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Chapter 33 DHCP Snooping option 82
Configuration
33.1 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.
33.1.1 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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