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 0.0.0.0. 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.

Read more: Using Linux as a Router