A logon window appears in Windows Server 2003 Service Pack 1 NLB when you try to browse the virtual NLB cluster name

If the following conditions are true, you may receive a logon window that prompts you for credentials:

• You try to browse the virtual Network Load Balancing (NLB) cluster name.
• You use a server that is a member of the NLB cluster.
• The NLB cluster is running Microsoft Windows Server 2003 Service Pack 1 (SP1).

After you type the appropriate credentials, you can view all shares, and you can access all shares. Also, you can browse the virtual NLB cluster name from any other client in the network without a problem.

This problem occurs because Windows Server 2003 SP1 includes a security feature that removes the last available authentication mechanism in NLB Manager. This security feature is an authentication loopback check that helps prevent man-in-the-middle (MITM) attacks on NTLM.

By default, loopback check functionality is turned on in Windows Server 2003 SP1. Also, the value of the DisableLoopbackCheck registry entry is set to 0 (zero).

Warning This workaround may make a computer or a network more vulnerable to attack by malicious users or by malicious software such as viruses. We do not recommend this workaround but are providing this information so that you can implement this workaround at your own discretion. Use this workaround at your own risk.
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
To work around this problem, set the DisableStrictNameChecking registry entry to 1. Then, use one of the following methods, as appropriate.

For more information about how to modify the "DisableStrictNameChecking" registry entry, click the following article number to view the article in the Microsoft Knowledge Base:

Method 1: Create the Local Security Authority host names that can be referenced in an NTLM authentication request

Re-enable the behavior that exists in Windows Server 2003 by setting the DisableLoopbackCheck registry entry in the following registry subkey to 1:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa

To do this, follow these steps on all NLB Nodes:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
3. Right-click MSV1_0, point to New, and then click Multi-String Value.
4. Type BackConnectionHostNames, and then press ENTER.
5. Right-click BackConnectionHostNames, and then click Modify.
6. In the Value data box, type the host name that is used for the NLB cluster, and then click OK.
7. Close Registry Editor, and then restart the computer.

Method 2: Disable the authentication loopback check

To do this, follow these steps for all the nodes:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
3. Right-click Lsa, point to New, and then click DWORD Value.
4. Type DisableLoopbackCheck, and then press ENTER.
5. Right-click DisableLoopbackCheck, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Close Registry Editor, and then restart the computer.

After you install security update 957097, applications such as SQL Server or Internet Information Services (IIS) may fail when making local NTLM authentication requests. For more information about how to resolve this issue, click the following article number to view the article in the Microsoft Knowledge Base: See the "Known issues with this security update" section of KB article 957097 for details about how to resolve the issue.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.