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                                  p0f 2

                    "Dr. Jekyll had something to Hyde"
                      passive OS fingerprinting tool
                               version 2.0.8

     (C) Copyright 2000 - 2006 by Michal Zalewski <>

                Various ports (C) Copyright 2003 - 2006 by:

                   Michael A. Davis <>
                      Kirby Kuehl <>
                     Kevin Currie <>

      Portions contributed by numerous good people - see CREDITS file.

      For a book on some interesting passive fingerprinting tips, see:

    **** HELP WITH P0F DATABASE: ****

0. Contents

  This document describes the concept and history of p0f, its
  command-line options and extensions, and goes into some detail about
  its operation, integration with existing solutions, and so on.

  Table of contents:

   1) What's this, anyway?
   2) Why would I want to use it?
   3) What's new then?
   4) Command-line
   5) Active service integration
   6) SQL database integration
   7) Masquerade detection
   8) Fingerprinting accuracy and precision
   9) Adding signatures
  10) Security
  11) Limitations
  12) Is it better than other software?
  13) Program no work!
  14) Appendix A: Links to OS fingerprinting resources

1. What's this, anyway?

  The passive OS fingerprinting technique is based on analyzing the
  information sent by a remote host while performing usual communication
  tasks - such as whenever a remote party visits your webpage, connects to 
  your MTA - or whenever you connect to a remote system while browsing the
  web or performing other routine tasks. In contrast to active fingerprinting 
  (with tools such as NMAP or Queso), the process of passive fingerprinting 
  does not generate any additional or unusual traffic, and thus cannot be 

  Captured packets contain enough information to identify the remote OS,
  thanks to subtle differences between TCP/IP stacks, and sometimes certain
  implementation flaws that, although harmless, make certain systems quite
  unique. Some additional metrics can be used to gather information about 
  the configuration of a remote system or even its ISP and network setup.

  The name of the fingerprinting technique might be somewhat misleading - 
  although the act of discovery is indeed passive, p0f can be used for
  active testing. It is just that you are not required to send any unusual
  or undesirable traffic, and can rely what you would be getting from
  the remote party anyway, in the course of everyday, seemingly innocuous

  To accomplish the job, p0f equips you with four different detection modes:

    - Incoming connection fingerprinting (SYN mode, default) - whenever
      you want to know what the guy or gal who connects to you runs,

    - Outgoing connection (remote party) fingerprinting (SYN+ACK mode) -
      to fingerprint systems you or your users connect to,

    - Outgoing connection refused (remote party) fingerprinting (RST+ mode) 
      - to fingerprint systems that reject your traffic,

    - Established connection fingerprinting (stray ACK mode) - to examine
      existing sessions without any needless interference.

  It is quite difficult to pinpoint who came up with this idea of passive
  SYN-based OS fingerprinting, though due credit must be given to Craig Smith, 
  Peter Grundl, Lance Spitzner, Shok, Johan, Su1d, Savage, Fyodor and other
  brave hackers who explored this and related topics in the years 1999 and 

  P0f was the first (and I believe remains the best) fully-fledged 
  implementation of a set of TCP-related passive fingerprinting techniques. 
  The current version uses a number of detailed metrics, often invented 
  specifically for p0f, and achieves a very high level of accuracy and detail;
  it is designed for hands-free operation over an extended period of time, 
  and has a number of features to make it easy to integrate it with other 

  Portions of this code are used in several IDS systems, some sniffer  
  software; p0f is also shipped with several operating systems and 
  incorporated into an interesting OpenBSD pf hack by Mike Frantzen, that 
  allows you to filter out or redirect traffic based on the source OS. 
  There is also a beta patch for Linux netfilter, courtesy of Evgeniy 
  Polyakov. In short, p0f is a rather well-established software at this 

2. Why would I want to use it?

  Oh, a number of uses come to mind:

    - Profiling / espionage - ran on a server, firewall, proxy or router, 
      p0f can be used to silently gather statistical and profiling information 
      about your visitors, users, or competitors. P0f also gathers netlink
      and distance information suitable for determining remote network
      topology, which may serve as a great piece of pre-attack intelligence.

    - Active response / policy enforcement - integrated with your server
      or firewall, p0f can be used to handle specific OSes in the most
      suitable manner and serve most appropriate content; you may also enforce 
      a specific corporate OS policy, restrict SMTP connections to a set of 
      systems, etc; with masquerade detection capabilities, p0f can be used
      to detect illegal network hook-ups and TOS violations. 

    - PEN-TEST - in the SYN+ACK, RST+, or stray ACK mode, or when a returning 
      connection can be triggered on a remote system (HTML-enabled mail with 
      images, ftp data connection, mail bounce, identd connection, IRC DCC 
      connection, etc), p0f is an invaluable tool for silent probing of a 
      subject of such a test.

