If you create a DBLink between a Unix server and a mainframe database how do you identify the actual user who submits a query to the Oracle mainframe database via the DBLink? The mainframe security group is concerned that we have no accountability as to who actually used the DBLink to view mainframe Oracle data.
If you're going from Oracle RDBMS to Oracle RDBMS, this question doesn't make sense since the DBLink would use whatever user you used when creating the link (connect to <username> identified by <password>). If you left out the "connect to" clause, it would be a current user link and would connect to the remote side using the same login credentials used to login to the local database.
If you're talking about connecting to a remote database that is non-Oracle via a gateway, I'm not sure since the remote side would use authentication mechanisms proprietary to that database vendor.