User's Manual

31-1
Chapter 31 DHCP option 82 Configuration
31.1 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.
31.1.1 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.
Len: the number of bytes in Sub-option Value, not including the two bytes in SubOpt segment and Len
segment.