Doubt regarding TCP Options negotiation Post: 302506104

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Special Forums IP Networking Doubt regarding TCP Options negotiation Post 302506104 by jake24 on Friday 18th of March 2011 01:39:36 PM

03-18-2011

Registered User

Quote:

Originally Posted by fpmurphy

Yes, renegotiation is possible but is often turned off for security reasons.

Thanks fpmurphy.

Could you elaborate more on this.
Once a connection is established, how would the re-negotiation be accomplished?
TCP implementations would be using special options meant only for use in the three-way handshake to turn on the corresponding feature.
E.g -
To enable Selective Acknowledgment, the option sent in the SYN segments is SACK PERMITTED. This, if successfully negotiated translates to SACK blocks in non-SYN segments.
Similarly, the usage of Big Window option requires special Big Window permitted option in the SYN segments. If successfully negotiated this translates to the actual Big Window option.

So can you, if possible explain to me as to how would options be turned off during the connection lifetime (in the non-SYN segments)

jake24

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synos

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)