# IPv4 - ARP

[TOC]

## 課程影片

`man` 可以參考 `arp(8)` 或 `arping(8)`。另外，netfilters 中有 ARP 的 hook，命令列工具方面，`nft` 也整合了舊有的 `arptables` 功能。所以那些 `*tables` 可以考慮直接參考新的 [`nft`](https://www.netfilter.org/projects/nftables/manpage.html) 中與 ARP 有關的功能。

另外，在 IPv6 中，在 IPv4 中的 ARP 該具有的功能整合進了 ICMPv6 中的 *Neighbor Discovery* 中

### arp for Showing Address Resolution Table in Linux

{%youtube ja787Yl01MY %}

### 第 7LMN 講 路由器運作原理以及網路互連技術 L07 12 13 14

{%youtube 4aRXm4ujh7g %}

## 目標：已知 IP，求 MAC 位址

## ARP 封包格式

### 請求方

        0        7        15       23       31
        +--------+--------+--------+--------+
        |       HT        |        PT       |
        +--------+--------+--------+--------+
        |  HAL   |  PAL   |        OP       |
        +--------+--------+--------+--------+
        |         S_HA (bytes 0-3)          |
        +--------+--------+--------+--------+
        | S_HA (bytes 4-5)|S_L32 (bytes 0-1)|
        +--------+--------+--------+--------+
        |S_L32 (bytes 2-3)|S_NID (bytes 0-1)|
        +--------+--------+--------+--------+
        |         S_NID (bytes 2-5)         |
        +--------+--------+--------+--------+
        |S_NID (bytes 6-7)| T_HA (bytes 0-1)|
        +--------+--------+--------+--------+
        |         T_HA (bytes 3-5)          |
        +--------+--------+--------+--------+
        |         T_L32 (bytes 0-3)         |
        +--------+--------+--------+--------+
        |         T_NID (bytes 0-3)         |
        +--------+--------+--------+--------+
        |         T_NID (bytes 4-7)         |
        +--------+--------+--------+--------+

### 回覆方

        0        7        15       23       31
        +--------+--------+--------+--------+
        |       HT        |        PT       |
        +--------+--------+--------+--------+
        |  HAL   |  PAL   |        OP       |
        +--------+--------+--------+--------+
        |         S_HA (bytes 0-3)          |
        +--------+--------+--------+--------+
        | S_HA (bytes 4-5)|S_L32 (bytes 0-1)|
        +--------+--------+--------+--------+
        |S_L32 (bytes 2-3)|S_NID (bytes 0-1)|
        +--------+--------+--------+--------+
        |         S_NID (bytes 2-5)         |
        +--------+--------+--------+--------+
        |S_NID (bytes 6-7)| T_HA (bytes 0-1)|
        +--------+--------+--------+--------+
        |         T_HA (bytes 3-5)          |
        +--------+--------+--------+--------+
        |         T_L32 (bytes 0-3)         |
        +--------+--------+--------+--------+
        |         T_NID (bytes 0-3)         |
        +--------+--------+--------+--------+
        |         T_NID (bytes 4-7)         |
        +--------+--------+--------+--------+