1-4Introduction to ARP Attack DetectionMan-in-the-middle attackAccording to the ARP design, after receiving an ARP response, a host adds the IP-to-MAC mapping ofthe sender into its ARP mapping table even if the MAC address is not the real one. This can reduce theARP traffic in the network, but it also makes ARP spoofing possible.In Figure 1-3, Host A communicates with Host C through a switch. To intercept the traffic between HostA and Host C, the hacker (Host B) forwards invalid ARP reply messages to Host A and Host Crespectively, causing the two hosts to update the MAC address corresponding to the peer IP address intheir ARP tables with the MAC address of Host B. Then, the traffic between Host A and C will passthrough Host B which acts like a “man-in-the-middle” that may intercept and modify the communicationinformation. Such attack is called man-in-the-middle attack.Figure 1-3 Network diagram for ARP man-in-the-middle attackARP attack detectionTo guard against the man-in-the-middle attacks launched by hackers or attackers, Switch 4210 Familysupport the ARP attack detection function. All ARP (both request and response) packets passingthrough the switch are redirected to the CPU, which checks the validity of all the ARP packets by usingthe DHCP snooping table or the manually configured IP binding table. For description of DHCPsnooping table and the manually configured IP binding table, refer to the DHCP snooping section in thepart discussing DHCP in this manual.After you enable the ARP attack detection function, the switch will check the following items of an ARPpacket: the source MAC address, source IP address, port number of the port receiving the ARP packet,and the ID of the VLAN the port resides. If these items match the entries of the DHCP snooping table orthe manual configured IP binding table, the switch will forward the ARP packet; if not, the switchdiscards the ARP packet.z With trusted ports configured, ARP packets coming from the trusted ports will not be checked,while those from other ports will be checked through the DHCP snooping table or the manuallyconfigured IP binding table.z With the ARP restricted forwarding function enabled, ARP request packets are forwarded throughtrusted ports only; ARP response packets are forwarded according to the MAC addresses in the
