Operation Manual – ARP
H3C S3100 Series Ethernet Switches
Chapter 1 ARP Configuration
1-6
II. ARP attack detection
To guard against the man-in-the-middle attacks launched by hackers or attackers,
S3100-EI series Ethernet switches support the ARP attack detection function. All ARP
(both request and response) packets passing through the switch 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 the manually configured IP binding table, refer to the DHCP snooping section
in the part discussing DHCP in this manual
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After you enable the ARP attack detection function, the switch will check the following
items of an ARP packet: 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 or the manual configured IP
binding table, the switch will forward the ARP packet; if not, the switch discards 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 manually configured IP binding table.
z
With the ARP restricted forwarding function enabled, ARP request packets are
forwarded through trusted ports only; ARP response packets are forwarded
according to the MAC addresses in the packets, or through trusted ports if the
MAC address table contains no such destination MAC addresses.
1.1.7 Introduction to ARP Packet Rate Limit
To prevent the man-in-the-middle attack, a switch enabled with the ARP attack
detection function delivers ARP packets to the CPU to check the validity of the packets.
However, this causes a new problem: If an attacker sends a large number of ARP
packets to a port of a switch, the CPU will get overloaded, causing other functions to fail,
and even the whole device to break down. To guard against such attacks, S3100-EI
series Ethernet switches support the ARP packets rate limit function, which will shut
down the attacked port, thus preventing serious impact on the CPU.
With this function enabled on a port, the switch will count the ARP packets received on
the port within each second. If the number of ARP packets received on the port per
second exceeds the preconfigured value, the switch considers that the port is attacked
by ARP packets. In this case, the switch will shut down the port. As the port does not
receive any packet, the switch is protected from the ARP packet attack.
At the same time, the switch supports automatic recovery of port state. If a port is shut
down by the switch due to high packet rate, the port will revert to the Up state after a
configured period of time.