the arp table is per interface, so it is possible to have the same
netwrok on multiple physical interfaces, tho with different source
ip address. one example would be a ethernet and a wlan interface.
the mac addresses on these mediums can differ (arp proxying taking
place).
so provide our source address on the interface we received the
request on.
the previous change used the ifcaddr; which is correct; but due to a
oversight in the kernel, had to match the ip of the arp entry.
source address will always work.
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