format of the operating system fingerprints
firewall and the
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 follows:
- The TCP window size.
- The IP time to live.
- 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.
- The class of operating system.
- The version of the operating system.
- The subtype of patchlevel of the operating system.
- The overall textual description of the operating system, version and
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.
- Allow any window size which is a multiple of the maximum segment size
- Allow any window size which is a multiple of the maximum transmission unit
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.
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:
- 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 ‘*’ value.
- the NOP option (NO Operation).
- 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.
- the Selective ACKnowledgement OK (SACKOK) option.
- 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
will match two NOPs. The fourth option
will match the SACKOK option. The fifth
will match another NOP. The sixth
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
will match the timestamp option with any
The TCP options in a fingerprint will only match packets with the exact same TCP
options in the same order.
field is the class, genre or vendor
of the operating system.
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.
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.
is a general description of the
operating system, its version, patchlevel and any further useful details.
The fingerprint of a plain 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 fingerprint could be:
65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3
# 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