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1Mounting 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>
6Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
7Updated 2006 by Horms <horms@verge.net.au>
8
9
10
11In 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/
14ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
15filesystem 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
17diskless system, and 'server' means the NFS server.
18
19
20
21
221.) Enabling nfsroot capabilities
23 -----------------------------
24
25In 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.
32
33
34
35
362.) Kernel command line
37 -------------------
38
39When 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:
43
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
54 If the `nfsroot' parameter is NOT given on the command line,
55 the default "/tftpboot/%s" will be used.
56
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.
61
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.
66
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 = 4096
71 wsize = 4096
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
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.
88
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.
93
94 The <autoconf> parameter can appear alone as the value to the `ip'
95 parameter (without all the ':' characters before). If the value is
96 "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
97 autoconfiguration will take place. The most common way to use this
98 is "ip=dhcp".
99
100 <client-ip> IP address of the client.
101
102 Default: Determined using autoconfiguration.
103
104 <server-ip> IP address of the NFS server. If RARP is used to determine
105 the client address and this parameter is NOT empty only
106 replies from the specified server are accepted.
107
108 Only required for NFS root. That is autoconfiguration
109 will not be triggered if it is missing and NFS root is not
110 in operation.
111
112 Default: Determined using autoconfiguration.
113 The address of the autoconfiguration server is used.
114
115 <gw-ip> IP address of a gateway if the server is on a different subnet.
116
117 Default: Determined using autoconfiguration.
118
119 <netmask> Netmask for local network interface. If unspecified
120 the netmask is derived from the client IP address assuming
121 classful addressing.
122
123 Default: Determined using autoconfiguration.
124
125 <hostname> Name of the client. May be supplied by autoconfiguration,
126 but its absence will not trigger autoconfiguration.
127 If specified and DHCP is used, the user provided hostname will
128 be carried in the DHCP request to hopefully update DNS record.
129
130 Default: Client IP address is used in ASCII notation.
131
132 <device> Name of network device to use.
133
134 Default: If the host only has one device, it is used.
135 Otherwise the device is determined using
136 autoconfiguration. This is done by sending
137 autoconfiguration requests out of all devices,
138 and using the device that received the first reply.
139
140 <autoconf> Method to use for autoconfiguration. In the case of options
141 which specify multiple autoconfiguration protocols,
142 requests are sent using all protocols, and the first one
143 to reply is used.
144
145 Only autoconfiguration protocols that have been compiled
146 into the kernel will be used, regardless of the value of
147 this option.
148
149 off or none: don't use autoconfiguration
150 (do static IP assignment instead)
151 on or any: use any protocol available in the kernel
152 (default)
153 dhcp: use DHCP
154 bootp: use BOOTP
155 rarp: use RARP
156 both: use both BOOTP and RARP but not DHCP
157 (old option kept for backwards compatibility)
158
159 Default: any
160
161
162nfsrootdebug
163
164 This parameter enables debugging messages to appear in the kernel
165 log at boot time so that administrators can verify that the correct
166 NFS mount options, server address, and root path are passed to the
167 NFS client.
168
169
170rdinit=<executable file>
171
172 To specify which file contains the program that starts system
173 initialization, administrators can use this command line parameter.
174 The default value of this parameter is "/init". If the specified
175 file exists and the kernel can execute it, root filesystem related
176 kernel command line parameters, including `nfsroot=', are ignored.
177
178 A description of the process of mounting the root file system can be
179 found in:
180
181 Documentation/early-userspace/README
182
183
184
185
1863.) Boot Loader
187 ----------
188
189To get the kernel into memory different approaches can be used.
190They depend on various facilities being available:
191
192
1933.1) Booting from a floppy using syslinux
194
195 When building kernels, an easy way to create a boot floppy that uses
196 syslinux is to use the zdisk or bzdisk make targets which use zimage
197 and bzimage images respectively. Both targets accept the
198 FDARGS parameter which can be used to set the kernel command line.
199
200 e.g.
