[MAILSCANNER] Reverse NDR attack.How to combat ? Any ideas ?

[Douglas Ward]

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I am using a postfix/MailScanner combination on our e-mail gateways.  We
use the relay_domain and relay_recipient options to only allow mail
through if it is first a valid domain and second if it is a valid
recipient on the domain.  It is a bit of manual work to keep the
recipient list up to date but they tell me pretty quickly if I forget! 
:)  With this setup postfix automatically rejects the message.  I don't
see many NDR's in the queue waiting to be processed.  I figure its either
that I haven't been dictionary attacked yet or postfix is doing a good
job!

On 9/20/05, Venkata Achanta <vachanta at gmail.com> wrote:
      >Given the reality of the reverse NDR style of spamming it
      won't be long
      before
      >blind-queue servers are generally regarded in the same light
      as open
      relays and
      >smurf amplifiers. Do yourself a favor and start fixing the
      problem now
      before
      >popular consensus agrees that this is a bad thing and
      blacklists are set
      up for
      >"open NDR relays" which include your server.

      Thanks Matt.didnt make sense until it became a reality on my
      mail servers
      this week. I agree with you 100%.

      Now if i go the route of accepting e-mail for only valid
      users, how do i
      mitigate the risk of Directory Harvest attack on a setup like
      mine ?can you
      throw some light on it as well ?

      Thanks again.

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