Apex domains cannot have CNAME records.
Question and response
Question
dig xtra.pl. NS
Answer
xtra.pl. 5 NS ns1.xtra.pl.
xtra.pl. 5 NS ns2.xtra.pl.
xtra.pl. 5 NS ns.xtra.pl.
Authority
The response has no authority records.
Additional
ns1.xtra.pl. 5 A 212.14.56.2
ns2.xtra.pl. 5 A 212.14.56.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0