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I have a COM domain with several subdomains. These subdomains have CNAME records pointing to ghs.google.com (they point to different Google App Engine apps). These have been in use with no problem for a few months.

Recently, however, I've started receiving reports from some users that they are no longer able to connect to my apps. I've discovered that nslookup subdomain.domain.com is failing for these users (for all of my subdomains). Many of these users are from a Singapore-based ISP called StarHub, though others are scattered across other ISPs in places as diverse as Argentina, India, and New Zealand. I haven't received any complaints from US-based users (the biggest segment of the user base). Most users seem unaffected by the problem (based on traffic levels).

Any ideas of what the problem is, or how I can fix it? I've tried contacting some of the ISPs but they've all been a bit unhelpful (insisting that the problem is on my end).

It is worth noting that the users experiencing problems are still able to connect directly via an IP address, or via the appname.appspot.com subdomain (provided by Google App Engine) - just not my subdomains anymore.

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If you'd told us the domain names, people could have looked up the information and checked it out for you. This is not secret information. You are already publishing it to the world. –  JdeBP Jun 1 '11 at 9:14
    
The domain is pocketgems.com; all of the subdomains seem equally affected - e.g., www.pocketgems.com. –  David Underhill Jun 1 '11 at 22:10
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2 Answers

Try googling for "dns health check" or "dns report", there are a number of automated tools that can traverse your DNS infrastructure and look for common problems. DNSStuff.com comes to mind but there are many others. Many domains are currently right now slightly out of spec or best practice, and for the most part this is not a problem but it may be affecting very strict resolvers.

See also this website: http://www.whatsmydns.net/ It can check your dns resolution from all over the world.

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+1 Thanks. whatmydns.net and the top hits for "dns health check" seem to report that everything is okay. I haven't tried DNSStuff.com yet since it requires sign-up, but it sounds like I should give it a try later. –  David Underhill Jun 1 '11 at 21:41
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Inspection of what you are publishing about that domain name reveals nothing that would cause query resolution to consistently fail. So you must go back to the StarHub customers (it being impossible to determine this externally) and get them to perform further diagnostic tests such as traceroute from within StarHub to 216.69.185.34 and 208.109.255.34 (which are where your GoDaddy content DNS servers currently are). One simple cause of such problems is an excessive hop count.

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+1 Interesting idea. We'll get back in touch with some of the customers experiencing the issue and see if the hop count looks excessive. Its strange that this only surfaced recently though. What would consider an "excessive" hop count? Presumably google might reveal some issues to deal with this if it does seem to be a problem. –  David Underhill Jun 4 '11 at 4:06
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