Apex domains cannot have CNAME records.
Question and response
Question
dig etf.RS. NS
Answer
etf.rs. 5 NS ns2.etf.rs.
etf.rs. 5 NS ns.etf.rs.
etf.rs. 5 NS ns.rcub.bg.ac.rs.
etf.rs. 5 NS ns1.etf.rs.
Authority
The response has no authority records.
Additional
ns.rcub.bg.ac.rs. 2 A 147.91.1.5
ns1.etf.rs. 5 A 147.91.8.6
ns2.etf.rs. 5 A 147.91.8.62
ns.rcub.bg.ac.rs. 2 AAAA 2001:4170:0:1:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0