ns6.o7.com has no CNAME records.
Question and response
Question
dig ns6.o7.com. NS
Answer
The response has no answer records.
Authority
o7.com. 300 SOA ns5.o7.com. support.cloud.rt.ru. 2016051535 600 10800 2419200 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0