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ch04: Address Resolutin Protocol

ARP

Previousch03: Internet ProtocolNextch05: Reverse Address Resolution Protocol

Last updated 4 years ago

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MindMap

ARP: Address Resolution Protocol

一个例子

流程:

  1. 应用程序调用gethostbyname通过hostname获得对应的IP地址;

  2. 应用程序请求TCP建立和IP的连接;

  3. TCP发送一个连接请求,使用IP地址来发送一个IP datagram;

  4. 如果目标主机在本地网络,就直接发送到对应的主机上。如果在远程的网络,就发送到下一跳的路由器上来转发。这两种情况都需要将IP datagram发送到一台主机或路由器上;

  5. 如果是以太网,那么需要知道对应主机的以太网地址MAC。IP是逻辑地址,MAC是物理地址。ARP就是根据IP获得MAC;

  6. ARP发送一个ARP请求,包含对应的IP地址,问”谁的IP地址是这个就告诉我你的MAC地址“;

  7. 目的主机收到这个ARP请求后,发送一个ARP相应,应答包括IP地址和MAC地址;

  8. 收到ARP应答之后,就可以将那个IP datagram发送到下一跳了;

  9. 发送IP datagram到目标主机或路由器。

ARP 缓存

还可以将一个主机所知道的IP到MAC的映射缓存起来,缓存时间是20分钟。

可以通过下面命令查看ARP缓存:

$ arp -a
? (169.254.0.34) at <incomplete> on eth1
? (10.233.145.193) at bc:16:65:fb:a7:ff [ether] on eth1
? (169.254.0.35) at <incomplete> on eth1
? (10.233.145.241) at ec:38:8f:7b:0b:0a [ether] on eth1

ARP包格式

格式如下图:

  • 前两个是以太网的目的地址和源地址。目的地址全是1的是广播地址;

  • Frame Type:两个字节,对于ARP请求和ARP响应,这个值是0x0806;

  • Hard Type:硬件类型,以太网的值是1;

  • Prot Type:协议类型,IP的值是0x0800;

  • Hard Size和Prot Size,分别是MAC地址和IP地址的大小,分别是6和4,单位是字节;

  • op:operation,ARP请求是1,响应是2,RARP请求是3,RARP响应是4;

  • 接下来就是源和目的的MAC地址以及IP地址,其中目的主机的MAC地址是空的,需要响应来填充。

image-20200611183838494
image-20200611184810137