[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-brotman-smtp-tlsrpt) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Using TLS in Applications                                    D. Margolis
Internet-Draft                                               Google, Inc
Intended status: Standards Track                              A. Brotman
Expires: December 16, 2018                                  Comcast, Inc
                                                         B. Ramakrishnan
                                                             Yahoo!, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                               M. Risher
                                                             Google, Inc
                                                           June 14, 2018


                           SMTP TLS Reporting
                     draft-ietf-uta-smtp-tlsrpt-23

Abstract

   A number of protocols exist for establishing encrypted channels
   between SMTP Mail Transfer Agents, including STARTTLS, DANE TLSA, and
   MTA-STS.  These protocols can fail due to misconfiguration or active
   attack, leading to undelivered messages or delivery over unencrypted
   or unauthenticated channels.  This document describes a reporting
   mechanism and format by which sending systems can share statistics
   and specific information about potential failures with recipient
   domains.  Recipient domains can then use this information to both
   detect potential attacks and diagnose unintentional
   misconfigurations.

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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 16, 2018.






Margolis, et al.        Expires December 16, 2018               [Page 1]


Internet-Draft                 SMTP-TLSRPT                     June 2018


Copyright Notice

   Copyright (c) 2018 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   4
   3.  Reporting Policy  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Example Reporting Policy  . . . . . . . . . . . . . . . .   7
       3.1.1.  Report using MAILTO . . . . . . . . . . . . . . . . .   7
       3.1.2.  Report using HTTPS  . . . . . . . . . . . . . . . . .   7
   4.  Reporting Schema  . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Report Time-frame . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Delivery Summary  . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Success Count . . . . . . . . . . . . . . . . . . . .   9
       4.2.2.  Failure Count . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Result Types  . . . . . . . . . . . . . . . . . . . . . .   9
       4.3.1.  Negotiation Failures  . . . . . . . . . . . . . . . .  10
       4.3.2.  Policy Failures . . . . . . . . . . . . . . . . . . .  10
       4.3.3.  General Failures  . . . . . . . . . . . . . . . . . .  11
       4.3.4.  Transient Failures  . . . . . . . . . . . . . . . . .  11
     4.4.  JSON Report Schema  . . . . . . . . . . . . . . . . . . .  11
     4.5.  Policy Samples  . . . . . . . . . . . . . . . . . . . . .  14
   5.  Report Delivery . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Report Filename . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .  16
     5.3.  Email Transport . . . . . . . . . . . . . . . . . . . . .  16
       5.3.1.  Example Report  . . . . . . . . . . . . . . . . . . .  17
     5.4.  HTTPS Transport . . . . . . . . . . . . . . . . . . . . .  18
     5.5.  Delivery Retry  . . . . . . . . . . . . . . . . . . . . .  19
     5.6.  Metadata Variances  . . . . . . . . . . . . . . . . . . .  19
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Message headers . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  Report Type . . . . . . . . . . . . . . . . . . . . . . .  19
     6.3.  +gzip Media Type Suffix . . . . . . . . . . . . . . . . .  20



Margolis, et al.        Expires December 16, 2018               [Page 2]


Internet-Draft                 SMTP-TLSRPT                     June 2018


     6.4.  application/tlsrpt+json Media Type  . . . . . . . . . . .  21
     6.5.  application/tlsrpt+gzip Media Type  . . . . . . . . . . .  23
     6.6.  STARTTLS Validation Result Types  . . . . . . . . . . . .  24
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  28
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix A.  Example Reporting Policy . . . . . . . . . . . . . .  30
     A.1.  Report using MAILTO . . . . . . . . . . . . . . . . . . .  30
     A.2.  Report using HTTPS  . . . . . . . . . . . . . . . . . . .  30
   Appendix B.  Example JSON Report  . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to establish secure SMTP sessions over TLS.  The protocol
   design uses an approach that has come to be known as "Opportunistic
   Security" (OS) [RFC7435].  This method maintains interoperability
   with clients that do not support STARTTLS, but means that any
   attacker could potentially eavesdrop on a session.  An attacker could
   perform a downgrade or interception attack by deleting parts of the
   SMTP session (such as the "250 STARTTLS" response) or redirect the
   entire SMTP session (perhaps by overwriting the resolved MX record of
   the delivery domain).

   Because such "downgrade attacks" are not necessarily apparent to the
   receiving MTA, this document defines a mechanism for sending domains
   to report on failures at multiple stages of the MTA-to-MTA
   conversation.

   Recipient domains may also use the mechanisms defined by MTA-STS
   [I-D.ietf-uta-mta-sts] or DANE [RFC6698] to publish additional
   encryption and authentication requirements; this document defines a
   mechanism for sending domains that are compatible with MTA-STS or
   DANE to share success and failure statistics with recipient domains.

   Specifically, this document defines a reporting schema that covers
   failures in routing, DNS resolution, STARTTLS negotiation, and both
   DANE [RFC6698] and MTA-STS [I-D.ietf-uta-mta-sts] policy validation
   errors, and a standard TXT record that recipient domains can use to
   indicate where reports in this format should be sent.  The report can
   also serve as a heartbeat that systems are successfully negotiating
   TLS during sessions as expected.





Margolis, et al.        Expires December 16, 2018               [Page 3]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   This document is intended as a companion to the specification for
   SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts], as well as
   adds reporting abilities for those implementing DANE [RFC7672].

1.1.  Terminology

   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.

   We also define the following terms for further use in this document:

   o  MTA-STS Policy: A mechanism by which administrators can specify
      the expected TLS availability, presented identity, and desired
      actions for a given email recipient domain.  MTA-STS is defined in
      [I-D.ietf-uta-mta-sts].

   o  DANE Policy: A mechanism by which administrators can use DNSSEC to
      commit an MTA to support STARTTLS and to publish criteria to be
      used to validate its presented certificates.  DANE for SMTP is
      defined in [RFC7672], with the base specification in [RFC6698]
      (updated in [RFC7671].

   o  TLSRPT Policy: A policy specifying the endpoint to which sending
      MTAs should deliver reports.

   o  Policy Domain: The domain against which an MTA-STS or DANE Policy
      is defined.  For MTA-STS this is typically the same as the
      envelope recipient domain [RFC5321], but when mail is routed to a
      "smarthost" gateway by local policy, the "smarthost" domain name
      is used instead.  For DANE the Policy Domain is the "TLSA base
      domain" of the receiving SMTP server as described in RFC7672 [1]
      and RFC6698 [2].

   o  Sending MTA: The MTA initiating the relay of an email message.

   o  Aggregate Report URI (rua): A comma-separated list of locations
      where the report is to be submitted.

