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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>
8Updated 2018 by Chris Novakovic <chris@chrisn.me.uk>
9
10
11
12In order to use a diskless system, such as an X-terminal or printer server
13for example, it is necessary for the root filesystem to be present on a
14non-disk device. This may be an initramfs (see Documentation/filesystems/
15ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/admin-guide/initrd.rst) or a
16filesystem mounted via NFS. The following text describes on how to use NFS
17for the root filesystem. For the rest of this text 'client' means the
18diskless system, and 'server' means the NFS server.
19
20
21
22
231.) Enabling nfsroot capabilities
24 -----------------------------
25
26In order to use nfsroot, NFS client support needs to be selected as
27built-in during configuration. Once this has been selected, the nfsroot
28option will become available, which should also be selected.
29
30In the networking options, kernel level autoconfiguration can be selected,
31along with the types of autoconfiguration to support. Selecting all of
32DHCP, BOOTP and RARP is safe.
33
34
35
36
372.) Kernel command line
38 -------------------
39
40When the kernel has been loaded by a boot loader (see below) it needs to be
41told what root fs device to use. And in the case of nfsroot, where to find
42both the server and the name of the directory on the server to mount as root.
43This can be established using the following kernel command line parameters:
44
45
46root=/dev/nfs
47
48 This is necessary to enable the pseudo-NFS-device. Note that it's not a
49 real device but just a synonym to tell the kernel to use NFS instead of
50 a real device.
51
52
53nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
54
55 If the `nfsroot' parameter is NOT given on the command line,
56 the default "/tftpboot/%s" will be used.
57
58 <server-ip> Specifies the IP address of the NFS server.
59 The default address is determined by the `ip' parameter
60 (see below). This parameter allows the use of different
61 servers for IP autoconfiguration and NFS.
62
63 <root-dir> Name of the directory on the server to mount as root.
64 If there is a "%s" token in the string, it will be
65 replaced by the ASCII-representation of the client's
66 IP address.
67
68 <nfs-options> Standard NFS options. All options are separated by commas.
69 The following defaults are used:
70 port = as given by server portmap daemon
71 rsize = 4096
72 wsize = 4096
73 timeo = 7
74 retrans = 3
75 acregmin = 3
76 acregmax = 60
77 acdirmin = 30
78 acdirmax = 60
79 flags = hard, nointr, noposix, cto, ac
80
81
82ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
83 <dns0-ip>:<dns1-ip>:<ntp0-ip>
84
85 This parameter tells the kernel how to configure IP addresses of devices
86 and also how to set up the IP routing table. It was originally called
87 `nfsaddrs', but now the boot-time IP configuration works independently of
88 NFS, so it was renamed to `ip' and the old name remained as an alias for
89 compatibility reasons.
90
91 If this parameter is missing from the kernel command line, all fields are
92 assumed to be empty, and the defaults mentioned below apply. In general
93 this means that the kernel tries to configure everything using
94 autoconfiguration.
95
96 The <autoconf> parameter can appear alone as the value to the `ip'
97 parameter (without all the ':' characters before). If the value is
98 "ip=off" or "ip=none", no autoconfiguration will take place, otherwise
99 autoconfiguration will take place. The most common way to use this
100 is "ip=dhcp".
101
102 <client-ip> IP address of the client.
103
104 Default: Determined using autoconfiguration.
105
106 <server-ip> IP address of the NFS server. If RARP is used to determine
107 the client address and this parameter is NOT empty only
108 replies from the specified server are accepted.
109
110 Only required for NFS root. That is autoconfiguration
111 will not be triggered if it is missing and NFS root is not
112 in operation.
113
114 Value is exported to /proc/net/pnp with the prefix "bootserver "
115 (see below).
116
117 Default: Determined using autoconfiguration.
118 The address of the autoconfiguration server is used.
119
120 <gw-ip> IP address of a gateway if the server is on a different subnet.
121
122 Default: Determined using autoconfiguration.
123
124 <netmask> Netmask for local network interface. If unspecified
125 the netmask is derived from the client IP address assuming
126 classful addressing.
