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PF.OS(5) NetBSD File Formats Manual PF.OS(5)
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pf.os -- format of the operating system fingerprints file
The pf(4) firewall and the tcpdump(8) program can both fingerprint the
operating system of hosts that originate an IPv4 TCP connection. The
file consists of newline-separated records, one per fingerprint, contain-
ing nine colon (`:') separated fields. These fields are as follows:
window The TCP window size.
TTL The IP time to live.
df The presence of the IPv4 don't fragment bit.
packet size The size of the initial TCP packet.
TCP options An ordered list of the TCP options.
class The class of operating system.
version The version of the operating system.
subtype The subtype of patchlevel of the operating system.
description The overall textual description of the operating sys-
tem, version and subtype.
The window field corresponds to the th->th_win field in the TCP header
and is the source host's advertised TCP window size. It may be between
zero and 65,535 inclusive. The window size may be given as a multiple of
a constant by prepending the size with a percent sign `%' and the value
will be used as a modulus. Three special values may be used for the win-
* An asterisk will wildcard the value so any window size will
S Allow any window size which is a multiple of the maximum seg-
ment size (MSS).
T Allow any window size which is a multiple of the maximum
transmission unit (MTU).
The ttl value is the initial time to live in the IP header. The finger-
print code will account for the volatility of the packet's TTL as it tra-
verses a network.
The df bit corresponds to the Don't Fragment bit in an IPv4 header. It
tells intermediate routers not to fragment the packet and is used for
path MTU discovery. It may be either a zero or a one.
The packet size is the literal size of the full IP packet and is a func-
tion of all of the IP and TCP options.
The TCP options field is an ordered list of the individual TCP options
that appear in the SYN packet. Each option is described by a single
character separated by a comma and certain ones may include a value. The
Mnnn maximum segment size (MSS) option. The value is the
maximum packet size of the network link which may
include the `%' modulus or match all MSSes with the
N the NOP option (NO Operation).
T the timestamp option. Certain operating systems
always start with a zero timestamp in which case a
zero value is added to the option; otherwise no value
S the Selective ACKnowledgement OK (SACKOK) option.
Wnnn window scaling option. The value is the size of the
window scaling which may include the `%' modulus or
match all window scalings with the `*' value.
No TCP options in the fingerprint may be given with a single dot `.'.
An example of OpenBSD's TCP options are:
The first option M* is the MSS option and will match all values. The
second and third options N will match two NOPs. The fourth option S will
match the SACKOK option. The fifth N will match another NOP. The sixth
W0 will match a window scaling option with a zero scaling size. The sev-
enth and eighth N options will match two NOPs. And the ninth and final
option T will match the timestamp option with any time value.
The TCP options in a fingerprint will only match packets with the exact
same TCP options in the same order.
The class field is the class, genre or vendor of the operating system.
The version is the version of the operating system. It is used to dis-
tinguish between different fingerprints of operating systems of the same
class but different versions.
The subtype is the subtype or patch level of the operating system ver-
sion. It is used to distinguish between different fingerprints of oper-
ating systems of the same class and same version but slightly different
patches or tweaking.
The description is a general description of the operating system, its
version, patchlevel and any further useful details.
The fingerprint of a plain OpenBSD 3.3 host is:
The fingerprint of an OpenBSD 3.3 host behind a PF scrubbing firewall
with a no-df rule would be:
16384:64:0:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3:!df:OpenBSD 3.3 scrub no-df
An absolutely braindead embedded operating system fingerprint could be:
65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3
The tcpdump(8) output of
# tcpdump -s128 -c1 -nv 'tcp == 2'
03:13:48.118526 10.0.0.1.3377 > 10.0.0.2.80: S [tcp sum ok] \
534596083:534596083(0) win 57344 <mss 1460> (DF) [tos 0x10] \
(ttl 64, id 11315, len 44)
almost translates into the following fingerprint
57344:64:1:44:M1460: exampleOS:1.0::exampleOS 1.0
pf(4), pf.conf(5), pfctl(8), tcpdump(8)
NetBSD 5.0 February 8, 2023 NetBSD 5.0