17 Feb
2014
17 Feb
'14
11:53 a.m.
Christopher Roberts <cjr(a)tridentgarages.co.uk> said, in message
20140217110647.GE12544(a)bootpolish.net:
That said, there are some disadvantages - there have been a couple of
occasions when that 15 minute delay has caused problems, it isn't RFC
compliant and I know some people consider grey listing to be evil.
AFAIK greylisting *is* RFC compliant. It's leveraging the standards
in a compliant manner to the disadvantage of non-compliant software
(i.e. spamware). RFC2821 says the following:
The sender MUST delay retrying a particular
destination after one
attempt has failed. In general, the retry interval SHOULD be at
least 30 minutes; however, more sophisticated and variable strategies
will be beneficial when the SMTP client can determine the reason for
non-delivery.
Retries continue until the message is transmitted or the sender gives
up; the give-up time generally needs to be at least 4-5 days. The
parameters to the retry algorithm MUST be configurable.
So a 15 minute temporary deferral shouldn't cause any problem with any RFC
compliant sender.
When we used it in Aber, we stuck with a full hour long deferral on the
basis this might give spammers time to end up in RBLs etc. Complaints
were rare, but there's nothing stopping people using a 1 minute deferral
period to reduce the delay (modulo the remote end's retry policy).
Entertainingly, I see there's now an RFC describing greylisting:
http://tools.ietf.org/search/rfc6647
The biggest justifiable evilness of greylisting is in expecting other
sites to do more work in getting mail to you. To block that vast amount
of spam, you're forcing legitimate sites to expend effort queuing
and retrying messages. Personally, I hold very little truck with
that argument. First off, the whitelisting that is part of the
greylisting process means that the vast majority of legit mail
doesn't end up being deferred. Secondly the argument works both
ways. My mail server has to do extra work to deliver to a site
implementing greylisting. Both my server and the remote server
benefit from the arrangement. I'd imagine the overheads are
negligible for all but the largest senders on the planet and,
given they're in it for the cash, having them bear a small
amount of extra hardware and network cost isn't something
that's going to make me cry.
Cheers,
Alun.
--
Alun Jones, auj(a)aber.ac.uk, 01970 622494
Gwasanaethau Gwybodaeth / Information Services
Prifysgol Aberystwyth / Aberystwyth University