127
128 Default: Determined using autoconfiguration.
129
130 <hostname> Name of the client. If a '.' character is present, anything
131 before the first '.' is used as the client's hostname, and anything
132 after it is used as its NIS domain name. May be supplied by
133 autoconfiguration, but its absence will not trigger autoconfiguration.
134 If specified and DHCP is used, the user-provided hostname (and NIS
135 domain name, if present) will be carried in the DHCP request; this
136 may cause a DNS record to be created or updated for the client.
137
138 Default: Client IP address is used in ASCII notation.
139
140 <device> Name of network device to use.
141
142 Default: If the host only has one device, it is used.
143 Otherwise the device is determined using
144 autoconfiguration. This is done by sending
145 autoconfiguration requests out of all devices,
146 and using the device that received the first reply.
147
148 <autoconf> Method to use for autoconfiguration. In the case of options
149 which specify multiple autoconfiguration protocols,
150 requests are sent using all protocols, and the first one
151 to reply is used.
152
153 Only autoconfiguration protocols that have been compiled
154 into the kernel will be used, regardless of the value of
155 this option.
156
157 off or none: don't use autoconfiguration
158 (do static IP assignment instead)
159 on or any: use any protocol available in the kernel
160 (default)
161 dhcp: use DHCP
162 bootp: use BOOTP
163 rarp: use RARP
164 both: use both BOOTP and RARP but not DHCP
165 (old option kept for backwards compatibility)
166
167 if dhcp is used, the client identifier can be used by following
168 format "ip=dhcp,client-id-type,client-id-value"
169
170 Default: any
171
172 <dns0-ip> IP address of primary nameserver.
173 Value is exported to /proc/net/pnp with the prefix "nameserver "
174 (see below).
175
176 Default: None if not using autoconfiguration; determined
177 automatically if using autoconfiguration.
178
179 <dns1-ip> IP address of secondary nameserver.
180 See <dns0-ip>.
181
182 <ntp0-ip> IP address of a Network Time Protocol (NTP) server.
183 Value is exported to /proc/net/ipconfig/ntp_servers, but is
184 otherwise unused (see below).
185
186 Default: None if not using autoconfiguration; determined
187 automatically if using autoconfiguration.
188
189 After configuration (whether manual or automatic) is complete, two files
190 are created in the following format; lines are omitted if their respective
191 value is empty following configuration:
192
193 - /proc/net/pnp:
194
195 #PROTO: <DHCP|BOOTP|RARP|MANUAL> (depending on configuration method)
196 domain <dns-domain> (if autoconfigured, the DNS domain)
197 nameserver <dns0-ip> (primary name server IP)
198 nameserver <dns1-ip> (secondary name server IP)
199 nameserver <dns2-ip> (tertiary name server IP)
200 bootserver <server-ip> (NFS server IP)
201
202 - /proc/net/ipconfig/ntp_servers:
203
204 <ntp0-ip> (NTP server IP)
205 <ntp1-ip> (NTP server IP)
206 <ntp2-ip> (NTP server IP)
207
208 <dns-domain> and <dns2-ip> (in /proc/net/pnp) and <ntp1-ip> and <ntp2-ip>
209 (in /proc/net/ipconfig/ntp_servers) are requested during autoconfiguration;
210 they cannot be specified as part of the "ip=" kernel command line parameter.
211
212 Because the "domain" and "nameserver" options are recognised by DNS
213 resolvers, /etc/resolv.conf is often linked to /proc/net/pnp on systems
214 that use an NFS root filesystem.
215
216 Note that the kernel will not synchronise the system time with any NTP
217 servers it discovers; this is the responsibility of a user space process
218 (e.g. an initrd/initramfs script that passes the IP addresses listed in
219 /proc/net/ipconfig/ntp_servers to an NTP client before mounting the real
220 root filesystem if it is on NFS).
221
222
223nfsrootdebug
224
225 This parameter enables debugging messages to appear in the kernel
226 log at boot time so that administrators can verify that the correct
227 NFS mount options, server address, and root path are passed to the
228 NFS client.
229
230
231rdinit=<executable file>
232
233 To specify which file contains the program that starts system
234 initialization, administrators can use this command line parameter.
235 The default value of this parameter is "/init". If the specified
236 file exists and the kernel can execute it, root filesystem related
237 kernel command line parameters, including `nfsroot=', are ignored.
