Netfilter是Linux 2.4.x引入的一个子系统，它作为一个通用的、抽象的框架，提供一整套的hook函数的管理机制，使得诸如数据包过滤、网络地址转换(NAT)和基于协议类型的连接跟踪成为了可能。(Netfilter is a subsystem introduced by Linux 2.4.x, which serves as a general, abstract framework that provides a complete set of management mechanisms for hook functions, making it possible for packet filtering, network address conversion (NAT) and protocol based connection tracking)