Dell S4048T-ON, Руководство по настройке

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However, DHCP release and decline packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls
below 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 snooping
binding table created using the ACK packets from the trusted port.
Dell#show ip dhcp snooping binding
Codes : S - Static D - Dynamic
IP 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
10.1.1.252 00:00:4d:57:e6:f6 172800 D Vl 10 Te 1/1
10.1.1.253 00:00:4d:57:f8:e8 172740 D Vl 10 Te 1/3
10.1.1.254 00:00:4d:69:e8:f2 172740 D Vl 10 Te 1/5
Total number of Entries in the table : 4
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.
310 Dynamic Host Configuration Protocol (DHCP)