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We've had some odd DNS problems over the past couple of days that I don't fully understand.

Some of our DNS names stopped resolving for some of our customers due to some 'unknown' server reconfiguration at our DNS provider.

The problem seemed to be intermittent i.e. stopped working and started working within a few minutes over a couple of days.

I'm no expert on DNS, but I'd have expected DNS caches to prevent this sort of thing from happening - when we need to change an IP address for a DNS record, it can take 24 hours to propogate, so how can our DNS provider be breaking name resolution intermittently for our customers so easily? Shouldn't the DNS caches kick in here?

We had a similar problem about a month ago when one of their nameservers 'decided to reload the DNS database from scratch' - this broke our name resolution too. Again, why didn't the caches satisfy the name resolution requests.

Any guesses would be appreciated. John

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2 Answers 2

You need to get a better understanding of how DNS works. DNS records don't propagate. If my DNS client or DNS server hasn't queried for your DNS records then they won't have them cached. If I then try to resolve your DNS records and your name servers are unavailable then I'll get nothing and I'll be unable to resolve your DNS records.

Only clients that have queried for your DNS records successfully will have your DNS records cached and only for the duration of the TTL of those records. Once the TTL for those records expires those clients will have to query anew... and if your name servers are unavailable then they'll get nothing.

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That was the point of the question - I don't have a good understanding of how DNS works, and I'd like to get a better one ;-) So are you saying that there aren't any DNS cache servers out there and that it's only clients that cache DNS entries? Because if so, that does help explain stuff and puts the 'blame' back on our DNS provider. We use a monitoring service to keep an eye on the domains and I guess they don't use DNS caches, but go directly to the hosting server otherwise the monitoring wouldn't be much use. –  John Francis Oct 24 '13 at 16:41
    
Gotcha. I hate the term propagation used in regards to DNS as it creates a misconception about how DNS works. A lot of people use the term propagation when they really mean caching. If I don't query for your DNS records then I won't have them. Once I do query for them then I'll cache them for the duration of the TTL. –  joeqwerty Oct 24 '13 at 16:44
    
I was using the term propogate to mean when the cache pulls new information from its information source server. I know there's no push going on here. –  John Francis Oct 24 '13 at 16:47
    
Still can't quite understand why we were seeing no/yes cycles all day for those domain name resolutions. Unless they set the TTL to something very low during maintenance. –  John Francis Oct 24 '13 at 16:50
    
Who hosts the DNS for these domains? Network Solutions was having problems earlier this week that caused sporadic outages for DNS for their customers. –  joeqwerty Oct 24 '13 at 16:55

Well, typically it'll depend on the records TTL time and the configuration of the caching DNS server. It'll be a different experience for every client because they're all using (most likely, anyway) different DNS servers. If client A is using their ISP's DNS server and hasn't visited your site in two days, but then makes a request for your site moments after your DNS provider made their mistake, the DNS server that client A is using will do a lookup in its cache for your domain, it will most likely find it, but it will then notice that it has passed the TTL, thus requiring a new lookup. But since your provider is having issues, it could very likely return with a cannot resolve error.

That's one possible scenario.

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DNSstuff shows TTL=172800 on those domains and we were getting no/yes lookups over the past two days over and over again. Our customers using the domains do so on a daily basis and had these intermittent issues. So I'm still a little confused. I'd have thought that once it 'came back' then the DNS cache should resolve the name for at least the next 48 hours. –  John Francis Oct 24 '13 at 15:45
    
Who is hosting the DNS for these domains? –  joeqwerty Oct 24 '13 at 15:47
    
M247.com (ex-WebTapestry) –  John Francis Oct 24 '13 at 16:43

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