egress interface index lookup through a route SACK_RAW socket call

[Martin]

How do I construct a routing raw socket call to retrieve the
outgoing interface index for a particular route that is destined out of
an interface other than Ethernet such as a point to point?
 
With Ethernet interfaces I was able to contruct a rt_msghdr with RTM_GET
and RTA_DST- retrieve the gateway IP (RTA_GATEWAY) for the route then
recursively call
the same function passing the gateway IP and the sock_addr_in
->s_port would return with the interface index.
 
But this does not work with PPP interfaces or other non-Ethernet
interfaces.  Any
insight to what i am lacking in the route request would be appreciated.
 
Thanks,
_Martin