The problem is a probably a common one; we have some external web applications (that is: reachable from the Internet) that uses some functionality from internal web service. These internal web service in turn calls a database. The problem probably arise in many SOA solutions since credentials will flow from server to server.
Since the users of the external applications may be anonymous or logged into a portal we can't send the user credentials all the way to the database.
So we thought that making the calls, to the internal web services, under system account would solve the problem. But then the double hop problem kicked in and the credentials will "disappear" when going to the database, resulting in the dreaded "User ('null')" error message from SQL Server.
So far the problem, the solution was to do a Impersonate when the internal web service executed. I first thought that this was impossible, since you actually have to change the credentials for the user running the current thread. However i was corrected via the following article which uses some Windows API to do a impersonation.
In our solution we are using that functionality in such way that the external web applications are making the request to the internal web applications under account that only has rights to run the web service. In that the request a SOAP Header is sent that describes the application and the type of operation request. The internal web service use the information in the SOAP Header to do a Impersonation of a suitable system account.
This works out very well indeed and is a much cleaner solution then others I've seen. The impact on the code is as small as sending the SOAP Header and then one using-statement in the internal web service to do the Impersonation.
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