2.  Related Technologies

   o  This document is intended as a companion to the specification for
      SMTP MTA Strict Transport Security [I-D.ietf-uta-mta-sts].

   o  SMTP-TLSRPT defines a mechanism for sending domains that are
      compatible with MTA-STS or DANE to share success and failure



Margolis, et al.        Expires December 16, 2018               [Page 4]


Internet-Draft                 SMTP-TLSRPT                     June 2018


      statistics with recipient domains.  DANE is defined in [RFC6698]
      and MTA-STS is defined in [I-D.ietf-uta-mta-sts].

3.  Reporting Policy

   A domain publishes a record to its DNS indicating that it wishes to
   receive reports.  These SMTP TLSRPT policies are distributed via DNS
   from the Policy Domain's zone, as TXT records (similar to DMARC
   policies) under the name "_smtp._tls".  For example, for the Policy
   Domain "example.com", the recipient's TLSRPT policy can be retrieved
   from "_smtp._tls.example.com".

   Policies consist of the following directives:

   o  "v": This document defines version 1 of TLSRPT, for which this
      value MUST be equal to "TLSRPTv1".  Other versions may be defined
      in later documents.

   o  "rua": A URI specifying the endpoint to which aggregate
      information about policy validation results should be sent (see
      Section 4, "Reporting Schema", for more information).  Two URI
      schemes are supported: "mailto" and "https".  As with DMARC
      [RFC7489], the policy domain can specify a comma-separated list of
      URIs.

   o  In the case of "https", reports should be submitted via POST
      ([RFC7231]) to the specified URI.  Report submitters MAY ignore
      certificate validation errors when submitting reports via https.

   o  In the case of "mailto", reports should be submitted to the
      specified email address ([RFC6068]).  When sending failure reports
      via SMTP, sending MTAs MUST deliver reports despite any TLS-
      related failures and SHOULD NOT include this SMTP session in the
      next report.  When sending failure reports via HTTPS, sending MTAs
      MAY deliver reports despite any TLS-related faliures.  This may
      mean that the reports are delivered in the clear.  Reports sent
      via SMTP MUST contain a valid DKIM [RFC6376] signature by the
      reporting domain.  Reports lacking such a signature MUST be
      ignored by the recipient.  DKIM signatures must not use the "l="
      attribute to limit the body length used in the signature.  The
      DKIM TXT record must contain the appropriate service type
      declaration, "s=tlsrpt", and if not present the receiving system
      SHOULD ignore reports signed using this record.

   The formal definition of the "_smtp._tls" TXT record, defined using
   [RFC5234] & [RFC7405], is as follows:





Margolis, et al.        Expires December 16, 2018               [Page 5]


Internet-Draft                 SMTP-TLSRPT                     June 2018


        tlsrpt-record     = tlsrpt-version 1*(field-delim tlsrpt-field)
                            [field-delim]

        field-delim       = *WSP ";" *WSP

        tlsrpt-field      = tlsrpt-rua /        ; Note that the
                            tlsrpt-extension    ; tlsrpt-rua record is
                                                ; required.

        tlsrpt-version    = %s"v=TLSRPTv1"

        tlsrpt-rua        = %s"rua="
                            tlsrpt-uri *(*WSP "," *WSP tlsrpt-uri)

        tlsrpt-uri        = URI
                            ; "URI" is imported from [RFC3986];
                            ; commas (ASCII 0x2C), exclamation
                            ; points (ASCII 0x21), and semicolons
                            ; (ASCII 0x3B) MUST be encoded

        tlsrpt-extension  = tlsrpt-ext-name "=" tlsrpt-ext-value

        tlsrpt-ext-name   = (ALPHA / DIGIT) *31(ALPHA /
                            DIGIT / "_" / "-" / ".")

        tlsrpt-ext-value  = 1*(%x21-3A / %x3C / %x3E-7E)
                            ; chars excluding "=", ";", SP, and control
                            ; chars

   If multiple TXT records for "_smtp._tls" are returned by the
   resolver, records which do not begin with "v=TLSRPTv1;" are
   discarded.  If the number of resulting records is not one, senders
   MUST assume the recipient domain does not implement TLSRPT.  If the
   resulting TXT record contains multiple strings (as described in
   Section 3.1.3 of [RFC4408]), then the record MUST be treated as if
   those strings are concatenated together without adding spaces.

   The record supports the abillity to declare more than one rua, and if
   there exists more than one, the reporter MAY attempt to deliver to
   each of the supported rua destinations.  A receiver MAY opt to only
   attempt delivery to one of the endpoints, however the report SHOULD
   NOT be considered successfully delivered until one of the endpoints
   accepts delivery of the report.

   Parsers MUST accept TXT records which are syntactically valid (i.e.
   valid key-value pairs separated by semi-colons) and implementing a
   superset of this specification, in which case unknown fields SHALL be
   ignored.



Margolis, et al.        Expires December 16, 2018               [Page 6]


Internet-Draft                 SMTP-TLSRPT                     June 2018


3.1.  Example Reporting Policy

3.1.1.  Report using MAILTO

            _smtp._tls.example.com. IN TXT \
                    "v=TLSRPTv1;rua=mailto:reports@example.com"

3.1.2.  Report using HTTPS

           _smtp._tls.example.com. IN TXT \
                   "v=TLSRPTv1; \
                   rua=https://reporting.example.com/v1/tlsrpt"

4.  Reporting Schema

   The report is composed as a plain text file encoded in the I-JSON
   format ([RFC7493]).

