Nolisting

Volume 50 of jgc's spam and antispam newsletter had a link
for Nolisting, 
Poor Man's Greylisting at 
ht
tp://www.joreybump.com/code/howto/nolisting.html .

Basically, the premise is set an MX with a high preference
pointing to a 
system that does not listen on port 25.  Broken mailers
would attempt to 
connect to it, fail, and not try a lower preference mail
exchanger.  A 
real mailer would fall back to a lower pref MX.

When I was first starting with spam filtering, I had 2
server with unequal 
preferences and it seemed that more garbage would head to
the lower 
preference exchanger, possibly thinking ti was an offsite
backup MX that 
wouldn't have as stringent a filter, or none at all.

What you would end up with is three classes of MX records:

1. One High priority record which point to an address tha
doesn't answer 
on SMTP.
2. Medium priority records that point to real mail servers
that acccept 
your mail.
3. One or more low priority records that point to addresses
that do not 
answer on SMTP.

What does the collective wisdom of the list think about
Nolisting, and the 
idea of a low preference MX record as well?

---
The Vista Content Protection specification could very well
constitute the 
longest suicide note in history  -- Peter Gutmann 
http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.
html

William Brown
Web Development & Messaging Services
Technology Services, WNYRIC, Erie 1 BOCES
(716)821-7285

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WBrowne1b.org wrote:

> Basically, the premise is set an MX with a high
preference pointing to a 
> system that does not listen on port 25.  Broken mailers
would attempt to 
> connect to it, fail, and not try a lower preference
mail exchanger.  A 
> real mailer would fall back to a lower pref MX.

[And add a low-preference non-functional MX too, to stymie
spammers]

[...]

> What does the collective wisdom of the list think about
Nolisting, and the 
> idea of a low preference MX record as well?

In practice, it will probably be moderately effective. 
However, I
would hesitate to have a non-functional host as my
most-preferred MX
machine.  The relevant RFC (2821) is a bit waffly:

   When the lookup succeeds, the mapping can result in a
list of
   alternative delivery addresses rather than a single
address, because
   of multiple MX records, multihoming, or both.  To provide
reliable
   mail transmission, the SMTP client MUST be able to try
(and retry)
   each of the relevant addresses in this list in order,
until a
   delivery attempt succeeds.  However, there MAY also be a
configurable
   limit on the number of alternate addresses that can be
tried.  In any
   case, the SMTP client SHOULD try at least two addresses.

It's not clear to me that an SMTP client that only ever
tries the
most-preferred MX host is in violation of the RFC.  (It's
violating a
SHOULD, but is it violating a MUST?)

Greylisting should be about as effective, but it won't
prevent mail
from a client that only ever tries the best MX host.

Regards,

David.
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On Thu, Feb 01, 2007 at 01:44:43PM -0500, David F. Skoll
wrote:

> Greylisting should be about as effective, but it won't
prevent mail
> from a client that only ever tries the best MX host.

Ah, that suggests perhaps greylisting on the most preferred
(and the
second-most), but also nolisting a least preferred - for
those spam
generators that just go to the least preferred address? 

Whit
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