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pf.os(7)

Standards, Environments, Macros, Character Sets, and miscellany
                                                                      pf.os(7)



NAME
       pf.os - format of the operating system fingerprints file

DESCRIPTION
       The  PF  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, con‐
       taining nine colon (':') separated fields. These fields are as follows:


         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 might be
       between zero and 65,535 inclusive. The window size might 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 might  be
       used for the window size:


       *    An asterisk will wildcard the value so any window size will match.


       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 transmis‐
            sion unit (MTU).




       The ttl value is the initial time to live in the IP header. The finger‐
       print 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    This is the 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       This is the NOP option (NO Operation).


       T[0]    This  is 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       This is the Selective ACKnowledgement OK (SACKOK) option.


       Wnnn    This is the 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:

         M*,N,N,S,N,W0,N,N,T



       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 dis‐
       tinguish 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 ver‐
       sion. It is used to distinguish between different fingerprints of oper‐
       ating 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.

EXAMPLES
       Example 1 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



       Example  2  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



       Example 3 Fingerprint of an  absolutely  braindead  embedded  operating
       system fingerprint could be:


         65535:255:0:40:.:DUMMY:1.1:p3:Dummy embedded OS v1.1p3



       Example 4 Output format of operating system fingerprints file



       Following is the output of tcpdump(1)


         # tcpdump -s128 -c1 -nv 'tcp[13] == 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)




       The above output almost translates into the following fingerprint:


         57344:64:1:44:M1460: exampleOS:1.0::exampleOS 1.0



SEE ALSO
       pf.conf(7), pfctl(8)

SOLARIS
       File has been introduced to Solaris as a part of firewall modernization
       project. The project brings slightly modified version of PF to Solaris.
       The  manual  page  has been tailored to match a PF feature set found on
       Solaris Operating System. The PF version is derived  from  OpenBSD  5.5
       release.



Oracle Solaris 11.4               27 Nov 2017                         pf.os(7)
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