Dell PowerConnect J-8216 Software Guide for JUNOS version 10.3 - Page 2628

Dell PowerConnect J-Series Ethernet Switch Complete Software Guide for Junos OS Address Resolution Protocol Sending IP packets on a multiaccess network requires mapping an IP address to an Ethernet media access control (MAC) address. Ethernet LANs use Address Resolution Protocol (ARP) to map MAC addresses to IP addresses. The switch maintains this mapping in a cache that it consults when forwarding packets to network devices. If the ARP cache does not contain an entry for the destination device, the host (the DHCP client) broadcasts an ARP request for that device's address and stores the response in the cache. ARP Spoofing ARP spoofing (also known as ARP poisoning or ARP cache poisoning) is one way to initiate man-in-the-middle attacks. The attacker sends an ARP packet that spoofs the MAC address of another device on the LAN. Instead of the switch sending traffic to the proper network device, it sends it to the device with the spoofed address that is impersonating the proper device. If the impersonating device is the attacker's machine, the attacker receives all the traffic from the switch that should have gone to another device. The result is that traffic from the switch is misdirected and cannot reach its proper destination. One type of ARP spoofing is gratuitous ARP, which is when a network device sends an ARP request to resolve its own IP address. In normal LAN operation, gratuitous ARP messages indicate that two devices have the same MAC address. They are also broadcast when a network interface card (NIC) in a device is changed and the device is rebooted, so that other devices on the LAN update their ARP caches. In malicious situations, an attacker can poison the ARP cache of a network device by sending an ARP response to the device that directs all packets destined for a certain IP address to go to a different MAC address instead. To prevent MAC spoofing through gratuitous ARP and through other types of spoofing, J-EX Series switches examine ARP responses through DAI. DAI on J-EX Series Switches DAI examines ARP requests and responses on the LAN and validates ARP packets. The switch intercepts ARP packets from an access port and validates them against the DHCP snooping database. If no IP-MAC entry in the database corresponds to the information in the ARP packet, DAI drops the ARP packet and the local ARP cache is not updated with the information in that packet. DAI also drops ARP packets when the IP address in the packet is invalid. The Junos OS for J-EX switches uses DAI for ARP packets received on access ports because these ports are untrusted by default. Trunk ports are trusted by default, so ARP packets bypass DAI on them. You configure DAI for each VLAN, not for each interface (port). By default, DAI is disabled for all VLANs. You can set an interface to be trusted for ARP packets by setting dhcp-trusted on that port. 2556