Set up split-brain DNS with Active Directory integrated zones

Derek Schauland explains how to configure conditional forwarding in Windows Server 2003 when a split-brain DNS set up is called for.

Many organizations may make use of two or more DNS servers — one for internal users and one for the rest of the world via the Internet. In addition, these organizations also use Active Directory integrated DNS to allow for easier management. In some cases it may be necessary to forward requests sent to these DNS servers differently, based on the location of the requester; this is what's known as split-brain DNS usage. This concept came about because some requests need to be forwarded differently if they go to the internal DNS server versus the request that would go to the publicly available DNS server, and Windows Server 2003 can handle this DNS behavior.

Let's look at split-brain DNS in Windows 2003 using Active Directory integrated DNS.

When does split-brain syndrome occur?

Organizations that have multiple internal domain names such as and can run into a problem with split-brain DNS because each domain requires DNS to work properly with Active Directory. If I'm a user in domain and I need to find a Web address or a computer at, there may be an issue when my DNS server tries to forward this request. The request goes to the DNS server for my domain, and the domain then passes it off to the public DNS servers for my company; when this happens, the DNS resource I'm looking for needs to be on the Internet for the server to find it. Because the resource I'm looking for is in a different internal domain, then there's no chance of the external DNS server finding it in a typical DNS scenario.

Windows server 2003 can combat this issue by using conditional forwarding. Conditional forwarding specifies that certain requests should not be forwarded to public servers, but instead forwarded directly to a specific server within your environment. This way, if I'm in the domain and I need to get to a resource in the domain, when the DNS request gets to my domain's DNS server, it can be sent directly to the other domain's DNS server. To the user, this is a seamless process that directs them to the correct resource.

How is conditional forwarding configured?

To configure conditional forwarding as an attribute of DNS, complete the following steps:

  1. On a Windows Server 2003 domain controller, open the DNS console.
  2. Right-click the DNS server you wish to work with and click Properties.
  3. Select the Forwarders tab of the DNS properties dialog box for the selected server.
  4. Click the New button to the right of the DNS domain list.
  5. Enter the domain name for which the conditional forwarders should be configured — for example, — and click OK.
  6. Click on the new domain forward that you just added in DNS domains list and type the IP address of the primary DNS server for that domain in the box below, labeled Selected Domains Forwarder IP Address List.
  7. Click the Add button.

Once you click OK on the DNS properties dialog box, the conditional forwarder for the domain you specified will be ready to go; however, you may want to restart the DNS service just to make sure everything is working. Note: For conditional forwarders to work, all the DNS servers in your Active Directory environment must run Windows Server 2003.

This simple change and setup can enable multiple subdomains to exist in your Active Directory environment.

Do you have other questions about Windows Server 2003 configuration?


Derek Schauland has been tinkering with Windows systems since 1997. He has supported Windows NT 4, worked phone support for an ISP, and is currently the IT Manager for a manufacturing company in Wisconsin.

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