Bind9 не отвечает на внешние запросы DNS, пока порт 53 открыт

Я установил dns-сервер bind9 на virtualmin и создал зону dns для следующего домена со следующими серверами имен.

domain = thecrystalsms.com Сервер имен ns5.crystalhost.net ns6.crystalhost.net

И ns5, и ns6 разрешаются правильно до 182.93.78.27

И на моем сервере, когда я запускаю следующую команду

host -t ns thecrystalsms.com

это показывает

thecrystalsms.com name server ns6.crystalhost.net.
thecrystalsms.com name server ns5.crystalhost.net.

Но когда я запрашиваю dns для thecrystalsms.com извне, он не работает.

Ниже приведены мои записи в зоне DNS.

$ttl 38400
@   IN  SOA ns5.crystalhost.net. root.ns5.crystalhost.net. (
            1559739241
            10800
            3600
            604800
            38400 )
@   IN  NS  ns5.crystalhost.net.
@   IN  NS  ns6.crystalhost.net.
thecrystalsms.com.  IN  A   182.93.78.27
www.thecrystalsms.com.  IN  A   182.93.78.27
ftp.thecrystalsms.com.  IN  A   182.93.78.27
m.thecrystalsms.com.    IN  A   182.93.78.27
localhost.thecrystalsms.com.    IN  A   127.0.0.1
webmail.thecrystalsms.com.  IN  A   182.93.78.27
admin.thecrystalsms.com.    IN  A   182.93.78.27
mail.thecrystalsms.com. IN  A   182.93.78.27
thecrystalsms.com.  IN  MX  5 mail.thecrystalsms.com.
thecrystalsms.com.  IN  TXT "v=spf1 a mx a:thecrystalsms.com ip4:182.93.78.27 ip4:182.93.78.27 ip6:fe80::250:56ff:fe8c:d4ad -all"
_dmarc.thecrystalsms.com.   IN  TXT "v=DMARC1; pct=100; ruf=mailto:postmaster@thecrystalsms.com; rua=mailto:postmaster@thecrystalsms.com; p=reject"

Не могу понять, в чем проблема.

Перезагружал сервер тоже несколько раз.

Выход netstat -an|grep :53 дает

tcp        0      0 182.93.78.27:53         0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN
tcp6       0      0 :::53                   :::*                    LISTEN
udp        0      0 182.93.78.27:53         0.0.0.0:*
udp        0      0 127.0.0.1:53            0.0.0.0:*
udp6       0      0 :::53                   :::*

Результаты сканера портов

Not shown: 840 closed ports, 142 filtered ports
PORT      STATE SERVICE
21/tcp    open  ftp
22/tcp    open  ssh
25/tcp    open  smtp
53/tcp    open  domain
80/tcp    open  http
110/tcp   open  pop3
143/tcp   open  imap
443/tcp   open  https
465/tcp   open  smtps
587/tcp   open  submission
993/tcp   open  imaps
995/tcp   open  pop3s
2222/tcp  open  unknown
8000/tcp  open  http-alt
8001/tcp  open  unknown
8002/tcp  open  teradataordbms
10000/tcp open  snet-sensor-mgmt
20000/tcp open  unknown
Nmap done: 1 IP address (1 host up) scanned in 20.40 seconds

Я запустил инструмент zenmap и нашел следующий результат

nmap -sS -sU -T4 -A -v thecrystalsms.com
Starting Nmap 7.70 ( https://nmap.org ) at 2019-06-11 12:02 Nepal Standard Time

NSE: Loaded 148 scripts for scanning.

NSE: Script Pre-scanning.

Initiating NSE at 12:02

Completed NSE at 12:02, 0.00s elapsed

Initiating NSE at 12:02

Completed NSE at 12:02, 0.00s elapsed

Initiating Ping Scan at 12:02

Scanning thecrystalsms.com (182.93.78.27) [4 ports]

Completed Ping Scan at 12:02, 0.75s elapsed (1 total hosts)

Initiating Parallel DNS resolution of 1 host. at 12:02

Completed Parallel DNS resolution of 1 host. at 12:02, 0.01s elapsed

Initiating SYN Stealth Scan at 12:02

Scanning thecrystalsms.com (182.93.78.27) [1000 ports]

Discovered open port 443/tcp on 182.93.78.27

Discovered open port 995/tcp on 182.93.78.27

Discovered open port 80/tcp on 182.93.78.27

Discovered open port 21/tcp on 182.93.78.27

Discovered open port 53/tcp on 182.93.78.27

Discovered open port 143/tcp on 182.93.78.27

Discovered open port 993/tcp on 182.93.78.27

Discovered open port 587/tcp on 182.93.78.27

Discovered open port 22/tcp on 182.93.78.27

Discovered open port 110/tcp on 182.93.78.27

Discovered open port 10000/tcp on 182.93.78.27

Discovered open port 20000/tcp on 182.93.78.27

Discovered open port 8007/tcp on 182.93.78.27

Discovered open port 465/tcp on 182.93.78.27

Discovered open port 8002/tcp on 182.93.78.27

Discovered open port 8000/tcp on 182.93.78.27

Discovered open port 2222/tcp on 182.93.78.27

Completed SYN Stealth Scan at 12:02, 2.04s elapsed (1000 total ports)

Initiating UDP Scan at 12:02

Scanning thecrystalsms.com (182.93.78.27) [1000 ports]

Discovered open port 53/udp on 182.93.78.27

Increasing send delay for 182.93.78.27 from 0 to 50 due to max_successful_tryno increase to 5

Increasing send delay for 182.93.78.27 from 50 to 100 due to max_successful_tryno increase to 6

Warning: 182.93.78.27 giving up on port because retransmission cap hit (6).

UDP Scan Timing: About 31.30% done; ETC: 12:04 (0:01:08 remaining)

UDP Scan Timing: About 37.70% done; ETC: 12:05 (0:01:41 remaining)

Кажется, у моего сервера тоже открыт порт udp. Кроме того, когда я ищу ns через командную строку, он показывает следующий результат

nslookup thecrystalsms.com 182.93.78.27

Server:  UnKnown
Address:  182.93.78.27

Name:    thecrystalsms.com
Address:  182.93.78.27

Но когда я запускаю nslookup thecrystalsms.com 8.8.8.8, я получаю

DNS request timed out.
    timeout was 2 seconds.
Server:  UnKnown
Address:  192.93.78.27

DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to UnKnown timed-out

Не удалось выяснить проблему с bind9 или где-то еще.