TITLE
    Mac OS X Server: NetBoot Theory of Operation
Article ID:
Created:
Modified: 		60080
3/9/99
8/4/00
TOPIC

    When NetBoot software is installed on a server, images of the Mac OS and the Applications folder are copied onto its hard disk. The Mac OS image contains the Mac OS system software that each of your NetBoot clients needs to start up. The Applications image contains application software for your users. These appear as two separate hard disks on the desktop after the computer has started up.

DISCUSSION

    Establishing a Network Connection

    When a user turns on a NetBoot client computer, the computer broadcasts a message to find the startup server program. The startup server program responds by sending information (such as an IP address) the client computer needs to establish a connection to the NetBoot server. This information is entered using the Setup Assistant, which asks for a range of IP addresses that can be used by NetBoot computers. The NetBoot server keeps track of the client's hardware Ethernet address, and associates it with the IP address and other information in a table. Each time the client starts up from a NetBoot server, this table is referenced to ensure the client receives the same IP address and other information at system startup time. Addresses in the NetBoot range are therefore not recycled when a computer shuts down. They are stored in the table for use when the computer starts back up. Be sure to include extra addresses in the range you specify for additional clients on your network, as addresses are reserved for each client by the association between the Ethernet adapter address and the assigned IP address. Once a connection is established between a client computer and the server, portions of the Mac OS ROM image are copied from the server to the client computer's memory. This is done using the Trivial File Transfer Protocol (TFTP).

    Note : The Mac OS ROM file must be present for NetBoot clients to start up successfully. When you use the installer, the Mac OS ROM file is installed on your disk. Be sure not to move or delete it. If you should add newer computers to your network over time, this file may need to be updated with a new ROM file to be compatible with your new computers.

    Next, the Mac OS ROM image that was copied to the client computer's memory connects to the image of the Mac OS and opens the System file. You see an icon on the screen indicating that the Mac OS is starting up. This is accomplished by a minimal Apple File Protocol (AFP) client (AppleShare-type) contained in the Mac OS ROM and stipulates that there must be an AFP server running on the Mac OS X Server used as the NetBoot server.

    NetBoot Process
    • NetBoot client is turned on, Open Firmware (OF) net-boot begins transmitting BOOTP requests
    • BOOTP server (bootpd on Mac OS X Server) receives request and looks for a host entry with the client's hardware Ethernet address
    • If no entry exists, the server creates a new client identity:
      • Finds an available IP address on the client's subnet
      • Assigns the client a unique number i, range 1..n
      • Creates an AFP login account
      • Creates a new host entry
    • Once an identity is established, the server: Creates an empty copy-on-write disk image file shadow of the system disk image created the first time and when size of shadowed file changes
    • Places startup parameters into a BOOTP reply, sends it to client
    • Open Firmware client receives BOOTP reply:
      • Stores the reply in the Open Firmware tree
      • Configures itself using the IP parameters in the reply
      • Downloads the startup file, Mac OS ROM, using TFTP
      • Passes control to Mac OS
    • Mac OS retrieves BOOTP reply from Open Firmware tree
    • The "network block driver" initializes itself using information in the reply:
      • Configures its IP stack
      • Logs into the AFP server using the AFP login identity
      • Connects to the system disk image copy-on-write and the applications disk image read-only
    • The remainder of Mac OS starts up as usual
    • Open Transport stack configures itself using BOOTP


    NetBoot Disk Image Files
    • NetBoot Server provides the client with three disk image files by default:
      • System read-only system disk image
      • Applications read-only applications disk image
      • System_s read/write system disk shadow of System

    System and Applications disk images are shared by all NetBoot clients. Each client gets its own shadow system disk image-an empty image file the same size as System-created by the BOOTP server when the client starts up. Files written to the System image are written to the shadow file and are therefore not maintained when a client is restarted.

    These images are stored on the Mac OS Extended format (HFS Plus) partition of the NetBoot server. If there is more than one HFS Plus partition on the server, the shadow images created for NetBoot users are automatically distributed between all HFS Plus volumes with available space.

    Each NetBoot client's shadow image takes up the full system disk image space for a short time. Prior to starting up the shadow image size decreases in size and increases as needed.

    There are two ways to start up a computer from a NetBoot server:
    • Open the Startup Disk control panel and click NetBoot HD.
    • Press and hold the "N" key as you start up the computer.

Document Information
Product Area: Mac OS System Software
Category: Mac OS X Server
Sub Category: NetBoot

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