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82 posts

Master Geek


Topic # 195281 13-Apr-2016 13:09
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Hello, if I create a brand new A record on our companies publically accessible DNS server,why does this not automatically resolve to the new value when someone on the internet enters that address is a web browser ?


I could understand if this was a change to an existing record i.e. the end user's local or intermediary DNS servers already had the old record cached so would have to wait until it timed out before renewing.


But I would have thought that if it was a brand new record, then the end user's local or intermediary DNS servers would query all the way to my DNS server to get the new record  (because it would have been cached anywhere) ?


Thanks for any help understanding this.

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Master Geek
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  Reply # 1532537 13-Apr-2016 13:21
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What TTL did you set the A record to? Also you might need to flush the DNS cache on any DNS servers along the way?



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  Reply # 1532538 13-Apr-2016 13:22
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I just tried it, using CloudFlare as my DNS server. 2 seconds after creating an A record I was able to resolve it. This is what I would expect.


Tell us more about who your DNS provider is, where the test is from. Maybe there's some aggressive caching done by whatever ISP you're with / DNS server you use.

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  Reply # 1532552 13-Apr-2016 13:43
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If you do a DNS lookup immediately BEFORE you create the record then the NOTFOUND may be cached on your browser, computer, router, ISP and probably more.


DNS does not "propagate" instead servers will cache the results for the specified TTL and request a new value from resolvers when the TTL expires. That's why sometimes you change an existing value it does not reflect instantaneously. It's not waiting for propagation, it's waiting for TTL expiration.


Now some ISPs have misconfigured servers that have their own TTL and ignore what DNS tells them - they might do this to save bandwidth checking external resolvers. 


Back to your question, it could be a lot of things happening.



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  Reply # 1532567 13-Apr-2016 14:20
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OpenDNS has a good tool to check what various DNS servers see for your domain/host:



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  Reply # 1532574 13-Apr-2016 14:34
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Its all based on TTL.


TTL means the dns server will cache your record for X amount of time set by the TTL. The dns server will not ask the authoritive server for that record while its cached.


If you are moving A records around all the time, set a low TTL.

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  Reply # 1532575 13-Apr-2016 14:46
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It can be caused by a number of things but mostly the SOA Record  which includes a large number of things like the serial number and retry/minimum time values for certain things.


If these SOA values are too high it can take a while for other name servers to know about the changes .


These values are set normally set to reasonable numbers so your DNS server is not hit every time someone makes a typo (which is what you are doing basically)


DNS can be a little confusing at times but if you want to learn all about it, read the O'Reilly DNS and BIND book or if you dare read the DNS RFC's (I don't know what the latest is these days but 1034/1035 should help )










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Wannabe Geek

  Reply # 1553719 16-May-2016 21:18
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if I know I am going to be making some changes to a DNS zone in a few days, I like to drop the TTL to something short, e.g. 1 hour. Then when I make the actual changes, hopefully they should propagate out quickly. Then I can raise the TTL back again at my leisure.

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