It is sometimes very useful to boot from a network, especially when you use a machine which has no local disk. In this case, you need to obtain a kind of Net Boot rom, such as a PXE rom or a free software package like Etherboot. Such a Boot rom first boots the machine, sets up the network card installed into the machine, and downloads a second stage boot image from the network. Then, the second image will try to boot an operating system actually from the network.
GRUB provides two second stage images, nbgrub and pxegrub (see Images). These images are the same as the normal Stage 2, except that they set up a network automatically, and try to load a configuration file from the network, if specified. The usage is very simple: If the machine has a PXE rom, use pxegrub. If the machine has an NBI loader such as Etherboot, use nbgrub. There is no difference between them except their formats. Since the way to load a second stage image you want to use should be described in the manual on your Net Boot rom, please refer to the manual, for more information.
However, there is one thing specific to GRUB. Namely, how to specify a configuration file in a BOOTP/DHCP server. For now, GRUB uses the tag `150', to get the name of a configuration file. The following is an example with a BOOTP configuration:
.allhost:hd=/tmp:bf=null:\ :ds=18.104.22.168 22.214.171.124:\ :sm=255.255.254.0:\ :gw=126.96.36.199:\ :sa=188.8.131.52: foo:ht=1:ha=63655d0334a7:ip=184.108.40.206:\ :bf=/nbgrub:\ :tc=.allhost:\ :T150="(nd)/tftpboot/menu.lst.foo":
Note that you should specify the drive name
(nd) in the name of
the configuration file. This is because you might change the root drive
before downloading the configuration from the TFTP server when the
preset menu feature is used (see Preset Menu).
See the manual of your BOOTP/DHCP server for more information. The exact syntax should differ a little from the example.