The "default IPv4 dns check" checks for the A record of WWW.WOODSTONE.NU and this via the server that you specify within the check entry.

If your DNS server is unable to retrieve that info (for example becaus the DNS servers responsible for the woodstone.nu domain are down), then you will get a down on the check.  This down means "we can't get the IP address for the A record www.woodstone.nu".

If you want to check that your DNS server is running without having to worry about eventual other upstream DNS server being down, then you should modify the "default IPv4 DNS check" and have it check for a A-record that is localy hosted on your DNS server.  You can do this via the PROTOCOL BUILDER (ctrl-P from within the mainscreen of Servers Alive)