pymilter/spfmilter.py

# A simple SPF milter.
# You must install pyspf for this to work.

# http://www.sendmail.org/doc/sendmail-current/libmilter/docs/installation.html

# Author: Stuart D. Gathman <stuart@bmsi.com>
# Copyright 2007 Business Management Systems, Inc.
# This code is under GPL.  See COPYING for details.

import sys
import os
import re
import Milter
import spf
import struct
import socket
import syslog
import anydbm
from Milter.config import MilterConfigParser
from Milter.utils import iniplist,parse_addr

syslog.openlog('spfmilter',0,syslog.LOG_MAIL)

class Config(object):
  "Hold configuration options."
  pass

def read_config(list):
  "Return new config object."
  cp = MilterConfigParser()
  cp.read(list)
  conf = Config()
  conf.socketname = cp.getdefault('milter','socketname', '/tmp/spfmiltersock')
  conf.miltername = cp.getdefault('milter','name','pyspffilter')
  conf.trusted_relay = cp.getlist('milter','trusted_relay')
  conf.internal_connect = cp.getlist('milter','internal_connect')
  conf.trusted_forwarder = cp.getlist('spf','trusted_relay')
  conf.access_file = cp.getdefault('spf','access_file',None)
  return conf

class SPFPolicy(object):
  "Get SPF policy by result from sendmail style access file."
  def __init__(self,sender):
    self.sender = sender
    self.domain = sender.split('@')[-1].lower()
    if access_file:
      try: acf = anydbm.open(access_file,'r')
      except: acf = None
    else: acf = None
    self.acf = acf

  def getPolicy(self,pfx):
    acf = self.acf
    if not acf: return None
    try:
      return acf[pfx + self.sender]
    except KeyError:
      try:
	return acf[pfx + self.domain]
      except KeyError:
	try:
	  return acf[pfx]
	except KeyError:
	  return None
  
class spfMilter(Milter.Milter):
  "Milter to check SPF.  Each connection gets its own instance."

  def log(self,*msg):
    syslog.syslog('[%d] %s' % (self.id,' '.join([str(m) for m in msg])))

  def __init__(self):
    self.mailfrom = None
    self.id = Milter.uniqueID()
    # we don't want config used to change during a connection
    self.conf = config

  # addheader can only be called from eom().  This accumulates added headers
  # which can then be applied by alter_headers()
  def add_header(self,name,val,idx=-1):
    self.new_headers.append((name,val,idx))
    self.log('%s: %s' % (name,val))

  def connect(self,hostname,unused,hostaddr):
    self.internal_connection = False
    self.trusted_relay = False
    self.hello_name = None
    # sometimes people put extra space in sendmail config, so we strip
    self.receiver = self.getsymval('j').strip()
    if hostaddr and len(hostaddr) > 0:
      ipaddr = hostaddr[0]
      if iniplist(ipaddr,self.conf.internal_connect):
	self.internal_connection = True
      if iniplist(ipaddr,self.conf.trusted_relay):
        self.trusted_relay = True
    else: ipaddr = ''
    self.connectip = ipaddr
    if self.internal_connection:
      connecttype = 'INTERNAL'
    else:
      connecttype = 'EXTERNAL'
    if self.trusted_relay:
      connecttype += ' TRUSTED'
    self.log("connect from %s at %s %s" % (hostname,hostaddr,connecttype))
    return Milter.CONTINUE

  def hello(self,hostname):
    self.hello_name = hostname
    self.log("hello from %s" % hostname)
    return Milter.CONTINUE

  # multiple messages can be received on a single connection
  # envfrom (MAIL FROM in the SMTP protocol) seems to mark the start
  # of each message.
  def envfrom(self,f,*str):
    self.log("mail from",f,str)
    self.mailfrom = f
    self.new_headers = []
    t = parse_addr(f)
    if len(t) == 2: t[1] = t[1].lower()
    self.canon_from = '@'.join(t)
    if not (self.internal_connection or self.trusted_relay) and self.connectip:
      rc = self.check_spf()
      if rc != Milter.CONTINUE: return rc
    return Milter.CONTINUE

  def envrcpt(self,f,*str):
    return Milter.CONTINUE

  def header(self,name,hval):
    return Milter.CONTINUE

  def eoh(self):
    return Milter.CONTINUE

  def eom(self):
    for name,val,idx in self.new_headers:
      try:
	self.addheader(name,val,idx)
      except:
	self.addheader(name,val)	# older sendmail can't insheader
    return Milter.CONTINUE

