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Re: icmp_unreachable uses wrong ip

To: Patrick McHardy <kaber@xxxxxxxxx>
Subject: Re: icmp_unreachable uses wrong ip
From: Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>
Date: Wed, 4 May 2005 10:29:51 +0200
Cc: Hasso Tepper <hasso@xxxxxxxxx>, "J. Simonetti" <jeroens@xxxxxxxxxxxxxxxxx>, netdev@xxxxxxxxxxx
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On Wed, May 04, 2005 at 01:37:40AM +0200, Patrick McHardy wrote:

> >>But when multiple addresses are used the result can be even more
> >>confusing. I don't like inconsistent behaviour, and this patch works
> >>sometimes and sometimes it doesn't.
> > 
> > I see no behaviour you can define as "it doesn't work". Purpose of this 
> > patch is to provide info about links (not addresses, you can't have this 
> > info) used to forward packets and it does the job. 
> Well, arguably it can be called "doesn't work" if addresses not used
> at all during transmit of the packet show up in traceroute.

That argument doesn't hold, since exactly the same situation occurs
if we use the outgoing address as we do now.

With asymmetric routing, the incoming interface, interface to
the destination, and the interface back to the source might all
be different, so we can end up with:

       path from r3 back to a
       |                   |
       V                   |
       a --- r1 --- r2 --- r3 --- r4 --- r5 --- b

The address of the 'upper' interface of r3 is likewise "not used
at all during transmit of the packet", but it is the address we
currently send the ICMP from.


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