ZyXEL Communications 802.11g ADSL 2+ 4-Port Security Gateway HW-D Series, User Manual

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P-662H/HW-D Series User’s Guide
172 Chapter 10 Firewalls
10.4.2 Types of DoS Attacks
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 91 Three-Way Handshake
Table 60 Common IP Ports
21 FTP 53 DNS
23 Telnet 80 HTTP
25 SMTP 110 POP3