Today one of my .es domains stopped to work. All websites on the same server are available, except one. I checked everything - my DNS servers are on, the problematic domain is active, everything is OK.
I tried some 20 proxy servers. Some 60% of them can open the website with problematic domain name but the others return errors something like "name lookup timed out"
I visited dnsreport.com website and when I entered my domain name, the test returned the following result:
Spain has a whois server. It is available via the web interface on www.nic.es [nic.es]. For some reason they don't provide a national whois service on port 4321 which can be queried from command line tools and remote processes though.
Some 60% of them can open the website with problematic domain name but the others return errors something like "name lookup timed out"
2 DNS servers are online, but the third: Error fetching SOA from ns3.mydnsserver.com So, if only 1 of 3 DNS servers have some problem, why the domain can not be resolved?
I think you are close to the solution. The IP addresses of your three DNS servers are assigned on a round robin base to the visitor's browsers. There is a chance of 1 in 3 that they will get the wrongly configured DNS server assigned. This matches with your test that about 60% of your proxy tests succeed but the other 40% fail. </added>