      Masquerade detection in SYN+ACK or RST+ modes can be also used to
      test for load balancers and so forth.
    - Network troubleshooting - RST+ mode can be used to debug network
      connectivity problems you or your visitors encounter.

    - Bypassing a firewall - p0f can "see thru" most NAT devices, packet
      firewalls, etc. In SYN+ACK mode, it can be used for fingerprinting
      over a connection allowed by the firewall, even if other types of
      packets are dropped; as such, p0f is the solution when NMAP and
      other active tools fail.

    - Amusement value is also pretty important. Want to know what this
      guy runs? Does he have a DSL, X.25 WAN hookup, or a shoddy SLIP 
      connection? What's Google crawlbot's uptime?

  Of course, "a successful [software] tool is one that was used to do 
  something undreamed of by its author" ;-)

3. What's new then?

  The original version of p0f was written somewhere in 2000 by Michal 
  Zalewski (that be me), and later taken over William Stearns (circa 2001). 
  The original author still contributed to the code from time to time, and 
  the version you're holding right now is his sole fault - although I'd like 
  William to take over further maintenance, if he's interested.

  Version 2 is a complete rewrite of the original v1 code. The main reason 
  for this is to make signatures more flexible, and to implement certain 
  additional checks for very subtle packet characteristics to improve 
  fingerprint accuracy. Changes include:


    - Option layout and count check,
    - EOL presence and trailing option data [*],
    - Unrecognized option handling (TTCP, etc),
    - WSS to MSS/MTU correlation checks [*],
    - Zero timestamp check,
    - Non-zero ACK in initial SYN [*],
    - Non-zero "unused" TCP fields [*],
    - Non-zero urgent pointer in SYN [*],
    - Non-zero second timestamp [*],
    - Zero IP ID in initial packet,
    - Unusual auxiliary flags,
    - Data payload in control packets [*],
    - SEQ number equal to ACK number [*],
    - Zero SEQ number [*],
    - Non-empty IP options.

    [*] denotes metrics "invented" for p0f, as far as I am concerned. Other 
    metrics were discussed by certain researchers before, although usually 
    not implemented anywhere. A detailed discussion of all checks performed
    by p0f can be found in the introductory comments in p0f.fp, p0fa.fp
    and p0fr.fp.

    As a matter of fact, some of the metrics were so precise I managed
    to find several previously unknown TCP/IP stack bugs :-) See
    doc/win-memleak.txt and p0fr.fp for more information.


    - Major performance boost - no more runtime signature parsing, added 
      BPF pre-filtering, signature hash lookups. All this to make p0f 
      suitable for being run on high-throughput devices,

    - Advanced masquerade detection for policy enforcement (ISPs,
      corporate networks),

    - Modulo and wildcard operators for certain TCP/IP parameters to make
      it easier to come up with generic last chance signatures for
      systems that tweak settings notoriously (think Windows),

    - Auto-detection of DF-zeroing firewalls,

    - Auto-detection of MSS-tweaking NAT and router devices,

    - Media type detection based on MSS, with a database of common
      link types,
    - Origin network detection based on unusual ToS / precedence bits,

    - Ability to detect and skip ECN option when examining flags,

    - Better fingerprint file structure and contents - all fingerprints
      are rigorously reviewed before being added.

    - Generic last-chance signatures to cover general OS characteristics,

    - Query mode to enable easy integration with third party software -
      p0f caches recent fingerprints and answer queries for src-dst
      combinations on a local stream socket in a easy to parse

    - Usability features: greppable output option, daemon mode, host
      name resolution option, promiscuous mode switch, built-in signature 
      collision detector, ToS reporting, full packet dumps, pcap dump
      output, etc,

    - Brand new SYN+ACK, RST+ and stray ACK fingerprinting modes for silent
      identifications of systems you connect to the usual way (web
      browser, MTA), or even systems you cannot connect to at all;
      now also with RST+ACK flag and value validator.

    - Fixed WSCALE handling in general, and WSS passing on little-endian,
      many other bug-fixes and improvements of the packet parser
      (including some sanity checks).

    - Fuzzy checks option when no precise matches are found (limited).

    - VLAN support.

  Sadly, this will break all compatibility with v1 signatures, but it's
  well worth it.

4. Command-line

  P0f is rather easy to use. There's a number of options, but you don't
  need to know most of them for normal operation:

  p0f [ -f file ] [ -i device ] [ -s file ] [ -o file ] [ -Q socket [ -0 ] ]
      [ -w file ] [ -u user ] [ -c size ] [ -T nn ] [ -e nn ] 
      [ -FNODVUKAXMqxtpdlRL ] [ 'filter rule' ]

  -f file   - read fingerprints from file; by default, p0f reads signatures 
              from ./p0f.fp or /etc/p0f/p0f.fp (the latter on Unix systems 
              only). You can use this to load custom fingerprint data. 
              Specifying multiple -f values will NOT combine several signature 
              files together.

