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  3. While hackers with motives make headlines, they represent less than 20% of all net- work security breaches. More common are inst

While hackers with motives make headlines, they represent less than 20% of all net- work security breaches. More common are inst

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问题     While hackers with motives make headlines, they represent less than 20% of all net- work security breaches. More common are instances of authorized users accidentally winding up where they should not be and inadvertently deleting or changing data. However, the Internet introduces another concern: some Internet surfers are bound to go where they have no business and, in so doing, threaten to wipe out data to which they should not have access.
    Before picking a firewall, companies need to adopt security policies. A security policy states who or what is allowed to connect to whom or what. You can group all users by department or classification. The better firewall products let you drag and drop groups in a graphical user interface (GUI) environment to define network security easily.
    Two methods are most often used together to establish an Internet firewall. They are application and circuit gateways, as well as packet filtering. With application and circuit gateways, all packets are addressed to a user-level application on a gate-way that relays packets between two points. With most application gateways, additional packet-filter machines are required to control and screen traffic between the gateway and the networks. A typical configuration includes two routers. With a bastion host that serves as the application gateway sitting between them.
    A drawback to application and circuit gateways is that they slow network performance. This is because each packet must be copied and processed at least twice by all the communication layers. Packet-filter gateways, which act as routers between two nets, are less secure than application gateways but more efficient. They are transparent to many protocols and applications, and they require no changes in client applications, no specific application management or installation, and no extra hardware.
    Using a single, unified packet-filter engine, all net traffic is processed and then for- warded or blocked from a single point of control. However, most packet filters are state- less, understand only low-level protocols, and are difficult to configure and verity. In addition, they lack audit mechanisms. Some packet filters are implemented inside routers, limiting computing power and filtering capabilities. Others are implemented as s9ftware packages that filter the packets in application-layer processes, an inefficient approach that requires multiple data copies, expensive delays and context switches and delivers lower throughput.
    So what’s a network administrator to do? Some vendors are developing firewalls that overcome many of these problems and combine the advantages of application gateways and packet filtering. These efficient, protocol-independent, secure firewall engines are capable of application-level security, user authentication, unified support, and handling of all protocols, auditing and altering. They are transparent to users and to system setup, and include a GUI for simple and flexible system management and configuration.  
Which of the following can be inferred from the passage concerning the networks using application and circuit gateways?
选项 A、Secure and speedy.
B、Secure but slow.
C、Insecure and slow.
D、Insecure but speedy.
答案B
解析 解决这个问题首先要仔细阅读第四段，(A drawback to application and circuit gateways is that they slow network performance...Packet-filter gateways...are less secure than application gateways but more efficient．这几句话一个说的是应用网关的缺点是速度慢，另外在介绍信息过滤器法时说过滤法不太安全。由此可以推断应用网关的特点是速度慢但比较安全
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