DNS issues with tools.ietf.org

a message of 8 lines which said:

Something seems wrong with the DNS of 'tools.ietf.org'.

Can you be more specific? It works for me except that one name server
does not actually exist (but it does not prevent the domain from

% zonecheck tools.ietf.org
ERROR: Unable to find nameserver IP address(es) for grenache.levkowetz.com

(NXDOMAIN, indeed)