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Date: Thu, 28 May 2015 13:36:45 +0200
From: Julian Kornberger <juliank@tzi.de>
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Subject: Re: Crash with GRE und IPFW fwd
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Am 28.05.2015 um 05:29 schrieb Julian Elischer:
> Julian, I am guessing (with no actual information) that something in gre
> processing is
> assuming that the packet id going where it normally would go. (and not
> where fwd is sending it).
>
> you really need to  use objdump or gdb  to look at the gre module and
> work out where
>
> gre_output+0x467
>
> is in the function. that should give you a clue on the problem.
>
> not too helpful I know...  but you have a reproducible case at least.
> kgdb may be your friend if you
> have a serial cable or firewire cable.

It is a multihomed machine behind two DSL routers. All packets leave the 
interface with the default route unless I use IPFW with fwd.
Is there another way to force packets to leave the correct interface?

The problem does also appear if I shutdown the second WAN interface.
This is the way the traffic goes:

re0 (internal network) -> fwd to gre1 -> fwd to gateway of re1 -> re1

If the traffic source is the machine an not the internal network it 
seems to cause no problems. I guess the problem is to use fwd twice.
I will do additional tests and I will try to debug this using a serial 
connection next week.

kind regards,
Julian