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328 | Dynamic Host Configuration Protocol
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• DHCP MAC Source Address Validation on page 328 verifies a DHCP packet’s source hardware
address matches the client hardware address field (CHADDR) in the payload.
• IP+MAC Source Address Validation on page 328 verifies that the IP source address and MAC source
address are a legitimate pair.
IP Source Address Validation
IP Source Address Validation (SAV) prevents IP spoofing by forwarding only IP packets that have been
validated against the DHCP binding table. A spoofed IP packet is one in which the IP source address is
strategically chosen to disguise the attacker. For example, using ARP spoofing an attacker can assume a
legitimate client’s identity and receive traffic addressed to it. Then the attacker can spoof the client’s IP
address to interact with other clients.
The DHCP binding table associates addresses assigned by the DHCP servers, with the port on which the
requesting client is attached. When IP Source Address Validation is enabled on a port, the system verifies
that the source IP address is one that is associated with the incoming port. If an attacker is impostering as a
legitimate client the source address appears on the wrong ingress port, and the system drops the packet.
Likewise, if the IP address is fake, the address will not be on the list of permissible addresses for the port,
and the packet is dropped.
DHCP MAC Source Address Validation
DHCP MAC Source Address Validation (SAV) validates a DHCP packet’s source hardware address
against the client hardware address field (CHADDR) in the payload.
FTOS Release 8.2.1.1 ensures that the packet’s source MAC address is checked against the CHADDR
field in the DHCP header only for packets from snooped VLANs.
IP+MAC Source Address Validation
IP+MAC Source Address Validation is available on platforms: c s
Task Command Syntax Command Mode
Enable IP Source Address Validation
ip dhcp source-address-validation
INTERFACE
Task Command Syntax Command Mode
Enable DHCP MAC Source Address
Validation.
ip dhcp snooping verify
mac-address
CONFIGURATION