We present FRAC, a Framework for Role-based Access Control in network file systems. FRAC is a reference monitor that controls the message flow between file system clients and servers. FRAC supports role hierarchies, user sessions, and static and dynamic separation of duty constraints. It also allows administrators to define dynamic policies based on access history and the environment, e.g., time of day. FRAC introduces a virtual control namespace (VCN) that provides an interface to query and update the state of the access control framework over the standard file system protocol. This namespace eliminates the need for executing specialized user agents either at the client or at the server. Therefore, FRAC does not require any modification to either the file system client or the file server. We have implemented FRAC for the widely deployed NFS protocol using FileWall, afile system proxy previously developed by us. Our experimental evaluation shows that FRAC imposes minimal overheads for the common file system operations.