how to know what DNS server is being used

[Matthew Seaman]

Robert Huff wrote:
> Chad Leigh -- Shire.Net LLC writes:
> 
>>  > On my registrars site I have two DNS servers listing. How would
>>  > I know that 1) both are working. 2) which one is being used.
>>  
>>  For #2, do you mean by the world at large?  Which one is being
>>  used when people look up your domain and hosts in your domain?
>>  Both of them should get used.  I don't know what algorithm is
>>  used but both will be used by people.
> 
> 	As I understand it, when presented with a list of N>1 possible
> nameservers standard resolver libraries will pick one at random and
> continue to use it until something changes (nameserver goes
> off-line, local cache is flushed, etc.).

On recent FreeBSD, the resolver actually iterates through the listed
nameserver lines in order, sending the query out to each in turn until
it gets a response.  It used to be that the resolver would wait for the
full 30s DNS timeout before trying the next server (hence the cry dreaded
by sysadmins everywhere that "the Internet is slow today"), but nowadays
if the resolver hasn't got an immediate answer it will initiate the second
and subsequent queries after a wait of some number of milliseconds and
then wait for a response from all of the queried servers.  Means that if
your first listed DNS server is down, users don't notice the delay before
the second server is queried.

	Cheers,

	Matthew

-- 
Dr Matthew J Seaman MA, D.Phil.                       7 Priory Courtyard
                                                      Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey         Ramsgate
                                                      Kent, CT11 9PW

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