tailieunhanh - wifi security phần 8

Một bảng địa chỉ MAC được ủy quyền được lưu trữ trong các điểm truy cập. Nó có hiệu quả trong việc giữ các hàng xóm mới làm quen ra khỏi một mạng mở. Tuy nhiên, địa chỉ MAC sẽ không bao giờ giữ bí mật và một card mạng có thể thay đổi địa chỉ của nó để phù hợp với địa chỉ của người khác. | . GATHERING INFORMATION ON A VICTIM 61 to induce action from the attached clients. WEPWedgie is the software tool which has implemented the attacks. There is no point in scanning for wireless hosts since they are already known by eavesdropping on wireless traffic. In addition to wireless hosts all wired hosts which have communicated with a wireless hosts are also known by their MAC address in the header of a frame. Scanning by inserting random MAC addresses as destinations and waiting for responses is useless as there are 248 different MAC addresses. Not all MAC addresses can belong to a physical network card but the amount is still too much. Sending out a frame for each of the 248 possible MAC addresses would take somewhere in the area of 3 000 years if frames are sent out at a rate of 2 900 frames second. If a packet for each possible IP address is attempted the time it takes is roughly 3 years. Fortunatly for a hacker the number of unused IP addresses is running out. To rectify the problem on a temporary basis Network Address Translation NAT is utilized in most multi-computer homes and small to medium size offices. With NAT most computers are assigned IP addresses in the ranges 24 and 16. Sending frames to the entire 16 range takes a minimum of 23 seconds at 2900 frames second. Since the replies are sent to the wireless client and the replies are encrypted there is a difficulty to determine which reply was to which request. The solution used by WEPWedgie and illustrated in Figure is to utilize an Internet return channel a helping host . ICMP echo requests are injected with the source address of the helping host. If the destination address of the request exists that computer will reply to the request but send the reply to the helping host. The helping host can view the reply with . tcpdump and will see the IP address of the client behind the firewall. However when scanning for computers IP addresses the attack will only

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