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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 8904

IETF                                                           A. Vesely
Internet-Draft                                          October 15, 2019
Intended status: Informational
Expires: April 17, 2020

              DNSWL Email Authentication Method Extension


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

   This document 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.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 17, 2020.

Copyright Notice

   Copyright (c) 2019 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Method Details  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  TXT Record Contents . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Security of DNSSEC Validation . . . . . . . . . . . . . .   6
     5.2.  Inherited Security Considerations . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 learn the trustworthiness of a mail relay in
   order 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.
   The present method can then be used only if the DNSxL record to be
   reported turns out to be a whitelisting one.

   Data conveyed in A and TXT records can be stored as method's
   properties.  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.

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2.  Method Details

   The following ptype.property items define how the data provided by
   the whitelist lookup can be saved.

   dns.zone:   DNSWL query root domain, which defines the meaning of the
               policy.ip property below.  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 in the usual way (explained, for example, in
               Section 3.3 of [RFC7208]).  See Section 3 for the
               resulting content and query options.
   dns.sec:    This is a generic property stating whether the relevant
               data was validated using DNSSEC ([RFC4033]).  For the
               present method, the relevant data consists of the
               reported policy properties above, or, if the method
               result is "none", their non-existence.  This property has
               three possible values:

               yes: DNSSEC validation confirms the integrity of data.
                    Section 5.1 considers how that is related to the DNS
               no:  The data is not signed.  See Section 5.1.
               na:  Not applicable.  No DNSSEC validation can be
                    performed, possibly because the lookup is run
                    through a different means than a security-aware DNS
                    resolver.  This does not necessarily imply less
                    security.  In particular, "na" is used if the data
                    was downloaded in bulk and then loaded on a local
                    nameserver --which is the case of an MTA querying a
                    local zone different from the reported dns.zone.
                    DNS errors, including validation errors, can also
                    report "na".  This is also the value assumed by

   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  If the MTA cannot interpret that value, that case
   results in a false positive.

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   The method has four possible results:

   pass:       The query successfully returned applicable records.  This
               result is usually accompanied by one or both the policy
               properties described above.  Agents unable to interpret
               those properties can still derive a positive value from
               the fact that the sender is whitelisted.
   none:       The query worked but yielded no A 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.

   Note that there is no fail result.

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
   signature ([RFC6376]) or an SPF check host ([RFC7208]).

   According to a DNSWL's policy, attributing responsibility of an IP
   address to an organization may require something more than a mere PTR
   record consistency.  If no domain names can be responsibly associated
   to a given IP, for example because the IP was added without direct
   involvement of the organization concerned, DNSWLs can use a subdomain
   of .INVALID ([RFC2606]) where the leftmost label hints at why an
   address is whitelisted.  For example, if the address was
   added by the list managers solely based on their knowledge, the
   corresponding TXT record might be AUTOPROMOTED.INVALID, so as to
   avoid to explicitly identify an entity who didn't opt-in.

   Following the example of Multicast DNS (see the second paragraph of
   Section 16 of [RFC6762]) names containing non-ASCII characters can be
   encoded in UTF-8 [RFC3629] using the normalization form canonical
   composition (NFC) as described in Unicode Format for Network

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   Interchange ([RFC5198]).  Inclusion of unaltered UTF-8 TXT values in
   the header entails an environment compatible with EAI [RFC6530].

   DNS 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 TXT records exist if
   a query for QTYPE A brought a result of none.

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          |        |         |
   | dnswl  | dns    | sec      | one of "yes" for  | active |       1 |
   |        |        |          | DNSSEC            |        |         |
   |        |        |          | authenticated     |        |         |
   |        |        |          | data, "no" for    |        |         |
   |        |        |          | not signed, or    |        |         |
   |        |        |          | "na" for not      |        |         |
   |        |        |          | applicable        |        |         |

                   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

5.1.  Security of DNSSEC Validation

   The dns.sec property is meant to be as secure as DNSSEC results.  It
   makes sense to use it in an environment where the DNSSEC validation
   can succeed.

   Section 7 of [RFC4033] examines various ways of setting up a stub
   resolver which either validates DNSSEC locally or trusts the
   validation provided through a secure channel.  For a different class,
   it is possible to set up a dedicated, caching, dnssec-enabled
   resolver reachable by the mail server through interprocess
   communication on  In such cases, the property dns.sec=yes
   corresponds to the Authenticated Data (AD) bit in the DNS response

   When the response contains no DNSSEC data, a security-aware resolver
   seeks a signed proof of the non-existence of a DS record, at some

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   delegation point.  If no error is returned, the zone is unsigned and
   dns.sec=no can be set.  Quoting the Security Considerations
   Section of [RFC3225]: The absence of DNSSEC data in response to a
   query with the DO bit set MUST NOT be taken to mean no security
   information is available for that zone as the response may be forged
   or a non-forged response of an altered (DO bit cleared) query.

   If the application verifies the DNSSEC signatures on its own, it
   effectively behaves like a validating stub resolver, and hence can
   set dns.sec correspondingly.

   When the data is downloaded in bulk and made available on a trusted
   channel without using DNSSEC, set dns.sec=na or not at all.  DNSWL
   who publish bulk versions of their data can also sign that data, for
   example using OpenPGP ([RFC4880]).  It is the responsibility of
   system administrators to authenticate the data by downloading and
   validating the signature.  The result of such validation is not
   reported using dns.sec.

5.2.  Inherited Security Considerations

   For DNSSEC, the considerations of Section 12 of [RFC4033] apply.

   All of the considerations described in Section 7 of [RFC8601] apply.
   That includes securing against tampering all the channels after the
   production of this Authentication-Results header field.

   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, and
   their format compatible with the computing environment.

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,

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

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   [RFC8601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 8601,
              DOI 10.17487/RFC8601, May 2019, <https://www.rfc-

6.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007, <https://www.rfc-

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,

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

   [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,

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/info/rfc6530>.

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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013, <https://www.rfc-

   [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-

   [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

   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 dns.sec=na
     policy.txt="fwd.example https://dnswl.example/?d=fwd.example"
   Received-SPF: fail (Address does not pass Sender Policy Framework)
   Received: from mailout.fwd.example (mailout.fwd.example [])
     (TLS: TLSv1/SSLv3,128bits,ECDHE-RSA-AES128-GCM-SHA256)
     by mta.example.org with ESMTPS; Thu, 03 Ocy 2019 19:23:11 +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].

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   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.

Author's Address

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

   Email: vesely@tana.it

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