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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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	                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	
170	nfsrootdebug
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	
178	rdinit=<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	
194	3.) Boot Loader
195	    ----------
196	
197	To get the kernel into memory different approaches can be used.
198	They depend on various facilities being available:
199	
200	
201	3.1)  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	
222	3.2) 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	
242	3.2) 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	
254	3.3) Using GRUB
255		When using GRUB, kernel parameter are simply appended after the kernel
256		specification: kernel <kernel> <parameters>
257	
258	3.4) 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	
267	3.5) 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	
276	3.6) 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	
292	4.) 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.
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