Router – Dual WAN Router w/Failover for < 50 IP Network Recommendation

A pair of Cisco ASA 5505's will do this in a number of different ways. The simplest would be to have an ASA plugged into each of your ISP connections, designate one as primary, then configure the secondary to kick in if the primary link goes down. This also gives you hardware redundancy which you won't get with a single server (which is pretty much all your Dual-WAN routers were).

Chapter 10 of the Cisco Press Cisco ASA book (ISBN: 978-1-58705-819-6) describes the config pretty well, and I am sure you can find a Cisco partner who will take you through it at reasonable cost.

You might also be able to do something with HSRP/VRRP on two routers connected via a switch, and a good network engineer. Better than me, as I can't tell you how to do it :-)

This is a little more complex since I'm assuming that your routers can't export their routes in a meaningful way. If you feel like digging into it, remember that when you send a packet locally the IP address is still the far-end host and the MAC address (on Ethernet, anyway) is based on the hosts route table and arp cache.

If you knock your software down a level and use the kernel's packet calls to form your message, you can write that IP packet with the MAC address of your choice. The packet will then use the router of your choice as the first hop. Use that to ping and monitor responses and you'll be able to test connectivity without changing system settings.

Of course, you'll need some elevated permissions in your program to accomplish this.

Otherwise, forced single-address routing table entries suggested by Evan will allow you to stay out of writing things that call on the kernel. For any method where the router's aren't complicit in telling you, you need to be in the DMZ otherwise you can't choose where your packets go unless you somehow end up with source routing enabled.