_netblocks3.mainloop.net has no CNAME records.
Question and response
Question
dig _netblocks3.mainloop.net. NS
Answer
The response has no answer records.
Authority
mainloop.net. 418 SOA ns1.mainloop.net. hostmaster.mainloop.net. 3959 600 100 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0