Dynamic ARP Inspection (DAI)
Address Resolution Protocol (ARP) provides IP-to-MAC (32-bit IP address into a 48-bit Ethernet address) resolution. ARP operates at Layer 2 (the data-link layer) of the OSI model. ARP provides the translation mapping the IP address to the MAC address of the destination host using a lookup table (also known as the ARP cache).
Several types of attacks can be launched against a host or devices connected to Layer 2 networks by "poisoning" the ARP caches. A malicious user could intercept traffic intended for other hosts on the LAN segment and poison the ARP caches of connected systems by broadcasting forged ARP responses. Several known ARP-based attacks can have a devastating impact on data privacy, confidentiality, and sensitive information. To block such attacks, the Layer 2 switch must have a mechanism to validate and ensure that only valid ARP requests and responses are forwarded.
Dynamic ARP inspection is a security feature that validates ARP packets in a network. Dynamic ARP inspection determines the validity of packets by performing an IP-to-MAC address binding inspection stored in a trusted database, (the DHCP snooping binding database) before forwarding the packet to the appropriate destination. Dynamic ARP inspection will drop all ARP packets with invalid IP-to-MAC address bindings that fail the inspection. The DHCP snooping binding database is built when the DHCP snooping feature is enabled on the VLANs and on the switch.
Figure 4-5a shows an example of an attacker attempting to spoof and hijack traffic for an important address (a default gateway in this example) by broadcasting to all hosts spoofing the MAC address of the router (using a gratuitous ARP). This will poison ARP cache entries (create an invalid ARP entry) on Host A and Host B, resulting in data being redirected to the wrong destination. Because of the poisoned entries, when Host A sends data destined for the router, it is incorrectly sent to the attacker instead. Dynamic ARP inspection locks down the IP-MAC mapping for hosts so that the attacking ARP is denied and logged.
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Figure 4-5a Dynamic ARP Inspection
The dynamic ARP Inspection (DAI) feature safeguards the network from many of the commonly known man-in-the-middle (MITM) type attacks. Dynamic ARP Inspection ensures that only valid ARP requests and responses are forwarded.
Figure 4-5b illustrates the DAI feature in action and shows how the intruder is blocked on the untrusted port when it is trying to poison ARP entries.
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Figure 4-5b DAI-in Action
DAI in a DHCP Environment
As mentioned earlier, DAI relies on the entries in the DHCP snooping binding database to verify IP-to-MAC address bindings. Configure each secure interface as trusted using the ip arp inspection trust interface configuration command. The trusted interfaces bypass the ARP inspection validation checks, and all other packets are subject to inspection when they arrive on untrusted interfaces.
Enable DAI on a per-VLAN basis by using the ip arp inspection vlan [vlan-range] command from the global configuration command.
Example 4-11 shows how to configure an interface as trusted and how to enable DAI for VLANs 5 through 10.
Example 4-11. DAI in a DHCP Environment Configuration Example
Switch(config)# interface GigabitEthernet1/0/1 Switch(config-if)# ip arp inspection trust Switch(config)# ip arp inspection vlan 5-10
DAI in a Non-DHCP Environment
In non-DHCP environments, because there is no DHCP snooping binding database, the DAI can validate ARP packets against a user-defined ARP ACL to map hosts with a statically configured IP address to their MAC address.
Use the arp access-list [acl-name] command from the global configuration mode on the switch to define an ARP ACL and apply the ARP ACL to the specified VLANs on the switch.
Example 4-12 shows how to configure an ARP ACL to permit ARP packets from host IP address 10.1.1.11 with MAC address 0011.0011.0011 and how to apply this ACL to VLAN 5 with the interface configured as untrusted.
Example 4-12. DAI in a Non-DHCP Environment Configuration Example
Switch(config)# arp access-list arpacl Switch(config-arp-acl)# permit ip host 10.1.1.11 mac host 0011.0011.0011 Switch(config-arp-acl)# exit Switch(config)# ip arp inspection filter arpacl vlan 5 Switch(config)# interface GigabitEthernet1/0/2 Switch(config-if)# no ip arp inspection trust
Use the show ip arp inspection vlan [vlan# or range] command to verify the configuration.
Rate Limiting Incoming ARP Packets
Because the switch CPU performs the DAI, there is a potential for an ARP flooding denial-of-service (DoS) attack resulting in performance degradation. To prevent this, ARP packets can be rate limited using the ip arp inspection limit command from the interface configuration mode to limit the rate of incoming ARP requests and responses. By default, 15 pps (packets per second) is allowed on untrusted interfaces; however, there is no limit on trusted interfaces. The burst interval is 1 second.
When the rate of incoming ARP packets exceeds the configured thresholds, the port is placed in the error-disabled state. The port will remain in this state until the user intervenes or the errdisable recovery cause arp-inspection interval [seconds] command is enabled, so that ports can automatically recover from this state after a specified timeout period.
Use the show ip arp inspection interfaces to display the trust state, the rate limit (pps stands for packets per second), and the burst interval configured for the interfaces.
Use the show ip arp inspection vlan [vlan# or range] command to display the DAI configuration and the operation state of the VLANs configured on the switch.
ARP Validation Checks
Specific additional checks can be performed on incoming ARP packets to validate the destination MAC address, the sender IP address in ARP requests, the target IP address in ARP responses, or the source MAC address. Use the ip arp inspection validate {[src-mac] [dst-mac] [ip]} command from the global configuration mode to enable these additional ARP validation checks.
Use the show ip arp inspection statistics command to display packet statistics on DAI-configured VLANs.