>By "gratuituous arp" I was really saying "gratuitous arp reply".
>The machine needs to send a packet of the type
>
> arp reply 1.2.3.5 is-at 0:40:5:42:d6:de
The ARP processing specified in RFC 826 says that if you have an entry for
the source IP address you update the hardware address no matter what the
opcode is (i.e. you can update your tables due to a request). Every IP
stack I've seen implements gratuitous ARP by sending a broadcast request
for itself. In normal ARP operation, replies are unicast so conceivably
an implementation that doesn't expect a broadcast reply might drop it.
Requests are normally broadcasted, and the ARP processing rules cause
broadcasted requests to update existing tables, so a broadcasted request
is a better choice for a gratuitous arp.
tcpdump hides too much information in an attempt to make things look
pretty; it doesn't show the fact that the MAC information is included
in a gratuitous ARP.
Bill
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