DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-11
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8904.
|
|
|---|---|---|---|
| Author | Alessandro Vesely | ||
| Last updated | 2019-11-08 (Latest revision 2019-10-15) | ||
| RFC stream | Independent Submission | ||
| Formats | |||
| IETF conflict review | conflict-review-vesely-authmethod-dnswl, conflict-review-vesely-authmethod-dnswl, conflict-review-vesely-authmethod-dnswl, conflict-review-vesely-authmethod-dnswl, conflict-review-vesely-authmethod-dnswl, conflict-review-vesely-authmethod-dnswl | ||
| Additional resources | |||
| Stream | ISE state | Response to Review Needed | |
| Consensus boilerplate | Unknown | ||
| Document shepherd | Eliot Lear | ||
| IESG | IESG state | Became RFC 8904 (Informational) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Adrian Farrel <rfc-ise@rfc-editor.org> |
draft-vesely-authmethod-dnswl-11
IETF A. Vesely
Internet-Draft October 15, 2019
Intended status: Informational
Expires: April 17, 2020
DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-11
Abstract
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
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 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
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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
response.
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
default.
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 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 192.0.2.38 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 127.0.0.1. In such cases, the property dns.sec=yes
corresponds to the Authenticated Data (AD) bit in the DNS response
header.
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,
<https://www.rfc-editor.org/info/rfc2606>.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
DOI 10.17487/RFC5782, February 2010, <https://www.rfc-
editor.org/info/rfc5782>.
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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-
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>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>.
[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,
<https://www.rfc-editor.org/info/rfc4033>.
[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-
editor.org/info/rfc4880>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<https://www.rfc-editor.org/info/rfc5198>.
[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>.
[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-
editor.org/info/rfc6762>.
[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
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.ip=127.0.10.1
policy.txt="fwd.example https://dnswl.example/?d=fwd.example"
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; 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
IT
Email: vesely@tana.it
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