   Aggregate reports contain the following fields:

   o  Report metadata:

      *  The organization responsible for the report

      *  Contact information for one or more responsible parties for the
         contents of the report

      *  A unique identifier for the report

      *  The reporting date range for the report

   o  Policy, consisting of:

      *  One of the following policy types: (1) The MTA-STS policy
         applied (as a string) (2) The DANE TLSA record applied (as a
         string, with each RR entry of the RRset listed and separated by
         a semicolon) (3) The literal string "no-policy-found", if
         neither a DANE nor MTA-STS policy could be found.

      *  The domain for which the policy is applied

      *  The MX host

   o  Aggregate counts, comprising result type, sending MTA IP,
      receiving MTA hostname, session count, and an optional additional
      information field containing a URI for recipients to review
      further information on a failure type.




Margolis, et al.        Expires December 16, 2018               [Page 7]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   Note that the failure types are non-exclusive; an aggregate report
   may contain overlapping "counts" of failure types when a single send
   attempt encountered multiple errors.  Reporters may report multiple
   applied policies (for example, an MTA-STS policy and a DANE TLSA
   record for the same domain and MX).  Because of this, even in the
   case where only a single policy was applied, the "policies" field of
   the report body MUST be an array and not a singular value.

   In the case of multiple failure types, the "failure-details" array
   would contain multiple entries.  Each entry would have its own set of
   infomation pertaining to that failure type.

4.1.  Report Time-frame

   The report SHOULD cover a full day, from 0000-2400 UTC.  This should
   allow for easier correlation of failure events.  To avoid a Denial of
   Service against the system processing the reports, the reports should
   be delivered after some delay, perhaps several hours.

   As an example, a sending site might want to introduce a random delay
   of up to four hours:

          func generate_sleep_delay() {
            min_delay = 1
            max_delay = 14400
            rand = random(min_delay,max_delay)
            return rand
          }

          func generate_report(policy_domain) {
            do_rpt_work(policy_domain)
            send_rpt(policy_domain)
          }

          func generate_tlsrpt() {
            sleep(generate_sleep_delay())
            for policy_domain in list_of_tlsrpt_enabled_domains {
              generate_report(policy_domain)
            }
          }

   A sending site might wish to introduce a random delay per destination
   site, up to four hours:








Margolis, et al.        Expires December 16, 2018               [Page 8]


Internet-Draft                 SMTP-TLSRPT                     June 2018


          func generate_sleep_delay() {
            min_delay = 1
            max_delay = 14400
            rand = random(min_delay,max_delay)
            return rand
          }

          func generate_report(policy_domain) {
            sleep(generate_sleep_delay())
            do_rpt_work(policy_domain)
            send_rpt(policy_domain)
          }

          func generate_tlsrpt() {
            for policy_domain in list_of_tlsrpt_enabled_domains {
              generate_report(policy_domain)
            }
          }

4.2.  Delivery Summary

4.2.1.  Success Count

   o  "total-successful-session-count": This indicates that the sending
      MTA was able to successfully negotiate a policy-compliant TLS
      connection, and serves to provide a "heartbeat" to receiving
      domains that reporting is functional and tabulating correctly.
      This field contains an aggregate count of successful connections
      for the reporting system.

4.2.2.  Failure Count

   o  "total-failure-session-count": This indicates that the sending MTA
      was unable to successfully establish a connection with the
      receiving platform.  Section 4.3, "Result Types", will elaborate
      on the failed negotiation attempts.  This field contains an
      aggregate count of failed connections.

4.3.  Result Types

   The list of result types will start with the minimal set below, and
   is expected to grow over time based on real-world experience.  The
   initial set is:








Margolis, et al.        Expires December 16, 2018               [Page 9]


Internet-Draft                 SMTP-TLSRPT                     June 2018


4.3.1.  Negotiation Failures

   o  "starttls-not-supported": This indicates that the recipient MX did
      not support STARTTLS.

   o  "certificate-host-mismatch": This indicates that the certificate
      presented did not adhere to the constraints specified in the MTA-
      STS or DANE policy, e.g.  if the MX hostname does not match any
      identities listed in the Subject Alternate Name (SAN) [RFC5280].

   o  "certificate-expired": This indicates that the certificate has
      expired.

   o  "certificate-not-trusted": This a label that covers multiple
      certificate related failures that include, but not limited to
      errors such as untrusted/unknown CAs, certificate name
      constraints, certificate chain errors etc.  When using this
      declaration, the reporting MTA SHOULD utilize the "failure-reason-
      code" to provide more information to the receiving entity.

   o  "validation-failure": This indicates a general failure for a
      reason not matching a category above.  When using this
      declaration, the reporting MTA SHOULD utilize the "failure-reason-
      code" to provide more information to the receiving entity.

4.3.2.  Policy Failures

4.3.2.1.  DANE-specific Policy Failures

   o  "tlsa-invalid": This indicates a validation error in the TLSA
      record associated with a DANE policy.  None of the records in the
      RRset were found to be valid.

   o  "dnssec-invalid": This would indicate that no valid records were
      returned from the recursive resolver.  The request returned with
      SERVFAIL for the requested TLSA record.

   o  "dane-required": This indicates that the sending system is
      configured to require DANE TLSA records for all the MX hosts of
      the destination domain, but no DNSSEC-validated TLSA records were
      present for the MX host that is the subject of the report.
      Mandatory DANE for SMTP is described in section 6 of [RFC7672].
      Such policies may be created by mutual agreement between two
      organizations that frequently exchange sensitive content via
      email.






Margolis, et al.        Expires December 16, 2018              [Page 10]


Internet-Draft                 SMTP-TLSRPT                     June 2018


4.3.2.2.  MTA-STS-specific Policy Failures

   o  "sts-policy-invalid": This indicates a validation error for the
      overall MTA-STS policy.

   o  "sts-webpki-invalid": This indicates that the MTA-STS policy could
      not be authenticated using PKIX validation.

4.3.3.  General Failures

   When a negotiation failure can not be categorized into one of the
   "Negotiation Failures" stated above, the reporter SHOULD use the
   "validation-failure" category.  As TLS grows and becomes more
   complex, new mechanisms may not be easily categorized.  This allows
   for a generic feedback category.  When this category is used, the
   reporter SHOULD also use the "failure-reason-code" to give some
   feedback to the receiving entity.  This is intended to be a short
   text field, and the contents of the field should be an error code or
   error text, such as "X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION".

