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IETF                                                           A. Vesely
Internet-Draft                                              May 18, 2019
Intended status: Informational
Expires: November 19, 2019


              DNSWL Email Authentication Method Extension
                    draft-vesely-authmethod-dnswl-08

Abstract

   This document describes an additional Email Authentication Method
   compliant with RFC 8601.  The method consists in looking up the
   sender'IP in a DNS whitelist.

   This document describes does not consider black lists.

Status of This Memo

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

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   This Internet-Draft will expire on November 19, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Method Details  . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  TXT Record Contents . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   One of the many checks that mail servers carry out is to query DNS
   whitelists (DNSWL, [RFC5782]).  The lookup is based on the connecting
   client's IP address, so this check can occur very early in an SMTP
   transaction.  The result can be used to counterweight policies that
   typically occur at early stages too, such as the Sender Policy
   Framework (SPF, the last paragraph of Appendix D.3 of [RFC7208] is
   illustrated in Appendix A).  In addition, the result of a DNSWL
   lookup can also be used at later stages; for example, a delivery
   agent can use it to estimate the spamminess of an email message.  The
   latter possibility needs a place to collect query results for
   downstream use, which is precisely what the Authentication-Results
   header field aims at providing.

   Results often contain additional data, encoded according to DNSWL-
   specific criteria.  The present method considers only whitelists
   --one of the major branches considered by [RFC5782].  In case of
   DNSxL, the boundary MTA (see [RFC5598]) which carries out the check
   and possibly stores the result, has to be able to discern at least
   the color of "x", which is required to make accept/reject decisions.

   Data conveyed in A and TXT records can be stored as result's
   parameters.  In effect, they are tantamount to local policies, albeit
   outsourced.  Downstream agents need to know DNSWL-specific encoding
   to understand the meaning of that data.  In order to smooth
   operations, this document endorses a usage of TXT fields consistent
   with other authentication methods.  Namely, to serve the domain name
   in the TXT record.

2.  Method Details

   The following ptype.property items define the relevant parameters
   where additional data can be stored.  They augment the "pass" result
   with information about the entry found.



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   dns.zone:   DNSWL query root domain, which defines the meaning of the
               result.  Note that an MTA can use a local mirror with a
               different name.  The name stored here has to be the best
               available reference for all foreseeable downstream
               consumers.  If the message is handed outside the internal
               network, dns.zone had better be the global zone.

   policy.ip:  The bit mask value received in type A response, in dotted
               quad.  Multiple entries can be arranged in a comma-
               separated list.

   policy.txt: The TXT record, if any.  Multiple records are
               concatenated as usual.  See Section 3 for the resulting
               content and query options.

   The result of the method states how the query did, up to the
   interpretation of the result.  In particular, some DNSBLs are known
   to return special codes to signal over quota, for example
   127.0.0.255.  If the result producer cannot interpret that value,
   that case results in a false positive.

   pass:       The query successfully returned applicable records.  The
               sender is whitelisted, up to differing interpretation.

   none:       The query worked but yielded no record, or returned
               NXDOMAIN, so the sender is not whitelisted.

   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 may produce a final result.

   permerror:  The DNS evaluation cannot work because test entries don't
               work, that is, DNSWL is broken, or because queries are
               overquota, e.g., a DNS RCODE of 5, commonly known as
               REFUSED, or a DNSWL-specific policy.ip was returned.  A
               later attempt is unlikely to produce a final result.
               Human intervention is required.

3.  TXT Record Contents

   According to [RFC5782], TXT records describe the reason why IP
   addresses are listed in a DNSWL.  The TXT record is useful if it
   contains the domain name(s).  The domain name would correspond to the
   DNS domain name used by or within the ADMD operating the relevant
   MTA, sometimes called the "organizational domain".  In that case, the
   authentication provided by this method is equivalent to a DKIM



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   signature ([RFC6376]) or an SPF check host ([RFC7208]).  When no
   domain names are known, some DNSWLs use a subdomain of .INVALID
   ([RFC2606]) where the leftmost label hints at why an address is
   whitelisted given that its operating organization is not known.  If
   the TXT record(s) contain non-ASCII characters, they need to be
   encoded as appropriate.

   Queries with a QTYPE of ANY may lead to inconsistent replies,
   depending on the cache status.  In addition, ANY is not "all", and
   the provisions for queries that have QTYPE=ANY ([RFC8482]) don't
   cover DNSxLs.  A mail server can issue two simultaneous queries, A
   and TXT.  Otherwise, a downstream filter can issue a TXT query on its
   own if it knows that an A query was successful, and that the DNSWL
   serves useful TXT records.  It is unlikely that a TXT record exist if
   a query for QTYPE A failed.

4.  IANA Considerations

   There is a registry of Email Authentication Methods.  The method
   described in this document is referred by Table 1, along with its
   ptype.property values.

