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Strange DNS issue.

Posted on 2009-04-02
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Last Modified: 2012-05-06
Hello,
I have run across one that has stumped me and I am scratching my head trying to fix it.  I have an AD domain with 2 DNS servers one is a backup to the other.  There is a website that some of my users frequent that helps them do their work.

This website is accesible from some computers but not others.  I started to track down the issue and realized that the address returns two different IP addresses when run against an NSLOOKUP.  I have not only run it against my two DNS servers but also against my providers DNS servers and it always returns with the same two IP addresses.  Which is not surprising since my DNS servers are doing lookups against my providers DNS servers.

What I can't figure out is why can some computers get to the site even though one of the ip addresses does not respond and other computers can't.  I first thought it was a Vista issue only because by chance I had found that all the Vista computers I had been on could not get to the site but the few that I had tested that could get to it were running XP.  I debunked that when I found a PC running XP that couldn't get to it as well.

Would creating a record for this domain on my DNS servers mitigate this problem?

I have flushed the DNS caches on all of the host machines that were not able to get to it and that did not help.

Any help is appreciated.

Thanks,

Jake
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Question by:ITPIP
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by:Kentrix70
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Could you please post the output of the reply you get in nslookup (mask the ip-adresses with xxx and yyy).
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by:ITPIP
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Is this what your looking for?

Server:  primarydns.mydomain.com
Address:  xxx.xxx.xxx.xxx

Non-authoritative answer:
Name:    www.problemdomain.com
Addresses:  xxx.xxx.xxx.xxx
          xxx.xxx.xxx.xxx
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Kentrix70 earned 200 total points
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I think this dns record use something called round robin, which means, that for load balancing purposes, is resolves
different ip-adresses from the same domain name.
The problem is, that if one of these ip-adresses is not responding, your pc's will sometimes get through
and sometimes don't.
And the local dns-cache on the pc does that, when it has resolved to a non-working ip-adress, it keeps that adress, and for that
reason will not get through to the webpage.

You could temporarily put the dnszone on your local dnsserver, and resolve the domainname to just the working ip-adress,
but keep in mind, that you the will not be able to resolve other hosts in the same domain, without adding them too.
For instance if the domain is called example.com, and you have a www.example.com and a www2.example.com adress,
they should both be put in your local zone to work.

On the other hand, it is unlikely that this is the case.
You might want to add an mx-record for the domains mailserver though, if someone sends an email to their domain.
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by:StrifeJester
StrifeJester earned 50 total points
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Depending on your organization size you can always add it to the hosts file which wouldn't cause all of the other issues with DNS.
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by:Kentrix70
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You can do that by start -> run and type notepad c:\windows\system32\drivers\etc\hosts

Then you can make an entry on a new line, like

xxx.xxx.xxx.xxx     www.problemdomain.com

where you use the ip-adress, which are working.

You should do that on every pc, and the problem should be solved until the company has 2 working ip-adresses,
then you might want to remove the entry again to regain the possibility of using the loadbalancing functionality
of roundrobin DNS.
If the provider changes the DNS record, you will not be able to access the server before you update the hosts file
or removes the entry, and let DNS take care of it.
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by:ITPIP
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Thanks for the input guys.  I thought both solutions were good ideas but centralizing it on the DNS server is the easier to manage of the two.  Adding the zone to my local DNS with a www record worked for everything except this wierd application the website provides for quotes.  It tries to connect to a url but fails like the page did before.  I only have problems on one PC with this though so it looks like my bigger problem is solved.  I am contacting the company to troubleshoot their software.  Thanks again for the help.
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