1 have 3 dns servers with names:
ns1.example1.com ns2.example1.com ns3.example1.com
ns1.example1.com is cPanel based server, while ns2 and ns3 are cPanel DNSOnly machines.
I also have a domain, example2.com . That domain name servers are:
ns1.example1.com ns2.example1.com ns3.example1.com
All subdomains of example2.com (*.example2.com) are also on these name servers.
Some time ago ns1 server went down, and after few minutes customers started complaining, that example2.com domain is unreachable. After some time ns1 server was back online, and complains stopped.
Now I'm trying to test how could that happen, so I stop "named" service on ns1. On my local machine, and on some of my servers, domain example2.com and all of subdomains still resolves correctly (maybe because of caching?). But when I use some dns checker like dnschecker.org , or any other, it says that some dns servers are not able to get example2.com domain IP (or any other subdomain).
The question is why turning off one of name servers is causing domain example2.com to be unreachable? Isn't it why I have 3 name servers to provide IP address when ns1 (or any other) is down?
When I'm using dig example2.com it still gets back with correct IP result. So is my domain reachable for everyone? Or not?
Answer
It seems that tld nameservers didn't have something that is called GLUE records. After setting it up in domain provider panel (and having to contact them to make it work) it seems it's working fine. Big thanks to Vadim as my answer was found when I noticed that nslookup can not resolve any ns*.example.com , that lead me straight to the point, and I got that idea from you!
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