Reference Guide

256 | Dynamic Host Configuration Protocol (DHCP)
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The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can
use this information to:
• track the number of address requests per relay agent; restricting the number of addresses available per
relay agent can harden a server against address exhaustion attacks.
• associate client MAC addresses with a relay agent to prevent offering an IP address to a client spoofing
the same MAC address on a different relay agent.
• assign IP addresses according to the relay agent. This prevents generating DHCP offers in response to
requests from an unauthorized relay agent.
The server echoes the option back to the relay agent in its response, and the relay agent can use the
information in the option to forward a reply out the interface on which the request was received rather than
flooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
DHCP Snooping
DHCP Snooping protects networks from spoofing. In the context of DHCP Snooping, all ports are either
trusted or untrusted. By default, all ports are untrusted. Trusted ports are ports through which attackers
cannot connect. Manually configure ports connected to legitimate servers and relay agents as trusted.
When DHCP Snooping is enabled, the relay agent builds a binding table—using DHCPACK messages—
containing the client MAC address, IP addresses, IP address lease time, port, VLAN ID, and binding type.
Every time the relay agent receives a DHCPACK on an trusted port, it adds an entry to the table.
The relay agent then checks all subsequent DHCP client-originated IP traffic (DHCPRELEASE,
DHCPNACK, and DHCPDECLINE) against the binding table to ensure that the MAC-IP address pair is
legitimate, and that the packet arrived on the correct port; packets that do not pass this check are forwarded
to the server for validation. This check-point prevents an attacker from spoofing a client and declining or
releasing the real client’s address. Server-originated packets (DHCPOFFER, DHCPACK, DHCPNACK)
that arrive on an untrusted port are also dropped. This check-point prevents an attacker from impostering
as a DHCP server to facilitate a man-in-the-middle attack.
Task Command Syntax Command Mode
Insert Option 82 into DHCP packets. For routers
between the relay agent and the DHCP server, enter
the
trust-downstream option.
ip dhcp relay information-option
[
trust-downstream]
CONFIGURATION
Configure the system to enable remote-id string in
Option 82.
ip dhcp relay information-option
[
remote-id]
CONFIGURATION