Error: 18452 Login failed for user ‘null‘, …

•          “Null” or ‘’ means that client windows token is not trusted

•          Kerberos authentication is not available, fallback to NTLM (not in a domain, SPN not registered)

•          Solution: Create the same account, password on the SQL Server

•          Delegation may be required (at least one hop exists)

•          Solution: use SQL Authentication or enable delegation

•          SQL Server service account cannot query domain

Error: 18456 Login failed for user ‘name‘

•          ‘Name’ means that principal is recognized, but not authorized to connect

•          “Login Failed for user 'NT Authority\ANONYMOUS' LOGON“

•          Reason: client is running under LocalSystem connects using NTLM

•          Solution: Create a dedicated account on both machines

•          “Login failed for user '<domain>\<machinename>$' “

•          Reason: client is LocalSystem or NetworkService

•          Solution: create login in SQL Server for client machine

•          “Login failed for user ‘machinename\User’“

•          Reason: client is local user and has no access

•          Solution: create login in SQL Server for ‘machinename\User’

•          “Login failed for user ‘user’“ (no “\”)

•          Reason: SQL Login cannot connect

•          Solution: Enable mixed mode

•          Note that ‘user’ can be ‘null’ if client windows machine is not trusted, i.e. even SQL authentication requires cross-NTbox windows authentication

•          SQL authentication can often be used as universal or the last resort solution to all these problems

Error: 18456, Severity: 14, State: XX.

•          Client is recognized, but not authenticated

•          State: XX is not reported back to the client

•          Written into ERRORLOG

see also blogs.msdn.com/sql_protocols/archive/2006/12/02/understanding-kerberos-and-ntlm-authentication-in-sql-server-connections.aspx