Re: [fw-wiz] A fun smackdown...

From: Marcus J. Ranum (mjr_at_ranum.com)
Date: 05/21/05

  • Next message: Marcus J. Ranum: "Re: [fw-wiz] Thoughts on the new Cisco ASA 5500 firewalls"
    To: Chuck Swiger <chuck@codefab.com>
    Date: Sat, 21 May 2005 12:58:02 -0400
    
    

    Chuck Swiger wrote:
    >By definition, the IETF is concerned with systems which interoperate over public networks using network-wide conventions and publicly documented standards. What people do with private machines or private networks is up to them, at least so long as they *don't* connect those machines to the Internet.

    You're completely ignoring the fundamental dilemma that I am trying
    to get you to confront. My position in a nutshell:
    - "Standards that don't take security into account are not internet-worthy"
    and you're asserting
    - "If you don't follow standards you break 'legitimate' traffic"

    The problem is that, since the standards don't take security into
    account, the traffic is not 'legitimate' - it's 'dangerous' and a
    security device can and SHOULD interfere with it.

    Maybe the first time someone invents a PMTUD denial of
    service attack you'll "get it."

    >A firewall which breaks ESMTP, or HTTP/1.1, or PMTUD to such machines (typically in a DMZ) significantly impacts legitimate access with questionable gains at best for security, and IMHO is a poor tradeoff.

    Well, since it's a matter of opinion, I don't agree with you. :)

    Let's look at another example. The RFCs for FTP include
    provisions for third party transfer. The PORT command could
    be connected to a different host than the client. Historically,
    that feature was never used. When I wrote the DEC SEAL
    FTP proxy* I realized that this could be used to issue arbitrary
    connections. So I deliberately broke from the RFC and put
    code in to sever a connection that was attempting this.
    So in your terms, since it was in the RFC, it was "legitimate"
    but by trashing all over the holy RFC I made networks much
    more secure.

    So, suppose you're running an older model Gauntlet firewall
    or a DEC SEAL. They trash all over the holy RFC by
    not even knowing what ESMTP is. Congratulations! If
    someone finds a vulnerability that has anything to do
    with ESMTP or any option that can be reached via that
    code path: you're protected.

    >And as for PMTUD, I'd be happy to see a better solution for MTU discovery, short of depending on all intermediate routers to handle IP fragmentation in an efficient and sane fashion. Do you have something better, Marcus...?

    It's kind of you to come to me for all the answers but I'm not a
    networking protocol designer - I'm a security system designer.
    So don't ask me how to implement something better than the
    current PMTUD. On the other hand, I can assert with some
    comfort that if I *did* implement some kind of PMTUD it'd be
    better than the current approach because it would take
    established security techniques and security into account
    in its design.

    mjr.
    (*Yes, hacker kiddies who think you invented FTP bouncing
    in 1995 you are completely wrong. Not only was I there
    first, I contacted the maintainers of BSD and had a check
    added to ruserok() so that the FTP server port was not
    treated as privileged...)

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  • Next message: Marcus J. Ranum: "Re: [fw-wiz] Thoughts on the new Cisco ASA 5500 firewalls"

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