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I have a device (a single-port terminal server that I use to emulate a serial port) that is accessible via two paths: a microwave T-1 (which connects the device to the internal network at my job) and a public Internet connection via the power company at the remote site. The T-1 goes down from time to time due to atmospheric conditions and whatnot, but the public Internet connection is also not 100% reliable.

I would like to set up a single IP address that would automatically send packets via the best path - either the T-1 or the public Internet connection. Is it possible to do this with a Linux box? I don't have the budget for a dedicated router.

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iproute does that, it can load balance the outgoing traffic, but you need two ip numbers for this. If you would like just one, you would need a BGP load balancing, so you would need to advertise on what ISP your ip number is, and you would have to ask them to do it. – Andrew Smith Jul 5 '12 at 17:03
You can use also DNS load balancing like GSLB or round-robin with two providers. – Andrew Smith Jul 5 '12 at 17:05

I wouldn't bother trying to make this work with one IP address. Just use Dynamic DNS and set your linux machine to update Dynamic DNS with the public IP address of most preferable path (I would guess the T1) until it isn't available.

Many DDNS providers don't like it if you send rapid updates, so if you have to fail over, you'd probably want to stick with that backup address until that path fails.

I use to do something similar at home, but there are many options for dynamic dns clients.

The reason I wouldn't bother trying to stick with 1 IP is because most public providers only allow you to announce address space through BGP in blocks no smaller than /24; you're probably looking at a /32 for every site.

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