802.11安全基础与实践

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"802.11 Security" 是一本由 Bob Fleck 和 Bruce Potter 合著，O'Reilly 出版的书籍，出版日期为2002年12月，ISBN为0-596-00290-4，共208页。本书旨在提供无线网络安全的理论和实践基础，特别是针对802.11b标准的安全问题。 802.11 Security 一书从802.11b的基本介绍开始，深入探讨了无线安全的各个方面，揭示了一些普遍存在的误解。作者们通过详细的技术讲解，使读者能够理解书中内容如何适用于具体的需求和场景。这本书对于网络工程师、安全工程师、系统工程师以及对部署基于802.11b系统的任何人都具有很高的参考价值，是他们在实施过程中不可或缺的参考资料。全书内容包括版权信息、前言、假设、读者定位、书本范围、书中使用的约定、其他信息来源、读者反馈、致谢等部分。第一部分“802.11 安全基础”由第一章“无线世界”构成，分为以下几个小节： 1.1 什么是无线？在这个小节中，作者解释了无线技术的基本概念，阐述了无线通信的优势和应用领域。 1.2 无线电传输介绍了无线信号如何在空气中传播，包括频率、波长和功率等相关概念。 1.3 内在不安全性讨论了无线通信天生存在的安全隐患，如信号的开放性、易被监听和干扰等问题。 1.4 802.11 标准详细解读了802.11标准，它是无线局域网（WLAN）最广泛采用的标准，涵盖了各种速度和功能的无线网络。 1.5 802.11 MAC 层结构分析了802.11标准中的介质访问控制（MAC）层，这是无线网络中控制数据传输的关键部分。 1.6 WEP Wired Equivalent Privacy（有线等效加密）是802.11早期的安全机制，旨在提供与有线网络相同的加密水平。 1.7 WEP 的问题揭示了WEP的漏洞和缺陷，包括密钥管理、初始化向量（IV）重复使用和弱加密算法等问题，这些都使得WEP在实际应用中容易受到攻击。 1.8 是否无解？尽管WEP存在严重的问题，但作者并未让读者陷入绝望。他们探讨了可能的解决方案和替代安全措施，为读者提供了前进的方向。通过本书，读者不仅可以了解802.11无线网络安全的基础知识，还能学习到如何评估和应对无线网络的潜在风险，从而更好地保护和设计安全的无线环境。

Chapter 1. A Wireless World

Wireless networking is revolutionizing the way people work and play. By

removing physical constraints commonly associated with high-speed

networking, individuals are able to use networks in ways never possible in

the past. Students can be connected to the Internet from anywhere on

campus. Family members can check email from anywhere in a house.

Neighbors can pool resources and share one high-speed Internet connection.

Over the past several years, the price of wireless networking equipment has

dropped significantly. Wireless NICs are nearing the price of their wired

counterparts. At the same time, performance has increased dramatically. In

1998, Wireless Local Area Networks (WLAN) topped out at 2Mb/s. In

2002, WLANs have reached speeds of 54Mb/s and higher.

Unfortunately, wireless networking is a double-edged sword. Wireless users

have many more opportunities in front of them, but those opportunities open

up the user to greater risk. The risk model of network security has been

firmly entrenched in the concept that the physical layer is at least somewhat

secure. With wireless networking, there is no physical security. The radio

waves that make wireless networking possible are also what make wireless

networking so dangerous. An attacker can be anywhere nearby listening to

all the traffic from your networkxxxmdashxxxin your yard, in the parking lot

across the street, or on the hill outside of town. By properly engineering and

using your wireless network, you can keep attackers at bay.

This chapter serves as an introduction to wireless networking and some of

the high-level security concerns. Building a secure wireless network requires

a wide breadth of knowledge; from the low-level aspects of radio

transmission to understanding how various applications interact with the

network. By understanding how all aspects of the network interact, you can

safely and freely use wireless networks.

1.1 What Is Wireless?

The term wireless means different things to different people. In general, the

term reflects any means of communication that occurs without wires. In this

Generally, the faster the frequency, the more information you can transmit

and receive. The method of encoding (or modulating) the data affects the

amount that can be transmitted. Some encoding techniques are more resilient

to errors but end up with lower data rates. Conversely, high data rate

modulation mechanisms may be more susceptible to outside interference.

1.2.2 Signal Strength

In the most general sense, the strength of a signal is the amplitude of its

radio waveform. The unit of measurement of amplitude (or power) of a radio

wave is a watt (W). WLAN devices typically transmit with a power of 30

milliwatts.

The strength of the signal decreases as it travels through its transmission

mediumxxxmdashxxxin this case, the air. This process of power loss is

called attenuation. The amount of attenuation varies depending on the

frequency of the signal and the medium through which it is traveling.

Amplifiers can be added along a transmission medium to add strength to a

signal.

The amount of power added due to amplification or removed due to

attenuation is usually expressed in decibels (dB). Decibels provide a relative

(logarithmic) measurement of signal strength. Each increase of three dB

indicates a doubling in strength. Decibels are convenient because values can

be added and subtracted from each other without need for complex

mathematics.

While a signal is being received by a radio, the signal is also subject to noise

at the same frequency as the signal. This noise may be due to environmental

causes such as cosmic radiation or man-made reasons such as microwave

ovens or even other radios operating in the same frequency. The ratio of the

power of the received signal versus the power of the received noise is called

the signal to noise ratio (SNR). The higher the SNR, the better the quality of

the data signal.

1.2.3 Antennas

Antennas are critical to radios sending and receiving radio waves. They turn

electrical impulses into radio waveforms and vice versa. There are hundreds

of different kinds of antennas, but they can be grouped roughly into two

categories: omni-directional and directional.

The name omni-directional implies that these antennas radiate

electromagnetic energy regularly in all directions. This is not the case. They

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