Routing is the glue that binds networks together to build internets. Without it, networks cannot communicate with each other. Configuring routing is an important task for the network administrator.
Minimal routing is required to communicate through the network interface to the directly attached network. These routes can be seen in the routing table where they show up as entries that do not have the G (gateway) flag set. On some systems, minimal routes are created by the ifconfig command when an interface is installed. On Linux systems, the route through the interface must be explicitly installed with a route command.
The route command is used to build a static routing table. Static routing is routing that is manually maintained by the network administrator. Routes are added to or removed from the routing table with the route command. The most common use for static routing is to install a default route.
Dynamic routing uses routing protocols to select the best routes and to update the routing table. There are many different dynamic routing protocols. The one that is available on most Unix systems is Routing Information Protocol (RIP). RIP is run by routed. routed builds the routing table from information received on the network and from information read from /etc/gateway.
gated is a software package that provides several more routing protocols for Unix systems, including advanced protocols such as Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP). gated is configured through the /etc/gated.conf file. The gated configuration commands are covered in Appendix B.
This is the last chapter on how to create the physical network connection. Once routing is installed, the system is capable of basic communication. In the next chapter, we begin the discussion of the various applications and services that are necessary to make the network truly useful.