Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-08

Network Working Group                                   S. Nurpmeso, Ed.
Internet-Draft                                             3 August 2026
Updates: 5321, 3207 (if approved)                                       
Intended status: Informational                                          
Expires: 4 February 2027

                    Secure SMTP/TLS SRV Announcement
                     draft-nurpmeso-smtp-tls-srv-08

Abstract

   This specification defines a DNS (RFC 1035) SRV (RFC 2782) record
   that announces TLS (RFC 9325) secured SMTP (RFC 5321, RFC 3207),
   optionally including Implicit TLS.

Status of This Memo

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   This Internet-Draft will expire on 4 February 2027.

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   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   2
   2.  SMTP/TLS SRV Service Name . . . . . . . . . . . . . . . . . .   3
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  STARTTLS  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Implicit TLS  . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Prioritized Server Selection  . . . . . . . . . . . . . .   4
   4.  Guidance for MTAs . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Guidance for Service Providers  . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   SMTP[RFC5321] uses MX[RFC974] RRs to discover how to create
   connections to MTAs ([RFC5598]).  The SMTP protocol default is
   unprotected, the selection of the optional and possibly unsupported
   STARTTLS[RFC3207] extension is subject to man-in-the-middle attacks.

   Moreover, no Implicit TLS SMTP protocol variant has ever been
   specified, despite noticeable achievable non-batchable packet
   roundtrip savings, and despite availability, or easy adoptability, of
   such a protocol variant in existing code bases.

   [RFC2782] defines a widely adopted DNS-based service discovery
   protocol.  [RFC6186] is a specification of SRV[RFC2782] records for
   the email protocols IMAP[RFC9051], POP3[RFC1939], and
   SUBMISSION[RFC6409].  This includes DNS service names for Implicit
   TLS protocol variants.

   This specification adds a SMTP/TLS service name for SRV[RFC2782]
   records.  These DNS RRs should be used in preference to MX records.
   Their presence signals availability of the STARTTLS SMTP extension,
   as well as optionally Implicit TLS on a dedicated port.

1.1.  Conventions and Terminology

   This document uses the terminology of The Open Group Standard Base
   Specifications, Issue 8, Volume 1, Chapter 1.6, Terminology.

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   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.

   The term "FOSS" refers to Free and Open Source Software.

2.  SMTP/TLS SRV Service Name

   The service name for TLS[RFC9325] enabled Secure[RFC3207]
   SMTP[RFC5321] is smtp-tls, the resulting DNS label _smtp-tls.

   STARTTLS
      A domain that publishes an according DNS SRV[RFC2782] resource
      record announces availability of Secure SMTP, namely the
      STARTTLS[RFC3207] SMTP service extension on the normal
      SMTP[RFC5321] port, specified by IANA as port 25.  The SRV RR port
      number shall be given as 25.

   Implicit TLS
      If the SRV RR port number is not 25, support for Implicit TLS on
      the specified port is announced.  The port number shall be given
      as 842.  Any other port number shall be treated as if port 25 was
      specified; special local policy rules may allow Implicit TLS on
      the given port.

      Servers shall not announce STARTTLS in the EHLO command response
      of an Implicit TLS connection, clients shall ignore it, if they do
      nonetheless.  Clients should not issue STARTTLS within an Implicit
      TLS connection; servers shall reply with code 554 if they do
      nonetheless, if enhanced status codes[RFC3463] are used, 5.5.1
      shall be used.

   DNS SRV RRs shall take priority over MX[RFC5321] ones: in the
   presence of a SRV RR no MX lookup should be performed, and no
   available MX RR shall be used.

   After a successful _smtp-tls DNS SRV lookup cleartext communication
   shall not be used.  Exceptions, for example "error-recovery"
   connections to the dedicated special "local-part" postmaster[RFC1123]
   (also see SMTP[RFC5321], section 4.5.1 Minimum Implementation), may
   be provided.  Servers and clients which make use of the _smtp-tls DNS
   SRV shall follow the guidelines of TLS[RFC9325].

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   If a DNS SRV lookup fails with NODATA negative caching[RFC2308]
   (sections 2.2 and 5) conditions, not only a maximum, but also a
   minimum cache time limit should be used in order to reduce excessive
   DNS SRV RR lookups.  (It may seem sensible to use the TTL of the
   domains MX or address RR, with a maximum limit, as via [RFC2308],
   section 5.)

