ES4626-SFP Management Guide.pdf

Chapter 12 DHCP option 82
Configuration
12.1 Introduction to DHCP 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. The Relay Agent adds option 82 (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 Relay Agent 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, according to the physical port information in the option. The application of
DHCP option 82 is transparent for the client.
12.1.1 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:
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