Check the SPN is Mapped to One User Only
There should ONLY be ONE user mapped to a given SPN. If you have two or more different users mapped to a given SPN record Kerberos authentication will ALWAYS FAIL. For more information see the following blog entry https://blogs.technet.microsoft.com/askds/2008/06/09/kerberos-authentication-problems-service-principal-name-spn-issues-part-2.
You can use the queryspn.vbs script from https://technet.microsoft.com/library/ee176972.aspx to quickly check that SPN is only mapped to one user account. For example, if we search for SPN
HTTP/proxy.diladele.lan@DILADELE.LAN the correct output will be one entry only:
c:\cscript queryspn.vbs HTTP/proxy* Microsoft (R) Windows Script Host Version 5.812 Copyright (C) Microsoft Corporation. All rights reserved. CN=squid,CN=Users,DC=diladele,DC=lan Class: user User Logon: squid -- HTTP/proxy.diladele.lan
If you have more than one user mapped to a given SPN you can remove the additional mappings by running the following command in the administrator’s command prompt on your domain controller. Please specify your own SPN and user name of course!
setspn -D HTTP/proxy.example.lan squid