本篇主要来介绍一下 什么是netfilter ，iptables 以及两者的关系 什么是ufw ，以及ufw与iptables的关系

先给出一段英文文献供同学们阅读学习

Traffic into or out of a computer is filtered through “ports,” which are relatively arbitrary （任意的）designations（名称） appended to（附加到） traffic packets destined for（注定要） use by a particular application.
By convention（公约；惯例）, some ports are routinely（例行公事地） used for particular types of applications. For example, port 80 is generally used for insecure web browsing and port 443 is used for secure web browsing.
Traffic to particular applications can be allowed or blocked by “opening” or “closing” (i.e. filtering) the ports designated for（指定为） a particular type of traffic. If port 80 is “closed,” for example, no (insecure) web browsing will be possible. The AntiVirus page might also be of interest.
The Linux kernel includes the netfilter（网络过滤器） subsystem（子系统）, which is used to manipulate（操纵） or decide the fate of network traffic headed into or through your computer. All modern Linux firewall solutions use this arbitrarys（任意的） system for packet filtering.
The kernel’s packet filtering system would be of little use to users or administrators without a user interface（接口） with which to manage it. This is the purpose of iptables. When a packet reaches your computer, it is handed off （移交）to the netfilter subsystem for acceptance, manipulation, or rejection based on the rules supplied to it via（通过）