[RndTbl] greylisting

Gilbert E. Detillieux gedetil at cs.umanitoba.ca
Mon Dec 4 14:43:57 CST 2006

On 2006-12-03 21:03, Trevor Cordes wrote:
> After domain keys, I implemented the milter-greylist that Gilbert was
> talking about.  It's pretty easy (on FC, with yum packages).
> Anyone care to compare notes?
> I've chosen the following values:
> timeout 25h
> greylist 6m
> autowhite 30d
> subnetmatch /24
> Does anyone think there could be an MTA retarded enough to have the
> queue retry time set to longer than 25h?  The greylist default was 5d,
> but that seems a bit excessive, or am I missing something here?

I think the reason for setting this to 5 days by default is that's the 
maximum time a message will typically stay queued up on a sending system 
before being bounced back to the sender.  In case things go very wrong, 
you don't want grey-listing to further contribute to delivery problems 
by forcing unnecessary retries.  So, setting the retry timeout to 5 days 
is a safe default.

That said, it's probably fine in most circumstances to set this shorter.
I don't think it buys you much, however, other than keeping the list of 
pending greylist entries down a bit.

> As for the greylist option, shouldn't 1m be enough to do the trick?
> Either a spammer will retry or it won't, how long you make it wait
> should not matter.  Of course, once everyone implements greylisting,
> then all spammers will simply start retrying -- it's only CPU cycles,
> and those keep getting cheaper.

So far, most spam sent out by botnets doesn't retry at all, so you could 
set this as short as you want without increasing spam input.  I suppose 
if spammers do start retrying, then a longer delay is perhaps more 
likely to keep them away than a short one, since the name of the game 
for spammers is to maximize volume while minimizing time.

And, according to an article I read, time is very much of the essence.
When spammers use botnets, it's often not their own - they rent them 
from hackers that control them, and they pay based on the amount of time 
they use.  So throughput does matter, and anything you can do to slow 
them down works in your favour.  That's perhaps why grey-listing is 
still so effective, and why they typically don't bother with retries.
It's not worth the time and effort.

> I suppose setting greylist to 30m or 60m or something might stop some
> spammers who are now waiting, but not very long.  But that's got to be
> rare.

Probably very rare right now.  I think most of the spam that does get 
through on retries is being sent through open relays which are "legit" 
mail servers.  In that case, the retry time doesn't really matter much.

> autowhite should be set pretty high IMO.

Prabably safest, and least disruptive to legit e-mail.  It might 
slightly increase the chance of repeat spam from the same addresses, 
though, but I'm not sure how much of a problem that would likely be in 

> My only complaint with the greylist concept is the fact that at the end
> of the conf file is:
> # This is a list of broken MTAs that break with greylisting.
> ...  Too bad there are MTA's out there that are so braindead.

Yeah, it's rather disconcerting.

Gilbert E. Detillieux		E-mail:	<gedetil at cs.umanitoba.ca>
Dept. of Computer Science	Web:	http://www.cs.umanitoba.ca/~gedetil/
University of Manitoba		Phone:	(204)474-8161
Winnipeg MB CANADA  R3T 2N2	Fax:	(204)474-7609

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