A common process in pentesting is to identify the operating system used by the host. Usually, this involves tools like hping or Nmap, and in most cases these tools are quite aggressive to obtain such information and may generate alarms on the target host. OS fingerprinting mainly falls into two categories: active OS fingerprinting and passive OS fingerprinting.

Active fingerprinting is the method of sending packets to a remote host and analyzing corresponding responses. In passive fingerprinting, it analyzes packets from a host, so it does not send any traffic to the host and acts as a sniffer. In passive fingerprinting, it sniffs TCP/IP ports, so it avoids detection or being stopped by a firewall. Passive fingerprinting determines the target OS by analyzing the initial Time to Live (TTL) in IP headers packets, and with the TCP window size in the first packet of a TCP session. The first packet of TCP session is usually either a SYN (synchronize) or SYN/ACK (synchronize and...