Quote:
The proxy is configured to send its traffic to a device on 'eth1's local subnet, so there's no question that the traffic should go out via 'eth0'.

I know you already retracted what you said, but I thought I'd add more. Insult to injury, I guess. Sorry.

First, I assume you meant those two interfaces to be the same.

How do you configure the proxy to send its traffic to a device on a particular interface? (Let's ignore the ultimate destination for now.) By having the best route for the packet pointed at that interface or using source routing. While there's nothing on his network, most likely, that would prevent source routing, there's nothing that will produce source routing. That leaves the normal best-route procedure. This would mean that to make it go there, it would need to be destined for that subnet, which isn't useful, it would need to be the default route, which means all traffic would go that way, or you'd need a static route, and there's not a practical way to figure out how to add such static routes, or to actually add them.

I'd like to point out that there's nothing in the IP specification that dictates that this is how hosts must work. That is, it would be perfectly within spec for a computer to determine where to send packets on a per-app basis. It's just that nothing like that actually exists outside specialty IP stacks for routers, Linux-based or otherwise.
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Bitt Faulk