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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 26 Configuring Dynamic ARP Inspection
Information About Dynamic ARP Inspection
because ARP allows a gratuitous reply from a host even if an ARP request was not received, an ARP
spoofing attack and the poisoning of ARP caches can occur. After the attack, all traffic from the device
under attack flows through the attacker’s computer and then to the router, switch, or host.
A malicious user can attack hosts, switches, and routers connected to you r Layer 2 network by poisoning
the ARP caches of systems connected to the subnet and by intercepting traffic intended for other hosts
on the subnet. Figure 26-1 shows an example of ARP cache poisoning.
Figure 26-1 ARP Cache Poisoning
Hosts A, B, and C are connected to the switch on interfaces A, B and C, all of which are on the same
subnet. Their IP and MAC addresses are shown in parentheses; for example, Host A uses IP address IA
and MAC address MA. When Host A needs to communicate to Host B a t the IP layer, it broadcasts an
ARP request for the MAC address associated with IP address IB. When the switch and Host B receive
the ARP request, they populate their ARP caches with an ARP binding for a host with the IP address IA
and a MAC address MA; for example, IP address IA is bound to MAC address MA. W hen Host B
responds, the switch and Host A populate their ARP caches with a binding for a host with the IP address
IB and the MAC address MB.
Host C can poison the ARP caches of the switch, Host A, and Host B by broadcasting forged ARP
responses with bindings for a host with an IP address of IA (or IB) and a MAC address of MC. Hosts
with poisoned ARP caches use the MAC address MC as the destination MAC address for traffic intended
for IA or IB. This means that Host C intercepts that traffic. Because Host C knows the true MAC
addresses associated with IA and IB, it can forward the intercepted traffic to those hosts by using the
correct MAC address as the destination. Host C has inserted itself into the traffic stream from Host A to
Host B, the classic man-in-the middle attack.
DAI is a security feature that validates ARP packets in a network. It intercepts, logs, and discards ARP
packets with invalid IP-to-MAC address bindings. This capability protects the network from certain
man-in-the-middle attacks.
DAI ensures that only valid ARP requests and responses are relayed. The switch performs these
activities:
• Intercepts all ARP requests and responses on untrusted ports
• Verifies that each of these intercepted packets has a valid IP-to-MAC address binding before
updating the local ARP cache or before forwarding the packet to the appropriate destination
• Drops invalid ARP packets
DAI determines the validity of an ARP packet based on valid IP-to-MAC address bindings stored in a
trusted database, the DHCP snooping binding database. This database is built by DHCP snooping if
DHCP snooping is enabled on the VLANs and on the switch. If the ARP packet is received on a trusted
interface, the switch forwards the packet without any checks. On untrusted interfaces, the switch
forwards the packet only if it is valid.
AB
C
Host A
(IA, MA)
Host B
(IB, MB)
Host C (man-in-the-middle)
(IC, MC)
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