Amavis not being called from postfix

[Patrick Ben Koetter]

* Dominic Raferd <dominic at timedicer.co.uk>:
> On Fri, 8 Feb 2019 at 17:17, Robert Moskowitz <rgm at htt-consult.com> wrote:
> ...
> > I would think I want to use MILTER.
> 
> I too am interested in running amavis as milter (Patrick has written
> some instructions in German), but the standard and more common setup,
> and the one which you are emulating at present, is as content filter
> instead, and I think you should focus on getting that working first.

There will be a MILTER section in the upcoming docs.



> As I understand it the disadvantages of this content filter
> post-queue(?) approach are
> - that mails are queued twice in the MTA [postfix] (once when it sends
> them out to amavis, and again when they return), with two different
> queue-ids, which is untidy and can make log tracking harder, and
> - that the response from amavis cannot be passed back to the client
> because the client has been told all is ok (250) when the email is
> passed over to amavis and before amavis has processed it - but usually
> you may not want to tell the client if you are quaranting or
> discarding the mail it has sent.

pre-queue::
    Mail is scanned while the client is still in session with the MTA. The MTA
    receives the verdict from amavis and can immediately respond to the
    client. If the server rejects the message it was never accepted. At least
    in Germany this is an important detail, because - from a legal point of
    view - one would have to deliver the message to the recipient once it has
    been accepted – even if it is spam or anything worse…
    -> Postfix: Use smtpd_milters or smtpd_proxy_filter for pre-queue handling
post-queue::
    The MTA accepts the message, queues it and sends it to amavis. The
    client/server session comes to an end. The client (sender) believes the
    message is on its way to the recipient. Amavis classifies the message and
    acts on the result. A late bounce may notify the sender, *but* for
    malware/spam envelope senders are usually forged. So you end up notifying
    the wrong person -> backscatter
    -> Postfix: Use content_filter for post-queue handling


> The main advantage over calling amavis as a milter is that it doesn't really
> matter how long amavis takes to process the mail, whereas with a milter you
> have the client still connected and waiting for a response. You can use
> something like $child_timeout = 20; so that amavis forces any child
> processes (esp. clamav) to give up after a given period - this isn't
> required with the content_filter approach.

If you use Postfix and want to use other milters, such as
open(dkim|spf|dmarc|...) together with amavis you *have* to use milter for
amavis, because smtpd_proxy_filter and milters don't work together. If you use
smtpd_proxy_filter a milter can't 'see' the content - only the session. That's
a technical limitation caused by Postfix compartmentalized architecture.

p at rick



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