Glossary
What is NXDOMAIN?
The DNS server reports that the queried domain does not exist. This differs from an empty response for a record type.
When you receive NXDOMAIN, check the spelling, whether you queried the correct subdomain and whether the name has been published in the DNS zone. You can also review the domain registration status. After a recent change, negative answer caching may cause resolvers to disagree for a while.
Example
In an illustrative scenario, typing ww.example.com instead of www.example.com can produce NXDOMAIN if that name has not been published.
Common issue
NXDOMAIN differs from NODATA, where only the requested record type is missing. A timeout or SERVFAIL also does not prove that the name does not exist.