Reference Guide

Source Address Validation
Using the DHCP binding table, FTOS can perform three types of source address validation (SAV).
Table 19. Three Types of Source Address Validation
Source Address Validation Description
IP Source Address Validation Prevents IP spoofing by forwarding only IP packets that
have been validated against the DHCP binding table.
DHCP MAC Source Address Validation Verifies a DHCP packet’s source hardware address
matches the client hardware address field (CHADDR) in
the payload.
IP+MAC Source Address Validation Verifies that the IP source address and MAC source
address are a legitimate pair.
Enabling 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 the DHCP servers assign, with the port on which the requesting client is
attached. When you enable IP source address validation 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 is not on the
list of permissible addresses for the port and the packet is dropped.
To enable IP source address validation, use the following command.
NOTE: If you enable IP source guard using the ip dhcp source-address-validation command and
there are 187 entries or more in the current DHCP snooping binding table, SAV may not be applied to all entries. To
ensure that SAV is applied correctly to all entries, enable the ip dhcp source-address-validation
command before adding entries to the binding table.
• Enable IP source address validation.
INTERFACE mode
ip dhcp source-address-validation
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 version 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.
• Enable DHCP MAC SAV.
CONFIGURATION mode
ip dhcp snooping verify mac-address
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