> I'm searching for an example userland source which takes an IP address
> as argument and returns the network interface which would be used to
> send a packet to the supplied IP address.
> 
> Do we have something like that somewhere in our sources? If we don't
> has anybody ever written something like that?

I just tried this, and it seems to work:

- Create a UDP socket
- bind() it to a local port
- connect() it to the "supplied IP address"
- getsockname() on the socket

The returned struct sockaddr from getsockname() will contain the
IP address of the interface that the supplied IP address will be
using.  From there, you should be able to figure out the interface name
by walking the interface list.

"No fuss, no muss".

--Ken