Sometimes, especially on a Small Business Server (SBS) machine, you will run into a situation where having more than one IP address linked to your server is very helpful.  This especially applies if you are only using one NIC in your machine and need to address it ‘logically’ by several IPs.  What situations is this useful for?  Well, how about a Database server application that wants it’s own independent IP address?  A SPAM filter/seperate Mail-Transfer-Agent (MTA)?  Of course, I probably don’t need to give you examples, you’re reading this because you already have a reason for multiple IPs on a single NIC and just want to know how to make it work properly!

The Problem:

You are correctly going into your NIC properties >> TCP/IPv4 properties >> Advanced Settings and adding your additional IP addresses.  That part is easy, and you’ve done it.  You can see the additional addresses in your DNS and DHCP MMC snap-ins.  So what’s the problem?

Let’s say your server is called “SERVER” (creative, huh?) and is at 192.168.1.1.  You have created two additional IP addresses at 192.168.1.5 and 192.168.1.10.  Now, you’ve gone into your DNS snap-in and statically assigned the following mappings:

192.168.1.5 >> “Server APP1”
192.168.1.10 >> “Server APP2”

Great!  That should be it, right?  WRONG!  Check your DNS snap-in after the next refresh interval and you’ll notice a few things…  First, your static mappings look like this now:

192.168.1.5 >> “Server APP1”
192.168.1.5 >> “SERVER”
192.168.1.10 >> “Server APP2”
192.168.1.10 >> “SERVER”

In addition, your reverse DNS entries will point back to the SERVER instead of the mappings you statically created.  How did this happen?  Well, your helpful server added these mappings for you assuming you really meant to have ALL your IPs point back to the server and just went momentarily insane when you setup your own mappings.  Let’s fix this…

Change DNS listener settings

The heart of the problem lies in the fact that the server wants to remain the primary DNS server for your domain, and it should.  However, it thinks that your additional addresses were an attempt to multi-home the system and thus, all addresses should continue to resolve to “SERVER”.  This is not the behaviour we want, so let’s shatter our server’s preconceived notions and fix DNS.

  1. Open your DNS MMC snap-in.
  2. Right-click on your server name and choose Properties.
  3. The first tab, Interfaces, shows us our problem.  You will see that your newly added IP addresses are automatically checked off!
  4. UNCHECK your newly created IP addresses, leaving only the original IP address.
  5. Click on Apply/OK.

Now our server is explicitly told that it should only serve DNS requests on the original IP address and not pay attention to the other, new, addresses.  Give it a sec, refresh the display and you’ll see that only your static mappings remain.  Plus, the reverse DNS will sort itself out too!

That’s it!  Enjoy having multiple ‘logical’ addresses on a single NIC!

Questions, comments, suggestions?  Hit me up below 🙂

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