Another approach to routing to nonlocal networks is to define a default route (or wildcard route) to a smart gateway. This gateway would then redirect information (using ICMP routing) to dynamically create a routing database. A default route consists of an entry in the kernel routing table that directs network traffic to hosts or networks not explicitly identified in the routing table.
You can create a default route at installation during the DNS client configuration phase, when you are prompted for a gateway address for the default route. You can also add or modify a default route after installation using the Client Manager or the Internet Manager. These managers add entries to the TCP/IP default configuration file tcp(SFF) (/etc/default/tcp), which are read by the tcp(ADMN) script that controls TCP/IP startup and shutdown.
You can also specify a default route using the
command, as in the following example:
/etc/route add default smart-gateway
The route command is used to manually manipulate routing tables. However, changes introduced with the route command are not preserved when TCP/IP is stopped and restarted. See the route(ADMN) manual page for more information.
The host uses the default route to the smart gateway as a last resort in routing packets to their destinations. Assuming the smart gateway will generate ICMP routing-redirect messages, the host will update its routing table entries based on the redirect messages received from the smart gateway.
This approach has certain advantages over using the routed daemon, but it is unsuitable in an environment where there are only bridges (for example, bridges do not generate ICMP routing-redirect messages). Further, if the smart gateway goes down, there is no way to maintain service except to alter the routing table manually using the route command.
The host always listens to, and processes, ICMP routing-redirect messages, and so it is possible to combine both of the above facilities. For example, the routed daemon might be used to maintain up-to-date information about routes to local networks, while employing the default routing techniques for distant networks.