “%s” token in the string, it will be
replaced by the ASCII-representation of the client’s
IP address.
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=
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
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”.
Default: Determined using autoconfiguration.
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.
Default: Determined using autoconfiguration.
the netmask is derived from the client IP address assuming
classful addressing.
Default: Determined using autoconfiguration.
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.
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.
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
Value is exported to /proc/net/pnp with the prefix “nameserver “
(see below).
Default: None if not using autoconfiguration; determined
automatically if using autoconfiguration.
See
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(if autoconfigured, the DNS domain)
nameserver(primary name server IP)
nameserver(secondary name server IP)
nameserver(tertiary name server IP)
bootserver(NFS server IP) /proc/net/ipconfig/ntp_servers:
(NTP server IP) (NTP server IP) (NTP server IP) and (in /proc/net/pnp) and and
(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=
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
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/admin-guide/serial-console.rst 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.
