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, containing
nine colon (`:') separated fields. These fields are as
window The TCP window size.
TTL The IP time to live.
df The presence of the IPv4 don't fragment
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
version and subtype.
The window field corresponds to the th->th_win field in the
and is the source host's advertised TCP window size. It may
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
* An asterisk will wildcard the value so any window
S Allow any window size which is a multiple of the
T Allow any window size which is a multiple of the
transmission unit (MTU).
The ttl value is the initial time to live in the IP header.
code will account for the volatility of the packet's
TTL as it traverses
The df bit corresponds to the Don't Fragment bit in an IPv4
tells intermediate routers not to fragment the packet and is
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
that appear in the SYN packet. Each option is described by
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
start with a zero timestamp in which
case a zero
value is added to the option; otherwise
no value is
S the Selective ACKnowledgement OK (SACKOK)
Wnnn window scaling option. The value is the
size of the
window scaling which may include the `%'
match all window scalings with the `*'
No TCP options in the fingerprint may be given with a single
An example of OpenBSD's TCP options are:
The first option M* is the MSS option and will match all
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 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 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
It is used to distinguish between different fingerprints of operating
systems of the same class and same version but slightly different
patches or tweaking.
The description is a general description of the operating
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
with a no-df rule would be:
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 > 10.0.0.0.2: 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(8) 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
typically twenty each and the usual sizes of the TCP options
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.
pf(4), pf.conf(5), pfctl(8), tcpdump(8)
OpenBSD 3.6 August 18, 2003
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