A UCS DC master should usually have itself configured as the primary name server. Check the config registry variables »nameserver1« through »nameserver3«: usually only one is set and contains the DC's IP address.
On a UCS DC master you have two choices for where DNS entries are lookup up: either in the OpenLDAP directory or in the Samba 4's LDAP directory. Both should contain nearly the same information, but it's possible that the synchronization between the two has been disrupted. Check with »univention-s4connector-list-rejected« for such unresolved conflicts.
Which of the two backends is used can be configured via the config registry variable »dns/backend«.
Queries which cannot be answered from that backend are then forwarded to the DNS server configured by the config registry variables »dns/forwarder1« through »dns/forwarder3«.