Linux can be a terrific "poor man’s" router. It takes a little more startup configuration than a typical hardware router, but once it is going, very little will stop it, at least in our experience. We’ve been using the router setup detailed below in production for six months now (a few months more worth of pilot-testing), and the only faults we’ve encountered were either service-provider related or due to water-soaked cables -- unbelievable, but true, and a really long story I won’t get into here!. | Using Linux as a Router Jonathan Feldman Linux can be a terrific poor man s router. It takes a little more startup configuration than a typical hardware router but once it is going very little will stop it at least in our experience. We ve been using the router setup detailed below in production for six months now a few months more worth of pilot-testing and the only faults we ve encountered were either service-provider related or due to water-soaked cables -- unbelievable but true and a really long story I won t get into here Concepts Since I m about to describe how to roll your own router it will help if you understand how most TCP IP routing works. Any router whether Cisco Proteon or Linux is based upon the premise that packets need to be forwarded. Why Because presumably the packets that come in on one interface are not local to the other interfaces in the router. Therefore the router in question must be able to take a packet look at its destination and forward it to the appropriate interface. How does the router map network destinations to interfaces Well just as every router needs to be able to forward packets so too it needs the ability to consult build and update a lookup table called a routing table that maps destination networks to interfaces. If a packet comes in for network X the routing table is consulted and the packet is dumped to the appropriate interface -- a local node that is either another gateway or the packet s final destination. If network X does not exist in the routing table the packet is dumped to the default destination also denoted by IP address . If you have not specified a default destination the packet is dropped and an ICMP Destination Unreachable message is sent back to the originating network client. How does the router maintain the routing table In two ways statically and dynamically. Static routes are added by you in a known and sometimes tedious fashion. Typically default routes are static routes. Dynamic routes are .