201 make bzdisk FDARGS="root=/dev/nfs"
202
203 Note that the user running this command will need to have
204 access to the floppy drive device, /dev/fd0
205
206 For more information on syslinux, including how to create bootdisks
207 for prebuilt kernels, see http://syslinux.zytor.com/
208
209 N.B: Previously it was possible to write a kernel directly to
210 a floppy using dd, configure the boot device using rdev, and
211 boot using the resulting floppy. Linux no longer supports this
212 method of booting.
213
2143.2) Booting from a cdrom using isolinux
215
216 When building kernels, an easy way to create a bootable cdrom that
217 uses isolinux is to use the isoimage target which uses a bzimage
218 image. Like zdisk and bzdisk, this target accepts the FDARGS
219 parameter which can be used to set the kernel command line.
220
221 e.g.
222 make isoimage FDARGS="root=/dev/nfs"
223
224 The resulting iso image will be arch/<ARCH>/boot/image.iso
225 This can be written to a cdrom using a variety of tools including
226 cdrecord.
227
228 e.g.
229 cdrecord dev=ATAPI:1,0,0 arch/x86/boot/image.iso
230
231 For more information on isolinux, including how to create bootdisks
232 for prebuilt kernels, see http://syslinux.zytor.com/
233
2343.2) Using LILO
235 When using LILO all the necessary command line parameters may be
236 specified using the 'append=' directive in the LILO configuration
237 file.
238
239 However, to use the 'root=' directive you also need to create
240 a dummy root device, which may be removed after LILO is run.
241
242 mknod /dev/boot255 c 0 255
243
244 For information on configuring LILO, please refer to its documentation.
245
2463.3) Using GRUB
247 When using GRUB, kernel parameter are simply appended after the kernel
248 specification: kernel <kernel> <parameters>
249
2503.4) Using loadlin
251 loadlin may be used to boot Linux from a DOS command prompt without
252 requiring a local hard disk to mount as root. This has not been
253 thoroughly tested by the authors of this document, but in general
254 it should be possible configure the kernel command line similarly
255 to the configuration of LILO.
256
257 Please refer to the loadlin documentation for further information.
258
2593.5) Using a boot ROM
260 This is probably the most elegant way of booting a diskless client.
261 With a boot ROM the kernel is loaded using the TFTP protocol. The
262 authors of this document are not aware of any no commercial boot
263 ROMs that support booting Linux over the network. However, there
264 are two free implementations of a boot ROM, netboot-nfs and
265 etherboot, both of which are available on sunsite.unc.edu, and both
266 of which contain everything you need to boot a diskless Linux client.
267
2683.6) Using pxelinux
269 Pxelinux may be used to boot linux using the PXE boot loader
270 which is present on many modern network cards.
271
272 When using pxelinux, the kernel image is specified using
273 "kernel <relative-path-below /tftpboot>". The nfsroot parameters
274 are passed to the kernel by adding them to the "append" line.
275 It is common to use serial console in conjunction with pxeliunx,
276 see Documentation/serial-console.txt for more information.
277
278 For more information on isolinux, including how to create bootdisks
279 for prebuilt kernels, see http://syslinux.zytor.com/
280
281
282
283
2844.) Credits
285 -------
286
287 The nfsroot code in the kernel and the RARP support have been written
288 by Gero Kuhlmann <gero@gkminix.han.de>.
289
290 The rest of the IP layer autoconfiguration code has been written
291 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
292
293 In order to write the initial version of nfsroot I would like to thank
294 Jens-Uwe Mager <jum@anubis.han.de> for his help.
1Mounting 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>
6Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@schottelius.org>
7Updated 2006 by Horms <horms@verge.net.au>
8
9
10
11In 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/
14ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt) or a
15filesystem 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
17diskless system, and 'server' means the NFS server.
18
19
20
21
221.) Enabling nfsroot capabilities
23 -----------------------------
24
25In 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.
