Date: Sat, 28 Oct 2006 15:08:09 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Robert Huff <roberthuff@rcn.com> Cc: FreeBSD Questions <questions@freebsd.org> Subject: Re: how to know what DNS server is being used Message-ID: <45436449.4020509@infracaninophile.co.uk> In-Reply-To: <17731.22324.706536.159980@jerusalem.litteratus.org> References: <20061028025626.GA39172@skytracker.ca> <2B8A4ACE-ACE3-44D0-B40B-26D96D43A4E8@shire.net> <17731.22324.706536.159980@jerusalem.litteratus.org>
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Robert Huff wrote:
> Chad Leigh -- Shire.Net LLC writes:
>=20
>> > 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.
>> =20
>> 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.
>=20
> 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 secon=
d
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
--=20
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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