Sequential IP id's (fwd)

[Haroon Meer <haroon () sensepost com>]

Hi

A while back antirez, in a post to Bugtraq, detailed a new Tcp portscan
method.

This method allows one to portscan a host, using spoofed packets, while
remaining totally invisible to the scanned host < almost as if u had a 6th
sense ;) >. 

The details of the scan (almost totally stolen from antirez's original
post) works as follows...

(A) When an open  tcp port recieves a SYN, it replies with a SYN|ACK
    When a closed tcp port recieves a SYN, it replies with a RST|ACK

(B) When a host recieves an unknown SYN|ACK, it replies with a RST
    When a host recieves an unknown RST,     it replies with nothing

(C) You can tell the number of packets a host is sending by reading the ID
value in the ip header

What this means....

We send 4 packets to our dummy host, to port 0, with no tcp flags set, and
make a note of the incoming ip id's

***************************************

Scanning Dumb Host (for Dumbness)
 33144
 33145
 33146
 33147

***************************************

If the incoming id's do not show a consistant increase, the host is not
dumb enough to suit our purposes, and the scan aborts.

If the incoming id's show a constant single increment, then it is safe to
assume that the dummy host is not ac
tively talking/communicating to any other host (at this point in time)     

We then send a spoofed packet (SYN) to our target host, on our target
port, on behalf of our Dummy.

***************************************

We Have a consistant 1 increment host
*** Injecting Spoofed Packet ***

***************************************

and once more track the incoming ip id's

***************************************

 33148
 33150
 33152
 33156

***************************************

Now, if the target port was closed, it would have replied with a RST, <as
mentioned in (A) earlier> and our Dummy would have responded with nothing
<as mentioned in B>
But, if the target port was open, it would have replied with a SYN|ACK
(A), causing our Dummy to reply with a RST. Dummy's ip id count, will now
increase, as it has been forced into conversation with Target.

***************************************

*** Yup looks like 22 is open on 196.10.XXX.38 *** 

***************************************

As mentioned before, all credit to antirez, for his initial discovery of
the scan... SixthSense.pl just automates (what still is) a tedious
process..

(transparent proxies etc, will have an effect on scan results)

The possibilities of remote OS detection as mentined by Ofir Arkin, may be
added later..

(SixthSense requires Net::RawIP, run ==> perl -MCPAN -e shell ==> install
Net::RawIP   )


Haroon Meer
+27 837866637
haroon () sensepost com
http://www.sensepost.com

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