<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Sorry for the short answer<div class="">case 1 and 2. </div><div class=""><br class=""><div class="">“Anycast resolvers” at ISP.</div><div class="">Client issues a query, </div><div class=""> no answer is returned within X ms, </div><div class=""> it repeats query hitting a DIFFERENT resolver possibly in different location, </div><div class="">now it gets the answer to first query </div><div class="">then it gets the answer to the second query. </div><div class=""><br class=""></div><div class="">Case 4: Real bad cache refresh code </div><div class=""><br class=""></div><div class="">Case 3: no clue</div><div class=""><br class=""></div><div class="">Olafur</div><div class=""><br class=""></div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Feb 3, 2015, at 4:09 PM, Matt Calder <<a href="mailto:calderm@usc.edu" class="">calderm@usc.edu</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><p class="">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.</p><p class="">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.</p><p class="">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 <span class="">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.</span></p><p class=""><b class="">Case 1.</b></p><p class="">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.</p><p class=""><b class="">Case 2.</b> </p><p class="">In all duplicate requests, all LDNS IPs are distinct and belong to Comcast but in different Comcast ASNs. </p><p class=""><b class="">Case 3.</b> </p><p class="">Many duplicate requests, all LDNS IPs are the same. </p><p class=""><b class="">Case 4.</b></p><p class="">Duplicate request once an hour through the same LDNS. This continues for days. </p><p class=""><br class=""></p><p class=""><b class="">Hypothesis I’ve imagined so far.</b></p>
<ul class="">
<li class="">DNS response packets are lost on their way back to the LDNS or to client so are re-requested</li>
<li class="">An LDNS may resolve on their own while also forwarding requests to load balanced counterparts or upstream/downstream resolvers to sync caches. </li>
<li class="">Browser/OS DNS cache is full/broken/non-existant so the measurement URLs are re-queried even after the warmup URLs.</li>
<li class="">Case 4 just seems like a straight up misbehaving resolver.</li></ul><p class="">If it helps, I am running BIND 9.9.6. </p><p class="">Appreciate any help! Thanks.</p></div>
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