Sorry for the short answer
case 1 and 2.
“Anycast resolvers” at ISP.
Client issues a query,
no answer is returned within X ms,
it repeats query hitting a DIFFERENT resolver possibly in different location,
now it gets the answer to first query
then it gets the answer to the second query.
Case 4: Real bad cache refresh code
Case 3: no clue
Olafur
> On Feb 3, 2015, at 4:09 PM, Matt Calder <[email protected]> wrote:
>
> Apologies, I am very new to DNS administration. My issue is that I have HTTP
> resource hostnames which are distinct across webpage accesses but are being
> resolved multiple times, often from LDNS resolvers in different networks. I
> am trying to understand why this is happening.
>
> I have a webpage that contains a resource with a unique hostname for each
> page load that used for some Javascript performance profiling. The hostname
> is made unique with a standard GUID. During the measurement, the browser
> should resolve the resource twice; once to induce DNS resolution and the
> second time to measure performance, assuming the DNS resolution is cached and
> doesn’t contribute to the end-to-end timing from our measurement.
>
> In my authoritative DNS logs, I see that there are many duplicate requests
> coming in for the same unique hostname. The A record TTL is short, only a few
> minutes and duplicate requests usually happen within seconds of each other.
> Sometime there are just a few extra, sometimes 10-15. Ideally, I would see
> only a single request per GUID, but at the moment only 51% of GUIDs see a
> single request from a single LDNS server. There are a few different patterns
> I’ve narrowed down and now I’m trying to understand what the possible causes
> of these duplicate requests are. In some examples, I use specific ISP names
> but these patterns are pretty common.
>
> Case 1.
>
> LDNS servers resolving the same GUID hostname are in different networks. In
> one case, 3/4 of the duplicates DNS requests come from an AT&T LDNS, the
> others were from COX.
>
> Case 2.
>
> In all duplicate requests, all LDNS IPs are distinct and belong to Comcast
> but in different Comcast ASNs.
>
> Case 3.
>
> Many duplicate requests, all LDNS IPs are the same.
>
> Case 4.
>
> Duplicate request once an hour through the same LDNS. This continues for
> days.
>
>
>
> Hypothesis I’ve imagined so far.
>
> DNS response packets are lost on their way back to the LDNS or to client so
> are re-requested
> An LDNS may resolve on their own while also forwarding requests to load
> balanced counterparts or upstream/downstream resolvers to sync caches.
> Browser/OS DNS cache is full/broken/non-existant so the measurement URLs are
> re-queried even after the warmup URLs.
> Case 4 just seems like a straight up misbehaving resolver.
> If it helps, I am running BIND 9.9.6.
>
> Appreciate any help! Thanks.
>
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