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       synos - guess operating system from TCP SYN fingerprint


       synos [--mtu mtu] [--db path] syn-fingerprint


       synos takes a SYN fingerprint, in the format described for the
       CLIENT_SYNFP environment variable in the avenger(1) man page, and
       outputs a guess as to the type of the client operating system.  synos
       makes use of the OpenBSD SYN fingerprint database (which is also
       repackaged with Mail Avenger).

       --mtu val
           Certain operating systems set the initial TCP window size based on
           the maximum transmission unit, or MTU, of the network.  For such
           operating systems, synos usually checks the window size using both
           the client’s MSS option plus 40 bytes (for TCP and IP headers), or
           a hard-coded MTU, which defaults to 1,500 bytes.  If either value
           works, the fingerprint is considered to match the operating system.
           You can change the value 1,500 by specifying this option.  A value
           of 0 tells synos to use only the value derived from the MSS option.

       --db file
           Specifies an alternate location for the SYN fingerprint database.


           Default location of SYN fingerprint database.


       avenger(1), asmtpd(8)

       The Mail Avenger home page: <>.

       The OpenBSD home page: <>.


       The operating system type is determined by heuristics that are not
       always reliable.  Moreover, not all operating systems can be
       distinguished.  The database may not even contain a client’s particular
       operating system and version.

       It is not hard to fool synos deliberately by changing TCP socket
       options or injecting raw packets onto the network.


       David Mazieres