pymilter/Milter/dsn.py

# Author: Stuart D. Gathman <stuart@bmsi.com>
# Copyright 2005 Business Management Systems, Inc.
# This code is under the GNU General Public License.  See COPYING for details.

# Send DSNs, do call back verification,
# and generate DSN messages from a template
# $Log$
# Revision 1.22  2011/03/18 20:41:31  customdesigned
# Python2.6 SMTP.close() fails when instance never connected.
#
# Revision 1.21  2011/03/03 05:11:58  customdesigned
# Release 0.9.4
#
# Revision 1.20  2010/10/11 00:29:47  customdesigned
# Handle multiple recipients.  For CBV or auto whitelist of multiple emails.
#
# Revision 1.19  2009/07/02 19:41:12  customdesigned
# Handle @ in localpart.
#
# Revision 1.18  2009/06/10 18:01:59  customdesigned
# Doxygen updates
#
# Revision 1.17  2009/05/20 20:08:44  customdesigned
# Support non-DSN CBV (non-empty MAIL FROM)
#
# Revision 1.16  2007/09/25 01:24:59  customdesigned
# Allow arbitrary object, not just spf.query like, to provide data for create_msg
#
# Revision 1.15  2007/09/24 20:13:26  customdesigned
# Remove explicit spf dependency.
#
# Revision 1.14  2007/03/03 18:19:40  customdesigned
# Handle DNS error sending DSN.
#
# Revision 1.13  2007/01/04 18:01:11  customdesigned
# Do plain CBV when template missing.
#
# Revision 1.12  2006/07/26 16:37:35  customdesigned
# Support timeout.
#
# Revision 1.11  2006/06/21 21:07:11  customdesigned
# Include header fields in DSN template.
#
# Revision 1.10  2006/05/24 20:56:35  customdesigned
# Remove default templates.  Scrub test.
#
## @package Milter.dsn
# Support DSNs and CallBackValidations (CBV).
#
# A Delivery Status Notification (bounce) is sent to the envelope
# sender (original MAIL FROM) with a null MAIL FROM (<>) to notify the
# original sender # of delays or problems with delivery.  A Callback Validation
# starts the DSN process, but stops before issuing the DATA command.  The
# purpose is to check whether the envelope recipient is accepted (and is
# therefore a valid email).  The null MAIL FROM tells the remote
# MTA to never reply according to RFC2821 (but some braindead MTAs
# reply anyway, of course).
#
# Milters should cache CBV results and should avoid sending DSNs
# unless the sender is authenticated somehow (e.g. SPF Pass).  However,
# when email is quarantined, and is not known to be a forgery, sending a DSN 
# is better than silently disappearing, and a DSN is better than sending
# a normal message as notification - because MAIL FROM signing schemes
# can reject bounces of forged emails.   Whatever you do, don't copy those
# assinine commercial filters that send a normal message to notify you
# that some virus is forging your email.
#
# <b>DSNs should *only* be sent to MAIL FROM addresses.</b>  Never send
# a DSN or use a null MAIL FROM with an email address obtained from
# anywhere else.
#
import smtplib
import socket
from email.Message import Message
import Milter
import time
import dns

