Editing ARP spoofing (section)

==Legitimate usage==
{{see also|Proxy ARP}}
The techniques that are used in ARP spoofing can also be used to implement redundancy of network services. For example, some software allows a backup server to issue a [[Address Resolution Protocol#ARP announcements|gratuitous ARP request]] in order to take over for a defective server and transparently offer redundancy.<ref>{{cite web | url = https://man.openbsd.org/carp.4 | title = OpenBSD manpage for CARP (4) | access-date = 2018-02-04 | archive-date = 2018-02-05 | archive-url = https://web.archive.org/web/20180205000848/https://man.openbsd.org/carp.4 | url-status = live }}, retrieved 2018-02-04</ref><ref>{{cite web | url = http://www.ultramonkey.org/3/ip_address_takeover.html | title = Ultra Monkey: IP Address Takeover | author = Simon Horman | access-date = 2013-01-04 | archive-date = 2012-11-18 | archive-url = https://web.archive.org/web/20121118031514/http://www.ultramonkey.org/3/ip_address_takeover.html | url-status = live }}, retrieved 2013-01-04</ref> Circle<ref>{{cite magazine | url = https://www.wired.com/2015/11/circle-with-disney-locks-down-kids-devices-from-afar | title = Circle with Disney Locks Down Kids Devices from Afar | magazine = Wired | access-date = 2016-10-12 | archive-date = 2016-10-12 | archive-url = https://web.archive.org/web/20161012230841/https://www.wired.com/2015/11/circle-with-disney-locks-down-kids-devices-from-afar | url-status = live | last1 = Barrett | first1 = Brian }}, retrieved 2016-10-12</ref> and CUJO are two companies that have commercialized products centered around this strategy.

ARP spoofing is often used by developers to debug IP traffic between two hosts when a switch is in use: if host A and host B are communicating through an Ethernet switch, their traffic would normally be invisible to a third monitoring host M.  The developer configures A to have M's MAC address for B, and B to have M's MAC address for A; and also configures M to forward packets.  M can now monitor the traffic, exactly as in a man-in-the-middle attack.