Apex domains cannot have CNAME records.
Question and response
Question
dig cs.tau.ac.il. NS
Answer
cs.tau.ac.il. 5 NS zeus-1.cs.tau.ac.il.
cs.tau.ac.il. 5 NS aristo.tau.ac.il.
cs.tau.ac.il. 5 NS sdns.tau.ac.il.
cs.tau.ac.il. 5 NS zeus-2.cs.tau.ac.il.
Authority
The response has no authority records.
Additional
sdns.tau.ac.il. 5 A 132.66.16.2
aristo.tau.ac.il. 5 A 132.66.32.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0