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Date:      Wed, 3 Jan 2001 00:04:58 -0500
From:      "C. Stephen Gunn" <csg@waterspout.com>
To:        Doug White <dwhite@resnet.uoregon.edu>
Cc:        freebsd-hackers@freebsd.org
Subject:   Re: ARP question.
Message-ID:  <20010103000458.A16838@waterspout.com>
In-Reply-To: <Pine.BSF.4.21.0101021400490.38463-100000@resnet.uoregon.edu>; from dwhite@resnet.uoregon.edu on Tue, Jan 02, 2001 at 02:02:03PM -0800
References:  <5.0.2.1.0.20010102023010.00a101f0@pop6.sympatico.ca> <Pine.BSF.4.21.0101021400490.38463-100000@resnet.uoregon.edu>

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On Tue, Jan 02, 2001 at 02:02:03PM -0800, Doug White wrote:

> > arp: 200.42.126.18 moved from 00:e0:7d:7b:53:f0 to 00:c0:df:f4:ac:05 on ed0
> 
> From personal experience, Linux has this nasty bad habit of broadcasting
> ARPs on all interfaces.  For this reason multihomed Linux boxes should be
> banned.
> 
> We got tired of it at my previous job and patched around it on the linux
> machine.

I've seen similar things from Linux.  In my experience it was an
ARP who-has request with the source IP of the other interface.
Linux apparently doesn't learn ARP addresses this way, but FreeBSD
takes note, which causes the problem mentioned above.

Is the Linux box a firewall/NAT box of some kind?

Having two interfaces on the same wire can be a problem.  FreeBSD's
ARP implementation gets rather upset about seeing the packets twice,
since the receive interface is significant.  Broadcast protocols 
aren't guaranteed to be idempotent.  Your mileage may vary.

 - Steve

--
C. Stephen Gunn                          URL: http://www.waterspout.com/
WaterSpout Communications, Inc.        Email: csg@waterspout.com
427 North 6th Street                   Phone: +1 765.742.6628
Lafayette, IN  47901                     Fax: +1 765.742.0646


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