(This summarizes a problem mentioned in other discussions.)
A recent enhancement to FortiOS, used in the FortiGate firewall, did not handle NTP DNS changes correctly. When DNS mapping changed FortiGate firewalls sent 10 second duration NTP bursts at rates that could exceed 20,000 requests/second. NTP Pool servers were impacted due to the use of DNS load balancing. Our team monitored three NTP pool servers and detected over 150 FortiGate devices sending NTP bursts.
FortiGate support identified the problem: Bug ID 607015
FortiGate support informed us that FortiOS 6.2.4, released on May 12, 2020, fixed the problem. Operators of the FortiGate firewall must install that software, it is not an automatic upgrade. We don’t know when the updates will be complete. Questions should be directed to FortiGate support.
We recommended that FortiGate apply for a Vendor Zone .