SMTP Require TLS Option

Internet Engineering Task Force                                J. Fenton
Internet-Draft                                          Altmode Networks
Intended status: Standards Track                          August 7, 2017
Expires: February 8, 2018

                        SMTP Require TLS Option


   The SMTP STARTTLS option, used in negotiating transport-level
   encryption of SMTP connections, is not as useful from a security
   standpoint as it might be because of its opportunistic nature;
   message delivery is prioritized over security.  This document
   describes a complementary SMTP service extension, REQUIRETLS.  If the
   REQUIRETLS option is used when sending a message, it asserts a
   request on the part of the message sender to override the default
   negotiation of TLS, either by requiring that TLS be negotiated when
   the message is relayed, or by requesting that policy mechanisms such
   as SMTP STS and DANE be ignored when relaying a high priority

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
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   This Internet-Draft will expire on February 8, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The REQUIRETLS Service Extension  . . . . . . . . . . . . . .   3
   3.  REQUIRETLS Semantics  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  REQUIRETLS Receipt Requirements . . . . . . . . . . . . .   5
     3.2.  REQUIRETLS Sender Requirements  . . . . . . . . . . . . .   5
       3.2.1.  Sending with TLS Required . . . . . . . . . . . . . .   5
       3.2.2.  Sending with TLS Optional . . . . . . . . . . . . . .   6
     3.3.  REQUIRETLS Submission . . . . . . . . . . . . . . . . . .   7
     3.4.  Delivery of REQUIRETLS messages . . . . . . . . . . . . .   7
   4.  Non-delivery message handling . . . . . . . . . . . . . . . .   7
   5.  Mailing list considerations . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  Passive attacks . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Active attacks  . . . . . . . . . . . . . . . . . . . . .   9
     7.3.  Bad Actor MTAs  . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Revision History  . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Changes since fenton-03 Draft . . . . . . . . . . . . . .  10
     9.2.  Changes Since -02 Draft . . . . . . . . . . . . . . . . .  10
     9.3.  Changes Since -01 Draft . . . . . . . . . . . . . . . . .  10
     9.4.  Changes Since -00 Draft . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
   means by which an SMTP server and client can establish a Transport
   Layer Security (TLS) protected session for the transmission of email
   messages.  By default, TLS is used only upon mutual agreement
   (successful negotiation) of STARTTLS between the client and server;
   if this is not possible, the message is sent without transport
   encryption.  Furthermore, it is common practice for the client to
   negotiate TLS even if the SMTP server's certificate fails to
   authenticate it.

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   Policy mechanisms such as DANE [RFC7672] and SMTP STS
   [I-D.ietf-uta-mta-sts] may impose requirements for the use of TLS for
   email destined for some domains.  However, such policies do not allow
   the sender to specify which messages are more sensitive and require
   transport-level encryption, and which ones are urgent and ought to be
   relayed even if TLS cannot be negotiated successfully.

   The default opportunistic nature of SMTP TLS enables several "on the
   wire" attacks on SMTP security between MTAs.  These include passive
   eavesdropping on connections for which TLS is not used, interference
   in the SMTP protocol to prevent TLS from being negotiated (presumably
   followed by eavesdropping), and insertion of a man-in-the-middle
   attacker taking advantage of the lack of server authentication by the
   client.  Attacks are described in more detail in the Security
   Considerations section of this document.

   The REQUIRETLS SMTP service extension allows the SMTP client to
   specify that a given message sent during a particular session MUST be
   sent over a TLS protected session with specified security
   characteristics, or conversely that delivery should be prioritized
   over ability to negotiate TLS.  For messages requiring TLS
   negotiation, it also requires that the SMTP server advertise that it
   supports REQUIRETLS, in effect promising that it will honor the
   requirement to enforce TLS transmission and REQUIRETLS support for
   onward transmission of those messages.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The REQUIRETLS Service Extension

   1.  The textual name of the extension is "Require TLS".

   2.  The EHLO keyword value associated with this extension is

   3.  One MAIL FROM option is defined by this extension.

   4.  Two new SMTP status codes are defined by this extension to convey
       error conditions resulting from failure of the client to
       negotiate a TLS connection with the required security and as a
       result of an attempt to send to a server not also supporting the
       REQUIRETLS extension.

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   In order to specify REQUIRETLS treatment for a given message, the
   REQUIRETLS option is specified on the MAIL FROM command when that
   message is transmitted.  With the exception of REQUIRETLS=NO
   (described below), this option MUST only be specified in the context
   of an SMTP session meeting the security requirements that have been

   o  The session itself MUST employ TLS transmission, unless the NO
      parameter is specified.

   o  Any server authentication requirements included as an option to
      the REQUIRETLS option (see below) MUST have been satisfied in
      establishing the current session.

   o  The SMTP server advertises that it supports REQUIRETLS.

