DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-03

IETF                                                           A. Vesely
Internet-Draft
Intended status: Informational                          January 20, 2016
Expires: July 23, 2016

              DNSWL Email Authentication Method Extension
                    draft-vesely-authmethod-dnswl-03

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  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     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, [RFC6471]).  The lookup is based on the sender's
   IP address, so this check can occur very early in an SMTP
   transaction, and can be used to counterweight policies that typically
   occur at early stages too, such as the Sender Policy Framework (SPF,
   [RFC7208]).  Nevertheless, the result of a DNSWL lookup can be used
   at later stages as well; 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 can contain more than pass/fail, but they are encoded
   according to list-specific criteria.  The present method considers
   only whitelists --one of the major branches exemplified by [RFC6471].
   That way, at least the meaning of pass/fail is clear and actionable.

   Additional query results can be stored as parameters.  In effect,
   they are tantamount to local policies, albeit outsourced, and as such
   they have to be interpreted locally, based on list-specific
   knowledge.

2.  Method Details

   dns.zone:   DNSWL query root domain, which defines the meaning of the
               result.  Note that if the MTA uses a local mirror, this
               can be different from the zone actually queried.  MTAs
               need to map query root domains to local zones in order to
               minimize external queries.

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

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   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.

   fail:       The query worked but yelded no record, or returned
               NXDOMAIN, so the sender is not whitelisted.

   temperror:  There was a transient problem with the query.

   permerror:  There is a setup problem, the query cannot work.  This
               possibly includes over quota cases interpreted by the
               result producer.

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.

   +--------+--------+----------+-------------------+--------+---------+
   | Method | ptype  | property | Value             | Status | Version |
   +--------+--------+----------+-------------------+--------+---------+
   | dnswl  | dns    | zone     | DNSWL publicly    | active |       1 |
   |        |        |          | accessible query  |        |         |
   |        |        |          | root domain       |        |         |
   | dnswl  | policy | ip       | type A query      | active |       1 |
   |        |        |          | result in dotted  |        |         |
   |        |        |          | quad              |        |         |
   | dnswl  | policy | txt      | type TXT query    | active |       1 |
   |        |        |          | result            |        |         |
   +--------+--------+----------+-------------------+--------+---------+

                   Table 1: Email Authentication Method

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   +-------+------------+----------------------------------------------+
   | 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.

   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>.

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   [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

   [RFC6471]  Lewis, C. and M. Sergeant, "Overview of Best Email DNS-
              Based List (DNSBL) Operational Practices", RFC 6471, DOI
              10.17487/RFC6471, January 2012,
              <http://www.rfc-editor.org/info/rfc6471>.

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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