1 .\" $OpenBSD: pf.os.5,v 1.8 2007/05/31 19:19:58 jmc Exp $
3 .\" Copyright (c) 2003 Mike Frantzen <frantzen@w4g.org>
5 .\" Permission to use, copy, modify, and distribute this software for any
6 .\" purpose with or without fee is hereby granted, provided that the above
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24 .Nd format of the operating system fingerprints file
30 program can both fingerprint the operating system of hosts that
31 originate an IPv4 TCP connection.
32 The file consists of newline-separated records, one per fingerprint,
36 These fields are as follows:
38 .Bl -tag -width Description -offset indent -compact
44 The presence of the IPv4 don't fragment bit.
46 The size of the initial TCP packet.
48 An ordered list of the TCP options.
50 The class of operating system.
52 The version of the operating system.
54 The subtype of patchlevel of the operating system.
56 The overall textual description of the operating system, version and subtype.
61 field corresponds to the th->th_win field in the TCP header and is the
62 source host's advertised TCP window size.
63 It may be between zero and 65,535 inclusive.
64 The window size may be given as a multiple of a constant by prepending
65 the size with a percent sign
67 and the value will be used as a modulus.
68 Three special values may be used for the window size:
70 .Bl -tag -width xxx -offset indent -compact
72 An asterisk will wildcard the value so any window size will match.
74 Allow any window size which is a multiple of the maximum segment size (MSS).
76 Allow any window size which is a multiple of the maximum transmission unit
82 value is the initial time to live in the IP header.
83 The fingerprint code will account for the volatility of the packet's TTL
84 as it traverses a network.
88 bit corresponds to the Don't Fragment bit in an IPv4 header.
89 It tells intermediate routers not to fragment the packet and is used for
91 It may be either a zero or a one.
95 is the literal size of the full IP packet and is a function of all of
96 the IP and TCP options.
100 field is an ordered list of the individual TCP options that appear in the
102 Each option is described by a single character separated by a comma and
103 certain ones may include a value.
106 .Bl -tag -width Description -offset indent -compact
108 maximum segment size (MSS) option.
109 The value is the maximum packet size of the network link which may
112 modulus or match all MSSes with the
116 the NOP option (NO Operation).
118 the timestamp option.
119 Certain operating systems always start with a zero timestamp in which
120 case a zero value is added to the option; otherwise no value is appended.
122 the Selective ACKnowledgement OK (SACKOK) option.
124 window scaling option.
125 The value is the size of the window scaling which may include the
127 modulus or match all window scalings with the
132 No TCP options in the fingerprint may be given with a single dot
135 An example of OpenBSD's TCP options are:
137 .Dl M*,N,N,S,N,W0,N,N,T
141 is the MSS option and will match all values.
142 The second and third options
147 will match the SACKOK option.
150 will match another NOP.
153 will match a window scaling option with a zero scaling size.
154 The seventh and eighth
156 options will match two NOPs.
157 And the ninth and final option
159 will match the timestamp option with any time value.
161 The TCP options in a fingerprint will only match packets with the
162 exact same TCP options in the same order.
166 field is the class, genre or vendor of the operating system.
170 is the version of the operating system.
171 It is used to distinguish between different fingerprints of operating
172 systems of the same class but different versions.
176 is the subtype or patch level of the operating system version.
177 It is used to distinguish between different fingerprints of operating
178 systems of the same class and same version but slightly different
183 is a general description of the operating system, its version,
184 patchlevel and any further useful details.
186 The fingerprint of a plain
190 16384:64:1:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3::OpenBSD 3.3
193 The fingerprint of an
195 host behind a PF scrubbing firewall with a no-df rule would be:
197 16384:64:0:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3:!df:OpenBSD 3.3 scrub no-df
200 An absolutely braindead embedded operating system fingerprint could be:
202 65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3
209 # tcpdump -s128 -c1 -nv 'tcp[13] == 2'
210 03:13:48.118526 10.0.0.1.3377 > 10.0.0.2.80: S [tcp sum ok] \e
211 534596083:534596083(0) win 57344 <mss 1460> (DF) [tos 0x10] \e
212 (ttl 64, id 11315, len 44)
215 almost translates into the following fingerprint
217 57344:64:1:44:M1460: exampleOS:1.0::exampleOS 1.0