Add SMTP AUTH support to Python smtpd library… can't override the method?

Question

So, I wanted to extend the Python smtpd SMTPServer class so that it could handle SMTP AUTH connections. Seemed simple enough...

So, it looked like I could just start like this:

def smtp_EHLO(self, arg):
    print 'got in arg: ', arg
    # do stuff here...

But for some reason, that never gets called. The Python smtpd library calls other similar methods like this:

        method = None
        i = line.find(' ')
        if i < 0:
            command = line.upper()
            arg = None
        else:
            command = line[:i].upper()
            arg = line[i+1:].strip()
        method = getattr(self, 'smtp_' + command, None)

Why won't it call my method?

After that, I thought that I could probably just override the entire found_terminator(self): method, but that doesn't seem to work either.

 def found_terminator(self):
     # I add this to my child class and it never gets called... 

Am I doing something stupid or...? Maybe I just haven't woken up fully yet today...

import smtpd
import asyncore

class CustomSMTPServer(smtpd.SMTPServer):

    def smtp_EHLO(self, arg):

        print 'got in arg: ', arg

    def process_message(self, peer, mailfrom, rcpttos, data):
        print 'Receiving message from:', peer
        print 'Message addressed from:', mailfrom
        print 'Message addressed to  :', rcpttos
        print 'Message length        :', len(data)
        print 'HERE WE ARE MAN!'
        return

    # Implementation of base class abstract method
    def found_terminator(self):
        print 'THIS GOT CALLED RIGHT HERE!'

        line = EMPTYSTRING.join(self.__line)
        print >> DEBUGSTREAM, 'Data:', repr(line)
        self.__line = []
        if self.__state == self.COMMAND:
            if not line:
                self.push('500 Error: bad syntax')
                return
            method = None
            i = line.find(' ')
            if i < 0:
                command = line.upper()
                arg = None
            else:
                command = line[:i].upper()
                arg = line[i+1:].strip()
            method = getattr(self, 'smtp_' + command, None)

            print 'looking for: ', command
            print 'method is: ', method

            if not method:
                self.push('502 Error: command "%s" not implemented' % command)
                return
            method(arg)
            return
        else:
            if self.__state != self.DATA:
                self.push('451 Internal confusion')
                return
            # Remove extraneous carriage returns and de-transparency according
            # to RFC 821, Section 4.5.2.
            data = []
            for text in line.split('\r\n'):
                if text and text[0] == '.':
                    data.append(text[1:])
                else:
                    data.append(text)
            self.__data = NEWLINE.join(data)
            status = self.__server.process_message(self.__peer,
                                                   self.__mailfrom,
                                                   self.__rcpttos,
                                                   self.__data)
            self.__rcpttos = []
            self.__mailfrom = None
            self.__state = self.COMMAND
            self.set_terminator('\r\n')
            if not status:
                self.push('250 Ok')
            else:
                self.push(status)

server = CustomSMTPServer(('127.0.0.1', 1025), None)

asyncore.loop()

Answer 1

You need to extend SMTPChannel -- that's where the smtp_verb methods are implemented; your extension of SMTPServer just needs to return your own subclass of the channel.

Answer 2

TL&DR: To add additional functionality to SMTPChannel you just need to declare a function, and then add it directly to smtpd.SMTPChannel

Explanation:

The SMTPChannel class is designed to respond to the commands that are entered by the user on the open port (typically port 25). The way it searches for which commands it can answer is based off 'Introspection' where it examines all the available attributes of the function.

Take note that the functions within SMTPChannel need to start with the "smtp_". For Example, if you wanted to respond to HELP you would create smtpd.SMTPChannel.smtp_HELP.

The Function below is from the source-code that details the introspection

class SMTPChannel(asynchat.async_chat):
  method = getattr(self, 'smtp_' + command, None)

CodeThatWorks

Step 1: Declare a FUNCTION that will be called

def smtp_HELP(self,arg):
  self.push("[8675309] GPT Answers to HELP")

Step 2: Add the following function to smtpd.SMTPChannel

class FakeSMTPServer(smtpd.SMTPServer):

"""A Fake smtp server"""
        smtpd.SMTPChannel.smtp_HELP = smtp_HELP

Step 3: Telnet to localhost 25 and test out

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 vics-imac.fios-router.home ESMTP Sendmail 6.7.4 Sunday 17 March 2019
HELP
[8675309] GPT Answers to HELP