ZyXEL 202H 91-003-194001B Manual Do Utilizador

P-202H Plus v2 User’s Guide

79

Chapter 8 Firewalls

There are four types of DoS attacks: 

1 Those that exploit bugs in a TCP/IP implementation.

2 Those that exploit weaknesses in the TCP/IP specification.

3 Brute-force attacks that flood a network with useless data. 

4 IP Spoofing.

5 "Ping of Death" and "Teardrop" attacks exploit bugs in the TCP/IP implementations of 

various computer and host systems. 

• Ping of Death uses a "ping" utility to create an IP packet that exceeds the maximum 

65,536 bytes of data allowed by the IP specification. The oversize packet is then sent to 
an unsuspecting system. Systems may crash, hang or reboot. 

• Teardrop attack exploits weaknesses in the re-assembly of IP packet fragments. As data is 

transmitted through a network, IP packets are often broken up into smaller chunks. Each 
fragment looks like the original IP packet except that it contains an offset field that says, 
for instance, "This fragment is carrying bytes 200 through 400 of the original (non 
fragmented) IP packet." The Teardrop program creates a series of IP fragments with 
overlapping offset fields. When these fragments are reassembled at the destination, some 
systems will crash, hang, or reboot. 

6 Weaknesses in the TCP/IP specification leave it open to "SYN Flood" and "LAND" 

attacks. These attacks are executed during the handshake that initiates a communication 
session between two applications.

Figure 27   Three-Way Handshake

Under normal circumstances, the application that initiates a session sends a SYN 
(synchronize) packet to the receiving server. The receiver sends back an ACK 
(acknowledgment) packet and its own SYN, and then the initiator responds with an ACK 
(acknowledgment). After this handshake, a connection is established.