Apex domains cannot have CNAME records.
Question and response
Question
dig gmbh. NS
Answer
gmbh. 5 NS v2n1.nic.gmbh.
gmbh. 5 NS v0n3.nic.gmbh.
gmbh. 5 NS v0n1.nic.gmbh.
gmbh. 5 NS v0n0.nic.gmbh.
gmbh. 5 NS v0n2.nic.gmbh.
gmbh. 5 NS v2n0.nic.gmbh.
Authority
The response has no authority records.
Additional
v0n0.nic.gmbh. 4804 A 65.22.32.7
v0n1.nic.gmbh. 4804 A 65.22.33.7
v0n2.nic.gmbh. 4804 A 65.22.34.7
v0n3.nic.gmbh. 4804 A 161.232.16.7
v2n0.nic.gmbh. 4804 A 65.22.35.7
v2n1.nic.gmbh. 4804 A 161.232.17.7
v0n0.nic.gmbh. 4804 AAAA 2a01:8840:22:0:0:0:0:7
v0n1.nic.gmbh. 4804 AAAA 2a01:8840:23:0:0:0:0:7
v0n2.nic.gmbh. 4804 AAAA 2a01:8840:24:0:0:0:0:7
v0n3.nic.gmbh. 4804 AAAA 2a01:8840:fa:0:0:0:0:7
v2n0.nic.gmbh. 4804 AAAA 2a01:8840:25:0:0:0:0:7
v2n1.nic.gmbh. 4804 AAAA 2a01:8840:fb:0:0:0:0:7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0