Recently, I got a power Failure in my Data Center and face an Active Directory/DNS Crash as well. I configured Active Directory and DNS as well to support my users and organization and AD start replication but after a day I notice that on nslookup there was a message Default Server: Unknown. Ooops! I was really, really worried that what happens to it. Obviously, I start troubleshooting and the solution was much unexpected to me.
You need to login your DNS Server and if you
haven’t setup your reverse lookup zone,
please do create it. If it’s already done than you need to create a PTR Record and point to 192.168.10.10
Server (in my example). After creating PTR Record or configuring Reverse lookup
zone, you will be able to see Server Name, as image below.
some cases, your DNS may behave differently like it shows exact Default
Server Name but on giving any website name (google.com), it show error
message like "request timed out. timeout was 2 seconds" etc. in this case you have to check your Firewall on DNS Server or any Firewall between your computer and DNS Server and you have to allow your DNS Server.
may notice that even disabling Firewall on DNS Server, Local Computer
or even allowing DNS Server in middle Firewall doesn't help then you
must check that your DNS Server have properly configured for Live DNS
Server's. Please double check Live DNS IP Address in DNS Forwarders and
hope these tips will help you, please show yourself in comments to improve the post.