Apex domains cannot have CNAME records.
Question and response
Question
dig do.de. NS
Answer
do.de. 4 NS ns1.mainns.de.
do.de. 4 NS ns2.mainns.eu.
do.de. 4 NS ns3.mainns.net.
Authority
The response has no authority records.
Additional
ns1.mainns.de. 4 A 192.174.68.21
ns2.mainns.eu. 4 A 176.97.158.21
ns3.mainns.net. 4 A 192.174.68.20
ns1.mainns.de. 4 AAAA 2001:67c:1bc:0:0:0:0:21
ns2.mainns.eu. 4 AAAA 2001:67c:10b8:0:0:0:0:21
ns3.mainns.net. 4 AAAA 2001:67c:1bc:0:0:0:0:20
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0