Hi Steve,
I have been wondering about Tiny Fragments
check for TCP for IPv6, as there can be many extension headers between the TCP
and the IPv6 standard header, we may have fragmented TCP packets?
The fragmentation I guess does not see if
it is fragmenting the inner TCP header or not(which is way inside the packet).
Thanks,
Vishwas
From: pmrn
[mailto:pmrn@mac.com]
Sent: Wednesday, February 16, 2005
10:34 PM
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:
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
Are such fragments discarded by the firewall in ISP(is it an option to
discard
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
Learn
like you will live for ever and Live like you will die tomorrow-Gandhi