title: Win32 sockets - Forcing ip packets to leave physical interfaces when sending to other local interfaces
body: <p>Summary: I'm trying to create sockets to pass data between two physical interfaces that exist on the same machine, and Win32 sockets always forwards the traffic directly in the kernel instead of pushing through the physical interfaces. Is there any way to disable this behavior, perhaps through device settings, registry tweaks, routing table shenanigans, or socket options? We're using Windows XP SP3. </p>
  
  <p>Some background. I'm attempting to build some completely automated IP tests to exercise our custom IPv4 equipment. We have a large lab of Windows XP machines, and individual physical ethernet interfaces for each device we're connecting to. Our devices are effectively ethernet routers each with their own IPs.</p>
  
  <p>We need to send data out our lab machines, through our devices, then back into the same computer. We will be sending Unicast and Multicast UDP, TCP, and broadcast IP traffic through the devices.</p>
  
  <p>We want (and likely need) the traffic to originate on the same machine it is destined to. 
  To do this, we configure two separate NICs each with their own IP on their own subnet, for instance NIC #1 with 10.0.0.1/24 and NIC #2 with 10.0.1.1/24. Our devices then act like simple passthrough routers, and have two interfaces, one on the 10.0.0.0/24 subnet, one on the 10.0.1.0/24 subnet, which they just forward packets back and forth from. </p>
  
  <p>To generate our data, we'd like to be able to use Win32 sockets, since it is well-understood, well-supported, what our customers are using, and would probably be the most rapid approach. Packet injection is probably feasible for UDP and broadcast IP, but very likely not so for TCP. I'd entertain ideas that used packet injection, but would strongly prefer standard Win32 sockets.</p>
  
  <p>As stated in the summary, the packets never leave the machine. I've googled like a madman and I've not found much. Any ideas?</p>