   An optional parameter to the REQUIRETLS MAIL FROM option specifies
   the requirements for server authentication that MUST be used for any
   onward transmission of the following message.  The parameter takes
   the form of either a single value or comma-separated list, separated
   from the REQUIRETLS option by a single "=" (equals-sign) character.
   If present, the parameter MUST take one or more of the following

   o  CHAIN - The certificate presented by the SMTP server MUST verify
      successfully in a trust chain leading to a certificate trusted by
      the SMTP client.  The choice of trusted (root) certificates by the
      client is at their own discretion.  The client MAY choose to use
      the certificate set maintained by the CA/B forum [citation needed]
      for this purpose.

   o  DANE - The certificate presented by the SMTP server MUST verify
      succesfully using DANE as specified in RFC 7672 [RFC7672].

   o  DNSSEC - The server MUST confirm that any MX record or CNAME
      lookup used to locate the SMTP server must be DNSSEC [RFC4035]
      signed and valid.

   o  NO - The SMTP client SHOULD attempt to send the message regardless
      of its ability to negotiate STARTTLS with the SMTP server,
      ignoring policy-based mechanisms, if any, asserted by the
      recipient domain.  Nevertheless, the client MAY negotiate STARTTLS
      with the server if available.  If the NO parameter is present, any
      other REQUIRETLS parameter MUST NOT be used.

   The CHAIN and DANE parameters are additive; if both are specified,
   either method of certificate validation is acceptable.  If neither

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   CHAIN nor DANE is specified, the certificate presented by the SMTP
   server is not required to be verified.

3.  REQUIRETLS Semantics

3.1.  REQUIRETLS Receipt Requirements

   Upon receipt of the REQUIRETLS option on a MAIL FROM command during
   the receipt of a message, an SMTP server MUST tag that message as
   needing REQUIRETLS handling with the specified option(s).  The manner
   in which this tagging takes place is implementation-dependent.  If
   the message is being locally aliased and redistributed to multiple
   addresses, all instances of the message MUST be tagged in the same

3.2.  REQUIRETLS Sender Requirements

3.2.1.  Sending with TLS Required

   When sending a message tagged with REQUIRETLS other than the
   REQUIRETLS=NO option, the sending (client) MTA MUST:

   1.  Look up the SMTP server to which the message is to be sent as
       described in [RFC5321] Section 5.1.  If the DNSSEC option is
       included in the message tag, the MX record lookups in this
       process MUST use DNSSEC verification and the response(s) MUST be
       DNSSEC-signed in order to ensure the integrity of the resource
       identifier [RFC6125] used to authenticate the SMTP server.

   2.  Open an SMTP session with the peer SMTP server using the EHLO
       verb.  The server MUST advertise the REQUIRETLS capability.

   3.  Establish a TLS-protected SMTP session with its peer SMTP server
       and authenticate the server's certificate with the specified
       authentication method as specified in [RFC6125] or [RFC6698] as

   4.  The SMTP client SHOULD also require that meaningfully secure
       cipher algorithms and key lengths be negotiated with the server.
       The choices of key lengths and algorithms change over time, so a
       specific requirement is not presented here.

   If any of the above steps fail, the client SHOULD issue a QUIT to the
   server and repeat steps 2-4 with each host on the recipient domain's
   list of MX hosts in an attempt to find a mail path that meets the
   sender's requirements.  If there are no more MX hosts or if the MX
   record lookup is not DNSSEC-protected and DNSSEC verification is
   required, the client MUST NOT transmit the message and MUST issue an

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   SMTP QUIT command to the server.  The client MAY send other,
   unprotected, messages to that server prior to issuing the QUIT if it
   has any.

   Following such a failure, the SMTP client MUST send a non-delivery
   notification to the reverse-path of the failed message as described
   in section 3.6 of [RFC5321].  The following status codes [RFC5248]
   SHOULD be used:

   o  DNSSEC lookup failure: 5.x.x DNSSEC lookup required

   o  REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed

   o  Unable to establish TLS-protected SMTP session: 5.7.10 Encryption

   Refer to Section 4. for further requirements regarding non-delivery

   If all REQUIRETLS requirements have been met, transmit the message,
   issuing the REQUIRETLS option on the MAIL FROM command with the
   required option(s), if any.