4.3.4.  Transient Failures

   Transient errors due to too-busy network, TCP timeouts, etc. are not
   required to be reported.

4.4.  JSON Report Schema

   The JSON schema is derived from the HPKP JSON schema [RFC7469] (cf.
   Section 3)






















Margolis, et al.        Expires December 16, 2018              [Page 11]


Internet-Draft                 SMTP-TLSRPT                     June 2018


 {
   "organization-name": organization-name,
   "date-range": {
     "start-datetime": date-time,
     "end-datetime": date-time
   },
   "contact-info": email-address,
   "report-id": report-id,
   "policies": [{
     "policy": {
       "policy-type": policy-type,
       "policy-string": policy-string,
       "policy-domain": domain,
       "mx-host": mx-host-pattern
     },
     "summary": {
       "total-successful-session-count": total-successful-session-count,
       "total-failure-session-count": total-failure-session-count
     },
     "failure-details": [
       {
         "result-type": result-type,
         "sending-mta-ip": ip-address,
         "receiving-mx-hostname": receiving-mx-hostname,
         "receiving-mx-helo": receiving-mx-helo,
         "receiving-ip": receiving-ip,
         "failed-session-count": failed-session-count,
         "additional-information": additional-info-uri,
         "failure-reason-code": failure-reason-code
         }
       ]
     }
   ]
 }


                            JSON Report Format

   o  "organization-name": The name of the organization responsible for
      the report.  It is provided as a string.

   o  "date-time": The date-time indicates the start- and end-times for
      the report range.  It is provided as a string formatted according
      to Section 5.6, "Internet Date/Time Format", of [RFC3339].  The
      report should be for a full UTC day, 0000-2400.






Margolis, et al.        Expires December 16, 2018              [Page 12]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   o  "email-address": The contact information for a responsible party
      of the report.  It is provided as a string formatted according to
      Section 3.4.1, "Addr-Spec", of [RFC5321].

   o  "report-id": A unique identifier for the report.  Report authors
      may use whatever scheme they prefer to generate a unique
      identifier.  It is provided as a string.

   o  "policy-type": The type of policy that was applied by the sending
      domain.  Presently, the only three valid choices are "tlsa",
      "sts", and the literal string "no-policy-found".  It is provided
      as a string.

   o  "policy-string": An encoding of the applied policy as a JSON array
      of strings, whether TLSA record ([RFC6698] section 2.3) or MTA-STS
      policy.  Examples follow in the next section.

   o  "domain": The Policy Domain is the domain against which the MTA-
      STS or DANE policy is defined.  In the case of Internationalized
      Domain Names ([RFC5891]), the domain MUST consist of the Punycode-
      encoded A-labels ([RFC3492]) and not the U-labels.

   o  "mx-host-pattern": The pattern of MX hostnames from the applied
      policy.  It is provided as a string, and is interpreted in the
      same manner as the "Checking of Wildcard Certificates" rules in
      Section 6.4.3 of [RFC6125].  In the case of Internationalized
      Domain Names ([RFC5891]), the domain MUST consist of the Punycode-
      encoded A-labels ([RFC3492]) and not the U-labels.

   o  "result-type": A value from Section 4.3, "Result Types", above.

   o  "ip-address": The IP address of the sending MTA that attempted the
      STARTTLS connection.  It is provided as a string representation of
      an IPv4 (see below) or IPv6 ([RFC5952]) address in dot-decimal or
      colon-hexadecimal notation.

   o  "receiving-mx-hostname": The hostname of the receiving MTA MX
      record with which the sending MTA attempted to negotiate a
      STARTTLS connection.

   o  "receiving-mx-helo": (optional) The HELO or EHLO string from the
      banner announced during the reported session.

   o  "receiving-ip": The destination IP address that was using when
      creating the outbound session.  It is provided as a string
      representation of an IPv4 (see below) or IPv6 ([RFC5952]) address
      in dot-decimal or colon-hexadecimal notation.




Margolis, et al.        Expires December 16, 2018              [Page 13]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   o  "total-successful-session-count": The aggregate count (integer,
      encoded as a JSON number) of successfully negotiated TLS-enabled
      connections to the receiving site.

   o  "total-failure-session-count": The aggregate count (integer,
      encoded as a JSON number) of failures to negotiate a TLS-enabled
      connection to the receiving site.

   o  "failed-session-count": The number of (attempted) sessions that
      match the relevant "result-type" for this section (integer,
      encoded as a JSON number).

   o  "additional-info-uri": An optional URI [RFC3986] pointing to
      additional information around the relevant "result-type".  For
      example, this URI might host the complete certificate chain
      presented during an attempted STARTTLS session.

   o  "failure-reason-code": A text field to include a TLS-related error
      code or error message.

   For report purposes, an IPv4 Address is defined via the following
   ABNF:

     IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
     dec-octet     = DIGIT                 ; 0-9
                   / %x31-39 DIGIT         ; 10-99
                   / "1" 2DIGIT            ; 100-199
                   / "2" %x30-34 DIGIT     ; 200-249
                   / "25" %x30-35          ; 250-255

4.5.  Policy Samples

   Part of the report body includes the policy that is applied when
   attemping relay to the destination.

   For DANE TLSA policies, this is a JSON array of strings each
   representing the RDATA of a single TLSA resource record as a space-
   separated list of its four TLSA fields; the fields are in
   presentation format (defined in [RFC6698] Section 2.2) with no
   internal spaces or grouping parentheses:

[
"3 0 1 1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513D747D1D085D",
"3 0 1 12350A337E6DB9C6123522D136A475638CC43E1ED424F8EEC8513D747D1D1234"
]

   For MTA-STS policies, this is an array of JSON strings that
   represents the policy that is declared by the receiving site,



Margolis, et al.        Expires December 16, 2018              [Page 14]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   including any errors that may be present.  Note that where there are
   multiple "mx" values, they must be listed as separate "mx" elements
   in the policy array, rather as a single nested "mx" sub-array.

