Whacky Iptables Wizardry

[Lucas J Barbuto <lucas@stabat.com>]

Hi All,

I'm wondering if I can pull a tricky move using Iptables and Linux
routing, one of you geniouses will be able to help me out I'm sure!

I have a router sitting in front of a subnet of 12 IP addresses.  One of
these subnet IPs is used as the main internet connection for an office.
We'd like to take this connection down for a few hours to do some
testing and maintenance, using a backup DSL connection in the mean time.

Is it possible for me to setup some rules on my router to say "All
traffic coming in looking for a certain IP (the office has a web server
and an SMTP server) should be re-routed to another IP (the DSL
connection).  The DSL of course is on a different subnet that I have no
control over.  I'm not sure if this is possible, can anyone give me
some pointers here?

On another topic (proxy ARP), I have a setup like this:

    the Internet

            |
            + network gw (238)
            |
            |
            + office gw (237)
            |
            |
            + another office gw (234)
            |

I'm using proxy ARP at 237 so that 238 knows how to get to 234, which
works, but I find that if the ARP cache entry on 238 expires, then 238
gets traffic from the Internet destined for 234 it doesn't get through.
My current work-around is to have a ping packet sent once a minute from
238 to 234 to keep the ARP cache fresh --- this is clearly not ideal ---
anyone know what's up with this?  238 is running as an ethernet bridge.

Regards,

Lucas