The xhost program is used to add and delete hosts to the list of machines that are allowed to make connections to the X server. This list of hosts permits access to any user on one of the named hosts. Because this mechanism is unable to permit or deny access to specific users, it provides only a rudimentary form of privacy control and security. It is only sufficient for a single user environment, although it does limit the worst abuses. Environments which require more sophisticated measures should use the hooks in the protocol for passing authentication data to the server. [Sch91]