Os subdomínios recebem o NXDOMAIN no Ubuntu Desktop 16.04

Question

Problema

Muitos subdomínios (ou todos?) como files.slack.com e guide.elm-lang.org get DNS_PROBE_FINISHED_NXDOMAIN no Chrome. Usando nslookup retorna:

Server:     127.0.1.1
Address:    127.0.1.1#53

** server can't find files.slack.com: NXDOMAIN

Pergunta (s)

1. Claro, como consertar o problema?
2. Eu pesquisei no Google e encontrei algumas soluções alternativas para os macacos que não funcionaram. Em geral, existe uma maneira sistemática de depurar o DNS?

Notas

• SO : Ubuntu Desktop 16.04
• Não tenho certeza se o problema está relacionado apenas a subdomínios
• Não me lembro de alterar as configurações padrão.

Mais detalhes :

dig slack.com output (funciona como esperado, eu acho):

; <<>> DiG 9.10.3-P4-Ubuntu <<>> slack.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39980
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;slack.com.         IN  A

;; ANSWER SECTION:
slack.com.      14  IN  A   143.204.93.160

;; Query time: 29 msec
;; SERVER: 127.0.1.1#53(127.0.1.1)
;; WHEN: Wed Oct 10 00:05:49 EET 2018
;; MSG SIZE  rcvd: 54

dig files.slack.com output (não funciona :():

; <<>> DiG 9.10.3-P4-Ubuntu <<>> files.slack.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 9614
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 2

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;files.slack.com.       IN  A

;; ADDITIONAL SECTION:
tunneling.      900 IN  SOA <REDACTED. Seems to DNS server of my ISP>. please_set_email.absolutely.nowhere. 3702 10800 3600 2419200 900

;; Query time: 60 msec
;; SERVER: 127.0.1.1#53(127.0.1.1)
;; WHEN: Wed Oct 10 00:07:29 EET 2018
;; MSG SIZE  rcvd: 144

No entanto, dig +trace files.slack.com funciona!:

// many many stuff then:
;; Received 698 bytes from 192.43.172.30#53(i.gtld-servers.net) in 82 ms

files.slack.com.    60  IN  A   52.84.158.79
slack.com.      172800  IN  NS  ns-1493.awsdns-58.org.
slack.com.      172800  IN  NS  ns-166.awsdns-20.com.
slack.com.      172800  IN  NS  ns-1901.awsdns-45.co.uk.
slack.com.      172800  IN  NS  ns-606.awsdns-11.net.
;; Received 197 bytes from 205.251.199.109#53(ns-1901.awsdns-45.co.uk) in 104 ms