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DNS - NSLOOKUP what is the meaning of the non-authoritative answer?

ayi

For some domains <code>nslookup</code> gives me a <code>Non-authoritative answer</code> section. What does this mean?
<pre><code class="lang-auto">Got answer:
HEADER:
opcode = QUERY, id = 3, rcode = NXDOMAIN
header flags: response, want recursion, recursion avail.
questions = 1, answers = 0, authority records = 1, additional =

QUESTIONS:
    www.example.com.SME, type = AAAA, class = IN
AUTHORITY RECORDS:
->  (root)
    ttl = 1787 (29 mins 47 secs)
    primary name server = a.root-servers.net
    responsible mail addr = nstld.verisign-grs.com

Non-authoritative answer:

------------
Name: example.com
Address: 93.184.216.34
Aliases: www.example.com

</code></pre>

ayi_2

Basically, it’s what the name says it is. An authoritative answer comes from a nameserver that is considered authoritative for the domain which it’s returning a record for (one of the nameservers in the list for the domain you did a lookup on), and a non-authoritative answer comes from anywhere else (a nameserver not in the list for the domain you did a lookup on).
It’s basically a distinction between a nameserver that’s an official nameserver for the domain you’re querying, and a nameserver that isn’t. Nameservers that aren’t authoritative are getting their answers second (or third or fourth…) hand - just relaying the information along from somewhere else.
So, for example, If I did an nslookup of <code>maps.google.com</code> right now, I would get a response from one of my configured nameservers. (Either from my ISP, or my domain.) It would come back as non-authoritative because neither my ISP’s nameservers, nor my own are in the list of nameservers for <code>google.com</code>. They aren’t Google’s nameservers, so they’re not the authoritative source that creates the NS records.
The list of authoritative nameservers for Google is below (from <a href="http://whois.internic.net">whois.internic.net</a>).
<blockquote></blockquote>
Domain Name: <a href="http://GOOGLE.COM">GOOGLE.COM</a>
Registrar: MARKMONITOR INC.
Whois Server: <a href="http://whois.markmonitor.com">whois.markmonitor.com</a>
Name Server: <a href="http://NS1.GOOGLE.COM">NS1.GOOGLE.COM</a>
Name Server: <a href="http://NS2.GOOGLE.COM">NS2.GOOGLE.COM</a>
Name Server: <a href="http://NS3.GOOGLE.COM">NS3.GOOGLE.COM</a>
Name Server: <a href="http://NS4.GOOGLE.COM">NS4.GOOGLE.COM</a>
Updated Date: 20-jul-2011
Creation Date: 15-sep-1997
Expiration Date: 14-sep-2020
If I changed my configured DNS server to one of the ones in that list, and then did an <code>nslookup</code> against <code>maps.google.com</code>, I’d get an authoritative answer back. Those servers are the authority, (or source) for what are valid names in Google’s domains, and what aren’t. All other nameservers, non-authoritative nameservers, get their NS records from the authoritative servers somewhere down the line.