A person with the SECURITY privilege can enable subsystems on a volume by using the /SUBSYSTEM qualifier on the MOUNT command. By default, subsystems are enabled only on the system disk. For other disks, you need to enable subsystems every time a volume is mounted.

In the following example, a security administrator uses the MOUNT command with the /SUBSYSTEM qualifier to enable the processing of Subsystem ACEs on device DUA0. Assume that this disk contains the subsystem with the identifier MEMBERS_SUBSYSTEM.

$MOUNT /SUBSYSTEM /SYSTEM DUA0: DOC WORK8

You can turn the processing of Subsystem ACEs on and off dynamically with the DCL command SET VOLUME /SUBSYSTEM. This command is especially useful for the system disk, which is not mounted using the MOUNT command.

Any person mounting a subsystem is responsible for knowing what is on the volume being mounted. Without this knowledge, an operator or system manager can inadvertently subvert system security. For example, it is easy for a user with privileges on one cluster to put an application holding a subsystem identifier on a volume and then take the volume to a naive operator on another cluster and request that it be mounted. Because the application holds an appropriate subsystem identifier, it feigns membership in a subsystem for which it is unauthorized. Therefore, mount volumes of only those users whom you trust, or thoroughly search a volume for Subsystem ACEs before you mount it with subsystems enabled.