[dns-operations] difference between several NS with several glue

[han feng]

>> example.com.	86400	NS 	a.example.net.
>> example.com.	86400 	NS 	b.example.net.
>> example.com. 	86400 	NS 	c.example.net.
>> 
>> a.example.net. 3600 A  1.1.1.1
>> b.example.net. 3600 A  2.2.2.2
>> c.example.net. 3600 A  3.3.3.3
>> 
>> and
>> 
>> example.com.	86400	NS 	a.example.net.
>> 
>> a.example.net. 3600 A  1.1.1.1
>> a.example.net. 3600 A  2.2.2.2
>> a.example.net. 3600 A  3.3.3.3
>> 
>> 
>> Since all the name servers are in example.net zone, these two configure both specify three name servers
>> for example.com zone and will be affected if example.net name server failed.  And the latter configuration 
>> will generate smaller package. But the first configuration seems more common, so what’s the benefit?
> 
> From what I understand, when 1.1.1.1 fails to respond, all of
> a.example.net will be considered bad, so 2.2.2.2 and 3.3.3.3 won't be
> queried at all, and a resolver will return a SERVFAIL.

AFAIK, Recursive name server has name server cache which store several entries, each of them includes
zone name
ip address
other information  including RTT, EDNS version, etc
So after the recursive name server get the NS records and related glue, both of the configuration should generate
similar name server entities, and only if all of them failed to answer the query, the recursive server will return SERVFAIL
to resolver. 

> 
> -- 
> Dave Warren
> http://www.hireahit.com/
> http://ca.linkedin.com/in/davejwarren
> 
> 
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