Please note that Michal is now actively developing version 2 of p0f; please look at http://lcamtuf.coredump.cx/p0f.shtml for more information and downloads. I have rpms of this branch below.
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--=-- p0f 2 --=-- "Dr. Jekyll had something to Hyde" passive OS fingerprinting tool version 2.0.4 (C) Copyright 2000 - 2004 by Michal Zalewski <firstname.lastname@example.org> Various ports (C) Copyright 2003 - 2004 by: Michael A. Davis <email@example.com> Kirby Kuehl <firstname.lastname@example.org> Kevin Currie <email@example.com> Portions contributed by numerous good people - see CREDITS file. http://lcamtuf.coredump.cx/p0f.shtml ********************************************************************* **** HELP WITH P0F DATABASE: http://lcamtuf.coredump.cx/p0f-help **** ********************************************************************* ----------- 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 6) Masquerade detection 7) Fingerprinting accuracy and precision 8) Adding signatures 9) Security 10) Limitations 11) Is it better than other software? 12) Program no work! 13) Appendix A: Exact output format 14) Appendix B: 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, connecs 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 detected. 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. To accomplish the job, p0f equips you with three 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. P0f was the first (and I believe remains the best) fully-fledged implementation of the passive fingerprinting technique. The current version uses a number of detailed metrics, often invented specifically for p0f, and achieves a very high level of accuracy and detail, 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 solutions. 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 point. ------------------------------ 2. Why would I want to use it? ------------------------------ Oh, a number of uses come to mind: - Profiling / espionage - run 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. - 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 or RST+ 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. - 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 contributes 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: NEW CORE CHECKS: - Option layout and count check, - EOL presence and trailing data [*], - Unrecognized options 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 auxilinary 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. IMPROVEMENTS: - Major performance boost - no more runtime signature parsing, added BPF pre-filtering, signature hash lookups - to make p0f suitable for running 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 form, - 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 and RST+ 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). 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 ] [ -w file ] [ -u user ] [ -c size ] [ -T nn ] [ -FNDVUKAXMqxtpdlRL ] [ '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. On some newer systems you might be able to specify 'any' to listen on all devices, but don't rely on this. -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). -w file - writes matching packets to a tcpdump snapshot; useful when you need to save the traffic in case it has to be verified or reviewed later on. Also useful if you encounter any parser bugs - data is being written prior to parsing. -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. You 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 events the usual way, but you can use -qKU to suppress any text output. Also see -c notes. From a shell script, you can query p0f using the p0fq tool provided in test/ subdirectory. 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. -c size - cache size for -Q and -M options. The default is 128, which is sane for a system with a moderate load (under 10 connections per second or such). 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 - chroot to this user's home directory after reading configuration data and binding to sockets, then 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 - do not report distances and link media. This option 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+ modes. -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. -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 only useful for fingerprint gathering. -q - be quiet - do not display banners. -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 administrators. -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 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, and still looks for a maintainer. -R - go into RST+ACK/RST mode. This option will fingerprint several different types of traffic, most importantly "connection refused" and "timeout" messages. It is similar to SYN+ACK mode, except that the program will now look for p0fr.fp. The mode is also called RST+. Please refer to p0fr.fp before using it. -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 to verify this revDNS result. Do not rely on the name alone. -C - perform collision check on signatures prior to running. This is an essential option whenever you add new signatures to the p0f.fp file, 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 masuqerade detection algotihm. 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. -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 value. 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 10.0.0.0 mask 255.0.0.0' 'dst port 80 and ( src host 188.8.131.52 or src host 184.108.40.206 )' 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). You cannot make the rule any broader, the optional filter expression can only narrow it down. You also can 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. In other cases, 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, 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 - OS genre, zero length if no match, detail - OS version, zero length if no match, dist - distance, -1 if unknown, link - link type description, zero length if unknown, tos - 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. 