  def close(self):
    return Milter.CONTINUE

  def check_spf(self):
    receiver = self.receiver
    for tf in self.conf.trusted_forwarder:
      q = spf.query(self.connectip,'',tf,receiver=receiver,strict=False)
      res,code,txt = q.check()
      if res == 'pass':
        self.log("TRUSTED_FORWARDER:",tf)
        break
    else:
      q = spf.query(self.connectip,self.canon_from,self.hello_name,
	  receiver=receiver,strict=False)
      q.set_default_explanation(
	'SPF fail: see http://openspf.org/why.html?sender=%s&ip=%s' % (q.s,q.i))
      res,code,txt = q.check()
    if res not in ('pass','temperror'):
      if self.mailfrom != '<>':
	# check hello name via spf unless spf pass
	h = spf.query(self.connectip,'',self.hello_name,receiver=receiver)
	hres,hcode,htxt = h.check()
	if hres in ('deny','fail','neutral','softfail'):
	  self.log('REJECT: hello SPF: %s 550 %s' % (hres,htxt))
	  self.setreply('550','5.7.1',htxt,
	    "The hostname given in your MTA's HELO response is not listed",
	    "as a legitimate MTA in the SPF records for your domain.  If you",
	    "get this bounce, the message was not in fact a forgery, and you",
	    "should IMMEDIATELY notify your email administrator of the problem."
	  )
	  return Milter.REJECT
      else:
        hres,hcode,htxt = res,code,txt
    else: hres = None

    p = SPFPolicy(q.s)

    if res == 'fail':
      policy = p.getPolicy('spf-fail:')
      if not policy or policy == 'REJECT':
	self.log('REJECT: SPF %s %i %s' % (res,code,txt))
	self.setreply(str(code),'5.7.1',txt)
	# A proper SPF fail error message would read:
	# forger.biz [1.2.3.4] is not allowed to send mail with the domain
	# "forged.org" in the sender address.  Contact <postmaster@forged.org>.
	return Milter.REJECT
    if res == 'softfail':
      policy = p.getPolicy('spf-softfail:')
      if policy and policy == 'REJECT':
	self.log('REJECT: SPF %s %i %s' % (res,code,txt))
	self.setreply(str(code),'5.7.1',txt)
	# A proper SPF fail error message would read:
	# forger.biz [1.2.3.4] is not allowed to send mail with the domain
	# "forged.org" in the sender address.  Contact <postmaster@forged.org>.
	return Milter.REJECT
    elif res == 'permerror':
      policy = p.getPolicy('spf-permerror:')
      if not policy or policy == 'REJECT':
	self.log('REJECT: SPF %s %i %s' % (res,code,txt))
	# latest SPF draft recommends 5.5.2 instead of 5.7.1
	self.setreply(str(code),'5.5.2',txt,
	  'There is a fatal syntax error in the SPF record for %s' % q.o,
	  'We cannot accept mail from %s until this is corrected.' % q.o
	)
	return Milter.REJECT
    elif res == 'temperror':
      policy = p.getPolicy('spf-temperror:')
      if not policy or policy == 'REJECT':
	self.log('TEMPFAIL: SPF %s %i %s' % (res,code,txt))
	self.setreply(str(code),'4.3.0',txt)
	return Milter.TEMPFAIL
    elif res == 'neutral' or res == 'none':
      policy = p.getPolicy('spf-neutral:')
      if policy and policy == 'REJECT':
        self.log('REJECT NEUTRAL:',q.s)
	self.setreply('550','5.7.1',
  "%s requires and SPF PASS to accept mail from %s. [http://openspf.org]"
	  % (receiver,q.s))
	return Milter.REJECT
    elif res == 'pass':
      policy = p.getPolicy('spf-pass:')
      if policy and policy == 'REJECT':
        self.log('REJECT PASS:',q.s)
	self.setreply('550','5.7.1',
		"%s has been blacklisted by %s." % (q.s,receiver))
	return Milter.REJECT
    self.add_header('Received-SPF',q.get_header(res,receiver),0)
    if hres and q.h != q.o:
      self.add_header('X-Hello-SPF',hres,0)
    return Milter.CONTINUE

if __name__ == "__main__":
  Milter.factory = spfMilter
  Milter.set_flags(Milter.CHGHDRS + Milter.ADDHDRS)
  global config
  config = read_config(['spfmilter.cfg','/etc/mail/spfmilter.cfg'])
  miltername = config.miltername
  socketname = config.socketname
  print """To use this with sendmail, add the following to sendmail.cf:

O InputMailFilters=%s
X%s,        S=local:%s

See the sendmail README for libmilter.
sample spfmilter startup""" % (miltername,miltername,socketname)
  sys.stdout.flush()
  Milter.runmilter("pyspffilter",socketname,240)
  print "sample spfmilter shutdown"