I have a little Linux (Ubuntu based) server with 2 network cards, which acts as a router, dhcp, dns server, firewall, etc. One network card is connected to the LAN, while the other is connected to my ISP. When the Internet link is down, I would like to redirect all http requests to my local Apache server, which would display a little webpage informing my LAN users, that their LAN connection is OK, the server itself is OK, only the ISP link is currently down. How would I do that?
I found no reliable way to detect if the Internet connection is down, other than periodically pinging a bunch of external hosts that are known to be up most of the time. (The status of the interface is UP all the time).
What tool should I use for redirection? Iptables? Something else?