问题 问答题

【说明】
某单位在部署计算机网络时采用了一款硬件防火墙,该防火墙带有3个网络接口,其网络拓扑如图7-9所示。


【问题1】
防火墙包过滤规则的默认策略为拒绝,图7-10给出防火墙的包过滤规则配置界面。若要求内部所有主机能使用IE浏览器访问外部IP地址202.117.118.23的Web服务器,为图7-10中(1)~(4)空缺处选择正确答案。


(1)

  • A.允许
  • B.拒绝
    (2)A.192.168.1.0/24B.211.156.169.6/30
  • C.202.117.118.23/24
    (3)A.TCPB.UDPC.ICMP
    (4)A.E3→E2B.E1→E3C.E1→E2

答案

参考答案:(1) A,或允许 (2) A,或192.168.1.0/2.4
(3) A,或TCP (4) C,或E1→E2

解析:这是一道要求读者掌握防火墙包过滤规则策略配置的分析理解题。本题的解答步骤如下。
①由试题中给出的关键信息“要求内部所有主机能……访问外部IP地址202.117.118.23的Web服务器”中,“能……访问”说明序号为1的包过滤规则策略为“允许”,即(1)空缺处的正确答案是选项A。
②试题中给出的关键信息“要求内部所有主机能……访问外部Ⅲ地址202.117.118.23的Web服务器”中的“内部所有主机”是指图7-9拓扑图中对应于区域C的计算机。由于防火墙与区域C相连接的接口 IP地址为192.168.1.1/24,其中“/24”表示子网掩码为255.255.255.0。因此区域C的网络组成的网段地址为192.168.1.0/24,即(2)空缺处应填入区域C的网段地址(192.168.1.0/24)。
③通常Web服务器是使用TCP/IP协议簇中的HTTP协议提供服务的,而HTTP协议是一个传输层基于TCP协议的应用层协议,因此(3)空缺处的“协议”应选择“TCP”。
④同理,由试题的关键信息“要求内部所有主机能……访问外部IP地址202.117.118.23的Web服务器”可知,该包过滤规则的控制方向是控制内部局域网传输给Internet网的IP包,即从防火墙的“E1”端口流入、“E2”端口流出的D数据包,因此(4)空缺处的“方向”应选择“E1→E2”。

问答题 简答题
单项选择题

On the first Earth Day, the U.S. was a poisoned nation. Dense air pollution blanketed cities like Los Angeles, where smog alerts were a fact of life. Dangerous pesticides like DDT were still in use, and water pollution was rampant—symbolized by raging fires on Cleveland’s Cuyahoga River. But the green movement that was energized by Earth Day— and the landmark federal actions that followed it—changed much of that. Today air pollution is down significantly in most urban areas, the water is cleaner, and even the Cuyahoga is home to fish again.

But if the land is healing, Americans may be sickening. Since World War Ⅱ, production of industrial chemicals has risen rapidly, and the U.S. generates or imports some 19 billion kg of them per day. These aren’t the sorts of chemicals that come to mind when we picture pollution—huge plants spilling contaminated wastewater into rivers. Rather, they’re the molecules that make good on the old "better living through chemistry" promise, appearing in items like unbreakable baby bottles and big-screen TVs. Those chemicals have a, habit of finding their way out of everyday products and into the environment—and ultimately into living organisms. A recent biomonitoring survey found traces of 212 environmental chemicals in Americans—including toxic metals, pesticides, etc. "It’s not the environment that’s contaminated so much," says the director of the Cincinnati Children’s Environmental Health Center. "It’s us."

As scientists get better at detecting the chemicals in our bodies, they’re discovering that even tiny quantities of toxins can have a potentially serious impact on our health—and our children’s future. Chemicals like bisphenol A (BPA) and phthalates—key ingredients in modern plastics—may disrupt the delicate endocrine system. A host of modern ills that have been rising unchecked for a generation—obesity, diabetes, attention-deficit disorder —could have chemical connections. "We don’t give environmental exposure the attention it deserves," says Dr. Philip Landrigan. "But there’s an emerging understanding that kids are uniquely susceptible to environmental hazards."

Washington has been slow to arrive at that conclusion. The Toxic Substances Control Act (TSCA), the 34-year-old vehicle for federal chemical regulation, has generally been a failure. The burden of proving chemicals dangerous falls almost entirely on the government. And the Environmental Protection Agency (EPA) has been able to issue restrictions on only a handful of chemicals and has lacked the power to ban even some dangerous cancer-causing substances.

But change is coming. The Obama Administration is taking a closer look at chemicals. More important, Congress may finally be ready to act. "We can’t permit this assault on our children’s health—and our own health—to continue," says Senator Frank Lautenberg.

The environmental chemicals in the U.S()

A. come from spilling contaminated wastewater

B. appear in everyday products

C. cause more contamination to the environment

D. have few sorts but serious impact