Boot Service Discovery Protocol


Boot Service Discovery Protocol is an Apple-developed, standards-conforming extension of DHCP. It allows Macintosh computers to boot from bootable images on a network instead of local storage media such as CD, DVD, or hard disk. The DHCP options used are the "vendor-specific information" option and the "vendor class identifier" option.
There are three versions of BSDP, though usually version 1.0 is used. All versions enable a client to choose from several bootable images offered by a server.
The of BSDP is Darwin's BOOTP server, which is part of Mac OS's NetBoot feature.

Description

Contents of DHCP Vendor Class Identifier

The DHCP server and client send a vendor class option that contains an ASCII-encoded string with three parts delimited by a / character. The first part is AAPLBSDPC, which advertises BSDP capability. The second part is the client's architecture. The third part is a system identifier. For example, an Intel-based iMac sends
AAPLBSDPC/i386/iMac4,1
as its vendor class. A list of Microsoft vendor classes can be found .

Contents of DHCP Vendor Specific Information Options

According to the DHCP Option carrying the number 43 is reserved for vendor specific information. This information is stored in the following format:
Code Len Vendor-specific information
+-----+-----+-----+-----+---
| 43 | n | i1 | i2 |...
+-----+-----+-----+-----+---
If the vendor wants to convey multiple options within this option field, this is done with encapsulated vendor-specific extensions. Vendor encapsulated extensions contain one or more concatenated fields. Each field consists of:
Byte PositionContent
0Type of field
1Length n of field
2 to n-2Data

The following table describes the possible field types. All numeric fields are interpreted as unsigned and Big Endian integers.
TypeMeaningData type
1Message Type8 Bit int
  • 0x00: none
  • 0x01: LIST
  • 0x02: SELECT
  • 0x03: error
2BSDP Version used16 Bit int
  • 0x0000: Version 0.0
  • 0x0100: Version 1.0
  • 0x0101: Version 1.1
    • 3Server IdentifierIP address of the server, one byte per component: c0 a8 64 01 represents 192.168.100.1
    4Server Priority16 Bit int
    5Port for Response16 Bit int
    6"boot image list path"String
    7ID of Standard Boot Image32 Bit int
    8ID of Selected Boot Image32 Bit int
    9List of Boot Images?
    10"netboot 1.0 firmware"?
    11Error List for Image Attribute?
    12Maximum Message Size?
    128"shadow mount path"String

    Here is possible to specify a network-accessible mount where data will be written after a successful boot. If this field is not specified and no storage medium is available locally on the client, then the boot process for Mac OS X is aborted. Officially, Mac OS X only supports AFP shadow mount paths. However, NFS may be used after a modifying of the startup files of the system.
    129"shadow file path"String
    130"machine name" String

    Example

    The following example illustrates the construction of the Vendor Encapsulated Option:
    0000 01 01 02 08 04 81 00 07 e5 82 0a 4e 65 74 42 6f 6f ..........NetBoo
    0010 74 30 30 31 t001
    The first field here, 01 01 02, means that the packet is a BSDP "SELECT" message. The 01 declares that field specifies the BSDP Message Type. The next 01 indicates that the field contents are one byte long — 02 is the code for "SELECT".
    The following 08 04 81 00 07 e5 means that the boot image with the ID 2164262885 is selected.
    Finally, 82 0a 4e 65 74 42 6f 6f 74 30 30 31 means that a string with 0x0a = 10 characters, namely "NetBoot001", is the name of the system to boot.