Internet DRAFT - draft-vesely-authmethod-dnswl
draft-vesely-authmethod-dnswl
IETF A. Vesely
Internet-Draft April 30, 2020
Intended status: Informational
Expires: November 1, 2020
DNSWL Email Authentication Method Extension
draft-vesely-authmethod-dnswl-16
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 is provided
for information in case the method is seen in the field, as well as
to suggest a useful practice and register the relevant keywords.
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
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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 November 1, 2020.
Copyright Notice
Copyright (c) 2020 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Method Details . . . . . . . . . . . . . . . . . . . . . . . 3
3. TXT Record Contents . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4.1. Email Authentication Methods . . . . . . . . . . . . . . 6
4.2. Email Authentication Property Type . . . . . . . . . . . 6
4.3. Email Authentication Result Names . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Over Quota Signaling . . . . . . . . . . . . . . . . . . 7
5.2. Security of DNSSEC Validation . . . . . . . . . . . . . . 7
5.3. Inherited Security Considerations . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Known Implementation . . . . . . . . . . . . . . . . 12
Appendix C. Future possibilities of the 'dns' ptype . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
One of the many checks that mail servers carry out is to query domain
name system whitelists (DNSWL). That method is fully discussed in
[RFC5782]. The DNS [RFC1034] lookup is based on the connecting
client's IP address, IPv4 or IPv6, and returns zero or more A
records. The latter are IPv4 IP addresses in the range 127.0.0.0/8.
Depending on the query, TXT records with varying content can also be
retrieved. Query examples are given in Appendix A.
Since the IP address is known as soon as the connection is accepted,
this check can occur very early in an SMTP transaction. Its 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 also 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 method described in this document considers
only whitelists --one of the major branches described by [RFC5782].
There are also black/block lists, DNSBL, and combined lists. Since
they all have the same structure, the abbreviation DNSxL is used to
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mean any. The core procedures of a mail transfer agent (MTA) tend to
be quite general, leaving particular cases to be handled by add-on
modules. In the case of combined lists, 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 each entry, as that
is required to make accept/reject decisions. This document provides
for storing the result when the DNSxL record to be reported is a
whitelisting one.
Data conveyed in A and TXT records can be stored as method's
properties. The meaning of such data varies widely at the mercy of
the list operator, hence the queried zone has to be stored as well.
Mail site operators who configure their MTAs to query specific DNWSLs
marry the policies of those lists, as, in effect, they become
tantamount to local policies, albeit outsourced. Downstream agents
who know DNSWL-specific encoding and understand the meaning of that
datum can use it to make delivery or display decisions. For example,
a mail filter which detected heuristic evidence of a scam can
counterweight such information with the trustworthiness score encoded
in the A response, so as to protect agains false positives. MUAs can
display those results, or use them to decide how to report abusive
messages, if configured to do so.
This document describes a usage of TXT fields consistent with other
authentication methods, namely to serve the domain name in the TXT
record. That way, a downstream filter could also consider whether
the sending agent is aligned with the author domain, with semantics
similar to [RFC7489].
At the time of this writing, this method is implemented by
[Courier-MTA]. An outline of the implementation is given in
Appendix B.
2. Method Details
The result of the method states how the query did, up to the
interpretation of the returned data.
The method has four possible results:
pass: The query successfully returned applicable records. This
result is usually accompanied by one or both of the
policy properties described below. Since the list is
configured as a DNSWL, agents unable to interpret list-
specifc 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.
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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. 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
over quota, e.g., a DNS RCODE of 5, commonly known as
REFUSED, or a DNSWL-specific property (policy.ip, defined
below) with the same meaning. A later attempt is
unlikely to produce a final result. Human intervention
is required.
Note that there is no "fail" result.
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. Setting dns.zone to
the global zone makes the result intelligible even if the
message is handed outside of the internal network.
policy.ip: The bit mask value received in type A response, in dotted
quad notation. Multiple entries can be arranged in a
quoted, comma separated list (quotes are necessary
because commas are not allowed in a token.)
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 5.2 considers how that is related to the DNS
response.
no: The data are not signed. See Section 5.2.
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
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security. In particular, "na" is used if the data
were 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.
