包过滤防火墙是一种通过软件检查数据包以实现系统安全防护的基本手段，以下叙述中，不正确的是（ ）。
- A.包过滤防火墙通常工作在网络层以上，因此可以实现对应用层数据的检查与过滤
- B.包过滤防火墙通常根据数据包源地址、目的地址、端口号和协议类型等标志设置访问控制列表实现对数据包的过滤
- C.数据包过滤用在内部主机和外部主机之间，过滤系统可以是一台路由器或是一台主机
- D.当网络规模比较复杂时，由于要求逻辑的一致性、封堵端口的有效性和规则集的正确性等原因，会导致访问控制规则复杂，难以配置管理
正确答案及解析
正确答案
解析
本题考查包过滤防火墙的基础知识。包过滤防火墙是一种通过软件检查数据包以实现系统安全防护的基本手段，数据包过滤用在内部主机和外部主机之间，过滤系统可以是一台路由器或是一台主机。通常通过查看所流经的数据包的包头来决定整个包的命运，可能会决定丢弃这个包，可能会接受这个包（让这个包通过），也可能执行其他更复杂的动作。具体来说，包过滤防火墙通常根据数据包源地址、目的地址、端口号和协议类型等标志设置访问控制列表实现对数据包的过滤。包过滤是在IP层实现的，包过滤根据数据包的源IP地址、目的IP地址、协议类型（TCP包、UDP包、ICMP包）、源端口、目的端口等包头信息及数据包传输方向等信息来判断是否允许数据包通过。当网络规模比较复杂时，由于包过滤防火墙要求逻辑的一致性、封堵端口的有效性和规则集的正确性等原因，会导致访问控制规则复杂，难以配置管理。
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