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RE: TCP small fragments

Cisco changed their acl language to include "fragment" as part of the
This paper explains what they did and why.


Fragments are not malformed. Unusual maybe but it really depends on
where your seeing them.

-----Original Message-----
From: owner-opsec@psg.com [mailto:owner-opsec@psg.com] On Behalf Of pmrn
Sent: Wednesday, February 16, 2005 10:04 AM
To: Greg Sayadian
Cc: Steven M. Bellovin; opsec@ops.ietf.org; Vishwas Manral
Subject: Re: TCP small fragments

I understood your point about Firewalls. Understand Prof. Bellovian's
point also. The point I was trying to make is that it is a malformed
packet and IMHO, all malformed packets are suspicious. I believe, Prof.
Bellovian published paper on this (not sure). Read it long time ago. 

It is a well known technique used by attackers to evade firewalls. All
malformed packets are suspicious in my opinion. You get them, can't stop
them and some are more harmful than others, in this case crashing hosts.

By the way who said Firewall is a Rock Solid security mechanism, it is
something better than nothing kind of thing. 


On Feb 16, 2005, at 10:05 AM, Greg Sayadian wrote: 

It is certainly possible with some routers to implement filtering based
on packet size. And as we know per RFC that valid packets have a minimum
size. So you can do things like filter on 40 byte SYN packets and drop,
count, log, etc. However some routers don't do this and will pass any
fragment with a MF bit set. This translates into firewall vendors as
well. To get the legitimate answer to your question you will need to
look at the specific device you are interested in and see how it reacts.


Steven M. Bellovin wrote: 
In message
<BB6D74C75CC76A419B6D6FA7C38317B2628B83@sinett-sbs.SiNett.LAN>, "Vis 
hwas Manral" writes: 
Hi Pall, 

We are not talking about right implementations of IP fragmentation. We
are tal 
king about what firewalls do in case of small fragments hwhich can be
caused b 
y an attack. 
Are such fragments discarded by the firewall in ISP(is it an option to

The problem is very well known in the firewall community. For that
matter, see RFC 1858, which documents it. I believe that most firewall
products handle it properly. 
--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb 

Greg Sayadian 

Pall Ramanathan 
Work: 678-9359670 
Mobile: 678-576-7105 


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