ns.ext-dc.com has no CNAME records.
Question and response
Question
dig ns.ext-dc.com. NS
Answer
The response has no answer records.
Authority
ext-dc.com. 203 SOA ns.ext-dc.de. hostmaster.ext-dc.de. 2025080601 28800 7200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0