  -i device - listen on this device; p0f defaults to whatever device libpcap 
              considers to be the best (and which often isn't). On some newer 
              systems you might be able to specify 'any' to listen on all 
              devices, but don't rely on this. Specifying multiple -i values 
              will NOT cause p0f to listen on several interfaces at once.

  -s file   - read packets from tcpdump snapshot; this is an alternate
              mode of operation, in which p0f reads packet from pcap
              data capture file, instead of a live network. Useful for
              forensics (this will parse tcpdump -w output, for example). 

              You can use Ethereal's text2pcap to convert human-readable
              packet traces to pcap files, if needed.

  -w file   - writes matching packets to a tcpdump snapshot, in addition to
              fingerprinting; useful when it is advisable to save copies of
              the actual traffic for review.

  -o file   - write to this logfile. This option is required for -d and 
              implies -t.

  -Q socket - listen on a specified local stream socket (a filesystem object, 
              for example /var/run/p0f-sock) for queries. One can later send a 
              packet to this socket with p0f_query structure from p0f-query.h,  
              and wait for p0f_response. This is a method of integrating p0f 
              with active services (web server or web scripts, etc). P0f will 
              still continue to report signatures the usual way - but you can 
              use -qKU combination to suppress this. Also see -c notes.

              A sample query tool (p0fq) is provided in the test/ 
              subdirectory. There is also a trivial perl implementation of 
              a client available; finally, test/p0fping.c can be used to
	      check the status of the socket prior to queries.

              NOTE: The socket will be created with permissions corresponding
              to your current umask. If you want to restrict access to this 
              interface, use caution.

              This option is currently Unix-only.
  -0          (In conjunction with -Q) Treat source port 0 in queries as
              a wildcard. This is useful when p0f query is constructed
	      from within a plugin to a program that does not provide
	      source port information (this holds true for some mail
	      filters, etc).
	      Note that some ambiguity is introduced: the response might
	      not refer to the exact connection the plugin is handling,
	      which may (seldom) cause misidentification of NATed hosts.

  -e ms     - packet capture window. On some systems (particularly on older Suns),
              the default pcap capture window of 1 ms is insufficient, and p0f
              may get no packets. In such a case, adjust this parameter to the
              smallest value that results in reliable operation (note that this
              might introduce some latency to p0f).

  -c size   - cache size for -Q and -M options. The default is 128, which
              is sane for a system under a moderate network load. Setting it 
              too high will slow down p0f and may result in some -M false 
              positives for dial-up nodes, dual-boot systems, etc. Setting it 
              too low will result in cache misses for -Q option. To choose the 
              right value, use the number of connections on average per the 
              interval of time you want to cache, then pass it to p0f with -c.

              P0f, when run without -q, also reports average packet ratio
              on exit. You can use this to determine the optimal -c setting.

              This option has no effect if you do not use -Q nor -M.

  -u user   - this option forces p0f to chroot to this user's home directory 
              after reading configuration data and binding to sockets, then to
              switch to his UID, GID and supplementary groups.
              This is a security feature for the paranoid - when running 
              p0f in daemon mode, you might want to create a new 
              unprivileged user with an empty home directory, and limit the 
              exposure when p0f is compromised. That said, should such a 
              compromise occur, the attacker will still have a socket he can 
              use for sniffing some network traffic (better than rm -rf /).

              This option is Unix-only.

  -N        - inhibit guesswork; do not report distances and link media. With
              this option, p0f logs only source IP and OS data.

  -F        - deploy fuzzy matching algorithm if no precise matches are
              found (currently applies to TTL only). This option is not
              recommended for RST+ mode.

  -D        - do not report OS details (just genre). This option is useful
              if you don't want p0f to elaborate on OS versions and such
              (combine with -N).

  -U        - do not display unknown signatures. Use this option if you want 
              to keep your log file clean and are not interested in hosts that 
              are not recognized.

  -K        - do not display known signatures. This option is useful when you
              run p0f recreationally and want to spot UFOs, or in -Q or -M 
              modes when combined with -U to inhibit all output.

  -q        - be quiet - do not display banners and keep low profile.

  -p        - switch card to promiscuous mode; by default, p0f listens 
              only to packets addressed or routed thru the machine it
              runs on. This setting might decrease performance, depending
              on your network design and load. On switched networks,
              this usually has little or no effect.

              Note that promiscuous mode on IP-enabled interfaces can be 
              detected remotely, and is sometimes not welcome by network 

  -t        - add human-readable timestamps to every entry (use multiple 
              times to change date format, a la tcpdump).

  -d        - go into daemon mode (detach from current terminal and fork into 
              background). Requires -o.

