Network Working Group J. Levine Internet-Draft Taughannock Networks Updates: 5321 (if approved) July 30, 2019 Intended status: Standards Track Expires: January 31, 2020 Update to Additional Registered Clauses in SMTP Received Headers draft-levine-additional-registered-clauses-03 Abstract SMTP servers add Received: trace headers to mail messages to track their progress This document updates the registration criteria for Additional Registered Clauses in those headers to Expert Review, and adds new clauses for Server Name Indication (SNI) and sending port number (PORT). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Levine Expires January 31, 2020 [Page 1] Internet-Draft Additional registered clauses July 2019 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. The Server Name Indication clause . . . . . . . . . . . . . . 2 3. The Port Number clause . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 4.1. Guidance for Designated Expert . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 6.2. Uninformative References . . . . . . . . . . . . . . . . 4 Appendix A. Change history . . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction SMTP servers add Received: trace headers to mail messages to track their progress. The syntax of those headers is defined in [RFC5321]. Each header can include optional Additional Registered Clauses that log information related to optional SMTP features. This document updates the registration criteria for Additional Registered Clauses in those headers to Expert Review, and adds a new clause for Server Name Indication (SNI). The headers can include Additional Registered Clauses that add information about optional SMTP features. 2. The Server Name Indication clause Server Name Indication or SNI [RFC6066] is an optional TLS feature that a TLS client can use to advise a server the name it expects the server to have. When used in the initial negotiation for a STARTTLS [RFC3207] session it enables the server to use a certificate with the identity that the client expects, as is recommended in [RFC7817] and is required for SMTP MTA-STS [RFC8461]. When a client presents a name using SNI, the server can log the name using the "sni" additional-registered-clause. IANA is requested to add one new entry to the additional-registered- clauses registry: "sni" - indicates the Server Name Indication was used. "name" - the name provided by SNI. It has the syntax of a Domain Name. Levine Expires January 31, 2020 [Page 2] Internet-Draft Additional registered clauses July 2019 3. The Port Number clause When several sending mail hosts send mail from behind a Network Address Translator (NAT), often the only way to tell which host sent the is to look up the sending port number in the NAT's logs. The server can log the sending port number using the "port" additional- registered-clause. IANA is requested to add one new entry to the additional-registered- clauses registry: "port" - indicates the mail client's port number "number" - the client's port number. It has the syntax of an integer. 4. IANA Considerations IANA is requested to update the Registration Procedure for the Additional-registered-clauses registry to Expert Review. The IESG will appoint the expert(s). 4.1. Guidance for Designated Expert The Designated Expert is expected to check that a proposed Additional-registered-clause has a specification that is stable and detailed enough to implement the clause and interoperate. The Expert should ensure that the clause name is reasonably related to the information it represents, that the contents of the clause are well- defined, and that any external references it depends on, e.g., a vocabulary of keywords, are stable and well-defined. 5. Security Considerations E-mail is subject to a vast range of threats and abuses. In a few circumstances, a new Additional-registered-clause might disclose information to a recipient that was otherwise unavailable. On the other hand, better logging usually makes it easier to diagnose failures and attacks. If the SNI information in a STARTTLS negotiation is logged, it may make it easier to determine the recipient(s) of the message if they are not already logged in trace headers. Levine Expires January 31, 2020 [Page 3] Internet-Draft Additional registered clauses July 2019 6. References 6.1. Normative References [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, February 2002, . [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, . [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, . 6.2. Uninformative References [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, . [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, . Appendix A. Change history 02 to 03 Add port numbers. What the heck. 01 to 02 Take out ESNI clause to prevent downref. 00 to 01 Add new ESNI clause. Fix many typos. 00 New draft Author's Address Levine Expires January 31, 2020 [Page 4] Internet-Draft Additional registered clauses July 2019 John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 Email: standards@taugh.com URI: http://jl.ly Levine Expires January 31, 2020 [Page 5]