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). --------------------------- 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: http://nk.puslapiai.lt/projects/p0f_db/ Jonas Eckerman has some tools to make it easier to move p0f output from one system to another, and then to run basic visualization: http://whatever.frukt.org/p0f-stats.shtml ----------------------- 7. Masquerade detection ----------------------- Masquerade detection (-M) works by looking at the following factors for all known signatures that belong to operating systems (and not userland tools like 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 occurence: /2 if more than half the cache size to the previous occurence 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 bet. 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 example: p0f -M -K -U -w evidence.bin -c 500 -l -V 'not src host my_ip' A quick demo: 220.127.116.11:20908 - OpenBSD 3.0-3.4 (up: 836 hrs) -> 18.104.22.168:80 (distance 6, link: GPRS or FreeS/WAN) 22.214.171.124:21154 - Linux 2.4/2.6 (NAT!) (up: 173 hrs) -> 126.96.36.199:80 (distance 6, link: GPRS or FreeS/WAN) 188.8.131.52:22003 - Windows XP Pro SP1, 2000 SP3 (NAT!) -> 184.108.40.206:80 (distance 6, link: GPRS or FreeS/WAN) >> Masquerade at 220.127.116.11: indicators at 69%. That was quite evident. 18.104.22.168:49030 - Windows 2000 SP2+, XP SP1 -> 22.214.171.124:80 (distance 10, link: ethernet/modem) 126.96.36.199:52942 - Windows 2000 SP4, XP SP1, patched 98 -> 188.8.131.52:80 (distance 12, link: ethernet/modem) >> Masquerade at 184.108.40.206: indicators at 43%. The host has a name of gateway.vlt.se, so once again, a good hit. Verbose output looks like this: >> Masquerade at 220.127.116.11/crawlers.looksmart.com: indicators at 26%. Flags: OS -far In this case, we have two different OSes (OS), but the time between two occurences is long enough to lowe 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 (when signature) xFW - FW flags differ (same signature) NAT1, NAT2 - NAT flags set (different signatures) FW1, FW2 - FW flags set (different signatures) FAST - timestamp delta too high TNEG - timestamp delta negative -time - timestamp delta within the norm -far - distant occurences Because the score is cummulative, it is possible to have exclusive flags set (e.g xNAT and NAT1) if there's a higher number of signatures for a specific IP in the cache. Masquerade information 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 detection. Note that a single host can be reported many times. The system reports immediately, but later on, the host might score a different result once new data arrives. 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, multiboot systems, etc. 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 can be detected by their own signatures, though. Masquerade detection will fail if all systems masqueraded have an identical configuration and network setup (which is very unlikely, even in a homogenous environment), or are never used at (roughly) the same time. NOTE: You can also try to combine -M with -A, 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 probably less reliable. ---------------------------------------- 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 realiability boost. More obscure modes, such as RST+, may be inherently less accurate or reliable - see section 10 for more details. NOTE: 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! 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 artifically 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 thing. 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. -------------------- 9. Adding signatures -------------------- Please refer to p0f.fp for more information about signatures and their format. You NEED to read this information carefully before changing or adding anything. Do consider submitting your signature to firstname.lastname@example.org, email@example.com, or connecting from the system to http://lcamtuf.coredump.cx/p0f-help/. We will be happy to incorporate this signature in the official release, and can help you make your signature more accurate. 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 and p0fr.fp files. You need to run p0f -A -C and p0f -R -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 http://lcamtuf.coredump.cx/mobp/ 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, 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 easy to audit. If you do not feel to comfortable, you can always use the -u option, which should mitigate the risk signigicantly. 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. The biggest threat is using -r option carelessly, as it enables the entire name resolution overhead, and introducing a potentially vulnerable DNS handling code, not to mention DoS potential. Other than that, 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. --------------- 11. Limitations --------------- 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 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. This is why I put stress on developing the SYN fingerprinting capability - but SYN+ACK database contributions and techniques are of course very welcome. Fingerprinting on a fully established (existing) TCP connection is currently not supported by p0f, because it is inherently less reliable, and seldom of any use. You can, however, implement this rather easily by tweaking the filter expression hardcoded in p0f.c and coming up with a new fingerprint database. 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 proven wrong one day. In other words, feel free to explore other 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 "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 cc 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. ----------------------- 14. Exact output format ----------------------- Following is a brief description of the output format of p0f v2. While it is recommended to use -Q mode for service integration, it might be still necessary to perform log post-processing, etc. Every line starts with an optional timestamp (enabled by -o or -t options). The format is as follows:
