User's Manual

Configuration Guide of XGS-5240-Series
38-1
Chapter 38 DHCP option 82
Configuration
38.1 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.
38.1.1 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: