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Microsoft

LoopbackCheck caused 401 only happens when you are browsing a local Web site using the fully qualified domain name (FQDN) or a custom host header and the web site is using Integrated Authentication. 

 

It is common to use System.Net.HttpWebRequest in a web application to connect to a web service and if the HttpWebRequest is hitting local web service and conditions mentioned above met, LoopbackCheck will also cause 401 for the System.Net.HttpWebRequest too.

 

To address LoopbackCheck caused 401 issue, use one of the two options below.

 

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

  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey:
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
  3. Right-click MSV1_0, point to
    New, and then click Multi-String Value.
  4. In the Name column, type
    BackConnectionHostNames, and then press ENTER.
  5. Right-click BackConnectionHostNames, and then click Modify.
  6. In the Value data box, type the CNAME or the DNS alias, that is used for the local shares on the computer, and then click OK.

    Note Type each host name on a separate line.

    Note If the BackConnectionHostNames registry entry exists as a REG_DWORD type, you have to delete the BackConnectionHostNames registry entry.
  7. Exit Registry Editor, and then restart the computer.

Method 2: Disable the authentication loopback check

Re-enable the behavior that exists in Windows Server 2003 by setting the DisableLoopbackCheck registry entry in the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa registry subkey to 1. To set the DisableLoopbackCheck registry entry to 1, follow these steps on the client computer:

  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey:
    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. Exit Registry Editor.
  8. Restart the computer.