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IETF                                                           A. Vesely
Internet-Draft                                            August 1, 2013
Intended status: Informational
Expires: February 2, 2014


              DNSxL Email Authentication Method Extension
                    draft-vesely-authmethod-dnswl-01

Abstract

   This document describes a method that can be registered within the
   Email Authentication Methods IANA registry created by RFC 5451.  The
   method consists in looking up a DNS white or black list, and
   interpreting any returned data.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 2, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Method Results  . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8







































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1.  Introduction

   One of the many checks that mail servers carry out is to query DNS
   white and black lists (DNSxL, [RFC5782]).  The semantics of DNSWL is
   similar to that of Vouch By Reference (VBR, [RFC5518]); that is, an
   external organization, trusted by the receiving mail transfer agent
   (MTA), vouches for the sender.

   Differently from VBR, the lookup is based on the IP address.  That
   allows it to occur very early in an SMTP transaction, and thus it can
   be used to counterweight policies that typically occur at those early
   stages too, such as the Sender Policy Framework (SPF, [RFC4408]).
   Nevertheless, the result of a DNSWL lookup is likely used at later
   stages as well; for example, a delivery agent can use it to estimate
   the spamminess of an email message.  Reusing the previously obtained
   result is more consistent than issuing multiple queries, and saves
   resources.

   The semantics of DNSBL often implies blocking any attempt to send
   mail, or even outright blocking any routing or peering from listed
   addresses.  However, in some cases mail is accepted despite a
   positive blacklist lookup.  In such cases, it is useful to record the
   result of the lookup, which can be done symmetrically, using the
   method described in this document.


2.  Method Results

   In this document, the acronyms DNSWL and DNSBL are used both to refer
   to a generic organization that publishes a DNS list, and to indicate
   the list itself.  The organization defines the DNS zone and the
   record type(s) to be queried, as well as the meaning of the listings
   and the procedures to maintain them.  DNSxL is used to indicate
   either or both those two, while dnsxl (lowercase) is the name of the
   method being defined here.

   As explained in "Combined IP Address DNSxL" (Section 2.3 of
   [RFC5782]), DNSxLs encode taxonomical details about the mail sender
   as bit masks of type A records.  The receiving MTA needs to determine
   whether the data returned is applicable.  In that case, it uses a
   uniformized rendering of that data as an outsourced extension of its
   local policy.  The properties, uniform across DNSxLs, that this
   document provides for are listed below:








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   policy.dnszone:  The name of the DNSxL, where the result is obtained
          from.

   policy.score:  This is a number in the range [-100, 100].  Zero (0.0)
          is for neutral, non-listed senders.  Positive values are for
          black lists.  Negative values are for white lists, with
          meaning roughly defined as:

          -0.1 only avoid outright blocking (e.g. free mail providers),

            -1 reduce chance of false positives,

           -10 make sure to avoid false positives but allow override for
               clear cases,

          -100 highly trusted sender, avoid override.

   policy.contact:  This is either a domain name or an abuse reporting
          address, which can be used as described in "Where to Send
          Reports" (Section 5.3 of [RFC6650]).

   The result proper of the dnsxl method is defined as follows:

   pass:       A query to a DNSWL completed, and a reply was returned,
               meaning the IP address is whitelisted.

   fail:       A query to a DNSBL completed, and a reply was returned,
               meaning the IP is blacklisted.

   neutral:    A query to a DNSBL completed, and a reply containing no
               answers was returned, meaning the IP is not listed.

   temperror:  The DNS evaluation could not be completed due to some
               error that is likely transient in nature, such as a
               temporary DNS error, e.g., a DNS RCODE of 2, commonly
               known as SERVFAIL, or other error condition resulted.  A
               later attempt might produce a final result.

   permerror:  The DNS evaluation could not be completed because of some
               kind of misconfiguration, e.g., a DNS RCODE of 3,
               commonly known as NXDOMAIN.  A later attempt is unlikely
               to produce a final result.


3.  IANA Considerations

   There is a registry of Email Authentication Methods created by
   RFC5451.  The method described in this document is referred by



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   Table 1, it has three ptype.Property values detailed in Table 2.

