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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 message to a client to associate a false MAC address with the
gateway address, which would blackhole all internet-bound packets from the client.
NOTE: Dynamic ARP inspection (DAI) uses entries in the L2SysFlow CAM region, a sub-region of SystemFlow. One CAM
entry is required for every DAI-enabled VLAN. You can enable DAI on up to 16 VLANs on a system. However, the ExaScale
default CAM profile allocates only nine entries to the L2SysFlow region for DAI. You can configure 10 to 16 DAI-enabled
VLANs by allocating more CAM space to the L2SysFlow region before enabling DAI.
SystemFlow has 102 entries by default. This region is comprised of two sub-regions: L2Protocol and L2SystemFlow.
L2Protocol has 87 entries; L2SystemFlow has 15 entries. Layer 2 protocols use six L2SystemFlow entries, leaving nine for
DAI. L2Protocol can have a maximum of 100 entries; you must expand this region to capacity before you can increase
the size of L2SystemFlow. This is relevant when you are enabling DAI on VLANs. If, for example, you want to enable DAI
on 16 VLANs, you need seven more entries; in this case, reconfigure the SystemFlow region for 122 entries using the
layer-2 eg-acl value fib value frrp value ing-acl value learn value l2pt value qos value
system-flow 122 command.
The logic is as follows:
L2Protocol has 87 entries by default and must be expanded to its maximum capacity, 100 entries, before L2SystemFlow
can be increased; therefore, 13 more L2Protocol entries are required. L2SystemFlow has 15 entries by default, but only nine
are for DAI; to enable DAI on 16 VLANs, seven more entries are required. 87 L2Protocol + 13 additional L2Protocol + 15
L2SystemFlow + 7 additional L2SystemFlow equals 122.
Configuring Dynamic ARP Inspection
To enable dynamic ARP inspection, use the following commands.
1. Enable DHCP snooping.
2. Validate ARP frames against the DHCP snooping binding table.
INTERFACE VLAN mode
arp inspection
To view entries in the ARP database, use the show arp inspection database command.
Dell#show arp inspection database
Protocol Address Age(min) Hardware Address Interface VLAN CPU
---------------------------------------------------------------------
Internet 10.1.1.251 - 00:00:4d:57:f2:50 Te 0/2 Vl 10 CP
Internet 10.1.1.252 - 00:00:4d:57:e6:f6 Te 0/1 Vl 10 CP
Internet 10.1.1.253 - 00:00:4d:57:f8:e8 Te 0/3 Vl 10 CP
Internet 10.1.1.254 - 00:00:4d:69:e8:f2 Te 0/50 Vl 10 CP
Dell#
To see how many valid and invalid ARP packets have been processed, use the show arp inspection statistics
command.
Dell#show arp inspection statistics
Dynamic ARP Inspection (DAI) Statistics
278
Dynamic Host Configuration Protocol (DHCP)