# 「明知故问」的单播 ARP 请求报文

IMPLEMENTATION: Four mechanisms have been used, sometimes in combination, to flush out-of-date cache entries.

(1) Timeout – Periodically time out cache entries, even if they are in use. Note that this timeout should be restarted when the cache entry is “refreshed” (by observing the source fields, regardless of target address, of an ARP broadcast from the system in question). For proxy ARP situations, the timeout needs to be on the order of a minute.

(2) Unicast Poll – Actively poll the remote host by periodically sending a point-to-point ARP Request to it, and delete the entry if no ARP Reply is received from N successive polls. Again, the timeout should be on the order of a minute, and typically N is 2.

[…]

Approaches (1) and (2) involve ARP cache timeouts on the order of a minute or less. In the absence of proxy ARP, a timeout this short could create noticeable overhead traffic on a very large Ethernet. Therefore, it may be necessary to configure a host to lengthen the ARP cache timeout.

arping -I wlan0 192.168.1.1


arping 命令在发送单播 ARP 请求报文之前，总是会先发送一个常见的广播 ARP 请求报文，不管当前 ARP 缓存中是否有对应主机的 MAC 地址。详细选项及说明请参考 arping(8)