How to tell if a DNS record is a top-level zone

By querying for the SOA of a DNS record, we can determine if the answer is the top level-zone or not.

Using the host command, set the query option to return the SOA record. If an answer is returned, the record is part of the top-level zone:

[email protected]:~ $ host -t soa wustl.edu
wustl.edu has SOA record ns00.ip.wustl.edu. noc.wustl.edu. 2012476365 10800 3600 2419200 900

Sub-domains can be top level zones:

[email protected]:~ $ host -t soa nts.wustl.edu
nts.wustl.edu has SOA record ns00.ip.wustl.edu. noc.wustl.edu. 2776394 10800 3600 2419200 900

If there is no SOA record, the DNS entry is a standard record and not a top-level domain:

[email protected]:~ $ host -t soa wugate.wustl.edu
wugate.wustl.edu has no SOA record

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