Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 492)

492 Chapter 12
TFTP and the Boot ROM File
NetBoot uses the Trivial File Transfer Protocol (TFTP) to send the boot ROM from the server
to the client. Installation of the NetBoot software on a server places the Mac OS 9 boot ROM
file in /Library/NetBoot/NetBootSPx/imagename.nbi/ (where x is the volume number and
imagename is the name of the NBI folder.) The file is called “Mac OS ROM.” For Mac OS X
images, Network Image Utility creates the boot ROM file (“booter”) at this location. The
NetBootSPx directory is automatically created as an NFS share point when you install
NetBoot on your server.
Instead of pointing the client directly to the location of the boot ROM file, NetBoot points to
a symbolic link stored in the directory /private/tftpboot/. The symbolic link references the
actual location of the Mac OS ROM file. This allows you to move the Mac OS ROM file, should
the need arise, by changing the symbolic link in /private/tftpboot/.
Disk Images
The read-only disk images contain the system software and applications used over the
network by the client computers. The name of a disk image file typically ends in .img or
.dmg. Disk Copy—a utility included with Mac OS X and Mac OS 9.2.2—can mount disk image
files as volumes on the desktop. With NetBoot, disk images mounted this way behave as
system startup disks.
You set up Mac OS 9 and Mac OS X disk images in slightly different ways. A preconfigured
Mac OS 9 disk image is provided for you on the CD named NetBoot, Mac OS 9. (The CD
contains four localized versions of the Mac OS 9 image: Tier 0: English, Japanese, French,
and German.) See “Installing the Mac OS 9 Disk Image” on page 497. You can modify the
Mac OS 9 disk image using NetBoot Desktop Admin. See “Modifying the Mac OS 9 Disk
Image” on page 498.
You use Network Image Utility to create Mac OS X disk images, using a Mac OS X install disc
as the “source.” See “Creating a Mac OS X Disk Image” on page 496.
Shadow Images
Many clients may read from the same system disk image, but whenever a client needs to
write anything back to its startup volume (such as print jobs and other temporary files),
NetBoot automatically redirects the written data to the client’s shadow image—a file hidden
from regular system and application software. The shadow image is what preserves the
unique identity of each computer during the entire time it is running off a NetBoot server
disk image. NetBoot transparently handles reading changed data from the shadow file, while
reading unchanged data from the shared system image. The shadow image is recreated at
boot time, so any changes made by the user to his or her startup volume are lost upon
restart. For instance, if a user saves a document to the startup volume, after a restart that
document will be gone. This behavior preserves the condition of the environment the
administrator set up. Therefore it is recommended that users have accounts on a file server
on the network to save their documents.