However, DHCP release and decline packets are allowed so that the DHCP snooping table can decrease in size. After the table usage fallsbelow the maximum limit of 4000 entries, new IP address assignments are allowed.To view the number of entries in the table, use the show ip dhcp snooping binding command. This output displays the snoopingbinding table created using the ACK packets from the trusted port.Dell#show ip dhcp snooping bindingCodes : S - Static D - DynamicIP Address MAC Address Expires(Sec) Type VLAN Interface================================================================10.1.1.251 00:00:4d:57:f2:50 172800 D Vl 10 Te 1/2/110.1.1.252 00:00:4d:57:e6:f6 172800 D Vl 10 Te 1/1/110.1.1.253 00:00:4d:57:f8:e8 172740 D Vl 10 Te 1/3/110.1.1.254 00:00:4d:69:e8:f2 172740 D Vl 10 Te 1/5/1Total number of Entries in the table : 4Dynamic ARP InspectionDynamic address resolution protocol (ARP) inspection prevents ARP spoofing by forwarding only ARP frames that have been validatedagainst 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 inits 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-MACmappings 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 protocolfield are strategically chosen by the attacker. For example, in an MITM attack, the attacker sends a client an ARP message containing theattacker’s MAC address and the gateway’s IP address. The client then thinks that the attacker is the gateway, and sends all internet-boundpackets 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 ableto 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, resultingin 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 gatewayaddress, which would blackhole all internet-bound packets from the client.Dynamic Host Configuration Protocol (DHCP) 301