   +--------+--------+----------+-------------------+--------+---------+
   | Method | ptype  | property | Value             | Status | Version |
   +--------+--------+----------+-------------------+--------+---------+
   | dnswl  | dns    | zone     | DNSWL publicly    | active |       1 |
   |        |        |          | accessible query  |        |         |
   |        |        |          | root domain       |        |         |
   | dnswl  | policy | ip       | type A response   | active |       1 |
   |        |        |          | received (or      |        |         |
   |        |        |          | comma-separated   |        |         |
   |        |        |          | list thereof)     |        |         |
   | dnswl  | policy | txt      | type TXT query    | active |       1 |
   |        |        |          | response          |        |         |
   +--------+--------+----------+-------------------+--------+---------+

                   Table 1: Email Authentication Method

   A new ptype, "dns" is introduced in Table 2.  It is meant to be used
   for properties related to the Domain Name System (DNS [RFC1034]).











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   +-------+------------+----------------------------------------------+
   | ptype | Definition | Description                                  |
   +-------+------------+----------------------------------------------+
   | dns   | [this doc] | The property being reported belongs to the   |
   |       |            | Domain Name System                           |
   +-------+------------+----------------------------------------------+

                Table 2: Email Authentication Property Type

   This method reuses four of the values already defined in the Email
   Authentication Result Names associated registry.  They are listed in
   Table 3.

   +---------+-----------+------------------------------------+--------+
   | Auth    | Code      | Specification                      | Status |
   | Method  |           |                                    |        |
   +---------+-----------+------------------------------------+--------+
   | dnswl   | pass      | Sender is whitelisted, up to       | active |
   |         |           | returned code interpretation       |        |
   | dnswl   | none      | NXDOMAIN or no record, sender is   | active |
   |         |           | not whitelisted                    |        |
   | dnswl   | temperror | Transient DNS error during the     | active |
   |         |           | query                              |        |
   | dnswl   | permerror | Query cannot work, human           | active |
   |         |           | intervention needed                |        |
   +---------+-----------+------------------------------------+--------+

                Table 3: Email Authentication Result Names

5.  Security Considerations

   All of the considerations described in Section 7 of [RFC8601] 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

   [RFC2606]  Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, DOI 10.17487/RFC2606, June 1999,
              <https://www.rfc-editor.org/info/rfc2606>.






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   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
              DOI 10.17487/RFC5782, February 2010, <https://www.rfc-
              editor.org/info/rfc5782>.

   [RFC8601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 8601,
              DOI 10.17487/RFC8601, August 2019, <https://www.rfc-
              editor.org/info/rfc8601>.

6.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009, <https://www.rfc-
              editor.org/info/rfc5598>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014, <https://www.rfc-
              editor.org/info/rfc7208>.

   [RFC8482]  Abley, J., Gudmundsson, O., Majkowski, M., and E. Hunt,
              "Providing Minimal-Sized Responses to DNS Queries That
              Have QTYPE=ANY", RFC 8482, DOI 10.17487/RFC8482, January
              2019, <https://www.rfc-editor.org/info/rfc8482>.

Appendix A.  Example
















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   Delivered-To: recipient@example.org
   Return-Path: <sender@example.com>
   Authentication-Results: mta.example.org;
     dkim=pass (whitelisted) header.i=@example.com
   Authentication-Results: mta.example.org;
     dnswl=pass dns.zone=list.dnswl.example
     policy.ip=127.0.10.1
     policy.txt="fwd.example http://fwd.example/s?s=100"
   Received-SPF: fail (Address does not pass Sender Policy Framework)
     client-ip=192.0.2.1;
     envelope-from="sender@example.com";
     helo=mailout.fwd.example;
     receiver=mta.example.org;
   Received: from mailout.fwd.example (mailout.fwd.example [192.0.2.1])
     (TLS: TLSv1/SSLv3,128bits,ECDHE-RSA-AES128-GCM-SHA256)
     by mta.example.org with ESMTPS; Mon, 04 Apr 2016 23:11:24 +0200
     id 00000000005DC044.000000005702D87C.000007FC

   Trace fields added at the top of the header by multiple agents at
   various stages during processing at the final MTA

   The message went through a third party, fwd.example, which forwarded
   it to the final MTA.  Such mail path was not arranged beforehand with
   the involved MTAs, it emerged spontaneously.  This message would not
   have made it to the target without whitelisting, because:

   o  the author domain published a strict SPF policy (-all),

   o  the forwarder did not alter the bounce address, and

   o  the target usually honors reject-on-fail, according to Section 8.4
      of [RFC7208].

   However, the target also implemented the last paragraph of
   Appendix D.3 of [RFC7208].  Rather than rejecting the message
   outright before DATA, the MTA received it, recorded the SPF fail
   result, and indicated the local policy mechanism which was applied in
   order to override that result.  Subsequent filtering detected no
   malware and verified DKIM [RFC6376].  It would still have been
   possible to reject the message, based on its content.  It is at these
   later stages, after receiving the body and also during delivery, that
   a deeper knowledge of the policy values obtained from dnswl.example
   can allow weighting that score against other factors.








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Author's Address

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

   Email: vesely@tana.it











































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