The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/nfsroot.txt

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    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 
    7 
    8 
    9 If you want to use a diskless system, as an X-terminal or printer
   10 server for example, you have to put your root filesystem onto a
   11 non-disk device. This can either be a ramdisk (see initrd.txt in
   12 this directory for further information) or a filesystem mounted
   13 via NFS. The following text describes on how to use NFS for the
   14 root filesystem. For the rest of this text 'client' means the
   15 diskless system, and 'server' means the NFS server.
   16 
   17 
   18 
   19 
   20 1.) Enabling nfsroot capabilities
   21     -----------------------------
   22 
   23 In order to use nfsroot you have to select support for NFS during
   24 kernel configuration. Note that NFS cannot be loaded as a module
   25 in this case. The configuration script will then ask you whether
   26 you want to use nfsroot, and if yes what kind of auto configuration
   27 system you want to use. Selecting both BOOTP and RARP is safe.
   28 
   29 
   30 
   31 
   32 2.) Kernel command line
   33     -------------------
   34 
   35 When the kernel has been loaded by a boot loader (either by loadlin,
   36 LILO or a network boot program) it has to be told what root fs device
   37 to use, and where to find the server and the name of the directory
   38 on the server to mount as root. This can be established by a couple
   39 of kernel command line parameters:
   40 
   41 
   42 root=/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 
   49 nfsroot=[<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 
   80 ip=<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 
  139 3.) Kernel loader
  140     -------------
  141 
  142 To get the kernel into memory different approaches can be used. They
  143 depend on what facilities are available:
  144 
  145 
  146 3.1)  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 
  172 3.2) 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 
  180 3.3) 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 
  188 3.4) 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 
  200 4.) 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.

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