My server name is server1.mycorp.com with ip 22.214.171.124. This is my ISPConfig server. There is another server, server.mycorp.com with ip 126.96.36.199. This is my cpanel/whm server on another host. the domain was registered at godaddy.com On godaddy I had set customer DNS for mycorp.com to ns1 and ns2, on GoDaddy I set glue record ns1 and ns2 point to 188.8.131.52. (which is my cpanel/whm server) on godaddy I also set glue record ns5 and ns6 to 184.108.40.206 (which is my ISPConfig server) On godaddy I set my other domain www.other.com's dns to ns5.mycorp.com and ns6.mycorp.com the above config was working fine. 1 week ago, I started to move my domains to namecheap. so mycorp.com is managed at namecheap now. I set custom DNS for mycorp.com to ns1.mycorp.com and ns2.mycorp.com (I forgot whether I manually set it or got transferred from godaddy) but I also manually set private dns (this is how namecheap calls glue record) to have ns1 and ns2.mycorp.com point to 220.127.116.11 and ns5/ns6.mycorp.com point to 18.104.22.168 then my domain www.other.com's dns is still ns5/ns6.mycorp.com It has been working fine until a couple hours ago, I woke up for something else and suddenly found out that www.other.com is not working, because dns error. I checked the ISPConfig server( which is also where I host other.com as well), it is up and running for 9 days, all services are up. I rebooted the server, still the same. So I contacted namecheap, and I was told the settings on their end is fine. But they told me ns5 and ns6.mycorp.com do not show A record IP. But I do have ns5 and ns6 pointed to 22.214.171.124 in my ISPConfig. So I don't know if customer DNS settings of mycorp.com plays a role or not, so I changed the custom dns settings for mycorp.com from ns1/ns2.mycorp.com to ns5/ns6.mycorp.com, but it didn't help. Now the strange thing is, I have other two servers, one is a cheap vps from a different company at a different city, another one is with in the same host company they all can ping and resolve www.other.com and ns5/ns6.mycorp.com But my windows machine (and namecheap CSRs machine) cannot. What is the problem? why it works on some of the machines but not others? Do you gues spot any issues in my configuration? Thanks!!!