Apex domains cannot have CNAME records.
Question and response
Question
dig tung.de. NS
Answer
tung.de. 5 NS ns4.webgo.de.
tung.de. 5 NS ns1.webgo.de.
tung.de. 5 NS ns2.webgo.de.
tung.de. 5 NS ns3.webgo.de.
Authority
The response has no authority records.
Additional
ns1.webgo.de. 5 A 89.44.8.112
ns2.webgo.de. 5 A 89.44.8.111
ns3.webgo.de. 5 A 89.44.8.124
ns4.webgo.de. 5 A 217.79.184.5
ns1.webgo.de. 5 AAAA 2a00:5fc1:0:120:89:44:8:112
ns2.webgo.de. 5 AAAA 2a00:5fc1:0:120:89:44:8:111
ns3.webgo.de. 5 AAAA 2a00:5fc1:0:120:89:44:8:124
ns4.webgo.de. 5 AAAA 2001:4ba0:ffff:81:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0