pf.os(5)

NAME

pf.os - format of the operating system fingerprints file

DESCRIPTION

The pf(4) firewall and the tcpdump(1) program can both fin

gerprint the
operating system of hosts that originate an IPv4 TCP connec

tion. The
file consists of newline-separated records, one per finger

print, containing 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 operat

ing 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 window size:

* An asterisk will wildcard the value so any window

size will

match.

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 fingerprint code will account for the volatility of the packet's

TTL as it traverses 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 function 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
options are:

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 MSS

es with the
`*' value.

N the NOP option (NO Operation).
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; other

wise no value
is appended.

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:

M*,N,N,S,N,W0,N,N,T

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 seventh and eighth N options will match two NOPs. And the

ninth and final
option T will match the timestamp option with any time val

ue.

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 operat

ing system.

The 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.

The subtype is the subtype or patch level of the operating

system version. It is used to distinguish between different finger

prints of operating systems of the same class and same version but slight

ly different
patches or tweaking.

The description is a general description of the operating

system, its
version, patchlevel and any further useful details.

EXAMPLES

The fingerprint of a plain OpenBSD 3.3 host is:

16384:64:1:64:M*,N,N,S,N,W0,N,N,T:OpenBSD:3.3::OpenBSD 3.3

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 finger

print could be:

65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3

The tcpdump(1) output of

# tcpdump -s128 -c1 -nv 'tcp[13] == 2'
03:13:48.118526 10.0.0.1.3377 > 10.0.0.0.2: S [tcp sum ok]

534596083:534596083(0) win 57344 <mss 1460> (DF) [tos 0x10]

(ttl 64, id 11315)

almost translates into the following fingerprint

57344:64:1:44:M1460: exampleOS:1.0::exampleOS 1.0

tcpdump(1) does not explicitly give the packet length. But

it can usually be derived by adding the size of the IPv4 header to the

size of the
TCP header to the size of the TCP options. The size of both

headers is
typically twenty each and the usual sizes of the TCP options

are:

mss four bytes.
nop 1 byte.
sackOK two bytes.
timestamp ten bytes.
wscale three bytes.

In the above example, the packet size comes out to 44 bytes.

SEE ALSO

tcpdump(1), pf(4), pf.conf(5), pfctl(8)

BSD August 18, 2003

BSD