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Based on kernel version 2.6.32. Page generated on 2009-12-11 16:22 EST.

1	Mounting the root filesystem via NFS (nfsroot)
2	===============================================
3	
4	Written 1996 by Gero Kuhlmann <gero[AT]gkminix.han[DOT]de>
5	Updated 1997 by Martin Mares <mj[AT]atrey.karlin.mff.cuni[DOT]cz>
6	Updated 2006 by Nico Schottelius <nico-kernel-nfsroot[AT]schottelius[DOT]org>
7	Updated 2006 by Horms <horms[AT]verge.net[DOT]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	
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	
128	  		Default: Client IP address is used in ASCII notation.
129	
130	  <device>	Name of network device to use.
131	
132			Default: If the host only has one device, it is used.
133				 Otherwise the device is determined using
134				 autoconfiguration. This is done by sending
135				 autoconfiguration requests out of all devices,
136				 and using the device that received the first reply.
137	
138	  <autoconf>	Method to use for autoconfiguration. In the case of options
139	                which specify multiple autoconfiguration protocols,
140			requests are sent using all protocols, and the first one
141			to reply is used.
142	
143			Only autoconfiguration protocols that have been compiled
144			into the kernel will be used, regardless of the value of
145			this option.
146	
147	                  off or none: don't use autoconfiguration
148					(do static IP assignment instead)
149			  on or any:   use any protocol available in the kernel
150				       (default)
151			  dhcp:        use DHCP
152			  bootp:       use BOOTP
153			  rarp:        use RARP
154			  both:        use both BOOTP and RARP but not DHCP
155			               (old option kept for backwards compatibility)
156	
157	                Default: any
158	
159	
160	
161	
162	3.) Boot Loader
163	    ----------
164	
165	To get the kernel into memory different approaches can be used.
166	They depend on various facilities being available:
167	
168	
169	3.1)  Booting from a floppy using syslinux
170	
171		When building kernels, an easy way to create a boot floppy that uses
172		syslinux is to use the zdisk or bzdisk make targets which use zimage
173	      	and bzimage images respectively. Both targets accept the
174	     	FDARGS parameter which can be used to set the kernel command line.
175	
176		e.g.
177		   make bzdisk FDARGS="root=/dev/nfs"
178	
179	   	Note that the user running this command will need to have
180	     	access to the floppy drive device, /dev/fd0
181	
182	     	For more information on syslinux, including how to create bootdisks
183	     	for prebuilt kernels, see http://syslinux.zytor.com/
184	
185		N.B: Previously it was possible to write a kernel directly to
186		     a floppy using dd, configure the boot device using rdev, and
187		     boot using the resulting floppy. Linux no longer supports this
188		     method of booting.
189	
190	3.2) Booting from a cdrom using isolinux
191	
192	     	When building kernels, an easy way to create a bootable cdrom that
193	     	uses isolinux is to use the isoimage target which uses a bzimage
194	     	image. Like zdisk and bzdisk, this target accepts the FDARGS
195	     	parameter which can be used to set the kernel command line.
196	
197		e.g.
198		  make isoimage FDARGS="root=/dev/nfs"
199	
200	     	The resulting iso image will be arch/<ARCH>/boot/image.iso
201	     	This can be written to a cdrom using a variety of tools including
202	     	cdrecord.
203	
204		e.g.
205		  cdrecord dev=ATAPI:1,0,0 arch/i386/boot/image.iso
206	
207	     	For more information on isolinux, including how to create bootdisks
208	     	for prebuilt kernels, see http://syslinux.zytor.com/
209	
210	3.2) Using LILO
211		When using LILO all the necessary command line parameters may be
212		specified using the 'append=' directive in the LILO configuration
213		file.
214	
215		However, to use the 'root=' directive you also need to create
216		a dummy root device, which may be removed after LILO is run.
217	
218		mknod /dev/boot255 c 0 255
219	
220		For information on configuring LILO, please refer to its documentation.
221	
222	3.3) Using GRUB
223		When using GRUB, kernel parameter are simply appended after the kernel
224		specification: kernel <kernel> <parameters>
225	
226	3.4) Using loadlin
227		loadlin may be used to boot Linux from a DOS command prompt without
228		requiring a local hard disk to mount as root. This has not been
229		thoroughly tested by the authors of this document, but in general
230		it should be possible configure the kernel command line similarly
231		to the configuration of LILO.
232	
233		Please refer to the loadlin documentation for further information.
234	
235	3.5) Using a boot ROM
236		This is probably the most elegant way of booting a diskless client.
237		With a boot ROM the kernel is loaded using the TFTP protocol. The
238		authors of this document are not aware of any no commercial boot
239		ROMs that support booting Linux over the network. However, there
240		are two free implementations of a boot ROM, netboot-nfs and
241		etherboot, both of which are available on sunsite.unc.edu, and both
242		of which contain everything you need to boot a diskless Linux client.
243	
244	3.6) Using pxelinux
245		Pxelinux may be used to boot linux using the PXE boot loader
246		which is present on many modern network cards.
247	
248		When using pxelinux, the kernel image is specified using
249		"kernel <relative-path-below /tftpboot>". The nfsroot parameters
250		are passed to the kernel by adding them to the "append" line.
251		It is common to use serial console in conjunction with pxeliunx,
252		see Documentation/serial-console.txt for more information.
253	
254		For more information on isolinux, including how to create bootdisks
255		for prebuilt kernels, see http://syslinux.zytor.com/
256	
257	
258	
259	
260	4.) Credits
261	    -------
262	
263	  The nfsroot code in the kernel and the RARP support have been written
264	  by Gero Kuhlmann <gero[AT]gkminix.han.de>[DOT]
265	
266	  The rest of the IP layer autoconfiguration code has been written
267	  by Martin Mares <mj[AT]atrey.karlin.mff.cuni.cz>[DOT]
268	
269	  In order to write the initial version of nfsroot I would like to thank
270	  Jens-Uwe Mager <jum[AT]anubis.han.de> for his help[DOT]
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