Reference Guide

Dynamic Host Configuration Protocol | 323
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
IP Source Address Validation validates the IP source address of an incoming packet against the DHCP
Snooping binding table. IP+MAC Source Address Validation ensures that the IP source address and MAC
source address are a legitimate pair, rather validating each attribute individually. IP+MAC Source Address
Validation cannot be configured with IP Source Address Validation.
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
Step Task Command Syntax Command Mode
1 Allocate at least one FP block to the ipmacacl
CAM region.
cam-acl l2acl CONFIGURATION
2 Save the running-config to the startup-config. copy running-config startup-config EXEC Privilege
3 Reload the system. reload EXEC Privilege
4 Enable IP+MAC Source Address Validation. ip dhcp source-address-validation
ipmac
INTERFACE