PLANET WGSW-50040 - Manual (Page 150)

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Chapter 20 DHCP Snooping option 82
Configuration
20.1 Introduction to DHCP Snooping option 82
DHCP option 82 is the Relay Agent Information Option, its option code is 82. DHCP option 82 is aimed at
strengthening the security of DHCP servers and improving the IP address configuration policy. Switch obtain
DHCP request packets(include DHCPDISCOVER, DHCPREQUEST, DHCPINFORM and DHCPRELEASE),
DHCP SNOOPING is addesd to option 82 by request packets (including the client’s physical access port, the
access device ID and other information), to the DHCP request message from the client then forwards the
message to DHCP server. When the DHCP server which supports the option 82 function receives the
message, it will allocate an IP address and other configuration information for the client according to
preconfigured policies and the option 82 information in the message. At the same time, DHCP server can
identify all the possible DHCP attack messages according to the information in option 82 and defend against
them. DHCP SNOOPING will peel the option 82 from the reply messages it receives, and forward the reply
message to the specified port of the network access device. The application of DHCP option 82 is transparent
for the client.
20.1.1 DHCP option 82 Message Structure
A DHCP message can have several option segments; option 82 is one of them. It has to be placed after other
options but before option 255. The following is its format:
Code: represents the sequence number of the relay agent information option, the option 82 is called so
because RFC3046 is defined as 82.
Len: the number of bytes in Agent Information Field, not including the two bytes in Code segment and Len
segment.
Option 82 can have several sub-options, and need at least one sub-option. RFC3046 defines the following
two sub-options, whose formats are showed as follows:
SubOpt: the sequence number of sub-option, the sequence number of Circuit ID sub-option is 1, the
sequence number of Remote ID sub-option is 2.