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# 1.6 Network Under Attack

Most Internet DoS attacks fall into one of three categories:

* Vulnerability attack. This involves sending a few well-crafted messages to a vulnerable application or operating system running on a targeted host. If the right sequence of packets is sent to a vulnerable application or operating system, the service can stop or, worse, the host can crash.
* Bandwidth flooding. The attacker sends a deluge of packets to the targeted host—so many packets that the target’s access link becomes clogged, preventing legitimate packets from reaching the server.
* Connection flooding. The attacker establishes a large number of half-open or fully open TCP connections at the target host. The host can become so bogged down with these bogus connections that it stops accepting legitimate connections.

In a distributed DoS (DDoS) attack, the attacker controls multiple sources and has each source blast traffic at the target. DDoS attacks leveraging botnets with thousands of comprised hosts are a common occurrence today. DDoS attacks are much harder to detect and defend against than a DoS attack from a single host.

Because packet sniffers are passive—that is, they do not inject packets into the channel—they are difficult to detect. So, when we send packets into a wireless channel, we must accept the possibility that some bad guy may be recording copies of our packets. As you may have guessed, some of the best defenses against packet sniffing involve cryptography.

The ability to inject packets into the Internet with a false source address is known as IP spoofing, and is but one of many ways in which one user can masquerade as another user. To solve this problem, we will need end-point authentication, that is, a mechanism that will allow us to determine with certainty if a message originates from where we think it does.


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