  -l	    - outputs data in line-per-record style (easier to grep).

  -A	    - a semi-supported option for SYN+ACK mode. This option will cause
              p0f to fingerprint systems you connect to, as opposed to systems
              that connect to you (default). With this option, p0f will look 
              for p0fa.fp file instead of the usual p0f.fp. The usual config 
              is NOT SUITABLE for this mode.

              The SYN+ACK signature database is sort of small at the moment, 
              but suitable for many uses. Feel free to contribute.
  -R        - a barely-supported option for RST+ mode. This option will 
              prompt p0f to fingerprint several different types of traffic, 
              most importantly "connection refused" and "timeout" messages.
	      This mode is similar to SYN+ACK (-A), except that the program
	      will now look for p0fr.fp. The usual config is NOT SUITABLE for
              this mode. You may have to familiarize yourself with p0fr.fp
              before using it.

  -O        - absolutely experimental open connection (stray ACK) 
              fingerprinting mode. In this mode, p0f will attempt to
              indiscriminately identify OS on all packets within an already
              established connection.

              The only use of this mode is to perform an immediate 
              fingerprinting of an existing session. Because of the sheer 
              amount of output, you are advised against running p0f in this
              mode for extended periods of time.
              The program will use p0fo.fp file to read fingerprints. The 
              usual config is NOT SUITABLE for this mode. Do not use unless
              you know what you are doing. NOTE: The p0fo.fp database is very 
              sparsely populated at the moment.

  -r	    - resolve host names; this mode is MUCH slower and poses some
              security risk. Do not use except for interactive runs or
              low traffic situations. NOTE: the option ONLY resolves
              IP address into a name, and does not perform any checks for
              matching reverse DNS. Hence, the name may be spoofed - do not
              rely on it without checking twice.

  -C        - perform collision check on signatures prior to running. This
              is an essential option whenever you add new signatures to
              .fp files, but is not necessary otherwise. 

  -L        - list all network interfaces. This option is Windows-only.

  -x        - dump full packet contents; this option is not compatible with
              -l and is intended for debugging and packet comparison only.

  -X        - display packet payload; rarely, control packets we examine
              may carry a payload. This is a bug for the default (SYN)
              and -A (SYN+ACK) modes, but is (sometimes) acceptable in
              -R (RST+) mode.

  -M        - deploy masquerade detection algorithm. The algorithm looks over 
              recent (cached) hits and looks for indications of multiple 
              systems being behind a single gateway. This is useful on routers 
              and such to detect policy violations. Note that this mode is 
              somewhat slower due to caching and lookups. Use with caution
              (or do not use at all) in modes other than default (SYN).

  -T nn     - masquerade detection threshold; only meaningful with -M,
              sets the threshold for masquerade reporting.

  -V        - use verbose masquerade detection reporting. This option
              describes the status of all indicators, not only an overall

  -v        - enable support for 802.1Q VLAN tagged frames. Available on
              some interfaces, on other, will result in BPF error.

  The last part, 'filter rule', is a bpf-style filter expression for
  incoming packets. It is very useful for excluding or including certain
  networks, hosts, or specific packets, in the logfile. See man tcpdump for 
  more information, few examples:

     'src port ftp-data'
     'not dst net mask'
     'dst port 80 and ( src host or src host )'

  The baseline rule is to select only TCP packets with SYN set, no RST, no 
  ACK, no FIN (SYN, ACK, no RST, no FIN for -A mode; RST, no FIN, no SYN
  for -R mode; ACK, no SYN, no RST, no FIN for stray ACK mode). You cannot 
  make the rule any broader (without cheating ;), the optional filter 
  expression can only narrow it down.

  You can also use a companion log report utility for p0f. Simply run 
  'p0frep' for help.

5. Active service integration

  In some cases, you want to feed the p0f output to a specific application to 
  take certain active measures based on the operating system (handle specific
  visitors differently, block some unwanted OSes, optimize the content served).

  As mentioned earlier, OpenBSD users can simply use the pf OS fingerprinting 
  implementation, a cool functionality coded by Mike Frantzen and based on 
  p0f methodology and signature database. This software allows them to 
  redirect or block OSes any way they want. Linux netfilter users can also 
  check out patches by Evgeniy Polyakov to get roughly the same stuff.

  In other setups, or if you do not feel like fiddling with the kernel, 
  you want to use the -Q option, and then query p0f by connecting to a 
  specific local stream socket and sending a single packet with p0f_query 
  struct (p0f-query.h), and receiving p0f_response. P0f, when running in -Q 
  mode, will cache a number of last OS matches, and when queried for a 
  specified host and port combination, will return what it detected. 
  Check test/p0fq.c for a clean example.