                       [
                       "version: STSv1",
                       "mode: testing",
                       "mx: mx1.example.com",
                       "mx: mx2.example.com",
                       "mx: mx.backup-example.com",
                       "max_age: 604800"
                       ]

5.  Report Delivery

   Reports can be delivered either as an email message via SMTP or via
   HTTP POST.

5.1.  Report Filename

   The filename is RECOMMENDED to be constructed using the following
   ABNF:

    filename        = sender "!" policy-domain "!" begin-timestamp
                      "!" end-timestamp [ "!" unique-id ] "." extension

    unique-id       = 1*(ALPHA / DIGIT)

    sender          = domain ; From the [RFC5321] that is used
                      ; as the domain for the `contact-info`
                      ; address in the report body

    policy-domain   = domain

    begin-timestamp = 1*DIGIT
                      ; seconds since 00:00:00 UTC January 1, 1970
                      ; indicating start of the time range contained
                      ; in the report

    end-timestamp   = 1*DIGIT
                      ; seconds since 00:00:00 UTC January 1, 1970
                      ; indicating end of the time range contained
                      ; in the report

    extension       = "json" / "json.gz"

   The extension MUST be "json" for a plain JSON file, or "json.gz" for
   a JSON file compressed using GZIP.



Margolis, et al.        Expires December 16, 2018              [Page 15]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   "unique-id" allows an optional unique ID generated by the Sending MTA
   to distinguish among multiple reports generated simultaneously by
   different sources within the same Policy Domain.  For example, this
   is a possible filename for a compressed report to the Policy Domain
   "example.net" from the Sending MTA "mail.sndr.example.com":

   "mail.sndr.example.com!example.net!1470013207!1470186007!001.json.gz"

5.2.  Compression

   The report SHOULD be subjected to GZIP [RFC1952] compression for both
   email and HTTPS transport.  Declining to apply compression can cause
   the report to be too large for a receiver to process (a commonly
   observed receiver limit is ten megabytes); compressing the file
   increases the chances of acceptance of the report at some compute
   cost.

5.3.  Email Transport

   The report MAY be delivered by email.  To make the reports machine-
   parsable for the receivers, we define a top-level media type
   "multipart/report" with a new parameter "report-type="tlsrpt"".
   Inside it, there are two parts: The first part is human readable,
   typically "text/plain", and the second part is machine readable with
   a new media type defined called "application/tlsrpt+json".  If
   compressed, the report should use the media type "application/
   tlsrpt+gzip".

   In addition, the following two new top level message header fields
   are defined:

   "TLS-Report-Domain: Receiver-Domain"

   "TLS-Report-Submitter: Sender-Domain"

   The "TLS-Report-Submitter" value MUST match the value found in the
   [RFC5321] domain from the "contact-info" from the report body.  These
   message headers MUST be included and should allow for easy searching
   for all reports submitted by a report domain or a particular
   submitter, for example in IMAP [RFC3501]:

   "s SEARCH HEADER "TLS-Report-Domain" "example.com""

   It is presumed that the aggregate reporting address will be equipped
   to process new message header fields and extract MIME parts with the
   prescribed media type and filename, and ignore the rest.  These
   additional headers SHOULD be included in the DKIM [RFC6376] signature
   for the message.



Margolis, et al.        Expires December 16, 2018              [Page 16]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   The [RFC5322].Subject field for report submissions SHOULD conform to
   the following ABNF:

       tlsrpt-subject = %s"Report" FWS               ; "Report"
                        %s"Domain:" FWS              ; "Domain:"
                        domain-name FWS              ; per [RFC6376]
                        %s"Submitter:" FWS           ; "Submitter:"
                        domain-name FWS              ; per [RFC6376]
                        %s"Report-ID:" FWS           ; "Report-ID:
                        "<" id-left "@" id-right ">" ; per [RFC5322]
                        [CFWS]                       ; per [RFC5322]
                                                     ; (as with FWS)

   The first domain-name indicates the DNS domain name about which the
   report was generated.  The second domain-name indicates the DNS
   domain name representing the Sending MTA generating the report.  The
   purpose of the Report-ID: portion of the field is to enable the
   Policy Domain to identify and ignore duplicate reports that might be
   sent by a Sending MTA.

   For instance, this is a possible Subject field for a report to the
   Policy Domain "example.net" from the Sending MTA
   "mail.sender.example.com".  It is line-wrapped as allowed by
   [RFC5322]:

              Subject: Report Domain: example.net
                  Submitter: mail.sender.example.com
                  Report-ID: <735ff.e317+bf22029@mailexample.net>

5.3.1.  Example Report





















Margolis, et al.        Expires December 16, 2018              [Page 17]


Internet-Draft                 SMTP-TLSRPT                     June 2018


      From: tlsrpt@mail.sender.example.com
          Date: Fri, May 09 2017 16:54:30 -0800
          To: mts-sts-tlsrpt@example.net
          Subject: Report Domain: example.net
              Submitter: mail.sender.example.com
              Report-ID: <735ff.e317+bf22029@example.net>
          TLS-Report-Domain: example.net
          TLS-Report-Submitter: mail.sender.example.com
          MIME-Version: 1.0
          Content-Type: multipart/report; report-type="tlsrpt";
              boundary="----=_NextPart_000_024E_01CC9B0A.AFE54C00"
          Content-Language: en-us

          This is a multipart message in MIME format.

          ------=_NextPart_000_024E_01CC9B0A.AFE54C00
          Content-Type: text/plain; charset="us-ascii"
          Content-Transfer-Encoding: 7bit

          This is an aggregate TLS report from mail.sender.example.com

          ------=_NextPart_000_024E_01CC9B0A.AFE54C00
          Content-Type: application/tlsrpt+gzip
          Content-Transfer-Encoding: base64
          Content-Disposition: attachment;
              filename="mail.sender.example!example.com!
                        1013662812!1013749130.json.gz"

          <gzipped content of report>

     ------=_NextPart_000_024E_01CC9B0A.AFE54C00--
     ...

   Note that, when sending failure reports via SMTP, sending MTAs MUST
   NOT honor MTA-STS or DANE TLSA failures.

5.4.  HTTPS Transport

   The report MAY be delivered by POST to HTTPS.  If compressed, the
   report SHOULD use the media type "application/tlsrpt+gzip", and
   "application/tlsrpt+json" otherwise (see section Section 6, "IANA
   Considerations").