The format is essentially the standard Unix date format. The next field is mandatory and specifies the source host: ip_address[/name]:port The /name part is appended only if -r option is provided, and there was a DNS record for this particular IP. Following is a ' - ' separator, and the name of the system with any additional remarks: System_name [ version (notes) ] The latter part is supressed by -D option. Notes usually describe signature number or other specifics. Then, there is a number of optional notes: (NAT!) - the system is behind MSS-tweaking NAT (NAT2!) - the system is behind MTU-tweaking NAT (ECN) - ECN enabled on the remote system (firewall!) - the system has DF cleared \[Name\] - provider name identified as..., OR \[tos XX\] - ToS set, but provider name not identified \[GENERIC\] - signature is a generic one \[FUZZY\] - fuzzy match \* - further details inhibited for this signature (up nnn hrs) - system uptime (refused) - RST+ mode: valid conn. refused packet (dropped) - RST+ mode: valid conn. dropped packet (invalid-NN) - RST+ mode: invalid RST packet (see p0fr.fp) If the signature is generic, or if -S option was used, the next line is: Signature: \[signature_data\] ...where signature_data follows the standard p0f.fp notation. If the system was not regognized, the format of the OS line is slightly different: UNKNOWN \[signature_data\] Where signature data follows the standard p0f.fp notation. All options except for GENERIC, * and (firewall!) can still follow the entry. Signature is not repeated in the next line regardless of -S. Unless -N is in effect, the next line is as follows: -> dst_ip:port ([distance NN, ]link: description) ...the distance part is present only when the signature was recognized properly. Link description is the name of the link type, or a special keyword "unspecified", or a special value "unknown-NNNN". The next line is present if -X mode is enabled and the packet has a payload. The line has the following format: # Payload: "...printable text..." Maximum length of the text to report is defined in config.h (PKT_MAXPAY). In -l mode, all line breaks are inhibited. If -x is enabled (not compatible with -l), the subsequent lines are in this format: \[XX\] nn nn nn nn .... | AAAA... Where XX is a hex address, nn is a hex sequence of byte values, and AAAA is human-readable contents of the packet. Additionally, when -M option is in effect, the data described in section 7 may be displayed. ---------------------------------------- 15. Links to OS fingerprinting resources ---------------------------------------- Recommended RFC reading: http://www.faqs.org/rfcs/rfc793.html - TCP/IP specification http://www.faqs.org/rfcs/rfc1122.html - TCP/IP tutorial http://www.faqs.org/rfcs/rfc1323.html - performance extensions http://www.faqs.org/rfcs/rfc1644.html - T/TCP extensions http://www.faqs.org/rfcs/rfc2018.html - TCP/IP selective ACK Practical information: Active ICMP fingerprinting: http://www.sys-security.com/html/papers.html Passive OS fingerprinting basics: http://project.honeynet.org/papers/finger/ http://www.linuxjournal.com/article.php?sid=4750 THC Amap, application fingerprinting: http://www.thc.org/releases.php Hmap, web server fingerprinting: http://wwwcsif.cs.ucdavis.edu/~leed/hmap/ Fyodor's NMAP, the active fingerprinter: http://www.nmap.org User-Agent information: http://www.siteware.ch/webresources/useragents/db.html Ident fingerprinting: http://www.team-teso.net/data/ldistfp-auth-fingerprints Other free tools known to have passive OS fingerprinting: http://ettercap.sourceforge.net/ - Ettercap (p0f v1) http://prelude-ids.org - Prelude IDS (p0f v1) http://www.w4g.org/fingerprinting.html - OpenBSD pf (p0f v2.0.1) http://cvs.netfilter.org/~checkout~/netfilter/patch-o-matic//base/osf.patch - Linux netfilter http://www-nrg.ee.lbl.gov/bro.html - Vern Paxson's / Holger Dreger's NIDS (p0f 2.0) http://www.raisdorf.net/projects/pfprintd - pfprintd http://siphon.datanerds.net - Siphon (very out of date) http://members.fortunecity.com/sektorsecurity/projects/archaeopteryx.html (Siphon w/GUI) http://r3b00t.itsec.pl/ - n0t and natdet (ripped off, AFAIC) http://toxygen.net/misc/ - masqdet (NAT detection only)
Bente Petersen wrote an article on p0f at:http://www.sans.org/resources/idfaq/p0f.php.
My sincere thanks to Marcus Ranum, security guru, for writing an article about p0f for Information Security Magazine. The article can be found at:http://www.infosecuritymag.com/2003/jun/cooltools.shtml.
Instructions for using p0f on windows 1) Download Cygwin The cygwin environment emulates a UNIX-like environment on tp of your windows machine. It is available for free from: http://www.cygwin.com/ (Bill adds: For reference, cygwin1.dll is in this directory for those that want it; it can be downloaded from ftp://mirror.mcs.anl.gov/pub/cygwin/release/cygwin in the most recent cygwin....tar.gz package) 2) Download WinPcap p0f also needs the WinPcap drivers installed on the system. These are also free and are available from http://winpcap.polito.it/ Once these are installed, make sure to place p0f.fp in your /etc directory in the cygwin environment or in the current directory and it should work fine. Thanks to Chris Connelly for getting it compile in the cygwin environment. If you have any trouble or have questions, feel free to contact him at firstname.lastname@example.org (Bill: As a side note, there was a (non-cygwin) port done in January 2001. If anyone is interested in updating the port to current p0f, please take a look at http://datanerds.net/~mike/dev/ . Many thanks to Michael Davis of Datanerds for the work.)
Name : p0f Relocations: (not relocatable) Version : 2.0.4 Vendor: Michal Zalewski <email@example.com> Release : 0 Build Date: Thu Jul 15 14:48:24 2004 Install Date: (not installed) Build Host: sparrow Group : Applications/Networking Source RPM: p0f-2.0.4-0.src.rpm Size : 195665 License: LGPL Signature : RSA/MD5, Thu Jul 15 14:48:24 2004, Key ID 012334cbf322929d Packager : William Stearns <firstname.lastname@example.org> URL : http://www.stearns.org/p0f/ Summary : passive OS fingerprinting tool Description : p0f performs passive OS fingerprinting technique bases on information coming from a remote host when it establishes a connection to our system. Captured packets contains enough information to determine OS - and, unlike active scanners (nmap, queSO) - it is done without sending anything to this host.
The files in this collection are part of William Stearns' software archive. If any of the links on this page do not work, you may be viewing an incomplete mirror. There is a complete list of the mirror sites at the starting page for this mirror and at the primary mirror.