Using DNS on Multihomed Computers, Part 2

Occasionally, you may wish to configure a client logging into a Windows server for two Network Interface Cards. Such Multihomed computers are likely to have issues when being set up for Windows' dynamic DNS servers. In this series, our stalwart MCSE explains the issues and how to confront them.

By Brien M. Posey | Posted May 2, 2001
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In Part 1 of this series, I explained how to prevent a multihomed computer from registering one of its adapters with a DNS server. Still, you might be wondering what to do if a server has already registered an undesirable name.

The trick to getting rid of the registration is to figure out where it has actually occurred. Regardless of how many network adapters exist on a system, they all use the same DNS server. Therefore, simply look at the systems TCP/IP properties sheet to get the address of the DNS server that the system is configured to use.

Once youve located the appropriate server, open the DNS console by selecting the DNS command from the DNS Servers Administrative Tools menu. When the console opens, navigate through the console tree to DNS | your DNS server name | your domain. As you do so, the column to the right will display a list of all of the systems registered in the DNS servers database. At this point, double check to make sure that the multihomed server is set to not register the address of one of its network cards. You can do this in the manner that I described in Part 1. (Remember that Windows 2000 uses dynamic DNS. Therefore, if you get rid of a systems DNS registration, but you havent told the system not to register the address of a network adapter, then the registration will eventually come back.)

Once youve confirmed that the system is set to not register the address of the network adapter, then you can select the name of the adapter from the list in the DNS console. Remember that a multihomed system will have two entries on the list, so make sure that you select the correct one. If you arent sure if youve made the appropriate selection, right click on your selection and select the Properties command from the resulting context menu. Doing so will display information about the DNS entry, including the corresponding IP address. Once youre sure that youve selected the correct entry, right click on it and select the Delete command from the resulting context menu. Youve now removed the entry from the DNS Server.

You may be wondering what happens if clients have cached the DNS entry before you deleted it, or if the entry has been cached by another DNS server. Keep in mind that cached copies are assigned a TTL (Time To Live) number. Once the TTL runs out, the entry is removed from the cache. If youre paranoid though or need to remove the entry from a certain clients cache, you can do so by entering the following command:

     IPCONFIG /FLUSHDNS
You can confirm that the DNS cache has been cleared by entering the command:
     IPCONFIG /DISPLAYDNS

As you can see, it isnt that difficult to get rid of undesirable DNS entries. Simply get rid of the entry from the main DNS server and the cached copies will eventually go away on their own (assuming that you dont flush the cache first).

Brien M. Posey is an MCSE who works as a freelance writer. His past experience includes working as the director of information systems for a national chain of health care facilities and as a network engineer for the Department of Defense. Because of the extremely high volume of e-mail that Brien receives, it's impossible for him to respond to every message, although he does read them all.

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