   The receiving system MUST return a "successful" response from its
   HTTPS server, typically a 200 or 201 HTTP code [RFC7321].  Other
   codes could indicate a delivery failure, and may be retried as per
   local sender policy.  The receiving system is not expected to process




Margolis, et al.        Expires December 16, 2018              [Page 18]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   reports at receipt time, and MAY store them for processing at a later
   time.

5.5.  Delivery Retry

   In the event of a delivery failure, regardless of the delivery
   method, a sender SHOULD attempt redelivery for up to 24hrs after the
   initial attempt.  As previously stated the reports are optional, so
   while it is ideal to attempt redelivery, it is not required.  If
   multiple retries are attempted, ideally they SHOULD be done with
   exponential backoff.

5.6.  Metadata Variances

   As stated above, there are a variable number of ways to declare
   information about the data therein.  If any of items declared via
   subject or filename disagree with the report, the report MUST be
   considered the authoritative source.

6.  IANA Considerations

   The following are the IANA considerations discussed in this document.

6.1.  Message headers

   Below is the Internet Assigned Numbers Authority (IANA) Permanent
   Message Header Field registration information per [RFC3864].

             Header field name:           TLS-Report-Domain
             Applicable protocol:         mail
             Status:                      standard
             Author/Change controller:    IETF
             Specification document(s):   this one


             Header field name:           TLS-Report-Submitter
             Applicable protocol:         mail
             Status:                      standard
             Author/Change controller:    IETF
             Specification document(s):   this one

6.2.  Report Type

   This document creates a new registry for "report-type" parameter to
   the Content-Type header field for the "multipart/report" top-level
   media type defined in [RFC6522].





Margolis, et al.        Expires December 16, 2018              [Page 19]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   The registry name is "Report Type Registry", and the procedure for
   updating the registry will be "Specification Required".

   An entry in this registry should contain:

   o  the report-type being registered

   o  one or more registered media-types that can be used with this
      report-type

   o  the document containing the registration action

   o  an optional comment

   The initial entries are:

   Report-Type: tlsrpt Media Type: application/tlsrpt+gzip, application/
   tlsrpt+json Registered By: [RFCXXXX] Comment: Media types suitable
   for use with this report-type are defined in Sections 6.4 and 6.5 of
   [RFCXXXX]

   Report-Type: disposition-notification Media Type: message/
   disposition-notification Registered By: [RFC8098] Section 10

   Report-Type: disposition-notification Media Type: message/global-
   disposition-notification Registered By: [RFC6533] Section 6

   Report-Type: delivery-status Media Type: message/delivery-status
   Registered By: [RFC3464] Appendix D

   Report-Type: delivery-status Media Type: message/global-delivery-
   status Registered By: [RFC6533] Section 6

6.3.  +gzip Media Type Suffix

   This document registers a new media type suffix "+gzip".  The GZIP
   format is a public domain, cross-platform, interoperable file storage
   and transfer format, specified in [RFC1952]; it supports compression
   and is used as the underlying representation by a variety of file
   formats.  The media type "application/gzip" has been registered for
   such files.  The suffix "+gzip" MAY be used with any media type whose
   representation follows that established for "application/gzip".  The
   media type structured syntax suffix registration form follows:

   Type name: GZIP file storage and transfer format

   +suffix: +gzip




Margolis, et al.        Expires December 16, 2018              [Page 20]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   References: [RFC1952][RFC6713]

   Encoding considerations: GZIP is a binary encoding.

   Fragment identifier considerations: The syntax and semantics of
   fragment identifiers specified for +gzip SHOULD be as specified for
   "application/gzip".  (At publication of this document, there is no
   fragment identification syntax defined for "application/gzip".)  The
   syntax and semantics for fragment identifiers for a specific "xxx/
   yyy+gzip" SHOULD be processed as follows:

       For cases defined in +gzip, where the fragment identifier
       resolves per the +gzip rules, then process as specified in
       +gzip.

       For cases defined in +gzip, where the fragment identifier does
       not resolve per the +gzip rules, then process as specified in
       "xxx/yyy+gzip".

       For cases not defined in +gzip, then process as specified in
       "xxx/yyy+gzip".

   Interoperability considerations: n/a

   Security considerations: GZIP format doesn't provide confidentiality
   protection.  Integrity protection is provided by and Adler-32
   checksum, which is not cryptographically strong.  See also security
   considerations of [RFC6713].  Each individual media type registered
   with a +gzip suffix can have additional security considerations.
   Additionally, GZIP objects can contain multiple files and associated
   paths.  File paths must be validated when the files are extracted; a
   malicious file path could otherwise cause the extractor to overwrite
   application or system files.

   Contact: art@ietf.org

   Author/Change controller: Internet Engineering Task Force
   (mailto:iesg@ietf.org).

6.4.  application/tlsrpt+json Media Type

   This document registers multiple media types, beginning with Table 1
   below.








Margolis, et al.        Expires December 16, 2018              [Page 21]


Internet-Draft                 SMTP-TLSRPT                     June 2018


    +-------------+----------------+-------------+-------------------+
    | Type        | Subtype        | File extn   | Specification     |
    +-------------+----------------+-------------+-------------------+
    | application | tlsrpt+json    |  .json      | Section 5.3       |
    +-------------+----------------+-------------+-------------------+
                    Table 1: SMTP TLS Reporting Media Type

   Type name: application

   Subtype name: tlsrpt+json

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations: Encoding considerations are identical to
   those specified for the "application/json" media type.  See
   [RFC7493].

   Security considerations: Security considerations relating to SMTP TLS
   Reporting are discussed in Section 7.

   Interoperability considerations: This document specifies format of
   conforming messages and the interpretation thereof.

   Published specification: Section 5.3 of this document.

   Applications that use this media type: Mail User Agents (MUA) and
   Mail Transfer Agents.

   Additional information:

                      Magic number(s):  n/a

                      File extension(s):  ".json"

                      Macintosh file type code(s):  n/a

   Person & email address to contact for further information: See
   Authors' Addresses section.

   Intended usage: COMMON

   Restrictions on usage: n/a

   Author: See Authors' Addresses section.





