It’s good to check with your Internet Service Provider (ISP) every once and a while to make sure there haven’t been any changes to their DNS servers.  I was recently working with a customer who was having a problem viewing a certain webpage.  Even though the webpage was valid, when the customer would try to access it, an “under construction” page would appear.  Troubleshooting revealed that when their DNS servers had to do a recursive query, they were getting invalid IP addresses from their ISP’s DNS servers.

I called the ISP’s tech support and the case was quickly escalated to tier 2.  When I received a call back, the engineer explained that the DNS servers that were entered for resolving recursive queries did not actually respond to queries, but were authoritative only. I was given a different pair of IP DNS server IP addresses to use for queries.  Once the change was made, the website displayed correctly.