view Documentation/nfsroot.txt @ 912:dd42cdb0ab89

[IA64] Build blktap2 driver by default in x86 builds.

add CONFIG_XEN_BLKDEV_TAP2=y to buildconfigs/linux-defconfig_xen_ia64.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
author Isaku Yamahata <yamahata@valinux.co.jp>
date Mon Jun 29 12:09:16 2009 +0900 (2009-06-29)
parents 831230e53067
line source
1 Mounting the root filesystem via NFS (nfsroot)
2 ===============================================
4 Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
5 Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
6 Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
7 Updated 2006 by Horms <horms@verge.net.au>
11 In order to use a diskless system, such as an X-terminal or printer server
12 for example, it is necessary for the root filesystem to be present on a
13 non-disk device. This may be an initramfs (see Documentation/filesystems/
14 ramfs-rootfs-initramfs.txt), a ramdisk (see Documenation/initrd.txt) or a
15 filesystem mounted via NFS. The following text describes on how to use NFS
16 for the root filesystem. For the rest of this text 'client' means the
17 diskless system, and 'server' means the NFS server.
22 1.) Enabling nfsroot capabilities
23 -----------------------------
25 In order to use nfsroot, NFS client support needs to be selected as
26 built-in during configuration. Once this has been selected, the nfsroot
27 option will become available, which should also be selected.
29 In the networking options, kernel level autoconfiguration can be selected,
30 along with the types of autoconfiguration to support. Selecting all of
31 DHCP, BOOTP and RARP is safe.
36 2.) Kernel command line
37 -------------------
39 When the kernel has been loaded by a boot loader (see below) it needs to be
40 told what root fs device to use. And in the case of nfsroot, where to find
41 both the server and the name of the directory on the server to mount as root.
42 This can be established using the following kernel command line parameters:
45 root=/dev/nfs
47 This is necessary to enable the pseudo-NFS-device. Note that it's not a
48 real device but just a synonym to tell the kernel to use NFS instead of
49 a real device.
52 nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
54 If the `nfsroot' parameter is NOT given on the command line,
55 the default "/tftpboot/%s" will be used.
57 <server-ip> Specifies the IP address of the NFS server.
58 The default address is determined by the `ip' parameter
59 (see below). This parameter allows the use of different
60 servers for IP autoconfiguration and NFS.
62 <root-dir> Name of the directory on the server to mount as root.
63 If there is a "%s" token in the string, it will be
64 replaced by the ASCII-representation of the client's
65 IP address.
67 <nfs-options> Standard NFS options. All options are separated by commas.
68 The following defaults are used:
69 port = as given by server portmap daemon
70 rsize = 1024
71 wsize = 1024
72 timeo = 7
73 retrans = 3
74 acregmin = 3
75 acregmax = 60
76 acdirmin = 30
77 acdirmax = 60
78 flags = hard, nointr, noposix, cto, ac
81 ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
83 This parameter tells the kernel how to configure IP addresses of devices
84 and also how to set up the IP routing table. It was originally called
85 `nfsaddrs', but now the boot-time IP configuration works independently of
86 NFS, so it was renamed to `ip' and the old name remained as an alias for
87 compatibility reasons.
89 If this parameter is missing from the kernel command line, all fields are
90 assumed to be empty, and the defaults mentioned below apply. In general
91 this means that the kernel tries to configure everything using
92 autoconfiguration.
94 The <autoconf> parameter can appear alone as the value to the `ip'
95 parameter (without all the ':' characters before) in which case auto-
96 configuration is used.
98 <client-ip> IP address of the client.
100 Default: Determined using autoconfiguration.
102 <server-ip> IP address of the NFS server. If RARP is used to determine
103 the client address and this parameter is NOT empty only
104 replies from the specified server are accepted.
106 Only required for for NFS root. That is autoconfiguration
107 will not be triggered if it is missing and NFS root is not
108 in operation.
110 Default: Determined using autoconfiguration.
111 The address of the autoconfiguration server is used.
113 <gw-ip> IP address of a gateway if the server is on a different subnet.
115 Default: Determined using autoconfiguration.
117 <netmask> Netmask for local network interface. If unspecified
118 the netmask is derived from the client IP address assuming
119 classful addressing.
121 Default: Determined using autoconfiguration.
123 <hostname> Name of the client. May be supplied by autoconfiguration,
124 but its absence will not trigger autoconfiguration.
126 Default: Client IP address is used in ASCII notation.
128 <device> Name of network device to use.
