SMTP TLS Reporting
draft-ietf-uta-smtp-tlsrpt-13

Document Type Active Internet-Draft (uta WG)
Last updated 2017-12-04
Replaces draft-brotman-smtp-tlsrpt
Stream IETF
Intended RFC status Proposed Standard
Formats plain text xml pdf html bibtex
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Leif Johansson
IESG IESG state I-D Exists
Consensus Boilerplate Yes
Telechat date
Responsible AD (None)
Send notices to Leif Johansson <leifj@sunet.se>
Using TLS in Applications                                    D. Margolis
Internet-Draft                                               Google, Inc
Intended status: Standards Track                              A. Brotman
Expires: June 7, 2018                                       Comcast, Inc
                                                         B. Ramakrishnan
                                                             Yahoo!, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                               M. Risher
                                                             Google, Inc
                                                        December 4, 2017

                           SMTP TLS Reporting
                     draft-ietf-uta-smtp-tlsrpt-13

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 attackers 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 https://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 June 7, 2018.

Margolis, et al.          Expires June 7, 2018                  [Page 1]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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
   (https://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 . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   4
   3.  Reporting Policy  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Example Reporting Policy  . . . . . . . . . . . . . . . .   6
       3.1.1.  Report using MAILTO . . . . . . . . . . . . . . . . .   6
       3.1.2.  Report using HTTPS  . . . . . . . . . . . . . . . . .   6
   4.  Reporting Schema  . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Report Time-frame . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Delivery Summary  . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Success Count . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Failure Count . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Result Types  . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.1.  Negotiation Failures  . . . . . . . . . . . . . . . .   8
       4.3.2.  Policy Failures . . . . . . . . . . . . . . . . . . .   8
       4.3.3.  General Failures  . . . . . . . . . . . . . . . . . .   9
       4.3.4.  Transient Failures  . . . . . . . . . . . . . . . . .   9
     4.4.  JSON Report Schema  . . . . . . . . . . . . . . . . . . .   9
   5.  Report Delivery . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Report Filename . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Email Transport . . . . . . . . . . . . . . . . . . . . .  13
       5.3.1.  Example Report  . . . . . . . . . . . . . . . . . . .  15
     5.4.  HTTPS Transport . . . . . . . . . . . . . . . . . . . . .  16
     5.5.  Delivery Retry  . . . . . . . . . . . . . . . . . . . . .  16
     5.6.  Metadata Variances  . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Message headers . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Report Type . . . . . . . . . . . . . . . . . . . . . . .  17
     6.3.  application/tlsrpt+json Media Type  . . . . . . . . . . .  17
     6.4.  application/tlsrpt+gzip Media Type  . . . . . . . . . . .  18

Margolis, et al.          Expires June 7, 2018                  [Page 2]
Internet-Draft                 SMTP-TLSRPT                 December 2017

     6.5.  STARTTLS Validation Result Types  . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Example Reporting Policy . . . . . . . . . . . . . .  24
     A.1.  Report using MAILTO . . . . . . . . . . . . . . . . . . .  24
     A.2.  Report using HTTPS  . . . . . . . . . . . . . . . . . . .  24
   Appendix B.  Example JSON Report  . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to establish secure SMTP sessions over TLS.  The protocol
   design is based on "Opportunistic Security" (OS) [RFC7435], which
   maintains interoperability with clients that do not support STARTTLS
   but means that any attacker who can delete 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) can perform a downgrade or interception attack.

   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, 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.

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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Margolis, et al.          Expires June 7, 2018                  [Page 3]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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

   o  MTA-STS Policy: A definition of the expected TLS availability,
      behavior, and desired actions for a given domain when a sending
      MTA encounters problems in negotiating a secure channel.  MTA-STS
      is defined in [I-D.ietf-uta-mta-sts].

   o  DANE Policy: A mechanism by which administrators can supply a
      record that can be used to validate the certificate presented by
      an MTA.  DANE is defined in [RFC6698].

   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.

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

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
      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-tlsrpt".  For example, for the Policy
   Domain "example.com", the recipient's TLSRPT policy can be retrieved
   from "_smtp-tlsrpt.example.com".

   Policies consist of the following directives:

   o  "v": This value MUST be equal to "TLSRPTv1".

   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.

Margolis, et al.          Expires June 7, 2018                  [Page 4]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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.  This may mean that the reports are delivered in the
      clear.  Additionally, 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 formal definition of the "_smtp-tlsrpt" TXT record, defined using
   [RFC5234] & [RFC7405], is as follows:

        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) and exclamation
              ; points (ASCII 0x21) 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-tlsrpt" are returned by the
   resolver, records which do not begin with "v=TLSRPTv1;" are

Margolis, et al.          Expires June 7, 2018                  [Page 5]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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, then the record MUST
   be treated as if those strings are concatenated together without
   adding spaces.

   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.

3.1.  Example Reporting Policy

3.1.1.  Report using MAILTO

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

3.1.2.  Report using HTTPS

           _smtp-tlsrpt.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

Margolis, et al.          Expires June 7, 2018                  [Page 6]
Internet-Draft                 SMTP-TLSRPT                 December 2017

         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

      *  An identifier for the policy (where applicable)

   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.

   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); 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.

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.

4.2.  Delivery Summary

4.2.1.  Success Count

   o  "success-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  "failure-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.

Margolis, et al.          Expires June 7, 2018                  [Page 7]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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:

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 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"
      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"
      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.  It should be noted that
      if the reporter's systems are having problems resolving
      destination DNS records due to DNSSEC failures, it's possible they
      will also be unable to resolve the TLSRPT record, therefore these
      types of reports may be rare.

