A mystery which can be easily solved. First we look at the opera-mini.net domain in the WHOIS database:
Domain Name: OPERA-MINI.NET
Registrar: GODADDY.COM, LLC
Sponsoring Registrar IANA ID: 146
Whois Server: whois.godaddy.com
Referral URL: http://www.godaddy.com
Name Server: NIC1.OPERA.COM
Name Server: NIC2.OPERA.NO
Name Server: NIC3.OPERA.COM
Name Server: NIC4.OPERA.COM
So then I'll simply ask nic1.opera.com for this name. And then we get to see something odd happening:
macron:/home/peter $ dig @nic1.opera.com adblockers.opera-mini.net
; <<>> DiG 9.9.9-P6 <<>> @nic1.opera.com adblockers.opera-mini.net
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 42687
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;adblockers.opera-mini.net. IN A
;; AUTHORITY SECTION:
opera-mini.net. 86400 IN SOA nic1.opera.com. hostmaster.opera.com. 2017021701 7200 900 604800 86400
;; Query time: 14 msec
;; SERVER: 185.26.183.160#53(185.26.183.160)
;; WHEN: Tue Feb 21 01:24:08 CET 2017
;; MSG SIZE rcvd: 115
As you can see it didn't give me an answer (which is odd because this is supposed to be the authoritive DNS server) because the server thinks I'm trying to do a recursive lookup. In other words: trying to look up a domain which isn't hosted by the server, yet it should be.
When I try to do a regular lookup (using my local DNS server) then I get something similar:
macron:/home/peter $ dig adblockers.opera-mini.net
; <<>> DiG 9.9.9-P6 <<>> adblockers.opera-mini.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 26962
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;adblockers.opera-mini.net. IN A
;; AUTHORITY SECTION:
opera-mini.net. 531 IN SOA nic1.opera.com. hostmaster.opera.com. 2017021701 7200 900 604800 86400
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Feb 21 01:28:37 CET 2017
;; MSG SIZE rcvd: 115
So bottom line: this name does not exist. But it also seems the DNS servers aren't properly configured either.