Checking Name Service

Name server problems are indicated when the “unknown host” error message is returned by the user’s application. Name server problems can usually be diagnosed with nslookup. Three features of nslookup are particularly important for troubleshooting remote name server problems. These features have the ability to:

  • Locate the authoritative servers for the remote domain using the NS query.

  • Obtain all records about the remote host using the ANY query.

  • Browse all entries in the remote zone using nslookup’s ls and view commands.

When troubleshooting a remote server problem, directly query the authoritative servers returned by the NS query. Don’t rely on information returned by non-authoritative servers. If the problems that have been reported are intermittent, query all of the authoritative servers in turn and compare their answers. Intermittent name server problems are sometimes caused by the remote servers returning different answers to the same query.

The ANY query returns all records about a host, thus giving the broadest range of troubleshooting information. Simply knowing what information is (and isn’t) available can solve a lot of problems. For example, if the query returns an MX record but no A record, it is easy to understand why the user couldn’t telnet to that host! Many hosts are accessible to mail that are not accessible by other network services. In this case, the user is confused and is trying to use the remote host in an inappropriate manner.

If you are unable ...

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