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DNS redundancy problem

Posted on 2014-02-24
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Last Modified: 2014-02-25
Hi,

I have faced a weird problem few times in past year, which I do not understand exactly.
I have server "farm" for webhosting, where all domains rely on 3 DNS servers. 2 of them are "real" servers, running each in separate datacenter for redundancy, while 3rd DNS is meant as last resort, in case of ....something. So I thought it is not so important where it is located, and I put it onto my home-server as a virtual machine. It runs 24/7/365 and has more than 99,9% uptime.
All 3 DNS servers are on Server 2008R2 technology.

All domains have ALL 3 DNS registered as their DNS servers at registrar.
Ok? Ok.

Now, if ONE of those DNS should fail, all domains should still resolve fine.
Even if 2 of 3 DNS fail, domains should still resolve. Right?

All this seems fine, but HUGE problems arrive upon rebooting or reinstalling some app on my home-server, and for some reason virtual machine does not start. As it starts most of times, I do not check it...because it 98% of times starts up normally.
But in those rare 2% cases, DNS 3 dose not start up...
...and after domain TTL expires, almost half of domains DO NOT RESOLVE ANYMORE! Not all of them, but random significant part.

I check DNS zones and records on primary DNS1 and secondary DNS2, and all records are there and manual resolving fine, if I query DNS server directly.
But most of internet DNS cache around the world is simply empty, without a clue about those domains.

Why is that happening?  
Why do not public DNS cache get filled with zone records from the rest of working DNS1 and DNS2 servers?
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Question by:Andrej Pirman
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2 Comments
 
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Accepted Solution

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RKnebel512 earned 500 total points
ID: 39884573
First, I would double check "IP config /all" on one of the computers that is having problems getting a DNS response back, just to make sure that all three are in there for DNS Servers.

failing that, it sounds to me like there is a problem with your first 2 DNS servers.  I think for some reason, they are not actually resolving and the computers are having to rely on the third one at all times.  That's fine when it's up, but not fine when it goes down.

I would ping the first 2 DNS servers by IP address to make sure your computer can communicate with them.  

If that works, I would try to resolve an address by pointing to one of your first DNS servers.  Do that by typing in:

nslookup     (This brings you into NSLookup mode)
server 192.168.1.53      (substitute the address for your DNS server.)
www.google.com     (or whatever name you want to resolve.)
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Author Comment

by:Andrej Pirman
ID: 39884949
Hi,

well, it is not a problem on MY computer only, but a global problem for domains that we host on our server farm.

In normal state, when all 3 DNS servers are up, all 3 DNS resolve every hosted domain just fine.
But when DNS 3 is down, and after TTL for domain expires....

...Oh, sjit! My bad, sorry!!!!
I just double-checked, which DNS is primary for some particular domains, and realized, that DNS 3 is set as PRIMARY DNS for some of the domains! That explains everything.

I have no idea, how DNS 3 became primary...must be a glitch in Control Panel software, as DNS 3 was never primary for any domain. Ok, problem solved.


Your suggestions were indeed of none help in my case, as I have tried all you suggest many times before. But still, thank you for effort, which I will still reward with points :)
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