Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-01
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draft-nurpmeso-smtp-tls-srv-01
Internet Engineering Task Force S. Nurpmeso, Ed.
Internet-Draft 31 October 2024
Updates: 3207 5321 5598 (if approved)
Intended status: Informational
Expires: 4 May 2025
Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-01
Abstract
It is described how DNS (RFC 1035) SRV (RFC 2782) records announce
availability of TLS (RFC 8446) enabled Secure SMTP (RFC 3207, RFC
5321).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 4 May 2025.
Copyright Notice
Copyright (c) 2024 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 (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. SMTP/TLS SRV Service Name . . . . . . . . . . . . . . . . . . 2
3. Example: service record . . . . . . . . . . . . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 3
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 3
6.1. Normative References . . . . . . . . . . . . . . . . . . 3
6.2. Informative References . . . . . . . . . . . . . . . . . 4
Appendix A. Rationale . . . . . . . . . . . . . . . . . . . . . 6
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC2782] defines a widely adopted DNS-based service discovery
protocol. [RFC6186] is a specification of [RFC2782] SRV records for
the email protocols IMAP[RFC9051], POP3[RFC1939], and
SUBMISSION[RFC6409]. SMTP[RFC5321] connections to MTAs ([RFC5598],
section 4.3.2) are yet excluded from their share on the widely
supported SRV record, and the email architecture security
considerations ([RFC5598], section 6.1) do not yet cover
explorability of transport security. The SMTP/TLS SRV announcement
can be queried before a connection to a discovered SMTP MX server is
established; it MAY be used to distribute SMTP load via the SRV
Priority and Weight mechanism further.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term "Implicit TLS" refers to the automatic negotiation of TLS
whenever a TCP connection is made on a particular TCP port that is
used exclusively by that server for TLS connections. The term
"Implicit TLS" is intended to contrast with the use of the STARTTLS
command in SMTP that is used by the client and the server to
explicitly negotiate TLS on an established cleartext TCP connection.
2. SMTP/TLS SRV Service Name
The service name for TLS[RFC8446] enabled Secure[RFC3207]
SMTP[RFC5321] is smtps, the resulting DNS label _smtps.
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STARTTLS
Whenever a domain publishes an according DNS SRV[RFC2782] resource
record it asserts availability of Secure SMTP, that is, of the
STARTTLS[RFC3207] SMTP service extension on the normal
SMTP[RFC5321] port (specified by IANA as port 25). The port
number MUST be given as 0.
Implicit TLS
If the port number of the published SRV resource record is not 0,
then the domain announces to support Implicit TLS on the given
port in addition to STARTTLS on the normal SMTP port. (Specifying
the IANA SMTP port results in a non-compliant service, and SHOULD
NOT be used on the internet.) The STARTTLS[RFC3207] extension
command MUST NOT be used during connections on an Implicit TLS
port.
3. Example: service record
Here are two examples, the first announcing to support the STARTTLS
SMTP service extension, the second specifying the port number 26 to
indicate an additional Implicit TLS port.
_smtps._tcp SRV 0 1 0 mail1.example.com.
_smtps._tcp SRV 0 1 26 mail2.example.com.
4. IANA Considerations
IANA is not expected to add a dedicated port for "Implicit TLS SMTP".
Shall it consider otherwise the author suggests port 26.
5. Security Considerations
This specification can be used to avoid downgrade attacks on the
opportunistic approach of STARTTLS, accomplished via the mechanism
used for many other IETF standardized protocols, most notably
[RFC2782] (IMAP, POP3, SUBMISSION). With its Implicit TLS capability
it grants the SMTP protocol the same level of confidentiality through
TLS[RFC8446] as is already standardized for the other email
protocols; the fewer necessary packet roundtrips aid, too. The
security of DNS[RFC1035] is out of scope for this specification, but
DNSSEC[RFC4033][RFC4034][RFC4035] and secure DNS
transport[RFC7858][RFC8094][RFC8310][RFC9250] etc exists.
6. References
6.1. Normative References
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
6.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
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[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
[RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
Access Protocol (IMAP) - Version 4rev2", RFC 9051,
DOI 10.17487/RFC9051, August 2021,
<https://www.rfc-editor.org/info/rfc9051>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.
[RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/info/rfc9460>.
[I-D.ietf-tls-svcb-ech]
Schwartz, B. M., Bishop, M., and E. Nygren, "Bootstrapping
TLS Encrypted ClientHello with DNS Service Bindings", Work
in Progress, Internet-Draft, draft-ietf-tls-svcb-ech-06,
21 October 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-svcb-ech-06>.
Appendix A. Rationale
Plenty of methods have been standardized by the IETF to perform
service TLS capability discovery for SMTP. They have in common that
they represent SMTP specific solutions to a problem that most, if not
all other protocols address via the means this document thus
reiterates for SMTP. They represent further development effort and
error surfaces. They also impose increased administration effort,
and, due to this, are inaccessible to a large number, if not the
majority, of private email server operators: either through
additional costs, the impossibility to set up a dedicated name
server, if so required, and, also as a superset of the former,
restrictive web interfaces that support only a minimal set of
DNS[RFC1035] resource record types. Finally they, especially looking
forward, disallow the business model where a provider manages
DNSSEC[RFC4033][RFC4034][RFC4035] for basic user DNS entries, created
via some kind of web interface.
There is for example MTA-STS[RFC8461] which requires SMTP operators
to install and maintain a HTTP protocol server, accessible on the
usual HTTP/S ports and the same domain as the email service, and
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subject to the now usual web server and DNS (traffic) attacks.
Dependent on the software this may require non-trivial and error
prone path access policy server rules.
Another example are the newer SVCB[RFC9460] DNS resource records.
They support a plethora of additional options, and one could say
that, with hindsight to TLS Encrypted Client
Hello[I-D.ietf-tls-svcb-ech], using SVCB would reduce DNS traffic.
SVCB, however, just like many other IETF designed DNS RR types, is
often not available in web interfaces providers make available to
normal end users. Furthermore, and that is the authors opinion, they
are problematic, especially regarding SMTP, in that they again would
represent a special approach for SMTP compared to other email
protocols, and that includes SUBMISSION[RFC6409] which effectively is
identical to SMTP. (The author believes email should be as easy as
possible to setup and use for end users.)
Also in the authors opinion we now start mixing the model, view,
controller paradigm with SVCB. Which leads to DANE for
SMTP[RFC7672]. Unfortunately DNSSEC[RFC4033][RFC4034][RFC4035] is
still consciously unsupported by major players as of today, and
provider web interfaces do often not allow users placement of the
necessary DANE resource records in addition, as of the time of this
writing. However, discovering a (TLS enabled) email service via SRV
records, thereafter checking for applicability of DNS-Based
Authentication of Named Entities (DANE)[RFC6698], possibly even Using
Raw Public Keys in Transport Layer Security (TLS)[RFC7250] would be a
bright future that addresses a lot of today's issues.
Appendix B. Acknowledgements
Thanks to Jan Ingvoldstad for remarks on the missing rationale.
Author's Address
Steffen Nurpmeso (editor)
Email: steffen@sdaoden.eu
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