The history of DHCP and DHCP options traces back to
the Bootstrap Protocol (BOOTP). BOOTP was used by a host to configure
itself dynamically during its booting procedure. A host could receive
an IP address and a file from which to download a boot program from
a server, along with the server’s address and the address of an
Included in the BOOTP packet was a vendor information field,
which could contain a number of tagged fields containing various
types of information, such as the subnet mask, the BOOTP file size,
and many other values. RFC 1497 describes the BOOTP Vendor Information Extensions.
DHCP replaces BOOTP; BOOTP is not supported on the firewall.
These extensions eventually expanded with the use of DHCP and
DHCP host configuration parameters, also known as options. Similar
to vendor extensions, DHCP options are tagged data items that provide
information to a DHCP client. The options are sent in a variable-length
field at the end of a DHCP message. For example, the DHCP Message
Type is option 53, and a value of 1 indicates the DHCPDISCOVER message.
DHCP options are defined in RFC 2132, DHCP Options and BOOTP Vendor Extensions.
A DHCP client can negotiate with the server, limiting the server
to send only those options that the client requests.