【Understanding and Configuring Dynamic ARP Inspection-the,ARP】
Table Of Contents
Understanding and Configuring Dynamic ARP Inspection
This chapter describes how to configure Dynamic ARP Inspection (DAI) on the Catalyst4500 series switch.
This chapter includes the following major sections:
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Note For complete syntax and usage information for the switch commands used in this chapter, refer to the Catalyst4500SeriesSwitch Cisco IOS Command Reference and related publications at
Overview of Dynamic ARP Inspection
Dynamic ARP Inspection (DAI) is a security feature that validates Address Resolution Protocol (ARP) packets in a network. DAI allows a network administrator to intercept, log, and discard ARP packets with invalid MAC address to IP address bindings. This capability protects the network from certain "man-in-the-middle" attacks.
This section contains the following subsections:
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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.
Consider the following configuration:
Figure34-1 ARP Cache Poisoning
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; for example, IP address IA is bound to MAC address MA. When HB responds, 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.
Dynamic ARP Inspection
To prevent ARP poisoning attacks such as the one described in the previous section, 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 that it is enabled on the VLANs and on the switch in question. In addition, DAI can also validate ARP packets against user-configured ARP ACLs in order to handle hosts that use statically configured IP addresses.
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.
Interface Trust state, Security Coverage and Network Configuration
DAI associates a trust state with each interface on the system. Packets arriving on trusted interfaces bypass all DAI validation checks, while those arriving on untrusted interfaces go through the DAI validation process. In a typical network configuration for DAI, all ports connected to host ports are configured as untrusted, while all ports connected to switches are configured as trusted. With this configuration, all ARP packets entering the network from a given switch will have passed the security check; it is unnecessary to perform a validation at any other place in the VLAN / network:
Figure34-2 Validation of ARP Packets on a DAI-enabled VLAN
Use the trust state configuration carefully.
Configuring interfaces as untrusted when they should be trusted can result in a loss of connectivity. If we assume that both S1 and S2 (in ) run DAI on the VLAN that holds H1 and H2, and if H1 and H2 were to acquire their IP addresses from S1, then only S2 binds the IP to MAC address of H1. Therefore, if the interface between S1 and S2 is untrusted, the ARP packets from H1 get dropped on S2. This condition would result in a loss of connectivity between H1 and H2.
