Sunday, February 15, 2009

Unauthorized (401.1) Exception when calling Sharepoint Web Services

We get this error when we work with moss farm deployment scenario that implements load balancing.
In fact, it's a security fix in adress resolution mechanism that prevent a machine to resolve the loopback address from accepting a connection unless that machine name matches the NETBIOS name

One solution is to use the server name instead of the load balanced IP.

Another solution is to modify configuration entries on all servers of our farm as described below:
  • Add the DisableLoopbackCheck registry entry discussed in this Microsoft KB article.
    Note: you will need to reboot your server before the DisableLoopbackCheck takes effect.
  • Be sure to add your load balanced host name for your web farm to the Hosts file on each front end web server. Use the loop-back IP address (
    This will ensure that each web server looks at itself to access the web services preventing any trips back out to the load-balancer - and possibly calling the web service another web server in the farm.
Mark Wagner posted on his blog an article that describes this issue :

See also Microsoft KB article :;EN-US;896861

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