Re: Mysterious ARP behavior on a tcp-ip ethernet


Jeff Mogul (mogul@navajo.stanford.edu)
31 Jul 1986 1540-PDT (Thursday)


[To summarize bjp's problem: he's got hosts sending out ARPs asking for the
 hardware address of IP host 192.12.120.255, and he wants to know why a host
 would ARP for what is supposed to be a broadcast address.]

Your problem is that you have two groups of hosts with different ideas about
what a broadcast address is. One group follows RFC919 and uses "all ones"
for broadcast; these are the "good hosts". The other group uses the "all
zeros" address that Berkeley put into 4.2BSD; these are the "bad hosts".
[Note that 4.3BSD is "good".] "Good hosts" know that there are bad hosts
out there and recognize, but never transmit, all-zeros broadcasts; that is
why they are not trying to ARP the 192.12.120.0 address that the bad hosts
are probably broadcasting to.

So, what you should do is determine which of your hosts are "bad" (any host
that sends an ARP request for 192.12.120.255 is bad; you might have others.)
Then, call up the manufacturers and demand that they fix their code to
conform to RFC919. If you are running 4.2BSD on a Vax, switch to 4.3BSD.

Whatever you do, NEVER EVER modify ARP code to respond to requests
for what turns out to be a broadcast address. Ever. Never, ever.
Not "hardly ever"; never.



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