pf.os.5

.\"	$OpenBSD: pf.os.5,v 1.8 2007/05/31 19:19:58 jmc Exp $
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.\" Copyright (c) 2003 Mike Frantzen <frantzen@w4g.org>
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.Dd August 18, 2003
.Dt PF.OS 5
.Os
.Sh NAME
.Nm pf.os
.Nd format of the operating system fingerprints file
.Sh DESCRIPTION
The
.Xr pf 4
firewall and the
.Xr tcpdump 1
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,
containing nine colon
.Pq Ql \&:
separated fields.
These fields are as follows:
.Pp
.Bl -tag -width Description -offset indent -compact
.It window
The TCP window size.
.It TTL
The IP time to live.
.It df
The presence of the IPv4 don't fragment bit.
.It packet size
The size of the initial TCP packet.
.It TCP options
An ordered list of the TCP options.
.It class
The class of operating system.
.It version
The version of the operating system.
.It subtype
The subtype of patchlevel of the operating system.
.It description
The overall textual description of the operating system, version and subtype.
.El
.Pp
The
.Ar 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
.Sq %
and the value will be used as a modulus.
Three special values may be used for the window size:
.Pp
.Bl -tag -width xxx -offset indent -compact
.It *
An asterisk will wildcard the value so any window size will match.
.It S
Allow any window size which is a multiple of the maximum segment size (MSS).
.It T
Allow any window size which is a multiple of the maximum transmission unit
(MTU).
.El
.Pp
The
.Ar ttl
value is the initial time to live in the IP header.
The fingerprint code will account for the volatility of the packet's TTL
as it traverses a network.
.Pp
The
.Ar 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.
.Pp
The
.Ar packet size
is the literal size of the full IP packet and is a function of all of
the IP and TCP options.
.Pp
The
.Ar 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 options are:
.Pp
.Bl -tag -width Description -offset indent -compact
.It Mnnn
maximum segment size (MSS) option.
The value is the maximum packet size of the network link which may
include the
.Sq %
modulus or match all MSSes with the
.Sq *
value.
.It N
the NOP option (NO Operation).
.It T[0]
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 is appended.
.It S
the Selective ACKnowledgement OK (SACKOK) option.
.It Wnnn
window scaling option.
The value is the size of the window scaling which may include the
.Sq %
modulus or match all window scalings with the
.Sq *
value.
.El
.Pp
No TCP options in the fingerprint may be given with a single dot
.Sq \&. .
.Pp
An example of
.Ox Ap s
TCP options are:
.Pp
.Dl M*,N,N,S,N,W0,N,N,T
.Pp
The first option
.Ar M*
is the MSS option and will match all values.
The second and third options
.Ar N
will match two NOPs.
The fourth option
.Ar S
will match the SACKOK option.
The fifth
.Ar N
will match another NOP.
The sixth
.Ar W0
will match a window scaling option with a zero scaling size.
The seventh and eighth
.Ar N
options will match two NOPs.
And the ninth and final option
.Ar T
will match the timestamp option with any time value.
.Pp
The TCP options in a fingerprint will only match packets with the
exact same TCP options in the same order.
.Pp
The
.Ar class
field is the class, genre or vendor of the operating system.
.Pp
The
.Ar version
is the version of the operating system.
It is used to distinguish between different fingerprints of operating
systems of the same class but different versions.
.Pp
The
.Ar subtype
is the subtype or patch level of the operating system version.
It is used to distinguish between different fingerprints of operating
systems of the same class and same version but slightly different
patches or tweaking.
.Pp
The
.Ar description
is a general description of the operating system, its version,
patchlevel and any further useful details.
.Sh EXAMPLES
The fingerprint of a plain
.Ox 3.3
host is:
.Bd -literal
  16384:64:1:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3::OpenBSD 3.3
.Ed
.Pp
The fingerprint of an
.Ox 3.3
host behind a PF scrubbing firewall with a no-df rule would be:
.Bd -literal
  16384:64:0:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3:!df:OpenBSD 3.3 scrub no-df
.Ed
.Pp
An absolutely braindead embedded operating system fingerprint could be:
.Bd -literal
  65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3
.Ed
.Pp
The
.Xr tcpdump 1
output of
.Bd -literal
  # tcpdump -s128 -c1 -nv 'tcp[13] == 2'
  03:13:48.118526 10.0.0.1.3377 > 10.0.0.2.80: S [tcp sum ok] \e
      534596083:534596083(0) win 57344 <mss 1460> (DF) [tos 0x10] \e
      (ttl 64, id 11315, len 44)
.Ed
.Pp
almost translates into the following fingerprint
.Bd -literal
  57344:64:1:44:M1460:	exampleOS:1.0::exampleOS 1.0
.Ed
.Sh SEE ALSO
.Xr tcpdump 1 ,
.Xr pf 4 ,
.Xr pf.conf 5 ,
.Xr pfctl 8