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Dynamic ARP Inspection
Dynamic address resolution protocol (ARP) inspection prevents ARP spoofing by forwarding only ARP
frames that have been validated against the DHCP binding table.
ARP is a stateless protocol that provides no authentication mechanism. Network devices accept ARP
requests and replies from any device. ARP replies are accepted even when no request was sent. If a client
receives an ARP message for which a relevant entry already exists in its ARP cache, it overwrites the
existing entry with the new information.
The lack of authentication in ARP makes it vulnerable to spoofing. ARP spoofing is a technique attackers
use to inject false IP-to-MAC mappings into the ARP cache of a network device. It is used to launch man-
in-the-middle (MITM), and denial-of-service (DoS) attacks, among others.
A spoofed ARP message is one in which the MAC address in the sender hardware address field and the IP
address in the sender protocol field 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 sniff all packets to and from the
client.
Other attacks using ARP spoofing include:
Broadcast An attacker can broadcast an ARP reply that specifies FF:FF:FF:FF:FF:FF as the
gateway’s MAC address, resulting in all clients broadcasting all internet-bound
packets.
MAC flooding An attacker can send fraudulent ARP messages to the gateway until the ARP cache
is exhausted, after which, traffic 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.
Dynamic Host Configuration Protocol (DHCP)
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