32
33
34
35
362.) Kernel command line
37 -------------------
38
39When 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:
43
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
54 If the `nfsroot' parameter is NOT given on the command line,
55 the default "/tftpboot/%s" will be used.
56
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.
61
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.
66
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 = 4096
71 wsize = 4096
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 <dns0-ip>:<dns1-ip>
83
84 This parameter tells the kernel how to configure IP addresses of devices
85 and also how to set up the IP routing table. It was originally called
86 `nfsaddrs', but now the boot-time IP configuration works independently of
87 NFS, so it was renamed to `ip' and the old name remained as an alias for
88 compatibility reasons.
89
90 If this parameter is missing from the kernel command line, all fields are
91 assumed to be empty, and the defaults mentioned below apply. In general
92 this means that the kernel tries to configure everything using
93 autoconfiguration.
94
95 The <autoconf> parameter can appear alone as the value to the `ip'
96 parameter (without all the ':' characters before). If the value is
97 "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
98 autoconfiguration will take place. The most common way to use this
99 is "ip=dhcp".
100
101 <client-ip> IP address of the client.
102
103 Default: Determined using autoconfiguration.
104
105 <server-ip> IP address of the NFS server. If RARP is used to determine
106 the client address and this parameter is NOT empty only
107 replies from the specified server are accepted.
108
109 Only required for NFS root. That is autoconfiguration
110 will not be triggered if it is missing and NFS root is not
111 in operation.
112
113 Default: Determined using autoconfiguration.
114 The address of the autoconfiguration server is used.
115
116 <gw-ip> IP address of a gateway if the server is on a different subnet.
117
118 Default: Determined using autoconfiguration.
119
120 <netmask> Netmask for local network interface. If unspecified
121 the netmask is derived from the client IP address assuming
122 classful addressing.
123
124 Default: Determined using autoconfiguration.
125
126 <hostname> Name of the client. May be supplied by autoconfiguration,
127 but its absence will not trigger autoconfiguration.
128 If specified and DHCP is used, the user provided hostname will
129 be carried in the DHCP request to hopefully update DNS record.
130
131 Default: Client IP address is used in ASCII notation.
132
133 <device> Name of network device to use.
134
135 Default: If the host only has one device, it is used.
136 Otherwise the device is determined using
137 autoconfiguration. This is done by sending
138 autoconfiguration requests out of all devices,
139 and using the device that received the first reply.
140
141 <autoconf> Method to use for autoconfiguration. In the case of options
142 which specify multiple autoconfiguration protocols,
143 requests are sent using all protocols, and the first one
144 to reply is used.
145
146 Only autoconfiguration protocols that have been compiled
147 into the kernel will be used, regardless of the value of
148 this option.
149
150 off or none: don't use autoconfiguration
151 (do static IP assignment instead)
152 on or any: use any protocol available in the kernel
153 (default)
154 dhcp: use DHCP
155 bootp: use BOOTP
156 rarp: use RARP
157 both: use both BOOTP and RARP but not DHCP
158 (old option kept for backwards compatibility)
159
160 Default: any
161
162 <dns0-ip> IP address of first nameserver.
163 Value gets exported by /proc/net/pnp which is often linked
164 on embedded systems by /etc/resolv.conf.
165
166 <dns1-ip> IP address of secound nameserver.
167 Same as above.
168
169
170nfsrootdebug
171
172 This parameter enables debugging messages to appear in the kernel
173 log at boot time so that administrators can verify that the correct
174 NFS mount options, server address, and root path are passed to the
175 NFS client.
176
177
178rdinit=<executable file>
179
180 To specify which file contains the program that starts system
181 initialization, administrators can use this command line parameter.
182 The default value of this parameter is "/init". If the specified
183 file exists and the kernel can execute it, root filesystem related
184 kernel command line parameters, including `nfsroot=', are ignored.
185
186 A description of the process of mounting the root file system can be
187 found in:
188
189 Documentation/early-userspace/README
190
191
192
193
1943.) Boot Loader
195 ----------
196
197To get the kernel into memory different approaches can be used.
