Additionally you can disable some ports on the package filtering level.

此外，您可以在包过滤层上禁用某些端口。

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Next, this paper discusses the package filtering and content filtering and describes the design and implementation of information filtering system.

接下来，本论文重点介绍了在信息过滤中主要使用的包过滤和内容过滤，并描述了信息过滤系统的设计与实现。

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The implementation is practical but with obvious disadvantage: the illegal user can easily notice the key recovery, and by data package filtering, the key recovery function can be screened.

该方法实现简单，但存在缺陷：由于附加块信息不同于正常通信过程，恶意用户容易察觉通信具有密钥恢复功能，使用数据包过滤方法，就可以完全屏蔽密钥恢复功能。

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