Nmap's OS detection can function without initiating a full TCP handshake by employing various TCP/IP stack fingerprinting techniques that analyze how different operating systems respond to specific, unconventional probe packets. These methods allow Nmap to infer the target system's operating system based on unique characteristics in its responses.
Key Techniques for OS Detection Without SYN Packets
-
FIN Probe:
Nmap sends a TCP packet with the FIN flag (or other non-SYN flags) to an open port. According to RFC 793, the correct behavior is to ignore such packets. However, many systems respond with a RST (reset) packet. The presence or absence of a response helps distinguish between different OS implementations.
-
TCP Flag Combinations (e.g., NULL, XMAS, and Custom Flags):
By sending packets with unusual combinations of TCP flags (like no flags for NULL scans or FIN, PSH, and URG for XMAS scans), Nmap observes how the target responds. Different operating systems handle these unconventional packets in varied ways, providing clues about their identity.
-
IP ID Sequence Analysis:
Nmap examines the IP Identification (IP ID) field in responses to probe packets. Operating systems generate IP IDs differently some increment sequentially, others randomly. Analyzing these patterns helps Nmap infer the OS type.
-
TCP Initial Sequence Number (ISN) Sampling:
By analyzing the Initial Sequence Numbers in TCP responses, Nmap can detect patterns unique to certain operating systems, as different systems have distinct methods for generating ISNs.
-
TCP Options and Window Size Analysis:
Nmap sends packets with various TCP options and examines the options and window sizes in the responses. The presence, order, and values of these options can be indicative of specific operating systems.
Nmap's ability to detect operating systems without sending SYN packets relies on analyzing subtle differences in how systems respond to unconventional network traffic. By leveraging these techniques, Nmap can accurately fingerprint remote systems while minimizing the likelihood of detection by intrusion detection systems or firewalls.
