D-Link DES-3550 Product Manual - Page 284

xStack® DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual Ethernet Header Gratuitous ARP Destination address Source address Ethernet H/W type Protocol H/W Protocol Operation type type address address length length Sender H/W address Sender protocol address Target H/W address Target protocol address (6-byte) (6-byte) (2-byte) (2-byte) (2-byte) (1-byte) (1-byte) (2-byte) (6-byte) (4-byte) (6-byte) (4-byte) FF-FF-FF-FF-FF-FF 00-20-5C-01-11-11 806 ARP reply 00-20-5C-01-11-11 10.10.10.254 00-20-5C-01-11-11 10.10.10.254 Table-5 A common DoS attack today can be done by associating a nonexistent or specified MAC address to the IP address of the network's default gateway. The malicious attacker only needs to broadcast ONE Gratuitous ARP to the network claiming it is the gateway so that the whole network operation will be turned down as all packets to the Internet will be directed to the wrong node. Likewise, the attacker can either choose to forward the traffic to the actual default gateway (passive sniffing) or modify the data before forwarding it (man-in-the-middle attack). The hacker cheats the victim's PC to think that it is a router and cheats the router to think it is the victim. As can be seen in Figure-5 all traffic will be then sniffed by the hacker but the users will not notice anything happening. Figure-5 269