H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 168)

23-2
Figure 23-1 Man-in-the-middle attack
Host A
Host B
IP_ A
MAC_ A
IP_B
MAC_B
IP_C
MAC_C
Host C
Forged
ARP reply
Forged
ARP reply
Device
ARP detection mechanism
With ARP detection enabled for a specific VLAN, ARP messages arriving on any interface in the VLAN
are redirected to the CPU to have their sender MAC and IP addresses checked. ARP messages that
pass the check are forwarded; otherwise, they are discarded.
1) ARP detection based on static IP Source Guard binding entries/DHCP snooping entries/802.1X
security entries/OUI MAC addresses
With this feature enabled, the device compares the sender IP and MAC addresses of an ARP packet
received from the VLAN against the static IP Source Guard binding entries, DHCP snooping entries,
802.1X security entries, or OUI MAC addresses to prevent spoofing.
After you enable this feature for a VLAN,
Upon receiving an ARP packet from an ARP untrusted port, the device compares the sender IP
and MAC addresses of the ARP packet against the static IP Source Guard binding entries. If a
match is found, the ARP packet is considered valid and is forwarded. If an entry with a matching IP
address but an unmatched MAC address is found, the ARP packet is considered invalid and is
discarded. If no entry with a matching IP address is found, the device compares the ARP packet's
sender IP and MAC addresses against the DHCP snooping entries, 802.1X security entries, and
OUI MAC addresses.
If a match is found in any of the entries, the ARP packet is considered valid and is forwarded. ARP
detection based on OUI MAC addresses refers to that if the sender MAC address of the received
ARP packet is an OUI MAC address, the packet is considered valid.
If no match is found, the ARP packet is considered invalid and is discarded.
Upon receiving an ARP packet from an ARP trusted port, the device does not check the ARP
packet.
2) ARP detection based on specified objects
You can also specify objects in ARP packets to be detected. The objects involve: