  Ensuring that information flowing through a network is secure from
manipulation and eavesdropping by unauthorized parties is an important task for
network administrators. Many cyber attacks rely on a lack of network-level
information flow controls to successfully compromise a victim network. Once an
adversary exploits an initial entry point, they can eavesdrop and move
laterally within the network (e.g., scan and penetrate internal nodes) to
further their malicious goals. In this paper, we propose a novel multilevel
security (MLS) framework to enforce a secure inter-node information flow policy
within the network and therein vastly reduce the attack surface available to an
adversary who has penetrated it. In contrast to prior work on multilevel
security in computer networks which relied on enforcing the policy at network
endpoints, we leverage the centralization of software-defined networks (SDNs)
by moving the task to the controller and providing this service transparently
to all nodes in the network. Our framework, MLSNet, formalizes the generation
of a policy compliant network configuration (i.e., set of flow rules on the SDN
switches) as network optimization problems, with the objectives of (1)
maximizing the number of flows satisfying all security constraints and (2)
minimizing the security cost of routing any remaining flows to guarantee
availability. We demonstrate that MLSNet can securely route flows that satisfy
the security constraints (e.g., >80% of flows in a performed benchmark) and
route the remaining flows with a minimal security cost.
