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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 EST.

1	Mounting the root filesystem via NFS (nfsroot)
2	===============================================
3	
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>
8	
9	
10	
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 Documentation/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.
18	
19	
20	
21	
22	1.) Enabling nfsroot capabilities
23	    -----------------------------
24	
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.
28	
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.
32	
33	
34	
35	
36	2.) Kernel command line
37	    -------------------
38	
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:
43	
44	
45	root=/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	
52	nfsroot=[<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	
81	ip=<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			if dhcp is used, the client identifier can be used by following
161			format "ip=dhcp,client-id-type,client-id-value"
162	
163	                Default: any
164	
165	  <dns0-ip>	IP address of first nameserver.
166			Value gets exported by /proc/net/pnp which is often linked
167			on embedded systems by /etc/resolv.conf.
168	
169	  <dns1-ip>	IP address of second nameserver.
170			Same as above.
171	
172	
173	nfsrootdebug
174	
175	  This parameter enables debugging messages to appear in the kernel
176	  log at boot time so that administrators can verify that the correct
177	  NFS mount options, server address, and root path are passed to the
178	  NFS client.
179	
180	
181	rdinit=<executable file>
182	
183	  To specify which file contains the program that starts system
184	  initialization, administrators can use this command line parameter.
185	  The default value of this parameter is "/init".  If the specified
186	  file exists and the kernel can execute it, root filesystem related
187	  kernel command line parameters, including `nfsroot=', are ignored.
188	
189	  A description of the process of mounting the root file system can be
190	  found in:
191	
192	    Documentation/early-userspace/README
193	
194	
195	
196	
197	3.) Boot Loader
198	    ----------
199	
200	To get the kernel into memory different approaches can be used.
201	They depend on various facilities being available:
202	
203	
204	3.1)  Booting from a floppy using syslinux
205	
206		When building kernels, an easy way to create a boot floppy that uses
207		syslinux is to use the zdisk or bzdisk make targets which use zimage
208	      	and bzimage images respectively. Both targets accept the
209	     	FDARGS parameter which can be used to set the kernel command line.
210	
211		e.g.
212		   make bzdisk FDARGS="root=/dev/nfs"
213	
214	   	Note that the user running this command will need to have
215	     	access to the floppy drive device, /dev/fd0
216	
217	     	For more information on syslinux, including how to create bootdisks
218	     	for prebuilt kernels, see http://syslinux.zytor.com/
219	
220		N.B: Previously it was possible to write a kernel directly to
221		     a floppy using dd, configure the boot device using rdev, and
222		     boot using the resulting floppy. Linux no longer supports this
223		     method of booting.
224	
225	3.2) Booting from a cdrom using isolinux
226	
227	     	When building kernels, an easy way to create a bootable cdrom that
228	     	uses isolinux is to use the isoimage target which uses a bzimage
229	     	image. Like zdisk and bzdisk, this target accepts the FDARGS
230	     	parameter which can be used to set the kernel command line.
231	
232		e.g.
233		  make isoimage FDARGS="root=/dev/nfs"
234	
235	     	The resulting iso image will be arch/<ARCH>/boot/image.iso
236	     	This can be written to a cdrom using a variety of tools including
237	     	cdrecord.
238	
239		e.g.
240		  cdrecord dev=ATAPI:1,0,0 arch/x86/boot/image.iso
241	
242	     	For more information on isolinux, including how to create bootdisks
243	     	for prebuilt kernels, see http://syslinux.zytor.com/
244	
245	3.2) Using LILO
246		When using LILO all the necessary command line parameters may be
247		specified using the 'append=' directive in the LILO configuration
248		file.
249	
250		However, to use the 'root=' directive you also need to create
251		a dummy root device, which may be removed after LILO is run.
252	
253		mknod /dev/boot255 c 0 255
254	
255		For information on configuring LILO, please refer to its documentation.
256	
257	3.3) Using GRUB
258		When using GRUB, kernel parameter are simply appended after the kernel
259		specification: kernel <kernel> <parameters>
260	
261	3.4) Using loadlin
262		loadlin may be used to boot Linux from a DOS command prompt without
263		requiring a local hard disk to mount as root. This has not been
264		thoroughly tested by the authors of this document, but in general
265		it should be possible configure the kernel command line similarly
266		to the configuration of LILO.
267	
268		Please refer to the loadlin documentation for further information.
269	
270	3.5) Using a boot ROM
271		This is probably the most elegant way of booting a diskless client.
272		With a boot ROM the kernel is loaded using the TFTP protocol. The
273		authors of this document are not aware of any no commercial boot
274		ROMs that support booting Linux over the network. However, there
275		are two free implementations of a boot ROM, netboot-nfs and
276		etherboot, both of which are available on sunsite.unc.edu, and both
277		of which contain everything you need to boot a diskless Linux client.
278	
279	3.6) Using pxelinux
280		Pxelinux may be used to boot linux using the PXE boot loader
281		which is present on many modern network cards.
282	
283		When using pxelinux, the kernel image is specified using
284		"kernel <relative-path-below /tftpboot>". The nfsroot parameters
285		are passed to the kernel by adding them to the "append" line.
286		It is common to use serial console in conjunction with pxeliunx,
287		see Documentation/serial-console.txt for more information.
288	
289		For more information on isolinux, including how to create bootdisks
290		for prebuilt kernels, see http://syslinux.zytor.com/
291	
292	
293	
294	
295	4.) Credits
296	    -------
297	
298	  The nfsroot code in the kernel and the RARP support have been written
299	  by Gero Kuhlmann <gero@gkminix.han.de>.
300	
301	  The rest of the IP layer autoconfiguration code has been written
302	  by Martin Mares <mj@atrey.karlin.mff.cuni.cz>.
303	
304	  In order to write the initial version of nfsroot I would like to thank
305	  Jens-Uwe Mager <jum@anubis.han.de> for his help.
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