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> 6 7 8 9If you want to use a diskless system, as an X-terminal or printer 10server for example, you have to put your root filesystem onto a 11non-disk device. This can either be a ramdisk (see initrd.txt in 12this directory for further information) or a filesystem mounted 13via NFS. The following text describes on how to use NFS for the 14root filesystem. For the rest of this text 'client' means the 15diskless system, and 'server' means the NFS server. 16 17 18 19 201.) Enabling nfsroot capabilities 21 ----------------------------- 22 23In order to use nfsroot you have to select support for NFS during 24kernel configuration. Note that NFS cannot be loaded as a module 25in this case. The configuration script will then ask you whether 26you want to use nfsroot, and if yes what kind of auto configuration 27system you want to use. Selecting both BOOTP and RARP is safe. 28 29 30 31 322.) Kernel command line 33 ------------------- 34 35When the kernel has been loaded by a boot loader (either by loadlin, 36LILO or a network boot program) it has to be told what root fs device 37to use, and where to find the server and the name of the directory 38on the server to mount as root. This can be established by a couple 39of kernel command line parameters: 40 41 42root=/dev/nfs 43 44 This is necessary to enable the pseudo-NFS-device. Note that it's not a 45 real device but just a synonym to tell the kernel to use NFS instead of 46 a real device. 47 48 49nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>] 50 51 If the `nfsroot' parameter is NOT given on the command line, the default 52 "/tftpboot/%s" will be used. 53 54 <server-ip> Specifies the IP address of the NFS server. If this field 55 is not given, the default address as determined by the 56 `ip' variable (see below) is used. One use of this 57 parameter is for example to allow using different servers 58 for RARP and NFS. Usually you can leave this blank. 59 60 <root-dir> Name of the directory on the server to mount as root. If 61 there is a "%s" token in the string, the token will be 62 replaced by the ASCII-representation of the client's IP 63 address. 64 65 <nfs-options> Standard NFS options. All options are separated by commas. 66 If the options field is not given, the following defaults 67 will be used: 68 port = as given by server portmap daemon 69 rsize = 1024 70 wsize = 1024 71 timeo = 7 72 retrans = 3 73 acregmin = 3 74 acregmax = 60 75 acdirmin = 30 76 acdirmax = 60 77 flags = hard, nointr, noposix, cto, ac 78 79 80ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf> 81 82 This parameter tells the kernel how to configure IP addresses of devices 83 and also how to set up the IP routing table. It was originally called `nfsaddrs', 84 but now the boot-time IP configuration works independently of NFS, so it 85 was renamed to `ip' and the old name remained as an alias for compatibility 86 reasons. 87 88 If this parameter is missing from the kernel command line, all fields are 89 assumed to be empty, and the defaults mentioned below apply. In general 90 this means that the kernel tries to configure everything using both 91 RARP and BOOTP (depending on what has been enabled during kernel confi- 92 guration, and if both what protocol answer got in first). 93 94 <client-ip> IP address of the client. If empty, the address will either 95 be determined by RARP or BOOTP. What protocol is used de- 96 pends on what has been enabled during kernel configuration 97 and on the <autoconf> parameter. If this parameter is not 98 empty, neither RARP nor BOOTP will be used. 99 100 <server-ip> IP address of the NFS server. If RARP is used to determine 101 the client address and this parameter is NOT empty only 102 replies from the specified server are accepted. To use 103 different RARP and NFS server, specify your RARP server 104 here (or leave it blank), and specify your NFS server in 105 the `nfsroot' parameter (see above). If this entry is blank 106 the address of the server is used which answered the RARP 107 or BOOTP request. 108 109 <gw-ip> IP address of a gateway if the server is on a different 110 subnet. If this entry is empty no gateway is used and the 111 server is assumed to be on the local network, unless a 112 value has been received by BOOTP. 113 114 <netmask> Netmask for local network interface. If this is empty, 115 the netmask is derived from the client IP address assuming 116 classful addressing, unless overridden in BOOTP reply. 117 118 <hostname> Name of the client. If empty, the client IP address is 119 used in ASCII notation, or the value received by BOOTP. 120 121 <device> Name of network device to use. If this is empty, all 122 devices are used for RARP and BOOTP requests, and the 123 first one we receive a reply on is configured. If you have 124 only one device, you can safely leave this blank. 125 126 <autoconf> Method to use for autoconfiguration. If this is either 127 'rarp' or 'bootp', the specified protocol is used. 128 If the value is 'both' or empty, both protocols are used 129 so far as they have been enabled during kernel configura- 130 tion. 'off' means no autoconfiguration. 131 132 The <autoconf> parameter can appear alone as the value to the `ip' 133 parameter (without all the ':' characters before) in which case auto- 134 configuration is used. 135 136 137 138 1393.) Kernel loader 140 ------------- 141 142To get the kernel into memory different approaches can be used. They 143depend on what facilities are available: 144 145 1463.1) Writing the kernel onto a floppy using dd: 147 As always you can just write the kernel onto a floppy using dd, 148 but then it's not possible to use kernel command lines at all. 149 To substitute the 'root=' parameter, create a dummy device on any 150 linux system with major number 0 and minor number 255 using mknod: 151 152 mknod /dev/boot255 c 0 255 153 154 Then copy the kernel zImage file onto a floppy using dd: 155 156 dd if=/usr/src/linux/arch/i386/boot/zImage of=/dev/fd0 157 158 And finally use rdev to set the root device: 159 160 rdev /dev/fd0 /dev/boot255 161 162 You can then remove the dummy device /dev/boot255 again. There 163 is no real device available for it. 164 The other two kernel command line parameters cannot be substi- 165 tuted with rdev. Therefore, using this method the kernel will 166 by default use RARP and/or BOOTP, and if it gets an answer via 167 RARP will mount the directory /tftpboot/<client-ip>/ as its 168 root. If it got a BOOTP answer the directory name in that answer 169 is used. 170 171 1723.2) Using LILO 173 When using LILO you can specify all necessary command line 174 parameters with the 'append=' command in the LILO configuration 175 file. However, to use the 'root=' command you also need to 176 set up a dummy device as described in 3.1 above. For how to use 177 LILO and its 'append=' command please refer to the LILO 178 documentation. 179 1803.3) Using loadlin 181 When you want to boot Linux from a DOS command prompt without 182 having a local hard disk to mount as root, you can use loadlin. 183 I was told that it works, but haven't used it myself yet. In 184 general you should be able to create a kernel command line simi- 185 lar to how LILO is doing it. Please refer to the loadlin docu- 186 mentation for further information. 187 1883.4) Using a boot ROM 189 This is probably the most elegant way of booting a diskless 190 client. With a boot ROM the kernel gets loaded using the TFTP 191 protocol. As far as I know, no commercial boot ROMs yet 192 support booting Linux over the network, but there are two 193 free implementations of a boot ROM available on sunsite.unc.edu 194 and its mirrors. They are called 'netboot-nfs' and 'etherboot'. 195 Both contain everything you need to boot a diskless Linux client. 196 197 198 199 2004.) Credits 201 ------- 202 203 The nfsroot code in the kernel and the RARP support have been written 204 by Gero Kuhlmann <gero@gkminix.han.de>. 205 206 The rest of the IP layer autoconfiguration code has been written 207 by Martin Mares <mj@atrey.karlin.mff.cuni.cz>. 208 209 In order to write the initial version of nfsroot I would like to thank 210 Jens-Uwe Mager <jum@anubis.han.de> for his help. 211