None of them are answering on PING or any other things like that. If you want to check them do it in one of this ways:
1. Go to http://www.kloth.net/services/dig.php and check them.
2. If you have a Windows System:
- open a CMD command line
- type NSLOOKUP
- type SERVER x.x.x.x (the IP Address of the Server)
- type www.google.com (you will see the IP Address of Google)
3. Go to NIRSOFT Website and download "QUICKDNS CHANGER", then add all DNS Server to this tool and use it on Windows to find the fastest. DNS Benchmark is also not 100% working.
Any other things which even just look likes an attack against the Server will be classified as an attack and rejected. If it's to much the client IP Address will be blacklisted.
Sorry for this but until I have a damn good provider who provides a real security for DNS it is a need to have this kind of security. DNS "ANY" requests are also almost disabled and to much of them will also cause an Blacklist of the client IP Address for a damn long time.
Remember to the 21st October 2016 where the half global Internet was down. It was an attack against one single wrong configured DNS Server and I don't want to be responsible if this would happens again because of my DNS Servers. The problem is not DNS, the problem is that some people think that this is so funny.
I don't have also a monitoring running on all servers because this will cost additional money but once a day I do a manual check for all of them.
They are working and for security and performance reasons it is required to reboot them once a day at 3:30 AM (local time).
Hope this help.