Ok, I stand corrected.
On Sun, 7 May 2000, Bill Fenner wrote:
> >By "gratuituous arp" I was really saying "gratuitous arp reply".
> >The machine needs to send a packet of the type
> > arp reply 126.96.36.199 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.
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