Arp Cache Poisoning - Cisco Catalyst 4500 Series Software Configuration Manual

About Dynamic ARP Inspection

ARP Cache Poisoning

You can attack hosts, switches, and routers connected to your Layer 2 network by "poisoning" their ARP
caches. For example, a malicious user might intercept traffic intended for other hosts on the subnet by
poisoning the ARP caches of systems connected to the subnet.
Figure 58-1
Figure 58-1
(IA, MA)
Hosts HA, HB, and HC 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 HA uses IP
address IA and MAC address MA. When HA needs to communicate to HB at the IP Layer, HA
broadcasts an ARP request for the MAC address associated with IB. As soon as HB receives the ARP
request, the ARP cache on HB is populated with an ARP binding for a host with the IP address IA and
a MAC address MA. When HB responds to HA, the ARP cache on HA is populated with a binding for
a host with the IP address IB and a MAC address MB.
Host HC can "poison" the ARP caches of HA and HB 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 HC intercepts that traffic. Because HC knows the true MAC addresses associated with
IA and IB, HC can forward the intercepted traffic to those hosts using the correct MAC address as the
destination. HC has inserted itself into the traffic stream from HA to HB, the classic "man in the middle"
attack.
Purpose of Dynamic ARP Inspection
To prevent ARP poisoning attacks, a switch must ensure that only valid ARP requests and responses are
relayed. DAI prevents these attacks by intercepting all ARP requests and responses. Each of these
intercepted packets is verified for valid MAC address to IP address bindings before the local ARP cache
is updated or the packet is forwarded to the appropriate destination. Invalid ARP packets are dropped.
DAI determines the validity of an ARP packet based on valid MAC address to IP address bindings stored
in a trusted database. This database is built at runtime by DHCP snooping, provided this feature is
enabled on VLANs and on the switch. In addition, in order to handle hosts that use statically configured
IP addresses, DAI can also validate ARP packets against user-configured ARP ACLs.
DAI can also be configured to drop ARP packets when the IP addresses in the packet are invalid or when
the MAC addresses in the body of the ARP packet do not match the addresses specified in the Ethernet
header.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
58-2
shows an example of cache poisoning.
ARP Cache Poisoning
A
HA
C
HC
(IC, MC)
Chapter 58
B
HB
(IB, MB)
Configuring Dynamic ARP Inspection