I assume you run bind. Write a script to go in ip-up.d that does something like:
1) dig @theispdomain | grep "IN NS" > afile 2) extract the IPs of the servers and sed them into the forwarder line in named.conf 3) ndc restart in ip-down.d you'll want to restore the named.conf to something 'normal' whatever that is :-) . Will add perhaps 10 s to your time to connect. There will be better way John
