Concept Guide

A spoofed ARP message is one in which the MAC address in the sender hardware address eld and the IP address in the sender protocol
eld are strategically chosen by the attacker. For example, in an MITM attack, the attacker sends a client an ARP message containing the
attacker’s MAC address and the gateway’s IP address. The client then thinks that the attacker is the gateway, and sends all internet-bound
packets to it. Likewise, the attacker sends the gateway an ARP message containing the attacker’s MAC address and the client’s IP address.
The gateway then thinks that the attacker is the client and forwards all packets addressed to the client to it. As a result, the attacker is able
to sni all packets to and from the client.
Other attacks using ARP spoong include:
Broadcast An attacker can broadcast an ARP reply that species FF:FF:FF:FF:FF:FF as the gateway’s MAC address, resulting
in all clients broadcasting all internet-bound packets.
MAC ooding An attacker can send fraudulent ARP messages to the gateway until the ARP cache is exhausted, after which,
trac from the gateway is broadcast.
Denial of service An attacker can send a fraudulent ARP messages to a client to associate a false MAC address with the gateway
address, which would blackhole all internet-bound packets from the client.
NOTE: Dynamic ARP inspection (DAI) uses entries in the L2SysFlow CAM region, a sub-region of SystemFlow. One CAM entry is
required for every DAI-enabled VLAN. You can enable DAI on up to 16 VLANs on a system.
Conguring Dynamic ARP Inspection
To enable dynamic ARP inspection, use the following commands.
1 Enable DHCP snooping.
2 Validate ARP frames against the DHCP snooping binding table.
INTERFACE VLAN mode
arp inspection
Examples of Viewing the ARP Information
To view entries in the ARP database, use the show arp inspection database command.
DellEMC#show arp inspection database
Protocol Address Age(min) Hardware Address Interface VLAN CPU
---------------------------------------------------------------------
Internet 10.1.1.251 - 00:00:4d:57:f2:50 Te 1/2 Vl 10 CP
Internet 10.1.1.252 - 00:00:4d:57:e6:f6 Te 1/1 Vl 10 CP
Internet 10.1.1.253 - 00:00:4d:57:f8:e8 Te 1/3 Vl 10 CP
Internet 10.1.1.254 - 00:00:4d:69:e8:f2 Te 1/5 Vl 10 CP
DellEMC#
To see how many valid and invalid ARP packets have been processed, use the show arp inspection statistics command.
DellEMC#show arp inspection statistics
Dynamic ARP Inspection (DAI) Statistics
---------------------------------------
Valid ARP Requests : 0
Valid ARP Replies : 1000
Invalid ARP Requests : 1000
Invalid ARP Replies : 0
DellEMC#
Dynamic Host
Conguration Protocol (DHCP) 341