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• 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 oering an IP address to a client spoong the same MAC address on a
dierent relay agent.
• assign IP addresses according to the relay agent. This prevents generating DHCP oers 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 ooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
To insert Option 82 into DHCP packets, follow this step.
• Insert Option 82 into DHCP packets.
CONFIGURATION mode
ip dhcp relay information-option [trust-downstream]
For routers between the relay agent and the DHCP server, enter the trust-downstream option.
• Manually reset the remote ID for Option 82.
CONFIGURATION mode
ip dhcp relay information-option remote-id
DHCP Snooping
DHCP snooping is a feature that protects networks from spoong. It acts as a rewall between the DHCP server and DHCP clients.
DHCP snooping places the ports either in trusted or non-trusted mode. By default, all ports are set to the non-trusted mode. An attacker
can not connect to the DHCP server through trusted ports. While conguring DHCP snooping, manually congure ports connected to
legitimate servers and relay agents as trusted ports.
When you enable DHCP snooping, 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 a trusted
port, it adds an entry to the table.
The relay agent checks all subsequent DHCP client-originated IP trac (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 checkpoint prevents an attacker from spoong a client and declining or releasing
the real client’s address. Server-originated packets (DHCPOFFER, DHCPACK, and DHCPNACK) that arrive on a not trusted port are also
dropped. This checkpoint prevents an attacker from acting as an imposter as a DHCP server to facilitate a man-in-the-middle attack.
Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE, DHCPNACK, or DHCPDECLINE.
DHCP snooping is supported on Layer 2 and Layer 3 trac. DHCP snooping on Layer 2 interfaces does not require a relay agent.
Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Line cards maintain a list of
snooped VLANs. When the binding table is exhausted, DHCP packets are dropped on snooped VLANs, while these packets are forwarded
across non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are made. However, DHCPRELEASE
and DHCPDECLINE packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls below the
maximum limit of 4000 entries, new IP address assignments are allowed.
NOTE
: DHCP server packets are dropped on all non-trusted interfaces of a system congured for DHCP snooping. To prevent
these packets from being dropped, congure ip dhcp snooping trust on the server-connected port.
Dynamic Host Conguration Protocol (DHCP) 259