3.2.2.  Sending with TLS Optional

   Messages tagged REQUIRETLS=NO are handled differently from other
   REQUIRETLS messages, as follows.  When sending a message tagged with
   REQUIRETLS=NO, the sending (client) MTA MUST:

   o  Look up the SMTP server to which the message is to be sent as
      described in [RFC5321] Section 5.1.

   o  Open an SMTP session with the peer SMTP server using the EHLO
      verb.  Attempt to negotiate STARTTLS if possible, and follow any
      policy published by the recipient domain, but do not fail if this
      is unsuccessful.

   o  If the server does not advertise the REQUIRETLS capability, send
      the message in the usual manner (without the REQUIRETLS option,
      because the server will not understand the option).

   o  If the server advertises the REQUIRETLS capability, send the
      message with the REQUIRETLS=NO option.

   Some SMTP servers that are configured to expect STARTTLS connections
   as a matter of policy may not accept messages in the absence of
   STARTTLS.  This MUST be expected, and a non-delivery notification
   returned to the sender.

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   Messages tagged with the REQUIRETLS=NO option will be sent without
   the option to SMTP servers not supporting REQUIRETLS.  REQUIRETLS=NO
   MAY therefore not persist through multiple email relay hops.

3.3.  REQUIRETLS Submission

   An MUA or other agent making the initial introduction of a message to
   SMTP has authority to decide whether to require TLS, and if so, using
   what authentication method(s).  It does so by issuing the REQUIRETLS
   option in the MAIL FROM command during message submission.  This MAY
   be done based on a user interface selection, on a header field
   included in the message, or based on policy.  The manner in which the
   decision to require TLS is made is implementation-dependent and is
   beyond the scope of this specification.

3.4.  Delivery of REQUIRETLS messages

   Messages are usually retrieved by end users using protocols other
   than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems.
   Mail delivery agents supporting REQUIRETLS SHOULD require that
   retrieval of messages requiring transport encryption take place over
   authenticated, encrypted channels.

4.  Non-delivery message handling

   Non-delivery ("bounce") messages usually contain important metadata
   about the message to which they refer, including the original message
   header.  They therefore MUST be protected in the same manner as the
   original message.  All non-delivery messages, whether resulting from
   a REQUIRETLS error or some other, MUST employ REQUIRETLS using the
   same authentication method(s) as the message that caused the error to

   It should be noted that the path from the origination of an error
   bounce message back to the MAIL FROM address may not share the same
   REQUIRETLS support as the forward path.  Therefore, users of
   REQUIRETLS (other than REQUIRETLS=NO) are advised to make sure that
   they are capable of receiving mail using REQUIRETLS at the same
   authentication method(s) as messages they send.  Otherwise, such non-
   delivery messages will be lost.

   If unable to send a bounce message due to a REQUIRETLS failure (the
   return path not supporting the TLS requirements in the original
   message), the MTA sending the bounce message MAY send a redacted non-
   delivery message to the postmaster of the domain identified in the
   envelope-From address identifying the message only by Message-ID and
   indicating the type of failure.  The original From, Return-path, To,

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   Sender, Cc, and related header fields MUST NOT be included in this

   Senders of messages specifying REQUIRETLS (other than REQUIRETLS=NO)
   are advised to consider the increased likelihood that bounce messages
   will be lost as a result of REQUIRETLS return path failure.

5.  Mailing list considerations

   Mailing lists, upon receipt of a message, originate new messages to
   list addresses, as distinct from an aliasing operation that redirects
   the original message, in some cases to multiple recipients.  The
   requirement to preserve the REQUIRETLS tag and options therefore does
   not extend to mailing lists.  REQUIRETLS users SHOULD be made aware
   of this limitation so that they use caution when sending to mailing
   lists and do not assume that REQUIRETLS applies to messages from the
   list operator to list members.

   Mailing list operators MAY apply REQUIRETLS requirements in incoming
   messages to the resulting messages they originate.  If this is done,
   they SHOULD also apply these requirements to administrative traffic,
   such as messages to moderators requesting approval of messages.

6.  IANA Considerations

   If published as an RFC, this draft requests the addition of the
   keyword REQUIRETLS to the SMTP Service Extensions Registry

   If published as an RFC, this draft also requests the creation of a
   registry, REQUIRETLS Security Requirements, to be initially populated
   with the CHAIN, DANE, DNSSEC, and NO keywords.

   If published as an RFC, this draft requests the addition of an entry
   to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
   Registry [SMTPStatusCodes] in the 5.7.YYY range to indicate lack of
   REQUIRETLS support by an SMTP server to which a message is being

   This section is to be removed during conversion into an RFC by the
   RFC Editor.

7.  Security Considerations

   The purpose of REQUIRETLS is to improve communications security for
   email by giving the originator of a message an expectation that it
   will be transmitted in an encrypted form "over the wire".  When used,
   REQUIRETLS changes the traditional behavior of email transmission,

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   which favors delivery over the ability to send email messages using
   transport-layer security, to one in which requested security takes
   precedence over delivery and domain-level policy.

