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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Mounting the root filesystem via NFS (nfsroot)
2===============================================
3
4Written 1996 by Gero Kuhlmann <gero@gkminix.han.de>
5Updated 1997 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
Nico Schottelius7e9dd122006-03-24 03:18:18 -08006Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
Horms64552a52006-07-10 04:43:58 -07007Updated 2006 by Horms <horms@verge.net.au>
Linus Torvalds1da177e2005-04-16 15:20:36 -07008
9
10
Horms64552a52006-07-10 04:43:58 -070011In order to use a diskless system, such as an X-terminal or printer server
12for example, it is necessary for the root filesystem to be present on a
13non-disk device. This may be an initramfs (see Documentation/filesystems/
Matt LaPlantefff92892006-10-03 22:47:42 +020014ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
Horms64552a52006-07-10 04:43:58 -070015filesystem mounted via NFS. The following text describes on how to use NFS
16for the root filesystem. For the rest of this text 'client' means the
Linus Torvalds1da177e2005-04-16 15:20:36 -070017diskless system, and 'server' means the NFS server.
18
19
20
21
221.) Enabling nfsroot capabilities
23 -----------------------------
24
Horms64552a52006-07-10 04:43:58 -070025In order to use nfsroot, NFS client support needs to be selected as
26built-in during configuration. Once this has been selected, the nfsroot
27option will become available, which should also be selected.
28
29In the networking options, kernel level autoconfiguration can be selected,
30along with the types of autoconfiguration to support. Selecting all of
31DHCP, BOOTP and RARP is safe.
Linus Torvalds1da177e2005-04-16 15:20:36 -070032
33
34
35
362.) Kernel command line
37 -------------------
38
Horms64552a52006-07-10 04:43:58 -070039When the kernel has been loaded by a boot loader (see below) it needs to be
40told what root fs device to use. And in the case of nfsroot, where to find
41both the server and the name of the directory on the server to mount as root.
42This can be established using the following kernel command line parameters:
Linus Torvalds1da177e2005-04-16 15:20:36 -070043
44
45root=/dev/nfs
46
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.
50
51
52nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
53
Horms64552a52006-07-10 04:43:58 -070054 If the `nfsroot' parameter is NOT given on the command line,
55 the default "/tftpboot/%s" will be used.
Linus Torvalds1da177e2005-04-16 15:20:36 -070056
Horms64552a52006-07-10 04:43:58 -070057 <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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070061
Horms64552a52006-07-10 04:43:58 -070062 <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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070066
67 <nfs-options> Standard NFS options. All options are separated by commas.
Horms64552a52006-07-10 04:43:58 -070068 The following defaults are used:
Linus Torvalds1da177e2005-04-16 15:20:36 -070069 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
79
80
81ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
82
83 This parameter tells the kernel how to configure IP addresses of devices
Horms64552a52006-07-10 04:43:58 -070084 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070088
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
Horms64552a52006-07-10 04:43:58 -070091 this means that the kernel tries to configure everything using
92 autoconfiguration.
Linus Torvalds1da177e2005-04-16 15:20:36 -070093
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.
97
Horms64552a52006-07-10 04:43:58 -070098 <client-ip> IP address of the client.
99
100 Default: Determined using autoconfiguration.
101
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.
105
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.
109
110 Default: Determined using autoconfiguration.
111 The address of the autoconfiguration server is used.
112
113 <gw-ip> IP address of a gateway if the server is on a different subnet.
114
115 Default: Determined using autoconfiguration.
116
117 <netmask> Netmask for local network interface. If unspecified
118 the netmask is derived from the client IP address assuming
119 classful addressing.
120
121 Default: Determined using autoconfiguration.
122
123 <hostname> Name of the client. May be supplied by autoconfiguration,
124 but its absence will not trigger autoconfiguration.
125
126 Default: Client IP address is used in ASCII notation.
127
128 <device> Name of network device to use.
129
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.
135
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.
140
141 Only autoconfiguration protocols that have been compiled
142 into the kernel will be used, regardless of the value of
143 this option.
