Apex domains cannot have CNAME records.
Question and response
Question
dig techfak.uni-bielefeld.de. NS
Answer
TechFak.Uni-Bielefeld.DE. 5 NS dns3.BelWue.DE.
TechFak.Uni-Bielefeld.DE. 5 NS auth-ext0.DNS.TechFak.NET.
TechFak.Uni-Bielefeld.DE. 5 NS auth1.DNS.techfak.uni-bielefeld.de.
TechFak.Uni-Bielefeld.DE. 5 NS dns1.BelWue.DE.
TechFak.Uni-Bielefeld.DE. 5 NS auth0.DNS.techfak.uni-bielefeld.de.
Authority
The response has no authority records.
Additional
dns1.belwue.de. 5 A 129.143.2.10
dns3.belwue.de. 5 A 129.143.253.133
auth0.dns.techfak.uni-bielefeld.de. 5 A 129.70.136.53
auth1.dns.techfak.uni-bielefeld.de. 5 A 129.70.132.13
auth-ext0.dns.techfak.net. 5 A 195.201.119.124
dns1.belwue.de. 5 AAAA 2001:7c0:0:0:0:0:53:10
dns3.belwue.de. 5 AAAA 2001:7c0:0:253:0:0:0:133
auth0.dns.techfak.uni-bielefeld.de. 5 AAAA 2001:638:504:2000:0:0:0:53
auth1.dns.techfak.uni-bielefeld.de. 5 AAAA 2001:638:504:2000:0:0:0:54
auth-ext0.dns.techfak.net. 5 AAAA 2a01:4f8:1c0c:7f8f:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0