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DNS multiple names to one IP on LAN

Posted on 2006-05-02
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Last Modified: 2010-04-18
On my internal LAN I have a DNS server.  I have a server named x that has an IP.  I need to be able to make an additional record using the same hostname but with a different IP address.  It seems to work OK I was just wondering if this could cause any issues.  Will it always check both DNS entries and go to the address that is online?
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Question by:wrwiii12
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by:feptias
ID: 16586176
A query to the DNS server will return both addresses (if two exist) in a list - one appears first in the list and then the other. The application that is using DNS will normally take the first address and use that. If you enable Round Robin then the sequence in which the addresses are returned should cahnge each time you make a new query.

Most applications that use DNS are not clever enough to select the one that is online. So you can use it load share but not to provide failover web servers for example.
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by:feptias
ID: 16586279
Further to my previous comment:
1. The utility nslookup is very handy for checking this sort of thing.
2. I just tried it on my Win 2k3 based DNS server and strangley, when the Round Robin option was ticked (it is on the Advanced tab of the properties of the DNS server in the DNS Management Console) the addresses always came back in the same order, but when I took the tick out they would come back in alternating order on each new lookup. So ticking the option did the opposite of what I'd expect.
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by:wrwiii12
ID: 16586642
Weird.  My situation is that I have a proxy spam filter in my DMZ and email server in the local LAN.  Everything is being natted from the internet to the DMZ and then from the DMZ to the LAN.  The spam filter uses the internal DNS.  Well the DNS has an entry in it for the email server using the LAN address.  The spam filter uses the DNS and gets the internal LAN address instead of the natted address.  It seems to be working ok with two entries to two different addresses but wanted to make sure I dont cause issues in the future.
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feptias earned 375 total points
ID: 16587121
hmm. It's a bit tricky to picture exactly what your setup is - there are different ways that a DMZ can be achieved and spam filters can use DNS for more than just locating the e-mail server they should be forwarding to. I can only make a couple of general points -
1. It is always best if you really understand what is happening in a system or its bound to cause you sleepless nights later (or your successor)
2. Don't muck about with the DNS Host records for the Domain Controller or you will surely make problems for yourself quite quickly. e.g. if your mail server is on the same PC as the Domain Controller then you'd best look for another solution.
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