DNS Server on Plesk is not responding to the calls because or port knocking: connection timed out; no servers could be reached

Symptoms

• Websites on the server are unavailable.

• DNS Server is not responding to the calls. They are failing due to timeout:

  # dig example.com @203.0.113.2
  ; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.62.rc1.el6_9.5 <<>> example.com @203.0.113.2
  ;; global options: +cmd
  ;; connection timed out; no servers could be reached

• If force TCP on the dig before asking, it is working:

  # dig example.com @203.0.113.2 +vc
  ; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.62.rc1.el6_9.5 <<>> example.com @203.0.113.2 +vc
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 22918
  ;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2
  ;; WARNING: recursion requested but not available

  ;; QUESTION SECTION:
  ;example.com. IN A

  ;; ANSWER SECTION:
  example.com. 86400 IN A 203.0.113.2

Cause

UDP traffic is not allowed on port 53.

Resolution

Check server firewall to ensure that incoming TCP and UDP traffic is allowed on port 53. If the issue persists, contact network administrator to allow TCP and UDP on intermediate firewalls as well.