Something like next hop match

Hello,

in my configuration there's a linux router which is dynamically learning routes via RIP from an isdn router connected to the "outside" interface of the router. The internet gateway is also conencted to that interface.

For some reason i don't want the packets which travel through the isdn router to get SNATted. Packets going out to the internet gateway must be SNATted.

Is there any way to match the packets which travel through the isdn router by something like a next hop match in the POSTROUTING table? Some years ago there was a nexthop match patch, but it never makes it into the netfilter project.

Any ideas?

Regards,
Markus