|PF.OS(5)||File Formats Manual||PF.OS(5)|
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 follows:
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:
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:
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 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 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.
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
|May 31, 2007||OpenBSD-6.9|