Remote OS Detection

Table of Contents

Introduction
Reasons for OS Detection
Determining vulnerability of target hosts
Tailoring exploits
Network inventory and support
Detecting unauthorized and dangerous devices
Social engineering
Usage and Examples
TCP/IP Fingerprinting Methods Supported by Nmap
Probes Sent
Sequence generation (SEQ, OPS, WIN, and T1)
ICMP echo (IE)
TCP explicit congestion notification (ECN)
TCP (T2T7)
UDP (U1)
Response Tests
TCP ISN greatest common divisor (GCD)
TCP ISN counter rate (ISR)
TCP ISN sequence predictability index (SP)
TCP IP ID sequence generation algorithm (TI)
ICMP IP ID sequence generation algorithm (II)
Shared IP ID sequence Boolean (SS)
TCP timestamp option algorithm (TS)
TCP options (O, 01–06)
TCP initial window size (W, W1W6)
Responsiveness (R)
IP don't fragment bit (DF)
Don't fragment (ICMP) (DFI)
IP initial time-to-live (T)
IP initial time-to-live guess (TG)
Explicit congestion notification (CC)
TCP miscellaneous quirks (Q)
TCP sequence number (S)
ICMP sequence number(SI)
TCP acknowledgment number (A)
TCP flags (F)
TCP RST data checksum (RD)
IP type of service (TOS)
IP type of service for ICMP responses (TOSI)
IP total length (IPL)
Unused port unreachable field nonzero (UN)
Returned probe IP total length value (RIPL)
Returned probe IP ID value (RID)
Integrity of returned probe IP checksum value (RIPCK)
Integrity of returned probe UDP length and checksum (RUL and RUCK)
Integrity of returned UDP data (RUD)
ICMP response code (CD)
IP data length for ICMP responses (DLI)
Fingerprinting Methods Avoided by Nmap
Passive Fingerprinting
Exploit Chronology
Retransmission Times
IP Fragmentation
Open Port Patterns
Understanding an Nmap Fingerprint
Decoding the Subject Fingerprint Format
Decoding the SCAN line of a subject fingerprint
Decoding the Reference Fingerprint Format
Free-form OS description (Fingerprint line)
Device and OS classification (Class lines)
Test expressions
OS Matching Algorithms
Dealing with Misidentified and Unidentified Hosts
When Nmap Guesses Wrong
When Nmap Fails to Find a Match and Prints a Fingerprint
Modifying the nmap-os-db Database Yourself

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