## Send DSN.
# Try the published MX names in order, rejecting obviously bogus entries
# (like <code>localhost</code>).
# @param mailfrom the original sender we are notifying or validating
# @param receiver the HELO name of the MTA we are sending the DSN on behalf of.
#	Be sure to send from an IP that matches the HELO.
# @param msg the DSN message in RFC2822 format, or None for CBV.
# @param timeout total seconds to wait for a response from an MX
# @param session Milter.dns.Session object from current incoming mail
#	session to reuse its cache, or None to create a fresh one.
# @param ourfrom set to a valid email to send a normal notification from, or
#	to validate emails not obtained from MAIL FROM.
# @return None on success or (status_code,msg) on failure.
def send_dsn(mailfrom,receiver,msg=None,timeout=600,session=None,ourfrom=''):
  """Send DSN.  If msg is None, do callback verification.
     Mailfrom is original sender we are sending DSN or CBV to.
     Receiver is the MTA sending the DSN.
     Return None for success or (code,msg) for failure."""
  user,domain = mailfrom.rsplit('@',1)
  if not session: session = dns.Session()
  try:
    mxlist = session.dns(domain,'MX')
  except dns.DNSError:
    return (450,'DNS Timeout: %s MX'%domain)	# temp error
  if not mxlist:
    mxlist = (0,domain),	# fallback to A record when no MX
  else:
    mxlist.sort()
  smtp = smtplib.SMTP()
  toolate = time.time() + timeout
  for prior,host in mxlist:
    try:
      smtp.connect(host)
      code,resp = smtp.helo(receiver)
      # some wiley spammers have MX records that resolve to 127.0.0.1
      a = resp.split()
      if not a:
        return (553,'MX for %s has no hostname in banner: %s' % (domain,host))
      if a[0] == receiver:
        return (553,'Fraudulent MX for %s: %s' % (domain,host))
      if not (200 <= code <= 299):
        raise smtplib.SMTPHeloError(code, resp)
      if msg:
        try:
          smtp.sendmail('<%s>'%ourfrom,mailfrom,msg)
        except smtplib.SMTPSenderRefused:
	  # does not accept DSN, try postmaster (at the risk of mail loops)
          smtp.sendmail('<postmaster@%s>'%receiver,mailfrom,msg)
      else:	# CBV
        code,resp = smtp.docmd('MAIL FROM: <%s>'%ourfrom)
        if code != 250:
          raise smtplib.SMTPSenderRefused(code, resp, '<%s>'%ourfrom)
        if isinstance(mailfrom,basestring):
          mailfrom = [mailfrom]
        badrcpts = {}
        for rcpt in mailfrom:
          code,resp = smtp.rcpt(rcpt)
          if code not in (250,251):
            badrcpts[rcpt] = (code,resp)# permanent error
        smtp.quit()
        if len(badrcpts) == 1:
          return badrcpts.values()[0]	# permanent error
        if badrcpts:
          return badrcpts
      return None			# success
    except smtplib.SMTPRecipientsRefused,x:
      if len(x.recipients) == 1:
        return x.recipients.values()[0]	# permanent error
      return x.recipients
    except smtplib.SMTPSenderRefused,x:
      return x.args[:2]			# does not accept DSN
    except smtplib.SMTPDataError,x:
      return x.args			# permanent error
    except smtplib.SMTPException:
      pass		# any other error, try next MX
    except socket.error:
      pass		# MX didn't accept connections, try next one
    except socket.timeout:
      pass		# MX too slow, try next one
    if hasattr(smtp,'sock'): smtp.close()
    if time.time() > toolate:
      return (450,'No MX response within %f minutes'%(timeout/60.0))
  return (450,'No MX servers available')	# temp error

class Vars: pass

# NOTE: Caller can pass an object to create_msg that in a typical milter
# collects things like heloname or sender anyway.
def create_msg(v,rcptlist=None,origmsg=None,template=None):
  """Create a DSN message from a template.  Template must be '\n' separated.
     v - an object whose attributes are used for substitutions.  Must
       have sender and receiver attributes at a minimum.
     rcptlist - used to set v.rcpt if given
     origmsg - used to set v.subject and v.spf_result if given
     template - a '\n' separated string with python '%(name)s' substitutions.
  """
  if not template:
    return None
  if hasattr(v,'perm_error'):
    # likely to be an spf.query, try translating for backward compatibility
    q = v
    v = Vars()
    try:
      v.heloname = q.h
      v.sender = q.s
      v.connectip = q.i
      v.receiver = q.r
      v.sender_domain = q.o
      v.result = q.result
      v.perm_error = q.perm_error
    except: v = q
  if rcptlist:
    v.rcpt = '\n\t'.join(rcptlist)
  if origmsg:
    try: v.subject = origmsg['Subject']
    except: v.subject = '(none)'
    try:
      v.spf_result = origmsg['Received-SPF']
    except: v.spf_result = None

  msg = Message()

  msg.add_header('X-Mailer','PyMilter-'+Milter.__version__)
  msg.set_type('text/plain')

  hdrs,body = template.split('\n\n',1)
  for ln in hdrs.splitlines():
    name,val = ln.split(':',1)
    msg.add_header(name,(val % v.__dict__).strip())
  msg.set_payload(body % v.__dict__)
  # add headers if missing from old template
  if 'to' not in msg:
    msg.add_header('To',v.sender)
  if 'from' not in msg:
    msg.add_header('From','postmaster@%s'%v.receiver)
  if 'auto-submitted' not in msg:
    msg.add_header('Auto-Submitted','auto-generated')
  return msg

if __name__ == '__main__':
  import spf
  q = spf.query('192.168.9.50',
  'SRS0=pmeHL=RH==stuart@example.com',
  'red.example.com',receiver='mail.example.com')
  q.result = 'softfail'
  q.perm_error = None
  msg = create_msg(q,['charlie@example.com'],None,
"""From: postmaster@%(receiver)s
To: %(sender)s
Subject: Test

Test DSN template
"""
  )
  print msg.as_string()
  # print send_dsn(f,msg.as_string())
  # print send_dsn(q.s,'mail.example.com',msg.as_string())