Stealth NS records are sent when the authoritative zone file has been delegated to a different nameserver.
For example, at the registrar level, I delegate to use the following nameservers for my domain, ‘massivedns.com’:
However, when a user queries the nameservers, they inform the user that the zone has been delegated elsewhere under the following nameservers:
The user then queries the delegated nameservers for the appropriate records and the lookup is completed.
Stealth NS records can also indicate a misconfiguration or discrepancy.
For example, Stealth NS records can be sent if only 2/3 of your ‘hypothetical’ nameservers are configured at the registry, and all 3/3 nameservers are assigned as authoritative within your zone file; this will cause 1/3 of your nameservers to be sent as a Stealth NS record.
Stealth NS records are achievable by using the NS Resource Record in your zone-file.