_nets.opendns.com has no CNAME records.
Question and response
Question
dig _nets.opendns.com. NS
Answer
The response has no answer records.
Authority
opendns.com. 2507 SOA auth1.opendns.com. noc.opendns.com. 1771788551 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0