DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-04

IETF                                                           A. Vesely
Internet-Draft
Intended status: Informational                             April 4, 2016
Expires: October 6, 2016

              DNSWL Email Authentication Method Extension
                    draft-vesely-authmethod-dnswl-04

Abstract

   This document describes an additional Email Authentication Method
   compliant with RFC 7601.  The method consists in looking up the
   sender'IP in a DNS whitelist.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Method Details  . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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, see in particular the last paragraph of Appendix D.3
   of [RFC7208]).  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".  It is required to make accept/reject decisions.

   Additional data can be stored as result's parameters.  In effect,
   they are tantamount to local policies, albeit outsourced.  Downstream
   agents can take advantage of DNSWL-specific knowledge to make finer
   decisions, such as render additional information or deliver to
   apposite folders.

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.

   dns.zone:   DNSWL query root domain, which defines the meaning of the
               result.  Note that an MTA can use a local mirror with a

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               different name.  The name stored here has to be the best
               available reference for all foreseable downstream
               consumers.

   policy.ip:  The bit mask value received in type A response, in dotted
               quad.  This entry can be repeated if the DNSWL returns
               multiple A records.

   policy.txt: The TXT record, if any.  Multiple records are
               concatenated.

   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.  IANA Considerations

   There is a registry of Email Authentication Methods created by
   RFC7601.  The method described in this document is referred by
   Table 1, along its ptype.property values.  A new "ptype" is
   introduced in Table 2.

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   +--------+--------+----------+-------------------+--------+---------+
   | 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 (may be  |        |         |
   |        |        |          | repeated)         |        |         |
   | dnswl  | policy | txt      | type TXT query    | active |       1 |
   |        |        |          | response          |        |         |
   +--------+--------+----------+-------------------+--------+---------+

                   Table 1: Email Authentication Method

   +-------+------------+----------------------------------------------+
   | ptype | Definition | Description                                  |
   +-------+------------+----------------------------------------------+
   | dns   | [this doc] | The property being reported is a domain name |
   +-------+------------+----------------------------------------------+

                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   | fail      | 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

4.  Security Considerations

   All of the considerations described in Section 7 of [RFC7601] apply.

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   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.  If
   they contain non-ASCII characters, they need to be encoded as
   appropriate.

5.  References

5.1.  Normative References

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

   [RFC7601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7601, DOI 10.17487/
              RFC7601, August 2015,
              <http://www.rfc-editor.org/info/rfc7601>.

5.2.  Informative References

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

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

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <http://www.rfc-editor.org/info/rfc7719>.

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

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

   Email: vesely@tana.it

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