摘要： 讨论了基于URL 的网页内容过滤器的设计与实现。通过对Linux 系统内核模块的分析，设计通过网络包截获、网络包过滤两个步骤进行过滤器的实现。在设计过程中，探讨并实现了URL 阻挡树模型、上下层通信的技术（上下层大规模数据传输和内核主动发信号给进程的技术）和包的截获技术。

关键词: 网页内容过滤器；网络包截获；网络包过滤；阻挡树

Abstract: The design and realization of net filter are discussed. By analyzing the kernel of Linux system, the realization of filter is designed by these two steps: package captured and package filtered. In the process of design, the model of URL tree, the communication skill between different layers (the skill regarding the large scale data transmission between different layers and the active sending signals from kernel to process) and the skill of package captured.

Key words: Net filter; Package captured; Package filtered; URL tree