Passive OS fingerprinting can identify attacks originating from a botnet. Network Administrators can configure the firewall to take action on a botnet attack by using information obtained from passive OS fingerprinting. Passive OS fingerprinting (POSFP) allows the sensor to determine the operating system used by the hosts. The sensor examines the traffic flow between two hosts and then stores the operating system of those two hosts along with their IP addresses. In order to determine the type of operating system, the sensor analyzes TCP SYN and SYN ACK packets that are traveled on the network. The sensor computes the attack relevance rating to determine the relevancy of victim attack using the target host OS. After it, the sensor modifies the alert's risk rating or filters the alert for the attack. Passive OS fingerprinting is also used to improve the alert output by reporting some information, such as victim OS, relevancy to the victim in the alert, and source of the OS identification. Answer: D is incorrect. A BPF-based filter is used to limit the number of packets seen by tcpdump; this renders the output more usable on networks with a high volume of traffic. Answer: B is incorrect. Recipient filtering is used to block messages on the basis of whom they are sent to. Answer: C is incorrect. IFilters are used to extract contents from files that are crawled. IFilters also remove application-specific formatting before the content of a document is indexed by the search engine.