Fault Tolerant Router

Fault Tolerant Router is a daemon, running in background on a Linux router or firewall, monitoring the state of multiple internet uplinks/providers and changing the routing accordingly. LAN/DMZ internet traffic (outgoing connections) is load balanced between the uplinks using Linux multipath routing. The daemon monitors the state of the uplinks by routinely pinging well known IP addresses (Google public DNS servers, etc.) through each outgoing interface: once an uplink goes down, it is excluded from the multipath routing, when it comes back up, it is included again. All of the routing changes are notified to the administrator by email.

via  Fault Tolerant Router