pre-smtpd (was: policy server for mail bombing control)

[Follow-ups to postfix-devel@postfix.org]

Of course the correct implementation is not to sleep in an smtpd
server process, but to handle all connections in a front end daemon
a la Michael Tokarev. This process drops connections from clients
that speak too early, and passes the remainder to Postfix.

The less elegant solution executes one Postfix process per connection.

The more elegant solution uses a to-be-developed hook to pass an
already accepted connection into a running smtpd process. This
would be a third connection management method for Postfix daemons.

1) Existing: invoke accept(2) on descriptor inherited from master
This is how most Postfix daemons work.

2) Existing: use open connection on stdin/stdout
This is how "/usr/sbin/sendmail -bs" invokes the smtpd program.

3) New: receive open connection via descriptor inherited from master
with a master entry like this:

smtp inet n - - - 1 pre-smtpd
-o smtpd_back_end=smtp-pass
smtp-pass pass n - - - - smtpd

The pre-smtpd server would be a multi-server process that
accepts all incoming SMTP connections and hands them off to a
regular smtpd process after some preliminary checks.

Wietse