Since DNS is a critical service, it helps to understand the basics of troubleshooting it:
- Is the server, computer, or device set up to use the proper DNS server? Many problems are related to the wrong information being delivered via an incorrectly defined DNS server.
- Are DNS services available on the defined DNS server? Check that the DNS service is running on the defined server. In Terminal, run the command telnet 53 (port 53 is the port used by DNS) and see whether a connection is made. After a successful connection, press Control-] and then type quit to close the connection).
- Are the proper DNS records available from the DNS server? Check all the pertinent records, forward and reverse, using Network Utility or command-line tools. Make sure that both the forward and reverse records are available and they match.
- Your computer might be caching old DNS information and needs to be flushed. To flush the DNS cache, run the command sudo discoveryutil udnsflushcache in the Terminal app.