About ARP Inspection and the MAC Address Table
For interfaces in a bridge group, ARP inspection prevents a “man-in-the-middle” attack. You can also customize other ARP settings. You can customize the MAC address table for bridge groups, including adding a static ARP entry to guard against MAC spoofing.
ARP Inspection for Bridge Group Traffic
By default, all ARP packets are allowed between bridge group members. You can control the flow of ARP packets by enabling ARP inspection.
ARP inspection prevents malicious users from impersonating other hosts or routers (known as ARP spoofing). ARP spoofing can enable a “man-in-the-middle” attack. For example, a host sends an ARP request to the gateway router; the gateway router responds with the gateway router MAC address. The attacker, however, sends another ARP response to the host with the attacker MAC address instead of the router MAC address. The attacker can now intercept all the host traffic before forwarding it on to the router.
ARP inspection ensures that an attacker cannot send an ARP response with the attacker MAC address, so long as the correct MAC address and the associated IP address are in the static ARP table.
When you enable ARP inspection, the ASA compares the MAC address, IP address, and source interface in all ARP packets to static entries in the ARP table, and takes the following actions:
If the IP address, MAC address, and source interface match an ARP entry, the packet is passed through.
If there is a mismatch between the MAC address, the IP address, or the interface, then the ASA drops the packet.
If the ARP packet does not match any entries in the static ARP table, then you can set the ASA to either forward the packet out all interfaces (flood), or to drop the packet.
The dedicated Management interface never floods packets even if this parameter is set to flood.
MAC Address Table for Bridge Groups
The ASA learns and builds a MAC address table in a similar way as a normal bridge or switch: when a device sends a packet through the bridge group, the ASA adds the MAC address to its table. The table associates the MAC address with the source interface so that the ASA knows to send any packets addressed to the device out the correct interface.
Because the ASA is a firewall, if the destination MAC address of a packet is not in the table, the ASA does not flood the original packet on all interfaces as a normal bridge does. Instead, it generates the following packets for directly connected devices or for remote devices:
Packets for directly connected devices—The ASA generates an ARP request for the destination IP address, so that it can learn which interface receives the ARP response.
Packets for remote devices—The ASA generates a ping to the destination IP address so that it can learn which interface receives the ping reply.
The original packet is dropped.