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Re: question on ptr rr

  • From: Paul Vixie
  • Date: Sat Feb 07 13:24:28 2004

garrett.allen@comcast.net writes:

> we are a stub network, injesting about 30k emails daily.  about a year
> ago we implemented a spam filtering product.  it works well.  recently we
> turned on the knob to enable it to do reverse lookups.  only the mild
> version, a reverse is made on the ptr rr for the ip address sending the
> email.  if it fails the spam filter issues a 421 and closes the
> connection.  unfortunately, we have 6 sites thus far that are
> legitimately trying to communicate with us but don't have ptr's
> associated with the ip address sending emails.  since it obviously isn't
> a requirement to have one is it generally accepted to do so?  any sense
> for how many end networks do and don't?

I've run all my mailers with aggressive PTR checks for about a year, and
while some of my guests aren't getting all the e-mail that's sent to them,
it's had no impact on me other than that periodically I have to tell some
remote postmaster that their PTR's are missing or that they don't match
the HELO hostname.  Invariably they fix it.

This is counter to RFC821, and also Jon's old mantra "be liberal in what
you accept and conservative in what you generate."  The trouble is, this
is information warfare now, not a grand experiment in interoperable
communications.  The bad guys aren't going to be conservative in what they
generate.  By being nonliberal in what I accept I keep 20,000 or more
malicious messages per day from hitting my personal inbox.  This brings
the load down to something postgresql and MH can actually cope with:

lartomatic=# select date(entered),count(*) from spam where date(entered)>'today'::date-'10 days'::interval group by date(entered) order by date(entered) desc;
    date    | count 
------------+-------
 2004-02-07 |  2691
 2004-02-06 |  2660
 2004-02-05 |  3181
 2004-02-04 |  3910
 2004-02-03 |  4712
 2004-02-02 |  4706
 2004-02-01 |  4866
 2004-01-31 |  4326
 2004-01-30 |  3712
 2004-01-29 |  2723
(10 rows)

I think the tipping point went by a while ago, and that anyone who wants
their e-mail to be accepted will make sure their mail relay has a PTR and
that that this PTR holds the same name used in the SMTP HELO command.
--
Paul Vixie