Apex domains cannot have CNAME records.
Question and response
Question
dig ipn.de. NS
Answer
ipn.de. 5 NS ns3.routing.net.
ipn.de. 5 NS ns2.routing.net.
ipn.de. 5 NS ns1.routing.net.
ipn.de. 5 NS nsh.ipn.de.
Authority
The response has no authority records.
Additional
nsh.ipn.de. 5 A 157.90.122.239
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0