Margolis, et al.          Expires June 7, 2018                  [Page 8]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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 June 7, 2018                  [Page 9]
Internet-Draft                 SMTP-TLSRPT                 December 2017

 {
   "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,
         "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.

   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].

Margolis, et al.          Expires June 7, 2018                 [Page 10]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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": A string representation of the policy, whether
      TLSA record ([RFC6698] section 2.3) or MTA-STS policy.  Examples:
      TLSA: ""_25._tcp.mx.example.com.  IN TLSA ( 3 0 1 \
      1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513D7 47D1D085D
      )"" MTA-STS: ""version: STSv1\nmode: report\nmx:
      mx1.example.com\nmx: \ mx2.example.com\nmx: mx.backup-
      example.com\nmax_age: 12345678""

   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 is the Punycode-encoded
      A-label ([RFC3492]) and not the U-label.

   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 is the Punycode-encoded
      A-label ([RFC3492]) and not the U-label.

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

Margolis, et al.          Expires June 7, 2018                 [Page 11]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   o  "total-failure-session-count": The aggregate number (integer) 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.

   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 as: 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

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:

Margolis, et al.          Expires June 7, 2018                 [Page 12]
Internet-Draft                 SMTP-TLSRPT                 December 2017

       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.

   "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 the gzip file of a report to the Policy
   Domain "example.net" from the Sending MTA "mail.sender.example.com":

   "mail.sender.example.com!example.net!1470013207!1470186007!001.json.g
   z"

5.2.  Compression

   The report SHOULD be subjected to GZIP 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"".

Margolis, et al.          Expires June 7, 2018                 [Page 13]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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
   filename and 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.

   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

Margolis, et al.          Expires June 7, 2018                 [Page 14]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   "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

      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.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.

Margolis, et al.          Expires June 7, 2018                 [Page 15]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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").

   A reporting entity SHOULD expect a "successful" response from the
   accepting HTTPS server, typically a 200 or 201 HTTP code [RFC7231].
   Other codes could indicate a delivery failure, and may be retried as
   per local policy.  The receiving system is not expected to process
   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 would be on a
   logarithmic scale.

5.6.  Metadata Variances

   As stated above, there are a variable number of ways to declare
   information about the data therein.  If it should be the case that
   these objects were to disagree, then the report data contained within
   the JSON body MUST be considered the authoritative source for those
   data elements.

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].

Margolis, et al.          Expires June 7, 2018                 [Page 16]
Internet-Draft                 SMTP-TLSRPT                 December 2017

             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 registers a new parameter "report-type="tlsrpt"" under
   "multipart/report" top-level media type for use with [RFC6522].

   The media type suitable for use as a report-type is defined in the
   following section.

6.3.  application/tlsrpt+json Media Type

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

    +-------------+----------------+-------------+-------------------+
    | 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.

Margolis, et al.          Expires June 7, 2018                 [Page 17]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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.

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

6.4.  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

Margolis, et al.          Expires June 7, 2018                 [Page 18]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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):  ".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

   Author: See Authors' Addresses section.

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

6.5.  STARTTLS Validation Result Types

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

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

Margolis, et al.          Expires June 7, 2018                 [Page 19]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   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.

   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  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,

Margolis, et al.          Expires June 7, 2018                 [Page 20]
Internet-Draft                 SMTP-TLSRPT                 December 2017

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

8.  References

8.1.  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-11 (work in progress),
              November 2017.

   [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>.

   [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>.

Margolis, et al.          Expires June 7, 2018                 [Page 21]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   [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>.

   [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>.

Margolis, et al.          Expires June 7, 2018                 [Page 22]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   [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>.

   [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>.

8.2.  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>.

   [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>.

   [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>.

Margolis, et al.          Expires June 7, 2018                 [Page 23]
Internet-Draft                 SMTP-TLSRPT                 December 2017

Appendix A.  Example Reporting Policy

A.1.  Report using MAILTO

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

A.2.  Report using HTTPS

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

Appendix B.  Example JSON Report

Margolis, et al.          Expires June 7, 2018                 [Page 24]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   {
     "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\r\nmode: report\r\n
                   mx: .mail.company-y.example\r\nmax_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": "98.136.216.25",
         "receiving-mx-hostname": "mx1.mail.company-y.example",
         "failed-session-count": 100
       }, {
         "result-type": "starttls-not-supported",
         "sending-mta-ip": "98.22.33.99",
         "receiving-mx-hostname": "mx2.mail.company-y.example",
         "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": "47.97.15.2",
         "receiving-mx-hostname": "mx-backup.mail.company-y.example",
         "failed-session-count": 3,
         "failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
       }]
     }]
   }

   Figure: Example JSON report for a 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.

Margolis, et al.          Expires June 7, 2018                 [Page 25]
Internet-Draft                 SMTP-TLSRPT                 December 2017

   Additionally 3 sessions failed due to
   "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED".

Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis (at) google (dot com)

   Alexander Brotman
   Comcast, Inc

   Email: alex_brotman (at) comcast (dot com)

   Binu Ramakrishnan
   Yahoo!, Inc

   Email: rbinu (at) yahoo-inc (dot com)

   Janet Jones
   Microsoft, Inc

   Email: janet.jones (at) microsoft (dot com)

   Mark Risher
   Google, Inc

   Email: risher (at) google (dot com)

Margolis, et al.          Expires June 7, 2018                 [Page 26]