198They depend on various facilities being available:
199
200
2013.1) Booting from a floppy using syslinux
202
203 When building kernels, an easy way to create a boot floppy that uses
204 syslinux is to use the zdisk or bzdisk make targets which use zimage
205 and bzimage images respectively. Both targets accept the
206 FDARGS parameter which can be used to set the kernel command line.
207
208 e.g.
209 make bzdisk FDARGS="root=/dev/nfs"
210
211 Note that the user running this command will need to have
212 access to the floppy drive device, /dev/fd0
213
214 For more information on syslinux, including how to create bootdisks
215 for prebuilt kernels, see http://syslinux.zytor.com/
216
217 N.B: Previously it was possible to write a kernel directly to
218 a floppy using dd, configure the boot device using rdev, and
219 boot using the resulting floppy. Linux no longer supports this
220 method of booting.
221
2223.2) Booting from a cdrom using isolinux
223
224 When building kernels, an easy way to create a bootable cdrom that
225 uses isolinux is to use the isoimage target which uses a bzimage
226 image. Like zdisk and bzdisk, this target accepts the FDARGS
227 parameter which can be used to set the kernel command line.
228
229 e.g.
230 make isoimage FDARGS="root=/dev/nfs"
231
232 The resulting iso image will be arch/<ARCH>/boot/image.iso
233 This can be written to a cdrom using a variety of tools including
234 cdrecord.
235
236 e.g.
237 cdrecord dev=ATAPI:1,0,0 arch/x86/boot/image.iso
238
239 For more information on isolinux, including how to create bootdisks
240 for prebuilt kernels, see http://syslinux.zytor.com/
241
2423.2) Using LILO
243 When using LILO all the necessary command line parameters may be
244 specified using the 'append=' directive in the LILO configuration
245 file.
246
247 However, to use the 'root=' directive you also need to create
248 a dummy root device, which may be removed after LILO is run.
249
250 mknod /dev/boot255 c 0 255
251
252 For information on configuring LILO, please refer to its documentation.
253
2543.3) Using GRUB
255 When using GRUB, kernel parameter are simply appended after the kernel
256 specification: kernel <kernel> <parameters>
257
2583.4) Using loadlin
259 loadlin may be used to boot Linux from a DOS command prompt without
260 requiring a local hard disk to mount as root. This has not been
261 thoroughly tested by the authors of this document, but in general
262 it should be possible configure the kernel command line similarly
263 to the configuration of LILO.
264
265 Please refer to the loadlin documentation for further information.
266
2673.5) Using a boot ROM
268 This is probably the most elegant way of booting a diskless client.
269 With a boot ROM the kernel is loaded using the TFTP protocol. The
270 authors of this document are not aware of any no commercial boot
271 ROMs that support booting Linux over the network. However, there
272 are two free implementations of a boot ROM, netboot-nfs and
273 etherboot, both of which are available on sunsite.unc.edu, and both
274 of which contain everything you need to boot a diskless Linux client.
275
2763.6) Using pxelinux
277 Pxelinux may be used to boot linux using the PXE boot loader
278 which is present on many modern network cards.
279
280 When using pxelinux, the kernel image is specified using
281 "kernel <relative-path-below /tftpboot>". The nfsroot parameters
282 are passed to the kernel by adding them to the "append" line.
283 It is common to use serial console in conjunction with pxeliunx,
284 see Documentation/serial-console.txt for more information.
285
286 For more information on isolinux, including how to create bootdisks
287 for prebuilt kernels, see http://syslinux.zytor.com/
288
289
290
291
2924.) Credits
293 -------
294
295 The nfsroot code in the kernel and the RARP support have been written
296 by Gero Kuhlmann <gero@gkminix.han.de>.
297
298 The rest of the IP layer autoconfiguration code has been written
299 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
300
301 In order to write the initial version of nfsroot I would like to thank
302 Jens-Uwe Mager <jum@anubis.han.de> for his help.