Configuring Defense Against ARP Spoofing Attacks

You can configure ARP entry fixing globally. If ARP entry fixing is enabled globally, all interfaces have this function enabled by default.

Configuring DAI on an access device can prevent MITM attacks and theft on authorized users' information. After DAI is configured, the device compares the source IP address, source MAC address, VLAN ID, and interface number in the received ARP packet with binding entries. If the ARP packet matches a binding entry, the device considers the ARP packet valid and allows the packet to pass through. If the ARP packet does not match a binding entry, the device considers the ARP packet invalid and discards the packet.

You can enable DAI in the interface view or the VLAN view. When DAI is enabled in an interface view, the device checks all ARP packets received on the interface against binding entries. When DAI is enabled in the VLAN view, the device checks the ARP packets received on all interfaces belonging to the VLAN against binding entries.

If you want to receive an alarm when a large number of ARP packets are generated, enable the alarm function for the ARP packets discarded by DAI. After the alarm function is enabled, the device will generate an alarm when the number of discarded ARP packets exceeds a specified threshold.

When ARP learning triggered by DHCP is enabled on the gateway, DAI can be enabled on the gateway.

This function is available only for DHCP snooping scenarios. The device enabled with DHCP snooping generates DHCP snooping binding entries when DHCP users go online. If a user uses a static IP address, you need to manually configure a static binding entry for the user. For details about the DHCP snooping configuration, see DHCP Snooping Configuration. For details on how to configure a static binding entry, see Configuring IPSG Based on a Static Binding Table.

After the DAI function is configured on the router, the port isolation and proxy ARP functions must be configured; otherwise, the DAI function does not take effect. For the configuration of port isolation, see Configuring Interface Isolation in the NetEngine AR Configuration Guide - Interface Management. For the configuration of proxy ARP, see Configuring Proxy ARP in the NetEngine AR Configuration Guide - IP Services.

If an attacker forges the gateway address to send ARP packets with the source IP address being the IP address of the gateway on the LAN, ARP entries on hosts in the LAN record the incorrect gateway address. As a result, all traffic from user hosts to the gateway is sent to the attacker and the attacker intercepts user information. Communication of users is interrupted.

If an attacker forges the gateway address to send ARP packets to other user hosts, ARP entries on the hosts record the incorrect gateway address. As a result, the gateway cannot receive data sent from the hosts. You can enable gratuitous ARP packet sending on the gateway. Then the gateway sends gratuitous ARP packets at intervals to update the ARP entries of authorized users so that the ARP entries contain the correct MAC address of the gateway.

The MAC address consistency check function for ARP packets prevents attacks from bogus ARP packets in which the source and destination MAC addresses are different from those in the Ethernet frame header. This function is usually configured on gateways.

This function enables the gateway to check the MAC address consistency in an ARP packet before ARP learning. If the source and destination MAC addresses in an ARP packet are different from those in the Ethernet frame header, the device discards the packet as an attack. If the source and destination MAC addresses in an ARP packet are the same as those in the Ethernet frame header, the device performs ARP learning.

After ARP packet validity check is enabled, the device checks the source MAC addresses in the ARP packet and Ethernet frame header, and discards the packets with inconsistent source MAC addresses.

To avoid the preceding problems, configure the strict ARP learning function on the gateway. This function allows the gateway to learn only ARP entries for ARP Reply packets in response to ARP Request packets that it has sent. In this way, the gateway can prevent most ARP attacks.

Strict ARP learning can be configured globally or in the interface view.

• If strict ARP learning is enabled globally, all interfaces on the device learn ARP entries strictly.
• If strict ARP learning is enabled in the interface view, only this interface learns ARP entries strictly.

When strict ARP learning is enabled globally and in the interface view simultaneously, the configuration on the interface takes precedence over the global configuration.

When strict ARP learning is enabled globally:

• If you run the arp learning strict force-disable command on a specified interface, strict ARP learning is forced to be disabled on the interface.
• If you run the arp learning strict trust command on a specified interface, strict ARP learning configured globally takes effect on the interface.

When many DHCP users connect to a network device, the device needs to learn and maintain many ARP entries. This affects device performance.

To address this issue, configure ARP learning triggered by DHCP on the gateway. When the DHCP server allocates an IP address for a user, the gateway generates an ARP entry for the user based on the DHCP ACK packet received on the VLANIF interface.

Before configuring ARP learning triggered by DHCP, ensure that DHCP snooping is enabled using the dhcp snooping enable command.

When both VRRP and DHCP relay are configured on the network, neither the dhcp snooping enable command nor the arp learning dhcp-trigger command can be configured on the VRRP master and backup devices.

You can also deploy DAI to prevent ARP entries of DHCP users from being modified maliciously.