> What I'm trying to do is this:  if the high speed connection goes down (which it seems to do
> frequently :-P ) I want iptables to default to the dialup connection.  And vice versa (I have
> iptables set up to use a particular link for a particular service).  Is this possible?  And if it is
> possible, how do I do it/where do I look to learn/etc.  Thanks.

Let's say that your internal interface is eth0, your WAN is eth1, and your modem is ppp0
You can set up your iptables rules so that instead of referring to "-i eth1" or "-i ppp0"
they refer to "-i ! eth0"

As far as changing your routes, that is not up to iptables, that is up to the way that you
programatically determine if the WAN interface is up.

-Ben.