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]