   The following considerations apply to STARTTLS other than the
   STARTTLS=NO option, since messages specifying that option are
   specifying less concern with transport security.

7.1.  Passive attacks

   REQUIRETLS is generally effective against passive attackers who are
   merely trying to eavesdrop on an SMTP exchange between an SMTP client
   and server.  This assumes, of course, the cryptographic integrity of
   the TLS connection being used.

7.2.  Active attacks

   Active attacks against TLS encrypted SMTP connections can take many
   forms.  One such attack is to interfere in the negotiation by
   changing the STARTTLS command to something illegal such as XXXXXXXX.
   This causes TLS negotiation to fail and messages to be sent in the
   clear, where they can be intercepted.  REQUIRETLS detects the failure
   of STARTTLS and declines to send the message rather than send it

   A second form of attack is a man-in-the-middle attack where the
   attacker terminates the TLS connection rather than the intended SMTP
   server.  This is possible when, as is commonly the case, the SMTP
   client either does not verify the server's certificate or establishes
   the connection even when the verification fails.  The REQUIRETLS
   CHAIN and DANE options allow the message sender to specify that
   successful certificate validation, using either or both of two
   different methods, is required before sending the message.

   Another active attack involves the spoofing of DNS MX records of the
   recipient domain.  An attacker having this capability could cause the
   message to be redirected to a mail server under the attacker's own
   control, which would presumably have a valid certificate.  The
   REQUIRETLS DNSSEC option allows the message sender to require that
   valid DNSSEC [RFC4033] signatures be obtained when locating the
   recipient's mail server, in order to address that attack.

   In addition to support of the DNSSEC option, domains receiving email
   SHOULD deploy DNSSEC and SMTP clients SHOULD deploy DNSSEC

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7.3.  Bad Actor MTAs

   A bad-actor MTA along the message transmission path could
   misrepresent its support of REQUIRETLS and/or actively strip
   REQUIRETLS tags from messages it handles.  However, since
   intermediate MTAs are already trusted with the cleartext of messages
   they handle, and are not part of the threat model for transport-layer
   security, they are also not part of the threat model for REQUIRETLS.

   It should be reemphasized that since SMTP TLS is a transport-layer
   security protocol, messages sent using REQUIRETLS are not encrypted
   end-to-end and are visible to MTAs that are part of the message
   delivery path.  Messages containing sensitive information that MTAs
   should not have access to MUST be sent using end-to-end content
   encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751].

8.  Acknowledgements

   The author would like to acknowledge many helpful suggestions on the
   ietf-smtp and uta mailing lists, in particular those of Viktor
   Dukhovni, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin,
   John Levine, Rolf Sonneveld, and Per Thorsheim.

9.  Revision History

   To be removed by RFC Editor upon publication as an RFC.

9.1.  Changes since fenton-03 Draft

   o  Wording improvements from Rolf Sonneveld review 22 July 2017

   o  A few copy edits

   o  Conversion from individual to UTA WG draft

9.2.  Changes Since -02 Draft

   o  Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
      suggestion on UTA WG mailing list.

   o  Additional guidance on bounce messages

9.3.  Changes Since -01 Draft

   o  Specified retries when multiple MX hosts exist for a given domain.

   o  Clarified generation of non-delivery messages

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   o  Specified requirements for application of REQUIRETLS to mail
      forwarders and mailing lists.

   o  Clarified DNSSEC requirements to include MX lookup only.

   o  Corrected terminology regarding message retrieval vs. delivery.

   o  Changed category to standards track.

9.4.  Changes Since -00 Draft

   o  Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
      parameter to better associate REQUIRETLS requirements with
      transmission of individual messages.

   o  Addition of an option to require DNSSEC lookup of the remote mail
      server, since this affects the common name of the certificate that
      is presented.

   o  Clarified the wording to more clearly state that TLS sessions must
      be established and not simply that STARTTLS is negotiated.

   o  Introduced need for minimum encryption standards (key lengths and

   o  Substantially rewritten Security Considerations section

10.  References

10.1.  Normative References

              Internet Assigned Numbers Authority (IANA), "IANA Mail
              Parameters", 2007,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <>.

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   [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,

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248,
              DOI 10.17487/RFC5248, June 2008,

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <>.

   [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, <>.

              Internet Assigned Numbers Authority (IANA), "Simple Mail
              Transfer Protocol (SMTP) Enhanced Status Codes Registry",
              2008, <

10.2.  Informative References

              Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", draft-ietf-uta-mta-sts-07 (work in progress), June

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,

              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,

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   [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,

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <>.

   [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,

Author's Address

   Jim Fenton
   Altmode Networks
   Los Altos, California  94024


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