3. TXT Record Contents
According to [RFC5782], TXT records describe the reason why IP
addresses are listed in a DNSWL. An example of a DNSWL whose TXT
records contain the domain name of the organization assignee of the
sending IP is given in Appendix B. The domain name would correspond
to the DNS domain name used by or within the administrative domain
(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]), if the DNSWL is trusted.
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 address, for example because the IP address 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
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.
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4. IANA Considerations
IANA maintains the "Email Authentication Parameters" registry with
several subregistries. IANA is requested to make assignments as set
out in the following sections.
4.1. Email Authentication Methods
IANA is requested to create four new entries in the "Email
Authentication Methods" registry as follows.
------+----------+------+--------+-------------------+------+-------
Method|Definition|ptype |property| Value |Status|Version
------+----------+------+--------+-------------------+------+-------
dnswl |[This.I-D]|dns |zone | DNSWL publicly |active| 1
| | | | accessible query | |
| | | | root domain | |
dnswl |[This.I-D]|policy|ip | type A response |active| 1
| | | | received (or a | |
| | | | quoted, comma | |
| | | | separated list | |
| | | | thereof) | |
dnswl |[This.I-D]|policy|txt | type TXT query |active| 1
| | | | response | |
dnswl |[This.I-D]|dns |sec | one of "yes" for |active| 1
| | | | DNSSEC | |
| | | | authenticated | |
| | | | data, "no" for | |
| | | | not signed, or | |
| | | | "na" for not | |
| | | | applicable | |
------+----------+------+--------+-------------------+------+-------
4.2. Email Authentication Property Type
IANA is requested to create a new entry in the "Email Authentication
Property Types" registry as follows.
+-------+------------+----------------------------------------------+
| ptype | Definition | Description |
+-------+------------+----------------------------------------------+
| dns | [This.I-D] | The property being reported belongs to the |
| | | Domain Name System |
+-------+------------+----------------------------------------------+
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4.3. Email Authentication Result Names
IANA is requested to create four new entries in the "Email
Authentication Result Names" registry as follows.
+-------------+-----------+---------------+--------+
| Auth Method | Code | Specification | Status |
+-------------+-----------+---------------+--------+
| dnswl | pass | [This.I-D] | active |
| | | | |
| dnswl | none | [This.I-D] | active |
| | | | |
| dnswl | temperror | [This.I-D] | active |
| | | | |
| dnswl | permerror | [This.I-D] | active |
+-------------+-----------+---------------+--------+
5. Security Considerations
5.1. Over Quota Signaling
Some DNSWLs which provide for free access below a given quota 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. It can accept messages that it would
otherwise reject. A DNSWL-specific module would realize the fact and
call for human intervention.
Returning an RCODE 5 (REFUSED) conveys the concept that the query is
"unauthorized", and human intervention required.
5.2. 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. According to 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 are 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.
5.3. 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>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8460] Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", RFC 8460,
DOI 10.17487/RFC8460, September 2018, <https://www.rfc-
editor.org/info/rfc8460>.
[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>.
[Courier-MTA]
"Courier Mail Server", <https://www.courier-mta.org/>.
[dnswl.org]
"DNSWL.ORG", <https://www.dnswl.org/>.
Appendix A. Example
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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=2001:db8::2:1;
envelope-from="sender@example.com";
helo=mail.fwd.example;
receiver=mta.example.org;
Received: from mail.fwd.example (mail.fwd.example [2001:db8::2:1])
(TLS: TLSv1/SSLv3,128bits,ECDHE-RSA-AES128-GCM-SHA256)
by mta.example.org with ESMTPS; Thu, 03 Oct 2019 19:23:11 +0200
id 00000000005DC044.000000005702D87C.000007FC
Trace fields at the top of the header
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]. Its behavior hinges on the following DNS
entries:
1.0.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.d.b.8.2.0.0.1.
list.dnswl.example.
IN A 127.0.10.1
IN TXT "fwd.example https://dnswl.example/?d=fwd.example"
DNS resource records for 2001:db8::2:1
(line breaks for editorial reasons)
If mail.fwd.example had connected from address 192.0.2.1, then the
query name would have been 1.2.0.192.list.dnswl.example. See full
description in [RFC5782]
At connection time, because the remote IP address is whitelisted, the
target MTA did not reject the message before DATA. Instead, it
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recorded the SPF fail result, and indicated the local policy
mechanism which was applied in order to override that result.
