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Nmap Network Scanning

Community Contributions

No matter how technically advanced a service detection framework is, it would be nearly useless without a comprehensive database of services against which to match. This is where the open source nature of Nmap really shines. The Insecure.Org lab is pretty substantial by geek standards, but it can never hope to run more than a tiny percentage of machine types and services that are out there. Fortunately experience with OS detection fingerprints has shown that Nmap users together run all of the common stuff, plus a staggering array of bizarre equipment as well. The Nmap OS fingerprint database contains more than a thousand entries, including all sorts of switches, WAPs, VoIP phones, game consoles, Unix boxes, Windows hosts, printers, routers, PDAs, firewalls, etc. Version detection also supports user submissions. Nmap users have contributed thousands of services. There are three primary ways that the Nmap community helps to make this an exceptional database: submitting service fingerprints, database corrections, and new probes.

Submit Service Fingerprints

If a service responds to one or more of Nmap's probes and yet Nmap is unable to identify that service, Nmap prints a service fingerprint like this one:

SF-Port21-TCP:V=3.40PVT16%D=9/6%Time=3F5A961C%r(NULL,3F,"220\x20stage\x20F
SF:TP\x20server\x20\(Version\x202\.1WU\(1\)\+SCO-2\.6\.1\+-sec\)\x20ready\
SF:.\r\n")%r(GenericLines,81,"220\x20stage\x20FTP\x20server\x20\(Version\x
SF:202\.1WU\(1\)\+SCO-2\.6\.1\+-sec\)\x20ready\.\r\n500\x20'':\x20command\
SF:x20not\x20understood\.\r\n500\x20'':\x20command\x20not\x20understood\.\
SF:r\n");

If you receive such a fingerprint, and are sure you know what daemon version is running on the target host, please submit the fingerprint at the URL Nmap gives you. The whole submission process is anonymous (unless you choose to provide identifying info) and should not take more than a couple minutes. If you are feeling particularly helpful, scan the system again using -d (Nmap sometimes gives longer fingerprints that way) and paste both fingerprints into the fingerprint box on the submission form. Sometimes people read the file format section and submit their own working match lines. This is OK, but please submit the service fingerprint(s) as well because existing scripts make integrating and testing them relatively easy.

For those who care, the information in the fingerprint above is port number (21), protocol (TCP), Nmap version (3.40PVT16), date (September 6), Unix time in hex, and a sequence of probe responses in the form r({<probename>}, {<responselength>}, "{<responsestring>}").

Submit Database Corrections

This is another easy way to help improve the database. When integrating a service fingerprint submitted for chargen on Windows XP or FooBar FTP server 3.9.213, it is difficult to determine how general the match is. Will it also match chargen on Solaris or FooBar FTP 2.7? Since there is no good way to tell, a very specific name is used in the hope that people will report when the match needs to be generalized. The only reason the Nmap DB is so comprehensive is that thousands of users have spent a few minutes each to submit new information. If you scan a host and the service fingerprint gives an incorrect OS, version number, application name, or even service type, please let us know as described below:

Upgrade to the latest Nmap (Optional)

Many Linux distributions and other operating systems ship with ancient versions of Nmap. The Nmap version detection database is improved with almost every release, so check your version number by running nmap -V and then compare that to the latest available from http://nmap.org/download.html. The problem you are seeing may have already been corrected. Installing the newest version takes only a few minutes on most platforms, and is valuable regardless of whether the version detection flaw you are reporting still exists. But even if you don't have time to upgrade right now, submissions from older releases are still valuable.

Be absolutely certain you know what is running

Invalid corrections can corrupt the version detection DB. If you aren't certain exactly what is running on the remote machine, please find out before submitting.

Generate a fingerprint

Run the command nmap -O -Pn -sSV -T4 -d --version-trace -p<port> <target>, where <port> is the port running the misidentified service on the <target> host. If the service is UDP rather than TCP, substitute -sUV for -sSV.

Send us your correction

Now simply submit your correction to us at http://insecure.org/cgi-bin/submit.cgi?corr-service. Thanks for contributing to the Nmap community and helping to make version detection even better!

Submit New Probes

Suppose Nmap fails to detect a service. If it received a response to any probes at all, it should provide a fingerprint that can be submitted as described above. But what if there is no response and thus a fingerprint is not available? Create and submit your own probe! These are very welcome. The following steps describe the process.

Steps for creating a new version detection probe

  1. Download the latest version of Nmap from http://nmap.org and try again. You would feel a bit silly spending time developing a new probe just to find out that it has already been added. Make sure no fingerprint is available, as it is better to recognize services using existing probes if possible than to create too many new ones. If the service does not respond to any of the existing probes, there is no other choice.

  2. Decide on a good probe string for recognizing the service. An ideal probe should elicit a response from as many instances of the service as possible, and ideally the responses should be unique enough to differentiate between them. This step is easiest if you understand the protocol very well, so consider reading the relevant RFCs and product documentation. One simple approach is to simply start a client for the given service and watch what initial handshaking is done by sniffing the network with Wireshark or tcpdump, or connecting to a listening Ncat.

  3. Once you have decided on the proper string, add the appropriate new Probe line to Nmap (see the section called “Technique Described” and the section called “nmap-service-probes File Format”). Do not put in any match lines at first, although a ports directive to make this new test go first against the registered ports is OK. Then scan the service with Nmap a few times. You should get a fingerprint back showing the service's response to your new probe. Send the new probe line and the fingerprints (against different machines if possible, but even a few against the same daemon helps to note differences) to the Nmap development list at . It will likely then be integrated into future versions of Nmap. Any details you can provide on the nature of your probe string is helpful as well. For custom services that only appear on your network, it is better to simply add them to your own nmap-service-probes rather than the global Nmap.

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