Nortel BSR222, Configuration, Page: 159 / 451

Chapter 10 Firewalls 159
Nortel Business Secure Router 222 Configuration — Basics
When computers communicate on the Internet, they use the client/server model,
where the server listens on a specific TCP/UDP port for information requests from
remote client computers on the network. For example, a Web server typically
listens on port 80. Note that, while a computer can be intended for use over a
single port, such as Web on port 80, other ports are also active and vulnerable to
attack by hackers.
Some of the most common IP ports are:
Types of DoS attacks
There are four types of DoS attacks:
• Those that exploit bugs in a TCP/IP implementation.
• Those that exploit weaknesses in the TCP/IP specification.
• Brute force attacks that flood a network with useless data.
• IP Spoofing.
1 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, and can cause systems to crash, hang, or reboot.
Teardrop attack exploits weaknesses in the reassembly 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. After these fragments are
reassembled at the destination, some systems crash, hang, or reboot.
21 FTP 53 DNS
23 Telnet 80 HTTP
25 SMTP 110 POP3