一篇比较全面、比较好的关于 iptables 的文章：A Deep Dive into Iptables and Netfilter Architecture | DigitalOcean。
讲解了所有的表和链都是从5️⃣个钩子函数中衍生出来的；表的存在是为了人为地体现出不同的调用优先级。
以及，链的遍历顺序：
- Incoming packets destined for the local system: PREROUTING -> INPUT
- Incoming packets destined to another host: PREROUTING -> FORWARD -> POSTROUTING
- Locally generated packets: OUTPUT -> POSTROUTING