Apex domains cannot have CNAME records.
Question and response
Question
dig dtag.de. NS
Answer
dtag.de. 5 NS pns.dtag.de.
dtag.de. 5 NS ns1.telekom.net.
Authority
The response has no authority records.
Additional
pns.dtag.de. 5 A 194.25.0.125
pns.dtag.de. 5 AAAA 2003:40:8000:0:0:0:0:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0