Linux 2.2 - BSD/OS 4.1 ARP incompatibility

From: David Luyer (
Date: Fri Sep 01 2000 - 20:28:00 EST

I'm seeing a problem between Linux 2.2 and BSD/OS 4.1 in the situation on one
of our backbones.

We have a number of Linux hosts on this backbone with a primary address in
the network a.b.c.0/24 and a secondary address in the network d.e.f.0/24.

We also have some BSD/OS 4.1 machines in this network with addresses in
d.e.f.0/24 only.

Now when the Linux box a.b.c.1 (with secondary address d.e.f.1) wants to
talk to the BSD/OS system d.e.f.2 it does

    a.b.c.1 arp who-has d.e.f.2

Which d.e.f.2 promptly ignores, presumably because the IP stack in BSD/OS
throws it away at a low level, or possibly simply because BSD/OS has no
idea where to send the ARP response.

The end result is that a.b.c.1 can't talk to d.e.f.2 because it does an ARP
from the wrong IP address and doesn't get a response.

Is this already fixed in 2.4 or it is something which needs investigation
and a patch?

According to the standards, who is in the wrong? To me it looks like Linux
is the misbehaved OS in this case.


David Luyer
Senior Network Engineer
Pacific Internet (Aust) Pty Ltd
Phone:  +61 3 9674 7525
Fax:    +61 3 9699 8693
Mobile: +61 4 1064 2258, +61 4 1114 2258        NASDAQ: PCNTF
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