| Internet-Draft | Secure SMTP/TLS SRV Announcement | October 2024 |
| Nurpmeso | Expires 5 April 2025 | [Page] |
It is described how DNS (RFC 1035) SRV (RFC 2782) records announce availability of TLS (RFC 8446) enabled Secure SMTP (RFC 3207, RFC 5321).¶
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[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.¶
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.¶
The service name for TLS[RFC8446] enabled Secure[RFC3207] SMTP[RFC5321] is smtps, the resulting DNS label _smtps.¶
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.¶
IANA is not expected to add a dedicated port for "Implicit TLS SMTP". Shall it consider otherwise the author suggests port 26.¶
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.¶