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

NAME

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

DESCRIPTION

     The packet filter firewall and the tcpdump(1) program can both fingerprint the operating system of hosts that originate an IPv4 TCP connec-
     tion.  The file consists of newline-separated records, one per fingerprint, containing 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 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.
	   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:

	   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 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 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

     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(1) output of

       # 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)

     almost translates into the following fingerprint

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

SEE ALSO

     pf.conf(5), pfctl(8), tcpdump(1)

BSD
								   May 31, 2007 							       BSD