DNS Slowness (Re: Diagnosing connection issue)

[Derek Martin]

On Tue, Feb 15, 2011 at 06:06:32PM -0500, edwardp-jjFNsPSvq+iXDw4h08c5KA at public.gmane.org wrote:
> This is one system with both OS' installed, it came pre-installed with=20
> 32-bit Windows Vista, then I installed 64-bit Ubuntu afterwards.
>=20
> I bought a new NIC (Netgear) this afternoon and it came right up in=20
> Linux.  The card (per the box) will work with Windows from 98SE all the=
=20
> way to and including Vista.
>=20
> As of now, the system has been up for 15 minutes and I have yet to see=20
> any connection loss.  :-)

Tangentially related:  Not too long a while back, I upgraded my old
desktop and installed windows 7 on it, and installed Fedora 14 running
in VirtualBox (I mostly play games these days on my home desktop, so
this arrangement makes the most sense).  Since then I've had a lot of
issues with DNS lookups timing out.  Like many people here, I run my
own DNS server on my network.  I found that as soon as a host expired
=66rom the cache, it was taking several seconds for DNS lookups to
complete, and often they would time out completely.

I did some investigation from Linux inside the VM, and noticed a few
things:

 - many/most of the sites I was visiting had TTLs of less than 5
   minutes (many only one or two minutes).  This led to the problem
   being a frequent one.
  =20
 - traceroute / MTR to various name servers involved looked fine;
   there was no significant packet loss or latency.

 - running dig against the site's DNS servers revealed no problems;
   the servers were invariably very responsive

Finally, I ran dig without specifying a name server, and found the
problem (a case of premature optimization, to be sure -- if I'd tested
the simple/normal case first, I'd have found the problem right away).
The machine was requesting both A and AAAA records; the A records
resolved essentially instantaneously, but the IPv6 records were taking
over 2s to resolve.  Solution: disable IPv6 in Windows.  DNS
resolution returned to normal.

I still do get occasional timeouts when hosts' entries in my name
server's cache expire, but these are more rare; and in those cases,
clearly there is some delay in resolution that's outside my home
network.  I can live with that.

--=20
Derek D. Martin    http://www.pizzashack.org/   GPG Key ID: 0xDFBEAD02
-=3D-=3D-=3D-=3D-
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