Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-02

Internet Engineering Task Force                         S. Nurpmeso, Ed.
Internet-Draft                                          29 December 2024
Updates: 3207 5321 (if approved)                                        
Intended status: Informational                                          
Expires: 2 July 2025

                    Secure SMTP/TLS SRV Announcement
                     draft-nurpmeso-smtp-tls-srv-02

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) to interested parties which aim to use it ("everybody"
   according to RFC 8314).

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 2 July 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  SMTP/TLS SRV Service Name . . . . . . . . . . . . . . . . . .   3
   3.  Example: service record . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 from the IETF point of view.  (Their usage is,
   however, real, for example for port distribution, and even cast in
   law, for example by the German De-Mail law.)  Also 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 establishing a connection to a
   discovered SMTP MX server; 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 por t 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.

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2.  SMTP/TLS SRV Service Name

   The service name for TLS[RFC8446] enabled Secure[RFC3207]
   SMTP[RFC5321] is smtps, the resulting DNS label _smtps.

   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.  (Using the
      IANA SMTP port results in a non-compliant service, and SHOULD NOT
      be used on the internet.)  If the STARTTLS[RFC3207] extension
      command is used during Implicit TLS connections, the SMTP reply
      code 550 MUST be returned; if enhanced status codes[RFC3463] are
      used, 5.5.1 SHOULD be used.  (This behaviour reflects how
      IMAP[RFC9051] deals.)

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

   The author does not expect IANA to add a dedicated port for "Implicit
   TLS SMTP", given that it has registered according ones for other
   email protocols, for example POP3S, no sooner but 1999, a quarter of
   a century ago.  The author nonetheless asks for assigning a dedicated
   port for Implicit TLS SMTP, and suggests port 26 (which, at the time
   of this writing, was already successfully used for that purpose) in
   the privileged port range.  The author wants to point out that the
   contra arguments given in section 7 of [RFC2595] that created
   according POP3S and IMAPS assignments in 1999 are contradicted by
   operational reality in the internet, and here that includes the IETF
   by means of [RFC8314].

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5.  Security Considerations

   This specification avoids 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; this is
   considered a value by itself, even for the possibly lesser sensitive
   MTA to MTA communication.  Implicit TLS reduces the number of packet
   roundtrips, that at a protocol stage where the widely used
   "pipelining" (RFC 2920) performance improvement extension cannot be
   used; it must be said that today these roundtrips are anachronistic
   (also environmentally): for example the over four million known DANE
   for SMTP[RFC7672] enabled servers at the time of this writing alone,
   which MUST use TLS by standard definition, likely generate (several)
   billion(s) of useless sequential and blocking roundtrip packets each
   and every day.  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.  Selection
   of the appropriate transport layer security protocol is out of scope
   for this specification, see for example TLS[RFC8446].

6.  References

6.1.  Normative References

   [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>.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, DOI 10.17487/RFC3463, January 2003,
              <https://www.rfc-editor.org/info/rfc3463>.

   [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

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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>.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <https://www.rfc-editor.org/info/rfc2595>.

   [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>.

   [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>.

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   [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>.

   [RFC8314]  Moore, K. and C. Newman, "Cleartext Considered Obsolete:
              Use of Transport Layer Security (TLS) for Email Submission
              and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
              <https://www.rfc-editor.org/info/rfc8314>.

   [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 permanent 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.  Regardless the fact that several
   of these extensions switch the SMTP protocol to a MUST usage of TLS,
   they still require the additional STARTTLS plus HELO roundtrips that
   for example [RFC8314] disregards; and even though that RFC explicitly
   refers to DANE for SMTP[RFC7672], giving no reason for why SMTP
   "requires a different approach", except for the TLS certificate
   discovery that could and should be hoped for as a general solution,
   including for clients for any sort of network protocol, DANE falls in

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   this category, too: what this specification tries to achieve is
   therefore not counteracting [RFC7672], but accompanying it; the
   author wants to state again that a (non-standardized) _smtp._tcp
   mechanism exists in active use.

   The other extension that [RFC8314] mentions, MTA-STS[RFC8461],
   requires SMTP operators to install and maintain an additional HTTP
   protocol server, accessible on the usual HTTP/S ports and the same
   domain as the email service.  Dependent on the software this may
   require non-trivial and error prone path access policy server rules.
   It is a complicated to implement approach that brings the complexity
   of another network protocol in the SMTP path; given that several
   different HTTP protocols exist, which in turn base upon several
   different transport protocols, the complexity and protocol is alien
   to SMTP.  The SVCB[RFC9460] DNS approach would not bring additional
   value to the problem in question, but on the other hand requires
   additional implementation efforts as email protocols do not make any
   use of it.

   Coming back to DANE for SMTP[RFC7672] it must be said that
   unfortunately DNSSEC[RFC4033][RFC4034][RFC4035] is still consciously
   unsupported by major players as of today.  Furthermore provider web
   interfaces do often not allow users creation of the necessary DANE
   TLSA resource records; not always can the answer be "run your own
   name server", also letting aside the administrative effort of doing
   so.  However, discovering an Implicit TLS enabled email service via
   the _smtp._tcp SRV record, 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 possibly all of today's
   issues.

Appendix B.  Acknowledgements

   Thanks to Jan Ingvoldstad for remarks on the missing rationale.
   Thanks to Jeremy Harris for hinting on MUSTs and, in practice, the
   rationale, and especially for a kickstart implementation in the
   widely used exim Open Source MTA.

Author's Address

   Steffen Nurpmeso (editor)
   Email: steffen@sdaoden.eu

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