Subsequent filtering verified DKIM [RFC6376].
At later stages, mail filters can reject or quarantine the message
based on its content. A deeper knowledge of the policy values
obtained from dnswl.example allows to interpret the values of
policy.ip and weight them against other factors so as to make better
decisions.
Appendix B. Known Implementation
Implementation details mentioned in this section have been stable for
several years. Yet, this description is necessarily superficial,
version dependent, and subject to change.
[Courier-MTA] can be configured to lookup DNSBL and DNSWL, with
similar command line switches:
-block=zone[=displayzone][,var[/n.n.n.n][,msg]]
-allow=zone[=displayzone][,var[/n.n.n.n[,]]]
zone is the zone to be queried.
displayzone is only used for -allow, it is the value to be set in the
dns.zone property.
var stands for the environment variable whose existence triggers a
special action. The default variable names result in a conventional
behavior implemented by Courier-MTA. By setting different
environment variables, users can customize the behavior.
Conventional behavior differs widely between -block and -allow. The
former rejects the message, the latter produces Authentication-
Results header fields.
The n.n.n.n IP address requires a precise A record response. If not
given, any response results in setting the corresponding variable.
If given, variables are set only if the response matches exactly.
Such syntax provides for a very limited interpretation of the
information encoded in A records. However, it is considered to be
too complicated already. Even specifying a range, an enumeration of
values, or a regular expression would require something beyond what a
normal user would be willing to manage.
Finally, the trailing message, which overrides the 5xx SMTP reply for
-block, is not used for -allow, except that its mere presence
requires to query TXT records to be registered in policy.txt.
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SPF is part of Courier-MTA's core. It is configured separately, and
provides for an "allowok" keyword to indicate to override rejection
in case of SPF failure and -allow whitelisting.
A customary whitelist is [dnswl.org]. It serves A records encoded as
follows:
1st octect: 127.
2nd octect: 0.
3rd octect: Category of business, 15 values.
4th octect: Trusworthiness/score, 4 values.
They also serve TXT records containing the domain name followed by an
URL pointing to further info about the relevant organization, such as
what other IP addresses of theirs are being whitelisted. They don't
use UTF-8.
dnswl.org provides for free registration and free access below
100,000 queries per day. They use the special return code
127.0.0.255 exemplified above to signal over quota. Although
Courier-MTA itself does not recognize it, it has a mail filter
(zdkimfilter, named after its main usage) where recognition of that
code, as well as that of trusworthiness in the 4th octect are hard-
coded.
Appendix C. Future possibilities of the 'dns' ptype
The description of the new ptype proposed in Section 4.2 just says
"The property being reported belongs to the Domain Name System".
That definition can broadly include any tag found in a domain's TXT
record. For example, auth methods designers can agree that within a
resinfo of a given method, any dns ptype refers to tags in the
relevant DNS record, unless otherwise specified. So one could have,
say:
Authentication-Results: example.com;
spf=pass smtp.mailfrom=example.net dns.sec=y;
dkim=pass header.i=@example.org header.b=jIvx30NG dns.s=tlsrpt
While dns.sec is defined above, albeit not for the spf method, the
use of tlsrpt in the DKIM record is exemplified in Section 3 of
[RFC8460]. The tag s= is part of the DKIM TXT record, not to be
confused with the selector s=, which is part of a DKIM-Signature.
Just like the latter can be reported as header.s because the DKIM
header field is in the message header, it may make sense to report
the former as dns.s because the DKIM DNS record is in the DNS.
Vesely Expires November 1, 2020 [Page 13]
Internet-Draft DNSWL email-auth-method extension April 2020
NOTE: This is only a hint at what may become a consistent naming
convention around the new ptype. In any case, any new property using
this ptype requires its own formal definition. This document does
NOT define the property dns.s=, let alone the service tlsrpt.
Author's Address
Alessandro Vesely
v. L. Anelli 13
Milano, MI 20122
IT
Email: vesely@tana.it
Vesely Expires November 1, 2020 [Page 14]
