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Asked: 2026-05-18T02:01:54+00:00

I wrote a tool running on a system (Win7) with two network interfaces, each

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I wrote a tool running on a system (Win7) with two network interfaces, each linked to a different subnet, each with its own gateway which is then linked to two separate distant networks (there are outgoing firewalls after each gateway). I’m initiating outgoing TCP connections via both NICs by using Socket.Bind (before doing Connect) to each relevant NIC’s IP address. First NIC is working fine, but for the second NIC, I’m getting SocketException: “A socket operation was attempted to an unreachable network”.

My original understanding was that since sockets are bound to concrete NIC’s local endpoint, which has its gateway defined, the connection should be routed to this gateway and therefore should work. However, it seems that source IP address is ignored and the routing is working according to local routing table (i.e. second NIC’s connect request goes to first, default, network and being rejected because it has wrong subnet).

Adjusting local routing tables helps, but it makes me wonder about the whole reasoning behind ability of the socket to bind to specific local IP.

Doing some extra reading, I found out that, indeed, there’s such thing as “source IP routing”, but it is disabled in Windows by default (via DisableIPSourceRouting registry setting), due to security reasons, as described, e.g. here:

Questions:

  1. If my original understanding was correct (i.e. Socket.Bind should be enough) – why it is not working without modifying routing tables?
  2. If my understand was NOT correct (i.e. Socket.Bind is ignored and routing is used) – what’s the point of having Socket.Bind? Why doing it at all?
  3. Also, I’d like to understand better, what is the actual risk of having source IP routing enabled (preferably with example of a possible exploit)?
  4. Any ideas of solving the requirement without manually modifying local routing table will be greatly appreciated.

Many thanks.

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1 Answer

  1. Editorial Team

    OK, after some reading, here are some high-level explanations on what’s happening. I still need to verify the below conclusions in my system. Apparently, local binding is typically ignored when selecting network interface. Instead, routing table is used for this. However, in Strong Host Model (default for Vista and newer, non-existant in XP), source IP is used as a ‘constraint’ in the routing table lookup.

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