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Routing Issue: Load Balancer with multiple external NICs

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Single Unix-based load balancer (LB) is configured with multiple network interfaces - 2x external and 1x internal. The LB sits in front of a DNS resolver. The issue I'm trying to solve is that all DNS responses come back to External Interface #1 even when the request comes through Interface #2. If the request came through Int #2, I need the response to come back through the same interface.

Update: Using iptables, but happy to consider alternatives.

Network Diagram

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In addition to a layer of routing to apply, the application, especially when it's handling UDP, must be multi-homed aware. So how is the LB configured? Does it bind twice, once per IP address on each external interface (and also, is each address routable only on the correct path or on any path?) ? Or else is it able to use features like IP_PKTINFO? I'm telling this just to say that this is in two parts: system (routing) fix + application fix. What's the application?
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