_external.a3bc.eu has no CNAME records.
Question and response
Question
dig _external.a3bc.eu. NS
Answer
The response has no answer records.
Authority
a3bc.eu. 10794 SOA ns1.a3bc.eu. noc.a3bc.eu. 2025112403 3600 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0