Even though many DNS servers use root hints for Internet name resolution, some use forwarders to link to an ISP's DNS server.
And if the ISP's DNS server goes down, Internet name resolution will cease to function as the entries in the resolver cache expire.
So it's critical to troubleshoot DNS problems as fast as possible. Here are10 of my favorite DNS troubleshooting techniques.
When DNS problems occur, one of the first things you should do is verify that the DNS server still has network connectivity.
If you can ping the host by IP address but not by name, check your DNS server to make sure that a Host (A) record exists for the host.
Without a Host (A) record, the DNS server will be unable to resolve the host's name.After you have determined that basic connectivity still exists, the next step is to determine the scope of the problem.Are Internet name resolutions failing or are local name resolutions failing too?About a week ago, someone called me because every time they would try to visit certain Web sites they were redirected to a malicious Web site instead.I initially suspected a DNS poisoning attack, but ruled out such an attack because only one computer was affected.DNS is one of the most essential services on any Windows network.Active Directory can't function without DNS, and it's is also used by any number of other network functions.The problem with this technique is that the DNS server has no way of knowing when one of the servers has failed.As a result, inbound traffic is still directed to all the servers in round robin fashion, even if one of those servers is offline.You should also try to ping the DNS server from a few random machines.Remember that ping will work only if you allow ICMP packets through the firewall on the machine you are pinging.