130 Default: If the host only has one device, it is used.
131 Otherwise the device is determined using
132 autoconfiguration. This is done by sending
133 autoconfiguration requests out of all devices,
134 and using the device that received the first reply.
136 <autoconf> Method to use for autoconfiguration. In the case of options
137 which specify multiple autoconfiguration protocols,
138 requests are sent using all protocols, and the first one
139 to reply is used.
141 Only autoconfiguration protocols that have been compiled
142 into the kernel will be used, regardless of the value of
143 this option.
145 off or none: don't use autoconfiguration (default)
146 on or any: use any protocol available in the kernel
147 dhcp: use DHCP
148 bootp: use BOOTP
149 rarp: use RARP
150 both: use both BOOTP and RARP but not DHCP
151 (old option kept for backwards compatibility)
153 Default: any
158 3.) Boot Loader
159 ----------
161 To get the kernel into memory different approaches can be used.
162 They depend on various facilities being available:
165 3.1) Booting from a floppy using syslinux
167 When building kernels, an easy way to create a boot floppy that uses
168 syslinux is to use the zdisk or bzdisk make targets which use
169 and bzimage images respectively. Both targets accept the
170 FDARGS parameter which can be used to set the kernel command line.
172 e.g.
173 make bzdisk FDARGS="root=/dev/nfs"
175 Note that the user running this command will need to have
176 access to the floppy drive device, /dev/fd0
178 For more information on syslinux, including how to create bootdisks
179 for prebuilt kernels, see http://syslinux.zytor.com/
181 N.B: Previously it was possible to write a kernel directly to
182 a floppy using dd, configure the boot device using rdev, and
183 boot using the resulting floppy. Linux no longer supports this
184 method of booting.
186 3.2) Booting from a cdrom using isolinux
188 When building kernels, an easy way to create a bootable cdrom that
189 uses isolinux is to use the isoimage target which uses a bzimage
190 image. Like zdisk and bzdisk, this target accepts the FDARGS
191 parameter which can be used to set the kernel command line.
193 e.g.
194 make isoimage FDARGS="root=/dev/nfs"
196 The resulting iso image will be arch/<ARCH>/boot/image.iso
197 This can be written to a cdrom using a variety of tools including
198 cdrecord.
200 e.g.
201 cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
203 For more information on isolinux, including how to create bootdisks
204 for prebuilt kernels, see http://syslinux.zytor.com/
206 3.2) Using LILO
207 When using LILO all the necessary command line parameters may be
208 specified using the 'append=' directive in the LILO configuration
209 file.
211 However, to use the 'root=' directive you also need to create
212 a dummy root device, which may be removed after LILO is run.
214 mknod /dev/boot255 c 0 255
216 For information on configuring LILO, please refer to its documentation.
218 3.3) Using GRUB
219 When using GRUB, kernel parameter are simply appended after the kernel
220 specification: kernel <kernel> <parameters>
222 3.4) Using loadlin
223 loadlin may be used to boot Linux from a DOS command prompt without
224 requiring a local hard disk to mount as root. This has not been
225 thoroughly tested by the authors of this document, but in general
226 it should be possible configure the kernel command line similarly
227 to the configuration of LILO.
229 Please refer to the loadlin documentation for further information.
231 3.5) Using a boot ROM
232 This is probably the most elegant way of booting a diskless client.
233 With a boot ROM the kernel is loaded using the TFTP protocol. The
234 authors of this document are not aware of any no commercial boot
235 ROMs that support booting Linux over the network. However, there
236 are two free implementations of a boot ROM, netboot-nfs and
237 etherboot, both of which are available on sunsite.unc.edu, and both
238 of which contain everything you need to boot a diskless Linux client.
240 3.6) Using pxelinux
241 Pxelinux may be used to boot linux using the PXE boot loader
242 which is present on many modern network cards.
244 When using pxelinux, the kernel image is specified using
245 "kernel <relative-path-below /tftpboot>". The nfsroot parameters
246 are passed to the kernel by adding them to the "append" line.
247 It is common to use serial console in conjunction with pxeliunx,
248 see Documentation/serial-console.txt for more information.
250 For more information on isolinux, including how to create bootdisks
251 for prebuilt kernels, see http://syslinux.zytor.com/
256 4.) Credits
257 -------
259 The nfsroot code in the kernel and the RARP support have been written
260 by Gero Kuhlmann <gero@gkminix.han.de>.
262 The rest of the IP layer autoconfiguration code has been written
263 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
265 In order to write the initial version of nfsroot I would like to thank
266 Jens-Uwe Mager <jum@anubis.han.de> for his help.