Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-00
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| Last updated | 2024-10-04 | ||
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draft-nurpmeso-smtp-tls-srv-00
Internet Engineering Task Force S. Nurpmeso, Ed.
Internet-Draft 2 October 2024
Intended status: Informational
Expires: 5 April 2025
Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-00
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 April 2025.
Copyright Notice
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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. Informative References . . . . . . . . . . . . . . . . . 3
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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. The SMTP/TLS SRV announcement can be queried
before a connection to a discovered SMTP MX server is established; it
MAY be used to further distribute SMTP load via the SRV Priority and
Weight mechanism.
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. Informative References
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[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>.
[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>.
[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>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[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>.
[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>.
[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>.
Author's Address
Steffen Nurpmeso (editor)
Email: steffen@sdaoden.eu
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