Dual Homed Linux - what am I doing wrong?

Dual Homed Linux - what am I doing wrong?

Post by Dan Peterk » Wed, 29 Jul 1998 04:00:00

I have a RH 5.0 Linux PC setup with two Ethernet interfaces configured and
connected to two different LANs. Each LAN has a way out to the Internet, and
I've disabled IP forwarding on Linux to prevent the PC from routing between
the two LANs (which is my intention). The routing table entries for the first
LAN (eth0) simply has the usual loopback and LAN entries, and a default route
pointing to the gateway to the Internet. This works just fine.

On the eth1 interface, I have set up the Cisco router on the second LAN to use
RIP to broadcast routes on the second LAN. I then configure Linux to run
routed with -q (listen only) so that it can pick up the RIP routes and know to
use the second interface for routes to networks serviced by the Cisco. This
isn't working.

When I start routed -q, I end up with a route to the serial side of the Cisco
(that's good) and a second default route pointing to the Cisco on eth1 is
created (not good). With two default routes, Linux seems to pick the first
available default route when it doesn't know where to go - hence, all my
traffic continues out the eth0 interface when a better path is available on

The route table never fills with routes known by the Cisco, so my question is
what am I doing wrong? Is this a known problem with Linux? Why is it creating
a second (and useless) default route instead of filling up with routes learned
via RIP?

Dan Peterka