BOOTP Vendor-Specific Area and Vendor Information Extensions
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The creators of the Boot Protocol realized that certain types of hardware might require additional information to be passed from the server to the client in order for the client to boot up. For this reason, they put into the BOOTP field format the 64-byte Vend field, also called the Vendor-Specific Area. Including this field makes BOOTP flexible, since it allows vendors to decide for themselves how they want to use the protocol, and to tailor it to their needs.
A client can use the Vend field by asking for certain types of information in the field when composing its request. The server can then respond to these requests, and may also include parameters it wants the client to have even if they were not requested. The original BOOTP protocol does not define any structure for the Vendor-Specific Area, leaving this up to each manufacturer to decide.
Obviously, there is nothing preventing a client made by one manufacturer from trying to send a request to a server made by another one. If each one is expecting the Vend field to contain something different, the results will be less than satisfactory. Thus, for the Vend field to be used properly, both devices must be speaking the same language when it comes to the meaning of this field. This is done by setting the first four bytes of the field to a special value. Each manufacturer chooses its own magic number for this four-byte subfield, which is also sometimes called a magic cookie.
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