Apex domains cannot have CNAME records.
Question and response
Question
dig rpc-extra.net. NS
Answer
rpc-extra.net. 4 NS server.rpc-extra.net.
Authority
The response has no authority records.
Additional
server.rpc-extra.net. 4 A 69.48.183.183
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0