Will milter-greylist solve my directory harvest attacks?

[Matt Kettler]

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dnsadmin 1bigthink.com wrote:
> Hello All,
> 
> I've implemented as much restriction, as tolerable by my users, within
> the MTA (sendmail) and still get some hammering directory harvest attacks.
> 
> Will milter-greylist help?

Somewhat, but you'll get hammered with a really large greylist database.

Really to deal with dictionary attacks there's a few quick sendmail features you
can use to help.

The BAD_RCPT_THROTTLE options is probably the most effective here. Here's a
quick sendmail.mc fragment for it:

#after 15 invalid recipients, start slowing them down with
#1 second sleeps
define(`confBAD_RCPT_THROTTLE',15)

You might also want to consider MAX_RCPTS_PER_MESSAGE, MAX_DAEMON_CHILDREN, and
CONNECTION_RATE_THROTTLE.

I'd also strongly suggest making sure that PRIVACY_FLAGS has either goaway or
novrfy,noexpn.

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