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PF.OS(5) File Formats Manual PF.OS(5)

pf.osformat 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 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 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.
Allow any window size which is a multiple of the maximum segment size (MSS).
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:

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 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:

  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 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[13] == 2'
  03:13:48.118526 > 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

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

May 31, 2007 OpenBSD-6.6