Margolis, et al.        Expires December 16, 2018              [Page 22]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   Change controller: Internet Engineering Task Force
   (mailto:iesg@ietf.org).

6.5.  application/tlsrpt+gzip Media Type

    +-------------+----------------+-------------+-------------------+
    | Type        | Subtype        | File extn   | Specification     |
    +-------------+----------------+-------------+-------------------+
    | application | tlsrpt+gzip    |  .gz        | Section 5.3       |
    +-------------+----------------+-------------+-------------------+
                    Table 2: SMTP TLS Reporting Media Type

   Type name: application

   Subtype name: tlsrpt+gzip

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations: Binary

   Security considerations: Security considerations relating to SMTP TLS
   Reporting are discussed in Section 7.  Security considerations
   related to gzip compression are discussed in [RFC6713].

   Interoperability considerations: This document specifies format of
   conforming messages and the interpretation thereof.

   Published specification: Section 5.3 of this document.

   Applications that use this media type: Mail User Agents (MUA) and
   Mail Transfer Agents.

   Additional information:

            Magic number(s):  The first two bytes are 0x1f, 0x8b.

            File extension(s):  ".gz"

            Macintosh file type code(s):  n/a

   Person & email address to contact for further information: See
   Authors' Addresses section.

   Intended usage: COMMON

   Restrictions on usage: n/a



Margolis, et al.        Expires December 16, 2018              [Page 23]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   Author: See Authors' Addresses section.

   Change controller: Internet Engineering Task Force
   (mailto:iesg@ietf.org).

6.6.  STARTTLS Validation Result Types

   This document creates a new registry, "STARTTLS Validation Result
   Types".  The initial entries in the registry are:

               +-------------------------------+-----------+
               | Result Type                   |   Desc    |
               +-------------------------------+-----------+
               | "starttls-not-supported"      |    4.3    |
               | "certificate-host-mismatch"   |    4.3    |
               | "certificate-expired"         |    4.3    |
               | "tlsa-invalid"                |    4.3    |
               | "dnssec-invalid"              |    4.3    |
               | "dane-required"               |    4.3    |
               | "certificate-not-trusted"     |    4.3    |
               | "sts-policy-invalid"          |    4.3    |
               | "sts-webpki-invalid"          |    4.3    |
               | "validation-failure"          |    4.3    |
               +-------------------------------+-----------+

   The above entries are described in section Section 4.3, "Result
   Types."  New result types can be added to this registry using "Expert
   Review" IANA registration policy.

7.  Security Considerations

   SMTP TLS Reporting provides transparency into misconfigurations or
   attempts to intercept or tamper with mail between hosts who support
   STARTTLS.  There are several security risks presented by the
   existence of this reporting channel:

   o  Flooding of the Aggregate report URI (rua) endpoint: An attacker
      could flood the endpoint with excessive reporting traffic and
      prevent the receiving domain from accepting additional reports.
      This type of Denial-of-Service attack would limit visibility into
      STARTTLS failures, leaving the receiving domain blind to an
      ongoing attack.

   o  Untrusted content: An attacker could inject malicious code into
      the report, opening a vulnerability in the receiving domain.
      Implementers are advised to take precautions against evaluating
      the contents of the report.




Margolis, et al.        Expires December 16, 2018              [Page 24]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   o  Report snooping: An attacker could create a bogus TLSRPT record to
      receive statistics about a domain the attacker does not own.
      Since an attacker able to poison DNS is already able to receive
      counts of SMTP connections (and, absent DANE or MTA-STS policies,
      actual SMTP message payloads), this does not present a significant
      new vulnerability.

   o  Ignoring HTTPS validation when submitting reports: When reporting
      benign misconfigurations, it is likely that a misconfigured SMTP
      server may also mean a misconfigured HTTPS server; as a result,
      reporters who required HTTPS validity on the reporting endpoint
      may fail to alert administrators about such misconfigurations.
      Conversely, in the event of an actual attack, an attacker who
      wished to create a gap in reporting and could intercept HTTPS
      reports could, just as easily, simply thwart the resolution of the
      TLSRPT TXT record or establishment of the TCP session to the HTTPS
      endpoint.  Furthermore, such a man-in-the-middle attacker could
      discover most or all of the metadata exposed in a report merely
      through passive observation.  As a result, we consider the risks
      of failure to deliver reports on misconfigurations to outweigh
      those of attackers intercepting reports.

   o  Reports as DDoS: TLSRPT allows specifying destinations for the
      reports that are outside the authority of the Policy Domain, which
      allows domains to delegate processing of reports to a partner
      organization.  However, an attacker who controls the Policy Domain
      DNS could also use this mechanism to direct the reports to an
      unwitting victim, flooding that victim with excessive reports.
      DMARC [RFC7489] defines a solution for verifying delegation to
      avoid such attacks; the need for this is greater with DMARC,
      however, because DMARC allows an attacker to trigger reports to a
      target from an innocent third party by sending that third party
      mail (which triggers a report from the third party to the target).
      In the case of TLSRPT, the attacker would have to induce the third
      party to send the attacker mail in order to trigger reports from
      the third party to the victim; this reduces the risk of such an
      attack and the need for a verification mechanism.

   Finally, because TLSRPT is intended to help administrators discover
   man-in-the-middle attacks against transport-layer encryption,
   including attacks designed to thwart negotiation of encrypted
   connections (by downgrading opportunistic encryption or, in the case
   of MTA-STS, preventing discovery of a new MTA-STS policy), we must
   also consider the risk that an adversary who can induce such a
   downgrade attack can also prevent discovery of the TLSRPT TXT record
   (and thus prevent discovery of the successful downgrade attack).
   Administrators are thus encouraged to deploy TLSRPT TXT records with
   a large TTL (reducing the window for successful application of



Margolis, et al.        Expires December 16, 2018              [Page 25]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   transient attacks against DNS resolution of the record) or to deploy
   DNSSEC on the deploying zone.

