DNS Query Behavior with Global Forwarders Statement

[Merton Campbell Crockett]

On 12 Aug 2008, at 23:12:18, Mark Andrews wrote:

>
>>> Is this an artifact of the -P2 changes or was the use of RTT dropped
>>> for some other reason?
>>
>> You didn't say which version you were running.


Our NMS systems tend to be running BIND 9.3.5-P1.  The -P2 rollout is  
in progress.  There is a smattering of Engineering name servers in the  
environment that are running BIND 9.4.2-P2.


>> I'd be quite surprised if this were an artifact of the -P1 and -P2
>> changes.  I'd be less surprised if it were a bug introduced in 9.5.0.
>>
>> --
>> Evan Hunt -- evan_hunt at isc.org
>> Internet Systems Consortium, Inc.
>
> 	RTT estimates work well with authoritative servers.
>
> 	The best I expect to get out of rtt and forwarders is whether
> 	the forwarder is up or not.  If there was some way to
> 	accurately work out which answers come from the cache and
> 	which first required recursion then rtt times would home
> 	in the closest forwarder.
>
> 	It takes 110ms to get a answer from A via F1 and F2.
>
> 			R 10ms F1 100ms A
> 			R 100ms F2 10ms A
>
> 	Cache talking to caches don't always get enough second level
> 	cache hits to make a statistical difference to the rtt estimate.


If I understand you correctly, BIND's RTT is based on how long it  
takes to get an answer to the DNS query.  The fact that a NMS and a  
regional name server are in the same rack has little bearing on the  
selection process if the majority of the time is spent querying the  
Internet.

This leads to another question.  Is the selection of forwarder in  
anyway affected by what is being asked for in the query?  If one  
regional name server could resolve queries related to CNN.COM  
significantly faster than the others, would it be favored for  
resolving queries related to that zone?

Also, should a regional name server be inaccessible due to a network  
outage how much of a delay might there be in using it after service is  
restored?


Merton Campbell Crockett
m.c.crockett at roadrunner.com