  The query structure (p0f_query) has the following fields (all
  values, addresses and port numbers are in machine's native endian):

    magic	- must be set to QUERY_MAGIC,
    id		- query ID, copied literally to the response,
    type        - query type (must be QTYPE_FINGERPRINT)
    src_ad	- source address,
    dst_ad	- destination address,
    src_port	- source port,
    dst_port	- destination port.

  The response (p0f_response) is as follows:

    magic	- must be set to QUERY_MAGIC,
    id		- copied from the query,
    type	- RESP_OK, RESP_BADQUERY (error), RESP_NOMATCH (cache miss),
    genre[20]	- OS genre, zero length if no match,
    detail[40]	- OS version, zero length if no match,
    dist	- distance, -1 if unknown,
    link[30]	- link type description, zero length if unknown,
    tos[30]	- ToS information, zero length if unknown,
    fw,nat	- firewall and NAT flags, if spotted,
    real	- "real" OS versus userland stack,
    score	- masquerade score (or NO_SCORE), see next section,
    mflags	- exact masquerade flags (D_*), see next section.

  There's also a special type of queries, where type = QTYPE_STATUS,
  and subsequent fields are irrelevant (should be zero); this returns
  a different structure:
    magic	- must be set to QUERY_MAGIC,
    id		- copied from the query
    type	- must be set to RESP_STATUS (or RESP_BADQUERY on error)
    version[16]	- p0f version
    mode	- p0f mode (ASCII character, same as in command-line options)
    fp_cksum	- checksum of the fingerprint file for versioning purposes
    cache	- cache size
    packets	- total number of packets analyzed
    matched	- total number of OSes recognized
    queries	- total number of queries handled
    cmisses     - cache misses (for cache size debugging)
    uptime	- process uptime in seconds

  The connection is one-shot. Always send the query and recv the
  response immediately after connect - p0f handles the connection in
  a single thread, and you are blocking other applications (until 
  timeout, that is, the timeout is defined as two seconds in config.h).

  As of today, there is no way to integrate p0f with other programs
  as a packet-parsing library. It would be trivial to implement this,
  but there are no volunteers at the moment :-)

6. SQL database integration

  At the very moment, p0f does not feature built-in database connectivity,
  although I am looking for a willing contributor to take care of it.
  In the meantime, however, you may use p0f_db utility authored by
  Nerijus Krukauskas:

  Jonas Eckerman has some tools to make it easier to move p0f output
  from one system to another, and then to run basic visualization:

7. Masquerade detection

  Masquerade detection (-M) works by looking at the following factors for
  all known signatures that belong to real operating systems (and not 
  userland tools such as scanners):

    - Differences in OS fingerprints for the same IP:
        -3 if the same OS
        +4 if different signature for the same OS genre
        +6 if different OS genres
    - NAT and firewall flags set:
        +4 if NAT flags differ for the same signature
        +4 if fw flags differ for the same signature
        +1 per each NAT and fw flag if signatures differ (max. 4)

    - Link type differences:
        +4 if media type differs

    - Distance differences:
        +1 if host distance differs

    - Timestamp scoring, if timestamps available:
        -1 if timestamp delta within MAX_TIMEDIF (config.h)
        +1 if timestamp delta past MAX_TIMEDIF
        +2 if timestamp delta negative (!)

    - Time from the previous occurrence:
        /2 if more than half the cache size to the previous occurrence

  The final score is reported as score * 200 / 25 (25 being the highest
  score possible) and reported as a percentage.

  The higher the value, the more likely the result is accurate. Since
  the situation when all indicators are up is rather unrealistic, the
  multiplier is 200, not 100, and you can get over 100% match ;-)
  Everything above 0% should be looked at, over 20% is usually a sure

  You can configure the reporting of matches by setting the threshold
  to a value different than zero with -T switch. -T 10 might be a good
  idea. If you're looking at a local network, you can define 
  DIST_EXTRASCORE to score distance differences much higher - it is
  unlikely for a local LAN to shrink or grow, but it's not uncommon for
  routing over the Internet to change. If you are unhappy with the
  scoring algorithm and do not want to modify the sources, you can use
  -V option to report the status of every masquerade indicator. In
  conjunction with -l, -V can be used to grep for the precise set
  of signatures you're interested in. 
  Every hit is prefixed with ">> ". Combine -M, -K and -U to report
  masquerade hits only (but it is recommended to still dump packets 
  with -w to be able to examine the evidence later on). A good

    p0f -M -K -U -w evidence.bin -c 500 -l -V 'not src host my_ip'
  A quick demo: - OpenBSD 3.0-3.4 (up: 836 hrs) 
    -> (distance 6, link: GPRS or FreeS/WAN) - Linux 2.4/2.6 (NAT!) (up: 173 hrs) 
    -> (distance 6, link: GPRS or FreeS/WAN) - Windows XP Pro SP1, 2000 SP3 (NAT!) 
    -> (distance 6, link: GPRS or FreeS/WAN)

  >> Masquerade at indicators at 69%.

