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Re: TCP small fragments

I agree that malformed packets are suspicious. But I disagree that you cannot stop them. That what my point. In my example the malformed packet was a 40 byte SYN, it could be anything. With some vendors (be it router or stateful firewall) you have the option of filtering on these types of packets so long as you know the signature. Other vendors however have no capability and pass anything with an IP address.


pmrn wrote:
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
        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 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

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Greg Sayadian

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


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