Logging SMTP connections with Twisted

Question

Python newbie here. I'm writing an SMTP server using Twisted and twisted.mail.smtp. I'd like to log incoming connections and possibly dump them when there are too many concurrent connections. Basically, I want ConsoleMessageDelivery.connectionMade() method to be called in the following, when a new connection is made:

class ConsoleMessageDelivery:
    implements(smtp.IMessageDelivery)

    def connectionMade(self):
        # This never gets called

    def receivedHeader(self, helo, origin, recipients):
        myHostname, clientIP = helo
        headerValue = "by %s from %s with ESMTP ; %s" % (myHostname, clientIP, smtp.rfc822date())
        # email.Header.Header used for automatic wrapping of long lines
        return "Received: %s" % Header(headerValue)

    def validateFrom(self, helo, origin):
        # All addresses are accepted
        return origin

    def validateTo(self, user):
        if user.dest.local == "console":
            return lambda: ConsoleMessage()
        raise smtp.SMTPBadRcpt(user)

class ConsoleMessage:
    implements(smtp.IMessage)

    def __init__(self):
        self.lines = []

    def lineReceived(self, line):
        self.lines.append(line)

    def eomReceived(self):
        return defer.succeed(None)

    def connectionLost(self):
        # There was an error, throw away the stored lines
        self.lines = None

class ConsoleSMTPFactory(smtp.SMTPFactory):
    protocol = smtp.ESMTP

    def __init__(self, *a, **kw):
        smtp.SMTPFactory.__init__(self, *a, **kw)
        self.delivery = ConsoleMessageDelivery()

    def buildProtocol(self, addr):
        p = smtp.SMTPFactory.buildProtocol(self, addr)
        p.delivery = self.delivery
        return p

Answer 1

connectionMade is part of twisted.internet.interfaces.IProtocol, not part of twisted.mail.smtp.IMessageDelivery. There's no code anywhere in the mail server implementation that cares about a connectionMade method on a message delivery implementation.

A better place to put per connection logic is in the factory. And specifically, a good way to approach this is with a factory wrapper, to isolate the logic about connection limits and logging from the logic about servicing SMTP connections.

Twisted comes with a few factory wrappers. A couple in particular that might be interesting to you are twisted.protocols.policies.LimitConnectionsByPeer and twisted.protocols.policies.LimitTotalConnectionsFactory.

Unfortunately, I don't know of any documentation explaining twisted.protocols.policies. Fortunately, it's not too complicated. Most of the factories in the module wrap another arbitrary factory to add some piece of behavior. So, for example, to use LimitConnectionsByPeer, you do something like this:

from twisted.protocols.policies import LimitConnectionsByPeer
...
factory = ConsoleSMTPFactory()
wrapper = LimitConnectionsByPeer(ConsoleSMTPFactory(...))
reactor.listenTCP(465, wrapper)

This is all that's needed to get LimitConnectionsByPeer to do its job.

There's only a little bit more complexity involved in writing your own wrapper. First, subclass WrappingFactory. Then implement whichever methods you're interested in customizing. In your case, if you want to reject connections from a certain IP, that would mean overriding buildProtocol. Then, unless you also want to customize the protocol that is constructed (which you don't in this case), call the base implementation and return its result. For example:

from twisted.protocols.policies import WrappingFactory

class DenyFactory(WrappingFactory):
    def buildProtocol(self, clientAddress):
        if clientAddress.host == '1.3.3.7':
            # Reject it
            return None
         # Accept everything else
         return WrappingFactory.buildProtocol(self, clientAddress)

These wrappers stack, so you can combine them as well:

from twisted.protocols.policies import LimitConnectionsByPeer
...
factory = ConsoleSMTPFactory()
wrapper = LimitConnectionsByPeer(DenyFactory(ConsoleSMTPFactory(...)))
reactor.listenTCP(465, wrapper)