  That was quite evident. - Windows 2000 SP2+, XP SP1 
    -> (distance 10, link: ethernet/modem) - Windows 2000 SP4, XP SP1, patched 98 
    -> (distance 12, link: ethernet/modem)

  >> Masquerade at indicators at 43%.

  The host has a name of, so once again, a good hit.

  Verbose output looks like this:

  >> Masquerade at indicators at 26%.
     Flags: OS -far

  In this case, we have two different OSes (OS), but the time between two
  occurrences is long enough to lower the score (-far). All -V flags are:

    OS		- different OS genres
    VER		- different OS versions
    LINK	- link type difference
    DIST	- distance differences
    xNAT	- NAT flags differ (same OS match)
    xFW		- FW flags differ (same OS match)
    NAT1, NAT2  - NAT flags set (different OSes)
    FW1, FW2	- FW flags set (different OSes)
    FAST	- timestamp delta too high
    TNEG        - timestamp delta negative
    -time	- timestamp delta within the norm
    -far	- distant occurrences

  Because the score is cumulative, it is possible to have mutually exclusive
  flags set (e.g xNAT and NAT1) whenever more than two signatures were taken
  into account when calculating the score. 

  Masquerade status and flags can be also retrieved via the query interface, 
  as noted in the section above.

  The functionality depends on keeping the fingerprint database clean and
  prefixing non-OS fingerprints (nmap, other scanner tools,
  application-induced TCP/IP stack behavior) with - prefix. Those
  fingerprints, as well as all the UNKNOWNs, are not used for masquerade 

  Note that a single host can be reported many times. The system reports
  immediately, but later on, the host might score higher once new data 
  arrives, and p0f will post a "correction" with a new, higher ranking.
  Use the highest result for a specific host, but also observe the 
  consistency of subsequent results.
  The solution uses a cyclic buffer also used in -Q mode (and affected by
  -c parameter). You should set the value to cache not more than an
  hour of traffic (and no less than a minute). Calculate the number of
  connections on average per the interval of time you wish to cache,
  then pass the value to p0f with -c.

  Setting -c too high will result in false positives for dial-up nodes or
  multiboot systems (of course, you sometimes want to detect the latter, 
  too). Setting it too low may miss some cases.

  The code detects NAT devices that do not rewrite packets (almost
  all packet firewalls). Ones that do rewrite packets (proxy firewalls)
  can, on the other hand, be detected by their own signatures.

  Masquerade detection will fail if all systems masqueraded have an
  identical configuration and network setup, uptimes and network usage
  (which is very unlikely, even in a homogeneous environment). A
  prerequisite for detection is that the systems are used at (roughly) the 
  same time, within the cache time frame.

  NOTE: The detector is most reliable and sensitive in the default (SYN) mode,
  and scores are adjusted to work well there; in other fingerprinting modes, 
  your mileage may vary. You can try to combine -M with -A (masquerade 
  detection on systems you connect to), which is only really useful for 
  detecting load balancers and other setups that map a single address to 
  several servers; or with -R, which can be used both for detecting load 
  balancers (RST) and normal incoming masquerade detection (RST+ACK), 
  although it's naturally less reliable and sensitive. Using -M with -O is 
  weird, but regrettably not prosecuted.

8. Fingerprinting accuracy and precision

  Version 2 uses some more interesting TCP/IP packet metrics, and should
  be inherently more accurate and precise. We also try to use common sense
  when adding and importing signatures, which should be a great
  reliability boost. More obscure modes, such as RST+ or stray ACK, may 
  and will be inherently less accurate or reliable - see section 10 for 
  more details - but are still far more sane than p0f v1.

  Link type identification is not particularly reliable, as some users tend
  to mess with their default MTUs for better (or worse ;-) performance.
  For most systems, it will be accurate, but if you see an unlikely value 
  reported, just deal with it.

  Uptime detection is also of an amusement value. Some newly released
  systems tend to multiply timestamp data by 10 or have other clocking
  algorithms. The current version of p0f does not support those differences
  over the entire database. I will try to fix it, until then, those boxes 
  would have an artificially high uptime.

  NAT detection is merely an indication of MSS being tweaked at some point.
  Most likely, the reason for this is indeed a NATing router, but there
  are some other explanations. Linux, for example, tends to mix up MTUs
  from different interfaces in certain scenarios (when, I'm not sure, but
  it's common and is probably a bug), and if you see a Linux box tagged as 
  "NAT", it does not have to be NATed - it might simply have two network 
  interfaces. P0f can still be a useful NAT detection tool (you can examine
  changing distances and OS matches for a specific host, too), simply don't
  rely on this flag alone.