144
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)
152
153 Default: any
Linus Torvalds1da177e2005-04-16 15:20:36 -0700154
155
156
Linus Torvalds1da177e2005-04-16 15:20:36 -0700157
Horms64552a52006-07-10 04:43:58 -07001583.) Boot Loader
159 ----------
160
161To get the kernel into memory different approaches can be used.
162They depend on various facilities being available:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700163
164
Horms64552a52006-07-10 04:43:58 -07001653.1) Booting from a floppy using syslinux
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166
Horms64552a52006-07-10 04:43:58 -0700167 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700171
Horms64552a52006-07-10 04:43:58 -0700172 e.g.
173 make bzdisk FDARGS="root=/dev/nfs"
Linus Torvalds1da177e2005-04-16 15:20:36 -0700174
Horms64552a52006-07-10 04:43:58 -0700175 Note that the user running this command will need to have
176 access to the floppy drive device, /dev/fd0
Linus Torvalds1da177e2005-04-16 15:20:36 -0700177
Horms64552a52006-07-10 04:43:58 -0700178 For more information on syslinux, including how to create bootdisks
179 for prebuilt kernels, see http://syslinux.zytor.com/
Linus Torvalds1da177e2005-04-16 15:20:36 -0700180
Horms64552a52006-07-10 04:43:58 -0700181 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700185
Horms64552a52006-07-10 04:43:58 -07001863.2) Booting from a cdrom using isolinux
187
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.
192
193 e.g.
194 make isoimage FDARGS="root=/dev/nfs"
195
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.
199
200 e.g.
201 cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
202
203 For more information on isolinux, including how to create bootdisks
204 for prebuilt kernels, see http://syslinux.zytor.com/
Linus Torvalds1da177e2005-04-16 15:20:36 -0700205
Linus Torvalds1da177e2005-04-16 15:20:36 -07002063.2) Using LILO
Horms64552a52006-07-10 04:43:58 -0700207 When using LILO all the necessary command line parameters may be
208 specified using the 'append=' directive in the LILO configuration
209 file.
210
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.
213
214 mknod /dev/boot255 c 0 255
215
216 For information on configuring LILO, please refer to its documentation.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700217
Nico Schottelius7e9dd122006-03-24 03:18:18 -08002183.3) Using GRUB
Horms64552a52006-07-10 04:43:58 -0700219 When using GRUB, kernel parameter are simply appended after the kernel
220 specification: kernel <kernel> <parameters>
Nico Schottelius7e9dd122006-03-24 03:18:18 -0800221
2223.4) Using loadlin
Horms64552a52006-07-10 04:43:58 -0700223 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.
228
229 Please refer to the loadlin documentation for further information.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700230
Nico Schottelius7e9dd122006-03-24 03:18:18 -08002313.5) Using a boot ROM
Horms64552a52006-07-10 04:43:58 -0700232 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700239
Nico Schottelius7e9dd122006-03-24 03:18:18 -08002403.6) Using pxelinux
Horms64552a52006-07-10 04:43:58 -0700241 Pxelinux may be used to boot linux using the PXE boot loader
242 which is present on many modern network cards.
243
244 When using pxelinux, the kernel image is specified using
Nico Schottelius7e9dd122006-03-24 03:18:18 -0800245 "kernel <relative-path-below /tftpboot>". The nfsroot parameters
246 are passed to the kernel by adding them to the "append" line.
Horms64552a52006-07-10 04:43:58 -0700247 It is common to use serial console in conjunction with pxeliunx,
248 see Documentation/serial-console.txt for more information.
249
250 For more information on isolinux, including how to create bootdisks
251 for prebuilt kernels, see http://syslinux.zytor.com/
Nico Schottelius7e9dd122006-03-24 03:18:18 -0800252
Linus Torvalds1da177e2005-04-16 15:20:36 -0700253
254
255
2564.) Credits
257 -------
258
259 The nfsroot code in the kernel and the RARP support have been written
260 by Gero Kuhlmann <gero@gkminix.han.de>.
261
262 The rest of the IP layer autoconfiguration code has been written
263 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
264
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.