DHCP Options, Option Format and "Option Overloading"
(Page 4 of 4)
Since DHCP relies so much more on the use of options than BOOTP did, the size of the Options field could theoretically grow quite large. However, since DHCP is using UDP for transport, the overall size of a message is limited. This could theoretically have led to a situation where a message might run out of room and be unable to carry all its options. Meanwhile, there are two spacious fields in the message format, SName and File, at 64 and 128 bytes respectively. These fields might not even be needed in some cases, because many devices use DHCP for getting a lease and parameters and not to download a boot image. Even if they are needed, they might be carrying much less information than their large fixed size allows.
To make better use of the total space in the message format, DHCP includes a special feature called option overloading, which allows these two fields to be used to carry more option fields instead of their conventional information. Use of this option is itself indicated through the use of a DHCP option, Option Overload, which tells a device receiving a message how to interpret the two fields. If option overload is used, the SName and/or File fields are read and interpreted in the same way as the Options field, after all options in the Option field are parsed. If the message actually does need to carry a server name or boot file, these are included as separate options (number 66 and number 67, respectively), which are variable-length and can therefore be made exactly the length needed. Again, see the next topic for details.
Incidentally, the creators of DHCP did recognize that even though vendor-independent options are important, a vendor might want to be able to send vendor-specific information just as the original BOOTP defined. To this end, they created a DHCP option called Vendor Specific Information. This option allows a vendor to encapsulate a set of vendor-specific option fields within the normal DHCP option structure.
In essence, you can think of this as a way of nesting a conventional BOOTP Vend field (of variable length) within a single DHCP option. Other DHCP options can be carried simultaneously, subject to overall message length limits. Note that this supplements an already-existing BOOTP option that allows reference to be made to a file containing vendor-specific information.
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