Object managers must be able to identify the SID of a client making a request when this SID is part of a security decision. It is also useful for clients to be able to identify the SID of a server to ensure that a service is requested from an appropriate server. Hence, the Flask architecture requires that the underlying system provide some form of client and server identification for inter-process communication (IPC). However, this feature is not complete without providing the client and server a means of overriding their identification. For instance, the need of a subject to limit its privileges when making a request on behalf of another subject is one justification for capability-based mechanisms . In addition to limiting privileges, overriding the actual identification can be used to provide anonymity in communications or to allow for transparent interposition, such as through a network IPC server connecting the client and server in a distributed system .
The Flask microkernel provides this service directly as part of IPC processing, rather than relying upon complicated and potentially expensive external authentication protocols such as those in Spring and the Hurd . The microkernel provides the SID of the client to the server along with the client's request. The client can identify the SID of the server by making a kernel call on the capability to be used for communication. When making an IPC request, the client can specify a different SID as its effective SID to override its identification to the server. The server can also specify an effective SID when preparing to receive requests. In both cases, permission to specify a particular effective SID is decided by the security server and enforced by the microkernel. Thus, the Flask microkernel supports the basic access control and labeling operations required for the architecture and it provides the flexibility needed for least privilege, anonymity or transparent interposition.