Dynamic ARP protection

On the VLAN interfaces of a routing switch, dynamic ARP protection ensures that only valid ARP requests and responses are relayed or used to update the local ARP cache. ARP packets with invalid IP-to-MAC address bindings advertised in the source protocol address and source physical address fields are discarded. For more information about the ARP cache, see “ARP Cache Table” in the multicast and routing guide.

ARP requests are ordinarily broadcast and received by all devices in a broadcast domain. Most ARP devices update their IP-to-MAC address entries each time they receive an ARP packet even if they did not request the information. This behavior makes an ARP cache vulnerable to attacks.

Because ARP allows a node to update its cache entries on other systems by broadcasting or unicasting a gratuitous ARP reply, an attacker can send his own IP-to-MAC address binding in the reply that causes all traffic destined for a VLAN node to be sent to the attacker's MAC address. As a result, the attacker can intercept traffic for other hosts in a classic "man-in-the-middle" attack. The attacker gains access to any traffic sent to the poisoned address and can capture passwords, e-mail, and VoIP calls or even modify traffic before resending it.

Another way in which the ARP cache of known IP addresses and associated MAC addresses can be poisoned is through unsolicited ARP responses. For example, an attacker can associate the IP address of the network gateway with the MAC address of a network node. In this way, all outgoing traffic is prevented from leaving the network because the node does not have access to outside networks. As a result, the node is overwhelmed by outgoing traffic destined to another network.

Dynamic ARP protection is designed to protect your network against ARP poisoning attacks in the following ways:

  • Allows you to differentiate between trusted and untrusted ports.

  • Intercepts all ARP requests and responses on untrusted ports before forwarding them.

  • Verifies IP-to-MAC address bindings on untrusted ports with the information stored in the lease database maintained by DHCP snooping and user-configured static bindings (in non-DHCP environments):

    • If a binding is valid, the switch updates its local ARP cache and forwards the packet.

    • If a binding is invalid, the switch drops the packet, preventing other network devices from receiving the invalid IP-to-MAC information.

DHCP snooping intercepts and examines DHCP packets received on switch ports before forwarding the packets. DHCP packets are checked against a database of DHCP binding information. Each binding consists of a client MAC address, port number, VLAN identifier, leased IP address, and lease time. The DHCP binding database is used to validate packets by other security features on the switch.

If you have already enabled DHCP snooping on a switch, you may also want to add static IP-to-MAC address bindings to the DHCP snooping database so that ARP packets from devices that have been assigned static IP addresses are also verified.

  • Supports additional checks to verify source MAC address, destination MAC address, and IP address.ARP packets that contain invalid IP addresses or MAC addresses in their body that do not match the addresses in the Ethernet header are dropped.

When dynamic ARP protection is enabled, only ARP request and reply packets with valid IP-to-MAC address bindings in their packet header are relayed and used to update the ARP cache.

Dynamic ARP protection is implemented in the following ways on a switch:

  • You can configure dynamic ARP protection only from the CLI; you cannot configure this feature from the WebAgent or menu interfaces.

  • Line rate—Dynamic ARP protection copies ARP packets to the switch CPU, evaluates the packets, and then re-forwards them through the switch software. During this process, if ARP packets are received at too high a line rate, some ARP packets may be dropped and will need to be retransmitted.

  • The SNMP MIB, HP-ICF-ARP-PROTECT-MIB, is created to configure dynamic ARP protection and to report ARP packet-forwarding status and counters.