  If you see link type identified as unknown-XXXX, try to Google for
  "mtu XXXX". If you find something reasonable, you might want update
  mtu.h and recompile p0f, and submit this information to me. Keep in
  mind some MTU settings are just arbitrary and do not have to mean a

  P0f also tries to recognize some less popular combinations of precedence 
  bits, type of service and so-called "must be zero" bit in TCP headers to
  detect certain origin ISPs. Many DSL and cable operators, particularly
  in Europe, tend to configure their routers in fairly unique ways in
  this regard. This, again, is purely of an amusement value. See tos.h
  for more information.

  P0f will never be as precise as NMAP, simply because it has to rely
  on what the host sends by itself, and can't check how it responds to
  "invalid" or tweaked packets. On the other hand, in the times of
  omnipresent personal and not quite personal firewalls and such,
  p0f can often help where NMAP is confused.

  Just like with any fingerprinting utility, active or passive, it is
  possible to change TCP/IP stack settings to either avoid identification,
  or appear as some other system - although some of the changes might
  require kernel-space hacking. There are no publicly available
  anti-p0f tools yet, although I expect them to appear at some point.

9. Adding signatures

  To avoid decreasing reliability of the database, you MUST read the 
  information provided at the beginning of p0f.fp carefully before touching 
  it in any way! If you are fiddling with p0fa.fp, p0fr.fp or p0fo.fp, read 
  all comments in those files IN ADDITION to the contents of p0f.fp. Those
  files provide a good technical primer, and document the format and
  subtleties of all the fingerprints.

  If you stumble upon a new signature, do consider submitting it to,, or connecting from the system to We will be happy to incorporate
  this signature in the official release, and can help you make your
  signature more accurate. The least popular the system is, the more valuable
  the signature; we have the mainstream covered quite well.

  Be sure to run p0f -C after making any additions. This will run a collision 
  checker and warn about shadowed or possibly incorrect signatures. This 
  happens more often than you'd think. The same applies to p0fa.fp, p0fr.fp 
  and p0fo.fp files. You need to run p0f -A -C, p0f -R -C or p0f -O -C to 
  verify their contents.

  Rest assured, you will sooner or later find something really surprising. You
  can look at tmp/ to see a current list of mysteries I've stumbled upon. The 
  museum at lists some other funky cases.
  By all means, I'd like to hear about other UFO sightings!

10. Security

  Running p0f as a daemon should pose a fairly low risk, compared to tcpdump 
  or other elaborate packet parsers (Ettercap, Ethereal, etc). P0f does not
  attempt anything stupid, such as parsing tricky high-level protocols. There 
  is a slight risk I screwed up something with the option parser or such, but 
  this code should be very easy to audit. If you do not feel too comfortable, 
  you can always use the -u option, which should mitigate the risk.

  General security precautions for operating p0f: 

  - Do not make p0f setuid, setgid or otherwise privileged when the caller
    isn't. Running it via sudo for users you do not trust entirely is also a 
    so-so idea.

  - Do not use -r option unless absolutely necessary, and only for short
    and supervised runs. The option introduces a bloated, potentially flawed
    libc DNS handling code, and has a DoS potential.

  - When running in -Q mode, you need to make sure, either by setting umask or 
    calling chmod/chown after launching p0f, to set correct permissions on the
    query socket - that is, unless you don't see a problem with your users 
    querying p0f, which isn't a great threat to the humanity.

  - Do not use world-writable directories for keeping the socket. Do not
    use world-writable directories for output files or configuration. Come
    to think about it, don't use world-writable directories for any purpose.

  - Don't panic.

11. Limitations

  There are several generic and some specific limitations as to what 
  passive fingerprinting and p0f can achieve.

  Proxy firewalls and other high-level proxy devices are not transparent
  to any TCP-level fingerprinting software. The device itself will be
  fingerprinted, not actual source hosts. There is some software that
  lets you perform application fingerprinting, this isn't it.

  Some packet firewalls configured to normalize outgoing traffic (OpenBSD pf 
  with "scrub" enabled, for example) will, well, normalize packets. Those
  signatures will not correspond to the originating system, and probably not 
  quite to the firewall either. Checkpoint firewall, in a fairly lame attempt
  to defeat OS fingerprinting, tweaks IP ID and TTL on outgoing packets; if
  you want to work around this problem, run p0f with -F option.

  In default mode, in order to obtain the information required for 
  fingerprinting, you have to receive at least one SYN packet initiating a 
  TCP connection to your machine or network. Note: you don't have to respond 
  to this particular SYN, and it's perfectly fine to respond with RST.
  For SYN+ACK fingerprinting, you must be able to connect to at least one
  open port on the target machine to actually get SYN+ACK packet. You 
  do not need any other ports, or the ability to send awkward, multiple
  or otherwise suspicious packets to the remote host (unlike with NMAP).
  Also note that SYN+ACK fingerprints are somewhat affected by the initial
  SYN on some systems.