8.  Privacy Considerations

   MTAs are generally considered public knowledge, however, the
   internals of how those MTAs are configured and the users of those
   MTAs may not be as public.  It should be noted that when providing a
   receiving site with information, it may reveal information about the
   sender's configuration, or even information about the senders
   themselves.  Consider that by sending a report, it might disclose
   your SSL library version as the inability to negotiate a session may
   be a known incompatbility between two library versions, or perhaps
   commonly used in a operating system release that is centered in a
   certain region.  The risk may be minimal, but should be considered.

9.  References

9.1.  Normative References

   [I-D.ietf-uta-mta-sts]
              Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", draft-ietf-uta-mta-sts-19 (work in progress), May
              2018.

   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
              RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
              <https://www.rfc-editor.org/info/rfc3492>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.



Margolis, et al.        Expires December 16, 2018              [Page 26]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, DOI 10.17487/RFC4408, April 2006,
              <https://www.rfc-editor.org/info/rfc4408>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008, <https://www.rfc-
              editor.org/info/rfc5234>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008, <https://www.rfc-
              editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008, <https://www.rfc-
              editor.org/info/rfc5322>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010, <https://www.rfc-
              editor.org/info/rfc5891>.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010, <https://www.rfc-
              editor.org/info/rfc5952>.

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <https://www.rfc-editor.org/info/rfc6068>.

   [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, <https://www.rfc-editor.org/info/rfc6125>.







Margolis, et al.        Expires December 16, 2018              [Page 27]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6522]  Kucherawy, M., Ed., "The Multipart/Report Media Type for
              the Reporting of Mail System Administrative Messages",
              STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
              <https://www.rfc-editor.org/info/rfc6522>.

   [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, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC6713]  Levine, J., "The 'application/zlib' and 'application/gzip'
              Media Types", RFC 6713, DOI 10.17487/RFC6713, August 2012,
              <https://www.rfc-editor.org/info/rfc6713>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
              editor.org/info/rfc7231>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015, <https://www.rfc-
              editor.org/info/rfc7493>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [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>.




Margolis, et al.        Expires December 16, 2018              [Page 28]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              DOI 10.17487/RFC3464, January 2003, <https://www.rfc-
              editor.org/info/rfc3464>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004, <https://www.rfc-
              editor.org/info/rfc3864>.

   [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov,
              "Internationalized Delivery Status and Disposition
              Notifications", RFC 6533, DOI 10.17487/RFC6533, February
              2012, <https://www.rfc-editor.org/info/rfc6533>.

   [RFC7321]  McGrew, D. and P. Hoffman, "Cryptographic Algorithm
              Implementation Requirements and Usage Guidance for
              Encapsulating Security Payload (ESP) and Authentication
              Header (AH)", RFC 7321, DOI 10.17487/RFC7321, August 2014,
              <https://www.rfc-editor.org/info/rfc7321>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <https://www.rfc-editor.org/info/rfc7469>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [RFC8098]  Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition
              Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098,
              February 2017, <https://www.rfc-editor.org/info/rfc8098>.

9.3.  URIs

   [1] Section 2.2.3

   [2] Section 3




Margolis, et al.        Expires December 16, 2018              [Page 29]


Internet-Draft                 SMTP-TLSRPT                     June 2018


Appendix A.  Example Reporting Policy

A.1.  Report using MAILTO

            _smtp._tls.mail.example.com. IN TXT \
                    "v=TLSRPTv1;rua=mailto:reports@example.com"

A.2.  Report using HTTPS

           _smtp._tls.mail.example.com. IN TXT \
                   "v=TLSRPTv1; \
                   rua=https://reporting.example.com/v1/tlsrpt"

Appendix B.  Example JSON Report

   Below is an example JSON report for messages from Company-X to
   Company-Y, where 100 sessions were attempted to Company Y servers
   with an expired certificate and 200 sessions were attempted to
   Company Y servers that did not successfully respond to the "STARTTLS"
   command.  Additionally 3 sessions failed due to
   "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED".






























Margolis, et al.        Expires December 16, 2018              [Page 30]


Internet-Draft                 SMTP-TLSRPT                     June 2018


   {
     "organization-name": "Company-X",
     "date-range": {
       "start-datetime": "2016-04-01T00:00:00Z",
       "end-datetime": "2016-04-01T23:59:59Z"
     },
     "contact-info": "sts-reporting@company-x.example",
     "report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be",
     "policies": [{
       "policy": {
         "policy-type": "sts",
         "policy-string": ["version: STSv1","mode: testing",
               "mx: *.mail.company-y.example","max_age: 86400"],
         "policy-domain": "company-y.example",
         "mx-host": "*.mail.company-y.example"
       },
       "summary": {
         "total-successful-session-count": 5326,
         "total-failure-session-count": 303
       },
       "failure-details": [{
         "result-type": "certificate-expired",
         "sending-mta-ip": "2001:db8:abcd:0012::1",
         "receiving-mx-hostname": "mx1.mail.company-y.example",
         "failed-session-count": 100
       }, {
         "result-type": "starttls-not-supported",
         "sending-mta-ip": "2001:db8:abcd:0013::1",
         "receiving-mx-hostname": "mx2.mail.company-y.example",
         "receiving-ip": "203.0.113.56",
         "failed-session-count": 200,
         "additional-information": "https://reports.company-x.example/
           report_info ? id = 5065427 c - 23 d3# StarttlsNotSupported "
       }, {
         "result-type": "validation-failure",
         "sending-mta-ip": "198.51.100.62",
         "receiving-ip": "203.0.113.58",
         "receiving-mx-hostname": "mx-backup.mail.company-y.example",
         "failed-session-count": 3,
         "failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
       }]
     }]
   }








Margolis, et al.        Expires December 16, 2018              [Page 31]


Internet-Draft                 SMTP-TLSRPT                     June 2018


Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis@google.com


   Alexander Brotman
   Comcast, Inc

   Email: alex_brotman@comcast.com


   Binu Ramakrishnan
   Yahoo!, Inc

   Email: rbinu@oath.com


   Janet Jones
   Microsoft, Inc

   Email: janet.jones@microsoft.com


   Mark Risher
   Google, Inc

   Email: risher@google.com





















Margolis, et al.        Expires December 16, 2018              [Page 32]


Html markup produced by rfcmarkup 1.127, available from https://tools.ietf.org/tools/rfcmarkup/