Modern critical infrastructure assets are exposed to security threats arising from their use of IP networks and the Domain Name System (DNS). This paper focuses on the health of DNS. Indeed, due to the increased reliance on the Internet, the degradation of DNS could have significant consequences for the critical infrastructure. This paper describes the Measuring Naming System (MeNSa), a framework designed to provide a formal methodology, metrics and tools for evaluating DNS health. Additionally, it proposes a process for aggregating health and security metrics to provide potential threat indicators. Results from a scenario-based experiment demonstrate the utility of the framework and aggregation metrics.