[Snort-devel] Evading Snort via splitting ACKs

Marc Norton marc.norton at ...402...
Tue Sep 24 06:12:03 EDT 2002


The Comer/Stevens book indicates the Final Ack of the 3 way handshake
may in fact have data with it, in fact his example code finishes the
connection and then passes the packet to a data handling routine to
process any associated data.  I believe TTCP in fact needs this feature
to operate, but I have not used that in a few years so I may off there.

The payload in the final ack of a connection handshake is not all that
unusual in my experience.  I find it very unusual to hear of an IP stack
accepting a TCP packet with data but without an ACK flag on an
established connection.  After all the ack field is crucial to
maintaining the proper and complete picture of session sequencing.  But,
alas the actual strictness of this requirement and it's implementation
is not covered well in the literature.   And, as you point out,
implementations are often found lacking.  If you checked out Stevens Vol
II let us know, it's an interesting point.

-----Original Message-----
From: snort-devel-admin at lists.sourceforge.net
[mailto:snort-devel-admin at lists.sourceforge.net] On Behalf Of David J.
Bianco
Sent: Monday, September 23, 2002 4:15 PM
To: Phil Wood
Cc: snort-devel at lists.sourceforge.net
Subject: Re: [Snort-devel] Evading Snort via splitting ACKs

On Mon, 2002-09-23 at 14:46, Phil Wood wrote:
> I do believe that you can formulate a packet that looks like tcp with
> the exception of an ack bit, and has as much data you can cram in to
> it or multiple fragments, up to 65,635 octets.  Also, a rule that
> expects the ack bit will no be exercised under those conditions.
> 
> However, a tcp implimentation that doesn't drop the segment and
silently
> return to await a properly formed tcp packet is a broken
implimentation.
> 
> Or, I don't understand RFC 793 all that well.
> 

I think you do understand RFC 793, but not all TCP implementors did.  My
informal testing has shown that Microsoft's TCP (at least in NT4, the
box I tested against) correctly failed to reply to my query.  But all 
Linux boxes processed it just fine.  I haven't tried Sun or HP yet,
or newer versions of Windows.  

I just rechecked RFC 793, and it confirms that the ACK flag isn't really
optional on an established connection, though I'd like to see what the
Stevens book has to say on the subject when I get home.

	David

-- 
David J. Bianco, GSEC		<bianco at ...1589...>
Thomas Jefferson National Accelerator Facility
GPG Fingerprint:   516A B80D AAB3 1617 A340  227A 723B BFBE B395 33BA

     The views expressed herein are solely those of the author and
	    not those of SURA/Jefferson Lab or the US DOE.




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