Default route in quagga doesn't work if specified using interface rather than address

Question

I have the following zebra.conf on my router box "A":

hostname nuclear-router
password password
enable password password

interface eth0
ip address 192.168.2.1/24
multicast
no shutdown

interface wlan0
ip address 192.168.1.2/24
multicast
no shutdown

#ip route 0.0.0.0/0 wlan0 # Does not work. 
ip route 0.0.0.0/0 192.168.1.1 # Works beautifully

eth0 is connected to a switch and contains the 192.168.2.0/24 range, and A is connected to my WRT54GL "B" (with IP 192.168.1.1, containing 192.168.1.0/24 range, DHCP serves IPs above .100) wirelessly. B is connected directly to my DSL modem.

The problem is that when I specify the default route using the interface name, I can't connect to the internet from A or any hosts behind it - I'm always hit with a "no route to host" error. Name lookup and pinging any local host (even in a different range) or the DSL modem works fine - it's only when going beyond the modem that things stop working. However, when I use the IP of B as the gateway, it works fine. I noticed that route output on A when the default route was specified using the interface alone had only an asterisk in the gateway column. I was under the impression that these approaches should be identical in practice, so though I got it working, I'd like to know what I'm misunderstanding (and/or misconfiguring). Why didn't the default route work when specified using just the interface?

All routers and the modem too have RIP (version 2) enabled, and of course the password isn't really the word "password".

Answer 1

While a default route might work in the desired manner when the destination is a point-to-point layer-2 link, it assuredly won't work to a broadcast interface like ethernet or wlan. Configurable routes are at OSI Layer 3 (IP numbers), and ought not to require MAC addresses (Layer 2), interface names (layer 1), or port numbers or packet types (layer 4+). Once the right destination IP number is provided for a route, the router will handle what MAC address to forward packets towards, and which interface that MAC is connected.

In summary, setting your default route to an interface name doesn't work because its not supposed to... If you don't know what IP address is supposed to handle forwarding of non-local traffic, then it's best not to have a default route set.

Answer 2

Seems to me you are confusing the hostname with the interface name. The name wlan0 is the name of the interface. The hostname is stored in the DNS and is what the lookup returns.

Most likely, what is happening is DNS is looking up the host wlan0 and coming up with the error no host found.