Apex domains cannot have CNAME records.
Question and response
Question
dig dsl.introweb.nl. NS
Answer
dsl.introweb.nl. 5 NS nsauth1.introweb.nl.
dsl.introweb.nl. 5 NS nsauth3.introweb.net.
dsl.introweb.nl. 5 NS nsauth2.introweb.nl.
Authority
The response has no authority records.
Additional
nsauth1.introweb.nl. 5 A 80.65.96.48
nsauth2.introweb.nl. 5 A 86.105.160.91
nsauth3.introweb.net. 5 A 62.165.127.221
nsauth1.introweb.nl. 5 AAAA 2001:1690:1:a:0:0:0:a
nsauth2.introweb.nl. 5 AAAA 2a0d:8600:1:0:0:0:0:5a
nsauth3.introweb.net. 5 AAAA 2001:1690:1:c:0:0:0:a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0