ZyXEL ADSL Prestige 662HW-61 130122.0662HW61 Manual Do Utilizador

Prestige 662H/HW Series User’s Guide

147

Chapter 13 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.

Table 39   Common IP Ports

21

FTP

53

DNS

23

Telnet

80

HTTP

25

SMTP

110

POP3