   [TO BE REMOVED: The registry is currently accessible here:
   http://www.iana.org/assignments/email-auth/email-auth.xhtml \
                                                   #email-auth-methods ]

                     +--------+------------+---------+
                     | Method | Defined    | version |
                     +--------+------------+---------+
                     | dnsxl  | [this rfc] |       1 |
                     +--------+------------+---------+

               Table 1: Method name, definition, and version

        +--------+----------+---------------------------+--------+
        | ptype  | Property | Value                     | Status |
        +--------+----------+---------------------------+--------+
        | policy | dnszone  | The origin of the results | active |
        | policy | score    | sender trustworthiness    | active |
        | policy | contact  | abuse reporting           | active |
        +--------+----------+---------------------------+--------+

                          Table 2: Method values

   In addition, this method reuses five of the values already defined in
   the Email Authentication Result Names associated registry.  They are
   listed in Table 3.

   [TO BE REMOVED: The registry is currently accessible here:
   http://www.iana.org/assignments/email-auth/email-auth.xhtml \
                                              #email-auth-result-names ]

                    +-----------+-----------+--------+
                    | Code      | Meaning   | Status |
                    +-----------+-----------+--------+
                    | pass      | Section 2 | active |
                    | fail      | Section 2 | active |
                    | neutral   | Section 2 | active |
                    | temperror | Section 2 | active |
                    | permerror | Section 2 | active |
                    +-----------+-----------+--------+

                          Table 3: Method results

   Finally, if at all possible, this document reserves the name dnswl,
   as detailed in Table 4.





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               +--------+------------+---------+----------+
               | Method | Defined    | version | Status   |
               +--------+------------+---------+----------+
               | dnswl  | [this rfc] |       1 | reserved |
               +--------+------------+---------+----------+

                       Table 4: Reserved method name


4.  Implementation Status

   [Note to RFC Editor: please remove this entire section before
   publication.]

   This section records the status of a known implementation of the
   method described in this document at the time of writing, based on a
   proposal described in "Improving Awareness of Running Code: The
   Implementation Status Section" ([RFC6982]).  See that document for
   further boilerplate that should have been copied here.

   OpenDKIM has optional DNSxL query support, and plans to implement
   this.

   Courier-MTA is a full-featured, mature mail server, first publicly
   released in May 2000.  A beta release in February 2013 introduced
   Authentication-Results in combination with DNS-based whitelists.  It
   made it to production in release 0.71, after one month testing, using
   the reserved method name.  End-user documentation of that feature is
   available online at
   http://www.courier-mta.org/couriertcpd.html#idp5867072

   In prior releases, only the -block option was present, and the
   Authentication-Results header field was handled by add-ons, not by
   the core implementation.  The -allow option was added so that black
   and white lists can be configured using mostly symmetrical syntax.
   Finally, an option was added to inhibit SPF reject-on-fail for
   whitelisted senders.

   It is not possible to know how many installations of Courier-MTA have
   enabled these new features.  However, no questions have been asked
   about them on the mailing list, yet.  Despite the amount of spam,
   there seems to be little traction for this kind of development.

   The only DNSWL known to have been used for this purpose is dnswl.org.
   See http://www.dnswl.org/.  With nearly 150K entries, it can make the
   email messages that get at least one authentication "pass" overreach
   a critical mass: It seems that subscribing to that list is easier,
   for some mail admins, than implementing other authentication methods.



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5.  Security Considerations

   All of the considerations described in Section 8 of
   [I-D.ietf-appsawg-rfc5451bis] apply.

   In addition, the usual caveats apply about importing text from
   external online sources.  Although queried DNSWLs are well known,
   trusted entities, it is suggested that TXT records be reported only
   if, upon inspection, their content is deemed actually actionable.


6.  References

6.1.  Normative References

   [I-D.ietf-appsawg-rfc5451bis]
              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status",
              draft-ietf-appsawg-rfc5451bis-10 (work in progress),
              July 2013.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2.  Informative References

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, April 2009.

   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
              February 2010.

   [RFC6650]  Falk, J. and M. Kucherawy, "Creation and Use of Email
              Feedback Reports: An Applicability Statement for the Abuse
              Reporting Format (ARF)", RFC 6650, June 2012.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              July 2013.







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Appendix A.  Example

      Authentication-Results: mta.example.com;
          dnswl=pass dns.zone=list.dnswl.example
          policy.ip=127.0.10.1
          policy.txt="example.org http://dnswl.example/s?s=100"


Author's Address

   Alessandro Vesely
   v. L. Anelli 13
   Milano, MI  20122
   IT

   Email: vesely@tana.it



































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