Linux.com offers a tutorial on using Linux to make a "failover router".
Today, it's hard to imagine an organization operating without taking advantage of the vast resources and opportunities that the Internet provides. The Internet's role has become so significant that no organization can afford to have its Net connection going down for too long. Consequently, most organizations have some form of a secondary or backup connection ready (such as a leased line) in case their primary Net connection fails. However, the process of switching over from the primary to the backup connection, if done manually by the system administrator, can take some time, depending upon how ready the backup setup is and on the availability of the administrator at the right moment. The process can even become a costly affair if the organization must buy dedicated routers for the purpose of automatic switchover. But there is an easy and cost-effective alternative -- setting up a Linux failover router.
In this article we will look at setting up an existing Linux machine as a failover router to provide quick and automatic switchover from a dead Internet connection (the primary connection) to one that is operational (the secondary connection).
To begin, you'll need a PC with any recent GNU/Linux distro installed. You'll also need three network cards to put into this Linux box. Two of the three network cards, say eth0 and eth1, will connect to the Internet routers/gateways of your primary ISP (say ISP1) and secondary ISP (say ISP2). The third network card, say eth2, will connect to your internal LAN.
