ns23.customer.level3.net has no CNAME records.
Question and response
Question
dig ns23.customer.level3.net. NS
Answer
The response has no answer records.
Authority
customer.level3.net. 1407 SOA ns1.Level3.net. dns.level3.net. 2019010900 7200 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0