Dynamic Arp Inspection - Dell Force10 S4810P Configuration Manual

High-density, 1ru 48-port 10gbe switch

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Table of Contents

View the number of entries in the table with the

displays the snooping binding table created using the ACK packets from the trusted port.

FTOS#show ip dhcp snooping binding

S - Static D - Dynamic

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Total number of Entries in the table : 4

Dynamic ARP Inspection

Dynamic 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, and 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

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.

show ip dhcp snooping binding

Expires(Sec)

Dynamic Host Configuration Protocol (DHCP) | 391

command. This output