238
239 A description of the process of mounting the root file system can be
240 found in:
241
242 Documentation/driver-api/early-userspace/early_userspace_support.rst
243
244
245
246
2473.) Boot Loader
248 ----------
249
250To get the kernel into memory different approaches can be used.
251They depend on various facilities being available:
252
253
2543.1) Booting from a floppy using syslinux
255
256 When building kernels, an easy way to create a boot floppy that uses
257 syslinux is to use the zdisk or bzdisk make targets which use zimage
258 and bzimage images respectively. Both targets accept the
259 FDARGS parameter which can be used to set the kernel command line.
260
261 e.g.
262 make bzdisk FDARGS="root=/dev/nfs"
263
264 Note that the user running this command will need to have
265 access to the floppy drive device, /dev/fd0
266
267 For more information on syslinux, including how to create bootdisks
268 for prebuilt kernels, see http://syslinux.zytor.com/
269
270 N.B: Previously it was possible to write a kernel directly to
271 a floppy using dd, configure the boot device using rdev, and
272 boot using the resulting floppy. Linux no longer supports this
273 method of booting.
274
2753.2) Booting from a cdrom using isolinux
276
277 When building kernels, an easy way to create a bootable cdrom that
278 uses isolinux is to use the isoimage target which uses a bzimage
279 image. Like zdisk and bzdisk, this target accepts the FDARGS
280 parameter which can be used to set the kernel command line.
281
282 e.g.
283 make isoimage FDARGS="root=/dev/nfs"
284
285 The resulting iso image will be arch/<ARCH>/boot/image.iso
286 This can be written to a cdrom using a variety of tools including
287 cdrecord.
288
289 e.g.
290 cdrecord dev=ATAPI:1,0,0 arch/x86/boot/image.iso
291
292 For more information on isolinux, including how to create bootdisks
293 for prebuilt kernels, see http://syslinux.zytor.com/
294
2953.2) Using LILO
296 When using LILO all the necessary command line parameters may be
297 specified using the 'append=' directive in the LILO configuration
298 file.
299
300 However, to use the 'root=' directive you also need to create
301 a dummy root device, which may be removed after LILO is run.
302
303 mknod /dev/boot255 c 0 255
304
305 For information on configuring LILO, please refer to its documentation.
306
3073.3) Using GRUB
308 When using GRUB, kernel parameter are simply appended after the kernel
309 specification: kernel <kernel> <parameters>
310
3113.4) Using loadlin
312 loadlin may be used to boot Linux from a DOS command prompt without
313 requiring a local hard disk to mount as root. This has not been
314 thoroughly tested by the authors of this document, but in general
315 it should be possible configure the kernel command line similarly
316 to the configuration of LILO.
317
318 Please refer to the loadlin documentation for further information.
319
3203.5) Using a boot ROM
321 This is probably the most elegant way of booting a diskless client.
322 With a boot ROM the kernel is loaded using the TFTP protocol. The
323 authors of this document are not aware of any no commercial boot
324 ROMs that support booting Linux over the network. However, there
325 are two free implementations of a boot ROM, netboot-nfs and
326 etherboot, both of which are available on sunsite.unc.edu, and both
327 of which contain everything you need to boot a diskless Linux client.
328
3293.6) Using pxelinux
330 Pxelinux may be used to boot linux using the PXE boot loader
331 which is present on many modern network cards.
332
333 When using pxelinux, the kernel image is specified using
334 "kernel <relative-path-below /tftpboot>". The nfsroot parameters
335 are passed to the kernel by adding them to the "append" line.
336 It is common to use serial console in conjunction with pxeliunx,
337 see Documentation/admin-guide/serial-console.rst for more information.
338
339 For more information on isolinux, including how to create bootdisks
340 for prebuilt kernels, see http://syslinux.zytor.com/
341
342
343
344
3454.) Credits
346 -------
347
348 The nfsroot code in the kernel and the RARP support have been written
349 by Gero Kuhlmann <gero@gkminix.han.de>.
350
351 The rest of the IP layer autoconfiguration code has been written
352 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
353
354 In order to write the initial version of nfsroot I would like to thank
355 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
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.