IETF A. Vesely
Internet-Draft
Intended status: Informational April 13, 2016
Expires: October 15, 2016
DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-05
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.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 October 15, 2016.
Copyright Notice
Copyright (c) 2016 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Vesely Expires October 15, 2016 [Page 1]
Internet-Draft DNSWL email-auth-method extension April 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Method Details . . . . . . . . . . . . . . . . . . . . . . . 2
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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". 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
Vesely Expires October 15, 2016 [Page 2]
Internet-Draft DNSWL email-auth-method extension April 2016
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.
Vesely Expires October 15, 2016 [Page 3]
Internet-Draft DNSWL email-auth-method extension April 2016
+--------+--------+----------+-------------------+--------+---------+
| 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
A new ptype, "dns" is introduced in Table 2. It is meant to be used
for properties related to the Domain Name System (DNS [RFC7208]),
whose value cannot be exactly derived from the relevant
authentication method specification.
+-------+------------+----------------------------------------------+
| ptype | Definition | Description |
+-------+------------+----------------------------------------------+
| dns | [this doc] | The property being reported is related 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
Vesely Expires October 15, 2016 [Page 4]
Internet-Draft DNSWL email-auth-method extension April 2016
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
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, DOI
10.17487/RFC5782, February 2010,
<http://www.rfc-editor.org/info/rfc5782>.
[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
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI
10.17487/RFC5598, July 2009,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7208>.
Vesely Expires October 15, 2016 [Page 5]
Internet-Draft DNSWL email-auth-method extension April 2016
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
policy.ip=127.0.10.1
policy.txt="example.org http://dnswl.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.
Vesely Expires October 15, 2016 [Page 6]
Internet-Draft DNSWL email-auth-method extension April 2016
Author's Address
Alessandro Vesely
v. L. Anelli 13
Milano, MI 20122
IT
Email: vesely@tana.it
Vesely Expires October 15, 2016 [Page 7]