3.  Examples

3.1.  STARTTLS

   An announcement for the STARTTLS SMTP service extension.

   _smtp-tls._tcp     SRV 0 0  25 mail.example.com.

3.2.  Implicit TLS

   An announcement of Implicit TLS, in addition to STARTTLS.

   _smtp-tls._tcp     SRV 0 0 842 mail.example.com.

3.3.  Prioritized Server Selection

   A multi-server scenario where the main server supports Implicit TLS
   and STARTTLS, whereas the backup server only supports STARTTLS.

   _smtp-tls._tcp     SRV 0 0 842 mail.example.com.
   _smtp-tls._tcp     SRV 1 0  25 backup.example.com.

4.  Guidance for MTAs

   If, after a successful _smtp-tls DNS SRV lookup that announces
   Implicit TLS support, a connection to the dedicated port fails, a
   fallback to IANA SMTP port 25 may be established, which shall use the
   STARTTLS SMTP extension.

   |  _Informative remark:_ This is meant to overcome two shortcomings:
   |  first the given port may be blocked along the network path; it may
   |  take time until the network adapts; whereas expected to be a rare
   |  event for the System Ports range ([RFC6335]), defining a recovery
   |  strategy seems useful.

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   |  Second DNS SRV lookups could return results unprotected by
   |  DNSSEC[RFC4033][RFC4034][RFC4035], or without perceived knowledge
   |  of whether DNSSEC was actually used, for example, when the DNS is
   |  accessed via some kind of furtherly unspecified intermediate proxy
   |  that needs to be trusted: in either case the possibility of DNS
   |  forge attacks exist; if the STARTTLS secured connection to the
   |  IANA SMTP port fails, the DNS result should be treated with
   |  maximum suspicion.  The mail log record may give useful insight.

   The section of [RFC6186] named "Guidance for MUAs" (section 4) in
   parts also applies to this specification.

5.  Guidance for Service Providers

   The equally named section of [RFC6186] (section 5) also applies to
   this specification.

6.  IANA Considerations

   IANA is asked to allocate port number 842 for the Implicit TLS
   operation mode of SMTP[RFC5321].  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].  A dedicated port enables administrators to
   apply strict policies, for example in firewalls.

7.  Security Considerations

   First of all, the equally named section of [RFC6186] (section 6) also
   applies to this specification.

   Due to fact that one and a half decade passed since RFC 6186 it
   follows a reiteration of the reasoning.  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[RFC9325] 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.

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   Implicit TLS reduces the number of packet roundtrips, that at a
   protocol stage where the widely used command pipelining[RFC2920]
   performance improvement extension cannot be used; these roundtrips
   are anachronistic: for example the about 4.2 million known DANE for
   SMTP[RFC7672] enabled domains 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 of their operation.

   The security of DNS[RFC1035] is out of scope for this specification,
   but DNSSEC[RFC4033][RFC4034][RFC4035] and secure DNS
   transport[RFC7858][RFC8094][RFC8310][RFC8484][RFC9250] etc exists.
   Selection of the appropriate transport layer security protocol is out
   of scope for this specification, please see for example TLS[RFC9325].

8.  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>.

9.  Informative References

   [RFC974]   Partridge, C., "Mail routing and the domain system",
              STD 10, RFC 974, DOI 10.17487/RFC974, January 1986,
              <https://www.rfc-editor.org/info/rfc974>.

   [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>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.

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   [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>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

   [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>.

   [RFC2920]  Freed, N., "SMTP Service Extension for Command
              Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
              September 2000, <https://www.rfc-editor.org/info/rfc2920>.

   [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>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

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   [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>.

   [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>.

   [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>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [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>.

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   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

Appendix A.  Acknowledgements

   Thanks to Jan Ingvoldstad.  Thanks to Jeremy Harris for spending time
   and revealing the many problems of early draft variants, as well as
   comments on how to do it better; very special thanks to him for a
   kickstart implementation of this very draft in the widely used FOSS
   MTA Exim.  Jeremy Harris, Viktor Dukhovni and Wietse Venema commented
   on the initial odd usage of port 0 for the STARTTLS discovery case.
   Thanks to Alex Brotman for hinting on the fallback strategy.  Thanks
   have to go to Jonas Stalder, also for finding IETF tooling bugs,
   despite his wishes not to be mentioned (anymore).

Author's Address

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

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