A string which opaquely encodes various aspects about a machine's operating system, hardware, and kernel attributes.

字符串，对器操作系统，硬件，和内核属性进行隐意编码。

Linux system kenel adopts the Netfilter mechanism ,which is a structural bottom frame for expanding various network service after 2.4 version.

Linux系统内核自2.4.版本之后采Netfilter制，它是在内核扩展各种网络结构化底层框架。