TCP/IP Stack Fingerprinting - TCP/IP Fingerprint Specifics

TCP/IP Fingerprint Specifics

Certain parameters within the TCP protocol definition are left up to the implementation. Different operating systems, and different versions of the same operating system, set different defaults for these values. By collecting and examining these values, one may differentiate among various operating systems, and implementations of TCP/IP. The TCP/IP fields that may vary include the following:

• Initial packet size (16 bits)
• Initial TTL (8 bits)
• Window size (16 bits)
• Max segment size (16 bits)
• Window scaling value (8 bits)
• "don't fragment" flag (1 bit)
• "sackOK" flag (1 bit)
• "nop" flag (1 bit)

These values may be combined to form a 67-bit signature, or fingerprint, for the target machine. Just inspecting the Initial TTL and window size fields is often enough in order to successfully identify an operating system, which eases the task of performing manual OS fingerprinting.