Provided by: mailavenger_0.8.5-1_amd64 bug


       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