Since you mentioned that the system was embedded Linux, see
http://lartc.org/howto/ in particular Chapter 11. Some combination of
packet marking and policy routing should get you taken care of.

Good luck,
Ed

Christian Gagneraud wrote:
>
> Finally, my problem is that i would like to simply force the reverse
> tunnel to use only ppp0. And at the same time i need the default route
> to go through eth0 (that is needed for the main programs running on this
> box)
>