  If you cannot establish a connection, but the remote party at least
  sends you RST+ACK back ("Connection refused"), you can use RST+ mode of 
  p0f (-R option), but be aware this mode is inherently less accurate
  and reliable, mostly because systems usually don't bother with
  putting any options in those packets, and they all look very similar.

  SYN+ACK fingerprinting is considered (by me) to be less accurate and
  sometimes dependent on the system that initiates the connection. Same goes
  for (again, experimental!) stray ACK fingerprinting. RST+ fingerprinting
  mode, on the other hand, is fairly reliable, but far less precise.
  This is why I put stress on developing the SYN fingerprinting capability - 
  but SYN+ACK, RST+ and stray ACK database contributions and tricks are 
  of course very welcome.
  Fingerprinting on a fully established (existing) TCP connection is now
  supported by p0f (since version 2.0.5), but the database contains very
  few entries, and the accuracy and applicability of this mode is not yet
  well established. Be prepared for this mode to produce excessive amounts
  of logs.
  What I'll be trying to do is to integrate a number of fingerprinting
  techniques, currently completely separate (SYN, SYN+ACK, ACK, FIN,
  RST, retransmission timing, etc) into a single solution for very
  high accuracy. But this is perhaps p0f 3.0.

12. Is it better than other software?

  Depends on what you need. As I said before, p0f is fast, lightweight,
  low-profile. It can be integrated with other services. It has a clean and 
  simple code, runs as a single thread and uses very little CPU power, works 
  on a number of systems (Linux, BSD, Solaris and probably others), has a 
  pretty detailed and accurate fingerprint database. Quite frankly, I
  doubt there is a program that offers better overall functionality or
  accuracy when it comes to passive fingerprinting, but I would not be
  surprised to be proved wrong one day. In other words, feel free to
  explore alternatives.

  Of the ones I know... is it better than Siphon? Yes. Ettercap? Yes, 
  version 2 is better than v1-derived fingerprinting in Ettercap. Besides, 
  it's simply different, and intended for a different range of applications.

  Version 1 of p0f did implement many novel fingerprinting metrics that were
  later incorporated in other software, but so did version 2 - and others are
  yet to catch up.

  As to other "current" utilities, you can use masqdet by Wojtek Kaniewski 
  as an alternative to p0f -M mode. On the web, you can also stumble upon 
  "n0t" and "natdet" utilities authored by a guy going by the nickname
  r3b00t, but these are just dumbed-down and inherently less reliable rip-offs
  closely inspired on p0f code. Your mileage may vary, but I recommend
  you to avoid them: they won't work any better.

13. Program no work!

  Whoops. We apologize. P0f requires the following to compile and
  run fine:

    - libpcap 0.4 or newer
    - GNU C Compiler 2.7.x or newer 
    - GNU make 3.7x or newer, or BSD make
    - GNU bash / awk / grep / sed / textutils (for p0frep only)

  For the Windows port requirements and instructions, please read 
  INSTALL.Win32 file.

  Not every platform is supported by p0f, and compilation problems do
  happen. Please let us know if you have any problems (or, better yet,
  managed to find a solution).

  If you find a system that is either not recognized, or is fingerprinted
  incorrectly, please do not downplay this and let us know.

  Platforms known to be working fine (regression tests not done on
  a regular basis, though):

    - NetBSD 
    - FreeBSD 
    - OpenBSD
    - MacOS X 
    - Linux (2.0 and up)
    - Solaris (2.6 and up)
    - Windows (see INSTALL.Win32)
    - AIX (you need precompiled BULL libpcap)

  If p0f compiles and runs, but displays "unknown datalink" or 
  "bad header_len" warnings, it is likely that your network interface type
  is not (yet) recognized. Let us know, it is easy to fix that once and
  for all users.

14. Links to OS fingerprinting resources

  Recommended RFC reading:  - TCP/IP specification - TCP/IP tutorial - performance extensions - T/TCP extensions - TCP/IP selective ACK

  Practical information:

    Active ICMP fingerprinting:

    Passive OS fingerprinting basics:

    THC Amap, application fingerprinting:

    Hmap, web server fingerprinting:

    Fyodor's NMAP, the active fingerprinter:

    User-Agent information:

    Ident fingerprinting:

  Other free tools known to have passive OS fingerprinting:

    P0f-based: - Ettercap (p0f v1) - Prelude IDS (p0f v1) - OpenBSD pf (p0f v2.0.1) - Linux netfilter (p0f v2.0) - Vern Paxson's / Holger Dreger's NIDS (p0f v2.0) - n0t and natdet (ripped off, AFAICT)

    Independent codebase: - pfprintd - Siphon (very out of date) (Siphon w/GUI) - masqdet (NAT detection only)