Passive security monitoring
The first form of security monitoring we will discuss is passive security monitoring. The name says it all—we passively look for security incidents. Passive refers to the way in which we gather the data. With passive security monitoring, we do not introduce additional traffic to a network (send out packets) or use up any additional resources from network-attached endpoints to collect the data we are interested in. Note, though, that creating a Switched Port Analyzer (SPAN) port on a switch does place additional load on this switch. In practice, passive monitoring almost always comes down to sniffing of network traffic. Passive security monitoring tools do their magic by capturing network packets via a network tap and scrutinizing the captured network packets for odd or known malicious content or behavior (explained in detail later in this chapter, in the Network packet capturing section). Passive security monitoring is the preferred method for ICS...
