H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 399)

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mode, all hosts on this subnet can receive the request, but only the requested host (namely, Host B)
will process the request.
4) Host B compares its own IP address with the destination IP address in the ARP request. If they are
the same, Host B saves the source IP address and source MAC address into its ARP mapping
table, encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.
5) After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP mapping table
for subsequent packet forwarding. Meanwhile, Host A encapsulates the IP packet and sends it out.
Usually ARP dynamically implements and automatically seeks mappings from IP addresses to MAC
addresses, without manual intervention.
Introduction to ARP Attack Detection
Man-in-the-middle attack
According to the ARP design, after receiving an ARP response, a host adds the IP-to-MAC mapping of
the sender into its ARP mapping table even if the MAC address is not the real one. This can reduce the
ARP traffic in the network, but it also makes ARP spoofing possible.
In Figure 40-3 , Host A communicates with Host C through Switch. To intercept the traffic between Host
A and Host C, the hacker (Host B) forwards invalid ARP reply messages to Host A and Host C
respectively, causing the two hosts to update the MAC address corresponding to the peer IP address in
their ARP tables with the MAC address of Host B. Then, the traffic between Host A and C will pass
through Host B which acts like a “man-in-the-middle” that may intercept and modify the communication
information. Such attack is called man-in-the-middle attack.
Figure 40-3 Network diagram for ARP man-in-the-middle attack
Host A
IP_A
MAC_A
Host B
IP_B
MAC_B
Host C
IP_ C
MAC_C
Switch
Invalid
ARP reply
Invalid
ARP reply
ARP attack detection
To guard against the man-in-the-middle attacks launched by hackers or attackers, the device supports
the ARP attack detection function. All ARP (both request and response) packets passing through the
device are redirected to the CPU, which checks the validity of all the ARP packets by using the DHCP
snooping table or the manually configured IP binding table. For description of DHCP snooping table and