In the audit process of Internet activities of users in a local area network, one of the most difficult problems is the problem of source authentication i.e. establishing securely the identity of the owner of the records. The problem cannot be solved by user authentication only because of the inherited security deficiencies in the structure of Internet protocols. To achieve secure and reliable source authentication, we propose an efficient proxy based architectural solution called UNIDES which detects and blocks unauthorized Internet access in realtime.