The business goal of Critical Security Control #9: Limitation and Control of Network Ports is “To limit potential vulnerabilities on systems by limiting unauthorized ports, protocols, & services on systems”. Sounds totally reasonable and something everyone should systematically monitor in their respective environments. How can this be accomplished, especially if this has not been an area of focus?
One strategy is capturing data about the network traffic in order to develop and maintain a rolling trend over time. A specific area to focus on is the Top Ports that were targeted on a daily basis. This data will readily highlight trends that can be investigated and help detect changes in traffic that may or many not be “normal". This daily baseline, when added to the last 30 days worth of the same data can easily be to highlight and compare traffic patterns. What could this look like for your network? As a practical example, below is a visualization of data that has been sent to the Internet Storm Center.