Apex domains cannot have CNAME records.
Question and response
Question
dig ruhr. NS
Answer
ruhr. 5 NS b.nic.ruhr.
ruhr. 5 NS c.nic.ruhr.
ruhr. 5 NS d.nic.ruhr.
ruhr. 5 NS a.nic.ruhr.
Authority
The response has no authority records.
Additional
a.nic.ruhr. 1450 A 194.169.218.155
b.nic.ruhr. 1450 A 185.24.64.155
c.nic.ruhr. 1450 A 212.18.248.155
d.nic.ruhr. 1450 A 212.18.249.155
a.nic.ruhr. 1450 AAAA 2001:67c:13cc:0:0:0:1:155
b.nic.ruhr. 1450 AAAA 2a04:2b00:13cc:0:0:0:1:155
c.nic.ruhr. 1450 AAAA 2a04:2b00:13ee:0:0:0:0:155
d.nic.ruhr. 1450 AAAA 2a04:2b00:13ff:0:0:0:0:155
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0