TID 3988333 says:

DisableDNSCheckup=1 (0 to disable) (Default=0)
//disables the persistent DNS checks that are made when a DNS server is down. Requires PXY014 or later.

I don't understand exactly what this does. Is there some time period that the DNS server doesn't respond to NBM after which NBM decides the server is down and doesn't check with it any more?

What happens if the DNS server was just brought down then back up? If this parameter is set to 1, does it mean that NBM immediately stops checking with that DNS server after the first time it doesn't get a reply and (when it was brought down) and won't check with it again when it comes back up?

I'm assuming that since the parameter is set to 1 in the example that that is the preferred state?