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steffen@sdaoden.eu

General Internet Engineering Task Force SMTP TLS Secure SMTP STARTTLS SRV 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).

Introduction defines a widely adopted DNS-based service discovery protocol. is a specification of SRV records for the email protocols IMAP, POP3, and SUBMISSION. SMTP connections to MTAs (, 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 (, 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.

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 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.

SMTP/TLS SRV Service Name The service name for TLS enabled Secure SMTP is smtps, the resulting DNS label _smtps.

STARTTLS

Whenever a domain publishes an according DNS SRV resource record it asserts availability of Secure SMTP, that is, of the STARTTLS SMTP service extension on the normal SMTP 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 extension command is used during Implicit TLS connections, the SMTP reply code 550 MUST be returned; if enhanced status codes are used, 5.5.1 SHOULD be used. (This behaviour reflects how IMAP deals.)

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.

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 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 .

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 (IMAP, POP3, SUBMISSION). With its Implicit TLS capability it grants the SMTP protocol the same level of confidentiality through TLS 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 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 is out of scope for this specification, but DNSSEC and secure DNS transport etc exists. Selection of the appropriate transport layer security protocol is out of scope for this specification, see for example TLS.

References Normative References Informative References

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 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 disregards; and even though that RFC explicitly refers to DANE for SMTP, 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 this category, too: what this specification tries to achieve is therefore not counteracting , but accompanying it; the author wants to state again that a (non-standardized) _smtp._tcp mechanism exists in active use. The other extension that mentions, MTA-STS, 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 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 it must be said that unfortunately DNSSEC 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), possibly even Using Raw Public Keys in Transport Layer Security (TLS) would be a bright future that addresses possibly all of today's issues.

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.