Updated: 2022/Sep/29

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

     pf.os - format of the operating system fingerprints file

     The NetBSD version of PF is obsolete, and its use is strongly
     discouraged.  Use npf(7) instead.

     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 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 operating system.
           description  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
           S    Allow any window size which is a multiple of the maximum
                segment 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 MSSes 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; otherwise 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:


     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)

NetBSD 9.99                     August 17, 2018                    NetBSD 9.99