While using FQDN kerbros fails to authenicate a user when opening c$ share on a Windows 7 Machine. The machines in question are members of domain yyy.zzz the machines authenicate when the machine_name.yyy.zzz is called. But does not when the FQDN is changes to machine_name.FFF.TTT.yyy.zzz. DNS has both entires in there tables. We have it this way for software development and when we deploy the machines to another environment. The error we are receiving is KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN. The Domain is running in Windows 2008 mod of operations.