Re: Discovering users by RCPT TO

From: Faisal Khan (
Date: 01/14/05

  • Next message: Gregory D. McPhee: "RE: DoS/DDoS Attack"
    Date: Fri, 14 Jan 2005 23:57:58 +0500

    Turning on Reverse DNS and Tarpitting helps for Dictionary Attacks.

    At 09:57 PM 1/14/2005, you wrote:
    >Hash: SHA1
    >I see spammers hitting my MTA daily with dictionary RCTP TO queries
    >and there isn't much you can really do against it; however I have been
    >thinking about a solution using real time blockers.
    >The idea is to monitor the logfile of the MTA, looking for a host
    >getting more than "X" failed destination addresses (I think 2 or 3 is
    >a nice entry threshold). Then when they reach the threshold their IP
    >gets put into a local DNS server that is used by the MTA to as a real
    >time blocker.
    >This wouldn't' require more than another RBL addition to the MTA and
    >then an external script tied to either bind or djbdns.
    >Vince Hoang wrote:
    >|On Thu, Jan 13, 2005 at 02:20:12PM -0500, Chris Buechler wrote:
    >|>I'd recommend disabling it unless you get flooded by such spam
    >|>attacks. I would probably consider it unnecessary information
    >|>disclosure, depending on the environment and reason (if any)
    >|>for doing it that way.
    >|Some MTAs allow permit you to drop the session after a certain
    >|number of failures, but that only slows down the dictionary
    >|You cannot disable RCPT TO because that is how the SMTP protocol
    >|designates the recipients.
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