05-19-2011
Maybe it has an MTU of less. Obsolete IP option 12 can send this during connect without the ICMP of PATH MTU Discovery.
IP, Internet Protocol
The MSS is the total remote buffer, the MTU is the packet max. Disregarding the chance of TCP options, a packet header is 20 IP + 20 TCP, so a packet sends MTU - 40, and the number of packets sent before ack is MSS/(MTU-40) including any partial. Since this is local, the ack round trip should be very short, so packets should not be MSS trimmed unless there is blocking downstream.
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synos(1) Mail Avenger 0.8.3 synos(1)
NAME
synos - guess operating system from TCP SYN fingerprint
SYNOPSIS
synos [--mtu mtu] [--db path] syn-fingerprint
DESCRIPTION
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).
OPTIONS
--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.
FILES
/usr/local/share/pf.os
Default location of SYN fingerprint database.
SEE ALSO
avenger(1), asmtpd(8)
The Mail Avenger home page: <http://www.mailavenger.org/>.
The OpenBSD home page: <http://www.openbsd.org/>.
BUGS
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.
AUTHOR
David Mazieres
Mail Avenger 0.8.3 2012-04-05 synos(1)